TiO2 Systems. I. Anatase

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Theoretical Study of NO Conversion on Ag/ TiO Systems. I. Anatase (100) Surface 2

Aliaksei Mazheika, Thomas Bredow, Oleg A. Ivashkevich, and Vitaly E. Matulis J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 15 Nov 2012 Downloaded from http://pubs.acs.org on November 15, 2012

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The Journal of Physical Chemistry

Theoretical study of NO conversion on Ag/TiO2 systems. I. Anatase (100) surface Aliaksei S. Mazheika*,†,‡, Thomas Bredow†, Oleg A. Ivashkevich‡, Vitaly E. Matulis‡ †

Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstr. 4, 53115 Bonn, Germany

‡

Research Institute for Physical Chemical Problems of the Belarusian State University, 14 Leningradskaya Str., 220050 Minsk, Belarus

Abstract A theoretical study of nitric oxide (NO) conversion on the anatase (100) surface covered with silver clusters has been performed. Two complementary approaches based on density-functional theory (DFT) have been applied, in which the electron density was expanded in plane waves and in atomcentered Gaussian-type orbitals, respectively. It was observed that the NO interaction with the surface occurs mainly via the N atom. Adsorption of NO on silver clusters or at the border between silver and the TiO2 surface is more exothermic than at the uncovered anatase surface. Therefore all stages of NO degradation proceed mainly on these active sites. Further adsorption of NO molecules leads to the formation of dimer species with previously adsorbed ones. Only acyclic cis-isomers of ONNO are formed according to the calculated energies. An analysis of electron density shows that the LUMO of adsorbed (NO)2 becomes partially occupied so that the adsorbed nitric oxide dimers are negatively charged. As a result of this charge transfer, the (NO)2 species are decomposed by breaking one or two N-O bonds, followed by formation and desorption of N2 or N2O. In the case of decomposition at silversurface boundaries, the main gas phase product is N2O, while on the silver cluster both N2 and N2O are formed. After the (NO)2 decomposition oxygen atoms remain on the surface and can further react with NO molecules from the gas phase leading to the formation of rather tightly bound nitrogen dioxide molecules.

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Introduction The enhanced interest to nitric oxide decomposition is caused by the fact that NO is one of the components of exhaust gases of car engines. Therefore, a considerable number of experimental works is devoted to nitric oxide conversion processes under various conditions (temperature, different reduction species). It was found, that nanoparticles of transition metals deposited on semiconductor surfaces show high catalytic activity for the above mentioned processes. The most frequently studied catalyst is alumina covered with silver nanoparticles1-6. Nitric oxide decomposition occurs already at room temperature, but the highest efficiency is obtained at 350-700 K depending on preparing methods and other conditions1-4. To provide comparability between laboratory experiments and real processes occurring in car engines and exhaust arrangements, usually decomposition is studied with certain amounts of molecular oxygen and reduction agents like hydrocarbons or carbon monoxide. Presence of these substances in the gas mixture significantly improves the NO degradation. The size of deposited silver particles plays a significant role in conversion processes. If silver is obtained in highly dispersed form, so that silver particles are actually single Ag+ ions, the selective reduction of NO to molecular nitrogen is observed in the presence of higher alkanes. Higher Ag loading leads to silver aggregation and favors the formation of N2O via NO reduction by hydrocarbons1. Among other catalysts the titania based composite systems are most widely studied. It is well known that TiO2 is characterized by a high photocatalytic activity under long-wave ultraviolet irradiation7,8. The photocatalytic activity can be improved by deposition of metal particles (Pd, Au, Ag) as in the case of Al 2O38-17. The experiments of thermal and photocatalytic NO-conversion with and without reducing agents show that the main products are also N2 or N2O11,18. Among the three most abundant TiO 2 modifications, anatase is the most efficient catalyst for nitric oxide decomposition. NO degradation on such catalysts occurs sometimes already at room temperature9,19. However, for some types of deposited particles (Au) alumina based systems have been found to be more efficient than titania-based catalysts. Irradiation significantly improves the reaction rate for TiO2-based catalysts. The presence of molecular oxygen leads in some cases to the formation of NO211,14. But in any case, the main products are N2 and some amount of N2O, and the presence of metal nanoparticles significantly enhances NO degradation. The improvement of catalytic properties by deposition of metal nanoparticles, particularly silver, is basically due to two factors. On the one hand, new catalytically active centers are formed at the border between nanoparticle and supporting surface, and on the other hand, the surface of the metal nanoparticles contains many unsaturated atoms. The average size of silver nanoparticles, for which the highest activity is observed, is 1-10 nm13,20,21. So, the relative amount of surface Ag atoms in such particles is rather high. The photocatalytic activity of metal/TiO2 systems is enhanced by the presence of additional energetic levels in the band gap of semiconductor, which can influence on the separation ACS Paragon Plus Environment

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of photogenerated electrons and holes. As it was shown in previous theoretical works for Ag/TiO 2 systems22-24, these additional eigenstates consist basically of silver orbitals. The aim of the present study was the investigation of catalytic properties of silver particles adsorbed on the TiO2 surface by means of quantum-chemical simulations of nitric oxide conversion from first principles. In this first part the anatase-based systems are considered, and in a second paper the conversion on rutile-based systems is presented25. Our models are based on previous studies of silver cluster adsorption on the anatase TiO2 (100) surface24. Although the thermodynamically most stable surface of anatase single crystals is (101), we have performed our investigations for the (100) surface, because it has been recently shown26, that this is one of the most abundant low-index facets on nanoparticles of industrially produced anatase. The largest adsorbed silver cluster considered in our previous work was an octamer. Its average size is ~0.5 nm. Consequently in the present study we chose the previously obtained most stable Ag8/TiO2 structure as starting point for NO adsorption. Computational Methods Density functional theory (DFT) calculations within the general gradient approximation (GGA) have been performed employing the PBE exchange-correlation functional27. This method provides accurate results for titania-based systems, in particular for Ag/TiO 222-24. For the reasons described below, we have also performed additional calculations with the Hartree-Fock/DFT hybrid functional PW1PW, in which 20 % Hartree-Fock exchange is mixed with the PWGGA exchange functional28. The PBE calculations were performed with the plane-wave (PW) program Quantum-Espresso31, PBE and PW1PW calculations were performed with the crystalline-orbital program package CRYSTAL0929. It is assumed that hybrid methods are more accurate than GGA functionals but at the same time they are computationally more demanding especially with plane waves. For this reason we performed only single-point calculations with the hybrid method. PW1PW optimizations for some selected structures showed that the energetic effect is rather small. Therefore no structure optimizations were carried out with PW1PW. The surface was simulated as a slab, based on a (2 × 2) supercell of the primitive surface cell with translations along diagonals of a (1 × 2) supercell in order to diminish lateral interactions between neighboring adsorbate-species. For simulations of NO decomposition at the border between cluster and surface, a larger (3 × 2) supercell was used. The translation vectors in this case were directed along the diagonals of a (3 × 1) supercell. The irreducible part of the first Brillouin zone was sampled with a (2 × 2 × 1) k-point Monkhorst-Pack (MP) mesh. The convergence behavior with respect to the number of kpoints was tested by using denser meshes (3 × 3 × 1) and (4 × 4 × 1). It was observed that the effect on calculated adsorption energies is relatively small. The multiplicity of the calculated systems was ACS Paragon Plus Environment

