X-ray PHoto-Emission Studies of Strong Metal-Support Interaction (SMSI): Metal Decoration and Electronic Effects

J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

X-ray Photoemission Studies of Strong Metal-Support Interaction (SMSI): Decoration and Electronic Effects

791

Metal

T. H. Fleisch 1 , A. T. Bel1 2, J. R. Regalbut03, R. T. Thomson 3, G. S. Lane 3, E. E. Wolf 3, and R. F. Hicks4 • 1 Amoco Corporation, P.O. Box 400, Naperville, Illinois 60566 2 University of California, Chemical Engineering, Berkeley, California 94720 3 University of Notre Dame, Chemical Engineering, Notre Dame, Indiana 46566 4 University of California, Chemical Engineering, Los Angeles, California 90024

ABSTRACT Metal-support and metal-promoter interactions have been studied by x-ray photoelectron spectroscopy. Decoration of the metal particles by metal oxide species is observed for Pd in contact with La203, Pt in contact with W0 3, and Pt in contact with Ti02• In every case where decoration occurs, there is also a negative shift in the apparent binding energy of the core electron levels of the metal after reduction in hydrogen at elevated temperatures. The shift can be as great as -0.7 eV relative to the bulk foil value. Charging of insulating catalyst powders makes it difficult to establish the binding energy reference level within the sample. However, analysis of the charging behavior suggests that the negative shift may result from a change in the electronic properties of the metal surface. INTRODUCTION Catalyst support materials not only provide large surface areas for dispersion of the active metal, but also affect the chemical and physical properties of the small metal crystallites formed. The effects of the support can be either indirect (parasitic), or direct (ref. 1).

An indirect effect is one in

which the support can add a catalytic function, or alter the structure of the metal crystallites, but the metal retains its own identity.

A direct effect,

also known as a Schwab effect, is one in which the support alters the electronic properties of the metal, so that the metal loses its own identity. We have been interested in elucidating the nature of direct support effects by photoelectron spectroscopy.

Unfortunately, interpretation of core line

shifts in the photoemission spectra of supported metal catalysts is not straightforward, and the results are often controversial.

For small metal

792

crystallites there exists the problem of distinguishing chemical shifts from changes in extraatomic relaxation (refs. 2-5).

Accordingly, we have always

chosen to work with large metal crystallites, greater than 25 angstroms in diameter, where differences in core hole screening can be clearly ruled out (ref. 6).

However, even with large metal particles, the results are obscured

by problems with sample charging and the assignment of the binding energy reference.

These effects are discussed further below.

We have studied by XPS a number of supported metal catalysts which are known to exhibit metal-support interactions. Pt/W0 3/Si02, and Pt/Ti0 2•

These catalysts are Pd/La203,

In previous studies, and in our own work, there is

convincing evidence that the metal becomes decorated by patches of support material (refs. 7-27).

This has led several researchers to propose that the

support effect is an indirect one, influencing the catalytic properties of the metal by blocking access of the reactants to the metal surface.

However, in

every case where decoration is observed, the XPS spectra show a negative shift in the apparent binding energy of the metal core lines.

This suggests that

metal decoration may be accompanied by a direct effect on the electronic properties of the metal.

While at the present time we do not fully understand the

cause of the. negative shifts, their occurrence in every system studied has convinced us that these shifts should be investigated further.

Our findings

are summarized below for Pd/La203, Pt/W0 3/Si02, and Pt/Ti0 2• Experimental 1. a.

Catalyst Preparation

Pd/Si0 2 and Pd/La203

The Pd/Si0 2 samples were prepared by incipient wetness impregnation of CabO-Sil HS-5 silica (300 m2/g) with a solution of H2PdC1 4 dissolved in 1 N HCL (ref. 19).

The Pd/La203 catalysts were made by addition of an aqueous sol-

ution of H2PdC1 4 to a slurry of La(OH)3 (15 m2/g).

After drying, all samples

were oxidized at 350°C for 2 hours in air, and reduced at 300°C for 3 hours in

H2 • b.

Pd/La203/H-ZSM5

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Two different methods of doping a Pd/H-ZSM5 catalyst with Laz03 were used to prepare samples with different degrees of contact between the Pd and Laz03 (refs. 21,22).

