First-Principle Investigations of the Interaction between CO, O2, and

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

pubs.acs.org/JPCA

First-Principle Investigations of the Interaction between CO and O2 with Group 11 Atoms on a Defect-Free MgO(001) Surface Kai Töpfer and Jean Christophe Tremblay* Freie Universität Berlin, Berlin, Germany ABSTRACT: In this contribution, we investigate the interaction between CO and O2 with metal atoms of group 11 deposited on a defect-free magnesium oxide surface using density functional theory with periodic point charge embedding. We present the first transversal study of the adsorption and coadsorption of CO and O2 on coinage metal adatoms deposited on metal oxide surfaces from the perspective of single-atom catalysis. Various analysis tools shed light on the binding situation of the metal atoms to the substrate as well as on the situation of the two molecules on the different metal centers. Our analysis demonstrates that cooperative electronic effects enhance the stability of CO upon coadsorption with O2 for all three metal centers. Our results also explain the lack of catalytic activity of group 11 metal atoms with respect to CO oxidation under thermal conditions as a competition between OC−O2 bond activation and surface diffusion, leading to metal atom agglomeration. Additionally, it is shown how coadsorption of CO and O2 on Au/Mg(001) could pave the way to singleatom photocatalysis.

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INTRODUCTION Since Haruta and co-workers observed the catalytic propensity of small gold clusters in the late 1980s,1−3 a large number of further investigations aiming at explaining the high activity of various gold clusters (see, e.g., special issue ref 4) have been published. For the catalytic CO oxidation reaction in a mixed CO/O2 atmosphere, the crucial step for the reaction was identified as the activation of the molecular O2 vibration, which is well investigated for negatively charged Aun− clusters in the gas phase.5−11 In particular, experiments have revealed a large O2 bond elongation on even-numbered Aun− anionic clusters.12 This is due to a partial electron transfer from the binding metal atom to the antibonding π-orbitals of O2, benefiting from the unpaired electron of the even-numbered Aun− clusters.13,14 Although they have not enjoyed the same popularity, small anionic silver clusters Agn−, with n = 7, 9, and 11 silver atoms, were found to exhibit a similar catalytic behavior regarding the CO oxidation in the gas phase.15 This activity can be traced back to the capacity of such small clusters to accommodate both reactants in a coadsorbed intermediate, leading to a mechanism reminiscent of Langmuir−Hinshelwood reactions. On the contrary, neutral gold and silver clusters show only a weak interaction with O2 in the gas phase, preventing any further reaction. Nevertheless, Xu and co-workers11 demonstrated in experiments supported by theoretical calculations the possibility of coadsorbing CO and O2 on group 11 atoms at low temperatures. Although no reaction could be induced by thermal excitation, the formation of CO2 was observed upon irradiation using light with a wavelength above 340 nm. For gold clusters deposited on a MgO(001) support, experiments record catalytic activity for the CO oxidation on © XXXX American Chemical Society

only defect-rich MgO(001) surfaces and for cluster sizes with more than 7 gold atoms. Limited catalytic activity is also observed for smaller cluster sizes, such as Au3−7, but not for Au1,2.16 In the presence of gaseous O2 that is necessary for the reaction, the number of defective sites on the magnesium oxide substrate is relatively small, but the importance of color centers in catalysis on larger gold clusters is well established.17−20 Their role in the catalytic cycle, in particular the importance of the charge state in the reactions, remains hotly debated. Following the hypothesis that Au1−3 clusters are too small to protect an oxygen vacancy in a mixed atmosphere, Amft and co-workers21 demonstrated by means of density functional theory (DFT) calculations that the catalytic oxidation of CO by Au3 on defectfree MgO(001) is sustainable. Later, we demonstrated from first-principle investigations of Au1 clusters that the O2 itself is not responsible for the breakdown of the catalytic cycle on F0 oxygen vacancies on the magnesium oxide surface but rather the remaining oxygen atom after an initial formation of a single CO2 molecule.22 To understand the CO oxidation reaction on gold atoms, it thus appears necessary to investigate its chemistry on defect-free substrates. In comparison, little is known about the catalytic behavior of other group 11 atoms on MgO(001). Interest in supported single-atom catalysts has surged in recent years (see ref 23 and references therein). One of the main objectives of single-atom catalysis is to reduce the amount of material required to perform a reaction and maximize the utilization of every metal atom, provided it remains catalytically Received: January 19, 2018 Published: February 1, 2018 A

