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A periodic DFT study on binding of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface and adsorption of CO on the TiO2 surface-supported Pd, Pt and Au
Applied Surface Science 258 (2012) 3298–3301
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A periodic DFT study on binding of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface and adsorption of CO on the TiO2 surface-supported Pd, Pt and Au Raina Wanbayor, Vithaya Ruangpornvisuti ∗ Department of Chemistry, Faculty of Science and Center for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 26 November 2011 Keywords: Binding energy Adsorption energy TiO2 supported Pd, Pt and Au Anatase TiO2 (0 0 1) CO adsorption Periodic DFT
a b s t r a c t Binding of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface and adsorption of CO on the TiO2 (0 0 1)supported Pd, Pt and Au were studied by means of periodic density functional theory calculations. The relative binding abilities of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface are in order: Pt Au ≈ Pd. The relative adsorption abilities of CO on the TiO2 (0 0 1)-supported Pd, Pt and Au for the surface coverage equal to 0.5 ML are in order: Pt > Au > Pd of which adsorption energies are −15.98, −12.97 and −3.53 kcal/mol, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been widely used as applications in photocatalysis [1–3], dye-sensitized solar cells [4–9] and gassensors [10–19]. In general, TiO2 exists in four crystalline forms, namely (i) anatase, (ii) rutile, (iii) brookite and (iv) TiO2 (B) structures [20–22] but adsorption abilities of their surfaces seem to be very different depending on their surface characteristics. Nevertheless, surface structures and characteristics of various types of TiO2 have been extensively studied [23,24]. TiO2 -supported transition-metal catalysts have been widely investigated in order to improve their good catalytic performance in CO oxidization [25–30]. There are many research works on investigating CO interaction and adsorption on clean TiO2 [30–35] and on TiO2 -supported metal surfaces [36–38]. Adsorptions of Pt, Pd and Au on TiO2 were theoretically studied and found that their binding energies are −47.04 [39], −50.27 [40] and −22.14 kcal/mol [41], respectively. The CO adsorbed on the Pd catalysts loaded on the TiO2 was studied [42]. It was found that CO adsorbed on Pt–Ti site of Pt/TiO2 is a physical adsorption with bridging configuration [38]. Nevertheless, adsorption energy of CO on the Pd/TiO2 and Pt/TiO2 has hardly ever been reported. The CO adsorptions on Au/TiO2 have been studied using IR spectroscopic
∗ Corresponding author. Tel.: +66 2 218 7644; fax: +66 2 254 1309. E-mail address: [email protected] (V. Ruangpornvisuti). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.085
measurement [43] and infrared reflection absorption spectroscopy (IRAS) method [44,45]. It has also been reported that oxidation of CO can occur on TiO2 (1 1 0)-supported Au nanoclusters at very low temperatures [46,47]. In this work, we have therefore studied binding abilities of single Pd, Pt and Au atoms on anatase TiO2 (0 0 1) surface with loading of 50% on the surface and adsorption of CO on anatase TiO2 (0 0 1)supported Pd, Pt and Au atoms with surface coverage of 0.5 ML using the periodic density functional theory (DFT) computations. Adsorptions of CO on TiO2 (0 0 1)-supported Pd, Pt and Au surfaces have been compared with their experiments and the results would be useful to describe and predict activities of TiO2 (0 0 1)-supported Pd, Pt and Au surfaces for adsorption and reaction with CO. 2. Computational details All DFT calculations of two-dimensionally periodic slab model have been carried out using the CRYSTAL06 computational code [48], based on the