Platinum-rhodium gauzes

In the converter, the gases react over the platinum/rhodium gauze catalyst. The gases leaving the gauze at 330 kPa and 865°C flow... 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

Metal location is but one of a number of applications for scanning electron microscope studies in catalysis. Other applications are the study of the morphology of platinum-rhodium gauzes used in the oxidation of ammonia and the poisoning of catalysts, in which the scanning electron microscope results show the location of poisons such as compounds containing sulfur, phosphorus, heavy metals, or coke relative to the location of the catalytic components. 

Nitric acid is S3mthesized commercially via two nitrogen oxides. First, ammonia is reacted with oxygen using a platinum-rhodium gauze catalyst to form nitric oxide, NO ... 

Figure 1 Final phase of installing a platinum/rhodium gauze pad in a nitric acid plant.

Figure 2 Changes of the surface morphology of a traditional woven platinum/rhodium gauze during use. The surface of fresh catalyst is smooth, but during use dendritic excrescences of alloy grow from the wire surface.

Figure 3 Micrograph of woven platinum/rhodium gauze after prolonged use in the Andrussow process. The gauze has a matte appearance, and the apertures are considerably smaller than in fresh gauze.

Figure 4 Schematic differences between woven gauzes and the new, knitted gauzes. The lower photograph is a micrograph of the surface developed on a knitted platinum/rhodium gauze after use in an ammonia oxidation plant.

To put these principles into practice, liquid ammonia is first vaporized by indirect heating with steam, and then filtered to reduce risk of catalyst contamination. This produces an ammonia gas stream at about 8 atm pressure without requiring mechanical compression. An air stream is separately compressed to about the same pressure, preheated to 200-300°C, and filtered prior to mixing with the ammonia (about 10%) gas stream immediately before conversion. This mixture is passed through the red hot platinum-rhodium gauze to produce a hot gas mixture of nitric oxide and water vapor plus the unreacted nitrogen and oxygen components of air (Fig. 11.5), with a yield efficiency (selectivity) under these conditions of about 95%. 

Platinum black, finely divided metal, is a well-established catalyst for hydrogenation and dehydrogenation reactions. Platinum-rhodium gauzes are used as catalysts for the large-scale oxidation of ammonia to nitric oxide. 

The platinum-rhodium gauzes with a diameter of up to 5 m are nowadays produced by knitting thin wires, usually with a wire diameter of 60-80 p,m. This corresponds to about 1000 knittings per cm and to a mesh opening of about 0.2 mm. 

Modem plants Platinum/rhodium Gauze 3-5 layers wire 0.075 mm. ... 

Since 1920 the platinum recpiired to produce a given amout of nitric acid has fallen to less than 30% of the earlier levels a resnlt of using catalysts with higher activity, greater selectivity, and longer operating fives. It is unlikely that better catalysts will be developed that can replace platinum/rhodium gauze. 

This process is very tricky to control in practice, because it is accompanied by a number of side reactions, which are also very fast for example, formation of nitrogen and water. These are minimized by careful reactor design and very fine temperature and flow rate control. Incredibly, the actual contact time in the reactor is a mere 10 s — a very fast reaction indeed The gases enter at 300 °C and leave at 900 °C due to the highly exothermic nature of the reaction. The catalyst for this step is a platinum-rhodium gauze. ... 

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