Tuning selectivity in the direct hydrogenolysis of PET plastic over Co catalysts through interfacial hydrogen spillover

Domain: chemistry
Published: 2026-04-13
Editorial outcome: minor_revision

Abstract
Polyethylene terephthalate (PET) plastic upcycling typically features two-step processes involving cascade depolymerization and functionalization to mixed products. Here, we realize the direct hydrogenolysis of PET over a Co@CoO heterogeneous catalyst, with high yield of either p-xylene (PX, >97 %) or 1,4-dimethylcyclohexane (1,4-DMC, >90 %). Pyrolysis of Co-MOF-71 yields carbon-supported Co nanoparticles partially encapsulated by CoO, whose metal/oxide interface facilitates hydrogen spillover. Experimental and computational investigations reveal Coδ+ atoms proximate to CoO particles selectively catalyze carboxylate C–O bond cleavage at 250 °C and 1 MPa H to yield PX. In contrast, hydrogen spillover from Co metal to CoO sites at 280 °C and 3 MPa H promotes selective ring hydrogenation to 1,4-DMC. Co/CoO-800 has excellent stability and efficacy for depolymerizing commercial PET plastics. Life cycle assessment indicates the PET-to-PX process offers negative CO emissions and outperforms fossil-fuel PX production. Direct hydrogenolysis over Earth-abundant catalysts offers a simple strategy for polyester waste upcycling.

Introduction
Polyethylene terephthalate (PET), with an annual, global production of 33 million tons, is extensively used in packaging and synthetic fibers, but is a major source of ‘white pollution’ being challenging to degrade naturally1. Microplastics resulting from waste PET are detrimental to marine and freshwater environments2 (e.g., through accumulation in the food chain), and also damage human health through oral inhalation, ingestion, or skin absorption3,4. The European Commission and the United States have set ambitious circular targets for plastics, especially PET, with 55 % and 50 % recycling goals by 2030, respectively to mitigate environmental pollution and health concerns5,6. Such goals also underpin the United Nations Environment Program's recent pledge to address planetary crises7. Strategies for recycling PET plastics, such as mechanical extrusion, enzymatic decomposition, and thermochemical treatment (e.g., methanolysis, hydrolysis, glycolysis, and pyrolysis), are hence of intense academic and commercial interest8,9. Recycling PET to its terephthalic acid or terephthalate monomers would promote a circular economy for plastics10,11. With existing technologies, only a small fraction of commodity plastics can be chemically recycled in an economically viable and energy-efficient manner12. Consequently, a critical goal moving forward is to develop advanced recycling methods that can effectively supplement or ultimately replace traditional chemical approaches13,14.

Chemocatalytic upcycling of PET into value-added chemicals or liquid fuels, notably aromatics as chemical intermediates and cycloalkanes as liquid organic hydrogen carriers, would also mitigate the environmental impact of waste plastics compared to traditional chemical recycling processes (e.g., alkaline hydrolysis and glycolysis)15, while furthering the development of carbon-neutral refineries12,16,17 for sustainable chemical manufacturing18. Such upcycling routes, using heterogeneous catalysts, offer high atom economy and low waste generation, with two-step PET conversion by cascade hydrolysis or methanolysis and hydrogenolysis, considered efficient and green routes to value-added chemicals in water or methanol, respectively (Fig. 1a)19-26. These polar solvents strongly interact with the polar functional groups in PET to weaken interchain interactions, enabling the production of monomers (for recycling) for subsequent hydrogenolysis to chemical intermediates19,20,25,27. However, the latter process is hindered by the co-production of oligomers and liquid PET that can also undergo hydrogenolysis, hindering access to a single target product11. Kinetic limitations of PET hydrolysis or alcoholysis also result in long reaction times (6-16 h) to achieve comprehensive hydrogenolysis28. Bifunctional catalysts, containing acid/base sites to promote PET depolymerization alongside metal sites for hydrogenolysis of resulting monomers, are a potential solution, but markedly complicate catalyst synthesis11,29. The direct, one-pot hydrogenolysis of PET (via C–O bond cleavage) would circumvent the preceding limitations and simplify the overall process, but requires significant improvements in catalyst activity and selectivity30,31.

A desirable catalyst for PET hydrogenolysis should: (i) be active for the dissociative adsorption of molecular hydrogen; (ii) offer a low barrier for high adatom diffusion to enable rapid transport to PET chains; (iii) feature redox active metals able to shuttle electrons and protons to PET for C–X bond hydrogenolysis; (iv) incorporate Earth-abundant elements to ensure economic viability at scale; and (v) deliver a high concentration of active sites to ensure a high specific activity under energy-efficient (modest temperature) operation. Cobalt (and its oxides) fulfill the first four criteria, and when dispersed in nanoparticle form on a carbon support also fulfills the fifth criterion, forming Earth-abundant reducible metal oxides in organic solvents at temperatures 200 °C32, conditions well-suited to PET hydrogenolysis. Cobalt is widely used in energy and environmental applications due to to its excellent redox properties33.

Here, we report a heterogeneous Co-based catalyst containing Co/CoO interfaces that promote reductive Cα–O bond cleavage, selectively upcycling PET into either PX or 1,4-DMC, depending on the reaction conditions and spatial distribution of hydrogen adatoms (Fig. 1b). Electronic polarization of the Co/CoO interface confines hydrogen dissociatively adsorbed over electron-deficient Co metal sites to direct Cα–O bond cleavage of PET to selectively produce PX at modest temperature and pressure. More forcing reaction conditions promote hydrogen adatoms spillover across the metal/oxide interface to CoO sites at which benzene ring hydrogenation (BRH) is favored to exclusively afford 1,4-DMC. Life cycle assessment (LCA) suggests this one-step PET-to-PX process would significantly reduce CO emissions compared to the current petroleum-to-PX process. Regulating hydrogen spillover across the Co/CoO interface is an effective strategy for upcycling diverse PET plastics into pure streams of high-value chemicals.

Results
Construction of active sites and interface structure

The parent columnar Co-MOF-71 precursor was prepared by hydrothermal treatment of terephthalic acid and cobalt nitrate hexahydrate, followed by pyrolysis in an argon atmosphere to obtain carbon-supported Co/CoO-n catalysts (n represents the pyrolysis temperature) (Fig. 2a). The Co/CoO catalytic system was selected primarily due to the ability of Co-based catalysts to exchange electrons with external species, leading to the existence of multiple oxidation states. The proportion of these states can be modulated by altering the redox conditions32. This characteristic of cobalt makes it particularly suitable for forming complexes with reactants, thereby facilitating redox processes33. Thermogravimetric analysis showed that the weight loss of Co-MOF-71, occurring almost entirely <600 °C, was accompanied by the decomposition of its ligands into an amorphous carbon support (Supplementary Fig. 1). Increasing the pyrolysis temperature from 700 °C to 900 °C drives the aggregation of Co cations, leading to the growth of Co nanoparticles (NPs) (Supplementary Figs. 2-3) and concomitant evolution of CO and H O34. The release of these oxygen-containing species promotes the surface oxidation of Co NPs to CoO.

Differential thermal analysis revealed a gradual exotherm >700 °C, possibly indicative of Co metal oxidation (Supplementary Fig. 1), however, powder X-ray diffractograms only exhibited reflections characteristic of fcc Co metal, indicating that any such CoO phase must be amorphous (Supplementary Fig. 4). Corresponding aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF STEM) images evidenced a low contrast, amorphous phase at the perimeter of Co NPs (Figs. 2b-d) consistent with the formation of a Co/CoO interface. The size of Co NPs and the amorphous phase both increased with pyrolysis temperature, particularly between Co/CoO-800 (Fig. 2c) and Co/CoO-900 samples (Fig. 2d), for which the metal phase grew from 8 nm to 22 nm. Pyrolysis at 900 °C resulted in the appearance of short-range lattice fringes corresponding to CoO around the metal particles (Fig. 2d and Supplementary Fig. 5).

