Catalytic Urea Decomposition

In the absence of water, urea was catalytically thermolyzed, yielding HNCO with high selectivity (Fig. 16.3a). In the presence of water, urea was efficiently 

10 % O2 in N2, total gas flow = 500 L/h at STP, GHSV = 91,000 h active masses 45 mg anatase T102, 52 mg Zr02, 48 mg H-ZSM-5 (H-MFI 27), 64 mg AI2O3, 55 mg S102. a Water-free experiments with 0.31 % ethanol in the gas phase, b Hydrolysis with 5 % water in the gas phase [58] 

TiOa H-ZSM-5 AI2O3 Z1O2 Si02 Zr02 Ti02 AI2O3 H-ZSM-5 Si02 

As for HNCO hydrolysis, Zr02 was most active for urea hydrolysis, but since Zr02 is sensitive to sulfiu- poisoning [54], Ti02 is the best for urea-SCR applications. 

The presented studies [56, 58] will not be the last on catalytic urea decomposition, yet the results already clearly show that urea thermolysis is catalyzed and that catalytic urea hydrolysis is much slower than the hydrolysis of pure HNCO. Taking into account the risk of urea-induced deactivation of the SCR catalyst [12], the ongoing trend for lower exhaust gas temperatures and the good properties of TiOi for byproduct decomposition [37], using a dedicated hydrolysis catalyst may be a good option for some urea-SCR applications. 

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Bernhard AM (2012) Urea evaporation and catalytic urea decomposition in the selective catalytic reduction of NOx. Dissertation No. 20813, ETH Zurich... 

The urea method can be applied to obtain nickel-modified catalytic filter with a uniform spatial distribution of nickel. The study of urea method showed that a reaction time of at least 6 h for urea decomposition is necessary and that a higher urea/nickel molar ration than 1.7 that led to less fixation of nickel precipitation. A maximum fixation of 75 % of the... 

In order to determine the catalytic characteristics of the encapsulated enzymes, we obtained the dependences of the stationary rate of substrate conversion on the substrate concentration. As an example, the curves of saturation of urease with urea in the reaction of urea decomposition are depicted in Figure 5. It can be seen that the dependences for urease in microcapsules, are generally similar to those for free enzyme, except small differences in the affinity constants. In particular, the Michaelis constant AM with respect to urea is 7.1 D2.2 mM for urease in microcapsules of eleven and seven layers, whereas the AM for free urease is 2.5 DO. mM. The maximal rate Umax for urease in microcapsules of eleven layers is 20% lower than that for urease in microcapsules of seven layers. The AM with respect to pyruvate for microcapsules containing LDH was not different from for free enzyme. 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

Enzymes are proteins of high molecular weight and possess exceptionally high catalytic properties. These are important to plant and animal life processes. An enzyme, E, is a protein or protein-like substance with catalytic properties. A substrate, S, is the substance that is chemically transformed at an accelerated rate because of the action of the enzyme on it. Most enzymes are normally named in terms of the reactions they catalyze. In practice, a suffice -ase is added to the substrate on which die enzyme acts. Eor example, die enzyme dial catalyzes die decomposition of urea is urease, the enzyme dial acts on uric acid is uricase, and die enzyme present in die micro-organism dial converts glucose to gluconolactone is glucose oxidase. The diree major types of enzyme reaction are ... 

Koebel, M. and Strutz, E.O. (2003) Thermal and Hydrolytic Decomposition of Urea for Automotive Selective Catalytic Reduction Systems Thermochemical and Practical Aspects, lnd. Eng. Chem. Res., 42, 2093. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

According to an O.S. amendment sheet, the procedure as described [1] is dangerous because the reaction mixture (dicyanodiamide and ammonium nitrate) is similar in composition to commercial blasting explosives. This probably also applies to similar earlier preparations [2]. An earlier procedure which involved heating ammonium thiocyanate, lead nitrate and ammonia demolished a 50 bar autoclave [3], TGA and DTA studies show that air is not involved in the thermal decomposition [4], Explosive properties of the nitrate are detailed [5], An improved process involves catalytic conversion at 90-200°C of a molten mixture of urea and ammonium nitrate to give 92% conversion (on urea) of guanidinium nitrate, recovered by crystallisation. Hazards of alternative processes are listed [6],... 

Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

This phase seems to be predominating in Zi, the sample obtained by solid state decomposition in presence of urea. This sample could have additional porosity, due to loss of CO2 during synthesis of and/or calcination. This could possibly result in exposure of additional catalytically active centers, making it highly active catalyst. The data in Table 1 illustrates the relative catalytic activity of various zinc oxide towards decomposition of propan-2-ol at 653"K at a contact time of 1.6 seconds. Under the above conditions all the catalyst showed dehydrogenation activity only the reactivity following the order Zi>Z2 4 Z5>Z3. 

The 1,5,2,4-dioxazadiazinedione system (35) has been prepared. However, these compounds decompose very readily. Thermal decomposition leads to carbon dioxide loss with the formation of isocyanates and nitrosoalkanes (Equation (1)). The compounds are stable to water, but are attacked by organic soluble nucleophiles, and reagents such as phenylmagnesium bromide, to yield products which arise from attack of the nucleophile at the carbonate carbonyl, followed by fragmentation (Equation (1)). Catalytic reduction of (35) leads to ureas <86JOC3355>. 

Catalytic activity of the same urea and its variations was examined in the Claisen rearrangement of 3-methoxyvinyl vinyl ether [20] (Table 9.6). Great rate acceleration was observed in the use of thiourea, even though completion of reaction could not be estimated due to slow decomposition of the catalyst under the reaction conditions used (run 6). 

Experiment 19.4 Catalytic decomposition of urea by the enzyme urease Some phenolphthalein solution is added to solutions of urea and methylurea which are then divided into three goblets (see illustration). The urea solution in the first glass serves as the reference. A suspension of urease is added to the second goblet containing urea solution as well as to the third containing methylurea solution. After a short while, the urea solution has a violet color due to formation of ammonia, while the methylurea solution remains unchanged. 

Several techniques have been considered to decrease NOx emission, such as selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR) with ammonia (NH3) or hydrocarbon, and direct catalytic decomposition of NO. The main disadvantage of the SCR process is the high cost associated with the consumption of reductants. Nevertheless, direct catalytic decomposition of NO without the addition of reducing agents is an effective and economical procedure to decrease NOx emission. Therefore, the direct decomposition of NO into N2 and O2 (2NO = N2 + O2) is the optimal way for NO removal, because the process is simple and there is no necessity for a reductant such as a hydrocarbon, NH3, or urea. 

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