There and Back Again: The Unique Nature of Copper in Ambient

Article pubs.acs.org/JPCC

There and Back Again: The Unique Nature of Copper in Ambient Pressure Dried-Silica Aerogels Tina Kristiansen,*,† Jon A. Støvneng,‡ Mari-Ann Einarsrud,§ David G. Nicholson,† and Karina Mathisen† †

Department of Chemistry, ‡Department of Physics, and §Department of Materials Science and Engineering, The Norwegian University of Science and Technology, 7491 Trondheim, Norway S Supporting Information *

ABSTRACT: We have previously reported the formation of framework single-site copper in silica ambient-pressure-dried (APD) aerogels and xerogels, for which the metal (2−11 wt %) was added during the sol−gel stage (Kristiansen, T.; Mathisen, K.; Einarsrud, M.-A.; Bjørgen, M.; Nicholson, D. G. J. Phys. Chem. C 2011, 115, 19260−19268). We here present a fundamental study on the formation of ultrasmall metal nanoclusters in hydrogen and complete restoration of the initial surroundings in nitric oxide/oxygen, using in situ X-ray absorption spectroscopy (XAS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We coupled density functional theory (DFT) with the Cu−Cu shell multiplicities and distances from extended X-ray absorption fine structure (EXAFS) to investigate the dimensions and structure dynamics of the copper nanoclusters. Their average cluster size did not exceed dimensions above 1.6 nm in either gel system up to 450 °C in hydrogen; however, a significant drop in the first Cu−Cu shell multiplicity suggests a change of morphology and structure. A subsequent treatment in nitric oxide/oxygen resulted in reoxidation to single-site copper(II) species. We believe the driving force for complete redispersion is framework vacancies containing acidic silanol clusters, created during cation removal as confirmed by DRIFTS. This unique reversibility of the copper(II) single sites establishes the silica aerogel and xerogel systems as highly functional supports for ultrasmall clusters and in redox applications.

1. INTRODUCTION Supported metal nanoclusters are desirable in numerous catalytic applications below 5 nm diameter, a size range where performance is highly altered by the effects of quantum size confinement.2−4 However, catalyst deactivation by sintering is a problem commonly encountered under working conditions.5 Eliminating the sintering problem is therefore crucial when considering new systems for heterogeneous catalytic processes. The choice of parent material and method of metal introduction are essential for achieving and stabilizing well-dispersed nanoclusters.6,7 Silica aerogels have been recognized as a novel highly porous carrier material for catalytically active nanospecies.8−11 Commercialization of silica aerogels has been restrained by expensive and hazardous methods of preparation. However, the cost-effective ambientpressure-drying (APD) method is believed to enable commercialization in the near future.12,13 The APD method conveniently yields hydrophobic inner surfaces, which can be coupled with incorporation of metal cations by addition during the sol−gel stage.1 Aerogels possess the flexibility and capacity to stabilize high amounts of single-site metal cations, which is regarded as a prerequisite to form ultrasmall metal particles.6 They also contain both micro- and mesopores, which raises the possibility of size-selective preparation of metal nanoclusters and stabilization induced by the porous gel system (Figure 1).14 Nanoscale (here 2−4 nm) copper catalysts are reported to show enhanced catalytic activity when compared to conventional copper catalysts consisting of larger particles. Lately, © 2012 American Chemical Society

Figure 1. Illustration of the silica aerogel structure containing mesopores and micropores and copper nanoclusters (in red).

copper nanoclusters have been introduced as a cost-effective alternative to noble metals in many industrial hydrogenation processes.15−17 Single-site copper(II), when incorporated into the silica aerogel framework, is a potential candidate for redox catalytic applications, such as CO oxidation and selective Received: June 6, 2012 Revised: August 21, 2012 Published: August 22, 2012 20368

dx.doi.org/10.1021/jp305549j | J. Phys. Chem. C 2012, 116, 20368−20379

The Journal of Physical Chemistry C

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Al2O3 sample pan and heated to 600 °C using a 3 °C/min heating rate, in hydrogen (5%) and argon with a flow rate of 20 mL/min. DRIFTS was carried out using a Bruker Vertex 80 with a LNMCT detector and a high temperature cell from Pike Technologies. The samples (5−15 mg) were loaded into a ceramic cup and heated to 450 °C using a 3 °C/min heating rate in hydrogen (5%) and helium with a flow rate of 15 mL/ min. Data were collected in situ in the range 4000−800 cm−1 with a 12 s delay between measurements. The sample scan time was 8 s, and the background scan time was 64 s. The spectra were normalized with respect to mass. 2.2. X-ray Absorption Spectroscopy. 2.2.1. Data Collection. Copper K-edge XAS data were collected at MaxLab, Sweden, in the transmission mode on beamline l811 at the Max-II ring. Max II offers an electron beam energy of 1.5 GeV and X-rays in the energy range 2.4−12 keV. These are extracted by a standard optical scheme consisting of a vertical collimating first mirror, double-crystal monochromator, and a second vertically focusing mirror. A multipole wiggler insertion device is used in this beamline to overcome the low energies of the electron beam.30 XAS data were also collected at the Swiss− Norwegian Beamlines (SNBL; BM01B) at ESRF, Grenoble, in the transmission mode. The white beam is collected from the storage ring by a bending magnet to the SNBL. The beamline is equipped with a channel cut Si(111) monochromator and double-crystal monochromator to select the desired wavelength. The spectral range is 4−70 keV. The initial and transmitted intensities I0 and It (31 cm) were detected by ion chamber detectors with lengths of 17 and 31 cm, respectively. The gas compositions were 80% N2 + 20% Ar and 75% N2 + 25% Ar. The ESRF provides an electron beam energy of 6 GeV and a maximum current of 200 mA.31 The in situ XAS data were collected using a stainless steel cell uniformly heated by two cartridges where temperature was measured by a thermocouple. The samples were ground and loaded in the 1.5 cm thick cell and were kept in place by glass wool on each side, thereby allowing gas to flow freely through the sample and cell. The cell was sealed with graphite windows of 0.2 mm thickness. The samples were heated to 450 °C using a ramp rate of 3 °C/min in 5% H2/helium (15 mL/min) during measurement of X-ray absorption near edge structure (XANES). The EXAFS scans were collected at selected temperatures. The samples were then cooled to room temperature in a NO/O2 mixture (0.4%/0.667%) with a total flow of 15 mL/min while changes were monitored by measuring XANES. EXAFS was collected at the end point. 2.2.2. Data Reduction. The XAS data were summed, normalized, and energy corrected relative to the metal foil (Cu foil K-edge = 8979 eV) using Athena, a program in the IFEFITT package.32 The K-edge absorption energy E0 on the XAS spectra of unknown samples was consistently positioned halfway up the absorption edge jump. XANES and EXAFS scans were normalized from 27 to 48 eV above the edge and from 150 to 800 eV above the edge, respectively. The data were carefully deglitched and truncated at the end of EXAFS scans when needed. The smooth background μ0(E) was checked and corrected to achieve the maximum overlap with total absorption μ(E). XANES is invaluable for distinguishing between the valence states of transition metals and for giving information on the electronic states and changes in the local environments. XANES spectra of copper valence states are characteristic;

