Effects of Cu over Pd based catalysts supported on silica or niobia

EFFECTS OF Cu OVER Pd BASED CATALYSTS SUPPORTED ON SILICA OR NIOBIA

M.N.S.C.Roma1^*, D.S.Cunha², G.M.Cruz¹ and A.J.G.Cobo3

¹Faculdade de Engenharia Química de Lorena, FAENQUIL, Rodovia Itajubá-Lorena,

km 74,5, 12600-000, Fax/Phone: [phone],

Lorena - SP, Brazil, E-mail: [e-mail]

² Laboratório Associado de Combustão e Propulsão (LCP), Instituto Nacional de

Pesquisas Espaciais (INPE), Cachoeira Paulista - SP, Brazil

³ Faculdade de Engenharia Química (FEQ), Universidade Estadual de Campinas,

UNICAMP, Campinas - SP, Brazil

(Received: January 6, 2000 ; Accepted: May 18, 2000)

Abstract - Palladium and palladium-copper catalysts supported on silica and niobia were characterized by H₂ chemisorption and H₂-O₂ titration. Systems over silica were also analyzed by transmission electron microscopy and EXAFS. The metallic dispersion decreased from 20% to 7% when the content of Pd was increased from 0.5wt.-% to 3wt.-% in monometallic catalysts. The addition of 3 wt.-% Cu to obtain Pd-Cu catalysts caused a remarkable capacity loss of hydrogen chemisorption. TPR analysis suggested an interaction between the two metals and EXAFS characterization of the catalyst supported on silica confirmed the formation of Pd-Cu alloy. Pd/Nb₂O₅ catalysts showed turnover numbers higher than those obtained with the Pd/SiO₂ systems in the cyclohexane dehydrogenation. However, the bimetallic catalysts showed very low turnover numbers.

Keywords: bimetallic catalysts, palladium-copper, niobia, silica, cyclohexane dehydrogenation.

INTRODUCTION

The combination of Group VIII-Group IB metals has been studied, Ni-Cu and Ru-Cu being the two systems of the most frequently investigated. The catalytic behavior of bimetallic systems is of interest since the influence of the second metal creates important changes on noble metals catalytic and adsorptive properties (Sinfelt et al., 1972). In the last years, an important number of studies have dealt with metal-metal as well as metal-support interactions found in Pd-Cu catalyst systems (Leon y Leon and Vannice, 1991; Pereira et al., 1993; Renouprez et al., 1997). In general, the strong metal-support interaction (SMSI) effect has a great influence on structure sensitive reactions but only a minor effect on structure insensitive reactions (Kunimori et al., 1990). Pereira et al. (1993) studied Pd-Cu bimetallic catalysts supported on niobia in the 1,3–butadiene hydrogenation and observed that the presence of copper inhibits the SMSI formation. However, small additions of Cu blocked the active sites and strongly decrease the turnover frequency.

The aim of this work was to study the behavior of palladium catalysts, modified by the addition of copper using two different supports, silica and niobia. The catalysts were characterized by hydrogen chemisorption, oxygen-hydrogen titration, transmission electron microscopy; temperature programmed reduction and extended X-ray absorption fine-structure. The catalytic systems were evaluated in the cyclohexane dehydrogenation reaction.

EXPERIMENTAL PART

Catalyst Preparation

The supported catalysts were prepared with two palladium loadings (0.5 wt.-% and 3 wt.-%) and constant copper content (3 wt.-%). The supports were silica (Aerosil 200 from Degussa, BET area: 200m²/g) and niobium pentoxide (Nb₂O₅-HY 340 from Companhia Brasileira de Metalurgia e Mineração – CBMM). The niobium pentoxide was calcined at 873 K for 3 h (BET area: 17 m²/g) before the impregnation. The metal precursors were palladium chloride (Aldrich, 99% p.a.) and copper sulphate (CuSO₄.5H₂O from CAAL, 99% p.a.). The precursors were deposited on support by incipient impregnation (and coimpregnation) method. After impregnation, the materials were dried in air at 373 K during two hours and then heated up to 573 K under N₂. At this temperature the materials were reduced by hydrogen flow during four hours.

