N2, formation

Thus, as shown already in Fig. 4.25, the rates of N2 and C02 formation are enhanced dramatically both with positive and with negative AUWr and A.67,68 As also already shown in Figures 4.51 and 4.52 and also in Figure 8.61 shown here, positive potential or current application leads to rate enhancement, p, values up to 125 for the rate of C02 formation and up to 50 for the rate of N2 formation (Figs. 4.51,4.52 and 8.61). 

This case has been already discussed in Chapter 2 (Fig. 2.3).69 The Rh film used is shown in Fig. 8.63 and exhibits inverted volcano behaviour,67 i.e. the rate of C02 and N2 formation is enhanced both with positive and with negative potentials. This is shown in Figure 8.65 and also in Figure 2.3 which depicts the rco2 and rN2 dependence on T of the unpromoted and electrochemically promoted Rh catalyst. The corresponding Tn2o vs T behaviour is shown in Figure 8.66. 

The reduction of NO by CO on Pt/p"-Al203 is another system exhibiting spectacular electrochemical promotion behaviour.7,23,24 Electrochemical supply of Na+ on the Pt catalyst surface can cause the rates of C02 and N2 formation (rCo2 and rN2)to increase by 48% and 1300%, respectively, over their values on a clean surface.23... 

Figure 9.13. Effect of pco on the rates of C02 and N2 formation at various imposed catalyst potentials on Pt/p"-Al203.23 Reprinted with permission from Academic Press.

The NO + CO reaction is only partially described by the reactions (2)-(7), as there should also be steps to account for the formation of N2O, particularly at lower reaction temperatures. Figure 10.9 shows the rates of CO2, N2O and N2 formation on the (111) surface of rhodium in the form of Arrhenius plots. Comparison with similar measurements on the more open Rh(llO) surface confirms again that the reaction is strongly structure sensitive. As N2O is undesirable, it is important to know under what conditions its formation is minimized. First, the selectivity to N2O, expressed as the ratio given in Eq. (7), decreases drastically at the higher temperatures where the catalyst operates. Secondly, real three-way catalysts contain rhodium particles in the presence of CeO promoters, and these appear to suppress N2O formation [S.H. Oh, J. Catal. 124 (1990) 477]. Finally, N2O undergoes further reaction with CO to give N2 and CO2, which is also catalyzed by rhodium. 

Figure 3.6. Example of the type of kinetic information available for the catalytic reduction of NO on rhodium single-crystal surfaces under atmospheric conditions. The data in this figure correspond to specific rates for C02, N20, and N2 formation over Rh(l 11) as a function of inverse temperature for two NO + CO mixtures PNO = 0.6 mbar and Pco — 3 mbar (A), and Pno — Pco = 4 mbar (B) [55]. The selectivity of the reaction in this case proved to be approximately constant independent of surface temperature at high NO pressures, but to change significantly below Pno 1 mbar. This highlights the dangers of extrapolating data from experiments under vacuum to more realistic pressure conditions. (Reproduced with permission from the American Chemical Society, Copyright 1995).

Complementary in-situ characterization of the surface species using infrared (IR) spectroscopy has provided information on the identity and coverage of the surface species involved in the NO catalytic reduction [56]. It was found that the changes observed in the surface coverages of NO and CO correlate well with the observed changes in N20 selectivity mentioned above below 635 K, where N20 formation is favored, NO is the major adsorbate on the surface, whereas above 635 K, where N2 formation is preferred,... 

Let us note that there is no need for any organic nitroso intermediate for N2 formation. Therefore, two adsorbed oxygen species Oads (ex-NO) are remaining, strongly adsorbed... 

At this level of discussion, some confusion can be done on the origin of N2 formation during what is often considered as the oxidation of RNOx releasing Nf. The present explanation has two parts ... 

For the sake of simplicity, a 0eO2—Zr02 (70/30) mixed oxide will be now used as material. This mixed oxide has been previously shown to be able to proceed to three-way catalysis, the general concept for N2 formation over a metal cation being the same NO decomposition and oxygen species scavenging, in stoichiometric conditions, by CO as reductant [10,11],... 

From 320°C on, methane starts being consumed leading directly to C02. The beginning of C02 formation corresponds to the maximum of N02 desorption. Steady-state results (figure not shown) confirm that simultaneously with C02 formation, N2 formation also occurs (deNOx process effectively starts at 320°C). Methane reacts with N02, probably leading to the formation of oxygenated species and NO [18] according to reaction ... 

Comparing the TPSR results obtained with Co-HFER and Co/Pd-HFER catalysts, it is possible to verify that the introduction of palladium has a major importance for the improvement of the catalytic performance. The presence of Pd species and the redistribution of cobalt oxide species with the formation of Co-oxo cations can have a major role as catalytic sites for the lower temperature activation of CH4 with N02 and N2 formation. A conversion of 80 % of NOx into N2 is obtained with the bimetallic catalyst. 

