Atomic nitrogen desorption

Figure 7.20. Coverage of atomic nitrogen on Fe(l 11) on the basis of Eqs. (57) and (68). Desorption in ultrahigh vacuum is observed as a symmetric TPD peak around 740 K. Note that at reaction conditions employingO.l bar(10 Pa) of nitrogen the surface will be largely covered by nitrogen atoms.

One of the striking conclusions to be drawn is that the quoted models are rather robust toward variation in particular input parameters. It turns out that under technical synthesis conditions, the surface vdll be largely covered by atomic nitrogen and that because of operation of the principle of detailed balance, a satisfactory description of the synthesis rate will be obtained if a good fit to the experimental thermal desorption data for N2 is used (32). This makes, of course, microkinetic modeling somewhat ambiguous and hence now the details of the rate-limiting step will have to be considered. 

The right-hand part of Fig. 7.7 corresponds to the second-order desorption of nitrogen atoms from a rhodium surface. As the desorption reaction corresponds to N -I- N —> N2 -I- 2 the rate is indeed expected to vary with A characteristic feature of second-order desorption kinetics is that the peaks shift to lower temperature with increasing coverage, because of the strong dependence of the rate on coverage. 

Less, but still significant, information is available on the surface chemistry of other nitrogen oxides. In terms of N20, that molecule has been shown to be quite reactive on most metals on Rh(110), for instance, it decomposes between 60 and 190 K, and results in N2 desorption [18]. N02 is also fairly reactive, but tends to convert into a mixed layer of adsorbed NO and atomic oxygen [19] on Pd(lll), this happens at 180 K, and is partially inhibited at high coverages. Ultimately, though the chemistry of the catalytic reduction of nitrogen oxide emissions is in most cases controlled by the conversion of NO. 

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

As with methanol desorption, a net weight loss was observed for the FeHo catalyst after ammonia desorption. This was caused by oxidation of the ammonia substrate to nitrogen and consequent catalyst reduction. The relative number of oxygen atoms removed was ca. 20 less than with methanol surface reduction. 

Early field ion emission studies of gas-surface interactions use field ionization mass spectrometry. Gas molecules are supplied continuously to the tip surface by a polarization force and by the hopping motion of the molecules on the tip surface and along the tip shank. These molecules are subsequently field ionized. The role of the emitter surface in chemical reactions is not transparent and has not been investigated in detail. Only in recent pulsed-laser stimulated field desorption studies with atom-probes are these questions addressed in detail. We now discuss briefly a preliminary study of reaction intermediates in NH3 formation in pulsed-laser stimulated field desorption of co-adsorbed hydrogen and nitrogen,... 

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