First Principles Calculations of the Adsorption Properties of CO and

6184

J. Phys. Chem. B 2002, 106, 6184-6199

First Principles Calculations of the Adsorption Properties of CO and NO on the Defective TiO2(110) Surface Dan C. Sorescu†,‡,§ and John T. Yates, Jr.*,§ U.S. Department of Energy, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, PennsylVania 15236, Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, and Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: NoVember 26, 2001; In Final Form: March 11, 2002

First-principles calculations based on spin-polarized density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO and NO molecules on the rutile (TiO2) (110) surface in the presence of oxygen vacancy sites. The calculations employ slab geometry and periodic boundary conditions with full relaxation of all atomic positions. We have identified several possible adsorption configurations at both Ti five-coordinated (Ti(5f)) and four-coordinated (Ti(4f)) sites, finding that adsorption binding energies are dependent on the defect density. Among these configurations the most stable have been found at the Ti(4f) sites in the case of the surface with missing bridging-oxygen rows. In this case both CO and NO molecules can bind either on-top of Ti(4f) atoms or in vertical or tilted bridge configurations to neighbor Ti(4f) sites. The highest binding energies we have determined are 36 kcal/mol for CO and 87.15 kcal/mol for NO, respectively, and correspond to tilted bridge molecular configurations along the [001] direction. The large increase of the binding energies on the defective surface relative to the full oxidized surface indicates that the adsorption on vacancy defect sites takes place through a predominantly chemisorption mechanism. Additional calculations performed for N2O and N2O2 molecules indicate that on the defective surface the adsorption at Ti(4f) is also preferred with maximum adsorption energies of 51.2 and 126.7 kcal/mol.

I. Introduction The chemisorption properties of titanium dioxide are of great interest to both fundamental research and technological applications, particularly in the fields of heterogeneous catalysis, photocatalysis,1-4 and for gas sensors.5 In the case of the rutile phase of TiO2, the (110) surface has been extensively investigated as this surface is thermodynamically the most stable.4 Upon heating at high temperatures other surface orientations will reconstruct leading to (110) facets. Among various molecular systems interacting with the TiO2(110) surface, the adsorption properties of CO and NO molecules have received particular attention as these molecules are involved in several important chemical processes. For example, knowledge of CO chemisorption is specifically important to the study of hydrogenation reactions where TiO2 serves as a support for transition metals6 and for the development of gas sensors,5 where competitive gas adsorption takes place. Additionally, the CO molecule can be used as a probe adsorbate7 to provide fundamental information about the gas-surface interactions, the adsorption sites, and the reactive dynamics on different surfaces. Similarly, the interaction of the NO molecule with the TiO2 surface is relevant for various technological applications such as pollution control, through catalytic reduction of NO, or for NO gas sensors.5 * Corresponding author. † U.S. Department of Energy, National Energy Technology Laboratory. ‡ Department of Chemical and Petroleum Engineering, University of Pittsburgh. § Surface Science Center, Department of Chemistry, University of Pittsburgh.

Several previous experimental studies have been dedicated to the analysis of the chemisorption properties of CO on the TiO2 surface.8-11 Among these, the early investigations have been performed on oxidized titanium foil.9 In this case it was found that CO was weakly adsorbed on the surface when the surface was dosed at 150 K. It was suggested that CO adsorption may occur at low-coordination cation sites with a desorption activation energy of 10.5 kcal/mol. More recently, Linsebigler et al.11 have analyzed in detail the adsorption properties of CO on the oxidized TiO2(110) surface. It was determined that in this case CO adsorbs at a temperature of 105 K on the Ti inplane lattice sites and completely desorbs from the surface at 225 K. In the limit of zero coverage, a binding energy of CO on the nondefective TiO2(110) surface of 9.9 kcal/mol was reported. Additionally, CO...CO repulsive interactions were observed as the coverage was increased. The problem of CO adsorption on the oxidized TiO2(110) surface has been also analyzed in several theoretical studies, including ours.12-15 The term “oxidized” refers to a TiO2 surface which contains no oxygen vacancy sites. The earlier investigations based on ab initio molecular-orbital cluster calculations12,14 or using periodic Hartree-Fock method13 have shown the existence of a weak interaction of CO with the oxidized surface. Depending on the theoretical model used, the binding energies were spread over a range of about 10 kcal/mol but were systematically higher than the experimental value of 9.9 kcal/ mol reported by Linsebigler et al.11 This overestimation of the binding energy can be due to several types of factors: (a) the smallness of the model cluster, (b) non consideration of the surface relaxation effects, and (c) the small basis set used or the lack of electron correlation in calculations performed at the

10.1021/jp0143140 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/29/2002

Adsorption Properties of CO and NO on TiO2(110) Hartree-Fock level. Moreover, in refs 12 and 13 no discussion of the basis set superposition error is given. We have investigated the problem of CO adsorption on the nondefective TiO2(110) surface using first-principles calculations based on density functional theory and the pseudopotential method and have studied the adsorption of CO molecules on the in-plane Ti cation sites of the rutile TiO2(110) oxidized surface.15 Our calculations employed slab geometry and periodic boundary conditions with full relaxation of all atomic positions. Our results indicate that the CO molecule is vertically adsorbed on the TiO2(110) surface with a clear preference for Ti-CO orientation compared with the Ti-OC configuration. In the first case the CO molecule is adsorbed at a distance of 2.32-2.37 Å from the 5-fold-coordinated Ti sites, while in the latter case the distance is larger, between 2.66 and 2.70 Å. At half coverage the adsorption energies of 11.1 and 2.71 kcal/mol have been determined for Ti-CO and Ti-OC binding configurations, in good agreement with the results based on temperatureprogrammed desorption experiments (9.9 kcal/mol).11 We have also found that at full coverage the adsorption energies decrease by 2-3 kcal/mol relative to the half coverage case due to the repulsion between the CO molecules. As the chemisorption properties on TiO2 surfaces are strongly dependent on the presence of defect sites, an important task of various experimental studies has been the identification of the role played by the oxygen vacancy sites upon the adsorption properties. Go¨pel at al.8 have found that in the temperature range 300-373 K CO adsorption occurs only in the presence of O-vacancy point defects and an isosteric heat of adsorption of 19 kcal/mol was estimated for CO on the TiO2(110) surface. Temperature-programmed desorption (TPD) experiments of COexposed TiO2 surfaces showed a small amount of CO2 desorbing. This result was explained as being due to an oxidation reaction between CO adsorbed on O-vacancy sites and an adjacent oxygen lattice site. Linsebigler et al.11 have also analyzed the role played by anion vacancy sites produced under controlled annealing conditions in a vacuum. Their results indicate that when adsorption takes place on a surface with oxygen vacancies, an increase in the desorption temperature of a portion of the adsorbed CO is observed, without any increase in CO chemisorption capacity. It was suggested that enhanced CO bonding occurs via two types of interactions: primary bonding occurs through the interaction of C end of molecule with Ti lattice sites, while the additional interaction is due to the O moiety of CO with the anion vacancy sites. Despite the significant interest stimulated by the various experimental studies for identifying the mechanism of CO interaction with O-vacancy sites on TiO2 surface, there have been only a few theoretical attempts to clarify such problems. Practically, only Kobayashi and Yamaguchi12 have considered this problem on the basis of low-level ab initio calculations in combination with small size cluster models. These authors found that the presence of O-vacancy defects strongly enhances the binding energy of CO, with values the range of 30-36 kcal/ mol. This effect was considered to be due to a large electron back-donation from surface Ti3+ ions to the CO molecule. Similar to CO, the interaction of NO with crystal surfaces or with powdered TiO2 was found to be highly dependent on the type of oxidized or defective surface and several molecular and dissociative adsorption states were identified.16-20 Sorescu, Rusu, and Yates have analyzed21 in great detail the chemisorption properties of NO on the oxidized TiO2(110) surface. The results of TPD measurements indicate that for NO exposures

