Interpretation of the EPR Spectra of Nitrogen-Containing Compounds

Teresa García , Paula Rubio-Marqués , Jose A. Vidal-Moya , Antonio Leyva-Pérez , Salem S. Al-Deyab ... Patrick J. Carl, Sara L. Isley, and Sara...
0 downloads 0 Views 175KB Size
Download PDF
8848

J. Phys. Chem. B 2000, 104, 8848-8854

Interpretation of the EPR Spectra of Nitrogen-Containing Compounds Adsorbed on Copper-Exchanged Zeolites Patrick J. Carl, Sandy L. Baccam, and Sarah C. Larsen* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: March 29, 2000

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate adsorption of a series of amines (ammonia, methylamine, triethylamine) on Cu-ZSM5 and Cu-Beta. Several changes were observed in the EPR spectra after adsorption of the amine: an increase in spectral broadening, a shift in the copper hyperfine features, and the appearance of nitrogen hyperfine splittings in the perpendicular region of the EPR spectrum for some of the samples. For samples that exhibited resolved nitrogen hyperfine couplings, the EPR spectra were easily interpreted in terms of the number of coordinated amine molecules. For samples in which resolved nitrogen hyperfine interactions were not observed, the interpretation of the EPR spectrum was more complex. A second approach for the interpretation of the EPR spectrum involved using an empirical model correlating the EPR parameters for a series of model compounds to the number of coordinated nitrogen atoms and the charge of the copper complex. This approach was very useful for determining the number of coordinated nitrogen atoms if the charge of the complex was known or vice versa. It was determined that four molecules of ammonia or methylamine bind to the Cu2+ centers in Cu-ZSM5 and Cu-Beta, but that only one molecule of triethylamine is able to bind to the Cu2+ centers in these zeolites. For cases in which the charge of the complex and the number of ligands are both unknown, there is more uncertainty in using this empirical approach to interpret the EPR spectra. This empirical approach was further assessed by application to the EPR spectra of NO adsorbed on Cu-ZSM5 and Cu-Beta.

Introduction Copper-exchanged zeolites exhibit unusual catalytic activity for the direct decomposition of NO and the selective catalytic reduction of NO with hydrocarbons (SCR-HC).1-6 Potential applications for copper-exchanged zeolite catalysts include diesel and lean-burn engines and stationary sources, such as power plants.1-6 Because the copper center is the active site for NO decomposition and SCR-HC of NO, it is important to understand how the structural and electronic environment of the copper center is affected by interactions with nitrogen ligands, such as NO and other nitrogen-containing reaction intermediates. Electron paramagnetic resonance (EPR) spectroscopy has been used extensively to probe the electronic environment of paramagnetic Cu2+ sites in zeolites, particularly in Cu-ZSM5 and Cu-Beta.7-16 These studies have provided valuable information about the electronic environment of the copper centers in Cu-ZSM5 and Cu-Beta. The framework of ZSM5 is composed of straight 10-ring, elliptical channels (pore dimension, 5.3 by 5.6 Å) running along the [010] direction and sinusoidal 10-ring, elliptical channels (pore dimension, 5.1 by 5.5 Å) along the [100] direction.17 The framework of zeolite Beta is similar in topology to that of ZSM5, but the pore size is larger. The framework of Beta is composed of straight 12ring channels (pore dimension, 5.5 by 5.5 Å) along the [001] direction and sinusoidal 12-ring, elliptical channels (pore dimension, 7.6 by 6.4 Å) along the [100] direction.18 The catalytic activities for the decomposition and SCR of nitrogen oxides are similar in Cu-ZSM5 and Cu-Beta. * Author to whom correspondence should be addressed. Fax: 319-3351279. E-mail: [email protected].

Several groups have studied the adsorption of nitrogencontaining compounds on copper-exchanged zeolites. For example, several groups have used EPR spectroscopy to study the interaction of ammonia with Cu-ZSM5.9,19,20 Changes in the EPR parameters after ammonia binding were reported, and on the basis of the observed nitrogen hyperfine splittings, it was concluded that four molecules of ammonia bind to the copper center.9,19 In other work, no nitrogen hyperfine features were resolved after adsorption of nitrogen-containing ligands.16,21 The present work was motivated by a desire to maximize the information that can be obtained from the EPR spectra of nitrogen-containing compounds adsorbed on copper-exchanged zeolites, particularly in the absence of observed nitrogen hyperfine splittings. In previous work, we reported the application of Peisach and Blumberg’s empirical model22 for Cu2+ EPR parameters to a series of copper-exchanged zeolites.23,24 The empirical model relates the EPR parameters, A| and g|, to the identity of the copper ligands and to the overall charge of tetragonal Cu2+ model complexes in frozen solution. Peisach and Blumberg’s empirical model has been applied extensively to copper proteins in order to identify nitrogen and sulfur ligands bound to the copper center.22,25 Analogously, these empirical correlations can be used to interpret the EPR spectra of copper-exchanged zeolites. Our previous work focused on the application of the empirical model to analysis of the EPR spectra of hydrated and dehydrated copper-exchanged zeolites.23,24 In the current study, EPR spectroscopy was used to investigate nitrogen ligation in copper-exchanged zeolites. The interactions of a series of amine compounds, ammonia, methylamine (MA), and triethylamine (TEA), with Cu-ZSM5 and Cu-Beta were

