J. Phys. Chem. B 2001, 105, 5497-5505
5497
DeNOx Reactions on MgO(100), ZnxMg1-xO(100), CrxMg1-xO(100), and Cr2O3(0001): Correlation between Electronic and Chemical Properties of Mixed-Metal Oxides J. A. Rodriguez,*,† M. Pe´ rez,‡ T. Jirsak,† L. Gonza´ lez,‡ A. Maiti,§ and J. Z. Larese† Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11953, Facultad de Ciencias, UniVersidad Central de Venezuela, Caracas 1020-A, Venezuela, and Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121 ReceiVed: February 17, 2001; In Final Form: April 11, 2001
The rational design of catalysts with a high efficiency for the destruction of NOx compounds (DeNOx process) is a major problem in environmental chemistry. The adsorption of NO, NO2, and N2O on MgO(100), Zn0.06Mg0.94O(100), Cr0.06Mg0.94O(100), and Cr2O3(0001) was studied using high-resolution photoemission and firstprinciples density-functional calculations. Important differences were found in the chemistry of the adsorbed NOx species, but in general our results show a clear correlation between the electronic properties and reactivity of the oxide surfaces. Systems that have occupied electronic states with a relatively low stability, Cr0.06Mg0.94O(100) and Cr2O3(0001), bond NO strongly and are able to induce the dissociation of NO2 and N2O at temperatures as low as 80 K. On these oxide surfaces, adsorbed NO is a main product in the dissociation of NO2, whereas adsorbed Nx species are produced upon decomposition of N2O. The trends in the behavior of ZnxMg1-xO(100) and CrxMg1-xO(100) illustrate a basic principle for the design of mixed-metal oxide catalysts in DeNOx operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix produce occupied electronic states located well above the valence band of the oxide. These hybrid dopant states lead to large adsorption energies for NOx species and facilitate N-O bond cleavage. The validity of this basic principle is confirmed after examining the bonding of NO and N2O to a series of TM0.06Mg0.94O(100) surfaces (TM ) Zn, Sn, Ni, Co, Fe, Mn, or Cr). The effects of different metal dopants on the electronic properties of MgO are discussed.
I. Introduction The destruction or removal of nitrogen oxides (DeNOx process) is a very important issue in environmental catalysis.1-4 Nitrogen oxides (NO, NO2, and N2O) are formed during combustion reactions in automotive engines and industrial plants when the nitrogen present in air/fuel mixtures reacts with oxygen.1,3 In addition, the burning of oil-derived fuels with N-containing impurities5 also leads to formation of nitrogen oxides.1,2 Subsequent release of the NOx combustion products into the atmosphere contributes to the generation of smog and constitutes a serious health hazard for the respiratory system.1 Furthermore, in the air, the NOx species undergo oxidation and react with water, producing the acid rain that corrodes monuments and kills vegetation.1 Thus, to reduce environmental pollution, it is necessary to use catalysts/sorbents to remove or trap the nitrogen oxides before they reach the atmosphere (DeNOx process).2-4 New environmental regulations call for a drastic reduction in the emission of NOx species from automobile engines in the United States.3 Similar laws are being enacted around the world. They emphasize the goal of more efficient technologies for DeNOx operations.2,3 There is still not a universally acceptable solution to this major problem in environmental chemistry.2-4 Metal oxides are frequently used in DeNOx processes.1-4,6,7 The metal elements form oxides that exhibit a large diversity * Corresponding author. FAX: 631-344-5815. E-mail:
[email protected]. † Brookhaven National Laboratory. ‡ Universidad Central de Venezuela. § Molecular Simulations Inc.
of electronic properties7 and can be useful as sorbents for trapping NOx species or catalysts for accelerating the cleavage of N-O bonds.1-4,6 For example, it has been found that MgO has the ability to trap the NO2 formed in automotive engines during the combustion of fuels under oxygen-rich conditions.8,9 A recent study at Brookhaven National Laboratory (BNL)10 has examined the catalytic decomposition of N2O over Zn- and Crdoped MgO catalysts prepared by a novel method.11 Pure MgO displayed a relatively low catalytic activity for the 2N2O f 2N2 + O2 reaction (see Figure 1). A large increase in catalytic activity was seen after doping MgO with small amounts of Zn or Cr.10 The CrxMg1-xO catalyst was 2.5 times more active than the best BNL produced MgO catalyst and over 10 times more active than commercial MgO.10 The fundamental reasons for the high catalytic activity of CrxMg1-xO are not well understood. Recently, a lot of attention has been focused on the preparation and characterization of mixed-metal oxides as catalysts for DeNOx operations.2,3,6,10-14 When dealing with the design of mixed-metal oxide catalysts, it is important to know how to choose the “right” combination of metals. This can be a very complex issue.2,15-18 The behavior of catalysts that consist of metal-doped MgO is a classic case.6,10,19-22 Metal-doped MgO is active as a catalyst for the oxidative coupling of methane to produce C2 hydrocarbons,6,19 the reforming of CH4 with CO2 or H2O,20 DeSOx,21,22 and DeNOx processes.6,10 Big variations in catalytic activity have been observed when changing the second metal or dopant agent.6,10,19-21 In general, the causes for such variations in catalytic activity are not clear. Changes in the electronic properties of the mixed-metal oxides could play an important role in this respect.17,18 In a mixed-metal oxide,
10.1021/jp010633f CCC: $20.00 © 2001 American Chemical Society Published on Web 05/18/2001
5498 J. Phys. Chem. B, Vol. 105, No. 23, 2001
Figure 1. Steady-state rate for the 2N2O f 2N2 + O2 reaction on MgO, ZnxMg1-xO, and CrxMg1-xO catalysts (x e 0.05). T ) 900 C, PN2O ) 0.2 atm, PHe ) 0.8 atm.10
