Characterization of Mixed-Metal Catalysts by Iron ... - ACS Publications

26 Characterization of Mixed-Metal Catalysts by Iron-57 and Ruthenium-99 Mössbauer

Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

Spectroscopy M . L. GOOD , M. D. PATIL, J. T . DONNER, and C. P. MADHUSUDAHAN 1

2

Division of Engineering Research, Louisiana State University, Baton Rouge, LA 70803

99

Iron-57 and Ru Mössbauer spectroscopy have been used to assess the chemical composition of an iron-ruthenium bimetallic catalyst system as a function of preparation and treatment. Supplementary data from ESCA spectroscopy and x-ray powder diffraction made it possible to completely characterize the material from the initial salt mixture through a hydrogen reduction and a subsequent calcination step. RuCl • xH O and FeSO • xH O were mixed in a 1:2 mole ratio in the solid state, slurried with a small amount of water, and evaporated to dryness. The product contained a mixture of iron(II) and iron(III) salts and some anhydrous RuCl . This material was reduced under flowing H at 400°C for 4 h. Surprisingly, the "reduced" product con tained ruthenium metal, RuO , bulk γ-Fe O (some FeS), and a fraction of small-particle paramagnetic γ-Fe O . After calcination the product was unchanged except for the loss of the FeS and an increase in the average particle size. The study indicates the special application of Ru Mössbauer spectroscopy to such solid-state problems and the extra versatility gained by "double Mössbauer labeling." 3

2

h

2

3

2

2

2

3

2

3

99

r

phe

complete physical a n d chemical

description

o f heterogeneous

m e t a l l i c catalysts a n d t h e r e l a t i o n s h i p b e t w e e n t h e i r p r o p e r t i e s a n d t h e i r c a t a l y t i c a c t i v i t y has b e c o m e t h e g o a l o f n u m e r o u s groups of i n v e s t i Current address: Universal Oral Products, Inc., Des Plaines, IL 60016. * Current address: Englehard Industries Division, Menlo Park, Edison, NJ 08817. 1

0065-2393/81 /0194-0553$05.00/0 © 1981 American Chemical Society In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

554 gators.

MOSSBAUER

S P E C T R O S C O P Y A N D ITS C H E M I C A L A P P L I C A T I O N S

T h e o v e r a l l t h e m e of t h e i r w o r k is to u n d e r s t a n d b e t t e r

interrelationships among

electronic

a n d c a t a l y t i c a c t i v i t y so t h a t m o r e effective rather than discovered.

the

structures, p h y s i c a l c h a r a c t e r i s t i c s , catalysts c a n be

designed

S i n c e most heterogeneous c a t a l y s t systems are

c o m p l e x solid-state m i x t u r e s of m e t a l a n d s u p p o r t , t h e c o m p l e t e

descrip

t i o n i n b o t h c h e m i c a l a n d p h y s i c a l terms is v e r y difficult, i f n o t i m p o s s i b l e to a c h i e v e .

H o w e v e r , d u r i n g the p a s t d e c a d e n e w p h y s i c a l tools

have

b e c o m e a v a i l a b l e to p r o b e these m a t e r i a l s , a n d o u r d e t a i l e d k n o w l e d g e of t h e i r m o l e c u l a r a n d solid-state structures is i m p r o v i n g at a r a p i d rate. E l e c t r o n m i c r o s c o p y has b e c o m e a r o u t i n e t o o l ( j o i n i n g t h e l o n g a v a i l a b l e Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

x-ray diffraction techniques)

f o r accessing t h e p h y s i c a l state of

hetero

geneous catalysts, a n d t h e i m p r o v e m e n t i n r e s o l u t i o n d o w n to a

few

angstroms has m a d e i t p o s s i b l e to "see" e v e n s m a l l clusters of m e t a l atoms highly dispersed

on

a support

material ( 1 , 2 ) .

New

and

improved

spectroscopic t e c h n i q u e s , p a r t i c u l a r l y E S C A , A u g e r , a n d m a g n e t i c reso nance

methods,

are n o w

used

r o u t i n e l y for

probing

the

electronic

p r o p e r t i e s a n d e l e m e n t a l c o m p o s i t i o n of t h e surface a n d b u l k of s u p p o r t e d a n d m i x e d - m e t a l catalysts (3,4,5).

A r e l a t i v e l y recent a d d i t i o n to t h i s

arsenal of p h y s i c a l m e t h o d s is M o s s b a u e r spectroscopy,

w h i c h can pro

v i d e u n i q u e i n f o r m a t i o n i n s p e c i a l cases i n v o l v i n g a m e t a l h a v i n g a nucleus

t h a t is

Mossbauer

active.

The

utility

of

this m e t h o d

has

p r o g r e s s e d to t h e p o i n t w h e r e M o s s b a u e r spectroscopy is i n c l u d e d i n a n y g e n e r a l d i s c u s s i o n of spectroscopic m e t h o d s for e v a l u a t i n g heterogeneous catalyst systems (6).

I n those cases w h e r e i t is a p p l i c a b l e , this t e c h n i q u e

has several advantages f o r heterogeneous c a t a l y s t c h a r a c t e r i z a t i o n . I t is b a s i c a l l y a solid-state m e a s u r e m e n t w h i c h i n the t r a n s m i s s i o n m o d e c a n p e n e t r a t e s o l i d substrates t h a t are o p a q u e to energies i n t h e o p t i c a l or v i b r a t i o n a l s p e c t r a l ranges, a n d i n t h e b a c k s c a t t e r m o d e c a n

provide

i n f o r m a t i o n a b o u t surface species. T h e m e t h o d p r o v i d e s c h e m i c a l s p e c i a tion information t h r o u g h "fingerprint" spectra, a n d structural a n d particle size information b y

analysis of

quadrupole

splitting parameters

and

m a g n e t i c s p e c t r a . U s e d i n c o n j u n c t i o n w i t h other m e t h o d s s u c h as E S C A , x-ray, a n d electron

diffraction, Mossbauer

spectroscopy

can

provide

d e f i n i t i v e c h e m i c a l a n d p h y s i c a l p a r a m e t e r s f o r heterogeneous c a t a l y s t systems, b o t h s u p p o r t e d a n d u n s u p p o r t e d .

P a r t i c u l a r l y significant meas

u r e m e n t s c a n b e m a d e i n those f e w cases w h e r e m i x e d - m e t a l catalysts c o n t a i n m o r e t h a n one M o s s b a u e r - a c t i v e n u c l i d e . T h i s c h a p t e r reports t h e results of s u c h a case w h e r e a n u n s u p p o r t e d catalyst system of i r o n a n d r u t h e n i u m w a s c h a r a c t e r i z e d as a f u n c t i o n of catalyst t r e a t m e n t .

Bimetallic Catalyst Systems—The Special Case of Ruthenium—Iron U n t i l q u i t e r e c e n t l y , m o s t b i m e t a l l i c o r a l l o y catalysts h a v e b e e n b u l k m a t e r i a l s w i t h l o w surface areas, u s u a l l y f o r m u l a t e d as p r e s s e d

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

26.

GOOD

E T

Mixed-Metal

AL.

p o w d e r s , w i r e s , or foils.