10.1021/jp0011995 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/19/2000

N-Containing Compounds on Cu-Exchanged Zeolites

J. Phys. Chem. B, Vol. 104, No. 37, 2000 8849

TABLE 1: ICP Results of Exchanged Zeolite Samples zeolite sample Cu-ZSM5 Cu-Beta a

source

exchanged with

Si/Al

exchange levela (%)

Zeolyst Corp Zeolyst Corp

Cu(NO3)2 Cu(NO3)2

16 18

40 64

Exchange level ) 2 × Cu/Al × 100%.

studied using EPR spectroscopy. The EPR spectra were fit using a previously described least-squares fitting program that included the effects of correlated g and A strain and rhombicity.24 The resulting EPR parameters (gzz and Azz) for copper-exchanged zeolites with nitrogen-containing ligands were analyzed using an empirical model relating the number of coordinated nitrogen atoms and the charge of the copper complex to the EPR parameters. The validity of the empirical approach was assessed, particularly for application to catalytic systems. The EPR spectra of nitric oxide (NO) adsorbed on the copper-exchanged zeolites ZSM5 and Beta were analyzed using this approach. Experimental Section Sample Preparation. The zeolites used in this study (NaZSM5 and NH4+-Beta) were purchased from Zeolyst and were not modified prior to exchange. The copper-exchanged zeolites were prepared by adding 5.0 g of the parent zeolite to 50 mL of a dilute copper solution [0.005 M copper(II) acetate for CuZSM5 and 0.05 M copper(II) nitrate for Cu-Beta], with subsequent stirring for 24 h at room temperature. The exchanged zeolite samples were then filtered, washed with 1.0 L of deionized water, and dried overnight in an oven at 363 K. Exchanged samples were characterized by X-ray powder diffraction and ICP-AES (inductively coupled plasma atomic emission spectroscopy) for elemental analysis. X-ray powder diffraction patterns were obtained using a Siemans D5000 diffractometer. Diffraction patterns agreed well with standard diffraction patterns for the parent zeolites. All of the samples were analyzed using ICP-AES (Perkin-Elmer Plasma 400) to determine the Si/Al ratios and the copper loading of the samples. The elemental analysis results are reported in Table 1. Prior to adsorption studies, the copper-exchanged zeolites were treated to remove zeolitic water. Two different pretreatment procedures were used. In the first pretreatment procedure, the in situ flow system described previously was used.23 This pretreatment consisted of placing 20 mg of sample in a quartz flow tube (i.d. ) 1.5 mm), heating the sample to 373 K in flowing helium UPC (99.999%, Air Products) for 1 h, and then heating the sample to 673 K for 1 h in flowing helium. The second pretreatment procedure was performed on a vacuum rack using standard EPR tubes with high-vacuum valves attached. This pretreatment consisted of evacuation of the sample (60 mg) at 10-3 Torr for 1 h and subsequent ramping of the temperature to 673 K over the course of 8 h, followed by maintenance of the sample at 673 K for 1 h under vacuum. Samples prepared by either of these pretreatment procedures are referred to as dehydrated. Gas Adsorption. Methylamine was adsorbed on the dehydrated zeolites (prepared using the first pretreatment procedure) by bubbling He at a rate of 5 cm3 min-1 through a solution of methylamine (40%, Fisher Scientific) at room temperature for 15 min. The sample was then purged with He at 5 cm3 min-1 for 5 min and cooled to 120 K for EPR data acquisition. Prior to ammonia, triethylamine, and nitric oxide adsorption, the copper-exchanged zeolite samples were dehydrated on a vacuum rack according to the second pretreatment procedure. Ammonia was introduced onto the dehydrated samples by

