Stepwise hydrogen spillover–engineered synergistic sites enable near-quantitative conversion of waste PET to p-xylene

Domain: neuroscience
Published: 2026-01-28
Editorial outcome: major_revision

Abstract
The valorization of plastic waste represents a major challenge of the 21st century due to its severe environmental impact. Here, we report a stepwise hydrogen spillover-constructed CuCo/CoO x catalyst that enables near-quantitative conversion of waste polyethylene terephthalate to p-xylene (>99.9%), significantly outperforming the performance of various Cu- and Co-based catalysts as well as previously reported noble metal catalysts. The stepwise hydrogen spillover induces the formation of partially phase-transformed Co0 species and abundant oxygen-vacancy-rich Co0/CoO x interfaces. The former enhances H2 dissociation efficiency, while the latter facilitates C-O bond activation in polyethylene terephthalate and regulates substrate-product adsorption equilibria, synergistically contributing to the exceptional catalytic performance. The catalyst demonstrates broad applicability to more than 30 real-world polyester plastics. Furthermore, techno-economic analysis reveals significant reductions in CO2 emissions and competitive processing costs. This breakthrough in near-quantitative polyethylene terephthalate conversion and stepwise hydrogen spillover-enabled active site construction offers valuable insights into plastic waste upcycling and the design of advanced heterogeneous catalysts.

Introduction
The continuous consumption of plastic materials has led to the accumulation of over 4.9 billion metric tons of plastic waste in landfills and natural ecosystems, a figure projected to rise to 12 billion metric tons by 20501,2. The environmental burden associated with plastic waste has intensified markedly3–9, with widespread pollution across terrestrial, marine, and atmospheric environments. Such widespread contamination facilitates the bioaccumulation of plastic particles within living organisms, thereby disrupting ecological systems and posing potential threats to human health10–14. Among various plastic types, polyethylene terephthalate (PET) represents the most extensively utilized polyester worldwide15, owing to its low density, high mechanical strength, excellent abrasion and chemical resistance, and cost-effectiveness. The global annual production of PET exceeds 82 million tons, primarily utilized in the fabrication of textiles and fibers, rigid packaging materials, disposable beverage bottles, and plastic films. PET accounts for approximately 8% of global plastic production and contributes around 12% to total solid plastic waste16–19.

The chemical upcycling of PET into high-value-added chemicals represents a critical pathway in advanced PET waste valorization. Conventional approaches such as hydrolysis20,21 and alcoholysis22,23 have been widely explored, while emerging methods such as hydrogenolysis and coupled alcoholysis-hydrogenolysis24 offer promising alternatives. Current studies on PET hydrogenolysis have revealed a wide distribution of products25–29, yet achieving near-quantitative yields of specific targeted compounds remains challenging, owing to the complex hydrogenation behavior of multiple functional groups. Among the potential products, p-xylene (PX) is of particular interest due to its relevance in three important industrial domains: high-end chemical manufacturing (e.g., polyester fibers), general-purpose solvents, and specialized applications in the pharmaceutical and laboratory sectors. Notably, these applications demand ultra-high purity PX, with rigorous control over trace impurities. This underscores the urgent need to develop highly selective catalytic systems for the PX production from PET. However, current reports on metal-based catalysts (e.g., Ru-, Pt-based systems) have shown limited success in achieving such selectivity, primarily due to undesired side reactions such as decarboxylation and aromatic ring hydrogenation27,30–33.

The C–O cleavage in hydrogenolysis requires Lewis acid-supported metal catalysts (M/MbOx), where synergistic catalysis at the interface between hydrogen dissociation at metallic (M0) sites and Lewis acid (MbOx)-facilitated C–O bond activation enables efficient transformation34–36. To maximize this synergistic catalysis, the construction of M/MOx systems composed of homologous transition metals is considered advantageous, owing to their favorable electronic interactions and band structure compatibility. Non-precious Co/CoOx has emerged as a promising candidate, leveraging the moderate hydrogen activation ability of metallic Co species and the strong oxygen affinity of CoOx for the activation of oxygen-containing substrates37,38. Co-based catalysts, such as Co/TiO2 38, Co–Fe–Al39, and Co-Cu-Al39, have demonstrated effective degradation of PET. Among these catalysts, Co/TiO2 and Co-Cu-Al exhibit relatively poor selectivity towards PX, and Co-Fe-Al exhibits high PX selectivity. However, there have been no reports yet on achieving highly selective conversion of PET into PX using Co-based catalysts within the single-metal category. Conventional high-temperature reduction techniques commonly employed in Co/CoOx preparation often lead to active site agglomeration and phase crystallization, thereby reducing the density of accessible active sites and weakening interfacial synergy40. Consequently, the rational design of structurally tailored Co/CoOx catalysts with abundant synergistic sites for the near-quantitative conversion of PET to PX remains highly desirable but has yet to be realized.

Hence, we report a unique stepwise hydrogen spillover-constructed CuCo/CoOx catalyst, enabling efficient PET conversion into PX with an near-quantitative yield (>99.9%) (Fig. 1). The stepwise hydrogen spillover is as follows: CuO is first reduced to Cu0, which initiates hydrogen spillover to reduce CoOx; the resulting Co0 with superior hydrogen activation ability drives a secondary spillover that accelerates CoOx reduction. Compared with monometallic Co systems, the stepwise hydrogen spillover allows the generation of partially phase-transformed Co0 species at lower temperatures, while simultaneously creating a high density of vacancy-rich Co0/CoOx interfaces. Partially phase-transformed Co achieves an optimal balance between hydrogen dissociation ability and active site density, markedly outperforming both fully transformed and untransformed states. The vacancy-rich Co0/CoOx interfaces exhibit enhanced C–O bond activation capacity and afford favorable adsorption/desorption of substrates and products. Additionally, we directly capture the signals of ester bond activation in PET, leading to the formation of an aldehyde-like intermediate, using in-situ FT-IR, providing unambiguous molecular-level evidence for ester bond activation and offering distinctive insights into the hydrogenolysis mechanism of PET. This unique catalyst demonstrates broad applicability across 28 real-world PET plastics, including single-component PET, PET with additives and PET-based mixed plastics, achieving near-quantitative PX in most cases. Furthermore, this catalyst demonstrates significant potential for the conversion of 5 other aromatic polyesters (e.g., PBT, PBAT) as well as aliphatic polyesters (e.g., PLA, PCL and PBS). Techno-economic analysis further confirms substantial CO2 emission reduction alongside cost competitiveness, highlighting its potential to reconcile environmental sustainability with industrial viability.

Results
Catalytic performance in hydrogenolysis of PET to PX
The catalytic performance of various Co-based catalysts, including Co/CoOx, CuCo/CoOx, NiCo/CoOx, RuCo/CoOx, and FeCo/CoOx, for the hydrogenolysis of PET was initially evaluated (Fig. 2a). The catalytic performance results follow this trend: CuCo/CoOx > Co/CoOx ≈ NiCo/CoOx > RuCo/CoOx > FeCo/CoOx. These results indicate that a significant enhancement in the hydrogenolysis activity of PET is observed only when Cu species are incorporated into the CoOx-based catalyst. For individual Co- or Cu-based catalysts supported on conventional supports (TiO2, SiO2 and Al2O3), the PX yield remained below 25%. Thermogravimetric analysis (TGA) (Figs. S1–4) and GC results (Figs. S5–7) suggest that the catalytic activity of Co/TiO2, Co/Al2O3, and Cu/SiO2 catalysts is exceptionally low, resulting in only limited depolymerization of PET, which largely retains its polymeric structure. These results indicate that catalysts primarily composed of metallic Co or Cu species exhibit limited catalytic activity, and these conventional supports have no obvious contribution. In contrast, the Co/CoOx catalyst achieved a significantly higher PX yield of 52.5%, suggesting a potential synergistic effect between metallic Co and CoOx species. Upon incorporation of Cu, the CuCo/CoOx catalyst achieved an near-quantitative PX yield exceeding 99.9%, almost two-fold improvement compared to Co/CoOx. To exclude the influence of CuO species, systematic evaluations were conducted on both Cu/CuOx and Co/CuOx. The results demonstrated that neither the Cu–CuOx synergy nor the Co–CuOx interaction exhibited appreciable catalytic activity. In summary, the superior performance observed in the CuCo/CoOx system can be unequivocally attributed to the synergistic cooperation between Co and CoOx species. The introduction of Cu may enhance the synergistic interaction between metallic Co and CoOx species by facilitating the formation of unique active sites, thereby contributing to the observed improvement in catalytic performance. Importantly, the catalytic performance for the PX production surpasses that of a series of noble metal catalysts as reported in the literature27,30–33.

