The active phase of iron catalysts for acetonitrile synthesis

In situ Mossbauer spectroscopy has been used to examine the active phase of two different reduced 4% Fe/SiO2 catalysts for acetonitrile synthesis from CO, H-2, and NH3. The Fe-0-containing, silica-supported catalyst starts wi th a finite initial activity and displays a maximum in activity with time t hat tracks the amount of the epsilon'-Fe2.2C phase, indicating it as the ac tive phase of the catalyst. The silica-supported catalyst containing Fe2+ o nly starts with a low activity and slowly rises to a stable activity but sh ows little change in the bulk structure of the catalyst. In this case, much of the change occurs on the surface of the catalyst and could not be monit ored with Mossbauer spectroscopy. An increase in the reducibility of iron w ith time was confirmed, however, with H-2 desorption experiments. For reduc ed 11.9% Fe/carbon, also known to have epsilon'-Fe2.2C as the active phase for acetonitrile synthesis, spectra collected at cryogenic temperatures aft er 3 and 10 min of reaction indicate the following order in carbide formati on: Fe-0-->epsilon'-->chi-->theta. Transient mass spectrometry used to moni tor the product stream in the first 5 min of reaction indicates that carbid e formation precedes product formation. Reactions carried out over rereduce d, postreaction 11.9% Fe/carbon indicate large-scale sintering during the f irst cycle of reaction. Mossbauer studies also show a rapid rearrangement o f the bulk structure of the rereduced used catalyst when it is reintroduced to the reaction environment. (C) 1999 Academic Press.