DOI: 10.1021/acs.jpca.8b00647 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

separates the inner cluster from the periodic array of formal point charges, taking values +2 for Mg2+ and −2 for O2−. This intermediate shell prevents an overly strong polarization of the electron density in the QM part by adjacent positive point charges.27 Convergence tests using the cluster adsorption energy and the HOMO−LUMO energy gap as criteria confirmed that the intermediate shell should count at least one layer between the inner local cluster and the array of point charges. Comparison of the vertical ionization potential of the cluster within the given setup (7.4 eV) and experimental results of a clean MgO(001) surface (7.15 ± 0.15 eV,32 7.110 eV33) shows a quantitative agreement and confirms the proper composition of the embedded Mg17O13 cluster. All calculated adsorption Eads(AB) and interaction Eint(AB) energies between fragment A and B are counterpoise corrected with the distinction that the adsorption energy includes fragment deformation using the optimized fragment energies (E′[A], E′[B]) while the interaction energy uses the unrelaxed fragment energies (E[A], E[B]):34

active. This strategy has already been proven successful, e.g., for CO oxidation on a single palladium atom supported by magnesium oxide,24 as well as on single metal atoms (Rh, Pd, Co, Cu, Ru, and Ti) on iron oxides.25,26 While supported gold clusters have demonstrated their potential for CO oxidation, single-atom catalysis remains either inefficient or impossible depending on the substrate. It is understood that reducing the sizes of supported metal clusters down to a single atom will affect their binding properties and their reactivity. The origin of the catalytic inertness of gold atoms on magnesium oxide must thus lie in the subtle details of its electronic structure on the substrate and its interaction with the reagents. In the following, we investigate the coadsorption behavior of CO and O2 on single atoms of group 11 metals (Cu, Ag, and Au) on terrace sites of a defect-free MgO(001) substrate to understand their binding behavior. The motivation for this transversal study is primarily to shed light on fundamental processes in single-atom catalysis. This knowledge can then help the design of more efficient catalysts, in which coinage metals could provide low-cost alternatives to more expensive metals. The choice of magnesium oxide as a substrate is otherwise justified by its simple experimental handling, which allows for the preparation of well-defined and regular surfaces. The study is based on density functional theory using the periodic electrostatic embedded cluster method (PEECM), where the metal atoms and part of the substrate are treated quantum mechanically and embedded into a periodic array of point charges to ensure proper description of the Madelung potential in the vicinity of the adsorbates.27 We first investigate the adsorption properties of the neutral metal atoms, as well as the absorption thereon of either CO or O2, to shed light on the different binding behaviors. We further study the inclination of precovered systems to take up either CO or O2 molecules to reach a coadsorbed state which could be favorable for the CO oxidation reaction. Compared to previous work on the topic, our investigations offer a global view of the CO adsorption behavior in a unified theoretical framework. Further, similar information for the O2 adsorption and its coadsorption with CO on coinage metal adatoms is not available.

−Eads(AB) = E CP[AB] − E′[A] − E′[B]

(1)

−E int(AB) = E CP[AB] − E[A] − E[B]

(2)

Following the notation of Martin and co-workers,35 the symbols in square brackets denote the chemical system or fragment considered, while the symbols in parentheses represent the presence of their respective basis functions. The counterpoise corrected energy ECP[AB] is calculated by adding the respective differences of the fragment energies in the complete (E[A(B)], E[B(A)]) and their own basis (E[A], E[B]) in the corresponding cluster conformation to the total energy of the system E[AB]. E CP[AB] = E[AB] + (E[A] − E[A(B)]) + (E[B] − E[B(A)])

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(3)

It was shown that, upon relaxation, the ions of the inner local part adjacent to the intermediate shell can lead to severe artifacts, i.e., the ions migrate in the array of point charges in an uncontrolled manner.27 Consequently, all ions of the MgO slab are kept frozen during structure optimization and vibrational analysis apart for the top 9 ions below the metal atom. Further, the harmonic frequencies are scaled with a factor of 0.97 to take into account anharmonic effects.36 All optimization and singlepoint calculations have been performed using the TURBOMOLE37−39 program package and the BFGS algorithm implemented in the atomic simulation environment40 (maximum force threshold: 0.01 eV/Å). Mulliken and natural41 population analysis were done using the implemented modules in the TURBOMOLE program package, whereas Bader charges were obtained by the Bader analysis alogrithm of the Henkelman group42 using electron density cube files of the clusters generated by ORBKIT.43 Predictions of bonding properties were based on the energy decomposition analysis implemented in TURBOMOLE.44 Reaction energy profiles are computed using the nudged elastic band method45 (spring constant: 0.1 eV/Å, if not mentioned otherwise) and the climbing image algorithm,46 for which the atomic simulation environment was used in combination with TURBOMOLE and the FIRE algorithm.47

