Structural and textural characteristics of Ce-containing mordenite and ZSM-5 solids and FT-IR spectroscopic investigation of the reactivity of NO gas adsorbed on them

The in situ interaction of nitric oxide (NO) gas on the cerium framework-substituted ZSM-5 and mordenite zeolites was studied by FT-IR spectroscopy. Samples of ZSM-5 and mordnite-containing cerium (7.5 wt.% expressed as CeO2) have been hydrothermally synthesized from starting gel upon which the introduction of cerium is being made during synthesis of zeolites. The morphological and textural characteristics of cerium-free and cerium-containing zeolites were studied using XRD, FT-IR in the TO range, and N2 adsorption at −196 °C. The results revealed that the insertion of cerium in both zeolites led to a decrease in their degree of crystallinity. Such a decrease was 36% for Ce-ZSM-5 while it was 18% for Ce-mordenite. The results indicated a significant decrease in nitrogen sorption capacity (BET) by 31% from 624 to 431 m2/g and also a decrease of the micropore volume by 27.8% from 0.579 to 0.418 cm3/g for Ce-ZSM-5. These results were found on the other extreme in case of Ce-mordenite. The presence of Ce did not affect the cell volume of ZSM-5 much, while it increased that of mordenite. Most of CeIV ions were embedded in the framework of mordenite, where they exposed as extra-framework CeO2 and cerium silicate in ZSM-5. The in situ interaction of NO on Ce-zeolites was studied using an FT-IR quartz cell. The adsorption of NO gas led to the formation of a series of nitrosyl species: N2O (2245 cm−1), NO+ (2160 cm−1), NO (1910 cm−1), N2O3 (1880, 1580 cm−1), (NO)2s,as (1844, 1734-1720 cm−1), NO2 (1630 cm−1) and ionic compounds were stable upon evacuation, i.e. nitrato and nitrito NOx− (x = 2-3) (1300-1500 cm−1). Such nitrosyl complexes were favorably formed on Ce-ZSM-5 than on Ce-mordenite due to facilitated intervention of the cerium couple (CeIII/CeIV) on the former than on the latter.