J. Phys. Chem. C 2007, 111, 6447-6453
6447
Characterization and Reactivity of Alumina-Supported Pd Catalysts for the Room-Temperature Hydrodechlorination of Chlorobenzene N. Seshu Babu, N. Lingaiah, Rajesh Gopinath, P. Siva Sankar Reddy, and P. S. Sai Prasad* Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India ReceiVed: September 8, 2006; In Final Form: February 21, 2007
A series of alumina-supported Pd catalysts were prepared by varying the metal loading between 0.5 and 5 wt. %, adopting the deposition precipitation (DP) method. These catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), pulse CO chemisorption, Brunauer-Emmett-Teller specific surface area measurement, and transmission electron microscopy. The catalytic properties of the catalysts were studied for the hydrodechlorination of chlorobenzene. The reaction was carried out in a continuous mode passing liquid chlorobenzene and gaseous hydrogen over a fixed bed of catalyst taken in a reactor operating at room temperature and atmospheric pressure. The catalysts with metal loading up to 2 wt. % demonstrated high dispersion, homogeneous distribution of active species with predominance of metal-support interaction and without any formation of β-PdH. Beyond 2 wt. % loading, the agglomeration of PdO took place forming bulk particles that reduced easily to metallic state displaying the characteristic negative peak in the TPR pattern. XPS measurements revealed the presence of electron deficient Pd species in the catalysts up to 2 wt. % Pd. The activity and stability of the catalysts are explained in terms of formation of the electron deficient Pd species (Pdn+) and the particle size of Pd on the surface of alumina.
1. Introduction Chlorinated hydrocarbons (CHCs) have been used in many chemical applications but their accidental release into the atmosphere poses a great environmental hazard. Various methods, like incineration and thermal destruction, have been adopted for converting these harmful chemical compounds into safer ones. However, as a result of incomplete oxidation these processes lead to the formation of more toxic products like dioxins and phosgene. One of the safe methods for the removal of chlorine from CHCs is the catalytic hydrodechlorination (HDC). HDC has emerged as a promising nondestructive technology not only for the conversion of toxic substances into safer compounds, but also for producing useful products from chlorinated wastes.1-4 Catalytic HDC has been reported in both liquid and vapor phases over supported noble metal (Pd,5-8 Pt,9-11 Rh,9,10 Ru10,12) and non-noble metal (Ni,13-15 Ni-Mo,16 Fe17,18) catalysts. Among all the Group VIII noble metals, Pd is identified as the best catalyst for liquid-phase HDC as it selectively replaces the chlorine component of the substrate with hydrogen5-8 at low temperatures and atmospheric pressure. Chloroarenes, particularly chlorobenzene,9,19 chlorophenols,20,21 and polychlorinated aromatics,22 have been treated in liquidphase. As reported by Yuan and Keane,23 HDC reaction is often controlled by inter- and intraparticle mass transfer resistances when the particle size of the catalyst exceeds 45 µm. This is particularly important when the reaction is conducted at high temperatures. In this respect, low temperatures are always preferable to carry out the reaction in kinetic regime. A few attempts reported in the literature on liquid-phase HDC are * Corresponding author. E-mail:
[email protected]. Fax: +91-402716 0921.
