Aluminum oxide, promoter effect

The effects of water-vapor and ammonia pretreatment on the initial rate of ammonia synthesis over Fe, Al O /Fe, and K/Al O /Fe surfaces can be summarized as follows. The presence of aluminum oxide promotes the restructuring of iron during the water-vapor pretreatment, but it inhibits the ammonia-induced restructuring. The presence of potassium shows no effect in the ammonia pretreatment and it inhibits water-vapor-induced restructuring of iron. These results suggest that to form the most active ammonia synthesis catalyst, the iron should first be restructured in ammonia before aluminum oxide is added. After aluminum oxide is added the surface should be treated in water vapor, and finally potassium should be added to serve as a promoter at high ammonia synthesis reaction conversions. 

He became intimately familiar with a wide range of catalytic materials—including aluminum oxide, silica, and clay, as well as nickel, platinum, zinc, and copper—and their role individually and as mixtures 111 effecting chemical transformation. One of Ipatieffs most important lines of research was his breakthrough work on the nature and mechanisms of catalytic promoters on organic reactions. 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

Catalyst Type - Nickel metal catalyst, sometimes promoted with copper, aluminum oxide, or sulfur, are commonly used in commercial hydrogenation. These catalysts are prepared by a variety of techniques, some proprietary to the catalyst supplier, to provide the surface activity necessary for the desired selectivity. Precious metals have been found to be effective hydrogenation catalysts, which are more active at lower temperatures and produce less trans-isomers. However, their use has been deterred by the initial cost and recovery problems associated with the minute quantities required. 

Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis The initial rate of ammonia synthesis has been determined over the clean Fe(l 11), Fe(lOO), and Fe(l 10) surfaces with and without aluminum oxide. The addition of aluminum oxide to the (110), (100), and (111) faces of iron decreases the rate of ammonia synthesis in direct proportion to the amount of surface covered [47]. This suggests that the promoter effect of aluminum oxide involves reaction with iron which cannot be achieved by simply depositing aluminum oxide on an iron catalyst. 

These results are consistent with earlier literature [63, 64] in which the effects of potassium on doubly promoted (aluminum oxide and potassium) catalysts were studied. It was shown that the turnover number for ammonia synthesis is roughly the same over singly (aluminum oxide) and double promoted iron when 1 atm reactant... 

Wyckoff and Crittenden (34) used x-ray line broadening to measure the crystal size of a reduced iron catalyst before and after heating it to 600 C. They shovjed that the promoted catalysts were stable to this temperature whereas the pure iron catalysts or those promoted only with K2 0 sintered badly and grew large crystals. This work was done before the development of a method for measuring surface area. It predicted properly, however, the stabilizing effect of both aluminum oxide and two promoters, aluminum oxide and potassium oxide. 

Other applications of lETS have included com-plexation reactions of CoBr2 and C0CI2 on silane-modified alumina [309], silylation of plasma-grown aluminum oxide [310], the effect of hydrogen on the growth of MgO [311], and adsorption of silane adhesion promoters on aluminum oxide [312]. 

Even though more potassium can be stabilized on the iron surface when aluminum oxide is coadsorbed, an enhancement in the promoter effect was not... 

It has been shown that inactive Fe(llO) and the slightly active Fe(lOO) surface can be restructured with water vapor in the presence of aluminum oxide to produce new surfaces with activities equivalent to Fe(lll) and Fe(211). The coadsorption of potassium with the aluminum oxide inhibits this restructuring. It seems likely that the formation of a potassium aluminate retards the interaction between aluminum oxide and the oxidized iron surface which is needed to change the morphology of the iron surface. Thus, the promotional effect of both aluminum oxide and potassium is eliminated if they are added together prior to the restructuring process on single-crystal catalysts. 

To realize fully the promotional effects of potassium and aluminum oxide, the promoters must be added at different times during the preparation of the active catalyst. " In Fig. 4.27, the partial pressure of ammonia in the reaction loop versus the time of reaction for clean Fe(lOO), restructured Alj,O /Fe(100), and restructured 0.25 MLK/25% Al O /Fe(100) where the potassium was added after the restructuring procedure is plotted (see the figure for conditions). The restructured Al ,Oy/Fe(100) surface with 0.25% Al O is about one order of magnitude more active than the clean Fe(lOO) surface when the surface areas of the samples are taken into account. The addition of 0.25 MLK to a restructured Al O /Fe(100) surface increases the activity of the restructured surface twofold at an ammonia partial pressure of 20torr. Thus the restructured 0.25 MLK/25% Alj Oy/Fe(100) surface is now 20 times more active than the clean Fe(lOO) surface. The only way to achieve this level of enhancement is to restructure the surface in the presence of aluminum oxide alone, and then to add the potassium. 

The presence of potassium on iron during ammonia pretreatment has no additional effect on the restructuring process when adsorbed alone or when coadsorbed with Al O. Thus, potassium does not seem to affect the structural promotion of ammonia synthesis catalyst either during ammonia or water-vapor pretreatment. However, the presence of potassium on a Al O /Fe surface, during water-vapor pretreatment (Section 4.6), inhibits restructuring. A likely explanation for this observation is that the formation of potassium aluminate blocks the interaction between iron oxide and aluminum oxide. ... 

SUMMARY OF THE PROMOTER EFFECTS OF POTASSIUM AND ALUMINUM OXIDE... 

The coadsorption of potassium with aluminum oxide inhibits the restructuring process. Thus, to realize the full promotional effects of both aluminum oxide and potassium, the surfaces should first be restructured with aluminum oxide alone and potassium should be added afterward. Using this procedure, the activity of the Fe(llO) surface can be increased by a factor of about 800 at an ammonia partial pressure of 20 torr (Pn2 = 5 atm and =15 atm). Restructuring leads to about a 400-fold increase in the activity of Fe(llO), and the addition of potassium contributes a further twofold increase. 

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