Andrew Crampton

The nature of the nano-catalyzed hydrogenation of ethylene, yielding benchmark information pertaining to the concept of structure sensitivity/insensitivity and its applicability at the bottom of the catalyst particle size-range, is explored with experiments on size-selected Ptn (n = 7--40) clusters soft-landed on MgO, in conjunction with first-principle simulations. As in the case of larger particles both the direct ethylene hydrogenation channel and the parallel hydrogenation--dehydrogenation… Show more

The nature of the nano-catalyzed hydrogenation of ethylene, yielding benchmark information pertaining to the concept of structure sensitivity/insensitivity and its applicability at the bottom of the catalyst particle size-range, is explored with experiments on size-selected Ptn (n = 7--40) clusters soft-landed on MgO, in conjunction with first-principle simulations. As in the case of larger particles both the direct ethylene hydrogenation channel and the parallel hydrogenation--dehydrogenation ethylidyne-producing route must be considered, with the fundamental uncovering that at the < 1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to the structure insensitivity found for larger particles. In this size-regime, the chemical properties can be modulated and tuned by a single atom, reflected by the onset of low temperature hydrogenation at T > 150 K catalyzed by Ptn (n ≥ 10) clusters, with maximum room temperature reactivity observed for Pt13 using a pulsed molecular beam technique. Structure insensitive behavior, inherent for specific cluster sizes at ambient temperatures, can be induced in the more active sizes, e.g. Pt13, by a temperature increase, up to 400 K, which opens dehydrogenation channels leading to ethylidyne formation. This reaction channel was, however found to be attenuated on Pt20, as catalyst activity remained elevated after the 400 K step. Pt30 displayed behavior which can be understood from extrapolating bulk properties to this size range; in particular the calculated d-band center. In the non-scalable sub-nanometer size regime, however, precise control of particle size may be used for atom-by-atom tuning and manipulation of catalyzed hydrogenation activity and selectivity. Show less

Ethylene hydrogenation catalyzed at 300 K by 1--1.5 nm nanoparticles of Ni, Pd and Pt supported on MgO(1 0 0) with a narrow size-distribution, as well as the deactivation under reaction conditions at 400 K, was investigated with pulsed molecular beam experiments. Ni nanoparticles deactivate readily at 300 K, whereas Pd particles deactivate only after pulsing at 400 K, and Pt particles were found to retain hydrogenation activity even after the 400 K heating step. The hydrogenation turnover… Show more

Ethylene hydrogenation catalyzed at 300 K by 1--1.5 nm nanoparticles of Ni, Pd and Pt supported on MgO(1 0 0) with a narrow size-distribution, as well as the deactivation under reaction conditions at 400 K, was investigated with pulsed molecular beam experiments. Ni nanoparticles deactivate readily at 300 K, whereas Pd particles deactivate only after pulsing at 400 K, and Pt particles were found to retain hydrogenation activity even after the 400 K heating step. The hydrogenation turnover frequency normalized to the number of particles exhibited the trend, Pt > Pd > Ni. The activity/deactivation was found to scale with the location of the particles' d-band centroid, ε, with respect to the Fermi energy of the respective metals calculated with density-functional theory. An ε closer to the Fermi level is indicative of a facile deactivation/low activity and an ε farther from the Fermi level is characteristic of higher activity/impeded deactivation. CO adsorption, probed with infrared reflection absorption spectroscopy was used to investigate the clusters before and after the reaction, and the spectral features correlated with the observed catalytic behavior. Show less

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8--15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and… Show more

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8--15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation--dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n≥10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity. Show less

In our previous paper [ Journal of the Electrochemical Society , 155 , P1 (2008) ] we reported on the impact of glass corrosion on establishing the electrocatalytic activity of fuel cell catalysts. It was shown that the leaching of glass constituents into the electrolyte is responsible for insufficiently reproducible measurements of the oxygen reduction reaction as well as the hydrogen oxidation reaction on polycrystalline platinum. In the present report we elucidate which glass constituents… Show more

In our previous paper [ Journal of the Electrochemical Society , 155 , P1 (2008) ] we reported on the impact of glass corrosion on establishing the electrocatalytic activity of fuel cell catalysts. It was shown that the leaching of glass constituents into the electrolyte is responsible for insufficiently reproducible measurements of the oxygen reduction reaction as well as the hydrogen oxidation reaction on polycrystalline platinum. In the present report we elucidate which glass constituents are leached into the electrolyte through the analysis of alkaline electrolytes in contact with Duran glass by inductively coupled plasma optical emission spectroscopy. By adding these constituents, i.e., silicates, borates, aluminates, and lead, separately to the electrolyte, we evaluate their individual impact on electrocatalytic measurements. The results presented in this study help to explain the effects seen in measurements in alkaline electrolyte with glass cells.
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Metal oxide thin films grown on metal single crystals are commonly used to model heterogeneous catalyst supports. The structure and properties of thin silicon dioxide films grown on metal single crystals have only recently been thoroughly characterized and their spectral properties well established. We report the successful growth of a three-dimensional, vitreous silicon dioxide thin film on the Pt(111) surface and reproduce the closed bilayer structure previously reported. The confirmation of… Show more

Metal oxide thin films grown on metal single crystals are commonly used to model heterogeneous catalyst supports. The structure and properties of thin silicon dioxide films grown on metal single crystals have only recently been thoroughly characterized and their spectral properties well established. We report the successful growth of a three-dimensional, vitreous silicon dioxide thin film on the Pt(111) surface and reproduce the closed bilayer structure previously reported. The confirmation of the three-dimensional nature of the film is unequivocally shown by the infrared absorption band at 1252 cm^-1. Temperature-programmed desorption was used to show that this three-dimensional thin film covers the Pt(111) surface to such an extent that its application as a catalyst support for clusters/nanoparticles is possible. The growth of a three-dimensional film was seen to be directly correlated with the amount of oxygen present on the surface after the silicon evaporation process. This excess of oxygen is tentatively attributed to atomic oxygen being generated in the evaporator. The identification of atomic oxygen as a necessary building block for the formation of a three-dimensional thin film opens up new possibilities for thin film growth on metal supports, whereby simply changing the type of oxygen enables thin films with different atomic structures to be synthesized. This is a novel approach to tune the synthesis parameters of thin films to grow a specific structure and expands the options for modeling common amorphous silica supports under ultra-high-vacuum conditions. Show less