Synergistic aluminum dual-atom sites and nickel nanoclusters for acetylene selective hydrogenation

Domain: chemistry
Published: 2026-03-06
Editorial outcome: major_revision

Abstract
Synergistic catalysis, where distinct active species collaboratively activate different substrates, provides a powerful strategy for achieving chemical transformations with enhanced efficiency. Although Al O and bulk aluminum species are widely employed as catalyst supports, they are seldom regarded as active centers, especially in hydrogenation. Here, we show that atomically dispersed Al species can catalyze acetylene conversion at elevated temperatures. Building on this insight, we have designed a synergistic catalyst featuring precisely controlled Al dual-atom sites paired with Ni nanoclusters, synthesized via a solid-transformation-coupled gas-adsorption strategy to overcome the typical activity-selectivity trade-off. Under mild, cost-effective conditions, this catalyst achieves nearly full acetylene conversion with ~90% ethylene selectivity and excellent long-term stability. In situ spectroscopy and theoretical calculations reveal a cooperative mechanism: Ni nanoclusters efficiently dissociate H into active hydrogen species (H*), while adjacent Al dual-atom sites shuttle the H* species to π-adsorbed acetylene, lowering the energy barrier for ethylene formation compared to over-hydrogenation and coke formation.

Introduction
Selective acetylene hydrogenation is a critical process in modern polyolefin industry upgrading for ethylene purification1,2, as it removes approximately 1% acetylene impurities generated mainly by catalytic cracking3,4. Although supported Pd-based catalysts have achieved significant industrial success5-7, their practical application is limited by the suboptimal activity/selectivity trade-offs and scarcity of precious metals8,9. This drives exploration of supported non-noble metal alternatives that offer cost-effectiveness and elemental abundance, though these systems often suffer from thermodynamically favored over-hydrogenation of ethylene and undesired polymerization on metal surface10,11. Meanwhile, conventional Al O supports, while widely utilized for their low cost, mechanical robustness and chemical stability, exhibit the limitation in enhancing catalytic performance as active species. Recent advances in support engineering through atomic-level modifications have emerged as promising strategies for catalyst optimization12-15. Despite their ability to unlock novel reaction pathways and deliver exceptional catalytic performance—thanks to unique Lewis acidity modulation and surface‐reconstruction effects—atomically dispersed aluminum sites remain largely unexplored.

Recent research advancements have demonstrated that atomically precise catalysts, particularly single-atom catalysts (SAC) and dual-atom catalysts (DAC), have emerged as promising candidates for various reactions, due to their precisely tunable electronic and geometric properties16-18, diverse configurations of adsorbed species19,20, and resulted unique performances21,22. Nevertheless, atomically dispersed sites often exhibit lower catalytic activity compared to their nanoparticle counterparts in scenarios where multiple active sites are required23. Therefore, further optimization of active sites is essential to achieve both high catalytic selectivity and activity. Synergistic catalysis can facilitate reactions between two activated intermediates, driven by distinct metal active sites acting on different substrates, thereby enhancing catalytic performance24-26. Based on this, some researchers have proposed efficient synergistic catalysis systems integrating the SAC and the metal nanoparticles/nanoclusters (NPs/NCs). This demonstrates the simultaneous integration and regulation of catalytic activity and selectivity, compensating for shortcomings of one-component catalysts27-29. However, research on synergistic catalysts that combine more flexible dual-atom sites with other types of catalytic sites is scarce, like the dual-atom and nanocluster/nanoparticle synergistic system, and the controllable fabrication methods for this system are still in their infancy. Moreover, despite the potential of synergistic catalysis, its application in complex reactions remains limited due to challenges such as undesired mono-catalytic pathways generating side products and the deactivation by metal detachment or aggregation30.

Inspired by the above insight, we were initially surprised to discover that, unlike the inert Al O support, atomic-level Al sites exhibit the ability to selectively activate acetylene into ethylene, despite their limited capacity for H dissociation. To enhance both selectivity and activity, we developed a two-step synthesis strategy, solid transformation coupled with gas adsorption, to construct the synergistic catalytic sites comprising Al dual-atom sites and Ni nanoclusters (Al -Ni /NCNT). This catalyst achieves up to 90% ethylene selectivity at nearly full conversion in selective acetylene hydrogenation, even under front-end (H -rich) reaction conditions, and no visible activity decay after 100 h of long-term testing. Our findings reveal that concerted Al decorating with Ni nanoclusters efficiently facilitates cleavage of C≡C and H-H bonds while retaining the C=C bond. Through in situ characterization and theoretical calculations, we demonstrate that Ni nanoclusters enhance the hydrogen activation, whereas Al dual-atom drives acetylene adsorption, conversion and active hydrogen transfer. Additionally, this work not only provides a strategy to overcome the trade-off between activity and selectivity and stability in Ni-based catalysts but also establishes a novel concerted catalysis integrating dual-atom sites with nanoclusters.

Results
Catalyst design and structural characterization
For the first time, we synthesized an Al dual-atom catalyst via a sublimation transformation strategy to explore the potential of atomic-level Al catalysts in the selective hydrogenation of acetylene. As shown in Fig. 1a, anhydrous aluminum chloride (AlCl ) and nitrogen-doped carbon nanotube (NCNT) were sequentially placed in two porcelain boats at 180 ℃ under an argon atmosphere. This setup facilitated the generation of Al Cl vapor from the sublimation of AlCl , which was subsequently captured by NCNT. The resulting Al Cl /NCNT composite was then collected and heated to 550 ℃ under a 5% hydrogen atmosphere to remove Cl ligands from the Al sites. This process effectively stabilized the Al dimers and facilitated their bonding with the N sites of the NCNT, resulting in the formation of a well-defined Al dual-atom catalyst (Al /NCNT). With the goal of improving practical utility, Al -Ni /NCNT synergistic catalysts were further fabricated through a gas adsorption method. In this process, the pre-synthesized Al /NCNT was exposed to a NiCl vapor stream for deposition, leading to the formation of Ni nanoclusters surrounded by Al dual-atom sites. Meanwhile, the catalysts composed exclusively of Al single-atom (denoted as Al /NCNT) and Ni nanocluster (denoted as Ni /NCNT) were also prepared for comparison. Inductively coupled plasma-optical emission spectroscopy (ICP-OES, Supplementary Table 1) reveals that Ni and Al contents in Al -Ni /NCNT are 0.28 wt.% and 0.43 wt.%, comparable to Ni nanoclusters and Al single-atom counterparts, respectively. In addition to the diffraction features from the NCNT support, the X-ray diffraction (XRD) pattern of Al -Ni /NCNT exhibits the weak characteristic diffraction peaks at 44.5o and 51.8o (PDF#04-0850, Fig. 1b), similar to those observed in Ni /NCNT. In contrast, no diffraction signals attributable to Al species are detected in the Al /NCNT and Al /NCNT samples (Fig. 1b). As shown in Supplementary Figs. 1-3, all catalysts maintain the characteristic tubular morphology of NCNT, with metallic nanoclusters uniformly confined within the NCNT support in Al -Ni /NCNT, and Ni /NCNT samples. Notably, the support properties remain unchanged, as evidenced by Raman spectra (Supplementary Fig. 4), in which the spectra display similar patterns with the characteristic D peak at 1338 cm-1 and G peak at 1562 cm-1, along with comparable I /I intensity ratios across Al -Ni /NCNT, Al /NCNT and Ni /NCNT catalysts. This consistency indicates that the carbon structures exhibit similar levels of disorder or defects. Meanwhile, the metallic Ni nanoclusters and Al dual-atom species shows no obvious Raman signals primarily due to their high polarizability, which hinders atomic vibrations from inducing measurable changes in polarizability31. Furthermore, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) imaging reveals that the dispersed Al single-atom and dual-atom sites in Al /NCNT and Al /NCNT are clearly observed on the NCNT support, highlighted by white circles in Fig. 1c, 1e and Supplementary Fig. 5. Following Ni deposition, as shown in Fig. 1d and Supplementary Fig. 5d, small nanoclusters with the lattice distances of 0.206 nm (corresponding to the Ni (111) plane) are observed, indicating a crystalline Ni structure with an average particle size of approximately 2.5 nm. Additionally, it is observed that while the Al distribution remains uniform, the Ni nanoclusters are segregated and surrounded by abundant Al dual-atom in Al -Ni /NCNT, as confirmed by the HAADF-STEM and EDS image (Fig. 1d and Supplementary Fig. 6).

X-ray absorption spectra (XAS) analysis was performed to elucidate the electronic and coordination structures of the active sites. As shown in Fig. 2a, the Al K-edge X-ray absorption near edge structure (XANES) result reveals that the absorption edges of Al -Ni /NCNT, Al /NCNT and Al /NCNT are positioned between Al O and Al foil, indicating that the average oxidation state of Al species is likely greater than 0 but less than +3. Moreover, compared to Al /NCNT and Al /NCNT, the absorption edge of Al -Ni /NCNT shifts to higher photon energies, indicating a higher electron deficiency for Al species in Al -Ni /NCNT, implying higher oxidation states. This conclusion is supported by the Al 2p XPS results, which reveal peak shifts toward higher binding energies (Supplementary Fig. 7). Notably, the absorption edge of Ni species in both Al -Ni /NCNT and Ni /NCNT samples aligns closer with that of Ni foil than NiO, indicating that the Ni species predominantly exist in a metallic state. Consistently, the absorption edge in Al -Ni /NCNT exhibits a negative shift (Fig. 2b), implying an increased electrons on Ni species, which suggests that there is an electron interaction between neighboring Ni nanocluster and Al dual-atom.

Based on this, Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) was utilized to investigate the coordination environment of Al species (Fig. 2c). The two prominent peaks for Al -Ni /NCNT and Al /NCNT appear at approximately 1.3 and 2.2 Å (without phase correction) corresponding to Al-N and Al-Al coordination, respectively32, whereas Al /NCNT exhibits only the former feature. Notably, the possibility of Al-Cl or Al-O coordination is excluded due to the higher peak position of M-Cl coordination at ~1.9 Å33, and the evidence of Al interaction with N species from XPS results (Supplementary Figs. 8 and 9). More importantly, the second peaks in Al -Ni /NCNT and Al /NCNT are both shorter than the typical Al-Al distance of ~2.4 Å observed in Al foil. Additionally, wavelet transforms analysis of Al EXAFS oscillations further corroborates the formation of Al dual-atom sites (Fig. 2g). The WT-EXAFS contour plot of Al -Ni /NCNT exhibits two intensity maxima at ~4.3 and ~4.9 Å-1, attributed to the Al-Al and Al-N interaction pathways, respectively. To further quantify the coordination environment of Al atoms, EXAFS fittings were performed. As shown in Fig. 2e, Supplementary Figs. 10-11 and Supplementary Table 2, the best-fit EXAFS result for Al -Ni /NCNT indicates an average Al-N coordination number of 3.9 with a bond length of 1.78 Å, while the Al-Al coordination number is 0.9 with a bond length of 2.32 Å, highly consistent with the local atomic configuration of Al /NCNT. These findings strongly corroborate that Al species in Al -Ni /NCNT exist as Al dual-atom rather than binding directly to Ni.

In contrast, the Ni K-edge EXAFS spectra of Al -Ni /NCNT exhibit a dominant peak at 2.1 Å with a shoulder at 1.4 Å (Fig. 2d), indicative of Ni-Ni and Ni-N coordination with a lower coordination number. This interpretation is reinforced by the EXAFS oscillation in k space, which exhibits shorter periods and weaker amplitudes compared to the referenced Ni catalyst and Ni foil, as confirmed by the wavelet transform spectra (Fig. 2h). EXAFS fittings (Fig. 2f, Supplementary Figs. 12-13 and Supplementary Table 3) further reveal a reduction in coordination numbers for both Ni-Ni and Ni-N pairs, suggesting the formation of smaller Ni nanoclusters interacting with the NCNT support.

Catalytic performance of Al -Ni /NCNT
The performances of acetylene hydrogenation in an excess ethylene stream were evaluated for all prepared catalysts with similar Al and Ni loadings using a fixed-bed reactor. The catalytic conversion and selectivity of Al -and Al- based samples supported on NCNT were first evaluated in acetylene selective hydrogenation, with the concentration of reactants and products as a function of temperature shown in Fig. 3. Remarkably, atomic-scale Al sites exhibit excellent activation ability of acetylene on the premise of excluding the interference of NCNT support (Supplementary Fig. 14) and reactor system (Supplementary Fig. 15), with Al /NCNT displaying better catalytic performance compared to single-atom Al counterpart (Fig. 3a). This behavior markedly contrasts with the inert α-Al O and Al powders, which is usually used as substrate to load Pd catalysts for selective hydrogenation in the industry. However, higher acetylene conversion for Al /NCNT catalysts requires the elevated reaction temperature (>240 °C in Fig. 3a and Supplementary Fig. 16) although exhibiting excellent ethylene selectivity (94.0%). Moreover, the synergistic catalytic system comprising Ni nanoclusters with exceptional hydrogen dissociation and Al dual-atom sites for acetylene activation was further evaluated. Various flow rates, H /hydrocarbon ratio, and temperatures were systematically tested to determine the optimal reaction conditions (Fig. 3 and Supplementary Table 4). As expected, Al -Ni /NCNT significantly enhances catalytic activity, achieving 99.98% conversion at 157 °C, far superior to Al /NCNT (49.75% @ 340 °C) and Ni /NCNT (99.93% @ 200 °C), as shown in Fig. 3a and Supplementary Figs. 17-18. More importantly, even at complete acetylene conversion, Al -Ni /NCNT maintains ethylene selectivity of 90.26% (Fig. 3b), with only minor formation of ethane (6.17% of selectivity) and oligomers (3.57% of selectivity), as depicted in Figs. 3c-d. In contrast, the Ni catalyst, which lacks adjacent Al dual-atom, exhibits markedly lower ethylene selectivity (43.70% at 99.93% conversion). Based on the temperature-dependent concentration profiles of acetylene, ethylene, ethane and oligomers (Supplementary Figs. 17-18), it is found that ethane and oligomers start to form over Al -Ni /NCNT catalyst when the temperature increases to 88 °C; however, their corresponding amounts are significantly lower than that of Ni catalyst. This difference may be attributed to the formation of ethylidene (=CHCH ) on Ni nanoclusters, resulting in excessive hydrogenation to ethane and polymerization into oligomers (Figs. 3c-3d). Meanwhile, the lower formation rate of oligomers of 0.29 mol mol -1 h-1 over Al -Ni /NCNT in Supplementary Fig. 19, compared to that of Ni /NCNT (1.72 mol mol -1 h-1) indicates that the construction of synergistic sites could suppress the deposition of carbonaceous species. These findings demonstrate that synergy between Al dual-atom sites and adjacent Ni nanoclusters not only drives the different product distribution but also enhances the activity and selectivity. With lower metal loading and reduced reaction temperature, this system outperforms most reported non-noble metal catalysts and even rivals precious metal catalysts (Supplementary Tables 5-7)34, making it highly promising for practical applications. Furthermore, the calculated reaction rate is 15.3 mol mol -1 h-1, approximately 2.4 times higher than Ni nanoclusters (Supplementary Fig. 20).

To clarify the kinetics advantages of the Al -Ni /NCNT synergetic catalyst, the intrinsic activity, characterized by apparent activation energy (E ) and reaction order was further analyzed under conditions below 15% conversion to eliminate the influence of mass or heat transfer. Firstly, kinetic measurements using the Arrhenius equation further affirm the intrinsic activity trend, with Al -Ni exhibiting a competitive advantage through a lower activation energy of 55.0 kJ mol-1 (Fig. 3e). The reaction order (n) for acetylene was also determined by plotting intrinsic activity versus pressure (Fig. 3f), yielding a value of 0.19 for Al dual-atom sites integrated with Ni nanoclusters, while a negative value (-0.39) was observed for the aggregated Ni species. This negative reaction order suggests that high acetylene coverage inhibits the reaction rate due to strong adsorption35. Additionally, the hydrogen reaction order, shown in Fig. 3g, exhibits a linear relationship, with values of 0.39 for Al -Ni /NCNT and 1.07 for Ni /NCNT. The near-zero reaction order indicates that the catalytic activity of Al -Ni is determined by the structure rather than hydrogen pressure36. This implies that the synergy between Al dual-atom sites and Ni nanoclusters effectively reduces competitive acetylene adsorption while enhancing hydrogen activation and dissociation.

