CO Adsorption and Oxidation on N-Doped TiO2 Nanoparticles - The

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CO adsorption and oxidation on N-doped TiO2 nanoparticles Tongxiang LIANG, Juan LIU, Limin Dong, Wenli Guo, and wensheng Lai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4001972 • Publication Date (Web): 04 Jun 2013 Downloaded from http://pubs.acs.org on June 5, 2013

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The Journal of Physical Chemistry

CO adsorption and oxidation on N-doped TiO2 nanoparticles LIU Juana, DONG Limina, GUO Wenlia, LIANG Tongxianga,b *, LAI Wenshengb a

Beijing Key Lab of Fine Ceramics, Institute of Nuclear and New Energy Technology,

Tsinghua University, Beijing 100084, China b

State Key Lab of New Ceramic and Fine Processing, Tsinghua University, Beijing

100084,China

Abstract In order to oxidize CO to CO2 during the process of steam pyrolysis nuclear graphite waste, the adsorption and oxidation of a CO molecule on undoped and N-doped TiO2 nanoparticles has been studied by first principles calculations, including the adsorption energies, bond lengths, local density of states (LDOSs), and the charge density difference (CDD) are calculated. In the adsorption process, two electrons transfer from CO to the particle resulting in the reduction of the Ti sites. CO2 and carbonate form during the adsorption process on the dangling oxygen atom. The CO over the undoped and N-doped nanoparticles forms CO2 by detaching the dangling oxygen atom. Less than 0.22 eV energy gaining can drive the CO2 away to a distance greater than 3.01 Å. Therefore, the CO molecule can be oxidized to CO2 by TiO2 nanoparticle and the physically adsorbed CO2 molecule can spontaneously dissociate at room temperature, apart from the energies released in the adsorption process. The adsorption energy for undoped particle is -0.93

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eV while that for N-doped particle is -2.47 eV or -2.52 eV. N-doped nanoparticles have stronger oxidation ability. The doped nitrogen site (N site) in N-doped nanoparticles is also a chemical adsorption site. N-doped anatase (101) surface have stronger adsorption ability than the undoped surface, but weaker than the N-doped anatase nanoparticles adsorption. The result show that the N-doped anatase particles can react with CO molecule more efficiently. Keywords: First principles calculation, N-doped, TiO2 nanoparticle, CO, nuclear graphite

1. Introduction Graphite has been widely used as a moderator, reflector, and fuel matrix in various types of nuclear reactors. After decommissioning, irradiated graphite represent radioactive waste, untill now about 250000 tons irradiated graphite has to be disposed as radioactive waste

[1]

. The radioactivity of the spent graphite comes from

many nuclides, including 60Co, 3H, 14C,

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Ni, 36Cl and

137

Cs. Most radioisotopes can

be removed by the purification methods that have been applied in graphite manufacture. However, due to the same chemical behavior as 12C, this does not seem to be appliable to 14C. 14C has a half-life of 5730 years and is a biologically hazardous substance, how to dispose

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C becomes the key issue for management of nuclear

graphite during decommissioning of graphite moderated reactors. There are several possible solutions: deep geological disposal, incineration of graphite with the exhaust of CO2 gas, or recycling. Deep geological disposal after encapsulation in cementitious

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materials is relatively technologically simple. But noncompliance of some graphite properties with waste acceptance criteria might impose additional difficulties or increase the price of disposal. Incineration means all

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C and huge volume of CO2

would be emitted to the atmosphere, this would face the biggest opposition of the public. In the current waste-management strategies for carbonaceous radioactive waste, recycling of nuclear graphite is of great practical value. However, it is very difficult to separate 14C from the graphite matrix. Several techniques such as bioseparation of 14C, thermal treatment and special decontamination techniques have been developed these years, among which steam pyrolysis is an effective technology for the remove of 14C [2]

. The mechanism of steam pyrolysis (see Fig. 1) for removing 14C is based on the

fact that most of

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C is located on the graphite surface, the inner surface of pores or

grain boundary, then it becomes 14CO2 and 14CO when oxidized by water or oxygen. Micro-oxidation of nuclear graphite by water vapor mainly consists of the following processes: (1) diffusion of O2, H2O molecules to the surface of the graphite; (2) adsorption of O2, H2O molecules on the surface of the graphite; (3) oxidation of C atoms on surface layer of the graphite; (4) gaseous products of CO2, CO molecules escape from the graphite, and further react with H2O and O2. The decontaminated graphite then can be recycled to fabricate new graphite. According to the mechanical parameters, recycled graphite is quite promising for application[3].

