Abstract
The role of surface oxygen species in the nucleation and reactions of metal nanoparticles on carbon surfaces has been explored using model systems based on graphite supported with DFT calculations. Features in the X-ray photoelectron spectra at characteristic binding energies of 532.6eV, 531.8eV, and 533.5eV were unambiguously assigned to hydroxyl, ketone, and ether groups after selective derivatization. Surfaces treated with nitric acid generate almost exclusively hydroxide groups which on heating to 573K transform into ketones and ethers. Gold nanoparticles deposited from an aurochloric acid solution show a better dispersion on the hydroxylated surface than on either the clean or the ketone-covered surface and whereas on the hydroxylated surface the adsorbed gold was reduced completely to Au0, a small component attributed to Au3+ was present after deposition at the ketone-/ether-covered surface. DFT calculations confirm that gold atoms adsorb more strongly to a hydroxylated step edge than one functionalised with ketone groups. The results give an insight into the effects of the acid washing of carbon catalysts before loading with the active metal components.
Highlights
Carbon surfaces are ubiquitous in heterogeneous catalysis with distinct advantages of weight, cost, and the ease with which functional materials supported by carbons can be recovered
Changes in the C(1s) spectrum are too small to be detected against the background carbon signal but there is a small increase in the N(1s) region with the development of a peak with a surface concentration corresponding to $1.4 Â 1014 cmÀ2 at a binding energy of 400.2 eV indicative of an amine rather than a nitrate
A possible explanation is that the water is not completely desorbed but rather reacts with the lattice in a concerted reaction with the hydrazine reaction; a hydroxyl group would be a possible assignment for the peak at 531.9 eV since its close proximity to the hydrazine group could be sufficient to shift the OH binding energy significantly from that of the hydroxyl group observed after HCl treatment
Summary
Carbon surfaces are ubiquitous in heterogeneous catalysis with distinct advantages of weight, cost, and the ease with which functional materials supported by carbons can be recovered. A great deal of research has been done studying metal deposition on graphite surfaces (Highly Ordered Pyrolytic Graphite, ‘‘HOPG’’) under vacuum where surface functionality is removed [3]. This ignores the role that surface functionality might play in the nucleation, stability, and chemistry of metal nanoparticles on carbon surfaces. We explore whether the chemistry is similar on nitric acid-treated HOPG surfaces and study the deposition onto these surfaces of more catalytically relevant coverages of gold nanoparticles from aqueous solution
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