XRD, XAS, and IR Characterization of Copper ... - ACS Publications

J. Phys. Chem. B 2000, 104, 8641-8651

8641

XRD, XAS, and IR Characterization of Copper-Exchanged Y Zeolite G. Turnes Palomino,*,† S. Bordiga, and A. Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniVersita` di Torino, I-10125 Via P. Giuria 7, Torino, Italy

G. L. Marra ENICHEM S. p. A., Centro Ricerche NoVara-“Istituto Guido Donegani”, Via G. Fauser 4, I-28100 NoVara, Italy

C. Lamberti* Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniVersita` di Torino, I-10125 Via P. Giuria 7, Torino, Italy, and INFM Sezione di Torino ReceiVed: February 14, 2000; In Final Form: June 19, 2000

Siting of copper ions in CuI-Y zeolite, prepared by gas-phase exchange of NH4-Y with CuCl, has been investigated employing XRPD, XAS and IR spectroscopies. An X-ray powder diffraction study of the zeolite in a vacuum shows that 23.4(2) cuprous ions are located at site I*, 6.1(3) at site II, and 11.5(3) at site II* (sites I* and II* are at the center of the plane of the six-membered ring connecting the hexagonal prism with the sodalite and the sodalite with the supercage, respectively). Addition of CO induces a relevant migration of copper ions from sites II and II* to a more exposed type II. EXAFS analysis shows that CuI ions in the outgassed zeolite are surrounded by 2.8(3) oxygen atoms of the zeolite framework, the average CuI-O distance being 1.99(2) Å. Both X-ray measurements and FTIR spectroscopy show that CO is adsorbed on the zeolite at room temperature with formation of carbonyl adducts. At liquid-nitrogen temperature and low CO pressure, two types of monocarbonyl species are observed, corresponding to CO adsorbed on copper ions located at sites II and II*. On increase of the CO pressure and subsequent formation of polycarbonylic species, cations at site II* move to the more exposed position II, and a single kind of tricarbonyl adducts is observed. IR spectroscopy also provides evidence for the interaction of NO with copper ions located at sites II and II*, which are the first sites able to adsorb up to two molecules of NO, whereas cations at site II*, because of their lower coordinative unsaturation, can only form CuI(NO) adducts. NO proves to be a sensitive probe not only for cuprous but also for cupric ions.

1. Introduction The production of undesired NO gas as a byproduct of chemical processes in several industrial factories (power plants) and civil utilities (internal combustion engines) represents a significant problem for modern societies. In the last two decades copper-exchanged zeolites have attracted great interest as catalysts for the direct conversion of NO into N2 and O21-9 and for the selective reduction of NO with ammonia and hydrocarbons.10-17 The early reports by Iwamoto and co-workers1-4 on the high activity of copper-exchanged ZSM-5 zeolites in the catalytic decomposition of nitrogen oxides prompted further research on this and analogous materials. Basic studies were aimed at elucidating the nature of the copper sites and at correlating structural data with redox chemistry18-25 and catalytic activity.5,8,9,19,26,27 However, despite the large amount of research being done in many laboratories, the NOx decomposition path is not yet fully understood. In fact, several reaction mechanisms have been hypothesized,5-9,28 but none of them satisfactorily takes into account the high activity of Cu-ZSM-5 and Cu* To whom correspondence should be addressed. † On leave from Departamento de Quı´mica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain.

mordenite compared to Cu-Y, where the copper concentration is about 1 order of magnitude larger. This means that the knowledge of the exact location of CuI and/or CuII cations in different zeolitic frameworks and of their local environment in a vacuum and in the presence of adsorbates is likely a key point in understanding the reaction mechanism. For this reason, we have performed a powder XRD and spectroscopic (XANES, EXAFS, and FTIR) characterization of a nearly fully Cuexchanged faujasite (Cu-Y) obtained by reacting NH4-Y with gas-phase CuCl. The results obtained by IR spectroscopy (using CO and NO as probes) were also compared for partially oxidized samples containing both CuI and CuII. Parallel studies of CuZSM-5 and Cu-mordenite have been reported recently.9,29,30 2. Experimental Section CuI-Y (Si/Al ) 2.7) was prepared by reacting NH4-Y (supplied by EniChem laboratories) with gaseous CuCl at 673 K (as detailed in ref 9). Two facts motivated the choice of this exchange procedure: (i) it allows a very high exchange level, and (ii) if care is taken to avoid contact with air, CuI is the only species introduced. The extent of ion exchange (∼90%) was approximately evaluated by comparison of the intensity of the O-H stretching band of bridged Si(OH)Al groups before