Subsequent optimization of reaction conditions was conducted to maximize catalytic performance. The results demonstrated that low temperature hindered the reaction progress, while excessively high temperature also adversely affected the PX yield. Thermogravimetric analysis (TGA) (Fig. S8) indicated that the weight loss profile of the solid residues obtained at 250°C did not correspond to typical PET degradation patterns (Fig. S4), which was attributed to undesired polymerization side reactions triggered by thermal overstress. Consequently, 230°C was identified as the optimal reaction temperature. In addition, the hydrogen pressure exhibited a strong correlation with catalytic activity. Under low H2 pressures, the reaction mixture remained turbid and could contain partially degraded PET fragments and oligomeric intermediates. The highest PX yield (>99.9%) was achieved at 2 MPa H2. However, further increasing the hydrogen pressure resulted in decreased PX yield. This decrease is likely associated with the over-hydrogenation of aromatic rings, resulting in the formation of cycloalkyl intermediates, which show non-planar configurations. Such configurations render the cleavage of the chemically robust C–O linkages more challenging, as previously reported31. Based on these findings, the optimal reaction conditions were determined to be 230°C and 2 MPa H2. Additionally, we optimized the solvent selection by evaluating several common organic solvents (tetrahydrofuran, cyclohexane, and 1,4-dioxane) as well as water. The results confirmed that 1,4-dioxane is the most promising solvent (for a detailed discussion, see Supporting Information below Table S1). Moreover, the catalyst demonstrated excellent reusability across four consecutive reaction cycles, maintaining consistently high PX yields (Fig. 2b), thereby highlighting its potential for practical applications.

To evaluate the purity of the product PX, a series of characterization techniques were performed. 1H nuclear magnetic resonance (1H NMR) analysis was performed on both the pure solvent and the reaction raw solution (Fig. 2c). The results demonstrated that, apart from the peaks corresponding to deuterated chloroform (CDCl3), solvent, and trace inherent impurities in solvent (Fig. 2c), only the distinct chemical shifts corresponding to PX were observed. Additionally, the H signals of PX at the α- and β-positions were maintained at 4:6 in solvents of varying purities, further confirming the high purity of the obtained PX and the absence of any by-products (Fig. S9). Consistently, gas chromatography (GC) analysis (Fig. 2e) showed a single product peak attributable to PX, after excluding the signals from the solvent (including residual peaks from needle washing) and the internal standard. TGA of the spent catalyst (Fig. 2d) showed no measurable mass loss, implying complete conversion of PET and the absence of any undecomposed polymer residues. In addition, XRD, Raman spectroscopy, XPS, TEM, and FTIR spectroscopy, conducted on the catalyst before and after the reaction (Figs. S11–S14, S10), revealed no significant structural changes and detectable accumulation of organic residues (for a detailed discussion, see Supporting Information below Figs. S11–S14, S10). In short, these results unambiguously confirm that the near-quantitative production of PX was successfully achieved.

To elucidate the degradation pathway of PET over CuCo/CoOx, we conducted the hydrogenolysis of PET under mild conditions (210°C, 2 MPa, 1 h), which successfully captured 2-hydroxyethyl 4-methylbenzoate (Fig. S15). Furthermore, the conversion of dimethyl terephthalate as a model substrate under milder conditions (190°C, 1 MPa, 1 h) yielded intermediates such as 4-methylbenzyl alcohol and methyl p-methylbenzoate (Fig. S16). These results confirm that the reaction initially involves the cleavage of ester bonds, generating p-methylbenzene intermediates (e.g., 2-hydroxyethyl 4-methylbenzoate and ethyl p-methylbenzoate). These intermediates are subsequently hydrogenated to form 4-methylbenzyl alcohol, which undergoes deoxygenation to produce PX. To further validate this pathway, we investigated the conversion of model intermediates, including bis(2-hydroxyethyl) terephthalate, terephthalic acid, 4-methylbenzyl alcohol and ethyl p-methylbenzoate (Fig. 2f). The results showed that bis(2-hydroxyethyl) terephthalate, 4-methylbenzyl alcohol and ethyl p-methylbenzoate were efficiently converted to PX, with the highest yield (99.9%) achieved using 4-methylbenzyl alcohol as the substrate. In contrast, terephthalic acid exhibited negligible conversion. These findings support a reaction pathway in which PET hydrogenolysis proceeds through ester bond hydrogenation to form 4-methylbenzyl alcohol, followed by deoxygenation, rather than via a terephthalic acid intermediate. Based on these observations, a three-step reaction pathway is proposed for PET hydrogenolysis as Fig. 2f: (i) initial depolymerization, (ii) asymmetric hydrogenolysis, and (iii) terminal dehydroxylation. Furthermore, GC analysis of the gaseous products revealed the formation of both methane and ethane (Fig. S17), indicating that the ethylene glycol segments of PET undergo hydrogenolysis and C–C bond cleavage in parallel with the main depolymerization pathway.

Catalyst characterizations
To elucidate the morphological and electronic structures of the designed catalysts, comprehensive characterization was performed. Inductively coupled plasma optical emission spectroscopy (ICP-OES) verified that the actual Cu loadings were in close agreement with the nominal values (Table S2, Fig. S18). TEM images of Co/CoOx (Fig. 3a, b) and CuCo/CoOx (Fig. 3d, e) at varying magnifications reveal the formation of smaller nanoparticles in the CuCo/CoOx sample, indicative of reduced crystallinity and increased structural disorder. High-resolution TEM (HR-TEM) image of Co/CoOx (Fig. 3c) displays well-resolved lattice fringes at 0.203 nm and 0.246 nm, corresponding to the (002) plane of hexagonal close-packed (hcp) Co and the (111) plane of CoO, respectively. An interfacial structure (Interface I) between metallic Co and CoOx is clearly observed. The HR-TEM image of CuCo/CoOx (Fig. 3f) demonstrates the simultaneous presence of the (111) plane of face-centered cubic (fcc) Co (0.205 nm) alongside the (002) and (101) planes of hcp-Co (0.203 nm and 0.191 nm, respectively). This coexistence provides direct evidence of a phase transformation within the CuCo/CoOx catalyst from hcp-Co to fcc-Co. Noteworthy, in addition to Interface I (hcp-Co/CoOx), two additional types of interfaces, namely abundant Interface II between fcc-Co and CoOx and Interface III between hcp-Co and fcc-Co, are observed. The appearance of Interface III (hcp-Co/fcc-Co) further suggests the partial phase transformation from hcp-Co to fcc-Co. Overall, HR-TEM analysis reveals that the incorporation of Cu induces partial phase transformation and lattice distortions, thereby promoting the formation of numerous interfaces, with Interface II (fcc-Co/CoOx) being particularly prevalent. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. 3g–j) confirms the homogeneous spatial distribution of Co, Cu, and O elements within CuCo/CoOx, indicating high elemental dispersion and structural uniformity.

To further understand the phase transformation, comparative XRD analyses were performed. The diffraction patterns of unreduced catalysts (Fig. S19) reveal negligible differences in the characteristic peaks of Co3O4 between the two samples, indicating that the phase transformation primarily occurs during the reduction process. As shown in Fig. 3k, the XRD patterns of the reduced catalysts exhibit distinct differences. The Co/CoOx sample displays characteristic diffraction peaks of Co (2θ=41.7, 44.8, 47.6, 75.9°) (PDF#05-0727). Notably, a discernible red shift of the metallic Co peak at 44.8°, corresponding to the (111) plane, is observed in CuCo/CoOx. This shift is attributed to a partial phase transformation from hcp-Co to fcc-Co during the reduction process41, consistent with the HR-TEM observations. To elucidate the influence of phase transformation on catalytic performance and to assess the implications of complete phase transition at elevated temperatures, the pre-reduced CuCo/CoOx catalyst was subjected to thermal treatment at 500°C under an inert Ar atmosphere. The structural characterizations of CuCo/CoOx-500 (Figs. S20–S24) revealed the complete disappearance of hcp-Co diffraction features, accompanied by a pronounced increase in the crystallinity of the fcc-Co phase (for a detailed discussion, see Supporting Information below Figs. S20–S24). Catalytic evaluations under optimized conditions showed a decrease in PX yield to 94.6% compared with the untreated CuCo/CoOx catalyst (Fig. S25). This decrease in activity is likely attributed to the competing effects of enhanced intrinsic activity associated with the fcc-Co phase and a reduced number of accessible active sites resulting from particle agglomeration. Importantly, it is evident that the introduction of Cu significantly decreases the crystallinity of the catalyst, likely due to the occurrence of lattice distortions and the formation of oxygen vacancies42. To identify the occurrence of lattice distortions and the formation of oxygen vacancies, several characterization techniques were systematically employed. Raman spectra (Fig. 3l) show that the bands at 186, 501 and 592 cm−1 are assigned to the F2g vibrational mode, corresponding to Co-O stretching vibrations associated with Co2+ species. The band at 461 cm−1 is attributed to the Eg mode of the Co3O4 spinel structure, arising from lattice vibrations induced by the synergistic interaction between Co3+ and Co2+ ions43. For the Co/CoOx catalyst, the A1g mode at 661 cm−1 is characteristic of octahedrally coordinated cobalt species (CoOx). Notably, in the case of the CuCo/CoOx catalyst, the A1g mode exhibits a red shift from 661 to 651 cm−1, indicating the presence of lattice distortion. This shift suggests that the incorporation of Cu species induces structural defects and intensifies lattice distortion44, thereby facilitating a partial phase transformation from hcp-Co to fcc-Co and promoting the generation of a greater number of oxygen-vacancy-rich Co0/CoOx interfacial sites. The Electron Paramagnetic Resonance (EPR) results (Fig. 3m) exhibit signals at g=2.003, which are attributed to unpaired electrons associated with oxygen vacancies45. Notably, the CuCo/CoOx catalyst displays a significantly stronger signal intensity compared to Co/CoOx, indicating a higher concentration of oxygen vacancies. The FTIR of pyridine adsorption show the three peaks that centered at 1596, 1581 and 1438 cm−1, which can be assigned to the Lewis acid sites on the surface46 (Fig. 3n). Notably, the peaks corresponding to Lewis acid sites exhibit substantially larger integrated areas on the CuCo/CoOx catalyst compared to Co/CoOx, clearly indicating a higher density of Lewis acid sites. This pronounced increase suggests the presence of a greater number of coordinatively unsaturated CoOx sites, which are closely associated with enhanced oxygen vacancy concentrations. Ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy (Fig. S26) shows that CuCo/CoOx exhibits significantly stronger light absorption than Co/CoOx, suggesting that Cu incorporation modulates the electronic band structure and promotes the formation of additional defect states associated with oxygen vacancies. Furthermore, XPS was performed to systematically investigate the surface electronic states of the catalysts. O1s spectrum was deconvoluted into three distinct components: lattice oxygen (OL) at 529.6 ± 0.1 eV, oxygen vacancies (Ov) at 531.3 ± 0.1 eV, and adsorbed oxygen species (Oa) at 533.4 eV (Fig. 3o). Quantitative analysis indicated a notable increase (Figure. S27) in the relative concentration of oxygen vacancies from 27.9% in Co/CoOx to 35.4% in CuCo/CoOx. In addition, Co 2p3/2 XPS (Fig. 3p) analysis revealed that the Co0/Co2+ ratio increased significantly from 0.19 to 0.60, indicating a more balanced distribution between Co0 and Co2+ in CuCo/CoOx47. Consequently, the population of interfacial active sites experienced a substantial increase. The Cu 2p XPS spectra (Figs. S28, S29) revealed a surface ratio of (Cu0+ Cu+)/Cu2+=1.73 for CuCo/CoOx47,48. The Cu LMM Auger spectra (Fig. S30) further showed a Cu0/Cu+ ratio of 1.14, confirming the substantial presence of Cu0 and Cu+ species on the surface of CuCo/CoOx48. The specific surface area and pore size of Co/CoOx and CuCo/CoOx were characterized using the Brunauer-Emmett-Teller method (Table S3). Compared to Co/CoOx, CuCo/CoOx exhibited a larger specific surface area, pore volume, and average pore size, favoring greater exposure of active sites and enhancing mass transfer during the reaction process. In summary, the above multi-technique characterizations provide compelling evidence that Cu incorporation induces lattice distortion, facilitates partial hcp-to-fcc phase transformation, and enhances the concentration of oxygen vacancy-rich interfacial Co-CoOx sites, all of which could contribute to the improved catalytic performance of the CuCo/CoOx system. Furthermore, the CuCo/CoOx catalyst still retained its structure after the reaction (for a detailed discussion, see Supporting Information below Figs. S10–13).

