CO Oxidation by Lattice Oxygen on V2O5 Nanotubes - The Journal of

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CO Oxidation by Lattice Oxygen on V2O5 Nanotubes Guo-min Zhu, Zhi-bei Qu, Gui-lin Zhuang, Qin Xie, Qiang-qiang Meng, and Jian-guo Wang* College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, China ABSTRACT:

Density-functional theory calculations were performed to investigate CO adsorption and oxidation on V2O5 nanotubes formed by rolling V2O5 (010) layers. CO adsorbs on both the inside and outside walls of V2O5 nanotubes by physisorption and chemisorption modes. The bidentate carbonate can form from CO chemisorption and decompose to CO2 on V2O5 nanotubes. CO oxidation into CO2 by the lattice oxygen, i.e., decomposition of carbonate, on the inside wall of V2O5 nanotubes requires less (0.21 vs 0.40 and 0.43 eV) and releases more energy (
0.17 vs
0.02 and
0.03 eV) than that on the V2O5 (010) single layer and the outside wall of V2O5 nanotubes. Our study shows that lattice oxygen of V2O5 participates in CO oxidation, and the confined environment is beneficial to CO2 formation.

1. INTRODUCTION Vanadium pentoxide (V2O5) has attracted special attention due to its layered structure and excellent physicochemical properties, which are of interest for a lot of applications (such as optical switches,1 chemical sensors,2 catalysts,3,4 solid-state batteries,5 ceramics6 and solar cells7). V2O5 is a kind of layered structure in which VO5 square pyramids are connected by sharing corners and edges. The interactions between these layers are rather weak, where the V
O distance is 2.79 Å. As with other layered structures (graphite, MoS2, BN, BC3),8
10 novel quasi-one-dimensional (1D) V2O5 nanoarchitectures, for example, V2O5 nanotubes (NTs), have been prepared. The synthesis methods of V2O5 NTs include hydrothermal,11 carbon NT template synthesis,12 template-based electrodeposition13 and oxidation of V4+ species.14 There is also a lot of experimental study on the electronic,15 magnetic,16 and optical properties of nanostructured vanadium oxide.17
20 The nanostructured vanadium oxide shows significantly enhanced capability for redox reactions,21,22 chemical sensors,23 and being positive electrodes in secondary Li batteries.24
26 In addition, it could be potentially used in novel nanoactuators27 and nonlinear optical limiters.28 Among many applications, catalytic oxidation of hydrocarbons, sulfur dioxide, nitric oxide, and CO have been widely studied.29
32 Nanostructured vanadium oxide shows catalytic oxidation properties r 2011 American Chemical Society

superior to those of the bulk due to the existence of more exposed lattice oxygen and low coordinated vanadium.33 Compared with lots of experimental research on nanostructured V2O5, only a few theoretical studies have been conducted. Ivanovskaya and Enyashin investigated the electronic properties of single-walled V2O5 NTs, scroll-like V2O5-based NTs, and single-walled VO2 NTs via the tight-binding band method.34,35 Petkov used a nontraditional approach (pair distribution function technique) to determine a real-size model for the V2O5 NTs.36 To the best of our knowledge, no theoretical studies have been conducted on the catalytic properties of CO oxidation on V2O5 NTs. In this study, we systematically investigate the geometric, electronic and catalytic CO oxidation properties of V2O5 NTs by means of density functional theory (DFT) calculations. Different diameters of armchair V2O5 NTs have been optimized, and the stability of these NTs is evaluated by strain energy of the NT and surface energy of the V2O5 (010) single layer. Especially, CO adsorption and oxidation with lattice oxygen on both the inside and outside walls of V2O5 NTs with a diameter of 12 Å

Received: March 20, 2011 Revised: June 26, 2011 Published: June 28, 2011 14806

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Figure 1. Optimized geometries of V2O5 NTs with different diameters. (a) Top and side view of bulk V2O5. (b
g) Cross section of armchair (n, n) (n = 3
8) V2O5 NTs.

have been investigated in detail and compared with those on the V2O5 (010) single layer.

