Pd-W and Pd-Co Bimetallic Catalysts’ Sulfur Resistance forSelective Hydrogenation

Pablo C. L’Argentiere and Nora S. Fıgoli*

INCAPE, Instituto de Investigaciones en Catalisis y Petroquımica (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, andW/V ) 0.002 g cm-3 has been studied. Catalysts were characterized byXPS, XRD, TEM, and hydrogen and carbon monoxide chemisorption. An increase in activityand sulfur resistance was observed after both cobalt and tungsten addition, which is ascribedto the formation of very active Pd-Co and Pd-W interfaces. A decorative effect of cobalt andtungsten on palladium is suggested.

Introduction

Palladium-based catalysts are highly active and se-lective in several important commercial reactions, suchas hydrocarbon hydrogenation. One of the major prob-lems with palladium is the high sensitivity to sulfurcompounds usually present in hydrogenation feedstocks(Boitiaux et al., 1987). In the 1980’s it was wellestablished that the addition of a second metal may bea useful way to increase the activity and the sulfurresistance of palladium for these reactions (Hegedus andMcCabe, 1981; Mallat et al., 1985).In previous papers, we have reported the influence

of Ni (L’Argentiere et al., 1993), Mn (L’Argentiere et al.,1995a), and La (Fıgoli et al., 1995) on palladium-basedcatalysts activity and sulfur resistance during selectivehydrogenation reactions; we have found that the inter-action of Pd with those metals creates and stabilizeselectron-deficient species (Pdn+) that modify not only theactivity but also the sulfur resistance of the catalysts.The purpose of this work is to study the modificationsproduced by W and Co on both the properties of Pd/Al2O3 catalyst and its resistance toward thiophene.Several papers have been published about the influenceof W on the behavior of platinum catalysts (Regalbutoet al., 1987; Contreras and Fuentes, 1996), but, to ourknowledge, less attention has been devoted to the effectof W and Co on the catalytic properties of palladium inhydrocarbon hydrogenation reactions. The catalyticproperties were studied using the selective hydrogena-tion of styrene to ethylbenzene as test reaction andthiophene as poisoning reagent. Styrene is a usefulmodel compound to investigate the catalytic perfor-mance of metal catalysts at a laboratory scale due tothe presence of two types of unsaturated groups (Ander-son et al., 1994). Thiophene is usually taken as a modelcompound for sulfur resistance studies (Palinko, 1994).

Experimental Section

Catalysts Preparation. Alumina CK 300, cylindersof 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 assupport. It was calcined in air at 773 K for 3 h prior tocatalyst preparation. An aqueous solution of Pd(NO3)2was used for impregnation in a suitable concentrationso as to obtain a catalyst containing 0.3% palladium.

The catalyst was then dried and calcined at 623 K for 3h within an air flow rate of 480 mL h-1 gcat-1 and afterthis reduced for 5 h at 393 K with a hydrogen flow rateof 480 mL h-1 gcat-1.Pd-W/Al2O3 was prepared by wet impregnation of the

Pd/Al2O3 catalyst with an aqueous H3PO4‚12WO3 solu-tion of an appropriate concentration so as to obtain aW/Pd atomic ratio of 6 during 1 h. After being washedwith bidistilled water, the solid was dried at 383 Kduring 24 h and calcined at 823 K during 7 h. Thecalcination temperature used assures the completeelimination 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 toobtain a Co/Pd atomic ratio of 6. The sample was driedat 383 K during 24 h, calcined at 823 K for 7 h, andreduced in hydrogen at 573 K during 5 h.Monometallic W/Al2O3 and Co/Al2O3 catalysts were

also prepared by wet impregnation of Al2O3 with aque-ous solutions of H3PO4‚12WO3 or Co(NO3)2‚6H2O, re-spectively, in appropriate concentrations so as to obtainthe same amount of metal as in the correspondingbimetallic catalysts.Catalyst Characterization. Dispersion. The dis-

persion was determined by hydrogen chemisorptionfollowing the method of Benson et al. (1973), in orderto ensure that the samples were free from the formationof â-PdH. In order to compare with the previous results,dispersion was also measured by CO chemisorptionfollowing the technique described by Martin et al. (1985)using a COads/Pd stoichiometry of 1.15/1 as previouslyreported (Fıgoli et al., 1995).X-ray Photoelectron Spectroscopy (XPS) Analysis.