expansion of the crystalline orbitals as a linear combination of a basis set consisting of atom centered Gaussian orbitals. The Kohn–Sham orbitals as Gaussian-type-orbital basis sets of double zeta quality as an 86-51G(3d) and an 8-411G contraction scheme have been respectively employed for the titanium [49] and oxygen [50] atoms on TiO2 (0 0 1) surface. Basis sets for carbon, oxygen, palladium, platinum and gold atoms employed in these calculations are a 631d1G [51], an 8411dG [52], a fitting effective core potential (ECP) of Kokalj et al. [53], Andrae et al. [54] and Hay
R. Wanbayor, V. Ruangpornvisuti / Applied Surface Science 258 (2012) 3298–3301
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Table 1 Binding energies of metals onto the anatase TiO2 (0 0 1) surface and adsorption energies of CO on the TiO2 (0 0 1)-supported metals with surface coverage of 0.5 ML and their selected geometrical parameters. Binding systems
Ebinding a
Geometrical paremeters
Adsorption system
Pd/TiO2
−32.06 −50.27b
˚ O–Pd = 2.227 A, Ti–Pd = 2.657 A˚
CO/[Pd/TiO2 ]
−3.53
Pt/TiO2
−61.19 −47.04c
˚ O–Pt = 2.057 A, Ti–Pt = 2.616 A˚
CO/[Pt/TiO2 ]
−15.98
CCO · · ·Pt = 2.041 A˚ Pt–C–O = 177.8◦ O2C –Pt–C–O = –178.5◦
Au/TiO2
−34.91 −22.14d
˚ O–Au = 2.456 A, Ti–Au = 2.515 A˚
CO/[Au/TiO2 ]
−12.97 −12.5e −10.9f
CCO · · ·Au = 2.176 A˚ Au–C–O = 164.5◦ O2C –Au–C–O = −150.0◦
a b c d e f
Eads a
Geometrical paremeters CCO · · ·Pd = 2.344 A˚ Pd–C–O = 143.3◦ O2C –Pd–C–O = −9.7◦
In kcal/mol. Periodic supercell model and DFT method, taken from Ref. [40]. Adsorbed on the anatase TiO2 (1 0 1) surface as a configuration of Pt bridging with O and Ti atoms, taken from Ref. [39]. Cluster model with O2c bridge, taken from Ref. [41]. Heat of CO adsorption on Au/TiO2 at 450 K, using infrared reflection absorption spectroscopy (IRAS) method, taken from Ref. [44]. Heat of CO adsorption on Au/TiO2 , measured for bulk Au/TiO2 (1 1 0)–[1 × 2] using the Clausius–Clapeyron method applied to isosteres of IRAS data, taken from Ref. [45].
The tolerances for geometry optimization convergence have been set to the default values [48] and the coulomb-exchange screening tolerances were set to (7, 7, 7, 7, 14). All slab calculations have been performed with a Monkhorst–Pack [59] k-point grid with shrinking factors (4, 4). There are two binding sites for the anatase TiO2 (0 0 1) surface: two-fold-coordinate O atom (O2C ) and five-fold-coordinate Ti atom (Ti5C ), see Fig. 1 and the Pd, Pt or Au atom binds with O2C and Ti5C as shown in Fig. 2. Binding energy (Ebinding ) for M-metal adsorption on the anatase TiO2 surface and adsorption energy (Eads ) for CO adsorption on the anatase TiO2 -supported M-metal are computed using Eqs. (1) and (2), respectively.
Fig. 1. The periodic slab model used for TiO2 (0 0 1) surface.
and Wadt [55], respectively. The hybrid functional, B3LYP including Becke’s three-parameter exchange [56] and Lee–Yang–Parr correlation [57], has been employed. ˚ and The optimized bulk lattice parameters, are a = 3.7365 A, ˚ to be compared to the experimental results of 3.872 c = 9.9811 A, ˚ respectively [58]. The Monkhorst–Pack scheme for and 9.616 A, 8 × 8 × 8 k-point mesh in the Brillouin zone was applied for anatase TiO2 crystal. In geometry optimizations of two-dimensionally periodic slab, the lattice constants were fixed at these values while the positions of all Ti and O atoms were allowed to relax. The loading of Pd, Pt and Au metals 50% on the anatase TiO2 (0 0 1) was modeled by binding the Pd, Pt and Au atoms on the surface, which is modeled as [1 × 1] slab with two Ti layers. The [1 × 1] slab with two Ti layers, which was modeled in our previous work [34] is shown in Fig. 1. The adsorption of CO on the TiO2 surface-supported metals is therefore the monolayer adsorption with surface coverage of 0.5 ML.
