Nitrogen adsorption on iron

Similarly to the concept of the biographical nonuniform surfaces mechanism (7.97) can be used to derive a rate expression in supposition of lateral interactions. Assuming that surface species other than chemisorbed nitrogen are present on the surface in inferior quantities, which is backed by experimental evidences showing that nitrogen adsorption on iron catalysts proceeds at a rate approximately equal to that of ammonia synthesis, the equilibrium constant of step 2 in eq. (7.97) can be expressed, following the general treatment, as... 

However, there still is no common sense about that the nitrogen adsorption on iron catalyst for ammonia S3mthesis is dissociation or not. 

The heats of adsorption of nitrogen on evaporated metal films of nickel and iron have been reported to decrease from 10 to 5 kcal./mole as the surface coverage increased from about 0.1 to 1.0 monolayer. Beeck (150) states that nitrogen is unsuitable for the evaluation of surface areas of evaporated iron and nickel films by the Brunauer-Emmett-Teller method because of its high heat of adsorption at 78°K., which would yield a value for monolayer adsorption too high by 50%. The author feels that this objection does not apply to the nitrogen adsorption on reduced electropolished planar copper plates for the following reasons. 

There is no particular a priori reason to suspect that oxygen production was any different from that in the modern ocean, except that there may have been constraints imposed by different availabilities of key nutrients such as phosphorus (Bjerrum Canfield 2002). Today, the availability of fixed nitrogen may constrain productivity on geological time scales (Falkowski 1997), but in the Archaean phosphorus removal by adsorption on iron oxides could have reduced P availability, significantly reducing productivity compared with today. 

The combination of the fact that more than one set of the energetics are able to predict rates of ammonia synthesis with reasonable accuracy, and the uncertainty as to whether the adsorption of nitrogen on a doubly promoted iron catalyst is activated, suggests that the detailed kinetics of nitrogen adsorption on a doubly promoted catalyst should be determined using modern surface-science techniques. 

Nitrogen adsorption experiments showed a typical t)q5e I isotherm for activated carbon catalysts. For iron impregnated catalysts the specific surface area decreased fix>m 1088 m /g (0.5 wt% Fe ) to 1020 m /g (5.0 wt% Fe). No agglomerization of metal tin or tin oxide was observed from the SEM image of 5Fe-0.5Sn/AC catalyst (Fig. 1). In Fig. 2 iron oxides on the catalyst surface can be seen from the X-Ray diffractions. The peaks of tin or tin oxide cannot be investigated because the quantity of loaded tin is very small and the dispersion of tin particle is high on the support surface. 

Multiwall carbon nanotubes (MWCNTs) have been synthesized by catalytic chemical vapor deposition (CCVD) of ethylene on several mesoporous aluminosilicates impregnated with iron. The aluminosilicates were synthesized by sol-gel method optimizing the Si/Al ratios from 6 to 80. The catalysts are characterized by nitrogen adsorption, X-ray diffraction, 27A1 NMR, thermogravimetric analysis (TGA) and infrared. The MWCNTs are characterized by TGA and transmission and scanning electron microscope. 

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... 

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 adsorption of nitrogen on tungsten 51) was the first nitrogen system to be studied (Fig. 12) by XPS subsequently nitrogen interaction with iron was studied (52, 53), and two distinct N(ls) peaks were observed at 80 K, one at about 405 eV and the other at 400 eV. At room temperature with both single crystals and polycrystalline iron surfaces only a single peak is... 

However, there is another consideration which has worried this author as early as 1937 when he and his coworkers measured the adsorption isotherm of nitrogen on nickel and iron at liquid oxygen temperature and found that the isotherm was essentially flat between 10-4 and 10 l mm. Hg pressure. It was about this time that the Emmet-Brunauer method was first made public, and off hand the two results appeared to be irreconcilable. This uncertainty was aggravated by the fact that the nitrogen adsorption isotherm as published by the author and his Coworkers could at that time not be extended to pressures higher than... 

It is apparent from the foregoing that the B.E.T. method will give values for the monolayer which are too large when applied to clean metal surfaces using a gas whose heat of adsorption is large relative to that in ordinary van der Waal s adsorption. With a heat of adsorption of 10,000 calories on iron, nitrogen is unsuitable for surface measurements of such metals by the B.E.T. method when absolute best values are of interest. 

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). 

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

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. 

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