Nitric oxide rhodium catalysts

A major step in the production of nitric acid [7697-37-2] (qv) is the catalytic oxidation of ammonia to nitric acid and water. Very short contact times on a platinum—rhodium catalyst at temperatures above 650°C are required. 

G. Fisher and co-workers, "Mechanism of the Nitric Oxide—Carbon Monoxide—Oxygen Reaction Over a Single Crystal Rhodium Catalyst," in M. 

However, reaction at 750°C to 900°C in presence of platinum or platinum-rhodium catalyst produces nitric oxide and water ... 

Currently, nitric acid is manufactured exclusively by catalytic oxidation of ammonia. Platinum or platinum-rhodium is an effective catalyst of this oxidation (Ostwald process). Three basic steps in such ammonia oxidation process are (1) oxidation of ammonia to form nitric oxide ... 

Nitric acid is produced industrially by the multistep Ostwald process, which involves (1) air oxidation of ammonia to nitric oxide at about 850°C over a platinum-rhodium catalyst, (2) rapid oxidation of the nitric oxide to nitrogen dioxide, and (3) disproportionation of N02 in water ... 

G. Fisher and co-workers, "Mechanism of the Nitric Oxide—Carbon Monoxide—Oxygen Reaction Over a Single Crystal Rhodium Catalyst," in M. J. Philips and M. Teman, eds., Proceedings of the 9th International Congress on Catalysis, Vol 3, Characterisation and Metal Catalysts, Chemical Institute of Canada, Ottawa, 1988. 

In this process (Fig. 1), the reactor contains a rhodium-platinum catalyst (2 to 10% rhodium) as wire gauzes in layers of 10 to 30 sheets at 750 to 920°C, 100 psi, and a contact time of 3 X 10"4 second. After cooling, the product gas enters the absorption tower with water and more air to oxidize the nitric oxide and hydrate it to nitric acid in water. Waste gases contain nitric oxide or nitrogen dioxide, and these are reduced with hydrogen or methane to ammonia or nitrogen gas. Traces of nitrogen oxides can be... 

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... 

The first reaction is run over platinum-rhodium catalysts at around 900°C (1,652°F). In the second and third stages, a mixture of nitric oxide and air circulates through condensers, where it is partially oxidized. The nitrogen dioxide is absorbed in a tower, and nitric acid sinks to the bottom. Nitric acid is mainly used to make ammonium nitrate, most of it for fertilizer although it also goes into the production of explosives. Nitration is used to manufacture explosives such as nitroglycerine and trinitrotoluene (TNT) as well as many important chemical intermediates used in the pharmaceutical and dyestuff industries. 

Ammonia reacts with air on platinum/rhodium alloy catalysts in the oxidation section of nitric acid plants. Nitric oxide and water are formed in this step according to Eq. (9.6). 

Cobalt oxide (C03O4) catalysts are being used in some nitric acid plants as an alternative to platinum-rhodium (Pt-Rh). They generate less N2O, cost less and have a longer campaign life than Pt-Rh gauzes. (A paper in 2000 reported a conversion rate of ammonia to nitrous oxide as low as 0.5% over cobalt oxide catalyst)222. 

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. 

Okay. These are the reactions we want to happen, and they do happen somewhat as the byproducts of the engine leave the exhaust. The problem is that they don t happen all that quickly, leaving bad stuff heading out into the atmosphere. That s why we use catalysts that lower the activation energies for these reactions and help them proceed at a rapid rate. A catalytic converter is inserted in the exhaust system of a car so that it receives the bad stuff after it s left the engine. The catalytic converter contains a ceramic core coated with very expensive metals—platinum, rhodium, and palladium—and has two sections. The first section deals with nitrogen oxide and nitric oxide. 

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 ... 

At the rhodium catalyst surface, nitric oxide is converted to nitrogen and oxygen. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

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%. 

Although the process configuration varies widely there are three main steps common to each, they are oxidation of ammonia to nitric oxide (NO), oxidation of NO to nitrogen dioxide (N02), and then absorption of N02 in water to produce nitric acid (Figure 3.11). The first step of the catalytic combustion of ammonia takes place over a catalyst consisting of platinum/rhodium (90 10) or platinum/rhodium/palladium (90 5 5). The reaction is very rapid and goes almost to completion as shown in Reaction 3.11. The reaction is one of the most efficient catalytic processes in industrial chemistry, having an extremely... 

Platinum is an effective oxidation catalyst for carbon monoxide and the complete oxidation of hydrocarbons. Palladium also promotes the oxidation of carbon monoxide and hydrocarbons but is more sensitive to poisoning than platinum in the exhaust environment. Both platinum and palladium promote the reduction of nitric oxide but are less effective than rhodium. In addition to the noble metals, three-way catalysts contain the base metal cerium and possibly other additives such as lanthanum, nickel or iron. These base metal additives are believed to improve catalyst performance by extending conversion during the rapid air-fuel ratio perturbations and help to stabilize the alumina support against thermal degradation. 

MECHANISMS OF THE CARBON MONOXIDE OXIDATION AND NITRIC OXIDE REDUCTION REACTIONS OVER SINGLE CRYSTAL AND SUPPORTED RHODIUM CATALYSTS ... 

Mechanisms of the Carbon Monoxide Oxidation and Nitric Oxide Reduction Reactions over Single Crystal and Supported Rhodium Catalysts High Pressure Rates Explained using Ultrahigh Vacuum Surface Science", G.B. Fischer, Se H. Oh, J.E. Carpenter, C.L. DiMaggio, S.J. Schmieg,... 

Dumpelmann, R., N.W. Cant and D.L. Trimm, 1995, Enhancement of the reaction of nitric oxide and carbon monoxide by hydrogen and water over platinum and rhodium-containing catalysts, in Catalysis and Automotive Pollution Control III, eds A. Frennet and J.-M. Bastin (Elsevier, Amsterdam) pp. 123-135. 

Although several methods for the preparation of nitric acid are theoretically available, only one finds much commercial use the direct oxidation of ammonia, an updated and improved version of the traditional Ostwald process. In this method, ammonia is heated and reacted with air over a catalyst, most commonly a mixture of rhodium and platinum metals. That reaction results in the formation of nitric oxide (NO), which is then converted to nitrogen dioxide (N02). The nitrogen dioxide reacts with water to form nitric acid. 

ENHANCEMENT OF THE REACTION OF NITRIC OXIDE AND CARBON MONOXIDE BY HYDROGEN AND WATER OVER PLATINUM AND RHODIUM-CONTAINING CATALYSTS... 

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