The interaction of NO, O2 and CO with Cu(710) and Cu(711)

Applied Sttrfac¢ Sciull¢c 55 (It192) 1 -t} Nt~rth-| lolland

~pplied

surface science

The interaction of NO, O~~ and CO with Cu(710) and Cu(711) A.R. Balkcncndc,

R. H o o g c n d a m ,

T. d c B e e r , O . L . J . G i j z c m a n

and J.W. Gcus

Di,hYi, Itl~lllutt'. Surl~l¢ ¢' .~'l'lt'Hc¢'I)H hH;IL l)lvnlrtmt'n/ el Itlr~rglt/zi{ ('hlmt.~trv. (;tilt ¢'fwl~ o/ Ulltl hi. 1' () [~ol SIK)5 I. 350,~ 771 Utn'~llt. Ncthlrhuul~

Rccciv,,:d fi Mity It/t/I; ;it:t.'¢fltldIor puhlit';lll~ul 29 ,~t'rdt'luher lqgl

"File kmcli¢~ ot the ad~t~rplitul ~11 NO and O2. and of the xub~cqucnt rcductiol~ ~ith ('O h;t~c bccn Mud.cd < m tile Mlppcd copper ~Llrlacu%('u(711 ) ~md ('u(7111). T h u ~urlille~ cxhd~it lcrra¢¢~,v~llh the (I(l(I) ",tructurc, ;II1L1,4cp,,~.~.tththe I I I 1) ;ll|tJ the ( I 1(I) •,tructur¢. ru,.pcctiv¢ly. (h~,¢,, wc r¢ ;¢thnitlcd at pY~.~Mllt.~r;tllllill!! Irtllll fl × I11 " 10 H) t P;t. ICIIIplF;IILII¢~r;lllgt.'d holll 3711to 67(1 K. Upon rt.'at.litHi,tilt.' ad~.t)rpliOn ~ilcs ~tlltl gt.t*lllL'tr~'arc ~illlihll to Ih~l,~c till ('tl(IUll) "l'hc reactivity, el lilt' ultlirt' ('It(Hit)) ,,urlat.'c i~ IIIt'rcilSCdby ;IINIul ~111urdt'r ~1 II);IgllittltlC tluc It) the prlst'ncc nl ,,top',. "['hc ori¢lllallOll o[ tilt' qlp', tllll~ ,,hghlly aI1¢¢1~ the kil)¢li¢~,.

I. Introduction

K n o w l e d g e of tile nleehanism, kinetics a n d ,structure scm, itivity of the intcractttm u l N O a n d C O with metal s u r f a c e s is essential for i m p r o v i n g o u r insiMu rote the w o r k i n g of a u t o m o t i v e exhaust catalyst,,i a n d o t h e r DENC)X processes. This f u n d a m e n t a l informalioft is lllosl a c c u r a t e l y obtained by ~,tudying the a p p r o p r i a t e r e a l t i t m s Oll single-crystal .,iurlaccs. l,~,eccntiy, wc r e p o r t e d t)n the (atl)sorption of N O on the low-index s u r f a c e s of Cu, i.e., Cut I 1 1), Cut IIR)) a n d Cut I 1(1). a n d on the s u b s e q u e n t r e d u c t i o n of the a d s o r b a t c with C'O at prcsr, urcs up to 1 Pa ill the t e m p e r a t u r e r a n g c f r o m 381) to 78(1 K [ 1,2] N O a d s o r p t i o n was o b s e r v e d to bc tti~sociativu on all thc~c surfaces, initially leaving oxygen illld m t r o g e n a d a t o m s at the surface. T h e initial sticking probability was a b o u t lO ~ at the most o p e n C u t [ 10) surf:tee, 111 '~ tit tile ('u(I1)11) stn'l';.lec ;mtl ll) ~' at the most densely p a c k e d C u ( l l I) surface. A f t e r s a t u r a t i o n of the surface layer ( a c c o l n p a ~ i e d by r e c o n s t r u c tion o| the surface), f u r t h e r dis~,ociation of N O leads to the p e n e t r a t i o n of oxygen a t o m s into stlbsttrfacc layers. N i t r o g e n at~mls do not a c c u m u late but a r c r e m o v e d from the s u r f a c e either as N, or a,, N2(). T h e r e a c t i o n probability for N O