H-ZSM5 was stirred into a solution of CHzCl z and enough

Pd(acac)z to yield 2 wt% Pd on H-ZSM5. ation and drying in air.

The solvent was removed by mild evacu-

A portion of the dried catalyst was impregnated to

incipient wetness by a solution of La(N0 3)3'

This yielded minimal contact

between Pd and La, because of the hydrophobic nature of Pd(acac)z.

Another

portion of the dried catalyst was heated in Oz for 3 hours at 400°C to give Pd~,

and was then impregnated with a solution of La(N0 3)3'

This yielded maxi-

mum contact between Pd and La. Both samples were oxidized at 350°C for 2 hours in air, and reduced at 300°C for 3 hours in Hz. c.

Pt/SiO z and Pt/W0 3/SiOz The Pt/SiO z and W0 3 promoted Pt/SiO z catalyst were prepared using a high

surface area silica (Harshaw, 600 mZ/g), HzPtCl s, and anhydrous ammonium tungstate, AAT ((NH 4)zW04, Aldrich), or ammonium metatungstate, AMT ((NH4)sHzWIZ049'4HzO, Climax Molybdenum Co.).

Since AMT is more

solubl~

in

water than AAT, it was used to prepare high W0 3 weight loadings. Low weight loadings of W0 3 were deposited on SiO z by incipient wetness impregnation of AAT.

For this series of promoted catalysts the amount of W0 3

varied, but the total amount of metal, Pt+W, remained constant at 5 wt%.

A

high weight loading of W0 3 was deposited on SiO z by incipient wetness impregnation of AMT.

To this 25 wt% W0 3 sample varying amounts of Pt were added.

After impregnation with AAT or AMT the samples were vacuum dried at 80°C for 2 hours, and oxidized at 700°C for 3 hours in air.

Platinum was then added by

incipient wetness impregnation of HzPtCl s, followed by vacuum drying at 80°C, oxidation at 300°C for 3 hours, and reduction at 425°C for 3 hours in a 95% Hz:5% He mixture. d.

Pt/TiO z The 5 wt% Pt/TiOz samples were prepared by incipient wetness impregnation

of either rutile or anatase TiO z (Degussa, 50 mZ/g) with a solution of HzPtCl s

794

(ref. 25). 2.

These samples were reduced at 200 0e and 500 0e for 2 hours in H2•

XPS Apparatus and Procedure

The XPS experiments were performed in a Hewlett-Packard 59508 ESeA spectrometer using monochromatic ALKa radiation (hv=1486.6 eV). The base pressure of the instrument was 1x10-g torr.

A Hewlett-Packard electron flood gun was

used to minimize surface charging effects.

The binding energy scale was ref-

erenced to the e ls peak at 284.6 eV for Pd/La203, to the Si 2p peak at 103.2 for Pd/Si02, Pd/La203/H-ZSM5, Pt/Si0 2, and Pt/W0 3/Si0 2, and to the Ti 2p peak at 458.7 eV for Pt/Ti0 2• The binding energies reported here are accurate to ±0.15 eV, unless otherwise noted. A dual reactor catalyst treatment system is attached to the XPS vacuum systern so that samples can be pretreated and transferred into the spectrometer without exposure to air.

For the reduced catalyst samples, an in-situ

reduction was always carried out at the desired temperature using a flow rate of 80 cm 3/min of H2 at 1 atm pressure. RESULTS 1.

Interactions between Pd and La203

Electron Binding Energy (eVI

Figure 1.

Palladium 3d Spectra of La203 promoted 0eand supported Pd catalysts after H2 reduction at 300 for 4 hours.

795

A comparison of the Pd 3d spectra for a Pd foil and the various supported and promoted Pd catalysts is shown in Figure 1. similar in all three cases.

The shape of'the spectra are

For the Pd/Si0 2 and Pd/H-ZSM5 catalysts, a shoul-

der on the high binding energy side of the peaks is sometimes observed, which can be removed by more thorough reduction of the catalyst.

The Pd 3d5/2 bind-

ing energy of 335.2 eV for Pd/Si0 2 and Pd/H-ZSM5 is in good agreement with the foil value of 335.1 eV. However, for both Pd/La203 and La203 promoted Pd/H-ZSM5, the Pd 3d5/2 binding energy shifts negatively by up to 0.7 eV.

