Supported PdCl2CuCl2 catalysts for carbon monoxide oxidation II. XAFS characterization

B ENVIRONMENTAL ELSEVIER

Applied Catalysis B: Environmental 7 (1996)

199-212

Supported PdCl,-CuCl, catalysts for carbon monoxide oxidation II. XAFS characterization Jae Sung Lee aT*,Sun Hee Choi a, Ki Dong Kim a, Masaharu Nomura b

Received 13 February 1995; revised 7 August 1995: accepted 7 August 1995

Abstract A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCI-CuC& was active for the oxidation of CO near room temperature. The structure of the catalyst was studied by X-ray absorption fine structure (XAFS). The active phase of Pd was a Pd” species containing chlorine and, probably, carbonyl ligands. Direct interaction of Pd-Pd or Pd-Cu was not detected. The active phase of copper was found to be solid Cu,Cl( OH)3 particles in agreement with the X-ray diffraction (XRD) results. The presence of Cu was essential to keep the Pd in the Pd” state during the reaction. The rates of CO oxidation measured at temperatures of 30-70°C showed a minimum at 40°C. which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature. Kr~words:

Carbon monoxide oxidation: PdU-CuClz/alumina;

XAFS; Structure

1. Introduction Carbon monoxide is a common air pollutant present in exhausts of many combustion systems and oxidation reactors. Its removal by oxidation using noble metal catalysts [ l-71 requires high reaction temperatures above 300°C and, as a result, consumes a large amount of energy to heat the vent stream to the reaction temperature. Metal oxide catalysts require much lower temperatures, yet are readily deactivated by water present in essentially all vent streams containing CO [ 8-101. * Corresponding author. Tel. 0926.3373/96/$15.00

( + 82-562)

2792266,

fax. ( + 82-562)

2795799

0 1996 Elsevier Science B.V. All rights reserved

SSD10926.3373(95)00043-7

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Recently, several new heterogeneous catalysts including Au/metal oxides [ 11,121, Pt-Sn [ 131, have been reported to be active for low temperature CO oxidation in the presence of water. In a companion paper [ 141, we demonstrated that in agreement with previous reports [ 15,161, a heterogenized Wacker system, in which an aqueous solution of PdCl,-CuCl, was loaded into the pores of high surface area alumina or carbon, oxidized CO near room temperature for an extended period as long as water was supplied with the reactants. The effects of catalyst composition and of reaction conditions were discussed. An interesting observation was made in the X-ray diffraction (XRD) study of the catalysts. Unlike homogeneous Wacker systems in which molecular Pd” and Cu” are active catalytic species [ 17-201, the active phase of copper in the heterogenized Wacker system was found to be solid Cu,Cl (OH), particles which were formed by an interaction between CuCl, and the support. The palladium species was not seen by XRD. In the present work, we performed an Xray absorption fine structure (XAFS) study in order to elucidate the structure and the chemical nature of the catalyst under the reaction conditions. Compared with XRD which provides information only about long range structural orders, XAFS is sensitive to local structure and bonding surrounding the X-ray absorbing atoms in the sample and should be more appropriate for this type of catalysts in indicating the average environment of most of the catalytic species. Furthermore, XAFS experiments were performed on catalyst samples contained in a controlled atmosphere cell under conditions close to those employed for the kinetic study. Hence, the results should be more relevant to real catalysts in contrast with those from the XRD experiments in which the samples were exposed to the atmosphere.

2. Experimental The procedures for catalyst preparation and catalytic reaction were described in a companion paper [ 141. Briefly, catalysts were prepared by impregnating a high surface area alumina (JRC; BET area, 298 m* g- ‘; pore volume, 0.72 cm3 g- ‘) by the incipient wetness technique with an aqueous solution of PdC12 and CuCl,. After impregnation, the catalyst was dried in room air for a day. The catalyst powders were pressed into a disc for XAFS experiments. The sample disc was placed in a controlled atmosphere spectroscopy cell with a Kapton window shown in Fig. 1. The pretreatments and oxidation of CO were performed in a flow reaction system for this sample. Feed gases of 1.O% CO in He and air were supplied through respective mass flow controllers with a total flow rate in gas hourly space velocity (GHSV, volume of gas at STP/volume of catalyst/h) of 7330 h-‘. The CO/He stream was directed through a water vapor saturator immersed in a constant temperature bath and fed to a reactor through a glass line warmed by a heating tape. After the treatment, the sample disc was transferred to the Kapton window end of the cell and sealed by a flame still under the same gas atmosphere as for the

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Kapton window

Sealed by flame *after pretreatment gas in

f

Catalyst wafer

Fig.

