Spinel solid solution catalysts

C. A. Leech and L. E. Campbell, in Spinel Solid Solution Catalysts for Automotive Applications , Am. Chem. Soc. Spring Meeting, Los Angeles, California, 1974. 

Spinel Solid Solution Catalysts for Automotive Applications... 

Chen M, Xu J, Su FZ, Liu YM, Cao Y, He HY, Fan KN (2008) Dehydrogenation of propane over spinel-type gallia-alumina solid solution catalysts. J Catal 256 293-300... 

When impregnated with Ce(N03)3 solution and calcined the resulting catalysts are extremely active towards SOx abatement (Table I, Figure 4). The solid solution spinel catalyst is nearly 25% more active than the stoichiometric spinel catalyst. This can be explained by assuming that -MgO- structural fragments of the spinel are the chemisorption active sites (1). There are more active sites in Mg2Al20c than in MgA1 04. We also see that the catalyst prepared by co-precipitation method is 4 times more active than the catalyst prepared by the thermal co-condensation method. In Zone D we see that the sulfated catalyst is very effectively reduced by H. Nearly 70% of the absorbed species is reduced within 2 min. of reduction. 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

There is an extensive literature devoted to the preparation of oxide and mixed-oxide catalysts. These compounds are often derived from hydroxides, though alternative processes are available, e.g., calcination of metals, oxalates etc. Some of the studies referred to above were undertaken to add to the understanding of the textural changes which accompany water removal during the preparation of catalytically-active phases of technological importance. Heating of mixed hydroxides, prepared by concurrent precipitation, yields spinels and other specific phases, including solid solutions, at lower temperatures than is possible with less-intimate mixtures. The... 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

The concentration of defects may increase at the interphase boundary during the preparation of mixed catalysts, and these would then display higher catalytic activity. Frequently the changes in catalytic activity of mixed contacts are due to formation of spinels. The effect of a polyphase system of this kind, consisting of metal oxide spinels (often as solid solutions), on the rates of various reactions is a complex problem. 

In all cases, the evolution of the spinel-type phases towards the smichiometric forms, which takes place with a consistent segregation of oxide phases, destroys diis system and leads to considerable modifications in the properties of the oxides obtained. In particular, for the Ni-rich samples an increase in the reducibility of the main fraction of die Ni " ions was observed, with a behaviour similar to that of free NiO, while for Mg-rich samples the reduction of the Ni " " ions is further hindered by die formation of NiO/MgO solid solutions, with reduction temperatures very similar to those reprated for nickel-magnesia catalysts. 

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