Evolution of Copper Nanospecies in the Synthesis Stages of MCM-41

Article pubs.acs.org/JPCC

Evolution of Copper Nanospecies in the Synthesis Stages of MCM-41-Type Mesoporous Molecular Sieves Corina M. Chanquía,*,†,‡,§ Elin L. Winkler,‡,§ María T. Causa,‡ and Griselda A. Eimer*,†,§ †

Centro de Investigación y Tecnología Química, Universidad Tecnológica Nacional, Facultad Regional Córdoba, Maestro López esq. Cruz Roja Argentina, 5016 Córdoba, Argentina ‡ Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Av. Ezequiel Bustillo 9500, R8402AGP, San Carlos de Bariloche, Río Negro, Argentina § CONICET, Avenida Rivadavia 1917, C1033AAJ, Buenos Aires, Argentina ABSTRACT: The redox behavior of copper nanospecies in the synthesis stages of MCM-41-type materials has been followed by ESR technique. These coppermodified mesoporous molecular sieves (Cu-MMS) were synthesized by direct incorporation method, and different treatments of the initial synthesis gel were evaluated. From ESR and UV−vis-DRS data, a synergistic approach about the distribution of copper species in the different catalysts has been made: the hydrothermal treatment favors the incorporation of isolated cooper species into the framework, whereas both the aging treatment and the lack of subsequent treatment promote the formation of copper clusters into the mesoporous channels. The Hamiltonian parameters g// ≈ 2.3 and g⊥ ≈ 2.06 have confirmed the presence of isolated Cu2+ ions in octahedral coordination with tetragonal distortion within siliceous framework, whereas the g ≈ 2.2 value was assigned to the presence of Cu2+ clusters. Only the catalyst aged at room temperature has presented additional signals characteristic of Cu2+ dimers. Finally, it is found that during the template removal process at 773 K the so-called “self-reduction” phenomenon of isolated cupric ions takes place: a 50−70% decrease in the ESR signal intensity was observed for the final Cu-MMS.

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reduction of both Cu2+ ions is achieved and followed by a spontaneous desorption of oxygen, giving rise to Cu+□Cu+ centers, where the □ symbol stands for a vacancy of an extralattice oxygen.4,15,16 The second mechanism, proposed by Larsen et al.,17 is based on the water elimination upon thermal treatment under vacuum and the formation of hydroxyl radicals (•OH) and O− ions. According to this mechanism, a fraction of copper is reduced, but a second fraction still remains as Cu2+ ions, in the form of Cu2+O− pairs. This mechanism was proposed on the basis of electron spin resonance (ESR) data, which showed that the intensity of the Cu2+ ESR signal diminishes upon thermal treatment under vacuum and is restored by simple addition of water even at room temperature. It is noteworthy that the paramagnetic Cu2+ ions are easily observed by ESR, whereas the diamagnetic Cu+ ions are ESRsilent. However, Lo Jacono et al.18 suggested that the loss of the ESR signal was not actually due to the reduction of Cu2+ ions but to changes in the coordination of the Cu2+ ions upon thermal treatment; this mechanism would give rise to ESRsilent species (such as dimeric Cu2+ moieties) without the formation of cuprous ions. Later, Turnes Palomino et al.8 proposed that the thermal reduction in Cu-exchanged ZSM-5

INTRODUCTION After the discovery of the copper-exchanged ZSM-5 zeolites activity for the NO direct decomposition1−5 and NOx reduction by hydrocarbons,6,7 a new set of studies on the chemistry of copper in different molecular sieves has been undertaken. These processes deserve considerable attention because nitrogen oxides are considered to be a major cause of air pollution. Recently several papers (experimental and computational) have presented information about the oxidation and coordination state of copper ions in zeolites, which is essential to understand the nature of the active centers in the working catalyst.8−13 In this sense, the main aim is to study the nature of the active sites and the physicochemical conditions to form the maximum amount of the same. Particular attention was focused on the study of the redox chemistry of copper ions inside the zeolite framework. For instance, it is well known that hydrated Cu2+ species in copper-exchanged zeolites undergo a progressive reduction upon thermal treatment under vacuum or in flowing inert gas (outgassing) at increasing temperatures.8,9,14 This reaction is usually called “self-reduction” processes. However, several discrepancies exist in the interpretation of the experimental results. Indeed, two main mechanisms were proposed to explain the self-reduction processes. The first mechanism consists of the condensation of two Cu(OH)+ neighboring species with elimination of a water molecule and subsequent formation of Cu2+O2−Cu2+ moieties. In a consecutive step, the © 2012 American Chemical Society

