Density Functional Calculations and IR Reflection Absorption

Jul 23, 2010 - [email protected]., †. Lehrstuhl ... Noèlia Pueyo Bellafont , Francesc Viñes , and Francesc Illas ... Yu Gong and John K...
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J. Phys. Chem. C 2010, 114, 13813–13824

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Density Functional Calculations and IR Reflection Absorption Spectroscopy on the Interaction of SO2 with Oxide-Supported Pd Nanoparticles Nicola Luckas,† Francesc Vin˜es,*,† Markus Happel,‡ Aine Desikusumastuti,‡ Jo¨rg Libuda,‡,§ and Andreas Go¨rling† Lehrstuhl fu¨r Theoretische Chemie, Lehrstuhl fu¨r Physikalische Chemie II, Erlangen Catalysis Resource Center, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: July 8, 2010

A systematic study on the interaction of sulfur dioxide (SO2) on BaO-supported Pd nanoparticles has been carried out using suitable models and state-of-the-art density functional (DF) calculations. Detailed information concerning the structure and energetics of the different conformations of adsorbed SO2 is provided as a function of coverage together with calculated infrared reflection absorption spectroscopy (IRAS) spectra. SO2 may adsorb on Pd(111) in several conformations, some active, η2-SbOa and η1-Sb, and others inactive in IRAS, η3-SaOaOa. SO2 is found to attach stronger to Pd nanoparticle edges and corners, a fact intimately related to catalyst poisoning by site blocking. On Pd nanoparticles, SO2 is found to preferably adopt adsorption conformations that depend on the specific region on the nanoparticle, thus adding site specificity to vibrational recognition. Molecular beam experiments and IRAS have been performed on a single-crystal-based Pd/BaAl2xO1+3x/NiAl(110) model NOx storage and reduction catalyst and its individual components. SOx formation on the oxide components, evolution of a SO2 multilayer, and adsorption of SO2 on BaO or Pd nanoparticles is linked to DF calculations. The effect of cation intermixing in the oxide support and overlap of absorption bands on the unequivocal discrimination of signals are discussed. 1. Introduction Sulfur-containing compounds are common impurities in gasoline and other oil-derived fuels.1,2 Once burned, sulfur oxide compounds like sulfur dioxide (SO2) are formed. Emissions of SO2 are a major problem for the environment since, when released into the atmosphere, sulfur dioxide can further oxidize and, through interaction with water steam, produce acid rain. Acid rain has a tremendous effect in vegetation degradation and corrosion of buildings, facilities, and monuments.3 The typical catalysts employed in postcombustion gas treatments, Pt-group metals (Pt, Pd, Rh, Ir, Ru, Os) as supported nanoparticles on porous oxides - suffer from a long-term deactivation by sulfur. This is enhanced by a rapid and complete decomposition of SO3 on metal nanoparticles and aggravated because some reaction products (O, S, and SOx) are extremely difficult to remove, as recently highlighted by means of density functional (DF) calculations and experiments.4 Sulfur-oxide compounds are well-known catalyst poisons in the chemical industry when using Pt-group metals and/or oxides.5 SO2 easily disproportionates (eq 1) on many metals and their alloys at room temperature,6-9 eventually forming SO3 (eqs 2 and 3) and SO4 species (eq 4) which may dissociate into O2 and atomic S (eq 5) typically above 450 K. * To whom correspondence should be addressed. E-mail: Francesc.Vines@ chemie.uni-erlangen.de. † Lehrstuhl fu¨r Theoretische Chemie. ‡ Lehrstuhl fu¨r Physikalische Chemie II. § Erlangen Catalysis Resource Center.

2SO2 f SO + SO3

(1)

SO f S + O

(2)

SO2 + O f SO3

(3)

SO3 + O f SO4

(4)

SO4 f S + 2O2

(5)

Atomic sulfur is strongly attached to metallic surfaces adsorption energies in the range of 320-560 kJ mol-1 - making its removal very difficult. Besides the effect of blocking of active sites, adsorbed sulfur atoms are known to strongly perturb the electronic structure of the underlying metal by withdrawing charge from the metal and inducing a strong reduction of the density of states near the Fermi energy level,10-12 which may spoil the catalytic activity. Theoretical results show that S atoms have a long-range steric repulsion effect,13,14 which explains why a single S atom is able to deactivate up to 10 surface metal atoms.14 This combination of undesirable effects translates into sulfur being among the most efficient sources of poison in catalysis.15,16 Many efforts are undertaken to reduce sulfur poisoning. For example, metal catalysts can be regenerated by reducing S atoms with H2 or by oxidizing them with O2. This may be appropriate in some cases, although in many others problems arise related to the reduction of the oxide support by hydrogen or oxidation of the metal functionalities by oxygen.15,16 Industrial operations frequently employ alkaline earth metal oxides (MO, M ) Mg, Sr, Ca, Ba) as SO2 scrubbers, storing it in the form of SOx species. Once the trap is saturated, it is replaced with a fresh

10.1021/jp105097z  2010 American Chemical Society Published on Web 07/23/2010

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load of material.2 This procedure generates waste and prevents continuous operation. SO2 poisoning and trapping plays an important role in automotive exhaust gas after treatment. Typical three-way catalysts also consist of Pt-group metal (Rh, Pt, Pd) nanoparticles supported on a porous oxide.17-19 These systems are intrinsically sensitive to sulfur poisoning. A special problem arises for modern lean-burn engines which operate under O2 excess.20 Here, so-called NOx storage and reduction (NSR) catalysts may be used,21 which contain Ptgroup metals and metal oxide (typically BaO). The latter stores NOx during fuel-lean operation periods in the form of nitrates.22 Unfortunately, NSR catalysts have the drawback that the MOs are quite sensitive to SO2 poisoning. Adsorption of COx, NOx, and SOx species on alkaline earth metal oxides is essentially connected to their basicity.23 Basicity increases in the order MgO < CaO < SrO < BaO, and so does the adsorption strength. Previous DF studies show that SOx species are remarkably stronger bound to any MO surface than the corresponding NOx or COx species.24,25 Thus, the effect of blocking active sites and storage sites seems to be the dominant mechanism of SOx poisoning on MOs, which agrees perfectly with the experimentally observed decrease in activity, which was found to be proportional to the SO2 concentration.26 It is noteworthy that SO2 is able to deactivate the catalyst at concentrations as low as ∼2.5 ppm, being stored by the MO mostly in the form of sulfite (SO32-), although sulfate (SO42-) species have also been detected on oxides at temperatures above 625 K or at exposures above 1 Langmuir (L) - 1 L corresponds to 1.33 × 10-6 mbar · s-1.27,28 Adsorption of SO2 on different oxides and metals has been extensively studied during recent years. However, as far as the oxides are concerned, most studies were carried out on powder catalysts.29-31 Their complex surface structure makes it rather difficult to clearly identify the adsorbates and reaction products formed upon SO2 adsorption on the catalyst surface. This fact leads to a controversial discussion in the literature with rather ambiguous assignments of adsorbates. A better understanding of S poisoning on Pt-group catalysts including supported metal nanoparticles would be valuable for the development of sulfurtolerant catalysts in general and, in particular, lean NOx traps.32 The most detailed information on reaction mechanisms and surface species is obtained experimentally through vibrational spectroscopy.33-35 The assignment of detected bands, however, is not trivial and often requires adequate theoretical calculations. In the present work we study SO2 adsorption on model NSR catalysts by means of state-of-the-art DF calculations and infrared reflection absorption spectroscopy (IRAS). Previous DF studies have mainly focused on single crystal surfaces such as Ir(111)36 and Pt(111).37-39 Here, we address a more realistic model NSR catalyst based on an experimentally ordered alumina film onto which Pd and BaO nanoparticles were deposited,40,41 although the latter form BaAl2xO1+3x particles upon annealing due to cation intermixing. These model systems were previously investigated by scanning tunnelling microscopy (STM)42,43 We studied SO2 adsorption on the full model NSR catalyst and on the individual components, allowing us to introduce certain features of complexity step by step without having to deal with the full complexity of the system in the first place. DF calculations carried out on both a slab and a nanoparticle model serve to interpret the experimental results and supply the demand for a systematic study on the interaction of SO2 on Pd. 2. Experimental and Computational Details 2.1. Experimental Methods. SO2 uptake experiments were carried out under ultrahigh vacuum (UHV) conditions for the

