Surface Chemistry of Formaldehyde on Rutile TiO2(011)-(2 x 1) Surface: Photocatalysis Versus Thermal-Catalysis

Investigating the surface chemistry of formaldehyde on the surface of TiO2 is important in understanding the thermal catalytic and photocatalytic reactions of formaldehyde on TiO2-involved catalysts. By combining thermal desorption spectroscopy and X-ray photoelectron spectroscopy, we studied the adsorption, thermo-induced surface reactions, and photo-induced surface reactions of formaldehyde on the rutile TiO2(011)-(2 x 1) surface. The dominant thermal-catalytic reaction is the formation of ethylene by a reductive carbon carbon formation reaction of formaldehyde adsorbed at the oxygen vacancy sites, and the dominant photocatalytic reaction is the formation of formate, assisted by the bridge O-2c sites, followed by carbon monoxide formation at elevated temperatures. The surface intermediates of formaldehyde reactions to ethylene and carbon monoxide on the rutile TiO2(011)-(2 x 1) surface were identified. The effect of the surface structure of the rutile TiO2(011)-(2 x 1) surface, particularly the oxygen vacancy, on the thermal-catalytic and photocatalytic activity toward formaldehyde was revealed by studying the coadsorption of water and formaldehyde. These results broaden our fundamental comprehension on the reaction mechanism of formaldehyde on the TiO2 surfaces.