Carbon Nanotubes Supported Mono- and Bimetallic Pt and Ru

Article pubs.acs.org/IECR

Carbon Nanotubes Supported Mono- and Bimetallic Pt and Ru Catalysts for Selective Hydrogenation of Phenylacetylene Chuang Li,† Zhengfeng Shao,† Min Pang,† Christopher T. Williams,‡ Xiongfu Zhang,† and Changhai Liang*,† †

Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States ABSTRACT: Deposition of Pt, Ru, Pt−Ru alloy, Ru@Pt, and Pt@Ru nanoparticles onto carbon nanotubes (CNTs) has been achieved by chemical reduction of the corresponding RuCl3·3H2O and/or H2PtCl6·6H2O by ethylene glycol in the presence of NaOH. The as-prepared catalysts were characterized by X-ray diffraction, H2-temperature programmed reduction, H2temperature programmed desorption, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Liquid-phase selective hydrogenation of phenylacetylene was used as a probe reaction to evaluate their catalytic performances. The as-prepared Pt, Ru, Pt−Ru alloy, Ru@Pt, and Pt@Ru nanoparticles fell in the range of 1.5−3.0 nm in diameter, and were uniformly dispersed on the CNTs. All the bimetallic catalysts displayed the characteristic diffraction peaks due to a Pt facecentered cubic structure, but the 2θ values were shifted to slightly higher ones, indicating the formation of alloy or core−shell structures. XPS analysis revealed that the catalysts contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV), and Ru(IV). The Pt@Ru/CNTs and Ru@Pt/CNTs core−shell catalysts showed different catalytic properties in selective hydrogenation of phenylacetylene from the Pt−Ru alloy and the mixed monometallic samples with the correspondingly identical composition.

1. INTRODUCTION Bimetallic catalysts have attracted considerable interest in heterogeneous catalysis from both a fundamental and an applied point of view, because their catalytic properties are superior to those of monometallic catalysts for many reactions.1−4 It is now widely acknowledged that the addition of the second metal provides a method of controlling the activity, selectivity, and stability of the catalysts in certain reactions by modifying electronic and/or surface or subsurface structures.5 Typical preparative methods for such bimetallic catalysts can be divided into coreduction6 and successive reduction.7 The coreduction method is simple, but gives a mixture of alloy and monometallic particles. Successive reduction of metal salts can be considered as one of the most suitable methods to prepare bimetallic catalysts with alloy or core−shell structures.6 Bimetallic Pt−Ru catalysts stand in a very interesting and well-studied class of materials because of their excellent catalytic properties in methanation, hydrogenolysis, and direct methanol fuel cells.8,9 However, the structure−property relationship is less well established because of ill-defined structures in alloy, core−shell and mixtures of monometallic particles. For example, Gonzalez et al.10,11 reported that supported Pt−Ru bimetallic particles could be formed by coimpregnation and pretreatment in H2, while Esteban et al.12 reported that partial phase segregation occurred in supported Pt−Ru catalysts under similar conditions. There also have been reports of a core−shell-type model with a Ru-rich core and a Pt-rich outer shell under similar conditions.13−15 Finally, there is disagreement on whether an oxidation treatment before reduction results in increased interactions between Pt and Ru.13−16 More recent reports on the Pt−Ru bimetallic catalyst © 2012 American Chemical Society

have focused on their application as electrocatalysts for hydrogen oxidation due to their high activity17−20 and high resistance to CO poisoning21 compared with conventional monometallic Pt. Selective hydrogenation with bimetallic Pt− Ru nanoparticles deposited on CNTs has already been reported.22−24 However, to the best of our knowledge, there are as of yet no reports on Pt−Ru bimetallic nanoparticles with different structures (i.e., core−shell, alloy, and mixtures of monometallic nanoparticles) in the selective hydrogenation of alkynes. Recently, CNTs have been considered to be potential supports and catalysts for heterogeneous catalysts.25 Some reactions, such as hydrogenation26 and electro-oxidation27 reactions have been investigated using the CNTs supported catalysts. Such catalysts, due to their unique structure feature, electronic properties, and interaction between the support and nanostructured metal particles, have shown the superior catalytic activity and selectivity in the above-mentioned reactions. However, well-dispersed metal particles on CNTs are very challenging, since there are relatively few sites available for anchoring the metal particles. In this work, we report on the synthesis of CNTs supported Pt, Ru, Pt−Ru alloy, Ru@Pt and Pt@Ru catalysts by the chemical reduction of RuCl3·3H2O and/or H2PtCl6·6H2O by ethylene glycol in the presence of NaOH. The Pt@Ru/CNTs and Ru@Pt/CNTs core−shell catalysts showed distinct catalytic properties in selective hydrogenation of phenylReceived: Revised: Accepted: Published: 4934

