Photodecomposition of Formic Acid on N-Doped and Metal-Promoted TiO2 Production of CO-Free H2

The photoinduced vapor-phase decomposition of formic acid was investigated on pure, N-doped and metal-promoted TiO2. The catalysts were characterized by bandgap determination, and by Fourier transformed infrared spectroscopy, the bandgap of N-doped TiO2 was narrowed by 0.5-1.02 eV. IR studies revealed that illumination of the HCOOH-TiO2 system initiated the decomposition of adsorbed formate species. On the IR spectra of metal-promoted TiO2 adsorbed CO attached to the metals was also detected. The photodecomposition of formic acid on pure TiO2 occurs to only a limited extent to yield H-2 and CO2 as the major products with a small amount of CO. Depending on the origin of TiO2 and on the preparation, N-doped TiO2 exhibited higher activity. Its efficiency is increased with the narrowing of the bandgap, a feature attributed to the prevention of electron-hole recombination. The deposition of noble metals on pure and N-modified TiO2 dramatically enhanced the extent of photodecomposition of formic acid. Pd/TiO2 was found to be the most active catalyst. Addition of water to formic acid completely eliminated the small amount of CO formed. Both the N-doped TiO2 and metal-promoted TiO2 + N samples exhibited photocatalytic effects even in visible light. The promoting effect of metals was explained by a better separation of charge carriers induced by illumination and by improved electronic communication between metal particles and TiO2.