RuO2·xH2O Supported on Carbon Nanotubes as a Highly Active

J. Phys. Chem. C 2008, 112, 11875–11880

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RuO2 · xH2O Supported on Carbon Nanotubes as a Highly Active Catalyst for Methanol Oxidation Hao Yu,* Kai Zeng, Xiaobo Fu, Yan Zhang, Feng Peng,* Hongjuan Wang, and Jian Yang School of Chemistry and Chemical Engineering, South China UniVersity of Technology, Guangzhou 510640, China ReceiVed: March 13, 2008

Highly dispersed ruthenium oxide clusters were synthesized on carbon nanotubes (CNTs) to form RuO2 · xH2O/ CNT catalyst by a homogeneous oxidation precipitation method. Methanol oxidation was carried out on RuO2 · xH2O/CNT at low temperatures for the production of diverse oxygenous products, such as methyl formate (MF) and dimethoxymethane (DMM). Unprecedented conversion rate of methanol, high up to 545 molMeOH (mol Rusurface)-1 h-1, was observed at 120 °C. The effect of structural water in RuO2 domains on the performances of the methanol oxidation reaction was investigated by annealing them in N2 at elevated temperatures. The dehydration of RuO2 clusters decreased the oxidation ability and changed the selectivity patterns. It was suggested that the behaviors of RuO2 during the annealing process may be strongly influenced by the interaction between RuO2 and CNTs. 1. Introduction Methanol oxidation can produce many chemical products of industrial importance, such as formaldehyde (FA), methyl formate (MF), and dimethoxymethane (DMM). About 30% of methanol in the world is converted into FA by oxidative dehydrogenation reaction over silver-based and ferric molybdate catalysts.1 Methyl formate, a versatile chemical intermediate, can be directly produced with high selectivity and yield, by catalytic methanol oxidation on V-Ti-O,2,3 Sn-Mo-O,4,5 and Bi-based mixed oxides.6 On ReOx7 and polyoxometalate Keggin clusters,8 the oxidation of methanol leads to high yield of DMM. A reaction pathway involving oxidation and dehydration reactions has been proposed for methanol oxidation,1 in which the oxidative dehydrogenation of methanol by breaking C-H bond is the rate-determining step. MF and DMM are sequentially formed via further oxidation and dehydration reactions. In this reaction scheme, MF and DMM are thermodynamically favored at low temperature. Thus, the oxidative dehydrogenation (ODH) of methanol at low temperature is of importance for producing diverse chemicals with tunable yield and low energy cost. Ruthenium oxides have been reported as an effective low temperature ODH catalyst, which can catalyze the reaction with significant conversion at about 120 °C.9 The outstanding capability of RuO2 for oxidation reactions has also been demonstrated in CO,10 alcohol,11,12 and cyclohexane.13 Many efforts have been made to enhance the low temperature activity of RuO2 catalyst. Liu et al.9 have investigated the ODH rate over RuO2 catalysts supported on diverse oxides. On the catalysts prepared by impregnation method with different supports, RuO2 domains with higher initial reduction rate would offer higher ODH rate. This situation has also been reported on V2O5 catalyst.14 A detailed investigation to the ZrO2supported RuO2 catalyst evidenced that the surface species in the form of RuO42- are more active for ODH reaction than RuO2, which is dominant at low Ru surface density and high dispersion.15 Recently, Zhan and Iglesia16 encapsulated RuO2 * Corresponding authors. E-mail: [email protected] (H.Y.); cefpeng@ scut.edu.cn (F.P.).

