Introduction of Co into the Vacant T-Atom Sites of SiBEA Zeolite as

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Introduction of Co into the Vacant T‑Atom Sites of SiBEA Zeolite as Isolated Mononuclear Co Species Rafal Baran,†,‡,§ Thomas Onfroy,‡,§ Sandra Casale,‡,§ and Stanislaw Dzwigaj*,‡,§ †

Faculty of Energy and Fuels, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30-059 Krakow, Poland Laboratoire de Réactivité de Surface, UMR 7197, Sorbonne Universités, UPMC Univ Paris 06, F-75005, Paris, France § Laboratoire de Réactivité de Surface, UMR 7197, CNRS, F-75005, Paris, France ‡

ABSTRACT: A CoSiBEA zeolite was obtained by a postsynthesis method which consisted of a treatment of tetraethylammonium BEA zeolite with nitric acid in the first step and introduction of Co ions into the vacant T atom sites of the SiBEA zeolite by reaction of aqueous Co(NO3)2 solution with silanol groups. The introduction of Co into SiBEA zeolite as a isolated mononuclear Co(II) was evidenced by XRD, FTIR, diffuse reflectance UV−vis, and XPS. Brønsted and Lewis acidity of HAlBEA, SiBEA, and CoSiBEA zeolites were investigated by FTIR spectroscopy using pyridine as the probe molecule. Reducibility of cobalt in CoSiBEA zeolites was investigated by temperature-programmed reduction of H2. Dispersion of metallic cobalt nanoparticles was determined by transmission electron microscopy.

1. INTRODUCTION

In this work, we have used the method of the introduction of cobalt into the vacant T atom sites of SiBEA zeolite, which allowed obtain Co-containing SiBEA zeolite with isolated mononuclear Co(II). This was evidenced by several physicochemical techniques.

Cobalt-based oxides belong to the materials that are often used in the processes important from the point of view of environmental protection and chemical industry. Co-containing catalysts have shown a very good performance in epoxidation processes,1,2 methane decomposition,3 NOx reduction,4−6 ammonia synthesis,7 ethanol reforming,8,9 and conversion of syngas into hydrocarbons.10−12 However, a major requirement for the catalysts preparation is to obtain a stable and high selective catalyst to hinder side reactions and to avoid fast deactivation. Currently, the most commonly used method for preparation cobalt-containing materials are conventional impregnation,13,14 ion exchange,15,16 precipitation at constant pH,8,17 and hydrothermal synthesis.18 Nevertheless, application of the above-mentioned methods leads to the preparation of catalysts with different forms of metal species, mainly in the extraframework position, poorly selective in many reaction tests. Recently,19,20 we have used a two-step postsynthesis method to introduce of metal into vacant T atom sites of SiBEA by reaction with silanol groups. As reported earlier,21,22 the postsynthesis method is very efficient way for the incorporation of metal into BEA zeolite as isolated mononuclear Co(II). © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. A TEABEA zeolite with atomic Si/Al ratio of 17 was dealuminated by a treatment with nitric acid solution (c = 13 mol L−1, 353 K) for 4 h. Obtained SiBEA with atomic Si/Al ratio of 1300 was washed several times with distilled water and dried at 368 K overnight, as reported earlier.19−21 SiBEA zeolite was contacted with an aqueous cobalt nitrate solution which concentration varied from 3.4 × 10−4 to 6.8 × 10−3 mol L−1 and stirred for 24 h at 298 K. Then, the suspension was stirred in evaporator under vacuum of a water pump for 2 h at 333 K until the water was evaporated. Pink cobalt-containing SiBEA samples after calcination at 773 K (100 K h−1) over 3 h were labeled as C-CoxSiBEA and after Received: June 26, 2014 Revised: August 5, 2014 Published: August 8, 2014 20445

