Rhenium ammonia synthesis

Fig. 7. Structure sensitivity in the ammonia synthesis over rhenium single-crystal faces. The turnover frequencies (TF) are given as NH3 molecules/cm2/s (PIotai = 20 atm, H2 N2 = 3 1, Tcryslai = 870 K). Schematics of the atomic structure of each surface are given above each bar. From Ref. 85.

Bottom figure Strong surface structure sensetivity of ammonia synthesis on rhenium single crystals. The symbols Cn indicate n-coordinated sites. Yield is expressed in arbitrary unitsl l. 

It was found during studies of ammonia synthesis on iron that the incorporation of a condenser downstream of the sample valve in the external circulation loop of the HPLP apparatus (Fig. 7), enabled the system to be run as a flow rather than a batch reactor. This is true for any reaction system where the reactants are more volatile than the products, since the condenser temperature can be adjusted to trap the products almost exclusively, allowing a nearly pure stream of reactants to impinge on the catalyst. In the case of ammonia synthesis, (where, next to the product, nitrogen at a partial pressure of 5 atm was the most condensable species) a slurry of isopentane (— 159.9 °C) was found to be the ideal condenser medium. During a study of rhenium-catalyzed ammonia synthesis the isopentane condenser was switched in periodically to reduce the ammonia partial pressure to below that at which it appeared to poison the catalyst. In this way, the rhenium was able to produce ammonia in excess of the amount usually leading to poisoning. 

Additional research, which supports the contention that highly coordinated surface sites are most active for ammonia synthesis, has been carried out on rhenium. In this study the rate of ammonia synthesis over the (0001), (lOlO), (1120), and (1121) faces were determined (Fig. 4.4). Here, as in the case of iron the face with the highest activity, the (1121) plane, is the surface which exposes the most highly coordinated sites. It is also shown in Fig. 4.4 that the (1121) face... 

Figure 4.4. The structure sensitivity of ammonia synthesis over rhenium single-crystal surfaces/ Schematics of the atomic structure of each surface is given above each bar.

The rate-limiting step in the decomposition reaction is a very sensitive function of the surface coverages of the various components and, in particular, of atomic nitrogen. Studies using both single-crystal and polycrystalline platinum and several other transition metals (tungsten, nickel, rhenium, and ruthenium ) have confirmed that the reaction follows the same pathway as the ammonia synthesis reaction on iron. All of the transition metals show broadly... 

From the viewpoint of competitive adsorption we can consider as well the sorption of precursors upon the synthesis of Pt-Be/Al203 catalysts from H2PtCl3 and rhenium acid [4] and of Pt-Ru/ 02 catalysts from the ammonia complexes of Pt and Ru [5]. In both cases precursors were in the same ionic form and adsorbed on the same sites. 

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