Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and

Article pubs.acs.org/JPCC

Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and Dependence on Oxide, Adsorbate Pair, and Density Functional Maxime Van den Bossche* and Henrik Grönbeck* Department of Physics and Competence Centre for Catalysis, Chalmers University of Technology, 412 58 Göteborg, Sweden S Supporting Information *

ABSTRACT: Open-shell molecules on metal oxide surfaces frequently display cooperative adsorption mechanisms, where pairs of adsorbates are significantly more stable than the isolated species. In this work, density functional theory is used to investigate the cooperative adsorption of OHH and NO2NO on rocksalt BaO(100), rutile TiO2(110), fluorite CeO2(111), and tetragonal PdO(101) surfaces. The OH and NO2 adsorbates are considered to be located at the metal sites, whereas H and NO are situated on the oxygen sites. Despite differences in adsorption mechanisms, the pairing is found to be consistently exothermic. The pairing is most pronounced on BaO(100) and weakest on PdO(101). OH and H form more stable pairs than do NO2 and NO. In all cases except CeO2(111), the hybrid HSE06 functional predicts a stronger pairing than the semilocal PBE functional. The absolute pairing energy is a convoluted measure of charge transfer energies, electrostatic interactions, and ionic relaxations, which is analyzed in detail using a thermodynamic cycle. Adsorbate pairing has marked effects on the reactivity, which is exemplified by studying the reaction of CO with OH on CeO2(111).

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INTRODUCTION

pairing, this entity donates electronic charge to the reductant that is formed by NO2 adsorption at cationic sites. The focus of the present work lies in the description of OHH and NO2NO pairing on four different metal oxide surfaces: rocksalt BaO(100), fluorite CeO2(111), rutile TiO2(110), and tetragonal PdO(101). BaO is chosen for its application as a NOx storage material16 and to represent the class of alkaline earth metal oxides, which have irreducible character. The three other oxides are reducible transition metal oxides with PdO being an important oxidation catalyst17 and CeO2 and TiO2 being widely used as catalyst support materials, while also possessing catalytic activity of their own. Additionally, the density functional dependence of the calculated pairing energy is investigated. As the adsorption of open-shell molecules often induces electronic defects in the oxide surface, considerable differences between (semi)local and higher rung exchange-correlation functionals can be expected.18 Here, results from a semilocal functional and a screened hybrid functional is compared. It is furthermore clear that the pairing affects the thermodynamics and kinetics of surface reactions. This is demonstrated using the CO + OH reaction on CeO2(111) as an example.

Understanding chemisorption on metal oxide surfaces is central to a host of scientific and technological applications, such as corrosion science, electrochemistry, gas sensing, and heterogeneous catalysis. Within catalysis, many oxidation catalysts are based on transition metals that are oxidized under operating conditions.1 The oxide surface chemistry is also important in adsorbate spillover phenomena when oxides are used as catalyst support materials.2 Metal oxides are furthermore used as storage materials, where ceria is a prototypical example used for oxygen buffering in automotive exhaust aftertreatment.3 One important aspect of chemisorption on metal oxide surfaces is the so-called “cooperative adsorption”, where specific adsorbate pairs display much higher stability as compared to the isolated species. This effect has previously been discussed mainly in theoretical studies of NOx adsorption on alkaline earth oxide surfaces.4−12 The theoretical predictions have later been confirmed in experimental studies on NO2 adsorption on BaO.13 Metiu et al.14 have discussed the cooperative adsorption in several other surface−adsorbate combinations, such as dissociated halogen hydrides on CeO2, adsorbed hydrogen and Au clusters on TiO2, and dissociated CH4 on ZrO2. Cooperative adsorption of, for example, CH3H and OHH pairs has also been found to be important in the reaction kinetics of methane oxidation over PdO(101).15 Cooperative adsorption is characterized by charge transfer between the adsorption sites, similar to redox reactions. For example, in the case of NO2NO adsorption on BaO(100), an oxidizing agent is produced upon NO adsorption at an anionic site. Upon © XXXX American Chemical Society

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COMPUTATIONAL METHOD Density functional theory (DFT) calculations are carried out using the plane-wave projector augmented-wave (PAW) code Received: December 20, 2016 Revised: March 22, 2017 Published: March 24, 2017 A

DOI: 10.1021/acs.jpcc.6b12789 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Information on Bulk Structures and Employed Surface Models

a

bulk crystal

lattice const.a (Å)

surface orientation

rocksalt BaO fluorite CeO2 rutile TiO2 tetragonal PdO

5.63 5.47 4.65; 2.97 3.10; 5.43

(100) (111) (110) (101)

surface unit cell

k-point mesh

stoichiometry per layer

× × × ×

3×3×1 3×3×1 3×3×1 4×4×1

BaO CeO2 TiO2 PdO

p(2 p(2 p(2 p(2

2) 2) 1) 1)

number of layers (fixed) 3 4 4 4

(1) (2) (2) (1)

Lattice constants calculated using the PBE functional.

