Ind. Eng. Chem. Res. 2005, 44, 6661-6667
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Effects of Support Surface Composition on the Activity and Selectivity of Pd/C Catalysts in Aqueous-Phase Hydrodechlorination Reactions Luisa Calvo, Miguel A. Gilarranz, Jose´ A. Casas, Angel F. Mohedano, and Juan J. Rodrı´guez* Ingenierı´a Quı´mica, Facultad de Ciencias, Universidad Auto´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain
Two different activated carbons were treated with nitric acid and were used as supports to prepare 0.5 wt % Pd/C catalysts. The supports were characterized by N2 adsorption, elemental analysis, X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) in order to study the modifications in the porous structure and in the surface chemistry composition. Both XPS and transmission electron microscopy (TEM) were employed to study the dispersion of Pd on the catalysts. Oxidation of the activated carbon led to a substantial increase in oxygen surface groups, whereas the porous structure was not modified significantly. All the catalysts were tested in the hydrodechlorination of 4-chlorophenol in aqueous phase at mild conditions. It was observed that an increase in the CO2/(CO + CO2) ratio obtained from TPD of the carbon supports is accompanied by a better dispersion of Pd and an increase in the catalyst activity and the selectivity toward cyclohexanol, which is less toxic than the other products, cyclohexanone and phenol. Introduction Activated carbon is an interesting material with potential applications as catalytic support due to its high porosity and surface area and its relatively low reactivity.1 The porosity and surface chemistry of the supports can be controlled during the manufacture of the activated carbon and by further thermal or chemical treatment providing materials with tailor-made properties. In addition to this, activated carbon supports have also been reported to play an important role in certain catalytic processes providing an efficient path for the reaction.1 The performance of activated carbons as catalytic supports has been found to be determined by both their porous structure and surface chemistry.2 In the case of metallic active phases supported on activated carbons, the surface chemistry is one of the factors determining the metallic dispersion and the resistance to sintering, being relevant the role of surface oxygen groups.3 The surface oxygen groups are considered to act as anchoring sites that interact with metallic precursors and metals increasing the dispersion, with CO-evolving complexes significantly implied in this effect.3,4 On the other hand, CO2-evolving complexes, mainly carboxylic groups, seem to decrease the hydrophobicity of the support improving the accessibility of the metal precursor during the impregnation step.3 Several methods based on reactions with oxidizing gases and oxidizing solutions have been described in the literature to generate oxygen surface complexes in activated carbons.5 In general, gas-phase treatments require high temperatures, which can produce important changes in the textural properties and the loss of part of the surface oxygen. Liquid-phase treatments can * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +34 914974048. Fax: +34 914973516.
be applied at variable severity causing changes of different grades in the textural properties and the content of oxygen surface complexes.6,7 The prevailing type of oxygen surface groups generated is highly dependent on the oxidation method employed. Thus, it has been shown that gas oxidation with N2O increases mainly the concentration of hydroxyl and carbonyl surface groups.8 Oxidation in the liquid phase with nitric acid and hydrogen peroxide increases especially the concentration of carboxylic acid groups and also fixes, to a much greater extent, CO-evolving groups such as ketones, quinones, and phenols.9 Industrial wastewater treatment has become one of the fields where metallic catalysts supported on activated carbon have found novel interesting applications, mainly in advanced oxidation processes.10,11 Likewise, it has been observed that the modification of the surface chemistry of carbon supports enhances the activity of Pt/C catalysts in the hydrogenation of benzene.4 Thus, the pretreatment of activated carbon supports can be stated as an important variable in the preparation of hydrogenation catalysts of this type. The hydrogenation of chlorinated pollutants in wastewaters, namely, hydrodechlorination, can be considered as an emerging technology of potential interest for the treatment of wastewaters containing highly toxic and refractory chlorinated organic contaminants. Nevertheless, there is still a lack of information on this potential application in the literature. In a previous work we have presented results on hydrodechlorination of 4-chlorophenol in the aqueous phase at low concentration using commercial and home-made Pd/C catalysts.12 Oxidation of the activated carbon support was found to enhance the catalytic activity. Selectivity was another important parameter attending to the different toxicity and biodegradability of the reaction products; thus the ecotoxicity values of both pollutants and reaction products are shown in Table 1.
