The study of chiral adsorption systems using synchrotron- based structural and spectroscopic techniques: stereospecific adsorption of serine on Au-modified chiral Cu{531} surfaces

We apply modern synchrotron-based structural techniques to the study of serine adsorbed on the pure andAumodified intrinsically chiral Cu{531} surface. XPS and NEXAFS data in combination with DFT show that on the pure surface both enantiomers adsorb in l4 geometries (with de-protonated b-OH groups) at low coverage and in l3 geometries at saturation coverage. Significantly larger enantiomeric differences are seen for the l4 geometries, which involve substrate bonds of three side groups of the chiral center, i.e. a three-point interaction. The l3 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 2ML Au the orientations of serine at saturation coverage are significantly different from those on the clean surface. In all cases, however, a l3 bond coordination is found at saturation involving different numbers of Au atoms, which leads to relatively small enantiomeric differences

Adsorption of methyl acetoacetate at Ni{111}: experiment and theory

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 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

Surface chemistry of alanine on Ni{111}

The adsorption of L-alanine on Ni{111} has been studied as a 10 model of enantioselective heterogeneous catalysts. Synchrotron-based X-ray 11 photoelectron spectroscopy and near-edge X-ray absorption fine structure 12 (NEXAFS) spectroscopy were used to determine the chemical state, bond 13 coordination, and out-of-plane orientation of the molecule on the surface. 14 Alanine adsorbs in anionic and zwitterionic forms between 250 and ≈320 K. 15 NEXAFS spectra exhibit a strong angular dependence of the π* resonance 16 associated with the carboxylate group, which is compatible with two distinct 17 orientations with respect to the surface corresponding to the bidentate and 18 tridentate binding modes. Desorption and decomposition begin together at 19 ≈300 K, with decomposition occurring in a multistep process up to ≈450 K. Comparison with previous studies of amino acid 20 adsorption on metal surfaces shows that this is among the lowest decomposition temperatures found so far and lower than typical 21 temperatures used for hydrogenation reactions where modified Ni catalysts are used

Investigations into chiral adsorption systems relevant to asymmetric heterogeneous catalysis on metal surfaces

Two chiral adsorption systems are investigated, adsorption of a known chiral modifier, alanine, onto Ni surfaces and the interaction of chiral homoserine on intrinsically chiral Cu {531}. Alanine is known to act as a chiral modifier in the enantioselective hydrogenation of ~-ketoesters, but the reaction mechanism has yet to be determined. Two approaches are employed to investigate how alanine acts as a chiral modifier. Adsorption onto single crystal Ni{111} and {11O} surfaces is studied by XPS and angle resolved NEXAFS. It was shown that in the chemisorbed layer, alanine adsorbs in an anionic state with a deprotonated carboxylate group on both surfaces. This arrangement was also observed for achiral glycine on Ni {Ill}. A triangular adsorption footprint is created with surface bonds between both carboxylate oxygen atoms and the amino nitrogen. The growth of zwitterionc multi layer alanine is also observed for both systems but on Ni{11O} the multi layer is shown to segregate into alanine crystallites with a Stranskiy-Krastanov-like morphology at 240 K. On Ni {111}, the tilt angle of the carboxylate group from the surface is 45' compared to 55' on the Ni {Il O}, determined by angle resolved NEXAFS. Decomposition of these amino acids is shown to proceed via initial cleavage of the Ca-COO bond to release CO2 followed by cleavage of the Ca-CH3 bond to ultimately produce (Hx)C=N and atomic carbon. On the Ni {11O} surface, these final decomposition products coexist until -700 K whereas on Ni {Ill} only atomic species exist on the surface after 450 K, for both alanine and glycine. Ultimately, the atomic N recombines to form N2 which is detected by TPD at mass 28 and 14 amu. The second approach involved alanine adsorption on polycrystalline Ni. Previous experiments to probe this system had either been performed in UHV on single crystal surfaces, or in solution based studies with high surface area catalyst particles. This complexity gap is a poignant problem in the attempt to understand the mechanistic details of this reaction. Polycrystals were seen as a bridge between these two systems since they offer several different surface terminations and boundaries, which are analogous to reactive, defect sites on catalyst particles. PEEM and LEEM were successfully used to identify the surface facets of polycrystalline Ni and allowed the application of several surface science techniques. The oxidation of poly crystalline Ni is successfully probed using PEEM and LEEM. For oxygen exposure at temperatures of 473 K and higher, spatially resolved NEXAFS spectra clearly identify NiO crystallites in the I urn range with areas of clean Ni surface between them. Certain boundary regions are devoid of crystalIites. For lower temperatures a uniform chemisorbed oxygen layer with a different spectroscopic signature is formed. The same technique was applied to study alanine adsorption. It is shown that alanine is capable of restructuring grain boundaries of polycrystalline Ni, possibly creating chiral facets, where the enantioselective reaction could occur. The adsorption of homoserine on chiral Cu{531} was performed to improve our understanding of the interactions between chiral molecules and chiral substrates, specifically, the influence of longer side chains. XPS and angle resolved NEXAFS experiments were performed to probe enantiomeric differences. Homoserine was found to exist on the surface in an anionic state involving a deprotonated carboxylate group and a neutral amino group. It was found that three surface bonds are always made, one from the amino nitrogen and one from each of the carboxylate oxygen atoms, creating a triangular footprint, typical for a-amino acids. Under certain conditions; 50% saturation coverage of D-homoserine and also 85 and 100% saturation coverage of L- homoserine, the alcohol side chain makes an additional bond to the surface via the oxygen atom, creating a rectangular footprint. Both enantiomers are shown to occupy both adsorption sites equally but a 12' rotational difference in adsorption between the two enantiomers is found to exist for low coverages.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Surface chemistry of alanine on Cu{111}: Adsorption geometry and temperature dependence

Adsorption of l-alanine on the Cu{111} single crystal surface was investigated as a model system for interactions between small chiral modifier molecules and close-packed metal surfaces. Synchrotron-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy are used to determine the chemical state, bond coordination and out-of-plane orientation of the molecule on the surface. Alanine adsorbs in its anionic form at room temperature, whilst at low temperature the overlayer consists of anionic and zwitterionic molecules. NEXAFS spectra exhibit a strong angular dependence of the π ⁎ resonance associated with the carboxylate group, which allows determining the tilt angle of this group with respect to the surface plane (48° ± 2°) at room temperature. Low-energy electron diffraction (LEED) shows a p(2√13x2√13)R13° superstructure with only one domain, which breaks the mirror symmetry of the substrate and, thus, induces global chirality to the surface. Temperature-programmed XPS (TP-XPS) and temperature-programmed desorption (TPD) experiments indicate that the zwitterionic form converts into the anionic species (alaninate) at 293 K. The latter desorbs/decomposes between 435 K and 445 K