Wire, catalytic

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). 

Metal deactivators (MD) act, primarily, by retarding metal-catalyzed oxidation of polymers they are, therefore, important under conditions where polymers are in contact with metals, e.g., wires and power cables. Metal deactivators are normally polyfunctional metal chelating compounds (e.g.. Table la, AO 19-22) that can chelate with metals and decrease their catalytic activity [21]. 

The hydrogen electrode consists of an electrode of platinum foil (approximately 1 X 1 X 0-002 cm) welded to a platinum wire which is fused into a glass tube. In order to increase its catalytic activity it is platinised by making it cathodic in a solution of chloroplatinic acid (2% chloroplatinic acid in 2 N HCl) frequently lead acetate is added to the solution (0-02%) and this appears to facilitate the deposition of an even and very finely divided layer... 

Type of Initiation Catalytic Cr Ni Metal Rings Spark Hot Wire Inert Shock... 

Type of Initiation Catalytic (Ni Cr oxides in coke granules) Spark Hot Wire Expl Shock... 

It has been recently found that direct electrical contact, via a metal wire, to the catalyst-electrode is not necessary to induce the effect of electrochemical promotion.8 11 It was found that it suffices to apply the potential, or current, between two terminal electrodes which may, or may not, be catalytically active. The concept appears to be very similar with that of the bipolar design used now routinely in aqueous electrochemistry. 

The so-called turbulent "V-shaped flames" are the flames anchored behind a rod or a catalytic wire in a flow where turbulence is generated by an upstream grid. Trinite et al. [7,40] and Driscoll and Faeth [41] have studied such flames. Instantaneous images of rare beauty have been obtained from which it is very clearly seen that the turbulent flame brush width is continuously increasing downstream of the stabilizing rod, see Figure 7.1.13. 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

GP 2] [R 3a] The performance of one micro reactor with three kinds of catalyst -construction material silver, sputtered silver (dense) on aluminum alloy (AlMg3), and sputtered silver on anodically oxidized (porous) aluminum alloy (AlMg3) -was compared with three fixed beds with the same catalysts [44]. The fixed beds were built up by hackled silver foils, aluminum wires (silver sputtered) and hack-led aluminum foils (anodically oxidized and silver sputtered), all having the same catalytic surface area as the micro channels. Results were compared at the same flow rate per unit surface area. 

GP 8[ [R 7[ The structure of the rhodium catalyst changed during operation. Owing to the microfabrication process (thin-wire pEDM), the surface of the micro channels was rough before catalytic use [3]. After extended operational use, small crystallites are formed, especially in oxygen-rich zones such as the micro channels inlet. Thereby, the surface area is enlarged by a factor of 1-1.5. 

Bulk catalytic materials, in which the gross composition does not change significantly through the material, such as platinum wire mesh. 

When a platinum wire (which may have been hot) was dipped for a flame test into a sintered funnel containing the air-dried complex, detonation occurred. This may have been due to heat and/or friction on a compound containing both strongly oxidising and reducing radicals. Avoid dipping (catalytically active) platinum wire into bulk samples of materials of unknown potential. 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

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