<br/>A combination of electron microscopy, X-ray and neutron spectroscopies and computational methods has provided new insights into the species present on the surface of freshly prepared and reduced precious metal (Pd and Pt) catalysts. For the fresh catalysts, the results show that in all cases, at least half of the surface is metallic or nearly so, with the remainder covered by oxygen, largely as hydroxide. Water is also present and is strongly held; weeks of pumping under high vacuum is insufficient to remove it. The hydroxyls are reactive as shown by their reaction with or displacement by CO and can be removed by hydrogenation.<br/>The relative amounts of hydrogen retained by a range of supported palladium catalysts have been investigated by a combination of electron microscopy and spectroscopic techniques, including incoherent inelastic neutron scattering. Contrary to expectation, the hydrogen capacity is not determined solely by the metal particle size, but it is a complex interaction between the particle size and its state of aggregation. The type of support also results in differences in the form of the residual hydrogen: whether it is α- or β-hydride phase, subsurface or in the threefold surface site. Our data on the supported catalysts reinforces what has only been seen previously with palladium black and our computational study provides confirmation of the empirical assignments. We also report the first observation of hydrogen adsorbed in the on-top site at the surface of b-PdH.<br/>While Pt is also a notable hydrogenation catalyst, it differs markedly from the Pd catalysts in that there is no bulk hydride, also surface species are present. Characterizing these has proved to be extraordinarily difficult and the nature of the adsorbed species has been debated for decades. We present new spectra that reproduce what has been seen previously but with better resolution and signal-to-noise ratio. We have observed both the Pt–H stretch and bend modes of the on-top site for the first time. Computational studies (both DFT and AIMD) of a hydrogen covered 1.1 nm Pt nanoparticle have enabled a complete assignment of the spectra and demonstrate that only on-top and bridging sites are present.<br/>

Characterization of the surface species on carbon supported Pd and Pt catalysts

2019 Spring National Meeting

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