Low-temperature catalyst

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). 

The most popular SCR catalyst formulations are those that were developed in Japan in the late 1970s comprised of base metal oxides such as vanadium pentoxide [1314-62-1J, V20, supported on titanium dioxide [13463-67-7] Ti02 (1). As for low temperature catalysts, NO conversion rises with increasing temperatures to a plateau and then falls as ammonia oxidation begins to dominate the SCR reaction. However, peak conversion occurs in the temperature range between 300 and 450°C, and the fah-off in NO conversion is more gradual than for low temperature catalysis (44). 

A study of reaction kinetics on a low-temperature catalyst (130) was performed under atmospheric pressure at 150, 175, 200, and 225°C. The composition of the catalyst was (moles) ... 

The form of the kinetic equation suggests that the reaction mechanism on the low-temperature catalyst does not differ from the mechanism on the high-temperature catalyst i.e., is described by scheme (343). 

The shift conversion involves two stages. The first stage employs a high-temperature catalyst, and the second uses a low-temperature catalyst. The shift converters remove the carbon monoxide produced in the reforming stage by converting it to carbon dioxide by the reaction... 

The most attractive monopropellants are those whose exothermic reaction or decomposition can be catalytically initiated. The employment of monopropellants requiring thermal ignition results in the undesirable addition of an ignition device to the propulsion system. The recent development of a low temperature catalyst for the decomposition of hydrazine has made hydrazine and hydrazine based compounds among the most attractive monopropellants for propulsion purposes. 

With the need to reduce olefins and aromatics in gasoline, C5/C6 isomerization has become increasingly important to a fuels refinery. The goal is to maximize gasoline octane with recycle flow schemes and with catalysts that are active at low temperatures. Catalysts also must have high selectivity to isomerization in order to... 

Shell-side effluent from the reforming exchanger is cooled in a waste-heat boiler, where HP steam is generated, and then flows to the CO shift converters containing two catalyst types one (4) is a high-temperature catalyst and the other (5) is a low-temperature catalyst. Shift reactor effluent is cooled, condensed water separated (6) and then routed to the gas purification section. C02 is removed from synthesis gas using a wet-C02 scrubbing system such as hot potassium carbonate or MDEA (methyl diethanolamine) (7). 

Reforming-exchanger effluent is cooled in a waste-heat boiler, where high-pressure steam is generated, and delivered to the CO shift converters containing two catalyst types One (4) is a high-temper-ature catalyst and the other (5) is a low-temperature catalyst. 

The second version has a S/C ratio of 2.5 and shift conversion with medium- and low-temperature catalysts, both copper-based. For C02 removal Selexol or aMDEA is chosen. The synthesis is performed at 140 bar with a Topsoe two-bed S 200 radial converter, followed by a single-bed radial S 50 converter (S 250 configuration). After the converters, high-pressure steam is generated. An additional proprietary item is the side-fired reformer. 

Low temperature catalysts These catalysts have the advantage of operating at lower temperatures, but are susceptible to sulfur and particulates. 

Low temperature units are particularly susceptible to fouling in high sulfur applications. Plus, some low temperature catalysts are susceptible to particulate plugging. 

The flue gas has to be reheated to the optimum temperature of the catalyst. Expensive regenerative heat exchangers are needed. Today, however, low-temperature catalysts are available [56]. 

Thus we may conclude that the low-temperature catalyst surface contains metal ions (electronic and ESR spectral data) in the form of complexes whose coordination sphere involves water and solvent molecules (IR spectral and elemental quantitative analysis). 

Some catalysts such as Ce-EMT, Ga-EMT and Cu-SAPO-34, although presenting a significant activity, seem to be non suitable for this application since the temperature range of activity is too high. However, if thermally stable, they could be associated with a low-temperature catalyst. 

Toshiba, in collaboration with Tokyo Electric Power Company, has developed a hybrid catalytic combustion. Here only a part of the fuel is converted heterogeneously on the catalyst. The system consists of a pre-combustion mixing zone, a low-temperature catalyst zone, and a gas-phase combustion zone. The fuel-air mixture is controlled to maintain the temperature of the catalyst below 800 C, because the catalyst is not stable above the temperature. More fuel is added downstream to attain the final combustion temperature. The function of the catalyst is to be a source of additional "pre-heat" to support the lean, homogeneous down-stream combustion. 

Amadeo and Laborde [51] analyzed five kinetic expressions for low-temperature catalysts two representing redox mechanism and three representing Langmuir-Hinshelwood model. Model I is proposed by Shchibrya et al. [2b]. Model II was a redox type model. Models III and IV were Langmuir-Hinshelwood type models that considered adsorption of four species (CO, H2O, CO2 and H2) and the final model only considered the adsorption of CO and CO2. The results of these authors indicated that only model HI described the reaction behaviour in the conditions investigated. 

The first reactor (high-temperature shift) is loaded with high-temperature catalyst, generally chromium-promoted iron oxide, which operates at 623—673 K (Ledjeff-Hey, Roes, Wolters, 2000). The second reactor (low-temperature shift) is loaded with low-temperature catalyst of copper-promoted zinc oxide, which operates at 473 K (Ledjeff-Hey et al., 2000). 

The well established activity of activated carbon as a low temperature catalyst for the oxidation of SO2 to SO3 coupled with our recent discovery of the solubility of sulfuric acid in various organic solvents spears to open up prospects for a novel sulfuric acid process suited to small installations and tunable to produce acid of any concentration for local use. 

The regeneration gas is produced by partial oxidation of natural gas, followed by the conversion of CO with steam on a low temperature catalyst or by reforming of natural gas, and is diluted typically to 2% hydrogen using steam as a carrier gas. 

The present work was undertaken to examine this possibility by trying a new method of low-temperature catalyst preparation. The method studied involves the adsorption of metal precursors on supports and the reduction by sodium tetrahydroborate solution for the preparation of supported platinum catalysts. The adsorption and reduction of platinum precursors are carried out at room temperature and the highest temperature during the preparation is 390 K for the removal of solvent. The activities of the catalysts prepared were examined for liquid-phase hydrogenation of cinnamaldehyde under mild conditions. Our attention was directed to not only total activity but also selectivity to cinnamyl alcohol, since it is difficult for platinum to hydrogenate the C=0 bond of this a, -unsaturated aldehyde compared to the C=C bond [2]. We examined the dependence of the catalytic activity and selectivity on preparation variables including metal precursor species, support materials and reduction conditions. In addition, the prepared catalysts were characterized by several techniques to clarify their catalytic features. The activity of the alumina-supported platinum catalyst prepared by the present method was briefly reported in a recent communication [3]. 

According to David Schryer, low-temperature catalysts constitute a whole new class of catalysts with abundant applications for the future. 

At present, the two major types of ammonia synthesis catalyst are the fused iron catalyst and the ruthenium catalyst, but the fused iron catalyst is still the primary catalyst in use. A fused iron catalyst may be classified in several ways according to the operating temperature as medium-temperature and low-temperature according to the state before use as pre-reduction and oxidized state or according to shape as irregular shape and regular shape. The ruthenium catalyst is a low temperature catalyst, but is not widely used because of its high price. 

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