Synthesis and Characterization of Nanocomposites with Strong

Sep 19, 2012 - Energy Laboratory, Corporate R&D Center, Samsung SDI Co., Ltd., 428-5, Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do. 446-577, Republ...
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Synthesis and Characterization of Nanocomposites with Strong Interfacial Interaction: Sulfated ZrO2 Nanoparticles Supported on Multiwalled Carbon Nanotubes Changchang Liu,† Sungchul Lee,‡ Dong Su,§ Zhiteng Zhang,† Lisa Pfefferle,† and Gary L. Haller*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8682, United States Energy Laboratory, Corporate R&D Center, Samsung SDI Co., Ltd., 428-5, Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Republic of Korea § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ‡

S Supporting Information *

ABSTRACT: We discuss the synthesis of a composite of ZrO2 nanoparticles supported on multiwalled carbon nanotubes (MWCNT). Using X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM), the ZrO2 were found to be 2−3 nm tetragonal crystalline nanoparticles. Strong interfacial interaction between the ZrO2 nanoparticles and the MWCNT surface was observed by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge and the oxygen K-edge, and this strong metal oxide/support interaction leads to small ZrO2 particle size and thermal stability. The ZrO2/MWCNT was converted into a solid acid catalyst by sulfation, and the properties of S-ZrO2/ MWCNT were studied. The nature of the acid sites was probed by S K-edge and Zr L-edges (L3, L2, L1) XANES as well as catalytic probe reaction of cyclohexane dehydrogenation/cracking. Such composites would be good candidates for potential catalysis applications in fuel cell electrodes and biomass processing.

1. INTRODUCTION

The increasing use of carbon materials (e.g., graphite, activated carbon, fullerenes, nanotubes, or graphene) as catalyst supports requires comprehensive understanding of their properties (e.g., surface area and porosity), particularly of the surface chemistry, through which the catalytically active phase (e.g., metal) interacts with the carbon support, and having such interaction, in return, affects the catalytic activity of the catalyst.7−10 It is the intrinsic characteristics of carbon materials that make them good catalyst supports. For example, the carbon structure is not only resistant to both acidic and basic media but also is stable at high temperatures (>1000 K). Moreover, the hydrophobic surface of carbon can be chemically modified to tailor its hydrophilicity, which is useful in anchoring the catalytically active phase.11,12 It is therefore important to study the surface chemistry in carbon-supported catalysts.13 The discovery of carbon nanotubes (CNTs)14 posed many promising possibilities in their application as catalyst supports to disperse and stabilize metal particles for heterogeneous catalysis.15 For one thing, given their conductive properties, CNTs are good candidates for fuel cell electrode catalysis.16,17 An early fuel cell application by Li et al.16 involved multiwalled

In heterogeneous catalysis, novel design of a catalyst that involves both nanoscale particles and catalyst support aims to achieve control over the particle size and/or particle size distribution and interaction with the support,1 which still remains challenging due to the complicated architecture and variation of the nanostructure. Introduced in 1978 by Tauster et al., the term “strong metal−support interactions” was used to describe the propensity of group VIII noble metals for the chemisorption of H2 and CO when these metals are supported on certain oxide substrates (e.g., SiO2 or Al2O3) and reduced at high temperature, in comparison with the diminished, if not completely eliminated, chemisorption when using TiO2 as the support.2,3 Such differences manifested by various metal− support systems may have multiple origins,4 and while there has been an extensive study of the effects of particle size for supported metal catalysts on metal−support interaction, dispersion, and catalytic activity, supported metal oxides are less studied.4 Regardless of the configuration of a complex composite system, however, it is acknowledged that the interaction between the nanoscale particles and the support exists only at the interfacial areas in the form of ionic or covalent chemical bonding or of the so-called “chemical glue” that is difficult to either define or characterize.4,6 © XXXX American Chemical Society

