IR STUDY OF CO CHEMISORBED ON METALLIC Cu DISPERSED IN ZnO

HAL Id: jpa-00223547

https://hal.archives-ouvertes.fr/jpa-00223547

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

IR STUDY OF CO CHEMISORBED ON METALLIC Cu DISPERSED IN ZnO

F. Boccuzzi, G. Ghiotti

To cite this version:

F. Boccuzzi, G. Ghiotti. IR STUDY OF CO CHEMISORBED ON METALLIC Cu DIS- PERSED IN ZnO. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-455-C10-458.

�10.1051/jphyscol:19831089�. �jpa-00223547�

JOURNAL DE

PHYSIQUE

Colloque ClO, supplkment a u n012, Tome 44, dkcembre 1983 page CIO-455

IR STUDY OF CO CHEMISORBED ON KETALLIC

Cu

DISPERSED I N ZnO

F. Boccuzzi and G. G h i o t t i

I s t i t u t o d i Chimica F i s i c a de2Z'Universitd d i Torino, c . s o M . d1Azeg2io 48,101 25 Torino, I t a Z y

Rbsum6. L ' a d s o r p t i o n de CO s u r Cu/ZnO a 6 t 6 6tudiBe p a r spectroscopic infra-rouge de t r a n s m i s s i o n . Deux v i b r a t i o n s d'dtirement de CO o n t Q t g observges : 2103 e t 2080 cm -1, l a premisre a t t r i b u g e

B

une espEce l i 6 e perpendiculairement b l a s u r f a c e , l a seconde ri une espece lice p a r a l l s l e m e n t a l a s u r f a c e .

A b s t r a c t . The a d s o r p t i o n of CO on Cu/ZnO h a s been s t u d i e d by i . r . t r a n s m i s s i o n spec- troscopy. The CO s t r e t c h i n g v i b r a t i o n s a r e observed a t 2103 and 2080 cm-'; t h e f i r s t i d e n t i f i e d w i t h an "on t o p " chemisorbed s p e c i e s , t h e second w i t h a s p e c i e s 1 y i n g " f l a t "

on t h e s u r f a c e .

Adsorption s t u d i e s of some simple molecules on d i f f e r e n t s i n g l e - c r y s t a l Cu s u r f a c e s havebeenperformed by s e v e r a l combined techniques (IRAS,ELS, LEEDS,UPS). A s a r e s u l t t h e r e i s now a r a t h e r d e t a i l e d d e s c r i p t i o n o f such a d s o r p t i o n and t h e r e f o r e i t seemed i n t e r e s t i n g t o s t u d y t h e a d s o r p t i o n on small Cu metal p a r t i c l e s using t h e i n s i g h t gained on s i n g l e - c r y s t a l s .

The Cu/znO supported samples were p r e p a r e d and c h a r a c t e r i z e d by P e t r i n i et a l . ( 1 ) . The copper c o n t e n t i s 3% i n Cu atoms. The mean p a r t i c l e s i z e , determined by 02 ad- s o r p t i o n a t 1 5 0 ~ ( 2 ) , i s %20

8,

i f a s p h e r i c a l shape o f t h e p a r t i c l e s and a O/Cu r a - t i o o f 1/2 a r e assumed.

I n t h i s work %re have s t u d i e d by i . r . t r a n s m i s s i o n spectroscopy t h e CO a d s o r p t i o n a t 77 and 300K on c l e a n Cu/ZnO s u r f a c e s and on 02 o r N20 p r e t r e a t e d ones. P r e a d s o r p t i o n of 02 h a s been performed a t 77 K and t h a t o f N20 a t 300 K, t h e s e being t h e condi

-

t i o n s t h a t a l l o w a n oxygen extended s u r f a c e coverage without bulk o x i d a t i o n . The sample d i s c s were i n s e r t e d i n a c e l l which a l l o w s t o perform thermal t r e a t m e n t s i n c o n t r o l l e d atmosphere o r i n vacuumand t o o b t a i n s p e c t r a a t d i f f e r e n t t e m p e r a t u r e s . The s p e c t r a were recorded w i t h a 580 B Perkin-Elmer spectrophotometer connected with a 3600 Kata S t a t i o n .

