reduction sulfur di ox over metal ox

8/11/2019 Reduction Sulfur Di Ox Over Metal Ox

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p p l i e d

c t l y s i s e

E L S EV I ER Applied Catalysis B: Environmental 4 (1994) 167-186

ed u c t i o n o f s u lfu r d i o x i d e b y ca rb o n m o n o x i d e to

e lem enta l su lfur ove r com po s i te ox ide cata lyst s

W e i L i u , A d e l F . S a r o f im , M a r i a F l y t z a n i - S t e p h a n o p o u l o s 1 '*

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 29 December 1993; revised 25 M arch 1994; accepted 25 M arch 1994

bs t r ac t

T h e c a t a ly s t a c t i v i ty o f f l u o r it e - ty p e o x i d e , s u c h a s c e r i a a n d z i r c o n i a, f o r t h e r e d u c t i o n o f s u l f u r

d i o x i d e b y c a r b o n m o n o x i d e t o e l e m e n t a l s u l f u r c a n b e s i g n i f ic a n t l y p r o m o t e d b y a c t i v e t r a n s i t io n

m e t a l s , su c h a s c o p p e r . M o r e t h a n 9 5 % e l e m e n t a l s u l f u r y i e l d , c o r r e s p o n d i n g to a l m o s t c o m p l e t e

s u l f u r d i o x i d e c o n v e r s i o n , w a s o b t a i n e d o v e r a C u - C e - O o x i d e c a ta l y s t w i t h a f ee d g a s o f s to i c h i o -

m e t r i c c o m p o s i t i o n ( [ C O ] / [ S O 2 ] = 2 ) a t t e m p e r a t u r e s a b o v e 4 5 0 C . T h i s ty p e o f m i x e d m e t a l o x id e

c a t a ly s t h a s s t a b l e a c t i v i ty a n d i s r e s i s ta n t to w a t e r a n d c a r b o n d i o x i d e p o i s o n i n g . X P S a n a l y s i s f o u n d

c o p p e r i n t h e C u - C e - O o x i d e i n a r e d u c e d o x i d a t i o n s t at e ( C u ~+ , Cu ) . The s t a b l e f l uo r i t e - t ype

s t ruc t u re , r e ga rde d a s t he ba c kb on e s t ruc t u re o f t he c a t a lys t , e x i s t e d i n bo t h t he f r e sh a nd t he spe n t

c a t a l ys t . Th e h i gh a c t i v i t y r e su l t e d f rom t he s t ron g i n t e ra c t i on o f t r a ns i t i on me t a l a nd f l uo r i t e ox i de .

Keywords:

Copp er-cerium oxide; Elemental sulfur production; Fluorite oxide; Reduction; Sulfur dioxide

I I n t r o d u c t i o n

De su l f u r i z a t ion o f c om bus t ion e xha us t ga se s i s p r e se n t ly a c os t ly p r oc e s s o f t e n

i n v o l v i n g c o m p l e x f l o w s h e et s a n d t h r o w - a w a y s o rb e n ts . D i r e ct ly r e d u c in g s u l-

f u r d iox id e to e l e m e n ta l su l f u r ove r a c a t a ly s t i s a tt r ac t ive , be c a use i t p r oduc e s a

sa l a b le p r oduc t w i thou t a ny so l id wa s t e t o d i spose o f . D i r e c t f l ue ga s r e duc t ion

p r oc e sse s inv o lve non - se l e c t ive c a t a ly s t s , thus c o nsu m ing l ar ge a mo un t s o f r e duc -

t a n t ( H2 o r C O) to r e a c t t he e xc e ss ox yge n in the e xha us t gas . Add i t iona l ly , t he se

p r oc e sse s r e qu i r e a n e x pe ns ive C la us p l a n t t o c om ple t e t he e l e me n ta l su lf u r r e c ov -

e r y [ 1 , 2 ] . O the r p r oc e s se s un de r de v e lop m e n t a r e two- s t a ge , d r y r e ge ne r a t ive f lue

ga s c l e a nup p r oc e s se s , w he r e by d i lu t e SO2 s t r ea ms a r e s c r ubbe d by r e a c t ion w i th

*Corresponding author.

~Present address: Departm ent of Ch emical Engineering, Tufts U niversity, M edford, MA 02155

0926-3373/94/ 07.00 19 94Elsevier Science B .V. All rights reserved

SSDIO926-3373 94)OOO19-B


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68 W . Liu e t a l . / A pp l i ed Ca ta ly s i s B : E nv i r onmenta l 4 1994) 167 -186

a me ta l ox ide so r be n t a nd the sp e n t so r be n t i s r e ge ne r a t e d fo r r e - u se w i th c on c om -

i t a n t e mis s ion o f a c onc e n t r a t e d S O 2 s t r e a m su i t a b l e f o r su l f u r r e c ove r y . T he

r e ge ne r a to r o f f - ga s ha s on ly a f r a c t i on o f t he f l ue ga s vo lume , a nd c on ta in s no

oxyge n . R e c ov e r y o f su l f u r fr om th i s t ype o f ga s i n a s ing l e - s ta ge c at a ly t i c c on -

ve r te r , a vo id ing the m u l t i s t a ge C la us p l a n t , w o u ld de c r e a se t he c os t a nd a c c e l e r a t e

the c om m e r c i a l i z a t i on o f d r y r e ge n e r a t i ve fl ue ga s c l e a nup p r oc e s se s .

T h e d i r ec t re d u c t i o n o f S O 2 b y C O t o e l e m e n t a l s u lf u r h a s l o n g b e e n k n o w n ,

bu t , no subs t a n t i a l c om m e r c i a l e xpe r i e nc e e x i s ts . T h e ove r a l l r e a c tions i nvo lv e d in

th is p roce ss a re a s fo l low s:

S O 2 + 2CO ~ 2CO 2 + 1 /xS x

C O + 1 / x S ,

~

C O S

2CO S + SO 2 ~ 2CO 2 + 3 /xS x

1)

2)

3)

w h e r e x = 2 - 8 o r h i g h e r . A t h i g h t e m p e r a t u re s , p r e d o m i n a n t l y g a s e o u s e l e m e n t a l

s u lf u r, 2 , p r o d u c e d t h r o u g h E q . ( 1 ) c a n re a c t w i t h c a r b o n m o n o x i d e t o fo r m

c a r bony l su l f i de , CO S , w h ic h ma y a ga in r e duc e S O 2 to e l e me n ta l su l f u r t h r ough

r e ac t io n ( 3 ) . H o w e v e r , C O S i s a m o r e to x i c c o m p o u n d t h a n S O 2 a n d i ts p r o d u c t i o n

shou ld be m in im iz e d in a su l f u r r e c ove r y p r oc e s s . I n a dd i t i on to t he f o r ma t ion o f

u n d e s i r ab l e C O S , t h e p r e s e n c e o f w a t e r v a p o r i m p u r i ty i n t h e f e ed g a s m a y h a v e a

m a jo r e f f e c t on b o th t he c a t a ly s t a c t i v i ty a nd p r odu c t s e l e ct i v i ty by po i s on in g the

c a ta l y st a n d / o r p r o m o t i n g t h e f o l l o w i n g r e a c ti o n s:

CO + H2 O ~ H2 + CO2

CO S + H2 O ~ H2 S + CO2

H 2 + [ S ] ~ H 2 S

3/xSx

2 H 2 0 ~ 2 H 2 S + SO 2

4)

5)

6)

7)

R e a c t i o n s ( 1 ) - ( 7 ) h a v e f a v o r a b le t h e r m o d y n a m i c s a t th e c o n d i t io n s o f in t e re s t

t o t h i s w or k ( a tmosphe r i c p r e s su r e , 300 to 700 C) . T he c ha l l e nge to r e a l i z e t he

one - s t a ge e le m e n ta l su l f u r re c ove r y s c he m e is t o de ve lo p a ca t a ly s t w h ic h i s no t

p o i s o n e d b y w a t e r o r o t h e r g a s i m p u r i ti e s a n d t o m a x i m i z e t h e y i e ld o f e l e m e n t a l

sulfur .

A lu m ina - su ppo r t e d t r a ns i ti on m e ta l s , suc h a s Cu , F e , N i , P d , e t c ., ha ve be e n

e x t e n si v e ly s t u d i e d f o r t h e r e d u c t i o n o f s u l f u r d i o x i d e b y c a r b o n m o n o x i d e [ 3 - 9 ] .

T he p r o duc t ion o f C O S usua l ly p r oc e e ds t o a subs t a n t i a l e x t e n t on the se c a ta ly s ts

a nd w a te r pa r t ia l ly p o i son s t he c a t a ly s t a c t iv i t y . L o w e r CO S f o r m a t ion w a s r e a li z e d

o n m i x e d o x i d e s o f e l e m e n t s f r o m t h e l a n t h a n i d e g r o u p a n d t h e g r o u p s o f I V A a n d

V A [ 1 0 - 1 2 ] , s u c h a s L a - T i - O . B u t , t h is t y p e o f c a ta l y st ty p i c al ly n e e d s t o b e

a c t iva t e d by r e duc ing ga se s a t h igh t e m pe r a tu r e . P e r ovs k i t e - type m ixe d ox ide s

[ 13 - 16 ] , A BO 3, c on ta in ing t ra ns i t ion me ta l s i n pos i t i on B a nd r a r e e a r th a nd /o r

a lka l ine e a r th e l e me n t s i n pos i t i on A , w e r e f ound a s a c t i ve c a t a ly s t s w i th h igh

se l e c ti v i ty t o e l e me n ta l su l f u r ove r CO S . H o w e v e r , t h is t ype o f c at a ly s t , f o r e xa m -


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W . L i u e t al . / A p p l i e d C a t a l ys i s B : E n v i r o n m e n ta l 4 1 9 9 4 ) 1 6 7 - 1 8 6 69

pie , La~ _xSrxCoO3 [ 15 ] , e ssent ia l ly los t i t s pe rovs ki te s t ruc ture a f te r a sh or t induc-

t io n p e r i o d u n d e r r e a c ti o n c o n d i t i o n s a n d b e c a m e a m i x t u r e o f m e t a l su l fi d es a n d

oxysu l f ide s wh ic h c o n t r ibu te d to th e c a t a ly s t a c t iv i ty a s sug ge s t e d by B a g l io [ 17 ] .