Received: March 12, 2012 Revised: September 19, 2012

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As described in a previous report from our laboratory,34 the synthesis of ZrO2/MWCNT followed a high-temperature grafting process of the precursor zirconium acetylacetonate, Zr(acac)4 (purchased from Sigma-Aldrich), onto the functionalized MWCNT. Ethanol was used as both the reducing agent (for highly oxidized functional groups on the MWCNT) and the solvent, and the reduction/grafting was accomplished simultaneously in a batch reactor (stainless steel autoclave with Teflon linear) at 20 atm and 180 °C for 6 h. The sample was filtered, washed, and dried overnight at 120 °C. A thermal treatment (annealing) was then performed in flowing He at 450 °C for 2 h to decompose the grafted precursor to ZrO2 particles. Bulk ZrO2 was purchased from Z-Tech Corporation with code SF-ULTRA0.5 as reference material. The bulk ZrO2 was physically mixed with functionalized MWCNT by grinding for 15 min with a mortar and pestle until the mixing was uniform (as indicated by color), with a weight loading of ZrO2 being 15 wt %. The ZrO2/MWCNT composite was then impregnated with aqueous (NH4)2SO4 (with molar ratio of S/Zr = 2 if all S was retained) and thermally treated again in order to form sulfated ZrO2/MWCNT (S-ZrO2/MWCNT). This thermal treatment was performed either (1) in flowing He at 450 °C for 2 h or (2) in flowing mixture of 0.5% O2/He for 10 min (4% O2/He mixed gas balanced with ultrahigh purity He, both purchased from Airgas). The commercial sulfated zirconia catalysts were purchased from Alfa Aesar. 2.2. Characterization Techniques. The catalysts were characterized through thermogravimetric analysis (TGA), physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) at the C and O K-edges (NEXAFS), S K-edge (XANES), Zr Kedge (XANES and EXAFS), and three Zr L-edges (XANES). The S-ZrO2/MWCNT solid acid catalysts were also characterized through the probe catalytic reaction of cyclohexane dehydrogenation/cracking. Thermogravimetric Analysis (TGA). The final weight loading of ZrO2 was measured using TGA, which was conducted on a Setaram Setsys 1750 instrument in flowing air (Ultra Zero grade, purchased from Airgas). During the measurement, the temperature was held at 200 °C for 30 min to remove the adsorbed water and ramped to 1000 °C at a rate of 10 °C/min and then held at 1000 °C for 30 min. A second ramp on the oxidized sample was repeated to establish the baseline. Physisorption. Nitrogen adsorption/desorption isotherms were measured using a Quantachrome Autosorb-3B. The specific surface area was calculated using the BET method. X-ray Diffraction (XRD). The XRD patterns were collected using the Bruker D8 Focus powder XRD with Cu Kα radiation at λ = 0.154 nm. Transmission Electron Microscopy (TEM). TEM was performed using a JEOL2100F microscope operated at 200 kV at the Center for Functional Nanomaterials, Brookhaven National Laboratory. X-ray Absorption Spectroscopy (XAS). XAS data were collected at different beamlines at National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). NEXAFS spectra at both C K-edge and O K-edge were taken at beamlines U7A and U4B. Each sample was pressed into a pellet on copper tape and mounted to the sample bar that was later inserted into an ultrahigh vacuum chamber for measurement. At the C K-edge, partial electron yield (PEY) data were

carbon nanotubes (MWCNT) as the support for a cathode catalyst (highly dispersed Pt nanoparticles) of a direct methanol fuel cell (DMFC), and this catalyst achieved higher activity of the cathode oxygen reduction reaction (ORR) and better performance of the DMFC compared to using commercial carbons as the support. Besides the utilization of CNT to attenuate catalyst poisoning through their high surface area (minimizing the use of precious metal), another strategy to increase the CO tolerance of the Pt catalysts is to either use a Pt-based alloy or a Pt/metal oxide composite,18−23 justified by the bifunctional mechanism24 and the electronic effect.25 More recently, emphasis has been placed on catalyst systems that combine Pt, metal oxide, and carbon, such as Pt-CeO2/C,26 Pt/ SnO2−CNT,27 and Pt-V2O5/C.28 Later on, ZrO2 was brought into this picture because (1) ZrO2 has been widely used as the support of Pt in low-temperature water-gas shift reactions (where CO reaction/tolerance is also a problem)29 and (2) when sulfated, S-ZrO2 becomes a proton conductor and a strong solid acid30 and Pt/S-ZrO2 would make a bifunctional catalyst. 31 Through comparison of the electrochemical oxidation of CO, Song et al.32 indicated that Pt-ZrO2/ MWCNT made possible a more facile CO oxidation than Pt/ MWCNT. This approach was amplified by Guo et al.33 by further modification of the ZrO2 through sulfation, forming PtS-ZrO2/MWCNT, and this catalyst demonstrated an even higher methanol electro-oxidation activity than Pt-ZrO2/ MWCNT or commercial Pt/C. Inspired by these studies, our attention is now drawn toward the interfacial interactions between the ZrO2 nanoparticles and the surface of MWCNT. In this work, we designed a synthesis route of ZrO2/MWCNT using a high-temperature grafting method followed by thermal annealing at moderate temperatures that creates stronger interfacial interaction between the nanoparticles and MWCNT. By probing such interfacial interactions using NEXAFS, tentative synthesis mechanisms are proposed. We also examined the crystalline phase and particle size of the ZrO2 to confirm our hypothesis that stronger interactions at the interface between the ZrO2 nanoparticles and the MWCNTs surface would result in smaller (better dispersed) and more thermally stable nanoparticles. Catalytic probe reactions (cyclohexane cracking) using sulfated ZrO2/MWCNT (S-ZrO2/MWCNT) catalysts were carried out, and preliminary results showed good activity.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The multiwalled carbon nanotubes (MWCNT) were purchased from Cheap Tubes Inc. (>95% purity). These MWCNT (or pristine MWCNT, as described later) have an outer diameter of 10−20 nm, an inner diameter of 3−5 nm, and a length of 10−30 μm, as described by the supplier. The pristine MWCNT were functionalized by nitric acid oxidation, using ∼68 wt % (15 M HNO3) nitric acid (purchased from J.T. Baker) in reflux at the boiling temperature (∼120 °C) for 2 h (different reflux times were applied for the purpose of studying the amount of surface oxygen containing groups introduced to the MWCNT surface). The mixture was then filtered and washed with 1000 mL of deionized water in five batches (5 × 200 mL) to remove excess acid and soluble oxidation products. The resulting material was then dried at 120 °C overnight in a thermostated oven and ground to fine powder. These functionalized MWCNT were used as the starting materials to prepare the ZrO2/MWCNT composites. B

