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Pt(1 1 1)
Surface Science 514 (2002) 394–403 www.elsevier.com/locate/susc
A density-functional study of the atomic structures and vibrational spectra of NO/Pt(1 1 1) H. Aizawa
a,*
, Y. Morikawa b, S. Tsuneyuki
c,1
, K. Fukutani d, T. Ohno
a
a
b
Computational Materials Science Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Institute for Computational Sciences (RICS), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan c Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan d Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-5805, Japan Received 12 December 2001; accepted for publication 14 February 2002
Abstract We performed ab initio plane-wave calculations for NO/Pt(1 1 1) using a slab model. The results show that at a low coverage of 0.25 ML, the fcc-hollow site is the most stable adsorption site. Our calculations at a higher coverage of 0.50 ML indicate that NO molecules prefer to be adsorbed at both fcc-hollow and atop sites rather than only at hollow sites. This adsorption arrangement is consistent with a recent scanning tunneling microscopy experiment. The calculations of the peak intensities of fcc-hollow and atop species reveal that the peak corresponding to the fcc-hollow species becomes very small in the presence of the atop species. This effect turns out not to be due to the well-known intensity-transfer effect derived from the dynamic dipole–dipole coupling, but to be related to a change of the electronic state of the adsorption system by the addition of the atop species. This conclusion warns experimentalists using vibrational spectroscopy that spectra they measure should be interpreted very carefully. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Nitrogen oxides; Platinum; Metallic surfaces; Chemisorption
1. Introduction The reduction of toxic nitric oxygen (NOx ) gas in the automobile exhaust is one of the most important reactions for preventing air pollution [1]. * Corresponding author. Tel.: +81-298-59-2616; fax: +81298-59-2601. E-mail address: [email protected] (H. Aizawa). 1 Present address: Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
This reaction along with the oxidation of hydrocarbons (HC) and carbon monoxide (CO) proceed efficiently with the use of the so-called three-way catalyst (TWC). TWC usually contains metal particles consisting of rhodium, platinum, and palladium deposited on an oxide support. Therefore it is no wonder that much effort has been devoted to the study of the simplest NOx molecule, that is, nitric oxide (NO), adsorbed on surfaces of those transition metals. Surprisingly, however, the adsorption site and geometry of such a simple system as NO on Pt(1 1 1) has long been misunderstood.
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 6 5 8 - 8
H. Aizawa et al. / Surface Science 514 (2002) 394–403
In 1980, Gland et al. observed electron energy loss spectra (EELS) for this system [2], and three years later Hayden measured infrared reflection absorption spectra (IRAS) [3]. The results were essentially the same in the following two respects: (1) At low coverages of NO, an N–O stretch peak developed around 1490 cm1 ; (2) as the coverage was increased, this peak started attenuating, and another N–O stretch peak began to develop around 1710 cm1 . The only difference between the two measurements was that at the saturation coverage, the peak around 1490 cm1 remained with a small intensity in the EELS, while it disappeared almost completely in the case of the IRAS. By comparison of the measured frequencies to those of a variety of nitrosyl complexes, the peaks at 1490 and 1710 cm1 were considered to be derived from bridge and atop species, respectively. Therefore, it has long been believed that at low coverages NO molecules are adsorbed at bridge sites of the Pt(1 1 1) surface, and as the coverage is increased they move from bridge sites to atop sites. In 1993, however, Materer et al. performed lowenergy electron diffraction (LEED) experiments [4], and proposed a model in which NO molecules are adsorbed at fcc-hollow sites at a coverage of 0.25 ML. Also, density-functional calculations with the generalized gradient approximations done by Ge et al. indicated that the fcc-hollow site is the most stable adsorption site at a coverage of 0.25 ML [5], supporting the LEED result. The studies by Materer et al. [4] and Ge et al. [5] did not explain the existence of a large peak around 1710 cm1 at high coverages in the previous vibrational spectroscopic data [2,3], which is apparently outside the frequency range for the three-fold hollow species. Recently, Matsumoto et al. measured scanning tunneling microsopy (STM) images [6,7] and high-resolution EELS (HREELS) data [7], according to which they proposed a structural model where the fcc-hollow and atop species coexist on the surface at a coverage of 0.5 ML. It is still unclear why there is only one major peak observed in the vibrational spectra [2,3,7] at high coverages, while there should be two corresponding to the fcc-hollow and atop species. In order to clarify its reason as well as to definitely determine the atomic structures of NO on Pt(1 1 1) at various
395
coverages, we performed ab initio calculations for NO/Pt(1 1 1) using a slab model and a plane-wave basis set. In Section 2, we describe the methods of calculations we employed. We first performed calculations for an NO coverage of 0.25 ML, because this is the simplest case consistent with the clear appearance of the observed p(2 2) LEED patterns. The results for 0.25 ML are shown in Section 3.1. We next tried calculations for higher coverages of 0.50 and 0.75 ML, the results of which are described in Sections 3.2 and 3.3, respectively. The discussion is given in Section 4, with a particular emphasis laid on the calculated peak intensities of vibrational spectra at the three coverages we studied. The conclusion is given in Section 5.
2. Methods of calculations All our calculations were carried out using a program package Simulation Tool for Atom Technology (STATE), which has been successfully applied for both metal and semiconductor surfaces [8–12]. The calculations were based upon the density-functional theory (DFT) [13] with the generalized gradient approximation (GGA) [14,15]. The PBE-type exchange-correlation functional was used [16]. Only valence electrons (5d and 6s for Pt, and 2s and 2p for N and O) were treated explicitly. Ultrasoft pseudo-potentials [17] were employed for the N 2p, O 2p, and Pt 5d states, while norm-conserving pseudo-potentials [18] were used for the other states. Wavefunctions were expanded by plane waves whose cutoff energy was taken to be 30.25 Ry. The pseudo-potential for Pt used in the present calculations yields the lattice and the bulk modulus of 257 constant of 3.95 A GPa for the Pt fcc crystal, in fair agreement with and 278 GPa. the experimental values of 3.92 A The bond distance, bond energy, and harmonic stretch frequency of an NO molecule calculated with the present pseudo-potentials for N and O are , 7.0 eV, and 1906 cm1 , respectively, again 1.17 A in good accord with the experimental values of 1.15 , 6.5 eV, and 1904 cm1 . Two-dimensional 4 4 A uniform k-point mesh was used to sample the surface Brillouin zone. The electron occupation
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numbers were smeared around the Fermi level according to the method of Methfessel and Paxton [19]. This method is essential in order to get precise atomic forces (and therefore phonon frequencies) when only a moderate number of k-points can be used [20]. We used a slab consisting of an NO molecule layer and three Pt layers. The NO molecule layer as well as the upper two Pt layers were allowed to relax in geometrical optimization procedures. The surface unit cell used has a periodicity of p(2 2), because the LEED experiments showed clear p(2 2) patterns [4]. Although a gasphase NO molecule is spin polarized, we did not take account of the spin-polarization effect in the calculations for the chemisorbed systems, because all previous calculations of NO on transition-metal surfaces (except for noble-metal surfaces such as Ag [21] where only weak adsorption of NO takes place) indicate that chemisorbed NO is not spin polarized due to a strong interaction between the adsorbate and the substrate [22–25]. 3. Results 3.1. For an NO coverage of 0.25 ML Table 1 shows the adsorption energies and N–O stretch frequencies calculated with one NO molecule put in the p(2 2) unit cell, corresponding to 0.25 ML. This coverage of 0.25 ML can be regarded as the low-coverage limit, because the adsorption energy of, e.g., the fcc-hollow species changes little from 2.09 to 2.16 eV when the coverage is decreased from 0.25 to 0.0625 ML. As shown in Table 1, the fcc-hollow site is the most stable adsorption site, and the hcp-hollow site is Table 1 The adsorption energies and N–O stretch frequencies of NO molecules adsorbed at several adsorption sites of the Pt(1 1 1) surface (This table also indicates whether the N–O axis of each adsorbed NO molecule is perpendicular to the surface or not.) Adsorption site
Adsorption energy (eV)
N–O frequency (cm1 )
N–O axis perpendicular?