the two metals can behave as “isolated units” that bring their intrinsic chemical properties to the system, or their behavior can be modified by the effects of metal/metal or metal/oxygen/ metal interactions.17,18 To improve the performance of mixedmetal oxide catalysts in DeNOx processes, one needs a molecular-level understanding of the chemistry of NOx species on the oxide surfaces. The modern techniques of surface science provide unique “tools” for addressing this issue.9,12,23-26 In this article, we use ultraviolet photoelectron spectroscopy (UPS) to study electronic properties of the MgO(100), ZnxMg1-xO(100), and CrxMg1-xO(100) systems. The chemistry of NO, NO2, and N2O on MgO(100), ZnxMg1-xO(100), CrxMg1-xO(100), and Cr2O3(0001) surfaces is investigated with synchrotron-based high-resolution photoemission. First-principles density functional calculations in a periodic supercell are used to examine the bonding of the NOx species to the oxide substrates. Our experimental and theoretical studies show a clear correlation between the electronic and chemical properties of the oxide surfaces. Trends in the chemical activity of ZnxMg1-xO(100) and CrxMg1-xO(100) reveal a basic principle in the design of mixed-metal oxide catalysts for DeNOx operations. II. Experimental and Theoretical Methods II.1 UPS and Photoemission Experiments. The UPS experiments described in section III were performed in a standard ultrahigh vacuum (UHV) system that combined two chambers equipped with instrumentation for UPS, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), ion scattering spectroscopy (ISS), and a quadrupole mass spectrometer.18 He-I (hν ) 21.2 eV) and Al KR (hν ) 1486.6 eV) radiation were used to acquire UPS and XPS spectra, respectively. The photoemission experiments were carried out in a UHV chamber that is part of the U7A beamline at the National Synchrotron Light Source (NSLS). This UHV chamber is equipped with optics for LEED, a hemispherical electron energy analyzer with multichannel detection for photoemission, and an X-ray source (Al or Mg KR radiation).27 In the experiments with the synchrotron beam, the N 1s and O 1s spectra were taken using photon energies of 480 and 625 eV, respectively. Since the oxides under study here are insulators or semiconductors, the effects of charging make it very difficult to carry out photoemission experiments with bulk single crystals on these materials. In particular, this is a very big problem if one is interested in obtaining reproducible absolute binding energies for the photoemission peaks. By working with ultrathin films (30-40Åinthickness)ofCr2O3(0001),MgO(100),ZnxMg1-xO(100),
Rodriguez et al. and CrxMg1-xO(100) grown on metal substrates, we prevented effects of charging in our photoemission experiments. The MgO(100), ZnxMg1-xO(100), and CrxMg1-xO(100) films were grown on a Mo(100) crystal,18,22,28 while a Pt(111) substrate was used for growing the Cr2O3(0001) films.22,29 MgO(100) and Cr2O3(0001) were grown following procedures described in detail in the literature.28,29 In short, high-purity (99.999%) Mg or Cr was vapor deposited on the corresponding metal substrates at 500600 K in a background O2 pressure of ∼2 × 10-6 Torr. In the case of MgO, this was followed by heating to 1200 K in 1 × 10-6 Torr of O2.18,28 The Cr2O3(0001) films were annealed at 600-700 K for 5 min after deposition.29 Core-level photoemission and low-energy electron diffraction (LEED) were used to verify the formation of MgO(100) and Cr2O3(0001).9,28,29 For preparing the ZnxMg1-xO(100) and CrxMg1-xO(100) systems, we followed a methodology18,22 that involved simultaneous dosing of Mg and the dopant agent (Zn or Cr) to a Mo(100) crystal at ∼600 K in an atmosphere of O2 (5 × 10-6 Torr), with annealing at 1000-1200 K (5 min) in the final step. The rates of deposition of Mg and the dopant agent were controlled in such a way that the average mole fraction of the dopant in the oxide films was in the range of 0.05-0.08% (metal basis). This was verified by measuring the relative intensity of the Mg and Zn or Cr core levels in photoemission. At these low concentrations, the Zn and Cr are dissolved within a matrix of MgO (probably occupying Mg sites in the lattice),10,20-22 and the ZnxMg1-xO(100) and CrxMg1-xO(100) films (x ) 0.050.08) exhibited a (1 × 1) LEED pattern similar to that of pure MgO(100) films. In experiments of ISS, the CrxMg1-xO(100) films (x ) 0.05-0.08) had a Cr concentration at the surface (0.10-0.15, metal basis) that was somewhat larger than the average Cr concentration for the “bulk” of the sample.18,30 For the ZnxMg1-xO(100) films (x ) 0.05-0.08), the Zn concentration at the surface measured with ISS was very close to the “bulk” concentration measured with XPS or photoemission. The core-level XPS spectra of the mixed-metal oxide films displayed peak positions characteristic of Mg2+, Zn2+, and Cr2+ ions.18,22,25,30 The oxide surfaces were exposed to NO, NO2, and N2O at 80 K. The adsorbates were dosed through a glass-capillary array positioned near the samples. The gas exposures are based on the ion gauge reading and are not corrected for the capillary array enhancement. The coverages of NOx species were estimated by normalizing the area under the N 1s peaks and comparing it to results of previous photoemission experiments. The NOx coverages are reported with respect to the number of Mg surface atoms in a MgO(100) surface; i.e., 1.1 × 1015 adatoms cm-2 corresponds to θ ) 1 monolayer (ML). II.2 Theoretical Methods and Models. The first-principles density functional (DF) calculations reported in sections III.1 and III.5 were performed using the commercial version of the CASTEP code31 available from Molecular Simulations Inc. CASTEP has an excellent track record in accurate prediction of geometry and energetics for oxide systems.9,18,26,31-33 In this code, the wave functions of valence electrons are expanded in a plane wave basis set with k-vectors within a specified energy cutoff Ecut. Tightly bound core electrons are represented by nonlocal ultrasoft pseudopotentials.34 Brillouin Zone integration is approximated by a sum over special k-points chosen using the Monkhorst-Pack scheme.35 The exchange-correlation contribution to the total electronic energy is treated in a spinpolarized generalized-gradient corrected (GGA) form of the local density approximation (LDA).36 In all calculations, the kinetic energy cutoff Ecut, and the density of the Monkhorst-Pack k-point mesh were chosen high enough to ensure convergence
DeNOx Reactions on Mixed-Metal Oxides
Figure 2. Schematic view of the four-layer slabs used to model MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100) surfaces. The unit cell of the mixed-metal systems contains only one zinc or chromium atom.
