Unraveling the Essential Role of Consecutive Protonation Steps in Photocatalytic CO 2 Reduction when Using Au Nanorods in a MOF

The proton-coupled electron transfer process (PCET) plays a crucial role in both natural and artificial photosynthesis, including CO₂ fixation chemistry. However, difficulties in capturing the transient intermediates generated during the protonation process impede the clarification of the fundamental mechanism behind photocatalytic CO₂ reduction. Herein, we report a general killing two birds with one stone strategy by spatially confining Au nanorods within a typical porphyrin metal-organic framework (MOF). Interestingly, 2.4-fold increase in CH₄/CO selectivity and 12-fold increase in CH₄ production were observed after loading of Au nanorods, indicative of a strengthened protonation process in the photocatalytic CO₂ reduction. More importantly, the plasmonic effect from Au nanorods simultaneously boosted the in situ Raman signals of *CO and *CHO intermediates on the Au-O-Zr active site. The evident protonation process was further clarified in a control H/D kinetic isotope experiment. This work highlights the significance of successive protonation steps for boosting CH₄ production in photocatalytic CO₂ reduction.