R e s u l t s and d i s c u s s i o n . CO a d s o r p t i o n a t 300 K on c l e a n s u r f a c e i s s l i g h t l y p r e s s u r e dependent: o n l y one band grows a t i n c r e a s i n g e q u i l i b r i u m p r e s s u r e (V=2100 an-', FWHM=12 cm-' 1 , no s h i f t was observed with i n c r e a s i n g coverage.

CO a d s o r p t i o n a t 77 K on c l e a n s u r f a c e g i v e s r i s e t o a more complex spectrum: t h r e e bands a r e p r e s e n t a t 2180, 2103, and 2080

cm-l .

The f i r s t one i s s l i g h t l y p r e s s u r e dependent (pmax= 3 t o r r ) and i t s frequency shows a r e d s h i f t with i n c r e a s i n g coverage and can be a s s i g n e d t o a eq CO chemisorbed on Zn i o n s ( 3 ) . The o t h e r two a r e i r r e v e r s i - b l e under o u t g a s s i n g a t 77 K: t h e 2103 cm-' i s s h a r p and s t r o n g (FWHM = l o an-') , t h e 2080 cm-l broad and weak (FWHM =40 an-'). Under o u t g a s s i n g a t 200 K t h e 2080 cm-I component completely d i s a p p e a r s and A t t h e same time t h e 2103 cm-I one s h i f t s t o 2100 cm-1.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831089

JOURNAL DE PHYSIQUE

-

Figure 1

2103

F i g . 1 shows t h e comparison between CO adsorbed a t 300 and 77 K ( a and b c u r v e s r e - s p e c t i v e l y ) f o r t h e maximum coverage and t h e d i f f e r e n c e spectrum b-a (curve c ) . I n f r a r e d r e f l e c t i o n experiments on C u monocrystals (4) i n d i c a t e t h a t , a t s a t u r a t i o n , t h e CO s t r e t c h i n g mode on Cu stepped s u r f a c e s (755)

,

( 3 1 1 ) , (211) f a l l a t 2103-21 10 an-', w h i l e t h e low-index p l a n e s g i v e CO bands a t 2070-2088 cm-I and have been a s c r i - bed t o "on top" bounded CO. The two bands of CO adsorbed on our samples f a l l i n these two r e g i o n s . A s r e g a r d t o t h e 2103 cm-I band, i t s p o s i t i o n , i n t e n s i t y and FWHM a r e c h a r a c t e r i s t i c of an"on topt'CO chemisorption. Moreover t h e p r e s s u r e and temperature ranges of i t s e x i s t e n c e on t h e s u r f a c e a r e t h e same a s shown by t h e o n t o p s p e c i e s adsorbed on monocrystals. A s f a r a s t h e 2080 cm-I band i s concerned, i t s FWHM, i n - t e n s i t y p r e s s u r e , and temperature dependence s u g g e s t a n a l t e r n a t i v e binding mode.

Angle-resolved photoemission experiments(5) on t h e stepped Cu (311) p l a n e have demon- s t r a t e d t h e e x i s t e n c e of a CO s p e c i e s i n which t h e molecule l i e s " f l a t " on t h e s u r f a - c e , p a r a l l e l t o t h e s t e p d i r e c t i o n , t h e r a n g e of p r e s s u r e and temperature of i t s e x i - s t e n c e being t h e same a s f o r our s p e c i e s r e l a t e d t o t h e 2080 cm-I band. Measurements of s u r f a c e p o t e n t i a l changes(4) with CO coverage a t 77 K show t h e e x i s t e n c e a t t h i s t e m - p e r a t u r e of two a d s o r p t i o n s t a g e s , which can be c o r r e l a t e d w i t h changes i n t h e LEED p a t t e r n s ( 4 ) . The second s t a g e was d i s c u s s e d i n terms of a d d i t i o n a l CO molecules bound bridged o r t i l t e d . Neither h y p o t h e s i s was supported by IRAS experiments ( 4 ) .