The effect of NO exposure time on the time at which the N2 and N2O signals attain a maximum is shown in Fig. 16. It is seen that the model of NO reduction predicts that N2 formation peaks about 0.5 s after the peak in the N2O formation and that the peak times for both products decline by about 0.5 s as the NO exposure time is increased from 5 to 30 s. These trends are in good agreement with the data. It should be noted that since a product analysis could be taken only once every 0.5 s, it was not possible to determine product peak positions with an accuracy of better than 0.5 s. Consequently, both the predicted difference between... 

The kinetic studies of this reaction on Pd(lll) at total pressure of 116 torr (CO/NO = 1.67) showed that the rate of the reaction increased above 550 K. This behavior was similar to that found at lower pressures, where it was attributed to the presence of energy barriers to NO dissociation and N2 formation. With regard to the high-pressure work, the presence of the isocyanate species did not lead to measurable changes in the reactivity compared to the studies carried out at low pressure. It, therefore, seems that the isocyanate species acts as little more than a spectator in the reaction. 

It is found that the eomplete conversion of CsHe is obtained at the temperature eorresponding to the maximum for N2 formation and to the start of NO2 formation. This suggests that the reduetion of NO could involve a ehemisorbed NO2 as a very reactive intermediate. 

On increasing the reaction temperature (e.g. to 250°C), the induction period observed at 150 °C disappears and N2 formation is observed immediately upon admission of H2 with no time delay, while NH3 evolution is seen corresponding to... 

It was shown that the Faradaic efficiency for N2 formation strongly depends on the electrode materials. The general scheme... 

The selectivity of the acrylonitrile formation with respect to ammonia is very low (<10%) for the molybdenum-based catalysts (mainly due to N2 formation) but very high (100%) for the Sn—Sb—(Fe) catalyst. This is in agreement with the results of the separate oxidation of ammonia, which only in the case of Sn—Sb—(Fe) demands a temperature above that of the propene ammoxidation. 

Boudart et al.20 studied nitrogen adsorption and ammonia decomposition on polycrystalline molybdenum and reported that the surface was predominantly covered with adsorbed N and some dissociated species such as NH2. N2 formation was thought to proceed through the recombination of adsorbed nitrogen on the surface above 645 K.22. Haddix et al.23 have reported that decomposition of NH3 adsorbed on y-Mo2N produces... 

As mentioned earlier, for the ammonia oxidation reaction carbides can also be characterized in terms of selectivity. It has been found that the selectivity towards N20 decreases in the sequence Cr7C3 > ZrC > Mo2C, WC > VC > TiC, TaC, NbC, HfC. This is similar to the pattern observed in activity (the obvious exception is ZrC). Here, with increasing q the selectivity to N20 decreases. This finding may be due to the fact that for N20 formation more oxygen-catalyst bonds need to be broken than for N2 formation.16... 

The difference in the number of oxygen-catalyst bonds cleaved upon formation of N2 and N20 should result in a larger activation energy for N20 formation in comparison with N2 formation and should lead to an increase in selectivity to N20 with temperature.16 Accordingly, for Cr7C3 (N2) = 5 kJ/mol (N20) = 160 kJ/mol. For all carbides studied the selectivity to NaO increases with temperature. 

The complete ammonia reaction has, among other things, been hard to elucidate because little is known about the fate of N2H3 radicals. Some uncertainty exists even as to the step which leads to N2 formation, although... 

Actually this is the quantum yield for N2 formation from N2O scavenging of the photoelectrons that escape primary geminate recombination. 

Predictions were completely analogous to those for the ribbon and are shown in Figure 5. Analogous predictions resulted for other gas compositions and wire sizes. As with the ribbon, prediction for n=5 agreed well with experimental observation when correction was made for loss by N2 formation during sampling. 

Product state analysis offers a flexible way to obtain detailed state resolved information on simple surface reactions and to explore how their dynamics differ from the behaviour observed for H2 desorption [7]. In this chapter, we will discuss some simple surface reactions for which detailed product state distributions are available. We will concentrate on N2 formation in systems where the product desorbs back into the gas phase promptly carrying information about the dynamics of reaction. Different experimental techniques are discussed, emphasising those which give fully quantum state resolved translational energy distributions. The use of detailed balance to relate recombinative desorption measurements to the reverse, dissociation process is outlined and the influence of the surface temperature on the product state distributions discussed. Simple low dimensional models which provide a reference point for discussing the product energy disposal are described and then results for some surface reactions which form N2 are discussed in detail, emphasising differences with the behaviour of H2. 

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