Reaction rates of ammonia synthesis

Fig. 1.18. DetAminarion of maximum reaction rate of ammonia synthesis (From Nielsen).

The reaction rate of ammonia synthesis depends on the rate of nitrogen chemisorption, the amoimt of adsorbed nitrogen (retardation), and the amount of adsorbed... 

In ammonia synthesis reaction, the dissociative adsorption of dinitrogen is the rate determining step. The electron donor type promoter, such as alkali metals, which can provide electrons to the Ru, is more propitious to activate the N=N bond in N2 molecule and thus enhance the dissociative adsorption of dinitrogen and increase the reaction rate of ammonia synthesis. The acidity and basicity of supports have important influence on the activity of the ruthenium catalysts without promoter. Acidic supports can easily accept electrons and decrease the activity of catalysts. Basic supports can easily provide electrons and enhance the dissociative... 

Structure Sensitivity over Re. As in the case of the Fe catalysts the rate of ammonia synthesis varies greatly over Re single crystal surfaces of different orientations. This phenomenon has been studied over the (0001), (loTo), (1120) and 0121) planes in a 3 1 Hp/N mixture at a total pressure of 20 atm. and a temperature or 870 K. Under these conditions these surfaces catalyze the reaction with relative rates of 1 94 920 2820 respectively, showing a range of activities even greater than that observed on Fe. 

From the data presented in the following tables, determine the rates of ammonia synthesis (moles NH3 produced per min per gcat) at 350°C over a supported ruthenium catalyst (0.20 g) and the orders of reaction with respect to dinitrogen and dihydrogen. Pressures are referenced to 298 K and the total volume of the system is 0.315 L. Assume that no ammonia is present in the gas phase. 

Predicting the Total Reaction Rate for an Ru Catalyst. Based on the information above, Honkala, Norskov and coworkers took on the extremely challenging task of seeking to predict the overall reaction rate for ammonia synthesis on a real catalyst with essentially no experimental input.35 To do so, they had to tackle two principal hurdles to connect their previous DFT results with a real material the fact that a real material is typically covered by a complex arrangement of adsorbates and the need to describe the number of step sites on a realistic catalyst quantitatively. An important note for an in depth reading of the paper by Honkala et al. is that much of the technical detail is included in the supplementary information associated with the online version of the paper. 

Ozaki et al. (33) compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen imder the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. 

Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis The initial rate of ammonia synthesis has been determined over the clean Fe(l 11), Fe(lOO), and Fe(l 10) surfaces with and without aluminum oxide. The addition of aluminum oxide to the (110), (100), and (111) faces of iron decreases the rate of ammonia synthesis in direct proportion to the amount of surface covered [47]. This suggests that the promoter effect of aluminum oxide involves reaction with iron which cannot be achieved by simply depositing aluminum oxide on an iron catalyst. 

The activity of the Fe(l 10) and Fe(lOO) surfaces for ammonia synthesis can also be enhanced to the level of Fe(l 11) by water-vapor pretreatments in the absence of aluminum oxide, but in this circumstance the enhancement in activity is only transient. Figure 7.18 shows the rate of ammonia synthesis as a function of reaction time for restructured Fe(llO) and Al Oy/FeCl 10) surfaces. Both surfaces have an initial activity similar to that of the clean Fe(lll) surface. The restructured AlvOy/Fe(l 10) surface maintains this activity for over 4 hr while the restructured Fe(l 10) surface loses its activity for ammonia synthesis within 1 hr of reaction. 

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions The changes in the apparent reaction order dependence in ammonia partial pressure suggest that to elucidate the effects of potassium on both iron single crystals and the industrial catalyst, it is necessary to understand the readsorption of gas-phase ammonia on the catalyst surface during ammonia synthesis The fact that the rate of ammonia synthesis is negative order in ammonia synthesis. Once adsorbed, the ammonia has a certain residence time (t) on the catalyst which is determined by its adsorption energy on iron [t cx tq exp (AH /RT)]... 

