J. Phys. Chem. C 2008, 112, 3827-3834
3827
Semihydrogenation of Phenylacetylene Catalyzed by Palladium Nanoparticles Supported on Carbon Materials Sonia Domı´nguez-Domı´nguez,† A Ä ngel Berenguer-Murcia,† Bhabendra K. Pradhan,‡ † A Ä ngel Linares-Solano, and Diego Cazorla-Amoro´ s*,† Departamento de Quı´mica Inorga´ nica, UniVersidad de Alicante, Ap. 99, San Vicente del Raspeig, E-03080 Alicante, Spain, and Columbian Chemicals Company, 1800 West Oak Commons Court, Marietta, Georgia 30062-2253 ReceiVed: NoVember 8, 2007; In Final Form: December 13, 2007
Palladium catalysts were prepared supporting palladium colloids on three types of carbon supports: multiwall carbon nanotubes (NTs), carbon black (CB), and an activated carbon (AC). These catalysts were tested in a reaction of great industrial interest such as the partial hydrogenation of phenylacetylene. The liquid-phase hydrogenation reaction of phenylacetylene was performed under very mild conditions (323 K, flow of H2 of 30 mL/min, 1 bar of pressure). The catalytic activities are very close to that of the homogeneous catalyst. A total conversion was achieved in all the Pd/C catalysts and also with very high selectivity toward styrene (higher than 95%). The carbon support produces differences between the catalysts. The AC provokes a Pd particle agglomeration, and the catalyst has the lowest activity and selectivity. The highest selectivity is obtained for the Pd/NT sample. The ease of manipulation of the Pd/NT catalysts is noteworthy, facilitating its recovery by filtration and its subsequent reutilization. It was found that neither the catalytic activity nor the selectivity decreased appreciably throughout five consecutive cycles of the Pd/NT sample. Furthermore, no Pd leaching of the samples occurred during the catalytic reactions.
1. Introduction Heterogeneous catalysis is of great practical importance in modern industry due to the numerous advantages it involves. To mention just a few, immobilization of the catalytic species on a suitable support avoids agglomeration of the active species during chemical reaction and enables easy catalyst recovery (and thus easy product separation), and the handling of the catalyst is greatly simplified. Carbon supports are regarded as highly interesting materials for heterogeneous catalysis applications due to their low cost, high thermal and chemical stability, the possibility of forming them into various shapes and sizes, and tunable textural properties such as surface area, porosity, and surface chemistry.1 It must be noted that the supporting material is a factor of great importance, since its interaction with the active species may greatly influence the properties of the resulting catalyst.2 In heterogeneous catalysis, one of the main aspects lies in the preparation of the catalyst itself. Despite the great number of existing and reported procedures,3 supported metallic catalysts present serious disadvantages concerning their synthesis. This is due to the fact that the size and composition of the catalytically active particles is difficult to control. In this sense, nanoparticle synthesis might be envisaged as a promising alternative to traditional methods, since it allows a very precise control of both the particle size and final composition of the resulting materials.4-6 Furthermore, the possibility of alloying two different metals and shaping them into nanoparticles is a very interesting feature that has been extensively studied during the past 20 years,6-8 because the incorporation of a second metal * To whom correspondence should be addressed. E-mail: cazorla@ ua.es. Fax: (+34) 965 903454. † Universidad de Alicante. ‡ Columbian Chemicals Co.
