Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
Rafał Pelka,
Rafał Pelka
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
Izabella Jasińska,
Izabella Jasińska
Grupa Azoty Zakłady Chemiczne “Police” S.A., 1 Kuźnicka Str., 72-010 Police, Poland
Zofia Lendzion-Bieluń
Zofia Lendzion-Bieluń
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
- The study presents a model demonstrating the thermodynamic parameters that govern the iron catalyst sintering process, focusing on stability aspects of nanocrystalline structures.
Catalyst Activation Mechanisms
- Potassium is identified as a critical electron and structure-forming promoter in the ammonia synthesis catalyst, enhancing active site formation and structural stability under synthesis conditions.
Temperature Effects on Morphology
- Catalyst samples showed irreversible structural changes after exposure to high temperatures (up to 973 K), confirming that sintering in nanocrystallites proceeds without reversal upon cooling.
Wetting and Surface Energy
- The role of wetting by promoter dipoles, especially potassium, was integral to surface energy adjustments, affecting specific surface areas and promoting sintering in the iron nanocrystallites.
Structural Integrity in Industrial Application
- Through controlled temperature treatments, the catalyst's structure can be maintained for industrial ammonia synthesis, ensuring consistent catalytic performance.
Abstract
The sintering of iron ammonia synthesis catalysts (nanocrystalline iron promoted with: Al2O3, CaO and K2O) was studied under a hydrogen atmosphere, in a temperature range of 773 to 973 K to obtain stationary states. The catalysts were characterized by measuring the nitriding reaction rate under an ammonia atmosphere at 748 K to obtain steady states and the measurement of specific surface area. Chemical processes were conducted in a tubular differential reactor enabling thermogravimetric measurements and the chemical composition analysis of a gas phase under conditions allowing experiments to be carried out in the kinetic region of chemical reactions. An extended model of the active surface of the iron ammonia synthesis catalyst was presented, taking into account the influence of the gas phase composition and process temperature. The surface of iron nanocrystallites was wetted using promoters in an exothermic process associated with the formation of the surface Fes-O- bond and the change in the surface energy of iron nanocrystallites. Promoters formed on the surface of iron nanocrystallites with different structures of chemisorbed dipoles, depending on the composition of the gas phase. The occupied sites stabilized the structure, and the free sites were active sites in the process of adsorption of chemical reagents and in sintering. Based on the bonding energy of the promoter oxides and the difference in surface energy between the covered and uncovered surfaces, the wetting abilities of promoters, which can be arranged according to the order K2O > Fe3O4 > Al2O3 > CaO, were estimated. By increasing the temperature in the endothermic sintering process, the degree of surface coverage with dipoles of promoters decreased, and thus the catalyst underwent sintering. The size distribution of nanocrystallites did not change with decreasing temperature. Only the equilibrium between the glass phase and the surface of iron nanocrystallites was then established.
