J . Phys. Chem. 1988, 92, 2512-2516
2512
Theoretical Study of the Interaction of a Single Water Molecule with Pt(ll1) and Pt( 100) Clusters. Influence of the Applied Potential Guillermina Estiu, Sergio A. Maluendes, Eduardo A. Castro, and Alejandro J. Arvia* Instituto de Investigaciones Fisicoquimicas Teciricas y Aplicades (INIFTA),Facultad de Ciencias Exactas. Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina (Received: May 1 I , 1987)
A molecular orbital study of the decomposition of adsorbed water on single-crystal Pt( 100) and Pt( 1 11) faces has been performed
by using the extended Huckel theory. Metal surfaces are simulated with different Pt, clusters, and applied potentials are introduced by shifting the valence band of the clusters. The different surface reactivities are revealed through the activation energies for the decomposition of adsorbed water on the (100) and the (1 1 1) faces. The reaction appears to be favored for cathodic applied potentials.
Introduction The problem of water at polarizable and nonpolarizable metal/aqueous electrolyte solution interfaces is of the utmost importance in electrochemistry. In recent years encouraging attempts have been made to confluence surface science, electrochemical techniques, and molecular orbital theory in order to achieve an understanding of molecular processes at electrode surfaces.'-3 To solve the real problem of water at electrode surfaces by molecular orbital theory appears to be at present in insurmountable task. However, new interesting information can already be obtained by restricting the metal phase to a cluster of a relatively small number of atoms and the adsorbed phase to a single water molecule. The interaction of water molecules with single-crystal metal surfaces has been extensively studied from the~reticall-~ and experimental4-' standpoints. This interaction, which plays an important role in many electrochemical reactions, is still not fully understood. Such is the case, for instance, for H atom electroadsorption/electrodesorption,apparently the simplest electron-transfer process in aqueous electrochemistry. According to data resulting from fast relaxation electrochemical techniques,* water seems to be one of the constituents of the actual complex reacting ensemble, participating in the process; the configuration of this reactant is potential dependent. Certainly this possibility can be either supported or rejected on the basis of molecular orbital theory calculations. This is precisely the main objective of this paper, in which the possible interactions of a water molecule on different platinum surfaces are discussed. For this purpose semiempirical approach that provides information about structures and adsorption and binding energies for the decomposition reaction without charge transfer, (H,O),d (H)ad (OH)ad,over R(111) and Pt(100) surfaces is used. Accordingly there is a quite similar adsorptive behavior of both surfaces, in agreement with previous theoretical calculation^,^ but from the data presented here, it appears that the most densely packed (1 11) surfaces are less favorable than the (100) ones for the adsorbed water decomposition reaction. Different surface reactivities apparently come out from different activation energies for the decomposition of adsorbed water. The activation energy can be modified by changing electrode potential, for both surface structures, although in any
-
+
(1) Anderson, A. B. Surf. Sci. 1981, 105, 159. (2) Itoh, H.; Ertl, G.; Kunz, A. B. Z . Naturforsch., A: Phys.. Phys. Chem. Kosmophys. 1981, 36A, 347. (3) Anderson, A. B.; Ray, N. K. J . Phys. Chem. 1982, 86, 488. (4) Creighton, J. R.; White, J. M. Surf. Sci. 1984, 136, 449. (5) Nyberg, C.; TengstAl, C. G. J. Chem. Phys. 1984, 80, 3463 (6) Spitzer, W.; Liith, A. Surf. Sci. 1982, 120, 376. (7) Andersson, S.; Nyberg, C.; Tengstil, C. G. Chem. Phys. Lert. 1984, 104, 305. (8) Bilmes, S.; Giordano, M. C.; Arvia, A. .I.J . Electround. Chem. Interfacial Electrochem. 1987. 225, 183. (9) Minot, C.; Van Hove, M. A,; Somorjai, G. A. Surf. Sci. 1982, 127,
441.
0022-3654/88/2092-2512SOI SO10 , I
,
TABLE I: Parameters Used in Extended Hiickel Calculations orbital
Hii,eV
exptl 1
exptl 2
cl
c2
5d Pt 6s Pt 6p Pt ISH 2s 0 2p 0
-12.590
6.013 2.554 2.554 1.300 2.275 2.275
2.616
0.6334
0.5512
-10.000 -5.475 -13.600 -32.300 -14.800
case it appears that the decomposition of water is always favored for cathodic applied potentials, that is, by decreasing the ionization potential of the uncharged metal surface.
