Adsorption of Methyl Acetoacetate at Ni{111}: Experiment and Theory

Abstract

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.

Adsorption of Aspartic Acid on Ni{100}: A Combined Experimental and Theoretical Study

“…DFT calculations were performed using plane-wave expansions of the wave functions, as implemented in the VASP code. , We used a functional based on the generalized gradient approximation (GGA) in the form of the revised Perdew–Burke–Ernzerhof (revPBE) exchange–correlation functional . To account for van der Waals interactions, we used the DFT-D3 method with Becke–Johnson damping. , This combination of functional and dispersion correction has been shown to be the most robust among GGA functionals in the description of main group thermochemistry, reaction kinetics, and noncovalent interactions , and has been used successfully in our previous studies of MAA adsorption on Ni surfaces. , However, we have checked that using the standard GGA functional by PBE with the same dispersion correction leads to, essentially, the same results in terms of relative adsorption energies for different configurations (the absolute adsorption energies with PBE-D3 were ∼0.1 eV less negative than those with revPBE-D3). As nickel is a ferromagnetic metal, all calculations included spin polarization.…”

“…We focus our discussion on the relative shifts of the O 1s levels of the oxygen atoms within the Asp molecule. In our previous studies of the MAA/Ni interface, the comparison of the simulated O 1s core levels with experimental XPS allowed us to identify the adsorption geometries. , Here, we chose the reference atom as the one yielding the lowest O 1s BE. In order to allow a visual comparison between experimental and theoretical data, spectra were modeled by sums of Gaussian peaks with equal width [full width at half-maximum (fwhm) = 1.25 eV, estimated from experimental data] and area, centered at the respective BE shifts.…”

“…In recent years, a significant body of research has been devoted to studying the molecular level interactions of these organic compounds with Ni{111}, {100}, and {110} single-crystal surfaces, employing surface science techniques such as temperature-programmed (TP) desorption, infrared spectroscopy, scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. These include studies of just the modifiers (TA, Glu, and Ala) − or reactants (MAA) , and coadsorption studies of reactants and modifiers (MAA with TA, Glu, and Asp) in vacuum and under (wet) reaction conditions. ,,− The latter research, carried out by Baddeley’s group, found strong evidence for a one-to-one interaction between the chiral modifier and MAA. Recent work from our group , has shown that MAA adsorbs on Ni{100} and Ni{111} as deprotonated enolate species, with bidentate coordination through the C–O groups.…”

“…These include studies of just the modifiers (TA, Glu, and Ala) − or reactants (MAA) , and coadsorption studies of reactants and modifiers (MAA with TA, Glu, and Asp) in vacuum and under (wet) reaction conditions. ,,− The latter research, carried out by Baddeley’s group, found strong evidence for a one-to-one interaction between the chiral modifier and MAA. Recent work from our group , has shown that MAA adsorbs on Ni{100} and Ni{111} as deprotonated enolate species, with bidentate coordination through the C–O groups. The molecular plane is tilted with respect to the surface plane, between 49 and 63°, thus breaking the local symmetry of the adsorption complex.…”

“…8,9,19−21 The latter research, carried out by Baddeley's group, found strong evidence for a one-to-one interaction between the chiral modifier and MAA. Recent work from our group 17,18 molecular plane is tilted with respect to the surface plane, between 49 and 63°, thus breaking the local symmetry of the adsorption complex. The role of a chiral modifier could simply be to ensure that the tilt is always in the same direction so that the reaction with surface hydrogen occurs from the same side for all MAA molecules; this would be enough to lead to enantiopure products.…”