Ebinding = EM/TiO2 − (EM + ETiO2 )
(1)
Eads = ECO/[M/TiO2 ] − (ECO + EM/TiO2 )
(2)
where EM/TiO2 is the total energy of M-metal adsorbed on the TiO2 surface. EM and ETiO2 are total energies of M-metal and the TiO2 surface, respectively. ECO/[M/TiO2 ] and ECO are total energies of the CO adsorbed on the TiO2 -supported M-metal and the isolated CO molecule, respectively. 3. Results and discussion Binding structures of Pd, Pt or Au atom onto the anatase TiO2 (0 0 1) surface of which the surfaces are respectively noted as Pd/TiO2 , Pt/TiO2 and Au/TiO2 are shown in Fig. 2. It shows that the metal atom binds with O2C and Ti5C of the TiO2 surface. The bond ˚ Pt/TiO2 lengths of the Pd/TiO2 (Pd–O = 2.227 and Pd–Ti = 2.657 A), ˚ and Au/TiO2 (Au–O = 2.456 and (Pt–O = 2.057 and Pt–Ti = 2.616 A) ˚ were obtained. It was found that bond lengths Au–Ti = 2.515 A) of the anatase TiO2 (0 0 1) surface-supported metals for binding with O2C atoms are in order: Au/TiO2 > Pd/TiO2 > Pt/TiO2 . On the other hand, the bond lengths of the anatase TiO2
˚ Fig. 2. The structures of anatase TiO2 (0 0 1)-supported (a) Pd, (b) Pt and (c) Au. Bond distances are in A.
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˚ Fig. 3. The adsorption structures of CO on the (a) Pd/TiO2 , (b) Pt/TiO2 and (c) Au/TiO2 surfaces with the monolayer surface coverage ( ML ) of 0.5 ML. Bond distances are in A.
(0 0 1) surface-supported metals for binding with Ti5C atoms are in order: Pd/TiO2 > Pt/TiO2 > Au/TiO2 . The binding energies of Au, Pt and Pd atoms with oxygen atom of the anatase TiO2 (0 0 1) surface are listed in Table 1. The relative binding abilities of the Au, Pt and Pd metal atoms on the anatase TiO2 (0 0 1) surface are in order: Pt (Ebinding = −61.19 kcal/mol) Au (Ebinding = −34.91 kcal/mol) ≈ Pd (Ebinding = −32.06 kcal/mol). The optimized structures of adsorption configuration of CO molecule on the Au/TiO2 , Pt/TiO2 and Pd/TiO2 surfaces are ˚ shown in Fig. 3 and their bond distances of CCO · · ·Pd = 2.344 A, CCO · · ·Pt = 2.041 A˚ and CCO · · ·Au = 2.176 A˚ were obtained, respectively. The relative adsorption abilities for CO adsorbed on the TiO2 (0 0 1)-supported Pd, Pt and Au atoms with the surface coverage of 0.5 ML are in order: Pt (Eads = −15.98 kcal/mol) > Au (Eads = −12.97 kcal/mol) Pd (Eads = −3.53 kcal/mol). The CO adsorption on the TiO2 (0 0 1)-supported Pd as compared with the adsorption energy of CO on clean TiO2 (0 0 1) surface (Eads = −9.73 kcal/mol, taken from our previous work [34]), it can be concluded that CO adsorption on the TiO2 (0 0 1)-supported Pd is weaker than on clean TiO2 (0 0 1) surface. The adsorption for CO adsorbed on the TiO2 (0 0 1)-supported Pt and Au atoms are slightly stronger than that on clean TiO2 (0 0 1) surface; their adsorption energies on the TiO2 (0 0 1)-supported Pt and Au atoms are stronger than that on clean TiO2 (0 0 1) surface by 6.25 and 3.24 kcal/mol, respectively. The adsorption energy of CO on the Au/TiO2 (0 0 1) surface (Eads = −12.97 kcal/mol) is in a good agreement with the experimental CO adsorption energies on Au/TiO2 ; adsorption strength obtained by IRAS method in terms of heat of adsorption (Hads = −18.3 kcal/mol) for CO adsorbed on 0.25 ML Au/TiO2 which the Au-nanocluster size is approximately 3 nm in diameter [44].
4. Conclusions Binding of single Pd, Pt and Au atoms on anatase TiO2 (0 0 1) surface with loading of 50% on the surface and adsorption of CO on anatase TiO2 (0 0 1)-supported Pd, Pt and Au atoms with surface coverage of 0.5 ML were investigated using DFT computations. It was found that bond lengths for the metals binding with O2C atoms of the anatase TiO2 (0 0 1) surface are in order: Au/TiO2 > Pd/TiO2 > Pt/TiO2 but for the metals binding with Ti5C atoms of the anatase TiO2 (0 0 1) surface are in order: Pd/TiO2 > Pt/TiO2 > Au/TiO2 . The relative binding abilities of the Au, Pt and Pd metal atoms on the anatase TiO2 (0 0 1) surface are in order: Pt Au ≈ Pd. The relative adsorption abilities for CO adsorbed on the TiO2 (0 0 1)-supported Pd, Pt and Au atoms with the surface coverage of 0.5 ML are in order: Pt > Au Pd.