of the computed structures and energetics. Since the DF calculations were performed at the GGA level, one can expect good predictions for the bonding energies of the NOx species on the oxide surfaces.36,37 Frequently, DF-GGA calculations predict adsorption energies within an accuracy of 5 kcal/ mol.9b,18,26,38 In any case, in this work our main interest is in qualitative trends in the energetics and not in absolute values. Previous works have shown that slabs of 3-4 layers provide a good representation of the MgO(100) surface.9,18,32,39 To model the oxide systems, we used a four-layer slab (see Figure 2) and the supercell approach.31a A vacuum of 12.5 Å was placed on top of the surface to ensure negligible interactions between the periodic images normal to the surface.9,26,33,38 The DF calculations predicted accurate lattice constants for bulk MgO and almost no reconstruction of the MgO(100) surface, in agreement with several experimental and theoretical investigations.32 To simulate ZnxMg1-xO(100) and CrxMg1-xO(100) surfaces, we replaced 25% percent of the Mg atoms in the first layer of the MgO(100) slab with Zn or Cr atoms. The overall concentration of the second metal in the unit cell of our models, see Figure 2, was 0.0625 (metal basis). This value is close to the average concentration of the ZnxMg1-xO(100) and CrxMg1-xO(100) films (x ) 0.05-0.08) examined experimentally, but the layer-bylayer distributions of Zn and Cr in these systems are different. Nevertheless, our slab models provide a very good representation of Zn and Cr atoms contained in a matrix of MgO.18,22,32 After optimizing the geometry of the bare Zn0.06Mg0.94O(100) slab, we found that the Zn atoms were relaxed upward by ∼0.01 Å with respect to the plane of Mg atoms in the surface (top layer). In the case of Cr0.06Mg0.94O(100), the Cr atoms were shifted downward by ∼0.07 Å with respect to the plane of Mg surface atoms. In these systems, the Mg-O distances were similar to those found in bulk MgO (∆ e 0.03 Å). No attempt was made to model the Cr2O3(0001) surface. This oxide surface can be Cr and/or O terminated,7,40 relaxation effects play an important role,40 and more experimental/ theoretical work is necessary to find a reliable representation of such type of system.41
J. Phys. Chem. B, Vol. 105, No. 23, 2001 5499
Figure 3. Density functional results for the density-of-states (DOS) of the occupied bands in MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). The zero of energy in the figure is not the vacuum level.31a
III. Results III.1 Electronic Properties of ZnxMg1-xO(100) and CrxMg1-xO(100). Figure 3 displays the calculated (DF-GGA) density-of-states for the occupied bands of MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). The valence band in MgO(100) contains mainly O 2p character (plus some Mg 3s character) and is located ∼11 eV above the O 2s levels. After adding zinc to magnesium oxide, Zn0.06Mg0.94O(100), states with Zn 3d character appear ∼1 eV below the {O 2p + Mg 3s} band, whereas states with Zn 4s character are located right above the top of the {O 2p + Mg 3s} band. Upon adding chromium to magnesium oxide, Cr0.06Mg0.94O(100), new occupied states with Cr 3d character are generated ∼4 eV above the {O 2p + Mg 3s} band. Differences in the electronic structure of the MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100) systems are clearly evident from Figure 3. The left panel in Figure 4 shows UPS spectra for MgO(100), Zn0.06Mg0.94O(100), and Cr0.07Mg0.93O(100) films. The valence spectrum for the MgO(100) film agrees very well with spectra reported in the literature for bulk MgO.7,42,43 In these spectra, the dominant O 2p band has a width of ∼ 6 eV42-44 and appears at ∼4 eV below the Fermi level (0 of binding energy).42,44 For the Zn0.06Mg0.94O(100) film, broad Zn-derived features are seen near binding energies of 12-10 eV (Zn 3d) and 5-3 eV (Zn 4s). In the case of the Cr0.07Mg0.93O(100) film, Cr-induced features appear at 3 and 1 eV. The qualitative trends in Figures 3 and 4 are similar: after doping MgO with the transition metals new states are created above the valence bands of the oxide, with the effects of Cr being more significant. For comparison in Figure 4, we also show valence photoemission spectra for Cr2O3(0001)45 and ZnO(0001)7,46 surfaces (right-side panel). In the valence spectrum of Cr2O3(0001), O 2p states appear from 8 to 3 eV with Cr 3d states from 2 to 0 eV.45 These binding energy positions are similar those found for the O 2p and Cr 3d states in Cr0.07Mg0.93O(100) and other CrOx systems.45 In the case of ZnO(0001) and Zn0.06Mg0.94O(100), the metal states
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Figure 4. Left-side: Valence UPS spectra for MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100) films. Right-side: Valence photoemission spectra for Cr2O3(0001)45 and ZnO(0001).7,46 He-I radiation was used to take the UPS spectra of pure and doped MgO, whereas the spectra of Cr2O3(0001) and ZnO(0001) were taken with photon energies of 50 and 240 eV, respectively.
appear at higher binding energy than the corresponding metal states in Cr2O3(0001) and Cr0.07Mg0.93O(100). As we will see in the following sections, the differences in the electronic properties of MgO(100), ZnxMg1-xO(100), CrxMg1-xO(100), and Cr2O3(0001) have a strong impact on the chemical reactivity of these oxides. III.2 Adsorption of NO. Thermal desorption spectra for NO on a MgO(100) single-crystal surface cleaved in a vacuum exhibit two clear peaks at 50-60 K (NO multilayer) and 7085 K (NO interacting with (100) terraces of the oxide surface), plus a weak broad feature from 90 to 120 K (NO interacting with defect sites of the surface).23,47 At the top of Figure 5 is shown a N 1s spectrum acquired after dosing NO to a MgO(100) film at 80 K (the lowest temperature that our sample manipulator could reach). A single peak is seen at ∼401.4 eV. This is the typical position found for NO molecules on metals48 and oxides.49 Therefore, we assign it to NO bonded to metal cations of MgO(100). After normalization of the area under the N 1s peak, we estimate that the surface was covered by ∼0.2 ML of NO. In the center and bottom of Figure 5 are shown the corresponding N 1s spectra for the adsorption of NO on Zn0.06Mg0.94O(100), Cr0.07Mg0.93O(100), and Cr2O3(0001). In each case, only one peak is seen and corresponds to NO chemisorbed on the metal cations of the oxide surface. From previous studies, it is known that NO chemisorbs intact on Cr2O3(111) films.50 Our photoemission results indicated that the chemistry of NO on the oxide surfaces of Figure 5 is simple. Under the conditions of these experiments, the molecule adsorbed and desorbed without decomposing or transforming into another NOx species on the surface. There was, however, a significant variation in the strength of the cation-NO bonds depending on the oxide substrate. For NO/MgO(100), no N 1s signal was left after heating the sample to 180 K. The other extreme was NO/Cr2O3(0001), where the adsorbate desorbed from 280 to 350 K. In the case of NO/Zn0.06Mg0.94O(100), desorption of NO was complete by 220 K. For NO/Cr0.07Mg0.93O(100), ∼60% of the NO desorbed below 180 K (i.e., Mg-bonded NO) and the other 40% between 280 and 350 K (i.e., Cr-bonded NO). Thus, the strength of the oxide T NO interactions increased in the sequence: MgO(100) < Zn0.06Mg0.94O(100) < Cr0.07Mg0.93O(100) ≈ Cr2O3(0001). Following a Redhead analysis,18,51 we estimate a difference of more than 10 kcal/mol between the
Figure 5. N 1s spectra acquired after adsorbing ∼0.2 ML of NO on MgO(100), Zn0.06Mg0.94O(100), Cr0.07Mg0.93O(100), and Cr2O3(0001) surfaces at 80 K. The electrons were excited using a photon energy of 480 eV.