dissociation d u r i n g t h e oxygen p e n e t r a t i o n staL',c was a b o u t [0 ' at all surfaee,~. R e d u c t i o n of the a d s o r b a t c with C O p r o c e e d s via r e a c t i o n of :tdsorbcd C O a n d a d s o r b e d oxygen, p e l l e t r a t e d oxygetl nl~grates to VdeZlllt surface site.,,. "rhc rate of r e d u c t i o n .,,trongly d e p e n d s (.)11 the actual oxygen surface c o v e r a g e a n d on tile rate of m i g r a t i o n of s u b s u r f a c e oxygen Io vacant sites. T h e obscr~,ed :lpparcnt activation e n e r g y was a b o u t 20 k J / r e e l . In the case of Cut 1(10) a n d C u ( [ l ( ) ) the effect e l :tdst)rbcd n i t r o g e n on the rate t)f r e d u c t i t m could be explained by siteblocking, i.e., n i t r o g e n blocks sites t)thcrwi~c avail:tblc lot t)xygcn or C O a d s o r p t i o n . T h e maximulll reaction probability at 57(1 K as a b o u t 10 ,i for Cu(l(l()) a n d 1(I ~ Ibr ('u(ll(I). Ill the c;mc o f ('u( I l i ) a n a d d i t k m a l effect of n i t r o g c n was obser'v,:d: n i t r o g e n c;lus¢ ~, the Cut I I 1) surfi,cc to bc Mcally r e c o n s t r u c t e d [1.3]. thus i n c r e a s i n g tile b o n d Ml'englll ~Jl .'Msorbcd oxygltl, alld lowering the r:tt¢ of rcductioll by dll o r d e r e l nlagni(ud¢. At ('u( 1 I I ) the tllaximunl r e d u c t i o n probability in the prep,once of nitroL',cn is ;iboul I() l:ronl the ;lJlov¢-nlentR]llCtt kitlCtils ;I .Mrong xtructurc scn,,itivity, cM~cci:tlly tot the initial stage of di~stlciativ¢ N O atl.~orption, is a p p a r c l | t . r h e r c f o r ¢ . il is intc~c,,tlrlg to ,,ludy thc,,c rcac-

(H(W-4132/92/$()].(I{) ' ]~P/2 I~hCViL't S¢lt.m.c Iluhli~llt.I ', |} V All right', r,..,~,.:r',,,:d

.,I R Ilal~em'mh' ct al / The mterm turn ot NO. O: ,rod ( ' 0 ~ttll ()d710] and ('ld7111

tions on less regularly ordered surfaces, that is, on surfaces exhibiting steps of kinks. It is generally thought thai the presence of steps or kinks may dramatically alter (increase) the reactivity of the surfaces. A ::umber of studio,,, on tile reaction of NO (and CO) on stepped and kinked surfaces of Pt showc(I the reactivity towards NO dissoeiati()n to decrease in ttlc order Pt(41(l)> Pt(21[))> Pt(10()) > Pt( I I I) [4]. FEM studies on a curved tip of Rh also showed that the kinked and stepped surfaces arc the most active, while the most densely packed surface was the least active [bl. In the present study, we will invcstiga)l(" the reactivity of two stepped surfaces, Cu(71()) ?nd Ct!('7111. Ideally, both unreconstructed surfaces exhibit terraces with the Cu(l(l()) structure, the length of the terraces being 6 unit cells and 3 unit cells, respectively. The Cu(71(I) surface exhibits steps with the more reactive ( I Ill) structure, the Cu(711 ) st|rface exhibits steps with the less reactive (111) structure, the height being half a unit cell in both c~,scs. However, it should be noted that the actual surfaces used arc not perfect stepped surfaces (misalignmcnt up to 1°) and arc not necessarily unreconstructed. This study mainly focusscs on the kinetics of the (ad)sorption of NO and O , , and the subsequcnt reduction with CO on Cu(711) and Cu(7 Ill), measured using cllipsomctry. The kinetics measured on stepped surfaces will bc discussed in relation to the kinetics reported for the lower index surfaces, especmlly CB[IOI}) and Cu(I Ill) [2].

