J. Phys. Chem. B 2001, 105, 3027-3033
3027
Alloying Effects on N-O Stretching Frequency: A Density Functional Theory Study of the Adsorption of NO on Pd3Mn (100) and (111) Surfaces D. Loffreda, F. Delbecq, D. Simon, and P. Sautet* Institut de Recherches sur la Catalyse, Centre National de la Recherche Scientifique, 2 AVenue Albert Einstein, F-69626 Villeurbanne Cedex, France, and Laboratoire de Chimie The´ orique, Ecole Normale Supe´ rieure de Lyon, 46 Alle´ e d’Italie, F-69364 Lyon Cedex 07, France ReceiVed: September 13, 2000
We present total energy and N-O stretching frequency calculations for the low-coverage adsorption of NO on palladium-manganese Pd3Mn (100) and (111) surfaces, on the basis of density-functional theory periodic calculations. A complete description of all the different adsorption sites and corresponding N-O vibrations is given and a theoretical interpretation of the experimental IR spectra is proposed. On both Pd3Mn (100) and (111) surfaces, the highly coordinated vertical adsorption sites are always energetically favored. The atop adsorption on the surface manganese atom is also a stable site. On Pd3Mn (100), a new horizontal dibridge site is reported. The adsorption on these palladium-manganese alloy surfaces is weaker than the adsorption on the pure corresponding palladium surfaces. The anharmonic N-O stretching frequencies on the Pd3Mn surfaces are shifted by 60-100 cm-1 toward the lower frequencies by comparison with the pure palladium surfaces. The weakening of the adsorption strength and the global shift for the N-O frequencies has been correlated with the presence of the surface manganese atoms, which play a predominant role for the electronic interactions between the magnetic NO molecule and the alloy periodic surface. An interpretation of the alloying effect on the strength of the N-O bond and the NO adsorption is proposed on the basis of a qualitative Mulliken population analysis. The empty states on the surface manganese atoms are responsible for an increased electron-transfer toward NO, and hence of the smaller vibrational frequency on the alloy compared to pure Pd. Indeed these empty states interact with the π*NO and push it below the Fermi level, resulting in a transfer from the “surface electron reservoir” toward the π*NO molecular orbital.
I. Introduction The improvement of the reactivity of metal catalysts by alloying several metals is an important phenomenon in heterogeneous catalysis. In the case of three-way catalysts, the main reactions are the oxidation of the carbon monoxide and the reduction of nitric oxides.1 The catalysts are composed of precious metals: palladium, rhodium and platinum. The oxidation of CO into CO2 is assured by palladium and platinum, while the reduction of NO into N2 is mainly activated by rhodium.1-3 The substitution of rhodium by a cheaper metal or metallic alloy having the same activity, is a major economic challenge. An alloy of palladium with more electropositive metal, cheaper than rhodium but active for dissociating NO, could be a potential substitute catalyst for this reduction. Indeed, recent studies of palladium clusters alloyed with manganese and supported on silica show an increase of the activity for the dissociation of NO compared to pure Pd.4,5 The catalysts have been prepared with various Pd/Mn atomic ratios. The characterization of these clusters reveals the existence of small alloy particles (close to 5 nm) and also small manganese oxide particles (about 1 nm) supported by silica. The bulk manganese concentration depends on the global manganese concentration in the material, and an important manganese segregation occurs on the surface layer of the bimetallic particles. For the catalytic conversion of NO and CO molecules, the optimal rate is obtained for the Pd65Mn35 particles.4 Two * Corresponding author. E-mail:
[email protected]. Fax: (+33) 4 72 44 53 99.
