- Email: [email protected]
Microscopic model of CO oxidation on Pt(1 1 1)
Surface Science 600 (2006) 2600–2607 www.elsevier.com/locate/susc
Microscopic model of CO oxidation on Pt(1 1 1) I.N. Yakovkin *, N.V. Petrova Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki 46, Kiev 03028, Ukraine Received 30 January 2006; accepted for publication 19 April 2006 Available online 12 May 2006
Abstract A new microscopic model, based on DFT/LDA modeling, is suggested for the Langmuir–Hinshelwood reaction of catalytic CO oxidation in coadsorbed O–CO layers on Pt(1 1 1). It has been found that only the oxygen atoms occupying threefold hollow sites of hcp type are chemically active. The potential barrier for the oxidation reaction significantly decreases due to changes in the adlayer oxygen states in the proximity to CO. The oxygen electronic density distribution is affected by approaching CO molecule which alters the oxygen position. Height of the barrier is estimated as 1.15 eV, which may be attributed to the upper limit of activation energy for the net reaction process. 2006 Elsevier B.V. All rights reserved. Keywords: Density functional calculations; Modeling; Catalytic oxidation; Platinum surface; Oxygen; Carbon monoxide
1. Introduction The surface catalytic reaction of CO oxidation on Pt surfaces is central to controlling CO emissions, and continues to demand greater understanding. Despite a great number of theoretical and experimental investigations, certain key features, required for a clear picture of the reaction on microscopic level, are still missing. In the present article we address these issues by modeling (on the DFT-LDA level) of the reaction path for the case of the reaction in the coadsorbed CO–O p(2 · 2) layer on Pt(1 1 1). Oxygen atoms on transition metal surfaces tend to occupy adsorption sites of the highest available coordination, which in the case of Pt(1 1 1) correspond to threefold sites [1]. At h = 0.25 (coverage here is defined with regard to the surface atomic concentration of the Pt(1 1 1)), oxygen films have a p(2 · 2) structure [2]. This structure is very stable and can be observed at temperatures up to 350 K [2]. The h = 0.25 is a saturation oxygen coverage, which means that further oxygen adsorption is suppressed. Nonetheless, adsorption of CO still can take place, probably, due to *
Corresponding author. E-mail address: [email protected] (I.N. Yakovkin).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.04.029
occupation of sites of different type – at low coverages, CO molecules occupy atop sites both when adsorbed on a clean Pt(1 1 1) and on the oxygen-covered surface [3,4]. The p(2 · 2) structure of the coadsorbed O–CO layer (which is observed at T < 300 K [3]) with hCO = hO = 0.25 is dictated by oxygen adatoms in the threefold hollow sites, while CO molecules occupy on-top adsorption sites in the oxygen cells [5]. The Langmuir–Hinshelwood reaction of CO oxidation on Pt(1 1 1) requires some 0.5–1.1 eV activation energy (as estimated from DFT simulations of pathways of the particles in the course of CO oxidation [6,7] and from experiments on the reaction kinetics [6,8–10]). This dictates therefore that the reaction occurs at temperatures in the range from 220 to 300 K (or higher), depending of the film structure [3]. The barrier for the oxidation reaction probably originates not from a direct repulsion between CO molecules and oxygen atoms, but, rather, from the strong bonding of the oxygen atoms with the surface [6]. For oxidation of a CO molecule, the oxygen atom should be activated, which can be achieved by moving the atom into an ‘‘active’’ site [1,3], however, conditions for such activation remain speculative. There is also some controversy with regard to the
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
2601
the reaction, because other considered configurations led to enormously high potential barriers inconsistent with activation energy derived from experiment. However, these simulations were performed using artificial constraints such as fixed distance between the C atom in CO molecule and O(1) adatom in the course of structural optimization at each reaction step [6,7]. As we show in the present article, another choice of the reaction coordinate allows for a more realistic description of the reaction steps and suggests that only those oxygen adatoms that are promoted to the hcp hollow sites on the Pt(1 1 1) surface can react with CO molecules. Fig. 1. The p(2 · 2) unit cell adopted in present simulations of CO oxidation. The threefold fcc and hcp sites on the Pt(1 1 1) surface are indicated.
course of reaction, which is reported to start at the boundaries of the ordered p(2 · 2) oxygen domains [11–13], but, on the other hand, can be observed over the net surface covered with the coadsorbed O–CO films [14]. This feature reveals an important role of mobility of oxygen atoms, which provides the movement of oxygen atoms to ‘‘active sites’’ where they become chemically active with regard to CO oxidation [1]. There are two types of the threefold hollow sites on Pt(1 1 1), favorable for oxygen adatom bonding, and these are the so-called fcc and hcp sites (Fig. 1). Other sites, such as bridge and atop, are strongly unfavorable (the barrier for diffusion of an oxygen adatom from the fcc hollow to bridge site is of 0.6–0.7 eV [1,15,16] ; atop position is unstable and the barrier is about 1.2 eV [1]), and therefore at low temperatures all oxygen adatoms will occupy the fcc sites which correspond to the lowest total energy (or, in other words, to the highest binding energy). It has been found that difference of binding energies for O adatoms in the fcc and hcp sites is surprisingly large (up to 0.47 eV [1]), as estimated through DFT calculations [1,16,17]. It was suggested [18] that the threefold hcp sites could serve as active sites in the oxidation reaction of CO. Due to thermal fluctuations, oxygen atoms can move from fcc into hcp sites and, thus, become chemically active. As follows from Monte Carlo simulations performed in the framework of this model [18], it is the enhanced mobility of oxygen atoms at the oxygen adlayer p(2 · 2) domain boundaries that results in CO oxidation just within this region, thus explaining propagation of the reaction front at temperatures about 200 K (obviously, the transition from fcc to hcp sites is favored for oxygen atoms just in vicinity of the boundaries of the domains). The issue of active sites, available for oxygen adatom, was studied in Ref. [1] in some detail. The authors [1] suggested that the reactivity of adsorbed O with regard to CO oxidation on Pt(1 1 1) surface increases accordingly to sequence fcc–hcp–bridge–atop of adsorption sites. The in-bridge site was suggested also from DFT/GGA simulations of the reaction course [6] to be the only relevant for
2. Method of calculations and control results The DFT/LDA semirelativistic calculations within a supercell (repeat-slab) approach using norm-conserving Troullier–Martins pseudopotentials [19,20] with exchangecorrelation part in PW92-CA form [21] were carried out using the ‘‘cold’’ Broyden–Fletcher–Goldfarb–Shanno minimization scheme as implemented in ABINIT code [22]. The norm-conserving pseudopotential, though providing very accurate approximation of wave functions, are known as rather ‘‘hard’’, that is, require many plane waves (which means a high energy cutoff) for good convergence of total energy. The test calculations have shown that 20 hartree (40 Ry) cutoff provides the total energy convergence better than 1 mRy for C and Pt bulk crystals. Also, equilib˚ , in rium lattice constant for Pt is found to be of 3.96 A ˚ experimental value as well as good agreement with 3.92 A ˚ , obtained in LDA and GGA calculations with 3.93–4.02 A [1,23]. For oxygen (which is known to be a ‘‘difficult case’’ in pseudopotential calculations [1,7,21,23]), some mRy convergence required about 24 Ha cutoff. With regard to ˚ , which the bondlength in the oxygen molecule (1.23 A ˚ ), convergence slightly exceeds experimental value 1.21 A ˚ was achieved already with 20 Ha cutoff. of 0.01 A Convergence of slab calculations with regard to the Brillouin zone (BZ) sampling was carefully verified by testing various sets of k-points. Specifically, the 4 · 4 · 1 Monkhorst–Pack [24] set was found to be quite sufficient for accurate calculations of total energies. It is reasonable, however, to reduce the number of k-points along the timeconsuming calculations of the reaction path. Such a restriction does not affect the final results due to cancellation of possible errors when the difference of energies is derived from calculations, performed with the same sets of k-points and energy cutoff. Hence, the present calculations have been performed with energy cutoff of 20 Ha (40 Ry) and 2 · 2 · 1 shifted set of k-points (giving two special points in the irreducible part of surface BZ; the same set of kpoints was used in Ref. [6]), and then verified, at critical points of the reaction path, with refined k-sets and 24 Ha cutoff. The choice of the surface unit cell was dictated by requirement of adequate description of the p(2 · 2) structure of oxygen and coadsorbed O–CO layers. With
2602
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
