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 ...
The recently reported interesting behavior of the EPR spin intensity as freshly prepared CuZSM-5 is dehydrated has been confirmed, and the interpretation offered in explanation has been tested by extending the data. Magnetic susceptibility and optical diffuse reflection spectroscopy have been used to show that the anomalous EPR data correspond to changes in symmetry rather than disproportionation of [CuOH] + into Cu + and Cu 2+ Oas H 2 O is removed, as previously suggested. Moreover, the relatively small negative Weiss constants suggest that antiferromagnetic coupling through the supposedly EPR silent conventional bridge species, [Cu 2+ -O 2--Cu 2+ ] 2+ , cannot be responsible for the relatively large changes in observed EPR spin intensity on dehydration. EPR measures selectively individual paramagnetic species and ignores those of low symmetry, while magnetic susceptibility integrates over all magnetic species in the sample. The changes in the EPR intensities were much larger than those observed by magnetic susceptibility. Furthermore, those from EPR occurred as the first H 2 O was removed, before any significant change in susceptibility occurred. The EPR data reflected those expected for the decrease in symmetry accompanying dehydration. Support for these views was found in the magnetic susceptibility and optical reflectance data. Evidence for CuO formation concomitant with dehydration was obtained.
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