Probing CuI-Exchanged Zeolite with CO: DFT Modeling and Experiment

cation, Cu being at site II and Na at site II in CuI-exchanged faujasite. A DRIFT ..... blue balls are Na, large gray balls are Cu, beige balls are Si...
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J. Phys. Chem. B 2006, 110, 16413-16421

16413

Probing CuI-Exchanged Zeolite with CO: DFT Modeling and Experiment Nicolas Jardillier, Enrique Ayala Villagomez, Ge´ rard Delahay, Bernard Coq, and Dorothe´ e Berthomieu* Laboratoire de Mate´ riaux Catalytiques et Catalyse en Chimie Organique, UMR CNRS-ENSCM-UM1 5618, FR 1878, ENSCM, 8, rue de l’Ecole Normale, 34296 MONTPELLIER Cedex 5, France ReceiVed: May 24, 2006; In Final Form: July 4, 2006

Addition of CO on Cu-exchanged zeolite was investigated by means of quantum chemical calculations based on density functional theory. The aim of this investigation was to get insights about changes of electronic properties of a copper site with zeolite composition by using a CO probe molecule. Calculated νCO frequency values show that various Si/Al ratios of faujasite zeolite reproduce the expected experimental decrease of the νCO values with decreasing Si/Al ratio. These calculations predict that H/Na ratio variations also induce changes in the νCO values. These results illustrate that different compositions of the zeolite change the electronic properties of copper that are reflected in the νCO frequency values. DFT results showed also that different structures and CO adsorption energies are obtained due to various Si/Al and H/Na ratios of the zeolite. Finally, these calculations evidence the possibility for CO to be connected at the same time to CuI and to a close Na cation, Cu being at site II and Na at site II in CuI-exchanged faujasite. A DRIFT experiment on two samples of faujasite, Cu(28)H(51)NaY and Cu(25)H(0)NaY, supports νCO displacements to higher energy values with increasing H/Na ratio.

1. Introduction By benefitting from catalytic activity of transition metal ionexchanged zeolites, several reactions were recently investigated and optimized for their potential in environmental applications. The removal of undesired NO from chemical plants exhaust gases being necessary and required, copper-exchanged zeolites have recently proven to be very efficient catalysts. Among them, CuY (Figure 1) exhibits the highest advantages and is operating in selective catalytic reduction processes since 2002 for NOx removal from the tail gases of nitric acid plants.1,2 The development of such materials was made possible by the understanding of the mechanism and a deeper knowledge of siting and properties of Cu cations in the zeolite.3 In this respect, CO is frequently used as a probe molecule to identify cationic sites in zeolite-based materials.4 Indeed, infrared (IR) spectroscopy of CO leads to an intense stretching νCO peak that is very sensitive to the bonding of the complex with the surface. The IR spectroscopy of adsorbed CO provides information about the nature of the metal, its coordination, and oxidation states.4-6 When CO is adsorbed on free CuI, the νCO stretching frequency value is higher than that of the isolated molecule (the most recent experimental value is 2178 cm-1 for [CuCO]+7 against 2143 cm-1 for isolated CO). Formation of carbonyl bonds in cation-exchanged zeolite is generally considered as the so-called “nonclassical carbonyls” hosted inside zeolitic nanocavities. The coordination of CO molecule with the cation is generally considered as the result of three contributions: the electrostatic interaction between the CO molecule and the field of the cation, the CO 5σ orbital donation to the cation and the metal dπ back-donation to the antibonding π* CO orbital. Depending on the nature of the cation, alkali, or transition metal, the relative importance of these three contributions varies, leading to characteristic νCO displacements. The electrostatic * Corresponding author. E-mail: [email protected].

Figure 1. Cation positions in Y zeolite. Site I is at the center of an hexagonal prism, site I′ is in a sodalite cage adjacent to an hexagonal face shared by an hexagonal prism and by a sodalite cage, site II is in a supercage adjacent to an hexagonal face of sodalite cage shared by a sodalite cage and a supercage, site II′ is in sodalite cage adjacent to an hexagonal face shared by a sodalite cage and a supercage, site II* is like site II, but displaced toward the supercage, and site III is in a supercage adjacent to a four-membered ring of a sodalite cage. According to a nomenclature of Palomino et al.,13 Sites I* and II* (not in Figure 1) are located at the center of the plane of the six-membered rings connecting hexagonal prisms with sodalite cages and supercages, respectively, and site II (not in Figure 1) is in the center of a sixmembered ring connecting a sodalite cage and a supercage, just inside the supercage.

