In the past decade, hydrogen evolution from photocatalytic alcohol oxidation on metal-loaded TiO2 has emerged as an active research field. While the presence of a metal cluster co-catalyst is crucial as a H2 recombination center, size and coverage effects on the catalyst performance are not yet comprehensively understood. To some extent, this is due to the fact that common deposition methods do not allow for an independent change in size and coverage, which can be overcome by the use of cluster sources and the deposition of size-selected clusters. This study compares size-selected Ni and Pt clusters as co-catalysts on a TiO2(110) single crystal and the resulting size- and coverage-dependent effects in the photocatalytic hydrogen evolution from alcohols in ultrahigh vacuum (UHV). Larger clusters and higher coverages of Ni enhance the product formation rate, although deactivation over time occurs. In contrast, Pt co-catalysts exhibit a stable and higher activity and size-specific effects have to be taken into account. While H2 evolution is improved by a higher concentration of Pt clusters, an increase in the metal content by the deposition of larger particles can even be detrimental to the performance of the photocatalyst. The acquired overall mechanistic picture is corroborated by H2 formation kinetics from mass spectrometric data. Consequently, for some metals, size effects are relevant for improving the catalytic performance, while for other co-catalyst materials, merely the coverage is decisive. The elucidation of different size and coverage dependencies represents an important step toward a rational catalyst design for photocatalytic hydrogen evolution.

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles.Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10 -5 Torr O2, but orders of magnitudes less effective at higher O2 pressures, where Pt is in an oxidic state.While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.

Metal-free organic perovskites such as [HMDABCO](NH 4 )I 3 ([HMDABCO 2+ ] = 1-methyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium) have recently emerged as sustainable high-performance ferroelectrics. To realize their application as a substitute for metal-based analogues in microelectronic devices, it is mandatory to establish a process for their fabrication in the form of thin films. Here, we investigate different deposition techniques such as spin, spray, and drop coating for their eligibility in the production of crystalline, dense, and smooth [HMDABCO](NH 4 )I 3 thin films. By systematically varying different process parameters, we show how to control the thin-film thickness, influence the preferred crystallite orientation, and obtain dense thin films, which were characterized by grazing incidence X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Our work presents the first deposition of [HMDABCO](NH 4 )I 3 as thin films and provides the experimental and scientific basis for further systematic investigations.