Breaking the activity-selectivity trade-off for acetylene semihydrogenation by Pd_2 dual-atom site

Domain: neuroscience
Published: 2026-03-25
Editorial outcome: minor_revision

Abstract
Acetylene semihydrogenation is a critical process in the polyolefin industry by selectively
removing trace acetylene from ethylene-rich reformate. However, this reaction is generally limited by the
inherent activity-selectivity trade-off due to the thermodynamic advantage of overhydrogenation. Herein,
we develop a facile and straightforward strategy to construct Pd dual-atom sites anchored on defect-rich
surface-graphitized nanodiamond (ND@G) via the solvent-mediated dispersion of palladium carboxylate
driven by the chelation of palladium precursors via carboxylate anion. Cs-corrected HAADF-STEM
images coupled with XAS analysis unambiguously manifest the successful architecting of Pd dual-atom
sites. Compared to Pd single-atom sites, the obtained Pd /ND@G sample demonstrates superior catalytic
performance in acetylene semihydrogenation, with the corresponding TOF values increased from 0.151
s-1 to 1.953 s-1, without the obvious decline of ethylene selectivity (93.2%, at full acetylene conversion).
C H /C H -TPD, H -D exchange reaction, isotope-labeled TPSR combined with DFT calculations
confirm the effective co-activation of C H /H on Pd dual-atom sites while maintaining the weak
adsorption of ethylene similar to that on its single-atom sites, which can break the activity-selectivity
trade-off in acetylene semihydrogenation.

Introduction
Ethylene, as a key bulk chemical, has a global annual production exceeding 200 million metric tons, with
the majority of them being utilized in the polymer industry.1, 2 It is typically produced through the catalytic
cracking of naphtha or light hydrocarbons.3, 4 Consequently, this ethylene-rich reformate inevitably
contains a small amount of acetylene impurity (0.5 – 2%), which can poison the Ziegler-Natta catalysts
used in the downstream polymerization of ethylene due to the very strong π coordinated bond between
Ti3+ and C H .5-9 Therefore, it is crucial to remove acetylene from the ethylene-rich reformate below 1
ppm before it is applied in the polymer industry.10, 11 So far, acetylene semihydrogenation is critical for
ethylene purification in polyethylene production via the selective removal of ethyne without
overhydrogenation.12, 13 However, architecting a cost-effective catalyst with high activity and ethylene
selectivity in acetylene semihydrogenation poses an intractable challenge due to the thermodynamic
advantage of overhydrogenation and the linear scaling relations constraints of adsorption energies
between acetylene and ethylene.5, 14

Currently, Cu-, Ni-, and Pd-based catalysts are extensively studied in the acetylene semihydrogenation
reaction.15-18 Among them, alumina-supported Pd-based catalysts, especially PdAg bimetallic nano-
catalysts, have been industrially implemented in the acetylene selective hydrogenation because of their
excellent hydrogenation activity.19, 20 Nevertheless, the ethylene selectivity is generally unsatisfied on Pd
nanoparticles (NPs) or clusters owing to their relatively strong adsorption of ethylene (di-σ bond mode)
and the presence of β-PdH .21-23 To improve the selectivity of ethylene in acetylene semihydrogenation,
forming an intermetallic compound or alloy is a common strategy by suppressing the strength of ethylene
adsorption and the formation of PdH due to the diluted surface Pd dispersion.24-26 However, the increase
of ethylene selectivity in the above strategy comes at the expense of reduced Pd atom utilization efficiency
due to the nonparticipation of many more bulk Pd atoms in acetylene semihydrogenation. In contrast,
single-atomization of active sites is another typical strategy to optimize the ethylene selectivity in
acetylene semihydrogenation because of only ethylene’s weak π-bond absorption and absence of β-PdH
on single-atom sites.27, 28 Nonetheless, the hydrogenation ability of Pd single-atom sites is significantly
decreased compared to their NP counterparts due to the insufficient hydrogen dissociation resulting from
the diminished metallicity.27, 29 Apart from the inadequate hydrogen activation, the activity of acetylene
semihydrogenation on Pd single-atom sites is also suppressed by the competitive adsorption of acetylene
and H due to its inability to simultaneously activate multiple reactants.12, 30 At present, dual-atom
catalysts have attracted extensive research attention because they not only possess the advantages of
single-atom catalysts, such as high atom utilization efficiency and highly uniform active sites, but also
exhibit a synergistic effect in their dual-atom structure, optimizing the interactions between the active
sites and multiple reactants or intermediates.31-33 This may be conducive to alleviating the competitive
adsorption/activation of multiple reactants and breaking the intrinsic linear scaling relations of adsorption
energies among reactants, intermediates, and products. However, the reported method for dual-atom
catalysts synthesis, including anion replacement deposition-precipitation, metal ion recognition, and
precursor-preselected, generally concerns cost-prohibitive reagents and intricate operation procedures,
remaining a huge challenge.31, 34-37

Herein, we report a facile and straightforward protocol for architecting a defect-engineered graphene
hybridized nanodiamond (ND@G) supported Pd dual-atom catalyst via the solvent-dependent dispersion
of palladium carboxylate due to the effective chelation of carboxylate anion in specific solvents.38-40 The
structure of Pd /ND@G was systematically investigated by high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) and X-ray absorption analysis (XAS), confirming the
successful construction of Pd dual-atom sites. The obtained Pd /ND@G sample shows excellent
performance in acetylene semihydrogenation reaction compared to its single-atom counterpart
(Pd /ND@G), lowering the total conversion temperature of acetylene from 180 to 100 oC and increasing
the corresponding TOF values from 0.151 s-1 to 1.953 s-1, showing dramatic improvement in acetylene
semihydrogenation activity without the obvious decline of ethylene selectivity (93.2%, at full acetylene
conversion). C H /C H -TPD, H -D exchange reaction, and isotope-labeled TPSR confirmed the
effective simultaneous-activation of C H /H on Pd dual-atom sites while maintaining the weak
adsorption of ethylene similar to that on its single-atom sites, which can break the activity-selectivity
trade-off in acetylene semihydrogenation. Density functional theory (DFT) calculations manifested the
advantages of Pd dual-atom in optimizing the interaction between active sites and C H /C H , and
breaking the intrinsic linear scaling constraints of their adsorption energies.