noteworthy. Felis et al.24 studied HDC of monoaromatic chlorophenols with 1-5 chlorine atoms on Ru/C catalyst at 298-353 K and at pressures ranging 3-5 bar. Roy et al.25 studied HDC of chlorophenols on Pdo/Al2O3, nickel powder, and raney type Ni-Al catalysts at ambient temperature and pressure, using different sources of active hydrogen species such as sodium borohydride and ammonium formate. Establishing the feasibility of HDC of 2,4-dichlorophenol on commercially available supported Pd and Rh catalysts, Yuan and Keane26 nicely brought out the influence of mass transfer at liquid/solid interface and intraparticle diffusion in limiting the reaction rate at higher metal loadings. The aqueous phase reaction was carried out in a stirred tank reactor at a temperature between 273 and 303 K. Calvo et al.27 studied HDC of 4-chlorophenol with hydrogen in aqueous phase on surface-modified activated carbon-supported Pd catalysts at 323-348 K and 2.4 bar and brought out the preferable surface groups on carbon that offer high conversion of the chlorocompound and enhanced selectivity to cyclohexanol. Hashimoto et al.28 studied the reaction on supported Pd catalysts, which contained both Lewis and Bronsted acid sites. A close look at the literature available on liquid-phase HDC reveals that the catalyst functionality of Pd cannot be generalized as the catalysts varied in their composition, nature of support, and reaction conditions. Furthermore, the present work aims at understanding the structure-activity relationships. One of the objectives of the present study is to establish this relationship by studying HDC of chlorobenzene on alumina-supported Pd catalysts at room temperature under atmospheric pressure and in a continuous mode. Catalyst deactivation is an important feature of HDC in both liquid and vapor-phase operations.29-40 HCl, obtained as a byproduct, strongly interacts with the active metal33-35 leading to poisoning. The other issues that need to be considered
10.1021/jp065866r CCC: $37.00 © 2007 American Chemical Society Published on Web 04/12/2007
6448 J. Phys. Chem. C, Vol. 111, No. 17, 2007 carefully in HDC include the method of catalyst preparation,29,30 the nature of support,9,10,29 the extent of metal loading39 and dispersion,9,29,30 the solvent used,11,12,40 and the presence of a second metal.17,35,37-39,41 Initially, researchers focused their attention on low-dispersed, high Pd-containing (up to 10 wt. %) catalysts31,34,41 to achieve the required stability. Deactivation was considerably reduced in these catalysts as the chloride evolved during the reaction diffused into the large Pd particle, reaching the support and transforming it into its the chlorinated form. By adopting the deposition-precipitation method for catalyst preparation (Pd-DP), Gopinath et al.42-43 have claimed that even high-dispersed systems (containing Pd of about 1 wt. %) also offer better stability. Pd-DP catalysts have also been successfully employed for various reactions such as methanol decomposition, CO hydrogenation, and hydrodechlorination reactions.44-46 However, there exists considerable ambiguity regarding the nature of active species formed, particularly with change in Pd loading. Shen et al.46 reported the formation of metallic Pd in their 3 wt. % Pd catalyst on Al2O3 support, whereas the formation of cationic Pd species (Pdn+) was observed on CeO2 and ZrO2 supports for the same loading of Pd.45 Detailed studies aiming at explaining this discrepancy are scarce. The other objective of the present investigation is to study the influence of metal loading on the nature of the active species formed and to observe the structure-activity relationship. Al2O3 supported Pd catalysts, varying in their metal content from 0.5 to 5% (w/ w), have been prepared by adopting the deposition-precipitation method. The catalysts have been characterized by various physicochemical techniques to establish the nature of Pd species formed at different metal loadings. HDC of chlorobenzene has been carried out on these catalysts in a continuous mode passing liquid chlorobenzene and gaseous hydrogen over a fixed bed of catalyst taken in a reactor, operating at ambient temperature and under atmospheric pressure. The activity data are interpreted in terms of the surface species formed. 2. Experimental Section 2.1. Catalyst Preparation. A series of alumina-supported (surface area measurement (SA): 220 m2/g) Pd catalysts,with varying Pd content in the range of 0.5-5 wt. %, were prepared by the deposition-precipitation method using Na2CO3 as the precipitating agent.45 The required quantity of alumina was first soaked in an acidified aqueous solution of PdCl2. Pd(OH)2 was exclusively precipitated on the support by the slow addition of 1 M Na2CO3 under effective agitation until the pH of the solution reached a value of 10.5. The catalyst mass was allowed to remain in the basic medium for 1 h, followed by filtration and washing with deionized water for several times until no chloride ion was detected, as confirmed by the AgNO3 test. The solid thus obtained was oven dried at 393 K for 6 h and then was calcined at 773 K for 5 h. The prepared catalysts were designated as Pd-0.5, Pd-1, Pd-2, Pd-3, and Pd-5 with the numeral denoting the nominal wt. % of the metal. The chemical composition of catalysts was determined by inductively coupled plasma/mass spectrometry (ICP-MS), using a Thermo star instrument, after being dissolved in HNO3 (1%) (1:250), using Au as internal standard. The samples were dissolved in aquaregia mixture and were dried over the hot plate at 373 K for 2 h to remove the residual impurities, and then standard solutions were prepared at 50 ppm level. 2.2. Characterization of Catalysts. 2.2.1. X-ray Diffraction. XRD patterns of the catalysts were recorded on a Rigaku