generally results in improved activity, selectivity, or lifetime in the final catalyst.9 Semihydrogenation (or partial reduction) of phenylacetylene is a very convenient process for catalyst optimization since it enables both evaluation of process design10 and the testing of hydrogenation catalysts11 since it can be performed under very mild conditions and is a reaction of great industrial importance. Phenylacetylene is an unwanted feedstock component in polystyrene production plants, and its removal to levels below 10 ppm is mandatory to avoid poisoning of the polymerization catalyst. In direct relation with the present study, there have been some previous works reported in the literature concerning the heterogeneous hydrogenation of phenylacetylene. Dell’Anna et al.12 reported the use of polymer-embedded Pd nanoparticles in the hydrogenation of many unsaturated compounds. The results using phenylacetylene as substrate at room temperature and under 1 bar of H2 showed little activity or selectivity, with TOF values under 0.02 s-1. Mandal et al.13 obtained very good activity values using zeolite-immobilized Pd and Pt nanoparticles. The reported selectivity values for styrene, however, never reached values over 35%, which would not be desirable from a practical point of view. A somewhat similar behavior was observed by Pellegatta et al.14 by using rhodium nanoparticles embedded in PVP at 70 °C and 7 bar of H2 or by Shephard et al.15 using Cu-Ru bimetallic clusters in mesoporous silicas as catalysts. Using Cu-Pd catalysts supported on pumice, Guzci et al.16 reported very high activities and selectivities around 90% carrying out the reaction under very mild conditions. Their results are similar to those obtained by Mastalir et al.17 using Pd nanoparticles in hydrotalcite. Despite the efforts dedicated to the study of this reaction, not many examples for heterogeneous phenylacetylene hydrogenation using carbon as supports can be found in the literature.18,19
10.1021/jp710693u CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008
3828 J. Phys. Chem. C, Vol. 112, No. 10, 2008 Consequently, the aim of the present work is to prepare and characterize catalysts based on Pd nanoparticles supported on three types of carbon materials (carbon nanotubes, carbon black, and an activated carbon). The prepared materials will be tested for their catalytic properties in the semihydrogenation of phenylacetylene under mild conditions and the results compared to those found in the literature. 2. Experimental Section 2.1. Pd Catalyst Preparation. The colloidal Pd nanoparticles were synthesized following the method described elsewhere,6 using ethylene glycol that reduces the Pd precursor by the socalled solvent method. The syntheses were performed under an Ar atmosphere by means of a Schlenk system. In a typical synthesis, the palladium precursor solution was prepared, in a two-necked, round-bottom flask, by adding 0.2245 g of palladium(II) acetate (Sigma-Aldrich, no. 205869, 98% pure) and 50 mL of 1,4-dioxane (Sigma-Aldrich, no. 533971, g99% pure) under vigorous stirring for 2 h, resulting in a dark-orange solution. In another two-necked, round-bottom flask, a solution containing 0.800 g of poly(n-vinylpyrrolidone) (Sigma-Aldrich, no. 234257, Mw ≈ 29 000) and 120 mL of anhydrous ethylene glycol (Sigma-Aldrich, no. 293237, g99% pure) was stirred and heated at 80 °C for 3 h. This solution was cooled to 0 °C by means of an ice bath. Both solutions were mixed under stirring to ensure homogenization. Immediately, the pH of the resulting mixture was adjusted to 9-10 by dropwise addition of a 1 M NaOH solution. The final solution was then heated at 100 °C under vigorous stirring. After a few minutes, the darkbrown colloidal Pd solution was formed. The heating was continued for 2 h, after which the colloidal suspension was cooled to room temperature. The prepared Pd nanoparticle colloids were treated with a large excess of acetone with the objective of purifying them.6 The flocculated metallic nanoparticles were separated from the polymeric protecting agent in the acetone phase by centrifugation. Finally, the purified Pd colloid was redispersed by very gentle stirring in a controlled volume of MeOH, so the Pd concentration in the resulting Pd nanoparticle/methanol dispersion (mg of Pd/mL of solution) was known. 