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Figure 1. (a) Sketch of a Pd nanoparticle ∼3 nm in size supported on an oxide surface (purple circles). The Pd nanoparticle exhibits large (111) facets and small (100) facets. It is modeled combining a Pd(111) slab of six layers (blue circles) to describe the inner part of (111) facets and a Pd79 nanoparticle for the description of edge, corner, and nearby sites (orange circles). (b) Experimental setup of the molecular beam/ IRAS instrument, and (c) experimental procedure and acquisition of IR spectra.

individual components and for the full Pd/BaO/Al2O3/NiAl(110) NSR catalyst. The molecular beam/time-resolved IRAS experiments were performed in an UHV apparatus that allows exposure of the sample surface to up to four effusive and one supersonic molecular beam. For the intensity calibration and alignment of the beam intensity a beam monitor was available. In addition, the system is equipped with a Fourier transform infrared spectrometer (Bruker IFS 66/v), two quadruple mass spectrometers, a vacuum transfer system, a high-pressure cell, and all necessary preparation tools. The SO2 beam (Linde AG, 99.0%) was generated from a supersonic expansion and modulated by a solenoid valve and a shutter (SO2 was expanded at room temperature; the backing pressure was held at approximately 1 bar). All measurements were carried out in fully remote-controlled sequences exposing the sample to pulses of SO2 ranging from 53 to 61 ms at an effective pressure of 1.25 × 10-5 mbar on the sample surface (equivalent to a beam flux of 2.8 × 1015 cm-2 · s-1) and a total exposure of 28 L. In between each SO2 pulse an IR spectrum was acquired at the corresponding dosing temperature (100 K), with a spectral resolution of 2 cm-1 and a typical acquisition time of 186 s. A schematic drawing of the experimental setup and procedure is given in Figure 1b and 1c, respectively. For preparation of the samples, the NiAl(110) single crystal was cleaned by several cycles of Ar+ sputtering and annealing in UHV up to 1300 K. During preparation, the sample was heated by radiative heating and electron bombardment. After

Interaction of SO2 with Oxide-Supported Pd Nanoparticles cleaning, two oxidation cycles were carried out (PO2 ) 2 × 10-6 mbar) at 550 K with subsequent annealing at 1135 K in order to form the ordered Al2O3 film on NiAl(110). Further details of the preparation procedure can be found elsewhere.44,45 The quality of the film was confirmed by low-energy electron diffraction (LEED) experiments, and the completeness of surface oxidation was proven by the absence of CO adsorption at 100 K. For preparation of the BaO particles, we proceeded as follows: First, Ba (rod, Alfa Aesar, 99%) was manually cleaned under an inert gas atmosphere in a glovebox and placed later into a Mo crucible. To prevent oxidation of the metal, Ba was covered with decane before it was mounted inside the evaporator. The decane is evaporated and removed without leaving any residuals during bake out of the machine and degassing of the evaporator. In previous XPS studies46 no carbon contamination was detected on the sample. For Ba deposition, a home-built Knudsen-type evaporator was used. Ba was deposited on Al2O3 at 300 K in UHV at typical rates ranging from 0.030 to 0.065 Å s-1 as calibrated using a quartz microbalance (an average Ba film thickness of 1 Å corresponds to 1.6 × 1014 atoms cm-2) and was subsequently oxidized by exposure to 1 × 10-6 mbar O2 for 900 s. For this study a film thickness of 20 Å Ba was used (3.2 × 1015 atoms cm-2). The BaO particles were stabilized by annealing in the presence of O2 (1 × 10-6 mbar) up to 800 K for 900 s before exposure to SO2. This procedure leads to the formation of barium-aluminate-like particles, BaAl2xO1+3x.47 Pd (wire, Alfa Aesar, 99.9%) deposition was performed using a commercial electron beam evaporator (Focus EFM3). The sample was biased to the same potential as the source to avoid ion damage. Typical deposition rates were around 5.7 × 1012 atoms cm-2 · s-1 (1 Å Pd corresponds to 6.8 × 1014 atoms cm-2) as calibrated using a quartz microbalance. For the underlying study, 4 Å Pd were deposited on the sample under UHV conditions. When depositing the Pd on the BaAl2xO1+3x particles, the Pd particles were stabilized according to a procedure described elsewhere.48 2.2. Computational Details and Models. Experimental nanoparticle sizes, ranging from a few to hundreds of nanometers, are far beyond what can be handled nowadays using highperformance supercomputers. Therefore, a split-and-win computational strategy has been applied, dividing an ideal Pd nanoparticle into handy parts (Figure 1a), which are optimized within the computational scheme described below. A slab model is used to study the chemical activity at the interior part of the facets and a Pd79 nanoparticle to model defect sites such as edges, corners, and nearby sites. BaO surfaces are modeled with a slab model. For further information on the models we refer to the Supporting Information. These models have extensively proven to provide accurate results for properties of species adsorbed on them.49-52 The present DF calculations were carried out with the help of the VASP code,53 carrying out periodic Kohn-Sham calculations. The projector-augmented plane-wave method has been used to represent the atomic cores.54 This representation of the core states allows one to obtain converged results with a cutoff kinetic energy of 415 eV for the plane wave basis sets. A Monkhorst-Pack grid has been used to select the special k points for numerical integrations in reciprocal space and been adapted to each system. They guarantee convergence of the energy to values within 0.1 kJ mol-1 as tested by using a denser grid. For Pd surface calculations a hexagonal unit cell and a gamma (Γ) centered k-point grid has been employed. Optimal grids of

