The Synthesis and Electrocatalytic Properties of Nonstoichiometric

9 The Synthesis and Electrocatalytic Properties of Nonstoichiometric Ruthenate Pyrochlores 1

2

3

H. S. HOROWITZ , J. M. LONGO , H. H. HOROWITZ , and J. T. LEWANDOWSKI

Downloaded by CORNELL UNIV on August 29, 2016 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch009

Corporate Research Science Laboratories, Exxon Research & Engineering Company, Annandale, NJ 08801

A new series of conductive, mixed metal oxides with the pyrochlore structure has been discovered and tested as electrocatalysts. They can be described by the general formula A [Ru A ]O where A = Pb or Bi, 0 < x < 1 and 0c 7-y 0

I

1

\

80

.... j

1

ν

p

+

\

Ρ+

^KSb0 \

Xs

u

3

V

υ

B i

PYROCHLORE^S.

}

B î

2°3

2°3

60

AMORPHOUS

40 —

-

20

1 12

I 10

1 14 —

1

p H —

F i g u r e 6. S y n t h e s i s p r o d u c t s as a f u n c t i o n of temperature and pH f o r t h e B i - R u - 0 s y s t e m i n KOH r e a c t i o n m e d i a . Time o f r e a c t i o n h e l d c o n s t a n t at 7 d a y s . "KSbO^" s i g n i f i e s t h e K S b 0 3 - r e l a t e d phase, Bi2Ru20y 33. "P" s i g n i f i e s t h e p y r o c h l o r e phase, Bi2[Ru2- Bi ]07y. e

x

x

Grasselli and Brazdil; Solid State Chemistry in Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


9.

HOROWITZ ET AL.

Nonstoichiometric Ruthenate Pyrochlores

F i g u r e 7. Equal weights o f Pb2 [Ru1.52Pb.33] 05.5. L e f t - Recovered by c o n v e n t i o n a l vacuum'fi l t r a t i o n "and d r y i n g . R i g h t - Produced by t h e s p r a y - f r e e z e / f r e e z e d r y t e c h n i q u e . Both have s u r f a c e areas of 78 m /g. 2


153

154

S O L I D STATE C H E M I S T R Y IN CATALYSIS

UJ X 1.0 Û£ (Λ >

O.9 oPb [Ru

"5 O.8 >

2

Pb

]0

> 5 8

6 > 5

• 1 5 % Pt on Carbon

O.7


l t 4 2

I

L

J

L

100

10

400 1000

mA/cm

F i g u r e 8 . S t e a d y - s t a t e p o l a r i z a t i o n curves f o r O 9 r e d u c t i o n i n 3M Κ 0 Η , 75°C on P b [ R u i 9Pb ç J°6.5* , ^ w/o P t on carbon. Catalyst lx>adin§? areι 6Ό and 50 mg/cms respectively. The surface areas of the pyrochlore and carbon-supported Pt are 67 m /g and 25 m / g , r e s p e c t i v e l y . Reproduced w i t h p e r m i s s i o n from R e f . ^ C o p y r i g h t 1983, The E l e c t r o c h e m i c a l S o c . I n c . 2

2

4

4+

0

an d 9

8

5

2

1.1

1"

1

1

1

.

1.0

—

O.9

X=O.11 X=O.33 X=O.58 X=O.82 X=O.94

O.8

ι

1

( 2 8 rru/g) ( 8 5 rru/g) (67 mVg) (60 mi/g) (81 mVg) — ' J

/

ι 10

1• 100

11 11 I

300

mA/cm

F i g u r e 9 . 0 r e d u c t i o n i n 3M KOH, 75°C on Pb2[Ru2- Pb x]o s a f u n c t i o n o f x . Scans at O.2 mV/sec. C a t a l y s t l o a d i n g i's 60 mg/cm . Reproduced w i t h p e r m i s s i o n from R e f . 1. Copyright 1983, The E l e c t r o c h e m i c a l S o c . I n c . 2

x

4

6

2


5

a

9.

155


HOROWITZ ET AL.

t h e c a t a l y s t s were q u i t e l o w ( R-COO" 4e

2

R-CH^CH " > RCOO" + COg 10e


2

=

Ο IL 6 . . R-C|CH -R' » R-COO + OOC-R' e

2

F i g u r e 12. T y p i c a l Alkali.