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In order to further oxidize CO, which exists in the C oxidation process, we consider TiO2 for CO catalytic oxidation process. As a photocatalytic material, TiO2 has been extensively studied and used in environmental and energy fields because of its strong oxidizing abilities for the decomposition of organic pollutants, chemical stability, nontoxicity and low cost. The photocatalytic properties of TiO2 are derived from the formation of photogenerated charge carriers (hole and electron) which occurs upon the adsorption of ultraviolet (UV) light corresponding to the band gap. Nitrogen-doped TiO2 can be developed to enable photocatalytic reactions using the visible range of the solar spectrum[4, 5]. The mechanism of the improvement of photocatalytic activity of dopants has been investigated by first principles theory[6, 7]. Nakata et al. find that the dimensionality associated with the structure of a TiO2 material can affect its photocatalysis [8]. A sphere with zero dimensionality has a high specific surface area, resulting in a higher rate of photocatalytic decomposition of organic pollutants

[9]

. Since TiO2 nanoparticles have large surface area and they are

relatively more open, the particles can accommodate a nitrogen atom easier, the doping formation energy for particles is lower than that of the bulk material calculated by first principles theory

[6].

Different size TiO2 nanoparticles electronic properties

have been calculated by tight-binding and density functional theory (DFT) [10-13]. A 72 atom TiO2 anatase nanoparticle model[11] is selected in this work to study the adsorption and oxidation properties of CO molecules by density-functional theory (DFT) calculations, and the doping effects are compared with the undoped nanoparticle. The adsorption energy, bond length, density of state (DOS), and charge

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density difference are analyzed. Also, we want to understand whether the presence of nitrogen impurities increases the adsorption properties of the surface, we have investigated the CO molecule adsorption on TiO2 surface. We focus on the anatase (101) surface for the reason that the (101) termination is the thermodynamically most stable low-index surface of anatase [14].

2.Calculation methods 2.1. Calculation detail First principles density functional theory (DFT) calculations are carried out to study the adsorption of a CO molecule on a stoichiometric TiO2 and the N-doped TiO2 anatase nanoparticles. The system energy calculations are performed using the Vienna ab initio simulation package (VASP) code[15, 16]. The projected augmented wave (PAW) method[17,

18]

is applied in this work. The generalized gradient

approximation (GGA) parameterized by Perdew, Burke and Enzerof (PBE)[19] are used to describe the potential. The core electrons are described by ultra-soft pseudopotentials[20], while the valence electrons are represented by plane wave basis sets with a cutoff energy of 400 eV. The blocked Davidson iteration scheme and the residual minimization method with direct inversion in the iterative subspace (RMM-DIIS)[21] are applied in the electronic relaxation calculations. The energy convergence criterion is set to 10−4 eV. In the ionic relaxations, the conjugate gradient method is used to minimize the Hellmann-Feynman forces with the energy convergence criterion of 10−3 eV.

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Spin-polarization is used in all calculations. A 20×15×30Å3 box contains 72 atom undoped or N-doped TiO2 nanoparticle is set in this calculations. The shortest distance to the neighbor particles is 11.4 Å in three directions, so the larger vacuum layer is sufficient to reduce the interaction between neighbor particles. The anatase (101) surface is modeled with a periodically repeated slab. A surface supercell containing 108 atoms of dimension 1×3 is considered. The optimized bulk lattice parameters are 3.74 Å and 9.48 Å for a and c, respectively, these are correspond to the experimental data and other calculation values [14, 22]. Three TiO2 layers are used, separated by a vacuum of 10 Å width. During geometry optimizations the atoms in the bottom layer are fixed. The energy calculations for particles are performed at the Γ point, for surface calculations the k-point is set to 4 in the periodical directions. The adsorption energy is defined as the follows:

Ead = Ematrix + CO − Ematrix − ECO

(1)

Where Ematrix+CO is the total energy of the system after adsorption, Ematrix is the energy of the particle or surface slab without any adsorbed gas molecule, and ECO is the energy of a carbon monoxide (CO) molecule. All of the energies for particles or surfaces are calculated in the same box, respectively. The adsorption process is exothermic if the adsorption energy is negative.

