Iron-ammonia catalyst surface measurement

The state of iron ammonia catalysts is dealt with in the following chapters, and x-ray, magnetic, and electric data will be discussed together with adsorption measurements. Information about the catalysts combined with kinetic experiments has led to a fairly good qualitative understanding of ammonia synthesis on iron catalysts, but owing to the extremely complicated nature of the catalyst surface during reaction, a quantitative treatment based on data of catalyst and reactants will not be attained in the near future. 

In the catalytic combination of nitrogen and hydrogen, the molecules lose their translational degrees of freedom by fixation on the catalyst surface. This drastically reduces the required energy of activation, for example, to 103 kj/mol on iron [100], The reaction may then proceed in the temperature range 250-400 °C. In 1972, it was discovered that electron donor - acceptor (EDA) complexes permit making ammonia with measurable reaction rate at room temperature. 

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. 

The well-known poisoning of the iron catalyst, used in ammonia synthesis, by minute amounts of water vapor or oxygen seems to be compatible with chemisorption measurements. According to Almquist and Black (85) only 10 to 15% poisoning of the total B.E.T. surface of this catalyst causes a decrease of its catalytic activity by about 70%. Since only a part of the catalyst surface actively chemisorbs hydrogen, and, probably, nitrogen, the area active for the formation of ammonia can also be expected to be a mere fraction of the total surface. 

Brunauer and Emmett 120), in their extensive studies on synthetic ammonia catalysts have concluded, by a comparison of the CO uptakes and nitrogen adsorption surface area measurements, that on pure iron at temperatures between —78 and — 183°C CO chemisorbs up to one molecule per surface atom. Beebe and Stevens 121) from measurements of differential heats of adsorption confirmed that chemisorption rather than physical adsorption was occurring in this system. 

Measuring Surface Promoter Distribution Attention has already been called to the use that has been made of surface area measurements in studying synthetic ammonia catalysts. It has been possible, for example, to show that certain promoters have a specific influence on the activity per unit surface area (4). Thus iron catalysts containing both K2O and AI2O3 as promoters have surfaces that are only about one-third as large as those containing only AhOs as promoter and yet are several fold more active under synthesis conditions. 

In 1935 Brunauer and Emmett [121] carried out the first successful attempt to determine - by means of isotherm adsorption of six different gases - the surface area of an iron synthetic ammonia catalyst. Later [122], in 1937, these authors determined the surface area of two different silica gels measuring adsorption isotherms of seven different gases.. In the above mentioned works the surface area was determined by extrapolating the middle linear sections of experimental isotherms to zero pressure in order to obtain the amount of gas required to cover the adsorbent surface with a monomolecular layer. On condition that the monomolecular layer was in a close-packed stage, the surface area was then evaluated from the monolayer adsorbed amount. Brunauer and Emmett [121,122] also proposed to determine the monolayer adsorption amount from the so-called point B of the experimental isotherm. It was assumed that this point corresponds to the inflection point and can be obtained from the beginning of the linear section of adsorption isotherms. 

The first use of chemisorption in the study of heterogeneous catalysis was introduced by Emmett during the study of iron-based catalyst for ammonia S3mthesis which used the chemical adsorption of CO and CO2 to measure the surface area of active Fe iron and promoters of K2O and AI2O3. He obtained the following instructive revelation Although content of promoters is very little, they cover most of the surface of the catalyst, which shows that the promoters tend to occupy the surface phase. Since then, many researchers have used chemisorption to study the effects of various components in the traditional iron catalyst, as well as the relationship between the mutative trends of various component and changes in activity... 

Zwietering et al. measured the amount of N— at the conditions of ammonia synthesis by weight analysis and it was 1% of equilibrium adsorption amount over iron catalyst containing 0.85% AI2O3 at 0.52-0.69 of surface N— coverage and 568 K of temperature. Aika and Ozaki further verified that H— was not a factor in the RDS, by the determination of label atom experiments. 

For example, Dmnesic et al measured the conversion at atmospheric pressure and the turnover munber of ammonia synthesis under conditions far from equilibrium over various iron catalysts supported on magnesium oxide. The number of active sites on the surface of these catalysts was determined by chemical titration of CO adsorption. Ammonia yield under experimental conditions is predicted by these microkinetic models, and the results calculated were four times higher at most. If it is taken into account that these catalysts without potassium promoter and exorbitant estimation for the number of active sites measured by CO chemisorption, this result could be considered as in fair agreement with experimental one. 

In order to reveal the intrinsic relation between the surface properties and textures with the hump-type activity curve, the specific surface area of both the ammonia synthesis fused iron catalyst with different iron oxides as precursors and their active components were measured by the means of low temperature physical adsorption of N2 and selective chemisorptions of CO, CO2 as shown in Table 3.17. 

For pure Fe304, at 444°C, when Kp = [H2]/[H20] = 5, Fe and Fes04 coexist. However, it is known from the experiment that at 444°C, [H2]/[H20] a 2,000, there are still measurable oxygen remained on the iron surface. It is confirmed from poisoning experiments on ammonia synthesis catalyst that a very low concentration of water vapor in synthesis gas causes the oxygen to be retained on the iron and reduces the catal3+ic activity. 

Shape factor is a basic parameter of catalyst particle, which is the basis of calculating pressure drop and thermal conductivity of catalyst beds. The fs of the regular particle can be directly calculated by the volume and surface area of particles the of the irregular particle cannot be obtained directly. In general, it can be confirmed by measuring the pressure drop of beds of particles e.g. for irregular fused iron catalyst for ammonia synthesis. 

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