10.1021/ie0503040 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/15/2005
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The catalytic hydrodechlorination of 4-chlorophenol with hydrogen was studied in the aqueous phase in a trickle bed reactor provided with control of temperature, pressure, liquid flow, and gas flow. A simplified scheme of the reaction setup is shown in Figure 1. The hydrodechlorination runs were carried out at 2.4 bar and 50-75 °C. The starting concentration of 4-chlorophenol was 100 mg/L with a flow rate of 0.5 mL/min. A weight of 0.5 g of catalyst was used in all the cases. Hydrogen was fed to the reactor diluted with nitrogen (50% vol.) at a total flow rate of 100 NmL/min. All the operating conditions were chosen according to a previous work,12 where it was reported that the hydrodechlorination on a Pd/C catalysts was a feasible method for the degradation of chlorophenols in aqueous solution at mild conditions. The analysis of the liquid stream from the reactor was performed by high-performance liquid chromatography/diode array detector (Prostar, Varian) using a C18 column as stationary phase and a mixture of acetonitrile and water (1:1, vol.) as the mobile phase, and by gas chromatography/ion trap mass spectrometry (Saturn 2100 T, Varian) using a 30 m long × 0.25 mm inside diameter capillary column (Meta ×5 Tracsil 5). Previous to GC/MS analysis, the aqueous samples were extracted with solid-phase cartridges (C18, Waters), which were eluted with acetonitrile. The ecotoxicity of the initial compounds and the reaction products was measured by means of the Microtox Acute Toxicity Test (SCI 500 Analyzer) using a freeze-dried preparation of the marine bacterium Vibrio fischeri as described in ISO-11348-3. The Pd/C catalysts were prepared by incipient wetness impregnation of activated carbon of 1-2-mm
particle size. The impregnation solution consisted on PdCl2 dissolved in 0.1 N HCl. All the catalysts were prepared with a palladium load of 0.5% (wt) from a solution volume exceeding by 30% the pore volume of the support. After impregnation the catalysts were dried overnight at 100 °C. Finally, the catalysts were calcinated for 3 h at 200 °C and then reduced with hydrogen at 100 °C for 1.5 h. Additional in situ reduction was carried out in the reactor at the operating temperature and pressure. Before the reaction, the catalyst was saturated with 4-chlorophenol to minimize the effect of adsorption and reduce the time required to reach a steady state. Two series of supports, made in the laboratory from activated carbons supplied by Chemviron (Ch series) and Merck (Mk series), were studied. In both series some of the activated carbon samples used as supports were subjected to an oxidative treatment with nitric acid. This treatment consisted in boiling 1 g of activated carbon in 10 mL of a 6 N HNO3 solution for 20 min as described by Prado-Burguete et al.3 After boiling, the samples were thoroughly washed with distilled water. The supports were then dried overnight in an oven at 100 °C. The porous structure of the activated carbon supports was characterized by N2 adsorption-desorption at 77 K (Autosorb-1 Quantachrome). The micropore volume corresponds to those pores with a size lower than 2 nm, while mesopore volume is associated with pores in the range of 2-8 nm. The t method was used to determine the external area, namely, the non-microporous area. Their chemical nature was studied by elemental chemical analysis, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Elemental chemical analyses were carried out with a Pelkin-Elmer 2400 CHNS series 2 analyzer. XPS analyses were performed with a Physical Electronics model 5700 apparatus with a Mg KR X-ray excitation source (1253.6 eV). TPD experiments were carried out by heating the samples up to 900 °C in He flow at a heating rate of 10 K/min and analyzing the CO and CO2 evolved with a non-dispersive infrared absortion analyzer (Ultramat 22, Siemens). XPS mapping technique was employed to study Pd dispersion. XPS elemental maps were generated from a 1 mm × 1 mm area of the catalysts. Morphology of the Pd particles was studied by transmission electronic microscopy (TEM) in a JEOL JEM2000 FX apparatus. Samples were dispersed ultrasonically in acetone and spread over a self-perforated microgrid.
Figure 1. Scheme of the reaction setup.
