(425f) Ternary Rare-Earth – Transition Metal Catalysts for Dry Reforming of Methane, Characterization of Final Structures and Sulfur Tolerance Evaluation

Rare earth oxide (REO) catalysts, doped with transition metals offer hydrocarbon conversion (e.g., dry and tar reforming) activities and selectivities that meet or exceed many other supported metal systems.¹ They also offer superior stability in the presence of sulfur poisons and sintering agents such as water.² We have explored the coking, sintering and sulfur tolerance of candidate materials for dry reforming, while also developing a mechanistic rationale for how surface sulfidation alters reforming and coke formation kinetics. Integrated experimental and computational efforts are furthering the design of next generation catalysts. Density functional theory (DFT) methods examine C-H and C-C bond activation and coke formation. Synthesis, testing, and characterization evaluate the computationally predicted catalyst performance and provide feedback and further hypothesis development to aid the computational efforts.

A rapid screening method based on DSC/TGA was established to identify the best metal additives (Ni, Ni/Co, Ru, Mn, Fe, etc) to the Ce/Zr and Ce/La systems based on simultaneous measurement of reforming and coking rates. Following rapid screening, select catalysts were examined at longer times on stream. To date, we have found that both Ni and Ni-Co containing catalysts outperform other dopant metals (Ru, Mn, Fe, etc.) in activity and coking resistance. Both can also maintain their activity over several days testing. Using Co with Ni as TM additives (at near 1:1 ratio) results in further Ce reduction to Ce³⁺, and lower rates of catalyst deactivation. Catalysts containing Ce-La oxides lacked practicality, partly due to excess reverse water-gas shift. Catalysts containing Ce-Zr oxides fared better, with Ce/Zr = 3 (molar) giving the best performance for Ni-based catalysts.

While Co is not needed to maintain activity under sulfur-free conditions, it does appear to be crucial to sulfur tolerance (tested 0.07 and 20 ppm sulfur in the feed). XANES on spent catalysts shows that while the bulk oxidation state of Ni in Ni/Co/Ce/Zr catalysts is reduced during the dry reforming reaction, to near zero-valent state, this bulk reduction is not accompanied by a change in activity. This in turn suggests that the Ni active sites are a distinct minority, associated with strong interaction with the Ce/Zr oxide. Other characterization evidence (XPS, XRD, chemisorption, Raman) supports this hypothesis.

1. R. Li, A. Roy, J. Bridges and K.M. Dooley, “Tar Reforming in Model Gasifier Effluents: Transition Metal/Rare Earth Oxide Catalysts,” Ind. Eng. Chem. Res. 53, 7999−8011 (2014)
2. M. Krcha, K. Dooley, and M. Janik, “Alkane Reforming on Partially Sulfided CeO₂ (1 1 1) Surfaces,” J. Catal., 330, 167-176 (2015)