fcc-hollow hcp-hollow Atop
2.09 1.92 1.61
1512 1539 1683
Yes Yes No
the second stable. The bridge site turned out to be an unstable site, that is, it does not correspond to an energy local minimum. The atop site corresponds to a local minimum, but with a much lower adsorption energy as compared to those of the hollow species. Recently, Feibelman et al. argued that well-converged DFT-GGA calculations predict wrong site preference for CO/Pt(1 1 1), that is, the fcc-hollow site adsorption instead of the experimentally established atop site adsorption [26]. We agree with them that there is a tendency of GGA calculations to overestimate the adsorption energy at a high coordination site more significantly than that at a low coordination site. We believe, however, that such a tendency results in a wrong prediction for the site preference only in subtle cases like CO/Pt(1 1 1), where the GGA energy difference between the fcc-hollow and atop adsorption is typically less than 0.2 eV [26]. In the present case of NO/Pt(1 1 1), the calculated energy difference between the fcc-hollow and atop adsorption is as much as 0.5 eV, and we consider this value to be large enough to lead to the conclusion that NO molecules are adsorbed at fcc-hollow sites in the low coverage regime up to 0.25 ML. Structural optimizations for the fcc- and hcphollow species resulted in the NO molecule perpendicular to the surface with the N-end down. Fig. 1 shows the top and side views of the fcchollow species. On the other hand, a structural optimization for the atop species resulted in a bent geometry rather than a linear geometry, as shown in Fig. 2. It is interesting to note that NO molecules adsorbed at atop sites of Pd(1 1 1) and Rh(1 1 1) have a bent and linear geometry, respectively, as verified by our calculations performed with the same scheme as the present one. So far, many researchers have believed that the atop NO species on Pt(1 1 1) (and probably also on Pd(1 1 1)) is perpendicular to the surface, mainly because there seemed to be no peak corresponding to the frustrated rotational mode in EELS observations [2, 27]. (In fact, there is such a peak as will be discussed in Section 4.) The adsorption energy of atop NO on Pt(1 1 1) calculated by Ge et al. is as low as 0.64 eV probably as a result of assuming a linear geometry [5]. Also, Loffreda et al. [28] performed calculations for NO on Pd(1 1 1) assuming
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Fig. 1. (a) The top and (b) side views of the NO molecules adsorbed at fcc-hollow sites of the Pt(1 1 1) surface at a coverage of 0.25 ML. The small black and grey spheres represent oxygen and nitrogen atoms, respectively. The large grey spheres represent Pt atoms. For clarity the surface Pt atoms are shown with a little darker spheres than the other Pt atoms. The dashed lines indicate the p(2 2) , respectively. The NO molecules are perpendicular to the surface. unit cells. The N–O and Pt–N distances are 1.22 and 2.07 A
Fig. 2. (a) The top and (b) side views of the NO molecules adsorbed at atop sites of the Pt(1 1 1) surface at a coverage of 0.25 ML. See , the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The N–O and Pt–N distances are 1.17 and 1.93 A respectively. The NO molecules are tilted from the surface normal. The tilt angles of the N–O and Pt–N bonds with respect to the surface normal are 52° and 3°, respectively. The N atom is almost right on top of a surface Pt atom.
a linear geometry of the atop species and obtained the N–O stretch frequency somewhat larger than the experimental value. If the tilt of the atop NO had been taken into account, the frequency would have been smaller because of the increased interaction of the anti-bonding 2p molecular orbitals of NO with the metal d orbitals, resulting in better agreement with the experiment. Now we turn to the calculated N–O stretch frequencies which are also listed in Table 1. The value obtained for the fcc-hollow species is 1512 cm1 , in good agreement with the experimental value of about 1490 cm1 . This fact further supports our conclusion that at low coverages NO first adsorbs at a fcc-hollow site of the Pt(1 1 1) surface. As far as the atop species is concerned, the calculated value of 1683 cm1 is reasonably close to the experimental value of about 1710 cm1 at high coverages. This indicates that the high-
frequency peak which appears at high coverages in the vibrational spectra actually corresponds to the presence of the atop species. Although the LEED study of Materer et al. [4] and the calculational study of Ge et al. [5] insisted that NO molecules are adsorbed at fcc-hollow sites at a coverage of 0.25 ML, they did not argue about the presence of the atop species. Here we would like to remind the readers of the traditional picture by Gland and Sexton [2], which is two-fold. One point is that at low coverages bridge sites of the Pt(1 1 1) surface are first occupied by NO molecules. This has been shown to be erroneous as mentioned above, and it has turned out that fcc-hollow sites instead of bridge sites are occupied in the low coverage regime. Such a misunderstanding originates from the conventional method of determining the adsorption site of NO at metal surfaces based on a comparison of the observed N–O stretch frequency
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of the adsorbate to those of metal nitrosyl compounds. The invalidity of such a methodology has been already pointed out by Loffreda et al. for NO on Pd(1 1 1) and Rh(1 1 1) [28]. The other, more important, point of the picture by Gland and Sexton [2] is that they claimed the bridge NO species (actually turning out to be the fcc-hollow species) to leave the bridge sites and move to atop sites as the NO coverage increases. According to the adsorption energetics shown in Table 1, however, there seems to be no reason why the fcchollow species move to atop sites as far as the coverage is lower than 0.25 ML, because the atop site is much less stable than the fcc-hollow site. Now it is clear that one of the most important problems to solve is why the atop species exist irrespective of the relative instability compared to the hollow species. In the next subsection, we tried calculations for a higher coverage of 0.50 ML by putting two NO molecules in the p(2 2) unit cell. 3.2. For an NO coverage of 0.50 ML Because the fcc- and hcp-hollow species are much more stable than the other species when one NO molecule is put in the p(2 2) unit cell, one would anticipate that only hollow sites will be occupied even when two molecules exist in the unit cell. In fact, there are many unoccupied hollow sites in the unit cell which await to be occupied. Our calculation, however, revealed that this is not the case. We tried two combinations of hollow
species, as shown in Fig. 3(a) and (b). In Fig. 3(a), only fcc-hollow sites are occupied, forming a p(2 1) periodicity. Even if the molecular arrangement has a p(2 1) periodicity, there will be three domains rotated by 120° with respect to each other, exhibiting a p(2 2) LEED pattern. In Fig. 3(b), fcc-hollow sites as well as hcp-hollow sites are occupied, forming a honeycomb network. We actually found that the structures of Fig. 3(a) and (b) are less stable by 0.22 and 0.39 eV per unit cell, respectively, than the combination of the fcchollow and atop species. This combination was originally proposed by Matsumoto et al. [7]. The top and side views of this most stable structure are illustrated in Fig. 4. Now we discuss why this structure is more stable than the ones consisting only of hollow species. Our calculations for 0.25 ML of NO show that fcc- and hcp-hollow adsorption induces significant surface atom relaxation, i.e., the expansion of the Pt-atom triangle supporting a three-fold hollow NO molecule. Therefore when hollow-site adsorption occurs at several neighboring places at a high-coverage surface, the expansion of one Pt-atom triangle conflicts with that of another Pt-atom triangle, resulting in appreciable energy loss. The most favorable structure shown in Fig. 4 does not involve such energy loss. Although the fcc-hollow site adsorption induces a large surface atom relaxation, the neighboring atop species does not, avoiding conflict. Recently, Matsumoto et al. observed STM images for NO/Pt(1 1 1) [6,7]. In their STM images,
Fig. 3. The top views of the two combinations of hollow site adsorption at a coverage of 0.50 ML: (a) fcc-hollow þ fcc-hollow (forming a 2 1 periodicity); (b) fcc-hollow þ hcp-hollow (forming a honeycomb structure). See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The adsorption energies of the NO molecules are (a) 3.41 eV/cell and (b) 3.24 eV/cell.
