Pd−W and Pd−Co Bimetallic Catalysts' Sulfur Resistance for Selective

Ind. Eng. Chem. Res. 1997, 36, 2543-2546

2543

Pd-W and Pd-Co Bimetallic Catalysts’ Sulfur Resistance for Selective Hydrogenation Pablo C. L’Argentie` re and Nora S. Fı´goli* INCAPE, Instituto de Investigaciones en Cata´ lisis y Petroquı´mica (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina

The effect of cobalt and tungsten addition to a Pd/Al2O3 catalyst on its catalytic activity, selectivity, and sulfur resistance during styrene selective hydrogenation at 353 K, 20 kg cm-2 hydrogen pressure, and W/V ) 0.002 g cm-3 has been studied. Catalysts were characterized by XPS, XRD, TEM, and hydrogen and carbon monoxide chemisorption. An increase in activity and sulfur resistance was observed after both cobalt and tungsten addition, which is ascribed to the formation of very active Pd-Co and Pd-W interfaces. A decorative effect of cobalt and tungsten on palladium is suggested. Introduction Palladium-based catalysts are highly active and selective in several important commercial reactions, such as hydrocarbon hydrogenation. One of the major problems with palladium is the high sensitivity to sulfur compounds usually present in hydrogenation feedstocks (Boitiaux et al., 1987). In the 1980’s it was well established that the addition of a second metal may be a useful way to increase the activity and the sulfur resistance of palladium for these reactions (Hegedus and McCabe, 1981; Mallat et al., 1985). In previous papers, we have reported the influence of Ni (L’Argentie`re et al., 1993), Mn (L’Argentie`re et al., 1995a), and La (Fı´goli et al., 1995) on palladium-based catalysts activity and sulfur resistance during selective hydrogenation reactions; we have found that the interaction of Pd with those metals creates and stabilizes electron-deficient species (Pdn+) that modify not only the activity but also the sulfur resistance of the catalysts. The purpose of this work is to study the modifications produced by W and Co on both the properties of Pd/ Al2O3 catalyst and its resistance toward thiophene. Several papers have been published about the influence of W on the behavior of platinum catalysts (Regalbuto et al., 1987; Contreras and Fuentes, 1996), but, to our knowledge, less attention has been devoted to the effect of W and Co on the catalytic properties of palladium in hydrocarbon hydrogenation reactions. The catalytic properties were studied using the selective hydrogenation of styrene to ethylbenzene as test reaction and thiophene as poisoning reagent. Styrene is a useful model compound to investigate the catalytic performance of metal catalysts at a laboratory scale due to the presence of two types of unsaturated groups (Anderson et al., 1994). Thiophene is usually taken as a model compound for sulfur resistance studies (Pa´linko´, 1994). Experimental Section Catalysts Preparation. Alumina CK 300, cylinders of 1.5 mm diameter with a BET surface area of 180 m2 g-1 and a pore volume of 0.52 mL g-1 was used as support. It was calcined in air at 773 K for 3 h prior to catalyst preparation. An aqueous solution of Pd(NO3)2 was used for impregnation in a suitable concentration so as to obtain a catalyst containing 0.3% palladium. * To whom correspondence should be addressed. Tel: 54 42 528062. Fax: 54 42 531068. E-mail: [email protected]. S0888-5885(96)00743-9 CCC: $14.00