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2.2. Structure of nanoparticles The structure of the 72 atom TiO2 nanoparticle is shown in Fig. 2(a) and 2(b). For the N-doped anatase particles, the nitrogen atom substitutes an oxygen atom in the TiO2 nanoparticle and introduces a hole in the particle. The empty state may be on the top of the valence band or inside the band gap as an impurity band[6]. N1 and N2 (see Fig. 2(b)) are the two substitutional positions which generate the impurity states inside the band gap while other substitutional positions generate empty state on the top of the valence band. The hole in the particle may oxidize the adsorbed small molecules. Two substitutional positions are chosen in the particle corresponding to the two types of doping effects. A N atom substitute the O atom at N1 position is one doping configuration and the other is a N atom substitute an O atom in the middle of the particle (see Fig. 3). The CO molecule may be adsorbed on all surface oxygen atom of the particle. After adsorption, the CO molecule forms strong chemical bonds with the particle at the dangling oxygen atom or the doped N atom adsorption site. Fig. 4 shows the absolute values of the adsorption energy on different surface oxygen atoms. The star-point is the adsorption energy for the dangling oxygen atom adsorption. CO is preferentially adsorbed on the dangling oxygen atom and doped N atom. Consequently, the adsorptions on the dangling oxygen atom and the doped nitrogen atom are mainly studied in this paper.

3. Results and discussion

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3.1. A CO molecule on a stoichiometric undoped anatase particle 3.1.1. Geometric optimization There exist only one adsorption configurations (type A) for a CO molecule on an undoped anatase particle (see Fig. 2(c)). The C atom in the CO molecule forms a C-O bond with the dangling O atom in the anatase particle. The adsorption energy is −0.93 eV by GGA (see Fig. 5). The bond length of the newly formed Ob-Cc bond is 1.19Å. The bond length of the Tia-Ob is 2.41Å and that of the Cc-Od bond is 1.17Å. Both bonds are longer than those before the adsorption, for example, they are 1.68Å and 1.14Å before the adsorption, respectively (see Table. 1). The dangling bond and the C-O bond are weakened after the adsorption, because the electrons transfer from the dangling bond of the particle and the CO molecule to the newly formed C-O bond between the CO molecule and the particle.

3.1.2. Electronic structure of the adsorption The local density of states (LDOSs), charge density difference (CDD) are calculated to further investigate the CO adsorption on anatase nanoparticles. The LDOSs for a CO molecule adsorbed on an undoped anatase particle at the dangling oxygen atom position are shown in Fig. 6. Panel (a) shows the LDOSs of the dangling oxygen atom and the nearest neighbor Ti atom before the adsorption. The LDOSs of the dangling oxygen atom and the neighbor Ti atom after adsorption are represented in panel (b). Compared to panel (a), the separation of the blue dash solid line and the red solid line means no mutual interactions between the two atoms, which indicate the

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breaking Tia-Ob bond. Panel (c) tells the LDOS of the dangling oxygen atom and the C atom in CO molecule after the adsorption. The large DOS overlap for these two atoms shows that the dangling oxygen atom of the particle and the carbon atom in the CO molecule form a chemical bond after the adsorption. Panel (b) and panel (c) represent the bond breaking and formation in the adsorption process. After the adsorption, the dangling oxygen atom is seized by the CO molecule resulting a distance of 2.41Å to the particle. The DOS (Fig. 6) and Charge Density Difference (Fig. 7(b)) analyses both indicate the broken chemical bonds. CO2 molecule is then adsorbed physically to the cationic site as the normal CO2 adsorption mode on oxides[23], with a distance of 2.41Å to the particle. 0.15eV energy difference appears as the distances increased to 3.01Å, that indicates the CO2 molecule can spontaneously dissociate at room temperature, and leaves the oxygen vacancy particle behind. The interaction would involve the oxidation of CO to CO2 and the reduction of a Ti site[24]:

CO + O 2 − → CO 2 + 2 e − Ti 4 + + 2 e − → Ti 2 +

or

2Ti 4 + + 2 e − → 2Ti 3 +

(2)

According to this redox mechanism, two electrons transfer from CO to the particle resulting in the reduction of the cationic sites. Ti4+ sites are reduced to either Ti3+ or Ti2+ by the transferred electrons. The delocalized nature of the DFT 9



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wavefunction cannot discern the reduced titanium sites, but these two oxidation states have been identified in sputtered anatase samples[25]. The charge density difference is defined as:

∆ ρ = ρ ( particle + CO ) − ρ ( particle ) − ρ (CO )

(3)

here, ρ(particle + CO) is the electron charge of the adsorption system, ρ(particle) and ρ(CO) are the electron charge density of the free particle without the CO molecule and the charge density of the free CO molecule without the particle. The charge transfer between the CO molecule and the anatase nanoparticle is shown in Fig. 7(a). The charge density calculation shows that the electron density at the center of the newly formed C=O bond increases. The electron charge transfers from the negative charge area. The charge transfer between the CO2 configuration and the oxygen vacancy particle is shown in Fig. 7(b). There exists no apparent charge transfer in these two parts. It suggests that the Tia-Ob bond is broken during the adsorption process and the CO2 adsorbed physically to the particle as the normal CO2 adsorption mode on oxides [23].

3.2. A CO molecule adsorbed on 72 atom N-doped anatase nanoparticles For the N-doped anatase nanoparticles, a nitrogen atom substitutes an oxygen atom in a TiO2 nanoparticle. Since the electron structure of a nitrogen atom is 2s22p3, while that of an oxygen atom is 2s22p4, one hole is introduced by the substitution. The

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empty state may be on the top of the valence band or inside the band gap as an impurity band [6]. Fig. 2(b) shows the doping positions. Substitutional positions in N1 and N2 generate the impurity states inside band gap while other substitutional positions generate empty state on the top of the valence band. Fig. 3(a) shows a possible configuration that may generate an impurity band; while a possible configuration which can generate a hole on the top of the valence band is described in Fig. 3(b). We choose these two particles to study the doping effect of the CO adsorption. The CO molecule may be adsorbed on all surface oxygen atoms of the particle. The absolute values of the adsorption energy on different oxygen atoms for the CO adsorption on the undoped particle and two N-doped particles are listed in Fig. 4. Panel (b) and panel (c) are the adsorption energies for the N-doped nanoparticles M1 and M2. There are three comments for this energy figure. First, the adsorption energy at the dangling oxygen site and doped N site are two energy favorable adsorption sites (see the stars and triangle points). In the undoped particle, the adsorption energy at the dangling oxygen site is much higher than that of all the other sites. Second, the adsorption energy for the N-doped particles is much higher than that for the undoped particle. Third, for N-doped particles, there are more active adsorption sites rather than only the dangling oxygen site, such as the N-doped site. Therefore, the adsorptions on the dangling oxygen atom and the doped N atom are investigated.

