TiO2 Systems. II. Rutile

Article pubs.acs.org/JPCC

Theoretical Study of NO Conversion on Ag/TiO2 Systems. II. Rutile (110) Surface Aliaksei S. Mazheika,*,†,‡ Thomas Bredow,† Oleg A. Ivashkevich,‡ and Vitaly E. Matulis‡ †

Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, University of Bonn, Beringstr. 4, 53115 Bonn, Germany ‡ Research Institute for Physical Chemical Problems of the Belarusian State University, 14 Leningradskaya Str., 220050 Minsk, Belarus ABSTRACT: Periodic density functional theory calculations of nitric oxide conversion on the Ag/rutile(110) surface have been performed and compared to the anatase(100) surface [Mazheika, A. S.; Bredow, T.; Ivashkevich, O. A.; Matulis, V. E. J. Phys. Chem. C. 2012, DOI: 10.1021/jp308393p]. Two complementary theoretical approaches were employed based on plane waves and on the linear combination of Gaussian-type orbitals, respectively. NO adsorption on the rutile(110) surface covered with Agn clusters (n = 2, 4, 8) was studied. For Ag8/rutile(110), two energetically most stable adsorption sites were found: at the border between the silver cluster and the surface and on the silver particles. In the absence of reducing agents, the further stages of NO decomposition are similar to those obtained on Ag/anatase(100). The adsorption of an additional NO molecule results in the formation of an acyclic cis-ONNO dimer. This is followed by the fragmentation of (NO)2 by breaking one or two N−O bonds. The main product of the on-cluster decomposition is molecular nitrogen, while at-border decomposition leads to the formation of N2O. In the on-cluster case, a rather stable dimer structure is formed, which is not directly decomposed. This intermediate can only be decomposed after isomerization to other dimer structures. Similar to the case of Ag/anatase(100), the Ag/rutile(110) surface is oxidized by remaining O atoms after NO decomposition. These O atoms can form NO2 according to an Eley−Rideal mechanism with other NO molecules adsorbed from the gas phase. From the calculated reaction energies, it is concluded that NO conversion on Ag/anatase(100) is more feasible than on Ag/rutile(110).

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INTRODUCTION In this article, we present results of a density functional theory (DFT) study of nitric oxide (NO) conversion on silver particles deposited on the rutile(110) surface. This paper represents part II of our study, while in part I the corresponding processes on Ag8/anatase(100) were studied.1 For nanoscopic and macroscopic rutile crystals, the thermodynamically most stable surface is (110).2 Deposition of silver atoms or clusters on this surface was investigated in a number of experimental3−6 and theoretical7−12 works. It was found that Agn clusters (n > 2) prefer to adsorb in the 3-fold hollow sites consisting of 5-coordinated Ti atoms (Ti5c) and 3coordinated O atoms (O3c) lying between the chains of bridging oxygen atoms (O2c). Two binding types of Ag particles with the surface have been postulated, the overlap of cluster’s HOMO with 3d-orbitals of Ti5c atoms and the interaction of silver molecular orbitals with surface eigenstates located on bridging O2c atoms. For the present study of NO conversion, the initial Agn/ rutile(110) structures (n = 2, 4, 8) were chosen as found in our previous study of Ag cluster adsorption on the rutile(110) surface.8 On these systems we have simulated the adsorption of one and two NO molecules. Possible subsequent stages of NO © XXXX American Chemical Society

conversion over silver clusters and at the border between silver clusters and rutile(110) were studied via NO dimerization and fragmentation. A Kohn−Sham orbital analysis has been carried out for selected structures including density of states (DOS), electronic density plots, and charge analysis. On this basis, binding mechanisms of intermediates with the Ag/rutile(110) surface were suggested.

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COMPUTATIONAL METHODS

The quantum chemical methods applied in the study of rutile systems were basically the same as in the case of anatase,1 the PBE functional as implemented in the plane-wave program Quantum-Espresso and in the crystalline-orbital program package CRYSTAL09. We will therefore present only differences and peculiarities of the rutile models. Periodic slab models were applied for simulations of the rutile(110) surface. NO and Agn adsorption was allowed only on one side of a slab. Due to the well-known oscillation of rutile thin-film properties with the number of layers,13,14 we have Received: September 5, 2012 Revised: November 9, 2012

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dx.doi.org/10.1021/jp308812q | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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obtained structures were reoptimized with a four-layer slab (also one fixed bottom layer) in the Γ-point approximation. Finally, the calculations were performed with a (2 × 2) Monkhorst−Pack k-point mesh for the four-layer slab. The basis sets for the LCAO calculations were taken from the CRYSTAL Website16 and augmented with additional diffuse functions to improve results for the rutile lattice parameters and to minimize the basis set superposition error (BSSE). Finally, we have chosen for Ti (20s12p4d) → [5s4p2d], for O (14s6p1d) → [4s3p1d], for Ag (27s18p11d) → [6s5p3d], and for nitrogen and oxygen of adsorbed species (11s6p2d) → [5s3p2d]. The lattice parameters with these basis sets were a = 4.617 Å, c = 2.999 Å, and u = 0.304, and BSSE was about 0.1−0.2 eV with PBE/LCAO. The geometry optimization was performed in several steps as well. Initial surface Agn/TiO2 structures were optimized using a four-layer slab fixing the bottom stoichiometric layer at the corresponding bulk coordinates and relaxing all other layers. For the simulation of NO conversion, first a three-layer slab was taken, in which only the top stoichiometric layer and the adsorbate species were allowed to relax, and the two other layers were fixed with atomic coordinates taken from the fourlayer Agn/TiO2 optimized structure. As a second step, the reoptimization of all obtained structures was performed with a four-layer slab. Due to the large number of basis functions, we simplified our model to decrease the computational effort. In the optimizations, the basis functions of Ti and O atoms of the third and fourth layers were replaced by Hay−Wadt effective core potentials with an ECP-411(31d)G basis17,18 and a modified Stuttgart−Dresden ECP2MWB (4s,5p) → [2s,3p] basis set,19 respectively. After optimization, single-point calculations of the obtained structures with the full basis sets were performed.

performed preliminary convergence tests regarding the number of layers in the slab models. The test system was the most stable adsorption structure of Ag2/rutile(110), in which the silver dimer is adsorbed parallel to the surface over a Ti5c atom between two O2c atoms of neighboring rows.8 Three to six stoichiometric layers were used, and varying numbers of bottom layers (1−3) were fixed. The results are presented in Table 1. Although full convergence of the calculated adsorption Table 1. Dependence of Adsorption Properties of Ag2/TiO2 on the Number of Triatomic Layers (m) in the m−n Slab Models (PBE/LCAO)a Ebb, eV r(Ag−Ag), Å

3−1

4−1

5−2

6−3

1.34 2.587

0.77 2.554

0.85 2.592

0.74 2.577

n is the number of fixed layers during geometry optimization. bBSSE corrected values.

