Comparison of Palladium and Platinum Water Gas Shift Kinetics Using Density Functional Theory Models

The Water Gas Shift (WGS) reaction can be either thermodynamically or kinetically limited, depending on process conditions. Improved catalysts are of particular interest at low temperatures where kinetic limitations dominate. In this work, density functional theory calculations were performed to calculate the binding energies, reaction energies, and activation barriers for the proposed WGS pathways on Pt and Pd(111). In addition to the previously published pathways, a new reaction involving the concerted formation of the carboxyl intermediate from water and CO is introduced. In general, binding energies and reaction energies are close on both surfaces. However, Pt has lower activation barriers. Since CO is important for WGS, the CO-CO interactions on Pd(111) were studied by the development of a cluster expansion. A cluster expansion is a polynomial function that allows for rapid prediction of the energy if a structure is given. At low coverages, CO adsorbs in HCP Hollow sites. As the CO coverage increases, CO begins to adsorb in FCC Hollow and Atop sites until the saturation coverage of 3/4 ML. Next grand canonical Monte Carlo simulations were performed to determine the coverage-dependent binding energy and temperature programmed desorption spectrum. Finally, a kinetic model was developed to analyze the different WGS reaction pathways, and metals. On both the Pt and Pd surfaces, WGS proceeds through the carboxyl intermediate with water dissociation being the rate determining step, but yields an unfavorably high CO coverage. Next, a coverage dependent binding energy of CO was included in the kinetic model. As the CO binding energy becomes less exothermic, WGS still proceeds through the carboxyl intermediate but carboxyl formation becomes rate determining. Also, the kinetic model shows that Pt is more active than Pd, independent of the CO binding energy. However, the calculated apparent activation barriers and rates do not agree with the experimental results of Pt and Pd supported on Al2O3, which suggest that the support plays an active role in catalyzing the WGS reaction. Since the support plays an important role in WGS catalysis, future studies should attempt to understand the promoting effect of the support and the support-metal interaction.