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determined by the adsorbed species, because the ground state of Ag8/TiO2 is a closed-shell singlet. The slab models contained four atomic layers according to previously performed convergence tests for the dependence of adsorption properties on the number of layers24. The catalytic processes were simulated on one site of the slab. Three top atomic layers were allowed to relax during geometry optimization and one bottom layer was held fixed at bulk geometry. In our previous study24 it was shown, that the depth of a local potential energy minimum sometimes depends on the basis set. The difference in binding energies between PW and LCAO approaches can be up to 50% for the same structure. Fortunately such a discrepancy between the computational approaches was obtained only for a few structures. In order to carefully check the reliability of our results for the investigation of NO decomposition, we have performed calculations of all considered systems applying two complementary approaches. The first one is based on the linear combination of atom-centered Gaussian type basis functions (LCAO) as implemented in the CRYSTAL program29. The basis functions were taken from our previous study: for Ti (21s13p4d)/[6s5p2d], for O (14s6p1d)/[4s3p1d], for Ag (27s18p11d)/[6s5p3d]. The basis sets of N and O were (11s6p2d)/[5s3p2d], which lead to a small basis set superposition error (BSSE). The estimated values of BSSE were usually in the range 0.1-0.2 eV. With this computational setup, the lattice parameters of anatase are obtained as a = 3.817 Å, c = 9.678 Å, in good agreement with experimental data30. The truncation of Coulomb and exchange integrals was set to 7, 7, 7, 7, 14 in logarithmic values. The convergence threshold of SCF procedure and geometry optimization was 10-6 a.u. For better convergence, the occupation of energetic states around the Fermi level was allowed to be fractional with a Gaussian spreading of 0.003 or 0.005 a.u. The calculation of energetic properties was performed using the values of total energies extrapolated to 0 K. The geometry optimization was carried out in two steps. In order to perform a scan of the potential energy landscape, the selected adsorbate structures were optimized with 3-layer slabs with fixed bottom layer, taking the atom positions from corresponding 4-layer slabs. Finally a reoptimization of all obtained structures with a 4-layer model was performed. No significant differences between the geometric parameters obtained with these two models were observed, with only a few exceptions. The Kohn-Sham orbitals were analyzed by a Mulliken population analysis. Also for some main intermediates and in particular for the most stable products of NO decomposition we have performed vibration analyses to verify these structures as minima on the potential energy surface. The optimized geometries obtained in the way described above indeed correspond to minima for the majority of cases, revealing a good suitability of the chosen computational setup for our purposes. In all other cases, more precise optimizations were carried out. In the second approach the crystal orbitals are expanded in plane waves (PW). The calculations were performed with the Quantum-Espresso code31. The cut-off energy of plane waves was chosen 27 ACS Paragon Plus Environment

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Ry as in our previous study. The atomic cores were described in terms of ultrasoft pseudopotentials 32: scalar relativistic ones for Ag and Ti with 11 and 12 valence electrons and non-relativistic ones for O and N with 6 and 5 valence electrons, respectively. With this computational setup, the parameters of bulk anatase elementary cell are obtained as a = 3.783 Å and c = 9.547 Å. The PBE/PW calculations were performed after corresponding PBE/LCAO calculations, taking the optimized coordinates obtained with CRYSTAL as starting point for Quantum-Espresso. The calculations with PBE/PW have been carried out also in several steps. All structures were initially optimized using the Г-point approximation. After that, the final optimization was performed with a (2 × 2) MP k-point mesh. The BFGS quasi-Newton algorithm was applied for geometry optimization 33. The vacuum gap between the slabs was chosen after preliminary tests in such a way, that the distance between nearest atoms of neighboring slabs (including adsorbates) was larger than 10 Å. Therefore, in all calculations the distance between TiO2 slabs was chosen as 15 Å. The SCF and optimization convergence thresholds were set to 10-6 Ry. The band occupation around Fermi level was also allowed to have smearing character with a Gaussian spreading of 0.0037 Ry. The atomic charges were calculated according to a Löwdin population analysis. Those structures, for which the two above mentioned approaches showed significantly different results, were additionally calculated employing denser k-point meshes. If this did not solve the problem, a third set of calculations was performed using the projector-augmented wave (PAW) method34 as implemented in VASP35. In this case the cut-off energy was taken as 400 eV as in our previous work. All other parameters were chosen to be the same as for the Quantum-Espresso program including energetic and electronic criteria. In the VASP calculations the electrostatic dipole correction was applied along the direction of the surface normal. Results and Discussion NO adsorption on the pure anatase (100) surface In order to decide if NO molecules prefer to adsorb on the uncovered anatase surface or on Ag/ TiO2, we have performed the calculations of NO adsorption on pristine titania. As it was found by Minot et al. using a plane wave approach36, the most stable adsorption site is the position over fivecoordinated titanium atoms Ti(5c) of the TiO2 (100) surface. In the most stable structure NO is bonded through the N atom (Ti-NO). We have calculated several structures in PBE/LCAO, which simulate the adsorption not only over a Ti(5c)-atom, but also over two-coordinated oxygen atoms O(2c). In these calculations the surface supercell was chosen as (1 × 2). However, the global minimum corresponds to the same adsorption structure Ti-NO as obtained previously. Another local minimum corresponds to NO adsorbed via the oxygen atom (Ti-ON). The binding energies are 0.28 and 0.15 eV, respectively, ACS Paragon Plus Environment

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rather close to the previously calculated data36. In the present study positive binding energies correspond to a stabilization of the adsorbate structure with respect to the isolated systems. Additional calculations of these structures with a larger (2 × 2) surface supercell within two complementary approaches have been performed. The binding energies increased in this case: 0.44 eV (PBE/LCAO)/ 0.30 eV (PBE/PW) for the structure Ti-NO and 0.32 eV (PBE/LCAO)/ 0.19 eV (PBE/PW) for the structure Ti-ON. Although the coverage dependence of Eb is rather large on the bare surface, we did not analyze this effect in more detail, because the focus of the present study is on NO adsorption on anatase (100) covered with silver. The increased distance between adsorbed nitric oxide molecules leads to an increase of the binding energies, indicating the presence of electrostatic repulsion between the parallel oriented NO molecules. NO adsorption on the Ag8/TiO2 system In the study of NO adsorption on Ag8/TiO2 two basic adsorption sides were considered: on the silver particle and at the border between the silver cluster and the titania surface. Initial structures were obtained taking into account the shapes of the singly occupied π*-orbital of nitric oxide and of the surface eigenstates near the Fermi level, that are expected to play the main role in chemical processes. Ten different initial structures were considered for geometry optimization. The most stable optimized structures are shown in Figure 1. Analyzing their binding energies (Table 1) we conclude, that the adsorption occurs preferably via the N-atom both on the silver cluster (structure (1a)) and at the border between Ag8 and TiO2 (1d). Corresponding binding energies calculated with the two complementary approaches are rather close to each other, 1.12 eV (PBE/LCAO)/1.25 eV (PBE/PW) for (1a) and 1.21 eV (PBE/LCAO)/1.08 eV (PBE/PW) for (1d). In the case of adsorption of nitric oxide over an adsorbed silver octamer, two structures were obtained with NO adsorbed via N-atom. Their binding energies differ by a small value of ~0.2 eV (1a and 1b). In the case of adsorption at the Ag8/TiO2 border the NO molecule additionally interacts with a surface Ti(5c) atom. In order to investigate the effect of exact exchange on the computed adsorption properties the hybrid method PW1PW was also applied to the most stable structures. The binding energies obtained with PW1PW from single-point calculations at the optimized PBE structures are in general rather close to the corresponding values obtained in PBE/LCAO and PBE/PW (Table 1). Compared to the adsorption of NO on the pure anatase (100) surface the interaction with supported silver particles is energetically more preferable. The interatomic N-O distance in NO adsorbed on Ag8/TiO2 is always larger compared to the gas phase, independent from the quantum-chemical approach and the adsorption site (Table 1). The Mulliken charge analysis shows that the adsorbed NO molecule is always negatively charged. This can be attributed to a charge transfer from the surface to the adsorbate that leads to a partial occupation of the π*-antibonding orbitals of NO which, in turn, leads to a weakening of the N-O bond. ACS Paragon Plus Environment