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13815 k points of 6 × 6 × 1 and 7 × 7 × 1 have been used for the Pd and BaO slab, respectively. Slabs have been initially constructed using a lattice parameter for the geometrically fixed layer that was previously optimized in a calculation of the bulk material. A space of 1 nm has been added in the vacuum direction to model the surface and avoid interactions between repeated slabs. The cuboctahedral Pd79 nanoparticle has been modeled by isolating it in a large cubic unit cell adding 1 nm vacuum in each direction to avoid interactions between neighboring repeated cells. Γ-point calculations were carried out for Pd79. In all calculations the Perdew-Wang55 91 (PW91) exchange correlation functional was used because it provides the best balanced results for the observables here studied (see discussion in the Supporting Information). Furthermore, it enables a more direct comparison with results of previous theoretical studies on similar systems, which also used the PW91 functional.25,36-39 Geometry optimizations were performed using a conjugated gradient algorithm and applying a Gaussian smearing of width 0.2 eV for the electron occupancy to speed up convergence; however, final energy values were corrected to 0 K (no smearing). In a first step an energy criterion of 0.01 kJ mol-1 was employed for the geometry convergence; however, a second geometry optimization employing a force criterion was performed when needed to ensure forces acting on atoms to be less than 0.03 kJ mol-1 nm-1. Previous studies and present tests do not reveal spin polarization of neither the models employed as substrates nor the adsorption of SO2 molecules on them. Thus, unless explicitly stated, all calculations were nonspin polarized. Harmonic frequencies were obtained through numerical calculation and diagonalization of the Hessian matrix taking into consideration only the adsorbate molecules. Negligible coupling with substrate phonons is expected since the latter exhibit distinctive lower frequencies, typically below 800 cm-1. The Hessian matrix is constructed by calculating energy changes due to independent displacements of 5 pm of every atom in each direction of the unit cell vectors. Smaller displacements up to 1 pm result in a small reduction of the SO2 stretching vibrational frequencies which is in all cases smaller than 6 wave numbers. Simulated IRAS spectra have been obtained, estimating the intensity of a band through that component of the change of the dipole moment accompanying the vibration that is directed along the surface normal direction. The spectra have been drawn by smoothing the peaks with a Gaussian function of 100 cm-1 half width. This procedure has been widely used in the past for calculations of adsorbate spectra on solid surfaces.56,57 Note that slight disagreements are found between calculated and experimental frequencies for gas-phase SO2 (see Supporting Information). For a thorough final comparison with experiments, calculated IR peaks have been scaled, adding for the difference between calculated harmonic and experimental anharmonic vibrational frequencies, which aims at correcting for anharmonicity effects and, to some extent, shortcomings of the employed density functional. A/B , of a species A, i.e., SO2, on a The adsorption energy, Eads given substrate B, i.e., Pd(111) and BaO(001) extended surfaces or Pd79 nanoparticles, is defined according to the following equation

EA/B ads ) -EA/B + (EA + EB)

(6)

where EA/B is the energy of the complete system where A is adsorbed on the substrate B and EA and EB are the energies of

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Figure 2. Simulated IRAS spectra for the most stable conformations of SO2 adsorbed on Pd(111). (Insets) Top and lateral views of the corresponding optimized structures. The intensities of the conformations with the SO2 molecular plane being parallel to the Pd(111) surface (lower two spectra) have been multiplied by a factor of N (×N) for better visualization.

the isolated species A and bare substrate B, respectively. Note that with this definition the adsorption energy is positive if the adsorbate is bound to the substrate, with larger adsorption energy indicating stronger bonding. 2.3. Computational Strategy and Nomenclature. Although the interaction of SO2 with BaO has been extensively studied by means of DF calculations,24,25 experimental studies are scarce26 compared with other MOs.27,28 The opposite situation is found for SO2 interaction with Pd surfaces or nanoparticles. Despite the significant amount of experimental data, mostly focused on determination of the local structure of adsorbed SO2 and the adsorbate-substrate interactions on Pd single-crystal surfaces,58-60 Pd thin films9,61,62 and Pd nanoparticles,4,63 there are only a few previous theoretical calculations addressing the interaction of SO2 with Pd substrates, in contrast to Pt, where its activity toward SOx species has been extensively and accurately studied by DF means.37-39 Previous calculations dealing with the interaction of SO2 with Pd were carried out with either semiempirical methods9,64 or first-principles methods applied on small clusters in the subnanometer particle size region.4,9,59,64 The latter did not systematically sample the complete adsorption energy landscape, and, moreover, the results must be handled with caution since the employed metal clusters were far from being in the scalable regime.52 Therefore, results can vary substantially when employing a different cluster and, in this sense, are not descriptive of nanometre-sized metal clusters. Detailed theoretical studies of SO2 on Pd nanoparticles are, to the best of our knowledge, nonexistent. Here, we want to provide a systematic and extensive study with detailed insights of the interaction of SO2 on Pd(111) and Pd79 nanoparticles by means of state-of-the-art DF methods, intending to cover all the hypersurface energy landscape and to provide a complete description of local and global minima. The existence of various complicated chemisorption configurations of SO2 on the substrates prompted us to use a nomenclature similar to that reported previously.37-39 Each adsorption configuration is initially defined by the number of atoms of the adsorbate which are directly bonded to the surface. This number occurs as a superscript attached to η, e.g., η2

denotes a molecule with 2 atoms bonded to the substrate. After this label those atoms of the molecule that are bound to the substrate are listed together with a subscript characterizing their bonding geometry. The subscript a indicates an atop site, b a bridge site, f an fcc 3-fold hollow site (with a Pd atom located at the second subsurface Pd layer), and h a hexagonal-closedpackage (hcp) 3-fold hollow site (with a Pd atom located at the first subsurface Pd layer). A superscript attached to the letter µ defines how many substrate atoms are needed, as first neighbors, to bind the adsorbed molecule. For µ3 situations, i.e., bonding to three substrate atoms, which in all cases are atoms forming a 3-fold hollow site, a tag fcc or hcp may be added to further specify the site in ambiguous situations. Symbols | and ⊥ specify whether the cutting line of the molecular plane of SO2 and the Pd(111) plane is parallel or perpendicular to a substrate Pd-Pd bond. The symbol ∠ indicates that the main symmetry axis of the molecule, C2V, is noticeably tilted with respect to the surface plane. In the case of Pd79 nanoparticle, the (111) facets exhibit nonequivalent Pd atoms. For these cases the SO2 atoms bonded to the surfaces carry a superscript indicating the position of the Pd atoms they are bonded to. A superscript i indicates bonding to a Pd atom in the interior of the facet, e to a Pd atom at an edge site, and c to a Pd atom at a corner site. Multiple superscript letters are used for bridging and 3-fold hollow cases. Similarly, a superscript specifies to which surface atoms SO2 is bonded on BaO, such that Ba designates bonding to a Ba atom and O to a surface oxygen atom. The adsorption conformation depicted in the top panel of Figure 2, e.g., is a η2-SbOa-µ3-fcc adsorption mode, in which two atoms of the SO2 molecule (η2) are bound to three substrate Pd atoms (µ3). In particular, the S atom is bridging two Pd atoms (Sb) and one oxygen atom is sitting atop of a substrate Pd atom (Oa). Therefore, the third oxygen atom is unbound to the substrate and is, in fact, pointing toward the vacuum. The overall molecule is lying over a fcc 3-fold hollow site, fcc tag. The number of possible conformations of SO2 on the Pd(111) extended surface is well above 30. When using Pd79 nanoparticles as a substrate the number of conformations is even several

Interaction of SO2 with Oxide-Supported Pd Nanoparticles TABLE 1: Optimized Adsorption Conformations of SO2 on a Pd(111) Slaba conformation η2-SbOa-µ3-fcc η2-SbOa-µ3-hcp η1-Sb-µ2-⊥ η3-SaOaOa-µ3-fcc η3-SaOaOa-µ3-hcp η3-SbObOb-µ4 η2-SaOb-µ3-fcc-∠ η1-Sf-µ3-fcc-| η2-SaOb-µ3-hcp-∠ η1-Sh-µ3-hcp-| η3-SaObOb-µ3 η3-SaOfOf-µ5 η3-SaOhOh-µ5 η1-Sa-µ1-∠ η1-Sa-µ1-| η1-Sa-µ1-⊥ η2-SaOa-µ2 η3-SbObOb-µ3-hcp η3-SbObOb-µ3-fcc η3-SfOhOh-µ5 η3-ShOfOf-µ5 η2-OaOa-µ2 η2-ObOb-µ3 η2-OfOh-µ5

Eads/ d(S-O1)/ d(S-O2)/ d⊥/ ∠(O-S-O)/ pm pm pm deg kJ mol-1 147 146 148 153 153 153 147 147 147 148 149 149 148 147 146 146 145 150 150 149 149 145 147 147