Results and Discussion Porous Structure. Figure 2 shows the 77 K N2 adsorption-desorption isotherms of the original and modified activated carbons. The surface area and pore volume values obtained from these isotherms are summarized in Table 2 where the nomenclature of the samples is also indicated. All the isotherms correspond to basically microporous solids, as revealed by the steep uptake at low relative pressure, although the slope of the nearly horizontal branch and the existence of hysteresis also indicate some contribution of mesoporosity. Despite the qualitative similitude of their porous structure, the two starting untreated carbons differ in their Brunauer-Emmett-Teller (BET) surface area and micropore volume values being both fairly higher for the Ch carbon. The treatment with nitric acid did not modify significantly those values.
Table 1. Ecotoxicity Values samples (100 mg/L)
EC50 (mg/L)
T.U.
4-chlorophenol phenol cyclohexanone cyclohexanol
1.9 15.9 11.6 18.5
53.1 6.3 8.6 5.4
This work tries to be a contribution to the knowledge of the role played by the nature of the carbon support, especially the oxygen surface groups and the porous structure, on the behavior of Pd/C catalysts in hydrodechlorination of chlorophenols in liquid phase at low concentration and mild conditions. Two different commercial activated carbons were used as received and after oxidation with nitric acid for catalysts preparation. Experimental Section
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Figure 2. N2 adsorption-desorption isotherms at 77 K of untreated and oxidized activated carbons. Table 2. Porous Structure of Untreated and Treated Activated Carbons mesopore vol BET external micropore area vol 2 < d < 8 nm area (cm3/g) (cm3/g) sample treatment (m2/g) (m2/g) MkU MkN ChU ChN
none HNO3 none HNO3
917 932 1392 1320
119 126 110 102
0.332 0.330 0.549 0.525
0.041 0.046 0.044 0.041
Different modifications of the porosity of activated carbons under the action of nitric acid treatment, in general at more severe conditions, have been reported in the literature. Aksoylu et al.4 and Choma et al.7 showed that nitric acid oxidation carried out at boiling temperature until dryness for 3 h caused an important decrease in BET surface area and micropore volume. In these conditions the decrease in BET area and micropore volume can reach about 50%.13 Gil et al.5 reported reductions in surface area and pore volume of 10-15% upon treatment with concentrated nitric acid at 25-60 °C. Comparable results were reached by El-Hendawy.14 The loss of total surface area and micropore volume has been related with the fixation of the new created oxygen groups at the entrance and/or on the walls of micropores, thus reducing the pore volume available for N2 uptake, as well as with the potential destruction of micropore walls.15,16 Surface Chemistry. The elemental chemical analyses and XPS results shown in Table 3 indicate a substantial modification in the chemical nature of the carbons surface as a consequence of the oxidation treatment. Both elemental and surface analysis (XPS) indicate that an important rise in nitrogen content takes place, which can be attributed to nitro groups resulting from HNO3 oxidation.14,16,17 An increase in the atomic oxygen concentration for both activated carbon series can be observed together with a simultaneous decrease in carbon content, indicating the functionalization of the carbon surface through the creation of carbon-oxygen bonds.6 The O/C ratio derived from XPS analyses became almost 1.5 and 1.9 times higher, respectively, for the Mk and Ch carbons, after the oxidative treatment. The C 1s and O 1s spectra were deconvoluted by using a sum of several Lorentzian-Gaussian functions with different binding energies. The width was fixed at half height equal between 1.5 and 2.5 eV.8,18 The C 1s spectrum was resolved into four individual component peaks representing graphitic carbon, carbon in alcohol
