Carbon Monoxide-Metal Oxide Interactions - Advances in Chemistry

13 Carbon Monoxide-Metal Oxide Interactions

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Surface Site Requirements for Electron Transfer Processes

KENNETH J. KLABUNDE, RICHARD A. KABA, and RUSSELL M. MORRIS Department of Chemistry, University of North Dakota, Grand Forks, ND 58202

Electron transfer from metal oxide surfaces to C O can be quite facile, occurring at room temperature. This process can be important as an initial C O activation step in metal oxide catalyzed reduction schemes. We have attempted to clarify what types of metal oxides interact (MO + C O -> MO . . . CO .) with C O in this way, and what surface features these active metal oxides possess. Only MgO, CaO, SrO, BaO, and ThO were electron transfer active. These oxides have in common the possession of both Lewis basic sites and one electron reducing site. It appears that C O is first adsorbed on Lewis base sites followed by slow migration to electron transfer reducing sites. The studies leading to this conclusion are discussed. +

-

2

A ctive Sites on Metal Oxides for Electron Transfer Processes. N u m e r o u s studies h a v e a p p e a r e d t h r o u g h o u t t h e l i t e r a t u r e d e a l i n g w i t h E S R studies of p a r a m a g n e t i c centers generated o n m e t a l o x i d e surfaces b y a v a r i e t y of m e t h o d s .

Scheme

1 s u m m a r i z e s some o f this w o r k b u t is

l i m i t e d to M g O , w h i c h serves as a n i l l u s t r a t i v e e x a m p l e . radiation

(neutrons

or y-rays)

High

energy

cause m a n y c r y s t a l l i n e defects

where

electrons c a n b e t r a p p e d to f o r m p a r a m a g n e t i c F o r F ' centers defects)

a n d S o r S ' centers

(surface

centers are " m e t a l excess" sites c a u s e d

defects)

(I).

The F

b y t h e presence

(bulk and S

of a n anion

v a c a n c y a n d t h e n e l e c t r o n t r a p p i n g . T h e r e a r e c o r r e s p o n d i n g V centers, w h i c h a r e c a t i o n vacancies t h a t t r a p p o s i t i v e holes.

I t is p r o b a b l e that

0-8412-0429-2/79/33-173-140$05.00/0 © 1979 American Chemical Society

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

13.

KLABUNDE

CO-Metal

E T AL.

Scheme 1.

141

Oxide Interactions

Paramagnetic Centers Generated on MgO

MgO


CO

less energy is r e q u i r e d to f o r m a c a t i o n v a c a n c y ( l e a d i n g to V c e n t e r s ) t h a n a n i o n vacancies ( l e a d i n g to S a n d F centers)

(1).

T h e c h e m i s t r y of the t r a p p e d electron centers ( S a n d F ) has b e e n s t u d i e d extensively w i t h a d d e d s m a l l m o l e c u l e s . a d d i t i o n has b e e n s t u d i e d i n some d e t a i l (2,3). t h a n M g O h a v e b e e n s t u d i e d s i m i l a r l y (4-10).

I n particular, oxygen

M a n y m e t a l oxides other E x t e n s i v e w o r k has s h o w n

that O " - , 0 ~ - , a n d 0 ' - c a n be obtained b y interacting the F a n d S 2

centers w i t h N 12,13).

3

2

0 (3), 0 , and N 2

2

0 ( w i t h 0 ~- present) respectively 2

(11,

T h e O " - species is p r e p a r e d p a r t i c u l a r l y w e l l b y U V i r r a d i a t i o n

of M g O i n the p r e s e n c e of H

2

(3), followed by N 0 addition. 2

Further-

m o r e , several groups h a v e b e g u n investigations of t h e c h e m i s t r y o f t h e


142

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

O " ' species f r o m t h e aspect o f surface a n i o n - m o l e c u l e reactions I n p a r t i c u l a r , a l k a n e d e h y d r o g e n a t i o n b y O - o n M g O (14) -

a n d ethylene oxide to f o r m H C = C ~ 2

b e e n s t u d i e d (15,16).

