Nitric oxide catalysts, temperature

The catalyst temperature is about 1100°C. Precious metal catalysts (90% Pt/10% Rh in gauze form) are normally used in the commercial processes. The converters are similar to the ammonia oxidation converters used in the production of nitric acid (qv) although the latter operate at somewhat lower temperatures. The feed gases to the converter are thoroughly premixed. The optimum operating mixture of feed gas is above the upper flammabiUty limit and caution must be exercised to keep the mixture from entering the explosive range. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

An example of a model rejection based upon temperature coefficients can be obtained from the single-site model considered by Ayen and Peters (A3) for the reduction of nitric oxide over a commercial catalyst ... 

An example of the first mechanism is provided by the oxidation of ammonia to nitric oxide over a platinum gauze catalyst, where reaction is initiated by raising the temperature of the gauze by some external means once reaction has been started in this way, it is self-sustaining (autothermal), heat being transferred to the cold reactants by conduction through the gauze and by radiation. 

Nitric oxide is capable of undergoing a variety of redox reactions, which are summarized as reduction potentials in Table 1, including disproportionation to N2O and NO2 (N2O3) or even to N2 and O2 with a zeolite catalyst [8]. At low temperature it can dimerize in a side-to-side fashion to form N2O2 (C2v) and, at higher temperature, pair electrons with NO2 to form N2O3 [18]. 

The reaction is exothermic (see Exercise 12.1), but, since it is very slow, a catalyst is necessary. Nitric oxide, once again, can serve as an oxygen carrier, as in the lead chamber process (Section 10.2) and in reaction 10.8, where (CH3)2S generated in the kraft process is converted to DMSO. Even so, at the elevated temperatures required, reaction 12.1 needs to be forced to completion by absorption of the steam in concentrated sulfuric acid or some other desiccant. In variants of the Deacon process, copper chloride acts as the catalyst or as an intermediate for chlorine regeneration. 

A similar activating effect is produced by S02 during pretreatment of the catalyst or by addition of S02 to the reagents in proportions between 0.1 and 25% of the zeolite weight (79). Important variations can also be produced by CS2, phenylmercaptans, nitrous and nitric oxides, nitro compounds and others (80, 81). Since most of these compounds are acidic, the problem of structural damage arises, especially at high temperatures. Information on this problem is scarce (56). 

Catalysts are of enormous importance, both in the chemical industry and in living organisms. Nearly all industrial processes for the manufacture of essential chemicals use catalysts to favor formation of specific products and to lower reaction temperatures, thus reducing energy costs. In environmental chemistry, catalysts such as nitric oxide play a role in the formation of air pollutants, while other catalysts, such as platinum in automobile catalytic converters, are potent weapons in the battle to control air pollution. 

Another important application of heterogeneous catalysts is in automobile catalytic converters. Despite much work on engine design and fuel composition, automotive exhaust emissions contain air pollutants such as unburned hydrocarbons (CxHy), carbon monoxide, and nitric oxide. Carbon monoxide results from incomplete combustion of hydrocarbon fuels, and nitric oxide is produced when atmospheric nitrogen and oxygen combine at the high temperatures present in an... 

At typical catalyst temperatures of 800°C to 940°C, nitric oxide (NO) is thermodynamically unstable and slowly decomposes into nitrogen and oxygen. Decomposition losses are minimized by avoiding excessive catalyst contact time and by rapidly cooling the gases as they exit the converter. To achieve ammonia conversions of 93% to 98% the catalyst contact time must be as short as 0.0010 to 0.0001 seconds104. 

Over AU/AI2O3 the reaction between carbon monoxide and nitric oxide is severely inhibited by oxygen, and this unfortunately militates against the promise of its use for three-way conversion at low temperature. Nevertheless the effectiveness of a gold-containing catalyst developed at the Anglo-American Research Laboratories in South Africa has been demonstrated (Figure 11.3).42 It consisted of 1% Au/CoOx in admixture with... 

Catalyst life depends on several variables. High-pressure oxidation operations (which also operate at higher temperatures, per Table 22.16) require more frequent catalyst regeneration. The presence of trace amounts of iron, calcium, MoS2 (lubricants), and phosphorus in the ammonia feed has been shown to shorten catalyst life. Deposits of iron oxides tend to catalyze the conversion of ammonia and oxygen to nitrogen and water, rather than to nitric oxide. These effects, as well as poor ammonia-air mixing and poor gas distribution across the catalyst, may reduce the yield by up to 10 percent 91,97... 

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