Dispersion-Enhanced Supported Pd Catalysts for Efficient Growth of

Mar 25, 2013 - The results revealed that the characteristics of CNTs were strongly correlated to the dispersion of active Pd species in the catalysts,...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/IECR

Dispersion-Enhanced Supported Pd Catalysts for Efficient Growth of Carbon Nanotubes through Chemical Vapor Deposition Tingting Sun, Guoli Fan, and Feng Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing, 100029, People’s Republic of China S Supporting Information *

ABSTRACT: Supported Pd nanoparticle catalysts over Mg−Al mixed metal oxides derived from layered double hydroxides were prepared by three different approaches, including coprecipitation, anion-exchange, and impregnation, in order to enhance the metal dispersion and thus facilitate the growth of uniform multiwalled carbon nanotubes (CNTs) via catalytic chemical vapor deposition of methane. The effects of preparation methods for supported Pd catalysts on the morphologies and microstructures of CNTs formed were investigated. The results revealed that the characteristics of CNTs were strongly correlated to the dispersion of active Pd species in the catalysts, and the structural defects and the diameter of CNTs were reduced with the increasing metal dispersion. Especially, the anion-exchange route for preparation of supported Pd catalyst was appropriate for the growth of uniform bamboo-like CNTs with fewer defects, which could be contributed to the presence of more uniform and smaller Pd0 nanoparticles highly dispersed over Mg−Al mixed-metal oxides.

1. INTRODUCTION Carbon nanotubes (CNTs) have been explored for various potential and promising applications in materials science, biology, electronics, energy conversion/storage, and catalysis,1−7 because of their unique structural, mechanical, and electrical properties. Catalytic chemical vapor deposition (CCVD) is the most effective and attractive way for largescale production of CNTs via catalytic pyrolysis of carboncontaining gases.8−11 In the CCVD, carbon atoms from hydrocarbon decomposition are deposited on the surfaces of nanosized transition-metal (iron, nickel, cobalt, and their alloys) particles supported on high-surface-area support materials (e.g., silica and alumina).12−14 Since the catalysts act as the seeds for the nucleation and growth of nanotubes by controlling the overall reaction with the hydrocarbon source, which involves the decomposition, diffusion and precipitation of carbon species,15 the type and the character of the utilized catalysts play an important role in the growth of CNTs with regular shapes and unique structures. Until now, there have been few reports on the catalytic performance of noble-metal catalysts (e.g., Pd, Pt and Au) for the growth of CNTs.16−19 The main reason is that during the growth of CNTs, noble metal nanoparticles are easy to aggregate, thus resulting in the formation of irregular CNTs with large amounts of defects and low carbon yield. Layered double hydroxides (LDHs, [M1−x2+Mx3+(OH)2]x+(An−)x/n·mH2O) are a class of synthetic highly ordered two-dimensional anionic clays, consisting of positively charged layers with charge-balancing anions between them.20,21 This class of materials has been widely used as catalysts, catalyst supports, stabilizers in polymer composites, drug delivery materials, adsorbents for wastewater treatment, molecular precursors for chemically tailored functional materials, and so forth.22−25 Especially, after calcination at intermediate temperatures (450−600 °C), LDHs lose their © 2013 American Chemical Society

layer structure and transform to mixed-metal oxides with high thermal stability and large surface area. In addition, highly dispersed active metal nanoparticles on metal oxide matrix can be obtained by reducing calcined LDHs containing desired metals either in the form of metal cations on the layers or in the form of metal complexes in the interlayers.26−31 Recently, it was reported that multi-walled CNTs could be synthesized by CCVD of acetylene over LDH-derived cobalt and nickel nanoparticle catalysts.32,33 A few works have been reported on the in situ growth of CNTs over LDHs in a fixed-bed reactor.34−36 On the other hand, nanocomposites of Pd and CNTs have large potential in various research fields. It was reported that Pd nanoparticle decorated bamboo-like CNTs as catalytic active materials exhibited excellent electrochemical metastability in pH ranges where the nanoparticles usually would be oxidized and inactivated.37 Also, Pd-decorated CNTs as heterogeneous catalysts effectively promoted hydrodehalogenation,38 carbon− carbon coupling reaction,39 and H2O2 reduction,40 and exhibited higher activity than a commercial Pd/C catalyst. In addition, Pd-decorated CNTs could be used as highly active catalysts for fuel cells and showed excellent ability as oxygen reduction agent.41 It is well-known that the formation of highly dispersed metal nanoparticles is necessary for the controllable growth of CNTs and the diameter of CNTs is closely related to the size of catalytically active particles.42 Therefore, controlling the dispersion and size of the active nanoparticles is crucial for the growth of uniform CNTs. Although the dispersion of active metal particles on supports is often discussed in the literature, Received: Revised: Accepted: Published: 5538