Received: November 6, 2011 Revised: February 6, 2012 Published: February 7, 2012 5376

dx.doi.org/10.1021/jp210652x | J. Phys. Chem. C 2012, 116, 5376−5382

The Journal of Physical Chemistry C

Article

catalysts were synthesized from a gel of molar composition: Si/ Cu = 240, TEAOH/Si = 0.3, CTABr/Si = 0.3, and 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 stirred 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 heat-treated at 358 K for 30 min to remove the ethanol used in the solution and produced in the hydrolysis of TEOS. This gel has been manipulated in three ways: without treatment, by aging treatment 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 (∼298 K) for 1 day. In the case of the hydrothermal-treated samples, the gel was transferred into a Teflon-lined stainless-steel autoclave and kept in a furnace at 373 K for 1 day under autogenous pressure. In the three cases, the final solids were filtered, washed with distilled water until pH ∼7, and dried at 333 K overnight. The color of these “as-synthesized” samples was light blue. To remove the template, we subjected the catalysts to desorption with N2 flow (45 mL/min) at 773 K, maintaining this temperature for 6 h. Later, calcination under dry air flow (45 mL/min) at 773 K maintaining this temperature for 6 h was performed to ensure a complete removal of organic compounds. The final color of these “calcined” samples was beige. The calcined samples exposed to atmospheric humidity are designed as “hydrated calcined” and the same dried at 473 K overnight as “dry calcined”. As it can be seen in Table 1, the samples were named

zeolites has two steps: The first part of the thermal treatment (between room temperature and 473 K) produces mainly the loss of water, without appreciable reduction of Cu2+ ions. The dehydration is accompanied by a rearrangement of the cupric species inside the zeolitic channels, where a fraction of Cu2+ ions becomes ESR-silent, as suggested by the presence of Cu2+ dimers. A further increase in the temperature (from 473 up to 673 K) causes the oxygen elimination and reduction of cupric ions to Cu+. In addition, these authors also study the reoxidation at different temperatures in oxygen atmosphere of the copper ions in Cu-ZSM5. The Cu-ZSM5 samples dehydrated at 673 K, mainly containing Cu+-ions, are not reoxidized by contact with molecular oxygen at room temperature. Instead, when a thermal treatment is performed at 673 K under “dry” conditions, an intense ESR spectrum corresponding to Cu2+ ions is observed. Besides, its ESR intensity is ∼60% smaller than that corresponding to as-synthesized materials (assumed as reference). Therefore, as it can be seen, the interpretation of the ESR experimental results of copper-modified zeolites even continue in discussion, whereas for the MCM-41-type mesoporous molecular sieves this subject has not been discussed so far. Previous studies have shown that metal-containing ordered mesoporous molecular sieves could be more effective catalysts than their amorphous counterparts, like metal-containing amorphous silica.19−21 These nanostructured materials arise from a supramolecular self-assembly process in the presence of ionic surfactants as templates during the mesophase formation. MCM-41 is a member of the M41S family, which possesses a hexagonal arrangement of uniformly sized bidimensional pores in the range from 2 to 10 nm; besides it has large and accessible internal areas around 1000 m2/g.22,23 Therefore, the synthesis of MCM-41 mesoporous materials, modified with transition metals, definitively opens up a new possibility for the catalysts with uniform pores in the mesoporous region. In particular, the copper-containing mesoporous molecular sieves (Cu-MMS) could be an effective catalyst for NO reduction because copper promotes NOx decomposition on various substrates. Besides, it is the most promising transition metal for de-NOx operations, competing favorably even with precious metal-based catalysts.24−26 Nevertheless, up to now, such mesoporous materials have been mostly tested in liquid-phase reactions of large molecules, where their mesoporosity is an obvious advantage.27−31 The nature and distribution of copper nanospecies in MCM41-type mesoporous molecular sieves as well as their morphological, structural, surface and chemical characteristics have been recently presented and discussed by us.32,33 The aim of the present article is to investigate the creation and evolution of copper nanospecies in the different synthesis stages of CuMMS prepared by direct incorporation methods of the metal in the initial synthesis gel. Such Cu-species evolution was followed by ESR measurements, and its distribution in the final solids was determined by synergistic approach between ESR and UV−vis-DRS techniques.