reasons were proposed to explain the improvement of the NO conversion rate with the presence of PdMn/SiO2 bimetallic samples: either a catalytic behavior due to mixed palladiummanganese sites, or a specific role of the manganese oxides close to the palladium particles. A recent surface science study about the adsorption of CO, NO, O2, and CO2 on the Pd(100)-Mn c(2×2) single-crystal alloy6 indicates that the CO molecule adsorbs molecularly without affecting the alloy structure, while NO dissociates easily at low temperature and small exposure. NO destroys the alloy structure by forming manganese oxides. For stronger NO exposures, molecular adsorption takes place at low temperature and the dissociation occurs by heating at 300 K. The adsorption of NO on manganese surfaces has been the subject of few investigations. On MnO oxides at 25 °C, the NO adsorption resulted in a slow oxidation of MnO to Mn2O3.7 The surface Mn2O3 species adsorbed NO, which desorbed almost unchanged on pumping at room temperature. Mn2O3 oxides adsorbed NO slowly only at 25 °C. Recently, Kapteijn et al.8 have observed vibrational bands with weak intensities at 1834 and 1865 cm-1 after the NO adsorption on MnOx/Al2O3. According to the frequency ranges of the pentacyanonitrosyl complexes Mn(NO)(CN)52- and Mn(NO)(CN)53- (with N-O vibrations at 1885 and 1725 cm-1), they have assigned these peaks to the existence of nitrosyl species with different coordination on the manganese atoms. The vibrational characterization of the NO adsorption on palladium-manganese particles has been studied by Trillat et
10.1021/jp003274h CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001
3028 J. Phys. Chem. B, Vol. 105, No. 15, 2001 al.9,10 The authors have compared the N-O stretching frequencies for the case of the adsorption on pure palladium catalysts supported on silica, and for the case of the adsorption on bimetallic catalysts Pd90Mn10 and Pd65Mn35. The IR spectrum shows new frequency bands compared to Pd. For the adsorption of NO on palladium, vibrational bands appear at 1740, 1640, and at 1560 cm-1. For NO on the Pd65Mn35 particles, five absorption bands have been observed; a narrow band at 1735 cm-1, a band less intense at 1700 cm-1 with a shoulder at 1680 cm-1, a larger and very intense band at 1640 cm-1 with a shoulder at 1620 cm-1, a large and intense band with three peaks at 1560, 1540, and 1510 cm-1, and finally a narrow and weak band at 1450 cm-1. The IR spectrum on the alloy hence shows additional peaks but the direct interpretation of these peaks is difficult and requires more information. Therefore, stretching frequency calculations would greatly improve the interpretation of the observed IR spectrum. The manganese segregation on the palladium-manganese alloy surface has been studied by LEED.11 The authors show that annealing of Mn films deposited on Pd (100) produces a Pd3Mn surface alloy with the Cu3Au structure. The surface of this alloy consists of a mixed 50-50 layer of Mn and Pd in a p(2×2) checkerboard like configuration. The second layer is a pure palladium layer, followed by a third mixed layer identical to the first one, and then the bulk is pure Pd. The chemisorption of NO on palladium surfaces has already been the subject of several theoretical studies.12-16 However, only a few calculations have been carried out for palladiummanganese alloy surfaces. The adsorption of NO and CO has been systematically studied with extended Hu¨ckel calculations on the palladium and palladium-manganese (100) and (111) surfaces.17 For NO and CO adsorptions on Pd (111) and (100), the high-coordination bridge, 3-fold and 4-fold hollow sites are favored. The multibonded sites are also more stable for the adsorption of CO and NO on Pd3Mn (111) and (100) but the adsorption energies and the N-O overlap populations are weaker. The magnetic properties of palladium-manganese alloys18 have been studied with density-functional theory (DFT). The alloy between a palladium atom and a more electropositive metal like manganese involves a small electron transfer from the electropositive metal to the palladium. A giant magnetic moment (4.2 µB) appears on the manganese atoms. The adsorptions of CO and NO on the Pd3Mn (100) surface have been studied with DFT calculations,19,20 without considering surface relaxation effects. CO prefers the atop palladium adsorption site with a low magnetic moment (0.14 µB), while NO prefers the atop manganese position on the surface with a high magnetic moment (4.56 µB). The adsorption of both molecules on palladium-manganese alloy surfaces is therefore favorable for the low-coordination sites. This differs from the extended-Hu¨ckel picture, but the energy differences between sites are not very large. On the pure palladium (100) and (111) surfaces, in contrast, the multibonded sites are preferred. NO keeps a high magnetic moment when adsorbed on a manganese atom (0.6 µB) and shows an antiferromagnetic coupling with the manganese magnetic moment. In these previous works, the calculation of the NO vibrational frequencies had not been carried out. In this study, we present total energy and N-O stretching frequency calculations for the low-coverage adsorption of NO on palladium-manganese Pd3Mn (100) and (111) surfaces, on the basis of density-functional theory periodic calculations. Surface relaxation is taken into account. A complete description
Loffreda et al.
Figure 1. Side (a, b) and top views (c, d) for the model (100) and (111) Pd3Mn surfaces. Pd atoms are indicated with white balls and Mn atoms with gray ones. The chemisorption sites considered in the calculations are also indicated for both surfaces.
