Nature and Location of Copper Nanospecies in Mesoporous

J. Phys. Chem. C 2010, 114, 1481–1490

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Nature and Location of Copper Nanospecies in Mesoporous Molecular Sieves Corina M. Chanquıá,† Karim Sapag,‡ Enrique Rodrı´guez-Castelloń,§ Eduardo R. Herrero,† and Griselda A. Eimer*,† Centro de InVestigacioń y Tecnologıá Quı´mica (CITeQ), UniVersidad Tecnolo´gica Nacional, Facultad Regional Co´rdoba, Maestro Lo´pez esq. Cruz Roja Argentina, 5016 Co´rdoba, Argentina, Instituto de Fı´sica Aplicada CONICET, Departamento de Fı´sica, UniVersidad Nacional de San Luis, Chacabuco 917, 5700 San Luis, Argentina, and Departamento de Quı´mica Inorgańica, Facultad de Ciencias, UniVersidad de Ma´laga, AVda. CerVantes no. 2, 29071 Ma´laga, Spain ReceiVed: October 1, 2009; ReVised Manuscript ReceiVed: December 9, 2009

Copper-mesoporous molecular sieves type MCM-41 have been successfully prepared by different direct incorporation methods of the metal in the initial synthesis gel. Various techniques including XRD, AAS, adsorption/desorption of N2, UV-vis-DR, TPR, and XPS were employed for the materials characterization. All of the materials exhibited a good structural regularity, even for loadings of copper of ∼10 wt %, besides high specific surface areas and pore volumes. An extended and detailed study about the nature of copper oxidation state as well as of the copper species distribution on the siliceous nanostructure has been made. It was found that copper is present on the mesoporous silica in the form of various species: isolated mononuclear Cuδ+ cations possibly in coordination with the lattice oxygen; linear oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters probably inserted into mesoporous channels; and bulky CuO oxide segregated of the structure. The distribution of copper species in the final solid depends on the preparation method used as well as of the copper content in the initial gel. Moreover, we infer the coexistence of the both copper oxidation states +1 and +2 (+δ) in the major species of these materials: isolated and oligonuclear copper species. Introduction The ultimate aim of the current nanotechonology is the development and design of materials and systems at the nanometric scale with controlled properties suitable to a concrete application. The success of this task requires the development of mechanical-physical and chemical procedures to obtain systems with desired shapes and compositions. In a nanometric scale, two important factors regulate the properties of a material: (a) the size effect, due to the increasing confinement of the electrons, and (b) the surface effect, as a consequence of the relative increase of the surface atoms with different chemical and structural topology. These two effects have a great influence on the total energy of the system and the condition of its final structure and shape. Because most of the atoms of the nanomaterial are located at the surface, the surface atoms play a crucial role in the energy minimization and, consequently, in the stabilization of the system. The minimization of the energy includes changes in the charge distributions that can differ from the surface to the bulk due to the quantum confinement of the electrons.1 In the area of the inorganic materials with controlled pore size distribution, one major success has been the synthesis of the mesoporous molecular sieves called M41S. These nanostructured materials arise from a supramolecular self-assembly process in the presence of ionic surfactants as templates during the mesophase formation, and its pore size can be tailored on the basis of the surfactant chain length and the synthesis conditions applied. In comparison with crystalline microporous * Corresponding author. Tel./fax: 54-351-4690585. E-mail: geimer@ scdt.frc.utn.edu.ar. † Universidad Tecnolo´gica Nacional. ‡ Universidad Nacional de San Luis. § Universidad de Ma´laga.

zeolites, the M41s materials break through the pore size limit and provide the opportunity to process bulky molecules larger than 1.2 nm in diameter.2 MCM-41 is the member of the M41s family, which possesses a hexagonal arrangement of uniformly sized monodimensional pores in the range from 2 to 10 nm besides large and accessible internal areas around 1000 m2/g. Because of the absence of active sites, the pure siliceous mesoporous molecular sieves are of limited use in catalysis. The incorporation of transition metals on the silicate framework is therefore necessary to make these materials as potential catalysts. There are many reports concerning the synthesis and characterization of Ti-MCM-41 catalysts with redox properties.3-6 We have previously reported7,8 that the Ti-MCM-41 mesoporous molecular sieves have a remarkable redox catalytic activity, particularly in olefin oxidation reactions with hydrogen peroxide as oxidant. In contrast, there are a very limited number of reports about Cu-substituted mesoporous catalysts for fine chemical processes.9 Copper is particularly interesting due to its special redox properties, polarizability, lower cost, and its higher reduction potential with respect to other transition metals.2,10,11 Copper catalysts have long been used in several industrial scale petrochemical catalytic processes, such as low-pressure methanol synthesis, steam reforming of methanol to produce H2 for fuel cells, low-temperature water-gas shift reaction, and CO oxidation.12 In the past decade, copper-containing zeolites and highly dispersed amorphous copper oxide systems have been extensively studied due to their exceptional activity in catalytic reactions considered as potential ways for abatement of major air pollutants, for example, NO decomposition, selective catalytic reduction of NOx by hydrocarbons (HC-SCR process), and decomposition of nitrous oxide.13 Nanoscale copper species can be expected to exhibit behavior different from that of bulk copper. Although copper nanospecies have received considerable

