NOx Adsorption on ATiO3(001) Perovskite Surfaces - American

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The Journal of Physical Chemistry

NOx Adsorption on ATiO3(001) Perovskite Surfaces Brita Abrahamsson and Henrik Grönbeck∗ Department of Applied Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden E-mail: [email protected]

Abstract Density functional theory calculations have been used to explore NOx adsorption on perovskite oxides surfaces ATiO3 (001) with A=Ca, Sr, Ba. NO adsorbs weakly on all facets with no apparent A-ion dependence, whereas NO2 adsorbs preferably over AO-terminated surfaces with adsorption energies that correlate with the ionization potentials of the alkaline earth atoms. Simultaneous adsorption of NO and NO2 is found to substantially enhance the stability of the adsorbates owing to an oxide mediated electron-pairing mechanism. Stabilization is predicted also for NO/O2 adsorption and it is suggested that presence of oxygen favors the formation of nitrite/nitrate pairs. It is found that the NOx adsorption properties can be modified by mixing alkaline earth cations in the perovskite framework. The results are put in context by comparison to NOx adsorption on the corresponding (001) facets of alkaline earth metal oxides and TiO2 (110). Keywords: Perovskite oxides, ATiO3 , NOx , Adsorption, Reactivity, DFT ∗

To whom correspondence should be addressed

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Introduction Metal oxides find diverse use in heterogeneous catalysis ranging from structural promoters to active sites and storage materials. 1,2 One prototypical example of a storage material is ceria which is used in automotive three-way catalysis to store and release oxygen as the composition of the exhaust fluctuates around the stochiometric point. Another type of storage materials are alkaline earth metal oxides, such as barium oxide, that are used to store nitrogen oxides in the NOx Storage and Reduction (NSR) catalyst which is a concept for NOx reduction in oxygen excess. 3 The NSR concept is built on a periodic lean/rich operation of the engine where NOx is stored (as nitrates) in BaO under lean conditions and subsequently released and reduced to N2 during short fuel rich pulses. One key design parameter in this type of catalysts is the stability of the nitrate phase which should decompose when the atmosphere is altered from lean to rich and current research directions involve reaction paths and storage properties of mixed metal oxides such as perovskites. 4–9 The ability to tune NOx adsorption properties is critical also in Selective Catalytic Reduction (SCR) of NOx using ammonia as a reducing agent. 10 To increase the low-temperature activity of such catalysts, one option is to introduce a NOx storage material upstream the reduction catalyst that capture NOx during the initial cold-start period. 11 The NOx adsorption energies should in this case be low enough to release the stored molecules when the catalyst has reached the operating temperature. In the present study, we have used Density Functional Theory (DFT) calculations to explore NOx adsorption on ATiO3 perovskite surfaces with A=Ca, Sr and Ba. Perovskite oxides are versatile materials that currently receive considerable attention thanks to applications within both microelectronics and catalysis. 12 One advantage with perovskite (ATiO3 ) oxides is the robust structural framework where the same type of structure may host different cations which may be used to tune chemical properties. In the ideal cubic structure, the oxygen anions form an octahedron with Ti in the center and the A-cations between the octahedrons. However, the ideal crystal structure requires an appropriate match between 2 ACS Paragon Plus Environment

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the ionic radii of the two cations in the structure. 12 For the alkaline earth titanates, this requirement is fulfilled for BaTiO3 and SrTiO3 , whereas the small radius of Ca2+ results in structural distortions of the octahedra which are accommodated by a larger unit cell. 13 The non-polar ATiO3 (001) surfaces have received considerable experimental and theoretical attention. 14–21 The surfaces have a rich structural phase diagram and SrTiO3 (001) has been observed with a range of different reconstructions. 15 However, ab initio thermodynamic studies indicate that the stoichiometric unreconstructed SrTiO3 (001) surface terminated with a SrO-layer is thermodynamically preferred over the entire SrTiO3 stability range. 18,19 Perovskites have over the years been explored as catalytic materials, where the work on CO oxidation is one early example. 22 With respect to fundamental adsorption studies on ATiO3 , most theoretical attention has been directed to water adsorption 23–28 on SrTiO3 (001) and BaTiO3 (001) where the stability of different surface termination as a function of water pressure has been investigated. Other adsorbates that have been explored are hydrogen, 29 oxygen 30 and carbon dioxide. 31 NOx adsorption has only received limited attention. 32,33 In Ref., 32 NO adsorption was investigated on SrTiO3 (001) within the Local Density Approximation (LDA) using (1×1) surface cells. It was suggested that NO adsorbs preferably on the TiO2 -terminated surface with an N-Ti bond. The same coverage was considered in Ref. 33 where calculations based on the Generalized Gradient Approximation (GGA) were used for NO adsorption on BaTiO3 (001). In Ref., 33 however, it was concluded that NO spontaneously dissociate and that the stable configuration is atomic N and O adsorbed atop the anion and cation site, respectively. On the experimental side, NO adsorption has been investigated on SrTiO3 (001) by Temperature Programmed Desorption (TPD) and a low adsorption energy was inferred from a TPD peak at 260 K 34 and 297 K. 35 The limited fundamental work on NOx adsorption on perovskite oxides is in sharp contrast to work performed on alkaline earth metal oxides which has been the subject of several theoretical investigations. 36–41 It has been 2− demonstrated that nitrites (NO− species (where Os is a surface oxygen) are 2 ) or [Os -NO2 ]

formed upon NO2 adsorption. 41 In Ref., 36 it was shown that subsequent adsorption of NO2