555

Catalysts

H o w e v e r , the ever i n c r e a s i n g n e e d f o r m o r e

efficient a n d m o r e selective catalysts has s t i m u l a t e d a n u m b e r of studies of b i m e t a l l i c m a t e r i a l s of h i g h surface area d i s p e r s e d o n a v a r i e t y of supports. M o s s b a u e r s p e c t r o s c o p y has b e e n e s p e c i a l l y effective i n d e t e r m i n i n g the c h e m i c a l states of these d i s p e r s e d m a t e r i a l s w h e r e techniques provide only l i m i t e d information. t h e p l a t i n u m - i r o n alloys o n g r a p h i t e ( 7 ) ;

I l l u s t r a t i v e examples

other are

t h e p a l l a d i u m - i r o n alloys o n

a l u m i n a ( 8 , 9 ) ; the r u t h e n i u m - i r o n alloys o n s i l i c a ( 1 0 ) ; a n d the n i c k e l i r o n alloys o n s i l i c a

(11,12,13).

I n e a c h case, o n l y i r o n

Mossbauer

s p e c t r o s c o p y w a s c a r r i e d out, a n d the c h e m i c a l state of the s e c o n d m e t a l Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

w a s i n f e r r e d f r o m t h e i r o n d a t a or d e t e r m i n e d b y other, u s u a l l y less definitive, methods. T h e M o s s b a u e r d a t a p r o v i d e d a d d i t i o n a l i n f o r m a t i o n of c a t a l y t i c significance b y e x h i b i t i n g s u p e r p a r a m a g n e t i c spectra associ a t e d w i t h the v e r y s m a l l p a r t i c l e s d i s p e r s e d o n the supports. A n analysis of this feature of the v a r i o u s spectra a l l o w e d the d e t e r m i n a t i o n of t h e p a r t i c l e sizes of

the active catalyst m a t e r i a l s .

The

success of

these

studies, a n d p a r t i c u l a r l y the i n f o r m a t i o n o b t a i n e d f r o m just the i r o n M o s s b a u e r spectra f o r the r u t h e n i u m - i r o n system, suggested t h a t s i m i l a r studies w h e r e b o t h metals w e r e s u s c e p t i b l e to M o s s b a u e r s p e c t r a l analysis m i g h t be

quite productive

and indeed, might have

the potential to

e l i m i n a t e c e r t a i n a m b i g u i t i e s left b y the fact t h a t so f a r o n l y the i r o n i n t h e b i m e t a l l i c m i x t u r e s has b e e n s t u d i e d . S e v e r a l classic studies u s i n g other M o s s b a u e r - a c t i v e n u c l e i to s t u d y alloys h a v e b e e n r e p o r t e d , a l t h o u g h i n g e n e r a l these h a v e b e e n d i r e c t e d t o w a r d u n d e r s t a n d i n g the e l e c t r o n i c effects i n p u r e m e t a l l i c alloys i n s t e a d of f o l l o w i n g the c h e m i c a l a n d p h y s i c a l changes that o c c u r d u r i n g t h e p r e p a r a t i o n a n d m a n i p u l a t i o n of catalyst m a t e r i a l s . T h e e l e c t r o n i c a n d m a g n e t i c p r o p e r t i e s of the h i g h - p r e s s u r e h e x a g o n a l phase of i r o n ( c phase) have been probed by alloying iron w i t h ruthenium and osmium, a n d o b s e r v i n g t h e i r o n M o s s b a u e r spectra (14,15,16).

S i m i l a r studies to

observe i n t e r n a l h y p e r f i n e fields a n d a l l o y electronic structures h a v e b e e n c a r r i e d out o n i r o n - r h o d i u m alloys alloys (19,20).

a n d other

(17,18)

ferromagnetic

A f e w systems h a v e b e e n i n v e s t i g a t e d b y p r o b i n g w i t h

other M o s s b a u e r n u c l e i . I r o n - p l a t i n u m alloys h a v e b e e n s t u d i e d b o t h i n t r a n s m i s s i o n a n d b a c k s c a t t e r experiments u s i n g t h e 9 9 - k e V t r a n s i t i o n in

1 9 5

Pt

(21,22,23).

T h e results of these

studies h a v e p r o v i d e d

the

necessary d a t a for c o n s t r u c t i n g a n d e v a l u a t i n g t h e o r e t i c a l m o d e l s of t h e e l e c t r o n i c s t r u c t u r e of these alloys at the a t o m i c l e v e l , a n d h a v e a l l o w e d t h e d e d u c t i o n of the m e c h a n i s m of t r a n s f e r r e d h y p e r f i n e fields i n these m a t e r i a l s . O n e s i m i l a r s t u d y has b e e n r e p o r t e d u s i n g the 7 3 - k e V M o s s bauer line in

1 9 3

I r for o b s e r v i n g the h y p e r f i n e

i r i d i u m alloys (24).

field

in iron-platinum-

T h e s e reports i n d i c a t e the p o t e n t i a l u t i l i t y of t h e

M o s s b a u e r effect i n p r o b i n g t h e c h e m i s t r y , m a g n e t i c

properties,

p a r t i c l e s t r u c t u r e of " d o u b l e - l a b e l e d " b i m e t a l l i c systems.


and

556

MOSSBAUER


T h e p r o v e n u t i l i t y of the " R u M o s s b a u e r p r o b e f o r assessing t h e c o m p l e x c h e m i s t r y of a v a r i e t y of r u t h e n i u m c o m p o u n d s

(25)

d e s c r i b i n g the c h e m i c a l transformations t h a t a c c o m p a n y

various treat

a n d for

ments of r u t h e n i u m species o n c a t a l y t i c supports ( 2 6 - 2 9 ) w o u l d i n d i c a t e t h a t i t m a y be a n u c l i d e of c h o i c e f o r e x a m i n i n g b i m e t a l l i c catalysts i n g e n e r a l . T h i s is q u i t e fortuitous since b o t h i r o n a n d r u t h e n i u m a r e k n o w n to

be

effective

F i s c h e r - T r o p s c h catalysts

(30),

and

their bimetallic

systems e x h i b i t i n t e r e s t i n g s e l e c t i v i t y i n h y d r o c a r b o n p r o d u c t i o n CO/H

(10,31)

2

systematic

and i n nitrogen

s t u d y of

both

T h i s c h a p t e r represents t h e first c o m p l e t e

s t u d y of a n u n s u p p o r t e d system w h e r e b o t h spectroscopy

from

Thus w e have begun a

(32).

i r o n - r u t h e n i u m b i m e t a l l i c c a t a l y s t systems,

supported and unsupported. Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

fixation

5 7

F e and " R u Mossbauer

h a v e b e e n a p p l i e d to the same samples.

i n f o r m a t i o n f r o m E S C A spectroscopy

Supplementary

a n d x - r a y d i f f r a c t i o n has m a d e i t

p o s s i b l e to define c o m p l e t e l y the solid-state reactions t h a t a c c o m p a n y t h e p h y s i c a l a n d c h e m i c a l t r e a t m e n t of the b i m e t a l l i c m a t e r i a l s . P r e l i m i n a r y results of a s i m i l a r s t u d y o n the b i m e t a l l i c system o n a z e o l i t e s u p p o r t h a v e b e e n r e p o r t e d separately

(33).

Details of the Acquisition and Interpretation of "Ru Mossbauer Spectra T h e e x p e r i m e n t a l c o n d i t i o n s necessary f o r the c o l l e c t i o n of quality

5 7

good-

F e M o s s b a u e r d a t a a n d m e t h o d s for t h e i r subsequent i n t e r p r e

t a t i o n are n o w w e l l k n o w n a n d easily accessible secondary

literature.

