The role of rhodium in the Andrussow process.

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Bicknell, C.R., 1997. The role of rhodium in the Andrussow process. PhD, Nottingham Trent University.

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Abstract

The Andrussow process is used industrially to synthesise hydrogen cyanide from methane, air and ammonia using a multilayer platinum-rhodium gauze catalyst. CH4 + NH3 + 1 1/2 O2 → HCN + 3 H2O

Oxygen conversion is normally complete. The reaction is, however, normally < 65% selective with by-products including H2 CO, CO2 and N2. When using a fresh platinum-rhodium catalyst, the initial selectivity to HCN is normally low but selectivity increases with time to reach a maximum of 65% (from methane) and then decays until renewal of the gauze pack is required. The catalyst pack undergoes extensive restructuring during the activation process. The role of rhodium is not well understood either in terms of peak selectivity or on its effect on activation. The above reaction was, therefore, studied by constructing and using a laboratory microreactor to

(a) measure the effect of rhodium concentration on peak selectivity.
(b) measure the effect of rhodium concentration on the activation of gauze catalysts.
(c) prepare samples of partially and fully activated catalyst for characterisation.

Microreactor results of fully activated catalysts show that the selectivity to HCN from methane and ammonia for the pure platinum catalysts is greater than for rhodium containing catalysts. Pure platinum catalysts are, however, less active than rhodium containing catalysts and require the furnace which surrounds the reactor tube to be at a higher temperature to reach peak selectivity. Although pure platinum catalysts are less active than platinum-rhodium catalysts, through changing the furnace temperature the maximum HCN yield for pure platinum catalysts, at 55% was 10% higher than the maximum yield of HCN for platinum-rhodium catalysts.

Activation studies show that pure platinum gauze catalysts have a high initial selectivity to HCN but show little further activation. Under the activation conditions investigated platinum-rhodium catalysts exhibit a low initial selectivity to HCN, but after an activation period of ca. 30 hours, have a selectivity within 2% of the selectivity of pure platinum catalysts.

Characterisation of catalysts throughout the activation process shows that the activation process is essentially independent of catalyst morphology. The surface rhodium concentration of unused platinum-rhodium gauze catalysts is above the bulk concentration but during the activation process the surface concentration of rhodium decreases to below the bulk concentration. The rhodium surface concentration after extensive activation is found to be within experimental error of published values for atomically clean alloy surfaces.

Characterisation of a catalyst removed from an industrial reactor shows that the formation of a multilayer thickness of carbon occurs and, due to a simple site blocking mechanism, may be a factor in the deactivation of platinum-rhodium catalysts under industrial conditions.