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leads to the formation of stable nitrite/nitrate pairs owing to an oxide mediated electron pairing effect. 41 The aim of the current work is to investigate the interaction of NOx with ATiO3 (001) surfaces. To explore the range of stabilities, special emphasis is put on different surface termination and co-adsorption. The results are compared to NOx adsorption over the corresponding alkaline earth (AE) metal oxides and TiO2 .

Computational Method DFT is used in an implementation with local basis functions 42 and the gradient corrected exchange-correlation functional according to Perdew, Burke and Ernzerhof (PBE). 43 The Kohn-Sham orbitals are expanded with atomic numerical basis functions that are stored on radial grids centered on each atom. 44,45 In particular, a double numerical basis set with polarization functions (dnp) is used for all atoms with a real-space cutoff of 6.1 ˚ A. Semicore pseudo potentials 46 are employed for Ti, Sr and Ba to describe the interaction between the valence electrons and the nuclei together with the inner-shell electrons. The number of electrons treated in the valence for Ti, Sr and Ba are 12, 10 and 10, respectively. The Kohn-Sham equations are solved self-consistently with an integration technique of weighted overlapping spheres centered on the atoms. The direct Coulomb potential is obtained by projection of the charge density onto angular-dependent weighting functions also centered at each atom. The Poisson equation can in this way be solved by one dimensional integrations. The calculations for the bulk and surface systems are performed with periodic boundary conditions and integration over the Brillouin zone is approximated by finite sampling. A thermal broadening of 0.13 eV is used to smear the Fermi discontinuity. The bulk systems are treated with a (6×6×6) k-point sampling. The BaTiO3 (001) and SrTiO3 (001) surface slabs are treated with a p(2×2) surface cell, whereas for the non-cubic CaTiO3 (001), a p(1×1) cell is used. The difference between the three perovskites is motivated by the larger unit cell for CaTiO3 . To capture structural relaxations in the oxide upon adsorption, it is


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important that the surface cell is sufficiently large. 24 The slabs are modeled by five atomic layers separated by vacuum regions of at least 20 ˚ A. A (3×3×1) k-point sampling is used for the surface slabs. The (001) facets of the alkaline earth metal oxides are also considered with (2×2) surface cells. In the case of rutile TiO2 (110), the surface cell was chosen to be (2×1). Structural optimization is performed with the three bottom layers fixed to the corresponding bulk structure. The geometries are considered optimized when convergence criteria of 10−5 Ha, 2×10−3 Ha/˚ A and 5×10−3 ˚ A are met in total energy, largest gradient and largest change in atomic position, respectively. Adsorption energies are calculated with respect to the bare surfaces and NO, O2 and NO2 in the gas-phase. Negative adsorption energies denote exothermic adsorption. Except for gas-phase and adsorbed O2 , where triplet states are considered, all systems are treated in the lowest possible spin-states being singlets or doublets. The gas-phase molecules NO, O2 and NO2 are treated with open boundary conditions. The N-O bond distance in NO is calculated to be 1.16 ˚ A which is close to the experimental value of 1.15 ˚ A. 47 The calculated bond distance of O2 (1.23 ˚ A) is a slight overestimation of the experimental value of 1.21 ˚ A. 47 The N-O distance in NO2 is instead underestimated as the calculated value is 1.19 ˚ A and the experimental distance is 1.21 ˚ A. 47 The calculated vibrational wavenumbers are within 35 cm−1 of the experimental values. 48

Results and Discussion Bulk and surface properties Perovskite oxides have the generic formula ABO3 where A is a divalent cation and B has an oxidation state of B4+ . The bulk crystal has a cubic structure with B in the center, A at the corners and O at the center of the facets, see Figure 1. A perfect cubic structure is obtained in cases where the radii of the constituting ions have proper proportions, i.e. 5 ACS Paragon Plus Environment


