The chemisorption of ethylene epoxide and carbonate on silver: A quantum-chemical study

Surface

426

Science 206 (1988) 426-450 North-HolIand. Amsterdam

THE CHEMISORI’TION OF ETHYLENE EPOXIDE AND CARBONATE ON SILVER: A QUAN~M-COMICAL STUDY Jo& A. RODRIGUEZ Department

Received

of Chemistry,

and Charles

indiono

1 April 1988; accepted

T. CAMPBELL

University, Bloomington, IN

for publication

9 August

*

47405% USA

1988

The che~so~tion of carbonate (COs) and ethylene epoxide (C,H,O) on silver has been examined employing semi-empirical quantum-che~~al calculations (INDO/S) and dusters of limited size (Ag,,). The results indicate that ethylene epoxide is an electron donor upon adsorption on Ag surfaces, with no contribution of the empty orbitals of the free molecule to the surface-adsorbate bond. Most of the charge transfer from ethylene epoxide toward the surface is from the occupied frontier orbitals: 2bt and 6a,. The CO, group appears as a net electron acceptor (n-donor, u-acceptor) when adsorbed on Ag(ll0). The influence of chemisorption effects upon the C-O, C-C and C-H bonds of ethylene epoxide, and the C-O bonds of carbonate are discussed. The bond order indices indicate that from the viewpoint of the substrate the Ag(Ss, 5p) orbitals are primarily responsible for the chemisorption of ethylene epoxide and carbonate on silver surfaces. For each adsorbed molecule, charge transfers and dipole moments important to the change in work function are presented. Based on these INDOfS results, the UPS spectra of ethylene epoxide and carbonate on Ag(ll0) and the NEXAFS spectrum of carbonate on Ag(ll0) are discussed and compared with experimental results.

1. Introduction The objective of the present work is to study from a qu~tum-che~~al point of view, the adsorption of carbonate (CO,) and ethylene epoxide (C,H,O) on silver surfaces. The adsorption of C,H,O on Ag is interesting from a catalytic viewpoint. The catalytic oxidation of ethylene to ethylene epoxide (2C,H, + 0, -+ 2C,H,O) is a several-billion-dollar per year industry [l], providing the necessary intermediate in the synthesis of ethylene glycol, which is used in polyester and antifreeze production The most common catalyst is silver supported on a-A1,03 [2]. The interaction of ethylene epoxide with an Ag(ll0) surface has been experimentally studied by means of thermal desorption spectroscopy (TDS) [3,4], low energy electron diffraction (LEED) [4], high resolution electron energy loss spectroscopy (HREELS) [5], ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) [3,4], and work * Alfred

P. Sloan Research

Fellow.

0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

function measurements [4]. To our knowledge, no previous quantum-chemicai work has appeared studying the adsorption of C,H,O on Ag(100) and Ag(ll0) surfaces, and little is known at a molecular orbital level of the chemisorption of ethylene epoxide on silver or other metal surfaces. Adsorbed CO, has been detected as the product of the reaction of CO, and adsorbed atomic oxygen on AgfllD) and Ag(111) surfaces f&7]. The adsorption of carbonate on Ag surfaces has been investigated by TDS ]6,7], LEED [8], HREELS f3], XPS 17,101, UPS [lO,llJt work function measurements [8,10], and near edge and surface extended X-ray absorption fine structure (NEXAFS and SEXAFS) [12]. No quantum-chemical work has been reported dealing with the chemisorption of CO, on any metal surface. We utilize here cluster models of Ag surfaces with the semi-empirical quantum-chemical method referred as INDO/S ff3-151, taking advantage of a recent par~ete~ation for second-row transition metals developed by the Zerner group [16]. To our knowiedge> this is the first application of INDO to chemisorption studies on transition metals beyond the first row. Our previous results [17-231 have shown that INDO/S is a good method for qualitativeIy addressing the mechanism of chemisorptive bond formation. In the past the INDO/S results for certain properties have compared very well with those obtained using much more sophisticated ab initio SCF methods [17,19,20,22] and experimental techniques [18,21]_ In particular, INDO/S is very good for providing: (1) a molecular orbital description of the ehemisorption bond, (2) the changes in atomic charges and bond orders which accompany chemisorptive bond formation, and (3) the influence of chemisorption upon the relative energy levels of the adsorbate molecular orbitals and its electronic spectra. On the other hand, INDO/S has not been proven to be good at geometry optimization, so its marriage with experimental results on adsorbate structure, such as provided by NEXAFS and SEXAFS here in the case of c~~nate/Ag~~l~)~ seems to be an optimum utilization of its capabilities. Our present study of the adsorption of C,H,O and CO, on silver surfaces serves as a further test of the utility in ehemisorption studies of this INDO/S approach and its new parameters for silver.

2. Theoretical method and models

The INDO/S method developed by Zerner et al. fl3-15f is used in the present work. The method has proven to be useful in the study of the adsorption of CO, HCOO, CN, OH, CO,, NH,, H&O, and H,O on copper surfaces [17-20,22-231, and CO, OH, COz, NH,, C,H,N, H.&O, HCOO, H,CO, and H,O on zinc oxide surfaces [20-231.

428 Table 1 Parameters Ionization

J.A. Rodriguez,

C. T. Campbell / The chemisorption of C, H,O and CO, on srluer

used for silver in calculations potentials

a)

(ev) ‘)

Orbital

- IS

- IP

- 1,

c, = Cp

7.31

3.62

12.51

1.5070

a) From ref. [16]. b, The numbers in parentheses are the coefficients ‘) Based on a 4d”5s’ configuration for silver.

exponents

in the double-zeta

Cd 4.9890(0.5576) 2.5837(0.5536)

b, b,

basis.

The parameters used for H, C, and 0 are those employed before and are listed in refs. [17,18]. Table 1 shows some of the parameters used for silver [16]. The 4d orbitals of silver were represented using a contracted double-zeta basis derived from the work of Clementi and Roetti [13,24]. For the Ag-bonding parameters (which are used in the evaluation of the resonance integrals), we employed the values [16]: /3,= & = - 1 eV and & = - 27.94 eV. The Slater-Condon factors of silver were calculated ab initio and scaled by a factor of 0.5 [16]. For the two-electron Coulomb integrals [14] of silver we used the values [16]: y,, = ys, = y,, = 5.25 eV; ysd= ypd= 6.96 eV; and ydd= 11.15 eV. For the weighting factors that are employed in the evaluation of the two-center one-electron integrals [ 131, we utilize the values recommended by Bacon and Zemer [15]: fpO= 1.267, fP,, = 0.640, f,, =fdO =fdlr =fds = 1. The bond-order indices that are reported in these INDO/S calculations were derived employing the expression proposed by Wiberg [25]. The reported charges and orbital populations are calculated by Mulliken population analysis [26] using the overlap matrix and the INDO-eigenvector matrix divided by the square root of the overlap matrix [27]. In this paper we refer to the density of states, which we have approximated using the discrete eigenvalue distribution calculated for the clusters, broadened into bands with a Gaussian convolution as described in ref. [28] (broadening parameter u = 0.15 eV). 2.2. Cluster models and geometries Fig. 1 shows the clusters employed to model different sites of the Ag(lOO) and Ag(ll0) surfaces. All the clusters have 18 Ag atoms arranged in two or three layers in a geometry identical to that in bulk Ag. The numbers in parentheses denote the number of Ag atoms in each layer. Clusters I and II have one atom in the third layer (not shown in fig. 1) just below atom #l. Cluster IV has five atoms in the third layer (not shown in fig. l), which are directly set below the five in the first layer. In the clusters, the distance