clusters in LTA zeolites via the so-called “boat in bottle” method. Due to the confinement effect of zeolite, the dispersion of RuO2 domains was significantly increased to 48%; therefore, the ODH rate was enhanced by 2.8-fold compared with that using SiO2-supported catalyst. However, the efficiency of the RuO2@LTA catalyst might be limited by the internal mass transfer resistance of micropores. In this paper, we demonstrate that the activity of RuO2 domains could be enhanced by dispersing them onto carbon nanotubes (CNTs),17 which offer large and highly active external surfaces18–20 and led to the improved dispersion of RuO2 without significant mass transfer resistance. The effect of annealing on CNT-supported RuO2 · xH2O nanoparticles was revealed in terms of their activity and selectivity. RuO2 · xH2O/CNT catalyst exhibited superior performance for the methanol oxidation reaction to those RuO2-based catalysts supported on oxides or inside zeolite. 2. Experimental Section The RuO2 · xH2O/CNT catalyst was prepared by a H2O2 homogeneous oxidation precipitation (HOP) method, as reported elsewhere.12 Multiwalled CNTs (Shenzhen Nanotech Port Co. Ltd.) were first functionalized by refluxing in 8 M nitric acid at 140 °C for appropriate times to introduce oxygen-containing groups. The functionalized CNTs were dispersed in RuCl3 aqueous solution and exposed to ultrasonic bath for 2 h. Then hydrogen peroxide aqueous solution (30%) was added dropwise at 80 °C to convert Ru(III) to RuO2 nanoparticles attached on CNTs. The resulting solid catalyst was separated by filtration and annealed at 100-800 °C in nitrogen for 8 h. A reference RuO2 catalyst supported on TiO2 (Degussa, P25) was prepared by the incipient wetness impregnation method.9 The methanol oxidation reaction was conducted in a gas-solids flowing micropacked bed reactor system. Helium was used as the balance gas. The flow rates of He and O2 were controlled by massflow controllers. The mixture of He and O2 passed through a methanol saturator to introduce methanol at desired partial pressure, and then the reactant was fed into a quartz

10.1021/jp804003g CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

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Figure 1. Effect of functionalization duration of CNTs in HNO3 on Ru dispersions of RuO2 · xH2O/CNT catalyst (c). TEM images of RuO2 (9.5 wt %) catalysts on functionalized for 2 h and pristine CNTs are shown in panels a and b, respectively. The white arrows in part b highlight two aggregates of RuO2 nanoparticles on pristine CNTs.

reactor with i.d. of 7 mm, in which 0.05-0.2 g of catalyst diluted with 0.4 g of silica was packed. The methanol oxidation was carried out at ambient pressure and temperature in the range from 30 to 150 °C. The composition of effluent gas was analyzed by a gas chromatographer equipped with a TCD detector and a FID detector. The gas sample was first analyzed by the TCD detector for FA, methanol, MF and DMM and then passed through a methanator packed with Ni catalyst at 350 °C to analyze COx in the FID detector. The carbon balance calculation is >90% for all the data. The catalysts were characterized by Fourier transform infrared spectroscopy (Bruker EQVINOX-55), X-ray diffraction (Rigaku, D/max-IIIA), transmission electron microscopy (JEOL JEM2010), thermal gravimetric analysis (Netzsch STA449C), and temperature-progarmmed reduction (TPR) and oxidation (TPO). The TPR tests of the catalysts were conducted in a flowing H2reduction system with a TCD detector. Typically, 50 mg sample was reduced by the gaseous mixture composed of H2 (5% vol) and N2 (95% vol) at a rate of 10 °C/min from 50 to 600 °C. The water in gas flow was removed by a column packed with 13X molecular sieve before entering the detector. TPO tests were performed in gas flow composed of O2 (25% vol) and N2 (75% vol) from room temperature to 800 °C. The dispersion of RuO2 domains was evaluated by measuring the dispersion of Ru crystallites using CO chemisorption at 40 °C after reduction of RuO2 in H2 at 300 °C for 1 h.

3. Results and Discussion The HOP method can effectively convert Ru(III) to Ru(IV) in the form of amorphous hydrous RuO2 without further thermal oxidation at elevated temperatures, which has been demonstrated by TEM, XPS, and XRD.12 RuO2 nanoparticles with a mean size of 1.4 nm were supported on CNTs with outer diameter of about 20-40 nm, as shown in Figure 1a. TEM observation demonstrated that no aggregates of RuO2 domains presents on the functionalized CNT surfaces. On the pristine CNTs without any surface oxidation processing, however, the HOP method leads to obvious aggregates of RuO2 nanoparticles, as shown in Figure 1b. Accordingly, the dispersion of RuO2 domains on CNTs significantly reduced from 39.8% (defined as the fraction of Ru atoms on surface detected by CO chemisorption in total Ru atoms) to 16.0% on functionalized and pristine CNTs, respectively. Since the functionalities on CNTs increase with oxidation degree of CNTs in the wet oxidation process, this result clearly demonstrates the contribution of oxygenous groups to the formation of highly dispersed RuO2 domains. The further insight to this process can be obtained by monitoring the state of surface groups. Figure 2 shows the FT-IR spectra of pristine CNTs, functionalized CNTs with nitric acid for 2 h, and the RuO2 · xH2O/CNT catalyst. Functionalization with nitric acid introduced carboxylic groups on CNTs as reflected by the CdO vibration at 1720 cm-1. However, the CdO peak at 1720 cm-1 was diminished significantly, after RuO2 was supported. Some authors suggested that the diminishing of CdO vibration in IR

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Figure 2. FT-IR spectra of pristine CNTs, functionalized CNTs by nitric acid, and RuO2 · xH2O/CNT catalyst. The arrow highlights the CdO vibration in carboxyl groups.