Notably, Al -Ni /NCNT demonstrates exceptional thermal and chemical stability under hydrogenation conditions, maintaining both activity and selectivity for at least 150 h at 142 °C, with no visible decline even upon regeneration (Fig. 3h). To investigate the nature and stability of this catalyst during reaction, XRD pattern and XAS analysis have been provided. As shown in Supplementary Figs. 21-22, the atomically-dispersed Al sites and Ni clusters of Al -Ni /NCNT could be maintained after the reaction; meanwhile, Al 2p XPS results (Supplementary Fig. 23) further indicate that the average oxidation state of Al species well keeps between 0 and +3, following the coking combustion and a subsequent hydrogen activation, which is basically in agreement with the fresh one. Additionally, the excellent resistance to carbon deposition is also confirmed (Supplementary Fig. 24)39. In contrast, Ni /NCNT shows a gradual decrease in ethylene selectivity, approximately with a loss of ~90% within the first five hours (Supplementary Fig. 25), and Raman measurements indicate severe coking (Supplementary Fig. 24)39.

Adsorption behavior and reaction mechanism
To investigate the role of different sites in Al -Ni /NCNT catalysts, the adsorption and hydrogenation processes were monitored by in situ FT-IR spectroscopy. As shown in Fig. 4, acetylene is rapidly adsorbed onto the catalyst surface, reaching equilibrium with characteristic bands at 3309, 3262, 1353 and 1303 cm-1. After purging with Ar, bands originating from gas-phase acetylene gradually disappear, while a new peak appears at 3280 cm-1 for Al -Ni /NCNT catalyst (Fig. 4a), indicating π-adsorbed acetylene40. When hydrogen is introduced, the intensity of π-binding reactant decreases significantly as reaction temperature rises (Fig. 4b), suggesting the efficient conversion of acetylene. Simultaneously, two characteristics at 1710 and 1606 cm-1 appear, corresponding to the vibrations of C=C and =CH in ethylene product in the π-bonded configuration41,42. The intensity of these peaks initially increases and then declines as the reaction temperature rises. More importantly, no bands originating from by-products, such as over-hydrogenated ethane (2940 and 2883 cm-1) and polymerized oligomers (1794 and 1750 cm-1), are detected in this process. This illustrates that the active sites, formed by Al dual-atom sites coupling with Ni nanoclusters in the Al -Ni /NCNT catalyst, promote the selective conversion of acetylene to ethylene. This process exhibits excellent desorption capability, as confirmed by C H -TPD results (Supplementary Fig. 26). Interestingly, in contrast to this behavior, acetylene is adsorbed on the Ni nanoclusters in dissociative and di-σ types (3250 and 1630 cm-1, Ni /NCNT in Fig. 4c), leading to the generation of by-products (Fig. 4d), consistent with the reported literature43. Comparing these two catalysts, it is evident that the introduction of Al dual-atom sites adjacent to Ni nanoclusters alters the adsorption configuration of reactants. Indeed, when using the Al dual-atom catalyst as a contrast, acetylene adsorbs in a π configuration on pristine Al dual-atom (Fig. 4e). This suggests that two adjacent Al species act as adsorption and activation sites for acetylene in Al -containing catalysts (Al -Ni /NCNT and Al /NCNT), facilitating the formation of weakly adsorbed vinyl intermediates (-CH=CH ). The hydrogenation of these intermediates is kinetically easier than C-C coupling44 (Figs. 4b and 4f).

Though Al dual-atom promote ethylene formation, the adsorption peak of acetylene on the Al /NCNT catalyst decreases slowly after hydrogen is introduced, indicating low activity (50% @ 340 °C) due to the weak ability of Al species to activate H , which agrees with H -TPD results (Supplementary Fig. 27). However, despite having the same adsorption configuration of reactants, the Al -Ni /NCNT drives the reaction faster (99.98% conversion @157 °C). This demonstrates that Ni nanoclusters more effectively dissociate H . In other words, integrating Al with Ni nanoclusters creates distinct roles: the Al dual-atom promote acetylene adsorption and activation in π binding, while Ni nanoclusters are responsible for the dissociation of H into active H*, transferring to the generated -CH=CH intermediates. This H transfer could be supported by a greater amount of H consumption being 230 μmol/g than 189 μmol/g required from Ni nanoclusters (Supplementary Fig. 27). Meanwhile, an experiment was conducted in which WO was mixed with Al -Ni /NCNT, and a distinct color change from canary yellow to dark blue was observed (Supplementary Fig. 28), confirming the occurrence of hydrogen spillover45. These integrated sites alleviate the competitive adsorption of reactants at a single site, preventing over-hydrogenation and polymerization, thus overcoming the trade-off between activity and selectivity. As a result, and as expected, ethane and oligomers are prominently detected in the Ni /NCNT catalyst, with no ethylene formation. This observation aligns well with temperature-programmed surface reaction (TPSR) results (Supplementary Fig. 29), which show that acetylene hydrogenation on Al -Ni /NCNT starts at a lower temperature (77 °C). The process exclusively produces ethylene as the anticipated product, without generating ethane or oligomers, in contrast to Al and Ni .

To further understand the high activity and selectivity towards ethylene of Al -Ni /NCNT compared to Ni bulk, density functional theory (DFT) calculations were performed. Based on experimental structural characterizations, two optimal atomic structure models were constructed with the Al and Ni nanoclusters (Figs. 2e and 2f). Several important pieces of information could be extracted in the free-energy profile of acetylene hydrogenation (Fig. 5a-5b, Supplementary Figs. 30 and 31). Primarily, C H molecule coordinates on Al sites in a π-complex manner with an adsorption energy of -0.53 eV. The resulting *C H species is hydrogenated in successive steps to easily produce vinyl intermediate as the precursors of ethylene products. As expected, the generated *CH CH intermediate adsorbs on Al sites in a π-adsorbed configuration, which facilitates the desorption step with an energy of 0.70 eV. Indeed, this desorption energy is lower than that of further hydrogenation (referred to as D-value, which is negative in Fig. 5c), suggesting that acetylene hydrogenation favors ethylene rather than ethane. However, acetylene adsorption on Ni sites occurs in a multi-bridged configuration with a strong binding energy of -2.88 eV, which favors conversion to ethane (namely the D-value is positive in Fig. 5c). Notably, the differential charge density plots in Supplementary Fig. 32 clearly reveal the distinct adsorption behaviors of C H and C H on the Al site and Ni (111) surface. On Al site, the charge redistribution is predominantly localized between the C-C bond and the surface, indicating a weak interaction. In the case of C H , only slight surface polarization is observed, suggesting limited electronic coupling and facile product desorption. In contrast, adsorption of both C H and C H on Ni (111) surface induce pronounced charge transfer, wherein the C π electrons strongly interact with Ni d orbitals, resulting in significant charge redistribution and stronger binding between the adsorbate and the surface. Consequently, the weak adsorption on Al facilitates the formation and release of C H , whereas the strong chemisorption on Ni (111) hinders C H desorption and reduces the selectivity for C H hydrogenation. Indeed, the Ni sites exhibit a significant increase in free energy during the *CH CH desorption step compared to its hydrogenation (Fig. 5d and Supplementary Fig. 33) regardless of whether the systems involve clusters or bulk catalysts. Specifically, the desorption energy of *CH CH on Ni sites of all sizes is markedly high, indicating strong adsorption. The subsequent hydrogenation of *CH CH readily occurs on both Ni clusters of various sizes and Ni (111) surface, with reaction energies ranging from -0.99 to -0.48 eV. This implies that the multi-bridged adsorption configuration stabilizes the *CH CH intermediate, rendering desorption energetically unfavorable and leading to poor ethylene selectivity. The above results are in good agreement with DRIFT analysis. Furthermore, the reaction energy for hydrogen dissociation has been calculated (Fig. 5c and Supplementary Fig. 34). Ni species display a stronger thermodynamic propensity for hydrogen activation than the Al site, which is energetically less favorable, demonstrating that the Ni (111) surface is highly effective at activating H . Thus, Ni sites exhibit stronger adsorption for C H and H compared to Al sites. However, catalytic performance is governed not only by the adsorption of reactants but also by the free-energy change associated with the rate-determining step (ΔG , namely activity) and the selectivity toward C H and C H . As calculated (Supplementary Fig. 35a), the ΔG value on Al sites is lower than Ni sites. Furthermore, *C H hydrogenation is thermodynamically less favorable compared to *C H desorption on Al sites, whereas it is more facile on Ni sites (Supplementary Fig. 35b). These thermodynamic analyses indicate that Al sites facilitate more favorable catalytic kinetics for the selective hydrogenation of acetylene, which is further supported by our calculated turnover frequency (TOF) values (Supplementary Fig. 36). Therefore, despite weaker adsorption of reactants, the superior intrinsic activity and enhanced selectivity of Al sites suggest that they serve as the primary active centers for acetylene selective hydrogenation.

To gain deeper insight into the adsorption behavior at Ni sites, the adsorption energies of *C H and *H under various *C H coverages were calculated (Supplementary Figs. 37-39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the adsorption energy of *C H shifts from initial -3 eV to +0.2 eV, indicating progressive surface saturation with adsorbed *C H species. In contrast, although *H adsorption energy exhibits a slight increase with increasing coverage, it remains negative, implying that *H species can still be generated from H dissociation. Therefore, we propose that while Ni sites exhibited strong adsorption towards *C H and *H, the favorable *C H adsorption mainly leads to high coverage. In contrast, the readily formed *H species could migrate to adjacent Al sites, where they participate in the hydrogenation of *C H . Based on the results from in situ spectroscopic characterizations and theoretical calculations, it could be regarded that the construction of the synergistic Ni nanoclusters and Al active centers offers excellent activity and selectivity, in which nickel species are responsible for hydrogen dissociation, while the Al sites trigger the spillover of active H* to react with π-bound acetylene and weaken the desorption of ethylene instead of simply modify Ni, inhibiting over-hydrogenation and polymerization.

Methods
Materials
All chemicals including nitrogen-doped carbon nanotube (NCNT, Macklin, >95%), anhydrous aluminum chloride (AlCl , Innochem, 99.99%), alumina (Al O , Macklin, 99.99%), aluminum powder (Aladdin, >99.5%) and anhydrous nickel chloride (NiCl , Innochem, 99%) were of reagent-grade quality. They were acquired from commercial suppliers and employed as received. Deionized water with a specific resistance of 18.25 MΩ·cm was used.

Preparation of Al -Ni /NCNT catalyst with Al dual-atoms and Ni clusters as comparison
AlCl (150 mg) and NCNT (50 mg) were placed in two separate porcelain boats, the former of which was in the upstream. The powders were heated up to 180 °C in a slow-flowing Ar stream (20 mL min-1) for 2 h to achieve the transfer and capture of Al Cl vapor, followed by 5% H /Ar treatment at 550 °C for 2 h to obtain dual-atom Al catalyst (Al /NCNT). Subsequently, a second vapor deposition step was further conducted, in which anhydrous NiCl (50 mg) and Al /NCNT (100 mg) were placed in a tubular furnace, with the former at the upstream position, followed by heating to 950 °C and maintaining for 1 h in Ar to obtain the final product, namely Al -Ni /NCNT. As comparison, Ni catalyst was acquired using the above preparation method but with 50 mg of anhydrous NiCl and 100 mg of NCNT.

Preparation of Al single-atom catalyst
NCNT (100 mg) were dispersed in 16 mL of deionized water via ultrasonication for 30 min to obtain a homogeneous suspension. Under vigorous stirring, 4 mL of an aqueous solution containing AlCl (15 mg) was slowly added dropwise to the NCNT suspension over 30 min. The mixture was then stirred at room temperature for 12 h. Following the reaction, the solid product was collected by centrifugation and dried for 12 h. The resulting powder was subsequently treated at 550 °C for 2 h under a flow of 5% H /Ar, yielding the Al /NCNT.

Characterizations
Morphologies of as-prepared samples were investigated using transmission electron microscopy (TEM, FEI Tecnai G2 20 STwin, operated at 200 kV), and high-resolution transmission electron microscopy (HRTEM, JEOL 2100, operated at 200 kV). Aberration-corrected high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) results were obtained on a JEOL JEM-ARM200F STEM with a spherical aberration corrector (operated at 200 kV). The loadings of Ni and Al were conveyed by the inductively coupled plasma optical emission spectroscopy (ICP-OES) on Shimadzu ICPE-9800. The crystalline structures of all catalysts were evaluated using Shimadzu XRD-6000 diffractometer (Cu Kα source, λ = 1.5418 Å). The N isothermal adsorption/desorption was performed on a Micromeritics ASAP 2460 system. Prior to N adsorption, the powders were degassed at 200 °C for 3 h under vacuum. Raman spectra were identified on an inVia Reflex RamanMicroscope. Thermogravimetric mass spectrometry (TG-MS) was conducted with TGA-8000 thermogravimetry and Clarus SQ8T mass spectrometer. H or C H temperature-programmed desorption measurements were conducted on Micrometrics Autochem II 2920 chemisorption system equipped with a thermal conductivity detector (TCD). About 100 mg of the sample was purged with pure Ar or He at 200 °C and then cooled down to room temperature. H or C H at a flow rate of 50 mL/min was introduced to the sample for 1 h to ensure a saturated adsorption, and then pure Ar or He was flowed to remove gas phase. The TPD profiles were subsequently obtained by increasing the temperature from 50 to 800 °C at a rate of 10 °C/min. Temperature-programmed surface reaction (TPSR) experiment was performed in a reaction cell. 0.2 g of sample was pretreated in N for 30 min. Then, the gas was switched to 0.31% C H /30.40% C H /6.20% H (balanced with nitrogen) with flow rate of 30 mL/min and heated from 40 to 380 °C for data recording. The products (m/z of C H , C H and C H being 26, 27 and 15, respectively) were measured via mass spectrometer. Diffuse reflectance infrared Fourier transform spectra (DRIFTS) were collected on a Bruker TENSOR II with an in-situ diffuse reflection accessory. Typically, about 50 mg of powders was purged with pure N in a reflection cell at 200 °C and then cooled down to room temperature. Thereafter, 5.0 vol% C H /N was introduced into the cell with a flow rate of 20 mL/min for 1 h to ensure the saturated adsorption of acetylene on the sample. Afterwards, pure N was flowed to remove gas phase to obtain pre-adsorbed acetylene. Then, 10 vol% H /N with a flow rate of 20 mL/min was flowed to the cell for hydrogenation of the pre-adsorbed acetylene, during which the temperature was increased gradually to 220 °C (for Al -Ni /NCNT and Ni catalysts) or 440 °C (for Al /NCNT). The spectra collected at different temperature were subtracted from background spectra at the corresponding temperature. X-ray absorption spectroscopy (XAS) measurement and data analysis: The Ni and Al K-edge XAS experiments were performed on on beamline 1W1B at the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). The Ni and Al K-edge data of different catalysts were recorded in a fluorescence mode and the references (NiO and Ni foil; Al O and Al foil) were recorded in a transmission mode. Data reduction, data analysis, and EXAFS fitting were performed with the Athena, Artemis, and IFEFFIT software packages.

Catalytic testing
Catalytic behavior was measured using a fixed-bed microreactor at 0.1 MPa and 40-440 oC. 0.2 g of catalyst was diluted with quartz sand (Aldrich, 40-70 mesh) and placed in the reactor of 8 mm. The introduced feed gas consisted of a 0.31% C H /30.40% C H /6.20% H /1% C H mixture diluted by nitrogen with a gas hourly space velocity (GHSV) of 9000 mL h-1 g-1. The reactant and product concentrations were analyzed by online Gas Chromatography (GC) with a flame ionization detector using a PLOT capillary column (50 m × 0.53 mm). Propane was used as an internal standard. Multiple data points were collected at different temperatures to ensure reproducibility. The selectivity to oligomers was calculated based on carbon balance (100 ± 0.5%). Acetylene conversion and ethylene selectivity were calculated as follows:
Acetylene conversion (%) = (C2H2(inlet)－C2H2(outlet)) / C2H2(inlet) (1)
Ethylene selectivity (%) = (C2H4(outlet)－C2H4(inlet)) / (C2H2(inlet)－C2H2(outlet)) (2)
In which C H (inlet), C H (inlet), C H (outlet) and C H (outlet) represent the concentration of reactant and product. Catalyst stability was performed for a long period at a constant temperature of 142 or 181 oC.

