The adsorption of L-alanine on Ni{111} has been studied as a model of enantioselective heterogeneous catalysts. Synchrotron-based X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to determine the chemical state, bond coordination, and out-of-plane orientation of the molecule on the surface. Alanine adsorbs in anionic and zwitterionic forms between 250 and ≈320 K. NEXAFS spectra exhibit a strong angular dependence of the π* resonance associated with the carboxylate group, which is compatible with two distinct orientations with respect to the surface corresponding to the bidentate and tridentate binding modes. Desorption and decomposition begin together at ≈300 K, with decomposition occurring in a multistep process up to ≈450 K. Comparison with previous studies of amino acid adsorption on metal surfaces shows that this is among the lowest decomposition temperatures found so far and lower than typical temperatures used for hydrogenation reactions where modified Ni catalysts are used. 65 With increasing coverage, alanine begins to bind as the zwitterion 66 in a bidentate mode, with surface saturation at 0.25 ML and both 67 forms of alanine coexisting on the surface. Desorption and 68 decomposition are seen to begin together at around 300 K, with 69 decomposition occurring in a multistep process below 450 K. 70 NEXAFS spectra exhibit strong angular dependence of the π* 71 resonances associated with the carboxylate group, allowing the 72 establishment of two distinct orientations with respect to the 73 surface, which are presumed to correspond to the bidentate and 74 tridentate binding modes.
Abstract. The hydrogenation of methyl acetoacetate (MAA) over modified Ni catalysts is one of the most important and best studied reactions in heterogeneous enantioselective catalysis. Yet, very little molecular-level information is available on the adsorption complex of the reactant.Here we report on a combined experimental and theoretical study of MAA adsorption on Ni{111}. XPS shows that the chemisorbed layer is stable up to 250 K; above 250 K decomposition sets in. In ultra-high vacuum conditions, multilayers grow below 150 K. DFT modelling predicts a deprotonated enol species with bidentate coordination on the flat Ni{111} surface. The presence of adatoms on the surface leads to stronger MAA adsorption in comparison with the flat surface, whereby the stabilization energy is high enough for MAA to 1 drive the formation of adatom defects at Ni{111}, assuming the adatoms come from steps.Comparison of experimental XPS and NEXAFS data with theoretical modeling, however, identify the bidentate deprotonated enol on the flat Ni{111} surface as the dominant species at 250 K, indicating that the formation of adatom adsorption complexes is kinetically hindered at low temperatures.
We apply modern synchrotron-based structural techniques to the study of serine adsorbed on the pure and Aumodified intrinsically chiral Cu{531} surface. XPS and NEXAFS data in combination with DFT show that on the pure surface both enantiomers adsorb in l 4 geometries (with de-protonated b-OH groups) at low coverage and in l 3 geometries at saturation coverage. Significantly larger enantiomeric differences are seen for the l 4 geometries, which involve substrate bonds of three side groups of the chiral center, i.e. a three-point interaction. The l 3 adsorption geometry, where only the carboxylate and amino groups form substrate bonds, leads to smaller but still significant enantiomeric differences, both in geometry and the decomposition behavior. When Cu{531} is modified by the deposition of 1 and 2 ML Au the orientations of serine at saturation coverage are significantly different from those on the clean surface. In all cases, however, a l 3 bond coordination is found at saturation involving different numbers of Au atoms, which leads to relatively small enantiomeric differences.
Low-energy and photoemission electron microscopy (LEEM/PEEM) enables the determination of facet planes of polycrystalline surfaces and the study of their chemical composition at the subµm scale. Using these techniques the early oxidation stages of nickel were studied. After exposing the surface to 20 L of oxygen at 373 K a uniform layer of chemisorbed oxygen was found on all facets. After oxygen exposure at 473 -673 K, small NiO crystallites are formed on all facets but not in vicinity of all grain boundaries. The crystallites are separated by areas of bare Ni without significant oxygen coverage.
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