Effect of the reaction medium on the physicochemical properties of the oxide monolith catalyst IK-42-1 for ammonia oxidation

The changes in the properties of the oxide monolith catalyst IK-42-1 for ammonia oxidation upon 3540-h-long operation as the second stage of a UKL-7 industrial reactor were studied by X-ray diffraction, chemical analysis, secondary ion mass spectrometry, temperature-programmed reduction, and IR spectroscopy. The spent catalyst shows a lower activity and a lower nitrogen oxide selectivity than the initial catalyst: the decrease in the NO yield is about 25%. As the catalyst operates, the state of its surface changes under the action of the reaction medium. The specific surface area of the catalyst decreases, and the total pore volume increases, which can cause a slight decrease in the ammonia conversion and diminish the mechanical strength of the monoliths. Under the ammonia oxidation conditions, there can be partial reduction of the catalyst surface and the local formation of a spinel structure (spinel-like defects). Oxygen adsorbed on these areas/defects is characterized by a high activation energy of desorption, which favors the reaction route yielding molecular nitrogen. According to IR spectroscopic data, the spent catalyst has a lower concentration of active, coordinatively unsaturated Fe²⁺ sites of adsorption, which are responsible for ammonia oxidation into NO. The decrease in the number of active sites can be due to both the breakdown of the solid solution of iron oxide in aluminum oxide during the reaction and the blocking of these sites by silicon, alkali and alkaline-earth metal, chromium, and rhodium atoms present on the surface of the spent catalyst.