I. Controlled atmosphere XAFS cell.

treatment. The XAFS spectra were taken in transmission mode for K-edges of Pd and Cu at beamline 7C or 10B of the Photon Factory in Tsukuba, Japan. They were analyzed by the UWXAFS 2.0 package and FEFF code (Version 3 and 5) licensed from University of Washington [ 211. The standard analysis procedure is described elsewhere in detail [ 22-241

I 24320

I 24340

I 24360

I 243x0

I 24m

Energy (eV)

Fig. 2. XANES of Pd K-edge: (a) Pd foil. (b) PdCIZ. (c) Pd(OCOCH,)2. (d) I.O3%Pd/AI,O,, fresh. (e) I .03%Pd/Al,O,, after reaction. (f) 1.7%Pd-5.62%Cu/AI,O,, fresh. (g) I .7%Pd-5,62%Cu/Al,O,, after reaction. CO oxidation conditions: T= 30°C. 1% CO, 2.9% O2 in He saturated with water at 30°C flowing at gas hourly space velocity of 7330 h-l.

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3. Results 3.1. X-ray absorption near edge structure (XANES) The XANES, the fine structure near the rising portion of an absorption spectrum, contains information on the chemical environment and bonding surrounding X-ray absorbing atoms. Fig. 2 compares the Pd K-edge XANES of catalyst samples with those of some Pd reference compounds. The absorption edge maximum in each spectrum corresponds to the allowed 1s + 5p transition, which merges into the continuum at higher energies. Pd foil (Fig. 2a) and PdC12 show easily differentiated shapes of their respective XANES. Fig. 2c of palladium acetate in comparison with Fig. 2b shows the effect of the anionic ligands on the XANES of Pdtt compounds. Pd/A1203 (Fig. 2d), prepared by impregnation of Al,O, with an aqueous PdCl* solution and drying at RT for a day, still showed a XANES similar to that of the PdCl, reference. When Pd/A1203 was exposed to a reacting gas mixture ( 1% CO, 2.9% 0, in He saturated with water at 30°C flowing at GHSV of 7330 hh ‘) for 1 h, its XANES (Fig. 2e) became similar to that of Pd foil. When CuCl, was loaded together with PdCl,, the original spectrum (Fig. 2f) remained unchanged even after exposure to the reaction conditions (Fig. 2g). The XANES also provides the edge position, which is defined as the energy giving the maximal slope in the rapidly rising portion of the absorbance vs. energy 0

6-

-6 -

? .’ \ .’ ‘\ _;’

-8x10~3 24320

I

24330

I

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24340

24350

24360

Energy (eV) Fig. 3. The second derivative of absorbance

to locate the position of Pd K-edge.

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(eV)

Fig. 4. XANES ofCu K-edge: (a) Cu foil. (b) CuzO. (c) CuO. (d) CuC12~2H,0. (e) CUE. (f) 5,79%Cul A120i, after reaction. (g) 1.7%Pd-5.62%Cu/A1201, after reaction. CO oxidation conditions were the same as in Fig. 2.

25

2

Radial distance (A) Fig. 5. RSFof Pd references. The Fourier transforms ( ,y) over the range of k between 2 and 12 k ’

were made using /?-weighted

Pd K-edge EXAFS oscillations

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0,. 3O’C

1.7%Pd-5.62%Cw

1

2

3

Radial distance (A) Fig. 6. RSF of Pd catalysts used for the reaction at different temperatures. The Fourier transforms were made using !&weighted Pd K-edge EXAFS oscillations (x) over the range of k between 2 and 12 A-‘. Other reaction conditions are the same as in Fig. 2.

