J. Phys. Chem. C 2008, 112, 16539–16549
16539
A Combined Density-Functional and IRAS Study on the Interaction of NO with Pd Nanoparticles: Identifying New Adsorption Sites with Novel Properties Francesc Vin˜es,† Aine Desikusumastuti,‡ Thorsten Staudt,‡ Andreas Go¨rling,§ Jo¨rg Libuda,*,‡ and Konstantin M. Neyman*,†,| Departament de Quı´mica Fı´sica and Institut de Quı´mica Teo`rica i Computacional (IQTCUB), UniVersitat de Barcelona, C/Martı´ i Franque`s 1, 08028 Barcelona, Spain, Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, Lehrstuhl fu¨r Theoretische Chemie and Interdisciplinary Center for Interface Controlled Processes, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, and Institucio´ Catalana de Recerca i Estudis AVanc¸ats (ICREA), 08010 Barcelona, Spain ReceiVed: May 15, 2008; ReVised Manuscript ReceiVed: August 1, 2008
Nanocrystalline particles expose special adsorption sites close to edges and corners, giving rise to novel adsorption and reaction properties. The spectroscopic identification of these sites represents a great challenge, however. Here, we present results of a combined experimental and theoretical study on the adsorption of NO on Pd nanoparticles, using infrared reflection absorption spectroscopy (IRAS) and calculations based on densityfunctional theory (DFT). This approach facilitates identification of the adsorption sites available on the nanoparticles and reveals detailed information on their bonding properties, on the vibrational parameters of NO adsorbed on these sites, and on their sequence of occupation. With respect to all these aspects, the adsorption behavior of NO on the Pd nanoparticles notably differs from any single crystal reference data available. The IRAS studies are performed on well-defined Pd nanoparticles supported on an ordered Al2O3 film on NiAl(110). The growth and structure of these particles has been characterized previously, predominately exposing (111) and a small fraction of (100) facets. Here, we systematically monitor the NO adsorption as a function of exposure in a temperature region between 100 and 300 K by means of time-resolved IRAS in combination with molecular beam (MB) dosing. We interpret these experimental data with the help of DFT calculations on the adsorption of NO on unsupported cuboctahedral Pdn clusters cut from Pd bulk and containing up to 140 atoms; for comparison, calculations of the reference adsorption complexes of NO on single-crystal Pd(111) surface have also been performed. NO molecules are shown to most favorably adsorb on hollow µ3-sites on (111) facets of Pdn clusters, closely followed by bridge µ2-sites at the edges between adjacent (111) facets. Both sites give rise to characteristic features in the vibrational spectrum and are populated sequentially. At higher coverage (and low temperature) on-top µ1-sites on the (111) facets begin to be occupied. At variance with the adsorption on Pd(111) surface, however, additional on-top-sites are available at the particle edges and corners, which reveal stronger NO adsorption. In spite of the strong adsorption in bridge (µ2) coordination geometry at edges, our calculations predict that intermolecular repulsion between adjacent µ2-NO species gives rise to the formation of mixed bridge/on-top structures at high coverage. Similarly to the bridge NO at particle edges, the edge- and corner-related µ1-NO species reveal characteristic vibrational frequencies, allowing for direct verification of this prediction by IRAS. The present results make possible the identification and monitoring of the occupation of specific sites on Pd nanoparticles by NO during adsorption and reaction processes. 1. Introduction Nanoparticles may exhibit strongly size-related adsorption and reaction properties. This is a statement that may almost appear trivial as seen from the rapid progress in research on nanostructured materials. Indeed, heterogeneous catalysis has been taking advantage of the properties of supported nanoparticles for a long time.1 On a more-or-less empirical basis, such material * To whom correspondence should be addressed. E-mail: joerg.libuda@ chemie.uni-erlangen.de and
[email protected]. † Universitat de Barcelona. ‡ Lehrstuhl fu ¨ r Physikalische Chemie II, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg. § Lehrstuhl fu ¨ r Theoretische Chemie and Interdisciplinary Center for Interface Controlled Processes, Friedrich-Alexander-Universita¨t ErlangenNu¨rnberg. | Institucio ´ Catalana de Recerca i Estudis Avanc¸ats (ICREA).
