In Situ X-ray Photoelectron Spectroscopy Detects Multiple Active Sites

Jun 18, 2019 - A direct route to convert methane into high-value commodities, such as methanol, with high selectivity is one of the primary challenges...
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Research Article Cite This: ACS Catal. 2019, 9, 6728−6737

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In Situ X‑ray Photoelectron Spectroscopy Detects Multiple Active Sites Involved in the Selective Anaerobic Oxidation of Methane in Copper-Exchanged Zeolites Luca Artiglia,*,†,‡ Vitaly L. Sushkevich,† Dennis Palagin,† Amy J. Knorpp,§ Kanak Roy,§ and Jeroen A. van Bokhoven*,†,§ †

Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland Laboratory of Environmental Chemistry, Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland § Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zürich, Switzerland

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S Supporting Information *

ABSTRACT: A direct route to convert methane into high-value commodities, such as methanol, with high selectivity is one of the primary challenges in modern chemistry. Copper-exchanged zeolites show remarkable selectivity in the chemical looping process. Although multiple copper species have been proposed as active, an in situ spectroscopic investigation is difficult, because of their similar fingerprints. We used ambient pressure X-ray photoelectron spectroscopy to investigate an actual powder sample. We could discriminate between different types of active species involved in the conversion of methane to methanol over two different copper-exchanged zeolites, namely, mordenite and mazzite. After activation at 400 °C in oxygen, we followed the reaction in situ at 200 °C, switching from methane to water, and followed by a second cycle with anaerobic activation. Our experimental results, combined with theoretical calculations, prove that Cu(II) sites bound to extra-framework oxygen are involved in the reaction, and that their structure, formation, and stabilization depend on the type of zeolite and on the Si/Al ratio. KEYWORDS: ambient pressure X-ray photoelectron spectroscopy, copper zeolites, methane to methanol, active sites, density functional theory, infrared spectroscopy

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mild temperature and pressure conditions.7 Typically, the process consists of activation of the sample in oxygen at high temperature (400−450 °C) followed by reaction with methane and extraction with water at a lower temperature, typically between 150 and 200 °C. More recently, isothermal routes have been described.8,9 Because the reacted methane is protected from further reaction through adsorption on the copper zeolite, some zeolite-based systems showed good selectivity; however, the productivity is very low. During the water desorption step, water may act both as an extraction solvent and as a source of oxygen in the direct conversion of methane to methanol over copper-exchanged mordenite.10,11 Other zeolite supports, namely, ZSM-5, 12,13 mazzite (MAZ),14,15 and chabazite (CHA),16−18 have also been used and showed good performance. On the basis of in situ spectroscopy, a two-electron redox mechanism involving the Cu2+/Cu+ redox couple is established.16,19,20 However, the

INTRODUCTION

One of the current challenges in the field of catalysis is to find an efficient way to convert methane to products that have a higher economic value.1−3 Considerable amounts of methane are found in shale gas and during the extraction of crude oil, and, due to the high cost needed either to transport or to convert it, are disposed or fully oxidized to carbon dioxide by means of flaring. A viable route to selectively convert methane to more valuable products, among which the most preferable is methanol, would be a successful strategy to get high-quality fuels and chemicals from a greenhouse gas. However, the direct conversion of methane to methanol (MtM) is difficult due to the higher reactivity of methanol compared to methane. Because natural enzymes, such as methane monooxygenase, convert methane selectively into methanol at ambient temperature and pressure, a lot of effort has been put into mimicking the enzyme performance4−6 for the applications in the field of heterogeneous catalysis. Thanks to their microporous structure, zeolites are suitable supports to stabilize welldispersed metal species. Iron- and copper-exchanged zeolites showed good selectivity in the MtM reaction carried out under © XXXX American Chemical Society

