Catalyst size matters: Tuning the molecular mechanism of the water–gas shift reaction on titanium carbide based compounds

The molecular mechanism of the water–gas shift reaction catalyzed by titanium carbide compounds was studied using a density functional approach. Three different catalyst models have been considered: the extended TiC(001) surface, the Ti8C12 MetCar, and a Ti14C13 nanoparticle. Adsorption of reactants, intermediates, and products occurs on different sites, demonstrating the chemical versatility of the TiC substrates. Thus, adsorption energies depend not only on the existence of low-coordinated sites, but also on the nature of atoms involved in the adsorption site. The two most likely molecular mechanisms, redox and associative, were considered. The first of these mechanisms involves complete water dissociation, whereas the second involves formation of the carboxyl (OCOH) intermediate. The catalytic activity was found to be highest for the TiC(001) surface, due to the overly strong adsorption of reactants and products on either Ti14C13 or Ti8C12. This has important consequences for the underlying chemistry, as evidenced by the corresponding reaction energy profiles, which show that the redox mechanism is the preferred route for the reaction occurring on the nanoparticles, whereas the carboxyl formation route is preferred for the reaction occurring above the TiC(001) surface. However, the calculated reaction rate constants indicate that the reaction will hardly occur on the former, whereas it is quite feasible on the latter. The present study suggests that TiC and similar transition metal carbides can be good catalysts for the water–gas shift reaction and can be potential substitutes for current low-temperature catalysts. In addition, the results point to a possible tuning to control the particle size or rate of steps.