J . Phys. Chem. 1993,97, 10372-10379
10372
In-situ X-ray Absorption Spectroscopy Studies of Hydride and Carbide Formation in Supported Palladium Catalysts James A. McCaulley Hoechst Celanese Research Division, Robert L. Mitchell Technical Center, 86 Morris Ave., Summit, New Jersey 07901 Received: May 4, 1993; In Final Form: July 16, 1993” Extended X-ray absorption fine structure (EXAFS) spectroscopy as used to characterize hydride and carbide phases in supported Pd catalysts. Transmission EXAFS measurements were made at room temperature on 5% Pd/C and 5% Pdly-AlzO3 catalysts. Combined EXAFS and transmission electron microscopy (TEM) results indicate that the average Pd particle diameter is 26 f 8 8, (or Pd dispersion is -45%) in both catalysts. Supported Pd particles in air-exposed catalysts were found to be -98% converted to a disordered Pd oxide; the remaining Pd is in metallic cores (average diameter is -6 A) inside the oxidized Pd particles. Catalysts were reduced in situ and cooled to 25 “ C in H2 (partial pressure of 26 Torr), yielding a hydride phase with a lattice expansion of 2.2 f 0.2%. The stoichiometry of the hydride phase, PdH, (x 0.44), is consistent with previous reports of decreased H2 sorption capacity, relative to bulk Pd, in supported Pd catalysts. Decomposition of the hydride phase yielded metallic Pd particles with a first-shell Pd-Pd coordination number of 9 i 1. Reaction of the reduced 5% Pd/C catalyst with 1% C2H4/Ar at 150 OC for 20 min generates a Pd carbide phase with an average Pd-Pd distance 1.2% larger than that in Pd metal. The PdC, phase has a maximum carbon 0.06, about half that of bulk PdC,tx 0.13, The decreased carbon content of the Pd carbide content, x phase in supported Pd catalysts is analogous to the decreased hydrogen content of the Pd hydride phase in Pd catalysts. PdC, can be distinguished from PdH, by its stability in the absence of H2. The rate of carbidization of 5% Pd/y-A1203 was found to be slower than that of 5% Pd/C. A detailed study of the kinetics of carbidization is needed to determine the cause. There is no significant shift of the Pd K-edge absorption of either PdH, or PdC,, relative to Pd metal, in these catalysts. The only noticeable difference between the near-edge spectra of interstitial phases and pure Pd is a decrease in the energy of the second resonance, a simple result of lattice expansion. The ability to detect Pd lattice expansion via the near-edge spectrum may be useful for time-resolved studies.
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Introduction The participation of bulk, or subsurface, hydrogen in heterogeneous reactions on supported metals has been inferred from a variety of indirect experiments, but the mechanisms are not well understood.’ Recently, Johnson et al. observed directly, for the first time, the participation of bulk hydrogen as a reactant in an elementary catalytic reaction, hydrogenation of CH3 adsorbedon Ni( 111).2 Supported Pd catalysts and Pd-containing bimetallic catalysts are used in a variety of commerical chemical processes: hydrogenation, hydrogenolysis, and oxidation. Because bulk hydrogen is both very soluble and mobile in Pd, it has long been believed to play an important role in catalytic reactions on Pd surfaces. Nearly 20 years ago, Pawlczewska reviewed the literature concerning catalytic activity of bulk hydrogen in Pd hydride phases.’ For example, Scholten and Konvolinka had studied ortho-para conversion and isotopeexchange in hydrogen catalyzed by Pd sponges and wire, and they obtained separate Arrhenius parameters for a-hydride and @-hydride phases.4 Studying hydrogenation of 1,3-butadiene by supported Pd/Au catalysts, Joke et al. found that the maximum yield of n-butane decreased with decreasinghydrogen content.5 At temperatures above those where @-PdHwas stable, the n-butane yield decreased dramatically. Conversely,Dus found that ethylenedoes not dissociatively chemisorb on @-PdHand concluded that hydrogen in bulk Pd @-hydrideis inactive in ethylene hydrogenation.6 It is not known whether such differences are caused by direct participation of bulk hydrogen as a reactant or by the different electronic structures of these phases. Nakamura and Yasui7 and Zaidi* used X-ray diffraction (XRD) to characterize Pd phases present after vapor-phase vinyl Abstract published in Aduance ACS Abstracts, September 15, 1993.
