Bifunctional Rhodium Cocatalysts for Photocatalytic Steam Reforming of Methane over Alkaline Titanate

Photocatalytic steam reforming of methane (PSRM; 2 H2O (g) + CH4 -> 4 H-2 + CO2) was examined over metal-loaded K2Ti6O13 photocatalysts. Although the production rate was improved by loading Pt cocatalyst on the K2Ti6O13 photocatalyst, unfavorable formation of CO and gradual deactivation of photocatalyst were observed. On the other hand, a Rh-loaded K2Ti6O13 sample showed two times higher activity than the Pt-loaded one did, and promoted the PSRM selectively without deactivation for many hours. In the highly active Rh-loaded photocatalyst, the Rh cocatalyst existed as a mixture of small metallic rhodium and large rhodium oxide particles. The photocatalytic activity tests for hydrogen evolution and oxygen evolution from each aqueous solution of sacrificial reagent (methanol and silver nitrate, respectively) revealed that the metallic rhodium particles and the rhodium oxide particles could function as cocatalysts preferably for reduction and oxidation, respectively. Also on a Na2Ti6O13 photocatalyst, a mixture of rhodium metal and oxide similarly enhanced the photocatalytic activity. Thus, it is suggested that the Rh cocatalyst on these alkaline titanates bifunctionally promoted the PSRM.