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reduction in flowing 10% H2/Ar (40 mL min−1) as red-CCoxSiBEA, with x = 0.4−2 wt % Co. 2.2. Techniques. XRD experiments were carried out on a PANalytical Empyrean diffractometer equipped with the Cu Kα radiation (λ = 154.05 pm) in the 2θ range of 5°−90°. Textural properties of zeolite materials were investigated by low-temperature (77 K) nitrogen sorption on an ASAP 2010 apparatus (Micromeretics). Samples, before analysis, were calcined under vacuum at 623 K for 5 h. The surface specific areas were determined by the BET method, whereas the micropore volumes were calculated with the Dubinin− Radushkevich equation. Acidic properties of zeolite samples were determined by adsorption of pyridine (Py) followed by infrared spectroscopy. First, the samples were prepared in form of self-supported wafers of ca. 10 mg cm−2 and transferred inside the IR cell. The activation procedure before pyridine sorption was as follows: (i) the wafers were calcined in a static atmosphere of O2 (∼3.0 × 104 Pa) at 723 K for 3 h and then outgassed under secondary vacuum at 573 K (10−3 Pa) for 1 h; (ii) these wafers were contacted at room temperature with gaseous Py (133 Pa) via a separate cell containing liquid pyridine. The spectra were recorded with a Bruker Vector 22 spectrometer (resolution 2 cm−1, 128 scans) after desorption from 423 and 573 K for 1 h. The final spectra were obtained after subtraction of the spectrum recorded before pyridine adsorption from spectrum recorded after pyridine adsorption. The concentration of Brønsted and Lewis acidic sites was estimated using parameters calculated by Emeis.23 Diffuse reflectance UV−vis spectra were recorded in air atmosphere on a Cary 5000 Varian instrument with polytetrafluoroethylene as reference. X-ray photoelectron spectroscopy (XPS) were carried out with Omicron (ESCA+) spectrometer, using an Al Kα (hν = 1486.6 eV). X-ray source was equipped with a flood gun. The area of the analyzed sample was ∼3 mm2. The powder samples were prepared for measurements by pressed them on an indium foil. Binding energy (BE) of Co, Si, and O was calibrated to the C 1s peak at 285.0 eV. Zeolite samples were outgassed at room temperature to a pressure of 10−7 Pa. All spectra were fitted with a Voigt function (a 70/30 composition of Gaussian and Lorentzian functions) in order to determine the number of components under each XPS peak. The TPR-H2 measurements were carried out on an AutoChem 2910 apparatus (Micromeretics) equipped with a thermal conductivity detector (TCD) in the temperature range of 298−1250 K with a linear heating rate of 7 K min−1, hydrogen stream (5% H2/Ar) flow of 40 cm3 min−1, and samples weight about 0.1 g. TEM studies of red-C-CoxSiBEA (C-CoxSiBEA after treatment in flowing mixture of 5% H2/Ar with a linear heating rate 7 K min−1 from 298 to 1250 K) were carried out using a JEOL JEM-100CXII electron microscope operated at an acceleration voltage of 100 keV. Samples for analysis were dispersed in ethanol, and a few drops of this suspension were put on carbon films on copper grids.

Figure 1. Isotherms of nitrogen adsorption at 77 K on HAlBEA, SiBEA, and Co1.0SiBEA. Full symbols: adsorption; empty symbols: desorption. For convenience, the data set for SiBEA and Co1.0SiBEA are shifted upward along the Y-axis.

Table 1. Textural Properties of HAlBEA, SiBEA, and Co1.0SiBEA specific surface area

micropores volume

samples

SBET (m2 g−1)

Vmic (cm3 g−1)