parameters as particle size and shape are optimized toward increased activity and selectivity. From the atomic-level point of view, these properties result from special sites exposed by supported nanoparticles that involve low-coordinated (thus, commonly more reactive) atoms at particle edges, corners, and other relevant sites. Particle size is often related to the particle shape, and in this way can affect the amount of various special surface sites available for interactions with reactants. Despite this general appreciation of the importance of the size-related properties of nanoparticles, it turns out to be one of the major challenges in surface chemistry and model catalysis to get insights into the origin of these effects at the atomic level. The reason is the intrinsic complexity of nanoparticle-based systems at the microscopic scale, requiring detailed knowledge of both the individual properties of a large number of different
10.1021/jp804315c CCC: $40.75 2008 American Chemical Society Published on Web 09/30/2008
16540 J. Phys. Chem. C, Vol. 112, No. 42, 2008 sites and, even more difficult, on the mutual interaction of adsorbates on these sites. From an experimental point of view, a more detailed knowledge would involve the spectroscopic identification of adsorbates on different sites of a nanoparticle, preferentially by means of vibrational spectroscopy due to the unprecedented “chemical resolution” of the method (see refs 2-5, for example). If the identification of reactants on the individual nanoparticle sites is successful, it opens up the possibility of monitoring the site-occupation under reaction conditions and obtaining site-specific information of the mechanism and the reaction kinetics on nanoparticles. Despite the beauty of the idea, there are only a few examples in which such attempts were successful.6,7 One of the main problems is that the assignment of IRAS peaks to specific adsorption sites on particles is by no means trivial. Most assignments are made by comparison with single crystal surfaces (cf. discussion in refs 8, 9), either based on experimental data or on first-principles calculations. However, it is apparent that some adsorption sites on a nanoparticle may have no equivalent single crystal counterpart, even if stepped and defect-containing surfaces were taken into account. In order to overcome this problem, we follow a combined experimental and theoretical approach. As a model system we choose the adsorption of NO on Pd nanoparticles, an adsorption system featuring considerable size-related structure dependencies (e.g. refs 9-14). In a first step, we performed systematic IRAS experiments during NO adsorption on a well-defined Pd/Al2O3 model system. The Pd crystallites, prepared under ultrahigh vacuum (UHV) conditions on an ordered Al2O3 film on NiAl(110)15-17 have been previously characterized in detail with respect to their structural and electronic properties (see, for example, refs 9, 18 and references therein). The system has also been subject to reactivity studies using time-resolved IRAS.7,19-21 The aim of the present study is to provide a solid basis for the spectroscopic identification of the adsorption sites on modelsupported Pd catalysts occupied by the NO probe. Toward this goal, we have performed systematic density-functional calculations on sufficiently large model Pd nanoparticles of different size in the range where their properties are scalable to the bulk. Thus, these models were chosen such that they closely resemble the surface properties of the larger particles investigated experimentally. A review of NO adsorption and reaction studies on metal surfaces is available.22 Earlier pertinent theoretical investigations were performed on small cluster models (e.g., refs 23-26) insufficiently accurate for reproducing adsorption energies of metals;27 recent first-principles studies of NO on Pd (e.g., refs 28-33) employed slab models. Despite very useful information obtained from calculations on extended surfaces, they do not describe the specific role of particle edges and corners. Sufficiently large cluster models, on the other hand, were shown to provide a realistic theoretical description of nanoparticles in model catalysts.34-40 We found here, in agreement with previous studies,34 that cuboctahedral Pd nanoparticles with ∼100 atoms can provide size-converged results for the adsorption of NO. These particles also expose adsorption sites near low-coordinated Pd atoms, which give rise to notable deviations from the single-crystal behavior. On the basis of a careful comparison between theory and experiment, we established a detailed scenario of the adsorption of NO on Pd nanoparticles, which includes site-specific variations of the adsorption energy, especially at the edges and corners, the sequence of site occupation, and the mutual interaction between adsorbed molecules at higher coverage.