XPS analyses were carried out on a Shimadzu ESCA750 electron spectrometer coupled to a Shimadzu ES-CAPAC 760 Data System. The C 1s line was taken asan internal standard at 285.0 eV and was used to correctpossible deviations caused by an electric charge of thesamples. The position of the maximum of the Pd 3d5/2peak was used to follow the superficial electronic stateof palladium. After reduction, the samples were intro-duced into the preparation chamber of the XPS equip-ment following an operation procedure previously de-scribed (Arcoya et al., 1990). The possibility that,during thermal treatments, the palladium particlesbecame decorated by the other metals species was

* To whom correspondence should be addressed. Tel: 54 42528062. Fax: 54 42 531068. E-mail: nfigoli@fiqus.unl.edu.ar.

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

S0888-5885(96)00743-9 CCC: $14.00 © 1997 American Chemical Society

investigated by ion sputtering using an Ar+ ion sourceat 3.5 keV. Before the Ar+ etching the samples wereheated in vacuum (823 K) in order to remove contami-nants from their surfaces. The determinations werecarried out in the preparation chamber of the XPSequipment for different periods of time, and after eachone, an XPS determination was performed in order todetermine the variation of the atomic ratios. Thisprocedure was repeated until a stationary state wasreached for the values of the metallic atomic ratios.Transmission Electron Microscopy (TEM) Determina-

tions. The Pd-Co/Al2O3 and Pd-W/Al2O3 catalystswere examined in a Jeol JMS-100 CS II electronmicroscope in order to determine the Pd particle size,following an experimental procedure previously reported(Figoli and L’Argentiere, 1989).X-ray Diffraction Spectroscopy (XRD). The crystalline

structure was analyzed by X-ray diffraction using Shi-madzu XD-D1 equipment in the 20-70° 2θ range, usingKR Cu radiation and /2/2/0.6 windows.Catalytic Activity and Selectivity. Catalytic activity

and selectivity were determined in a stirred tank reactoroperated at 22 kg cm-2 hydrogen pressure,W/V ) 0.002g cm-3, 353 K and stirring velocity 700 rpm; 1 g ofcatalyst was used for each experiment. No diffusionallimitations were observed under these conditions. Dur-ing each run the reactor was connected to a hydrogencylinder, and a constant pressure was maintainedthroughout the experiment. The selective hydrogena-tion of styrene to ethylbenzene was used as a testreaction. As previously reported (L’Argentiere et al.,1990), ethylbenzene can be further hydrogenated toethylcyclohexane, and polymerization of styrene can alsooccur as a side reaction. A 5% styrene in toluenesolution (I) or the same solution with the addition ofeither 100 ppm thiophene (II) or 300 ppm thiophene (III)were used as feed. Reactants and products werechromatographically analyzed by means of a flameionization detector and a DC 200 column. The catalyticactivity and selectivity to ethylbenzene were calculatedfrom the chromatographic data; benzene was added tothe reaction mixture as an internal standard to followthe polymerization consecutive reaction.

Results

In Table 1 are presented the hydrogen chemisorptioncapacities of the catalysts. Palladium dispersion for thePd/Al2O3 catalyst is 24%, calculated from either hydro-gen or carbon monoxide chemisorption data. Thehydrogen chemisorption capacity of the Pd-Co/Al2O3and Pd-W/Al2O3 catalysts reduced at 393 K was verylow (32 and 29 µmol‚gcat-1, respectively), and in the caseof Pd-W/Al2O3 it decreased dramatically when thereduction temperature was raised up to 773 K. Asimilar result was obtained for carbon monoxide chemi-sorption. Furthermore, an attempt to determine thepalladium particle size by TEM failed because of theinterference of the support (Jose-Yacaman et al., 1995).Consequently, in Table 1 the catalytic activities arepresented as the kinetic constants, assuming a zeroorder kinetics (Fıgoli et al., 1995), as turn-over frequen-cies could not be calculated. The monometallic W/Al2O3and Co/Al2O3 catalysts were absolutely inactive for thereaction studied under the conditions previously men-tioned.Table 2 presents the XPS results for the fresh and

used catalysts. The Pd 3d5/2 binding energy for the freshand used monometallic Pd/Al2O3 corresponds to Pd0. A