2.2. Synthesis of Supported Pd Nanoparticles on Carbon Materials. In this work, three carbon materials were used as supports for the preparation of the Pd/C catalysts: multiwall carbon nanotubes (sample NTs, NanoBlack) and carbon black (sample CB, CD6026), from the Columbian Chemicals Co., and an activated carbon (sample AC), prepared from a Spanish anthracite by chemical activation with potassium hydroxide.20 The Pd nanoparticles were supported on the carbon materials using the impregnation method. First, the appropriate volume of the palladium nanoparticle/methanol dispersion was mixed with the carbon material. All Pd/C catalysts were prepared so as to have a final catalyst loading of 1 wt %. The suspension was then gently stirred at room temperature for 3 days. After this, the suspension was transferred to an oven at 60 °C until the methanol was evaporated. The collected solid was washed with a mixture of ethanol and water (50/50%, v/v) several times. Finally, the carbon support containing the deposited Pd nanoparticles was dried overnight. The nomenclature used for the prepared catalysts was Pd/NTs, Pd/CB, and Pd/AC, depending on the carbon material on which the Pd nanoparticles were deposited. 2.3. Carbon Supports and Pd/C Catalyst Characterization. Porous texture characterization of the carbon supports and the synthesized Pd/C catalysts was carried out by physical adsorp-
Domı´nguez-Domı´nguez et al. tion of N2 at 77 K and CO2 at 273 K, using volumetric adsorption equipment (Autosorb-6, Quantachrome), after outgassing the samples at 523 K under vacuum for 4 h. The apparent surface area, SBET, was calculated by the BET equation. The external surface, Sext, excluding the area corresponding to micropores, was calculated by de Boer’s t method.21 The total micropore volume (pore size below 2 nm), VDRN2, was determined by the application of the Dubinin-Radushkevich equation to the N2 adsorption isotherm at 77 K, using the range of relative pressure, P/P0, from 0.005 to 0.14 for the analysis. The total pore volume, Vtotal, was obtained at P/P0 ) 0.95. The volume of mesopores with pore sizes between 2 and 7.5 nm, Vmeso, was calculated by the difference between the volume of N2 adsorbed at P/P0 ) 0.7 and that at P/P0 ) 0.2.22 The volume of narrow micropores (mean pore size lower than 0.7 nm), VDRCO2, was calculated from CO2 adsorption at 273 K, using the DR equation and for relative pressures below 0.025.23-25 The average pore size, dp, was estimated from Vtotal and SBET using the following equation:
dp ) 4Vtotal/SBET
(1)
The densities of the adsorbed phases used for the calculations were 0.808 and 1.023 g/mL for N2 and CO2,23 respectively. The palladium contents in the prepared catalysts were determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES), in a Perkin-Elmer Optima 4300 ICPOES spectrometer. Before analysis, the samples were treated in concentrated HNO3 for 48 h to dissolve the metallic nanoparticles. The Pd loadings were obtained from the emission intensities by means of a calibration curve. An average of three analyses was done to calculate the Pd content present in the samples. Powder XRD patterns were collected on a Seifert 2002 diffractometer, using the Cu KR radiation (λ ) 0.1542 nm). The 2θ range scanned was from 2° to 90°, with a step size of 0.1°. Metal crystallite sizes were calculated from the broadening of the Pd(111) peak 2θ ) 40.12° by using Scherrer’s equation. The Pd/C catalysts were characterized by means of transmission electron microscopy (TEM) using a JEOL JEM-2010 hightilt instrument operating at 200 kV with a structural spatial resolution of 0.5 nm. High-resolution transmission electron microscopy studies were performed in a JEOL JEM-3011 electron microscope operating at 300 kV with a structural resolution of 0.16 nm. The samples were prepared by suspending the prepared Pd/C catalysts in ethanol and sonicating for 1 h. A drop of the suspension was deposited on a carbon-coated copper grid, followed by drying at ambient conditions. The average value of the palladium particle diameters (dTEM) was calculated by the following equation:
dTEM )
∑nidi ∑ni
(2)
where ni (ni > 100) is the number of particles of diameter di. The catalyst dispersion from TEM (DTEM) was estimated by assuming spherical particle geometry and with the equation
6MFsite DTEM ) 1021 FPdNdTEM
(3)
where M is the atomic mass of the palladium (106.42 g/mol), Fsite the palladium surface site density (12.7 atoms/nm2), FPd the palladium density (12.02 g/cm3), N Avogadro’s constant (6.022 × 1023 mol-1), and dTEM the average palladium particle
Semihydrogenation of Phenylacetylene
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3829 where 1.27 × 1019 is the number of surface atoms per square meter of the polycrystalline palladium surface. The metal dispersion (DCO), i.e., the fraction of exposed Pd, is given by
DCO ) Ns/Nt
Figure 1. Nitrogen adsorption-desorption isotherms at 77 K of carbon supports and prepared Pd/C catalysts.