plot. Since the edge position is directly related to the binding energy of the ejected electron during the absorption process, it moves to a higher energy as the oxidation state of the absorbing atom increases. As shown in Fig. 3, the edge position is obtained as the energy that gives the first zero in the second derivative of absorbance. The edge position determined for Pd foil was 24341 eV and those of PdCl* and Pd-Cu/AlZ03 before and after the reaction fell between 24344 eV and 24345 eV. Hence, we can conclude that Pd in the catalyst sample remained in the Pd” state. The Cu K-edge XANES are compared in Fig. 4a-c for Cu foil, Cu,O, and CuO, respectively, as references. As discussed elsewhere [ 251, the absorbance maximum is assigned to the allowed 1s -+ 4p transition. Below the maximum are subsidiary peaks and shoulders reflecting transitions to empty orbitals according to the dipole selection rule. The small peak at 8981 eV in Fig. 4a and b is due to the Is + 4s transition. This peak is expected for Cue which has an empty 4s orbital. The presence of the sharper peak in Cu,O reflects the tendency of the CL? compound to assume trigonal or distorted tetrahedral geometry which allows s-p mixing [ 261. A very weak peak below the edge at 8976 eV in CuO represents the quadruple allowed 1s + 3d transition, which serves as a signature of a Cu” compound since there is no 3d hole in Cue or Cu’ compounds. The 1s + 4s peak in Cu” compounds appears at 8985 eV as a shoulder with much reduced intensity. The edge positions of these

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k (A-‘) Fig. 7. Curve fitting of Pd EXAFS for 1.7%Pd-5.62%Cu/Al~O, references.

used for the reaction at 30°C with different Pd

references were determined to be 8977.9 eV, 8979.5 eV, and 8984.0 eV for Cu foil, Cu,O, and CuO, respectively, and again reflected the shift to a higher energy as the oxidation number was raised. The XANES of CuCl, .2H,O (Fig. 4d) and Cu( OH) 2 (Fig. 4e) were very similar in edge position (8984.0 eV) and the presence of the pre-edge peak, indicating that Cu in both compounds was present as CL?. However, the maximal absorbance for Cu(OH), occurred at a higher energy than that for CuCI, .2H,O. According to XANES, both CU/AI,O~ (Fig. 4f) and Pd-Cu/AlT03 (Fig. 4g) exposed to the reaction mixture contained CL?’species. Yet, both supported samples showed multiple maxima in absorbance unlike any other reference copper compounds. 3.2. Extended X-ray absorptionjine

structure (EXAFS)

The small oscillations in absorbance present 100-1000 eV above the absorption edge are isolated from background absorption and Fourier-transformed to yield the radial structure function (RSF). To a first approximation, the peak position in RSF corresponds to an interatomic distance between absorbing and surrounding scatterer atoms displaced from the true distance by a phase shift and the peak intensity is correlated to the average coordination number for the atom at the distance.

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Table I Results of the EXAFS curve fitting for

1.7%Pd-5.62%Cu/A120q

catalysts

Absorber

Catalyst

Scatterer

N”

Pd

fresh after reaction at 30°C

Cl Cl co Cl co Cl co

4.9 3.7 I .5 2.3 1.0 4.7 0.5

2.30 2.31 2.1 I 2.33 2.08 2.29 2.33

0.0008 - 0.0058 0.0057 - 0.0056

0.14

OH Cl OH Cl OH

5.3 0.6 3.4 I .o 3.2 I .o 1.2 3.7

I .99 2.1 I I .96 2.26 I .96 2.27 I .94 2.20

0.0120 0.0055 0.0057 0.0179 0.0063 0.0133 0.002 I 0.0210

0.02

after reaction at 40°C after reaction at 70°C

CU

fresh after reaction at 30°C after reaction at 40°C

Cl after reaction at 70°C

OH Cl

0.0054 0.0035

0.0 I 0.03

-0.0010

0.03

0.01 0.01 0.01

’ N= coordination number. ’ R= interatomic distance. ’ A uL = Debye-Wailer factor. ’ i = the goodness-of-tit parameter

