Role of Water in Methanol Photochemistry on Rutile TiO2(110)

Photochemistry of the molecularly and dissociatively adsorbed forms of methanol on the vacuum-annealed rutile TiO2(110) surface was explored using temperature-programmed desorption (TPD) both with and without coadsorbed water. Methoxy, and not methanol, was confirmed as the photochemically active form of adsorbed methanol on this surface. UV irradiation of methoxy-covered TiO2(110) led to depletion of the methoxy coverage and to formation of formaldehyde and a surface OH group. Coadsorbed water did not promote either molecular methanol photochemistry or thermal decomposition of methanol to methoxy. However, terminal OH groups (OHt), prepared by coadsorption of water and oxygen atoms, thermally converted molecularly adsorbed methanol to methoxy at 120 K thus enabling photoactivity. While chemisorbed water molecules had no influence on methoxy photochemistry, water molecules hydrogen-bonded in the second layer to bridging oxygen (O-br) sites inhibited methoxy photodecomposition to formaldehyde. From this, we conclude that O-br sites accept protons from methoxy to form formaldehyde. These results provide new fundamental understanding into the hole-scavenging role of methanol in photochemical processes on TiO2-based materials and into how water influences this photochemistry.