Regeneration
The used Al -Ni /NCNT catalyst by oxidative treatment to remove oligomers, following a protocol adapted from industrial practices used at Jam domestic petrochemical for acetylene hydrogenation37,38. Firstly, a nitrogen stream at a flow rate of 100 mL min-1 was fed to the reactor at 115 °C for 60 min. The temperature was then increased to 165 °C while maintaining the nitrogen flow to stabilize the catalyst bed. In the next step, in addition to nitrogen, water vapor (H O (g)) at a flow rate of 0.02 mL min-1 was injected as the temperature was raised from 165 to 400 °C, and this temperature was maintained for 90 min to wash out and reform light hydrocarbons. Subsequently, the heavy hydrocarbons prone to coking were burnt by introducing oxygen with the flow rate gradually increased from 5 to 40 mL min-1 at 500 °C. Following this step, nitrogen was purged through the reactor for 130 min to displace residual oxygen. Then, a subsequent hydrogen stream at 10 mL min-1 was introduced to reduce the oxidized Al and Ni species back to their fresh states. Finally, nitrogen was passed through the system until the bed temperature decreased to ambient conditions, yielding the regenerated Al -Ni /NCNT catalyst.

Computational details
Density Functional Theory (DFT) calculations were conducted using Vienna Ab initio Simulation Package (VASP)46, the version number of which is 5.4.1. Interactions between the ionic core and valence electrons were modeled complementing the projector-augmented wave (PAW) method.47,48 The Perdew-Burke-Ernzerhof (PBE) exchange-correlation within the framework of the generalized gradient approximation (GGA) functional was employed.49 The convergence criteria for force and energy were set to 0.02 eV/Å and 10⁻6 eV, respectively. The Brillouin zone was sampled in k-space using a Monkhorst-Pack k-point grid mesh of 3×3×1 for geometric optimization. A 15 Å vacuum layer was used to reduce virtual interactions between slabs along the z direction. Considering the presence of a magnetic element (Ni) in the system, the spin-polarized effect was incorporated. The work of Kresse and Hafner has indicated that surface magnetism is vital for an accurate quantitative analysis of adsorption energy.50

The carbon-carbon bond length within graphene was optimized to 1.42 Å and the in-plane lattice constant of graphene to 2.64×2.64 Å2, while the Ni crystal lattice constant was refined to 3.52 Å, both values closely matching with established experimental values.51,52 For Ni surface, a p (4×4) four-layer supercell was used, while fixing the bottom two layers and relaxing the upper two layers. While a p (6×6) monolayer graphene was utilized to build the Al , and a p (8×8) graphene supercell was used to support the Ni , Ni , and Ni clusters. Elemental steps for acetylene hydrogenation were considered as listed below:
*+C H (g) →*C H (3)
*+ H (g)+*C H →*C H +*H (4)
*C H +*H →*C H (5)
*H (g)+*C H →*C H +*H (6)
*C H +*H →*C H (7)
*H (g)+*C H →*C H +*H (8)
*C H +*H →*C H (9)
*H (g)+*C H →*C H +*H (10)
*C H +*H →*C H (11)
*C H →*+C H (12)
Where * represents a surface site, the reaction energy along elemental steps of acetylene hydrogenation is calculated to investigate the reaction process, especially the selectivity, that is proceeding the *C H desorption for C H production and subsequent hydrogenation for C H . Besides, adsorption energies are calculated as below, which is used to evaluate the adsorption ability:
∆E = E -E -E (13)
Where ∆E is the adsorption energy, E is the total energy of the adsorbates adsorbed on the surface, E and E are the energies of the isolated slab and molecule, respectively.

The free energy of all species involved in acetylene selective hydrogenation is calculated below:
G = E+E +PV-TS (14)
Where G represents Gibbs free energy, while E and E are the electronic energy via DFT calculations and zero-point energies, respectively. The pressure-volume (PV) contribution is considered negligible53. The temperature (T) is considered as 298 K, and S is the entropy.

The dissociation energy of H is calculated in the following equation:
E = E -E -E (15)
E represents the dissociation energy, where E is the total energy of the catalyst after adsorbing two H atoms, E is the energy of the clean catalyst surface, and E is the energy of the gas-phase H molecule. This energy reflects the difficulty of H dissociation on the catalyst surface. a more negative value indicates that H molecules dissociate more easily on the surface.

The desorption energy of C H (E ) is calculated below:
E =E +E -E (16)
Here, E represents the energy of the C H molecule in the gas phase, and E denotes the total energy of the catalyst surface with an adsorbed C H molecule. E represents the desorption energy required for the desorption of C H from the catalyst surface; a more negative value of E indicates that C H can desorb more easily from the surface.

The hydrogenation energy of C H (E ) is calculated below:
E = E -E - E (17)
Where E represents the total energy of the catalyst surface with an adsorbed C H species, and E denotes the energy required for the hydrogenation of C H to form C H . A more negative E value indicates that the hydrogenation of C H to C H is more favourable.