0022-3654/93/2091- 10372$04.00/0
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acetate synthesis. Both observed expansion of the Pd lattice in Pd catalysts treated with C2H4 at 150 OC and concluded that a Pd hydride phase (8-PdH,, x I0.60) had been created. Nakamura and Yasui found that this “hydride” phase poisoned hydrogenation of ethylene but not vinyl acetate formation; i.e., it improved the selectivity of vinyl acetate synthesis. Noting that @-PdH,is unstableunder the conditions of the earlier XRD studies, Ziemecki et al. showed that interstitial Pd carbide phases (PdC,, x I0.13) were formed when Pd was exposed to C2H4 at high temperat~re.~.’~ Two carbide phases, analogous to the a- and @-hydridephases, were found.10 They also found that formation of PdCo.13 completely suppressed absorption of hydrogen. For x < 0.13, a ternary phase, PdC,H,, can be formed.l’J2 If absorbed hydrogen is active in catalysis by Pd, then interstitial carbon, by its suppression of hydrogen absorption, will likewise influence the activity and selectivity of Pd-containing catalysts. Zaidi has detected Pd carbide after acetoxylationof ethylene and correlated loss of activity with carbide formation.13 Ouchaib et al. suggested that interstitial carbon might be responsible for “stabilization” of supported Pd catalysts for selective hydrogenation of butadiene.14 Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful tool for characterizing the local structure of supported metals in catalysts.15 As a probe of short-range order, EXAFS provides complementary information to that obtained from X-ray diffraction. An application where EXAFS may be uniquely valuable is identifying interstitial phases in bimetallic catalysts. X-ray diffraction cannot distinguish between interstitital and substitutional alloying in supported Pd-containing bimetallic catalysts if the face-centered-cubic (fcc) structure of Pd is preserved. EXAFS can easily distinguish between substitutional or interstitial alloying. Before one attempts to 0 1993 American Chemical Society
Hydride and Carbide Formation in Pd Catalysts 30
The Journal of Physical Chemistry, Vol. 97,No. 40, 1993 10373
I
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DIAMETER (nm) 30
n
8 20 Z
0
Figure 2. Schematic diagram of in-situ catalyst reactor. Inset shows a removable sample holder with Kapton tape windows.
5 m
En
10
0 2
4
6
0
10
DIAMETER (nm) Figure 1. Particle size distribution histograms of 5% Pd/C and 5% Pd/ y-AlzO3 catalysts. Particles smaller than 20 A were not observable with the instrument used here. The number-averaged particle diameters are 35 and 34 A, respectively.
characterize hydrideor carbide phases in Pd-containingbimetallic catalysts, it is desirable to study interstitial phases in supported Pd catalysts by EXAFS. There are several published EXAFS and XRD studies relevant to the results presented here. Yokoyama et al. reported the temperature dependence of Pd K-edge EXAFS of supported Pd catalysts in the metallic state.l6 Several groups have reported EXAFS studies of in-situ reduction and hydride formation in supported Pd cataly~ts.l~-~l Mutschele and Kirchheim22and Eastman et a l . 2 3 ~reported ~~ studies of hydride formation in unsupported Pd nanocrystals. Their results show that the phase diagram of bulk PdH, cannot be used to predict hydridingbehavor in nanocrystalline Pd particles. EXAFS studies of unsupported “bulk” Pd carbide have also been rep0rted.~*-2~Here, I present room temperature EXAFS studies of hydride and carbide phases formed in situ in Pd supported on y-A1203and activated carbon. Experimental Section
Pd (5 wt %) supported on steam-activated peat carbon (- 1000 m2/g) and Pd (5 wt%) supported on high-purity y-A1203(-300 m2/g) were purchased from Johnson Matthey. Both catalysts were 325 mesh powders (particle size -45 pm). A JEOL 1OOCX transmissionelectron microscope, operated at 100 keV, was used to characterize the Pd particle size distributions of the catalysts. Particle size histograms (see Figure 1) were obtained by measuring a total of -300 particles from three micrographs for each catalyst. Particles smaller than 20 A were not observable in these