exposing each sample to ammonia gas (Matheson) for 1 h at room temperature to an equilibrium pressure of 50 Torr. TEA was adsorbed on the dehydrated zeolites by exposing each sample to TEA (99.9%, Fisher Scientific) vapor for 1h at room temperature to an equilibrium pressure of 80 Torr for Cu-ZSM5 and 153 Torr for Cu-Beta. The samples were then evacuated at 10-3 Torr for 5 min and sealed in an EPR tube fitted with a high-vacuum valve. NO was adsorbed on the dehydrated samples by exposure to NO (99% Matheson) gas at room temperature for 1h to a final equilibrium pressure of 115 Torr for Cu-ZSM5 and 142 Torr for Cu-Beta. Experimental Apparatus. CW EPR (continuous-wave EPR) spectra were acquired using a Bruker EMX61 EPR spectrometer equipped with a PC for spectrometer control and data acquisition. A Bruker ER41111 variable-temperature unit with a temperature range of 110-673 K was used to heat and cool the sample. Typical EPR spectral parameters were: X-band frequency ) 9.43 GHz, modulation amplitude ) 1 G, and modulation frequency ) 100 kHz. The magnetic field and microwave frequency were measured using a Hall probe and a frequency counter, respectively. Least-Squares Fitting Program. Experimental EPR spectra were fit using second-order perturbation equations26,27 for axial and rhombic A and g, including both of the isotopes of Cu (63Cu and 65Cu), combined with a simplex least-squares fitting routine.28 In some experimental EPR spectra, it was possible to resolve the different isotopes of copper in the low-field hyperfine region; therefore, the effects of both isotopes of Cu on the hyperfine lines were included. Fit parameters for the axial case included A|, g|, A⊥, g⊥, and a Gaussian broadening factor. Fit parameters for the rhombic case included Azz, Ayy, Axx, gzz, gyy, gxx, and a Gaussian broadening factor. For each case, a parameter representing correlated g and A strain was included. This strain parameter controlled the deviation of the g factor in a Gaussian manner from the normal g factor (∆g ) strain × σnormal and gi ) gnormal + ∆g). The slopes of the best-fit straight lines of A| vs g| and A⊥ vs g⊥ for the model compounds used by Peisach and Blumberg 22 were then used to calculate an A factor for each of the g factors (Ai ) ∆g × slope + Anormal). Separate parameters for each region of the spectrum were used, the lowfield region (Azz and gzz) and high-field region (Ayy, Axx, gyy, and gxx). Because some samples contained two resolved copper sites, the fitting program was modified to include a set of adjustable parameters for each of the two resolved copper sites and another adjustable parameter representing the relative concentration of each site. Results Ammonia, Methylamine, and Triethylamine Adsorption on Copper-Exchanged Zeolites. The EPR spectra acquired at 120 K after adsorption of ammonia, MA, and TEA on CuZSM5 and Cu-Beta are presented in Figures 1 and 2, respectively. For comparison, the EPR spectra of pretreated CuZSM5 and Cu-Beta are included in Figures 1D and 2D, respectively. The EPR spectra of pretreated Cu-ZSM5 and CuBeta, prior to adsorption, exhibit four hyperfine features in the low- and high-field regions of the spectra (Figures 1D and 2D). The hyperfine features are characteristic of a copper nucleus (I ) 3/2) interacting with the unpaired electron (S ) 1/2) of the Cu2+ ion (d9 electronic configuration). As discussed previously, the EPR spectra were analyzed using a least-squares fitting routine to determine the best-fit values of g and A.23,24 A typical example of the agreement between the experimental EPR spectrum and the least-squares fit to the spectrum is shown in

8850 J. Phys. Chem. B, Vol. 104, No. 37, 2000

Carl et al.

TABLE 2: EPR Fitted Parametersa for Zeolite Samples sample

Azz (A|) (MHz)

Axx (A⊥) (MHz)

Cu-ZSM5 (A) (B) Cu-Beta AM/Cu-ZSM5 MA/Cu-ZSM5 TEA/Cu-ZSM5b NO/Cu-ZSM5b AM/Cu-Beta MA/Cu-Beta TEA/Cu-Betab NO/Cu-Beta

488 504 512 535 521 488 370 516 540 500 365

34 75 39 10 93 50 20 78 89 50 40

Ayy (MHz)

4

16

gzz (g|)

gxx (g⊥)

2.305 2.263 2.316 2.256 2.255 2.323 2.400 2.258 2.266 2.295 2.388

2.056 2.053 2.067 2.061 2.080 2.060 2.090 2.068 2.079 2.060 2.079

gyy

2.062

2.056

| strain

⊥ strain

broadening (G)

0.002 0.002 0.001 0.008 0.019 0.020 0.001 0.011 0.009 0.030 0.021

0.001 0.001 0.002 0.028 0.007 0.010 0.001 0.026 0.003 0.010 0.040

34 34 25 99 67 80 50 99 97 80 44

a Estimated errors are (0.001 for g (g ), (5 MHz for A (A ), (0.005 for g (g and g ), and (10 MHz for A (A and A ). b Parameters were | zz | zz ⊥ xx yy ⊥ xx yy determined by spectral simulation.

Figure 1. EPR spectra of Cu-ZSM5 after adsorption of (A) triethylamine (TEA), (B) methylamine (MA), and (C) ammonia. For comparison, the EPR spectrum of dehydrated Cu-ZSM5 is shown in D. Inset: Expansion of the hyperfine region from 2500 to 3250 G. All EPR spectra were recorded at 120 K.

Figure 3 for MA adsorbed on Cu-Beta. The EPR parameters obtained from the least-squares fits to the EPR spectra in Figures 1 and 2 are reported in Table 2. After adsorption of the amine compounds on Cu-ZSM5 and Cu-Beta, several changes were observed in the EPR spectra: an increase in spectral broadening, a shift in the copper hyperfine features, and the appearance of nitrogen hyperfine features in the perpendicular region of the EPR spectrum for some of the samples. The Gaussian broadening of the EPR spectra of CuZSM5 and Cu-Beta after adsorption of amines was significantly larger than the Gaussian broadening of the EPR spectra of the pretreated samples. The Gaussian broadening of the pretreated samples ranged from 25 to 34 G, whereas the broadening of the samples after adsorption of nitrogen-containing compounds ranged from 44 to 99 G (see Table 2). In addition, a systematic increase in broadening with mI (-3/2, -1/2, 1/2, and 3/2) was observed after amine adsorption. The systematic increase in spectral broadening with mI has previously been attributed to correlated g and A strain in copper zeolite systems and in frozen solutions of copper complexes.24,29 The correlated g and A strain is indicative of sample heterogeneity.29 A shift in the low-field copper hyperfine features in the EPR spectra was also observed after adsorption of ammonia and MA on Cu-ZSM5 and Cu-Beta (insets to Figures 1 and 2). The shift corresponds to an increase in A| and a decrease in g| relative to the EPR parameters of the dehydrated zeolites. This change

Figure 2. EPR spectra of Cu-Beta after adsorption of (A) triethylamine (TEA), (B) methylamine (MA), and (C) ammonia. For comparison, the EPR spectrum of dehydrated Cu-Beta is shown in D. Inset: Expansion of the hyperfine region from 2500 to 3250 G. All EPR spectra were recorded at 120 K.

