Effect of Ba and K addition and controlled spatial deposition of Rh in Rh/Al2O3 catalysts for CO2 hydrogenation

• Rh/alumina with Ba- and K-additives were synthesized by flame spray pyrolysis.
• Two nozzle system controls Rh deposition on alumina or additive (Ba, K).
• CO2 hydrogenation over Rh/alumina and Ba-containing Rh/alumina afford CH4.
• K addition to Rh/alumina results in CO formation.

The effect of Ba and K addition to Rh/Al2O3 catalysts for CO2 hydrogenation was investigated. Catalysts with preferential deposition of 1 wt% of Rh either on the alumina support or the Ba or K component were prepared by the two nozzle flame spray pyrolysis method and characterized by nitrogen adsorption, CO chemisorption combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and scanning transmission electron microscopy. XRD and thermoanalysis combined with mass spectroscopy indicated that the Ba existed mainly as BaCO3, while K was present in the form of KHCO3 and KOH. DRIFTS combined with CO adsorption measurements revealed a strikingly different CO adsorption behavior of Ba- and K-containing catalysts.The pure Rh/Al2O3 catalyst as well as the Ba-containing catalysts showed a high selectivity to CH4 below 500 °C with a maximum yield at 400 °C. Above 400 °C, the reverse water gas shift reaction leading to CO and H2O started to become dominant, in accordance with thermodynamics. In contrast, the K-containing catalysts produced no CH4, all CO2 was directly converted to CO in the entire temperature range (300–800 °C). Preferential deposition of the Rh on the additive components (Ba, K) or the alumina support had comparably little effect on the catalytic behavior. The CO2 capturing additives, Ba and K, did not result in enhanced catalytic performance, probably due to the relatively high stability of the corresponding carbonates at the optimal reaction temperatures.

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