Babu et al. Diffractometer by using Ni-filtered CuKR radiation (λ ) 1.5405 Å). The measurements were recorded in steps of 0.045° with a count time of 0.5 s and a 2θ range of 10-80°. Identification of the crystalline phases was made with the help of JCPDS files. 2.2.2. Temperature-Programmed Reduction (TPR). TPR of the catalysts was carried out in a flow of 10% H2/Ar mixture gas at a flow rate of 30 mL/min with a temperature ramp of 10 K/min. Before the TPR run, the catalysts were pretreated in Ar gas at 573 K for 2 h. The hydrogen consumption was monitored using the thermal conductivity detector of a gas chromatograph (Varian, 8301). 2.2.3. Pulse CO Chemisorption. Room-temperature CO chemisorption was carried out on a pulse adsorption apparatus. In a typical experiment, the catalyst was first oxidized in a 10% O2/He mixture at 573 K for 30 min and subsequently reduced in a 10% H2/He gas at the same temperature, flushing with pure He in between. The CO adsorption capacity was then obtained by the number of pulses required to saturate the total surface of the catalyst at room temperature. 2.2.4. Surface Area and Pore Volume. The specific surface areas and pore volumes of the catalyst samples were determined by N2 adsorption at 77 K. Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) equations were used for the determination of surface area and pore volumes, respectively. Prior to the adsorption measurements, the samples were dried at 423 K for 2 h. 2.2.5. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were conducted on a KRATOS AXIS 165 with a DUAL anode (Mg and Al) apparatus using the MgKR anode. The nonmonochromatized Al-KR X-ray source (hν ) 1486.6 eV) was operated at 12.5 kV and 16 mA. Before acquisition of the data, the sample was outgassed for about 3 h at 373 K under a vacuum of 1.0 × 10-7 Torr to minimize surface contamination. The XPS instrument was calibrated using Au as the standard. For energy calibration, the carbon 1s photoelectron line was used. The carbon 1s binding energy was taken as 285 eV. Charge neutralization of 2 eV was used to balance the charge up of the sample. The spectra were deconvoluted using Sun Solaris-based Vision-2 curve resolver. The location and the full width at half-maximum value for the species were first determined using the spectrum of pure sample. Symmetric Gaussian shapes were used in all cases. Binding energies for identical samples were generally reproducible within (0.1 eV. 2.2.6. Transmission Electron Microscopy (TEM). The morphological features of the catalysts were monitored using a JEOL JEM 2000EXII transmission electron microscope, operating between 160 and 180 kV. The specimens were prepared by dispersing the samples in methanol using an ultrasonic bath and evaporating a drop of resultant suspension onto the lacey carbon support grid. For estimation of particle size, at least more than 100 individual particles were counted for each catalyst. The mean particle sizes are quoted as both a number average diameter
dhn )
∑i nidi / ∑i ni
and a surface area-weighted diameter (dhs)
dhs )
∑i nidi3 / ∑i nidi2
where ni is the number of particles of diameter di, and ∑ini > 100. 2.3. Activity Measurements. The gas-phase HDC reaction was performed under atmospheric pressure at room temperature. In a typical experimen,t about 1.0 g of catalyst was suspended between two quartz wool plugs and was reduced in a flow of
Alumina-Supported Pd Catalysts
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6449
Figure 1. XRD patterns of unreduced Pd/Al2O3 catalysts.
hydrogen (30 mL/min) at 573 K for 3 h prior to the reaction. After bringing the temperature of the catalyst bed to the required level (303 K), chlorobenzene (S.D. Fine Chemicals, AR grade) was fed into the preheater portion of the reactor at a flow rate of 2 mL/h by means of a microprocessor-based feed pump (Braun Corporation, Germany). The H2 flow was kept at a molar ratio of 10:1 of H2 to chlorobenzene. The products were analyzed by gas chromatography using a flame ionization detector with a 10% carbowax 20 M column. The particle size of the catalysts was fixed based on the influence of the initial rates below a value corresponding to the British Standards Sieving (BSS) mesh of 85/100. This ensured the reaction to be under chemical control regime.26 The percentage conversion of chlorobenzene was defined as follows:
% Conversion of chlorobenzene ) 100 (moles of chlorobenzene fed - moles of chlorobenzene remaining)/mol of chlorobenzene fed 3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. XRD Studies. Powder XRD patterns of the calcined Pd/Al2O3 catalysts are shown in Figure 1. The diffraction patterns showed the amorphous nature of Pd in samples up to 2 wt. %. However, there could be crystallites of PdO that were below the detection limit of the technique. At higher loadings of Pd, the PdO phase appeared (2θ ) 34.7, 54.8°) due to an increase in crystallite size. All the patterns exhibited lines related to the alumina support. Figure 2 shows the XRD patterns of the catalysts subjected to reduction. These patterns also revealed the amorphous nature of Pd due to homogeneous distribution on the support at lower loadings. In the Pd-3 and Pd-5 catalysts, the major diffraction signals of R-Pd phase were observed at 2θ values of 40.1 and 68.1°, respectively, which agreed with those reported in PDF No. 40-1043. The intensity of the peaks also increased due to the increase in crystallite size of Pd. The average particle size estimated by XRD measurements for Pd-5 catalyst is found to be 5.1 nm (Table 1), which agrees with the generalization that XRD can only detect the particle size above 4 nm. 3.1.2. BET Surface Area. The surface area of the catalysts decreased with metal loading from 0.5 to 5.0. wt. %. (Table 2). The decrease was less pronounced up to Pd-2 catalyst. But the variation was relatively more significant at higher Pd loadings due to the formation of larger particles that could have blocked the pores. A continuous decrease in pore volume of alumina
Figure 2. XRD patterns of reduced Pd/Al2O3 catalysts.