Figure 3. Comparison of (A) experimental EPR spectrum and (B) corresponding best-fit EPR spectrum for methylamine adsorbed on CuBeta.

in hyperfine parameters after amine adsorption can be interpreted using the empirical correlation between g| and A| observed for model compounds and will be discussed later in this paper. Adsorption of TEA causes a very small change in g| and A| when compared to the EPR parameters of dehydrated CuZSM5 and Cu-Beta (Table 2). The high degree of strain present

N-Containing Compounds on Cu-Exchanged Zeolites

J. Phys. Chem. B, Vol. 104, No. 37, 2000 8851

Figure 5. EPR spectra of NO adsorption on (A) Cu-ZSM5 and (B) Cu-Beta. Inset: Expansion of the hyperfine region from 2500 to 3250 G. EPR spectra were recorded at room temperature.

Figure 4. EPR spectra of Cu-ZSM5 samples after adsorption of (A) ammonia and (B) triethylamine. Insets: Second derivative of the hyperfine region from 3150 to 3450 G. All EPR spectra were recorded at 120 K.

in the spectrum of TEA/Cu-Beta resulted in a decrease in the resolution of the hyperfine features and prevented accurate fitting of the experimental spectra. Therefore, the EPR parameters reported in Table 2 for TEA/Cu-Beta were obtained through visual inspection and spectral simulation. The presence of nitrogen hyperfine splittings in the EPR spectrum often clearly indicates the nature of the Cu2+-nitrogen interaction. Nitrogen hyperfine features in the perpendicular region (high-field) were observed after adsorption of ammonia and TEA on Cu-ZSM5. The second derivative of the EPR spectra of ammonia/Cu-ZSM5 and TEA/Cu-ZSM5 are shown in Figure 4. The second derivative of the ammonia/Cu-ZSM5 spectrum revealed the presence of nine hyperfine lines in the perpendicular region of the spectrum with a spacing of 14 G. This is consistent with four nitrogen nuclei (I ) 1) interacting with a single unpaired electron (S ) 1/2), as observed previously by other groups.9,19 Similarly, TEA/Cu-ZSM5 exhibited three hyperfine lines with a spacing of 21 G in the perpendicular region, which suggests one nitrogen nucleus interacting with the unpaired electron. No hyperfine features in the perpendicular region were resolved for the MA/Cu-ZSM5 spectrum or for any of the Cu-Beta samples. The presence or absence of motional broadening at room temperature and low temperature (∼120 K) can also provide important information about the system. As has been observed in the room-temperature EPR spectra of hydrated Cu-exchanged zeolites,9,12,30 the room-temperature EPR spectra (not shown) of ammonia and MA adsorbed on Cu-ZSM5 and Cu-Beta are broad and featureless, suggesting the presence of motional broadening. The motional broadening can be eliminated and resolved copper hyperfine splittings can be observed by recording the EPR spectra at 120 K, as was done in this study. The EPR spectra of dehydrated Cu-ZSM5 and Cu-Beta exhibit well-resolved copper hyperfine features even at room temperature, and this has been interpreted as indicating that the copper complex is bound to the oxygen atoms in the zeolite lattice.9 Similarly, resolved copper hyperfine splittings are observed in the room-temperature EPR spectra of TEA adsorbed on Cu-

ZSM5 and Cu-Beta. There are two possible explanations. Either the TEA/Cu2+ complex is bound to zeolitic oxygen atoms or the TEA/Cu2+ complex is too large to undergo motion in the pores of ZSM5 and Beta. Because the nitrogen hyperfine splitting indicates coupling to one nitrogen nucleus, the most plausible explanation for the lack of motional broadening is that the [Cu2+(TEA)] complex is bound to the oxygen atoms of the zeolite lattice. Nitric Oxide Adsorption on Copper-Exchanged Zeolites. EPR spectra shown in Figure 5 were recorded after adsorption of nitric oxide (NO) on Cu-ZSM5 and Cu-Beta. The six evenly spaced signals with small intensities are due to a manganese impurity in the sample cavity. The EPR spectra were collected at room temperature, and broadening due to motional effects was not observed. This suggests that the Cu2+ complex formed is immobile and most likely coordinated to the lattice oxygen atoms of the zeolite framework. The EPR spectrum of NO/Cu-Beta indicates the presence of one species, whereas the spectrum of NO/Cu-ZSM5 indicates the presence of two distinct Cu2+ species. The low signal-to-noise ratio of the NO/ Cu-ZSM5 EPR spectrum prevented the fitting of the spectrum to obtain the EPR parameters for both species. The EPR parameters were obtained for the more intense of the two species through visual inspection and spectral simulation. The EPR parameters for the EPR spectra in Figure 5 are reported in Table 2. After NO adsorption, an increase in g| and a decrease in A| were observed relative to the values for the dehydrated samples. No nitrogen hyperfine features in the perpendicular region were observed in either EPR spectrum. The Gaussian broadening of the EPR spectra for NO/Cu-ZSM5 and NO/Cu-Beta was larger than the Gaussian broadening observed for the dehydrated samples and smaller than the Gaussian broadening reported for the samples with ammonia, MA, and TEA adsorbed on CuZSM5 and Cu-Beta. Discussion General Features of the EPR Spectra of NitrogenContaining Compounds Adsorbed on Cu-ZSM5 and CuBeta. The room-temperature EPR spectra of hydrated Cuexchanged zeolites ZSM5 and Beta are broad and featureless. However, the EPR spectra recorded at 120 K exhibit clearly resolved copper hyperfine features. The differences in broadening of EPR spectra collected at room temperature and 120 K

