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.

An atomic layer deposition process is used to modify size-selected Pt7/alumina model catalysts by Sn addition, both before and after Pt7 cluster deposition. Surface science methods are used to probe the effects of Sn-modification on the electronic properties, reactivity, and morphology of the clusters. Sn addition, either before or after cluster deposition, is found to strongly affect the binding properties of a model alkene, ethylene, changing the number and type of binding sites, and suppressing decomposition leading to carbon deposition and poisoning of the catalyst. Density functional theory on a model system, Pt4Sn3/alumina, shows that the Sn and Pt atoms are mixed, forming alloy clusters with substantial electron transfer from Sn to Pt. The presence of Sn also makes all the thermally accessible structures closed shell, such that ethylene binds only by π-bonding to a single Pt atom. The Sn-modified catalysts are quite stable in repeated ethylene temperature programmed reaction experiments, suggesting that the presence of Sn also reduces the tendency of the sub-nano clusters to undergo thermal sintering.

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.

In most electrochemical syntheses, reactions are happening at or near the electrode surface. For catalyzed reactions, ideally, the electrode surface would solely contain the catalyst, which then simplifies purification and lowers the amount of catalyst needed. Here, a new strategy involving phthalocyanines (Pc) to immobilize catalysts onto carbon electrode surfaces is presented. The large π structure of the Pc enables adsorption to the sp2-structure of graphitic carbon. TEMPO-modified Pc were chosen as a proof of concept to test the new immobilization strategy. It was found that the TEMPO-Pc derivatives functioned similarly or better than the widely used pyrene adsorption method. Interestingly, the new TEMPO-Pc catalyst appears to facilitate a cascade reaction involving both the anode and the cathode. The first step is the generation of an aryl aldehyde (anode) followed by the reduction of the aryl aldehyde in a pinacol-type coupling reaction at the cathode. The last step is the oxidation of a hydrobenzoin to create benzil. This work demonstrates the unique ability of electrochemistry and bifunctional catalysts to enable multistep chemical transformations, performing both reductive and oxidative transformations in one pot.