T h u s no

technique w i l l be presented.

i n the p r i m a r y a n d

detailed background

material on

this

H o w e v e r , s i m i l a r i n f o r m a t i o n f o r the " R u

system is n o t r e a d i l y a v a i l a b l e , a n d a s u b s t a n t i a l o u t l i n e of t h e p r o b l e m w i l l b e p r e s e n t e d here for the readers' c o n v e n i e n c e . and

1 0 1

R u M o s s b a u e r spectroscopy

A r e v i e w of

"Ru

t h r o u g h 1973 is a v a i l a b l e , a n d is

r e c o m m e n d e d r e a d i n g f o r the serious e x p e r i m e n t a l i s t

(25).

T h e 9 0 - k e V M o s s b a u e r t r a n s i t i o n i n " R u w a s first d i s c o v e r e d

by

Kistner a n d co-workers w h o reported the spectrum for a m e t a l absorber at 85 K (34).

T h e s e w o r k e r s o b s e r v e d a s i n g l e - l i n e resonance of

0.37-

m m / s l i n e w i d t h w i t h a c a l c u l a t e d l i f e t i m e f o r t h e 9 0 - k e V l e v e l of 8 10"

9

s as a l o w e r l i m i t .

k n o w n v a l u e f o r the 1 4 - k e V M o s s b a u e r l e v e l i n u s i n g the d e l a y e d - c o i n c i d e n c e ±

1) X

(both

10" s (35). 9

source

5 7

Fe.

Subsequent w o r k

m e t h o d p r o d u c e d a l i f e t i m e v a l u e of

(20

F u r t h e r experiments at l i q u i d - h e l i u m t e m p e r a t u r e

and absorber)

n u c l e a r parameters

X

T h i s value compared favorably w i t h the w e l l -

of

the

provided detailed information about Mossbauer

t r a n s i t i o n (36,37).

r u t h e n i u m a l l o y e x h i b i t e d a w e l l - r e s o l v e d , 18-line, m a g n e t i c

An

the iron-

hyperfine

s p e c t r u m t h a t w a s u n i q u e l y fit b y a m o d e l a s s u m i n g a m i x e d


dipole-

26.

GOOD

E T

quadrupole

AL.

Mixed-Metal

557

Catalysts

( M 1 - E 2 ) transition from I =

analysis p r o d u c e d a v a l u e of

3/2

to I —

5/2.

Further

—0.19 db 0.05 f o r the m a g n e t i c

g-factor

( g i ) of the 9 0 - k e V state a n d a v a l u e of 2.7 db 0.6 for t h e E 2 / M 1 m i x i n g r a t i o S , w i t h t h e assignment of a n e g a t i v e s i g n to the m i x i n g p a r a m e t e r 8. 2

A

close e x a m i n a t i o n of

the

q u a d r u p o l e - s p l i t spectra of

[ R u ( C H ) ] and ruthenium dioxide ( R u 0 ) 5

5

2

2

ruthenocene

i n d i c a t e d t h a t the o b s e r v e d

d o u b l e t w a s a c o n s e q u e n c e of t h e d o m i n e n c e of the n u c l e a r q u a d r u p o l e m o m e n t of the e x c i t e d 3 / 2 state, w i t h a v a l u e of Qi/Qo

>

3.

These

e a r l y studies b y K i s t n e r a n d c o - w o r k e r s p r o v i d e d the necessary n u c l e a r p a r a m e t e r s a n d the v e r i f i c a t i o n of l a r g e c h e m i c a l i s o m e r shifts f o r n o n Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

m e t a l l i c absorbers that l e d to the e x p l o i t a t i o n of " R u M o s s b a u e r spectra to elucidate the complex unique compounds

c h e m i s t r y of r u t h e n i u m systems.

Thus both

a n d v a r i o u s m i x t u r e s of c h e m i c a l interest c a n

be

addressed. Experimentally, good-quality

" R u M o s s b a u e r spectra are difficult

a n d expensive to o b t a i n . T h e 16-day " R h p r e c u r s o r of t h e e x c i t e d state of " R u w a s first p r o d u c e d b y 1 0 - M e V p r o t o n b o m b a r d m e n t of m e t a l l i c ruthenium

i n the c y c l o t r o n .

ruthenium

(12.7%

"Ru).

"Ru)

Sources

can be

prepared

from

or from metal enriched w i t h " R u

natural (>98%

T h e n a t u r a l m e t a l sources are o n l y u s a b l e b e t w e e n t w o a n d six

w e e k s after b o m b a r d m e n t

because of short- a n d l o n g - l i v e d

1 0 1

R h con

t a m i n a n t s . H i g h specific a c t i v i t y sources h a v i n g l o n g e r u s e f u l lives c a n b e p r e p a r e d f r o m a n e n r i c h e d target b y c h e m i c a l l y s e p a r a t i n g t h e r e s u l t i n g " R h a n d c o - p r e c i p i t a t i n g i t w i t h a s m a l l a m o u n t of n a t u r a l r u t h e n i u m m e t a l ( 3 6 , 3 7 ) . W h e r e r u t h e n i u m m e t a l serves as the host l a t t i c e , single resonance l i n e w i d t h s close to the n a t u r a l l i n e w i d t h h a v e b e e n a c h i e v e d , a l t h o u g h the m e t a l has a h e x a g o n a l c r y s t a l structure. T h e a c t u a l p r e p a r a t i o n of the source is s o m e w h a t of a n art r a t h e r t h a n a science,

and

q u a l i t y has b e e n a v a r i a b l e f r o m source to source, e v e n f r o m t h e same vendor.

I n e v e r y case, the h i g h - e n e r g y ( 9 0 - k e V )

Mossbauer transition

a n d the l o w D e b y e - W a l l e r factors f o r most r u t h e n i u m c o m p o u n d s

(as

c o m p a r e d to r u t h e n i u m m e t a l ) h a v e m a d e i t necessary to o b t a i n " R u Mossbauer

spectra at l o w

temperatures, preferably

having both

the

source a n d a b s o r b e r c o o l e d to 4.2 K . T h e l i f e t i m e of the e x c i t e d state of " R u a n d the m a g n i t u d e of the c h e m i c a l i s o m e r shifts are c o m p a r a b l e to those of

5 7

F e , thus m a k i n g i t p o s s i b l e to u t i l i z e

5 7

F e instrumentation

w i t h o u t m o d i f i c a t i o n , except f o r t h e g a m m a detector

w h i c h must

be

o p t i m i z e d for the h i g h e r e n e r g y 9 0 - k e V x-ray. M o s t w o r k e r s h a v e u s e d large

glass

or

metal liquid-helium Dewars, where

the

spectrometer

v e l o c i t y d r i v e m o t o r is m o u n t e d v e r t i c a l l y o n top of the D e w a r

and

a t t a c h e d to the source v i a a l o n g d r i v e r o d e x t e n d i n g i n t o the l i q u i d h e l i u m b a t h (36-39).

F o r most r e p o r t e d spectra, a N a l ( T l ) s c i n t i l l a t i o n

detector has b e e n u s e d ( a 3 - m m t h i c k c r y s t a l is o p t i m a l ) to assay t h e


558

MOSSBAUER


9 0 - k e V g a m m a r a y s , a l t h o u g h a g e r m a n i u m - l i t h i u m d r i f t e d detector c a n provide improved resolution (26). t h e u s u a l w a y , either u s i n g a n

5 7

V e l o c i t y c a l i b r a t i o n is p r o v i d e d i n

F e source a n d p u r e i r o n f o i l

absorbers

o r u s i n g laser i n t e r f e r o m e t r y f o r absolute v e l o c i t y measurements All

isomer

shift values h a v e

been

r e p o r t e d w i t h respect

(40).

to m e t a l l i c

r u t h e n i u m , a l t h o u g h this m a t e r i a l is subject t o some u n r e s o l v e d q u a d r u pole

splitting.