Received: March 25, 2019 Revised: May 24, 2019 Published: June 18, 2019 6728

DOI: 10.1021/acscatal.9b01223 ACS Catal. 2019, 9, 6728−6737

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ACS Catalysis structure of the active site/s involved in the mechanism is still under debate. Spectroscopic techniques such as X-ray absorption (XAS), infrared (IR), Raman, and UV−vis are sensitive to the local structure, but due to the complex spectral features with different components superimposed, careful analysis is needed to provide conclusive assignments.7,21 For example, on the basis of resonance Raman spectroscopy and DFT, Schoonheydt et al. identified the active sites as mono(μoxo) dicopper, where the formal oxidation state of copper is +2.12,22 By contrast, Grundner et al. proposed a trinuclear copper−oxo cluster, the structure of which was suggested on the basis of theoretical calculations and in agreement with extended X-ray absorption fine structure (EXAFS).23,24 Also, monocopper species nucleating in the 8-membered ring of SSZ-13 have been investigated by means of density functional theory (DFT) calculations, and Cu−OH+ has been suggested as the active sites.25 In general, DFT calculations are a promising tool to prescreen all the possible active species formed and reacting in the zeolite framework and can be used to explain the experimental results.26 In the present work, we investigated three different copper-exchanged zeolites samples, namely, 4.3 wt % Cu-mordenite (Cu-MOR, Si/Al = 6, hereafter named Cu-MOR(6)), 1.2 wt % Cu-mordenite (CuMOR, Si/Al = 46, hereafter denoted Cu-MOR(46)), and 4.6% Cu-mazzite (Cu-MAZ, Si/Al = 4, hereafter named CuMAZ(4)), by means of in situ ambient pressure X-ray photoelectron spectroscopy (APXPS).27,28 After activation in oxygen at 400 °C, photoemission spectra were acquired while switching to methane, then to water at 200 °C, then again to methane after reactivation of the samples in helium at 400 °C. Combining XPS with theoretical calculations and infrared spectroscopy of adsorbed nitrogen monoxide, we show that different copper species are involved in the reaction. Their nature and reactivity are influenced by the zeolite support and by the Si/Al ratio.

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RESULTS AND DISCUSSION Spectroscopic Characterization and Simulation of the Electron Binding Energy. The synthesis of the three samples used in this work and the experimental procedures adopted to investigate them by means of XPS are reported in the Experimental Section. Figure 1a,b shows photoemission spectra acquired in situ on Cu-MOR(6) and Cu-MAZ(4) under a constant pressure of 1.0 mbar. The experimental procedure is in agreement with the stepwise procedure adopted in the methane reactivity tests.11,15 All the Cu 2p spectra were normalized to the same number of iterations; thus, they share the same y-axis. The intensity of the spectra of Figure 1a,b changes as a function of the gas dosed. This can be explained by the interaction of photons and photoelectrons with the different gaseous environment. As an example, it has been reported that oxygen absorbs more X-rays than methane in the soft X-ray energy range,29 in agreement with our results showing that the Cu 2p spectra acquired in oxygen are less intense and more noisy than the ones acquired in methane. Although the copper loading was similar, the signal of Cu 2p is more intense on MOR than on MAZ. Because XPS is a surface sensitive technique and the kinetic energy of the Cu 2p photoelectrons was fixed (the same excitation energy, hν = 1250 eV, was used to acquire all the photoemission spectra),30,31 the data suggest that more copper is found on the surface of Cu-MOR(6). This is further shown by the Cu/Si and Cu/Al ratios as reported in Figure S1. The areas of the

Figure 1. (a, b) Cu 2p photoemission spectra of Cu-MOR(6) and Cu-MAZ(4) zeolites, respectively, acquired at 1.0 mbar with an excitation energy hν = 1250 eV. (c, d) Deconvolution of the Cu 2p3/2 photoemission peaks of Cu-MOR(6) and Cu-MAZ(4), respectively.

photoemission signals, normalized to the relative sensitivity factors, show higher relative fractions of copper in the MOR sample. Because the copper weight percentage on the two samples is almost the same, such a difference may be due to the different geometry and pore size of the zeolites as well as their morphology, which affects the measurements by lowering the penetration depth of photoelectrons and thus the relative atomic percentage of detected copper in the Cu-MAZ(4) sample. Another hypothesis to explain such behavior is based on the calculated Si/Al ratios (Figure S1). The Si/Al ratio of MOR stabilizes around 3 after the first reaction with methane, and it oscillates around 4 in MAZ. While in the former case it is lower than expected (the nominal Si/Al ratio of mordenite should be 6), it matches the expected value (4) in MAZ. This suggests that MAZ has a homogeneous composition whereas the MOR sample shows aluminum surface enrichment. Because copper species are coordinated to oxygens bound to aluminum ions in the zeolite framework, the aluminum surface enrichment in Cu-MOR(6) can explain the observed larger amount of copper than on Cu-MAZ(4). 6729

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ACS Catalysis Table 1. Calculated XPS Cu 2p3/2 binding energies

