NH3 formation

A formate ester can be cleaved selectively in the presence of an acetate [(MeOH, 1 eflux) ordil. NH3 (formate is 100 times faster than an acetate) ] or benzoate ester (dil. NHg). ... 

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

Figure 14 shows the computed responses for 0fj, 0q, and oh These coverages increase slowly during NO adsorption but rise rapidly and pass through a maximum when H2 is added to the flow. The responses for 0jjh and 0 u are similar in shape to the response for 0qjj but are significantly smaller in magnitue and, hence, have not been shown in Fig. 14. It is significant to note that the maxima in 0q and 0jj occur at times which coincide with the time at which the rates of N2 and N2O reach a maximum. Similarly, the maxima in 0qh and 0 appear at times nearly identical to the times at which the rates of H2O and NH3 formation reach a maximum.

Early field ion emission studies of gas-surface interactions use field ionization mass spectrometry. Gas molecules are supplied continuously to the tip surface by a polarization force and by the hopping motion of the molecules on the tip surface and along the tip shank. These molecules are subsequently field ionized. The role of the emitter surface in chemical reactions is not transparent and has not been investigated in detail. Only in recent pulsed-laser stimulated field desorption studies with atom-probes are these questions addressed in detail. We now discuss briefly a preliminary study of reaction intermediates in NH3 formation in pulsed-laser stimulated field desorption of co-adsorbed hydrogen and nitrogen,... 

Lastly, non-elementary several-stage reactions are considered in Chapters 8 and 9. We start with the Lotka and Lotka-Volterra reactions as simple model systems. An existence of the undamped density oscillations is established here. The complementary reactions treated in Chapter 9 are catalytic surface oxidation of CO and NH3 formation. These reactions also reveal undamped concentration oscillations and kinetic phase transitions. Their adequate treatment need a generalization of the fluctuation-controlled theory for the discrete (lattice) systems in order to take correctly into account the geometry of both lattice and absorbed molecules. As another illustration of the formalism developed by the authors, the kinetics of reactions upon disorded surfaces is considered. 

The kinetic equations are useful as a fitting procedure although their basis - the homogeneous system - in general does not exist. Thus they cannot deal with segregation and island formation which is frequently observed [27]. Computer simulations incorporate fluctuation and correlation effects and thus are able to deal with segregation effects but so far the reaction systems under study are oversimplified and contain only few aspects of a real system. The use of computer simulations for the study of surface reactions is also limited because of the large amount of computer time which is needed. Especially MC simulations need so much computer time that complicated aspects (e.g., the dependence of the results on the distribution of surface defects) in practice cannot be studied. For this reason CA models have been developed which run very fast on parallel computers and enable to study more complex aspects of real reaction systems. Some examples of CA models which were studied in the past years are the NH3 formation [4] and the problem of the universality class [18]. However, CA models are limited to systems which are suited for the description by a purely parallel ansatz. 

The equations are written for the specific reaction of NH3 formation as a fully general approach would be unwieldy. The modification of the approach to other reactions is not trivial but could be done following the outline below. Another application of the formalism to a very complex reaction system (CO + O2 on a Pt/Sn disordered catalyst) is demonstrated, as well as the generality of the stochastic ansatz [28],... 

In [4] we have introduced a CA model for the NH3 formation which accounts only for a few aspects of the reaction system. In our simulations the surface was represented as a two-dimensional square lattice with periodic boundary conditions. A gas phase containing N2 and H2 with the mole fraction of t/N and j/h = 1 — j/n, respectively, is above this surface. Because the adsorption of H2 is dissociative an H2 molecule requires two adjacent vacant sites. The adsorption rule for the N2 molecule is more difficult to be described because experiments show that the sticking coefficient of N2 is unusually small (10-7). The adsorption probability can be increased by high energy impact of N2 on the surface. This process is interpreted as tunnelling through the barrier to dissociation [32]. 

Twenty-six moles of NH3 and 19 moles of H2 are produced per mole of cluster in 4 hr. The catalytic activity of [Mo-Fe]m is superior to that of [4-Fe]n with respect to NH3 formation, and in both cases, higher pH favors the fonnation of NH3. The maximum current efficiency using [Mo-Fe]m catalyst in MeOH/thf is 97%. 

The escape of NH3 was also investigated by Luo et al. [152,153], They reported that the concentration of NH3 in desorbed H2 increased with the desorption temperature. For the (2LiNH2 + MgH2) system the NH3 concentration was found to be 180 ppm at 180°C and 720 ppm at 240°C. The capacity loss after 270 cycles at a temperature of 200°C was 25%, with 1/3 of the loss due to NH3-formation. They concluded that more research is needed to determine the cause for the remaining capacity loss. 

To predict the potential concentration of metal-ammine complexes in solution, one needs to understand the relationship between pH and NH3 formation. Consider the equation... 

The equilibrium favors NH3 formation at low temperature and high pressure. In order to achieve acceptable reaction rates and conversion to NH3, the reaction is carried out over an Fe or Fe203 catalyst at 500 °C and ca. 10 Pa. 

Many techniques have been applied to examine the reaction pathways of the NO-CO and NO-H2 reactions and to elucidate the reaction mechanisms. In this section some of the relevant results are discussed with emphasis on the reaction mechanism. Both the NO-CO and NO-H2 reactions are discussed here since it is likely that the mechanisms of N2 and N2O formation are independent of the type of reducing agent. Note that CO dissociation is not considered to be involved in the mechanism. However, dissociative adsorption of NO into N and O adatoms is an important process on the relevant metals, as discussed in Section 11.A. Possible mechanisms of N2, N2O, and NH3 formation can then be evaluated on the basis of the following hypothetical mechanisms involving all the possible elementary steps in which NOaus, Nads, Oads, COads, and Hads can participate ... 

An interesting effect of the surface structure was also found for Pt by Zemlyanov et al. (82, 83). HREELS and TDS confirmed that NO desorbs completely from Pt(111) without dissociation. However, N2 and NH3 formation were fo und upon heating the NO-covered Pt(l 11) surface in a hydrogen atmosphere. HREELS results were interpreted in terms of formation of HNOads- It was proposed that NH3 and N2 are formed without direct dissociation of NO ... 

The results summarized previously show that the surface structure and the presence of coadsorbed species have a large effect on the relative stability of NHads and NH2,ads- However, more detailed studies are required to understand the relative stabilities of N H and NH2 on noble metal surfaces. In conclusion, there is ample evidence that process VI is a major mechanism for NH3 formation. The intermediates NH have been identified. The only possible exception may be Pt(lll). 

Similar studies of co-adsorption of ethylene and NO have also been reported Many of the interactions observed for N and C deriving from NO and CO are seen in this case. However, since no oxygen derives from dissociation of ethylene (as it does for CO), the observed formation of CO must be due to a reaction with ceria derived oxygen. The presence of H also opens the possibility of NH3 formation, which is seen in small amounts. 

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