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the A-O distance is roughly

√ 2 times larger than the B-O distance. 12 This condition is

fulfilled for BaTiO3 and SrTiO3 but not for CaTiO3 where the small Ca2+ radius makes the perfect cubic structure unstable with respect to distortions. 13 Experimentally, a tetragonal structure has been observed for CaTiO3 with four formal units in each cell, see Figure 1. In this structure, the oxygen octahedra are tilted with respect to each other to compensate for the small A-cation radius. 13 The calculated bulk parameters are collected in Table 1. The lattice constants of SrTiO3 and BaTiO3 are in good agreement with the experimental values of 3.905 and 4.012 ˚ A, respectively. 49 The slight expansion is a known effect of the applied approximation to the exchange-correlation functional. 50 Also the calculated lattice constants of CaTiO3 compare favorably with the experimental report of 5.38 (a), 5.44 (b) and 7.64 (c) ˚ A. 51 The perovskites show a clear trend in formation energy and bulk modulus. BaTiO3 has the highest formation energy and lowest bulk modulus. The formation energy is calculated with respect to TiO2 and the corresponding alkaline earth (AE) metal oxide. The lattice parameters for the AE oxides are calculated as references. The AE oxides adopt the rocksalt structure with two intertwined cation and anion fcc lattices. The calculated lattice constants 4.83, 5.22 and 5.63 ˚ A, for CaO, SrO and BaO, respectively, are in good agreement with experimental values (4.81, 5.16 and 5.54 ˚ A) 52 as well as previous theoretical reports. 50 The lattice constants for TiO2 are calculated to be 2.98 and 4.67 ˚ A, which should be compared with the experimental values of 2.96 and 4.58 ˚ A. 53 Table 1: Calculated lattice parameters (a,b,c) in ˚ A, formation energy (Ef ) in eV and bulk modulus (B) in GPa. The effective cation radii (rA ) are included for reference. 54

CaTiO3 SrTiO3 BaTiO3

a (b,c) 5.41 (5.51, 7.70) 3.95 4.04

Ef -0.93 -1.27 -1.39

B rA 1.00 176 1.16 145 1.36

We have studied the ATiO3 (001) facets which are stable for ATiO3 under equilibrium


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conditions. 18,19 The surface can be terminated either with an AO or a TiO2 layer, see Figure 1. The two terminations have previously been calculated to have similar surface energies 16 and both terminations are considered here. Surface slabs of ATiO3 (001) can be constructed to be either symmetric or asymmetric with respect to the center of the slab. With a symmetric slab (odd number of layers), the termination of the surface is the same on both sides of the slab. An asymmetric slab (even number of layers) have different terminations on the two sides. Although the stoichiometry is preserved in each layer, the slightly more covalent character of the Ti-O bond as compared to the A-O bond 16 results in different dipole moments perpendicular to the surface for the two terminations. In order to prevent an electric field in the vacuum region, we chose to perform the calculations with symmetric slabs. Following the method of Heifets et al., 16 the surface energies are calculated as a sum of cleavage and relaxation terms where the difference in the surface energies originates from differences in the relaxations. The surface energy for the AO-terminated surfaces are calculated to be 74 (Ca), 73 (Sr) and 63 meV/˚ A2 (Ba). The corresponding values for the TiO2 ˚2 , respectively. The values are in good agreeterminated surfaces are 74, 75 and 58 meV/A ment with previous reports 55 and the lowest surface energy is calculated for BaTiO3 (001). A similar trend applies for the AE oxide surfaces. 50 The atomic displacements of the atoms in the two top layers are presented in Table 2 together with data from previous reports using the same functional for SrTiO3 (001) with a seven layer slab. 55 The anions and cations relax differently on the two terminations. On the TiO2 -terminated surface, there is an inward relaxation for both ions which simply is related to loss of bonds at the surface. The situation is more interesting on the AO-terminated surface where the oxygen anion relaxes outwards on CaTiO3 (001) and SrTiO3 (001) whereas the relaxation is inward on BaTiO3 (001). This relaxation is related to the somewhat strained Ti-O bond distance in the case of Ca and Sr perovskites, being 1.95 and 1.97 ˚ A, respectively. The distances could be compared with the average Ti-O distance in TiO2 which is 1.99 ˚ A. The larger lattice constant for BaTiO3 results in an inward relaxation for both surface ions.



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Our results are in good agreement with the previous reports which justifies the use of a five layer slab. Table 2: Relative atom displacements in percent of the lattice parameter for the two top layers. Two terminations (Term) are considered.

Term AO

TiO2

Layer 1 1 2 2 1 1 2 2

Atom A O Ti O Ti O A O

CaTiO3 This Study -5.69 0.50 1.67 1.26 -1.84 -0.87 3.74 0.36

SrTiO3 This Study Ref. 55 -4.69 -4.60 0.51 1.00 1.57 1.30 0.77 0.67 -2.54 -1.88 -0.70 -0.57 3.14 2.75 0.32 0.45

