Adsorption Configuration and Dissociative Reaction of NH3 on Anatase

Mar 26, 2009 - National Center for High-Performance Computing, No. 28, Nan-Ke ... National Sun-Yat-Sen UniVersity, Kaohsiung 804, Taiwan. Hsin-Tsung ...
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J. Phys. Chem. C 2009, 113, 6663–6672

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Adsorption Configuration and Dissociative Reaction of NH3 on Anatase (101) Surface with and without Hydroxyl Groups Jee-Gong Chang* National Center for High-Performance Computing, No. 28, Nan-Ke Third Road, Hsin-Shi, Tainan 744, Taiwan

Shin-Pon Ju* and Ching-Sheng Chang Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun-Yat-Sen UniVersity, Kaohsiung 804, Taiwan

Hsin-Tsung Chen* National Center for High-Performance Computing, No. 28, Nan-Ke Third Road, Hsin-Shi, Tainan 744, Taiwan ReceiVed: NoVember 4, 2008; ReVised Manuscript ReceiVed: January 17, 2009

This study investigates the possible adsorption configurations and dissociative reactions of NH3 on the anatase (101) surface by employing the first principles calculations. In addition, the hydroxyl group effect is also included to study how this effect influences the adsorption and the dissociative reactions. Without the presence of the hydroxyl group, the most stable adsorbate is the bidentate adsorbate Ti-N-O (Eads ) 44.9 kcal/mol), and the second is the bidentate adsorbate Ti-(H)N-O (Eads ) 40.8 kcal/mol). NH3 can also be adsorbed on 5c-Ti, forming H3N-Ti, which is the third most stable adsorbate (Eads ) 27.5 kcal/mol). The hydroxyl group present on the surface has the effect of significantly enhancing the adsorption of the monodentate adsorbates H2N-Ti and HN-Ti. However, such a presence only slightly enhances the bidentate adsorbate Ti-N-O. In addition, the adsorption energy increases as the number of hydroxyl groups on the surface increases. The hydroxyl group also has the effect to diminish the adsorption for bidentate adsorbate Ti-(H2)N-O and monodentate adsorbate H-O, or to simultaneously enhance and diminish adsorption for Ti-(H)N-O depending on the location and number of the hydroxyl groups. However, the effect of the hydroxyl group on these two bidentate adsorbates (Ti-(H2)N-O and Ti-(H)N-O) is not as significant as for monodentate adsorbates (H2NTi and HN-Ti). Two reaction pathways are found to reach two final products, Ti-N-Oc2+3(H-O) and N-Tic3+3(H-O), with the energetics of 89.0 and 83.2 kcal/mol, respectively. In addition, the maximum reaction energy barriers required to reach these two final products are 76.0 kcal/mol for the pathway where H2N-Ti dissociates into Ti-(H)N-O, and 126.9 kcal/mol for the pathway where HN-Ti dissociates into N-Ti. All of the reactions, except the forming of H3N-Ti, are endothermic. The hydroxyl group was found to lower or raise the energetics. The energetics of H2N-Ti+H-O and HN-Ti+2(H-O) are significantly lowered; however, the energetics of Ti-(H)N-O+2(H-O) and Ti-N-O+3(H-O) are slightly raised, as compared to those energetics without the presence of the hydroxyl group. Finally, the reaction pathway to N-Ti+3(H-O) is only found when considering the hydroxyl group effect. Introduction Titanium dioxide (TiO2) material has proven to be very robust for use in the renewable-energy industry in such applications as hydrogen conversion1-3 or solar cells4-6 due to its promising photocatalytic7 and photovoltaic8-10 properties. In addition, other new applications are now being discovered and explored.11-13 The adsorption efficiency of TiO2 film is very low in solar energy conversion due to the wide band gap of the TiO2 film, the adsorption band of which falls in the UV range.14 To improve its adsorption efficiency, a tandem material on the TiO2 film is usually developed to broaden the adsorption band including the range of the visible light. A well-known example is Gra¨tezl’s black dye,15 the black dye sensitizer on the surface of the TiO2 film, which significantly increases the adsorption efficiency.16-18 However, one drawback of using organic mate* To whom correspondence should be addressed. (J.-G.C.) E-mail: [email protected]. (S.-P.J) E-mail: [email protected]. (H.-T.C.) E-mail: [email protected].

rial such as this black dye to serve as the sensitizer is its low resistance to oxidation. Thus, it is very important for use in solar cell applications to find a more durable inorganic material that has the same broadband adsorption feature. N-doped TiO2 (TiO2-xNx) powders have been shown to have a noticeable improvement over pure TiO2 under visible light in their optical adsorption and level of photocatalytic activities in the visible range.19,20 In a method similar to that of the solar cell fabrication of the N-doping of TiO2 film,21 Wang et al.22 have succeeded in depositing InN thin films by OMCVD (organometallic chemical vapor deposition) on TiO2 nanoparticles using HN3 as an N-precursor.23,24 Though its dissociation on some specific surfaces might be low, HN3 possesses highly explosive and toxic characteristics. Conversely, NH3 is relatively stable and one of the most highly produced inorganic chemicals to serve as the N- precursor for most of the N-doped TiO2.25-29 Thus, it is very important to understand the dissociative reaction of NH3 on the TiO2 surface. In related research in literature, Onal et al.30 have

