Photolysis of HCOOH over Rh Deposited on Pure and N-Modified TiO2

Abstract  

The photo-induced vapor-phase decomposition of formic acid was investigated on pure, N-doped and Rh-promoted TiO₂. The bandgap of TiO₂ was narrowed by 0.82–1.04 eV as a result of the incorporation N into TiO₂. Adsorption of formic acid on pure TiO₂ produced strong absorption bands due to formate species, the intensity of which decreased by illumination. The photodecomposition of formic acid on pure TiO₂ at 300 K occurs to only a limited extent: on N-doped TiO₂, however, it is enhanced by a factor of 2–4. The N-modified TiO₂ catalyzes the photoreaction even in the visible light, which is attributed to the prevention of electron–hole recombination. The deposition of Rh on TiO₂ markedly increased the extent of photodecomposition. The conversion is complete in 200 min, while the extent of decomposition reaches only ~30% on pure TiO₂. The effect of Rh is explained by a better separation of charge carriers induced by illumination and by enhanced electron donation to the adsorbed formate species. On TiO₂ samples both the dehydrogenation and dehydration reactions occurred, on Rh/TiO₂ only a trace amount of CO was formed. Addition of water to formic acid eliminated this CO, but exerted no other influence on the occurrence the photoreaction.