151 151 147 153 153 153 155 147 155 148 149 149 148 148 146 146 149 150 150 149 149 150 147 147

178 177 179 211 210 214 218 185 214 181 229 229 229 224 222 221 208 218 218 222 225 223 272 268

115 115 117 111 111 114 113 116 113 115 117 117 117 118 118 119 116 114 115 115 115 115 117 117

121 118 118 108 108 96 81 80 79 78 75 71 70 66 64 63 60 36 36 27 27 24 3 2

a Geometrical parameters like bond lengths d, molecular angles ∠, perpendicular distance of S with respect to the Pd(111) surface d⊥, and adsorption energies Eads are listed for each adsorption conformation. See Table S1 in the Supporting Information for gas-phase SO2 reference data and Figure 2 for sketches of the 5 most stable conformations.

times higher. Most of the conformations however correspond to weak adsorption situations unlikely to be observed in the experiments under UHV conditions. As a strategy to save computational time, the Pd(111) extended surface has been used for screening all possible conformations, and only the 5 most stable ones on Pd(111) have been further studied on Pd79. Fortunately, adsorption of SO2 on BaO(001) has been extensively and accurately studied by DF means.25 In this particular system, a systematic sampling of adsorption conformations has been avoided, and only the most likely conformations reported previously have been recalculated for the sake of comparison. 3. Results: DF Calculations 3.1. SO2 on Pd(111). Adsorption of SO2 on the extended Pd(111) surface has been systematically studied, sampling all possible adsorption sites and molecular conformations. Table 1 summarizes the most important geometric parameters as well as the calculated adsorption energies for each conformation which turned out to be a local minimum. Note that all other possible conformations, not listed in Table 1, spontaneously evolved upon geometry relaxation into one of the listed adsorption geometries representing the local minima. Inspection of the geometries reveals that in general when SO2 is adsorbed the S-O bonds are elongated, resulting in final bond lengths in the range of 146-155 pm. The angle ∠(S-O-S) is slightly reduced to values of 111-118°. The most stable site corresponds to a η2-SbOa-µ3-fcc situation, described in the previous section. The adsorption energy of 121 kJ mol-1 in this conformation is slightly higher than the reported adsorption energy of 118 kJ mol-1 for the same conformation on Pt(111) for the same coverage of θ ) 0.11 monolayers (ML) calculated using the same xc functional.37 The coverage, θ, is defined as the ratio between adsorbed species and the number of surface Pt atoms (or BaO units). However, comparison with the work

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13817 of Lin et al.37 is affected by slightly different plane wave sets, geometries, and k-point meshes. In this sense, the difference is well within the DF accuracy of ∼10 kJ mol-1; therefore, no different behavior can be stated between Pd and Pt at this point. In any case, both metals seem to be more sulfur resistant than Ir, on whose (111) surface SO2 is adsorbed adopting a η3-SaOaOa-µ3-fcc mode with an adsorption energy of 168 kJ mol-1.36 The four next most stable adsorption sites for SO2 on Pd(111) match those reported for adsorption on Pt(111), although there are small differences. For instance, on Pd(111) the difference in adsorption energy between the most stable and the second stable adsorption site, a η2-SbOa-µ3-hcp situation (Figure 2), is only 3 kJ mol-1, whereas this difference is noticeable bigger, ∼11 kJ mol-1, for Pt(111). This difference points to a stronger steric repulsion of the first subsurface metal layer in Pt, maybe due to the longer Pt-Pt bond length,65 which makes the hollow sites bigger and, in this sense, the electronic density of the subsurface Pt layer becomes less screened. Besides, the third most stable site on Pd, a η1-Sb-µ2-⊥ adsorption configuration, see Figure 2, is only 3 kJ mol-1 destabilized with respect the most stable case, while on Pt it is more than 10 kJ mol-1 destabilized.37 This means that even at low coverage all these three sites may be accessible for SO2 molecules. Finally, situations where SO2 is laying flat on the surface (Figure 2) seem to be as much destabilized on Pd as on Pt. The rest of the adsorption conformations on Pd(111) show much smaller adsorption energies, and their occupancy is unlikely. Current results for the most stable conformations are in excellent agreement with the experimental spectroscopic data reported on the structure of SO2 adsorbed on Pd(111).59-62 These X-ray absorption fine structure (XAFS) and near-edge XAFS studies determined that the molecular plane is normal to the surface and that a bridge site is occupied by an S atom. The bond lengths, experimentally obtained at a saturation coverage of θ ) 0.33 ML, are 143 and 148 pm, noticeably shorter than the present calculated ones of 145 and 151 pm at θ ) 0.33 ML. However, if we correct the distances by subtracting the elongation of 2 pm already present in gas-phase SO2(g), the calculated values perfectly agree with the experimental values and fall within the accuracy range of the XAFS methods of (3 pm. Finally, the angle between the surface normal and the molecular symmetry axis matches the experimental value of 25°, and the optimized distance of the S atom to bridging Pd atoms of 228 pm is in line with the experimental value of 223 ( 3 pm. Present calculations discard previous calculated structures as most-likely conformations. In this sense, the η2-SbOb-µ3-hcp reported previously to be the most stable structure according to DF calculations with the XR xc functional59 is, at the present level of calculation, unstable and evolved spontaneously into the η2-SbOa-µ3-hcp structure. Other ab initio and semiempirical studies9,59,64 reported η1-Sa, η1-Sh, and η2-OO coordination modes as suitable adsorption structures, but according to the present study they are either not adsorbing (η2-OO) or much less stable than the most preferred conformation (η1-Sa and η1-Sh structures). Putting aside the predictive failure of semiempiric methods due to their lack of electronic structure/bonding description, our belief is that the incongruent previous theoretical structures are due to the use of small clusters that do not enable a meaningful global minima search, since in all cases the cluster sizes ranged from 5 to 25 Pd atoms, which is far from the sizeindependent regime.52

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Figure 3. Simulated IRAS spectra for increasing coverage of SO2 on a Pd(111) single crystal for the η2-SbOa-µ3-fcc conformation.

Figure 2 depicts the simulated IRAS spectra fingerprints for the 5 most stable conformations of SO2 adsorbed on Pd(111) that are reported in Table 1. According to the spectra, it would be possible to distinguish between η2-SbOa, η1-Sb, and η3-SaOaOa coordination modes, both exhibiting diverse features at different wave numbers. η2-SbOa modes present two IRAS-active vibrations of equal band intensity located around 948-950 and 1179-1197 cm-1. For η1-Sb according to the metal surface selection rule (MSSR)66 only the symmetric stretching mode is IRAS active and is located at 1013 cm-1. Since in η3-SaOaOa conformations the SO2 molecule is planar to the surface, no significant intensity of the symmetric stretching is observed when imposing the MSSR66 and, according to this, SO2 molecules adopting such an adsorption conformation would be transparent to IRAS methods. Although no previous vibrational spectroscopic data is available for Pd(111), the present results resemble the proposed η2-SO structure and experimental electron energy loss spectroscopy data of SO2 adsorbed on Pd(100), which, at a submonolayer coverage at 115 K, presents two defined bands58 similar to those of the η2-SbOa conformation on Pt(111). The simulated IRAS spectra of Figure 2, however, refer to a θ ) 0.11 ML scenario. For a better comparison with the experiments, a coverage dependence IRAS for SO2 adsorbed on Pd(111) single crystals has been simulated in Figure 3 for the η2-SbOa-µ3-fcc adsorption mode. Since a complete monolayer of SO2 has been experimentally reported60 to be achieved at θ ) 0.33 ML, three sets of calculations (θ ) 0.11, 0.25, and 0.33 ML) have been carried out to consider the range from low coverage to the saturation limit. Additional SO2 adsorbed on a Pd(111) single crystal has been experimentally reported9,60,67 to adsorb in a SO2 multilayer, i.e., weakly physisorbed SO2, which starts desorbing at 120 K. The spectra for a coverage of 0.33 ML have been obtained by placing three SO2 molecules on a (3 × 3) slab. Coverage of 0.25 ML has been obtained by placing one SO2 molecule on a (2 × 2) slab.68 The spectra show the effect of lateral intermolecular vibrational coupling and/or weakening of the interaction with the substrate, which is translated in blue shifts of up to 34 cm-1 for the asymmetric SO2 stretching. The effect is markedly reduced (11 cm-1) for the bending mode. A strange behavior is observed for the symmetric mode, with a red shift at a coverage of 0.33 ML