Table 1. Treatment Conditions and Synthesis Stages of Cu-MMS

a

catalyst

treatment type

synthesis stage

Cu-M/HT-as Cu-M/HT-hc Cu-M/HT-dc Cu-M/AT-as Cu-M/AT-hc Cu-M/AT-dc Cu-M/WT-as Cu-M/WT-hc Cu-M/WT-dc

hydrothermal

as synthesized hydrated calcineda dry calcinedb as synthesized hydrated calcineda dry calcinedb as synthesized hydrated calcineda dry calcinedb

aging

without

By exposure to atmospheric humidity. bOvernight at 473 K.

Cu-M/TT-ss, where “TT” indicates the treatment type applied at the synthesis gel (i.e., hydrated calcined, HT; aging treatment, AT; without treatment, WT) and “ss” indicates the synthesis stage of the catalyst (i.e., as-synthesized, as; hydrated calcined, hc dry calcined, dc). Characterization Techniques. The materials were characterized by electronic spin resonance (ESR), ultraviolet− visible diffuse reflectance spectroscopy (UV−vis-DRS), and X-ray photoelectron spectroscopy (XPS). The Cu content in the catalysts was determined by inductively coupled plasma mass spectrometry (ICP-MS) using an AA Varian Spectra spectrophotometer. The ESR measurements were performed in an ESP-300 Bruker spectrometer, operating at a frequency close to 9.4 GHz (X-band) and between 100 and 300 K. To improve the signal-to-noise ratio, all of the spectra presented in this work were measured at 130 K. UV−vis-DR spectra of the materials were recorded using a Perkin-Elmer precisely Lambda

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EXPERIMENTAL METHODS Catalyst Preparation. The copper-containing mesoporous molecular sieves (Cu-MMS) were prepared using cetyltrimethyl ammonium bromide (CTABr, Aldrich) as template. Tetraethoxysilane (TEOS, Fluka ≥98%) 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 the addition of a tetraethylammonium hydroxide (TEAOH, Sigma-Aldrich) 20 wt % aqueous solution. The 5377



