Hydrogenation has been widely employed in solar-driven photocatalysis, to enhance the light absorption and photon-to-electron (PE) conversion of semiconductors. However, there still remain controversies in this field, such as the precise roles of bulk and surface oxygen vacancies (VO). Furthermore, the influence of hydrogenation on surface reactions in photocatalysis at gas-solid interface has been rarely discussed. Herein, gray CeO2 was prepared by treating pristine CeO2 in H2/Ar, presenting surface plasma resonance (SPR) like visible light absorption. No modifications on microstructural surface disordering or valence band edge were detected in gray CeO2. Hydrogenation induced abundant VO inside both the bulk lattice and the surface layer. Gray CeO2 presented much enhanced performance as well as improved water resistance in photocatalytic oxidation of gaseous hydrocarbons. Additional post-annealing in air was applied to remove surface VO and discolored the material. With series of characterization including XPS, Raman, EPR and in-situ FTIR, it was found bulk VO mostly contributes to the enhanced PE conversion, while surface VO is mainly responsible for the improved water resistance. Our results unambiguously confirmed hydrogenation as an efficient approach to simultaneously optimize oxide semicomductors in various elementary steps of photocatalysis.