The oxidation state of cobalt was also investigated by hydrogen temperature-programmed reduction (H -TPR) and X-ray photoelectron spectroscopy (XPS). H -TPR profiles for all Co/CoO-n catalysts exhibited low (~100-300 °C) and high temperature (~300-800 °C) reduction features, associated with the stepwise reduction of Co O →CoO→Co (α and β) and of CoO→Co (γ), respectively (Fig. 2e)35-37. The readily reducible α and β peaks are reported to promote CO hydrogenation over Co/CeO 38. The low-temperature reduction peak in H -TPR (akin to PET hydrogenolysis conditions) indicates that the catalyst adsorbs hydrogen atoms, inducing reconstruction of CoO on metal particle surfaces. This likely results in a dynamic distribution of cobalt between metal and metal oxide phases that enhances activity39. Compared to the initial Co/CoO-800 surface predominantly comprising amorphous CoO (Supplementary Fig. 6a), AC-HAADF STEM imaging of Co/CoO-800 after reduction at 250 °C for 1 h under a flow of 10 vol% H /Ar reveals a polycrystalline stacked morphology, indicating surface reorganization (Supplementary Fig. 6b). EDS mapping also shows a local depletion of oxygen coincident with lattice contraction and gully formation on metal particles (Supplementary Figs. 6c and 6d). These results provide direct evidence for the reduction and restructuring of cobalt oxide under a hydrogen atmosphere that may expose additional active sites (Supplementary Fig. 6e). The γ peak appears at a higher temperature than for bulk CoO (Supplementary Fig. 7), indicative of a strong metal-support interaction (SMSI)39. Although the intensities of α and β peaks increase with pyrolysis temperature due to enhanced CoO formation, the overall hydrogen consumption shows a corresponding decrease, which we attribute to sintering of Co metal NPs at higher temperatures (Supplementary Fig. 2 and Table S1). The increase in CoO content with pyrolysis temperature between 700-900 °C is seemingly counter-intuitive, but reflects the release of oxygen-containing vapours at these temperatures, notably CO and H O from decomposition of the carboxylic acid linkers in the parent MOF34, that oxidise Co clusters formed from the agglomeration of metallic Co nodes. Differential thermal analysis reveals a small exotherm between 650-850 °C consistent with Co metal oxidation (Supplementary Fig. 1). Cobalt oxidation proceeds the formation of amorphous carbon, that thermogravimetric analysis indicates is completed ~600 °C (Supplementary Fig. 1), and continues until graphitisation commences at ~1000 °C. At temperatures <600 °C, partial pyrolysis entails complex physicochemical processes, including the escape of guest molecules that are inefficient oxidants40. XPS reveals that the near-surface carbon content exhibits only minimal increase during pyrolysis between 700-900 °C (from 89 atom% to 92 atom%, Supplementary Table 2). Electron microscopy indicates that cobalt particles throughout this pyrolysis regime are predominantly located on the outer surface of the carbon matrix, with no evidence of particle encapsulation by a carbon layer (Figs. 2b-d).

SMSI effects are reported for carbon-supported Co species in thermal-41,42 and electrocatalysis43,44, predominantly reflecting electron transfer dictated by differences in work function. Oxygen-containing functions increase the work function of reduced graphene oxides45, promoting electron transfer from supported metals34. Conversely, the lower work function of graphene favors electron transfer from Co metal to the support46. Oxygen lost from the carbon matrix at higher pyrolysis temperatures, apparent from an increase in the surface C:O atomic ratio (Supplementary Table 2), is therefore expected to lower the support work function and increase the binding energy of Co metal NPs.The Co0 2p binding energy of ~778.2 eV in all Co/CoO-n catalysts was slightly lower (~0.1-0.2 eV) than that for bulk Co metal, consistent with electron transfer from Co NPs to a peripheral CoO phase (Supplementary Fig. 8). Peak fitting revealed the surface Co0 content decreased from 24 % in Co/CoO-700 to 7 % in Co/CoO-900, confirming that higher pyrolysis temperatures promoted cobalt oxidation and metal/oxide interface formation (Fig. 2e and Supplementary Table 2). Oxygen 1s XPS spectra revealed a simultaneous increase in a 529.8 eV peak attributed to lattice oxygen in CoO (Fig. 2f), consistent with oxidation of Co atoms by CO /H O liberated during pyrolysis of the MOF. A strong peak was also observed at ca. 531.6 eV attributed to abundant hydroxyl groups formed on the C support during pyrolysis.

The morphology of the Co/Co-800 catalyst, as visulaised by HAADF STEM at the 500 nm scale (Supplementary Fig. 9), indicates that the original macroscopic framework is largely preserved in the pyrolyzed Co-MOF; beneficial for dispersing active sites and hence maximizing the Co/CoO interface. The local structure of Co NPs in the Co/CoO-800 sample was further explored by high-resolution TEM, revealing lattice fringes predominantly consistent with the (111) plane of Co metal (0.204 nm spacing, Fig. 3a). These metallic NPs were partially encapsulated by short-range, ordered domains exhibiting lattice fringes consistent with (200) planes of CoO (0.212 nm), to form a Co metal/oxide interface (see area I in Fig. 3a and Supplementary Fig. 10). Selected area electron diffraction (SAED) confirmed that Co/CoO-800 displayed features characteristic of both Co and CoO (Fig. 3a inset), while a fast Fourier transform of the SAED image of area II in Fig. 3a is ascribed to a projection along the (110) plane of fcc Co metal (also see Supplementary Fig. 11). Electron energy loss spectroscopy (EELS, Supplementary Fig. 12) chemical mapping of an individual nanoparticle identified Co (blue) signal throughout, with higher intensity in the center (Fig. 3b), whereas O (green) was confined within ~1 nm of the external surface, highlighting the metal/oxide interface. The spatial distribution (EELS line) of C, O, and Co in various nanoparticles consistently revealed a Co-rich (metal) core, with O (as CoO) and C (from the pyrolyzed MOF) decorating the external surface (Supplementary Fig. 13). Additional spectral analysis from different regions of the annular dark field (ADF) image of a nanoparticle affirmed a Co-enriched core and O-enriched surface (Fig. 3c). Note that the Co and O energy losses in region 2 are respectively red-shifted and blue-shifted compared to their counterparts in region 1 (Fig. 3c inset), consistent with electron transfer from Co to O atoms at the surface. AC-HAADF STEM imaging of Co/CoO-800 also visualizes nanoparticles embedded in the C matrix (Supplementary Fig. 14)47,48. Computational modeling of an oxygen-deficient CoO cluster (Co O ) placed on a Co(111) surface enables computing of charge redistribution in the metal atoms upon formation of the metal/oxide interface (Supplementary Fig. 15). Bader charge analysis evidences electron accumulation on the cluster (yellow arrows) (Fig. 3d and Supplementary Table 3), in accordance with the EELS energy spectra (Fig. 3c), due to electron depletion at Co atoms in direct contact with the CoO cluster (Supplementary Fig. 16 and Supplementary Table 4).

Depth profiling of nanoparticles in Co/CoO-800 by Ne+ low-energy ion scattering (LEIS) spectroscopy revealed weak Co signals in the atomically thin (0.02 nm) selvedge, which may be due to the wrapping of the CoO layer (Fig. 3e). Progressive removal of this overlayer by Ne+ ions resulted in a rapid increase in the Co signal with sputtering depth (Fig. 3e inset), with a maximum attained at ~1.2 nm depth (similar to the thickness of the CoO overlayer observed by EELS, Fig. 3b), indicating that it corresponds to uncovering of the Co/CoO interface. Analogous sputtering and LEIS using lighter He+ ions confirmed the presence of C, O, and Co within the surface of the Co/CoO-800. The measured Co:O relative intensity was <<1 (the expected stoichiometry for CoO), suggesting that the oxygen signal predominantly arises from the C matrix (Supplementary Fig. 17). The Co intensity and Co:O relative intensity both increased with sputtering depth (up to at least 0.3 nm), whereas the C intensity was unchanged, consistent with: (i) the presence of oxygen-containing functional groups in the carbon layer; and (ii) CoO clusters decorating the surface of Co NPs. Ar+ ion sputtering was also used in combination with XPS to examine the Co/CoO-800 interface. Removing ~4 nm of the outer surface increased the overall Co 2p XPS signal, predominantly due to an increase in Co metal (778.2 eV, Fig. 3f), and was accompanied by a concomitant decrease in the O 1s signal associated with lattice oxygen in CoO (Supplementary Fig. 18). These observations collectively evidence the presence of a Co/CoO interface formed by oxide domains at the surface of metallic NPs.

The local electronic structure and chemical environment of Co within Co/CoO-800 were also studied by Co K-edge transmission X-ray absorption spectroscopy (XAS). Co/CoO-800 exhibited a similar X-ray absorption near-edge spectrum to that of a Co metal foil (Supplementary Fig. 19), with a pre-edge feature at ~7710 eV assigned to a 1s→3d hybrid electron transition, and at 7720-7740 eV assigned to a 1s→4p electron transition. However, the slight shift to the higher energy of the pre-edge feature and a slight increase in white line (~7728 eV) intensity, for Co/CoO-800 versus the foil, indicates partial oxidation and the presence of a low concentration of Co2+/3+ species49. The similar white line energies of Co/CoO-800 and Co foil are suggestive of CoO (Co2+) rather than Co O (Co2+,3+), for which a significant shift to higher energy is anticipated. Corresponding extended absorption X-ray fine structure (EXAFS) spectra confirmed that the average local environment of Co atoms within Co/CoO-800 and in the foil were dominated by Co nearest neighbours (Supplementary Fig. 20 and Supplementary Table 5), with a Co–Co bond distance of ~2.5 Å (Fig. 3g), albeit the shell coordination number in the catalyst was lower (8.9 versus 12). A weak Co–O shell at 2.05 Å could also be fitted for Co/CoO-800, with a coordination number of 0.89, consistent with the presence of small CoO domains in accordance with XRD, TEM, and EELS.