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6185 less than 1.1 × 1014 molecules/cm2, NO adsorbs weakly and desorbs at ∼127 K. The experimental activation energy for NO desorption was found equal to 8.4 kcal/mol in the limit of zero coverage. Above a critical NO exposure of 5.5 × 1014 molecules/cm2, partial conversion of NO to N2O is observed yielding N2O desorption processes at ∼169 and ∼250 K. The weak interaction between the NO molecule and the TiO2(110) surface has been also revealed from first-principles calculations based on density functional theory (DFT) and the pseudopotential method in which NO molecules are adsorbed at the inplane Ti cation sites.21 At half coverage the adsorption energies of 10.51 and 5.75 kcal/mol have been calculated for Ti-NO and Ti-ON binding configurations, respectively, in good agreement with the experimental results. At full coverage the adsorption energies were found to decrease by about 1.501.75 kcal/mol relative to the half coverage case. The lack of large chemical effects indicates that the adsorption takes place through a predominantly physisorption mechanism on the perfect surface. Wu et al.22 have also analyzed the chemisorption properties of NO on the oxidized TiO2 surface using ab initio methods in conjunction with small cluster models. In their calculations the Ti-O bonds of the clusters have been maintained fixed at the values corresponding to the real crystal. Their results support the fact that NO is weakly bound on the surface with a preference for the Ti-NO configuration. However, the calculated adsorption energies of 3.75 and 2.00 kcal/mol for TiNO and Ti-ON geometries are significantly smaller than the experimental reported adsorption energies,21 indicating some limitations of the model used. Both NO and N2O species have been observed not only on the oxidized but also on the reduced or defective surface.16,18-20 Particularly it was observed that on the annealed (reduced) surfaces, NO bonds weakly in a molecular adsorption state while N2O is formed as a reduction product. Moreover, the N2O yield was found to be highly dependent on the surface defect density. These facts indicate that formation of N2O species is mediated by the bridging-oxygen vacancies. So far the problem of NO adsorption on the TiO2 defective surface has received little theoretical attention. Only Wu et al.22 have made a first attempt to characterize different adsorption configurations of NO on a defective surface. On the basis of cluster calculations with up to two Ti atoms and frozen Ti-O bond lengths, they concluded that the most stable adsorption configurations are at Ti3+ sites, created as a result of surface oxygen defects. These configurations correspond to the on-top and vertical bridge binding configurations of NO with the O atom oriented toward the surface. As indicated by the above studies the chemisorption and the dissociation properties of small molecules on the TiO2 surface are strongly influenced by the presence of defective sites. Consequently, it is important to provide a better characterization of these sites and of the changes in chemisorption properties of various molecules at these sites. For this reason, in the present study we extend our previous theoretical investigations15,21 of the chemisorption properties of CO and NO molecules on the oxidized nondefective TiO2(110) rutile surface by considering the role played by the defective sites. Among various types of structural defects observed on the TiO2 surface,4 namely, point defects, interstitial defects, and planar defects, in the present study we will limit ourselves to the case of point defects caused by simple oxygen vacancies. Given the lower probability of formation of the in-plane oxygen atoms defects on the TiO2(110) surface it is expected that the major role will be played

6186 J. Phys. Chem. B, Vol. 106, No. 24, 2002 in this case by vacancies formed as a result of bridging oxygen desorption.23 Such vacancies can be easily produced by annealing at temperatures above 500 K and the vacancy coverage increases with increasing annealing temperatures.18 II. Computational Method The computational method employed in the present study is similar to that which we have previously used to analyze the adsorption properties of CO and NO on the perfect TiO2(110) surface.15,21 Basically, the periodic nature of the surface is considered in the present simulations by the aid of a tridimensional model. Within this model, the surface is simulated by supercells repeated periodically in all three directions. Repetition of the simulation box in the plane of the slab creates an infinite slab, while periodicity in the direction perpendicular to the slab creates an infinite stack of slabs. By separating each slab from its neighbors by a vacuum layer, the otherwise unphysical interactions between slabs in a direction perpendicular to the surface are made negligible. The calculations performed in this study were done using the VASP code.24-26 This program evaluates the total energy of periodically repeating geometries based on density-functional theory and the pseudopotential approximation. In this case only the valence electrons are represented explicitly in the calculations, the electron-ion interaction is described by fully nonlocal optimized ultrasoft pseudopotentials similar to those introduced by Vanderbilt.27,28 Such pseudopotentials allow the use of a considerably smaller plane wave cutoff energy than would be needed with standard norm-conserving pseudopotentials. Periodic boundary conditions are used with the occupied electronic orbitals expanded in a plane-wave basis. The expansion includes all plane waves whose kinetic energy p2k2/2m < Ecut where k is the wave vector, m the electronic mass, and Ecut is the chosen cutoff energy. This cutoff energy is chosen to ensure the convergence with respect to the basis set. In all our calculations we have used a cutoff energy of 495 eV. The calculations have been done using the spin-polarized Perdew-Wang 91 (PW91) generalized gradient-corrected exchange-correlation functional.29 Throughout calculations the total spin of the system has been allowed to relax. The final adsorption energies reported in this study correspond to the most stable states among different spin states. The Brillouin zone was sampled with the lowest-order Monkhorst-Pack30 set of two k-points, with the component parallel to the surface normal set to zero. The minimization of the electronic free energy was performed using an efficient iterative matrix-diagonalization routine based on a sequential band-by-band residuum minimization method (RMM)25,26 or, alternatively, based on preconditioned band-byband conjugate-gradient (CG) minimization.31 The optimization of different atomic configurations was performed on the basis of a conjugate-gradient minimization of the total energy. III. Results and Discussion A. Preliminary Tests for Bulk TiO2, Oxidized and Defective TiO2(110) Surfaces, and for Isolated CO and NO Molecules. The first step in our computational studies was represented by the analysis of the performances of the set of ultrasoft pseudopotentials provided in VASP code in conjunction with PW91 exchange-correlation functional to describe the equilibrium configurations of bulk TiO2, of the oxidized and defective bare surfaces, and of the isolated CO and NO molecules. The corresponding results obtained from these tests are presented in Appendices 1-3. In this section we will summarize the main conclusions of these tests.

Sorescu and Yates For the cutoff energy of 495 eV, the optimized lattice parameters of bulk TiO2 are a ) 4.659 Å and c ) 2.974 Å with the internal parameter u ) 0.3048. These data differ by only 1.42%, 0.5%, and 0.0%, respectively, from the corresponding experimental values,32 indicating a good prediction of TiO2 crystallographic parameters. The TiO2(110) surface was modeled as a 2 × 1 slab with five layers containing 20 Ti and 40 O atoms (see Figure 1a). The surface unit cell has dimensions of x2a × 2c along [1h10] and [001] directions, where a and c are the crystallographic dimension of the bulk TiO2 unit cell. As a result of relaxation it was found that significant displacements of surface ions take place, particularly along the direction [110] normal to the surface. These findings confirm quantitatively the previous results obtained by Bates et al.33 and semiquantitatively the LDA data by Ramamoorthy et al.34 Our results also agree for the great majority of surface atoms with data obtained by X-ray diffraction measurements.35 The largest differences have been noticed in the case of the bridging oxygen atoms where larger relaxations have been obtained experimentally compared to those predicted by our calculations. In the case of defective surface we have considered two models. The first one corresponds to removal of alternate bridging oxygens and has been denoted as 1Ob (see Figure 1b). The second model refers to removal of all bridging oxygen atoms in the system (see Figure 1c). This model was denoted as 2Ob. In the case of the defective 1Ob surface we found that the major relaxations are along the [110] direction. However, there are additional small displacements of the in-plane oxygens along the [001] direction leading to a lowering of the symmetry of the surface. For the 2Ob surface model, the major relaxations take place for the in-plane and subsurface oxygen atoms, which are moving predominantly along the [110] direction. These results confirm the previous findings of Lindan et al.36 based on a 1 × 1 reduced surface model. Finally, our tests for the equilibrium geometries of CO and NO molecules indicate that at the cutoff energy of 495 eV the calculated values are in reasonable agreement with the corresponding experimental values37 with errors below 1.5% and 2.0%, respectively. For both these molecules, the calculated vibrational frequencies are within 1.0% of the experimental values. From the results of these tests if can be concluded that our theoretical approach is able to give a good description for both the bulk TiO2, for the bare surface, and for the isolated molecules of interest. Such characteristics are required conditions for an accurate description of the interaction of CO and NO molecules with either oxidized or defective TiO2(110) surfaces. B. Adsorption of the CO Molecule on TiO2(110). The analysis of the adsorption properties of CO on TiO2(110) surface has been performed for both the oxidized surface as well as for the surface with bridging oxygen vacancies. In this last case we have considered both the 1Ob and 2Ob defective surfaces (described in the Appendix) corresponding to one, respectively, two bridging oxygen atoms per surface unit cell removed (see Figures 1b and 1c). The study of the adsorption properties of CO on the oxidized surface is similar to the one previously reported by us15 and has been repeated here to determine the variability of the corresponding results with the size of the slab (increased in this study from three to five layers) and with the new set of ultrasoft pseudopotentials instead of hard norm-conserving pseudopotential, previously used. In a number of instances, particularly

Adsorption Properties of CO and NO on TiO2(110)

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6187

Figure 1. (a) The 2 × 1 configuration of the slab used in calculations corresponding to the TiO2(110) oxidized surface. Inserts (b) and (c) correspond to 1Ob and 2Ob bridging-oxygen defective models described in text. The simulation cells are extended for display purposes.