CLUSIER I

Ag(lO0): a-top site Ag(9:8:1)

r

CLUSTER Ag(llO): a-top Ag(9:8:1)

II

site

Y

X

CLUSTER III Ag(llOf

:

bridge

Ag(12:6)

sites

CLUSTER IV Ag(ll0) : a-top ri t-e A~(s:a:s)

Fig. ‘1. Schematic diagram of the clusters used to model different adsorption sites on Ag(100) and Ag(ll0). The notation Ag(9 : 8 : 1) specifies haw many Ag atoms are in each of the first three layers, respectively. The full circles represent the silver atoms in the first layer of the cluster, and the dashed circles the atoms in the second layer. Ethylene epoxide and carbonate were adsorbed on the Ag atoms numbered 1 and 2 (see text).

between Ag nearest-neighbors

f291,

was taken equal to 2.89 A as in the bulk metal

We have studied the adsorption of C,H,O on atop sites of Ag(100) and Ag(ll0) surfaces, and on the short-bridge and long-bridge sites of Ag(ll0). There is no experimental information above the exact geometry of C,H,O on any metal surface. On Ag(llO) [4] and other metal surfaces [30], work function changes and UPS data suggest that C2H,0 is bonded via the oxygen atom, with the moIecutar axis nearly perpendicular to the surface. Intuition suggests that this type of adsorption geometry should maximize the interaction between the 6a,, Zb, (approximately a lone-pair of axygen) and 3b, occupied frontier orbitals of C,H,Q and the adsorption site. XPS [3] and HREELS [5] data, together with a small heat of adsorption, indicate that the C,H,O molecular geometry is barely perturbed by adsorption on an Ag(ll0) surface. Based on these facts, we have studied the adsorption of C,H,O bonded to the surface

through its oxygen atom, with its molecular (C,) axis perpendicular to the surface, and with the same geometry as in gas phase [31a]. C,H,O adsorption on atop sites of Ag(100) and Ag(llO) was examined by locating the molecule such that the oxygen atom sits directly above the Ag atom numbered 1 in clusters I and II. For adsorption on atop sites of Ag(ll0) we found essentially the same results when the C,H,O molecule was adsorbed with its molecular (C-C-O) plane parallel or pe~endicul~ to the rows of surface Ag atoms. Cluster III was used to model adsorption on short- and long-bridge sites of Ag(ll0). For adsorption on a short-bridge site, the molecule was adsorbed with its oxygen atom centered between the Ag-atoms numbered 1, in such a way that the “bridge” was either contained in the C,H,O molecular plane (A conformation in fig. 2) or perpendicular to it (B conformation in fig. 2). For adsorption on long-bridge sites, we inverted cluster III and adsorbed the molecule on the bridge between the Ag-atoms numbered 2, with the molecular plane in a B conformation (see fig. 2). In all the cases we used an oxygen-tosilver nearest-neighbor distance of 2.10 A. This distance was estimated using the metallic radius of Ag [29] and the covalent radius of oxygen [29], and it is close to those observed for atomically adsorbed oxygen on Ag(ll0) (2.06 A, [32]), and for bulk Ag,O (2.05 A, [22]). (Some variation of this distance was studied to insure that our general results are not influenced by this choice of distance.) Recent NEXAFS and SEXAFS studies of carbonate on Ag(110) by Brader, Hillert, Puschmann, Haase and Bradshaw 1121 indicate that the molecule is adsorbed with its molecular plane parallel to the surface (rt 15O) and that it maintains the D3,, symmetry of the free CO:- ion (at least with respect to the a-reasonances). The exact adsorption geometry of carbonate was not determined in those studies, although a Ag-0 distance of 2.8 A was determined from the SEXAFS results. The authors suggest that this unusually long bond length is related to the strongly ionic nature of the adsorbate (by favorable comparison to the length of 2.73 A in AgNO,). They note that, if this distance corresponds to the nearest-neighbor separation, then the atop site geometries such as shown in fig. 2 would agree with the SEXAFS amplitude ratio observed with, however, no preferred azimuthal orientation [12]. We have studied the two local adsorption geometries shown in fig. 2, using Ag-0 nearest-neighbor distances of 2.80 A. For the C-O bond length of adsorbed carbonate we have used a range of five values between 1.27 A (the value for the CO:ion 1541) and 1.42 A (an average of the bond length observed for single C-O bonds in organic molecules [31]). Properties such as the bond orders, (I- and rr-populations, and atomic and molecular charges were slightly affected by the variation of the C-O bond length. Although there were no great changes for this range of bond lengths, the best agreement between the calculated and the experimental UPS spectra of CO, on Ag(ll0) was obtained for a C-O bond length of 1.42 A. The numerical values reported in section 3.3

J.A. Rodriguez,

C.T. Campbell / The chemisorption of C, H,O and CO, on silver

ETHYLENE

431

Ef'OXIDE

SHORT BRIDGE

LONG BRIDGE

A AND B CONFORMATIONS

B

CONFORMATION

CARBONATE

C

CONFORMATION

D

CONFORMATION

Fig. 2. Different adsorption geometries studied in the present work for ethylene epoxide and carbonate. (Only the nearest Ag atoms in the cluster are shown here, although more atoms are used in the calculations as described in the text.) The C,H,O molecule is adsorbed with its oxygen atom down and its molecular (C,) axis perpendicular to the surface. For carbonate, the molecular plane is parallel to the surface.

for adsorbed CO, correspond to this C-O distance. To model the CO, adsorption site we used cluster IV (adsorption on the Ag atom #l). This 18-atom Ag cluster simulates a Ag(ll0) surface with a (1 X 2) reconstruction as has been observed experimentally [12] for the system CO,/Ag(llO). (Here we assume a missing-row model for this reconstruction.) Notice that all of the Ag atoms to which carbonate bonds have all of their nearest-neighbor Ag atoms. For comparison, some studies were also carried out by adsorbing the

432

J.A. Rodriguez,

C. T. Campbell / The chemisorption of C, H,O and CO, on silver

CO, molecule on cluster II (adsorption on the Ag atom #l) no (1 X 2) reconstruction.