Reducibility of the Co/CoO-800 sample was further investigated by temperature-programmed near-ambient pressure XPS (NAP-XPS) under 0.5 mbar hydrogen. The changes in the Co 2p signal prior to 250 °C are a direct result of the catalyst’s dynamic reduction process (Fig. 3h). Above 250 °C, the Co3+ (~782 eV) signal diminishes, and the Co signal shifts markedly toward lower binding energy, confirming the onset of reduction from high-valence to low-valence cobalt. Cobalt oxides underwent rapid reduction at >370 °C to Co metal (~778 eV) which dominated the surface above 400 °C, coincident with the loss of lattice oxygen at ~529.8 eV from the corresponding O 1s spectra (Supplementary Fig. 21). Note that H -TPR and NAP-XPS are in good agreement >300 °C (γ peak in Fig. 2e and 778 eV peak in Fig. 3h), wherein reduction kinetics will be fast, with both techniques identifying the complete reduction of cationic Co species to the metal. In-situ XRD under 1 bar of 10 vol% H /Ar (Fig. 3i and Supplementary Fig. 22) revealed a systematic shift to a lower angle and an increase in intensity of the Co(111) and Co(200) reflections at 44.2° and 51.5° at temperatures >250 °C, indicative of lattice expansion and metal NP sintering, respectively. In-situ NAP-XPS enables real-time monitoring of dynamic surface state evolution in metals/oxides during hydrogen spillover50. Time-resolved NAP-XPS measurements at 100 °C reveal a gradual shift of the Co binding energy in cobalt-oxygen species toward lower values over time (Fig. 3j). Since CoO is unlikely to undergo hydrogen dissociation and self-reduction at this temperature, the observed changes indicate that hydrogen atoms spill over onto the cobalt-oxygen species surface, thereby reducing their valence state (Fig. 3k). Concurrently, the gradual increase in oxygen binding energy (~529.5 eV) of CoO over time provides further evidence for the hydrogen spillover process (Supplementary Fig. 23).

Catalyst testing

The performance of Co catalysts was initially evaluated for the direct hydrogenolysis of PET to PX at 250 °C and 1.0 MPa H in a n-dodecane solvent. This solvent was chosen due to its ease of separation from the water by-product, high solubility for the desired PX and 1,4-DMC products, and high boiling point that permitted lower reaction pressures and thereby lower reactor costs and improved process safety. Experiments using a 1,4-dioxane solvent proved ineffective.

Pure fcc CoO and (600 °C pre-reduced) Co NPs were essentially inert (Supplementary Table 6, entries 1, 2), whereas MOF-derived Co/CoO-n catalysts exhibited a parabolic dependence of PX yield on pyrolysis temperature, with a maximum of 98 % PX for Co/CoO-800 (Fig. 4a and Supplementary Table 6, entries 3-5). In contrast, a physical mixture of pure CoO and Co NPs was inert for PET hydrogenolysis (Supplementary Table 6, entry 6), evidencing the importance of the metal/oxide interface. Notably, the yields for both the CoO/C and Co/C catalysts in PET hydrogenolysis were low (<10 %), indicating poor activity. This stark contrast highlights the unique specificity of MOF-pyrolyzed Co/CoO catalysts for this reaction (Supplementary Table 6, entries 7, 8). The optimal PX yield for the Co/CoO-800 catalyst is attributed to its high degree of surface oxidation and relatively small size of Co/CoO NPs (high surface area for reaction), while only trace PET intermediates underwent BRH hydrogenation to 1,4-DMC. The CoO content (determined from O 1s XPS) was calibrated against hydrogen consumption corresponding to the H -TPR low-temperature reduction peak. A strong correlation was observed between this calibrated CoO content and PX yield (Supplementary Fig. 24). This evidences that all surface CoO participates as active sites under reaction conditions through the dynamic process proposed above (Supplementary Fig. 6), contributing to the high catalytic performance relative to the recent literature (Supplementary Table 7). Investigation into the catalyst loading revealed that 10 mol% Co/Co-800 (relative to PET) delivered optimal reaction performance (Supplementary Fig. 25). From the structure of PET, it is evident that C8 liquid products such as PX and 1,4-DMC can only be obtained from the terephthalic acid fragment of the polymer. The high C8 yields indicate that such fragments do not readily undergo decarboxylation, and hence that any gaseous products predominantly arise from decomposition of the ethylene glycol fragment of the polymer. The gas/liquid product molar ratio of ~2 confirms this postulate (Supplementary Table 6). PET has a high melting point (245-260 °C), and hence it is challenging to perform liquid phase hydrogenolysis at 240 °C (below its melting point), resulting in a steep decrease in PX yield to only 16 % (Supplementary Table 6, entry 9). At 250 °C and hydrogen pressures >1 MPa, PET intermediates underwent partial hydrogenation to 1,4-DMC (Fig. 4b); BRH was faster at 280 °C, being the dominant pathway >2 MPa H , such that PET hydrogenolysis afforded >90 % 1,4-DMC in only 30 min at 3 MPa H (Fig. 4a, Supplementary Table 6, entry 10). Notably, under optimal reaction conditions, PET hydrogenolysis efficiency exceeds 97 % with near-total conversion to the target product (Supplementary Table 6, entries 4, 10). Under suboptimal conditions, however, lower carbon efficiency primarily stems from PET depolymerization into oligomers (rather than actual carbon loss). Enhanced BRH is expected at higher hydrogen pressures due to the resulting higher coverage of H adatoms on Co metal sites and concomitant spillover to CoO active sites to form semihydrogenated PX intermediates. Hydrogen coverage effects are widely reported to influence product selectivity in thermal and electrocatalysis51-53.

The Co/CoO-800 catalyst was used for further mechanistic studies of PET hydrogenolysis. For convenience, the carboxyl carbon on the terephthalate moiety in PET is defined as C and that carbon on the ethylene glycol moiety as C . Gel permeation and gas chromatography confirmed that hydrogenolysis proceeded by the continuous cleavage of long polymer chains into smaller chains over the first hour of reaction (Supplementary Fig. 26), with subsequent rapid cleavage of the latter yielding PX as a secondary product over the second hour (Supplementary Fig. 27). In contrast to PET hydrogenolysis, which predominantly yielded PX, direct hydrogenolysis of dimethyl terephthalate afforded a mixture of PX and 1,4-DMC (Fig. 4c). It is suggested that Co/CoO-800 preferentially activates C–O rather than C –C bond, despite the C –C bond dissociation energy (BDE) of 340.5 kJ/mol being less than that of C–O bonds (C –O: 398.1 kJ/mol, C –O: 346.5 kJ/mol)54. This is supported by two key observations: dimethyl terephthalate (DMT) was not detected by GC-MS, nor were its characteristic methyl proton signals observed in 1H NMR spectra (Fig. 4d). This selectivity originates from the partially filled d-band of metallic Co, which readily interacts with the lone-pair electrons of oxygen. The molecular structure of PET suggests that the active sites have difficulty in binding carbon atoms of the ethylene glycol moiety (Supplementary Fig. 28), thereby preventing effective activation of the C –C bonds and impeding their cleavage. Hydrogenolysis of terephthalic acid reactively-formed by C –O bond cleavage would favour a high PX, consistent with that observed in the liquid phase for PET hydrogenolysis (Fig. 4c), but would also typically afford significant ethane, inconsistent with our observation of methane as the primary gaseous product (>93 %) (Fig. 4a and Supplementary Fig. 29). Based on relevant literature55,56, we hypothesize that PET hydrogenolysis occurs by C –O cleavage (Fig. 4f). The intermediate 2-hydroxyethyl 4-methylbenzoate (HMBE) was detected within the PET reaction system (Supplementary Fig. 30). These results collectively confirm that PET hydrogenolysis initiates via C –O bond cleavage. The product distribution of ethylene glycol hydrogenolysis was monitored at different time intervals under the experimental conditions to investigate the reaction pathway (Supplementary Fig. 31). The results revealed that ethylene glycol was completely converted within 10 min, with the methane yield reaching approximately 90 %. The methanol yield first increased and then decreased with prolonged reaction time, indicating its formation via hydrocracking of ethylene glycol and subsequent conversion57. Note that the aldehyde group generated by C –O cleavage is unstable, and quickly undergoes HDO; this may explain why the 1H NMR spectrum of the PET hydrogenolysis solution after reaction at 250 °C and 1 MPa H for 30 min only revealed terminal methyl H atoms connected to benzene rings, but no methylene and methine H atoms (Fig. 4d). At 1 MPa H , HMBE underwent C –O cleavage to form 4-methylbenzaldehyde (4-MBA), which subsequently converted to PX rapidly and near-quantitatively at 250 °C (Fig. 4c); this PX product was stable over a further 1 h reaction.

