10229
J. Phys. Chem. 1994,98, 10229-10236
Oxygen Adsorption on Potassium Yidong Shao and Jan A. K. Paul* KTH/Royal Institute of Technology, Physics III, $100 44 Stockholm, Sweden Received: May 9, 1994; In Final Form: June 20, 1994@
Quantum chemical calculations of oxygen adsorption on potassium are presented. The surface was modeled by clusters of six atoms and the electronic structure evaluated in the Hartree-Fock and Hartree-FockSlater schemes. Adsorbed intermediates were identified by comparisons between calculated data and spectroscopic information, mainly from photoemission and vibrational spectroscopies. Low doses of oxygen at cryogenic temperatures lead to the formation of surface 0- and higher doses to subsurface 0 2 - . The 0 2 species dissociate to bulk 0- upon gentle annealing. Further annealing initiates the formation of a threedimensional oxide. Lattice relaxation is critical to the understanding of atomic adsorption as well as molecular orientation.
1. Introduction
Bulk
Oxygen adsorption on alkali metals and the subsequent formation of a bulk oxide are of interest as a model system for oxidation. The simplicity of free-electron metals and the range of computational methods developed for the surfaces of such metals make it an ideal test ground for theoretical models.'S2 Adsorption on an alkali metal operates via an apparently simple mechanism which still embodies the essentials of the interaction at more technologically important surface^.^ A uniform background model has to be revised, since the considerable energy released by the alkali-oxygen interaction makes 3D compound formation inevitable and the issue of lattice distortion an obvious one. Alkali metals are not unique in this respect but follow the pattern of other easily oxidized and mechanically soft metals. Lattice distortion during the penetration of the first atomic layer is a crucial step in bulk ~ x i d a t i o n .The ~ short range of any perturbation of the electron gas and the necessity to include discrete atoms increase the chances that alternative theoretical approaches may become ~ompetitive.~ Oxidation of metallic surfaces is important for many technological applications such as corrosion inhibition and adhesion. The importance for catalysis is limited to a few cases where oxidized metal supports are impregnated. One example is steel coated with alumina as catalytic converters for the automotive industry. Modifications of catalysts by alkali metals are all but rare, but these additives are commonly deposited from an aqueous solution and oxidized at high temperatures to remove water, ligands, or counterions. The reactants of the subsequent catalytic process cannot be expected to reduce the alkali metal oxide to a metallic state. Characterization of deposited oxides with respect to dispersion and the formation of bimetallic compounds is still important groundwork for catalysk6 The present work addresses oxygen adsorption on potassium. We do this by calculating the electronic structure of cluster models by different quantum chemical schemes. Previous work has used fi-ee-electronmodels and the Kohn-Sham method. Our approach is to identify surface intermediatesby comparisons between calculated results and spectroscopic data. We calculate vibrational intensities and frequencies, ionization potentials, and total energies for a set of potassium-oxygen clusters with characteristic geometries. Thereafter we identify intermediates by least-square fits to experimental data and proceed to discuss
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* Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, August 1, 1994.
0022-365419412098-10229$04.50/0
reaction coordinates and, to some extent, compound formation on alkali-modified surfaces.
2. Computational Methods The potassium surface was represented by medium-sized clusters of atoms (Figure 1). Adsorption at open sites was modeled by a six-atom cluster centered around a bcc(100) fourfold hollow site. We depict this cluster as 'bulk'. Terminal 0 1994 American Chemical Society
10230 J. Phys. Chem., Vol. 98, No. 40, 1994
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Figure 3. Calculated ionization potentials (a, top) and vibrational spectra (b, bottom) for atomic oxygen at terminal surface coordination or peneuating into the bulk at an open site. The bulk data are calculated either with a rigid lanice or with lattice relaxation. The inset shows from left to right cuts through the surface, the rigid bulk, and the flexible bulk clusters. These geometries represent energy minima.