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Fig. 4. (a) The top and (b) side views of the most stable molecular arrangement (fcc-hollow þ atop) at a coverage of 0.50 ML. See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The atop NO molecules are tilted from the surface normal. The tilt angles of the N–O and Pt–N bonds with respect to the surface normal are 49° and 4°, respectively. The adsorption energies of the NO molecules are 3.63 eV/cell, i.e., 1.82 eV per molecule.
there is a p(2 2) background hexagonal lattice consisting of dark features as well as a p(2 2) array comprised of bright features. The dark features are considered to represent fcc-hollow species, because reducing the local coverage by electronstimulated desorption results in the appearance of an area where only the dark features are observable. Then there are two kinds of triangles consisting of neighboring three dark features in the hexagonal array. One is centered at a hcp-hollow site, and the other is centered at an atop site. Since in their STM images almost all the bright features are seen only at the centers of one kind of triangle, the coexistence of fcc-hollow species with either hcp-hollow species or atop species can be possible. Considering the difference in brightness between the dark and bright features, the height of the two kinds of NO molecules should be quite different. Matsmoto et al. reported the height difference to [7]. Our structural optimization revealed be 0.4 A that in the structure of Fig. 4 the oxygen and , nitrogen atoms are located by 0.40 and 0.86 A respectively, higher at the atop site than at the fcc-hollow site. In the case of the structure of Fig. 3(b) where the fcc- and hcp-hollow species coexist, the height differences of the oxygen and , respecnitrogen atoms are only 0.04 and 0.05 A tively, which would result in very small contrast in the STM images. Therefore the structure of Fig. 4 where fcc-hollow and atop species coexist at the surface is quite consistent with the observed STM images. Still, this structure seems inconsistent with the vibrational spectra where the low-frequency
peak corresponding to the fcc-hollow species is greatly attenuated or even disappears at high coverages. Our calculations solve this contradiction, as will be discussed in detail in Section 4. 3.3. For an NO coverage of 0.75 ML With the conclusion that the structure shown in Fig. 4 is the most stable at a coverage of 0.50 ML, it is natural to think that if NO molecules are added further they will adsorb at the hcp-hollow sites located at the center of the triangle consisting of the occupied fcc-hollow sites. In fact, the STM image by Matsumoto et al. exhibits a few bright features considered to correspond to such hcphollow species [7]. The structure obtained as a result of a structural optimization is illustrated in Fig. 5. In this structure, the hcp-hollow species moves somewhat upward as compared to the case of Fig. 3(b), and the oxygen and nitrogen height differences between the fcc- and hcp-hollow species , respectively. Therefore it become 0.08 and 0.09 A is likely that the hcp-hollow species are observed as a little brighter features than the fcc-hollow species. Molecular arrangements similar to Fig. 5 have been recently proposed for CO/Rh(1 1 1) [29–32], NO/Rh(1 1 1) [33,34], and NO/Ru(0 0 0 1) [35], although the atop CO or NO is perpendicular to the surface in all the three cases. The former two systems had long been believed to consist of one bridge and two near-atop species based on vibrational spectra and LEED analyses [36–38], and it is
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Fig. 5. The most likely molecular arrangement (fcc-hollow þ atop þ hcp-hollow) at a coverage of 0.75 ML. See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The adsorption energies of the NO molecules are 4.68 eV/cell, i.e., 1.56 eV per molecule.
only recently that the correct structure consisting of one atop and two hollow species as shown in Fig. 5 has been proposed. The misleading features of vibrational spectra for NO as well as CO chemisorbed on metal surfaces will be discussed in detail in the next section. In the structure of NO/Pt(1 1 1) shown in Fig. 5, the adsorption energies of hcp-hollow, atop, and fcc-hollow species are calculated to be 1.06, 1.54, and 2.09 eV, respectively. Since the adsorption energy of the hcp-hollow species is as low as 1 eV, we consider that this species corresponds to the low-temperature feature around 200 K seen in the thermal desorption spectra [39]. As discussed by Materer et al. [4] and Matsumoto et al. [7], exposure of the Pt(1 1 1) surface to NO at a low temperature followed by annealing to 220–250 K yields a well-ordered p(2 2) structure. Probably annealing removes the weakly bound hcp-hollow species causing some disorder, and achieves a wellordered surface as shown in Fig. 4. Fukutani et al. performed experimental studies of photo-induced desorption of NO/Pt(1 1 1) [40]. When laser was irradiated onto an as-adsorbed surface (like Fig. 5) and an annealed surface (like Fig. 4), they found that the rotational-state population of the desorbed NO molecules exhibits Boltzmann and non-Boltzmann distributions, respectively. Also, Song et al. observed that NO molecules are photo-dissociated by irradiating
laser onto a Pt(1 1 1) surface at low NO coverages like Fig. 1 [41]. Therefore, we consider that strongly bound fcc-hollow species are not desorbed but dissociated by laser irradiation, while less-strongly bound atop and hcp-hollow species are photo-desorbed. A non-Boltzmann distribution is observed when bent atop species are desorbed, while Boltzmann distribution is obtained when perpendicular hcp-hollow species are desorbed. In fact, there are several theories claiming that a non-Boltzmann rotational-energy distribution of the desorbed species results from a bent geometry in their adsorbed state [42,43].
4. Discussion In this section we discuss the origin of the misleading features of vibrational spectra observed for NO as well as CO chemisorbed on transitionmetal surfaces. Table 2 shows the calculated frequencies and peak intensities of the normal modes for NO/Pt(1 1 1) at the three coverages we studied. The peak intensity of a normal mode in vibrational spectroscopy is known to be proportional to its squared dynamic dipole moment [8,10,11]. In the case of EELS, the peak intensity also depends on the frequency [10,11], which was neglected in the present work. The dynamic dipole moment of a normal mode was calculated from the change in the work function difference between the two sides of a slab when the adsorbate atoms were displaced according to the normal mode. Only the normal modes whose squared dynamic dipole moments are larger than 0.003 (in arbitrary units) are listed in Table 2. Before discussing the N–O stretch modes, we briefly comment on the other modes. The results for the cases of 0.25 ML reveal that the Pt–NO stretch frequency is almost the same for the fcchollow (326 cm1 ) and atop (301 cm1 ) species. At high coverages, the Pt–NO stretch modes of the hollow species have almost zero intensity probably due to the electronic-state effect which will be discussed below for the N–O stretch modes. In the observed EELS data [2,7], as the coverage was increased a peak with a frequency of about 450 cm1 started developing together with the N–O
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Table 2 The normal-mode frequencies (in cm1 ) and the peak intensities (i.e., squared dynamic dipole moments, in parentheses) of NO/Pt(1 1 1) at various coverages Coverage (ML)
0.25
0.25
0.50
0.50
0.50
0.75
Adsorption Site
fcc-hollow
atop
fcc-hollow þ atop
fcc-hollow þ atop (atop fixed)
fcc-hollow þ atop (hollow fixed)
fcc-hollow þ atop þ hcp-hollow
Structure
Fig. 1
Fig. 2
Fig. 4
Fig. 4
Fig. 4
Fig. 5
Pt–NO stretch 1 Pt–NO stretch 2 NO rotation N–O stretch 1 N–O stretch 2 N–O stretch 3
326 (0.005) 301 (0.010)
300 (0.009)
301 (0.010)
293 (0.006)
483 (0.005)
472 (0.003) 1447 (0.147)
472 (0.003)
484 (0.005) 1463 (0.020) 1540 (0.111) 1708 (1.208)
1512 (1.171) 1683 (2.089)
1703 (1.475)
1459 (0.363) 1699 (1.259)
Only the modes with intensities larger than 0.003 are shown. ‘‘Pt–NO stretch 1’’ and ‘‘Pt–NO stretch 2’’ represent the Pt–NO stretch modes of the fcc-hollow and atop species, respectively. The Pt–NO stretch modes of the hollow species become very small in intensity in the presence of the atop species. ‘‘NO rotation’’ represents the frustrated rotational mode of the atop NO species. The corresponding modes of the hollow NO species have zero intensity because of the perpendicular geometry. ‘‘N–O stretch 1’’ and ‘‘N–O stretch 2’’ correspond to the N–O stretch modes of the hollow species, while ‘‘N–O stretch 3’’ corresponds to that of the atop species. See text for the significance of the columns with ‘‘(atop fixed)’’ and ‘‘(hollow fixed)’’.
