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Photocatalytic oxidation dynamics of acetone on TiO2: tight-binding quantum chemical molecular dynamics study
Applied Surface Science 244 (2005) 541–545 www.elsevier.com/locate/apsusc
Photocatalytic oxidation dynamics of acetone on TiO2: tight-binding quantum chemical molecular dynamics study Chen Lva, Xiaojing Wanga, Govindasamy Agalyaa, Michihisa Koyamaa, Momoji Kuboa,b, Akira Miyamotoa,c,* a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Received 21 June 2004; accepted 13 September 2004 Available online 6 January 2005
Abstract The clarification of the excited states dynamics on TiO2 surface is important subject for the design of the highly active photocatalysts. In the present study, we applied our novel tight-binding quantum chemical molecular dynamics method to the investigation on the photocatalytic oxidation dynamics of acetone by photogenerated OH radicals on the hydrated anatase TiO2 surface. The elucidated photocatalytic reaction mechanism strongly supports the previous experimental proposal and finally the effectiveness of our new approach for the clarification of the photocatalytic reaction dynamics employing the large simulation model was confirmed. # 2004 Elsevier B.V. All rights reserved. PACS: 31.70.Hq; 71.15.Pd; 82.65.+r; 82.50.Hp Keywords: Titanium dioxide; Acetone; Excited state dynamics; Photocatalytic reaction dynamics; Tight-binding quantum chemical molecular dynamics
1. Introduction
* Corresponding author. Tel.: +81 22 217 7233; fax: +81 22 217 7235. E-mail address: [email protected] (A. Miyamoto).
The environmental pollution by harmful organic compounds is a severe problem in all of the ecosystem functions. Acetone is one of the most serious air pollutants in indoor environments, and its catalytic decomposition to less harmful compounds has been explored intensively. Heterogeneous photocatalytic
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.159
542
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
oxidation [1,2] is an attractive technology for the decomposition of organic compounds, such as acetone. In photocatalysis, photoinduced molecular transformation and reaction take place on the photocatalyst surface when the reactant is adsorbed under UV irradiation [3]. Many researchers have reported that TiO2 is an effective photooxidation catalyst for these organic compounds [4]. The activity of the photocatalyst might be dependent on the surface conditions, and water has been reported to enhance photocatalytic oxidation of other hydrocarbons, such as formic acid over TiO2 surface [5]. It is proposed that the photogenerated OH radicals, which are produced from water adsorbed on the TiO2 surface, are highly reactive species and important oxidizing agent. Water controls the kinetics of the overall oxidation process. Thus, the identification of OH radical species on the surface is important to understand photooxidation mechanism of organic compounds. On the other hand, first-principles methods, such as time-dependent density functional theory (TDDFT) and differential self-consistent-field-based DFT (DSCF-DFT) are employed to evaluate the excited states for small molecules. However, these methodologies request huge computational costs, and hence realistic large model including the photocatalysts cannot be calculated by the above methodologies. Therefore, in order to solve the above problem, recently we have successfully developed a novel tightbinding quantum chemical molecular dynamics program [6,7], which is based on our original tightbinding approximations, and extended it for the excited state calculations. Hence, in the present study, our tight-binding quantum chemical molecular dynamics method was applied to the investigation on the excited states dynamics of OH radical produced on the hydrated anatase TiO2 surface. Furthermore, the photocatalytic oxidation dynamics of acetone by OH radical produced on the hydrated anatase TiO2 surface was also investigated.
