Platinum alloy ammonia oxidation gauze

The need to reduce metal losses during ammonia oxidation led to the development of a number of specialized technologies. The first was the substitution of alloy gauzes for pure platinum gauzes. Handforth and Tilley (143) demonstrated that platinum gauzes containing 10-20% Rh lost far less metal than pure platinum screens and were actually somewhat more active than pure... 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

For many catalysts, the major component is the active material. Examples of such unsupported catalysts are the aluminosilicates and zeolites used for cracking petroleum fractions. One of the most widely used unsupported metal catalysts is the precious metal gauze as used, for example, in the oxidation of ammonia to nitric oxide in nitric acid plants. A very fast rate is needed to obtain the necessary selectivity to nitric oxide, so a low metal surface area and a short contact time are used. These gauze s are woven from fine wires (0.075 mm in diameter) of platinum alloy, usually platinum-rhodium. Several layers of these gauze s, which may be up to 3 m in diameter, are used. The methanol oxidation to formaldehyde is another process in which an unsupported metal catalyst is used, but here metallic silver is used in the form of a bed of granules. 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts, whose active ingredients are major amounts of other substances called promoters, which increase the activity. Examples of supported catalysts are the automobile-muffler catalysts mentioned above, the platinum-on-alumina catalyst used in petroleum reforming, and the vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

A development in catalyst support systems in which half of the 5-10% rhodium-platinum alloy gauzes were replaced by nonnoble metal supports or by ordinary metal catalysts gave cost economies without adversely affecting operating efficiency [47]. More recently, ammonia oxidation in a two-bed system (Pt gauzes followed by monolithic oxide layers) gave nearly the same ammonia conversion while reducing platinum losses by 50% [48]. 

Nowadays, while reaction conditions are more or less the same, seven to ten layers of gauze are rrsed to ensitre that the gauze is imiformly heated, and to avoid carbon deposition. The process operates at about 70% conversion and reqitires rapid cooling. As in ammonia oxidation, the siuface of the platinum alloy is etched as it is activated. Metal foil catalysts have often been formed from expanded sheets... 

The first step in the process is the heterogeneous, highly exothermic, gas-phase catalytic reaction of ammonia with oxygen (Reaction 2). The primary oxidation of ammonia to nitric acid (over a catalyst gauze of 9 l platinum/rhodium alloy) proceeds rapidly at process temperatures between 900-970°C. 

Metal wires and screens are used as fixed-bed catalysts in which reactants are passed through the openings in the gauze, the size of which is defined by the mesh and wire diameter (see Fig. 10A). Gauzes composed of an alloy of platinum and rhodium catalyze the air oxidation of ammonia to nitric oxide, which is subsequently converted to nitric acid, and the production of hydrogen cyanide from ammonia, air, and methane. Formaldehyde production by... 

Wire Gauzes. Wire gauzes are commonly used in the oxidation of ammonia and hydrocarbons. A gauze is a series of wire screens stacked one on top of another (Figure 11-12). The wire is typically made out of platinum or a platinum-rhodium alloy. The wire diameter ranges between 0.004 and 0.01 cm. 

Ammonia is oxidized using a mixture of ammonia gas (9-11%) in air which is passed through multiple layers of fine platinum-rhodium alloy gauze (Eq. 11.35). 

Platinum/rhodium alloy gauze is used as a catalyst in the selective oxidation of ammonia during nitric acid production and in the production of hydrogen cyanide. The wire in the gauze is only a few thousandths of an inch in diameter woven at 80 wires per inch. Several layers of gauze, up to about 8 ft in diameter, are used. 

The oxidation of ammonia, however, was less economic at high pressure than at atmospheric pressure because the burner temperatures had to be increased in order to achieve the same selectivity. This led to shorter catalyst life as the metal gauzes deteriorated more rapidly and the loss of platinum became uneconomic. The 90% platinum/10% rhodium alloy introduced by DuPont solved this problem by reducing platinum loss by 50% and also improving selectivity. DuPont also found that the loss of platinum was proportional to the oxygen content of the gas mixture. It was realized that the smface of the catalyst etched as it was activated and the wires were covered by tinsel. 

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