November 28, 2012 February 16, 2013 March 25, 2013 March 25, 2013 dx.doi.org/10.1021/ie3032795 | Ind. Eng. Chem. Res. 2013, 52, 5538−5547

Industrial & Engineering Chemistry Research

Article

h to obtained final mixed-metal oxides (denoted as IM-PdLDO). 2.1.2. Growth of CNTs. CNTs were synthesized in a quartz tube inside a horizontal furnace equipped by CCVD of methane. About 50 mg catalyst was loaded in a ceramic boat and placed in the middle of a horizontal furnace, which was heated to 900 °C at a rate of 5 °C/min under a flowing N2 (as protecting gas, flow rate: 80 standard-state cm3/min, sccm). Subsequently, CH4 was introduced into the furnace with a flow rate of 60 sccm. After 30 min, the flow of CH4 was turn off and then the furnace was cooled to room temperature. The resultant black powder was collected from the ceramic boat. The obtained carbon products over IE-Pd-LDO, CP-Pd-LDO, and IM-Pd-LDO samples were denoted as IE-C, CP-C, and IM-C, respectively. The yield of carbon is defined as

the effective solutions to enhance the metal dispersion are rarely reported. Aiming at a combination of outstanding characteristics of LDH materials and good catalytic properties of noble metals, in this work, a series of supported Pd-based catalysts over Mg−Al mixed-metal oxides derived from LDH materials are prepared, and active Pd species were introduced through different approaches, i.e., coprecipitation, ion exchange, and impregnation, to obtain highly dispersed Pd nanoparticle catalysts for the growth of CNTs via CCVD of methane. The effects of preparation method for supported Pd catalysts on the morphologies and microstructures of CNTs were investigated, and a correlation between the metal dispersion degree and the catalytic performance for the growth of CNTs was discussed. To our knowledge, the study of the catalytic performance of supported Pd-based catalysts over Mg−Al mixed-metal oxides derived from LDH materials for the growth of CNTs has not been reported previously.

yield (%) =

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. 2.1.1. Preparation of Catalyst Precursors. Coprecipitation. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and Na2PdCl4 with a Mg2+/Al3+ molar ratio of 3.0 and a Al3+/PdCl42‑ molar ratio of 20 were dissolved in 100 mL of deionized water with the total cation concentration of 0.05 M. Subsequently, the 100 mL of alkali solution of NaOH (0.08 M) and Na2CO3 (0.25 M) was added dropwise into a salt solution under vigorous stirring at room temperature. The pH value of the solution was adjusted to 10.0 by further titration of alkali solution. The precipitate was washed by four dispersion and centrifugation cycles in water, and finally dried under vacuum at 70 °C for 12 h. The solid was denoted as CP-PdLDH. As-prepared solid was calcined at 500 °C for 4 h to obtain final mixed-metal oxides (denoted as CP-Pd-LDO). Anion Exchange. Mg(NO3)2·6H2O and Al(NO3)3·9H2O with a Mg2+/Al3+molar ratio of 3.0 were dissolved in 100 mL of CO2-free deionized water with the total cation concentration of 0.05 M under N2 atmosphere. Subsequently, the above salt solution was titrated with an alkali solution of NaOH and NaNO3 ([NO3−] = 2[Al3+], [OH−] = 1.6([Mg2+] + [Al3+])) under vigorous stirring at room temperature until pH = 10.0. Then the aqueous solution was aged at 60 °C for 6 h under N2 atmosphere. The resulting suspension was centrifuged and washed with deionized water for five times, and then dried under vacuum at 70 °C for 12 h to obtain MgAl-NO3−-LDH precipitate. Under N2 atmosphere and vigorous magnetic stirring, a certain amount of Na2PdCl4 with the Al3+/PdCl42− molar ratio of 20 was dissolved in 100 mL of decarbonized water containing MgAl-NO3−-LDH precipitate. The pH of the mixture was held constant at ∼6.5 by simultaneous addition of 0.1 M nitric acid solution. The exchange process was kept at 60 °C for 24 h. The precipitate was washed by four dispersion and centrifugation cycles in water, and finally dried under vacuum at 70 °C for 12 h. The solid was denoted as IE-Pd-LDH. Asprepared solid was calcined at 500 °C for 4 h to obtained final mixed-metal oxides (denoted as IE-Pd-LDO). Impregnation. MgAl-CO 32−-LDH according to above procedure for CP-Pd-LDH in the absence of Na2PdCl4. Supported Pd-based catalyst precursor was prepared by traditional incipient wetness impregnation with Na2PdCl4 aqueous solution with the Al3+/PdCl42− molar ratios of 20. The slurry was dried overnight at 80 °C. The solid was denoted as IM-Pd-LDH. As-prepared solid was calcined at 500 °C for 4