catalytic reduction of NOx in the presence of a hydrocarbon reductant (HC-SCR-DeNOx). It is assumed that the single-site configuration is a prerequisite to stabilize copper(I)/copper(II) redox pair during HC-SCR-DeNOx.18,19 Over-reduction to metallic state by the feed reductant is still a challenge. Once metallic, reoxidation often results in irreversible formation of bulk oxides rather than redispersing to initial isolated surroundings.20,21 The ability of copper to redisperse, and thus restore, the copper(I)/copper(II) redox pair is believed to depend on the cluster size and interaction with the carrier material.22,23 X-ray absorption spectroscopy (XAS) is the technique of choice for probing the local surroundings of a transition metal in amorphous carrier materials during catalytic reaction conditions. XAS is an element-specific local probe which offers valuable information about the target element and its local chemical environment, such as valence state, local symmetry, multiplicities (coordination numbers), and shell distances. Long-range order is not a prerequisite, making XAS an invaluable technique not only to our systems, but to most heterogeneous catalysts and other amorphous materials as well. Within XAS, extended X-ray absorption fine structure (EXAFS) is considered to be a vital tool for determining cluster sizes down to the subnanometer range, and not least also for detecting morphological changes.24,25 The average cluster size can be calculated on the basis of the obtained nearest-shell multiplicity and shell distances.26−28 The cumulant expansion method can be applied within EXAFS to correct for static disorder caused by the reaction conditions. This size range is beyond the limits of X-ray diffraction (XRD) and in many cases also transmission electron microscopy (TEM).29 We report here a coupled in situ XAS, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) study on the formation and temperature-dependent growth of metallic copper nanoclusters from single-site copper(II) species in silica aerogel and xerogel systems. The redox properties of copper in these systems were investigated by subsequent treatment in a nitric oxide/oxygen rich mixture, by capturing the reoxidation of the metallic copper nanoclusters, and by correlating the speciation with the aerogel and xerogel structure.

2. EXPERIMENTAL SECTION 2.1. Preparation, Temperature-Programmed Reduction (TPR), and DRIFTS. The silica aerogels were prepared by the APD coprecursor method by adding the silylation agents HMDSO (hexamethyldisiloxane, Fluka) and HMDS (hexamethyldisilazane, Fluka) into silicic acid (8 wt %) which contained appropriate amounts of copper(II) nitrate (Cu(NO3)2·2.5H2O, Merck) for aerogels and copper(II) acetate (Cu(CH3COOH)2·H2O, Merck) for the xerogel. The preparation and characteristics in detail are given in a previous report.1 In the present paper, two aerogels containing 1.8 and 10.9 wt % copper were chosen for study, in addition to a single xerogel with 11.3 wt % copper. The samples are denoted Cuag-2, Cuag11, and Cuxg-11, respectively. Plain gel analogues were also included as references for the in situ DRIFTS studies. Temperature-programmed reduction (TPR) accompanied by thermogravimetric analysis−differential scanning calorimetry (TGA−DSC) was carried out using a Perkin-Elmer thermogravimetric analyzer (TGA7) combined with a Jupiter STA 449C connected to a QMS 403C Aëlolos mass spectrometer from Netzsch. The samples (5−15 mg) were loaded into an 20369



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hence keeping the EXAFS analysis more exact. The Debye− Waller factor 2σ2 and NCu−Cu were not affected. Hence, keeping these parameters constant appeared to result in approximations well within the uncertainties. The best-fit C3 value to the refined RCu−Cu was used while introducing C4. Using the same process, the goodness of fit was mapped by varying C4 while refining the nearest shell multiplicity NCu−Cu. The highly correlated 2σ2 was kept constant in this part of the analysis to minimize the amount of free parameters. 2σ2 was not affected significantly by the variation of C4 and refined NCu−Cu, checked by frequent test refinements. The parameter validity of introducing higher order cumulants C3 and C4 to the analysis was justified by calculating the reduced chi squared (χ2) in EXCURV98. Conventional least-squares refinements were carried out on reoxidized copper gels at room temperature as follows: the nearest absorber−backscatterer pair was introduced first and refined with correlation until stable fits were obtained. The next-nearest and also a third absorber−backscatterer shell were introduced, following the same procedure. To verify the type of backscattering atoms in the third shell, Fourier filtering was used to isolate/extract the contribution from the third shell from 2.8 to 3.2 Å in R space. It was transformed back to k space and refined with the following absorber−backscatterer pairs Cu···Cu, Cu···Si, and Cu···O, where the best quality of fit and k value of the amplitude maximum in the k3·χexp(k) revealed the most probable backscattering element. 2.3. Computational Methods. 2.3.1. Density Functional Theory. Density functional calculations were performed with the ADF/BAND program, release 2012.39 The Kohn−Sham equations were solved within the generalized gradient approximation, using Vosko, Wilk, and Nusair’s parametrization of the correlation energy within the local density approximation40 and the gradient corrections to the exchange and correlation energy proposed by Perdew et al.41 A triple-zeta (ζ) basis set with two polarization functions (TZ2P) was used to describe the copper valence electrons, whereas a double-ζ basis set was applied to the frozen core which includes the 1s, 2s, and 2p orbitals. The overall integration accuracy was set to 6.0. For the BAND calculations for the body-centered-cubic (bcc) and face-centered-cubic (fcc) copper crystals, a KSPACE value of 5 was chosen. The nearly metallic behavior of the fairly large copper clusters (80 atoms) implies a small HOMO−LUMO energy gap and consequently certain challenges concerning convergence of the self-consistent field (SCF). The augmented Roothaan−Hall method (ARH), recently implemented in ADF, was used in the most problematic cases.42 2.3.2. EXAFS Modeling. The modulation of the absorption coefficient, χ(k), was calculated for each of the DFT optimized Cu80 clusters, using backscattering amplitude and phase shift data from McKale et al. for values of k between 2 and 20 Å−1.43 For each atom in a cluster, the contribution from atoms within a radius of 5 Å was included, and a Debye−Waller factor 2σ2 = 0.015 Å2 and the step size 0.1 Å−1 were used. The resulting χ(k) was Fourier transformed after weighting with k3.