BaTiO3 This study -1.90 -0.92 1.48 1.11 -2.80 -0.25 2.31 0.43

Adsorption of NO and NO2 The results for isolated NO and NO2 adsorption on AO- and TiO2 -terminated surfaces are collected in Table 3. Adsorption on the perovskites is compared to adsorption on the AE oxides and titania. Selected structures for the Sr-containing systems and TiO2 (110) are shown in Figure 2. On the AO-terminated perovskites, NO adsorbs with N bonded to a surface anion (Os ) with a bond strength of about 0.8 eV. The Os -ON distance is long and shows a clear dependence on A-cation. The longest distance is predicted for CaTiO3 and the shortest for SrTiO3 . The difference correlates with the difference in bond strength. Despite the weak bond, the internal N-O distance is significantly elongated with respect to the gas-phase value of 1.16 ˚ A. There are clear differences in the adsorption properties as compared to the AE oxides. The bond strength on the AE oxides correlates clearly with the ionization potentials of the alkaline earth atoms. 56 The weakest bond (0.81 eV) is calculated for CaO(001), whereas the bond on BaO(001) is as large as 1.46 eV. The Os -NO distance correlates with the bond


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strength; the shortest distance is calculated for BaO(001). A substantial elongation of the internal N-O bond distance is predicted also for the AE-oxides. 38 NO binds preferably to the Ti4+ site on the TiO2 -terminated perovskites, see Figure 2. The Ti-NO bond distance is in all cases long and the internal N-O distance is not altered with respect to the gas phase value. The configurations on the perovskites resembles that over TiO2 (110). The adsorption energies on the TiO2 -terminated CaTiO3 and SrTiO3 surfaces are smaller than on the AO-terminated cases. The reversed situation applies for BaTiO3 which is related to a somewhat larger hybridization between the Ti 3dyz,xz and the 2π ∗ orbital of NO. It is interesting to analyze the marked difference in the internal N-O bond distance for adsorption over perovskite surfaces terminated with either an AO- or a TiO2 -layer. Despite a weak bond on both types of surfaces, the character of the bond is different. On the AOterminated perovskites (and the AE-oxides), NO is adsorbed over an O2− site and the filled pz -orbital interacts with the antibonding 2π ∗ of NO. Although the net orbital contributions are bonding between the adsorbate and the surface, it results in a weakening of the NO bond which is consistent with the elongated N-O bond distance. The adsorption energy is a balance between the deformation energy of the bonding fragments and the molecule-surface interaction. The total energy of NO with a bond length of 1.31 ˚ A is 0.75 eV higher than in the optimal gas phase structure. On the TiO2 -terminated perovskites, NO interacts with the Ti 3d orbitals. This state is formally empty and the net bonding is mainly electrostatic. Although well-defined adsorption geometries are calculated for NO adsorption, it should be noted that the potential energy surface is flat and NO should be mobile over the surfaces at elevated temperatures. It has previously been proposed that NO spontaneously dissociate over AO-terminated BaTiO3 (001). 33 We have investigated this scenario and do not find any local energy minimum for dissociated NO. Our result is in accord with TPD measurements over SrTiO3 35 where no indications of NO dissociation is observed. Moreover, integration of the Polanyi-Wigner equation yields a desorption temperature of 275 K which is in close



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agrement with the experimental values of 260 K 34 and 297 K. 35 NO2 is known to adsorb on oxide surfaces in three distinctly different conformations; 36,38 i) bridging two cation sites in a bidentate fashion, ii) close to a planar configuration which could be interpreted as monodentate adsorption to a cation site and iii) an (Os NO2 )2− species. The different conformations have previously been predicted to have similar stabilities on AE-oxides. 38 NO2 is preferably adsorbed in the bidentate bridge configuration on all AO-terminated perovskites, see Figure 2. The adsorption energies scale with the ionization energy of A and the highest adsorption energy is calculated for BaTiO3 . The ONO-A bond distance follow the size of the cation and the internal O-N distance is extended by 0.05 ˚ A with respect to the gas-phase value. The O-N-O angle is markedly decreased as compared to the gas-phase angle which is calculated to be 133◦ . The reduced angle signals a charge transfer to the adsorbate approaching the structure of a nitrite which has an angle of 115◦ . The trend in the adsorption energy between the AO-terminated perovskites, follows the energetics of AE-oxides although the adsorption energies are higher over the AE-oxides. The increased bonding is accompanied by a more pronounced charge transfer. Comparing the AO-terminated perovskites and the AE-oxides, there are interesting differences with respect to the adsorption configurations. On the AE-oxides, NO2 prefers the bidentate structure on CaO, whereas the monodentate is favored over SrO and BaO. The preference for the monodentate conformation is related to the formation of nitrite (NO− 2 ) species. As for the AE-oxides, the potential energy surface for NO2 is flat and the three different structures have similar stability. In agreement with the NO adsorption, the bond strength for NO2 over the TiO2 -terminated surfaces is clearly smaller than for the AO-terminated case. The molecule is adsorbed in a bidentate fashion bridging two Ti-cations. The ONO-Ti bond distance is similar for all three systems and the large O-N-O angle signals a small charge transfer to the adsorbate. The adsorption configuration on the TiO2 -terminated perovskites is close to that on TiO2 (110) which has an adsorption energy similar to that of CaTiO3 . The difference in NO2 adsorption


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energies between the two terminations is related to the ability of the surfaces to transfer an electron to the adsorbate and, thus, the difference in workfunction. Owing to a reversed dipole moment at the surface, the TiO2 -terminated surfaces have higher workfunctions than the corresponding AO-terminated surfaces. 57 For BaTiO3 (001), the workfunctions are calculated to be 6.26 and 3.11 eV, respectively. Table 3: Adsorption energies (Ea ) and selected structural properties for NO (upper part) and NO2 (lower part) adsorbed on the investigated surfaces. The energy is given in eV and the distances in ˚ A. Xs denote surface atom being either a surface anion (O), an alkaline earth cation (A) or a titanium cation (Ti).