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where / Z and / h correspond to the interactions of the spin S with the external magnetic field (Zeeman interaction) and with its nuclear spin I (hyperfine interaction), respectively, being I(Cu) = 3/2. The parameters g and A are second-order tensors. For tetragonal symmetry, the principal values of the g and A tensors are g⊥ = gx = gy, g// = gz and A⊥ = Ax = Ay, A// = Az. In addition, it can be assumed that the principal axes for g and A are parallel to each other. As the materials studied in this work are powder samples, the ESR spectrum consists of the sum of the spectra corresponding to crystallites oriented in all of the possible space directions. Therefore, an asymmetric absorption line is obtained with two anomalies corresponding to resonances of crystallites oriented perpendicular (the maximum absorption) and parallel (minimum absorption) to H0. As a consequence, in the derivative spectra shown in Figure 1, two groups of peaks are clearly observed centered in the H// and H⊥; from these values, g// and g⊥ were obtained. This Figure also indicates the splitting of the absorption resonance due to the hyperfine interaction, which allows us to determine the A parameter. Using the experimentally determined parameters (summarized in Table 2): g⊥ = 2.064(5), g// = 2.31(1), A⊥ = 27(2) G, A//= 166 (3) G, ΔH⊥= 8 G, and ΔH//= 30 G, the spectrum of Cu-M/HT-as catalyst was simulated. The simulated spectrum is showed in Figure 1b. This simulation presents a well-defined adjust, which confirms the existence of isolated Cu2+ ions in octahedral coordination with tetragonal distortion within the siliceous structure. In this work, these isolated Cu2+ species will be called α center. Figure 2A shows the ESR spectra of the catalysts synthesized by hydrothermal treatment at 373 K for the different synthesis stages (as-synthesis, hydrated, and dried calcined). All of the spectra were normalized by the cooper content (in grams) of each sample, calculated from the ICP-MS measurements. As it can be seen in Figure 2A, the calcined samples show a remarkable decrease in the ESR signal. Moreover, a further reduction of the ESR signal was observed in the dried calcined sample. The Cu-M/HT-hc spectrum can be also described by the spin Hamiltonian (1) corresponding to isolated Cu2+ ions located in tetragonal symmetry sites with octahedral coordination (α species).34 The experimentally determined parameters are g⊥ = 2.070 (5), g// = 2.38(2), and A//= 135 (5) G, whereas A⊥ could not be resolved due to the intrinsic broad line width. In the case of the Cu-M/HT-dc sample, because of the low ESR signal-to-noise ratio, just an estimation of the gyromagnetic factor could be made, resulting in g ≈ 2.07. Because the ESR absorption intensity is proportional to the number of resonant paramagnetic species,35 the Cu2+ relative concentration at the different synthesis stages can be quantified. The ESR intensity (IESR) was calculated by the double integral of the derivative ESR absorption spectra. Therefore, the inset of Figure 2A shows the evolution of IESR for the different synthesis stages of Cu-M(HT) catalysts. It is noteworthy that after the calcination, only the 30% of the Cu2+ initial concentration contributes to the ESR signal. According to the literature data,8,9 this result is probably due to the autoreduction of the majority of the Cu2+ ions to Cu+ during N2 desorption at 773 K, followed by a partial Cu+-reoxidation during the air-calcination process at 773 K. The mentioned autoreduction mechanism is consistent with the light-turquoise color of the as-synthesized sample, which is characteristic of hydrated Cu2+ ions, with respect to the beige color for the calcined samples. Figure 2B,C shows the ESR spectra corresponding to the CuM/WT (without treatment) and Cu-M/AT (aging treatment)

35 spectrophotometer. 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−Monk 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 8 software. Curve-fitting calculations were useful for determining the location of the bands and their areas; the fitting confidence was χ2 ≤ 0.0005 and R2 ≥ 0.99. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer.

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RESULTS AND DISCUSSION The ESR spectroscopy is a suitable technique to perform the characterization of the sitting-coordination of 3d cations in molecular sieves. In magnetic 3d ions, the energy degeneration of the Hund state is partially lifted when the ions are doped on material thorough electrostatic interactions between the 3d electrons and the lattice anions. The resulting electronic configuration depends on the ion site symmetry. In a cubic field, the five 2D orbitals are split into two energy levels:34 (a) three orbitals t2g: xy ; xz; yz oriented parallel to the (110) axis and (b) two orbitals eg: x2−y2 ; 3r2−z2 oriented in the direction of the (100) axis. The Cu2+ 3d9 case may be treated as a spin S = 1/2 hole (positive charge) in the 3d10 complete shell, where a minimum electrostatic energy is obtained for the eg orbitals. Then, the Jahn−Teller effect acting on this configuration splits the two eg levels distorting the original cubic field. The type of distortion will determine which of the two orbitals is occupied. The evolution of the Cu2+ species on the Cu-MMS, in the different synthesis stages, was studied by the ESR technique. To obtain well-resolved ESR spectra, the Si/Cu molar ratio in the initial synthesis gels was adjusted to 240. Figure 1a shows the

Figure 1. (a) ESR spectrum of the Cu-M/HT-as sample measured at 130 K, where the H//, H⊥, and the splitting of the absorption resonance due to the hyperfine interaction are signaled. (b) Simulated powder spectrum.