Catalyst Characterization

(a) Hydrogen Chemisorption(HC) And Oxygen-Hydrogen Titration (HT)

Adsorption experiments were conducted in a volumetric chemisorption apparatus (Roma, 1999). Catalysts samples in the range from 0.5 to 1.5 g were placed in an evacuable gas cell. Then, the catalysts were reduced in 50 cm³/min hydrogen for one hour at 573 K. After reduction, the catalysts were evacuated at the same reduction temperature at 1.10^-5 torr (1 torr =133N/m²) for one hour. Then, the catalysts were cooled to 343 K and hydrogen pressure increased to about 100 torr, which was determined as the maximum pressure according to previous work (Roma, 1999). The adsorption temperature was chosen to avoid b-PdH_x formation, according to Aben (1968). These isotherms were consistent with the expected variation of the a®b phase transition pressure for this adsorption temperature (Ragaini et al. 1994). The amount of irreversible adsorbed hydrogen (HC) was measured by the difference between the total and the reversible uptakes, and the obtained value represents H chemisorbed on the Pd surface since no irreversible adsorption occurs neither on the supports nor on the Cu⁰. Absorption of hydrogen into Pd is reversible under these conditions (Boudart and Hwang, 1975). The palladium dispersion (D), defined as the ratio of Pd surface atoms to the total number of the Pd atoms, was determined for each catalyst assuming the usual ratio H/Pd*=1 (Aben, 1968), where Pd* is the symbol used to Pd surface atoms. After hydrogen chemisorption, the catalyst was evacuated at 43 K under 1.10^-5 torr for one hour, helium was used to calculate the dead volume. Then, the catalyst was cooled down to room temperature and evacuated for 30 min before the oxygen adsorption. After O₂ adsorption at room temperature, the catalyst was evacuated for 30 min and heated to 343 K. Irreversible hydrogen uptake was determined using the same procedure as for hydrogen chemisorption. The amount of hydrogen consumed at titration experiment should be three times higher than the amount of hydrogen chemisorbed, according to the following equations:

Pd* + ½ O₂® Pd*-O

(1)

Pd*-O + 3/2 H₂® Pd*-H + H₂O

(2)

(b) Temperature-Programmed Reduction (TPR)

After oxidation of the reduced catalysts at 573 K during 3 hours, the samples were purged under nitrogen flow, and cooled by immersing the reactor into a dry ice-ethanol bath (~213 K). At about 213 K the gas flow was switched to 2% H₂/N₂ (30 cm³/min) and increased in the rate of 10 K/min up to 773 K. Throughout the TPR, hydrogen uptake was monitored by a computer interfaced with a thermal conductivity detector. A trap of molecular sieve ensured the water retention, which was removed prior to the detector, so that only hydrogen consumption or desorption was measured.

(c) Transmission Electron Microscopy (TEM)

TEM measurements were made on a JEOL microscope (JSM 840 A). Samples were suspended in water and a drop of suspension placed on a carbon-coated copper grid. The electron microscope was operated at magnifications in the region of 80,000-100,000x (further magnification, 350,000x, was achieved by photographic enlargement). Particle size distributions and the average size of metallic particles were obtained by statistical treatment of the micrographs using about 400 particles per catalyst.

(d) Extended X-ray Absorption Fine-Structure (EXAFS)

An X-ray absorption fine-structure spectroscopy beamline has been installed and commissioned at a bending-magnet source at LNLS (Laboratório Nacional de Luz Síncroton in Campinas, Brazil). The LNLS synchrotron radiation source is composed of a 1.37 GeV storage ring and reached 100 mA (Tolentino et al., 1998). The method of analysis used for the EXAFS of a binary system is based on procedures developed by Sinfelt et al.(1981). In the present study, the EXAFS was obtained by X-ray absorption spectroscopy in the transmission mode at the Pd K-edge (24350eV) and the Cu K-edge (8979 eV). The normalized EXAFS function, c (k ), was calculated from the measured X-ray absorption spectrum above the absorption threshold (at energy E₀) for palladium. All experiments were performed under ambient conditions, the samples were placed in the EXAFS sample holder (3 to 11 mm of thickness) with Kapton windows; the thickness of the samples was adjusted so that the edge jump was @1.5. The EXAFS data were weighted by k³and Fourier transformed (FT) over a range of k between 3 and 13 Å^-1. EXAFS oscillations from the first coordination shell were filtered by inverse FT between 1.7 and 3 Å, and then fitted with the experimental phases and amplitudes of Pd-Pd, Cu-Cu, and Pd-Cu bonds. The amplitude and phase functions were determined from the reference materials, Pd and Cu foils and a Pd₅₀Cu₅₀ alloy. The theoretical values for the Pd-O and Cu-O bonds were obtained from McKale Tables (McKale et al., 1988).