Acknowledgements The authors would like to acknowledge the financial support from the Research, Development and Engineering (RD&E) Fund through The National Nanotechnology Center (NANOTEC), The National Science and Technology Development Agency (NSTDA), Thailand (Project No. P-11-00409) to Chulalongkorn University. The Royal Golden Jubilee (RGJ) grant, number PHD/0244/2549 supported by TRF and the postdoctoral fellowship (Ratchadaphisek Somphot Endowment Fund) provided through the Graduate School, Chulalongkorn University, Thailand, granted to Dr. Raina Wanbayor are gratefully acknowledged. References [1] Z.B. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, J. Phys. Chem. B 102 (52) (1998) 10871. [2] M. Grätzel, Nature 414 (6861) (2001) 338. [3] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515. [4] B. Oregan, M. Grätzel, Nature 353 (6346) (1991) 737. [5] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Gratzel, Nature 395 (6702) (1998) 583. [6] J.M. Macak, H. Tsuchiya, A. Ghicov, P. Schmuki, Electrochem. Commun. 7 (11) (2005) 1133. [7] K. Shankar, J. Bandara, M. Paulose, H. Wietasch, O.K. Varghese, G.K. Mor, T.J. LaTempa, M. Thelakkat, C.A. Grimes, Nano Lett. 8 (6) (2008) 1654. [8] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Solar Energy Mater. Solar Cells 90 (14) (2006) 2011. [9] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2) (2006) 215. [10] J.H. Tang, K. Prasad, R. Sanjines, F. Levy, Sens. Actuators B: Chem. 26 (1995) 71. [11] V. Guidi, M.C. Carotta, M. Ferroni, G. Martinelli, L. Paglialonga, E. Comini, G. Sberveglieri, Sens. Actuators B: Chem. 57 (1–3) (1999) 197. [12] G.C. Mather, F.M.B. Marques, J.R. Frade, J. Eur. Ceram. Soc. 19 (6–7) (1999) 887. [13] M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, K.G. Ong, Nanotechnology 17 (2) (2006) 398. [14] O.K. Varghese, D.W. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes, Adv. Mater. 15 (7–8) (2003) 624. [15] O.K. Varghese, G.K. Mor, C.A. Grimes, M. Paulose, N. Mukherjee, J. Nanosci. Nanotechnol. 4 (7) (2004) 733. [16] S. Yoriya, H.E. Prakasam, O.K. Varghese, K. Shankar, M. Paulose, G.K. Mor, T.J. Latempa, C.A. Grimes, Sens. Lett. 4 (3) (2006) 334. [17] G.K. Mor, M.A. Carvalho, O.K. Varghese, M.V. Pishko, C.A. Grimes, J. Mater. Res. 19 (2n) (2004) 628. [18] G.K. Mor, O.K. Varghese, M. Paulose, C.A. Grimes, Sens. Lett. 1 (1) (2003) 42. [19] G.K. Mor, O.K. Varghese, M. Paulose, K.G. Ong, C.A. Grimes, Thin Solid Films 496 (2006) 42. [20] R.L. Penn, J.F. Banfield, Am. Miner. 84 (1999) 871. [21] S. Bakardjieva, V. Stengl, L. Szatmary, J. Subrt, J. Lukac, N. Murafa, D. Niznansky, K. Cizek, J. Jirkovsky, N. Petrova, J. Mater. Chem. 16 (2006) 1709. [22] D. Yang, H. Liu, Z. Zheng, Y. Yuan, J.-c. Zhao, E.R. Waclawik, X. Ke, H. Zhu, J. Am. Chem. Soc. 131 (2009) 17885. [23] H. Ariga, T. Taniike, H. Morikawa, R. Tero, H. Kondoh, Y. Iwasawa, Chem. Phys. Lett. 454 (2008) 350. [24] T.L. Hsiung, H.P. Wang, H.P. Lin, J. Phys. Chem. Solids 69 (2008) 383. [25] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [26] M. Valden, S. Pak, X. Lai, D.W. Goodman, Catal. Lett. 56 (1998) 7. [27] A. Kolmakov, D.W. Goodman, Surf. Sci. 490 (2001) L597.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A periodic DFT study on binding of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface and adsorption of CO on the TiO2 surface-supported Pd, Pt and Au Raina Wanbayor, Vithaya Ruangpornvisuti ∗ Department of Chemistry, Faculty of Science and Center for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 26 November 2011 Keywords: Binding energy Adsorption energy TiO2 supported Pd, Pt and Au Anatase TiO2 (0 0 1) CO adsorption Periodic DFT