COMPUTATIONAL DETAILS The defect-free nanostructured surfaces are studied by means of cluster embedding within an unrestricted density functional theory framework using the UB3LYP hybrid functional.28,29 Following Pacchioni and co-workers, a small symmetric cluster containing the metallic impurity, Mg17O13, surrounded by an intermediate pseudopotential region is embedded into a periodic array of point charges (PEECM).27 The inner local part of the cluster that is treated quantum mechanically (QM) containing the metal atom is described using 6-311G and 631+G* basis sets on Mg and O, respectively. Inclusion of diffuse functions for the oxygen atoms of the substrate was found to affect the energies only marginally (∼1%). The adsorbate atoms C and O are described by a basis set of valence triple-ζ quality (6-311+G*). The Ag and Au atoms are represented using the 28 and 60 relativistic electron pseudopotentials of Andrae and co-workers, respectively,30 and the associated valence triple-ζ basis, to which one additional f polarization function (TZVPP) was added. Cu is represented by an all-electron TZVPP basis. An intermediate shell in the QM part composed of Mg2+ and 2− O , described by a 10-electron pseudopotential due to Hay and Wadt31 on Mg2+ and a point charge of −2 on O2−, B


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RESULTS AND DISCUSSION Adsorption Behavior. Figure 1 shows the adsorption energies Eads of Cu (0.53 eV), Ag (0.24 eV), and Au (0.52 eV)

bond that is weaker than those for Cu and Au. When compared to literature values of the adsorption energy from periodic calculation at the GGA level with the PBE functional (0.75, 0.26, and 0.74 eV for Cu, Ag, and Au, respectively),50 our results agree reasonably well for Ag but predict a weaker adsorption strength for Cu and Au. The discrepancies are potentially due to the choice of hybrid functional in the present work, which are less affected by self-energy errors than GGA functionals. The atomic charges obtained by Bader and natural bond order analysis predict a weak charge transfer character, consistent with the behavior reported in the literature for similar systems.18,48,49 On the contrary, Mulliken charges predict a larger charge transfer from the surface system to the metal adsorbate. It was shown that Mulliken charges can converge slowly with the size of the atomic basis.52 Upon examination of the chemistry of a single supported metal atom, the height of the diffusion barrier along the surface will determine the tendency of atoms to aggregate into larger clusters instead of undergoing a catalytic cycle. Theoretical calculations predict the diffusion barrier of Au atoms on clean MgO(001) terraces to be about 0.2−0.3 eV,18,53 which is in agreement with EPR experiments.54 These EPR experiments also reveal that formation of larger Au particles requires an additional 0.2 eV compared to the diffusion barrier. Using the PEECM embedding scheme, we performed NEB calculations on a Mg24O18 cluster (vertical ionization potential: 6.7 eV) with two equivalent O2− adsorption sites. The diffusion barrier for Au from one O2− surface ion to the adjacent and equivalent site is found to be 0.27 eV, in great agreement with the experiment. Similar NEB calculations for Ag and Cu result in diffusion barriers of 0.15 and 0.44 eV, respectively. The trends in the diffusion barriers are reasonable in light of the adsorption strength of the respective metal atoms and the literature values (Ag, 0.10 eV, and Cu, 0.45 eV) obtained from periodic density functional theory calculations using LDA and GGA functionals.53,55 Due to the low diffusion barrier (0.15 eV) compared with the adsorption energy (0.24 eV), aggregation of Ag atoms on the MgO(001) support will probably be impossible to prevent in practice. For each molecular species, CO and O2, geometry optimization was initiated by adding the adsorbates in various slightly asymmetric configurations on top of the metal cluster, as well as at different sites on the surface itself. The equilibrium structures, confirmed by vibrational analysis, are presented in Figure 2 for CO and in Figure 3 for O2. The CO molecule is seen to adsorb perpendicularly to the surface and laterally displaced with respect to the position of the copper atom. On the other hand, adsorption on Ag and Au behaves similarly, with CO adopting a moderately bent conformation on top of

Figure 1. Depth of the surface O2− ion below the metal (top panel) and of the metal atoms (second panel) from the default surface plane, as well as the adsorption energy (third panel) and the atomic charge (bottom panel) of the metal atoms at a defect-free MgO(001) surface. Mulliken (left column, dotted line), natural (center column, dash− dotted line), and Bader charges (right column, dashed line) are given for each metal type.

atop the O2− ion and their structural properties, as well as their partial charge calculated using different methods. Transition metals interact rather weakly with the Lewis acidic sites (Mg2+) and moderately more strongly with the basic sites (O2−) of the MgO(100) surface. The interaction strength originates almost entirely from polarization effects without any significant charge transfer.18,48,49 As already discussed in the literature,50,51 the interaction strength can be understood from the mixing of the metal orbitals of the ns and (n−1)d shells. For the isolated atoms, the energy difference between the singly occupied ns and the fully occupied degenerated (n−1)d atomic orbitals of Cu (1.27 eV) and Au (1.63 eV) is much lower than that for silver (3.56 eV). This indicates that a low mixing should be expected for Ag on the MgO(001) support, correlating with a

Figure 2. Energetically most favorable structure of CO adsorbed on a single metal atom (Cu left, Ag center, Au right) atop a clean MgO(100) surface slab. C


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Figure 3. Energetically most favorable equilibrium structure of O2 adsorbed on a single metal atom (Cu left, Ag center, Au right) atop a clean MgO(100) surface slab.