micrographs. The number-averaged particle diameters for 5% Pd/C and 5% Pdly-Al203 were 35 and 34 A, respectively, corresponding to a dispersion of -40%. Anhydrous PdO (99.9%) and Pd metal foil (>99.9%, 25 pm thick), used as reference materials for EXAFS data analysis, were obtained from Johnson Matthey. Catalyst powder was packed in a sample holder, with path length x = 10 or 12 mm, and sealed with Kapton tape. The Pd K-edge step height, Apx, was 1.O and -2.5 for C-supported and y-Al203-supported catalysts; the difference reflects the powder densities. Anhydrous PdO samples were prepared on adhesive tape, four (Apx = 1.4) or six (Apx = 2.1) layers, and sealed in plastic bags with CaS04 dessicant. X-ray absorptionexperimentswere performed during four visits to the X-11A beamlineof the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory?* Thestorage ring operated at 2.5 GeV with current between 110 and 230 mA. The monochromator was operated with two flat Si( 11 1) or Si(3 11) crystals; the entranceslit was 0.25 mm high. The monochromator was not detuned to eliminate harmonics because the bending magnet source produces neglible flux at the third harmonic of the Pd K-edge energy and the Ar ionization chamber detectors are transparent at that energy. The calculated monochromator resolution at the Pd K-edgeis 5 eV. During several experiments, the absorption spectrum of a Pd foil (25 pm thick) was measured (at -300 K) stimultaneously with the catalyst samples to accurately determine the position of the absorption edges in XANES spectra of the catalysts. All X-ray absorption measurements were performed in transmission mode, using flowing Ar ionization chambers to measure the incident intensity and the intensities transmitted through the catalyst samples and through the Pd reference foil. Except for one experiment at 150 OC,all EXAFS measurements were made with the samples at 298 f 2
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K. The absorption cell, depicted schematically in Figure 2, was machined from a block of aluminum. It contains two wells for cartridge heaters, channels for water cooling, and gas feed lines to a removable sample holder. Catalyst powder is sealed in a sample holder with Kapton tape, which limits operation to -200
McCaulley
10374 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 100
I
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05 A
& x
106 I
I
1
A
ni Y
o
-0 5
(b)/\
0
=
e
100
I/----? 0
1
I
I
10
20
U I
30
t (min) Figure 3, Approximate temperature schedules for in-situ gas treatment of Pd catalysts: (a) reduction in 3.5% H2/Ar, (b) hydride decomposition
in pure Ar, (c) carbide formation in 1% CzHd/Ar.
OC and atmospheric pressure. The sample holder has entrance and exit holes for gas flow that are press fit against matching holes in the body of the cell. The cell is then sealed with Kapton tape, and a slight positive pressure of reactant minimizes air leakage into the sample holder; no oxidation of reduced samples by air leaks has been observed. The cell can quickly (2-5 min) be cooled to room temperature after treatment at high temperature. The temperature was measured with a type J or K thermocouple and controlled with an Omega temperature controller. Treatments are performed at atmospheric pressure with the appropriate gas mixture flowing through the cell; the effluent is vented to an exhaust hood. To minimize safety concerns, nonflammable gas mixtures were used in these experiments: for reduction, 3.5% H2 in He or Ar, and for carbidization, 1% C2H4 in Ar. Catalysts were usually first analyzed as-received, having been exposed to air. Approximate temperature schedules for subsequent in-situ treatment cycles are depicted in Figure 3. The first treatment cycle performed on each sample was to reduce the Pd in a mixture of 3.5% H2 in Ar. The temperature of the cell was raised to 100 OC for 5 min with the H2/Ar gas mixture flowing, then cooled to -25 O C . The hydride phase was formed and EXAFS data collected with a partial pressure of hydrogen, P H 2 = 26 f 1 Torr. This pressure is sufficient to generate the &hydride phase in bulk Pd. Although several experiments showed that reduction of Pd oxides can be done at room temperature, to ensure fast, reproducible reduction the temperature was increased to 100 OC. Purging the cell with pure Ar at room temperature destroys the Pd hydride phase, but a brief heating (100 "C) cycle was performed to ensure complete removal of hydrogen. The Pd carbide phase was produced by heating a catalyst to 150 "C in a 1% C2H4/Ar mixture for 20 min. Bulk PdC, is stable in inert gases up to -600 OC but decomposes rapidly in H2 or 0 2 at 150 OC.10