10.1021/jp000584r CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000

8642 J. Phys. Chem. B, Vol. 104, No. 36, 2000 and after the exchange (results not shown for the sake of brevity). Powder diffraction patterns were collected at 80 K using an Oxford Instruments cryostream on the powder diffraction beam line BM1631 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The beam line was used with a collimating mirror before the Si(111) water-cooled, doublecrystal monochromator, set to deliver a wavelength of λ ) 0.629 92(1) Å, calibrated using the NIST Si standard 640b. The optional focusing mirror after the monochromator was not used. The sample capillary was spun on the axis of the diffractometer, while the detector bank was scanned from 2θ ) 2 to 2θ ) 60° at a rate of 0.5°/min at low angle and at a rate of 0.1°/min at high angle. Data were collected in a continuous scanning mode, with the electronic scalers and the 2θ encoder reading around 6 times per second. High-angle regions were scanned more than once to improve the statistical quality of the pattern. The total data acquisition lasted ca. 8 h for each sample. The detector bank consists of nine scintillation counters, each behind a separate Ge(111) analyzer crystal, with the nine crystals mounted on a single rotation stage. The separation between each channel is close to 2°. Consequently, nine diffraction patterns, offset from each other by around 2°, are measured simultaneously. The 2θ angle of the central channel is taken as the nominal 2θ value of the diffractometer arm. Following data collection, the counts from the nine channels collected at the various positions during the scans are analyzed, taking account of the exact separation between the channels, the different detector efficiencies, and the decrease in the beam current during the scan, to produce the equivalent normalized step scan, which is more suitable for analysis by standard programs. Rejection of the harmonics λ/3, λ/4, etc. transmitted by the monochromator and the analyzer crystals is achieved by setting the electronic windows on the detector electronics so that they accept only counts from the fundamental λ. For powder XRD measurements, zeolite powder, activated under dynamic vacuum at 873 K for 2h, was transferred (in vacuo) into a quartz capillary with a 1-mm diameter. The capillary was sealed and then mounted on the sample spinner on the axis of the diffractometer, which maximizes the number of crystallite orientations presented to the incoming radiation and minimizes the effect of any preferred orientation in the sample. For the experiment concerning the effect of CO adsorption, 40 Torr of CO were dosed before the capillary was sealed. X-ray absorption measurements were carried out using synchrotron radiation of the BM8 GILDA32 beamline at ESRF (Grenoble, France), which was equipped with an Si(331) channel-cut monochromator. The Cu K-edge XANES and EXAFS spectra were recorded in the transmision mode, using N2- and Ar-filled ion chambers for measuring the incident (I0) and the transmitted (I1) beams, respectively. The photon energy was calibrated from the spectrum of a copper foil, assigning 8979 eV to the pre-edge peak. Spectra were recorded using sampling steps of 0.2 eV for XANES and a variable sample step, giving ∆kmax ) 0.05 Å-1, for EXAFS. An integration time of 2.0 s per point was used in both cases. The energy resolution was of ca. 0.5 eV. XAFS measurements at RT were performed by using a metal cell specifically designed to allow measurements under controlled atmosphere. The Cu-Y sample was activated 2 h at 873 K under dynamic vacuum and then put into contact with carbon monoxide at room temperature. For both types of experiment (CuI-Y in vacuo and after interaction with CO) three EXAFS spectra were recorded under the same experimental conditions.

Palomino et al. Extracted χ(k) were averaged before the EXAFS data analysis. The standard deviation calculated from the averaged spectra was used as an estimate of the statistical noise for the evaluation of the error associated with each structural parameter. The experimental χ(k) function was extracted from absorption data as described in detail in ref 33. The main contributions to the Fourier transform modulus were filtered in order to obtain nearest neighbor Cu shell. These filtered contributions were analyzed using programs developed by Michalowicz,34 following standard procedures.35 For IR measurements two experimental procedures (I and II) were followed. In the first, a thin self-supporting wafer of the zeolite was prepared and activated in situ (in a specially designed IR cell as already described in ref 9) under dynamic vacuum at 873 K for 2 h without exposure to air, thereby obtaining a Cu-Y containing exclusively copper(I) ions. This procedure is the same followed for XRPD and XAFS measurements. In the second procedure, the nearly fully exchanged CuI-Y zeolite powder, prepared in a separate vessel following procedure I, was exposed to air, compressed in the form of pellets suitable for IR experiments, and reactivated under dynamic vacuum at 873 K for 2 h inside the IR cell. The IR cell used in the two experiments allowed in situ high-temperature treatments, gas dosage, and low-temperature measurements. The exposure to air is sufficient to induce a partial oxidation of CuI, so the samples prepared following procedure II contain, after reactivation at 873 K, a mixture of CuI and CuII ions. Spectra were recorded at 2 cm-1 resolution, on a Bruker IFS 66 FTIR spectrometer. Although the IR cell was permanently cooled with liquid nitrogen, the actual sample temperature (under the IR beam) was likely to be ca. 100-110 K. 3. Results and Discussion 3.1. Faujasite Structure and Cationic Distribution. The tridimensional structure of zeolite Y is generated by connecting sodalite units with hexagonal prisms to give a framework characterized by large, empty cavities (supercages) with a diameter of about 13 Å. It is recognized36 that cations are mainly located in a few well-defined sites named, following the nomenclature of Smith,37 I, I′, II, II′ and III. Site I is located at the center of the hexagonal prisms. Cations located at this site have octahedral coordination and are almost completely inaccessible to guest molecules. Because of their high coordination (six oxygen ligands), these sites are usually highly populated. Site I′ is located above the base of the prism inside the sodalite cage. These cations are three-coordinated (to three oxygen atoms of the base of the prism) and are accessible only to molecules able to penetrate through a six-membered ring from the supercages into the sodalite cavities. This penetration is not possible for molecules such as CO or N238 but occurs for H2.39 Because of Coulombic repulsion, the simultaneous occupation of adjacent I and I′ sites is forbidden. Sites II′ and II are in the center of the six-membered rings that connect the sodalite cage with the supercage, just inside the sodalite cage and the supercage, respectively. Cations located in these sites are also coordinated to three oxygens of the ring. Also in this case the simultaneous occupation of adjacent sites, II and II′ is forbidden. Finally, site III is located in the supercage near the four membered-rings. Only sites II and site III, are accessible to CO and NO molecules; note, however, that site III is occupied only at low Si/Al ratios (X zeolites). XRD has been widely used to determine the locations and occupancies of extraframework ions in the different exchanged forms of faujasites.36 To the best of our knowledge, until now,

XRD, XAS, and IR Characterization of Copper-Exchanged Y Zeolite

J. Phys. Chem. B, Vol. 104, No. 36, 2000 8643

TABLE 1: Atomic Parameters Resulting from the Rietveld Refinement of the Zeolite at 80 K in Vacuoa atom

x

y

z

Uiso (Å)

occupancy actor

site

number of atoms per unit cell

Si Al O1 O2 O3 O4 Cu (II) Cu (I*) Cu (II*)

0.1238(1) 0.1238(1) 0.1038(2) 1.0010(3) 0.1768(2) 0.1732(3) 0.2360(8) 0.0377(1) 0.2171(4)