or ether groups, carbonyl groups, and carboxyl or ester groups. The deconvolution of O 1s spectra revealed the presence of five peaks corresponding to quinones and carbonyl groups, lactones, carboxylic anhydrides, phenol, and ether groups, carboxylic acid, chemisorbed water, and occluded CO and/or CO2.6,8 The area of all the peaks corresponding to oxygen-containing functional groups increased after the oxidation treatment, being particularly clear the case of phenol, ether, lactone, carboxylic anhydride, carbonyl, and quinone groups. These results are in good agreement with the XPS observations reported in the literature for activated carbons oxidized with nitric acid under similar conditions.8 The deconvolutions showed that the starting untreated activated carbons exhibited important differences in the distribution of oxygen surface groups, as it can be appreciated from the XPS results summarized in Table 4. The main differences can be found in the relative amounts assigned to the O 1s spectra, with a particularly higher importance of the peaks assigned to phenol, ether, lactone, and also carboxylic anhydride in the case of the Ch carbon. For the oxidized carbons, changes in the distribution of oxygen surface groups were observed mainly in the O 1s spectra. The peaks assigned to phenol, ether, lactone, and carboxylic anhydride increased in the case of MkN, whereas minor changes were found for ChN. TPD experiments were carried out to achieve a deeper characterization of the surface oxygen groups. Carboxylic acids and lactone groups are known to evolve as CO2 upon heating and carboxylic anhydride produces both CO2 and CO, whereas CO derives from phenols, ethers, carbonyls, and quinones.6 There is some controversy in the literature with respect to the assessment of TPD peaks; however some general trends have been observed. Carboxylic acids eliminated as CO2 usually appear between 200 and 240 °C, while lactones and carboxylic anhydrides are desorbed at higher temperatures. Those peaks are generally centered at 240-370 °C and 410-480 °C, respectively. CO desorption takes place at higher temperatures than CO2. Thus, the desorption of carboxylic anhydride groups is assigned to temperatures between 400 and 560 °C, phenols above 600 °C, carbonyls at 720-810 °C, and quinones at about 850 °C.8,19 Figures 3 and 4 show the CO and CO2 TPD curves obtained for the activated carbons studied. The important incorporation of oxygen to the activated carbons upon oxidation can be observed from the significantly higher evolution of CO and CO2 from the oxidized samples. The overall amounts of evolved CO and CO2, calculated by integration of the TPD curves, can be seen in Table 5. These values confirm that the HNO3 treatment gives rise to important modifications in the composition of the surface of the activated carbons where oxygen groups are created in fairly significant amounts. It can be observed that the oxidation leads to a greater generation of CO- than CO2-evolving surface oxygen groups. As in the case of XPS spectra, the TPD curves can be deconvoluted to evaluate the amount of CO and CO2 evolved from different oxygen surface groups. Thus, relevant peaks were observed in the oxidized carbons between 175 and 250 °C and around 400 °C for the CO2 evolution profile, which can be attributed to carboxylic acids and carboxylic anhydrides, respectively. For both carbons, the amount of CO evolved above 700 °C was
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Table 3. Elemental Chemical Analysis and XPS Surface Analysis of Untreated and Treated Activated Carbons elemental chemical analysis
surface analysis by XPS
sample
C (%)
N (%)
S (%)
C 1s (%)
O 1s (%)
MkU MkN ChU ChN
89.48 76.34 94.32 84.46
0.50 1.19 0.25 0.70
0.11 0.15 0.09 0.03
93.8 90.3 94.5 89.5
6.2 8.7 5.3 9.5
N 1s (%)
S 2p (%)
O/C
0.2 0.2 0.1
0.066 0.096 0.056 0.106
0.8 0.4
Table 4. Relative Concentration of Oxygen Functionalities from Deconvolution of XPS C 1s and O 1s Spectraa deconvolution of C 1s peak (% of peak) sample
C-C
alcohol, ether
carbonyl
ester, carboxylic
MkU MkN ChU ChN
52.9 (284.3 eV) 56.5 (284.5 eV) 60.2 (284.5 eV) 57.0 (284.5 eV)
27.7 (288.5 eV) 24.4 (285.8 eV) 23.1 (285.8 eV) 23.6 (285.9 eV)
10.8 (288.4 eV) 10.4 (288.3 eV) 8.8 (288.6 eV) 10.4 (288.3 eV)
8.7 (290.9 eV) 8.7 (290.8 eV) 7.9 (290.8 eV) 9.0 (290.7 eV)
deconvolution of O 1s peak (% of peak) sample
carbonyl, quinone
phenol, ether, lactone, carboxylic anhydride
carboxylic acid
chemisorbed water
CO, CO2
MkU MkN ChU ChN
25.1 (531.1 eV) 27.4 (531.4 eV) 25.8 (531.6 eV) 32.2 (531.5 eV)
26.6 (532.5 eV) 32.8 (532.7 eV) 33.1 (532.4 eV) 26.8 (532.8 eV)
22.3 (533.3 eV) 20.5 (534.0 eV) 26.1 (533.7 eV) 18.7 (534.0 eV)
20.2 (535.5 eV) 19.3 (535.8 eV) 15.0 (536.8 eV) 17.2 (535.4 eV)
5.8 (537.3 eV)
a
5.0 (538.1 eV)
Values in brackets indicate the binding energy assigned to different oxygen surface groups. Table 5. Data from TPD Profiles for Total Evolved CO and CO2 sample
CO2 evolved (µmol/g)
CO evolved (µmol/g)
CO2/(CO + CO2) ratio
MkU MkN ChU ChN
23 1250 137 1477
357 4393 357 5821
0.060 0.22 0.28 0.20
Table 6. Data from the Deconvolution of TPD Profiles for Evolved CO
Figure 3. CO-releasing curves from TPD of the activated carbons.