EXAFS data were analyzed with University of WashingtonNaval Research Labs (UW-NRL) EXAFS analysis programs.29 Data analysis parameters are presented in Table I. The EXAFS function, x(k), was extracted from the absorption spectrum by linear pre-edge background subtraction, subtraction of a smooth,
5
10
15
20
k (k') Figure 4. /$-weighted EXAFS of 5% Pd/A1203 before (dotted line) and after (solid line) in-situ reduction. The latter was acquired with 3.5% H2/Ar flowing in the in-situ cell; the Pd was in a hydride phase. TABLE I: EXAFS Data Analysis Parameters parameter shell value 24 350 eV Eo -200 to 1200 or 1400 eV data range pre-edge linear fit range -180 to -30 eV step-height normalization range 50 to 1200 or 1400 eV no. of cubic spline segments 4 k weighting 293 2.5 to 17 or 19 A-l range of FT Hanning % 20% range of inverse FT Pd-Pd 1.5 to 3.4 A Pd-O 0.8 to 2.2 A Hanning % Pd-Pd 21% Pd-0 28% range of least-squaresfit Pd-Pd 3.0 to 17 or 19 A-1 Pd-0 3.0 to 14.0 A-1 AEO both -4.0to 5.0 eV segmented cubic spline background above the edge, and step height normalization. The photoionization threshold, Eo, was initially assumed to beat the first inflection point in theXANES spectrum. Because experimental standards, PdO and Pd metal, were employed, energy-dependent normalization was not done. The number of segments in the cubic spline fit was varied to minimize intensity in the Fourier transform (FT) magnitude plot below 1 A, while ensuring that the nearest-neighbor peak (either Pd-0 or Pd-Pd) was not attenuated. The x(k) data were weighted by k",with 1 In I 3. Representative k2 x(k) plots are presented in Figure 4. The knx(k) data were Fourier transformed to produce pseudoradial distribution functions (PRDFs) such as those shown in Figure 5. Here, "pseudo" refers to the fact that these have not been corrected for phase shifts, and therefore the peak positions do not reflect the actual distances of the various coordination shells. The EXAFS contribution from a single coordination shell was isolated (Fourier filtered) by back-transforming over a limited range of R-space. For example, the EXAFS from the first PdPd coordination shell was isolated and inverse transformed to obtain k3xl(k), where the subscript 1 refers to the first Pd-Pd shell. The Fourier-filtered EXAFS from a selected coordination shell, knxi (k),was least-squares fitted in k-space using experimental standards. For analysis of Pd-Pd data, Pd foil at -300 K was used: N = 12, R = 2.7505 A, and u2 = 0.00629 A2.30 For analysis of the first Pd-0 coordination shell, PdO powder at -300 K was used: N = 4, R = 2.024 A, and u2 was assumed to be 0.005 A2.31 Representative fits are shown in Figure 6 for metallic Pd and Pd carbide phases supported on activated carbon. Coordination numbers measured by EXAFS are considered accurate to +20%; internuclear distances usually have a quoted absolute uncertainty of f0.01 A. The precision of the results presented below is within
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10375
Hydride and Carbide Formation in Pd Catalysts
TABLE II: EXAFS Results for the First Pd-O Coordination
50
Shell in 5%Pd/C and 5%Pd/y-AlzOJ I support N R (A)
Catalysts’ A$ (A21
carbon avg alumina
avg
3.9 3.9 3.4 3.7 4.3 4.0 3.7 3.8 4.0
* 0.3
* 0.3
2.021 2.041 2.001 2.021 2.0 19 2.026 2.029 2.020 2.024
* 0.020 0.005
+0.0041 +0.0039 +0.0028 0.0036 +0.0030 +0.0028 +0.0020 +0.0022 0.0025
AEo (eV) +3 0 0
-
0.0007
0 +2 0 0
* 0.0005
-
a Samples were exposed to air. The Au2 values are relative to the assumed value 0.0050 A2.