2. METHOD All DFT calculations were performed with the DMol3 module37 in Materials Studio. The generalized gradient approximation (GGA) with Perdew, Burke and Emzerhof (PBE)38 function was used to describe the exchange-correlation (XC) effects. The double numerical basis set with polarization functions (DNP) and a real space cutoff of 5.0 Å was used. The sizes of the DNP basis sets are comparable with the 6-31G** basis set and believed to be much more accurate than Gaussian basis sets of the same size.39,40 A series of armchair ((n, n), n = 3
8) V2O5 NTs were built and optimized. For CO adsorption and oxidation, a single layer (1  3) V2O5 (010) and an armchair V2O5 (5, 5) NT supercell were applied. The lengths of the a, b, and c lattices of V2O5 (5, 5) NTs are 40, 40, and 7.14 Å, in which the axial direction is 2 times that of the primitive unit cell of V2O5 NTs. The minimum distance between opposing sidewalls of neighboring V2O5 NTs is about 18 Å, which can render the interactions among repeating slabs negligible. The Monkhorst-Pack41 k-point sampling of the bulk V2O5, V2O5 (010) surface, and V2O5 NT is 4  4  4, 4  4  1, and 1  1  4, respectively. Transition states of the reactions were searched by constraining the distance between CO and lattice oxygen of V2O5 surface or NTs and relaxing all the other degrees of freedom.42
44 For all of the calculations, the convergence in energy and force was set to 10
5 Ha and 2  10
3 Ha/Å. The absorption energy of CO was used to characterize the strength of bonding of CO and defined as Ead = E(CO/V2O5)
E(CO)
E(V2O5), where E(CO/V2O5), E(CO), and E(V2O5) are the total energies of the combined system, the gaseous CO, and the clean V2O5 system. 3. RESULTS AND DISCUSSION 3.1. The Geometric Properties of V2O5 NTs. The orthorhombic crystal V2O5 (space group Pmmn) is built up by stacking

two-dimensional (2D)-like layers along (010) and is composed of distorted VO5 pyramids. The calculated lattice parameters (a = 11.52, b = 4.74, and c = 3.57 Å) of bulk V2O5 are in good agreement with the experimental values (a = 11.51, b = 4.37, and c = 3.56 Å) and other theoretical calculated values (a = 11.65, b = 4.69, and c = 3.57 Å).45 There are three structurally nonequivalent types of oxygen centers in the crystal V2O5: single coordinated vanadyl O(1), double coordinated O(2), and bridging oxygen O(3) triply coordinated to vanadium atoms (Figure1a). Similar to other layered structures, the V2O5 (010) single layer can be rolled into a cylinder forming sandwich-like NTs. After the formation of V2O5 NTs, there are two kinds of O(1) (Oin and Oout), where Oin and Oout represent the single coordinated vanadyl O(1) on the inside and outside wall of the tube, respectively. Of course, three different types of V2O5 NTs (chiral, armchair and zigzag) can be obtained by the rolling of a single V2O5 (010) layer. In this study, we focus on the armchair NTs and investigate the activity of lattice oxygen inside and outside V2O5 NTs. Figure 1 shows the optimized structures of bulk V2O5 and armchair (n, n) (n = 3
8) V2O5 NTs. Similar to the experimentally observed ZnO NTs,46,47 the cross sections of (n, n) V2O5 NTs have polygonal rather than standard circular shape: they are triangle, quadrangle, pentagon, hexagon, heptagon, and octagon from the side view of V2O5 (n, n) (n = 3
8) NTs. Recently, “fullerene-like” V2O5 nanoparticles have been observed with clear polygon geometrical shapes.48 The distances between two single coordinated oxygens inside the tube (Oin) in the radial direction of these structures are about 3.2 Å, which are nearly the same with those of bulk V2O5. On the contrary, the distances between two Oout oxygens in the radial direction are dependent on the diameters of these NTs. The smallest distance between two Oout oxygens is 4.6 Å in a V2O5 (8, 8) nanotube. The optimized V
Oin bond lengths within the inner shell are nearly the same as that on the outer shell, which is about 0.01 Å smaller than the bulk V
O(1). The V
O(3) bond length of the nanotube is increased by 0.01 Å compared with that of the planar 14807

