The effect of gas environment (H2, O2) on the structural properties of small iridium metal particles supported on gamma-Al2O3 as determined by EXAFS

The effect of gas environment (H2, O2) on the structural

properties of small iridium metal particles supported on

gamma-Al2O3 as determined by EXAFS

Citation for published version (APA):

Koningsberger, D. C., Zon, van, F. B. M., Kip, B. J., & Sayers, D. E. (1986). The effect of gas environment (H2,

O2) on the structural properties of small iridium metal particles supported on gamma-Al2O3 as determined by

EXAFS. Journal de Physique. Colloque, 47(C8), 255-259. https://doi.org/10.1051/jphyscol:1986847

DOI:

10.1051/jphyscol:1986847

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Published: 01/01/1986

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C0110que

ca,

supp1ément au n" 12, Tome 47, décembre 1986 CS-2.55

THE EFFECT OF GAS ENVIRONMENT (H₂, O₂) ON THE STRUCTURAL PROPERTIES OF

SMALL IRIDIUM METAL PARTICLES SUPPORTED ON ~-A120) AS DETERMINED BY EXAFS

D.e. KONINGSBERGER, F.B.M. DUIVENVOORDEN, B.J. KIP

and

D.E. SAYERS'

Laboratory for Inorganic Chemistry and Catalysis, Eindhoven

University of Technology, P.O. Box

513,

NL-5600

MB

Eindhoven,

The Netherlands

'Department of

Physics, North Carolina state University,

Raleigh, NC 27695-8202, U.S.A.

Abstract

EXAFS measurements have been carried out on an 0.8 wt\ Ir/Y-Alp3 cata-lyst after the followlng treatments: 1) reductlon at 773 K. 2) evacuatlon at 623 x. 3) 02 adsorption at 77 K. 4) suhsequent heatlng to 100 1<' The tent~tlve

results show tbat chemisorbed hydrogen influences the structure of the metal par-Ucles and their interaction wUh the support. Oxygen adsorptlon at 77 x (most probab1y physisorption) seems to produce the same kind of metal-oxygen bands as detected for the metal-support interface. when the metal crystallites are under vacuum. Subsequent heating to 100 K resulted in the onset of oxygen chemisorption.

1. Introduction

Structural properties of small metal particles . supported on non-interacting substrates (mylar. rare gas sol1ds) have been extensively studied with EXAFS [1-3]. Kxperiments ware performed under vacuum showing contractions of the nearest-neighbaur distance and a decrease of the Debye temperature of the metal particle due to the softening of the phonon spectrum [4.5]. EXAFS exper1ments on dispersed metal catalysts (supported on stronger lnterecting suhstrates. e.g. Y-AlZ03' Ti02_ Si02) have been mostly carrled out after reduction with the metallic partieles covered with chemisorbed hydrogen [6-10]. Under these cond1tions contraction of the metal-metal coordinatlon distance has not been observed. This can be due to the lnfluence of the chemisorbed hydrogen or the interaction of the metallic partiele with the support. Tentative EXAFS results have been reported for Pt/NaY (zeol1te) [11] and Rh/Alp3 [12) catalysts under vacuum showing a contractlon of the flrst neighbaur coordination distance.

Chemisorbed hydrogen influences the electronic properties of the metal [13] which in turn may change the metal-support interaction. The metal-sup-port interaction has been studied with EXAFS for Rh/AIP3 [8.9] and Pt/AlZ03 [10] catalysts with the metal partieles covered wlth chemisorbed hydrogen.

Here we present the preliminary results obtained for a llighly dlspersed 0.8 wt\ Ir/Y-AIP3 catalyst with and without chemisorbed hydrogen. Removal of chemisorbed hydrogen seems to influence the metal-support oxygen bands. Oxygen physisorption at 77 K was carried out in order to compare the metal-oxygen bands formed hy thls adsorption process with the metal-support oxygen bands. The onset of tbe oxygen chemlsorptlon process was further studied with SXAFS.

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2. Experimental

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An 0.8 wt% lr/Y-A1203 catalyst was prepared by incipient wetting the support (y-A120]. Ket jen type 000-1.5 E. surface area 200 m2 g-1 • pore volume 0.6 m2 g- I ) with an aqueous solution of IrC13'x

Hl0.