2. Computational methods Our tight-binding quantum chemical molecular dynamics program Colors was used for the excited states dynamics calculations. Our tight-binding
approximation realizes 5000 times acceleration as compared to the conventional first-principles molecular dynamics calculations and hence large simulation model including photocatalysts can be employed. In this program, various parameters are used to accelerate the calculation speed; however, all the parameters in the Colors program are determined on the basis of the density functional theory calculation results in order to realize high accuracy. In our tight-binding approximations, the total energy, E, is expressed by Eq. (1): E¼
N X 1 i¼1
þ
2
mi v2i þ
O CC X
ek
k¼1
N X N N X N X Zi Zj e2 X þ Erepuls ðrij Þ rij i¼1 j > i i¼1 j > i
(1)
where mi is the atomic weight, vi the atomic velocity, e the elementary electric charge, and rij is the internuclear distance. Z is the atomic charge obtained by the electronic states calculations. The first, second, third, and fourth terms in Eq. (1) represent the kinetic energy, the summation of the eigenvalues for all occupied orbitals, the Coulombic interaction energy, and the short-range exchange-repulsion energy, respectively. The short-range exchange-repulsion term Erepuls is represented by Eq. (2): aij rij Erepuls ðrij Þ ¼ bij exp (2) bij where aij and bij are parameters related to the size and stiffness of the atoms.
3. Results and discussion 3.1. Excited states dynamics of TiO2 surface with dissociatively adsorbed water The singlet and triplet excited state energies of the anatase TiO2 were calculated by using Colors. Here, a periodic bulk TiO2 model including 48 atoms was employed. The vertical singlet to singlet excitation energy was calculated in terms of the formula of E (singlet) = 2E (mixed) E (triplet). The excitation energy that corresponds to the electron transfer from HOMO to LUMO is 3.24 eV, which is close to the experimental band gap energy of 3.2 eV [8]. To the
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
543
Fig. 1. Tight-binding quantum chemical molecular dynamics simulation on the excited state dynamics of hydrated anatase TiO2 surface at (a) 0 step and (b) 300 step.
best of our knowledge, there are no other computational methods, which can simulate the excited state dynamics of such large periodic systems because of their huge computational cost. Hence, we performed the extended application of our Colors program to the investigation on the excited states dynamics of anatase TiO2 surface with dissociatively adsorbed water. Here, we employed the hydrated anatase TiO2(0 0 1) surface for the above purpose. Fig. 1 shows the snapshots of the excited states dynamics of the hydrated anatase TiO2 surface at (a) 0 step and (b) 300 step at 298 K. The light was introduced to this system by promoting the electron from HOMO to LUMO orbital. From Fig. 1(b), it was found that OH radicals were formed at 300 step. Fig. 2 illustrated the change in the bond length between the surface Ti atom and OH radical. It is clear that the Ti–OH bond length increased with time. Finally, the Ti–OH distance ˚ , which indicates the Ti–OH was elongated to 2.0 A bond-breaking and the OH radical formation.
Fig. 2. Change in the Ti–OH bond length of hydrated anatase TiO2 surface at the excited state obtained by tight-binding quantum chemical molecular dynamics method.
Experimentally, it was already pointed out that when UV is irradiated, the electron–hole pair is generated, and then the hole reacts with surface OH groups and OH radicals is produced. Our simulation results for the excited states dynamics of the hydrated anatase TiO2 surface strongly supports the above experimental results. 3.2. Photocatalytic oxidation dynamics of acetone on excited TiO2 surface The photocatalytic oxidation dynamics of the acetone molecule with an OH radical on the anatase TiO2 surface at 298 K was simulated by using our Colors program. Fig. 3 depicted the snapshots of the photocatalytic reaction dynamics of the acetone with an OH radical on the anatase TiO2 surface at (a) 0 step, (b) 500 step, and (c) 1500 step. It was found that one C–H bond length of the acetone was increased with time and completely broken at 500 step (Fig. 3(b)). Moreover, new bond formation between the dissociated H of the acetone and O of the OH radical was observed at 1500 step (Fig. 3(c)). Fig. 4(a and b) shows the change in the above C–H and O–H bond lengths, respectively, which illustrate the detailed mechanism of the above C–H bond-breaking and O–H bond-formation. Accordingly, the formation of H2O molecule and CH2 COCH3 radical was found at 1500 step (Fig. 3(c)). Therefore, our results strongly support the experimental proposal that in the presence of adsorbed water, reactive OH radical initiate the photooxidation of acetone. Finally, we confirmed that our new tight-binding quantum chemical molecular dynamics program is very effective to simulate the photocatalytic reaction dynamic employing large simulation model.