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ACS Catalysis Table 1. continued

deconvolution were fixed throughout the data processing and are reported in Table S1. In the following, monovalent and divalent copper will be named Cu(I) and Cu(II), respectively. The different screening experienced by 2p electrons in Cu(I) and Cu(II) ions leads to a negative shift of kinetic energy of

The Cu 2p signal shows a complex line shape, indicating that different copper species are copresent. To get a more quantitative idea, a deconvolution of the main photoemission peak (Cu 2p3/2) is shown in Figure 1c,d for Cu-MOR(6) and Cu-MAZ(4), respectively. The parameters used during the 6731

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Figure 2. FTIR spectra of absorbed nitrogen monoxide over (a) Cu-MOR(6) and (b) Cu-MAZ(4).

the photoemission peaks.32 This translates into a positive shift of the binding energy (BE) passing from Cu(I) to Cu(II). Conversely, XPS does not allow a direct discrimination between metallic copper, Cu(0), and Cu(I). Because the Auger peak associated with the L3M4,5M4,5 transition is sensitive to the local electronic structure of Cu(0) and Cu(I), the Auger parameter (AP) is used to discriminate between them.33−35 Photoemission spectra in Figure 1c,d show three different peaks, centered at 932.7, 934.2, and 936.3 eV. The former can be assigned to Cu(I), proven by the AP of 1849.5 eV calculated after fully reducing in helium a Cu-MOR(6) sample (Figure S2).35,36 In agreement with other literature reports, the peaks at 934.2 and at 936.3 eV are assigned to Cu(II) ions having different coordination.37−39 A BE of 933.5−934.5 eV is commonly found in cupric oxides.32,37−39 In the case of copper-exchanged zeolites, slightly positively shifted peaks in the 935.5−936.5 eV BE range have been reported,37−40 but their assignment is still a matter of debate. Contarini at al. differentiate between cupric ions with a tetrahedral and octahedral coordination in the supercage of zeolites X and Y.37 They show that photoemission peaks in the 934 eV BE range correspond to tetrahedrally coordinated Cu(II), whereas the octahedrally coordinated Cu(II) ions are found at BE values higher than 936 eV. Sainz-Vidal et al., combining XPS and UV−vis spectroscopy, assign a high BE peak (936.3 eV) to the mono(μ-oxo) dicopper sites that form only after calcination at a high temperature of Cu-MOR.39 In an attempt to estimate the shift of the electron BE as a function of the copper oxidation state and local structure, we carried out theoretical calculations of the electron BE of Cu 2p electrons in various possible active species, following standard procedures of calculating ionization energies from the respective core levels.41 The results are shown in Table 1 and in Table S2. The calculated BE values of copper in different reference bulk compounds (932.3 eV for metallic copper, 932.5 eV for Cu(I) oxide, and 934.2 eV for Cu(II) oxide) are shown in the first three rows and are in good agreement with the literature.36 In the case of Cu-MOR, the simulated BE of mono(μ-oxo) dicopper sites,12 having a formal oxidation state of +2, is 934.3 eV, whereas it shifts to 936.1 eV for bis(μ-oxo) dicopper sites. Such an effect can be explained by the change of the formal oxidation state to +3, although it is not yet clear whether copper really oxidizes or a Cu(II)−