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collected at −40 VDC bias and aligned to the π* transition at 285 eV of an inner carbon mesh reference; at the O K-edge, PEY data were collected at −40 VDC bias and aligned to the I0 (the incident beam) inherent dip at 531.2 eV. At U4B, total electron yield (TEY) data were obtained at the C K-edge and aligned the same way as used at U7A. All NEXAFS spectra were normalized to the edge jump. Both S K-edge and three Zr Ledge (L3, L2, L1) XANES were measured at beamline X15B. Each sample was brushed homogeneously onto a stripe of ∼1 cm × 2 cm Kapton tape, which was then mounted to a Hepurged sample chamber. The fluorescence signal was recorded on a 500 eV range that covers the XANES of all four edges. Zr K-edge XANES and EXAFS data were collected at beamline X18B, aligned to Zr foil, and normalized to the edge jump. Each sample was pressed into a pellet and was scanned ex situ by transmission absorption. 2.3. Catalytic Reactions. The acid catalytic activity of SZrO 2 /MWCNT catalysts was probed by the catalytic dehydrogenation and cracking reaction of cyclohexane. The temperature-programmed reaction experiments were carried out in the gas phase at 1 atm pressure and space velocity of 1240 mL (STP) cyclohexane per gram of catalyst per hour.

Figure 1. Qualitative evidence of the introduction of OCG on the MWCNT surface after nitric acid treatment for different times through the increase in the amount of surface C−O bonding in C K-edge NEXAFS. The inset is an expansion of the C−O bonding region.

2a). The same nominal loading yields higher final loading for the 5 h-treated MWCNT than for the 2 h-treated MWCNT, indicating more OCG bonding sites on the 5 h-treated MWCNT. In Figure 2b, for the same final ZrO2 loading, the BET surface area of ZrO2/MWCNT is higher using the 5 htreated MWCNT as the support compared to the 2 h-treated MWCNT. The rationalization is that longer acid treatment introduces more sidewall damage on the MWCNT and therefore opens up the cylindrical pores of the carbon tubes.37 All samples characterized in the remainder of the paper used 2 h acid treatment as the standard. To study the structure of ZrO2 nanoparticles formed in the ZrO2/MWCNT composites, we first performed powder XRD on a series of references (Figure 3a) and the ZrO2/MWCNT composites of various ZrO2 loadings (Figure 3b). Commercially available monoclinic zirconia (ZrO2)m powder and sulfated tetragonal ZrO2 catalyst support (S-ZrO2)t were tested, and the XRD patterns correlate very well with what has been reported in the literature.41,42 Both bulk materials have sharp peaks in their XRD patterns, indicating large particle size. Amorphous ZrO2 does not exhibit diffraction peaks but can be thermally converted to ordered structures.43 This is the observation for the grafted Zr precursor. It is X-ray amorphous as prepared and crystalline after a 450 °C annealing. On the basis of XRD, we could differentiate between the monoclinic and tetragonal structure of the ZrO2 nanoparticles supported MWCNT. They do not have the thermodynamic most stable monoclinic structure of pure ZrO2. While cubic ZrO2 (111)c and (200)c diffraction peaks coincide with the tetragonal (111)t and (002)t (200)t peaks, the cubic structure is only stable at high temperature (>2650 K) or when doped with stabilizing ions such as Y, so we assume that the particles we analyze are tetragonal.44 In Figure 3b, the peak at 2θ = 26° is the (002) diffraction for MWCNT, at 2θ = 30° for tetragonal ZrO2 (111)t, and at 2θ = 35° for tetragonal ZrO2 (002)t and (200)t, with both diffraction peaks line-broadened by the small particle size. Figure 3b shows that both the ZrO2 (111)t and the (002)t (200)t peaks grow in intensity as the ZrO2 loading increases from a low loading of ∼15 wt % to a high loading of ∼40 wt %. We also noticed that before thermal annealing, no distinctive diffraction patterns for ZrO2 are visible. In order to form