TABLE 1: Surface Area Characterization Results for Carbon Supports and Prepared Pd/C Catalysts Sext VDRN2 Vtotal Vmeso VDRCO2 dp SBET sample (m2/g) (m2/g) (cm3/g) (cm3/g) (cm3/g) (cm3/g) (nm) NTs Pd/NTs CB Pd/CB AC Pd/AC
253 177 214 136 2879 1343
245 177 179 128 99 52
0.10 0.07 0.10 0.06 1.19 0.56
0.63 0.48 0.33 0.27 1.37 0.66
0.11 0.08 0.07 0.06 0.10 0.06
0.05 0.04 0.09 0.03 0.71 0.44
10.0 10.8 6.2 7.9 1.9 2.0
diameter obtained from TEM measurements (nm). Hence, replacing all the previous values, we determined the metal dispersion (DTEM) directly using the following equation:26
DTEM ≈ 0.9/dTEM (nm)
(4)
CO chemisorption analyses were carried out by volumetry with an automatic adsorption system (ASAP 2020 Chemisorption from Micromeritics) to evaluate the dispersion (DCO) and metal particle size (dCO). In a typical experiment, about 150 mg of the catalyst sample was placed in a U-shaped microreactor of 10 mm i.d. and 180 mm long quartz reactor. The sample was first pretreated with a N2 flux at 383 K for 1 h and then heated at 423 K under a H2 flow at a heating rate of 10 K/min. Next, the sample was evacuated at 10-6 Torr for 4 h at 423 K and cooled to 308 K. Irreversible CO uptakes were obtained from the total and reversible adsorption isotherms taken in the pressure range 0.8 × 104 to 65 × 104 Pa. Using the difference between the total and the reversibly adsorbed CO, the number of exposed Pd metal atoms was calculated, assuming a chemisorption stoichiometry of Pd/CO ) 2.18,27,28 Hence, the number of exposed Pd atoms per gram of catalyst on the surface, Ns, is equal to 2 times the CO molecules that were irreversibly chemisorbed per gram of catalyst. The total metal surface area, S, is given by
S ) (Ns/1.27 × 1019) m2 g-1 catalyst
(5)
(6)
where Nt is the total number of Pd atoms per gram of catalyst. The mean particle diameter by chemisorption of CO, dCO, is obtained by using the same equation used for the calculations in TEM analyses (eq 4). 2.4. Catalytic Tests. The liquid-phase hydrogenation of phenylacetylene was performed as described elsewhere.6 The prepared Pd/C catalyst (approximately 150 mg), containing the desired amount of palladium, and the appropriate amount of MeOH (90 mL) were added to a 250 mL three-neck flask. A glass stopper was set in one of the necks, and the system was purged with Ar for 30 min. The flask was then purged with H2 for a further 30 min. After purging, 10 mL (0.089 mol) of phenylacetylene was added to the reaction mixture, and a continuous H2 bubbling (30 mL‚min-1) through the liquid phase was regulated by means of a needle valve. This time was taken as the start of the catalytic reaction. The final volume of the solution in the flask was always 100 mL. All reactions were performed up to a 100% conversion, and this was taken as the end of the reaction. Agitation was carried out with a magnetic stirring bar (800 rpm). The reactions were performed at a temperature of 323 K. Samples were withdrawn at the desired times, diluted to a 1:100 ratio with methanol, and injected by means of a microsyringe for their analysis on a gas chromatograph (HP 6890) using a capillary column (HP-1 cross-linked methylsiloxane). Finally, reutilization of the sample Pd/NT was investigated, in terms of activity and selectivity, in five consecutive reactions. 3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 shows the N2 adsorption isotherms of the carbon materials before and after supporting the Pd nanoparticles. We observe that the isotherms obtained for the activated carbon (AC and Pd/AC) are of type I, according to the IUPAC classification,29,30 indicating that this carbon support is essentially microporous. Isotherms of the carbon nanotubes (NTs and Pd/NTs) and carbon black (CB and Pd/CB) are of type IIb, according to the IUPAC classification.29,30 The porosity of the NTs consists mainly of the inner hollow cavities the pores formed by interaction of isolated NTs.31,32 The CB is mainly formed by agglomerates of strongly bonded particles with some surface heterogeneity. They are formed by concentric, surface parallel orientated graphite layers and exhibit a slight microporosity.33 The calculated SBET, Sext, VDRN2, Vtotal, Vmeso, VDRCO2, and dp are presented in Table 1. According to the data listed in Table 1, the BET surface areas of Pd/NTs, Pd/CB and Pd/AC decreased by 30%, 36%, and 53%, respectively, compared to that of the corresponding carbon support. On the other hand, the Sext of the Pd/C catalysts also decreased compared to that of the corresponding support, as occurred with SBET. NTs and CB have an external surface, Sext, that is very close to the BET
