Combined Experimental and Theoretical Study of Methyl Acetoacetate Adsorption on Ni{100}

The enantioselective
hydrogenation of methyl acetoacetate (MAA)
over modified Ni-based catalysts is a key reaction in the understanding
of enantioselective heterogeneous catalysis as it represents the only
example of this class of reactions catalyzed by base metals. Yet,
there is very little molecular-level information available about the
adsorption complex formed by the reactants on Ni surfaces. Here, we
report a combined experimental and theoretical study of the adsorption
of MAA on the Ni{100} surface. X-ray photoelectron spectroscopy shows
that MAA forms stable multilayers at low temperatures, which desorb
between 200 and 220 K. At higher temperatures a single chemisorbed
layer is formed, which decomposes between 300 and 350 K. Density functional
theory modeling predicts an enolate species with bidentate coordination
as the most stable chemisorbed species. Comparison of photoelectron
spectroscopy and X-ray absorption data with simulations using this
adsorption model show good qualitative and quantitative agreement.
The molecular plane is tilted with respect to the surface plane by
about 50°. This breaking of symmetry provides a mechanism for
the enantioselective hydrogenation.