Iron catalysis chemisorption

An indication of growing interdisciplinary interest in the field is illustrated in a review on new perspectives in surface chemistry and catalysis by Roberts (.160), who discussed the interaction of N2 with iron surfaces. In so doing, he referred to the Fe (N2) , matrix Mdssbauer work of Barrett and Montano (7), which showed that molecular nitrogen only bonds to iron when the latter is present as a dimer. As the chemisorption studies (161) indicated that N2 is absorbed on singleatom sites, Roberts suggested (160), of the matrix data (7), "if this is correct, then our assignment of the N(ls) peak at 405 eV to end-on chemisorbed N2 will require further investigation. Other reviews that consider matrix-isolation techniques for chemisorption simulation are collected in footnote a. 

H. Topsbe, N. Topsbe, H. Bohlbro, and J. A. Dumesic, Supported iron catalysts Particle size dependence of catalytic and chemisorptive properties, Proc. 7th Int. Congress Catalysis, edited by T. Seyama, K. Tanabe (Kondansha, Tokyo), p. 247 (1981). 

The heat of chemisorption, which must be low in order to enable catalysis to take place, may even be negative. In various sections we have seen that endothermic chemisorption may play an important role (Secs. V,9, VI,3,4,5, and X,4). Figure 40 shows that surface contaminations can promote endothermic chemisorption. In nickel, as in iron, hydrogen atoms can be dissolved endothermically. It is highly probable that dissolved hydrogen atoms react from the metal phase with chemisorbed hydrocarbons. 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

Deactivation by sulfur has been explained by the withdrawing of electrons from the catalyst surface. It has also been shown that sulfur inhibits the dissociation of CO on iron surfaces l]. The deliberate partial poisoning of iron/manganese cataly.sts with sulfur has been used to shift the product selectivity towards short-chain hydrocarbons. At higher sulfur concentrations (0.7 mg S/g catalyst) the activity is significantly decreased and the olefin selectivity increased [82]. Sulfur poisoning of nickel catalysis has recently been shown to inhibit the chemisorption of hydrogen 83.84). 

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... 

In recent years, much attention has been focused on electrochemical studies of metalloporphyrins, not only as mimetic compounds of the iron porphyrin unit in heme proteins but also as potential electrocatalysts . Metalloporphyrins have been found to be applicable in both homogeneous and heterogeneous catalysis - and, because oxygen can be reduced directly through a 4-electron pathway on some transition metal porphyrins, catalysis in the heterogeneous electrochemical oxygen reduction reaction has received particular attention The application of metalloporphyrins to heterogeneous electrocatalysis requires their attachment to solid electrodes which can be realized based on chemisorption, chemical reactions with previously functionalized electrodes, chemical reactions with a functionalized polymer, incorporation of the porphyrin with the polymer film and electrochemical polymerization. 

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