8852 J. Phys. Chem. B, Vol. 104, No. 37, 2000 have been attributed to the mobility of the Cu2+ hydrated complexes in the zeolite channels.9,12,30 Similarly, after adsorption of ammonia and MA on dehydrated Cu-Beta and CuZSM5, the room-temperature EPR spectra were broad and featureless, whereas the low-temperature EPR spectra showed the characteristic copper hyperfine features. In previous studies, it was suggested that the ammonia binds to the copper center forming a mobile copper-amine complex in Cu-Y analogous to the hydrated Cu2+ complex.31 The [Cu(NH3)42+] complex formed is free to move around in the zeolite channels, resulting in the observed broadening of the room-temperature EPR spectrum. This motional broadening was not observed in the EPR spectra of TEA/Cu-ZSM5 and TEA/Cu-Beta, which implies that the observed complex is not free to move around, either because of its size or because it is bound to the zeolite lattice. Further information regarding the identity of the EPR-active species can be obtained through analysis of the nitrogen hyperfine interaction observed in the perpendicular region after ammonia and TEA adsorption on Cu-ZSM5. The EPR spectrum of ammonia adsorbed on Cu-ZSM5 shows nine resolved nitrogen hyperfine features in the perpendicular region. This indicates four equivalent nitrogen nuclei (I ) 1) interacting with the unpaired electron with a coupling constant of 14 G. The kinetic diameter of NH3 is 2.6 Å,32 which would result in a complex with a kinetic diameter of ca. 6 Å (assuming a Cu-N bond length ca. 2 Å) for [Cu(NH3)4]2+. A complex of this size would be limited to two possible locations in zeolite Beta and one possible location in ZSM5. The most likely sites for the Cu2+-ammonia complex in Cu-Beta would be in the larger sinusoidal channels or at the intersection of the two channels. In ZSM5, the size of the channels is too small to accommodate the Cu2+-ammonia complex; however, the free volume at the intersection of the channels would be sufficient to accommodate the Cu2+-ammonia complex. The lack of motional broadening and the presence of three hyperfine features in the EPR spectrum of TEA/Cu-ZSM5 both suggest a Cu2+-TEA complex that is bound to the zeolite framework. After dehydration of the Cu-exchanged ZSM5 and Beta samples, the Cu2+ centers are presumably located at the channel intersections in a square-planar [Cu(Ozeo)4] or squarepyramidal coordination [Cu(Ozeo)4(OH)]. The charge of these proposed complexes is not included because the charge of the zeolitic oxygen atoms is not known. The observed nitrogen hyperfine features in the perpendicular region of the EPR spectrum of TEA/Cu-ZSM5 indicate the interaction of one nitrogen nucleus with the Cu2+ center, suggesting a copper complex, such as [Cu2+(TEA)]. Considering the kinetic diameter of triethylamine (7.8 Å),32 it would be unlikely for more than one TEA molecule to be able to coordinate to the zeolite-bound Cu2+ center within the constrained environment of the zeolite. This zeolite-bound complex would be immobile and would not be expected to exhibit motional broadening, consistent with our experimental observations. The Use of Empirical Correlations to Interpret the EPR Spectra of Amines Adsorbed on Copper-Exchanged Zeolites. By using Cu2+ tetragonal model compounds with well-defined structures and charges, Peisach and Blumberg developed empirical correlations between g| and A| and the charge and identity of ligands for Cu2+ tetragonal model compounds. A plot of A| vs g| for the model compounds produced a linear correlation in which the position of each data point is dependent on the number of ligands in the first coordination sphere and the overall charge of the copper complex. Peisach and Blumberg applied this model to Cu2+ systems containing oxygen, nitrogen,

Carl et al.