The possibility of 4-MBA hydrogenation to a 4-methylbenzyl alcohol (4-MB-A) intermediate can be discounted as the latter formed significant 1,4-DMC under our hydrogenolysis conditions (Fig. 4c). At 150 °C, over 95 % of 4-MBA is converted to 4-MB-A (Supplementary Table 8), indicating that the direct conversion of 4-MBA to PX (bypassing 4-MB-A) must be favored under specific reaction conditions. We propose that at elevated temperatures, kinetics favor the rapid hydrodeoxygenation of adsorbed 4-MBA before it can desorb as 4-MB-A, aligning with analogous mechanisms documented in the literature58. Coexisting cyclohexane and benzene ring hydrogen atoms in the 1H NMR spectrum of the reaction mixture after PET hydrogenolysis at 280 °C and 3 MPa H for 5 min suggest that such forcing conditions drive partial BRH of PET oligomers (Fig. 4e). Such partially saturated oligomers may undergo further C –O cleavage on the cyclohexane side to generate an unstable 4-methylcyclohexane-1-carbaldehyde (4-MCHC) after removal of ethylene glycol, which likely hydrogenates to 4-methyl-1-cyclohexanemethanol (4-MCHM) prior to HDO producing 1,4-DMC. Control experiments confirmed that 4-MCHC underwent almost complete hydrogenation to 4-MCHM at only 220 °C (Fig. 4c and Supplementary Table 8), with subsequent heating to 280 °C promoting HDO in only 5 min (Fig. 4c). Observation of trace HMBE, 2-hydroxyethyl 4-methylcyclohexane-1-carboxylate (HMCE) and 4-MCHM in the liquid product mixture by gas chromatography further evidenced 4-MCHM as an intermediate to 1,4-DMC during PET hydrogenolysis (Supplementary Fig. 32). The major product of C –O cleavage on the benzene ring side of 4-MBA was PX, which formed little 1,4-DMC even at 280 °C and 3 MPa H for 5 min. Similarly, using PX as a substrate under more forcing reaction conditions, the yield of 1,4-DMC was less than 25 % (Supplementary Table 8). PX conversion to 1,4-DMC is thus challenging, whereas HDO of 4-MCHC and 4-MCHM readily yields 1,4-DMC (Fig. 4c). The primary pathway for PET conversion to 1,4-DMC is thus partial depolymerization and HDO of terminal aldehyde groups in oligomers, followed by partial BRH and complete C –O cleavage on the cyclohexane side, and finally HDO of the remaining terminal aldehyde (Fig. 4f).

Catalytic mechanism

Ethyl 4-methylbenzoate (Supplementary Fig. 33), which has a similar repeat unit to PET, was selected as a model substrate for density functional theory (DFT) calculations of hydrogenolysis reactions over a Co O /Co(111) model surface containing a range of Co sites (Supplementary Fig. 34). Reaction energies for C–O bond cleavage of ethyl 4-methylbenzoate at interface electron-deficient Co (Coδ+), CoO (Co2+), and Co(111) (Co0) sites were computed. Adsorption of ethyl 4-methylbenzoate at interfacial Coδ+ sites was favored (–1.20 eV, Fig. 5a) compared to the Co2+(–0.8 eV, Supplementary Fig. 35) and Co0 sites (–0.5 eV, Supplementary Fig. 36). In all cases, adsorption occurred through the carbonyl O atom. In contrast, dissociative chemisorption of molecular H was favored over the bare metal surface which exhibited the strongest absorption (ca. –1.1 eV) and lowest energy barrier (0.43 eV, Supplementary Fig. 37). Co-adsorption of a hydrogen adatom (*H) alongside ethyl 4-methylbenzoate was exothermic for all sites, with interfacial Coδ+ sites offering the most stable configuration (-1.69 eV), being 46 % and 126 % lower in energy than Co2+ and Co0, respectively. At low concentrations, confinement of *H species to interfacial sites is thus expected. The energy barrier for C –O bond activation was lower than that for the C –O bond at all three sites (Figs. 5a-b, and Supplementary Figs. 35 and 36), indicating that the ester bond is more easily cleaved in carboxylates, consistent with experimental observation of 4-MBA and 4-MCHC, rather than the ether group to yield terephthalates (Fig. 4c). While the energy barrier difference between a hydrogen atom attacking the ethylene glycol carbon and the carboxyl carbon is negligible (0.47 eV vs. 0.43 eV), the energetic cost for subsequent bond breaking diverges significantly (Fig. 5a, Supplementary Fig. 38). Specifically, cleaving the C –C bond requires >28 % additional energy compared to cleaving the C –O bond. Consequently, the hydrogenolysis of PET is kinetically favored to proceed via C –O bond cleavage, a finding consistent with our experimental observations (Figs. 4c and 4d). Activation of the C –O bond may be facilitated by protonation of the carboxyl carbon to form a relatively stable carbocation intermediate, which is associated with a lower barrier than protonation of the carboxyl oxygen. The lowest energy pathway for activation of the C –O bond and HDO of the resulting 4-MBA monomer to PX was observed over Coδ+ sites (versus the metal or oxide) (Figs. 5b-c, Supplementary Figs. 35 and 36); PET hydrogenolysis to PX can be entirely performed at Coδ+ sites. In summary, electron-deficient Coδ+ sites at the interface with CoO clusters can catalyze the selective depolymerization of PET to PX through C –O cleavage and subsequent HDO, using hydrogen adatoms spilled over from Co metal sites (Fig. 4f).

Insight into the selective depolymerization of PET to 1,4-DMC was also gained from DFT calculations of the transition state energy barrier for BRH in ethyl 4-methylbenzoate (Fig. 5d and Supplementary Fig. 39). Considering conjugation between the benzene ring and carbonyl group, the meta-C of the benzene ring is most vulnerable to hydrogenation. For all Co sites, the BRH of ethyl 4-methylbenzoate has an energy barrier of >0.68 eV, i.e., ring hydrogenation is more energetically demanding than C –O bond cleavage. This prediction is supported by experiments which show that BRH and resultant 1,4-DMC production is only significant at 280 °C (Supplementary Table 6, entries 4 & 10). C –O cleavage of the saturated product of ethyl 4-methylbenzoate BRH, 4-methyl cyclohexanecarboxylic acid ethyl ester, has a lower energy barrier (0.43 eV), and hence aromatic saturation is likely the rate-controlling step in 1,4-DMC production from PET (Supplementary Figs. 40 and 41). Co-adsorbed *H lowered the energy barrier for BRH over Co metal, from 0.62 eV in the case of single adatom to 0.41 eV for a *H-saturated surface (Supplementary Fig. 42), consistent with the experimentally observed increase in 1,4-DMC yield with hydrogen pressure (Fig. 4b). Nevertheless, high 1,4-DMC yields require both higher reaction temperature (to activate aromatic rings) and pressure (to provide a sufficient *H coverage on the catalyst surface). At 300 K, BRH was always more energetically challenging than C –O bond cleavage (Fig. 5e). However, BRH may be susceptible to the phase of the reactants59, and hence the DFT simulation temperature was increased from 300 K to the experimental temperature (553 K, sufficient to induce vaporise smaller molecules). As anticipated, the energy barriers of BRH and C –O bond cleavage decreased over all Co sites at the higher temperature, but surprisingly, the barrier for BRH over the Co2+ sites dropped by 64 %, falling below that for C –O bond cleavage (Fig. 5f). It should be noted that since the energy barrier required for the ring-hydrogenation of PX is as high as 1.05 eV over Co metal (Supplementary Fig. 43), explaining the need for the higher reaction temperature (280 °C) to effect PET hydrogenolysis to 1,4-DMC (Fig. 4c). Synergy between different Co active sites enables the highly selective conversion of PET to PX or 1,4-DMC. Dissociatively chemisorbed H at Co0 liberates atomic hydrogen that spillsover to interfacial Coδ+ sites to catalyse partial hydrogenolysis of PET (C –O cleavage) and HDO of the terminal aldehyde group to form oligomers (Fig. 4f). At higher temperature and pressure, the increased concentration of hydrogen adatoms at the metal-oxide interface drives their spillover to Co2+ sites in CoO clusters, promoting partial BRH of the resultant oligomers, complete C –O cleavage on the cyclohexane side, and subsequent HDO of the reactive intermediates at Coδ+ sites to 1,4-DMC.

Catalyst reusability and scope of PET commodities

As Co/CoO-800 was strongly magnetic, it was easily separated and recycled for PET hydrogenolysis. There was a 6 % decrease in the yields of desired products (either PX at 250 °C and 1 MPa, or 1,4-DMC at 280 °C and 3 MPa) over four reaction cycles (Fig. 6a, Supplementary Table 9). Physicochemical properties of the post-reaction catalyst also closely resembled those of the as-prepared materials (Supplementary Fig. 44). The Co2+ content exhibited little variance (<2 %) between as-prepared and post-reaction catalysts (Supplementary Fig. 44c), confirming that CoO was not significantly reduced to Co metal during reaction. This is consistent with H -TPR, H -XRD, and NAP XPS data that evidence complete CoO reduction requires 600 °C, far higher than our hydrogenolysis temperatures. The ~200 °C reduction peak in H -TPR (Fig. 2e) indicates some low-temperature reducibility, suggesting that oxygen from CoO may participate in the catalytic cycle, but this may be replenished by oxygen-containing species generated during PET hydrogenolysis.

