CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300730

Spectroscopy and Redox Chemistry of Copper in Mordenite Pieter Vanelderen,[a] Julie Vancauwenbergh,[a] Ming-Li Tsai,[b] Ryan G. Hadt,[b] Edward I. Solomon,*[b] Robert A. Schoonheydt,*[a] and Bert F. Sels*[a] Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved dif-

fuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu2 + sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm1, and quantified with TPR. Its full description and true location in MOR is currently being investigated.

1. Introduction Cu2 + sites in zeolites have been studied intensively and they have attracted particular interest for their potential in heterogeneous catalysis.[1] Cu2 + -zeolites are, for instance, used as active catalysts for a range of reactions, such as the decomposition of nitrogen oxides,[2–13] the selective hydroxylation of methane and benzene to methanol[14–20] and phenol[21–25] respectively, and the oxidative carbonylation of alcohols.[26–33] In addition, a lot of spectroscopic data on Cu2 + -containing zeolites are available in literature. Indeed, Cu2 + -zeolites are excellent probes to study the coordination chemistry of transition metal ions (TMIs), mainly with the combination of UV/Vis-NIR and EPR spectroscopy and theoretical approaches.[34–50] Location of TMIs in zeolites and their coordination is thus established for many zeolite topologies and zeolite compositions.[1] Often these spectroscopic studies focused on low transition metal contents,[35–38, 51] while the best catalytic results were mainly reported with high Cu2 + loadings. Despite the complex[a] P. Vanelderen, J. Vancauwenbergh, Prof. Dr. R. A. Schoonheydt, Prof. Dr. B. F. Sels Center for Surface Chemistry and Catalysis, KULeuven Kasteelpark Arenberg 23, 3000 Leuven (Belgium) E-mail: [email protected] [email protected] [b] Dr. M.-L. Tsai, R. G. Hadt, Prof. Dr. E. I. Solomon Department of Chemistry, Stanford University Stanford, CA 94305 (USA) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300730.

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ity, we believe that an appropriate combination of spectroscopy, reactivity and theoretical studies will offer unique opportunities to identify novel active sites, also in highly loaded Cuzeolite samples.[2, 15, 40, 52, 53] The identification of the peroxo precursor site, namely m-(h2 :h2) peroxo dicopper(II) species [Cu2(O2)]2 + , and the bent [Cu2O]2 + core in Cu2 + -rich ZSM-5 for the selective oxidation of methane is an important recent example of such collaborative work.[16–18] Interestingly, the constrained surroundings imposed by the rigid ZSM-5 zeolite lattice apparently provide a unique environment that results in this novel reactivity. Cu-mordenite (CuMOR), along with Cu-ZSM-5, is among the best catalysts within the range of Cu-zeolites tested, but it is less well studied. Because of the increasing interest in CuMOR,[20, 54] intense research on highly loaded copper MOR is a requisite for a more fundamental understanding. A better view on the copper speciation and its coordination chemistry in Cu2 + -rich MOR will allow for the development of structural contributions to its intriguing catalytic/redox properties. There is some spectroscopic information available on CuMOR with varying Cu content. A combined UV/Vis–NIR/EPR study at low Cu loadings for instance revealed one d–d band with a maximum around 13 600 cm1 and two EPR signals with the following spin Hamiltonian parameters:[40, 42, 55] 1

g :gjj ¼ 2:32, Ajj ¼ 169  104 cm1 and g? ¼ 2:07, A? ¼ 9  22  104 cm1 ChemPhysChem 2014, 15, 91 – 99

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g :gjj ¼ 2:27, Ajj ¼ 191  104 cm1 and g? ¼ 2:09, A? ¼ 18  42  104 cm1

At low Cu loadings only signal 1g is present; as the loading increases signal 2g appears. The only change in the d–d region of the spectrum with the Cu2 + increase is some line broadening. Kucherov et al. assigned signal 1g to a square pyramidal environment and signal 2g to a square planar coordination environment of Cu2 + .[40] De Tavernier et al. used the Ca2 + sites as determined by XRD for the assignment of the Cu2 + signals (Figure 1).[44, 56, 57] On the basis of a ligand field analysis, they