Based on the aforementioned characterization results, it is evident that the incorporation of Cu induces a substantial decrease in overall crystallinity, resulting in a more amorphous structural nature that may facilitate the exposure of additional surface active sites. More notably, two critical and distinctive structural features have been identified: 1) the partial phase transformation of hcp-Co to fcc-Co; 2) the introduction of lattice distortion that generates oxygen vacancy-rich Co0/CoOx interfaces, with the fcc-Co0/CoOx interfaces being predominant. Since the influence of the distinctive structural characteristics on catalytic performance has been exhibited, these findings naturally lead to two fundamental scientific questions that will be systematically addressed in subsequent investigations: First, what are the formation mechanisms underlying these unique structural features? Second, what are the molecular-level structure-activity relationships governing this catalytic system?

Identification of stepwise hydrogen spillover
To elucidate the formation mechanism of the unique active sites structure, a comprehensive investigation was conducted by integrating H2-temperature programmed reduction (H2-TPR) (Fig. 4a), hydrogen spillover-controlled experiments (Fig. 4b) and density functional theory (DFT) calculations (Fig. 4c). The H2-TPR profile of Co/CoOx exhibits a minor reduction peak at 200–300°C, corresponding to the reduction of Co3+ to Co2+, and a dominant peak at 300–400°C, assigned with the complete reduction of Co2+ to Co0. Upon Cu incorporation, the entire reduction peaks shift downward by approximately 100°C. This shift is primarily attributed to hydrogen spillover, which may proceed via two possible pathways: 1) Cu0-mediated spillover: CuO is first reduced to metallic Cu0, followed by the migration of dissociated hydrogen species (H*) from Cu0 to adjacent CoOx, thereby accelerating its reduction; 2) Co0-mediated spillover: CoOx undergoes initial partial reduction to Co0, and the resulting metallic Co0, which subsequently promotes further hydrogen dissociation and then spillover to surrounding CoOx domains. To elucidate the difference in reducibility between CuO and Co3O4, both oxides were individually supported on inert (SiO2) and reducible (TiO2) supports, and subjected to H2-TPR. In all cases, CuO exhibited a consistently lower reduction temperature than Co3O4, likely due to its lower Gibbs free energy of formation. Therefore, it can be concluded that the Cu-mediated spillover pathway is thermodynamically more favorable. Further insight was gained from the H2-TPR analysis of a physically mixed Co/CoOx and Cu/TiO2 (Cu/SiO2) sample in a 1:1 mass ratio, which showed only a modest downward shift in reduction temperature, suggesting weaker interactions and diminished spillover efficiency compared to the integrated CuCo/CoOx system.

Then, the WO3 coloration experiments show that pronounced changes in color upon exposure to unreduced Co/CoOx and CuCo/CoOx catalysts, with WO3 transitioning to yellow-green and deep blue, respectively (Fig. 4b), reflecting markedly enhanced hydrogen spillover after Cu incorporation. In contrast, unreduced Cu/SiO2 exhibits negligible chromatic change, whereas a physical mixture of unreduced Cu/SiO2 and Co/CoOx in a 1:1 mass ratio induced only limited color variation, indicating the reduced hydrogen spillover efficiency due to limited contact between Cu and CoOx. These stark contrasts provide compelling evidence for intensive hydrogen spillover phenomena during the reduction of CuCo/CoOx in agreement with H2-TPR results. Furthermore, the reduced catalysts exhibit discernible hydrogen spillover effects, although the associated coloration is markedly diminished compared to their pre-reduced state. This decrease in chromatic intensity provides further evidence for substantial hydrogen spillover occurring during the reduction process.

To further understand the hydrogen spillover phenomena, DFT calculations were carried out to systematically evaluate both the hydrogen adsorption energies and dissociation energy barriers on metallic Cu0 and Co0 surfaces with distinct crystallographic structures (Fig. 4c, Figs. S31–33). Based on lattice parameter variations identified by XRD and predominant surface orientations observed via TEM, atomistic models were constructed for hcp-Co(101), fcc-Co(111), and Cu(111) surfaces to simulate representative hydrogen activation sites. The calculations revealed that hydrogen exhibits stronger chemisorption on Co0 surfaces compared to Cu0. More importantly, the energy barrier for H2 dissociation on fcc-Co was found to be as low as 0.05 eV, significantly lower than that on hcp-Co (0.18 eV) and Cu (0.48 eV), indicating facile hydrogen activation on fcc-Co. Furthermore, the H2 dissociation process on fcc-Co is more exothermic (−0.87 eV) compared to that on hcp-Co (−0.83 eV) and Cu (−0.55 eV), indicating a thermodynamically more favorable activation on the fcc-Co surface. These results provide a mechanistic rationale for the experimentally observed hydrogen spillover process: although Cu0 acts as the initial site for H2 dissociation and primary spillover, the emergence of Co0 species, especially fcc-Co species, enables a more efficient secondary spillover due to its superior capability of H2 dissociation.

Building upon these findings, a stepwise hydrogen spillover mechanism is proposed (Fig. 4d). Initially, CuO species are preferentially reduced to metallic Cu0 sites capable of dissociating H2 into active H* species. These H* species subsequently migrate to adjacent CoOx regions, promoting the reduction of a small amount of CoOx to Co0. The formed Co0 sites, particularly those with fcc-structure, then act as secondary spillover centers, further facilitating hydrogen activation and migration for the reduction of more CoOx species. In summary, the unique stepwise hydrogen spillover process not only facilitates the partial phase transformation of hcp-Co into fcc-Co, but also promotes the formation of oxygen vacancy-rich Co0/CoOx interfaces, among which the fcc-Co0/CoOx interfaces are dominant.

Unique roles of CuCo/CoOx and Co/CoOx
Subsequently, the focus lies in explaining why the active sites constructed via the aforementioned strategies exhibit such distinctive catalytic efficiency. Due to the inherent difficulty in precisely quantifying the number of active sites on the catalyst surface, kinetic orders for the substrate and H2 could not be determined. Nevertheless, within the range of kinetic testing conditions, a systematic investigation was conducted to examine the relationship between product yield and both substrate concentration and hydrogen pressure. The results indicate that both factors exhibit comparable levels of influence, as demonstrated by the similar slopes obtained from fitting the data in Fig. 5a. This convergence suggests that hydrogen partial pressure and substrate concentration exert equally significant and synergistic effects on catalytic efficiency. Considering the critical role of reactant adsorption and activation in heterogeneous catalysis, particular focus was placed on investigating the adsorption and activation behaviors of H2 and PET on the CuCo/CoOx catalyst via temperature-programmed desorption (TPD) analysis.