TABLE 2: Pd Content (wt %) by ICP Analysis and CO and TEM Adsorption Characterization Results of the Prepared Pd/C Catalysts sample
Pd content (wt %)
particle size, dTEM (nm)
dispersion, DTEM
dispersion, DCO
metallic surface area (m2 g-1 of catalyst)
metallic surface area (m2 g-1 of Pd)
particle size, dCO (nm)
Pd/NTs Pd/CB Pd/AC
0.66 0.88 0.65
2.5 2.6 5.6
0.35 0.34 0.16
0.33 0.31 0.09
0.96 1.19 0.22
146 136 34
2.7 2.9 10.0
3830 J. Phys. Chem. C, Vol. 112, No. 10, 2008
Figure 2. XRD patterns of the three Pd/C samples: (a) Pd/NTs, (b) Pd/CB, (c) Pd/AC.
surface area, SBET, as expected in materials with low or no microporosity. In the case of the AC, it has a very small external surface area because most of its surface is associated with micropores. The total, micropore, and mesopore volumes of the three catalysts also decrease with respect to those of the supports, but this phenomenon was especially notorious for Pd/AC. The decrease of pore volumes for the prepared catalysts indicates that a fraction of pores are blocked by the supported Pd nanoparticles. The blocking of pore volume was previously observed in Pd/C catalysts prepared using other procedures.34 The results of the ICP analysis for the Pd contents of the three prepared Pd/C samples are summarized in the first column of Table 2. It can be observed that the initially targeted 1 wt % Pd loading was not really achieved. Therefore, the rest of the metal was lost during the synthesis procedure, probably during
Domı´nguez-Domı´nguez et al. the washing step, where Pd nanoparticles not bound strongly enough were washed off the carbon support. Figure 2 shows XRD patterns of the three synthesized Pd/C samples. Except for the XRD pattern of Pd/AC, no Pd0 diffraction peak was detected, suggesting that small Pd particles, not large enough to be detected by the XRD technique, may be present on the carbon surface in the other two Pd/C samples. Therefore, the Pd particle sizes of Pd/NTs and Pd/CB are smaller than 3-4 nm. In the case of the Pd/AC sample, in which the calculation of the particle size was possible applying Scherrer’s equation, the estimated average size of the Pd crystallites was 6.3 nm. This observation, subjected to a significant experimental error due to the very small peak height, is in agreement with TEM analysis, as is explained next. To directly compare the metal dispersion of Pd/C catalysts, TEM and HRTEM analyses were also made. Figure 3 shows TEM micrographs of (a) Pd/NTs, (b) Pd/CB, and (c) Pd/AC. Both Pd average particle sizes and metal dispersion results of the three Pd/C catalysts obtained from TEM analyses are shown in the second and third columns of Table 2. The obtained average particle sizes were (a) 2.5 ( 0.6 nm, (b) 2.6 ( 0.7 nm, and (c) 5.6 ( 1.6 nm, which are quite consistent with the size of the Pd particles deduced from XRD analysis. Furthermore, the particle sizes of the Pd nanoparticles of Pd/NT and Pd/CB catalysts are very similar to the average particle size of the Pd particles of the colloid (2.4 ( 0.5 nm6), but are larger in the Pd/AC catalyst. Thus, the particles seem to grow during the step of purification or deposition on the activated carbon support, because this support is essentially microporous with an average pore size below the size of the Pd colloidal particles and with a low external surface. As can be observed in the HRTEM micrographs of Figure 4, the Pd nanoparticles are spherical and reasonably monodisperse. The HRTEM results also show the high crystallinity of the Pd
Figure 3. TEM micrographs and their corresponding histograms of the particle sizes of the three Pd/C samples: (a) Pd/NTs, (b) Pd/CB, (c) Pd/AC.