The definition of the D-value is as follow:
D value = E -E (18)
The more negative this value is, the easier the catalyst is to remove the C H molecules, and the more difficult hydrogenation occurs.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Liu et al. reported a synergistic catalytic strategy by integrating two-functional Al dual-atom sites (Al2) and Ni nanoclusters (NiNC) to enable highly selective hydrogenation of acetylene. In this system, as authors reported, the Al sites weakly adsorb acetylene and catalyze the subsequent hydrogenation process, while the Ni sites facilitate hydrogen spillover for its subsequent hydrogenation. The resulting Al2-NiNC/NCNT catalyst demonstrated excellent selectivity toward ethylene, maintaining approximately 90% selectivity and exhibiting outstanding long-term stability without noticeable activity loss after 100 hours of continuous operation. Compared to strategies in previous studies focusing on the modifications in single-atom catalysts (SACs), nanoparticle systems, or support engineering (Chem. Rev. 2020, 120, 683-733; JACS 2023, 145, 26728-26735), this work presents a novel intergrade strategy through combining Al2 and NiNC in carbon supports, significantly suppress the over-hydrogenation to ethane. Notably, the strategy is compelling due to its effective integration of distinct active sites, rather than relying on precise structural or electronic modulation. Based on this, I suggested this work can be considered for acceptance, but I still have several questions on this work for further revision as follows.\n1. In this work, I primarily interested in why the acetylene can adsorbed on Al sites for subsequent hydrogenation, rather than Ni sites. As we know, the Ni sites can enable strong adsorption compared to Al sites, which would appear to be more energetically favorable for acetylene adsorption.\n2. Figure 5 presents DFT calculated results, including reaction energies, adsorption energies, and notably the D value, which is important for elucidating the underlying mechanism. Given that these results serve different roles in illustrating the reaction mechanism, detailed definitions and corresponding descriptions should be provided in the main text to aid reader comprehension.\n3. Figure 5 provides thermodynamic results to explore the role of Ni and Al sites in hydrogenation of acetylene, but it is not clear to describe particularly given the energy values dependent on DFT methodologies. I would recommend including additional electronic structure analyses, which could provide more direct insight into the interaction between acetylene and the active sites.\n4. The atom labeling in the visualizations of models appears to be missing or insufficient. Clear labeling of key atoms is crucial for readers.\n5. As author calculated, the behavior on various scale of Ni models were studied to understand the role of Ni sites. But the size of catalysts can affect the catalysis, which is commonly reported (J. Catal. 2023, 425, 70–79). Therefore, the discussion on particle size effect should be considered, whether the size effect is not affected the results, especially the H2 dissociation and ethylene adsorption.\n6. Compared to Al2O3, Al2/NCNT and Al1/NCNT have significantly improved activity. It is recommended to supplement the performance experiment of NCNT to rule out the interference of the support on atomically precise Al catalysts.\n7. The conversion should be controlled below 15% for Arrhenius plot measurement. This is critical to validate the reliability of the activation energy and reaction order. The authors should clarify this point. Meanwhile, the authors claim that atomically precise Al catalysts exhibit good activity for the acetylene hydrogenation, but missing the sufficient kinetic analysis, especially for Al1 catalysts, which is important for catalytic mechanism.", "reviewer_2": "The authors of this manuscript propose an intriguing concept, where atomically dispersed Al sites (in pairs) work in concert with Ni nanoclusters to achieve high selectivity for acetylene hydrogenation, an industrially relevant reaction. The use of atomically dispersed Al on carbon nanotubes is novel, however I would like to know if the catalyst will survive any oxidation which may be required to burn off the oligomers in an industrial setting.\nAlso, I have a few concerns about the manuscript which should be addressed by the authors:\n1) Metallic Ni is not a selective catalyst for this reaction and leads to over hydrogenation. So, if you have any metallic clusters exposed, they will lower the selectivity of the reaction. How do you keep the Ni from doing the hydrogenation (non-selectively) since it is a more active catalyst than the Al. From the schematic diagram of the synthesis, in Figure 1, it appears that the Ni is added after the addition of the Al. So, the Ni is exposed to the gas phase and can potentially catalyze the non-selective over hydrogenation to ethane.\n2) The AC-STEM images need revision.\na. First, Figure 1c is too dark. It needs to be brightened to be able to see clearly. Also, a corresponding image without the circles needs to be included side by side with Figure 1c, so the reader can see clearly the basis for choosing to circle the atoms that are being highlighted.\nb. Next, the authors circle some bright objects on the surface of the Ni nanoparticles in Figure 2d. As in comment 2a above, the authors need to show in the supporting information this image along with one without the circles. And they need to explain the basis for concluding there are Al atoms on the surface of the Ni. Al is much lighter than Ni, furthermore similar bright objects are also seen on the Ni catalyst (Figure 1e). So, without some other evidence, perhaps XPS, or LEIS and also EDS, we cannot be sure that the Ni is indeed covered by the Al atoms.\n3) The analysis of the EXAFS via fitting the data does not show any evidence for Ni nearest neighbors to the Al, or vice versa. But this is implied in Figure 1d. Do the Ni clusters expose metallic Ni atoms to the gas phase? If so, what stops them from doing the non-selective hydrogenation of ethylene to ethane. Are the Ni clusters physically distant from the Al atoms on the carbon.\n4) There are several aspects of the catalytic measurements that need clarification:\na. It is stated that the Al2/NCNT catalyst shows no influence of temperature on the conversion or selectivity. Does the reactor system have any background reactivity? It seems odd that there is increase in rate with temperature.\nb. In the supporting information, they need to show more details of the reaction products, for instance the concentrations of acetylene, ethylene and ethane as a function of temperature, or a function of space velocity, so the reader can see when the formation of ethane starts and what happens as acetylene is fully consumed. Figure 3 only shows conversion and selectivity. It is important to see all the products and to understand how much ethane is being produced.\nc. Did they quantify the rate of formation of oligomers, or green oil during their long term tests.", "reviewer_3": "This manuscript reports the synthesis of Al dual-atom sites and Ni clusters (ca. 2.5 nm average size) on nitrogen-doped carbon nanotubes (Al2-NiNC-NCNT) and its catalytic activity for the selective semi-hydrogenation reaction of acetylene in ethylene streams. The corresponding blanks (Al2-NCNT, NiNC-NCNT and also a Al-NCNT solid catalyst) are also prepared and tested. The reactivity shows that while Al2-NiNC-NCNT and NiNC-NCNT are not far in catalytic activity (confirmed by the similar activation energy), the former is more selectivity, since apparently Al avoids over-hydrogenation of acetylene to ethane (and coupling reactions) on the Ni cluster sites. The latter is due to a much higher adsorption of acetylene on the Ni sites than in Al, favouring the undesired further reactions. If this is so, it means that Al overrules Ni when activating acetylene, and this is something that is not clear in the study. It could be that H2 competes strongly with acetylene on Ni sites but, in this case, the catalytic activity of NiNC-NCNT would be much lower, and it is not, just the selectivity.\nAnother point that makes the key claim of the study, i.e. that the symbiosis between dual Al sites (perform all the process except H2 dissociation) and Ni (H2 dissociation) triggers the reaction, to be doubtable is the absence of any significant characterization of the catalyst during or after reaction. The only data given are just a Raman, where any information on the metal sites cannot be obtained. The atomically-dispersed Al sites could be rapidly destroyed (aggregated, sublimated) under the thermal reaction conditions, even the Ni clusters could further aggregate or disperse. If the main advance of the paper is the originality of the catalytic sites and the mechanism, the nature and stability of the fresh catalyst during reaction must be clearly demonstrated.\nWith that said, other issues could be considered less important, but nor for the high standards of Nat. Commun.:\n1) Catalytic results: The conversion of acetylene is given in rough numbers (100%), that means 99%, 99.99%? The industrial process requires less than 1 ppm in the final stream, although <10 ppm can be acceptable at the academic level. This has to be measured and explain how is measured (exact mass balance, not with a 0.5% error). A 10% of ethane is very much, the industrial process requires less than 2%, otherwise the economic penalty is huge.\n2) Catalyst nature: Why using CNTs if the support structure does not have any influence on the formation of the metal sites, as demonstrated by the Raman experiments? In this case, a simple N-doped carbon should be enough. Is N doping really essential? One could argue that O instead of N is stabilizing the dual sites (even perhaps Cl). The difference if XAS between N and O is very low. N XPS should help here. Any evidence that all the Cl is gone? It could still be on the catalyst.\nThe Al1-catalyst is made by impregnation in water solution, and no characterization is given. The likely structure here is Al2O3 entities, thus comparison with the Al2-catalyst is quite doubtable.\nIn any case, any Al-N or Al-O single atom site is quite prone to decompose under H2 atmospheres and severe heating, thus shedding more doubts on the catalyst stability under reaction conditions.\nFigure 1b (PXRD) does not guarantee the formation of Ni aggregates, it looks the opposite.\n3) Reaction conditions: The reaction temperatures employed here (>150 ºC, >300 ºC for only Al) are extremely high for this reaction. The optimized reaction conditions employ 20:1 H2:ethylene, this is much more than standard front-end conditions.\nNon-precious metal catalysts can perform the reactions under milder reaction conditions, see for instance J. Am. Chem. Soc. 2018, 140, 8827–8832 (this article should be cited and included in the comparative Table). This comparative Table is partially informative, it is true that hundreds of catalytic systems have been published, but the selection here is quite random, and the commercial catalyst (Pd-Al2O3) operates at much milder reaction conditions (35-70 ºC) than indicated in the Table.\n4) Reaction mechanism: The characterization (HR-TEM) claims that the dual Al sites are nearby the Ni clusters, surrounding them. There is not clear evidence of this (EDS mapping, Al-Ni reduced distances in XAFS, the XPS shift is very doubtable). In any case, H-spillover from Ni to Al sites would not require any spatial approximation, and that has not been considered. Please check (a simple test with WO3 will confirm the H spillover).\nMinor comment: The meaning of TPSR should be specified (not said neither in the main text or SI).\nIn conclusion, although recognizing that the activation of acetylene on atomically-dispersed Al sites for the semi-hydrogenation reaction is of academic interest, the study here does not demonstrate that this is occurring, since the stability of the Al site during reaction and a significant catalytic performance beyond that of the Ni sites, is not demonstrated."}, "author_response": "Responsetoreviewers\nReviewer#1\nLiu et al. reported a synergistic catalytic strategy by integrating two-functional Al\ndual-atom sites (Al ) and Ni nanoclusters (Ni ) to enable highly selective\nhydrogenation of acetylene. In this system, as authors reported, the Al sites weakly\nadsorb acetylene and catalyze the subsequent hydrogenation process, while the Ni\nsites facilitate hydrogen spillover for its subsequent hydrogenation. The resulting\nAl -Ni /NCNT catalyst demonstrated excellent selectivity toward ethylene,\nmaintaining approximately 90% selectivity and exhibiting outstanding long-term\nstability without noticeable activity loss after 100 hours of continuous operation.\nCompared to strategies in previous studies focusing on the modifications in\nsingle-atom catalysts (SACs), nanoparticle systems, or support engineering (Chem.\nRev. 2020, 120, 683-733; JACS 2023, 145, 26728-26735), this work presents a novel\nintergrade strategy through combining Al and Ni in carbon supports, significantly\nsuppress the over-hydrogenation to ethane. Notably, the strategy is compelling due to\nits effective integration of distinct active sites, rather than relying on precise structural\nor electronic modulation. Based on this, I suggested this work can be considered for\nacceptance, but I still have several questions on this work for further revision as\nfollows.\nComment1:\nIn this work, I primarily interested in why the acetylene can adsorbed onAl sites for\nsubsequent hydrogenation, rather than Ni sites. As we know, the Ni sites can enable\nstrong adsorption compared toAl sites, which would appear to be more energetically\nfavorablefor acetylene adsorption.\nReply:\nWe gratefully acknowledge the reviewer for their insightful questions regarding the\nactive sites. As correctly pointed out, Ni sites exhibit stronger adsorption for *C H\n2 2\nand *H compared to Al sites (Figure 5c), resulting in preferential adsorption of C H\n2 2\nand H on Ni sites. However, catalytic performance is governed not only by the\n2\nadsorption of reactants but also by the free-energy change associated with the\nrate-determining step (ΔG , namely activity) and the selectivity toward C H and\nRDS 2 4\nC H .As calculated (Figure S35a), theΔG value onAl sites is lower than Ni sites.\n2 6 RDS\nFurthermore, *C H hydrogenation is thermodynamically less favorable compared to\n2 4\n*C H desorption on Al sites, whereas it is more facile on Ni sites (Figure S35b).\n2 4\nThese thermodynamic analyses indicate that Al sites facilitate more favorable\ncatalytic kinetics for the selective hydrogenation of acetylene, which is further\nsupported byour calculated turnover frequency (TOF) values (Figure S36).Therefore,\ndespite weaker adsorption of reactants, the superior intrinsic activity and enhanced\nselectivity of Al sites suggest that they serve as the primary active centers for\nacetyleneselective hydrogenation.\nTo gain deeper insight into the adsorption behavior at Ni sites, we calculated the\nadsorption energies of *C H and *H under various *C H coverages (Figure\n2 2 2 2\nS37-S39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the\nadsorption energy of *C H shifts from initial -3 eVto +0.2 eV, indicating progressive\n2 2\nsurface saturation with adsorbed *C H species. In contrast, although *H adsorption\n2 2\nenergy exhibits a slight increase with increasing coverage, it remains negative,\nimplying that *H species can still be generated from H dissociation. Therefore, we\n2\npropose that while Ni sites exhibited strong adsorption towards *C H and *H, the\n2 2\nfavorable *C H adsorption mainly leads to high coverage. In contrast, the readily\n2 2\nformed *H species could migrate to adjacent Al sites, where they participate in the\nhydrogenationof *C H .\n2 2\nFigureS35a.Thefreeenergyoftherate-determiningstep(RDS)ontheAl dual-atomsiteandNi\n2\n(111)surface;b.ThedesorptionenergyandhydrogenationenergyoftheC H moleculeontheAl\n2 4 2\ndual-atomsiteandNi(111)surface.\nFigure S36. Logarithmic turnover frequencies (log (TOF)) of Ni (111) andAl dual-atom sites at\n2\nthereactiontemperatureof430K.\nFigure S37. Adsorption energies of *C H and *H on the Ni (111) surface under different C H\n2 2 2 2\ncoverages.\nFigureS38.Atomicvisualizationof*C H ontheNi(111)surfaceatdifferent*C H coverages.\n2 2 2 2\nFigureS39.Atomicvisualizationof*HontheNi(111)surfaceatdifferent*C H coverages.\n2 2\nRevisionsmade inthemain text:\nIndeed, the Ni sites exhibit a significant increase in free energy during the\n*CH CH desorption step compared to its hydrogenation (Figure 5d and S33)\n2 2\nregardless of whether the systems involve clusters or bulk catalysts. Specifically, the\ndesorption energy of *CH CH on Ni sites of all sizes is markedly high, indicating\n2 2\nstrong adsorption. The subsequent hydrogenation of *CH CH readily occurs on both\n2 2\nNi clusters of various sizes and Ni (111) surface, with reaction energies ranging from\n-0.99 to -0.48 eV. This implies that the multi-bridged adsorption configuration\nstabilizes the *CH CH intermediate, rendering desorption energetically unfavorable\n2 2\nand leading to poor ethylene selectivity. These above results are in good agreement\nwith DRIFT analysis. Furthermore, the reaction energy for hydrogen dissociation has\nbeen calculated (Figure 5c and S34). Ni species display a stronger thermodynamic\npropensity for hydrogen activation than the Al site, which is energetically less\n2\nfavorable, demonstrating that the Ni (111) surface is highly effective at activating H .\n2 2 2\nHowever, catalytic performance is governed not only by the adsorption of reactants\nbut also by the free-energy change associated with the rate-determining step (ΔG ,\nRDS\nnamely activity) and the selectivity toward C H and C H . As calculated (Figure\n2 4 2 6\nS35a), the ΔG value on Al sites is lower than Ni sites. Furthermore, *C H\nRDS 2 4\nhydrogenation is thermodynamically less favorable compared to *C H desorption on\n2 4\nAl sites, whereas it is more facile on Ni sites (Figure S35b). These thermodynamic\nanalyses indicate that Al sites facilitate more favorable catalytic kinetics for the\nselective hydrogenation of acetylene, which is further supported by our calculated\nturnover frequency (TOF) values (Figure S36). Therefore, despite weaker adsorption\nof reactants, the superior intrinsic activity and enhanced selectivity ofAl sites suggest\nthat they serve as theprimary active centers foracetylene selective hydrogenation.