TABLE 1: Details of H2 Consumption Values of Pd/Al2O3 Catalysts H2 consumption H2 consumption Pd particle (µmol/gm) ratio size (nm) catalyst (actual Pd wt % Tmax Tmax Tmax CO by ICP) 373 K 673 K (373 K/673 K) chemisorption XRD TEM Pd-0.5 (0.46) Pd-1 (0.92) Pd-2 (1.86) Pd-3 (2.72) Pd-5 (4.20)
33.1 74.8 166.5
13.8 10.2 8.5
2.39 7.33 19.5
1.3 1.7 2.9 3.5 4.2
5.1
1.5 2.2 4.0 4.9
(from 0.84 cm3/g of Pd-0.5 to 0.64 cm3/g of Pd-5) was also observed with an increase in metal loading. 3.1.3. TPR. The TPR profiles of the catalysts are shown in Figure 3. The patterns revealed important information about the nature of reduction of Pd species. At lower Pd loadings, the catalysts (Pd-0.5 to Pd-2) exhibited a positive reduction peak with its maximum at ∼373 K and a broad reduction peak between 623 and 673 K. The H2 consumption peak at 373 K can be ascribed to the reduction of support-interacted PdO species. It clearly demonstrated that smaller Pd particles were generated as long as the surface interaction existed. Gaspar and Dieguaz47 have reported active component-support interaction in the DP method of catalyst preparation. The interaction was initiated during the calcination process, leading to the formation of (Pd-O-Al) species, especially when PdCl2 was used as the precursor. In the case of catalysts prepared from PdCl2, the formation of a species of the type PdxOyClz48 is also reported with its reduction temperature lying in the region 350-750 K. However, the possibility of formation of this complex is remote as the TPR patterns of Pd-3 and Pd-5 did not reflect its presence. The high-temperature reduction peak could be a result of reduction of a two-dimensional (2D) PdO surface phase, as reported by Barrera et al.49 in the case of catalysts with low concentrations of Pd. Lieske and Volter50 studied a 0.6 wt.% Pd/Al2O3 catalyst prepared from Pd(NO3)2 and treated in oxygen in the temperature range of 773-1173 K. A 2D surface phase named as [PdO]SC was proposed to be formed during the oxidation at high temperatures. This species was stated to be responsible for spreading of Pd and more stable for reduction than the low-temperature PdO. Ruckenstein and Chen51 also reported the spread of Pd particles when heated in oxygen and the extent of spreading was found to be dependent on the temperature and crystallite size. At 623 K, smaller crystallites exhibited spreading whereas the larger ones did not. It is
6450 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Babu et al.
TABLE 2: Physico-Chemical Properties of Pd/Al2O3 Catalysts catalyst
Pd loading (wt. %) (determined by ICP analysis)
surface area (m2/g)
pore volume (mL/g)
CO uptake (mL/g)
dispersion (%)
specific metal area (m2/g)
γ-Al2O3 Pd-0.5 Pd-1.0 Pd-2.0 Pd-3.0 Pd-5.0
0.46 0.92 1.86 2.72 4.20
220 209 205 202 190 185
0.87 0.84 0.79 0.73 0.71 0.64
0.86 1.27 1.47 1.83 2.10
88 66 38 32 26
1.82 2.68 3.11 3.87 4.44
a
particle size (nm) d2b d1a 1.3 1.7 2.9 3.5 4.2
1.5 2.2 4.0 4.9
d1: Particle size measured by CO chemsiorption. b d2: Particle size calculated by TEM measurements.