Cu (0.65 eV) and Au (0.49 eV) with a moderate adsorption strength and weakly on Ag (0.18 eV). These adsorption energies and the associated metal−CO bond distances correlate well with the variation of the covalent radius of the respective metal support (1.32, 1.45, and 1.36 Å for Cu, Ag, and Au, respectively61). The strong red-shift of the CO stretch frequency even in the weakly bound Ag/MgO(001) case (vs 2169.8 cm−1 for CO in the gas phase56) is frequently reported in experimental54 and theoretical60 publications. This is due to the back-donation of an electron in the π* orbital of the CO molecule. It indicates that an initially negatively charged metal atom can serve as an electron donating group to stabilize the metal−CO bond. Generally, O2 is more strongly bound to the metal atoms, although adsorbing at about the same distance from the impurity. This is in stark contrast with the gas phase compounds of the neutral M−O2 type, where they are found to be weakly adsorbed for Ag and Au (0.13 and 0.02 eV, respectively) or metastable (−0.07 eV for Cu). The large frequency red-shift and bond elongation with respect to the gas phase values (ν = 1580.2 cm−157 and re = 1.208 Å62) show that the adsorbed molecule is closer to the anionic species (ν = 1145 cm−1 for O2−63). This indicates an electron transfer from the metal adsorbed on MgO(100) to the O2 adsorbate, as is the case for CO but to a lesser extent. The net electron shift toward the adsorbate is confirmed by both the natural and Bader charges, as well as the difference density maps. The bottom panels in Figure 4 show the atomic charges in the minimal energy configurations for the different systems. To help correlate this information with the structural and thermodynamical properties of the different systems, the adsorption energies (Eads) of CO and O2 on the three metal atoms and characteristic bond distances (d(M−CO)) are also depicted. While all three types of atomic charges yield a positive charge on the metal atom, the Mulliken charges of the metal atoms are found to be higher in most cases (with the exception of O2@Cu/Mg(001)). Mulliken charges also yield a bond polarization that is weaker than the natural and Bader charges for O2 and CO. In the latter case, the sign of the bond polarization is even reversed. These differences cast doubt on the usefulness of Mulliken charges to understand the binding situation in such systems. All three types of atomic charges similarly find a negative charge on O2, which confers a strong ionic character to the M−O2 bond. An important bond polarization in the molecule is observed upon adsorption on all three metal atoms, which is predicted by the natural charge analysis to be smaller than that of the density-based Bader analysis. Despite the inclusion of diffuse functions to describe the oxygen atoms of the substrate, the latter could be a

the atom. O2 is adsorbed more centrally but strongly bent away from the center. On all three metallic impurities, O2 is found with one oxygen atom bound to the metal center. The energetically favorable electronic doublet state results from the initial triplet state of O2 and the doublet state of the metal center. While O2 lies almost parallel to the surface atop the copper atom, adsorption on the Ag and Au atoms yields a slightly bent structure. The conformations where the CO and O2 adsorbates point toward the Mg2+ or O2− surface ion have a similar energy. This is due to the large distance between the adsorbates and the MgO(100) surface. On all substrates, CO and O2 are preferentially oriented toward the Mg2+ rather than toward the O2− surface ion. For CO, the energetic stabilization amounts to 3, 1, and 1 meV on Cu/MgO(001), Ag/ MgO(001), and Au/MgO(001), respectively. For O2, the total energy rises by 6, 3, and 3 meV, respectively. In all cases, the energetically unfavorable orientation corresponds to the transition state for the rotational motion of the adsorbate. Within the uncertainty of the current computation setup, the rotation of the adsorbed moieties on top of the metal atoms can thus be considered barrier-less. Important energetic and structural parameters for the molecular adsorption behavior on top of the different metal atoms are summarized in Table 1, along with internal vibrational frequency shifts upon adsorption. CO adsorbs on Table 1. Counterpoise Corrected Adsorption Energies Eads of CO and O2 on the Respective Optimized Surface, as Well as Characteristic Structural Parameters (X = C, O) and the XO Stretch Frequency Difference Δν(XO) from the Neutrally Charged Gas Phase Value (CO: 2169.8 cm−1;56 O2: 1580.2 cm−157)a Eads (eV)

Δν(XO) (cm−1)

EBSSE (eV)

d(X−O) (Å)

d(M−X) (Å)