(e) Cyclohexane Dehydrogenation Reaction

The catalysts were tested in the cyclohexane dehydrogenation reaction at 573 K by using a differential dynamic microreator at atmospheric pressure. Prior to any activity measurement the catalysts were reactivated at 573 K for 2 h under hydrogen flow. The reactant mixture was obtained from one saturator-condenser system containing cyclohexane at 285 K through hydrogen flow using Phydrogen/Pcyclohexane ratio of 14. The total flow was in the range of 100 to 120 cm³/min, depending whether the kind of the supported catalyst used (silica or niobia). Reaction products were analyzed by gas chromatography (FID detector). Under these conditions, 573 K was the optimal temperature to obtain a sufficient conversion without reaching the thermodynamic limitation (80% at 573 K, Rogemond et al., 1997). The conversion values were limited to 10% and benzene was the only product formed.

RESULTS AND DISCUSSION

Determination of Average Size of Metallic Particles

Table 1 shows the number of active sites per gram of catalyst (Y), palladium dispersion (D), and average diameter of metallic particles (dp) determined by gas chemisorption, hydrogen chemisorption (HC) and oxygen-hydrogen titration (HT) and by transmission electron microscopy (TEM).

The obtained results using the two methods of gas chemisorption were similar and showed deviation of ±5%. As can be seen the dispersion of monometallic Pd catalysts decreased when the loading of Pd increased, showing about 20% and 7% of dispersion for the catalysts containing 0.5wt.-% and 3 wt.-% of Pd, respectively. Although the supports had different surface areas (17 m²/g for niobia and 200 m²/g for silica), the dispersion values seen not be depended on the support used. These results suggest different interactions between the Pd precursor and the supports, and this could explain the same dispersion found for the catalyst supported on niobia in spite of its low surface area.

The average particle size for the catalysts determined by chemisorption and TEM measurements were consistent. It should be noted that the apparatus used for the analysis by TEM had poor magnification power (350,000x), which made difficult to detected size of particles minor than 3nm, especially for catalysts containing low Pd loading. Therefore, the particle size value for 0.5% Pd/SiO₂ catalysts could be lower than 5 nm, allowing to assume that the most correct value should be the one obtained by hydrogen chemisorptions (~ 4.1 nm).

The addition of Cu on Pd modified the hydrogen adsorption properties of the noble metal, the results of hydrogen chemisorbed on the bimetallic catalyst was remarkably decreased. Results obtained by TEM showed the formation of large particles for the bimetallic catalyst supported on silica (~ 18.4 nm).

Catalytic Activity

Table 2 shows the specific velocity (V) and turnover frequency (TOF) values obtained in the cyclohexane dehydrogenation reaction at 573 K using Pd monometallic catalysts. Under these conditions, bimetallic catalysts showed no activity. Also, Cu supported catalysts were inactive in this reaction.

It is well known from the literature that the cyclohexane dehydrogenation is considered as structure insensitive in the case of supported metal catalysts (Cusumano et al., 1966) and it was observed that the activity could change with the support (Aramendia et al., 1995). As can be seen when Pd was supported over niobia the activity was increased (2.1± 0.2 s^-1), while the turnover number for Pd catalysts supported on silica was constant (1.7 s^-1). This performance for Pd/Nb₂O₅ catalysts suggest an interaction metal-support, which could promote an activity increase.

In order to determine the influence of niobia on catalyst activity, tests were performed with 3% Pd/Nb₂O₅ catalyst in the temperature range from 543 K to 583 K. The apparent activation energy was found to be close to 67 kJ/mol, showing an expected value when noble metal is used under these conditions (Rogemond et al, 1997).

The bimetallic catalysts showed no activity in the cyclohexane dehydrogenation and this behavior could be attributed either to Pd-Cu interaction or to Cu in excess which could cover completely the Pd.

Pd-Cu Interaction

Some TPR profiles obtained with silica supported reduced catalysts (oxidized before this analysis) are presented in Figure 1. The TPR profile of 3% Pd/SiO₂ catalyst showed only one peak with a maximum at 340 K, but data from the literature indicates that palladium oxide reduces at room temperature or lower than that. The reduction profile for 3% Cu/SiO₂ catalyst is also shown in Fig.1 where it can be noticed one shoulder at 520 K and a maximum peak detected at 550 K.

In the case of bimetallic catalysts 3%Pd -3%Cu/SiO₂ (Fig.1) a large region of hydrogen consumption between 350 K and 575 K can be observed with a maximum at 525 K. Anderson and Pratt (1985) attributed the existence of interaction between the two metals when the TPR profile of the bimetallic catalyst is different from the linear combination of the two single component profiles.

