(650a) Towards the Design of Sulfur-Tolerant CO2-Reforming Catalysts

Mixed metal oxide catalysts, combining late transition metals (TMs) doped into earth oxides (REOs), offer hydrocarbon conversion (e.g., tar reforming) activities and selectivities that meet or exceed supported metal systems,¹ while also offering continued catalytic performance in the presence of sulfur poisons and sintering agents such as water.² These materials also have potential as sulfur tolerant CO₂-reforming or tri-reforming catalysis. The objectives of the proposed work are to acquire a “proof of principle” data set towards the development of sulfur- and water-tolerant (especially the former) TM-doped rare-earth oxysulfide catalysts for high temperature reforming, and 2) develop a mechanistic rationale for how surface sulfidation alters reforming and coke formation kinetics. Integrated experimental and computational efforts will provide a mechanistic rationale for designing sulfur- and water-tolerant catalysts. Density functional theory (DFT) methods examine C-H and C-C bond activation and coke formation over partially sulfided metal oxide surfaces, identifying materials promising for retaining, or possibly enhancing, conversion in the presence of sulfur. Synthesis, testing, and characterization evaluate the computationally predicted catalyst performance and provide feedback and further hypothesis development to aid the computational efforts.

To date, we have found that Ce-Zr supports at the 1:1 or higher elemental ratio are superior to the Ce-La supports at similar elemental ratios. The latter are relatively inactive. We have also found that coking rates in the absence of water are strongly dependent upon stoichiometric equivalence between CO₂ and CH₄. Structurally, both the Ce-Zr and Ce-La families of supports show no post-mortem evidence of separate TM (Ni, Mn, Fe) or TM oxide phases, at least for TM:REO ratios kept below thresholds determined in past work on tar reforming. Characterizations of the used catalysts have included XANES, XAFS, Raman, XRD and chemisorption techniques.

Extended coking rate measurements by DSC-TGA show that for the TM-doped REO catalysts a coking plateau is attained, while for simpler Co- and Ni-supported Al₂O_3 catalysts the coking rates are nonzero and almost constant. This suggests that regardless of their short-term performance, the latter family of catalysts supported on Al₂O₃ will ultimately deactivate more rapidly.

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)