ESR spectrum of the Cu-M/HT-as catalyst measured at 130 K. This spectrum can be described by the following spin Hamiltonian: / = /z + /h = μBS ·g ·H0 + S ·A ·I

(1) 5378



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Table 2. ESR Parameters of the Different Magnetic Resonant Species: Isolated Cu2+ (α), Cu2+ Clusters ( β), and Exchange Coupled Cu2+ Dimers (δ) α

β

catalyst

g⊥

g//

A⊥ (G)

A// (G)

Cu-M/HT-as Cu-M/HT-hc Cu-M/HT-dc Cu-M/WT-as Cu-M/WT-hc Cu-M/WT-dc Cu-M/AT-as Cu-M/AT-hc Cu-M/AT-dc

2.064(5) 2.070(5) 2.07 (2) 2.065(5) 2.07(2) 2.07 (2) 2.065(5) 2.07(2) 2.07 (2)

2.31(1) 2.38(2)

27(2)

166 (5) 135 (5)

2.29(2)

27 (2)

169 (5)

2.29 (1)

27(2)

166(5)

δ

g

g1

g2,3

g4

6.25(3) 5.86(3)

3.03(2) 2.40 (2) 2.36(2)

1.57 (2)

2.18(5) 2.24(2) 2.21(2) 2.18(2) 2.19(2)

Figure 2. ESR spectra measured at 130 K of the hydrated calcined Cu-M/HT (A), without treatment Cu-M/WT (B), and aging treatment Cu-M/AT (C) samples. The different synthesis stages: as-synthesized, hydrated calcined, and dry calcined are labeled by “as”, “hc”, and “dc”, respectively. The α, β, and δ labels indicate the resonance lines assigned to isolated Cu2+ ions, Cu2+ clusters, and Cu2+ dimers, respectively. The insets show the evolution of the ESR signal intensity (IESR).

indicate that the α species have higher reactivity compared with the Cu2+ clusters. Finally, Figure 2C shows the ESR signal around g ≈ 2 corresponding to the Cu-M/AT catalysts. These spectra are more complicated because they are formed by the overlapping of several absorption resonances. The evolution of the α and β species with the synthesis stages is similar to that previously described for the Cu-M/HT and Cu-M/WT samples: whereas the ESR signal of the isolated ions diminishes after the calcination, the signal of the Cu2+ clusters remains almost unchanged. In addition, in Figure 3, another three resonance lines at Hr ≈ 1150, 2850, and 4260 G can be observed besides the α and β lines. These signals can be assigned to the resonance of exchange coupled pairs of Cu2+ ions.39−41 These exchange-coupled dimers are described by the following Hamiltonian

catalysts for their different synthesis stages, respectively. In the case of the Cu-M/WT samples, the ESR spectra can be described by two resonant centers, α and β. The α center presents a similar behavior to that of the copper ions in the CuM/HT samples and can be assigned to isolated Cu2+ ions located in tetragonal symmetry sites. The corresponding ESR parameters for the Cu-M/WT-as sample, obtained from the spin Hamiltonian eq 1, are presented in Table 2. After the thermal treatments, the α centers follow a similar evolution to that observed for the copper ions in the Cu-M/HT system: (i) their ESR signal shows a remarkable decrease after the calcination and (ii) a further reduction of the ESR intensity is observed in the dried sample (Cu-M/WT-dc). Besides the resonance of the isolated Cu2+ ions, a second ESR line (β center) is observed. This β resonance corresponds to a single broad (ΔH ≈ 150 G) line center at g ≈ 2.2, which is more evident after the calcination. Similar ESR signals are usually observed for Cu2+ clusters, where variations in the Cu ligand field broaden the absorption lines, and, as a consequence, the hyperfine splitting is smeared-out, besides the line becomes more symmetric due to the higher degree of disorder.36−38 It is noteworthy that, whereas the ESR intensity of the α species decreases after the calcination, the same remains almost constant in the case of the β-species; these behaviors could