I n a dd i t ion , t he wa te r v a po r e f f e c ts on th e se c a t a ly s t s ha ve n o t be e n r e po r t e d .

A r e d o x r e a c t i o n m e c h a n i s m i n v o l v i n g t h e p a r t i c i p a t i o n o f s u r f a c e o x y g e n /

o x y g e n v a c a n c y h a s b e e n p r o p o s e d f o r t h e S O2 r e d u c ti o n b y C O [ 1 2 , 1 5 ] . W e

r e c e n t ly r e po r t e d tha t c e r iu m o x ide , a f luo r i t e ox ide w e l l kn ow n f o r i ts h igh oxy ge n

va c a nc y c onc e n t r a t ion a nd mob i l i t y , i s a n a c t ive c a t a ly s t a nd i t s a c t iv i ty c a n be

f u r t h e r e n h a n c e d b y d o p i n g i t w i t h l a n t h a n u m o x i d e [ 1 8 ] . H o w e v e r , a l ka l in e a n d

r a r e e a r th ox ide dopa n t s d id no t impr ove the r e s i s t a nc e o f c e r i a t owa r ds wa te r

po i son ing . I n the p r e se n t s tudy , w e ha ve m od i f i e d the c e r i a w i th t r a ns i ti on me ta l s ,

e xa m ine d o th e r oxyg e n - ion c o ndu c t ing ma te r i a ls a s po te n t i al c at a ly s t s , a nd e va l -

ua t e d the c a t a ly s t a c tiv i ty a nd se l e c t iv i ty a s a f un c t ion o f c a t aly s t t ype a nd ope r a t ing

c ond i t ions , i nc lud ing the e f f e c t o f w a te r va po r .

2 Experimental

2 1 Apparatus and procedure

Al l c a t a ly s ts we r e t e s t e d in a l a bo r a to r y - sc a l e , pa c ke d b e d f low r e a c to r , c ons i s t i ng

o f a 0 .6 c m I .D . X 50 c m long q ua r t z t ube w i th a po r ou s qua r tz f r i t p l a c e d a t t he

midd le f o r suppor t ing the c a t a ly s t . The r e a c to r t ube wa s he a t e d by a L indbe r g

f u r na c e . Th e r e a c t ion t e mpe r a tu r e wa s m on i to r e d by a qua r t z t ube - she a the d K- type

t h e r m o c o u p l e p l a c e d a t th e t o p o f t h e p a c k e d b e d a n d c o n t r o l le d b y a W i z a r d

te m pe r a tu r e c on t r o l l e r . The r e a c t ing ga se s , a l l c e r ti f ie d c a l ib r a t ion ga s mix tu r e s

w i t h h e l i u m M a t h e s o n ) , w e r e m e a s u r e d w i t h ro t a m e t er s a n d m i x e d p r i o r t o t h e

r e a c to r in l e t. The r e su l t i ng ga s m ix tu r e f lowe d down w a r d th r ough the pa c ke d be d .

W a t e r v a p o r w a s i n t r o d u c e d w i t h h e l i u m b u b b l i n g t h r o u g h a h e a t e d w a t e r b a th .

T h e p r e s s u re d r o p o f g a s f l o w i n g th r o u g h t h e a s s e m b l y w a s s m a l l. T h u s , e x p e ri -

m e n t s w e r e c a r r i e d ou t a t ne a r ly a tm osph e r i c p r e s su r e . A c o ld t r a p in s t al l e d a t t he

o u t l e t o f t h e r e a c to r w a s u s e d t o c o n d e n s e o u t t h e e l e m e n t a l s u lf u r fr o m t h e p r o d u c t

ga s s t r e a m. The p r oduc t ga s , f r e e o f su l f u r a nd pa r t i c u l a t e s , wa s a na lyz e d by a

H P 5 8 8 0 A g a s c h r o m a t o g r a p h G C ) w i t h a t h e r m a l c o n d u c t i v i ty d e t e c to r T C D ) .

H e l i u m w a s u s e d a s t h e G C c a rr ie r a n d r e fe r e n c e ga s , e a c h a t 3 0 c m 3 / m i n . T h e

de te c to r a nd ove n t e m pe r a tu r e s we r e se t a t 200C a nd 60C , r e spe c tive ly . A 1 /4

i n. O . D . X 6 ft . l o n g p a c k e d c o l u m n o f C h r o m o s i l 3 1 0 f r o m S U P E L C O ) p r o v i d e d

goo d se pa r a t ion o f C O, C O2 , C OS , H2S , C S 2, a nd S O2 unde r t he se c ond i tions . Th e

T C D s h o w e d l in e a r r e sp o n s e t o a l l t h e c o m p o u n d s m e n t i o n e d ab o v e a n d a d e t e c ti o n

l im i t o f le s s t h a n 1 0 0 p p m b y v o l u m e .

A f r e sh c a t a ly s t wa s e v a lua t e d by a two - s t e p te s t p r oc e dur e . In the f i rs t s t ep , t he

r e a c t a n t ga s mix tu r e wa s in t r oduc e d in to the r e a c to r a t a bou t 500C a nd the r e a c t ion

te m pe r a tu r e wa s r a i s e d by a bou t 50C inc r e m e n t s a ft e r a s te a dy - s ta t e r e a c t ion w a s


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17 W. Liu et aL /Applied Catalysis B: Environmental 4 1994) 167-186

reached at ea ch tem perature. The react ion l ight-off temperature was thus identified,

at which the reaction occurred with SO2 conver sion exceed ing 90 . In the second

step, the react ion temperature was lowered in steps of about 50C from a high

temperature unti l conversion was significant ly decreased. Hysteresis effects

betwee n the l ight-off and fal l -off branc hes were fol low ed in this sequence of steps.

The effect of water on the catalyst act ivi ty and select ivi ty was examined by

adding about 2 by volu me of water vapor into the reactant gas mixture. Unless

specifically noted, catalyst tests were typical ly perform ed with 150 mg catalyst

loade d in the reactor, gas flow-rate set at 100 scc m and com pris ing 1 SO2, 2

CO by volume, and balance hel ium. The contac t t ime was 0 .09 g s /c m 3 (STP).

Each catalyst had a different packing dens i ty so that the space veloci ty varied with

the individual catalyst tested. S ulfur dioxide conversion, X-SO2, and elemental

sulfur yield, Y-sulfur, are defined by fol lowing equations:

X-SO2 = { [SO2]o - [SO2] }/[S O2 ]o

Y-s ul fur = IS] / [SO2]o

where [SO2]o and [SO2] are the inlet and outlet sulfur dioxide concentrat ions,

respect ively, while [ S ] is the outlet elemen tal sulfur concentration calculated from

the material balance of ca rbon an d sulfur, and occasional ly checked by t i t rat ion of

the sulfu r collec ted in the cold trap.

2 2 Catalyst preparation

Both b ulk com posite and im pregna ted oxide catalysts were prepared in this study.

The com posite catalysts were p repared by coprecipi tat ing aqueous salt solut ions of

the metals with am m on ium carbonate. Th e precipi tates were washed twice with hot

deionized water and then dried for about 12 hours at 110C. The dried samples

were further heated for a few hours in stagnant air at 650C. Th e part icle sizes used

for tests were -35 + 100 me sh (420o 149 m m ) except for tests with zirconia catalysts

where -250 m esh ( < 63 mm ) part icles were used. The sup ported catalysts were

prepared by conve ntional wet impre gnation of the support with aqueous sal t solu-

t ions of the metals. Th e slurry of the support and solut ion was degassed in vacuu m

so that the sal t solut ion ful ly fi l led the pores o f the support during im pregnation.

Then, the excess solut ion was fil tered. The w etted samples were dried for two days

at room temperature and then heated for four hours at 6500700C in a muffle

furnace. The metal con tent in the impreg nated catalysts was determ ined with induc-

t ive ly coupled p lasma (ICP) a tomic emiss ion spect rometry (Perkin-Elmer P lasma

40) . The

Cu/T-AI203

catalyst was prepared by reduction of

CuO/ y-AI203

with

10 C O /H e at 300C. The materials used in catalyst preparat ion are l isted in

Table 1.