Figure 4. Calculated ionization potentials (a, top) and vibrational spectra (b, bottom) for molecular oxygen at terminal surface wordination. The inset shows from left to right cuts through surface clusters with 02 bound in horizontal, tilted, and vertical orientations. These geometries represent energy minima. Bond lengths are given in Table 1.
coordination was modeled by another six-atom cluster from the bcc(100) surface, this time with one atom in the center surrounded by four atoms in the plane and one atom in a second plane. This cluster is named ‘surface’. The above two clusters illustrate typical coordination at the surface and in the bulk and should not be seen exclusively as representing atop and fourfold hollow adsorption at K(100). Bulk potassium oxides obey well-defined crystalline smctnres with long range order. To model these smchues is beyond the scope of this paper. We model the first crucial steps in oxidation and surface penetration. An improved model can be achieved by embedding procedures, but our problem lies at the boundiuy between the regimes of isolated adsorbates, where a free-electron potential could be used around the cluster and the ionic salt, for which a Madelung potential should be added. A screened Madelung potential, where the screening is represented by the remaining free-electron density, offers one way to improve the present treatment. Electron loss measurements would provide the necessary information to generate this potential, but we have found no experimental data which give both ‘one-electron’ binding energies and plasmon loss energies. As a consequence we have chosen not to apply any embedding procedure. The starting geometry of all calculations was given by the K-K distance of the ordered crystal. This geometry was kept
constant for all ‘rigid‘ clusters. In a second series of calculations the K-K distances of ‘flexible’ clusters were allowed to adapt, first to the forces of the empty cluster and later to any given configurationof adsorbate atoms. The empty potassium clusters, either ‘rigid‘ or ‘flexible’, plus free molecular oxygen served as a reference point for adsorption energies. All calculations for ‘bulk‘ clusters were doubled with both a rigid and a flexible lattice. The potential energies of the flexible cluster still neglect the energy required to move the substrate atoms in a surrounding lattice. The issue of substrate flexibility is very relevant for oxygen adsorption, since the strong potassium-oxygen interaction leads to the formation of a 3D compound, the oxide. Penetration of the first atomic layer is an important step in this process. No flexible clusters were used for ‘surface’ coordination. Lattice relaxation is linked to penetration into the bulk and less crucial for atop adsorption. Convergence with cluster size has been tested, both with smaller clusters and, in some cases, with a more extended model. We have found that the relative differences between different surface s t ~ c t u r e are s well represented already by a potassium dimer. Multiply coordinated species are affected by an additional potassium atom atop the ‘bulk‘ cluster, thus modeling the interior, but this represents a later stage in the oxidation. We model atomic as well as molecular oxygen adsorption on the above clusters. No restrictions of K-0 and 0-0
J. Phys. Chem., Vol. 98, No. 40, 1994 10231
Oxygen Adsorption on Potassium distances were set, but certain symmetry restrictions apply. These restrictions are illustrated in Figure 1. Atomic adsorption was modeled for the ‘surface’ cluster and for both the rigid and the flexible ‘bulk’ clusters. The atom was in all cases restricted to a position along the cluster symmetry axis. Molecular adsorption at the surface was modeled in vertical, tilted, and horizontal orientations, and bulk coordination was modeled in bridged, horizontal, and vertical positions (Figure 1). These geometries represent all high-symmetry Orientations for which penetration can be expected. The bridged orientation is an azimuthal position bridging two potassium, atoms, and the horizontal orientation is 45” rotated with respect to the bridged position. Finally, the vertical orientation assumes that the molecular axis lies along the cluster symmetry line. The geometry was optimized in a commercially available Hartree-Fock computational scheme using a modest STO-3G basis set, the only one available for potassium atoms.7 This step provided the first input to our evaluation of adsorption geometries. In a second step we calculate ionization potentials (IP’s) for each adsorbate in a Hartree-Fock-Slater (HFS) LCAO discrete variational method (DVM) scheme, with numerical basis functions and the X a exchange and correlation potential, and compare these data with measured valence band photoemission data.8 The comparison allows for a constant rigid shift throughout all clusters and is thus sensitive to orbital separations rather than to an absolute match between calculated and measured peak positions. Orbital energies are broadened by a Gaussian profile, with the peak height proportional to the projected occupation on the adsorbate. The statistical l/2 electron approach of the HFS method has proven to be very useful for ionization potential calculations.8 This code uses numerical basis functions, but the pointwise integration is not accurate enough for total energies.8 In the third and final step we calculate vibrational energies and intensities analytically, again