\nTo gain deeper insight into the adsorption behavior at Ni sites, the adsorption\nenergies of *C H and *H under various *C H coverages were calculated (Figure\n2 2 2 2\nS37-S39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the\nadsorption energy of *C H shifts from initial -3 eVto +0.2 eV, indicating progressive\n2 2\nsurface saturation with adsorbed *C H species. In contrast, although *H adsorption\n2 2\nenergy exhibits a slight increase with increasing coverage, it remains negative,\nimplying that *H species can still be generated from H dissociation. Therefore, we\n2\npropose that while Ni sites exhibited strong adsorption towards *C H and *H, the\n2 2\nfavorable *C H adsorption mainly leads to high coverage. In contrast, the readily\n2 2\nformed *H species could migrate to adjacent Al sites, where they participate in the\nhydrogenation of *C H . Based on the results from in situ spectroscopic\n2 2\ncharacterizations and theoretical calculations, it could be concluded that the\nconstruction of the synergistic Ni nanoclusters and Al active centers offers excellent\n2\nactivity and selectivity. In this system, nickel species facilitate H dissociation, while\n2\nthe Al sites promote the spillover of active H* species, enabling their reaction with\n2\nπ-bound acetylene and suppressing ethylene desorption, thereby inhibiting\nover-hydrogenationand oligomerization.\nComment2:\nFigure 5 presents DFT calculated results, including reaction energies, adsorption\nenergies, and notably the D value, which is important for elucidating the underlying\nmechanism. Given that these results serve different roles in illustrating the reaction\nmechanism, detailed definitions and corresponding descriptions should be provided in\nthemain textto aid reader comprehension.\nReply:\nWe appreciate the reviewer’s suggestion on computational details. As the reviewer\nadvised, we added the related description in the main text regarding definitions of D\nvalues, reaction energy,adsorption as follows.\nRevisionsmade intheSupporting Information:\nThe free energy of all species involved in acetylene selective hydrogenation is\ncalculated below:\n=  +     +PV−  \nWhere represents Gibbs free energy, while and are the electronic energy\nvia DFT  calculations and zero-point energies, re  spective  l  y  .  The pressure-volume ( )\ncontribution is considered negligible. The temperature ( ) is considered as 298 K,PanVd\nSis theentropy. \nThedissociation energy of H is calculated in thefollowing equation:\n2\n=  ∗2  −  ∗ −   2\nrepresents the dissociation energy, where is the total energy of the catalyst\naf  t  e  r adsorbing two H atoms, is the energy  o∗f2  the clean catalyst surface, and\nis the energy of the gas-phase  H∗ 2 molecule. This energy reflects the difficulty of  H  2 2\ndissociation on the catalyst surface. a more negative value indicates that H molecules\n2\ndissociatemore easily onthesurface.\nThedesorption energy of C H ( ) iscalculated below:\n2 4\n\nHere, represents the e  n  e  r  gy=o  f∗th+e  C  2 H2  4 4m−o  le∗c  u2  le4in the gas phase, and\ndenotes   t2h  e4 total energy of the catalyst surface with an adsorbed C 2 H 4 mol  e∗c  u2l  e4.\nrepresents the energy required for the desorption of C H from the catalyst\ndesorption 2 4\n\nsurface; a more negative value of indicates that C H can desorb more\ndesorption 2 4\neasily from the surface. \nThehydrogenation energy of C H ( )is calculated below:\n2 4\nℎ  \n1\nℎ   =  ∗  2  5 −  ∗  2  4 −   2\n∗  2  5\nC H species, and denotes the energy required for the hydrogenation of\n2 5 hydrogenation\n\nC H to form C H . A more negative value indicates that the\n2 4 2 5 hydrogenation\nhydrogenationof C H to C H is morefavou  rable.\n2 4 2 5\nThedefinition ofthe D-value isas follow:\nThe more negative thDisvvaaluluee=is  ,  t  h  e    e  a   s  ie−r  th ℎ e    c  a  t  a  ly   s  t   is to remove the C H\n2 4\nmolecules, and themoredifficulthydrogenation occurs.\nComment3:\nFigure 5 provides thermodynamic results to explore the role of Ni and Al sites in\nhydrogenation of acetylene, but it is not clear to describe particularly given the energy\nvalues dependent on DFT methodologies. I would recommend including additional\nelectronic structure analyses, which could provide more direct insight into the\ninteractionbetween acetylene and the activesites.\nReply:\nWe appreciate the reviewer’s insightful comments. The differential charge density\nplots presented in Figure S32 clearly reveal the distinct adsorption behaviors of C H\n2 2\nand C H on the Al site and Ni (111) surface. On Al site, charge redistribution is\n2 4 2 2\npredominantly localized between the C-C bond and the surface, indicating a weak\ninteraction. In the case of C H , only slight surface polarization is observed,\n2 4\nsuggesting limited electronic coupling and facile product desorption. In contrast,\nadsorption of both C H and C H on Ni (111) surface induces pronounced charge\n2 2 2 4\ntransfer, wherein the C π electrons exhibit strong interactions with Ni d orbitals,\nresulting in significant charge redistribution and stronger binding between the\nadsorbate and the surface. Consequently, the weak adsorption on Al facilitates the\n2\nformation and release of C H , whereas the strong chemisorption on Ni (111) hinders\n2 4\nC H desorption and reduces theselectivity for C H hydrogenation.\n2 4 2 4\nFigure S32. Differential charge density of C H and C H adsorbed on (a) Al and (b) Ni (111).\n2 2 2 4 2\nYellowandcyanregionsdenoteelectronaccumulationanddepletion,respectively.\nRevisionsmade inthemain text:\nAs expected, the generated *CH CH intermediate adsorbs on Al sites in a\n2 2 2\nπ-adsorbed configuration, which facilitates the desorption step with an energy of 0.70\neV. Indeed, this desorption energy is lower than that of further hydrogenation\n(referred to as the \"D-value\", which is negative in Figure 5c), suggesting that\nacetylene hydrogenation favors ethylene rather than ethane. However, acetylene\nadsorption on Ni sites occurs in a multi-bridged configuration with a strong binding\nenergy of -2.88 eV, which favors conversion to ethane (namely the D-value is positive\nin Figure 5c). Notably, the differential charge density plots in Figure S32 clearly\nreveal the distinct adsorption behaviors of C H and C H on theAl site and Ni (111)\n2 2 2 4 2\nsurface. On Al site, the charge redistribution is predominantly localized between the\n2 4\nonly slight surface polarization is observed, suggesting limited electronic coupling and\nfacile product desorption. In contrast, adsorption of both C H and C H on Ni (111)\n2 2 2 4\nsurface induce pronounced charge transfer, wherein the C π electrons strongly interact\nwith Ni d orbitals, resulting in significant charge redistribution and stronger binding\nbetween the adsorbate and the surface. Consequently, the weak adsorption on Al\n2\nfacilitates the formation and release of C H , whereas the strong chemisorption on Ni\n2 4\n(111)hinders C H desorption andreduces theselectivity forC H hydrogenation.\n2 4 2 4\nComment4:\nThe atom labeling in the visualizations of models appears to be missing or\ninsufficient.Clear labeling ofkey atoms iscrucial for readers.\nReply:\nWe thank the reviewer for this valuable comment. The atom labeling to the related\nfigure caption for clarity has been added.\nRevisionsmade inthemain text:\nFigure 1. Synthesis principles and catalytic properties. (a) Schematic illustration of\nthe synthesis of theAl -Ni /NCNT synergistic catalyst (grey: C; blue: N; yellow:Al;\nlight blue: Ni); (b) XRD patterns;Aberration-corrected HAADF-STEM images of (c)\nAl /NCNT,(d)Al -Ni /NCNTand (e) Ni /NCNT.\n2 2 NC NC\nFigure 2.Analysis of electronic structure and coordination environments. (a)Al and (b)\nNi K-edge XANES spectra; Fourier-transformed (c) Al and (d) Ni K-edge EXAFS\nspectra; (e)Al and (f) Ni K-edge EXAFS fitting in R space ofAl -Ni /NCNT (grey:\nC; blue: N; pink: Al; blue grey: Ni); (g) Al K-edge wavelet transform (WT)-EXAFS\nplots of Al foil and Al -Ni /NCNT; (h) Ni K-edge wavelet transform (WT)-EXAFS\nplotsofNi foil andAl -Ni /NCNT.\nFigure 5. Reaction mechanism of acetylene hydrogenation. (a) Free energy profile of\nselective acetylene hydrogenation on Al with Ni nanoclusters as contrast; (b)\nEnergies of hydrogen dissociation and D-value standing for energy difference\nbetween C H desorption and its further hydrogenation; (d) Reaction energy for\n2 4\nethylene desorption and hydrogenation on various Ni sites on Ni Ni and Ni and\n19, 34 52\nNi (111).\nComment5:\nAs author calculated, the behavior on various scale of Ni models were studied to\nunderstand the role of Ni sites. But the size of catalysts can affect the catalysis, which\nis commonly reported (J. Catal. 2023, 425, 70-79). Therefore, the discussion on\nparticle size effect should be considered, whether the size effect is not affected the\nresults, especially theH dissociation and ethylene adsorption.\nWesincerely thank for the reviewer’s comments on particle size effect.The particle\nsize effect is important for catalysis as the activities of catalytic sites are related to the\ncoordinated environment (Science 2015, 350,185; J. Catal. 2023, 425, 70).As shown\nin Figure S31 and 5d, Ni clusters of different sizes were constructed and analyzed,\nrevealing that the adsorption energy varies with the size. The adsorption energy of\n*C H remains notably low, indicating strong adsorption on Ni sites (Figure S33). In\n2 4\ncontrast, the subsequent hydrogenation of *C H readily occurs on both Ni clusters of\n2 4\nvarious sizes and the Ni (111) surface, with reaction energies ranging from -0.99 to\n-0.48 eV. These results suggest that, for this reaction, Ni sites lead to\nover-hydrogenationrather than theselective formation ofethylene.\nRevisionsmade inthemain text:\nIndeed, the Ni sites exhibit a significant increase in free energy during the\n*CH CH desorption step compared to its hydrogenation (Figure 5d and S33)\n2 2\nregardless of whether the systems involve clusters or bulk catalysts. Specifically, the\ndesorption energy of *CH CH on Ni sites of all sizes is markedly high, indicating\n2 2\nstrong adsorption. The subsequent hydrogenation of *CH CH readily occurs on both\n2 2\nNi clusters of various sizes and Ni (111) surface, with reaction energies ranging from\n-0.99 to -0.48 eV. This implies that the multi-bridged adsorption configuration\nstabilizes the *CH CH intermediate, rendering desorption energetically unfavorable\n2 2\nand leading to poor ethylene selectivity.\nComment6:\nCompared toAl O ,Al /NCNTandAl /NCNThave significantly improved activity.\n2 3 2 1\nIt is recommended to supplement the performance experiment of NCNT to rule out\ntheinterferenceofthe support onatomically preciseAl catalysts.\nReply:\nThanks for your constructive comment. The catalytic performance of NCNT\nsupport was evaluated in the absence of any metal specie. As shown in Figure S14,\ntheNCNTsupport exhibited negligible catalyticactivity under thesameconditions.\nFigureS14.AcetyleneconversionoftheNCNT.\nRevisionsmade inthemain text:\nRemarkably, atomic-scale Al sites exhibit excellent activation ability of acetylene\non the premise of excluding the interference of NCNT support (Figure S14) and\nreactor system (Figure S15), with Al /NCNT displaying better catalytic performance\n2\ncompared to single-atomAl counterpart (Figure 3a).\nComment7:\nThe conversion should be controlled below 15% for Arrhenius plot measurement.\nThis is critical to validate the reliability of the activation energy and reaction order.\nThe authors should clarify this point. Meanwhile, the authors claim that atomically\npreciseAl catalysts exhibit good activity for the acetylene hydrogenation, but missing\nthe sufficient kinetic analysis, especially for Al catalysts, which is important for\n1\ncatalyticmechanism.\nReply:\nWe appreciate the reviewer’s insightful comments. Indeed, the previously\ncalculated activation energies and reaction orders were determined under conditions\nlimited below 15% conversion, which have been replenished in the revised\nmanuscript.\nThe activation energy, reaction orders of acetylene and hydrogen onAl /NCNT have\n1\nbeen supplemented in Figure 3e-g as below.\nFigure 3. Catalytic performance of acetylene hydrogenation. (e) Arrhenius plots; Reaction\norderof(f)acetyleneand(g)hydrogen,respectively.\nRevisionsmade inthemain text:\nTo clarify the kinetics advantages of the Al -Ni /NCNT synergetic catalyst, the\nintrinsic activity, characterized by apparent activation energy (E ) and reaction order\na\nwas further analyzed under conditions below 15% conversion to eliminate the\ninfluence of mass or heat transfer. Firstly, kinetic measurements using the Arrhenius\nequation further affirm the intrinsic activity trend, with Al -Ni exhibiting a\ncompetitive advantage through a lower activation energy of 55.0 kJ mol-1 (Figure 3e).\nThe reaction order (n) for acetylene was also determined by plotting intrinsic activity\nversus pressure (Figure 3f), yielding a value of 0.19 forAl dual-atom sites integrated\nwith Ni nanoclusters, while a negative value (-0.39) was observed for the aggregated\nNi species. This negative reaction order suggests that high acetylene coverage inhibits\nthe reaction rate due to strong adsorption. Additionally, the hydrogen reaction order,\nshown in Figure 3g, exhibits a linear relationship, with values of 0.39 for\nAl -Ni /NCNT and 1.07 for Ni /NCNT. The near-zero reaction order indicates that\nthe catalytic activity of Al -Ni is determined by the structure rather than hydrogen\npressure. This implies that the synergy between Al dual-atom sites and Ni\nnanoclusters effectively reduces competitive acetylene adsorption while enhancing\nhydrogen activation and dissociation.\nReviewer#2:\nThe authors of this manuscript propose an intriguing concept, where atomically\ndispersed Al sites (in pairs) work in concert with Ni nanoclusters to achieve high\nselectivity for acetylene hydrogenation, an industrially relevant reaction. The use of\natomically dispersed Al on carbon nanotubes is novel, however I would like to know\nif the catalyst will survive any oxidation which may be required to burn off the\noligomers inan industrial setting.\nReply:\nWe sincerely appreciate your helpful comment. As suggested, we regenerated the\nused Al -Ni /NCNT catalyst by oxidative treatment to remove oligomers, following\na protocol adapted from industrial practices used at Jam domestic petrochemical for\nacetylene hydrogenation (J. Nat. Gas Sci. Eng. 2016, 34, 1382; Chem. Eng. J. 2012,\n198-199, 491). Firstly, a nitrogen stream at a flow rate of 100 mL min-1 was fed to the\nreactor at 115 °C for 60 min. The temperature was then increased to 165 °C while\nmaintaining the nitrogen flow to stabilize the catalyst bed. In the next step, in addition\nto nitrogen, water vapor (H O (g)) at a flow rate of 0.02 mL min-1 was injected as the\n2\ntemperature was raised from 165 to 400 °C, and this temperature was maintained for\n90 min to wash out and reform light hydrocarbons. Subsequently, the heavy\nhydrocarbons prone to coking were burnt by introducing oxygen with the flow rate\ngradually increased from 5 to 40 mL min-1 at 500 °C. Following this step, nitrogen\nwas purged through the reactor for 130 min to displace residual oxygen. Then, a\nhydrogen stream at 10 mL min-1 was introduced to reduce the oxidized nickel species\nback to metallic Ni. Finally, nitrogen was passed through the system until the bed\ntemperature decreased to ambient conditions, yielding the regenerated\nAl -Ni /NCNTcatalyst.\nRaman and XAS results in the figure below demonstrate that the industrial\nregeneration process could effectively remove the oligomers and coking from the\nsurface of Al -Ni /NCNT catalyst, while the atomically dispersed Al sites and Ni\nnanoclusters remain intact. This is further supported by the catalytic performance of\nthe regenerated Al -Ni /NCNTcatalyst, which is basically consistent with that of the\ncorresponding fresh catalyst.\nFigureRamanspectraoftheregeneratedAl -Ni /NCNT.\nFigure S22 Fourier-transformed (a) Al and (b) Ni K-edge EXAFS spectra with fitting of spent\nAl -Ni /NCNTafterregeneration.\nFigure Acetylene conversion and ethylene selectivity over fresh and regenerated Al -Ni /NCNT\ncatalysts.\nRevisionsmade inthemain text:\nNotably, Al -Ni /NCNT demonstrates exceptional thermal and chemical stability\nunder hydrogenation conditions, maintaining both activity and selectivity for at least\n150hat 142°C, withnovisibledecline even uponregeneration (Figure 3h).\nRevisionsmade intheSupporting Information:\nThe used Al -Ni /NCNT catalyst by oxidative treatment to remove oligomers,\nfollowing a protocol adapted from industrial practices used at Jam domestic\npetrochemical for acetylene hydrogenation (J. Nat. Gas Sci. Eng. 2016, 34, 1382;\nChem.Eng. J. 2012, 198-199,491). Firstly, anitrogen stream at a flowrate of 100mL\nmin-1 was fed to the reactor at 115 °C for 60 min. The temperature was then increased\nto 165 °C while maintaining the nitrogen flow to stabilize the catalyst bed. In the next\nstep, in addition to nitrogen, water vapor (H O (g)) at a flow rate of 0.02 mL min-1\n2\nwas injected as the temperature was raised from 165 to 400 °C, and this temperature\nwas maintained for 90 min to wash out and reform light hydrocarbons. Subsequently,\nthe heavy hydrocarbons prone to coking were burnt by introducing oxygen with the\nflow rate gradually increased from 5 to 40 mL min-1 at 500 °C. Following this step,\nnitrogen was purged through the reactor for 130 min to displace residual oxygen.\nThen, a hydrogen stream at 10 mL min-1 was introduced to reduce the oxidized nickel\nspecies back to metallic Ni. Finally, nitrogen was passed through the system until the\nbed temperature decreased to ambient conditions, yielding the regenerated\nAl -Ni /NCNTcatalyst.\nAlso, I have a few concerns about the manuscript which should be addressed by the\nauthors:\nComment1:\nMetallic Ni is not a selective catalyst for this reaction and leads to over\nhydrogenation. So, if you have any metallic clusters exposed, they will lower the\nselectivity of the reaction. How do you keep the Ni from doing the hydrogenation\n(non-selectively) since it is a more active catalyst than the Al. From the schematic\ndiagram of the synthesis, in Figure 1, it appears that the Ni is added after the addition\nof the Al. So, the Ni is exposed to the gas phase and can potentially catalyze the\nnon-selectiveoverhydrogenation to ethane.