Copper-based spinels

Supported copper-chromium oxide catalysts. The non-modified support after its thermal treatment at 773 K, if coated with a 7% (Cu+Cr) mixture, seems to contain a series of spinel-type phases on the base of the support and copper chromite structures as well. Then diffractogramms of the sample are characterized by distorted lines of the support only. After thermal treatment at 1273 K, there coexist a-Al203 with the increased cell parameter and aluminium-copper-chromium spinel with a = 8.098 A, which is typical for Cu(Ali 8Cro.2)04 composition. No lines of copper (+1), i.e. Cu2Cr204 and CU2AI2O4, are observed. 

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

When 2 and 7% of (Cu+Cr) mixture are introduced, the cerium modified support pretreated at 773 K, exhibits only its own lines, while when 15% of (Cr+Cu) is supported, the lines of roughly dispersed copper oxide are observed. According to X-ray patterns, the lines intensity of the spinel-based solid solutions increases with the increase of the supported components concentration. At 1273 K, all the studied samples reveal the presence of a-Al203 with the increased cell parameter and the spinel-type solid solution with a parameter typical for Cu(Alx,8Cro.2)04. Concentration of components introduced determine a-Al203 content. The content of a-oxide is considerably lower in the sample promoted with a 7% (Cu+Cr) mixture than in the pure alumina. 

For example, calcium copper titanate - CaCu3Ti40i2 (CCTO), ruthenium oxide RUO2 cobalt-nickel spinels, carbon-based ceramic composite films... 

Preparation by coprecipitation forms two active phases. This is based on two different precursors and a precipitating agent. As examples, we consider the preparation of copper chromite and Co-Cu spinel, using different coprecipitation methods. 

It must be pointed out that the main increase in catalytic activity takes place for Cat B, in which only 20% of zinc ions is substituted by copper ions. The methanol productivity of this catalyst is similar to the best values reported in the literature, if based on kg of catalyst (33- 34), but clearly better if calculated on the basis of kg of copper, thus indicating the formation of very active copper-containing centers. XRD powder patterns of the catalysts after reaction show the presence until a Cu/(2u+Zn ration 0.5 only of cubic spinel-type phases, while for copper-rich samples the main phases identified are metallic copper (Cat E) and Cu20 (Cat F). Furthermore, from Table 2, it is possible to observe that for these last catalysts practically all the theoretical CuO is extracted by NH4OH/NH4NO3, while for Cat B and C values very similar to those of the calcined samples are obtained. 

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