sample
carboxylic anhydride (µmol/g)
phenol (µmol/g)
carbonyl (µmol/g)
quinone (µmol/g)
MkU MkN ChU ChN
120 145 negligible 501
84 1133 28 1688
94 2245 134 2410
59 870 195 1222
Table 7. Data from the Deconvolution of TPD Profiles for Evolved CO2
Figure 4. CO2-releasing curves from TPD of the activated carbons.
increased as a result of the oxidation treatment. The amounts of CO and CO2 assessed to different oxygen surface groups are reported in Tables 6 and 7, respectively. By consideration of the acidic nature of the CO2evolving groups and the relatively neutral and/or basic character of CO-evolving groups, it can be stated that the surface of the ChU carbon has much more acidic
sample
carboxylic acid (µmol/g)
lactone (µmol/g)
carboxylic anhydride (µmol/g)
MkU MkN ChU ChN
6 324 83 337
12 474 54 617
5 452 negligible 523
character than that of MkU. The HNO3 treatment substantially increases the CO2/(CO + CO2) ratio of Mk carbon, achieving values close to that of ChN. For both activated carbon series, the oxidation increased the amount of evolved CO and CO2 by 1-2 orders of magnitude and a similar trend can be observed for the individual oxygen surface groups in most cases. These results are in good agreement with those reported by other authors. Aksoylu et al.4 and Figuereido et al.8 stated that the oxidation with nitric acid leads to drastic increases in both CO- and CO2-releasing groups. In addition to the increase in the absolute amount of oxygen surface groups, some important modifications in the relative distribution of the different functional groups take place. Thus, upon oxidation the percentage of carbonyl group doubles for Mk series, whereas for Ch
Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6665 Table 8. Concentration of Pd in the Catalysts as Determined by XPS catalyst
Pd (%, w/w)
Pd/C atomic ratio
MkU-Pd MkN-Pd ChU-Pd ChN-Pd
0.41 0.16 0.15 0.18
0.0044 0.0018 0.0016 0.0020
series the most significant changes observed are the increase in the relative percentage of phenol group and the decrease in that of quinone group. The results obtained in the characterization of the catalysts prepared from the different supports indicated that no significant modifications in the amount and distribution of oxygen surface groups took place during the catalysts’ preparation procedure. Only a slight loss of BET area and pore volume occurred during impregnation, which can be attributed to partial blockage of pores due to the deposition of Pd. The results for the surface atomic concentration of Pd determined by XPS are shown in Table 8. Data given correspond to the percentage of palladium atoms in a layer of the surface with a depth of few atomic distances. It can be seen that the metal concentration is basically the same in all the catalysts, except for catalyst MkU-Pd, whose atomic concentration of Pd is substantially higher. A similar trend can be observed in Pd/C ratio. XPS only accounts for the concentration in the most external layers of the material analyzed. Therefore, the comparison between the value of 0.41% obtained for MkU-Pd and the values of 0.15-0.18 obtained for the rest of the catalysts
suggests that in the former the deposition of Pd takes place mainly in the external surface. This would be indicative of a poorer Pd dispersion in MkU-Pd catalyst. XPS mapping technique was employed to determine the Pd surface composition. The XPS intensity data were displayed, using a bilinear interpolation scheme, as three-dimensional chemical images with a pseudo-color scale representing variations in elemental surface composition. XPS mapping (Figure 5) for catalysts MkU-Pd and MkN-Pd show a higher homogeneity in the Pd response for the latter, suggesting a better dispersion. The oxidation of the Mk carbon seems to contribute to the dispersion by both the reduction in the hydrophobicity of the carbon surface, which would improve the penetration of the impregnation solution into the pores,3 and the creation of groups for a more homogeneous anchorage of Pd particles. The low values obtained for the concentration of Pd on the external surface for the ChU-Pd and ChN-Pd catalysts indicate a good distribution of Pd within the structure of the support. The slight increase from 0.15 to 0.18% in Pd concentration upon oxidation of the support has a low significance. However the mapping indicates that oxidation leads to a more homogeneous distribution of Pd on the surface of the catalyst. The TEM micrographs of Pd catalysts shown in Figure 6 serve to illustrate the nature of the metal dispersion. The results are in agreement with XPS mapping. Thus, MkU-Pd showed the poorer dispersion with much larger particles than the rest of the catalysts studied. For this catalyst even agglomerates of Pd
Figure 5. Elemental mapping for Pd in catalysts: (a) MkU-Pd; (b) MkN-Pd; (c) ChU-Pd; (d) ChN-Pd.