E l e c t r o - O x i d a t i o n s on L e a d R u t h e n a t e

CH =CH(CH ) COO" " > "OOC(CH ) COO" + COg 2

2

8

10e

2

=

8

+

OOC(CH ) COO" 7% 2

7

CH —CH

D—

2

2

CH COO -8e 2

/

- CH-CH.COOH

> HOO(

2

HOOC Figure 13.

O x i d a t i o n of S o l u b i l i z e d U n s a t u r a t e s .

85%


SOLID STATE CHEMISTRY IN CATALYSIS


160

ΙΑ Figure 14. Ruthenate.

Rates of

Oxidation

°κ

of Various

Organics

on


Lead

9.

HOROWITZ ET AL.


161


Summary and D i s c u s s i o n of R e s u l t s A new f a m i l y o f h i g h c o n d u c t i v i t y , m i x e d m e t a l o x i d e s having t h e p y r o c h l o r e c r y s t a l s t r u c t u r e has been d i s c o v e r e d . T h e s e compounds d i s p l a y a v a r i a b l e c a t i o n s t o i c h i o m e t r y , as g i v e n by E q u a t i o n 1. The a b i l i t y t o s y n t h e s i z e t h e s e m a t e r i a l s i s h i g h l y d e p e n d e n t upon t h e l o w t e m p e r a t u r e , a l k a l i n e s o l u t i o n p r e p a r a t i v e t e c h n i q u e t h a t has been d e s c r i b e d ; the r e l a t i v e l y low thermal s t a b i l i t y o f t h o s e p h a s e s where an a p p r e c i a b l e f r a c t i o n o f t h e B - s i t e s are o c c u p i e d by post t r a n s i t i o n element c a t i o n s p r e c l u d e s t h e i r s y n t h e s i s i n p u r e f o r m by conventional s o l i d state reaction techniques. The d a t a summarized i n t h i s paper have e s t a b l i s h e d t h a t t h e o x i d e p y r o c h l o r e s under d i s c u s s i o n s u b s t a n t i a l l y r e d u c e the a c t i v a t i o n energy overvoltages a s s o c i a t e d w i t h oxygen e l e c t r o c a t a l y s i s . S p e c i f i c a l l y , i t i s found t h a t t h e s e c a t a l y s t s , i n aqueous a l k a l i n e media near ambient t e m p e r a t u r e , a r e s u p e r i o r t o any o t h e r o x y g e n e v o l u t i o n c a t a l y s t and a r e e q u a l i n p e r f o r m a n c e t o the b e s t known oxygen r e d u c t i o n c a t a l y s t s . As b i d i r e c t i o n a l o x y g e n e l e c t r o c a t a l y s t s , t h e y appear t o be unmatched. I t has been demonstrated t h a t t h e s e same m a t e r i a l s a r e a b l e t o p e r f o r m t h e r e l a t i v e l y r a r e r e a c t i o n o f c l e a v a g e o f o l e f i n s and s e c o n d a r y a l c o h o l s , k e t o n e s and g l y c o l s t o t h e c o r r e s p o n d i n g c a r b o x y l a t e s w i t h h i g h y i e l d s and s e l e c t i v i t i e s . There are s e v e r a l p h y s i c a l and chemical c h a r a c t e r i s t i c s o f t h e s e o x i d e p y r o c h l o r e s which may c o n t r i b u t e t o t h e i r h i g h e l e c t r o c a t a l y t i c a c t i v i t y . The p r e v i o u s l y d e s c r i b e d a l k a l i n e s o l u t i o n s y n t h e s i s t e c h n i q u e ( 6 , 7 ) has p r o v i d e d t h e s e m a t e r i a l s w i t h s u r f a c e a r e a s t y p i c a l l y r a n g i n g f r o m 50 t o 200 m / g . T h u s , one o f t h e b a s i c r e q u i r e m e n t s f o r an e f f e c t i v e e l e c t r o c a t a l y s t has been s a t i s f i e d : t h e e l e c t r o c a t a l y t i c a c t i v i t y i s n o t l i m i t e d by the u n a v a i l a b i l i t y of c a t a l y t i c a l l y a c t i v e s u r f a c e s i t e s , as i s so o f t e n t h e c a s e w i t h metal and mixed metal o x i d e s . Since a l l the c a t a l y s t s i n v e s t i g a t e d i n t h i s study d i s p l a y m e t a l l i c or near m e t a l l i c c o n d u c t i v i t y , the a d d i t i o n a l b a s i c r e q u i r e m e n t o f m i n i m i z i n g ohmic l o s s e s w i t h i n t h e c a t a l y s t and between t h e c a t a l y s t and c u r r e n t c o l l e c t o r has a l s o been s a t i s f i e d . W h i l e h i g h s u r f a c e a r e a and m e t a l l i c c o n d u c t i v i t y are b e n e f i c i a l t o e l e c t r o c a t a l y s i s , t h e y do n o t a l o n e e x p l a i n t h e h i g h c a t a l y t i c a c t i v i t y . We s p e c u l a t e t h a t the v a r i a b l e oxygen s t o i c h i o m e t r y o f t h e p y r o c h l o r e l a t t i c e , and the m u l t i p l e v a l e n c e s t a t e s o f t h e c a t i o n s , p a r t i c u l a r l y the r u t h e n i u m , are e s s e n t i a l t o t h e c a t a l y t i c a c t i v i t i e s of these pyrochlores. N o b l e m e t a l p y r o c h l o r e s were o r i g i n a l l y c o n s i d e r e d by the a u t h o r s as prime oxygen e l e c t r o c a t a l y s t c a n d i d a t e s b e c a u s e o f t h e i r a b i l i t y t o accommodate oxygen v a c a n c i e s i n up t o one seventh of the anion s i t e s . Changes i n t h e a v e r a g e v a l e n c e o f t h e r u t h e n i u m can be expected to accommodate any s u c h s t o i c h i o m e t r y changes in Pb2(Ru2- Pb )06 5. The e x i s t e n c e o f h i g h e r and lower v a l e n t o x i d e s ( e i t h e r s u r f a c e or b u l k ) i n the p o t e n t i a l range o f i n t e r e s t a p p e a r s t o be a c h a r a c t e r i s t i c o f many oxygen e l e c t r o c a t a l y s t s such as P t , Ag, Ni ( f o r oxygen e v o l u t i o n ) , A u , e t c . The r e a s o n s f o r t h i s have n e v e r been e x p l a i n e d e x a c t l y , although the a b i l i t y o f the s u r f a c e t o i n t e r a c t w i t h or adsorb the p o t e n t i a l i n t e r m e d i a t e peroxide or h y d r o p e r o x i d e i o n on i t s s u r f a c e i s o f t e n i n v o k e d . 2

