Catalytic reductions, nitric acid with ammonia

The Pd +Cu )-catalysed reduction of nitric oxide by carbon monoxide (i.e., C0 + 2N0 C02+N20), which takes place in hydrochloric acid is believed to involve a Cu -Pd -NO species. The catalytic reduction of nitric oxide with ammonia by aqueous [Cu(NH3)4] + has been compared with those in which a Y-type zeolite was employed as a support. ... 

Nitric oxide is the simplest thermally stable odd-electron molecule known and, accordingly, its electronic structure and reaction chemistry have been very extensively studied. The compound is an intermediate in the production of nitric acid and is prepared industrially by the catalytic oxidation of ammonia (p. 466). On the laboratory scale it can be synthesized from aqueous solution by the mild reduction of acidified nitrites with iodide or ferrocyanide or by the disproportionation of nitrous acid in the presence of dilute sulfuric acid ... 

F. T. Austen said that hydrogen burns with an intense white flame in the vapour of nitric aoid. H. Gall and W. Manohot said that the catalytic reduction of nitric acid in aq. soln. in the presence of spongy platinum yields ammonia and nitrous aoid. If water be gradually added to the fuming acid, the reddish-brown colour changes... 

Selective Catalytic Reduction (SCR) is normally used in new nitric acid plants. In this process ammonia reacts with nitric oxide and nitrogen dioxide but only to a lesser extent with oxygen to selectively reduce the NOx compounds to N2. The reactions are shown below97,104 ... 

Postformation nitrogen oxide emission control measures include selective catalytic and noncatalytic reduction with ammonia, which between them are used by some 900 power station installations worldwide [51]. The catalytic removal methods are 70-90% efficient at NOx removal, but are more expensive to operate than the noncatalytic methods which are 30-80% efficient. Ammonia or methane noncatalytic reduction of NOx to elemental nitrogen is also an effective method which is cost-effective for high concentration sources such as nitric acid plants (Chap. 11). NOx capture in packed beds is less expensive, but this method is not particularly effective [23]. It is also not a very practical method either for utilities or for transportation sources. Two-stage scrubbing has also been proposed as an effective end-of-pipe NOx control measure. The first stage uses water alone and the second uses aqueous urea. 

Selective non catalytic reduction (SNCR) with NH3 is limited to industrial boilers in consequence of the relatively narrow temperature range for the reaction. Selective catalytic reduction (SCR) by ammonia has high efficiency and it can be used for many stationary sources, especially for nitric acid plants [1], and it is based on the catalytic pairing of nitrogen atoms, one fi-om nitric oxide, one fi om ammonia. This method, however, is imsuitable for small sources and vehicles. As far as automotive emission is concerned nonselective catalytic reduction (NSCR) by hydrocarbons, CO and H2 from the exhaust stream has been reported over various catalysts recently [1,3,4]. 

Selective Catalytic Reduction (SCR) [17] - Ammonia is used as a reducing agent over a catalyst to convert the NOx gases to nitrogen. These processes can reduce the concentration of NO in tail gas to 100 ppmv and lower. Existing plants with medium-pressure absorption can be economically revamped using this process. The capital cost of the selective catalytic reduction (SCR) process to remove NO from a concentration of 500-200 ppmv would increase the capital costs of a dual-pressure nitric acid plant by 2% and operational costs by US 0.30/tonne of nitric acid. The SCR method needs ammonia for reduction of NOx- The following reactions occur in the process of selective reduction of NO ... 

Briiggemann TC, Przybylski M-D, Balaji SP, Kell FJ (2010) Theoretical investigation of the mechanism of the selective catalytic reduction of nitrogen dioxide with ammonia on H-form zeolites and the role of nitric and nitrous acids as intermediates. J Phys Chem C 114 6567-6587... 

The Thermal De-NOx process was developed by Richard Lyon at Exxon in the early 1970 s and patented in 1975 [1]. It is one of three SNCR (selective non-catalytic reduction) schemes for nitrogen oxides (the others are RAPRENOx, or cyanuric acid injection, and urea injection). Such after-treatment processes are commonly used on stationary combustion systems to control NOx emissions. The Thermal De-NOx process uses ammonia as the additive, and the complex reaction by which the ammonia reacts with nitric oxide has a number of fascinating properties that have prompted considerable research over the past 15 years or so. 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

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