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Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 Table 9. Results of the Catalytic Activity Experiments selectivity at 75 °Ca
conversion (%) catalyst
50 °C
75 °C
MkU-Pd MkN-Pd ChU-Pd ChN-Pd
85.4 96.9 96.0 95.7
86.8 97.5 99.0 98.3
a
Figure 6. TEM micrographs of Pd catalysts: (a) MkU-Pd; (b) MkN-Pd; (c) ChU-Pd; (d) ChN-Pd.
particles were observed, which could be associated to the small number of oxygen surface groups available for anchorage. This fact could favor the growth of the particles versus the germination process.20 Catalysts prepared with oxidized supports showed small and homogeneous particles of Pd that were well dispersed on the activated carbon. The increase in the content in oxygen-bearing groups in catalysts MkN-Pd and ChNPd led to a higher number of nucleation centers to anchor the metallic precursor. In that way, dispersion was improved. It should be noticed that the catalyst ChU-Pd showed small and well-dispersed particles even though the amount of oxygen surface groups was not very high. Consequently, the presence of some particular oxygen complexes may play an important role in the final Pd dispersion being of relevance those that evolve as CO2. A higher content in carboxylic acid and lactone groups compared to MkU-Pd was observed in ChU-Pd catalyst, giving rise to a decrease of hydrophobicity. Thus, the diffusion of the metal precursor toward the support would be enhanced, giving rise to a better Pd dispersion. In addition to this, it has been reported that nitric acid oxidation causes the reduction of critical adsorption sites or impurity centers, which are probable sites for Pd particles growing.21 Catalysts Activity and Selectivity. Pd carbon catalysts were tested in the hydrodechlorination of 4-chlorophenol; the results of the activity experiments are shown in Table 9. In these experiments, the activity results shown remained constant after 24-28 h on stream. The oxidative treatment of Mk carbon led to a significant enhancement of the catalyst activity, whereas a negligible decrease is observed in the case of Ch-Pd. The increase in activity can be considered of importance
phenol cyclohexanone cyclohexanol 0.426 0.186 0.213 0.015
0.338 0.447 0.308 0.023
0.236 0.366 0.479 0.963
Percent mass/total mass of organic compounds.