3.2.1. A CO molecule adsorbed on the dangling oxygen atom in N-doped

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nanopaticles. For the substitution positions N1 and N2, in Fig. 2(b), the N doping atom introduces an impurity band inside the band gap [6]. The adsorbed CO molecule on the dangling oxygen atom of the particle forms a new C-O bond, see Fig. 3(a). The adsorption energy for this type is −2.47 eV (see Fig. 5), that is 1.54 eV lower than the undoped system (−0.93 eV). So, for N-doped particle, the CO adsorption is much stronger than on the undoped particles. The LDOSs for the CO adsorption on the M1 particle are shown in Fig. 8(a). The Tia-Ob bond is weakened after the adsorption. From the large overlap in panel (c) we can see the newly formed Ob-Cc bond. The charge density difference calculations also shows the newly formed O-C bond and the broken Ti-O bond (see Fig. 9(a) and Fig. 9(b)). The doped N atom introduces a hole in the particle that can activate the dangling oxygen atoms and increase their oxidation ability. The CO can be oxidized more easily by the hole-assisted anatase[26]. That also agrees with the work that N-doped nanoparticles have stronger ability to oxidate NO molecule[27]. For the other 21 possible substitutional configurations except N1 and N2 substitutional sites, the empty state is on the top of the valence band and the hole mainly distributes on the nitrogen atom and the two dangling oxygen atoms. We choose one of these substitutional configurations to investigate the doping effect of the CO adsorption on M2 particle (see Fig. 3(b)). Unlike M1, there exists two adsorption configurations on the dangling oxygen atom in M2 particle, typed M2A and M2B (see Fig. 3(b) and Fig. 3(c)). The

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adsorption energy for M2A is −2.52 eV (see Fig. 5), and it is 170% lower than the adsorption energy for the undoped particle. The dangling bond and the bond length of the CO molecule are elongated after the adsorption because of the electrons transfer to the newly formed bonds. Similar to M1A, the LDOSs and CDD calculation also indicate the newly formed chemical bond and the broken bond (see Fig. 8 and Fig. 9). For M1A and M2A, the energy differences are 0.22eV and 0.03eV after increasing the distance between CO2 and the O-vacancy nanoparticle to 3.01 Å respectively. There exists little interation between two atoms with a distance of 3.01 Å. It means that after gaining energy about 0.22 eV or 0.03 eV the CO2 breaks away from the particle. This energy gaining can be achieved at room temperature apart from the energies (-2.47 eV and -2.52 eV) released in the adsorption process. Based on these results, we conclude that the CO molecule can be oxidized to CO2 by TiO2 nanoparticle and the CO2 can spontaneously dissociate at room temperature. The carbonate formation is also appeared in the adsorption procedure. It involves the dangling oxygen and the neighbor oxygen atoms in interaction with the CO molecule. The adsorption energie is −2.95 eV. Three C-O bonds are 1.20 Å, 1.34 Å, 1.40 Å, respectively. The mechanism of formation of carbonate species can be explained by following formulas:

CO + 2 O 2 − → CO3 2 − + 2 e − Ti 4 + + 2 e − → Ti 2 +

or

2Ti 4 + + 2 e − → 2Ti 3 + 13


(4)


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Two electrons from CO transfer to the particle to reduce the Ti sites.

3.2.2. A CO molecule adsorbed near the N site of N-doped nanoparticles. The CO molecule forms one bond with the doped N atom in two kinds of doped nanoparticles (see Fig. 10). Adsorption energies are −3.28 eV and −2.45 eV, respectively, both describe stronger interaction than the undoped nanoparticle (-0.93 eV), which means the particles have strong ability to adsorb CO molecule because of the empty states in it. The bond lengths are listed in Table 2. C-O bonds are 1.17 Å and 1.18 Å after the adsorption, which are much longer than the free CO molecule bond (1.14 Å). It can be concluded that C-O bonds in CO molecules are weakened after the adsorption, while the N-C chemical bonds form in this process.

3.3 CO molecule adsorbed on undoped and N-doped anatase (101) surface The configuration for a CO molecule on undoped anatase (101) surface after relaxation is shown in Fig. 11. In this case, no significant structural variation is observed upon adsorption. CO keeps a distance of 3.26 Å (see Table 3) from the surface, and the Cc-Od bond in CO molecule is only slightly (about 0.01 Å) elongated with respect to the corresponding C-O bond. The calculated adsorption energy for this configuration is +1.14 eV, which means an endothermic process. We investigate N-doped system in which one O atom per supercell is substituted by one N atom, this corresponds to a nitrogen concentration of about 1.4%

