Chemisorption on iron

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...

AES can be used for studying specific crystal faces. For example, nitrogen chemisorption on iron has been shown to be much stronger on the more open Fe(lll) crystal faces than on the more compact Fe(100) and Fe(110) faces (see Figure 5.16). 

The facile dissociative adsorption of CO on transition metals at low temperatures has been demonstrated by XPS or pulse techniques for Ti, V, Cr and Mn (96] and at elevated temperatures for Ni, Co and Ku with Fc as the borderline case [96, 97J. A more detailed study by Somorjai for Pt (111) surfaces showed that dissociation occurs at the step sites only, and once these are filled, carbon monoxide is absorbed moiccularly [98]. All of the XPS studies on chemisorption on iron, except at very low temperatures, are indicative of dissociative surpikm being the first step in Fischer-Tropsdi reactions (99 101). However, photoelectron spectroscopy has so far not delineated a logical sequence of precursors and intermediates 1102. ... 

We note that the use of N2 chemisorption on iron, for example, really amounts to measuring the atoms per active site as a function of FE. This procedure is merely an alternative to the one we have adopted, in which we reason about the number of sites involving a given configuration of atoms per unit of exposed atoms. This quantity has been called the Taylor fraction (367). Similar concepts have been discussed recently by Carberry (368). 

When I joined Texaco in 1948, one of the company s major research efforts was in the production of synthetic fuels by an iron catalyzed Fischer-Tropsch reaction. At that time there was fear of a worldwide oil shortage. The oil fields in Arabia had been discovered but their potential was not yet realized. Because of this company interest, my first projects were studies of CO chemisorption on iron. The most significant of these studies were carried out in collaboration with Dr. A.N. Webb. 

The study on the pattern of nitrogen chemisorption is very important for the understanding of reaction mechanism of ammonia synthesis. But the chemisorption of nitrogen on the surface of transient metals is very complex. Taking chemisorption on iron catalyst for ammonia synthesis as an example, it was found that the formation rate of ammonia depends on pre-adsorption temperature if the adsorption of nitrogen was preceded first on the surface of activated catalyst, and then hydrogen was passed on surface of pre-adsorbed nitrogen. It is considered that there exist two adsorption patterns The first one is of L-type which appeared at 200°C of pre-adsorption temperature, in which the formation rate of ammonia was proportional to second one is of H-type which appeared at 400°C 40°C of... 

A promising technique for the study of the catalytic activity of alloys is the study of chemisorption on iron overlayers on single crystals of other metals such as Fe/Ru [375], Fe/Re [376], and Fe/W [377]. 

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... 

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. 

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. 

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 ]).

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

The heats of chemisorption of hydrogen on nickel and iron are nearly identical and decrease from about 30,000 calories for the sparsely covered surface to about 18,000 calories for the completely covered surface. The heat of chemisorption on tungsten decreases from 45,000 calories to about 13,000 calories as a function of surface coverage (Roberts, also Beeck and coworkers). The lower values of Frankenburg for higher... 

There are some indications in literature, however, that may endorse the non-dependence. In Sec. VII,6 we learned that the chemisorption of oxygen on iron was not restricted to the surface only iron atoms (ions) moving on top of the chemisorbed oxygen atoms (ions) bring about an... 

In all cases where an activation energy is already present at 0 = 0 (N2 on iron, H2 on contaminated metal surfaces see Sec. V,9) it increases with increasing 0 values. The increase of the activation energy is slower than the decrease of the heat of chemisorption. An examination of Fig. 37 shows that this should be so. The maxima in the potential curves are shifted to the left and the minima of the curves are either at the same distance from the surface—as we have assumed in our figure—... 

Figure 38 gives the heats of chemisorption of nitrogen on iron films, recently published by Bagg and Tompkins (363), and we may compare the decrease of this curve with the increase shown by the curve of Fig. 36. 

The heat of chemisorption of hydrogen, adsorbed on iron that has previously been covered with nitrogen up to 0 = 0.18, is, indeed, lower than the heat of chemisorption of hydrogen adsorbed on a clean surface 395). The heat of chemisorption of CO on an iron film partly covered with nitrogen is also lower than on a clean film, but Bagg and Tompkins 395) found that hydrogen when adsorbed on a film partly covered with CO shows a higher heat of adsorption than when adsorbed on a clean film. 

Fig. 39. Potential curves indicating the chemisorption on and the dissolution in iron having a noncontaminated surface.

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