Surface Species in Photocatalytic Methanol Reforming on Pt/TiO₂(110): Learning from Surface Science Experiments for Catalytically Relevant Conditions

Abstract

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.

Light‐Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

“…Both the two possible mechanisms have a same net reaction, that is, one photon absorbed by the photocatalyst generates one H 2 molecule and the corresponding carbonyl compound. This is in agreement with previously reported results [54,71] and the current‐doubling effect [55–58] …”

“…Report regarding light‐driven splitting of liquid alcohols can be tracked back to the pioneering work of Pichat and co‐workers in 1981 [62] . Since then, various semiconductor‐based photocatalysts have been developed, and most of them are metal‐modified titanium dioxide (TiO 2 ) which are only active under irradiation of ultraviolet (UV) light [54,62–73] . One potential issue regarding using UV light is that undesirable by‐products might be obtained due to the high energy of the UV light.…”

“…Loading metal particles on the surfaces of semiconductors can significantly enhance the activity of the pure semiconductors in LDAS, which could be attributed to the formation of the Schottky barrier [46,48] and the catalytic properties of the metals. The metal particles, also called co‐catalysts, are transition metals such as Pt, [32,65,66,71,74,75] Pd, [32,74,75] Ru, [32,75] Au, [67,69] Ni [29,30,63,73] . Their crystal structures are usually face‐centered cubic (fcc) structures (Figure 2).…”

“…Rutile TiO 2 (110) is a readily available, one of the best understood crystalline materials (Figure 4); [76–79] therefore, it has been widely used as the semiconductor to elucidate fundamental mechanisms in LDAS [54,65,71–73,80–82] . Kollmannsberger and co‐workers investigated light‐driven splitting of methanol, ethanol, cyclohexanol, benzyl alcohol, and tert‐butanol on Pt cluster‐modified TiO 2 (110) single crystal under ultra‐high vacuum (UHV).…”

Chemical reactions on surfaces for applications in catalysis, gas sensing, adsorption-assisted desalination and Li-ion batteries: opportunities and challenges for surface science

“…132 On the other hand, the use of surface-science techniques could provide useful insights for a detailed comprehension of the photocatalytic processes. 133 Researchers focused their attention on the modification of 2D materials for improving the photocatalytic efficiency, including the design of the structure and morphology, 134,135 doping and vacancy engineering, 136 and integration with other semiconductors or metals. 137,138 Surface science can play a pivotal role in terms of the analysis of photoactive sites and to unveil reaction mechanisms and intermediates, as well as the optimal operational conditions to get the final conversion.…”