x

x

e


162

S O L I D STATE C H E M I S T R Y IN CATALYSIS


In t h e c a s e o f l e a d r u t h e n a t e , t h e o x y g e n n o n - s t o i c h i o m e t r y c o n c e p t c a n be d e v e l o p e d f u r t h e r by c o m b i n i n g i t w i t h t h e known r e a c t i o n s of t h e v a r i a b l e v a l e n c e r u t h e n i u m . I t has been shown i n t h i s work t h a t t h e s e same c a t a l y s t s can c l e a v e c a r b o n - c a r b o n double bonds ( 3 ) i n a manner a n a l o g o u s t o t h a t o f osmium and r u t h e n i u m tetroxiïïe (II). I t i s known (12) t h a t 0$04 (and presumably RUO4) c l e a v e o l e f i n s v i a complexes w i t h T h e s t r u c t u r e :

•C

R'

•C

R

1 The f a c t t h a t t h e s e c a t a l y s t s can c a r r y out t h e v e r y same c l e a v a g e s u g g e s t s t h a t t h e y c a n f o r m m o i e t i e s s i m i l a r t o t h e t e t r o x i d e on t h e i r s u r f a c e s . By a n a l o g y , i t i s p o s s i b l e t h a t the r u t h e n i u m atoms at t h e s u r f a c e of the c r y s t a l l a t t i c e can r e a c t w i t h oxygen m o l e c u l e s so as t o form t h e s e same s u r f a c e complexes:

—0, Ru

—0'

h

/

%

T h e s e c a n be r e d u c e d s t e p w i s e t o r e g e n e r a t e t h e s t a r t i n g s t r u c t u r e :

-O.

