Transition metal carbides as active phase and as support in catalysis: Insights from first principles theoretical modelling

Identificador:  TDX:2619

Handle: https://hdl.handle.net/20.500.11797/TDX2619

Autors:  Asara, Gian Giacomo

Resum:
Carbides of the early transition metals (TMC) exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals. Alcohol conversion, hydrogenation of olefins and many others important reactions demonstrated the applicability of these compounds for industrial processes. It is also known that small noble metal nanoparticles (NMNPs) show high catalytic activity despite of the poor reactivity or inertness of the bulk metal. Additionally, recent investigations pointed out that supporting TMCs polarize the electron density of adsorbed NMNPs increasing the catalytic activity respect to more traditional metal oxide supports. These recent discoveries inspired the work reported in this thesis using state-of-the-art quantum chemical techniques. We studied CO, CO2, H2, H2O molecules adsorbed on TiC and on small gold clusters adsorbed thereon. We considered the (001) extended surface, terraces, monatomic steps and kink defective sites. The reactivity of adsorbed molecules on the clean TiC (001) surface and on two gold clusters, Au4 and Au6, adsorbed thereon were also studied. Energy barriers calculated for methane or formaldehyde formation from syngas, on the clean TiC (001) surface were by far too high and those processes are unviable on the clean support. Gold clusters supported by TiC (001) show higher catalytic activity but the reaction continues to be highly hindered. However water gas shift reaction is predicted to be fast on the Au4/TiC(001) system, overtaking catalysts normally used in industry. Recent experiments show that Ni, Cu and Au clusters are strongly perturbed upon adsorption on TMC resulting in extremely active catalysts. We investigated the interaction of those atoms with the delta phase of the MoC catalyst. The interaction is stronger for the lowest coverage considered, the relaxation of the surface important and the activity is predicted to increase in the order Ni>Cu >Au. Finally, we have studied possible non-polar reconstructions of the (001) surface of Mo2C focusing on the cleavage energy, proportional to the stability of each type of termination. The non-polar reconstructions decreased the calculated cleavage energy, confirming the applicability of the classical Tasker’s concepts for oxides to TMCs.