Reaction pathway for ethanol steam reforming on a Ni/SiO2 catalyst including coke formation

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 H-2 production by ethanol steam reforming has been studied in a fluidized bed reactor. This catalyst allows obtaining total conversion above 500 degrees C, with a steam/ethanol molar ratio of 6 and a space time of 0.138 g(catalyst)h/g(ethanol). Catalyst deactivation in the 300-500 degrees 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 degrees 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 degrees C, it has a low effect on catalyst deactivation because it does not block metal sites. Above 600 degrees 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 degrees 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. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.