10.1021/jp9094529  2010 American Chemical Society Published on Web 01/04/2010

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attention in many fields, few researchers have discussed the characterization of nanoscale copper species less than 4 nm in diameter.14 At the present, many efforts have been directed to elucidate the main reasons for the peculiarity of the Cu-zeolites in comparison with other copper-containing systems or with other transition metal ions. However, coordination, location, and aggregation degree of copper in zeolites are still not well understood.15 In particular, the nature of copper species on mesoporous silica has not been discussed so far. The aim of this Article is the characterization of the textural, structural, and surface properties of copper-containing mesoporous molecular sieves (Cu-MMS) of type MCM-41. The attention is focused on the nature, location, and distribution of copper species in mesoporous silica generated from direct incorporation methods of the metal in the initial synthesis gel, through a synergic analysisoftheXRD,AAS,N2 adsorption/desorption,UV-vis-DR, TPR, and XPS techniques. Specifically, the synthesis variables such as reaction time, temperature, and type of treatment of initial gel as well as copper content in the materials have been studied. Experimental Section Catalyst Preparation. The copper-containing mesoporous molecular sieves (Cu-MMS) were prepared using cetyltrimethyl ammonium bromide (CTABr, Aldrich) as template. Tetraethoxysilane (TEOS, Fluka g98%) and copper(II) nitrate (Cu(NO3)2 · 3H2O, Anedra) were used as the Si and Cu sources, respectively. The pH of the synthesis was adjusted to 13 by addition of a tetraethylammonium hydroxide (TEAOH, Sigma-Aldrich) 20 wt % aqueous solution. The catalysts were synthesized from a gel of molar composition: Si/Cu ) 20-60-240, TEAOH/Si ) 0.3, CTABr/Si ) 0.3, H2O/Si ) 60. In a typical synthesis, TEOS and Cu(NO3)2 · 3H2O were vigorously mixed for 30 min. Next, 25 wt % solution of CTABr in ethanol and 70% of the TEAOH were added dropwise; this mixture was continuously stirring for 3 h. Finally, the remaining TEAOH and the water were further added dropwise to the milky solution of light blue color, which was then heated at 85 °C for 30 min to remove ethanol used in solution and produced in the hydrolysis of TEOS. This gel has been manipulated in three ways: without any subsequent treatment, by aging at room temperature, and by hydrothermal treatment. In the case of the samples aged at room temperature, the gel was transferred into a black plastic bottle filled and kept at room temperature (∼25 °C) for 1 and 3 days. In the case of the samples hydrothermal treated, the gel was transferred into a Teflon-lined stainlesssteel autoclave and kept in an oven at 100 °C for 1 and 3 days under autogenous pressure. In the three cases, the final solids were filtered, washed with distilled water until pH ≈ 7, and dried at 60 °C overnight. The color of the as-synthesized samples was light turquoise. The template was evacuated from the samples by heating (2 °C/min) under N2 flow (45 mL/min) at 500 °C for 6 h and subsequent calcination at 500 °C for 6 h under dry air flow (45 mL/min). The final color of the calcined samples was brown, which turns to beige for the lower copper content. Table 1 details all of the synthesized samples with their corresponding treatment conditions and composition of the synthesis gel. The samples are named by letters, and the treatment conditions as well as the Cu content are indicated in parentheses. Characterization Techniques. The materials were characterized by powder X-ray diffraction (XRD), nitrogen adsorption/ desorption, UV-vis diffuse reflectance (UV-vis-DR) spectroscopy, temperature-programmed reduction (TPR), and X-ray

Chanquıá et al. TABLE 1: Treatment Conditions and Initial Gel Composition of Cu-MMS treatment conditions sample

type

temperature (°C)

time (days)

Si/Cua

A (0d-60) B (RT-1d-60) C (RT-3d-60) D (HT-1d-60) E (HT-3d-60) F (HT-1d-240) G (HT-1d-20)

without aging aging HT HT HT HT

25 25 100 100 100 100

0 1 3 1 3 1 1

60 60 60 60 60 240 20

a

Molar ratio in the initial synthesis gel.

