An EXAFS study of the formation of small metal aggregates on MgO using organometallic compounds as precursors

An EXAFS study of the formation of small metal aggregates

on MgO using organometallic compounds as precursors

Citation for published version (APA):

Koningsberger, D. C., Lamb, H. H., Kirlin, P. S., Kelly, M. J., & Gates, B. C. (1986). An EXAFS study of the formation of small metal aggregates on MgO using organometallic compounds as precursors. Journal de Physique. Colloque, 47(C8), 261-264. https://doi.org/10.1051/jphyscol:1986848,

https://doi.org/10.1051/jphyscol;1986848

DOI:

10.1051/jphyscol:1986848 10.1051/jphyscol;1986848 Document status and date: Published: 01/01/1986 Document Version:

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AN EXAFS STUDY OF THE FORMAT ION OF SMALL METAL AGGREGATES ON MgO USING ORGANOMETALLIC COMPOUNDS AS PRECURSORS

D.C. KONINGSBERGER*, H.H. LAMB, P.S. KIRLIN, M.J. KELLY and B.C. GATES

Center for Cata1ytic Science and Techno1ogy, Department of Chemica1 Engineering, University of De1aware, Newark, DE 19716, U.S.A.

*Laboratory for Inorganic Chemistry and Cata1ysis, Eindhoven University of Techno1ogy, NL-5600 MB Eindhoven, The Netherlands

Abstract:

EXAFS measurements have been carried out on very small osmium and rhenium metal particles supported on MgO. The metal particles were formed via decornposition

and subsequent reduction in hydrogen of metal carbonyl clusters.

1. Introduction

Starting with molecular metal carbonyl clusters, extremely small and weIl defined metal particles can be prepared, allowing a detailed study of the

structure of the metal particle and the metal-support interface. The surface structures resulting from decornposition of the metal carbonyl clusters are ~onic

complexes with metal cations bound to oxygen anions of the support. Information of the metal-cation oxygen-anion bonds in these ionic complexes can be obtained with EXAFS. [1] By a subsequent controlled reduction procedure, the formation of the metal particle and the generation of the metal-support interface can be studied in great detail with the EXAFS technique. Here we present the preliminary results of an EXAFS study of the formation of small osmünn and rhenünn metal particles supported on ;~gO using [OS10C(CO)24JZ- and H3Re3(CO) 12 as precursors.

2. Experimental

A stabIe

[O~10C(CO)Z4]Z-

cluster was prepared by treating

H~OsCI6

irnpregnated IvlgO (1.3 wH Os) with a mixture of CO/HZ = 1/1 at Z75 C for 5 hr. [Z] A subsequent reduction was carried out in flowing HZ at 3000

e

for 1 hr. A H3Re3(CO)12 cluster was deposited on

MgO

(Z wt% Re) from a hexane solution under vacuum.

Reduction was carried out at 500°C in flowing HZ for 1 hr.

The treatments of the catalysts and the EXAFS experiments were pel'formed in an EXAFS in situ cello The samples were pressed into a t}lin self-supporting wafel' with llil optimum thickness to give 6~x ~ 1.5 - Z.O at the corresponding metal

L111-edee. The EXAFS experiments were carried out at liquid nitrogen temperature with the catalysts exposed to 1 atm. HZ. The EXAFS spectra were recorded at EXAFS station C-2 of the CHESS laboratory at Cornell (ring energy 5 GeV, ring currents ZO-40

mA).

C8-262

3. Data Analysis mld Results

JOURNAL DE PHYSIQUE

EXAFS oscillations in k space are obtained from the x-ray absorption spectrum by a cubic spline background subtraction and norrnalization by means of division by the hight of the edge. [3] Phase shift and backscattering functions extracted from EXAFS data on Pt-foil and Na2Pt(OH)6 have been used to analyze the 050_-050

and Osn+_02- EXAFS contributions. The sUltability of using these platinum standards for analyzing the osmium EXAFS spectra has been discussed LTl rl]. Re-foil and

Re03 have been taken as standards for the Reo-Reo and Ren+-02- absorber-scatterer pair, respectively.

Fig. la and d present the raw EXAFS oscillations in k space of the reduced Re(2)/MgO and Os(1.3)/MgO samples. A k3 foreward followed by an inverse Fourier transforrn can isolate the first metal-metal coordination shell from higher coordination shells. The inverse Fourier transforrn contains also the low ~

contributions present in the same r-range used for the inverse Fourier

transforrn. Owing to the high quali ty of the EXAFS data and the large k range of the backscattering amplitude of osmium and rhenium the first shell metal-metal

EXAFS can be reliably separated fro~ these low ~ contributions by fitting in k space starting at about k

=

7

(Ä- ).

The fitting procedure results in the 0

following first shell metal-metal coordination parameters: N

=

5.5, R

=

2.74 A, 60 2 = 0.0019 Ä2 (ReO-ReO) and N= 4.8, R = 2.62 Ä, 60 2 = 0.0027 Ä2 (050_{-050 ).}

The presence of low ~ contributions in the experirnental data can now be

demonstrated by using a k1 Fourier transforrn which is corrected for the metal-metal phase and backscattering amplitude. [4]. Fig. lb, c, e and f s~or the marnitudes and irnagi~arY parts of Fourier transforrns (Os: k1, 6k

=

3.2 - 14 A- ; Re: k , 6k

=

3.5 - 13 A-l) of the experirnental data and the EX~S functions calculated with the metal-metal parameters given above. The differences between the solid and the dotted lLTleS in the r range of the corresponding first shell metal-metal peak are due to the low ~ scatterers present in this r range. By subtracting the calculated first shell metal-metal EXAFS function from the eÀ~erimentaldata a difference file is obtained which contains all low ~-contributions and, if present, higher shell metal-metal contributions. To detect metal-support oxygen contributions the difference files for both samples have been Fourier transforrned with a correction made for the metal-oxygen phase shift (see Fig. 2a and b).