0.9474(1) 0.9474(1) -x x x x x x x

0.0356(1) 0.0356(1) 0 0.1484(3) 0.9743(3) 0.3218(4) x x x

0.013(1) 0.013(1) 0.014(3) 0.055(5) 0.029(4) 0.024(4) 0.019(2) 0.019(2) 0.019(2)

0.73 0.27 1.00 1.00 1.00 1.00 0.19(1) 0.732(6) 0.36(1)

192i 192i 96h 96g 96g 96g 32e 32e 32e

140.2 51.8 96 96 96 96 6.1(3) 23.4(2) 11.5(3)

a Values of Rwp and Rp parameters are 9.11% and 6.95%, respectively, and reduced χ2 ) 1.066 for 47 variables. The refined cell parameter is a ) 24.7373(6) Å [V ) 15 138(1) Å3]. The total number of CuI cations located in the unit cell is 41.0(5).

no XRD studies on a fully CuI-exchanged Y zeolite have been published. Gallezot et al. published, in 1972,40 a powder X-ray analysis of partially exchanged copper(II)-sodium and copper(II)-hydrogen Y zeolites. In both cases cupric ions occupy sites I and I′ with higher affinity for site I′. In a later work (1975), Maxwell and de Boer41 attempted to obtain more detailed structural information on Cu(II)-exchanged faujasites by performing single-crystal studies on the hydrated and dehydrated forms of this zeolite. They found that, in the dehydrated form, copper ions were located at sites I, I′, II, II′ and III, with the following cation distribution: 1.5(2) [I], 14.2(2) [I′], 5.3(2) [II], and 0.8(2) [II′]. Although their X-ray study cannot be directly compared with the powder diffraction studies of Gallezot et al.,40 as the copper-exchange levels and dehydration temperatures are different, it is worth noting that the distributions of copper ions over sites I and I′ were very similar. Also in this case, copper(II) cations showed a strong preference for site I′. These authors also pointed out the existence of two kinds of sites I′ and II, named I′A, I′B, IIA and IIB, respectively. Sites I′A and IIA are very close to the plane of the hexagonal windows connecting the prisms with sodalite cages and the sodalite cages with the supercages, respectively, which is to say, positions very similar to those found in this contribution for CuI-Y and called by us I* and II* (vide infra). Sites I′B and IIB correspond to positions obtained by displacements of cations from sites I′A and IIA into the sodalite cage and into the supercage, respectively. According to these results, Cid et al.42 published in 1980 a work about a copper(II)-sodium-hydrogen faujasite where copper(II) ions occupied mainly sites II and a second kind of sites located in the plane of the hexagonal window between the sodalite cage and the supercage, site II* according with our nomenclature (vide infra). 3.2. Powder XRD Structure Refinement. The Rietveld refinement43 was performed using the software package GSAS (Generalized Structure Analysis System).44 The refinement of the framework atoms in space group Fd3hm was done by imposing restraints on the T(Si/Al)-O and O-O distances. The technical details and the strategy adopted for the structure refinement are the same of those described in ref 45. Na-Y was used as the starting structure model.46 The bare framework (without cations) was refined during the first iteration cycles. The starting coordinates of the extraframework cations were obtained by examination of peaks found in the difference Fourier map. In the final refinement, we succeeded in locating 41.0(5) of the 51.8 extraframework ions present in the unit cell (calculated from the Si/Al ratio). This result is quite satisfactory considering that a significant number of the remaining ions to be located are H+ ions (about 5 per unit cell), which are still present after the exchange procedure adopted in this investigation (see Experimental Section) and which are undetectable from

Figure 1. Observed, calculated, and difference profiles and reflection positions in low and high 2θ regions (parts a and b, respectively) of the CuI-Y zeolite in a vacuum.

our XRPD data. The results of the final refinement of the sample in vacuo are shown in Table 1. Figure 1 shows the observed and calculated patterns, their difference, and the positions of the diffraction peaks of the sample outgassed at 873 K. The quality of the Rietveld refinements is confirmed by the low Rwp and Rp parameters (9.11% and 6.95% respectively). The refined cell parameter is a ) 24.737 3(6) Å [V ) 15 138(1) Å3]. Our analysis locates 23.4(2) copper ions in site I*, 6.1(3) in site II, and 11.5(3) in site II*. By sites I* and II*, we mean two sites, not included in the conventional site nomenclature,36,37 located at the center of the plane of the six-membered rings connecting hexagonal prisms with sodalite cages and sodalite cages with supercages, respectively (positions marked in Figure 2). Note that CuI* and CuII* are in the plane of an equilateral triangle formed by three framework oxygens, while CuII forms a pyramid with a triangular basis of oxygens.

8644 J. Phys. Chem. B, Vol. 104, No. 36, 2000

Figure 2. Representation of the locations of the cations for CuI-Y in vacuo and their migration upon adsorption of CO. White sphere represents site I*, black sphere represents site II*, and gray spheres represent site II.

From these results, we can infer that, unlike alkali cations, cuprous ions exhibit a great affinity toward the trigonal planar symmetry (sites I* and II*). This kind of coordination has also been observed in CuII-Y40-42 (see previous section) and in other transition-metal-exchanged zeolites such as Co(II)-, Ni(II)-, Zn(II)-, and Mn(II)-exchanged Y and A zeolites.47-52 However, in contrast with copper cations, these divalent transition metal ions show greater affinity for site I, in agreement with the fact that the more highly charged ions occupy the more shielded positions (highest coordination numbers). For instance, in a dehydrated Mn(II)-exchanged type Y zeolite, Mn(II) ions are located in the hexagonal oxygen window between the sodalite and the supercage only if site I (the center of the hexagonal prisms) is occupied by other cations.51 Also in nearly fully exchanged dehydrated nickel(II) faujasite,52 Ni2+ cations are preferentially located inside the hexagonal prisms at site I. In conclusion, in most of dehydrated transition-metal-exchanged Y zeolites, site I is the preferred site, because, in this position, the ion can acquire the highest coordination number. As mentioned in the previous section, copper(II) prefers site I′ even if it is also occupying site I. This difference in site preference of copper(II) cations compared to other divalent transition metal ions was explained by Gallezot et al.40 and by Maxwell and de Boer41 as being due to the preference of Cu(II) cations toward sites of specific symmetry (rather than toward sites with highest coordination number). As we have seen before, in CuI-Y zeolite, site I is not occupied at all. This is not unexpected, as the stereochemistry of mononuclear complexes of copper(I) is dominated by low coordinations. In fact, if four coordination (tetrahedral geometry) predominates, then 2-fold-coordinated (linear) and 3-fold-coordinated (trigonal planar) complexes will also provide a significant contribution to its stereochemistry.53-55 Another factor that can justify why no significant charge density has been detected in site I is that, because some of the H+ ions have not been exchanged, site I could be preferentially occupied by them. The conditions adopted for powder XRD measurements on the zeolite/CO system (see Experimental Section) were chosen in order to ensure the nearly exclusive presence of well-defined Cu+(CO)3 adducts formed on Cu(I) located at the accessible sites of the supercage. Figure 3 shows the observed and calculated patterns, their difference, and the positions of the diffraction peaks of the zeolite/CO system (Rwp ) 10.91 and