CaTiO3 SrTiO3 BaTiO3 CaTiO3 SrTiO3 BaTiO3 CaO SrO BaO TiO2 CaTiO3 SrTiO3 BaTiO3 CaTiO3 SrTiO3 BaTiO3 CaO SrO BaO TiO2

Term AO AO AO TiO2 TiO2 TiO2 AO AO AO TiO2 Term AO AO AO TiO2 TiO2 TiO2 AO AO AO TiO2

Xs O O O Ti Ti Ti O O O Ti Xs A-A A-A A-A Ti-Ti Ti-Ti Ti-Ti A-A A A Ti-Ti

Ea r1 (Xs -NO) -0.77 1.97 -0.86 1.43 -0.77 1.51 -0.54 2.39 -0.59 2.33 -1.02 2.31 -0.81 1.80 -1.15 1.52 -1.46 1.48 -0.51 2.49 Ea r3 (Xs -NO2 ) -0.90 2.48 -1.11 2.65 -1.27 2.89 -0.45 2.35 -0.61 2.33 -0.95 2.42 -1.09 2.51 -1.44 2.83 -1.96 2.99 -0.43 2.47

r2 (NO) 1.20 1.31 1.28 1.16 1.16 1.16 1.23 1.31 1.31 1.15 r4 (NO2 ) 1.24 1.25 1.25 1.22 1.24 1.23 1.25 1.27 1.28 1.22

α1 (Xs -NO) 108.8 109.4 108.8 135.1 138.5 141.0 107.9 108.7 109.5 131.1 α2 (ONO) 123.6 120.5 119.8 127.7 126.3 128.1 120.5 114.6 113.2 130.7

To conclude this section, we note that the adsorption energies on the AO-terminated surfaces are in all cases higher than for the TiO2 -terminated surfaces. Given the similarity in surface energies of the pristine surfaces, molecular NO and NO2 adsorption could stabilize the AO-terminated situation.



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Simultaneous adsorption of NO and NO2 Simultaneous adsorption of NO and NO2 on the ATiO3 -surfaces is performed to investigate possible stabilization owing to co-adsorption. Such effects have previously been reported for NOx adsorption on AE-oxides 36,58 and found to originate from surface mediated electron pairing. 41 The pairing effect could be present between different adsorbate combinations where charge is transferred from one adsorption site to another. For example, the co-adsorption of NO and NO2 could be enhanced if charge is transferred from the Os site where NO is adsorbed to the NO2 . In this way, two nitrites are formed on the surface. This mechanism could, in principle, be active with different electron acceptors where, for example, also O2 could accommodate excess charge. Structures for pair-adsorption on SrTiO3 (001), SrO(001) and TiO2 (110) are shown in Figure 2. Adsorption energies for all investigated systems are reported in Table 4 and Figure 3. In similarity with isolated NO2 adsorption, the energy difference between different configurations of NO2 is small, however, the O-N-O angle is reduced in presence of NO. Co-adsorption have a dramatic effect on the Os -NO structure. In all cases, this species becomes close to a nitrite structure. The Os -NO bond length is reduced and the Os -N-O angle approaches 115◦ . For some cases, the Os -NO species detaches from the surface and the distance between Os and Ti in the second layer is markedly increased. In the case of SrTiO3 (001), the Ti-Os distance increases from 2.26 to 3.17 ˚ A. Co-adsorption is found to have pronounced effects on the adsorption energies (Figure 3). Taking the example of SrTiO3 (001), the adsorption energy for simultaneous adsorption is 2.79 eV which should be compared to the adsorption energies of isolated NO (0.86 eV) and NO2 (1.11 eV). Thus, the effect of electron pairing is 0.82 eV. The total (NO+NO2 ) adsorption energy increases from Ca to Ba for all types of systems. The absolute enhancement energy owing to pairing varies considerably between the systems where the largest effect (1.11 eV) is calculated for AO-terminated BaTiO3 (001). Several factors affect the stabilization energy for NO/NO2 co-adsorption. One is the 12 ACS Paragon Plus Environment

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Table 4: Adsorption energies (Ea ) and selected structural properties for co-adsorbed NO and NO2 . The energy is given in eV and the distances in ˚ A. Os denotes surface anion.

