Iron chemisorption

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.)...

Wang L S, Wu FI and Desai S R 1996 Sequential oxygen atom chemisorption on surfaces of small iron clusters Phys. Rev. Lett. 76 4853... 

Fischer-Tropsch ohgomerization of CO -1- H9 to make hydrocarbons and oxygenated compounds was originally catalyzed by cobalt, which forms the active carbonyl, but now iron promoted by potassium is favored. Dissociative chemisorption of CO has been observed in this process. 

Figure 2. I. Spatial distribution of the main orbitals of N2 involved in molecular chemisorption on iron promoted by potassium (K or K20). Arrows indicate the direction of transfer of electron density.5...

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

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. 

A Japanese group reacted iron vapor with nitric oxide at 77 K (6). Two different species were observed, with Pno at 1800 and 1720 cm , that were assigned as NO species adsorbed on oxidized and metallic iron, respectively. Although no evidence was presented as to the nu-clearity of the products, the authors considered the species to be models for the chemisorption of NO on iron surfaces. 

Carbon monoxide chemisorption was used to estimate the surface area of metallic iron after reduction. The quantity of CO chemisorbed was determined [6J by taking the difference between the volumes adsorbed in two isotherms at 195 K where there had been an intervening evacuation for at least 30 min to remove the physical adsorption. Whilst aware of its arbitrariness, we have followed earlier workers [6,10,11] in assuming a stoichiometry of Fe CO = 2.1 to estimate and compare the surface areas of metallic iron in our catalysts. As a second index for this comparison we used reactive N2O adsorption, N20(g) N2(g) + O(ads), the method widely applied for supported copper [12]. However, in view of the greater reactivity of iron, measurements were made at ambient temperature and p = 20 Torr, using a static system. 

Iron surface areas from CO chemisorption and N2O reaction... 

Iron particle size from X-ray line broadening and percentage-reduction/CO-chemisorption ... 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

Figure 3.20 UPS spectra of CO chemisorbed on iron show that the 5a orbital has shifted down to higher binding energy as a result of chemisorption. CO largely desorbs from clean iron upon heating to 390 K. Potassium enhances the bond between CO and the metal and promotes the dissociation of CO at higher temperatures (adapted from Broden et al. [51 ]).

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

The physical conditions to effect a satisfactory extent of reaction are fairly severe, but are needed to overcome the enormous strength of the nitrogen triple bond. The role of the iron is essential chemisorptive adsorption of nitrogen occurs on the surface of the iron, with charge being donated from the N=N bond to the surface of the iron. As a result, less electron density remains between... 

But why do we heat the mixture to effect reaction We will perform a thought experiment imagine immersing iron into nitrogen at different temperatures. We will see the effects of the relative differences between physisorption and chemisorption. 

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). 

Fig. 8. CO reactivity pattern at 290 K dissociative chemisorption occurs to the left of the heavy line. With iron, dissociation is comparatively slow at this temperature, and so is a borderline case.

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