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THE ADSORPTION SITE OF FORMATE (HCOO) ON COPPER SURFACES
M. Crapper, C. Riley, D. Woodruff
To cite this version:
M. Crapper, C. Riley, D. Woodruff. THE ADSORPTION SITE OF FORMATE (HCOO) ON COPPER SURFACES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-487-C8-490.
�10.1051/jphyscol:1986891�. �jpa-00226222�
THE ADSORPTION SITE O F FORMATE (HCOO) ON COPPER SURFACES
M . D . CRAPPER, C . E . R I L E Y ( ' ) and D.P. WOODRUFF
P h y s i c s Department, U n i v e r s i t y o f ' w a r w i c k , GB-Conventry CV4 7 A L , G r e a t - B r i t a i n
Gs gt~ides SMAFS prgcgentes de l a nature de formiate sur Cu(100) e t Cu(ll0) ont condui ?I de diffgrentes concluq,ions. En particulier l a distance Cu-O sur Cu(l03) e s t trouvde t r g s longue (-2.3A) e t e s t crue come 6vidence pour une bond de surface anomale. En r6alanlysant l e s deux s @ c t r e s par l ' u t i l i s a t i o n de l a sinulation de m l t i c o q i l l e atomique, l e s resultats n ~ n t r e n t y e l a structure e s t , en f a i t , t r e s ,similaire pour l e s deux surfaces evec une distance voisine Cu-O d'environ 1.97A, qui e s t similaire que c e l l e t r o u d e dans l e formiate de cuivre anhydre. L'assignation precedente resulte de ll&hec de l a f i l t r a t i o n de Fourier de l a "contribution du premier voisin" pour s6parer l e s e f f e t s de deux liaison similaires pour c e t t e coquille atomique. Le problem se pose par le dgPlacement de l a distcmce 00 dans l e forri~iate e t de l a distance Cu-Cu 2 l a surface qi force les atomes d 'oxycjgne adopter les s i t e s d 'adsorption de faible s+tr ie .
Previous SMAFS studies of the structure of f o r m t e on Cu(l00) and Cu(l.10) have led t o very different conclysions . In particular the Cu-O distance on Cu (100) was found t o be very long (-2.3A) and believed t o be evidence for an ano om lo us' surface bond. Reanalysis of both s e t s of data using a m l t i s h e l l sinulation shows that the structure is, in f a c t , yery similar on both surfaces with a Cu-O nearest neighbour distance of about 1.97A, similiar t o that in anhydrous copper £ornate.
The previous assignmnt arises from the failure of the Fourier f i l t e r i n g of the 'nearest neighkmur contribution' t o resolve the effects of t w o similar bondlengths t o t h i s s h e l l . The problem arises because of the mismatch of the 0-0 distance in f o r n ~ t e and the Cu-Cu distance on the surface which forces the oxygen atom t o adopt low s y r m t r y adsorption s i t e s .
1. Introduction
The great m j o r i t y of structural studies of adsorbates on surfaces, both by surface MAFS and by other nlethods, has been on atomic adsorbates. Of far greater general interest in relation to catalytic properties of surfaces, hotever, is the study of mlecular adsorbate species. The recent application of SMAFS and NFh'AE'S t o the structure of a c a t a l y t i c intermediate, f o r m t e (HCOO) , on copper surfaces is therefore an important s t e p i n the study of adsorbate s t r u c t u r e s . The two independent studies performed, however, one on Cu (100) 121 and t h e other on Cu (110) 13 I*] have led t o m r k d l y different conclusions. On Cu (110) the f o r m t e
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986891
JOURNAL D E PHYSIQUE
species is found t o be positioned atop a.top layer copper atom with a Cu-O nearest neighbour distance of a p p r o x h t e l y 1.97A, essentially identical t o that found i n copper formates. The e a r l i e r study on Cu(100), however, concludes that on t h i s surface the formate sits in a,cross-bridge position with a Cu-O nearsst neighbour distance of approximtely 2.31A. This very long Cu-0 bondlength (-0.3A longer than other known Cu-O nearest neighbour bondlen t h s ) has been rationalised in terms of repulsive forces a t the cross-bridged sitefir2151 and was thought t o be an example of a novel molecule-surface bond.
Here we show t h a t by reanalysing the o r i g i n a l d a t a using a multishell simulation technique, rather than relying on nearest neighbour Fourier f i l t e r i n g , the discrepancy in conclusions for the two surfaces can be resolved. W e conclude that both the adsorption s i t e and bondlength for f o r m t e on Cu(l00) a r e similar t o that found on Cu(ll0). The difference i n interpretation arises from the fact that the Fourier f i l t e r i n g approach f a i l s t o describe adequately the fact that there are two or more s l i g h t l y different near neighbour atoms contributing t o the " f i r s t "
shell. This arises because the mismatch of the molecular 0-0 distance and the substrate Cu-Cu distance forces the oxygen atom into low symmtry s i t e s . Such problems a r e l i k e l y t o be cornmn i n molecular adsorption systems but can be overcone by careful modelling calculations.