in which there is

3. Results 3.1. Electronic properties

of the bare clusters

In this section we present and examine some of the electronic properties that INDO/S predicts for the diatomic Ag, molecule (as a test) and for the bare Ag-clusters of fig. 1. Table 2 shows the calculated energies with INDO/S for the valence molecular orbitals (MOs) of free Ag, (re = 2.48 A [34], ‘2: ground state). For comparison are also listed the results of an ab initio SCF calculation that includes electron correlation but not relativistic effects [35]. The agreement between the INDO/S results and those of the ab initio calculation is reasonable (differences 5 1.7 ev). In INDO/S the 4d orbitals appear spread out in a range of energy (- 2.50 eV) that is larger than in the ab initio calculation (- 1.30 ev). The HOMO of the Ag, molecule is somewhat (- 1.7 eV) more stable in the INDO/S calculation than in the ab initio calculation. Using Koopmans’ theorem, INDO/S predicts a value of 7.14 eV for the first ionization potential of the Ag, molecule. This value agrees very well with the experimental value of 7.56 eV [34] and with the calculated ionization potential of - 8 eV reported using a semi-empirical pseudopotential with electron correlation and relativistic effects [36]. An analysis of the atomic-orbital coefficients in the MOs of Ag, showed that there was almost no rehybridization, with the occupied MOs being almost pure 5s or 4d functions. We found that the chemical bond between the two Ag atoms (Ag-Ag bond order = 1.222) was mainly (- 80%) due to the interaction between the 5s orbitals. All these results are in agreement with previous ab initio SCF calculations [35,37] for the Ag, molecule. Table 2 Ag, molecule: energy of valence MOs (eV) Orbital

Ab initio SCF ‘)

This work, INDO/S

5SO” 5s0, 4d% 4d ?T~ 4dS” 4d% 4dli, 4d%

0.54 a’ - 5.45 Q - 13.29 - 13.58 - 13.84 - 13.95 - 14.28 - 14.60

- 3.07 a’ - 7.14 b’ - 11.88 - 12.40 - 13.04 - 13.24 - 13.77 - 14.37

a) LUMO.

b, HOMO.

‘) From ref. [35].

J.A. Rodriguez,

C. T. Campbell / The chemisorption of C, H,O and CO, on silver

433

b

Ag(4d)-band

CLUSTER

II

Ag(5s)-band

Ag (4d)

-band

CLUSTER

,,I

Ag(5s)-band

ci

Ag(4d)-band CLUSTER

Ag (5s)

I”

-band

I -18

-16

-14 ENERGY

Fig. 3. Density II; (c) cluster

-12

-10

-8

-6

(ev>

of states calculated with INDO/S for the bare Ag clusters: (a) cluster I: (b) cluster III; and (d) cluster IV. Only occupied orbitals are included. The energies are reported with respect to the vacuum level.

Fig. 3 shows the density of occupied states (DOS) calculated with INDO/S for the bare clusters of fig. 1. Not shown in this figure is the position of the empty orbitals. For all the bare Ag clusters we found that the LUMO was - 3 eV above the HOMO. Table 3 displays the results for some calculated electronic properties (using Koopmans’ theorem) of each cluster. In the bottom row of table 3 are presented some experimental values for the bulk metal. We use the term “work function” here to refer to the first-ionization potential of the clusters, which we have equated with the energy of the HOMO employing Koopmans’ theorem. The values predicted in our models for the “work function” are - 2 eV larger than the experimental value for the bulk metal. This discrepancy is caused, in part, because we are neglecting the relaxation energy that accompanies the ionization process, and in part because

434

J.A. Rodriguez, C. T. Campbe@ / The c~ernisor~~~a~ of C, H,O and CO, on siluer

Table 3 Electronic

properties

of the bare Ag clusters

“Work function” Cluster Cluster Cluster Cluster

I II III IV

Experimental Bulk Ag a) From

7.02 6.63 6.38 6.62

- 4.60 a)

ref. [43].

(eV)

Occupied s-bandwidth

Separation top d-bottom

4.23 3.83 3.53 4.34

1.21 2.01 2.53 1.61

-

b, From ref. 1381.

s

Separation top dFermi level

d-bandwidth

5.44 5.85 6.06 5.95

3.39 3.12 2.52 3.27

3.9 b’* 4.0 F’

3.2 c’, 3.5 b,

=) From ref. 1411.

the electronic properties of metal clusters of size around 18 atoms are still somewhat different from those of the bulk metal (39,401. Thus, a direct comparison is not completely valid; and, the experimental values in table 3 should be taken as a limit for infinite cluster size. In our models we found that there was a small separation between the occupied 5s and 4d bands of Ag, whereas bulk band calculations for the metal have shown overlap [42]. In general, in the 4d band, all our orbitals presented at least 90% d character, and small 5s and/or 5p character. In the 5s band, we found some molecular orbitals with small amounts (less than 10%) of .5p and/or 4d character. The DOS of the Ag clusters (fig. 3) are in very good agreement with the results of SCF-X(Y-SW calculations of 4 to 16 atom Ag clusters [44], which show 5s levels extending from 0 to 4.5 eV below the Fermi level (Er), slightly separated from the 4d levels, which appear in the range between 5 and 8.5 eV below E,. In the experimental spectra of bulk Ag [4,10], the 4d levels appear closer to the 5s levels and to the Fermi level than in the present INDO/S results for Ag clusters. A similar discrepancy has been previously observed between the experimental UPS spectra of Cu and the results of INDO/S [22] and ab initio SCF 140,451 calculations for Cu clusters. In those cases the discrepancy was attributed to the fact that the final-state relaxation energy of the valence Cu(3d) levels is considerably larger than that of the valence Cu(4s) levels [22,40,45]. Based on this and on the similarities between the electronic properties of Ag and Cu, a similar argument is probably valid to explain the difference in energy position for the valence Ag(4d) levels between the INDO/S results using Koopmans’ theorem and the experimental spectra. 3.2. Adsorption

of ethylme

epoxide

Table 4 displays the calculated energies with INDO/S for the valence molecular orbitals (MOs) of free (gas phase) ethylene epoxide. C,H,O has C,,

Table 4 Free ethylene epoxide molecule: energy of valence MOs and ionization potentials Orbital

Valence MOs energies (ev)

- ionization potentials (eV)

Ab initio SCF *)

Green’s function ‘)

Ab initio SCF b,

7%

54

7.24 6.23

4% 2bl 6a, la2 3bz Sal lb, 2b, 4% 3%

6.fO 0 - 12.32 - 12.31 s) - 15.05 - 14.10 - 17.82 - 19.54 -23.72 -25.54 -38.86

a> From ref. [SS], dJ From ref. [51].

This work, lNDO,‘S

-

12.28 12.23 s) 14.97 x4.71 17.81 19.49 -

5.78 3.50 -

2.90 0 10.93 8) 12.90 13.84 16.89 18.94 20.71 25.24 25.39 51.79

- 10.59 - 11.64 “- 13.88 - 14.07 - 16.42 - 17.48 -21.41 - 22-04 - 32.33

HAM/3

d,

Experimental ‘)

_ _ - 10.59 - 11.97 - 14.21 - 14.00 - 16.03 - 17.66 -21.23 - 22.60 - 31.85

- -

10.57 11.85 14.00 14.00 16.60 17.40 20.80 22.00 32.50

b, From ref. [47]. ‘) From refs. [49,50]. c, From ref. [SO]. r, LUMO. 8) HOMO.