Fig. 5 compares RSF of Pd reference samples in which each sample could be easily differentiated by the position (distance) of the main peak. In agreement with XANES, RSF also showed that PdCl* in the fresh Pd/A1203 had changed to metallic Pd during CO oxidation at 30°C. On the other hand, Pd-Cu/A1203 maintained its PdCl,-like structure during the reactions up to 70°C as shown in Fig. 6. Interestingly, the catalyst used for the reaction at 40°C showed the minimum peak intensity and the peak was located at a noticeably longer distance relative to the rest of the samples. Furthermore, there was a significant intensity at the distance where the metallic Pd-Pd peak appeared. The RSF of other catalysts showed only a single peak of any significant intensity between radial distances of 1 and 10 ‘. In order to acquire quantitative structural information including interatomic distances and coordination numbers, the main peak in RSF (radial distance ranges of 1.2-2.4 A for Pd and 1.O-2.3 A for Cu) was isolated from the rest of the spectrum and back-Fourier transformed to obtain the EXAFS oscillation due only to the scatterer at that distance. As shown in Fig. 7 as an example for Pd-Cu/Al,O,, the single shell EXAFS data could be fitted to the theoretical EXAFS equation [ 271. A good fit was achieved assuming the catalyst contained PdCl,. When the catalyst was assumed to contain carbonyl ligands in addition to Cl, the quality of fitting improved. For this curve fitting, the reference Pd( CO), spectrum was synthesized theoretically by using the FEFF code. Numerical results of the curve fitting are summarized in Table 1. The quality of fitting represented by the parameter 2 (the squared deviation of experimental data from the model spectrum) [ 241 was generally good except for the catalyst used for the reaction at 40°C.

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cu roll

cue

. CuCI, 2H,O - - - Cu(OH), -

5 79%Cu/AI,O,,

fresh

Radial distance (A) Fig. 8. RSFof Cu references. The Fourier transforms (x) over the range of k between 2 and 12 k ‘.

were made using k’-weighted

Cu K-edge EXAFS oscillations

The RSF of Cu reference samples are compared in Fig. 8. Unlike Pd, Cu/A1203 showed similar RSF before and after CO oxidation which had a significant peak between 2.5 and 3.0 A in addition to the major peak. There was no reference compound that matched Cu/Al,O, in the positions of its two significant peaks. The RSF of Pd-Cu/AlZ03 after CO oxidation at different temperatures are shown in Fig. 9. They are very similar to Cu/Al,O, and the peak intensity decreased as the reaction temperature was raised. The major peak of Pd-Cu/AlZ03 used for the reaction at 30°C was isolated and back-Fourier transformed. The resulting EXAFS did not correlate with either that of CuCl, . 2H20, Cu( OH),, or CuCl,, as shown in Fig. 10. A good fit was obtained only when the copper species was assumed to contain both Cl and OH. Coordination numbers and interatomic distances obtained from the curve fitting procedure are summarized in Table 1. The negative values of Debye-Wailer factor for Pd-CO bond appear to be due to statistical errors. Fig. 11 compares the RSF of monometallic Pd/A1203 or Cu/Al,O, with bimetallic Pd-Cu/A&O, catalyst right after preparation. The spectra were very similar, indicating that similar species were present in both monometallic and bimetallic catalysts in their fresh state. However, changes in peak intensity from monometallic to bimetallic catalysts occurred in opposite directions; Pd gained intensity while Cu lost it. Since the peak intensity in both Pd and Cu edge spectra of the fresh catalyst is mainly due to Cl (Table 1)) the result suggests that Cl in CuCl, has been transferred to Pd during the preparation of Pd-Cu/Alz03 catalyst.

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Radial distance (i) Fig. 9. RSF of Cu catalysts used for the reaction at different temperatures. The Fourier transforms were made using /&weighted Cu K-edge EXAFS oscillations (x) over the range of K between 2 and 12 i ‘, Other reaction conditions are the same as in Fig. 2.

4. Discussion In a companion paper [ 141, we discussed the effects of catalyst composition and reaction conditions on the low-temperature oxidation of CO over a heterogenized Wacker system in which the pores of alumina or carbon were filled with an aqueous PdCl,-CuCl, solution. Unlike homogeneous Wacker systems in which molecular Pd” and Cu” species are active catalytic species, the XRD study indicated that the active phase of copper in the heterogenized system was solid Cu2Cl (OH) 3 particles which were formed by an interaction between CuCl, and the support. The palladium species was not seen by XRD. In the present XAFS study, the average structure and the chemical nature of the catalyst under the reaction conditions were elucidated. Both XANES and EXAFS indicated that the active Pd species was a Pd” species containing chlorine and probably CO as ligands. Thus, the active Pd species in the heterogeneous Wacker system is similar to the one known to exist in its homogeneous counterpart [ 17-201. The presence of the CO ligand was suggested by the results of the EXAFS curve fitting, the quality of which improved when Pd was assumed to have both Cl and CO ligands rather than Cl alone. The difference between the two fittings is not great probably because CO is a much weaker scatterer