(14,15).

a n d ethylene

a n d H C — C H 0 " radicals have 2

2

0 , C O , a n d ethylene f o r m 0 ~ - , C 0 " - , a n d 2

3

C H ~ - a c c o r d i n g t o N a c c a c h e (16). 2

II

4

2

T h e r e is m u c h f a s c i n a t i n g c h e m i s t r y

s t i l l t o b e i n v e s t i g a t e d r e g a r d i n g these k i n d s of i n t e r a c t i o n s . A n u m b e r o f other m o l e c u l e s h a v e b e e n a l l o w e d to i n t e r a c t w i t h t h e F a n d S centers.

Even C 0

2

c a n b e r e d u c e d b y t h e centers

(17,18,19).

S i m i l a r l y , t h e r m a l l y a c t i v a t e d a n d U V - a c t i v a t e d M g O exposed to S 0

2

causes t h e f o r m a t i o n of t w o different S 0 ~ - r a d i c a l s ( 2 0 ) . A l s o , H S ~ Downloaded by IOWA STATE UNIV on January 23, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch013

2

2

c a n b e f o r m e d b y a d d i t i o n of H S to U V - a c t i v a t e d M g O (21).

2

Thus, it

2

appears that t h e c h e m i s t r y of O " - o n M g O a n d F a n d S centers o n M g O or other oxides are r i c h areas f o r investigations, a n d E S R is a p o w e r f u l t o o l to use i n these studies

(22).

G e n e r a l l y , i t appears that U V - a c t i v a t e d M g O is s i m i l a r i n c h e m i s t r y to y - r a y o r n e u t r o n - b o m b a r d e d M g O ( o r other o x i d e s ) , a l t h o u g h there is s t i l l r o o m f o r m o r e d e t a i l e d studies r e g a r d i n g U V w a v e l e n g t h d e p e n d e n c e a n d " q u a n t u m y i e l d " studies. H o w e v e r , there are gross differences b e t w e e n t h e a c t i v e sites o n M g O g e n e r a t e d t h e r m a l l y

(nonparamagnetic)

a n d those g e n e r a t e d b y i r r a d i a t i o n t e c h n i q u e s ( p a r a m a g n e t i c ) .

Thermally

g e n e r a t e d sites are s t i l l " r e d u c i n g sites" i n t h e sense that e l e c t r o n transfer to a d d e d m o l e c u l e s takes p l a c e . A t t h e same t i m e , h o w e v e r , b a c k b o n d i n g o r b a c k d o n a t i o n c a n a l l o w t h e r e d u c e d m o l e c u l e to d r a i n b a c k

charge

d e n s i t y to t h e o x i d e surface b u t s t i l l m a i n t a i n a p a r a m a g n e t i c b o n d i n g p i c t u r e . T h e s e a c t i v e sites, w h a t e v e r t h e y l o o k l i k e , are selective. 0

2

Thus,

does n o t react w i t h t h e r m a l l y generated a c t i v e sites o n M g O t o y i e l d

p a r a m a g n e t i c species

( 1 , 2 3 ) , n o r does C 0

2

form paramagnetic

w h e n e x p o s e d to t h e r m a l l y a c t i v a t e d M g O , a l t h o u g h C 0

2

species

is a d s o r b e d

( n o n p a r a m a g n e t i c a l l y ) at r o o m t e m p e r a t u r e to t h e extent o f near m o n o l a y e r coverage

(1).

I n a f a s c i n a t i n g p a p e r , T e n c h a n d N e l s o n discuss t h e r m a l l y g e n e r a t e d a c t i v e sites o n M g O , a n d t h e i r e l e c t r o n transfer t o a d s o r b e d n i t r o c o m p o u n d s to f o r m p a r a m a g n e t i c a n i o n r a d i c a l s ( 2 4 ) . A c c o r d i n g t o p r o t o n s p l i t t i n g a n i s o t r o p y , i t w a s c o n c l u d e d t h a t n i t r o b e n z e n e lies flat o n t h e M g O surface after a c c e p t i n g t h e electron.