symmetry. In this work, the C,H,O MOs are labeled in a way that is consistent with a coordinate system in which the z-axis is the C, axis of the mofecule, and the y.r plane is the molecular plane. For a picture of the C,H,U MC& we refer the reader to refs. f46,47& Of particular interest for the results below are the 2b,, 6a, and 3b, orbit&. They are high in energy and have important contributions from the atomic orbitals (AOs) of the oxygen: 2b, = 77% of 0(2p,), 6a, = 20% of 0(2p,), and 3b, = 47% of 0(2p,). Both characteristics make them ideal for bonding with the Ag surface. For comparison with the MO energies that INDO/S Predicts for free C,H,U in table 4 are listed the C,H,U MO energies of two different ab initio SCF calculations [47,48], the experimental ionization potentials (IPs) of C2H,U I50], and calculated IPs with a many-body Green’s function theory [49,X)] and with the semi-empirical HAM/3 (hydrogenic atoms in molecules) method [51f. For orbitals with energy higher than 26 eV, the agreement between the results of INDO/S and those of the ab initio methods is reasonably good (differences generally < 1.5 ev). For the 3a, orbital the agreement is very poor. However this is not a serious problem, because this inner valence {deep lying) orbital in any case is so stable that it is almost not involved in making che~~~~on bonds. As will be described below, it is a gene& feature of INDU/S that the inner valence orbitals are over-stabilized. The order that INDO/S predicts for the ionization potentials of the C,H,O MOs (using Koopmans’ theorem) is the same as the order calculated for the IPs using a Green’s function method, Fig. 4 shows the DOS calculated with INDO/S for C,H,O adsorbed on atop and bridge sites of C,H,O. In the figures we can observe the 4d and 5s

A-TOP CLUSTER

a

Ag(4d)-band

CzH40/Ag
Ag (4d)

-band

SHORT-BRIDGE SITE A CONFORMATION CLUSTER It, 2 17,

n

E

CONFORMAT,ON CLUSTER 111

iii ifi cl

LONG-BRIDGE SITE B CONFORMATION INVERTED CLUSTER

-35

-30

-25

III

-20

ENERGY

-15

-10

-

j

(eV>

Fig. 4. Density of states calculated with INDO/S for ethylene epoxide adsorbed on Ag(ll0): (a) C,H,O on an atop site of cluster II; (b) C,H,O in an A conformation on a short-bridge site of cluster III; (c) C,H,O in a B conformation on a short-bridge site of cluster III; and (d) C,H,O in a B conformation on a long-bridge site of inverted cluster III. For an explanation of the adsorption geometries see section 2.2. Only occupied orbitals are included in the DOS. The energies are reported with respect to the vacuum level.

bands of the metal and several of the occupied orbitals that are derived from C,H,O MOs. The energies of those occupied orbitals of the C,H,O/cluster systems which showed significant C,H,O character (2 10%) are presented in detail in fig. 5. For simplicity, we have labeled the C,H,O-derived orbitals according to the orbitals in the free moIecule from which they are derived. (We

Fig. 5. Energies calculated with INl3OjS for the occupied valence MOs of free ethylene epoxide and for the occupied orbitals of the C,H,O/Ag systems with significant C,H,O character. The numbers in parentheses indicate the percentage of original MO character in the derived cluster MO. The energies are reported with respect Co the vacuum level. The results for adsorption of C,H,O on atop sites of Ag(100) were within 0.2 eV of those reported in this figure for C2H4CI adsorbed on atop sites of Ag(ll0). In both cases the order of MO energies was identical. Far comparison with the INDO/S results, also presented here are the C,H.+O peak positions in the experimental UPS spectrum of C,H,O/Cu(llOf f5?]. The experimental values have been shifted so that the 3b, levef lines up with tke cab&ted 3b, eigenwlue in tke long-bridge site. The shift is necessary due to final state relaxation energy effects, which we are negiecting when using Koopmans’ theorem for comparison.

will apply this type of labeling also for adsorbed carbonate.) The numbers in parentheses in fig. 5 indicate the percentage of C2W,0 character in each derived orbital. It is possible to see in this figure that all the CzH,O MOs are stabilized upon adsorption. The magnitude of this stabilization is probably overestimated and an artifact of the INDO approbations. For example, in the cases of CO and NH, on Cu(100) where ab initio SCF calculations were available for comparison, we found that the adsorbate-derived orbitals were overstabilized by _ 1 eV [17,22]. Generally, we trust these overall stabilizations predicted by INDO/S only qualitatively, and we will pay more attention here to relatioe changes in energies among the orbitals, for which INDO/S tends to give good results for adsorbates [17-221,

438

J.A. Rodriguez, C. T. Campbell / The chemisorption of C, Hd 0 and CO, on silver

It is interesting to consider the relative stabilizations that the upper valence MOs of C,H,O (2b,, 6a,, la,, 3b,) suffer upon adsorption. In fig. 5, for all the adsorption geometries, it is possible to observe that with respect to the la, orbital, the 2b,, 6a, and 3b, orbitals present a downward shift of between 1 and 3 eV. This relative shift is caused by the fact that the 2b,, 6a, and 3b, orbitals, which have an important contribution from the oxygen AOs, overlap much better with the adsorption sites than the la, orbital, which does not have any contribution from oxygen AOs (see for example, MO pictures in refs. [46,47]). According to their energy the C,H,O derived MOs can be ordered in the following groups, where within each group the energies are close enough ( < 1.5 eV) to expect that they would show up as a single, unresolved peak in photoemission: 2b,, 6a,, la, > 3b, > 5a,, lb, > 2b, > 4a,. A coefficient analysis of the occupied MOs of the C,H,O/silver systems revealed that the occupied orbitals of ethylene epoxide mix with 5s, 5p and 4d orbitals of Ag upon adsorption. The same kind of analysis showed that the contribution of the virtual orbitals of C,H,O to the surface-adsorbate bond was insignificant. No occupied MO with significant 4b,, 7a,, 5b,, 3b,, 2a,, 6b, or 8a, character was found (always much lower than 1%). The atomic and molecular charges for free and adsorbed ethylene epoxide are listed in table 5. For free C,H,O ab initio SCF calculations [48] give a charge of +O.l88e on the hydrogens, -0.156e on the carbons, and -O&t& on the oxygen. The INDO/S results are in reasonable agreement with these values. In order to test the transfer of charge that INDO/S predicts in a Ag-0 bond, we calculated the charge transfer in the 22 state of the diatomic AgO molecule (re = 2.07 A, [52]). We found a charge transfer from silver to oxygen of 0.62e. This compares reasonably well with the value of 0.79e reported as a result of ab initio SCF calculations [52]. In table 5 it is seen that C,H,O behaves as an electron donor upon adsorption on silver. A MO coefficient analysis showed that most of the charge transferred toward the surface comes from the 2b, and 6a, orbitals, and that the transfer of charge from the metal toward the empty orbitals of ethylene epoxide is negligible. The form of bonding between C,H,O and the Ag surfaces is essentially due to transfer of electrons from the occupied upper-valence C,H,O MOs toward the surface, plus the corresponding electrostatic interaction between the dipole moment of the molecule ( - 1.9 D, [53]) and the corresponding charge distribution of the substrate. Table 5 also displays the calculated bond order indices for free C,H,O and for C,H,O adsorbed on silver surfaces. The bonds between the hydrogens and the surfaces, and between the carbons and the surfaces were insignificant, so they are not listed. Ethylene epoxide was bonded to the surfaces only through the oxygen atom. For the atop sites, “0-Ag,,” represents the bond index between the oxygen and the nearest-neighbor metal atom on which the C,H,O molecule is adsorbed. For adsorption on the bridge sites of Ag(llO),

J.A. Rodriguez,C.T. Campbell/

Thechemisorption ofC, H,U and CO, on doer

439

Table 5 Ethylene epoxide: charges and bond orders Species

Charges (e)

qc

40

‘k,H,O

0.071

0.047

- 0.377

0.000

0.113

0.030

-0.157

0.355

0.117 0.120 0.122 0.114

0.025 0.017 0.012 - 0.023

-0.138 -0.116 - 0.010 0.021

0.380 0.398 0.502 0.431

C-H

C-C

C-O

0-Ag,,

Free C,H,O

0.972

0.998

0.975

_

AgWO) atop

0.971

1.003

0.922

0.635

0.729

A&l 10) atop SB ‘) A Conf. SB a1 B Conf. LB b, B Conf.

