Reduced Transition Metal Catalysts on Support

One of the original disclosures of such catalyst preparation was as follows [308]. A transition metal nitrate solution is used to saturate a charcoal bed, previously leached with nitric acid. The charcoal bed is then heated to temperatures high enough to decompose the nitrate to the oxide. 

Following the heating, the oxide is reduced with H2 or NaBH4. Sometimes, powdered AI2O3 or M0O3-AI2O3 is added prior to reduction. 

For an Al203-Si02 support, a 3% metal nitrate solution is used. The saturated bed is heated to 500°C to decompose the nitrate. This is followed by a reduction. Many modifications of the above procedure developed since. 

Preparations based on molybdenum oxide are called Standard Oil Co. catalysts, while those based on chromium oxide are known as the Phillips Petroleum Co. catalysts. Following is a description of a preparation of a molybdenum based catalyst. Alumina is saturated with a solution of ammonium molybdate and then subjected to heating in air at 500-600°C. The oxide that forms is reduced with hydrogen at 430 80°C. Reducing agents like CO, SO, or hydrocarbons are also used. Hydrogen, however, is preferred at pressures of approximately 75 psi. The catalyst may contain between 5 and 25% of the molybdenum compound dispersed on the surface [309]. 

A typical chromium based catalyst is prepared similarly. It is usually supported by a 9 1 of Si02 AI2O3 [347] carrier. Either CrCNOsjs 9H2O or CrOs solutions in nitric acid are used to impregnate the support. The nitrates are decomposed in air at 400-1,000°C [310]. Optimum chromium content in these catalysts is 2-3%. 

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