Vin˜es et al. 2. Experimental and Computational Details 2.1. Experimental. All IRAS experiments were performed in a newly developed UHV apparatus for TR-IRAS (timeresolved IRAS)/MB (molecular beam) measurements at the University Erlangen-Nuremberg. The setup allows for up to four effusive molecular beams and one supersonic molecular beam to be superimposed on the sample surface. Additionally, the system is equipped with a FTIR (Fourier-transformed IR) spectrometer (Bruker IFS66/v), a beam monitor which allows alignment and intensity calibration of the beams, two quadrupole mass spectrometers (QMS), a vacuum transfer system with high pressure cell, and all necessary preparation tools (evaporators, gas dozer, quartz microbalance, LEED/Auger, ion gun, etc.). The NO beam (Linde, 99.5%) was generated from an effusive beam dozer and modulated by a valve system (see Figure 1a). All measurements were performed by means of fully remotecontrolled sequences [interfacing and programming: National Instruments (NI), Laboratory View] (see Figure 1b). In order to systematically cover a large exposure range, dosing is performed by pulsing NO at variable beam intensities typically in the range between 8 × 1012 cm-2 s-1 (equivalent pressure: 3 × 10-8 mbar) and 2 × 1015 cm-2 s-1 (equivalent pressure 7 × 10-6 mbar), followed by acquisition of IR spectra (spectral resolution of 2 cm-1, typical acquisition times of 38 s). In order to prepare the Al2O3 film on the NiAl(110) surface, the latter was cleaned by numerous cycles of sputtering and vacuum annealing, followed by two cycles of oxidation in 10-6 mbar O2 at 550 K and UHV annealing at 1135 K. The detailed procedure is described elsewhere.15-17 The quality of the film was checked by LEED (low-energy electron diffraction), and complete oxidation of the surface was proven by the absence of CO adsorption at 100 K. Pd particles (>99.95%, Goodfellow) were grown by physical vapor deposition at a sample temperature of 300 K using a commercial evaporator (Focus, EFM 3). The flux was calibrated by a quartz microbalance. The amount of Pd deposited corresponds to a nominal layer thickness of 4 Å. This is equivalent to a Pd atom density of 2.7 × 1015 atoms cm-2 (1 Å Pd corresponds to a density of 6.8 × 1014 atoms cm-2). In order to avoid damage by ion bombardment, the sample was biased during Pd evaporation. Before the adsorption experiment, the Pd particles were stabilized by extended exposure to oxygen (1200 s, 10-6 mbar, sample temperature 500 K) followed by surface oxygen removal by CO titration above the desorption temperature of CO (1200 s, 10-6 mbar, sample temperature 500 K). For a further discussion of this procedure we refer to the literature.41 2.2. Computational Details. Density-functional calculations were performed with the help of the VASP code,42 an implementation of a periodic plane-wave variant of the Kohn-Sham method. Interactions of valence electrons with the atomic cores were described by the projected augmented planewave method.43 A kinetic energy cutoff of 415 eV for the planewave basis set was employed throughout. The lack of a universal exchange-correlation (xc) functional that provides sufficiently accurate representation of various observables of heavy-element systems44,45 prompted us to examine several xc approximations in order to find out which of them provides the most accurate adsorption energies and vibrational frequencies for the NO/Pd systems. The xc functionals employed were either within the local density approximation (LDA) [by Vosko-Wilk-Nusair46 (VWN)] or the generalized gradient approximation (GGA) [by Perdew-Wang47 (PW91), Perdew-Becke-Erzenhof48 (PBE), and the revised form of PBE49 (RPBE)]. Geometry optimization
Interaction of NO with Pd Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16541
Figure 1. NO adsorption experiments on Pd nanoparticles on Al2O3/NiAl(110): experimental setup (a); the data acquisition scheme for the coverage dependent IR reflection absorption spectra (b); an STM image 150 nm × 150 nm (ref 56) of the Pd nanoparticles on Al2O3/NiAl(110) (c); sketch of a Pd nanoparticle on Al2O3/NiAl(110), indicating the types of available adsorption sites (d).