3. RESULTS AND DISCUSSION It has been reported in the literature that creating oxygencontaining groups (OCG) on the surface of a carbon support improves the dispersion of catalysts, and this can be achieved by oxidation using reagents such as HNO3, H2SO4/HNO3, H2O2, KMnO4, or ozone.13,35−37 In our catalyst preparation, we followed the procedure (with a modified treatment time) of the acid treatment reported previously by our group13 to functionalize the MWCNT, i.e., using concentrated nitric acid (15 M HNO3) refluxing at its boiling point. After the acid treatment, three major functionalities are believed to be introduced into the CNT surface: hydroxyl (C−OH), carbonyl (CO), and carboxyl (COOH) groups, as verified by XPS and chemical derivatization (CD-XPS) studies.37 We used C K-edge NEXAFS to probe the local bonding environments of C associated with O on the MWCNT surface by monitoring the PEY signal38 as a function of the incident photon energy. As seen in Figure 1, and in accordance with the literature,39 the π* (C ring) transition lies at 285 eV and the σ* (C ring) transition lies at 292−294 eV; the approximate range for the π* and σ* transitions of C−O bonding appears between 287 and 291 eV. In this region, there is a distinctive increase at 288.9 eV for the MWCNT after HNO3 treatment in comparison with the pristine MWCNT, and this indicates the success in introducing OCG onto the surface of MWCNT. The π* and σ* C−O transitions are unresolved. The extent of the incorporation of OCG during the oxidation process increases linearly with increasing weight percentage of HNO3 used,40 and our results indicate that longer oxidation time also enhances the amount of OCG introduced (Figure 1). We treated the pristine MWCNT with HNO3 of the same concentration at three different treatment times30 min, 2 h, and 8 hand the intensity of the C−O peak in the C K-edge NEXAFS increases with increasing oxidation time. Using these surface OCG as attachment points, we incorporated Zr species onto the MWCNT surface to construct the nanocomposites using a grafting synthesis followed by thermal annealing, as previously reported.34 We grafted Zr(acac)4 precursor on MWCNT functionalized by 2 and 5 h with the nominal loading of ZrO2 ranging from ∼15 wt % to as high as 67 wt % (Figure C

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The final loading of ZrO2 affects its particle size in the resulting ZrO2/MWCNT composites. Particle size analysis was performed based on the XRD patterns of ZrO2/MWCNT of various loadings (Figure 3b), all annealed at 450 °C. We applied a three-peak Gaussian fitting to deconvolute the three peaks at 2θ = 26°, 2θ = 30°, and 2θ = 35° and used the line broadening at the ZrO2 tetragonal (111)t peak in the Scherrer equation to calculate the particle size of ZrO2. The correlation between particle size and final loading is illustrated in Figure 5. A slight increase in the particle size is observed when the ZrO2 loading increases and the particles are 2−3 nm. This particle size calculation is complemented by HRTEM images, as shown in Figure 6. A detailed study of particle size and size distribution as determined by XRD, TEM, and small-angle Xray scattering will be published elsewhere.45 At moderate annealing temperature (450 °C), both the low loading (∼15 wt %) and the high loading (∼40 wt %) ZrO2/MWCNT have crystalline ZrO2 particles of the size of 2−3 nm, but for the higher loading sample the particles are more densely packed and agglomerated. This suggests that the ZrO2 particles are more thermally stable against sintering when the loading remains low. Using the particle size given by XRD (Figure 5) and the surface area by BET (Figure 2b), i.e., 2.04 nm and 220 m2/g for 15 wt % ZrO2/MWCNT, we calculated the approximate coverage of ZrO2 particles (interfacial area based on hemispheric particles) on MWCNT to be ∼15%. The particle size analysis indicates that the surface of functionalized MWCNT appears to stabilize only a limited amount of ZrO2 particles. In other words, when the ZrO2 loading is high, the nanoparticles become more mobile and begin to sinter more severely. Moreover, the tetragonal phase of ZrO2 is thermodynamically unfavorable but is known to be stabilized by sulfur as a surface species (as in S-ZrO2).30 It is possible that the interface interaction of the nanoparticles of ZrO2 with MWCNT stabilizes the tetragonal structure as is the case for S-ZrO2, the tetragonal reference in Figure 3a. However, it is more likely that this is simply an inherent property of nanoparticles of ZrO2. That is, Radha et al.46 have concluded that tetragonal ZrO2 becomes thermodynamically stable at room temperature for particle sizes less than 28 ± 6 nm for a hydrated surface and less than 34 ± 5 nm for anhydrous ZrO2. Both of the limits are greater than the particle sizes observed in this study so we conclude that it is the stabilization of small particle size by the MWCNT that determines the tetragonal structure. In order to probe the interfacial interactions between the MWCNT surface and the ZrO2 particles, we measured C Kedge NEXAFS and focused on the C and O molecular orbitals, as shown in Figure 7. Five spectra at the C K-edge are shown in Figure 7a,b. These spectra have been normalized to the edge jump. Our attention is focused on the intensity and binding energy of the peaks in the region of 287−291 eV, which are assigned to the C−O bonds π* and σ* transitions (Figure 7b). No surface OCG are detected on the pristine MWCNT, but this peak (288.9 eV) becomes apparent after oxidation by nitric acid. A physical mixture of the functionalized MWCNT and bulk ZrO2 (at the same mass ratio as the synthesized ZrO2/ MWCNT composite) has very similar intensity and binding energy for the C−O bonding (note that the O in the bulk ZrO2 does not contribute to the spectrum unless it is bonded with C), as would be expected because interaction between the large crystallites of ZrO2, when ground with MWCNT, would be surprising. After grafting Zr(acac)4 to the MWCNT, a