of all the different adsorption sites and corresponding N-O vibrations is given in order to propose a theoretical interpretation of the experimental IR spectra. The presence of manganese atoms plays a predominant role for the electronic interactions between the magnetic NO molecule and the alloy periodic surface. An interpretation of the alloying effect on the strength of the N-O bond and the NO adsorption is proposed on the basis of a qualitative orbital interaction picture. II. Methodology A. Surface and Chemisorption Models. The Pd3Mn alloy crystallizes in a disordered Cu3Au-like fcc structure above 800 K and in a Al3Zr-like tetragonal structure under this temperature.21,22 In the fcc structure, the unit cells are separated by antiphase frontier domains, giving a unit cell 4 times larger (16 atoms). For the calculation, we have considered a simplified Cu3Au-like fcc unit cell with 3 Pd atoms and 1 Mn atom in the cell. The lattice vector has been fixed to the calculated DFT optimum (3.96 Å). The model surfaces have a p(2×2) structure with four metallic atoms per layer, and they are shown in Figure 1. There are two types of (100) planes for a bulk termination of such a Pd3Mn alloy. The first plane (A) has four palladium atoms and, to respect the alloy stoechiometry, the second plane (B) has two Pd and two Mn atoms in alternating positions. So the model surface can be terminated either at plane A (surface A) or at plane B (surface B). Surface A has no Mn atom at the surface, while surface B shows a 50% Mn concentration in the top layer. Hence surface B corresponds well with the experimental segregation of Mn6 and with the LEED results.11 In this study, we have therefore focused on the adsorption of NO on surface B (Figure 1a,c). For the (111) terminations, all planes
Alloying Effects on N-O Stretching Frequency are equivalent in the bulk with a nominal Pd3Mn composition and a bulk termination structure has been considered as a model (cf. Figure 1b,d). Both surfaces have been modeled with a fourlayer thick slab. The NO adlayer corresponds to a (2×2) NO (0.25 monolayer (ML)] structure and the adsorption has been done only on one side of the slab. Four equivalent vacuum layers with ideal bulk interspace have been added to the metallic slab in order to separate one slab with its periodic image in the z direction. During the geometry optimizations, the nitrogen and oxygen coordinates have all been optimized, and the two uppermost metallic layers have been relaxed. B. Theoretical Method. The geometry optimizations and the N-O stretching vibrations have been performed using the Vienna ab initio simulation program (VASP).23-25 The KohnSham equations of the density-functional theory are solved with the generalized gradient approximation (GGA) exchangecorrelation functional proposed by Perdew and Wang.26 The electron-ion interaction is described by efficient ultrasoft pseudopotentials, so that the plane-wave expansion can be truncated at a cutoff energy of 400 eV. Brillouin-zone integrations have been performed on a grid of 5 × 5 × 1 MonkhorstPack special k-points for all the (2×2) NO structures (0.25 ML). A spin unrestricted approach has been used since giant magnetic moments have been found for palladium-manganese Pd3Mn (100) and (111) surfaces.18-20 The Mulliken population analyses have been computed for the comparison of the bridge adsorption sites on Pd (100) and Pd3Mn (100) surfaces with the Amsterdam density-functional code for periodic structures (ADF-band).27 The geometries of the systems have been fixed to the optimum determined by VASP for the (2×2)-1 NO (0.25 ML) on both surfaces, the slab being restricted to the three uppermost layers. A spinunrestricted computation of the density was also used. The atoms were modeled by a frozen core up to the 3d orbitals for Pd, up to the 2p orbitals for Mn, and up to the 1s orbital for the N and O atoms. Relativistic effects are not included in the calculations. The valence basis set (4s4p4d5s for Pd, 3s3p3d4s for Mn, 2s2p for N and O) are of double-ζ quality. The basis corresponds to a combination of numerical atomic orbitals (NAO) and of Slatertype orbitals (STO), with an additional 5p hybridization orbital for Pd, 4p for Mn, and a 3d polarization function for N and O atoms. The accuracy for numerical integration was set to 10-4. Nine irreducible k-points were used in the first Brillouin zone. C. Stretching Frequency Calculation Scheme. The stretching frequency calculation technique has already been detailed in previous studies.28-30 Since the calculation of the analytic second partial derivatives of the total energy is not implemented in the VASP program, the N-O stretching frequency calculations require us to explore manually the potential energy surface. The vibrational treatment is greatly simplified by neglecting the coupling, first between the surface phonons and the N-O stretching vibrations, and second between the N-O stretching and bending vibrations, since the N-O stretching frequencies are much harder than all the other normal vibration modes. During the potential energy surface exploration, the metallic slab is fixed to the optimized geometry of the considered adsorption site, and the equilibrium N-O direction, respective to the surface, is also fixed. The N-O bond is stretched, keeping fixed the center of mass of the molecule. The potential energy curves are directly fitted with a Morse-shape potential, developed at the third order around the equilibrium state. The force constants of the fitted potential are linked to the third-order developed Morse potential constants by a system of nonlinear equations that can be analytically solved. The inversion of this
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3029
Figure 2. Most stable chemisorption structures for NO on a Pd3Mn (100) surface (a-c) and on a Pd3Mn(111) surface (d-g). Bond distances (Å), and the eventual tilt angle of the NO molecule (in deg, with respect to surface normal) are given.