Figure 1. Top views of the studied oxide surfaces. Black lines indicate the employed surface unit cells. The considered anionic (cationic) adsorption sites are denoted with stars (circles). The atomic color coding is Ba (green), Ce (yellow), O (red), Ti (blue-gray), and Pd (gray). The VESTA software22 was used for rendering the structural models.

VASP19−21 with a kinetic energy cutoff of 450 eV. PAW potentials are applied for all elements with the semicore states in Ba and Ti included in the valence. Hence, the different elements are treated with the following valences: Ba (10), Ce (12), C (4), H (1), N (5), O (6), Pd (10), Ti (12). The calculations are carried out in a spin polarized manner whenever unpaired spins are expected (i.e., the gas phase molecules and the isolated adsorbates). Additional information regarding the computational setup for the slab calculations is given in Table 1. Structural models of the surfaces are shown in Figure 1. Volume relaxations are performed with a higher kinetic energy cutoff, namely 700 eV. Local optimizations are considered to be converged when all forces are below 0.05 eV/ Å. For the semilocal density functional calculations, the functional by Perdew, Burke, and Ernzerhof (PBE)23 is applied. Because of the electronic self-interaction error (SIE) in semilocal functionals spurious delocalization of electronic states may take place. For metal oxides, this has repeatedly been demonstrated in the description of localized Ti3+ and Ce3+ states in TiO2 and CeO2, respectively.18 More realistic descriptions of the electronic structure are typically obtained using DFT+U or hybrid Hartree−Fock density functional methods.18 Despite the larger computational cost, the latter approach is chosen here, as hybrid functionals are able to reduce the SIE without introducing Hubbard U parameters. The hybrid DFT calculations are carried out using the functional suggested by Heyd, Scuseria, and Ernzerhof (HSE06).24−26 HSE06 uses the PBE expression for the correlation energy and a 25% admixture of Fock exchange into the PBE exchange energy. Without loss of accuracy, the exchange potential is furthermore screened using error functions to lower the computational cost for extended systems. We find that reoptimization using the hybrid functional is required (see Table S2 in the Supporting Information). The description of the electronic structure and the relaxed geometry are heavily intertwined, which is manifested in lattice distortions that accompany the localization of defect states. In

case the defect states are delocalized when using the PBE functional, the hybrid functional relaxations are started after prerelaxing the geometry using the PBE+U approach with a U value of 5 eV. To limit the computational cost, the HSE06 energies (using the full k-point mesh) are evaluated at the geometries obtained using a reduced k-point grid for the Fock operator, comprising only the Γ-point. It should be noted that the PBE lattice constants have been used throughout the calculations. The reported adsorption energies ΔEads are calculated as the electronic energy difference between the adsorbate-covered surface and the bare surface with the adsorbate in the gas phase. The pairing energies ΔEpair, which reflect the stabilization by cooperative adsorption, are computed as the energy differences between the paired and unpaired adsorbates, according to ΔEpair,A − B = (EA + B + slab + Eslab) − (EA + slab + E B + slab) (1)

The pairing energies depend on the considered adsorption configurations for the isolated adsorbates and the adsorbate pair. The values reported here pertain to OH and NO2 coordinated to metal cations, whereas H and NO to are adsorbed at oxygen anions. In addition to the electronic structure calculations, electrostatic modeling of adsorbate−adsorbate interactions has been performed to aid the analysis of the calculated pairing energies. In these calculations, electrostatic energies are computed based on the Poisson equation using a simplified representation of the adsorbate charge distributions and the dielectric screening by the oxide surface. The employed scheme is based on the methods by Pasquarello et al.27,28 and Freysoldt et al.29−31 and is further described in the Supporting Information.

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RESULTS AND DISCUSSION The HSE06 geometries of the paired adsorbates are displayed in Figure 2 and the calculated adsorption energies of the isolated adsorbates and stabilization upon pairing are shown in Figure 3 (numerical values are listed in Table S1 of the B


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The Journal of Physical Chemistry C

Figure 2. Top views of geometries of the paired adsorbates, after ionic relaxation using the HSE06 functional. The atomic color coding is as in Figure 1 and with H in white and N in purple.

endothermic with respect to physisorption. On BaO(100), charge transfer only occurs between the H and NO adsorbate and the single O atom to which they are bound. The electronic configuration of these adsorbates can be described as (Os− H)2− and (Os−NO)2−. As for OH and NO2, several differences are apparent between the functionals. PBE does not yield chemisorbed NO on PdO(101) as a local minimum on the potential energy

