(45d) Ni-Fe Binary Metal Catalysts for Hydrogen Production Via Auto-Thermal Reforming of Ethanol

Auto-thermal reforming (ATR) of bio-ethanol is a promising process for hydrogen production, because of its self-sustainment for reaction heat. Ni-based alumina supported catalysts has been extensively studied in ATR of ethanol; Hydrogen selectivity, coke deposition, as well as carbon-containing product distribution are main challenges to be addressed.

Ni-Fe binary catalysts have been studied, and show a higher activity and stability in ATR of ethanol for hydrogen production. Over an optimized Ni-Fe/Al2O3 catalyst, hydrogen selectivity has been increased by a factor of 70.52 %, 81.47 %, and 47.44 % at 400 ?aC, 500 ?aC and 600 ?aC, respectively, compared with the un-promoted catalyst. Moreover, the optimized catalyst shows a significantly improved stability without obvious loss in catalytic performance during a durability test at 600 ?aC, while the catalyst without iron promoter performs poorly with remarkable decrease (about 60 %) in hydrogen selectivity within 20 hours. For carbon selectivity, meanwhile, higher carbon dioxide yield has been achieved over iron promoted catalysts, i.e. 54.3 % (promoted sample) vs. 9.6 % (un-promoted sample). For the ratio of dioxide to monoxide, iron promotes a higher selectivity of dioxide than that of monoxide, with an approximately double gain in the ratio from 0.93 to 1.82, which can be attributed to the high activity of iron in the water-gas shift reaction (WGSR). Furthermore, over the un-promoted catalyst, the dehydration of ethanol is dominant in ATR of ethanol; as a result, ethylene becomes the main carbonaceous product and reached 76.7 % on carbon basis, which can result in carbon deposit on the catalyst surface via polymerization and dehydrogenation. By comparison, only about 2.5 % of ethylene selectivity has been found over the promoted sample, and coke deposition is constrained. The zirconia and ceria contained Ni-Fe binary catalysts also show a comparable performance, and the lower selectivity to carbon monoxide and ethylene are observed, which shows the improvement in WGSR and the restrain on dehydration of ethanol.

The characterization results indicate that the active component in Ni-based catalysts is the partially reduced spinel phase NiAl2O4, which favors the ethanol dehydration reaction route, and results in a high ethylene selectivity and a low hydrogen selectivity at low temperatures. Within the Ni-Fe binary catalysts, the mixed crystals in spinel phase as NiAl2O4-FeAl2O4 are formed, where the catalysts' structure and electronic properties are modified: NiAl2O4 crystal is distorted because of heterogeneous iron atoms coordination, and more defect sites are formed, which results in changes in reduction as well as reactants adsorption and activation, and in an improvement of activity. Meanwhile, FeAl2O4 phase in mixed crystals promotes the ethanol dehydrogenation reaction. Furthermore, more acid sites on Al2O3 surface are also covered by these new species. Consequently, the synergistic effect of the NiAl2O4-FeAl2O4 mixed crystal induces the reaction network into the dehydrogenation route, which is coupled with a high activity of partially reduced NiAl2O4 for acetaldehyde transformation through steam reforming, partial oxidation, and water-gas shift reaction processes. This route preference explains the high selectivity to hydrogen and C1 products of Ni-Fe binary metal catalysts.