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

Abstract

A cobalt rhenium catalyst active for ammonia synthesis at 400 °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 °C, whereas reduction of rhenium starts at 300 °C. The reduction of rhenium is near complete below 400 °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.

XAS investigation of silica aerogel supported cobalt rhenium catalysts for ammonia decomposition

“…Linear combinations of the CoRe-gel in 5% ammonia at 600 1C (ESI, † S10) using the as-synthesised CoRe-gel and corresponding Co-/Re-foil show that the active material cannot be expressed as a combination of the two. The white line feature at 7726.5 eV at the Co K-edge was also reported for CoRe 25,54 and CoPt in NaY zeolite, 55 CoPT/C 56 where bimetallic interaction is suggested. Therefore, the XANES strongly indicate bimetallic Co-Re interaction when the CoRe-gel is active for ammonia decomposition.…”

“…This difference was also observed in the unsupported CoRe material. 25,29 The results of the EXAFS analysis (Table 3 and Fig. 5, 8) of pre-treated CoRe-gel supports the MCR-analysis confirming an 80-90% reduction.…”

“…However, the in situ XRD pattern (ESI, † S15) shows small broad reflections confirming that the CoRe-phase is present as nanoparticles and a larger size is expected. 25,61 By comparison, particles (2 nm from TEM) were reported by Guczi et al 53 for CoRe deposited on monolithic SiO 2 . Guczi et al 53,62 also report the requirement of higher temperatures (4450 1C) for complete reduction of the CoRe-phase which is likely to be related to different particle sizes and metal-support interactions.…”

“…23 In the context of bimetallic systems, notwithstanding the fact that rhenium is a scarce metal, it is interesting to note that cobalt together with rhenium has been reported to exhibit a synergistic effect with lower reduction temperatures for both metals being achieved. [24][25][26][27][28] We have observed that unsupported Co-Re possesses ammonia decomposition activities comparable to Ru/CNT 29 which, given that it possesses a very low surface area, suggests that successfully supporting the active phase might lead to further improvement in catalyst performance. Nanoparticles confined in micropores generally experience growth restriction due to pore confinement and enhanced bimetallic interaction.…”