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C. Lv et al. / Applied Surface Science 244 (2005) 541–545
Fig. 3. Tight-binding quantum chemical molecular dynamics simulation of photocatalytic reaction dynamics of acetone and OH radical on hydrated anatase TiO2 surface at (a) 0 step, (b) 500 step, and (c) 1500 step.
Fig. 4. Change in the (a) C1–H1 and (b) O1–H1 bond lengths during the photocatalytic reaction dynamics of acetone and OH radical on hydrated anatase TiO2 surface (positions of C1, H1, and O1 are shown in Fig. 3(b)).
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
4. Conclusion
References
In the present study, we applied our Colors program to the investigation on the excited state dynamics of the hydrated anatase TiO2 surface and observed the formation of OH radical. Furthermore, the photocatalytic oxidation dynamics of the acetone on the anatase TiO2 surface were successfully simulated and our simulation results strongly support the previous experimental proposal. Finally, we confirmed that our new tight-binding quantum chemical molecular dynamics program is a very effective tool to investigate the photocatalytic reaction dynamics employing large simulation model.
[1] [2] [3] [4] [5] [6]
[7] [8]
545
J. Peral, D.F. Ollis, J. Catal. 136 (1992) 554. R.M. Alberici, W.F. Jardim, Appl. Catal. B 14 (1997) 55. M. Trillas, J. Peral, X. Domenech, Catal. Lett. 55 (1998) 129. G. Lu, A. Linsebigler, J.T. Yates Jr., J. Phys. Chem. 99 (1995) 7626. D.S. Muggli, S.A. Keyser, J.L. Falconer, Catal. Lett. 55 (1998) 129. K. Sasata, T. Yokosuka, H. Kurokawa, S. Takami, M. Kubo, A. Imamura, T. Shinmura, M. Kanoh, P. Selvam, A. Miyamoto, Jpn. J. Appl. Phys. 42 (2003) 1859. M. Elanany, P. Selvam, T. Yokosuka, S. Takami, M. Kubo, A. Imamura, A. Miyamoto, J. Phys. Chem. B 107 (2003) 1518. J.K. Burdett, T. Hughbanks, G.J. Miller, J.W. Richardson Jr., J.V. Smith, J. Am. Chem. Soc. 109 (1987) 3639.
Photocatalytic oxidation dynamics of acetone on TiO2: tight-binding quantum chemical molecular dynamics study Chen Lva, Xiaojing Wanga, Govindasamy Agalyaa, Michihisa Koyamaa, Momoji Kuboa,b, Akira Miyamotoa,c,* a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Received 21 June 2004; accepted 13 September 2004 Available online 6 January 2005
Abstract The clarification of the excited states dynamics on TiO2 surface is important subject for the design of the highly active photocatalysts. In the present study, we applied our novel tight-binding quantum chemical molecular dynamics method to the investigation on the photocatalytic oxidation dynamics of acetone by photogenerated OH radicals on the hydrated anatase TiO2 surface. The elucidated photocatalytic reaction mechanism strongly supports the previous experimental proposal and finally the effectiveness of our new approach for the clarification of the photocatalytic reaction dynamics employing the large simulation model was confirmed. # 2004 Elsevier B.V. All rights reserved. PACS: 31.70.Hq; 71.15.Pd; 82.65.+r; 82.50.Hp Keywords: Titanium dioxide; Acetone; Excited state dynamics; Photocatalytic reaction dynamics; Tight-binding quantum chemical molecular dynamics
1. Introduction
* Corresponding author. Tel.: +81 22 217 7233; fax: +81 22 217 7235. E-mail address: [email protected] (A. Miyamoto).