T h e s e authors discuss three

p o s s i b i l i t i e s f o r t h e m e c h a n i s m of t h e e l e c t r o n transfer, o r i n p a r t i c u l a r t h e exact surface site r e q u i r e d . T h e y r e p o r t e d that a d s o r p t i o n of H C0

2

2

0 or

( n o n p a r a m a g n e t i c a l l y ) c o m p l e t e l y i n h i b i t s t h e e l e c t r o n transfer t o

nitro compounds.

T h e y c o n s i d e r three site p o s s i b i l i t i e s : ( 1 ) t r a n s i t i o n

m e t a l i m p u r i t y ions; ( 2 ) electrons t r a p p e d at i n t r i n s i c defects ( F o r S centers);

a n d ( 3 ) lattice o x y g e n ions o n t h e surface.

T h e y reject

(1)

b e c a u s e o f t h e v e r y l o w concentrations of t r a n s i t i o n m e t a l ions v s . spins o b s e r v e d a n d ( 2 ) b e c a u s e F a n d S centers are E S R o b s e r v a b l e a n d


13.

KLABUNDE

ET

CQ-Metdl

AL.

Oxide

143

Interactions

essentially none are present o n t h e r m a l l y a c t i v a t e d M g O . T h e y c o n c l u d e that ( 3 ) is the m e c h a n i s m , w h i c h means that a n O i o n m u s t g i v e u p one r

e l e c t r o n to the n i t r o c o m p o u n d .

N o w the reaction O

e x o t h e r m i c i n the gas phase b y ~ 6.5 e V (24,25,26), lattice the 0

O" +

r

e

"

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

=

T h u s , the ease w i t h w h i c h O =

s

field.

gives u p a n e l e c t r o n w i l l d e p e n d greatly

r

o n its degree of c o o r d i n a t i o n . c o o r d i n a t i o n of the 0

*

but i n the M g O

T e n c h a n d N e l s o n c o n c l u d e that i f t h e

i o n is correct, t h e n c o m p o u n d s of e l e c t r o n affinity

of 0.7 or greater c a n a c c e p t a n electron ( s u c h as n i t r o c o m p o u n d s ) , b u t m o l e c u l e s of < 0.5 e V ( s u c h as 0 ) Downloaded by IOWA STATE UNIV on January 23, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch013

2

cannot

(24).

T h u s , i n t h e r m a l l y a c t i v a t e d M g O , a n d p r e s u m a b l y i n other oxides t h e r m a l l y a c t i v a t e d that d o not s h o w F a n d S centers, w e h a v e a c o m p l e t e l y different t y p e of a c t i v e center t h a n i n i r r a d i a t e d samples ( F a n d S u n p a i r e d e l e c t r o n c e n t e r s ) . W h a t does this t h e r m a l l y g e n e r a t e d a c t i v e site l o o k l i k e ?

A n d w h a t does the " h o l e " left b e h i n d l o o k l i k e ?

Very

l i t t l e has b e e n established c o n c e r n i n g these questions, e x c e p t t h a t i t is v e r y i m p r o b a b l e that t r a n s i t i o n m e t a l i m p u r i t i e s p l a y a r o l e

(23,24,27,

28,29). Electron Transfer to C O from Thermally Activated M g O and T h 0 . 2

T h e classic w o r k of L u n s f o r d a n d J a y n e i n 1966 first s h o w e d that C O adsorption on thermally activated M g O yielded a paramagnetic

species

T h i s p a p e r r e p o r t e d that C O interacts i n s u c h a w a y as to p r o d u c e

(30).