Theoretical Methods A cluster approach was used for modeling Pt( 100) and Pt( 11 1) surfaces. This approximation furnishes an acceptable description of surface phenomena'-l' because generally surface interactions are mainly dominated by local interactions. Cluster calculations have been carried out by using the extended Huckel method (EHM).l2,l3 The off-diagonal Hamiltonian matrix elements were derived from the diagonal terms using the weighted H, f0rmu1a.I~ Each cluster was geometrically constructed by maintaining a constant Pt-Pt bond length of 2.77 A. This value, which has been used in previous Pt, studies,' agrees with the Pt-Pt distance in the bulk metal.'* The Pt-0 bond length was set at 1.7 A for all the adsorption geometries. This theoretically predicted interatomic distance provides reliable results for the Pt-H,O interaction.' For higher coordination sites an increasing Pt-0 distance should be expected. However, as the results remained unchanged to slight lengthening, the aforementioned bond length was taken as a constant throughout the present calculations. Anyway, water coordination taking place in a hollow site implies a location of the 0 atom closer to the surface than when taking place in a top site. For a water molecule an O H bond length of 1 .O A and an angle of 104.5' are assumed. Parameters used in the calculations are those given in ref 10 and 11 and are summarized in Table I. Surface Resulting from the Metal Cluster Approach The cluster approach requires a clear definition of both the cluster size and boundary. There is agreement in the related literature about criteria for cluster size. Surface interactions have a localized character, (10) Bigot, B.; Minot, C. J . Am. Chem. SOC.1984, 106, 6601. (11) Minot, C.; Bigot, B.; Hariti, A. J . Am. Chem. Soc. 1986, 108, 196. (12) Hoffmann, R. J . Chem. Phys. 1963, 39, 1397. ( 1 3) Howell, J.; Ressi, A,; Wallace, D.; Haraki, K.; Hoffmann, R. QCPE, 1977, No. 344. (14) Ammeter, J. H.; Biirgi, H. B.; Thibeault, J. C.; Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 3686. (1 5 ) Handbook of Chemistry and Physics, 55th ed.; CRC Press: Cleveland, OH, 1973.
- 1988 American Chemical Societv
Water Interaction with Pt( 11 1) and Pt( 100) Clusters
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2513
-12
I
C.
Figure 1. Metal clusters used to model different Pt surface sites. Full lines represent the topmost surface layer and dotted lines the second metal layer; (a) cluster used to model a top site, a bridge site, and a (3-1) hollow site of a Pt(ll1) surface; (b) cluster used to model a (3-3) hollow site of Pt( 11 1); (c) cluster used to model a top site, bridge site, and a hollow site of a Pt(100) surface.
therefore only a few nearest-neighbor metal atoms influence the bonding of adsorbates, and the use of relatively small clusters can be justified. However, there is not a single criterion about how many atoms have to be included to describe a metal surface. In some cases a monolayer is built up with only a few atoms.' Although this could be questionable when one deals with the description of highly coordinated hollow sites, as no difference between (3-1) and (3-3) hollow sites for a (111) surface can be noticed in the absence of a second layer of atoms, it has been proved to be a good approximation for selecting the adsorption site and the adsorbatemetal bond length. Bilayers of metal atoms are more extensively ~ ~ e d ,but ~ it s appears ~ ~ - ~more ~ reasonable to select the cluster size for each particular adsorption system. Many models have been proposed to deal with the cluster boundary problem. Perhaps the most widely used approximation consists of embedding the cluster in hydrogen atoms,Ie2l but the large number of atoms involved turns out to be a computational disadvantage. Another approximation consists in describing the nonnearest atoms by their valence s electrons, but this approach only gives good results when adsorbate and substrate electronegativities are quite ~ i m i l a r . ~ For the purpose of the present work the size of the cluster has to be sufficiently large to have the periodicity of the entire metal surface and to describe an equivalent environment for each possible adsorption site. This means that none of the metal atoms directly involved in adsorption bondings should be affected by boundary effects. Due to the higher coordination number of the (1 11) surface, the corresponding cluster requires a number of atoms smaller than that for the cluster describing the (100) surface to avoid boundary effect. Thus, a PtI9cluster for the Pt( 11 1) surface and a Pt,, cluster for the (100) surface were chosen (Figure 1). Surface site-adsorbate interactions are considered by selecting surface sites that are, in principle, free of boundary effects. Once the cluster size has been set, it remains unchanged for all the calculations. This criterion is essentially similar to that earlier applied22for the study of the interaction of H with Fe(100), although in this (16) Gavezzotti, A,; Simonetta, M. Surf. Sci. 1980, 99, 453. (17) Ortoleva, E.;Simonetta, M. J . Mol. Struct. (THEOCHEM) 1978, 149, 161. (18) Andzelm. J. S u r f . Sci. 1981. 108. 561. (19j Caballo1,'R.; Igcal, J.; Rubio, J.'J. Mol. Strucr. (THEOCHEM) 1986. 136. 303. (20) Barone, V.;Lelj, F.; Iaconis, E.; Illas, F.; Russo, N.; Jounou, A. J . Mol. Struct. (THEOCHEM) 1986, 136, 313. (21) Barone, V.;Lelj, F.; Iaconis, E.; Illas, F.; Russo, N. J . Mol. Struct. (THEOCHEM) 1986, 139, 277. ( 2 2 ) Blyholder, G.; Head, J.; Ruette, F. Surf. Sci. 1983, 131, 403.