Palomino et al. Rp ) 8.22). The results of the final refinement are summarized in Table 2. The major result deriving from this experiment is that the adsorption of CO at 77 K deeply modifies the entire cationic distribution, with the new distribution being as follows: 14.8(2) ions at site I*, 4.2(2) at site I′, and 22.4(3) at site II. Notice that the total number of copper cations located in the unit cell, 41.4(4) is in good agreement with the number obtained on the sample not in contact with CO, 41.0(5). It is also worth emphasizing that, in the presence of a high CO pressure, site II* is not occupied at all, while the occupancy of site II increases considerably and that of site I* decreases. All of these facts indicate that adsorption of CO is accompanied by cation migration (see Figure 2). This result is not unexpected as many authors have already reported that the adsorption of various molecules on different faujasite-type zeolites has a similar effect. For instance, it is now well-established that the dehydration of faujasites (corresponding to loss of H2O ligands) is accompanied by a migration of the cations from the large cavities to more shielded positions.56-59 A similar migration, but in the opposite direction, usually occurs when molecules (like water, ammonia, pyridine, butadiene, etc.) are adsorbed on the dehydrated zeolite.40,60-65 For the CO/CuY, there is spectroscopic evidence that the adsorption of CO on CuI-Y zeolite is accompanied by a gradual migration of Cu+ cations from positions not exposed to the supercages (sites I, I′, and II′) toward sites II.66-70 These spectroscopic data have now been confirmed by our powder XRD results, which also provide a quantification of the cation displacement upon interaction with CO. The more exposed CuI ions located at site II exhibits a migration of 0.167(2) Å toward the center of the supercage, while the more shielded ones, at site II*, undergo a much more relevant displacement of 0.975(1) Å. It has been recently reported, from EXAFS data, that the interaction of CO with CuI cations in ZSM-5 moves CuI cations 0.16(4) Å away from their original positions upon formation at RT of the CuI(CO)2 adduct.71 With cuprous cations in ZSM-5 in a rather low coordination state, the value obtained by EXAFS on that system should be compared with the movement of copper ions located at site II in Cu(I)-Y zeolite. A later EXAFS study,72 performed at liquid-nitrogen temperature, has provided evidence that the EXAFS signal of the CuI(CO)3 adducts formed in ZSM-5 can be simulated without consideration of the contribution of the framework oxygens. This indicates that, at low temperature, when tricarbonyl complexes are formed, the displacement of cuprous ions from the zeolite walls is even larger. The present XRD data are thus in full agreement with the previous EXAFS findings. 3.3 XAS Spectroscopy. An alternative way to obtain information about the coordination state of counterions in zeolites is provided by XAS spectroscopy. Because of its atomic selectivity and sensitivity to both the local environment and the oxidation state of the selected atomic species, several groups have used EXAFS and XANES techniques to characterize the oxidation and aggregation states of the copper species present in zeolites, their coordinative state, and the effect of adsorbing molecules. See for example refs 24, 25, 29, 73, and 74 (and references therein) for Cu-ZSM-5; refs 30, 73, 75, and 80 for Cu-mordenite; and refs 33, 73, and 76 for Cu-Y. Figure 4 shows the XANES region of the X-ray absorption spectra of CuI-Y in a vacuum and after interaction with CO; the corresponding spectra of the reference compounds Cu2O (CuI) and CuO (CuII) are also reported for comparison. In the spectrum (obtained in vacuo) of the sample activated at 873 K, we can distinguish a sharp and intense peak at 8983 keV and a broad absorption around 8993 keV. Both bands are ascribed77,78



TABLE 2: Atomic Parameters Resulting from the Rietveld Refinement of the Zeolite at 80 K in CO Atmospherea atom

x

y

z

Uiso (Å)

occupancy actor

site

number of atoms per unit cell

Si Al O1 O2 O3 O4 Cu (II) Cu (I*) Cu (I′)

0.1255(1) 0.1255(1) 0.1020(2) -0.0007(2) 0.1746(2) 0.1761(2) 0.2399(2) 0.0433(2) 0.0791(7)

-0.0519(1) -0.0519(1) -x x x x x x x

0.0357(1) 0.0357(1) 0 0.1452(2) -0.0321(3) 0.3241(2) x x x

0.018(1) 0.018(1) .012(1) 0.025(2) 0.032(2) 0.035(2) 0.063(2) 0.063(2) 0.063(2)

0.73 0.27 1.00 1.00 1.00 1.00 0.699(8) 0.463(7) 0.130(6)

192i 192i 96h 96g 96g 96g 32e 32e 32e

140.2 51.8 96 96 96 96 22.4(3) 14.8(2) 4.2(2)

a Values of Rwp and Rp parameters are 10.91% and 8.22%, respectively, and reduced χ2 ) 1.1827 for 47 variables. The refined cell parameter is a ) 24.642(1) Å [V ) 14 962(2) Å3]. The total number of CuI cations located in the unit cell is 41.4(4).