Figure 6. Graph showing the correlation of the EPR parameters g| and A| for a series of nitrogen-containing Cu2+ tetragonal model compounds. The EPR parameters for Cu2+ model compounds containing four nitrogen atoms (4N) (2) and one nitrogen atom and three oxygen atoms (1N3O) (9) ligands are plotted. The bar indicates the charge of the model complex within each series. Bars are shown for two other cases, three nitrogen atoms and one oxygen atom (3N1O) and two nitrogen atoms and two oxygen atoms (2N2O), but for clarity the data has been omitted. (The data were obtained from ref 22 and are listed in Table S1.)

and sulfur ligands.22 Recently, we revisited the work of Peisach and Blumberg to analyze the EPR data of copper-exchanged zeolites.23,24 For this study, the relevant correlation plots for the EPR parameters of nitrogen-containing model compounds are shown in Figure 6. A high degree of overlap was observed in the correlation plots for the various numbers of coordinated nitrogen atoms. For example, the EPR parameters for the model compounds with four coordinated nitrogen atoms (4N, triangles) and with one coordinated nitrogen atom and three coordinated oxygen atoms (1N3O, squares) are plotted in Figure 6, along with bars to represent the charge on the Cu2+ model complexes. The bars representing the charge of Cu2+ model complexes with two and three nitrogen atoms are also shown in Figure 6 to illustrate the degree of overlap between the different groups of model compounds. For clarity, all of the data points are not shown. The substantial overlap between the different groups of complexes (4N, 3N1O, etc.) suggests that this method will have limited utility for using the EPR parameters alone to determine the number of coordinated nitrogen atoms and the charge of the Cu2+ complex. The uncertainty of this analysis can be improved by considering a situation in which the charge of the copper complex is known but the number and/or identity of the ligands is not known. Figure 7 illustrates how the EPR parameters systematically change with ligand identity at a constant copper-complex charge of +2. In this group, the model compound with four oxygen ligands has the largest g| and the smallest A|, whereas the model complex with four coordinated nitrogen atoms has the largest A| and the smallest g|. Therefore, if the charge of the copper complex is known, the number and identity of the coordinated ligands can be ascertained from a graph analogous to Figure 7. To further analyze the EPR data from this study and to evaluate the usefulness of the empirical correlations, the EPR parameters (g| or gzz and A| or Azz) from Table 2 for Cu-Beta and Cu-ZSM5 after adsorption of ammonia, MA, and TEA were plotted in Figure 8. For comparison, the bars from Figure

N-Containing Compounds on Cu-Exchanged Zeolites

Figure 7. Graph showing the correlation of the EPR parameters g| and A| for a series of nitrogen-containing Cu2+ tetragonal model compounds with a constant charge of 2+. (The data were obtained from ref 22 and are listed in Table S2.)

Figure 8. Graph showing the correlation of g| (gzz) and A|| (Azz) for dehydrated Cu-ZSM5 and Cu-Beta after adsorption of various nitrogen-containing ligands. The bars indicate the ranges of g| and A| and the charges of the relevant Cu2+ model compounds for comparison. The g| and A| parameters are given in Table 2. Ammonia/Cu-Beta (O), MA/Cu-Beta (0), TEA/Cu-Beta (∆), NO/Cu-Beta (]), ammonia/Cu-ZSM5 (b), MA/Cu-ZSM5 (9), TEA/Cu-ZSM5 (2), and NO/Cu-ZSM5 ([).

6 indicating the charges of the Cu2+ model complexes with different ligands are also shown in Figure 8. The data points for ammonia and MA adsorption on Cu-ZSM5 and Cu-Beta are clustered around the 4N and +2 charge model complex region. This is consistent with the observed nitrogen hyperfine splittings for ammonia/Cu-ZSM5. The nine hyperfine features that were observed after ammonia adsorption on Cu-ZSM5 (Figure 4A) suggest that four nitrogen nuclei interact with the unpaired electron. Therefore, on the basis of the nitrogen hyperfine and the correlation plot in Figure 8, the copper complexes were identified as [Cu(NH3)4]2+ and [Cu(MA)4]2+. These complexes are free to move around in the zeolite channels, as indicated by the observed broadening of the EPR spectra at room temperature. The interpretation of the EPR parameters for the samples of TEA on Cu-ZSM5 and Cu-Beta are more challenging because of the overlap of the 1N3O and 2N2O regions of correlation plot shown in Figure 6. Because the charge of the TEA/CuZSM5 or TEA/Cu-Beta complex is not known, the data in