a

properties is not achieved even for the six-layer slab, the fourlayer slab model provides binding energies rather close to those for the six-layer model. Therefore, to reduce computational cost, all calculations of NO conversion both with plane waves (PW) and with linear combinations of atomic Gaussian-type orbitals (LCAO) were carried out with four-layer slabs. To minimize the lateral interaction between the adsorbates, the surface was expanded to a (3 × 2) supercell with translation vectors directed along the diagonals of a (3 × 1) supercell. For some structures simulating NO conversion at the border between cluster and surface, a larger (4 × 2) supercell was used with translations along diagonals of a (2 × 2) supercell. The irreducible part of the first Brillouin zone was sampled with an automatically generated (2 × 2 × 1) Monkhorst−Pack k-point mesh. For some systems, convergence tests with denser meshes (3 × 3 × 1) and (4 × 4 × 1) were performed, but it was found that the (2 × 2 × 1) mesh is accurate enough. The cutoff energy for the PW wave function was chosen as 30 Ry according to preliminary tests. An increase to 40 Ry did not change adsorption properties significantly. For this basis set the lattice parameters of rutile with PBE are very close to experimental data:15 a = 4.624 Å, c = 2.958 Å, and u = 0.305. For elimination of spurious interaction between slabs, the vacuum distance was set to 18 Å. The geometry optimization was performed in several steps. All structures were initially optimized with a three-layer slab (one bottom fixed layer with bulk parameters) in the Γ-point approximation. After that, the

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RESULTS AND DISCUSSION NO Adsorption. The adsorption of NO on rutile(110) was previously studied both theoretically20−23 and experimentally.23,24 It was found that on the perfect rutile(110) surface NO molecules prefer to adsorb over Ti5c atoms, which are located between chains of bridging O2c atoms. In the case of the partially reduced rutile surface containing oxygen vacancies, NO adsorbs O-down at the defect sites with rather high binding energies, in this way “healing” the surface. The highest calculated binding energy obtained with a stoichiometric three-layer slab was 0.46 eV by adsorption of NO via the Natom on a (1 × 2) surface supercell and 0.39 eV on a (1 × 1)

Figure 1. Adsorption structures of NO on the Agn/rutile(110) surface (n = 2, 4, 8). B



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Table 2. Binding Energies and Interatomic Distances of Adsorbed NO on Agn/TiO2(110) (n = 2, 4, 8) for Structures Shown in Figure 1a NO@Ag2/TiO2 EbLCAOb e,V EbPW, eV EbPW1PW/LCAOb, eV r(N−O) LCAO, Å r(N−O) PW, Å a

NO@Ag4/TiO2

NO@Ag8/TiO2

1a

1b

1c

1d

1e

1f

1g

1h

1i

0.58 0.55

1.05 0.84

0.92

0.80

1.19 1.20

1.20

1.20

0.26 0.19 0.07 1.21 1.21

0.75 1.14 1.27 1.23 1.28

0.71 1.22 1.08 1.21 1.23

0.21

1.23 1.21

0.77 0.74 0.22 1.20 1.21

1.21

Interatomic distance of NO in the gas phase is 1.16 Å (GTO)/1.17 Å (PW). bBSSE corrected values.

cell.23 In the present study, positive values of the adsorption energy indicate stabilization with respect to the separate systems. We performed similar calculations with our fourlayer slab model and the (1 × 2) surface supercell. An extended (6 × 2) Monkhorst−Pack k-point grid was applied. With PBE/ PW the calculated adsorption energy of NO over bridging O2c atoms is 0.05 eV, indicating that there is almost no interaction. The adsorption on Ti5c is more exothermic, and the binding energy Eb is 0.23 eV (PBE/PW)/0.18 eV (PBE/LCAO). In this case, NO is bent to the rutile(110) surface by ∼45°, but the interatomic N−O distance almost does not change during adsorption. The binding energy differs from the values found in previous studies, possibly because of the different number of atomic layers in the rutile slabs and probably also due to different cutoff energies and k-point meshes. Compared to the anatase(100) surface (Eb = 0.30 eV (PBE/PW)/0.44 eV (PBE/ LCAO)), the binding energy on rutile is considerably smaller. The next step of our study of nitric oxide adsorption was the simulation of NO interaction with the Ag2/rutile(110) system. The most stable adsorption structure of the silver dimer was found in our previous work.8 Ag2 is situated between two bridging O2c atoms over a Ti5c atom parallel to the surface. NO prefers to adsorb over the silver dimer N-down perpendicular to the surface (Figure 1a). During NO adsorption the distance between the silver atoms increases by 0.97 Å. The binding energy of NO on the supported silver dimer is 0.55 eV (PW)/0.58 eV (LCAO) (Table 2), much higher than in the case of adsorption on the pure rutile(110) surface. The study of NO adsorption on Ag4/rutile(110) was carried out with PBE/LCAO. The Ag4/rutile(110) structure was taken from our previous study8 as well. Three initial configurations were considered for NO adsorption. In all optimized structures, NO is above the silver tetramer without interaction with the surface (Figure 1b−d). The Ag4 cluster did not significantly change its shape during adsorption of NO with the exception of structure (1d). However, this is not the most stable adsorption structure: its binding energy is 0.80 eV (PBE/LCAO), whereas in the case of structure (1b) the binding energy is higher, 1.05 eV (PBE/LCAO)/0.84 eV (PBE/PW) (Table 2). The latter values are close to the corresponding adsorption energies obtained for a fully optimized gas-phase NO/Ag4 system (0.96 eV (PBE/PW)/1.01 eV (PBE/LCAO)), where NO is bonded via the N-atom to a Ag3c lying in the vertex of Ag4.25,26 Similar calculations of the gas-phase NO/Ag4 system but with fixed geometry of Ag4 taken from structure (1b) give higher binding energies, 1.36 eV (PBE/PW)/1.44 eV (PBE/LCAO). The difference between the fully and the partially optimized NO/ Ag4 system is basically due to the different energies of the Ag4 cluster, which is higher for the pyramidal shape (corresponding to 1b). If we compare the partially fixed NO/Ag4 system with