CaTiO3 SrTiO3 BaTiO3 CaTiO3 SrTiO3 BaTiO3 CaO SrO BaO TiO2

Term Ea r1 (Os -NO) AO -2.14 1.50 AO -2.82 1.29 AO -3.15 1.33 TiO2 -1.66 1.70 TiO2 -1.87 1.35 TiO2 -2.44 1.35 AO -2.26 1.42 AO -3.07 1.37 AO -3.85 1.35 TiO2 -1.93 1.67

r2 (NO) α1 (Os -NO) 1.17 113.7 1.24 115.1 1.24 113.4 1.14 111.9 1.19 119.2 1.19 119.2 1.23 112.4 1.24 113.6 1.26 113.6 1.14 112.0

r4 (NO2 ) α2 (O-N-O) 1.26 115.1 1.28 113.6 1.28 140.0 1.27 117.9 1.28 119.9 1.26 120.4 1.28 119.9 1.28 113.9 1.28 114.4 1.27 118.6

stability of the adsorbed isolated NO and NO2 . A weak binding energy for isolated NO can in the presence of co-adsorbed NO2 be enhanced by the charge transfer and the formation of a stable Os NO nitrite. The efficient charge transfer from the Os -site renders the pz -2π ∗ interaction purely bonding. Note that the possibility of charge transfer changes the adsorption configuration completely for the TiO2 -terminated surfaces. Isolated NO is adsorbed on the Ti-cations whereas it prefers an Os site in the presence of NO2 . For the isolated NO2 adsorption, the stability in the absence of a co-adsorbate is given by the ability to transfer an electron to the adsorbed NO2 forming a nitrite. This depends in part on the workfunction of the surfaces which scales for the AO-terminated perovskites (and AE-oxides) with the ionization potential of the A-cations. The highest workfunction is calculated for CaTiO3 (CaO) and the lowest for BaTiO3 (BaO). 57 The strong dependence on workfunction for the NO2 adsorption energies is reduced by co-adsorption of NO as this adsorbate facilitates the charge transfer. In addition to the electronic reasons for the interaction between the surface and the adsorbates, the stabilization of the (Os -NO/NO2 ) pair depends on the ability of the surface to structurally accommodate the two nitrites. The structural relaxations are large for the AO-terminated pervoskites and the AE-oxides. One measure of the relaxation is the deformation energy, thus, the difference between the energy for the bare slab and the slab in the structure with adsorbates. For pair adsorption on AO-terminated BaTiO3 (001), TiO2 13 ACS Paragon Plus Environment


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terminated BaTiO3 (001) and BaO(001), this energy is 1.94, 1.81 and 1.50 eV, respectively. The pairing mechanism, where the stability of adsorbed NO and NO2 is enhanced by charge transfer from the Os -NO site to an electron acceptor, can be realized with other molecular acceptors. In connection to NOx storage in oxygen excess, O2 is an interesting alternative. The (experimental) electronic affinity of O2 is only 0.4 eV which is considerably lower than 2.3 eV for NO2 . 47 Here, we have investigated co-adsorption of NO and O2 on AOterminated BaTiO3 (001). Isolated molecular oxygen is weakly adsorbed on an A-A bridge site on BaTiO3 (001). The adsorption energy is calculated to be 0.65 eV, and the O-O bond distance to be 1.27 ˚ A. The bond distance should be compared with the gas-phase value which is calculated to be 1.23 ˚ A. When NO is co-adsorbed with O2 , the total adsorption energy is 2.00 eV, which corresponds to a stabilization of 0.58 eV. The O-O bond distance is in the coadsorbed configuration elongated to 1.33 ˚ A indicating a superoxo species. The results show that the pairing effect is present also with O2 as the electron acceptor. Although the NO/O2 stabilization is smaller than the NO/NO2 stabilization, the action could be important for the understanding of NO storage in AE-oxides and perovskites in absence of noble metal for NO oxidation. The initial activation of molecular oxygen towards a superoxo-species could be important in the subsequent reaction with NO to form NO2 . The consorted mechanism in the stabilization is underlined by the fact that the difference in stabilization between NO/O2 and NO/NO2 not simply is given by the difference in electron affinity of the electron acceptor which is 1.9 eV. The electron transfer for the NO/O2 and NO/NO2 systems are illustrated in Figure 4 where the charge density differences are compared with the case of isolated NO adsorption. Adsorption of isolated NO is manifested through a slight depletion of the pz orbital whereas the 2π ∗ orbital gains charge. The charge transfer from the Os site is clearly enhanced in the presence of an electron acceptor. Charge is transferred to the antibonding (2π ∗ ) orbital of O2 which rationalizes the elongated O-O bond distance for the co-adsorbed case. Presence of an adsorbate with high electron affinity (NO2 ) makes the charge transfer complete. In