0.971 0.964 0.967 0.930

1.002 0.997 1.007 0.988

0.921 0.867 0.903 0.889

0.619 0.441 0.531 0.395

0.766 0.942 1.124 1.166

qH

Free CaH,O

AgWW atop AgUlO) atop SB a) A Conf. SB ‘) B Conf. LB b, B Conf. Species

Bond orders ‘)

O-Cluster

” SB = short-bridge site. b, LB = long-bridge site. ‘) Refers to bond order between oxygen and orre Ag-atom nearest-neighbor.

“0-AgNN” represents the bond index between the oxygen and one of the Ag atoms of the bridge site. In all the cases, “O-cluster” represents the total bond order index of oxygen with the whole cluster. A comparison of the values of the “O-cluster” bond order suggests that adsorption on the bridge sites results in a stronger bond than on atop sites. The C-C and C-H bonds of C,H,O are almost not affected upon adsorption, and there is a small weakening of the C-O bond. As noted above, most of the charge transfer to the surface upon chemisorption comes from the 2b, and 6a, orbitals. The 2b, orbital (approximately an oxygen lone pair) is slightly antibonding with respect to the C-O bonds, and the 6a, orbital is bonding. Therefore, a slight weakening of the CO bonds is expected upon chemisorption and indeed observed here. The lack on increased population of the C*H,O virtual orbitals upon adsorption, which are strongly antibonding with respect to the C-H, C-C and C-O bonds, avoids any further weakening of the bonds in the adsorbate. An analysis of the atomic-orbital components of the bond order indices [25] between ethylene epoxide and the different sites of the Ag surfaces (i.e.

440

J.A. Rodriguez,

CT. CampbeH / The cbe~isarptian

of C,H,O

and CO, on silver

adsorbate(Ag(5s), adsorbate(Ag(5p) and adsorbate(Ag(4d)). showed that from the viewpoint of the substrate the chemisorption bond is mostly due to the Ag(5s) and Ag(5p) orbitals. In general, we found that the percentages of participation of the Ag orbitals in the adsorption bond were as follows: 5s = 25% 5p = 71% and 4d = 4%. 3.3. Adsorption

of carbonate

Table 6 lists the calculated energies with INDO/S for the valence MOs of free CO,and free CO:- (both with D,, symmetry and C-O bond lengths = 1.27 A [54]). The MOs are labeled in a way that is consistent with a coordinate system in which the z-axis is the C, axis of the molecule, and the xy plane is the molecular plane. According to their symmetry with respect to the molecular plane, the valence MOs can be classified as u or 9r orbitals. The 7~orbitals are the occupied lay and le” orbitals and the empty 2a’,’ orbital. All the other MOs are of u symmetry. In free CO, the four highest occupied MOs (le” and 4e’) and the LUMO (la’,), are essentially (2 96%) centered only on the oxygens and nonbonding with respect to the C-O bonds. For comparison with the MO energies that INDO/S predicts for free CO:-, table 6 also lists the CO:- MO energies from an early ab initio SCF calculation [54]. For the occupied upper valence orbitals (energies 2 - 10 eV) of CO:-, INDO/S gives energies that are overstabilized between 1.5 and 2.8 eV with respect to the ab initio values. For the 3a; and 2e’ orbitals (inner valence orbitals) the overstabilization in INDO/S compared to ab initio is much worse. A similar problem has been observed in calculations for ethylene epoxide in this work (see section 3.2) and for a series of molecules previously Table 6 Free CO, and CO:-

: energy of valence MOs (eV)

CO3

co;-

This work, INDO/S

This work, INDO/S 28.54(5e’) 20.55(5a;) 14.82(2a;‘) ‘) 1.91(la$) ‘) 1.07(&‘) 1.03{1e”) -5.36(1a;‘) - 5.65(3e’) -6.51(4a;) - 28.54(2e’) - 38.05(3a;)

14.77(5e’) 6.79(5a;) l.l2(2a;‘) - 5.9O(la;) ‘) - 11.93(4e’) b, - 13.44Qe”) - 19.39(1a;) - 19.54(3&) - 20.62(4ai) - 42.22(2e’) - 51.72(3a;) *) From ref. [54].

b, HOMO.

=) LUMO.

Ab initio SCF ”

3.7(la;) 3.1(le”) 2.3(4e’) - 2.8(3e’) - 2.9(la;‘) - 4.9(4a;) - 20.2(2e’) - 24.6(3a;)

b,

J.A. Rodriguez,

C. T. Campbell / The chemisorption of C, H,O and CO, on silver

- COI/AS(l t 2

*--TOP c

441

Ag(4d)-band

10) SITE

CONFORM~TTION

_ CLUSTER

I”

t;

Ag(5s)-band ,

4&l@ la; ii-

4a;

3s’

la ;

k&J/

1

LLfYI,

-20

-16 ENERGY

-12

-6

(eV)

Fig. 6. Density of states calculated with INDO/S for carbonate adsorbed in a C conformation on Ag(ll0) (cluster Iv). Only occupied orbitals are included. The energies are reported with respect to the vacuum level. The results for carbonate adsorbed in a D conformation were very similar to those shown here for a C conformation.

examined: CO [17], HCOO- [18], CO, [23], NH, [21], H&O [21], OH [23], H,O [22], C,H,N [21] and ZnO clusters [20,21,23]. All this evidence together suggests that there is a permanent overstabilization in the energies that INDO/S predicts for inner valence MOs with energies below -25 eV. This defect is not a serious problem for the types of properties we address in chemisorption calculations, because in all cases the inner valence orbitals are so stable that they are almost completely uninvolved in making chemisorption bonds. Fig. 6 shows the DOS calculated with INDO/S for CO, adsorbed in the C conformation on an atop site of cluster IV (Ag(ll0)). The DOS results for the D conformation (not shown) were very similar to those displayed for the C conformation. The energies of those occupied orbitals of the CO,/cluster IV system which showed significant CO, character (2 10%) are presented in detail in fig. 7. For simplicity, we have labeled the CO,-derived orbitals according to the orbitals of the the free molecule (D,, symmetry) from which they are derived. It is clear that the la; orbital, which is the LUMO of free CO,, is occupied for adsorbed carbonate. This fact is not surprising when one observes in table 6 the very low energy of this orbital in the free CO, group (5.9 eV below vacuum according to INDO/S). According to their energies, the CO,-derived MOs in the adsorbed system can be ordered in the following groups, where within each group the energies are close enough together ( * 0.7 eV) to expect that they would show up as a single, unresolved band in photoemission: la;, 4e’, le” > lay, 3e’ > 4a; > 2e’ > 3a;. Only the first three of these groups fall in the range typically explored in HeI/II UPS.