0.07

1.170 (1.2058) 1.160 (1.1658) 1.169

1.874 (1.8358) 2.180 (2.4158) 2.044

−337

(1.20,59 1.18260)

(2.05,59 2.03860)

(−299,60 − 31854)

0.09

1.337

1.872

−466

0.07

1.319

2.090

−455

0.09

1.310

2.062

−450

CO Cu@ 0.65 MgO (0.8458) Ag@ 0.18 MgO (0.0758) Au@ 0.49 MgO (0.78,59 0.5160)

0.04 0.06

−328 −315

O2 Cu@ 1.70 MgO Ag@ 0.91 MgO Au@ 0.97 MgO a

Available literature values appear in parentheses. D


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Figure 4. Structural and thermodynamical properties for a metal−XO (X = C, O) adsorbed on a defect-free MgO(001) surface. Top left panel: metal−CO bond distance; top right panel: O2 bond distance; central panels: XO adsorption energies. In the bottom panels, the Mulliken (left column, dotted line), natural (center column, dash−dotted line), and Bader charges (right column, dashed line) are given for the metal atom (◇), metal bound X atom (▽), and terminal O (△) atom.

Figure 5. Electron density difference map (isodensity of ±0.005 au) of the energetically most favorable structure for CO adsorbed on a single metal atom (Cu left, Ag center, Au right) atop a clean MgO(100) surface. Blue denotes an increased electron density in the equilibrium structure compared to that in the fragments, while red shows the decreased electron density.

Figure 6. Electron density difference map (isodensity of ±0.005 au) of the energetically most favorable structure of O2 adsorbed on a single metal atom (Cu left, Ag center, Au right) atop a clean MgO(100) surface. Blue denotes an increased electron density in the equilibrium structure compared to that of the fragments, while red shows the decreased electron density.

consequence of an insufficient polarization of the cluster used in the present simulations, as investigated below.

The electron density difference maps, presented in Figure 5 for CO and in Figure 6 for O2, show the grid-based difference E


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Figure 7. Energy decomposition analysis for the CO and O2 molecules adsorbed on M/MgO(001) (M = Cu, Ag, Au). The adsorption energy Eads is split into its attractive contributions (top bar chart), including electrostatic interaction ΔEelstat, orbital relaxation ΔEorb, and electron correlation ΔEcorr, as well as into its repulsive contributions (bottom bar chart), including the exchange−repulsion energy ΔEexrep, the counterpoise correction term ΔECP, and the preparation energy for the fragment deformation ΔEprep. The lowest bar chart indicates the color code for the respective energy contributions.

counterpoise correction (ΔECP) and the preparation energy of the fragments (ΔEprep = Eint − Eads), are shown in Figure 7. CO is found to interact moderately with the deposited Cu and Au atoms, while its interaction with Ag/MgO(001) is rather weak. The different ratios of the electrostatic ΔEelstat and orbital relaxation ΔEorb contributions indicate a stronger ionic bond character of the OC−Cu/MgO(001) interaction, while CO on Au/MgO(001) is bonded more covalently. For the Ag/ MgO(001) system, the contributions are much smaller than those of the two other metals. This adsorption behavior correlates with the covalent radii of the metal atoms and the distance of the adsorbate from the metal center. Since orbital relaxation energy strongly depends on the MO overlap of the different fragments, the larger metal−CO distance in the silver case results in a smaller energy gain from the contribution of ΔEorb. The larger atomic radius of Ag (1.45 Å)61 compared to that of Cu (1.32 Å) and Au (1.36 Å) also implies a softer, more diffuse bond between the MOs of the metal and the carbon atom and a smaller MO overlap. To test this hypothesis, we artificially reduced the Ag−CO bond distance to that of the Au/MgO(001) system. A significant orbital relaxation energy gain was observed but still did not reach the magnitude of the other two systems. This is in line with the picture offered by the electron difference maps, which predict a stronger hybridization for this type of metal center. Therefore, a slightly more covalent bond is observed. O2 is more strongly adsorbed on the metal atom than CO mainly because of the reduced bond distance, which increases the relative importance of the orbital overlap contributions. This confers a more covalent character to the bond. The singly occupied MO of the metal center has a strong ns character and forms a σ-bond to the π* orbital perpendicular to the surface. The overlap is thus maximized by the angled conformation between the metal and O2 molecule at the equilibrium geometry. The remaining π* orbital forms π-bonds with the energetically lower lying MOs of (n−1)d character. One further reason for the stronger interaction of O2 with the metal centers might be the reduced Pauli repulsion due to the spin state of the system. Indeed, one of the electrons in the two π* SOMOs of O2 oriented perpendicularly to the O2 bond axis and the ns1 electron of the respective supported metal atom are in different spin states and do not contribute to the Pauli repulsion energy. As a result the O2 adsorbate has an exchange−repulsion energy

