The Influence of the Reaction Conditions on the Photocatalytic Gas-Phase Conversion of Methanol with Water Vapor over Pt/SrTiO3 in a Continuously Operated Flow Reactor

Abstract

Continuous methanol photooxidation in the gas phase is a promising method to produce valuable chemicals like formaldehyde or methyl formate in addition to hydrogen under mild conditions. The influence of the reaction conditions on the selectivity of methanol oxidation to formaldehyde is studied using a heated flat-plate flow photoreactor illuminated by an LED array (λmax = 368 nm) and Pt-modified SrTiO3. A combination of online analytical methods allowed to quantify all gaseous products during extended time-on-stream (> 48 h TOS). The selectivity to formaldehyde is found to be primarily determined by the residence time and the process temperature. At a low methanol to water ratio, methanol conversion and evolution of CO2 are favored, whereas the light intensity primarily influenced the apparent quantum yield from 5.1 to 1.8% at 9.36 to 52.93 mW cm−2, respectively, and the methanol conversion thus determining the economic efficiency of the process. Operation temperatures higher than 110 °C resulted in a strong deactivation of the catalyst while simultaneously the formation of CO at the expense of formaldehyde selectivity is favored. This study demonstrates the importance of understanding the influence of relevant reaction conditions and the potential of selective photocatalytic gas-phase oxidation of small molecules.