2. Experimental The experiments were carried out in an U H V system (base pressure Ill 7 Pa) equipped with facilities for cllipsometry, AES, L E E D and At-ion sputtering [1]. f h c crystals were obtained by aligni,g a copper single-crystal rod using Lauc back-reflection X-ray analysis. The crystals werc spark-cut within I ° from the desired direction, and subsequently mechanically polished. Cleaning was carried out t)y Ar-ion bombardment ( E = 511~ eV, PA, = 5 × lit-:' Pa, J = 2 × 10-: A / m - ' ) and annealing at 720 K for 15 rain.

Ellipsometry is based on the polarization tr.'msl~.')rmati,.m which occurs upon reflection of polarized light lrom a surface. This transformation is expressed with the ellipsomctric parameters A and 4,. which are related to the complex dielectric constant (or the complex refractive index) of the surface layer. Upon adsorption or reaction the complex dielectric constant changes, giving rise to changes in A and ~b ( 6 A and 6V~, respectively) [6]. Ellipsometry was carried out using a rotating analyzer, the angle of incidence of the laser beam (wavelength 632.8 nm) wits 69 °. Auger spectra were recorded in the derlvativc medc E d N ( E ) / d E using a cylindrical mirror analyzer with an on-axis electron gun. The anode current was 51111/~A at an electron energy of 2000 eV, the modulation voltage wits 10 V peak-topeak. The peak-to-peak heights at 38~; cV (N), 512 cV (O1 and 920 cV (Cu), are dcnor=d its h N, h() and hco. The folhwing gases were used: NO (99.9%). O , (99.995%), CO (99.997%) and Ar (q9.999()c,,¢ ),

3. Results and discussion 3.1. EaT~osure o f N O or O : 3.2. El/ip.~ometric measteremettts

Cu(710) and Cu(711) wcrc exp,)sed to NO or O , at pressures varying from 6 × 1() " to ().3 Pa, the crystal temperature varied from 370 to (~7[) K. The changes in the e!lipsometric parameters 6A and 6~h upon exposure of NO or O: to Cu(711) arc represented in fig. 1. Fig. 2 shows the changes in ,,SA upon exposure to Cu('Tl0). It will be shown in the next section that ~A is proportional to the amount of adsorbed species present either on or penetrated into the crystal. The parameter 8~b appears to be sensitive to species adsorbed on tile surface only. The shape of the curves is roughly similar to the shape observed for Cu(100)[2]. In the case ,,f NO exposure a relatively fast increase of (5..1 up to 0.5 ° is followed by a slower increase up to ..IA=0.8 ° . whereas in the case of O. exposure the increase in relatively fast up to 0.8 °. With Cu(710) some irregularities are observed

A R. Balkt',u'mh' ,'t al

The mtcta('tton o f NO. 0., and ( • 0 .',ill ("ld (710J and ( , f T / l )

15

1.5 NO-Cu(711) 8.%. 8~1:

•"

520 K

8.%. f i t } ,

O.)-Cu(711 )

(deg)

424 K

• "

(deg)

1" lo

0

0S ..me...,......

...........

''•

.........

8~ 5W

(a)

(b)

[

15

20

30

t

40

--) O 2 expo~.urc (Pa s)

--) NO exposure {Pa sl

F i g 1. rid and ~Sd, '.'cry, w, cxpo,,urc ot N(.) or (.): to C t d T l l ) at 420 K

b e t w e e n 8..1 = 0.8 o ;,nd 8A = 1.2 °. T h e s e will be discussed further on. For 8.1 > 1.2 ° a constant further increase of 8 3 is observed. T h e rate of c h a n g e of fiA did not d e p e n d on the previous

e x p o s u r e (i.e., N O

e x p o s u r e p r i o r to O : e x p o s u r e ,

or O., exposure prior to NO exposure)• The change in 8~h is roughly proportional to that ol 6A up to fiA = 0 . 5 ° , a flattening is ob-

1.5

..." NO-Cu(710)

•

~

....."

o:-C~,:" o'

. ....,,.,-"

626 K o_

1"

••"

1"

~o-"

: > 575 K

~."

