Photothermal catalytic CO2 hydrogenation, a promising sunlight-powered carbon-negative technology, has been attracting increasing attention due to its unique ability to convert CO2 and green hydrogen into valuable chemicals and fuels on a large scale under mild conditions. Among various reaction routes, the photothermal catalytic reverse water-gas shift (RWGS) reaction provides a sustainable way for producing CO, a key feedstock for the Fischer-Tropsch synthesis. However, this reaction pathway is often accompanied by CO2 methanation reaction arising from the over-hydrogenation of adsorbed *CO, leading to the limited selectivity. One effective strategy to improve CO selectivity is to design metal single-site catalysts (SSCs), which not only maximize the utilization of metal atoms but also weaken *CO binding by suppressing π-backbonding, thus mitigating the competing methanation reaction. Unfortunately, SSCs generally encounter high energy barriers when simultaneously activating multiple reactants, resulting in low activity for the RWGS reaction. Currently, breaking the activity-selectivity trade-off to achieve both high activity and selectivity in the photothermal catalytic RWGS reaction is highly desirable yet remains a challenging task in this field.
Recently, the research group of professor Zheng Hu at the Key Laboratory of Mesoscopic Chemistry of MOE has proposed a concept of decoupling the activity-selectivity trade-off in the photothermal catalytic reverse water-gas shift (RWGS) reaction by hydrogen spillover-assisted dual-site synergy. This concept is demonstrated through a hybrid catalyst constructed by immobilizing abundant Ru single sites and trace Ru clusters onto high-efficiency photothermal support of N-doped hierarchical carbon nanocages (hNCNC). Theoretical calculations reveal that the Ru-N4 sites are highly active and selective for the RWGS reaction, contingent on the efficient migration of dissociated *H species to adjacent C atoms of Ru. Combined with a series of advanced characterizations, we confirm that Ru single sites dominate CO2 hydrogenation to CO, whereas Ru clusters facilitate H₂ activation and supply hydrogen species to adjacent single sites via spillover over hNCNC. Leveraging this synergistic interaction, the hybrid catalyst achieves an exceptional CO production rate of 3.1 mol·gRu-1·h-1 and selectivity over 98%. This mechanism shows universal applicability as demonstrated by the effective promotion of CO2 hydrogenation of Ru single sites by other typical hydrogen-spillover-active metal clusters, e.g., Pt and Pd clusters. This design concept liberates the potential to overcome the longstanding activity-selectivity trade-off in hydrogenation reactions.
Figure 1. DFT computational insights into the promotional effect of Ru clusters (Ru4) on single sites (Ru-N4) in CO2 hydrogenation reaction. a) CO adsorption energy. b) Gibbs free energy of *COOH and *HCOO intermediates on Ru-N4. c) Gibbs free energy diagram of CO2 hydrogenation on Ru-N4 with different reaction pathways. d) Gibbs free energy diagram of H2 dissociation and migration. e) Schematic illustration of the rational design of hybrid Ru catalyst.
Figure 3.Photothermal catalytic CO2 hydrogenation performances of Russ/hNCNC, Russ+cluster/hNCNC and Rucluster/hNCNC.a), b) CO production rates (a) and CO selectivity (b) under different light irradiations. The error bars represent the relative deviation obtained from parallel experiments. c), d) Stability test of CO production rates (c) and CO selectivity (d) under 3.0 W·cm-2 light irradiation. e) Photographs of the reaction system under outdoor sunlight irradiation. This test was conducted on the Nanjing University campus from 10:15 to 10:45 local time on 21 July 2024. The solar irradiation of the reaction system is augmented by a Fresnel lens. f) Comparison of catalytic performances of Russ+cluster/hNCNC between sunlight condition and Xe lamp (3.8 W·cm-2) condition.
Figure 4. Catalytic mechanism analysis.a)Arrhenius plots drawn from temperature-dependent RCO. The error bars represent the relative deviation obtained from parallel experiments. b)-d) CO2-TPD, H2-TPD and CO-TPD profiles, respectively. Inset in c) is the H-D exchange profiles.e) In situ DRIFTS spectra for catalytic CO2 hydrogenation over Russ+cluster/hNCNC at different temperatures.f) Schematic illustration of the CO2 hydrogenation process on Russ+cluster/hNCNC.
The related paper entitled “Decoupling Activity-Selectivity Trade-off in Photothermal Catalytic CO2 Hydrogenation: A Hydrogen Spillover-Assisted Dual-Site Synergy Mechanism” has been published on Angewandte Chemie International Edition on June 20, 2025 (Paper link: https://onlinelibrary.wiley.com/doi/10.1002/anie.202508090, DOI：10.1002/anie.202508090). Ph.D. student Zixuan Sun is the first author. Prof. Hongwen Huang and Prof. Zheng Hu from our department are co-corresponding authors. This work was jointly supported by the National Key Research and Development Program of China (No. 2021YFA1500900), the National Natural Science Foundation of China (Nos. 22475097, 22479073, 22322902), and the Natural Science Foundation of Jiangsu Province, Major Project (BK20212005).