increasing the number of atoms in the unit cell the computational time significantly increases, so that it is desirable to use as thin slab as possible while keeping the most important structure and electronic properties unchanged with respect to those of the surface of a bulk (semi infinite) crystal. To study the degree of influence of the thickness of the slab on surface properties, test calculations were carried out for 3- and 4-layer Pt(1 1 1) slabs. In the course of optimization of the surface structure, atoms of two (the outmost and next to the surface) Pt(1 1 1) layers were allowed to move while remaining layers were kept fixed. Optimization of the atomic positions leads to a small contraction (about 0.5%, in good agreement with the low energy electron diffraction (LEED) results [25] and similar calculations [1]) of the topmost Pt(1 1 1) surface layer with respect to related interplane distance in a bulk Pt crystal ˚ ). This surface relaxation is largely independent on (2.26 A the thickness of the slab and, probably, results from the close-packed structure of the Pt(1 1 1) surface. The distribution of electronic density at the surface remains essentially the same for slabs with 4, 3 and even 2-layer thickness. This means that electronic structure of the Pt(1 1 1) surface, due to quite effective screening, can be well reproduced already with a ‘rigid’’ (that is, with fixed positions of atoms) bilayer slab, which was used for purposes of preliminary search of possible reaction paths. Then, all calculations of optimal atomic positions of adsorbed species and variations of the potential energy in the course of CO oxidation were performed for CO–O coadsorbed films on the 3-layer Pt(1 1 1) substrate. In these simulations, atoms of two Pt(1 1 1) layers were allowed to move while the bottom layer was kept fixed to provide necessary rigidity of the slab. Control calculations have shown that results of the present simulations do not depend significantly neither on the thickness of the slab, nor on variation of the vacuum gap, which was taken ˚. about 7 A It is well established that CO molecules on Pt(1 1 1) are oriented with the long molecular axis perpendicular to the surface with the carbon end down (see, e.g., [26] and references therein). Concerning the type of adsorption sites occupied by CO on the Pt(1 1 1), the most recognized is the model of occupation of the atop sites at low coverages (up to h = 3.33 [27]) followed by occupation of the bridge sites. Redistribution of CO molecules over the sites with increasing coverage results in equal occupations of atop and bridge sites at h = 0.5 [27,28]. However, recent DFT calculations (both LDA and GGA) predict the threefold sites to be the most favorable, in disagreement with experimental data. The origin of this feature of DFT is not clear [23], and in our calculations for hCO = 0.25 the threefold sites also occur favored (by 0.05 eV) with regard to the atop sites. For purposes of modeling CO adlayer structures, the surface unit cell was chosen accordingly with the c(4 · 2) lattice (that is characteristic for CO overlayer on Pt(1 1 1) at h = 0.5 [29]). Orientation of the CO molecule along the surface normal is found to be stable. Specifically, if the molecule was tilted and shifted, it returned back to
(a)
(b)
p
Fig. 2. The (2 · 3) rect–2CO structure on the Pt(1 1 1) surface, obtained by optimization of the film (a). This structure is inconsistent with LEED data [2,27,29] because of position of the CO molecule in the threefold site – correct position of the molecule (accordingly to Ref. [32]) is shown in panel (b).
the on-top position, which, as thus, corresponds to a local minimum in total energy. Estimated length of the C–O ˚ , consistent with bond in adsorbed molecule is of 1.156 A ˚ 1.15 A value obtained in earlier calculations [1,30]. The adsorption energy (binding energy between the CO molecule and Pt(1 1 1) surface) for the atop position is of 1.9 eV, in agreement with experimental value 1.67 eV [31] and results of other calculations (1.87 eV [1]). For hCO = 0.5, p optimization of the film leads to formation of the (2 · 3) rect–2CO structure. However, obtained unit cell with CO molecule in the threefold site (Fig. 2a), as well as generally recognized position of the CO molecule in the center of the unit cell, is inconsistent with the LEED data [2,27,29]. Indeed, the LEED pattern observed in experiment corresponds to an asymmetric unit cell, such as shown in Fig. 2b (for detailed discussion, see Ref. [32]). This inability of DFT simulations to correctly predict the CO structures might be attributed to insufficient account of many-body effects, in particular, peculiarities of screening which lead to Friedel oscillations and related indirect lateral interaction between chemisorbed CO molecules [32]. The p(2 · 2)O/Pt(1 1 1) structure can be accomplished with oxygen adatoms in threefold sites of the fcc or hcp types. The latter has found to be strongly favorable – the estimated binding energy for oxygen in the fcc site is for 0.42 eV exceeds that for hcp site. The fcc and hcp sites can be distinguished only by position of Pt atoms of the next substrate layer. Nonetheless, the difference of oxygen binding energies in the fcc and hcp adsorption sites is very significant (consistent with earlier studies [15–17]). As is discussed below, this difference leads to different chemical activity of adsorbed oxygen atoms. 3. Results: the reaction path for CO oxidation 3.1. Structure of CO–O coadsorbed film and reaction coordinate For coadsorption of oxygen atoms and CO molecules, the film structure is best characterized in the case of CO
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
adsorption on the Pt(1 1 1) surface with previously adsorbed oxygen layer with the p(2 · 2) film structure. The on-top site in the center of the rhombic unit cell is strongly favored for the CO molecule with regard to other available sites. CO adsorption does not reconstruct the oxygen-induced p(2 · 2) surface lattice, and, at saturation, hCO = hO = 0.25. This structure of the coadsorbed film is suitable as starting configuration for modeling of the reaction of CO oxidation. The choice of the rhombic surface unit cell, shown in Figs. 1 and 3, allows for the most convenient description of two possible disposition of oxygen atoms in the fcc and hcp sites (Fig. 3), while CO molecules in both structures occupy the on-top site in the center of the cell. These two structures are remarkably symmetric, which the feature facilitates treatment of the reaction path and allows for a direct comparison of binding energies for oxygen adatoms in the fcc and hcp sites. The difference in total energies for these two configurations is found to be of 0.43 eV, consistent with results of calculations for the p(2 · 2)O/Pt(1 1 1). At low temperature, oxygen adatoms in the p(2 · 2) structure of the coadsorbed film will occupy, preferentially, the fcc sites, because this configuration corresponds to minimal total energy. CO molecules are believed to be significantly more mobile than O adatoms, probably, due to smoother potential relief for CO molecules. One can imagine that a CO molecule at random obtains some thermal excitation with momentum directed towards O adatom. For the p(2 · 2) structure, there are three equivalent such directions, and we will consider that along the diagonal of the rhomb, marked by dashed line in Figs. 1 and 3.
O(1) fcc
(a)
CO O(1) hcp
(b)
CO Fig. 3. The p(2 · 2) structure of coadsorbed O–CO films on Pt(1 1 1) for hCO = hO = 0.25 with two possible disposition of oxygen atoms: (a) in the fcc and (b) hcp threefold hollow sites. The CO molecule occupies the atop site in the center of the cell. The reaction coordinate for the first stage of the reaction is defined by position of the C atom on the diagonal of the rhombic cell, marked by dashed line.
2603
As the CO molecule approaches the O(1) adatom, redistribution of electronic density (which may be considered in terms of forming/breaking chemical bonds) results in formation of (chemisorbed) CO2 molecule, so that, at the first glance, the O(1)–C distance could be taken as the reaction coordinate. Just this way was adopted in Ref. [6] and, though in a somewhat more sophisticated way, also in Ref. [7]. However, optimization of the atomic positions, performed at each step along the reaction coordinate with an artificial constraint (such as fixed O(1)–C distance in Ref. [6]) results, both in Ref. [6] and Ref. [7], in unrealistic reaction paths. To make sure of this, one may want consider carefully any intermediate step of the reaction path, suggested in Ref. [6]. Thus, in the course of the optimization with fixed O(1)–C distance the O(1) adatom got shifted into bridge site, which position was suggested to be essential for the reaction because of related decrease of the barrier [6]. If fact, such configuration would never end in formation of CO2 molecule because of the strong repulsion between CO and O(1). In other words, further decrease of the distance between the species would require either another ‘‘coherent’’ thermal excitation with corresponding momentum, which is obviously improbable, or artificial forced constrain in simulations. Hence, the formation of the CO2 molecule is possible only when CO molecule approaches O(1) adatom along a straight line (or with minor deviations from the line connecting C and O(1) atoms). The only realistic way to describe the reaction path is consideration of step-by-step movements of the C atom along a straight line towards O(1) while both oxygen atoms are allowed to adjust their positions at each step. A reasonable choice of the reaction coordinate therefore is position of the C atom on the diagonal of the rhombic unit cell, as depicted in Fig. 3. Calculated potential energy plots for two positions of the oxygen adatom: in the fcc (dashed line) and hcp (solid line) hollow sites are shown in Fig. 4. Zero potential corresponds to the ground state configuration of the film with CO molecules in the centers of the unit cells and oxygen atoms in the fcc sites. The reduced coordinate xred is given with regard to corresponding corners of the cell for both cases (see Fig. 3), so that decreasing xred corresponds to moving CO towards O(1). The coordinate of the C atom along the normal to the surface was not fixed to allow the carbon atom to adjust its distance from the surface in the course of reaction. If position of the O(1) adatom is kept fixed, the barrier for the CO molecule moving along the diagonal of the rhombic surface unit cell occurs too high thus precluding formation of the CO2 molecule [6]. It is a small shift of the O(1) adatom that results in some redistribution of electronic density (or weakening bonding with Pt substrate) which, in turn, significantly decreases the lateral repulsion between CO and O species. Bonding with the substrate for O(1) adatom in the hcp site is substantially weaker than in the fcc site, and this feature is essential for the reaction. In contrast to the case of O(1) in the fcc site, approaching
2604
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
sorbed state of CO2 was also reported in Ref. [6], but, for unknown reason, with molecular plane parallel to the surface.