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The Journal of Physical Chemistry C sheet. The V
O(2) bond lengths of the nanotube and the planar sheet are nearly the same. The diameters of these V2O5 NTs range from 6 to 24 Å. The calculated strain energies (the energy difference of per V2O5 subunit between a nanotube and the corresponding single V2O5 (010) layer characterizes the chemical stability of tubular structures) nearly follow an A/D2 behavior as in carbon nanotube, as shown in Figure 2, suggesting that the V2O5 NTs follow a classical elasticity law49 (A = 8.4 eVÅ2/unit). The strain energy of V2O5 NTs is smaller than some other transition metal oxide NTs, for example, TiO2 NTs, but larger than that of carbon NTs with similar diameters.50 The strain energy of V2O5 NTs of 24 Å diameter reduces to the same value of a carbon nanotube of 15 Å diameter.51 The calculated surface energy of V2O5 (010) is 0.06 J/m2, which is much smaller than several other D-metal oxides, such as TiO2 (110) (0.89 J/m2)52 and SnO2 (110) (1.04 J/m2).53

Figure 2. Calculated strain energy of V2O5 NTs (energy per V2O5 unit relative to the single V2O5 (010) layer) as a function of tube diameters (D) in Å.

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3.2. CO Adsorption. Two kinds of CO adsorption modes are identified in this study. CO weakly interacts with the V2O5 (010) single layer and V2O5 (5, 5) NTs in physisorption, as shown in Figure 3a
c. The calculated adsorption energy of CO physisorption is
0.16 eV on the V2O5 (010) single layer, and
0.10 and
0.14 eV on the inside and outside walls of V2O5 (5, 5) NTs. The C
O bond length (1.140 Å) is nearly the same as the calculated value of gaseous CO (1.143 Å). The nearest distance between CO and V2O5 is more than 3.0 Å for all of investigated systems. CO can strongly chemisorb on both the inside and outside walls of V2O5 NTs (Figure 3e,f), where the adsorption energy is
0.84 and
1.16 eV, respectively. CO chemisorbs on the bridge of two Oin in the axial and radial direction or two Oout only in the axial direction for V2O5 NTs. Similarly, CO can strongly chemisorb on the V2O5 (010) single layer (Figure 3d) where the adsorption energy is
0.90 eV. Therefore, the strong CO chemisorption leads to the formation of O-bridging carbonate, which is found among a lot of metal oxides.54
56 It confirms the structure of bidentate CO3 species on V2O5 proposed by Hirota by using heavy oxygen O18 as the tracer to investigate the oxidation of CO on a V2O5 catalyst.57 The strong adsorption induces significant structural changes. The three O
C
O angles are slightly distorted from the ideal 120° of gaseous carbonate to 123° for O
C
Oin and 114° for Oin
C
Oin, respectively. The C
O bond (1.20 Å) is only slightly elongated from that of free CO, but the C
Oin bond (1.36 Å) is relatively lengthy. The length of V
Oin bonding with CO is much longer (1.81 Å) than the V
Oin (1.59 Å) without bonding with CO. The distance between two Oin atom changes from 3.45 Å to 2.29 Å after CO chemisorption on the inside wall of V2O5 (5, 5) NTs. The carbonate configuration on the outside wall and the V2O5 (010) single layer are quite similar with that on the inside one. The density of states (DOS) of the physisorbed CO is very similar to that of free CO and no overlap appears between CO and V2O5, while the DOS of the chemisorbed CO shows several dispersed peaks between
9.0 eV and
2.0 eV, which implies that some states of CO delocalize to lower energy level bands

Figure 3. Optimized structures of CO physisorbed (a,b,c) and chemisorbed (d,e,f) on the inside (c,f) and the outside (b,e) wall of V2O5 (5, 5) NTs and the surface (a,d). Bond lengths are shown in units of Angstroms. 14808

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Figure 4. The DOS for (a,c) CO and (b,d) V2O5 (5, 5) NTs by chemisorption (a,b) and physisorption (c,d) modes. The PDOS of different species (e
h) for CO chemisorption: (e) carbon and oxygen in CO, (f) carbon and Oin (bonded with CO), (g) vanadium (not bonded with CO) and Oin (not bonded with CO), and (h) vanadium (bonded with CO) and Oin (bonded with CO).

