In situ X-ray absorption spectroscopic studies of unsupported and supported CoRe during ammonia synthesis/ammonia decomposition

Abstract

In situ X-ray absorption spectroscopy (XAS) studies are crucial for studying the local environment, oxidation state and speciation of transition metals during thermal pre-treatments and during working conditions of catalysts. This work studies unsupported and supported bimetallic CoRe during hydrogen production via ammonia decomposition with focus on the speciation and synergistic effects of bimetallic CoRe with in situ X-ray Absorption Spectroscopy (XAS). The effect of carrier, pre-treatments and Co:Re-ratio on the CoRe-speciation were investigated

Supporting transition metals in microporous materials may promote bimetallic interaction and prevent particle growth. Cobalt and rhenium were supported in silica aerogels by including the elements in the synthesis during the sol-gel stage. Introduction of rhenium into silica aerogels and microporous supports were found to increase in the presence of cobalt.

In situ XAS studies showed a synergistic effect between cobalt and rhenium which resulted in lower reduction temperatures for both elements. The synergistic relationship between cobalt and rhenium was also observed during the annealing step. Cobalt was found to prevent the volatilisation of rhenium upon annealing while rhenium was promoting the dispersion of particulate CoOx upon annealing. The annealing step further promotes formation of synergistic Co-Re pairs in the structure enhancing reducibility and bimetallic interaction. Annealing prior to reduction and ammonia decomposition, imposed growth limitations of inactive monometallic rhenium at higher metal contents (8.5 wt.% Co and 9.2 wt.%) in the CoRe supported on silica aerogels.

Catalytic activity studies of CoRe-gel showed that the temperatures where the conversion of ammonia was 50% (T50-temperature) was 445°C. Increasing the metal content of cobalt and rhenium with equimolar amounts was found to improve the ammonia conversion from 35% to 95% at 550°C. The microporous and mesoporous CoRe-gel was compared with other more conventional microporous and/or mesoporous materials such as ion exchanged/impregnated microporous materials (SAPO-5, AlPO-5, SAPO-34 and AlPO-34) and mesoporous materials (SBA-15 and mesoporous SAPO). Aerogels containing CoNi and NiRe were prepared in order to evaluate the effect of using nickel instead of cobalt or rhenium.

In this work new insight into the synergistic effect and size control of CoRe supported in silica aerogels for ammonia decomposition has been achieved. Evidence from in situ XAS studies of the effect of thermal pre-treatments concerning CoRe speciation is presented.