Catalyst characterizat ion was perfo rm ed by X-ray pow der diffract ion on a R igaku

300 X-ray diffractometer for crystal l ine phase identificat ion, by N2 desorpt ion on

a Micromeri t ics FlowSorb II 2300 apparatus for BET surface area measurement ,


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W. Liu et al. / Applied Catalysis B: Environmental 4 1994) 167-186

Table 1

Materials inven tory used for catalys t preparat ion

171

Materials Source

Chemicals

Cer ium ni t ra t e hexahydra te , conta in ing 1 .38 wt .- Aldr i ch

l a n t h a n u m

Cer ium ace ta te hydra te , 99 .9

Cob al t l ni t ra te hydrate , 99.99

T e r b i u mm ni t rate pentahydrate , 99.9

Yttr ium ni t rate pentahydrate , 99.9

Hy droge n hexa chloro plat inate hydrate , A.C.S. grade

C h r o m i u mm nitrate, 98.5

C o p p e ru nitrate, A.C.S. grade

Ga dol in ium nit rate , 99.9

Zi rconium dichlor ide oxide , 99 .9

Am mo nium carbona te , A .C .S . grade

Nickel ni t ra te , A.C.S. grad e

M a n g a n e s en nitra te tetrahydrate, 98

Support

CeO2

CeO2 particle sizes: 500 to 150 k~m

CeOz surface area: 24 m2/g

CeO2 void fract ion: ca . 0 .12 cm 3/g

CeO2 packed dens i ty : 0 .95 g / cm 3

y-A12O3 LaRoche

y-AI203 part ic le s ize: 72/zm average

7-Ah_O3 surface area: 3 00 m 2/g

y-AI203 void fraction: ca . 1.0 cm 3/g

y-A1203 packe d densi ty: 0.6 g /c m 3

Aldr i ch

Aldr i ch

Aldr i ch

Aldr i ch

Aldr i ch

J o h n s o n M a t th e y

Johnson Mat they

J o h n s o n M a t t h ey

Johnson Mat they

Fish er Scientific

Fisher Scientific

F luka

Therm al decomposi tion of cerium acetate a t 750C

and by scann ing e l ec tron m icroscopy SE M ) on a Cambridge S tereoscan 2 40 MK .3

for catalyst morp holog y observation. Elem ental d istr ibution wa s exam ined by scan-

n ing transmission e l ectron microscopy ST EM ) us ing a Vacuum Generators

HB 603 instrument , wh i l e X-ray photoe lec tron spec troscopy X PS ) o f the cata lyst

surface w as p erform ed with a Perkin-Elmer 51 00 instrument. For STEM analysis ,

the f ine catalyst pow der w as em bed ded in a resin m atrix, the resin wa s ul tramicro-

tom ed to s l ices o f 80 to 120 nm , and the s l ices w ere supported on a nickel gr id and

coated with carbon.

3 . Re su l t s

3.1. Transition m eta l im pre gna ted ceria catalysts

Fig . 1 show s the l i gh t -of fc urv es o f su lfur d iox ide react ion wi th carbon mon oxide

ove r the transition m etal-imp regnated ceria catalysts. T he l igh t-o ff temperatures

are 500C on the Cu/CeO2 and Ni /CeO2 cata lyst s , 550C on the Pt /CeO2, and


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172

W . L i u e t a L A p p l i e d C a t a l y s i s B : E n v i r o n m e n t a l 4 1 9 9 4 ) 1 6 7 - 1 8 6

C u / C e O 2

0 .8 N i / C e O 2

P t / C e O 2

0 . 6

Mn/Ce02

001420 rn C o lC e02

o

C r / C c O 2

A C e O 2

4 5 0 5 0 0 5 5 0 6 0 0 6 5 0

R e a c t i o n T e m p e r a t u r e ( C )

F i g . I . L i g h t - o f f e h a v i o r o n t h e t r an s it i on e t a l - i m p r e g n a t e d e r i a a t al y st s S O 2 , 2 C O , 9 7 H e ) .

6 0 0 C o n t h e C o / C e O 2 a n d M n / C e O 2 . A l i g ht - o ff e m p e r a t u r e h i g h e r t h a n 6 0 0 C

w a s f o u n d f o r u n m o d i f i e d c e r i a a n d f o r C r / C e O 2 c at a l y s t s . T h u s , a d d i t i o n o f

t r a n s i t i on m e t a l s s i g n i f i c a n t l y l o w e r e d t h e l i g h t -o f f e m p e r a t u r e o f c e r ia . T h e f al l-

o f f b e h a v i o r o n t h e s e c a t a l y s ts a s w e l l a s w a t e r v a p o r e f f e c t s a r e p l o t t e d i n F i g . 2.

I n t h e s e t es ts , e r i a a n d C r / C e O 2 w e r e a c t i v a t e d b y r e d u c t i o n w i t h I 0 C O / H e a t

6 0 0 C . T h e r e a c t i o n s t a rt s t o f al l o f f a r o u n d 5 5 0 C o n c e r i a a n d b e l o w 5 0 0 C o n

t h e o t h e r c a t a l y s t s . h e C r / C e O 2 s h o w s a l o w f a i l - o ff e m p e r a t u r e , a l t h o u g h i t w a s

d if fi cu lt t o a c t i v a t e i t i n t h e r e a c t i o n m i x t u r e ( F i g . I ) . H o w e v e r , t h i s c a t a l ys t

c o m p l e t e l y d e a c t i v a t e d u p o n a d d i t i o n o f w a t e r a t 5 5 0 ( 2 a s d i d c er i a . T h e o t h e r

catalysts w ere st i l l act ive in the presence of w ater. The sulfur yield at 51 0C was

high er on the Cu-, Co- and Ni- impregnated c er ia catalysts ca. 0.7) than on the

T

0 8

0

4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0

R e a c t io n T e m p e r a t u r e ( C )

F i g . . F a l l - o f f e h a v i o r n d w a t e r a p o r e f f e c t n t h e t r a ns i t io n e t a l - i m p r e g n a t e d e r i a a t al y st s . [ ] ) C u / C e O 2 ,

( ( ~) N i / C e O 2 , ( A ) P t / C e O 2 , ( V ) M n / C e O 2 , ( ( D ) C o / C e O 2 , ( : ~ ) C r / C e O 2 , ( ( D ) C a O 2 . O p e n s y m b o l s f or r y

g a s ( 1 S O 2 , 2 C O , 9 7 H e ) , fi lle d y m b o l s f or w e t g a s ( 1 S O 2 , 2 C O , 2 H 2 0 , 9 5 H e ) .


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T able 2

T r a n s i ti o n m e t a l - i m p r e g n a t e d c e r i a c a t a l y s ts

173

C a t a l y s t L i g h t - o f f

C o m p o s i t i o n S u r f a c e a r e a ( m S / g ) Tgo,~ ( C )

Y - s u l f u r / X - S O 2

D r y g a sd ( 5 1 0 C ) W e t g a se ( 5 1 0 o c )

C e O 2 2 4 > 6 0 0 0 . 0 4 / 0 . 1 2 0 . 0 / 0 . 0

C u / C e O 2 ~ 2 1 . 4 5 0 0 0 . 9 7 / 0 . 9 9 0 . 7 3 / 0 .9 1

N i / C e O 2 a 2 2 . 7 5 0 0 0 . 9 7 / 0 . 9 8 0 . 6 9 / 0 . 7 8

P t / C e O z b 2 2 .9 5 5 0 0 . 6 6 / 0 . 6 8 0 . 4 2 / 0 . 4 4

M n / C e O 2 a 2 0 .8 6 0 0 0 . 9 5 / 0 . 9 7 0 . 6 0 / 0 . 6 6

C o / C e O 2 a 2 2 .5 6 0 0 0 . 9 6 / 0 . 9 8 0 . 7 3 / 0 . 8 8

C r / C e O 2 a 1 6 .3 > 6 0 0 0 . 9 6 / 0 . 9 7 0 . 0 / 0 . 0

~ CeO2 wa s imp regna te d wi th 0 . 5 M me ta l n i t ra t e s o lu t ion . T he a tomic t rans i t ion me ta l con ten t in the re s u l t ing

ca ta lys t

n M / n M + n C e )

100 ) i s abou t 13 .

b C eO 2 w a s i m p r e g n a t e d w i t h 1 0 m g P t / c m ~ h y d r o g e n h e x a c h l o r o p l a t i n a teT hydra te s o lu t ion .

C R e ac ti on t e m p e r a t u r e a t w h i c h 9 0 c o n v e r s i o n o c c u r r e d d u r in g t e m p e r a t u r e - ri s e t e s t.

a R e a c t in g g a s m i x t u r e c o n s i s t e d o f 1 S O 2 , 2 C O , b a l a n c e h e l iu m .

2 H 2 0 w a s a d d e d i n to r e a c t i n g g a s m i x t u r e .

f S u l fu r y i e l d / S O 2 c o n v e r s i o n ; s e e t e x t .

M n - a n d P t - im p r e g n a t e d c e ri a 0 . 4 - 0 . 6 ) . T a b l e 2 li st s t h e n u m e r i c a l v a l u e s o f

s u l f u r y i e l d a n d S O 2 c o n v e r s i o n i n t e s t s w i t h t h e d r y a n d w e t r e ac ta n t g a s m i x t u r e s

s h o w n i n F i g s . 1 a n d 2 . I t i s s e e n t h a t i n th e d r y g a s a l m o s t a l l t h e s u lf u r d i o x i d e

w a s c o n v e r t e d t o e l e m e n t a l s u l f ur . H o w e v e r , t h e e l e m e n t a l s u lf u r y i e ld d e c r e a s e d

f r o m a t y p i ca l 0 . 9 6 d r y g a s ) t o 0 . 4 - 0 . 7 w e t ga s ) d u e t o t h e f o r m a t i o n o f h y d r o g e n

su l f ide .

F i g . 3 s h o w s t h e v a r i a t i o n o f g a s e o u s p r o d u c t d i s t r i b u t i o n w i t h t i m e - o n - s t r e a m

o n t h e C u / C e O 2 c a t a ly s t a t 51 0 2 i n t h e p r e s e n c e o f w a t e r . F o r m a t io n o f H 2 S

12000

10000.

8000

6000 ~

4000

2000

0

12_ _ ~_._ tlD_ t~ Q ~

L ~ d ~ . . _ ~ a P .. _ 4 1 ~ - - ~ 4 1 . -- . - - O .

C O S H 2 S

SO2 Elem ental Sul fur

[] CO2/2

0 5 10

Time on Stream

hr)

F i g . 3 . L o n g - t e r m t e s t o f C u / C e O 2 c a t a ly s t i n w e t g a s ( 1 S O 2 , 2 C O , 2 H 2 0 , 9 5 H e ) , 1 0 0 s e e m , 5 0 5 C ,

3 0 7 m g c a t a l y s t l o a d in g .