in a Hartree-Fock scheme, and compare these data with vibrational ~ p e c t r a . ~Extended basis sets and polarization functions have been tested for certain clusters, but we found that the characteristic differences between different adsorption geometries are maintained, though the numerical values of e.g. vibrational frequencies will shift less than 50 cm-’, when introducing more elaborate methods. We consider it to be a safe identification if all three steps point at the same geometrical arrangement, and we hope that this approach will deter us from making conclusions which are method dependent. We display all polarizations of vibrational lines without restrictions set by ‘surface selection rules’ for dipole excitations. The motivation is not a neglect of physics but an illustration of the fact that we recognize our clusters to be examples of local atomic coordination. The clusters themselves can be oriented in any way with respect to the macroscopic surface normal. We also understand that more advanced computational schemes will add to the accuracy of the calculations, but our aim with the present work is merely to determine trends and point at characteristic differences between well-defined adsorption geometries. The converged geometries of each cluster are shown in insets in Figures 2-7. In the insets we have chosen to use atomic radii defined by touching spheres for potassium and the ionic radius for oxygen. This procedure reveals the crowded conditions in the bulk. We have also removed one or two potassium atoms to show the geometry of the adsorbate. Both smaller clusters and more extended ones have been tested. Trends in adsorption energies, ionization potentials, and vibrational frequencies are maintained independent of cluster size; however, we do acknowledge the need to use extended
clusters to mimic the free-electron character of potassium. Physically the use of limited cluster sizes can be justified by the rapid screening of the perturbation induced by a chemisorbed species.
3. Results 3.1. Atomic Oxygen. Figure 2 shows that the adsorption energy of atomic oxygen varies considerably with cluster geometry. Penetration into the bulk in combination with lattice distortion is favorable and gives a more stable system than any configuration of molecularly adsorbed oxygen. Figure 3 gives ionization potentials and vibrational spectra for atomic oxygen in three potassium clusters. The geometrical arrangements of the converged clusters are shown in an inset. The large peak close to the Fermi level corresponds to the 0 2p orbital, and the smaller peak, to the 0 2s orbital. We observe that penetration into the bulk shifts both orbitals to larger binding energies in agreement with the results from uniform background modelsG4Lattice distortion will only have a minor additional influence on orbital energies, hardly observable in experimental photoemission (Figure 3a), but a considerable effect on vibrational energies (Figure 3b). The frequency shift between surface oxygen and oxygen in a rigid bulk lattice is not significant, given the level of approximation in the calculations, but the intensity shift is dramatic. This reflects the very effective screening inside the image plane. The inset shows a considerable relaxation of the lattice as a result of K-0 interaction. Matrix isolation provides complementary reference data for identification of surface intermediates. Such data give a main vibrational line at 384 cm-’ for KO and 502 cm-l for K20,9 to be compared with the calculated frequencies of Figure 3b. 3.2. Molecular Oxygen. Figure 2 shows that surface coordinated dioxygen is less stable than 02 bound in the bulk. Again a flexible bulk lattice is advantageous and the vertical and horizontal orientations are more favorable than the bridged coordination. 3.2.1. Surface Adsorption. The ionization potentials of surface coordinated dioxygen are not very sensitive to the orientation of the molecule (Figure 4a). In contrast to the IP’s the vibrational spectra can hardly be recognized as originating from the same molecule in three different orientations (Figure 4b). If we compare with Figure 3b, we again recognize the very strong intensity observed for species adsorbed on the outside of the image plane. 3.2.2. Bulk Coordination. Figure 5 -7 show calculated IF”s and vibrational spectra for three orientations of molecular oxygen in bulk potassium. Spectra are displayed for both rigid and flexible lattices. The set of peaks is obviously similar for all clusters, but the orbital energies do reflect the local coordination. A general trend is that lattice relaxation moves the highest molecular orbital away from the Fermi level. This downward shift is not rigid across all levels, which means that orbital separation will be a valid tool for identification of adsorbed intermediates. All vibrational intensities of 0 2 in bulk potassium will be weak. The cluster approach may be a poor starting point for calculations of dynamic properties, but we again state that our aim is not to calculate oxygen species adsorbed in or on a semiinfinite metal. Instead we try to illuminate the effects of the local geometry. With these restrictions in mind we can observe that the vibrational energies of multiply coordinated dioxygen can vary considerably. The insets show large lattice flexibility for bridged and vertical orientations, but surprisingly enough, the vibrational spectrum of horizontal oxygen is most affected by relaxation.