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To elucidate the binding mechanism between NO and Ag8/TiO2, we have calculated the Density-of-States (DOS) for the two most stable structures (1a) and (1d). Since the electronic structures obtained with PW and LCAO do not differ significantly, we present here only the DOS calculated with PBE/PW (Figure 2). The valence band of anatase consists mainly of oxygen 2p orbitals, whereas the titanium 3d orbitals make significant contribution to the conduction band. Deposition of silver particles results in the appearance of additional occupied eigenstates lying predominantly in the valence band and in the band gap24. New energetic eigenstates with NO contribution appear in the DOS during nitric oxide adsorption. They lie mainly in the bottom of the valence band and around the Fermi level (Figure 2). For further analysis we have calculated the electron densities of these eigenstates. Comparing the shapes of the densities of NO@Ag 8/TiO2 with the states of Ag8/TiO2 near the Fermi level and with the shape of the singly occupied π*-orbital of NO (Figures 2a-c) it can be concluded that these new levels are linear combinations of the above mentioned eigenstates of NO and Ag8/TiO2 surface. Such an interaction of occupied and partially occupied eigenstates leads to the previously mentioned charge transfer to the NO molecule. In the case of adsorption at the border between cluster and surface, there is additional interaction of the NO π*orbital with surface d-orbitals of Ti(5c) atoms (Figure 2b). Taking into account the negligible difference in binding energies of on-cluster and at-border adsorption structures, it can be deduced that the stabilization energy due to the latter interaction type is very small. This conclusion is confirmed by the fact that the binding energies of NO adsorbed on pure anatase (100) surface, where only this last type of interaction takes place, are also small. The adsorption of nitric oxide molecule via the N atom is more preferable than via the oxygen atom. This is typical both for the present systems and for the pure silver (111) surface as it was shown by King et al.37. The reason for this phenomenon is the different distribution of electronic density around oxygen and nitrogen atoms. As it was shown above, the antibonding NO π*-orbital plays a significant role in the adsorption process. This MO consists of p-orbitals of N and O atoms. The contribution of the N2p orbitals is greater than that of the O2p orbitals (Figure 2c). Therefore the overlap of surface orbitals is larger near the N atom compared to the O atom of NO. For this reason, adsorption via nitrogen is energetically more preferable. In previous quantum chemical studies of NO interaction with silver particles in the gas phase it was obtained that the binding energies lie within the range of 0.44-0.52 eV 38,39. The corresponding values of a fully optimized Ag8+NO system in the gas phase calculated with our methods are rather close, 0.58 eV (LCAO)/0.57 eV (PW) (Figure 3). The binding energy of a gas phase Ag 8+NO, in which the geometric parameters of the silver octamer were taken from structure (1a) and held fixed during geometry optimization while only N and O were allowed to relax, is only slightly higher, 0.69 eV (LCAO)/0.66 eV (PW). But these values are still considerably lower than those of adsorption ACS Paragon Plus Environment

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structure (1a) (1.12 eV (LCAO)/1.25 eV (PW)). The influence of the TiO 2 surface is evident. The observed difference between binding energies in the gas-phase and at the surface is the result of positive charging of adsorbed silver clusters [24]. A detailed analysis has shown that during NO adsorption almost no charge transfer occurs from the surface to the cluster and further to nitric oxide. The electron density is transferred from the supported cluster to NO and the entire charge of NO@Ag8 is the same as the charge of adsorbed Ag8 without nitric oxide. In order to verify this idea, we have studied the gas-phase system (NO@Ag8)+ with fixed geometry of octamer, because the charge of adsorbed clean Ag8 is approximately (+1). The binding energy obtained with PBE/LCAO is 1.00 eV, much higher than for the neutral gas-phase system but closer to structure (1a). Therefore in order to accurately describe this charge transfer, the surface should always be explicitly taken into account in studies of chemical processes on supported particles. NO dimerization As it was obtained both experimentally and theoretically, nitric oxide molecules form dimeric species on silver surfaces37,40. Similar processes can take place also on the Ag/TiO2 surface, in particular in the absence of any additives or impurities from the gas phase. We have performed corresponding calculations of nitric oxide dimerization. The most stable NO@Ag8/TiO2 structures were taken as initial structures and one more NO molecule was added in different ways. Two different isomers of nitric oxide dimer can be formed. The first one is either a cis- or a trans-conformation of the ONNO-molecule, and the second one is a cyclic ONON molecule with four formal N-O bonds. In the gas phase the most stable molecule is the cis-isomer ONNO 41. Pure GGA methods do not provide good results for both geometric and energetic properties of the gas phase (NO)2 molecule. For instance, with PBE the most stable isomer is trans-(NO)2 and its N-N binding energy value is 4-5 times higher than the experimentally observed one. In previous theoretical studies it was found that the best computational approaches in this case are post-HF methods (CASPT2 etc.), which accurately reproduce experimental bond lengths, angles and binding energies 41,42. But these methods are still not implemented into standard codes for solid state systems and, in addition, are computationally highly demanding. However, for nitric oxide dimers some hybrid DFT methods provide satisfactory results concerning both geometry and energetics. According to the above mentioned PW1PW method, the most stable dimer structure is the cis-isomer and the binding energy is about 5 kJ/mol compared to the experimental value of 9-10 kJ/mol. Therefore, in order to verify the PBE results, for all PBE optimized structures have been done single-point calculations with PW1PW. In the present study we have considered both possibilities of NO dimerization. First, the formation of cyclic ONON molecules was studied, because as it was previously found, such species are rather stable on the pure rutile (110) surface43. The dimerization was simulated by addition of a NO ACS Paragon Plus Environment