peroxide forms. The tricopper sites show two BE values,23 i.e., 934.8 and 935.9 eV. In this case, copper ions having two different coordination environments coexist. Isolated Cu2+ ions coordinated to the 8-membered ring (8-MR) cage of MOR show a negative shift of the BE (933.5 eV) of 0.8 eV as compared to that of mono(μ-oxo) dicopper sites, due to the absence of extra-framework Cu−O bonds (bridging oxygen in the dicopper species that facilitate partial charge transfer onto oxygen atoms). A significant positive shift (to 935.6 eV) is observed upon formation of copper(II) hydroxide (Cu−OH+) in the 8-MR. The calculated electron BEs for copper species in Cu-MAZ show the same behavior as in MOR. Cu2+ ions in the 6-MR have a BE of 934.1 eV. The value shifts by 1.0 eV in the case of mono(μ-oxo) dicopper sites and further shifts to 935.8 eV after the formation of Cu(OH)+. Bis(μ-oxo) dicopper and tricopper species also exhibit a positive shift of BE compared to Cu2+, at 936.0 eV and 934.7−936.7 eV (copper ions having two different coordination environments), respectively. On both zeolite samples, the simulated BE trend is the following: BE(Cu+ ions) < BE(Cu2+ ions) < BE(Cu(II) bound to extraframework oxygen). Although in the cases of individual Cu2+ ions, mono(μ-oxo)dicopper species, bis(μ-oxo) dicopper species, tricopper species, and Cu−OH+ monomers copper atoms have the same formal oxidation state of +2, the calculated BE value shifts due to the electron withdrawing effect of extra-framework oxygen atoms bound to copper. Crucially, it indicates that various Cu(II) species can exhibit a range of BE values within ≈934−936 eV. In the case of CuMOR the calculated BE values range from 933.5 to 936.1 eV, while in the case of Cu-MAZ the corresponding range is 934.1−936.7 eV. As the magnitude of such an electronic density shift is strongly dependent on the chosen level of theory,42 the assignment of the experimentally observed XPS peaks to individual geometrical configurations of the active sites must be verified by experimental evidence. To get a deeper insight into the structure of the copper sites present in the Cu-MOR and Cu-MAZ samples, we used infrared spectroscopy of adsorbed nitrogen monoxide. The NO+ cation is isoelectronic to carbon monoxide and is extremely sensitive toward the copper oxidation state and the local coordination, revealing different characteristic bands in the spectrum.43,44 Figure 2 represents the IR spectra of CuMOR(6) and Cu-MAZ(4) taken during the gradual adsorption 6732

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Figure 3. Ratios between the different components of the Cu 2p3/2 (934.2, 936.3, sum of 934.2 and 936.3 eV) and the Cu 2p3/2 total area as a function of the reaction conditions (p = 1.0 mbar) on Cu-MOR(6) (a) and on Cu-MAZ(4) (b).

mixture of sites. This is in line with XPS spectra, showing the peaks at 934.2 and 936.3 eV, assigned to Cu(II) species. In an attempt to clearly assign the peaks at ∼934 and ∼936 eV in XPS spectra, we characterized a Cu-MOR(46) sample, the high Si/Al of which enables preferential stabilization of active Cu−OH+ monomers.11 This means that, among all the Cu(II) bound to extra-framework oxygen (Cu−OH+, mono- and bis(μ-oxo) dicopper and tricopper sites), almost exclusively Cu−OH+ is formed (Al site proximity is required to form copper dimers and trimers). Such a sample is active in the MtM reaction, but it shows low methanol production due to the low active copper loading. Because only Cu(II) species bound to extra-framework oxygen are active in the oxidation of methane, in Cu-MOR(46) we would expect a lower relative intensity of the infrared and XPS lines corresponding to Cu(II) bound to extra-framework oxygen compared to CuMOR(6).11,46 The infrared spectra of adsorbed nitrogen monoxide in Figure S4 show intense bands at 1908 cm−1 due to the NO adsorption over cupric monomers, similar to the ones observed previously. Only a small shoulder at 1950 cm−1 is observed, confirming the presence of mostly copper monomeric sites (including Cu−OH+), in this sample, although with a lower relative amount than on Cu-MOR(6). Consistently, the photoemission spectra of Cu 2p (Figure S5) show a sharp peak at 934.2 eV, due to Cu2+ ions coordinated to the zeolite framework, the peak at 932.7 eV assigned to the Cu(I), and a weak peak at 936.3 eV. The fraction of 936.3 eV species evaluated from the fitting of Figure S5b is 10% in CuMOR(46), whereas it is around 55% in the case of CuMOR(6) and Cu-MAZ(4) (Figure 3). This proves that the photoemission peak at 936.3 eV is associated with Cu(II) species bound to extra-framework oxygen active in the MtM reaction. Recent experimental results about the reaction mechanism of MtM on copper-exchanged zeolites prove that the Cu(I)/