Supporting Information). The unpaired adsorbates adopt geometries that are qualitatively similar to those of the paired species. Aside from the previously mentioned distortions of the metal oxide lattice, the PBE geometries are approximately similar to the ones obtained using HSE06. Here we first describe the adsorption of the isolated adsorbates and the stabilization upon pairing. The influence on the reactivity is discussed by taking CO + OH in CeO2 as an example and the section is closed by a general comparison between the two functionals. Single Molecule Adsorption. OH and NO2 Adsorption. For OH and NO2 the most stable configurations on the cationic adsorption sites involve coordination of the O atoms to one or two metal ions. Using the HSE06 functional, these adsorbates are either strongly bound as OH− and NO2− anions (BaO, PdO) or weaker bound in a radical-like electronic state (CeO2, TiO2). Adsorption in an anionic configuration is accompanied by the formation of an O 2p hole localized on a nearby surface oxygen atom (BaO), or a hole in the Pd 4d band (PdO). Two important differences arise when applying the PBE functional instead of HSE06. First, the O 2p holes in the BaO(100) surface become delocalized over several O atoms. On the TiO2(110) surface, the charge transfer from the metal oxide to the adsorbed OH increases with PBE, leading to an electronic configuration characterized as adsorbed OH− and a delocalized hole in the O 2p band. Bonding of both OH and NO2 to the metal oxide surfaces is significantly stronger with PBE, as compared to HSE06. H and NO Adsorption. The preferred adsorption configurations of H and NO on anionic sites also vary across the different oxide surfaces. On the reducible oxides, surface hydroxides (Os−H)− and nitrites (Os−NO)− are formed, involving transfer of one electron to the metal d (or f) states. On TiO2(110), however, NO adsorption to the anionic site is of physisorbed character without significant charge transfer between the adsorbate and the surface. NO can, however, also adsorb to the anionic site in a nitrite configuration on TiO2(110) (with formation of a Ti3+ ion) but this is

Figure 3. Calculated adsorption and pairing energies. Color coding: green for BaO(100), yellow for CeO2(111), blue for TiO2(110), and gray for PdO(101). PBE and HSE06 results are colored in lighter and darker shades, respectively. C


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The Journal of Physical Chemistry C surface but instead predicts physisorbed NO with a long Os− NO distance of 1.8 Å. Furthermore, when charge is transferred to the metal d states upon adsorption, HSE06 predicts charge localization (with the formation of single Ti3+ and Ce3+ ions), whereas within PBE delocalization over several metal ions occurs. Finally, the adsorption strengths for H and NO are significantly decreased with the PBE functional, as compared to the HSE06 results. Cooperative Adsorption. The increase in total adsorption energy upon pairing is discussed in the following sections. After taking OHH pairing on CeO2(111) as an example, a thermodynamic cycle is applied to evaluate the different contributions to the pairing energies. The results from the analysis are subsequently utilized to discuss the variation of the stabilization as a function of adsorbate pair and oxide surface. Example: OHH Pairing on CeO2(111). The electronic and geometrical structure before and after the pairing of OH and H adsorbed on the CeO2(111) surface are displayed in Figure 4.

place. This oxide-mediated pairing restores the original oxidation state of the metal oxide surface. The adsorbates undergo significant geometrical changes as a consequence of the attractive electrostatic interactions. While this example is representative of adsorbate pairing on the surface of a reducible oxide, there are differences compared to irreducible oxides such as BaO. The main difference is the character of the defect charge in the case of unpaired H and NO on the anionic adsorption site Os. In the absence of reducible metal ions, the defect charge remains localized at the adsorption site and resembles the HOMO level of (Os−H)2− and (Os−NO)2−, respectively. Thermodynamic Cycle. In order to analyze the different contributions to the calculated pairing energies, a thermodynamic cycle is constructed, which is schematically represented in Figure 5. The cycle starts with the relaxed isolated adsorbates and ends with the relaxed adsorbate pair and a bare surface unit. The intermediate steps are devised to allow a separation between electronic effects and contributions arising from ionic relaxations. This scheme is advantageous in the sense that only additional single-point calculations are required, and that periodic boundary conditions can be used throughout. OH and NO2 are in this scheme denoted by “A”, whereas H and NO are denoted by “B”. The first step in the cycle is the addition (removal) of an electron to the structure with adsorbed A (B). The positions of the nuclei are kept unchanged in this step, hence the corresponding changes in the total energy contain contributions from the vertical electron affinities (EAs) and ionization potentials (IPs). The sum of the energy changes for adsorbed A and B is denoted the “charge transfer energy” ΔEct: ΔEct = (E A− − EA ) + (E B+ − E B)

(2)