10.1021/jp809724r CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

6664 J. Phys. Chem. C, Vol. 113, No. 16, 2009 investigated the adsorption and dissociation of NH3 on (101) and (001) TiO2 anatase surfaces using the cluster models of Ti2O9H10 and Ti9O33H30, either totally fixed or partially relaxed in order to approach the surface model. They found that the adsorption energy and the geometry of NH3 depend on surface relaxation. The whole dissociative reaction of NH3 is not fully explored. Yamazoe et al.31,32 investigated the mechanisms of photo-oxidation of NH3 over a TiO2 catalyst by using experimental methods. They found that the N2 could be formed from the photo-oxidation of NH3 in the presence of O2 under UV irradiation, showing a potential method of reducing ammonia pollution at low temperature. These notable results suggest that the conditions of our study, which details the dissociative reaction of NH3 on TiO2, can be informative to other studies such as that by Yamazoe et al. and can serve as a valuable foundation for other substantial works. Because of the pervasiveness of moisture (water) in the environment, hydroxyl groups are likely to form on any kind of surface. Hydroxyl groups relate to the split of H2O into H and OH; in such a TiO2 system, the former H is adsorbed on 2c-O, and the latter OH fills the O vacancy of the TiO2 surface defect.33-40 The free energy barrier related to the formation of these hydroxyl groups is only 0.11 eV.33b Thus, the natural formation of hydroxyl groups on the surface occurs as a result of the surface defects as well as the presence of moisture itself, because the H2O splits and forms two hydroxyl groups, as suggested by its lower reaction barrier. A recent publication34 has pointed out that the surface properties might be significantly modified by the hydroxyl group: it found that the hydroxyl group can facilitate O2 adsorption on the TiO2 surface not only on the nearby 5c-Ti but also on the distant 5c-Ti (which the authors note as the long-range effect). The adsorption energy of O2 is dependent on its distance to the hydroxyl group (which we recognize as the geometry effect, an effect neither emphasized nor named by the authors of the paper). The hydroxyl group present on the surface might enhance or diminish the adsorption of other molecules on the surface41,42 In addition to the effects of the surface defects or environmental moisture, the hydroxyl group can also form in some instances of dissociative adsorption, in which the H atom is dissociated from the reactant and then adsorbed on the 2c-O on the TiO2 surface, for example, HN342 and B(OH)3.41 Thus, in the investigation of the reaction mechanism, the effect of a hydroxyl group, which is coadsorbed with another adsorbate, must be considered not only because its presence could enhance or diminish the adsorption of the adsorbate but also because different configurations and locations of the hydroxyl group affect the adsorption of the adsorbate. These two effects contribute to the final reaction pathways; however, they are seldom mentioned in the existing literature. The objective of this article is to study the adsorption configurations and dissociative reaction of NH3 on the TiO2 anatase (101) surface by the first principle calculation based on the density functional theory (DFT). The influence of the hydroxyl group on the adsorption of the NH3 fragment is also presented. Finally, the potential energy surface (PES) is also included in order to understand the detailed reaction pathway of the dissociative adsorption of NH3 on the TiO2 surface. Computational Model and Method Figure 1 shows the schematic diagram of the TiO2 anatase (101) surface and an NH3 molecule. For the TiO2 anatase structure, the large white bead and the smaller red bead represent the Ti and O atoms, respectively. For the NH3 molecule, the

Chang et al.

Figure 1. (a) Geometric model for the anatase (101) surface with the NH3 molecule and (b) location notation for 2c-O.

larger blue bead and the smaller white bead represent the N and H atoms, respectively. The anatase structure of the TiO2 surface has 2- and 3-fold-coordinate O atoms and 5- and 6-fold coordinate Ti atoms,43 as indicated by 2c-O, 3c-O, 5c-Ti, and 6c-Ti, respectively. Usually, the 2c-O and 5c-Ti atoms are more reactive than the 3c-O and 6c-Ti atoms due to their unsaturated coordination. The slab model is adopted in the present study to simulate the interaction between the TiO2 solid phase and the NH3 gas phase. This model takes into account the periodic condition along the TiO2 surface. A vacuum space, which exists in the direction perpendicular to the TiO2 surface, is assumed to contain gas phase molecules. Extending the model in the x-, y-, and z-directions leads to the creation of an infinite slab. Imposing a vacuum space of 12.63 Å in the slab model prevents the gas phase from interacting with the lowest layer of the TiO2 surface and allows the TiO2 surface to relax so as to achieve the required surface conditions. In the current study, the supercell contains 24[TiO2]units, the dimensions of which are 7.57 Å × 10.24 Å × 23.61 Å, as shown in Figure 1. In this study, geometrical structures are optimized using the Vienna ab initio Simulation Package (VASP)44-47 and are calculated on the basis of density functional theory. The generalized gradient approximation (GGA)48,49 used for the total energy calculations is that of the Perdew-Wang 1991 (PW91) formulation.48 The core pseudo potentials supplied with VASP are used for the present calculations. The ten 3p, 3d, and 4s electrons of each Ti atom and the six 2s and 2p electrons of each O atom are explicitly considered. The spin-polarization effects are also included in the calculations for the open-shell systems. Regarding the periodic condition along the TiO2 surface, the valence electrons are expanded by a plane-wave basis set. The plane wave expansion includes all plane waves with kinetic energies smaller than the chosen cutoff energy, pK2/ 2m < Ecut (Ecut set as 450 eV). This ensures convergence with respect to the basis set. The Brillouin zone is sampled with the chosen Monkhorst-Pack k-points,50 which ensures the convergence of the whole system. The transition states are found using

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TABLE 1: Heat of Reaction for Different NH3 Reactions reaction

∆Ereaction (kcal/mol)

NH3(g) f NH2(g) + H(g) NH3(g) f NH(g) + 2H(g) NH3(g) f N(g) + 3H(g)

114.3a 209.5b 295.2c

TABLE 2: Location Notation for Single, Double, and Triple Hs Adsorbed on 2c-Os 2c-O location

a ∆E reaction(NH3 f NH2 + H) ) E(NH2) + E(H) - E(NH3). ∆E reaction(NH3 f NH + 2H) ) E(NH) + 2E(H) - E(NH3). c ∆E reaction(NH3 f N + 3H) ) E(N) + 3E(H) - E(NH3). b

the climbing-image nudged elastic band method (CI-NEB).51,52 All transition states are checked for the presence of the imaginary vibration frequency. The adsorption energy is calculated for all possible adsorbates, including NH3 or its fragments, on the clean surface (without the presence of hydroxyl group) as follows:

Eads)Emole + Esurface - Emole/surface

(1)

where Emole is the energy of an isolated molecule, which represents NH3 or its fragments (NHx, x < 3, N and H), which is calculated by placing the associated gas molecule into a 10.0 Å cubic cell, Esurface is the energy of a clean TiO2 anatase (101) surface, and Emol/surface is the total energy of the same molecule adsorbed together on the surface with the TiO2 anatase (101) surface. Note that a positive value for Eads represents the fact that NH3 or its fragment can form bonds with the anatase (101) surface, and a negative or zero value for Eads indicates that NH3 or its fragment cannot be adsorbed on the anatase (101) surface in this particular configuration. For the presence of the hydroxyl group on the surface, where the H atom is adsorbed on the 2c-O, the adsorption energy of the adsorbate present in a hydroxyl group coadsorption configuration is calculated by the following eq (2), instead of eq 1.