Figure 4. TEM image of RuO2 · xH2O/CNT catalyst annealed in N2 at 600 °C. The inset shows the HRTEM image of RuO2 nanocrystals on CNTs.

Figure 3. XRD patterns of RuO2 · xH2O/CNT catalyst annealed in N2 at 400, 600, and 800 °C.

spectrum is caused by the formation of ester species between -OOH on the CNT surface and -OH on hydrous RuO2.21 This result indicates that the surface carboxylic groups play an important role in adsorption of Ru ions and nucleation of RuO2 nanoparticles, as predicted in our previous work.12 The dispersion of catalytic domains can also be reflected by XRD pattern, because the monolayer or very small clusters of catalytic domains will give no response in powder diffraction patterns. Since the as-prepared RuO2 is amorphous, the sample must be annealed at high temperature to transform the amorphous hydrates to rutile crystallites above 300 °C for XRD measurement. Figure 3 shows the XRD patterns of RuO2 (9.5%)/ CNT annealed in nitrogen. Even in the sample annealed at 600 °C, no RuO2 peaks were detected, indicating that the RuO2 domains are either highly dispersed or thermally stable. This was demonstrated by high resolution TEM (HRTEM) as shown in Figure 4. Annealing in N2 at 600 °C converted amorphous RuO2 · xH2O nanoparticles into RuO2 nanocrystals, as shown in the inset of Figure 4. However, the nanocrystals distributed on CNTs evenly and no aggregates or large particles were observed. Toebes et al.22 has reported that the metallic Ru nanoparticles can be stable on CNTs in reducing atmosphere at elevated temperatures. Our result demonstrates that CNTs also perform well to stabilize oxide nanoparticles on their surface. It should be noted that the higher temperature would induce undesired reduction of RuO2 domains into Ru by carbon.23 As shown in Figure 3, Ru peaks were detected in the sample annealed at 800 °C. Thus, the annealing process has been controlled below 400 °C for the investigation of catalytic performances. Table 1 shows the comparison among RuO2 catalysts supported on different supports. The reference catalyst RuO2/TiO2

gave a similar activity and selectivity pattern to those reported in literature.9 Under same conditions, the conversion rate and TOF of RuO2 · xH2O/CNT annealed at 100 °C are much higher than those of oxide-supported and zeolite-encapsulated catalysts. On RuO2(5.0 wt %)/CNT, the TOF was over 3-fold higher than that of RuO2 within LTA cage, which is the most active catalyst of low temperature methanol oxidation in literature to our knowledge,16 partly due to the elimination of mass transfer resistance. The TOF reached unprecedented 545 molMeOH (mol Rusurface)-1 h-1, suggesting a promising efficient heterogeneous process of methanol oxidation for chemical products. Reducing the loading of RuO2 favored to increase the catalytic activity. The TOF for the rate-determining ODH reaction was improved by ∼2-fold when the loading decreased from 9.5% to 5%. The higher intrinsic activity of heterogeneous catalysts with low loading is usually attributed to the improved dispersion of active sites. Although the CO chemisorption gave closed Ru dispersion values for catalysts with different loadings, more dispersive RuO2 particles characterized by smaller sizes and less neighbors were observed on low-content catalysts, as shown in Figure 5. The average diameter of RuO2 domains reduced from 1.4 ( 0.5 nm to 1.1 ( 0.3 nm. (The histogram of RuO2(9.5 wt %)/ CNT can be found in our previous work for comparison.12) This indicates that the activity of RuO2 for the ODH reaction is sensitive to their sizes; therefore, the high dispersion is desired for the optimum catalyst. Above results also indicate the limitation of CO chemisorption method in evaluating the dispersion of RuO2. Because RuO2 has to be reduced to Ru at 300 °C for CO chemisorption, the dispersion of RuO2 will be distorted due to the possible aggregation during reduction, particularly in the cases with high dispersion values. Thus, we always present the dispersion degree with either chemisorption or TEM statistics results in this work. To justify the feasibility of RuO2 · xH2O/CNT catalyst for the oxidative reaction, the thermal stability of CNTs in the catalyst in oxidative reactants has to be considered. TPO profiles of purified CNTs and RuO2 · xH2O/CNT catalyst are shown in Figure 6. The oxidation peak of CNTs shifted from 640 to 570 °C when RuO2 existed, indicating that RuO2 effectively

