Iron surface area

Fe/MgO catalysts with 5 to 30 mol % Fe have been prepared by impregnation and coprecipitation. Their reducibility has been measured and a comparison made of their Fe° surface areas. Catalysts prepared via coprecipitation yielded larger iron areas than those via impregnation. The activity and selectivity of the reduced catalysts for the hydrogenation of propanenitrile at 20-30 bar and 473 K and of ethanenitrile at 1 bar and 508 K have been determined. The most active catalysts are those prepared by coprecipitation and they show high selectivity for primary amines. The activity for ethanenitrile hydrogenation correlates with the iron surface area. 

Iron surface areas from CO chemisorption and N2O reaction... 

Effect of Iron Concentration. Among the factors that influence kohs, the effect of the amount of iron surface area that is accessible to the contaminant has received the most attention. This effect is most often described by a linear relationship ... 

The rale of the Boudouard reaction is mcreased by struciuial promoters proportional to the increase of the iron surface area (85). Fleetronic promoters not only enhance the catalyst activity but atscarbon deposition (57). This effect can be controlled by the addition of SiO . Thus, in order to minimize carbon depcKirion during Fischer Tropsch synthesis, it is necessary to control the catalyst basicity (851. 

Catalyst Preparation The industrial catalyst is prepared by the reduction of iron oxide, Fe304 (94 wt%). It is in the shape of small porous particles with a surface area in the range of 10-15 m /g. Additives that improve its performance include AUO (2.3 wt%), K2O (0.8 wt%), and often CaO (1.7 wt%), MgO (0.5 wt%), and Si02 (0.4 wt%). Al, Mg, Ca, and Si oxides stabilize the pore structure and the surface structure of the iron catalyst K2O, although decreases the iron surface area somewhat still greatly increases the ammonia yield at 613 K from 0.2 mol% to 0.34 mol%. 

No. Promoters (mass fraction)% Space velocity/ h-i Outlet NH3/% Total surface area/ m2. g-l Surface area without promoter/ (m2.g-l) Ks =kT Total surface area Ks =kT Iron surface area Change of effusion work Ay /eV... 

The inhibition effect of promoters on the methanation. R is commonly believed that for fused iron catalyst, AI2O3 increases iron surface area (structural effect), while K2O donates electrons to iron atom, and increases electron density and enhances the activity of ammonia synthesis reaction (electronic effect). For the supported ruthenium catalysts, the effect of promoters on performances becomes more complex due to the existence of support. ARhough there are a lot of studies on the role of promoter for ammonia synthesis reaction, the chemical state, the distribution and the mechanism are still unclear. The role of promoters include covering chemisorption s site, donating electron to active metal, direct interacting with the adsorption intermediate and electrostatic field and so For supported... 

The observation that the Al ,Oy/Fe(l 10) and Al Oy/Fe(100) becomes as active as the Fe(l 11) surface for ammonia synthesis suggests that new crystal orientations are being created upon restructuring the Al ,O3,/Fe(110) and Al,O /Fe(100) surfaces in water vapor. A suggested increase in the surface area cannot account for the enhancement in rate, since it has been shown that about 40% less carbon monoxide adsorbs on restructured Al cO /Fe(110) and Alj,O /Fe(100) relative to the clean respective surfaces (i.e., the iron surface area actually decreases). 

Calculations relating adsorption properties measured on clean single-crystal surfaces at (very) low pressure with actual rates of ammonia synthesis at high pressures described later in this chapter require some measurement of the free metallic iron surface area of the reduced synthesis catalysts. The free metallic iron surface area is generally calculated from the extent of chemisorption of carbon monoxide. In view of the importance of the free metallic iron surface area in work dealing with the mechanism of ammonia synthesis, it is important to review the adsorption of carbon monoxide on different iron surfaces reduced under different conditions. 

The most common method for the estimation of the surface area of a metallic phase in a supported catalyst is by measuring the extent of the hydrogen adsorption. With iron surfaces, however, the adsorption of hydrogen often does not proceed consistently. The free iron surface area is therefore usually calculated from the extent of adsorption of carbon monoxide, measured at 90 K. In contrast to adsorption of hydrogen, which has a low boiling point, physical adsorption of carbon monoxide by oxidic surfaces present in the catalysts is appreciable at 90 K. The extent of the adsorption is therefore measured twice after the first measurement, the catalyst is evacuated at 195 K and the extent of adsorption is determined again. The amount of carbon monoxide chemisorbed by the iron surface is assumed to be the difference between the values obtained from the two adsorption isotherms. ... 

The ammonia synthesis rate per iron surface area was calculated in this way. The iron surface area of the commercial Topspe KMIR catalyst was determined by the carbon monoxide chemisorption method, assuming that each carbon monoxide molecule titrates two iron atoms (this method gives a rather higher iron surface area than usually accepted). After integration of the rate equation for a piston-flow reactor, values for the ammonia concentration in the exit gas were obtained in very good agreement with the experimental values as shown in Fig. 4.14. However, the authors themselves prefer not to stress such agreement, in view of the approximations introduced and because of the uncertainties in some of the input data. 

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