Figure 2. Comparison of the effects of different nitric acid treatment times (2 and 5 h) on (a) nominal and final ZrO2 loadings in wt %; (b) BET surface area of the resulting ZrO2/MWCNT.

crystalline ZrO2 nanoparticles, thermal annealing is required to decompose the ligand-exchanged acetylacetonate residue, zirconyl species, and perhaps amorphous ZrO2. In the work reported by Song et al.,32 they used a sol−gel method to prepare a ZrO2/MWCNT composite that served as the support for further Pt deposition, and their MWCNT was found to be covered by either a layer of amorphous ZrO2 or ZrO2 nanoparticles of 11−12 nm, depending on the molar ratio of Pt:ZrO2. In our annealed ZrO2/MWCNT composites, the XRD patterns indicate tetragonal structured ZrO2 crystalline particles. The TEM result of ∼15 wt % annealed ZrO2/ MWCNT is shown in Figure 4, where a narrow size distribution is observed from the low-magnification image of Figure 4a. Figure 4b shows a high-resolution TEM (HRTEM) image of the nanoparticles. The selected area fast Fourier transform (FFT) from one nanoparticle, shown in the inset of Figure 4b, can be indexed as the [100] orientation of the tetragonal ZrO2. HRTEM image from a [100] oriented single particle is a good match to the ZrO2 tetragonal structure model (Figure 4c−e), and our HRTEM result is consistent with the XRD result. D

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Figure 3. XRD patterns of (a) comparison between MWCNT, commercial ZrO2 (monoclinic), and S-ZrO2 (tetragonal) and 24 wt % ZrO2/ MWCNT; (b) ZrO2/MWCNT with various loadings (normalized to the MWCNT diffraction peak).

lower binding energy with a narrower peak shape, relative to functionalized MWCNT. This is interpreted as evidence for C−O−Zr bonding (or at least strong interactions between these three elements) where the shift in the C−O binding energy results from C bonded to Zr through O. It is noted that when the synthesis procedure is repeated on functionalized MWCNT without the Zr(acac)4 precursor, the intensity of the peak at 288.9 eV is essentially removed by the same thermal treatment see Figure 7c. That is, the C−O bond is thermally stabilized by formation of a C−O−Zr bond between the MWCNT and ZrO2 particles. The above synthesis of ZrO2/MWCNT combined the reduction of various OCG by ethanol to hydroxyls and grafting of Zr(acac)4 to the resulting hydroxyls with ethanol as the solvent. There will be some degree of ligand exchange of acetylacetonate by ethanol:

significant increase is observed in the peak intensity of C−O and a small shift to lower binding energy (288.6 eV). The structure of the grafted complex is not known, but the following reaction is expected (which may be repeated, depending on the availability of additional hydroxyl or other reactive functional groups at the reaction sites on the MWCNT): −C−O−H + Zr(acac)4 → −C−O−Zr(acac)3 + Hacac

The increased intensity and the shift may result from the acetylacetonate ligands (residue from the precursor after ligand exchange) or ethoxy ligands (resulting from ligand exchange with the solvent), which have not yet been decomposed completely through thermal annealing. No crystalline ZrO2 particles have yet been formed (as shown by XRD), and the Zr complex is expected to be predominantly isolated species with some combination of C−O−, H−O−, and acetylacetonate ligands. After annealing, the C−O peak returns to an intensity comparable to functionalized MWCNT but is shifted 0.4 eV to

Zr(acac)4 + EtOH → Zr(acac)3 (EtO) + Hacac E

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Figure 4. TEM images at different magnifications of ∼15 wt % ZrO2/ MWCNT thermally annealed at 450 °C in He for 2 h. (a) An image of ×100 000. (b) HRTEM image; the inset is a selected area fast Fourier transform (FFT) from one single nanoparticle of (b). The unit cell model of ZrO2 tetragonal structure is shown at (c), and its [100] projection is shown at (d), with Zr in green and O in red. A comparison between the same nanoparticle of (b) and the [100] projection of unit cell model is shown in (e).

Figure 6. HR-TEM images at different length scales (10 nm, 5 nm) of (a, b) ∼15 wt % ZrO2/MWCNT thermally annealed at 450 °C in He for 2 h and (c, d) ∼40 wt % ZrO2/MWCNT thermally annealed at 450 °C in He for 2 h.

