We report gas-phase photocatalytic CO2 reduction with Cu2O/TiO2 photocatalysts of varying surface passivation. With taurine adsorbed on the photocatalyst surface, the CH4 production rate increases and CO production rate decreases compared to photocatalysts with no ligand treatment. When ethylenediamine is present on Cu2O/TiO2 photocatalysts, CO selectivity enhances significantly. In situ Fourier transform infrared spectroscopy and density functional theory calculation reveal that the surface ligands on the photocatalyst surface alter the binding strength of reaction intermediates (e.g., CO) on surface atoms of the photocatalysts. Our findings cast light on a new design principle for the catalyst surface in selective CO2 conversion.