Single-Site Copper by Incorporation in Ambient Pressure Dried Silica

ARTICLE pubs.acs.org/JPCC

Single-Site Copper by Incorporation in Ambient Pressure Dried Silica Aerogel and Xerogel Systems: An X-ray Absorption Spectroscopy Study Tina Kristiansen,*,† Karina Mathisen,† Mari-Ann Einarsrud,‡ Morten Bjørgen,† and David G. Nicholson† † ‡

Department of Chemistry, The Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway Department of Material Science and Engineering, The Norwegian University of Science and Technology, Sem Sælandsvei 5, 7491 Trondheim, Norway ABSTRACT: Copper incorporated silica aerogels and xerogels were prepared by adding the metal during the sol
gel stage and applying the ambient pressure drying (APD) method. The materials were characterized using inductively coupled mass spectrometry (ICP-MS), the Brunauer
Emmet
Teller (BET) method, X-ray diffraction (XRD), thermograviometric analysis
differential scanning calorimetry (TGA
DSC), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The introduction of copper led to changes in the aerogel architecture, seen by considerable expansion of average pore size, while maintaining a hydrophobic nature. The valence state and local chemical environment of copper was studied by X-ray absorption spectroscopy (XAS) before and after annealing to evaluate the interaction with the gel systems. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra showed that copper was divalent and present in a tetragonally distorted environment in both aerogels (1.8
11 wt %) and xerogels (11 wt %) before and after annealing, seen by 4 + 2 Cu
O distances at 2 Å and 2.25
2.35 Å. EXAFS also showed a third and fourth shell contribution attributed to one or more Cu 3 3 3 Si backscattering pairs at 2.9
3.2 Å. This confirmed that copper was coordinated to siloxy groups in the gels. Copper(II) oxide was not formed during annealing in any of the materials in this study, even at relatively high copper loadings. Clearly, the silica aerogels and xerogel showed great flexibility and capacity for copper cation uptake. The sol
gel incorporation route combined with the APD drying yielded a highly porous hydrophobic material able to stabilize singlesite copper cations up to 450
C, a prerequisite for optimal and stable catalytic performance.

1. INTRODUCTION Silica gels are amorphous inorganic networks composed of interconnected silica particles typically 2
10 nm in size. Aerogels feature mainly mesopores in the range 2
50 nm and exhibit high surface areas, high degrees of porosity, low densities, and other properties that make them promising for a growing number of applications.1 Xerogels are denser silica gels that consist mainly of micropores (below 2 nm) with surface areas comparable to and exceeding the surface areas of crystalline microporous material analogues.2,3 Silica gels are very versatile as the microstructure and porosity can be tailored to fit a specific application by altering processing conditions.4 Silica aerogels have been recognized as potential carriers for catalytically active transition metals in the form of isolated/well-distributed cations, welldispersed metallic nanoparticles or particulate metal oxides.5 Copper species, both as particulate oxides or isolated cations, are active for a number of red-ox processes, such as CO oxidation, oxidation of hydrocarbons, and selective catalytic reduction of NOx (SCR DeNOx).6
8 The Cu2+/Cu+ red
ox pair is believed to be responsible for the high activities reported for the SCR DeNOx process in the presence of a hydrocarbon reductant.3,9
11 It is assumed that red
ox reversibility of single-site copper cations is crucial for the catalytic removal of NOx, which requires evaluation of both the choice of carrier and method of metal introduction. r 2011 American Chemical Society

The commonly used methods of introducing copper cations to commercial silica substrates are adsorption, wet incipient impregnation, or precipitation methods.7,12 The literature agrees that these methods often lead to low distributions of the active phase, and low interaction between the metal and support, thus resulting in particulate oxide formation.7,13 During the ion exchange method, copper cations in solution are grafted on the surface siloxy groups at high pH. However, the low number of initial surface siloxy groups restricts the resulting amount of copper introduced by this method to a maximum of 1
2 wt %. The preferred method of choice for obtaining high distribution, high loading, and increased stability seems to be incorporation by the sol
gel route.14
18 In this introduction method the copper cations are added to the sol before gelation during preparation of the silica gel. However, site preference and local environment for metal cations introduced by this method are not yet fully understood. The preparation of sol
gel incorporated copper in silica aerogels and xerogels by acid catalysis of silicon alkoxides has been reported but with varying degrees of success.14
18 Received: June 22, 2011 Revised: August 12, 2011 Published: August 19, 2011 19260

dx.doi.org/10.1021/jp2058782 | J. Phys. Chem. C 2011, 115, 19260–19268

The Journal of Physical Chemistry C

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Table 1. Preparation Conditions for Silica Aerogels and Xerogelsa molar ratio of precursors gel Cuag-1