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the case of NO2 , the excess electron occupies an orbital which is bonding with respect to the two oxygen atoms. The results from the charge density difference analysis is consistent with the calculated Mulliken charges. 59 When only O2 is adsorbed on BaTiO3 (001), the molecule is negatively charged by -0.36 electrons. The excess charge is increased to -0.47 electrons when O2 is co-adsorbed with NO. The corresponding charges on adsorbed NO2 are -0.55 and -0.69 electrons, respectively. The formation of two nitrites upon simultaneous adsorption of NO and NO2 is corroborated by the charge on Os -NO which is calculated to be -0.72 electrons. The Mulliken charge analysis demonstrate also the difference in the bonding of NO to AOand TiO2 -terminated BaTiO3 (001). NO is positively charged (0.09) on the TiO2 -terminated surface, whereas it is negatively charged (-0.51 electrons) on the AO-terminated surface. NOx is known to be stored as nitrites and nitrates in BaO. 36 In order to investigate the thermodynamic driving force towards this situation from gas-phase NO and O2 , we have calculated the enthalpy diagram for BaTiO3 (001) [and SrTiO3 (001)]. NO is adsorbed on the surface with an energy of -0.77 [-0.86] eV. Subsequent O2 adsorption is stabilized via electron pair formation and the total adsorption energy is -2.00 [-1.81] eV. Adsorption of a second NO molecule and formation of two nitrites on the surface stabilizes the system by 3.40 [3.33] eV. Disproportionation into a nitrite/nitrate pair further stabilizes the system by 0.35 [0.15] eV. The pronounced stabilization in the last steps is owing to both the oxidation of NO into NO2 and NO3 and the electron-pairing mechanism. The gain in energy upon formation of the nitrite/nitrate pair is crucial given the entropic loss adsorbing two NO and O2 at elevated temperatures, which is about 3.65 eV at 300◦ C. The differences in the enthalpy diagram between BaTiO3 (001) and SrTiO3 (001) is consistent with the more pronounced pairing effects on the AO-terminated BaTiO3 (001). The clear difference between the paired configurations for the perovskite systems opens up the possibility to modify the adsorption by the choice of A-cation. To this end, we have explored the possibility with equal amount of Sr and Ba ions in the cubic perovskite structure.



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The lattice parameter of the optimized lattice with symmetric distribution of A-cations is 4.00 ˚ A. This value is an average between the lattice constants of SrTiO3 and BaTiO3 . The total adsorption of (NO/NO2 ) is calculated to be 3.08 eV on the AO-terminated perovskite, whereas the corresponding value for the TiO2 -terminated surface is 1.89 eV. The adsorption energies are for both terminations between the values for SrTiO3 and BaTiO3 which indicates the possibility to tune the adsorption properties.

Conclusions In the present work, density functional theory calculations have been used to explore NO and NO2 adsorption on perovskite oxides surfaces ATiO3 (001) with A=Ca, Sr, Ba. The results have been compared with the adsorption onto the corresponding alkaline earth metal oxides and TiO2 . NO is found to adsorb weakly on all surfaces. For the perovskites, the adsorption site changes with the surface termination. In similarity with the alkaline earth oxides, NO adsorbs on an oxygen site on the AO-terminated surfaces, whereas a Ti-cation site is preferred over the TiO2 -terminated surfaces. The adsorption energy is stronger for NO2 and correlates with the ability of the surface to transfer one electron to the adsorbates. An oxide mediated electron pairing favors simultaneous adsorption of NO and NO2 which is predicted to substantially increase the stability of the adsorbates. This is in agreement with previous reports for NOx adsorption on the alkaline earth oxides. It is found that the NOx adsorption properties can be tuned by mixing alkaline earth cations.

Acknowledgement Financial support from the Swedish Research Council and the Chalmers Areas of Advance Nano and Transport is acknowledged. The calculations have been performed at C3SE (Göteborg) through a SNIC grant.


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Electronic and Chemical Properties of Mixed-Metal Oxides: Adsorption and Reaction of NO on SrTiO3 (100). J. Chem. Phys. 2003, 118, 6562–6571. (35) Azad, S.; Szanyi, J.; Peden, C. H. F.; Wan, L. Q. Adsorption and Reaction of NO on Oxidized and Reduced SrTiO3 (100) Surfaces. J. Vacuum Sci. Techn. As 2003, 21, 1307–1311. (36) Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. NOx Storage on BaO(100) Surface from First Principles: A Two Channel Scenario. J. Phys. Chem. 2002, 106, 137–145. (37) Karlsen, E. J.; Nygren, M. A.; Petterson, L. G. M. Comparative Study on Structures and Energetics of NOx , SOx , and COx Adsorption on Alkaline-Earth-Metal Oxides. J. Phys. Chem. B 2003, 107, 7795–7802. (38) 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. (39) Branda, M. M.; Valentin, C. D.; Pacchioni, G. NO and NO2 Adsorption on Terrace, Step, and Corner Sites of the BaO Surface from DFT Calculations. J. Phys. Chem. B 2004, 108, 4752–4758. (40) Broqvist, P.; Grönbeck, H.; Fridell, E.; Panas, I. Characterization of NOx Species Adsorbed on BaO: Experiment and Theory. J. Phys. Chem. B 2004, 108, 3523–3530. (41) Grönbeck, H.; Broqvist, P.; Panas, I. Fundamental Aspects of NOx Adsorption on BaO. Surf. Sci. 2006, 600, 403–408. (42) We have used the program Dmol3 version 7.0. (43) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868.