of nitrogen monoxide over the oxygen-activated samples. Two groups of bands within the regions 1900−2000 and 1700− 1850 cm−1 can be identified and assigned to Cu(II) and Cu(I) sites, respectively. The latter ones are formed via partial autoreduction of Cu(II) species upon the treatment in vacuum at elevated temperature and are also observed with XPS.45 According to the in situ XAS, autoreduction of Cu(II) in CuMOR(6) starts at about 350 °C and the Cu(I) fraction is about 7% at 400 °C (Figure S3), pointing to the high stability of the Cu(II) species located in the zeolite pores. The adsorption of small doses of nitrogen monoxide onto activated Cu-MOR(6) leads to the appearance of the band at 1804 cm−1 due to the Cu(NO)+ mononitrozyls, while in the case of CuMAZ(4) the positions of this band shift to 1795 cm−1. At higher nitrogen monoxide coverage, bands at 1826 and 1730 cm−1, assigned to asymmetric and symmetric vibrations in Cu(NO)2+ dinitrozyls, develop in both Cu-MOR(6) and CuMAZ(4) spectra. In contrast, the spectral region associated with the vibrations of Cu(NO)2+ mononitrozyls differs significantly depending on the sample studied. In the case of Cu-MOR(6), the most intense band at 1908 cm−1 is accompanied by at least two shoulders at 1950 and 1995 cm−1. The first band was previously assigned to the adsorption of NO over the monomeric Cu(II) sites, such as Cu2+ and Cu−OH+, while the high-frequency shoulders are due to the oligomeric Cu(II)-oxo species, comprising two or more copper atoms.11,46−48 These results are consistent with the data extracted from XPS, showing that at least two types of Cu(II) species are present in Cu-MOR(6) after activation in oxygen. Adsorption of nitrogen monoxide on Cu-MAZ(4) reveals the presence of two distinct bands corresponding to Cu(NO)2+ mononitrozyls. The position of the bands does not allow unambiguous attribution to certain copper−oxo sites; however, in combination with the analysis given for the Cu-MOR(6) sample, one can conclude that, upon activation, copper forms a 6733

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required, after dosing water, to partially reoxidize Cu(I) to Cu(II). This may indicate that energy (in a form of high temperature or/and pressure) is required to promote the reaction between Cu(I) and water. A second reaction cycle was performed after cooling down the samples to 200 °C. Dosing methane leads again to a decrease of the Cu(II) fraction, due to reduction to Cu(I) by the partial oxidation of methane. As observed in the first reaction cycle, the intensity of the 936.3 eV photoemission peak reduces significantly, proving that the methane oxidation happens on such species. In particular, a larger reduction extent is observed in Cu-MOR(6). This may be due to the presence of different relative amounts of active sites in the two zeolites. Indeed, the formation of active mono(μ-oxo) dicopper is well-documented on Cu-MOR, whereas it is still a matter of debate on Cu-MAZ.51 On the basis of the reactivity tests, MAZ is more selective than MOR in the MtM reaction, and this could also explain the lower copper reduction extent (more copper is reduced on MOR to produce CO and CO2).15 Dosing water after the second reaction cycle with methane leads to a slightly positive increase of the Cu(II) fraction on Cu-MOR(6), in tune with the results of the first reaction cycle, whereas no relevant changes are observed on Cu-MAZ(4). This result may also indicate that different relative amounts of active sites display the same BE in the two zeolites. Specifically, as it is known that mono(μ-oxo) dicopper species form preferentially on Cu-MOR(6), XPS suggests that such sites are efficiently regenerated using water as an oxidant and after hightemperature activation.

