Ultrahigh-vacuum and ambient-pressure characterisation of nickel model catalysts for enantioselective hydrogenation reactions

Authors: Panayiotis Tsaousis (University of Reading)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: January 2018

Abstract: The adsorption of (R,R) tartaric acid (TA) on clean and oxidised Ni{100}, (S)- alanine and MAA on Ni{100} and the co-adsorption of (S)-alanine and water on Ni{110} was investigated, under UHV conditions, to obtain fundamental insights into the enantioselective sites of the chirally modified nickel catalyst, which causes the asymmetric hydrogenation of β-ketoesters. The TA/Ni{100} and (S)- alanine/Ni{100} system was also investigated under ambient pressure conditions. The characterisation of these adsorbed complexes was performed using XPS, NEXAFS, TPD, LEED and DFT. The temperature of the crystal, the dosing rate of the TA molecule and its surface coverage influence the chemical state and adsorption geometry of TA on Ni{100} (µ4/TA2- or µ2/µ3/HTA- ) with the latter being favoured, also, at elevated pressures of H2 and H2O. Deposition of TA on oxidised Ni{100}, causes the formation of tartrate species which fully decompose on the nickel surface at T>650 K (200 degrees higher than on clean Ni{100}). (S)-alanine chemisorbs on Ni{100} and Ni{110}, in its anionic and neutral form with coexistence of zwitterionic species which might not be a part of the chemisorbed layer, since these zwitterionic species are dominating the multilayer regime. The presence of PH2= 6.3 mbar destabilises thermally the alanine molecule on Ni{100} and cause the formation of neutral and (perhaps zwitterionic species) of alanine. The presence of multilayer water does not influence the decomposition temperature of alanine on Ni{110} (Tdecomposition≈400-420 K) but causes the formation of zwitterionic species. Finally DFT, XPS and NEXAFS results suggest that MAA adsorbs on Ni{100} in a tilted bidentate enolate geometry. The full decomposition of MAA on Ni{100} occurs at ∼ 350 K. The contribution of these studies into the understanding of the mechanism of the chiral modification of the nickel catalyst is thoroughly discussed in the thesis.

Subject Areas: Chemistry

Instruments: B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS

Other Facilities: Elettra; MAX-Lab; BESSY II

Added On: 13/08/2020 09:19

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