Figure 4. Normalized XANES spectra of CuI-Y in a vacuum and CuI-Y after interaction with CO, Cu2O, and CuO.

Figure 3. As in Figure 1, after adsorption of CO.

to the 1s f 4p transition of CuI ion, thus indicating that copper ions are present as Cu(I). The lack of the 1s f 3d transition (at 8978 keV), typical of copper(II),79 also supports this interpretation, although the absence of the absorption at 8978 keV does not exclude the presence of a minor fraction of CuII ions (because its characteristic peak at 8978 keV is intrinsically weak and can be overshadowed by the intense 8983 keV signal). UVvis spectra (not shown for the sake of brevity) of the Cu-Y sample prepared following procedure I did not show any relevant absorption band in the 500-1250 nm region (where d f d transitions of CuII would appear),29,33 thus showing that copper ions are present mainly as CuI. Figure 4 also shows the effect of the CO adsorption on the XANES spectrum of CuI-Y. Note that, by dosing CO, (i) the intensity of the band at 8983 keV sharply decreases and shifts toward lower energy, (ii) a new band grows up at 8981 keV, and (iii) a significant variation in the near-edge structure occurs.

These observations indicate that, at room temperature, CO interacts with CuI ions with modification of the coordination state of the metal ion. Similar results has also been obtained dosing CO on CuI-ZSM-5,29,74 and on CuI-MOR.30,80 Figure 5 summarizes the results obtained from the EXAFS data analysis on both systems (Cu-Y activated in a vacuum and in interaction with CO). The k3-weighted Fourier transform (FT) in the range 2.9-13.5 Å-1 on the χ(k) functions of CuI-Y in a vacuum (solid-line curve) is shown, without any phase correction, in Figure 5b. The main peak at ∼1.6 Å represents the first coordination shell, which is typical of Cu-O distances. This contribution was filtered in the 1.06-2.06 Å range and then fit to a model of Cu(I) ions coordinated to N oxygen atoms using phases and amplitudes extracted from the Cu2O reference spectrum. The numerical values thus obtained for the coordination number of Cu(I) ions (N) and for the Cu-O bond length (R) were N ) 2.8(3) and R(Cu-O) ) 1.99(2) Å. The relative Debye-Waller factor was σ ) 0.092(9) Å, taking σ ) 0.068 Å for Cu2O. These values are, within the experimental errors, the same as were obtained in two previous experiment, both published in 1997, on CuI-Y samples prepared in similar way by us33 [N ) 2.8(3) and R(Cu-O) ) 1.97(2) Å] and by Fo¨rster and Hatje81 [N ) 2.9 and R(Cu-O) ) 1.99 Å]. Note that a


Figure 5. (a) From top to bottom: kχ(k) function for the experimental CuI-Y zeolite in a vacuum (solid line) and after interaction with CO (dotted line), and the first-shell filtered signal and corresponding best fit. (b) k3-weighted (phase-uncorrected) FT functions of CuI-Y zeolite in a vacuum and after interaction with CO (solid and dotted line, respectively. (c) Comparison between experimental and fitted first-shell contribution observed in r space (modulus and imaginary parts).

coordination number of 2.8 is in complete agreement with the cation distribution found with powder XRD: in all three sites (II, I* and II*), the copper ions are 3-fold-coordinated. On the contrary, the average distance obtained by EXAFS [1.99(2) Å] seems to be rather underestimated with respect to the Cu-O distances obtained by powder XRD: 2.032(5), 2.002(5), and 2.21(1) Å for cuprous cations located in sites I*, II*, and II, respectively. In fact, by weighting the three Cu-O distances with the corresponding number of atoms per unit cell, an average Cu-O distance of 2.05 Å is obtained by XRPD, which is definitively larger than 1.99(2). We will now show that this discrepancy is only apparent. To fulfill this task, we computed a simulated EXAFS spectrum of the first coordination shell around copper ions located in sites I*, II*, and II using the local Cu-O distance obtained by XRPD as input. Corresponding ∆E and σ values have been arbitrarily fixed as 0 eV and 0.08 Å, respectively, while the corresponding coordination numbers have been defined as N ) 3 weighted with the relative fractional abundances of copper, which are 0.571, 0.281, and 0.149 for sites I*, II*, and II, respectively (again taken from XRPD; Table 1). The simulated contributions of the three sites are compared in Figure 6 (first three spectra from top). The very small difference in the Cu-O distance observed in sites I* and II* (2.03 vs 2.00 Å) implies that the corresponding two simulated EXAFS signals will be constructively added in nearly all of the 2.9-13.5 Å-1 k interval (where the experimental data have been fit). The same does not hold for the simulated EXAFS signal of Cu ions located in site II. In fact, a relevantly higher Cu-O distance causes an important phase shift with respect to the signals coming from Cu hosted in sites I* and II*, resulting in an incoherent contribution to the overall signal. This view clearly emerges from the results of a one-shell fit on the sum

Palomino et al.

Figure 6. Simulated EXAFS spectra of the contribution to the overall signal of copper cations located in, from top to bottom, site I*, II*, and II. In the simulation, R and N values have been obtained from the XRPD data reported in Table 1, while, for the three sites, ∆E and σ values have been arbitrarily fixed as 0 eV and 0.08 Å, respectively. The sum of the three simulated spectra was than fit using a single shell of oxygen scatterers. The quality of the fit can be appreciated by comparing the two overlapped bottom curves, while the results are reported in Table 3.