Semihydrogenation of Phenylacetylene
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3831
Figure 4. HRTEM micrograph of (a) Pd/NT and (b) Pd/CB catalysts.
nanoparticles and, moreover, that each particle has its own random crystallographic orientation. Pd catalyst dispersion (DCO) on the prepared Pd/C samples and the mean Pd particle size (dCO) were calculated from the irreversible CO chemisorption measurements, and the results are summarized in the last four columns of Table 2. They are, again, in agreement with those obtained from XRD and TEM,
although the particle size for sample Pd/AC is somewhat larger. The heat treatment of the sample to 423 K prior to the CO chemisorption measurement produces additional Pd agglomeration when the support is the AC, whereas the increase in particle size is not important for the CB and NTs (see the TEM micrograph of the Pd/AC catalyst after CO chemisorption in Figure 5). As explained above, the AC used is essentially
3832 J. Phys. Chem. C, Vol. 112, No. 10, 2008
Domı´nguez-Domı´nguez et al.
Figure 5. TEM micrograph and the corresponding histogram of the particle size of the Pd/AC sample after the chemisorption of CO.
microporous, and the average pore size is smaller than the Pd particle size (Table 1), which causes most of the Pd particles to remain in the external surface, which is lower in this material compared to the NTs and CB, thus facilitating the metal agglomeration. 3.2. Catalytic Tests. As a catalytic test of synthesized Pd/C materials, the activity for hydrogenation of phenylacetylene was measured. The reaction was performed under the same conditions of the previous work.6 Furthermore, with the purpose of ensuring that the hydrogenation tests are performed under a regime without mass transfer limitations, previous experiments were carried out with different stirring rates, hydrogen flows, and catalyst masses. Thus, at an agitation rate of 700 rpm and
a hydrogen flow of 30 mL min-1, different Pd/NT catalysts with increasing amounts of palladium loading, between 0.7 and 2.2 wt % Pd, were used in the catalytic tests. The reaction rates were measured for applying the Koros-Nowak criterion,35 and the results are shown in Figure 6. These Pd/NT catalysts were previously characterized by CO chemisorption to ensure the correct application of the aforementioned criterion (no change in the catalyst dispersion). This was done to ensure the correct application of the aforementioned criterion. In the four Pd/NT samples, the dispersion was around 0.3. As can be observed in Figure 6, the reaction rates and the palladium loading in the catalysts are directly proportional, so it is proven that the catalytic tests are measured in the kinetic regime.
Semihydrogenation of Phenylacetylene
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3833
Figure 6. Reaction rate vs palladium content (wt %) in different Pd/ NT catalysts.
Figure 7. Conversion (full symbols)/selectivity (hollow symbols) graphs of the first cycle of the three Pd/C samples: Pd/NTs, Pd/CB, and Pd/AC.