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molecule from the gas phase with an adsorbed NO on Ag8/TiO2. Four structures have been examined: in two of them the dimers were placed above the silver octamer (from initial 1a and 1c), and in the two others the dimers were placed at the border between Ag 8 and surface. In all cases local minima have been found on the potential energy surface and the cyclic shape of the dimers was retained. In all cases the dimerization process was endothermic. Dimerization energy calculated with PBE/LCAO lies in the range -1.58 to -1.81 eV without BSSE correction. The binding energies of cyclic ONON-molecules adsorbed at the Ag8/TiO2 surface are also negative in all cases, ranging from -1.03 to -1.71 eV. Based on these results, the formation of cyclic nitric oxide dimers over/at silver particles on the anatase(100) surface is thermodynamically not feasible, therefore we excluded these dimer structures from our further study. The second possible way of dimerization is the formation of cis-ONNO-dimers. We have performed simulations of such processes in the same way as for the formation of cyclic isomers, approaching a NO molecule from the gas phase to an adsorbed NO. In this way, ten different initial dimer structures have been built (Figure 4). In the optimized structures (NO) 2 cis-isomers are quite stable both on silver particles and at the border between silver and TiO2 surface. This is verified by the exothermic dimerization reactions and by positive values of the binding energies of (NO)2 with the Ag8/TiO2 surface (Table 2). The latter values were calculated with PBE with reference to a gas-phase trans-isomer of (NO)2, which has lower energy than the cis-isomer in GGA methods. The only unstable situations correspond to structures (4c) and (4g) (Figure 4). In these cases the second NO molecule is decomposed during geometry optimization, the fracture of one N-O bond and subsequent desorption of a N2O molecule. The complementary approaches used in this study, PBE/LCAO and PBE/PW, provide similar results for most of the dimer structures. But in contrast to NO@Ag8/TiO2 systems, there are exceptions (Table 2). In the following we analyzed structure (4b) in more detail since here we found the most pronounced energy difference (ΔrE) between PBE/LCAO and PBE/PW, about 0.4 eV. Geometric parameters of this structure according to PW and LCAO are rather close. The only difference is the interatomic distance N-N: 1.49 Å (PW) and 1.60 Å (LCAO). We excluded the possibility of different local minima obtained by the different approaches by mutually starting optimizations with the minimum structures of the other method, and by more precise geometry optimization. The binding energies of (NO)2 with the Ag8/TiO2 surface are also quite different in this case (Table 2). Therefore the difference of ΔrE is due to various depth of the local minima on the potential energy surfaces for structure (4b) according to the different methods, because the binding energies of NO in the (2b) structure calculated in PW and LCAO are close (Table 1). The DOS of structure (4b) obtained with PW and LCAO is rather similar (Figure 5a). The energy gap between two neighboring eigenstates in the center of the band gap, however, is different (1.9 eV (LCAO) and 0.5 eV (PW)), and different ACS Paragon Plus Environment

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contributions of silver and (NO)2 are found in some eigenstates. This might not be the only reason for the energy difference, because similar effects have been observed in the DOS of other structures without significant differences in binding energies. The discrepancy is not due to insufficient basis sets because such phenomenon was obtained only for structure (4b). Calculations performed with a denser (3 × 3 × 1) k-point mesh did not change the situation; in the case of LCAO the Eb of (NO)2 even decreased for 0.23 eV comparing to (2 × 2 × 1) mesh. Therefore in order to clarify the situation we have performed additional calculations with the third PBE/PAW (VASP) approach. The initial geometry was taken from preliminary optimized calculations with PBE/PW. The PBE/PAW binding energy of (NO)2 in the (4b) structure is 1.84 eV, that is closer to LCAO results (Table 2). Increasing the cut-off energy up to 800 eV virtually does not change the binding energy (1.82 eV). Another alternative parameter, which helps to estimate the depth of minima, is the formation energy of structure (4b) with reference to Ag8/TiO2 and two gas phase NO. The calculated formation energies are 2.69 eV (PBE/PW)/ 2.45 eV (PBE/PAW)/ 2.23 eV (PBE/LCAO), slightly different from results of the Eb of (NO)2 due to different gas-phase NO dimerization energies obtained with the applied methods. Thus, the energetic effect of NO dimerization leading to (4b) is presumably in the range between 1.27 and 1.67 eV. In this particular case pronounced differences due to the different basis functions, numerical integration procedures and treatment of core electrons in CRYSTAL and Quantum-Espresso become apparent. In order to investigate the effect of exact exchange we re-calculated the dimerization energies with PW1PW in the same way as described before. The hybrid method gives slightly different values compared to PBE (Table 2). But apart from (4a) they all are positive in agreement with the GGA approach. In the case of formation of (NO)2 at the Ag8/TiO2 border, the energy change obtained with PW1PW is even higher than with PBE. The zero value of (4a) is probably due to the omitted geometry re-optimization at PW1PW level. The geometric parameters of all systems containing (NO)2 calculated with PBE/PW and PBE/LCAO are rather similar. In all cases an elongation of the N-O distance in the dimers was observed after co-adsorption of the second NO molecule. In structures (4a) and (4b) the elongation is 0.01-0.02 Å, in (4d) and (4e) it is much larger, 0.10-0.16 Å. In the case of structure (4f), the change of the N-O bond length of the initially adsorbed NO is ~0.2 Å, which can be the result of additional binding of the oxygen atom with a neighboring surface Ti-atom. The electron density analysis was carried out in the same way as for NO@Ag8/TiO2. We have analyzed two structures with dimers adsorbed on silver cluster – via nitrogens (4b) and via oxygens (4e), and the structure (4f) with (NO)2 at the Ag8/TiO2 border. In the DOS of these systems (Figure 5) the eigenstates formed by the nitric oxide dimer and silver orbitals are located mainly in the band gap region. The analysis of the electron density of the occupied level near the Fermi energy of (4b) shows ACS Paragon Plus Environment