Eads)Emole + Esurface/nH - Emole/surface/nH

(2)

The term Esurface/nH, the second term of the right-hand side of eq 2, represents the energy of the surface including the n number of H that have adsorbed on the 2c-O of the surface to form the n hydroxyl group. Similarly, the third term of Emol/surface/nH is the total energy of the surface including n hydroxyl groups and the NH3 or its fragment coadsorbed elsewhere on the surface. Note that if there is no effect of the hydroxyl group on the adsorption of the molecule in a coadsorption configuration, then the adsorption energies calculated by above two eqs 1 and 2 will be the same. Results and Discussion We have carried out verification to access the accuracy of the present model, and the parameters we used are appropriate. The whole procedure is similar to our former calculation,42 and all of the data obtained are all consistent with the former literature.41,53-59 Thus, we leave the details in the Supporting Information. Finally, Table 1 lists the reaction energy required to dissociate NH3(g) into NH2(g) + H(g), NH(g) + H2(g), and N(g) + H3(g). The dissociation energies are later used to calculate the reaction energetics. Adsorption of NH3 and Its Fragments with and without Hydroxyl Groups. In order to investigate the influence of the different locations of the hydroxyl group on the adsorption of NH3 fragment, in the following subsection we have listed in Table 2 all of the possible locations when a single H, two Hs,

H-O label arrangement 2(H-O) label arrangement 3(H-O) label arrangement a

a

a1 I b1 I, II c1 I, II, III

a2 III b2 I, III c2 I, II, IV

a3 IV b3 II, IV c3a I, III, IV

a4a II b4 III, IV c4a II, III, IV

a4 and a1 are symmetrical; c4 and c3 are also symmetrical.

and three Hs are present on the 2c-Os, where I, II, III, and IV represent different 2c-O locations as indicated in Figure 1b. To get the final arrangement of the 2c-O locations, where one-, two- and three-Hs may be adsorbed (forming one, two, and three hydroxyl groups), we also have eliminated some locations that show the same effect with each other while the periodic boundary or symmetrical conditions are considered. For example, for two Hs adsorbed on two 2c-O locations, there are a total of six different permutations, i.e., C24 ) 6; however, since for any particular case, there will be a number of symmetrical arrangements, we have included only one for simplicity’s sake. For example, of the six possible for b3, the I and IV arrangement pattern is identical to II and IV; similarly, the arrangement of I and III are identical to I and II in b1. Thus, only four distinct arrangements occur for two Hs adsorbed on two 2c-O locations. Note that the letters of a1, a2, a3, and so on will appear on the superscript of the adsorbate; for example, H2N-Tib2 denotes the H2N-Ti adsorbate, while two other Hs (hydroxyl groups) are present at location I and location III (referenced from Table 2), or may be simply denoted as the b2 arrangement. On the contrary, no superscript appearing on the adsorbate represents the adsorption on a clean surface. Tables 3 and 4 show the adsorption configurations and the adsorption energy of NH3 and its fragments on the anatase (101) surface. The numbers in the first column of each adsorbate are also designated in Figures 2 and 3 as an easy reference to its adsorption configuration (the top view is provided in the Supporting Information). Note that in both figures the atoms at the bottom right are repeated from the upper right. In Figures 2 and 3, we use yellow to represent the hypothetical presence of the coadsorption of H (hydroxyl group). For example, the H2N-Tib2 denotes the properties, such as adsorption energy and bond length, obtained for H2N-Ti, which includes the two hydroxyl groups present at location I and location III. Note that H2N-Tib2 does not in fact denote the physical configuration including two hydroxyl groups. For NH3, it can form the monodentate H3N-Ti adsorbate on the surface with an adsorption energy of 27.5 kcal/mol and with a Ti-N bond length of 2.260 Å. In addition, the three H atoms of NH3 form the three hydrogen bonds between the three nearest neighbors of 2c-Os. There bond lengths are 2.685 Å, 2.656 Å, and 2.649 Å. Besides this adsorbate, NH3 also can form the less strongly bonded adsorbate of H3N...O, where two H atoms of NH3 are attracted by the two 2c-Os on the surface forming the double hydrogen bonds, in which the distances between the H and 2c-O are 2.240 Å and 2.530 Å, respectively, as depicted in configuration 2 in Figure 2. The adsorption energy of this adsorbate of H3N...O is only 4.1 kcal/mol. For the NH2 fragment, the N atom of NH2 is adsorbed on 5c-Ti, forming the monodentate H2N-Ti adsorbate with a N-Ti bond length of 2.178 Å and an adsorption energy of 19.4 kcal/ mol. When one hydroxyl group is present in the H2N coadsorption configuration, denoted as H2N-Tia1 (a1 can be refer-

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TABLE 3: Possible Adsorption Configurations of NHx (x ) 1-3) and Their Adsorption Energy on the Anatase (101) Surface NH3 configuration

r(Ti-N) (Å)

1. H3N-Ti

r(N-O) (Å)

N-HO2c (Å)

O2c-Ti6c (Å)

2.260

27.5

r(H1-O1)

r(H2-O2)

2.240

2.530

2. O... H3N

Eads (kcal/mol)

N-HO2c

O2c-Ti6c

Eads 4.1

NH2 configuration

r(Ti-N)

3. 4. 5. 6. 7. 8. 9.