TABLE 2: Optimized Adsorption Conformations of SO2 on a (111) Facet of Pd79a conformation η2-Siib Oia-µ3-fcc η2-SbieOac-µ3-fcc η2-Sicb Oea-µ3-fcc 3 i η2-Sce b Oa-µ -fcc η2-Siib Oea-µ3-hcp η2-Sieb Oia-µ3-hcp η2-Sicb Oca-µ3-hcp 3 i η2-Scc b Oa-µ -hcp η1-Sbii-µ2-⊥ η1-Sieb -µ2-⊥ η1-Sicb -µ2-⊥ 2 η1-Sce b -µ -⊥ 2 η1-Scc b -µ -⊥ η3-SiaOiaOia-µ3-fcc η3-SiaOcaOea-µ3-fcc η3-SacOaiOae-µ3-fcc η3-SeaOiaOca-µ3-fcc η3-SiaOiaOea-µ3-hcp η3-SiaOiaOia-µ3-hcp η3-SiaOcaOca-µ3-hcp η3-ScaOcaOia-µ3-hcp

d(S-O1)/ d(S-O2)/ d(S-Pd)/ ∠(O-S-O)/ Eads/ pm pm pm deg kJ mol-1 146 147 147 147 146 147 147 147 147 147 147 147 147 151 153 153 152 153 150 153 153

151 153 151 151 151 151 149 151 148 148 148 147 147 152 153 151 154 154 153 154 152

227 227 226 226 225 228 228 228 226 227 227 226 227 226 228 225 226 227 225 229 226

113 113 115 115 115 114 116 114 116 116 116 118 118 111 111 113 112 111 114 111 112

130 144 134 141 135 125 127 124 131 131 131 151 138 111 136 131 137 122 104 142 134

a Interatomic bond lengths, d, molecular angles, ∠, and adsorption energies, Eads, are listed for each adsorption conformation.

compared to lower coverages. This peculiar behavior may originate simply from the most direct interaction of oxygen atoms of different SO2 molecules at this coverage compared to situations at lower coverage (see Figure 3). 3.2. SO2 on Pd79. The adsorption sites corresponding to the five most stable ones of SO2 on Pd(111) slab found in the previous section have been systematically investigated on a Pd79 nanoparticle in order to obtain insights into the effects of defects such as corners, edges, and nearby sites on the adsorption and vibrational spectra of SO2 on experimental Pd model catalysts. Table 2 summarizes the most important geometric parameters and adsorption energies for the optimized structures. After careful examination of the results, it seems that the effect of defect sites on the adsorption of SO2 is small in terms of geometric structures. All the studied cases exhibit variations of interatomic SO2 distances and molecular angles compared to the corresponding sites of SO2 on Pd(111) below 2 pm and 2°, respectively.69

Interaction of SO2 with Oxide-Supported Pd Nanoparticles

Figure 4. Simulated IRAS spectra for selected conformations of SO2 2 2 ie c 3 2 ce i 3 adsorbed on Pd79: (a) η1-Sce b -µ -⊥, (b) η -Sb Oa-µ -fcc, (c) η -Sb Oa-µ 2 3 c c i 3 1 fcc, (d) η3-SiaOcaOca-µ3-hcp, (e) η1-Scc -µ -⊥, (f) η -S O O -µ -hcp, (g) η b a a a Siib -µ2-⊥, (h) η2-Siib Oia-µ3-fcc. (Inset) Top and lateral views of a twolayer cut of the (111) facet.

Note that a Pd79 nanoparticle seems to be a meaningful model for the discrimination of low-coordinated sites and sites of the interior of the facet since similar adsorption energy values are obtained when comparing sites at the interior of a (111) facet and those on the Pd(111) slab. For instance, adsorption energy values for the η2-SbiiOai-µ3-fcc, η1-Sbii-µ2-⊥, and η3-SaiOaiOai-µ3fcc sites, which are located in the interior of the Pd79 (111) facet, are only 3-13 kJ mol-1 higher than the corresponding Pd(111) slab counterparts and thus in excellent agreement with the complementary model and almost within chemical accuracy. Note that the remaining difference can arise for many simple reasons. On one hand, there is no steric repulsion between adsorbates in Pd79, thus favoring a stronger interaction with the substrate Pd atoms. On the other hand, small residual size effects may partly explain a slightly enhanced activity on Pd79 nanoparticles compared to Pt(111) extended systems. Although an overall stabilization of adsorbates at metal nanoparticles edges and corners has been reported for the adsorption of many composites,50,70 it seems not to be the case for SO2 on Pd, since only selected conformations are stabilized significantly beyond the effects described in the previous 2 paragraph. In particular, the η1-Sce b -µ -⊥ adsorption conformation (Figure 4) seems to exhibit the major stabilization, 33 kJ mol-1, with respect to the Pd(111) slab model. Accordingly, bridge sites at particles edges connecting adjacent (111) facets would be rapidly populated when exposing Pd nanoparticles to SO2. The implications for a catalytic deactivation of Pd catalysts by SO2 via site blocking are obvious, because such low-coordinated