2.5
O
ΔE (eV)
2.0
(1)
in fcc
3.2. Distributions of electronic density
1.5
2.25 1.0 0.5
1.15
O
(1 )
in hcp
CO2 0.43
0.0 0.25
0.30
0.35
x red
0.40
0.45
0.50
Fig. 4. Potential energy versus position of the C atom on the diagonal of the unit cell for two positions of the oxygen adatom: (i) in the fcc (shortdash line) and (ii) hcp (solid line) hollow sites. Zero potential corresponds to the ground state configuration of the film with CO molecules in the centers of the unit cells and oxygen atoms in the fcc sites. The reduced coordinate xred is given with regard to corresponding corners of the cell for both cases, so that decreasing xred corresponds to moving CO towards O(1). (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
Detailed picture of the reaction course can be gained by consideration of distributions of electronic density, presented in Fig. 5. Due to found behavior of the species to remain in the normal plane that intersects the surface at the rhomb diagonal, it is convenient to present evolution of charge density distribution by contour maps in this plane. The starting point corresponds to equilibrated O(1) in the hcp hollow site and CO molecule on top of the Pt atom (Fig. 5a). The CO molecule is oriented along the sur˚ . The C–Pt face normal, and the C–O bond length is 1.15 A ˚ bond length is of 1.85 A, so that both values are consistent with experiment and earlier calculations [1,30]. With moving C atom towards O(1), CO molecule becomes tilted. Surprisingly, it occurs tilted towards the O(1) adatom, that is just opposite to what could be ex-
a
CO molecule can perform much more significant shift of the O(1) adatom from the hcp site resulting in effective decrease of lateral repulsion. The estimated activation barrier for reaction in the case of O(1) adatom in the hcp site is of 1.15 eV, which is consistent with value of 1.05 eV calculated in Ref. [6]. The critical point is position of C atom just at the threefold site on the diagonal of the rhombic unit cell (with reduced coordinates 0.333, 0.667 with regard to the a1, a2 lattice vectors; note that this corresponds to the reduced coordinate xred = 0.333 along the diagonal). Oxygen adatoms in the fcc threefold hollow site are strongly bounded to the surface, so that it is difficult, for incoming CO molecule, to shift such an oxygen atom from its initial equilibrium position. This leads to relatively strong lateral repulsion (Fig. 4, dashed line) and, as thus, the barrier for the reaction is too high for the reaction. In fact, the height of the barrier, about 2.2 eV, makes this way improbable (recall that desorption energy of CO is about 1.5 eV, so that the once excited CO molecule would rather evaporate than react with O(1)). In simulations, nonetheless, it is possible to force the CO molecule to come closer to the oxygen adatom. Then, as the distance between C and O(1) decreases sufficiently to form C–O(1) bond, redistribution of electrons results in drop down of the potential and formation of a chemisorbed CO2 molecule. This drop of the potential reveals related decrease of the repulsion between the species, so that the C atom can be released to allow the CO2 molecule to find optimal position and configuration. Worth noting is that chemisorbed CO2 molecule has a triangle configuration with O–C–O angle of 134, lying in the plane normal to the Pt surface. This result is in line with VASP simulations by Eichler and Hafner [7]. The chemi-
O O1
b
C
Pt
c
d
e
f
Fig. 5. Distributions of valence charge density in the (1 1 0) plane that intersects the surface at the diagonal of the rhombic unit cell (see Fig. 3) along the course of CO oxidation on Pt(1 1 1). Spacing between successive contours is of 0.0375 electron/bohr3.
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
3.3. Desorption of CO2 It should be noted that in fact the drop of potential left to the critical point in Fig. 4 does not correspond to the reaction path. Indeed, this would be the case only if at this stage of reaction the molecule could exchange its kinetic energy with a cold substrate (which, however, must be hot enough to excite the CO molecule). Nonetheless, it is interesting to consider the possibility of the chemisorbed state of CO2 molecule, which in reality might be achieved by adsorbing CO2 from a gas phase at low temperature, and to simulate desorption of the molecule. Modeling of the desorption of CO2 was accomplished using the distance from the surface (z coordinate with origin in the plane passing through the centers of topmost atoms of Pt(1 1 1) surface) as the reaction coordinate. The first model experiment was performed by moving the CO2 molecule ‘‘as is’’, that is, with fixed triangle configuration, pertinent to the chemisorbed state (shown in the left insert in Fig. 6), away from the surface. The potential energy, calculated for this model desorption, increases dramatically as the molecule moves away from the surface (Fig. 6, red dashed line). Such a dependence of the poten-
2.5 2.0 1.5
ΔE (eV)
pected. This feature indicates that repulsion between the O(1) and O(CO) is rather weak if any, and, probably, directed bonds of C atom can reveal their importance. Indeed, the shift of C atom results in redistribution of binding electrons so that the bond with Pt atom occurs tilted (Fig. 5b), and such ‘‘rotation’’ may cause corresponding tilting of the bond with oxygen atom. As the C atom occurs at the critical point (near the threefold hollow fcc site, Fig. 5c), charge distribution changes dramatically. Forming O(1)–C bond is accompanied by diminishing of O(1) bonding with Pt atom, and thus obtained O(1)–C–O complex corresponds to chemisorbed CO2 molecule (Fig. 5d). Key to the reaction mechanism is behavior of the O(1) adatom as the distance from CO molecule decreases. The repulsion from CO makes the O(1) adatom ‘‘climbing’’ on the ‘‘back’’ Pt atom (Fig. 5b– d). The carbon atom, in turn, moves slightly downwards and, on further moving along the rhomb diagonal, tends to break the O(1)–Pt bond. It is interesting to follow a kind of switching between positions of the O(1) and O(CO) atoms (Fig. 5e and f) which relates to rotation of the CO2 so that the O(1)–C bond becomes tilted while the O(CO) atom occurs close to the surface in vicinity of the fcc threefold site. In fact, this configuration reproduces remarkably well the position of chemisorbed CO2 molecule, obtained in the simulation for initial O(1) disposition in fcc site. Worth noting that the Pt atom in the back provides the barrier for the O(1) atom, which is essential for possibility to decrease the O(1)–CO distance – otherwise, the O(1) adatom would be unavoidably pushed into the neighboring threefold hollow site by forthcoming CO molecule. This is why the reaction will never occur for O(1) in a bridge site on Pt(1 1 1), as it was suggested by earlier simulations [6].
2605
1.0 0.5 0.0
-0.5 2.0
2.5
3.0
3.5
4.0
zC (A) Fig. 6. Potential energy versus distance from the Pt(1 1 1) surface in the course of desorption of CO2 (solid line). The chemisorbed CO2 molecule has the triangle configuration (shown in the insert by the map of electronic density), and if it were evaporated with fixed shape (related potential is marked by red dashed line) it would require enormous energy because of significant dipole moment of the triangle molecule, which results in attraction with its image in the substrate. However, in the course of desorption, the CO2 molecule becomes linear, and therefore the potential energy decreases dramatically. When the linear molecule is moved towards the surface, the potential energy, shown by the blue dashed line, has a ˚ . (For interpretation of the references in shallow minimum about z = 3 A color in this figure legend, the reader is referred to the web version of this article.)