In the next step of the cycle, the charge states are retained but the geometries are distorted so as to match the geometry adopted by the relaxed adsorbate pair. The sum of the total energy differences is denoted by ΔEdistort. We note that both the structure of the oxide lattice and of the adsorbates are changed in this step. In the third step, the energies of the isolated, charged adsorbates are compared to the energy of the relaxed adsorbate pair and the bare surface. In all structures, the ionic positions correspond to those of the matching atoms in the geometry of the relaxed adsorbate pair. As the charge transfer between the adsorbates has already been enforced, and no geometrical changes are introduced except for merging the adsorbates in the same unit cell, the change in total energy ΔEpair‑elec contains contributions from attractive electrostatic interactions between the two adsorbate configurations. This interpretation is supported by model calculations based on classical electrostatics, described in the Supporting Information. The final step in the cycle constitutes an additional geometrical relaxation ΔErelax, corresponding to the lowering of the total energy upon relaxation of the bare surface cell. Because of the use of charged supercells, the calculated ΔEct and ΔEpair‑elec energies depend on the size of the employed supercell. Only the sum of both terms is insensitive to the dimensions of the supercell and is meaningful in this context. To estimate the contribution from electrostatic adsorbate− adsorbate interactions to ΔEct + ΔEpair‑elec, the electrostatic interaction ΔENN‑elec within a nearest-neighbor adsorbate pair is evaluated using the electrostatic model described in the Supporting Information. The remainder (ΔEct + ΔEpair‑elec −

Figure 4. Left: HSE06 projected density of states (PDOS) on the cerium cations (orange), oxygen anions (red), the adsorbate atoms (green), and the adsorbate-induced defect states (blue, filled) for (co)adsorption of H and OH on CeO2(111). In the case of adsorbed H, the defect state consists of a next-nearest neighbor Ce3+ ion, whereas for adsorbed OH the hole state is shared between the adsorbate and a surface oxygen atom. The energy scales are calculated relative to the highest occupied state and the plots are shifted so as to align the valence bands. Right: structural models together with an isosurface of the HSE06 magnetization density (isolevel equal to 5 × 10−3 e−/Å3). Atomic color coding is as in Figure 2.

The projected density of states (PDOS), as well as the magnetization density isosurfaces, show the presence of electronic defects in the case of the isolated adsorbates. Upon H adsorption, charge is transferred from the adsorbate to a next-nearest Ce atom at the surface, which is reduced from the +4 to the +3 oxidation state. In the case of adsorbed OH, a partial charge transfer occurs from a nearby surface oxygen atom to the adsorbate. Upon placing the two adsorbates next to each other, charge transfer between the adsorption sites takes D


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Figure 5. (a) Schematic representation of the thermodynamic cycle for the analysis of the pairing energies. “M” and “O” represent cationic and anionic adsorption sites of the metal oxide surface, respectively. “A” denotes OH and NO2 adsorbates, whereas “B” denotes adsorbed H and NO. (b) Calculated energy differences for each step in the thermodynamic cycle. Color coding as in Figure 3.

ΔENN‑elec) can then be considered to mostly correspond to the (vertical) energy for realizing the transfer of charge between the isolated adsorbates. This cycle has similarities to the cycle employed by Schneider et al.4 One important difference is, however, the preservation of the periodic boundary conditions throughout the cycle. This allows for consistent usage of the same electronic structure code and computational setup, which avoids approximations and difficulties associated with embedded cluster calculations. Another difference is that the charge transfer takes place entirely in the adsorbed state, instead of proceeding via desorption of the adsorbates, followed by gas phase charge transfer and subsequent readsorption of the charged molecules. The current approach appears to be more meaningful, because there is usually little correspondence between the EAs and IPs for adsorbed species and those of the corresponding gas phase molecules. This is because the differences in electronic structure between the neutral and charged species in the gas phase is typically of very different character than on the surface. The bonding in NO, for example, changes to that of nitrosonium (NO+) upon ionization in the gas phase. When NO is bonded to a surface oxygen atom, however, the electronic configuration in the neutral state is closer to (Os−NO)− or (Os−NO)2− (see above). Ionization then leads to either reoxidation of the metal oxide or to a transition to (Os−NO)−. In other words, it is important to consider that the adsorbate pairing is oxide-mediated in the sense that the pairing is accompanied by healing of the

electronic defects in the oxide created by the adsorption of the unpaired adsorbates. Two limitations of the present cycle, however, are that the separation of the electrostatic contributions is approximate and that the charge transfer process is not divided into the two corresponding halfreactions. Comparing OHH and NO2NO. Comparing results obtained for the same oxide surface and with the same functional, the pairing energies are calculated to be more exothermic for OHH pairs than for NO2NO pairs (see Figure 3, bottom panel). The largest difference in exothermicity is calculated for BaO(100) (∼1.8 eV), whereas the smallest is found for PdO(101) (∼0.1 eV). Figure 5 shows that the terms in the thermodynamic cycle may contribute in opposite manners to the difference in the pairing energy for OHH and NO2NO. First, the ΔEct + ΔEpair‑elec term is found to be more exothermic for OHH as compared to NO2NO. The calculated ΔENN‑elec term shows to a large extent that this is because attractive electrostatic interactions between oppositely charged adsorbates are significantly more pronounced for OHH pairs in comparison to NO2NO pairs. This is directly related to the adsorbate sizes and adsorption configurations. The OHH pair allows for a much closer distance between positive and negative charge centers than for the NO2NO pair (see Figure 2). In addition, also a more exothermic charge transfer process between the isolated adsorbates is contributing to the total difference in ΔEct + E