photoelectron spectroscopy (XPS). The Cu content in the final solid products was determined by atomic absorption spectroscopy (AAS) using an AA Varian Spectra spectrophotometer. The samples were previously digested using HF and HNO3 in a 2:1 ratio, and then were diluted with distilled water until 4.5 mL of solution. XRD patterns were collected in air at room temperature on a Philips PW 1729 diffractometer using Cu KR radiation of wavelength 0.15418 nm. Diffraction data were recorded in the 2θ ) 1-8°/34-40° ranges at an interval of 0.02°, and a scanning speed of 0.3°/min was used. The interplanar distance (d100) was obtained by the Bragg law using the position of the first X-ray diffraction line. The lattice parameter (a0) of the hexagonal unit cell can be calculated by a0 ) (2/(3)1/2)*d100. The mean crystalline sizes of the CuO oxides were calculated by applying Scherrer equation using the FWHM (full width at half-maximum) values of the most intense peaks. Specific surface area, pore size distribution, and total pore volume were determined from N2 adsorption/desorption isotherms obtained at 77 K using a Micromeritics ASAP 2010 (Accelerated Surface Area and Porosimetry System). The surface area was determined by the Brunauer, Emmett, and Teller (BET) method, and the pore size distribution was determined by the Barrett, Joyner, and Halenda (BJH) method, based on the Kelvin equation and obtained for the adsorption branch. The primary mesoporous volume was estimated by the alpha plot method, the total pore volume by the Gurvischt rule at P/P0 ≈ 0.98, and the secondary mesoporous volume by the difference. UV-vis-DR spectra of the materials were recorded using a Perkin-Elmer precisely - Lambda 35 spectrophotometer, to which a diffuse reflectance chamber Labsphere RSA-PE-20 with an integrating sphere of 50 mm diameter and internal Spectralon coating is attached. Once the solid samples were compacted in a Teflon sample holder to obtain a sample thickness of ∼2 mm, their spectra were recorded at 100 nm/min in the range 200-900 nm. Spectral grade BaSO4 was used as the reference material. The spectra were taken in air at room temperature, and the data were automatically transferred according to the Kubelka-Munk equation: f(R) ) (1 - R∞)2/2R∞. The original spectra obtained for the calcined samples have been fitted by three bands using the NLSF (Nonlinear Least Squares Fitter) Wizard of OriginPro 7.5 software. Curve-fitting calculations were useful for determining the location of the bands and their areas; the fitting confidence was χ2 e 0.0005 and R2 g 0.99. The reducibility of the calcined copper catalyst was measured by the TPR experiments in the Chembet 3000 of the Quantachrome Instruments. In these experiments, the samples were heated at a rate of 10 K/min in the presence of H2 (5% H2/N2 flow, 20 mL/min STP), and the reduction reactions were monitored by the H2 consumption. The original patterns have been fitted by three bands using the NLSF (Nonlinear Least Squares Fitter) Wizard of OriginPro 7.5 software. Curve-fitting calculations were again useful for

Nanocopper Species in Mesoporous Molecular Sieves

Figure 1. XRD patterns of the samples synthesized under different treatment types.

determining the location of the bands and their areas corresponding; the fitting confidence was χ2 e 0.0006 and R2 g 0.998. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with nonmonochromatic Mg KR radiation (300 W, 15 kV, 1253.6 eV) for the analysis of photoelectronic signals of O 1s, Si 2p, and Cu 2p and multichannel detector. Spectra of powdered samples were recorded with the constant pass energy values at 29.35 eV, using a 720 µm diameter analysis area. During data processing of the XPS spectra, binding energy values were referenced to the C 1s peak (284.8 eV) from the adventitious contamination layer. Short acquisition time of 10 min was used to examine C 1s, Cu 2p, and CuLMM XPS regions to avoid, as much as possible, photoreduction of Cu(II) species. The PHI ACCeSS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss-Lorentz curves, to determine the binding energy of the different element core levels more accurately. The error in the BE was estimated to be ca. (0.1 eV. Results and Discussion Influence of the Treatment Conditions of the Synthesis Gel. The XRD patterns of the Cu-MMS prepared with a molar ratio Si/Cu of 60 and different treatment conditions of the synthesis gel are shown in Figure 1. All of the materials exhibit an intense low-angle reflection between 1.5° and 3.5° corresponding to distance between planes (100) and a weak reflection between 4° and 6° due possibly to the overlapping of the diffraction peaks corresponding to planes (110) and (200). These signals are characteristic of materials with MCM-41 type mesoporous structure, which remained stable under calcination. Contrary to what some authors said2 regarding the loss of structural order that can cause the use of direct synthesis method to introduce metal species in a mesoporous network, all of the materials obtained by us showed a well-defined structure. However, there is a noticeable difference between the XRD patterns of the samples synthesized from gels aged at room temperature and from gels hydrothermally treated at 100 °C. A minor half-height width and a higher intensity of the main peak