Cohen

and Kalvius

(41)

a n d the N a t i o n a l Research

C o u n c i l A d H o c P a n e l o n M o s s b a u e r D a t a (42)

have recommended

the

use of K 4 [ R u ( C N ) ] • 3 H 0 as a n i s o m e r shift s t a n d a r d . T h i s c y a n i d e 6

2

c o m p l e x has the a d v a n t a g e of a s y m m e t r i c r u t h e n i u m site t h a t p r o v i d e s Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch026

a s i n g l e , n a r r o w l i n e just s l i g h t l y b r o a d e r t h a n t h e n a t u r a l l i n e w i d t h ; h o w e v e r , i t suffers f r o m a l o w recoil-free f r a c t i o n t h a t r e q u i r e s l o n g r u n t i m e s to p r o v i d e d a t a w i t h s u i t a b l e statistics. T y p i c a l " R u M o s s b a u e r spectra are of o v e r a l l p o o r e r q u a l i t y t h a n analogous

5 7

F e spectra b e c a u s e

of the s m a l l e r recoil-free f r a c t i o n a n d t h e n e e d f o r l a r g e samples 5 0 - 1 0 0 m g of r u t h e n i u m p e r c m

2

signal-to-noise ratios i n the o b s e r v e d

are t y p i c a l l y r e q u i r e d f o r

(i.e.,

adequate

spectrum).

T h e r e l a t i v e l y l a r g e i s o m e r shifts c o m p a r e d to t h e o b s e r v e d l i n e w i d t h s r e p o r t e d for " R u M o s s b a u e r s p e c t r a i m p l i e d t h a t t h e t e c h n i q u e s h o u l d have

significant p o t e n t i a l f o r

providing chemical

information

about

r u t h e n i u m systems. T h i s w a s a n e x c i t i n g p r o s p e c t i n v i e w of t h e c o m p l e x c h e m i s t r y of r u t h e n i u m , a n element

w h i c h exhibits formal

oxidation

states f r o m zero to p o s i t i v e e i g h t i n a l a r g e v a r i e t y of c o m p o u n d s

and

w h i c h has a t e n d e n c y to f o r m m u l t i n u c l e a r complexes w i t h m i x e d - m e t a l o x i d a t i o n states, m e t a l - m e t a l b o n d s , a n d b r i d g i n g l i g a n d s . T h e

mecha

n i s m s of c o m p l e x r u t h e n i u m reactions a n d the s t r u c t u r e of r u t h e n i u m complexes

have

t a x e d t h e skills a n d i n s i g h t of

numerous ruthenium

chemists since t h e e a r l y e x p e r i m e n t s o n the m a t e r i a l i n t h e l a t t e r p a r t of the last c e n t u r y . T h u s t h e r e w a s a n a l r e a d y e s t a b l i s h e d c l i e n t e l e f o r the a p p l i c a t i o n of " R u M o s s b a u e r spectroscopy Kistner a n d co-workers

(36,37)

to c h e m i c a l p r o b l e m s .

p r o v i d e d the first c o r r e l a t i o n of

M o s s b a u e r p a r a m e t e r s w i t h c h e m i c a l b o n d i n g b y r e l a t i n g the splitting i n R u 0

2

and ruthenocene

doublet

to the s i g n a n d m a g n i t u d e of

e l e c t r i c field g r a d i e n t at t h e r u t h e n i u m n u c l e u s i n these t w o

"Ru the

compounds.

T h e d o u b l e t s p e c t r u m r e p o r t e d f o r these c o m p o u n d s is t y p i c a l for r u t h e n i u m spectra of systems e x p e c t e d to h a v e q u a d r u p o l e i n t e r a c t i o n i n t h e absence of

magnetic

hyperfine interactions.

E a c h component

of

the

d o u b l e t consists of a n u n r e s o l v e d t r i p l e t w h i c h is a c o n s e q u e n c e of t h e I =

3 / 2 -> I =

5 / 2 transition, where the large

s i m p l e q u a d r u p o l e s p l i t t i n g of the e x c i t e d I =

Q 3 / 2 / Q 5 / 2

a l l o w s the

3 / 2 state to p r e d o m i n a t e .

F o r spectra of this t y p e , the i s o m e r shift u s u a l l y is r e p o r t e d as the center of the d o u b l e t , a n d the d o u b l e t s p l i t t i n g is r e f e r r e d to as A E . T h i s s i m p l e Q


26.

GOOD

E T

Mixed-Metal

A L .

559

Catalysts

e x t r a c t i o n of M o s s b a u e r p a r a m e t e r s is o v e r - s i m p l i f i e d for v e r y values and

precise

b u t is q u i t e p r a c t i c a l f o r r o u t i n e c h e m i c a l analysis

(43,44,45),

fingerprinting

of p a r t i c u l a r r u t h e n i u m species.

I s o m e r shift trends i n several series of r u t h e n i u m c o m p o u n d s h a v e been reported. co-workers

T h e first extensive s t u d y b y W a g n e r , indicated that a monatomic

(46)

Mossbauer,

shift

occurred w i t h increasing oxidation number.

This indicated a positive

nuclear factor

the a s s u m p t i o n

(AR /R)

for

2

" R u based

on

that

s-electron d e n s i t y at the r u t h e n i u m n u c l e u s i n c r e a s e d as the s h i e l d i n g w a s r e d u c e d w i t h i n c r e a s i n g o x i d a t i o n states. was


and

increase i n i s o m e r

further substantiated b y

calculated

the

total charge

Clausen, Prados, a n d density

at t h e

the

d-electron

This assumption Good

(38)

who

r u t h e n i u m nucleus

using

r e s t r i c t e d H a r t r e e - F o c k w a v e f u n c t i o n s of the s-electron d e n s i t y at the n u c l e u s for specific

o x i d a t i o n states.

B o t h o f these

studies

identified

m o r e subtle c h e m i c a l effects b y o b s e r v i n g t h a t t h e e l e c t r o n d e r e a l i z a t i o n effected b y c e r t a i n l i g a n d s s u c h as n i t r o s y l ( N O ) c o u l d b e s e m i - q u a n t i +

t a t i v e l y d e s c r i b e d b y c o m p a r i n g i s o m e r shifts w i t h i n a h o m o l o g o u s series of c o m p o u n d s .

T h i s c o n c e p t has b e e n e x p l o i t e d f u r t h e r b y r e l a t i n g t h e

i s o m e r shifts i n a series of r u t h e n i u m ( I I )

pentaamines

([Ru(NH ) X] 3

± n

5

where X = N O , C O , S 0 , N , pyrazine, C H C N , C H C N , pyridine, and +

2

2

3

6

5

NH )

to the r e l a t i v e a- a n d 7r-bonding c a p a b i l i t i e s of t h e s i x t h l i g a n d

(47).

The

3

utility

o f the " R u

Mossbauer

parameters

for

describing

c h e m i c a l b o n d i n g a n d s t r u c t u r e i n s i m p l e r u t h e n i u m c o m p l e x e s c a n best be i l l u s t r a t e d b y e x a m i n i n g the values o b t a i n e d f o r a series of r u t h e nium (II)-cyanide

c o m p l e x e s (48).

