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Reaction pathway for ethanol steam reforming on a Ni/SiO<inf>2</inf> catalyst including coke formation

Abstract:

The effect operating conditions (temperature, space time, steam/ethanol molar ratio, ethanol partial pressure and time on stream) have on the activity and stability of a Ni/SiO2 catalyst for H2 production by ethanol steam reforming has been studied in a fluidized bed reactor. This catalyst allows obtaining total conversion above 500°C, with a steam/ethanol molar ratio of 6 and a space time of 0.138 gcatalysth/gethanol. Catalyst deactivation in the 300-500°C range is due to coke deposition, whose nature (determined by TPH and TPO analysis) mainly depends on reaction temperature. The coke deposited at 300°C is amorphous and blocks metallic sites, whereas at higher temperatures the coke is mainly filamentous and, although its content increases as reaction temperature is raised to 500°C, it has a low effect on catalyst deactivation because it does not block metal sites. Above 600°C the decrease in coke content due to gasification is noticeable, although at this temperature an incipient Ni sintering is observed, which is significant at 700°C. Based on the results at zero time on stream and on the deactivation results, a kinetic scheme has been proposed that explains the formation of the different types of coke and their relationship with reaction conditions. In this kinetic scheme, the intermediate compounds (ethylene, acetaldehyde and acetone) and ethanol (adsorbed as ethoxy ions) are identified as encapsulating coke precursors. CO (by means of Boudouard reaction) and CH4 (to a lesser extent, by decomposition) are the precursors of the filamentous coke.