Encouraged by the high activity, selectivity and recyclability of the Co/CoO-800 catalyst, this was then tested against a range of PET consumer goods, including a food grade beverage bottle (cut to 1 x 1 x 0.3 cm), cable insulation film (cut to 1 x 1 x 0.3 cm), filter (cut to 2 x 2 x 0.012 cm), and retractable flame retardant woven mesh (cut into 1 x 0.2 cm cylindrical shape) (Supplementary Table 10). At 250 °C and 1 MPa H , the 3 h yields of PX from the bottle and film were 94 % and 92 %, respectively. Slightly lower PX yields of 91 % and 92 % were achieved for the filter and woven mesh (inner ring of Fig. 6b), attributed to the presence of additives. At 280 °C and 3 MPa H , the 1 h yields of 1,4-DMC ranged from 83 % to 88 % (outer ring of Fig. 6b), and the gas-liquid product molar ratio is also around 2. Phthalocyanine blue was also explored as a model contaminant of waste plastic.9 Although pigment addition to PET adversely affected the PX yield, the decrease of 6 % fall for 1 % addition was modest and demonstrated the robustness of a hydrogenolysis route (Supplementary Fig. 45). With PX as the target product, both the carbon balance of terephthalic acid in PET and that of 1,4-DMC reach high levels: over 93 % and 86 %, respectively. This indicates that direct hydrogenolysis of different PET materials maintains high carbon balance efficiency. Commercial PET could thus be ultraselectively converted to either PX or 1,4-DMC by simply changing the reaction conditions. Hydrogenolysis of (equal masses) of these commercial PET products was also performed under the preceding two sets of reaction conditions, yielding 91 % PX or 80 % 1,4-DMC accordingly, demonstrating the feasibility of direct hydrogenolysis for the upcycling of blended PET plastics into valuable chemicals (See Supplementary Information for preprocessing steps).

Life cycle assessment (LCA) and Technoeconomic Analysis (TEA)

The global warming potential (GWP) is a generally accepted metric to quantify the total impact of greenhouse gas emissions from a specific process, product, or activity over a specified period, usually expressed in equivalent CO . The calculation of GWP involves converting the emissions of different greenhouse gases into the equivalent amount of CO that would exert the same warming effect over a defined timeframe. Here, we choose 100 years as the timeframe for GWP calculation. Consistent PX yield was observed in sub-kilogram-scale (100 g) PET hydrogenolysis experiments, demonstrating the potential for scaling up this production strategy. Fig. 6c shows the process flow of PX hydrogenolysis from which the GWP value for producing 1 kg of PX was calculated. As PET raw materials come entirely from nature, the GWP value is dominated by contributions from the Co/CoO-800 catalyst and hydrogenolysis equipment (Fig. 6d). Currently, the catalyst has proven effective over three reaction cycles; additional recycling would indirectly further mitigate GWP. Our results are compared against LCA data for the production of PX through toluene methylation60, bio-EtOH61, and petro-PX using various methodologies62-64. Assuming the processing of waste plastics, our GWP value of –1.12 kg CO -eq. is much lower than other methodologies (equating to 5.8 kg CO -eq·kg–1 versus 74.6 kg·CO -eq·kg–1 and 91.3 kg·CO -eq·kg–1 for alkaline hydrolysis and glycolysis)15.

According to S&P Global market insights, the U.S. recycled PET market is experiencing a downturn. Platts, part of S&P Global Commodity Insights, last assessed post-consumer PET curbside bales at 8.5 cents/lb (ca. 187 USD/ton) FOB Chicago July 11, marking its first time prices have fallen to single digits since Sept. 27, 2023, according to Platts data. Southeast bales were assessed at 9 cents/lb (ca. 198 USD/ton) EXW Southeast July 17, marking the first time prices have fallen to single digits since Platts started assessing them on Feb. 2, 202465. Using cost data for 2020, PX produced by direct PET hydrogenolysis is estimated to cost 935 USD/ton, slightly higher than the current European market price of 911 USD/ton66. This represents an approximately sevenfold increase in value relative to the cost of the mixed, bundled PET feedstock (140 USD/ton). Catalyst manufacturing (29 %), the hydrogenolysis process (29 %), and bundled waste PET (26 %) are the major contributors to the overall PX production cost (Supplementary Table 11 and Supplementary Figs. 46-47).

Based on the optimal Co loading of 10 mol% relative to PET (assuming a typical molecular weight of ~20,000 g/mol), the catalyst cost of ~296 USD per ton of PET product (equating to ~15 kg of cobalt nitrate and a similar mass of TPA) is comparable to that of typical high loading 3d transition metal hydrogenation catalysts such as Ni/Al O . The precursors to Co/CoO-800 also have long-term steady prices, and the catalyst stability over multiple recycles would mitigate operating expenses for new catalyst and the disposal of old catalyst batches. Note that only a small amount of TPA (relative to the vast scale of PX production) is, entirely coincidentally, required as a catalyst precursor. Our TEA demonstrates that hydrogenolysis of recycled PET is economically sustainable, with resulting TPA cost-competitive with that derived from fossil resources (~1020 USD/ton) and much cheaper than recycled TPA (1,750 USD/ton)67, which typically commands a ~40 % market premium. Hydrogenolysis of waste PET would therefore offer environmental and cost benefits for PX production.

Methods
Preparation of cobalt-based catalysts. The Co/CoO-n catalysts were derived from a Co-MOF-71. In a typical synthesis, 12 mmol of terephthalic acid and 2 g of polyvinylpyrrolidone were dissolved in a round-bottomed flask containing 80 mL of N,N-dimethylformamide (DMF). To this solution, 0.2 M cobalt nitrate in DMF (40 mL, 1.17 wt% Co) was added dropwise, and the mixture was subsequently stirred at 300 rpm at 120 °C for 8 h under reflux. The resulting turbid liquid was transferred to an autoclave and statically heated at 120 °C for 40 h. The resulting solid-liquid mixture was then centrifuged, repeatedly washed with DMF and methanol, and dried in vacuo at 60 °C overnight to afford Co-MOF-71. The obtained Co-MOF-71 was then pyrolyzed under Ar at different temperatures (n = 700, 800, or 900 ºC, heating ramp 5 °C.min-1) for 3 h to derive Co/CoO-n catalysts. CoO and Co NPs were prepared by pyrolysis of hydrated cobalt nitrate at 400 ºC under argon and reduction of the pyrolysed cobalt nitrate at 600 ºC in 10 vol% H /Ar, respectively, for activity comparison with Co/CoO-n.

Catalyst characterization. Low-energy ion scattering (LEIS) of samples was performed on a Qtac 100 instrument. Ne (5 keV, 1.6 nA) and He (3 keV , 6 nA) ions were used to sputter samples (with a range from 2000 μm × 2000 μm to 300 μm × 300 μm) and profile the resulting surface elemental composition, respectively. In-situ near-ambient pressure XPS (NAP-XPS) was performed using a NAP-XPS instrument equipped with a temperature-controlled laser heating device produced by SPECS company. A micro-focused high-performance monochromatic Al K X-ray source was utilized as the photon source. A typical test in a pure hydrogen atmosphere of 0.5 mbar, at a temperature of 100 °C, recorded the binding energies of Co 2p, O 1s, and C 1s every 10 min for 40 min. Then, a laser heating program was used to raise the temperature to 500 °C, and the Co 2p, O 1s, and C 1s spectra of the sample were collected at intervals of 50 °C. In-situ XRD characterization was performed on a BRUKER D8 ADVANCE diffractometer equipped with an Anton-Parr XRK900 sample cell under 1 bar 10 vol% H /Ar atmosphere. The sample was heated from 100 °C to 600 °C in a 10 vol% H /Ar atmosphere at a heating rate of 10 °C/min. After reaching the specified temperature measurement point and stabilizing for 2-3 min, diffractograms were obtained at a scanning rate of 10 °/min over 10-80°.

Catalytic performance evaluation. Catalytic performance in direct PET hydrogenolysis was studied in a 15 mL magnetically stirred Hastelloy autoclave. Generally, 120 mg PET, 10 mol% Co catalyst relative to PET (~22 mg), and 3 mL n-dodecane solvent were added to the autoclave, which was subsequently flushed with hydrogen and then pressurized (0.5-4 MPa). Stirring at 600 rpm, sufficient to overcome external diffusion limitations, was commenced when the reactor reached the desired reaction temperature (230-290 °C). After the reaction, gaseous products were analyzed using an Agilent 7890A gas chromatograph equipped with a flame ionization detector (FID) and thermal conductivity detector (TCD); see Supplementary Information for further details. Liquid products were analysed using the FID and a HP-5 capillary column (Agilent J&W 30 m × 0.25 mm × 0.25 µm). The FID and injector temperatures were 300 °C and 285 °C, respectively, with n-hexadecane as an internal standard. Product yield was calculated using Eq. 1, where m is the mass of the product observed, and m is the theoretical maximum mass of the product.