10.1021/jp063190u CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006

16414 J. Phys. Chem. B, Vol. 110, No. 33, 2006 interactions of CO adsorbed on the cation can be strongly modified by the environment, i.e., the zeolite and the neighboring cations of the zeolite. These interactions decrease the charge and the electrostatic potential of the cation on which CO is bonded. In addition to the reduction of the attractive terms, there is also an increase of the repulsive term originating from Pauli repulsion.4 Metal-exchanged zeolite properties are different depending on the zeolite structure and on its chemical composition. In CuIY, the occupation level of Cu cations at the different possible sites can be affected by the presence of cocations such as Na, Ca, Ba, and protons.8 Depending on the zeolite, the Si/ Al ratio can be varied as well as exchangeable cations. Precisely, in CuIY zeolites, a low Si/Al ratio and high density of exchangeable cations can be obtained. There are several possibilities of coordination for CO with cationic centers in CuI- and Na-exchanged zeolites. It is generally accepted that, in CuI-exchanged zeolites, CO adsorbs on isolated CuI species via C, leading to the formation of CuI(CO)n(n)1,2,3) species. There is neither experimental evidence for O-bonded CO with CuI in zeolites4 nor formation of isocarbonyl [CuOC]+ in the gas phase.7 In contrast, it was shown that CO coordinates to Na ions in zeolite according to different modes: either through the C end or through the O end. Most of the experimental studies considered isolated CuI species in zeolites to be responsible of νCO signals in the IR region, between 2080 and 2160 cm-1.4 In YCuICO zeolite, two νCO bands are frequently observed at ca. 2155-2160 and 21302143 cm-1.4,9-12 IR spectroscopy of CO has been used to probe the CuI location in Y zeolites. Borovkov et al.10 proposed that CuI at site I′ and at site II (Figure 1) can interact with CO, leading to a stretching νCO at 2160 and 2145 cm-1, respectively. In contrast, Turnes Palomino et al., based on combined XRD and IR experiments on CuIY before and after addition of CO at low temperature and low CO pressure,13 assigned the νCO at 2159 and 2143 cm-1 to adducts involving CuI placed at site II and II*, respectively, according to their nomenclature CuI being originally at sites I*, II*, and II.13 Both Borovkov et al.10 and Turnes Palomino et al.13 showed that the adsorption of CO induces a migration of some of the copper cations into the supercage. The migration of Cu following CO adsorption provided experimental evidence for displacement of cations upon addition of ligands. Besides these statements, recent studies reported that it is not obvious that any defined C-O stretching band is due to CO molecules adsorbed on CuI at different cationic sites. Indeed, it has been recently concluded that IR spectroscopy does not allow discrimination of different CuI sites simply on the basis of the νCO.14 In this respect, theoretical modeling can bring additional insights to a better understanding of the νCO values for adsorbed CO in CuI-exchanged zeolites. A lot of computational studies were devoted to identify the location of cations in the zeolite framework based on IR modeling of probe molecules on metal active sites. Most of these studies were based on the computing of IR spectra of adsorbed probe molecules on metal ions at only one position. On the basis of the comparison of metal ions at various positions, it has been recently concluded that CO stretching frequencies are not sitespecific in FERCuICO zeolite.15 There are no quantum chemical studies regarding IR spectra of transition metal ion-exchanged zeolite containing explicitly more than one cation. In the present work, we have considered it crucial to investigate how stretching νCO values of YCuICO depend on the chemical composition of the Y zeolite. One can expect then to better understand the

Jardillier et al. displacements of the νCO signal when CuIY zeolite composition is changed. This study required to compute IR spectra of CO adsorbed on metal-exchanged zeolites containing various Si/ Al ratios and explicitly more than one cation. Computational chemistry has made significant advances and allows accurate normal modes in the IR region of materials. To obtain the most accurate calculations, a higher level of theory and largest models are required. The present study is aimed at investigating qualitatively the influence of the zeolite composition, Si/Al ratio, and the nature of neighboring cations (Na and H) on the stretching frequency of CO adsorbed on CuIY. Quantum calculations based on density functional theory (DFT) were chosen for this study because it has been shown to yield a good description of systems containing transition metal species and to give harmonic frequencies in good agreement with experimental data. Because previous studies have shown that an electronic redistribution all over the material occurs in CuY systems,16-18 special attention is paid to understand more about the role of the zeolite upon a metal active site and to investigate how a νCO signal from incoming CO adsorbed on CuIY can be used to probe changes in the electronic properties of the whole CuIY material. In particular, CO adsorption on CuIY containing various cocation ratios was investigated because, to our knowledge, no experimental nor computational studies are available. As Y zeolites with Si/Al > 1 are not periodic and contain more than 600 atoms per unit cell, a cluster model approach has been chosen to model copper in Y zeolite. Model clusters large enough to possess various Al contents, Al distributions, and cocations were chosen. This paper is organized as follows. First, the νCO stretching signal of CO adsorbed on NaY is calculated using DFT to validate our model cluster. Then νCO of CO adsorbed on the copper site in CuIY is calculated in order to study how CO can be used to probe changes in the electronic properties of the zeolite depending on various Si/Al and Na/H ratios and distributions. The study of the zeolite structure upon CO addition on a metal active site and changes in the metal-CO binding energies in Y zeolite with Si/Al and H/Na ratios and Al distributions are reported. Finally, this approach from quantum chemical modeling is complemented with diffuse reflectance infrared Fourier transform (DRIFT) experiments of CO adsorbed on real CuIY materials. 2. Methodology 2.1. Models. Two small model clusters, [4Na]4Si,4Al and [3Na]5Si,3Al, and a large model [12Na]30Si,12Al cluster were cut from a NaY zeolite structure with a Si/Al ratio of 2.5. The model clusters are labeled as follows (Figure 2): cations and protons are in brackets and Al and Si tetrahedra are in subscript characters. This NaY structure was previously optimized using a molecular mechanic approach and periodic boundary conditions (cvff-aug-ionic force field).19 Dangling bonds were saturated by H atoms. These atoms were fixed during geometry optimization, whereas all the other atoms were relaxed. The model clusters containing copper and protons were built by changing Na cations for CuI or protons. In the large model cluster, different Al distributions were obtained by changing positions of selected Al and Si atoms. The Al distributions at copper sites in large models are labeled as follow: 1-3, 1-4, and 1-3-5 (Figure 3). To get more realistic positions for protons in model clusters, the deepest molecular electrostatic potential (MEP) wells were searched on the anionic Cu cluster.20 A series of model clusters was then available for the study of CO adsorption on a copper or sodium ion at site II of a Y zeolite (Figures 2 and 3).

Probing CuI-Exchanged Zeolite with CO

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16415

Figure 3. CuI at site II in Y zeolites. Large model clusters [CuI, 9H, 2Na]30Si,12Al with various Al distributions. Small gray balls are H, large blue balls are Na, large gray balls are Cu, beige balls are Si, brown balls are Al, and red balls are O. Figure 2. CuI and Na at site II in Y zeolites. Small model clusters and one large model cluster with various Si/Al and H/Na ratios. Small gray balls are H, large blue balls are Na, large gray balls are Cu, beige balls are Si, brown balls are Al, and red balls are O.