for the oxidized surface, the dependence of the adsorption energy on CO coverage was also investigated by placing one or two CO molecules in each repeating simulation box. Thus CO coverages of θ ) 0.5 and θ ) 1.0 can be studied with these models. Additionally, we have analyzed the variation of the chemisorption properties when adsorption takes place through the C or O ends of the molecule. For all configurations considered we calculated the adsorption energies according to the expression:

Eads ) (N*Emolec + Eslab - Etot)/N

(1)

where Emolec is the energy of the isolated NO molecule in its equilibrium position, Eslab is the total energy of the slab, Etot is the total energy of the adsorbate/slab system, and N is the total number of CO molecules in the unit cell, i.e., N ) 1 or 2. A positive Eads corresponds to a stable adsorbate/slab system. In calculating the energies of the bare slab and of the moleculeslab systems the same Brillouin-zone sampling has been used. The values of the calculated binding energies and the corresponding geometric parameters for CO molecules adsorbed on oxidized, 1Ob and 2Ob defective surfaces are given in Table 1. Some representative atomic configurations are presented in Figure 2. For the oxidized surface (entries 1-4 in Table 1) we see that the most stable configuration corresponds to CO perpendicular to the plane of the surface with the C atom oriented toward the surface (see Figure 2a). At half coverage the Ti-CO configuration is more stable by 6.83 kcal/mol relative to the Ti-OC orientation. In the first case the equilibrium Ti-C distance is

2.372 Å versus a Ti-O distance of 2.533 Å for the Ti-OC orientation. When the coverage is increased from θ ) 0.5 to θ ) 1 the adsorption energy decreases by 2.6 kcal/mol for TiCO and 2.3 kcal/mol for Ti-OC configurations. This behavior is mainly due to the increase of the CO...CO repulsion interactions. In our previous studies15 we have shown that the intermolecular CO...CO interactions are very small for distances larger than 6 Å. At half coverage the separation distance between neighbor CO molecules is about 5.95 Å. This implies that the lateral interactions between molecules at half coverage are small. Consequently, the calculated binding energy for the half coverage case can be considered as a close estimation of the true zero-coverage binding energy. By comparing the present sets of results with the experimental data obtained by Linsebigler et al.11 we note that the calculated adsorption energy of 9.04 kcal/mol is in very good agreement with the value of 9.9 kcal/ mol determined experimentally in the limit of zero coverage. Our previous theoretical results15 involving a 3 layer slab model yielded an adsorption energy of 11.1 kcal/mol. The second set of configurations we have analyzed correspond to adsorption of CO on the defective 1Ob surface (see entries 5-7 in Table 1). In this case two main configurations have been considered. The first one corresponds to adsorption at the Ti(5f) site while the second one takes place at the bridging oxygen vacancy site, denoted herewith as @Ob. As can be seen from the data provided in Table 1 both binding energies remain small with values of 4.66 kcal/mol at the Ti(5f) site and 7.40 kcal/ mol at the @Ob vacancy site. The reverse orientation of CO at the @Ob defective site is even less favorable than the one with

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Sorescu and Yates

TABLE 1: Calculated Equilibrium Distances and Angles and the Corresponding Adsorption Energies for CO Molecule on TiO2(110) Surface at Different Coverages Θa system/modelb

Θ

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Ox+CO (5f,v) Ox+CO (5f,v) Ox+OC (5f,v) Ox+OC (5f,v) 1Ob+CO (5f,v) 1Ob+CO (@Ob,v) 1Ob+OC (@Ob,v) 2Ob+CO (5f,v) 2Ob+CO (4f,v) 2Ob+CO (4f,br,v) 2Ob+CO (4f,br,t)

0.5 1.0 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5

12. 13.

2Ob+OC (4f,br,v) 2Ob+OC (4f,br,t)

0.5 0.5

1. 10. 11. 12.

bond lengths (Å) r(Ti-C) r(Ti-C) r(Ti-C) r(Ti-C) r(Ti-C) r(Ti-C) r(Ti-O) r(Ti-C) r(Ti-C) r(Ti-C) r(Ti1-C) r(Ti2-C) r(Ti2-O) r(Ti-O) r(Ti1-O) r(Ti2-O) r(Ti2-C)

2.372 2.394 2.533 2.587 2.424 2.469 2.760 2.305 2.077 2.323 2.060 2.259 2.213 2.275 2.078 2.194 2.205

Eads (kcal/mol)

Fig. 2

r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O) r(C-O)

1.139 1.140 1.147 1.148 1.141 1.153 1.150 1.147 1.168 1.170 1.212

θ(Ti-C-O) θ(Ti-C-O) θ(Ti-O-C) θ(Ti-O-C) θ(Ti-O-C) θ(Ti-O-C) θ(n-O-C) θ(Ti-C-O) θ(Ti-C-O) θ(Ti-C-O) θ(n-C-O)

bond angles (deg.) 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0 59.6

9.04 6.44 2.21 -0.17 4.66 7.40 2.68 6.05 26.77 24.01 35.96

a

r(C-O) r(C-O)

1.165 1.225

θ(n-O-C) θ(n-O-C)

180.0 180.0

19.21 20.32

b c d e f

system

q(C)

q(O)

q(Ti)c

Ox+CO (5f) 2Ob+CO (4f) 2Ob+CO (br) 2Ob+CO (tilt-4f)

0.00 -0.48 -1.11 -1.44

-0.38 -0.38 -0.72 -0.90

3.78 3.54 3.49, 3.48 3.70, 3.48

a Selected configurations depicted in Figure 2 are indicated in the last column. In a number of instances the Mulliken charges on C, O and Ti sites involved in the direct binding of the CO molecule are also indicated. b Notations CO and OC pertain to the orientations of the CO molecule on the rutile surface, i.e. with C and O atoms toward the surface. Entries 1-4 (Ox) correspond to the oxidized surface, entries 5-7 (10b) correspond to the defective surface with one bridging oxygen removed, entries 8-13 (20b) correspond to the defective surface with all the bridging oxygen atoms removed. The abbreviations used are: Ox-oxidized surface, 10b-defective surface with one bridging oxygen removed, 20b-defective surface with two bridging oxygen atoms removed, 5f and 4f-refer to the Ti 5-folded and 4-folded sites, br-bridging configuration between two Ti(4f) sites, @Ob-adsorption configuration at the bridging oxygen vacancy site, v-verticle adsorption configuration, t-tilted adsorption configuration. The symbol n used to denote n-C-O or n-O-C angles refers to angles formed by the C-O bond with the surface normal. c The indicated charge corresponds to the Ti site at which the binding takes place. In the case of bridge configurations two sets of charges are indicated corresponding to the two binding Ti sites.

C atom toward the surface by 4.72 kcal/mol. These data indicate that a single bridging oxygen vacancy has a relatively small effect on the binding energy of CO. Practically the molecule is weakly bound in a physisorption state. For these configurations the spin state is triplet as is the case for the 1Ob defective surface without CO adsorbed on it. The major structural changes we noticed for these states are some large relaxations of the atoms surrounding the vacancy, particularly involving the in-plane and subsurface oxygen atoms. For example, when CO adsorption takes place at the Ti(5f) site, the subsurface O(s1′) atoms move significantly toward the surface such that the Ti(6f)-O(s1′) bonds decrease from 2.091 to 1.883 Å. Additional relaxations take place for the in-plane oxygen atoms which move toward the vacancy by 0.02 Å while the subsurface O(s2′) atoms move upward by a similar distance. The displacement of subsurface O(s1′) atoms is slightly smaller but still significant in the case when adsorption takes place at the @Ob defective site with a variation of the Ti(6f)-O(s1′) bonds of 0.11 Å relative to the nondefective surface. Finally, in the case of adsorption on the defective 2Ob surface new adsorption sites beside Ti(5f), become accessible at Ti(4f) ions. We have identified three major adsorption configurations for CO molecule: a first configuration corresponds to CO molecule adsorbed on-top of the Ti(4f) site, the second one corresponds to CO adsorbed perpendicular on the surface in a bridge configuration with neighbor Ti(4f) ions, and finally a bridge, tilted configuration of CO between the neighbor Ti(4f) sites (see entries 9-13 in Table 1 and Figures 2d-2f). Both orientations of CO molecule with C or O atoms toward the surface have been analyzed in this case. From the data provided

in Table 1 it can be observed that the adsorption energies for configurations with C atoms toward the surface are significantly more stable than those with the O toward the surface. For the C-down orientations the calculated adsorption energies with values between 26.77 and 35.96 kcal/mol are significantly larger than those obtained on the oxidized surface indicating a significant coupling between CO molecule and the surface ions. We find that the most stable configuration is a bridge state in which CO bond makes an angle of about 60° with the surface normal. The significant binding is also indicated by the total spin value which decreases from S ) 2 for the 2Ob surface to S ) 1 when CO molecule adsorbs in this titled bridge configurations. In this case the C atom is separated by the neighbor Ti(4f) sites by 2.060 and 2.259 Å, respectively, while the shortest Ti(4f)-O distance (for the O atom of CO molecule) is 2.213 Å. For the case of CO configurations with the O atom toward the surface the interaction with surface atoms becomes also significant relative to the oxidized surface. As in the C-down case we find that for the Ti-OC orientation, the tilted bridge configuration is also the most favorable with an adsorption energy of 20.32 kcal/mol. Additionally, the spin state corresponds to a triplet state as in the case of C-down adsorption configurations. Overall, these sets of values indicate that adsorption on Ti(4f) sites is significantly stronger relative to the one at Ti(5f) sites with a preferred bridge orientation in which CO molecule is titled relative to surface normal and has the C atom toward the surface. C. Bonding Mechanism for CO on TiO2(110). Further insight in the type of bonding mechanism for CO on the TiO2(110) surface can be obtained by analyzing the distribution of