_ _ l _ ~ Q _ ~ _

15 20 25 30


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a c c o rd i n g t o r e a c t io n s 4 ) a n d 6 ) c o n s u m e d s o m e re d uc ta n t C O , r e s u lt in g i n a

s u b - s t o i c h i o m e t r i c a m o u n t f o r t h e r e d u c t i o n o f S O 2 . T h e r a t i o o f H 2 S t o S O 2

concentra t ion in the produc t gas wa s around s to ich iome tr ic , wh ich m ean s that near ly

c o m p l e t e c o n v e r s i o n o f t h e i n l e t C O t o o k p l a c e . F i g . 3 a l s o i n d i c a t e s t ha t t h e C u /

CeO2 ca ta lys t i s s tab le under the reac t ing con di t ions , s in ce n o apparent deac t iva t ion

w a s o b s e r v e d d u r i n g a 2 5 - h o u r r u n . T h i s p r o m i s in g c a t a ly s t s y s t e m w a s u s e d f o r

d e t a il e d s t u d i e s o f t h e S O 2 r e a ct i on w i t h C O .

3 .2 . E f f e c t o f c opp e r on ten t on the c a t a l y s t ac t i v i ty an d s e l e c ti v i ty o f the C u

C e O s y s te m

T w o s e r ie s o f C u - C e - O c a t a ly s t s, l i s te d i n T a b l e 3 , w e r e p re p ar ed b y c o p r e c ip -

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

content on the ca ta lys t ac t iv i ty and se l ec t iv i ty . The La-doped cer ia and a lumina-

suppor ted copper ca ta lys t s are a l so inc luded in Table 3 for compar i son . The bulk

C u - C e - O c o m p o s i t e c a t a l y s t t y p i c a l l y h a s a s u r f a c e a r e a a r o u n d 3 0 m 2 / g , w h i l e

the impregnated cer ia ca ta lys t has a sur face area s l ight ly lower than the CeO2

s u pp o rt 2 4 m 2 / g ) . T h e C u / A 1 2 0 3 c a t a l ys t s h a v e m u c h h i g h e r s u r f a ce a re a c a .

Table 3

V a r ia ti on o f c a t a l y st a c ti v i ty a n d s e l e c t i v i t y w i t h c a t a ly s t c o m p o s i ti o n i n t h e C u - C e - O s y s t e m

C a t a l y s t Y - s u l f u r / X - S 0 2

C o m p o s i t i o n S u r f a c e a r e a m 2 / g )

D r y

g a sa W e t g a se

4 6 5 C 5 1 0 C 4 7 0 C 5 1 0 C

Composite

ca t a l y s t a

L a o. lC e o.9 0 x 2 8 . 5 0 . 0 6 / 0 . 2 0 0 . 9 7 / 1 . 0 0 - 0 . 0 / 0 . 0

C u o. oi Ce o .9 9 (L a )O x 5 7 . 0 0 . 9 7 / 1 . 0 0 0 . 9 7 / 1 . 0 0 0 . 6 5 / 0 . 8 1 0 . 6 6 / 0 . 9 0

C u o.o sC eo .9 5 L a ) O x 5 7 . 0 0 . 8 3 / 0 . 9 2 0 . 9 5 / 0 . 9 8 0 . 5 3 / 0 . 7 1 0 . 6 1 / 0 . 8 7

Cuo.15Ceo.85 L a ) Ox

3 0 . 7 0 . 9 7 / 0 . 9 9 0 . 9 7 / 1 . 0 0 0 . 6 9 / 0 . 9 0 -

C u o . 2 C e o .s ( L a )O x 3 7 .1 0 . 8 6 / 0 . 8 9 0 . 9 3 / 0 . 9 6 0 . 7 2 / 0 . 8 9 0 . 7 3 / 0 . 9 1

C u o 2 5 Ce o. 75 (L a )O x 3 0 . 2 0 . 9 3 / 0 . 9 7 0 . 9 4 / 1 . 0 0 0 . 6 4 / 0 . 8 4 0 . 6 9 / 0 . 9 2

C u o.a 5C eo .6 s L a ) O x 3 3 . 8 0 . 9 3 / 0 . 9 8 0 . 9 4 / 1 . 0 0 0 . 6 7 / 0 . 8 8 -

hnpregnated catalyst b

C e O 2 s u p p o r t 2 4 . 0 0 / 0 . 0 0 . 0 4 / 0 . 1 2 - 0 . 0 / 0 . 0

3 . 5 w t . - C u O J C e O 2 2 2 . 0 0 . 9 3 / 0 . 9 7 0 . 9 4 / 1 . 0 0 0 . 6 0 / 0 . 7 7 0 . 7 3 / 0 . 9 3

7 . 8 w t . - C u O x / C e O 2 2 1 . 0 0 . 8 4 / 0 . 9 2 0 . 9 5 / 0 . 9 9 0 . 5 6 / 0 . 7 3 0 . 6 8 / 0 . 9 1

2 1 w t . - C u O / ~ , - A I 2 0 3 1 3 7 0 . 3 8 / 0 . 4 2 0 . 5 6 / 0 . 5 7 - -

C u / ' y - A I 2 0 3c 1 3 7 - 0 . 7 7 / 0 . 8 1 - 0 . 3 8 / 0 . 6 1

aCata lys ts were prepared by coprec ip ita t ion . La ) den otes that the cer ium nitrate precursor used conta ined about

1.38 wt . - lanthanum.

hCata lys ts we re prepared by im pregnation ( w t . - d e n o te s th e w e ig h t p e r ce n t o f c o p p e r ) .

cPrepared by reduc ing the 2 1 w t . - C u O / T - A I 2 0 3 mater ia l with 1 0 C O / H e a t 3 0 0 C .

a R e a c tin g g a s m ix tu r e c o n s i s te d o f

1 SO 2, 2 C O ,

balance he l ium.

~ 2 H 2 0 w a s a d d e d in to r e a c t in g g a s m i x t u r e .

r S u lfu r y i e ld /S O 2 c o n v e r s io n ; s e e t ex t .


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175

1 3 0 m 2 / g ) d u e t o t h e y - a l u m i n a s u p p o r t. S in c e th e SO c onve r s ion inc r e ase s ve r y

f a s t w i th t he r e a c t ion t e m pe r a tu r e , on ly e x pe r ime n ta l re su l ts a t r e la t i ve ly l ow t e m-

pe r a tu r e s a r e i nc lu de d in Ta b le 3 t o c om pa r e t he a c t iv it y a nd se l e c tiv i ty . Th e l i gh t -

o f f t e m pe r a tu r e on a l l t he C u- C e - - O c a t a ly s t s wa s a pp r ox ima te ly 500C , wh i l e t he

l i g h t - o f f t e m p e r a t u r e s o n t h e Cu/AI203 c a t a ly s t s a nd La - dope d c e r ia we r e 560C

a nd 610C , r e spe c t ive ly .

T h e i m p r e g n a t e d an d b u l k C u - C e - O c a ta l y st s s h o w s i m i l a r o v e r al l b e h a v i o r a n d

the c a t a ly s t a c t iv i t y i s no t s e ns i t i ve to t he c opp e r c on te n t i n t he c a ta ly s t. Ho we v e r ,

t he r e i s a t r e nd f o r t he s e l e c t iv i t y t owa r d e l e m e n ta l su l f u r t o de c r e a se a s t he c oppe r

c on te n t i nc r e a se s . I n t he p r e se n c e o f 2 wa te r , hyd r oge n su lf ide i n s t e a d o f c a r bony l

s u l f i d e b e c o m e s a m a j o r b y p r o d u c t o v e r t h e C u - C e - O c a t a l y s t . T h e e l e m e n t a l

su l f u r y i e ld d r ops f r om a typ i c a l 95 in t he d r y ga s t o a bou t 70 in t he we t ga s .

T h e

Cu/T-A1203

ca ta lys t has a l i t t l e h igher ac t iv i ty than the

CuO / y-A1203, bu t,

bo th c a t a ly s ts a r e mu c h l e s s a c t ive tha n the C u - C e - O c a t aly s ts . A l so , i n c on t r a s t

t o th e C u - C e - O , t h e c o p p e r / a l u m i n a ca t al y s t f a v or s t h e f o rm a t i o n o f C O S i n t h e

p r e se nc e o f wa te r a s ha s b e e n r e po r t e d in the l i te r a tu r e [ 8 ] a nd ve r i fi e d in t h is

wor k .

3 3 Zircon ate an d zirconia catalysts

Zi r c on iu m ox ide h a s t h e f l uo r i t e -type st r uc tu r e o f c e r i a a nd i s s imi l a r t o c e r ia i n

m a n y p h y s i c a l p ro p e r ti e s, s u c h a s h i g h o x y g e n m o b i l it y a n d o x y g e n i o n v a c a n c y

c onc e n t r a t i on , bu t , z i r c on ia i s m uc h m or e d i f f i c u lt t o r e duc e tha n c e r ia . Y t t r i um-

s t ab i li z ed z i r c o n i a is a w e l l k n o w n o x y g e n i o n c o n d u c t o r [ 1 9 ] . M i x e d o x i d e s o f

l a n tha n ide e l e m e n t s a nd z i r c on ium w e r e r e po r t e d by B a ja r s [ 10 ] a s c a ta ly s ts f o r

su l fur d ioxide reduc t ion by CO. In addi t ion , Zr2Ln2OT- type z i rcona tes

( L n = l a n t h a n i d e e l e m e n t f r o m L a t o T b ) o f p y r o c h l o r e s tr u ct u re , n o t a b ly ,

Gd2Zr 207 , a r e kno w n a s a no th e r c la s s o f oxyg e n ion c onduc to r s [ 20 ] .