.382.,

1.0 ....

o ,••,,°

•

,o°

0.5

0

Ib)

(a) 0

40

---) N O exposure (Pa s) Fig. 2. 53 vcr~,u~,¢xpo~.urt: of NO or O: to ('()(?Hi) al f)20 K.

50

Its1

- " ) (-)2 L,X['W~urL' (1','1 r-,)

A R Ihd~,+ncmh' ct NI / 77h+int,'ra,tu.I of NO. O, ,,Id ( ' 0 wittl ('l,( 71O) and (', ( 711)

served for 6J >0.5 °. At higher tempcr;:tures (above 500 K) a small incrcasc in ,,5#J is observed again when &J > 1.0°. As with the low-index surfaces, no indication for molecular adsorption of NO was obtained: 8.J and 6@ were not influenced by evacuation or electron bombardment, reduction of the adsorba:e did not affect the amount of nitrogen present, and upon heating no oxygen removal was observed. The similar shape of the curves of ,.53 and 8~/, versus the exposure for the stepped surfaces and the low-index surfaces [2] indicates that the same processes are taking place. NO is adsorbed dissociatively, leaving adsorbed nitrogen

and oxygen atoms at the surface (up to ,5.3 = 0.8°). Subsequently, for &A > 0.8°-1.2 °, NO dissociation results in the penetration of oxygen alums into subsurhicc layers (as in the case of the lowindex surhJces this was established with Ar ion sputter profiles), while nitrogen is dcsorbcd (as N, or N20).

I10+hM "hcu

0.3

0.2

/

Cu(711)

t

3.3. Calibration o f surfiv'e cotcragc

In order to correlate ,~J to the amount and

nature of adsorbed species, a calibratk)n has to be performed, using nuclear reaction analysis (NRA) [7]. it has been established that &J is proportional to the total amount of oxygen, either present ad,~orbcd at the crystal surface of penetrated into subsurface layers, irrespective of the low-index surface exposed. In the case of Cut 1(]0), the surface Coverage saturates tit ~ g = ().8°, corresponding to a fractional surface coverage of 0 = 0.52 :L 0.()5. ('Fhis value is higher than the one quoted in our previous paper [2] due to a better analysis of the N R A data.) Additional calibration using AES, yielded that ,5..1 is equally sensitive to adsorbed oxygen and adsorbed n:trogcn [2]. Thc hitter is also evident for C u t 7 1 1 ) a n d Cut710) from fig. 3. In thc el, so of Cut711) (fig. 3~,) the curve iff the At, get ratio h()/h(.. (proportion:ll tt) the amount of adstirbcd oxygen) versus ,~J in the

to) ee • ee O (02)

575K ~ae°° ~°°N+O(NO

o o ° o O (NO e ~ o D o ~ g86•

0.1

~6

" •'"

O0

i

o

(b) ho+hN0.3 II NO-Cut710) B 37gK m=,g II I=•B hcu / a S7eK = o o •

0.2J

1~

[ 0.1 'i

~ Boa •o

I@:~'

0.0

"0

• aa" e°° iio• e o I o_.°=lp e alf.~._.

0.0

o, •= N+OII

0.5

"~,,••

1.0

Fig. 3. h~./htu, h o / h ( u and lib, + h ~ . ) / h t .

N(NO:

> ~, (deg)

ho0 3

•

•

(c)

O2.Cu(710) +K

.

~ o ,'o.

• 526K a • = 623K ee~o== = 0.1

•

g

"

0.0 0.0

f,

1.5 20 0.5 1.0 15 > 6~ (deg) - - > ~a (deg) ',ersu~~J: (,0 Cu(711),cxpw,urc of N(3 or t):: (h) ('u(71()).cxpu.,urc.f NO; (O C~l(71(I).cxpo~u,'¢Of (~,