of the electron density at the equilibrium geometry with that of the unrelaxed fragments (MgO(001) + metal atom + adsorbate). The points are distributed on a Cartesian grid with an equidistant grid spacing of 0.045 Å. All density difference maps reveal a strong polarization of the O2− ion located below the metal atom upon adsorption of the molecule. Another similarity is the increased electron density in the antibonding π-orbitals of the CO or O2 adsorbates (blue contours) resulting from the electron back-donation of the metal atoms. The major distinctions in the character of the bonds are found in the area of the metal atom, where the Cu/ MgO(001) and Ag/MgO(001) systems show a clear decrease in electron density. The Au/MgO(001) systems, however, show an additional polarization similar to that of the surface O2− ion. From all the metal atoms studied, the lowest atomic charges on the metal atoms are predicted for the adsorption of CO and O2 on Au/MgO(001). While the O2@Au/MgO(001) system shows an electron shift away from the metal−O bond to the uncoordinated sites of the Au atom, the CO@Au/ MgO(001) system shows an increased electron density in the area between the metal atom and the negatively charged O2− surface ion. In general, it can be observed that adsorption on Au/MgO(001) involves a stronger hybridization of the molecule with the surface, which also has strong repercussions on the surface−metal bond. This is particularly marked for the oxygen molecule, and it is bound to affect the chemistry on the metal center. Energy decomposition analysis (EDA) can be used to provide a more detailed picture of the adsorption behavior of CO and O2 on the different supported metal atoms. EDA separates the uncorrected interaction energies Eint (without regarding fragment deformation) into their electrostatic (quasiclassical Coulomb) interaction ΔEelstat, exchange−repulsion ΔEexrep, orbital relaxation ΔEorb, and electron correlation ΔEcorr contributions. Note that the TURBOMOLE implementation of the EDA relies on the resolution-of-identity approximation, leading to total energies that are slightly different compared to those from the computational setup used in the rest of the paper. The energy differences remain unaffected at a chosen level of accuracy. Further details and deviations about the energy contributions can be found in refs 44 and 64. The different contributions to the interaction energy, as well as the F


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Figure 8. Different conformers for the coadsorbed CO and O2 on Cu@MgO. The total energy ΔE and activation barriers for the conformer conversion (italic) are given in electronvolts. The peaks in the IR spectra belong to calculated vibrational modes of the respective conformers.

Figure 9. Different conformers for the coadsorbed CO and O2 on Ag@MgO. Same key as in Figure 8.

lower than that of CO for Cu and Au. For the Ag/MgO(001) system, the strongly decreased bond distance from Ag−CO to Ag−O2 may also be responsible for the absolute increase of the Pauli repulsion contribution, despite this favorable spin configuration. In all cases, the energy gained from orbital relaxation confers a more covalent character to the metal− molecule bonds, which drastically stabilizes the systems. Mixed Coadsorption. In this work, we investigate the potential for the catalytic CO oxidation on supported metal atoms via a Langmuir−Hinshelwood mechanism. To this end, both reactants must be adsorbed simultaneously on the surface system before the O2 bond breaking occurs. Alternative mechanisms of the Eley−Rideal type have revealed activation barriers on the order of 1 eV and will not be investigated

further. Considering the much larger affinity of the metal atoms toward oxygen, the O2 molecule is first placed in its equilibrium position on the various M/MgO(001) systems, and the CO molecule is subsequently brought in from different orientations, either from the gas phase or along the surface. The resulting mixed coadsorbed structure (labeled A) obtained upon geometry optimization is depicted in Figures 8, 9, and 10 for the three supported metal atoms. Further, the energy relative to the global minimal structure, the activation barriers for the transformation between the conformers, and the vibrational frequencies with their predicted IR intensities are also reported. In conformer A, the O2 is adsorbed on top of the metal atom and CO is trapped in a local physisorption minimum on a neighboring Mg2+ surface ion. The weak interaction of CO on G


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Figure 10. Different conformers for the coadsorbed CO and O2 on Au@MgO. Same key as in Figure 8.