Results
Characterization. Pd /ND@G and Pd /ND@G samples were prepared via the impregnation method,
utilizing the solvent-dependent dispersion of palladium carboxylate, followed by a sequential
pretreatment of O -H at 200 oC. It is reported that palladium acetate (Pd(OAc) ) exists as a monomer
species in acetic acid (HAc) solution, whereas palladium trifluoroacetate (Pd(TFA) ) forms a dimer in hot
acetone (CP) solution (Fig. 1a).38-40 The Pd cluster supported on ND@G was synthesized by a deposition-
precipitation strategy41, 42. The detailed procedures are listed in the supporting information. To remove
surface ligands of the as-obtained samples, the Pd /ND@G and Pd /ND@G were subjected to sequential
pretreatment with air and hydrogen at 200 oC, which is confirmed by the temperature programmed
oxidation (TPO) of Pd(OAc) and Pd(TFA) (Supplementary Fig. 1). The complete removal of
carboxylate ligands was confirmed by Fourier transform infrared spectroscopy, as evidenced by the
absence of their characteristic stretching vibration frequencies (Supplementary Fig. 2).40 Hereafter, the
notations of Pd /ND@G, Pd /ND@G, and Pd /ND@G refer to samples subjected to a sequential O -H
pretreatment. No obvious diffraction peaks for Pd are detected in the X-ray diffraction (XRD) patterns of
Pd /ND@G, Pd /ND@G, and Pd /ND@G, implying the high dispersion of Pd active sites
(Supplementary Fig. 3).41, 43 The actual loading of Pd in the above three samples was determined by
inductively coupled plasma atomic emission spectrometry, almost in line with their nominal loadings
(Supplementary Table 1).

To further elucidate the geometric structure of Pd sites at the atomic level, Cs-corrected HAADF-STEM
(Cs-HAADF-STEM) examination was performed with samples of Pd /ND@G, Pd /ND@G, and
Pd /ND@G. With atomic resolution, it was found that only Pd single atoms exist in the Pd /ND@G
sample, without the presence of Pd clusters or NPs, indicating the single-atom dispersion of Pd active
sites highlighted in yellow cycles (Fig. 1b-c and Supplementary Fig. 4). As regards Pd /ND@G, a large
number of Pd-Pd dual atom pairs are observed on the ND@G surface, as highlighted in the red rectangles.
In contrast, some single-atom Pd sites are also detected due to the decomposition of palladium carboxylate
during the ligand removal procedure (Fig. 1d-e and Supplementary Fig. 5). The Pd :Pd site ratio was
determined to be 51:49, based on the identification of 175 Pd dual-atom pairs and 179 Pd single-atom
sites via directly counting their site numbers in Cs-corrected HAADF-STEM images, indicating
approximately 33% of Pd carboxylate dimers in Pd /ND@G-No treatment sample decomposing into Pd
single-atom sites during calcination.44 The mean distance between Pd-Pd in Pd dimers is about 2.75 Å,
confirmed by their intensity profiles, demonstrating the formation of Pd dual-atom geometry (Fig. 1f-g
and Supplementary Fig. 6-7).35, 37 Similarly, no Pd clusters or NPs are present in Pd /ND@G, manifesting
the stable Pd sites without sintering under pretreatment. As a reference, the Cs-corrected HAADF-STEM
images of Pd /ND@ and its size distribution are shown in Supplementary Figs. 8 and 9. Pd active sites,
including single-atom, subnanometer clusters, as well as a small amount of Pd NPs, are present in the
Pd /ND@G sample.

To obtain deep insight into the electronic structure and local coordination geometry of Pd species in the
samples of Pd /ND@G and Pd /ND@G, the adsorption of Pd K-edge was collected for XAS analysis. Fig.
1h shows the normalized X-ray absorption near-edge spectra (XANES) of Pd K-edge in Pd /ND@G and
Pd /ND@G samples, located in the range of Pd foil and PdO, indicating the electron-deficient state of Pd
species.27, 30 Notably, the adsorption threshold of Pd K-edge in Pd /ND@G is lower than that of
Pd /ND@G, denoting the increased metallicity of Pd entity compared to Pd single-atom, namely
enhanced electron state density.35 This enhanced electron state density of Pd dimers is also evidenced by
the negative binding energy shift of the Pd 3d electron in the XPS spectra compared to that of Pd /ND@G.
(Supplementary Fig. 10). To further investigate the geometric construction of Pd species, extended X-ray
absorption fine structure (EXAFS) spectroscopy was performed. For Pd /ND@G sample, only a distinct
peak centered at 1.6 Å assigned to the Pd-C/O first-shell scattering was observed, whereas no signal
corresponding to Pd-Pd scattering at 2.5 Å was detected (Fig. 1i).28, 45 According to the Pd K-edge fitting,
Pd /ND@G shows a coordination number (CN) of 2.67±0.37 with a bond distance of 2.03 Å
(Supplementary Fig. 11 and Supplementary Table 2). The above results confirmed the single-atom
dispersed Pd sites. In contrast, a main peak located at 1.6 Å, accompanied by a weak band at 2.5 Å, is
present in the Pd /ND@G sample, implying the presence of both Pd-C/O and Pd-Pd scattering (Fig.1i).35,
37 In addition to Pd-C/O oscillation, Pd /ND@G also exhibits a Pd-Pd coordination with CN of 0.5±0.19
based on the Pd K-edge fitting (Fig. 1j, Supplementary Fig. 11, and Supplementary Table 2). Given that
the average CN of Pd-Pd in Pd /ND@G sample represents the weighted average of the Pd-Pd coordination
numbers in Pd and Pd sites (C.N.(Pd-Pd) = n(Pd )/[n(Pd )+n(Pd )] Eq. 1, where Pd exhibits zero Pd-
Pd coordination and Pd has a Pd-Pd coordination number of 1), it is concluded that the site number ratio
of Pd to Pd is 1:1, consistent with the value derived from microscopic statistical analysis of Cs-corrected
HAADF-STEM images. These consequences, combined with Cs-corrected HAADF-STEM images of
Pd /ND@G, unambiguously demonstrated the formation of Pd dual-atom pair sites (Fig. 1d-g and
Supplementary Fig. 5-6).