Figure 2. The main adsorption configurations of CO on oxidized and defective TiO2 (110) surface. (a) adsorption at Ti(5f) site on oxidized surface; (b) adsorption at Ti(5f) on 10b defective surface; (c) adsorption at @Ob site for 1Ob defective model; (d) adsorption on-top of Ti(4f) site; (e) bridge vertical configuration between two Ti(4f) sites; (f) bridge tilted configuration between two Ti(4f) sites. The view corresponding to plots (a-c) is along [001] direction while the view for plots (d-f) is along [11h0] direction, respectively.

the electron density in the molecule-slab system. For the case of the oxidized surface we have shown previously15 that adsorption of CO does not produce any significant change in the electronic distribution. Consequently, in the present study we will focus on the case of bonding on defective surface in the particular case of 2Ob model. In Figures 3a and 4a we present a contour plot of the electron densities in a plane perpendicular to the (110) surface which passes through the CO molecule and Ti(4f) sites for the case of vertical adsorption on top of Ti(4f) site or for the tilted configuration between these sites. In these figures the [110] direction corresponds to the vertical axis while the horizontal axis is parallel to the [001] direction. As can be seen in both figures, the valence electron density is no longer concentrated only on C and O atoms (as we have noticed for the oxidized surface15) but also in the region between the C and the Ti atoms. Also, while on to bottom side of the slab the Ti atoms are almost invisible, while in the upper part of the slab the Ti atoms are significantly more visible. The adsorption of CO causes changes in the electronic distribution, particularly in the region between the C atom and the Ti(4f) sites underneath (see Figure 3a) or nearby neighbor Ti(4f) sites (see Figure 4a). To investigate more precisely the effect of molecular adsorption on the change of electron distribution we have also calculated the difference electron density maps. These are calculated by subtracting from the electron density of the adsorbent-adsorbate system the electron densities of both the TiO2(110) slab and of the CO isolated molecules, with the relative position corresponding to the adsorbed configuration. These differential maps are shown in Figures 3b and 4b.

As can be seen the distortion effects are localized basically in the region of the adsorption sites and their magnitudes are large with values up to 0.25 electrons/Å3 in the case of adsorption at Ti(4f) sites or 0.40 electrons/Å3 for the tilted adsorption configuration between neighbor Ti(4f) sites. When adsorption occurs there is a clear increase of the electronic charge in the region between carbon and titanium atoms. In the case of vertical adsorption on top of Ti(4f) site there is also a large quadrupolar distortion of the neighbor Ti (4f) sites. In the case of tilted configuration, practically the entire region between Ti(4f) sites and the tilted CO molecule is polarized with significant charge variations on Ti and C atoms. These large electrostatic distortions correspond to a chemical binding mechanism. The charge redistribution effects can be more clearly understood on the basis of the evaluation of Mulliken charges of different atoms of the system. It is known that these types of charges can provide a qualitative description of the bonding character. These quantities have been determined using the formalism developed by Segall et all.38 which is incorporated in the CASTEP package.39 Selective variations of the individual atomic Mulliken charges for C and O atoms as well as for the Ti atoms involved directly in the bonding are presented in Table 1. From these values it can be seen that when adsorption takes place at Ti(4f) sites a significant charge transfer to the CO molecule takes place with a corresponding depletion of the charge on Ti sites involved in binding. This charge transfer effect is highly correlated with the increase of the binding energy, such that for the adsorption in a bridge, tilted configuration between Ti(4f) sites the CO molecule is most negatively


Sorescu and Yates

Figure 3. A contour plot of the valence electron density (units of electrons/Å3) (a) and the difference between the electron density shown in (a) and the one resulting from the superposition of the TiO2(110) slab and the isolated CO molecule (b) in the case of adsorption on-top of Ti(4f) sites for 2Ob defective surface. The contours are taken along (001) plane containing the CO molecules and the surface Ti(4f) sites. The [110] direction is parallel to the vertical axis.

charged. Additionally, in contradistinction to the oxidized case where there are no changes of the CO equilibrium bond distance upon adsorption, we notice that on the defective surface, particularly on the 2Ob surface adsorption significantly alters the CO equilibrium bond. From the data provided in Table 1 we observe a continuous increase of the CO bond length with the increase of the binding energy for on-top, bridge or tilted configurations on the 2Ob surface. These findings support the existence of a strong surface bonding due to significant charge transfer from Ti atoms to CO molecule. D. Adsorption of the NO Molecule on TiO2(110) Surface. The adsorption of NO was investigated in a manner similar to the one described for the case of CO molecule. Particularly we have considered the oxidized as well as the defective surfaces. The calculated binding energies of a NO molecule on the oxidized surface are given in Table 2 (see entries 1-3). We have analyzed both orientations of NO molecule with either end toward the surface. The results obtained indicate that the most stable configuration corresponds to a tilted geometry of NO molecule with N atom toward the surface. In this case the molecule is adsorbed at about 2.459 Å from the Ti(5f) site with a tilt angle Ti-N-O of about 128°. This configuration has an adsorption energy of 8.06 kcal/mol and is more stable by about 3.5 kcal/mol than the Ti-ON geometry. We note that for the N-down configuration the potential energy is shallow and other equilibrium configurations with NO molecule out of the (1h10)plane are also possible (see entry 2 in Table 2).

These results confirm our previous findings about the adsorption of NO on oxidized TiO2(110) surface.21 However, in the present study the values of the calculated adsorption energies, particularly for the Ti-NO configuration, are slightly smaller by 2.4 kcal/mol then those we previously reported.21 This change brings the current calculated adsorption energy of 8.0 kcal/mol in even better agreement with the experimental activation energy for NO desorption from nondefective TiO2(110) of 8.4 kcal/mol.21 When considering the adsorption on a defective surface we notice a strong influence on the adsorption properties at Ti(5f) sites. For example, as indicated in Table 2 (see entries 4-6) in the case of 1Ob defective surface the adsorption energies increase significantly to about 19.0 kcal/mol for the Ti-NO orientation and to about 8.2 kcal/mol for the Ti-ON orientation. The increased coupling between NO molecule and the surface is also evidenced by the spin state, which changes from triplet to doublet upon adsorption. For this surface we found that in the equilibrium configuration the NO molecule is almost perpendicular to the surface. Additionally, there is a slight elongation of about 0.03 Å for the NO bond relative to the geometry found on the oxidized surface. The binding energy for NO adsorbed at the Ti(5f) site is even higher in the case of 2Ob defective surface (entry 9 in Table 2 and Figure 5b). In this case values around 25 kcal/mol are obtained for the TiNO configurations and of 15 kcal/mol for Ti-ON configura-



Figure 4. A contour plot of the valence electron density (units of electrons/Å3) (a) and the difference between the electron density shown in (a) and the one resulting from the superposition of the TiO2(110) slab and the isolated CO molecule (b) in the case of adsorption in a tilt configuration between Ti(4f) sites on 2Ob defective surface. The contours are taken along (001) plane containing the CO molecules and the surface Ti(4f) sites. The [110] direction is parallel to the vertical axis.