shift to higher binding energies in the Pd 3d5/2 peakposition for both bimetallic catalysts can be observed;in the case of Pd-W/Al2O3, this shift is lower as thereduction temperature increases. The presence of phos-phorus was not detected by XPS in the Pd-W/Al2O3catalyst. According to the XPS results for the catalystspoisoned with feeds II and III, the poisoning speciesadsorbed is mainly molecular thiophene, as has previ-ously been reported (L’Argentiere et al., 1995b), therebyindicating that thiophene has not been hydrogenolyzedin the reaction conditions; this result is in accordancewith the fact that H2S and butane have not been foundeither in the reaction products or in the gas phase. TheCo/Pd and W/Pd superficial atomic ratios are higherthan the theoretical values, thus suggesting the pos-sibility of a decorative effect of Pd by Co and W species,respectively. The S/Pd atomic ratios indicate that notall palladium atoms have been affected by the poisonmolecules; thus, a residual activity after poisoning ispossible.Table 3 presents the XPS results after different times

of a 3.5 keV Ar+ bombardment for the Pd-Co/Al2O3 andPd-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

Table 1. Hydrogen Chemisorption Capacity, H2 chem, ofthe Fresh Catalyst and Activity (Expressed as theKinetic Constant, k) and Selectivity to Ethylbenzene (SE)for Pd/Al2O3, Pd-Co/Al2O3, and Pd-W/Al2O3 CatalystsReduced at Different Temperatures (Tred), UsingUnpoisoned and Poison-Containing Feeds

catalyst Tred (K)H2 chem

(µmol‚gcat-1) feedk × 105(s-1) SE (%)

Pd/Al2O3 393 339I 2.72 100II 1.72 82III 0.98 75

Pd-Co/Al2O3 393 32I 5.30 100II 5.30 100III 5.00 99

Pd-W/Al2O3 393 29I 5.00 100II 5.00 100III 4.90 100

573 11I 5.20 100II 5.20 100III 5.10 100

773 0I 4.80 99II 4.80 98III 4.80 98

Table 2. XPS Results for the Different Catalysts

catalystTred(K) feed

Pd 3d5/2(eV)

S2p(eV)

Co/Pd(at./at.)

W/Pd(at./at.)

S/Pd(at./at.)

Pd/Al2O3 393 335.1I 335.1II 335.2

Pd-Co/Al2O3 393 335.6 7.0I 335.7 7.0III 335.8 164.4 6.9 0.08

Pd-W/Al2O3 393 335.8 6.7I 335.9 6.7III 336.1 164.3 6.8 0.08

573 335.7 6.7I 335.8 6.7III 335.9 164.5 6.7 0.08

773 335.3 6.7I 335.4 6.7III 335.6 164.4 6.7 0.07

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

present, as can be expected from the WOx concentrationof the catalyst, well below the monolayer (Salvati et al.,1981).

Discussion

The absence of signals in the 23 < 2θ < 24.5 regionof the XRD spectra of Pd-W/Al2O3 is indicative thattungsten oxide is present as an amorphous phase. Thedecrease in hydrogen chemisorption when increasingreduction temperature could indicate a decorative effectof WOx species on palladium.The results presented in Table 1 clearly show that

the bimetallic catalysts are more active and sulfurresistant than the monometallic one. It can also be seenthat the Pd-W/Al2O3 and Pd-Co/Al2O3 catalysts aremore selective in the presence of poison than Pd/Al2O3.The increase in activity cannot be ascribed to theindependent effect of either W or Co because W/Al2O3and Co/Al2O3 are not active for styrene selective hydro-genation. The Pd-W/Al2O3 catalyst is slightly moreactive and sulfur resistant after reduction at 573 K. Itis also noticeable that the monometallic Pd/Al2O3 cata-lyst is severely poisoned using feed II (containing 100ppm thiophene), while the other catalysts keep theiractivity 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, andconsidering the preparation conditions, an increase inthe Pd binding energy (BE) can be attributed to anelectron transfer from Pd to either W or Co rather thanto the formation of Pd-Co and Pd-W bonds. Hence,Co and W interact with Pd, modifying its electronicproperties. For other systems, such as Pd-Ni(L’Argentiere et al., 1993) and Pd-Mn (L’Argentiere etal., 1995a), such increase in the Pd BE was associatedto a decrease in catalytic activity. The increase inactivity over Pd-W/Al2O3 and Pd-Co/Al2O3 could bedue to the existence of an interface Pd-Co and Pd-WOx more active for the styrene-selective hydrogena-tion. Farbokto et al. (1993) have also assigned theincrease in activity in the Pt/WO3-Al2O3 system, mostlybased on changes in reactivity rather than on structuraldeterminations, to the existence of a very active inter-face. Parvulescu et al. (1994), studying the Pd-Fe/Al2O3system using EPR and Mossbauer spectroscopy, foundthat the interaction of Pd-Fe, due to the “d” characterof the non-platinum metal, led to the formation of somecenters of high electron density, which may be respon-sible for the higher activity of this catalyst comparedwith the monometallic Pd/Al2O3. While a quantitativeeffect for the promotional action of W and Co overcatalytic activity and sulfur resistance has been pre-

