Surface-dependent pathways for formaldehyde oxidation and reduction on TiO 2(001)

Abstract

The reactions of formaldehyde on differently prepared surfaces of a TiO 2(001) single crystal were investigated by temperatureprogrammed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Surfaces which had been ion-bombarded and annealed at low temperature, up to ca. 650 K, were oxygen deficient and contained Ti cations in lower oxidation states than + 4, in addition to Ti 4+ cations. TPD after formaldehyde adsorption on these surfaces showed coincident desorption of methanol and formaldehyde in two temperature domains, at ca. 370 K and ca. 550 K. In contrast on fully oxidized surfaces (obtained by annealing at 750 K and above), formaldehyde and methanol desorbed only at ca. 370 K. Formaldehyde reacted on TiO 2(001) surfaces via two different routes. The first was direct reduction of formaldehyde to methanol on the reduced surfaces, as evidenced by the presence of methoxide species at 286.3 eV in the C(1s) spectrum at 250 K following formaldehyde adsorption. Part of the formaldehyde was totally decomposed to C(a), H(a), and O(a); H(a) was involved in the reduction of formaldehyde to methanol, C(a) was oxidized to CO and CO 2 while O(a) oxidized reduced titanium sites as indicated by the decrease of the XPS Ti(2p 3 2 ) lines corresponding to Ti x+ ( x < 4). The second principal pathway for reaction of formaldehyde involved a Cannizzaro-type reaction and was only observed on fully oxidized surfaces. This reaction was responsible for the coincident formation of methoxide (286.3 eV) and formate (289.5–289.7 eV) species observed by XPS at room temperature.