Sulphuric acid platinum catalyst

Platinum is used as a catalyst for nitric and sulphuric acid production, in petroleum refining and in catalytic mufflers to control air pollution. Platinum salts can cause respiratory complaints, asthma, and platinosis , an allergic response. Allergic dermatitis may also result from exposure to soluble platinum salts and once subjects have been sensitized it generally precludes continued occupational exposure at any level. The 8 hr TWA OEL for platinum metal is 5 mg/m but for soluble platinum salts it is only 0.002 mg/m. Handling precautions must include containment where possible, ventilation, personal protection, and the screening out of individuals who have become sensitized. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

Although the commercial manufacture of sulphur trioxide and sulphuric acid by the catalytic process has attained success only in comparatively recent years, a patent was acquired in 1831 by P. Phillips of Bristol for the production of sulphuric acid in this way, the suggested catalyst being platinum.4 The commencement of the twentieth century saw the main difficulties overcome and the installation of an economical and commercial process in Germany. Since then the number of plants has increased largely and various modifications have been introduced in many countries. 

Concentrated sulphur acid evaporation and dehydration is performed in a group of two heat exchangers with important exchange surface (up to 1 340 m2) (HX-208). The S03/S02 decomposition reactor (HX-209) is a set of five reactors with two reactive zones. The first one, with a temperature of 875 K requires a platinum catalyst and the second one an iron-oxide catalyst. The operating temperature in the second zone increases up to 1125 K. Due to operating conditions (temperature, chemical composition), these three devices require a nickel-iron-chromium alloy. Then sulphur trioxide recombination reactor consists of a packed column (HX-210). Required investment for S03 conversion is estimated about EUR(08) 508.6 M. 

A prototype receiver-reactor for the solar decomposition of sulphuric acid has been developed and qualified. The concept of a volumetric receiver-reactor has been proven feasible in practice. High conversion almost up to the maximum achievable can be achieved with a platinum catalyst. Further investigations with a less expensive catalyst, iron oxide, provided evidence of the potential of such material. 

Platinum is also an excellent catalyst for performing oxidations. This property is used on an enormous scale in the preparation of sulphuric acid by the oxidation of sulphur dioxide, and of nitrates by the oxidation of ammonia. Also, like most other metals, platinum assists the decomposition of many organic compounds by heat. 

When spongy platinum or even the compact foil is boiled with sulphuric acid containing ammonium sulphate, sulphur dioxide is formed, and some free nitrogen evolved. The platinum apparently acts as a catalyst by alternately yielding the disulphate and free metal, thus ... 

The production of sulphuric acid was commercialised in the mid-18th century. In the so-called lead chamber process the oxidation of SO2 into SO3 was catalysed by NO. The acid produced is not very concentrated. The raw material used was elemental sulphur from Sicily. Later, pyrite was used because of its lower price. One of the consequences was a much higher impurity level in the feed to the reactor. As early as 1831 a process was patented in which SO2 was oxidised in the presence of finely divided platinum. The commercial application, however, was strongly delayed due to technical difficulties, the major one being catalyst poisoning. 

The primary oxidation products were passed through several layers of platinized asbestos and platinum wire formed into a star pattern. A series of experiments on the combustion of diphenyl sulphoxide showed that at 850°C and in the presence of a platinum catalyst sulphur is quantitatively converted into sulphur dioxide with no sulphur trioxide being produced. The oxidation temperature can be increased to 1200°C if a vanadium catalyst is used. The water is absorbed in a tube containing calcium sulphate, which helps to prevent the formation of sulphurous acid. Carbon dioxide and sulphur dioxide were concentrated in a U-shaped trap cooled with liquid nitrogen, and were subsequently analysed by GC at 92°C using a 6-m column filled with dinonyl phthalate. The content of sulphur in the sample was derived from the sulphur dioxide peak area with due regard to the weight of the sample and the calibration coefficient. 

Platinum is largely used in the chemical industry as a catalyst in various processes. Every chemist thinks immediately of the contact process of the sulphuric acid industry and the classic researches of Knietsch in 1901. A healthy stimulus to the investigation of platinum catalysts was afforded by the placing on the market... 

The reaction is exothermic and the conditions ate controlled to keep the temperature at an optimum 450°C. Formerly, platinum catalysts were used but vanadium-vanadium oxide catalysts are now mainly employed (although less efflcient, they are less susceptible to poisoning). The sulphur trioxlde is dissolved in sulphuric acid H2SO4 + SO3 — H2S2O7 and the oleum is then diluted. 

Hogarth MP, Munk J, Shukla AK, Hamnett A. Performance of carbon-cloth bound porous-carbon electrodes containing an clectrodeposited platinum catalyst towards the electrooxidation of methanol in sulphuric acid electrolyte. J Appl Electrochem 1994 24 85-8. 

Key words fuel, hydrogen, methane, methanol, biogas, alkaline fuel cell (AFC), polymer electrolyte fuel cell (PEFC), phosphoric acid fuel cell (PAFC), platinum, catalyst, degradation, sulphur, carbon monoxide, poisoning, particulates. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

These cells operate only with hydrogen as the anode fuel and, moreover, the hydrogen must be pure since sulphur compounds and carbon monoxide adversely affect the performance of the Pt catalyst. Each cell consists of two teflon-bonded gas diffusion electrodes on a porous conducting support (see Fig. 10.21). At both anode and cathode the catalyst is platinum particles dispersed on carbon and a recent success has been a reduction in Pt loading from 10 mg cm to 0.75 mg cm ". The electrolyte is concentrated phosphoric acid absorbed onto a solid matrix and the cell operates at 200°C to improve the electrode kinetics. The cells are then mounted in stacks to increase the power output. 

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