Synthesis pressure, synthesis temperature, space velocity, inlet gas composition, and catalyst particle size all affect ammonia synthesis. LeChatelier s principle helps explain how synthesis pressure affects the synthesis of ammonia. As the ammonia reaction takes place, there is a decrease in volume. Thus, raising the pressure increases the equilibrium percentage of ammonia and accelerates the reaction rate. The ammonia synthesis reaction is exothermic therefore, higher temperatures increase reaction rates and thermal degradation of the catalyst. But the equilibrium amount of ammonia decreases with an increase in temperature. Space velocity, the ratio of the volumetric rate of gas at standard conditions to the volume of the catalyst, decreases the... 

The Haber-Bosch catalytic process for production of ammonia is perhaps an invention that had the most dramatic impact on the human race (Ritter 2008). The inexpensive iron-based catalyst for ammonia synthesis, which replaced the original, more expensive osmium and uranium catalysts, made it possible to produce ammonia in a substantially effective manner. The objective here was not improvement in selectivity but higher reaction rates for rapid approach to the equilibrium conversion at the specified temperatme and pressme. Higher rates meant lower catalyst volume and smaller high-pressme reactors. The iron catalyst was improved by addition of several promoters such as alkali metals. In contrast to this simple single reaction case of ammonia synthesis, most organic reactions are complex with multiple pathways. 

Table 1.19 Rate of ammonia synthesis reaction on rare earth metal inter-metallic compounds ...

The study on the pattern of nitrogen chemisorption is very important for the understanding of reaction mechanism of ammonia synthesis. But the chemisorption of nitrogen on the surface of transient metals is very complex. Taking chemisorption on iron catalyst for ammonia synthesis as an example, it was found that the formation rate of ammonia depends on pre-adsorption temperature if the adsorption of nitrogen was preceded first on the surface of activated catalyst, and then hydrogen was passed on surface of pre-adsorbed nitrogen. It is considered that there exist two adsorption patterns The first one is of L-type which appeared at 200°C of pre-adsorption temperature, in which the formation rate of ammonia was proportional to second one is of H-type which appeared at 400°C 40°C of... 

Besides, it is possible, for the first time, to propose what type of surface structure was responsible for the enhanced rate of reaction. Clearly, the surface atoms or active sites involved become more abundant as the cluster size grows. Since we deal with clusters, with ill-defined crystal faces, it is better to describe the surface structure in terms of the number of surface atoms Cj with coordination number equal to i, rather than in terms of Miller indices used for large crystals. Dumesic et showed the parallel between enhanced values of turnover rates of ammonia synthesis and the following three phenomena. 

It can be seen from Fig. 6.63 that the rate of isotopic equilibration reaction is higher than the ammonia synthesis reaction without Sm20s. At the same time, with the addition of Sm203, the rate of isotopic equilibration reaction declines a lot, while the rates of ammonia synthesis reaction are all higher. This indicates that the effect of hydrogen is very evident at this time, which is a problem that cannot be ignored on Ru catalyst. 

Increase of reaction temperature on one hand accelerates the reaction speed but on the other hand, decreases the equilibrium ammonia concentration. The change of temperature affects the total rate of ammonia synthesis positively and negatively. Therefore there is an optimum reaction temperature at which the reaction velocity is the fastest and the synthesis efficiency is the highest. 

To model a catalytic reaction, some knowledge of the elementary reaction steps must be assumed. For ammonia synthesis it is usually accepted that the dissociative chemisorption of nitrogen is the rate-limiting step, a process which requires two adjacent open sites on the catalyst surface. " Using Langmuir-Hinshelwood kinetics the rate of ammonia synthesis (denoted by r) can be written as... 

In Fig. 4.25, the rate of ammonia synthesis versus % free iron surface, as determined by carbon monoxide TPD (see experimental section), is shown graphically. The rate of ammonia synthesis decreases roughly in proportion to the amount of iron covered by the aluminum oxide and potassium. The only mechanism for this reduction in rate is site-blocking, which occurs during initial reaction conversions (Pnhj ranges from 0 torr to 3 torr during this measurement). 

Figure 4.25. The rate of ammonia synthesis decreases roughly in proportion to the amount of iron covered by potassium and aluminum oxide at initial reaction conversions. ...

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