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Molecular simulation for gas adsorption at tio2 (rutile and anatase) surface
NanoStmchued
Pergamon PI1 SO9659773(99)00134-S
MOLECULAR
Materials, Vol. 12, pp. 357-360, 1999 Elsevier Science Ltd D 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/99/$-see front matter
SIMULATION FOR GAS ADSORPTION (RUTILE AND ANATASE) SURFACE
AT TiO,
Hsin-Fu Lin’, Hong-Ming Lin’, Shaw-Ling Hsu’ ‘Dept. of Materials Engineering, Tatung Institute of Technology, Taipei 104, Taiwan, R.O.C. ‘Polymer Scienceand Engineering, University of Massachusetts,Amherst, MA, U. S. A.
Abstract -- In this study, the simulationfor gas adsorption on l?O, was performed using the “SORPTION” module in the software of molecular simulation. The properties of gas adsorption at the surface of Ti02 are predicted by Monte-Carlo method. The openforce field theory is usedto calculate the interaction energiesbetween CO gas and rutile or anatase Ti02 surface. Existence of diflerent energy peaks in energy analysis suggeststhat adsorption of CO gas occurs at various atomic sites of r-utile or anatase TiO, surface. 01999 Acta Metallurgica Inc. INTRODUCTION Several studies on synthesis, characteristics, properties of nanocrystalline (NC) TiO, particles have been published [l-3], and titanium dioxide has been used in a great variety of applications, as well. Especially, titanium dioxide is an intriguing support material in heterogeneouscatalysis, and it hasbeen usedasa semiconductorsurface in sensorapplications. It is the purposeof this study to build up the databasefor nanocrystalline gas sensorsof multifunctions and good selectivity. To evaluate adsorption properties of NC r-utile or anataseTiOz particle, molecular simulation is employed to calculate the binding energiesof CO gason TiOz surfaceat different temperaturesand pressuresin this study. THEORETICAL
Simulation
SIMULATION
Techniques
In this study, simulation is performed using “SORPTION” module in molecular simulation software of Cerius 2-1.6 (Molecular Simulation Inc.). The properties of gas adsorption at surface of TiO, are predicted by Monte-Carlo method[4], and the interaction energiesbetween gas molecules and TiO, surface are calculated by the theory of open force field [5]. Equilibrium is achieved when the temperatureand the chemical potential of the gas inside the framework are equal to the temperature and the chemical potential of the free gas outside the framework. The initial configuration contains no adsorbate molecules; each subsequentconfiguration is generatedby one of four steps:(i) create a molecule, (ii) destroy a molecule, (iii) translate, (iv) rotate, for which the acceptance criteria are different. Each generatedconfiguration is accepted or rejected using a Metropolis algorithm [6] basedon the configuration energy change. 357
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
Energy Calculation Energy calculation in this simulation is confined to inter-molecular or non-bond energies. Since both the framework and the adsorbates are held internally rigid during the simulation, no Thus, only van der Waals and valence energy terms are required in the energy calculation. Coulomb energy of the adsorbate/framework system are calculated. Adsorbate/framework electrostatics is evaluated using the Ewald summation method[7], which accelerates the longrange Coulomb calculation. Since the non-periodicity of the adsorbates precludes the use of the Ewald summation method, adsorbate/adsorbate electrostatics is evaluated directly.
Variables for Simulation Small periodic framework models of TiO, crystal are built up first by arranging 6 x 6 x 6 superlattice of rutile unit cell and 6 x 6 x 3 of anatase due to the working space limitation of so&ware. These two models are then made nearly spherical in shape by 15% facetting along { 1 lo} planes. To calculate Coulomb energy, partial net charges[8] are placed on each atom of the frameworks. Since the framework structure must be of neutral charge, average charge is set to be zero, and partial effective charges are consequently obtained. In addition, charge equilibration [9] is applied to calculate partial charges on models of gas adsorbate [lo]. The simulation of adsorption properties of CO gas on NC rutile and anatase TiO, surface is performed at different temperatures of 473, 573, 673, 773, 873 K and pressures of 0.1, 0.5, 1.O kPa in this study. RESULTS
AND DISCUSSION
The mean loading curves show the average number of adsorbate molecules that are packed within the pores after equilibrium is achieved, which is equivalent to adsorption isotherms, as shown in Figure l(a) and (b). For adsorption of CO gas on rutile or anatase TiO, surface, the adsorption capacity increases with increasing pressures but decreases with increasing temperatures.