Cu

Si:H2O:HMDS:HMDSO (wt %) 1:49:2.5:2.2

10.9

Ag-1 Cuag-2

1:46:1.1:1

8.1

Ag-2 Cuag-3

1:49:1.1:1

1.8

Ag-3

SSAb

ÆPdæc

Vcd

2

(m /g)

(nm)

(cm3/g)

379

5.10

0.54

689

3.05

0.47

655

4.90

1.05

568

3.05

0.47

722

4.70

0.98

662

3.30

0.59

515 550

2.90 3.30

0.44 0.44

SA:H2O:NaOH Cuxg Xg

1:182:1.1

11.3

a

Molar ratio of silicic acid (SA), water, silylation agents (HMDS and HMDSO), Cu loading, specific surface area (SSA), average pore size ÆPdæ, and Cumulative Pore Volume (Vc). b Calculated from the BET method from nitrogen adsorption isotherm. c Calculated from the BJH method based on the nitrogen desorption isotherm. d Calculated from BJH method at the relative pressure p/p0 = 0.98 on the nitrogen desorption isotherm.

Base catalysis is far superior for incorporating copper by the sol
gel route, yielding higher copper loadings, as reported by Mohahan et al.19 With respect to overall cost of preparation, cheap silicon derivates like water
glass can replace commonly used silicon alkoxides, without affecting the gel quality.2 Aerogels are conventionally prepared by supercritical drying, which is expensive and time-consuming. The ambient pressure drying (APD) method is considered a simpler and more advantageous route to silica aerogels, and a more realistic solution with respect to commercialization.20
23 Subcritical drying with minimum pore shrinkage is achieved by surface modification and solvent exchange, using silylation agents and organic solvents, resulting in hydrophobic internal surfaces. We present here a structural study on copper cations in silica aerogel and xerogel systems. X-ray absorption spectroscopy (XAS) is used to study the nature of the copper species in silica aerogel and xerogel systems from low to high loading before and after annealing in air to 450
C. To our knowledge, this is the first report on copper incorporated silica ambient pressure dried aerogels derived from water
glass.

2. EXPERIMENTAL SECTION 2.1. Preparation. Diluted Na2SiO3 (water
glass, SigmaAldrich) containing 8 wt % SiO2 was ion-exchanged with Amberlite 120H+ (Fluka Chemika) in a column with a 1:1 volume proportion. The pH changed from 13 to 2
2.5 during the formation of silicic acid (SA, H2SiO3), as expected from previous reports.24,25 Subsequently, appropriate amounts of copper(II) nitrate (Cu(NO3)2 3 2.5H2O, Merck) for aerogels and copper(II) acetate (Cu(CH3COOH)2 3 H2O, Merck) for the xerogel were added to the SA and stirred vigorously for a few minutes to obtain homogeneity. Aerogels were prepared by the APD co-precursor method by adding the silylation agents HMDSO (hexamethyldisiloxane) and HMDS (hexamethyldisilazane) purchased from Fluka Chemika.20,21 HMDS produced ammonia (NH3) during surface silylation and caused an abrupt increase in pH to 8
10, causing gelation to occur within 1 min. The xerogels