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(i.e.,TiO2-xNx with x=0.028). Finazzi et al. have reported that nitrogen prefers substitutional subsurface sites, in particular the O3c-bridging position is the most stable substitutional sites [28]. Therefore, subsurface substitutional N site is considered to investigate the CO adsorption in this section. Fig. 12 shows the configurations of CO on anatase (101) surface. CO locates above the surface as an initial state, while it seizes the O2c atom to form a CO2 molecule after relaxation (see Fig. 12(b)). The adsorption energy during this process is -1.54 eV, it means a more stable configuration compared to the undoped surface adsorption, but not as stable as the N-doped nanoparticle adsorption (-2.47 eV and -2.52 eV). The Ob-Cc bond is 1.18 Å, that is nearly the same length (1.17 Å) as CO2 molecule calculated in this work, and the adsorption gives rise to an increase of the CO bond from 1.14 Å to 1.16 Å, thus the Ob-Cc-Od part can be treated as CO2 molecule. The Tia-Ob bond elongated from 1.91 Å to 2.92 Å (see Table.3), it suggests that CO2 have no significant interaction with the O-vacancy surface. It can be concluded that N-doped anatase (101) surface have stronger adsorption ability than the undoped surface, but weaker than the N-doped anatase nanoparticles adsorption.

4. Conclusions The adsorption and oxidation of a CO molecule on 72 atom undoped and N-doped anatase nanoparticles has been studied by first principles calculations. The CO molecule over the undoped and N-doped nanoparticles forms CO2 by detaching

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the dangling oxygen atom. Two electrons transfer from CO to the particle resulting in the reduction of the Ti sites. The CO2 molecules are located near the particles with a distance of 2.41 Å, 2.28 Å, 2.37 Å, respectively. Little interaction proved by DOS and CDD analyses exists between the CO2 and the O-vacancy particle. Therefore, the CO2 can be treated as a physical adsorption on the particle. Less than 0.22 eV energy gaining would drive the CO2 away to a distance greater than 3.01 Å . This means CO molecule can be oxidized to CO2 by TiO2 nanoparticle and the CO2 can spontaneously dissociate at room temperature apart from the energies released in the adsorption process. Adsorption energy is -0.93 eV for undoped configuration while that is -2.47 eV or -2.52 eV for N-doped nanoparticle. The more energy released, the stronger the response. This means N-doped TiO2 nanoparticles have stronger oxidation ability. Besides the dangling oxygen atom, the doped nitrogen site (N site) in N-doped nanoparticles is a chemical adsorption site as well. Adsorption energies are -3.28 eV and -2.45 eV, both describe stronger interaction than the undoped nanoparticle (-0.93 eV). N-doped anatase (101) surface have stronger adsorption ability than the undoped surface, but weaker than the N-doped anatase nanoparticles adsorption. Based on these results, it can be concluded that the N-doped anatase particles are more efficient to oxidize CO molecule in the irradiated nuclear graphite decontamination process. Author Information *Corresponding author. Tel.: +86-10-89796111,

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E-mail address: [email protected]

Acknowledgment This work is supported by the National Natural Science Foundation of China (Grand No.21271114); Tsinghua University independent research and development fund (20111080982).

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[14] Lazzeri, M.; Vittadini, A.; Selloni A. Structure and energetics of stoichiometric

TiO2 anatase surfaces. Phys. Rev. B 2001, 63,155409 . [15] Kresse, G. and Hafner, J. Ab. initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558 – 561 . [16] Kresse, G. and Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B 1994, 49, 14251 – 14269 . [17] Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953 – 17979. [18] Kresse, G. and Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758 – 1775 . [19] Perdew, J. P.; Burke, K., and Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865–3868. [20] Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 1990, 41, 7892–7895 . [21] Pulay, P. Convergence acceleration of iterative sequences, the case of scf iteration. Chemical Physics Letters 1980, 73(2), 393 – 398 . [22] Burdett, J.; Hughbanks, T.; Miller, G.; Richardson, J.; Jr. and Smith, J. Structural-electronic relationships in inorganic solids: powder neutron diffraction studies of the rutile and anatase polymorphs of titanium dioxide at 15 and 295 K .J. Am. Chem. Soc.1987, 109, 3639-3646. [23] Freund, H.-J. and Roberts, M. Surface chemistry of carbon dioxide. Surface

