Ammonia synthesis surface restructuring

Figure 7.20, Ammonia TPD following ammonia synthesis from restructured Al,0 /Fe(100) surfaces exhibit low-temperature peaks similar to those of Fe(l 11) and Fe(211). Thus, restructuring by water vapor creates active C7 sites [38].

Treatment of the (110), (100), and (111) faces of iron with 20torr of water vapor causes surface restructuring. The restructured Fe(llO) and Fe(lOO) surfaces become as active as the clean Fe(lll) surface for ammonia synthesis. The restructured Fe(lll), however, exhibits a slight decrease (about 5%) in activity when compared to the clean Fe(lll) surface. The restructured (110), (100), and (111) surfaces reconvert to their unrestructured orientations within one hour of ammonia synthesis and lose their increase in activity. 

The same restructuring on the Fe(llO), Fe(lOO), and Fe(lll) surfaces can be performed with water vapor in the presence of aluminum oxide. In this case, 20 torr of water vapor restructures the Al fO /Fe(100) and only 0.4 torr of water vapor is needed to restructure the AUO3,/Fe(110) surface so that they become as active as the Fe(lll) face in ammonia synthesis. The restructured AlxOy/Fe(110) and Al O3,/Fe(100) surfaces maintain their activity for longer than four hours in the ammonia synthesis conditions in contrast to the Al O -free surface. The formation of iron aluminate in the region near to the iron surface is invoked to explain the stability of the restructured A1 03,/Fe surfaces. 

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

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. 

Figure 7.17. Rates of ammonia synthesis over clean iron single crystals and water-induced restructured Al,0 /Fe surfaces. Restructuring conditions are given in the figure (38J.

AlfOy/Fe surfaces pretreated with water vapor prior to ammonia synthesis is shown in Figure 7.17. The initially inactive A1 0y/Fe(l 10) surface restructures and becomes as active as the Fe(lOO) surface after a 0.05-torr water vapor treatment and as active as the Fe(lll) surface after a 20-torr water-vapor pretreatment. This is about a 400-fold increase in the rate of ammonia synthesis compared with clean Fe(llO) [37]. The activity of the AljOv/Fe(100) surface can also be enhanced to that of the highly active Fe(l 11) surface by utilizing a 20-torr water-vapor pretreatment, and this high activity is maintained indefinitely as in the case for the restructured A1 0y/Fe(l 10). Little change in the activity of the Fe(lll) surface is seen experimentally when it is treated in water vapor in the presence of Al O,. 

The activity of the Fe(l 10) and Fe(lOO) surfaces for ammonia synthesis can also be enhanced to the level of Fe(l 11) by water-vapor pretreatments in the absence of aluminum oxide, but in this circumstance the enhancement in activity is only transient. Figure 7.18 shows the rate of ammonia synthesis as a function of reaction time for restructured Fe(llO) and Al Oy/FeCl 10) surfaces. Both surfaces have an initial activity similar to that of the clean Fe(lll) surface. The restructured AlvOy/Fe(l 10) surface maintains this activity for over 4 hr while the restructured Fe(l 10) surface loses its activity for ammonia synthesis within 1 hr of reaction. 

Characterization of the Restructured Surfaces The observation that the AlfOv/Fe(l 10) and Al,Oy/Fe(100) become as active as the Fe(l 11) surface for ammonia synthesis suggests that new crystal orientations are being created upon restructuring the Al,Ov/Fe(l 10) and Al,0v/Fe(100) surfaces in water vapor. A sug-... 

Figure 7.18. Deactivation of the restructured Fe(l 10) surface occurs within 1 hr while the restructured AltO /Fe(l 10) surface maintains its activity under ammonia synthesis conditions [38].

TPD of ammonia from iron single-crystal surfaces following high-pressure ammonia synthesis proves to be a sensitive probe of the new surface binding sites formed upon restructuring. Ammonia TPD spectra for the four clean surfaces are shown in Figure 7.19. Each surface shows distinct desorption sites. The Fe(llO) surface displays one desorption peak ( 2) with a peak maximum at 658 K. Two desorption peaks are seen for the Fe(lOO) surface 2 1 3) 556 K and 661 K. 

EfTecisof Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis. 471... 

Al,xOy/Fe(l 10) surface maintains this activity for over four hours while the restructured Fe(llO) surface loses its activity for ammonia synthesis within one hour of reaction. 