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I 10

I 12

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k ( A-‘) Fig. 10. Curve fitting of Cu K-edge EXAFS for 1.7%Pd-5.62%Cu/Al,0y different Cu references.

used for the reaction

at 30°C with

than Cl and its coordination numbers were much smaller than those of Cl (Table 1) . The Pd-Cl distance was the same as the distance in PdCl* (2.30 A), and was not affected by the presence of CO ligand. Thus, our EXAFS results alone may not lead to a definitive conclusion regarding the presence of CO ligand, yet they at least support the report of Choi and Vannice [ 161 who observed PdClCO species via infrared spectroscopy. As mentioned, the phase of Cu was identified as solid Cu,Cl( OH) 3 particles by XRD. The XAFS also identifies the same phase in catalysts under the reaction conditions as follows. First, XANES spectra had multiple maxima unlike either CuCl, or Cu( OH),. An origin of the multiple peaks is sometimes the presence of the metal atom in different structural environments. Indeed, in Cu,Cl(OH),, one half of Cu is surrounded by 40H + 2Cl and the remainder by 50H + Cl [ 281. If all Cu exists in the same environment consisting of OH and Cl ligands, a single maximum is expected. Second, EXAFS spectra showed the presence of the higher shells, indicating the presence of a long range order. Curve fitting of EXAFS yielded coordination numbers and Cu-Cl and Cu-OH distances. In homogeneous Wacker system, molecular Pd” and Cu” species are known to be active catalytic species. Naturally, it has been envisaged that, for supported PdCl,-CuCl,, essentially the same molecular catalytic complexes are contained in a liquid phase within the internal pore structure of the support [ 15,161. This appears

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0

i;i ! i : ;: :;

2

4

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Radial distance (A)

2

4

6

Radial distance (A) Fig. I 1. RSF of fresh l.O3%Pd/Al,O, Al,O, and 1.7%Pd-5.62%Cu/Al,O,

and 1.7%Pd-5.62%Cu/A120, from Cu K-edge EXAFS (b).

from Pd K-edge EXAFS (a) and 5.79%Cu/

to be the case for palladium, yet not to be the case for copper which involves a solid component. The solid Cu,Cl(OH), phase must be in large particles as indicated by sharp XRD peaks [ 141. The phase appears to be formed by an interaction of CuCl, and alumina irrespective of the presence of Pd and to remain unchanged during the reaction. The role of copper in Wacker chemistry is to reoxidize Pd’, which is formed from the reduction of Pd” by CO, to active Pd” to sustain a catalytic cycle. This role of copper was nicely demonstrated in this work. Fig. 2 and Fig. 5 showed that Pd” in monometallic Pd/A1203 is readily converted to Pd” upon exposure to the reaction mixture at 30°C. The conversion of Pd did not take place for bimetallic Pd-Cu/Al,O, catalysts (Fig. 2 and Fig. 6).

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When CuCl, is converted to Cu,Cl( OH) 3r a part of Cl is lost and replaced by OH from the support. Fig. 11 suggests that Cl is transferred to Pd. It has been shown that maintaining a certain level of Cl in the catalyst is important for the maintenance of catalytic activity of the heterogenized Wacker system [ 1.51. Apparently, it is Pd that needs Cl in its surrounding and a possible source of the required extra Cl is CuCl,. In a companion paper [ 141, the activity of the catalyst towards CO conversion showed a minimum at 40°C. The unusual temperature dependence was attributed to a consequence of the complicated interplay between thermal acceleration of the reaction rates and decreased water content as the reaction temperature is increased. As discussed in the paper, depletion of water in the catalyst decreases the rate of CO oxidation. In addition to this influence of water on the reaction rates, XAFS showed that the structure of the active palladium species in the catalyst used for the CO oxidation at 40°C was unusual. As shown in Fig. 6, the catalyst showed the minimum intensity of the Pd-Cl peak in RSF and the peak was located at a noticeably longer distance relative to the rest of the catalysts. Furthermore, there was a significant intensity at the distance where the metallic Pd-Pd peak appeared. As a result, the curve fitting of this sample gave the unusually high value of 2 as shown in Table 1. All these results suggest that the catalyst contains inactive Pd” species in addition to Pd”. No such a behavior was observed for Cu. The reason for the unusual behavior of Pd is not clear.

Acknowledgements The traveling expense for XAFS experiments Source.

was provided by Pohang Light

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