Platinum, ammonia conversion

Dual-Pressure Process. Dual-pressure processes have a medium pressure (ca 0.3—0.6 MPa) front end for ammonia oxidation and a high pressure (1.1—1.5 MPa) tail end for absorption. Some older plants still use atmospheric pressure for ammonia conversion. Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance. Platinum losses are significantiy lower and production mns are extended by a longer catalyst life. Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance. Due to the spHt in operating conditions, the dual-pressure process requires a specialized stainless steel NO compressor. 

A development in catalyst support systems in which half of the 5-10% rhodium-platinum alloy gauzes were replaced by nonnoble metal supports or by ordinary metal catalysts gave cost economies without adversely affecting operating efficiency [47]. More recently, ammonia oxidation in a two-bed system (Pt gauzes followed by monolithic oxide layers) gave nearly the same ammonia conversion while reducing platinum losses by 50% [48]. 

Surface-science studies succeeded to identify many of the molecular ingredients of surface catalyzed reactions. Each catalyst system that is responsible for carrying out important chemical reactions with high turnover rate (activity) and selectivity has unique structural features and composition. In order to demonstrate how these systems operate, we shall review what is known about (a) ammonia synthesis catalyzed by iron, (b) the selective hydrogenation of carbon monoxide to various hydrocarbons, and (c) platinum-catalyzed conversion of hydrocarbons to various selected products. 

Because oxidation of ammonia on a nonplatinum catalyst differs substantially from the process on a platinum catalyst, the use of nonplatinum catalysts is restricted. In the republics of the QS two-step catalysts are widely used. One or several platinum gauzes are used as the first step, and a bed of nonplatinum oxide catalyst is used as the second step. The platinum step offsets the drawbacks of the nonplatinum step. A significant share of the ammonia burners in the CIS operate with the two-step catalysts. Reportedly, this allows the platinum input to be reduced by 40%-50% and platinum losses to be reduced by 15%-30% under equal conditions. The ammonia conversion efficiency is considered to be approximately the same as with conventional catalysts [8,9]. 

There is little opportunity to improve further the Oswald nitric acid process. The overall efficiency of ammonia conversion into acid is in the range of 949fr 96%, The process is self-sufficient in energy supply and can even export steam. Up to 80% of platinum catalyst losses can now be recovered. Capital investments were significantly reduced by the development of high-pressure absorption and with the design of efficient and reliable compressors and expanders. Economies of scale improve little for single-train units with capacities above 1,000 tpd. 

Conversion of Ammonia. Ammonia [7664 1-7] mixed with air and having an excess of oxygen, is passed over a platinum catalyst to form nitric oxide and water (eq. 10). The AH g = —226 kJ/mol of NH consumed (—54 kcal/mol). Heats of reaction have been derived from heats of... 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

New aluminophosphate oxynitrides solid basic catalysts have been synthesised by activation under ammonia of an AIPO4 precursor. When the nitrogen content increases, XPS points out two types of nitrogen phosphorus bonding. The conversions in Knoevenagel condensation are related to the surface nitrogen content. Platinum supported on aluminophosphate oxynitride is an active catalyst for isobutane dehydrogenation. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

Conversion of substituted nitrobenzenes to the arylhydroxylamine is easily achieved by reduction in neutral or slightly acid solution. In the first classical experiments, Haber [35] used a platinum cathode and ammonia ammonium chloride buffer and die process was improved by Brand [57] using either a nickel or silvered copper cathode in an acetate buffer. The hydroxylamine can also be obtained from reduction in dilute sulphuric acid provided tire temperature is kept below 15° C to suppress furtlier reduction [58]. This electrochemical route to arylhydroxylamines due to Brand is superior to the chemical reduction using zinc dust and ammonium chloride solution. The latter process is known to give variable yields depending on... 