a r a d i c a l w i t h u n i a x i a l a n i s o t r o p y of the g factor,

and with bonding

s i m i l a r to that b e l i e v e d to exist i n m e t a l c a r b o n y l s

(strong

7r-acceptor

c h a r a c t e r i s t i c of C O ) . T h e y o b s e r v e d values of 2.0021 f o r g u a n d 2.0055 f o r gjL. It w a s p r o p o s e d t h a t the r a d i c a l f o r m e d w a s a c t u a l l y n e u t r a l or e v e n s l i g h t l y p o s i t i v e o w i n g to the strong d o n a t i o n of a n e l e c t r o n p a i r to the M g O , a n d r e c e i v i n g i n r e t u r n a n e l e c t r o n i n a n a n t i b o n d i n g ir o r b i t a l . F u r t h e r , L u n s f o r d a n d J a y n e (30)

b e l i e v e d that i r o n i o n i m p u r i t i e s w e r e

necessary a n d w e r e r e l a t e d to the electron transfer process this is t h o u g h t not to be true

(currently

(27).

B r e y a n d c o - w o r k e r s e x t e n d e d these studies to t h e r m a l l y a c t i v a t e d T h 0 , a n d o b s e r v e d several p a r a m a g n e t i c species after C O a d d i t i o n ( 2 9 ) . 2

T h e s e authors

o u t l i n e several

models

for the

a d s o r p t i o n process,

as

d e s c r i b e d i n S c h e m e 2. T h e y b e l i e v e that ( A ) is f o r m e d b y C O a d s o r p t i o n o n a site w i t h a p r e e x i s t i n g surface defect that has

immediately

a v a i l a b l e a n u n p a i r e d e l e c t r o n ( s i m i l a r to the L u n s f o r d m o d e l (30)). believe ( B )

and ( C )

They

are f o r m e d b y s l o w d i f f u s i o n of defects i n T h 0

2

w h i c h finally r e a c h the surface a n d react w i t h a d s o r b e d C O to p r o b a b l y form

b r i d g e d C O r a d i c a l species.

A fourth E S R

s i g n a l is

probably

a t t r i b u t e d to a C O - species ( D ) , a l t h o u g h o v e r a l l the species w o u l d +

p r o b a b l y b e n e a r l y n e u t r a l . A n d w i t h C O - , ( D ) c o u l d interact w i t h +

toformCO/- (E)

(29).


0

=

144

INORGANIC

Scheme 2.

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

Assignment of CO Surface Structures to Observed ESR Signal (29)

II

II

C

C

^

0

=

_—>

CO~


D

O

.0.

.0

\\

//

C C

II c

B

C

M e r i a u d e a u , Breysse, a n d C l a u d e l ( 3 1 ) also s t u d i e d C O o n T h 0

2

w i t h p a r t i c u l a r emphasis o n t h e p o s s i b l e f o r m a t i o n of C O since t h e y h a d +

p r o p o s e d this species earlier as a n i n t e r m e d i a t e i n t h e c a t a l y t i c o x i d a t i o n of C O ( 3 2 ) .

1 3

C O as w e l l as

1 2

C O experiments w e r e c a r r i e d o u t w i t h

the c o n c l u s i o n that t h e o n l y species t h e y c a n ascribe to a C O species has a n a x i a l s y m m e t r i c s i g n a l s i m i l a r to ( C ) i n S c h e m e 2. U s i n g t h e s p i n densities o b t a i n e d f r o m t h e i r

1 3

C O work, M e r i a u d e a u a n d co-workers

( 3 1 ) w e r e a b l e to c a l c u l a t e that the C O r a d i c a l species has its u n p a i r e d e l e c t r o n i n a n o r b i t a l of h i g h p c h a r a c t e r a n d spends 15 o r 3 3 % of its t i m e a r o u n d t h e c a r b o n a t o m ( t w o different p a r a m a g n e t i c C O absorbates). T h e i r C O - s u r f a c e b o n d i n g interpretation depends o n the fact that m a r k e d c h a n g e i n e l e c t r i c a l c o n d u c t i v i t y of t h e T h 0 s o r p t i o n w a s f o u n d . T h e y b e l i e v e that t h e C O - T h 0

2

2

a

during C O chemielectronic exchange

results i n the f o r m a t i o n of s l i g h t l y p o s i t i v e C O adsorbates a n d a surface b e c o m i n g n e g a t i v e as a w h o l e to a d e p t h of a f e w a t o m i c layers.