Yield (%) = m /m × 100

(1)

After the reaction, 5 mL of hexafluoroisopropanol was added to the reaction mixture to dissolve the PET oligomers. Since the catalysts were magnetic, they were easily separated from the product. Solvent and low-boiling products were then separated from the solid product by rotary evaporation. Separated oligomers were dissolved in a mixture of deuterated chloroform and deuterated trifluoroacetic acid (3:1) and analyzed by 1H NMR. The weight-average molecular weight of the PET oligomers was determined by gel permeation chromatography equipped with an HFIP gel chromatography column (300 x 7.5 mm, Agilent) with hexafluoroisopropanol as the eluent at 35 °C and 1 mL min-1. The molecular weight of the PET oligomers was calculated using polystyrene standards of different molecular weights.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "The manuscript employs a Co/CoO catalyst for PET hydrogenolysis; however, the rationale for selecting this specific\ncatalyst system is not clearly articulated. In addition, the catalyst characterization raises several questions. During catalyst\npreparation, the reported CoO fraction increases as the pyrolysis temperature increases from 700 to 900 °C. The authors are\nencouraged to clarify the basis for this trend and the conditions under which it is expected. The final catalyst also appears to\ncontain a carbon layer derived from MOF pyrolysis. Quantification of the carbon content (e.g., wt% or surface coverage)\nwould be helpful, along with discussion of whether—and to what extent—this carbon layer may limit access to Co/CoO\nactive sites during hydrogenolysis.\nIn the model hydrogenolysis reaction, the authors report that CoO is converted to Co₃O₄ under reductive conditions (0.3\nmbar H₂). This transformation is counterintuitive under reducing atmospheres and would benefit from additional verification\nand clarification. Specifically, the authors should explain the experimental basis for assigning Co₃O₄ formation, address the\npossibility of re-oxidation during handling or characterization, and reconcile the observation with expected redox chemistry\nunder the stated conditions. Otherwise, the basic redox chemistry rules must be re-written.\nThe proposed hydrogenolysis mechanism also warrants reconsideration and clearer presentation. The terminology used to\ndescribe “α carbon” and “β bond” is difficult to follow and does not align with common conventions; adopting standard\nchemical nomenclature (or providing a clear schematic definition) would improve readability and reduce ambiguity.\nMechanistically, the manuscript rules out Cβ–Cβ and Cβ–O scission and instead favors Cα–O cleavage. This preference\nappears inconsistent with a straightforward bond dissociation energy (BDE) argument and is presented without sufficient\njustification. If the authors propose that Cα–O cleavage dominates, they should provide stronger supporting evidence (e.g.,\nkinetic trends, isotope labeling, product evolution vs time, or operando/controlled experiments) and explicitly explain why\nlower-BDE pathways are suppressed on this catalyst.\nRelatedly, the manuscript proposes that ethylene glycol is formed and subsequently cracked to methanol and then methane.\nThis sequence is not adequately supported in the current dataset and may be chemically nontrivial under the reported\nconditions. If ethylene glycol cracking is central to the proposed pathway, additional evidence is needed to demonstrate that\nethylene glycol can convert to the reported C1 products under the same reaction conditions (e.g., control experiments using\nethylene glycol as the sole substrate, time-resolved product distributions, or mechanistic probes).\nAlternative chain-scission pathways should also be considered more explicitly. Cleavage at Cβ–Cβ and/or Cβ–O positions\ncould plausibly generate acid-containing intermediates. The observation of trace cyclohexanecarboxylic acid, as reported by\nthe authors, may be consistent with formation of such intermediates and appears difficult to reconcile with the proposed\npathway as currently written. A more balanced discussion that evaluates competing mechanisms in light of the full product\nslate would strengthen the credibility of the mechanistic conclusions.\nFinally, the tone of the manuscript could be made more measured. Several statements appear overly promotional while\ndismissing other chemical recycling approaches. A more neutral presentation—emphasizing evidence-based comparisons\nand acknowledging practical considerations such as hydrogen cost, process complexity, and scalability constraints—would\nlikely be more persuasive to readers. In addition, the manuscript would benefit from careful editing to correct typographical\nand presentation issues (e.g., misspellings such as “aove” for “above” and unclear or error-prone figure captions, including\nFigure 3).", "reviewer_3": "Chemical recycling of PET is the most viable one among all the waste plastics. In this manuscript, Wang and co-authors\nprepared Co/CoO composite catalysts and applied in the conversion of PET to p-xylene, 1,4-dimethylcyclohexanol and\nmethane/ethane. Although the result is convincing and encouraging, I still have concerns on the following issues.\n1. The hydrogen spill over is proposed and shown in the calculations. Is there experimental evidence?\n2. Co and CoOx were compared with the specially synthesized Co/CoO catalyst. This could be a difference of metal\ndispersion. Normally, an oxidation layer would form on Co with exposure to air, making a similar Co/CoO hybrid composites.\nSo I suggest authors to prepare an activated carbon supported Co, and compare its catalytic activity.", "reviewer_4": "The manuscript demonstrates the conversion of assorted PET plastics into high-value chemicals (p-xylene or 1,4-\ndimethylcyclohexane) of high efficiency (>90% yield) through precise modulation of hydrogen spillover at Co/CoO interfaces.\nIt offers dual scientific insights by deepening the mechanistic knowledge of C–O bond cleavage in PET and unveiling a\nbroadly applicable upcycling approach, paving the way for sustainable plastic waste valorization.\nIn response to the reviewers' concerns regarding PET-to-PX conversion, the authors have provided a comprehensive and\nconvincing reply, with their arguments grounded in life cycle assessment, the 12 principles of green chemistry, and the\nconceptual unity of waste plastics and biomass. Additionally, in response to the referees' queries regarding the necessity of\nusing pyrolyzed Co-MOF to prepare Co/CoO composites, the authors have supplied a more detailed justification as well.\nOverall, the authors have adequately addressed the reviewers' concerns, and the manuscript can be considered for\npublication after several minor issues are addressed.\n1. The core advantage lies in the MOF pyrolysis approach, which maintains the overall morphology while achieving superior\nactive site dispersion and interfacial area, unlike the limitations inherent in conventional preparation routes. To further\nsubstantiate the rationale of this synthetic route, it is recommended that electron microscopy images depicting the overall\ncatalyst morphology be provided. Such evidence would visually confirm the preservation of the MOF's macroscopic\nframework after pyrolysis, thereby highlighting the distinct advantages of this approach.\n2. The manuscript describes the hydrogenolysis of PET, establishing through control experiments that the aldehyde group,\nrather than the hydroxyl group, is the intermediate functional group. The authors need to determine whether the direct\nformation of PX from 4-MBA (bypassing 4-MB-A) is governed by the catalyst itself or the reaction conditions. Therefore, to\nclarify this point, the reviewer suggests specifically testing for the potential formation of 4-MB-A by lowering the reaction\ntemperature during the hydrogenolysis.\n3. To enhance the timeliness of the discussion, please incorporate relevant studies published within the past two years. In\naddition, the color scheme in Figure 2i appears inconsistent with the other figures. Please adjust it to better align with the\noverall visual style."}, "author_response": "Response to Reviewers\nManuscript ID: NCOMMS-25-98072A\nTitle: Tuning selectivity in the direct hydrogenolysis of PET plastic over Co catalysts through interfacial hydrogen spillover\nAuthors: Baoyu Wang et al\nWe thank the reviewers for their valuable comments and constructively respond to these in a point-by-point fashion below. Note\nthat changes to the manuscript are highlighted in yellow.\nReviewer #2\n1. The manuscript employs a Co/CoO catalyst for PET hydrogenolysis; however, the rationale for selecting this specific catalyst\nsystem is not clearly articulated. In addition, the catalyst characterization raises several questions. During catalyst preparation,\nthe reported CoO fraction increases as the pyrolysis temperature increases from 700 to 900 °C. The authors are encouraged to\nclarify the basis for this trend and the conditions under which it is expected. The final catalyst also appears to contain a carbon\nlayer derived from MOF pyrolysis. Quantification of the carbon content (e.g., wt% or surface coverage) would be helpful, along\nwith discussion of whether—and to what extent—this carbon layer may limit access to Co/CoO active sites during hydrogenolysis.