For heterogeneously catalyzed reactions with more than one key surface intermediate, it is likely that multiphase catalysts have a significant advantage over conventional monophase catalysts since each phase can potentially be adjusted independently to activate a key reaction step. At the same time, our understanding of bifunctional multiphase systems is relatively poor. It is the objective of this paper to significantly enhance our molecular understanding of heterogeneous catalysis at the three-phase boundary (TPB) of a gas-phase, a reducible oxide surface, and a noble metal cluster. In particular, we intend to illustrate the specific role of the TPB in determining the activity and selectivity of TiO2 and CeO2 supported Pt catalysts for the water-gas shift (WGS, CO + H2O -> CO2 + H2) reaction.
We performed an ab initio atomistic thermodynamics analysis of the Pt/TiO2 (110) interface in various gas-phase environments and conclude: (1) In an oxidizing atmosphere, oxygen vacancies in TiO2 should not play any role and the surface Pt atoms will likely be oxidized. (2) Under WGS reaction conditions, formation of oxygen vacancies at the Pt/TiO2 interface is thermodynamically favorable and the Pt clusters are surface oxygen free. Furthermore, surface Pt atoms are covered by CO and only interfacial Pt atoms are available for reaction.
Next, we investigated the mechanism of the WGS at the Pt/TiO2 (110) interface. The redox and carboxyl pathways were studied and the results can be summarized as follows: Pt sites away from the TPB primarily act as a CO reservoir. The oxide support plays an essential role in the O-H bond breaking and H2 formation process. CO poisoning is reduced on Pt atoms at the TPB. Water activation and dissociation at the TPB becomes exothermic and is facile. Finally, there seems to be no significant advantage of the TPB for COOH dissociation which is another key step in the carboxyl pathway on Pt(111) surfaces.