J. Phys. Chem. B, Vol. 104, No. 37, 2000 8853 Figure 7 are not particularly helpful. Using the correlation plot in Figure 8 alone, it is difficult to ascertain whether one or two TEA molecules are adsorbed on Cu-ZSM5 and Cu-Beta. The EPR parameters of TEA/Cu-ZSM5 and Cu-Beta correspond to the region of the model compounds with one coordinated nitrogen atom (1N3O) with a charge between 0 and +1 and at the end of the bar representing model compounds with two coordinated nitrogen atoms (2N2O). Additional information, such as the observation of nitrogen hyperfine interactions and line broadening, can be used to further determine the identity and number of the coordinated nitrogen atoms. The analysis of observed hyperfine features in the EPR spectrum of TEA/CuZSM5 (Figure 4B) indicates one coordinated nitrogen atom. This is reasonable given the size of TEA relative to the pore sizes for Cu-ZSM5 and Cu-Beta. Therefore, the complex is identified as [Cu(TEA)]2+. Because motional broadening is not observed, it is likely that the [Cu(TEA)]2+ complex is also bound to the oxygen atoms of the zeolite lattice. The Use of Empirical Correlations to Interpret the EPR Spectra of NO Adsorbed on Copper-Exchanged Zeolites. In previous EPR studies of NO adsorbed on Cu-ZSM5, a copper nitrosyl [Cu(NO)1+] complex was observed at very low NO concentrations (∼0.5 Torr NO).33 The EPR parameters for the copper nitrosyl complex were g| ) 1.87, g⊥ ) 2.0053, A| ) 187.5 G, and A⊥ ) 176.6 G. Nitrogen hyperfine splittings were also observed, which indicated the presence of one NO/Cu center. In a separate study at higher NO concentrations and under flowing conditions, it was reported that the EPR signal intensity of Cu-ZSM5 initially decreased in the presence of NO because of the formation of a diamagnetic Cu2+NO complex.12 After a 20-min exposure to 4% NO/He followed by a helium purge, the signal intensity had increased again. This was attributed to the formation of paramagnetic species, such as Cu2+NO2- or Cu2+NO3-. Analysis of the EPR parameters was not reported. In this study, samples with relatively high NO concentrations (∼100-145 Torr) were examined. The EPR parameters of the samples with NO adsorbed on Cu-ZSM5 and Cu-Beta are significantly different from the EPR parameters observed for the amine compounds adsorbed on Cu-ZSM5 and Cu-Beta. When A| and g| are plotted on the correlation plot in Figure 8 (filled and open diamonds), the data points lie well below the other data points, i.e., higher g| and lower A| relative to EPR parameters for the amines adsorbed on Cu-ZSM5 and CuBeta. No nitrogen hyperfine splittings were observed, and EPR spectra recorded at different temperatures suggest that motion of the copper complex does not occur. On the basis of these observations and the EPR parameters, the EPR signal is attributed to a [Cu2+(NO2-)] complex. On the basis of this assignment, the charge of the overall charge of the complex should be +1, with a decrease due to binding to the oxygen atoms of the zeolite framework. However, the EPR parameters from the experimental spectrum are outside the range of expected EPR parameters for Cu2+ complexes bound to one nitrogen atom and three oxygen atoms (1N3O). Another possibility is that the complex that is formed is [Cu2+NO+], which would have an overall +3 charge (potentially decreased by bonding to zeolitic oxygen atoms). This would more convincingly explain the position of the EPR parameters of NO/ Cu-ZSM5 and Cu-Beta on the correlation plot in Figure 8. IR bands for [Cu2+NO+] on Cu-ZSM5 have been reported previously.34 The EPR spectra of NO adsorbed on Cu-ZSM5 and CuBeta are challenging to interpret because the system is so complex. It is well-known that most copper-exchanged zeolites

8854 J. Phys. Chem. B, Vol. 104, No. 37, 2000 contain both Cu2+ and Cu1+ after standard pretreatment procedures. Several FTIR studies of Cu-ZSM5 have shown that numerous nitrogen-containing surface species are formed after exposure to NO.33-36 Many of these species result from reactions of NO with the copper centers even at room temperature. Further complexity is added because of fact that NO and Cu2+ are both paramagnetic. Considering all of these factors, it is perhaps not surprising that it was difficult to definitively identify the copper complex that was responsible for the observed EPR signal. Implications of This Work on Interpreting the EPR Spectra of Catalytic Systems. The best strategy for EPR spectral interpretation is to combine all available spectral data, such as information about motional broadening, the empirical correlation of the EPR parameters, and nitrogen hyperfine splittings. If nitrogen hyperfine couplings are resolved, the spectral interpretation is usually straightforward. If nitrogen hyperfine couplings are not resolved, the spectral interpretation is more challenging. In this paper, the use of the observed empirical correlations of the EPR parameters for model compounds has been assessed. It was found that the empirical correlations can be useful, particularly in cases where the charge of the copper complex or the identity of the ligands are known, such as ammonia adsorption. However, the spectral interpretation can still be ambiguous even after application of these ideas, particularly for cases in which no nitrogen hyperfine is observed and neither the charge nor the identity of the nitrogen-containing ligand is known, such as NO adsorption on Cu-ZSM5 and CuBeta. The methods outlined in this paper may be more amenable to systems in which the surface chemistry is less complex and more is known about the identification of the ligands. For example, the catalytic combustion of domestic odors, such as trimethylamine, occurs on copper-exchanged zeolites.37 In this system, EPR could be used to monitor the binding and reaction of trimethylamine at copper centers. Conclusions EPR spectroscopy was used to investigate the adsorption of a series of amine compounds, ammonia, methylamine, and triethylamine, on the copper-exchanged zeolites Cu-ZSM5 and Cu-Beta. The resulting EPR spectra were analyzed by considering changes in spectral broadening and changes in the EPR parameters after adsorption of the amine. For samples that exhibited resolved nitrogen hyperfine couplings, the EPR spectra were easily interpreted in terms of the number of coordinated amine molecules. For samples in which resolved nitrogen hyperfine interactions were not observed, the interpretation of the EPR spectrum was more complex. The use of an empirical model relating the number of coordinated nitrogen atoms and the charge of the copper complex to g| and A| was evaluated as a method for interpreting EPR spectra of nitrogen-containing compounds adsorbed on copper-exchanged zeolites. Analysis of the series of amine compounds adsorbed on Cu-ZSM5 and Cu-Beta suggested that this empirical correlation could be used successfully to determine the number of coordinated nitrogen atoms if the charge of the complex is known and vice versa. This is an important result, particularly for systems in which the nitrogen hyperfine splittings are not resolved in the EPR spectrum. For cases in which the charge of the complex and the number of ligands are both unknown, there is more uncertainty in using this empirical approach for interpreting the EPR spectra. The best strategy for EPR spectral interpretation is to combine all available spectral information, as outlined in this paper. This approach was demonstrated by the application

