Iron single-crystal surface

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. 

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. 

Effects of Potassium on Ammonia Synthesis Kinetics Extensive research has been completed in which the effects of potassium on ammonia synthesis over iron single-crystal surfaces of (111), (100), and (110) orientations [59] have been determined. The apparent order of ammonia and hydrogen for ammonia synthesis over iron and K/Fe surfaces has been determined in addition to the effect of potassium on the apparent activation energy Ef) for the reaction. In all the experiments, potassium was coadsorbed with oxygen because only about 0.15 ML of potassium coadsorbed with oxygen is stable under ammonia synthesis conditions (20... 

D.R. Strongin and G.A. Somorjai. The Effects of Potassium on Ammonia Synthesis over Iron Single Crystal Surfaces. J. Catal. 109 51 (1988). 

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

Hlfcci of Potassium on the Di.ssociaiivc ChcmiM)rption of Nitrogen on Iron Single-Crystal Surfaces in UHV. 475... 

Figure 7.18. (a) Rates of ammonia synthesis over five iron single-crystal surfaces with different orientations (111), (211), (100), (210), and (110). (b) Schematic representations of the idealized surface structures of the (111), (211), (100), (210), and (110) orientation of iron single crystals. The coordination of each surface atom is indicated. Adapted from Spencer et... 

The next stages of the strategy will now consist of direct investigation of the various possible surface species under UHV conditions which might be present on well-defined iron single-crystal surfaces, in order to develop (hopefully) a complete microscopic picture describing the overall reaction. 

Interaction of Nitrogen with Iron Single-Crystal Surfaces... 

Below room temperature ammonia adsorbs nondissociatively on clean iron single-crystal surfaces with a rather high sticking coefficient.The structure model proposed for the ordered phase formed on Fe(llO) is reproduced in Fig. 3.11. Coupling to the surface occurs through the nitrogen atom and mainly by... 

A SURFACE SCIENCE AND CATALYTIC STUDY OF THE EFFECTS OF ALUMINUM OXIDE AND POTASSIUM ON THE AMMONIA SYNTHESIS OVER IRON SINGLE-CRYSTAL SURFACES... 

In this section a large amount of work which has been performed outside the Lawrence Berkeley Laboratory (except for Section 4.4.4) is summarized. The structure and bonding strength of potassium will be discussed along with the effect of potassium on the dissociative chemisorption of nitrogen on iron single-crystal surfaces. 

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)... 

AMMONIA-INDUCED RESTRUCTURING OF IRON SINGLE-CRYSTAL SURFACES... 

Rates of ammonia synthesis over initially clean (110), (100), and (111) faces of iron are shown in Fig. 4.28, along with the rates obtained after the treatment of the Fe(llO), Fe(lOO), and Fe(lll) crystals with 5 torr of ammonia at 723 K for 30 minutes prior to the ammonia synthesis reaction. The Fe(llO) and Fe(lOO) surfaces show large increases in activity for ammonia synthesis (Fig. 4.28) after ammonia pretreatment. The initially inactive Fe(llO) face becomes about half as active as the clean Fe(l 11) surface. The activity of the Fe(lOO) surface, for ammonia synthesis, increases by a factor of four and the Fe(lll) surface shows only a small decrease in activity after ammonia pretreatment. Scanning electron micrographs of the ammonia pretreated iron single-crystal surfaces are shown in Fig. 4.29. They clearly show that ammonia pretreatment drastically changes the surface structure of the (110), (100), and (111) faces of iron. 

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