tial indicates significant dipole moment of the triangle molecule, which results in attraction with its image in the substrate. This suggestion was verified in the second model ˚ experiment, when the C atom was fixed at some z = 3.5 A position while oxygen atoms were allowed to move. As a result of optimization, the CO2 molecule became linear (with zero dipole moment) and oriented parallel to the surface (see the right insert in Fig. 6). Changing configuration, in turn, resulted in significant decrease in potential energy. Then, in the third model experiment, the linear molecule was moved backwards (that is, to the surface). The shallow ˚ of the potential energy, shown by minimum about z = 3 A blue dashed line in Fig. 6, reveals a physisorbed state of the CO2 molecule. Further approach to the surface, however, is unfavorable for the linear molecule, which is evident from increase of the potential energy. Hence, chemisorbed CO2 molecule has the triangle configuration, whereas desorption should be accompanied by changing shape and desorbed CO2 molecule becomes linear. Modeling of the net process of evaporation was conducted by step-by-step moving the molecule away from the surface and performing optimization of positions of oxygen atoms with fixed z coordinate of carbon. The potential energy, derived from these simulations, is shown in Fig. 6 by a solid line. The potential barrier for CO2 desorption originates from braking bonds with the Pt(1 1 1) surface, while subsequent decrease of potential energy is caused by changing shape of the molecule. The inverse process, that is, chemisorption of CO2 molecule on the Pt(1 1 1) surface, requires significant activation
2606
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
energy, which can be estimated by the height of the barrier measured from the right side of the potential plot in Fig. 6 (i.e., for CO2 molecule far from surface). It must be difficult to obtain the chemisorbed state of CO2, because, on the one hand, kinetic energy should be sufficient to overcome the barrier while, on the other hand, afterwards the molecule should exchange kinetic energy with the surface. Now the net LH reaction path for CO oxidation on Pt(1 1 1) can be suggested. At temperatures above 200 K some of oxygen atoms will be promoted to the hcp sites thus becoming chemically active with regard to oxidation of CO molecules. Position of the oxygen atoms in the hcp sites is metastable and therefore they can stay in this state for some time sufficient for interaction with CO. Worth noting is that the energy of promotion of oxygen atoms to the hcp sites does not directly give rise to the potential barrier of the reaction itself, because the promotion and reaction processes are time-separated. Then, a CO molecule, obtaining some thermal excitation with momentum directed towards O(1) adatom in the hcp site, can overcome the potential barrier, provided that its kinetic energy is sufficient. Height of the barrier occurs significantly decreased due to shift of the O(1) adatom, which, in turn, leads to decrease of lateral repulsion with the CO molecule. It should be noted that the height of the barrier for the reaction, obtained in present model calculations, corresponds, in fact, to the ‘‘upper limit’’ of the barrier, because at final temperatures, pertinent to activation of the reaction in nearly perfect p(2 · 2) CO + O layers on Pt(1 1 1) (T = 300 K [3]), the O(1) atom will be already in some excited (vibration) state. This will obviously facilitate the required shift of the O(1) adatom from the hcp site thus decreasing the potential barrier. Another reason why in experiment the barrier can be lower is the starting configuration with regard to CO position. For example, if some position of CO molecule other than in the on-top site in the perfect p(2 · 2) structure (which in reality must be disordered at T = 300 K) were chosen as starting point for simulations, the barrier would be evidently diminished, as may be derived from analysis of the potential plot in Fig. 4. Having the potential barrier passed, forming CO2 molecule will increase its kinetic energy, as suggested in Ref. [33]. From our modeling, performed using a ‘‘cold’’ optimization, it is impossible to derive a detailed picture of how the molecule exchanges its kinetic energy with the Pt(1 1 1) surface and gets some momentum towards vacuum, so we can only guess that this might be accomplished by collisions with Pt atoms and facilitated by the ‘‘spring release’’ effect when desorbing CO2 molecule changes its shape from triangular to linear. 4. Conclusion Our DFT/LDA simulations elucidate, on a microscopic level, essential features of the course of classical Langmuir–Hinshelwood (LH) reaction of catalytic CO oxida-
tion on the Pt(1 1 1) surface. Key to the reaction mechanism is the initial position of oxygen adatom in the threefold hollow site of hcp type and decrease of the lateral repulsion with approaching CO molecule due to redistribution of the electronic density as the O adatom is displaced. Thus, the oxygen atom in the hcp site is bounded substantially weaker than in the fcc site, which makes it more flexible and therefore more reactive. On the other hand, oxygen adatoms in other possible positions on the Pt(1 1 1) surface (such as bridge or atop) cannot readily contribute to the oxidation reaction, because such oxygen atoms unavoidably will be repelled by approaching CO molecules. The calculations have been performed for a particular starting configuration that is the p(2 · 2) structure and stoichiometric coverages of CO and O species on Pt(1 1 1), but the main conclusion should hold also for other cases of the LH reaction between CO molecules and chemisorbed oxygen atoms. References [1] M. Lynch, P. Hu, Surf. Sci. 458 (2000) 1. [2] P.R. Norton, J.A. Davies, T.E. Jackman, Surf. Sci. Lett. 122 (1982) L593. [3] J. Yoshinobu, M. Kawai, J. Chem. Phys. 103 (1995) 3220. [4] A. Szabo´, M. Kiskinova, J.T. Yates Jr., J. Chem. Phys. 90 (1989) 4604. [5] S. Volkening, J. Wintterlin, J. Chem. Phys. 114 (2001) 6382. [6] A. Alavi, P. Hu, T. Deutsch, P.L. Silvestrelli, J. Hutter, Phys. Rev. Lett. 80 (16) (1998) 3650. [7] A. Eichler, J. Hafner, Phys. Rev. B 59 (1999) 5960. [8] J. Wintterlin, S. Volkening, T.W. Janssens, T. Zambelli, G. Ertl, Science 278 (1997) 1931. [9] C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, J. Chem. Phys. 73 (1980) 5862. [10] G. Ertl, Surf. Sci. 299 (1994) 742. [11] J.L. Gland, E.B. Kollin, Surf. Sci. 151 (1985) 260. [12] V.K. Medvedev, Yu. Suchorski, J.H. Block, Appl. Surf. Sci. (76–77) (1994) 136. [13] H.H. Rotermund, S. Jakubith, A. von Oertzen, G. Ertl, J. Chem. Phys. 91 (1989) 4942. [14] S. Akhter, J.M. White, Surf. Sci. 171 (1986) 527. [15] P.J. Feibelman, Phys. Rev. B 56 (1997) 10532. [16] A. Bogicevic, J. Stro¨mquist, B.I. Lundqvist, Phys. Rev. B 57 (1998) R4289. [17] P.J. Feibelman, E. Stefanie, M. Thomas, Phys. Rev. Lett. 77 (1997) 2257. [18] N.V. Petrova, I.N. Yakovkin, Surf. Sci. 578 (2005) 162. [19] N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. [20] M. Fucks, M. Scheffler, Comp. Phys. Commun. 119 (1999) 67. [21] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244; D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 48 (1980) 567. [22] X. Gonze, J.-M. Beuken, R. Caracas, F. Detraux, M. Fuchs, G.-M. Rignanese, L. Sindic, M. Verstraete, G. Zerah, F. Jollet, M. Torrent, A. Roy, M. Mikami, Ph. Ghosez, J.-Y. Raty, D.C. Allan, Comput. Mat. Sci. 25 (2002) 478. [23] P.J. Feibelman, B. Hammer, J.K. Norskov, F. Wagner, M. Scheffler, R. Stumpf, R. Watwe, J. Dumesic, J. Phys. Chem. B 105 (2001) 4018. [24] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [25] N. Materer, U. Starke, A. Barbieri, R. Doll, K. Heinz, M. Van Hove, G.A. Somordjai, Surf. Sci. 325 (1995) 207. [26] N.D. Lang, S. Holloway, J.K. Nørskov, Surf. Sci. 150 (1985) 24; K.L. Kostov, P. Jakob, D. Menzel, Surf. Sci. 377–379 (1997) 802.
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607 [27] H. Steininger, S. Lehwald, H. Ibach, Surf. Sci. 123 (1982) 264; B.N.J. Persson, R. Ryberg, Phys. Rev. B 40 (1989) 20173; D.C. Skelton, D.H. Wei, S.D. Kevan, Surf. Sci. 320 (1994) 77. [28] J.J. Mortensen, B. Hammer, J.K. Norskov, Surf. Sci. 414 (1998) 315; Y. Morikawa, J.J. Mortensen, B. Hammer, J.K. Norskov, Surf. Sci. 386 (1997) 67.
[29] [30] [31] [32] [33]
2607
G. Ertl, M. Neumann, K.M. Streit, Surf. Sci. 64 (1977) 393. D. Ogletree, M. Van Hove, G.A. Somordjai, Surf. Sci. 173 (1986) 351. Y.Y. Yeo, L. Vattuone, D. King, J. Phys. Chem. 106 (1997) 391. N.V. Petrova, I.N. Yakovkin, Surf. Sci. 519 (2002) 90. K.-H. Allers, H. Pfnuer, P. Feulner, D. Menzel, J. Chem. Phys. 100 (1994) 3985.