the supported adsorption system (1b), the binding energy of NO@Ag4/TiO2 is smaller, although geometric parameters of the NO@Ag4 fragment are very close. This is probably the result of a charge transfer from the silver tetramer to the TiO2 surface. Therefore, there is less electron density around Ag4, and the overlap of the cluster orbitals with NO orbitals is smaller. The decrease of binding energies in the series (1b)− (1d) can be explained in a similar way. For instance, the electron density between two Ag3c atoms of the adsorbed silver tetramer (Figure 1c) is smaller than that of the gas-phase tetramer (for more details see ref 8). As a result, the overlap of NO orbitals with silver eigenstates is smaller, and the bond is weaker. The Mulliken population analysis showed that NO molecules are negatively charged in all considered configurations (−0.27 to −0.35 |e|). The spin density is also located mainly on NO. The observed charge transfer from the surface to NO leads to the occupation of antibonding π*-orbitals resulting in a weakening of the N−O bond (Table 2). It is well-known that the highest activity of Ag/TiO2 catalysts is observed for deposited silver particles of average size 1−10 nm.27,28 The silver octamer with size about ∼0.5 nm is a reasonable approximation of the smallest particles in the real catalysts. Therefore, the energetically most stable Ag8/rutile(110) structure was taken from our previous work8 for the present simulations. As in the case of anatase,1 two principal adsorption sites have been considered: on the silver octamer and at the border between silver and supporting TiO2. Six initial structures have been built: three simulating adsorption over Ag8 (parallel and perpendicular to the cluster) and three structures with NO at the border. They were constructed in a similar way as in the case of Ag8/anatase(100).1 The binding can occur through the overlap of Ag8/TiO2 orbitals lying around the Fermi energy with the partially occupied NO π*orbital. The optimized structures are presented in Figure 1e−1i. In some cases, different initial structures converged to the same final structure during optimization. Only in one case the two methods resulted in different geometries: in structure (1i), obtained with PBE/LCAO, NO has a horizontal position parallel to Ag8/rutile(110), whereas during geometry optimization with PBE/PW this structure was converted into a perpendicular orientation to the surface (1e). In the most stable adsorption structures, NO is situated Ndown above Ag8 and at the border between the cluster and surface (Table 2). In the case of at-border adsorption, a discrepancy between PBE/PW and PBE/LCAO results is observed. The calculated adsorption energies differ by 0.4−0.5 eV, Eb = 0.75 eV/0.71 eV (PBE/LCAO), and 1.14 eV/1.22 eV (PBE/PW) for (1g) and (1h), respectively. This is not due to different local minima since the geometric parameters of structures (1g) and (1h) calculated with both methods are quite close. The calculated density of states (DOS) and C



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Figure 2. DOS diagrams of NO adsorption structures on Ag8/rutile(110) (1h) (a), (1g) (b), and (1e) (c) (see Figure 1) and electron density plots for selected eigenstates of these structures. (a), (b) Left: the DOS calculated in LCAO. Right: in PW. (d) The shapes of some energetic levels and the DOS of Ag8/rutile(110) and the π*-orbital of NO. The energy zero is set to the Fermi level.

projected DOS (pDOS) of structure (1h) are also very similar for both methods (Figure 2a). The only distinction is that there are three spin-up and three spin-down occupied eigenstates in the band gap with PBE/LCAO, whereas with PBE/PW there

are four spin-up and three spin-down levels in the same region below the Fermi energy. The number of spin-down energetic levels with significant NO contribution in the bottom of the conduction band is larger in PBE/LCAO than in PBE/PW. D



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To study the binding mechanism in more detail, a similar analysis as for anatase(100) was performed. The most stable adsorption structures are (1e) with NO adsorbed on Ag8 and (1h) at the Ag8/rutile(110) border. In the case of structure (1e), the highest occupied eigenstate consists mainly of the antibonding π*-orbital of NO and orbitals of surface Ti atoms (Figure 2c). However, due to steric hindrance, there is no binding between these atoms. The HOMO of adsorbed Ag8 has negligible contributions in this state. The lower-lying level at −0.53 eV has contributions from the same NO orbital and from the HOMO-2 of adsorbed Ag8. The eigenstate at −1.11 eV is formed by the HOMO-1 of adsorbed Ag8 (Figure 2d) and π*NO. In the case of adsorption structure (1h), the lowest unoccupied level is also formed by the Ag8-HOMO and the π*orbital of NO, which overlaps with surface Ti 3d-orbitals (Figure 2a). The next lower-lying eigenstates consist of the NO π*-orbital and HOMO-2 of Ag8 with antibonding (at −0.82 eV) and bonding overlap (at −1.15 eV, not shown). Therefore, a change of the energetic ordering of the Ag8-MOs is observed during adsorption of NO on Ag8/rutile(110). The charge analysis shows that adsorbed NO is negatively charged in all cases. In the case of adsorption on Ag8, the charge is −0.31 to −0.53 (PBE/LCAO)/ −0.63 (PBE/PW) and for atborder adsorption is −0.49 to −0.53 (PBE/LCAO)/−0.37 to −0.57 (PBE/PW). The same phenomenon was observed in the case of anatase(100) and NO@Ag4/rutile(110), and it is determined by the occupation of the singly occupied π*-orbital of NO leading to the elongation of N−O bonds (Table 2). Considering the series of NO adsorption on uncovered rutile(110) and on Agn/rutile(110) with n = 2, 4, 8, an increase of binding energies for the most stable adsorption structures is observed (based on PBE/PW): for pure rutile 0.23 eV, for the dimer 0.55 eV, for the tetramer 0.84 eV, and for the octamer 1.22 eV. It is evident that deposited silver particles have a significant influence on NO adsorption. This is due to additional interactions of NO orbitals with Agn orbitals. Increasing the number of Ag atoms in a cluster, the number of Ag atomic orbitals taking part in the binding also increases providing the increase of overlap with NO. On the other hand, the charge on silver clusters supported by rutile(110) increases in the order Ag2−Ag4−Ag8: +0.37, +0.84, +0.98 (PBE/LCAO). The adsorbed silver clusters are Lewis acids, whose acidity enhances with increasing number of atoms. In this context, NO is regarded as a Lewis base. The NO@Agn/rutile(110) binding energies (Table 2) partially correlate with the charges of adsorbed silver clusters. Therefore, for the simulation of the next stages of NO decomposition on Ag/rutile(110), only the largest and most active Ag8 cluster was chosen. Comparing the binding energies of NO on Ag8/rutile(110) and Ag8/anatase(100), Eb is higher for anatase in the case of adsorption on the silver octamer (anatase: 1.12 eV (PBE/ LCAO)/1.25 eV (PBE/PW), rutile: 0.77 eV (PBE/LCAO)/ 0.74 eV (PBE/PW)), whereas in the case of at-border adsorption, they are rather similar (anatase: 1.21 eV (PBE/ LCAO)/1.08 eV (PBE/PW), rutile: 0.77 eV (PBE/LCAO)/ 1.22 eV (PBE/PW)). The reason for different binding energies for rutile and anatase supported on-cluster adsorption is probably the result of enhanced stability of the Ag8/rutile(110) structure, and on the other hand, in different binding mechanisms, in the case of anatase, there is a direct bonding overlap of supported Ag8−HOMO with the π*-orbital of NO, which is not considerably pronounced for rutile. The preference

