Gauze, catalytic

Catalytic properties are dependent on physical form, principally the exposed surface area which is a function of particle size. Industrial PGM catalysts are in the form of finely divided powder, wine, or gauze, or supported on substrates such as carbon or alumina (see Catalysis Catalysts, supported). 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Fast catalytic reac tions that must be quenched rapidly are done in contac t with wire screens or thin layers of fine granules. Ammonia in a 10% concentration in air is oxidized by flowthrough a fine gauze catalyst made of 2 to 10% Rh in Pt, 10 to 30 layers, 0.075-mm (0.0030-in) diameter wire. Contact time is 0.0003 s at 750°C (1,382°F) and 7 atm (103 psi) followed by rapid quenching. Methanol is oxidized to formaldehyde in a thin layer of finely divided silver or a multilayer screen, with a contact time of 0.01 s at 450 to 600°C (842 to 1,112°F). 

Chemical and catalytic. This grade of platinum is for conversion to catalysts, gauzes and chemical compounds. Spectrographic analysis is employed to control the presence of trace impurities harmful in these applications. 

By far the most important use of the platinum metals is for catalysis. The largest single use is in automobile catalytic converters. Platinum is the principal catalyst, but catalytic converters also contain rhodium and palladium. These elements also catalyze a wide variety of reactions in the chemical and petroleum industry. For example, platinum metal is the catalyst for ammonia oxidation in the production of nitric acid, as described in Pt gauze, 1200 K... 

Fig. 7a. (a) A packed bed reactor, (b) A multitubular packed bed reactor, (c) A catalytic gauze reactor. 

The catalytic bed (70 cm ), supported by a metallic gauze, is located in the reforming section. Water is fed to the reactor at the bottom of the metallic gauze. The temperature inside the reactor is monitored by four thermocouples one (Tcomb) is located on the SiC foam and the other three (T ref L, T ref M, T ref H) are located at 25, 50 and 75%, respectively, of the catalytic bed height to provide the reactor temperature axial profile. Moreover, additional thermocouples monitor... 

The phenomenon under consideration was studied systematically in the beginning of the 19th century. In 1815, Davy performed experiments that dealt with catalytic combustion on platinum gauzes. The term catalysis , however, was introduced by Berzelius in 1836. He first defined a catalyst (Berzelius, 1836) as a compound, which increases the rate of a chemical reaction, but which is not consumed during the reaction. This definition was later amended by Ostwald (1853-1932) in 1895 to involve the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly decreased A catalyst is a substance that increases the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction. It was more than a century after Berzelius first definition that Marcel Prettre s introduced the notion of yield The catalyst is a substance that increases the rate of a chemical transformation without modifying the yield, and that is found intact among the final products of the reaction. ... 

The catalytic oxidation of ammonia by air over platinum gauze at 900 °C gives nitric oxide (reaction 9.12), which is then oxidized to nitric acid by air and liquid water in a nitrous gas absorber (reactions 9.13 and 9.14) ... 

Thermally induced elimination of fluorine occurs if polyfluorocycloalkanes, -cyclohexadienes or some heterocyclic perfluoro compounds arc passed over a nickel or iron gauze at 500-700cC (examples are summarized in Table 7). Within a certain time the catalytic activity of the metal decreases and it has to be regenerated by passing hydrogen over it at 300-600JC. The yields... 

Fig. 11. Used Pt/Rh gauze (10% Rh) catalyst with rearranged surface structure (20X). Loss of catalytic metal and catalyst gauze strength during ammonia oxidation is a serious industrial concern (146). 

When a catalytic reaction takes place on a nonporous metal surface the coupling between the exothermic chemical reaction and the transport effects may also give rise to multiple steady states. Apparently, in the realm of chemical reaction engineering the first experimental observation of multiple steady states was done just for the catalytic wire problem [see Tamman (29), Davies (30), and Buben (5/)]. Catalytic gauzes consisting of wire screens or layers of metal pills (e.g., the silver crystals) are used for a number of industrially important catalytic reactions as, e.g., synthesis of... 

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. 

Ozone decomposition in airplanes Selective catalytic reduction of NOx Arrays of corrugated plates Arrays of fibers Gauzes Ag Methanol -> formaldehyde Pt/Rh NO production from ammonia HCN production from methane Foams Catalytic membranes reactors... 

The catalytic packing MULTIPAK (147) applied in this case study consists of corrugated wire gauze sheets and catalyst bags of the same material assembled in alternate sequence. Sufficient mass transfer between gas and liquid phase is... 

Critical effects in CO oxidation over Pt catalysts were obtained [33, 34, 63-85] in various catalytic systems over wires, foils and gauzes, on single pellets and fixed beds, in isothermal and adiabatic reactors (differential and integral). The literature also reported the oscillating behaviour of the homogeneous oxidation of CO [86, 87]. 

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. 

Includes catalytic NOx abatement to 200 ppm v and average ammonia conversion efficiency over the entire gauze run length. 

Wire Gauzes Wire screens are used for very fast catalytic reactions or reactions that require a bulk noble metal surface for reaction and must be quenched rapidly. The nature and morphology of the gauze or the finely divided catalyst are important in reactor design. Reaction temperatures are typically high, and the residence times are on the order of milliseconds. 

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