TABLE 3: TOF and Selectivity toward Styrene Values of the First Cycle of the Three Pd/C Catalysts and of the Five Cycles of Reutilization of the Pd/NT Catalyst catalyst
cycle
TOF (s-1)
selectivity (%)
Pd/NTs Pd/NTs Pd/NTs Pd/NTs Pd/NTs Pd/CB Pd/AC
1 2 3 4 5 1 1
0.96 0.96 0.92 0.97 0.97 0.86 0.81
97.4 96.7 96.9 94.5 95.7 96.9 95.3
The conversion and the selectivity graphs of the first cycle of the catalytic test of the three Pd/C catalysts are shown in Figure 7. From the conversion curve and, as was observed in other similar works36,37 and in our previous work on homogeneous catalysis,6 the reaction is zeroth order in phenylacetylene. Nevertheless, the conversion curve of the Pd/AC catalyst is different from those of the other two Pd/C catalysts. Thus, in the curve of Pd/AC, two different slopes are observed, because the adsorption of the reactants and products in the microporosity might have an influence on the kinetics of the process. TOF and selectivity results of the first cycle of the catalytic test at the previously mentioned experimental conditions of the three Pd/C catalysts (Pd/NTs, Pd/CB, Pd/AC) are shown in Table 3. TOF values were obtained by dividing the reaction rates by the number of palladium sites per gram of catalyst (calculated from the dispersion by TEM analysis). In the case of Pd/AC, the reported TOF value corresponds to the lowest reaction times. Pd/C heterogeneous catalysts and the Pd homogeneous catalyst prepared from the same colloidal suspension have similar catalytic properties in the semihydrogenation
of phenylacetylene. Thus, their TOF numbers are similar to that for the homogeneous catalyst (TOF ) 1.01 s-1 6). Additionally, total conversions were achieved in all the Pd/C catalysts, with very high selectivity toward styrene (around 96%). Although the lowest catalytic activity is obtained for the Pd/AC catalyst, which is the one with the highest Pd particle size, the TOF is similar for all the Pd/C catalysts, which is in agreement with the description of the hydrogenation of phenylacetylene to styrene as a structure-insensitive reaction,16,36 at least in the particle size range that we have studied (between 2.5 and 5.6 nm). On the other hand, the highest selectivity is obtained for the Pd/NT catalyst (above 97%). Moreover, the values of activity and selectivity presented in this work are similar and, in some cases, higher than those reported in the literature for Pd supported on different types of materials (SiO2, MCM-41, pumice, ...).16,36,38-40 From an applied point of view, the easiness of manipulation of the Pd/NT sample compared to Pd/CB is noteworthy, facilitating its recovery by filtration and, thus, its reutilization. In fact, Table 3 also shows the five consecutive cycles of reutilization results of the sample Pd/NTs. It was found that neither the catalytic activity nor the selectivity decreased appreciably in any of the five cycles. Furthermore, the conversion and the selectivities of the hydrogenation reaction curves could be reproduced in the five cycles. Moreover, no Pd leaching of the samples occurred during the catalytic reactions. This was confirmed by ICP analysis. By this analysis, the Pd content of the Pd/NT sample after the fifth cycle was confirmed to be 0.65 wt %. With the purpose of verifying the selective hydrogenation of the Pd/C catalysts for phenylacetylene over styrene, an experiment was performed with Pd/NTs at the same conditions as the other catalytic tests but with 90% styrene and 10% phenylacetylene as initial reactants. It was observed that, until the phenylacetylene was completely hydrogenated, the styrene did not start to react. Then, the styrene was not hydrogenated in the presence of the phenylacetylene, in agreement with previous results reporting that hydrogenation of alkynes over palladium is governed by the strong adsorption of the alkyne.41 This is due to the higher adsorption enthalpy of the alkyne and the specificity of the interaction of the Pd sites with the carboncarbon triple bonds, which causes the ratio of the surface coverage of the alkyne to be very high until the alkyne disappears, producing either the displacement of the alkene from the metal surface or the blocking of the active sites for its readsorption.39,41 4. Conclusions In this paper we describe the preparation of palladium heterogeneous catalysts supported on different carbon materials, using palladium colloids (prepared by the reduction-by-solvent method) and three types of carbon supports: carbon NTs, CB, and an AC, together with their catalytic testing in the partial hydrogenation of phenylacetylene. The prepared catalysts have proved to be very active and selective under very mild conditions (323 K, flow of H2 of 30 mL/min, ambient pressure), showing results similar to those of Pd nanoparticles in the homogeneous phase. These results are also very interesting in terms of selectivity, reaching values very close to 96%. Comparing the three carbon materials used, the microporous AC used has important disadvantages since Pd particles remain out of the microporosity, thus facilitating the agglomeration, and because the adsorption of the reactants and products affect the kinetics of the process.