.4 R Ball+(.p..,uh, el ++I /

77wi, tt'ra( lio+++:?'NO. O: and ('() .~th ('u¢7lOI and ('u+ 7111

case of 0 2 exposure, coincides with that of (h o + versus (5.1 in the case of NO exposure. At higher crystal temperatures, these curves tqattcn for (5..1 > 0.8 °. This is due to the different depth sensitivity of ,$A and AES (about 100 and I nm, respectively. The flattening indicates that the surface coverage is saturated ill a.~ = t).g °, (hi) + h r ~ ) / h t . = 0.23, ill good ilgrecment with the saturation coverage obscrvcd for the Cu(10t}) surface, which exhibits im almost CClual atomic surface density. In the case of Cut?Ill) tile curves oi (h o + h r ~ ) / h ( , , versus +53 are temperature-dependent (figs. 3b and 3c). The curve:, are lineer up to (hi> + h r ~ ) / h t , = 0.15, IrOlll (,/I o + h N ) / h t u ~ O. 15 ( h o + h ~ ) / h ( . --.. 1123:1 temperature-dependent dc, lilt, m from lineurity is observed ill the hitrher temperatures. This is especially evident for O , c:q)osure (fig. 3c), upt)n NO exposure the effect is less pronot!nccd (fig. 3b). The curve m e a s m e d at 370 K coincides with the curves obtained for Cut711). The oh,served tenlper:lture d e p c n d e n o ! was equal 6,)r two different orientations t)t' the Cut710) surf:tee, viz. the steps at the surface parallel to the phme of incidence of the laser beam, and tile steps perpendicular to tile pl:mc of incidence. A n o l h e r type of temperature tlepel~dellee was observed t')y Habraken lind Bt')t')tsma [8] when exposing O , to Cu(l ltD. "l'he sensitivity of (5~ versus the (submonol.'wer) amount of adst)rhed oxygen not only d e p e n d e d on tile temperature:, tlut list) on the direction of the troughs at the (110) surface with rcspc('f to the ph,nc uf incidence of tile la~er beam. Also, tile observed opti('d anisotropy of ,5.3 upon reduction is different for Ctl(lll)) ;ind ('u(710). Ill tile ease ol ('u(110) a :h;wt~e iwl the :ullount of ad+orhed t')xygt2n yields a ch:tllge Ill (~..~ Ct)lrespt)lldil'lg to tile calibr:ltion ctn'~,d at tile Ienlpel:lttlr¢ of rcactioll, i.e., O , adsorption at high temperature caused :l larger change in ,$A than tile sUbSeqdelll complete reductioll at h)wcr temperature [8]. However. with Cut7110) tile observed challgc iJ'~ ~..1 upon reduction was alwa!)s equal to tile ell:lllge ill t<)~ e:lused by the preceding atlsorption stage. A str.'wghtforward cxplimatit)n ill terms of surface anisolrol')y, electron dcnsily distributio)l ;rod chemical bond-

h.,~)/hc, ,

5

ing c.nnot be given. In tile R)lMwing. tilt: rates of change of ,53 in tile case of submonohtyer amounts are sealed to the Iow-tem[3erilU, lr¢ equivalent, which agrees wcll with tile curve observed for Cu(7I 1). The ilnIOUllt of adsorbed n i t r o g e n atoms. h N / h c , ,. is ob~,erved to increase tip to (5.J = ().N ° ill the ease of C u ( 7 l l ) . With Cu(71t)), h r ~ / h ( , u increases tip to (SJ = t).0 °. Further expt)sute rcr,ults in a decrease, depending on tile temperalure. This is again similar to the behaviour of the (I00) plane. 3, 4. L I - E I ? e.~1)t "imt'nt.~

When studyin~ the adsorption with low ¢nel~y electron diffraction (LEt:.DL the folk)wing observatlons were made. The (I × I) patterns of the clean surfacer, showed : 'F filing of the spots, as is expected for stepper, surfaces. Exposure of either NO or 0 2 up to cSA = 0.5 ° did not change this pattern, except t'or :i,, increase in tile diffuse background. Ill the case of Cut710) a (~2 x ~/_~)R45" pattern is o[')serx'ed at ~$~ = 0.8 °. This does not t.'hange tlpon further exposure. In the ease of Cut711) vcry faint spots on a rather illtense background were observed. Studying tile structur:d changes of vicinal surfaces associated with Cut ltll)). Boulliard c t a l . [9,10] showed these stiff,ices to facet into (410) and (Ill{l) ,t,;faees ttpoll 0 2 adsorptiotl. Ill the ease of Cu(7l +) these facets arc linked by (511) facets. This may account fi.)r the ahsellee of a clear L E E D pattern lot Cu(7f l) exposed to O , or NO ill tile present ¢ilse.