M1 − M 2 × 100 M2 × P

(1)

where M1 is the mass of carbon products, M2 the initial mass of calcined samples, and P the weight percentage (wt %) of Pd in the calcined samples, as determined by ICP-ES. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were performed on a Shimadzu Model XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu Kα source (λ = 0.15418 nm). Elemental analysis for metal ions in samples was performed using a Shimadzu Model ICPS-75000 inductively coupled plasma emission spectrometry (ICP-ES) system. Solutions were prepared by dissolving the samples in dilute hydrochloric acid. Scanning electron microscopy (SEM) observations were investigated using a Hitachi Model S-4700 apparatus with an applied voltage of 20 kV. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) observations were carried out on a JEOL Model JEM-2100 electron microscope operated at 120 and 200 kV, respectively. Temperature-programmed reduction (TPR) of the samples was characterized using a Micromeritics ChemiSorb 2720 system. The sample (80 mg) was placed in a quartz U-tube reactor. Before reduction, the precursor was degassed under flowing argon at 200 °C for 2 h. The sample then was reduced in a stream of 10% v/v H2/Ar (40 mL/min total flow) with a heating rate of 10 °C/min up to 500 °C. The hydrogen consumption was monitored continuously, using a thermal conductivity detector (TCD). H2−O2 titration of the sample was carried out on a Micromeritics ChemiSorb 2720. After TPR for the calcined sample, the sample reactor was cooled to 120 °C and maintained for 1 h in a stream of 10% v/v H2/Ar. Then, the sample reactor was flushed with argon gas (40 mL/min total flow) for 30 min. The same flushing under argon was performed between each titration cycle. Subsequently, pulses of oxygen were introduced until the full saturation of the catalyst was achieved. The chemisorbed oxygen was then titrated by hydrogen. Afterward, a second oxygen titration was carried out to confirm the volume of H2. The dispersion of Pd (Dis, %) on the catalysts was calculated by the following equation:43 Dis (%) = 5539

2 × M × VTH × 10−3 × 100 3 × 22.4 × W × P

(2)

dx.doi.org/10.1021/ie3032795 | Ind. Eng. Chem. Res. 2013, 52, 5538−5547

Industrial & Engineering Chemistry Research

Article

Figure 1. XRD patterns of (A) Pd-containing catalyst precursors (CP-Pd-LDH (pattern a), IE-Pd-LDH (pattern b), IM-Pd-LDH (pattern c)) and (B) calcined precursors (CP-Pd-LDO (pattern d), IE-Pd-LDO (pattern e), and IM-Pd-LDO (pattern f)).

where M is the formula weight of Pd (MPd = 106.42 g mol−1), VTH the volume of H2 used for the titration of O2 (mL), W the mass of catalyst (g), and P the weight percentage of Pd in the sample as determined by ICP-ES. N2 adsorption−desorption isotherms of the samples were obtained on a Micromeritics Model ASAP 2020 sorptometer apparatus at 77 K. The total specific surface areas were evaluated from the multipoint Brunauer−Emmett−Teller (BET) method, and the mesopore size distribution and average pore diameter were determined by the Barrett−Joyner− Halenda (BJH) method applied to the desorption isotherms. Thermogravimetric and differential thermal analysis (TGDTA) was carried out in air on a HCT-2 thermal analysis system produced locally. Samples of 9.0−10.0 mg were heated from 30 °C to 800 °C at a temperature ramp of 5 °C/min. Raman spectra were recorded at room temperature on a microscopic confocal Raman spectrometer (Jobin−Yvon Horiba HR800) using an Ar+ laser of 532 nm wavelength as an excitation source. The laser, with 10 mW output power, was focused on the surface.

