Theoretical Analysis of the Adsorption of Late Transition-Metal Atoms on the (001) Surface of Early Transition-Metal Carbides

The interaction of atoms of Groups 9, 10, and 1 1 with the (001) surface of TiC, ZrC, VC, and delta-MoC has been studied by means of periodic density functional calculations using slab models. The calculated values of the adsorption energy are rather large, especially for Groups 9 and 10 elements (E-ads = 3-6 eV), but without clear trends along the series. Nevertheless, the analysis of the interaction at different sites indicates that the adsorbed atoms will be relatively mobile. Many of the admetals are electronically perturbed upon interaction with the carbide surfaces. Co, Ni, Cu, and Rh adatoms get positively or negatively charged, depending on the nature of the carbide substrate. Ir, Pd, Pt, and Au adatoms are always negatively charged. An analysis of the Bader charges for the most stable sites provides strong evidence that the most negative charge on the adatoms corresponds to the interaction with ZrC, followed by TiC. In the case of VC and delta-MoC, the charge on the adsorbed atoms may he slightly positive and of the same order for both carbides. The effect of the underlying carbide is large; with ZrC and TiC being predicted as the supports with the largest effect on the electronic structure of the adsorbed atoms with direct implications for the use of these systems in catalysis.