( C N ) ] ; IS = - 0 . 2 5 m m / s ; A E e

Q

These parameters are: (1) = 0.0 m m / s ; ( 2 )

2 H 0 ; IS = + 0 . 0 3 m m / s ; A E = 0.49 m m / s ; a n d ( 3 ) 2

Q

K [Ru4

K [Ru(CN) NO] • 2

5

K [Ru(CN) N0 ] 4

5

2

• 2 H 0 ; IS = - 0 . 4 0 m m / s ; A E = 0.35 m m / s . F i r s t , t h e i s o m e r shift f o r 2

Q

the h e x a c y a n o c o m p l e x is s i g n i f i c a n t l y l a r g e r t h a n the c o m p a r a b l e f o r the r u t h e n i u m ( I I ) - h e x a a m i n e

value

—0.25 vs. —0.92 m m / s ,

(46,47,49),

i n d i c a t i n g the effective

d e r e a l i z a t i o n of t w o d-electrons

f r o m the t

l e v e l of r u t h e n i u m ( I I )

i n t o the c y a n i d e o r b i t a l s . N o t e t h a t t h e i s o m e r

6

2g

shifts decrease i n the o r d e r of l i g a n d s u b s t i t u t i o n as N O > C N ' > N 0 ~ , +

a n d the q u a d r u p o l e s p l i t t i n g is greater f o r N O increase i n i s o m e r shift for N O

(from

+

2

t h a n for N 0 " . T h e l a r g e

+

2

—0.25 to + 0 . 0 3 m m / s )

f r o m the l a r g e , extra T r - d e l o c a l i z a t i o n of d-electrons f r o m t h e t

6

2g

results configu

r a t i o n i n t o the ? r * o r b i t a l of N O . T h e decrease of the i s o m e r shift i n +

t h e N 0 " case ( f r o m 2

—0.25 to —0.40 m m / s )

is a t t r i b u t a b l e to t h e f a c t

t h a t N 0 ~ is a p o o r 7 r - d e l o c a l i z a t i n g l i g a n d as c o m p a r e d 2

A E

q

to C N " .

The

v a l u e s i n d i c a t e the s y m m e t r y of the e l e c t r i c field g r a d i e n t a b o u t t h e

r u t h e n i u m n u c l e u s i n e a c h case, t h a t i s , A E electronic configuration i n R u ( C N )

6

4

Q

" ; and A E

= 0.0 for t h e Q

symmetric

for [ R u ( C N ) N O ] 5


2 +

>

560 AE

MOSSBAUER

q

for [ R u ( C N ) N 0 ] 5

2

4 +


b e c a u s e of l a r g e r d i s t o r t i o n c a u s e d b y greater

d e r e a l i z a t i o n i n t h e R u — N O b o n d as c o m p a r e d to t h e R u — N 0

2

entity.

T h i s t y p e of analysis has n o w b e e n a p p l i e d to m a n y s i m p l e r u t h e n i u m c o m p l e x e s a n d to some b i n u c l e a r systems (50,51).

T h e success of t h e

" R u spectral parameters i n elucidating such chemical b o n d i n g problems has l e d to the use of the t e c h n i q u e to define c h e m i c a l species i n v o l v e d i n solid-state catalysts, as m e n t i o n e d e a r l i e r i n this c h a p t e r . T h e p r o b l e m o u t l i n e d here is y e t another instance of t h e u n i q u e c o n t r i b u t i o n to m a d e b y M o s s b a u e r spectroscopy

be

to the u n d e r s t a n d i n g of r u t h e n i u m


c h e m i s t r y , b o t h i n c o m p l e x m o l e c u l a r systems a n d i n c a t a l y t i c m a t e r i a l s . Experimental Materials. Ferrous sulfate (hydrated) was purchased from Fischer Sci entific Co. and ruthenium trichloride trihydrate from Engelhard Industries. Hydrogen gas was obtained from Matheson (grade = 99.9%). All materials were used without subsequent purification. Preparation of the Catalyst Material. Ruthenium trichloride trihydrate and ferrous sulfate (hydrated) were mixed in a 1:2 mole ratio in the solid state. A few drops of sulfuric acid and a few grains of ascorbic acid (to maintain the ferrous state by oxidation retardation) were added to the mixture. A minimum amount of distilled water was added to prepare a reasonable slurry, and the ingredients were thoroughly mixed. The mixture was heated in a porcelain dish on a steam bath for 2 h with constant stirring. The residue was filtered, washed with water, and dried in a vacuum desiccator at a pressure of approxi mately 10" torr for 5 h. A small sample of the resulting material was removed for analysis and designated F e - R u (initial). The remainder of the F e - R u (initial) material was placed in a porcelain boat and reduced in flowing hydrogen at 400°C for 4 h. During reduction, copious amounts of white fumes were evolved. The reduced sample was cooled to room temperature in a stream of hydrogen. Again a small sample was re moved for analysis. This reduced residue was designated F e - R u (reduced). The larger portion of the F e - R u (reduced) sample was exposed to air at room temperature for 24 h. A small sample was removed for analysis and designated as F e - R u (exposed). The remainder of the material was heated to 400°C in air for 4 h. The sample was cooled slowly to room temperature and designated as F e - R u (calcined). Physical Methods. The ruthenium and iron Mossbauer spectra were obtained with an Austin Science Associates Mossbauer spectrometer operating in the constant-acceleration mode. Calibration of the spectrometer was accom plished by laser interferometery using the Austin Science Associates ruby laser system (38, 40). Isomer shifts for ruthenium were referenced to the single-line resonance of pure (99.9%) ruthenium metal powder. An NBS-certified iron foil was used for the reference standard for the iron isomer shift. The physical arrangement of the Mossbauer spectrometer and the Kontes/ Martin glass liquid-helium Dewar for studying ruthenium catalyst samples has been described previously (26). Sample and absorber were immersed in the liquid helium at 4.2 K. For the results reported here, a scintillation detector with a 3-mm thick Harshaw Nal ( T l ) crystal was used. The ruthenium source 3


26.

GOOD

E T

AL.

Mixed-Metal

561

Catalysts

was approximately 8 m C i of 16-day " R h contained i n a host lattice of ruthe nium metal. T h e source was prepared b y the N e w E n g l a n d Nuclear C o r p . of Boston, M A by separating the " R h activity from an enriched " R u target that h a d been proton bombarded i n the Oak Ridge National Laboratory cyclotron. T h e " R h activity was subsequently coprecipitated w i t h 8 m g of natural r u thenium metal and annealed at 1000°C i n hydrogen for 20 m i n . T h e source, as received, exhibited a linewidth of 0.22-0.25 m m / s for a natural ruthenium metal absorber. T h e iron Mossbauer data were collected w i t h the source at room tempera ture and the absorber at liquid-nitrogen (77 K ) temperature. Corrections for second-order Doppler shifts were not made. T h e source was 54.3 m C i of C o i n a host lattice of rhodium metal purchased from the Spire Corp., Bedford, M A . This source exhibited a linewidth of 0.28 m m / s for a K F e ( C N ) • 3 H 0 absorber. D a t a reduction for both the iron and ruthenium spectra was carried out by a conventional least-squares Lorentzian line shape program on a D E C P D P - 1 0 or P e r k i n - E l m e r Interdata 8/32 computer. T h e x-ray photoelectron spectra were taken on a P H I - 5 4 8 E S C A / A E S spectrometer. Powder samples were pressed onto a pure indium foil, and spectra were scanned w i t h the neutralizer (zero kinetic energy electrons) on. T h e samples were sputtered w i t h argon ions, and the indium peaks were used as internal reference standards for the assignment of sample b i n d i n g energies. X - r a y powder diffraction spectra were taken on a Phillips' diffractometer equipped w i t h an X R G - 3 0 0 0 x-ray generator and an A P D - 3 5 0 0 data controller and processor. The scan rate was 2°/min. Average surface areas for the catalyst particles i n the power samples were determined using the gas-adsorption method. A continuous gas flow on a P e r k i n - E l m e r Shell M o d e l 2 1 2 D spectrometer was used. Measurements were made i n triplicate for each sample and the B E T equation was used for the average surface area calculation ( 5 2 ) . 5 7


