Studies of the surface species formed from nitric oxide on copper zeolites

Abstract

The surface chemistries of CuZSM-5 and of CuY preparations have been compared, using spectra of adsorbed N O taken under dynamic flow conditions at various temperatures between 173 and 723 K. Flow microbalance experiments were made with NO/He mixtures. At N O decomposition temperatures, a curious time dependence was observed where adsorption on the fresh catalyst increased rapidly, passed through a maximum, and then decreased again as it reached a steady state. In the steady state, the weight of CuZSM-5 was greater than its initial weight. Under favorable circumstances this increment was sufficiently large to permit observation of surface species present by IR. It was also shown that these species could be fully desorbed by flushing with pure He. Similar behavior was observed with CO, but in this instance the final weight was less than the initial weight. The weight loss with CO at 773 K (after flushing) corresponded almost exactly to a one-electron reduction of the copper ions (Cu2+ to Cu+). The spectra obtained after various pretreatments emphasized the differences between the CuY and CuZSM-5. At 173 K only the bands corresponding to chemisorption on Cu2+ appeared on the fully oxidized CuY, and only the bands corresponding to chemisorption on Cu+ appeared following reduction. With CuZSM-5, both sets of bands appeared in both instances, including those for the dinitrosyl species. The same bands appeared at higher temperatures, where oxidation occurred at rates that were temperature and pressure dependent. At 373 K and above, only species bound to Cu2+ were detected on CuY, whereas with CuZSM-5 bands for the mononitrosyls on both Cu+ and Cu2+ appeared, as well as those for adsorbed N02, nitrito, nitrato, and nitro species. The superior catalytic behavior of CuZSM-5 may be the result of its ability tostabilize thesespecies. To aid in interpretation, a briefspectroscopicstudy of the chemisorption of NO2 was carried out. The relevance of these findings to the mechanism of N O decomposition is discussed. IntroductionSince the discovery by Iwamoto and co-workers* that the decomposition of NO was catalyzed at high conversions by CuZSM-5 zeolites, increasing interest in this field has been fueled by various environmental concerns. In spite of substantial effort in several laboratories, however, a truly satisfactory catalyst has not yet been developed. Because the catalysts usually cannot be operated at full conversion, NO2 is produced from unreacted NO and the 0 2 formed during reaction.2 Moreover, the obvious procedure of increasing the conversion to the levels required to avoid this problem by raising the temperature is ineffective because the Arrhenius plots tend to a maximum at around 800 K.lb3 The origin of this unfortunate circumstance is not understood.Various catalysts have been investigated with a view to developing more active catalysts or finding ones of sufficient activity to operate without development of this maximum,l so far without success. CuZSM-5 remains the best catalyst known for this purpose; it is ...

FTIR, XPS and TPR studies of N2O decomposition over Cu-ZSM-5

“…On the activated Cu-ZSM-5 sample that was preheated up to 500°C in Ar gas, the N 2 O sticking probability is equal to unity, few N 2 O molecules were detected (both in gaseous phase and in adsorbed form) and little IR absorption was observed at typical N 2 O frequencies According to the literature, the adsorption of pure nitrogen on Cu-ZSM-5 at <300°C could result in a narrow band at 2158 cm 1 . 9 Thus, the 2161 cm 1 band in the present study can be assigned to N 2 adsorbed on copper ions.…”

Spectroscopic Identification of Adsorbed Intermediates Derived from the CO+H2O Reaction on Zeolite-Encapsulated Gold Catalysts

“…When the reaction time increased, all the bands showed an increase in intensity. The strong and sharp CO band at 2192 cm −1 , assigned to the monodentate carbonyl bound to Au + highly dispersed in ZSM-5 channels (23), showed considerable stability throughout the reaction for over 3 h. It has been reported that the carbonyl species of some amorphous oxides such as TiO 2 -, MgO-, and Fe 2 O 3 -supported Au (24) are readily eliminated by evacuation at 300 K; however, it is of interest to find that the carbonyl species at 2192 cm −1 is stable, even after the gas phase evacuation at 358 K. A similar result was obtained for the Cu/Na-ZSM-5 system that established the stability of the Cu + -CO species compared with the corresponding analogue in the Cu/NaY zeolite catalyst (25). Nevertheless, there are other carbonate species that evolved at 1730 and 1400 cm −1 on the surface of Au/Na-ZSM-5 with time.…”

Plasma-Assisted Catalytic Reduction of NOx

“…Many studies suggest that lean-NO x SCR proceeds via oxidation of NO to NO 2 by oxygen, followed by the reaction of the NO 2 with hydrocarbons [3][4][5][6][7][8][9][10][11][12][13]. On catalysts that are not very effective in catalyzing the equilibration of NO+O 2 and NO 2 , the rate of N 2 formation is substantially higher when the input NO x is NO 2 instead of NO.…”