system finally gives an estimation of the Morse potential constants. For the case of the Morse potential, the Schro¨dinger equation resolution is exact and the anharmonic stretching frequency can be directly computed. The exact expression of the anharmonic stretching frequency (νan) can be divided into a simple harmonic frequency (νha) and a second term that corresponds to the corrective anharmonic constant (νc). III. Geometric, Energetic and Magnetic Properties A. Adsorption on Pd3Mn (100). A large number of adsorption sites have been considered (see Figure 1). The surface presents two types of top sites on a palladium or on a manganese atom, one bridge site with a mixed Pd-Mn composition, and one 4-fold hollow site with two Pd and two Mn atoms (cf. Figure 1c). By distortion of the 4-fold hollow site, a pseudo 3-fold hollow site (Pd2Mn) can be formed. Stable NO structures correspond to a vertical molecule (N atom interacting with the surface) or to a NO molecule parallel to the surface (cf. Figure 2a). For the vertical structures, a tilt of the molecule has been tested, but it was energetically favorable only in the case of the atop Pd site. The calculation results are summarized in Table 1. The comparison of all the optimized adsorption sites shows that the horizontal NO adsorption is among the most stable ones (cf. Table 1 and Figure 2a) with an adsorption energy of -1.83 eV. The structure can be seen as a dibridge geometry, each atom of the molecule being close to the bridge site. The “ditop” case is less stable by ∼0.3 eV and will not be considered. The vertical bridge and pseudo 3-fold hollow geometries (cf. Figure 2b,c) are very close in energy (-1.82 eV). The 4-fold hollow site is less stable by less than 0.1 eV. The top site on Mn is not far in energy, while the other top sites are clearly less favored. The tilt of the molecule toward the surface is slightly stabilizing for the palladium top site (the equilibrium Pd-N-O bond angle is 142°), with an energy gain of 0.05 eV with respect to the vertical situation. These results are not completely in agreement with previous DFT calculations, which showed that the manganese top site (-1.80 eV) was more stable than the 2-fold bridge site (-1.62 eV) and the 4-fold hollow site (-1.52 eV).20 However, in this study, a different surface lattice parameter was used
3030 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Loffreda et al.
TABLE 1: N-O Equilibrium Distances (Å), Adsorption Energies Eads (eV), Harmonic N-O Stretch Frequencies (νha) (cm-1) and Anharmonic N-O Stretch Frequencies (νan) (cm-1) for the Adsorption of NO on a Pd3Mn(100) Surface (p(2×2)-NO Structure with a Coverage of 0.25 ML)a (2 × 2)-NO (0.25 ML) top dN-O Eads νha νan
on Pd
tilted/Pd
on Mn
bridge
4-fold hollow
pseudo 3-fold hollow
dibridge
1.18 -1.32 1765 1738
1.19 -1.37 1717 1684
1.20 -1.69 1715 1688
1.21 -1.82 1573 1540
1.25 -1.75 1366 1322
1.24 -1.81 1453 1421
1.32 -1.83 1026 992
gas phase 1.17 1882 1861
a Experimental frequencies for NO on Pd3Mn catalyst particles are 1735, 1680-1700, 1620-1640, 1510-1540-1560 and 1450 cm-1.9,10 The experimental NO gas phase value is 1903 cm-1.31
TABLE 2: N-O Equilibrium Distances (Å), Adsorption Energies Eads (eV), Harmonic (νh.) (cm-1), and Anharmonic N-O Stretch Frequencies (νa.) (cm-1) for the NO Adsorption on Pd (100) Surface, at the Coverage 0.25 ML (p(2×2)-NO Structure)a p(2 × 2)-NO 0.25 ML
dN-O Eads νha νan
top
bridge
4-fold hollow
1.176 -1.30 1830 1804
1.197 -2.15 1675 1644
1.228 -2.17 1495 1467
atoms
pseudo 3-fold hollow
gas phase
1.219 -2.19 1527 1499
1.174 1882 1861
a Experimental N-O stretch frequencies on Pd (100) are 1660 and 1515 cm-1 34 or 1678 and 1492 cm-1.35 The experimental NO gas phase value is 1903 cm-1.31
TABLE 3: Mean Interlayer Distance (Å) ∆zPd1-Pd2 between the First- and Second-Layer Palladium Atoms and ∆zMn1-Pd2 between the First-Layer Manganese Atoms and the Second-Layer Palladium Atoms for the Bare Slab Pd3Mn (100) and for the Adsorption of the NO Molecule on a Dibridge Site on Pd3Mn (100)
∆zPd1-Pd2 ∆zMn1-Pd2
TABLE 4: Atom-Projected Spin Polarizations (Gr-Gβ, in Electrons) for a Three-Layer Thick Slab and for the Bridge Site Adsorption of NO on Pd3Mn and Pd (100) Surfaces at the Coverage 0.25 ML (p(2×2)-NO Structure)a
Pd3Mn (100) clean slab
di-bridge NO on Pd3Mn (100)
1.99 1.86
2.00 1.92
