Abstract
In the present work, a simplified model of the Fe(111) surface’s promoter-oxide system was investigated in order to experimentally verify the previously proposed and known models concerning the structure and chemical composition of the surfaces of iron nanocrystallites in the ammonia-synthesis catalyst. It was shown that efficient oxygen diffusion from metal oxides to the clean Fe(111) iron surface took place even at temperatures lower than 100 °C. The effective wetting of the iron surface by potassium oxide is possible when the surface is covered with oxygen at temperatures above 250 °C. In the TOF-SIMS spectra of the surface of iron wetted with potassium, an emission of secondary FeOK+ ions was observed that implies that potassium atoms are bound to the iron surface atoms through oxygen. As a result of further wetting the iron surface with potassium ions, a heterogeneous surface structure was formed consisting of a thin K2O layer, next to which there was an iron-oxide phase covered with potassium ions. Only a limited increase in calcium concentration was observed on the Fe(111) iron surface upon sample annealing at up to 350 °C. As a result of wetting the iron surface with calcium ions, an oxide solution of CaO-FexOy was formed. In the annealing process of the sample containing alumina, only traces of this promoter diffusing to the iron surface were observed. Alumina formed a solution with a passive layer on the iron surface and under the process conditions (350 °C) it did not wet the pure iron (111) surface. The decrease in Fe+-ion emission from the Fe-Ca and Fe-Al samples at 350 °C implies a reduction in the oxygen concentration on the sample surface at this temperature.
Highlights
IntroductionThe interactions of an iron surface with the hardly reducible oxides K2 O, Al2 O3 , CaO have been the subject of many studies attempting to understand their influence on the elementary processes occurring on the surface of the iron catalyst used in the ammoniasynthesis reaction [1,2,3].Based on research with AES (Auger Electron Spectroscopy) [4] and TPD (Temperatureprogrammed desorption) methods [5,6,7] that were carried out on models of monocrystalline iron samples under high vacuum conditions, it was found that potassium bonded directly to surface iron atoms that had completely desorbed from the iron surface at temperatures below 200 ◦ C [8,9].Further studies on the interaction of potassium with iron revealed that the FeOK structures on the surfaces of iron nanocrystallites were thermally stable due to the co-adsorption of potassium with oxygen [5,8,10,11,12], the promoting effect of potassium on the nitrogen-adsorption rate was reduced [5]
Based on research with AES (Auger Electron Spectroscopy) [4] and TPD (Temperatureprogrammed desorption) methods [5,6,7] that were carried out on models of monocrystalline iron samples under high vacuum conditions, it was found that potassium bonded directly to surface iron atoms that had completely desorbed from the iron surface at temperatures below 200 ◦ C [8,9]
The effect of potassium oxide on the catalytic activity of iron increased in monocrystalline iron [12] and the iron catalyst [14] with the increasing ammonia pressure in the system
Summary
The interactions of an iron surface with the hardly reducible oxides K2 O, Al2 O3 , CaO have been the subject of many studies attempting to understand their influence on the elementary processes occurring on the surface of the iron catalyst used in the ammoniasynthesis reaction [1,2,3].Based on research with AES (Auger Electron Spectroscopy) [4] and TPD (Temperatureprogrammed desorption) methods [5,6,7] that were carried out on models of monocrystalline iron samples under high vacuum conditions, it was found that potassium bonded directly to surface iron atoms that had completely desorbed from the iron surface at temperatures below 200 ◦ C [8,9].Further studies on the interaction of potassium with iron revealed that the FeOK structures on the surfaces of iron nanocrystallites were thermally stable due to the co-adsorption of potassium with oxygen [5,8,10,11,12], the promoting effect of potassium on the nitrogen-adsorption rate was reduced [5]. The interactions of an iron surface with the hardly reducible oxides K2 O, Al2 O3 , CaO have been the subject of many studies attempting to understand their influence on the elementary processes occurring on the surface of the iron catalyst used in the ammoniasynthesis reaction [1,2,3]. It was shown that under the conditions of ammonia synthesis on the surfaces of single iron crystals, potassium could be stabilized in an amount corresponding to a maximum of 15% of the monolayer, which caused the reaction rate to double [12]. The effect of potassium oxide on the catalytic activity of iron increased in monocrystalline iron [12] and the iron catalyst [14] with the increasing ammonia pressure in the system
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