The catalyst was then dried and calcined at 623 K for 3 h within an air flow rate of 480 mL h-1 gcat-1 and after this reduced for 5 h at 393 K with a hydrogen flow rate of 480 mL h-1 gcat-1. Pd-W/Al2O3 was prepared by wet impregnation of the Pd/Al2O3 catalyst with an aqueous H3PO4‚12WO3 solution of an appropriate concentration so as to obtain a W/Pd atomic ratio of 6 during 1 h. After being washed with bidistilled water, the solid was dried at 383 K during 24 h and calcined at 823 K during 7 h. The calcination temperature used assures the complete elimination of phosphorus from the catalyst (Song et al., 1992). The material was finally reduced at 393, 573, or 773 K during 5 h. Pd-Co/Al2O3 was prepared by wet impregnation of the Pd/Al2O3 catalyst with an aqueous solution of Co(NO3)2‚6H2O of an appropriate concentration so as to obtain a Co/Pd atomic ratio of 6. The sample was dried at 383 K during 24 h, calcined at 823 K for 7 h, and reduced in hydrogen at 573 K during 5 h. Monometallic W/Al2O3 and Co/Al2O3 catalysts were also prepared by wet impregnation of Al2O3 with aqueous solutions of H3PO4‚12WO3 or Co(NO3)2‚6H2O, respectively, in appropriate concentrations so as to obtain the same amount of metal as in the corresponding bimetallic catalysts. Catalyst Characterization. Dispersion. The dispersion was determined by hydrogen chemisorption following the method of Benson et al. (1973), in order to ensure that the samples were free from the formation of β-PdH. In order to compare with the previous results, dispersion was also measured by CO chemisorption following the technique described by Martin et al. (1985) using a COads/Pd stoichiometry of 1.15/1 as previously reported (Fı´goli et al., 1995). X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS analyses were carried out on a Shimadzu ESCA 750 electron spectrometer coupled to a Shimadzu ESCAPAC 760 Data System. The C 1s line was taken as an internal standard at 285.0 eV and was used to correct possible deviations caused by an electric charge of the samples. The position of the maximum of the Pd 3d5/2 peak was used to follow the superficial electronic state of palladium. After reduction, the samples were introduced into the preparation chamber of the XPS equipment following an operation procedure previously described (Arcoya et al., 1990). The possibility that, during thermal treatments, the palladium particles became decorated by the other metals species was © 1997 American Chemical Society

2544 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997

investigated by ion sputtering using an Ar+ ion source at 3.5 keV. Before the Ar+ etching the samples were heated in vacuum (823 K) in order to remove contaminants from their surfaces. The determinations were carried out in the preparation chamber of the XPS equipment for different periods of time, and after each one, an XPS determination was performed in order to determine the variation of the atomic ratios. This procedure was repeated until a stationary state was reached for the values of the metallic atomic ratios. Transmission Electron Microscopy (TEM) Determinations. The Pd-Co/Al2O3 and Pd-W/Al2O3 catalysts were examined in a Jeol JMS-100 CS II electron microscope in order to determine the Pd particle size, following an experimental procedure previously reported (Figoli and L’Argentie`re, 1989). X-ray Diffraction Spectroscopy (XRD). The crystalline structure was analyzed by X-ray diffraction using Shimadzu XD-D1 equipment in the 20-70° 2θ range, using KR Cu radiation and /2/2/0.6 windows. Catalytic Activity and Selectivity. Catalytic activity and selectivity were determined in a stirred tank reactor operated at 22 kg cm-2 hydrogen pressure, W/V ) 0.002 g cm-3, 353 K and stirring velocity 700 rpm; 1 g of catalyst was used for each experiment. No diffusional limitations were observed under these conditions. During each run the reactor was connected to a hydrogen cylinder, and a constant pressure was maintained throughout the experiment. The selective hydrogenation of styrene to ethylbenzene was used as a test reaction. As previously reported (L’Argentie`re et al., 1990), ethylbenzene can be further hydrogenated to ethylcyclohexane, and polymerization of styrene can also occur as a side reaction. A 5% styrene in toluene solution (I) or the same solution with the addition of either 100 ppm thiophene (II) or 300 ppm thiophene (III) were used as feed. Reactants and products were chromatographically analyzed by means of a flame ionization detector and a DC 200 column. The catalytic activity and selectivity to ethylbenzene were calculated from the chromatographic data; benzene was added to the reaction mixture as an internal standard to follow the polymerization consecutive reaction. Results In Table 1 are presented the hydrogen chemisorption capacities of the catalysts. Palladium dispersion for the Pd/Al2O3 catalyst is 24%, calculated from either hydrogen or carbon monoxide chemisorption data. The hydrogen chemisorption capacity of the Pd-Co/Al2O3 and Pd-W/Al2O3 catalysts reduced at 393 K was very low (32 and 29 µmol‚gcat-1, respectively), and in the case of Pd-W/Al2O3 it decreased dramatically when the reduction temperature was raised up to 773 K. A similar result was obtained for carbon monoxide chemisorption. Furthermore, an attempt to determine the palladium particle size by TEM failed because of the interference of the support (Jose´-Yacama´n et al., 1995). Consequently, in Table 1 the catalytic activities are presented as the kinetic constants, assuming a zero order kinetics (Fı´goli et al., 1995), as turn-over frequencies could not be calculated. The monometallic W/Al2O3 and Co/Al2O3 catalysts were absolutely inactive for the reaction studied under the conditions previously mentioned. Table 2 presents the XPS results for the fresh and used catalysts. The Pd 3d5/2 binding energy for the fresh and used monometallic Pd/Al2O3 corresponds to Pd0. A