Fronl tile observations deserihcd above, it is apparent that tile nature of adsorption on tile Cut711) and Cll(71()) surface is similar to that on Cu(III(D. Mouolayer coverage is saturitted at ~5..1 --~ 0.8 ° (0 = 11.5) lind :ldst')rptioll is acconlpanied by ordering ill the ;ldsorbatc layer. 3.5. K i n e t i c s

The rcaclioll prohability for tile dissociation of NO arid O 2, ohlained Irtlnl tile slope of the curves ,)f ,SA versus tile exposure, has I~een deter. lllilled ill dif|erent Stll')lllOI1Olily~'l surface cover-

A.R. Balkenende et al. / The interact#hi of NO. 0_, u,ul ( ' 0 witll Ctd710) and ('u( 711)

100

_~ 10"4.

(a)

a~

02,/Cu(711 ) O2~Cu(7tO)

~ 10-t.

•

~

g

•O

0 ~1 D •

N 10 2.

I

10-3. 1.4

"

•a

80

8

o

J~O/Cu(711

(b)

NO-Nacl+Oad+ OBpen/Cu e ='= o ~. 10-5, o° o ~ ' o #, o o oo o

J~

o'o

10.6,

02"Oad+Open/Cu

r*~Jff~/Cu(71C 10.7

1.8

2.2 2,6 • 1000/T (K 1)

14

18

22 26 ) 1000/T(KI )

Fig. 4. Arrhenius plot of (a) the initial sticking coefficient of adsorption of O or O,, and (h) tile reaction probability during oxygen penetration.

ages, and after penetration of oxygen atoms. For both surfaces, the initial rate of dissociative adsorption of NO as well as that of 0 2 are represented in an Arrhenius plot (fig. 4a). F;g. 5 shows the sticking coefficient (relative to the initial sticking coefficient) as a function of the surface coverage. The curve for Cu(100) is also included in this figure. Although the three curves look almost identical the value of s(0) for Cu(100) at this temperature is in fact an order of magnitude lower than that for the stepped surfaces, which both have the same value of ;(0). Thus the step orientation does not influence the coverage dependence of the adsorption kinetics, it merely increases the absolute value of the sticking coefficient. In the case of NO dissociation the rates at the stepped surfaces are even about two crders of magnitude larger than those observed for c',l(1O0). As is evident from fig. 5, the relalively high rates

1.0 s(e) s(0)

Q

323 K Cu(10o) • Cu(711;

u

*B

t0.5

-i

•

Cu(710)

D

0.0 0.0

I

o .'2

U n

o .'4

>o Fig. 5. Sticking coefficient of O, versus surface coverage at Cut711), Cu(710), and Cu(100).

at the stepped copper surfaces arc not only obtained for the initial reactivity but are also observed at higher surface coverages. Therefore, in the case of submonolayer coverages, the presence of steps or kinks at the Cu(100) surface drastically increases the ratc of both O2 and NO dissociation compared with these rates a, a perfect Cu(100) surface, irrespective of the morphology of the steps and the relatively low abundance of the steps. An Arrhenius plot of he rate of oxygen penetration from either NO or 0 2 is shown in fig. 4b. The reaction probabilities for NO as well as for O2 dissociation are then the same as those observed on the low-index surfaces. The dissociation reactions are thus not structure-sensitive at this stage. The apparent activation energy is about zero for NO dissociation and about 2() kJ/tool for 0 2 dissociation. When 0 2 is exposed to Cu(100) with adsorbed nitrogen present the apparent activation energy is also about 20 kJ/tool. However, in the case of 0 2 exposure to an oxygen-covered Cu(100) ~urface an apparent activation energy of 35 kJ/tool is observed for temperatures higher than 470 K, while an apparent activation energy of 8 kJ/mol is observed at lower temperatures [2]. Noticably, also the (V~ × 2 ~ ) R 4 5 ° pattern is only observed for exposure of 0 2 to oxygen-covered Cu(100). This seems to be due to differences in the extent of surface ordering and not to the nature of the ordering, since it has been shown that a structural relationship exists between the reconstructed Cu(100) surface upon 0 2 adsorption and the Cu(410) sur-

A.R. Balkenende et aL / The i, lwraetion tV NO. O , and C O widt Cu( 710) a n d Cu( 711) 1.C m

0.8 "'.'-%..