leading to conformers A, but conformers B remain accessible from the latter by low energy barriers, as will be discussed below. From conformer B, it is possible to define two similar seeds for structure optimization by moving the two molecules on either side of the metal centers (conformer D) and by further rotating the O2 away from the CO molecule (conformer C). Subsequent geometry optimization reveals a drastically different coadsorption behavior on the three metal atoms. Compared to conformers C and D, the total energy of conformers B (where both adsorbates are parallel and on the same side of the metal atom) is energetically unfavorable at the Cu center and energetically equivalent on the Ag system. This conformation even becomes the most favorable by a fair margin (0.32 eV vs conformer C) for the adsorption on Au/MgO(001). This can be explained on the basis of the decreasing interaction strength of O2 from Cu/MgO(001) to Ag/MgO(001) to Au/ MgO(001), combined with an increase of the metal−O2 distance. This trend is opposite that of the relation between the metal adatom covalent radius and the CO and O2 adsorption strengths discussed above, which points at an important cooperative effect in the coadsorption scenario. For coadsorption on Ag/MgO(001), conformers B, C, and D are found to be energetically equivalent within the uncertainty of the present methodology. Interestingly, the adsorption sites of O2 in conformers C and D are stable only due to the presence of the nearby CO molecule. After removal of the CO moiety and reoptimization of the structure, the O2 adsorbate is found to invariably relax back to the equilibrium structure. Such a cooperative effect, where CO (O2) enhances the adsorption of O2 (CO), was already reported for negatively charged gold clusters in the gas phase.65 An EDA of the interaction energy of O2 in conformers C and D with and without the CO coadsorbate was done to evaluate the importance of the cooperative effect. In all cases, the CO coadsorbate leads to a stronger covalent character between the metal atom and O2, as indicated by a larger proportion of the orbital relaxation energy in the attractive

the metal adatom has only marginal influence on the metal−O2 interaction, which stays comparable to the pure O2 adsorption. This structure is also the least energetically stable local minimum of those reported in the present work for all three metal centers. The weak adsorption of CO in conformer A suggests that only a marginal influence will be observed on the adsorption state of O2. On the other hand, the adsorption energy of O2 shows a large decrease compared to the monoadsorbed system. The dominant contribution is due to fragment deformation in the CO molecule. Because of the large impact of fragment deformation on the adsorption energies, it appears preferable to use the interaction energies to compare the adsorption strength of CO or O2 between the coadsorbed and the respective monoadsorption cases. The interaction energy allows us to separate the contributions of fragment deformation from electronic effects. Isolating the interaction energy yields a more meaningful comparison of the importance of electronic effects between the different bonding situations. For example, the impact of CO coadsorption on the state of O2 in conformer A results only in a slight decrease of the interaction energy compared to the molecular adsorption. In all cases, the large distance between the O2 and CO molecules would effectively prevent direct reaction, rendering this structure somewhat uninteresting from a catalytic perspective. Alternatively, the structure optimization can be initiated by first adsorbing CO in its equilibrium geometry on top of the metal centers and by having an O2 molecule approach from the gas phase or along the surface. Subsequent geometry optimization leads to the conformers labeled B in Figures 8−10, which are 0.16, 0.26, and 0.58 eV more stable than conformers A for Cu, Ag, and Au, respectively. Similar but energetically less favorable conformers B′ (+0.25, +0.17, and +0.08 eV relative to conformers B for Cu, Ag, and Au, respectively), where O2 is rotated around the OC−O2 bond and points away from the surface, were not investigated further in the present work. Due to the larger affinity of the metal center for the oxygen molecule than for CO, this initial condition is likely to be statistically less represented than those H


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The Journal of Physical Chemistry A contributions. As a direct consequence, the cooperative effect is also beneficial for the CO molecule. The reaction barriers between all conformations presented in Figures 8−10 have been calculated by NEB calculations using five images between the respective conformers. In the cases where the reaction path was not smooth, further NEB calculations using five images between the images surrounding the transition state were performed. In most cases, the converged transition state of the smoother NEB calculation does not differ qualitatively from the coarser NEB calculations, as often observed when using the climbing image algorithm.46 The resulting activation energies are given on the reaction arrows associated with the different transformations. The lowest activation barriers are observed between the conformers of the Cu/MgO(001) coadsorbed system. It can be expected that, even at low temperatures, there will be little kinetic hindrance for the system to relax toward the energetically most favorable conformations. On the Ag/MgO(001) surface, conformers B, C, and D are close in energy and the activation barriers for the conformational changes are similar to or higher than the diffusion barrier of a free Ag atom on the MgO(001) surface (0.14 eV). Hence, it is likely that agglomeration of Ag atoms to form larger silver clusters will compete with possible conformational transformations of the adsorbates. The weak binding situation and the very flat potential energy landscape on Ag/MgO(001) will ultimately nullify its catalytic significance. For the Au/ MgO(001) system, conformer B appears to be the energetically most favorable. The relatively high activation barriers from conformers A and C (via D) to conformer B (0.31 and 0.32 eV, respectively) are similar to the diffusion barrier of a free Au atom on the MgO(100) surface (0.27 eV). As for the Ag/ MgO(001) case, this might hinder transformation between the conformers in practice, also leading to a decrease in the catalytic activity. In the IR spectra reported for the various conformers in Figures 8−10, the black and blue spectral lines denote the internal stretch frequency of CO and O2, respectively. The red colored spectral lines below 600 cm−1 correspond to vibrations of the adsorbate and of the nine nearest MgO surface ions. As the mass of the metal center increases from copper to gold, the vibrational density of states in this region of the spectrum becomes further dominated by higher vibrational frequency modes between 500 and 600 cm−1 and the lower energy components disappear. For all conformations and metal atoms, the frequency of the O2 molecule (blue) remains almost unchanged. This is in stark contrast with the frequency of the internal CO stretch (black), which is increasingly red-shifted from Cu to Ag and Au but only in conformation B. This indicates a stronger degree of electron back-donation in the π* orbital of the CO molecule. This correlates with the increase in the OC−O2 stretch frequency, depicted as a green line at the conformation of B (between 600 and 900 cm−1). The OC−O2 stretch vibration describes the proximity of the activated reagents, i.e., the reaction coordinate along which the catalytic formation of CO2 would proceed. Experimentally, the infrared spectrum of coadsorbed CO/O2 on a single Au atom in the gas phase has also shown a vibrational band in this range (850 cm−166 and 842 cm−111). The mode was first predicted as a specific OC−O2 vibration in a conformation similar to that of conformer B of the coadsorbed species on Au/MgO(001).66 This hypothesis was later confirmed by a joint experimental and theoretical investigation.11