First, H2-TPD analysis (Fig. 5b) reveals two distinct desorption peaks for both Co/CoOx and CuCo/CoOx catalysts. The low-temperature peak is attributed to the adsorption of hydrogen on metallic species, while the high-temperature peak is likely associated with hydrogen spillover processes. Notably, the CuCo/CoOx catalyst exhibits a significantly larger total desorption area compared to Co/CoOx, suggesting a higher density of active sites available for H2 adsorption and activation. This observation is consistent with previous DFT calculations (Fig. 4c), which demonstrate that the fcc-Co phase present in CuCo/CoOx exhibits superior hydrogen adsorption and dissociation capabilities compared to the hcp-Co phase in Co/CoOx. In addition, XRD analysis (Fig. 3k) reveals reduced overall crystallinity in the CuCo/CoOx catalyst, implying a greater abundance of exposed active sites. Moreover, the high-temperature desorption peak for CuCo/CoOx shifts to a substantially higher temperature, indicating stronger binding or stabilization of spillover-formed hydrogen species. This shift is corroborated by WO3 coloration experiments using both Co/CoOx and CuCo/CoOx catalysts (Fig. 4b), which confirm enhanced hydrogen spillover capability on CuCo/CoOx. The resulting Co–OH species formed via spillover exhibit improved thermal stability, likely due to the presence of abundant oxygen vacancies. These vacancies may facilitate hydrogen retention and stabilization, thereby accounting for the elevated desorption temperature observed49,50.

The time course of ethyl p-methylbenzoate conversion revealed that CuCo/CoOx exhibits higher activity consistently. In contrast, when Co/CoOx achieved a yield of 50% at 6 h, the reaction progressed minimally thereafter with the increased yield of PX, implying the existence of competitive adsorption between PX and the substrate (Fig. 5c). The TPD of ethyl p-methylbenzoate was conducted after in situ reduction of Co/SiO2, Cu/SiO2 at 500°C, and CuO at 145°C to obtain Co0, Cu0, or CuOx sites (Fig. 5d). The results show negligible adsorption of ethyl p-methylbenzoate on all catalysts. In contrast, both Co/CoOx and CuCo/CoOx exhibit significant adsorption, with the desorption peak area for CuCo/CoOx being 3.5-fold larger than that for Co/CoOx. This suggests a notably higher density of substrate adsorption sites, primarily located on CoOx, rather than Cu0 or CuOx. Cu species (whether Cu0 or CuOx) do not act as the primary active sites in the catalytic process, but instead function as components that significantly contribute to the formation of the oxygen vacancy-rich Co0/CoOx interface active sites through stepwise hydrogen spillover. The enriched coordinatively unsaturated CoOx sites in CuCo/CoOx account for its abundant active sites.

To investigate the role of unsaturated CoOx at the Co/CoOx interfaces facilitating substrate activation and validate the relationship between unsaturated CoOx species and substrate adsorption capacity, Co3O4 was reduced at 250°C and 350°C to obtain CoOx species with varying degrees of coordination saturation (denoted as Co/CoOx-250 and Co/CoOx-350, respectively), distinct from the Co/CoOx sample. XPS (Fig. S34), EPR (Fig. S35), Raman (Fig. S36), and pyridine-IR analyses (Fig. S37) confirm that Co/CoOx reduced at 300°C possesses the most abundant unsaturated CoOx species, which are closely associated with enhanced oxygen vacancy concentrations51. Subsequently, in-situ FTIR adsorption-desorption experiments (Fig. S38) and TPD experiments (Fig. S39) further reveal a strong positive correlation between substrate adsorption-activation and the presence of unsaturated CoOx species.

In order to investigate the role of different metals in active site construction and their structure-activity relationships, we systematically characterized a series of MCo/CoOx (M=Ru, Fe, Ni, Cu, Co) catalysts (Figs. S40–S43) (for a detailed discussion, see Supporting Information below Figs. S38–S43). The findings are as follows: Fe and Ni are incapable of inducing hydrogen spillover. Although Ru can initiate hydrogen spillover to promote the reduction of CoOx, excessive reduction inhibit the formation of low-crystallinity fcc-Co and low-coordinated, oxygen vacancy-rich CoOx species. In contrast to other MCo/CoOx catalysts, the introduction of Cu plays a critical role in initiating the stepwise hydrogen spillover process. This process not only promotes the partial phase transition from hcp-Co to fcc-Co but also facilitates the formation of oxygen vacancy-rich Co0/CoOx interfaces, with fcc-Co0/CoOx interfaces being the predominant species. As we have robustly demonstrated, the synergy between Co0 (particularly fcc-Co) and oxygen vacancy-rich CoOx species at the abundant Co0/CoOx interfacial sites significantly enhances the catalytic activity.

In addition, comparative TPD studies of PX revealed distinct differences in product adsorption strengths (Fig. 5e). PX exhibited notably weaker adsorption on CuCo/CoOx, which facilitates its rapid desorption and promotes continuous catalytic turnover. In contrast, on Co/CoOx, PX displayed much stronger adsorption, with its desorption temperature nearly coinciding with that of ethyl p-toluate. This overlap implies significant competitive adsorption between the substrate and the product. Accumulation of PX significantly suppresses the reaction progress, consistent with the observed reaction profiles. These findings, together with prior characterizations and TPD analyses of H2 and substrate adsorption-desorption, support a synergistic mechanistic model. The lattice distortion within CuCo/CoOx reduces crystallinity and promotes the formation of fcc-Co domains, which exhibit enhanced hydrogen dissociation capabilities. Concurrently, interfacial CoOx sites with abundant vacancies induced by lattice distortion promote substrate adsorption and product desorption. These processes are further promoted by interfacial hydrogen spillover, collectively contributing to the superior catalytic performance of CuCo/CoOx.

To gain deeper insight into the catalytic mechanism at the molecular level, a series of in situ FTIR spectroscopy experiments was conducted using methyl benzoate as a model compound (Fig. 5f–i). The characteristic band observed at 1729 cm−1 is assigned to the C=O stretching vibration, while the peaks at 1278 cm−1 and 1110 cm−1 correspond to the asymmetric and symmetric C–O stretching vibrations, respectively52. Notably, all three bands exhibited enhanced intensity on the CuCo/CoOx catalyst compared to Co/CoOx, indicating a higher density of active adsorption sites. Temperature-programmed in situ FTIR adsorption-desorption experiments on both catalysts revealed contrasting vibrational shifts: with increasing temperature, the C–O bands exhibited red shifts, whereas the C=O band underwent distinct blue shifts. This inverse shift behavior strongly suggests that the C–O bond, rather than the C=O bond, is preferentially adsorbed and activated. The preferential activation of the C–O bond is proposed to alter the electronic environment of adjacent functional groups, resulting in strengthened (blue-shifted) C=O vibrations. Importantly, on the CuCo/CoOx catalyst, additional IR bands emerged at 1694 cm−1, closely matching those of aldehyde species53, and their intensities increased progressively with temperature. In contrast, no such bands were observed on Co/CoOx, indicating a superior ability of CuCo/CoOx to activate and cleave the C–O bond in ester groups. This provides compelling mechanistic evidence that on CuCo/CoOx, the initial activation of the C–O bond leads to the formation of aldehyde-like intermediates. To investigate the differences in adsorption behavior between the substrate and the solvent 1,4-dioxane on CuCo/CoOx, we performed TPD and in-situ FT-IR experiments using 1,4-dioxane as a probe molecule (Fig. S44). The results showed an absence of desorption peaks or infrared signals corresponding to 1,4-dioxane, indicating negligible adsorption on the catalyst surface. These findings suggest that the catalyst can selectively adsorb and activate the substrate, and 1,4-dioxane remains chemically inert and stable as the solvent throughout the reaction process. Subsequently, in-situ hydrogenolysis was initiated by introducing hydrogen gas over the CuCo/CoOx catalyst. As the reaction proceeded, the characteristic vibrational signals of ester groups gradually diminished, suggesting smooth and continuous conversion. However, no detectable signals corresponding to aldehyde intermediates were detected during this process, implying their rapid conversion into subsequent products. Their highly transient nature renders them difficult to capture by in situ spectroscopic techniques.

Based on the aforementioned results, the catalytic mechanism over CuCo/CoOx is proposed (Fig. 5j). The process initiates with the adsorption and activation of the C–O bond in PET on the oxygen-vacancy-rich CoOx interface. Subsequently, H2 is dissociated on metallic sites, predominantly fcc-Co species, generating active H* species that spill over to adjacent CoOx surfaces. These H* species attack the activated C–O bonds, resulting in their cleavage and the formation of aldehyde intermediates. The C=O group within the aldehyde is further activated and hydrogenated on CoOx to form a hydroxyl intermediate, which then undergoes C–O bond cleavage to yield the final product PX. Finally, PX desorbs from the catalyst surface, completing the catalytic cycle. In short, the distinctive catalytic role of CuCo/CoOx includes that compared to Co/CoOx, CuCo/CoOx exhibits superior H2 dissociation due to the presence of fcc-Co phases. The oxygen-vacancy-rich interfacial CoOx species not only facilitates the activation of C–O bonds in PET but also modulates the adsorption equilibrium between the substrate and product, thereby continuously driving the reaction forward.