Figure 3. TPR patterns of alumina-supported Pd catalysts.
noteworthy to observe the absence of this species in Pd-3 and Pd-5 catalysts, possibly due to an increase in particle size. The H2 consumption values of Al2O supported Pd catalysts estimated by TPR measurements were shown in the Table 1. The spreading of PdO species depends on the calcination temperature and particle size of PdO species as also reported in the literature.50 However in the present case, TPR profiles displayed high-temperature H2 consumption peak at 673 K, which is ascribed to the 2D PdO species, and whose presence depends on the particle size of Pd. At lower Pd particle size, the spreading of PdO species is more, whereas the spreading is not noticed at higher Pd particle size. To explain the extent of Pd spreading with Pd particle size, integration of each H2 consumption peak at Tmax 373 and 673 K is done and estimated the exact amount of H2 consumed. The estimated H2 consumption value for Pd-0.5, Pd-1, and Pd-2 catalysts is consistent with theoretical H2 consumption values. The results inferred that the area under the high-temperature reduction peak, which pertinent to 2D PdO phase, is decreased with an increase of Pd particle size. The ratio of two H2 consumption peaks at Tmax 373 and Tmax 673 K is calculated and this ratio increased with Pd loading, as indicated in Table 1. It shows that density of surface-interacted Pd species increased with Pd loading up to 2 wt % Pd, with simultaneous decrease in surface density of 2D PdO species. In the case of Pd-3 and Pd-5 catalysts, the other interesting feature of the TPR patterns was the observation of a negative peak in the region of 348-373 K. The negative peak is a characteristic of decomposition of β-PdH phase that forms during H2 flushing before the start of TPR run. It has been widely accepted that the formation of β-PdH is associated with the presence of large Pd particles, which are easily reducible to metallic Pd.41 Sandoval et al.52 also observed such a lowtemperature desorption peak at 333 K and assigned it to hydrogen derived from β-PdH decomposition. Furthermore, CO chemisorption (Table 1) revealed formation of particles of size
Figure 4. XPS spectra of reduced Pd/Al2O3 catalysts.
of the order of 4 nm. The evidence for the formation of larger particles can be inferred from the XRD patterns of these catalysts (Figure 1) where enhanced crystallinity of PdO could be seen. 3.1.4. Pulse CO Chemisorption. CO chemisorption gives the information about metal dispersion and particle size of the catalysts. The gas uptake, along with the corresponding values of particle size and dispersion, are given in Table 2. Catalysts up to Pd-2 showed high dispersion (lower particle size) suggesting homogeneous distribution of active component on the support. The Pd dispersion decreased as the loading increased with a consequent increase in particle size.27 These larger particles inhibit interaction with the support. The TPR results also support these findings. The decrease in metal dispersion is also substantiated by the increase in intensity of PdO phase in the XRD patterns of Pd-3 and Pd-5 catalysts. 3.1.5. XPS. The binding energies of electrons determined by XPS provide useful information on the oxidation states of different elements. The electronic properties of Pd metal in Pd/ Al2O3 catalysts can be explained by this technique. The XPS spectra of the catalysts are presented in Figure 4. The values of
Alumina-Supported Pd Catalysts binding energy (BE) of Pd 3d5/2 for Pd-0.5, Pd-1.0, and Pd-2.0 catalysts, after subjecting them to hydrogen reduction at 573 K, were found to be in the range of 336.0-336.8 eV. These values are higher compared with the binding energy of metallic Pd 3d5/2, reported as 335.0 eV.47,53 The higher BE values of these catalysts are characteristic of the presence of electron deficient Pdn+ species whose valency is close to +1.43 Shen and Matsumura also observed similar values of binding energies for supported palladium catalysts prepared by the DP method.44,45 It is known that the DP method facilitates metal-support interaction between Pd and Al2O3 thus leading to easy electron transfer from Pd to Al2O3. This allows Pd to acquire a positive charge forming cationic Pd species. In the case of Pd-3 and Pd-5 catalysts, as the Pd content increased the binding energy of Pd 3d5/2 was found to be 335.2 eV, corresponding to metallic Pd species.53,54 At higher Pd content, a facile formation of PdO with substantially reduced metal-support interaction takes place that can be reduced easily to metallic Pd. By adopting the DP method, it was also possible to eliminate residual chloride content on the surface. The XPS peak intensity also increased gradually as Pd loading increased. This significant observation made from XPS measurements supports the formation of metallic Pd species in Pd-3 and Pd-5 catalysts. The presence of a negative peak due to formation of β-PdH in the TPR patterns and the low dispersion values obtained by CO chemisorption also support the XPS results. These observations suggest that after a definite metal content on Al2O3, Pd changes its morphology from interacted cationic species to noninteracted bulk species. In the present case, this critical Pd content is about 2 wt.