The TPR profiles for the catalysts supported on niobia are presented in Figure 2. For Pd/Nb₂O₅, there is a hydrogen uptake at 275 K that can be attributed to a reduction of palladium oxide, and the small hydrogen consumption at 550 K could be associated to the niobia reduction. The TPR profile obtained with Cu/Nb₂O₅ showed a low hydrogen uptake at 550 K and high H₂ consumption at 725 K. The bimetallic catalyst presented two large peaks at 370 K and 575 K. The comparison of the three systems revealed that the reduction of palladium oxide occurred at higher temperature when Cu was presented, however the reduction of copper was facilitated when Pd was presented.

According to Gates (1992) the most helpful physical characterization method is EXAFS spectroscopy. This method takes the advantage of backscattering of photoelectrons emitted from atoms absorbing X-radiation giving precise structural information about the atoms in the immediate neighborhood of the absorbing atom. In this work, X-ray absorption measurements were performed for Pd K-edge (24350 eV) and these were the first results obtained at Laboratório Nacional de Luz Sincroton using XAFS beamline higher than 15000 eV (Tolentino et al., 1998). The EXAFS functions of 3% Pd/SiO₂ and the bimetallic systems were presented in Figure 3 and their Fourier transforms are displayed in Figure 4.

Theoretical fittings for the experimental data in the inverse Fourier transforms are shown in Figures 5 and 6 for 3%Pd/SiO₂ and 3%Pd-3%Cu/SiO₂ catalysts, respectively.

The fitted parameters are listed in Table 3. In the presentation of EXAFS results, the average number of neighbor atoms forming bonds with a central X-ray absorbing atom is generally referred to as coordination number N. In the present study, Pd atoms are absorbers. The fitting of parameters such as bond lengths (R) and coordination numbers (N) for Pd-Pd and Pd-Cu bonds showed a good quality as can be seen in Figures 5 and 6 (and low values for Debye-Waller factors, Ds²).

The monometallic catalyst (3% Pd/SiO₂) showed values for coordination number (N = 7.4) and the distance Pd-Pd (R = 2.73 Å) in agreement with the results published by Faudon et al.(1993) using Pd supported on silica systems. The low coordination number (N = 7.4) obtained for the catalyst when compared to the standard Pd foil (N =12 ) indicated that Pd atoms are dispersed on the support.

The main results were those obtained for the bimetallic system (3%Pd-3%Cu/SiO₂) which showed that Pd atoms were bonded with both Cu (N_Pd-Cu = 1.4) and Pd atoms (N_Pd-Pd = 6.4). These results confirmed the formation of bimetallic particles Pd-Cu, and the existence of an alloy rich in Pd.

Further studies are still necessary to confirm whether the surface of bimetallic particles is enriched with Cu atoms. In our preliminary studies at Cu K-edge (Roma, 1999) was observed that Cu atoms were bonded with Pd (N_Cu-Pd = 0.28) and oxygen atoms (N_Cu-O = 4.6). These results showed that total coordination number for Cu (N_T = 4.88) was lower than Pd (N_T = 7.8), that could indicate the existence of an enrichment surface in Cu atoms. The Cu oxidation occurred under atmospheric air since the sample holder was not suitable to perform this analysis in controlled-atmosphere. However, fittings for adjusting experimental data for EXAFS at Pd K-edge was not necessary for Pd-O bonds, showing that palladium was not oxided.

CONCLUSIONS

The present study showed that the Pd reduction was easier attained when this element was supported on niobia rather than silica. The Pd/Nb₂O₅ systems were more active than Pd/SiO₂ in the cyclohexane dehydrogenation reaction. The addition of copper on palladium supported on silica and niobia carriers changed the adsorptive and physicochemical properties of palladium. Each element influenced the reducibility of the other, palladium had a positive effect while copper had an inhibitory effect on the reduction. The lack of activity for the bimetallic systems in the cyclohexane dehydrogenation reaction can be attributed both to the surface of the bimetal particles which had Cu atoms in excess and Pd-Cu interaction as confirmed by EXAFS.

ACKNOWLEDGEMENTS

We would like to thank Dr Hélio Tolentino and Drª. Maria do Carmo Martins Alves at LNLS, as well as Dr. Pedro K. Kiyohara and Drª. Marina Silveira at Instituto de Física-USP and Dr. Gustavo Paim Valença at FEQ-UNICAMP, for their collaborations.

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To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  16 Mar 2001
• Date of issue
  Dec 2000

History

• Received
  06 Jan 2000
• Accepted
  18 May 2000