NH3 oxidation

As already mentioned in section 2.5.1.4, oxygen is dissociatively adsorbed on most metals even below room temperature. Thus under conditions of technological interest (e.g. in the NH3 oxidation reaction) the... 

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

Table 3.3 Catalytic activity of supported lb metal catalysts for NH3 oxidation [70].

CU/AI2O3, and AU/AI2O3 catalysts and the effects of Ii20 and CeO addition [69]. However, the additives caused a decrease in the N2 selectivity but remarkably improved the catalytic activity, in particular, a decrease in Tso over 200°C in the case of gold. Gold catalysts have a potential for NH3 oxidation at lower temperature if a proper kind of support metal oxides is selected. 

Fig. 3. Comparison of average NH3 oxidation rate of Iwt. %, 3wt. %, lOwt. % and 15wt. % VOx supported on Ti02 at 348 K over 3 h.

Fig. 1 Effect of temperature on NH3 oxidation. Closed symbols electric finnace, open symbols microwave heating,, O NH3 conversion, A, A N2 selectivity,, NO selectivity, BO N2O selectivity.

In the absence of O2, NO reduction continued, however at a rate about ten times lower than that in the presence of O2. During 20 h experiments NO conversion remained constant. On O2 addition, the catalytic activity increased with O2 content in the mixture up to about 1000 ppm, and changed little thereafter. We noticed that increasing the O2 concentration caused NO conversion to become lower than that of NH3, probably due to changes in the stoichiometry of the overall reaction (the NO/NH3 ratio passed from 1.5 to 1). Catalytic tests of NH3 oxidation with O2 yielded high selectivity to N2 (66-90%), which decreased with the higher loading catalysts. 

Pb(110) at 77 K and warming to 140 K with (b) electron energy loss spectrum confirming the presence of surface hydroxyls at 160K when molecularly adsorbed water has desorbed. Both the oxide overlayer at Pb(110) and the atomically clean surface are unreactive to water. H abstraction was effected by transient Os states, which were also active in NH3 oxidation. (Reproduced from Refs. 40, 42). 

Mass and heat transfer between the bulk fluid phase and the external catalyst surface can have an affect on reaction rates, and hence the selectivity, because of modified concentration and temperature driving forces. Such effects are unimportant for porous catalysts, but are significant for catalysis by non-porous metallic gauzes (for example, in NH3 oxidation referred to in Sect. 6.1.1). 

Thus in this example, the prod t indicated is fed into the next stage, where it is reacted with other species (H2O, N2, O2, toluene). Different temperatures and pressures are usually used in each stage to attain optimum performance of that reactor. For example, NH3 synthesis requires very high pressure (200 atm) and low temperature (250°C) because it is an exothermic reversible reaction, while NH3 oxidation operates at lower pressurse (-10 atm) and the reaction spontaneously heats the reactor to -800°C because it is strongly exothermic but irreversible. Formation of hquid HNO3 requires a temperature and pressure where liquid is stable. 

This is frequently the required mode of operation for fast oxidation reactions because the heat release is too fast to provide efficient heat exchange. Most combustion processes are nearly adiabatic (your home furnace and your automobile engine), and many catalytic oxidation processes such as NH3 oxidation in HNO3 synthesis are nearly adiabatic. 

This mode is used industrially for exothermic reactions such as NH3 oxidation and in CH3OH synthesis, where exothermic and reversible reactions need to operate at temperatures where the rate is high but not so high that the equilibrium conversion is low. Interstage cooling is frequently accomplished along with separation of reactants from products in units such as water quenchers or distillation columns, where the cooled reactant can be recycled back into the reactor. In these operations the heat of water vaporization and the heat removed from the top of the distillation column provides the energy to cool the reactant back to the proper feed temperature. 

As will be discussed further in this chapter, there is now much evidence to suggest that NO is an obligatory intermediate in the denitrification pathway. Furthermore, there is evidence that NH3 nitrifiers can synthesize the denitrification apparatus in addition to the nitrification apparatus and that the former system can produce NO and N2O (also N2 in at least one case) from nitrite under low partial pressures of O2. It is possible therefore that NO may be an intermediate in the denitrification activity of nitrifiers and so arise as a secondary consequence of NH3 oxidation. NO can also be ptoduced by nondenitrifying organisms under certain conditions. For example, NO can be slowly produced by the anaerobic reduction of nitrite, but only in absence of nitrate, by a variety of enteric bacteria. Some of the NO can be further reduced to N2O. 

The parameter estimates associated with the fitting in Fig. 36 well compare with other estimates for NH3 oxidation over V-based SCR catalysts reported in the literature. 

The following reactions were included in the kinetic model NH3 adsorption (R3 in Table V), NH3 desorption (R4 in Table IV), NH3 oxidation (R5 in Table IV) and standard SCR (R6 in Table V). Mass balances for adsorbed ammonia and nitrogen now include the standard SCR reaction. Moreover, the mass balance of gaseous NO was introduced, too... 

Ammonia oxidation under low pressures, as a method of transferring this reaction into the kinetic region, is inapplicable in the case of the Co304 catalyst since, at such temperatures as 700°C and low 02 pressures, Co304 decomposes with the formation of CoO. In order to obtain information on the kinetic of NH3 oxidation on Co304, we studied limiting loads at which the catalyst is extinguished (166, 167). The experiments were performed at pressures from 1 to 9 atm. A catalyst pellet was placed in a vertical tube of a diameter such that the cross-section occupied by the pellet comprised one-half of the cross-section of the tube. This was an imitation of conditions in the bed of pellets. The gas mixture at the inlet was at room temperature the stream of the mixture was directed downward. 

This mechanism is a constituent of the mechanisms for various catalytic reactions, e.g. for NH3 oxidation [233]. 

Experiments using Pb cathodes in an undivided cell (NH3 oxidation as the anode process) indicate rapid deactivation of the cathodes. By optimizing the electrolyte, the operating times can be increased to about 40 hours without deactivation 395). However, this operating time is still far from sufficient from the industrial point of view. 

Figure 2 shows that this new reaction has a temperature window, i.e. it occurs only in a narrow range of temperatures. If the temperature is too high, the NH3 oxidizes to form NO. If the temperature is too low, little or no reaction occurs. As Figure 2 also shows, adding some H2 to the NH3 shifts this temperature window to lower values but does not make it any wider. 

The presence of a PtRh gauze catalyst catalyzes the reactants along the NO pathway with a selectivity of 98%. Therefore although the free energy is more favorable and the equilibrium constant for the N2 reaction is 105 times greater, the highly selective PtRh catalyst promotes the NH3 oxidation reaction to NO. In contrast the presence of Pd favors the N2 product. In each case the catalyst respects the equilibrium constant but directs the reactants to specific products. 

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