\nWe apologise for not clearly articulating the rationale for our catalyst selection which was based on the following design criteria:\n(i) active for the dissociative adsorption of molecular hydrogen; (ii) a low barrier for high adatom diffusion to enable rapid\ntransport to PET chains; (iii) redox active metals able to shuttle electrons and protons to PET for C-X bond hydrogenolysis;\n(iv) Earth-abundant elements to ensure economic viability at scale; and (v) a high surface concentration of active sites to ensure a\nhigh specific activity under energy-efficient (modest temperature) operation. Cobalt (and its oxides) fulfill the first four criteria,\nand when dispersed in nanoparticle form on a carbon support also fulfills the fifth criterion, forming Earth-abundant reducible\nmetal oxides in organic solvents at temperatures 200 °C (Chem. Mater. 2014, 26, 2086), conditions well-suited to PET\nhydrogenolysis. Cobalt is widely used in energy and environmental applications due to to its excellent redox properties (Appl.\nCatal. 2023, 661, 119254). Four years ago, Hensen and co-workers demonstrated that the metal-oxide interface in cobalt\nnanoparticles could break the metal’s conventional structure sensitivity for CO hydrogenation, resulting in a striking rate-\n2\nenhancements (Nat. Catal. 2022, 5, 1051). We speculated that such Co-CoO interfaces could also promote ether hydrogenolysis\nversus the pure metal or oxide. In 2025, Zhang and co-workers demonstrated that tuning the valence of Co species was critical\nfor H O dissociation during the water-gas shift reaction (WGSR, Nat Commun 2025, 16, 736), with oscillations in the cobalt\n2\nelectron density behavior driving sequential reduction and oxidation reactions analogous to those required for direct PET\nhydrogenolysis. Experimental and computational studies of CoO surfaces reveal low barriers for the diffusion of H adatoms and\ntheir consequent high activity for the WGRS (Nat Commun 2023, 14, 6851) and nitrate reduction (Nat Commun 2022, 13, 1129).\nIn summary, we hoped that a catalyst containing a high concentration of Co-CoO interfacial sites would facilitate rapid H\n2\ndissociation and subsequent atomic H spillover to promote C -O bond cleavage. These points are now made explicit in the\nα\nexpanded introductory text.\nAction: Manuscript amended\nThe reviewer correctly notes that the CoO content of our catalyst increases with pyrolysis temperature between 700-900 °C.\nThis seemingly counter-intuitive observation reflects the release of oxygen-containing vapours at these temperatures, notably\nCO and H O (Nat Commun 2017, 8, 340) from decomposition of the carboxylic acid linkers in the parent MOF, that oxidise Co\n2 2\nclusters formed from the agglomeration of metallic Co nodes. Differential thermal analysis reveals a small exotherm between\n650-850 °C consistent with Co metal oxidation (Supplementary Fig. 1). Cobalt oxidation proceeds the formation of amorphous\ncarbon, that thermogravimetric analysis indicates is completed ~600 °C (Supplementary Fig. 1), and continues until\ngraphitisation commences at ~1000 °C. At temperatures <600 °C, partial pyrolysis entails complex physicochemical processes,\nincluding the escape of guest molecules that are inefficient oxidants (J Am Chem Soc 2018, 140, 15336). XPS reveals that the\nnear-surface carbon content exhibits only minimal increase during pyrolysis between 700-900 °C (from 89 atom% to 92 atom%,\nSupplementary Table 2). Electron microscopy indicates that cobalt particles throughout this pyrolysis regime are predominantly\n1\nlocated on the outer surface of the carbon matrix, with no evidence of particle encapsulation by a carbon layer (Figs. 1b-d).\nDepth-profiling by low-energy ion scattering (LEIS) using He⁺ ions also reveals a continuous increase in the Co signal with\nsputtering depth (Supplementary Fig. 17), whereas the C signal remains constant, indicating the absence of any carbon\noverlayer. We therefore conclude that active sites at the Co/CoO interface are readily accessed by reactants. These aspects are\nnow commented on in the revised text.\nAction: Manuscript + ESI amended\n2. In the model hydrogenolysis reaction, the authors report that CoO is converted to Co₃O₄ under reductive conditions (0.3 mbar\nH₂). This transformation is counterintuitive under reducing atmospheres and would benefit from additional verification and\nclarification. Specifically, the authors should explain the experimental basis for assigning Co₃O₄ formation, address the\npossibility of re-oxidation during handling or characterization, and reconcile the observation with expected redox chemistry\nunder the stated conditions. Otherwise, the basic redox chemistry rules must be re-written.\nThe reviewer raises an important issue that we have addressed by repeating our NAP-XPS experiment at a higher H pressure\n2\n(0.5 mbar, revised Fig. 2h). It is now clear that the cobalt binding energy is essentially constant at ~780 eV for temperatures\n<350 °C, indicative of Cox+, shifting to lower binding energy (~778 eV) at higher temperature consistent with reduction to Co0\n(Fig. 2h) and the loss of lattice oxygen (at ~529.8 eV) in the corresponding O 1s spectra (Supplementary Fig. 21). We believe\nthat the increase in higher binding energy between 250-350 °C previously observed using 0.3 mbar H (attributed to\n2\ndisproportionation of CoO  Co O + Co, Chem Mater 2014, 26, 2086) ) reflected a limited hydrogen supply. The manuscript\n3 4\nand SI have been revised accordingly to correct the previous experimental artifact. Good agreement is now apparent between\nthe catalyst reducibility determined by NAP-XPS and that measured by H -TPR (Fig. 2h versus Fig. 1e).\n3. The proposed hydrogenolysis mechanism also warrants reconsideration and clearer presentation. The terminology used to\ndescribe “α carbon” and “β bond” is difficult to follow and does not align with common conventions; adopting standard\nchemical nomenclature (or providing a clear schematic definition) would improve readability and reduce ambiguity.\nMechanistically, the manuscript rules out Cβ–Cβ and Cβ–O scission and instead favors Cα–O cleavage. This preference appears\ninconsistent with a straightforward bond dissociation energy (BDE) argument and is presented without sufficient justification.\nIf the authors propose that Cα–O cleavage dominates, they should provide stronger supporting evidence (e.g., kinetic trends,\nisotope labeling, product evolution vs time, or operando/controlled experiments) and explicitly explain why lower-BDE pathways\nare suppressed on this catalyst.\nAs recommended, we have modified Scheme 1 and Fig. 3f to more clearly label the α and β carbons and associated bonds that\nlie outside of the aromatic ring in the PET structure. Although the reviewer is correct that the C -C bond has a slightly lower\nβ β\ndissociation energy than the C -O bond, there was no evidence for the former: the resulting expected dimethyl terephthalate\nα\n(DMT) product was not detected by GC-MS or 1H NMR (Fig. 3d); and experiments using DMT as a substrate showed a product\ndistribution distinct from that observed from PET hydrogenolysis. The absence of reactively-formed ethane from PET\nhydrogenolysis also discounted possible C -O cleavage. Rather, control control experiments and DFT calculations suggest that\nβ\nour catalyst preferentially activates the C -O bond (due to stronger adsorption of the oxygen atom) in PET due to an interaction\nα\nbetween oxygen lone-pair electrons and partially filled d-band of metallic Co. DFT calculations identify a favored pathway\ninvolving carboxyl carbon protonation which stabilises the transition state for C -O cleavage resulting in a lower barrier than\nα\nthat for oxygen protonation (C –O cleavage). New DFT calculations show that although the energy barrier difference between a\nβ\nhydrogen atom attacking the ethylene glycol carbon and the carboxyl carbon is negligible (0.47 eV vs. 0.43 eV), the energetic\ncost for subsequent bond breaking diverges significantly (Fig. 4a, new Supplementary Fig. 38). Specifically, cleaving the C -\nβ\nC bond requires >28 % additional energy than cleaving the C -O bond. Hydrogenolysis of PET by C -O bond cleavage is thus\nβ α α\npredicted to be kinetically favourable. Experiments using 4-methylbenzaldehyde (an intermediate formed by C -O bond cleavage)\nα\nas the substrate yielded a liquid product matching that from PET hydrogenolysis (Supplementary Table 8), consistent with 2-\n2\nhydroxyethyl-4-methylbenzoate as the critical intermediate undergoing C -O bond cleavage. These results collectively confirm\nα\nthat PET hydrogenolysis initiates via C –O bond cleavage.\nα\nAction: Manuscript + ESI amended\n4. Relatedly, the manuscript proposes that ethylene glycol is formed and subsequently cracked to methanol and then methane. This\nsequence is not adequately supported in the current dataset and may be chemically nontrivial under the reported conditions. If\nethylene glycol cracking is central to the proposed pathway, additional evidence is needed to demonstrate that ethylene glycol\ncan convert to the reported C1 products under the same reaction conditions (e.g., control experiments using ethylene glycol as\nthe sole substrate, time-resolved product distributions, or mechanistic probes).