Catalytic performances. Subsequently, the catalytic performance of the as-obtained Pd-based catalysts
with single-atom, dual-atom, or cluster active sites was systematically evaluated for the semi-
hydrogenation of acetylene under C H /H -rich co-feed, which is an intractable challenge due to the facile
overhydrogenation of ethylene.46-48 The acetylene conversion over Pd /ND@G is below 10% at 60 oC and
reaches complete conversion only at high temperatures above 180 oC, showing inadequate catalysis of the
Pd single-atom sites in acetylene semihydrogenation due to its inability to simultaneously activate C H
and H despite its superior ethylene selectivity (Fig. 2a and 2b).27, 30 In contrast, the Pd cluster sites exhibit
excellent activity in acetylene semihydrogenation with total conversion of C H even at 60 oC (Fig. 2a).
However, the ethylene selectivity over Pd /ND@G is even lower than -500% at 90 oC, implying
consumption of much C H in the co-feed gas due to overhydrogenation (Fig. 2b).49, 50 Compared to
Pd /ND@G and Pd /ND@G, the complete conversion of acetylene over Pd /ND@G decreased from 180
oC to 100 oC with the ethylene selectivity up to 93.2%, indicating significantly increased activity of Pd
dual-atom sites in acetylene semi-hydrogenation owing to its’ increased electronic state density and
synergistic effect in Pd geometry structures (Fig. 1 and 2a-b). Notably, the ethylene selectivity of Pd
dual-atom sites at full C H conversion is comparable to that of the Pd single-atom sites, which can also
be indirectly verified by their activity in the ethylene hydrogenation reaction (Supplementary Fig. 12).
Apart from the excellent activity and selectivity in acetylene semihydrogenation, the long-term stability
of Pd /ND@G is tested at 70% conversion or full conversion of acetylene (Fig. 2c and Supplementary
Fig. 13). It can be observed that the Pd /ND@G sample exhibits remarkable stability during acetylene
semihydrogenation, without any obvious decline in acetylene conversion (Fig. 2c) or ethylene selectivity
(Supplementary Fig. 13). Additionally, Pd /ND@G sample after long-term stability test, labeled as
Pd /ND@G-used, was characterized with Cs-corrected HAADF-STEM. The Cs-corrected HAADF-
STEM images revealed only Pd dual-atom sites and Pd single-atom sites, with no observation of Pd
clusters or nanoparticles (Supplementary Fig. 14). Actually, the high ethylene selectivity during the long-
term stability test also indirectly excluded the presence of Pd clusters or nanoparticles due to their
tendency to over-hydrogenation (Supplementary Fig. 13). This is primarily attributed to the abundant
carbon defects on the ND@G surface, which enable the robust anchoring of the Pd dual-atom site,
evidenced by their change in the I /I ratio of the Raman spectra (Supplementary Fig. 15). Furthermore,
to elucidate the role of the ND@G support in stabilizing Pd dual-atom sites, Pd , Pd , and Pd sites
supported on silica and α-alumina (commonly used supports in industry) were evaluated in the acetylene
semihydrogenation. The catalytic performance of Pd₁, Pd₂, and PSd sites on both supports was similar:
complete acetylene conversion was achieved at 80°C, but the selectivity remained below -150%,
indicating over-hydrogenation of ethylene (Supplementary Fig. 16). This is primarily because the surfaces
of silica and alumina supports lack specific sites capable of strongly anchoring Pd₁ or Pd₂, leading to the
sintering of single-atom and dual-atom sites into clusters or particles.

To further quantitatively investigate the differences in catalytic performance of various Pd active sites in
acetylene semihydrogenation, the TOF value of converted acetylene and ethylene space-time yield at 80
oC was calculated (Fig. 2d). The Pd dual-atom sites exhibit superior catalytic performance in acetylene
semihydrogenation compared to the Pd single-atom sites with a 12.9-fold increase in the TOF values
(1.953 s-1 vs 0.151 s-1), indicating distinguishable enhancement in catalytic activity. The specific activity
of Pd /ND@G is compared to the data reported in published literature, surpassing most results (Fig. 2e
and Supplementary Table 3). Although Pd /ND@G shows a much higher TOF value than that of
Pd /ND@G, the space-time yield of ethylene on the Pd cluster is as low as -120.5 mol g -1h-1,
which means a large amount of ethylene is consumed in the co-feed gas (Fig. 2d). Besides, the apparent
kinetic activation energy barriers based on Arrhenius plots reveal that Pd /ND@G shows a lower
activation energy compared to that of Pd /ND@G ( 25.3 kJ/mol vs 43.3 kJ/mol), which further verifies
the superior activity of acetylene semihydrogenation on Pd dual-atom sites (Fig. 2f).

Mechanism study. To elucidate the underlying reason for the notable difference in the catalytic
performance among various Pd sites, the temperature-programmed experiments are conducted to
investigate the adsorption, activation, and co-activation behaviors of the reactants/products (C H , C H ,
and H ) on Pd , Pd , and Pd sites. C H - and C H - TPD measurements are conducted to probe the
adsorption/desorption ability of acetylene and ethylene on the obtained Pd-based catalysts (Fig. 3a-b).
The main desorption peaks of acetylene on Pd /ND@G, Pd /ND@G, and Pd /ND@G samples are
centered at 82, 88, and 88 oC, respectively, which may be assigned to the π-bonded C H (Fig. 3a).30, 51
Apart from the main desorption peak at 88 oC, a shoulder peak at 125 oC is also observed on Pd clusters,
which may be attributed to the di- bond adsorption of acetylene (Fig. 3a).29, 52 Compared to Pd /ND@G,
the higher desorption peak observed on Pd dual-atom sites indicates a stronger binding affinity. In
contrast, ethylene remains physically adsorbed on both Pd and Pd sites due to its complete removal
during the stabilization of the baseline at room temperature, which is confirmed by the lack of desorption
peaks in the range of 40-200 oC (Fig. 3b). The C H /C H -TPD results reveal that Pd dual-atom sites
exhibit enhanced acetylene adsorption while preserving the physical adsorption of ethylene, which may
contribute to its high activity and ethylene selectivity in acetylene semihydrogenation.

In addition to C H /C H adsorption, the H -D exchange reaction is conducted to probe the H activation,
a factor influencing the catalytic performance of acetylene semihydrogenation (Fig. 3). The H -D
exchange reaction attains equilibrium on Pd /ND@G even at room temperature, verified by the constant
concentration of HD, indicating the facile dissociation of H , which accounts for its propensity for
overhydrogenation (Fig. 2b, 3c, Supplementary Fig. 12, and Supplementary Fig. 17). In contrast, Pd
single-atom sites show weak hydrogen dissociation ability, confirmed by the balanced conversion
temperature of 120 oC, in line with its low activity in acetylene semihydrogenation (Fig. 2a, 3c, and
Supplementary Fig. 17). For Pd dual-atom sites, the HD ion current at equilibrium is achieved at a
temperature of 80 oC, indicating its increased H dissociative activation (Fig. 3c and Supplementary Fig.
17). Besides, the isotope-labeled TPSR measurements are performed to explore the competitive
adsorption-activation between C H and H (Fig. 3d and Supplementary Fig. 18). The isotopic tracer
experiment is conducted by initially flowing a mixture of H , D , and He over a Pd-based sample, followed
by substitution of He with the same acetylene flow to maintain the total feed rate, excluding the influence
of gas flow rate on HD concentration. The intensity of competitive adsorption-activation between C H
and H can be quantitatively assessed by monitoring the evolution of HD concentration after acetylene
introduction. The HD signal intensity over Pd /ND@G is virtually eliminated upon switching the He flow
to acetylene, demonstrating that the competitive adsorption-activation between C H and H on Pd single-
atom sites significantly inhibits its catalytic activity in acetylene semihydrogenation (Fig. 3d and
Supplementary Fig. 18). In contrast, the intensity of the HD signal on the Pd dual-atom or Pd cluster
still remains notably considerable despite a reduction due to the competitive adsorption of C H and H ,
indicating its significant alleviation. Additionally, the ethylene and ethane peak intensities observed in
the isotope-labeled TPSR experiment on Pd-based samples align well with their corresponding catalytic
performance in acetylene semihydrogenation (Supplementary Fig. 18).