FREE

CO

Caloulatsd --

MO energies

WZXG;

-11

-12

_-

4s’

-13

9 t

--

len

-18

1a; --

3e’

-20 47s; -

--2t Fig. 7. Energies c&ulated with INDOJS for the occupied valence MOs of free carbonate (in T&, symmetry, rc_o --I.27 A [54]) and for the occupied orbitals of the CO3/&(110) systems with significant CO, character. The energies are reported with respect to the vacuum level. Not shown in this figure is the LIMO (la; orbital) of the free CO, group, which appears at an energy of - 5.90 eV. For comparison with the INDOJS results, aiso presented here are the carbonate peak positions in the e~~~~rnent~ UPS spectrum of CU3JAg(I10) [l&11]. The experimental values have been shift& so that the top la; level I&s up with the cakxlated energy for the la; orbit& in a I3 ~nfo~at~on. The shift is ne-cessary due to finaI state reIaxa&n energy effects, &ich we are neglecting when using Konpmans’ theorem for comparison.

For carbonate adsorbed in C and D conformations on cluster IV, we found that the empty orbitals with strong CO, character appeased at energies of +1.2 eV (- 80% 2a”; character), f 6,7 eV (- 75% 5a;) and 4-13.4 eV (- 98% Se’) with respect to the vacuum level.

J.A. Rodriguez,

C. T. Campbell / The chemisorption of C_,H,O and CO, on silver

443

Table 7 Adsorption of carbonate Species

Charges (e)

e and P populations (electrons)

4c

40

4c0,

0

57

0.930 0.745

- 0.310 -0.915

0.000 - 2.000

16.000 18.000

6.000 6.000

C Conf.

0.687

- 0.857

17.697

5.160

D Conf.

0.687

-

- 0.836

17.675

5.161

Free CO, Free CO:-

CO,/&WO)

Species

Free CO, Free CO: CD,/Ag(llO) C Conf. D Conf.

0.502 0.521 0.543 0.490

‘) b’ s) b,

Bond order indices c-o

0-Ag,

O-A&

O-Cluster

C-Ag,

1.283 1.303

-

_ -

_ -

_

1.121 1.190 1.219 1.143

‘) b, ‘) b,

0.228 0.339 0.368 0.266

‘) b, ‘) b,

0.461 0.175 0.029 0.403

‘) b, ‘) b,

0.725 0.574 0.482 0.713

‘) b, a) b,

C-Cluster

CO,-Cluster

_

_

0.209

0.238

2.111

0.209

0.238

2.146

‘) Oxygen atom numbered 1 in fig. 2. b, For each of the two oxygen atoms numbered 2 in fig. 2.

A coefficient analysis of the occupied MOs of the CO,/cluster IV systems showed that the LUMO and the occupied orbitals of CO, mix with 5s, 5p and 4d orbitals of Ag upon adsorption. No occupied MO with significant 2a;‘, 5a; and 5e’ character was found (always much lower than 1%). The atomic and molecular charges for free and adsorbed CO, are listed in table 7. The charge distribution predicted by INDO/S for free CO:is in reasonable agreement with those obtained using ab initio MO calculations [54,55], and also with those calculated using methods based on electronegativities and on the cohesive energy of ionic solids [55]. Carbonate appears as a net electron acceptor upon adsorption on Ag(ll0) (when referring to neutral, free CO,). An analysis of the valence u and rr populations listed in table 7 shows that when adsorbed, CO, is a strong a-donor and a strong u-acceptor. The mechanism of CO, chemisorption involves charge transfer from the occupied u and 7~ orbitals of CO, toward the silver surface (a- and r-donation), and charge transfer from the metal toward the previously empty la; orbital of neutral, free CO, (a-backdonation). In table 7 we show the calculated bond order indices for free CO, and for CO, adsorbed on C and D conformations on Ag(llO) (cluster IV). The terms

444

J.A. Rodriguez, C.T. Campbell / The chemisorption of C, H,O and CO, on silver

“0-Ag,“, “0-Ag,” and “O-cluster” represent, respectively, the bond order indices between one oxygen atom and one of the Ag atoms numbered 1 or 2 in cluster IV (see fig. 1) and between that oxygen atom and the whole cluster. In a similar way, the terms “C-Ag,” and “C-cluster” refer to the bond orders between carbon and Ag atom number 1 and between that carbon atom and the whole cluster. A comparison of the bond orders shows that the CO, chemisorption bond involves important contributions from the central Ag atom at the adsorption site and from its two Ag nearest-neighbors in the topmost atomic plane. Most of the chemisorption bond strength comes from bonding between these three Ag atoms (i.e., those labelled 1 and 2 in fig. 1, cluster IV) and the three oxygen atoms of the carbonate. Upon adsorption, there is a small weakening of the C-O bonds (compared to neutral, free CO,). An analysis of the atomic-orbital components of the bond order indices between carbonate and the Ag(ll0) surface showed that CO, was chemisorbed mostly due to the contribution of the Ag(5s) and Ag(5p) orbitals (from the substrate’s point-of-view). In general, we found that the contributions of the Ag orbitals in the chemisorption bond were approximately: 5s = 20%, 5p = 76% and 4d = 4%. This is very similar to the case of ethylene oxide, which is a net electron donor (see above). The results presented in this section above were obtained by adsorbing CO, on cluster IV. This cluster was chosen in order to represent an Ag(ll0) surface with a (1 x 2) reconstruction [12]. It should be pointed out that we also studied CO, adsorption on cluster II (without (1 x 2) reconstruction), obtaining almost identical results to those reported above.

4. Discussion 4.1. Ethylene

epoxide

The INDO/S results indicate that ethylene epoxide is an electron donor upon adsorption on Ag surfaces, with no contribution of the empty orbitals of the molecule to the surface-C,H,O bond. For the orientation of this adsorbate chosen here, both the direction of the permanent dipole of the free molecule (- 1.9 D [53], pointing down) and the adsorption charge transfer calculated here (- 0.4e, toward the surface) indicate [59] that we should expect a substantial decrease in the work function of the Ag surface upon adsorption of C,H,O. Experimental results indeed show an - 1.4 eV decrease in the work function of Ag(ll0) [4] and other metal surfaces [30] when C,H,O is adsorbed. This helps substantiate the adsorbate orientation studied here. Experimental evidence [5] shows that the adsorption of ethylene epoxide on Ag(ll0) is promoted by the presence of atomic oxygen on the surface (i.e., its heat of adsorption increases). This phenomena can be explained using previ-