Table 1. Hydrogen Chemisorption Capacity, H2 chem, of the Fresh Catalyst and Activity (Expressed as the Kinetic Constant, k) and Selectivity to Ethylbenzene (SE) for Pd/Al2O3, Pd-Co/Al2O3, and Pd-W/Al2O3 Catalysts Reduced at Different Temperatures (Tred), Using Unpoisoned and Poison-Containing Feeds catalyst

Tred (K)

H2 chem (µmol‚gcat-1)

393

339

Pd/Al2O3

Pd-Co/Al2O3

393

Pd-W/Al2O3

feed

k × 105 (s-1)

SE (%)

I II III

2.72 1.72 0.98

100 82 75

I II III

5.30 5.30 5.00

100 100 99

I II III

5.00 5.00 4.90

100 100 100

I II III

5.20 5.20 5.10

100 100 100

I II III

4.80 4.80 4.80

99 98 98

32

393

29

573

11

773

0

Table 2. XPS Results for the Different Catalysts catalyst Pd/Al2O3

Tred Pd 3d5/2 (K) feed (eV) 393 I II

Pd-Co/Al2O3 393 I III Pd-W/Al2O3

393 I III 573 I III 773 I III

335.1 335.1 335.2 335.6 335.7 335.8 335.8 335.9 336.1 335.7 335.8 335.9 335.3 335.4 335.6

S2p (eV)

164.4 164.3 164.5 164.4

Co/Pd W/Pd S/Pd (at./at.) (at./at.) (at./at.)

7.0 7.0 6.9

0.08 6.7 6.7 6.8 6.7 6.7 6.7 6.7 6.7 6.7

0.08 0.08 0.07

shift to higher binding energies in the Pd 3d5/2 peak position for both bimetallic catalysts can be observed; in the case of Pd-W/Al2O3, this shift is lower as the reduction temperature increases. The presence of phosphorus was not detected by XPS in the Pd-W/Al2O3 catalyst. According to the XPS results for the catalysts poisoned with feeds II and III, the poisoning species adsorbed is mainly molecular thiophene, as has previously been reported (L’Argentie`re et al., 1995b), thereby indicating that thiophene has not been hydrogenolyzed in the reaction conditions; this result is in accordance with the fact that H2S and butane have not been found either in the reaction products or in the gas phase. The Co/Pd and W/Pd superficial atomic ratios are higher than the theoretical values, thus suggesting the possibility of a decorative effect of Pd by Co and W species, respectively. The S/Pd atomic ratios indicate that not all palladium atoms have been affected by the poison molecules; thus, a residual activity after poisoning is possible. Table 3 presents the XPS results after different times of a 3.5 keV Ar+ bombardment for the Pd-Co/Al2O3 and Pd-W/Al2O3 catalysts reduced at 393 K. No peaks were detected by XRD at 23 < 2θ < 24.5, indicating that the tetragonal phase of WO3 is not

Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2545 Table 3. XPS Results for the Pd-Co/Al2O3 and Pd-W/ Al2O3 Catalysts Reduced at 393 K, after Ar+ Etching Using an Ion Source Operating at 3.5 keV, during Different Periods of Time catalyst

time (s)

Pd/Al (at./at.)

Co/Al (at./at.)

Pd-Co/Al2O3

0 120 240 600 0 120 240 600

0.03 0.04 0.05 0.05 0.03 0.04 0.06 0.06

0.21 0.15 0.10 0.10

Pd-W/Al2O3

W/Al (at./at.)