the toluene-dissolved Zr(acac)4 solutions have been properly degassed (with N2 flow to remove any O2 and/or H2O), the only oxygen remaining to react with in the grafting of Zr(acac)4 would have been already bonded to the C of the MWCNT. As a comparison of Figures 7a and 7b, which uses the synthesis with the simultaneous reduction and grafting, the sequential reduction and grafting synthesis gives the same pattern of intensity changes and energy shifts in the C K-edge NEXAFS, as shown in Figure 8. This assures that the liquid phase decomposition of Zr(acac)4 is not a major concern, and the more expedient combination of reduction and grafting can be used to form covalently bonded surface Zr complexes that can be thermally decomposed into nanoparticles of ZrO2. While the overall pattern of intensity and shift in energy are the same as seen in Figure 7, we note that the energies are consistently higher by 0.1−0.5 eV. The latter value appears to be outside of measurement error (the resolution was about 0.1 eV). While the spectra in Figures 7 and 8 were both run on U7A (but not on the same day), if we assume a 0.25 eV offset in the absolute energy, there would be good agreement. In any case, the relative energies are in very good agreement and so are the intensities. In Figure 9, catalysts with the 10, 15, 30, and 40 wt % loadings were compared at the O K-edge. The spectra of these catalysts are normalized to the edge jump. Reference spectra of bulk ZrO2 and functionalized MWCNT are shown in the inset of Figure 9. A linear combination of the two references provides a reasonable fit to the observed spectra; details are provided in the Supporting Information. On the basis of the theory and experiments for the first row transition metal oxides, discussed by Chen et al.,47 the four peaks in the reference spectra of Figure 9 can be assigned by analogy with TiO2. The two most intense peaks at lowest energy are assigned to the transition from O 1s to the Zr 4d−O 2pπ (absorption edge) and the Zr 4d−O 2pσ molecular orbitals of the Zr−O bond, in the order

Figure 5. Particle size of ZrO2 in the ZrO2/MWCNT composites with various loadings based on the Scherrer calculations of XRD patterns. Each sample was thermally annealed at 450 °C.

Suppose there was some degree of decomposition of either of these precursors in solution prior to grafting to MWCNT and/ or some oligomer of the Zr−O unit was formed in the liquid phase. This might result in the formation of some Zr precursor that could be physically deposited on the MWCNT so that a species of ZrO2 not bonded to MWCNT could be formed. Thus, we devised a second synthesis route that separated the reduction of various OCG by ethanol at 180 °C and used toluene as the solvent for Zr(acac)4 grafting at 110 °C. The pure hydrocarbon solvent limits side reaction that might produce a Zr−O oligomer in oxygen-containing solvents, such as ethanol. Once both the MWCNT toluene suspension and F

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Figure 8. C K-edge NEXAFS of ZrO2/MWCNT grafted in toluene, as comparison with the NEXAFS results of synthesis in ethanol; (b) is the 287−291 eV region (C−O bonds) of (a).

Figure 7. C K-edge NEXAFS of ZrO2/MWCNT grafted in ethanol in comparison with pristine MWCNT, functionalized MWCNT, and a physical mixture of bulk ZrO2 with functionalized MWCNT. (b) is the 287−291 eV region (C−O bonds) of (a). (c) C K-edge NEXAFS of functionalized MWCNTs after thermal treatment at 450 °C in He for different times. Note that these data were obtained at a different beamline (U4B instead of U7A and TEY detection instead of PEY) but were still comparable to the data in (a) and (b). There is a noticeable background for pristine MWCNT in the 287−291 eV region, in the U4B data, not seen in the U7A data (compare with Figure 1).

Figure 9. O K-edge NEXAFS of thermally treated ZrO2/MWCNT of various ZrO2 loadings (10, 15, 30, and 40 wt %).

2pπ orbitals. The peak at 540.6 eV reflects the surface oxygen on MWCNT, a 1s O transition to a C−O σ* orbital, and this peak is absent in the bulk ZrO2 reference. Through comparison among the four ZrO2/MWCNT samples, we can see that with

of increasing energy. The two higher energy peaks (unresolved) are assigned to 1s O transition to Zr 5s−O 2pσ and Zr 5p−O G

