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J. Phys. Chem. 19!23,97, 1204-1212
Studies of the Surface Species Formed from NO on Copper Zeolites Jozsef Valyont and W. Keith Hall' Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: September 29, 1992; In Final Form: November 23, 1992
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 C O 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.
Introduction
Since the discovery by Iwamoto and co-workers* that the decomposition of NO was catalyzed at high conversions by CuZSM-5 zeolites, increasinginterest in this field has been fueled by various environmentalconcerns. 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 02 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.lb3The 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 superior to Cu-mordenite and much better than C U Y . ~The order of activity is the reverse of that expected on the basis of either the accessibility of gases to sites within the pore system (diffusion constraints) or the Cu loading per unit volume. The reasons for this are also not understood. Interestingly, a similarsituation was reported earlier' for NO reduction with CO over FeZSM-5, FeM, and FeY zeolites. In recognition of this rather mysterious behavior, Li and Hall3 undertook a rather detailed kinetic study of these systems with the idea that a better understanding of how the catalyst operates might suggest means of improvement. The gross kinetics proved quite simple and could be explained by a redox mechanism in which the catalyst is oxidized by NO and reduced through the spontaneousdesorption of 02.The data suggested that a small but finite concentration of vacant sites is maintained by an On leave from Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-I525Budapest, Hungary. To whom all corrcspondence should be addressed.
0022-3654/93/2097- 1204504.00/O
equilibriumbetween sites populated with oxygen and the ambient 0 2 pressure at the reaction temperature. Still problems remained: (a) how can the oxygen atoms be deposited on two sites relatively far apart on reoxidation of the reduced catalyst and how can two N atoms get together to form NZon decomposition of NO; (b) on spontaneous desorption of 0 2 how can atoms held on sites remote from each other combine to form 0 2 ; and (c) is lattice oxygen involved in the transport mechanisms required for (a) and (b)? The kinetic data suggested that N-N bond formation might be accomplished if the individual Cu+sites acted as templates for formation of dinitrosyl species which could then decompose into N20, leaving the oxygen behind.5 But this is a two-electron process, and the Cu ions undergo a one-electron redox. Thus, two Cu ions would appear to be required. It is not clear how this can be accomplished in high Si/Al materials where the aluminum T sites are relatively far apart. With a view to shedding light on some of these questions, Valyon and Hall6 undertook an infrared study of the chemisorption of NO from flowing gas streams in contact with a catalyst platelet maintained at temperatures between 173and 573 K. The spectra obtained at higher temperatures were, however, too weak to interpret. This was unfortunate since the maximum temperature was just below that where NO decompositionbecomes measurable. Indeed, Li and Armor7 have shown that reduced sites may be oxidized at 273 K freeing N2, but 0 2 is not released at temperatures below about 623 K. Ir) the experiments reported herein, an improved techniquehas been used to obtain reliable spectra under steady-state operating conditions. From previous EPR8-'8 and IR1*639*'9-22 studies much is known about the positions and redox properties of Cu cations in CuY and CuZSM-5 zeolites and the binding of NO to the copper ions. These results have shown that nitrosyl complexes can be formed and stabilized in these catalysts on both Cu2+and Cu+ ions. In the present work CuY and CuZSM-5 catalysts were studied under comparable conditions to see whether their different activities in 0 1993 American Chemical Societv
Surface Species Formed from NO on Cu Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 6, 1993 1205
NO decomposition are reflected by the different properties of the nitrosyl complexes in thesystems. It was found that the CuZSM-5 is much better at stabilizing the reaction intermediates which may be required for the desired transformations. Recently, Shelefz3 has questioned whether redox chemistry is involved at all in NO decomposition. He has suggested instead that all of the chemistry occurs on Cu2+ sites. Hall and Valy0n2~ have summarized the evidence favoring redox chemistry but have pointed out conditions under which Shelef could be correct. In the present paper these possibilities are further investigated. A possible role for NO2 in this chemistry has been recognized.
773 K
212.0
_L
673 K
.
Experimental Section Catalysts. Samples were prepared from the sodium form of the zeolites using conventional base-exchange procedures. The sample identification is given by cationic form, zeolite type, Si/ A1 ratio, and percent exchanged, e.g., CuZSM-5-26-166. The unit cell compositions of the preparations identified as CuZSM5-26-166, CuZSM-5-14-114, and CuY-2.5-80 were Nal.35Cu2.9~(A102)3.~~(Si02)9 N2 a. o. 4z ~o , Cu3.63(AlO2)6.36(Si02)89.64 and . Na10.72Cu22~9(A102)~6 47(SiO2)I 35.53, respectively. Thus, the number of T sites per unit cell was taken as 96 and 192 for the ZSM-5 and Y zeolites, respectively. Further characterization data are given elsewhere3-3' for the catalysts reported herein. MicrobalanceExperimeots. A Cahn microbalance (MMel RG2000) was used in the flow mode. Weight changes with time in various (reducing or oxidizing) gases were recorded at various temperatures and under reaction conditions. A detailed description of the experimental setup and operation has been r e p ~ r t e d . ~ . ~ Gases used were of >99.99% purity, and premixed gases were prepared from componentgases matching the same specification. He and 0 2 passed through activated 4A molecular sieve traps at 78 K before use. The NO/He mixtures were passed through a dry ice trap to remove possible traces of NO2. Otherwise, the premixed gases were used without further purification. The weights recorded were in the presence of the ambient gas atmospheres and corrected for buoyancy. IR Measurements. The IR cell was a modified version of that of Basu et al.25 It could be used to record spectra in the temperature range 173 K < T < 773 K. Self-supporting wafers varying between 10 and 20 mg/cm2 thickness were pressed, fixed to a tungsten or a stainless steel heating mesh, and positioned on a nickel support between the CaF2windows of the cell. Heating or cooling of the mesh controlled the wafer temperature. Heating was electrical and cooling was with liquid nitrogen or dry iceisopentane slush. Applying simultaneous cooling and electric heating, thermal equilibrium could be maintained at any desired temperature between 173 and 273 K. The temperature was measured with a chromel-alumel thermocouple in contact with the wafer on the side opposite from the mesh. Temperature fluctuations were within f 2 K. To improve heat transfer between the heating/cooling mesh and the wafer, NO/He or N02/He mixtures were introduced into the cell at a total pressure of 1 atm, or the premixed gases were passed through the cell in a continuous flow at a rate of -30 cm3 (NTP) min-I. Since the wall of the stainless steel container of the coolant was in direct contact with thegasesin thecell, theuse of liquid nitrogen as coolant limited the maximum N O pressure in the cell to about 0.1 Torr, Le., to its vapor pressure at 78 K. In some experiments a modified version of the cell was used. The coolant container and the heating mesh were substituted by a single copper rod of about 2.5-cm diameter with a window drilled for the IRbeam. It was slotted perpendicular to the window to hold the wafer in the closed sample chamber between the IR windows.26 The wafer was heated or cooled indirectly by heating electrically or cooling the part of the copper rod out of the closed chamber. Using this cell, the temperature of the sample could
Figure 1. Flow microbalance studies of the CuZSM-5-26-166 catalyst. Lined-out weights are represented by horizontal bars except for those for CO reduction, where weight varied continuously. All data werecorrected for buoyancy. The gas flowing over the catalyst is identified above these bars, and the treatment times (in minutes) are given below (ov = overnight). Pure He and 0;!,4% CO in He, 4% NO in He, and 1% NO;! in He were used.
be decreased to