The catalyst was dried in air at 395 K (heating rate 2 K min-I) for 16 h followed by direct reduction in flowing H2 at 773 K for 5 h (heating rate 5 K mln-I). After this treatment the catalyst was passivated with 02 at room temperature and stored for further use. Hydrogen chemisorption measurements resulted in H/"

=

2.6. indi-cating highly dispersed metal partieles. A further characterization of th is cata-lyst with different physical methods has been described in [14).

The treatments of the catalyst prior to the EXAFS experiments were performed in an EXAFS in situ cello The sample was pressed into a self-supporting wafer with an optimum thicl<:ness to give.t.11X 1.5-2.0 at the iridium LlII-edge. The following treatments were carried out:

1) Rereduction in flowing H2 at 473 K for 1 h (producing the same reduced state as obtained aft er the first reduction at 773 K); 2) Evacuation at 623 K for 2 h. P - 10-5 Pa; 3) 02 adsorption (10 5 Pa) at 77 K; 4) Heating under 02 to 100 K.

EXAFS data were collected after every treatment at 77 K. The EXAFS spectra were recorded at the EXAFS station of beamline X-IHI of the NSLS at Brool<:haven

(ring energy 2.5 GeV. ring currents 50-100 mA).

3. Results and Discussion

EXAFS oscillations in I<: space are obtained from the x-ray absorption spectrum by a cubic spline bacl<:ground subtraction and normalization by means of division by the he1ght of the edge [8]. The EXAFS data for all samples are shown in Fi;}. 1. The imaginary part of the Fourier transforms of the EXAFS data are displayed in Fig. 2. The imaginary part contains amplitude (viz. coordination number and disorder) and phase (viz. coordination distance) information. The Fourier transforms are corrected for the metal-metal phase and bacJescattering amplitude. obtalned from EXAFS experiments on Pt foil. The suitability of using this platlnum standard for analyzing nearest and next nearest neighbours has been discussed in [15].

Bl' comparing the experimentaI EXAFS data obtained aft er reduction (Fig. la) and af ter evacuation (Fig. lb). it can be seen that the amylitude of the EXAFS oscillations in Fig. lb is lower at high I<: values (k>8 A- ). This indicates a larger disorder for the iridium metal particles after evacuation. "This also causes a lower amplitude of the corresponding Fourier transform (compare in Fig. 2a the solid line wlth the dotted line). A full analysis will mal<:e clear whether a decrease in coordination number (= decrease in average particle size) has also to be tal<:en into account to explain the lower amplitude of the Fourier transform for the samplé after evacuation. An increase of the Debye-Walter factor is in line with a softening of the phonon spectrum due to the removal of the che-misorbed hydrogen from the surface of the metal particles. The IrG-Ir G first coordination shell peal<:s in the Fourier transform at the right distance due to the applied phase correction. It can be clearly noticed in Fig. 2a that evacua-tion induces a contract ion of the IrG-Ir G coordinaevacua-tion distanee.

The asymmetry at lew rvalues (1. 5<r<2. 5 A) in the 1<:1 weighted Fourier "transforms in Fig. 2a is most probably caused by contributions of metal-support bonds analogous to the resul ts obtained for Rh/Al 203 [8.9] and Pt/A1203 [10] catalysts. It is obvious from Fig. 2a that the phase of the imaginary part of the Fourier transform at low rvalues changes after evacuation. indicating a change in coordination distance of the metal-support honds.

oxygen adsorption at 77 K changes the low Je part of the EXAFS spectrum (com-pare Fig. lb with Fig. Ic), which might be due to oxygen nearest neighbours. A clear picture can be obtained from the Fourier transforms of the corresponding EXAFS data (compare Fig. 2b the solid line with the dotted line). The asymmetri-cal part at low rvalues (1.5<r<2.5 A) increases in amplitude while the phase of the whole transform remains the same. From th is one can directly conclude that

C&-257

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"'0-2

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3

3

2

a

2

b

1

1

0

0

-1

-1

-2

-2

20

16

12

8

4

k(l-l)

o

3

2

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4

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x10-2

2

Figure 1. EXAFS data for the 0.8 wt\ Ir/y-A1203 catalyst: a) reduction at 773 K. bl evacuation at 623 K. c} 02 adsorption at 77 K. d} subsequent heating to 100 K.

the oxygen adsorption process at 17 K leads to oxygen neighbours at the surface of the iridium metal particles with the same type of metal-oxygen bonds as present in the metal-support interface af ter evacuation.