2.2. Calculations. B3LYP and PBE functionals were considered for the quantum chemical calculations based on DFT. All the calculations using the B3LYP functional as well as one calculation of a small model cluster using the PBE functional were carried out with the Gaussian 03 program.21 The extended Wachters basis set (8s6p3d) contracted according to (62111111/ 511112/311) was used for copper and a standard 6-31G(d) basis was used for the other atoms. The basis sets were chosen to be large enough to avoid BSSE corrections.21,22 Geometries were fully optimized with H atoms used to saturate dangling fixed bonds. Convergence criteria were 3 × 10-4 for the gradient and 1 × 10-8 for the energy. Unscaled values were used for the analytical frequency values and for zero-point vibrational energy corrections. No frequency calculation was performed for the large model cluster using the B3LYP calculation. Conversions from ∆U to ∆H ) ∆U + ∆(pV) use the ideal gas law to calculate ∆(pV) ) RT∆n, where ∆n is the change in the number of gas-phase molecules in the reaction. Thermal corrections were calculated for the evaluation of reaction enthalpies ∆H at 298.15 K using standard statistical mechanical formulas in the independent mode, harmonic oscillator, and rigid rotor approximations by using our DFT results. The enthalpy has been evaluated according to the development given in McQuarrie.23 Three large clusters (Figure 3) were calculated using the gradient-corrected density functional level using the PBE exchange correlation functional, and the calculations were carried out with the deMon program. All-electron DZVP quality

basis sets were used for all the atoms. The associated auxiliary basis sets used to fit the density for all the atoms was A2.24 Convergence criteria were 1 × 10-3 for the gradient and 5 × 10-7 for the energy. The new partial Hessian25 numerical vibrational analysis approach available in the deMon program20 was used for the three large clusters containing CO to reduce the computational time. This partial vibrational analysis approach is suitable for these structures because only C and O atoms are involved in the νCO vibrational mode. Normal modes and ZPE (zero-point energy) were not computed for large model clusters without CO. 2.3. Experiment. The preparations of the CuHNaY catalysts were previously reported.26 Briefly, Cu(25)H(0)NaY (wt % Si ) 22.0, wt % Al ) 8.8, wt % Na ) 5.9, wt % Cu ) 2.5) was obtained by pouring NaY (2 g; Su¨d Chemie, Si/Al ≈ 2.55, SBET ≈ 700 m2 g-1) into the desired amount of a Cu(NO3)2 solution (≈0.01 mol L-1, pH 5) and stirring for 24 h at 298 K. Cu(28)H(51)NaY (wt % Si ) 26.7, wt % Al ) 10.2, wt % Na ) 1.85, wt % Cu ) 3.4) was obtained by pouring NH4Na(25)Y (2 g) into the desired amount of a Cu(NO3)2 solution (0.01 mol L-1, pH 5) and stirring for 24 h at 298 K. NH4Na(25)Y was obtained previously by exchange of NaY with a desired amount of NH4NO3 for 24 h at reflux. The solids were then separated from the liquid phase by centrifugation, washed with deionized water (30 mL), and centrifuged again. The samples were dried for 1 h in an oven at 353 K and then calcined overnight at 723 K in air (60 cm3 min-1). CO adsorption was studied by DRIFT spectroscopy with a Bruker IFS 55 spectrometer equipped with a SPECAC reacting cell. After reduction at 500 °C in a flowing 3% H2/Ar gas, the catalyst was cooled to 298 K under He flowing gas. Then the

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Jardillier et al.

TABLE 1: Calculated and Experimental Stretching νCO Values, νCO Shifts Related to Isolated CO, and Structure Parametersa entry

method

structure

νCO cm-1

∆(νCO-νCOgas) cm-1

d6mr-M Å

dM-C Å

a b c d e f g h i j k l m n o p

B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP PBEb PBE PBEc PBE PBE PBE exp exp

CO [CuICO, 3H]4Si,4Al [CuICO, 2H, Na]4Si,4Al [CuICO, H, 2Na]4Si,4Al [CuICO, H, 2Na]′4Si,4Al [CuICO, 3Na]4Si,4Al [NaCO, H, 2Na]4Si,4Al [NaOC, H, 2Na]4Si,4Al [CuICO, 2Na]5Si,3Al [CuICO, 11Na]30Si,12Al COgas [CuICO, 2Na]5Si,3Al COgas 1-3-5[CuICO, 9H, 2Na]30Si,12Al 1-3[CuICO, 9H, 2Na]30Si,12Al 1-4[CuICO, 9H, 2Na]30Si,12Al COgas YCuICO12

2208 2225 2219 2191 2189 2176 2238 2185 2202

+17 +11 -17 -19 -32 +30 -23 -6

0.818 1.058 0.975 0.866 0.926 0.438 0.431 0.995 0.829

1.845 1.818 1.820 1.820 1.819 2.676 2.540 1.821 1.855

-21

0.995

1.785

-15 -6 -9

0.841 0.944 0.962

1.831 1.825 1.812

exp

XCuICO12

exp exp

YNaCO29 YNaOC29

2124 2103 2112 2097 2105 2102 2143 2160 2140 2155 2130 2171 2122

dC-O Å 1.138 1.134 1.138 1.142 1.142 1.144 1.134 1.14 1.14 1.147 1.149 1.157 1.151 1.155 1.154 1.155

+17 -3 +12 -13 +28 -21

a M-C is the distance between the adsorbed metal site (CuI or Na) and C of CO, d 6mr-M is the distance between the metal site and the plan defined by a triangle containing three O atoms centered toward the six-membered ring. b Calculated with the calculation criteria reported for [CuICO, 2Na]5Si,3Al (see Section 2.2). c Calculated with the calculation criteria reported for largest clusters [CuICO, 9H, 2Na]30Si,12Al (see Section 2.2).