.
(a) r-utile Figure 1: Adsorption
(b) anatase
isotherms for CO interaction with (a) r-utile and (b) anatase.
.
FOURTH INTERNATIONAL
(l)P=O.l
CONFERENCE
kPa
ON NANOSTRUCTURED
MATERIALS
(l)P=O.l
359
kPa
(2) P = 0.5 kPa
(2) P = 0.5 kPa
(3) P = 1.0 kPa
(3) P = 1.O kPa
Figure 2 (a): Energy distribution for different CO gas pressures on t-utile TiO,.surface.
Figure 2 (b): Energy distribution for different CO gas pressures on anatase TiO,.surface.
Energy analysis shows energy distribution in form of number of accepted configurations versus the interaction energy over the selected range. The energy distribution of adsorbates CO on either t-utile or anatase TiO, particle can be apparently divided into two portions[ 111, as shown in Figures 2(a) and 2(b). The energy distribution suggests that there is a stronger chemisorption between CO gas and anatase TiO, than that of r-utile TiO,. There is large number of configurations for anatase structure appearing in higher interaction energy range. That indicates the anatase structure of TiO, has a greater catalytic activity or selectivity for CO gas than that of t-utile structure of TiO,.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
The results also show the energy peaks are shifted from lower interaction energy to higher interaction energy while the temperature is increased, because it needs more interaction energy for gas molecules to react with TiO, surface. Meanwhile, it can be observed that energy peaks diminish gradually with increasing temperature. This could be attributed to thermal desorption or fluctuation at higher temperatures due to escape of adsorbed gas molecules after obtaining energy larger than binding energy. In addition, each energy peak also can be considered as one preferred binding position. In other word, existence of different energy peaks in the energy distribution infers that adsorption of CO gas molecules might occur at various binding sites. CONCLUSIONS The simulation results can be used semi-quantitatively to compare the adsorption properties of CO gas on the rutile and anatase TiO, surfaces. From the adsorption isotherms, it is found that the amount of CO gas adsorption increases as the pressure is increased, while it decreases as the temperature is raised. The energy distribution suggests that CO gas is reacted with anatase structure by a strong chemical bond than that of rutile structure. The results indicate the anatase structure of TiO, has a greater catalytic activity or selectivity for CO gas than that of rutile structure of TiO,. ACKNOWLEDGMENTS We would like to thank the National Science Council, Republic of China for financial support through Contract Number NSC 86-22 16-E-036-013. REFERENCES 1. R. W. Siegel, S. Ramasamy, H. Hahn, Li Zongquan and Lu Ting, J. Mcztex Res., 3(6), 1367(1988). 2. H. Hahn, J. Logas and R. S. Averback, J. Mutex Res., 5(3), 609( 1990). 3. U. Balachandran, N. G. Eror, J. Mater: Sci., 23,2676( 1988). 4. K. Binder, ‘Monte Carlo Methods in Statistical Physics”, Springer-Verlag, New York (1986). 5. A. K. Rappt, C. J. Casewit, K. S. Colwell, W. A. Goddard III and W. M. Skiff, J. Am. Chem. Sot., 114, 10024(1992). 6. N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth and A.H. Teller, J. Chem. Phys., 21(6), 1087(1953). 7. N. Karasawa and W. A. Goddard III, J. Phys. Chem., 93,7320( 1989). 8. B. Silvi, N. Fourati, R. Nada and C. R. A. Catlow, J. Phys. Chem. Solids, 52(8), 1005(1991). 9. A. K. RappC and W. A. Goddard III, J. Phys. Chem., 95,3358( 1991). 10. L. Pauling, “The Nature of the Chemical Bond’, 3” edition, Cornell University Press (1960). 11. A. Mandelis, “Physics, Chemistry and Technology of Solid State Gas Sensor Devices”, John Wiley & Sons, Inc. (1993)
Pergamon PI1 SO9659773(99)00134-S
MOLECULAR
Materials, Vol. 12, pp. 357-360, 1999 Elsevier Science Ltd D 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/99/$-see front matter
SIMULATION FOR GAS ADSORPTION (RUTILE AND ANATASE) SURFACE
AT TiO,
Hsin-Fu Lin’, Hong-Ming Lin’, Shaw-Ling Hsu’ ‘Dept. of Materials Engineering, Tatung Institute of Technology, Taipei 104, Taiwan, R.O.C. ‘Polymer Scienceand Engineering, University of Massachusetts,Amherst, MA, U. S. A.