adsorption energies of NO on Mg and Cr sites of MgO(100) and Cr0.07Mg0.93O(100). III.3 Adsorption of NO2. It is known that at moderate temperatures and high coverages nitrogen dioxide can undergo disproportionation reactions (2NO2 f NO3 + NO) on MgO(100),9 ZnO(0001),46 and other oxide surfaces.52 Since we are mainly interested in examining pure cation T NO2 interactions, we studied the adsorption of NO2 on MgO(100), Zn0.06Mg0.94O(100), Cr0.07Mg0.93O(100), and Cr2O3(0001) at 80 K and low coverages. Figure 6 displays N 1s spectra obtained after depositing ∼0.2 ML of NO2 on the oxide surfaces. For NO2 on MgO(100), a single peak is found near 404 eV that corresponds to chemisorbed NO2.9 A similar peak is seen for NO2 on Zn0.06Mg0.94O(100). On the other hand, the Cr0.07Mg0.93O(100) surface is much more reactive and nearly half of the NO2 dissociates into NO and possibly O adatoms.9 Finally, for NO2 on Cr2O3(0001), there is a complete transformation of the adsorbate into NO, as has been reported for the NO2/Cr2O3(111) system.50 There is a good correlation between the electronic (Figure 4) and chemical properties (Figure 6) of the oxides: the surfaces that exhibit occupied states with a relatively low stability, Cr0.07Mg0.93O(100) and Cr2O3(0001), display the highest activity for breaking N-O bonds. A key issue for the design of mixedmetal oxide catalysts is the fact that Cr is able to induce occupied electronic states with a low stability after orbital hybridization within a matrix of MgO. III.4 Adsorption of N2O. Figure 7 shows N 1s spectra for the adsorption of ∼0.15 ML of N2O on MgO(100), Zn0.06Mg0.94O(100), Cr0.08Mg0.92O(100), and Cr2O3(0001) at 80 K. The spectrum for N2O on MgO(100) exhibits two peaks. These peaks at ∼404.5 and 407.9 eV have similar intensities and match the two N 1s positions reported for N2O adsorbed on oxides.49 This assignment is also supported by the fact that both peaks disappeared simultaneously upon heating to 200 K. When the heating of N2O/MgO(100) was done in front of a mass spectrometer, no signal was detected for NO and N2 that could
DeNOx Reactions on Mixed-Metal Oxides
J. Phys. Chem. B, Vol. 105, No. 23, 2001 5501 TABLE 1: Adsorption of NO: DF-GGA Results bond lengths (Å) metal-Na metal-Oa N-O free NO MgO(100)b η1-N on Mg η1-O on Mg Zn0.06Mg0.94O(100)b η1-N on Zn η1-O on Zn Cr0.06Mg0.94O(100)b η1-N on Cr η1-O on Cr
ads energy (kcal/mol)
1.15 2.40 2.58
1.16 1.15
6 4
2.24
1.18 1.17
9 6
1.83
1.21 1.19
24 15
2.21 1.77
a Mg in MgO, Zn in Zn0.06Mg0.94O, and Cr in Cr0.06Mg0.94O. b θNO) 0.25 ML.
Figure 6. N 1s spectra for the adsorption of ∼0.2 ML of NO2 on MgO(100), Zn0.06Mg0.94O(100), Cr0.07Mg0.93O(100), and Cr2O3(0001) surfaces at 80 K. A photon energy of 480 eV was used to take this spectra.
Figure 7. N 1s spectra taken after adsorbing ∼0.15 ML of N2O on MgO(100), Zn0.06Mg0.94O(100), Cr0.07Mg0.93O(100), and Cr2O3(0001) surfaces at 80 K. The electrons were excited using a photon energy of 480 eV.
not be attributed to the cracking pattern of N2O in this instrument. In practical terms, the MgO(100) surface is inactive for breaking N-O and N-N bonds in N2O. This is reminiscence of the results obtained above for NO on MgO(100). To see DeNOx chemistry for NO or N2O on MgO(100), this oxide
surface has to contain a substantial amount of defect sites or imperfections.9b,53,54 In the case of N2O/Cr0.08Mg0.92O(100) and N2O/Cr2O3(0001), a third N 1s peak appears near 399 eV that can be assigned to Nx species49 produced by the decomposition N2O on Cr centers of these oxide surfaces at 80 K. For N2O/ Zn0.06Mg0.94O(100), there was no dissociation of N2O upon adsorption at 80 K, as found for N2O/MgO(100), but during the heating of N2O/Zn0.06Mg0.94O(100) from 80 to 250 K a signal for desorption of N2 was detected in the mass spectrometer that could not be attributed to the cracking of N2O in the instrument. In summary, our adsorption/reaction studies indicate that the activity of the mixed-metal oxides for breaking N-O bonds in N2O increases in the following sequence: MgO(100) < Zn0.06Mg0.94O(100) < Cr0.07Mg0.93O(100). This trend agrees qualitatively with studies for the catalytic decomposition of N2O at 600-900 C (for an example, see Figure 1), which show CrxMg1-xO as a very good catalyst and pure MgO as a relatively poor catalyst.10 No N 1s signal was observed for the MgO(100), Zn0.06Mg0.94O(100), and Cr0.08Mg0.92O(100) surfaces of Figure 7 upon heating to the elevated temperatures (>600 C) typically used in DeNOx operations. III.5 Bonding of NO, N2O, and NO2 to the Oxide Surfaces. First-principles density-functional (DF) calculations were used to study the bonding interactions of the NOx species on MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). Table 1 lists DF results for the adsorption of NO on the slabs of Figure 2. NO molecules (0.25 ML) were placed above the first layer of the slabs bonded to metal sites via the nitrogen (η1-N) or oxygen (η1-O) atom. The resulting structures were optimized allowing the movement of the adsorbate and atoms in the top two layers of each slab. On all the oxide surfaces, bonding of NO through the N atom was energetically more stable than bonding through the O. On MgO(100), the DF-GGA calculations predict an NO adsorption energy of 6 kcal/mol, which is close to the value of 3-5 kcal/mol observed experimentally.47,55 For NO on MgO(100), the bonding involves weak adsorbate-surface electrostatic interactions, and there is no substantial mixing of the frontier orbitals of the molecule with the bands of the oxide. A similar phenomenon has been observed in theoretical studies for CO on MgO(100).56,57 Larger NO adsorption energies are calculated after doping MgO with Zn or Cr. In this respect, the Cr0.06Mg0.94O(100) system is particularly interesting. The key to the chemical activity of CrxMg1-xO(100) is in the occupied Cr 3d states which are much less stable than the valence bands of pure magnesium oxide (Figures 3 and 4). These states mix well with the orbitals of NO leading to a large adsorption energy and a significant elongation in the N-O bond. On metal centers of an oxide, NO2 can bond via nitrogen (η1-N) or the oxygens (η2-O,O) as shown in Figure 8. The molecule was bonded in these configurations (θNO2 ) 0.25 ML)
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Figure 8. Bonding conformations for NO2 on MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). The metal atoms are represented by small (Zn or Cr) and large (Mg) dark spheres.