ous theoretical results for 0 adsorbed on silver clusters [56] and our present results for C,H,O/Ag(llO). Ab initio pseudopotential SCF-CI calculations [56] show that atomic oxygen is a strong electron acceptor (charge = - 1.7e) upon adsorption on a 26-atom Ag cluster. Thus, we can expect that adsorbed oxygen atoms create Ag sites on the surface with partial positive charge, which should facilitate the electron donation to these sites. Our results predict that ethylene epoxide will be an electron donor on Ag(llO), and its interaction should therefore be strengthened by the presence of nearby oxygen adatoms. There should also be a stabilizing through-space interaction between the partial negative charge on the oxygen adatom and the partial positive charge on the ethylene epoxide molecule. The results in table 5 indicate that the C-C and C-H bonds of C2H,0 are almost not affected upon adsorption, and that there is a very small weakening of the C-U bond, These results are consistent with vibrational spectra of adsorbed ethylene epoxide on Agflla) [S], which show no clear shifts in the C-H stretching frequencies or other observable modes compared to the free molecule, except for an - 45 cm-’ decrease in the mode associated with ring deformation (and therefore C-O stretching). At the same time, a small heat of adsorption (- 10 kcal/mol, 13-51) suggests that the molecular structure of C,H,O is perturbed very little by the adsorption process. It is useful to note that this adsorbate does not dissociate upon heating, but desorbs completely in a molecular form [3-51. The UPS spectrum of C,H,U on Ag(l10) has been reported [4]. Unfortunately, that spectrum was sufficient to clearly resolve only two peaks. However the same authors reported the angle-resolved UPS spectrum of C,H,O on Cu(llO), where five peaks could be clearly resolved due to lower overlap with the metal bands [57,58]. Since the chemisorption of C,H,O on Ag(ll0) and Cu(ll0) are thought to be very similar ]30,58], we will use the richer results for Cu(ll0) against which to compare our present calculated eigenvalue spectra. These peaks in the experimental spectrum are shown in fig. 5, along with their assignments based on angle-resolved UPS experiments with polarized light [57], For comparison to our calculated eigenvalues, we have shifted the experimental spectrum so that the 3b, level (i.e., the strongest feature in the experimental spectrum) lines up with the calculated 3b, eigenvalue in the long-bridge site. (Such a shift is necessary due to the effects of final-state relaxation energy, which are neglected when using Koopmans’ theorem for comparison_) As can be seen, the ordering of the calculated orbital groups is the same as the experimental assignment, except for the la, orbital. However, the basis for the assignment of this la, peak is not clearly presented in the interpretation of the experimental spectra [57]. For a C,H,O molecule oriented with the C, axis perpendicular to Cu(ll0) (C,, symmetry), emission perpendicular to the surface from orbit& with a2 symmetry is forbidden by dipole selection rules [62]. Note that the spectra for C,H,O/Cu(llO) were

indeed measured with detection perpendicular to the surface [5’7]. For the case of C,H,O/Fe(lOO), these same authors point out that the large peak which they similarly had attributed to a combination of la, and 3b, emissions is actually dominated by emission from the 3b,, so that intensity from the la, orbital was never cleariy identified in that UPS spectrum [30]. An interesting feature of the experimental spectrum in fig. 5 is that electron emission from the 6a, orbital appears at lower binding energy than that from the 2b, orbital. fn the calculated results this is true only for C, z-I,0 adsorption in a B conformation on short and long bridge sites of Ag(l10). This comparison supports preferential adsorption of C,H,O in these geometries. However, this implication has to be taken with caution because in all cases the 6a, and 2b, orbitals appear in a range of energy of less than 0.8 eV, and their position in the UPS spectrum could therefore be inverted due to small differences in final-state relaxation energy (which are neglected in the calculated results). The stronger stabilization observed for the 2b, orbital when C,H,O is adsorbed in a B conformation on bridge sites is due to much better overlap between this orbital (approximately a lone-pair on oxygen, perpendicular to the molecular plane) and the Ag atoms at the adsorption site. 4.2. Carbmate

The INDO/S results for adsorbed carbonate show that, when adsorbed on Ag(llO), the CO, group is a net electron acceptor (a-donor, u-acceptor). In inorganic compounds a formal charge of -2e is usually assigned to the carbonate ligand [54,55,60]. The large electron affinity of CO, in the gas phase (2.69 eV, [61]) indicates that the molecule should be an acceptor upon adsorption, in agreement with our results. Based on the calculated extent of charge transfer, we can expect 1591 that adsorbed CO, should substantially increase the work function of Ag(ll0). This expectation is consistent with experimental measurements, which show an increase in the work function between clean Ag(ll0) and Ag(ll0) with CO,,, [8,10]. The results listed in table 7 show that adsorbed carbonate is bonded to the Ag(ll0) surface mainly through the oxygen atoms, and that all the C-O bonds have a bond order index of around one. The chemistry of carbonate on Ag(ll1) [7] and Ag(ll0) f6,8] indicates that the adsorbed molecule is stable on these surface in vacuum until a temperature of - 450 K, at which it decomposes to produce oxygen adatoms and CO2 desorption. The carbonate peaks in the experimental UPS spectrum of CO,/Ag(llO) are shown in fig. 7, along with their assignment based on angle-resolved UPS experiments [ll]. For comparison to our calculated eigenvalues, we have shifted the experimental spectrum so that the top la; level lines up with the calculated lal, eigenvalue in a D conformation. (The shift is necessary due to

J.A. Rodriguez, C. T. CarnF~[~ / The chemisofptio~

of C, H&J and CO, on silver

447

the final state relaxation energy which is neglected when using Koopmans’ theorem here for comparison.) As can be seen, the ordering of the calculated orbital groups agrees very well with the experimental assignment [ll]. Notice in figs. 5 and 7 that the calculated valence eigenvalue spectrum of both ethylene epoxide and carbonate on Ag(ll0) cover a somewhat broader energy window that the experimental photoelectron spectrum. This was also the general case for other adsorbates we have calculated with INDO/S [17-223, where a compression of the energy scale by - 15-30% gives a much better quantitative comparison to experimental UPS The same can be said concerning gas-phase molecules [17-231. This is at least partially due to the fact that Koopmans’ theorem, which is inherently assumed in this comparison, neglects the fact that deeper-lying levels will be more efficiently stabilized in the final-state relaxation process accompanying photoemission. A systematic error in INDO/S eigenvalues may also contribute to this problem. In any case, it is useful to remember this energy scaling when comparing experimental UPS spectra to INDO/S eigenvalues using Koopmans’ theorem. The NEXAFS spectra of the carbonate species on Ag(ll0) present three resonances at the oxygen K edge and two resonances at the carbon K edge [12]. These resonances have been tentatively assigned I121 to electronic transitions into the unoccupied orbitals in order of increasing energy: 2a;’ < 5a’, < 5e’. We observed an identical ordering of energies for the three groups of empty orbitals with strong carbonate character in the CO,/Ag(llO) system: 2a;/ < 5ai < 5e’ (see section 3.3). In the NEXAFS spectra, the 5ai and 5e’ resonances lie - 6.8 and 10.8 eV, respectively, above the 2a;’ resonance. This compares reasonably well to the INDO/S eigenvalue separations of 5.5 and 12.3 eV between these same unoccupied orbitals. Of course, small differential final-state relaxation energies are neglected in this comparison. Results of SEXAFS experiments for CO, on Ag(ll0) suggest that a possible geometry for adsorption of carbonate is at the atop sites of the Ag(ll0) surface [123. Our INDO/S results show that carbonate adsorbed on these sites will have similar UPS and NEXAFS spectra to those observed experimentally (see above). No other possible sites were considered in this paper for adsorption of carbonate on Ag(ll0) due to the lack of experimental information about the adsorption geometry. For all the absorption geometries examined for C,H,O and CO, on silver surfaces, most of the chemisorption bond was due to Ag(5s) and Ag(5p) orbitals (from the substrate’s point-of-view), with a very small contribution of the Ag(4d) orbitals. A similar fact was observed for the atomic orbitals of Cu, when a series of molecules was adsorbed on different sites of Cu surfaces [17,23]. A general characteristic for these adsorbates on noble metals is that the valence d orbitals of the surface mix significantly with MOs of the adsorbate upon chemisorption. However, this mixing of the valence d orbitals does not produce any appreciable (< 5%) contribution to the surface-ad-

448

J.A. Rodriguez, C. T. CampbeIf /

sorbate bond due to cancellation parts.