HAlBEA SiBEA Co1.0SiBEA

626 612 595

0.26 0.25 0.24

micropore volume (0.24−0.36 cm3 g−1) which is typical of BEA zeolites24,25 (Table 1). The two-step postsynthesis method applied for sample preparation did not influence textural properties because mesoporosity formation has not been observed after dealumination and introduction of cobalt into SiBEA support. Evidence of contraction and/or expansion of zeolites structure may be established by XRD measurements from the position of the diffraction peak (302) at 2θ = 22.5°−22.6°, within a given series of zeolite samples.26−28 The d302 spacing, calculated from the corresponding 2θ value, decreases from 3.956 Å (HAlBEA) (with 2θ of 22.45°) to 3.919 Å (SiBEA) (with 2θ of 22.66°) suggesting a matrix contraction, consistent with the dealumination of zeolite BEA. Introduction of Co into SiBEA resulted in increase of the d302 spacing, from 3.919 Å (SiBEA) (with 2θ of 22.66°) to 3.935 Å (Co0.4SiBEA) (with 2θ of 22.57°) and to 3.945 Å (Co2.0SiBEA) (with 2θ of 22.51°) (Figure 2). Those changes in position of the main diffraction reflex might suggest expansion of the BEA structure and introduction of cobalt ions into the vacant T atom sites of the framework of SiBEA zeolite according to our earlier investigation on VSiBEA29 and MoSiBEA30 zeolites. Furthermore, all diffractograms are characteristic of zeolite BEA. Amorphization and appearance of other crystalline phases did not take place, suggesting that removal of aluminum by nitric acid treatment did not affect zeolite structure.

3. RESULTS AND DISCUSSION 3.1. XRD and FT-IR Evidence for Incorporation of Cobalt into the Framework of SiBEA. Figure 1 shows the nitrogen adsorption/desorption isotherms of HAlBEA, SiBEA, and Co1.0SiBEA type I according to IUPAC. HAlBEA, SiBEA, and Co1.0SiBEA have BET surface area (612−626 m2 g−1) and 20446

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Figure 2. XRD patterns recorded at room temperature of HAlBEA, SiBEA, Co0.4SiBEA, Co1.0SiBEA, and Co2.0SiBEA.

Figure 4. FTIR spectra recorded at room temperature of HAlBEA, SiBEA, and Co2.0SiBEA after adsorption of pyridine (133 Pa) for 1 h at room temperature and desorption at 423 K for 1 h.

Table 2. Amounts of Brønsted and Lewis Acidic Centers in HAlBEA, SiBEA, and Co2.0SiBEA SiBEA

Co2.0SiBEA

Brønsted acidic centers (μmol g−1)

samples

temp (K) HAlBEA 423

334

15

15

Lewis acidic centers (μmol g−1)

573 423 573

239 135 137

8 2 1

0 177 111

related to aluminum in framework and extraframework position, respectively, at 3608 cm−1 attributed to bridging acidic hydroxyls Si−O(H)−Al,31,32 at 3745 and at 3735 cm−1 attributed to isolated external silanol and isolated internal silanol, respectively, and a broad band at 3540 cm−1 attributed to H-bonded SiO−H groups.33 Treatment of parent HAlBEA zeolite with nitric acid led to disappearance of the three bands at 3781, 3660, and 3608 cm−1 related to zeolite aluminum species, whereas two broad bands at 3705 and 3520 cm−1 appeared, which is proof of the formation of vacant T atom sites associated with silanol groups in places of removed aluminum species. Contact of SiBEA zeolite with cobalt nitrate solution resulted in the decreasing of the intensity of the infrared bands at 3735, 3705, and 3520 cm−1 due to specific reaction of hydroxyl groups (Figure 3), located in vacant T atom sites, with the cobalt ions leading to the introduction of cobalt into SiBEA as isolated framework mononuclear Co(II) species. 3.2. Nature and Strength of Acidic Centers Determined by FTIR. To investigate acidic properties of zeolites, FTIR spectroscopy with pyridine sorption was applied.

Figure 3. FTIR spectra recorded at room temperature of HAlBEA, SiBEA, and Co2.0SiBEA in the vibrational range of the OH group.

The FTIR technique was applied to perform the changes in environment and nature of hydroxyl groups present in BEA zeolite before and after dealumination step as well as after incorporation of Co into zeolite structure. The FTIR spectrum of HAlBEA contains six characteristic bands of hydroxyl groups (Figure 3): at 3781 and 3660 cm−1 20447

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Figure 5. DR UV−vis spectra recorded at ambient atmosphere of Co1.0SiBEA, C-Co1.0SiBEA, and red-C-Co1.0SiBEA.