catalysts were put in contact with CO/He flow (1/99, CO purity >99.95%,). The spectra were collected at increasing time exposure by accumulation of 200 scans (resolution 2 cm-1). CO/He flow rate was adjusted at 10 cm-3 min-1 (4.09 mmol min-1). Therefore, less than two minutes of flowing CO/He (1: 99) were theoretically required to saturate the sample. 3. Results and Discussion 3.1. Stretching νCO of CO Adsorbed on CuIY. Previous studies have shown that the model cluster approach can be useful for interpreting the adsorption energy of CO on alkali.27,28 Small cluster models (Figure 2) were used in order to get a valuable and easy tool for studying parameters that may induce a νCO change of CO adsorbed on CuI. The size of the model clusters was chosen large enough to simulate significant variation of Si/Al and H/Na ratios. The study was extended to larger cluster models in order to investigate the effect of different Al distributions around the copper active site (Figure 3). Cu at site II has been considered in the present study because experimental studies led to the conclusion that most of Cu cations are located at this site. Because of the large range of values reported in the literature,4,9-12 calculated νCO values for CO adsorbed on Cu were compared to the recent experimental values12 (Table 1). Comparison of CO Adsorbed on Na and on CuI in Y Zeolite. First the adsorption of CO on [3Na, H]4Si,4Al used to model NaY (Figure 1) is considered. Species corresponding to CO adsorbed on alkali metal ions such as Na and Li are characterized by high-frequency bands, while transition metal ions are characterized by low-frequency bands. For an easy comparison of the results obtained for Na and Cu clusters, the approximation was made that CO adsorbs only on Na at site II, and the model considered [3Na, 1H]4Si,4Al as having a Si/Al ratio of 1, which actually corresponds to a X zeolite. The calculated absolute values of νCO for free CO and in the cluster [NaCO, H, 2Na]4Si,4Al strongly differ from experimental values. The unique νCO band for CO on NaY is reported at 2171 cm-1 from experiment,29 whereas it is calculated at 2238 cm-1 (Table 1). This large discrepancy was expected because the empirical

scaling factors had not been used, while it is generally used to compensate harmonic approximation that constitutes one of the most important parts of the gap between experimental and theoretical frequency values. As shown in Table 1, the calculated shift of νCO in the gas phase compared to CO adsorbed on NaY at site II is +30 cm-1, in very good agreement with the experimental shift of +28 cm-1. Adsorption of CO on NaY is known also to lead to the formation of Na-OC species from O-end adsorption. The calculation on [NaCO, H, 2Na]4Si,4Al with O-end adsorption predicts a red-shift of νCO frequency value of -23 cm-1 against -21 cm-1 from experiment. The gap calculated between [NaCO, H, 2Na]4Si,4Al (C-end) and [NaOC, H, 2Na]4Si,4Al (O-end) is 53 cm-1, in very good agreement with the value of 49 cm-1 reported in the literature.4 These results show that present model clusters and DFT calculations lead to reliable values. Then, the adsorption of CO was studied on [Cu, H, 2Na]4Si,4Al with CuI at site II. We studied the effect of increase of acid strength of the zeolite by increasing the Si/Al ratio30 that is also known to induce a shift of νCO to higher frequency values for CO adsorbed on CuI-exchanged zeolites.4 The νCO is shifted by -17 cm-1 in [CuCO, H, 2Na]4Si,4Al (C-end) (Table 1, entry d) with respect to COgas. Two νCO bands for CO adsorbed on CuIY and on CuIX are generally obtained experimentally. Recent results12 reported a small red-shift (-3 and -13 cm-1) with respect to COgas for the lowest normal mode in CuIY and CuIX, respectively. The calculated red-shift of -17 cm-1 (Table 1, entry d) is larger than these experimental values. Nevertheless, the calculated νCO frequency offset value between [NaCO, H, 2Na]4Si,4Al and [CuCO, H, 2Na]4Si,4Al reproduces the expected decrease of the νCO, although this decrease is overestimated. The calculated deviation is -47 cm-1 between [NaCO, H, 2Na]4Si,4Al and [CuCO, H, 2Na]4Si,4Al (Table 1, entries g and d) compared to the experimental shifts between NaX and CuIY and NaX and CuIX of -11 and -16 cm-1 for high frequencies and -31 and -41 cm-1 for low frequencies (Table 1).12 CO Adsorbed on CuI in Y Zeolite with Different Si/Al Ratio. Going further into the investigation of the reliability of our

Probing CuI-Exchanged Zeolite with CO

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16417

TABLE 2: Comparison of Calculated Geometrical Parameters Depending on the Calculation Method and on the Size of the Model Clustera [CuICO, 2Na]5Si,3Al B3LYP

[CuICO, 11Na]30Si,12Al PBE

B3LYP

distances in Å

with CO

without CO

∆

with CO

without CO

∆

with CO

without CO

∆

Cu-O1 Cu-O2 Cu-O3 Cu-O4 Cu-O5 Cu-O6 Cu-C CdO CuCO CONa′ CuCONa′ O-Na′ Cu-Na′ Na′-Na′′ Na′′′-Na′′′′

2.059 3.427 3.431 3.633 2.998 2.092 1.821 1.14 174.13

1.988 2.696 2.029 3.234 3.354 3.242

0.071 0.731 1.402 0.399 -0.356 -1.15

2.044 3.452 3.456 3.661 2.998 2.068 1.785 1.157 173.31

1.954 2.646 1.976 3.312 3.487 3.375

0.09 0.806 1.48 0.349 -0.489 -1.307

2.128 2.905 2.08 3.108 2.769 3.158 1.855 1.147 168.80 133.45 2.73 2.732 5.112 5.323 6.105

2.03 2.999 2.029 3.141 2.442 3.042

0.098 -0.094 0.051 -0.033 0.327 0.116

5.726 7.067 6.100

-0.614 -1.744 0.005

a

5.890

5.966

-0.076

5.890

5.971

-0.081

∆ are distance variations in Å.

