Cesium-promoted rhenium catalysts supported on alumina for ammonia synthesis

Abstract

Alumina-supported rhenium catalysts were prepared and tested for ammonia synthesis. Ammonium perrhenate was used as the precursor of the supported rhenium catalysts and γ-Al 2O 3 (100 m 2 g −1) was impregnated with this compound in an aqueous solution. NH 4ReO 4/Al 2O 3 was reduced with pure hydrogen at 823 K for 2 h, and then the sample was activated with the reactant gas or pure hydrogen prior to ammonia synthesis. The catalyst, which was activated with the reactant gas, was found to contain not only Re metal, but also Re nitride, which is more active than Re metal. The cesium promoter (Cs:Re = 1:1) was added to the catalyst by the impregnation method; the resulting activity was significantly higher than that of the non-promoted catalyst. H 2 chemisorption and N ads+H 2 TPSR experiments revealed that the Re area was decreased by the addition of cesium, whereas the onset of NH 3 formation during the TPSR experiment was not affected. XRD and elemental analyses revealed that the alkali addition has some drawbacks: the formation of cesium perrhenate and the decomposition of rhenium nitride. The ammonia synthesis rates over supported rhenium catalysts were found to increase under high-pressure. Kinetic studies based on power-law rate expressions indicated that the unpromoted Re surfaces were covered by H ads and NH x,ads . By the addition of cesium, the degree of ammonia poisoning was not changed, but the inhibition by hydrogen was removed. In summary, cesium is identified as a very efficient promoter for supported Re catalysts.