www.chemphyschem.org sites b and g. The square-planar Cu2 + coordination 2g was assumed to be located in site a, balanced by only one framework Al. The possibility of an extra-lattice oxygen ligand was also considered for the latter site.[45, 59–61] The group of Schoonheydt and Pierloot used a quantum chemical approach to study the coordination of Cu2 + in sixmembered rings (6-MRs) of zeolite A, Y, ZK4, MOR and ZSM-5. The 6-MRs were geometry optimized with DFT and the spectra were calculated on the optimized structures with CASPT2. The conclusions of those studies were:[35, 42, 43, 62] · Cu2 + attempts to acquire its maximum coordination number of 4. · In this 4-fold coordination, Cu2 + preferentially coordinates with O atoms belonging to the Al tetrahedron and as a result, the 6-MR is strongly distorted. · The number of Al tetrahedra in the 6-MR determines the EPR signals, in combination with the crystallographic site, namely, the a, b and g sites. Thus, EPR signals 1g and 2g were assigned to mononuclear Cu2 + coordinated in 6-MRs with 2 and 1 Al atoms, respectively. However, there are still several essential missing links in our interests of CuMOR:

Figure 1. MOR structure with crystallographic sites E(a), A(b) and C(g) indicated in black.

proposed site A for signal 1g and site E for 2g signal. Wichterlova et al. introduced for the first time the a, b and g sites for the pentasil zeolites in general.[58] These sites are shown in Figure 2 for MOR, ZSM-5 and ferrierite. Site a is an elongated six-membered ring composed of twofold connected five-membered rings in the main channel of MOR (site E) and in the straight channel of ZSM-5. Site b is the twisted eight-membered ring of the MOR pocket (site A) and a six-membered ring at the intersection of the straight and the sinusoidal channel in ZSM-5. And finally site g is a complex boat-shaped site, which corresponds to site C in MOR, as it is formed by five and six rings in the sinusoidal channels of ZSM-5. The 1g signal in ZSM-5 is suggested to correspond to a square pyramidal Cu2 + coordination in the vicinity of two framework Al atoms in

· Up to now, the two Cu2 + species are distinguished only based on their EPR spectra. They do not have a distinct signature of d–d transitions in the visible region of the DR spectra. · CuMOR is easily autoreduced. Thus, Cu2 + is reduced to Cu + at high temperature in vacuo or in inert atmosphere. It is unclear which Cu2 + species is the most susceptible to this autoreduction phenomenon. · In CuZSM-5 a band at 22 700 cm1, assigned to a bent [Cu OCu]2 + species, is responsible for the selective oxidation of CH4 to CH3OH. As CuMOR is also active in selective methane oxidation, there is a rising interest in the copper speciation of highly loaded CuMOR. One would like to know whether dimeric Cu2 + species, similar to those in Cu-ZSM-5, can be found in CuMOR. We have therefore undertaken an EPR/UV/Vis-NIR study of CuMORs with different Cu loadings, complemented with a temperature programmed reduction (TPR) study, to answer the above questions.

2. Results 2.1. UV/Vis-NIR and EPR Spectroscopy

Figure 2. Local framework structures for pentasil zeolites: Mordenite (MOR), Ferrierite (FER) and MFI. Reproduced with permission from Elsevier.[58]

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Two CuMOR samples with different Cu loadings were prepared, namely CuMOR0.43 and CuMOR0.06, with the subscript value denoting the atomic Cu/Al ratio. More details on the elemental composition are presented in the Experimental Section. The very low copper-loaded sample (CuMOR0.06) only exhibits the 1 g feature and this sample is occasionally used as a reference in this study. Both as-synthesized CuMOR samples have the ChemPhysChem 2014, 15, 91 – 99

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Figure 3. CuMOR sample (CuMOR0.43) after standard treatment (c) or after an extra oxidation step (a). The EPR spectra (a) are perfectly overlapping. The corresponding UV/Vis-NIR spectra exhibit an extra CT band at 22 200 cm1 after the extra oxidation step (b). The arrow indicates the spectral changes when the sample is repeatedly autoreduced (500 8C in He) and oxidized (250 8C in O2).