(2.85 Å for ADF-band) and the surface relaxation effects were not taken into account. Upon chemisorption on the Pd3Mn (100) surface, the NO bond is elongated, in a moderate way for the vertical geometries (maximum +0.08 Å), but in a really significant way for the horizontal dibridge chemisorption (+0.15 Å). The calculated Mn-N and Mn-O distances are generally shorter than the Pd-N and Pd-O bond lengths. Hence, a large variety of sites are close in energy for NO on the Pd3Mn (100) surface, and diffusion of the molecule should be very easy. The comparison with the pure Pd surface (Table 2) shows that on Pd the chemisorption energy is ∼0.4 eV larger but that the horizontal dibridge geometry is not stable. The most stable site on Pd (100) is the pseudo 3-fold hollow one, with a vertical NO molecule. However, the 4-fold hollow and the bridge sites are not far in energy. The influence of surface relaxation for NO chemisorption on Pd3Mn (100) can be detailed for example on the most stable horizontal dibridge chemisorption. The average interlayer distances are indicated in Table 3. Only the values between first and second layers are significantly modified compared to the bulk separation (1.98 Å) and are given in the table. In the case of the bare slab, the Pd1-Pd2 separation is only slightly increased, while the surface Mn atom is strongly pushed downward. This results in a buckling of the clean surface layer by 0.13 Å. This phenomenon has been observed experimen-
Pd 3rd Mn 3rd Pd 2nd surface surface layer layer layer Pd (0.14)b Mn (4.56)b NO (1)c
Pd3Mn (100) 0.16 Pd (100) 0.053
4.53 ***
0.15 0.068
0.14 0.039
4.07 ***
-0.58 -0.015
a The total spin polarization of the slab is 17.83 electrons for Pd Mn 3 (100) and 0.63 electrons for Pd (100)) b Spin polarizations (electrons) c for the Pd3Mn (100) bare slab. Spin polarization (electrons) for the gas-phase NO molecule.
tally.11 The authors have suggested a buckling value of 0.2 Å. The geometric relaxation is hence correctly reproduced by VASP. Upon NO dibridge chemisorption, the Mn atom is shifted up while the ∆zPd1-Pd2 interlayer spacing remains close to the ideal bulk value. Hence, the z separation between Pd and Mn in the first layer is reduced to 0.08 Å by NO adsorption. This can be seen as a consequence of bond order conservation, since the Mn atom acquires a strong interaction with the O atom, then weakening the interaction with the other atoms of the slab. The chemisorption of NO on Pd3Mn (100) modifies the magnetic properties of the molecule and of the surface atoms. The individual atom spin polarization calculated with the ADFband program using the geometry optimized with VASP are reported in Table 4. The same chemisorption mode, vertical NO on a bridge site, is compared on Pd3Mn (100) and on Pd(100). In the case of the adsorption on Pd3Mn (100), the NO molecule keeps a strong spin polarization (-0.58 electron, the negative sign indicating the antiferromagnetic coupling with the Mn moment). This contrasts with the Pd (100) surface where chemisorbed NO shows a negligible spin polarization. The spin polarization on the surface manganese decreases (4.07 electrons compared to 4.56 electrons for the bare slab). A small spin polarization is induced on the Pd atoms of the slab by alloying with Mn and is not affected by NO chemisorption. The result agrees with previous calculations, although a different geometry was used.20 B. Adsorption on Pd3Mn (111). The Pd3Mn (111) surface shows a large variety of potential binding sites. Two top sites on Pd or Mn can again be considered (cf. Figure 1d). Three a priori nonequivalent bridge sites are possible. Two pure Pd2 bridge sites and one mixed Pd-Mn bridge site. Four different 3-fold hollow sites can be considered. Two of them have a Pd3 composition at the surface layer, while the other two are of Pd2Mn type. The additional site diversity comes from the nature of the metallic atom in the second and the third layer of the slab. The Pd3h and Pd2Mnh sites are located above a manganese or a palladium atom of the first sublayer (cf. Figure 2d,f, respectively) and the Pd3f and Pd2Mnf sites are located above a manganese or a palladium atom of the second sublayer (cf. Figure 2g,e, respectively). In contrast with the (100) surface,
Alloying Effects on N-O Stretching Frequency
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3031
TABLE 5: N-O Equilibrium Distances (Å), Adsorption Energies Eads (eV) and Harmonic (νha) (cm-1) and Anharmonic N-O Stretch Frequencies (νan) (cm-1) for the NO Adsorption on Pd3Mn (111) Surface, at the Coverage 0.25 ML (p(2×2)-NO Structure)a (2 × 2)-NO (0.25 ML) top dN-O ads
νha νan a
3-fold hollow
tilted/Pd
on Mn
bridge
Pd2Mnh
Pd3h
Pd2Mnf
Pd3f
gas phase
1.188 -1.26 1718 1688
1.198 -1.57 1713 1690
1.200 -1.48 1633 1600
1.224 -1.85 1495 1467
1.214 -2.11 1574 1544
1.225 -1.91 1502 1460
1.215 -1.75 1555 1526
1.174 1882 1861
See Table 1 for experimental values.