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that it is formed by interaction of the (NO)2 LUMO with orbitals of the silver octamer (mostly the HOMO). The lower-lying level at -0.46 eV consists of the (NO)2 HOMO and silver orbitals as well (Figures 5a, d). Thus, the partial occupation of the NO dimer LUMO takes place after adsorption of (NO)2 on Ag8/TiO2. This results in a negative charge on the adsorbed nitric oxide dimer: q=-0.52 (LCAO)/-1.14 (PW). In the case of dimer structure (4e), the binding mechanism is different (Figure 5b). In this case the unoccupied level near the Fermi energy consists of the HOMO-1 of the NO dimer and silver cluster molecular orbitals, whereas the lower-lying eigenstate at -0.50 eV consists also of the HOMO-1 of (NO)2 and another MO of adsorbed Ag8. The eigenstates in the band gap at lower energies (around 1 eV) have contributions of the HOMO of (NO)2. Therefore the NO dimerization on Ag8/TiO2 does follows a different mechanism compared to the gas phase where new molecular orbitals appear in the order shown in Figure 5d. An interchange of the MO ordering takes place: the HOMO energy is decreased and the HOMO-1 energy is increased. Nevertheless, the net effect of the orbital interactions is a charge transfer from the Ag cluster to the NO dimer. The charge on (NO) 2 is q=-0.90 (LCAO)/1.12 (PW). If the nitric oxide dimer is located at the Ag8/TiO2 border, a band gap state at -0.42 eV is formed by the HOMO of (NO)2 and a Ag8 molecular orbital. Evidently, this eigenstate is the result of superposition of the π*-orbital of the initially adsorbed NO (Figure 2b, +0.01 eV) with a π*-orbital of the second NO molecule. The contributions of silver orbitals change during dimerization. The single NO molecule interacts with the HOMO of Ag8, whereas in the final dimer structure the HOMO-1 of Ag8 interacts with the NO π*-orbital. HOMO and LUMO of (NO)2 are interchanged as in the case of dimer structure (3e) and are lowered to -1.02 eV (Figure 5c). The nitric oxide dimer is negatively charged, q=-1.06 (LCAO)/-0.71 (PW), as a result of partial LUMO occupation. In contrast to the dimerization over the silver octamer, there is additional overlap of dimer molecular orbitals with dorbitals of surface Ti atoms. The charge analysis of all (NO)2@Ag8/TiO2 structures shows that adsorbed nitric oxide dimers are always negatively charged ~(-1). The electron density is localized mainly on the oxygen atoms. This charge transfer leads to changes of bond strengths and bond lengths in the dimer compared to a neutral gas-phase molecule. To verify this statement, additional calculations of cis-N2O2 in the gas phase have been performed at ab initio level of theory (CCSD(T)). The basis set was aug-cc-pVDZ for both N and O atoms. This method provides good results for the geometry of nitric oxide dimers41. The calculated N-N and N-O bond lengths of neutral cis-N2O2 are 2.23 Å and 1.17 Å, respectively. In the case of the gas-phase anion cis-N2O2¯, the N-N bond is much shorter (1.43 Å) and the N-O bond is longer (1.26 Å). The charging of the nitric oxide dimer strengthens the N-N bond and weakens the NO bond. ACS Paragon Plus Environment

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Although most of the obtained (NO)2@Ag8/TiO2 structures are rather stable, transitions between them are possible from the energetic point of view. The conversion of (NO)2 from the upright N-down geometry to an upright O-down structure is relevant for the decomposition process as will be discussed below. The reaction energies for transitions from (4a) to (4d) and (4e) are 0.39 eV (LCAO)/0.36 eV (PW) and 0.58 eV (LCAO)/0.39 eV (PW), respectively. Transitions from (4b) to (4d) and (4e) are mainly exothermic, -0.05 eV (LCAO)/-0.34 eV (PW) and 0.14 eV (LCAO)/-0.32 eV (PW), respectively. In the last cases different values of transition energies according to LCAO and PW are determined by different relative total energies of structure (4b) obtained with both methods. Taking into account the PAW results, these transitions are slightly endothermic, close to the PW values. Again we can only provide a range for the reaction energies. Due to their high computational cost, the calculations of kinetic barriers have not been carried out. The “precursors” and “products” of the considered isomerization are energetically close and their geometry parameters do not differ significantly, thus, it can be assumed that the barriers are not large. In addition, previous calculations of kinetic barriers of similar transitions between nitric oxide dimers adsorbed on the Ag (111) surface have shown that the potential energy surface is rather flat, and the barrier for inverting the nitric oxide dimer is below 0.2 eV37. Therefore, it is assumed that on the Ag/TiO 2 surface inverting of (NO)2 takes place already at room temperature. Attention should be also paid to structures (4c) and (4g). After co-adsorption of a NO molecule the formed dimer decays during geometry optimization leading to the formation of N 2O. In both cases the N2O molecule is physisorbed on the surface. Corresponding adsorption energies were calculated only with PBE/LCAO, 0.64 eV for (4c) and 0.18 eV for (4g). The data presented in Table 2 are calculated with reference to N2O in the gas phase and clean O@Ag8/TiO2 surface. Decomposition of (NO)2 and possible further processes It has been mentioned before that as a result of charge transfer from the Ag/TiO 2 surface to (NO)2, the N-O bonds become weaker. In some cases this leads to N-O bond fracture and subsequent desorption of N2O already during geometry optimization (i.e. barrier-free). In other cases the (NO)2@Ag8/TiO2 structures correspond to local minima on the potential energy surface and are relatively stable. For such systems we have performed simulations of (NO) 2 decomposition. First of all we have considered processes, in which one N-O bond is broken and N2O is desorbed from the surface. The O atoms remain on Ag8/TiO2. Such processes can take place only if (NO)2 is situated in a suitable way. For instance, in (3a) and (3b) the dimer is bonded via two N atoms that geometrically prevents N2O desorption. Only after rearrangement of the dimers, back reaction with protruding Oatoms can be avoided. Possible ways of (NO)2 decomposition are presented in Figure 6. Processes of elimination of ACS Paragon Plus Environment

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N2O from the silver octamer (Figures 6a, b) are exothermic with ΔrE ranging from 0.29-0.51 eV (PBE/ LCAO)/ 0.27-0.67 eV (PBE/PW). The elimination of N2O from the border is a slightly endothermic process according to PBE/PW or almost athermic in PBE/LCAO (Table 3). The discrepancy of results (-0.02 eV (PBE/LCAO) vs. -0.31 eV PBE/(PW)) is caused by different values of ΔrE ((NO)2 → O + N2O), because the relative total energies with the reference to (NO)2 and O-atom of corresponding adsorption systems (NO)2@Ag8/TiO2 and O@Ag8/TiO2 are very close (Tables 2 and 3). The calculated in CCSD(T) value of ΔrE ((NO)2 → O + N2O) is 1.53 eV, that is closer to PBE/LCAO value (1.86 eV) than to PBE/PW one (2.14 eV). So, the process of elimination of N2O according to mechanism 6e is supposedly almost athermic, and it can proceed in inverse directions. An alternative way of decomposition is followed by desorption of oxygen while N 2O remains on the surface. The oxygen atom can be eliminated by another O atom from an adjacent nitric oxide dimer at a neighboring cluster, or by a reducing species if such substances are included into the catalytic system. Such processes are usually unlikely because of their high endothermicity25. But there is one exception – the dimer structure (4f) with a (NO)2 fragment at the Ag8/TiO2 border. Our calculations indicate that if the bottom O atom of the nitric oxide dimer is eliminated, the formed N 2O molecule desorbs into the gas phase as well. The corresponding reaction energy is negative: -1.69 eV (PBE/LCAO)/-1.35 eV (PBE/PW) with reference to O2, N2O and the pure Ag8/TiO2 surface. If the top O-atom of (NO)2 is eliminated, the other N-O bond of the remaining N2O fragment is also broken. An N2 molecule is desorbed whereas the bottom O-atom remains adsorbed on the TiO2 surface in the same place as it is shown in Figure 6e. The energy connected with this process (with reference to O2 molecule formation) is 0.31 eV (PBE/LCAO)/ -0.06 eV (PBE/PW). The difference of energy changes of this process and of the process described in the previous paragraph with N2O elimination is the formation energy of nitrous oxide calculated with PBE, which has a positive value and is almost exactly the same with PBE/LCAO and PBE/PW. Taking into account the CCSD(T) result of ΔrE ((NO)2 → O + N2O) it can be concluded that the reaction is slightly exothermic. We therefore conclude that such a process is one of possible ways of (NO)2 decomposition at the border between an Ag cluster and the TiO2 surface. In the presence of reducing agents the energetic effect will depend on the reductant. The third considered way of (NO)2 decomposition is the simultaneous fracture of two N-O bonds. For some dimer structures (4a, 4b) this can in principle occur followed by formation and desorption of molecular oxygen. However, according to our calculations these processes are highly endothermic and will therefore not take place. For other structures, in which O atoms of NO dimers are bonded with Ag/TiO2 surface atoms, the decomposition can occur with elimination of molecular nitrogen into the gas phase. Such processes are presented in Figures 6c and 6d. They are characterized by rather large energetic effects, 0.95-1.93 eV (Table 3), which are even larger than those for ACS Paragon Plus Environment