stretch mode of the atop species around 1710 cm1 . Since there was no other peak which developed together, one might consider that the peak around 450 cm1 corresponds to the Pt–NO stretch mode of the atop species. Our calculations, however, reveal that this peak corresponds to the NO frustrated rotational mode which is dipole active only when NO has a bent geometry. Now we turn to the main issue of the N–O stretch modes. As can be seen from Table 2, the N–O stretch mode of the fcc-hollow species at a coverage of 0.25 ML has a frequency of 1512 cm1 , and its intensity is 1.171. At a coverage of 0.50 ML where fcc-hollow species coexist with atop species, the peak intensity of the above mode is decreased down to as small as 0.147. This magnitude is about one tenth of that for the peak of the atop species. This is surprising because the fcchollow species exist at the surface as much as the atop species. The calculations reproduce very well the experimental fact that at high coverages the peak intensity of the low-frequency N–O stretch mode was greatly diminished [2,7] or even disappeared [3] in the vibrational spectra, the phenomenon misleading researchers for a long time. Note that the disappearance of the low-frequency N–O
stretch mode in IRAS has been also reported for NO/Ru(0 0 0 1), although the mode can be observed with a small intensity by EELS just as in the case of NO/Pt(1 1 1). It was only recently proposed that the disappearance of the peak in IRAS might result from the presence of surface contaminants and other imperfections [44]. In fact, the low-frequency N–O stretch mode which could not be detected in old IRAS studies for NO/Ru(0 0 0 1) has been observed as a small peak in a recent refined IRAS study [44]. EELS seems less sensitive to such contaminants and imperfections. Such misleading features of vibrational spectra as discussed above are not limited to NO/Pt(1 1 1) but can occur for a variety of systems with CO or NO chemisorbed at transition-metal surfaces. In fact, as briefly commented in Section 3.3, the structure of CO/Rh(1 1 1) has also been misunderstood for a long time due to such a misleading feature of its vibrational spectrum. At the saturation coverage of this system, Dubois et al. observed two C–O stretch peaks at 1870 and 2070 cm1 , which were considered to correspond to bridge and near-atop species, respectively [36]. Since the peak area of the former mode is about a
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half of that of the latter mode, they concluded that two near-atop species and one bridge species exist in the p(2 2) unit cell. As mentioned in Section 3.3, the correct structure determined only recently by X-ray related techniques [29,32] consists of one atop and two hollow species, as shown in Fig. 5. This example also indicates that peak intensities of vibrational spectra are almost impossible to interpret without the help of theoretical calculations. It has been well known that if there are two modes with their frequencies close to each other (typically the frequency difference of the order of 10 cm1 ), the intensity of the lower-frequency peak is transferred to that of the higher-frequency peak. This phenomenon is due to the dynamic dipole– dipole coupling and called the ‘‘intensity transfer’’ effect. Such an effect, however, is very unlikely to take place between the modes with a frequency difference of about 200 cm1 as in the present cases. In order to clarify the origin of the calculated results for NO/Pt(1 1 1), we calculated the N–O stretch frequency and peak intensity for the fcc-hollow species without taking account of the motion of the atop species at the 0.50 ML structure of Fig. 4. By performing such procedures, we can obtain the frequency and intensity without the effect of the dynamic dipole–dipole coupling between the fcc-hollow and atop species. The calculated frequency and intensity are 1459 cm1 and 0.363, respectively. This implies that the peak intensity of the N–O stretch mode of the fcc-hollow species is reduced from 1.171 to 0.363 just by the presence of the atop species, even without the effect of dynamic dipole–dipole coupling. The effect of dynamic dipole–dipole coupling only reduces the intensity from 0.363 to 0.147. We also performed the opposite calculations, i.e., calculations of the N–O stretch frequency and peak intensity for the atop species without taking account of the motion of the fcchollow species. The results are 1699 cm1 and 1.259. The peak intensity of the atop species is also dramatically reduced from 2.089 to 1.259 by the presence of the fcc-hollow species. Due to the intensity transfer effect, however, the higher-frequency atop peak gains some intensity from the lower-frequency fcc-hollow peak, resulting in a
slight increase from 1.259 to 1.475 in the 0.50 ML structure of Fig. 4. We consider that the large reduction from 1.171 (2.089) to 0.363 (1.259) for the fcc-hollow (atop) species is due to a change in the electronic structure of the whole adsorbed system by the addition of the atop (fcc-hollow) species. For example, electron transfer between the NO molecules and the substrate Pt surface might be promoted as the different species come to exist at the surface, and the charged states of NO molecules might be changed resulting in a change of the dynamic dipole moment. Further analysis of the electronic structure of the adsorbed system is clearly in order.
5. Conclusion Our calculations for NO/Pt(1 1 1) have definitely determined that at coverages up to 0.25 ML fcc-hollow sites are occupied by NO, and as the coverage is increased over 0.25 ML the atop species come to coexist with the fcc-hollow species. Although the measured vibrational spectra seem inconsistent with this picture, our calculations reproduce them very well, indicating that there is no inconsistency. We have revealed that the peak intensities of the N–O stretch modes for NO molecules adsorbed at different sites of transition-metal surfaces sometimes exhibit quite misleading behaviors, warning experimentalists to interpret their vibrational spectroscopic data very carefully.
Acknowledgements We wish to thank M. Matsumoto at Institute of Industrial Science, University of Tokyo, and M. Scheffler, H.-J. Freund and T. Kl€ uner at FritzHaber-Institut der Max-Planck-Gesellschaft for valuable discussions. The calculations were performed using the Numerical Materials Simulator (NEC SX-5) at National Institute for Materials Science and Hitachi SR-8000 at Institute for Solid State Physics, University of Tokyo. This research is supported by ACT-JST.
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[22] M.-H. Tsai, K.C. Hass, Phys. Rev. B 51 (1995) 14616. [23] K.C. Hass, M.-H. Tsai, R.V. Kasowski, Phys. Rev. B 53 (1996) 44. [24] B. Hammer, J.K. Nørskov, Phys. Rev. Lett. 79 (1997) 4441. [25] B. Hammer, Phys. Rev. Lett. 83 (1999) 3681. [26] P.J. Feibelman, B. Hammer, J.K. Nørskov, F. Wagner, M. Scheffler, R. Stumpf, R. Watwe, J. Dumesic, J. Phys. Chem. B 105 (2001) 4018. [27] M.E. Bartram, B.E. Koel, E.A. Carter, Surf. Sci. 219 (1989) 467. [28] D. Loffreda, D. Simon, P. Sautet, Chem. Phys. Lett. 291 (1998) 15. [29] A. Beutler, E. Lundgren, R. Nyholm, J.N. Andersen, B. Setlik, D. Heskett, Surf. Sci. 371 (1997) 381. [30] M. Gierer, A. Barbieri, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 391 (1997) 176. [31] A. Beutler, E. Lundgren, R. Nyholm, J.N. Andersen, B. Setlik, D. Heskett, Surf. Sci. 396 (1998) 117. [32] E. Lundgren, X. Torrelles, J. Alvarez, S. Ferrer, H. Over, A. Beutler, J.N. Andersen, Phys. Rev. B 59 (1999) 5876. [33] Y.J. Kim, S. Thevuthasan, G.S. Herman, C.H.F. Peden, S.A. Chambers, D.N. Belton, H. Permana, Surf. Sci. 359 (1996) 269. [34] I. Zasada, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 418 (1998) L89. [35] M. Stichler, D. Menzel, Surf. Sci. 391 (1997) 47. [36] L.H. Dubois, G.A. Somorjai, Surf. Sci. 91 (1980) 514. [37] M.A. Van Hove, R.J. Koestner, J.C. Frost, G.A. Somorjai, Surf. Sci. 129 (1983) 482. [38] C.-T. Kao, G.S. Blackman, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 224 (1989) 77. [39] R.J. Gorte, L.D. Schmidt, J.L. Gland, Surf. Sci. 109 (1981) 367. [40] K. Fukutani, Y. Murata, R. Schwarzwald, T.J. Chuang, Surf. Sci. 311 (1994) 247. [41] M.-B. Song, M. Suguri, K. Fukutani, F. Komori, Y. Murata, Appl. Surf. Sci. 79/80 (1994) 25. [42] F.M. Zimmermann, W. Ho, Phys. Rev. Lett. 72 (1994) 1295. [43] Y. Murata, K. Fukutani, J. Mol. Struct. 352–353 (1995) 519. [44] P. Jacob, M. Stichler, D. Menzel, Surf. Sci. 370 (1997) L185.
