Theoretical Investigation of the Water-Gas Shift Reaction At the Three Phase Boundary of Ceria Supported Platinum Metal Clusters

The Water-Gas Shift (WGS: CO+H2O→CO2+H2) reaction is a key step in hydrogen fuel processing for mobile fuel cell applications. Since the reaction is equilibrium-limited and exothermic, high conversions are favored by low temperatures. However, conventional low-temperature shift catalysts are not active enough. Reducible oxide supported small noble metal clusters have shown excellent activity as low temperature WGS catalysts and it is generally believed that these catalysts are bifunctional. In other words, the reaction occurs at the three-phase boundary (TPB) of the noble metal, the reducible oxide, and the gas phase. The small noble metal cluster adsorbs/activates the CO molecules and the reducible oxide, in this study ceria, activates the water molecules to provide necessary hydroxyl groups in the vicinity of the metal cluster for further reaction. While many experimental observations suggest that the TPB is the active site of ceria supported noble metal clusters, no systematic theoretical investigation of the WGS reaction mechanism at the TPB of these catalysts has been reported that could unambiguously prove this hypothesis. In this computational study, density functional theory (DFT+U) has been used to study the ceria (111) surface and the effect of small platinum clusters on the reducibility of the surface. Then, constraint ab initio thermodynamic calculations have been performed to determine a meaningful catalyst model for a systematic kinetic study of the platinum-ceria interface. Next, different reaction mechanisms have been investigated from first principles and microkinetic reactor models based on parameters obtained from DFT+U and transition state theory are developed to study the effect of temperature and partial pressure of the gas phase environment and to compare various reaction mechanisms to experimental data. To conclude, the importance of the TPB of ceria supported Pt clusters for the WGS reaction is theoretically verified and a redox pathway involving the creation of oxygen vacancies at the Pt/ceria interface is identified as the most dominant reaction pathways at relevant experimental conditions