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1483 of reflection, as well as more intense reflections for higher Miller indices, reveal the higher degree of structural ordering reached by the sample synthesized under hydrothermal treatment during one day (sample D). However, a longer hydrothermal synthesis time caused a partial loss of the structural ordering of the mesoporous solid. Moreover, an aging at room temperature of the untreated sample (sample A) causes a light loss in the structural regularity (samples B and C), which is almost not affected by the aging time. The large angle XRD results in the region between 34° and 40° are shown as the inset in Figure 1. All of the samples exhibit two peaks at 35.6° and 38.8° typical of the CuO presence.2,13,16-18 The size of these CuO particles for all of the samples, calculated from the XRD reflections according to the Scherrer equation, was between 17 and 26 nm.19 However, a point worthy of noting is that even if all of the samples were developed with an identical copper content in the synthesis gel, those aged at room temperature present a higher diffraction peaks area corresponding to the CuO phase. This feature suggests that when the samples do not undergo hydrothermal treatment, a greater relative amount of CuO nanoparticles can be formed. Finally, no diffraction peaks at major angles, corresponding to Cu2O phase (2θ ) 36.44° and 42.33°) and Cu0 phase (2θ ) 43.3° and 50.5°), could be observed in these samples. Figure 2 shows the N2 adsorption/desorption isotherms with their corresponding BJH pore size distribution of the more representative calcined samples with the same relation Si/Cu ) 60; the corresponding physical parameters are collected in Table 2. It must be noted that for comparisons of different samples the isotherm curves have been shifted in the y-axis. All of the samples exhibit type IV isotherms20 typical of mesoporous structures with an inflection at relative pressure P/P0 ≈ 0.2-0.3 characteristic of capillary condensation inside the conventional mesopores present in MCM-41 structure (primary or structural mesopores). Such inflection provides a measure of the distribution range of the pore size of these materials.21 The adsorption isotherm for the sample treated hydrothermally featured a narrow step of capillary condensation, which provides clear evidence of their narrowly defined diameter range for mesoporous channels of this sample; meanwhile, the adsorption isotherm of sample synthesized without treatment and aged at room temperature exhibited a more broad capillary condensation step, which indicates a bigger range in the distribution of the pores size.21 In fact, sample D showed the narrowest BJH mesopores size distribution. Additionally, all of the samples synthesized exhibited a pronounced increase in the adsorption branch at relative pressures about 0.85, which could be due to a capillary condensation in secondary mesopores,22,23 and hysteresis loops that resemble H4-type24 with a sharp decrease in the desorption branch at P/P0 ≈ 0.45-0.5. In Table 2 can be seen the secondary mesoporous volume for the samples. According to some authors,25 the nonstructural porosity consists of large cavities eventually interconnected and accessible through necks, which have an average diameter smaller than those of the main voids. This secondary mesoporosity could possibly contribute to the greater range of the distribution pore diameter of these materials. Moreover, it is possible to suggest that some CuO nanoparticles could be trapped inside such secondary mesopores present in these samples. As it can be observed in Table 2, all of the samples show surface areas above 1000 m2/g and total pore volumes around 1 cm3/g, which is typical of mesoporous materials. It is noteworthy that the materials synthesized by us have wall thicknesses much larger than the typical MCM-41 materials

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Figure 2. N2 adsorption/desorption isotherms and BJH pore size distribution of the samples synthesized under different treatment types.

TABLE 2: Structural Parameters of the Samples Synthesized under Different Treatment Types Vtotale (cm3/g) sample

d(100)a (nm)

a0b (nm)

DBJHc (nm)

twd (nm)

primary

secondary

SBET f (m2/g)

A (0d-60) B (RT-1d-60) D (HT-1d-60)

3.54 3.52 3.53

4.08 4.06 4.08

2.35 2.45 2.64

1.73 1.61 1.44

0.705 0.635 0.876

0.375 0.322 0.390

1120 1026 1158

Interplanar spacing (100). b Lattice parameter. c BJH pore diameter. d Wall thickness calculated by a0 - Dp. e Pore volume. f BET specific surface area. a

Figure 3. UV-vis diffuse reflectance spectra of the samples synthesized under different treatment types and Si/Cu molar ratio of 60; experimental spectra, thick solid lines; deconvoluted sub-bands, thin lines; assignments: - isolated Cuδ+, - - - oligomerics [Cuδ+ · · · Oδ- · · · Cuδ+]n species, · · · CuO bulky oxides.

(∼0.8 nm),26 which possibly would confer greater stability to the material under hydrothermal conditions. On the other hand, it should be noted here that the presence of segregated CuO phase (extra-bulk) seems not to influence the high surface area of these materials. This is possibly due to the relatively little amount of these nanoparticles evidenced by the much lower intensity of the XRD signal at high angle with respect to the signal at low angle (Figure 1). The UV-vis-DR spectra were recorded to understand the coordination environment of Cu species in the MCM-41 type structure. The UV-vis-DR spectra for the calcined samples prepared with a molar ratio Si/Cu of 60 in the initial synthesis gel and different treatment conditions (samples A, B, and D) are shown

in Figure 3. The Si/Cu atomic ratio and the overall metal content in the final solid as well as the distribution of copper species, determined by UV-vis-DR, are presented in Table 3. The original spectra have been deconvoluted into three sub-bands to facilitate the assignment to the different Cu species. The first maximum at about 290 nm could be related to the charge-transfer between mononuclear Cuδ+ ion and oxygen. According to the literature data,27 the main assignments of the DRS absorption maxima in the UV range correspond to the Cu2+ r O2- ligand-metal charge transfer (LMCT) between oxygen and isolated Cu2+ ions (250280 nm and 310 or 350 nm) and the 3d10-3d9s1 transition of Cu+ (UV up to 320 nm). It is well-known that the relative stabilities of Cu(I) and Cu(II) states in a solid framework strongly depend on


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TABLE 3: Chemical Composition and Copper Species Relative Distribution in Cu-MMS Synthesized under Different Treatment Types distribution of copper species isolated cations a

a

sample

Cu content [wt %]

Si/Cu

A (0d-60) B (RT-1d-60) D (HT-1d-60)

1.43 2.5 3.6

73.02 41.538 28.69

a

oligomeric species

bulky oxides

% area

% Cu

% area

% Cu

% area

% Cu

36.5 33 52.9

0.52 0.825 1.90

35.8 18 32.70

0.51 0.45 1.18

27.7 49 14.40

0.4 1.225 0.52

In the final solid.

