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Catalysis Today, 12 (1992) 401-407 Elsevier Science Publishers B.V., Amsterdam
IDENTIFICATION
OF CHEMISORBED
MOLECULES
N.V. RICHARDSON
Surface Science Research Centm, University of Liverpool, Liverpool l.69 3BX, England, U.K.
SUMMARY This contribution aims to briefly contrast the surface science techniques, based on electron SIXC~TOSCODV,which can be used for the chemical identification of chemisorbed molecules. The d%fiiulties’&ociated with x-ray or UV photoelectron spectroscopies, which arise because of the low resolution relative to the spectral range, are compared with vibrational spectroscopies in general and electron energy loss spectroscopy (EELS) in particular. It is further shown that EELS can give, in addition, structural information such as adsorption site and adsorbate orientation. The chemisorption of formic acid, benxoic acid and succinic anhydride am taken as examples. INTRODUCTION Since the 196Os, the growth of surface science and an increasing awareness of its wide ranging applications has stemmed from the development of a bewildering array of experimental techniques. Bewildering, largely because they are mainly referred to by their acronyms. Broadly speaking, the methods can be characterized as being structural tools (“where are the atoms?“) such as LEED and, SEXAFS, techniques of elemental analysis (“what atoms are present?“), such as AES and XPS and techniques which address more precisely the chemical nature of the surface (“what is the bonding?“) such as angle-resolved UPS and EELS. Of course, this is a broad brush categorisation and there are many cross-linkages e.g. SEXAPS can give structural information related to a particular element at a surface. Often one of the hardest areas to resolve, in chemisorption studies in particular, has been that of bonding within a modified, chemisorbed molecule ( intra-adsorbate bonding), between the adsorbed species and the substrate and, most difficult of all, between adsorbed species (interadsorbate bonding). It has usually been necessary to rely on techniques which are either destructive, such as SIMS or TPRS, or to relatively indirect methods, such as XPS chemical shifts. XPS’and AES when applied to chemical identification or bonding information suffer from the low ratio of the available energy range in which the information can be carried to the energy resolution
of the technique.
Even in favourable cases this ratio reaches only -15 and
considerable reliance has to be placed on curve synthesis or deconvohttion methods. In addition, little is available in terms of an independent data base transferable from other phases of matter. These difficulties have contributed, with a few notable exceptions, to the inhibition of surface science to move away from the small molecule (or CC/NO!!) problems. In principle, UPS offered an opportunity to derive chemical information more directly and with a somewhat better database of gas-phase UPS spectra to draw on. However, the width
1992 Elaevier Science Publishers B.V.
402
of individual features (-0.3 eV) determined by the lifetime of the excited state rather than inherent problems in the instrumentation, compamd to the available energy range (typically - 15 eV) leads to a ratio (vide W,WU) of - 40 - 50. Add to thisthateach molecule contributes several bands in this energy region and the difliculties associated with more complex adsorb4 molecules and coadsorbate systems becomes apparent. Vibrational spectroscopic techniques provide a way out of this dilemma. There is an enormous database of vibrational frequency information compiled from gas, liquid, solution and solid state measumments for even the most complex systems. They address the bonding between atoms directly and the concept of group frequencies, which can identify specific units in a complex system, is well developed. Thirdly, the energy range to resolution ratio is vastly superior perhaps reaching -100 for EELS and more than 500 for IR and Raman spectroscopies. Experhrmmtal advances and the availability of comn~&4
instruments for EELS, the development
of FT techniiues for surface IR studies and major instrumental improvements, directed at the Raman sensitivity problem, have all served to bring vibrational spectroscopies to the attention of surface scientists and scientists with a surface or interface problem For the remainder of this presentation, I propose to illustrate, using initially the room temperature interaction of formic acid with a nickel surface, how EELS can give chemical information from a measurement of vibrational frequencies, as well as, orientational information, and, in this case though not generally, bonding site identification
through application of
symmetry based selection rules. In the final section, I hope to show that extension of these ideas to more complex molecular adsorbate systems is possible.
DCOO/Ni(llO)
E=8eV, Specular 300K
0
500
1000
1500
Energy
2000
Loss
2500
3000
3500
(cm-l)
Fig. 1. EELS spectrum of a Ni(ll0) surface exposed. at room temperature, to 6L of formic acid. The spectrum obtained in the specular direction at a primary energy of 8 eV, is indicative of a surface formate species
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Exposure of a clean Ni( 110) surface at room mmpemmm to 6L of deuterated formic acid, givesrisetoac(2x2)LEDpatteanandtheEELSspecmunshowninFig.l[l].Thaearcno features associated with O-D (or indeed O-H) stretching vibrations, suggesting that the acidic proton has been lost and a deuterated formate species formed This is confirmed by the mom detailed comparison (given in Table 1) of vibrational frequencies of both the hydrogen and deuterium containing species with those of the corresponding bulk sodium formate species. Although the angle-resolved UPS (figure 2) resulting from exposure of Cu to formic acid at room temperature is clearly different to that of formic acid condensed on
Cu at lower
temperatures and indeed is compatible with that of a chemisorbed formate species [2,3], a co&dent assignment, on the basis of the UPS data alone, is not possible. This is because UPS spectra of a bulk formate is not available for comparison and one must rely heavily on the use of molecular orbital calculations for the detailed assignment of both the gas-phase formic acid and admrbed fonnate spectra [Z].