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The Journal of Physical Chemistry C ΔEpair‑elec. Second, the distortion term (ΔEdistort) is typically more endothermic for OHH as compared to NO2NO, especially in the case of TiO2(110). On CeO2(111), however, the reverse is true. Finally, the ionic relaxation term (ΔErelax) is more exothermic for NO2NO pairs, except for the TiO2(110) surface. As a result, it can be concluded that the origin of the more exothermic OHH pairing energy is of electronic origin, that is, due to an electronically more advantageous charge transfer process (ΔEct + ΔEpair‑elec). The larger contribution of ionic relaxation to the total pairing energies in the case of NO2NO (ΔEdistort + ΔErelax), however, reduces the difference in the total pairing energies. This difference contains significant contributions both from relaxations of the oxide lattice and from changes in the adsorbate geometries upon pairing. As an illustration of the latter, ionic relaxation effects are calculated to contribute by −0.64 eV to the adiabatic EA of gas phase NO2 but only by −0.04 eV in the case of OH (using the PBE functional). We find that the attractive adsorbate−adsorbate interactions are of importance to the total pairing energy. This has been recognized previously,4 and is reminiscent of the role of the lattice energy in the Born−Haber cycle for the cohesive energy of ionic solids. An interesting implication is that the adsorbate pairing can be expected to become less exothermic as the A−B distance is increased. This has previously been observed for various adsorbates on the PdO(101) surface15 and is here also found to be significant for OHH pairing on the BaO(100) surface (see the Supporting Information). The loss of attractive adsorbate−adsorbate interactions, however, appears to be significantly compensated by ionic relaxation effects, which explains the lack of distance dependence previously found for NOx pairing on BaO(100).9 The final remark concerns the usefulness of the sum of the EA of gas phase A and the IP of gas phase B as a descriptor for the pairing energy of adsorbed A and B. Previous studies have argued that this energy difference needs to be sufficiently small for adsorbate pairing to occur.4,5 The present comparison between OHH and NO2NO pairs, however, shows that this criterion is not required. The EAs of NO2(g) and OH(g) amount to 2.2732 and 1.83 eV,33 respectively, and the IPs of NO(g) and H(g) to 9.2634 and 13.60 eV,35 respectively, wrongly suggesting that NO2NO pairing should be stronger than OHH pairing. Another counterexample is BaOterminated BaTiO3(100),10 where O2−NO pairing is calculated to be only slightly less exothermic than NO2NO pairing, despite the significantly lower EA of O2(g), namely 0.45 eV.36 One reason why the gas phase IP−EA difference has limited descriptive power is the previously mentioned qualitative differences between the pairing-induced changes in the electronic structure in the gas phase and adsorbed state, as well as electrostatic effects. Comparing the Different Oxides. For each adsorbate pair, using the same functional the exothermicity of the adsorbate pairing is generally found to be ordered as BaO(100) ≃ TiO2(110) > CeO2(111) > PdO(101) (see Figure 3, bottom panel). In contrast to the previous sections, the observed trend appears to be difficult to relate to particular stages of the thermodynamic cycle in Figure 5. This result is in fact not surprising, considering that the studied metal oxide surfaces show considerable variety in terms of crystal structure, metal− oxygen bond lengths, reducibility, ionicity, dielectric screening ability, and lattice distortion behavior.

One possible rule, however, is that the pairing effect can be expected to be small for conducting metal oxides. This is apparent in the results for PdO(101) using the PBE functional, where no Kohn−Sham gap is present. Ruthenium dioxide, which is a true conducting transition metal oxide, appears to follow the same behavior. The OHH and NO2NO pairing energies (calculated on a 3-layer (2 × 1)-RuO2(110) slab using the PBE functional) amount to −0.05 and 0.00 eV, respectively. Also in this case, it is likely that stabilization by attractive electrostatic interactions is counteracted by ionic distortion, as previously discussed in the context of the distance dependence of the pairing energy. Another example is that the cooperative adsorption of OHH pairs on MgO(100) is severely reduced when a thin MgO(100) slab is considered which is supported on a Ag(100) substrate.37 Influence on Reactivity. As cooperative adsorption represents a stabilization of the adsorbates, it can be expected that the reactivity of an adsorbed species changes depending on whether or not it is paired to another adsorbate. To illustrate the relationship between adsorbate pairing and reactivity, the associative reaction of an adsorbate “X” with another adsorbate “Z” is taken as an example, with X being either unpaired (“unp”) or paired (“p”) to a third adsorbate “Y”. The change in reactivity upon pairing can be analyzed following the cycle