case it appears that Felz clusters are not sufficiently large to account for the same environment for each adsorption site. The following step is to select the energetically favored adsorption sites for each adsorbate taking part in the surface reaction.
Interaction of a Water Molecule with Different P t ( l l 1 ) and Pt( 100) Adsorption Sites So that the energetically most favorable adsorption configurations for a single water molecule and its decomposition products on the metal surface can be calculated, three different adsorption sites on a Pt(100) surface are considered (Figure IC), namely onefold (on top), twofold (bridge), and higher coordinated (hollow) sites. Similarly, four types of sites on a Pt( 11 1) surface can be distinguished, as depending on the relative position of the second layer two different hollow sites can be defined (Figure 1, parts a and b). When the water molecule interacts with different adsorption sites on a Pt surface, the HOH plane always remains perpendicular to the single-crystal surface, with the 0 atom end coupled to the metal atom and the H atoms symmetrically placed away. Calculated binding energies for both Pt(100) and Pt( l l l ) (Table 11) indicate that water bonds on a onefold site. These results agree with previous theoretical calculations,'s2 and the geometry for the adsorbed state is supported by experimental data.23*24The present results also show that water molecule upper-energy levels are very near to the energy level of the bottom of the s-d band of Pt( 111) (Figure 2 ) . This fact produces the stabilization of the adsorbed system as previously pointed out in the literature.] The metal acts mainly a s a n acceptor and water donor through t h e nonbonding orbitals. As seen from Table I1 and Figure 2, the top site is favored because it allows a better overlapping between the metal band and directional water orbitals like 3a,. Likewise, as depicted in Figure 2 , the lowest unoccupied molecular orbital of the naked cluster turns out to be the highest occupied one after water adsorption (Figure 2). The atomic orbital population indicates a (23) Ibach, H.; Lehwald, S . Surf.Sci. 1980, 91, 187. (24) Sexton, B. Surf.Sci. 1980, 94, 435.
Estiu et al.
2514 The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 TABLE I11 Changes in Energy for Different Steps of Water Decomposition Reaction' Pt(ll1) H20
+
applied potential, eV energy change, eV adsorbed water intermediate state I b transition stateC intermediate state 2d final statee
Pt(100)
+ H2O
-0.5
0.0
+0.5
-0.5
0.0
+0.5
0.1962 -0.1 114 -1.0225 -0.6660 2.2860
0.4181 0.0120 -0.9962 -0.8848 1.9724
0.6099 0.1957 -0.8747 -0.9448 1.7901
0.3206 0.0704 -0.6714
0.5739 0.2642 -0.5936
0.7910 0.46 12 -0.4 760
1.7997
1.6000
1.4683
'Initial state: Infinite distance between water and metal surface. Values are given for different surface charging. bOH bond parallel to the metal surface. E O Hbond (parallel to the metal surface) stretched 0.4 A. d O H bond (parallel to the metal surface) stretched 0.6 A. CDissociatedwater; H and OH in the en&etically favorable adsorption sites.
+
E le" 1
-
-:+
-
AEleVI
I
-''I +t
T
I -
-121
301
-...... -