This

creates e n e r g y - b a n d c u r v a t u r e near the surface a n d accounts f o r t h e change i n electrical conductivity (31). Thermally Activated Alkaline

E a r t h Oxides

(MgO, CaO, S r O ,

BaO) as Catalysts. I t is c u r i o u s that t h e r m a l a c t i v a t i o n of a l k a l i n e e a r t h oxides, besides p r o m o t i n g e l e c t r o n transfer processes, also leads to m o r e a c t i v e h y d r o g e n a t i o n a n d i s o m e r i z a t i o n catalysts. I n a n e x t r e m e l y signific a n t p a p e r , H a t t o r i , T a n a k a , a n d T a n a b e ( 3 3 ) d e s c r i b e M g O as a h y d r o g e n a t i o n catalyst a n d s h o w t h a t i t a p p a r e n t l y operates i n a n i o n i c m o d e (H

2

-> H + H " o n t h e s u r f a c e ) that a l l o w s D +

2

u p t a k e i n t o 1,3-butadiene

w i t h o u t H - D s c r a m b l i n g to f o r m c i s - 2 - b u t e n e - l , 4 - d . 2

Alkaline


earth

13.

KLABUNDE

CO-Metal

ET A L .

Oxide

145

Interactions

oxides s h o w e d a n a c t i v i t y o r d e r o f C a O > S r O > M g O > B a O > > B e O f o r this r e a c t i o n w h e n a c t i v a t e d b e t w e e n 8 0 0 ° - 1 0 0 0 ° C also b e h y d r o g e n a t e d temperatures.

(34). Alkenes can

( 3 5 ) b y these oxides b y u s i n g h i g h e r r e a c t i o n

O t h e r processes h a v e also b e e n s t u d i e d w i t h the a l k a l i n e

e a r t h oxides as catalysts: i s o m e r i z a t i o n of alkenes (36,37,38), t i o n o f styrene (39,40), exchange

and H - D 2

2

(28).

Results and Downloaded by IOWA STATE UNIV on January 23, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch013

polymeriza-

esterification o f b e n z a l d e h y d e (41),

Discussion

W h a t types o f surface sites are i n v o l v e d i n these e l e c t r o n transfer a n d c a t a l y t i c processes o n t h e r m a l l y a c t i v a t e d a l k a l i n e e a r t h

oxides?

T a n a b e a n d h i s c o - w o r k e r s b e l i e v e that L e w i s base sites a n d o n e - e l e c t r o n " r e d u c i n g sites" a r e b o t h i m p o r t a n t a n d h a v e s h o w n f o r C a O that o n e t y p e o f site c a n b e f a v o r e d o v e r the other b y different t h e r m a l a c t i v a t i o n procedures

(39,42).

W e h a v e u s e d T a n a b e ' s i d e a i n p r i n c i p l e f o r t h e q u a n t i t a t i v e s t u d y of the n u m b e r of r e d u c i n g sites o n M g O . T h e measurements w e r e m a d e b y d e t e r m i n i n g t h e n u m b e r of spins o f C H N 0 ~ - f o r m e d o n M g O samples 6

5

2

after e a c h h a d b e e n heat t r e a t e d at a different t e m p e r a t u r e a n d t h e n c o o l e d to 2 5 ° C .

(400°-1000°C)

T h e samples w e r e a g e d f o r several w e e k s to

i n s u r e c o m p l e t e r a d i c a l a n i o n f o r m a t i o n . T h e s e experiments w e r e c a r r i e d o u t t o : ( 1 ) d e t e r m i n e i f t h e n u m b e r of r e d u c i n g sites g e n e r a t e d b y heat treatment

of M g O i n v a c u o c h a n g e d w i t h c h a n g e

i n temperature,

as

T a n a b e h a d f o u n d f o r heat treatment of C a O i n a i r ; a n d ( 2 ) t o d e t e r m i n e i f the C H N 0 ~ 6

5

2

formation paralleled C O ' - formation o n identically

treated M g O samples.