Thus, the presence of an unoccupied level in PBE/LCAO, in contrast to the same level occupied in PBE/PW, is most probably responsible for the decrease of binding energy. Similar analysis of adsorption structure (1g) shows that the DOS is also similar with PBE/PW and PBE/LCAO (Figure 2b). The only difference is that two spin-up energetic levels lying in the band gap are very close to each other in PBE/PW (−0.45 and −0.40 eV), whereas in PBE/LCAO they are separated by 0.19 eV (−0.48 and −0.29 eV). The relative contributions of silver and of NO orbitals in the level at −0.45 eV (PBE/PW)/−0.48 eV (PBE/LCAO) are also different. However, in this case the occupation of energetic states does not play a significant role. In our study of NO@Ag8/anatase(100)1 and of Agn/anatase(100),29 the reason for the sometimes different adsorption energies was assumed to be differently implemented algorithms of geometry optimization in Quantum Espresso and CRYSTAL09 and in technical details of the DFT implementation. To clarify the situation, calculations with a third method (PBE as implemented in the plane-wave program VASP using the projector-augmented wave (PAW) approach, cutoff 400 eV) have been performed. The binding energies of NO for structures (1g) and (1h) were obtained as 1.10 and 1.13 eV, respectively. Single-point calculations with increased cutoff energy 800 eV in PAW did not show significant differences in adsorption energy for structure (1g), 1.09 eV, but in the case of (1h) Eb became much larger (1.51 eV). The PBE/PAW results confirm those obtained with PBE/PW for NO@Ag8/TiO2. Additional single-point calculations with the more accurate but also computationally more demanding hybrid method PW1PW were performed for almost all considered systems. The PW1PW results basically confirm those obtained with PBE except system (1e) (Table 2). Because there is only one exception, we assume that the difference of ∼0.5 eV is a result of the fact that the PBE geometry of (1e) does not correspond to a local minimum of the PW1PW potential surface. For structure (1e), we compared the binding energy of NO on Ag8/rutile(110) (0.77 eV (PBE/LCAO)/0.74 eV (PBE/ PW)) to NO/Ag8 systems in the gas phase. The binding energy of fully optimized NO/Ag8 is 0.58 eV (PBE/LCAO)/0.51 eV (PBE/PW), rather close to previously calculated values.25,30 For fixed Ag8 taken from the optimized (1e) structure, Eb is similar, 0.57 eV (PBE/LCAO)/0.64 eV (PBE/PW). Thus, in contrast to the corresponding results for NO@Ag4/rutile(110), the supported Ag8 cluster has enhanced affinity to NO, similar to adsorption of NO on Ag8/anatase(100).1 The titanium dioxide surface affects adsorption of NO on supported silver particles independent of the polymorphous modification and size of cluster. The binding energy increases with respect to gas-phase Ag8 clusters due to additional electrostatic interaction of negatively charged NO with positively charged Ag8(ads). The charge of adsorbed octamer without NO is the same as the charge of the NO@Ag8(ads) fragment of the (1e) structure, +0.98 (PBE/LCAO). The study of (NO+Ag8)gas+ with fixed geometry of Ag8 according to structure (1e) gave a binding energy of 0.92 eV, which is even higher than for NO adsorbed on Ag8/rutile(110). With this numerical experiment, we probed the effect of deformation and polarization of the Ag cluster by the surface, which should be explicitly taken into account in theoretical model calculations. Apparently, the rearrangement of the silver atoms due to bonding with the rutile surface and the charging of the adsorbed Ag cluster enhance the reactivity toward NO. E



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Figure 3. Reactions of dimerization of NO on Ag8/rutile(110) through coadsorption of another nitric oxide molecule.

2) supercell with translations along diagonals of (2 × 2), the distance is larger than 6 Å. The reaction energies of (NO)2 formation are presented in Table 3. These values indicate that all considered processes are

of adsorption via the N-atom is explained for rutile in the same way as for the anatase Ag/TiO2(100) surface.1 NO Dimerization. As discussed for the anatase system,1 the coadsorption of additional nitric oxide molecules is the only possible further process in the absence of any impurities or reductants in a catalytic system. If a second NO is adsorbed close to a previously adsorbed molecule, a dimer species can be formed. The direct formation of dimers from two NO molecules can be obtained in several ways: following the formation of cyclic structures with four formal N−O bonds and with the formation of the cis- or trans-isomer of the acyclic ONNO structure with an N−N bond. The formation of cyclic (NO)2 species is a highly endothermic process on the Ag8/anatase(100) surface, but on the perfect rutile (110) such structures are more stable.23 To study the formation of cyclic dimers on Ag/rutile(110), we have considered three dimer structures constructed from initial NO@Ag8/TiO2 systems (1e), (1g), and (1h), thereby simulating dimerization on-cluster and at-border. The calculations were performed with PBE/LCAO. Similar to the case of anatase, these structures were not decomposed during geometry optimization, indicating that they represent local minima on the potential energy surface. However, dimerization is highly endothermic in all cases. Without BSSE correction, calculated reaction energies ΔrE are −1.72 eV (1e), −2.27 eV (1g) for at-border adsorption, and −2.13 eV (1h) for on-cluster adsorption. According to these results and those for anatase(100),1 NO dimerization with formation of cyclic forms does not take place on or at silver particles deposited on both titania surfaces. Four NO@Ag8/TiO2 structures were taken for the study of the formation of acyclic ONNO molecules. In these structures, NO was placed on the silver octamer (1e, 1f) and at the border (1g, 1h). Corresponding dimerization reactions with optimized final structures are shown in Figure 3. Starting from the initial structure with NO adsorbed O-down on the silver octamer, two ways are possible (3b and 3c). In (3b) the Ag8 has Cs symmetry, whereas structure (3c) contains no symmetry element. It should also be mentioned that the dimer structures, in which (NO)2 is situated at the border, were additionally calculated with an extended surface supercell (4 × 2) to study the lateral interaction between adsorbate species. In the standard (3 × 2) supercell the distance between neighboring (NO)2−Ag8 is less than 5 Å, which is rather short. For the (4 ×

Table 3. Dimerization Energies (ΔrE) and Binding Energies (Eb) of Adsorbed (NO)2 on Ag8/TiO2(110) for Structures Shown in Figure 3a ΔrELCAOb, eV ΔrEPW, eV ΔrEPW1PWb, eV EbLCAOc, eV EbPW, eV

3a

3b

3c

3d

3e

1.18 1.29 1.17c 1.67 1.44

2.32 2.04 2.21 2.02b 1.50

2.26 2.02 2.05 1.88b 1.48

1.70/2.25c 1.82/2.22 1.83/ 2.28/2.26 2.38/2.68

1.51/ 1.52/1.49 1.80/ 1.97/1.78 2.16/2.06

a

Values separated by a slash (/) are calculated for different surface supercells (3 × 2)/(4 × 2), respectively. bBSSE corrected values. c Without BSSE.

exothermic. Some open-shell systems of the (4 × 2) surface supercell that contained more than 200 atoms and additionally ghost functions could not be calculated with the LCAO approach due to technical limitations of the CRYSTAL program. For those structures which we managed to calculate, significant lateral interaction (0.4−0.5 eV) was found only for the reaction starting from (3d). The dimer formation is more exothermic on the larger supercell possibly due to smaller electrostatic interaction and overlap of wave function between neighboring adsorbate species. The dimerization energies calculated by the complementary methods are similar for most cases, with the exception of (3b) and (3c) reactions (Table 3). For both dimer structures, the binding energies of (NO)2 calculated with PBE/PW and PBE/LCAO differ by 0.4−0.5 eV. However, the geometric parameters of (NO)2−Ag8 are very close in both methods; therefore, the discrepancy cannot be explained by different local minima. As for the previously mentioned cases, the situation was clarified by a third type of approach, PBE/PAW. The PBE/PAW binding energies of the dimer with the surface are 1.98 eV for (3b) and 1.87 for (3c). These values are close to the PBE/LCAO results. The corresponding reaction energies (shown in Figure 3b and 3c) are about 2.2 eV. As it was discussed in the paper devoted to NO conversion on anatase(100),1 the PBE method does not provide accurate results for (NO)2 formation in the gas phase. Therefore, additional single-point calculations were performed F