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the increase in the loading, the 536.5 eV intensity increases and there is a consistent decrease in the remaining amount of surface OCG (540.6 eV), and the change in these two intensities creates an isosbestic point between them, as is expected when one species is reacting to form a second species (see also Figure S1(f) in Supporting Information). This indicates the tendency of an increased coverage of surface OCG (the attachment sites for Zr) on the MWCNT (in the case of 40 wt % ZrO2/MWCNT, the OCG peak at 540.6 eV is essentially removed). The changes in intensity with increased coverage are indicated by the arrows in Figure 9. We therefore hypothesize that the available surface OCG could only provide limited amount of attachment sites for the grafting of Zr, and once the ZrO2 consumes all of the OCG bonding sites, such as in the case of high loading ZrO2/ MWCNT, some of the ZrO2 particles are simply physically deposited, i.e., with less or no interfacial interactions with the MWCNT surface. This also explains why the ZrO2 particles of high loading ZrO2/MWCNT annealed at 450 °C have more mobility and are more subject to sintering (i.e., less thermally stable).45 The ZrO2/MWCNT composite was also characterized via EXAFS at the Zr K-edge. Figure 10a compares the XANES spectrum of 15 wt % ZrO2/MWCNT composite with the spectra of references including Zr foil, Zr(acac)4, and bulk ZrO2. The white line intensity of a metal oxide at its absorption edge is a reflection of the density of empty d-orbital states at the Fermi level on the metal, and the apparent loss of d-orbital electrons (behaving as if the metal is oxidized, but in the case of Zr(IV), just a reflection of electron density shift in the Zr−O bonds) results in higher white line intensity, and vice versa. In our case, the precursor Zr(acac)4, ZrO2/MWCNT, and bulk ZrO2 all have the Zr formal oxidation states of 4+ (in comparison to Zr0 in the metal foil). However, the electron density around the Zr cation centers varies with their surrounding chemical structures. This variation in Figure 10a indicates that the electron density (or deficiency) of the Zr(IV) ion in the 15 wt % ZrO2/MWCNT catalyst is in-between that of Zr(acac)4 and that of bulk ZrO2. In other words, the conjugating π effect of the four acetylacetonate ligands in Zr(acac)4 is more electron-withdrawing than that of the C rings on MWCNT (in the case of ZrO2/MWCNT), which, in turn, has a more electron-deficient Zr(IV) center than the Zr center in bulk ZrO2. After Fourier-transforming the EXAFS data to R space, the spectra provide qualitative information on the coordination environment of Zr in the ZrO2/MWCNT composites, as shown in Figure 10b. To retain a higher S/N, the Fourier transform (FT) was calculated from the k2-weighted EXAFS oscillations in the range k = 3.0−13.3 Å−1 (where k is the photoelectron wavenumber), and the window of the backFourier transform was R = 1.1−2.3 Å and has been phase corrected. In Figure 10b, the FT of Zr foil gives the bond length of Zr−Zr at R = 3.23 Å, and the peak corresponding to the first coordination shell of O atoms differs in position for the Zr(acac)4 reference and those of the 15 wt % ZrO2/MWCNT and bulk ZrO2. In contrast to bulk ZrO2, the FT of ZrO2/ MWCNT composite does not have the second and third coordination shells of Zr and O atoms at R = 3.40 Å, which is the result of the small particle size of the ZrO2 in the composite.48 The peaks at R < 1 Å are due to low-frequency oscillation in the atomic background and therefore have no physical meaning. A quantitative fitting of the first (oxygen)

Figure 10. (a) Zr K-edge XANES on the precursor Zr(acac)4, 15 wt % thermally treated ZrO2/MWCNT composite, and the bulk ZrO2 reference; (b) Fourier transforms based on k2-weighted corresponding EXAFS signals.

coordination sphere is complicated by a near-neighbor environment of eight oxygen atoms with two distinct but similar bond lengths. We have not yet attempted a quantitative fitting but rely on the visual second neighbor (Zr−O−Zr) scattering which disappears in our nanoparticles on MWCNT (see Figure 10b). Study of Sulfated ZrO2/MWCNT Composites (S-ZrO2/ MWCNT). We impregnated sulfate species with aqueous (NH4)2SO4 onto ZrO2/MWCNT with a S/Zr ratio of 2, and the 450 °C thermal treatment was repeated afterward to react the sulfate with the Zr surface to form the acid sites. In commercial synthesis this reaction is performed in air at 600− 650 °C, and at temperatures below 800 °C, sulfur-containing Zr(OH)4 is transformed to tetragonal zirconia.30 Because of the fact that MWCNT would be consumed by oxidation at high temperature, annealing in an inert atmosphere (flowing He) was used to decompose the (NH4)2SO4 precursor. The MWCNT, however, can withstand a mild oxidation at 450 °C, e.g., 10 min in a 0.5% O2 balanced with He. We investigated the S K-edge XANES in S-ZrO2/MWCNT, but since the S Kedge (2472 eV) is bracketed by the three L-edges of Zr (Zr L3H