were prepared by applying NaOH as a base catalyst. All samples were immersed in n-hexane (VWR Int.) for 3 h at 35
C in closed vessels. The drying procedure was as follows: 18 h at 65
C, 3 h at 85
C, and finally 2 h at 150
C with a heating rate 5
C/min. The procedures of this preparation without addition of copper were previously described by Bhagat et al.20,21 The samples were annealed to 450
C in air with a heating rate 5
C/min for 30 min. Three aerogels prepared from copper(II) nitrate with varying copper contents were studied with inductively coupled mass spectrometry (ICP-MS), the Brunauer
Emmet
Teller (BET) method, X-ray diffraction (XRD), thermograviometric analysis
differential scanning calorimetry (TGA
DSC), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and XAS. These were denoted Cuag-1, Cuag-2, and Cuag-3. The copper gels prepared for this work were selected from several series of gels that were prepared for screening of optimal preparation conditions. A high content of copper (>10 wt %) was only achieved by a high molar ratio of silylation agents, whereas a lower molar ratio resulted in a higher porosity. However, plain gels were prepared to compare with the copper containing gel analogues and for comparison of different Si: HMDS:HMDSO molar ratios. A xerogel with high loading of copper was prepared for comparison with copper(II) acetate, denoted Cuxg. Xerogels containing low contents of copper cations were prepared; however, gels with low loadings showed porosities similar to that of the plain xerogel. One copper xerogel with high copper loading was included in this study for comparison with the copper aerogels. The gels containing high amounts of copper are of particular importance, due to focus on the structural and thermal stability and also the influence on the gel structure. The preparation details are given in Table 1. 2.2. Characterization. Elemental analysis by ICP-MS was carried out by placing dried gels in vessels (18 mL volume) prewashed with MQ-water (Milli-q). The samples were then digested with hydrofluoric acid HF (45% v/v) + HNO3 (67% v/v) for 40 min by Milestone UltraClave under high pressure. The dilution was carried out to obtain approximately 43 mg of Si/L and 0.1 mg/L of HNO3 before a final 2 h digestion. The analysis was carried out in a double focusing magnetic sector field High Resolution Element 2 ICP-MS. The final results in μg/L of silicon and copper were converted to Si:Cu molar composition. The copper loadings were calculated relative to the weight of the dried gels. Porosity measurements were performed on a Tristar 3000 Micromeritics surface area and porosimetry analyzer at liquid nitrogen temperature. Samples were degassed at 250
C for 24 h prior to measurements. Specific surface area was calculated by the Brunauer
Emmet
Teller (BET) model on the adsorption isotherm from 5 points of relative pressure.26 Pore sizes were calculated using the Barret
Joyner
Halenda (BJH) equation on the nitrogen desorption isotherm, and pore volumes were calculated at the nitrogen desorption isotherm p/p0 = 0.98.27 Themograviometric analyses were carried out using a PerkinElmer thermograviometric analyzer (TGA7) combined with Jupiter STA 449C connected to a QMS 403C A€elolos mass spectrometer from Netzsch. A 7
10 mg sample was loaded in an Al2O3 sample pan and heated to 650
C with a heating rate 5
C/min under a constant air flow at 80 mL/min air and 20 mL/min protective argon gas. 19261

dx.doi.org/10.1021/jp2058782 |J. Phys. Chem. C 2011, 115, 19260–19268

The Journal of Physical Chemistry C Powder X-ray diffraction (PXRD) analyses were performed using a Siemens D-5005 diffractometer operated at 30 kV and 30 mA, a scintillation counter, and Ni-filtered Cu Kα radiation. The diffractograms were collected using a constant 2 mm slit opening and a step size of 0.06
covering a range of 10
60
2θ with a counting time of 0.25 s per step. DRIFT spectroscopy was carried out using a Bruker Vertex 80 with a LN-MCT detector and a high temperature cell from Pike Technologies. A 3
5 mg sample was loaded into a ceramic cup and annealed in synthetic air to 450
C using a 5
C/min ramp rate. Data were collected in situ in the range 850
4000 cm
1 with 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. 2.3. X-ray Absorption Spectroscopy. 2.3.1. Data Collection. Copper k-edge XAS data was collected at Max-Lab, in transmission mode at beamline 1811 at the Max-II ring. Max II offered electron beam energies of 1.5 GeV and X-rays in the energy range 2.4
12 keV. These were extracted by a standard optical scheme consisting of a vertical collimating first mirror, double-crystal monochromator, and a second vertically focusing mirror.28 A multipole wiggler insertion device was used in this beamline to overcome the low energies of the electron beam. XAS data were also collected at the Swiss
Norwegian Beamline (BM01B) at ESRF, Grenoble, France, in transmission mode. The white beam was collected from the storage ring by a bending magnet to SNBL. The beamline was equipped with a channel cut Si(111) monochromator and double-crystal monochromator to select the desired wavelength. The available spectral range was 4
70 keV. The initial and transmitted intensities I0 and It (31 cm) were detected by ion chamber detectors with the lengths 17 and 31 cm, respectfully. The gas compositions were 80% N2 + 20% Ar and 75% N2 + 25% Ar. The ESRF provided electron beam energies of 6 GeV and maximum current 200 mA.29 The asprepared and annealed samples were ground and placed in aluminum sample holders sealed with Kapton tape. 2.3.2. Data Reduction. The XAS data were summed, normalized, and energy corrected from the metal foil (Cu-foil k-edge = 8979 eV) using Athena, a program in the IFEFITT package.30 The k-edge absorption energy E0 on the XAS spectra of unknown samples was consistently placed halfway up the absorption edge jump. X-ray absorption near edge structure (XANES) scans were normalized from 27 to 48 eV above the edge, and extended X-ray absorption fine structure (EXAFS) scans were normalized from 150 to 800 eV above the edge. 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). Reference compounds were used to approximate the amplitude reduction parameter (AFAC) of the unknown samples. Linear combination (LC) fitting procedures of XANES were used to select the reference compound with a chemical environment that shows most resemblance to the unknown samples, thus estimating the more probable AFAC. The fitting procedures were carried out in Athena consistently. The data were fitted from
20 eV prior to the edge and +60 eV above the edge. Reference compounds were copper foil, copper(I) oxide, copper(II) oxide, and copper(II) tutton (Cu(NH4)2(SO4)2 3 6H2O). The relevance to using a standard was determined by several linear combination fittings with a varying number of standards and evaluation of the statistical goodness of fit. This fit parameter