characteristic d–d spectrum of [Cu(H2O)6]2 + in their hydrated form with band maximum at 12 500 cm1, slightly asymmetric towards lower wavenumbers. The presence of water is evidenced by the vibrational (n + d) combination band at 5200 cm1 and the 2n overtone at 7000 cm1 (Figure S1). Figure 3 displays the EPR and DR spectra of the Cu-rich CuMOR0.43 after standard treatment (full line), namely calcination in O2 at 450 8C followed by He treatment at 500 8C, and after subsequent O2 dosage at 250 8C (dashed line) (see the Experimental Section for details). The EPR spectra look identical for both treatments, showing the two characteristic EPR signals, namely 1g and 2g, as presented in the Introduction. The DR spectra are clearly different. After standard treatment one observes the d–d band envelope in the 10 000-20 000 cm1 region with maxima around 13 600 cm1 and 16 750 cm1. Illdefined low- and high-frequency shoulders are also seen. In the UV region O!Cu2 + charge transfer (CT) bands are present. A subsequent treatment in O2 at 250 8C results in the same d– d and O!Cu2 + CT spectrum, but with an additional feature at 22 200 cm1. The latter is accompanied by weak absorption at 6500 cm1. The absorption at 22 200 cm1 is likely similar to the 22 700 cm1 band observed in CuZSM-5, which was assigned to a bent dimeric [CuOCu]2 + species. It can thus be suggested that there are two monomeric Cu2 + species characterised by their EPR signals and d–d band spectrum with maxima at 13 600 cm1 and 16 750 cm1 in the Cu-rich CuMOR0.43 sample, while the extra 22 200 cm1 is due to an EPR silent Cu species, tentatively assigned as a binuclear Cu species that is under further study.

22 200 cm1 band disappears and O2 evolves, as ascertained by MS analysis (Figures S2 A and B, Supporting Information). Addition of O2 at 250 8C restores the 22 200 cm1 feature. The interaction of O2 with some type of Cu in CuMOR0.43 is thus a reversible process. The two mononuclear Cu2 + species in CuMOR0.43 with their diagnostic EPR and diffuse reflectance spectroscopy (DRS) characteristics are not reduced by the additional He treatment. According to literature these species autoreduce on hydrated CuMOR samples directly after synthesis. Thus, only then does autoreduction lead to Cu + with disappearance of associated spectroscopic signatures in DRS and EPR.[63, 64] H2 Temperature-Programmed Reduction CuMOR0.43 after standard treatment was subjected to temperature programmed reduction (TPR) with H2 from RT to 900 8C (at 10 8C min1). The data are shown in Figure 4. The H2 TPR plot clearly reveals three H2 consumption maxima at temperatures of 180 8C, 430 8C and 800 8C. During the first reduction peak, Cu2 + ions are reduced to Cu + accompanied by the formation of protons. Their forma-

2.2. Redox Behavior Autoreduction When CuMOR0.43 with the characteristic 22 200 cm1 band is subjected to an additional He treatment up to 500 8C, the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 4. H2 consumption during TPR of CuMOR0.43 after standard treatment. The colour of the sample before and after each H2 consumption peak is shown.

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CHEMPHYSCHEM ARTICLES tion is reflected by the appearance of SiOHAl vibrations in the NIR spectrum (Figure S10).[65–68] Subsequently, Cu + is reduced to Cu0 metallic clusters during the second H2 reduction peak. The area of peak 2 is 0.7 times that of the area of peak 1, while unity is expected. However, at 400 8C H2O desorption starts (Figure S9), due to dehydroxylation of the MOR lattice. This might influence the Cu + !Cu0 reduction, but needs further investigation.[69–73] EPR and DRS spectra were monitored after the 180 8C reduction peak and compared with the spectra before this reduction step. The new EPR and DRS spectra are shown in Figure 5. Upon reduction, the 2g j j = 2.27 signal of the monomeric Cu species, that is, the monomeric Cu2 + species that only appears in highly loaded copper MOR, completely disappeared in the EPR spectrum along with the disappearance of the 16 750 cm1 band in the electronic spectrum. Interestingly, the mononuclear Cu2 + with 1g j j = 2.32, which is predominantly present in MOR with low Cu loadings, is not reduced, while the d–d band at 13 600 cm1 also remains. These data allow us to unambiguously couple the EPR and UV/Vis data of the two different monomeric Cu2 + species, namely species I and II. We may thus conclude that two mononuclear Cu2 + species exist in Cu-rich CuMOR with the following spectroscopic fingerprints: · Species I: 13 600 cm1 d–d band maximum; g j j = 2.32, A j j = 169  104 cm1 and g ? = 2.07, A ? = 9–22  104 cm1 · Species II: 16 750 cm1 d–d band maximum; g j j = 2.27, A j j = 191  104 cm1 and g ? = 2.09, A ? = 18–42  104 cm1