A density-functional study of the atomic structures and vibrational spectra of NO/Pt(1 1 1) H. Aizawa
a,*
, Y. Morikawa b, S. Tsuneyuki
c,1
, K. Fukutani d, T. Ohno
a
a
b
Computational Materials Science Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Institute for Computational Sciences (RICS), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan c Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan d Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-5805, Japan Received 12 December 2001; accepted for publication 14 February 2002
Abstract We performed ab initio plane-wave calculations for NO/Pt(1 1 1) using a slab model. The results show that at a low coverage of 0.25 ML, the fcc-hollow site is the most stable adsorption site. Our calculations at a higher coverage of 0.50 ML indicate that NO molecules prefer to be adsorbed at both fcc-hollow and atop sites rather than only at hollow sites. This adsorption arrangement is consistent with a recent scanning tunneling microscopy experiment. The calculations of the peak intensities of fcc-hollow and atop species reveal that the peak corresponding to the fcc-hollow species becomes very small in the presence of the atop species. This effect turns out not to be due to the well-known intensity-transfer effect derived from the dynamic dipole–dipole coupling, but to be related to a change of the electronic state of the adsorption system by the addition of the atop species. This conclusion warns experimentalists using vibrational spectroscopy that spectra they measure should be interpreted very carefully. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Nitrogen oxides; Platinum; Metallic surfaces; Chemisorption
1. Introduction The reduction of toxic nitric oxygen (NOx ) gas in the automobile exhaust is one of the most important reactions for preventing air pollution [1]. * Corresponding author. Tel.: +81-298-59-2616; fax: +81298-59-2601. E-mail address: [email protected] (H. Aizawa). 1 Present address: Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
This reaction along with the oxidation of hydrocarbons (HC) and carbon monoxide (CO) proceed efficiently with the use of the so-called three-way catalyst (TWC). TWC usually contains metal particles consisting of rhodium, platinum, and palladium deposited on an oxide support. Therefore it is no wonder that much effort has been devoted to the study of the simplest NOx molecule, that is, nitric oxide (NO), adsorbed on surfaces of those transition metals. Surprisingly, however, the adsorption site and geometry of such a simple system as NO on Pt(1 1 1) has long been misunderstood.
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 6 5 8 - 8
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In 1980, Gland et al. observed electron energy loss spectra (EELS) for this system [2], and three years later Hayden measured infrared reflection absorption spectra (IRAS) [3]. The results were essentially the same in the following two respects: (1) At low coverages of NO, an N–O stretch peak developed around 1490 cm1 ; (2) as the coverage was increased, this peak started attenuating, and another N–O stretch peak began to develop around 1710 cm1 . The only difference between the two measurements was that at the saturation coverage, the peak around 1490 cm1 remained with a small intensity in the EELS, while it disappeared almost completely in the case of the IRAS. By comparison of the measured frequencies to those of a variety of nitrosyl complexes, the peaks at 1490 and 1710 cm1 were considered to be derived from bridge and atop species, respectively. Therefore, it has long been believed that at low coverages NO molecules are adsorbed at bridge sites of the Pt(1 1 1) surface, and as the coverage is increased they move from bridge sites to atop sites. In 1993, however, Materer et al. performed lowenergy electron diffraction (LEED) experiments [4], and proposed a model in which NO molecules are adsorbed at fcc-hollow sites at a coverage of 0.25 ML. Also, density-functional calculations with the generalized gradient approximations done by Ge et al. indicated that the fcc-hollow site is the most stable adsorption site at a coverage of 0.25 ML [5], supporting the LEED result. The studies by Materer et al. [4] and Ge et al. [5] did not explain the existence of a large peak around 1710 cm1 at high coverages in the previous vibrational spectroscopic data [2,3], which is apparently outside the frequency range for the three-fold hollow species. Recently, Matsumoto et al. measured scanning tunneling microsopy (STM) images [6,7] and high-resolution EELS (HREELS) data [7], according to which they proposed a structural model where the fcc-hollow and atop species coexist on the surface at a coverage of 0.5 ML. It is still unclear why there is only one major peak observed in the vibrational spectra [2,3,7] at high coverages, while there should be two corresponding to the fcc-hollow and atop species. In order to clarify its reason as well as to definitely determine the atomic structures of NO on Pt(1 1 1) at various
395
coverages, we performed ab initio calculations for NO/Pt(1 1 1) using a slab model and a plane-wave basis set. In Section 2, we describe the methods of calculations we employed. We first performed calculations for an NO coverage of 0.25 ML, because this is the simplest case consistent with the clear appearance of the observed p(2 2) LEED patterns. The results for 0.25 ML are shown in Section 3.1. We next tried calculations for higher coverages of 0.50 and 0.75 ML, the results of which are described in Sections 3.2 and 3.3, respectively. The discussion is given in Section 4, with a particular emphasis laid on the calculated peak intensities of vibrational spectra at the three coverages we studied. The conclusion is given in Section 5.
2. Methods of calculations All our calculations were carried out using a program package Simulation Tool for Atom Technology (STATE), which has been successfully applied for both metal and semiconductor surfaces [8–12]. The calculations were based upon the density-functional theory (DFT) [13] with the generalized gradient approximation (GGA) [14,15]. The PBE-type exchange-correlation functional was used [16]. Only valence electrons (5d and 6s for Pt, and 2s and 2p for N and O) were treated explicitly. Ultrasoft pseudo-potentials [17] were employed for the N 2p, O 2p, and Pt 5d states, while norm-conserving pseudo-potentials [18] were used for the other states. Wavefunctions were expanded by plane waves whose cutoff energy was taken to be 30.25 Ry. The pseudo-potential for Pt used in the present calculations yields the lattice and the bulk modulus of 257 constant of 3.95 A GPa for the Pt fcc crystal, in fair agreement with and 278 GPa. the experimental values of 3.92 A The bond distance, bond energy, and harmonic stretch frequency of an NO molecule calculated with the present pseudo-potentials for N and O are , 7.0 eV, and 1906 cm1 , respectively, again 1.17 A in good accord with the experimental values of 1.15 , 6.5 eV, and 1904 cm1 . Two-dimensional 4 4 A uniform k-point mesh was used to sample the surface Brillouin zone. The electron occupation
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numbers were smeared around the Fermi level according to the method of Methfessel and Paxton [19]. This method is essential in order to get precise atomic forces (and therefore phonon frequencies) when only a moderate number of k-points can be used [20]. We used a slab consisting of an NO molecule layer and three Pt layers. The NO molecule layer as well as the upper two Pt layers were allowed to relax in geometrical optimization procedures. The surface unit cell used has a periodicity of p(2 2), because the LEED experiments showed clear p(2 2) patterns [4]. Although a gasphase NO molecule is spin polarized, we did not take account of the spin-polarization effect in the calculations for the chemisorbed systems, because all previous calculations of NO on transition-metal surfaces (except for noble-metal surfaces such as Ag [21] where only weak adsorption of NO takes place) indicate that chemisorbed NO is not spin polarized due to a strong interaction between the adsorbate and the substrate [22–25]. 3. Results 3.1. For an NO coverage of 0.25 ML Table 1 shows the adsorption energies and N–O stretch frequencies calculated with one NO molecule put in the p(2 2) unit cell, corresponding to 0.25 ML. This coverage of 0.25 ML can be regarded as the low-coverage limit, because the adsorption energy of, e.g., the fcc-hollow species changes little from 2.09 to 2.16 eV when the coverage is decreased from 0.25 to 0.0625 ML. As shown in Table 1, the fcc-hollow site is the most stable adsorption site, and the hcp-hollow site is Table 1 The adsorption energies and N–O stretch frequencies of NO molecules adsorbed at several adsorption sites of the Pt(1 1 1) surface (This table also indicates whether the N–O axis of each adsorbed NO molecule is perpendicular to the surface or not.) Adsorption site
Adsorption energy (eV)
N–O frequency (cm1 )
N–O axis perpendicular?
