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.

In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110).

The plasmonic behavior of size-selected supported silver clusters is studied by surface second harmonic generation spectroscopy for the first time. A blue shift of ∼0.2 eV in the plasmon resonance is observed with decreasing cluster size from Ag55 to Ag9. In addition to the general blue shift, a nonscalable size-dependence is also observed in plasmonic behavior of Ag nanoclusters, which is attributed to varying structural properties of the clusters. The results are in quantitative agreement with a hybrid theoretical model based on Mie theory and the existing DFT calculations.

Mechanisms in heterogeneous photocatalysis have traditionally been interpreted by the band-structure model and analogously to electrochemistry. This has led to the establishment of ‘band-engineering’ as a leading principle for the discovery of more efficient photocatalysts. In such a picture, mainly thermodynamic aspects are taken into account, while kinetics are often ignored. This holds in particular for chemical kinetics, which are, other than those for charge carrier dynamics, often not at all considered for the interpretation of the catalysts’ photocatalytic performance. However, while being usually neglected in photocatalyis, they are a traditional and powerful tool in thermal catalysis and are still applied with great success in this field. While surface science studies made substantial contributes to thermal catalysis, analogous studies in heterogeneous photocatalysis still play only a minor role. In this review, the authors show that the photo-physics of defined materials in well-defined environments can be correlated with photochemical events on a surface, highlighting the importance of well-characterized semiconductors for the interpretation of mechanisms in heterogeneous photochemistry. The work focuses on contributions from surface science, which were obtained for the model system of a titania single crystal and alcohol photo-reforming. It is demonstrated that only surface science studies have so far enabled the elucidation of molecularly precise reaction mechanisms, the determination of reaction intermediates and assignment of reactive sites. As the identification of these properties remain major prerequisites for a breakthrough in photocatalysis research, the work also discusses the implications of the findings for applied systems. In general, the results from surface science demonstrate that photocatalytic systems shall also be approached by a perspective originating from heterogeneous catalysis rather than solely from an electrochemical point of view.

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.

In this ultrahigh vacuum study, temperature-programmed desorption, Auger electron spectroscopy, and ex-situ atomic force microscopy are used to evaluate the surface chemistry of ethanol on the GaN(0001) surface. Ethanol undergoes dehydration and dehydrogenation reactions on the GaN(0001) surface to a larger extend than on the TiO 2 (110) surface. This enhanced reactivity is attributed to a higher amount of metal-bound ethoxy. In addition, molecular H 2 has been identified as a byproduct of the ethanol dehydrogenation to acetaldehyde. We attribute the reactivity, including the formation of molecular hydrogen, to the combination of wurtzite structure and nitride chemistry, since surface amines are considered to be less stable than surface hydroxyls on other model oxides.

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.