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was reported for each fit procedure by the statistical R-factor, defined as   N 1 exp ðjχi ðkÞ
χth ðkÞjÞ 100% ð1Þ R ¼ i 3 σi i

∑

2.3.3. EXAFS Least-Squares Refinements. EXAFS data refinements were carried out with EXCURV98,31 which conducted the curve fitting of the theoretical χth(k) to the experimental χexp(k) using the curved wave theory. 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 wavenumber k range 1
13 using a k3 weighting scheme. To approximate AFAC for the selected reference compounds, the refinements were carried out by keeping coordination numbers fixed at known values, whereas bond lengths, the correction parameter to E0 (EF), and Debye
Waller factors were refined. During the procedures the value of AFAC was set to 1. Finally, AFAC was refined and transferred to the EXAFS data of the unknown samples. Conventional least-squares refinements were carried out on the copper containing gels as follows: The nearest absorber
backscatterer pair was introduced first and refined with correlation off. The next-nearest and also a third absorber
backscatterer shell were introduced, following the same procedure. In some cases it was necessary to introduce a fourth shell. To verify the type of backscattering atoms in the third and fourth shells, Fourier filtering was used to extract the contribution from 2.8 to 3.2 Å in R space. It was transformed back to k space and refined with the following absorber
backscatterer pair Cu 3 3 3 Cu, Cu 3 3 3 Si, and Cu 3 3 3 O, where the best quality of fit and the k value of the amplitude maximum of the k3 weighted χexp(k) revealed the most probable backscattering element.

3. RESULTS 3.1. Observations. The addition of copper nitrate and copper acetate leads to color change of the transparent sols to turquoise blue and azure blue, respectively. Turquoise is characteristic of the hexaaquacopper(II) complex ion [Cu(OH2)6]2+. Gelation time is significantly longer for plain aerogels compared to the times for the copper aerogels, which are 30 ( 10 min and maximum 1 min, respectively. Also, the gelation time decreases with increasing copper content. Plain aerogels are jelly-like, and copper gels tend to be gradually more granulated. After addition of HMDS the sol turns royal blue due to formation of copper tetraammine [Cu(NH3)4 3 2H2O]2+. The pH ranges from 8 to 10 at this point. During gelation the color changes to a paler blue. As surface modification and solvent exchange commenced, a royalblue phase appears to be repelled from the gel, both trapped underneath and some over the gels. After a few hours the gels enter the organic phase, floating on top of the royal blue phase. The royal blue color of the expelled pore fluid indicates that copper was added in excess and thus indicates that only a certain amount of copper can be incorporated into the aerogels. There is no apparent sign of shrinkage of the aerogels during drying, and the copper species seem well-dispersed on the basis of the color of the final gels. The final plain silica gels tend to be more glasslike than their respective powder-like copper gels. Color changes are observed during annealing in air to 450
C of the copper gels. As-prepared gels are pale blue or blue depending on copper 19262

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Figure 1. Nitrogen adsorption/desorption isotherms where the quantity of nitrogen is plotted against relative pressure p/p0: Copper gels (solid line) and plain gel analogues (
9
).

content. At 250
300
C, all gels expect Cuag-3 turn olive green; Cuag-3 does not change color before 300
C to pale green, in accordance with the copper content of this aerogel system. The gelation point for Cuxg is 10
15 min, when the pH is around 6.5. There is no clear difference in wet gel consistency compared with that of the plain xerogel. Cuxg shrinks drastically during drying and exhibits a blue
greenish color, which remains during annealing. 3.2. Porosity and Chemical Analysis. The final Si:Cu molar ratio and calculated copper loading for the copper aerogels and the single copper xerogel are included in Table 1. Maximum copper loading for the aerogel is obtained for Cuag-1 with 10.9 wt %, whereas Cuag-2 and Cuag-3 exhibit loadings of 8.1 wt % and 1.8 wt %, respectively. Maximum copper loading for the xerogel sample was 11.3 wt % for Cuxg. Nitrogen adsorption/desorption (NAD) isotherms for copper gels and plain gel analogues are shown in Figure 1. The copper aerogels show type IV isotherms, characteristic for mesoporous materials (2
50 nm). The plain gels exhibit type I isotherms, characteristic for micropores (