Difference spectra of pyridine adsorbed on HAlBEA, SiBEA, and Co2.0SiBEA are shown in Figure 4. In the case of HAlBEA zeolite two bands related to pyridinium cations are seen at 1546 and 1638 cm−1, indicating the presence of Brønsted acidic sites. Furthermore, the presence of Lewis acidic sites is also confirmed because of appearance of bands at about 1622, 1611, 1491, and 1455 cm−1 which are typical for coordinately bonded pyridine molecules according to data on CrSiBEA,34 SiBEA, and VSiBEA.35 Pyridinium cations (bands at 1546 and 1638 cm−1) and pyridine bonded to Lewis acidic sites (bands at 1622, 1491, and 1455 cm−1) are present in the IR spectra even after outgassing of HAlBEA at 573 K (Table 2), suggesting strong character of Brønsted and Lewis acidic sites in this zeolite. In contrast, for SiBEA very low intense bands at 1638, 1546, and 1491 cm−1 are observed, indicating the presence of only very little amounts of Brønsted and Lewis acidic sites (Figure 4 and Table 2). The introduction of cobalt in SiBEA resulted in arising of the band at 1450 cm−1, as shown for Co2.0SiBEA in Figure 4. The formation of new Lewis acidic sites is due to the presence of isolated pseudotetrahedral Co(II) species in Co2.0SiBEA zeolite. 3.3. Isolated Pseudotetrahedral Co(II) Evidenced by DR UV−vis and XPS. DR UV−vis spectra of fresh Co1.0SiBEA (Figure 5) pink sample contain two characteristic bands at 470 and 515 nm attributed to cobalt species in octahedral environment in line with recent investigation on CoAPO-536 and CoMOR37 materials. Besides these bands, two others are observed at 585 and 640 nm related to Co(II) species in distorted pseudotetrahedral coordination, in agreement with earlier work on cobalt-containing zeolites.4,38,39 So, it seems that in fresh Co1.0SiBEA zeolite two types of cobalt species occur simultaneously. Calcination of pink Co 1.0 SiBEA sample led to the disappearance of bands at 470 and 515 nm, consistent with change of color into intense blue of C−Co1.0SiBEA due to dehydratation and creation of very stable isolated, pseudotetrahedral Co(II) species incorporated into the SiBEA frame-

Figure 6. XP spectrum recorded at room temperature of Co 2p (A), Si 2p (B), and O 1s (C) core level of C-Co1.0SiBEA zeolite. 20448

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work. The broad band at 300 nm is likely to be related O2− → Co(II) charge transfer (CT) transition.40 The DR UV−vis spectrum of red-C-Co1.0SiBEA (after exposure to moist air) contains two bands at 595 and 670 nm characteristic of pseudotetrahedral Co(II) and a lowintensity band at 515 nm related to octahedral Co(II) species. It shows that cobalt is present in red-C-Co1.0SiBEA zeolite as stable pseudotetrahedral Co(II) species. The XPS measurements were carried out in the BE regions corresponding to O 1s, Si 2p, and Co 2p to investigate the cobalt species present in CoSiBEA zeolite. The XP spectrum of C-Co1.0SiBEA zeolite (Figure 6) exhibits doublet of signals corresponding to the spin−orbit coupling of the Co 2p region. The main signals of Co 2p3/2 at 780.5 eV and of Co 2p1/2 at 796.0 eV may be assigned to Co(II), most probably present as pseudotetrahedral Co(II) species incorporated into framework of zeolite, in line with studies on CoZSM-541 and Co/BEA zeolites and CoSiO4.42 The two remaining peaks at 785.5 and 800.9 eV are related to satellite arised due to presence of shakeup electrons typical of cobalt(II) species. The BE value at 102.4 eV in the Si 2p spectrum is characteristic of Si−O−Si bonds present in zeolite structure and very close to that reported earlier for CoZSM5 and CoERI zeolites.43 An additional XPS peak at 103.8 eV might be related

Figure 7. TPR experiment of H2 consumption (5% H2/Ar) for CCo1.0SiBEA and C-Co2.0SiBEA.