TABLE 3: Cu-O Bond Distance (R), Coordination Number (N), Relative Debye-Waller Factor (σ), and Energy Shifts (∆E) (i) Obtained from the One-Shell Fit of the Experimental Data, (ii) Imposed to Create the Simulated EXAFS Signal for Cu Located in the Three Different Crystallographic Sites Observed by XRPD, and (iii) Obtained from a One-Shell Fit of the Sum of the Three Simulated Contributionsa R (Å)

N

σ (Å)

One-Shell Fit on the Experimental Data average exp. values 1.99(2) 2.8(3) 0.092(9)

site I* site II* site II

Simulations of the Single Contributions from Cu in the Different Crystallographic Sites 2.00 1.712 0.080 2.03 0.842 0.080 2.21 0.446 0.080

∆E (eV) +0.8(9)

0.0 0.0 0.0

One-Shell Fit on the Sum of the Simulated Spectra average th. values 2.02 2.743 0.092 -0.7 a Note that the last four rows of Table 3 refers to simulations and to the fit of a simulated EXAFS spectrum. This is the reason coordination numbers up to the third decimal have been reported. The actual ability of EXAFS in determining N from an experimental spectrum is much lower, typically not better than (10% (see esd given in the first row).

of the three simulated spectra: N ) 2.743, R ) 2.02 Å, and σ ) 0.092 Å (see bottom curves in Figure 6 and Table 3). This clearly means that the contribution to the overall EXAFS signal of the small fraction of Cu ions hosted in site II has no influence on the average Cu-O distance and only results in an increment of the fitted Debye-Waller factor of static origin and in a systematic underestimation of the actual average coordination number N. On the basis of these considerations, it is now evident that XRPD and EXAFS give, within the experimental errors, comparable values for the average Cu-O distance. In comparison with the spectrum obtained in vacuo, the adsorption of CO (Figure 5, dotted curve) brings about significant


Figure 7. IR spectra at nominally liquid-nitrogen temperature of CO adsorbed on CuI-Y prepared following procedure I. The successive formation of mono-, di-, and tricarbonyl adducts is indicated with different colors (white, light gray, and black, respectively). Equilibrium pressures in the 0.1-10 Torr range (1 Torr ≈ 133.3 Pa).

changes, especially on the signal intensity and on the radial distribution. We have not tried a detailed analysis of the EXAFS signal after CO adsorption because the dotted curve in Figure 5 is the averaged sum of the contribution of copper(I) ions not interacting with CO (sites located in the sodalite) and of the CuI(CO)2 adducts located in the supercages (vide infra). In fact, the simultaneous presence of different copper species gives a superposition of EXAFS signals of different amplitudes, periods, phase shifts, and thermal disorder whose analysis is not significant or could even be misleading. The same problem was found for CuI-MOR.30 Notice how the interaction with CO at room temperature is associated with a consistent reduction of the first-shell peak in the pseudo-radial distribution around copper (Figure 5b). This fact has already been explained in terms of a consistent destructive interference between the contribution of the oxygen atoms of the framework and the contribution of the carbon atom of the CO molecule.71 3.4. IR Spectroscopy. To obtain more detailed information on the structural and electronic properties of the copper(I) ions in Y zeolite, the IR characterization of the sample prepared following procedure I (see Experimental Section) and containing mainly CuI was performed, using CO and NO as probe molecules. Analogous experiments were done on samples prepared following procedure II, containing both CuI and CuII ions, with the aim of learning more about the spectroscopy of CuII. CO Adsorption. CO is one of the most-used probe molecules for testing surfaces, and it usually provides information about the oxidation and coordination states of the ions. In this context, numerous infrared studies have been devoted to CO adsorption on copper-exchanged zeolite catalysts.9,18,66-69,75,80-85 Figure 7 shows the IR spectra in the C-O stretching region of carbon monoxide adsorbed, at liquid-nitrogen temperature,


on the CuI-Y sample prepared following procedure I. The IR spectra of CO adsorbed on the system prepared following procedure II are very similar (and hence are not reported for sake of brevity). This result is not unexpected because the affinity of CuII toward CO is negligible, and hence, no CuII(CO) complexes can be observed on this partially oxidized sample. At the lowest CO dosage, the spectrum shows two bands at 2159 and 2143 cm-1, which, according to data reported in the literature,68,69,82,85 are assigned to two CuI monocarbonyl species located in two different extraframework sites of the zeolite. This result completely agrees with the previously discussed XRPD data, which showed the existence of two positions accessible to CO molecules, namely, sites II and II*. Bands at nearly the same frequencies were reported by our group for CO adsorbed on CuI-ZSM-529 (2157 cm-1), on CuIMOR30 (2159 cm-1), and on CuI-β (2157 cm-1)86 zeolites and assigned, in all cases, to monocarbonyl CuI‚‚‚CO complexes formed between CO and copper cations placed in the main channels of the zeolites. As the coordinative unsaturation of cations located at sites II is similar to that of cations placed in the main channels of both ZSM-5 and mordenite, we assign the band at higher frequency (2159 cm-1) to the C-O stretching vibration of carbon monoxide interacting with CuI ions located at this site. The absorption at 2143 cm-1 has been consequently assigned to CuI‚‚‚CO adducts formed between CO and copper(I) cations placed at site II* (which are more tightly bonded to the framework and in a less-exposed position). Both bands are also observed in spectra taken at room temperature and do not disappear even after outgassing at 298 K: this indicates that, regardless of the original position of exposed centers, the interaction of CO with cuprous ions is always quite strong. Whereas, at the lowest CO equilibrium pressure (Figure 7), the two bands have similar intensity, upon an increase in the CO pressure, the intensity of the absorption assigned to CO adsorbed on site II* preferentially gains intensity. Notice that the last white spectrum in Figure 7, corresponding to the formation of Cu+(CO) adducts at all accessible sites of the supercage, exhibits a low-frequency band that has an intensity approximately twice that of the component at high frequency, in fair agreement with the 1.9 Cu(site II*)/Cu(site II) ratio obtained from powder XRD measurements. To explain both the vibrational properties and the stability of the Cu+(CO) complexes, it is necessary to recall that the interaction of transition metal ions with CO is dominated by chemical effects: σ donation (from the CO 5σ orbital) and d-π back-donation. The withdrawal of electronic density from the CO 5σ orbital (which has weak antibonding character) leads to the strengthening of the C-O bond and to a shift in the frequency of the C-O stretching vibration to higher wavenumber. On the other hand, the d-π back-donation increases in the density on the CO 2π antibonding orbital should be associated with a decrease in the strength of the C-O bond and a bathochromic shift in the υC-O. Both contributions strengthen the bond between the metal ion and the carbon atom. In our case, the high stability of the copper monocarbonyl formed, together with the small shifts in υC-O (relative to that of free gas CO molecules), indicate that the actual frequency of the stretching vibration of the CO molecules coordinated to the CuI cations is determined by the balance between the two contributions: σ donation and π back-donation. Several authors have reported that the Cu-CO bond has an important σ component,87 although there is also both experimental and theoretical evidence of π back-donation.88,89


Palomino et al.

