An In Situ XAS Study of the Cobalt Rhenium Catalyst for Ammonia Synthesis

作者：Mathisen Karina^*; Kirste Karsten Granlund; Hargreaves Justin S J^*; Laassiri Said; McAulay Kate; McFarlane Andrew R; Spencer Nicholas A

来源：Topics in Catalysis, 2018, 61(3-4): 225-239.

DOI：10.1007/s11244-018-0892-7

• Ammonia
• Cobalt
• Rhenium
• In situ XAS

A cobalt rhenium catalyst active for ammonia synthesis at 400 degrees C and ambient pressure was studied using in situ XAS to elucidate the reducibility and local environment of the two metals during reaction conditions. The ammonia reactivity is greatly affected by the gas mixture used in the pre-treatment step. Following H-2/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H-2/N-2 pre-treatment. In situ XAS at the Co K-edge and Re L-III-edge show that cobalt initiates reduction, undergoing reduction between 225 and 300 degrees C, whereas reduction of rhenium starts at 300 degrees C. The reduction of rhenium is near complete below 400 degrees C, as also confirmed by H-2-TPR measurements. A synergistic co-metal effect is observed for the cobalt rhenium system, as complete reduction of both cobalt and rhenium independently requires higher temperatures. The phases present in the cobalt rhenium catalyst during ammonia production following both pre-treatments are largely bimetallic Co-Re phases, and also monometallic Co and Re phases. The presence of nitrogen during the reduction step strongly promotes mixing of the two metals, and the bimetallic Co-Re phase is believed to be a pre-requisite for activity.

• 出版日期2018-4