Thermal Control of Selectivity in Photocatalytic, Water-Free Alcohol Photoreforming

The selective oxidation of alcohols has attracted a great deal of attention. While most photocatalytic studies focus on the generation of hydrogen from alcohols, there is also a great potential to replace inefficient thermal reaction pathways (as e.g. the formox process) by light-driven reactions. In this work we focus on the photoreforming of methanol, ethanol, cyclohexanol, benzyl alcohol, and tert-butanol on well-defined Pt-x/TiO2(110) under UHV. It is found that, with the exception of tert-butanol, alcohol oxidation can produce the respective water-free aldehydes and ketones along with the formation of stoichiometric molecular hydrogen with 100% selectivity. While alpha-H-containing alcohols usually exhibit only a disproportionation reaction with the release of H-2, another reaction pathway is detected for methanol (and to a much lower extent benzyl alcohol) to yield the respective ester, methyl formate (or benzyl benzoate, respectively). The formation of this product occurs via a consecutive photoreaction and is strongly influenced by temperature. In general, higher temperatures lead to a higher selectivity toward formaldehyde, as product desorption is favored over the consecutive photoreaction. For tert-butanol two parallel photoreactions occur. In addition to the splitting of a C-C bond yielding a methyl radical, hydrogen, and acetone, dehydration to isobutene is observed. The branching ratios of both reaction pathways can be strongly controlled by temperature, by changing the reaction regime from adsorption to desorption limited. The high selectivities toward aldehydes are attributed to the absence of O-2 and water, which inhibits an unwanted overoxidation to acids or CO/CO2. This study shows that photocatalysis under such conditions provides a prospective approach for a highly selective and water-free aldehyde production under mild conditions.