sented, the specific nature of the sites and why theyexhibit higher activity are still under study. Thehighest sulfur resistance of catalysts presenting suchan interface may be due to electronic effects, althougha geometric effect caused by the lower possibility of thepoisoning molecule adsorption on the very active Pd-Co 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 evaluatedwith the poisoned feeds II and III the selectivity toethylbenzene of the monometallic catalyst decreasesubstantially, while that of the bimetallic catalystsremains practically constant. The decrease of selectivityof the monometallic catalyst may be understood con-sidering that the adsorption of thiophene may cause apartial electronic deficiency (Biloen et al., 1980); this ispossible considering the electron-acceptor properties ofsulfur on thiophene. Other authors (Marecot et al.,1993) stated that these electron transferences canmodify the relative rates of superficial competitivereactions, thus modifying the selectivity. In the caseof the bimetallic catalysts the interaction of the activesites with thiophene is weaker, and hence the superficialcompetitive reactions caused by the poison adsorptionoccur to a much lesser extent; hence, the selectivity toethylbenzene remains practically constant in the pres-ence of the poison.The very low hydrogen and carbon monoxide chemi-

sorption capacities of the bimetallic catalysts reducedat 393 K and their absence at higher reduction temper-atures can be ascribed to a decorative effect of Co andWOxmoieties over Pd. In the case of Pd-W/Al2O3, thisdecorative effect increases with reduction temperature;the same was previously found by other authors forPt-W (Contreras and Fuentes, 1996) and Pd-La (Ar-coya et al., 1990) systems. However, the existence of ageometrical effect induced by palladium during impreg-nation of the second species cannot be neglected. Butthe existence of the decorative effect may be alsosuggested from the Ar+ sputtering results presented inTable 3. An increase in the Pd/Al ratios and a decreasein the Co/Pd andW/Al ratios, respectively, could be seenuntil a stationary state was achieved at 240 s; thecomposition remains constant when the ion bombard-ment goes on up to 600 s. It is well established in theliterature (Betz and Wehner, 1984) that ion bombard-ment induces structural and chemical effects that maycause important changes in the superficial compositionas well as in the electronic properties of the speciesremaining on the surface. Segregation and preferentialsputtering have been detected and quantitatively char-acterized by XPS in certain systems (Gonzalez-Elipe etal., 1989). However, and with these limitations in mind,the data displayed in Table 3 seem to support the ideaof a decoration of the Pd-active species. Probably, thelayers decorating Pd crystallites are thick enough toblock hydrogen and carbon monoxide chemisorption butnot thick enough to completely block the Pd XPS signal.

Conclusion

Bimetallic Pd-W/Al2O3 and Pd-Co/Al2O3 catalystsare more active and sulfur resistant than Pd/Al2O3 inthe selective hydrogenation of styrene to ethylbenzene.The Pd-W/Al2O3 and Pd-Co/Al2O3 catalysts are alsomore selective in the presence of thiophene poison thanthe monometallic one. The increase in activity cannotbe ascribed to the independent effect of either W or Cobecause W/Al2O3 and Co/Al2O3 are not active for the

Table 3. XPS Results for the Pd-Co/Al2O3 and Pd-W/Al2O3 Catalysts Reduced at 393 K, after Ar+ EtchingUsing an Ion Source Operating at 3.5 keV, duringDifferent Periods of Time

catalyst time (s)Pd/Al(at./at.)

Co/Al(at./at.)

W/Al(at./at.)

Pd-Co/Al2O3 0 0.03 0.21120 0.04 0.15240 0.05 0.10600 0.05 0.10

Pd-W/Al2O3 0 0.03 0.20120 0.04 0.16240 0.06 0.11600 0.06 0.11

Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2545

reaction studied. The observed increase in activity andsulfur resistance may be ascribed to the formation ofvery active Pd-Co and Pd-W interfaces. A decorativeeffect of cobalt and tungsten on palladium is suggested.

Acknowledgment

The experimental assistance of C. A. Mazzaro andEng. M. G. Canon are greatly acknowledged, as well ofthe donation of a XPS equipment by JICA.

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Received for review November 25, 1996Revised manuscript received April 4, 1997

Accepted April 7, 1997X

IE960743M

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

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