HCO,-/Cu(llO}
hv=30eV
14
12 10
8
Binding
6
Energy
eV
Fig. 2. UPS spectrum of the surface formate species adsorbed on Cu( 110) at room temperature. The spectrum was obtained using the SRS at the SERCs Daresbury Laboratory with 30 eV photons. The vibrational ERLS da@ like the UPS data. also contains clues to the orientation of the formate species at the metal surface, by application of symmetry based selection rules [ 11.In the EELS spectra of figure 1, collected in the specular direction. where the dipole mechanism dominates the energy loss pnxess, the only internal modes with significant intensity are the va bandat2186cm-l.thesymmrricvCObandat1306cnrlandthe60CObandat750cnrl.All three modes are chsractetised by a dynamic dipole aligned parallel to the C2 axis of the DC00 species, i.e. parallel to the CD bond. Since the dipole-scattering
selection rule for metal
substrates demands that active modes are those with a dipole change papendicular to the Ni
404
TABLE 1 Comparison of the vibrational fmquencies of the chemisotbed formate species with those of bulk sodium folmate [41. Na(HC02)
Na(DCQ,)
782
766
1081
927
1363
1339
1395
1040
HCO2/Ni(llO)
DC@/Ni{ 1101
Assignment
242
v(Ni-Ni)
403
395
vs(Ni-O)
766
750
s(m) x(C-H,C-D)
839 1347
1306 1008
1597
1597
1557
1525
2847
2137
2863
2186
v&W &c-H,C-D) va(CW v(C-H,C-D)
surface. This is perhaps mom precisely confirmed by the observation that the asymmetric vco stretching mode expected -1530 cm-l is a very strong featum of the bulk sodium formate IR spectrum [4] but, with a dynamic dipole perpendicular to the CH ( CD) axis, is very weak in the spectrum of the chemisorbed species. Indeed the weak feature at 1525 cm-l could even be assigned as an overtone of the &o band. The lower frequency features in the spectrum at 395 cm-l and 242 cm- 1 also show dipole activity and can be assigned to the frustrated translation of the foxmate species perpendicular to the surface (i.e. symmetric wt-0 stretch) and a feature from the top of the Ni phonon band, to which I will return shortly. Off-specular spectra reveal other bands expected by the impact scattering mechanism but forbidden in dipole excitation. In particular, the in-plane b bending mode at 1010 cm-l is observed when the planar scattering geometry is aligned along the close-packed Ni rows but absent when aligned along the azimuth. The out-of-plane WD bonding mode at 810 cm-l shows the reverse behaviour. Since the impact scattering rules select the modes which are polarised in the scattering plane, this is clear evidence that the plane of the formate species is aligned along the Ni cl lO> rows [l]. The assignment of the UPS chemisorbed foitnate shows that application of selection rules is less easy because of the number of overlapping bands. Nevertheless, the feature at 8.1 eV below Et is assigned to the single b molecular orbital, which is based on oxygen orbitals. It is absent in normal emission when the radiation is polarised in the cl lO> azimuth but appears strongly at high emission angles in the aximuth when the pohuisation vector of the light also lie in this azimuth. These results also indicate an orientation of the formate plane with the metal atom rows [2,3]. ItisonlytheNi(llO)surfacethatformicacidadsoxbstoproduceanorderedLEED
405
0 @
Ni/Cu 0
cl C .riiii:. :::::::: ::$$i’
H
0
Fig. 3. The ~(2x2) arrangement of formate ions on a Ni(ll0) surface. The C-H axis is perpendicular to the surface and the plane of the ion is aligned along the close packed metal IDWS. pattern. The occutrence of a ~(2x2) structum reduces the size of the surface Brillouin zone and has the effect of folding the S point at the corn= of the clean-surface Brillouin zone back to the centre. Specular EELS probes the centre of the Brillouin zone, the r point, and, if the new substrate phonon modes folded back from S have the appropriate symmetry to be dipole active, they will appear in the spectrum. The symmetry of these modes, however, depends on the adsorption site of the molecule so observation of dipole active phonon mcdes can. in principle,
8eV, Specular
600000
t . . . 0
”
1..
lOi30
”
2000
”
1”’
3000
.
“.
4000
Energy Loss km-l)
Fig. 4. EELS spectnun of a Cu (110) surface expose+ at room temperature, to benzoic acid. The specaum obtained in the specular direction at a pnmary energy of 8 eV, is indicative of a surface benzoate species.
