Hydrodechlorination of Carbon Tetrachloride to Chloroform in the

Active carbon, alumina, silica, and sepiolite were used as supports and Ru, Rh, Pd, Pt, and Ni as the metallic phase. The catalysts were characterized...
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Ind. Eng. Chem. Res. 2000, 39, 2849-2854

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Hydrodechlorination of Carbon Tetrachloride to Chloroform in the Liquid Phase with Metal-Supported Catalysts. Effect of the Catalyst Components Luisa Ma. Go´ mez-Sainero, Antonio Corte´ s, Xose´ L. Seoane,* and Adolfo Arcoya Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain

The effect of the nature of catalyst components has been analyzed in the hydrodechlorination of carbon tetrachloride to chloroform. Active carbon, alumina, silica, and sepiolite were used as supports and Ru, Rh, Pd, Pt, and Ni as the metallic phase. The catalysts were characterized by CO chemisorption, TPR, and XPS. Reaction was carried out at 373 and 413 K in a slurry semibatch reactor. Dispersion of the catalysts containing different metals are very similar, while the values corresponding to Pd supported on different carriers follow the trend Pd/Al2O3 > Pd/ carbon > Pd/SiO2 ≈ Pd/sepiolite. The XPS analysis showed that Pd, Ru, and Rh are homogeneously distributed on the carbon, Pt is preferentially deposited on the external surface, and Ni is inside the pores. On the other hand, Pd on silica or alumina is mainly located on the external surface of the support. Pd/C is highly active and selective to chloroform, but Pd supported on the other carriers is less active and leads the reaction toward hexachloroethane. Hydrodechlorination activity of carbon-supported catalysts are in the order Pd/C . Pt/C > Rh/C > Ru/C > Ni/C, all of them being very selective toward chloroform. The results of the reaction are interpreted in terms of the spillover of hydrogen on carbon, the distribution of the metal on the support, and the density of states of the metals. 1. Introduction Up to recent years, carbon tetrachloride was mainly used as an intermediate in the production of fluorochlorocarbon compounds. After the Montreal Protocol of 1987, which engages the signatory countries to eradicate, in the year 2000, the manufacture and use of the fluorochlorocarbonates,1 the market of tetrachloromethane has been progressively declining. Additionally, this compound was classified as a group IV material at the London Conference in 1990, to be banned due to its potential impact on ozone layer depletion. Since carbon tetrachloride is a byproduct in several chlorine industrial processes, it must be eliminated. Incineration is the ongoing process for this purpose, but it is a very expensive method and does not completely avoid the pollution concern. In contrast, re-evaluation of CCl4 by selective hydrodechlorination to chloroform, a growing intermediate in the production of fluorinated polymers, reveals one of the more desirable methods to reduce or eliminate its disposal problem. Some patents2-4 and papers5-9 concerning the hydrodechlorination of carbon tetrachloride to chloroform were published. Most of these processes are performed in the gas phase over supported-platinum catalysts, giving high yields, but they are not entirely satisfactory because they use excessive amounts of hydrogen, the reaction products are difficult to recover, and hot spots formation causes catalyst damage. In previous patents10,11 we have described a process of hydrodechlorination of carbon tetrachloride to produce chloroform with selectivities close to 80%, at CCl4 conversion levels of 95%, using a 0.5 wt % palladium/ carbon catalyst in a liquid-phase reactor. In this system, * To whom correspondence should be addressed. Tel.: 3491-5854804. E-mail: [email protected]. Fax: 34-91-5854760.