no stable geometry with the NO parallel to the surface was found, so that only structures with vertical NO are presented. The geometry optimizations (cf. Table 5) show that the 3-fold hollow sites are stable and energetically favored. The Pd3h 3-fold hollow site is the most stable with an adsorption energy of -2.11 eV. Other 3-fold hollow sites are less stable by at least 0.2 eV, the least stable one being the Pd3f one (-1.75 eV). Hence the most stable site is pure Pd3 at the surface. However, the presence of the Mn atom in the sublayer is important since the other Pd3f site is 0.36 eV less stable. Only one bridge site corresponds to a local minimum for the NO chemisorption (cf. Figure 1d). The two other bridge sites are not stable and evolve into the Pd3h and Pd2Mnf 3-fold hollow sites. The bridge site is not favored on this surface (-1.48 eV), and it is even less stable than the atop manganese site (-1.57 eV). The atop palladium site is clearly unfavorable (-1.26 eV), and the molecule tilts toward the surface with a Pd-N-O bond angle of 133°. These results do not agree with previous Hu¨ckel calculations that showed a preferred bridge site (-3.35 eV) with respect to the Pd3h 3-fold hollow position (-3.29 eV).17 The main reason is the absence of spin polarization in the extended Hu¨ckel calculations. The chemisorption on Pd3Mn (100) and (111) surfaces can be compared. Overall, the adsorption is more stable on the (111) surface, the Pd3h 3-fold hollow site on (111) being 0.3 eV lower than the dibridge site on Pd3Mn (100). Therefore, the (111) facets of clusters should be first populated if diffusion across edges is easy. In more details, the low coordination sites (top and bridge) are more stable on the (100) surface, while the high coordination sites are more stable on the (111) surface. If we compare the Pd-N and Mn-N distances between the 3-fold hollow site on the (111) surface and the pseudo 3-fold one on (100), the Pd-N is shorter on the (111) while the Mn-N is longer. Hence the contribution of Mn to the bond with the adsorbate is reduced on the (111) surface. Despite the stronger chemisorption on the (111) surface, the N-O bond distance is longer on the (100) surface, especially in the case of the dibridge site. Moreover, no stable horizontal chemisorption has been found on the (111) surface, a qualitative indication of reduced activity for NO bond dissociation. Compared to the pure (111) surface, the chemisorption on the alloy is again weaker.29 However, the effect is twice smaller on the (111) surface compared to the (100) case. Therefore, several effects (NO bond distance, metal-N distances, change in chemisorption energy) indicate that the influence of the alloying with Mn is smaller on the (111) surface compared to the (100) surface. This could be related to the different Mn concentrations in the surface plane on these models. IV. Vibrational Frequency Analysis The vibrational properties of the NO molecule on palladiummanganese surfaces had never been studied theoretically. The
Figure 3. Experimental infrared spectrum for NO chemisorbed at 300 K on Pd65Mn35 clusters supported on SiO2 (from ref 9 and 10). Calculated NO stretch frequencies for the various sites of Pd3Mn (100) and (111) surfaces are indicated for comparison.
experimental literature gives also little information. First results on Pd65Mn35 clusters supported on SiO2 are provided by a recent study (cf. Figure 3).9-10 The clusters are rather large, so it can be assumed that the spectra are dominated by the contribution of (111) and (100) facets. Experiments are conducted in catalytic conditions where the coverage is higher than the 0.25 ML used in the calculations. Therefore, not only the most stable sites are populated but also less favored sites, such as top sites. Hence the spectra show contributions from a mixture of chemisorption sites, and insights from theoretical calculations can be used to sort them. The calculated frequencies are given in Tables 1 and 5 for the Pd3Mn (100) and Pd3Mn (111) surfaces, respectively. For the Pd3Mn (100) surface, the DFT calculations indicate that the bridge and the 3-fold pseudohollow vertical adsorptions are very close in energy to the horizontal dibridge structure. The estimated anharmonic frequencies are 1540 cm-1 for the bridge site and 1421 cm-1 for the 3-fold pseudohollow site. The calculated value for the bridge chemisorption would fit with the observed peak at 1540 cm-1 on the infrared spectrum (band IV in Figure 3). Furthermore, the theoretical frequency for the 3-fold pseudohollow site could be associated with the lowest experimental value at 1450 cm-1 (band V). The top sites, tilted on the palladium atom and vertical on the manganese atom, correspond to the respective anharmonic wavenumbers 1684 and 1688 cm-1, which appear in the experimental band 16801700 cm-1 (band II). The vertical palladium top site on the (100) surface is associated with an anharmonic frequency of 1738 cm-1, corresponding to the band I of the observed spectrum. Such a vertical species could be formed at high coverage. For the most stable horizontal dibridge chemisorption structure, the N-O anharmonic stretching frequency has been estimated to 992 cm-1. This frequency is clearly different from all the other calculated frequencies and indicates a strongly weakened NO bond. The experimental observation of this band is impossible due to strong contributions of the SiO2 support in this range.