tions. In both these situations the NO molecule is either perpendicular or nearly perpendicular to the surface. On the 2Ob surface we have evaluated not only the adsorption on top of Ti(5f) sites but also the bridge binding between two neighbor Ti(5f) sites. In this particular case both vertical or tilted bridge configurations (see Figures 5c and 5d) have been analyzed and we have observed (see entries 10 and 11 in Table 2) that the bridge tilted structure is the most strongly bound to the surface with an adsorption energy of 35.0 kcal/mol. The corresponding spin state is doublet. In this configuration the NO molecule is tilted in the (001) plane with an angle between the vertical axis and the molecular NO axis of 70.4°. The increase of the adsorption energy takes place with an unusual large elongation of the NO bond, which reaches an equilibrium value of 1.311 Å. Beside the Ti(5f) site, in the case of 1Ob surface new active sites become available. In this case we have analyzed the adsorption of NO at the @Ob site (see Figure 5a). As indicated in Table 2 (see entries 7 and 8) both configurations with either N or O atoms oriented toward the surface are important with adsorption energies of 36.7 and 25.8 kcal/mol, respectively. In these configurations the equilibrium NO distances are significantly stretched by 0.05-0.09 Å relative to geometries seen for the oxidized surface. Finally, in the case of 2Ob defective surface we have identified a third set of configurations corresponding to adsorption on top of the Ti(4f) sites or bridged between these sites

(see entries 12-13 and 15-17 in Table 2). On top of the Ti(4f) sites NO adsorbs by 49.0 kcal/mol in Ti-NO configuration (see Figure 5e) and by 22.5 kcal/mol in Ti-ON orientation. Correspondingly, the Ti-N and Ti-O separations decrease significantly relative to adsorption at Ti(5f) case from 2.459 and 2.670 Å to about 1.883 and 1.945 Å, respectively. However, as in the case of CO molecular adsorption, the largest adsorption energies have been obtained for the case of bridge tilted configurations of NO between Ti(4f) sites (see Figures 5f). Inthis case the adsorption energies are quite high with values of 87.1 and 69.0 kcal/mol for the two orientations of NO with N or O toward the surface. In both these case the spin state is doublet. E. Bonding Mechanism for NO on TiO2(110). We present in Figures 6a and 7a the contour plots of the electron densities in a plane perpendicular to the (110) surface which passes through the N and O atoms for two important adsorption configurations: adsorption on top of Ti(4f) site and the bridge tilted configuration between Ti(4f) sites. In these figures the [110] direction corresponds to the vertical axis while the horizontal axis is parallel to the [001] direction. Additionally, we represent in Figures 6b and 7c the difference electron density maps obtained by subtracting from the electron density of the adsorbent-adsorbate system the electron densities of the slab and individual molecule. For both these configurations there is a significant perturbation of the electronic charge in the region between the N atom and Ti(4f) sites. In particular we can see from the charge difference maps depletion of the charge on Ti


Sorescu and Yates

TABLE 2: Calculated Equilibrium Distances and Angles and the Corresponding Adsorption Energies for the NO on TiO2(110) Surface at Θ ) 0.5 Coveragea system/modelb 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Ox+NO (5f,t)* Ox+NO (5f,t)** Ox+ON (5f,t) 1Ob+NO (5f,t) 1Ob+NO (5f,v) 1Ob+ON (5f,v) 1Ob+NO (@Ob,v) 1Ob+ON (@Ob,v) 2Ob+NO (5f,t) 2Ob+NO (5f,br,v) 2Ob+NO (5f,br,t)

12. 13.

2Ob+NO (4f,v) 2Ob+NO (4f,br,t)

14. 15. 16. 17.

2Ob+ON (5f,t) 2Ob+ON (4f,v) 2Ob+ON (4f,br,v) 2Ob+ON (4f,br,t)

1. 10. 12. 13. 18.

bond lengths (Å) r(Ti-N) r(Ti-N) r(Ti-O) r(Ti-N) r(Ti-N) r(Ti-O) r(Ti-N) r(Ti-O) r(Ti-N) r(Ti-N) r(Ti1-N) r(Ti2-N) r(Ti2-O) r(Ti-N) r(Ti1-N) r(Ti2-N) r(Ti2-O) r(Ti-O) r(Ti-O) r(Ti-O) r(Ti1-O) r(Ti2-O) r(Ti2-N)

2.459 2.446 2.670 1.927 1.932 1.984 2.181 2.145 1.983 2.198 1.831 2.282 1.946 1.883 1.834 2.033 1.948 1.975 1.945 2.105 1.911 2.861 2.736

bond angles (deg.)

Eads (kcal/mol)

Fig. 5

r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O) r(N-O)

1.163 1.164 1.172 1.197 1.195 1.215 1.209 1.267 1.194 1.209 1.311

θ(Ti-N-O) θ(Ti-N-O) θ(Ti-O-N) θ(Ti-N-O) θ(Ti-N-O) θ(Ti-O-N) θ(Ti-N-O) θ(Ti-N-O) θ(Ti-N-O) θ(Ti-N-O) θ(n-N-O)

127.6 130.6 130.7 173.1 180.0 180.0 134.8 132.5 174.8 137.5 70.4

8.06 8.02 4.15 18.85 19.01 8.16 36.76 25.85 25.16 15.30 35.00

b c d

r(N-O) r(N-O)

1.204 1.310

θ(Ti-N-O) θ(n-N-O)

180.0 63.0

49.02 87.15

e f

r(N-O) r(N-O) r(N-O) r(N-O)

1.219 1.235 1.269 1.348

θ(Ti-N-O) θ(Ti-N-O) θ(Ti-N-O) θ(n-N-O)

175.5 180.0 180.0 67.5

15.36 22.52 35.49 69.02

a

system

q(N)

q(O)

q(Ti)c

Ox+NO (5f) 2Ob+NO (5f) 2Ob+NO (4f) 2Ob+CO (tilt-4f) 2Ob+OC (tilt-4f)

-0.04 -0.96 -0.81 -1.96 -1.39

-0.17 -0.32 -1.45 -1.87 -1.17

3.83 3.42 3.61 3.97, 3.77 3.95, 3.86

a Selected configurations depicted in Figure 5 are also indicated in the last column. b Notations NO and ON pertain to the orientations of the NO molecule on the rutile surface, i.e. with N and O atoms toward the surface. The abbreviations used are: Ox-oxidized surface, 1Ob-defective surface with one bridging oxygen removed, 2Ob-defective surface with two bridging oxygen atoms removed, 5f and 4f-refer to the Ti 5-folded and 4-folded sites, br-bridging configuration between two Ti (4f) sites, @Ob-adsorption configuration at the bridging oxygen vacancy site, v-verticle configuration, t-tilted configuration, *-configuration in (1h10) plane; (**)-configuration out of (1h10) plane.

atoms involved in bonding with a corresponding increase of the total charge on the NO molecule. The charge transfer between the surface and adsorbed NO can be also seen from the values of Mulliken charges indicated in Table 2 for selective configurations. We observe that the defective 2Ob surface has a strong effect on the amount of charge transferred to the NO molecule. Moreover, among various adsorption configurations the amount of charge transferred to NO molecule is directly correlated with the strength of adsorption such that for the tilted bridge configurations this amount is the largest. These findings suggest that the presence of defects on the TiO2(110) surface changes significantly the type of adsorption mechanism from physisorption to chemisorption. In this process some significant geometric changes take place, particularly for Ti-N or Ti-O distances. Additionally, there is a continuous elongation of the N-O bond lengths with the strength of the binding energies. For example, in the case of 1Ob+NO(@Ob,v) structure the NO bond increases to 1.267 Å while for 2Ob+NO(4f,br,tilt) structure this bond reaches 1.310 Å. These data clearly suggest a continuous weakening of the NO bond with the increase of the surface binding energy. Such a process will favor the dissociation of the NO molecule and its reaction with other NO molecules. F. Adsorption of N2O and N2O2 Species. In the previous experimental studies17 it was found that on the annealed, defective TiO2 surfaces, N2O was formed by a reduction process. Consequently we have extended in the present work our analysis

to include the adsorption properties of this molecule. The corresponding geometric and energetic data calculated for this species on either the oxidized or defective 2Ob surfaces are given in Table 3, while selective atomic configurations are depicted in Figure 8. The results obtained on the oxidized surface indicate that adsorption of N2O could take place through a vertical configuration in which the Ti-N-N-O orientation is slightly more stable than the Ti-O-N-N geometry. The adsorption energies for these two configurations are small with values of 5.79 and 2.30 kcal/mol (see entries 1 and 2 in Table 3). Consequently, such species can be found on the surface only at low temperatures. In the case of 1Ob structure we have also considered the possibility for a bridge binding between Ti(5f) site and the @Ob sites (see Figure 8a). Our results (see entry 3 in Table 3) indicate that N2O can adsorb in such a configuration with an adsorption energy of 20.56 kcal/mol. Finally, when the adsorption takes place on the defective 2Ob surface, on top of the Ti(4f) site the adsorption energies increase but remain small with values of 8.93 kcal/mol for the Ti-NN-O configuration and 4.64 kcal/mol for the Ti-O-N-N structure. However, for this surface some other adsorption configurations become important, particularly involving neighbor Ti(4f) sites. For example we have identified two such configurations as presented in Figures 8b and 8c. The first one corresponds to a chairlike structure of N2O between two neighbor Ti(4f) sites while the second one is a symmetric bridge



Figure 5. Selective adsorption configurations of NO on oxidized and defective TiO2 (110) surface: (a) adsorption at @Ob site for 10b defective model; (b) adsorption at Ti(5f) site for 2Ob defective model; (c) bridge vertical configuration between two Ti(5f) sites on 2Ob defective model; (d) bridge tilted configuration between two Ti(5f) sites for 2Ob defective model; (e) adsorption on-top of Ti(4f) site for 2Ob defective model; (f) bridge tilted configuration between two Ti(4f) sites for 2Ob defective model. The view corresponding to plots (a-c) is along [001] direction while the view for plots (d-f) is along [11h0] direction, respectively.