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decomposition with elimination of an N2O molecule for the same dimer structures. The differences of energy changes calculated with PBE/LCAO and PBE/PW have the same cause as in previously discussed cases. For instance, during geometry optimization of the initial 2O@Ag8/TiO2 structure (6d) with PBE/LCAO, a rather high-lying local minimum was found, whereas a corresponding optimization with PBE/PW lead to a different, lower-lying minimum. The re-optimization of the final PBE/PW structure with PBE/LCAO resulted in a lower minimum structure than after the first optimization, as shown in Table 3. The differences in reaction energies for the formal reaction (NO) 2 → 2O + N2 and ΔfE(1/2О2) (Table 3) obtained with PBE/LCAO and PBE/PW also affect the calculated energy changes for the considered processes. Results of single-point reference calculations with PW1PW/LCAO are rather close to the obtained with PBE (Table 3) and confirm the general trends. After desorption of N2O and N2 molecules oxygen atoms remain adsorbed on the surface Ag8/TiO2. This corresponds to the oxidation of Ag/TiO2 surface, in particular of the silver particles, during NO conversion. The binding energies of oxygen atoms are rather high (Table 3), even higher than a half of the formation energy of O2 molecule. Thus, desorption of molecular oxygen is unlikely. The same situation is observed in the case of 2O@Ag8/TiO2 structures (Figures 6c, d). Desorption of both O atoms with formation of an oxygen molecule is also a highly endothermic process (Table 3). Presumably such an oxidation of silver particles on TiO2 surface will lead to a decrease of its catalytic activity because Ag particles are active centers and play the role of charge donors for adsorbed species. One possible way to eliminate such undesirable oxidation is addition of reducing agents into the catalytic system, which could remove O atoms from silver. Indeed, in experimental studies it was shown that the presence of CO or hydrocarbons significantly improves catalytic and photocatalytic NO conversion9-12,14,18,19. But it should be also taken into account that after desorption of N 2 or N2O new nitric oxide molecules can be adsorbed on the On@Ag8/TiO2 surface and react with the remaining O atoms to form NO2 species according to an Eley-Rideal mechanism. We have performed simulations of such processes as well because they could provide a way to eliminate O atoms from the surface. Only those structures were considered, for which the hypothetical formation of NO 2 followed by its desorption was an exothermic process, i.e. the difference E (NO2(g) + Ag8/TiO2) – E (NO(g) + O@Ag8/TiO2) was negative. Only two structures correspond to this criterion. In both of them an O atom is situated on the silver octamer (Figure 7). In addition, we have considered one structure with two O atoms on Ag8. In all considered systems spontaneous desorption of NO2 during geometry optimization was not observed. Local minima on the potential energy surface were found that were lower than the initial system On@Ag8/TiO2 and gas phase NO, and also lower than Ag 8/TiO2 and NO2. Geometry optimization was performed with both PBE/LCAO and PBE/PW independently from each other. Therefore the obtained geometry parameters of the final structures differ in some cases, and the ACS Paragon Plus Environment

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reaction energies ΔrE are also different. Only after starting the PBE/LCAO optimization with the PBE/ PW NO2@Ag8/TiO2 minimum structure closer agreement was obtained. Thus, the adsorption of new NO molecules from the gas phase and their reaction with oxygen atoms remaining after decomposition of (NO)2 on Ag/TiO2 provides the formation of rather tightly bound NO2 molecules. Nitrogen dioxide is also a poisonous gas, and its formation is not desirable. According to experiments, the addition of reducing substances (CO, hydrocarbons) can prevent such process9-12,14,18,19. Energetic cycles of NO-conversion on anatase Ag8/TiO2(100) surface Concluding all considered stages of nitric oxide conversion on Ag/TiO2 it can be stated that these processes take place predominantly on adsorbed silver particles or at their border with the anatase (100) surface. These reactions are energetically preferable over the corresponding processes on the uncovered anatase (100) surface. In Figure 8 energy diagrams of all discussed processes are presented with reference to Ag8/TiO2 and NO. The most stable products of NO conversion are the systems where an oxygen atom is situated at the border between the silver octamer and the anatase surface. Such structures can be obtained in different ways, a) Adsorption of NO on silver cluster via the O-atom, then dimerization after co-adsorption of another NO molecule and finally decomposition of the dimer with formation of N2O; b) Adsorption of nitric oxide molecule on Ag8 via the N-atom, dimerization as in (a), inversion of (NO)2 to an upright O-down position on the silver cluster, and finally decomposition as in (a); c) Adsorption of nitric oxide at the border between cluster and surface. Depending on the way of adsorption (via O or N) further stages can proceed differently. In the case of adsorption through the O-atom the formed (NO)2 is decomposed into Oads and N2O. In the case of the more stable NO adsorption via the N atom and NO co-adsorption the quite stable dimer structure (4f) arises. Its decomposition into Oads and N2O is a thermoneutral process and can proceed reversibly. But the final O@Ag8/TiO2 structure can be isomerized to another structure with lower energy, which can facilitate the entire process. The other way of decomposition of the (4f) dimer structure is more complicated and requires extra-species, such as reductants or oxygen atoms from neighboring dimers, which could tear off an O-atom from (NO)2. After elimination of this oxygen atom, the other N-O bond of N2O fragment is also broken resulting in the desorption of N2. Although such a mechanism is less common compared to the others, it is slightly exothermic and therefore energetically more feasible for decomposition of (NO)2@Ag8/TiO2. Besides final structures with the oxygen atom at the Ag8/TiO2 border, there are also rather stable structures with two O atoms adsorbed on the silver cluster. They are products of (NO) 2 decomposition on Ag8 with desorption of molecular nitrogen. This mechanism is most likely if the nitric oxide dimer does not move to the border, and decomposition takes place only on the silver ACS Paragon Plus Environment