2.178 1.937 1.904 1.904 2.108 2.117 2.109

H2N-Ti H2N-Tia1 H2N-Tia2 H2N-Tia3 Ti-(H2)N-O Ti-(H2)N-Oa1 Ti-(H2)N-Oa3

r(N-O)

N-HO2c 2.487 3.017 3.872

1.407 1.409 1.424

4.113 3.089

O2c-Ti6c

Eads

1.858 2.076 2.038 2.098 1.866 2.149 2.154

19.4 54.3 56.1 52.3 12.8 6.1 8.5

O2c-Ti6c

Eads

1.872 2.061 2.008 2.100 2.099 2.080 2.007 1.864 2.118 2.155 2.116 2.147 2.163

13.3 40.0 40.3 35.7 78.2 67.8 72.1 40.8 39.5 41.4 37.1 39.9 42.9

NH configuration

r(Ti-N)

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

2.247 2.012 1.866 1.899 1.711 1.721 1.698 1.998 1.990 2.025 2.084 1.992 2.016

HN-Ti HN-Tia1 HN-Tia2 HN-Tia3 HN-Tib2 HN-Tib3 HN-Tib4 Ti-(H)N-O Ti-(H)N-Oa1 Ti-(H)N-Oa3 Ti-(H)N-O3cb1 Ti-(H)N-Ob1 Ti-(H)N-Ob3

r(N-O)

1.891 2.850 3.951 2.650 2.507 2.769 1.415 1.434 1.428 1.488 1.435 1.428

TABLE 4: Possible Adsorption Configurations of N and H Fragments and Their Adsorption Energy on the Anatase (101) Surface N configuration 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

N-Tia1 N-Tia2 N-Tia3 N-Tib2 N-Tib4 N-Tic3 Ti-N-O Ti-N-Oa1 Ti-N-Oa3 Ti-N-Ob1 Ti-N-Ob3 Ti-N-Oc2

r(Ti-N) (Å) 2.024 2.004 1.986 1.790 1.769 1.674 2.008 1.900 1.904 1.880 1.883 1.885

r(N-O) (Å)

1.307 1.344 1.353 1.360 1.364 1.373

N-HO2c (Å)

O2c-Ti6c (Å)

Eads (kcal/mol)

2.042 2.475 3.914 2.036 2.928 2.404

2.080 1.998 2.096 2.076 2.731 2.098 1.852 2.100 2.114 2.142 2.127 2.143

24.7 26.3 19.8 50.4 41.2 61.2 44.9 48.4 48.9 54.5 56.7 55.4

4.408 2.866 4.21 3.317 2.835 H

configuration

r(O-H)

35. 36. 37. 38. 39.

0.972 0.968 0.969 0.971 0.971

H-O H-Oa1 H-Oa3 H-Ob1 H-Ob3

r(N-O)

N-HO2c

N-HO2c

O2c-Ti6c

Eads 55.5 48.3 47.7 43.6 45.1

enced from Table 2), the adsorption energy becomes 54.3 kcal/ mol, which is an increase of 34.9 kcal/mol in adsorption energy as compared to that of H2N-Ti, showing the significant influence of the hydroxyl group effect on the adsorption of the H2N-Ti

4.666 2.590 1.949 4.668 2.908

adsorbate. As compared to the bond length of Ti-N between these two adsorbates, it is found that the bond length changes from 2.178 Å for the clean surface H2N-Ti to 1.937 Å for H2NTa1, which becomes shorter when the hydroxyl group is present. In addition, comparison of the bond length of O2c-Ti6c, where O2c represents the location that H is adsorbed upon (i.e., the O atom of the hydroxyl group). The O2c-Ti6c bond length of the adsorbate of H2N-Tia1 also becomes longer (2.076 Å) as compared to that of H2N-Ti (1.858 Å). This is because the presence of H forms a strong bond between 2c-O and thus weakens the bond between 2c-O and 6c-Ti. Finally, for the adsorbate of H2N-Tia1, the distance between the N atom of NH2 and the H atom of the hydroxyl group, denoted as N-HO2c, is less than 3.0 Å, indicating that a possible hydrogen bond forms between H and N atoms (see configuration 4). To investigate the effect of the hydroxyl group present in different locations, the additional adsorption configuration 5 and 6, listed in Table 3 and pictured in Figure 2, shows the same NH2 adsorbate but with the hydroxyl group present at different locations in the arrangement as H2N-Tia2 and H2N-Tia3. Considered together with H2N-Tia1, the adsorption energy among them is highest for H2N-Tia2, at 56.1 kcal/mol; however, the energy difference between them is very small, as is the change of the N-Ti bond length. This also illustrates that bonding characteristics are not significantly influenced by the presence of the hydroxyl group at different locations. Observing the adsorbate configurations 4, 5, and 6 in Figure 2, it is clear that different locations of the hydroxyl group also changes the orientation of the H2N-Ti adsorbate and leads to the formation of a different number and location of hydrogen bonding. For

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Figure 2. Possible adsorption configurations of NHx (x ) 1-3) on the anatase (101) surface.