TABLE 3: Calculated Equilibrium Distances and Angles and the Corresponding Adsorption Energies for the N2O Molecules and N2O2 Isomers on TiO2(110) Surface at Θ ) 0.5 Coveragea system/modelb

bond lengths (Å)

1.

Ox+N2O (5f,v)

r(Ti-N1)

2.432

2.

Ox+ON2 (5f,v)

r(Ti-O)

2.593

3.

1Ob+N2O (br,5f-@1Ob)

4.

2Ob+N2O (4f,v)

r(Ti4f-N1) r(Ti5f-O) r(Ti-N1)

2.110 2.005 2.134

5.

2Ob+N2O (br,sym)

6.

2Ob+N2O (br,chair)

7.

Ox+N2O2(5f,sym)

8.

2Ob+N2O2(4f,sym)

9.

2Ob+ONNO (4f,sym)

10.

2Ob+ONNO(br,5f-@1Ob)c

r(Ti1-N2) r(Ti2-N2) r(Ti1-N1) r(Ti2-N2) r(Ti-N1) r(Ti-N2) r(Ti1-N1) r(Ti-N2) r(Ti1-O) r(Ti2-O) r(Ti1-N1) r(Ti2-N2)

2.117 2.171 2.037 2.062 2.376 2.361 1.944 1.911 2.037 2.155 1.944 1.911

bond angles (deg.)

r(N2-O) r(N1-N2) r(N2-O) r(N1-N2) r(N2-O) r(N1-N2) r(N2-O) r(N1-N2) r(N2-O) r(N1-N2) r(N2-O) r(N1-N2) r(N-O) r(N1-N2) r(N-O) r(N1-N2) r(N-O) r(N1-N2) r(N-O) r(N1...N2)

1.193 1.137 1.198 1.139 1.337 1.255 1.205 1.152 1.320 1.224 1.235 1.264 1.172 1.909 1.194 2.329 1.396 1.247 1.194 2.329

Eads (kcal/mol)

Fig. 8

θ(Ti-N-O)

180.0

5.79

θ(Ti-N-O)

180.0

2.30

θ(N1-N2-O)

118.5

20.56

θ(Ti-N-O)

180.0

8.93

θ(N1-N2-O)

136.2

51.12

c

θ(N1-N2-O)

132.9

48.99

b

θ(N1-N2-O2) θ(N2-N1-O1) θ(N1-N2-O2) θ(N2-N1-O1) θ(N1-N2-O2) θ(N2-N1-O1) θ(N1-N2-O) θ(N1-N2-O)

107.2 107.4 92.2 91.0 116.1 116.1 125.9 128.9

12.93

a

44.99

e

126.76

f

-25.90

d

a Selected configurations depicted in Figure 8 are also indicated in the last column. b Notations N O and ON , respectively N O and ONNO, 2 2 2 2 pertain to the orientations of the N2O and N2O2 molecules with N and respectively O atoms toward the surface. The superscript abbreviations used represent: (v)-vertical configuration; (t)-tilted configuration, (*)-configuration in (1h10) plane; (**)-configuration out of (1h10) plane. c The adsorption energy was calculated with respect to oxidized surface and isolated N2O molecule.

structure between these sites. In both these cases the adsorption energies are high with values of 49 and 51 kcal/mol, respectively.

Besides the independent adsorption configurations of NO molecules, we have found in our previous study21 that at full coverage the N2O2 species can be formed. Among several


Sorescu and Yates

Figure 6. A contour plot of the valence electron density (units of electrons/Å3) (a) and the difference between the electron density shown in (a) and the one resulting from the superposition of the TiO2(110) slab and the isolated NO molecule (b) in the case of adsorption on top of Ti(4f) sites for 2Ob defective surface. The contours are taken along (001) plane containing the NO molecules and the surface Ti(4f) sites. The [110] direction is parallel to the vertical axis.

different N2O2 isomers analyzed, we determined that the most stable has a cis-ONNO configuration formed between two Ti(5f) sites with a binding energy of 13.6 kcal/mol in the singlet state.15 In the present work we reconsider the adsorption of the N2O2 molecular species for both the oxidized and as well as for the 2Ob defective surfaces. As indicated in Table 3 (see entry 7), on the oxidized surface the calculated adsorption energy in the present study is 12.93 kcal/mol, slightly smaller than the value determined in our previous study of 13.6 kcal/mol.21 On the 2Ob defective surface however, the adsorption energies are significantly increased. In particular when molecular binding takes place between Ti(4f) sites with N atoms down (see Figure 8e) the adsorption energy relative to 2Ob surface and the isolated N2O2 molecule is found equal to 44.9 kcal/ mol. In this configuration the N-N bond is significantly elongated to 2.329 Å as a result of strong interaction with Ti(4f) sites. The corresponding spin state is triplet. When the binding of the N2O2 molecule takes place with O atoms toward the surface we have identified a very stable configuration as illustrated in Figure 8f. In this case the oxygen atoms occupy positions similar to those of the bridging oxygens in the oxidized surface. The overall adsorption energy of this structure is quite high with a value of 126.7 kcal/mol. In this configuration the N-O and N-N bonds of N2O2 are stretched by 0.22 and 0.74

Å, respectively, relative to the values found for isolated N2O2 molecule. Finally, we have analyzed the adsorption of the N2O2 molecule between Ti(5f) and @Ob sites in the case of the 1Ob defective surface (see Figure 8d). Our results indicate that relative to the 1Ob surface and the isolated N2O2 molecule such an adsorption configuration has a high adsorption energy of 85.3 kcal/mol. However, relative to the oxidized surface and the isolated N2O molecule the adsorption energy is -25.90 kcal/ mol. These results indicate that once an N2O2 molecule adsorbs at the @Ob site the structure becomes unstable. As a result an N2O molecule is desorbed together with formation of a lattice bridging oxygen. It is important to note that the molecular complex ONNO can be also obtained as a result of the interaction of a NO molecule with another NO molecule already adsorbed at the @Ob site. These results indicate that the N2O molecule can be easily formed on defective surface. These conclusions also support the previous experimental data18 which indicate that on defective surface, N2O is formed as a reduction product. IV. Conclusions We have performed plane-wave pseudopotential DFT calculations to investigate the adsorption of CO and NO molecules



Figure 7. A contour plot of the valence electron density (units of electrons/Å3) (a) and the difference between the electron density shown in (a) and the one resulting from the superposition of the TiO2(110) slab and the isolated NO molecule (b) in the case of adsorption in a bridge tilt configuration between Ti(4f) sites on 2Ob defective surface. The contours are taken along (001) plane containing the NO molecules and the surface Ti(4f) sites. The [110] direction is parallel to the vertical axis.