To shed light on the catalytic potential of the different systems, NEB calculations for the desorption of CO2 were performed for conformer B, yielding reaction barriers of 0.65, 0.72, and 1.19 eV for the Cu, Ag, and Au systems, respectively. These high activation energies compared to the diffusion barrier of the metal atoms on the surface will render this catalytic oxidation reaction thermally unaccessible at the expense of metal agglomeration. On the other hand, the wellisolated frequency of the OC−O2 mode in conformation B could potentially be excited by an infrared light source, thereby activating directly and specifically the reaction coordinate. In view of the thermodynamical properties of the mixed coadsorbed states on the three metal centers and the associated activation barriers for the internal transformations, this strategy would appear viable only on the Au/MgO(001) system. Although a similarly favorable situation is found for the Ag/ MgO(001) surface, the presence of (at least) two other species at the same energy and the low barrier for Ag agglomeration would reduce the efficiency of such a photocatalytic reaction.

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CONCLUSION In conclusion, we have presented first-principle simulations of the interaction between CO and O2 with metal atoms of group 11 deposited on a defect-free magnesium oxide surface. The investigations were performed using density functional theory using a hybrid functional with a periodic point charge embedding scheme to simulate the electrostatic effects of an infinite ionic surface on the reactive center at a low dilution limit. The simulations revealed that initial adsorption of the metal atom on the MgO(001) surface leaves the reaction center almost neutral. The weak adsorption energy for the Ag atom is comparable with the diffusion barrier, and agglomeration into silver clusters would be impossible to prevent. On all three metal centers, O2 adsorbs much more strongly than CO, in an atop configuration, where the two atoms of the molecule have a similar negative charge. The adsorption of CO leads to a strong polarization of the CO bond, where the carbon atom is almost neutral and the oxygen atom carries a large negative charge. The metal−adsorbate bond is seen to be dominated by an electron back-donation from the metal in the π* orbitals of the molecules. Polarization of the substrate oxygen atom below the metal center is responsible for the stabilization of the positively charged metal impurity in both monoadsorbed cases. From electron difference densities and energy decomposition analysis, the adsorption is seen to involve a larger degree of hybridization among the adsorbate, the surface, and the metal center for the Au/MgO(001) substrate. A few representative structures for the mixed coadsorption of CO and O2 on the different metal atoms have been presented, with structure optimization initiated with different rational guesses to sample the energy landscape for the catalytic CO oxidation following a Langmuir−Hinshelwood mechanism. A multitude of structures interconnected by small barriers were found on all metal supports. For the Cu/MgO(001) system, conformations where the two reagents are found on opposite sides of the metal center appear to be favored, such that the distance between the reagents would prevent further reaction. The Ag/MgO(001) support has a largely flat energy landscape with barriers larger than the metal diffusion barrier. Due to its weak interaction with the surface and lack of favorable potential topology, the supported silver system also appears to have limited potential for catalysis. Finally, the Au/MgO(001) system favors the coadsorption of CO and O2 in a I


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conformation where the reagents are in a reactive orientation. The somewhat higher interconversion activation energies, comparable to the gold atom diffusion barrier, would nonetheless prevent CO oxidation under thermal conditions. On the other hand, the proximity of the two adsorbates, their advantageous orientation, and the well-isolated frequency of the OC−O2 vibrational mode could pave the way to single-atom photocatalysis via the selective infrared activation of the reaction coordinate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Deutsche Forschungsgemeinschaft through Project TR 1109/2-1. REFERENCES

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First-Principle Investigations of the Interaction between CO, O2, and

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