www.chemphyschem.org If CuMOR0.06 with low Cu content is now treated with the standard procedure, the DR spectrum only shows one maximum at 13 600 cm1 (Figure S3 a) and the corresponding EPR spectrum with 1g j j = 2.32 (Figure S3 b), in agreement with earlier findings.[42] These features are thus due to the presence of species I, and its presence is irrespective of an additional treatment with O2. In other words, contact of O2 with species I does not lead to the formation of an observable new Cu species in Cu-poor MOR, in contrast to the redox behaviour of Cu-rich MOR. Interestingly, the H2 TPR experiment on this CuMOR0.06 sample (Figure S11) shows that H2 consumption starts at higher temperatures when compared to CuMOR0.43, indicating that this particular mononuclear Cu2 + species (species I; with 1 g j j = 2.32) is more difficult to reduce. This is also confirmed by the lack of H2 consumption in a H2 titration experiment at 180 8C (Figure S7). This information confirms the spectroscopic fingerprinting of the two mononuclear Cu2 + species, but also shows the difference in redox behaviour of species I and II. The second H2 reduction peak (above 400 8C) is quite complex. In the course of this reduction metallic copper clusters are formed, which is evidenced by the plasmon band in the UV/Vis spectrum at 18 000 cm1 (Figure S4) and the corresponding pink colour of the sample (Figure 4).[74–77] The pink colour did not disappear when the sample was exposed to air. This is an interesting phenomenon, since copper nanoparticles are reported generally to be extremely sensitive to oxygen.[78] The surprisingly high stability of the plasmon band might be an interesting feature for future applications, for example, in biosensing and catalysis.[79–81] Simultaneously, H2 consumption at the second reduction peak occurs with the release of water. The origin of the water is not only related to the redox chemistry occurring in the second TPR signal. If, after the first reduction peak, the flow is switched from H2 to He, significant amounts of water were analysed by MS while heating the sample above 400 8C (Figure S9). However, no plasmon band appeared after the latter treatment, which indicates that H2 at about 400 8C is necessary to form metallic copper clusters. Quantification of the hydrogen consumption in the second reduction peak is quite complex, because more than one H2 consumption mechanism is occurring. In addition to Cu reduction, there is also significant physisorption of H2 on the metallic copper. The physisorption phenomenon is apparent by pulsing H2 over the sample after the second peak. If the pulse frequency was very rapid, no additional H2 consumption was observed after each successive H2 pulse. Additional amounts of H2 were retained on the sample each time a long He flush was applied between each H2 pulse (Figure S8). It is thus important to take into account such physisorption phenomenon when interpreting and quantifying TPR spectra of Cu zeolites. TPR Quantification

Figure 5. EPR spectra (a) and corresponding zoom of the d–d transition domain (b) of CuMOR0.43 after standard treatment (c) and the same sample after additional reduction in H2 at 180 8C (a).

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The first reduction peak at 180 8C is also present when CuMOR0.43 is subjected to the extra oxidation step (in O2 at 250 8C), but the signal is much more intense, as evident from the signal intensity in Figure 6. The integrated area of the TPR curve is indeed 7 to 8 times larger for the oxidized Cu sample. ChemPhysChem 2014, 15, 91 – 99

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www.chemphyschem.org 3. Discussion Previous interpretations of the spectroscopic fingerprints of Cu-rich CuMOR were only based on one unresolved d–d band envelope between 8000 and 18 000 cm1 and two EPR signals (see Table 2). However, through carefully handling the preparation and treatments of the CuMOR0.43, we obtained resolution

Table 2. Assignments of Cu sites in the literature. Figure 6. H2 consumption during TPR in the region 65 to 220 8C after standard treatment (c), and after standard treatment followed by the oxidation step (a).

Water formation can be observed (in the NIR spectrum, Figure S10) during this reduction step on the oxidized samples, indicating that a different H2 reduction mechanism occurs when compared to that of the isolated Cu2 + ions. A hypothetical [CuOCu]2 + complex is reduced, but the exact nature of this complex is under investigation. The exact amount of H2 consumed in the 180 8C peaks was measured through an additional chemisorption experiment with H2 pulses at 180 8C (Figures S5–S6), assuming one hydrogen molecule reduces two Cu2 + into Cu1 + . The results are summarized in Table 1. The data show that 0.07 mmol g1 Cu2 +