336.0

336.0

IReduction: H2/300oC/3 Hours)

;-

(Pd/La203)

/

s3

335.6

>

e'

335.2

Q)

c w

'"

c

'0c iii

334.8

N

in ;5l -0

c,

334.4

I

oj ;}

r------~-_,

0.80 ~

e' Q) c

0.40

335.6

>

0.60 X

o

.0 0

w

335.2

iii

334.8

c =g'"

N

•

in ;5l 0.20

334.0 0:----A.. l:'=0--2O-'----~30-----l4O

0.00

~

334.4 334.0

~_--'----_~_~_......J

o

0.20

0.40

0.60

0.80

XeD, IRatio of adsorbed CO to surface Pd atoms at stauration coverage)

Dpd(%1

Figure 2. The dependence of the CO adsorption capacity X ,and the Pd 3d5/2 b~H~ing energy on the Pd dispersion of the Pd/La203 catalysts.

Figure 3. The dependence of the Pd 3d5/2 binding energy on the CO adsorption capacity of the Pd/La203 catalysts. Correlation between metal decoration and electronic interaction.

The negative shift of the Pd 3d5/2 binding energy is proportional to the weight loading of Pd on the La203 support, and inversely proportional to the Pd dispersion (refs. 19,20).

A similar decrease in CO adsorption capacity is

observed to follow the changes in metal loading and dispersion (ref. 27). Shown in Figure 2 is the dependence of the Pd 3d5/2 binding energy and the CO adsorption capacity on Pd dispersion for the Pd/La203 samples. tion capacity, XCOs

=

The CO adsorp-

CO molecule/Pd surface atom, is constant at 0.85 for

Pd/Si0 2, regardless of the Pd weight loading or dispersion (ref. 27).

Con-

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versely, for Pd/Laz03 the CO uptake declines from 0.6 to zero as the dispersion declines from 30% to 8%.

The decrease in XCos is attributable to

decoration of the Pd by the support.

The data shown in Figure 2 reveal the

close correspondence between the negative binding energy shift and the decoration of the metal particles. In Figure 3 the data have been replotted to show the dependence of the negative binding energy shift on XCos'

These data indicate that the Pd 3d5/2

binding energy approaches an asymptotic value of 334.4 eV for complete coverage of the metal particles by the support, i.e., XCos

=

O. This binding

energy is 0.7 eV below the Pd foil value. The La203-promoted Pd/H-ZSM5 samples also evidence a close correlation between the decoration of the Pd particles and the negative binding energy shift.

For the sample in which poor contact between Pd and La203 is achieved

a dispersion of 5.4% is obtained by CO chemisorption.

For the sample in which

good contact between Pd and La203 is achieved a dispersion of 3.5% is obtained.

Palladium surface concentrations derived from the XPS spectra are

1.2% for the former sample and 0.35% for the latter.

These data suggest that

the sample with good contact between Pd and La203 is decorated by the La203. As expected, the Pd 3d5/2 binding energy for the sample with good contact is 334.6 eV, while that for the sample with poor contact is 335.3 eV (ref. 37). In a recent study, Rieck and Bell (ref. 21,22) prepared La203 promoted Pd/Si0 2 catalysts by the same techniques as described above for Pd/H-ZSM5. They observed identical catalytic properties for the La203 supported Pd and the La203 in close contact with Pd on Si0 2• These effects were again attributable to the decoration of the metal particles by the rare earth oxide. The negative shift is extremely air sensitive.

Shortest exposure to air

causes the Pd 3d5/2 binding energy to revert to 335.1 eV, the metal foil value.

Thus, only when samples can be reduced in-situ, and transferred in

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vacuum to the analysis stage, can the binding energy shift be detected. 2.

Interaction between Pt and WO a

The interaction of platinum with tungsten oxide has been widely studied because of the occurrence of hydrogen spillover from Pt to WO a (ref. 1).

The

hydrogen reduces the WO a producing hydrogen tungsten bronzes, HxWO a (refs. 28,29).

The decoration of Pt by WO a is observed by electron microscopy

to occur after catalyst oxidation at 700°C (ref. 25).