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(44) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508–517. (45) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756–7764. (46) Delley, B. Hardness Conserving Semilocal Pseudopotentials. Phys. Rev. B 2002, 66, 155125–1–155125–9. (47) Lide, D. R., Ed. Handbook of Chemistry and Physics, 71st ed.; CRC Press, Inc., 19901991. (48) The calculated (measured) stretch vibration for NO is 1893 (1904) cm−1 . The calculated (measured) stretch vibration for O2 is 1547 (1580) cm−1 . The calculated (measured) stretch vibration for NO2 are 741, 1338 and 1641 (750, 1318 and 1641) cm−1 . (49) Moreira, R. L.; Dias, A. Comment on ”Prediction of Lattice Constant in Cubic Perovskites”. J. Phys. Chem. Solids 2007, 68, 1617–1622. (50) Broqvist, P.; Grönbeck, H.; Panas, I. Surface Properties of Alkaline Earth Metal Oxides. Surf. Sci. 2004, 554, 262–271. (51) Cockayne, E.; Burton, B. P. Phonons and Static Dielectric Constant in CaTiO3 from First Principles. Phys. Rev. B 2000, 62, 3735–3743. (52) Cortona, P.; Monteleone, V., A Ab Initio Calculations of Cohesive and Structural Properties of the Alkali-Earth Oxides. J. Phys.: Cond. Matter 1996, 8, 8983–8994. (53) Abrahams, S. C.; Bernstein, J. L. Rutile: Normal Probability Plot Analysis and Accurate Measurement of Crystal Structure. J. Chem. Phys. 1971, 55, 3206–3211. (54) Ahannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Crystallogr. 1969, 25, 925–945. 22 ACS Paragon Plus Environment

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(55) Heifets, E.; Eglits, R. I.; Kotomin, E. A.; Maier, J.; Borstel, G. Ab Initio Modeling for Surface Structure for SrTiO3 Perovskite Crystals. Phys. Rev. B 2001, 64 . (56) The experimental ionization energies for the alkaline earth atoms are 6.11, 5.69 and 5.21 eV for Ca, Sr and Ba, respectively. 60 (57) The calculated work functions for AO-terminated ATiO3 are 4.28, 3.89 and 3.11 eV for Ca, Sr, Ba respectively. The corresponding values for the TiO2 -terminated surfaces are 5.89, 6.06 and 6.26 eV, respectively. (58) 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. (59) Note that the relative values in the Mulliken analysis are more reliable than the absolute ones. (60) Kramida A.; Ralchenko Yu.; Reader J.; NIST ASD Team, NIST Atomic Spectra Database (ver. 5.2), [Online]. Available: http://physics.nist.gov/asd [2015, June 17]. National Institute of Standards and Technology, Gaithersburg, MD. 2014.



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Figure 1: Structural models of the cubic ATiO3 perovskite bulk structure (a) together with the tetragonal bulk structure for CaTiO3 (b). (c) shows the first layer of an AO-terminated (001) surface, whereas (d) shows the first layer of a TiO2 -terminated surface. Atomic color codes: A-atom (green), calcium (blue), titanium (grey) and oxygen (red).


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Figure 2: Top-row from left to right: NO adsorbed on AO-terminated SrTiO3 (001), TiO2 terminated SrTiO3 (001), SrO(001) and TiO2 (110). Mid-row from left to right: NO2 adsorbed on AO-terminated SrTiO3 (001), TiO2 -terminated SrTiO3 (001), SrO(001) and TiO2 (110). Bottom-row from left to right: Co-adsorption of NO and NO2 adsorbed on AO-terminated SrTiO3 (001), TiO2 -terminated SrTiO3 (001), SrO(001) and TiO2 (110). Atomic color codes: Strontium (green), titanium (grey), nitrogen (blue) and oxygen.



5 Pair effect NO2 NO

4

AE-oxides

AO-term. Energy (eV)

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3

TiO2-term. TiO2

2 1 0

Ca

Sr

Ba

Ca

Sr

Ba

Ca

Sr

Ba

Figure 3: Total adsorption energy for co-adsorbed NO and NO2 . The total energy is divided into energy contributions from isolated NO and NO2 adsorption and stabilization owing to electron pairing.


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Figure 4: Charge density difference analyses for NO (a), NO/O2 (b) and NO/NO2 (c) adsorbed on BaTiO3 (001). Blue and yellow iso-surfaces correspond to charge gain and depletion, respectively.

Figure 5: Enthalpy diagram for 2NO+O2 adsorption on AO-terminated BaTiO3 (001) and, in parenthesis, SrTiO3 (001). The energies are given with respect to NO and O2 in the gas-phase and a pristine surface.



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NOx Adsorption on ATiO3(001) Perovskite Surfaces - American

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