Figure 6. Top panel: cycle showing the relationship between the reaction energies for a species Z with either paired or unpaired X. Bottom panel: energy diagram for the cycle applied to the reaction of CO and OH on CeO2(111) with energy differences calculated using the HSE06 functional.

shown in Figure 6 (top panel). The difference in the reaction energy is given by p unp ΔEreac − ΔEreac = ΔEpair,XZ − Y − ΔEpair,X − Y

(3)

In case the transition state is similar for the paired and the unpaired OH reactant, it can be expected that the difference in activation energy follows the difference in reaction energy, according to the Brønsted−Evans−Polanyi relation. A practical example of this type of process is the reaction of CO (= Z) with OH (= X) on a ceria surface, relevant to, that is, water−gas shift or preferential CO oxidation catalysis.38−40 F


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comparatively small. The minor variations of the ΔENN−elec term indicate that the electrostatic interactions between the adsorbates are similar in both functionals, despite the reduced screening ability of the studied oxide materials with HSE06 as compared to PBE (see the Supporting Information). The origin of the functional dependence of the ΔEct + ΔEpair‑elec term hence needs to be sought in the differences in EAs and IPs of the adsorbates. The remaining terms (ΔEdistort and ΔErelax), however, all show the opposite functional dependence: these terms are more exothermic with HSE06 as compared to PBE. The observed functional dependence hence mainly originates from the stronger contributions of ionic relaxations to the HSE06 pairing energies, as compared to PBE. Likely reasons for the importance of the ionic relaxations are the more pronounced distortions of the oxide lattice upon localization of the defect charge using HSE06, which are absent in the final, paired configuration.

When the carboxyl (COOH) reaction product is considered to be adsorbed on Ce sites, the change in reaction energy upon OH pairing with H adsorbed on an oxygen site is equal to the difference in the pairing energy of COOHH and OHH pairs. Using the HSE06 functional, the COOHH pairing is calculated to be significantly weaker than the OHH pairing (−0.42 versus −1.66 eV). The reaction of CO is consequently 1.24 eV more exothermic with unpaired OH than with OHH (reaction energies of −0.75 eV and +0.49 eV, respectively), as shown in Figure 6 (bottom panel). The COOH reaction product can also be considered to bind to an oxygen (Os) site (with the formation of bicarbonate and a Ce3+ ion) in which case the change in reaction energy upon OHH pairing will be similar to the OHH pairing energy itself. This is because minor stabilization is expected from pairing of Os−CO2H and Os−H species, as charge transfer from both species to the Ce 4f states will be preferable to charge transfer between the two adsorbates. This example shows that the formation and dissociation of adsorbate pairs can be crucial to understanding reactions on metal oxide surfaces (and possibly other semiconducting materials), as has previously been recognized, for example, for methane oxidation over PdO(101).15 In certain cases, such as processes limited by heterolytic dissociation reactions, altering the stabilization by adsorbate pairing may allow enhancement of the catalytic activity. In this respect, cooperative adsorption has similarities to the effects of aliovalent doping.41,42 Instead of charge transfer to or from another adsorbate, charge transfer takes place between the adsorbate and the dopant-induced electronic defect, provided that the electronic structure is not restored by charge compensation. Functional Dependence. The previously mentioned tendencies toward overdelocalization using the semilocal PBE functional are known to be directly related to the selfinteraction error (SIE), which is reduced upon partial admixture of Fock exchange. Regarding the observed differences in adsorption energies between the hybrid and the semilocal functional, several factors may be contributing, such as (1) localization of the defect state, (2) differences in the metal-to-oxygen charge transfer within the oxide, and (3) changes in the screening ability of the oxide. In addition, changes in the contribution from ionic relaxation can be expected as a consequence of the different descriptions of the bonding. This is particularly evident when the defect state is localized with HSE06 and delocalized with PBE, because such charge localization is accompanied by significant distortions of the oxide lattice. Reduction of a Ti4+ or Ce4+ ion to the 3+ state, for example, leads to elongation of the bonds to its oxygen neighbors. When comparing results obtained for each adsorbate pair on the same oxide surface, the pairing energies are found to be usually more exothermic by about 0.1−0.6 eV using the HSE06 functional as compared to the PBE functional (see Figure 3, bottom panel). One exception is the CeO2(111) surface, where both OHH and NO2NO pairing is calculated to be slightly less exothermic with HSE06. Similar to the previous section, each component of the thermodynamic cycle (see Figure 5) shows a functional dependence that is qualitatively similar across the oxides and adsorbate pairs, whereas different components can have opposite signs. The ΔEct + ΔEpair‑elec term, for example, is consistently less exothermic with HSE06 as compared to PBE, except for the PdO(101) surface where this term is

