Acid temperature control alloys

Platinum/tin catalysts are rrtade by impregnating the support with tin chloride and calcining at 500°C, before irrrpregnation with chloroplatirtic add. The catalyst is then dried, oxidiied, and reduced. Temperature control is necessary to avoid reduction of the tin chloride to metallic tin, which would then alloy with the platinum. Tin oxide cotttbines with acid sites on the support artd increases resistance to deactivatiom Typical catalyst formulations are shown in Table 6.19. 

The simplest and most thoroughly studied solutions are those based on phosphoric acid at low temperatures (<35°C) which alone can fulfil all three requirements of acid solvent, film former (as metal phosphate) and diffusion agent by virtue of its viscosity. Thus copper and its main alloys of brasses and bronzes can be very effectively electropolished in 60-70% orthophos-phoric acid with the temperature maintained below 35°C under other conditions copper passivates or dissolves freely under mass transfer controlled conditions, but by varying the conditions appropriately polishing can be continued under mild agitation. An annotated polarisation curve is given in Fig. 11.7 readers are referred to recent studies for more detailed 2ispects " . 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

The nitric oxide was determined as usual—by absorption in water and titration. Control experiments on the combustion of the gas at low temperature and of analysis according to Kjeldahl (reduction of nitric and nitrous acids to NH3 by the action of MgAl-alloy in alkaline solution, distillation and absorption of NH3 by acid) convinced us that the gas contained no noticeable traces of sulphur and that the acidity as ordinarily determined depends on nitric and nitrous acids. 

Nitration reactions are carried out in closed vessels that are provided with an agitating mechanism and means for controlling the reaction temperature. The nitration vessels are usually constructed of cast iron and steel, but often acid-resistant alloys, particularly chrome-nickel steel alloys, are used. 

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