calculations, we modeled clusters with different Si/Al ratios. Increasing Si/Al ratio is known to induce an increase of the acidity of the zeolite that is characterized by a displacement of the νCO adsorbed on Cu to higher frequency values. Several experiments reported that νCO bands are shifted to lower frequency values from CuIY (Si/Al > 1) to CuIX (Si/Al ) 1).4,12 The lowest frequency band at 2140 cm-1 in CuIY is at 2130 cm-1 in CuIX, and the highest frequency band at 2160 cm-1 in CuIY is shifted to 2155 cm-1 in CuIX.12 Two clusters with two different Si/Al ratios were chosen: [Cu, 3Na]4Si,4Al and [Cu, 2Na]5Si,3Al (Figure 2). These two clusters were selected because both of them contain Na as cocation. Calculated values of νCO showed a red-shift of -26 cm-1 going from cluster [CuCO, 2Na]5Si,3Al (entry i) to [CuCO, 3Na]4Si,4Al (entry f) (Table 1). In this case also, despite a calculated gap value of -26 cm-1, larger than the experimental value of -5 and -10 cm-1, the calculated data follow the experimental trend (Table 1). In conclusion, cluster models associated to B3LYP calculations follow nicely the experimental trends with a very good accuracy for CO adsorbed on Na and with a good qualitative agreement for CO adsorbed on Cu. B3LYP and PBE functionals are known to overestimate the π back-donation between copper and CO in zeolite.31 It has already been shown that it is still a challenge to reproduce accurately π back-donation for quantum calculations.7 This would explain why larger deviations from experimental data are calculated for Cu than for Na. In addition, the size of the model cluster is probably more crucial for Cu than for Na. Indeed, large deviations between small and larger clusters are calculated in the structures for clusters containing copper (see Section 3.2 and Table 2). CO Adsorbed on CuY with Different H/Na Ratios. We investigated then the influence of the nature of extraframework cations on νCO for CO adsorbed on CuIY. In a previous study,26 it was shown that large H/Na ratio increased the catalytic activity of the NO SCR by NH3. A computational study of various complexes showed that this effect can be partly explained by an increase of the “lability” of the proton as H/Na ratio of the material increases.26 In addition, it was shown that the zeolite plays the role of a reservoir of electronic charge.17 In the present study, we have considered that computational studies can be very useful to model CO adsorption on CuIY with different H/Na ratios to predict how changes in the electronic properties of CuY are reflected in the νCO vibration mode. As reported in Table 1 (entries f, e, d, c, b), B3LYP calculations show that higher H/Na

ratios induce an increase of νCO, from 2176 to 2225 cm-1. The two isomers [CuICO, H, 2Na]4Si,4Al and [CuICO, H, 2Na]′4Si,4Al (Table 1, entries d and e), which only differ by the position of one Na and H, led to similar νCO values (Table 1). Now, considering all the small clusters (Table 1, entries b-f,i), either a blue-shift (models [CuCO, 3H]4Si,4Al and [CuCO, 2H, Na]4Si,4Al ) or a red-shift (models [CuCO, H, 2Na]4Si,4Al and [CuCO, 3Na]4Si,4Al) is calculated (Table 1). The largest gap reaches 49 cm-1 (gap between entries f and b in Table 1), which is probably overestimated. Indeed, considering experimental νCO frequency values (2155-2160 and 2130-2143 cm-1),4,9-12 the largest gaps are expected to be around 10 cm-1. The calculated effects of the change of H/Na ratio on the νCO frequency shift of CO adsorbed on CuI are most probably too large. Nevertheless, on the basis of the present calculations, one may predict that increasing the H/Na ratio induces a change in the νCO to higher frequencies. CO Adsorbed on CuY with Different Al Distributions. Different Al distributions around the Cu site have also been considered by employing extended model clusters (Figure 3). To decrease the computational time, most of the Na cocations were exchanged for H in the models. In addition, we used the PBE functional and a partial Hessian vibrational analysis approach.25 Whatever the model cluster, a red-shift of νCO is always found. Depending on the Al distribution in the model cluster, the red-shift of νCO is slightly different and reaches a maximum of 9 cm-1. Considering the 1-3 and 1-4 Al distributions, the νCO deviation is only of 3 cm-1 (Table 1, entries o and p), a value considered as being within the range of calculation error. A larger deviation is found for the 1-3-5 [CuICO, 9H, 2Na]30Si,12Al cluster (Table 1, entry n), which would indicate that larger numbers of Al in the first coordination of copper modify the νCO frequency value. However, the gap between the values is rather small. In addition, the calculated deviations remain very small in comparison with those provided by different Si/Al and H/Na ratios. On the basis of previous νCO frequency calculations on CO adsorbed on copperexchanged zeolites,31 one cannot expect a νCO gap as small as 9 cm-1 to be significant using the present calculations. Thus additional calculations are required to conclude definitively if different Al distributions change or not the νCO values and thus the electronic properties of CuI in CuIY. To evaluate the influence of the methodology on the results, we compared the νCO values calculated using both B3LYP and