\nReply:\nWe thank the reviewer for their insightful comments regarding the potential\nnon-selective hydrogenation behavior of Ni clusters. To address this, we performed\ncalculations of *C H adsorption on Ni surfaces under different coverages, as shown\n2 2\nin Figure S37-S39. To further elucidate the strong adsorption of Ni sites, we\ncalculated the adsorption energies of *C H and *H at various *C H coverages. As\n2 2 2 2\nthe *C H coverage increases from 0.125 to 0.875 ML, the adsorption energy of\n2 2\n*C H rises from -3.0 eV to 0.2 eV, indicating that the surface could be covered by\n2 2\n*C H species. In addition, although the adsorption energy of *H slightly increases\n2 2\nwith coverage, it remains negative, suggesting that *H species could still be generated\nfrom H dissociation. Therefore, we infer that while Ni sites exhibit strong adsorption\n2\ntoward *C H and *H, the favorable *C H adsorption mainly leads to high surface\n2 2 2 2\ncoverage. In contrast, the readily generated *H species could migrate to adjacent Al\nsites, where *C H hydrogenation preferentially occurs. This mechanism ensures that\n2 2\ntheNi surface does not dominatethenon-selective hydrogenation process.\nFigure S37. Adsorption energies of C H and H on the Ni (111) surface under different C H\n2 2 2 2\ncoverages.\nFigureS38.Atomicvisualizationof*C H ontheNi(111)surfaceatdifferent*C H coverages.\n2 2 2 2\nFigureS39.Atomicvisualizationof*HontheNi(111)surfaceatdifferent*C H coverages.\n2 2\nRevisionsmade inthemain text:\nIndeed, the Ni sites exhibit a significant increase in free energy during the\n*CH CH desorption step compared to its hydrogenation (Figure 5d and S33)\n2 2\nregardless of whether the systems involve clusters or bulk catalysts. Specifically, the\ndesorption energy of *CH CH on Ni sites of all sizes is markedly high, indicating\n2 2\nstrong adsorption. The subsequent hydrogenation of *CH CH readily occurs on both\n2 2\nNi clusters of various sizes and Ni (111) surface, with reaction energies ranging from\n-0.99 to -0.48 eV. This implies that the multi-bridged adsorption configuration\nstabilizes the *CH CH intermediate, rendering desorption energetically unfavorable\n2 2\nand leading to poor ethylene selectivity. These above results are in good agreement\nwith DRIFT analysis. Furthermore, the reaction energy for hydrogen dissociation has\nbeen calculated (Figure 5c and S34). Ni species display a stronger thermodynamic\npropensity for hydrogen activation than the Al site, which is energetically less\n2\nfavorable, demonstrating that the Ni (111) surface is highly effective at activating H .\n2 2 2\nHowever, catalytic performance is governed not only by the adsorption of reactants\nbut also by the free-energy change associated with the rate-determining step (ΔG ,\nRDS\nnamely activity) and the selectivity toward C H and C H . As calculated (Figure\n2 4 2 6\nS35a), the ΔG value on Al sites is lower than Ni sites. Furthermore, *C H\nRDS 2 4\nhydrogenation is thermodynamically less favorable compared to *C H desorption on\n2 4\nAl sites, whereas it is more facile on Ni sites (Figure S35b). These thermodynamic\nanalyses indicate that Al sites facilitate more favorable catalytic kinetics for the\nselective hydrogenation of acetylene, which is further supported by our calculated\nturnover frequency (TOF) values (Figure S36). Therefore, despite weaker adsorption\nof reactants, the superior intrinsic activity and enhanced selectivity ofAl sites suggest\nthat they serve as theprimary active centers foracetylene selective hydrogenation.\nTo gain deeper insight into the adsorption behavior at Ni sites, the adsorption\nenergies of *C H and *H under various *C H coverages were calculated (Figure\n2 2 2 2\nS37-S39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the\nadsorption energy of *C H shifts from initial -3 eVto +0.2 eV, indicating progressive\n2 2\nsurface saturation with adsorbed *C H species. In contrast, although *H adsorption\n2 2\nenergy exhibits a slight increase with increasing coverage, it remains negative,\nimplying that *H species can still be generated from H dissociation. Therefore, we\n2\npropose that while Ni sites exhibited strong adsorption towards *C H and *H, the\n2 2\nfavorable *C H adsorption mainly leads to high coverage. In contrast, the readily\n2 2\nformed *H species could migrate to adjacent Al sites, where they participate in the\nhydrogenation of *C H . Based on the results from in situ spectroscopic\n2 2\ncharacterizations and theoretical calculations, it could be concluded that the\nconstruction of the synergistic Ni nanoclusters and Al active centers offers excellent\n2\nactivity and selectivity. In this system, nickel species facilitate H dissociation, while\n2\nthe Al sites promote the spillover of active H* species, enabling their reaction with\n2\nπ-bound acetylene and suppressing ethylene desorption, thereby inhibiting\nover-hydrogenationand oligomerization.\nComment2:\nTheAC-STEM images need revision.\na. First, Figure 1c is too dark. It needs to be brightened to be able to see clearly.Also,\na corresponding image without the circles needs to be included side by side with\nFigure 1c, so the reader can see clearly the basis for choosing to circle the atoms that\nare being highlighted.\nb. Next, the authors circle some bright objects on the surface of the Ni nanoparticles\nin Figure 1d. As in comment 1a above, the authors need to show in the supporting\ninformation this image along with one without the circles. And they need to explain\nthebasis for concluding there areAl atoms onthe surface ofthe Ni.Al is much lighter\nthan Ni, furthermore similarbright objects are also seen ontheNi catalyst (Figure 1e).\nSo, without some other evidence, perhaps XPS, or LEIS and also EDS, we cannot be\nsurethat theNi is indeed covered by theAl atoms.\nReply:\nWe sincerely appreciate your helpful comment. Figure 1c has been brightened to be\nable to see clearly. A corresponding image without the circles has been provided in\nFigure S5, so the reader could see clearly the basis for choosing to circle the atoms\nthat are being highlighted. Meanwhile, the original version of Figure 1d without the\ncircles has been added in Figure S5.As shown in EDS images presented in Figure S6,\nAl atoms are well distributed onthesurrounding of Ni species.\nFigure 1 Aberration-corrected HAADF-STEM images of (c) Al /NCNT (d) Al -Ni /NCNT (e)\n2 2 NC\nAl /NCNT.\n2 2 NC\nAl /NCNTwithoutthecirclesand(d)Ni /NCNT.\nFigureS6TheEDXelementmapsofAl -Ni /NCNTcatalyst.\nRevisionsmade inthemain text:\nAdditionally, it is observed that while the Al distribution remains uniform, the Ni\nnanoclusters are segregated and surrounded by abundant Al dual-atom in\nAl -Ni /NCNT, as confirmed by the HAADF-STEM and EDS image (Figure 1d and\nS6).\nComment3:\nThe analysis of the EXAFS via fitting the data does not show any evidence for Ni\nnearest neighbors to theAl, or vice versa. But this is implied in Figure 1d. Do the Ni\nclusters expose metallic Ni atoms to the gas phase? If so, what stops them from doing\nthe non-selective hydrogenation of ethylene to ethane. Are the Ni clusters physically\ndistantfrom theAl atoms onthecarbon.\nReply:\nThanks for your constructive comment. Based on EDS results (Figure S6), the Ni\nclusters are surrounded by the Al atoms. However, no chemical bonding is observed\nbetween Al atoms and Ni species, as evidenced by the absence of Ni-Al bonding in\ntheEXAFS analysis based ondatafitting.\nIndeed, Ni clusters expose metallic Ni atoms to the gas phase. When Ni clusters are\nsurrounded by theAl atoms, theΔG value, representing the free energy change of\nRDS\nthe rate-determining step, on Al sites is lower than that on Ni sites (Figure S35a).\nMoreover, compared to *C H desorption, *C H hydrogenation is less favorable on\n2 4 2 4\nAl sites but more facile on Ni sites (Figure S35b). These thermodynamic results\nindicate that Al sites exhibit more favorable catalytic kinetics for the selective\nhydrogenation of acetylene, which is further supported by our calculated turnover\nfrequency (TOF) values (Figure S36). Therefore, despite the stronger adsorption on\nNi sites, the higher activity observed on Al sites suggests that they serve as the\nprimary active centers for acetyleneselective hydrogenation.\nFigureS6TheEDXelementmappingofAl -Ni /NCNTcatalyst.\nFigureS35a.Thefreeenergyoftherate-determiningstep(RDS)ontheAl dual-atomsiteandNi\n2\n(111)surface;b.ThedesorptionenergyandhydrogenationenergyoftheC H moleculeontheAl\n2 4 2\ndual-atomsiteandNi(111)surface.\nFigure S36. Logarithmic turnover frequencies (log (TOF)) of Ni (111) andAl dual-atom sites at\n2\nthereactiontemperatureof430K.\nComment4:\nThere are several aspects ofthecatalytic measurements that need clarification:\na. It is stated that the Al /NCNT catalyst shows no influence of temperature on the\n2\nconversion or selectivity. Does the reactor system have any background reactivity? It\nseems oddthat there is.\nReply:\nThanks for your question. The performance evaluation results of the reactor system\nhave been presented in Figure S15. In the absence of any catalysts, the reactor system\nexhibits no reactivity. Regarding the increase in reaction rate with temperature over\nthe Al /NCNT catalyst, we apologize for the previous description, which may have\n2\ncaused confusion; thishas been revised accordingly in theupdated manuscript.\nFigureS15Reactivityofreactorsystem.\nRevisionsmade inthemain text:\nRemarkably, atomic-scale Al sites exhibit excellent activation ability of acetylene\non the premise of excluding the interference of NCNT support (Figure S14) and\nreactor system (Figure S15), with Al /NCNT displaying better catalytic performance\n2\ncompared to single-atomAl counterpart (Figure 3a).This behavior markedly contrasts\nwith the inert α-Al O and Al powders, which is usually used as substrate to load Pd\n2 3\ncatalysts for C selective hydrogenation in the industry. However, higher acetylene\n2\nconversion for Al /NCNT catalysts requires the elevated reaction temperature\n2\n(>240 °C in Figure 3a and S16) although exhibiting excellent ethylene selectivity\n(94.0%).\nb. In the supporting information, they need to show more details of the reaction\nproducts, for instance the concentrations of acetylene, ethylene and ethane as a\nfunction of temperature, or a function of space velocity, so the reader can see when\nthe formation of ethane starts and what happens as acetylene is fully consumed.\nFigure 3 only shows conversion and selectivity. It is important to see all the products\nand to understand howmuch ethane is being produced.\nReply:\nWe sincerely appreciate your helpful comments. The concentrations of acetylene,\nethylene, ethane and oligomers as a function of temperature have been supplemented\nin Figure S17-S18. It is found that ethane starts to form overAl -Ni /NCNT catalyst\nwhen the temperature increases to 88 °C. When acetylene is fully consumed, the\nethane is further generated; however, the corresponding amount is significantly lower\nthan that ofNi /NCNTcatalyst.\nNC\nBased on your suggestion, the selectivity of all products have been presented in\nFigure 3b-d. As expected, Al -Ni /NCNT significantly enhances catalytic activity,\nachieving 99.98% conversion at 157 °C, far superior toAl /NCNT(49.75%@ 340 °C)\n2\nand Ni /NCNT (99.93% @ 200 °C) (Figure 3a and Figure S17-S18). More\nNC\nimportantly, even at complete acetylene conversion, Al -Ni /NCNT maintains\nethylene selectivity of 90.26% (Figure 3b), with only minor formation of ethane\n(6.17%of selectivity) andoligomers (3.57% ofselectivity) in Figure 3c-d.\nFigure S17. The concentrations of (a) acetylene, (b) ethylene (c) ethane and (d) oligomers as a\nfunctionoftemperatureoverAl -Ni /NCNT.\nFigure S18. The concentrations of (a) acetylene, (b) ethylene (c) ethane and (d) oligomers as a\nfunctionoftemperatureoverNi /NCNT.\nNC\nFigure 3. Catalytic performance of acetylene hydrogenation. (a) Conversion as a function of\nreaction temperature;Selectivity of (b)C H , (c) C H and(d) oligomers as a function of reaction\n2 4 2 6\ntemperature;(e)Arrheniusplots;(f,g)Reactionorderofacetyleneandhydrogen,respectively;(h)\nDurabilitytestonAl -Ni /NCNTat142°C.(Reactioncondition:ahydrogentoacetyleneratioof\n20:1; space velocity = 9000 mLg−1 h−1; atmospheric pressure. Regeneration method is originated\nfromtheJamdomesticpetrochemicalforacetylenehydrogenation38,39).\nRevisionsmade inthemain text:\nThe catalytic conversion and selectivity of Al - and Al - based samples supported\n1 2\non NCNT were first evaluated in acetylene selective hydrogenation, with the\nconcentration of reactants and products as a function of temperature shown in Figure\n3.\nAs expected, Al -Ni /NCNT significantly enhances catalytic activity, achieving\n99.98% conversion at 157 °C, far superior to Al /NCNT (49.75% @ 340 °C) and\nNC\nimportantly, even at complete acetylene conversion, Al -Ni /NCNT maintains\nethylene selectivity of 90.26% (Figure 3b), with only minor formation of ethane\n(6.17%of selectivity) and oligomers (3.57%of selectivity), as depicted in Figure 3c-d.\nIn contrast, the Ni catalyst, which lacks adjacent Al dual-atom, exhibits markedly\nNC\nlower ethylene selectivity (43.70% at 99.93% conversion). Based on the\ntemperature-dependent concentration profiles of acetylene, ethylene, ethane and\noligomers (Figure S17-S18), it is found that ethane and oligomers start to form over\nAl -Ni /NCNT catalyst when the temperature increases to 88 °C; however, their\ncorresponding amounts are significantly lower than that of Ni catalyst. This\nNC\ndifference may be attributed to the formation of ethylidene (=CHCH ) on Ni\n3\nnanoclusters, resulting in excessive hydrogenation to ethane and polymerization into\noligomers (Figure 3c-3d).\nc. Did they quantify the rate of formation of oligomers, or green oil during their long-\nterm tests.\nReply:\nWesincerely thank the reviewer forthis comment.Theoligomer formation rate was\ncalculated at complete acetylene conversion (Figure S19), where Al -Ni /NCNT\nexhibits an oligomers formation rate of 0.29 mol mol -1 h-1. This is\noligomers Ni\nsignificantly lower than that of Ni /NCNT (1.72 mol mol -1 h-1), indicating\nNC oligomers Ni\nthat the construction of synergistic sites can effectively suppress the deposition of\ncarbonaceous species.\nFigureS19TheformationratesofoligomersoverNi /NCNTandAl -Ni /NCNTatcomplete\nNC 2 NC\nacetyleneconversion\nRevisionsmade inthemain text:\nMeanwhile, the lower formation rate of oligomers of 0.29 mol mol -1 h-1\noligomers Ni\noverAl -Ni /NCNT in Figure S19, compared to that of Ni /NCNT (1.72 mol\nmol -1 h-1) indicates that the construction of synergistic sites could suppress the\nNi\ndepositionof carbonaceous species.\nReviewer#3:\nThis manuscript reports the synthesis of Al dual-atom sites and Ni clusters (ca. 2.5\nnm average size) on nitrogen-doped carbon nanotubes (Al -Ni -NCNT) and its\ncatalytic activity for the selective semi-hydrogenation reaction of acetylene in\nethylene streams. The corresponding blanks (Al -NCNT, Ni -NCNT and also\nAl-NCNT solid catalyst) are also prepared and tested. The reactivity shows that while\nAl -Ni -NCNT and Ni -NCNT are not far in catalytic activity (confirmed by the\nsimilar activation energy), the former is more selectivity, since apparently Al avoids\nover-hydrogenation of acetylene to ethane (and coupling reactions) on the Ni cluster\nsites.The latter is due to a much higher adsorption of acetylene on the Ni sites than in\nAl, favoring the undesired further reactions. If this is so, it means thatAl overrules Ni\nwhen activating acetylene, and this is something that is not clear in the study. It could\nbe that H competes strongly with acetylene on Ni sites but, in this case, the catalytic\n2\nactivity ofNi -NCNTwould be muchlower,and it isnot, justthe selectivity.\nNC\nAnother point that makes the key claim of the study, i.e. that the symbiosis between\ndualAl sites (perform all the process except H dissociation) and Ni (H dissociation)\n2 2\ntriggers the reaction, to be doubtable is the absence of any significant characterization\nof the catalyst during or after reaction. The only data given are just a Raman, where\nany information on the metal sites cannot be obtained. The atomically-dispersed Al\nsites could be rapidly destroyed (aggregated, sublimated) under the thermal reaction\nconditions, even the Ni clusters could further aggregate or disperse. If the main\nadvance of the paper is the originality of the catalytic sites and the mechanism, the\nnature and stabilityof thefresh catalyst during reaction must beclearly demonstrated.\nWith that said, other issues could be considered less important, but nor for the high\nstandards of Nat. Commun.:\nReply:\nWe appreciate the reviewer’s insightful comments. These thermodynamic analyses\nindicate that Al sites facilitate more favorable catalytic kinetics for the selective\nhydrogenation of acetylene, which is further supported by our calculated turnover\nfrequency (TOF) values (Figure S36). Therefore, despite weaker adsorption of\nreactants, the superior intrinsic activity and enhanced selectivity of Al sites suggest\nthat they serve as theprimary active centers foracetylene selective hydrogenation.\nTo gain deeper insight into the adsorption behavior at Ni sites, we calculated the\nadsorption energies of *C H and *H under various *C H coverages (Figure\n2 2 2 2\nS37-S39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the\nadsorption energy of *C H shifts from initial -3 eVto +0.2 eV, indicating progressive\n2 2\nsurface saturation with adsorbed *C H species. In contrast, although *H adsorption\n2 2\nenergy exhibits a slight increase with increasing coverage, it remains negative,\nimplying that *H species can still be generated from H dissociation. Therefore, we\n2\npropose that while Ni sites exhibited strong adsorption towards *C H and *H, the\n2 2\nfavorable *C H adsorption mainly leads to high coverage. In contrast, the readily\n2 2\nformed *H species could migrate to adjacent Al sites, where they participate in the\nhydrogenationof *C H .\n2 2\nSimilar activation energies are observed for Al -Ni /NCNT and Ni /NCNT\ncatalysts in the kinetic regime, specifically at low acetylene conversion (< 15%).\nHowever, the main challenge of selective hydrogenation of acetylene in C2 streams\nlies in the strong competitive adsorption of acetylene, ethylene, and hydrogen at high\nconversion (particularly near complete conversion), which is governed not only by\nkinetics but also by thermodynamics. As expected, in this situation, the catalytic\nactivity of Ni /NCNT is indeed lower, with complete acetylene conversion achieved\nNC\nat 200 °C compared to 157 °C foe the Al -Ni synergistic catalysts. Such a\ntemperature gap is quite substantial in industrial process (Appl. Surf. Sci. 2022, 604,\n154497).\nBesides, additional and more comprehensive post-reaction characterizations have\nbeen provided in response to the reviewer's comments, including XRD, XPS, and\nXAS spectra, as well as regeneration testing. As shown in Figure S21-S23, XRD,\nXAS and XPS results reveal that the atomically-dispersed Al sites and Ni clusters\ncould be well maintained under the thermal reaction conditions. Consequently,\nAl -Ni /NCNT catalyst also exhibits good stability in a long-period running, even\nregeneration.