hence, the XANES of relevant reference copper compounds can be used to identify and quantify mixtures by the method leastsquares linear combination.33−35 The reference compounds copper(II) Tutton (Cu(NH4)2(SO4)2·6H2O), copper(I) oxide, and copper foil were used to approximate the amplitude reduction parameter (AFAC) of the unknown samples. The valence state of the major component from XANES least-squares linear combination (LC) fitting in Athena dictated the choice of reference to estimate the value of AFAC. LC was also used to identify the copper species present during the course of reduction and reoxidation. As a divalent reference the fully oxidized asprepared copper gels were also included in LC for each sample, while copper(I) oxide and copper foil were chosen as monovalent and zerovalent references, respectively. The data were fitted from −20 eV below the edge and 30 eV above the edge. The maximum number of standards was set to three, and the most relevant were chosen on the basis of the statistical goodness of fit. This fit parameter was reported for each fit procedure by the statistical R-factor, defined as N

R=

∑[ i

1 exp (|χ (k) − χi th (k)|]·100% σi i

(1)

2.2.3. EXAFS Least-Squares Refinements. EXAFS leastsquares refinements were carried out using EXCURV98, which conducts the curve fitting of the theoretical χth(k) to the experimental χexp(k) using the curved wave theory.36 The calculation of ab initio phase shifts for the expected neighboring elements also took place in EXCURV98. The least-squares refinements were carried out in the wavenumber k range 2−13 Å−1 using a k3 weighting scheme. In order to obtain a value for AFAC for the reference compounds, the refinements were carried out keeping shell multiplicities N fixed at known values, while shell distances R, the correction parameter to E0 (EF), and the Debye−Waller factor 2σ2 were refined. During this process the value of AFAC was set to 1. Finally, AFAC was refined and transferred to the EXAFS data of the unknown samples. Note that, for all reported parameters, errors quoted on refined interatomic distances are statistical and the true experimental errors on refined distances are ±0.01 Å. For the Debye−Waller factor the true error on this parameter is ±10%. For the data collected in situ, EXAFS analyses were limited to the nearest Cu−Cu shell throughout the refinements. This is due to increasing uncertainties on going from the next-nearest shell to those beyond. Refinements were limited to a single parameter, while the rest were kept constant. The in situ EXAFS data were analyzed stepwise: first with conventional least-squares refinements based on a Gaussian pair distribution function (PDF). Subsequently, the cumulant expansion method was applied in order to test whether high static disorder makes the model-independent analysis necessary.37,38 The cumulant expansion method was applied within EXCURV98 by introducing higher order cumulants to the best-fit parameters from conventional refinements. All parameters, except Cu−Cu shell distances RCu−Cu, were kept constant while a third cumulant C3 was introduced (the fourth cumulant C4 was set to zero). The goodness of fit was mapped by varying C3 during the refinements of RCu−Cu. EF was hardly affected by the small changes in C3 and the refined RCu−Cu, checked frequently by carrying out test refinements of EF during the process. Although EF is highly correlated with RCu−Cu, this is an accepted approximation to make the number of free parameters low,

3. RESULTS 3.1. Temperature-Programmed Reduction (TPR). TPR measurements, while running thermogravimetric analysis (TGA) in hydrogen rich flow, are shown in the Supporting Information, where plain gel analogues are included as references. DSC and online mass spectrometry (MS) of hydrogen and water are shown in Figure 2. 20370



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Figure 2. (left) DSC during H2-TPR to 600 °C. (right) Water and hydrogen measured by online MS. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2.

The reduction of copper in Cuag-11, seen by hydrogen consumption, takes place within the temperature range 230− 270 °C. DSC shows two maxima at 240 and 260 °C, which means that the events are exothermic. Reduction of copper in this temperature range has been assigned to isolated siloxycoordinated copper species in mesoporous silica analogues.44 Water and ammonia, present as extraframework species, desorb exclusively from the copper-containing gels prior to and during reduction.1 Copper species in Cuxg-11 are reduced in the range 200−280 °C. In Cuag-2, hydrogen appears to be consumed during two temperature ranges, namely, 240−300 and 280−440 °C. This is consistent with stepwise reduction. 3.2. In Situ XAS. 3.2.1. In Situ XANES: Reduction in Hydrogen. Selected XANES and the first derivatives of the spectra collected in situ during heating to 450 °C in hydrogen are shown in Figure 3. Copper foil and copper(I) oxide are included here as references for the respective valence states. Results of the least-squares linear combination (LC) of XANES are shown in Figure 4, using the as-prepared sample, copper(I) oxide, and copper foil as divalent, monovalent, and zerovalent references, respectively. Figure 5 shows XANES of the copper gels in metallic state together with the first derivatives of the spectra collected at 300, 350, and 450 °C, while the spectrum of copper foil is shown for comparison. The copper species in Cuag-11 reduced directly from divalent to the metallic state in the range 235−300 °C, confirmed by the edge position. The presence of copper(I) species by its characteristic pre-edge 1s−4p (4s) was not captured within the time frame of the XANES scans.33,35 The reduction of copper in Cuxg-11 proceeds via copper(I) to the metallic state in the range 235−325 °C, illustrated by LC. Similarly, for Cuag-2, the monovalent state had a significant presence in the temperature range 300−400 °C, and reduced further to the metallic state upon heating to 400−450 °C. The complete reduction to metallic state in the range 300−325 °C for high loading, as opposed to 450 °C for low loading, reflects the close vicinity of the isolated copper cations in the 11 wt % loaded gels vs highly isolated copper cations in Cuag-2.45−47

Figure 3. Selected normalized XANES and the first derivatives in H2. The copper foil and copper(I) oxide are also included as reference compounds. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2.