4

Results

and

6

2

Discussion

T h e i n t e n t of this s t u d y w a s to evaluate o u r a b i l i t y to c h a r a c t e r i z e i n d e t a i l the c h e m i c a l ( a n d p h y s i c a l ) transformations t h a t

accompany

the v a r i o u s p r e p a r a t i v e steps i n f o r m u l a t i n g a b u l k b i m e t a l l i c catalyst. T h e c h o i c e of the p a r t i c u l a r s t a r t i n g m a t e r i a l s w a s s o m e w h a t a r b i t r a r y , other t h a n the fact t h a t w e w a n t e d to i n v e s t i g a t e a defined i r o n - r u t h e n i u m system. T h e r a t i o n a l e for the p a r t i c u l a r m e t a l salts u s e d w a s b a s e d o n our p r e v i o u s experience w i t h R u C l

3

• * H 0 on a l u m i n a a n d silica 2

supports ( 2 6 ) , a n d the fact t h a t o x i d a t i o n - r e d u c t i o n reactions of F e S 0

4

• x H 0 o n zeolites h a d b e e n i n v e s t i g a t e d extensively b y other w o r k e r s 2

(53).

I t has b e e n r e c o g n i z e d t h a t the anions associated w i t h the m e t a l

i n the i n i t i a l m i x c a n i n f l u e n c e the final p r o d u c t s ( 5 4 ) , a n d f u t u r e studies are p l a n n e d w h e r e t h e c h e m i c a l transformations are m o n i t o r e d f u n c t i o n of i n i t i a l a n i o n ( s )

present.

T h e actual treatment

as a

procedures

w e r e c h o s e n to c o r r e s p o n d to those u s e d p r e v i o u s l y i n o u r l a b o r a t o r y for the o x i d a t i o n - r e d u c t i o n of s u p p o r t e d r u t h e n i u m catalysts o n a l u m i n a , s i l i c a , a n d zeolites (26,28).

P r o d u c t s at e a c h stage of t r e a t m e n t w e r e

c h a r a c t e r i z e d as f u l l y as p o s s i b l e , as o u t l i n e d later. R u t h e n i u m - 9 9 a n d


562 5 7

MOSSBAUER


F e M o s s b a u e r spectra w e r e o b t a i n e d for e a c h s a m p l e , a n d t h e results

are g i v e n i n T a b l e s I a n d I I a n d F i g u r e s 1-5.

ESCA

spectra for a l l

samples, b o t h as r e m o v e d f r o m the r e a c t i o n sequence a n d after

extensive

s p u t t e r i n g w i t h a r g o n ions (to p r o b e the b u l k properties of the m a t e r i a l s ) , are s h o w n i n F i g u r e 6 a n d 7. X - r a y d i f f r a c t i o n patterns f o r the i n d i v i d u a l samples are d i s p l a y e d i n F i g u r e 8. Fe-Ru

(Initial).

The

Mossbauer

t h a t it contains a m i x t u r e of i r o n ( I I I )

d a t a for

this s a m p l e

and iron (II)

indicates

and a ruthenium

e n t i t y w i t h a n isomer shift of —0.61 m m / s a n d a A E v a l u e of 0.69 m m / s .


Q

Table I. Sample* Fe-Ru (initial)

Fe-Ru (reduced)

Iron-57 Mossbauer Parameters of Bimetallic Catalysts

Iron-Ruthenium

Absorption

Comments

Lines"

0

0.01;

Peak 1

2.63 ±

0.01

Fe

I S = 1.32 2.63 =b 0.01

Peak 2 Peak 3

0.97 0.00:

0.01 0.01

Fe

I S — 0.49 ± 0.01; AE 0.97 ± 0.01

Peak 1

8.39 ±

0.02

Peak 2

4.85 ±

0.02

Peak Peak Peak Peak

3 4 5 6

2.56 1.32 0.49 -0.27

± =b db ±

0.03 0.02 0.02 0.02

Peak Peak Peak Peak Peak

7 8 9 10 11

-1.22 -2.60 -3.92 -4.71 -8.22

± ± ± ± ±

0.02 0.03 0.03 0.02 0.02

8.49 ±

0.02

2 +

AE

=

Q

3 +

—

Q

y-FesOa (magnetic) I S = 0 . 0 7 : 0.02; H I — 522 k G F e S (magnetic) I S — 0.50 db 0.02; H I = 281 k G

y - F e 0 (supermagnetic) I S = 0.11 ± 0 . 0 2 ; AE = 0.76 ± 0.02 2

3

Q

Fe-Ru (calcined)

Peak 1 Peak Peak Peak Peak

2 3 4 5

Peak 6 Peak 7 Peak 8

4.89 1.37 0.45 -0.12

± db ± ±

0.02 0.05 0.02 0.02

y - F e 0 (magnetic I S = 0.02; H I = 525 k G 2

3

y - F e 0 (supermagnetic) I S = 0.16 =b 0.02; AE 0.57 ± 0.02 2

0.09 :

3

Q

=

- 1 . 2 4 ± 0.03 - 4 . 7 0 db 0.02 - 8 . 3 0 =b 0.02

° See text for preparative conditions. Error estimates are from computer least^squares fits to Lorentzian lines. Pre cision (reproducibility) from two mirror-image spectra is ± 0.04 mm/s. IS == Isomer shift in mm/s; Ai£ = quadrupole splitting in mm/s; HI = inter nal field in kG calculated from Mossbauer magnetic splitting. 6

0

Q


26.

GOOD

E T

Mixed-Metal

AL.

563

Catalysts

Table II. Ruthenium-99 Mossbauer Parameters of Iron—Ruthenium Bimetallic Catalysts


Linewidth (mm/s)

Isomer Shift* (mm/s)

Sample* Fe-Ru (initial)

P e a k l -0.27 Peak 2 -0.96

± 0.01 ± 0.01

0.38 0.38

0.01 0.01

Fe-Ru (reduced)

Peakl 0.01 Peak 2 -0.48

± 0.01 ± 0.01

0.21 0.39

0.01 0.02

Fe-Ru (exposed)

Peakl 0.01 Peak 2 - 0 . 4 9

± 0.01 ± 0.01

0.22 0.36

0.01 0.02

Fe-Ru (calcined) R u (metal)

P e a k l -0.02 Peak 2 - 0 . 4 8 0.00

± 0.01 ± 0.01 ± 0.00

0.24 0.39 0.21

Ru0

Peak 1 0.02 Peak 2 - 0 . 4 9

± 0.01 ± 0.01

0.38 0.38

-0.67

± 0.05

0.35

2

/?-RuCl

3

± ± ± ±

0.01 0.02 0.01 0.01 0.01 0.05 A #

Q

—

0.79 ±

0.05

See text for preparative conditions. Error estimates are from computer least-squares fits to Lorentzian lines, Precision (reproducibility) from two mirror-image spectra is ± 0.04 mm/s. a

b

A n a l t e r n a t i v e assignment for the r u t h e n i u m s p e c t r u m w o u l d be

two

r u t h e n i u m species, b o t h w i t h no q u a d r u p o l e s p l i t t i n g a n d a n isomer shift of

—0.27 a n d —0.96 m m / s , r e s p e c t i v e l y .