the troubles encountered in the gas-phase reaction are minimized or do not exist at all. To our knowledge, studies devoted to the catalytic hydrodechlorination of carbon tetrachloride in the liquid phase are not available. In view of the potential interest of this process, a research program was undertaken and in this paper the influence of both the type of support and the nature of group VIII metals on the catalytic performance is discussed. Catalysts have been characterized by TPR, CO chemisorption, and XPS and their activity has been analyzed in terms of the different physical properties of the support and the electronic structure of the metal. 2. Experimental Section 2.1. Catalyst Preparation. Four catalysts containing 1 wt % Pd were prepared by incipient wetness impregnation of four different supports (particle size ) 50-100 µm) with PdCl2 anhydrous aqueous solutions: active carbon (C, Erkimia CF-IV, 1.200 m2 g-1), SiO2 (Girdler G-57, 300 m2 g-1), γ-Al2O3 (Girdler T-126, 220 m2 g-1), and sepiolite (SEP, Tolsa; 3SiO2‚2MgO, 140 m2 g-1). After allowing to dry overnight at 393 K, the impregnated supports were reduced under hydrogen flow (500 L h-1 kgcat-1) at 523 K for 3 h. To examine the effect of calcination, the samples 0.97% Pd/SiO2, 0.97% Pd/Al2O3, and 0.95% Pd/SEP were heated in air at 573 K for 3 h. To evaluate the effect of the nature of the metal, another four catalysts containing 1 wt % of Pt, Ru, and Rh and 5 wt % of Ni on active carbon were also prepared following the same impregnation procedure. The precursors used were H2PtCl6‚6H2O, RuCl3‚3H2O, RhCl3‚ 3H2O, and NiCl2‚6H2O, respectively. Catalysts were reduced at 523 K except for Ni, which was reduced at 623 K.

10.1021/ie990892f CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000

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2.2. Catalyst Characterization. Metal content (Met, atom g-1) was determined by induced coupled plasma (ICP). The precursors’ reducibility was analyzed by temperature-programmed reduction (TPR) in a flow system with a thermal conductivity detector. Samples of 0.5 g were outgassed at 373 K in a flow of Ar. After cooling at 273 K, the samples were stabilized under a flow of 5% of H2 in Ar (30 cm3 min-1). The TPR profiles were registered when the samples were at 10 K min-1 from 273 to 773 K. The number of exposed metal atoms (Mes, atom g-1) on the catalysts was determined by CO chemisorption using a dynamic pulse technique.12 Samples of 0,5 g were evacuated under an Ar flow of 30 cm3 min-1 at 573 K for 2 h. After the samples cooled at 298 K, pulses of CO (50 µL) were introduced into the argon stream until saturation of the sample was obtained. Prior to use, each gas was carefully purified. From the volume of CO chemisorbed, Mes was calculated assuming the stoichiometry Pds/CO ) 1.15/1 and Mes/CO ) 1/1 for the other metals.13 The dispersion (D, %) is defined as D ) 100Mes/Met. The surface of the catalysts was analyzed by X-ray photoelectron spectroscopy (XPS) with a VG Escalab 200R system, using Mg KR radiation (1253.6 eV). After reduction, the samples were introduced into the sample holder under isooctane, which was removed during pumping in the preparation chamber of the XPS equipment. From the integrated intensities and by comparison of the area of the peaks after background subtraction and specific corrections14 and from use of the atomic sensitivity factors published by Wagner et al.15 for a given emission, the surface atomic ratios of the different species were obtained. 2.3. Catalytic Activity. The catalytic activity measurements in the hydrodechlorination of carbon tetrachloride (TTCM) were performed in a conventional unit with a slurry semibatch reactor. Liquid TTCM and catalyst were charged in the required proportion and the hydrogen was continuously fed upstream. At the same time, the reaction temperature and pressure were adjusted. The gaseous effluent of the reactor containing H2, hydrogen chloride, methane, and chlorinated hydrocarbons was successively passed through a water absorption column to remove the HCl and a cold trap at 193 K to retain the volatile chlorinated products. The volume of the exit gas was measured by a flowmeter. After 15 min of reaction, a sample of the reactor liquid was taken, weighed, and analyzed by GC in a SE-30 (30%)/Chromosorb column packet. The organic condensate at 193 K was also weighed and its composition determined with the same chromatographic column. This condensate, however, contains important amounts of volatile chlorinated products, so that liquid injection in the gas chromatograph is rather difficult and inaccurate. The analysis was then made by taking several gas samples before the cold trap during the run. Samples of the exit gas were also taken to analyze methane. In this case, an active carbon packet column was used. The HCl collected in the absorption column was titrated with Na(OH) and the result taken into account in the mass balance. To follow the change of catalytic activity with time, successive runs of 15 min were made for 2.5 h. The reaction conditions were as follows: temperature ) 373 and 413 K, pressure ) 3 MPa, hydrogen flow ) 400 cm3 min-1, CCl4 volume ) 220 mL, catalyst weight ) 10.8 g, and catalyst particle

Table 1. Characterization Results of the Catalysts Supported on Different Carriersa Pd/A* (atom/atom) catalyst

D (%)

bulk

XPS

0.98% Pd/C 0.96% Pd/SiO2 0.97% Pd/SiO 1.01% Pd/Al2O3 0.97% Pd/Al2O3 0.98% Pd/SEP 0.95% Pd/SEP

18.1 7.0 9.8 20.0 22.1 6.2 7.0

0.0011 0.0056

0.0013 0.0960

0.0048

0.0870

0.0080

0.1720

a

D, dispersion; A*, C, Si, or Al atoms.

