Hydrogen production by reforming of bioethanol

Hydrogen is the most abundant element in the Universe, but its abundance on Earth is modest. Moreover, with few exceptions, it does not exist in its native state. It is essentially present in water, fossil hydrocarbons and biomass, in association with oxygen and/or carbon. It is widely used in industry and refining, with requirements in the order of 50 to 60 MT/year. However, its use as an energy source remains marginal. Currently, it is prepared industrially by steam reforming of fossil hydrocarbons (natural gas, naphtha) or, to a lesser extent, by water electrolysis. Other alternative processes are also possible. The main problem with steam reforming is that it produces considerable quantities of CO ₂ . In reality, the reaction extracts hydrogen from the hydrocarbon and water, but a significant proportion of the carbon is transformed into carbon dioxide. To avoid this, one solution has been to replace the fossil fuel source with a biomass-derived molecule that is CO ₂ neutral overall, the carbon dioxide produced during steam reforming being recycled by plants during photosynthesis. Much effort has gone into using bioethanol to produce hydrogen. The aim of this article is to review the use of ethanol in the manufacture of hydrogen by steam reforming. The process is catalytic. As the reaction is endothermic, it is carried out at high temperature, which requires the development of particularly resistant catalysts. Rhodium is the metal of choice, but its performance depends considerably on the substrate used. Part of this article is devoted to the process using high-purity refined ethanol. However, the use of raw bioethanol is economically more viable. Catalysts that are stable in "pure" ethanol are no longer stable when switching to bioethanol. It was therefore necessary to develop catalysts resistant to bioethanol impurities. The RhNi system deposited on yttrium-doped alumina proved highly effective. Industrial applications of ethanol steam reforming are still few and far between, and pose the problem of CO _{2 neutrality} . Numerous factors can alter the CO ₂ balance, and it is essential to consider the life cycles of the process as a whole, starting with the cultivation of the plants producing bioethanol. Despite highly controversial balances, it seems that hydrogen production by bioethanol reforming remains highly advantageous in terms of the CO ₂ balance if 2nd-generation bioethanol is involved.