The environmental pollution by harmful organic compounds is a severe problem in all of the ecosystem functions. Acetone is one of the most serious air pollutants in indoor environments, and its catalytic decomposition to less harmful compounds has been explored intensively. Heterogeneous photocatalytic
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.159
542
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
oxidation [1,2] is an attractive technology for the decomposition of organic compounds, such as acetone. In photocatalysis, photoinduced molecular transformation and reaction take place on the photocatalyst surface when the reactant is adsorbed under UV irradiation [3]. Many researchers have reported that TiO2 is an effective photooxidation catalyst for these organic compounds [4]. The activity of the photocatalyst might be dependent on the surface conditions, and water has been reported to enhance photocatalytic oxidation of other hydrocarbons, such as formic acid over TiO2 surface [5]. It is proposed that the photogenerated OH radicals, which are produced from water adsorbed on the TiO2 surface, are highly reactive species and important oxidizing agent. Water controls the kinetics of the overall oxidation process. Thus, the identification of OH radical species on the surface is important to understand photooxidation mechanism of organic compounds. On the other hand, first-principles methods, such as time-dependent density functional theory (TDDFT) and differential self-consistent-field-based DFT (DSCF-DFT) are employed to evaluate the excited states for small molecules. However, these methodologies request huge computational costs, and hence realistic large model including the photocatalysts cannot be calculated by the above methodologies. Therefore, in order to solve the above problem, recently we have successfully developed a novel tightbinding quantum chemical molecular dynamics program [6,7], which is based on our original tightbinding approximations, and extended it for the excited state calculations. Hence, in the present study, our tight-binding quantum chemical molecular dynamics method was applied to the investigation on the excited states dynamics of OH radical produced on the hydrated anatase TiO2 surface. Furthermore, the photocatalytic oxidation dynamics of acetone by OH radical produced on the hydrated anatase TiO2 surface was also investigated.
2. Computational methods Our tight-binding quantum chemical molecular dynamics program Colors was used for the excited states dynamics calculations. Our tight-binding
approximation realizes 5000 times acceleration as compared to the conventional first-principles molecular dynamics calculations and hence large simulation model including photocatalysts can be employed. In this program, various parameters are used to accelerate the calculation speed; however, all the parameters in the Colors program are determined on the basis of the density functional theory calculation results in order to realize high accuracy. In our tight-binding approximations, the total energy, E, is expressed by Eq. (1): E¼
N X 1 i¼1
þ
2
mi v2i þ
O CC X
ek
k¼1
N X N N X N X Zi Zj e2 X þ Erepuls ðrij Þ rij i¼1 j > i i¼1 j > i
(1)
where mi is the atomic weight, vi the atomic velocity, e the elementary electric charge, and rij is the internuclear distance. Z is the atomic charge obtained by the electronic states calculations. The first, second, third, and fourth terms in Eq. (1) represent the kinetic energy, the summation of the eigenvalues for all occupied orbitals, the Coulombic interaction energy, and the short-range exchange-repulsion energy, respectively. The short-range exchange-repulsion term Erepuls is represented by Eq. (2): aij rij Erepuls ðrij Þ ¼ bij exp (2) bij where aij and bij are parameters related to the size and stiffness of the atoms.
3. Results and discussion 3.1. Excited states dynamics of TiO2 surface with dissociatively adsorbed water The singlet and triplet excited state energies of the anatase TiO2 were calculated by using Colors. Here, a periodic bulk TiO2 model including 48 atoms was employed. The vertical singlet to singlet excitation energy was calculated in terms of the formula of E (singlet) = 2E (mixed) E (triplet). The excitation energy that corresponds to the electron transfer from HOMO to LUMO is 3.24 eV, which is close to the experimental band gap energy of 3.2 eV [8]. To the
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
543
Fig. 1. Tight-binding quantum chemical molecular dynamics simulation on the excited state dynamics of hydrated anatase TiO2 surface at (a) 0 step and (b) 300 step.
best of our knowledge, there are no other computational methods, which can simulate the excited state dynamics of such large periodic systems because of their huge computational cost. Hence, we performed the extended application of our Colors program to the investigation on the excited states dynamics of anatase TiO2 surface with dissociatively adsorbed water. Here, we employed the hydrated anatase TiO2(0 0 1) surface for the above purpose. Fig. 1 shows the snapshots of the excited states dynamics of the hydrated anatase TiO2 surface at (a) 0 step and (b) 300 step at 298 K. The light was introduced to this system by promoting the electron from HOMO to LUMO orbital. From Fig. 1(b), it was found that OH radicals were formed at 300 step. Fig. 2 illustrated the change in the bond length between the surface Ti atom and OH radical. It is clear that the Ti–OH bond length increased with time. Finally, the Ti–OH distance ˚ , which indicates the Ti–OH was elongated to 2.0 A bond-breaking and the OH radical formation.