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10232 J. Phys. Chem. Vol. 98, No. 40, 1994
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Figure 5. Calculated ionization potentials (a. top) and vibrational spectra (b, bottom) for molecular oxygen in bulk coordination with the molecular axis bridging two potassium atoms. The inset shows 0 2 bound in a bridged orientation in a rigid (left) and flexible (right) cluster. Bond lengths are given in Table 1.
Matrix isolation identifies two coordination compounds of dioxygen and potassium.I0 KO2 gives a main line at 1108 cm-l and a smaller band at 308 cm-l and (K02)2hands at 1098 and 1104 cm-l. We observe that these frequencies are similar to those of the 'horizontal' cluster. 3.3. 0 1s Binding Energies. Oxygen 1s binding energies are displayed in Figure 8. Atomic oxygen gives the lowest biudiug energies. We also note the large range between surfacecoordinated dioxygen and 0 2 hound in a distorted lattice. This merely reflects the charge transfer. Table 1 presents a wellknown correlation between ionisity, vibrational frequency, and bond lengthLLas well as our own calculated 0-0 distances, and the charge transfer and the bond length are reflected in the vibrational frequency and also in the 0 1s binding energy. In fact, a reasonable estimate of the force constant or the core level position can be made from the occupation of the antibonding x-orbital or the charge of the molecule. Table 1 indicates that vw is an excellent fingerprint of the presence and state of adsorbed molecular oxygen. Unfortunately experimental data are not readily available because dipole excitations inside the image potential are screened by the electron gas.
4. Discussion 4.1. Experimental Data: Identification of Intermediates.
Our calculations model the adsorption of isolated species on
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Figure 6. Calculated ionization potentials (a, top) and vibrational spectra (b. bottom), for molecular oxygen in bulk coordination with
the molecular axis horizontally positioned bemeen potassium atoms. The inset shows O2bound in a horizontal orientation in a rigid (left) and flexible (right) cluster. Bond lengths are given in Table 1. pure potassium, and the experimental data chosen for comparisons have to he screened accordingly. Potassium are almost exclusively prepared by evaporation. The large mobility makes it crucial to quench the condensate to prevent coalescence and inhomogeneous dispersion in the thin films. Alkali-promoted oxidation of an underlying substrate is not representative of bulk potassium. We have carefully examined published data, and only spectra which are truly representative of the surface of bulk potassium are ~ e l e c t e d . ' ~ -We ~ ~ have also screened the published spectra for contaminants, which would indicate the formation of hydroxides or carbonates, two commonly found artifacts in studies of 'dry'oxidation. Quenched overlayers are polycrystalline, which again underlines that our clusters represent characteristic local coordination and not exclusively the K(100) surface. The overlayers are not randomly oriented or amorphous but arranged as arrays of well-defined planes with distinct kinematic LEED I-V peaks, provided that the underlying substrate is smooth and the temperature kept below ca. 150 K?6 Condensed layers of dioxygen exposed to potassium vapor constitute a separate class of data which may he used for reference, although the validity of the surface chemistry for the present problem may he hard to judge. Oxygen reactive adsorption on potassium involves atomic and molecular entities, and both singly or doubly charged species have been discussed.L2-25Condensed overlayers of neutral 0 2 will not be considered, since these structures are found
Oxygen Adsorption on Potassium
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Atomic 0 in flexible bulk
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Figure 9. Comparison between calculated ionization potentials and experimental photoemission spectra for atomic oxygen. The calculated curve comes from one atom bound at a bulk site in a flexible lattice. The experimental data show initial low exposures at cryogenic temperaNre and after annealing."