Table 1. Amount (mmol g1) of Cu reduced by H2 at 180 8C. Standard treatment

Extra oxidation step

Difference

0.07

0.50

0.43

is reduced in CuMOR0.43 after standard treatment, while 0.50 mmol g1 Cu2 + is reduced in the oxidized CuMOR0.43 sample. While only mononuclear Cu2 + species II (with 2g j j = 2.27 and d–d band maximum at 16 750 cm1) is reduced by H2 at 180 8C, the additional consumption of H2 leads to the disappearance of the characteristic 22 200 cm1 band in the electronic absorption spectrum. The difference in the amount of Cu2 + reduced in both chemisorption experiments is 0.43 mmol g1 and thus corresponds to the amount of binuclear Cu (and potentially other autoreducible EPR-silent Cu species). We note that the amount of EPR active copper is the same before and after the additional O2 activation step. Taking into account a total amount of 0.71 mmol of Cu per gram zeolite (dry weight), as ascertained by ICP, a total of 0.21 mmol Cu2 + per gram zeolite may be considered as non-reduced by H2 at 180 8C. This fraction mainly contains the mononuclear Cu2 + species I (with 1g j j = 2.32 and 13 600 cm1 d–d transition), but the presence of some other non-autoreducible Cu2 + or non-oxidizable Cu + species cannot be excluded. Since the spectral intensities of the 1g j j = 2.32 and 2g j j = 2.27 signals are roughly the same, here we approximate equal amounts of both species.

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EPR signal

Kucherov et al.[40] Wichterlova et al.[45, 82]

De Tavernier et al.[44]

Delabie et al.[42]

2.27

square planar with 1 Al square pyramidal with 2 Al

Site A[a]

5 MR (site C) or 6 MR (site E) with 1 Al 6 MR (site E) with 2 Al

2.32

Site E[a]

[a] Irrespective of Al content.

of the d–d region of the electronic spectrum into two contributions (see Figure 3 b). The interpretation of our data in terms of number of Al tetrahedra is appealing. Cu species I (with 1g j j = 2.32 and d–d transition at 13 600 cm1) interacts with 2 Al’s and is therefore more difficult to reduce, in accordance with our reduction experiments under H2 at 180 8C. Cu species II (with 2g j j = 2.27 and d–d transition at 16 750 cm1) is readily reduced and this observation fits the weaker interaction with only one Al. This Cu2 + ion type has also been reported to readily reduce by reaction with CO at elevated temperatures.[35] The reduction occurs with the selective disappearance of the EPR signal at 2 g j j = 2.27, along with the d–d transition envelope at 16 750 cm1. Therefore both spectroscopic features unambiguously belong to the same mononuclear Cu2 + species. The difference in ligand field transitions of both EPR active species can be understood as follows: For an approximately tetragonal CuII site, A j j is given by Equation (1): Ajj ¼ Pd ½ðk4=7Þa2 þ 3=7Dg? þ Dgjj 

ð1Þ

where Pd is 396  104 cm1, k is the Fermi contact term, a2 is the metal covalency of the 3dx2y2 molecular orbital, which decreases as the covalency of the species increases. Using the experimental g and A values of species I and II, Equation (1) can effectively decouple the contributions from Dg and Cu covalency to the DA j j between the two species. Assuming no change in Cu covalency, the change in g values from species II to species I (2.27 to 2.32 and 2.09 to 2.07) contribute 17  104 cm1 to DA j j between species II and I, which is in reasonable agreement with the experimental DA j j (22  104 cm1). Therefore the change in A j j reflects the change in the g j j value, which is the orbital dipolar contribution to the hyperfine interaction. Thus, there is little change in covalency between species I and II. The deviation of the g j j value from that of the ChemPhysChem 2014, 15, 91 – 99

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free electron (2.0023) can be expressed by Equation (2): gjj ¼ 2:0023ð8la2 b2 Þ=ðE xy E x2y2 Þ

ð2Þ

where l(CuII) = 830 cm1, a2 and b2 are the metal covalencies of the 3dx2y2 and 3dxy molecular orbitals, respectively. Since the hyperfine change shows that there is essentially no change in Cu covalency between the two species, the ratio of the Dg j j for the two species can be used to predict the ratio of the two 3dxy !3dx2y2 transition energies [Eq. (3)]:

Table 3. Calculated d–d transitions of Cu2 + in MOR topology in accordance with the proposed positions of the square planar species II in Figure 7. Energy levels of the d orbitals in a one hole system are presented in cm1, dx2y2 : 0 (taken as reference).[42] Site E1