Abstract -- In this study, the simulationfor gas adsorption on l?O, was performed using the “SORPTION” module in the software of molecular simulation. The properties of gas adsorption at the surface of Ti02 are predicted by Monte-Carlo method. The openforce field theory is usedto calculate the interaction energiesbetween CO gas and rutile or anatase Ti02 surface. Existence of diflerent energy peaks in energy analysis suggeststhat adsorption of CO gas occurs at various atomic sites of r-utile or anatase TiO, surface. 01999 Acta Metallurgica Inc. INTRODUCTION Several studies on synthesis, characteristics, properties of nanocrystalline (NC) TiO, particles have been published [l-3], and titanium dioxide has been used in a great variety of applications, as well. Especially, titanium dioxide is an intriguing support material in heterogeneouscatalysis, and it hasbeen usedasa semiconductorsurface in sensorapplications. It is the purposeof this study to build up the databasefor nanocrystalline gas sensorsof multifunctions and good selectivity. To evaluate adsorption properties of NC r-utile or anataseTiOz particle, molecular simulation is employed to calculate the binding energiesof CO gason TiOz surfaceat different temperaturesand pressuresin this study. THEORETICAL
Simulation
SIMULATION
Techniques
In this study, simulation is performed using “SORPTION” module in molecular simulation software of Cerius 2-1.6 (Molecular Simulation Inc.). The properties of gas adsorption at surface of TiO, are predicted by Monte-Carlo method[4], and the interaction energiesbetween gas molecules and TiO, surface are calculated by the theory of open force field [5]. Equilibrium is achieved when the temperatureand the chemical potential of the gas inside the framework are equal to the temperature and the chemical potential of the free gas outside the framework. The initial configuration contains no adsorbate molecules; each subsequentconfiguration is generatedby one of four steps:(i) create a molecule, (ii) destroy a molecule, (iii) translate, (iv) rotate, for which the acceptance criteria are different. Each generatedconfiguration is accepted or rejected using a Metropolis algorithm [6] basedon the configuration energy change. 357
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
Energy Calculation Energy calculation in this simulation is confined to inter-molecular or non-bond energies. Since both the framework and the adsorbates are held internally rigid during the simulation, no Thus, only van der Waals and valence energy terms are required in the energy calculation. Coulomb energy of the adsorbate/framework system are calculated. Adsorbate/framework electrostatics is evaluated using the Ewald summation method[7], which accelerates the longrange Coulomb calculation. Since the non-periodicity of the adsorbates precludes the use of the Ewald summation method, adsorbate/adsorbate electrostatics is evaluated directly.
Variables for Simulation Small periodic framework models of TiO, crystal are built up first by arranging 6 x 6 x 6 superlattice of rutile unit cell and 6 x 6 x 3 of anatase due to the working space limitation of so&ware. These two models are then made nearly spherical in shape by 15% facetting along { 1 lo} planes. To calculate Coulomb energy, partial net charges[8] are placed on each atom of the frameworks. Since the framework structure must be of neutral charge, average charge is set to be zero, and partial effective charges are consequently obtained. In addition, charge equilibration [9] is applied to calculate partial charges on models of gas adsorbate [lo]. The simulation of adsorption properties of CO gas on NC rutile and anatase TiO, surface is performed at different temperatures of 473, 573, 673, 773, 873 K and pressures of 0.1, 0.5, 1.O kPa in this study. RESULTS
AND DISCUSSION
The mean loading curves show the average number of adsorbate molecules that are packed within the pores after equilibrium is achieved, which is equivalent to adsorption isotherms, as shown in Figure l(a) and (b). For adsorption of CO gas on rutile or anatase TiO, surface, the adsorption capacity increases with increasing pressures but decreases with increasing temperatures.