October 12, 2011 February 16, 2012 March 16, 2012 March 16, 2012 dx.doi.org/10.1021/ie202342a | Ind. Eng. Chem. Res. 2012, 51, 4934−4941

Industrial & Engineering Chemistry Research

Article

(XRD) analysis of the samples was carried out using a Rigaku D/Max-RB diffractometer with Cu Kα monochromatized radiation source, operated at 40 KV and 100 mA. The particle size and distribution of the samples were analyzed by transmission electron microscopy (TEM) (Philips CM200, 200 kV). Powder samples were ultrasonicated in ethanol and dispersed on copper grids covered with a porous carbon film. Energy dispersive X-ray spectroscopy was also performed during the same microscopy. Elemental mapping was conducted under STEM mode with the EDX detector as recorder. The oxidized CNTs were dried in an oven at 70 °C for 2 h under dynamic air before further measurements. Temperatureprogrammed desorption (TPD) was carried out in a horizontal quartz tube reactor with an inner diameter of 10 mm. Helium (99.9999%, 30 sccm) was used as carrier gas. An online infrared detector (Binos) was used to detect CO and CO2. The Binos was calibrated, and the measurement range is 0−4000 ppm for both gases. Typically, about 200 mg was used for each TPD measurement. The sample was heated from room temperature to 1100 °C at a heating rate of 2 °C min−1, and then held at that temperature for 1 h before cooling down to room temperature. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer equipped with a monochromatized Al KR source. The data were collected using an aperture of 4, a 70° takeoff angle, and a pass energy of 50 eV. The position of the C 1s peak (284.6 eV) was used to correct the binding energies for all catalysts for possible charging effects. The catalyst powders were attached to conductive carbon tape for the XPS measurements. For each catalyst, a survey spectrum was collected before high-resolution spectra were recorded. H2-temperature programmed reduction was conducted using 0.150 g sample. The sample was first flushed by an Ar (of 99.999% purity) stream at 353 K for 60 min to clean its surface, and then cooled down to room temperature, followed by switching to a 5% H2−Ar as reducing gas (30 mL min−1) to start the TPR measurement from 298 to 973 K. H2-temperature programmed desorption was conducted using 0.050 g sample. Each sample was heated at a rate of 10 K/min to 160 °C in a flow of 5% H2−Ar, and kept at this temperature for 1 h. After cooling to RT at a rate 10 K/min, the sample was kept at room temperature for 0.5 h. Then the 5% H2−Ar flow was switched to pure Ar for a period of 0.5 h. Finally, the catalyst was heated at a rate of 10 K/min and the H2-TPD curve was recorded. 2.3. Hydrogenation of Phenylacetylene. Liquid-phase selective hydrogenation of phenylacetylene was carried out in a 50 cm3 closed vessel at controlled temperature. Each catalyst was activated in an ultrapure hydrogen stream at 300 °C for 2 h, followed by cooling to room temperature. Approximate 0.1000 g of the catalyst was placed in the reactor with 10 mL of ethanol and 0.5344 g of phenylacetylene solution. The vessel was filled with H2 to 0.40 MPa and vented three times to remove the air. Then the reactor was then filled with H2 to 0.40 MPa. The reaction was carried out at 50 °C for 1 h with stirring. The products were analyzed by a 7890IIgas chromatograph using an FID detector with a SE-54/52 capillary column. A known amount of n-octane solution was used as the internal standard.