on the oxidized and defective TiO2(110) surface. The main conclusions of this study can be summarized as follows: (1) The presence of bridging-oxygen defects on TiO2(110) produces significant electronic and structural changes on the TiO2 surface. For the 1Ob surface the in plane oxygen ions move outward by 0.34 Å and the subsurface oxygen ion (Os1′) below the oxygen vacancy which are pulled upward by 0.37 Å. The largest vertical relaxations seen for the 2Ob model take place for the in-plane oxygens (moved upward by 0.44 Å) and subsurface oxygens positioned below Ti(5f) sites (moved upward by 0.20 Å). The excess spin distribution due to removal of bridging oxygen is localized predominantly on subsurface Ti atoms for the 1Ob surface and on both Ti(4f) and Ti(5f) sites for the 2Ob surface. (2) CO adsorbs molecularly on the oxidized surface at Ti(5f) sites with a weak adsorption energy of 9.04 kcal/mol. The TiCO configuration is preferred relative to the reverse Ti-OC configuration. The calculated adsorption energy is in very good agreement with experimental value of 9.9 kcal/mol determined by Linsebigler et al.11 (3) On the defective TiO2 surface various equilibrium CO configurations at Ti(5f) and Ti(4f) have been identified. Among them the most important have been noticed for the 2Ob surface and correspond to adsorption of CO at Ti(4f) sites in an ontop, bridge, and tilt configurations, with adsorption energies ranging from 24.01 to 35.96 kcal/mol. A continuous elongation of the CO bond is seen with the increase of the adsorption energy on different defective sites. These effects, together with

large modifications of the valence electron distribution, indicate an enhanced back-donation mechanism to the empty π* orbitals of CO. (4) NO adsorbs weakly on the oxidized TiO2(110) surface with a preferential orientation Ti-NO. We have evaluated an adsorption energy at Ti(5f) sites of 8.06 (4.53) kcal/mol at half and full coverage. This adsorption energy is in very good agreement with the experimental value of 8.4 kcal/mol obtained recently based on low-temperature TPD investigations.19 (5) Both 1Ob and 2Ob defective surfaces have a very strong effect on the adsorption properties of NO. We have identified several adsorption configurations at Ti(5f) and Ti(4f) in either vertical (on-top), bridge or tilt configurations. Among these the strongest bound configurations correspond to a tilted bridge geometry between two neighbor Ti(4f) sites. As in the CO case a significant elongation of NO bond length with the increase of adsorption energy takes place. The adsorption mechanism is predominant covalent. (6) The N2O species are only weakly bound in vertical configurations on either Ti(5f) or Ti(4f) sites. However, more strongly bounded configurations involving a bridging binding to two neighbor Ti(4f) sites have been identified. Additionally, we have also observed the possibility of a bridge adsorption configuration of N2O between Ti(5f) and @Ob sites. (7) The adsorption energy of the cis-ONNO species has been characterized on oxidized and defective surface. On the oxidized case the adsorption energy is equal to about 13 kcal/mol and corresponds to a structure symmetrically bound from two


Sorescu and Yates

Figure 8. Selected adsorption configurations of N2O and N2O2 on oxidized and defective TiO2 (110) surface: (a) bridge binding configuration of N2O between Ti(5f) and @Ob sites; (b) chairlike bridge structure of N2O between two neighbor Ti(4f) sites; (c) symmetric adsorption configuration of N2O between two neighbor Ti(4f) sites; (d) bridge binding configuration of ONNO between Ti(5f) and the @Ob sites; (e) bridge adsorption configuration of N2O2 at two neighbor Ti(4f) sites; (f) bridge adsorption configuration of ONNO at two neighbor Ti(4f) sites.

adjacent Ti(5f) sites. This energy is significantly increased to about 45 and 127 kcal/mol for the case when adsorption takes place at Ti(4f) with either N or O atoms oriented toward the surface. On 1Ob surface it is found that interaction of N2O2 with the @Ob defective site leads to formation of a lattice (bridging) oxygen and formation of N2O which is desorbed. (8) Taken together, we find that larger enhancements in the binding energy of CO and NO occur when oxygen vacancy defect sites are present on the TiO2(110) surface. Nearly quantitative agreement between theoretical and experimental binding energies on the nondefective surface have now been obtained. Acknowledgment. This work was supported by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the Army Research Office under Grant DAAD19-01-1-0619. We are grateful for supercomputer time allocations at Pittsburgh Supercomputer Center and ARL MSRC, Aberdeen Proving Ground. Appendices A1. Results on Bulk TiO2. The computational method used in the present work to optimize the unit cell for bulk rutile is similar to the one previously presented by us based on normconserving pseudopotentials.15 In the present study we will restrict ourselves to indicate only the final results obtained using the current set of ultrasoft pseudopotentials as provided with the VASP code.22-24 The relaxation of the unit cell was done with respect to the independent lattice parameters a and c, and

the internal parameter u of the tetragonal (space group P42/ mnm) rutile unit cell. In these calculations a number of 2 sampling K-points were used which were generated using the Monkhorst-Pack scheme28 with mesh parameters 2 × 2 × 4 along the three reciprocal lattice vectors. At 495 eV cutoff energy the optimized lattice parameters are a ) 4.659 Å, c ) 2.974 Å, and u ) 0.3048. A convergence within 0.01 eV for the total energy of the unit cell has been found at this cutoff energy. The calculated lattice values differ by 1.42%, 0.5% and 0.0%, respectively, from the experimental values aexp ) 4.594 Å, cexp ) 2.958 Å, and uexp ) 0.305.32 The analysis of these results indicates that the agreement between the calculated and the experimental crystallographic values is good. Additionally, the calculated geometrical parameters agree well with our previous reported values based on norm-conserving pseudopotentials15 as well as with the results reported by Bates et al.33 obtained using a method similar to the one used in the present work. A2. Results on Slab Geometry. The second series of optimizations considered in this study were related to relaxation calculations performed on the oxidized and defective TiO2(110) surfaces. The corresponding structure of the stoichiometric (110) surface is shown in Figure 1a. The calculations have been done in supercells with dimensions 2c × x2a along [001] and [1h10] directions where a and c are the crystallographic dimensions of the bulk TiO2 unit cell. The surface unit contains two types of coordinated Ti atoms, i.e., 5-fold and 6-fold, marked as Ti(5f) and Ti(6f) in Figure 1a. Above the plane of the surface Ti atoms, there are 2-fold coordinated O atoms, which form chains parallel



TABLE 4: The Ionic Displacements from the Bulk Terminated Structure of TiO2 for the Oxidized (110) Surface and the Corresponding 1Ob and 2Ob Defective Surfacesa oxidized label

[110]

Ti(5f) Ti(5f ′) Ti(6f) Ti(6f ′) O(b) O(p) O(p′) O(s1) O(s1′) O(s2) O(s2′)

-0.17 (-0.16 ( -0.17 (-0.16 ( 0.05) 0.18 (+0.12 ( 0.05) 0.18 (+0.12 ( 0.05) -0.02 (-0.27 ( 0.08) 0.16 (0.05 ( 0.05) 0.34 0.02 (0.05 ( 0.08) 0.02 (0.05 ( 0.08) 0.00 (0.00 ( 0.08) 0.00 (0.00 ( 0.08)

0.05)b

1Ob surface

2Ob surface

[1h10]

[110]

[1h10]

[001]

[110]

[1h10]

[001]

0.00 0.00 0.00 0.00 0.00 -0.06 (-0.16 ( 0.08) -0.02 0.00 0.00 0.00 0.00

-0.02 -0.08 0.04 0.04 0.03 0.34 -0.02 0.05 0.37 0.15 0.11

0.00 0.00 0.00 0.00 0.00 -0.02 0.44 0.02 0.02 0.00 0.00

0.00 0.00 -0.15 0.13 0.01 0.02 -0.07 -0.00 -0.00 0.00 0.00

0.02 0.02 -0.12 -0.12

0.01 0.01 0.01 0.01

-0.01 -0.01 0.01 0.01

0.44 0.00 0.06 0.06 0.20 0.20

-0.07

0.00

0.02 0.02 0.00 0.00

0.01 0.01 0.02 0.02

a The indicated displacements are given in Å. The atomic labels are those indicated in Figure 1 for the oxidized surface. b The values in parentheses are the X-ray surface diffraction data from ref 35.

to the bulk [001] direction. These O atoms are called bridging O atoms and are marked as O(b) in Figure 1a. In every surface unit there are 3-fold coordinated O atoms, which connect the 6-fold and 5-fold coordinated Ti atoms and lie in the same plane with these atoms. These atoms called in-plane oxygens are marked O(p) in Figure 1a. It can be observed that the (110) rutile surface structure can be considered as being composed of planes containing two Ti atoms and two O atoms, separated by planes containing only O atoms. The TiO2 surface model used in the present set of calculations is that of an infinite stack of slabs with periodic boundary conditions in which each slab is separated from its neighbors by a vacuum layer. An important problem associated with such surface models is the choice of the slab thickness and the corresponding vacuum width. Bates et al.33 have performed a systematic study based on first-principles density functional theory calculations and the pseudopotential method of the surface energetics and structure of TiO2 as function of these two parameters. Their results indicate that vacuum widths larger than 4 Å are sufficient to converge the surface energy to within 0.01 J/m2. Additionally, they concluded that accurate predictions of the surface displacements can be obtained for a slab thickness larger than four layers. In the present study we have used a slab containing five layers with all the bridging oxygen atoms on the surface present. This slab considered as a 2 × 1 surface cell contained in the case of the oxidized surface 20 Ti and 40 O atoms. The vacuum width was chosen equal to 10.0 Å. This vacuum width ensures not only the convergence of the surface energy but also minimal interaction effects between the molecules adsorbed on one surface of the slab and the neighbor slab. Every atomic configuration for either oxidized or defective surfaces was determined by relaxing respective systems to equilibrium (see Figure 1). In the case of defective surface we have analyzed two main types of defects, namely a surface with a single O-bridge vacancy (1Ob) in the simulation box (see Figure 1b) and one with two O-bridge vacancies (2Ob) (see Figure 1c). For comparison we have also analyzed the case when one O-in plane vacancy (1Op) is produced (not shown). Due to the periodic boundary conditions used in these calculations the 1Ob defect model (see Figures 1b) corresponds to a row along [001] direction with bridging oxygen atoms separated by vacancies, giving a vacancy density of a half a monolayer. Similarly, the second type of defects model (2Ob), shown in Figure 1c corresponding to a system with all bridging-oxygen ions removed, give a density of surface vacancies of one monolayer. We notice that this is only one of the several structural models