S e ve r a l z ir c ona te s ( C e , Tb , G d) a nd z i r c on ia -ba se d ox ide s we r e p r e pa r e d a nd

te s t e d in t h i s w or k u nd e r s im i l a r c ond i t i ons a s t he c e r i a- ba se d c a t aly s ts . The r e su l ts

a r e sum m a r i z e d in Ta b le 4 . Th e z i r c ona te s ha ve su r f a c e a re a s f r om a bou t 30 to 50

m Z / g . Y - d o p e d z i rc o n i a h a s m u c h h i g h e r s u r fa c e a re a t h a n t h e C u - d o p e d z i rc o n ia ,

wh ic h ma y r e f l e c t t he known s t a b i l i z a t ion e f f e c t y t t r i um impa r t s t o t he z i r c on ia

c r ys t a l s t ruc tu r e . Th e z i r c on ia - ba se d c a ta ly s ts c o ns i s t la r ge ly o f me so por e s ( t e ns

o f A in d i a me te r ) . The r e f o r e , sma l l pa r t i c l e s , l e s s t ha n 63 /xm, we r e u se d in t he

t e s ts o f t he Z r Oz - ba se d c a t a ly s t s t o o ve r c om e po r e d i f f u s ion r e s is t a nce . The l i gh t-

o f f t e mpe r a tu r e ove r t he C e , Th a nd G d- z i r c ona te s wa s a r ound 700 (2 , m uc h h igh e r

tha n tha t f o r c e ri a . Af t e r t he r e a c t ion w a s in i t i a te d a t h igh t e m pe r a tu r e , how e ve r ,

t he z i r c ona te c a t a ly s t c ou ld m a in t a in i t s h igh a c t iv i ty a s t he r e a c t ion t e mp e r a tu r e

wa s lo we r e d to 470C . The p e r f o r m a nc e o f the Gd2Zr207 c a t a ly s t sy s t e m is show n

in F ig . 4 . M or e tha n 95 e l e m e n ta l su l f u r y i e ld wa s ob ta ine d ove r t he t e mp e r a tu r e

r a nge 470 to 700C in t he d r y ga s. Af t e r t he a dd i t ion o f 2 vo l . - wa te r in to t he

r e a c t a n t ga s a t 560C , t he Ce2Zr207 a n d Th2Zr2 7 ca ta lys ts were quick ly deac t i -


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T a b l e 4

R e d u c t i o n o f s u l f u r d i o x i d e b y c a r b o n m o n o x i d e o v e r z i r c o n a t e a n d z i r c o n i a c a ta l y s t s

C a t a l y s t L i g h t - o f f Y - s u l f u r / X - S O 2 a

C o m p o s i ti o n a S u r f ac e a r e a ( m 2 / g ) T o o ~ ( C ) D r y g a sh ( 5 1 0 o c ) W e t g as C ( 5 1 0 0 C )

G d 2 Z r 2 0 7 3 7 . 8 7 0 0 0 . 9 5 / 0 . 9 9 0 . 0 / 0 . 0

T b 2 Z r 2 0 7 5 0 . 8 7 0 0 0 . 9 7 / 0 . 9 9 -

C e 2 Z r 2 0 7 3 7 . 0 7 0 0 0 . 9 6 / 1 . 0 0 -

( G d 2Z r 2) o. ss C u oa s O x 2 8 . 2 5 1 0 0 . 9 7 / 0 . 9 9 0 . 5 9 / 0 . 8 1

Z ro .g Y o . O x 4 1 . 7 6 1 0 0 . 5 4 / 0 . 5 8 -

[ Zro.gYo.1 ] o.ssCuo.150x 6 5 . 0 5 1 0 0 . 8 7 / 0 . 9 0 -

Z r o. sC u o .2 0 , 1 7 .8 5 1 0 0 . 9 2 / 0 . 9 6 0 . 7 0 / 0 . 8 7

a C a t a ly s t s w e r e p r e p a r e d b y c o p r e c i p i ta t i o n .

b R e a c t i n g g a s m i x t u r e c o n s i s t e d o f 1 S O 2 , 2 C O ,

c 2 H 2 0 w a s a d d e d i n to r e a c t i n g g a s m i x t u r e .

d S u l f u r y i e l d / S O 2 c o n v e r s i o n ; s e e t e x t .

b a l a n c e h e l i u m .

vated; only the Gd2Zr207 cata lys t remained act ive . As the temperature was

decre ased to 510C, th i s ca ta lys t wa s a l so deac t ivated in the presence of wa ter F ig .

4) . Therefore , Gd2Zr207 i s the most ac t ive among the three z i rconate ca ta lys t s

t e st ed . The ac t iv i ty o f th i s ca t a lys t can b e p rom oted by coppe r a s i nd i ca t ed by t he

da t a o f Tab l e 4 .

Th e yt t r ia-d op ed zirc on ia catalyst , Zro.9Yo. Ox, has a 6 10C l ight-off tem pera ture,

s ame as t he La-do ped ce r i a . The l igh t -o f f t empera tu re was l owered t o 510C by t he

a d d it io n o f c o p p er . T h e s l ig h tl y l o w e r SO 2 c o n v e r s io n o b s er v e d o n t h e C u - Z r Y ) -

O c a ta l y st T a b l e 4 ) c o m p a r e d t o C u - C e L a ) - O a t t h e s a m e r e a c ti o n t e m p e ra t u re

Tab le 3 ) m ay be due t o a po re d i f fus ion l im i t a ti on in t he fo rmer ca t a lys t syst em.

Ove ral l , z i rcon ia is s im i lar to ce r ia in i ts ca ta ly t ic proper t ies for the redu ct ion of

su l fu r d iox ide by ca rbon mon ox ide .

1 .00- / ~ W 0W = ~l

r

.60-

O O oo T

o . o o .

- . . . . . . . . . - - . . . . . . . . .

400 450 500 550 600 650 700 750

Reaction Tem perature C )

F i g . 4 . S O 2 R e d u c t i o n b y C O o v e r t h e G d 2 Z r 2 0 7 c a t a l y s t ( 1 S O 2 , 2 C O , 9 7 H e ) .

>,

X


11/20

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177

1

0 8 t

0.6

0.4-

0.2-

0

450

,3% CO 2 added

I l I ~ I

500 550 600

R e a c t io n T e m pe r a t ure C )

650

Fig. 5 . Ef fec t o f CO 2 o n catalyst activity and selectivity ( 1 % S O 2 , 2 % C O , 3 % CO2, 94% He). X-SO2: (1 7)

Lao.lCeo.9Oi.95, (O ) CU/CeO2 Y-sulfur: closed symbols (11, 0) .

3 4 . E f f e c t o f c a r b o n d i o x i d e o n t h e c a t a l y t ic a c t iv i ty a n d s e l e c t i vi t y

Carbon d iox ide i s p roduced by t he p resen t r eac ti on and m ay a l so be p resen t i n

the feed gas s t ream. T hus , i ts ef fect on cata lys t ac t iv i ty and se lect iv i ty was br ief ly

check ed i n th i s work . A s show n in F ig . 5 , add i ti on o f abou t 3% ca rbon d iox ide in to

the reactant gas m ixture a t 510(2 a lm ost com plete ly suppressed the react ion on the

Ceo.gLao.lOx cata lys t. T he SO2 conv ers ion dro pped f rom about 1 .0 to 0 .1 , w hi le the

sul fur y ie ld decre ase d f rom 0 .96 to t race amounts . Th e cata lys t ac t iv i ty was res tored

in t he p resenc e o f ca rbon d iox ide on ly w hen t he t empera tu re w as ra i s ed t o 600C .

How ever , t he e l em en ta l su lfu r y i e ld was dec reased t o 0 .81 because CO2 lowered

the SO2 convers ion and p rom oted t he p roduc t i on o f ca rbony l su lf ide (F ig . 5 ) . In

cont rast , addi t ion of CO2 at 510C did not supp ress the react ion on the 7 .8 w t .-%

Cu /CeO 2 c ata lys t. This ca ta lys t can m ain ta in it s ac t iv i ty even a t the low er temp er-

a ture o f 465C . S u l fu r y i e ld was dec reased by a f ew percen t a s a r e su lt o f p romoted

COS p roduc t ion and l ower S O2convers ion . These r esu l t s show tha t copper can

enhance t he r es i st ance o f t he c e r i a ca t a lys t t o ca rbon d iox ide po i son ing .

Table 5

Variation of the Cuo.l~Ceo.ss(La)Ox surface area with thermal treatment

Fresh catalyst Tested l2 h in dry Tested 37 h in wet Tested l8 h in dry Calcined l7 h in air

gasa at ca. 500C gash at ca. 500C gas at ca. 750C at ca, 7500C

Sg (m2/g) 30.7 26.4 26.8 22.8 22.6

"Dry reactant gas mixture consisted of 1% SO2, 2% CO, balance helium.

b2% H20 was added into the dry reactant gas mixture.


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178

W . L i u e t a l . / A p p l i e d C a t a l y s i s B : E n v i r o n m e n t a l 4 1 9 9 4 ) 1 6 7 - 1 8 6

1 . 0 0

0.90

. 0 . 8 0 -

O 0 .70-

X

0 . 6 0

//

X - S 0 2

[ ]

0

Y - s u ~ u r

1

M

A

6 1 0 C

5 6 0 C

A 510 C

0.50 i

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Contact Time(g.s/cc(STP))

Fig. 6 . Ef fec t of contac t t ime on SO: convers ion a t va r ious t emperatures : 1 SO2 , 2 CO, 97 He ;

Cuo.15Ceo.s5 L a ) Ox cataly st.

3.5. Bu lk Cuo.15Ceo.ss La)Ox ca taly st

The Cuo.15Ceo.85(La)Ox ca ta lys t wa s chos en for fu r the r s tud ies . This ca ta ly s t

s h o w e d n o a p p a r e n t d e a c t i v a t i o n d u r i n g a 3 5 - h o u r r u n in t h e p r e s e n c e o f 2 w a t e r

a t 4 7 0 C . T h e v a r i a ti o n o f s u r fa c e a r e a w i t h t e m p e r a t u r e i s s h o w n i n T a b l e 5 .

Va r i a t io n o f c o n v e r s i o n w i t h c o n t a c t ti m e a t d i f f e r e n t t e m p e r a t u r e s i s p l o t t e d in

F i g . 6 . T h e c o n v e r s i o n s t a r t s t o d e c l i n e f r o m 0 . 9 9 a t c o n t a c t t i m e s s h o r t e r t h a n

1.2

. ~ 0 . 8

g 0.6-

.~.