acetylene compared with the Pt−Ru alloy, and a mixed monometallic sample with the same metal composition.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Commercial multiwalled CNTs (above 95% purity) with the diameter of 20−40 nm and the length of 1−2 μm were obtained from Shenzhen Nanotech. Port. Co. Ltd.. The amorphous carbon content is no more than 3%. All the reagents used for the experiments were of analytical grade. The CNTs (1.0 g) was refluxed in 100 mL of 0.2 M HNO3 and 0.6 M H2SO4 mixture for 4 h. After refluxing, the mixture was cooled to room temperature, and filtered. Then the CNTs were washed repeatedly using distilled water and dried at 120 °C for 24 h. The Ru@Pt core−shell nanoparticles were synthesized by using a sequential polyol process. RuCl3·3H2O was initially reduced in refluxing glycol in the presence of NaOH. The resulting Ru nanoparticles (about 1.5 nm) were subsequently coated with Pt by adding H2PtCl6·6H2O to the Ru/glycol colloid and slowly heating to 198 °C.28−30 The Pt@Ru core− shell nanoparticles were also synthesized by using a sequential polyol process, with H2PtCl6·6H2O being initially reduced in refluxing glycol in the presence of NaOH. The resulting Pt nanoparticles (about 1.8 nm) were subsequently coated with Ru by adding RuCl3·3H2O to the Pt/glycol colloid and slowly heating the mixture to 198 °C. The Pt−Ru alloy nanoparticles were synthesized via coreduction of the RuCl3·3H2O and H2PtCl6·6H2O with glycol and NaOH as the stabilizer at 160 °C.28,29 Monometallic Pt and Ru nanoparticles were prepared from the corresponding H2PtCl6·6H2O and RuCl3·3H2O, respectively, using a slighty modified previously published procedure.31−35 To make a physical mixture of monometallic Pt and Ru nanoparticles, the separated colloids were mixed. The catalysts were then prepared by the addition of the CNTs. The resulting solution was cooled to 80 °C, at which point dilute hydrochloric acid was added to adjust the pH value to below 3, in order to reduce the concentration of the glycolate that acts as a stabilizer for the colloid. This solution was then heated to 140 °C, and stirred for 1 h. After the mixture was cooled to room temperature, the obtained black product was filtered, washed, and dried. The metal loading is controlled by adjusting the oCNTs/Pt polyol ratios. Label, composition, and structure of the as-prepared CNTs supported sample were listed in Table 1. 2.2. Characterization. Fourier transform infrared (FT-IR) spectra were collected at room temperature on a Nicolet Impact 410 with a resolution of 4 cm−1. X-ray diffraction Table 1. Label, Composition, And Structure of the AsPrepared CNTs Supported Samples Pt/Ru molar ratio label Ru@Pt3 Ru@Pt Pt−Ru alloy Pt@Ru3 Pt@Ru Pt + Ru Pt1 + Ru3

structure Rucore−Ptshell Rucore−Ptshell Pt−Ru alloy Ptcore−Rushell Rucore−Ptshell physical mixture physical mixture

designed (Pt + Ru) loading

designed

XPS

1% 1% 1% 1% 1% 1%

3:1 1:1 1:1 1:3 1:1 1:1

73:27 45:55 47:53 26:74 51:49 52:48

1%

1:3

28:72

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dx.doi.org/10.1021/ie202342a | Ind. Eng. Chem. Res. 2012, 51, 4934−4941


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3. RESULTS AND DISCUSSION Since CNTs have a hydrophobic surface, they tend to aggregate in polar solvent. To deposit a large amount of metal nanoparticles with uniform distribution, the surface of the CNTs must be modified. One common method used to modify the CNTs surfaces is oxidation treatment. Figure 1 shows FT-

Figure 1. FT-IR spectra of CNT (a) and CNTs after oxidation (b).