Microscopic model of CO oxidation on Pt(1 1 1) I.N. Yakovkin *, N.V. Petrova Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki 46, Kiev 03028, Ukraine Received 30 January 2006; accepted for publication 19 April 2006 Available online 12 May 2006
Abstract A new microscopic model, based on DFT/LDA modeling, is suggested for the Langmuir–Hinshelwood reaction of catalytic CO oxidation in coadsorbed O–CO layers on Pt(1 1 1). It has been found that only the oxygen atoms occupying threefold hollow sites of hcp type are chemically active. The potential barrier for the oxidation reaction significantly decreases due to changes in the adlayer oxygen states in the proximity to CO. The oxygen electronic density distribution is affected by approaching CO molecule which alters the oxygen position. Height of the barrier is estimated as 1.15 eV, which may be attributed to the upper limit of activation energy for the net reaction process. 2006 Elsevier B.V. All rights reserved. Keywords: Density functional calculations; Modeling; Catalytic oxidation; Platinum surface; Oxygen; Carbon monoxide
1. Introduction The surface catalytic reaction of CO oxidation on Pt surfaces is central to controlling CO emissions, and continues to demand greater understanding. Despite a great number of theoretical and experimental investigations, certain key features, required for a clear picture of the reaction on microscopic level, are still missing. In the present article we address these issues by modeling (on the DFT-LDA level) of the reaction path for the case of the reaction in the coadsorbed CO–O p(2 · 2) layer on Pt(1 1 1). Oxygen atoms on transition metal surfaces tend to occupy adsorption sites of the highest available coordination, which in the case of Pt(1 1 1) correspond to threefold sites [1]. At h = 0.25 (coverage here is defined with regard to the surface atomic concentration of the Pt(1 1 1)), oxygen films have a p(2 · 2) structure [2]. This structure is very stable and can be observed at temperatures up to 350 K [2]. The h = 0.25 is a saturation oxygen coverage, which means that further oxygen adsorption is suppressed. Nonetheless, adsorption of CO still can take place, probably, due to *
Corresponding author. E-mail address: [email protected] (I.N. Yakovkin).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.04.029
occupation of sites of different type – at low coverages, CO molecules occupy atop sites both when adsorbed on a clean Pt(1 1 1) and on the oxygen-covered surface [3,4]. The p(2 · 2) structure of the coadsorbed O–CO layer (which is observed at T < 300 K [3]) with hCO = hO = 0.25 is dictated by oxygen adatoms in the threefold hollow sites, while CO molecules occupy on-top adsorption sites in the oxygen cells [5]. The Langmuir–Hinshelwood reaction of CO oxidation on Pt(1 1 1) requires some 0.5–1.1 eV activation energy (as estimated from DFT simulations of pathways of the particles in the course of CO oxidation [6,7] and from experiments on the reaction kinetics [6,8–10]). This dictates therefore that the reaction occurs at temperatures in the range from 220 to 300 K (or higher), depending of the film structure [3]. The barrier for the oxidation reaction probably originates not from a direct repulsion between CO molecules and oxygen atoms, but, rather, from the strong bonding of the oxygen atoms with the surface [6]. For oxidation of a CO molecule, the oxygen atom should be activated, which can be achieved by moving the atom into an ‘‘active’’ site [1,3], however, conditions for such activation remain speculative. There is also some controversy with regard to the
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
2601
the reaction, because other considered configurations led to enormously high potential barriers inconsistent with activation energy derived from experiment. However, these simulations were performed using artificial constraints such as fixed distance between the C atom in CO molecule and O(1) adatom in the course of structural optimization at each reaction step [6,7]. As we show in the present article, another choice of the reaction coordinate allows for a more realistic description of the reaction steps and suggests that only those oxygen adatoms that are promoted to the hcp hollow sites on the Pt(1 1 1) surface can react with CO molecules. Fig. 1. The p(2 · 2) unit cell adopted in present simulations of CO oxidation. The threefold fcc and hcp sites on the Pt(1 1 1) surface are indicated.
course of reaction, which is reported to start at the boundaries of the ordered p(2 · 2) oxygen domains [11–13], but, on the other hand, can be observed over the net surface covered with the coadsorbed O–CO films [14]. This feature reveals an important role of mobility of oxygen atoms, which provides the movement of oxygen atoms to ‘‘active sites’’ where they become chemically active with regard to CO oxidation [1]. There are two types of the threefold hollow sites on Pt(1 1 1), favorable for oxygen adatom bonding, and these are the so-called fcc and hcp sites (Fig. 1). Other sites, such as bridge and atop, are strongly unfavorable (the barrier for diffusion of an oxygen adatom from the fcc hollow to bridge site is of 0.6–0.7 eV [1,15,16] ; atop position is unstable and the barrier is about 1.2 eV [1]), and therefore at low temperatures all oxygen adatoms will occupy the fcc sites which correspond to the lowest total energy (or, in other words, to the highest binding energy). It has been found that difference of binding energies for O adatoms in the fcc and hcp sites is surprisingly large (up to 0.47 eV [1]), as estimated through DFT calculations [1,16,17]. It was suggested [18] that the threefold hcp sites could serve as active sites in the oxidation reaction of CO. Due to thermal fluctuations, oxygen atoms can move from fcc into hcp sites and, thus, become chemically active. As follows from Monte Carlo simulations performed in the framework of this model [18], it is the enhanced mobility of oxygen atoms at the oxygen adlayer p(2 · 2) domain boundaries that results in CO oxidation just within this region, thus explaining propagation of the reaction front at temperatures about 200 K (obviously, the transition from fcc to hcp sites is favored for oxygen atoms just in vicinity of the boundaries of the domains). The issue of active sites, available for oxygen adatom, was studied in Ref. [1] in some detail. The authors [1] suggested that the reactivity of adsorbed O with regard to CO oxidation on Pt(1 1 1) surface increases accordingly to sequence fcc–hcp–bridge–atop of adsorption sites. The in-bridge site was suggested also from DFT/GGA simulations of the reaction course [6] to be the only relevant for
2. Method of calculations and control results The DFT/LDA semirelativistic calculations within a supercell (repeat-slab) approach using norm-conserving Troullier–Martins pseudopotentials [19,20] with exchangecorrelation part in PW92-CA form [21] were carried out using the ‘‘cold’’ Broyden–Fletcher–Goldfarb–Shanno minimization scheme as implemented in ABINIT code [22]. The norm-conserving pseudopotential, though providing very accurate approximation of wave functions, are known as rather ‘‘hard’’, that is, require many plane waves (which means a high energy cutoff) for good convergence of total energy. The test calculations have shown that 20 hartree (40 Ry) cutoff provides the total energy convergence better than 1 mRy for C and Pt bulk crystals. Also, equilib˚ , in rium lattice constant for Pt is found to be of 3.96 A ˚ experimental value as well as good agreement with 3.92 A ˚ , obtained in LDA and GGA calculations with 3.93–4.02 A [1,23]. For oxygen (which is known to be a ‘‘difficult case’’ in pseudopotential calculations [1,7,21,23]), some mRy convergence required about 24 Ha cutoff. With regard to ˚ , which the bondlength in the oxygen molecule (1.23 A ˚ ), convergence slightly exceeds experimental value 1.21 A ˚ was achieved already with 20 Ha cutoff. of 0.01 A Convergence of slab calculations with regard to the Brillouin zone (BZ) sampling was carefully verified by testing various sets of k-points. Specifically, the 4 · 4 · 1 Monkhorst–Pack [24] set was found to be quite sufficient for accurate calculations of total energies. It is reasonable, however, to reduce the number of k-points along the timeconsuming calculations of the reaction path. Such a restriction does not affect the final results due to cancellation of possible errors when the difference of energies is derived from calculations, performed with the same sets of k-points and energy cutoff. Hence, the present calculations have been performed with energy cutoff of 20 Ha (40 Ry) and 2 · 2 · 1 shifted set of k-points (giving two special points in the irreducible part of surface BZ; the same set of kpoints was used in Ref. [6]), and then verified, at critical points of the reaction path, with refined k-sets and 24 Ha cutoff. The choice of the surface unit cell was dictated by requirement of adequate description of the p(2 · 2) structure of oxygen and coadsorbed O–CO layers. With
2602
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
