OCR Text |
Show 1.6.9 to those of Fig. 3. For decreasing values of A the curves again show an upward rather than a downward shift of the optimum temperature for NO reduction. Also, the curves do not reproduce the relatively small dependence on A of the reduction in NO obtained at low temperatures. It is of interest to compare the value given in Eq. (20) for P k2/(kjk3> with the results of Fenimore [121 Fenimore used k3 = 3 x 10^ cm3gmole"^s""^, and found from a comparison with his experimental data that k2/k^ = 10 ± 5. The resulting value of P k2/(k^k3) is 3.4 ± 1.7 ppm atm s, agreeing within error bounds with Eq. (20). IV. RATE LIMITATION SET BY THE FORMATION OF OH. It was found in the preceding section that the OH-concentration in the model considered is well below its equilibrium value. This implies that the assumption [OH] = constant should be reconsidered. At the higher temperatures, the ratio COH3/[OH3eq can be expected to drop below unity initially, and to return towards unity as reactions <i)-(3) approach completion. At the lowest temperatures, the OH-concenctration might be approximately constant as long as the NH3~concentration is approximately constant. However, there arises another difficulty at the lowest temperatures: each OH-radical produced would essentially provide for the reduction of one NO-molecule, so that the reduction would be independent of the NH3-concentration. This is contrary to experimental evidence (see Fig. 3). In order to resolve these difficulties, the following revisions were incorporated in the model: 1. The rate at which OH-radicals are produced was represented by ([OH]eq -COH])/TQH» where T Q H IS a characteristic time for the replenishment of OH. 2. Following Lyon and Benn [10] and others [13], [15], [16] and [20], reaction (1) was replaced by NH2 • NO -> (1 - ocXN2 • H20) + (N2 • H + OH) (la) Here, oc is an adjustable parameter with a value between 0 and 1. For a - 0, Eq. (la) reduces to Eq. (1). As indicated in the introduction, the term with oc represents a global reaction, equivalent to channel 1(A) of [131 3. Following Miller, Branch and Kee [13], the H-atoms formed in reaction (la) were assumed to contribute to the formation of NH2 via NH3 * H -»NH2 + H2. (3a) Inclusion of reactions (3a) and (la) with « ^ 0 provides a mechanism to account for the |