Carl et al. of this strategy to the interpretation of the EPR spectra of NO adsorbed on Cu-ZSM5 and Cu-Beta. Acknowledgment. S.C.L. gratefully acknowledges the support of NSF (CTS-99-73431) and the University of Iowa. The authors also thank Dr. Russell Larsen for helpful discussions. Supporting Information Available: The EPR parameters plotted in Figures 6 and 7 along with relevant references are available in Table 1S and Table 2S. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95, 3727-3730. (2) Iwamoto, M.; Furukawa, H.; Kagawa, S. Catalytic Decomposition of Nitric Monoxide oVer Copper Ion-Exchanged Zeolites; Elsevier Publishing Company: New York, 1986. (3) Li, Y.; Armor, J. N. Appl. Catal. B: EnViron. 1993, 3, L1-L11. (4) Shelef, M. Chem. ReV. 1995, 95, 209-225. (5) Li, Y.; Armor, J. N. Appl. Catal. B: EnViron. 1992, 1, L21-L29. (6) Li, Y.; Hall, W. K. J. Phys. Chem. 1990, 94, 6145-6148. (7) Dedecek, J.; Sobalik, Z.; Tvaruzkova, Z.; Kaucky, D.; Wichterlova, B. J. Phys. Chem. 1995, 99, 16327-16337. (8) Attfield, M. P.; Weigel, S. J.; Cheetham, A. K. J. Catal. 1997, 170, 227-235. (9) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 4174-4179. (10) Kucherov, A. V.; Kucherova, T. N.; Slinkin, A. A. Catal. Lett. 1991, 10, 289-296. (11) Kucherov, A. V.; Slinkin, A. A.; Kondrat’ev, D. A.; Bondarenko, T. N.; Rubinstein, A. M.; Minachev, K. M. Zeolites 1985, 5, 320-324. (12) Larsen, S. C.; Aylor, A.; Bell, A. T.; Reimer, J. A. J. Phys. Chem. 1994, 98, 11533-11539. (13) Schoonheydt, R. A. Catal. ReV.-Sci. Eng. 1993, 35, 129-168. (14) Kucherov, A. V.; Gerlock, J. L.; Jen, H.-W.; Shelef, M. J. Catal. 1995, 152, 63-69. (15) Kucherov, A. V.; Hubbard, C. P.; Shelef, M. J. Catal. 1995, 157, 603-610. (16) Oliva, C.; Selli, E.; Ponti, A.; Correale, L.; Solinas, V.; Rombi, E.; Monaci, R.; Forni, L. J. Chem. Soc., Faraday Trans. 1997, 93, 26032608. (17) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nature 1978, 272, 437-438. (18) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. Zeolites 1988, 8, 446-452. (19) Sendoda, Y.; Ono, Y. Zeolites 1986, 6, 209-212. (20) Biglino, D.; Li, H.; Erickson, R.; Lund, A.; Yahiro, H.; Shiotani, M. Phys. Chem. Chem. Phys. 1999, 1, 2887-2896. (21) Yu, J.-S.; Kevan, L. J. Phys. Chem. 1994, 98, 12436-12441. (22) Peisach, J.; Blumberg, W. E. Arch. Biochem. Biophys. 1974, 165, 691-708. (23) Carl, P. J.; Larsen, S. C. J. Catal. 1999, 182, 208-218. (24) Carl, P.; Larsen, S. C. J. Phys. Chem. B 2000, 104, 6568-6575. (25) Valko, M.; Morris, H.; Mazur, M.; Telser, J.; McInnes, E. J. L.; Mabbs, F. E. J. Phys. Chem. B 1999, 103, 5591. (26) Chasteen, N. D. Vanadyl(IV) EPR Spin Probes: Inorganic and Biochemical Aspects. In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1981; Vol. 3, p 53. (27) Toy, A. D.; Chaston, S. H. H.; Pilbrow, J. R.; Smith, T. D. Inorg. Chem. 1971, 10, 2219-2225. (28) Press: W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes in C, 2nd ed.; Cambridge University Press: New York, 1992. (29) Froncisz, W.; Hyde, J. S. J. Chem. Phys. 1980, 73, 3123-3131. (30) Kucherov, A. V.; Slinkin, A. A. J. Phys. Chem. 1989, 93, 864867. (31) Naccache, C.; Taarit, Y. B. Chem. Phys. Lett 1971, 11, 11. (32) Pauling, L. Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, New York, 1960. (33) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510-520. (34) Hoost, T. E.; Laframboise, K. A.; Otto, K. Catal. Lett. 1996, 37, 153-156. (35) Aylor, A.; Larsen, S. C.; Reimer, J. A.; Bell, A. T. J. Catal. 1995, 157, 592. (36) Jang, H.-J.; Hall, W. K.; d'Itri, J. L. J. Phys. Chem. 1996, 100, 9416-9420. (37) Kuwabara, H. Chem. Lett. 1992, 947-950.