TABLE 2: Adsorption of NO2: DF-GGA Results bond lengths (Å) metal-Na metal-Oa N-O free NO2 MgO(100)b η1-N on Mg η2-O,O on Mg η2-O,O on Mg,Mg Zn0.06Mg0.94O(100)b η1-N on Zn η2-O,O on Zn η2-O,O on Zn,Mg Cr0.06Mg0.94O(100)b η1-N on Cr η2-O,O on Cr η2-O,O on Cr,Mg O on Cr, NO on Mg
ads energy (kcal/mol)
1.21 2.31 2.33 2.28
1.22 1.23 1.25
19 22 28
2.19 2.12 2.21c
1.24 1.26 1.27 1.25c
27 32 36
1.97 1.89 2.16c 1.65
1.26 1.30 1.32 1.26c 1.16
36 40 46
2.07
1.94
2.42
49
a
Mg in MgO, Zn in Zn0.06Mg0.94O, and Cr in Cr0.06Mg0.94O. b θNO2 ) 0.25 ML c Bond lengths for the oxygen of NO2 bonded to Mg.
to the slabs in Figure 2. Table 2 displays the corresponding calculated adsorption energies and optimized geometries. On all the oxide surfaces, bonding through the oxygens is more stable than via N. This is the reverse of the behavior seen for NO adsorption above. An identical difference is found when comparing the forms of bonding of NO and NO2 on metal surfaces.58,59 On pure MgO(100), NO2 forms stronger adsorption bonds than NO (compare to Table 1). The partially occupied 6a1 orbital of NO2 is a better electron acceptor (i.e., more stable) than the 2π orbitals of NO, see Figure 9, and interacts better with the occupied bands of MgO.60 Very strong NO2 T oxide interactions are seen on Zn0.06Mg0.94O(100) and Cr0.06Mg0.94O(100). For NO2/Zn0.06Mg0.94O(100), the adsorption energies of NO2 on Zn are comparable to those calculated previously for the NO2/ZnO(0001) system.61 On Cr0.06Mg0.94O(100), the DF calculations give very large NO2 adsorption energies, and, when compared to MgO(100) and Zn0.06Mg0.94O(100), this is the only oxide surface on which partial dissociation (NO2,gas f NOads + Oads) is more exothermic than molecular chemisorption. In the cases of MgO(100) and Zn0.06Mg0.94O(100), the Mg-O and Zn-O bonds are not strong enough to induce the dissociation of chemisorbed NO2. These predictions agree well with the trends seen in the experimental data of Figure 6, where NO2 dissociates spontaneously only on CrxMg1-xO(100). To induce the dissociation of the adsorbate in NO2/MgO and NO2/ZnxMg1-xO, one has to put energy into these systems.
Figure 9. Energy range covered by the bands of pure MgO32 and Cr2O3.27 The empty and occupied states are indicated by dashed and solid lines, respectively. For comparison, we also include the positions for the dopant levels in Cr0.07Mg0.93O(100) and Zn0.06Mg0.94O(100), from Figure 4, plus molecular orbital energies for NO, NO2, and N2O.9,62 All the energies are reported with respect to the vacuum level.
TABLE 3: Adsorption of N2O: DF-GGA Results metal-Na free N2O MgO(100)b η1-N on Mg η1-O on Mg η2-O,N on Mg,Mg Zn0.06Mg0.94O(100)b η1-N on Zn η1-O on Zn η2-O,N on Zn,Mg Cr0.06Mg0.94O(100)b η1-N on Cr O on Cr, N2 on Mg
bond lengths (Å) ads energy metal-Oa N-O N-N (kcal/mol) 1.21
1.16
2.55
1.21 1.21 1.21
1.16 1.16 1.16
3 3 4
2.23 2.24 2.56c
2.28
1.21 1.21 1.21
1.16 1.16 1.17
8 7 12
1.81 2.54c
1.24 1.66
1.21 1.14
22 63
2.57 2.54 2.59
a
Mg in MgO, Zn in Zn0.06Mg0.94O, and Cr in Cr0.06Mg0.94O. b θN2O ) 0.25 ML. c Mg-N bond length.
N2O is a linear molecule that, in principle, can bond to oxide substrates with its main axis either parallel (η2-N,O and η3N,N,O) or perpendicular (η1-O and η1-N) to the surface. Table 3 lists results obtained after adsorbing N2O (θN2O ) 0.25 ML) on the slabs of Figure 2 in the configurations shown in Figure 10. As in the cases of NO and NO2 adsorption, we allowed full relaxation of the adsorbed N2O molecules and the atoms in the first two layers of the slab. The bonding interactions between N2O and MgO(100) are very weak. N2O is not a radical molecule like NO or NO2, and its frontier orbitals62 match poorly in energy with the bands of MgO (see Figure 9, HOMO very stable, LUMO at high energy). Essentially, there is no mixing of the frontier orbitals of N2O with the bands of MgO, and the weak adsorption bond is due to adsorbate T oxide electrostatic interactions. After doping MgO with zinc, a substantial increase in the adsorption energy of N2O is predicted. On Zn0.06Mg0.94O(100), a η2-O,N bonding configuration is clearly more stable
DeNOx Reactions on Mixed-Metal Oxides
J. Phys. Chem. B, Vol. 105, No. 23, 2001 5503
Figure 10. Bonding conformations for N2O on MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). The metal atoms are represented by small (Zn or Cr) and large (Mg) dark spheres.