406
lead to a determination of the adsorption site [l,S]. In the case of the Ni(ll0)
surface, it is
fortunate that an extensive study of the clean surface phonons has been carried out by Ibach et al. [6]. One can be confident, therefore, that the low-frequency feature at 242 cm-l corresponds to the out-of-phase motion of neighbouring Ni atoms along the Ni cl lO> close-packed rows. This is induced to be dipole active only if the formate ion occupies a short-bridge site i.e. the oxygen atoms lie above the Ni atoms in these rows in a ~(2x2) arrangement, as shown in figure 3. This adsorption site on Cu( 110) and Ni( 110) has been deduced from photoelectron diffraction and SEXAFS measurements, though not without some initially false conclusions from SEXAFS data [71.
Succinic
Anhydride
! Cu(ll0)
6000
8cccoO-
600000 -
.e
4000
2
2
fi 400000 CI 2OOil 200000 -
0
"r
I..,
0
..
I,
1000
...
1..
2000
“‘.I
“1 3000
Energy Loss km-l)
Fig.5. EELS spectrum of a Cu(ll0) surface exposed, at room temperature, to succinic anhydride. The spectrum obtained in the specular direction at a primary energy of 4 eV, is indicative of a surface carboxylate species.
The adsorption of the formate species on a Ni(ll0)
surface provides a neat, textbook
example of the use of EELS in a rather complete determination of adsorbate species, orientation and binding site. In principle, however, these ideas can be extended in a straightforward way to the analysis of more complex adsorbate systems with the confidence that one can rely on the very extensive IR data which is available. As an example of this, I can refer to the spectra of the more complex but related carboxylate species which arises when benzoic acid is adsorbed at room temperature on a Cu(ll0) surface, shown in figure 4. Once again features are recognisable which arise from the metal phonon band at 234 cm-l which would indicate a similar adsorption site with the oxygen atoms bonded to size Cu atoms and a symmetric vcu_o mode at 492 cm-l both of which are dipole active. Other dipole active modes are again the 80~0 bending mode, this time at 702 cm-l and the symmetric vco stretch at 1428 cm-l indicating an OCO plane
perpendicular
to the Cu surface. The clear observation of a low frequency shoulder at 400 cm-l,
assigned to the asymmetric Cu-0 stretching mode and the feature at 1605 cm-l asymmetric C-G stretching male both in the offqecular
spectra suggest that the GCG aligmnent with the metal
rows is the same as that of the formate species on Ni(ll0).
For a more complete determination
of the structure it would be necessary to know the orientation of the aromatic ring with respect to the metal atom rows. In principle, the spectral activity of several modes in the off-specular regime could provide this information at 1320 cm-l in the off-specuhu spectrum and the absence of a strong in-phase C-H out-of-plane bending mode at -780 cm-l suggest that the aromatic plane is also aligned with the aximuth. Fiiy,
in this series of related molecules, we have been attempting to determine the
chemical identity of the adsorbed species which is produced following exposure of a Cu( 110) surface to carboxylic anhydrides, such as phthalic anhydrlde and succinic anhydride. Figure 5 shows the RELS specmun for succinic anhydride adsorbed at room temperature on a Cu( 110) surface. There is no vm feature to be seen either in the specular or any off-specular direction. The spectrum from a condensed layer of molecules sees this feature clearly. Instead, a strong, dipole active band at 1420 cm-l is observed very reminiscent of the symmetric va stretch of a carboxylate species. Clearly the bonding to the metal substrate is occurring through the anhydrlde ring as would be expected. The degree to which ring opening has occurred or indeed whether some disso&ion
has taken place is not yet clear. An XPS comparison of the condensed and
chemisorbed species in the Cl, region also shows that there is a major perturbation to the carbonyl part
ofthe molecule.
In conclusion,
an attempt has been made to show that vibrational
spectroscopic
techniques have a great deal to offer in terms of identifying the chemical aspects of gas-surface interactions. Gf course. the most powerful solution to problems of chemical identification at surfaces and understanding of surface bonding follows an input from several experimental methods. Hopefully, some recognition of tbat is present also in the w& described here. ACKNOWLEDGEMENTS Dr. T.S. Jones and Miss M.R. Ashton are thanked for their assistance with the work presented in this paper. Mr. M.O. Schweitzer is thanked for his help with the synchrotron radiation measurements of the formate species on copper. REFERENCES 1 2
T.S. Jones, MR. Ashton and N.V. Richardson, J. Chem, Phys., 90 (1989) 7564-7576. 2 Lmdn.,mdn.~~, A.M. Bradshaw and G.P. Wrlhams, Surf. Ser. 185 (1987) 75-87
3 4 5 6
P. Hofmanu and D. l+nxel, Surf. Sci. 191(1987) 353-366. R. Fonteyne, Naumwssenschaft, 31(1943) 411-422. T.S. Jones, N.V. Richardson and A.W. Joshi, Surf. Sci., 207 (1988) L948-L953. S..ehwald, F. Wolf, H. Ibacb, B.M. Hall and D.L. Mrllq Surf. Ser., 192 (1987) 131-
7
D.P: Woodruff, CF. McConville, A.L.D. Rilcoyne. Th. Lindner, J. Somers, M. Surman, G. Paolucci and A.M. Brad&w, Surf. Sci. 201(1988) 228-244 and references therein.