~ 0.4-

0.2-

0

w ~ v

3 0

m m m

cos]

[ s o ]

[C02] /2

0 90 120 150 180

Time-on-Stream(rain)

Fig. 7. Act iv at ion profile of Cuo.j5Ceo.ss(La)Ox catalys t a t 510C ( 1 SO2, 2.06 CO, bala nce hel iu m ).


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W. Liu et al . / Applied Catalysis B: Environmental 4 1994) 167-186 179

2000

1600-

1 2 0 0

I

8 0 0 -

4 0 0 .

0

Cuo.15Ceo.g5 La)O , 510 C

~ k ~

Cu0.27x0,80x , 550C

0 30 60 9 0 120 150

Time-on-S~eam min)

Fig. 8 . COS Ev olu t ion profi le dur ing 2 CO /He scavenging of spent ca ta lys ts .

a b o u t 0 . 04 5 g s / c m 3 S T P ) a t 5 1 0 C . N o e f fe c t o f c o n t a c t t i m e o n s el e ct iv i ty

wa s obse r ve d .

A f r e sh c a t a ly s t c a n be a c t iva t e d e i the r by p r e - r e duc t ion o r s t a rt i ng the r e a c t ion

a t h igh tem pera ture . F ig . 7 s ho w s the ac t iva t io n prof i le of the Cuo.15Ceo.85 La)O~

c a ta ly s t a t 5 1 0 C i n t h e r e a c t in g g a s a t m o s p h e r e . T h e c o n s u m p t i o n o f C O i n t h e

150O0

c

N

T

S t 2 0 0 0

S

E

C

8000

3000

u

i . . . . . . , , , | . . . . . . . . . . . . . . i . . . . . . .

I 0 2 0 3 0 4 0 5 0

a T I ~ A

( a ) b ~ m e

8 0 7 0 8 0 9 0 t 0 0

o t 1l iT

Fig. 9 . X-ray d i ff rac tograms of the 7.8 wt - CuOx/CeO2 ca ta lys t (a ) b e fore use; (b ) a f t e r use.


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180

W. L iu et a l . A pp l i ed Cata lysis B: Envir onmenta l 4 1994) 167-186

4 n m

I b

a) before use O Ce

Cu

100 nm

b) after use O S

Ce Cu

Fig. 10. STEM elemental mapping of the 7.8 wt.- CuOx/CeO2 catalyst (a) before use; (b) after use.

t ransit ion region is more than what is required to reduce SO2, which means that

addi t ional CO was con sum ed to reduc e the ca ta lyst and /or adsorbed oxidants on

the catalyst surface. During activation, a signif icant am oun t of CO S was observed.

As the ac t iva t ion proceeded , COS decreased to zero and about 50 min la ter COS

increase d to a s teady-s tate level of 300 ppm. F ig. 8 shows the CO S ev olution profi le


15/20

W. Liu et al ./ Applied Catalysis B: Environmental 4 1994) 167-186 181

10

Z

a )

8 -

6 -

4

2

O . i ~ . i . . i i . . . . i . . . . i . . . . i . . . . i . . . .

9 ~ 9 6 0 9 4 0 9 ~ 9 0 0 8 8 0 8 6 0 8 4 0 8 2 0 8 0 0

Binding Energy, eV

10

8 C u 2 0 o r C u

r e f . 2 1

Z 4 - :

0

l l l

9 4 9 9 4 4 9 3 9 9 3 4 9 2 9

Bi n di ng En erg y ad ju sted to C1 s a t 28 4 . 6 eV ) , e V

Fig. 11. XPS of freshCuo.]sCeo.8~(La)Ox atalyst(M g, 15 kV, 300 W) (a) survey,one-minute cquisition;(b)

foay-minute cquisition.

w h e n a g a s w i t h 2 C O i n H e w a s u s e d t o sc a v e n g e a s p e n t c a t a ly s t a f t e r t h e sy s t e m

h a d b e e n f l u s h e d w i t h h e l i u m . T h e C O S l e v e l d e c r e a s e d e x p o n e n t i a ll y w i th t i m e .

3.6. Characterization o f the Cu C e O catalysts

X - r a y p o w d e r d i f f r ac t io n a n a l y s es o f t h e f r e sh c o m p o s i te C u - C e - O c a ta ly s ts

i de n ti fi e d t w o c r y s t a l pha s e s , f l uo r i t e - t ype a nd c op pe r ox i de , i n c a t a l y s ts c on t a i n i ng

ove r 20 a t. - c opp e r , a nd on l y t he f l uo r i te s t r uc t u r e i n t he c om pos i t e s c on t a i n i ng

le ss t h a n 2 0 a t. - C u . A l s o in t h e Y - d o p e d a n d C u - d o p e d z i r c o n ia c a t a ly s t s, o n l y

t h e f l u 0 ri te - ty p e s tr u c t u r e w a s f o u n d . T w o s m a l l c o p p e r o x i d e p e a k s w e r e f o u n d i n


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182

W. Liu et al ./ Applied Catalysis B: Environmental 4 1994) 167-186

Table 6

Binding energies of elements in the Cuo.15Ceo.as(La)O~catalyst determined by XPS (Mg, 15 kV, 300 W)

Element Fresh (eV) Used (eV) Assignmentb

Cu 2p3/2 932.7 932.4 Cu, Cu20, copper sulfide

Ce 3d5/2 881.9 882.4 CeO2

0 ls 529.0 529.3 (72%) Metal oxide (CeO2)

531.7 (28%) Metal sulfate, carbonate

S 2p - 162.6 (74%) Copper sulfide

168.0 (26%) Metal sulfate

~Peak area percent.

bAssignmentbased on the data in ref. [211.

the fresh 7.8 wt.- Cu OJ Ce O2 cata lyst as show n in Fig. 9, but, these two peaks

disappeared in the used catalyst. T he distinct fluorite-type diffraction pattern w as

found in both the used and fresh Cu-C e--O catalysts, whic h indicates that the fluorite

oxide was stable in the present reaction conditions.

STEM analysis of the C u-impregnated ceria identif ied many large copper oxide

particles (ca. 100 nm ) in the fresh catalyst, but relatively smaller particles on the

used one as seen in Fig. 10. In Fig. 10, the left-hand -side top and botto m pictures

are the dark field image o f the fresh catalyst matrix and the bright field image o f

the used one, respectively, whil e the right-hand-side pictures are the corresponding

elemental mapp ing im ages. Sulfur found on the used catalyst was always associated

with copper. ST EM analyses of the fresh Cu--Ce-O comp osite catalyst found the

copper (oxid e) as small particles (a few nm ) and large agglomerates ( > 10 nm ),

the latter increasing in population with the copper content. A higher number of

small copper particles was found unifor mly dispersed on the used com posite catalyst

and they we re also extensively associated with sulfur.

Fig. 11 shows the X P sp ectra of the fresh Cuo.tsCeo.as(La)O~ catalyst. Com par-

ison with standard spectra [ 21 ] finds the Cu 2p satellite for the fresh catalyst similar

to cuprous ox ide (C u2 0) or metallic coppe r and Ce 3d satellites for both fresh and

used catalysts similar to bulk ceria. Th e C u 2p spectra of the CuO/~/-A1203 are

identical to CuO. B ut, the Cu 2p XPS of the CuOx/CeO2 is between the Cu O and

reduced CuO (Cu20 or Cu). The binding energies of the Cuo.15Ceo.85(La)Ox

catalyst are summ arized in Table 6. The doub le S 2p XPS peaks of the used catalyst

Table 7

Surface compositional analysis of Cu0.1~Ceo.ss(La)O~ catalyst by XPS

Atomic ratio Cu/Ce S/Cu

Bulk material 0.176 0

Fresh catalyst 0.133 0

Used catalyst 0.364 1,59

1 inin Ar + sputtering 0.206 1.36

7 rain Ar + sputtering 0.151 1.15

Based on stoichiometry.


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W. L iu e t a l . /Ap pl i e d Cata lys is B: Env i ronm enta l4 1994) 167-1 86

83

i nd i c a t e two k inds o f su l f u r : m e ta l su l f a t e a nd su l f ide c on t r ibu t ing to t he h igh a nd

low b ind ing e ne r g i e s , r e spe c t ive ly . Th e su l f ide pe a k i s a m a jo r pe a k . S imi l a rly , two

O 1s p e a k s w e r e f o u n d i n t h e X P S o f t h e u s e d c a ta ly s t. T h e p e a k o f h i g h e r b in d i n g

e ne r gy c a n be a s s igne d to su l f a t e o r c a r bona te , wh i l e the pe a k o f low b ind ing e ne r gy

i s a s s igne d to m e ta l ox ide . T he qu a l i ta t i ve su r f a c e c om pos i t i ona l a na lys i s o f t he

Cuo.~sCeo.85 L a) O~, ca ta lys t b y X PS is g iv en in T able 7 . T he sur face wa s enr ich ed

i n c o p p e r a ft e r u s e . T h e C u / C e a n d S / C u r a ti o s d e c r e a se d b y a rg o n i o n s p u t te r in g

o f t h e s u r f ac e , b u t t h e S / C u d i d n o t r e a ch a z e ro v a l u e b e c a u s e o f i n t er fe r e n c e f r o m

the c a t a ly s t po r e s .

4 . D i s c u s s i o n

P r e v i o u s s t u d ie s h a v e s u g g e s t e d t h a t th e r e d u c ti o n o f s u l fu r d io x i d e b y c a r b o n

m o n o x i d e p r o c e e d s v i a a re d o x m e c h a n i s m [ 1 2 , 1 5, 1 8 ].