increasing the number of atoms in the unit cell the computational time significantly increases, so that it is desirable to use as thin slab as possible while keeping the most important structure and electronic properties unchanged with respect to those of the surface of a bulk (semi infinite) crystal. To study the degree of influence of the thickness of the slab on surface properties, test calculations were carried out for 3- and 4-layer Pt(1 1 1) slabs. In the course of optimization of the surface structure, atoms of two (the outmost and next to the surface) Pt(1 1 1) layers were allowed to move while remaining layers were kept fixed. Optimization of the atomic positions leads to a small contraction (about 0.5%, in good agreement with the low energy electron diffraction (LEED) results [25] and similar calculations [1]) of the topmost Pt(1 1 1) surface layer with respect to related interplane distance in a bulk Pt crystal ˚ ). This surface relaxation is largely independent on (2.26 A the thickness of the slab and, probably, results from the close-packed structure of the Pt(1 1 1) surface. The distribution of electronic density at the surface remains essentially the same for slabs with 4, 3 and even 2-layer thickness. This means that electronic structure of the Pt(1 1 1) surface, due to quite effective screening, can be well reproduced already with a ‘rigid’’ (that is, with fixed positions of atoms) bilayer slab, which was used for purposes of preliminary search of possible reaction paths. Then, all calculations of optimal atomic positions of adsorbed species and variations of the potential energy in the course of CO oxidation were performed for CO–O coadsorbed films on the 3-layer Pt(1 1 1) substrate. In these simulations, atoms of two Pt(1 1 1) layers were allowed to move while the bottom layer was kept fixed to provide necessary rigidity of the slab. Control calculations have shown that results of the present simulations do not depend significantly neither on the thickness of the slab, nor on variation of the vacuum gap, which was taken ˚. about 7 A It is well established that CO molecules on Pt(1 1 1) are oriented with the long molecular axis perpendicular to the surface with the carbon end down (see, e.g., [26] and references therein). Concerning the type of adsorption sites occupied by CO on the Pt(1 1 1), the most recognized is the model of occupation of the atop sites at low coverages (up to h = 3.33 [27]) followed by occupation of the bridge sites. Redistribution of CO molecules over the sites with increasing coverage results in equal occupations of atop and bridge sites at h = 0.5 [27,28]. However, recent DFT calculations (both LDA and GGA) predict the threefold sites to be the most favorable, in disagreement with experimental data. The origin of this feature of DFT is not clear [23], and in our calculations for hCO = 0.25 the threefold sites also occur favored (by 0.05 eV) with regard to the atop sites. For purposes of modeling CO adlayer structures, the surface unit cell was chosen accordingly with the c(4 · 2) lattice (that is characteristic for CO overlayer on Pt(1 1 1) at h = 0.5 [29]). Orientation of the CO molecule along the surface normal is found to be stable. Specifically, if the molecule was tilted and shifted, it returned back to
(a)
(b)
p
Fig. 2. The (2 · 3) rect–2CO structure on the Pt(1 1 1) surface, obtained by optimization of the film (a). This structure is inconsistent with LEED data [2,27,29] because of position of the CO molecule in the threefold site – correct position of the molecule (accordingly to Ref. [32]) is shown in panel (b).
the on-top position, which, as thus, corresponds to a local minimum in total energy. Estimated length of the C–O ˚ , consistent with bond in adsorbed molecule is of 1.156 A ˚ 1.15 A value obtained in earlier calculations [1,30]. The adsorption energy (binding energy between the CO molecule and Pt(1 1 1) surface) for the atop position is of 1.9 eV, in agreement with experimental value 1.67 eV [31] and results of other calculations (1.87 eV [1]). For hCO = 0.5, p optimization of the film leads to formation of the (2 · 3) rect–2CO structure. However, obtained unit cell with CO molecule in the threefold site (Fig. 2a), as well as generally recognized position of the CO molecule in the center of the unit cell, is inconsistent with the LEED data [2,27,29]. Indeed, the LEED pattern observed in experiment corresponds to an asymmetric unit cell, such as shown in Fig. 2b (for detailed discussion, see Ref. [32]). This inability of DFT simulations to correctly predict the CO structures might be attributed to insufficient account of many-body effects, in particular, peculiarities of screening which lead to Friedel oscillations and related indirect lateral interaction between chemisorbed CO molecules [32]. The p(2 · 2)O/Pt(1 1 1) structure can be accomplished with oxygen adatoms in threefold sites of the fcc or hcp types. The latter has found to be strongly favorable – the estimated binding energy for oxygen in the fcc site is for 0.42 eV exceeds that for hcp site. The fcc and hcp sites can be distinguished only by position of Pt atoms of the next substrate layer. Nonetheless, the difference of oxygen binding energies in the fcc and hcp adsorption sites is very significant (consistent with earlier studies [15–17]). As is discussed below, this difference leads to different chemical activity of adsorbed oxygen atoms. 3. Results: the reaction path for CO oxidation 3.1. Structure of CO–O coadsorbed film and reaction coordinate For coadsorption of oxygen atoms and CO molecules, the film structure is best characterized in the case of CO
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
adsorption on the Pt(1 1 1) surface with previously adsorbed oxygen layer with the p(2 · 2) film structure. The on-top site in the center of the rhombic unit cell is strongly favored for the CO molecule with regard to other available sites. CO adsorption does not reconstruct the oxygen-induced p(2 · 2) surface lattice, and, at saturation, hCO = hO = 0.25. This structure of the coadsorbed film is suitable as starting configuration for modeling of the reaction of CO oxidation. The choice of the rhombic surface unit cell, shown in Figs. 1 and 3, allows for the most convenient description of two possible disposition of oxygen atoms in the fcc and hcp sites (Fig. 3), while CO molecules in both structures occupy the on-top site in the center of the cell. These two structures are remarkably symmetric, which the feature facilitates treatment of the reaction path and allows for a direct comparison of binding energies for oxygen adatoms in the fcc and hcp sites. The difference in total energies for these two configurations is found to be of 0.43 eV, consistent with results of calculations for the p(2 · 2)O/Pt(1 1 1). At low temperature, oxygen adatoms in the p(2 · 2) structure of the coadsorbed film will occupy, preferentially, the fcc sites, because this configuration corresponds to minimal total energy. CO molecules are believed to be significantly more mobile than O adatoms, probably, due to smoother potential relief for CO molecules. One can imagine that a CO molecule at random obtains some thermal excitation with momentum directed towards O adatom. For the p(2 · 2) structure, there are three equivalent such directions, and we will consider that along the diagonal of the rhomb, marked by dashed line in Figs. 1 and 3.
O(1) fcc
(a)
CO O(1) hcp
(b)
CO Fig. 3. The p(2 · 2) structure of coadsorbed O–CO films on Pt(1 1 1) for hCO = hO = 0.25 with two possible disposition of oxygen atoms: (a) in the fcc and (b) hcp threefold hollow sites. The CO molecule occupies the atop site in the center of the cell. The reaction coordinate for the first stage of the reaction is defined by position of the C atom on the diagonal of the rhombic cell, marked by dashed line.
2603
As the CO molecule approaches the O(1) adatom, redistribution of electronic density (which may be considered in terms of forming/breaking chemical bonds) results in formation of (chemisorbed) CO2 molecule, so that, at the first glance, the O(1)–C distance could be taken as the reaction coordinate. Just this way was adopted in Ref. [6] and, though in a somewhat more sophisticated way, also in Ref. [7]. However, optimization of the atomic positions, performed at each step along the reaction coordinate with an artificial constraint (such as fixed O(1)–C distance in Ref. [6]) results, both in Ref. [6] and Ref. [7], in unrealistic reaction paths. To make sure of this, one may want consider carefully any intermediate step of the reaction path, suggested in Ref. [6]. Thus, in the course of the optimization with fixed O(1)–C distance the O(1) adatom got shifted into bridge site, which position was suggested to be essential for the reaction because of related decrease of the barrier [6]. If fact, such configuration would never end in formation of CO2 molecule because of the strong repulsion between CO and O(1). In other words, further decrease of the distance between the species would require either another ‘‘coherent’’ thermal excitation with corresponding momentum, which is obviously improbable, or artificial forced constrain in simulations. Hence, the formation of the CO2 molecule is possible only when CO molecule approaches O(1) adatom along a straight line (or with minor deviations from the line connecting C and O(1) atoms). The only realistic way to describe the reaction path is consideration of step-by-step movements of the C atom along a straight line towards O(1) while both oxygen atoms are allowed to adjust their positions at each step. A reasonable choice of the reaction coordinate therefore is position of the C atom on the diagonal of the rhombic unit cell, as depicted in Fig. 3. Calculated potential energy plots for two positions of the oxygen adatom: in the fcc (dashed line) and hcp (solid line) hollow sites are shown in Fig. 4. Zero potential corresponds to the ground state configuration of the film with CO molecules in the centers of the unit cells and oxygen atoms in the fcc sites. The reduced coordinate xred is given with regard to corresponding corners of the cell for both cases (see Fig. 3), so that decreasing xred corresponds to moving CO towards O(1). The coordinate of the C atom along the normal to the surface was not fixed to allow the carbon atom to adjust its distance from the surface in the course of reaction. If position of the O(1) adatom is kept fixed, the barrier for the CO molecule moving along the diagonal of the rhombic surface unit cell occurs too high thus precluding formation of the CO2 molecule [6]. It is a small shift of the O(1) adatom that results in some redistribution of electronic density (or weakening bonding with Pt substrate) which, in turn, significantly decreases the lateral repulsion between CO and O species. Bonding with the substrate for O(1) adatom in the hcp site is substantially weaker than in the fcc site, and this feature is essential for the reaction. In contrast to the case of O(1) in the fcc site, approaching
2604
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
sorbed state of CO2 was also reported in Ref. [6], but, for unknown reason, with molecular plane parallel to the surface.
