Catalytic dehydrogenation of ethylene on size-selected Pt n (n = 4, 7, 8) clusters deposited on the surface of Al 2 O 3 was studied experimentally and theoretically. Clusters were mass-selected, deposited on the alumina support, and probed by a combination of low energy ion scattering, temperature-programmed desorption and reaction of C 2 D 4 and D 2 , X-ray photoelectron spectroscopy, density functional theory, and statistical mechanical theory. Pt 7 is identified as the most catalytically active cluster, while Pt 4 and Pt 8 exhibit comparable activities. The higher activity can be related to the cluster structure and particularly to the distribution of cluster morphologies accessible at the temperatures and coverage with ethylene in catalytic conditions. Specifically, while Pt 7 and Pt 8 on alumina have very similar prismatic global minimum geometries, Pt 7 at higher temperatures also has access to single-layer isomers, which become more and more predominant in the cluster catalyst ensemble upon increasing ethylene coverage. Single-layer isomers feature greater charge transfer from the support and more binding sites that activate ethylene for dehydrogenation rather than hydrogenation or desorption. Size-dependent susceptibility to coking and deactivation was also investigated. Our results show that size-dependent catalytic activity of clusters is not a simple property of single cluster geometry but the average over a statistical ensemble at relevant conditions.

Size--selected supported clusters of transition metals can be remarkable and highly tunable catalysts. A par--ticular example is Pt clusters deposited on alumina, which have been shown to dehydrogenate hydrocarbons in a size--specific manner. 1 Pt 7 , of the three sizes studied, is the most active and therefore like many other catalysts, deactivates by coking during reactions in hydrocarbon--rich environments. Using a combination of experiment and theory, we show that nano--alloying Pt 7 with boron modifies the alkene--binding affinity to reduce coking. From a fundamental perspective, the comparison of experimental and theoretical results shows the importance of considering not simply the most stable clus--ter isomer, but rather the ensemble of accessible structures as it changes in response to temperature and reagent cover--age.

Bimetallic catalysts provide the ability to tune catalytic activity, selectivity, and stability. Model catalysts with size-selected bimetallic clusters on well-defined supports offer a useful platform for studying catalytic mechanisms, however, producing size-selected bimetallic clusters can be challenging. In this study, we present a way to prepare bimetallic model (Pt n B m /alumina) cluster catalysts by depositing size-selected Pt 7 clusters on an alumina thin film, then selectively adding boron by exposure to diborane and heating. The interactions between Pt 7 /alumina and diborane were probed using temperature-programmed desorption/reaction (TPD/R), X-ray photoelectron spectroscopy (XPS), low energy ion scattering (ISS), plane wave density functional theory (PW-DFT), and molecular dynamic (MD) simulations. It was found that the diborane exposure/heating process does result in preferential binding of B in association with the Pt clusters. Borated Pt clusters are of interest because they are known to exhibit reduced affinity to carbon deposition 1 in catalytic dehydrogenation. At high temperatures, theory, in agreement with experiment, shows that boron tends to migrate to sites beneath the Pt clusters forming Pt-B-O suf bonds that anchor the clusters to the alumina support.

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis, through differential AC photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of sub-bandgap states in the TiO 2 , and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly resulting in Pauli blocking, and are thereby undetectable under DC or CW excitation. In the method demonstrated here, the incident light is modulated with an optical chopper while the photocurrent is measured with a lock-in amplifier. Thin (5nm) films of TiO 2 deposited by atomic layer deposition (ALD) on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent photocurrent spectra, with a broad peak centered around 2.0eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak around 2.0eV, corresponding to the sub-band gap emission associated with these defect sites. As a control, alumina (Al 2 O 3) films exhibit no PL or PC over the visible wavelength range. The AC photocurrent plotted as a function of electrode potential, shows a peak around-0.4 to-0.1V vs. NHE, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photo-excitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or 3 the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 µsec. We believe this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.