than η1-N or η1-O configurations. In Table 3, the Cr0.06Mg0.94O(100) system displays the largest reactivity toward N2O. On this system, η1-O and η2-O,N bonding coordinations are not stable as local minima in the energy map, and the N2O molecule spontaneously dissociates into O and N2 in agreement with the experimental results in Figure 7. A η1-N configuration is stable on Cr0.06Mg0.94O(100), but a Cr f N2O charge transfer63 reduces the N-N-O angle from 180 to 138° due to population of the LUMO of the molecule.62 This big structural change in N2O was not observed on MgO(100) and Zn0.06Mg0.94O(100) surfaces. In the Cr0.06Mg0.94O(100) system, the bonding interactions between adsorbate and oxide were clearly dominated by mixing (or hybridization) of the frontier orbitals of N2O with the bands of the surface. The photoemission and DF results described up to this point show a very good correlation between the electronic and chemical properties of MgO(100), Zn0.06Mg0.94O(100), and Cr0.06Mg0.94O(100). Figure 9 compares the band energies of bulk Cr2O3 27 and MgO32 with the molecular orbital energies of NO, NO2, and N2O.9,62 To induce DeNOx reactions and N-O bond breaking on an oxide surface, one needs a strong interaction between the occupied bands of the oxide and the empty orbitals (N-O antibonding) of the NOx species. In general, this type of interactions are facilitated if the occupied states of a surface have a relatively low stability.18,64-66 In this respect, Cr2O3 is better suited than MgO and, thus, exhibits a larger reactivity toward NO, NO2, and N2O. Since MgO is an inexpensive material, it is desirable to find ways for increasing its chemical activity.2,6,10,21 In principle, this can be accomplished by doping the oxide with a second metal. The basic idea is to find metal dopants that upon hybridization within a matrix of MgO are able to produce occupied electronic states located well above the MgO valence band. The trends in the behavior of ZnxMg1-xO(100) and CrxMg1-xO(100) illustrate a fundamental principle for the design of DeNOx catalysts. The Cr 3d levels in CrxMg1-xO(100) are energetically well positioned for responding to the presence of adsorbates. The results of DF calculations indicate that occupied states with a low stability also can be obtained after doping magnesium oxide with Co, Fe, or Mn. Plots for the calculated DOS of Co0.06Mg0.94O(100), Fe0.06Mg0.94O(100), and Mn0.06Mg0.94O(100) are shown in Figure 11. Dopant derived states appear at 2-3 eV above the MgO valence band. In contrast, more stable dopant states were found after doping with Zn (Figure 3), Ni,18,32 Cu, and Sn. The top panel in Figure 12 displays the relative position of the dopant levels in a series of TM0.06Mg0.94O(100) systems (TM ) Zn, Sn, Ni, Co, Fe, Mn, or Cr). In these mixed-metal
Figure 11. Density functional results for the density-of-states (DOS) of the occupied bands in Co0.06Mg0.94O(100), Fe0.06Mg0.94O(100), and Mn0.06Mg0.94O(100). The zero of energy in the figure is not the vacuum level.31a
oxides, the instability of the TM 3d band increases as one moves from right to left in the 3d series. The effects of an earlytransition metal on the electronic properties of MgO are stronger than those of s,p metals such as Zn or Sn. The predicted trends agree with experimental results of valence photoemission for Zn0.06Mg0.94O (Figure 4), Ni0.06Mg0.94O,18 and Cr0.07Mg0.93O (Figure 4). The bottom panel in Figure 12 shows the values calculated (DF-GGA) for the adsorption energy of NO on the TM0.06Mg0.94O(100) mixed-metal oxides. Clearly, there is a relationship between the energy position of the dopant electronic levels and the chemical reactivity of a mixed-metal oxide. This relationship is not linear, because one must also consider the number of states with low stability provided by a dopant agent. In addition, several factors can affect the strength of a metal-NO bond.64,66,67 For N2O on Fe0.06Mg0.94O(100) and Mn0.06Mg0.94O(100), DF studies showed dissociative adsorption (O + N2) as occurred on a Cr0.06Mg0.94O(100) surface. All these results stress the importance of having an element in the mixed-metal oxide that has occupied states with a low stability. A similar correlation between electronic and chemical properties is found after examining the reactivity of a series of molybdates (AMoO4, A ) Mg, Fe, Co, Ni) toward H2 and H2S,17 and the chemisorption of CO18 or the dissociation/activation of CH4 and SO2 on TMxMg1-xO systems.22,68 It appears that this type of correlation can be useful as a general criterion for designing or choosing active mixed-metal oxide catalysts. A dopant agent can enhance the chemical activity of a host oxide by facilitating the formation of O vacancies.69-71 For example, doping of ceria (CeO2) with Zr induces a relatively minor narrowing of the band gap (∼0.6 eV), and no occupied states appear well above the host valence band,71 but the presence of Zr makes easier a loss of oxygen, which leads to the appearance of Ce 4f occupied states ∼2 eV above the CeO2 valence band and a subsequent increase in DeNOx activity.71 Cerium can undergo facile switching between “4+” and “3+”
5504 J. Phys. Chem. B, Vol. 105, No. 23, 2001
Rodriguez et al. transition metal on the electronic properties of MgO are stronger than those of s,p metals such as Zn or Sn. Acknowledgment. The authors thank J. Evans and H. Lee for many interesting conversations on the behavior of metaldoped MgO. The research carried out at Brookhaven National Laboratory was supported by the US Department of Energy (Divisions of Chemical and Materials Science) under contract DE-AC02-98CH10086. M. Pe´rez thanks ENRI for a travel grant (99-FT026) that made possible a work-visit to BNL. J.A. Rodriguez is grateful to the American Chemical Society for supporting a trip to Venezuela that allowed the development of this research project. References and Notes
Figure 12. Top panel: DF-calculated average position for the dopant levels in TM0.06Mg0.94O(100) surfaces (TM ) Zn, Sn, Ni, Co, Fe, Mn, or Cr). The values reported are with respect to the top of the MgO valence band. Bottom panel: Calculated (DF-GGA) adsorption energy for NO on the TM0.06Mg0.94O(100) surfaces.