DFT calculations. To investigate the underlying mechanism and reveal the catalytic origin of the Pd
dual-atom sites in the semi-hydrogenation of acetylene, we conducted density functional theory (DFT)
calculations. First, two atomically dispersed catalyst models (Pd /ND@G and Pd /ND@G) were
constructed based on coordination number characteristics derived from EXAFS fitting. These correspond
to a single Pd atom anchored at a dual-carbon-defect site (a defect region where three adjacent carbon
atoms are missing) in graphene (Pd /ND@G) and a Pd dual-atom stabilized at a tri-carbon-vacancy site
(Pd /ND@G), respectively. The optimized structure of Pd /ND@G shows that the Pd atoms exhibit Pd-
C bonds (coordination number, CN = 2.5; bond length, R = 1.96 Å) and an intradimer Pd-Pd bond (R =
2.55 Å). In addition, a Pd(111) unit cell comprising 36 Pd atoms was constructed to represent the
Pd /ND@G sample based on the size distribution of the Pd cluster (Fig. 1i-1j, Supplementary Fig. 9, 11,
19, and Supplementary Table 2). Geometric optimization results show that the structural parameters, such
as coordination numbers and bond lengths, of the constructed models agree well with experimental values
(Supplementary Table 2 and Table 4). Bader charge analysis reveals that the charge transfer trend of Pd
in Pd /ND@G is consistent with the results of XPS and XANES, which further confirms the increased
metallicity of the Pd site compared to that of Pd (Fig. 1h, Supplementary Fig. 10, and Table 4).

Fig. 4 shows the reaction pathway of acetylene hydrogenation over Pd /ND@G, Pd /ND@G, and
Pd /ND@G samples. The calculated adsorption energies of acetylene become increasingly negative
across the order: Pd /ND@G (–1.05 eV), Pd /ND@G (–1.11 eV), and Pd /ND@G (–2.46 eV,
Supplementary Fig. 20), indicating stronger adsorption and higher activation capability, consistent with
the C H -TPD results (Fig. 3a). Accordingly, the energy barriers for the rate control step reveal a distinct
activity trend in acetylene semihydrogenation activity. The highest transition state energy, corresponding
to the competitive adsorption/activation of C H and H (TS1), on Pd /ND@G is the lowest (–1.85 eV),
followed by Pd /ND@G (1.04 eV), and is highest on Pd /ND@G (1.93 eV, Fig. 4). This difference in
transition state energy between the above three samples indicates that the activity follows the order:
Pd /ND@G > Pd /ND@G > Pd /ND@G, in line with their reaction activity (Fig. 2a, 2d, and 2f). However,
despite the superior activity, Pd /ND@G exhibits poor ethylene selectivity due to its higher desorption
energy (–1.65 eV) than the energy barrier for further hydrogenation to C H (–2.37 eV), implying that
ethylene tends to undergo over-hydrogenation to ethane rather than desorb, leading to poor selectivity
(Fig. 2b). This high desorption energy of ethylene on Pd /ND@G is consistent with its C H -TPD profile
(Fig. 3b). In contrast, on both Pd /ND@G and Pd /ND@G, the ethylene desorption energy is lower than
the barrier for C H formation, in line with the physical adsorption of ethylene on both Pd sites, confirming
the high selectivity toward ethylene (Fig. 2b and 3b). Furthermore, the H -D exchange reaction pathway
reveals a substantially lower energy barrier on Pd /ND@G (0.96 eV) than on Pd /ND@G (1.66 eV),
further indicating the superior kinetic performance of the Pd₂ dual-atom site in hydrogen activation (Fig.
3c, Supplementary Fig. 14 and 21). DFT calculations demonstrated the advantages of Pd dual-atom in
optimizing the interaction between active sites and C H /C H , and overcoming the intrinsic linear scaling
constraints of their adsorption energies, which can break the activity-selectivity trade-off in acetylene
semihydrogenation.

