Promotional effects and mechanism of second cations on activity and stability of Co-MOR for nitrous oxide decomposition: UV-Vis spectroscopy and EXAFS analysis

In the current study, a series of second components, such as Ce, K, La, Ni, Mo and In, were introduced into Co-MOR catalyst and their effect on the catalytic performance for N₂O decomposition was examined under identical conditions. It is found the incorporation of In not only improved the activity, but also significantly improved the durability of Co ions in MOR, whereas the introduction of La, Mo and K only enhanced the durability. Besides, K introduction had detrimental effect on the activity. The effects of In, La, Mo and K on the formation of Co species in the bimetallic catalyst, especially their transformation during the process of CoIn-MOR preparation were systematically examined using X-ray diffraction (XRD), H₂-temperature program reduction (H₂-TPR), UV-Vis spectroscopy, and extended X-ray absorption fine structure (EXAFS) analysis. Based on the characteristic results the promotional effect of In on the activity is mainly attributed to the higher content of β type Co ions after the addition of In, which are the active sites for N₂O decomposition. The promotional effect of La and Mo on the stability is mainly attributed to the higher content of Co ions located at the γ sites after addition of La and Mo. The EXAFS results indicate that the β type Co ions in CoIn-MOR and γ type Co ions in CoLa-MOR and CoMo-MOR exhibit stronger coordination to framework oxygens, preventing its relocation into CoO_x clusters thus the activity could be kept. It is found that the exchange of In enhanced the thermodynamic affinity of Co to the MOR zeolites and re-located from Co₃O₄ particles into the α sites of zeolite channel and subsequently stabilized on the β sites after calcinations.