(R,R)-tartaric acid on Ni(110):: the dynamic nature of chiral adsorption motifs

Nickel surfaces modified by optically active tartaric acid have been shown to be very effective in heterogeneous enantioselective hydrogenation reactions, but the performance of the catalysts is critically determined by the preparation conditions. Crucially, the enantioselective performance is linked directly to the presence of the chiral modifiers, yet little is known about the nature of the chiral adsorbate on the modified surface. Recently, model chirally modified surfaces created by the adsorption of the well-known modifier, (R,R)-tartaric acid, on a Cu(110) single crystal surface, have been shown to exhibit a variety of surface phases. In this paper, we turn from the Cu(110) surface to the Ni(110) surface, which is the metal most commonly used in the successful catalytic system. The chemical nature and orientation of the adsorbed chiral species were analysed by Fourier transform reflection absorption infrared spectroscopy (FT-RAIRS), while the two-dimensional order of the adlayer was investigated by scanning tunneling microscopy (STM). This work shows that on Ni(110), too, a complex adsorption phase diagram exists, in which different bonding, molecular forms, and orientation of the chiral molecules are adopted, from the intact biacid form at T = 90 K, to the singly deprotonated monotartrate form between the temperature range of 170-270 K, to the coverage-dependent behaviour at T > 270 K, where the doubly deprotonated bitartrate species is preferred at low coverage, while the monotartrate species is preferred at high coverages. This versatility of structural and chemical form and its delicate dependence on preparation conditions give insight into the behaviour of the real catalyst system where small changes in formulation conditions can lead to significant loss of enantiomeric excess (ee). (C) 2004 Elsevier Inc. All rights reserved.