Reactivity of Transition-Metal Compounds from Electronic Structure

Doctoral thesis, 2009

Transition-metal carbides (TMC's), nitrides, and sulfides belong to the class of materials known as transition-metal compounds (TMX's). Besides having intriguing properties, these materials are relevant for, e.g., growth and catalysis. A fundamental understanding of the reactivity of TMX surfaces is far less established than that of pure transition metal (TM) surfaces. The results of this thesis shed new light on the reactivity of TMX's. Extensive density-functional theory calculations are performed, combining substrates from the set {ScC, TiC, VC, ZrC, NbC, MoC, TaC, WC, TiN} with adsorbates from the set {H, B, C, N, O, F, Al, Si, P, S, Cl, NH, NH2, NH3}. This allows a mapping of adsorption properties, whose characteristic trends are rationalised in terms of a concerted-coupling model (CCM). In the CCM, the adsorbate frontier orbitals interact with two types of surface resonances (SR's), a TM-localised SR (TMSR) and several C-localised SR's (CSR's). The foundation of the CCM is enabled by a systematic analysis of the electronic structure, including various types of density of states, in combination with examinations of the Kohn-Sham orbitals. The developed framework makes it possible to identify a single measurable descriptor, the mean energy of the TMSR, for atomic and molecular adsorption, dissociation, and catalytic activity on TMC surfaces. The generality of this descriptor is demonstrated by applications to TM surfaces, and to ligand and defect effects. The existence of a single descriptor implies that the Bronsted-Evans-Polanyi relation and scaling relations, valuable for screening of new catalysts, apply. One-dimensional metallic surface states localised at the edges of MoS2 nanoparticles are also studied. Based on electronic structure analyses, the existence, origin, and consequences of the experimentally observed magnetism of the nanoparticles under different environmental conditions are discussed. A correlation between the edge electronic structure and the magnetism is established. A screening study of the steam reforming reaction on TMC surfaces is performed using the CCM as a tool. The results show that TMC's provide a wide spectrum of reactivities, from too reactive, via suitable, catalysts to too inert, just as TM's. Also, the importance of chemical bonds for the stability and thus the relevance of adsorption models for understanding the crucial first steps in nucleation and growth of, e.g., TiX/Al2O3 multilayer coatings are shown. Taken together, the findings of this thesis show that the versatility of TMX systems and the understanding of the underlying adsorption mechanisms point towards the possibility of tuning the reactivity of TMX's.