Figure 8. TEM images and histograms of Co(0) nanoparticles distribution in red-Co1.0SiBEA and red-Co2.0SiBEA. 20449

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Further studies are underway on CoSiBEA zeolites to determinate their catalytic properties in SCR of NO with ammonia.

to silicon in Si−O−Co linkage involved in formation of Co(II) isolated species in Co1.0SiBEA zeolite. Furthermore, in the O 1s region four different XP bands appear with the most intense at 531.8 eV due to oxygen in Si− O−Si structure of zeolite and at 531.2 eV probably attributed to bridging oxygen between Si and Co in Si−O−Co linkages, in line with recent study on Fischer−Tropsch Co/BEA catalysts.44 The latter XPS band suggests that cobalt ions are incorporated into the SiBEA framework. 3.4. Reducibility of the Cobalt Species Determined by TPR. The reducibility of cobalt species in C-CoSiBEA zeolite has been determined by H2-TPR in the wide range of temperatures. The TPR patterns of C-Co1.0SiBEA and CCo2.0SiBEA (Figure 7) exhibit only one peak with maximum at 1160 and 1105 K, respectively. This reduction temperature, much higher than observed for other Co-containing zeolites like Co-USY45 and Co-H-ZSM-5,46 suggests very strong interaction of cobalt with SiBEA matrix and high stability of cobalt species. Furthermore, in TPR patterns other peaks in lower temperature region related to extraframework cobalt clusters and/or cobalt oxide are not observed. Thus, TPR data are in agreement with XRD, XPS, and DR UV−vis results, indicating that only one kind of cobalt species is present in C-CoSiBEA, the most probably isolated pseudotetrahedral cobalt(II) incorporated into the zeolite framework. 3.5. TEM Results. Transmission electron microscopy images were taken on reduced C-CoSiBEA samples in order to determine influence of preparation procedure on dispersion and size of Co(0) particles. TEM images and histograms of particles distribution of red-C-Co 1.0 SiBEA and red-CCo2.0SiBEA are shown in Figure 8. They reveal uniform distribution of Co(0) particles in red-C-CoSiBEA samples with their average size of 8 nm. A very uniform distribution of Co(0) nanoparticles with almost absence of nanoparticles bigger than 15 nm is related to use the two-step postsynthesis procedure for preparation Cocontaining CoSiBEA zeolites. Cobalt present in C-CoSiBEA as isolated pseudotetrahedral Co(II) in framework positions was transformed, after reduction, into small and well-dispersed metallic particles. Zeolite with such well-dispersed Co(0) nanoparticles may be very interesting material for application in catalytic processes: Fischer−Tropsch synthesis, dry reforming of methane, or methane decomposition. In contrast, the application of conventional wet impregnation procedure for preparation Co-containing BEA zeolite led to a much broader distribution of nanoparticules Co(0) with higher size (results not shown).

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], tel + 33 1 44 27 21 13 (S.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by the National Science Center “PRELUDIUM” UMO-2012/07/N/ST5/00171 (R.B., S.D.). REFERENCES

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CONCLUSIONS The two-step postsynthesis method is an excellent tool to obtain cobalt species in the framework of SiBEA as a mononuclear pseudotetrahedral Co(II). Isolated pseudotetrahedral Co(II) species are assigned as a new Lewis acid site in CoSiBEA zeolite. TPR patterns of C-CoSiBEA contain one peak at 1105−1160 K that could be attributed to reduction of isolated pseudotetrahedral Co(II) into Co(0). XPS, XRD, and TPR results reveal that only one kind of framework pseudotetrahedral Co(II) is present in CoSiBEA. Co(0) nanoparticles obtained by the reduction of isolated pseudotetrahedral Co(II) species are small and homogeneously dispersed in zeolite structure. 20450

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dx.doi.org/10.1021/jp506375v | J. Phys. Chem. C 2014, 118, 20445−20451