These highly isolated copper species can also explain the stabilization of copper(I) over the wide temperature range (300−450 °C) observed for this sample. 3.3. Metallic Copper Nanoclusters: Size and Dynamics. 3.3.1. EXAFS Least-Squares Refinements with the Cumulant Expansion Method. Higher order cumulants are introduced into the EXAFS equation in order to address nonharmonic contributions to the PDF, thereby adjusting initially under- or overestimated distances and multiplicities. Since the goodness of fit of χth(k) to χexp(k) is significantly improved using higher order cumulants, the static disorder is an important factor. The given standard errors to the refinements were also reduced, accompanied by the improving quality of fit. A detailed description of the results and figures of the EXAFS least-squares refinements using the cumulant expansion method and conventional refinement results is available in the Supporting Information. The results from the extensive least-squares refinement procedures on the first Cu−Cu shell, using the cumulants C3 and C4, are listed in Table 1. The nearest Cu−Cu shell distances and multiplicities as a function of temperature are shown in Figure 6, which essentially describes the results given below. For Cuag-11 at 300 °C 8.0 Cu−Cu distances are found at 2.56 Å, which appear to be fairly expanded relative to those in bulk metallic copper (2.55 Å). The best fit for C3 at 7 × 10−3 Å3 shows that the conventionally derived distances are under20371



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higher static disorder here. We note that after heating to 350 °C we find a significant drop to 6.2 shell distances, which contracted to 2.53 Å. Whereas C4 drops to −4.5 × 10−4 Å4, indicating a further broadening of the PDF, C3 decreases to 8 × 10−3 Å3. Finally, at 450 °C, we obtain 5.7 shell distances at 2.53 Å. Accompanying the drop in multiplicity and shell distances, a consistent increase in C3 is found at 1 × 10−2 Å3, while C4 also consistently decreases to −4 × 10−5 Å4. For Cuag-2 the goodness of fit was not greatly improved as it decreased slightly from 40.2 to 39.7% when introducing appropriate C3 and C4 for EXAFS collected at 450 °C. The low fit improvement questions the necessity of C3 and C4 in this sample, due to a low degree of static disorder and noisy data. The increase of the reduced χ2 does not justify the introduction of additional two new parameters; however, the presence of static disorder cannot be ruled out for this sample, either. From conventional least-squares refinements we obtained 8.0 Cu−Cu shell distances at 2.534 Å. 3.3.2. Density Functional Theory and EXAFS Modeling. The EXAFS experiments suggest the formation of copper nanoclusters with 75−80 atoms. In an attempt to address questions related to particle shape and underlying crystal symmetry, various Cu80 clusters were optimized with DFT calculations. Initial geometries were constructed by cutting bonds along various planes of both fcc and bcc Cu crystals. In Table 2, binding energies per atom as well as cluster dimensions are reported for five such clusters. Corresponding results for bulk fcc and bcc copper crystals have been included for comparison. On average, the binding energy per atom is only about 0.6 eV lower in the Cu80 clusters than in the bulk crystals. The most stable of the optimized Cu80 clusters is a triangular pyramid with Td symmetry, an fcc-like structure, and a diameter of about 1.1 nm. Only marginally less stable is the somewhat flattened hexagonal bipyramid with D2h symmetry, a bcc-like structure, with a height of 0.7 nm and a diameter of about 1.6 nm. A two-layer fcc-based disk with C2h symmetry (diameter 1.6 nm) and two bcc-based rods have slightly higher energies, but 33−69 meV per atom could be compensated by a stabilizing environment. For the discussion of Cu−Cu shell multiplicities and the EXAFS results, we will focus on the two fcc-based clusters, with Td and C2h symmetry, as well as the bccbased cluster with D2h symmetry, shown in Figure 7. For these three clusters, the Cu−Cu shell distance distributions are shown in Figure 8. The Cu−Cu distance distribution is much broader for the bcc D2h than for the fcc Td and C2h clusters. Clearly, these distance distributions prevent a unique theoretical definition of the first Cu−Cu shell multiplicity, for the comparison with EXAFS results. This is illustrated by plotting the multiplicities as a function of an upper radial distance Rmax (Figure 8). Clear trends are seen for the average multiplicities, where Td (fcc) increases earlier than C2h (fcc), followed by D2h (bcc), as expected. Comparing the k3·χDFT(k) to k3·χexp(k) of Cuxg-11 and Cuag-11 is not straightforward (available in the Supporting Information). However, the Fourier transform of the experimental EXAFS spectra of Cuag-11 and Cuxg-11 at 300, 350, and 450 °C and theoretical EXAFS for the low-energy Cu80 clusters are shown in Figure 9. The Fourier transformed k3·χDFT(k) EXAFS spectra possess a significant loss of amplitude in first shell and third shell for the bcc D2h cluster, which is in response to the lower Cu−Cu multiplicities than those of the fcc-like clusters. The broad distribution of shell distances seen for the bcc-like D2h cluster is consistent with the broadening of the PDF (the Debye−Waller

Figure 4. Least-squares linear combination of XANES during heating in H2 to 450 °C and cooling to 25 °C in NO/O2.

Figure 5. Normalized XANES and the first derivatives at 300, 350, and 450 °C in H2. Copper foil is included as a reference. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2.

estimated, as is the attendant multiplicity. The latter was adjusted by a C4 value of −1.5 × 10−4 Å4. At 350 °C the Cu− Cu shell multiplicity drops significantly to 6.9. This is accompanied by a decrease in C4 to −2 × 10−4 Å4, which is due to a larger broadening of the PDF. In contrast, the shell distances and C3 are unaffected. At 450 °C we find the 6.9 shell distances contracting to 2.55 Å, and the accompanying C3 increases to 1 × 10−2 Å3. The multiplicity and C4 are both stable during increasing temperature. Compared to the conventional analysis these results follow the same trends, just shifted by C3 and C4. For Cuxg-11 at 300 °C the best fit was also obtained by 8.0 Cu−Cu shell distances at 2.56 Å. Similarly, C4 is −1.5 × 10−4 Å4 but accompanied by C3 at 1 × 10−2 Å3, implying a somewhat 20372



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Table 1. Least-Squares Refinements of EXAFS at 300, 350, and 450 °C in H2 Using Higher Order Cumulants C3 and C4a T (°C)

sample b

300

Cuag-11 Cuxg-11b Cuag-11b Cuxg-11b Cuag-11b Cuxg-11b Cuag-2c

350 450

RCu−Cu (Å)

NCu−Cu

C3·10−3 (Å3)

C4·10−4 (Å4)

R (%)/FI

2.560(3) 2.559(3) 2.561(3) 2.534(3) 2.554(3) 2.539(3) 2.552(4)

8.0(3) 8.0(3) 6.9(2) 6.2(2) 6.9(2) 5.7(2) 8.0(4)

7 10 7 8 10 10 4

−1.5 −2.0 −2.0 −4.5 −2.0 −4.0 −1.5

41.74/0.00135 30.21/0.00111 36.63/0.00104 36.70/0.00121 38.27/0.00121 39.11/0.00126 39.71/0.00129

a The least-squares refinements were carried out in wavenumber Δk = 2−13 Å−1 using a k3 weighting scheme. Refined parameters: nearest Cu−Cu shell distances RCu−Cu in Å and nearest Cu−Cu shell multiplicities NCu−Cu. The Debye−Waller factor 2σ2 and EF (E0 correction, eV) were fixed and transferred from conventional least-squares refinements available in the Supporting Information. The goodness of fit R (%) and fit index (FI) are given. The standard deviation in the last significant digit as calculated by EXCURV98 is given in parentheses. The parameter validity of introducing higher order cumulants C3 and C4 to the analysis were justified by calculating the reduced χ2 in EXCURV98. bAFAC = 0.951 (copper foil). cAFAC = 0.939 (copper foil).