SEM micrographs of restructured Fe(llO) shows none of the features associated with the restructured Al ,O /Fe(110) surface. Figure 4.18a shows a (110) surface which has been restructured with 20 torr of water vapor. No LEED pattern is obtainable from this surface. Its appearance is different than the restructured iron single crystals which had aluminum oxide present. None of the crystallite structures associated with the restructured Al O3 /Fe(110) surface are present. Figure 4.18b shows the restructured Fe(llO) surface after one hour of ammonia synthesis. The features are now gone and the surface has no activity toward... 

Figure 4.18. SEM of the restructured Fe(llO) surface (a) after a 20torr pretreatment in water vapor (b) after one hour of ammonia synthesis. Note that the features smooth out under ammonia synthesis conditions.

Temperature-programmed desorption of ammonia from iron single-crystal surfaces after high-pressure ammonia synthesis proves to be a sensitive probe of the new surface binding sites formed upon restructuring. Ammonia TPD spectra for the four clean surfaces are shown in Fig. 4.19. Each surface shows distinct desorption sites. The Fe(llO) surface displays one desorption peak with a peak maximum at 658 K. Two desorption peaks are seen for the Fe(lOO) surface p2 and P ) at 556 K and 661 K. The Fe(lll) surface exhibits three desorption peaks Pi, P2, and p ) with peak maxima at 495 K, 568 K, and 676 K, and the Fe(211) plane has two desorption peaks P2 and P ) at 570 K and 676 K. Temperature-programmed desorption spectra for the AljcO /Fe(110), A1 03,/Fe(100), and A1 0 /Fe(lll) surfaces restructured in 20torr of water vapor are shown in Fig. 4.20. A new desorption peak, P2 develops on the restructured Al fOy/Fe(110)... 

The clean Fe(llO), Fe(lOO), and Fe(lll) surfaces restructured with 20ton of water vapor produce the same TPD spectra as the Al O restructured surfaces. Deactivation of the (100) and (110) clean restructured iron surfaces is rapid under the conditions of ammonia synthesis, and the P2 peaks become equivalent in intensity to those on the respective clean surfaces within one hour of ammonia synthesis. 

While 20torr of water vapor was needed to restructure clean iron single crystals, only 0.4 torr of water vapor is needed to restructure an Al O /Fe surface. It therefore seems that Al O provides an alternate and apparently more facile mechanism for the migration of iron. Upon reduction, metallic iron is left in a highly active orientation [such as Fe(lll) and Fe(211)] for the ammonia synthesis reaction, and the Al O stabilizes the active iron, since if the Al O were not present the iron would move to positions coincident with the bulk periodicity (see Fig. 4.22 for a schematic representation of the restructuring). 

The same coverages of potassium coadsorbed with two monolayers of aluminum oxide on the Fe(llO), Fe(lOO), and Fe(lll) surfaces hindered the restructuring process in water vapor (see Section 4.6). As increasing amounts of potassium were coadsorbed, more aluminum oxide was detected by AES after water pretreatments of 20 torr, and less restructuring of the iron occurred (rates of ammonia synthesis over these surfaces were less than the rates on those surfaces which were restructured with aluminum oxide alone). There is a one-to-one ratio between aluminum oxide and potassium on the surface and, in the case where one monolayer of potassium was deposited on two monolayers of aluminum oxide, AES showed that no aluminum oxide or potassium migrated from the iron surface after a 20 torr water-vapor pretreatment and restructuring of the surface failed to occur. 

Aluminum oxide is found to block the iron surface which could otherwise dissociate ammonia. This blocking of the restructuring process by Alj,Oy is in sharp contrast to the case where aluminum oxide catalyzes the restructuring of iron in the presence of water vapor prior to ammonia synthesis (Section 4.6). In this circumstance iron oxide is found to migrate on top of the aluminum oxide overlayer as a result of the oxidizing environment (water vapor). The major driving force for this structural transformation is most likely compound formation between iron oxide and aluminum oxide. 

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. ... 

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. 

Pretreating iron single crystals in high pressures of ammonia prior to ammonia synthesis have been shown to induce a surface restructuring. Both the Fe(llO) and Fe( 100) surfaces are found to approach the Fe( 111) activity after ammonia pretreatment. Treatment of the Fe(l 11) surface in ammonia causes a surface transformation to Fe(211). The presence of aluminum oxide on the iron surface inhibits ammonia-induced restructuring and potassium shows no observable effect. 

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