Fig. 2. Change in the Ti–OH bond length of hydrated anatase TiO2 surface at the excited state obtained by tight-binding quantum chemical molecular dynamics method.
Experimentally, it was already pointed out that when UV is irradiated, the electron–hole pair is generated, and then the hole reacts with surface OH groups and OH radicals is produced. Our simulation results for the excited states dynamics of the hydrated anatase TiO2 surface strongly supports the above experimental results. 3.2. Photocatalytic oxidation dynamics of acetone on excited TiO2 surface The photocatalytic oxidation dynamics of the acetone molecule with an OH radical on the anatase TiO2 surface at 298 K was simulated by using our Colors program. Fig. 3 depicted the snapshots of the photocatalytic reaction dynamics of the acetone with an OH radical on the anatase TiO2 surface at (a) 0 step, (b) 500 step, and (c) 1500 step. It was found that one C–H bond length of the acetone was increased with time and completely broken at 500 step (Fig. 3(b)). Moreover, new bond formation between the dissociated H of the acetone and O of the OH radical was observed at 1500 step (Fig. 3(c)). Fig. 4(a and b) shows the change in the above C–H and O–H bond lengths, respectively, which illustrate the detailed mechanism of the above C–H bond-breaking and O–H bond-formation. Accordingly, the formation of H2O molecule and CH2 COCH3 radical was found at 1500 step (Fig. 3(c)). Therefore, our results strongly support the experimental proposal that in the presence of adsorbed water, reactive OH radical initiate the photooxidation of acetone. Finally, we confirmed that our new tight-binding quantum chemical molecular dynamics program is very effective to simulate the photocatalytic reaction dynamic employing large simulation model.
544
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
Fig. 3. Tight-binding quantum chemical molecular dynamics simulation of photocatalytic reaction dynamics of acetone and OH radical on hydrated anatase TiO2 surface at (a) 0 step, (b) 500 step, and (c) 1500 step.
Fig. 4. Change in the (a) C1–H1 and (b) O1–H1 bond lengths during the photocatalytic reaction dynamics of acetone and OH radical on hydrated anatase TiO2 surface (positions of C1, H1, and O1 are shown in Fig. 3(b)).
C. Lv et al. / Applied Surface Science 244 (2005) 541–545
4. Conclusion
References
In the present study, we applied our Colors program to the investigation on the excited state dynamics of the hydrated anatase TiO2 surface and observed the formation of OH radical. Furthermore, the photocatalytic oxidation dynamics of the acetone on the anatase TiO2 surface were successfully simulated and our simulation results strongly support the previous experimental proposal. Finally, we confirmed that our new tight-binding quantum chemical molecular dynamics program is a very effective tool to investigate the photocatalytic reaction dynamics employing large simulation model.
[1] [2] [3] [4] [5] [6]
[7] [8]
545
J. Peral, D.F. Ollis, J. Catal. 136 (1992) 554. R.M. Alberici, W.F. Jardim, Appl. Catal. B 14 (1997) 55. M. Trillas, J. Peral, X. Domenech, Catal. Lett. 55 (1998) 129. G. Lu, A. Linsebigler, J.T. Yates Jr., J. Phys. Chem. 99 (1995) 7626. D.S. Muggli, S.A. Keyser, J.L. Falconer, Catal. Lett. 55 (1998) 129. K. Sasata, T. Yokosuka, H. Kurokawa, S. Takami, M. Kubo, A. Imamura, T. Shinmura, M. Kanoh, P. Selvam, A. Miyamoto, Jpn. J. Appl. Phys. 42 (2003) 1859. M. Elanany, P. Selvam, T. Yokosuka, S. Takami, M. Kubo, A. Imamura, A. Miyamoto, J. Phys. Chem. B 107 (2003) 1518. J.K. Burdett, T. Hughbanks, G.J. Miller, J.W. Richardson Jr., J.V. Smith, J. Am. Chem. Soc. 109 (1987) 3639.