Cu(II) redox couple is involved, ruling out the participation of Cu(III) in the reaction.19 Our photoemission data, combined with theoretical calculations, suggest that different Cu(II) species coexist after activation in oxygen at 400 °C and yield the Cu 2p BE in the range 934−936 eV. The experimental results suggest that the XPS peak at ∼934 eV corresponds to the Cu2+ ions coordinated to the zeolite framework oxygens, while the peak at ∼936 eV includes mono(μ-oxo) dicopper, bis(μ-oxo) dicopper, tricopper species, and Cu−OH+. This is qualitatively in line with theoretical BE values showing a positive shift for Cu(II) species bound to extra-framework oxygen compared to individual Cu2+ ions. In Situ XPS During MtM Reaction. Figure 3 shows the Cu(II) fraction under different reaction conditions, evaluated from the deconvolution of the Cu 2p3/2 photoemission data of Cu-MOR(6) and Cu-MAZ(4) (Figure 1c,d). The activation of the samples in oxygen at 400 °C leads to the formation of a relevant amount of Cu(II), approximately 88% in the case of Cu-MOR(6) and 72% in the case of Cu-MAZ(4). In both cases, the main contribution to such a percentage comes from the 936.3 eV component, corresponding to Cu(II) species bound to extra-framework oxygen, which are considered the active sites responsible for the MtM reaction by different research groups.10−12,19,25,26 The spectra acquired in the presence of methane display a relevant decrease of the Cu(II) fraction, in good agreement with the reduction of Cu(II) to Cu(I) in concert with the partial oxidation of methane. Such a decrease is mostly due to the disappearance of the 936.3 eV component both on Cu-MOR(6) and on Cu-MAZ(4), and it proves that Cu(II) species bound to extra-framework oxygen are actively involved in the oxidation of methane. Once again, we would like to stress that Cu−OH+ monomers, Cu(II) dimers, and trimers would show the same spectral features. While as a two-electron mechanism19,49 MtM can take place on dimers and trimers, it is not yet clear whether Cu−OH+ monomers are active or just precursors of other species. The results in Figure 3 show that Cu(II) species bound to extraframework oxygen are reacting but do not give any clear indication about their nature. Reactivity data show that the CuMOR(46) sample is oxidizing a small amount of methane, and this may demonstrate that Cu−OH+ is active.11 Therefore, in agreement with recent literature reports, we can speculate that methane can be oxidized on neighboring Cu−OH+ sites.50 Also, the 934.2 eV photoemission peak, assigned to Cu2+ coordinated to the framework of the zeolite, shows small oscillations as a function of the reaction environment. We believe that these sites behave as spectators during the reaction; thus, the oscillations could be simply a result of the deconvolution procedure. Figure 3 shows that dosing water at 200 °C does not significantly change the Cu(II) fraction, although the fraction of 936.3 eV species increases in both samples. Specifically, it increases more in the case of Cu-MAZ(4). As this peak is associated with Cu(II) bound to extra-framework oxygen, we can speculate that the formation of active sites for the MtM using water as an oxidant is favored on copper-exchanged MAZ. The first cycle of the stepwise MtM reaction ends reactivating the samples in helium at 400 °C, and after this treatment a relevant increase of the Cu(II) fraction is observed (from 50% to approximately 60% on both samples), although the percentage of Cu(II) achieved after activation in oxygen is not regained. This behavior suggests that, under the conditions of the XPS measurement, high-temperature activation is

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CONCLUSIONS In summary, we have shown that APXPS measurements, combined with theoretical calculations and IR, are a powerful method to detect, discriminate, and understand the nature of active species formed on copper-exchanged zeolite samples. Thanks to the microporous structure of zeolites, different types of Cu(II) sites can form upon activation of the samples in oxygen, specifically, Cu2+ ions, mono(μ-oxo) dicopper, bis(μoxo) dicopper, tricopper sites, and Cu(OH)+, giving XPS signals within the range 934−936 eV. It was shown that a high binding energy signal is typical of copper species having extralattice oxygen, which leads to the abstraction of electrons from copper atoms and the positive shift of the binding energy. In contrast, the fully charge-balanced copper ions reveal the signal around 934 eV. The evolution of Cu-MOR(6) and CuMAZ(4) samples during the MtM reaction can be followed in situ with photoemission, reproducing the relevant reaction conditions. The results show that mostly Cu(II) sites bound to extra-framework oxygen are reacting to oxidize methane and that water acts as an oxidant, partially restoring the Cu(II) fraction after high-temperature activation. Moreover, after reaction with methane, high BE species formed on CuMOR(6) react to a larger extent than on Cu-MAZ(4). This suggests that different relative amounts of active species form on different supports, thus affecting the selectivity toward the MtM reaction. Our work demonstrates that combining in situ spectroscopic techniques with theoretical calculations is an efficient approach to understand how a sample performs under reaction conditions. Such an approach will be useful to design, tailor, and develop novel actual materials. 6734