C a t - [ ] [ 'SO2 ~ C a t - O + S O 8 )

C a t - O + C O ~ C a t - [ ] + C 0 2 9 )

C a t - [ ] + S O ~ C a t - O + S 1 0)

Ac c or d in g ly , the r e a c t ion i s i n i t i a te d by the c r e a t ion o f a n oxy ge n va c a nc y . A s

a n ox yge n va c a n c y i s c r e a te d , S O2 dona te s on e oxyg e n to tha t va c a nc y to f o r m a

S O g r o u p . T h e S O m a y b e m o b i l e o n t h e s u r f ac e u n t i l it f in d s a n o t h e r v ac a n c y to

dona te i t s oxyge n o r a va c a nc y ma y mig r a t e t o a ne ighbor ing s i t e t o a c c e p t i t s

oxyg e n . H ig h o xy ge n m ob i l i t y i n t he c a t a ly s t wo u ld f a c il i ta t e t he ox yge n t r a ns fe r

on the su r f a c e o r f r om the bu lk to t he su r f a c e . The r e f o r e , oxyge n va c a nc y a nd

m ob i l i t y s e e m to be im por t a n t p r ope r t i e s f o r a n a c t ive c at a ly s t . The a c t ive c a t aly s ts

p r e v i o u s l y r e p o r t e d [ 1 3 - 1 6 ] , L a - T i - O a n d L a - - C o -O p e r o v s k it e m i x e d o x id e s ,

ge ne r a l ly pos se s s suc h p r ope r t ie s . C e r i a a nd z i r c on ia s tud ie d in th i s w or k a r e w e l l

kn ow n f o r t he i r oxy ge n v a c a nc y a n d mo b i l i t y p r ope r t i e s [ 19 ] . C e r i a ha s a ve r y

s tab le f luor i te - type s t ruc ture , w hi le the f luor i te s t ruc ture of z i rconia can be s tab i l ized

by th e u se o f ox ide dopa n t s . The oxy ge n va c a nc y in a f luo r i te ox ide i s c re a t e d by

r e p la c i n g t h e m e t a l i o n C e a , Z r 4 ) in th e la t t ice wi th o the r d i - o r t r i -va len t m e ta l

ions . R a r e e a r th z i r c ona te s Ln2Zr 207) ha v ing oxyg e n m ob i l i t y p r ope r t i e s [20 ]

we r e a l so f oun d to be a c t ive c a t aly s t s i n t h i s wor k . A l l t he se r e su l ts suppo r t t he

p r e s e n t m e c h a n i s t i c a r g u m e n t .

H o w e v e r , t h e o x y g e n v a c a n c y c a n b e t a k e n u p b y o t h e r o x y g e n - c o n t a i n i n g

mole c u le s , suc h a s C O2 o r H20 . The s t r onge r t he a s soc i a t i on o f t he se impur i ty

ox ida n t s w i th t he va c a n c y , t he m or e the y w i l l i nh ib i t t he r e a ct ion . Thu s , c re a t ion

o f ox yge n va c a nc ie s o n th e su r f a c e i s a ke y s t ep . I nc o r po r a t ion o f a lka l ine a nd r a r e

e a r th do pa n t ox ide s i n to t he c e r i a l a t ti c e e nha nc e s t he ox yge n v a c a nc y c onc e n t r a t ion

a nd m ob i l i t y [ 19 ] . B u t , t he c r e a t e d va c a nc ie s a r e u sua l ly c a ppe d by a n ou t s ide

oxy ge n a n d the s t r on g va c a nc y - d opa n t i on a s soc i a t i on st a b il iz e s t he c a pp ing

o x y


18/20

1 8 4 W . L i u e t a L / A p p l i e d C a t a l ys i s B : E n v i r o n m e n ta l 4 1 9 9 4 ) 1 6 7 - 1 8 6

g e n . T h e c a p p i n g o x y g e n h a s to b e r e m o v e d t o b r in g a b o u t o x y g e n v a c a n c y a n d

in i ti a t e t he r e dox r e a c t ion . F o r c e ri a , t e m pe r a tu r e - p r og r a m m e d e du c t ion by hyd r o -

g e n [ 2 2 ] h a s f o u n d t h at t h e s u r fa c e c a p p i n g o x y g e n c a n b e r e m o v e d a t a b o u t 5 00 C .

H i g h e r r e d u c t i o n t e m p e r a t u r e m a y b e n e e d e d i n t h e p r e s e n c e o f S O 2 d u e t o i ts

s t r ong bond ing to the su r f a c e . I t i s a l so known tha t C O a dso r p t ion on c e r i a i s

i n h i b i t e d b y w a t e r [ 2 3 , 2 4 ] . T h e i n t r o d u c t i o n o f t r a n s i t i o n m e t a l s m a y p r o v i d e

su r f a c e s it e s f o r C O a dso r p t ion a n d f a c il i ta t e the r e du c t ion o f t he f luo r i te ox ide

s u r fa c e t h r o u g h s t r o n g m e t a l - s u p p o r t i n t e ra c t io n a s f o u n d i n t h e P t C e O 2 [ 22]

sys t e m:

C a t - M - - C O + C a t - O - o C a t - M + C a t - [ ] + C O 2 (1 1)

C a t - [ ] c a n f u r t h e r r e a c t a c c o r d i n g t o t h e r e d o x r e ac t io n s c h e m e ( 8 ) - ( 1 0 ) . T h i s

a s sum pt ion wa s va l ida t e d by the f a c t t ha t t he c a ta ly t ic a c t iv i ty o f c e r ia o r z i r c on ia

a nd r e s i s t a nc e to wa te r o r c a r bon d iox ide po i son ing we r e s ign i f i c a n t ly e nha nc e d

b y t h e a d d i t i o n o f t r a n si t io n m e t a ls , s u c h a s c o p p e r . X P S s t u d ie s s h o w e d t h a t c o p p e r

i n th e C u - C e - O c a ta l y st t e n d s t o b e s t a b i li z ed i n a re d u c e d o x i d a t io n s ta t e ( C u 2 0

o r C u ) i n s te a d o f C u O , w h i l e i t is w e l l k n o w n t h a t C O s t ro n g l y a d s o rb s o n C u 1+

i on . P r e l im i n a r y t e m p e r a t u r e - p r o g r a m m e d r e d u c t io n e x p e r i m e n t s i n d i c at e d t h a t

a d d i t io n o f c o p p e r c a n i n it ia t e t h e r e d u c t io n o f s u r fa c e c a p p i n g o x y g e n o f c e r i a a t

low t e m pe r a tu r e . Th e se da t a su gge s t t ha t t he r e is a s t r ong in t e r a c tion be tw e e n c opp e r

( o x i d e ) a n d c e ri a. F o r t h e C u - C e - O s y s t e m , b o t h c o p re c i p it a t io n a n d i m p r e g n a t i o n

p r e pa r a t ion m e tho ds ga ve r i se t o a n a c t ive c a ta ly st . The c a t a ly s t a c t iv i ty wa s n o t

se ns i t i ve to the c oppe r c on te n t , bu t t he s e l e c t iv i ty t e nde d to de c r e a se w i th the

c o p p e r c o n t e n t. X R D a n a l y si s i d e n ti fi e d t h e C u O p h a s e w h e n t h e c o p p e r c o n t e n t

w a s o v e r 2 0 a t . S T E M a n a ly s i s r e v e a le d b i m o d a l si z e d i s tr i b u ti o n o f c o p p e r o n

c er ia . W e p o s t u l a t e t h a t o n l y a c e r t a in a m o u n t o f c o p p e r i s n e e d e d t o p r o m o t e t h e

c a ta ly t i c p r ope r t i e s o f c e r ia , a nd the e xc e ss c op pe r ( o x ide ) c a n f o r m a gg r e ga te s.

P r e se n t ly , we do no t ha ve e nough da ta t o c l a r i f y the c oppe r pa r t i c l e s i z e e f f e c t .

F u r the r wo r k to e luc ida t e t he in t e r a c tion o f c oppe r w i th c e r i a is unde r w a y .

As r e po r t e d in the l i t e r a tu r e [ 15 ,17 ] , t he t r a ns i ti on m e ta l w i l l l i ke ly be su l f ide d

by the e l e m e n ta l su l f u r p r oduc t :

C a t - M + S x --->C a t - M S x ( 1 2)

C o p p e r su l fi d e w a s f o u n d o n a u s e d C u - C e - O c a ta l y st b y X P S s u r fa c e c o m p o s i -

t i ona l a na lys i s , bu t wa s no t de t e c t e d by XR D a na lys i s , poss ib ly be c a use i t i s

d i spe r se d in c e r i a i n a mor phous f o r m o r be c a use c oppe r su l f ide c r ys t a l s d id no t

g r ow on the s t a b le c e r i a su r f a c e . The f luo r i t e ox ide showe d h igh s t a b i l i t y i n t he

p r e se n t s tudy . F o r e xa m ple , c e r i a ke p t i ts f l uo r it e - type cr ys t a l s t r uc tu r e a f t e r a 48 h

t e s t r u n . S T E M e l e m e n t a l m a p p i n g s h o w e d t h a t t h e s u l f u r o n t h e c a t a l y s t w a s

e x te ns ive ly a s soc ia t e d w i th c oppe r . The f o r ma t ion o f c oppe r su l f ide e x t r a c t e d

c opp e r f r om the bu lk so tha t t he su r f a c e wa s e n r i c he d in c opp e r on the u se d c a t a ly s t.