Figure 7. Model Cu80 clusters, optimized with DFT: fcc-like Td and C2h and bcc-like D2h.

Figure 6. Average nearest Cu−Cu shell multiplicities NCu−Cu and distances RCu−Cu in H2 from EXAFS least-squares refinements. Cuag11 (green); Cuxg-11 (red); Cuag-2 (blue).

Table 2. DFT Results for Cu80 Clusters and Bulk fcc and bcc Crystals crystal structure

symmetry

binding energy per atom (eV)

fcc

Td

2.936

pyramid

fcc

C2h

2.867

two layer disk

bcc

D2h

2.933

bcc

Ci

2.902

bcc

C1

2.882

deformed hexagonal bipyramid rod (“3 × 4 × 7 − 4”) rod (“3 × 3 × 9 − 1”)

fcc bcc

fcc bcc

3.507 3.530

morphology

height, width, length (nm)

Ns/Nba

1.05, 1.09, 1.26 0.23, 1.49, 1.60 0.71, 1.23, 1.66

7

0.41, 0.62, 2.07 0.49, 0.72, 2.21

9

Nb = 0 5.67

9

Figure 8. Radial distributions of Cu−Cu multiplicity (three histograms), and average first shell multiplicity as a function of Rmax (lower right diagram), for DFT optimized Cu80 clusters with fcc structure (Td and C2h) and bcc structure (D2h).

a

Ns/Nb is the ratio between the number of interior (bulk) and surface atoms.

factor here is at 0.015 Å2). There is a clear shift to a lower average shell distance for the bcc-like D2h cluster, in comparison to the fcc-like clusters, which are centered at 2.55 Å in consistency with bulk copper metal. 3.4. In Situ XANES: Reoxidation in Nitric Oxide/ Oxygen. Figure 10 shows a selection of normalized XANES collected in situ during cooling in a nitric oxide/oxygen rich flow. The least-squares linear combination of XANES illustrates the approximate mixture of valence states in Figure 4, using the

as-prepared sample, copper(I) oxide, and copper foil as divalent, monovalent, and zerovalent references, respectively. Cuag-11 reoxidized to copper(II) by nitric oxide/oxygen rich feed, and coexisted with copper(I) over a wide temperature range. Reoxidation is complete at room temperature. In contrast, Cuxg-11 is directly reoxidized from metallic to copper(II) immediately after the switch to nitric oxide/oxygen 20373



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partly a result of the Jahn−Teller effect, which is the cause of the well-known tetragonal distortion of the octahedral local geometry of copper(II) compounds. It is also believed to be partly a response to the degree of covalency of equatorial interactions.49 Cuag-2 immediately reoxidizes partly to copper(I) and copper(II) on switching to the nitric oxide/oxygen mixture. During cooling, the reoxidation process ceases, and copper(I) and copper(II) species coexist at room temperature, consistent with the 1s−4p (4s) pre-edge characteristic for copper(I). LC fitting indicates that this sample consists approximately of species containing 45% copper(II) and 55% copper(I). 3.5. EXAFS: Identifying Reoxidized Copper Species. Figure 11 shows the quality of fit of the k3 weighted χth(k) to Figure 9. Fourier transforms of experimental EXAFS k3·χexp(k) at 300−450 °C in H2. (a) Cuag-11; (b) Cuxg-11; (c) theoretically generated EXAFS k3·χDFT(k) of Cu80 with fcc (Td and C2h) and bcc (D2h), optimized with DFT.

Figure 11. Experimental () and least-squares refined (---) EXAFS k3·χ(k) and Fourier transforms of as-prepared (red) and reoxidized (blue) copper gels. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2; (d) copper(II) oxide; (e) copper Tutton.

the χexp(k) and the Fourier transformed spectra of the asprepared and reoxidized copper gels. The divalent reference compounds copper(II) oxide and copper Tutton salt are included for comparison. The results from EXAFS least-squares refinements are listed in Table 3, which also includes the data for the as-prepared gels. Copper species in the 11 wt % loaded gels are divalent and have tetragonally distorted octahedral environments in which oxygen is the nearest and next-nearest neighbors. The two Cu− O shell distances range from 1.94 to 1.98 Å and from 2.27 to 2.35 Å. The best fit NCu−O of the oxygen shells were approximately 4 + 2 in Cuxg-11 and 4 in Cuag-11. The nextnearest NCu−O could not be determined in Cuag-11 due to unstable fit, which can be due to antiphase behavior in the low k-regime.50 In Cuag-2 the best fit NCu−O were 3.5 + 1.4, while RCu−O1 was 1.93 Å, assigned to the coexistence of copper(II) and copper(I) environments.19,20 Refinements of the third shell

Figure 10. Normalized XANES (left) and first derivatives (right) during cooling in NO/O2. Copper foil and copper(I) oxide are included as references. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2.

prior to cooling. The presence of copper(I) during reoxidation cannot be ruled out; however, within the time frame of the XANES scan it was not captured. The XANES features of the reoxidized copper(II) species in the 11 wt % loaded gels indicate a chemical environment similar to the as-prepared 4 + 2 tetragonally distorted octahedral environment of two coordinated oxygen shells. The shoulder feature at 8985− 8986 eV, characteristic for Cu(II) compounds, is present for all copper gels.48 The intensity of the shoulder is believed to be 20374



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Table 3. EXAFS Least-Squares Refinements of the Samples in the As-Prepared and Reoxidized Statea R (Å) sample Cuag-11

b

Cuxg-11b

Cuag-2c

2σ2 (Å2·10−2)

N

EF (eV)/R (%)

shell

as-prep

reoxid

as-prep

reoxid

as-prep

reoxid

as-prep

reoxid

Cu−O Cu−O Cu···Sid Cu−O Cu−O Cu···Sid Cu···Sid Cu−O Cu−O Cu···Sid Cu···Sid

2.004(5) 2.32(1) 3.195(7) 2.019(6) 2.31(1) 2.94(4) 3.21(4) 2.004(5) 2.33(1) 2.93(3) 3.33(2)

1.981(5) 2.283(2) 3.165(1) 1.990(4) 2.32(2) 3.17(1)

4.2(2) 2.6(4) 1.9(3) 3.9(2) 2.2(6) 0.7(4) − 4.0(2) 3.3(5) 4.3(6) 4.3(1)