Sodium-liquid ammonia reduction of the parent bicycle gave a smooth conversion to the 4,5,6,7-tetrahydro derivative 255 (R = H).165 Reduction of the pyridine ring to give 255 was also observed during hydrogenation of 2-hydroxypyrazolo[l,5-a] pyridine (240 R = OH) over platinum oxide.192... 

Careful inspection of the reported photocatalytic reactions may demonstrate that reaction products can not be classified, in many cases, into the two above categories, oxidation and reduction of starting materials. For example, photoirradiation onto an aqueous suspension of platinum-loaded Ti02 converts primary alkylamines into secondary amines and ammonia, both of which are not redox products.34) ln.a similar manner, cyclic secondary amines, e.g., piperidine, are produced from a,co-diamines.34) Along this line, trials of synthesis of cyclic imino acids such as proline or pipecolinic acid (PCA) from a-amino acids, ornithine or lysine (Lys), have beer. successfuL35) Since optically pure L-isomer of a-amino acids are available in low cost, their conversion into optically active products is one of the most important and practical chemical routes for the synthesis of chiral compounds. It should be noted that l- and racemic PCA s are obtained from L-Lys by Ti02 and CdS photocatalyst, respectively. This will be discussed later in relation to the reaction mechanism. 

J. Tafel electrolyzed a soln. of 0-4 grm. of nitric acid and 20 c.c. of 50 per cent, sulphuric acid, using 10 sq. ems. of cathode surface and 2-4 amps, at 0°. The product of the reduction is largely dependent on the nature of the metal used as electrode. Some results are indicated in Table XXVII. With platinum, no ammonia or hydroxylamine was formed, and with palladium the reduction is extremely slow. Hie chief products of the reduction are hydroxylamine and ammonia. The largest proportion of the hydroxylamine is formed when mercury is used as cathode, and the conversion of the nitric acid into this can be carried out almost quantitatively. With lead electrodes, about 40 per cent, of the nitric acid is converted into hydroxylamine, and with copper electrodes only about 15 per cent. if the copper be in the form of a spongy mass, only about one per cent, of the acid is transformed into hydroxylamine, the remainder being reduced to ammonia. When... 

Tafel1 showed that a mercury cathode or one of amalgamated lead gives the best results. At a platinum cathode very little reduction takes place, and the products are ammonia and hydroxylamine. According to Tafel, a lead cathode gives a 40 per cent, conversion of nitric acid to hydroxylamine, but with a copper ielectrode only 15 per cent, reduction to this substance takes place, whilst much ammonia is formed. 

Cobalt oxide (C03O4) catalysts are being used in some nitric acid plants as an alternative to platinum-rhodium (Pt-Rh). They generate less N2O, cost less and have a longer campaign life than Pt-Rh gauzes. (A paper in 2000 reported a conversion rate of ammonia to nitrous oxide as low as 0.5% over cobalt oxide catalyst)222. 

The manufacture of nitric acid by the oxidation of ammonia on platinum-type metal gauzes uses a technology which has change little since its first introduction in 1902. Although the conversion proceeds with an efficiency in excess of 90%, the loss of... 

Volumetric Methods.—Nickel may be conveniently estimated volu-metrically in the absence of cobalt, copper, silver, gold, and the platinum metals by means of potassium cyanide.4 The solution containing the nickel is, if acid, neutralised with ammonia and some ammonium sulphate is added to render the indicator more sensitive. A little ammonia is now added, and a few drops of potassium iodide and silver nitrate. The solution becomes turbid in consequence of the precipitation of silver iodide. The liquid is now titrated with potassium cyanide solution until the turbidity just disappears. The reaction consists in converting the nickel salt into the double cyanide, Ni(CN)a.2KCN, after which any excess of potassium cyanide attacks the silver iodide, yielding the soluble double cyanide, AgCN.KCN. The disappearance of the turbidity therefore indicates the complete conversion of the nickel salt. A slight correction is necessary for the silver introduced. 

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