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

Abstract

Photochemistry of the molecularly and dissociatively adsorbed forms of methanol on the vacuum-annealed rutile TiO 2 (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 TiO 2 (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 (OH t ), 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 TiO 2 -based materials and into how water influences this photochemistry.