Atomically dispersed rhodium catalysts supported on xSm 2 O 3 −yCeO 2 − Al 2 O 3 supports were synthesized and applied in the methane steam reforming (MSR) reaction. Scanning transmission electron microscopy and MSR catalytic tests show that the dispersion of the metal, and thus the catalytic performance, depends on the support composition. The 12CeO 2 −Al 2 O 3 support confers partial stability to rhodium, leading to the coexistence of atomically dispersed and nanosized rhodium particles during MSR at 773 K. A reaction mechanism with CH 4 being activated on the atomically dispersed rhodium and CO being formed on the nanoparticles is proposed. A lower rate caused by the formation of carbon species is seen in the presence of atomically dispersed rhodium and in the absence of nanoparticles as CO formation is hindered. KEYWORDS: methane steam reforming, atomically dispersed Rh catalysts, CeO 2 , Sm 2 O 3 , scanning transmission electron microscopy, Rh/Al 2 O 3 -promoted catalysts

a b s t r a c tSorption enhanced CO 2 methanation is a complex process in which the key challenge lies in the combined optimization of the catalyst activity and water adsorption properties of the zeolite support. In the present work, improved nickel-based catalysts with an enhanced water uptake capacity were designed and catalytically investigated. Two different zeolite frameworks were considered as supports for nanostructured Ni, and studied with defined operation parameters. 5Ni/13X shows significantly increased, nearly three-fold higher, operation time in the sorption enhanced CO 2 methanation mode compared to the reference 5Ni/5A, likely due to its higher water sorption capacity. Both catalysts yield comparable CO 2 conversion in conventional CO 2 methanation (without water uptake). Regeneration of the catalysts performance is possible via a drying step between methanation cycles under both reducing and oxidizing atmospheres; however, operation time of 5Ni/13X increases further after drying under air.

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