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13819 sites represent the main catalytic active centers of Pd and other Pt-group metal catalysts in the decomposition or combustion of sulfur oxide compounds.4,71 Concerning the nanoparticle edge connecting adjacent (111) and (100) facets, surprisingly, a flat conformation at an hcp hollow site, η3-SiaOcaOca-µ3-hcp conformation, see Figure 4, is the most stable one compared to other flat or up-right situations at the same site, in fact, 15 kJ mol-1 more stable than the equivalent up-right conformation over the same site. The sites connecting adjacent (111) and (100) facets would be occupied in the second place when exposing to SO2. For the interior of the facets, up-right conformations seem to be the preferred ones, as observed in the last section. In this sense, the final scenario for SO2 adsorption implies three different adsorption modes, each one being the most stable one in only one specific region of Pd nanoparticles. Moreover, the calculated IRAS fingerprints, see Figure 4, exhibit characteristic features for each of the three particular conformations, which might help in their possible spectroscopic differentiation. It has to be pointed out that for planar SO2 conformations, being stable at Pd particle edges at (111)/(100) facet junctions, the IR bands have very low intensity due to the MSSR. However, the amount of these sites is expected to be quite small compared to sites at other (111)/(111) borders or to sites on (111) facets. Due to these reasons, these sites are very unlikely to be resolved from background noise by means of vibrational spectroscopy. On the contrary, when SO2 is adsorbed at Pd particle edges connecting adjacent (111) facets, it would exhibit, according to the present calculations, a sharp and differentiated peak at 1045 cm-1, which is not overlapping with signals from up-right SO2 at the interior region of (111) facets. The latter signals are located at 941 and 1195 cm-1 at a coverage of ∼0.08 ML, in this sense very similar to the values obtained for the Pd(111) slab. Note that a second feature for η1-Sbce-µ2-⊥ is appearing at 1234 cm-1, however not as the dominant peak. This peak might be resolved as well, although it could overlap with the asymmetric stretching of up-right conformations of SO2 at the interior of the facets, especially at saturation coverage, as seen in Figure 3. Higher coverage calculations at particle edges placing two neighboring SO2 molecules - result in minimal changes to the structure (below 1 pm) and blue shifts of ∼15 cm-1. Note in passing that adsorption on small (100) facets has been ignored because the amount of these sites is expected to be rather small on supported Pd nanoparticles.72 3.3. SO2 on BaO(001). The calculated geometries and adsorption energies for adsorption of SO2 on the BaO(001) surface are listed in Table 3. Adsorption of SO2 has been extensively studied in the past by DF and ab initio calculations.23-25,27 In the present study, only selected adsorption conformations have been optimized for the sake of comparison with previous studies. Attempts to optimize other structures proposed in the past23,27 as stable conformations on other MO showed that, for BaO(001), they were, in each case, unstable and spontaneously evolved barrierless to one of the two minima indentified for SO2/ BaO(001) and listed in Table 3. The present DF results match those of Karlsen et al. carried out employing the B3LYP xc functional and an embedded Ba5O5 cluster24 as those authors reported a η3-SOa OBaBa OBaBa -µ5 adsorpb b 3 O Ba Ba 3 tion conformation being more stable than a η -Sa Oa Oa -µ mode (Figure 5). According to present results, the energy difference is only 11 kJ mol-1. Unfortunately, in the work of Karlsen et al., no exact value of this difference was given.24 Moreover, the found optimal adsorption geometry differs from that reported by Karlsen et al. only by 2 pm in the SO2

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TABLE 3: Optimized Adsorption Conformations of SO2 on BaO(001)a conformation

d(S-O1)/pm

d(S-O2)/pm

d(S-Os)/pm

d(O-Ba)/pm

Ba 3 η -SOa OBa a Oa -µ BaBa 5 η3-SOa OBaBa O -µ b b

152 153

152 153

165 164

274

3

a

d(O-BaBa)/pm

∠(O-S-O)/deg

Eads/kJ mol-1

276

111 108

250 262

Bond lengths, d, molecular angles, ∠, and adsorption energies, Eads, are listed for the two most stable adsorption conformations.

Figure 5. Simulated IRAS spectra for the two most stable conformations of SO2 on BaO(001). (Insets) Lateral and top views of the corresponding optimized structures. Dashed and pointed lines indicate the intensities of vibrations parallel to BaO(001) surface directions, which, for visualization purposes, have been enlarged 10 times.

interatomic distances and by 4° for the molecular angle. A significant difference, however, is found for the distance between the S atom and the substrate O atom (Os), which was larger by 20 pm in the previous study. This longer distance is in line with a much smaller adsorption energy using the embedded cluster model of 195 kJ mol-1. Such a discrepancy may have its origin in the limitations present when using an embedment to simulate the effect of long-range interactions with other substrate cations/anions. In fact, in the present calculations the resulting adsorption mode can be thought of as a pseudo-SO3 molecule (sulfite) formation, if we take into account that the S-Os bond length is comparable to other S-O bond lengths, in juxtaposition to the situation of a BaO-chemisorbed SO2 reported by Karlsen et al.24 The disagreement concerning geometries is not found when comparing our present results with those of Schneider.25 In both works slab models are employed with periodic boundary conditions and, thus, go beyond a simplistic description of longrange interactions in the substrate. As a consequence, an excellent agreement with previous results is found. For instance, not only differences in the BaO lattice constants but also SO2adsorbed interatomic distances are always within 2 pm. The SO2 molecular angle agrees to 4°. The adsorption energy for -1 Ba 3 is very close to the the η3-SOa OBa a Oa -µ with ∼260 kJ mol present value. However, no record is found concerning the η3SOa ObBaBaObBaBa-µ5 mode in Schneider’s work. In any case, we expect that since the adsorption energies for both modes are high, SO2 can get kinetically trapped in one or the other mode, and thus, a mixture of both adsorption states will be present, especially at low temperatures. In addition, the differences in vibrational spectra between the two adsorption modes are minor, and probably experiments could hardly distinguish between the two situations (see Figure 5). Here, those parallel to surface components are plotted as well, as they can be of interest, see below.

4. Results: IRAS Experiments 4.1. SO2 Adsorption on Al2O3/NiAl(110). Figure 6a shows the spectra obtained during SO2 adsorption on a pristine Al2O3 layer at 100 K. An absorption feature is apparent at 966 cm-1 together with two dominant features located at 1152 and 1340 cm-1. The absorption band located at 966 cm-1 was previously assigned to sulfite species (SO32-) on alumina,73 also in line with the results presented by Goodman et al.74 Spontaneous sulfite formation may occur at this low temperature on other oxides27 as shown also in the last section. Therefore, one may tentatively assign the band at 966 cm-1 to a sulfite species on Al2O3. Note that in a previous study75 combining diffuse reflectance infrared Fourier transform spectroscopy and DF calculations it was shown that sulfate species adsorbed on γ-Al2O3 exhibit a dominant vibrational peak at ∼950 cm-1. In this sense, it might be difficult to differentiate between sulfate species (SO42-) and sulfite, although formation of higher oxidized species appear unlikely under the present conditions. Note that in previous work the sulfates were formed after exposing the Al2O3 to SO2 in an excess of O2 at temperatures above 200 °C.75,76 Otherwise, sulfate formation has been reported to occur only at higher temperatures and in the presence of O2.77,78 These corresponding species give rise to absorption bands in the IR spectra located at higher wave numbers. In fact, bands at 1178 and 1346 cm-1 were previously assigned to bidentate sulfate species adsorbed on γ-Al2O3.75 In general, it has been reported that bands below 1100 cm-1 correspond to SO32- species whereas bands located between 1100 and 1400 cm-1 arise from SO42- species.77-79 However, this statement does not always apply and seems to depend on the alumina surface structure and the dentition mode. In fact, previous DF calculations show that sulfate species can exhibit, at high coverage, features around 1000 cm-1.75 Nevertheless,

Interaction of SO2 with Oxide-Supported Pd Nanoparticles

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13821

Figure 6. Spectra obtained during SO2 adsorption on the different samples at 100 K: (a) Al2O3/NiAl(110), (b) 20 Å BaO/Al2O3/NiAl(110), (c) 4 Å Pd/Al2O3/NiAl(110), and (d) 4 Å Pd on 20 Å BaO/Al2O3/NiAl(110).

these previous results were obtained using small clusters out of the scalable regime, and hence, calculations on slab models may significantly deviate concerning vibrational frequencies. The two well-resolved and dominant features in the spectra located at 1152 and 1340 cm-1 correspond well to symmetric (νs) S-O and asymmetric S-O stretches (νas) of gas-phase SO2 located at 1147 and 1351 cm-1, respectively.80 Accordingly, the observed bands correspond to condensed multilayers of SO2 on alumina. Note, however, that the barrier between the physisorbed

and the chemisorbed state was calculated to be on the order of kBT ) 0.5 kcal mol-1.27 In this sense, a scenario in which only physisorption takes place is unlikely, since the physisorbed state would immediately “flip-over” into a more stable chemisorbed state, even at very low temperature. More likely is a situation in which multilayers of SO2 are physisorbed on a chemisorbed monolayer. The broad background and weak features between 1000 and 1150 cm-1 would support this argument, since sulfites were reported to appear in this region.27,74,77 However, due to

