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

Karina Mathisen,Kate Mcaulay,Nicholas A Spencer,Andrew R Mcfarlane,Justin S J Hargreaves,Said Laassiri,Karsten Granlund Kirste

doi:10.1007/s11244-018-0892-7

Karina Mathisen, Kate Mcaulay + Show 5 more

Open Access

https://doi.org/10.1007/s11244-018-0892-7

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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 H2/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H2/N2 pre-treatment. In situ XAS at the Co K-edge and Re LIII-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 H2-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.

Highlights

Since the development of the Haber Bosch Process at the beginning of the twentieth Century, considerable effort has been made to reduce the severity of the process by developing new efficient catalytic materials operational under milder reaction conditions [1,2,3,4,5]
Results from EXAFS analysis of references prepared by ammonolysis (CoReyNx, CoNx and R eNx) are shown in Table 1 and Figure S.2 confirming that the process has led to the reduction to the metallic state in all three samples
These numbers do confirm bimetallic Co–Re mixing occurs in the sample, as relatively high multiplicity Co–Co and Re–Re absorption pairs are found during refinements, one cannot exclude the possibility that pure Co and Re phases occur in the sample

Summary

Introduction

Since the development of the Haber Bosch Process at the beginning of the twentieth Century, considerable effort has been made to reduce the severity of the process by developing new efficient catalytic materials operational under milder reaction conditions [1,2,3,4,5]. The current industrial catalyst which is based on promoted iron is operated at high pressure Amongst the materials developed as alternatives to the iron based system, Re based catalysts display comparatively high catalytic activity [7]. The catalytic activity of such Re based catalysts was found only to be stabilized when cobalt was added as a promoter to the bulk Re (atomic ratio of 4:1 Re:Co), whereas rapid deactivation over time was observed in the case of the non-promoted system [8]. The stability of the cobalt rhenium phase was correlated in the literature to the formation of a stable active rhenium nitride phase upon the ammonolysis step employed in its preparation. More recent work from McAulay et al [9] has demonstrated that highly active cobalt rhenium catalysts can be prepared without an ammonolysis stage. Post-reaction analysis did not reveal any relevant structural changes in cobalt rhenium upon the different pre-treatments,

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