0.20 0.16 0.11 0.11

present, as can be expected from the WOx concentration of the catalyst, well below the monolayer (Salvati et al., 1981). Discussion The absence of signals in the 23 < 2θ < 24.5 region of the XRD spectra of Pd-W/Al2O3 is indicative that tungsten oxide is present as an amorphous phase. The decrease in hydrogen chemisorption when increasing reduction temperature could indicate a decorative effect of WOx species on palladium. The results presented in Table 1 clearly show that the bimetallic catalysts are more active and sulfur resistant than the monometallic one. It can also be seen that the Pd-W/Al2O3 and Pd-Co/Al2O3 catalysts are more selective in the presence of poison than Pd/Al2O3. The increase in activity cannot be ascribed to the independent effect of either W or Co because W/Al2O3 and Co/Al2O3 are not active for styrene selective hydrogenation. The Pd-W/Al2O3 catalyst is slightly more active and sulfur resistant after reduction at 573 K. It is also noticeable that the monometallic Pd/Al2O3 catalyst is severely poisoned using feed II (containing 100 ppm thiophene), while the other catalysts keep their activity even when a feed containing 300 ppm thiophene (III) is used. According to the XPS results, Pd in Pd/ Al2O3 is in the metallic state after reduction at 393 K, as can be expected for a catalyst prepared using Pd(NO3)2 as metal precursor. For the other catalysts, and considering the preparation conditions, an increase in the Pd binding energy (BE) can be attributed to an electron transfer from Pd to either W or Co rather than to the formation of Pd-Co and Pd-W bonds. Hence, Co and W interact with Pd, modifying its electronic properties. For other systems, such as Pd-Ni (L’Argentie`re et al., 1993) and Pd-Mn (L’Argentie`re et al., 1995a), such increase in the Pd BE was associated to a decrease in catalytic activity. The increase in activity over Pd-W/Al2O3 and Pd-Co/Al2O3 could be due to the existence of an interface Pd-Co and PdWOx more active for the styrene-selective hydrogenation. Farbokto et al. (1993) have also assigned the increase in activity in the Pt/WO3-Al2O3 system, mostly based on changes in reactivity rather than on structural determinations, to the existence of a very active interface. Paˆrvulescu et al. (1994), studying the Pd-Fe/Al2O3 system using EPR and Mo¨ssbauer spectroscopy, found that the interaction of Pd-Fe, due to the “d” character of the non-platinum metal, led to the formation of some centers of high electron density, which may be responsible for the higher activity of this catalyst compared with the monometallic Pd/Al2O3. While a quantitative effect for the promotional action of W and Co over catalytic activity and sulfur resistance has been pre-

sented, the specific nature of the sites and why they exhibit higher activity are still under study. The highest sulfur resistance of catalysts presenting such an interface may be due to electronic effects, although a geometric effect caused by the lower possibility of the poisoning molecule adsorption on the very active PdCo and Pd-W interface cannot be neglected. As presented in Table 1, all of the catalysts are 100% selective in the absence of thiophene. When evaluated with the poisoned feeds II and III the selectivity to ethylbenzene of the monometallic catalyst decrease substantially, while that of the bimetallic catalysts remains practically constant. The decrease of selectivity of the monometallic catalyst may be understood considering that the adsorption of thiophene may cause a partial electronic deficiency (Biloen et al., 1980); this is possible considering the electron-acceptor properties of sulfur on thiophene. Other authors (Mare´cot et al., 1993) stated that these electron transferences can modify the relative rates of superficial competitive reactions, thus modifying the selectivity. In the case of the bimetallic catalysts the interaction of the active sites with thiophene is weaker, and hence the superficial competitive reactions caused by the poison adsorption occur to a much lesser extent; hence, the selectivity to ethylbenzene remains practically constant in the presence of the poison. The very low hydrogen and carbon monoxide chemisorption capacities of the bimetallic catalysts reduced at 393 K and their absence at higher reduction temperatures can be ascribed to a decorative effect of Co and WOx moieties over Pd. In the case of Pd-W/Al2O3, this decorative effect increases with reduction temperature; the same was previously found by other authors for Pt-W (Contreras and Fuentes, 1996) and Pd-La (Arcoya et al., 1990) systems. However, the existence of a geometrical effect induced by palladium during impregnation of the second species cannot be neglected. But the existence of the decorative effect may be also suggested from the Ar+ sputtering results presented in Table 3. An increase in the Pd/Al ratios and a decrease in the Co/Pd and W/Al ratios, respectively, could be seen until a stationary state was achieved at 240 s; the composition remains constant when the ion bombardment goes on up to 600 s. It is well established in the literature (Betz and Wehner, 1984) that ion bombardment induces structural and chemical effects that may cause important changes in the superficial composition as well as in the electronic properties of the species remaining on the surface. Segregation and preferential sputtering have been detected and quantitatively characterized by XPS in certain systems (Gonza´lez-Elipe et al., 1989). However, and with these limitations in mind, the data displayed in Table 3 seem to support the idea of a decoration of the Pd-active species. Probably, the layers decorating Pd crystallites are thick enough to block hydrogen and carbon monoxide chemisorption but not thick enough to completely block the Pd XPS signal. Conclusion Bimetallic Pd-W/Al2O3 and Pd-Co/Al2O3 catalysts are more active and sulfur resistant than Pd/Al2O3 in the selective hydrogenation of styrene to ethylbenzene. The Pd-W/Al2O3 and Pd-Co/Al2O3 catalysts are also more selective in the presence of thiophene poison than the monometallic one. The increase in activity cannot be ascribed to the independent effect of either W or Co because W/Al2O3 and Co/Al2O3 are not active for the