4. Discussion

The rhenium particles created by the reduction procedure carried out in this work show a nearest neighbour coordination distance of 2.74 A equal to the value of rhenium bulk metal. The rhenium-rhenium coord~ation nurnber of 5.5 indicates an average (spherical) partical size of about 10 A consisting of about 15 metal atorns. From these results one can infer that the applied reduction treatment causes the agglomeration of approxirnately 5 Re} molecular entities.

The Fourier transforrn (corrected for tlle Re-O phase shift) of the difference file (Fig. 2a) shows an inteference of at least two contributions. These

contributions most probably arise from scatterers present in the metal-support interface (i.e. oxygen and/or magnesium). A full analysis of the difference file will be perforrned LTl due course and published separately.

The reducticn of the [OS10C(CO)24]2- cluster results in particles with an osmium-osmium coordination distance of 2.62 A which is significagtly shorter than the corresponding distance in the [OS10C(CO)24]2- cluster (2.84 A). [5] This

indicates that the osmium carbonyl clusters are transforrned by the reduction process into osmium metal aggregates. The average 050- Osocoordination number (N

=

4.8) is

(within the error of ± 0.2) the same as found for the osmium carbonyl cluster, indicatLTlg the presence of spherical particles consisting of 10 atorns. Clearly, the reduction process does not lead to any sintering of the osmium metal aggregates. The Fourier transforrn of the difference file (Fig. 2b) shows for 1<R<3 a much more

cornplica~ed imaginary.part a~ fo: the rhenium sample. The most probable explanation

for the lnterference m the lIDagmary part is a superposition of peaks due to the same type of scatterers as present for the Re/MgO sample with peaks arising from

20 10 EXAFS I:

~

d I1 /' I '~

!\ \

,

'\ i! f 'I I \

f\

I~ !~

A/\... I

11

\) \J

VV \'

I '", \ : , ; I I I' i

i

I \ \

I

:L

~

\1 \ \1 _klk1 ) 1 -1

o

1 -2

o

20 a x10-2 2 1 0 -1 I 1

I

-2

~

-3 0 10 x10-1 x1Q-1 I FT! b 10 IFTI

~

e 15 I' 10 r

I:

" ,I , I ₅ I '\ I '

ft

I I

~

5 ,I

~

, \ i :

:.. : \p

f

I I~ - (,:

~r"

\

r\

rIA) ~'/';\'~A) ~ \/\~,

I ./

/",- .~r'.,.'\/ \ / \)Ij _";?,L, ' v _ - " ' ' . _ 0 .1 " .... -, _ . ' . .•o . 0 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 x10-1 X 10-1 15

1

I

10 Im FT _c 10 5 5 0 0 -5 -5 -10 -15 rIAl _-10 r(A) 0 1 2 3 4 5 6 7 8 0 2 3 4 5 6 7 8

Figure 1. Data for the Re/MgD Îa~b~c) and the Os/MgO Îd~e~f) catalygts. Raw EXAFS spectra Îq~d). Fourier transform (k1~ Re : 6k

=

3.5 - 13 A-1~ Os : 6k

=

3.2 - 14 A-1~ metal-metal phase and amplitude corr.) of experimental data (solid lines) and calculated l~-~P EXAFS Îdotted lines). (b,e) Magnitude. Îc~f) Imaginary part.

C8-264 JOURNAL DE PHYSIQUE b 5 10x1Q-1 FT -5 r(Al .10-I---,.---,---.---,.---.--.,...--~_J

o

1 2 3 4 5 6 7 8 7 8 6 5 2 3 4 FT

o

1 (K11 , , Re ~k - 3.5 - 13

A-

1 ,... OS : ~k

=

3.4 - 11 ....~-1~

metal-oxygen phase corr.) of the difference file (see text). (a) Re/MgO, (b) Os~~gO. Magnitude : solide ~ine, Imaginary part: dotted line.

Figure 2. Fourier transform

a residual Os-C

=

0* contributioIl. As 05-0 phase corrected Fourier transforw of ~1

Os-C

=

0*

entity gives an imaginary part which peaks pqsitive at about 1.9 A (C) and almast negative at about 3 Ä (0*) [lJ. Tne C and ~ peak positions are indicated in Fig. 2b. From this result one c~~ conclude that the reduction conditions for the osmium sample as applied in this work does not lead to a complete remaval of all carbonyl ligands. A more complete reduction procedure is necessary befare the

scatterers present in the Os/MgO interface can be fully analyzed from the resulting EXAFS spectrum.

Acknowledgement. Tnis work was supported by agrant from the National Science Foundation (CPE-8317140). Tne authors like to acknowledge the assistance of the staff of the Cornell Synchrotron.

Re fe rences

[lJ Duivenvoorden, F.B.M., Koningsberger, D.C., Uh, Y.S. and Gates, B.C., Accepted for publication J.Am. Chem. Soc.

[2J Lamb, H.H. et al., Accepted for publication J. Chem. Soc., Chem. Commun.

[3J van Zon, J.B.A.D., Koningsberger, D.C., van 't Blik, H.F.J., and Sayers, D.E., J. Chem. Phys. 82 (1985) 5742.

[4] Koningsberger, D.C., Sayers, D.E., Solid State lonics, 16 (1985) 23.

[5] Cook, S.L., Evans, J., Greaves, G.N., Johnson, B.F.G., Lewis, J., Raithby, P.R., WelIs, P.B. and Worthington, P., J.Chem. Soc., Olem. Commun. (1983) 777.