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Figure 7. Calculated ionization potentials (a, top) and vibrational spectra (b, bottom) for molecular oxygen in bulk cwrdination with the molecular axis vertically positioned midway between metal atoms. The inset shows 0 2 b u n d in a vertical orientation in a rigid (left) and flexible (right) cluster. Bond lengths are given in Table 1.
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Figure 8. Relative 0 Is binding energies for all clusters. A lower binding energy means an orbital closer to the Fermi level. Most clusters with bound dioxygen are charactenzed by one single 0 Is peak. The bound in a vertical orientation in a rigid potassium only exception is 0%
cluster exclusively at cryogenic temperatures well below 77 K. Additional complexity comes from 3D rearrangements of atomic oxygen into potassium monoxide (KzO),peroxide (KzO)?, superoxide (KO?), or trioxide (KZO~).~' Ionization potentials are not readily available for all of these oxides. Nevertheless, experimental data show that two factors alone, temperature and oxygen coverage or activity, govem the state of potassium-
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Figure 10. Comparison between calculated vibrational spectra and experimental electron energy loss spectra for initial low doses at cryogenic temperature.lS The calculated data come fmm an oxygen atom adsorbed at a terminal surface site and from molecular oxygen adsorbed in a bidentate position.
adsorbed oxygen. The problem is that neither factor readily can be transferred from one experimental chamber in one laboratory to a different setup in a different lab, but we will try to divide our search for intermediates accordingly. 4.1.1. Low Exposures at Low Temperature. Low doses of oxygen at 77-100 K give a dominant valence band peak 2.7 eV below the F e d level and some ill-defined structure at higher binding energies (Figure 9 and refs 12-14). Core level spectra show a double-peak smcture with a dominant band at 528-529 eV and a second hand at 531-532 eV.12J4.15This shows that two species are present. The 528 eV peak is associated with the dominant valence band peak, and both these featnres reveal atomic adsorption. Figures 3a and 8 show that neither the 0 1s core level nor the 0 2p position are very sensitive monitors of the coordination of the atomic species. The separation between the 0 2s and 0 2p structures could reveal some information, but an unfortunate overlap with the K 3p peak limits further use of this tool. Adsorption energies obviously favor bulk coordination (Figure 2). Low doses of oxygen give one dominant vibrational peak at 250-280 cm-I for an Auger W O ratio of l:l.16 The 1:l ratio is merely a relative measure of oxygen concentration. The lowfrequency points toward surface coordination (Figure 10).
10234 J. Phys. Chem., Vol. 98, No. 40, 1994
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TABLE 1: 0-0 Bond Lengths and Stretch Frequencies for Potassium-Bound Dioxygen cluster 0-0 bond length (8) YOO (cm-') free 1.22 1555 horizontal, surface 1.22 1546 tilted, surface 1.22 1548 vertical, surface 1.22 1523 horizontal in K, rigid 1.46 883 horizontal in K, flexible 1.44 1042 bridged in K, rigid 1.22 1552 bridged in K, flexible 1.40 1150 vertical in K, rigid 1.25 1381 vertical in K, flexible 1.49 979 neutral free molecule" 1.21 1580 singly valent free ion" 1.33 1097 doubly valent free ion" 1.49 802 Surface species also give more intense vibrational bands than bulk species, since screening increases the surface sensitivity of vibrational spectroscopies based on dipole scattering. The best agreement is found for surface-coordinated horizontal 0 2 , synonymous with bidentate coordination. This assignment also gives a rational explanation for the 0 1s band at 531-532 eV and the ill-defined structure in valence band photoemission spectra. A best fit assignment thus shows that low exposures at low temperatures lead to bulk atomic oxygen in combination with surface-bound molecules. This assignment is further supported by annealing sequences discussed below and by data for adsorption on an ordered monolayer. The molecular surface species is only stable at low temperatures. 4.1.2. High Exposures at Low Temperature. The illdefined structure below the 0 2p peak develops upon further exposure into a spectrum with multiple peaks (Figure 11 and refs 12- 14,17- 19). This spectrum is a fingerprint of molecular adsorption. Parallel to this increase, the 532 eV peak becomes the dominant core level structure. A 3-4 eV shift compared with the atomic adsorption peak at 528-529 eV can only be explained by molecular adsorption (Figure 8). The multiple peaks in the valence band are assigned and the positions compared with calculated energies of molecular oxygen in Figure 11. The flexible cluster with a horizontally oriented 0 2 , axis in bulk potassium gives the best agreement with experimental data.
