Mixed gold/platinum group metal supported catalysts have recently demonstrated their efficiency for catalyzing various reactions [1]. Moreover, Pd-Au is presently the sole gold-based catalyst used in a large-scale chemical process, namely the production of vinyl acetate monomer.

In this study, a Pd₇₀Au₃₀(111) single-crystal surface has been used as a model catalytic system. From low energy electron diffraction, Auger electron spectroscopy and low energy ion scattering spectroscopy, this surface was found unreconstructed and its first atomic layer composed of 75 at.% Au [2]. The reactivity of Pd and Pd-Au surfaces was investigated in a static reaction cell (pressures up to 100 Torr) coupled to an ultra-high vacuum preparation setup. In the case of the CO oxidation reaction, gold has a negative effect on the Pd activity, by lowering the probability of oxygen chemisorption [3]. Besides, a study of the H₂+O₂ reaction reveals that the activity and stability differences between Pd and Pd-Au crystals also arise from their different abilities to absorb hydrogen and form surface hydrides [4].

From a practical point of view, the most interesting effect of the addition of Au to Pd was observed for the hydrogenation of 1,3-butadiene (C₄H₆). The presence of gold is seen to increase the selectivity toward butenes (C₄H₈) up to almost 100% over the whole range of butadiene conversions, and to significantly lower the rate of (unwanted) n-butane (C₄H₁₀) formation from butene hydrogenation [2]. On the basis of density functional theory calculations (VASP program), the adsorption structures of butadiene and its partially hydrogenated products (butenes) have been modeled on various Pd₃Au(111) surfaces, in order to rationalize these results. In realistic conditions of temperature and pressure, the DFT calculations indicate that the adsorption of butenes is more destabilized by the increased surface content of gold than the adsorption of butadiene. Hence, the change of selectivity has been associated with the higher ability of Pd-Au surfaces to desorb the partially hydrogenated products, hence lowering the butene hydrogenation rate.

References

[1] D.T. Thompson, Platinum Metals Rev. 48 (2004) 169

[2] L. Piccolo, A. Piednoir, J.C. Bertolini, Surf. Sci. 592 (2005) 169

[3] A. Piednoir, M.A. Languille, L. Piccolo, A. Valcarcel, F.J.C.S. Aires, J.C. Bertolini, Catal. Lett. 114 (2007) 110

[4] L. Piccolo, A. Piednoir, J.C. Bertolini, Surf. Sci. 600 (2006) 4211