16418 J. Phys. Chem. B, Vol. 110, No. 33, 2006 PBE functionals. The absolute νCO value (2103 cm-1) of cluster model [CuCO, 2Na]5Si,3Al (Table 1, entry l) is closer to the experimental values than those calculated with the B3LYP functional. However the calculated νCO shift from COgas using PBE functional is much larger, -21 cm-1 against -6 cm-1, using B3LYP for [CuCO, 2Na]5Si,3Al (Table 1, entries l and i). This result shows that calculated νCO values and shifts strongly depend on the calculation method. Cluster size effect has been also considered as well. The comparison of the structures between the small cluster [CuCO, 3Na]5Si,3Al and the large cluster [CuCO, 11Na]30Si,12Al showed that, using the same B3LYP functional, the C-O distance is different in the two clusters (1.140 and 1.147 Å, respectively) (Table 1, entries i and j). Thus a different νCO frequency is expected for the large cluster. As will be shown in the next paragraph (Section 3.2), in small and large clusters, the geometry of the active site as well as the positions of cations are slightly different. On the basis of the present DFT calculations of νCO of CO adsorbed on model clusters used to model CuI-exchanged faujasite, one can conclude that these calculations reproduce the general trends obtained experimentally and predict changes of the νCO values with different Si/Al and H/Na ratios. 3.2. Structure of Copper Site after CO Adsorption. Experimental results clearly indicated that the adsorption of CO on isolated copper in CuIY modifies both copper position and coordination.10,13,32 Using X-ray diffraction, a migration of Cu from its initial position to a new position was observed upon CO adsorption.10,13 Change in the coordination of copper by CO adsorption was deduced from the decrease of the Cu-O peak intensity in EXAFS and the increase in the Cu-O atomic bond lengths.32 The geometries of model clusters were compared in order to get insights about changes in the structures before and after addition of CO on copper site. Calculations showed that an increase of the νCO value is always correlated with a smaller C-O bond distance, in agreement with previous results.33 The same tendency was obtained for large model clusters. As reported in Table 2, variations of Cu-O bond distances after CO adsorption strongly depend on the size of the model cluster. To evaluate changes of copper position, we have considered the distance between copper and the plane defined by a triangle containing the 3O atoms pointing toward the center of the sixmembered ring. This distance is labeled d6mr-M (Table 1), where the triangle defines the “plane” of the six-membered ring. For various Si/Al, H/Na and distributions of the cations, the calculated values of d6mr-Cu are in the range of 0.82 to 1.06 Å with CO adsorbed on copper. Without CO, this d6mr-Cu distance is close to zero. The comparison of the two clusters [CuCO, 2Na]5Si,3Al and [CuCO, 11Na]30Si,12Al shows that the d6mr-Cu value depends on the cluster size (d6mr-Cu value is 0.995 Å in the [CuCO, 2Na]5Si,3Al model against 0.829 Å in the [CuCO, 11Na]30Si,12Al model). This result illustrates, if necessary, that the environment of the zeolite has clearly a large influence on the position of the Cu site. Nevertheless, a large d6mr-Cu distance is also calculated for large model clusters, and in the same range of values (0.829-0.962 Å) as for small models (Table 1). The calculated d6mr-Cu distances are in good agreement with the copper displacement of 0.975 Å upon CO adsorption obtained from X-ray diffraction.13 These calculations confirm that when copper is in the plane of the six-membered ring, addition of CO induces its migration out of this plane. It has been shown from experiments that CO adsorption on CuIY induces not only a migration of Cu but also a decrease of the coordination of copper with the zeolite.13 The same evolution

Jardillier et al.

Figure 4. Adsorption of CO on model cluster [CuICO, H, 2Na]4Si,4Al, [NaCO, H, 2Na]4Si,4Al, [NaOC, H, 2Na]4Si,4Al, and [CuICO, 11Na]30Si,12Al, using B3LYP calculations. Small gray balls are H, large blue balls are Na, large gray balls are Cu, beige balls are Si, brown balls are Al, green balls are C, and red balls are O.

was found from calculation of CO adsorption on model clusters: one of the three short Cu-Ozeolite bond lengths becomes larger (Table 2). Whereas there is no experimental evidence for any displacement of Na cation upon CO adsorption, a slight migration of Na upon adsorption of CO is calculated using DFT. Considering the distance d6mr-Na between the Na cation and the plane previously described as containing the three O pointing toward the center of the six-membered ring, the calculated d6mr-Na value is 0.438 Å for CO adsorbed on [3Na, H]4Si,4Al (Table 1, entry g). However, in contrast to copper, before CO adsorption, the d6mr-Na distance is 0.149 Å. Thus the migration of Na upon CO addition is small (around 0.3 Å). The addition of CO did not change the coordination of Na, as indicated by the small variations of the calculated Na-O bond distances, the largest calculated variation being only 0.07 Å. A detailed analysis of the geometries provides evidence that cocations can be also perturbed by the addition of CO. As reported in Table 2 for Na′, Na′′, Na′′′, and Na′′′′ (Figure 4), depending on their position, small or large changes of the positions of the Na cocations were calculated before and after CO addition in cluster [CuICO, 11Na]30Si,12Al. This result suggests that CO addition induces not only a migration of copper, but also a rearrangement of cocation positions in the zeolite structure. This result clearly indicates that depending on the incoming ligand, a rearrangement of the Cu site and of the structure could occur. In [CuICO, 11Na]30Si,12Al, the M-C-O angle is 168.80° against 174.13° for the small model cluster (Table 2). The study of this large cluster [CuCO, 11Na]30Si,12Al reveals the possibility for CO to be bonded to Cu via the C-end and slightly bonded to Na via the O-end atom (Figure 4). The proximity of one Na at site III in the [CuICO, 11Na]30Si,12Al model induces a slight bending of the Cu-C-O angle. The short CuNa-OC bond distance (2.732 Å) and a small C-ONa angle (∼133.5°) allows the interaction between Na and O atom of CO bonded to Cu. As a result of its lower framework Si/Al atomic ratio, Y zeolite has a much higher density of ionexchangeable sites than ZSM-5 and mordenite zeolites. This

Probing CuI-Exchanged Zeolite with CO

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16419

TABLE 3: Calculated νCO and νCO Shifts Values and Adsorption Enthalpya method

model

B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP PBE exp exp exp

[CuICO, 2H, Na]4Si,4Al [CuICO, H, 2Na]4Si,4Al [CuICO, H, 2Na]′4Si,4Al [CuICO, 3Na]4Si,4Al [NaCO, H, 2Na]4Si,4Al [NaOC, H, 2Na]4Si,4Al [CuICO, 2Na]5Si,3Al [CuICO, 2Na]5Si,3Al YCuICO YNaCO YNaOC

a

∆H0K kJ/mol

adsorption enthalpy ∆H298.15K kJ/mol

-112 -99 -86 -92 -28 -23 -104 -145

-110 -98 -81 -88 -21 -20 -100 -139 -100 -21, -2838

Adsorption enthalpy for the reaction CuIY + CO f YCuICO.

