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 T–O range, and N2 adsorption at 196 8C. 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 cm1), NO+ (2160 cm1), NO (1910 cm1), N2O3 (1880, 1580 cm1), (NO)2s,as (1844, 1734–1720 cm1),
NO2 (1630 cm1) and ionic compounds were stable upon evacuation, i.e. nitrato and nitrito NOx
(x = 2–3) (1300–1500 cm1). 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. |