IR spectra of the raw CNTs and the CNTs after oxidation (oCNTs). It is found that after oxidation in acid, some additional bands appear at 1690, 1434, 1256, and 1167 cm−1. The peak at 1690 cm−1 can be assigned to a CO stretching vibration of carboxyl or carbonyl groups.36 Those at 1434 cm−1 and 1170 cm−1 are associated with −NO2−37 and the stretching vibration of C−O,38 respectively. Moreover, the intensity of the 1256 cm−1 band due to C−O groups is significantly enhanced. Table 2 shows textural characteristics of the CNTs before and

Figure 2. TPD profiles of CNTs and oCNTs samples.

chemisorbed carboxylate groups (below 250 °C), carboxylic acid (310 °C), carboxylic anhydride (420 °C), and lactone (580 °C), while CO is formed from decomposition of ketone and aldehyde (below 300 °C), carboxylic anhydride (420 °C), phenol and ether (700 °C), and pyrone (830 °C). The CO2 spectra show that oxidation with HNO3−H2SO4 increases the CO2 evolution mainly at low temperatures (from 200 to 450 °C). This results from the decomposition of carboxylic acid groups. However, the oxidation also introduces, to a lesser extent, carboxylic anhydrides (CO and CO2 released from 400 to 600 °C) and lactones (CO2 released from 550 to 700 °C). Large amounts of phenols and carbonyls (CO released at high temperatures) are also introduced by this treatment. Figure 3 shows typical TEM images of the oCNTs supported Pt−Ru catalysts. It can be seen that for all samples the Pt−Ru nanoparticles were formed uniformly on the external walls of the oCNTs. Well-dispersed Pt−Ru nanoparticles in spherical shape can be found in all microregions in the sample view on the TEM grid. More than 200 randomly chosen nanoparticles were measured to ensure statistically significant representation of the nanoparticle sizes. The mean nanoparticle size (diameter) for each catalyst was calculated. The Pt@Ru and Ru@Pt samples show a mean particle size of 2.7 and 2.4 nm from Figure 3 panels a and b correspondingly, which are larger than those of monometallic Pt (1.8 nm) and Ru (1.5 nm) from Figure 3 panels d and e, respectively. Both Pt@Ru and Ru@Pt present a typical {111} lattice fringe clearly on the highresolution transmission electron microscopy images in Figure 4a,b, which is consistent with the result in ref 28. TEM energydispersive X-ray spectroscopy point analysis of Pt@Ru and Ru@Pt on the randomly chosen nanoparticles shows that each sample has a main content of Pt and a small amount of Ru. To confirm the distinct existence of Ru in the Pt−Ru alloy/ oCNTs, X-ray mapping of Pt−Ru alloy/oCNTs was taken (Figure 5). The results show that this method has deposited a

Table 2. Textural Characteristics of CNTs and oCNTs sample

surface area/m2/g

pore volume/cc/g

pore diameter/nm

CNTs oCNTs

103 138

0.73 0.89

2.7 3.5

after oxidation. It is found that after oxidation in acid, surface area, pore volume, and pore diameter of CNTs have increased. Combined with FT-IR and BET, the surface defects and functional groups can be effectively created under the strongly oxidizing conditions. TPD is becoming popular for characterization of oxygencontaining groups on the surface of carbon materials. In this technique, all oxygenated surface groups are thermally decomposed releasing CO and/or CO2 at different temperatures. The nature of the surface groups can be assessed by the decomposition temperature and the type of gas released, and their respective amounts are determined by the areas of the component peaks. The major difficulty lies in identifying each surface group individually, because TPD profiles show overlapping CO and CO2 peak. It is widely confirmed that CO2 evolution results from the decomposition of carboxylic acids at low temperature, and lactones at higher temperatures; carboxylic anhydrides originate both CO and CO2; phenols and carbonyl/quinone groups originate CO. Figure 2 shows the TPD spectra of the CNTs and oCNTs samples. An increase in the amount of surface oxygen-containing groups is evidenced by the larger amounts of CO and CO2 released. On the basis of the literature,39 CO2 is formed from decomposition of 4936



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Figure 4. High-resolution TEM images of Pt@Ru/oCNTs (a), Ru@ Pt/oCNTs (b), and Pt−Ru alloy/oCNTs (c).