The chemisor~~io~ of C, H,O and CO., on sifoer

between bonding and antibonding counter-

The authors would like to thank A.M. Bradshaw and M.C. Zerner for communication of results prior to publication. This work was partially supported by the US Department of Energy, Office of Basic Energy Science, Chemical Science Division. All of these calculations were performed on the VAXll-780 computer at the Chemistry Department of Indiana University, whose purchase was partially sponsored by the National Science Foundation under Grant Nos. CHE-83-09446 and CHE-84-05851.

Note added in proof The calculations above for carbonate on Ag(ll0) are based upon the adsorption geometry proposed by Brader et al. [12] on the basis of NEXAFS and SEXAFS studies. In that model the C-O bond length of adsorbed carbonate was not specified 1121. We tested C-O distances between 1.27 A (the value for the CO:- ion 1541) and 1.42 A (an average of the bond length observed for single C-O bonds in organic molecules [31]). The numerical values reported in section 3.3 correspond to a C-O distance of 1.42 A, which gave the best agreement between the calculated and the experimental UPS spectra of CO, on Ag(ll0). Properties such as the bond orders, u-, and s-populations, and atomic and molecular charges were slightly affected by the variation of the C-O bond length. Since submission of this paper, a new work by Madix et al. studying the structure of surface carbonate on Ag(ll0) has appeared [63]. Using NEXAFS, these authors found that the molecular plane of surface carbonate is parallel to the surface (in good agreement with that reported by Brader et al. [12]). By comparison of the energy of the C(ls) + u* transition for CO,/Ag(llO) with that for bulk CdCO,, Madix et al. conclude that the C-O bonds in adsorbed carbonate are all 1.29 & 0.04 A in length [63]. Using the models of figs. 1 and 2 (see section 2), we performed INDO/S calculations for carbonate adsorbed with C-O dis!ances of 1.29 A (assu~~g always Ag-0 nearest-neighbor distances of 2.80 A [12]). We found that the molecular charges, and (I- and n-populations were very similar (variations < 0.05 electrons) to those reported in table 7 for C-O distances of 1.42 A. The bond orders were within 0.1 of those in table 7, following the same qualitative trends. In the calculated UPS spectra, we observed nine CO,-derived peaks. These peaks can be grouped in three different sets, each set with similar

J.A. Rodriguez, C. T. Campbell / The ehem~o~ptjo~

5j

C,H,U

and CO, on silver

449

composition and close energy (within 1 eV) to those in fig. 7. For the CO,/Ag(llO) systems the calculated separations between the empty orbitals with strong CO, character were as follows: 2a’,’ - 5a; = - 6.2 eV, and 2a’,’ 5e’ = - 13.8 eV. (The order of absolute MO energies was: 2a’,’ < 5a; < 5e’.) These values compare reasonably well to the separations observed between the 5a; and 2a;’ (- 6.8 ev), and the Se’ and 2a’; (- 10.8 ev) resonances in the NEXAFS spectrum of CO~/Ag(llO) [12]. Finally, the discussion presented above concerning the bonding mechanism of CO, on Ag(ll0) is valid for all C-O bond distances between 1.27 and 1.42 A.

References [I] (a) J.C. Zomerdijk and M.W. Hall, Catalysis Rev. - Sci. Eng. 23 (1981) 163; (b) CT. Campbell, ACS Symp. Ser. 288 (1985) 210. [2] (a) W.M.H. Sachtler, C. Backx and R.H. van Santen, Catalysis Rev. - Sci. Eng. 23 (1981) 127; (b) R.A. van Santen and H.P.C.E. Kuipers, Advan. Catalysis. 35 (1987) 265. [3] CT. Campbell and M.T. Paffett, Surface Sci. 177 (1986) 417. [4] B. Kruger and C. Benndorf, Surface Sci. 178 (1986) 704. [S] C.Backx, C.P.M. de Groot, P. Biloen and W.M.H. Sachtler, Surface Sci. 128 (1983) 81. [6] M. Bowker, M.A. Barteau and R.J. Madix, Surface Sci. 92 (1980) 528. [7] C.T. Campbell, Surface Sci. 157 (1985) 43. [8] M.A. Barteau and R.J. Madix, J. Chem. Phys. 74 (1981) 4144. [9] E.M. Stuve, R.J. Madix and B.A. Sexton, Chem. Phys. Letters 89 (1982) 48. [lo] M.A. Barteau and R.J. Madix, J. Electron Spectrosc. Related Phenomena 31 (1983) 101. [ll] K.C. Prince and G. Paolucci, J. Electron Spectrosc. Related Phenomena 37 (1985) 181. [12] M. Bader, B. Hillert, A. Puschmann, J. Haase and A.M. Bradshaw. Europhys. Letters 5 (1988) 443. [13] W.P. Anderson, W.D. Edwards and M.C. Zerner, Inorg. Chem. 25 (1986) 2728. 1141 M.C. Zemer, G.H. Loew, R.F. Kirchner and U.T. Mueller-Westerhoff, J. Am. Chem. Sot. 102 (1980) 589. (1.5) A.D. Bacon and M.C. Zemer, Theoret. Chim. Acta 53 (1979) 21. [16) M.C. Zerner, to be published. [I7) J.A. Rodriguez and C.T. Campbell, J. Phys. Chem. 91 (1987) 2161. [18] J.A. Rodriguez and C.T. Campbell, Surface Sci. 183 (1987) 449. [19] J.A. Rodriguez and C.T. Campbell, Surface Sci. 185 (1987) 299, [20] J.A. Rodriguez and C.T. Campbell, J. Phys. Chem. 91 (1987) 6648. [21] J.A. Rodriguez and C.T. Campbell, Surface Sci. 194 (1988) 475. 1221 J.A. Rodriguez and C.T. Campbell, Surface Sci. 197 (1988) 567. [23] J.H. Rodriguez, Langmuir 4 (1988) 1006. [24] E. Clementi and C. Roetti, At. Data Nucl. Data Tables 14 (1974) 177. 1251 (a) K.A. Wiberg, Tetrahedron 24 (1968) 1083; (b) I. Mayer, Intern. J. Quantum Chem. 29 (1986) 73. [26] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1841. (271 (a) J.E. Bloor and Z.B. Maksic, Mol. Phys. 22 (1971) 351; (b) D.D. Shillady, F.P. Billingsley and J.E. Bloor, Theoret. Chem. Acta 21 (1971) 1; (c) J.E. Bloor and Z.B. Maksic, J. Chem. Phys. 57 (1972) 3572. [28] H.L. Yu, Phys. Rev. B 15 (1977) 3609.

450

J.A. Rodriguez,

C. T. Campbell / The chemisorption

of C, H,O and CO, on silver

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