CuCo/CoOx for real-world plastic waste conversion and techno-economic assessment
To evaluate the practical applicability of the CuCo/CoOx catalyst, its performance was systematically assessed for the conversion of 28 real-world PET plastics, including single-component PET, PET with additives and PET-based mixed plastics involving various feed ratios (Fig. 6a–c, Figs. S45–S49 and Tables S4, S5). Following simply shearing, all tested feedstocks can be efficiently converted into PX over CuCo/CoOx (Fig. 6a). To investigate the impact of typical additives present in waste plastics, a range of representative compounds was examined54 (Fig. 6c). The majority of these additives (1–4 and 7 in Fig. 6c) exhibited negligible influence on catalytic activity, maintaining PX yields above 99.9%. Some additives (5 in Fig. 6c) induced only mild inhibition, with yields retained at ~95%, whereas a few specific species (6, 8 in Fig. 6c) exerted more pronounced poisoning effects. Furthermore, the CuCo/CoOx catalyst demonstrates remarkable potential for the selective conversion of PET in mixed plastics, enabling its high-selectivity conversion to PX without interference from other plastics (Fig. 6b). Furthermore, this catalyst demonstrates significant potential for the conversion of 5 other aromatic polyesters (e.g., PBT, PBAT) as well as aliphatic polyesters (e.g., PLA, PCL, PBS) (for a detailed discussion, see Supporting Information below Tables S4, S5). These results highlight the intrinsic robustness and tolerance of the CuCo/CoOx system toward real-world impurities.

To benchmark the techno-economic and environmental advantages of the PET-to-PX process, a comprehensive comparison was conducted against conventional naphtha-to-PX production, with both routes maintaining elemental mass balance (Figs. S50, S51). In terms of economic efficiency, the PET-to-PX pathway exhibits a lower production cost of ¥5.90 per kg PX and a higher profit margin of ¥3.30 per kg PX, representing a 106.3% increase compared to the conventional naphtha-based routes. A life cycle assessment (LCA) performed using Simpro and Aspen simulations further indicates a significantly reduced global warming potential (GWP) of 0.45 kg CO2 eq/kg PX, reflecting a 30.7% decrease in emissions (Fig. 6d). A comprehensive life-cycle carbon footprint analysis, including feedstock acquisition, transportation, and production, further confirms the considerable carbon reduction potential of this upcycling strategy (Fig. 6d). Additional environmental advantages include the valorization of by-product gases (methane and ethane) for natural gas generation, high-purity PX output, and exceptional catalyst durability, which allows for multiple recycling cycles. Detailed parameter settings, economic comparisons, environmental evaluation results and in-depth discussion are provided in Supporting Information (Figs. S50–S53 and Tables S6–S88). Collectively, these findings establish the CuCo/CoOx-catalyzed PET-to-PX conversion as a viable, economically attractive, and environmentally superior alternative to traditional petroleum-based PX production, offering a sustainable route for plastic waste valorization and fossil resource mitigation.

Methods
Synthesis of catalysts
The Co3O4 supports were synthesized with a precipitation method using cobalt nitrate as the synthetic precursor37. Typically, 17.46 g Co(NO3)2·6H2O and 6.63 g (NH4)2CO3 were dissolved in 200 mL deionized water, respectively. Then the solution of (NH4)2CO3 was added slowly into the solution of Co(NO3)2·6H2O with vigorous stirring to form a precipitate until the pH ≈ 9. After that, the resulting suspension was stirred at 65°C for 1 h and then allowed to maintain at room temperature for 16 h. After filtration and washing with deionized water multiple times, ensuring the pH reached 7, the solid phase was dried under vacuum at 100°C for 12 h and finally calcined in air at 500°C for 4 h to obtain Co3O4 supports. The CuO supports were synthesized in the same way.

CuCo/CoOx was prepared by a simple impregnation method. Detailly, the Co3O4 support was added to a 5 mL aqueous solution containing 0.3802 g Cu(NO3)2·3H2O. After stirring at room temperature overnight to ensure the uniform dispersion of Cu, the obtained sample was dried at 80°C for 16 h, and then calcined at 500°C in the air for 3 h.

Cu/TiO2, Co/TiO2, Cu/SiO2 and Co/SiO2 were synthesized in the same way as CuCo/CoOx with supports changed. All catalysts should be reduced at 300°C for 3 h under flowing H2 (10% H2-Ar mixed gas) before reaction.

Catalytic test and product analysis
The detailed reaction conditions are described in the figure captions and table footnotes. In a typical reaction with PET conversion as an example, 0.1 g of PET balls, 0.1 g of catalyst, and a solvent mixture of 10 mL of 1,4-dioxane were loaded into a stainless-steel autoclave reactor in a 50 mL stainless steel batch reactor equipped with a Polytetrafluoroethylene (PTFE) liner and magnetic rotor. After the reactor was purged with H2 three times and charged to the target H2 pressure (2 MPa), the reaction was conducted at the target temperature (230°C) with a magnetic stirring speed of 600 rpm. Upon completion, the reactor was cooled naturally. The quantitative analysis of reacting products was performed using gas chromatography (GC-2060) with a flame ionization detector equipped with HP-5 capillary columns. The column temperature began at 50°C (held for 5 min) and was then increased to 280°C at a heating speed of 10°C min−1 (held for 10 min); the total running time was 50 min. The liquid solution was separated from the solid catalyst by centrifugation and was directly analyzed when 1,4-dioxane was used as a solvent. Biphenyl was used as an internal standard for the quantification of the liquid products. The PX yield was calculated following the Eq. (1):

n (mol) PX
(yield) = PX (1)
n (mol) aromatic ring moles of PET

Characterizations and in situ reaction studies
The crystal structure of the catalyst was analyzed via X-ray diffraction with CuKα (λ=0.15406 nm) radiation (XRD, D8 ADVANCE, Bruker Co., Germany) at 40 kV and 40 mA. The morphology of catalysts was examined by transmission electron microscopy (TEM) with an FEI Talos 200X (USA) field emission electron microscope. The actual Cu loading in the sample was detected by a Thermo ScientificTM iCAPTM PRO XP ICP-OES. Raman spectra were recorded using a HORIBA XploRA PLUS equipped with an Ar+ laser (532 nm wavelength). Surface composition and chemical states of the products were characterized by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versaprobe III, Japan) equipped with monochromatic Al Kα radiation, with binding energies corrected using the C 1s peak at 284.8 eV as a reference. Defects in catalysts were determined using electron paramagnetic resonance (EPR, MEX-nano, Bruker, Germany). Optical properties were obtained using a UV-vis spectrophotometer (UV-2600, Shimadzu, Japan). The 1H nuclear magnetic resonance (1H NMR) spectrum was recorded on an AV 500 MHz Fourier-transform superconducting nuclear magnetic resonance spectrometer. Thermal gravimetric analysis (TGA) of the catalyst was conducted on a HS-TGA-101 Thermogravimetric Analyzer. Fourier-transform infrared (FTIR) spectroscopy on the catalyst was performed on a FOLI 10-R Fourier-transform infrared spectrometer.

FTIR of pyridine adsorption was recorded on the FOLI 10-R Fourier-transform infrared spectrometer with an in-situ cell and MCT detector. Following the dissolution of a 5 mg sample in cyclohexane, the solution was applied dropwise onto a 160 mg KBr pellet and uniformly spread. The sample was in situ reduced at a H2 (10% H2-Ar mixed gas) atmosphere at 300°C for 30 min, and then heated at 400°C for 30 min under vacuum in order to remove the physically adsorbed water. After cooling to 100°C, a background scan was performed. Pyridine vapor was then introduced into the cell at room temperature for 20 min. Subsequent evacuation was performed at 100°C for 30 min followed by spectral acquisitions. The spectra presented were obtained by subtracting the spectra recorded before and after pyridine adsorption.

In-situ FTIR measurements of methyl benzoate adsorption were collected with the FOLI 10-R Fourier-transform infrared spectrometer with an in-situ cell and MCT detector. Firstly, the catalysts were pretreated in the in-situ cell in a H2 (10% H2-Ar mixed gas) atmosphere at 300°C for 30 min and then switched to an Ar atmosphere for 30 min to eliminate surface adsorbed substances. Then the background scan of the sample was performed. Methyl benzoate was bubbled into the in-situ cell under an Ar atmosphere for 40 min. Next, the cell was purged with Ar at 35°C for 30 min, and the adsorption spectra of methyl benzoate were recorded. Finally, the temperature was increased to 300°C, and the adsorption spectra of methyl benzoate were recorded at increasing temperature. The in-situ reaction was conducted following the same procedure with H2 (10% H2-Ar mixed gas) introduced during the heating process.

Overlayer formation of the catalysts was investigated using hydrogen temperature-programmed reduction (H2-TPR) on VDsorb-92i. 50 mg of catalyst was treated at 300°C for 30 min under flowing Ar at 30 sccm. After cooling to room temperature, the catalyst was exposed to 10% H2-Ar (30 sccm) and heated to 800°C at a ramp rate of 10°C/min to determine the reducibility of the catalysts.

The absorption and desorption of H2 and ethyl p-methylbenzoate on the catalysts were investigated using temperature-programmed desorption (TPD) on VDsorb-92i. 50 mg of catalyst was loaded into a quartz U-reactor and pre-treated at 300°C for 30 min in a flowing 10% H2-Ar (30 sccm) to simulate the in-situ reduction process. After cooling to room temperature, the catalyst was heated to 300°C for 30 min under flowing Ar at 30 sccm to eliminate surface adsorbed substances. After the temperature dropped to 30°C, the H2 or ethyl p-methylbenzoate was bubbled onto the catalysts for 40 min under a flow of Ar. The sample was then purged by Ar for 40 min to remove the physically adsorbed substances, and the temperature was heated to 800°C at a heating rate of 10°C·min−1.