/ = /z + /h + /D + /Ex

viz. ,

/ = g μBH(S1z + S2z) + A(S1I1 + S2I2) + ∑ DijS1zS2z − JS1S2 i,j=x ,y,z

(2) 2+

where S1, S2, I1, and I2 are the spin operators of the two Cu ions and the corresponding nuclear operators, respectively, Dij are the dipolar tensors, and J is the exchange constant. When the 5379



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Figure 4. Cu 2p core level photoelectron profile of the hydrated calcined Cu-M/HT catalyst.

short acquisition time of 10 min was used to examine the Cu 2p and CuLMM XPS regions to avoid, as much as possible, the photoreduction of the Cu2+ species. Therefore, the Cu 2p region shows two peaks at 934.5 and at 954.7 eV assigned to the doublet Cu2+ 2p3/2 and 2p1/2 respectively, along with two shakeup satellite peaks at ∼941.7 and ∼962.3 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 an np (ligand) → 3d (metal) transition,42 which is impossible for Cu+ and Cu0 species that have filled d levels but it is mainly a characteristic of bivalent copper.43 In addition, this spectrum also exhibits two peaks at ∼931.7 and ∼952.1 eV, which are attributable to the doublet Cu 2p3/2 and Cu 2p1/2 levels for Cu+ species. It is noteworthy that these latter peaks are more intense than those corresponding to Cu2+ ions; the relative proportion of Cu+ with respect to Cu2+ is 80/20, which is obtained from deconvolution of the XPS signal. In concordance with ESR results, the XPS technique confirms that the copper is present in this Cu-M/HT-hc material, in both oxidation states Cu+/ Cu2+, with a higher relative proportion of Cu1+.32 Finally, to inquire about the distribution of Cu-species in the MCM-41 type structure, the UV−vis-DR spectra of the hydrated calcined Cu-MMS synthesized under the three different types of treatment are shown in Figure 5. The original spectra have been deconvoluted into three sub-bands to facilitate the assignment to the different copper species. According to a detailed study about the UV−vis-DR spectra of Cu-MMS reported by us elsewhere, these bands can be assigned to the following copper species:7,11,32,44−51 (1) isolated mononuclear Cuδ+ cations possibly in coordination with lattice oxygen, related to the sub-band between 250 and 400 nm; (2) linear oligonuclear [Cuδ+···Oδ−···Cuδ+]n clusters like chains possibly inserted into mesoporous channels, related to the sub-band between 400 and 600 nm; and (3) bulky CuO oxides segregated of the siliceous structure, related to the sub-band between 600 and 800 nm. To obtain a rough estimation of the wt % 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 ICP-MS). The overall metal content in the

Figure 3. ESR spectra corresponding to the hydrated and dry Cu-M/ AT calcined sample measured at 130 K. The resonances of the α, β, and δ centers are signaled with arrows.

anisotropic interactions are vanishing small (D ≪ J) the first-order equation for the resonance fields is Hr = hν/g μB ± [J − A(m I1 − m I2)]/2g μB ± φ /g μB, i = 1 − 4 φ = [J + A(m I1 − m I2)]/2

(3)

Therefore, in this approximation, the four (ΔMS = ±1) transitions are reduced to three: two of them overlap at hν/g μB and the other two are located at (hν ± J)/g μB. The resonances assigned to the Cu2+ dimers are signaled in Figure 3 (δ species), where it can be noticed that the lines are better resolved in the dried sample, probably due to the thinner line width. The insets of Figure 2B,C show the double integral of the ESR spectra for the Cu-M/WT and Cu-M/AT samples. In the first system, the IESR diminishes up to a 47% after the calcination. Instead, in the Cu-M/AT samples, the IESR is reduced to a 55% after the calcination treatment and then increases after the drying. These results would be consistent with the expected reduction of the isolated Cu2+ ions to Cu+, whereas the Cu2+ clusters would be more stable during the desorption with N2. The reason of the largest reduction in the IESR parameter for the Cu-M/HT system is probably due to the fact that this material mostly presents isolated Cu2+ ions. In summary, the ESR results indicate that the hydrothermal treatment promotes the incorporation of isolated cooper species into the framework, whereas the system without treatment and the gel aging at room temperature favor the formation of copper clusters into the mesoporous channels. Moreover, after the calcination, the isolated Cu2+ ions are reduced to Cu+, whereas the Cu2+ clusters remain almost unchanged. Usually, XPS is a powerful technique at superficial level to explore the oxidation state of the transition metals with localized valence d orbitals due to the different energies of the photoelectrons. The assignment of the XPS signals to the different copper states in MCM-41 type mesoporous molecular sieves was already discussed by us elsewhere.32 To assert the coexistence of the Cu+/Cu2+ species in the Cu-M/HT-hc sample, its Cu 2p core level spectrum is showed in Figure 4. A 5380