was performed using a conjugated gradient algorithm until forces acting on each atom became less than 0.3 eV/nm. To speed up convergence of the Kohn-Sham self-consistent process, a Gaussian smearing of 0.2 eV has been applied, but the final energy values were extrapolated to 0 K (no-smearing). Previous studies50 and present tests do not reveal any noticeable spinpolarization effect for either the substrate Pd models or the adsorption complexes of NO molecules on them. Thus, all calculations (except for the free NO molecule) were non-spinpolarized. We defined the calculated adsorption energy Eads of an NO probe molecule on Pd substrates in terms of the total energies of the adsorption system, ENO/Pd, the adsorbate, ENO, and the substrate, EPd, as Eads ) -ENO/Pd + (ENO + EPd); according to this definition, positive energy values correspond to favorable adsorption. If not stated otherwise, numerical calculation of the Hessian matrices followed by diagonalization to obtain harmonic frequencies was performed, taking into account only displacements of N and O atoms.51 The computed harmonic NO stretching vibrational frequencies can be directly linked to the experimental IRAS spectra. The reference Pd(111) surface was represented by six-layer slabs. Positions of atoms in the bottom three layers were kept frozen as optimized for Pd bulk, whereas the other three layers (closer to adsorbates) were allowed to relax completely. An intralayer vacuum space of 1 nm thickness was introduced to avoid interactions between repeated slabs. The following grids of k-points have been used for slab surface cells of different sizes: 18 × 18 × 1, surface cell (1 × 1), 9 × 9 × 1 (2 × 2), 6 × 6 × 1 (3 × 3), 4 × 4 × 1 (4 × 4). For Pdn nanoparticles, calculations were performed at the Γ-point only, with 1 nm of vacuum in each direction of the unit cell to avoid interactions between nanoparticles in adjacent unit cells (see also section 3.3). For benchmarking our calculated data, we first computed isolated (gas-phase) NO molecules (Γ-point, spin-polarized) and
TABLE 1: Characteristic Observablesa Calculated Using Various Exchange-Correlation (xc) Functionals for Pd Bulk, Gas Phase NO(g), and NO Adsorbed at the Coverage 1/9 ML on an fcc Site of a Pd(111) Slab in Comparison with the Experimental Values NO(g) xc VWN PW91 PBE RPBE exptl
NO/Pd(111)
Eads, ν(N-O), Pd bulk d(N-O), ν(N-O), cm-1 kJ mol-1 d(Pd-Pd), pm pm cm-1 273 280 280 282 275b
116 117 117 117 115c
1959 1923 1922 1912 1903d
309 223 216 183 179e
1653 1580 1586 1570 1570f
a d(Pd-Pd) is the nearest distance between Pd atoms in the bulk for a unit cell parameter optimized using the respective xc; d(N-O) is the distance between the N and O atoms of NO molecule; ν(N-O) is the harmonic stretching N-O vibrational frequency; Eads is the NO adsorption energy. b Reference 52. c Reference 53. d Harmonic value from ref 54. e Reference 10. f Measured in ref 55 at the lowest coverage θ ) 0.26 ML; the experimental value for θ ) 0.33 ML equals 1590 cm-1. One ML coverage corresponds to one adsorbed NO molecule per Pd atom of the (111) surface.
