Application of TOF-SIMS Method in the Study of Wetting the Iron (111) Surface with Promoter Oxides

• Published in: Molecules (Basel, Switzerland) Switzerland), 2022-01, Vol.27 (3), p.648
• Main Authors: Arabczyk, Walerian, Rogowski, Jacek, Pelka, Rafał, Lendzion-Bieluń, Zofia
• Format: Article
• Language: English
• Subjects: Adsorption; Aluminum oxide; Ammonia; Calcium; Calcium ions; Calcium oxide; Catalysts; Chemical composition; Decomposition; diffusion; Experiments; Ion emission; Ions; Iron; Lime; monocrystalline iron; Oxides; Potassium; Potassium oxides; surface; Surface structure; TOF-SIMS; Wetting
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules27030648

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 K O 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-Fe O 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.