Towards a Chemisorption Model for Transition-Metal Nitrides and Carbides from Density-Functional Theory: A TiN and TiC Case Study
Abstract: Since the 1960's, a number of models for chemisorption on metals has been presented, most notably the Anderson-Grimley-Newns model for metals and the d-band model of Hammer and Nørskov for transition metals. The intention of this thesis is to explore the adsorption on transition-metal nitrides and carbides, technologically interesting materials (in, e.g., cutting tools, microelectronics, and bio-implants, and as promising catalyst materials) due to their combination of metallicity, covalency, and ionicity. The aim of this study is to identify and understand the key microscopic mechanisms behind the adsorption on the polar and reactive (111) surfaces of titanium nitride and carbide, which have long been considered prototypes for the early transition-metal nitrides and carbides. The chemisorption of atoms from the first three periods in the periodic table (H, B, C, N, O, F, Al, Si, P, S, and Cl) on the TiN(111) surface is studied in a systematic manner by density-functional theory (DFT) and compared with corresponding results on TiC(111). The adsorption on the TiX's (X = C, N) is found to be more complex and versatile than on transition-metal surfaces. The trends in adsorption energies Eads have similar characteristics on both the Ti-terminated TiX(111) surfaces. The Eads trends for period 2 and 3 adatoms show a great variation and are found to vary along the periods in a Sabatier-like pyramidic way, with an extraordinarily strong chemisorption for the O atom. The preference for adsorption on TiC vs. TiN varies depending on the adsorbate element, with TiC showing a stronger chemisorption than TiN for all elements except B and Al. This variety in the calculated adsorption energies is explained with the concerted-coupling model (CCM), which originally was proposed for adsorption on the Ti-terminated TiC(111) surface and here is shown to be generalisable to adsorption on Ti-terminated TiN(111). The model describes the chemisorption to be based on the concerted action of two different types of couplings: (i) between the adatom state and a Ti-localised surface resonance (TiSR), and (ii) between the adatom state and X-localised surface resonances (XSR's). The CCM is also able to explain the similarities and differences in the Eads trends between TiN(111) and TiC(111). In addition, it is shown that the theoretical stabilities in vacuum of the N- and Ti-terminated TiN(111) surfaces are very close. Therefore, adsorption on the N-terminated TiN(111) surface is also considered, revealing an ``inverted'' pyramid-like adsorption energy trend for period 2 and 3 adatoms and indicating a concerted-coupling mechanism on this surface as well.
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