Reduced platinum sponge

In the case of injection of [ NJ-NHs into an ammonia-free He flow over the reduced platinum sponge [Fig. 5(b)], the injected ammonia also travels through the catalyst bed. Gas-phase analysis showed that only NH3 is detected at the reactor outlet, indicating that ammonia does not dissociate under these conditions. The PEP image shows the weak binding of... 

Ammonia oxidation is conducted on a pre-oxidised platinum sponge catalyst. Figure 20 shows the conversion and selectivity at 373 K. The same selectivity characteristics as on the reduced platinum sponge catalyst are observed (Fig. 14). Thus, a high oxygen surface coverage does not favour initial nitrous oxide formation. The main difference with the reduced platinum sponge is the faster deactivation of the pre-oxidised catalyst below 413 K. 

A pre-oxidised catalyst deactivates much faster than reduced platinum sponge. Ammonia adsorption and dissociation are accelerated by the presence of oxygen. Thus, the NHx species cover much faster the platinum surface. The concentration profiles for nitrogen and nitrous oxide do not change, which indicates that the reaction mechanism is not changed for the pre-oxidised catalyst. 

Reduced cobalt, platinum sponge, and copper are not suitable for aldehyde reduction. 

In the majority of cases, the last step in the preparation of catalytically active metals is a reduction. The precursor is very frequently an oxide. An oxychloride is the real precursor of active platinum and some noble metals if chlorometal complexes (e.g. chloroplatinic acid) are used. It may be advantageous to use still other precursors and to reduce them directly without any intermediary transformation to oxide. On the other hand, nearly all catalytic metals are used as supported catalysts. The only notable exception is iron for ammonia synthesis, which is a very special case and then the huge body of industrial experience renders scientific analysis of little relevance. The other important metals are Raney nickel, platinum sponge or platinum black, and similar catalysts, but they are obtained by processes other than reduction. This shows the importance of understanding the mechanisms involved in activation by reduction. 

Completely anhydrous black powder. Readily soluble in aqua regia. Instantly reduced by H3 at ordinary temperatures, liberating heat and forming gray platinum sponge. Decomposes at atmospheric pressure (560°C) 2 PtO = 2 Pt + O3. 

Several methods are employed for the purification of the single platinum metal salts, for instance re-crystallization of Na2[Pt(Cl)6] with oxidative hydrolysis of the impurities of Pd, Rh, Ir, etc. which together with base-heavy metals are removed as insoluble hydroxides. Generally the metals are obtained, initially as sponge or powder, by conversion of their salts through thermal decomposition or by reduction of aqueous solutions of the ammonium-chloro complexes. Os which has been oxidized to the volatile 0s04 and Ru have to be reduced by H2. 

We will have to suppose that the lead sponge occurring at the lead cathode reduces the nitrobenzene to aniline. Men-tionable quantities of lead sulphate cannot be found, since this is continually reconverted to lead sponge by the freed hydrogen ions. This process is analogous to the one previously published by me3 in which a hydro-alcoholic solution of nitrobenzene acidified with sulphuric acid gives aniline when a zinc cathode is used. Considerable quantities of zinc sulphate do not occur. At a platinum cathode, under the same conditions, no aniline is formed, but azoxybenzene and hydrazobenzene or benzidine form. This has been confirmed by Haussermann. 4... 

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