2.5
O
ΔE (eV)
2.0
(1)
in fcc
3.2. Distributions of electronic density
1.5
2.25 1.0 0.5
1.15
O
(1 )
in hcp
CO2 0.43
0.0 0.25
0.30
0.35
x red
0.40
0.45
0.50
Fig. 4. Potential energy versus position of the C atom on the diagonal of the unit cell for two positions of the oxygen adatom: (i) in the fcc (shortdash line) and (ii) hcp (solid line) hollow sites. Zero potential corresponds to the ground state configuration of the film with CO molecules in the centers of the unit cells and oxygen atoms in the fcc sites. The reduced coordinate xred is given with regard to corresponding corners of the cell for both cases, so that decreasing xred corresponds to moving CO towards O(1). (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
Detailed picture of the reaction course can be gained by consideration of distributions of electronic density, presented in Fig. 5. Due to found behavior of the species to remain in the normal plane that intersects the surface at the rhomb diagonal, it is convenient to present evolution of charge density distribution by contour maps in this plane. The starting point corresponds to equilibrated O(1) in the hcp hollow site and CO molecule on top of the Pt atom (Fig. 5a). The CO molecule is oriented along the sur˚ . The C–Pt face normal, and the C–O bond length is 1.15 A ˚ bond length is of 1.85 A, so that both values are consistent with experiment and earlier calculations [1,30]. With moving C atom towards O(1), CO molecule becomes tilted. Surprisingly, it occurs tilted towards the O(1) adatom, that is just opposite to what could be ex-
a
CO molecule can perform much more significant shift of the O(1) adatom from the hcp site resulting in effective decrease of lateral repulsion. The estimated activation barrier for reaction in the case of O(1) adatom in the hcp site is of 1.15 eV, which is consistent with value of 1.05 eV calculated in Ref. [6]. The critical point is position of C atom just at the threefold site on the diagonal of the rhombic unit cell (with reduced coordinates 0.333, 0.667 with regard to the a1, a2 lattice vectors; note that this corresponds to the reduced coordinate xred = 0.333 along the diagonal). Oxygen adatoms in the fcc threefold hollow site are strongly bounded to the surface, so that it is difficult, for incoming CO molecule, to shift such an oxygen atom from its initial equilibrium position. This leads to relatively strong lateral repulsion (Fig. 4, dashed line) and, as thus, the barrier for the reaction is too high for the reaction. In fact, the height of the barrier, about 2.2 eV, makes this way improbable (recall that desorption energy of CO is about 1.5 eV, so that the once excited CO molecule would rather evaporate than react with O(1)). In simulations, nonetheless, it is possible to force the CO molecule to come closer to the oxygen adatom. Then, as the distance between C and O(1) decreases sufficiently to form C–O(1) bond, redistribution of electrons results in drop down of the potential and formation of a chemisorbed CO2 molecule. This drop of the potential reveals related decrease of the repulsion between the species, so that the C atom can be released to allow the CO2 molecule to find optimal position and configuration. Worth noting is that chemisorbed CO2 molecule has a triangle configuration with O–C–O angle of 134, lying in the plane normal to the Pt surface. This result is in line with VASP simulations by Eichler and Hafner [7]. The chemi-
O O1
b
C
Pt
c
d
e
f
Fig. 5. Distributions of valence charge density in the (1 1 0) plane that intersects the surface at the diagonal of the rhombic unit cell (see Fig. 3) along the course of CO oxidation on Pt(1 1 1). Spacing between successive contours is of 0.0375 electron/bohr3.
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
3.3. Desorption of CO2 It should be noted that in fact the drop of potential left to the critical point in Fig. 4 does not correspond to the reaction path. Indeed, this would be the case only if at this stage of reaction the molecule could exchange its kinetic energy with a cold substrate (which, however, must be hot enough to excite the CO molecule). Nonetheless, it is interesting to consider the possibility of the chemisorbed state of CO2 molecule, which in reality might be achieved by adsorbing CO2 from a gas phase at low temperature, and to simulate desorption of the molecule. Modeling of the desorption of CO2 was accomplished using the distance from the surface (z coordinate with origin in the plane passing through the centers of topmost atoms of Pt(1 1 1) surface) as the reaction coordinate. The first model experiment was performed by moving the CO2 molecule ‘‘as is’’, that is, with fixed triangle configuration, pertinent to the chemisorbed state (shown in the left insert in Fig. 6), away from the surface. The potential energy, calculated for this model desorption, increases dramatically as the molecule moves away from the surface (Fig. 6, red dashed line). Such a dependence of the poten-
2.5 2.0 1.5
ΔE (eV)
pected. This feature indicates that repulsion between the O(1) and O(CO) is rather weak if any, and, probably, directed bonds of C atom can reveal their importance. Indeed, the shift of C atom results in redistribution of binding electrons so that the bond with Pt atom occurs tilted (Fig. 5b), and such ‘‘rotation’’ may cause corresponding tilting of the bond with oxygen atom. As the C atom occurs at the critical point (near the threefold hollow fcc site, Fig. 5c), charge distribution changes dramatically. Forming O(1)–C bond is accompanied by diminishing of O(1) bonding with Pt atom, and thus obtained O(1)–C–O complex corresponds to chemisorbed CO2 molecule (Fig. 5d). Key to the reaction mechanism is behavior of the O(1) adatom as the distance from CO molecule decreases. The repulsion from CO makes the O(1) adatom ‘‘climbing’’ on the ‘‘back’’ Pt atom (Fig. 5b– d). The carbon atom, in turn, moves slightly downwards and, on further moving along the rhomb diagonal, tends to break the O(1)–Pt bond. It is interesting to follow a kind of switching between positions of the O(1) and O(CO) atoms (Fig. 5e and f) which relates to rotation of the CO2 so that the O(1)–C bond becomes tilted while the O(CO) atom occurs close to the surface in vicinity of the fcc threefold site. In fact, this configuration reproduces remarkably well the position of chemisorbed CO2 molecule, obtained in the simulation for initial O(1) disposition in fcc site. Worth noting that the Pt atom in the back provides the barrier for the O(1) atom, which is essential for possibility to decrease the O(1)–CO distance – otherwise, the O(1) adatom would be unavoidably pushed into the neighboring threefold hollow site by forthcoming CO molecule. This is why the reaction will never occur for O(1) in a bridge site on Pt(1 1 1), as it was suggested by earlier simulations [6].
2605
1.0 0.5 0.0
-0.5 2.0
2.5
3.0
3.5
4.0
zC (A) Fig. 6. Potential energy versus distance from the Pt(1 1 1) surface in the course of desorption of CO2 (solid line). The chemisorbed CO2 molecule has the triangle configuration (shown in the insert by the map of electronic density), and if it were evaporated with fixed shape (related potential is marked by red dashed line) it would require enormous energy because of significant dipole moment of the triangle molecule, which results in attraction with its image in the substrate. However, in the course of desorption, the CO2 molecule becomes linear, and therefore the potential energy decreases dramatically. When the linear molecule is moved towards the surface, the potential energy, shown by the blue dashed line, has a ˚ . (For interpretation of the references in shallow minimum about z = 3 A color in this figure legend, the reader is referred to the web version of this article.)