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cluster. Summary and conclusions DFT calculations of nitric oxide conversion on Ag8/anatase(100) surface were performed with methods employing complementary types of basis functions. The first one is based on the linear combination of atom-centered Gaussian functions, in the other one the basis functions are plane waves. In most cases the two approaches gave results in close agreement with each other indicating a relatively high reliability of the results. Exceptions were discussed in detail in this study, and, when necessary, the calculations were repeated with a third approach based on projector-augmented waves. The reaction mechanisms of NO conversion were investigated by calculating the energies of possible intermediates. It was obtained, that nitric oxide molecules prefer to adsorb on Ag particles or at the border between silver and the anatase surface compared to the pure and stoichiometric anatase(100) surface. All mechanisms found in this study proceed on two active sites: on silver clusters and at the cluster/surface border. A number of stable adsorption structures of NO on Ag 8/TiO2 were identified. Their Kohn-Sham wave function analysis indicated, that the binding occurs through the overlap of the antibonding π* orbital of NO with the HOMO of the adsorbed silver octamer. As a result of the partial charge transfer into the antibonding orbital the N-O bond is weakened. In the case of adsorption at the Ag8/TiO2 border there is additional but small interaction of the NO π* orbital with 3d-orbitals of surface Ti atoms. The NO molecule adsorbs preferably via the N atom because the electronic density of the π* orbital is higher than on the O atom. The TiO2 surface significantly affects NO adsorption on supported Ag8 due to cluster charging and polarization, providing additional electrostatic interaction. As next step simulations of NO conversion were performed. After co-adsorption of another NO dimerization takes place, both on silver clusters and at the border. The formation of a cyclic (NO)2 isomer with four N-O bonds is highly endothermic, while the formation of acyclic cis-dimers with one N-N bond is in all cases an exothermic process. The binding mechanism between (NO) 2 and Ag8/TiO2(100) is more complicated than in the case of one NO molecule and depends on the structure. Basically the interaction is due to the overlap of HOMO-1 or HOMO of adsorbed Ag 8 with HOMO-1, HOMO or LUMO of a dimer. Such binding leads to a charge transfer from Ag8/TiO2 to (NO)2 resulting in a weakening of the N-O bonds. If (NO)2 is oriented with O atoms down to the surface or at the border the dimers can be decomposed in two ways: either breaking one N-O bond and desorption of N2O, or breaking two N-O bonds and desorption of N2. After both processes quite strongly bound oxygen atoms remain on Ag8/TiO2 corresponding to an oxidation of the surface. Subsequent adsorption of new NO molecules from the gas phase near the remaining O atoms and formation of NO2 according to an Eley-Rideal mechanism is possible. The last species are quite tightly bound with the Ag 8/TiO2 ACS Paragon Plus Environment

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surface and poison the catalyst. To avoid such processes, the addition of reductants (CO, hydrocarbons) into the gas phase mixture is desired. The two complementary approaches, PBE/LCAO and PBE/PW, give almost equal results for most of the considered structures except a few. There the discrepancy is presumably the result of different implementations of geometry optimization algorithms and different numerical methods for calculations of integrals in CRYSTAL and Quantum-Espresso. For clarifying of situation the third approach PBE-PAW has been applied in these cases. The single-point calculations performed with the more accurate hybrid DFT method confirm the general trend of NO conversion obtained with pure GGA functionals. Acknowledgments. This work was supported by the German Academic Exchange Service (DAAD) and by the World Federation of Scientists, which are gratefully acknowledged.

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References (1) Shimizu, K.-I.; Shibata, J.; Yoshida, H.; Satsuma, A.; Hattori, T. Appl. Catal. B: Env. 2001, 30, 151. (2) He, H.; Yu, Y. Catal. Today. 2005, 100, 37. (3) Li, J.; Ke, R.; Li, W.; Hao, J. Catal. Today. 2008, 139, 49. (4) Arve, K.; Hernández Carucci, J.R.; Eränen, K.; Aho, A.; Murzin, D.Y. Appl. Catal. B: Env. 2009, 90, 603. (5) Ingelsten, H.H.; Hellman, A.; Kannisto, H.; Grönbeck, H. J. Mol. Catal. A: Chem. 2009, 314, 102. (6) Mhadeshwar, A.B.; Winkler, B.H.; Eiteneer, B.; Hancu, D. Appl. Catal. B: Env. 2009, 89, 229. (7) Hoffmann, M.R.; Martin, S.T.; Choi, W.; Bahnemann, D.W. Chem. Rev. 1995, 95, 69. (8) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (9) Arve, K.; Adam, J.; Simakova, O., Čapek, L.; Eränen, K.; Murzin, D.Y. Top. Catal. 2009, 52, 1762. (10) Halasi, Gy.; Kecskeméti, A.; Solymosi, F. Catal. Lett. 2010, 135, 16. (11) Yu, Y.-H.; Su, I-H.; Wu, J.C.S. Env. Tech. 2010, 31, 1449. (12) Wang, J.A.; Cuan, A.; Salmones, J.; Nava, N.; Castillo, S.; Morán-Pineda, M.; Rojas, F. Appl. Surf. Sci. 2004, 230, 94. (13) Zhang, F.; Jin, R.; Chen, J.; Shao, C.; Gao, W.; Li, L.; Guan, N. J. Catal. 2005, 232, 424. (14) Irfan, M.F.; Goo, J.H.; Kim, S.D.; Hong, S.C. Chemosph. 2007, 66, 54. (15) Ivanova, E.; Mihaylov, M.; Thibault-Starzyk, F.; Daturi, M.; Hadjiivanov, K. J. Mol. Catal. A: Chem. 2007, 274, 179. (16) Wu, Z.; Sheng, Z.; Liu, Y.; Wang, H.; Tang, N.; Wang, J. J. Hazard. Mat. 2009, 164, 542. (17) Chien, S.-H.; Kuo, M.-C.; Lu, C.-H.; Lu, K.-N. Catal. Today. 2004, 97, 121. (18) Bowering, N.; Croston, D.; Harrison, P.G.; Walker, G.S. Int. J. Photoen. 2007, 90752. (19) Nguyen, L.Q.; Salim, C.; Hinode, H. Appl. Catal. A.: Gener. 2008, 347, 94. (20) Grünert, W.; Bruckner, A.; Hofmeister, H.; Claus, P. J. Phys. Chem. B. 2004, 108, 5709. (21) Claus, P.; Hofmeister, H. J. Phys. Chem. B. 1999, 103, 2766. (22) Mazheika, A. S.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Mol. Struct. (THEOCHEM). 2010, 942, 47. (23) Mazheika, A. S.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Mol. Struct. (THEOCHEM). 2010, 950, 46. (24) Mazheika, A. S.; Bredow, T.; Matulis, Vitaly E.; Ivashkevich, O. A. J. Phys. Chem. C. 2011, 115, 17368. (25) Mazheika, A. S.; Bredow, T.; Ivashkevich, O. A., Matulis, Vitaly E. J. Phys. Chem. C., to be published. (26) Feldhoff, A.; Mendive, C.; Bredow, T.; Bahnemann, D. ChemPhysChem. 2007, 8, 805. (27) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B. 1996, 54, 16533. (28) Bredow, T.; Gerson, A. R. Phys. Rev. B 2000, 61, 5194. (29) Dovesi, R.; Orlando, R.; Civalleri, B.; Roetti, R.; Saunders, V. R.; Zicovich-Wilson, C. M. Z. Kristallogr. 2005, 220, 571. Dovesi, R.; Saunders, V. R.; Roetti, R.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. Crystal09; University of Torino: Torino, 2009. (30) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W. Jr.; Smith, J. V. J. Am. Chem. Soc. 1987, 109, 3639. (31) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; Fabris, S.; Fratesi, G.; de Gironcoli, S.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; 18 ACS Paragon Plus Environment

