Platinum alloy catalysts are evolving in a diversified direction

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In the ammonia oxidation process for nitric acid production, the ammonia oxidation rate, selectivity, platinum consumption, and service life are the main indicators for evaluating the catalytic properties of platinum mesh. Together with high-temperature strength, creep resistance, and poisoning resistance, they serve as parameters to assess the performance of platinum mesh in practical applications. Platinum possesses excellent catalytic properties, chemical stability, and high-temperature oxidation resistance, making it the preferred catalytic material in the ammonia oxidation process. Pure platinum has relatively poor mechanical strength. Platinum particles can be easily carried away by gases. Therefore, platinum alloys with high mechanical strength and a higher activity than pure platinum are widely used. Platinum-catalyzed alloys have evolved from pure Pt to Pt–Rh binary alloys, then to Pt–Pd–Rh ternary alloys, as well as multi-component alloys such as Pt–Pd–Rh–Ru and Pt–Pd–Rh–Re. By continuously improving their composition and catalytic activity, the platinum group metal resources have been utilized efficiently, thereby reducing the cost of platinum alloy catalysts and nitric acid production. Binary alloys include those of platinum and rhodium, as well as those of platinum and palladium or platinum and iridium. Among them, the Pt–Rh alloy containing rhodium exhibits good workability and high strength at high temperatures, and is therefore used as a catalyst alloy. Commonly used binary alloys include Pt-5Rh, Pt-7Rh, Pt-8Rh, and Pt-10Rh, among which Pt-10Rh has the best overall properties and is the most widely used. The service life of binary alloys is generally between 3 months and 1 year. They possess excellent high-temperature performance and creep resistance, as well as high mechanical strength. It has strong resistance to poisoning, but more rhodium oxides are formed. Platinum and rhodium are scarce and expensive; reducing their usage has always been a direction for improving platinum mesh. Pd is introduced into the Pt—Rh binary alloy. It can provide solid solution strengthening, reducing the amount of platinum and rhodium used as well as platinum volatilization losses. Reduce the cost of platinum mesh. The ternary alloys Pt-4Pd-3.5Rh and Pt-5Pd-5Rh have been widely used in China, the United States, and the former Soviet Union. The service life of ternary alloys ranges from 6 months to 1 year. Compared with binary alloys, they have a similar oxidation rate, lower costs, and a longer service life; however, their high-temperature performance and creep resistance are poor, their resistance to poisoning is weak, and their resistance to adhesion is good. To further save Pt and Rh, matrix strengthening elements are introduced into the ternary alloy. Tetralloy and pentalloy can be formed. The composition of the quaternary alloy is Pt—Pd—Rh—M, where M mainly consists of elements such as Au, Ru, Ce, Re, etc. The enhanced plow catalyst and conventional platinum-type quaternary catalysts developed by the Kunming Institute of Precious Metals, which are modified with the rare noble metal Re (with composition: (Pd) = 5%–15%, w(Rh) = 1.5%–3.5%, w(Re) = 0.01%–0.5%, and Pt accounting for the remainder), show a higher ammonia oxidation rate of 1–1.5% compared to traditional Chinese ternary alloys and imported binary catalysts, as well as a higher nitric acid production efficiency of 3–6%. The platinum consumption is reduced by about 20–28%, while the ammonia consumption is reduced by 3%. 6%, with improved corrosion resistance, anti-adhesion properties, and resistance to poisoning, in atmospheric-pressure nitric acid production plants. It has a service life of over 2 years; in medium-pressure devices, its service life is over 7 months. This catalyst has been put into industrial use and has obtained a Chinese invention patent. The 48.5 Pt-35Pd-6.5 Rh-10Au alloy developed by the American company Engelhard, which uses a high amount of palladium to replace platinum, was used at 600 kPa and 900 degrees Celsius; the oxidation rate reached 96%, resulting in a 46% reduction in the use of platinum and a 35% reduction in the use of rhodium. A catalytic alloy of 81Pt-3.5Rh-15Pd-0.5Ru developed by the former Soviet Union. It can save 7% to 9% of Pt. The ammonia oxidation rate, platinum consumption rate, cost, resistance to oxidative corrosion, as well as the anti-adhesion and anti-toxication properties of quaternary alloy meshes are significantly better than those of binary and ternary alloy meshes, and their service life is also much longer. Research on five-element alloys is not yet fully developed; the Pt-10Pd-2.5Rh-0.05Ce-0.5Au catalytic alloy, jointly developed by the Kunming Institute of Precious Metals and Jinchuan Non-ferrous Metals Corporation, is one such example. Compared with the ternary alloy Pt-4Pd-3.5Rh, it can save 6% Pt and 28% Rh, but it has not yet been put into industrial use.