Computational methods
The spin-polarized density functional theory (DFT) calculations were performed by VASP (version number: 5.4.4)55, using the Perdew–Burke–Ernzerh (PBE) functional within the generalized gradient approximation (GGA)56. The VDW correction (PBE-D3) was also included. The core-valence electron interaction was described by the projector augmented wave (PAW) method57, and the valence electronic states were expanded in the plane-wave basis sets with a cutoff energy of 450 eV. According to the experimental characterization results, the surface configurations of Cu(111), hcp-Co(101) and fcc-Co(111) were modeled separately as the p(2×2), p(2×2) and p(2×2) periodic slabs, and the vacuum between slabs was 15 Å to exclude the mutual influence. During structural optimization, the bottom two layers were fixed, and all other atoms were fully relaxed. For these surface slabs, a 2×2×1 k-point mesh was used. The transition states were searched using a constrained optimization scheme58. The force threshold for the optimization and transition state search was 0.05 eV/Å. The adsorption energy of surface species was defined via the following formula (2):

Eads (X) = EX surf − Esurf − EX (2)

where Esurf, EX, and EX surf are the energies of the optimized catalyst surface, X in the gas phase, and X adsorbed on the catalyst surface, respectively. The more negative the Eads (X) value is, the more strongly the species X binds to the surface. Note that the energy reference of the H atom is half that of H2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors report a high 99% conversion of PET to p-xylene using a CuCo/CoOx catalyst. While the catalyst design is conceptually interesting, I believe the study is not suitable for publication in Nature Communications. A major limitation lies in the absence of well-defined active sites, which complicates any detailed mechanistic investigation. Furthermore, the transformation primarily involves C–O bond cleavage—a reaction pathway that is relatively facile and well-documented. The reaction was also demonstrated on a tiny scale, using a 1:1 catalyst-to-polymer ratio, which raises concerns about the scalability and economic feasibility of the process. Although the authors compare the production cost of p-xylene from this PET upcycling route to the conventional naphtha-based process and claim a nearly 50% cost reduction, this conclusion does not take into account the high catalyst loading, thereby limiting the validity of the economic analysis and the practical viability of the approach.\nAlso, the 1H-NMR contains impurities; use of HPLC-grade solvent is advisable. Also, the 1H-NMR peak for p-xylene is unclear, and the integration for the two types of protons is not shown. The 1H-NMR spectra appears quite blurry, making it difficult to assess data quality. Also, how is the impurity peak in Dioxane solvent not visible in GC-MS spectra?\nIt is quite intriguing that the catalyst can cleave the C-O bond of PET, but no solvent decomposition is observed. How? Solvent also has a C-O bond. Please clarify.\nThe author provides a detailed explanation suggesting that copper doesn’t actively participate in the reaction but instead plays a synergistic role, evidenced by the red shift of the A1g mode from 661 to 651 cm⁻¹, indicating lattice distortion and the creation of oxygen vacancies. This claim is further supported by XPS and TPR studies. However, one question still remains why copper? Have the authors tested other metals for comparison? Also, the copper XPS data is missing; most of the characterization focuses only on cobalt. This raises the possibility that copper might be directly involved in the mechanistic pathway, which hasn't been thoroughly ruled out.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "The authors present an interesting catalyst system for the hydrogenolysis of PET to paraxylene with high selectivity. The significance of this reaction is clear. It produces pure para xylene without contamination from ring hydrogenation, ring cleavage, or over-reduction, which is an advantage over previously reported precious metal system. The catalysts are well characterized, and the role of copper in promoting the reduction of Co3O4 is supported by both experimental data and DFT. The authors also show that para xylene desorbs more easily from CuCo/CoOx than from Co/CoOx, which improves turnover and selectivity. However, i believe that major revisions are required, outlined below:\nthe paper focuses mainly on catalyst characterization, while the actual catalytic work is rather limited, actually only one reaction was studied. Since the authors have already developed and characterized this catalyst in detail and understand why it works well, it would be more convincing if they tested it on additional substrates. For example, they could try polybutylene terephthalate (PBT) or polybutylene adipate terephthalate (PBAT, now commonly used in China to replace LDPE).\nfurther, it would also be interesting how aliphatic polyesters such as PLA would react with their system.\nAlthough the authors show that the catalyst works with post-consumer PET containing additives and dyes, it would also be useful to test it with mixed plastic streams. Real plastic waste often contains HDPE, LDPE, and PP from bottle caps, labels, and cups. Understanding how these plastics affect the catalyst’s performance would help assess its practicality. It would also be interesting to test a mixture of polyesters, such as PET and PLA. One major hurdle in plastics recycling is the difficulty of sorting plastics, especially those used for similar applications (for example, PET and PLA are both used for disposable cups). If the catalytic system can produce xylene and light alkanes from PET and PLA in a mixed-polyester stream, it would strongly support its potential for real-world application. Also, PLA is commonly mixed with PBAT in straws used in most large coffee brands in china.\nMore concerning is the supporting data for the environmental and economic claims. The information provided for product quantification, process modeling, LCA, and TEA is seriously lacking. These issues make it impossible to evaluate the impact and reproducibility of the work. Since environmental and economic factors are a key motivation for converting plastic waste into chemicals, these sections are critical to the significance of the paper. Major revisions are required to provide adequate data to support the TEA and LCA, or these sections should be removed, and the paper should be reevaluated based only on the catalysis.\nThe authors claim that their process reduces the cost and global warming potential of producing p-xylene compared to the conventional naphtha-based route. However, the supporting data are missing, and the analysis cannot be published in its current form.\nThe life cycle assessment is not reproducible or verifiable. There is no life cycle inventory provided in the main text or the supplementary information. Key assumptions such as hydrogen source, electricity mix and transport distances are not described. The emission factors of the materials are not given, and no data sources are cited. the authors also do not cite where the GWP values for p-xylene from naptha come from. No sensitivity analysis or uncertainty assessment is provided. Since hydrogen production and energy mix can strongly affect the LCA outcome, this lack of detail makes the LCA results unreliable.\nThe techno-economic analysis also lacks key details. The authors report a PX production cost of $413 per ton, but no cost breakdown is given. We do not know the assumed plant scale, catalyst lifetime, utility costs, or operating schedule. No performance indicators such as payback time, internal rate of return, or capital investment are reported. Therefore, the claim that this route is cheaper than the conventional process is unsupported.\nA process flow diagram is included but is purely schematic. No data are included in the diagram or in any table in the supplementary information. It is unclear whether mass and energy balances were done, and solvent or heat recovery is not discussed. As it stands, the process diagram does not help readers understand the system or evaluate the LCA and TEA.\nMinor comment:\n1: In Figure 2e, the FTIR spectra show two flat lines. It is unclear what the purpose of this figure is. The authors state that the spectra show no accumulation of organic materials and no structural changes to the catalyst. However, a flat FTIR does not provide useful information about the structure. The claim that the catalyst maintains structural integrity is not supported by this data. it world make more sense to show TEM before and after reaction.\n2: The NMR figure could possibly be larger, and the ppm range is a bit too broad. it would make more sense to focus on the range between 0-10 ppm, and possibly include integrals and peak values.", "reviewer_4": "The manuscript presents a novel stepwise hydrogen spillover strategy utilizing a CuCo/CoOₓ catalyst for the near-quantitative conversion of waste PET to p-xylene. The design involves Cu⁰-assisted hydrogen activation and Co⁰-facilitated secondary spillover, promoting active site formation and oxygen-vacancy-rich interfaces. The catalyst performs well on real plastic waste, with promising techno-economic and environmental metrics. My comments are as below,\n• The authors claim this to be the first hydrogenolysis system achieving >99% p-xylene yield. However, similar Co-based catalysts with comparable efficiency have been previously reported. The authors are encouraged to better position their work by providing a more critical comparison with existing literature.