Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: A trend study of surface electronic factors

摘要

This study explores atomic and molecular adsorption on a number of earlytransition-metal carbides (TMC's) by means of density-functional theorycalculations. Trend studies are conducted with respect to both period and groupin the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC,delta-MoC, TaC, and WC and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3.Trends in adsorption strength are explained in terms of surface electronicfactors, by correlating the calculated adsorption energy values with thecalculated surface electronic structures. The results are rationalized with useof a concerted-coupling model (CCM), which has previously been appliedsuccesfully to the description of adsorption on TiC(111) and TiN(111) surfaces[Solid State Commun. 141, 48 (2007)]. First, the clean TMC(111) surfaces arecharacterized by calculating surface energies, surface relaxations, Badercharges, and surface-localized densities of states (DOS's). Detailedcomparisons between surface and bulk DOS's reveal the existence oftransition-metal localized SR's (TMSR's) in the pseudogap and of severalC-localized SR's (CSR's) in the upper valence band on all considered TMC(111)surfaces. Then, atomic and molecular adsorption energies, geometries, andcharge transfers are presented. An analysis of the adsorbate-induced changes insurface DOS's reveals a presence of both adsorbate--TMSR and adsorbate--CSR'sinteractions, of varying strengths depending on the surface and the adsorbate.These variations are correlated to the variations in adsorption energies. Theresults are used to generalize the content and applications of the previouslyproposed CCM to this larger class of substrates and adsorbates. Implicationsfor other classes of materials, for catalysis, and for other surface processesare discussed.