TABLE IIk EXAES Results for the First Pd-Pd Coordination Shell h 5%Pd/C Catalysts state N R(A) uz(A2) &(eV) as loaded 4.6 2.753 0.0079 -3.0 ~
Figure 5. PRDFs of (a) air-exposed and (b) reduced 5% Pdly-AlzOs catalyst and (c) Pd foil. The data for the reduced catalyst were acquired with the sample in pure Ar,Le., in the metallic state. Note the loss of the Pd-O peak and the greatly increased intensity of the Pd-Pd peak that
hydride metallic
accompanies in-situ reduction. carbide carb. 40min ’metallic”
3.9 3.8 8.9 8.6 9.1 9.2 8.8 8.8 8.6 8.2 8.3 8.6 8.3
2.762 2.750 2.826 2.818 2.813 2.767 2.764 2.758 2.793 2.803 2.790 2.797 2.802
0.0070 0.0077 0.0089 0.0086 0.0090 0.0076 0.0074 0.0076 0.0093 0.0091 0.0090 0.0093 0.0091
~~~
-4.0 0.0 -3.0 -4.0 +5.0 -3.0 -4.0 +5.0 -3.0 -4.0 +5.0 +5.0 -4.0
internuclear distance. The parameter A& is the difference between ionization threshold energies of the reference material and the unknown; this parameter was varied in integer steps to minimize the variance of the fit.
n Y
,
X
PJ -
Y
0
5
10
k 12
15
20
(i-’)
c
I
t -12
1
I
0
5
k
I
I
I
10
15
20
(i-’)
Figure 6. Representativeleast-squaresfit to the Fourier-filteredEXAFS of the f i t Pd-Pd coordination shell in 5% Pd/C catalyst in (a) the metallic state and (b) in a carbide phase. Note the decreased EXAFS amplitude of the carbide phase.
these limits. The EXAFS Debye-Waller factor, u2, is a measure of the combined thermal (vibrational) and static variation of the
Results Figure 4 presents typical x ( k ) data (weighted by kZ to show the high-k region) obtained for a sample of 5% Pd/r-A1203 before and after in-situ reduction. The data on the reduced sample were acquired with 26 Torr of Hz in the cell; the Pd was hydrided. Figure 5 presents PRDFs of air-exposed and reduced (metallic state) samplesof 5% Pd/yAlzOa catalyst, showing the conversion of Pd oxide to metallic Pd. Note the similarity of the PRDFs of the reduced catalyst and bulk Pd. For air-exposedsamples, EXAFS from both the nearest Pd-O and Pd-Pd coordinations shells were analyzed. The range of the inverse Fourier transforms for the Pd-O and Pd-Pd shells are given in Table I. Figure 6 shows representativefits to the Fourierfiltered EXAFS of the first Pd-Pd coordination shell, k3xl(k), in a reduced (metallic state) 5% Pd/C catalyst and a carbided sample of 5% Pd/C. EXAFS results for the Pd-O coordination shell are presented in Table 11. The first-shell Pd-Pd results for the 5% Pd/C and 5% PdlyAlzO3 catalysts are presented in Tables I11 and IV, respectively. Results of repeated independent experiments,performed during four visits to NSLS, agree within the expected precision and accuracy claimed for EXAFS. See Table V for a summary of the averaged results from repeated independentexperiments. Figure 7 shows how the Pd-Pd distance (plottedas R/Rm, whereR-is thePd-Pddistancein themetallic state) varies following in-situ treatment cycles. Note the small lattice expansion after ‘carbidization” of 5% Pd/y.AlzO3, indicating that little or no carbon was incorporated in the Pd lattice. Figure 8 summarizes the EXAFS Debye-Waller (DW) factors (presented as u2/uZma,where ~2~ is the D-W factor in themetallicstate). Notethat formationofhydrideandcarbide phases is accompanied by an increased D-W factor. Figures 9 and 10 present Pd K-edge XANES spectra of 5% Pd/C and 5% Pdlr-AlzO3 in hydride,metallic, and carbide states.