Table 2. Hydrodechlorination Activity of the Pd-Supported Catalysts at 373 Ka catalyst

Xb (%)

TOF (h-1)

STCMc (%)

STTCAc (%)

SHCAc (%)

0.98% Pd/C 0.96% Pd/SiO2 0.97% Pd/SiO2 1.01% Pd/Al2O3 0.97% Pd/Al2O3 0.98% Pd/SEP 0.95% Pd/SEP

15.1 4.8 5.1 4.6 4.0 1.6 0.8

328 2753 2068 886 719 1015 463

96.0 4.2 9.8 8.7 17.5 0.0 0.0

0.0 2.1 2.0 4.3 2.5 0.0 0.0

4.0 93.7 88.2 87.0 80.0 100.0 100.0

a Reaction conditions: catalyst weight, 4.2 g; particle size, 50100 µm; catalyst concentration, 19.0 g/LTTCM; pressure, 3 MPa; H2 flow, 400 cm3 min-1; reaction time, 90 min. b TTCM conversion. c Selectivity.

size ) 50-100 µm. Under these conditions, using the overall rate equation for a first-order reaction in a slurry reactor,17

-RH )

Ci (1/kg,lag,l) + (1/kl,cacm) + (1/ksmη)

(1)

We have experimentally verified that both gas-liquid and external and internal liquid-solid diffusion limitations were absent and that a perfect solid-liquid-gas mixing was attained.17 In eq 1, Ci (mol of H2 LTTCM-1) is the concentration of H2 in the liquid CCl4, ac (dm2 g-1) the external surface area of the catalyst particle, ag,l (m2 mTTCM-3) the gas bubble surface area, kg,l (dm h-1) the gas-liquid mass-transfer coefficient, kl,c (dm h-1) the liquid-solid mass-transfer coefficient, ks (LTTCM gcat-1 h-1) the specific reaction rate, and m the catalyst concentration (g LTTCM-1). 3. Results and Discussion 3.1. Influence of the Support. Characterization results of palladium catalysts supported on the different carriers are given in Table 1 and reaction data at 373 K after 90 min on stream in Table 2. Chloroform (TCM), hexachloroethane (HCA), tetrachloroethylene (TTCE), and traces of methane were obtained as reaction products. Conversion of carbon tetrachloride (X, %) is the percentage of the TTCM charged in the reactor which is transformed and the selectivity to a product i (Si, %) is the number of TTCM molecules transformed into i per 100 molecules of TTCM converted. Palladium supported on carbon is by far more active and selective to chloroform than the catalysts containing inorganic supports, which preferentially favor the formation of hexachloroethane. Hydrodechlorination of TTCM in a slurry reactor needs a great liquid-solid interface; i.e., the liquid must widely wet the solid. Thus, the high performance of the carbon catalyst is likely due to the fact that carbon has the highest surface area with

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hydrophobic groups, so that the carbon particles are able to form an interface with the nonpolar carbon tetrachloride wider than the other supports. The result is that a very favorable balance of surface tension and interface energy seems to be reached for TTCMcarbon.18 The hydrophilic nature of the inorganic supports, in contrast, hinders the formation of a high liquid-solid contact surface with organic liquids and strongly restrains the interaction reactant-metal active site. In fact, it was experimentally observed that the carbon particles in contact with liquid TTCM give a “pseudo-homogeneous” suspension, while with the inorganic supports the phases are more segregated and the contact is rather poor. For this reason the more dispersed catalysts (1.01% Pd/Al2O3 and 0.97% Pd/ Al2O3) are not the most active (see Tables 1 and 2). In this sense, it is not casual that most of the reactions carried out in a gas-solid-liquid system use metal carbon catalysts.19 Another outstanding result is the high selectivity of 0.98% Pd/C to chloroform, in contrast with those of Pd/ inorganic supports, showing again the important role of the support in this reaction. A support effect was also observed by Kim et al.6 in this reaction, carried out in the gas phase, using Pt-supported catalysts. The authors found that the chloroform and hexachloroethane selectivities change in the range 76.1-4.3% and 4.491.2%, respectively, when the support is changed. Hydrodechlorination of carbon tetrachloride to chloroform occurs by the interaction of H and CCl4 activated species adsorbed on the active metal sites (*Pd):20