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Figure 4. DOS diagrams of dimer (NO)2 adsorption structures on Ag8/rutile(110) 3a (a), 3b (b), and 3e (c) (see Figure 3) and electron density plots for selected eigenstates of these structures. (d) The shapes of some molecular orbitals of (NO)2 in the gas phase. The energy zero corresponds to the Fermi level.

of the HOMO of (NO)2 overlapping with the HOMO of the silver octamer. The partial occupation of its LUMO results in charging of (NO)2: q = −0.21 (LCAO)/−0.51 (PW). This is the same binding mechanism as found for (NO)2@Ag8/ anatase(100).1 In the case of structure (3b), however, another binding principle was found. Several eigenstates of this system are found in the bottom of the conduction band and do not contain any contributions of silver or nitric oxide (Figure 4b). This is caused by charge transfer from lower-lying occupied silver molecular orbitals overlapping with filled MOs of (NO)2 to Ti atoms. The eigenstates in the band gap at −1.29 and −0.95 eV have contributions from silver orbitals, which overlap with HOMO and HOMO-1 of (NO)2. Such an interchange of the energetic ordering of (NO)2 molecular orbitals was also found for anatase.1 It takes place because according to symmetry the overlap of lower-lying silver orbitals is more preferable with higher-lying (NO)2 orbitals and vice versa. In the case of at-border structure (3e), the Fermi level lies below the bottom of the conduction band. The energy levels at −0.53 and −0.91 eV consist of both LUMO of (NO)2 and HOMO-1 and HOMO-2 of Ag8 (Figures 2d and 4c). In such a way, the previously unoccupied MO of (NO)2 is partially occupied. This leads to a negative charge on (NO)2: q = −0.86 (LCAO)/ −0.59 (PW) for the (4 × 2) surface supercell and −1.02 (LCAO)/−0.62 (PW) for (3 × 2). Additionally, as in all other similar cases, there is an overlap of (NO)2-MOs with surface Ti 3d orbitals in the dimer structure (3e). Comparing the dimerization at silver particles supported on rutile(110) and anatase(100), it can be concluded that the dimerization mechanism and the binding of (NO)2 with Ag8/ TiO2 are rather similar for both surfaces. Concerning the binding mechanism, the change in energetic ordering of dimer MOs occurs for both rutile and anatase, and one molecular orbital of (NO)2 can make contributions to different occupied eigenstates located in the band gap. The energy changes during

with the hybrid method PW1PW that gave closer agreement with experiment for gas-phase (NO)2. However, for the supported Ag8 clusters, the PW1PW/LCAO reaction energies confirm those obtained in PBE/LCAO (Table 3). As was shown for anatase-based systems,1 the (NO)2 species adsorbed N-down on the silver octamer have to be inverted to O-down isomers for further proceeding of dimer fragmentation. Dimer transitions (3a) → (3b) and (3a) → (3c) are exothermic with energy change 0.65 eV (PBE/LCAO)/0.06 eV (PBE/ PW). Taking once more PBE/PAW results as reference, the value of 0.65 eV seems to be more reliable. Of course, to estimate exactly the possibility of such a transition, the activation barriers should be calculated. These calculations were not performed due to their high computational cost. However, taking into account a previous study of NO conversion on the Ag(111) surface, where a barrier of only ∼0.2 eV was found,31 such an isomerization of (NO)2 seems to be feasible even at rather low temperatures. The analysis of the binding mechanisms was performed in a similar way as for the previously discussed structures. Only the DOS calculated with PBE/PW is presented because the electronic structure obtained with PBE/LCAO is similar. For anatase(100) it was found that the formation of (NO)2 on the Ag/TiO2 surface proceeds almost in the same way as NO dimerization in the gas phase.1 To identify the binding mechanism for rutile, three dimer systems were analyzed: in two of them (NO)2 was situated on silver octamer and bound via N and via O atoms (Figures 3a and 3b), and in the third one (3e) (NO)2 dimer lay at the border between the cluster and TiO2. As before, we considered eigenstates lying in the band gap and around the Fermi level (Figure 4), which consist mainly of silver and (NO)2 orbitals. In the case of dimer structure (3a), the occupied eigenstate near the Fermi level is formed by the former LUMO of (NO)2 and of surface orbitals (Figure 4a). The lower-lying eigenstate near −0.59 eV consists G



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Figure 5. Reactions of oxygen elimination from (NO)2@Ag8/rutile(110) systems.

Figure 6. Schemas of possible reactions of nitric oxide dimer degradation on the Ag8/rutile(110) surface with elimination of N2O or N2 molecules.

formation of 1/2 O2 and N2O. These processes were simulated only with PBE/LCAO. Corresponding structures were modeled based on the five initial (NO)2@Ag8/TiO2 structures shown in Figure 3. After oxygen elimination from those systems, in which (NO)2 is placed on the silver cluster, the N2O molecule did not desorb to the gas phase during geometry optimization (Figure 5). The reaction energies were always negative, ranging from −1.81 to −1.94 eV (without BSSE correction). The binding energies of N2O on the Ag8 cluster were also negative (−0.18 to −0.31 eV without BSSE). Therefore, although local energy minima of N2O on Ag8/ rutile(110) were found, on-cluster decomposition is endothermic. For structures with (NO)2 adsorbed at the cluster/surface border, the situation is different. After O atom elimination from the dimer structure (3d), formation of N2O is energetically unfavorable. In this case the second N−O bond gets broken

various types of dimerization are, as a rule, higher for rutile than for anatase. The highest dimerization energies are obtained for reactions proceeding over the silver octamer, in which (NO)2 is initially bonded with supported Ag8 via O atoms. In the case of at-border adsorption, no immediate decomposition of the dimer was observed for rutile in contrast to anatase, due to a higher stability of supported (NO)2. (NO)2 Decomposition. In all considered (NO)2@Ag8/ rutile(110) configurations the NO dimers are negatively charged. As it was found in the study of anatase,1 such charging of adsorbed (NO)2 results in the elongation and the weakening of N−O bonds. For all considered rutile systems we have simulated the next stages of (NO)2 decomposition by breaking one or two N−O bonds. First, we considered decomposition processes where one N− O bond is broken and an oxygen atom is released leading to the H



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Table 4. Reaction Energies for (NO)2 Decomposition (ΔrE) and Binding Energies (Eb) of On (n = 1, 2) with Ag8/TiO2(110) Shown in Figure 6a ΔrELCAOb, eV ΔrEPW, eV ΔrEPW1PWb, eV Eb(On)LCAOc, eV Eb(On)PW, eV a

