Magnetite ammonia catalysts Surface

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

The experiment was performed inside a UHV chamber with a dynamic atmosphere of 8 X 10" mbar hydrogen. The result is shown in Fig. 2.17, in which the water evolution detected by a QMS is compared to the evolution of the metallic character of the surface, as expressed by the intensity at the Fermi edge monitored by He I UPS (for the shape of the whole spectra, see Section 2.7). The water evolution curve indicates two steps in the reduction process, but only the second step leads to the formation of metallic iron in the region near the surface. This evidence provides further strong support for the suggested two-step nucleation mechanism found for the reduction of magnetite in its modified form, which is now suggested as a model for the activation of the ammonia synthesis catalyst. 

Promoted magnetite in its reduced state, used as an ammonia synthesis catalyst, undergoes deactivation for a number of reasons. Like other high-surface area structures, it may sinter. Species present in the gas stream may chemisorp upon the catalytic surface and thereby poison it, and furthermore, access to the pore system may be blocked by deposits. These processes of sintering, poisoning, and fouling are discussed below. 

In its active state, an ammonia synthesis catalyst is a high-density, medium-surface area composite, which is produced by the reduction of promoted magnetite (see Chapter 2). With a resulting surface area of about 10 m g and a bulk density around 2.8 g cm", an approximate surface area of 2-3 x 10 m per m is available. On a microscopic scale this corresponds to an average crystallite size of about 20-30 nm. 

It can be calculated that a H2S/H2 ratio of 4.7 x 10 is required to establish the Fe/FeS equilibrium at TOOK, using data from Barin et but a H2S/H2 ratio of only 10 to affect the surface properties as extrapolated from Grabke s data. Similarly, a H2O/H2 ratio of 0.15 is required to establish the equilibrium between magnetite and iron at 700 K, but only ppm levels of water are required to affect the catalytic activity of an ammonia synthesis catalyst. 

Alkali promoters can also inhibit ammonia adsorption. In the case of magnetite catalysts there is indirect evidence that this is caused by neutralization of the acidic alumina phase. A similar effect is also observed on alumina-supported transition metals. ESCA evaluation of ruthenium catalysts demonstrated a significant increase in the ammonia bonding on alumina supports that could be reversed in the presence of alkali promoters. This is also apparent from the marked changes in the reaction order in ammonia for ruthenium on a variety of acidic and basic supports (Table 9.7). However, recent surface-science studies (discussed in detail in Chapter 5)... 

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