4.2(2) − 3.0(3) 4.1(1) 2.4(4) 2.8(4)

1.5(2) 2.3(1) 0.9(4) 1.0(2) 4.2(9) 0.6(1) − 1.0(1) 1.8(3) 3.1(4) 0.4(1)

1.4(1) 3.2(1) 1.4(3) 1.2(1) 3.4(9) 1.3(3)

−4.2(3) 33.4

−5.8(3) 31.65

−6.5(4) 37.1

−6.2(2) 26.84

1.7(2) 4 (1) 2.0(8)

−6.1(3) 26.3

−5.5(4) 31.56

1.938(6) 2.304(3) 3.179(1)

3.4(2) 2.3(4) 4.2(5)

The least-squares refinements were carried out in wavenumber Δk = 2−13 Å−1 using a k3 weighting scheme. The refined parameters are shell elements, shell distances R in Å, EF (energy correction of E0) in eV, multiplicity N, Debye−Waller factor 2σ2 in Å2, and goodness of fit R in percent. The standard deviation in the last significant digit as calculated by EXCURV98 is given in parentheses. Errors quoted on refined interatomic distances are statistical, and the true experimental errors on refined distances are ±0.01 Å. For the Debye−Waller factor the true error on this parameter is ±10%. bAFAC = 0.919 (copper Tutton). cAFAC = 0.979 (copper Tutton). dVerified from Fourier filtering. a

(around 3 Å) defining copper, silicon, and oxygen as the third shell contributors clearly indicated that silicon is the third shell element, by a physically meaningful result and a stable fit. The 2.8−4.2 Cu···Si shell distances range from 3.16 to 3.18 Å, accompanied by sufficiently low 2σ2 below 0.02 Å2. Fourier filtering was used in the range 2.8−3.2 Å to validate silicon as the third shell contributor. The proposed backscatterer pair Cu···Si was therefore fitted by least-squares refinements and compared to the goodness of fit obtained for Cu···O and Cu···Cu. Figure 12 shows the contribution to the total χ(k) for the third shell from 2.8 to 3.2 Å for the copper gels, compared to the theoretical contributions from silicon, oxygen, and copper for multiplicity at 4 and 2σ2 = 0.015 Å2. The resulting shapes of the filtered χexp(k) are not in accordance with copper being the backscattering element, because the amplitude maxima are positioned at approximately 4 Å−1. This attributes

the third shell backscattering element to silicon in all samples. Judging from these results, copper has redispersed to its initial isolated framework surroundings coordinated with siloxy end groups. 3.6. In Situ DRIFTS: Reduction in Hydrogen and Reoxidation in Nitric Oxide/Oxygen. Figure 13 shows, in detail, the spectral regions 3800−3500 and 3200−2900 cm−1 covering the infrared υOH, υNH, and υCH modes at selected

Figure 12. Fourier filtered experimental and theoretical EXAFS of the shell with Δr = 2.8−3.2 Å. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2.

Figure 13. Selected DRIFTS spectra collected during heating in H2 and cooling in NO/O2. (a) Cuag-11; (b) Cuxg-11; (c) Cuag-2. 20375



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Figure 14. Schematic illustration of the proposed course of reduction in H2 (a−d), change of morphology, and the subsequent reoxidation/ redispersion of copper to the initial surroundings in the aerogel in NO/O2 (e−h).

4. DISCUSSION Fundamental studies on silica aerogels containing single-site guest cations incorporated for catalytic purposes have not been previously reported. Whereas in situ XAS probes the metal species present during the different stages of reaction, in situ DRIFTS targets the surface interactions between the metal and the parent material on a molecular level. Figure 14 illustrates the proposed course of reduction (Figure 14a−d) and reoxidation (Figure 14e−h) of the copper guest atoms incorporated into the silica aerogel surroundings. The formation of metal nanoclusters commences around 200−250 °C as confirmed by the LC of XANES and TPR (Figure 4). The removal of copper cations from the gel framework creates vacancies consisting of negatively charged siloxy groups, which are neutralized by proton spillover from the reduction (Figure 14b). Copper is initially bonded to two to four siloxy/silanol groups (Table 3); hence these are in close proximity and can form at least three-membered mutually hydrogen-bonded hydroxyls, also known as silanol clusters.51 In situ DRIFTS of the copper-containing gels confirms the formation of trisilanols (3180 cm−1) and tetrasilanols (3140 cm−1) which appear during reduction (Figure 13).51 The absorption bands attributed to silanol clusters were not present for the plain gel analogues (not shown), confirming origination from guest cation removal. These silanol clusters interact strongly through poly hydrogen bonding, which leads to enhanced Brønsted acidity relative to monobonded geminal, vicinal, or free silanols.51,53 We observe a dynamic interaction between the single-site guest cation and the silica gel host systems in hydrogen, shown by complete copper reduction at 300−325 °C for high loadings and 450 °C for low loading. The EXAFS and XANES features are inconsistent regarding the degree of reduction to the metal. Whereas LC of XANES indicates incomplete reduction, both the edge position and EXAFS reflect complete reduction to the metallic state. However, care must be taken when comparing XANES features to those of bulk metal for ultrasmall metallic nanoclusters (below 2 nm), due to a lack of multiple scattering. This originates from a longer photoelectron mean free path in

temperatures during the reduction in hydrogen and subsequent reoxidation in nitric oxide/oxygen. An overview of the spectral region 3800−2900 cm−1 is available in the Supporting Information. At low temperatures (25−150 °C) the presence of extraframework species (water/ammonia) is seen exclusively on the copper gel surfaces in the range 3700−3280 cm−1.1 All the aerogels exhibit fairly intense bands at 2963 and 2900 cm−1 which are assigned to symmetric and asymmetric υ modes of CH3, respectively. The extraframework species and water desorb upon the reduction of copper in the range 200−250 °C, and the absorption band in the range 3715−3700 cm−1 which is assigned to free silanols becomes more intense and distinct. These groups are weakly acidic and may interact with a variety of adsorbed species through hydrogen bonds. A broad band which appears in the range 3200−3100 cm−1 is assigned to poly-hydrogen-bonded trisilanols and tetrasilanols, exclusive to copper aerogels.51 As the metal nanoclusters form (300−450 °C), the intensity of the free silanol band decreases in the copper aerogels, whereas in the plain gels and Cuxg-11 it increases as is expected due to dehydration and possible dehydroxylation.52 This is accompanied by an absorption band appearing at 3615 cm−1 for Cuag-11, which is assigned to mono-hydrogen-bonded vicinal silanols or germinal silanols, ruling out dehydroxylation. This band was not seen in Cuag-2 or Cuxg-11; hence dehydroxylation of interacting silanols cannot be excluded.52 In Cuag-2, however, all silanol bands decrease in intensity in response to heating in hydrogen. The subsequent treatment in a nitric oxide/oxygen rich flow resulted in an instant elimination of mono-hydrogen-bonded vicinal/geminal silanols (3615 cm−1) in Cuag-11. However, the absorption band reappeared during cooling and was also present in Cuag-2 at this stage. During cooling the υOH mode assigned to silanol clusters (3200−3100 cm−1) clearly loses intensity in Cuag-11 and is absent for Cuag-2 when room temperature is reached. The number of free silanols decreases in the copper gels during cooling, similar to the plain gel analogues. 20376