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CONCLUSIONS We have explored the effect of cooperative adsorption on a series of oxide surfaces relevant to catalytic applications for two different adsorbate pairs, using two different functionals. Our results illustrate that this phenomenon is general and is not restricted to adsorption of NOx species on alkaline earth oxides, which has been studied in detail previously. The analysis using the thermodynamic cycle indicates that both the intrinsic redox properties of the adsorbates, electrostatic interactions, and ionic relaxation effects are important to understand the total stabilization upon pairing. Additionally, the calculated pairing energy displays a sensitivity to the employed density functional, suggesting that this property may not be sufficiently accurately described at the level of semilocal DFT. The magnitude of the stabilization and its effect on the reactivity of adsorbates emphasize the importance of cooperative adsorption for understanding and modeling various chemical processes on oxide surfaces. The general nature of the phenomenon stresses the limitations in describing reaction kinetics on oxide surfaces using traditional mean-field models, which neglect spatial correlations in the adlayer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12789. (i) Tables with the adsorption and pairing energies and the effects of HSE06 relaxation, (ii) an illustration of the structures used in the different stages of the thermodynamic cycle, (iii) a description of model electrostatics calculations, and (iv) an analysis of the distance dependence (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Maxime Van den Bossche: 0000-0002-0938-0156 Henrik Grönbeck: 0000-0002-8709-2889 Notes

The authors declare no competing financial interest. G


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(19) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (20) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (21) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251− 14269. (22) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207−8215. (25) Heyd, J.; Scuseria, G. E. Efficient Hybrid Density Functional Calculations in Solids: Assessment of the Heyd-Scuseria-Ernzerhof Screened Coulomb Hybrid Functional. J. Chem. Phys. 2004, 121, 1187−1192. (26) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: Hybrid Functionals Based on a Screened Coulomb Potential [J. Chem. Phys. 118, 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906. (27) Giustino, F.; Pasquarello, A. Theory of Atomic-Scale Dielectric Permittivity at Insulator Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 144104. (28) Komsa, H.-P.; Pasquarello, A. Finite-Size Supercell Correction for Charged Defects at Surfaces and Interfaces. Phys. Rev. Lett. 2013, 110, 095505. (29) Freysoldt, C.; Neugebauer, J.; Van de Walle, C. G. Fully Ab Initio Finite-Size Corrections for Charged-Defect Supercell Calculations. Phys. Rev. Lett. 2009, 102, 016402. (30) Freysoldt, C.; Neugebauer, J.; Van de Walle, C. G. Electrostatic Interactions Between Charged Defects in Supercells. Phys. Status Solidi B 2011, 248, 1067−1076. (31) Freysoldt, C.; Grabowski, B.; Hickel, T.; Neugebauer, J.; Kresse, G.; Janotti, A.; Van de Walle, C. G. First-Principles Calculations for Point Defects in Solids. Rev. Mod. Phys. 2014, 86, 253−305. (32) Ervin, K. M.; Ho, J.; Lineberger, W. C. Ultraviolet Photoelectron Spectrum of Nitrite Anion. J. Phys. Chem. 1988, 92, 5405−5412. (33) Smith, J. R.; Kim, J. B.; Lineberger, W. C. High-Resolution Threshold Photodetachment Spectroscopy of OH−. Phys. Rev. A: At., Mol., Opt. Phys. 1997, 55, 2036−2043. (34) Reiser, G.; Habenicht, W.; Müller-Dethlefs, K.; Schlag, E. W. The Ionization Energy of Nitric Oxide. Chem. Phys. Lett. 1988, 152, 119−123. (35) Lide, D. R. Handbook of Chemistry and Physics, 81st ed.; CRC Press, 2000. (36) Ervin, K. M.; Anusiewicz, I.; Skurski, P.; Simons, J.; Lineberger, W. C. The Only Stable State of O−2 Is the X2Πg Ground State and It (Still!) Has an Adiabatic Electron Detachment Energy of 0.45 eV. J. Phys. Chem. A 2003, 107, 8521−8529. (37) Honkala, K.; Hellman, A.; Grönbeck, H. Water Dissociation on MgO/Ag(100): Support Induced Stabilization or Electron Pairing? J. Phys. Chem. C 2010, 114, 7070−7075. (38) Huang, X.; Beck, M. J. Metal-Free Low-Temperature Water-Gas Shift Catalysis over Small, Hydroxylated Ceria Nanoparticles. ACS Catal. 2015, 5, 6362−6369. (39) Burch, R. Gold Catalysts for Pure Hydrogen Production in the Water-Gas Shift Reaction: Activity, Structure and Reaction Mechanism. Phys. Chem. Chem. Phys. 2006, 8, 5483−5500. (40) Davó-Quiñonero, A.; Navlani-García, M.; Lozano-Castelló, D.; Bueno-López, A.; Anderson, J. A. Role of Hydroxyl Groups in the Preferential Oxidation of CO over Copper Oxide-Cerium Oxide Catalysts. ACS Catal. 2016, 6, 1723−1731.