The conclusions drawn from the results mentioned above are further supported by the following results. Heating the sample under 02 to 100 K gives rise to a significant change in the EXAFS spectrum. The spectrum (Fig. ld) is now dominated by oxygen as backscatterer with a different (shorter) coordination distance. This can be seen clearly in Fig. 2c. The amplitude of the low r part (due to oxygen) of the Fourier transform largely increases accompanied by a change in phase (dif-ferent metal-oxygen distance). The metal-'metal part of the Fourier transform

(2.5<r<3 A) decreases in amplitude showing the onset of the oxidation of the metal particles.

In conclusion. the results presented here definitely show the influence of chemisorbed hydrogen on the structural proper ties of small metal particles. Che-misorbed hydrogen also influences the metal-support interaction. Oxygen physi-sorption at 17 K produces the same type of metal-oxygen bonds as present in the metal-support interface of naked oxide-supported metallic particles. A complete analysls of the EXAFS data 15 in progress and wl11 be publlshed elsewhere.

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b

rOl.)

6

8

6 x10-

1

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0

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-4

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-6

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0

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-8

-12

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-6

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Figure 2. Imaginary parts of the Fourier transforms (kl. Ak= 3-12 A-I. phase and amplitude corr.) of the BXAFS data obtained af ter different treatments. a) Solid line: reduction at 773 K. Dotted line: evacuation at 623 K; b) solid line: evacuation at 623 K. Dotted line: 02 adsorption at 77 K; c) Solid line: 02 adsorption at 77 K. Dotted line: subsequent heating to 100 K.

Acknowledqment

one of us (DBS) likes to acknowledge grant DB-A505-80ERI0742 provided by the Department of Energy for the development of beamline X-IIA (NSLS).

References

[1] Ap,ü. G.• Hamilton. J.F .• 5tohr. J •• and Thompson. A.. Phys. Rev. Lett. 43 (979) 165.

[2] Montano. P.A .• Shenoy. G.K .. Morrison. T.r. and Schulze. W.• in 'EXAFS and Neae Edge Steucture 111'. Eds. K.O. Hodgson. B. Hedman. and J.E. Penner-Hahn (springer-Verlag. Berlin. 1984) p. 231.

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[3) Balerna. A.. Bernieri. E.• Picozz1. P •• Reale. A.• Santuee1. S .• Burattini. E.. and Mobilio. S.• Phys. Rev. Ral (1985) 5058. [4) Balerna. A.• and Mobilio. S .• Phys. Rev. B34 (1986).

[5) Delley. B.• EUis. D.E .• Freeman. A.J .• Baerends. E.J .• and Post. 0 .• Phys. Rev. B27 (1983) 2132.

[6) Via. G.H .• Sinfelt. J.H., arid Lytle. F.W., J. Chem. Phys. 71 (1979) 690. [7) Lagarde. P .• Murata. T.• Vlaic. G.• Freund, E.• Dexpert, H.• and

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[8) Van Zon. J.B.A.D .• Koningsberger. O.C .• Van Ot Blik. H.F.J .• and sayers. D.E .• J. Chem. Phys. 82 (1985) 5742.

[9) Koningsberger. O.C .• Van Zon. J.B.A.D .• Van Ot Blik. H.F.J .• Visser. G.J .• Prins. R.• Mansour. A.N .• sayers. D.E .• Short. D.R .• and Katzer. J.R .• J. Phys. Chem. 89 (1985) 4075.

[10) Koningsberger. O.C .• and sayers, D.E .• Solid state lonies 16 (1985) 23. [11] Moraweek. B.• and Renouprez. A.J .• Surf. Sei. 106 (1981) 35.

[12] Van 't Blik. H.F.J., Van Zon, J.B.A.D .• Koningsberger. O.C .• and Prins. R., J. Mol. Catal. 25 (1984) 379.

(13] Lytle. F.W., Greegor, R.B .• Marques. E.C .• Sandstrom. D.R .• Via. G.H., and Sinfe1t. J.H .• J. Catal. 95 (1985) 546.

[14) Kip. B.J., Van GrondeUe. J .• Hartens. J.H.A .• and Prins. R.• accepted for pub1ication in Appl. Catal.

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