Photocatalytic hydrogenation of carbon dioxide (CO2) is a promising technology to mimic photosynthesis by the conversion of solar energy into methanol (CH3OH) for the development of sustainable energy. This technology not only can potentially cut down the atmospheric CO2, but also reduce the world's dependence on fossil fuel. Using a novel twin reactor, the hydrogen, which is directly generated from the water splitting, could be utilized to hydrogenate CO2 into a desirable fuel and simultaneously reduce greenhouse gas. Remarkable result has been observed by using a gaseous mixture of CO/CO2 to yield CH3OH under artificial sunlight, compared with the one using solely CO or CO2. Although direct hydrogenation of CO to form CH3OH on either Pt/CuAlGaO4 or Pt/SrTiO3:Rh photocatalysts is not likely to occur, CO still plays an important role on the generation of either H-2 via water-gas shift reaction or methyl formate (HCOOCH3), the intermediate product of CH3OH. Nevertheless, adding too much CO as the co-feed will adversely decrease the yield of CH3OH. A possible mechanism of CO/CO2 photo-hydrogenation over Pt/CuAlGaO4 and Pt/SrTiO3:Rh photocatalysts is proposed to explain the key experimental trends observed as well as the species involved during the reaction. (C) 2015 Elsevier Ltd. All rights reserved.