Two routes to formaldehyde from formic acid on TiO2(001) surfaces

Two routes were observed for formaldehyde formation from formic acid on TiO2(001) single crystal surfaces. The first, on reduced surfaces, involves the reduction of formic acid to formaldehyde accompanied by the oxidation of surface Ti cations. Surface oxidation was evident from the increase in the portion of the Ti (2p(3/2)) signal corresponding to Ti4+ and the decrease in that corresponding to Ti-x+(0<x<4) in XPS. XPS after formic acid adsorption on both reduced and oxidized TiO2 surfaces showed the presence of formate species having a C(1s) binding energy of 289.3 eV; these species are formed by dissociative adsorption of formic acid at room temperature. A second route for formaldehyde formation, operative on fully oxidized surfaces, involves bimolecular coupling of two formates. This reaction occurs only on the reconstructed, fully oxidized surface; the {114}-faceted TiO2(001) structure (obtained by annealing at 900 K or above), as previously reported by Kim and Barteau [Langmuir 6 (1990) 1485].