.
(a) r-utile Figure 1: Adsorption
(b) anatase
isotherms for CO interaction with (a) r-utile and (b) anatase.
.
FOURTH INTERNATIONAL
(l)P=O.l
CONFERENCE
kPa
ON NANOSTRUCTURED
MATERIALS
(l)P=O.l
359
kPa
(2) P = 0.5 kPa
(2) P = 0.5 kPa
(3) P = 1.0 kPa
(3) P = 1.O kPa
Figure 2 (a): Energy distribution for different CO gas pressures on t-utile TiO,.surface.
Figure 2 (b): Energy distribution for different CO gas pressures on anatase TiO,.surface.
Energy analysis shows energy distribution in form of number of accepted configurations versus the interaction energy over the selected range. The energy distribution of adsorbates CO on either t-utile or anatase TiO, particle can be apparently divided into two portions[ 111, as shown in Figures 2(a) and 2(b). The energy distribution suggests that there is a stronger chemisorption between CO gas and anatase TiO, than that of r-utile TiO,. There is large number of configurations for anatase structure appearing in higher interaction energy range. That indicates the anatase structure of TiO, has a greater catalytic activity or selectivity for CO gas than that of t-utile structure of TiO,.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
The results also show the energy peaks are shifted from lower interaction energy to higher interaction energy while the temperature is increased, because it needs more interaction energy for gas molecules to react with TiO, surface. Meanwhile, it can be observed that energy peaks diminish gradually with increasing temperature. This could be attributed to thermal desorption or fluctuation at higher temperatures due to escape of adsorbed gas molecules after obtaining energy larger than binding energy. In addition, each energy peak also can be considered as one preferred binding position. In other word, existence of different energy peaks in the energy distribution infers that adsorption of CO gas molecules might occur at various binding sites. CONCLUSIONS The simulation results can be used semi-quantitatively to compare the adsorption properties of CO gas on the rutile and anatase TiO, surfaces. From the adsorption isotherms, it is found that the amount of CO gas adsorption increases as the pressure is increased, while it decreases as the temperature is raised. The energy distribution suggests that CO gas is reacted with anatase structure by a strong chemical bond than that of rutile structure. The results indicate the anatase structure of TiO, has a greater catalytic activity or selectivity for CO gas than that of rutile structure of TiO,. ACKNOWLEDGMENTS We would like to thank the National Science Council, Republic of China for financial support through Contract Number NSC 86-22 16-E-036-013. REFERENCES 1. R. W. Siegel, S. Ramasamy, H. Hahn, Li Zongquan and Lu Ting, J. Mcztex Res., 3(6), 1367(1988). 2. H. Hahn, J. Logas and R. S. Averback, J. Mutex Res., 5(3), 609( 1990). 3. U. Balachandran, N. G. Eror, J. Mater: Sci., 23,2676( 1988). 4. K. Binder, ‘Monte Carlo Methods in Statistical Physics”, Springer-Verlag, New York (1986). 5. A. K. Rappt, C. J. Casewit, K. S. Colwell, W. A. Goddard III and W. M. Skiff, J. Am. Chem. Sot., 114, 10024(1992). 6. N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth and A.H. Teller, J. Chem. Phys., 21(6), 1087(1953). 7. N. Karasawa and W. A. Goddard III, J. Phys. Chem., 93,7320( 1989). 8. B. Silvi, N. Fourati, R. Nada and C. R. A. Catlow, J. Phys. Chem. Solids, 52(8), 1005(1991). 9. A. K. RappC and W. A. Goddard III, J. Phys. Chem., 95,3358( 1991). 10. L. Pauling, “The Nature of the Chemical Bond’, 3” edition, Cornell University Press (1960). 11. A. Mandelis, “Physics, Chemistry and Technology of Solid State Gas Sensor Devices”, John Wiley & Sons, Inc. (1993)