Pd bulk (unit cell of four atoms, Γ-centered Monkhorst-Pack k-point grid 18 × 18 × 18) using various xc functionals (Table 1). Not surprisingly, calculated GGA bond distances are longer than the respective LDA ones, which almost perfectly agree with the experimental data52,53 with only a minor underestimation of the nearest interatomic distances for Pd bulk, by 2 pm, and a minor overestimation of the bond length of gas-phase NO molecule, by 1 pm. Thus, the LDA xc functional provides the most accurate geometric parameters for these two systems. The situation becomes different for the computed stretching frequency of an NO molecule. In fact (Table 1), all GGA xc functionals under scrutiny perform well, and the RPBE func-
16542 J. Phys. Chem. C, Vol. 112, No. 42, 2008
Vin˜es et al.
tional gives the harmonic vibrational frequency of the gas-phase NO, which exceeds the experimental value54 solely by less than 10 cm-1. Further comparison has been carried out for adsorption energies and vibrational frequencies of NO adsorbed on fcc and hcp µ3-hollow sites on a (3 × 3) Pd(111) slab, computed with the same set of xc functionals. (Hereafter we use the notation fcc for octahedral hollow sites and hcp one for tetrahedral hollow sites.) The computed adsorption energies and NO vibrational frequencies listed in Table 1 again reveal, as expected, that LDA overestimates the bonding. GGA xc functionals provide notably more accurate adsorption energies, and in particular RPBE values are in excellent agreement with experimental values10 on Pd(111). Also, at a coverage θ ) 0.25 ML the RPBE vibrational frequency of NO, 1586 cm-1, lies within 20 cm-1 of the experimental anharmonic value 1570 cm-1, measured at θ ) 0.26 ML.55 Thus, the RPBE functional appears to provide a balanced description of both the adsorption energies and vibrational frequencies of NO on Pd. These calculated values of observables can be directly compared to experimental data, without the need of scaling or readjustment. Unless explicitly stated otherwise, all calculations discussed in the following were performed using the RPBE xc functional. 3. Results and Discussion 3.1. Structure and Properties of the Pd Nanoparticles on Al2O3/NiAl(110). Before discussing the adsorption of NO, we briefly summarize the most relevant previous results on the structure of the Pd nanoparticles on Al2O3/NiAl(110), mainly as a basis for the development of suitable model for the DFT calculations in sections 3.3 and 3.4. The system has been previously investigated in detail.9,41,56,57 A scanning tunneling microscopy (STM) image of the Pd particles on the alumina film is displayed in Figure 1c.56 Upon deposition of Pd at a surface temperature of 300 K, the Pd aggregates preferentially nucleate at oxide domain boundaries and other defects on the oxide film. At larger coverage, the metal forms well-shaped three-dimensional crystallites. At the Pd coverage used in this work, we obtain particles with an average diameter of 5.5 nm, at a density of around 1012 cm-2. These particles cover about 20% of the support. On the average, the Pd particles contain about 3000 atoms each. Particles sizes in this range are typical for supported noble metal catalysts. However, such crystallites are by far larger than systems that could presently be calculated at a rigorous density-functional level. In order to design suitable theoretical models for the supported Pd nanocrystallites (section 3.3), we reconsider experimental results on the morphology of the latter in more detail. Previous STM studies have shown that the Pd particles grow in (111) orientation.41,56 Accordingly, the crystallite top is represented by a large (111) facet, and the sides consist of rather large (111) and smaller (100) facets. As a result, 80 to 90% of the particle surface is terminated by (111) facets (cf. also ref 58). Note that stabilization of the model system by cycles of O2 and CO treatments (see section 2) has only a minor effect on the particles morphology, whereas the oxide support film is modified substantially.41,56 A model Pd nanocrystallite is depicted schematically in Figure 1d. On the basis of this model, we may identify different types of sites that could be responsible for the specific adsorption properties of the nanocrystallites and, potentially, can be manifested via characteristic vibrations of adsorbed molecules. Sites within the central area of large (111) facets and, to a smaller extent, (100) facets should exhibit adsorption properties
Figure 2. NO adsorption experiments on Pd nanoparticles on Al2O3/ NiAl(110). IR reflection absorption spectra of the NO stretching frequency region taken during NO adsorption at a sample temperature of 100 K.