SCHEME 1

With increasing CO equilibrium pressure, new IR absorption bands (grey spectra in Figure 7) grow at 2178 (with a shoulder at 2168 cm-1) and at 2148 cm-1 (with shoulder at ca. 2135 cm-1). These facts strongly suggest that copper monocarbonyl complexes add a second ligand to yield CuI(CO)2 adducts. We assign the bands at 2178 and 2148 cm-1, and the corresponding shoulders at 2168 and 2135 cm-1, to the symmetric and asymmetric stretching modes of the CuI(CO)2 adducts formed on sites II and II*, respectively. Finally, the formation of CuI(CO)3 is evident in the spectra at highest CO equilibrium pressures (black spectra in Figure 7). These spectra show three main IR absorption bands at 2188, 2165, and 2138 cm-1, which correspond to the three C-O stretching frequencies of CuI(CO)3 adducts. The fact that the CuI(CO)3 complexes are associated with three different υC-O values implies that the CO ligands are not fully equivalent (otherwise, only two bands should be present). This assignment suggests a symmetry of the complex rather lower than C3V. It is, however, worth noting that the assignment of the band at 2138 cm-1 to a third, low-frequency, component of the CuI(CO)3 complex has recently been questioned on the basis of a combined low-temperature EXAFS/XANES study72 on CuI(CO)3 complexes formed inside CuI-ZSM-5.53 That work supports the thesis of CuI(CO)3 complexes in C3V symmetry (thus having only two IR active bands). Scheme 1 depicts the foregoing spectral changes upon stepwise formation of the different CuI carbonyls as the CO equilibrium pressure is increased. Because the 2138 cm-1 band could not be assigned unambiguously, Scheme 1 reports that component in brackets. It is worth noting that, whereas at low CO pressure, two families of monocarbonyl complexes (with clearly different C-O stretching frequencies) are present (already interpreted as carbonyl species formed on sites II and II*), at the highest pressures, only one family of tricarbonyl adducts is observed. This result is in agreement with XRPD data previously reported, which indicate that CO adsorption is accompanied by migration of cations from I* and II* positions toward site II. According with the powder XRD analysis, at high CO pressure, all cations accessible to CO are located at site II, justifying the IR results obtained. Borovkov et al.69,85 have published a diffuse reflectance FTIR study of the CO adsorption on copper-exchanged Y zeolites. After a complete analysis of fundamental, combination, and overtones modes of the metal carbonyls, they conclude that, in accordance with our data, at low CO pressure, two kinds of monocarbonylic complexes are formed (located at sites II and II′). However, in contrast with our results, they conclude that only some of the monocarbonyl species, particularly those located at site II, are transformed into dicarbonyl species,

Figure 8. IR spectra of increasing doses of NO adsorbed at liquidnitrogen temperature on CuI-Y prepared following procedure I. Equilibrium pressures in the 0.01-2 Torr range.

whereas copper ions located at site II′ do not form polycarbonyls because of steric limitations. The two sets of results are not completely comparable because experimental conditions are different. In fact, whereas Borovkov et al.69,85 performed their experiments at room temperature, we did both XRPD and IR measurements at liquid-nitrogen temperature. In any case, we consider that the combination of XRPD and IR results presented in this work demonstrate that, under our experimental conditions, CO induces the migration of some of the copper ions into the supercage where they can form polycarbonyl species. An important fraction of copper(I) ions remains, however, in positions not accessible to CO, in agreement with the results of Borovkov et al.69,85 NO Adsorption. The discovery of the high activity of CuZSM-5 for the direct decomposition of NO to N2 and O2 has stimulated the use of NO as a molecular probe for exploration of the nature and reactivity of copper cations in these systems.6,8,9,22,24,28 In relation to this issue, it is worth underlining that, unlike CO, NO has proved to be a sensitive molecular probe not only for CuI but also for CuII ions, and hence, it can be used to explore the valence of copper-exchanged zeolites. The IR spectra of increasing doses of NO adsorbed, at 77 K, on Cu-Y prepared following both procedures, I and II (see Experimental Section) are shown in Figures 8 and 9. For the sample containing only CuI ions (prepared following procedure I), we can observe that, at low equilibrium pressure (first two spectra), two bands, around 1815 and 1792 cm-1, are present. When the pressure increases, the band at 1815 cm-1 decreases, while the band at 1792 cm-1 becomes very intense, and two new bands simultaneously develop at 1826 and 1730 cm-1. A comparison with literature data for NO adsorbed on copper zeolites6,8,9,22,24,28 and with our previous IR results of CO


Figure 9. IR spectra of increasing doses of NO adsorbed at liquidnitrogen temperature on CuI,II-Y prepared following procedure II. Equilibrium pressures in the 0.01-2 Torr range.

