(753b) Combined DFT and Microkinetic Modeling Study of the Water-Gas Shift Reaction At the Three Phase Boundary of Pt/TiO2 Catalysts

The objective of this work is 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 supported Pt catalysts for the water-gas shift (WGS, CO + H₂O ↔ CO₂ + H₂) reaction. Plane wave DFT calculations with periodic slab models have been carried out using the VASP program. Prior to the mechanistic study, we performed a systematic ab initio atomistic thermodynamics study of the Pt/oxide interface in order to identify the structure and composition of the catalytic site under WGS reaction conditions. The insights obtained from this analysis were used to design the catalyst model for the mechanistic study. The mechanism of WGS reaction at the Pt/TiO₂ interface has then been investigated by considering the following pathways: (i) redox pathway, (ii) associative carboxyl pathway, and (iii) associative carboxyl with redox regeneration route. Analysis of a microkinetic model containing all these pathways indicated the CO promoted redox pathway to be the most favorable pathway with the lowest activation barrier and highest rate. The calculated apparent activation barrier (9.0 kcal/mol) for this pathway is in good agreement with the experimental value of 10.8 kcal/mol. Campbell’s degree of rate control analysis predicted that the water dissociation process at the oxygen vacancy is rate-limiting. In general, this study revealed that the Pt sites away from the TPB likely act primarily as a CO reservoir and the oxide support plays an essential role in O-H bond breaking and H-atom transfer to the metal surface. But overall, the TPB seems to primarily act as a unique site with unique metal and oxide sites.