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Science Reports 1996, 25(8), 225 – 273. [24] Mguig, B.; Calatayud, M.; and Minot, C. CO oxidation over anatase TiO2-(001). Journal of Molecular Structure: THEOCHEM 2004, 709(13), 73 – 78 . [25] Tanner, R.; Liang, Y.; and Altman, E. Structure and chemical reactivity of adsorbed carboxylic acids on anatase TiO2 (001). Surface Science 2002, 506(3), 251 –271 . [26] Wanbayor, R.; Deak, P.; Frauenheim, T. and Ruangpornvisuti, V. First principles theoretical study of the hole-assisted conversion of CO to CO2 on the anatase TiO2 (101) surface. The Journal of Chemical Physics 2011, 134(10), 104701. [27] Liu, J.; Liu, Q.; Fang, P.; Pan, C. and Xiao, W. First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles. Applied Surface Science 2012, 258(20), 8312 – 8318. [28] Finazzi, E.; Valentin, D.; Selloni, A.; Pacchioni, G. First Principles Study of Nitrogen Doping at the Anatase TiO2 (101) Surface. J. Phys. Chem. C 2007, 111, 92759282 .

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Tables

Table 1

Bond lengths for a CO molecule on the dangling oxygen atom. Unit in Å Type

Tia-Ob

Ob-Cc

Cc-Od

Cc-Oe

Undoped Non-adsorbed

1.68

-

1.14

-

Undoped(A)

2.41

1.19

1.17

-

Non-adsorbed

1.73

-

1.14

-

M1(A)

2.28

1.16

-

Non-adsorbed

1.79

1.14

-

M2(A)

2.37

1.19

1.17

-

M2(B)

1.93

1.34

1.20

1.40

N-doped

Table 2

1.19

-

Bond lengths for a CO molecule on the doped N atom. Unit in Å Type

N-C

C-O

M1(N)

1.24

1.17

M2(N)

1.23

1.18

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Table 3

Page 22 of 32

Bond lengths for a CO molecule on undoped and N-doped anatase (101)

surface. Unit in Å Tia-Ob

Type

Ob-Cc

Cc-Od

Undoped Non-adsorbed

1.88

-

1.14

adsorbed

1.88

3.26

1.15

Non-adsorbed

1.91

-

1.14

adsorbed

2.92

1.18

1.16

N-doped

Figure Captions: Figure 1: Schematic drawing of steam pyrolysis. Figure 2: Undoped TiO2 anatase nanoparticles. (a) Front view of a 72 atom undoped particle. (b) Side view of the particle, N1 and N2 are the two substitutional positions that generate the impurity states while other substitutional positions generate empty state on the top of the valence band. (c) A CO molecule adsorbed on the undoped particle. The CO molecule forms one bond with the dangling O atom on the particle. Figure 3: CO molecule on the dangling oxygen atom in N-doped anatase nanoparticles. (a)The doping N atom generates an impurity band inside the band gap and the CO molecule forms one bond with the dangling oxygen atom. (b) The doping

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N atom generates a hole on the top of the valence band. CO forms one bond with the dangling oxygen atom. (c) Same doping position as (b), the CO molecule forms carbonate configuration with the dangling oxygen and the neighbor oxygen atom. Figure 4: The adsolute values of the adsorption energy of a CO molecule on different surface oxygen adsorption sites of the anatase particle. Star-point is the adsorption energy at the dangling oxygen site. Red-triangle-point is the adsorption energy at the doped N site. Black-triangle-point represent those positive adsorption energies at oxygen sites. Panel (a): Adsorption energies of a CO molecule on undoped nanoparticle. Because of the symmetry, there are only 24 different adsorption sites. Panel (b): Adsorption energies of a CO molecule on the N-doped nanoparticle M1. Panel (c): Adsorption energies of a CO molecule on N-doped nanoparticle M2. Figure 5: Adsorption energies (in GGA) of a CO molecule on different nanoparticles. Figure 6: LDOS for the adsorption of a CO molecule on 72 atom undoped particles. The vertical lines are Fermi energies. (a) LDOS for the adsorption on undoped nanoparticle. Panel(a): LDOS of the dangling oxygen atom of the particle and the neighbor Ti atom before adsorption. Panel (b): LDOS of the dangling oxygen atom of the particle and the neighbor Ti atom after adsorption. Panel (c): LDOS of the dangling oxygen atom of the particle and the C atom in the CO molecule after adsorption. Figure 7: Charge density difference (CDD) for a CO molecule adsorbed on a 72 atom undoped anatase paricle. (a) CDD between the Ob-Cc bond. The orange color