a b s t r a c t Binding of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface and adsorption of CO on the TiO2 (0 0 1)supported Pd, Pt and Au were studied by means of periodic density functional theory calculations. The relative binding abilities of Pd, Pt and Au on the anatase TiO2 (0 0 1) surface are in order: Pt Au ≈ Pd. The relative adsorption abilities of CO on the TiO2 (0 0 1)-supported Pd, Pt and Au for the surface coverage equal to 0.5 ML are in order: Pt > Au > Pd of which adsorption energies are −15.98, −12.97 and −3.53 kcal/mol, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been widely used as applications in photocatalysis [1–3], dye-sensitized solar cells [4–9] and gassensors [10–19]. In general, TiO2 exists in four crystalline forms, namely (i) anatase, (ii) rutile, (iii) brookite and (iv) TiO2 (B) structures [20–22] but adsorption abilities of their surfaces seem to be very different depending on their surface characteristics. Nevertheless, surface structures and characteristics of various types of TiO2 have been extensively studied [23,24]. TiO2 -supported transition-metal catalysts have been widely investigated in order to improve their good catalytic performance in CO oxidization [25–30]. There are many research works on investigating CO interaction and adsorption on clean TiO2 [30–35] and on TiO2 -supported metal surfaces [36–38]. Adsorptions of Pt, Pd and Au on TiO2 were theoretically studied and found that their binding energies are −47.04 [39], −50.27 [40] and −22.14 kcal/mol [41], respectively. The CO adsorbed on the Pd catalysts loaded on the TiO2 was studied [42]. It was found that CO adsorbed on Pt–Ti site of Pt/TiO2 is a physical adsorption with bridging configuration [38]. Nevertheless, adsorption energy of CO on the Pd/TiO2 and Pt/TiO2 has hardly ever been reported. The CO adsorptions on Au/TiO2 have been studied using IR spectroscopic
∗ Corresponding author. Tel.: +66 2 218 7644; fax: +66 2 254 1309. E-mail address: [email protected] (V. Ruangpornvisuti). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.085
measurement [43] and infrared reflection absorption spectroscopy (IRAS) method [44,45]. It has also been reported that oxidation of CO can occur on TiO2 (1 1 0)-supported Au nanoclusters at very low temperatures [46,47]. In this work, we have therefore studied binding abilities of single Pd, Pt and Au atoms on anatase TiO2 (0 0 1) surface with loading of 50% on the surface and adsorption of CO on anatase TiO2 (0 0 1)supported Pd, Pt and Au atoms with surface coverage of 0.5 ML using the periodic density functional theory (DFT) computations. Adsorptions of CO on TiO2 (0 0 1)-supported Pd, Pt and Au surfaces have been compared with their experiments and the results would be useful to describe and predict activities of TiO2 (0 0 1)-supported Pd, Pt and Au surfaces for adsorption and reaction with CO. 2. Computational details All DFT calculations of two-dimensionally periodic slab model have been carried out using the CRYSTAL06 computational code [48], based on the expansion of the crystalline orbitals as a linear combination of a basis set consisting of atom centered Gaussian orbitals. The Kohn–Sham orbitals as Gaussian-type-orbital basis sets of double zeta quality as an 86-51G(3d) and an 8-411G contraction scheme have been respectively employed for the titanium [49] and oxygen [50] atoms on TiO2 (0 0 1) surface. Basis sets for carbon, oxygen, palladium, platinum and gold atoms employed in these calculations are a 631d1G [51], an 8411dG [52], a fitting effective core potential (ECP) of Kokalj et al. [53], Andrae et al. [54] and Hay
R. Wanbayor, V. Ruangpornvisuti / Applied Surface Science 258 (2012) 3298–3301
3299
Table 1 Binding energies of metals onto the anatase TiO2 (0 0 1) surface and adsorption energies of CO on the TiO2 (0 0 1)-supported metals with surface coverage of 0.5 ML and their selected geometrical parameters. Binding systems
Ebinding a
Geometrical paremeters
Adsorption system
Pd/TiO2
−32.06 −50.27b
˚ O–Pd = 2.227 A, Ti–Pd = 2.657 A˚
CO/[Pd/TiO2 ]
−3.53
Pt/TiO2
−61.19 −47.04c