The presence o f " f l a t " CO moleaules could be t h e r e a s o n of t h i s discrepancy. I n f a c t molecules l y i n g f l a t on t h e s u r f a c e would n o t b e seen i n i . r . a s t h e d i p o l e would be e f f e c t i v e l y screened by t h e m e t a l . I t was however suggested t h a t t h e "metal s e l e c - t i o n r u l e " should apply o n l y a t m e t a l p a r t i c l e s l a r g e r t h a n 20

8

^(6). I n o u r c a s e t h e d i s p e r s i o n of t h e m e t a l p a r t i c l e s i s s u f f i c i e n t t o weaken t h e s e l e c t i o n r u l e . We propose t o a s s i g n t h e 2080 cm-I band t o a s p e c i e s of t h i s t y p e .

The two s p e c i e s show a d i f f e r e n t behaviour w i t h r e s p e c t t o t h e two d i f f e r e n t o x i d i - z a t i o n t r e a t m e n t s .

F i g . 2 shows t h e comparison between t h e i . r . s p e c t r a of t h e CO adsorbed a t 77 K on a c l e a n sample (curve a ) and on a sample p r e v i o u s l y c o n t a c t e d with 02 a t 77 K and sub- s e q u e n t l y evacuated (curve b ) ; curve c i s t h e d i f f e r e n c e spectrum (a-b).

Two e f f e c t s a r e produced by t h e p r e a d s o r p t i o n of oxygen: i ) t h e broad band a t 2 0 8 0 cm-' i s missing; ii) t h e band a t 2103 i s s h i f t e d t o h i g h e r frequency (2116 cm-I), i t s FWHM and i n t e n s i t y being almost u n a f f e c t e d .

The N,O pre%dsorption a t 300 K ( n o t shown f o r b r e v i t y ) produces t h e d e p l e t i o n o f t h e 2080 cm-I band and a v e r y small e f f e c t on t h e 2103 cm-I one ( a s h i f t from 2103 t o 2105 cm-"ithout changes i n i t s i n t e n s i t y ) .

The " f l a t " s p e c i e s i s d e s t r o y e d by t h e two o x i d a t i o n p r o c e s s e s i l l u s t r a t e d , while t h e w o n t o p " s p e c i e s i s s t r o n g l y p e r t u r b e d by 0 2 i n t e r a c t i o n a t 77 K and only s l i g h t l y p e r t u r b e d by N20 i n t e r a c t i o n .

A scheme which a l l o w s t o e x p l a i n a l l t h e above d a t a i s :

0 !i

C t 1 ^{C t I?}

-C,U v O - ~ ~ u - c u - c u c o ^-?u% F

Cu-GI-9 _CU-CU- - - ^cu-cu-cu%

300K

'CU-CU-

7 7 K

XU- CU -

-cu-0

'ci-cu-80 cu-cu-

0 GX+

c t

-?I?

Cu-Cu " _'cu- ^{,$ 6}

u- u ,o,

'Cu-Cu-

C10-458 JOURNAL

DE

_PHYSIQUE

In thescheme,the "on t o p " s p e c i e s have been assumed t o be c o o r d i n a t e d t o a Cu atom l o c a t e d a t t h e e x t e r n a l edge of a s t e p , t h e " f l a t " s p e c i e s c o o r d i n a t e d t o t h e f i r s t uncoordinated Cu atom on t h e i n t e r n a l edge.