\nFigure S36. Logarithmic turnover frequencies (log (TOF)) of Ni (111) andAl dual-atom sites at\n2\nthereactiontemperatureof430K.\nFigure S37. Adsorption energies of C H and H on the Ni (111) surface under different\n2 2\nC H coverages.\n2 2\nFigureS38.Atomicvisualizationof*C H ontheNi(111)surfaceatdifferent*C H coverages.\n2 2 2 2\nFigureS39.Atomicvisualizationof*HontheNi(111)surfaceatdifferent*C H coverages.\n2 2\nFigureS21XRDpatternofspentAl -Ni /NCNT.\nFigure S22 Fourier-transformed (a) Al and (b) Ni K-edge EXAFS spectra with fitting of spent\nAl -Ni /NCNT.\nFigureS23Al2pspectraofpost-reactionAl -Ni /NCNT.\nFigure 3h. Durability test on Al -Ni /NCNT at 142 °C. (Reaction condition: a hydrogen to\nacetylene ratio of 20:1; space velocity = 9000 mL g−1 h−1; atmospheric pressure. Regeneration\nmethodisoriginatedfromtheJamdomesticpetrochemicalforacetylenehydrogenation38,39).\nRevisionsmade inthemain text:\nIndeed, the Ni sites exhibit a significant increase in free energy during the\n*CH CH desorption step compared to its hydrogenation (Figure 5d and S33)\n2 2\nregardless of whether the systems involve clusters or bulk catalysts. Specifically, the\ndesorption energy of *CH CH on Ni sites of all sizes is markedly high, indicating\n2 2\nstrong adsorption. The subsequent hydrogenation of *CH CH readily occurs on both\n2 2\nNi clusters of various sizes and Ni (111) surface, with reaction energies ranging from\n-0.99 to -0.48 eV. This implies that the multi-bridged adsorption configuration\nstabilizes the *CH CH intermediate, rendering desorption energetically unfavorable\n2 2\nand leading to poor ethylene selectivity. These above results are in good agreement\nwith DRIFT analysis. Furthermore, the reaction energy for hydrogen dissociation has\nbeen calculated (Figure 5c and S34). Ni species display a stronger thermodynamic\npropensity for hydrogen activation than the Al site, which is energetically less\n2\nfavorable, demonstrating that the Ni (111) surface is highly effective at activating H .\n2 2 2\nHowever, catalytic performance is governed not only by the adsorption of reactants\nbut also by the free-energy change associated with the rate-determining step (ΔG ,\nRDS\nnamely activity) and the selectivity toward C H and C H . As calculated (Figure\n2 4 2 6\nS35a), the ΔG value on Al sites is lower than Ni sites. Furthermore, *C H\nRDS 2 4\nhydrogenation is thermodynamically less favorable compared to *C H desorption on\n2 4\nAl sites, whereas it is more facile on Ni sites (Figure S35b). These thermodynamic\nanalyses indicate that Al sites facilitate more favorable catalytic kinetics for the\nselective hydrogenation of acetylene, which is further supported by our calculated\nturnover frequency (TOF) values (Figure S36). Therefore, despite weaker adsorption\nof reactants, the superior intrinsic activity and enhanced selectivity ofAl sites suggest\nthat they serve as theprimary active centers foracetylene selective hydrogenation.\nTo gain deeper insight into the adsorption behavior at Ni sites, the adsorption\nenergies of *C H and *H under various *C H coverages were calculated (Figure\n2 2 2 2\nS37-S39). As the coverage increases from 0.125 to 0.875 monolayer (ML), the\nadsorption energy of *C H shifts from initial -3 eVto +0.2 eV, indicating progressive\n2 2\nsurface saturation with adsorbed *C H species. In contrast, although *H adsorption\n2 2\nenergy exhibits a slight increase with increasing coverage, it remains negative,\nimplying that *H species can still be generated from H dissociation. Therefore, we\n2\npropose that while Ni sites exhibited strong adsorption towards *C H and *H, the\n2 2\nfavorable *C H adsorption mainly leads to high coverage. In contrast, the readily\n2 2\nformed *H species could migrate to adjacent Al sites, where they participate in the\nhydrogenation of *C H . Based on the results from in situ spectroscopic\n2 2\ncharacterizations and theoretical calculations, it could be concluded that the\nconstruction of the synergistic Ni nanoclusters and Al active centers offers excellent\n2\nactivity and selectivity. In this system, nickel species facilitate H dissociation, while\n2\nthe Al sites promote the spillover of active H* species, enabling their reaction with\n2\nπ-bound acetylene and suppressing ethylene desorption, thereby inhibiting\nover-hydrogenationand oligomerization.\nNotably, Al -Ni /NCNT demonstrates exceptional thermal and chemical stability\nunder hydrogenation conditions, maintaining both activity and selectivity for at least\n150 h at 142 °C, with no visible decline even upon regeneration (Figure 3h). To\ninvestigate the nature and stability of this catalyst during reaction, XRD pattern and\nXAS analysis have been provided. As shown in Figure S21-S22, the\natomically-dispersed Al sites and Ni clusters of Al -Ni /NCNT could be well\nmaintained after the reaction. Meanwhile, Al 2p XPS results (Figure S23) further\nindicate that the average oxidation state of Al species well keeps between 0 and +3.\nThat is to say, the atomically-dispersed Al sites and Ni clusters could be well\nmaintained under the thermal reaction conditions, with excellent resistance to carbon\ndeposition (Figure S24). In contrast, Ni /NCNT shows a gradual decrease in\nNC\nethylene selectivity, approximately with a loss of ~90% within the first five hours\n(Figure S25),and Raman measurements indicatesevere coking (Figure S24).39\nRevisionsmade intheSupporting Information:\nThe used Al -Ni /NCNT catalyst by oxidative treatment to remove oligomers,\nfollowing a protocol adapted from industrial practices used at Jam domestic\npetrochemical for acetylene hydrogenation (J. Nat. Gas Sci. Eng. 2016, 34, 1382;\nChem.Eng. J. 2012, 198-199,491). Firstly, anitrogen stream at a flowrate of 100mL\nmin-1 was fed to the reactor at 115 °C for 60 min. The temperature was then increased\nto 165 °C while maintaining the nitrogen flow to stabilize the catalyst bed. In the next\nstep, in addition to nitrogen, water vapor (H O (g)) at a flow rate of 0.02 mL min-1\n2\nwas injected as the temperature was raised from 165 to 400 °C, and this temperature\nwas maintained for 90 min to wash out and reform light hydrocarbons. Subsequently,\nthe heavy hydrocarbons prone to coking were burnt by introducing oxygen with the\nflow rate gradually increased from 5 to 40 mL min-1 at 500 °C. Following this step,\nnitrogen was purged through the reactor for 130 min to displace residual oxygen.\nThen, a hydrogen stream at 10 mL min-1 was introduced to reduce the oxidized nickel\nspecies back to metallic Ni. Finally, nitrogen was passed through the system until the\nbed temperature decreased to ambient conditions, yielding the regenerated\nAl -Ni /NCNTcatalyst.\nComment1:\nCatalytic results: The conversion of acetylene is given in rough numbers (100%),\nthat means 99%, 99.99%? The industrial process requires less than 1 ppm in the final\nstream, although <10 ppm can be acceptable at the academic level. This has to be\nmeasured and explain how is measured (exact mass balance, not with a 0.5% error).A\n10% of ethane is very much, the industrial process requires less than 2%, otherwise\ntheeconomic penalty is huge.\nReply:\nWe sincerely thank you for your attention to this detail. The rough conversion of\nacetylene has been modified as the specific value (99.98% for Al -Ni /NCNT\ncatalyst and 99.93% for Ni /NCNT catalyst). As accurately determined by online\nNC\nGas Chromatography using an internal standard method, the initial concentration of\nacetylene was detected as 3109 ppm at 25 °C and the final acetylene concentration\ndecreased to 0.8 ppm at 157 °C for the Al -Ni /NCNT catalyst, which satisfies the\nindustrialrequirement (less than 1 ppm)in thefinal stream.\nBased on the plot of acetylene conversion and selectivity as the function of\ntemperature, it is found that the ethyleneselectivity could be higher than 98% at lower\nconversion (<75%). That is to say, the generated ethane is less than 2%, and thus\npossesses the high economy. This also complies with the operating at lower\nconversion in the industrial process to achieve high selectivity of the target product.\nMoreover, in this manuscript, we also emphasize higher conversion of acetylene, at\nthis situation of which the ethylene selectivity could still be maintained at >90% to\nachieve ~90% of yield, which is reported as typical values for some Pd industrial\ncatalysts (J. Am. Chem. Soc. 2018, 140, 8827; III. US patents WO 03/106020, 2003,\nand US0137433, 2005). However, it is still difficult to simultaneously achieve\ncompleteconversion ofacetylene and >98% ofethylene selectivity at theindustry.\nRevisionsmade inthemain text:\nAs expected, Al -Ni /NCNT significantly enhances catalytic activity, achieving\n99.98% conversion at 157 °C, far superior to Al /NCNT (49.75% @ 340 °C) and\nNC\nIn contrast, the Ni catalyst, which lacks adjacentAl dual-atom, exhibits markedly\nNC\nlowerethylene selectivity (43.70% at 99.93%conversion).\nHowever, despite having the same adsorption configuration of reactants, the\nAl -Ni /NCNTdrivesthe reaction faster (99.98%conversion @157°C).\nComment2:\nCatalyst nature: Why using CNTs if the support structure does not have any\ninfluence on the formation of the metal sites, as demonstrated by the Raman\nexperiments? In this case, a simple N-doped carbon should be enough. Is N doping\nreally essential? One could argue that O instead of N is stabilizing the dual sites (even\nperhaps Cl). The difference if XAS between N and O is very low. N XPS should help\nhere.Any evidence that all theClis gone? It could stillbe onthecatalyst.\nTheAl -catalyst is made by impregnation in water solution, and no characterization\n1\nis given. The likely structure here is Al O entities, thus comparison with the\n2 3\nAl -catalyst is quite doubtable. In any case, anyAl-N orAl-O single atom site is quite\n2\nprone to decompose under H atmospheres and severe heating, thus shedding more\n2\ndoubts onthecatalyst stabilityunder reaction conditions.\nFigure 1b (PXRD) does not guarantee the formation of Ni aggregates, it looks the\nopposite.\nReply:\nThanks for your constructive question. Based on the referenced literature (Dalton\nTrans. 2022, 51, 10898), metallic Ni nanoclusters and Al dual-atom species typically\nexhibit no obvious Raman signals, primarily due to their high polarizability, which\nmakes it difficult for atomic vibrations to induce a change in polarizability. In our\nsituation, as shown in Figure S4, although the Raman spectra display similar patterns\nwith the characteristic D and G peaks, along with comparable I /I intensity ratios\nD G\nacross Al -Ni /NCNT, Al /NCNT and Ni /NCNT catalysts, it does not mean that\nthe support structure has not influence on the formation of the metal sites due to the\npossibility of metal having no obvious Raman signals. To enhance the rigor of the\nmanuscript, therelevant discussions havebeen revised accordingly.\nCompared to a simple N-doped carbon, CNT is generally reported to have larger\nspecific surface areas, which could facilitate the dispersion of active metal sites and\nthereby prevent agglomeration (Nat. Catal. 2023, 6, 818). Meanwhile, N-doped CNT\ncould provide abundant nitrogen coordination sites for stabilizing active metal atoms\n(Nat. Synth. 2024, 3, 1427; Nat. Chem. 2020, 12, 764; Nat. Catal. 2025, 8, 248).\nConsequently, NCNT with high specific surface area is employed as the support for\nloading Ni nanoclusters and anchoring theatomically dispersedAl species.\nNC 2\nRegarding the coordination environment of Al species, Cl ligands stabilizing the\ndual sites can be ruled out, as M-Cl coordination typically appears at higher radial\ndistances, generally around 1.9 Å (Angew. Chem. Int. Ed. 2024, 63, e202401373).\nFurthermore, no Cl signal is detected in the XPS results of in Al /NCNT and\nWith respect to Al-O coordination, Al K-edge X-ray absorption near-edge structure\n(XANES) spectra (Figure 2) could exhibit a higher oxidation state for Al species,\nparticularly characteristic of Al O . In contrast, the Al valence state of Al -Ni/NCNT\n2 3 2\nis lower, ranging from 0 to +3. Meanwhile, the N 1s XPS spectrum of\nAl -Ni /NCNT in Figure S9 exhibits a positive binding energy shift (~0.6 eV)\ncompared tothat of NCNT(399.7 eV), suggesting theinteraction ofAl with Nspecies.\nTaken together, these results indicate the absence of Al-O coordination and confirm\nthat nitrogen atoms serve as the primary coordinating ligands stabilizing the Al dual\nsites.\nThe Al catalyst was prepared via wet impregnation in an aqueous solution. No\n2 3 1\n(Figure 1b). Furthermore, the aberration-corrected high-angle annular dark-field\nscanning transmission electron microscopy (AC HAADF-STEM) imaging reveals\nwell-dispersed Al single-atom sites as isolated bright spots on the NCNT support in\nAl /NCNT, highlighted by white circles in Figure 1e. Fourier-transform extended\ncoordination environment of Al species (Figure 2c). Aprominent peak forAl /NCNT\n1\nappears at approximately 1.3 Å corresponding toAl-N coordination33, while noAl-Al\ncoordination appears. These above results suggest the generation of Al single atoms\nrather thanAl O .\n2 3\nIn Figure 1b, the intensity of XRD peaks associated with Ni nanoclusters is low.\nTwo factors might account for this phenomenon: lower metal loading below the\ndetection limit or smaller particle size of Ni species (Science. 2024, 385, 295).\nMeanwhile, to confirm the formation of Ni aggregates, we performed HAADF-STEM\ncharacterization over Al -Ni /NCNT and Ni /NCNT catalysts, as shown in Figure\n1d and S5d. In both samples, Ni nanoclusters could be observed with an average\nparticle sizeofapproximately 2.5nm.\nBesides, additional and more comprehensive post-reaction characterizations have\nbeen provided in response to the reviewer's comments, including XRD, XPS, and\nXAS spectra, as well as regeneration testing. As shown in Figure S21-S23, XRD,\nXAS and XPS results reveal that the atomically-dispersed Al sites and Ni clusters\ncould be well maintained under the thermal reaction conditions. Consequently,\nAl -Ni /NCNT catalyst also exhibits good stability in a long-period running, even\nregeneration.\nFigure 1 Aberration-corrected HAADF-STEM images of (c) Al /NCNT (d) Al -Ni /NCNT (e)\n2 2 NC\nAl /NCNTwithoutthecircles.\n2 2 NC\nAl /NCNTwithoutthecirclesand(d)Ni /NCNT.\nFigureS8Cl2pXPSspectraofAl\n2\n-Ni\nNC\n/NCNTandAl\n2\n/NCNT.\nFigureS9N1sXPSspectraofAl\n2\n-Ni\nNC\n/NCNTandNCNT.\nFigureS21XRDpatternofspentAl -Ni /NCNT.\nFigure S22 Fourier-transformed (a) Al and (b) Ni K-edge EXAFS spectra with fitting of spent\nAl -Ni /NCNT.\nFigureS23Al2pXPSspectraofspentAl -Ni /NCNT.\nFigure 3h. Durability test on Al -Ni /NCNT at 142 °C. (Reaction condition: a hydrogen to\nacetylene ratio of 20:1; space velocity = 9000 mL g−1 h−1; atmospheric pressure. Regeneration\nmethodisoriginatedfromtheJamdomesticpetrochemicalforacetylenehydrogenation38,39).\nRevisionsmade inthemain text:\nIn contrast, no diffraction signals attributable to Al species are detected in the\nAl /NCNTandAl /NCNTsamples (Figure 1b).\n2 1\nFurthermore, aberration-corrected high-angle annular dark-field\nscanning\ntransmission electron microscopy (AC HAADF-STEM) imaging reveals that the\ndispersedAl single-atom and dual-atom sites inAl /NCNT andAl /NCNT are clearly\n1 2\nobserved ontheNCNTsupport, highlighted bywhitecircles in Figure 1c,1e and S5.\nBased on this, Fourier-transform extended X-ray absorption fine structure\n(FT-EXAFS) was utilized to investigate the coordination environment of Al species\n(Figure 2c). The two prominent peaks for Al -Ni /NCNT and Al /NCNT appear at\napproximately 1.3 and 2.2 Å (without phase correction) corresponding to Al-N and\nAl-Al coordination, respectively32, whereas Al /NCNT exhibits only the former\n1\nfeature.\nNotably, the support properties remain unchanged, as evidenced by Raman spectra\n(Figure S4), in which the spectra display similar patterns with the characteristic D\npeak at 1338 cm-1 and G peak at 1562 cm-1, along with comparable I /I intensity\nD G\nratios across Al -Ni /NCNT, Al /NCNT and Ni /NCNT catalysts. This consistency\nindicates that the carbon structures exhibit similar levels of disorder or defects.\nMeanwhile, the metallic Ni nanoclusters and Al dual-atom species shows no obvious\nRaman signals primarily due to their high polarizability, which hinders atomic\nvibrations from inducing measurable changes inpolarizability31.\nNotably, the possibility ofAl-Cl orAl-O coordination is excluded due to the higher\npeak position of M-Cl coordination at ~1.9 Å33, and the evidence of Al interaction\nwithN species from XPSresults (Figure S8and S9).\nIn addition to the diffraction features from the NCNT support, the X-ray diffraction\n(XRD) pattern ofAl -Ni /NCNT exhibits the weak characteristic diffraction peaks at\n44.5o and 51.8o(PDF#04-0850, Figure 1b), similar to those observed in Ni /NCNT.\nNC\nIn contrast, no diffraction signals attributable toAl species are detected, in agreement\nwith the pattern of atomically dispersed Al /NCNT and Al /NCNT samples (Figure\n2 1\n1b).\nFollowing Ni deposition, as shown in Figure 1d and S5d, small nanoclusters with\nthe lattice distances of 0.206 nm (corresponding to the Ni (111) plane) are observed,\nindicating a crystalline Ni structure with an average particle size of approximately 2.5\nnm.\nNotably, Al -Ni /NCNT demonstrates exceptional thermal and chemical stability\nunder hydrogenation conditions, maintaining both activity and selectivity for at least\n150 h at 142 °C, with no visible decline even upon regeneration (Figure 3h). To\ninvestigate the nature and stability of this catalyst during reaction, XRD pattern and\nXAS analysis have been provided. As shown in Figure S21-S22, the\natomically-dispersed Al sites and Ni clusters of Al -Ni /NCNT could be well\nmaintained after the reaction. Meanwhile, Al 2p XPS results (Figure S23) further\nindicate that the average oxidation state of Al species well keeps between 0 and +3.\nThat is to say, the atomically-dispersed Al sites and Ni clusters could be well\nmaintained under the thermal reaction conditions, with excellent resistance to carbon\ndeposition (Figure S24). In contrast, Ni /NCNT shows a gradual decrease in\nNC\nethylene selectivity, approximately with a loss of ~90% within the first five hours\n(Figure S25),and Raman measurements indicatesevere coking (Figure S24).39\nComment3:\nReaction conditions: The reaction temperatures employed here (>150 ºC, >300 ºC\nfor only Al) are extremely high for this reaction. The optimized reaction conditions\nemploy 20:1 H : ethylene, this is much more than standard front-end conditions.