2(Pd-H)* + *(CCl4-Pd) f 3*Pd + CHCl3 + HCl (2) We want to point out that *(CCl4-Pd) does not represent a CCl4 molecule associatively adsorbed on the metal active site, but it merely symbolizes a reactive “adsorbed intermediate species”, without going into the detail of its molecular structure. Reactions (2) and (3), therefore, have here only a stoichiometric rather than a mechanistic significance. The species *(CCl4-Pd) is also probably the precursor of hexachloroethane:

2(Pd-H)* + 2*(CCl4-Pd) f 4*Pd + C2Cl6 + 2HCl (3) It is believed that, under our experimental conditions, the high selectivity of 0.98% Pd/C to chloroform is greatly due to the hydrogen spillover onto the carbon surface, a phenomenon widely invoked21,22 in hydroprocessing reactions on metal/carbon catalysts. Thus, part of the hydrogen adsorbed and dissociated on *Pd sites may migrate through the carbon surface, ensuring that other more separated *(CCl4-Pd) species can form chloroform. The high capability of carbon to spillover hydrogen in comparison with other supports is at present well established and it has been reported that the surface concentrations of H on Pd/SiO2, Pd/Al2O3, and Pd/C are ≈1012, ≈1012, and ≈1016 (atom cm-2), respectively, and that the diffusion coefficient of the H atoms on the carbon surface may become about 103 times higher than those on the other supports.23 Therefore, spillover would also increase the reaction rate because it decreases the H coverage of the Pd surface and facilitates that new H2 molecules can be adsorbed and dissociated.

It is little likely, however, that spillover differences can explain by themselves alone that Pd supported on alumina, silica, or sepiolite forms hexachloroethane almost exclusively, if we consider that 0.98% Pd/C and 1.01% Pd/Al2O3 have very similar degrees of dispersion. Reaction (3) is statistically less probable than reaction (2) because two very close *(CCl4-Pd) entities are required to form one molecule of C2Cl6. On the other hand, the stoichiometry of H2/CCl4 needed to form chloroform (2/1) is higher than that to form hexachloroethane (2/2). Obviously, for a similar metal dispersion, the vicinity of the active *Pd sites will depend not only on the surface area but also on the distribution of the metal in the support. In such a way that, for a homogeneous distribution, the higher the specific surface area, the higher the separation of the *Pd sites. It is known that XPS provides information about the chemical composition of the catalyst surface and metal dispersion, but in certain conditions it can also supply information about the distribution of the metal throughout the support. Thus, following the literature,24,25 we have used the atomic surface ratios measured by XPS to determine the Pd distribution in the catalyst grains. According to Angevine et al.26 for a monatomically dispersed and uniformly distributed metal, the XPS Imetal/Isupport ratio should be proportional to the nominal surface density of metal, calculated from the number of metal atoms (NM) and the total surface area of the support (SBET). Values of Imetal/Isupport higher than those predicted by the nominal surface density indicate metal migration to the outer surface, such as it was found by Fillimonov et al.27 for Pd/lanthana and Pd/yttria and for Rh on small carbon pores.24 On the other hand, values below those predicted suggest either metal particle growth or preferential deposition of metal inside the small pores, as was shown for supported Pt26 and Pd27 catalysts. The XPS results show (Table 1) that the Pd/C atomic ratio in 0.98% Pd/C is similar to the bulk ratio obtained by chemical analysis, thus suggesting that Pd is homogeneously deposited on both the external and internal surface of the carbon. Different results were obtained, however, with 0.96% Pd/SiO2, 0.98% Pd/SEP, and 1.01% Pd/Al2O3 catalysts, for which the Pd/Si (both in 0.96% Pd/SiO2 and 0.98% Pd/SEP) and Pd/Al atomic ratios, measured by XPS, are higher than the bulk ratios. In these catalysts, most of the palladium is preferentially deposited on the external surface of the supports, where the particles will be nearer than those in 0.98%Pd/C. Since on the outer surface of the grain there is sufficient H for reaction (2) to take place, the fact that reaction (3) is predominant adds weight to the idea that the major formation of hexachloroethane requires neighboring *Pd sites. 3.2. Influence of the Metal. Catalyst Characterization. Characterization results of CO chemisorption and XPS are summarized in Table 3 and TPR profiles are depicted in Figure 1. The palladium, platinum, and nickel catalysts show a single reduction peak, indicating that their precursors are reduced in one step. The TPR profiles of 0.95% Pt/C and 4.86% Ni/C are similar to those reported in the literature for Pt and Ni supported on amorphous carriers. In contrast, the TPR profile of 0.98% Pd/C (Figure 1) exhibits a broad peak at a higher temperature than those reported for other Pd catalysts,28,29 suggesting the existence of a strong interaction Pd2+ T carbon. The reduction profile of 0.96% Ru/C