Site C1

Site C2

dx2y2 : 0 dxy : 11 623 dxz : 13 291 dyz : 13 685 dz2 : 17 311 Average: 13 998

dx2y2 : 0 dxy : 11 042 dxz : 12 080 dyz : 12 637 dz2 : 16 780 Average: 13 180

dx2y2 : 0 dxy : 11 020 dxz : 12 118 dyz : 12 742 dz2 : 16 252 Average: 13 033

E xy ðspecies IIÞ=E xy ðspecies IÞ ¼ Dgjj ðspecies IÞ=Dgjj ðspecies IIÞ ð3Þ The ratio of Exy(species II)/Exy(species I) derived from Dg j j is 1.19. This predicts that the ligand field (LF) transition should shift down in energy in species II relative to I by ~ 2580 cm1. This is in reasonable agreement with experiment provided by the DRS LF transition envelope (a decrease of 3150 cm1). Therefore, the lower LF transition energy leads to a larger g j j value and this in turn lowers the A j j in species I, and there is little change in the covalency between these species. Note that higher resolution spectrosocpic methods such as MCD will be helpful to resolve and assign individual d–d transitions of the two species. The d–d transition band maximum of species II at 16 750 cm1 is located at a high wavenumber and the corresponding g j j at 2.27 is low for a Cu2 + core surrounded by four O atoms in the first coordination sphere. The calculations of Delabie et al. indicated that 5 MR (sites C) and 6 MR (site E) are most plausible to host this Cu species. Three geometry-optimized structures are shown in Figure 7 and the calculated d–d transitions are collected in Table 3. Note that all proposed Cu sitings show gzz = g j j values of about 2.27. The highest calculated energy transition is in the range of 16 250–17 300 cm1 and it is striking how close this value is to our experimental data for species II, namely the 16 750 cm1 d–d band is connected to the g j j = 2.27 signal. The agreement of spectroscopic data points to an almost perfect square planar coordination—point group D4h—of Cu2 + in site E (6MR) and sites C (5MR). In these coordination sites the dxz and dyz orbitals form a degenerate pair with an energy separation corresponding to only 600 cm1. Note that we used this criterion to assign the site as square planar. The CuO distances were calculated from the geometries and summarized in Table 4. The spread in the Cu

Figure 7. Most plausible geometry optimized copper siting or an EPR signal around g j j = 2.27 according to Pierloot et al.[42]

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Table 4. Corresponding calculated CuO bond lengths [nm] of square planar species II in MOR.[42] Site E1

Site C1

Site C2

0.201 0.206 0.192 0.197 0.199 (average) 0.014 (DCuO)

0.192 0.191 0.202 0.205 0.198 (average) 0.014 (DCuO)

0.195 0.195 0.205 0.205 0.200 (average) 0.010 (DCuO)

O bond distances is indeed small in accordance with a quasiperfect square planar coordination. The spectroscopic features of species I, d–d band maximum around 13 600 cm1 and g j j = 2.32, point to a weaker ligand field and a deviation from almost perfectly square planar to square pyramidal coordination. The calculated transition energies are too high (see ref. [42]) when compared to the observed value. Nevertheless models that come close are E2 (1 Al), E6 and E8 (2 Al). Our preference goes to E6 and/or E8 because Cu2 + species I prefers sites with 2 Al’s. Note that site E6 has previously been eliminated as a possibility based on the Takaishi and Kato rule.[83, 84] However, this rule has yet to be rigorously proven.[85] It is noteworthy that E6 is also a favourable candidate for species I based on the EPR analysis, as the calculated energy of the 3dxy !3dx2y2 transition shifts down by ~ 2500 cm1 compared to species II (i.e., C1, C2 or E1), which is quite similar to the experimental difference between the d–d transition envelope maxima between the two species. It is interesting to compare the experimentally observed redox behavior to that predicted by ligand field arguments. The square planar geometry of species II favors the oxidized over the reduced state, while the more distorted ligand field environment of species I favors the reduced over the oxidized state. Therefore, from a ligand field perspective, species I should be easier to reduce than species II. However, we observe the opposite here. This is a direct experimental demonstration that the redox behavior of these two mononuclear Cu species in highly loaded CuMOR is governed by electrostatic interactions with the negatively charged Al ions in the zeolite lattice. Interestingly, the EPR-silent Cu fraction with an absorption feature at 22 200 cm1 is analogous to the species with a 22 700 cm1 CT band observed in O2-treated Cu-ZSM-5. This ChemPhysChem 2014, 15, 91 – 99

