Water oxidation as a multi-electron transfer and endothermal reaction has been considered to be the bottleneck of solar-driven water splitting into oxygen and hydrogen. Herein, an alternative approach for solar energy conversion is developed by coupling H2 generation with the selective oxidation of alcohol. In a closed redox system containing molecular ruthenium catalyst (RuCat) and Pt modified g-C3N4 (Pt-g-C3N4) composite, hydrogen production is concomitant with the oxidation of benzyl alcohols to aldehydes with over 99% selectivity in the presence of visible light and pure water. By contrary, the system lacking molecular catalyst only exhibits low to moderate selectivities towards aldehydes. The remarkably improved selectivity is attributed to the formation of highly active Ru(IV) = O intermediate through efficient hole transfer from g-C3N4 to RuCat.