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Table 3. Dispersion and XPS Results of the Metal-/ Carbon-Supported Catalysts and Magnetic Susceptibility of the Metalsa Me/C (atom/atom) catalyst

D (%)

bulk

XPS

106χ emu/atom

0.98% Pd/C 0.95% Pt/C 0.96% Ru/C 0.94% Rh/C 4.86% Ni/C

18.1 19.5 15.0 20.6 7.0

0.0011 0.0006 0.0012 0.0011 0.0100

0.0013 0.0021 0.0006 0.0011 0.0016

557 189 43 102 ferro

a

D, dispersion; Me, metal; χ, magnetic susceptibility.

Figure 2. Conversion of carbon tetrachloride as a function of time at 373 K.

Figure 1. TPR profiles of the carbon-supported catalysts.

shows two partially overlapped peaks that probably correspond to the two reduction steps: Ru3+ f Ru2+ f Ru0. For 0.94% Rh/C, the two steps Rh3+f Rh2+ f Rh0 are suggested by the presence of a peak at 378 K and a shoulder at 413 K. Additional TPR experiments showed that, except for 4.86% Ni/C, all samples were completely reduced at 523 K. So this temperature was selected for reducing the catalysts, except for 4.86% Ni/C, which was reduced at 623 K to ensure its complete reduction. Metal dispersion of 0.98% Pd/C, 0.95% Pt/C, 0.96% Ru/ C, and 0.4% R/Ch is very similar, with values between 15% and 20%. For 4.86% Ni/C a lower dispersion (D ) 7%) is obtained. These values correspond to particle sizes ranging from 5.3 nm for rhodium to 14 nm for nickel, calculated using a spherical particle model. Since the metal/carbon atomic ratio is an estimate of catalyst dispersion, a parallelism exists between the experimental Me/C XPS ratios and the respective dispersion values evaluated by CO chemisorption. On the other hand, since the dispersion values are similar, comparing the values of the external Me/C ratios determined by XPS with the bulk ratios obtained by chemical analysis (Table 3), one can deduce, as was done in the previous section, that Pd, Ru, and Rh are homogeneously distributed throughout the support, while Pt is preferentially deposited on the outside and most of the Ni is located inside the pores. Catalytic Activity. Conversion of carbon tetrachloride at 373 and 413 K is shown in Figures 2 and 3, as a

Figure 3. Conversion of carbon tetrachloride as a function of time at 413 K. Table 4. Hydrodechlorination Activity of the Metal-/Carbon-Supported Catalysts at 373 and 413 Ka T ) 373 K catalyst

Xb (%)

TOF (h-1)

0.98% Pd/C 15.1 3281 0.95% Pt/C 3.2 1221 0.96% Ru/C T ∼0 0.94% Rh/C 0.6 115 4.86% Ni/C 0.0 0.0

T ) 413 K

STCMb SHCAc (%) (%) 96 100 T 100

4 0 0 0

Xb (%)

TOF (h-1)

