Interaction of Water with TiO2-Supported Small Platinum Nanoclusters: Geometric and Electronic Structure vs Electrocatalytic Activity

Abstract Ptn clusters with n = 1, 2, 6, 8, 9, 10, 13, 19, and 29 atoms supported on titania have been investigated with respect to their electrocatalytic performance of the hydrogen evolution reaction (HER) and compared to the catalytic activity of larger nanoparticles (d ≈ 230 nm), bulk platinum, and the bare TiO2 support. Temperature-programmed desorption measurements of D2O indicate that the Ptn clusters undergo geometric transitions from one to two at n = 9 and from two- to three-layer structures at n = 19 atoms. This allows the number of accessible surface sites to be estimated, making the intrinsic activity of a catalytic active center accessible. The cyclovoltammetric measurements in 0.1 M H2SO4 reveal significant mass transport limitation, some of which could be captured for by considering diffusion current densities using a simple, planar Nernst diffusion layer. This makes it possible to determine the kinetic current densities and, based on knowledge of the number of active centers, to compare and correlate the intrinsic catalytic activity of the clusters with each other and with the behavior of the nanoparticles and bulk sample. However, it should be noted that the values of the kinetic current density are still underestimated, which is particularly true for the nanoparticles and the bulk sample, because mass transport effects have not been fully accounted for. Nevertheless it turned out that the onset potentials indicate an increase in catalytic activity with increasing metallic character of the clusters, which is confirmed by comparison with the energy gaps determined by ultraviolet photoelectron spectroscopy. Accordingly all clusters showed less intrinsic activity than the nanoparticles and bulk sample. In terms of the total mass related current density, however, all clusters with n > 9 were more efficient catalysts due to the more effective use of material. In this context Pt19 stood out as the most active catalyst among the clusters by a factor of 4 compared to the next active cluster Pt29. However, one major problem with the use of clusters is their lack of stability. It has been proven that samples with Pt clusters degrade, with a significant portion of the deposited material detaching from the support during electrocatalysis.