10376 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
McCaulley 1.3 I
TABLE Iv: EXAFS Results for the First Pd-Pd Coordination Shell in 5%Pdly-AlzOJ Catalysts state N R(A) uZ(A2) A&(eV) as loaded
hydride
hydr. 150 OC metallic
'carbide"
carb. 40 min
3.6 3.6 4.0 3.8 8.2 6.0 7.6 9.4 10.0 9.5 7.6 8.1 10.4 10.1 9.6 8.8 8.4 7.8 7.0 9.8 9.6
2.740 2.749 2.751 2.757 2.818 2.818 2.816 2.814 2.750 2.755 2.744 2.750 2.752 2.753 2.765 2.758 2.774 2.763 2.751 2.750 2.782
0.0076 0.0075 0.0083 0.0080 0.0084 0.0079 0.0083 0.0082 0.0093 0.0071 0.0074 0.0074 0.0071 0.0072 0.0078 0.0078 0.0084 0.0081 0.0078 0.0069 0.0084
I
-3.0
-4.0 -3.0 0.0 -3.0 0.0 -3.0 +5.0 -3.0 -3.0
-4.0 0.0 -3.0 +5.0 -3.0
I
t
0.9 0
-4.0 0.0 0.0
I
I
1
2
3
TREATMENT CYCLE Figure 8. Variation of the normalized Debye-Waller factor with the in-situ treatment cycle. The cycles are as follows: 0, air-exposed, 1, hydride formation; 2, metallic state; 3, carbide formed.
0.0 +5.0 +5.0
TABLE V: Summary of EXAFS Results from Replica Measurements Presented in Tables III and IV' support state N R (A) a2 (A*) as loaded hydride metallic carbide as loaded hydride metallic carbide
carbon
alumina
4.1 f 0.4 8.9 f 0.2 8.9 0.2 8.4 f 0.2 3.8 f 0.2 7.8 f 1.4 9.1 f 1.2 8.6 f 1.1
*
2.755 f 0.006 2.819 f 0.006 2.763 f 0.004 2.795 f 0.007 2.749 f 0.007 2.816 f 0.002 2.751 f 0.004 2.760 0.009
0.0075 f 0.0005 0.0088 i 0.0002 0.0075 f 0.0004 0.0091 f 0.0002 0.0078 f 0.0004 0.0082 f 0.0002 0.0072 f 0.0002 0.0078 f 0.0005
Error limits are 1u sample standard deviations.
-40
-20
20
0
E-E,
60
40
a0
(eV)
Figure 9. Pd K-edge XANES spectra of 5% Pd/C catalyst in hydride, metallic, and carbide states. 1
0.99
I
I
1
0
1
2
3
3
0.5
7 Legend
TREATMENT CYCLE
HYdM
Metal
Figure 7. Variation of the normalized Pd-Pd distance with the in-situ treatment cycle. The cycles are as follows: 0, air-exposed; 1, hydride formation; 2, metallic state; 3, carbide formed.
-
-
The energies of the first two absorption maxima (1s 5p,pd and 1s 4f res0nances)3~above the absorption edge are presented in Table VI.
Discussion The average Pd-O distance in air-exposed 5% Pd/C and 5% Pdly-AlzO3 catalysts (2.021 f 0.020 and 2.024 f 0.005 A) is identical to that of crystalline PdO (2.024 A). The Pd-O coordination number (3.7 f 0.3 and 4.0 f 0.3) is likewise nearly identical to that of bulk PdO, suggesting that most of the Pd is oxidizedupon exposure toair at room temperature. These results
.................. 'Carbide'
0
-40
-20
0
E-E,
I
I
I
20
40
60
80
(eV)
Figure 10. Pd K-edge XANES spectra of 5% Pdly-AlzOl catalyst in hydride, metallic, and "carbide" states.