allows copper cations to be closer to Na and CO to interact both with Cu at site II and Na at site III, even if the interaction between CO and Na is small. This study shows that there is a change of the structure of the cationic adsorption site before and after CO addition and that this change is sensitive to the surrounding cations. 3.3. Energetics of CO Adsorption on CuIY Zeolite with Different Si/Al and H/Na Ratio. The energetics of CO adsorption has been studied in several zeolites in order to get a better knowledge about adsorption properties and selectivities of zeolites. CO adsorbed on CuIY is highly stable due to a strong interaction between CO and CuI. Even at high temperature, CO remains adsorbed on CuIY,6 whereas HY and NaY materials do not adsorb CO under the same conditions.34 A heat of CO adsorption of -80 kJ/mol at very low coverage was recently reported for CuIY containing Na.34 This experimental value can be compared to the present one calculated for CO adsorption on all the cluster models containing Na cocations. Only small clusters were considered for this study. The calculated values show that there is not always a correlation between shifts in the νCO and heat of adsorption (Table 3). Using the small models and a B3LYP functional, adsorption energy values are between -81 and -110 kJ mol-1, against -100 kJ mol-1 from experiment (Table 3). The highest value is thus in good agreement with experimental value. These results suggest that different experimental values of adsorption energies should be found for CO adsorption on CuIY zeolites structures of various elemental compositions. The heat of adsorption is -100 kJ mol-1 with B3LYP and reaches a value of -139 kJ mol-1 using PBE for the same cluster [Cu, 2Na]5Si,3Al. This result was expected since it has been previously shown that the PBE functional leads to overestimated binding energies between Cu and ligands for copper-ligand complexes when compared to the B3LYP functional.35 Because of the strong interaction between CO and CuI, the heat of adsorption of CuICO in CuIY is high. In contrast, when CO is mainly bonded by electrostatic interaction as with Na, the heat of formation is low and CO adsorbed on NaY is only detected at very low temperature. The calculated heat of CO adsorption on NaY using B3LYP leads to a value of -21 kJ/mol at 298.15 K against -21-(-28) kJ/ mol from experiment.4 The present calculated value is comparable to these experimental values. B3LYP calculation for O-end adsorption of CO on NaY is -20 kJ mol-1. This value is slightly smaller than the value calculated for C-end adsorption by -4 kJ mol-1. For C- and O-end adsorption of CO on NaZSM-5, a slight decrease was also reported from experiment, from -28 to -24 kJ mol-1, respectively.29 Considering the O-end adsorption of CO to CuIY, the calculated binding energy of CO is -10 kJ mol-1 at 298.15 K using the model cluster [CuI, H,

Figure 5. DRIFT spectra in the ν(C-O) region of Cu(25)H(0)NaY (a) and Cu(28)H(51)NaY (b) after time exposure to a flowing 1% CO/ He mixture.

2Na]4Si,4Al and the B3LYP calculation. This value is lower than the adsorption values of CO on Na+ and confirms that adsorption of CO via the O atom cannot be detected at room temperature using standard IR analysis. The present study illustrates that adsorption properties are markedly affected by the nature of exchanged cations and their distribution and thus by the structure of the zeolite. 3.4. DRIFT Spectra of Adsorption of CO on CuIY Zeolite with Different Si/Al and H/Na Ratio. One of the objectives of the present work is to identify how quantum chemical modeling can contribute to the understanding of IR spectra of adsorbed CO. For that purpose, DRIFT experiments of CO adsorption on two CuIY materials were carried out (Si/Al ∼ 2.5, H/Na: 0 and 3). CuIY were obtained from reduction of the starting materials by H2/Ar (3/97) at 500 °C (see Experimental Section). We have indeed demonstrated clearly by temperatureprogrammed reduction experiments8,26 on the same and/or similar CuY catalysts that: (i) Cu in these solids is only composed of CuII cationic or oxocationic species at exchanging sites, and (ii) treatment at 500 °C in H2/Ar (3/97) only transforms CuII into CuI without any further reduction to Cu0. Figure 5 shows the IR spectra in the region 2100-2200 cm-1 for CO adsorption, as a function of time, under flowing CO/He. In agreement with most studies (see references in ref 4 for a review), three absorption bands were assigned to monocarbonyl (bands at ca. 2140 and 2160 cm-1) and dicarbonyl (band at 2178 cm-1) species. The achievement of full CO coverage (higher band intensity) is very fast on Cu(H)Y (less than 5 min), but occurs at a much lower rate on CuI(Na)Y. Two explanations can be proposed for this phenomenon: a lower intrinsic CO adsorption rate on a CuI site in a protonic environment in comparison with a sodic one and the initial occupation (before exposure to CO) by CuI of hidden sites. From IR spectroscopy9,10,13,36-38 and in situ XRD13 studies, it was indeed concluded that a migration of CuI occurs upon CO exposure. It was also claimed that a significant fraction of CuI did not migrate in the presence of Na.9 From CO-TPD experiments,26 we indeed found an accessibility to CO (CO/Cu) of 0.34 and 0.78 on CuINaY and CuIHY, respectively. These observations prompt us to propose that the rate to achieve a full intensity of IR band on CuINaY system is slow because it is kinetically driven by the migration of CuI species from hidden to accessible sites for CO adsorption. We are inclined to favor the second explanation.

16420 J. Phys. Chem. B, Vol. 110, No. 33, 2006

Jardillier et al. These results suggest also that increasing H/Na ratios induce an increase of νCO. Preliminary DRIFT experiments confirm this prediction because νCO normal modes are shifted to higher energy values for higher H/Na ratio. DFT calculations clearly show a change in the active site position when CO adsorbs on copper and also on Na. The position of cocations can be also perturbed. For zeolites containing low Si/Al ratio, calculations showed the possibility for Cu and Na to be both connected to CO: CO can be bonded to Cu via C and can be slightly bonded to Na via O atom. Finally, on the basis of the present calculations, adsorption energy values for CuIY are expected to depend on the chemical composition of the zeolite, typically on the Si/Al and H/Na ratio. These results illustrate also the strong interaction between the zeolite and the cations and support the hypothesis that CuIY catalysts behave as a supermolecule.

Figure 6. Experimental ν(C-O) value in function of time exposure to a flowing 1% CO/He mixture for Cu(25)H(0)NaY (open symbol) and Cu(28)H(51)NaY.

