Switching Photocatalytic CO₂ Reduction Path via Controlled Surface Hydroxylation in GO/CoTPP Hybrids

Confronting the dual challenges of carbon neutrality and sustainable energy, photocatalytic CO₂ reduction requires precise control over product selectivity. This study demonstrates that surface hydroxyl (-OH) density serves as a molecular switch for reaction pathways in graphene oxide/cobalt tetraphenylporphyrin (GO/CoTPP) hybrids. By tuning the reduction degree of GO supports via gradient hydrazine hydrate treatment (0-85%), we constructed catalysts with controlled -OH concentrations. Systematic characterization confirmed progressive removal of oxygen functionalities and enhanced hydrophobicity with increasing reduction severity. Remarkably, the 8.5% reduced GO/CoTPP catalyst achieved optimal CO production (62.01 μmol g^-¹ h^-¹, 4.1 times enhancement) with 100% selectivity, while suppressing CH₄ and H₂ by-products. In-situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and kinetic solvent isotope effect (KSIE) experiments elucidated a triple regulatory mechanism: (1) moderate -OH density enhances structural integrity and facilitates water molecule adsorption on the catalyst surface, optimizing CO selectivity facilitated via proton-coupled electron transfer (PCET); (2) excessive -OH groups trigger competitive hydrogen evolution and overhydrogenation; and (3) insufficient -OH coverage shifts the mechanism to stepwise proton-electron transfer (PTET), increasing energy barriers. This work establishes a critical hydroxylation threshold for pathway control, providing new design principles for selective CO₂ photoreduction catalysts.