Methods
Materials. Nanodiamond (ND) powders were purchased from Beijing Grish Hitech Co. Ltd. China. Palladium acetate
(Pd(OAc) ) and palladium trifluoroacetate (Pd(TFA) ) were purchased from Aladdin. Acetone (CP) and acetic acid (HAc)
were purchased from Sinopharm Chemical Reagent Co. Ltd. China. All chemicals were analytical grade.
Preparation of ND@G. The ND powders were calcined at a temperature of 1300 K for 4 hrs under an argon flow with
a rate of 100 mL/min. Upon cooling to room temperature, a graphene hybridized nanodiamond (ND@G) was obtained.
This sample was successively purified by washing with concentrated hydrochloric acid and water. Ultimately, the purified
ND@G powders were dried in a vacuum at 80 oC overnight.
Preparation of Pd /ND@G and Pd /ND@G. The Pd /ND@G and Pd /ND@G samples are prepared by the
impregnation method. 200 mg of purified ND@G powder was impregnated into 2 mL of a certain concentration of
Pd(OAc) in acetic acid solution and Pd(TFA) in a hot acetone solution (0.1 mg /mL). This slurry was stirred at room
temperature for 24 h, followed by drying under vacuum at 80 oC overnight. Finally, the obtained samples were pretreated
with air at 200 oC for 1 hr, followed by reduction with 10 vol% H /He at 200 oC for 1 hr, denoted as Pd /ND@G and
Pd /ND@G.
Preparation of Pd /ND@G. The Pd /ND@G sample was prepared by the deposition-precipitation method. 200 mg of
ND@G powder was dispersed into 30 mL of deionized water in a 100 mL round-bottom flask by ultrasonic treatment
for 30 mins. The pH value of the above suspension was adjusted to 10 by dropping a certain amount of Na CO solution
(0.25 M). 12.5 μL of Pd(NO ) solution containing 16 mg /mL was added to 3mL deionized water, followed by tuning
its pH value with 3 drops of Na CO solution (0.25 M). Subsequently, the Pd solution was added dropwise into the
ND@G suspension under stirring at 100 oC for 1h. After cooling to room temperature, the sample was filtered and washed
several times with deionized water. Finally, the powder was dried in a vacuum oven at 80 oC overnight. The obtained
Pd-based catalyst was pretreated 10% H at 250 oC for 1 hr, denoted as Pd /ND@G.
Catalysts characterization. The Pd loading of Pd /ND@G, Pd /ND@G, and Pd /ND@G samples was determined by
an inductively coupled plasma atomic emission spectrometer (ICP-AES). The powder X-ray diffraction (XRD) patterns
of the catalysts were recorded on a X’ Pert PRO diffractometer operated at 40 kV and 40 mA using a Cu Kα radiation
source (λ = 0.15432 nm), with a scanning rate of 10°/min. X-ray photoelectron spectroscopy (XPS) measurements were
conducted on an ESCALAB 250 instrument equipped with an Al Kα X-ray source (1489.6 eV, 150 W, 50.0 eV pass
energy). The binding energies in XPS were referenced to the C 1s binding energy at 284.8 eV. Cs-corrected HAADF-
STEM images were recorded by a JEOL JEM ARM 200CF cold-field-emission scanning transmission electron
microscope operated at 200 kV. Temperature-programmed desorption (TPD) of C H / C H was performed in a quartz-
bed flow reactor equipped with an online mass spectrometer (MS, Pfeiffer OMNIstar) for gas mixture analysis. The
150 mg of sample was first pretreated under a 10 vol.% H /He mixture at 200 °C for 1 hr, followed by cooling to room
temperature in a He flow. After cooling, a gas mixture of 1 vol.% C H or C H balanced with He was introduced into
the reactor for 3 hours at 25 °C (flow rate = 20 mL/min) to saturated adsorption. Subsequently, the gas flow was switched
to pure He flow (flow rate = 10 mL/min) for purging the physically adsorbed C H /C H . The mass spectrometer
continuously monitored the signal intensities at m/z = 26 (C H ), 28 (C H ), 18 (H O), and 4 (He). Once the MS signals
stabilized, the sample was heated under a He flow (10 mL/min) to 350 oC at a rate of 2 °C/min while continuously
recording the mass spectrum. H -D exchange experiments were performed in a quartz-bed flow reactor with an online
mass spectrometer (MS, Pfeiffer OMNIstarTM) to evaluate gas mixtures. Typically, a certain amount of sample
(Pd /ND@G and Pd /ND@G 10 mg; Pd /ND@G using 5 mg) mixed with 500 mg of quartz sand was reduced with 10
vol.% H in He at 200 oC for 1 h (flow rate = 20 mL/min) and cooled to room temperature in He flow. Then, the mixture
gas of 10 vol.% H /He (20 mL/min), 5 vol.% D /He (20 mL/min) in He was introduced into the reactor and kept for 30
mins until the baseline was stable (flow rate = 20 mL/min). The sample was heated under 10 vol.% H , 5 vol.% D in He
flow (flow rate = 20 mL/min) from room temperature to 200 oC. The intensities for the m/z values equaling 2, 3, and 4
were continuously recorded for H , HD, and D /He, respectively. Isotope-labeled TPSR experiments were also performed
on a quartz-bed flow reactor with an online mass spectrometer (MS, Pfeiffer OMSNIstarTM). Typically, a certain amount
of samples (Pd /ND@G and Pd /ND@G 10 mg; Pd /ND@G using 5 mg) mixed with 500 mg of quartz sand were
pretreated with 10 vol.% H balanced with He at 200 oC for 1 h. Then, the gas flow was switched to pure He during the
cooling process. After cooling to 50 oC, the He flow was continuously introduced for some time, followed by switching
to 10 vol.% H (20 mL/min) and 5 vol.% D (20 mL/min). After some time, the 10 mL/min pure He was also introduced
to the reactor for a while. Subsequently, the pure He flow was replaced by a 1 vol.% C H at 10 mL/min. The intensities
of m/z values of 2 (H ), 3 (HD), 4 (D +He), 26 (C H ), 28 (C H ), and 30 (C H ) were continuously recorded by an
online mass spectrometer (MS, Pfeiffer OMNIstarTM). X-ray absorption spectroscopy (XAS) measurements were
performed at the Argonne Advanced Photon Source, 10-ID beamline under operating conditions of 3.5 GeV and 250 mA,
using a Si(111) double-crystal monochromator. The X-ray beam size is 500µm horizontal × 1000µm vertical.
Catalytic test. The acetylene semihydrogenation performances of the Pd-based catalysts were evaluated in a quartz-bed
flow reactor with a certain amount of samples (20 mg for Pd /ND@G and Pd /ND@G or 10 mg for Pd /ND@G to
maintain a consistent content of palladium) mixed with 500 mg of quartz sand. Prior to testing, the prepared Pd-based
samples were pretreated sequentially with oxygen (20% O /N ) and hydrogen (10% H /He) at 200 oC for 1 h, respectively.
After cooling to room temperature, a mixture gas of 1 vol.% C H , 10 vol.% H , and 20 vol.% C H balanced with He
was introduced with a flow rate of 20 mL/min, followed by temperature rise testing. The gas hour space velocity (GHSV)
of Pd /ND@G and Pd /ND@G is 60000 mL/g/h, while GHSV for Pd /ND@G is 120000 mL/g/h/. The concentrations
of reactants and products were periodically analyzed online using an Agilent 7890 A Gas Chromatograph equipped with
a HP-PLOT AL/S capillary column and a flame ionization detector (FID).
Acetylene conversion, ethylene selectivity, TOF values of acetylene, space-time yield, and specific activity were
calculated as the following formulas:
c (C H ) - c (C H )
Conver. ( 𝐶 𝐻 ) = x 100% Eq.(2)
c (C H )
Select. = (1- (c (C H ) + 2c (C olefin)) / (c (C H ) - c (C H ))) x 100% Eq.(3)
TOF (s−1)= (GHSV x 𝑐 (𝐶 𝐻 ) x Cover.(C H )) / (22.4 x 1000 x 3600 x 𝐶𝑎𝑡. 𝑀 x 𝐷 𝑃𝑑 x (𝑚 x 𝑤𝑡%))
Space−time yield (mol∙ g−1 ∙h−1)= (GHSV x Cover.x Select.) / (22.4 x 1000 x 𝑚 𝐶𝑎𝑡.)
Specific Activity (mol−1 ∙ mol−1∙ min−1)= TOF x 60
Where c(C H ) is the concentration of acetylene; m , wt% are the weight of the catalyst tested and the loading of Pd.
Similarly, the ethylene hydrogenation performances of Pd /ND@G and Pd /ND@G were evaluated in the same reactor.
Before testing, the prepared Pd-based samples were pretreated sequentially with oxygen (20% O /N ) and hydrogen (10%
H /He) at 200 oC for 1 h, respectively. After cooling to room temperature, a mixture gas of 0.5 vol.% C H , 5 vol.% H