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Figure 3. Possible adsorption configurations of of N and H fragments on the anatase (101) surface.

example, for configuration 4, there is one hydrogen bond between H and N atoms; for configuration 5, there are two hydrogen bonds between two Hs and two 2c-Os (bond lengths: 2.887 Å and 2.875 Å); and for configuration 6, there are none. This is why adsorption energy decreases in the same order: H2NTia2, H2N-Tia1, and H2N-Tia3. In addition to the monodentate adsorbate forming for the NH2 fragment, bidentate adsorbate such as Ti-(H2)N-O also forms. The adsorption energy of this adsorbate is 12.8 kcal/mol, which is less stable than the monodentate adsorbate of H2N-Ti. The lower adsorption energy of the bidentate adsorbate of Ti(H2)N-O is due to one coordination remaining for the NH2. The N-O bond length is 1.407 Å, and the Ti-N bond length is 2.108 Å, a little shorter (0.070 Å) than that of the monodentate adsorbate of H2N-Ti. While the hydroxyl group is present for the a1 arrangement, the adsorption energy of Ti-(H2)N-Oa1 becomes 6.1 kcal/mol, which is less stable than that of the Ti(H2)N-O. For this adsorbate, Ti-(H2)N-O, the presence of the

hydroxyl group weakens the NH2 to form a bidentate adsorbate of Ti-(H2)N-O; the effect of the presence of the hydroxyl group decreases the adsorption energy, which is opposite when compared to NH2, which forms a monodentate adsorbate of H2N-Tia1. Comparing the effect of the presence of the hydroxyl group in different arrangements, we see there is only a slight difference of 2.4 kcal/mol in adsorption energy between configuration 8, Ti-(H2)N-Oa1, and configuration 9, Ti-(H2)N-Oa3. Similarly, the Ti-N and N-O bond lengths between these two adsorbates are only slightly different. Note that there is no Ti-(H2)N-Oa2 adsorbate because the O (2c-O) of the hydroxyl group already binds with the N atom of NH2. The NH fragment, similarly, also can form the monodentate adsorbate of HN-Ti with an adsorption energy of 13.3 kcal/ mol, which is less than that of the monodentate adsorbate of H2N-Ti. As a consequence, the Ti-N bond length of HN-Ti is 2.247 Å and is longer than that of H2N-Ti. Similar to that

Adsorption Configuration and Dissociative Reaction previously mentioned, configurations of 11, 12, and 13 in Table 3, which are labeled with superscripts of a1, a2, and a3, respectively, are HN-Ti while one hydroxyl group is present at three different arrangements (locations). Comparing the adsorption energy of these three adsorbates of HN-Tia1, HN-Tia2, and HN-Tia3 to that of HN-Ti on the clean surface, it can be found that the presence of the hydroxyl group will enhance the maximum difference of NH adsorption by 27.0 kcal/mol, which is the energy difference between HN-Tia2 and HN-Ti. Comparing the three adsorbates HN-Tia1, HN-Tia2, and HN-Tia3, it is found that the influence of the different locations of the hydroxyl group on the adsorption energy is insignificant. For the three N-Ti bond lengths of these three adsorbates, the HN-Tia1 is the longest (2.012Å) among the three. This is because the H of the hydroxyl group forms the hydrogen bond with the N of NH molecule, as is shown in configuration 11 in Figure 2, as well as in Table 3, which shows that N-HO2c is the shortest (1.891 Å) among the three. Configurations 14, 15, and 16 show the adsorbates of HNTib2, HN-Tib3, and HN-Tib4, respectively, indicating the HN-Ti adsorbed with two hydroxyl groups on the surface. Note that the monodentate adsorbate of HN-Tib1 does not exist, but HNTib1 becomes the bidentate adsorbate of Ti-(H)N-O3cb1 after geometric optimization, listed as configuration 20 in Table 3 and Figure 2. A comparison of the adsorption energy for configurations 14, 15, and 16 in decreasing order is HN-Tib2 (78.2 kcal/mol), HN-Tib4 (72.1 kcal/mol), and HN-Tib3 (67.8 kcal/mol), where the maximum energy difference between the first and third is 10.4 cal/mol. This indicates that the adsorption energy is influenced by the different locations of the hydroxyl group on the surface. The adsorption energy is smallest for HNTib3 because the two hydroxyl groups are located at positions II and IV, far away from 5c-Ti, where the NH is adsorbed. For the other two adsorbates, the two hydroxyl groups are located at positions I and III for HN-Tib2 and III and IV for HN-Tib4. The maximum enhancement in adsorption energy with the addition of one hydroxyl group is found between HN-Tia3 and HN-Tib2, with an enhancement of 42.5 kcal/mol. The NH molecule can form a bidentate adsorbate Ti-(H)N-O on the clean surface, as shown by configuration 17. Its adsorption energy is 40.8 kcal/mol, which shows that this bidentate adsorbate is more stable than that of the monodentate adsorbate of HN-Ti. In addition, the N-Ti bond length is 1.998 Å, shorter than that of HN-Ti (2.247 Å). Configurations 18 and 19 show the bidentate adsorbate while one hydroxyl group is in a1 and a3 arrangements, respectively. Note that the lack of the hydroxyl group in the a2 arrangement is because the O (2cO) of the hydroxyl group already binds with the N atom of NH. Comparing the adsorption energy of these two adsorbates to that of Ti-(H)N-O on the clean surface, the enhancement of adsorption energy is not significant while one hydroxyl group is present. In addition, the influence of different locations of the hydroxyl group is also insignificant. The difference of the adsorption energy between these a1 and a3 arrangements is 1.9 kcal/mol, which arises from the hydrogen bonding between the H and N atoms in the a3 arrangement, where the H-N distance is 2.590 Å. Configurations 21 and 22 show the same bidentate adsorbates with two hydroxyl groups in b1 and b3 arrangements. Note that the bidentate adsorbate configurations of Ti-(H)N-Ob2 and Ti(H)N-Ob4 are not shown because the O (2c-O) of the hydroxyl group already bonds with the N atom of NH. Comparing the adsorption energy of adsorbates Ti-(H)N-Ob1 and Ti-(H)N-Ob3 (both with two hydroxyl groups), Ti-(H)N-Oa1 and Ti-(H)N-