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TABLE 1: Oxidation Activity of RuO2 · xH2O/CNT

catalysts RuO2/CNTa (5 wt % RuO2) RuO2/CNTa (7.5 wt % RuO2) RuO2/CNTa (9.5 wt % RuO2) RuO2/TiO2 (9.5 wt % RuO2) Ru@LTA16 (4.7 wt % Ru)

turnover rate O2 conversion MeOH turnover for ODHc rate [molMeOH SCO2 SFA SMF SDMM [molMeOH temperature pressure pressure rateb [molMeOH (kPa) (kPa) (mol Rutotal)-1 h-1] (mol Rusurface)-1 h-1] (mol Rusurface)-1 h-1] (%) (%) (%) (%) (°C) 120 120 120 120 120

4 4 4 7 4

9 9 9 20 9

218d 192 135 31.5 82

545 480 339 n.m.e 171

381 355 201 n.m.e 120

0 0 0 0 7

47 54 27 15 35

33 28 47 85 52

19 18 26 0 6

a Annealed in N2 at 100 °C. b Conversion rate of methanol taken at conversion of ∼20% unless otherwise specified. c Turnover rate of ODH reaction. The formation of every molecular of MF or DMM needs a single ODH event. d 38% conversion. e Not measured due to the unknown dispersion.

Figure 5. TEM image (a) of RuO2 · xH2O(5%)/CNT catalyst. The size distribution of RuO2 domains on CNTs is shown in panel b.

Figure 6. TPO profiles of functionalized CNTs and RuO2 (9.5 wt %)/ CNT catalyst.

catalyzed the oxidation of carbon, as observed in the Ru-carbon black mixture.24 However, no obvious COx formation was detected at low temperature region. Therefore CNTs would be safe to be used as supports for the low-temperature ODH reaction. Insight into the catalytic behavior of RuO2 is quite difficult to obtain due to the dependences on various structural factors, such as cluster size and water of crystallization. Some authors have reported that the oxidation ability of RuO2 is structuresensitive and the coordinatively unsaturated sites on RuO2(110) are active.25–28 On the other hand, the ability to transport electron

and proton of RuO2 strongly depends on the ordering of RuO6 octahedra and the content of structural water, which lead to the dependence of property of RuO2 on the annealing temperature.29,30 In earlier reports on RuO2 catalysts, the as-prepared catalysts were usually annealed at >300 °C to form oxides.9,15 However, RuO2 nanoparticles tend to aggregate and form larger crystallites at the elevated temperatures in air,30–32 which will decrease the dispersion of catalytic sites (see Supporting Information Figure S1). As mentioned hereinbefore, RuO2 is stable as annealed in N2. The TEM images and corresponding histograms of RuO2 cluster sizes of catalysts annealed at different temperatures are shown in Figure 7. The histogram of the sample annealed in 100 °C can be found in our previous work for comparison.12 After annealing at 400 °C, the average size of RuO2 clusters only increased by ∼0.3 to 1.7 nm, demonstrating the resistance to sintering of the RuO2/CNT catalyst. This provided an opportunity to investigate the effect of content of structural water with weak dependence of size effect. As shown in Table 2, the activities of the samples decreased with annealing temperature, though their dispersions and particle sizes had no obvious changes. This result indicates that the hydrate of RuO2 is more active than the anhydrous RuO2 for ODH reaction. Despite the same average size of RuO2, the most serious decrease of activity occurred in the range from 100 to 200 °C. Since the water of RuO2 clusters loses in the temperature range from 190 to 500 °C in N2 (see the TGA curves in

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Figure 7. TEM images of RuO2/CNT catalysts annealed in N2 at 200 (a), 300 (b), and 400 (c) °C. The right-hand panels show the corresponding histograms of RuO2 cluster sizes.