similar to those on the corresponding single crystal surfaces. In contrast, edges between (111) facets and, less abundant, edges between (111) and (100) facets, are expected to give rise to deviations from the single crystal references. Sites both directly at the edges and on facets in close proximity to edges may have notably modified properties. Finally, we have to take into account the particle corners, which for smaller particles may expose sites sufficiently abundant to be detected. In the following sections we will first investigate the IR spectra of NO adsorbed on the Pd particles (section 3.2), then establish suitable models and suitable approaches for the theoretical description of the NO adsorption (section 3.3) and, finally, identify adsorbed NO on specific sites based on a comparison of experiment and theory (section 3.4). For completeness, it should be noted that in addition to the sites mentioned above there are boundary sites at the metal-oxide interface. These could neither be identified spectroscopically (even if present, their IRAS “fingerprints” manifested by adsorbates in geometries nearly parallel to the surface are probably hindered by stronger signals caused by the metal sites) nor described using the present nanoparticle models treated without explicit accounting for the oxide support. Thus, one has to await very demanding accurate size-converged future calculations on supported nanoparticles that may provide information on properties and detectability of such boundary sites. 3.2. IRAS of NO on the Pd Nanoparticles on Al2O3/ NiAl(110). We have systematically acquired IR spectra of NO on Pd/Al2O3/NiAl(110) as a function of NO exposure at sample temperatures of 100 and 300 K; see Figures 2 and 3. In this
Interaction of NO with Pd Nanoparticles
Figure 3. NO adsorption experiments on Pd nanoparticles on Al2O3/ NiAl(110). IR reflection absorption spectra of the NO stretching frequency region taken during NO adsorption at a sample temperature of 300 K.
work, we focus on the low exposure range between 0 and 30 L (1 L (Langmuir) corresponds to 10-6 Torr). In this interval of exposures, the interaction is largely restricted to molecular adsorption on the Pd particles themselves. At larger exposures, a low probability reaction channel involving the Al2O3 support becomes relevant, giving rise to the formation of a variety of nitrogen-oxo species, as shown previously.20 There is no indication that Pd nanoparticles are oxidized, in contrast with similar experiments with NO2 where oxidation of Pd is observed.59 Focusing on adsorption at 100 K (Figure 2), we observe a weak band at 1530 cm-1, which first increases in intensity and shifts to 1555 cm-1 with increasing NO exposure. At exposures >3 L, however, the band again decreases in intensity, while a second feature appears around 1580 cm-1. Together with the lower frequency peaks, characteristic bands develop at higher frequency. Starting from exposures as low as 0.1 L, a peak appears at 1645 cm-1, which similarly to the lower frequency features first increases and later decreases in intensity, simultaneously blue-shifting to 1675 cm-1. A further weak band around 1630 cm-1 develops at high exposure. At exposures larger than 1.5 L, the dominating bands are those in the region above 1700 cm-1. Here, a weaker feature at 1735 cm-1 develops first, which is, however, rapidly obscured by a fast growing very intense band at 1750 cm-1. At exposures around 30 L, the latter band develops a high-frequency shoulder around 1770 cm-1. Upon adsorption at 300 K (Figure 3), we observe a similar behavior concerning the lower frequency region between 1500 cm-1 and 1650 cm-1. Initially, we detect a weak band at 1520