Carbon monoxide adsorp-

tion on reduced Pt/WO a/Si0 2 catalysts provide evidence for WO a decoration of Pt.

For the catalysts containing 25 wt% WO a, Pt crystallite sizes calculated

from the CO uptakes range from 300 to 400 angstroms in diameter.

Conversely,

Pt crystallite sizes calculated from x-ray diffraction line broadening range from 100 to 150 angstroms in diameter.

XPS measurements of surface composi-

tion further confirm Pt decoration by WO a•

Both the Pt 4f7/2 peak intensity

and the Pt/Si atomic ratios decrease with increasing loading of tungsten.

46

Figure 4.

36 26 Electron Binding Energy leVI

Tungsten 4f spectra of WO~ promoted Pt/Si0 2 catalysts after H2 reduction at 400 C for 3 hours.

The reduction of W0 3 by hydrogen spillover can be quantitatively followed by XPS.

As shown in Figure 4, W+ s and W+ 4 can be clearly identified after

reduction in H2 (ref. 9).

Reduction of the W0 3 is extensive, extending as far as 1000 angstroms from the Pt particles (ref. 30). The W+ s signal is indic-

798

ative of the formation of HxW0 3 , while the W+ 4 signal is indicative of W0 2 • The W0 2 phase is only formed at high Pt and Wloadings (ref. 23). Table 1 provides a summary of Pt 4f7/2 binding energies measured for the W0 3 doped Pt/Si0 2 catalysts.

For the tungsten oxide free samples the binding

energy is constant at 71.1 eV, in agreement with the foil value (ref. 31). However, for several of the tungsten oxide doped samples, the Pt binding energy is 0.4 to 0.6 eV below that for the bulk metal.

Thus, decoration of Pt

by partially reduced oxides of tungsten causes a negative shift in the Pt 4f7/2 binding energy. TABLE 1 Pt 4f7/2 electron binding energies and Pt dispersion of reduced catalysts (Si 2p at 103.2 eV is reference) W0 3 free l

low W0 3

high W0 3

Wt% Pt

Opt

4f7/2

OPt

4f7/2

OPt

4f7/2

1.2 2.5 3.8 5.0

23 18 13 7

70.9 70.9 70.8 70.8

8 7 6 5

70.9 70.2 70.4

3 4 3 4

70.6 70.6 70.4 70.2

I

2

2

Pt metal foil 70.9±0.1 (eV), Measured by CO chemisorption (percent). 3.

Interaction between Pt and Ti0 2

Changes in the catalyst surface composition following reduction of Pt/Ti0 2 are shown in Table 2.

Reduction of the sample at 200°C and at 500°C causes a

continuous decline in the surface atomic ratio of Pt/Ti from 0.12 to 0.09 to 0.06. TABLE II Changes in XPS surface composition of 5% Pt/Ti0 2 upon Reduction in H2 * Treatment As received 200°C, 2 hr 500°C, 2 hr

* OPt

Atom Percent Ti Pt 19.3 20.3 22.2

Atomic Ratio Pt/Ti

2.4 1.8 1.4

0.12 0.09 0.06

12%, measured following 200°C reduction.

Figure 5 presents Pt 4f7/2 and Ti 2p spectra for these same samples.

The as

received samples contain Pt metal as well as Pt0 2 (shaded region in the fig-

799

ure).

After reduction at 200°C there is still some platinum oxide present,

but this is all converted to the metal during the 500°C treatment.

The high

temperature reduction also leads to a shift in the Pt 4f7/2 binding energy to 70.7 eV, a negative shift of 0.4 eV relative to bulk metallic platinum.

Other

Pt/Ti0 2 catalysts were investigated by XPS following high temperature reduction.

In many cases the negative shift is no more than 0.2 eV, which is

close to the experimental error of the measurement.

The small shift, the dif-

ficulty of completely reducing the Pt particles, and problems with transferring samples to the electron spectrometer without air exposure, may explain why negative shifts have not been observed in some studies of SMSI Pt/Ti0 2 (ref. 32).

Ti 2p

Pt 4f

4&8.7

70.7

As Received

90

75

60

472

462

Electron Binding Energy (eV)

Figure 5.

