Size and Coverage Effects of Ni and Pt Co-Catalysts in the Photocatalytic Hydrogen Evolution from Methanol on TiO2(110)

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 H-2 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 H-2 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 H-2 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.