A second interesting observation can also be commented on regarding the shift to higher frequencies of the two IR bands at ca. 2140 and 2155 cm-1 from CuINaY to CuIHY, respectively (Figure 5); this shift is more sensitive on the low-frequency band. This behavior is in agreement with the predictions of the calculations on model clusters (Table 1) upon substitution of Na for H. Now the question can be asked regarding potential and limits of the calculations on model clusters to understand and interpret the experimental IR spectra. There is no doubt that the calculations predict the blue-shift of νCO upon substitution of Na for H (vide supra). On the basis of present preliminary results obtained using DRIFT spectroscopy of adsorbed CO versus time on the two samples Cu(28)H(51)NaY and Cu(25)H(0)NaY, changes in the νCO frequency values support changes of the electronic properties of the material with different H/Na ratios (Figure 6). The conclusion is far from straightforward regarding the assignment of bands at 2140 and 2155 cm-1 to CO adsorbed on Cu at a given site. Indeed, to assign the two bands at 2140 and 2155 cm-1, DFT calculations on large clusters featuring all the possible accessible sites III and II are required. The present study shows that this is not sufficient because the νCO value depends on the zeolite composition. Furthermore, DFT calculations on a small cluster [CuICO, 2Na]5Si,3Al (entry i, Table 1) and a large cluster [CuICO, 11Na]30Si,12Al (entry j, Table 1) show a significant increase of the CdO bond length (from 1.14 to 1.149 Å). This result suggests that a red-shift of the νCO value is expected due to the CO bond length increases for CO connected at the same time to copper and to close sodium. This result highlights that cocations can play an important role upon the position and the electronic properties of the transition metal. Thus, to calculate with a high accuracy the absolute νCO value, the present DFT results indicate that cluster model containing explicitly cocations could be essential in order to model CO adsorption on CuI in zeolites. 4. Conclusion CO is frequently used to probe CuI in zeolite, and in the present work, it has been used to probe changes in the electronic properties of copper-exchanged zeolites using DFT calculations. Calculated νCO stretching mode adsorbed on a metal active site in CuIY, using model clusters, shows that νCO value depends not only on the Si/Al ratio but also on the Na/H ratio. These results confirm the experimental increase of the νCO with higher Si/Al ratio, i.e., with an increasing acidity of the framework.

Acknowledgment. These calculations were carried out on the IBM SP4 computers of the CINES (Centre Informatique National de l’Enseignement Supe´rieur) in Montpellier (France) and of the IDRIS (Institut des Ressources en Informatique Scientifique) in Orsay (France). We gratefully acknowledge conversations with Dr. Francesco Di Renzo. References and Notes (1) Coq, B.; Delahay, G.; Neveu, B.; Peudpiece, J. B.; Kieger, S. Process for the Removal from Gases of Nitrogen Oxides NOx by Selective Catalytic Reduction (SCR) Using Ammonia over Zeolite Catalysts Not Causing the Formation of Nitrogen Protoxide. U.S. Patent, 6,221,324, 2001. (2) Berthomieu, D.; Delahay, G. Catal. ReV.sSci. Eng. 2006, in press. (3) Coq, B.; Delahay, G.; Durand, R.; Berthomieu, D.; AyalaVillagomez, E. J. Phys. Chem. B 2004, 108, 11062. (4) Hadjiivanov, K. I.; Vayssilov, G. N. Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as an IR Probe Molecule. In AdVances in Catalysis; Gates, B. C., Knozinger, H., Eds.; Elsevier; New York, 2002; Vol. 47, p 307. (5) Garrone, E.; Rodriguez Delgado, M.; Otero Arean, C. Trends Inorg. Chem. 2001, 7, 119. (6) Garrone, E.; Otero Arean, C. Chem.l Soc. ReV. 2005, 34, 846. (7) Zhou, M.; Andrews, L.; Bauschlicher, C. W., Jr. Chem. ReV. 2001, 101, 1931. (8) Kieger, S.; Delahay, G.; Coq, B. Appl. Catal., B 2000, 25, 1. (9) Borovkov, V. Y.; Karge, H. G. J. Chem. Soc., Faraday Trans. 1995, 91, 2035. (10) Borovkov, V. Y.; Jiang, M.; Fu, Y. J. Phys. Chem. B 1999, 103, 5010. (11) Rakic, V. M.; Hercigonja, R. V.; Dondur, V. T. Microporous Mesoporous Mater. 1999, 27, 27. (12) Datka, J.; Kozyra, P. J. Mol. Struct. 2005, 744-747, 991. (13) Palomino Turnes, G.; Bordiga, S.; Zecchina, A.; Marra, G. L.; Lamberti, C. J. Phys. Chem. B 2000, 104, 8641. (14) Bolis, V.; Barbaglia, A.; Bordiga, S.; Lamberti, C.; Zecchina, A. J. Phys. Chem. B 2004, 108, 9970. (15) Bludsky, O.; Silhan, M.; Nachtigall, P.; Bucko, T.; Benco, L.; Hafner, J. J. Phys. Chem. B 2005, 109, 9631. (16) Berthomieu, D.; Goursot, A.; Ducere, J.-M.; Delahay, G.; Coq, B.; Martinez, A. Stud. Surf. Sci. Catal. 2001, 135, 2618. (17) Berthomieu, D.; Krishnamurty, S.; Coq, B.; Delahay, G.; Goursot, A. J. Phys. Chem. B 2001, 105, 1149. (18) Goursot, A.; Coq, B.; Fajula, F.; Berthomieu, D.; Delahay, G. J. Catal. 2003, 216, 324. (19) Cerius2: Molecular Modeling Software for Materials Research; Biosym Technologies/Accelerys: San Diego, CA, 1993. (20) Koester, A. M.; Flores, R.; Geudtner, G.; Goursot, A.; Heine, T.; Patchkovskii, S.; Reveles, J. U.; Vela, A.; Salahub, D. Program deMon, version 1.1.0; NRC: Ottawa, Canada, 2003. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.

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