TABLE 2: Effects of Annealing in N2 on the Dispersion, Cluster Size, Activity, and Selectivity of RuO2(9.5 wt %)/CNT Catalyst annealing temperature (°C) 100 200 300 400

dRuO2b (nm) 1.4 ( 0.5 1.4 ( 0.3 1.6 ( 0.5 1.7 ( 0.5

dispersionc (%)

conversion ratea [molMeOH (mol Rutotal).-1 h-1]

turnover rate for ODHd [molODH (mol Rusurface).-1 h-1]

39.8 41.5 45.4 35.7

158 100 124 79

232 147 171 141

EobsODH (kJ/mol)

SCO2 (%)

SFA (%)

SMF (%)

SDMM (%)

48.0 ( 0.9

0 0 0 0

17 22 25 27

34 41 52 66

49 37 22 6

60.9 ( 3.7

a Reaction conditions: 120 °C, 7 kPa methanol, 20 kPa O2, conversion in 13∼25%. b By TEM method. c The fraction of Ru atoms on surface detected by CO chemisorption in total Ru atoms. d Turnover rate of ODH reaction. The formation of every molecular of MF or DMM needs a single ODH event.

Supporting Information Figure S2), the fact that the major change of activity takes place in the range from 100 to 200 °C implies that the surface oxygenous species, typically in the form of -OH groups associated with Ru-O,33 play a vital role in the ODH reaction. The activation energy of ODH reaction increased by ∼10 kJ/mol from 48 to 60.9 kJ/mol, after the RuO2 · xH2O/CNT catalyst was annealed at 300 °C in N2. On most of the reported oxide catalysts, the ODH reaction has an apparent activation energy of ∼85 kJ/mol.1 In our experiment, the usage of RuO2 · xH2O/CNT catalyst remarkably lowered the activation energy. Furthermore, the structural water in RuO2 clusters decreases the activation energy, therefore favoring the high activity of this catalyst. The selectivity of ODH reaction was investigated in the conversion range 13-25%, since the selectivity of consequential

reaction strongly depends on extent of reaction. On the N2annealed samples, the selectivity to DMM deceased and that to MF increased with increasing annealing temperature. In ODH reaction of methanol, the moderate basicity favors the formation of MF and the acidicity favors the formation of DMM.1 Toebes et al.22 have reported that elevated temperature will cleave the surface functionalities, thus influence the reactivity of supported catalytic particles. An X-ray adsorption study of CNT supported Pt nanoparticles from Zhang et al.34 suggests that the behavior of nanoparticles may be tuned by the distance between particle and carbon, which gradually decreases with elevated temperature due to the desorption of chemiadsorbed hydrogen. It can be reasonably expected that the annealing would cleave the functionalities on CNTs and intensify the interaction between CNTs and RuO2. Thus, the abundant delocalized electrons on

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Yu et al. and a TGA curve of RuO2 (9.5 wt %)/CNT catalyst is shown in Figure S2. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. H2-TPR of RuO2 · xH2O/CNT catalysts annealed in N2 at 100, 200, and 300 °C.

CNT walls may enhance the nucleophilicity of RuO2 domains, and favor the selectivity to MF. This is supported by TPR profiles of RuO2 · xH2O/CNT annealed in N2. As shown in Figure 8, the reduction peaks of N2 annealed samples shifted toward low temperature side as increased annealing temperature. This can be explained by the promotion of H2-reduction of oxides by the electron-rich environment on CNTs. Although the precise nature of the RuO2-CNT interaction is yet to be elucidated on current stage, these results indicate that the interaction between catalyst and CNTs might be complicated, and the selectivity can be tuned by proper annealing process. 4. Conclusions It was demonstrated that highly dispersed RuO2 · xH2O nanoclusters can be uniformly supported on functionalized CNTs by homogeneous oxidation precipitation method. Carboxyl groups on the surface of functionalized CNTs play a crucial role to obtain high dispersion. The improved dispersion of RuO2 domains and the surface oxygenous species associated with water of hydration play important roles, and lead to an unprecedented activity for ODH of methanol at low temperature. Annealing process also intensifies the interaction between RuO2 and CNTs, which increase the selectivity to MF and decrease that to DMM. Although the precise nature for this is yet to be elucidated, it is suggested that RuO2 · xH2O/CNT can be used as an effective catalyst for methanol oxidation, and the selectivity can be tuned by proper annealing process. Acknowledgment. We thank Ms. Lin Hu and Dr. Qiang Zhang in the Department of Chemical Engineering, Tsinghua University, for the help on the TEM measurements. This work was supported by the Guangdong Provincial Science and Technology Project (No. 2006A10903002) and Guangzhou Civic Science and Technology Project (No. 2007Z3-D2101). Supporting Information Available: TEM image of RuO2 · xH2O/CNT annealed in air at 300 °C is shown in Figure S1

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