F i g u r e 1 is a p l o t of r e l a t i v e C O ' - f o r m a t i o n v s .

absolute C H N 0 ' - f o r m a t i o n , b o t h v s . t e m p e r a t u r e of heat treatment. 6

5

2

N o t e t h e excellent production.

c o r r e l a t i o n of C H N 0 " 6

5

production with C O " -

2

W e b e l i e v e this is s t r o n g e v i d e n c e that t h e same t y p e of

r e d u c i n g sites a r e i n v o l v e d i n b o t h s y s t e m s — C O a n d C H N 0 . 6

5

It

2

r e m a i n s to b e seen i f o t h e r a l k a l i n e e a r t h oxides w i l l b e h a v e s i m i l a r l y a n d i f other m o l e c u l e s w i l l b e h a v e s i m i l a r l y o n these oxides. T h e s e d a t a also s u p p o r t o u r c o n t e n t i o n that t r a n s i t i o n m e t a l i m p u r i t i e s a r e n o t r e s p o n s i b l e f o r f o r m a t i o n of the r a d i c a l species o b s e r v e d . W e h a v e d e t e r m i n e d surface areas of the heat-treated M g O samples b y B E T m e t h o d s . T h e values r a n g e d f r o m 213 m / g to 83 m / g ( 8 3 m / g 2

2

2

at 3 0 0 ° C heat treatment, 213 at 4 0 0 ° , 139 at 5 0 0 ° , 140 at 6 0 0 ° , 130 at 7 0 0 ° , 134 at 8 0 0 ° , 129 at 9 0 0 ° , a n d 122 at 1 0 0 0 ° C ) .

D u r i n g the experi-

m e n t i t appears t h a t at 4 0 0 ° - 5 0 0 ° C t h e m a i n p o r t i o n s o f H 0 , 0 , C 0 , 2

2

2

a n d other gases a r e d e s o r b e d i n v a c u o . T h e surface area becomes l o w e r w i t h h i g h e r heat treatment a n d t h e n stabilizes at a b o u t 5 0 0 ° C , a n d this is w h e r e m a x i m u m r a d i c a l f o r m a t i o n a c t i v i t y w a s f o u n d .


146

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

40 :r

o


20

400

600

Heat Treatment

1000

800

Temperature

Figure 1. Relative radical concentration (spins/g) in the MgO/CO (circles) system and absolute radical concentration (spins/g) of the MgO/nitrobenzene (triangles) system as a function of the heat treatment temperature of MgO. NB. The scales for the two systems are different; also some samples have not reached their maximum radical growth value, especially in the MgO/CO system. K n o w i n g t h e surface areas, w e w e r e a b l e to c a l c u l a t e c r y s t a l l i t e sizes a n d t h e r e b y d e t e r m i n e t h e ratio o f M g O surface to M g O i n t e r n a l as 0.12. A l s o k n o w i n g t h e n u m b e r of spins of C H N 0 ~ - f o r m e d , w e w e r e 6

5

2

a b l e to c a l c u l a t e t h e n u m b e r of M g O surface m o l e c u l e s t o C H N 0 ~ - at 6

5

2

a p p r o x i m a t e l y 1 5 : 1 ( f o r t h e 5 0 0 ° C heat t r e a t e d s a m p l e ) . A l s o , a l t h o u g h m o r e d a t a is s t i l l n e e d e d , w e estimate t h e M g O surface m o l e c u l e to C O " f o r m a t i o n as a p p r o x i m a t e l y 4 0 0 : 1 f o r t h e same

M g O sample.

These

results d e m o n s t r a t e that these e l e c t r o n transfer processes are major surface processes of s i g n i f i c a n t i m p o r t a n c e a n d n o t s i m p l y c a u s e d b y m i n o r effects and/or small impurities. Is M g O u n i q u e i n these e l e c t r o n transfer processes?

