Iron catalysts, adsorption

Where E is appreciable, adsorption rates may be followed by ordinary means. In a rather old but still informative study, Scholten and co-workers [130] were able to follow the adsorption of N2 on an iron catalyst gravimetrically, and reported the rate law... 

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

Calculate the entropy of adsorption A 2 for several values of d for the case of nitrogen on an iron catalyst. Use the data of Scholten and co-workers given in Section XVIII-4B. 

The reactant is adsorbed on the catalyst s surface. As a reactant molecule attaches to the surface of the catalyst, its bonds are weakened and the reaction can proceed more quickly because the bonds are more easily broken (Fig. 13.36). One important step in the reaction mechanism of the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on the iron catalyst and the weakening of the strong N=N triple bond. 

Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst ((7 = 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (55). 

In the original derivation of (305), it was supposed (40) that the nitrogen adsorption equilibrium on the catalyst follows the logarithmic isotherms (i.e., that the surface is evenly nonuniform). In this case y — 0 and, according to (143) and (164), m — a, n — / . Experiments with iron catalyst promoted with A1203 and K20 gave m = 0.5. This was interpreted as a = 0.5 (93). 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

Tibbetts, G. G., Devour, M. G. Rodda, E. J. 1987 An adsorption-diffusion isotherm and its application to the growth of carbon filaments on iron catalyst particles. Carbon. 25, No. 3, 367-375. 

Both type-4 and type-B adsorption on iron have a poisoning effect on the H2 and D2 exchange on iron catalysts at very low temperatures... 

The active Fe is formed from the magnetite through a reduction produced by the reactant mixture both A1203 and CaO are structural promoters which preserve the high surface area of the active iron catalyst [5], The K influences the activity per unit area of the Fe by enhancing of the velocity of dissociative nitrogen chemisorption by increments of the adsorption energy [129],... 

Brunauer and Emmett,2 however, take the view that, on porous iron catalysts, the first effect of van der Waals adsorption is to cover the surface with a layer one molecule thick. In the case of several permanent gases, and also of carbon dioxide and butane, if the adsorption isotherms are measured not too far above the boiling-point of the gases, the first layer is complete at 50 mm. pressure or less. If the pressure is raised up to atmospheric, further quantities are adsorbed, and there appears a nearly linear relation between the pressure, and the amount adsorbed in excess of the first monomolecular layer but the increase of adsorption, as the pressure is raised above that at which the first layer is complete, is much more gradual than the increase with pressure, at low pressures, before the surface is completely covered.3... 

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. 

Taylor (37) describes the shape of adsorption isobars for hydrogen on an iron catalyst and curves showing desorption, readsorption when the temperature is increased stepwise. These observations are best explained by the existence of an a priori heterogeneity. This heterogeneity may well be connected with the existence of abnormal lattice distances on the surface. 

Bulatnikov et al. (340) studied the effects of promoters on sulfur resistance of iron by measuring the amount of radioactive H2S adsorbed on iron catalysts promoted with A1203 and/or K20. They reported irreversible deactivation of Fe promoted with A1203, A1203 + K20, or KzO after 0.8, 1.5, and 5 monolayers of sulfur had been adsorbed. In other words, the presence of K20 was responsible for increasing sulfur adsorption capacity, although it was not clear upon which portion of the surface sulfur had adsorbed. It was also reported that A1203 was necessary to prevent volatilization of K20. 

Accurate and reproducible Mossbauer spectra of supported iron catalysts require the prevention of the adsorption of impurities including oxygen and water onto the highly reactive sample surface. Cells designed to allow chemical reactions and in vacuo pretreatments at temperatures up to 673 K while the Mossbauer spectrum is being recorded have recently been reported and represent a significant and important development in the application of Mossbauer spectroscopy to catalytic and surface studies (109-111). 

Upon the addition of electronic promuiers to iron catalysts, an increase of the heat of adsorption of CO results, as well as a decrease of the heat of adsorption of 11 . and the electron affinity of iron is lowered. Tills leads to an increase... 

The liydroxycarbene mechanism is supported mainly by observation of IR absorption bands suggesting enolic complexes of the type M=CH(OH) if CO/Hj is chemisorbed on iron catalysts [112. 162, 1631. Sorption measurements showed that chemisorbed CO/Hj mixtures are always released in a I 1 ratio, even when gas mixtures of a different composition were used for adsorption [164]. Based on mass spectrometry formaldehyde was shown to be the primary reaction product (16SJ. 

In applying these equations, the authors assumed that the adsorption of nitrogen on the iron catalyst in the presence of an ammonia-hydrogen mixture is the same as it would be when at a nitrogen pressure equivalent to the existing partial pressure of ammonia and hydrogen in the gas mixture. Thus, since the equilibrium constant for ammonia synthesis is... 

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