Unlocking Efficient CO2-to-CH4 Photoreduction by Optimizing Protonation Pathways

Understanding and engineering the rate-determining step (RDS) is key to unlocking the concurrent high activity and selectivity required for efficient CO2-to-CH4 conversion. Herein, we employ mesostructured UiO-66-NH2 (denoted as MUiO) as a model photocatalyst to investigate the pathway control in CO2 photoreduction. Combining macroscopic product and microscopic intermediate kinetic isotope effect (KIE) studies and density functional theory (DFT) calculations, we demonstrate that loading Pd clusters onto MUiO triggers a decisive switch from normal to inverse KIE, thus steering the dominant reaction pathway from CO toward CH4 production and lowering the activation barrier of the RDS from 1.88 to 1.12 eV (from *CH3O → CH4 to *CO → *CHO). The integration of framework and Pd sites promotes high product rate and selectivity by driving RDS through a dual function: enhanced interfacial proton supply by Pd clusters and suppressed unproductive *CO desorption through a kinetic trap for CO within the pores. Consequently, the optimal Pd-MUiO catalyst exhibited a 4.6-fold increase in CH4/CO selectivity and a 62.8-fold improvement in CH4 production relative to the pristine MUiO, surpassing previously reported analogous photocatalysts. This study offers a compelling strategy for tailoring CO2 photoconversion to CH4 by rationally engineering the RDS.