%. This is an important observation of the present work. A positive peak in the TPR patterns of low Pd-containing catalysts also reveals that the cationic Pd is stabilized on an alumina surface. Stabilization of Pd species on Al2O3 was also reported by Juszczyk et al.55 According to them, if we assume that on the surface of Al2O3 the top O2- ion of the octahedron is missing, the remaining part of O2- skeleton forms a square pyramid and that alumina stabilizes Pdn+ ions in these vacant octahedral holes. 3.1.6. TEM. The morphological observations of the Pd catalysts can be explained by TEM technique. The micrographs, shown in Figure 5, reveal that the Pd particles are flat revealing strong metal-support interaction. By combined XPS and TEM studies, Sandoval et al.52 also observed similar nature of the particles. At higher loadings, the particles were more spherical. More than 100 particles were examined in the TEM micrographs to calculate the volume/area average particle diameter. The average particle sizes of Pd of these catalysts were comparable with the particle size calculated from CO chemisorption and XRD data (Table 1). 3.2. Activity Measurements. The catalytic properties of Pd/ Al2O3 were studied for the continuous hydrodechlorination of chlorobenzene at a reaction temperature of 300 K. The main product was benzene and a small amount of cyclohexane was also formed. The formation of chlorocyclohexane was not observed. The activity patterns are presented in Figure 6. Initially, all the catalysts showed appreciable conversions. However, during the time on stream analysis the activity varied, reaching their respective steady-states after 6 h of operation. Catalysts with low Pd content (Pd-0.5 and Pd-1), however, did not reach perfect steady-state. The activity taken after 6 h of reaction (expressed in terms of conversion), taken for the sake of comparison, increased with an increase in Pd loading up to 2 wt % and then started to decrease. A comparison of the TPR
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6451
Figure 5. TEM micrographs of reduced Pd/Al2O3 catalysts (a) Pd-1, (b) Pd-2, (c) Pd-3, and (d) Pd-5.
6452 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Figure 6. Activity patterns of Pd/Al2O3 catalysts.
Babu et al. is superior to the latter. Aramendia et al.56,57 reported a steadystate activity of about 60% for their best catalyst. However, the catalyst was tested at about 4 bar pressure and 313 K in a liquid-phase batch reactor with and without the addition of NaOH to the catalyst. Hashimoto et al.28 evaluated their Pd/ Al2O3 catalyst in a fixed bed flow reactor at 298 K and reported a chlorobenzene conversion much less than the present catalyst. The initial decline in activity can be explained as due to rapid deactivation by HCl adsorption. HCl formed in the reaction poisons the active sites of Pd leading to inhibition for further hydrogenolysis of chlorobenzene. The slow deactivation, or better stability, of Pd-3 and Pd-5 catalysts can be explained on the basis of particle size. Literature studies, reported earlier, also revealed that larger Pd particles offer better resistance to deactivation from the chloride species.40,41,56 The CO chemisorption study of the present investigation disclosed increased particle size in Pd-3 and Pd-5 catalysts (3.5 and 4.2 nm, respectively). They show higher stability possibly due to the diffusion of chloride ions into the bulk. It can be construed that two factors govern the activity and stability. The cationic Pd offers higher activity and the larger particles size helps attain better stability. It appears that a balance between the two has reached at a Pd content of 2 wt. % in the case of Pd-2 catalyst. 4. Conclusions
Figure 7. Variation of TOF with loading on Pd/Al2O3 catalysts.
pattern obtained on the catalysts revealed that the area percent of the second high-temperature peak continuously decreases with an increase in Pd content up to 2 wt %. The area estimated for hydrogen consumptions corresponding to two peaks are given in Table 1. Whereas that for the low-temperature peak increased with loading, it is note worthy to understand that the conversion is proportional to the area of the first hydrogen consumption peak. Therefore, the reduction temperature was fixed at 573 K, and the CO chemisorption data obtained under these conditions was used for the calculation of turnover frequency (TOF) values. TOF is calculated as µmol-1 gm-1 sec-1 and whose formula as given below.