˚ O–Pt = 2.057 A, Ti–Pt = 2.616 A˚
CO/[Pt/TiO2 ]
−15.98
CCO · · ·Pt = 2.041 A˚ Pt–C–O = 177.8◦ O2C –Pt–C–O = –178.5◦
Au/TiO2
−34.91 −22.14d
˚ O–Au = 2.456 A, Ti–Au = 2.515 A˚
CO/[Au/TiO2 ]
−12.97 −12.5e −10.9f
CCO · · ·Au = 2.176 A˚ Au–C–O = 164.5◦ O2C –Au–C–O = −150.0◦
a b c d e f
Eads a
Geometrical paremeters CCO · · ·Pd = 2.344 A˚ Pd–C–O = 143.3◦ O2C –Pd–C–O = −9.7◦
In kcal/mol. Periodic supercell model and DFT method, taken from Ref. [40]. Adsorbed on the anatase TiO2 (1 0 1) surface as a configuration of Pt bridging with O and Ti atoms, taken from Ref. [39]. Cluster model with O2c bridge, taken from Ref. [41]. Heat of CO adsorption on Au/TiO2 at 450 K, using infrared reflection absorption spectroscopy (IRAS) method, taken from Ref. [44]. Heat of CO adsorption on Au/TiO2 , measured for bulk Au/TiO2 (1 1 0)–[1 × 2] using the Clausius–Clapeyron method applied to isosteres of IRAS data, taken from Ref. [45].
The tolerances for geometry optimization convergence have been set to the default values [48] and the coulomb-exchange screening tolerances were set to (7, 7, 7, 7, 14). All slab calculations have been performed with a Monkhorst–Pack [59] k-point grid with shrinking factors (4, 4). There are two binding sites for the anatase TiO2 (0 0 1) surface: two-fold-coordinate O atom (O2C ) and five-fold-coordinate Ti atom (Ti5C ), see Fig. 1 and the Pd, Pt or Au atom binds with O2C and Ti5C as shown in Fig. 2. Binding energy (Ebinding ) for M-metal adsorption on the anatase TiO2 surface and adsorption energy (Eads ) for CO adsorption on the anatase TiO2 -supported M-metal are computed using Eqs. (1) and (2), respectively.
Fig. 1. The periodic slab model used for TiO2 (0 0 1) surface.
and Wadt [55], respectively. The hybrid functional, B3LYP including Becke’s three-parameter exchange [56] and Lee–Yang–Parr correlation [57], has been employed. ˚ and The optimized bulk lattice parameters, are a = 3.7365 A, ˚ to be compared to the experimental results of 3.872 c = 9.9811 A, ˚ respectively [58]. The Monkhorst–Pack scheme for and 9.616 A, 8 × 8 × 8 k-point mesh in the Brillouin zone was applied for anatase TiO2 crystal. In geometry optimizations of two-dimensionally periodic slab, the lattice constants were fixed at these values while the positions of all Ti and O atoms were allowed to relax. The loading of Pd, Pt and Au metals 50% on the anatase TiO2 (0 0 1) was modeled by binding the Pd, Pt and Au atoms on the surface, which is modeled as [1 × 1] slab with two Ti layers. The [1 × 1] slab with two Ti layers, which was modeled in our previous work [34] is shown in Fig. 1. The adsorption of CO on the TiO2 surface-supported metals is therefore the monolayer adsorption with surface coverage of 0.5 ML.
Ebinding = EM/TiO2 − (EM + ETiO2 )
(1)
Eads = ECO/[M/TiO2 ] − (ECO + EM/TiO2 )
(2)
where EM/TiO2 is the total energy of M-metal adsorbed on the TiO2 surface. EM and ETiO2 are total energies of M-metal and the TiO2 surface, respectively. ECO/[M/TiO2 ] and ECO are total energies of the CO adsorbed on the TiO2 -supported M-metal and the isolated CO molecule, respectively. 3. Results and discussion Binding structures of Pd, Pt or Au atom onto the anatase TiO2 (0 0 1) surface of which the surfaces are respectively noted as Pd/TiO2 , Pt/TiO2 and Au/TiO2 are shown in Fig. 2. It shows that the metal atom binds with O2C and Ti5C of the TiO2 surface. The bond ˚ Pt/TiO2 lengths of the Pd/TiO2 (Pd–O = 2.227 and Pd–Ti = 2.657 A), ˚ and Au/TiO2 (Au–O = 2.456 and (Pt–O = 2.057 and Pt–Ti = 2.616 A) ˚ were obtained. It was found that bond lengths Au–Ti = 2.515 A) of the anatase TiO2 (0 0 1) surface-supported metals for binding with O2C atoms are in order: Au/TiO2 > Pd/TiO2 > Pt/TiO2 . On the other hand, the bond lengths of the anatase TiO2
˚ Fig. 2. The structures of anatase TiO2 (0 0 1)-supported (a) Pd, (b) Pt and (c) Au. Bond distances are in A.