Figure 5. Schematic energy diagrams for CO oxidation by lattice oxygen on the (a) surface, (b) outside wall, and (c) inside wall of V2O5 (5, 5) NTs. The reference system is CO + V2O5 (5, 5) NTs for the inside and outside nanotube. The reference system is CO + V2O5 surface for the calculation of the surface. The optimized structures of the transition state of CO2 formation by CO reaction with the lattice oxygen are (d) on the surface, (e) on the outside wall, and (f) on the inside wall of V2O5 (5, 5) NTs. The carbon atom, lattice oxygen, oxygen in carbon monoxide, and vanadium atoms are represented by gray, red, pink, and white balls, respectively. Bond lengths are shown in units of angstroms. 14809

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The Journal of Physical Chemistry C due to the formation of new chemical bonds between CO and V2O5, as shown in Figure 4a,c. After CO chemisorption, the DOS peaks of V2O5 in the vicinity of the Fermi level down shift about 1.8 eV and thereby induce the valence states of the Oin (bonded) anchor at the Fermi level (Figure 4b,d). The partial density of states (PDOS) analysis for carbonate, i.e., CO chemisorption, is shown in Figure 4e
h. The states of lattice oxygen Oin (bonded) below the Fermi level are well integrated with that of carbon, coupled with the similarity of PDOS between oxygen in CO and Oin bonded with CO, which can identify the formation of a covalent bond between C and Oin (Figure 4e,f). However, the bond is not stronger than that of C
O, owing to the existence of weaker overlapped peaks between C and Oin at
2 eV below the Fermi level. On the other hand, it is observed that three mixed PDOS peaks exist at the Fermi level for C, Oin, and V, respectively, as shown in Figure 4f,h, which suggests that the delocalized chemical bond may appear in the three atoms and the valence electrons of CO may transfer to the V atom via the Oin atom. Generally, chemisorption of CO can effectively influence the nature of the V
Oin bond and thereby pave a way for the formation of CO3. The Hirshfeld charge population analysis for CO, carbonate (including CO and two bonded single coordinated oxygen Oin), and bonded vanadium are also analyzed. For CO physisorption, it is observed that the adsorbed CO is charge neutral, which indicates no charge transfers between CO and V2O5 NTs. However, after CO chemisorption, the charge of CO3 inside and outside the tube is totally
0.21 and
0.18 e, respectively. The charge of Oin (bonded with CO) and Oin (not bonded with CO) is
0.14 and
0.17 e; obviously, the charge transfer occurs between the V and Oin. 3.3. CO Oxidation. We further investigate CO oxidation by the lattice oxygen of a V2O5 (010) single layer and V2O5 (5, 5) NTs, i.e., the decomposition of carbonate, as shown in Figure 5. It can be seen that the reaction energy (
0.02 vs
0.03 eV) and reaction barrier (0.40 vs 0.43 eV) on the V2O5 (010) single layer are nearly the same as that on the outside wall of V2O5 (5, 5) NTs. However, on the inside wall of V2O5 (5, 5) NTs, the formation of CO2 and an oxygen vacancy from carbonate decomposition is exothermic by
0.17 eV; especially, the reaction barrier is 0.21 eV, which is half of that on the V2O5 (010) single layer and the outside wall of V2O5 (5, 5) NTs. Therefore, CO oxidation by the lattice oxygen on the V2O5 (010) single layer and the outside and inside walls of V2O5 (5, 5) NTs into CO2 release
0.92,
1.19, and
1.01 eV, respectively, compared with gaseous CO and clean V2O5 systems. The transition state in these three cases is nearly the same, which is a bent CO2 structure, as shown in Figure 5a
c. The distance of the dissociated Osurface
C and Oout
C is 1.91 (Figure 5a) and 1.96 Å (Figure 5b), which is larger than the distance of the dissociated Oin
C (1.80 Å Figure 5c). Therefore, we think that the confined environment is beneficial to CO2 formation. In our previous study, we found that O2 adsorption on the confined inside wall of ZnO NTs was much stronger than that on the outside one.58 In fact, the formation of CO2 by the decomposition of carbonate is an important reaction pathway for CO oxidation on a series of metal (for example, Au59) and metal oxide (for example, CeO2 and RuO2).55,56,60 Our study also provides a solid evidence that the lattice oxygen of V2O5 participates in the CO oxidation, which is the first theoretical study to confirm a series of experimental results that CO3 species exist and CO reacts with the lattice oxygen on V2O5 catalyst.

