Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.

Proton adsorption on metallic catalysts is a prerequisite for efficient hydrogen evolution reaction (HER). However, tuning proton adsorption without perturbing metallicity remains a challenge. A Schottky catalyst based on metal-semiconductor junction principles is presented. With metallic MoB, the introduction of n-type semiconductive g-C N induces a vigorous charge transfer across the MoB/g-C N Schottky junction, and increases the local electron density in MoB surface, confirmed by multiple spectroscopic techniques. This Schottky catalyst exhibits a superior HER activity with a low Tafel slope of 46 mV dec and a high exchange current density of 17 μA cm , which is far better than that of pristine MoB. First-principle calculations reveal that the Schottky contact dramatically lowers the kinetic barriers of both proton adsorption and reduction coordinates, therefore benefiting surface hydrogen generation.

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The conversion of ethylene to ethylidyne on Pt(111) has been studied using density functional periodic slab model calculations. Similar to our recent investigation of this reaction on Pd(111), we considered the following three mechanisms: (M1) ethylene f vinyl f ethylidene f ethylidyne; (M2) ethylene f vinyl f vinylidene f ethylidyne; (M3) ethylene f ethyl f ethylidene f ethylidyne. We systematically compared three coverages of the adsorbate, 1/3, 1/4, and 1/9. Our calculations show that the typical barriers of hydrogenationdehydrogenation reactions on Pt(111), 19-92 kJ mol -1 , are slightly lower than those on Pd(111), 25-120 kJ mol -1 . The barriers of direct 1,2-H shift reactions are much higher, above 160 kJ mol -1 . The surface coverage notably affects the relative barriers of the reactions, by up to 30 kJ mol -1 . Mechanisms M1 and M2 are expected to be competitive. As the barriers of the three elementary steps of mechanism M3 are lower or comparable to the rate-limiting barriers of the other two mechanisms, M3 could be operative when a sufficient concentration of surface hydrogen is present. However, at such conditions one expects the formation of ethane rather than that of ethylidene. On the basis of our calculated vibrational frequencies and reaction barriers, we suggest that an intermediate identified in recent vibrational spectroscopic studies of the title reaction is possibly not ethylidene but perhaps vinyl.

On Pd(111), thermal activation of ethylene has been reported to yield ethylidyne. Using more approximate models, a plausible three-step mechanism, ethylene f vinyl f ethylidene f ethylidyne, was recently proposed for this process on the basis of DFT calculations. We employed more elaborate computational models and characterized the thermodynamics and kinetics of the mechanism of ethylene conversion to ethylidyne on Pd(111). We carried out density functional slab-model studies for three coverages of the adsorbate, 1/3, 1/4, and 1/9. The resulting refined potential energy landscape turned out to differ notably from that reported previously: our calculated barriers for the various elementary steps are significantly lower than those of previous studies, and we determined the overall process to be exothermic, in contrast to earlier computational results. We show that the three-step mechanism is thermodynamically and kinetically feasible on Pd(111), with the dehydrogenation of ethylene to vinyl being the rate-limiting step at all coverages considered. Direct conversion of ethylene to ethylidene is unlikely due to a very high barrier. Coverage effects have been found important. At high coverage, the rate-limiting first reaction barrier is ∼50 kJ mol -1 above the desorption energy of ethylene, whereas at low coverages the two energies become comparable.

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We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

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