A r e alkaline

e a r t h oxides u n i q u e ? W e c a r r i e d o u t studies o n a w i d e r a n g e of oxides, carbonates, h y d r o x i d e s , a n d sulfides l o o k i n g f o r a c t i v i t y f o r f o r m a t i o n of p a r a m a g n e t i c species w i t h C O . T h e s e w e r e a l l heat t r e a t e d a t 6 0 0 ° C o r as h i g h as t h e i r m e l t i n g p o i n t o r d e c o m p o s i t i o n p o i n t w o u l d a l l o w . T a b l e I lists t h e c o m p o u n d s s t u d i e d . I t c a n b e seen that o n l y t h e a l k a l i n e e a r t h oxides a n d T h 0

2

w e r e f o u n d t o b e a c t i v e . W h a t d o these h a v e i n

c o m m o n that t h e other m a t e r i a l s s t u d i e d d o not?

M u c h previous w o r k


13.

KLABUNDE

ET

CO-Metal

AL.

Oxide

147

Interactions

i n d i c a t e s that these a l l possess f a i r l y strong L e w i s base surface sites as w e l l as " r e d u c i n g sites" (24, 42-46). L e w i s a c i d sites o n its surface.

I n a d d i t i o n to these, T h 0

2

also has

N o n e of the other materials possess basic

a n d r e d u c i n g sites together ( 4 7 ) .

F u r t h e r m o r e , since the C O r a d i c a l

f o r m a t i o n is s l o w c o m p a r e d w i t h C O a d s o r p t i o n a n d since L e w i s a c i d s s u c h as C 0 believe (0

2

(27)

p o i s o n the surface t o w a r d s a n y C O r a d i c a l f o r m a t i o n , w e that C O a b s o r p t i o n first takes p l a c e o n L e w i s base sites

sites) i n a p p r o x i m a t e m o n o l a y e r coverage,

=

m u c h like Tench

and

N e l s o n (24) d e s c r i b e for H 0 o r C 0 . W e also b e l i e v e that once the C O 2

2


is a d s o r b e d o n basic sites, s l o w m i g r a t i o n of C O to r e d u c i n g sites takes place

(23,27).

T h e o r d e r of a c t i v i t y f o r the C O r a d i c a l f o r m a t i o n is

M g O > C a O > SrO > B a O (very weak)

(27).

A s p r e v i o u s l y m e n t i o n e d , it has b e e n suggested b y earlier w o r k e r s (30)

that i r o n i m p u r i t i e s p l a y a n i m p o r t a n t p a r t i n the f o r m a t i o n of C O

r a d i c a l s o n M g O . T h e d a t a i n T a b l e I suggests that this is not the case f o r o u r samples

(i.e., i t seems u n l i k e l y that i r o n i m p u r i t i e s w o u l d o n l y

l e a d to r a d i c a l f o r m a t i o n i n the a l k a l i n e e a r t h oxides a n d t h o r i u m o x i d e ) . F u r t h e r m o r e , o u r d e t e r m i n a t i o n of the i r o n content of the v a r i o u s samples ( M g O , 0.0018% F e ; C a O , 0.0005% F e ; a n d SrO, 0.0019% F e )

certainly

d i d n o t reflect the o b s e r v e d o r d e r of a c t i v i t y of the a l k a l i n e e a r t h oxides t o w a r d s C O r a d i c a l f o r m a t i o n discussed earlier.

T h u s , w e b e l i e v e i t is

u n l i k e l y that i r o n i m p u r i t i e s p l a y a role i n the present w o r k . T h e E P R s i g n a l of the a d s o r b e d C O r a d i c a l o n M g O , F i g u r e 2, shows a n u n s y m m e t r i c g v a l u e . T h e s i g n a l is not d e s t r o y e d u n t i l the s a m p l e is h e a t e d to > 1 5 0 ° C .