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the low k regime (0−150 eV); hence XANES probes the local environment up to 2 nm.54−56 LC of XANES is therefore useful for monitoring the course of reduction and reoxidation, but EXAFS and the edge position are more accurate for determining the reduced fraction in the case of small clusters. For samples Cuxg-11 and Cuag-11 we obtained RCu−Cu = 2.56 Å and NCu−Cu = 8 at 300 °C, and assuming spherically shaped clusters, this corresponds to an average cluster diameter of 1.4 nm ± 10% (75−80 atoms). This correlation is based on the work of de Graaf et al., where particle sizes were estimated from first shell multiplicities in the Ramses software, developed by Voogt et al.57,58 Similar dimensions are obtained for the clusters formed in Cuag-2 at 450 °C. The cluster dimensions are well below the average pore diameter in the aerogels, which is 5.1 and 4.7 nm for Cuag-11 and Cuag-2, respectively;1 hence the nanoclusters can reside in micropores present in both systems (Figure 1). The pore size distribution in the aerogels is large and shows a significant fraction of micropores, which can induce growth limitations. The cluster size is independent of copper loading; thus we obtained ultrasmall clusters for 1.8−11 wt % copper loading, which can be explained by the initial single-site state. We believe the second factor preventing sintering is interaction between the metal and gel surface acidic vacancies seen by DRIFTS (Figure 13). Due to the cluster size regime and harsh conditions in this study, we applied the cumulant expansion method during EXAFS refinements of the metallic nanoclusters. The EXAFS equation assumes the Gaussian pair distribution function (PDF), thus leaving out highly disordered systems in typical in situ conditions, such as highly dispersed nanoparticles in reactive atmospheres and elevated temperatures.38,59 Conventional EXAFS analysis on asymmetric local bonding environments, exhibiting non-Gaussian PDFs, typically results in underestimated bond lengths and coordination numbers. These shortcomings can be addressed by using the cumulant expansion method, which essentially quantifies the deviation from a Gaussian shape for the PDF.60−62 This modelindependent method has to be used with care since it introduces two additional free parameters, which is thoroughly validated during the analysis. The cumulant expansion method has proved to be an invaluable tool for these systems, seen by high fit improvements during the refinement procedure in Cuag-11 and Cuxg-11 (available in the Supporting Information), also validated by the comparative reduced χ2 test. The high values of C3 and C4 show that the closest Cu−Cu shell distances and multiplicity were significantly underestimated by the conventional EXAFS equation. Clearly, the ultrasmall nanoclusters comprise a considerable amount of surface atoms and are strongly affected by the harsh conditions to which they are exposed. Cu−Cu shell distances in Cuag-11 and Cuxg-11 are considerably expanded (2.56 Å) compared to copper foil (2.55 Å) upon formation of metallic nanoclusters at 300 °C (Figure 6). We suggest this expansion is due to water adsorbed on the copper nanocluster surface. Our previous study shows that water and ammonia are present in significant amounts as extraframework species in the aerogels, correlating with copper loading.1 Water desorption during formation of copper nanoclusters is seen exclusively for copper-containing gels by online MS during H2-TPR (Figure 2). Cuxg-11 has a hydrophilic surface; hence it contains a relatively high amount of water, however less in the plain gel analouge.1 It is therefore conceivable that water desorbs from the copper nanoclusters.

Adsorption and dissociation of reactive molecules are known to lead to expansion of RMe−Me due to π- or σ-type charge transfer interactions.63,64 This phenomenon is detectable by EXAFS in very small metal nanoclusters, due to the large fraction of surface atoms. Chen et al. reported on Cu−Cu shell expansion of hydrogen-reduced clusters (2−4 nm) coinciding with water being added to the feed.65 Copper is increasingly more active in the water gas shift reaction as particle size decreases, correlating with the ability to dissociate adsorbed water.15,16 Since the nanoclusters in this study are smaller (1−2 nm), adsorption, and possibly dissociation, of water is expected to expand Cu− Cu shell distances to a noticeable degree. Increasing the temperature further to 350 and 450 °C causes major drops in shell multiplicity in both systems as captured by EXAFS. Additionally, higher values of C3 are necessary to correct for an increasing static disorder, which correlates with increasing surface exposure.66 Destabilization of metal nanocluster surfaces, as a response to desorption of reactive gases, has been reported to cause radical changes in morphology, and those consisting of 100 atoms or less display strong variations in first shell multiplicity as a function of shape.67,68 Hence, the drops in multiplicities observed by EXAFS indicate a change in cluster morphology, which we suggest originates from water desorption from all copper-containing samples, confirmed by online MS during TPR. We therefore applied DFT to calculate energies, first shell multiplicities, shell distances, and generated theoretical EXAFS for clusters with a selection of symmetries and structures (fcc and bcc). We based our calculations on clusters comprised of 80 atoms only, based on EXAFS results and estimates from ref 57 from initial cluster formation at 300 °C. Based on cluster stability, we suggest Td symmetry (pyramid-shaped) for nanoclusters formed at 300 °C. The calculated height, width, and length of 10.5 × 10.9 × 12.6 Å are consistent with cluster size estimations by de Graaf et al.57 It is apparent from Figure 8 that the average multiplicity and shell distances rely heavily on the choice of Rmax of the calculated clusters, due to geometry optimizations in DFT calculations. Both flattening of the fcc cluster from Td to C2h and the phase change to bcc (D2h) yield lower theoretical average multiplicities for Rmax ranging from 2.5 to 2.7 Å. We suggest that cluster flattening from Td to C2h/ D2h symmetry due to water removal occurs for Cuxg-11 and Cuag-11 upon heating from 300 to 350 °C (Figure 8). We also note that the drop in shell multiplicity is accompanied by a significant shortening of the shell distances (from 2.56 to 2.53 Å) in the xerogel, whereas the shell distances remain expanded (2.56 Å) in the aerogel system. Interestingly, our DFT calculations in this size regime show comparable energies for Cu80 clusters with fcc and bcc structures when applying symmetries such as Td, C2h, or D2h. Moreover, the radial distribution of neighbors in Cu80 clusters of bcc structure, relative to the fcc structure (up to 2.8 Å), is significantly larger and centered toward lower shell distances and multiplicities (Figure 8). This can explain observed changes for Cuxg-11 above 350 °C, where a contraction of nearest Cu−Cu shell distances is accompanied by a major drop in multiplicity, suggesting both cluster flattening and a transformation to a bcclike structure. We find a high value of the fourth cumulant C4 for Cuxg-11 at temperatures above 350 °C, which confirms a significant broadening of the PDF (Figure 9 and Table 1). This is further confirmed by the theoretically generated PDFs of Cu80 with fcc (Td and C2h) and bcc (D2h) structures, showing a significant reduction of amplitude in the first and third shells for 20377