Figure 5. Representative STEM dark-field images and X-ray maps of C, Pt, Ru for Ru−Pt alloy/oCNTs. Figure 3. Representative TEM images, histograms of particle size distribution, and EDX of Pt@Ru/oCNTs (a), Ru@Pt/oCNTs (b), Pt−Ru alloy/oCNTs (c), Pt/oCNTs (d), and Ru/oCNTs (e).

large amount of Pt and Ru nanoparticles with uniform distribution in the surface of oCNTs. Moreover, the amount of Pt is almost the same as the amount of Ru. Figure 6 shows typical XRD patterns of 20 wt % Pt/oCNTs, Ru@Pt/oCNTs, Pt−Ru alloy/oCNTs, Pt@Ru/oCNTs, Ru/ oCNTs, and oCNTs. The peaks at 26.5 o, 42.4 o, 54.7 o, and 77.4° could be assigned to the hexagonal graphite structures (002), (100), (004), (110) for Ru/oCNTs and oCNTs.40 There were no obvious diffraction peaks of metallic Ru, probably due to the poor crystallization in polyol-synthesis. It has been shown in previous studies that Pt−Ru alloys takes the face-centered cubic (fcc) structure of Pt if the Ru content is below 60 wt %.41 The XRD patterns displayed the (111), (200), (220), and (311/222) reflections, confirming that the nanoparticles have taken the fcc structure. The Pt (111)

Figure 6. XRD patterns of oCNTs, Ru/oCNTs, Pt@Ru/oCNTs, Ru@Pt/oCNTs, Pt−Ru alloy/oCNTs, and Pt/oCNTs. 4937



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diffraction peaks for the Pt−Ru alloy catalysts shifted to higher position compared withthose of the Pt/oCNTs, which can be interpreted as the evidence of an alloy.29 Diffraction peaks near 38° and 44° arising from Ru are not observed, possibly because Ru has entered the Pt lattice and formed the Pt−Ru alloy or Ru exists in the amorphous form.42−45 The Pt (111) diffraction peaks of Ru@Pt catalysts are very similar with those of Pt/ oCNTs, which is probably due to core−shell cluster with Pt being outside. The Pt (111) diffraction peaks of Pt@Ru catalysts shifted to a higher position compared with those of Pt/oCNTs, which is ascribed to the insertion of Ru into the Pt lattice. H2-TPR study of the catalyst provided useful information about its reducibility. Figure 7 shows the H2-TPR profile of the Figure 8. H2-TPD profiles of CNTs supported Pt, Pt−Ru alloy, Ru@ Pt, Pt@Ru, and Ru catalysts.

desorption peaks in all the catalysts. The H2-TPD profile of the supported Ru catalyst is consistent with that of the Pt@Ru catalyst. Similarly, the profile of the supported Pt catalyst is consistent with that of the Ru@Pt catalyst. This gives an indirect proof that these materials basically have a consistent core−shell structure. Combined with TEM and XRD, all the data points to a core−shell structure for Ru@Pt and Pt@Ru nanoparticles, which is totally different from that of the Pt−Ru alloy. Figure 9 panels a and b show the XPS spectra of the Pt 4f and Ru 3p, respectively, for the Pt−Ru alloy colloids with 1.8 nm in size. The colloids were deposited on oCNTs, resulting in Figure 7. H2-TPR profiles of oCNTs supported Pt catalyst.

Pt catalyst supported on oCNTs. From Figure 7, it can be seen that there is two H2 consumption peaks in the temperature range of 25 to 120 °C representing the reduction of highly dispersed PtO2 species in the Pt/oCNTs catalyst. These TPR peaks may be ascribed to the continuous multistep singleelectron reduction of Ptn+ species. The high specific H2 consumed amount (i.e., the amount of hydrogen consumed due to reduction of Ptn+ in the unit mass of Pt) indicated the high percentage of the Ptn+ species reducible to lower valence in the total Pt amount, with a H2/Pt molar ratio basically equal to the stoichiometry governed by the equation PtO2 + 2H2 → Pt + 2H2O. However, two bigger H2 consumption peaks are obtained at about 250 and 300 °C in the TPR profile of Pt/ oCNTs. Zhang et al.46 reported that dispersed [Pt(OH)4Cl2]2‑ species were reduced at a higher reduction temperature (250 °C). Marceau et al.47 indicated the presence of oxy-chlorinated Pt species, which were reduced at 290 °C, while Hwang and Yeh48 reported that dispersed PtOxCly species were reduced at a higher reduction temperature (350 °C), while the biggest H2 consumption peak is obtained at about 600 °C in the TPR profile of Pt/oCNTs, considering that the hydrogenation of some surface carbon by H-adspecies (which would lead to consumption of part of H-adspecies and formation of C1−2hydrocarbons) could occur at temperatures of 600 °C and above.49 H2-TPD is one of the effective methods to characterize active sites of a catalyst. In the H2-TPD profile, the different positions of peaks correspond to the different adsorption states of hydrogen on the surface of the catalyst. H2-TPD profiles of oCNTs-supported Pt, alloy, Ru@Pt, Pt@Ru, and Ru catalysts are shown in Figure 8. It can be seen that there are three H2