the nature and vicinity of the ligand atoms.28 The coexistence of isolated Cu+ and Cu2+ ions in zeolitic frameworks has just been reported.27,29,30 Moreover, some authors have attributed the presence of the isolated Cu+ species in MCM-41 to the autoreduction of the Cu2+ to Cu+ by dehydration during the calcination thermal treatment.31,32 Recently, new copper-oxygen systems have attracted great interest due to the coexistence of Cu species with both oxidation states (1+ and 2+).33 In conclusion and according to these latter comments, the presence of Cu+ species in our materials cannot be discarded. Consequently, and judging by the UV-vis absorption in the 250-400 nm range, we suggest the coexistence of both isolated Cu+ and Cu2+ ions (Cuδ+), which was then evidenced by XPS (as discussed below). Jiang et al.30 tried to explain this phenomena in Cu-VSB-5 materials with a mechanism proposed by Larsen et al.34 According to this mechanism, when the sample is calcined, two Cu2+OH- groups can be dehydrated to form Cu+ and Cu2+O-. Thereby, Cu2+ and Cu+ could be detected by XPS in the calcined samples. Given the light turquoise color of the as-synthesized samples, characteristic of hydrated copper(II) cations, with respect to the brown color for the calcined samples, the mentioned autoreduction-dehydration mechanism results as plausible for our samples. Such mononuclear species could be possibly occupying silica framework isolated positions in coordination with the lattice oxygen. On the other hand, the second maximum at 480 nm may be related to the presence of copper oxide structures like linear chains with different electronic states of copper ions [Cuδ+ · · · Oδ- · · · Cuδ+]n.15,35 Quantum chemical calculations have proven the occurrence of LMCT bands in this region for copper clusters with low coordination, such as linear chains [Cu2+ · · · O2- · · · Cu2+]n.30,33-38 Moreover, these calculations show that the energies of the oxygen p-orbitals and the copper d-orbitals are close in the structures with coordination number of copper and oxygen equal to two and formal oxidation states [Cu2+O2-]. As a result, the approaching of oxygen p-orbitals to orbitals of Cu2+ ion makes possible an internal self-reduction (Cu2+O2- f Cu+O- intrachain transitions without external action) leading to structures with mixed-valence copper ions (Cu2+ and Cu+).15,35,39 Therefore, such pairs of Cu2+ and Cu+ ions located close to each other are to result from partial reduction of copper cations in these chains [Cu2+ · · · O2- · · · Cu2+]n. In addition, Cu2+ · · · Cu+ intervalence transition bands have been reported to occur in this range.15,35 We suppose that such oligonuclear species like chains, with mixed oxidation states (Cu2+/Cu+), could be stabilized inside the mesoporous channels by the decompensation of charge caused in the silica framework due to the incorporation of isolated Cuδ+ species. Finally, it is known that the pure CuO oxide shows a strong UV-vis-DR absorption band around 750 nm, which is due to the 2Eg f 2T2 g spin-allowed transition of the Cu2+ ions in the octahedral symmetry.40,41 Next, the third maximum observed around 700 nm (Figure 3) may be associated with this d-d

transition of Cu with a tetragonally distorted octahedral environment in CuO.27,29,30,40 Therefore, the formation of CuO phase could be detected in our samples, as it was also evidenced by XRD. It can be concluded from the obtained results that copper is present on the mesoporous silica in the form of various species: isolated mononuclear Cuδ+ cations, assigned to sub-band between 250 and 400 nm; linear oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters, assigned to sub-band between 400 and 600 nm; and bulky CuO oxides, assigned to sub-band between 600 and 800 nm. As it can be seen in Table 3, the sample synthesized under hydrothermal treatment showed higher incorporation of overall copper in the final solid. In addition, the distribution of Cu species also depends on the nature of treatment applied to the initial synthesis gel. To obtain a rough estimation of the percentage of the different Cu species in the samples, the percentage of each sub-band area with respect to the total area of the experimental spectrum has been multiplied by the overall Cu content (accurately determined by AAS) (Table 3). Being aware that the obtained values in Table 3 are inaccurate to a certain degree due to the deconvolution procedure, they are nevertheless regarded to be helpful for a comparison of different samples. The results shown in the table indicate that the hydrothermal treatment promotes the incorporation of both the isolated species Cuδ+ into the framework and the oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters into the channels. Meanwhile, an aging of the initial gel at room temperature favors the formation of extra bulk CuO. Influence of Copper Content. The XRD patterns of the CuMMS samples prepared with different copper content under hydrothermal treatment for 1 day are shown in Figure 4. All of the samples exhibit a main diffraction peak corresponding to the (100) plane and a weak diffraction peak between 4° and 6° possibly due to the overlapping of the signals of the planes (110) and (200). As it was mentioned above, these signals are characteristic of materials with well-defined MCM-41 type structure. Contrary to what some authors said,12,42 the copper content increasing in the initial gel did not affect substantially the ordering of the materials synthesized by us. It is noteworthy that even for copper content around 10 wt %, a good structural regularity and high specific surface area were obtained. However, the lower structural order for sample F could be related to thermodynamic reasons, but this point is even under discussion. On the other hand, the large angle XRD results in the region between 34° and 40° are shown as the inset in Figure 4. All of the samples exhibit two peaks at 35.6° and 38.8° typical of the CuO phase.2,13,16-18 Nevertheless, very weak and broad reflections were observed, which are characteristic of very small CuO particles. An approximate estimation of the CuO particle dimensions, performed from the broadening of the peak at 2θ ) 35.6° by the Scherrer formulas,13 had again values in the range between 17 and 26 nm for samples G and D. As it can