TOF )
Rate of reaction No. of active Pd sites*
*No. of Pd active mole is determined by CO chemisorption method. This assumption may not cause considerable deviation from TOF, as the percentage ratios of the second peak, namely 12 and 5% corresponding to 1 and 2 wt % Pd catalysts, were very small. The relationship between turnover frequency, TOF of the catalysts, and their exact Pd loading is shown in Figure 7. This plot also suggests that the turnover frequency increases linearly up to 2 wt.% of Pd and decreases appreciably beyond this loading. A comparison of the activity between Pd-2 and those reported in literature reveals that the performance of the present catalyst
Al2O3 supported Pd catalysts prepared by deposition precipitation method exhibit metal-support interaction at low loading; Pd exhibits in its cationic form. Bulk Pd particles are formed at high loading (Pd-3 and Pd-5) due to reduced interaction with the support leading to the formation of Pd species that can be reduced easily to metallic state. Catalysts with cationic Pd (Pdn+) exhibit higher activity compared to those having metallic Pd (Pd0) on their surface. However, resistance to deactivation by HCl poisoning increases with an increase in Pd loading. A balance between the activity and stability can be observed at 2 wt. % of Pd loading. The catalyst with optimum Pd loading shows considerably high activity for the room-temperature hydrodechlorination of chlorobenzene. References and Notes (1) Hagh, B. F.; Allen, D. T. Catalytic Hydrodechlorination. In InnoVatiVe Hazardous Waste Treatment Technology; Freeman, H. M., Ed.; Technomic: Lancaster, PA, 1990; Vol. 1. (2) Bijan, F. H.; Allen, D. T. Chem. Eng. Sci. 1990, 45, 2695. (3) Zanaveskin, L. N.; Averyanov, V. A.; Treger, Y. A. Russ. Chem. ReV. 1996, 65, 617. (4) Moreau, C.; Jaffre, J.; Savenz, C.; Genste, P. J. Catal. 1990, 122, 448. (5) Coq, B.; Ferrat, G.; Figueras, F. J. Catal. 1986, 101, 434. (6) Fung, S. C.; Sinfelt, J. H. J. Catal. 1987, 103, 220. (7) Coq, B.; Cognion, J. M.; Figueras, F.; Tournigant, D. J. Catal. 1993, 141, 21. (8) Urbano, F. J.; Marinas, J. M. J. Mol. Catal., A. 2001, 173, 329. (9) Prati, L.; Rossi, M. Appl. Catal., B 1999, 23, 135. (10) Benitez, J. L.; Angel, G. React. Kinet. Catal. Lett. 2000, 70, 67. (11) Ukisu, Y.; Miyadera, T. J. Mol. Catal., A 1997, 125, 135. (12) Kulkarni, P. P.; Deshmukh, S. S.; Kovalchuk, V. I.; d’Itri, J. L. Catal. Lett. 1999, 61, 161. (13) Menini, C.; Park, E.-J. S.; Tavoularis, G.; Keane, M. A. Catal. Today. 2000, 62, 355. (14) Murthy, K. V.; Patterson, P. M.; Jacobs, G; Davis, B. H.; Keane, M. A. J. Catal. 2004, 223, 74. (15) Zanaveskin, L. N.; Aver’yanov, V. A. Russ. Chem. ReV. 1998, 67, 713. (16) Hagh, B. F.; Allen, D. T. Chem. Eng. Sci. 1990, 45, 2695. (17) Lingaiah, N.; Sai Prasad, P. S.; Rao, P. K.; Berry, F. J.; Smart, L. E. Catal. Commun. 2002, 3, 391. (18) Mishakov, I. V.; Chesnokov, V. V.; Buyanov, R. A.; Pakhomov, N. A. Kinet. Catal. 2001, 42, 543. (19) Lingaiah, N; Uddin, M.; Muto, A; Sakata, Y. J. Chem. Soc., Chem. Commun. 1999, 1657.