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6669 Oa3 (both with only one hydroxyl group), and Ti-(H)N-O, we find the adsorption energy is only slightly influenced by the presence of the additional hydroxyl group. Configurations 23 to 28 show the monodentate adsorbate for the N fragment, N-Ti, with the presence of one, two, and three hydroxyl groups, which are shown in Table 4 and whose related configurations are shown in Figure 3. The monodentate adsorbate of N-Ti on the clean surface does not exist, but rather exists as the bidentate adsorbate of Ti-N-O, shown as configuration 29. The adsorption energy of the N-Ti adsorbate is 24.7, 26.3, and 19.8 kcal/mol with one hydroxyl group present in the a1, a2, and a3 arrangements, respectively, as shown in configurations 23, 24 and 25, respectively. The adsorption energy among the three is smallest at N-Tia3 because of the lack of the hydrogen bonding between the N and H atom. This can be observed in the N-HO2c distance; for the a3 arrangement, it is 3.914 Å, whereas for the a1 and a2 arrangements, it is 2.042 Å and 2.475 Å, respectively. Only two configurations exist with two hydroxyl groups on the surface: configuration 26 of N-Tib2 with an adsorption energy of 50.4 kcal/mol and configuration 27 of N-Tib4 with an adsorption energy of 41.2 kcal/mol. (Both have in common an N, which can be monodentately adsorbed on 5c-Ti, with one of the two hydroxyl groups located at location III.) The adsorption energy difference of the N-Ti adsorbate with different H locations of b2 and b4 arises from hydrogen bonding between N and H (see N-HO2c distance in Table 4) in addition to the hydroxyl group location effect. Finally, there is only one configuration with three hydroxyl groups on the surface, occurring in the c3 arrangement, i.e., N-Tic3. The adsorption energy is 61.2 kcal/mol, which is highest among all arrangements of the hydroxyl group. In addition, the N-Ti bond length is also the shortest at 1.674 Å. The adsorption energy increases as the number of hydroxyl groups present on the surface increases. For the N fragment, the maximum energy difference between the presence of one and two hydroxyl groups is 30.6 kcal/mol (configurations 26 and 25). Similarly, the maximum energy difference between two hydroxyl groups and three hydroxyl groups is 20.0 cal/mol (configurations 28 and 27). Configurations 29 to 34 also show the N fragment, forming the bidentate adsorbate of Ti-N-O with and without the hydroxyl group present on the surface. The adsorption energy for this adsorbate on the clean surface, i.e., Ti-N-O, is 44.9 kcal/mol. In addition, the N-Ti and N-O bond lengths are 2.008 Å and 1.307 Å, respectively. For the presence of the one hydroxyl group, there are only two possible adsorbates of Ti-N-Oa1 and Ti-N-Oa3 with adsorption energies of 48.4 and 48.9 kcal/mol, respectively. Similarly, the lack of an a2 arrangement is because the 2c-O is already occupied by the N bond. The Ti-N bond length of a1 and a3 is shorter than on the clean surface, with Ti-N-Oa1 at 1.900 Å and Ti-N-Oa3 at 1.904 Å as compared to 2.008 Å for Ti-N-O. Similarly, there are two adsorbates with two hydroxyl groups, Ti-N-Ob1 and Ti-N-Ob3, whose adsorption energies are 54.5 and 56.7 kcal/mol, respectively. The Ti-N bond lengths for these two adsorbates also become shorter than those for adsorbates with only one hydroxyl group, at 1.880 Å for Ti-N-Ob1 and 1.883 Å for Ti-N-Ob3. Finally, configuration 34 of Ti-N-Oc2 shows the adsorbate with three hydroxyl groups. The adsorption energy of Ti-N-Oc2 is 55.4 kcal/mol. Comparing the adsorption energy for all Ti-N-O adsorbates, as the number of hydroxyl groups increases, the adsorption energy of this bidentate adsorbate also increases, although the influence is not significant. For example, the adsorption energy increases by only 4 kcal/mol from Ti-N-O on the clean surface to Ti-N-Oa3 and

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Chang et al.

Figure 4. Potential energy surface of NH3 on the anatase (101) surface in which the dashed line shows the energetics without the hydroxyl group effect.

by only 7.8 kcal/mol from Ti-N-Oa3 to Ti-N-Ob3. In fact, the adsorption energy is even observed to decrease 1.3 kcal/mol from Ti-N-Ob3 to Ti-N-Oc2. The increase in the adsorption energy is more significant as the hydroxyl group increase from one to two. On the contrary, the adsorption energy difference is very small as the hydroxyl group increase from two to three on the surface. Finally, for the H fragment, the adsorption energy for the H adsorbed on 2c-O of the clean surface is 55.5 kcal/mol, and the O-H bond length is 0.972 Å. Note that H is adsorbed at 2c-O, which is located at III, leaving fewer possible arrangements from Table 2: a1 and a3, and b1 and b3. When one hydroxyl group is present, there are two H adsorbates of H-Oa1 and H-Oa3 with adsorption energies of 48.3 and 47.7 kcal/mol, respectively. The difference of the adsorption energy between the adsorbate of H-Oa3 and that on the clean surface is 7.8 kcal/mol. Finally, for the adsorbates H-Ob1 and H-Ob3, where two hydroxyl groups are present on the surface, adsorption energies are 43.6 and 45.1 kcal/mol, respectively. The maximum difference of the adsorption energy between H-Ob1 and H-Oa1 is 4.7 kcal/mol. Potential Energy Surface and Reaction Mechanism. With the Hydroxyl Group. The potential energy surface starts from NH3(g) plus a TiO2(101) surface, which is set as zero energetic reference, as shown in Figure 4. Next, the gas phase of NH3 is adsorbed on 5c-Ti, forming the monodentate adsorbate of H3NTi with an energetic of -27.5 kcal/mol. H3N-Ti forms two