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represents positive value of the electron tranfer. The blue color represents the negative value area. The electron density increases at the center of the newly formed O-C bond, while the decrease area shows the weakened Ti-O bond. (b) CDD between the Tia-Ob bond. It shows a physical adsorption between CO2 and the particle. Figure 8: LDOS for the adsorption of a CO molecule on 72 atom N-doped particles. The vertical lines are Fermi energies. (a) LDOS for the adsorption on M1 nanoparticle. Panel (a): LDOS of the dangling oxygen atom of the particle and the neighbor Ti atom before adsorption. Panel (b): LDOS of the dangling oxygen atom of the particle and the neighbor Ti atom after adsorption. Panel (c): LDOS of the dangling oxygen atom of the particle and the C atom in the CO molecule after adsorption. (b) LDOS for the adsorption on M2 nanoparticle. Panel (a), (b) and (c) represent similar results as (a). Figure 9: Charge density difference (CDD) for a CO molecule adsorbed on a 72 atom N-doped anatase paricle. (a) CDD between the Ob-Cc bond in M1A. The orange color represents positive value of the electron tranfer. The blue color represents the negative value area. The electron density increases at the center of the newly formed O-C bond, while the decrease area shows the weakened Ti-O bond. (b) CDD between the Tia-Ob bond in M1A. It shows a physical adsorption between CO2 and the particle. (c) CDDbetween the Ob-Cc bond in M2A. (d) CDD between the Tia-Ob bond in M2A. Figure 10: CO molecule on the doped N site in N-doped anatase nanoparticles. (a) The doping N atom generates an impurity band inside the band gap and the CO molecule forms one bond with the doped N atom. (b) The doping N atom generates a

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hole on the top of the valence band. CO forms one bond with the doped N atom. Figure 11: CO molecule on the undoped anatase (101) surface. (a) Side view of the adsorption configuration. (b) Top view of the adsorption configuration. Figure 12: CO molecule on the subsurface N-doped anatase (101) surface. (a) Initial state of the relaxation. (b) CO forms one bond with the O2c after relaxation.

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Page 26 of 32

Figures Figure 1: TiO2

dry cooler calciner steam 1 1, 2, 3: 0.1MHNO3 4, 5: 4MNaOH exhaust

Ar

5

4

3

Figure 2：：

N2

(a) front view

N1

(b) side view

26


d c a b

(c) undoped A

2

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Figure 3：：

b c d

a

a

bc

Ob C c Od

d

Tia Of

(a) M1A

(b) M2A

Figure 4：：

27


(c) M2B


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5：：

Figure 6：：

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Figure 7：：

(a) Undoped A (Ob-Cc)

(b) Undoped A (Tia-Ob)

Figure 8：：

(a) M1A

(b) M2A

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Page 30 of 32

Figure 9：：

(a) M1A (Ob-Cc)

(b) M1A (Tia-Ob)

(c) M2A (Ob-Cc)

(d) M2A (Tia-Ob)

Figure 10：：

O N C C N

(a) M1N

(b) M2N

30


O

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Figure 11:

c d a

b

(a) Side view

(b) Top view

Figure 12:

c b

d c b

d

a

a N

N

(a) Initial state

(b) After relaxation

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TOC:

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CO Adsorption and Oxidation on N-Doped TiO2 Nanoparticles - The

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