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the broad and overlapping bands, a distinct assignment is not straightforward. Finally, the small features located at 1180 and 1270 cm-1 are, at present, difficult to assign. Possibly, they may arise from adsorption at defect sites, such as vacancies or steps. The Al2O3/NiAl(110) system merely serves as a reference system, in order to identify new features which correspond to BaO or Pd functionalities. Note that no theoretical background has been done for the Al2O3 system due to the complexity of this model surface. 4.2. SO2 Sdsorption on BaAl2xO1+3x/Al2O3/NiAl(110). The spectra acquired during SO2 adsorption on a sample where 20 Å BaO was deposited on Al2O3/NiAl(110) are shown in Figure 6b. Here, a strong intermixing occurs between BaO and Al2O3 during preparation of the sample. This intermixing leads to formation of a barium-aluminate phase (BaAl2xO1+3x). In this sense, the absorption bands associated with SO2 adsorption may arise from SO2 interacting with either Ba2+ or Al3+ centers. Indeed, the spectra look very similar to the ones obtained during SO2 adsorption on pristine Al2O3/NiAl(110) (Figure 6a). One discerns again an absorption band appearing at 966 cm-1 together with two dominating features at 1155 and 1340 cm-1. However, additional weak features appear between 1000 and 1250 cm-1. The two dominant absorption bands at 1155 and 1340 cm-1 are assigned to symmetric (νs) and asymmetric (νas) S-O stretch vibrations of a SO2 multilayer, respectively. As discussed before, the band located at 966 cm-1 can be assigned to sulfite species on alumina. The strong intensity of the band despite the deposition of 20 Å of Ba on the Al2O3 film could be explained by the strong intermixing of BaO with Al2O3 during sample preparation. However, previous studies found absorption bands located at 978, 1045, 1140, and 1247 cm-1 corresponding to bulk sulfate species on alumina and/or BaO.81,82 Hence, the band at 966 cm-1 could also be interpreted as representing the lowest frequency feature of surface sulfates, although sulfate formation appears unlikely under the present working temperature. The present DF calculations show that SO2 may adsorb on BaO(001) in two different planar modes (see Figure 5). Both modes exhibit, at coverage 0.125 ML, vibrational frequencies located at 957-969 and 1005-1034 cm-1. These results are in line with those obtained by Schneider et al.27 on the similar system MgO, in which the authors assigned bands at 960 and 1030 cm-1 as sulfite-like. In this sense, SO2 adsorbed on pure BaO and Al2O3 environments would exhibit low-frequency vibrations which would in every case match the experimental value of 966 cm-1. Moreover, the intensity of the band at 966 cm-1 seems to be suppressed when having BaAl2xO1+3x particles instead of pristine Al2O3. This could be related to a more prominent intensity of SO2 stretching modes on alumina than on BaO-like sites. Last but not least, the broad feature between 1030 and 1100 cm-1 was not present in the pristine Al2O3 system and therefore belongs to the BaAl2xO1+3x particles. Tentatively, the region can be associated with two asymmetric stretching vibrations of SO2 adsorbed on BaO (compare Figure 5). Unfortunately, these modes have nearly zero intensity in the IRAS spectra, with those polarized nearly parallel to the surface components being the dominant ones. Thus, the observed feature would arise from molecules adsorbed on facets being nonparallel to the alumina surface. Previous STM images showed that the model surface exhibits a degree of surface corrugation, which would support this assignment.42,43 Note that present DF frequencies underestimate the experimental ones, although coverage effects and Ba2+/Al3+ intermixing could contribute to the discrepancies.

Luckas et al. 4.3. SO2 Adsorption on Pd/Al2O3/NiAl(110). Now we turn our attention to the catalyst systems where Pd nanoparticles are involved. In Figure 6c the spectra obtained during SO2 adsorption at 100 K on 4 Å Pd on Al2O3/NiAl(110) are depicted. An absorption band located at 983 cm-1 occurs together with two dominant features at 1150 and 1337 cm-1. As discussed above, the band at 983 cm-1 could most likely be assigned to a sulfite species on alumina. The two most intense bands at 1150 and 1337 cm-1 are close to the frequencies of the SO2(g) vibrations (νs and νas) normally located at 1147 and 1351 cm-1, respectively.80 According to the DF calculations presented in section 3.2, three distinct sites are available for SO2 adsorption on Pd nanoparticles: (i) particle edges connecting adjacent (111) facets, (ii) edges connecting (111) and (100) facets, and (iii) the interior of the (111) facets. The IRAS fingerprints for SO2 adsorbed on Pd nanoparticles were calculated to exhibit characteristic features for each of the three particular conformations, which may help in their spectroscopic differentiation. Note, however, that at (111)/(100) edges SO2 is most strongly adsorbed in a planar adsorption geometry. Therefore, it should be transparent to IRAS due to the MSSR.66 When SO2 is adsorbed at edges connecting adjacent (111) facets a sharp feature should be apparent in the spectrum at 1045 cm-1, which should not be overlapping with signals from upright SO2 at the interior region of the (111) facets (941 and 1195 cm-1). Unfortunately, the high degree of complexity of the spectra shown in Figure 6c does not allow a clear identification of the species. The reason for this, in part, is the general weakness and width of the absorption features. We conclude that on the present complex catalyst surface it will in general be rather difficult to differentiate between absorption bands arising from different Pd nanoparticles sites and from species on the oxide support. In fact, adsorption as η2-SbOa at the interior of the (111) facets of Pd nanoparticles would give rise to similar features in a range (920-970 cm-1) similar to the alumina-related bands. The SO2 asymmetric stretching which would appear between 1180 and 1215 cm-1 overlaps as well with potential features of SO2 adsorbed on alumina. The broad and weak band between 1040 and 1100 cm-1 could be linked to SO2 adsorbed at (111) junctions or the interior of (111) facets of Pd nanoparticles adopting a η2-Sb-⊥ adsorption mode. Its vibrational frequency, ranging from 1015 to 1045 cm-1, is however slightly below the band, although coverage effects would compensate for such small disagreement. 4.4. SO2 Adsorption on Pd/BaAl2xO1+3x/Al2O3/NiAl(110). The spectra obtained during SO2 adsorption at 100 K on 4 Å Pd deposited on top of 20 Å BaO on Al2O3/NiAl(110) are shown in Figure 6d. Again, intermixing between BaO and Al2O3 occurs before Pd deposition, leading to formation of a barium-aluminate phase. In the spectra a broad absorption region between 970 and 1100 cm-1 is apparent with two distinct bands at 970 and 1090 cm-1. At higher exposure, two dominant features located at 1150 and 1335 cm-1 appear. Similar to other cases, these last two bands match with νs and νas of SO2(g) 80 and, therefore, are assigned to multilayer SO2. The bands at 970 and 1090 cm-1 correspond to sulfite and/ or sulfate species on the oxide support.77,79 A distinct assignment is problematic, however, as in previous cases. The origin of these peaks may be multiple. In fact, DF predicts two absorption bands for SO2 on BaO(001), one located between 957 and 969 cm-1 and another between 1005 and 1034 cm-1. Coveragedependent dipole interaction could lead to a blue shift of these bands. These results could explain the two experimental signals