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˚ Fig. 3. The adsorption structures of CO on the (a) Pd/TiO2 , (b) Pt/TiO2 and (c) Au/TiO2 surfaces with the monolayer surface coverage ( ML ) of 0.5 ML. Bond distances are in A.
(0 0 1) surface-supported metals for binding with Ti5C atoms are in order: Pd/TiO2 > Pt/TiO2 > Au/TiO2 . The binding energies of Au, Pt and Pd atoms with oxygen atom of the anatase TiO2 (0 0 1) surface are listed in Table 1. The relative binding abilities of the Au, Pt and Pd metal atoms on the anatase TiO2 (0 0 1) surface are in order: Pt (Ebinding = −61.19 kcal/mol) Au (Ebinding = −34.91 kcal/mol) ≈ Pd (Ebinding = −32.06 kcal/mol). The optimized structures of adsorption configuration of CO molecule on the Au/TiO2 , Pt/TiO2 and Pd/TiO2 surfaces are ˚ shown in Fig. 3 and their bond distances of CCO · · ·Pd = 2.344 A, CCO · · ·Pt = 2.041 A˚ and CCO · · ·Au = 2.176 A˚ were obtained, respectively. The relative adsorption abilities for CO adsorbed on the TiO2 (0 0 1)-supported Pd, Pt and Au atoms with the surface coverage of 0.5 ML are in order: Pt (Eads = −15.98 kcal/mol) > Au (Eads = −12.97 kcal/mol) Pd (Eads = −3.53 kcal/mol). The CO adsorption on the TiO2 (0 0 1)-supported Pd as compared with the adsorption energy of CO on clean TiO2 (0 0 1) surface (Eads = −9.73 kcal/mol, taken from our previous work [34]), it can be concluded that CO adsorption on the TiO2 (0 0 1)-supported Pd is weaker than on clean TiO2 (0 0 1) surface. The adsorption for CO adsorbed on the TiO2 (0 0 1)-supported Pt and Au atoms are slightly stronger than that on clean TiO2 (0 0 1) surface; their adsorption energies on the TiO2 (0 0 1)-supported Pt and Au atoms are stronger than that on clean TiO2 (0 0 1) surface by 6.25 and 3.24 kcal/mol, respectively. The adsorption energy of CO on the Au/TiO2 (0 0 1) surface (Eads = −12.97 kcal/mol) is in a good agreement with the experimental CO adsorption energies on Au/TiO2 ; adsorption strength obtained by IRAS method in terms of heat of adsorption (Hads = −18.3 kcal/mol) for CO adsorbed on 0.25 ML Au/TiO2 which the Au-nanocluster size is approximately 3 nm in diameter [44].
4. Conclusions Binding of single Pd, Pt and Au atoms on anatase TiO2 (0 0 1) surface with loading of 50% on the surface and adsorption of CO on anatase TiO2 (0 0 1)-supported Pd, Pt and Au atoms with surface coverage of 0.5 ML were investigated using DFT computations. It was found that bond lengths for the metals binding with O2C atoms of the anatase TiO2 (0 0 1) surface are in order: Au/TiO2 > Pd/TiO2 > Pt/TiO2 but for the metals binding with Ti5C atoms of the anatase TiO2 (0 0 1) surface are in order: Pd/TiO2 > Pt/TiO2 > Au/TiO2 . The relative binding abilities of the Au, Pt and Pd metal atoms on the anatase TiO2 (0 0 1) surface are in order: Pt Au ≈ Pd. The relative adsorption abilities for CO adsorbed on the TiO2 (0 0 1)-supported Pd, Pt and Au atoms with the surface coverage of 0.5 ML are in order: Pt > Au Pd.
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