Selective photocatalytic CO2 reduction to CH4 over Pt/In2O3: Significant role of hydrogen adatom

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The selectivity of photocatalytic CO2 reduction to CH4 can be enhanced over Pt-decorated semiconductors, which is commonly attributed to the fact that a Schottky junction is formed, thus enhancing photoinduced electron lifetime. However, it is difficult to understand why the yield of CO—the other product of photocatalytic CO2 reduction—is decreased only in terms of photoinduced electron lifetime. In this work, the mechanism of Pt-promoted CH4 formation was probed again in a gas-solid system of photocatalytic CO2 reduction over highly-dispersed-Pt decorated In2O3 nanorods (Pt/In2O3). It was found that the presence of Pt modulates the surface property of In2O3 due to electronic and steric effect, resulting in a loss of HCO3−, b-CO32− and m-CO32− species for the coadsorption of CO2 with H2O. However, this is not directly related with the high CH4 selectivity and low CO yield on the Pt/In2O3 photocatalysts. Photocatalytic reductions of CO, HCOOH, CH2O, and CH3OH as well as photocatalytic CO2 reduction over photocatalysts with different H2 uptakes confirm that H adatoms derived from H2 or H2O dissociation on Pt play a key role in the formation of CH4. Low H2 dissociation barrier on Pt and weak HPt bond facilitate the bonding of C in CO2 with H, thus restraining CO production. In other words, the metallic Pt acts as atomic hydrogen reservoir that supplies sufficient and readily available protons for CH4 formation over Pt-decorated semiconductors. The present work offers a new window to explore non-noble metals or their alloys with stability in air and high dissociation ability to H2 or H2O as a replacement of Pt for CO2 photoreduction to CH4.