balanced with He was introduced with a flow rate of 40 mL/min, followed by ascending temperature testing. The gas
hour space velocity (GHSV) of Pd /ND@G and Pd /ND@G is 120000 mL/g/h. The concentrations of reactants and
products were periodically analyzed online using an Agilent 7890 A Gas Chromatograph equipped with a HP-PLOT
AL/S capillary column and a flame ionization detector (FID). The ethylene conversion was calculated as the following
formulas:
Conver. (𝐶 𝐻 ) = (c(C H ) - c(C H )) / c(C H ) x 100% Eq.(7)
Computational Details: 
All density functional theory calculations were performed using the VASP package(VASP.6.3.2)53. The projector
augmented wave (PAW) pseudopotential was employed to describe the interactions between nuclei and valence electrons,
and the exchange-correlation functional was treated within the generalized gradient approximation using the PBE
functional54, 55. A plane-wave energy cutoff of 400 eV was applied56. To accurately describe the electronic structures of
different systems, differentiated spin settings were adopted: spin-restricted calculations were used for the metallic Pd(111)
surface, while spin-polarized calculations were applied for the Pd /ND@G and Pd /ND@G catalyst models, which may
possess localized magnetic moments57. The Brillouin zone was sampled with k-point grids of 4×4×1 for Pd(111) and
2×2×1 for Pd /ND@G and Pd /ND@G58. The energy convergence threshold for electronic self-consistent iteration was
set to 10⁻⁵ eV, and van der Waals interactions were corrected using the DFT-D3 method59. During structural optimization,
the convergence criterion for interatomic forces was set to 0.03 eV/Å. Transition states were located using the NEB and
dimer methods60, 61. To better reflect realistic reaction conditions (1 bar, 80 °C, 1% acetylene concentration),
thermodynamic corrections were applied to the species along the reaction pathways, with the corresponding correction
values provided in Supplementary Table 5.
The adsorption energy (𝐸 ) was calculated as:
𝐸 =𝐸 −𝐸 −𝐸 Eq.(8)
where 𝐸 is the total energy of the adsorbate on the substrate, 𝐸 is the energy of the substrate, and 𝐸
is the energy of the isolated adsorbate.
The Gibbs free energy at finite temperature and pressure (𝐺 ) was calculated as:
𝐺 =𝐸 +∆𝐺 Eq.(9)
where 𝐸 is the DFT-calculated energy at 0 K, and ∆𝐺 is the thermodynamic correction term, which accounts
for the effects of temperature and pressure on the adsorption energy, enabling accurate prediction of reaction free energies
under experimental conditions.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports the synthesis, characterization and application as a catalyst for the semi-hydrogenation reaction of acetylene in ethylene streams of Pd1, Pd2 and Pd clusters on nanodiamond-supported graphene. The results show that the Pd2 catalyst is much more active and selective than the Pd1 catalyst, while the Pd clusters lead to over-hydrogenation. The explanation for this is the higher adsorption of the acetylene starting material and the lower adsorption of the ethylene product (also starting material) in Pd2, with lower but enough H2 dissociation, thus enabling the selective semi-hydrogenation reaction. In contrast, the Pd1 catalyst does not dissociated H2 at a convenient rate, while the Pd clusters over-hydrogenate ethylene after an stronger adsorption and H2 dissociation. These results are in line with previous works on the suitability of dual atoms to catalyze this reaction (i.e. ref 12 in the main text, which deals with Pd-Au dual atoms and that, indeed, is the second more active catalyst after the one here reported according to Figure 2e). Moreover, I think that the catalytic activity of the Pd2 dual atoms is clearly underestimated in the manuscript (see comments below). Overall, despite the study still needs some revision, the message is remarkable and publication is recommended. Issues to be addressed before publication:\n- The amount of Pd2 sites is clearly lower than Pd1 sites in the Pd2-catalyst: The TEM images (Figures 1d-e) show a high amount of single atoms and the EXAFS spectrum (Figure 1i) confirms the major presence of Pd1 atoms. My suggestion here is: Estimate the relative Pd1/Pd2 ratio with histograms, and then recalculate the data for Pd2. In this way, the real TOF value is much higher (if all Pd atoms were taken for the calculations). This recalculation is extensible to other values that are just the average (XANES energy, the Pd2 are more metallic than commented; Activation energy, the Ea is much lower for Pd2, probably near to that in ref. 18; XPS,… and so on). It is a pity that cleaner samples of Pd2 could not be achieved, but this approximation should be fine at this stage.\n- The used catalyst sample (at least of Pd2) should be characterized, to confirm that the Pd2 dual sites are still there after reaction. This is mandatory for this kind of study.\n- The role of the support is not commented. The nanodiamond-hybridized graphene solid is not the most common (and affordable) support for a catalyst. Any possibility that other supports might perform similarly and that the Pd1/Pd2/Pd cluster effect here observed is reasonably general?\n- The text mentions that 5 ppm is the limit for acetylene in the industrial process. From my understanding, it is much lower (<1 ppm), but perhaps the authors could clarify this. When the study talks about >90% selectivity (without full conversion), the text should remark that this value is quite far from any industrial standard.\n- Figure 3d caption, no need to explain the experiment there, please place in methods. The TOC graphic is not understandable with references in there.", "reviewer_2": "The manuscript presents a compelling study on overcoming the long-standing activity–selectivity trade-off in acetylene semihydrogenation, a key purification step in polyolefin production. The authors report a solvent-mediated strategy for constructing Pd₂ dual-atom sites on defect-rich surface-graphitized nanodiamond (ND@G), and demonstrate that these di-nuclear motifs provide substantially enhanced acetylene conversion rates while maintaining high ethylene selectivity. The work combines advanced electron microscopy, XAS, isotope-labeled TPSR, and DFT calculations to elucidate the co-activation mechanism of C₂H₂/H₂ on Pd₂ sites. Overall, the study is scientifically interesting and potentially impactful. However, several issues need to be addressed to reinforce the mechanistic claims and improve the clarity and rigor of the presentation.\n1. Provide full experimental conditions for acetylene semihydrogenation\n2. Clarify the degree and type of defects on ND@G that anchor Pd₂ sites.\n3. Improve figure readability, particularly HAADF-STEM and XAS fitting plots.\n4. Include error bars and specify the number of replicates for catalytic data.", "reviewer_3": "This manuscript, entitled “Breaking the activity-selectivity trade-off for Acetylene Semihydrogenation by Pd2 Dual-Atom Site”, reported a facile and straightforward strategy to construct Pd2 dual-atom sites anchored on defect-rich surface-graphitized nanodiamond (ND@G) via the solvent-mediated dispersion of palladium carboxylate driven by the carboxylate anion chelation, which demonstrated superior catalytic performance in acetylene semihydrogenation, with the corresponding TOF values increased from 0.151 s-1 to 1.352 s-1, without the obvious decline of ethylene selectivity (93.2%). The authors carried out a systematic characterization (Cs-corrected HAADF-STEM, XAS, C2H2/C2H4-TPD, H2-D2 exchange reaction, isotope-labeled TPSR, and DFT calculations), and confirmed the effective co-activation of C2H2/H2 on Pd2 dual-atom sites while maintaining the weak adsorption of ethylene similar to that on its single-atom sites, which can break the activity-selectivity trade-off in acetylene semihydrogenation. The experiment is well designed and the obtained result is quite interesting. Thus, I recommend accepting this manuscript after a minor revision. Comments:\n1. The authors argued that the Pd species were anchored on the carbon defect in ND@G. Can the change in the ID/IG ratio of the Raman spectra from these samples demonstrate the binding of metal sites to carbon defects?