fcc-hollow hcp-hollow Atop
2.09 1.92 1.61
1512 1539 1683
Yes Yes No
the second stable. The bridge site turned out to be an unstable site, that is, it does not correspond to an energy local minimum. The atop site corresponds to a local minimum, but with a much lower adsorption energy as compared to those of the hollow species. Recently, Feibelman et al. argued that well-converged DFT-GGA calculations predict wrong site preference for CO/Pt(1 1 1), that is, the fcc-hollow site adsorption instead of the experimentally established atop site adsorption [26]. We agree with them that there is a tendency of GGA calculations to overestimate the adsorption energy at a high coordination site more significantly than that at a low coordination site. We believe, however, that such a tendency results in a wrong prediction for the site preference only in subtle cases like CO/Pt(1 1 1), where the GGA energy difference between the fcc-hollow and atop adsorption is typically less than 0.2 eV [26]. In the present case of NO/Pt(1 1 1), the calculated energy difference between the fcc-hollow and atop adsorption is as much as 0.5 eV, and we consider this value to be large enough to lead to the conclusion that NO molecules are adsorbed at fcc-hollow sites in the low coverage regime up to 0.25 ML. Structural optimizations for the fcc- and hcphollow species resulted in the NO molecule perpendicular to the surface with the N-end down. Fig. 1 shows the top and side views of the fcchollow species. On the other hand, a structural optimization for the atop species resulted in a bent geometry rather than a linear geometry, as shown in Fig. 2. It is interesting to note that NO molecules adsorbed at atop sites of Pd(1 1 1) and Rh(1 1 1) have a bent and linear geometry, respectively, as verified by our calculations performed with the same scheme as the present one. So far, many researchers have believed that the atop NO species on Pt(1 1 1) (and probably also on Pd(1 1 1)) is perpendicular to the surface, mainly because there seemed to be no peak corresponding to the frustrated rotational mode in EELS observations [2, 27]. (In fact, there is such a peak as will be discussed in Section 4.) The adsorption energy of atop NO on Pt(1 1 1) calculated by Ge et al. is as low as 0.64 eV probably as a result of assuming a linear geometry [5]. Also, Loffreda et al. [28] performed calculations for NO on Pd(1 1 1) assuming
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Fig. 1. (a) The top and (b) side views of the NO molecules adsorbed at fcc-hollow sites of the Pt(1 1 1) surface at a coverage of 0.25 ML. The small black and grey spheres represent oxygen and nitrogen atoms, respectively. The large grey spheres represent Pt atoms. For clarity the surface Pt atoms are shown with a little darker spheres than the other Pt atoms. The dashed lines indicate the p(2 2) , respectively. The NO molecules are perpendicular to the surface. unit cells. The N–O and Pt–N distances are 1.22 and 2.07 A
Fig. 2. (a) The top and (b) side views of the NO molecules adsorbed at atop sites of the Pt(1 1 1) surface at a coverage of 0.25 ML. See , the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The N–O and Pt–N distances are 1.17 and 1.93 A respectively. The NO molecules are tilted from the surface normal. The tilt angles of the N–O and Pt–N bonds with respect to the surface normal are 52° and 3°, respectively. The N atom is almost right on top of a surface Pt atom.
a linear geometry of the atop species and obtained the N–O stretch frequency somewhat larger than the experimental value. If the tilt of the atop NO had been taken into account, the frequency would have been smaller because of the increased interaction of the anti-bonding 2p molecular orbitals of NO with the metal d orbitals, resulting in better agreement with the experiment. Now we turn to the calculated N–O stretch frequencies which are also listed in Table 1. The value obtained for the fcc-hollow species is 1512 cm1 , in good agreement with the experimental value of about 1490 cm1 . This fact further supports our conclusion that at low coverages NO first adsorbs at a fcc-hollow site of the Pt(1 1 1) surface. As far as the atop species is concerned, the calculated value of 1683 cm1 is reasonably close to the experimental value of about 1710 cm1 at high coverages. This indicates that the high-
frequency peak which appears at high coverages in the vibrational spectra actually corresponds to the presence of the atop species. Although the LEED study of Materer et al. [4] and the calculational study of Ge et al. [5] insisted that NO molecules are adsorbed at fcc-hollow sites at a coverage of 0.25 ML, they did not argue about the presence of the atop species. Here we would like to remind the readers of the traditional picture by Gland and Sexton [2], which is two-fold. One point is that at low coverages bridge sites of the Pt(1 1 1) surface are first occupied by NO molecules. This has been shown to be erroneous as mentioned above, and it has turned out that fcc-hollow sites instead of bridge sites are occupied in the low coverage regime. Such a misunderstanding originates from the conventional method of determining the adsorption site of NO at metal surfaces based on a comparison of the observed N–O stretch frequency
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of the adsorbate to those of metal nitrosyl compounds. The invalidity of such a methodology has been already pointed out by Loffreda et al. for NO on Pd(1 1 1) and Rh(1 1 1) [28]. The other, more important, point of the picture by Gland and Sexton [2] is that they claimed the bridge NO species (actually turning out to be the fcc-hollow species) to leave the bridge sites and move to atop sites as the NO coverage increases. According to the adsorption energetics shown in Table 1, however, there seems to be no reason why the fcchollow species move to atop sites as far as the coverage is lower than 0.25 ML, because the atop site is much less stable than the fcc-hollow site. Now it is clear that one of the most important problems to solve is why the atop species exist irrespective of the relative instability compared to the hollow species. In the next subsection, we tried calculations for a higher coverage of 0.50 ML by putting two NO molecules in the p(2 2) unit cell. 3.2. For an NO coverage of 0.50 ML Because the fcc- and hcp-hollow species are much more stable than the other species when one NO molecule is put in the p(2 2) unit cell, one would anticipate that only hollow sites will be occupied even when two molecules exist in the unit cell. In fact, there are many unoccupied hollow sites in the unit cell which await to be occupied. Our calculation, however, revealed that this is not the case. We tried two combinations of hollow
species, as shown in Fig. 3(a) and (b). In Fig. 3(a), only fcc-hollow sites are occupied, forming a p(2 1) periodicity. Even if the molecular arrangement has a p(2 1) periodicity, there will be three domains rotated by 120° with respect to each other, exhibiting a p(2 2) LEED pattern. In Fig. 3(b), fcc-hollow sites as well as hcp-hollow sites are occupied, forming a honeycomb network. We actually found that the structures of Fig. 3(a) and (b) are less stable by 0.22 and 0.39 eV per unit cell, respectively, than the combination of the fcchollow and atop species. This combination was originally proposed by Matsumoto et al. [7]. The top and side views of this most stable structure are illustrated in Fig. 4. Now we discuss why this structure is more stable than the ones consisting only of hollow species. Our calculations for 0.25 ML of NO show that fcc- and hcp-hollow adsorption induces significant surface atom relaxation, i.e., the expansion of the Pt-atom triangle supporting a three-fold hollow NO molecule. Therefore when hollow-site adsorption occurs at several neighboring places at a high-coverage surface, the expansion of one Pt-atom triangle conflicts with that of another Pt-atom triangle, resulting in appreciable energy loss. The most favorable structure shown in Fig. 4 does not involve such energy loss. Although the fcc-hollow site adsorption induces a large surface atom relaxation, the neighboring atop species does not, avoiding conflict. Recently, Matsumoto et al. observed STM images for NO/Pt(1 1 1) [6,7]. In their STM images,
Fig. 3. The top views of the two combinations of hollow site adsorption at a coverage of 0.50 ML: (a) fcc-hollow þ fcc-hollow (forming a 2 1 periodicity); (b) fcc-hollow þ hcp-hollow (forming a honeycomb structure). See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The adsorption energies of the NO molecules are (a) 3.41 eV/cell and (b) 3.24 eV/cell.
