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 α-H-containing alcohols usually exhibit only a disproportionation reaction with the release of H2, 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 O2 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.

The photocatalytic H2 evolution on co-catalyst loaded titania is interpreted by a new mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction.

Photocatalytic hydrogen evolution from methanol is a standard test reaction for photocatalyst materials. Surprisingly, the exact chemical mechanism is still widely discussed in the literature. In order to disentangle photochemical from thermal reaction steps and gain insights on the atomic level, we use a Pt cluster-loaded TiO2(110) photocatalyst in very well-defined environments. Using Auger electron spectroscopy, temperature-programmed desorption/reaction, isotopic labeling, and isothermal photoreactions, it is possible to identify the surface species present on the catalyst under photocatalytic conditions. Furthermore, an initial conditioning of the photocatalyst is observed and attributed to thermal dehydrogenation of methanol to CO species on the cluster. The analysis of the isothermal photoreactions reveals that the photo-oxidation kinetics are not significantly affected by cocatalyst loading. The observed conversion and product distribution of formaldehyde and methyl formate can be rationalized with kinetic parameters gained from the bare TiO2(110) crystal. The work leads to a detailed mechanistic understanding of the surface species and paves the way for an educated microkinetic modeling approach, which may be extended to a variety of noble metal cocatalysts and other TiO2 modifications.

According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α‐H leads to a C−C‐bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α‐position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long‐chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co‐catalysts may also play a decisive role.