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Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J.Phys.:Condens.Matter. 2009, 21, 395502. (32) Vanderbilt, D. Phys. Rev. B. 1990, 41, 7892. (33) Broyden, C. G. IMA J. Appl. Math. 1970, 6, 76-90. (34) Blöchl, P. E. Phys. Rev. B. 1994, 50, 17953–17979. (35) Kresse, G.; Furthmüller, J. Phys. Rev. B. 1996, 54, 11169–11186. (36) Mguig, B.; Calatayud, M.; Minot, C. Surf. Rev. Lett. 2003, 10, 175. (37) Liu, Z.-P.; Jenkins, S.J.; King, D.A. J. Am. Chem. Soc. 2004, 126 (23), 7336. (38) Grönbeck, H.; Hellman, H.; Gavrin, A. J. Phys. Chem. A. 2007, 111, 6062. (39) Matulis, Vitaly E.; Palagin, D. M.; Mazheika, A. S.; Ivashkevich, O. A. Comp. Theor. Chem. 2011, 963, 422. (40) Brown, W.A., King, D.A. J. Phys. Chem. B. 2000, 104, 2578. (41) Sayós, R.; Valero, R., Anglada, J.M.; González M. J. Chem. Phys. 2000, 112, 6608. (42) Glendening, E.D., Halpern A.M. J. Chem. Phys. 2007, 127, 164307. (43) Sorescu, D.C.; Rusu, C.N.; Yates, J.T.Jr. J. Phys. Chem. B. 2000, 104, 4408.

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TABLE 1: Binding energies (eV) and interatomic distances (Å) of adsorbed NO on Ag8/TiO2(100) for structures shown in Figure 1. 1a 1b 1c 1d 1e EbLCAO, a, eV

1.12

0.89

0.69

1.21

0.68

EbPW, eV

1.25

1.01

0.79

1.08

0.55

EbPW1PW/LCAO, a, eV

1.09

0.95

0.59

0.86

0.69

r(N-O)LCAO, Å

1.19

1.21

1.20

1.23

1.26

r(N-O) PW, Å 1.20 1.22 1.21 1.22 a - BSSE corrected values The interatomic distance of NO in the gas phase is 1.16 Å (LCAO)/ 1.17 Å (PW)

1.23

TABLE 2: Energy changes in reactions of dimerization (ΔrE) and binding energies of adsorbed (NO)2 on Ag8/TiO2(100) for structures shown in Figure 4. 4a

4b

4c

4d

4e

4f

4g

ΔrELCAO, a, eV

0.67

1.27

3.74

1.43

1.62

2.17

3.90

ΔrEPW, eV

0.73

1.67

3.38

1.55

1.58

2.15

3.61

ΔrEPW1PW/LCAO, a, eV

~0.0

0.76

4.19b

1.15

1.56

2.61

3.95b

EbLCAO, eV

1.36b

1.81a

1.82b

2.02b

2.64b

1.67

2.53

EbPW, eV 1.28 1.99 1.65 a b - BSSE corrected values; - without BSSE Eb(ON-NO) = 0.41 eV (LCAO)/0.70 eV (PW)/ eV (PW1PW/LCAO)

TABLE 3: Changes of energy for reactions of dimer (NO)2 decomposition (ΔrE) and binding energies of On (n=1, 2) with Ag8/TiO2(100) shown in Figures 6 and 4. 6a 6b 6c 6d 6e 6c 6g ΔrELCAO, a, eV

0.51

0.29

1.93

0.95

-0.02

ΔrEPW, eV

0.67

0.27

1.60

1.33

-0.31

ΔrEPW1PW/LCAO, a, eV

0.41

0.42

2.21

0.92

-0.07

Eb(Оn)LCAO, b, eV

4.31

4.10

2.35c

1.59c

4.48

5.98

5.73a

Eb(Оn)PW, eV 4.46 4.06 1.85c 1.60c 4.37 5.62 5.60 a - BSSE corrected values; b - without BSSE; c – binding energy the with reference to molecular O2 ΔfE(1/2О2)LCAO = 3.08 eV, ΔfE(1/2О2)PW = 3.32 eV


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Figure 1. Adsorption structures of NO on Ag8/TiO2(100) surface. Figure 2. DOS diagrams of NO adsorption structures on Ag8/TiO2(100) 1a (a) and 1e (b) (see Figure 1) and electron density plots for selected eigenstates of these structures. (c) The shapes of an occupied eigenstate of Ag8/TiO2(100) near the Fermi level and the π*-orbital of NO. The energy zero is set to the Fermi level. Figure 3. The interaction of NO with Ag8: supported by TiO2 and in the gas phase with partial and full optimization. Figure 4. The reactions of dimerisation of NO on Ag8/TiO2(100) through coadsorption of another nitric oxide molecule. Figure 5. DOS diagrams of dimer (NO)2 adsorption structures on Ag8/TiO2(100) 4b (a), 4e (b) and 4f (c) (see Figure 4) and electron density plots for selected eigenstates of these structures. (a) Top-diagram according to PBE/LCAO, bottom-diagram – PBE/PW. (d) The shapes of some molecular orbitals of (NO)2 in the gas phase. The energy zero is set to the Fermi level. Figure 6. Schemas of possible reactions of nitric oxide dimer degradation on Ag8/TiO2(100) surface. Figure 7. The processes of adsorption of NO on On@Ag8/TiO2(100) with formation of NO2 molecule. Figure 8. Energetic diagrams of NO conversion with the reference to free Ag8/TiO2(100) surface and gas phase NO. (top) On silver cluster, (bottom) at the border between cluster and surface.



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Figure 1.


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-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

30

Ag-up pDOS NO-up pDOS Total-up DOS

20

a)

DOS

10

0

10

Ag-down pDOS NO-down pDOS Total-down DOS

20

30 -8

-7

-6

-5

-4

-3

-2

-1

0

1

2

-2

-1

0

1

2

-0.44 eV (spin-up)

E, eV -8

-7

-6

-5

-4

-3

30

Ag-up pDOS NO-up pDOS Total-up DOS

20

10

b)

DOS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

10

+0.01 eV (spin-down)

20

Ag-down pDOS NO-down pDOS Total-down DOS 30 -8

-7

-6

-5

-4

-3

-2

-1

0

1

2

E, eV

c) eigenstate of π*-orbital Ag₈/TiO₂(100) adjoining to Fermi level Figure 2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3.


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Figure 4.



a)

b)

c)

Ag p-DOS N2O2 p-DOS

50

Ag p-DOS N2O2 p-DOS

50

Total DOS

DOS

DOS

Total DOS

0

0 -8

-7

-6

-5

-4

-3

-2

-1

0

1

-8

2

-7

-6

-5

-4

E, eV

50

-3

-2

-1

0

1

2

E, eV

Ag p-DOS N2O2 p-DOS Total DOS

DOS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

-0.50 eV

-0.01 eV

-1.02 eV

-0.42 eV

0 -7

-6

-5

-4

-3

-2

-1

0

1

2

E, eV

d) -0.46 eV

-0.01 eV

HOMO-1

HOMO

Figure 5.


LUMO

LUMO+1

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Figure 6.



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Figure 7.


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Figure 8. 29 ACS Paragon Plus Environment


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Table of contents


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TiO2 Systems. I. Anatase

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