Water-assisted reactions

The combination of NOx trapping materials with NH3-SCR catalysts for the NOx treatment from mobile lean-burn engines has been reported. Particular attention has been paid in the mechanism of ammonia emission and reactivity toward NOx abatement in NSR process. For the first point, two reaction paths are proposed in the literature. In the presence of hydrogen during the rich pulses of the LNT regeneration, ammonia can be formed by direct reaction with the previously stored NOx. When CO is use as the reductant agent, water-assisted reaction, by hydrolysis of intermediate isocyanate species, is suggested. In the presence of water and carbon dioxide in the gas mixture, both reaction pathways co-exist due to direct and reverse water gas shift reaction. Ammonia is thereafter involved in the NOx reduction mechanism, by a sequential route in which NH3 reacts faster with NOx to yield N2 compared with its own formation rate. It is found that both the nature and the content of the basic element as well as the redox properties of the support interfere in NH3 yield. 

Based on the analogy to model systems, the following assumptions concerning gas-phase and water assisted proton transfer reactions in the DNA bases seems to be well justified. We expect that the calculated non-water-assisted gas-phase rate constants for both forward and reverse reactions will never reach the values which characterize any appreciable rate at room temperature. In other words, the value of the proton transfer barrier for non-water-assisted reactions (see Tables 12 - 16) is too large in both directions to be reached with any really... 

Some adverse reactions, such as dry mouth, episodes of orthostatic hypotension, and drowsiness, may need to be tolerated because drug therapy must continue. Nursing interventions to relieve some of these reactions may include offering frequent sips of water, assisting the patient out of the bed or chair, and supervising all ambulatory activities. 

Scheme 7.7 Water-based microwave assisted reactions...

These profiles clearly show that in the gas phase the alkylations of both ammonia and water by o-QM are assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism), since S4 and S5 TSs are favored over their uncatalyzed counterparts (SI and S2) by 5.6 and4.0 kcal/mol [at the B3LYP/6-311 + G(d,p) level], respectively. In contrast, the reaction with hydrogen sulfide in the gas phase shows a slight preference for a direct alkylation without water assistance (by 0.8 kcal/mol). 

The bulk solvent effect on the reaction energy, described by the lower portion of Scheme 2.3, significantly modifies the relative importance of the uncatalyzed and water-assisted alkylation mechanism by o-QM in comparison to the gas phase. 

FIGURE 2.2 Optimized TS geometries of the cytosine alkylation reaction by o-QM, without and with water assistance. Bond lengths (in A) and activation Gibbs free energies (in kcal/mol) in the gas phase and in aqueous solution (in parentheses) at the B3LYP/6-311 +G(d,p)// B3LYP/6-31G(d) level of theory with respect to the reactants have been taken from Ref. [14]. 

Jacobs, G., Patterson, P.M., Graham, U.M., Crawford, A.C., Dozier, A., and Davis, B.H. 2005. Catalytic links among the water-gas shift, water-assisted formic acid decomposition, and methanol steam reforming reactions over Pt-promoted thoria. J. Catal. 235 79. 

The oxyl radicals resulting from the decomposition of organic peroxides are energy-rich intermediates and undergo a number of rapid reactions (Chap. 7). Here, it is briefly recalled that the tertiary oxyl radicals undergo facile (3-fragmentation [reaction (52)], and the primary and secondary oxyl radicals an equally fast water-assisted 1,2-H shift [reaction (53)]. 

This process must be water assisted (see also Elford and Roberts 1996), since this reaction requires a high activation energy in the gas phase (Batt et al. 1981 ... 

The results of a multiple isotope effect study of the acid-catalysed hydrolysis of HC02Me (Scheme 5) have provided a detailed picture of the transition-state structure for the reaction, which involves one hydronium ion and two water molecules (15).8 DFT calculations on the water-assisted neutral hydrolysis of MeC02Et indicated that a stepwise process involving four molecules of H20 is energetically favoured.9... 

A systematic ab initio investigation of the water-assisted decomposition of chloro-methanol, dichloromethanol, and formyl chloride as a function of the number of water molecules (up to six) building up the solvation shell has been reported.33 The decomposition reactions of the chlorinated methanols and formyl chloride are accelerated substantially as the reaction system involves additional explicit coordination of water molecules. 

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