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Figure 4. XRD patterns of the samples modified with different copper content.

be observed through the relative areas of the high angle diffraction peaks, a higher content of copper in the initial gel favors the formation of CuO oxide particles likely on the external surface of the solid. Additionally, these CuO nanoparticles could also be probably trapped inside secondary mesopores, contributing to the total amount of the CuO species and decreasing their number on the external surface of the Cu-MMS grains. On the other hand, no diffraction major peaks corresponding to Cu2O phase (2θ ) 36.44° and 42.33°) and Cu0 phase (2θ ) 43.3° and 50.5°) could be observed in our samples. Figure 5 shows the N2 adsorption/desorption isotherms and the respective distributions of pores size of the samples prepared with different copper content under hydrothermal treatment for 1 day. It is noteworthy that for comparisons of different samples the isotherms have been shifted in the y-axis. The corresponding physical parameters are collected in Table 4. As it can be observed, all of the samples exhibit type IV isotherms typical of mesoporous structures with an inflection at relative pressure P/P0 ≈ 0.2-0.3, characteristic of capillary condensation inside the conventional mesopores of the MCM-41 structure. It is noteworthy that the adsorption isotherm of sample F exhibits a broad capillary condensation step, indicating a bigger range in the distribution of the pores size. Such a feature is also evidenced by the low intensity and larger width of the peak in the curve of pores size distribution.21 Additionally, this sample exhibits a pronounced hysteresis loop H4-type in IUPAC classification, with a sharp decrease in desorption branch at P/P0 ≈ 0.45-0.5, which may evidence the existence of ink-bottle pores, while G and D samples show a small hysteresis loop, which may evidence the presence of cylindrical pores. Finally, all samples show a parallelism between the adsorption and desorption branches, which is characteristic of slit pores.24 On the other hand, all of the samples synthesized exhibited a pronounced increase in the adsorption branch at relative pressures about 0.85, which could be due to a capillary condensation in secondary mesopores, as has been mentioned above. In Table 4 can be seen the secondary mesoporous volume for the samples modified with different copper content. Finally, it is possible to suggest that some CuO nanoparticles could be probably trapped inside such secondary mesopores present in these samples.

Chanquıá et al. Although the incorporation of Cu decreases the specific surface area, all of the catalysts possess large BET surface areas above 800 m2/g, which is characteristic of the MCM-41 materials.2 Three causes may account for the surface area reduction of the sample prepared with higher Cu content (sample G): the intrapore formation of enough small CuO nanoparticles (no detectable by XRD) and/or [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters finely dispersed within the mesopores, which is evidenced by the apparent increase in the wall thickness; the introduction of Cuδ+ ions into the MCM-41 framework, which lead to an increase in density of the composites; and, finally, the blocking of pores of the meso-structure by bulky CuO oxides.2 Although MCM-41 walls are amorphous, if their chemical composition is modified by adding a certain amount of an element different from Si, it is expected that the average unit cell parameter will be affected. This technique is generally accepted as an additional way of establishing the element location in the silicate framework, or as extra framework species.7,22,43 For our samples, as the content of Cu is increased (Table 4), the main peak corresponding to plane (100) shifted toward lower diffraction angles, and an increase in the parameter a0, consistent with a probable incorporation of Cu into de siliceous structure, was observed. Thus, due to the differences in the ionic radius of Cuδ+ (Cu1+ ) 0.91 Å and Cu2+ ) 0.87 Å) and Si4+ (0.41 Å), the substitution of the larger Cuδ+ ion in place of Si4+ invariably should distort the geometry around Cu from an ideal Td. Therefore, the length of Si-O-Cu bond different from that of Si-O-Si should certainly lead to some structure deformation and consequently contribute to the decrease of surface area. On the other hand, the slight increase in the pore diameter for sample D with respect to sample F could be attributed to the longer length of the Si-O-Cu bond with respect to the Si-O-Si bond due to the incorporation of the metal into the framework. However, it is noteworthy that the pore size for sample G prepared with the higher Cu content does not increase with respect to that for sample D. This feature could possibly be due to the larger amount of metal clusters and/or nano oxides inside the mesoporous channels, which would lead to an apparently thicker wall for sample G. The UV-vis-DR spectra of samples synthesized with different copper content with hydrothermal treatment at 100 °C for 1 day are shown in Figure 6. The original spectra have been deconvoluted into three sub-bands to obtain the relative distribution of copper species. These bands have been assigned to three copper species: isolated mononuclear Cuδ+ cations related to the sub-band between 250 and 400 nm, linear oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters related to the sub-band between 400 and 600 nm, and bulky CuO oxides related to the subband between 600 and 800 nm. Table 5 shows an estimation of the percentage of the different Cu species in the samples. Distribution of these Cu species depends on the quantity of copper in the initial gel. Judging by the relative percentage of sub-bands area, the mononuclear Cuδ+ ions seems to play a dominant role in the case of the samples prepared with low Cu content (samples D and F). Meanwhile, the increase of Cu content favors the formation of oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters and CuO. Furthermore, it is notable the marked increase in the relative proportion of CuO for the sample synthesized with the highest Cu content. The reducibility of copper species in the MMS was investigated by temperature programmed reduction (TPR) experiments, and the corresponding profiles are displayed in Figure 7. This figure shows the normalized H2 consumption curves, obtained by dividing the H2 consumption signal by the amount of catalyst


J. Phys. Chem. C, Vol. 114, No. 3, 2010 1487

Figure 5. N2 adsorption/desorption isotherms and BJH size distribution of the samples modified with different copper content.