6a

6b

6c

6d

6e

6f

6g

∼0.0 0.58 ∼0.0

∼0.0 0.61 ∼0.0

∼0.0 0.40 0.07 0.79d 0.59d

1.17 1.27 2.51 1.72d 1.43d

−0.08/0.08 −0.32/−0.20 −0.36/ 4.01/3.97 3.56/3.58

- /1.29 - /1.46

1.19/0.75 1.16/0.59 0.21/ 5.50/4.65b 5.26/4.99

3.91b 3.81

- /4.88 - /5.23

Values separated by a slash (/) are calculated for different surface supercells (3 × 2)/(4 × 2), respectively. bBSSE corrected values. cWithout BSSE. Binding energy with reference to molecular O2; ΔfE(1/2O2)LCAO = 3.08 eV, ΔfE(1/2O2)PW = 2.87 eV.

d

(NO)2 with desorption of N2O (Figure 6g) is an exothermic process as well. The calculated reaction energy depends on the distance between neighboring adsorbate species: it decreases with increasing size of the surface supercell (Table 4). A possible explanation is that there is covalent bonding or attractive dipole−dipole interaction between the adsorbates. As a result, the total energy of the system increases with increasing distance. It should also be mentioned that final structures (5c) without N2 and (6g) are the same. A third way of (NO)2 decomposition is the simultaneous break of both N−O bonds with elimination of molecular nitrogen. Corresponding reactions were considered for dimer systems (3b) and (3c) (Figures 6c, d). The energy change is ∼0.0 eV (PBE/LCAO)/0.40 eV (PBE/PW) for (6c) and 1.17 eV (PBE/LCAO)/1.27 eV (PBE/PW) for (6d). Taking into account the possibly too high PBE/PW energies of the initial dimer systems, the decomposition pathway of (6c) is thermoneutral and therefore unlikely. Fragmentation occurs most probably according to the pathway (6d). This is the second identified reaction mechanism that includes N2 formation during NO conversion, in addition to at-border decomposition with formation of O2. However, the first mechanism is more complicated and requires the presence of atomic oxygen close to (NO)2 or additional reducing agents in the catalytic system. So, it can be expected that the formation of molecular nitrogen during nitric oxide conversion on the Ag/rutile(110) surface without reductants occurs basically on the adsorbed silver particles. In the case of anatase, a similar way of N2 formation on silver particles was obtained. Single-point calculations of (NO)2 fragmentation on Ag/ rutile(110) with the hybrid method PW1PW confirm the trends obtained with PBE (Table 4). Although the reaction energies in cases (6d) and (6g) do not match those obtained by GGA, the general trend is the same. We did not calculate the larger (4 × 2) surface supercells with PW1PW because of the large computational effort. Further Possible Stages. After elimination of N2 or N2O from Ag8/rutile(110) during nitric oxide conversion, O atoms remain adsorbed. In all considered On@Ag8/TiO2 structures, the binding energy of the O atoms is higher than 1/2Eb(O2) (Table 4). Therefore, no desorption of oxygen atoms with formation of gas-phase O2 is expected independent from the number of O atoms adsorbed on the silver cluster or at the cluster/surface boundary. In this way, the NO conversion on Ag/TiO2 results in the oxidation of the surface by atomic oxygen. The same effect was found for anatase.1 One of the possible further stages is the adsorption of additional NO molecules from the gas phase on these oxygen atoms with formation of NO2 molecules according to an Eley−Rideal mechanism. In the case of anatase, it was found that such processes can take place.1 Therefore, we have performed

with formation of N2 which desorbs into the gas phase; the oxygen atoms remain adsorbed (Figure 5c). The corresponding reaction energy is 0.99 eV (PBE/LCAO)/0.77 eV (PBE/PW) on the (4 × 2) surface supercell with reference to the gas-phase O2. It should also be mentioned that the same result was found for a similar structure on the anatase(100) surface. In the case of structure (3e), the elimination of an O atom is a slightly endothermic process: ΔrE is −0.63 or −0.35 eV without BSSE correction. The N2O molecule can either be desorbed or remains adsorbed with a binding energy of 0.50 eV (PBE/ LCAO) (Figure 5b, left). According to these results, the border between the silver cluster and the titania surface is the only place where a nitrous oxide molecule can be adsorbed on Ag8/ TiO2. In a similar way, a hypothetical reaction was studied, in which two N−O bonds were broken in structure (3a) and an O2 molecule was released to the gas phase, whereas N2 remained on the silver cluster. However, this process was found to be highly endothermic (ΔrE = −2.54 eV). The formal reaction (NO)2 → N2 + O2 cannot occur in this way. An alternative way of (NO)2 fragmentation is followed by desorption of N2O from the surface, whereas O atoms remain adsorbed. Such reactions are possible from the geometrical point of view for those systems, in which (NO)2 is connected via oxygen to the silver cluster (Figures 3b, c) or is situated at the cluster/surface border (Figures 3d, e). Corresponding processes are shown in Figure 6. Structure (3a) with protruding O atoms of (NO)2 was not considered. Its formation is, however, not a limiting step because, as it was shown in a previous section, its isomerization to an O-down structure is an exothermic process with assumingly low energetic barrier. The dimer structures with (NO)2 placed on Ag8 (3b and 3c) relax to a rather stable O@Ag8/TiO2 system after desorption of N2O (Figures 6a, b). The reaction energies are 0.0 eV (PBE/ LCAO)/0.6 eV (PBE/PW) (Table 4). This pronounced discrepancy is due to the different energies of the initial (NO)2@Ag8/TiO2 systems because the O@Ag8/TiO2 structures calculated in the two complementary methods have close relative and also binding energies (Table 4). Taking into account the problematic calculations of dimer structures with the PAW method, it may be concluded that the more reliable reaction energy is the LCAO value (close to zero). If (NO)2 is placed at the cluster/surface border (3e), its decomposition may proceed in two ways, one where the oxygen atom migrates on the silver cluster (6e) and the other where the O atom remains at the border (6f). The first mechanism is slightly endothermic (PBE/PW) or thermo-neutral (PBE/LCAO), independent of the size of the surface supercell, but the second one is quite exothermic (Table 4). Therefore, the second way is supposed to be the only possible mechanism of (NO)2 decomposition for the dimer structure (3e). Concerning another at-border dimer structure (3d), the fragmentation of I



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Figure 7. Processes of adsorption of NO on On@Ag8/rutile(110) with formation of the NO2 molecule.

Table 5. Energy Changes in Reactions of NO2 Formation (ΔrE) and Binding Energies (Eb) of NO2 with Ag8/TiO2(110) Shown in Figure 7 7a , eV ΔrE ΔrEPW, eV Eb(NO2)LCAOb, eV Eb(NO2)PW, eV LCAOa

a

1.16 0.30 2.00 2.91

b

7b

7c

7d

7e

7f

7g

1.04 0.41 1.97 3.02

0.60 1.23 0.70 2.45

1.42 1.55 1.63 2.53

0.86 1.54 0.76 2.96

1.60 1.97 1.85 3.15

2.88 2.37 1.72 2.49

BSSE corrected values. bWithout BSSE. ΔrE(NO + O → NO2) = 4.16 eV (LCAO)/2.40 eV (PW).