3032 J. Phys. Chem. B, Vol. 105, No. 15, 2001 Such a low frequency for NO has, however, been observed on a Rh (100) surface.33 It would be therefore useful to characterize the NO adsorption on Pd3Mn single-crystal surfaces with vibrational techniques in order to compare the experimental observations with the theoretical predictions. For the NO adsorption on the Pd3Mn (111) surface, the most stable Pd3h 3-fold hollow site has an anharmonic N-O stretching frequency of 1544 cm-1. This value belongs to the observed band close to 1540 cm-1 (band IV). The anharmonic frequency of the slightly less stable Pd2Mnf hollow site is estimated at 1460 cm-1, which agrees with the experimental band at 1450 cm-1 (band V). Two comments arise here. First, sites with the same coordination can yield NO frequencies different by ∼100 cm-1. Sites incorporating a Mn atom, such as the Pd2Mnf hollow site, are associated with a lower frequency. This is a general behavior. For another example, the mixed Pd-Mn bridge site on Pd3Mn (100) is associated with a frequency of 1540 cm-1, while the bridge site on pure Pd (100) shows a frequency of 1640 cm-1 (cf. Table 2). Second, as a consequence, it is not possible to clearly separate the spectrum in bands associated with various site coordinations. The peak at 1540 cm-1, for example, can correspond either to a mixed Pd-Mn bridge site or to a pure Pd3 hollow site. On the (111) surface, the top site corresponds to NO frequencies similar to that obtained on the (100) surface. Let us now summarize the interpretation of the spectrum that can be obtained from the calculations. The high-frequency band I can only be attributed to a NO atom atop a Pd atom. Two assignments are possible for the second band II, 50 cm-1 lower in frequency: atop Mn or tilted atop Pd. The atop Mn situation is more probable due to its better stability. Band III (from 1610 to 1650 cm-1) is more delicate. It can be associated with Pd2 type bridge sites. Such sites are absent in our model of the Pd3Mn (100) surface, but on the pure Pd (100) surface, the calculated NO frequency for the bridge site is 1640 cm-1. On the alloy (111) surface, the Pd2 bridge site gives a frequency of 1600 cm-1, at the limit of the considered band. The broad nature of the band can be associated with the various environments of such a Pd2 site on the particle. Band IV shows several peaks, and it is a region of overlap between bridge and hollow sites: mixed bridge site on a (100) surface (1540 cm-1) or pure Pd3 hollow site on a (111) surface (1544 or 1526 cm-1). The lowfrequency band V is clearly linked with a 3-fold mixed site (Pd2Mn) either on the (100) or on the (111) plane. V. Mulliken Population Analysis A simple comparison of the stretching frequencies between the NO adsorption on Pd (100) in the p(2×2)-1 NO (0.25 ML) structure (Table 2), the NO adsorption on Pd (111) in the (x3×x3)R30°-1 NO (0.33 ML) structure,29 and the NO adsorption on Pd3Mn (100) and (111) surfaces points out that the vibrations are systematically softer on the palladiummanganese surfaces. Indeed, for the Pd (100) surface, the anharmonic stretching frequencies of the top, bridge and 4-fold hollow sites are respectively 1804, 1644, and 1467 cm-1 (cf. Table 2). On the Pd3Mn (100) surface, the corresponding anharmonic frequencies for the vertical palladium top, bridge, and 4-fold hollow sites are weaker (1738, 1540, and 1322 cm-1). On the Pd (111) surface the anharmonic frequencies for the bridge and 3-fold fcc hollow sites are 1667 and 1574 cm-1.29 On the Pd3Mn (111) surface, the corresponding anharmonic frequencies are 1600 cm-1 for the bridge site and 1460-1544 cm-1 for the 3-fold hollow sites. The associated modes are hence softer once again, and this is especially the case for the mixed Pd-Mn sites.
Loffreda et al. TABLE 6: Electronic Populations of the NO π*x and π*y Molecular Orbitals for r Spin, β Spin, and Total r + β, for the Bridge NO Adsorption on the Pd3Mn (100) Surface ((2×2)-NO 0.25 ML Structure) and on the Pd (100) Surface (p(2×2)-NO 0.25 ML Structure)a bridge NO on Pd (100) π*x π*y π*x + π*y
bridge NO on Pd3Mn (100)
R spin
β spin
R+β
R spin
β spin
R+β
0.37 0.26 0.63
0.38 0.27 0.65
0.75 0.53 1.28
0.29 0.19 0.48
0.55 0.49 1.05
0.84 0.68 1.53
a The x direction corresponds to the bridge site axis, and the y direction is normal to the bridge.