TABLE 4: Structural Parameters of the Cu-MMS Modified with Different Copper Content Vtotale (cm3/g) sample

a

d(100) (nm)

a0 (nm)

DBJH (nm)

tw (nm)

primary

secondary

SBET f (m2/g)

G (HT-1d-20) D (HT-1d-60) F (HT-1d-240)

3.77 3.53 3.33

4.36 4.08 3.85

2.63 2.64 2.46

1.73 1.44 1.39

0.628 0.876 0.751

0.344 0.390 0.205

834 1158 1112

b

c

d

a Interplanar spacing (100). b Lattice parameter. c BJH pore diameter. d Wall thickness calculated by a0 - Dp. e Pore volume. f BET specific surface area.

Figure 6. UV-vis diffuse reflectance spectra of the samples modified with different copper content; experimental spectra, thick solid lines; deconvoluted sub-bands, thin lines; assignments: - isolated Cuδ+, - - - oligomerics [Cuδ+ · · · Oδ- · · · Cuδ+]n species, · · · CuO bulky oxides.

employed in these experiments. To get more insight into the TPR results, the profiles were deconvoluted. The peak positions and their contributions derived from deconvolution are summarized in Table 6. The original TPR profiles can be deconvoluted into at least two peaks for samples D and G. Many authors informed that the segregated CuO would be reduced to Cu0 by H2 in one step at about 310 °C.2,44,45 Marchi et al.45 reported that the values of the temperature maximum and the peak width increase with increasing CuO particle size. Next, the two peaks present in our samples in the 200-450 °C range could be assigned to particles of different sizes. The first peak would be possibly due to the reduction of very small CuO nanoparticles and/or [Cuδ+ · · · Oδ- · · · Cuδ+]n nanoclusters inserted in the pores. When the copper content increases, the agglomeration of larger particles is more probable.12,46 Therefore,

the notable increase in the peak area at higher temperature for sample G can be attributed to the presence of larger CuO particles, detectable by XRD (Figure 4). Such a feature can also be inferred from Table 6. On the other hand, it clearly appears from the TPR profiles that the peak areas for the sample G are larger, indicating the greater abundance of CuO nano-oxides. This conclusion is in agreement with AAS data (Table 4) and DR-UV-vis analysis (Table 5). It should be noted that sample F did not present consumption of H2 in these experiments, which is corroborating the very low presence of CuO nano-oxides and/ or [Cuδ+ · · · Oδ- · · · Cuδ+]n nanoclusters, as evidenced in the UV-vis-DR analysis (see Table 5). Finally, the absence of reduction peaks at temperatures above 450 °C could confirm the strong interaction of Cuδ+ isolated species with the framework, which are not reduced even at high temperatures.

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Chanquıá et al.

TABLE 5: Chemical Composition and Copper Species Relative Distribution in Cu-MMS Modified with Different Copper Content distribution of copper species isolated cations a

a

sample

Cu content [wt %]

Si/Cu

G (HT-1d-20) D (HT-1d-60) F (HT-1d-240)

9.45 3.6 0.657

10.15 28.69 160.78

a

oligomeric species

bulky oxides

% area

% Cu

% area

% Cu

% area

% Cu

38.4 52.9 69.5

3.63 1.90 0.46

36.90 32.70 22.20

3.49 1.18 0.15

24.70 14.40 8.30

2.33 0.52 0.047

In the final solid.

Figure 7. TPR profiles of samples modified with different copper content; solid lines are experimental curves, and dashed lines are deconvoluted curves.

TABLE 6: TPR Peak Positions and Concentrations of CuO Nanoparticles relative concentration of CuO nanoparticles (%)

Figure 8. Cu 2p core level photoelectron profile of the samples modified with different copper content.

TABLE 7: BE Values (in eV) and Surface Si/Cu and O/Si Atomic Ratios sample

O 1s

Si 2p

Cu 2p3/2

Si/Cu

O/Si

933.1(64%) 935.1(36%) 933(62%) 935(38%) 932.3(80%) 934.5(20%)

61

2.24

228

2.26

433

2

sample

peak 1 (∼310 °C)

peak 2 (∼350 °C)

G (HT-1d-20)

532.9

103.8

G (HT-1d-20) D (HT-1d-60) F (HT-1d-240)

39 88

61 12

D (HT-1d-60)

532.6

103.4

F (HT-1d-240)