Energetic Cycles of NO Conversion on Ag8/Rutile(110). In the present study we obtained the stages of nitric oxide conversion on Ag/rutile(110) in a stepwise manner. To summarize all above-described mechanistic steps, the corresponding energy diagrams with reference to the free Ag8/ rutile(110) surface and gas-phase NO are given in Figure 8. Similar to the case of anatase, two basic cycles can be identified. The first one involves the reactions which occur on the silver cluster, and the second one involves reactions at the border between the cluster and surface. Concerning the first cycle, the conversion starts with the N-down adsorption of NO on Ag8. Then another NO molecule is adsorbed, and dimerization takes place. (NO)2 is rather stable with respect to elimination of O2 or N2O. However, the N-down adsorbate structure can be converted into an O-down configuration with a presumably low barrier (vide supra). O-down dimer structures can also arise from dimerization of previously O-down adsorbed NO on the silver cluster, which are less stable though. The next stage of conversion is the decomposition of O-down adsorbed (NO)2 which may occur in two different ways: with elimination of N2O or N2. The second way which results in the formation of a 2O−Ag8 structure on rutile(110) is energetically preferable. The elimination of N2O is less likely. The other cycle of NO decomposition proceeds in the same order starting from adsorption of NO at the border between the silver cluster and surface. In contrast to the first cycle, the difference in adsorption energies via N or via O is not significant. As a result, the subsequent dimerization occurs in both ways, and further decomposition does not require transformation between various dimer structures as was discussed in the first cycle. The main product of dimer fragmentation following the second cycle is N2O. The formation of N2 is also possible by an elimination of O from (NO)2 by an O atom of a neighboring (NO)2 or by any reductant species if present in the catalytic system. In any case, after desorption of N2 or N2O, the oxygen atoms remain adsorbed on the silver cluster or at the Ag8/surface boundary.

simulations of corresponding reactions for Ag8/rutile(110) covered with O atoms as well. All On@Ag8/TiO2 systems were considered. The initial NO2@Ag8/TiO2 structures were calculated with both PBE/PW and PBE/LCAO methods independently. In all considered systems, nitrogen dioxide molecules remained adsorbed during geometry optimization (Figure 7). NO2 molecules are usually bonded with silver clusters via only one oxygen atom or sometimes additionally through the nitrogen atom. In one case of at-border site (7a), both O atoms of NO2 are bonded to Ti5c surface atoms adjacent to the supported silver octamer. Energetic data are presented in Table 5. The discrepancies for reaction and binding energies obtained with PBE/PW and PBE/LCAO are caused by partially different geometric parameters. This is the result of different implementations of geometry optimization algorithms in Quantum−Espresso and CRYSTAL. For instance, the reoptimization of the final PBE/PW structures with PBE/ LCAO resulted usually in new minima with close energetic and geometric parameters found with PBE/PW and vice versa. However, in some cases geometric parameters of optimized PBE/LCAO and PBE/PW structures were the same, although the energetic parameters were different. These differences are attributed to technical details of the DFT implementation in the two programs. The formation of NO2 according to an Eley−Rideal mechanism is a typical process on Ag/TiO2 surfaces independent from the modification. The adsorption of NO2 at or near silver particles supported by titania is also an energetically favorable process independent from the adsorption site and the presence of preadsorbed O atoms because all adsorption energies of NO2 are positive (Table 5). To avoid NO2 formation after NO conversion, the remaining oxygen atoms must be removed. As it was discussed in the paper devoted to anatase, the addition of reducing agents such as carbon monoxide or hydrocarbons into catalytic system seems to be most efficient.1 J



Article

processes were observed both on Ag8 and at the Ag8/surface border. For both rutile and anatase, (NO)2 adsorbed at the Ag8/surface border can be decomposed by elimination of a single oxygen atom by a reducing agent or by another O atom of nearby adsorbed nitrogen oxide species, resulting in the formation of N2. On the basis of the relative stability of the final product On@ Ag8/TiO2 with respect to Ag8/TiO2 and NO, it can be concluded that conversion occurs preferentially on the anatase surface with an energy gain of approximately 4.0−4.3 eV per two decomposed NO molecules. On rutile(110) this reaction energy is only between 3.4 and 3.6 eV. We suggest that this is the reason for the experimentally observed enhanced catalytic activity of anatase and its mixtures with rutile compared to pure rutile.

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SUMMARY AND CONCLUSIONS

A quantum-chemical study of nitric oxide adsorption on rutile(110) and Agn/rutile(110) (n = 2, 4, 8) systems and its stepwise decomposition have been performed using density functional theory and periodic slab models but two complementary types of basis functions, plane waves, and atom-centered functions. In most cases the two approaches gave results in close agreement with each other, thus confirming the theoretical results. Cases where discrepancies were observed are discussed in detail. It was shown that NO adsorption on the uncovered rutile (110) surface and on Ag2/ TiO2 is only slightly exothermic, while increasing the number of silver atoms in the adsorbed cluster results in a significant enhancement of binding. This is presumably caused by the presence of larger positive charges on the cluster and larger electron density around them. Two active sites of Ag/TiO2 were considered: on the supported silver cluster and at the border between the cluster and surface. The most stable adsorption structures of NO on the supported silver clusters were calculated. On the basis of a Kohn−Sham orbital analysis, the binding mechanism of nitric oxide with the surface was discussed. The energetic levels lying around the Fermi energy and in the band gap, which have contributions of silver and nitric oxide, were analyzed in detail. It was shown that for NO@Ag8/TiO2 systems the level near the Fermi energy consists mainly of a singly occupied antibonding π*-orbital of NO, which in the case of at-border NO adsorption overlaps additionally with d-orbitals of surface Ti atoms. In contrast to anatase, the contribution of HOMO of adsorbed Ag8 is negligible for Ag8/rutile(110). The band gap states consist of the same NO orbital overlapping with occupied molecular orbitals of adsorbed Ag8 (HOMO-1, HOMO-2), whose contribution is much more significant. The next step of the decomposition reaction is the coadsorption of additional NO molecules and dimerization leading to formation of an acyclic cis-ONNO. Depending on the structure, the levels near and below the Fermi energy are formed by HOMO-1, HOMO, and LUMO of (NO)2 overlapping with occupied molecular orbitals of the silver octamer. As a result, adsorbed (NO)2 is always negatively charged leading to a weakening of the N−O bonds. The overlap with orbitals of surface Ti atoms in the case of at-border adsorption does not result in any significant increase of binding energy. It was also shown that the isomerization of dimers adsorbed N-down on Ag8 to O-down structures is exothermic, which is important for proceeding of further decomposition.

Figure 8. Energetic diagrams of NO conversion with the reference to the free Ag8/rutile(110) surface and gas-phase NO. (Top) On silver cluster. (Bottom) At the border between cluster and surface.

