Formation and properties of carbon/coke as nanostructured materials and deactivating agent in the ethanol steam reforming over a catalyst derived from Ni-Al spinel

Abstract

The ethanol steam reforming (ESR) on a Ni/Al 2 O 3 catalyst derived from the complete reduction of NiAl 2 O 4 spinel provides high H 2 yield with low greenhouse gas emissions due to a significant co-production of solid carbon under certain conditions. The impact of temperature and steam/ethanol (S/E) ratio on the carbon formation has been studied both theoretically and experimentally (by ESR tests in a fluidized bed reactor, in the 450–650 °C range and with S/E ratios in the 0–9 range), identifying the main precursors and formation routes. The carbon has been characterized by scanning electron and transmission electron microscopies (SEM and TEM), Raman spectroscopy, X-ray diffraction (XRD) and temperature programmed oxidation (TPO) to define its dual role as nanostructured material (with commercial interest) or as catalyst deactivation agent (coke), depending on operating conditions. The amorphous carbon covering the active sites has a deactivating role and prevails at 650 °C (< 0.9 g/(g catalyst)), whereas nanostructured carbon (nanotubes with no significant effect on deactivation) prevails below 600 °C and a stable production of 34 g/(g catalyst)) is obtained at 500 °C and S/E ratio of 3. The carbon crystallinity and structural purity are enhanced as the temperature increases and the S/E ratio decreases. The analysis by temperature programmed gasification (TPG) has emphasized the potential of the gasification above 650 °C to valorize the carbon/coke into syngas (without the CO 2 emissions obtained by combustion). The results are interesting for the scale-up of a versatile combined process (ESR + carbon/coke gasification) that integrates carbon valorization into syngas, while minimizing the CO 2 emissions.

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