proposed for the defective surface and has been previously analyzed theoretically by Ramamoorthy et al.34 and Lindan et al.36 This model is also known as the empty row model or the missing row model.34,36 By considering as reference the total energy of the relaxed oxidized surface we find that the energy of the 1Ob relaxed slab plus and energy of an isolated oxygen atom is about 6.6 eV less stable than the one for the oxidized system. A similar calculation performed for the 1Op vacancy leads to an energetic difference of about 8.0 eV. These results indicate that formation of an in-plane O defect (1Op) requires an additional 1.6 eV energy relative to the formation energy of the 1Ob defect. Given this large energetic difference between the two types of defects in the present study we have not considered further the 1Op defects. Once a 1Ob defect is produced, the energy of the 2Ob structure plus the one of an isolated oxygen atom is less stable by 7.3 eV. This value is only 0.6 eV larger than the one required to produce the 1Ob defect starting from the oxidized structure. More insight about specific relaxations of the surface atoms for the oxidized 1Ob and 2Ob surfaces can be gained by analyzing the data in Table 4. For the oxidized surface, the dominant relaxations are normal to the surface. Particularly, the 5-fold- and 6-fold-coordinated Ti ions move in and out of plane by 0.17 and 0.18 Å, respectively, the bridging oxygens move into the surface by 0.02 Å, and the in-plane oxygens move out of surface by 0.16 Å. Also, only the in-plane oxygen atoms experience displacements along [1h10] axis by 0.05 Å. There are no relaxations along the [001] direction for any of the ions. These results determined for the case of a slab with 5 layers are only slightly different from those obtained previously by Bates et al.33 In Table 4 we also indicate for the case of oxidized surface the results obtained based on X-ray diffraction studies.35 We note that for both predicted and experimental sets of values, the sign of atomic displacements is the same. Excepting the bridging oxygen atoms, generally there is also a quantitative agreement between the two sets of values. In the case of bridging oxygens, however, the experimental displacement is -0.27 ( 0.08 Å while the predicted value is -0.02 Å. A similar large difference for the bridging oxygen atoms has been observed in previous theoretical studies based on either GGA33 or LDA34 methods. The nature of this difference is not clearly understood. It has been previously suggested33 that a possible explanation for this difference is due to the fact that calculations give the nuclear positions while X-ray diffraction data correspond to the electron density distribution. In the case of the bridging oxygen atoms the polarization of the valence electrons might be


Sorescu and Yates

Figure 9. Plots of the spin density in the (1h10) plane containing Ti(5f) and Ti(6f) atoms for the case of the 1Ob (a and b) and 2Ob (c and d) defective models.

responsible for the observed discrepancy between the theoretical and experimental results. In the case of the defective 1Ob surface the major relaxations observed (see Table 4 and Figure 1b) are along [110] and [001] directions. Among these the largest displacements are seen perpendicular on the surface for the in plane oxygens ion which moves outward by 0.34 Å and the subsurface oxygen ion (Os1′) below the oxygen vacancy which is pulled upward by 0.37 Å. We note that the neighbor subsurface oxygen (Os1) is displaced significantly less than Os1′ ion by 0.05 Å. Beside these vertical relaxations the Ti(6f) ions, which were 6-fold coordinated in the oxidized surface, have opposite displacements along [001] direction with slightly different amounts of -0.15 and 0.13 Å. Additionally, there are small displacements of the in-plane oxygens along [001] direction leading to a lowering of the symmetry of the surface. Focusing now on the reduced 2Ob surface (see Figure 1c) we observe that the 4-folded Ti(4f) ions (resulted from Ti(6f) on the oxidized surface by desorption of bridging oxygens) have opposite displacements relative to the case of oxidized structure, i.e., they are moved toward the bulk by -0.12 Å. The largest vertical relaxations seen for the 2Ob surface model take place for the in-plane oxygens (0.44 Å) and subsurface oxygens (0.20 Å) positioned below Ti(5f) sites. These relaxations are quite similar to those determined previously by Lindan et al.36 using a 1 × 1 reduced surface model. For example, for the Ti(6f) and O(p) ions the vertical displacements determined in their study are -0.11 and 0.39 Å. These values compare well with our values of -0.12 and 0.44 Å, respectively. As removal of a neutral oxygen ion leaves two electrons which previously occupied O(2p) levels it is important to analyze

the spin distribution for both the 1Ob and 2Ob defective surfaces. In the first situation there are two excess electrons while in the second case four excess electrons per cell. In Figure 9 we have represented the spin densities in planes perpendicular to the surface along [001] direction passing through the surface Ti(5f) ions (see Figures 9a and 9c) and, respectively, through the surface Ti(6f) ions (see Figures 9b and 9d) for the case of 1Ob and 2Ob defective models, respectively. We observe that in both cases the excess electrons are localized on Ti ions. However, the particular distribution is different for the two surfaces. For the 1Ob model, the largest excess electrons are localized on subsurface Ti(6f) ions. On 2Ob defective surface the largest amplitude of spin density is seen on surface Ti(4f) and Ti(5f) sites. Consequently, it is expected that the strongest interactions of NO and CO molecules with will take place at Ti(4f) sites followed by interactions at Ti(5f) sites. Moreover, due to the additional unpaired electron it can be predicted that NO molecules will be chemisorbed more strongly than CO molecules at these sites. A3. Calculations on the CO and NO Molecules. The accuracy of the ultrasoft pseudopotentials has been also tested by calculation of the equilibrium properties of CO and NO molecules. The geometries of these molecules have been determined from calculations with the respective molecules isolated in large cubic boxes of length 10 Å. For these calculations a single k-point at the Γ point has been used with a cutoff energy of 495 eV. The corresponding equilibrium bond lengths were found equal to d(C-O) ) 1.1452 Å and d(N-O) ) 1.1742 Å. These values differ by 1.5% and 2.0% from the corresponding experimental values of 1.1283 and 1.1508 Å.37

Adsorption Properties of CO and NO on TiO2(110) We have also calculated the fundamental vibrational frequencies of CO and NO molecules isolated in a 10 Å cubic cell. The force constant was determined by fitting a third-order polynomial to the total energy as a function of small displacements of the C-O interatomic distance. From these calculations the vibrational frequencies of ν(C-O) ) 2174 cm-1 and ν(NO) ) 1895 cm-1 have been determined which overestimates by 1.4% and 1.0%, respectively, the experimental values of 2143 and 1875 cm-1.37 A final test performed was related to evaluation of the atomization energies for the CO and NO molecules. Using the calculated energies of individual atoms and zero-point energy corrections determined on the basis of the calculated vibrational frequencies we have determined an atomization energy of 259.7 kcal/mol for CO and 157.7 kcal/mol for NO, respectively. These results are in very good agreement with experimental values of 259.3 and 152.9 kcal/mol, respectively.40 References and Notes (1) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (2) Photocatalysis-Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley Interscience: New York, 1989. (3) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: England, 1993. (4) Linsebigler, A. L.; Lu G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (5) Huusko, J.; Lantto, V.; Torvela, H. Sensors Actuators 1993, B1516, 245. (6) Vannice, M. A.; Sudhaker, C. J. Phys. Chem. 1984, 88, 2429. (7) Yates, J. T., Jr. Surf. Sci. 1994, 299, 731. (8) Go¨pel, W.; Rocker, G.; Feierabend, R. Phys. ReV. 1983, B28, 3427. (9) Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1985, 89, 5240. (10) Beck, D. D.; White, J. M.; Ratcliffe, C. T. J. Phys. Chem. 1986, 90, 3132. (11) Linsebigler, A.; Lu, G.; Yates, J. T., Jr. J. Chem. Phys. 1995, 103, 9438. (12) Kobayashi, H.; Yamaguchi, M. Surf. Sci. 1989, 214, 466. (13) Fahmi, A.; Minot, C. J. Organomet. Chem. 1994, 478, 67.

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First Principles Calculations of the Adsorption Properties of CO and

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