532.8

103.2

Usually, XPS is a powerful technique at superficial level to explore the oxidation state of the transition metal compounds with localized valence d orbitals, due to the different energies of the photoelectrons.47-49 The Cu2+ has mainly d9 character, while the Cu+ is expected to have a full 3d shell. The Cu 2p core level spectra of the Cu-MMS samples prepared with different Cu loadings after 10 min of irradiation are shown in Figure 8. As it is evident from the curve-fitted spectra, the Cu 2p region shows two peaks at ∼934.7 (2) and at ∼954.7 eV (4) assigned to Cu2+ 2p3/2 and 2p1/2, respectively, along with two shakeup satellite peaks at ∼943.20 and ∼963.6 eV. It is known that these satellite signals are attributed to an electron transfer from a ligand orbital to a d orbital of the metal. This transition is a np (ligand) f 3d (metal) transition,50 which is impossible for Cu+ and Cu0 species that have filled d levels, but is mainly a characteristic of bivalent copper.49 In addition, these spectra also exhibited two peaks at ∼932.96 (1) and ∼952.83 eV (3), which are attributable to Cu 2p3/2 and Cu 2p1/2 levels of Cu+ species. Moreover, as it can be seen, these peaks, (1) and (3), are more intense than those corresponding to Cu2+ ions. It is necessary to note that the Cu+ ion is difficult to distinguish from zero-valence copper by XPS. However, we do not believe there is zero-valence copper in the catalyst due to

the calcination process performed under the air conditions; besides the Cu0 signal is not observed in XRD patterns. Therefore, the XPS results showed copper to be present in both oxidation states Cu+ and Cu2+. Table 7 summarizes the binding energy values and surface Si/Cu and O/Si atomic ratios as well as the quantification of both species Cu+ and Cu2+ considering the Cu 2p3/2 region, for the samples modified with different copper contents. The O 1s peak position at ∼532.8 eV is as expected for silica. Likewise, 103.6 eV for the Si 2p peak is in agreement with SiO2-type material.51 The position of these latter peaks for all of the CuMMS samples did not substantially change. On the other hand, the surface Si/Cu atomic ratios are rather higher than those obtained from AAS measurements (see Table 4), indicating a higher atomic concentration of copper species on the internal surface of these materials. Finally, to obtain some inference about the nature of the copper species present in the Cu-MMS, the following analysis was conducted. According to the literature,52-54 the copper may suffer reduction in the vacuum chamber under X-ray irradiation, and consequently a short exposure time is mandatory to avoid the photoreduction of Cu2+ species. As an example, the XPS


J. Phys. Chem. C, Vol. 114, No. 3, 2010 1489 Acknowledgment. G.A.E. and K.S. are CONICET Researchers; C.M.Ch. received a CONICET Doctoral Fellowship. This work was supported by the CONICET, the UTN-FRC, the UNSL of Argentina, and UMA of Spain. We thank J. Bazań Aguirre (UTN-FRC Student) for valuable help on some experimental activities and M. Amaral (INFAP-CONICETUNSL) for his help in recording N2 adsorption data. Finally, thanks to the project MAT2009-10481 Ministerio de Ciencia y Tecnologıá (Spain) for financial support. References and Notes

Figure 9. Cu 2p core level spectra for sample D after ∼10 and ∼30 min of X-ray irradiation.

spectra of sample D after 10 and 30 min of X-ray irradiation are shown in Figure 9. As it can be seen, the Cu 2p3/2 signal corresponding to Cu+ species (∼933 eV) is notably increased after 30 min of irradiation with respect to the Cu 2p3/2 signal corresponding to Cu2+ (∼935 eV). Moreover, the notable increase observed for the ratio between the intensity of the Cu 2p3/2 peak at 933 eV and the shakeup satellite peaks is logically due to the absence of satellite signals for the monovalent copper. Next, we suggest that the increase in the intensity of the peak at ∼933 eV is probably caused by the reduction of Cu2+ to Cu+ in the segregated CuO nano-oxides. At the same time, the remaining Cu2+, even under these photoreduction conditions, could be attributed to Cu2+ isolated species and/or [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters, more strongly interacting with the framework. Conclusions Copper-mesoporous molecular sieves type MCM-41 with various compositions have been prepared through a successful direct incorporation method of the metal in the initial synthesis gel either by aging at room temperature or by hydrothermal treatment. All of the materials exhibited a good structural regularity, even for loadings of metal of ∼10 wt %, besides specific surface areas of around 1000 m2/g and pore volume ∼1 cm3/g, which are typical of these mesoporous materials. It was found that copper is present on the mesoporous silica in the form of various species: isolated mononuclear Cuδ+ cations possibly in coordination with the lattice oxygen; oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters like chains possibly inserted into mesoporous channels; and bulky CuO oxides, segregated of the structure, on the external surface and/or probably trapped also inside the secondary mesopores. By analyzing the UV-vis-DR and XPS techniques, we infer the coexistence of Cu states +1 and +2 (+δ) in these materials. Depending on the preparation method used, samples with different distribution of copper species could be obtained: the hydrothermal treatment promoted the incorporation of both the isolated species Cuδ+ and the oligonuclear [Cuδ+ · · · Oδ- · · · Cuδ+]n clusters; meanwhile, an aging of the initial gel at room temperature favored the formation of bulky CuO. TPR analysis showed that an increase in the copper content in the initial gel promoted the generation of larger CuO particles.

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Nature and Location of Copper Nanospecies in Mesoporous

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