41.1 8929 7.1 2707 0.1 25 1.1 211 0.0 0.0

STCMc SHCAc (%) (%) 92 100 100 100

8 0 0 0

a Reaction conditions: catalyst weight, 4.2 g; particle size, 50100 µm; catalyst concentration, 19.0 g/LTTCM; pressure, 3 MPa; H2 flow, 400 cm3 min-1; reaction time, 90 min. b TTCM conversion. c Selectivity.

function of time on stream. In all cases, a linear dependence is found, indicating that at least up to a conversion level of about 45% the kinetics of the reaction reasonably follows an overall zero-order law. For this reason, to compare the activity of the different catalysts, the values of conversion, turnover frequency (TOF, h-1), and selectivity were taken after 90 min of reaction (Table 4). We must point out the high hydrodechlorination activity of palladium as compared with the other metals. Platinum shows a lower activity, although the difference is attenuated when the TOF is used to evaluate the activity. Both catalysts are very selective to chloroform, although some hexachloroethane is always formed with 0.98% Pd/C while it is not detected with the platinum catalyst. Finally, the low activity of rhodium and ruthenium and the lack of activity of nickel

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on carbon are noticeable. Similar results have been obtained by Balko et al.30 and Barto´k31 with powdered noble-metal catalysts in hydrodehalogenation reactions carried out in the liquid phase and by Ohnishi et al.29 in the hydrodechlorination of 1,1,2-trichloro-1,2,2-trifluoroethane in the gas phase. High performances, however, have been reported for the hydrodechlorination of carbon tetrachloride with platinum catalysts5,7 working in the gas phase. Hydrodechlorination of chlorocarbon compounds consists of the substitution of chlorine atoms by the corresponding hydrogen atoms and the stoichiometric formation of hydrogen chloride:

CxHyClz + uH2 f CxH(y+v)Cl(z-v) + (2u-v)HCl (4) This reaction requires the dissociation of the H2 molecule, the hydrogenolysis of the C-Cl bond, and the formation of H-C and H-Cl bonds. In principle, all transition metals with incomplete d orbitals, and particularly noble metals of group VIII, are capable of activating such bonds because they present both hydrogenolysis and hydrogenation-dehydrogenation functions.32 In fact, as mentioned above, the catalysts commonly used for hydrodehalogenation of chloro- and chlorofluorocarbons are supported mono- or bimetallic catalysts.5,6,9,11,29,30,33,34 We have seen, however, that in the liquid phase palladium is far more active than the other metals. According to the data in Tables 3 and 4, this behavior cannot be attributed to either the dispersion or the distribution of the metal on the support and, therefore, the observed differences must be related rather to the electronic properties. Since the particle size is higher than 2 nm, we can assume that the average surface electronic properties correspond to those of the massive metal.31 Thus, we can use the local density of unoccupied states at the Fermi level, N(EF), to characterize the intrinsic electronic structure of these metals. The density of states measures the number of quantum states available for bonding reactants.36 Actually, N(EF) determines the donor-acceptor capacity of the metal, so that the lower the density of states, the greater must be the electron-donor character of a molecule to be adsorbed on the metal. This property has been used by various authors to explain the catalytic activity of group VIII metals.37-40 As long as the values of N(EF) are not easily available, we have used the magnetic susceptibility of the metals (χ, emu/atom), i.e., the magnetic moment per atom per field unit, which depends on N(EF),36,41

M ) 2µ0µB2N(EF)H ) χH

(5)

where µ0 is the vacuum magnetic permeability, µB the Bohr magneton, M the magnetization intensity or magnetic moment per atom, and H the applied magnetic field strength. The χ values given in Table 3 have been taken from ref 42. Assuming that the CCl4 molecule is heterolytically dissociated and adsorbed by the terminal electron-donor Cl-,20 the higher the unoccupied density of states of the metal, the higher the activity. Except for nickel, from the values of χ (N(EF)), one can expect that the reactivity sequence is Pd > Pt > Rh > Ru. This is just the order we have experimentally found for the activity, as shown in Table 4. This trend confirms the validity of our interpretation.