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EXPERIMENTAL SECTION Synthesis of Materials. Zeolite MOR (mordenite, CBV10A, SiO2/Al2O3 = 13, in sodium form, and CBV90A, SiO2/Al2O3 = 92, in proton form) was purchased from Zeolyst International. Zeolite MAZ (mazzite, SiO2/Al2O3 = 8.6) was synthesized and cation-exchanged according to the procedure described elsewhere.15 The following procedure was used to ion-exchange for MOR and MAZ: 10 g of zeolite was stirred in 500 mL of 0.05 M solutions of copper(II) nitrate trihydrate (>99%, Sigma-Aldrich) at 323 K overnight. The suspension was then filtered at room temperature and rinsed with 300 mL of deionized water. This procedure was repeated twice. The precursor was dried overnight at 393 K in a drying oven and subsequently calcined in air at 773 K for 4 h with a heating ramp of 5 K min−1. Ambient Pressure X-ray Photoelectron Spectroscopy. APXPS measurements were carried out at the X07DB In Situ Spectroscopy beamline at the Swiss light source synchrotron. The powder samples were dispersed in Millipore water and drop-cast on silver foil (Aldrich, 99.99% purity). The samples were mounted on a manipulator and introduced in the solid− gas interface endstation, which allows precise dosing of gas/gas mixtures under flow conditions.27,52 Ultrapure gases were dosed by means of mass flow controllers and pumped away with a tunable diaphragm valve connected to a root pump. This allows the dosing of relevant gas flows and a precise control of the pressure during the experiments. The pressure, monitored by means of Baratron measurement heads, was stabilized at 1.0 mbar. The samples were heated using a tunable IR laser (976 nm, max power 25 W) hitting the back of the sample, and the temperature was monitored with a Pt100 sensor. Photoemission spectra were acquired with linearly polarized light, using excitation energy of 1250 eV. After alignment of the sample with the photon beam at the focal distance of the analyzer, the sample was investigated acquiring all the photoemission peaks in a sequence while being exposed to a specific gas at a stable temperature. Due to the low intensity of the Cu 2p signal, several acquisitions were necessary, approximately within a 60 min time span. This gave us the possibility to check the reproducibility of the acquired data and to exclude beam damage effects. The peaks were aligned using multiple “standard” reference peaks to correct for charging. In particular, Si 2p (103.4 eV) and C 1s (284.6 eV) were used. After subtraction of a Shirley background, the Cu 2p3/2 peaks were fitted using Voigt shaped peaks. The position, shape, and full width at half-maximum were constrained during the fitting procedure. Infrared Spectroscopy of Adsorbed Nitrogen Monoxide. IR spectra were recorded on a Thermo Nicolet iS50 FTIR spectrometer equipped with DTGS detector at a 4 cm−1 optical resolution and with 128 scans. Prior to the measurements, the samples (20 mg) were pressed in self-supporting discs, placed into the IR cell attached to the vacuum line, and activated in three steps: (1) evacuated at 673 K for 2 h, (2) oxidized in oxygen (300 Torr) at 673 K for 1 h, and (3) additionally evacuated at 673 K for 40 min to remove the traces of water and oxygen. A low-temperature vacuum cell cooled with liquid nitrogen was used for nitrogen monoxide (NO) adsorption measurements. Calibrated aliquots of the gas were introduced into the cell, and the spectra were collected immediately. Pressure was measured by a Pfeiffer gauge. Difference spectra were obtained by the subtraction of the

spectra of the activated samples from the spectra of samples with the adsorbate. The subtraction was performed using the OMNIC 9.3 software package. Density Functional Theory Calculations. All theoretical calculations in this work have been performed with the allelectron full-potential DFT code FHI-aims within the periodic boundary conditions model.53,54 Electronic exchange and correlation were treated on the hybrid functional level with the PBE0 functional.55 All geometry optimizations were done with the “tier2” atom-centered basis set using “tight” settings for numerical integrations. Tkatchenko−Scheffler dispersion correction56 has been used to account for the van der Waals energies arising from the attraction between induced dipoles formed due to charge fluctuations in the interacting species. For comparison, the energetic differences between various Cu(II) species were also systematically recomputed on the quantum mechanical level within the coupled cluster approach using single and double substitutions from the Hartree−Fock determinant (CCSD)57 with a cc-pVDZ functional,58 without ever obtaining any qualitative changes that would conflict with the conclusions deduced from the standard PBE0 calculations (see Table S3). All CCSD calculations were done with the cluster models of the active sites in the corresponding pore of the zeolite, using the Gaussian16 suite.59

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01223. Additional results including ratios calculated from photoemission spectra, fitting parameters, binding energies, and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Luca Artiglia: 0000-0003-4683-6447 Vitaly L. Sushkevich: 0000-0002-3788-8969 Dennis Palagin: 0000-0001-5251-3471 Kanak Roy: 0000-0003-0802-7710 Jeroen A. van Bokhoven: 0000-0002-4166-2284 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.P. acknowledges the use of the computing facilities of the Swiss National Supercomputing Centre (CSCS) within the s878 project, as well as the use of the Merlin5 cluster computer at Paul Scherrer Institute. V.L.S. thanks the ESI platform of Paul Scherrer Institute for financial support.

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