different adsorbates H2N-Tia1 + H-O and H2N-Tia2 + H-O when the H is dissociated from H3N-Ti, with energetics of 4.5 and 2.7 kcal/mol, respectively. In these two dissociation processes, one pathway to H2N-Tia1 + H-O proceeds without the energy barrier. On the contrary, the other pathway to H2N-Tia2 + H-O requires overcoming an energy barrier of 30.6 kcal/mol. For further H dissociation from the adsorbate of H2N-Tia1 + H-O, there are two possible pathways: one forms the Ti-(H)N-Ob1 + 2(H-O) adsorbate with an energetic of 66.5 kcal/mol, and the other one forms the adsorbate of HN-Tib2 + 2(H-O) with an energetic of 28.1 kcal/mol. The former dissociative reaction requires an energy barrier of 76.0 kcal/mol through TS2-1, and the latter reaction requires a lower energy barrier of 37.2 kcal/ mol through TS2-2. In addition, there is also a possible pathway where H is dissociated from the adsorbate H2N-Tia2 + H-O to the same adsorbate HN-Tib2 + 2(H-O), a process of 25.4 kcal/ mol heat of reaction without a well-defined transition state. Finally, note that there are two dissociation processes without transition states: from H3N-Ti(a) to H2N-Ti(a)a1 + H-O(a) and H2N-Ti(a)a2 + H-O(a) to HN-Ti(a)b2 + 2(H-O(a)). One possible reason for these is the condition they have in common where the dissociative H goes to its nearest neighbor of 2c-O. In addition, in their initial state, the H atom has formed a hydrogen bond to 2c-O (see Figure 2 for configurations 1 and 5). For further H dissociation, 26.5 kcal/mol is required to dissociate the adsorbate Ti-(H)N-Ob1 + 2(H-O) through TS3-1

Adsorption Configuration and Dissociative Reaction

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6671

to become the final adsorbate Ti-N-Oc2 + 3(H-O), with an energetic of 89.0 kcal/mol. However, a higher energy of 126.9 kcal/mol is required to dissociate the similar adsorbate Ti-(H)NOb2 + 2(H-O) through TS3-2 to become the final adsorbate TiN-Oc3 + 3(H-O), with an energetic of 83.2 kcal/mol, lower than that of Ti-N-Oc2 + 3(H-O). Finally, we have compared the NH3 dissociation reaction with those in the existing literature,60 which investigated the reaction on different metal surfaces, e.g., Fe(100), Ni(100), and Cr(100). The dissociation reaction of NH3(g) f NH2 + H f NH + 2H f NH + 3H, where the dissociative NH3 fragments at each stage form the stable adsorbates, is an endothermic reaction on the Ni (100) surface, which is similar to that on the TiO2 (101) surface of this study. On the contrary, NH3 decomposition is an exothermic reaction on Fe (100) and Cr (100) surfaces. Without the Hydroxyl Group. In Figure 3, the dashed line shows the energetics without the hydroxyl effect, which assumes that the adsorption of the NH3 or its fragment is not influenced by the presence of the hydroxyl group. Here, we omit the CINEB calculation to find the transition state because the actual energy between any two states is included the hydroxyl effect during the VASP geometric optimization. The energetics are calculated by the adsorption of the related molecule on the clean surface and the related gas dissociation reaction of NH3, NH2, or NH. For example, the energetic of H2N-Ti + H-O is calculated by summing the following three reactions:

NH3(g) f NH2(g) + H(g), ∆H ) 114.3 kcal/mol (3) H(g) + TiO2 f H-O, ∆H ) -55.5 kcal/mol

(4)

H2N(g) + TiO2 f H2N-Ti, ∆H ) -19.4 kcal/mol (5) obtaining 39.4 kcal/mol, where all the values, ∆H, in eqs 3, 4, and 5 can be found in Tables 1 and 3. Note that if the value in eq 3 is replaced with the negative value of the adsorption energy of H2N-Tia1, we will get 4.5 kcal/mol, which is the energetic of H2N-Tia1 + H-O when the hydroxyl effect is considered. Comparing these two energetics, it is clear that the energy is lower when one hydroxyl group is present on the surface. Adopting a procedure similar to that described above, we can find that the energy of HN-Tib2 + 2(H-O), where two hydroxyl groups are present, is lower than that of the clean surface. The energy of HN-Tib2 + 2(H-O) is 57.1 kcal/mol lower than that of HN-Ti + 2(H-O) because it represents the increase (64.9 kcal/mol) in the adsorption energy of HN-Tib2 together with the decrease (-7.2 kcal/mol) in the adsorption energy of the second dissociated H adsorbed on 2c-O for a value of 57.7 kcal/ mol. (Note that the 0.6 kcal/mol difference arises from different calculation approaches: PES uses the total energy between the specific and reference states to obtain the energy, whereas here, we use the summation of the dissociation and adsorption reactions.) A comparison of the energetics for Ti-(H)N-O + 2(H-O) with and without the presence of the hydroxyl group shows that the energetics without the hydroxyl is lower than that with the hydroxyl. The reason for this is the decrease in the adsorption energy of the second dissociated H adsorbed on 2c-O in addition to the already-adsorbed hydroxyl group (as in eq 8), together with the decrease in the adsorption energy of NH(g), which adsorbed on the surface to form the bidentate adsorbate of H2NTib1 in addition to the two hydroxyl groups already formed on

the surface (see eq 9). Two decreases in adsorption energy will raise the energetics. The detailed reactions are as follows:

NH3(g) f NH(g) + 2H(g), ∆H ) 209.5 kcal/mol (6) H(g) + TiO2 f H-O, ∆H ) -55.5kcal/mol

(7)

H(g) + TiO2′ H-Oa3 + H-O, ∆H ) -47.7kcal/mol (8) HN(g) + TiO′′2 f Ti-(H)N-Ob1 + 2(H-O), ∆H ) -39.9kcal/mol