agree well with those of Nandi et al. for air-exposed Pd/SiO* catalysts (with dispersion 50-80%).18 This is not, however, sufficient evidence to show that the oxide formed is crystalline. The nearest Pd-Pd distance in the air-exposedcatalysts (2.755 f 0.006 and 2.749 f 0.007 A) is essentially identical to that of
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10377
Hydride and Carbide Formation in Pd Catalysts
TABLE VI: Energies, Relative to the First Inflection Point in the XANES Spectrum of Pd Foil, of the Fmt Two Absorption Maxima in XANES Spectra of Pd Catalysts
carbon alumina
hydride metallic
16.6 16.8
carbide hydride metallic carbide
17.5 16.8 17.6 18.4
38.1 39.8 39.1 38.0 40.2 40.5
bulk Pd metal (2.7505 A). This indicates the presence of two phases: a metallic Pd core and an oxidized outer layer. There is also a small peak near 3.1 A in the PRDFs of air-exposed samples (see Figure 5) from the first Pd-Pd distance in PdO. However, in &weighted EXAFS from crystalline PdO, the contribution of the first Pd-Pd shell is stronger than that of the first Pd-0 shell. In the EXAFS from air-exposed Pd catalysts, the contribution from the Pd-Pd coordination shell of the Pd oxide layer is weak relative to the Pd-O shell, indicating that the oxide layer is disordered beyond the Pd-0 coordination shell, Le., amorphous. This conclusion differs from that of Nandi et al. who, based only on Pd-0 first-shellEXAFS results, concluded that crystalline PdO was formed.’* After reduction, purging the cell with pure Ar at 100 OC (or even 25 “C) destroys the hydride phase, yielding metallic Pd. The average Pd-Pd distance of the metallic-state samples is 2.749 A, in excellent agreement with the distance in bulk Pd. The coordination number, 9, agrees well with that found by Davis et al. for 5% Pd/yA120sn20 The D-W factor of the supported metallic Pd (0.0072 f 0.0002 A2) is 0.0009 A2 larger than that of bulk Pd, in fair agreement with that of Davis et al., whoobtained Au2 = 0.0003 A2. Our absolute D-W factors differ by 0.0006 A, an 8% discrepancy. A larger static EXAFS D-W factor in nanocrystalline Pd was also reported, but not quantified, by Eastman et al.23 Yokoyama et al. reported a much larger value (u2 = 0.0125 A2 at 295 K) for smaller (15 A) Pd particles supported on Si02.16 By comparing the Pd-Pd coordination numbers before and after in-situ reduction, one can estimate the thickness of the oxidized layer. The average Pd-Pd coordination number of the air-exposed catalysts, on both supports, is 4.0 f OS, indicating an average spherical core diameter of 6 f 1 A.33 For average coordinationnumbers approaching 12 (in fcc metals), the accuracy of particle sizes measured by EXAFS is poor. The Pd-Pd coordination number of reduced (metallic state) catalysts, 9 f 1, corresponds to an average Pd particle diameter, 18 f 8 A, or a dispersion of -50%. TEM measurements yielded a particle size of -25 A (or dispersion of -40%), but particles smaller than 20 A were not observable with the instrument used here. The EXAFS results should provide a good lower limit on the average Pd particle size, and the TEM results a good upper limit. I therefore conclude that the average Pd particle size is 26 f 8 A, corresponding to a Pd dispersion of -45%. Particles of this size would have a Pd-Pd coordination number of -9.5, in reasonable agreement with the EXAFS result. The estimated thickness of the Pd oxide layer in air-exposed samples is 10 A. About 98% of the Pd is oxidized. This explains why the average Pd-0 coordination number is -4 in air-exposed samples; most, but not all, of the Pd is converted to a disordered Pd oxide. In-situ reduction with 3.5% H2/Ar at 100 OC quickly destroys the Pd oxide phase formed during exposure to air (see Figure 5). If the sample is cooled to 25 OC in 3.5% H2/Ar (26 Torr Hz), a Pd hydride phase is formed, as is revealed by a lattice expansion (2.0 f 0.3% and 2.4 f 0.2% for 5% Pd/C and 5% PdlyAl2O3) and an increase in u2 (17 h 6% and 14 4% for 5% Pd/C and 5% Pd/r-A1203). See Figures 7 and 8. The partial pressure (26 Torr) of H2 exceeds the minimum pressure (- 18 Torr) required
-
*
to form 6-PdH0.ain Pd black,Mbut the lattice expansionobserved here (-2.2%) is significantly smaller than that in &Pd&.@, 3.5%. It has been shown that supported Pd catalysts (and nanocrystalline Pd powders) absorb less hydrogen in the &phase than bulk forms.23*24*3S-37 The average lattice expansion (2.2%) observed in these 5% Pd catalysts indicates a Pd hydride stoichiometry, PdH0.u. This agrees well with the recent results of Bonivardi and Baltanas” but is in poorer agreement with those of Hwang and B~udart.’~Using EXAFS, Davis et al. found a 3.74latticeexpansion for a 5% Pd/yAlzOp sample (Pddispersion 21%) in -750 Torr of H2 and inferred a hydride phase, 8-PdH0.65.~~ Their result for a catalyst having lower dispersion than those studied here also agrees well with the results of Bonivardi and Baltanas. EXAFS, being a probe of short-range order, cannot distinguish between a pure &phase havng x < 0.6 and a mixture of a-phase (x
