(406b) Characterization of Ni-Substituted Pyrochlore Catalysts for the Dry Reforming of CH4

Authors: 
Haynes, D. J., National Energy Technology Laboratory
Shekhawat, D., National Energy Technology Laboratory
Berry, D., Department of Energy
Spivey, J., Louisiana State University
Pyrochlores are mixed metal oxides that have proven to be effective catalysts for the reforming of hydrocarbon fuels. These materials are thermally stable at the demanding conditions associated with fuel reforming, and have a modifiable structure which can incorporate various dopants to improve their catalytic activity. One example is a lanthanum zirconate pyrochlore (La-Zr). The Zr framework has been partially substituted by Rh to disperse the catalytic metal into small and stable clusters that are resistant to both carbon and sulfur poisoning [1]. Furthermore, alkaline metals (e.g. Ca) can be doped into the A-site for La to introduce oxygen vacancies which can increase lattice oxygen mobility, while also providing surface basicity that promotes the gasification of carbon.

Ni is an attractive catalytic reforming metal, because of its well-known specific activity, and low cost. However, Ni is problematic due to its low sintering temperature, and propensity to form carbon. This study will evaluate the substitution of different levels of Ni (1 vs 3 wt%) into a La-Zr pyrochlore to determine whether the pyrochlore structure is able to stabilize the Ni under conditions required for dry reforming of CH4. The materials will be synthesized by a variation of the Pechini method [2], which is a well-known procedure to obtain mixed oxide solid solutions. The materials will be characterized by various methods to determine the location of the Ni, and how this will relate activity stability, and particularly carbon deposition. Traditional characterization methods like XRD, temperature programmed reduction, BET, and Raman are used. In addition, an analysis using by Cameca using a LEAP 5000 atom probe provided 3-D images with corresponding compositional analysis on each Ni containing material. The atom probe results (see Figure 1 for results on the 1 wt% substituted catalyst), and XRD results both show Ni is not soluble in the pyrochlore structure (for either weight loading). Instead, crystallization elicits the exsolution of Ni to the surface and grain-boundary regions of the pyrochlore, which forms NiO. At the elevated temperatures, and with high enough Ni loading (3 wt%), some of the Ni forms pocket of mixed-phases with the excess La (e.g. LaNiO3). The Ni distribution at the surface and grain boundary regions suggests these enriched regions with metallic Ni would be the active component of the catalyst for dry reforming.