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Figure 12. Comparison between calculated vibrational spectra and experimental electron energy loss spectra.ls Similar spectra are observed for high doses without annealing and low doses annealed to 240 K. The calculated data come from 0- and 0 2 - , both adsorbed at bulk sites in a flexible lattice. The separation of the zU2p and og2p peaks is more pronounced in the horizontal and bridged orientations than either in the vertical bulk geometry or for surface coordination (Figures 4-7). The adsorption energies favor a flexible lattice (Figure 2). We acknowledge that our calculations only model a limited set of idealized coordinations, but the best fit procedure hints at the flexible horizontal geometry as a realistic one (Figure 1). The net molecular charge is -(0.66 - 0.82), which we identify as belonging to a superoxide 0 2 - species. The superoxide assignment is in complete harmony with the position of unoccupied bands as probed by recent experiments.20,21 The new structure in the valence band and the 0 1s binding energy of 532 eV12J4,15are associated with a dominant vibrational band at 330-350 cm-1.15916 The vibrational spectra are somewhat puzzling because high doses at low temperature, which evidently result in molecular adsorption, gives the same vibrational energy as annealing of low doses. Photoemission spectroscopy clearly shows that annealing leads to dissociation. One solution to the above riddle has already been discussed; Le., vibrational spectroscopy probes over species than photoemission spectroscopy. Evidently the surface is covered by a mixture of atomic and molecular intermediatesprior to annealing and increased doses tilts the balance in favor of molecularly bound species. Our calculations offer another possible solution (Figure 12). The vibrational fingerprints of bulk coordinated oxygen, atomic or molecular, may simply overlap, since both give vibrational structure around 350 cm-l. The 350 cm-l peaks are not seen at low exposures and low temperatures simply because surface-bound species give much more intense vibrational bands in dipole scattering. To summarize, high doses of oxygen on potassium at cryogenic temperatures lead to subsurface molecular oxygen in combination with subsurface atomic oxygen. 4.1.3. Annealing. Annealing of molecularly adsorbed oxygen to around 200 K shifts the dominant 0 1s level from 532 to 528 eV,12J5typical for atomic absorption. The multiplepeak valence band structure is suppressed in favor of a single peak closer to the Fermi level (Figure 9 and refs 12-14). This is undisputable evidence of dissociation. A weak structure also appears at Ef - E = 13.5 eV, Le. above the K 3p structure. This could be an image of the 0 2s level and indicate a more narrow separation between the 0 2p and 0 2s orbitals compared with low exposures at low temperatures (Figure 9). In our calculation such an effect is observed when the lattice relaxes (Figure 3a). The 0 2s level is still partly obstructed by the K
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Oxygen Adsorption on Potassium '- I
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Figure 13. Potential energy surfaces for oxygen adsorption on rigid and flexible potassium clusters. Atomic adsorption is indicated by solid (rigid cluster) and dashed (flexible) lines. Molecular adsorption energy minima are given by filled (rigid) and open (flexible) squares. The marks for vertical 02 indicate the position of the innermost atom. The arrows mark the shift accompanying lattice relaxation.