adsorption leads to the assignment of the bands at 1815 and 1792 cm-1 (observable at low coverages) to two different CuI‚ ‚‚NO adducts. The corresponding wavenumber values for NO/ Cu-ZSM-59,29 and NO/Cu-mordenite86 systems are 1812 and 1813 cm-1, respectively. As was done before for CO, we assigned the 1815 cm-1 band to the mononitrosyl adducts formed between NO and copper cations located at site II (whose environment should be similar to that found in the main channels of ZSM-5 and mordenite zeolites) and the absorption at 1792 cm-1 to the CuI‚‚‚NO adducts located at site II*. The new bands at 1826 and 1730 cm-1, which dominate at higher NO pressures, are assigned (by analogy with Cu-ZSM-5 and Cu-mordenite) to the symmetric (1826 cm-1) and asymmetric (1730 cm-1) stretching of the CuI(NO)2 dinitrosylic species located at site II. The parallel decrease in the 1815 cm-1 band further confirms this assignment. Copper ions placed at site II* (at least the majority of them) are able only to interact with one NO molecule, not showing any tendency to form dinitrosylic adducts even at the highest pressure, as indicated by the fact that the corresponding IR absorption continues to grow on continuing increases in the NO pressure. Valyon and Hall8 gave a different explanation for the three bands present at high pressure. They assigned the 1796 cm-1 band to copper mononitrosyls that are partly transformed into dinitrosylic species at high NO pressure, giving rise to two IR absorptions at 1825 and 1732 cm-1 for symmetric and asymmetric stretching, respectively. We think that the demonstrated existence for two different sites accessible to NO molecules and the presence of the two mononitrosylic species at low NO coverages demonstrate the validity of our hypothesis. Probably, as happened with CO, in the case of copper ions located at site II*, the addition of a second molecule must be accompanied by a migration of copper ions from these positions toward the


supercage. NO, being a weaker base than CO, is not able to induce this migration. Now, for the IR spectra of NO adsorbed on the system exposed to the atmosphere and hence partly oxidized (Figure 9), we note that, in addition to the bands in the 1850-1650 cm-1 range, already discussed and assigned to Cu(I) nitrosyl adducts, the spectra show a new group of absorptions in the 1975-1850 cm-1 range. In agreement with results on Cu-ZSM5,9,28 these new bands can be assigned to two CuII mononitrosylic adducts located at the two available sites in the supercage, II and II* (1955 and 1923 cm-1 bands, respectively). These results demonstrate that, as expected, the exposure of CuI-Y zeolite to air oxidizes an important fraction of cuprous ions to cupric ions and that the successive outgassing treatments in a vacuum at 873 K are not enough to re-reduce all of the formed cupric ions and restore the original situation. Note that, on passing from the fully reduced system (Figure 8) to the partly oxidized one (Figure 9), the relative intensities of the bands in the 1850-1650 cm-1 range (CuI nitrosyl adducts region) change. In particular, the intensity of the absorption assigned to the CuI‚ ‚‚NO adduct located at site II* is more affected than the bands associated with CuI dinitrosyl adducts on site II. This result suggests that divalent copper ions are located at site II*. Portions of the spectra of NO adsorbed on a Cu-exchanged Y zeolite, prepared following procedure II, shown in Figure 9, have already been published in a recent communication.91 On that occasion, we did not attempt a detailed assignment of the bands because we centered our attention only on the unusually strong and well-defined overtones signals in the 4000-3500 cm-1 region whose unusual intensity was explained in terms of scattering effects. To verify this hypothesis, we recently made diffuse IR reflectance measurements on this system, and as expected, we obtained well-defined overtone signals in this region. However, the intensity of these overtones was lower than expected on the basis of the transmission experiments. Similar results have been reported by Borovkov et al.69,85 on CO/Cu-Y systems. Thanks to the suggestions of Prof. M. Suhm (Institut fu¨r Physikalische Chemie, Universita¨t Go¨ttingen) we now believe that the unusually high intensity of overtone signals observed on our Cu-Y zeolites in the transmission mode is partially due to nonlinear effects of the MCT detector, which can add spurious intensity at twice the frequency of the fundamental modes. These effects are particularly relevant in the case of highly scattering samples (as in the case of the Cu-Y prepared by us). 4. Conclusions CuI-Y zeolite, prepared via the gas-phase reaction of H-Y with CuCl, has been investigated with powder XRD, XAFS, and IR spectroscopies. It is found that a nearly total H+/Cu+ exchange can be obtained if contact of the sample with atmospheric oxygen is avoided. XRPD analysis reveals that, under vacuum conditions, cuprous ions occupy three kind of sites, two (II and II*) located in the supercage and accessible to adsorbates such as CO or NO and the third in a more internal and inaccessibile position. Adsorption of CO on CuI sites with formation of carbonylic species is accompanied by a migration of cations from sites I* and II* to more-exposed positions (sites II). Both XAFS and IR spectroscopies show that the sample adsorbs CO at room temperature. The IR spectra of adsorbed CO and NO allow us to confirm that two accessible sites are present in the supercage and that a displacement of cations from sites II* to sites II occurs at low temperature in the presence of a high pressure of CO. In addition, NO proves to be a sensitive probe of both cuprous and cupric ions.

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XRD, XAS, and IR Characterization of Copper-Exchanged Y Zeolite (90) In fact, in all studied cases [CuI(CO)3 in CuI-Y, CuI-ZSM-5,9,29 CuI-mordenite,30 and CuI-β86), the low-frequency band lies in the 21352146 cm-1 interval, i.e., very close to the broad components that develop around 2138 cm-1 when high CO pressures are dosed inside nearly all zeolites.38 That band is due to CO molecules nonspecifically interacting with a cation (i.e., only weakly interacting with the zeolite walls through dispersive forces, as also observed upon dosing CO on the cation-free


silicalite) and has been already labeled as the liquidlike component (see, e.g.: Lamberti, C.; Bordiga, S.; Cerrato, G.; Morterra, C.; Scarano, D.; Spoto G.; Zecchina, A. Comput. Phys. Commun. 1993, 74, 119. Lamberti, C.; Morterra, C.; Bordiga, S.; Cerrato G.; Scarano, D. Vib. Spectrosc. 1993, 4, 273). (91) Turnes Palomino, G.; Bordiga, S.; Zecchina, A. Phys. Chem. Chem. Phys. 1999, 1, 2033.

XRD, XAS, and IR Characterization of Copper ... - ACS Publications

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