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species was assigned to a mono (m-oxo) dicopper core [CuO Cu]2 + using resonance Raman (rR). Also various mixed-valence copper sites on Cu-ZSM-5 have been suggested in literature,[53] but we have no indication of their existence based on EPR data. The geometric and electronic structure of the species with the 22 200 cm1 band in CuMOR is not yet known. Studies involving isotope labeling and rR spectroscopy are currently underway. Nevertheless, it is evident that such EPR-silent Cu species may undergo reversible redox steps, and thus they have an intrinsic autoreduction capacity. This happens through the loss of O2, leaving 4 e for the reduction of 4Cu2 + to 4Cu + . The intrinsic capacity to undergo redox cycles is crucial, for example, in the direct N2O decomposition or oxidation reactions with O2 or N2O. The mononuclear copper species I and II will be active only if reducing agents such as methanol or NH3 are used to complete the redox cycle. This is for instance the case in the selective catalytic reduction (SCR) of NO. It is interesting to mention that mononuclear copper sites have recently been appointed as the active site for the SCR of NO in Cu-chabazite zeolite.[86, 87] The amount of autoreduced Cu2 + in Cu-rich CuMOR that we correlate with the formation of binuclear copper species (or potentially another autoreducible EPR-silent Cu species) is estimated on the basis of H2 titration at 180 8C [Eq. (4)]: 2 Cu2þ þ H2 ! 2 Cuþ þ 2 Hþ

ð4Þ

It is striking that the binuclear autoreducible fraction takes about 60 % of the total Cu content, that is, 0.43 mmol g1 of a total of 0.71 mmol g1 in the case of the fully exchanged CuMOR0.43. This number is noteworthy and agrees with a recent extended X-ray absorption fine-structure spectroscopy (EXAFS) study of Van Bokhoven et al., who suggested that 50 % of the total Cu content changed oxidation state during the partial oxidation of methane.[54] Note that this amount is by far more than that of active dicopper species determined traditionally by methanol extraction after methane reaction.[14, 19] Both the EXAFS and our chemisorption experiments suggest that quantifying active copper sites in methane oxidation is better done by determining the amount of converted CH4. CH4 titration in a well-designed chemisorption experiment thus seems a more accurate approach to perform active-site quantification in future experiments. It has been reported that usually up to 80 % of the total Cu2 + content undergoes autoreduction and in some cases even an almost complete autoreduction has been observed for Cu-rich ZSM-5.[64, 88, 89] It should be noted that this is only possible when the mononuclear EPR active Cu2 + sites are also autoreduced, in contrast to our results on CuMOR. However, we have experienced that consis-

tent synthesis and pretreatment conditions including temperature, gas compositions and contact times are a prerequisite to reproduce the redox chemistry of Cu zeolites, which of course makes the comparison of results from different research groups deceptive. Following our synthesis methods and pretreatment protocols, three predominant Cu2 + fractions are thus present in the Cu-rich sample CuMOR0.43 and their redox behaviour was studied. The characteristic spectroscopic features of two mononuclear Cu2 + sites were unambiguously identified and correlated with their coordination at different sites in the zeolite, whereas the predominant Cu fraction is tentatively assigned to an EPR-silent binuclear Cu species, which is formed upon an extra oxidation of CuMOR0.43 at 250 8C in O2 atmosphere. The latter is able to subsequently release O2 in inert atmosphere at elevated temperature. The Cu species and their main characteristics are summarized in Table 5.

4. Conclusions Three Cu sites have been identified in Cu-rich MOR. Two are fully characterized: a mononuclear square planar Cu2 + with a g j j value of 2.27 and a d–d transition at 16 750 cm1 and a mononuclear Cu2 + with a more distorted square pyramidal coordination with a g j j value of 2.32 and d–d transition at 13 600 cm1. Their redox behaviour is governed by electrostatic interactions with negatively charged Al ions in the zeolite lattice, as opposed to their ligand field environment. Knowledge of the spectroscopic characteristics led to a refinement of the Cu sites and their coordination characteristics in CuMOR.[35] The identified monomeric Cu2 + species make up almost 30 % of the total Cu fraction. The dominant Cu species is likely a binuclear copper site, but other autoreducible EPRsilent Cu species cannot be ruled out. All or a fraction of these EPR-silent Cu species exhibit an absorption feature at ~ 22 000 cm1. While this parallels the reactive species in CuZSM-5, its nature in MOR still needs to be defined. However, in highly loaded CuMOR we can now quantify its concentration at  60 % of the Cu sites. Remaining research questions are: 1) finding experimental evidence of the siting of CuII species 1 and 2. This will allow for the refinement of the theoretical models handled in this paper. 2) Determining whether the 0.43 mmol g1 of EPR-silent Cu is indeed due to one binuclear [CuOCu]2 + species or whether the 0.43 mmol g1 is a conglomerate of different EPR-silent Cu species.