Table 1. Analytical, Structural, and Textural Data for Different Samples sample d003 Pda Disb BET surface areab pore volumeb pore diameterb mean crystallite size of Pd particlesc yield of carbon ID/IG for CNTs

IE-Pd-LDH

CP-PdLDH

IM-PdLDH

0.858 nm 3.05 wt % 65.0% 103 m2/g 0.25 cm3/g 9.3 nm 24 nm

0.782 nm 2.88 wt % 52.6% 128 m2/g 0.35 cm3/g 11.2 nm 34 nm

0.787 nm 3.0 wt % 31.7% 121 m2/g 0.42 cm3/g 13.8 nm 28 nm

1920% 0.89

1590% 1.32

1040% 1.37

a

Determined by ICP-ES. bDegree of dispersion of Pd in calcined samples. cCalculated by means of the Scherrer equation.

with the oxygen atoms of the interlayer CO32− anions.46,47 It is well-known that the LDHs are generally thermodynamically unfavorable, with respect to mixtures of metal hydroxides.48 Therefore, in the present preparation system for the CP-PdLDH sample, the addition of Na2CO3 can facilitate the formation of the well-crystalline LDH phase with the intense exposed surface facets (e.g., (003), (006), (012)). Correspondingly, the negatively charged PdCl42− anions only are adsorbed on the edges of LDH platelets through interfacial electrostatic interaction, which has little impact on the structure of LDH phase. Obvious differences are observed for IE-Pd-LDH sample, with the corresponding (003) and (006) reflections shifting toward lower 2θ angles. This is because the intercalation of PdCl42− anions with a large radius of ∼0.167 nm into the interlameller space results in an expanded interlayer structure of the LDH with the basal spacing of ∼0.858 nm and the lowered crystallinity with the weak (012) reflection.48 However, the basal spacing (d003) for LDH phase in IM-Pd-LDH is ∼0.787 nm, which is slightly larger than that for CP-Pd-LDH, indicative of the presence of interlayer CO32− ions. This is because PdCl42− anions are mainly impregnated

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. Figure 1 shows the XRD patterns of different Pd-containing catalyst precursors derived from LDH materials, while Table 1 summarizes the structural and analytical data of samples. All samples exhibit the typical characteristic reflections for (003), (006), and (012) crystal planes of hydrotalcite-like materials in layered structures (JCPDS File Card No. 38-0487), which correspond to the basal spacing and its higher-order diffractions.44,45 The peaks at ∼60° 2θ arise from the (110) and (113) reflections. The basal spacing (d003) for CP-Pd-LDH sample is ∼0.782 nm, in agreement with the value reported for CO32−-intercalated LDH compound. It demonstrates that the intercalation of PdCl42− anion into the interlameller space of LDH does not take place in the CP-Pd-LDH sample, mainly due to the fact that chargebalancing CO32− anions have a high affinity to the LDH layers, since hydroxyl groups on the layers can form hydrogen bonds 5540