Figure 9. Typical X-ray photoelectron spectra of the Pt−Ru alloy catalyst. 4938



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actual loadings of 6 wt % of Pt and 3 wt % of Ru. The less intensive Ru 3p region was analyzed instead of the main Ru 5d region, as the latter overlays with the carbon 1s region. The results obtained by deconvoluting the XPS spectra are summarized in Table 3. The Pt core level region was Table 3. Binding Energies of Pt and Ru Species Obtained from Curve-Fitted XPS Spectra for oCNTs Supported Pt− Ru Alloy Catalysts species Pt

Ru

orbital/ spin

binding energy (eV)

assignment

relative concentrations (%)

4f7/2 4f5/2 4f7/2 4f5/2 4f7/2 4f5/2 3p1/2 3p3/2 3p1/2 3p3/2

71.1 74.4 72.1 75.4 74.5 77.7 462.4 484.8 466.1 488.6

Pt metal Pt metal PtO PtO PtO2 PtO2 Ru metal Ru metal RuO2 RuO2

29.5 14.4 18.4 10.5 16.9 10.3 45.8 15.9 36.6 1.7

Figure 10. Hydrogenation of phenylacetylene on the as-prepared catalysts.

The conversion of phenylacetylene over the Pt−Ru alloy/ oCNTs is lower than that for the Pt1+Ru1/oCNTs, while the selectivity to styrene is essentially unchanged. The lower conversion over the former sample is likely due to the insertion of Ru into the Pt lattice, which changes the Pt unit cell lattice, and influences the adsorption of styrene. This behavior therefore suggests that a Pt−Ru alloy was successfully formed, as opposed to a mixture of monometallic Pt and Ru nanoparticles. The conversion over Pt1@Ru3/oCNTs is higher than Pt1+Ru3/oCNTs, while the selectivity to styrene is the same. This suggests that the Pt1@Ru3/oCNTs was indeed prepared, again as opposed to mixed monometallic Pt and Ru nanoparticles. Finally, the conversion over Pt1@Ru3/oCNTs is higher than Ru/oCNTs, which could be explained by the core effect that modifies the electronic structure and thus changes the adsorption of styrene. The novel nanoparticles with a Ru/ Pt core and a Pt/Ru shell (Ru@Pt and Pt@Ru core−shell nanoparticle) stand extraordinarily in some catalytic reactions, for example, the selective hydrogenation of phenylacetylene, distinguishing them from the Pt−Ru alloy or monometallic Pt/ Ru mixtures with identical loading and composition in terms of catalytic properties. To investigate whether the catalysts become irreversibly deactivated, the Pt/oCNTs and Ru@Pt3/oCNTs catalysts were used to test the recyclability. Table 4 details the experimental results for the recyclability of the catalyst. From Table 4, the catalytic activities of the two catalysts were almost the same after the first round. After the fifth round, the Pt/oCNTs catalyst still had a good catalytic activity (97%) and selectivity