Changes in Pt 4f and Ti 2p spectra of a 5% Pt/Ti0 2 (rutile) catalyst upon reduction in H2•

DISCUSSION The results show for three systems, Pd:la20a, Pt:WO a, and Pt:Ti0 2, that negative apparent binding energy shifts occur whenever the metal particles become covered by a thin layer of oxide.

The dispersion of all catalyst sam-

ples studied is below 50%, which is indicative of metal particle sizes that exhibit "bulk-like" photoemission (ref. 6).

Therefore, these samples should

800

not exhibit particle size induced binding energy shifts.

What then do the

negative shifts mean? The explanation of the negative shifts lies in understanding the binding energy reference level for insulators.

The surface potential of a sample in

an electron spectrometer depends on the electron current emitted from its surface, the electron current striking its surface, and the current flowing to it through the spectrometer sample holder (refs. 33,34).

Two extreme conditions

exist depending on the insulating properties of the sample:

(1) when the

resistance is low, the sample is neutralized by electron flow from the sample holder.

The Fermi level of the sample equals that of the spectrometer, and

the binding energy is given by the standard relationship, EB = hv-E k,ln - ~ sp , where hv is the photon energy, Eki n is the photoelectron kinetic energy, and ~sp

is the spectrometer work function.

(2) When the resistance is extremely

high, the sample is neutralized by electron flow from the vacuum.

The sample

charges such that its vacuum level is raised to the potential of the incident electron beam,

~e'

The electron beam potential is proportional to the flood

gun voltage, if stray secondary electrons are minimized by using a monochromatic x-ray source and a low gun voltage (ref. 34).

For this second case, the

binding energy is given by

~loc

EB=hv-Ekin-~loc+~e'

function of the sample surface.

where

is the local work

Intermediate sample resistivities can also

exist in which the measured binding energy is influenced partly by the electron current from the flood gun, and partly by th€ spectrometer current through the sample holder. If case (1) is obeyed,the observed binding energy shift can only be ascribed to a change in the energy separation between the metal core electron level and the valence band edge.

Such a situation is unlikely, given that the

metal-support interaction occurs only at the surface, and the electrons which fill the valence band are delocalized throughout the large metal particles (ref. 35). If case (2) is obeyed, the observed binding energy shift can be ascribed to a difference in the local work function of the sample.

This is true provided

801

the flood gun voltage, and in turn,

~e

are constant throughout the experiment.

Since a carbon ls or silicon 2p internal reference is used, and the sample has a common vacuum level, not a common Fermi level, the comparison of binding energies includes both the support work function and the metal work function. Assuming the support work function does not change, an apparent negative binding energy shift is in reality a positive work function change of the metal. Decoration of the metal particles by a thin, partially reduced coating of oxide could be responsible for this work function change.

More specifically

there must be an electronic interaction between the surface metal atoms and the partially reduced support or promoter metal ions which might be best described as a rehybridization between the d9 metal (Pt,Pd) orbitals and the dOX metal ion orbitals where ox is typically equal or less than one (such as d 1 for W+5). Any contribution of the spectrometer current towards sample neutralization will reduce and possibly eliminate the effects of the sample work function. The spectrometer current is affected by the thickness of the sample, the way it is mounted in the spectrometer, and the conductivity of the sample during photoemission.

The conductivity of the catalyst sample is influenced by the

size of the support particles, the degree of contact between the support particles, and the amount of metal deposited.

Thus, unless the XPS experiments

are done with extreme care, it will be difficult to measure reproducibly shifts in the metal work function due to metal-support effects.

Experiments

are now underway which should allow us to carefully control charging effects, such that the electronic nature of the metal-support interactions can be unambiguously established (ref. 36).

CONCLUSION The decoration of metal crystallites by supports or promoters has been demonstrated in many studies of supported metallic catalysts.

Exactly the oppo-

site is true of electronic interactions, with little or not direct experimental evidence available.