oxidation states.72 Magnesium does not have this property and O-vacancy formation is not responsible for the chemistry seen on Zn0.06Mg0.94O(100) and Cr0.06Mg0.94O(100) surfaces. For these systems, core-level XPS spectra showed peak positions characteristic of Mg2+, Zn2+, and Cr2+ ions.18,22,30,46 The high chemical activity of CrxMg1-xO(100) is a direct consequence of the electronic states induced by chromium. IV. Conclusions After adsorbing NO, NO2, and N2O on MgO(100), Zn0.06Mg0.94O(100), Cr0.06Mg0.94O(100), and Cr2O3(0001), a correlation was found between the electronic and chemical properties of the oxide surfaces. Systems that have occupied electronic states with a relatively low stability, Cr0.06Mg0.94O(100) and Cr2O3(0001), bond NO strongly and are able to induce the dissociation of NO2 and N2O at temperatures as low as 80 K. On these oxide surfaces, adsorbed NO is a main product of the dissociation of NO2, whereas adsorbed Nx species are produced by the decomposition of N2O. The trends in the behavior of ZnxMg1-xO(100) and CrxMg1-xO(100) illustrate a basic principle for the design of mixed-metal oxide catalysts in DeNOx operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix produce occupied electronic states located well above the valence band of the oxide. These hybrid dopant states lead to large adsorption energies for NOx species and facilitate N-O bond cleavage. Occupied states with a low stability can be obtained after doping MgO with Cr, Mn, and Fe. The effects of an early
(1) Stern, A. C.; Boubel, R. W.; Turner, D. B.; Fox, D. L. Fundamentals of Air Pollution, 2nd ed.; Academic Press: Orlando, FL, 1984. (2) (a) EnVironmental Catalysis; Armor, J. N., Ed.; ACS Symposium Series No 552; American Chemical Society: Washington DC, 1994. (b) Workshop on Catalytic Technologies for EnVironmentally Benign Processes; Apesteguia, C. A., Resasco, D. Eds.; Catal. Today, 2000, 62, 133. (3) (a) Taylor, K. C. Catal. ReV. Sci. Eng. 1993, 35, 457. (b) Shelef, M.; Graham, G. W. Catal. ReV. Sci. Eng. 1994, 36, 433. (c) Shelef, M.; McCabe, R. W. Catal. Today, 2000, 62, 35. (4) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH: New York, 1997. (5) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Dekker: New York, 1991. (6) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: New York, 1989. (7) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (8) Schneider, W. F.; Hass, K. C., private communication. (9) (a) Rodriguez, J. A.; Jirsak, T.; Sambasivan, S.; Fischer, D.; Maiti, A. J. Chem. Phys. 2000, 112, 9929. (b) Rodriguez, J. A.; Jirsak, T.; Kim, J.-Y.; Larese, J. Z.; Maiti, A. Chem. Phys. Lett. 2000, 330, 475. (10) Larese, J. Z. to be published. (11) Kunnmann, W.; Larese, J. Z. “Method for the Generation of Variable Density Metal Vapors which Bypasses the Liquid Phase,” US Patent No. 6179897. (12) (a) Dmowski, W.; Egami, T.; Gorte, R.; Vohs, J. Physica B 1996, 221, 420. (b) Putna, E. S.; Bunluesin, T.; Fan, X. L.; Gorte, R. J.; Vohs, J. M.; Lakis, R. E.; Egami, T. Catal. Today, 1999, 50, 343. (13) (a) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439. (b) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kaspar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173. (c) Fornasiero, P.; Balducci, G.; Kaspar, J.; Meriani, S.; Di Monte, R.; Graziani, G. Catal. Today 1996, 29, 47. (14) Gao, Y.; Herman, G. S.; Thevuthasan, S.; Peden, C. H. F.; Chambers, S. A. J. Vac. Sci. Technol. A 1999, 17, 961. (15) Recent AdVances in Catalytic Materials; Rodriguez, N. M., Soled, S. L., Hrbek, J., Eds; MRS Symposium Proceedings Vol 497; Materials Research Society: Pittsburgh, PA, 1998. (16) Symposium on the Characterization of Mixed-Metal Oxide Catalysts; 215th National Meeting of the American Chemical Society, Dallas, TX, March-April, 1998. (17) (a) Rodriguez, J. A.; Hanson, J. C.; Chaturvedi, S.; Maiti, A.; Brito, J. L. J. Chem. Phys. 2000, 112, 935. (b) Ibid, J. Phys. Chem. B 2000, 104, 8145. (18) Rodriguez, J. A.; Jirsak, T.; Pe´rez, M.; Gonza´lez, L.; Maiti, A. J. Chem. Phys. 2001, 114, 4186. (19) (a) Wu, M.-C.; Truong, C. M.; Coulter, K.; Goodman, D. W. J. Am. Chem. Soc. 1992, 114, 7565. (b) Lunsford, J. H. Catal. Today 1990, 6, 235. (c) Nagaoka, K.; Karasuda, T.; Aika, K.-I. J. Catal. 1999, 181, 160. (20) (a) Tomishige, K.; Chen, Y.-G.; Fujimoto, K. J. Catal. 1999, 181, 91. (b) Chen, Y.; Tomishige, K.; Yokoyama, K.; Fujimoto, K. Applied Catal. A: General 1997, 165, 335. (21) Fisher, I.; Zhang, L.; Wu, Y.; Evans, J.; Lee, H., private communication. (22) (a) Rodriguez, J. A.; Jirsak, T.; Pe´rez, M.; Chaturvedi, S.; Kuhn, M.; Gonza´lez, L.; Maiti, A. J. Am. Chem. Soc. 2000, 122, 12362. (b) Rodriguez, J. A.; Pe´rez, M.; Jirsak, T.; Gonza´lez, L.; Maiti, A. Surf. Sci. 2001, 477, L279. (23) Freund, H.-J. Faraday Discuss. 1999, 114, 1. (24) (a) Goodman, D. W. Chem. ReV. 1995, 95, 523. (b) Barteau, M. Chem. ReV. 1996, 96, 1413. (25) (a) Campbell, C. T. Surf. Sci. Reports 1997, 27, 1. (b) Campbell, C. T. Curr. Opin. Solid State Mater. Sci. 1998, 3, 439.
