Platinum sponge catalyst

The temperature dependence of the conversion and product formation in the ammonia oxidation over a platinum sponge catalyst is investigated. Positron emission profiling experiments demonstrate that below 413K, the catalyst deactivates due to the poisoning of the catalyst surface, mainly by... 

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. 

Fig. 20. Ammonia oxidation reaction performed at 373 K on a pre-oxidised platinum sponge catalyst (a) concentration of N2, N2O and H2O versus time for (b) conversion of NH3 and O2 versus time (GHSV = 5600 hr , NH3/O2 = 2/1.5, flow = 46.5cm /min.).

In 1839 Kuhlmann described ammonia oxidation to produce nitrogen oxides for nitric acid production using a platinum sponge catalyst at 300 C. At the same time he was also granted a patent for the oxidation of sulfur dioxide and used the process in his factoiy at Loos. He was apparently unaware of the Phillips patent granted in the United Kingdom, but he attempted to make sulfuric acid with a platinum catalyst. 

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. 

This reaction can also be brought about without the influence of solar radiation by means of catalysts such as platinum sponge, vegetable carbon, animal charcoal, etc. 

Derivation (1) By passing hydrogen with bromine vapor over warm platinum sponge which acts as a catalyst (2) As a by-product in the bromination of organic compounds. 

Cavallito (66) reports that neither it nor 3-benzoxypyridine was hydrogenated in the presence of Willstatter s palladium sponge catalyst (67). Under low pressure conditions in ether or dioxane Raney nickel and platinum oxide were ineffective. However, other examples show that reduction takes place readily under a variety of conditions. Biel used Raney nickel at 125° and 50 atm (68), excellent yield of 6-propyl-3-hydroxypiperidine resulted from reduction of the pyridine in acetic acid with platinum oxide (69). Ruthenium in the conversion of 3-hydroxypyridine in aqueous solution gave very high yield of the corre-... 

The catalysts were prepared by co-impregnation of y-alumina from Jolmson Matthey with solutions of platinum sponge dissolved in HCl and of RI1CI3 in H2O. Calcination at 600°C did not affect greatly the stmcture of y-Al203 despite sintering, whereas at 1600°C in air the AI2O3 was exclusively of the alpha type as determined by XRD. 

The two methods of achieving high surface may be illustrated on the classic catalyst, platinum thus, platinum black is the subdivided solid, while platinum sponge is the porous form. 

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... 

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. 

J. Reiset and N. A. E. Millon investigated the action of platinum sponge, pumice, and charcoal on the oxidation or decomposition of many solid or fused substances (tartaric acid, sugars, butter, ammonium and urea nitrates, etc.). They concluded that the process occurs in two stages in the first the action of the contact substance is confined to une influence acceleratrice in the second the force de contact exerts an influence speciale , which may alter the course of a reaction occurring without a catalyst. 

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