Alumina-Supported Pd Catalysts (20) Matatov-Meytal, Y.; Sheintuch, M. Ind. Eng. Chem. Res. 2000, 39, 18. (21) Shindler, Y.; Matatov-Meytal, Y.; Sheintuch, M. Ind. Eng. Chem. Res. 2001, 40, 3301. (22) Ukisu, Y.; Miyadera, T. Appl. Catal., B 2003, 40, 141. (23) Yuan, G.; Keane, M. A. Chem. Eng. Sci. 2003, 58, 257. (24) Felis, V.; De Bellefon, C.; Fouilloux, Pierre.; Schweich, D. Appl. Catal., B 1999, 20, 91. (25) Roy, H. M; Wai, C. M; Yuan, T.; Kim, J. K.; Marshall, W. D. Appl. Catal., A 2004, 271, 137. (26) Yuan, G.; Keane, M. A. Catal. Commn. 2003, 4, 195. (27) Calvo, L;, Gilarranz, M. A.; Casas, J. A.; Mohedano, A. F.; Rodriguez, J. J. Appl. Catal., B. 2006, 67, 68. (28) Hashimoto, Y.; Uemichi, Y.; Akimi, Ayame. Appl. Catal., A 2005, 223, 89. (29) Schuth, C.; Reinhard, M. Appl. Catal., B 1998, 18, 215. (30) Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jimenez, C.; Lafont, F.; Marinas, A.; Marinas, J. M.; Urbano, F. J. J. Catal. 1999, 187, 392. (31) Gopinath, R.; Rao, K. N.; Sai Prasad, P. S.; Madhavendra, S. S.; Narayanan, S.; Vivekanandan, G. J. Mol. Catal., A 2002, 181, 215. (32) Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jimenez, C.; Lafont, F.; Marinas, A; Marinas, J. M.; Urbano, F. J. J. Mol. Catal., A 2002, 184, 237. (33) Aramendia, M. A.; Burch, R.; Garcia, I. M.; Marinas, A.; Marinas, J. M.; Southward, B. W. L.; Urbano, F. J. Appl. Catal., B 2001, 31, 163. (34) Bodnariuk, P.; Coq, B.; Ferrat, G; Figueras, F. J. Catal. 1989, 116, 459. (35) Gampine, A.; Eyman, D. P. J. Catal. 1998, 179, 315. (36) Jujjuri, S.; Ding, E.; Shore, S. G.; Keane, M. A. Appl. Organomet. Chem. 2003, 17, 493. (37) Berry, F. J.; Smart, L. E.; Sai Prasad, P. S.; Lingaiah, N.; Rao, P. K. Appl. Catal., A 2000, 204, 191. (38) Lingaiah, N.; Sai Prasad, P. S.; Rao, P. K.; Smart, L. E.; Berry, F. J. Appl. Catal., A 2001, 213, 189. (39) Halligudi, S. B.; Devassay, B. M.; Ghosh, A.; Ravikumar, V. J. Mol. Catal., A 2002, 184, 175.
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6453 (40) Lassova, L.; Lee, H. K.; Hor, T. S. A. J. Mol. Catal. A: Chem. 1999, 144, 397. (41) Berry, F. J.; Smart, L. E.; Sai Prasad, P. S.; Lingagaih, N.; Rao, P. K. Appl. Catal., A 1993, 101, 41. (42) Gopinath, R.; Lingaiah, N.; Sreedhar, B.; Suryanarayana, I.; Sai Prasad, P. S.; Obuchi, A. Appl. Catal., B 2003, 46, 587. (43) Gopinath, R.; Lingaiah, N.; Seshu Babu, N.; Suryanarayana, I.; Sai Prasad, P. S.; Obuchi, A. J. Mol. Catal., A. 2004, 223, 289. (44) Matsumura, Y.; Okumura, M.; Usami, Y.; Kagawa, K.; Yamashita, H.; Anpo, M.; Haruta, M. Catal. Lett. 1997, 44, 189. (45) Shen, W.-J.; Ichihashi, Y.; Okumura, M.; Matsumura, Y. Catal. Lett. 2000, 64, 23. (46) Shen, W.-J.; Okumura, M.; Matsumura, Y.; Haruta, M. Appl. Catal., A 2001, 213,225. (47) Gaspar, A. B.; Dieguez, L. C. Appl. Catal., A 2000, 201, 241. (48) Contescu, C.; Macovei, D.; Craiu, C.; Teodorescu, C.; Schwarz, J. A. Langmuir 1995, 11, 2031. (49) Barrera, A.; Viniegra, M.; Fuentes, S.; Gabriela, D. Appl. Catal., B 2005, 56, 279. (50) Lieske, H.; Voelter, J. J. Phys.Chem. 1985, 89, 1841. (51) Chen, J. J.; Ruckenstein, E. J. Phys.Chem. 1981, 85, 1606. (52) Sandoval, V. H.; Gigola, C. E. Appl. Catal., A 1996, 148, 81. (53) Briggs, D.; Seah, M. P. Practical Surface Analysis. In Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Seah, M. P., Ed.; Wiley: New York, 1990; Vol. 1. (54) Bozon-Verduraz, F.; Omar, A.; Escard, J.; Pontvianne, B. J. Catal. 1978, 53, 126. (55) Juszczyk, W.; Karpinski, Z.; Ratajczykowa, I.; Stanasiuk, Z.; Zielinski, J.; Sheu, L. L.; Sachtler, W. M. H. J. Catal. 1989, 120, 68. (56) Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jimenez, C.; Lafont, F; Marinas, A; Marinas, J. M.; Urbano, F. J. J. Catal 1999, 187, 392. (57) Aramendia, M. A.; Burch, R.; Garcia, I. M.; Marinas, A.; Marinas, J. M.; Southward, B. W. L.; Urbano, F. J. Appl. Catal., B 2001, 31, 163.