\nThe reviewer makes an excellent suggestion, and we have now investigated the reactivity of ethylene glycol under\nhydrogenolysis conditions (250 °C and 1 MPa H Supplementary Fig. 30). Ethylene glycol underwent complete conversion\n2,\nwithin 10 min, with methanol as the primary product which underwent subsequent deoxygenation to methane (90 % yield).\nThis observation supports our proposed reaction pathway and is consistent with prior literature reports (Phys. Chem. Chem. Phys.\n2013, 15, 9043) that such temperatures drive the rapid conversion of methanol (from ethylene glycol cracking) into methane.\nThis observation is now discussed in the revised text.\nAction: Manuscript + ESI amended\n5. Alternative chain-scission pathways should also be considered more explicitly. Cleavage at Cβ–Cβ and/or Cβ–O positions could\nplausibly generate acid-containing intermediates. The observation of trace cyclohexanecarboxylic acid, as reported by the\nauthors, may be consistent with formation of such intermediates and appears difficult to reconcile with the proposed pathway\nas currently written. A more balanced discussion that evaluates competing mechanisms in light of the full product slate would\nstrengthen the credibility of the mechanistic conclusions.\nMany apologies for the confusion caused by our previous misidentification of \"cyclohexanecarboxylic acid\" in the manuscript;\nthe ion chromatogram actually corresponds to 2-hydroxyethyl 4-methylbenzoate (HMBE). At 280 °C, the detected liquid\nproducts, HMBE, 2-hydroxyethyl 4-methylcyclohexane-1-carboxylate (HMCE), and 4-methyl-1-cyclohexanemethanol (4-\nMCHM), arise from common reaction pathway (HMBE → HMCE → 4-MCHM), and all derive from an initial C -O bond\nα\ncleavage. The absence of any carboxylic acid products discounts possible pathways involving C -O bond cleavage.\nβ\nThe manuscript has been corrected accordingly.\nAction: Manuscript amended\n6. Finally, the tone of the manuscript could be made more measured. Several statements appear overly promotional while\ndismissing other chemical recycling approaches. A more neutral presentation—emphasizing evidence-based comparisons\nand acknowledging practical considerations such as hydrogen cost, process complexity, and scalability constraints—would\nlikely be more persuasive to readers. In addition, the manuscript would benefit from careful editing to correct typographical\nand presentation issues (e.g., misspellings such as “aove” for “above” and unclear or error-prone figure captions, including\nFigure 3).\nWe appreciate the reviewer’s comment and have revised the manuscript to moderate our tone to be more objective regarding\nalternative chemical recycling approaches. A detailed comparison between PET hydrogenolysis-based recycling and alternative\nmethods is now included, that explicitly recognises the limitations of our process in terms of hydrogen cost, process complexity,\nand scalability. Apoligies for the typographical/presentation issues; we have more carefully proofread the manuscript to address\nthese and improve clarity.\nAction: Manuscript amended\n1. The hydrogen spill over is proposed and shown in the calculations. Is there experimental evidence?\nThe reviewer raises an important issue. Experimental evidence for hydrogen spillover derives from the in-situ NAP-XPS\nmeasurements that enable real-time monitoring of surface state evolution in metals/oxides (ACS Catal. 2021, 11, 3159). At\n100 °C, Co 2p XP spectra the gradual reduction of CoO to Co metal as a function of reaction time (notably >15 min, Fig. 2j).\nx\nAs CoO is extremely unlikely to undergo hydrogen dissociation and self-reduction at this temperature, this observation\nx\nevidences the spillover of hydrogen adatoms from molecular H through dissociatively chemisorbed at neighbouring Co metal\n2\nsites. A concomitant increase in the binding energy of lattice oxygen in CoO (from ~529.2 eV to 529.5 eV, Supplementary Fig.\nx\n23) is also consistent with oxygen vacancy formation arising from spillover (Surf. Sci. 2021, 705, 121761).\nAction: Manuscript + ESI amended\n2. Co and CoOx were compared with the specially synthesized Co/CoO catalyst. This could be a difference of metal dispersion.\nNormally, an oxidation layer would form on Co with exposure to air, making a similar Co/CoO hybrid composites. So I suggest\nauthors to prepare an activated carbon supported Co, and compare its catalytic activity.\nThe reviewer raises an important issue regarding reference catalysts. We have now prepared and evaluated CoO/C and Co/C\nanalogues for PET hydrogenolysis under our standard conditions. Both showed poor activity (<10 % PX yield, Supplementary\nTable 6) confirming the advantage of the MOF-derived Co/CoO catalyst which we attribute to a unique ensemble of active site,\nlikely formed through by redox cycles during gas evolution from ligand decomposition during pyrolysis. The CoO/C catalyst\nwas synthesized by impregnating activated carbon with cobalt nitrate, followed by high-temperature pyrolysis. XRD confirmed\nthe presence of fcc CoO crystallites (Supplementary Fig. 46a), which H -TPR revealed were were more difficult to reduce\n2\n(530 °C, Supplementary Fig. 46b) than bulk CoO /C (380 °C, Supplementary Fig. 7), indicating a strong support interaction\nfor the former that hinders activation of molecular hydrogen and hence PET hydrogonolysis. A Co/C catalyst was prepared from\nCoO/C by its reduction in H at 600 °C for 2 h and subsequent air exposure. XRD and TEM revealed large fcc Co metal\n2\ncrystallites in the resulting Co/C (20 nm to 100 nm diameter, Supplementary Fig. 46a,d), with XPS analysis evidencing both\nCo metal and CoO surface species (Supplementary Fig. 46c). The poor performance of Co/C versus Co/CoO-800 suggests\nx\nthat the co-existence of surface cobalt metal and oxide species is not sufficient for catalytic PET hydrogenolysis, with MOF\npyrolysis offering a higher metal dispersion and optimal metal-oxide interface. This important conclusion is now discussed in\nthe manuscript.\nAction: Manuscript + ESI amended\nReviewer #4:\n1. The core advantage lies in the MOF pyrolysis approach, which maintains the overall morphology while achieving superior\nactive site dispersion and interfacial area, unlike the limitations inherent in conventional preparation routes. To further\nsubstantiate the rationale of this synthetic route, it is recommended that electron microscopy images depicting the overall catalyst\nmorphology be provided. Such evidence would visually confirm the preservation of the MOF's macroscopic framework after\npyrolysis, thereby highlighting the distinct advantages of this approach.\nAs suggested by the reviewer we now include an AC HAADF-STEM image of the Co/Co-800 catalyst which better visualizes\nthe size and distribution of Co containing nanoparticles throughout the carbon support (Supplementary Fig. 9). This image\nconfirms that the pyrolyzed Co-MOF largely retains its parent macroscopic structure, conducive to achieving a high dispersion\nof active sites/metal-oxide interfacial area. CoO/C and Co/C catalysts synthesized using activated carbon as a support result in\nlarger and less uniform nanoparticles (Supplementary Fig. 46) that are ineffective for PET hydrogenolysis. This is now\ndiscussed in the revised manuscript.\nAction: Manuscript + ESI amended\n4\n2. The manuscript describes the hydrogenolysis of PET, establishing through control experiments that the aldehyde group,\nrather than the hydroxyl group, is the intermediate functional group. The authors need to determine whether the direct formation of PX\nfrom 4-MBA (bypassing 4-MB-A) is governed by the catalyst itself or the reaction conditions. Therefore, to clarify this point, the\nreviewer suggests specifically testing for the potential formation of 4-MB-A by lowering the reaction temperature during the\nhydrogenolysis.\nAs suggested by the reviewer we have now performed a reaction at 150 °C using 4-MBA as the substrate to confirm whether its\nconversion to 4-MB-A (versus PX) is favoured at lower temperature. Complete conversion of 4-MBA to 4-MB-A was indeed\nobserved at 150 °C (>95 % yield, Supplementary Table 8), however this reaction pathway that can be discounted at 250 °C as\nthe latter would form significant 1,4-DMC under our hydrogenolysis conditions which was not observed from 4-MBA (Fig. 3c).\nWe propose that elevated temperatures favour the rapid hydrodeoxygenation of adsorbed 4-MBA before it can desorb as\nreactively-formed 4-MB-A, consistent with a previously proposed reaction mechanism (J. Am. Chem. Soc. 2024, 146, 15309).\nAction: Manuscript + ESI amended\n3. To enhance the timeliness of the discussion, please incorporate relevant studies published within the past two years. In addition,\nthe color scheme in Figure 2i appears inconsistent with the other figures. Please adjust it to better align with the overall visual\nstyle.\nWe have updated references throughout the manuscript to incorporate more recent studies from the past two years. Additionally,\nthe color scheme in Fig. 2i has been adjusted to ensure better consistency with the overall visual style of the text.\nAction: Manuscript + ESI amended\n5"}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed my concerns and I have no more questions.", "reviewer_3": "In the revised manuscript, authors have adequately addressed all my previous concerns and revised the content. I think the\ncurrent version can be published.", "reviewer_4": "The author has resolved all my concerns, and I recommend this work be published."}, "author_response": ""}]

Decision letter