Abstract
This short review reports on spinel-type mixed oxides as catalysts for the transformation of biomass-derived building blocks into chemicals and fuel additives. After an overview of the various methods reported in the literature for the synthesis of mixed oxides with spinel structure, the use of this class of materials for the chemical-loop reforming of bioalcohols is reviewed in detail. This reaction is aimed at the production of H2 with intrinsic separation of C-containing products, but also is a very versatile tool for investigating the solid-state chemistry of spinels.
Highlights
Spinel oxides with the general formula AB2 O4 are chemically and thermally stable materials suitable for several applications, including catalysis.The ideal stoichiometric spinel structure is assumed by oxides with average cation charge of 2.33, like, for instance, magnetite Fe3 O4, with one divalent and two trivalent Fe cations
The formation of small particles at high temperature is possible by reducing the time spent at the synthesis temperature, for instance in methods of solution combustion or spray pyrolysis
Consecutive utilization of CoFe2 O4, CuFe2 O4 and Cu0.5 Co0.5 Fe2 O4 ferrospinels as looping materials resulted in higher amounts of produced hydrogen which surpass the value obtained over the reference material—Fe3 O4 ; increasing the total tos from 20 to 60 min leads to the decreasing of H2 /COx ratio, which in its turn affects the final purity of the target gas—H2
Summary
Spinel oxides with the general formula AB2 O4 are chemically and thermally stable materials suitable for several applications, including catalysis. The distribution of different cations in the A and B sites essentially depends on their crystal field stabilisation in coordination four or six The effect of this distribution on catalytic properties is not negligible. B sites have been considered to represent the most effective catalytic sites [1] This assumption has somehow to be qualified, as the internal charge transfers needed to close a catalytic cycle imply both site B and site A, and both sites are involved in oxygen mobility. Reduction of the oxide can reach complete destruction of the spinel the material is used as can oxygen carrier in a significantly modifies the distribution of thephase sites when [2]. The incorporation of cations with higher positive charge significant effects on catalytic activity.
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