Interaction of SO2 with Oxide-Supported Pd Nanoparticles at 970 and 1090 cm-1 as being due to the symmetric and asymmetric S-O stretching vibrations of SO2 adsorbed on BaO(001), thus strongly supporting our assignment of the 970 cm-1 band to a SOx species on the oxide. However, from DF one would also expect absorption bands due to SO2 on the Pd nanoparticles, in particular bands located at 920-970, 1180-1215, and 1015-1045 cm-1 (see section 3.2). However, in these regions these bands perfectly overlap with the corresponding ones for SOx on BaO and Al2O3.77,79 Therefore, we conclude that all assignments given in the literature should be treated with utmost care. Most likely, the more pronounced peak at 1090 cm-1 is due to the combination of features coming from SO2 adsorbed on BaO, either adopting Ba 3 3 O Ba Ba 3 η3-SOa OBa a Oa -µ or η -Sa Oa Oa -µ conformations, and SO2 adsorbed on Pd nanoparticles, in a η2-Sb-⊥ adsorption mode. 5. Conclusions Here, we systematically studied the interaction of SO2 on BaO-supported Pd nanoparticles using suitable models and stateof-the-art DF calculations. The study provides detailed information concerning the structure and energetics of the different adsorption conformations of SO2, either on Pd(111) extended surfaces or on Pd nanoparticles. IRAS fingerprints are provided for every adsorption conformation. In addition, the effect of coverage on the structure, energetics, and vibrational frequencies has been explicitly studied. Since BaO functionalities serve as an SO2 trap in NSR catalysts, the interaction of sulfur dioxide with BaO(001) has been studied as well. According to the DF results, SO2 may adsorb on Pd(111) in many different conformations. Those most stable include situations in which the S atom is bridging two surface Pd atoms, having one - η2-SbOa-µ3 - or none - η1-Sb-µ2-⊥ - of the O atoms bound to a surface Pd atom. In these cases, the molecular plane is perpendicular to the Pd(111) surface. Nonetheless, situations in which the molecular plane is parallel to the surface - η3-SaOaOa-µ3 - are also remarkably stable. Their orientation, however, makes the direct detection of these species by IRAS impossible. On the contrary, the nonplanar adsorption modes can, in principle, be easily differentiated by IRAS, as they show distinct vibrational features, including a sharp peak at ∼1015-1040 cm-1 for the η1-Sb-µ2-⊥ adsorption situations and two peaks at 945-970 (νs) and 1180-1215 cm-1 (νas) for the η2-SbOa-µ3 conformations. SO2 attaches strongly to defect sites on Pd nanoparticles, such as edges and corners, as shown by systematic calculations of the most likely adsorption conformations on a Pd79 nanoparticle. An enhancement of the adsorption strength of up to 33 kJ mol-1 at defect sites such as edges and corners is found. This has evident consequences on catalysts containing Pt-group nanoparticles, due to preferential site blocking. In detail, different conformations are found to be most stable on different particle sites: η2-SbOa-µ3-fcc adsorption geometries are the most stable ones at the interior of (111) facets. η1-Sb-µ2-⊥ is the most stable one at edges connecting (111) facets. Finally, planar η3-SaOaOaµ3 is most stable at edges connecting (111) and (100) facets. In summary, due to the different vibrational fingerprints of each adsorption mode, their identification would in principle provide both molecular and site-specific spectroscopic information. Theoretical information has been used to assist in interpretation of IRAS spectra related for SO2 adsorption on a Pd/ BaAl2xO1+3x/NiAl(110) model NSR catalyst and on its individual components, i.e., pristine Al2O3/NiAl(110), Pd/Al2O3/NiAl(110), and BaAl2xO1+3x/NiAl(110). Features in the frequency region around ∼970 cm-1 are due to sulfite formation and appear for all systems, and this may be considered as rather unspecific.

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13823 Both the deposition of Pd and BaO to the alumina support induces emergence of a broad weak band between 1040 and 1100 cm-1. In the case of BaO, the band is associated to the νas vibration of sulfite species. The low intensity is due to polarization of the mode mainly parallel to the surface. On Pd nanoparticles, the broad feature is associated mainly to a η1Sb-µ2-⊥ adsorption geometry of SO2 either in the interior of (111) facets or at the junction between vicinal (111) facets. In general, the width and low intensity of all bands prevents an unequivocal assignment. In addition, the νs mode of SO2 adsorbed in η2-SbOa-µ3 geometry overlaps with sulfite bands on BaAl2xO1+3x and the νas mode overlaps with signals already present on alumina. These findings illustrate that definite band assignments of adsorbed SO2 and surface sulfites on oxidesupported noble metal catalysts should be treated with utmost care. Acknowledgment. F.V. thanks the Alexander von Humboldt Foundation for financing his postdoctoral grant. M.H. gratefully acknowledges financial support of the “Fonds der Chemischen Industrie”. N.L., F.V., and A.G. gratefully acknowledge computational time provided by the Regionales Rechenzentrum Erlangen. The authors also acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the ERACHEM program (“NanoFunC” project), additional support from the DFG within the Excellence Cluster “Engineering of Advanced Materials” (www.eam.uni-erlangen.de) in the framework of the excellence initiative, and support from the COSTD41 Action Program. Supporting Information Available: Structural and computational details about the models used in this study; discussion addressing the question of which approximation for the exchangecorrelation functional provides results closest to the experimental results together with a table summarizing structural, energetic, and vibrational properties of SO2 and bulk BaO and Pd. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Speight, J. G. The chemistry and Technology of Petroleum; Dekker: New York, 1991. (2) Slack, A. V.; Holliden, G. A. In Sulfur Dioxide RemoVal from Waste Gases; Noyes Data Corp.: Park Ridge, NJ, 1975. (3) Kerminen, V.-M.; Pirjola, L.; Boy, M.; Eskola, A.; Teinila, K.; Laasko, L.; Asmi, A.; Hienola, J.; Lauri, A.; Vainio, V.; Lehtinen, K.; Kulmala, M. Atmos. Res. 2000, 54, 41. (4) Rashkeev, S. N.; Ginosar, D. M.; Petkovic, L. M.; Farrell, H. H. Catal. Today 2009, 139, 291. (5) Rodriguez, J. A.; Hrbek, J. Acc. Chem. Res. 1999, 32, 719. (6) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Hrbek, J. J. Am. Chem. Soc. 1998, 120, 11149. (7) Pole`ik, M.; Wilde, L.; Haase, J.; Brena, B.; Cocco, C.; Comelli, G.; Paolucci, G. Phys. ReV. B 1996, 53, 13720. (8) Rodriguez, J. A.; Jirsak, T.; Hrbek, J. J. Phys. Chem. B 1999, 103, 1966. (9) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S. J. Chem. Phys. 1999, 110, 3138. (10) Rodriguez, J. A.; Kuhn, M.; Hrbek, J. Chem. Phys. Lett. 1996, 251, 13. (11) Wilke, S.; Scheffler, M. Phys. ReV. Lett. 1996, 76, 3380. (12) Rodriguez, J. A.; Chaturvedi, S.; Jirsak, T. Chem. Phys. Lett. 1998, 296, 421. (13) Lang, N. D.; Holloway, S.; Nørskov, J. K. Surf. Sci. 1985, 150, 24. (14) Goodman, D. W. Appl. Surf. Sci. 1984, 19, 1. (15) Oudar, J.; Wise, H. DeactiVation and Poisoning of Catalysts; Dekker: New York, 1991. (16) Ertl, G.; Kno¨zinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH: New York, 1997. (17) Taylor, K. C. Catal. ReV. Sci. Eng. 1993, 35, 457.

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