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edge at 2223 eV, Zr L2-edge at 2307 eV, and Zr L1-edge at 2532 eV), we measured the data of the entire region to probe both elements (S and Zr) at the same time (Figure 11a,b). Sulfur K-edge XANES (S-XANES) have been utilized as fingerprints for determination and quantification of different S forms in the petroleum industry.49 By comparing a series of S compounds, it was established that the absorption edges progressively shift about 10−12 eV to higher energy as the formal oxidation states of S increase from −2 to +6.50,51 We measured the absorption edge by taking the lowest energy maximum of the first derivative of S-XANES. As a control experiment, we used the same impregnation procedure to add (NH4)2SO4 onto functionalized MWCNTs. No ZrO2 exists in this system; hence, no absorption occurs at the Zr L-edges. As seen in Figure 10a, a strong absorption edge for S6+ (in SO42−) can be seen for the impregnated MWCNTs sample (the absorption edge for (NH4)2SO4 lies at 2482.4 eV) but is absent after the sample is thermally treated. This indicates that sulfate species interacted only with MWCNT is subject to decomposition and removal during heating. Further evidence shows that this thermal treatment removes the sulfate species that are interacting with the MWCNT but not with Zr (at least not completely). In Figure 10b, three samples are compared with a commercial S-ZrO2 catalyst: (1) ZrO2/MWCNT impregnated with (NH4)2SO4, (2) a thermal treatment of (1) in 0.5% O2/He for 10 min, and (3) a thermal treatment of (1) in He for 2 h. Assuming that all the Zr content remains the same, we normalized the intensity of Zr L3-edge and observed the difference in S content by qualitatively comparing the S Kedge peak intensity. We see that the 0.5% O2/He treatment for 10 min retains more sulfate species than the He treatment, but this may also be caused by the difference in the length of treatment time. The S content was confirmed by the quantitative chemical analysis by Galbraith Laboratories, Inc. (see Table 1). A preliminary test for acid catalysis has been performed using the dehydrogenation and cracking of cyclohexane as the probe reaction. The reaction was carried out in the gas phase at atmospheric pressure at a space velocity of 1240 mL (STP) cyclohexane per gram of catalyst per hour. Under these conditions, there was measurable conversion (∼2%) at 500 °C for the S-ZrO2/MWCNT catalyst thermally treated at 450 °C in He (∼0.5 wt % S); on the same catalyst thermally treated at 450 °C in 0.5% O2/He, the 500 °C conversion was about 20% and the 450 °C conversion was about 5% (but immeasurable for the catalyst thermally treated in an inert atmosphere), as shown in Figure 11c. The increased activity can be mostly attributed to the fact that the 0.5% O2/He treated catalysts retain 4 times as much S as the He treated ones.

4. CONCLUSION In summary, we have developed a synthesis procedure for ZrO2/MWCNT composites of various ZrO2 loadings. Using line-broadening fitting based on XRD and confirmed by HRTEM, we have shown that the ZrO2 nanoparticles in the ZrO2/MWCNT composite are tetragonal crystals, and their particle size is 2−3 nm for both the low loading (∼15 wt %) and high loading (∼40 wt %) ZrO2/MWCNT. Thermal annealing at 450 °C results in a monatomic increase in particle size with increasing loading. This indicates that the availability of surface OCG on MWCNT introduced by nitric acid treatment is consumed at high loading such that some ZrO2 could lack strong interaction with the MWCNT surface and are

Figure 11. Scans covering the three Zr L-edges and the S K-edge to compare (a) functionalized MWCNTs impregnated with (NH4)2SO4 before and after thermal treatment at 450 °C in He for 2 h. (b) SZrO2/MWCNT before thermal treatment, thermal treatment in 0.5% O2/He for 10 min, and thermal treatment in He for 2 h; the commercial bulk S-ZrO2 catalyst was used as reference. All spectra are aligned to the intensity of the peak at the Zr L3 edge. (c) Preliminary catalytic results using S-ZrO2/MWCNT catalysts through cyclohexane cracking. The ZrO2 loading is 15 wt % in all S-ZrO2/MWCNT. I

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Table 1 no. S3 S4 S5 S6 a

description

sulfur content (wt %)a

S-ZrO2/MWCNT, before thermal treatment S-ZrO2/MWCNT, thermal treatment in He S-ZrO2/MWCNT, thermal treatment in 0.5% O2/He S-ZrO2, commercial catalyst

6.45 0.511 1.94 2.22

The ZrO2 loading is 15 wt % in all S-ZrO2/MWCNT.

formed only by physical deposition. Strong interfacial interactions between ZrO2 nanoparticles and MWCNTs were probed using C K-edge NEXAFS and complemented by O Kedge NEXAFS. The ZrO2/MWCNT can be converted to a strong solid acid, S-ZrO2/MWCNT, by sulfate impregnation. Using Zr L-edge and S K-edge XANES, we probed the effect of two different thermal treatments (i.e., thermal treatment in He and 0.5% O2/He). The latter retrains more sulfur, and this results in higher conversion of cyclohexane in the probe reaction of cyclohexane dehydrogenation/cracking.

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ASSOCIATED CONTENT

S Supporting Information *

O K-edge NEXAFS of the spectra in Figure 9 can be modeled approximately as a linear combination of two reference spectra (also shown in Figure 9) of bulk ZrO2 and functionalized MWCNT; the reference spectra do not include the perturbation of the interface interactions between MWCNT and ZrO2 and thus improve as the weight loading increases (presumably because the interface contribution decreases); the weight fraction of the O K-edge of ZrO2, required for a good fit is a reasonably linear function of zirconia loading, as would be expected. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the DOE, Office of Basic Energy Sciences, Grant DE-FG02-05ER15732, and AFOSR MURI, Grant FA9550-08-0309, for financial support. We thank colleagues at the Brookhaven National Laboratory, National Synchrotron Light Source, in particular the beamline managers Bruce Ravel of X23A2; Syed Khalid and Nebojsa Marinkovic of X18B; Daniel Fisher and Cherno Jaye of U7A; Dario Arena of U4B; and Paul Northrup of X15B for their onsite help. We also thank Wei Zhang, the postgraduate associate in our group for his contribution. TEM was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886.

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