C a O behaves q u i t e s i m i l a r l y . W a t e r a d d i t i o n at

r o o m t e m p e r a t u r e to the C O r a d i c a l M g O system i m m e d i a t e l y causes a change i n the E P R s i g n a l (see Table I.

Figure 2).

T h e a v a i l a b l e data suggest

Oxides, Hydroxides, Carbonates, and Other Materials Tested for A c t i v i t y for Formation of Paramagnetic Species"

Active MgO CaO SrO BaO Th0

2

Non-Active BeO N a 0 (220 ° C ) Na0H(5%):Si0 N a O H (250°C) N a O O C H (190°C) Na C0 2

2

2

A1 0 2

3

2

3

3

a

2

2

2

2

2

; 2

2

3

2

2

8

Al 0 (45%):Si0 SiO Ti0 FeS(7%):Si0 2

F e O ( 0 . 0 5 % ) : A l O :'3 Ge0 ZnO ZrO La 0 Ce0 Nd 0 Eu 0 Dy 0 U0 /U 0 PbO Sn0 3

2

3

2

3

3

2

5

2

°A11 materials were heat treated at 600°C unless otherwise indicated i n vacuo (10 Torr) for 12-18 hr before exposing to 150 Torr C O . -6



1

= 2.0053

%

Figure 2. EPR spectra of the CO-derived radical in the MgO/CO system. MgO heat treated at 800° C. (A) After ca. two weeks exposure to 160 Torr CO; (B) MgO/CO system, spectra (A), upon exposure to ca. 25 Torr D 0.

g


13.

KLABUNDE

that the H

2

E TA L .

CO-Metal

Oxide

149

Interactions

0 ( o r D 0 ) is a d s o r b e d i n the v i c i n i t y of t h e a d s o r b e d r a d i c a l , 2

c h a n g i n g its e n v i r o n m e n t a n d thus t h e E P R s i g n a l rather t h a n f o r m i n g a new radical. I n later p u b l i c a t i o n s w e w i l l d e s c r i b e some C O r a d i c a l c h e m i s t r y o n a l k a l i n e earth oxides w i t h reagents s u c h as 0 , H 2

2

,H 0 , D 0 , and 2

2

organics. A l s o , w e are testing M g O a n d C a O i n m o d e l c o m p o u n d r e d u c t i o n sequences to d e t e r m i n e i f r a d i c a l processes s u c h as those d e s c r i b e d here are a c t u a l l y i m p o r t a n t i n the C O - H 0 r e d u c i n g m e d i u m . 2


Relationship Electron

to Catalysis, Coal Liquefaction,

and

Movement in Metal Oxides

U n d e r s t a n d i n g i n d e t a i l t h e c h e m i s t r y of C O a n d C O - H 0 m i x t u r e s 2

over m e t a l o x i d e surfaces has c o n s i d e r a b l e i m p o r t a n c e f o r a n u m b e r of reasons: pure H

( 1 ) C O - H 0 is a m o r e effective r e d u c i n g m e d i u m r e l a t i v e to 2

2

f o r r e d u c t i o n a n d d e p o l y m e r i z a t i o n o f l o w r a n k coals

m e t a l oxides a n d o t h e r m i n e r a l s c a n a c t as i n s i t u catalysts 51,52);

where

(48,49,50,

( 2 ) C O - H 0 is better t h a n H f o r t h e r e d u c t i o n o f some o r g a n i c 2

f u n c t i o n a l groups (53,54);

2

( 3 ) some m e t a l oxides, e s p e c i a l l y basic oxides,

are u s e d as p r o m o t o r s i n F i s c h e r - T r o p s c h c a t a l y t i c c h e m i s t r y a n d t h e i r role is n o t u n d e r s t o o d m e c h a n i s t i c a l l y (55,56,57); are o f t e n u s e d as catalysts f o r t h e C O + H

2

a n d ( 4 ) m e t a l oxides

0

Carbon Monoxide-Metal Oxide Interactions - Advances in Chemistry

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