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16543 cm-1, blue-shifting and splitting into features at 1575 and 1530 cm-1. These bands are rapidly followed by the appearance of a band at 1640 cm-1 (at exposures >0.4 L), which blue-shifts to 1657 cm-1 with increasing NO dose. Finally, a weak band at 1730 cm-1 appears at exposures exceeding 3 L. One of the main differences to adsorption at 100 K is the absence of the very strong band at 1750 cm-1. At this point it is reasonable to compare the above results with adsorption data on single crystal surfaces. In thermal desorption studies on Pd(111), three desorption features are observed at 257 K, 285 K, and 510 K.60,61 For the high temperature peak, desorption occurs from a disordered phase at coverages up to θ ) 0.33 ML.61,62 Vibrational spectroscopy in this coverage range shows a single band continuously shifting from 1540 cm-1 to 1590 cm-1 with increasing coverage.10,11 Upon saturation at 300 K, a single band at 1589 cm-1 is detected.61,62 It is essential to note that under UHV conditions, higher coverage can be obtained at cryogenic temperatures only. The corresponding behavior is quite complex, involving successive formation of numerous ordered superstructures. At θ ) 0.50 ML, an ordered c(2 × 4) phase gives rise to an absorption band between 1610 cm-1 and 1620 cm-1.55,61-63 For the maximum coverage at 100 K (θ ) 0.75 ML), a (2 × 2) structure is observed which is characterized by two bands at 1589-1600 cm-1 and 1735-1750 cm-1.10,55,61,62 Structures at intermediate coverages, such as the ordered (8 × 2) antiphase domain boundary structure at θ ) 0.625 ML, give rise to more complex vibrational spectra (peaks at 1590 cm-1, 1615 cm-1, 1736 cm-1, 1744 cm-1).63 Typically, the vibrational features in the different regions are assigned to on-top, bridging, and hollow NO species, depending of the vibrational frequency. However, such simple rules of thumb, which in older studies arose from the interpretation of vibrational spectra on the basis of nitrosyl compounds, may give rise to misassignments, as for example pointed out by Brown and King for NO on different transition metal surfaces.22 Here, calculations may help to shed light on the local adsorption geometries. Based on DFT calculations,28,29,31,32 it was concluded that NO occupies fcc hollow sites at low coverage, a mixture of fcc and hcp sites at θ ) 0.50 ML, and on-top sites in addition to the mixture of fcc and hcp sites at θ ) 0.75 ML. Because 80-90% of the Pd particle surface of the Pd/Al2O3/ NiAl(110) model system is terminated by (111) facets, close similarities with vibrational spectra of NO/Pd(111) are expected. Indeed, several observed features may be interpreted accordingly. At low coverage, the band at 1530 cm-1 (Figure 2) can be attributed to NO on 3-fold hollow sites of Pd(111) (fcc and/ or hcp, indistinguishable experimentally). At high NO coverage, the peaks at 1750 cm-1 and 1585 cm-1 (Figure 2) can be associated with on-top NO on (111) facets and with NO on 3-fold hollow sites, similar to the (2 × 2) superstructure at θ ) 0.75 ML on Pd(111) (see above). However, there are also differences that cannot be explained by the Pd(111) reference. Among them are (i) the appearance of all peaks between 1640 and 1680 cm-1, including the splitting of the bands between 1500 and 1600 cm-1 and 1600 and 1700 cm-1 at high coverage, and (ii) the appearance of a band at 1735 cm-1 at 300 K (the shoulder at 1770 cm-1, which is also not observed on Pd(111), can be associated with the low temperature decomposition on Al2O320). In order to account for these differences, we may in principle invoke the influence of the Pd(100) minority facets. On the Pd(100) single crystal, a p(4 × 2) superstructure is formed (θ ) 0.25 ML) at elevated temperature (420 K) showing a
16544 J. Phys. Chem. C, Vol. 112, No. 42, 2008 simple vibrational spectrum with a single band at 1492 cm-1.64 At lower temperatures a second band at 1653 cm-1 emerges, until at θ ) 0.50 ML a (22 × 22)R45° superstructure gives rise to a single band at 1678 cm-1. Using high-resolution photoelectron spectroscopy, it was shown that in the case of the low coverage structures (θ ) 0.25-0.30 ML), 4-fold hollow sites are occupied, whereas at higher coverage (θ ) 0.50 ML), NO adsorbs on bridge sites exclusively.65 Thus, we could attribute the bands between 1630 and 1675 cm-1 to bridge bonded NO on (100) facets. However, the abundance of these facets is low and the orientation of the Pd particles is such that the (100) facets are tilted with respect to the surface normal (of the upper largest (111) facet, Figure 1d). As a consequence of the metal surface