3834 J. Phys. Chem. C, Vol. 112, No. 10, 2008 Regarding the CB and NTs, the easiness of manipulation of the NTs must be emphasized, which is an interesting advantage to recover and reuse the catalyst. Furthermore, it was found that neither the catalytic activity nor the selectivity decreased appreciably in up to five consecutive cycles of the sample Pd/ NTs. ICP analysis has shown that no Pd leaching occurred during the cycles of reutilization, showing the strong interaction between the Pd nanoparticles and the carbon support. Thus, the heterogeneous catalysts whose preparation is reported in this study could be used in other different reactions of industrial importance. Acknowledgment. We thank the Spanish Ministerio de Educacio´n y Ciencia (Project CTQ2006-08958/PPQ), EU (FEDER program), and Generalitat Valenciana (Grant ARVIV/ 2007/063) for financial support. References and Notes (1) Radovic, L. R.; Rodrı´guez-Reinoso, F. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1997; pp 243-358. (2) Stevenson, S. A.; Dumesic, J. A.; Baker, R. T. K.; Ruckenstein, E. Metal-support interactions in catalysis, sintering, and redispersion; Van Nostrand Reinhold: New York, 1987. (3) Haber, J.; Block, J. H.; Delmon, B. Pure Appl. Chem. 1995, 67, 1257. (4) Richard, D.; Couves, J. W.; Thomas, J. M. Faraday Discuss. Chem. Soc. 1991, 92, 109. (5) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455. (6) Domı´nguez-Domı´nguez, S.; Berenguer-Murcia, A.; Cazorla-Amoro´s, D.; Linares-Solano, A. J. Catal. 2006, 243, 74. (7) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517. (8) Raja, R.; Golovko, V. B.; Thomas, J. M.; Berenguer-Murcia, A.; Zhou, W. Z.; Xie, S. H.; Johnson, B. F. G. Chem. Commun. 2005, 2026. (9) Roma´n-Martı´nez, M. C.; Cazorla-Amoro´s, D.; de Miguel, S.; Scelza, O. J. Jpn. Pet. Inst. 2004, 47, 164. (10) Vergunst, T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 2001, 40, 2801. (11) Huang, X.; Wilhite, B.; McCready, M. J.; Varma, A. Chem. Eng. Sci. 2003, 58, 3465. (12) Dell’Anna, M. M.; Gagliardi, M.; Mastrorilli, P.; Suranna, G. P.; Nobile, C. F. J. Mol. Catal. A 2000, 158, 515. (13) Mandal, S.; Roy, D.; Chaudhari, R. V.; Sastry, M. Chem. Mater. 2004, 16, 3714. (14) Pellegatta, J. L.; Blandy, C.; Colliere, V.; Choukroun, R.; Chaudret, B.; Cheng, P.; Philippot, K. J. Mol. Catal. A 2002, 178, 55. (15) Shephard, D. S.; Maschmeyer, T.; Sankar, G.; Thomas, J. M.; Ozkaya, D.; Johnson, B. F. G.; Raja, R.; Oldroyd, R. D.; Bell, R. G. Chem.s Eur. J. 1998, 4, 1214.
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