\n• In Fig. 2a, the Co/TiO₂, Co/Al2O3, Cu/SiO2 catalysts shows no detectable p-xylene production. It remains unclear whether this implies complete inactivity or formation of side products. The authors should clarify if any alternative products were formed and provide corresponding GC or GC-MS data to support this.\n• The manuscript mentions a p-xylene yield of 52.55% over the Co/CoOₓ catalyst in the main text. However, this data point appears to be missing in Fig. 2.\n• The proposed mechanism involving TPA and Bis-(2-hydroxyehyl) terephthalate lacks sufficient clarity. The identification of 4-methylbenzyl alcohol does not conclusively establish the pathway for PET conversion. A more detailed mechanistic explanation is required, additional intermediate tracking, and literature validation.\n• While TEM images are provided, the particle size distribution is not quantified. This analysis is critical for understanding the structure–activity relationship, especially considering the discussion around active site density and agglomeration.\n• The authors attribute activity loss to a balance between enhanced intrinsic activity (fcc-Co phase) and reduced accessible active sites due to agglomeration. This explanation is reasonable but requires experimental support such as particle size evolution, sintering analysis.\n• Figure label “Fig. 3 I” appears to be a typographical error and should be corrected to “Fig. 3i.\n• The manuscript discusses the involvement of Cu⁰ in the reaction mechanism; however, the corresponding XPS data for Cu⁰ is not presented. Please include this to substantiate the claim. Additionally, referencing previous XPS studies for Co and Cu species would strengthen the assignment of oxidation states.\n• The surface area and pore size distribution of the catalysts are not reported. These parameters are essential for evaluating catalyst accessibility.\n• The authors use 1,4-dioxane as the reaction solvent, which is considered toxic and non-green. This choice raises sustainability concerns. Please justify this selection and discuss potential alternatives that align better with green chemistry principles."}, "author_response": "Response to Reviewers’ Comments\nReviewer #1: In this work, the authors report a high 99% conversion of PET to p-xylene using a\nCuCo/CoOx catalyst. While the catalyst design is conceptually interesting, I believe the study is not\nsuitable for publication in Nature Communications.\nResponse 1-0. We sincerely appreciate your constructive comments, which have significantly\nimproved the quality of this manuscript. In response to your comments, we have made the following\nsubstantial revisions:\n1. We have conducted additional experiments to further elucidate the H₂ activation on Co⁰ species,\nparticularly fcc-Co at the interface, the substrate activation on the oxygen vacancy-rich CoOₓ\nspecies at the interface, and the hydrogen spillover from Co⁰ to CoOₓ sites at the interface. These\nfurther support the conclusion that the oxygen vacancy-rich Co⁰/CoOₓ interface serves as the\nactive site for the PET hydrogenolysis.\n2. We have elaborated on the two main innovations of this work: 1) For the first time, we directly\ncaptured the ester bond activation signal in PET via in situ FT-IR, observing the aldehyde-like\nintermediate. This provides clear molecular-level evidence for ester bond activation and offers\nnew insights into the mechanism of PET hydrogenolysis. 2) We propose a stepwise hydrogen\nspillover strategy to construct unique interfacial synergistic sites composed of identical metal\nspecies.\n3. We have conducted additional experiments to further reduce the catalyst-to-feed mass ratio and\nadded a detailed economic analysis to demonstrate the economic viability of our system.\n4. We have used a higher purity solvent and standardized the ¹H-NMR testing to further confirm the\nhigh purity of the product.\n5. Through FT-IR spectra and TPD experiments with probe molecules, we confirm that our catalyst\ndoes not adsorb 1,4-dioxane but can specifically adsorb and activate the C-O bonds in PET.\n6. To highlight the key role of Cu in catalyst construction, we have added characterization and\nexperimental data for comparison catalysts doped with various metals. The results indicate that\nCu species play a crucial role in inducing stepwise hydrogen spillover at the active sites.\nFurthermore, we have supplemented the Cu XPS and Auger spectra to clarify its electronic state\nand have confirmed through probe molecule experiments that Cu species are not the main active\nsites, thus reinforcing the explanation of the catalytic mechanism.\nIn conclusion, we have thoroughly addressed all of your concerns in a detailed, point-by-point\nmanner. The specific revisions can be found in responses 1-1 to 1-6. We believe that the revised\nmanuscript has been significantly improved and now meets the publication standards of Nature\nCommunications.\nComment 1-1: A major limitation lies in the absence of well-defined active sites, which complicates\nany detailed mechanistic investigation.\nResponse 1-1: Thanks for your comments. Although the original manuscript identified the oxygen\nvacancy–rich Co⁰/CoOₓ interface as the true active site for PET hydrogenolysis, the explanation\nprovided may have lacked sufficient clarity. In the revised version, we offer a more thorough\ndiscussion and present extensive additional experimental evidence to reinforce and clarify this\nconclusion. A detailed analysis of the well-defined active sites and the catalytic mechanism is\npresented below, with newly added content highlighted in yellow:\nAs comprehensively demonstrated in the original manuscript, the stepwise hydrogen spillover–\nengineered CuCo/CoOₓ catalyst features low crystallinity, a partial phase transformation from hcp-\nCo to fcc-Co, and abundant oxygen vacancy–rich Co/CoOₓ interfaces, with fcc-Co/CoOₓ interfaces\npredominating. The hydrogenolysis of PET to PX on CuCo/CoOₓ primarily proceeds through three\nsteps: (1) activation of hydrogen on Co⁰ sites, (2) adsorption and activation of the substrate on CoOₓ,\nand (3) spillover of dissociated hydrogen species from Co⁰ to CoOₓ sites at Co/CoOₓ interfaces.\nRegarding hydrogen activation, zero-valent metal species typically serve as the active sites.\nH₂-TPD profiles (Fig. R1) indicate that CuCo/CoOₓ exhibits superior hydrogen adsorption and\ndissociation performance compared to Co/CoOₓ. Furthermore, DFT calculations (Fig. R2) reveal that\nthe energy barriers for H₂ dissociation on Co⁰, particularly the fcc-Co phase, are significantly lower\nthan those on Cu⁰. Therefore, Co⁰ species, especially fcc-Co at the interface, are identified as the\nprimary active sites responsible for H₂ activation during PET hydrogenolysis, rather than Cu⁰.\nFig. R1 (Fig. 5b in the manuscript) H -TPD profiles of CuCo/CoO and Co/CoO .\n2 x x\nFig. R2 (Fig. 4c in the manuscript) Calculated adsorption energies of H and dissociation energy of H on Cu (111),\n2 2\nhcp Co (101) and fcc Co (111).\nRegarding substrate activation, we conducted TPD experiments using ethyl p-methylbenzoate as a\nprobe molecule on a series of catalysts enriched in CuOₓ, Cu⁰ and Co⁰ sites, respectively (Fig. R3).\nNo evident desorption peaks were observed for these CuOₓ, Cu⁰ and Co⁰-rich catalysts, whereas\ndistinct desorption peaks appeared for both Co/CoOₓ and CuCo/CoOₓ catalysts. These observations\nindicate that CoOₓ is the primary active site for substrate adsorption. Notably, CuCo/CoOₓ exhibits\nsignificantly higher adsorption capacity for the probe molecule compared to Co/CoOₓ. Oxygen\nvacancies in Lewis acid catalysts (CuCo/CoOₓ and Co/CoOₓ) could play a critical role in the\nadsorption and activation of C-O bonds due to their enhanced oxophilicity1, 2. To further elucidate the\nrole of oxygen vacancies, a series of characterizations including Raman, EPR, pyridine-IR, XPS,\nXAS and UV-Vis spectroscopy were performed to compare the oxygen vacancy concentrations of\nCo/CoOₓ and CuCo/CoOₓ (Fig. R4a-f). These results consistently revealed a higher oxygen vacancy\nconcentration in CuCo/CoOₓ, confirming that Cu incorporation significantly increases the oxygen\nvacancy density in CoOₓ at the Co/CoOₓ interfaces. This enhancement is attributed to a decrease in\nthe formation energy of oxygen vacancies at the interface, which is induced by the high interfacial\nstrain. Therefore, we propose that the oxygen vacancy–rich CoOₓ species at the Co/CoOₓ interfaces\nserve as the primary active sites for the adsorption and activation of substrates.\nFig. R3 (Fig. 5d in the manuscript) Ethyl p-methylbenzoate-TPD profiles of CuCo/CoO and Co/CoO .\nx x\nFig. R4 (Fig. 3j-3o in the manuscript and Fig. S26, S27 in the supporting information) Raman (a), EPR (b),\npyridine-IR (c), O 1s XPS (d), soft X-ray absorption spectroscopy (e) and UV-Vis (f) spectroscopy of\nCuCo/CoOₓ and Co/CoOₓ catalysts.