Table 5. Overview of Cu speciation in Cu-rich MOR0.43. Absorption Maximum [cm1]

gj j

Assignment

Amount of Cu2 + [mmol g1]

% of Total Amount

13 600 16 750 22 200

2.32 2.27 EPR silent

distorted coordination in 6 MR with 2 Al square planar in 5 or 6 MR with 1 Al Binuclear, unknown location

< 0.21 0.07 0.43

< 30 10  60

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Experimental Section

Acknowledgements

Sample Preparation The MOR (Sd-Chemie; Si/Al = 5) was exchanged in a sodium nitrate (NaNO3) solution, prior to copper cation exchange. NaCuMOR samples with two different CuII loadings were prepared by ionic exchange for 24 h in an aqueous Cu(CH3CO2)2.H2O solution at ambient temperature. The samples were washed and dried at 80 8C. The Cu and Al contents were determined by an ICP analysis and the analytical results are given in Table 6.

Table 6. Synthesized CuMOR samples. The total amount of copper (ICP analysis) and Cu/Al ratio are indicated. Sample Symbol

Cu2 + [mmol g1]

Cu/Al Ratio

CuMOR0.43 CuMOR0.06

0.71 0.11

0.43 0.06

Sample Treatments The CuMOR was pelletized to obtain grain sizes between 0.25 and 0.5 mm diameter and brought into a quartz flow cell. The latter is equipped with a window and a side arm for in situ UV/Vis-NIR DRS and EPR measurements. The samples were subjected to following treatments:

· Standard pretreatment: Calcination in pure O2 at 450 8C for 2 h (5 8C min1), followed by He treatment at 500 8C overnight. · Oxidation in O2 : Standard pretreatment, followed by pure O2 treatment at 250 8C.

H2 TPR and H2 Pulse Experiments H2TPR experiments were performed in a cylindrical quartz tube by cooling down the CuMOR sample in helium after one of the above-mentioned treatments and subsequently heating it in a 5 % H2 in He flow up to 900 8C (10 8C min1). Ideal plug flow conditions were assured by applying the criteria introduced by Monti and Baiker.[90] H2 pulse experiments were done in the same setup. After the pretreatment the sample is cooled to 180 8C and brought into a helium flow. Pulses of pure H2 were obtained by means of a sixway valve with a sample loop (5 ml or 20 ml). H2 TPR experiments were also performed in a quartz flow cell with a window and EPR side arm, in order to measure UV/Vis and EPR before and after each H2 reduction peak.

Spectroscopic Measurements In situ EPR and UV/Vis-NIR were performed in a specially designed quartz flow cell with a window and a side arm for respectively UV/ Vis-NIR DRS and EPR measurements. DRS spectra were recorded on a Varian Cary 5000 UV/Vis-NIR spectrophotometer at room temperature. They were measured against a halon white reflectance standard in the range 4000–50 000 cm1. EPR spectra operating in Xband with a microwave power of 20 mW were recorded at 120 K with a Bruker ESP 300E instrument in a rectangular TE104 cavity.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

This work was performed within the framework of FWO (G.0596.11), IAP (Belspo), ERIC (European Union), Methusalem (long-term structural funding by the Flemish Government) projects, and supported by National Science Foundation Grants CHE0948211 (to E.I.S.). M.-L.T. received supports from the Postdoctoral Research Abroad Program sponsored by the National Science Council, Taiwan (R.O.C.) and R.G.H. acknowledges a Gerhard Casper Stanford Graduate Fellowship and the Achievement Rewards for College Scientists (ARCS) Foundation. Keywords: copper · electron paramagnetic resonance · reduction · uv/vis–nir · zeolites

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Received: August 7, 2013

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