Ammonia, silica-supported

Chemical reduction. The injection of ammonia reduces NO emissions by the reduction of NO , to nitrogen and water. Although it can be used at higher temperatures without a catalyst, the most commonly used method injects the ammonia into the flue gas upstream of a catalyst bed (typically vanadium and/or tin on a silica support). 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

A silica-supported Sn—V—P—O catalyst (Sn/V/P = 1/9/3) was investigated by Onsan and Trimm [244]. Working with a flow reactor at about 520°C, a maximum selectivity of 75% to acrylonitrile was reached at a contact time of ca. 230 g sec l-1 and an oxygen/propene/ammonia ratio of 2/1/1.75. The authors assume that the six principal products (acrylonitrile, acetonitrile, HCN, CO, C02, N2) are formed by six parallel reactions and in the first instance apply power rate equations. A more detailed analysis reveals that a Langmuir—Hinshelwood type rate equation, surface reaction being rate-determining, properly describes the production of acrolein plus acrylonitrile from propene, viz. 

In the reduction of nitrite, Horold found that supports with lower surface areas showed improved selectivity for nitrogen over ammonia. The same study found that silica supports had much higher selectivity (i.e. much less ammonia formation) than alumina supports. (Horold et al. 1993)... 

TiSi 3 TiSi 5 TiSi 7 TiSi TiO, Figure 9.7 Acid site distributions of ALD silica-supported Ti02 determined by ammonia adsorption at 353 K. The labels 1, 3,... 

Figure 9.19 Differential heats of ammonia adsorption over a silica support and silica-supported vanadia catalysts prepared by ALD (filled symbols) and Impregnation (open symbols). VS-A6 and VS-16 on one hand, and VS-A12 and VS-llO on the other hand, have comparable vanadia contents.

The best yield reported in the literature is 13.3%, with selectivity of about 60% obtained with silica-supported K-promoted iron oxide catalysts modified by amines [43c]. The same catalyst is inactive in propene oxidation with air. However, the use of ammonia/air mixtures leads to a considerably enhanced conversion with respect to air only, with 60% selectivity for the epoxide. This observation suggests a mechanism whereby ammonia is first oxidized to nitrous oxide, which subsequently produces the active oxygen species for epoxidation. 

T. Shikada, K. Fujimoto, T. Kuniigi, and H. Tominaga, "Reduction of Nitric Oxide with Ammonia on Silica Supported Vanadium Oxide Catalysts", J. Chem. Tech. Biotechnol. 1983, 22A, 446-454. 

The process by BioCatalytics is comparable in terms of starting material and product but favors the use of isolated, silica-supported, immobilized L-aspar-tase. This process was claimed to have a higher productivity than a comparable whole cell process and takes place in a plug flow reactor, which is fed with 168 and ammonia solution. The immobilized enzyme is stable and keeps half of its initial activity for approximately half a year. The high activity can best be described by the fact that a single kilogram of enzyme produces 10,000 to more than 100,000 kg of 169, making it one of the most efficient biocatalytic processes known [144]. 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Kiel JHA, Edelaar ACS, Prins W, van Swaaij WPM (1991) Performance of silica-supported copper oxide sorbents for SOx/NOx-removal from flue gas II. Selective catalytic reduction of nitric oxide by ammonia. Appl. Catal. B. 1 41-60... 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

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