\nIt is well established that a higher concentration of oxygen vacancies corresponds to a greater\nabundance of low-coordination metal species1, 3. To further investigate how oxygen vacancy–rich\nCoOₓ species at the CuCo/CoOₓ interfaces facilitate substrate activation, and to validate the\nrelationship between unsaturated CoOₓ species and substrate adsorption capacity, Co₃O₄ was reduced\nat 250 °C and 350 °C to obtain CoOₓ species with varying degrees of coordination saturation (denoted\nas Co/CoOₓ–250 and Co/CoOₓ–350, respectively), distinct from the Co/CoOₓ–300 sample (Co/CoOₓ\nin the manuscript). XPS analysis (Fig. R5) revealed a significant increase in the Co⁰/Co²⁺ ratio with\nincreasing reduction temperature, from 0.05 to 1.96, suggesting that the higher reduced temperatures\ngenerate significantly more Co⁰ species, which could correspondingly reduce the number of\nunsaturated CoOₓ sites. While this trend enhances hydrogen dissociation, excessive Co⁰ species can\nlead to a shortage of CoOₓ species, thereby hindering substrate adsorption. On the other hand, the\nlower reduced temperatures produce fewer Co⁰ species and the remaining CoOₓ species exhibit higher\nvalence states and approach a coordination-saturated configuration. Thus, the oxygen vacancy\nconcentrations in Co/CoOₓ–250 and Co/CoOₓ–350 are inferred to be significantly lower than in\nCo/CoOₓ–300. This hypothesis was corroborated by EPR, Raman, and pyridine-IR analysis (Fig. R6\na-c). EPR data revealed that the oxygen vacancy concentration in Co/CoOₓ-250 and Co/CoOₓ-350 is\nsignificantly lower than in Co/CoOₓ-300. Raman spectroscopy further confirmed this trend, with only\nthe A1g characteristic peak of Co/CoOₓ-300 exhibiting a distinct redshift, indicating a higher oxygen\nvacancy concentration induced by lattice distortion. Additionally, the FTIR of pyridine adsorption\nrevealed that Co/CoOₓ–300 has considerably higher pyridine adsorption capacities compared to\nCo/CoOₓ–250 and Co/CoOₓ–350, suggesting a substantial increase in Lewis acid sites. This\npronounced increase suggests the presence of a greater number of coordinatively unsaturated CoOₓ\nsites, which are closely associated with enhanced oxygen vacancy concentrations. Then, TPD\nexperiments (Fig. R7b) using ethyl p-methylbenzoate as a probe molecule further confirmed that\nsubstrate adsorption on Co/CoOₓ-250 and Co/CoOₓ-350 is much weaker than on Co/CoOₓ-300.\nTemperature-programmed in situ FTIR adsorption-desorption experiments (Fig. R7a) also exhibited\nextremely weak adsorption on Co/CoOₓ-250 and Co/CoOₓ-350, different from the adsorption on\nCo/CoOₓ-300, revealing a strong positive correlation between the adsorption-activation of substrates\nand Lewis acid sites. Notably, the observed obvious aldehyde-like intermediates on CuCo/CoO than\nx\nthat on Co/CoOₓ-300 (Fig. R8), as discussed in the manuscript, confirms the stronger activation of\nester groups. Therefore, we confirm that the oxygen vacancy–rich CoOₓ species at the Co/CoOₓ\ninterfaces serve as the true active sites for substrate adsorption and activation.\nFig. R5 (Fig. S34 in the supporting information) XPS spectra of Co 2p3/2 in Co/CoO -250, Co/CoO -300, and\nx x\nCo/CoO -350.\nx\nFig. R6 (Fig. S35-S37 in the supporting information) (a) EPR results of Co/CoO -250, Co/CoO -300,\nx x\nCo/CoO -350 and CuCo/CoO catalysts. (b) Raman spectra of Co/CoO -250, Co/CoO , Co/CoO -350 and\nx x x x x\nCuCo/CoO . (c) The FIIR spectra of adsorbed pyridine on Co/CoO -250, Co/CoO -300, Co/CoO -350 and\nx x x x\nCuCo/CoO .\nx\nFig. R7 (Fig. S38, S39 in the supporting information) (a) In situ methyl benzoate-absorbed FTIR spectra under\nAr on Co/CoO -250, Co/CoO -300, Co/CoO -350 and CuCo/CoO . (b) Ethyl p-methylbenzoate-TPD profiles\nx x x x\nof Co/CoO -250, Co/CoO -300, Co/CoO -350 and CuCo/CoO .\nx x x x\nFig. R8 (Fig. 5g, h in the manuscript) In situ methyl benzoate-absorbed FTIR spectra in the presence of Ar on\n(a) CuCo/CoO and (b) Co/CoO .\nx x\nAdditionally, the hydrogen spillover ability of the CuCo/CoOₓ and Co/CoOₓ catalysts was\nevaluated using WO₃ color change experiments. As shown in the WO₃ color change experiments (Fig.\nR9), pale yellow WO₃ was reduced to blue grey tungsten oxide species by spillover active hydrogen\nspecies on CuCo/CoOₓ. However, on Co/CoOₓ, the color of reduced WO₃ shifted only to a lighter\nyellowish gray, indicating that the hydrogen spillover capabilities of Co/CoOₓ were significantly\nweaker than those of CuCo/CoOₓ. The above results indicate that abundant oxygen vacancy-rich\nCo/CoOₓ interfaces, especially fcc-Co/CoOₓ interfaces, of CuCo/CoOₓ can significantly boost the\nhydrogen spillover from Co0 to CoOₓ at the interfaces during the hydrogenolysis of PET.\nFig. R9 (Fig. 4b in the manuscript) Hydrogen spillover experiments of reduced Co/CoO and CuCo/CoO .\nx x\nIn short, we have successfully constructed well-defined active sites (oxygen vacancy–rich\nCo⁰/CoOₓ interface) and elucidated the catalytic roles: 1) hydrogen activation on Co⁰, particularly\nthe fcc-phase Co⁰ species; 2) substrate adsorption and activation on the oxygen vacancy–rich CoOₓ\nspecies at the Co/CoOₓ interfaces; 3) activated substrate can be hydrogenated by the active hydrogen\nspecies, which is from the spillover from Co⁰ to CoOₓ sites at the interface.\nRevision 1-1:\n“To investigate the role of unsaturated CoOₓ at the Co/CoOₓ interfaces facilitating substrate\nactivation and validate the relationship between unsaturated CoOₓ species and substrate adsorption\ncapacity, Co O was reduced at 250 °C and 350 °C to obtain CoOₓ species with varying degrees of\n3 4\ncoordination saturation (denoted as Co/CoOₓ–250 and Co/CoOₓ–350, respectively), distinct from the\nCo/CoOₓ sample. XPS (Fig. S34), EPR (Fig. S35), Raman (Fig. S36), and pyridine-IR analyses (Fig.\nS37) confirm that Co/CoOₓ reduced at 300 °C possesses the most abundant unsaturated CoOₓ species,\nwhich are closely associated with enhanced oxygen vacancy concentrations51. Subsequently, in-situ\nFTIR adsorption-desorption experiments (Fig. S38) and TPD experiments (Fig. S39) as a probe\nmolecule further reveal a strong positive correlation between substrate adsorption-activation and the\npresence of unsaturated CoOₓ species.” (See page 18 of the Manuscript).\n“…The XPS spectra (Fig. S34) of the Co 2p₃/₂ peak in the Co/CoO -250, Co/CoO , and Co/CoO -350\nx x x\nsamples revealed a significant increase in the Co⁰/Co²⁺ ratio with higher reduction temperatures,\nrising from 0.05 to 1.96. This indicates that higher reduction temperatures lead to a substantially\nlarger number of Co⁰ species, which in turn reduce the number of unsaturated CoOₓ sites. While this\ntrend favors hydrogen dissociation, an excessive amount of Co⁰ species may deplete CoOₓ species,\nthereby hindering substrate adsorption. In contrast, lower reduction temperatures result in fewer Co⁰\nspecies, with the remaining CoOₓ species exhibiting higher oxidation states and approaching a\ncoordination-saturated configuration. As a result, the oxygen vacancy concentrations in the\nCo/CoOₓ-250 and Co/CoOₓ-350 catalysts are expected to be considerably lower than in the Co/CoOₓ\ncatalyst reduced at 300 °C.” (See page 66 of the Support Information).\n“The EPR spectra (Fig. S35) exhibit signals at g = 2.003, attributed to unpaired electrons associated\nwith oxygen vacancies. Notably, the CuCo/CoO catalyst displays a significantly stronger signal\nx\nintensity compared to the Co/CoOx-250, Co/CoO , and Co/CoO -350 samples, indicating a higher\nx x\nconcentration of oxygen vacancies, consistent with the XPS findings. The oxygen vacancy\nconcentration is ranked as follows: CuCo/CoO > Co/CoO > Co/CoO -350 > Co/CoO -250.” (See\nx x x x\npage 67 of the Support Information).\n“The Raman spectra (Fig. S36) of the Co 2p₃/₂ peaks in the Co/CoO -250, Co/CoO , and Co/CoO -\nx x x\n350 catalysts show bands at 186, 501, and 592 cm⁻¹, corresponding to the F₂g vibrational mode of\nCo–O stretching vibrations associated with Co²⁺ species. The band at 461 cm⁻¹ is attributed to the Eg\nmode of the Co₃O₄ spinel structure, resulting from lattice vibrations induced by the synergistic\ninteraction between Co³⁺ and Co²⁺ ions. For the Co/CoOₓ-250 and Co/CoOₓ-350 catalysts, the A₁g\nmode at 667 cm⁻¹ is characteristic of octahedrally coordinated Co species (CoO₆). Notably, the A₁g\ncharacteristic peak of Co/CoOₓ shows a distinct redshift (from 667 to 661 cm⁻¹) compared to\nCo/CoOₓ-250 and Co/CoOₓ-350, indicating a higher concentration of oxygen vacancies induced by\nlattice distortion.” (See page 68 of the Support Information).\n“The FTIR spectra (Fig. S37) of pyridine adsorption on the Co/CoO -250, Co/CoO -300, Co/CoO -\nx x x\n350, and CuCo/CoO catalysts reveal that CuCo/CoO exhibits considerably higher pyridine\nx x\nadsorption capacities than Co/CoOₓ-250, Co/CoO , and Co/CoO -350, suggesting a significant\nx x\nincrease in Lewis acid sites. This pronounced increase implies the presence of a greater number of\ncoordinatively unsaturated CoOₓ sites, which are closely associated with enhanced oxygen vacancy\nconcentrations. The concentration of Lewis acid sites follows the order: CuCo/CoO > Co/CoO >\nx x\nCo/CoO -350 > Co/CoO -250. These findings are consistent with the patterns observed in the EPR\nx x\nresults” (See page 69 of the Support Information).\n\"Temperature-programmed in situ FTIR adsorption-desorption experiments (Fig. S38) further\nrevealed that the adsorption strength of methyl benzoate follows the order: CuCo/CoO > Co/CoO >\nx x\nCo/CoO -350 ≈ Co/CoO -250. Additionally, TPD experiments using ethyl p-methylbenzoate as a\nx x\nprobe molecule (Fig. S38) confirmed that the substrate adsorption strength on the Co-based catalysts\nis ranked as: CuCo/CoO > Co/CoO > Co/CoO -350 ≈ Co/CoO -250. These findings establish a\nx x x x\nstrong positive correlation between substrate adsorption and activation and the concentration of\nLewis acid sites (coordinatively unsaturated CoOₓ species). Therefore, oxygen vacancy–rich CoOₓ\nspecies at the Co/CoOₓ interfaces are identified as the active sites responsible for substrate\nadsorption and activation."}]

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