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4. CONCLUSIONS In summary, by utilizing DFT calculations we investigated the geometric, electronic, and catalytic CO oxidation properties of V2O5 NTs. The formation of V2O5 NTs is relatively easier than some other transition metal oxides due to the smaller strain energy of the nanotube and the smaller surface energy of the V2O5 (010) single layer. Two kinds of CO adsorption modes (physisorption and chemisorption) are identified on the V2O5 (010) single layer and the inside and outside walls of V2O5 NTs. CO2 can be obtained by the decomposition of carbonate, which is formed from CO chemisorption. The confined environment is a benefit to CO2 formation due to the smaller reaction barrier on the inside wall than that on the V2O5 (010) single layer and the outside wall of V2O5 (5, 5) NTs. Of course, many vanadium oxide-based NTs are multiwalled and have been fabricated using organic templates such as amines or hydroxides, which can be found between the walls of NTs. The presence of an organic part may certainly influence the formation, adsorption, and catalytic properties of V2O5 NTs. In the future, we will investigate the effect of the organic part on the structure and properties of V2O5 NTs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC-20906081), Zhejiang Provincial Natural Science Foundation of China (ZJNSF- R4110345), and the New Century Excellent Talents in University Program. ’ REFERENCES 00 (1) Beke, S.; Giorgio, S.; Kor€ osi, L.; Nanai, L.; Marine, W. Thin Solid Films 2008, 516, 4659. (2) Ding, N.; Liu, S. H.; Chen, C. H.; Lieberwirth, I. Appl. Phys. Lett. 2008, 93. (3) Asim, N.; Radiman, S.; Yarmo, M. A.; Golriz, M. S. B. Microporous Mesoporous Mater. 2009, 120, 397. (4) Penner, S.; Klotzer, B.; Jenewein, B. Phys. Chem. Chem. Phys. 2007, 9, 2428. (5) Huang, B. Y.; Cook, C. C.; Mui, S.; Soo, P. P.; Staelin, D. H.; Mayes, A. M.; Sadoway, D. R. J. Power Sources 2001, 97 (8), 674. (6) Zheng, C. M.; Zhang, X. M.; Qiao, Z. P.; Lei, D. M. J. Solid State Chem. 2001, 159, 181. (7) Menezes, W. G.; Reis, D. M.; Benedetti, T. M.; Oliveira, M. M.; Soares, J. F.; Torresi, R. M.; Zarbin, A. J. G. J. Colloid Interface Sci. 2009, 337, 586. (8) Saito, R.; Takeya, T.; Kimura, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B 1998, 57, 4145. (9) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (10) Liu, C. J.; Burghaus, U.; Besenbacher, F.; Wang, Z. L. ACS Nano 2010, 4, 5517. (11) Niederberger, M.; Muhr, H. J.; Krumeich, F.; Bieri, F.; G€unther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995. (12) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (13) Wang, Y.; Takahashi, K.; Shang, H. M.; Cao, G. Z. J. Phys. Chem. B 2005, 109, 3085. (14) Vera-Robles, L. I.; Campero, A. J. Phys. Chem. C 2008, 112, 19930.

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