3p level, and adsorption on other alkali metals gives a more well-defined peak.22 As discussed in the previous section, annealing also shifts the dominant vibrational band of low exposures from 250280 to 330 cm-', which agrees with atomic oxygen in a flexible bulk l a t t i ~ e . ' ~ ,There ' ~ is no evidence that molecularly bound oxygen survives annealing, and consequently there is nothing left of the previous ambiguity about the origin of the vibrational band. The vibrational band comes from atomic oxygen in a flexible lattice. Our different computational schemes give an excess electron population of 0.31-0.56 on oxygen at this site, and we identify the adsorbate as an 0- ion. A population analysis is different from the formal ionic bonding, but the above figures indicate the formation of singly valent species. Our total energy calculations also speak in a favor of a bulk coordination with lattice relaxation (Figure 2). Further annealing alters the valence orbital, indicating a large rearrangement of atoms and the formation of a three-dimensional structure of potassium oxide. Three-dimensional structures are beyond the scope of our calculations and will not be discussed further. 4.2. Reaction Paths: Importance of Lattice Relaxation. Lattice relaxation is crucial for all stages of oxygen adsorption on potassium. Figure 13 shows potential energy surfaces for adsorption of atomic oxygen on 'bulk' sites on rigid and flexible lattices and also energy minima for molecular oxygen in two orientations. Eventually, these calculations mimic penetration of the interior of an isolated cluster and neglect the distortion of the surrounding lattice in a bulk material, but some general aspects can still be derived. Neither atomic nor molecular adsorption shows any significant penetration barrier at the first layer of metal atoms. A more densely packed surface resists distortion more vigor~usly.~ The extreme is adsorption at a terminal site, which of course is impregnable. In the present case lattice mobility will always give a shorter K-0 distance and always increase the charge transfer, thereby affecting the vibrational frequency and the 0 1s core level position. We have no evidence that directional bonding plays a major role for chemisorption, either associative or dissociative. On the contrary, modeling based solely on the electron density catches the essentials of the interaction between oxygen and a potassium metal. 4.3. Implications for Alkali Modified Surfaces. The data of Figures 3-8 can be used to identify potassium-oxygen
compound formation on alkali-promoted surfaces, more so if data exist for several spectroscopies. Below we demonstrate this procedure for oxygen adsorbed on Ru(0001)/K(d3 x d3 - R300).23-25 Low doses of oxygen at cryogenic temperatures result in a single 0 1s peak at 531 eV and a multiple-peak valence band s p e ~ t r u m . ~Both ~ - ~features ~ agree with molecular adsorption. This species shows an intense vibrational band at 240 cm-1.23,25 The previous discussion and Figures 4 and 10 indicate local bidentate coordination atop potassium. The orientation of this complex with respect to the surface normal obviously will have to consider selection rules for dipole excitation. Compared with the case of bulk potassium, the absence of a second layer of potassium atoms suppresses subsurface atomic oxygen and hinders dissociation. Both higher coverage and annealing lead to oxygen-ruthenium interaction.
5. Conclusions Quantum chemical calculations on cluster models of oxygen adsorption on potassium have been performed. On the basis of comparisons between our theoretical predictions for different adsorption geometries and experimental data, we arrive at the following steps for the initial steps for oxidation. Oxygen adsorbs both dissociatively and molecularly on potassium at cryogenic temperatures and small doses. The atoms occupy subsurface sites, and the molecules, bidentate surface sites. Spectroscopically the atoms are identified by a single 0 2p peak 2.7 eV below the Fermi level and a 0 1s binding energy of 528 eV, and the molecules, by a vibrational band at around 250 cm-'. Increased doses lead to subsurface molecular adsorption in a flexible lattice. The bound species are characterized as a superoxide 0 2 - . The molecular species are identified by multiple peaks in the valence band below the 0 2p peak of atomic oxygen, a 0 1s peak at 532 eV, and a vibrational band around 330 cm-'. The adsorbed molecules dissociate upon annealing to around 200 K. The result is singly valent 0- ions coordinated in a distorted potassium lattice. These species are again characterized by a strong 0 2p peak but also a weak 0 2 structure. The 0 1s binding energy is 528 eV, and the dominant vibrational band, again at 350 cm-'. This work clearly demonstrates that the procedure for the identification of oxygen intermediates preferably should start with functional groups from photoemission spectra. The coordination of these groups, once identified, can in a subsequent step be derived from vibrational spectra. Core level energies provide a valuable way to verify the identification process.
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