CO-Oad(+Open)/Cu(710)

CO-OaoJCu(71 1) "% "-% "°°%° %.

T . 620 K

"°.

~

1" "'-

07.

\

527 K

%° °-.. °. -°Oo

0.4

\

o. "'-

°

o.

(a) • --'-"'__

o..

"o

~•.

--_ - ~.~t..

•

-..

-"

"

.-

(b)

- .i

"o.o

"--

"" t"""

3

....

" .....

4

--.".

---~t'.

8

•--~CO expo.-;ule ~P,I .~) --~CO exposure (Pa s) Fig. 6. hA versus CO exposure after different exposures of O., to Cu(710) or Cu(711).

face [11,12], while the high-index surfaces associated with Cu(l()0) tend to tacet into (410) and (1(10) [9,11)]. Surface ordering will be less when also nitrogcn atoms are adsorbed. This can be

anticipated from the different LEED patterns, which are observed for Cu(100) with adsorbed nitrogen and with adsorbed oxygen, respectively

[11,13].

1.0¸ "~'~ -~..~_._~.~_

SO-Oad(+Open)/Cu(71 O) "~

T. 623 K

~'~

0.5

\

6N = 015

T m 561 K eN=015

(~ T

o%.

--..

\\

-~

•---...

CO-Nad+Oad/CU(711)

-%°°."

%

T

0.8 "..'.'._..

0.4

...

". O.o

o

;.'3.'-. ~.=..

"%..

.-.,--~ .

• o o •..'?..-

. . . . So~.oo°..°~...o . . . . . . . . .

.-.

.".

•

o

" ..

(a)

(b) J

20 40 --). CO exposure (Pa s)

L

25

50 ~ C O exposure (Pa s)

Fig. 7. &J vcr~u~ ('O expo~,ur¢ after different ¢xl)osures of NO "~ Cu(TI0) (~r Cu(711).

A.R. Balkenende et at. / The interaction of NO. O, aml CO with Cu (710) and Cu (711) ~_~,10"4

3.6. Reduction with C O

Reduction was carried out at C O pressures varying from 10 -~ to 10-~ Pa and at temperatures between 370 and 670 K. Typical plots of ~A versus the exposure of C O after previous exposure to O , up to ~SA =0.5(b3, ~SA = 0 . 8 °, and ~A --- 1.0° (only for Cu(710)) are shown in fig. 6 for both surfaces. Fig. 7 shows the reduction of these surfaces after N O exposure. It was established previously that at small C O pressures first the net reduction of sub:~,~rface oxygen is observed, followed by the reduction of adsorbed oxygen [2,14]. Since reduction proceeds via reaction of adsorbed oxygen with adsorbed CO. the rate can be expressed as: d O o / d t = -- kP(.oOot;,d)( Omax "- 0tOt;,d)),

(1)

in which 00 is the total a m o u n t of oxygen present, expressed as a n u m b e r of monolayer equivalents, 0r~;,x is the fraction of sites available for C O and oxygen adsorption. In the case of Cu(710) the observed plots point to 0m,,x = 0.5 (~SAm,,,, = 0.8°): a steady rate down to ~iA = 0.8 ° is observed when ,,SAioi,:, > 1.0°. a~, Sshaped profile is observed for (~g'~initi;,I ~" 0"80° and when 8A~.,,~,~ = 0.5 ° the rate of reduction is already high initially. In the case of Cu(711) the observations are similar for reduction after NO exposure. However, when O , has been exposed previously, 0m,,x seems to be 0.3 (~A~.;, x = 0.5°). In the case of Cu(100) a value of 0,~,,~ = 0.3 was also observed (both for N O and O , exposure). This was explained by the need of ensembles for the adsorption of CO, which are larger than a single oxygen adsorption site [2,15]. Evidcntly, the presence of steps at the surface increases the n u m b e r of sites available for C O adsorption. At Cu(100) and Cu(110) the effect of adsorbed nitrogen on the rate of reduction could be explained by site-blocking, i.e., nitrogen blocks sites otherwise available to C O or oxygen, thus lowering 0m:,~. A similar explanation is valid in the case of Cu(710). In the case of Cu(711 ) the decrease of the rate of reduction in the presence of adsorbed nitrogen was too large to be explained by siteblocking only. From the steepest slope in the plots of ~SA

