Magnetite catalyst inhibition

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)... 

Activation of the catalyst is carried out in a H2/N2 mixture at moderate pressure. The process must be carefully controlled in order to ensure the development of maximum surface area. The rate of reduction is controlled by adjusting the temperature the lower the reduction rate, the better the development of the micro porous structure (r = 100 to 150 A). Exposure to water/steam of the reduced part of the catalyst will cause sintering and loss of activity and counter-diffusion of water should be minimised by operating at highest possible gas velocities. Water is also reported to inhibit the magnetite reduction rate (15). It is removed by condensation before the exit gas is... 

The limitations of magnetite-based plants are well known. Operation around 80 bar already requires large catalyst volumes significant pressure reductions, even with more active catalysts, are highly unlikely. The catalyst is also severely inhibited... 

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

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