Ammonia synthesis over iron

Bare S R, Strongin D R and Somoqai G A 1986 Ammonia synthesis over iron single crystal catalysts—the effects of alumina and potassium J. Phys. Chem. 90 4726... 

While chlorine is a poison for the ammonia synthesis over iron, it serves as a promoter in the epoxidation of ethylene over silver catalysts, where it increases the selectivity to ethylene oxide at the cost of the undesired total combustion to C02. In this case an interesting correlation was observed between the AgCl27Cl ratio from SIMS, which reflects the extent to which silver is chlorinated, and the selectivity towards ethylene oxide [16]. In both examples, the molecular clusters reveal which elements are in contact in the surface region of the catalyst. 

Unfortunately, these requirements have not yet fully been met for any catalytic reaction, although for some simple catalytic reactions reasonable approaches are known. Such reactions are the oxidation of CO over a supported Rh catalyst [46,47], ammonia synthesis over iron [48, 49], and the HCN synthesis over a Pt gauze catalyst. More recently Wolf [50] carried out a micro-kinetic analysis of the primary reaction steps in the oxidative coupling of methane and also related the rate... 

Table 7.3.1 CHAPTER 7 ivncroKineTic Analysis nr e.aiaiyiic Heantinn< 1 A proposed mechanism of ammonia synthesis over iron catalysts. ig /... 

Describe the main differences in the kinetics of ammonia synthesis over iron catalysts compared to ruthenium catalysts. 

The rate used here is what has been called the nominal turnover rate (4), moles reacted per second per mole of sites, taken as surface atoms for metal catalysts. These sites are measured, for example, by hydrogen chemisorption, electron microscopy, X-ray diffraction, and magnetic methods. Of course, only a fraction of these sites may be active, but this fraction has to be learned by kinetic experiments and is subject to change as new kinetic results become available (6). This fraction seems to be known for ammonia synthesis over iron and can be measured by nitrogen chemisorption. 

Effects of Potassium on Ammonia Synthesis Kinetics Extensive research has been completed in which the effects of potassium on ammonia synthesis over iron single-crystal surfaces of (111), (100), and (110) orientations [59] have been determined. The apparent order of ammonia and hydrogen for ammonia synthesis over iron and K/Fe surfaces has been determined in addition to the effect of potassium on the apparent activation energy Ef) for the reaction. In all the experiments, potassium was coadsorbed with oxygen because only about 0.15 ML of potassium coadsorbed with oxygen is stable under ammonia synthesis conditions (20... 

D.R. Strongin and G.A. Somorjai. The Effects of Potassium on Ammonia Synthesis over Iron Single Crystal Surfaces. J. Catal. 109 51 (1988). 

In addition to the BET equation, Paul Emmett made enduring contributions to the experimental determination of gas-solid equilibria and the understanding of ammonia synthesis over iron-based catalysts. He also pioneered the development of selective chemisorption methods to estimate the surface composition of multicomponent catalysts and the use of tracer methods to explore the mechanism of Fischer-Tropsch synthesis and catalytic cracking. 

The experimental work of the past 50 years leads to the conclusion that the rate-determining step in ammonia synthesis over iron catalysts is the chemisorption of nitrogen. The question as to whether the nitrogen species involved on the surface is molecular or atomic is still not conclusively resolved, though, in my opinion, the direct participation of nitrogen in an atomic form seems more likely than that in molecular form. 

The initial method for the measurement of a was to determine the rate at approaching equilibrium. Taking ammonia synthesis over iron catalyst as an example,tracer is N. Assuming elementary step sequence of the reaction as follows ... 

A SURFACE SCIENCE AND CATALYTIC STUDY OF THE EFFECTS OF ALUMINUM OXIDE AND POTASSIUM ON THE AMMONIA SYNTHESIS OVER IRON SINGLE-CRYSTAL SURFACES... 

The first attempt to use the simultaneous solution approach for ammonia synthesis over iron catalysts gave estimated reaction rates several orders of magnitude lower than those found in practice. This is discussed in the chapter on iron kinetics, but can be attributed to inadequacies in the underlying database, in particular the use of low-coverage adsorption data. The closest approach to a mechanistically acceptable model has come from studies into ammonia decomposition on platinum. " The characteristics of this reaction over many transition metals are ... 

Strongin DR, Somoijai GA. The effects of potassimn on ammonia synthesis over iron singlecrystal surfaces. J Catal 1988 109 51-60. 

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