selection rule (MSSR), the corresponding absorption signal should be strongly attenuated.2 Although intensities in IRAS spectra do not directly reflect the abundance of the corresponding species because of differences in the dynamic dipole moment and dipole coupling effects,2,66 it appears unlikely that the strong bands in the region between 1600 and 1700 cm-1 primarily originate from NO on (100) facets. Therefore, we recently argued that a tentative assignment to NO at defect sites such as particle edges or steps would be more plausible.7,19,21 Following analogous arguments, a similar assignment was previously suggested for CO adsorption on the same particles6,67 and later confirmed by theoretical calculations.35 Another argument for the assignment to NO at particle edges and defects stems from studies on stepped surfaces, for which a similar band appears in the spectral region between 1655 and 1670 cm-1.10,11,68 It is noteworthy that the band at 1645 cm-1 is detected already at low exposure, indicating a high adsorption energy at these defect- or edge-related sites, being only slightly smaller than the adsorption energy for NO on hollow sites on the (111) facets. Another surprising observation is the appearance of the peak at 1735 cm-1 upon adsorption at 300 K. In contrast, the binding energy of on-top NO on Pd(111) is too small to allow detection of this species at 300 K. This implies the presence of defect sites, which allow adsorption in the on-top geometry with strongly enhanced adsorption energy. In the following, we aim at the identification of these nanoparticle-specific adsorption sites on the basis of our results of density-functional calculations for single NO molecules on Pdn nanoparticles and comparison to the experimental IRAS data. 3.3. Nanoparticle Models and Benchmark Calculations. We have chosen cuboctahedral Pdn crystallites cut by (111) and (100) planes from Pd bulk and consisting of 79, 116, and 140 atoms (see Figure 4) to model NO adsorption on the abovedescribed experimentally studied supported Pd nanoparticles. Metal nanoparticles in this size range have been proven to feature scalability of the properties to the bulk Pd and the sizeconverged reactivity of interior areas of the (111) facets with respect to that of extended Pd(111) terraces.34,45,50,69,70 Our first benchmark was to determine which part of the Pdn substrate nanoparticles needs to be included in the geometry relaxation induced by NO adsorption. To this end we studied the adsorption of a single NO molecule on the Pd79 crystallite using the most bond-overestimating xc functional VWN and placing NO either on the fcc site in the center of a (111) facet or on-top of a Pd atom at one of the nanoparticle corners (see Figure 4). We have started by freezing the optimized bare Pd79 cluster and allowing relaxation of only the NO adsorbate. In further optimization steps, positions of both the adsorbate and a gradually increasing number of the neighboring Pd atoms were
Vin˜es et al.
Figure 4. Models of NO adsorption on bridge-on-edge and on-top corner sites of Pdn particles of increasing size: Pd, turquoise; N, yellow; O, red. Computed adsorption energy values (kJ mol-1, plain font) along with the NO stretching frequencies (cm-1, italics) are shown.
relaxed until the whole Pd79 species was included in the geometry optimization (see Supporting Information, Table S1). We found that the adsorption energy value for NO at high- and low-coordinated sites, when relaxing the second shell of Pd neighbors (Pd atoms at e0.4 nm around the atom N), deviates by less than 5 kJ mol-1 from the fully optimized result. Restricting relaxation to the second shell of Pd neighbors thus enables the reduction of computer time by a factor of 2, essentially without accuracy loss. This approach is expected to be even more accurate for the adsorption interactions, which are somewhat weaker at our standard RPBE level. The second question is related to the size of the vibrational coupling of the NO stretching mode with substrate phonons and with vibrational modes of nearby NO adsorbates. Since the smallest frequency of the N-O stretching mode (∼1500 cm-1) is far above the Pd phonon modes (