due to the high toxicity of 4-chlorophenol in comparison to that of the reaction products. The porous structure of the supports cannot explain by itself the different behavior of the catalysts. For instance, the catalyst MkN-Pd shows an activity similar to that of catalyst ChN-Pd although the former has a significantly lower BET area and pore volume. Most of the BET area of these carbons corresponds to micropores, probably with a low significance with regard to the catalytic activity. A look at the external (i.e., non-micropous) surface area reveals that the values are quite comparable and even somewhat higher for the MkN carbon. This external surface is much more accessible to Pd, and then most of this metal must be distributed in that part of the porous structure, which thus plays the important role relative to the catalyst activity. Likewise, the increase in the overall amount of oxygen surface groups not always resulted in a higher activity. Thus, there are no significant differences in catalytic activity between the catalysts ChU-Pd and ChN-Pd, although the oxidation treatment led to a very important increase of the oxygen surface groups. These results confirm that the type of oxygen surface groups created are relevant for Pd dispersion and catalytic activity. From the TPD results, it can be seen that a higher proportion of oxygen groups desorbed as CO2, i.e., a higher value of the CO2/(CO + CO2) ratio, affects positively to the activity of the catalysts. In this sense, for the Mk carbon series the oxidation led to an increase of the CO2/(CO + CO2) ratio from 0.060 to 0.22 and to an increase of 4-chlorophenol conversion from 86.8 to 97.5% at 75 °C. For the Ch carbon series, the untreated support already exhibited a high CO2/(CO + CO2) ratio, which would be consistent with the high activity of the ChU-Pd catalyst. The differences observed in catalytic activity are in agreement with the results on Pd distribution and dispersion commented before (XPS mapping and TEM), with the activity higher for the catalyst with a better dispersion and with a narrower particle-size distribution. The role of individual oxygen surface groups is difficult to asses and would require further work, but it can be seen that carboxylic acid groups could be among the ones that contribute to the increase of activity.18 Thus, it can be observed that the MkU-Pd catalyst provided the lowest conversion, as this is the one with a lower concentration of carboxylic acid groups. Carbonyl and quinone groups can also be of importance since they are in higher proportion in all the catalysts of high activity (MkN, ChU, and ChN). Groups such as phenols and carboxylic anhydrides do not seem to play an important role in that respect. An important characteristic of the catalysts of higher activity (MkN-Pd, ChU-Pd, and ChN-Pd) is that they also showed a higher selectivity toward cyclohexanol, which is the less toxic product in the reaction pathway for chlorophenol hydrodechlorination, as can be seen in Table 1 where the ecotoxicity values were shown. Therefore it can be considered as more easily biodegrad-
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able than phenol and cyclohexanone.22 It is interesting to point out that the reactions carried out with the catalysts whose supports were treated with nitric acid provided the lowest proportion of phenol in the reaction products. This fact indicates that the improved dispersion of Pd attained upon the creation of oxygen surface groups in the support is of relevance to achieve a more complete hydrogenation. Likewise, the role of the surface groups of carbon supports has been reported to be of importance since they can provide an efficient pathway for the reaction.4 The influence may be related to the adsorption properties of the catalyst; thus the more polar surface generated by oxidation can be of importance. Thus the results of this work suggest that a high content in carboxylic acid groups may be responsible for a high selectivity toward cyclohexanol, as it can be appreciated in the case of the ChN-Pd catalyst. Likewise, the selectivity toward phenol diminishes when the amount of carboxylic acid groups increases. Conclusions The treatment of activated carbons with nitric acid led to a higher content in oxygen surface groups, whereas the porous structure was only slightly modified. TPD results showed both a greater generation of COand CO2-evolving groups, although the increase in carboxylic acid, lactone, and carboxylic anhydride groups associated to CO2 can be considered of special relevance. As a result of oxidation, the dispersion of Pd on the surface of the catalysts prepared was improved. This was particularly clear for the Mk carbon series where the increase of the CO2/(CO + CO2) ratio upon oxidation was very important. The better dispersion led to a higher activity of the catalysts. A higher CO2/(CO + CO2) ratio from TPD of the carbon supports showed to be associated to an enhancement of the catalysts activity, with evidence of the important role of carboxylic acid groups and also of carbonyl and quinones. These groups also could be of importance to improve the selectivity toward the end chain reaction product cyclohexanol, which is less toxic than the other products phenol and cyclohexanone. Acknowledgment We greatly appreciate financial support from the Spanish MCYT through the Project PPQ2000-1763CO3-01 and from the Comunidad Auto´noma de Madrid (Project 07M/0039/2002). We express our gratitude also to Aguas de Valencia S. A. for kindly providing samples of Chemviron activated carbons. Literature Cited (1) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. Carbon Materials as Adsorbents in Aqueous Solutions. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; Marcel Dekker. New York, 2000; Vol 27. (2) Rodrı´guez-Reinoso, F. The role of carbon materials in heterogeneous catalysis. Carbon 1998, 36, 159-175. (3) Prado-Burguete C.; Linares-Solano, A.; Rodrı´guez-Reinoso, F.; Salinas-Martı´nez de Lecea, C. The effect of oxygen surface groups of the support on platinum dispersio´n in Pt/carbon catalysts. J. Catal. 1989, 115, 98-106.
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Received for review March 3, 2005 Revised manuscript received May 23, 2005 Accepted June 14, 2005 IE0503040
