(55c) Optimization of Pyrochlore Catalysts for the Dry Reforming of Methane

The dry reforming of methane (DRM) using CO₂ has long been considered a viable method for converting methane from geologic or biological sources into syngas, which can then been readily converted to liquid fuels or used in the production of polymers and related chemicals.  More recently this reaction has gained greater attention because it involves the conversion of two green house gases into useful fuels and chemicals and because it offers the possibility of converting land locked methane gas into 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.  In order to determine which combination of A and B metals 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 reactions on these catalyst materials.  This combinatorial computational study is guiding experimental efforts to pyrochlore catalyst materials with enhanced activity for syngas production and lower rates of catalyst deactivation.