The E S C A

spectra of

the

s a m p l e i n d i c a t e that b o t h c h l o r i d e a n d sulfate are present i n the surface a n d b u l k of the s a m p l e . T h u s t h e r u t h e n i u m entity ( i e s )

present c o u l d

b e a r u t h e n i u m ( I I I ) - c h l o r i d e c o m p o u n d or a m i x t u r e of the h e x a h a l i d e complexes value(s)

of

ruthenium (II)

are c o m p a t i b l e w i t h

and

ruthenium ( I V ) .

either of these t w o

isomer

shift

possibilities

The

(25).

H o w e v e r , the m a t e r i a l is i n s o l u b l e i n w a t e r a n d a l c o h o l , w h i c h is c h a r a c teristic of the i n s o l u b l e , a n h y d r o u s t r i h a l i d e s s u c h as « - R u C l . 3

We

do

not h a v e M o s s b a u e r p a r a m e t e r s f o r « - R u C l for c o m p a r i s o n , b u t t h e v a l u e s 3

f o r / ? - R u C l are s h o w n i n T a b l e I I a n d i n F i g u r e 2. T h e x - r a y d i f f r a c t i o n 3

p a t t e r n s h o w n i n F i g u r e 8 is i n c o n c l u s i v e for F e - R u

( i n i t i a l ) since n o

definitive assignments c o u l d be m a d e . T h u s , t h e most p r o b a b l e c h e m i c a l c o m p o s i t i o n of the d r i e d F e - R u ( i n i t i a l ) s a m p l e is a m i x t u r e of i r o n ( I I ) and iron (III) anhydrous R u C l

salts, p r o b a b l y 3

mixed

chlorides

a n d sulfates, a n d

an

species.

F e - R u (Reduced).

T h e c h e m i c a l c h a r a c t e r of t h e s a m p l e c h a n g e d

d r a s t i c a l l y after the F e - R u ( i n i t i a l ) w a s r e d u c e d i n h y d r o g e n at 4 0 0 ° C , as is i n d i c a t e d i n the M o s s b a u e r a n d E S C A spectra. A first t r i a l f o r t h e d a t a analysis of the

5 7

F e M o s s b a u e r spectra fit the d a t a to e i g h t lines as

s h o w n i n F i g u r e 3. H o w e v e r , the fit w a s not p a r t i c u l a r l y g o o d a n d o t h e r


564

MOSSBAUER

S P E C T R O S C O P Y A N D ITS C H E M I C A L

APPLICATIONS

s m a l l u n r e s o l v e d p e a k s a r e c l e a r l y v i s i b l e . A r e r u n o f these d a t a r e s o l v e d the s p e c t r u m i n t o 11 lines w i t h t h e p e a k positions l i s t e d i n T a b l e I . T h e d a t a c o u l d b e sorted into t w o s i x - l i n e m a g n e t i c spectra a n d a p a r a m a g netic doublet.

T h e lower intensity six-line doublet w i t h a n internal

field

of 281 k G is most p r o b a b l y F e S ( 5 6 , 5 7 ) . T h i s a s s i g n m e n t is s u b s t a n t i a t e d b y t h e E S C A d a t a w h i c h e x h i b i t t w o s u l f u r peaks o n t h e surface of t h e r e d u c e d s a m p l e , o n e w h i c h c a n b e assigned to sulfate a n d t h e o t h e r t o sulfide ( 5 8 ) . A f t e r s p u t t e r i n g o n l y t h e sulfide p e a k r e m a i n s , i n d i c a t i n g the presence of a sulfide i n t h e b u l k . T h e m o r e intense s i x - l i n e s p e c t r u m w a s first t h o u g h t t o b e a - F e 0 2

3


w i t h the center d o u b l e t r e p r e s e n t i n g a f r a c t i o n o f s m a l l p a r t i c l e s t h a t a r e p a r a m a g n e t i c (11, 56). N o c o n f i g u r a t i o n c o u l d b e o b t a i n e d f r o m t h e x - r a y d i f f r a c t i o n d a t a since t h e y c o n t a i n e d o n l y a s m a l l n u m b e r of resonance p e a k s , i n d i c a t i n g v e r y s m a l l p a r t i c l e sizes. G a s a b s o r p t i o n measurements c o n f i r m e d this f a c t b y s h o w i n g t h a t t h e average p a r t i c l e size w a s a b o u t 300 A o r less as c a l c u l a t e d f r o m t h e surface area o f 18.6 m / g - H o w e v e r , 2

t h e x - r a y d i f f r a c t i o n p a t t e r n f o r t h e c a l c i n e d samples, as discussed l a t e r , i n d i c a t e d the presence o f y - F e 0 2

3

rather than « - F e 0 . 2

3

T h e " R u Mossbauer spectrum for the reduced sample w a s p a r t i c u l a r l y i n t e r e s t i n g i n t h a t t h e r u t h e n i u m salt i n t h e i n i t i a l s a m p l e d i d n o t r e d u c e a l l t h e w a y t o r u t h e n i u m m e t a l as h a d b e e n o b s e r v e d p r e v i o u s l y for r u t h e n i u m trichloride supported o n silica a n d a l u m i n a or a r u t h e n i u m c a t i o n e x c h a n g e d onto a Y - t y p e zeolite (26,28).

T h e ruthenium spectrum

c o u l d b e a n a l y z e d as t w o signals, o n e r a t h e r intense p e a k a t a p p r o x i m a t e l y zero v e l o c i t y (as c o m p a r e d to a r u t h e n i u m m e t a l s t a n d a r d ) a n d

100.00 i

98.00

g

96.00

^

z

94.00

5 Q.

92.00

f

UJ

f

-4.0

-3.0

-2.0

-1.0

0

DOPPLER VELOCITY

Figure 1.

1.0

IN

2.0

3.0

MM/SEC

Iron-57 Mossbauer spectrum of Fe-Ru

(initial)


4.0

26.

GOOD E T A L .

Mixed-Metal

565

Catalysts

loaoo

i

99.90

(A

" Y

A /

99.80

1 99.70 99.00 »z g s

z ^

7

' ' * • • • • '

v

"

FE-RU

99.50

\f

99.40

-3.0

-2.0

-1.0

y

(INITIAL)

0

1.0

2.0

34


OOPPLER VELOCITY IN MM/SEC

•»

*

4i

+

+

-5.0

-4.0

-3.0

-2.0

-1.0

+

0

+

+

+

1.0

2.0

3.0

*

*

4.0

5.0

DOPPLER VELOCITY IN MM/SEC

Figure 2.

Ruthenium-99

Mossbauer spectrum of Fe-Ru

(initial)

a d o u b l e t w i t h p a r a m e t e r s c h a r a c t e r i s t i c of R u 0 . N o t e t h a t o n e h a l f o f 2

the R u 0

2

d o u b l e t is b u r i e d i n t h e m o r e intense m e t a l p e a k .

appears that t h e c h e m i c a l c o m p o s i t i o n of t h e r e d u c e d

y - F e 0 , small-particle paramagnetic y - F e 0 , F e S (small 2

3

2

3

Thus it

sample is b u l k component),

r u t h e n i u m m e t a l , a n d R u 0 . T h i s a s s i g n m e n t is f u r t h e r c o r r o b o r a t e d b y 2

the Mossbauer a n d x-ray data cited later for the calcined sample. Fe-Ru

(Exposed).