\n2. The authors carried out the isotope-labeled TPSR to investigate the competitive adsorption-activation between C2H2 and H2 over Pd-based active sites. It is plausible that the intensity of competitive adsorption-activation between C2H2 and H2 can be quantitatively assessed by monitoring the evolution of HD concentration after acetylene introduction (Fig. 3d). However, the hydrogenation of C2H2 by H2 or D2 could also decrease the concentration of H2 and D2, resulting in the decline of HD intensity. How could the author explain this problem?\n3. Apart from the isotope-labeled TPSR experiment, are there any other evidence confirming the detrimental influence of the competitive adsorption-activation between C2H2 and H2 on the activity of acetylene semihydrogenation?\n4. There are some typos in this manuscript. A notable example is the inconsistent formatting of the milliliter unit in the caption of Fig. 3 ('mL' vs. 'ml')."}, "author_response": "Response: We sincerely thank Reviewer 1 for the thorough review and positive and encouraging comments on our manuscript. We have carefully considered all the issues raised by the reviewer and have addressed them point-by-point in the revised manuscript. We believe that these revisions have further improved the clarity and quality of our work.\nComment 1: The amount of Pd sites is clearly lower than Pd sites in the Pd -catalyst: The TEM images (Figures 1d-e) show a high amount of single atoms and the EXAFS spectrum (Figure 1i) confirms the major presence of Pd atoms. My suggestion here is: Estimate the relative Pd /Pd ratio with histograms, and then recalculate the data for Pd . In this way, the real TOF value is much higher (if all Pd atoms were taken for the calculations). This recalculation is extensible to other values that are just the average (XANES energy, the Pd are more metallic than commented; Activation energy, the Ea is much lower for Pd , probably near to that in ref. 18; XPS,… and so on). It is a pity that cleaner samples of Pd could not be achieved, but this approximation should be fine at this stage.\nResponse: Thank you for your insightful suggestion. We acknowledge that this profound point had been overlooked in our original analysis. We firstly performed a microscopic statistical analysis by directly counting the numbers of Pd and Pd sites in Cs-corrected HAADF-STEM images. The Pd :Pd site ratio was determined to be 51:49, based on the identification of 175 Pd dual-atom pairs and 179 Pd single-atom sites in Figs. 1d-e and S5 (Nat. chem., 2011, 3, 634-641.). Furthermore, we quantified the Pd /Pd ratio through a macroscopic approach, utilizing the average Pd-Pd coordination number (C.N.) derived from EXAFS fitting curves of the Pd /ND@G sample. This approach is valid since the average C.N. of Pd-Pd in Pd /ND@G sample represents the weighted average of the Pd-Pd coordination numbers of Pd and Pd sites. The formula is as follows:\n𝐶.𝑁.(𝑃𝑑 −𝑃𝑑) = 𝑛(𝑃𝑑 )/[𝑛(𝑃𝑑 )+𝑛(𝑃𝑑 )] (𝐸𝑞.1)\nwhere Pd exhibits zero Pd-Pd coordination and Pd has a Pd-Pd coordination number of 1. The n(Pd ) and n(Pd ) represent the number of Pd and Pd sites, respectively. Based on the above formula, we confirmed that the site number ratio of Pd /Pd was calculated to be approximately 1:1, which agrees well with the microscopic statistical analysis approach based on the Cs-corrected HAADF-STEM images. The site number ratio of Pd to Pd indicates that approximately 33% of the Pd₂ carboxylate dimers in the Pd /ND@G-No treatment sample decomposed during calcination, a process for ligand removal, to form Pd₁ single-atom sites.\nBased on the site number ratio of Pd to Pd , we recalculated the TOF value for Pd sites (1.953 s-1). However, recalculating the XPS binding energy and XANES absorption threshold of Pd dual-atom sites based on the Pd /Pd ratio is unreliable, because we have not found any such computational precedents in the literature on dual-atom catalysis. As regards the recalculation of the apparent activation energy for Pd /ND@G (Ea), we believe that the value of Ea would remain unchanged upon recalculation despite its increase in the intercept of the lnr - 1000/T plot. The reason is as follows.\nAs derived from the reaction rate of acetylene semihydrogenation (Eq. 1) and Arrhenius equations (formula 2), the population of active sites, affecting the pre-exponential factor (A), only impacts the intercept of the ln(r) - 1000/T plot, while the intrinsic property of the active sites dictates the slope, which represents the apparent activation energy (formula 3).\n𝑟 = 𝑘𝑝𝛼 𝐶2𝐻2 𝛽 ∙𝑝 𝐻2 (𝐸𝑞.2)\n𝑘 = 𝐴𝑒 −𝐸𝑎 𝑅𝑇 (𝐸𝑞.3)\n𝐸 𝑎 𝑙𝑛𝑟 = − + 𝑙𝑛𝐴 +𝛼𝑙𝑛𝑝 𝑅𝑇 𝐶2𝐻2 +𝛽𝑙𝑛𝑝 𝐻2 (𝐸𝑞.4)\nWhere r, k, p , α, p , β, A, e, Ea, R, and T represent the reaction rate, rate constant, acetylene pressure, acetylene reaction order, hydrogen pressure, hydrogen reaction order, pre-exponential factor (related to the number of active sites), the base of the natural logarithm, activation energy, ideal gas constant, and thermodynamic temperature.\nComment 2: The used catalyst sample (at least of Pd ) should be characterized to confirm that the Pd dual sites are still there after reaction. This is mandatory for this kind of study.\nResponse: We appreciate the reviewer for this valuable suggestion. We have investigated the size distribution of Pd /ND@G-used via Cs-corrected HAADF-STEM examination. The Cs-corrected HAADF-STEM images show a large number of Pd dual-atom pairs with some single-atom Pd sites (Fig.R1). However, no clusters or nanoparticles were observed in the Pd /ND@G-used images. This can also be confirmed by the results of the long-term stability test (Fig. 2c and S13). The persistently high ethylene selectivity demonstrated that no sintering or agglomeration of Pd₂ dimers into clusters/nanoparticles occurred in the Pd₂/ND@G-used sample.\nComment 3: The role of the support is not commented. The nanodiamond-hybridized graphene solid is not the most common (and affordable) support for a catalyst. Any possibility that other supports might perform similarly and that the Pd /Pd /Pd cluster effect here observed is reasonably general?\nResponse: Thank you for this good suggestion. The ND@G support is a novel type of material first reported by our group, characterized by a defect-rich graphitized nanodiamond surface that can anchor single-atom sites, dual-atom sites, and clusters, thereby endowing them with superior stability (Nat. Catal., 2025, 8, 436-447; Nat. Catal., 2022, 5, 485-493). To elucidate the role of the ND@G support in acetylene semi-hydrogenation, we investigated the catalytic performance of Pd /Pd /Pd supported on the commonly used supports in industry (silica and α-alumina). The catalytic performance of Pd₁, Pd₂, and Pd sites on both supports was similar: complete acetylene conversion was achieved at 80°C, but the selectivity remained below -150%, indicating over-hydrogenation of ethylene (Fig. R2). This is primarily because the surfaces of silica and alumina supports lack specific sites capable of strongly anchoring Pd₁ or Pd₂, leading to the sintering of single-atom and dual-atom sites into clusters or particles. Some literature studies have shown that defect sites on the surface of supports are essential for anchoring dual-atom sites in catalysis (Chem, 2024, 10, 3342-3363; J. Am. Chem. Soc. 2024, 146, 24945-24955).