4. Conclusions Palladium supported on carbon seems to be the best catalyst for hydrodechlorination of carbon tetrachloride to chloroform in the liquid phase, due to the singular physicochemical properties of their two components. Carbon provides a high capacity to form a “pseudohomogeneous” suspension with liquid carbon tetrachloride, which increases the solid-liquid interfacial area, together with a high surface area, which allows a uniform distribution of the metal active sites. On the side of palladium, its high density of unoccupied states at the Fermi level promotes the adsorption of the CCl4 molecule by its slightly electron-donor terminal Cl-, leading to the formation of CHCl3. The reaction is highly favored by the H2 spillover on the carbon surface. Acknowledgment This research was carried out under a contract subscribed by CSIC and ERKIMIA S.A. Partial support by CICYT, Spain (MAT98-1021), is gratefully acknowledged. L.M.G.S. thanks the Spanish Ministerio de Educacio´n y Ciencia for a Ph.D. fellowship and to ERKIMIA S.A. (ERCROS Group) for financial assistance. Literature Cited (1) Armor, J. N. Environmental Catalysis. Appl. Catal. B 1992, 1, 221. (2) Mullin, Ch. R.; Wymore, C. E. Hydrogenolysis of Carbon Tetrachloride and Chloroform. U.S. Patent 3.579.596, 1971. (3) Holbrook, M. T.; Harvey A. D. Vapour Phase Hydrogenation of Carbon Tetrachloride. U.S. Patent 5.105.032, 1992. (4) Dogimont, Ch.; Franklin, J.; Janssens, F.; Schoebrechts, J. P. Process for the Production of Chloroform from Carbon Tetrachloride, Catalytic Compositions and Process for Obtaining Them. U.S. Patent 5.146.013, 1992. (5) Weiss, A. H.; Gambhir, B. S.; Leon, R. B. Hydrodechlorination of Carbon Tetrachloride. J. Catal. 1971, 22, 245. (6) Kim, S. Y.; Choi, H. C.; Yang, O. B.; Lee, K. H., Lee, J. S.; Kim, Y. G., Hydrodeclorination of Tetrachloromethane over Supported Pt Catalysts. J. Chem. Soc., Chem. Commun. 1995, 2169. (7) Choi, H. C.; Choi, S. H.; Yang O. B.; Lee, J. S.; Lee, K. H.; Kim, Y. G. Hydrodechlorination of Carbon Tetrachloride over Pt/ MgO. J. Catal. 1996, 161, 790. (8) Yakov, X. W.; Letuchy, A.; Eyman D. P. Catalytic Hydrodechlorination of CCl4 over Silica Supported PdCl2-Containing Molten Salt Catalysts: The Promotional Effects of CoCl2 and CuCl2. J. Catal. 1996, 161, 164. (9) Choi, H. C.; Choi, S. H.; Lee, J. S.; Lee, K. H.; Kim, Y. G. Effects of Pt Precursors on Hydrodechlorination of Carbon Tetrachloride over Pt/Al2O3. J. Catal. 1997, 166, 284. (10) Tijero, E.; Sule´, J. M.; Corte´s, A.; Seoane, X. L.; Arcoya, A. A Process for the Manufacture of Chloroform. U.S. Patent 5.208.393, 1993. (11) Tijero, E.; Sule´, J. M.; Corte´s, A.; Seoane, X. L.; Arcoya, A. Method for Producing Chloroform. E.U. Patent 0460138, 1995. (12) Brooks, C. S.; Kehrer, V. J. Chemisorption of Carbon Monoxide on Metal Surfaces by Pulse Chromatography. Anal. Chem. 1969, 41 (1), 103. (13) Moss, R. L. Preparation and Characterization of Supported Metal Catalysts. In Experimental Methods in Catalytic Research; Anderson, R. B., Dawson, P. T., Eds.; Academic Press: New York, 1976; Vol. 2, p 71. (14) Ott, G. L.; Fleisch, T. H.; Delgass, W. N. Fischer-Tropsch Synthesis over Freshly Reduced Iron-Ruthenium Alloys. J. Catal. 1979, 70, 394. (15) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Empirical Atomic Sensitivity Factors for Quantitative Analysis by Electron Spectroscopy for Chemical Analysis. Surf. Interface Anal. 1981, 3 (5), 211. (16) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: Englewood Cliffs, NJ, 1986; Chapter 12, p 597.

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Received for review December 10, 1999 Revised manuscript received May 15, 2000 Accepted May 16, 2000 IE990892F