\nconditions, see for instance J. Am. Chem. Soc. 2018, 140, 8827–8832 (this article\nshould be cited and included in the comparative Table). This comparative Table is\npartially informative, it is true that hundreds of catalytic systems have been published,\nbut the selection here is quite random, and the commercial catalyst (Pd-Al O )\n2 3\noperates at much milderreaction conditions (35-70 ºC)than indicated in theTable.\nReply:\nWe gratefully acknowledge the valuable comments. Based on the reported\nreferences (Tables S5 and S6), the atomically dispersed non-noble metal catalysts\ngenerally require a higher reaction temperature than industrial Pd-based catalysts\n(Tables S7), owing to the relatively weak hydrogen activation capability. In this work,\nthe Al /NCNT and Al /NCNT catalysts operated at higher temperature are employed\n1 2\nas reference samples. Upon construction of the synergistic Al -Ni /NCNT catalyst,\nthe reaction temperature could be effectively decreased, reaching a level comparable\nto that of most catalysts reported in current references, such as J. Am. Chem. Soc.\n2018, 140, 8827. Inevitably, a trade-off between energy consumption and catalyst cost\nneeds to be considered during industrial scaling-up.Although thereaction temperature\nremains higher than that of commercial Pd catalysts, the development of non-noble\nAl -Ni /NCNTcatalyst in thiswork could significantly reduce thecost of catalysts.\nThe optimized H :C H ratio of 20:1 was employed based on the reported\n2 2 2\nreferences (Nat Commun. 2021, 12, 5770; ACS Catal. 2021, 11, 6073) and the\nindustrial conditions (Ethylene Industry 2009, 21, 62), falling within the range of\nfront-end condition of China Petroleum & Chemical Corporation (10-30%\nhydrogenation/0.5-1% acetylene). Meanwhile, based on your suggestion, the\nreference (J. Am. Chem. Soc. 2018, 140, 8827) has been added to the main text and\nTable S6, as well as the comparative data in the supplementary tables have been\nupdated, which is systematically classified as Ni-based catalysts (Table S5), Cu-based\nand Fe-based catalysts (TableS6)as well as Pd-based catalysts (TableS7).\nTableS5.PerformanceofNi-basedcatalystsreportedinreferences\nCatalysts Loading(%) M C H :H :C H T(℃) Conv. Sel.\n2 2 2 2 4\namount (vol.%) (%) (%)\n(mg)\nAl -Ni /NCNT 0.28 0.56 1:20:99 157 99.98 90.26\nNi Cu /g-C N 10 3.1 4.65 0.5:5:25 170 100 90\n1 2 3 4\nNiGa11 10 5 1:10:20 180 70 75\nNiCu/MMO12 10 10 0.33:0.66:34.5 160 100 70\nNi-Cu/r-Al O 13 4.7 14.1 1:10:99 135 100 86\n2 3\nNi@CeO 14 1.5 - 0.9:1.9:33.15 200 65 100\n3 0.7\nOCNT15\nAgNi /SiO 16 1.09 0.327 1:20:79 220 99 65\n0.5 2\nCuNi /SiO 16 0.41 0.123 1:20:0 300 100 60\n0.125 2\nNi/g-C N -T17 1.31 0.655 1:10:20 175 100 84\n3 4\nNiCu/ZrO 18 4.6 1.15 0.5:5:25 170 98 86\nn\nNa-Ni@CHA21 3.5 7 0.5:8:5 180 100 97\nNi MoS/Al O 22 3.5 17.5 0.15:3.03:15 125 100 90\n1 2 3\nNi Sb 23 10.7 32.1 0.5:2.5:30 260 100 93\n1 2\nTableS6.PerformanceofCu-basedandFe-basedcatalystsreportedinreferences\nM\nloading C H :H :C H Conv. Sel.\n2 2 2 2 4\nCatalysts amount T(℃)\n(wt%) (vol.%) (%) (%)\n(mg)\nCuZn/NC224 1.15 - 0.33:0.66:33 180 97 97.5\nCu /C N 10 8.1 12.15 0.5:5:25 200 <10 <60\n1 3 4\nCu /ND@G25 0.5 0.5 1:10:20 200 100 93\n2 3\nCu /ND@G27 0.25 0.5 1:10:20 200 95 98\n0.16 x28\nCuPd /SiO 29 4.96 1.488 1:20:20 160 100 85\n0.006 2\nCuBi/SiO (HTR)30 - - 0.5:10:20 100 100 91.1\naacetyleneconcentrationinanethylene(1mL/min,2bar)andH (2mL/min,4bar)flow\nCatalysts Loading(%) Mamount C H :H :C H T Conv. Sel.\n2 2 2 2 4\n(mg) (vol.%) (℃) (%) (%)\nPd/Al O 35 2.58 1.032 1:2:99 138 100 88\n2 3\nPd Cu /ND@G36 0.09 0.027 1:10:20 110 100 92\n1 1\nPd /TiO 37 0.146 0.0219 1:10:20 120 100 >50\n1 2\nPd Au @38 2.9 0.87 1.1:9.1:89.8 100 100 89\n1 1\nPd @Cu-SiW39 0.41 1.23 0.5:5:50 120 100 93\n8 44\nPd/Bi O /TiO 41 2.5 0.75 1:20:20 44 90 91\n2 3 2\nPd /N-graphene42 2.3 1.15 1:20:20 125 99 94\nPd /ND@G44 0.11 0.033 1:10:20 180 100 90\nPdIn/MgAl O 46 2 0.5 0.5:5:50 90 96 92\n2 4\nPd@SOD47 0.099 0.099 1:10:0 150 99.8 94.5\nPd@C/CNT48 1.5 0.075 0.5:3:20 150 93 70\nPd Sn/C49 0.97 2.716 1:2:99 160 97.5 91\n3 4\nISA-Pd/MPNC53 0.0433 0.0866 0.5:5:50 120 100 80\nPd-SAs@ZIF-854 0.16 0.00256 0.5:5:50 120 96 93.4\nPd /C N 55 0.5 - 0.5:1:25 115 99 83\n1 3 4\nPd/Ni(OH) 56 0.005 - 0.65:5:50 105 80 70\nPd/MgO58 0.16 0.032 1:10:20 140 100 70\nPd /ZnO59 0.01 50 2:20:40 100 100 80\n1 2\nPd/MgAl O 61 0.1 0.01 1:5:20 120 96 87\n2 4\nPd -N /CNT62 0.0092 0.0046 2:4:50 40 83 98\n1 8\nPd /GDY63 0.4 - 0.33:0.66:32.8 160 100 97\nPdAg/Al O 65 0.0003 - 0.738:0.738:64.594 45 90 80\n2 3\nReferences\n10. 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Dehghani O, Rahimpour M and Shariati A. An experimental approach on industrial Pd-Ag\nsupported α-Al O catalyst used in acetylene hydrogenation process: mechanism, kinetic and\n2 3\ncatalystdecay.Processes.2019,7,136.\nComment4:\nReaction mechanism: The characterization (HRTEM) claims that the dual Al sites\nare nearby the Ni clusters, surrounding them. There is not clear evidence of this (EDS\nmapping, Al-Ni reduced distances in XAFS, the XPS shift is very doubtable). In any\ncase, H-spillover from Ni toAl sites would not require any spatial approximation, and\nthat has not been considered. Please check (a simple test withWO will confirm the H\n3\nspillover).\nReply:\nWe gratefully acknowledge the reviewer’s insightful comments. As shown in EDS\nimages, theNi clustersaresurrounded bytheAl atoms.\nFigureS6TheEDXelementmapsofAl -Ni /NCNTcatalyst\nThanks for your constructive comment. We have conducted the hydrogen spillover\ntest using WO as the indicator material. In detail, WO is stuck to Al -Ni /NCNT\n3 3 2 NC\nusing the reported method from the reference (Angew. Chem. Int. Ed. 2024, 63,\ne202316319). As shown in Figure S28 below, the mixture of WO and\nreaction, which confirms theoccurrence of hydrogen spillover.\nFigureS28.ColorchangeofWO andAl -Ni /NCNTmixtures\n3 2 NC\nRevisionsmade inthemain text:\nAdditionally, it is observed that while the Al distribution remains uniform, Ni\nnanoclusters are segregated and surrounded by abundant Al dual-atom in\nAl -Ni /NCNT, as confirmed by the HAADF-STEM and EDS image (Figure 1d and\nS6).\nMeanwhile, an experiment was conducted in which WO was mixed with\nobserved (Figure S28), confirmingtheoccurrence ofhydrogen spillover45.\nComment5:\nMinor comment: The meaning of TPSR should be specified (not said neither in the\nmain text orSI).\nReply:\nThanks very much for your attention to this detail. TPSR stands for\ntemperature-programmed surface reaction, which has been specified inthe main text.\nRevisionsmade inthemain text:\nAs a result, and as expected, ethane and oligomers are prominently detected in the\nNi /NCNT catalyst, with no ethylene formation. This observation aligns well with\nNC\ntemperature-programmed surface reaction (TPSR) results (Figure S29), which show\nthat acetylene hydrogenation onAl -Ni /NCNTstarts at alower temperature (77°C).\nRevisionsmade intheSupporting Information:\nTemperature-programmed surface reaction (TPSR) experiment was performed in a\nreaction cell. 0.2 g of sample was pretreated in N for 30 min. Then, the gas was\n2\nswitched to 0.31% C H /30.40% C H /6.20% H (balanced with nitrogen) with flow\n2 2 2 4 2\nrate of 30 mL/min and heated from 40 to 380 °C for data recording. The products\n(m/z of C H , C H and C H being 26, 27 and 15, respectively) were measured via\n2 2 2 4 2 6\nmass spectrometer.\nComment6:\nIn conclusion, although recognizing that the activation of acetylene on\natomically-dispersed Al sites for the semi-hydrogenation reaction is of academic\ninterest, the study here does not demonstrate that this is occurring, since the stability\nof the Al site during reaction and a significant catalytic performance beyond that of\ntheNi sites, isnot demonstrated.\nReply:\nThanks for your constructive comment. As suggested in your comments, we have\nconducted additional experiments including HAADF-STEM and XAS\ncharacterizations of the Al -Ni /NCNT catalyst after reaction, hydrogen spillover\nand soonto improvethis manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have carefully reviewed the revised manuscript and the authors’detailed responses to my comments. They have made substantial and satisfactory efforts in addressing all the raised issues. The manuscript has been significantly improved and is now clear, compelling, and ready for publication.\nTherefore, I am pleased to recommend its acceptance in its current form.", "reviewer_2": "The authors have revised the manuscript and have addressed my concerns adequately.", "reviewer_3": "I recognize that the authors have made an effort to answer to all Reviewer´s requirements, including mine. However, two main issues still puzzle me. Regarding characterization of the spent catalyst, the only technique shown here that gives real information about the isolated Al atoms is the XAS, but the spectra are presented in a very general way, without comparing with the fresh one. This has to be done. Second, and more important, one of the Reviewers insightfully asked about the regeneration of the solid catalyst with O2, to burn the residual carbonaceous molecules. The authors claim that the isolated Al sites are able to bear the treatment with O2 at 500 ºC without oxidation. This is completely unbelievable, if so, this is the discovery here: A solid that keeps Al unoxidized at 500 º in O2. Catalysis with O2 should be tested! Trying to be positive here, the only explanation that comes to my mind in order to accept that the catalyst preserves structure and activity after treatment with O2 at 500 ºC, is that the Al sites are originally oxidized and that, under the H2 conditions and with the Ni sites continuously supplying H atoms by spillover during reaction, the Al sites become active for acetylene. However, the most plausibly explanation is that Al has noting to do with acetylene activation, but exerts some kind of modification on Ni. All these arguments should be considered by the authors before publication."}, "author_response": "Response to reviewers\nReviewer #1\nI have carefully reviewed the revised manuscript and the authors’detailed responses to\nmy comments. They have made substantial and satisfactory efforts in addressing all the\nraised issues. The manuscript has been significantly improved and is now clear,\ncompelling, and ready for publication. Therefore, I am pleased to recommend its\nacceptance in its current form.\nReply:\nWe are deeply grateful for your positive assessment and confirmation that our revised\nmanuscript is acceptable for publication.\nReviewer #2\nThe authors have revised the manuscript and have addressed my concerns adequately.\nReply:\nWe are deeply grateful for your positive assessment.\nReviewer #3\nI recognize that the authors have made an effort to answer to all Reviewer´s\nrequirements, including mine. However, two main issues still puzzle me. Regarding\ncharacterization of the spent catalyst, the only technique shown here that gives real\ninformation about the isolated Al atoms is the XAS, but the spectra are presented in a\nvery general way, without comparing with the fresh one. This has to be done.\nReply:\nThanks for your valuable comments and critical questions. Based on the suggestion\nof reviewer, XAS results of fresh and spent Al -Ni /NCNT have been compared in\nFigure S22, in which both exhibit the similar coordination features, indicating that the\natomically-dispersed Al sites and Ni clusters could be well maintained under the\nthermal reaction conditions.\na b\nAl-N\nNi-Ni\nAl-Al\n3 (\nk)\nT\nk c fresh 3 (\nk)\nT\nk c fresh\nF F\nspent spent\n0 2 4 6 0 2 4 6\nR (Å) R (Å)\nFigure S22 Fourier-transformed (a) Al and (b) Ni K-edge EXAFS spectra of spent Al -Ni /NCNT\ncompared with fresh ones (grey: C; blue: N; pink: Al; blue grey: Ni)\nSecond, and more important, one of the Reviewers insightfully asked about the\nregeneration of the solid catalyst with O , to burn the residual carbonaceous molecules.\n2\n500ºC without oxidation. This is completely unbelievable, if so, this is the discovery\nhere: A solid that keeps Al unoxidized at 500ºC in O . Catalysis with O should be tested!\n2 2\nTrying to be positive here, the only explanation that comes to my mind in order to\naccept that the catalyst preserves structure and activity after treatment with O at 500\n2\nºC, is that the Al sites are originally oxidized and that, under the H conditions and with\n2\nthe Ni sites continuously supplying H atoms by spillover during reaction, the Al sites\nbecome active for acetylene. However, the most plausibly explanation is that Al has\nnothing to do with acetylene activation, but exerts some kind of modification on Ni. All\nthese arguments should be considered by the authors before publication.\nReply:\nSincerely apologies for the ambiguous description that led to the misinterpretation.\nIn the previous response and revised manuscript, we described the spent catalysts as\nbelow:\n“Al -Ni /NCNT demonstrates exceptional thermal and chemical stability under\nhydrogenation conditions, maintaining both activity and selectivity for at least 150 h at\n142 °C, with no visible decline even upon regeneration (Figure 3h). To investigate the\nnature and stability of this catalyst during reaction, XRD pattern and XAS analysis have\nbeen provided. As shown in Figure S21-S22, the atomically-dispersed Al sites and Ni\nclusters of Al -Ni /NCNT could be maintained after the reaction. Meanwhile, Al 2p\nXPS results (Figure S23) further indicate that the average oxidation state of Al species\nwell keeps between 0 and +3, which is basically in agreement with the fresh one.\nAdditionally, the excellent resistance to carbon deposition is also confirmed (Figure\nS24)”.\n“Raman and XAS results in the figure below demonstrate that the industrial\nregeneration process could effectively remove the oligomers and coking from the\nsurface of Al -Ni /NCNT catalyst, while the atomically dispersed Al sites and Ni\nnanoclusters remain intact. This is further supported by the catalytic performance of\nthe regenerated Al -Ni /NCNT catalyst, which is basically consistent with that of the\ncorresponding fresh catalyst”.\nNotably, the industrial regeneration process not only introduces oxygen to remove\nheavy hydrocarbons as the precursor of coking, but also employs subsequent nitrogen\nand hydrogen purging to eliminate residual oxygen and fully reduce the oxidized active\nspecies back to their fresh states. These could be confirmed by XAS and XPS spectra\n(Figure S22 and S23). Specifically, the active Al species in the fresh catalyst exhibit a\nvalence state intermediate between 0 and +3, which can be deeply oxidized to higher\noxidation states (e.g., inert Al3+) during regeneration. Upon subsequent exposure to H ,\n2\nthese inert Al species are reduced and reactivated, forming sites capable of acetylene\nadsorption, which could be supported by both spectroscopic evidence and DFT\ncalculations (Figures 4-5 and S35-S39).\na b\nAl-N\nNi-Ni\nAl-Al\n3 (\nk)\nT\nk c fresh 3 (\nk)\nT\nk c fresh\nF F\nspent spent\n0 2 4 6 0 2 4 6\nR (Å) R (Å)\nFigure S22 Fourier-transformed (a) Al and (b) Ni K-edge EXAFS spectra of spent Al -Ni /NCNT\ncompared with fresh ones (grey: C; blue: N; pink: Al; blue grey: Ni)\nAl 2p\n75.0 eV\n)\nu.\na.\n(\ny\nt\nsi\nn\ne\nt\nn\nI\n78 76 74 72\nBinding Energy (eV)\nFigure S23 Al 2p XPS spectra of spent Al -Ni /NCNT\nTo make it better understand for readers, the corresponding discussions have been\nmodified as below.\nRevisions made in the main text:\nThe regeneration method originates from Jam domestic petrochemical's process for\nacetylene hydrogenation and primarily involves coke combustion through the\nintroduction of oxygen at 500 °C, followed by a hydrogen treatment step to reduce the\noxidized Al and Ni species back to their fresh states37,38.\nNotably, Al -Ni /NCNT demonstrates exceptional thermal and chemical stability\nunder hydrogenation conditions, maintaining both activity and selectivity for at least\n150 h at 142 °C, with no visible decline even upon regeneration (Figure 3h). To\ninvestigate the nature and stability of this catalyst during reaction, XRD pattern and\nXAS analysis have been provided. As shown in Figure S21-S22, the atomically-\ndispersed Al sites and Ni clusters of Al -Ni /NCNT could be maintained after the\nreaction; meanwhile, Al 2p XPS results (Figure S23) further indicate that the average\noxidation state of Al species well keeps between 0 and +3, following the coking\ncombustion and a subsequent hydrogen activation, which is basically in agreement with\nthe fresh one. Additionally, the excellent resistance to carbon deposition is also\nconfirmed (Figure S24).\nBased on the results from in situ spectroscopic characterizations and theoretical\ncalculations, it could be regarded that the construction of the synergistic Ni nanoclusters\nand Al active centers offers excellent activity and selectivity, in which nickel species\n2\nare responsible for hydrogen dissociation, while the Al sites trigger the spillover of\n2\nactive H* to react with π-bound acetylene and weaken the desorption of ethylene\ninstead of simply modify Ni, inhibiting over-hydrogenation and polymerization.\nRevision made in the Supporting Information:\nThe used Al -Ni /NCNT catalyst by oxidative treatment to remove oligomers,\nfollowing a protocol adapted from industrial practices used at Jam domestic\npetrochemical for acetylene hydrogenation1. Firstly, a nitrogen stream at a flow rate of\n100 mL min-1 was fed to the reactor at 115 °C for 60 min. The temperature was then\nincreased to 165 °C while maintaining the nitrogen flow to stabilize the catalyst bed. In\nthe next step, in addition to nitrogen, water vapor (H O (g)) at a flow rate of 0.02 mL\n2\nmin-1 was injected as the temperature was raised from 165 to 400 °C, and this\ntemperature was maintained for 90 min to wash out and reform light hydrocarbons.\nSubsequently, the heavy hydrocarbons prone to coking were burnt by introducing\noxygen with the flow rate gradually increased from 5 to 40 mL min-1 at 500 °C.\nSubsequently, the heavy hydrocarbons prone to coking were burnt by introducing\noxygen with the flow rate gradually increased from 5 to 40 mL min-1 at 500 °C.\nFollowing this step, nitrogen was purged through the reactor for 130 min to displace\nresidual oxygen. Then, a subsequent hydrogen stream at 10 mL min-1 was introduced\nto reduce the oxidized Al and Ni species back to their fresh states. Finally, nitrogen was\npassed through the system until the bed temperature decreased to ambient conditions,\nyielding the regenerated Al -Ni /NCNT catalyst."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The questions have been properly answered, the manuscript seems ready for publication."}, "author_response": ""}]

Decision letter