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Fig. 4. (a) The top and (b) side views of the most stable molecular arrangement (fcc-hollow þ atop) at a coverage of 0.50 ML. See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The atop NO molecules are tilted from the surface normal. The tilt angles of the N–O and Pt–N bonds with respect to the surface normal are 49° and 4°, respectively. The adsorption energies of the NO molecules are 3.63 eV/cell, i.e., 1.82 eV per molecule.
there is a p(2 2) background hexagonal lattice consisting of dark features as well as a p(2 2) array comprised of bright features. The dark features are considered to represent fcc-hollow species, because reducing the local coverage by electronstimulated desorption results in the appearance of an area where only the dark features are observable. Then there are two kinds of triangles consisting of neighboring three dark features in the hexagonal array. One is centered at a hcp-hollow site, and the other is centered at an atop site. Since in their STM images almost all the bright features are seen only at the centers of one kind of triangle, the coexistence of fcc-hollow species with either hcp-hollow species or atop species can be possible. Considering the difference in brightness between the dark and bright features, the height of the two kinds of NO molecules should be quite different. Matsmoto et al. reported the height difference to [7]. Our structural optimization revealed be 0.4 A that in the structure of Fig. 4 the oxygen and , nitrogen atoms are located by 0.40 and 0.86 A respectively, higher at the atop site than at the fcc-hollow site. In the case of the structure of Fig. 3(b) where the fcc- and hcp-hollow species coexist, the height differences of the oxygen and , respecnitrogen atoms are only 0.04 and 0.05 A tively, which would result in very small contrast in the STM images. Therefore the structure of Fig. 4 where fcc-hollow and atop species coexist at the surface is quite consistent with the observed STM images. Still, this structure seems inconsistent with the vibrational spectra where the low-frequency
peak corresponding to the fcc-hollow species is greatly attenuated or even disappears at high coverages. Our calculations solve this contradiction, as will be discussed in detail in Section 4. 3.3. For an NO coverage of 0.75 ML With the conclusion that the structure shown in Fig. 4 is the most stable at a coverage of 0.50 ML, it is natural to think that if NO molecules are added further they will adsorb at the hcp-hollow sites located at the center of the triangle consisting of the occupied fcc-hollow sites. In fact, the STM image by Matsumoto et al. exhibits a few bright features considered to correspond to such hcphollow species [7]. The structure obtained as a result of a structural optimization is illustrated in Fig. 5. In this structure, the hcp-hollow species moves somewhat upward as compared to the case of Fig. 3(b), and the oxygen and nitrogen height differences between the fcc- and hcp-hollow species , respectively. Therefore it become 0.08 and 0.09 A is likely that the hcp-hollow species are observed as a little brighter features than the fcc-hollow species. Molecular arrangements similar to Fig. 5 have been recently proposed for CO/Rh(1 1 1) [29–32], NO/Rh(1 1 1) [33,34], and NO/Ru(0 0 0 1) [35], although the atop CO or NO is perpendicular to the surface in all the three cases. The former two systems had long been believed to consist of one bridge and two near-atop species based on vibrational spectra and LEED analyses [36–38], and it is
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Fig. 5. The most likely molecular arrangement (fcc-hollow þ atop þ hcp-hollow) at a coverage of 0.75 ML. See the caption of Fig. 1 for the explanation of the spheres and dashed lines shown. The adsorption energies of the NO molecules are 4.68 eV/cell, i.e., 1.56 eV per molecule.
only recently that the correct structure consisting of one atop and two hollow species as shown in Fig. 5 has been proposed. The misleading features of vibrational spectra for NO as well as CO chemisorbed on metal surfaces will be discussed in detail in the next section. In the structure of NO/Pt(1 1 1) shown in Fig. 5, the adsorption energies of hcp-hollow, atop, and fcc-hollow species are calculated to be 1.06, 1.54, and 2.09 eV, respectively. Since the adsorption energy of the hcp-hollow species is as low as 1 eV, we consider that this species corresponds to the low-temperature feature around 200 K seen in the thermal desorption spectra [39]. As discussed by Materer et al. [4] and Matsumoto et al. [7], exposure of the Pt(1 1 1) surface to NO at a low temperature followed by annealing to 220–250 K yields a well-ordered p(2 2) structure. Probably annealing removes the weakly bound hcp-hollow species causing some disorder, and achieves a wellordered surface as shown in Fig. 4. Fukutani et al. performed experimental studies of photo-induced desorption of NO/Pt(1 1 1) [40]. When laser was irradiated onto an as-adsorbed surface (like Fig. 5) and an annealed surface (like Fig. 4), they found that the rotational-state population of the desorbed NO molecules exhibits Boltzmann and non-Boltzmann distributions, respectively. Also, Song et al. observed that NO molecules are photo-dissociated by irradiating
laser onto a Pt(1 1 1) surface at low NO coverages like Fig. 1 [41]. Therefore, we consider that strongly bound fcc-hollow species are not desorbed but dissociated by laser irradiation, while less-strongly bound atop and hcp-hollow species are photo-desorbed. A non-Boltzmann distribution is observed when bent atop species are desorbed, while Boltzmann distribution is obtained when perpendicular hcp-hollow species are desorbed. In fact, there are several theories claiming that a non-Boltzmann rotational-energy distribution of the desorbed species results from a bent geometry in their adsorbed state [42,43].