tial indicates significant dipole moment of the triangle molecule, which results in attraction with its image in the substrate. This suggestion was verified in the second model ˚ experiment, when the C atom was fixed at some z = 3.5 A position while oxygen atoms were allowed to move. As a result of optimization, the CO2 molecule became linear (with zero dipole moment) and oriented parallel to the surface (see the right insert in Fig. 6). Changing configuration, in turn, resulted in significant decrease in potential energy. Then, in the third model experiment, the linear molecule was moved backwards (that is, to the surface). The shallow ˚ of the potential energy, shown by minimum about z = 3 A blue dashed line in Fig. 6, reveals a physisorbed state of the CO2 molecule. Further approach to the surface, however, is unfavorable for the linear molecule, which is evident from increase of the potential energy. Hence, chemisorbed CO2 molecule has the triangle configuration, whereas desorption should be accompanied by changing shape and desorbed CO2 molecule becomes linear. Modeling of the net process of evaporation was conducted by step-by-step moving the molecule away from the surface and performing optimization of positions of oxygen atoms with fixed z coordinate of carbon. The potential energy, derived from these simulations, is shown in Fig. 6 by a solid line. The potential barrier for CO2 desorption originates from braking bonds with the Pt(1 1 1) surface, while subsequent decrease of potential energy is caused by changing shape of the molecule. The inverse process, that is, chemisorption of CO2 molecule on the Pt(1 1 1) surface, requires significant activation
2606
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607
energy, which can be estimated by the height of the barrier measured from the right side of the potential plot in Fig. 6 (i.e., for CO2 molecule far from surface). It must be difficult to obtain the chemisorbed state of CO2, because, on the one hand, kinetic energy should be sufficient to overcome the barrier while, on the other hand, afterwards the molecule should exchange kinetic energy with the surface. Now the net LH reaction path for CO oxidation on Pt(1 1 1) can be suggested. At temperatures above 200 K some of oxygen atoms will be promoted to the hcp sites thus becoming chemically active with regard to oxidation of CO molecules. Position of the oxygen atoms in the hcp sites is metastable and therefore they can stay in this state for some time sufficient for interaction with CO. Worth noting is that the energy of promotion of oxygen atoms to the hcp sites does not directly give rise to the potential barrier of the reaction itself, because the promotion and reaction processes are time-separated. Then, a CO molecule, obtaining some thermal excitation with momentum directed towards O(1) adatom in the hcp site, can overcome the potential barrier, provided that its kinetic energy is sufficient. Height of the barrier occurs significantly decreased due to shift of the O(1) adatom, which, in turn, leads to decrease of lateral repulsion with the CO molecule. It should be noted that the height of the barrier for the reaction, obtained in present model calculations, corresponds, in fact, to the ‘‘upper limit’’ of the barrier, because at final temperatures, pertinent to activation of the reaction in nearly perfect p(2 · 2) CO + O layers on Pt(1 1 1) (T = 300 K [3]), the O(1) atom will be already in some excited (vibration) state. This will obviously facilitate the required shift of the O(1) adatom from the hcp site thus decreasing the potential barrier. Another reason why in experiment the barrier can be lower is the starting configuration with regard to CO position. For example, if some position of CO molecule other than in the on-top site in the perfect p(2 · 2) structure (which in reality must be disordered at T = 300 K) were chosen as starting point for simulations, the barrier would be evidently diminished, as may be derived from analysis of the potential plot in Fig. 4. Having the potential barrier passed, forming CO2 molecule will increase its kinetic energy, as suggested in Ref. [33]. From our modeling, performed using a ‘‘cold’’ optimization, it is impossible to derive a detailed picture of how the molecule exchanges its kinetic energy with the Pt(1 1 1) surface and gets some momentum towards vacuum, so we can only guess that this might be accomplished by collisions with Pt atoms and facilitated by the ‘‘spring release’’ effect when desorbing CO2 molecule changes its shape from triangular to linear. 4. Conclusion Our DFT/LDA simulations elucidate, on a microscopic level, essential features of the course of classical Langmuir–Hinshelwood (LH) reaction of catalytic CO oxida-
tion on the Pt(1 1 1) surface. Key to the reaction mechanism is the initial position of oxygen adatom in the threefold hollow site of hcp type and decrease of the lateral repulsion with approaching CO molecule due to redistribution of the electronic density as the O adatom is displaced. Thus, the oxygen atom in the hcp site is bounded substantially weaker than in the fcc site, which makes it more flexible and therefore more reactive. On the other hand, oxygen adatoms in other possible positions on the Pt(1 1 1) surface (such as bridge or atop) cannot readily contribute to the oxidation reaction, because such oxygen atoms unavoidably will be repelled by approaching CO molecules. The calculations have been performed for a particular starting configuration that is the p(2 · 2) structure and stoichiometric coverages of CO and O species on Pt(1 1 1), but the main conclusion should hold also for other cases of the LH reaction between CO molecules and chemisorbed oxygen atoms. References [1] M. Lynch, P. Hu, Surf. Sci. 458 (2000) 1. [2] P.R. Norton, J.A. Davies, T.E. Jackman, Surf. Sci. Lett. 122 (1982) L593. [3] J. Yoshinobu, M. Kawai, J. Chem. Phys. 103 (1995) 3220. [4] A. Szabo´, M. Kiskinova, J.T. Yates Jr., J. Chem. Phys. 90 (1989) 4604. [5] S. Volkening, J. Wintterlin, J. Chem. Phys. 114 (2001) 6382. [6] A. Alavi, P. Hu, T. Deutsch, P.L. Silvestrelli, J. Hutter, Phys. Rev. Lett. 80 (16) (1998) 3650. [7] A. Eichler, J. Hafner, Phys. Rev. B 59 (1999) 5960. [8] J. Wintterlin, S. Volkening, T.W. Janssens, T. Zambelli, G. Ertl, Science 278 (1997) 1931. [9] C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, J. Chem. Phys. 73 (1980) 5862. [10] G. Ertl, Surf. Sci. 299 (1994) 742. [11] J.L. Gland, E.B. Kollin, Surf. Sci. 151 (1985) 260. [12] V.K. Medvedev, Yu. Suchorski, J.H. Block, Appl. Surf. Sci. (76–77) (1994) 136. [13] H.H. Rotermund, S. Jakubith, A. von Oertzen, G. Ertl, J. Chem. Phys. 91 (1989) 4942. [14] S. Akhter, J.M. White, Surf. Sci. 171 (1986) 527. [15] P.J. Feibelman, Phys. Rev. B 56 (1997) 10532. [16] A. Bogicevic, J. Stro¨mquist, B.I. Lundqvist, Phys. Rev. B 57 (1998) R4289. [17] P.J. Feibelman, E. Stefanie, M. Thomas, Phys. Rev. Lett. 77 (1997) 2257. [18] N.V. Petrova, I.N. Yakovkin, Surf. Sci. 578 (2005) 162. [19] N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. [20] M. Fucks, M. Scheffler, Comp. Phys. Commun. 119 (1999) 67. [21] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244; D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 48 (1980) 567. [22] X. Gonze, J.-M. Beuken, R. Caracas, F. Detraux, M. Fuchs, G.-M. Rignanese, L. Sindic, M. Verstraete, G. Zerah, F. Jollet, M. Torrent, A. Roy, M. Mikami, Ph. Ghosez, J.-Y. Raty, D.C. Allan, Comput. Mat. Sci. 25 (2002) 478. [23] P.J. Feibelman, B. Hammer, J.K. Norskov, F. Wagner, M. Scheffler, R. Stumpf, R. Watwe, J. Dumesic, J. Phys. Chem. B 105 (2001) 4018. [24] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [25] N. Materer, U. Starke, A. Barbieri, R. Doll, K. Heinz, M. Van Hove, G.A. Somordjai, Surf. Sci. 325 (1995) 207. [26] N.D. Lang, S. Holloway, J.K. Nørskov, Surf. Sci. 150 (1985) 24; K.L. Kostov, P. Jakob, D. Menzel, Surf. Sci. 377–379 (1997) 802.
I.N. Yakovkin, N.V. Petrova / Surface Science 600 (2006) 2600–2607 [27] H. Steininger, S. Lehwald, H. Ibach, Surf. Sci. 123 (1982) 264; B.N.J. Persson, R. Ryberg, Phys. Rev. B 40 (1989) 20173; D.C. Skelton, D.H. Wei, S.D. Kevan, Surf. Sci. 320 (1994) 77. [28] J.J. Mortensen, B. Hammer, J.K. Norskov, Surf. Sci. 414 (1998) 315; Y. Morikawa, J.J. Mortensen, B. Hammer, J.K. Norskov, Surf. Sci. 386 (1997) 67.
[29] [30] [31] [32] [33]
2607
G. Ertl, M. Neumann, K.M. Streit, Surf. Sci. 64 (1977) 393. D. Ogletree, M. Van Hove, G.A. Somordjai, Surf. Sci. 173 (1986) 351. Y.Y. Yeo, L. Vattuone, D. King, J. Phys. Chem. 106 (1997) 391. N.V. Petrova, I.N. Yakovkin, Surf. Sci. 519 (2002) 90. K.-H. Allers, H. Pfnuer, P. Feulner, D. Menzel, J. Chem. Phys. 100 (1994) 3985.