Previous work

on reduced

ruthenium metal

dispersed o n a zeolite support indicated that the material w a s quite r e a c t i v e a n d c h a n g e d c h e m i c a l c o m p o s i t i o n o n exposure t o a i r , e v e n a t r o o m t e m p e r a t u r e ( 2 8 ) . T o test t h e r e a c t i v i t y of t h e m i x e d - m e t a l system, the F e - R u

( r e d u c e d ) s a m p l e w a s exposed t o a i r a t r o o m t e m p e r a t u r e .

T h e Mossbauer a n d E S C A spectra indicated that the sample composition d i d not change from that reported earlier for the reduced sample. A g a i n this b e h a v i o r is q u i t e different f r o m t h a t o b s e r v e d f o r r u t h e n i u m a l o n e o n a s u p p o r t system.


566

MOSSBAUER

Fe-Ru

(Calcined).


The

5 7

F e a n d " R u Mossbauer parameters

t h e c a l c i n e d s a m p l e are g i v e n i n t h e T a b l e s a n d i n F i g u r e s 3 a n d 5.

for The

m a i n differences b e t w e e n these s p e c t r a a n d those f o r t h e r e d u c e d s a m p l e are t h e a b s e n c e of t h e F e S p a t t e r n a n d t h e m o r e c l e a r l y r e s o l v e d p e a k s for y - F e 0 . 2

3

N o t e t h a t t h e p a r a m a g n e t i c f r a c t i o n of

the i r o n

oxide

r e m a i n s . T h e t r e a t m e n t w i t h o x y g e n a p p a r e n t l y t r a n s f o r m s a n y F e S to y-Fe 0 2

3

a n d p r o d u c e s p a r t i c l e s of l a r g e r sizes. T h e a v e r a g e surface area

of t h e F e - R u ( c a l c i n e d ) p a r t i c l e s w a s r e d u c e d to 6 m / g , w h i c h c o r r e 2


sponds to a n average p a r t i c l e size of 900 A . T h e E S C A spectra s u b s t a n -

1

I

-12.0

-10J0

-&0

-OX

-4.0

H H T

-2.0

0

'

1

2.0

4.0

1

6.0

8.0

10.0

12.C

DOPPLER VELOCITY IN MM/SEC

Figure

3. Iron-57 Mossbauer spectra for Fe-Ru (reduced) and (calcined). (Spectrum for iron foil is shown for comparison.)


Fe-Ru

26.

GOOD

E T

Mixed-Metal

A L .

567

Catalysts


\FeS

+

+ + + -12.0

-10.0

+ +

-8.0

+ +

-6.0

+

-4.0

+

4.

-2.0 -4-—+—4— 0 2.0

4.0

+

6.0

+

+

* 4 .

8.0

+

10.0

4.

12.0

OOPPLER VELOCITY IN MM/SEC

Figure 4. Iron-57 Mossbauer spectrum of Fe-Ru (reduced) rerun with 11 resolved lines. (Note that most of the magnetically split spectrum for FeS can be resolved although it is a minor component of the total spectrum.)

t i a t e d the loss of F e S since t h e s p u t t e r e d samples i n F i g u r e 7 s h o w

no

sulfide peaks i n the b u l k s a m p l e . the

I t is i n t e r e s t i n g that the " R u M o s s b a u e r spectra are u n c h a n g e d

by

calcination.

of

The

spectra

still can

be

assigned

to

a mixture

r u t h e n i u m m e t a l a n d R u 0 . T h e s e assignments of the c h e m i c a l species 2

of t h e b i m e t a l l i c m a t e r i a l are e n h a n c e d b y the o b s e r v e d x - r a y d i f f r a c t i o n p o w d e r patterns.

S i n c e t h e average p a r t i c l e sizes w e r e i n c r e a s e d , the

p o w d e r p a t t e r n is n o w c l e a r l y assignable as s h o w n i n F i g u r e 8. c o r r e s p o n d i n g to r u t h e n i u m m e t a l , R u 0 , a n d y - F e 0 2

T h e assignment of y - F e 0 2

3

2

3

Peaks

are c l e a r l y v i s i b l e .

is p a r t i c u l a r l y significant since t h e M o s s b a u e r

p a r a m e t e r s c o u l d h a v e b e e n assigned to the m o r e c o m m o n « - F e 0 2

3

(56).

H o w e v e r , the x - r a y peaks c o u l d not b e fit to those r e p o r t e d f o r a - F e 0 2

b u t d i d c o r r e s p o n d w e l l to those r e p o r t e d for y - F e 0 2

3

3

(59).

Summary T h e d a t a o u t l i n e d i n this c h a p t e r c l e a r l y i n d i c a t e the u t i l i t y

of

M o s s b a u e r spectroscopy as a p r o b e for assessing the c h e m i c a l c o m p o s i t i o n of solid-state catalyst m a t e r i a l s a n d f o r f o l l o w i n g t h e i r solid-state reactions. T h e a b i l i t y to " d o u b l e l a b e l " a b i m e t a l l i c s y s t e m is p a r t i c u l a r l y effective. T h e a d v a n t a g e of s u p p l e m e n t a r y d a t a s u c h as E S C A s p e c t r a a n d x - r a y



568

MOSSBAUER

*— -3.0

SPECTROSCOPY

»

»

-2.0

-1.0

A N D ITS C H E M I C A L

+

+

0

1.0

APPLICATIONS

*— 2.0

+

3.0

00PPLER VELOCITY IN MM/SEC

Figure Fe-Ru

5. Ruthenium-99 Mossbauer spectra for Fe-Ru (reduced) and (calcined). (Spectrum for pure Ru0 is shown for comparison.) 2



GOOD

E T A L .

300

Mixed-Metal

250 2O0 60 BINDING ENERGY (tV)

Catalysts

569

Figure 6. ESC A spectra for Fe-Ru samples as a function of treatment

A L L SAMPLES ARE SPUTTERED FOR T E N MINUTES

F E - R U (CALCINED)

FE-RU(EXPOSED)

B/NCHNG ENERGY (eV)

Figure 7. ESC A spectra for Fe-Ru samples as a function of treatment. (Samples were first sputtered with argon ions for 10 min to remove surface contamination.)


570

MOSSBAUER

SPECTROSCOPY AND

ITS

CHEMICAL

APPLICATIONS

FE-RU (INITIAL)

47

45

40

35 26

30

25

21


FE-RU (REDUCED)

47

45

) • .1. )• •

40

35 20

25

30

FE-RU (CALCMED)

Ru

****

Ru

r 45

Figure 8.

40

35 20

25

30

X-ray powder patterns for the Fe-Ru treatment

samples as a function

of

d i f f r a c t i o n p a r a m e t e r s i n p r o v i d i n g c o m p l e m e n t a r y details f o r t h e

com

p l e t e c h a r a c t e r i z a t i o n of

dem

s u c h solid-state systems

also has b e e n

onstrated. T h e a c t u a l c h e m i c a l n a t u r e of t h i s p a r t i c u l a r i r o n - r u t h e n i u m b i metallic

system

is m o s t

i n t e r e s t i n g i n l i g h t of

i n f o r m a t i o n f o r r e l a t e d systems.

previously

reported

T h e overall reaction scheme can

s u m m a r i z e d as f o l l o w s : steam bath

Fe S0 n

4

+ Ru ^Cl -zH 0 m

3

> Fe , Fe 1 1

2

1 1 1

+

Ru iCl (anh) n

3

100°C in air

H

2

400°C 4h

O -400°C 2

Ru, Ru0 , y-Fe 0 2

2

3

Characterization of Mixed-Metal Catalysts by Iron ... - ACS Publications

Recommend Documents