ACKNOWLEDGMENTS We thank Brita Abrahamsson for initial calculations on BaO(100) and TiO2(110). Financial support is acknowledged from the Swedish Research Council, Chalmers Areas of Advance Nano and Transport and COST action CM1104. The calculations were performed at C3SE (Göteborg) and PDC (Stockholm) via a SNIC Grant. The Competence Centre for Catalysis (KCK) is hosted by Chalmers University of Technology and is financially supported by the Swedish Energy Agency and the member companies: AB Volvo, ECAPS AB, Haldor Topsøe A/S, Scania CV AB, Volvo Car Corporation AB, and Wärtsilä Finland Oy.

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REFERENCES

(1) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis, 1st ed.; Studies in Surface Science and Catalysis; Elsevier Science, 1989; Vol. 45. (2) Conner, W. C.; Falconer, J. L. Spillover in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 759−788. (3) Gorte, R. J. Ceria in Catalysis: From Automotive Applications to the Water-Gas Shift Reaction. AIChE J. 2010, 56, 1126−1135. (4) Schneider, W. F.; Hass, K. C.; Miletic, M.; Gland, J. L. Dramatic Cooperative Effects in Adsorption of NOx on MgO(001). J. Phys. Chem. B 2002, 106, 7405−7413. (5) Miletic, M.; Gland, J. L.; Hass, K. C.; Schneider, W. F. FirstPrinciples Characterization of NOx Adsorption on MgO. J. Phys. Chem. B 2003, 107, 157−163. (6) Schneider, W. F. Qualitative Differences in the Adsorption Chemistry of Acidic (CO2, SOx) and Amphiphilic (NOx) Species on the Alkaline Earth Oxides. J. Phys. Chem. B 2004, 108, 273−282. (7) Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. NOx Storage on BaO(100) Surface from First Principles: a Two Channel Scenario. J. Phys. Chem. B 2002, 106, 137−145. (8) Broqvist, P.; Panas, I.; Grönbeck, H. The Nature of NOx Species on BaO(100): An Ab Initio Molecular Dynamics Study. J. Phys. Chem. B 2005, 109, 15410−15416. (9) Grönbeck, H.; Broqvist, P.; Panas, I. Fundamental Aspects of NOx Adsorption on BaO. Surf. Sci. 2006, 600, 403−408. (10) Abrahamsson, B.; Grö n beck, H. NO x Adsorption on ATiO3(001) Perovskite Surfaces. J. Phys. Chem. C 2015, 119, 18495−18503. (11) Markovits, A.; Mguig, B.; Calatayud, M.; Minot, C. Spin Localization for NO Adsorption on Surface O Atoms of Metal Oxides. Catal. Today 2006, 113, 201−207. (12) Lu, N.-X.; Tao, J.-C.; Xu, X. NO Adsorption and Transformation on the BaO Surfaces from Density Functional Theory Calculations. Theor. Chem. Acc. 2014, 133, 1−14. (13) Yi, C.-W.; Kwak, J. H.; Szanyi, J. Interaction of NO2 with BaO: From Cooperative Adsorption to Ba(NO3)2 Formation. J. Phys. Chem. C 2007, 111, 15299−15305. (14) Metiu, H.; Chrétien, S.; Hu, Z.; Li, B.; Sun, X. Chemistry of Lewis Acid-Base Pairs on Oxide Surfaces. J. Phys. Chem. C 2012, 116, 10439−10450. (15) Van den Bossche, M.; Grönbeck, H. Methane Oxidation over PdO(101) Revealed by First-Principles Kinetic Modeling. J. Am. Chem. Soc. 2015, 137, 12035−12044. (16) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.-i.; Tanizawa, T.; et al. The New Concept 3-Way Catalyst for Automotive Lean-Burn Engine: NOx Storage and Reduction Catalyst. Catal. Today 1996, 27, 63−69. (17) Weaver, J. F. Surface Chemistry of Late Transition Metal Oxides. Chem. Rev. 2013, 113, 4164−4215. (18) Pacchioni, G. Modeling Doped and Defective Oxides in Catalysis with Density Functional Theory Methods: Room for Improvements. J. Chem. Phys. 2008, 128, 182505. H


Article

The Journal of Physical Chemistry C (41) Hu, Z.; Li, B.; Sun, X.; Metiu, H. Chemistry of Doped Oxides: The Activation of Surface Oxygen and the Chemical Compensation Effect. J. Phys. Chem. C 2011, 115, 3065−3074. (42) Sun, X.; Li, B.; Metiu, H. Methane Dissociation on Li-, Na-, K-, and Cu-Doped Flat and Stepped CaO(001). J. Phys. Chem. C 2013, 117, 7114−7122.

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