Ammonia conversion surface reaction rate

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

The detailed variation in reaction rate with reactant pressures and surface composition has been examined at 200 and at 400 °C. The production of N 2 coincided quantitatively with the intensity of the AES N (390 V) peak the NO production rate correlated well with the intensity of the AES O (510 V) peak. At 200 °C the rate of nitrogen formation was first order in oxygen pressure but independent of NH3 pressure. Conversely at 400 °C the nitric oxide formation rate was first order in ammonia pressure above 4 x 10 Torr. Desorption experiments during the reaction proved the surface species were N atoms and O atoms respectively. 

The influence of ammonia on the partial (amm)oxidation of propene was studied over the iron antimony oxide catalyst (Sb/Fe = 2) at 375 °C (see Figure 5). The yield of the partial (amm)oxidation products acrylonitrile plus acrolein decreased with increasing ammonia partial pressure. The yield of the combustion products CO and CO2 first decreased and then increased with increasing ammonia partial pressure. The opposing trends for the yield of both product groups resulted in a complex behaviour of the conversion of propene as a function of the partial pressure of ammonia. The rate of formation of the partial (amm)oxidation products can be easily modelled as a surface reaction ocupying one or two active sites, and ammonia occupying one of the sites. 

Figure 3.8 shows why high temperature is needed for the anunonia synthesis to proceed rapidly. At 300 K, the adsorbed N-containing species are so stable with respect to NH in the gas phase that the surface will be completely covered as soon as a small amount of anunonia is formed. This means that there is no place for to dissociate and the rate is extremely low. Only at temperatures around 700 K does the thermodynamic sink associated with adsorbed NH disappear. At this high temperature, however, the reaction is uphill in free energy (endergonic). In order to push the equilibrium toward some ammonia conversion, the pressures of and need to be high (see also Figure 3.1). We will discuss the influence of pressure in the following.

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

Surface-science studies succeeded to identify many of the molecular ingredients of surface catalyzed reactions. Each catalyst system that is responsible for carrying out important chemical reactions with high turnover rate (activity) and selectivity has unique structural features and composition. In order to demonstrate how these systems operate, we shall review what is known about (a) ammonia synthesis catalyzed by iron, (b) the selective hydrogenation of carbon monoxide to various hydrocarbons, and (c) platinum-catalyzed conversion of hydrocarbons to various selected products. 

---