2546 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997

reaction studied. The observed increase in activity and sulfur resistance may be ascribed to the formation of very active Pd-Co and Pd-W interfaces. A decorative effect of cobalt and tungsten on palladium is suggested. Acknowledgment The experimental assistance of C. A. Ma´zzaro and Eng. M. G. Can˜o´n are greatly acknowledged, as well of the donation of a XPS equipment by JICA. Literature Cited Anderson, J. A.; Daza, L.; Damyanova, S.; Fierro, J. L. G.; Rodrigo, M. T. Hydrogenation of styrene over nickel/sepiolite catalysts. Appl. Catal. A: General 1994, 113, 75. Arcoya, A.; Seoane, X. L.; Fı´goli, N. S.; L’Argentie`re, P. C. Relationship between sulphur resistance and electronic state of the metal on supported Palladium catalysts. Appl. Catal. 1990, 62, 35. Benson, J. E.; Hwang, H. S.; Boudart, M. Hydrogen-oxygen titration method for the measurement of supported palladium surface areas. J. Catal. 1973, 30, 146. Betz, G.; Wehner, G. K. Sputtering by particle bombardment II; Behrish, R., Ed.; Springer: Berlin, 1984; Chapter 2. Biloen, P.; Helle, J. N.; Verbeek, H.; Dautzenberg, F. M.; Sachtler W. M. H. The role of rhenium and sulfur in platinum based hydrocarbon-conversion Catalysts. J. Catal. 1980, 63, 112. Boitiaux, J. P.; Cosyns, J.; Verna, F. Poisoning of hydrogenation catalysts. How to cope with this general problem? Stud. Surf. Sci. Catal. 1987, 34, 105. Contreras, J. L.; Fuentes, G. A. Effect of tungnsten on supported platinum catalysts. In 11th International Congress on Catalysis, Studies in Surface Science and Catalysis; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Ed. Elsevier Science B.V.: Amsterdam, 1996; Vol. 101, p 1195. Farbotko, J. M.; Garin, F.; Girard, P.; Maire, G. Reactions of labelled hexanes on Pt-WO3/Al2O3 catalysts. J. Catal. 1993, 79, 185. Figoli, N. S.; L’Argentie`re, P. C. Selective Hydrogenation Catalysts. Influence of the Support on the Sulphur-Resistance. Catal. Today 1989, 5, 403. Fı´goli, N. S.; L’Argentie`re, P. C.; Arcoya, A;. Seoane, X. L. Modification of the properties and sulfur resistance of a Pd/ SiO2 catalyst by La addition. J. Catal. 1995, 155, 95. Gonza´lez-Elipe, A. R.; Munuera, G.; Espinos, J. P. Compositional changes induced by 3.5 keV Ar+ ion bombardment in Ni-Ti oxide systems. Surface Sci. 1989, 220, 368. Hegedus, L. L.; McCabe, R. W. Catalyst Poisoning. Catal. Rev. 1981, 23, 377. Jose´-Yacama´n, M.; Dı´az, G.; Go´mez, A. Electron microscopy of catalysts; the present, the future and the hopes. Catal. Today 1995, 23, 161.

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Received for review November 25, 1996 Revised manuscript received April 4, 1997 Accepted April 7, 1997X IE960743M

X Abstract published in Advance ACS Abstracts, May 15, 1997.