The m i n calculations presented here a r e based on the EXCURVE computer program[6r7] a t t h e Science and Engineering Research C o u n c i l ' s Daresbury Laboratory. The program allows the EXAFS t o be calculated for a specified s h e l l structure of scattering atoms using a 'curved wave' formulation. Its u t i l i t y has been established in t h e study of a wide variety of bulk rraterials, and most recently for surface E M S . In addition t o the structural parameters (bondlengths and nurrber (or effective number) of atom in each s h e l l ) a number of non-structural parameters are also involved; the vibrational amplitudes, i n e l a s t i c scattering mean-free-path, energy zero for energy-photoelectron wavevector conversion, overall attenuation factor t o account for m y body effects and scattering phase s h i f t s . I n t h e p r e s e n t c a l c u l a t i o n s we have a t t e m p t e d t o f i t a s many of t h e s e non-structural parameters a s possible by f i t t i n g t o EXAFS data for a known Cu-O model compound, i n t h i s case Cu2G. 181 In particular w h v e fixed the scattering phase s h i f t s and energy zero. Small changes in the energy zero, Eo of a t most a few eV might be permitted t o account for chemical differences[gI betwen the bulk oxide and surface f o r m t e but here we explore the consequences of no adjustnent.
The approach has therefore been t o use these optimised non-structural Fararneters t o compute t h e EXAFS for d i f f e r e n t p o s s i b l e copper surface/formate adsorption geometries, and compare these with t h e r e s u l t s of t h e previous experimental studies. These calculations have included not only the scattering from substrate copper atoms, but a l s o s c a t t e r i n g from t h e other oxygen atom i n t h e surface formate. A t low energies and i n the appropriate polarisation geomtry, t h i s inclusion proves important. The precision of any deductions concerning 0-0 scattering is marred, however, by the f a c t t h a t 0-0 scattering in the Cu20 m d e l compound is likely t o be influenced by linear chain m l t i p l e scattering
S Othe 0 scattering phase s h i f t s cannot be s a t i s f a c t o r i l y optimised.
3. Results and Discussion
Figs. 1 and 2 show comparisons of the experimental data for the Cu(ll0) and (100) a d s o r p t i o n systems, a t normal and g r a z i n g incidence, compared with calculations based on the adsorption s i t e s proposed in the original, single s h e l l Fourier f i l t e r i n g , anal es. The (110) data have a rather short range due t o experinental problem[?4] but t h e comparisons of theory and experilrent a r e generally satisfactory and appear t o confirm the original analysis. By contrast, t h e comparisons of f i g . 2 f o r t h e (100) s u r f a c e show poor agreement. In particular, although the norm1 incidence (&go0) data are broadly in agreement,
the grazing incidence data show peaks and valleys which are essentially completely
a single s h e l l f i l t e r i n g approach indicates t h a t similar unphysical changes i n Eo (by -10 - 15eV) a r e needed i n t h i s method. We therefore believe the previously proposed s t r u c t u r a l model f o r f o r m t e on Cu(1CK)) t o be unsatisfactory and have t r i e d a variety of s i t e s and bondlengths t o determine the best f i t . This is found t o involve t h e f o r m t e i n an atop configuration but with the o r m t e plane along a
<~cK)> azimuth, with a CU-O nearest neighbur distance of 1.99 6 . Fig. 3 shows the r e s u l t s of t h e calculation for t h i s s i t e . We see generally good agreement in absolute energy location and amplitudes of a l l features for both incidence angles using the s m (previously fixed) Eo value.
Inspection of t h e experimental data for t h e (100) surface (figs. 2 and 3) ,
together with some s i n p l e considerations regarding t h e p o l a r i s a t i o n d i r e c t i o n dependence of t h e MAFS i n d i f f e r e n t adsorption s i t e s , allows us t o understand vhy t h i s s i t e gives a generally superior f i t t o the data. Because of the mismatch of the 0-0 spacing i n f o m t e and the Cu-Cu spacing on C u ( l O ) , adsorption of f o m t e i n any high symmetry s i t e leaves t h e 0 a t o m i n low symmetry s i t e s . There w i l l therefore always be a t l e a s t tw similar C u 4 bondlengths contributing t o the 'nearest neighbour' s c a t t e r i n g s h e l l . The experimental data show c l e a r differences i n t h e EXAFS recorded a t t h e tw d i f f e r e n t incidence angles wfiich c l e a r l y indicate t h a t these d i f f e r e n t contributions t o the near neighbour s h e l l have d i f f e r e n t r e l a t i v e importance as t h e polarisation direction changes. This, in turn, -lies t h a t the two (or more) near neighbour bonds have s i g n i f i c a n t l y d i f f e r e n t angles, P ,
r e l a t i v e t o t h e surface normal. Such a s i t u a t i o n occurs i n the 'diagonal atop
50 1M) 150
Phatodectron Ewrpy I eV
