XPS study of the surface composition of modified nickel-cobalt powder catalysts for enantioselective ethyl acetoacetate hydrogenation

The surface cobalt concentration in the powder bimetallic Ni-Co catalysts (ratio Co/Ni ≈ 0.25) was shown by X-ray photoelectron spectroscopy to be 2–4 times that of the volume content. Both Ni and Co exist in mixed valent states, viz., M⁰ and M²⁺, even after catalyst reduction. After ethyl acetoacetate hydrogenation, the cobalt content on the surface increases 16 times compared to that in the bulk. In addition, the metals are partially oxidized, and the amount of Ni⁰ and Co⁰ in the surface layers of the catalyst decreases 2–3 times. It was assumed that the increase in the cobalt content indicates an increase in the amount of cobalt complexes that involve the modifier, substrate, and reaction product, and which shield the nickel active sites. As a result, the enantioface differentiating ability of the catalyst decreases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2265–2268, November, 2007.