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. Effects of illumination time on the formation of CO2 and CO on pure and N-doped TiO2 in the photodecomposition of HCOOH.