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Figure 5. UV−vis diffuse reflectance spectra of the hydrated calcined catalyst synthesized under different types of treatment and with Si/Cu molar ratio of 240. The thick solid lines correspond to the experimental spectra and the thin lines correspond to the deconvoluted sub-bands assigned to:  Cuδ+-isolated; −−− [Cuδ+···Oδ−···Cuδ+]n clusters; and --- CuO bulky oxides.

removal of the template from the as-synthesized Cu-MMS. This behavior was assigned to the autoreduction of the Cu2+ ions to Cu+ during N2 desorption at 773 K. It is noteworthy that after the thermal treatment the ESR intensity corresponding to the isolated Cu2+ species decreases and it remains almost constant for the Cu2+ clusters case; this fact could indicate that the isolated Cu2+ species have higher reactivity compared with the Cu2+ clusters. From a synergistic approach between ESR and UV−vis-DRS techniques, it was found that the hydrothermal treatment promotes the incorporation of isolated cooper species into the framework, whereas the system without gel treatment as well as a gel aged at room temperature favor the formation of copper clusters into the channels.

final solid as well as the distribution of copper species are presented in Table 3. As it can be seen, the sample synthesized Table 3. Chemical Composition and Copper Species Relative Distribution in Cu-MMS Synthesized under Different Treatment Types distribution of copper species δ+

Cu -isolated catalyst Cu-M/ WT-hc Cu-M/ AT-hc Cu-M/ HT-hc

Cuδ+ clusters

CuO

Cu content (wt %)

area %

Cu %

area %

Cu %

area %

Cu %

0.26

0.40

0.10

0.45

0.12

0.15

0.04

0.46

0.29

0.13

0.49

0.23

0.22

0.10

0.66

0.61

0.40

0.32

0.21

0.07

0.05

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.M.C.); [email protected]. edu.ar (G.A.E.). Tel/Fax: 54-351-4690585.

under hydrothermal treatment showed higher incorporation of overall copper in the final solid. Then, it could be concluded that the distribution of Cu species depends on the nature of treatment applied to the initial synthesis gel regardless of the copper content employed.32 The results showed in the Table 3 also indicate that the hydrothermal treatment promotes the incorporation of isolated Cuδ+ species into the framework, whereas an aging of the initial gel at room temperature favors the formation of oligonuclear [Cuδ+···Oδ−···Cuδ+]n clusters into the channels and the bulky CuO nanoparticles, which it is agreement with the ESR results.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by CONICET, UTN-FRC, and CABCNEA of Argentina. We thank Dr. H. Thomas and Lic. N. Firpo (CINDECA-UNLP, Argentina) for DRUV−vis data ́ and Dr. E. Rodriguez-Castelló n (UMA, Españ a) for the XPS measurement.

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CONCLUSIONS ESR studies were utilized to elucidate the evolution of copper nanospecies in the synthesis stages of MCM-41 type materials prepared by the direct incorporation method under different treatments of the initial synthesis gel: hydrothermal at 373 K, without treatment, and aging at room temperature. All of the catalysts present ESR signals corresponding to isolated Cu2+ ions located in tetragonal symmetry sites in octahedral coordination within siliceous framework. However, the materials prepared without gel treatment as well as by gel aging at room temperature presented additional ESR signals characteristic of Cu2+ clusters and Cu2+ dimers. A 50−70% decrease in the ESR signal intensity was observed after the

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