(643a) Optimization of Pyrochlore Catalysts for the Dry Reforming of Methane

The dry reforming of methane (DRM) using CO₂has long been considered a viable method for converting methane from geologic or biological sources into syngas, which can then be readily used in the production of a variety of chemicals and particularly liquid fuels that can more readily be shipped via pipeline.

   Though dry reforming holds great promise, the high temperatures required for the reaction have made it very difficult to find catalysts that exhibit high activity for extended periods.  Several factors often lead to the deactivation of these catalysts: the sintering of active metals, the structural rearrangement of the catalyst support causing a reduction in surface area, and the accumulation of carbon on the catalyst surface. To date, many catalyst materials have been investigated for this reaction; for example, unsupported transition metal carbides and sulfides, supported group VIII metals, and more recently perovskites and hydrotalcites have received attention. In this study, however, we have chosen to develop optimized pyrochlore catalyst materials.

   Pyrochlores are crystalline oxides having high thermal stability and a general formula of A₂B₂O₇, where A represents a rare-earth metal and B represents a transition metal. Initial experimental efforts by others showed that pyrochlores are active for DRM but the tested catalysts exhibited poor long term stability; however, more recent data suggests that this trend in deactivation may not be applicable to all pyrochlores. La₂Zr₂O₇(LZ) is a pyrochlore structure which has shown good long term stability, so that efforts have been made to tailor its catalytic properties, showing Rh as a promising dopant to enhance catalytic performance for DRM. In order to determine which concentration of Rh dopant yields a pyrochlore with the greatest stability and activity, we are using first principles methods employing Density Functional Theory (DFT) to calculate the structural stability, localized charge, vacancy distribution, and transition state energies for the reactions on this catalyst material. To date, DFT simulations are reported for pyrochlore structures but most of them deal with bulk properties of these catalysts. To our knowledge, the work done by Mantz (2011) is the only one that deals with the interactions of a species with pyrochlore surfaces.

   Preliminary results on surface energy for the 2% Rh doped LZ pyrochlore have shown the planes (011) and (111) as the most stable thermodynamically. Thus, adsorption energies for key species of the reaction mechanism are computed for both planes, showing the Rh as a prevalent active site on the plane (111). Currently, the interactions of the species with this surface are being studied which leads to obtain a micro-kinetic model that describes the reaction mechanism. This model will allow us to validate the simulation results against experimental data and then proceed to suggest ways to enhance activity for syngas production and lower rates of catalyst deactivation.