From the calculated energies of the final On@Ag8/TiO2 (n = 1, 2) systems, it is difficult to judge which cycle is thermodynamically preferred, in particular taking into account the sometimes wide range of values obtained with LCAO and PW. Comparison of Rutile and Anatase Surfaces for NO Decomposition. Comparing the calculated mechanisms of NO decomposition on rutile and anatase, it can be concluded that in general the conversion occurs in a similar way on both polymorphs. The adsorption energies of NO at-border Ag8/ TiO2 are very close for rutile and anatase. In the case of oncluster adsorption, corresponding values are higher for anatase. However, we have to take into account that our slab models are not large enough to ensure convergence of the calculated properties so that an uncertainty of the absolute values remains there. NO dimerization on rutile and anatase occurs also in a similar way and with similar reaction energies. (NO)2 adsorbed N-down on Ag8 is rather stable for both rutile and anatase surfaces. These structures have to be converted into O-down isomers to take part in the decomposition reactions. The fragmentation of (NO)2 follows the same mechanism on both considered TiO2 surfaces. (NO)2 adsorbed at the Ag8/titania border is more stable on anatase than on rutile. Its decomposition with direct formation of N2O is an athermic process on anatase. Decomposition of the same dimer structure on rutile is an exothermic process. On the other hand, in the case of anatase, in contrast to rutile, it was found that some dimers spontaneously decompose after formation. Such K



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(7) Giordano, L.; Pacchioni, G.; Bredow, T.; Sanz, J. F. Surf. Sci. 2001, 471, 21−31. (8) Mazheika, A. S.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Mol. Struct. (THEOCHEM). 2010, 942, 47−54. (9) Mazheika, A. S.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Mol. Struct. (THEOCHEM). 2010, 950, 46−52. (10) Mazheika, A. S.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Mol. Struct. (THEOCHEM). 2009, 909, 75−78. (11) Pillay, D.; Wang, Y.; Hwang, G. S. Catal. Today. 2005, 105, 78− 84. (12) Pillay, D.; Hwang, G. S. J. Mol. Struct. (THEOCHEM). 2006, 771, 129−133. (13) Bredow, T.; Giordano, L.; Cinquini, F.; Pacchioni, G. Phys. Rev. B 2004, 70, 035419(1−6). (14) Hameeuw, K. J.; Cantele, G.; Ninno, D.; Trani, F.; Iadonisi, G. J. Chem. Phys. 2006, 124, 024708(1−8). (15) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W., Jr.; Smith, J. V. J. Am. Chem. Soc. 1987, 109, 3639−3646. (16) http://www.crystal.unito.it/Basis_Sets/Ptable.html (accessed Nov 19, 2011). (17) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−312. (19) Bergner, F.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431−1441. (20) Abad, J.; Böhme, O.; Román, E. Surf. Sci. 2004, 549, 134−142. (21) Abad, J.; Böhme, O.; Román, E. Langmuir 2007, 23, 7583− 7586. (22) Hadjiivanov, K.; Knözinger, H. Phys. Chem. Chem. Phys. 2000, 2, 2803−2806. (23) Sorescu, D. C.; Rusu, C. N.; Yates, J. T., Jr. J. Phys. Chem. B 2000, 104, 4408−4417. (24) Sorescu, D. C.; Yates, J. T., Jr. J. Phys. Chem. B 2002, 106, 6184−6199. (25) Grönbeck, H.; Hellman, H.; Gavrin, A. J. Phys. Chem. A 2007, 111, 6062−6067. (26) Matulis, Vitaly E.; Ivashkevich, O. A.; Gurin, V. S. Comput. Lett. 2004, 1, 1−6. (27) Grünert, W.; Bruckner, A.; Hofmeister, H.; Claus, P. J. Phys. Chem. B 2004, 108, 5709−5717. (28) Claus, P.; Hofmeister, H. J. Phys. Chem. B 1999, 103, 2766− 2775. (29) Mazheika, A. S.; Bredow, T.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Phys. Chem. C 2011, 115, 17368−17377. (30) Matulis, Vitaly E.; Palagin, D. M.; Mazheika, A. S.; Ivashkevich, O. A. Comput. Theor. Chem. 2011, 963, 422−426. (31) Liu, Z.-P.; Jenkins, S. J.; King, D. A. J. Am. Chem. Soc. 2004, 126 (23), 7336−7340.

As in the case of anatase, further stages of (NO) 2 decomposition on Ag8/rutile(110) occur by breaking of one or two N−O bonds and elimination of nitrous oxide or molecular nitrogen. During decomposition on the silver cluster the main product is N2, while at the border the main product is N2O. (NO)2 decomposition at the Ag8/surface border occurs without isomerization and is more exothermic. For one atborder structure, another way of decomposition was found, via elimination of an O atom and subsequent N2 formation. A similar reaction was also found for this dimer structure on the Ag8/anatase(100) surface. The effect of lateral interaction between neighboring adsorbates was also studied. For selected structures, it was shown that the binding energies decrease with increasing distance between adsorbed NnOm-Ag8 species by increasing the supercell size, indicating attractive covalent or electrostatic interaction. As for anatase, oxygen atoms remain adsorbed after desorption of N2 or N2O on silver particles or at the cluster/ surface border. Other NO molecules can be adsorbed from the gas phase on these O-atoms, and nitrogen dioxide molecules are formed in such a way according to an Eley−Rideal mechanism. NO2 molecules are rather strongly bound to Ag/ rutile(110). A way to prevent nitrogen dioxide formation during NO conversion is the addition of reductants (CO, hydrocarbons) into the catalytic system, which could reduce the oxidized surface and enhance decomposition of nitric oxide molecules. The mechanisms of nitric oxide conversion over rutile(110) and anatase(100) Ag/TiO2 are the same. The only distinctions found in our studies are different NO adsorption energies, which were found to be higher on anatase; the (NO)2 species appearing at the border between Ag8 and anatase are slightly more stable than on rutile; from the thermodynamic point of view, NO conversion on anatase is somewhat more preferable, caused by lower relative energies of final On@Ag8/TiO2 structures.

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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 Support from the German Academic Exchange Service (DAAD) and from the World Federation of Scientists is gratefully appreciated.

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REFERENCES

(1) Mazheika, A. S.; Bredow, T.; Ivashkevich, O. A.; Matulis, Vitaly E. J. Phys. Chem. C 2012, DOI: 10.1021/jp308393p. (2) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Soc. Rev. 2008, 37, 2328−2353. (3) Tong, X.; Benz, L.; Chrétien, S.; Kemper, P.; Kolmakov, A.; Metiu, H.; Bowers, M. T.; Buratto, S. K. J. Chem. Phys. 2005, 123, 204701−204706. (4) Tong, X.; Benz, L.; Kolmakov, A.; Chrétien, S.; Metiu, H.; Buratto, S. K. Surf. Sci. 2005, 575, 60−68. (5) Benz, L.; Tong, X.; Kemper, P.; Lilach, Y.; Kolmakov, A.; Metiu, H.; Bowers, M. T.; Buratto, S. K. J. Chem. Phys. 2005, 122, 081102− 081105. (6) Mohamed, H. H.; Dillert, R.; Bahnemann, D. J. Chem. Phys. C 2011, 115 (24), 12163−12172. L


TiO2 Systems. II. Rutile

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