C o p p e r s u l fi d e m a y p l a y a s i m i l a r ro l e t o c o p p e r i n p r o m o t i n g t h e r e d u c ib i li ty o f

t h e f lu o r it e o x i d e . B u t , t h e d e p o s i t e d s u l fu r m a y r e ac t w i t h C O t o f o r m C O S t h r o u g h

f o l low ing e qua t ion :


19/20

W . L i u e t a l. / A p p l i e d C a t a l y s i s B : E n v i r o n m e n t a l 4 1 9 9 4 ) 1 6 7 - 1 8 6 85

C a t - M S x + C O --->C a t - M + C O S ( 1 3 )

T h e r e f o r e , a n a d s o r b e d C O m o l e c u l e c a n p i ck u p o x y g e n t h r o u g h E q . ( 9 ) a n d

s u lf u r t h r o u g h E q . ( 1 3 ) . W e b e l i e v e th a t C O S f o r m a t i o n is th e r e s u lt o f t h e s e t w o

c om pe t i t i ve p r oc e s s e s a nd i t p r e va i l s in the f o l l ow i ng t w o c a s e s : ( i ) w h e n e xc e s s

CO e x i s t s a nd t he o x i da n t i s no t s u f f i c i e n t t o ox i d i z e i t, t he e x t r a CO r e a c t s w i t h

s u lf u r t o f o r m C O S ; ( i i ) w h e n t h e s u r f a c e o x y g e n is m o r e s t r o n g l y b o u n d t h a n

s u lf u r, t h e a d s o r b e d C O p i c k s u p s u l fu r to f o r m C O S . T h i s r e a s o n in g i s e v i d e n c e d

by t he CO S e vo l u t i on p r o f i l e s s how n i n F i g s . 7 a nd 8 . I n F i g . 7 , CO S f o r m a t i on

p r oc e e de d s i gn i f i c a n t l y a t i n it ia l r e a c t i on t i m e s w he n t he c a t a l y s t w a s no t a c t i va t e d

o r the s u r f a c e oxy ge n w a s no t m ob i l e , de c r e a s i ng t o z e r o a t f u l l y a c t i va t e d s t at e ,

w h i l e a s m a l l a m o u n t o f C O S w a s f o r m e d a t a l at e r r e a c t io n t im e . T h e l a tt e r m u s t

b e t h e re s u l t o f r e a c ti o n o f a s m a l l a m o u n t o f C O w i t h t h e s u l fu r d e p o s i te d o n t h e

c a t a l y s t s u r f a c e . T he r e a c t i on o f CO a nd t he de po s i t e d s u l f u r is f u r t he r il l u s tr a t e d

b y F i g. 8, w h e r e C O S e v o l u t i o n p r o f il e s a r e s h o w n f o r 2 C O / H e s c a v e n g i n g o f

t h e u s e d c a t a ly s t s f o l lo w i n g a h e l i u m f lu s h. A s im p l e e x p o n e n t ia l d e c a y o f C O S ( t )

pred ic ted by as su m ing a fo rm at ion ra te r co s = ks [ S ] sur fPco f it s we l l the da ta of F ig .

8.

5 C on c lu s ion

I n t h is w or k , w e h a ve f ou nd t ha t f l uo r i t e - t ype ox i de s , s uc h a s c e r i a a nd z i r c on ia ,

a r e a c t iv e c a t a l y s ts f o r t h e r e d u c t i o n o f s u l f u r d i o x i d e b y c a r b o n m o n o x i d e w i t h

h i gh s e l e c t i v i t y t o e l e m e n t a l s u l f u r ove r c a r bony l s u l f i de . T he a c t i v i t y o f t he s e

o x i d e s m a y r e s u l t f r o m t h e i r h i g h o x y g e n v a c a n c y c o n c e n t r a t i o n a n d m o b i l i t y

p r o p e rt ie s w h i c h a r e n e e d e d f o r a r e d o x r e a c t i o n m e c h a n i s m . A d d i t io n o f a c t iv e

t r a ns i t i on m e t a l s s uc h a s c oppe r t o t he f l uo r i t e ox i de s i gn i f i c a n t l y l ow e r e d t he

r e a ct io n l i g h t - o f f t e m p e r a t u r e a n d e n h a n c e d t h e c a t a ly s t r e si s ta n c e t o w a r d w a t e r

a n d c a r b o n d i o x i d e p o is o n in g . A n a l y s e s o f t h e C u - C e - O s y s te m s h o w e d t h a t th e

f luo r i te c r y s t a l s t r uc t u r e o f c e r i a i s s t ab l e a t t he p r e s e n t r e a c t i on c ond i t i ons a nd

c o p p e r is s t a b il iz e d i n t h e c e r i a m a t r ix . A n a c t i v e C u - C e - O c a t a ly s t c a n b e p r e p a r e d

by e i t he r c op r e c i p i t a t i on o r i m p r e gna t i on . T he c a t a l y s t a c t iv i t y is no t s e ns i t i ve t o

t he c o p p e r c o n te n t . T h e C u - C e - O c o m p o s i t e s u rf a c e w a s e n r i ch e d in c o p p e r a n d a

s u r f a c e c op pe r s u l f ide w a s f ou nd i n t he u s e d c a t a l y s t . F u r t h e r c ha r a c t e r iz a t i on a nd

s y s te m a t i c c o m p o s i t io n s t u d ie s o f t h is c a t a ly s t s y s t e m a r e u n d e r w a y t o c l a r if y t h e

t ype o f m e t a l - f l uo r i t e ox i de i n t e r a c t i on t ha t l e a ds to t he e nha nc e d a c t i v i t y r e po r t e d

here .

A c k n o w l e d g e m e n t s

W e t h a n k D r . A . J . G a r r a t t - R e e d f o r t h e S T E M a n d M r . J . M a r ti n f o r t h e X P S

a n a ly s e s. T h i s w o r k h a s b e e n f i n a n ci a ll y su p p o r t e d b y t h e U S D e p a r t m e n t o f E n e r g y

u n d e r th e U n i v e r s i ty C o a l R e s e a r c h P r o g r a m , G r a n t N o . D E - F G 2 2 - 9 2 P C 9 2 5 3 4 .


20/20

186 W . L i u e t a l. / A p p l i e d C a t a l ys i s B : E n v i r o n m e n t a l 4 1 9 9 4 ) 1 6 7 - 1 8 6

e f e r e n c e s

[ 1 ] J .B. Pfeiffer , Sulfu r Rem ova l and R ecov ery Industr ia l Processes , Ad v. Chem . Ser. , No. 139, Am . Che m.

SOC., W ashing ton, DC, 1975.

[2] K .V. Kwong, R .E . Mers sner and C .C . Hong, in P roc . of the SO2 Cont ro l Symposium, W ashington , DC,

Decem ber 3-6 , 1991, Vol . 2 , Ses s ion 5B , Paper 8 .

[3] P .R . Ryason and J . Hark ins , J . A i r Pollu t. C ont ro l Assoc ., 17 196 7) 796-7 99.

[4] S .E. Khalafal la , E.F. Foers ter and L.A. Haas , Ind. Eng . Chem., Prod. Res . Dev. , I0 19 71 ) 133-1 37.

[5 ] S .E . Kh a la fa l la and L .A. Haas , in J .B . P fe i f fe r Ed i tor ) , Su l fur Rem ova l and Recov ery Indus t r ia l P roces ses ,

Adv . Chem . Ser., No. 139, Am . Chem . Soc., Wa shing ton, DC , 1975, pp. 60--64.

[61 L.A. Haas and S.E. Khalafal la , J . Catal. , 29 19 73 ) 264 -26 9.

[71 L .A. Haas and S .E . Kha la fa l la , J . Ca ta l. , 30 197 3) 45 1-45 9.

[8] R . Quer ido and W.L. Shor t , Ind . Eng. Chem. , P roces s Des . Dev., 12 197 3) 10-18 .

[9] V .C . Okay and W.L. Shor t , Ind . Eng. Chem., P rocess Des . Dev. , 12 197 3) 291-29 4.

[ 10] L . Ba ja rs , US Pa tent 3 978 200 197 6) .

[11] J . Happol , M.A. Hn atow , L. Bajars and M. Kundrath, Ind. Eng . Chem., Prod. Res . Dev. , 14 19 75 ) 15 4-

158.

[ 121 J . Happel , A.L. Leon, M.A . Hnatow an d L. Bajars , Ind. Eng . Chem., Prod. Res . Dev. , 16 19 77 ) 150-154.

[ 13] J.M. Whelan , US Pa tent 4 081 519 197 7) .

[ 14] D .B . Hibber t and R .H. Cam pbe l l , Appl . Catal ., 41 198 8) 273-28 7.

[ 15 ] D .B . Hibber t and R .H. Cam pbe l l , Appl . Ca ta l. , 41 198 8) 289-29 9.

[ 16] D.B. Hibbert , Catal. Rev. Sei . Eng . , 34 19 92 ) 3 91-4 08.

[17] J .A. Bagl io, Ind. Eng . Che m. Prod. Res . Dev., 21 19 82 ) 38--41.

[ 18 ] W . L iu and M . F ly tzani -S tephanopoulos , in J.N . A rmo r Ed i tor ) , Envi ronm enta l Ca ta lys is , A m. Chem . Soc .,

Symp. Ser ., Vol . 552 , Am. Chem . Soe. , Washington , DC, 1994, pp . 375-392 .

[ 19] P . Hag em uller and W . Van G ool , Sol id Electrolytes , Acad emic Press , New York, 1978.

[20 ] H.L. TuUer and P.K. Moo n, Mater . Sci . Eng rg. B I, 19 88 ) 171-191.

[ 21 ] C .D. W agner , W.M . R igg , L .E . D avi s , J .F . M oulder and G.E . M ui lenberg , Hand book of X -ray Photoe lec tron

Spectroscopy, Perkin-E lmer, Ed en Prair ie, M N, 1978.

[22 ] H.C. Yao and Y.F.Y. Yao, J . Catal. , 86 19 84 ) 254 -26 5.

[231 C. Li, Y. Sakata , T. Arai , K. Do ma n, K. M aruy a and T. Onishi , J. Chem. So t . , Farad ay Trans . 1, 85 19 89 )

9 2 9 - 9 4 3 .

[24] C. Li , Y. Sakata , T. Arai , K. Do me n, K. M aruy a and T. On ishi , J . Che m. Soc. , Farada y Trans . 1, 85 19 89 )

1451-1461.

reduction sulfur di ox over metal ox

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