dx.doi.org/10.1021/ie3032795 | Ind. Eng. Chem. Res. 2013, 52, 5538−5547

Industrial & Engineering Chemistry Research

Article

and deposited on the surface of MgAl-LDH crystallites, thus leading to the similar intensities of (003), (006) and (012) planes to those for the CP-Pd-LDH sample. In all cases, no characteristic reflections indexed to palladium-containing crystallized phase are observed. SEM micrographs of Pdcontaining catalyst precursors depict that, in each case, the sample is composed of densely packed platelike particles with visible edges and a thickness of ∼20 nm (Figure S1 in the Supporting Information). Especially, the platelet-like nature of the large crystallites is clearly apparent in the IE-Pd-LDH sample, indicative of uniformity of the product with a wellcrystalline structure. XRD patterns of the calcined Pd-containing samples (Figure 1B) show the complete transformation from the hydrotalcite to the metal oxide phase. Three intense reflections are observed in each case, which are almost at the same positions as (111), (200), and (220) reflections of crystalline MgO phase (periclase).49 This indicates that the layer hydroxyl groups and interlayer anions have decomposed upon heating at elevated temperature. Meanwhile, some Al3+ ions may take the place of Mg2+ and exist in the crystal lattice of MgO and the others react with oxygen to form amorphous alumina during the calcination process. Also, all samples do not exhibit any distinct XRD peaks corresponding to the PdO phase, because of the low Pd content and/or small particle size. It suggests that the Pd atoms are well-dispersed on LDH-derived Mg−Al mixed-metal oxide matrix. In addition, note that the intensities of (111), (200), and (220) reflections of crystalline MgO phase decrease gradually from IE-Pd-LDO to CP-Pd-LDO and IMPd-LDO, indicative of the reduced particle size or crystallinity of MgO-like products. From Figure S2 in the Supporting Information, it can be seen that after the thermal decomposition at 500 °C, the morphology of the primary LDH crystals has been destroyed. Large quantities of aggregates of small particles are formed in the IE-Pd-LDO sample. Compared with those in the IE-Pd-LDO sample, the larger particles are observed in the CP-Pd-LDO and IM-Pd-LDO samples. Furthermore, the mean crystallite size of MgO-like phase, estimated by means of the Scherrer equation, increases as follows: IE-Pd-LDO (6.3 nm) < CP-Pd-LDO (10.2 nm) < IM-Pd-LDO (14.0 nm). Here, well-crystalline LDH phases in CP-Pd-LDH and IM-Pd-LDH samples can be transformed easily to a MgO-like phase, since the ordering of the resulting Mg-containing oxide crystallites within the initial platelet is maintained without the incorporation of Pd species into the interlayer domains, thus leading to the larger particle size of the MgO-like phase. Correspondingly, smaller Pd nanoparticles are more easily formed and more highly dispersed on the MgO-like metal oxide with smaller particle size by an in situ reduction of IE-Pd-LDO precursor, probably favoring the formation of uniform CNTs. Figure 2 displays the TPR profiles of the calcined Pdcontaining samples. In each case, a characteristic negative peak at the low temperature of ∼77 °C, similar to those for Pdcontaining catalysts reported in the literature,50,51 is ascribed to the decomposition of β-PdHx formed during H2 contact at room temperature. All samples exhibit another reduction peak located at a temperature region from 250 °C to 450 °C, which can be attributed to the reduction of Pd2+ species related to PdO phase to Pd0 species. It is noted that the present reduction peak occurs at a much higher temperature than those for PdO reported previously in the literature (ca. 50−70 °C).52 It suggests the presence of an extensive interaction between Pd

Figure 2. TPR profiles of IE-Pd-LDO (curve a), CP-Pd-LDO (curve b), and IM-Pd-LDO (curve c).

oxide species and the support.53 In the case of LDH-derived samples, Pd2+ species can interact with near groups through the Pd−O bonds polarized by Al3+ or Mg2+ ions in the metal oxides formed by the decomposition of LDH phase, thus strongly hindering the reduction of Pd2+ species. Meanwhile, note that the reduction peak shifts toward the higher temperatures from IE-Pd-LDO (338 °C) to CP-Pd-LDO (349 °C) and IM-PdLDO (359 °C). The difference in the reducibility can be attributed to the varying metal dispersion. As shown in Table 1, the increase in the Pd dispersion degree follows: IM-Pd-LDO (31.7%) < CP-Pd-LDO (52.6%) < IE-Pd-LDO (65.0%). Since the higher metal dispersion can lead to the exposure of more defect sites at the surface, the dissociation of dihydrogen may proceed more easily in the reduction. As a result, the IE-PdLDO sample having the highest Pd dispersion degree exhibits the best reducibility. Figure 3 shows the N2 adsorption−desorption isotherms and corresponding BJH pore size distribution curves of the calcined

Figure 3. N2 adsorption−desorption isotherms of (−●−) IM-PdLDO, (−▲−) CP-Pd-LDO, and (−▼−) IE-Pd-LDO. Inset shows the pore size distributions. 5541

dx.doi.org/10.1021/ie3032795 | Ind. Eng. Chem. Res. 2013, 52, 5538−5547

Industrial & Engineering Chemistry Research

Article

Figure 5a, a large amount of one-dimensional tubular carbon nanostructures with a smooth external surface and uniform diameter grown over the IE-Pd-LDO sample are found to distribute densely in a wide field. Close-up SEM micrograph reveals that these carbon nanostructures are multiwalled CNTs with the hollow cores, small outer diameters (∼30−45 nm), and lengths from several micrometers to tens of micrometers (see Figure 5b). Besides straight CNTs, few solenoid-like threedimensional helical structured CNTs were formed and the statistical percentage of helical structured CNTs produced is estimated to be