Q.

A

•

•

•

Cu(710) Cu(711)

g Io-S

10-6. __

~4

1:7

~;o

~;3

2'6

> 1000fT (K "1)

Fig. 8. Arrhcniu~ plot of the maximum rcaclion probability of the reduction of (ad)~,orbcd oxygen from Cu(711) and Cu(711)). versus C O exposure, the reduction probability is obtained. The reaction probabilities in the absence of adsorbed nitrogen ale given as a function of the reciprocal t e m p e r a t u r e in fig. 8. W h e n comparing the reduction probabilities in the absence of nitrogen with those observed at Cu(100) they are found to be an order of m a g n i t u d e larger.

4. Conclusions (1) The nature and mechanism of the adsorption of N O or O , and of the subsequent reduction with CO, on the stepped copper surfaces Cu(710) and Cu(711) are similar to those on the low-index surfaces of copper. (2) The adsorption sites and geometry are similar to those on Cu(100) (the abundant terraces on the stepped surfaces). (3) The reactivity of a perfect Cu(100) surface is increased considerably by the presence of steps or kinks, irrespective of their structure and orientation. This holds for the a d s o r p t i o n / d e c o m p o s i tion of O : and N O for coverages up to about 0.5 monolayers and the reduction of the oxidized surfaces by CO. Incorporation of oxygen from either O , or N O into the crystal is not increased.

References Ill A.R. Balkenend¢. O.L.J. Gijzeman and J.W. Geus. Appl. Surf. Sci. 37 (1~)8~))l~9.

A R. Bulkencnde t't aL / 7711'interaction of NO, 0 2 and CO with Cuf710) aud Cu(7111 [2] A.R. Bulkem:ndc. H. dun Daas. M. lluisman. O.LJ. Gijzuman and J.W. Geus. AppL Surf. Sci. 47 (1991) 341. [3] V. Higgs. P. Hollins. M.E. Pemhle and J. Pritchard. J. Electron Spectrosc. Rel. Phen. 39 (1987) i37. [4] LM. Gohndrone. Y.O. Park and RI. Mascl. J. Catal. 95 (1985) 244. [5] |I.C.A.M. Hcndrix and BE. Nieuwenhuys, Surf. Sci. 175 (19861 185. 16] R.M.A. Azzum and NM. Bashara. Ellipsomet~ and Polarized Lignt (North-Holhmd. Amsterdam. 1977). [7~ M. WiegeL A.R. Balkencndu. G.WR. Leibhrandt, F.II.P.M. Habrakcn. OL.J. Gijzeman and J.W. Geus, Surf. Sci. 254 (1991) L42g.

[8] F.flP.M. Hahrakun and G A BtxJtsma. Surf. Sci. 87 (1979) 333. [9] J.C. Boulliard, J.L. Domange and MP. Sotto. Surf. Sci. 165 (1986) 434. [10] J.C. Boulliard and M.P. Sotto. Surf. Sci. 182 (t98b) 20(I. f i l l H.C. Zeng. R.A. McFarlune. R.N.S Sodhi and K.A.R. Mitchell. Can. J. Chem. 66 (1988) 2054. [12] E. Legrand-Bonnyns and A. Ponslet. Surf. Sci. 53 (1975) 675. [t3] H.C. Zeng and K A R . Mitchell. Langmuir 5 (1989) 829. [14] O.P. van Pruissen, M M M . Dings and O.LJ, Guzeman, Surf. Sci. 179 (1987) 377. [15] J.W. Evans. Surf. Sci. 214 (1989) 315.