(710g) Reaction Mechanism for the Dry Reforming of Methane on Rh Doped Pyrochlore Catalysts: A Combined DFT and Ssitka Approach

The dry reforming of methane (DRM) using CO₂has long been considered a viable method for converting methane from geological 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 DRM 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 study 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 suggest 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 the role of Rh in the reaction performance and understand how the reaction proceeds, we are using first principles methods employing Density Functional Theory (DFT) and Steady State Isotopic Transient Kinetic Analysis (SSITKA). Computational methods are used to analyze structural stability, species adsorption, and calculate transition state energies for the reactions on this catalyst material, so that the main reaction pathway can be elucidated. SSITKA was used to identify carbon migration among the reactants and products, quantify the surface residence time and the concentration of active surface intermediates; thus, the reaction mechanism for DRM on this catalyst surface is explained in detail using both computational and experimental tools.

   Simulations show that inclusion of Rh decreases activation barriers, including the barrier for the rate limiting CHO dehydrogenation step, which makes the plane (111) catalytically active for DRM. Results also show that the limiting reaction step is on the CH₄dehydrogenation path, which agrees with experimental observations. In addition, SSITKA experiments confirm the migration of carbon species amongst reactants which can be related to the findings from DFT simulations. The calculated surface residence times ( < 1 s) evidence the low coverage of surface intermediates during reaction. Furthermore, we were able to correlate this surface residence times with the bed temperature during reaction.

   Computational data are employed in a micro-kinetic model for a batch reactor, which outputs the partial pressure profile for the gas phase species and the coverage profile for the adsorbates with respect to residence time. This model provides then a deeper understanding of the DRM reaction mechanism on the catalyst.