4. Discussion In this section we discuss the origin of the misleading features of vibrational spectra observed for NO as well as CO chemisorbed on transitionmetal surfaces. Table 2 shows the calculated frequencies and peak intensities of the normal modes for NO/Pt(1 1 1) at the three coverages we studied. The peak intensity of a normal mode in vibrational spectroscopy is known to be proportional to its squared dynamic dipole moment [8,10,11]. In the case of EELS, the peak intensity also depends on the frequency [10,11], which was neglected in the present work. The dynamic dipole moment of a normal mode was calculated from the change in the work function difference between the two sides of a slab when the adsorbate atoms were displaced according to the normal mode. Only the normal modes whose squared dynamic dipole moments are larger than 0.003 (in arbitrary units) are listed in Table 2. Before discussing the N–O stretch modes, we briefly comment on the other modes. The results for the cases of 0.25 ML reveal that the Pt–NO stretch frequency is almost the same for the fcchollow (326 cm1 ) and atop (301 cm1 ) species. At high coverages, the Pt–NO stretch modes of the hollow species have almost zero intensity probably due to the electronic-state effect which will be discussed below for the N–O stretch modes. In the observed EELS data [2,7], as the coverage was increased a peak with a frequency of about 450 cm1 started developing together with the N–O
H. Aizawa et al. / Surface Science 514 (2002) 394–403
401
Table 2 The normal-mode frequencies (in cm1 ) and the peak intensities (i.e., squared dynamic dipole moments, in parentheses) of NO/Pt(1 1 1) at various coverages Coverage (ML)
0.25
0.25
0.50
0.50
0.50
0.75
Adsorption Site
fcc-hollow
atop
fcc-hollow þ atop
fcc-hollow þ atop (atop fixed)
fcc-hollow þ atop (hollow fixed)
fcc-hollow þ atop þ hcp-hollow
Structure
Fig. 1
Fig. 2
Fig. 4
Fig. 4
Fig. 4
Fig. 5
Pt–NO stretch 1 Pt–NO stretch 2 NO rotation N–O stretch 1 N–O stretch 2 N–O stretch 3
326 (0.005) 301 (0.010)
300 (0.009)
301 (0.010)
293 (0.006)
483 (0.005)
472 (0.003) 1447 (0.147)
472 (0.003)
484 (0.005) 1463 (0.020) 1540 (0.111) 1708 (1.208)
1512 (1.171) 1683 (2.089)
1703 (1.475)
1459 (0.363) 1699 (1.259)
Only the modes with intensities larger than 0.003 are shown. ‘‘Pt–NO stretch 1’’ and ‘‘Pt–NO stretch 2’’ represent the Pt–NO stretch modes of the fcc-hollow and atop species, respectively. The Pt–NO stretch modes of the hollow species become very small in intensity in the presence of the atop species. ‘‘NO rotation’’ represents the frustrated rotational mode of the atop NO species. The corresponding modes of the hollow NO species have zero intensity because of the perpendicular geometry. ‘‘N–O stretch 1’’ and ‘‘N–O stretch 2’’ correspond to the N–O stretch modes of the hollow species, while ‘‘N–O stretch 3’’ corresponds to that of the atop species. See text for the significance of the columns with ‘‘(atop fixed)’’ and ‘‘(hollow fixed)’’.
stretch mode of the atop species around 1710 cm1 . Since there was no other peak which developed together, one might consider that the peak around 450 cm1 corresponds to the Pt–NO stretch mode of the atop species. Our calculations, however, reveal that this peak corresponds to the NO frustrated rotational mode which is dipole active only when NO has a bent geometry. Now we turn to the main issue of the N–O stretch modes. As can be seen from Table 2, the N–O stretch mode of the fcc-hollow species at a coverage of 0.25 ML has a frequency of 1512 cm1 , and its intensity is 1.171. At a coverage of 0.50 ML where fcc-hollow species coexist with atop species, the peak intensity of the above mode is decreased down to as small as 0.147. This magnitude is about one tenth of that for the peak of the atop species. This is surprising because the fcchollow species exist at the surface as much as the atop species. The calculations reproduce very well the experimental fact that at high coverages the peak intensity of the low-frequency N–O stretch mode was greatly diminished [2,7] or even disappeared [3] in the vibrational spectra, the phenomenon misleading researchers for a long time. Note that the disappearance of the low-frequency N–O
stretch mode in IRAS has been also reported for NO/Ru(0 0 0 1), although the mode can be observed with a small intensity by EELS just as in the case of NO/Pt(1 1 1). It was only recently proposed that the disappearance of the peak in IRAS might result from the presence of surface contaminants and other imperfections [44]. In fact, the low-frequency N–O stretch mode which could not be detected in old IRAS studies for NO/Ru(0 0 0 1) has been observed as a small peak in a recent refined IRAS study [44]. EELS seems less sensitive to such contaminants and imperfections. Such misleading features of vibrational spectra as discussed above are not limited to NO/Pt(1 1 1) but can occur for a variety of systems with CO or NO chemisorbed at transition-metal surfaces. In fact, as briefly commented in Section 3.3, the structure of CO/Rh(1 1 1) has also been misunderstood for a long time due to such a misleading feature of its vibrational spectrum. At the saturation coverage of this system, Dubois et al. observed two C–O stretch peaks at 1870 and 2070 cm1 , which were considered to correspond to bridge and near-atop species, respectively [36]. Since the peak area of the former mode is about a
402
H. Aizawa et al. / Surface Science 514 (2002) 394–403
half of that of the latter mode, they concluded that two near-atop species and one bridge species exist in the p(2 2) unit cell. As mentioned in Section 3.3, the correct structure determined only recently by X-ray related techniques [29,32] consists of one atop and two hollow species, as shown in Fig. 5. This example also indicates that peak intensities of vibrational spectra are almost impossible to interpret without the help of theoretical calculations. It has been well known that if there are two modes with their frequencies close to each other (typically the frequency difference of the order of 10 cm1 ), the intensity of the lower-frequency peak is transferred to that of the higher-frequency peak. This phenomenon is due to the dynamic dipole– dipole coupling and called the ‘‘intensity transfer’’ effect. Such an effect, however, is very unlikely to take place between the modes with a frequency difference of about 200 cm1 as in the present cases. In order to clarify the origin of the calculated results for NO/Pt(1 1 1), we calculated the N–O stretch frequency and peak intensity for the fcc-hollow species without taking account of the motion of the atop species at the 0.50 ML structure of Fig. 4. By performing such procedures, we can obtain the frequency and intensity without the effect of the dynamic dipole–dipole coupling between the fcc-hollow and atop species. The calculated frequency and intensity are 1459 cm1 and 0.363, respectively. This implies that the peak intensity of the N–O stretch mode of the fcc-hollow species is reduced from 1.171 to 0.363 just by the presence of the atop species, even without the effect of dynamic dipole–dipole coupling. The effect of dynamic dipole–dipole coupling only reduces the intensity from 0.363 to 0.147. We also performed the opposite calculations, i.e., calculations of the N–O stretch frequency and peak intensity for the atop species without taking account of the motion of the fcchollow species. The results are 1699 cm1 and 1.259. The peak intensity of the atop species is also dramatically reduced from 2.089 to 1.259 by the presence of the fcc-hollow species. Due to the intensity transfer effect, however, the higher-frequency atop peak gains some intensity from the lower-frequency fcc-hollow peak, resulting in a
slight increase from 1.259 to 1.475 in the 0.50 ML structure of Fig. 4. We consider that the large reduction from 1.171 (2.089) to 0.363 (1.259) for the fcc-hollow (atop) species is due to a change in the electronic structure of the whole adsorbed system by the addition of the atop (fcc-hollow) species. For example, electron transfer between the NO molecules and the substrate Pt surface might be promoted as the different species come to exist at the surface, and the charged states of NO molecules might be changed resulting in a change of the dynamic dipole moment. Further analysis of the electronic structure of the adsorbed system is clearly in order.
5. Conclusion Our calculations for NO/Pt(1 1 1) have definitely determined that at coverages up to 0.25 ML fcc-hollow sites are occupied by NO, and as the coverage is increased over 0.25 ML the atop species come to coexist with the fcc-hollow species. Although the measured vibrational spectra seem inconsistent with this picture, our calculations reproduce them very well, indicating that there is no inconsistency. We have revealed that the peak intensities of the N–O stretch modes for NO molecules adsorbed at different sites of transition-metal surfaces sometimes exhibit quite misleading behaviors, warning experimentalists to interpret their vibrational spectroscopic data very carefully.
Acknowledgements We wish to thank M. Matsumoto at Institute of Industrial Science, University of Tokyo, and M. Scheffler, H.-J. Freund and T. Kl€ uner at FritzHaber-Institut der Max-Planck-Gesellschaft for valuable discussions. The calculations were performed using the Numerical Materials Simulator (NEC SX-5) at National Institute for Materials Science and Hitachi SR-8000 at Institute for Solid State Physics, University of Tokyo. This research is supported by ACT-JST.
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