(9)

where the number of primes at superscript of TiO2 represents the number of hydroxyl groups on the TiO2 surface. Similarly, a comparison of the energetics of Ti-N-O + 3(HO) with and without hydroxyl groups shows that the energetics on the clean surface is also lower than that with the hydroxyl group. The reason for this is (a) the decrease in the adsorption energies of the second and third dissociated Hs adsorbed on 2c-Os and (b) the increase in the adsorption energy of N(g) adsorbed on the surface that forms the bidentate adsorbate TiN-Oc2. In total, this leads to an increase in energetics as compared to that on the clean surface. Conclusions In this article, we have employed the first principles calculations based on the DFT with GGA and the plane-wave method to investigate the adsorption configurations and dissociative reactions of NH3 on the anatase (101) surface. In addition, the hydroxyl effect is also included to study how this effect influences the adsorption and the dissociative reactions. Without the presence of a hydroxyl, adsorption on the clean surface can take the form of both monodentate adsorbates H3N-Ti, H2NTi, HN-Ti, and H-O, and bidentate adsorbates Ti-(H2)N-O, Ti(H)N-O, and Ti-N-O. Among these (with the exception of H-O), the most stable are the bidentate adsorbates of Ti-N-O with an adsorption energy of 44.9 kcal/mol, Ti-(H)N-O with an adsorption energy of 40.8 kcal/mol, and the monodentate adsorbate H3N-Ti with an adsorption energy of 27.5 kcal/mol. The hydroxyl group present on the surface was found in different adsorbates to either enhance or diminish the adsorption of these adsorbates. Significant enhancement in adsorption is found mostly for the monodentate adsorbates H2N-Ti and HNTi where the adsorption energy increases as the number of hydroxyl groups increases. In addition, while the N does not exist for the monodentate adsorbate of N-Ti on the clean surface, it instead forms several monodentate adsorbates while at least one hydroxyl group is present, for example, N-Tia1, N-Tib2, and N-Tic3. For bidentate adsorbates, the effect on adsorption is not as significant as for monodentate adsorbates such as H2N-Ti and HN-Ti. Slight enhancement occurs for Ti-N-O, slight diminishing occurs for Ti-(H2)N-O, while both slight enhancement and diminishment occurs for Ti-(H)N-O. The dependence of adsorption energy on the different location of hydroxyl groups was not found to be significant. We found two reaction pathways to reach two final products: one is Ti-N-Oc2 + 3(H-O), and the other is N-Tic3 + 3(H-O), with energetics of 89.0 and 83.2 kcal/mol, respectively. In addition, the maximum reaction energy barrier required to reach these two final products are 76.0 kcal/mol, obtained from the pathway H2N-Tia1 + H-O to Ti-(H)N-Ob1 + 2(H-O), and 126.9

6672 J. Phys. Chem. C, Vol. 113, No. 16, 2009 kcal/mol, obtained from the pathway HN-Tib2 + 2(H-O) to N-Tic3 + 3(H-O). All of the reactions, except the forming of H3N-Ti, are endothermic. The hydroxyl group was found to lower or raise the energetics. The energetics of H2N-Ti + H-O and HN-Ti + 2(H-O) are significantly lowered; however, the energetics of Ti-(H)N-O + 2(H-O) and Ti-N-O + 3(H-O) are slightly raised, as compared to those energetics without the presence of the hydroxyl group. The reaction pathway to N-Ti + 3(H-O) is only found when considering the hydroxyl group effect. This example illustrates that the reaction pathway can only be predicted when the hydroxyl group effect is taken into account. Finally, it should be noted that the energy barriers for the dissociation of NH3 to smaller fragments are very high. Thermally, those processes are not possible and are irrelevant to practical applications. Accordingly, one needs plasma or UV irradiation to generate NHx (x < 3), for example. Acknowledgment. We gratefully acknowledge the financial support provided for this study by the National Science Council, Republic of China, under Grant No. NSC 96-2221-E-492-008 and NSC 97-2221-E-492-003-MY2, and for the use of CPUs at the National Center for High-Performance Computing in Hsinchu, Taiwan. In addition, we are also thankful for the financial support from the National Center for Theoretical Sciences, Taiwan, during the short term visit. Finally, we are greatly indebted to Professor M. C. Lin for fruitful discussions and his input to this research. Supporting Information Available: Verification for the parameters and the surface model used in VASP, and the top view of the configurations listed in Figures 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Moon, S.-C.; Matsumura, Y.; Kitano, M.; Matsuoka, M.; Anpo, M. Res. Chem. Intermed. 2003, 29, 233. (2) Kitano, M.; Tsujimaru, K.; Anpo, M. Top. Catal. 2008, 49, 4. (3) Kitano, M.; Takeuchi, M.; Matsuoka, M.; Thomas, J. M.; Anpo, M. Catal. Today 2007, 120, 133. (4) Gra¨tzel, M. Nature 2001, 414, 338. (5) Robertson, N. Angew. Chem., Int. Ed. 2008, 47, 1012. (6) Millington, K. R.; Fincher, K. W.; King, A. L. Sol. Energy Mater. Sol. Cells 2007, 91, 1618. (7) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (8) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (9) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (10) Kalyanasundaram, K.; Gra¨tzel, M. Coord. Chem. ReV. 1998, 77, 347. (11) Stebbins, J. F. Chem. Mater. 2007, 19, 1862. (12) Hoffmann, A. A.; Dias, S. L. P.; Rodrigues, J. R.; Pavan, F. A.; Benvenutti, E. V.; Lima, E. C. J. Brazil. Chem. Soc. 2008, 19, 943. (13) Yadav, B. C.; Srivastava, R.; Dwivedi, C. D. Philos. Mag. 2008, 88, 1113. (14) Nozik, A. J. Annu. ReV. Phys. Chem. 1978, 29, 189. (15) O’Reganoulos, B.; Gra¨tzel, M. Nature 1991, 353, 737. (16) Chatterjee, D.; Patnam, V. R.; Sikdar, A.; Joshi, P.; Misra, R.; Rao, N. N. J. Hazard. Mater. 2008, 156, 435. (17) Hart, J. N.; Cheng, Y.-B.; Simon, G. P.; Spiccia, L. J. Nanosci. Nanotechnol. 2008, 8, 2230. (18) Kitao, O.; Sugihara, H. Inorg. Chim. Acta 2008, 361, 712. (19) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483.

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