Insights into the Reaction Routes for H2 Formation in the Ethanol Steam Reforming on a Catalyst Derived from NiAl2O4 Spinel

Abstract

This work describes the satisfactory performance of a Ni/Al₂O₃ catalyst derived from NiAl₂O₄ spinel in ethanol steam reforming and focuses on studying the prevailing reaction routes for H₂ formation in this system. NiAl₂O₄ spinel was synthesized using a coprecipitation method and reduced at 850 °C to obtain a Ni/Al₂O₃ catalyst. The spinel structure and catalyst were characterized using XRD, TPR, N₂ physisorption, NH₃ adsorption and TPD, TPO, SEM, and TEM. The experiments were carried out in a fluidized-bed reactor at 500 or 600 °C and different space-time values, using pure ethanol, ethanol-water, pure ethylene, or ethylene-water feeds. The reaction takes place through two paired routes activated by each catalyst function (metal and acid sites) whose extent is limited by the selective catalyst deactivation. The results evidence that at the beginning of the reaction the main route for the formation of H₂ and carbon (nanotubes) is the dehydration of ethanol on acid sites followed by decomposition of ethylene on the Ni-Al₂O₃ interface. This route is favored at 500 °C. After the rapid deactivation of the catalyst for ethylene decomposition, the route of H₂ formation by steam reforming of ethanol and water gas shift reactions over Ni sites is favored. The morphology of the carbon deposits (nanotubes) allows the catalyst to maintain a notable activity for the latter pathways, with stable formation of H₂ (during 48 h in the experiments carried out). At 600 °C, the extent of the gasification reaction of carbon species lowers the carbon material formation. The high formation of carbon material is interesting for the coproduction of H₂ and carbon nanotubes with low CO₂ emissions.