We have shown conclusively that negative

802

shifts in the apparent binding energies of the metal core levels occur whenever the metal particles are decorated by partially reduced ter oxide moieties.

support~of

promo-

Through carefully planned photoemission experiments it is

hoped that the electronic interactions in these systems may finally be revealed. REFERENCES 1 M. Boudart, and Djega-Mariadassou, G., "Kinetics of Heterogeneous Catalytic Reactions," Princeton University Press, Princeton, NJ (1984). 2 K. S. Kim and Winograd, N., Chern. Phys. Lett. 30, (1975) 91. 3 M. G. Mason, Gerenser, L. J., and Lee, S.-T., Phys. Rev. Lett. 39 (1977) 288. 4 Mason, M. G., Phys. Rev. B 27, 748 (1983). 5 P. H. Citrin, and Wertheim, G. K., Phys. Rev. B. 27 (1983) 3176. 6 Y. Takasu, Unwin, R., Tesche, B., Bradshaw, A. M., and Grunze, M., ·Surf. Sci. 77 (1978) 219. 7 S. J. Tauster, and Fung, S. C., J. Catal. 55, (1981) 28. 8 S. J. Tauster, J. Fung, S. C., Baker, R. T. K., and Horsley, J. A., Science 211 (1981) 1121. 9 J. A. Horsley, J. Amer. Chern. Soc. 101 (1979) 2870. 10 B.-H. Chen and White, J. M., J. Phys. Chern. 86 (1982) 3534. 11 J. M. Hermann, J. Catal. 89 (1984) 404. 12 J. M. Hermann, Gravelle-Rumeau-Maillot, M., and Gravelle, P. C., J. Catal. 104 (1987) 136. 13 Y.-W. Chung, Xiong, G., and Kao, C.-C., J. Catal. 85 (1984) 237. 14 J. Santos, Phillips, J., and Dumesic, J. A., J. Catal. 81 (1983) 147. 15 C. C. Kao, Tsai, S. C., and Chung, Y.W., J. Catal. 73 (1982) 136. 16 D. N. Belton, Sun, Y.-M., and White, J. M., J. Phys. Chern. 88 (1984) 5172. 17 J. A. Dumesic, Stevenson, S. A., Sherwood, R. D., and Baker, R. T. K., J. Catal. 99 (1986) 79. 18 S. D. Cameron, and Dwyer, D. J., Surf. Sci. 176 (1986) L857. 19 R. F. Hicks, Yen, Q. J., Bell, A. T., and Fleisch, T. H., Appl. Surf. Sci. 19 (1984) 315. 20 T. H. Fleisch, T. H. Hicks, R. F., and Bell, A. T., J. Catal. 87 (1984) 398. 21 J. S. Rieck, and Bell, A. T., J. Catal. 99 (1986) 262. 22 J. S. Rieck, J. S., and Bell, A. T., J. Catal, 99 (1986) 278. 23 J. R. Regalbuto, Fleisch, T.H., and Wolf, E. E., J. Catal., accepted. 24 R. T. Thomson, Fleisch, T. H., and Wolf, E. E., J. Catal., to be submitted. 25 J. R. Regalbuto and Wolf, E. E., J. Catal., submitted. 26 G. S. Lane and Wolf, E. E., J. Ctal., in press. 27 R. F. Hicks, Yen, Q.-J., and Bell, A. T., J. Catal. 89 (1984) 498. 28 M. Boudart, Vannice, M. A., and Beson, J. E., J. Phys. Chern. 6 (1969) 171. 29 T. H. Fleisch and Mains, G. J., J. Chern. Phys. 76 (1982) 780. 30 T. H. Fleisch and Abermann, R., J. Catal. 50 (1977) 268. 31 T. H. Fleisch and Mains, G. J., J. Phys. Chern. 90 (1986) 5317. 32 T. Huizinga, Van T. Blik, H. F. J., Vis, J. C., and Prins, R., Surf. Sci. 135 (1983) 580. 33 A. Dilks, "Electron Spectroscopy: Theory, Techniques, and Applications," Vol. 4, Brundle, C. R., and Baker, A. D., Eds., Academic Press, NY (1981). 34 R. T. Lewis, and Kelly, M. A., J. Electron. Specrosc. Rel. Phenom. 20 (1980) 105. 35 W. H. Chen, White, J. M., and Ekerdt, J. G., J. Catal. 99 (1986) 293. 36 R. F. Hicks, and Fleisch, T. H., in preparation. 37 T. H. Fleisch, R. T. Thomson, and E. E. Wolf, in preparation.