\nComment 4: The text mentions that 5 ppm is the limit for acetylene in the industrial process. From my understanding, it is much lower (<1 ppm), but perhaps the authors could clarify this. When the study talks about >90% selectivity (without full conversion), the text should remark that this value is quite far from any industrial standard.\nResponse: We appreciate the reviewer for this professional suggestion. We have revised the description for the upper limit of acetylene concentration in the polymerization-grade ethylene gas. It should be noted that the high ethylene selectivity (93.2%) was achieved at complete acetylene conversion, a point not specified in the abstract because of word limits. We have revised this part.\nComment 5: Figure 3d caption, no need to explain the experiment there, please place in methods. The TOC graphic is not understandable with references in there.\nResponse: Thank you for this good suggestion. We have deleted the excessive explanation in the caption of Fig. 3d, as well as the TOC.\nResponse: We are deeply grateful to Reviewer 2 for their time and for providing these insightful and constructive comments, which have significantly helped us to enhance the quality of our manuscript. We have carefully addressed all the points raised below and believe that the manuscript has been substantially improved as a result.\nComment 1: Provide full experimental conditions for acetylene semihydrogenation\nResponse: Thank you for this good suggestion. We have added the reaction conditions for acetylene semihydrogenation in the caption of Fig. 2 and the method part.\nComment 2: Clarify the degree and type of defects on ND@G that anchor Pd₂ sites.\nResponse: We appreciate the reviewer's question. Through density functional theory (DFT) calculations, we clarify that the Pd dual-atom sites are anchored on ND@G with a three-carbon vacancy defect (a defect region where three adjacent carbon atoms are missing). As detailed in Table S4, the optimized structure of Pd /ND@G shows that the Pd atoms exhibit Pd-C bonds (coordination number, CN = 2.5; bond length, R = 1.96 Å) and an intradimer Pd-Pd bond (R = 2.55 Å). These results are in good agreement with the experimental characterization data (Table S2), where the Pd atoms display Pd-O/C bonds (CN = 2.6; R = 2.09 Å) and Pd-Pd bonds (R = 2.75 Å), fully verifying the rationality of the theoretical model.\nComment 3: Improve figure readability, particularly HAADF-STEM and XAS fitting plots.\nResponse: Thank you for this good suggestion. We have improved the resolution of figures, especially HAADF-STEM images and XAS fitting plots for better visual readability.\nComment 4: Include error bars and specify the number of replicates for catalytic data.\nResponse: We appreciate the reviewer for this good suggestion. We have included error bars in the figure of catalytic data and specified the number of replicates in the caption of Fig. 2.\nResponse: We are truly grateful to the reviewer for their positive evaluation of our work and their valuable time in providing these constructive comments to help us further improve the quality and clarity of our manuscript. We have carefully considered all the points raised and have revised the manuscript accordingly. Our point-by-point responses to the specific issues are detailed below. All changes in the revised manuscript are highlighted in red for easy identification.\nComment 1: The authors argued that the Pd species were anchored on the carbon defect in ND@G. Can the change in the I /I ratio of the Raman spectra from these samples demonstrate the binding of metal sites to carbon defects?\nResponse: Thank you for this good question. We have carried out the Raman spectroscopy examination. The Raman spectra revealed a sequential decrease in the I /I ratio for Pd /ND@G, Pd /ND@G, and Pd /ND@G, indicating that anchoring Pd dual-atom sites or Pd clusters consumes more carbon defects than Pd single-atom sites (Fig. R3).\nComment 2: The authors carried out the isotope-labeled TPSR to investigate the competitive adsorption-activation between C H and H over Pd-based active sites. It is plausible that the intensity of competitive adsorption-activation between C H and H can be quantitatively assessed by monitoring the evolution of HD concentration after acetylene introduction (Fig. 3d). However, the hydrogenation of C H by H or D could also decrease the concentration of H and D , resulting in the decline of HD intensity. How could the author explain this problem?\nResponse: Thank you for this good question. Actually, the addition reaction of acetylene with hydrogen or deuterium gas will reduce the concentration of hydrogen and deuterium, leading to a decrease in HD signal intensity. However, since the reaction gases introduced are 10 mL/min of 1% acetylene, 20 mL/min of 10% hydrogen, and 10 mL/min of 5% deuterium, with a ratio of C H : D : H at 1:10:20, even if acetylene is completely converted, the impact on the concentrations of hydrogen and deuterium is minimal and does not cause a significant reduction in HD signal intensity. Therefore, the notable decline in HD intensity is due to the competitive adsorption and activation between H₂ and acetylene.\nComment 3: Apart from the isotope-labeled TPSR experiment, are there any other evidence confirming the detrimental influence of the competitive adsorption-activation between C H and H on the activity of acetylene semihydrogenation?\nResponse: Thank you for this good question. The influence of competitive adsorption and activation between acetylene and hydrogen on catalytic performance, initially indicated by isotopic labelling studies, can also be demonstrated by the reaction profiles of acetylene semi-hydrogenation and H₂-D₂ exchange. Specifically, the complete conversion of acetylene on the Pd /ND@G and Pd /ND@G catalysts occurs at elevated temperatures of 180 °C and 100 °C, respectively, notably exceeding those at which the H₂-D₂ exchange reaches equilibrium on the same samples (120 °C and 80 °C, respectively), which is due to the detrimental influence of the competitive adsorption-activation between C H and H on the activity of acetylene semihydrogenation. Additionally, the DFT calculations reveal that the transition state corresponding to the competitive adsorption-activation between acetylene and hydrogen on the Pd /ND@G and Pd /ND@G surfaces exhibits the highest energy barrier among all the elementary reaction steps, and thus constitutes the rate-determining step of acetylene semihydrogenation.\nComment 4: There are some typos in this manuscript. A notable example is the inconsistent formatting of the milliliter unit in the caption of Fig. 3 ('mL' vs. 'ml').\nResponse: Thank you for this good suggestion. We have thoroughly proofread the manuscript to eliminate typos."}, {"round": 2, "reviewer_comments": {"reviewer_1": "All the issues have been correctly addressed. The manuscript is ready for publication.", "reviewer_2": "The manuscript can be published as its current version.", "reviewer_3": "After carefully check the response to the comments raised by the reviewer. I feel that authors have well readdressed the response for the reviewer's comments, now, this manuscript can be accepted for the publication of nature communications."}, "author_response": "Response: Many thanks to Reviewer 1 for the previous valuable comments and publication recommendation on this work, which helped us to improve the quality of our manuscript. We are glad that the responses are satisfactory to you. Thanks very much for your insightful comments again.\nResponse: Thank you for your positive feedback and for recommending our manuscript for publication. We are very pleased to hear that you find it acceptable in its current version.\nResponse: Thank you very much for your time and for your encouraging comments. We are delighted to receive your recommendation for publication without further revisions. We truly appreciate your valuable assessment of our work."}]

Decision letter