deconvoluted, as described by Bancroft et al.,50 while other literature51−55 was referred to for deconvolution of the Ru 3p region. The mean free path of X-rays in XPS for Pt and Ru is around 1.5 nm, suggesting that the XPS data for the small particles analyzed here represent all the particles, as opposed to only surface species. It is noteworthy that the survey of XPS spectra did not show the presence of Cl, as no peak at 198 eV, typical for the highest intensity Cl 3p core level, was observed. The XPS data suggest that 44% of the Pt is present as zerovalent Pt metal, while 29% is present as divalent Pt oxide (PtO). The remaining fraction (27%) of the Pt is present in a higher oxidation state, likely as platinum dioxide (PtO2). The deconvolution of the Ru 3p core level region was more difficult, due to its low intensity and increased noise. However, a large fraction (ca. 60%) of the Ru appears to be in the form of Ru metal, and a small fraction, ca. 40%, is present in a higher oxidation state such as RuO2. It is unknown whether the “partial” oxidation of both Pt and Ru takes place when the carbon nanotube-supported Pt−Ru alloy colloids are dried in air and/or during incomplete reduction of the noble metal precursor salts. The amount of RuO2 seems to be very large, possibly explaining the absence of Ru in the XRD pattern as well as the possibility that the RuO2 may be amorphous. To compare the activity of the core−shell nanoparticles with that of the alloys and monometallic nanoparticles, the activity and selectivity of catalysts were tested in the selective hydrogenation of phenylacetylene. Figure 10 shows that the phenylacetylene conversions are 98, 88, 38, 78, 3, 77, and 26%, respectively, over Pt/oCNTs, Ru1@Pt3/oCNTs, alloy/oCNTs, Pt1@Ru3/oCNTs, Ru/oCNTs, Pt1+Ru1/oCNTs, and Pt+Ru3/ oCNTs catalysts. The corresponding selectivities to styrene are 86, 88, 91, 91, 95, 89, and 90%, respectively. Ru/oCNTs is not an active catalyst for selective hydrogenation of phenylacetylene although it shows high selectivity to styrene. The Pt/oCNTs catalyst shows the highest phenylacetylene conversion and middle selectivity to styrene. And the Ru@Pt3/oCNTs shows superior activity to the Ru/oCNTs, and superior selectivity to styrene to the Pt/oCNTs, suggesting a core−shell structure in the latter with Pt being the outside shell.

Table 4. The Recycling of Pt/oCNTs and Ru@Pt3/oCNTs in the Selective Hydrogenation of Phenylacetylenea catalyst Pt/oCNTs

Ru@Pt3/oCNTs

run

conversion (%)

selectivity to ST (%)

1 2 3 4 5 1 2 3 4 5

98 97 99 96 97 88 89 85 86 88

86 87 86 87 85 88 88 88 89 88

a

Reaction conditions: 0.10 g of catalyst, 10 mL of ethanol, 0.5344 g of phenylacetylene, 0.40 MPa H2, 50 °C, and 1 h.

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to styrene (85%), and the Ru@Pt3/oCNTs catalysts also had a high phenylacetylene conversion (88%) and selectivity to styrene (88%). No irreversibly deactivation was observed in the selective hydrogenation of phenylacetylene.

4. CONCLUSIONS Through the use of the same precursors and protocols, Pt−Ru bimetallic nanoparticles with different structures (i.e., core− shell, alloy, and mixtures of monometallic NP) can be prepared selectively. The prepared Pt, Ru, Pt−Ru alloy, Ru@Pt, and Pt@ Ru nanoparticles fell in the range of 1.5−3.0 nm in diameter, and were uniformly dispersed on the CNTs. All the bimetallic catalysts exhibited the main characteristic peaks of fcc crystalline Pt, but the 2θ values were shifted to slightly higher ones compared to that of Pt/oCNTs catalyst, indicating the formation of alloy or core−shell structures. Despite the presence of Pt(II), Pt(IV), and Ru(IV) in the bimetallic catalysts, the XPS spectra were all dominated by Pt(0) and Ru(0). The Pt@Ru/CNTs and Ru@Pt/CNTs core−shell catalysts showed different catalytic properties in the selective hydrogenation of phenylacetylene from the Pt−Ru alloy and the mixed monometallic samples with the correspondingly identical composition. The strategies and results presented in this article would be very useful in guiding the fabrication and characterization of core−shell nanoparticles with extremely thin shells and in understanding the unique properties of these nanomaterials.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-411-84986056. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21073023) and Project Based Personnel Exchange Program with CSC and DAAD. C.T.W. thanks Dalian University of Technology for a “Sea-sky” professorship.

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REFERENCES

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