3

2e+2H?0— —

U

—0

—0

0 \ R^ N

0

V

c

/ 20H-

+20H-

Rtj —0^

—0

.OH y

V

^OH

—0'

T h i s o r a s i m i l a r s e t o f r e a c t i o n s t e p s would a v o i d t h e f o r m a t i o n o f hydrogen p e r o x i d e c o n s i s t e n t w i t h the r e s u l t s of the r o t a t i n g d i s k e l e c t r o d e experiment. W h i l e t h e mechanism above i s s p e c u l a t i v e , we p r e f e r i t t o o t h e r p o s s i b i l i t i e s i n v o l v i n g - R u - O - O - R u - m o i e t i e s , because (1) the e l e c r o o r g a n i c o x i d a t i o n s must i n v o l v e s i n g l e Ru atoms, due t o t h e s t e r i c r e q u i r e m e n t s o f the c l e a v a g e r e a c t i o n ; (2) t h e c a t a l y t i c a c t i v i t y o f n o n - s t o i c h i o m e t r i c l e a d r u t h e n a t e s i s s u r p r i s i n g l y i n s e n s i t i v e to d i l u t i o n o f t h e Ru atoms w i t h Pb, whereas h i g h s e n s i t i v i t y w o u l d be expected i f t h e mechanism i n v o l v e d p a i r s o f n e i g h b o r i n g Ru atoms; and (3) t h e n e a r e s t Ru-Ru d i s t a n c e i n t h e l a t t i c e i s about 3 . 5 A - p r o b a b l y too l o n g f o r a b r i d g i n g p e r o x i d e group.


9.

HOROWITZ E T A L .

163


Acknowledgments The a u t h o r s w o u l d l i k e t o t h a n k Β . B o w l i n g and H. B r a d y f o r a s s i s t a n c e i n the s y n t h e s i s and m a t e r i a l s c h a r a c t e r i z a t i o n o f t h e c a t a l y s t s u s e d i n t h i s s t u d y ; J . Haberman, K. S t r o h m a i e r , and L. Y a c u l l o f o r t h e e l e c t r o c h e m i c a l m e a s u r e m e n t s ; and G. Dupre and K. Rose f o r o r g a n i c product a n a l y s e s .


Literature Cited

1.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., Mat. Res. Bull. 1981, 16, 489-496.

2.

Horowitz, H.S.; Longo; J.M.; Horowitz, H.H., J. Electrochem. Soc. 1983, 130, 1851-1859.

3.

Horowitz, H.H.; Horowitz, H.S.; Longo, J.M., In "Proceedings of the Symposium on Electrocatalysis" O'Grady, W.E.; Ross, P.N. Jr.; Will, F.G., Eds.; The Electrochemical Society, Inc.: Pennington, N.J., 1982; pp. 285-290.

4.

Sleight, A.W., Inorg. Chem. 1968, 7, 1704.

5.

McCauley, R.A., J. Appl. Phys. 1980, 51, 290.

6.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., 4 129 525, 1978.

7.

Horowitz, H.S.; Longo, J.M.; Lewandowski, J.T., In "Inorganic Syntheses"; Holt, S.L. Ed.; Wiley-Interscience: N.Y., 1983; pp. 69-72.

8.

Beyerlein, R.A.; Horowitz, H.S.; Longo, J.M.; Leonowicz, M.E.; Jorgensen, J.D.; Rotella, F.J., J. Solid State Chem. 1984, 51, 253.

9.

Pourbaix, M.; "Atlas of Electrochemical Equilibria in Aqueous Solutions"; Pergamon Press: New York City, 1966.

U. S. Patent

10.

Abraham, F.; Nowogroki, G.; Thomas, D. C. R. Acad. S. Paris 1974, 278C, 421.

11.

Djerassi, C.; Engle, R.R., J. Am. Chem. Soc. 1953, 75, 3838.

12.

Criegee, R., Annalen 1936, 522, 75.

RECEIVED

December

17, 1984


The Synthesis and Electrocatalytic Properties of Nonstoichiometric

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