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configuration through the formation of ultrasmall metal nanoclusters and complete restoration of the initial surroundings. With the coupling of DFT with the shell distances and multiplicities from EXAFS, we arrive at average metallic copper cluster dimensions which do not exceed 1.6 nm in both high loaded gels (11 wt %) and in the low loaded gel (1.8 wt %) up to 450 °C in hydrogen. This is well below the average pore diameter in the aerogels and in the xerogel; hence the nanoclusters can reside in micropores in both systems. The stabilization of the ultrasmall nanoclusters is independent of the degree of mesoporosity and the gel surface inertness. An important factor preventing sintering is the strongly indicated interaction between the metal and acidic silanol clusters, formed during cation removal. Sintering of the copper nanoclusters did not occur in this study. In contrast, further heating led to significantly reduced Cu−Cu shell multiplicities, confirmed by EXAFS, signifying a morphology change in the aerogel and a possible transformation to a bcc-like structure in the xerogel. During the final treatment in nitric oxide/oxygen, flexible copper−gel surface redox interplay by the acidic silanol clusters was observed, resulting in complete redispersion of copper(I) and copper(II) to single-site configuration. The incorporation of single-site copper in high amounts (up to 11 wt %), the stabilization of ultrasmall nanoclusters in silica aerogels, and the complete restoration of the single-site copper(I/II) make them desirable as multipurpose catalysts.

the latter, accompanied by shell shortening and broadening. This correlates with the observed changes in the first and third shells in the Fourier transforms of Cuxg-11 at 450 °C (Figure 9). For Cuag-11, the expansion of shell distances is more consistent with an fcc structure, but the drop in multiplicity suggests a change in morphology, from Td to C2h at temperatures above 300 °C (Figures 8 and 9). There are significant differences in surface structures of aerogels and xerogels, as framework vacancies containing acidic silanol clusters are created only in the aerogel. The aerogel exhibits hydrophobicity, meaning that water is only present in connection with copper(II) cations, whereas the xerogel is hydrophilic and contains up to 10 wt % water.1 The desorption of water from the copper nanoclusters may enable an increased interaction with the silanol clusters, which reside in close vicinity on the aerogel inner surface (Figure 14d). This is consistent with the smaller drop in shell multiplicity, while shell distances remain expanded. Hence, the aerogel surface chemistry constitutes a driving force for significantly flatter cluster morphology, for instance C2h (disk-shaped). By contrast, the hydrophilic xerogel does not possess these silanol clusters, and following water desorption from the copper nanoclusters and the internal surfaces, only hydrophilic silanol and strained siloxane-covered surfaces remain to interact with copper.52 The observed contraction of Cu−Cu shell distances is caused by a lack of water interaction;69 hence we believe the drop in multiplicity for Cuxg-11 originates from a phase transformation (fcc to bcc) and flattening from Td to D2h symmetry. Treating the copper nanoclusters in nitric oxide/oxygen reveals a pronounced interaction with the aerogel surface, by stabilized mixtures of copper(I) and copper(II) in Cuag-11 and Cuag-2 starting from 450 to 100 °C (Figures 4 and 10). At room temperature copper is completely reoxidized to the divalent state in both systems with high loading. However, in Cuag-2 a near 50:50 mixture of copper(I) and copper(II) is stabilized at room temperature. Complete redispersion to single sites during reoxidation is confirmed by EXAFS, which shows recoordination to three to four siloxy/silanol groups. The careful refinements and application of Fourier filtering of the third shell rules out Cu···Cu interaction, known to be a clear indication of oxide formation.20 The Brønsted acidic silanol clusters, formed on the aerogel surface upon reduction, are reduced in intensity or disappear entirely during reoxidation of copper (DRIFTS, Figure 13). We believe the redispersion is governed by a dynamic interplay between copper and the silanol cluster containing vacancies residing on the aerogel surfaces. On the basis of our results, we propose a general mechanism (Figure 14e−h) for simultaneous reoxidation and redispersion: Nitric oxide and oxygen adsorb on copper nanoclusters, which results in reoxidation of metallic copper to copper(I)/copper(II) species under the formation of reduced NxOy species. The acidic protons originating from the silanol clusters are exchanged with copper cations, leading to recoordination to siloxy/silanol groups and the restoration of the tetragonally distorted octahedral environment. The protons and adsorbed oxygen species consequently may form water, which readsorb on the gel surfaces, as seen by DRIFTS.

■

ASSOCIATED CONTENT

S Supporting Information *

TGA−DSC measurements of the copper gels and the plain gel analogues in H2, conventional EXAFS least-squares refinements in the range 300−450 °C in H2, figures describing the EXAFS refinement procedures using the cumulant expansion method in detail, a comparison of k3·χDFT(k) and k3·χexp(k) of samples in the range 300−450 °C in H2 and the spectral regions covering 3800−2950 cm−1 for some selected DRIFT spectra collected during heating in hydrogen and cooling in nitric oxide/oxygen, and from DFT: XYZ coordinates (Å) and total of the ADF energy (hartree). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the Norwegian University of Science and Technology and the Norwegian Research Council for grants supporting the Swiss−Norwegian Beamlines (SNBL), and the assistance of the SNBL Project team (H. Emerich, W. van Beek, O. Safonova) is very much appreciated. We thank the ESRF for beam time. We would also like to thank Max-Lab for the beam time at beamline 1811 and S. Carlson and co-workers for the assistance. We are also grateful for the assistance of S. Forselv during in situ DRIFTS, and M. Bjørgen and S. Svelle for the helpful discussion regarding the DRIFTS results.

■

5. CONCLUSION The complementary techniques in situ XAS and in situ DRIFTS allowed for a fundamental study of sol−gel introduced copper in silica aerogel and xerogel systems, from single-site

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