DeNOx Reactions on Mixed-Metal Oxides (26) Sorescu, D. C.; Rusu, C. N.; Yates, J. T. J. Phys. Chem. B 2000, 104, 4408. (27) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S. J. Chem. Phys. 1999, 111, 8077. (28) (a) Wu, M.-C.; Truong, C. M.; Goodman, D. W. Phys. ReV. B 1992, 46, 12688. (b) Dohna´lek, Z.; Kimmel, G. A.; Joyce, S. A.; Ayotte, P.; Scott Smith, R.; Kay, B. D. J. Phys. Chem. B 2001, 105, 3747. (29) Zhang, L.; Kuhn, M.; Diebold, U. Surf. Sci. 1997, 375, 1. (30) Evans, J.; Wu, Y.; Lee, H., to be published. (31) (a) Payne, M. C.; Allan, D. C.; Arias, T. A.; Johannopoulus, J. D. ReV. Mod. Phys. 1992, 64, 1045. (b) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Int. J. Quantum Chem. 2000, 77, 895. (32) Rodriguez, J. A.; Maiti, A. J. Phys. Chem. B 2000, 104, 3630. (33) Refson, K.; Wogelius, R. A.; Fraser, D. G.; Payne, M. C.; Lee, M. H.; Milman, V. Phys. ReV. B 1995, 52, 10823. (34) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (35) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (36) White, J. A.; Bird, D. M. Phys. ReV. B 1994, 50, 4954. (37) (a) Ziegler, T. Chem. ReV. 1991, 91, 651. (b) van Santen, R. A.; Neurock, M. Catal. ReV.-Sci. Eng. 1995, 37, 557. (38) Sorescu, D. C.; Yates, J. T. J. Phys. Chem. B 1998, 102, 4556. (39) (a) Ferro, Y.; Allouche, A.; Cora, F.; Pisani, C.; Girardet, C. Surf. Sci. 1995, 325, 139. (b) Chen, L.; Wu, R.; Kioussis, N.; Zhang, Q. Chem. Phys. Lett. 1998, 290, 255. (c) Mejias, J. A.; Marquez, A. M.; FernandezSanz, J.; Fernandez-Garcia, M.; Ricart, J. M.; Sousa, C.; Illas, F. Surf. Sci. 1995, 327, 59. (d) Scamehorn, C. A.; Harrison, N. M.; McCarthy, M. I. J. Chem. Phys. 1994, 101, 1547. (40) (a) Rohr, F.; Baumer, M.; Freund, H.-J.; Mejias, J. A.; Staemmler, V.; Muller, S.; Hammer, L.; Heinz, K. Surf. Sci. 1997, 372, L291. (b) Rohr, F.; Baumer, M.; Freund, H.-J.; Mejias, J. A.; Staemmler, V.; Muller, S.; Hammer, L.; Heinz, K., 1997, 389, 391. (41) Baxter, R.; Reinhardt, P.; Lopez, N.; Illas, F. Surf. Sci. 2000, 445, 448. (42) Lee, Y. C.; Tong, P.; Montano, P. A. Surf. Sci. 1987, 181, 559. (43) Onishi, H.; Egawa, C.; Aruga, T.; Iwasawa, Y. Surf. Sci. 1987, 191, 479. (44) Gu¨nster, J.; Liu, G.; Kempter, V.; Goodman, D. W. Surf. Sci. 1998, 415, 303. (45) Rodriguez, J. A.; Chaturvedi, S.; Kuhn, M.; van Ek, J.; Diebold, U.; Robbert, P. S.; Geisler, H.; Ventrice, C. A. J. Chem. Phys. 1997, 107, 9146. (46) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Dvorak, J. J. Mol. Catal. A 2001, 167, 47. (47) Wichtendahl, R.; Rodriguez-Rodrigo, M.; Ha¨rtel, U.; Kuhlenbeck, H.; Freund, H.-J. Phys. Stat. Sol. A 1999, 173, 93. (48) Jirsak, T.; Kuhn, M.; Rodriguez, J. A. Surf. Sci. 2000, 457, 254, and references therein. (49) Overbury, S. H.; Mullins, D. R.; Huntley, D. R.; Kundakovic, L. J. Catal. 1999, 186, 296. (50) Xu, C.; Hassel, M.; Kuhlenbeck, H.; Freund, H.-J. Surf. Sci. 1991, 258, 23. (51) Redhead, P. A. Vacuum 1962, 12, 203.
J. Phys. Chem. B, Vol. 105, No. 23, 2001 5505 (52) Rodriguez, J. A.; Jirsak, T.; Liu, G.; Hrbek, J.; Dvorak, J.; Maiti, A., manuscript in preparation. (53) Manuscript to be published. (54) Kolmakov, A.; Stultz, J.; Goodman, D. W. J. Chem. Phys. 2000, 113, 7564. (55) Kim, J.-Y.; Larese, J. Z., manuscript to be published. (56) Soave, R.; Pacchioni, G. Chem. Phys. Lett. 2000, 320, 345. (57) Snyder, J. A.; Alfonso, D. R.; Jaffe, J. E.; Lin, Z.; Hess, A. C.; Gutowski, M. J. Phys. Chem. B 2000, 104, 4717. (58) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 2578. (59) Jirsak, T.; Kuhn, M.; Rodriguez, J. A. Surf. Sci. 2000, 457, 254. (60) (a) The charges in NO/MgO(100) and NO2/MgO(100) were calculated using the DMol3 code32,60b and the Mulliken approach.60c DMol3 uses local basis functions defined in a numerical grid.60b Our calculations employed numerical basis sets of double-ζ quality plus polarization functions to describe the valence orbitals of N, O, and Mg. For the DMol3 calculations, we used nonlocal DF theory with Becke-88 for exchange and Perdew-91 for correlation.32 The results of DMol3 gave a Mulliken charge of almost zero (< -0.1e) for NO on the MgO(100) surface, whereas the calculated charge for adsorbed NO2 was in the range of -0.2 to -0.4e depending on the adsorbate geometry. In gas phase, the experimental electron affinity of NO2 is ∼1.7 eV larger than that of NO.60d,e (b) Delley, B.; Shefer, J.; Woike, T. J. Chem. Phys. 1997, 107, 10067. (c) Szabo, A.; Ostlund N. S. Modern Quantum Chemistry; McGraw-Hill: New York, 1989. (d) Refaey, K. M. A. Int. J. Mass Spectrom. Ion Phys. 1976, 21, 21. (e) Leffert, C. B.; Jackson, W. M.; Rothe, E. W. J. Chem. Phys. 1973, 58, 5801. (61) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. B 2000, 104, 319. (62) Peyerimhoff, S. D.; Buenker, R. J. J. Chem. Phys. 1968, 49, 2473. (63) Using the DMol3 code and the Mulliken method,60 we calculated a charge of almost zero for N2O on MgO(100) in η1-O, η1-N and η2-O,N conformations. In contrast, a charge of ∼ -0.3e was calculated on N2O when the molecule was bonded to Cr0.06Mg0.94O(100) in a η1-N configuration. (64) Hoffmann, R. Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures; VCH: New York, 1988. (65) Shustorovich, E.; Baetzold, R. C. Science 1985, 227, 876. (66) (a) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. ReV. Lett. 1996, 76, 2141. (b) Hammer, B.; Nørskov, J. K. AdV. Catal., in press. (67) (a) Rodriguez, J. A. Surf. Sci. 1990, 226, 101. (b) Rochefort, A.; Fournier, R. J. Phys. Chem. 1996, 100, 13506. (c) Illas, F.; Lo´pez, N.; Ricart, J. M.; Clotet, A.; Conesa, J. C.; Ferna´ndez-Garcı´a, M. J. Phys. Chem. B 1998, 102, 8017. (68) Rodriguez, J. A.; Pe´rez, M.; Evans, J.; Gonza´lez, L.; Marquez, A.; Maiti, A., manuscript in preparation. (69) de Carolis, S.; Pascual, J. L.; Pettersson, L. G. M.; Baudin, M.; Wojcik, M.; Hermansson, K.; Palmquist, A. E. C.; Muhammed, M. J. Phys. Chem. B 1999, 103, 7627. (70) Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. J. Phys. Chem. B 2000, 104, 11110. (71) Liu, G.; Rodriguez, J. A.; Hrbek, J.; Dvorak, J. Peden, C. H. F. J. Phys. Chem. B, manuscript submitted. (72) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439.