To understand the origin of this global shift (60-100 cm-1) for the N-O stretching frequencies toward the lower frequencies on the Pd3Mn (100) and (111) surfaces, we focus on the comparison between the NO bridge adsorption on the Pd (100) and Pd3Mn (100) surfaces. A mixed Pd-Mn site is considered on the alloy and compared with a pure Pd2 site on Pd. This case is analyzed in detail on the basis of a Mulliken electronic population and density of states analysis. As it is well-known, the electronic population of the π*NO orbital is a key point for the strength of the NO bond. Table 6 reports the calculated occupations of the R and β π* spinorbitals, when NO is chemisorbed on pure Pd (100) or on Pd3Mn (100). Compared to the gas phase where it is occupied by one electron, π*NO gains some electronic population on the surface, in clear relation with the observed lowering of the stretch frequency. Moreover, this excess population is larger on Pd3Mn by 0.25 electron compared to Pd, explaining the further shift to lower frequencies on the alloy. The behavior of R and β spin electron is however not similar. For π*NO, the occupation of the β spin-orbital is strongly increased compared to Pd, while the occupation of the R spinorbital is slightly decreased. These changes in the electronic transfer when Pd3Mn is compared to Pd(100) have already been pointed out.20 They can be qualitatively understood by comparing the density of states of the surface metals atoms for the clean surfaces. These density of states projected on surface atoms for Pd(100) and Pd3Mn(100) are recalled in Figure 4. In the case of Pd (100) (cf. Figure 4a), both spin bands are positioned below the Fermi level and almost completely occupied. Upon chemisorption, these d orbitals interact with the partially occupied π*NO orbitals, resulting in an increase of the population of these orbitals. In the case of Pd3Mn (cf. Figure 4b,c), the density of states projected on the Pd atom is not significantly modified, the energy position of the center of the d band being only slightly shifted. The projection on the Mn atom shows a completely different behavior. In the alloy, the Mn atom has a very strong spin polarization. The R band is almost completely occupied, with a d band center lower than that of the Pd atom. It therefore interacts less efficiently with the π*NO orbital, explaining the smaller R occupation for this orbital on Pd3Mn. In contrast, the β band is almost empty, its center being positioned 1.4 eV above the Fermi level. It strongly interacts with the partially filled π*NO orbitals. The bonding combination for this interaction is pushed below the Fermi level, so that both π*NO and the involved Mn orbitals gain electronic population. This explains the stronger electronic transfer toward π*NO in the case of the β spin for the alloy. This influence of the β spin on the electron transfer is dominating, resulting in a net increase of the π*NO population. In a surprising way, empty states on the Mn atom are responsible for an increased electron-transfer toward NO, and
Alloying Effects on N-O Stretching Frequency
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3033 on the Mn atom are responsible for this increased electron transfer. Indeed, these vacant states have an energy close to that of the π*NO orbitals, so that the coupling is strong and the bonding combination is pushed down below the Fermi level. From the calculation, all observed vibrational frequencies can be interpreted. Moreover, a new chemisorption structure is found. Indeed, the most stable situation for NO on a Pd3Mn surface is a horizontal dibridge structure. In this case, the NO bond is significantly elongated, and a low NO frequency is calculated. This band cannot be seen in the experiment due to the presence of strong signal from the silica support.4,5 Such a chemisorption structure with an activated NO bond could have important consequences for the NO bond dissociation reactivity on the alloy surface.32 This example shows that, although the situation is rather complex, vibrational methods combined with theoretical calculations can give detailed information on the molecular chemisorption on alloy surfaces. The possibility to perform the frequency measurements in a pressure of gases opens fascinating possibilities for the understanding of elementary reaction mechanisms at complex alloy surfaces. References and Notes
Figure 4. Spin polarized density of states projected on the surface atom of Pd (100) (a) and on the surface Pd atom (b) or the surface Mn atom (c) of Pd3Mn (100) for the clean surfaces. Only the d band is shown. Energies in eV are referenced to vacuum and the Fermi level is indicated by a vertical line. The dotted arrow indicates the position of the center of the band.
hence of the smaller vibrational frequency on the alloy compared to pure Pd. These empty states interact with the π*NO and push it below the Fermi level, resulting in a transfer from the “surface electron reservoir” toward the π*NO molecular orbital. VI. Conclusions The interpretation of vibrational spectra for adsorbates on bimetallic alloy surfaces is a complex problem. The number of different surface sites is large, and the frequencies depend both on the coordination of the site (hollow, bridge, etc.) and on the chemical nature of the metal atoms. Theoretical calculations can now yield accurate and useful insights in order to help the assignment of bands. The results for NO chemisorbed on Pd3Mn (100) and Pd3Mn (111) surfaces show that sites containing a Mn atom give a lower NO stretch frequency. The variation is ∼100 cm-1 and is of the same order of magnitude as the change associated with a modified coordination of the surface site (such as going from a bridge to a hollow site). Therefore, it is not possible to clearly separate the spectrum in regions associated with different metallic coordinations for the adsorbate, as it is usually done for pure metal surfaces, but these regions now overlap. The lower NO stretch frequency for the sites containing a Mn atom can be explained by a larger electronic transfer toward the π*NO orbitals. In a rather counterintuitive way, empty d states
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