I n f a c t t h e o r e t i c a l c a l c u l a t i o n s ( 7 ) p r e d i c t t h a t atoms a t t h e e x t e r n a l edge of m e t a l s u r f a c e s a r e r e l a t i v e l y e l e c t r o p o s i t i v e i f compared with t h o s e of non stepped s u r - f a c e s . On t h e c o n t r a r y t h e atoms a t t h e i n t e r n a l edges a r e r e l a t i v e l y e l e c t r o n e g a - t i v e i f compared w i t h t h o s e of non stepped f a c e s . Therefore t h e f i r s t would be t h e most s u i t a b l e t o g i v e "on t o p " s p e c i e s , while t h e second would b e t t e r s t a b i l i z e a f l a t l y i n g geometry, being more e f f i c i e n t i n donating e l e c t r o n s t o t h e CO 271 o r b i t a l . T h i s scheme a l l o w s u s t o e x p l a i n t h e d i f f e r e n t e f f e c t of t h e t w o o x i d i z a t i o n i n t e r a c - t i o n s . When an 02 i n t e r a c t i o n i s performed t h e more a c t i v e s i t e s , i . e . t h e e x t e r n a l edge atoms, decompose t h e 02 molecule and t h e oxygen atoms remain chemisorbed on t h e s e s i t e s ; t h e subsequent CO "on tbp" chemisorption r e v e a l s a Cu s i t e s t r o n g l y modified a s a consequence o f a charge t r a n s f e r fbom t h e copper t o t h e o x y g e n ( 8 ) . W h e n t h e o x i d i z a t i o n i n t e r a c t i o n i s performed w i t h t h e N z O , i f t h e decomposition o c c u r s

a s shown i n t h e scheme, t h e oxygen atom i s l e f t on t h e f i r s t Cu atoms of t h e i n - t e r n a l edge and t h e r e f o r e t h e subsequent CO chemisorption on t h e e x t e r n a l edge atoms i s o n l y s l i g h t l y pertubed.

I n e i t h e r case t h e oxygen does n o t a l l o w t h e formation of t h e f l a t s p e c i e s . Since a l l t h e s p e c t r o s c o p i c evidences of CO on Cu can be i n t e r p r e t e d w i t h t h e wroposed scheme,we can i n f e r t h a t almost a l l t h e Cu exposed atoms a r e p r e s e n t a s e x t e r n a l edge atoms o r a s i n t e r n a l edge atoms. T h i s f a c t can be a consequence o f t h e v e r y mild c o n d i t i o n of p r e p a r a t i o n t h a t produces rough o r "incomplete" m i c r o c r y s t a l s .

References.

1 ) P e t r i n i C . , Montino F . , Bossi A., G a r b a s s i F., " S c i e n t i f i c b a s e s f o r t h e prepara- t i o n of heterogeneous c a t a l y s t s " . Paper H 5 , Louvain-La Neuve (1982).

2) V a s i l e v i c h A . A . , ShapiroG.P.,Alekseev A.M., Semenova T.A., Markina M . I . , V a s i l ' e v a T.A., Budkina O . G . , K i n e t . i K a t a l .

2,

(19761, 1363.

3) g r i f f i n G . L . and Yates J.T. Jr., J. Chem. Phys. 77, (19821, 3751.

-

4) P r i t c h a r d J . , Surface S c i . ,

2,

f 1 9 7 9 ) , 231.

5) Shinn N . D . , Trenary M . , Ec C l e l l a n M.R. and Mc F e e l y F . R . , J. Chem. Phys.,

2,

(1981), 3142.

6) G r e e n l e r R.G., S n i d e r D.R., W i t t D. and S o r b e l l o R.S., Surface S c i . ,

118

(19821, 415.

7) Primet M . , B a s s e t J . M . , Mathieu M.V.,Prettre M . , J . C a t a l . ,

29,

(1973), 213.

8) Krohl-Urban B., Wagner H . , and Butz R., S u r f a c e S c i . ,

93,

(1980), 423.