Partial Oxidation of Methane on Anatase and Rutile Defective TiO2

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Partial Oxidation of Methane on Anatase and Rutile Defective TiO Supported Rh Cluster#A Density Functional Theory Study 2

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Dan Guo, and Gui-Chang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07489 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Journal of Physical Chemistry

Partial Oxidation of Methane on Anatase and Rutile Defective TiO 2 Supported Rh4 Cluster：A Density Functional Theory Study

Dan Guo 1 , and Gui-Chang Wang*,1,2 (1 College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Educ ation) and Collaborative Innovation Center of Chemical Science and Engineering(Tianjin), Nankai University, Tianjin 2

300071, P. R. China; State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China)

*Corresponding author: Gui-Chang Wang. E-mail: [email protected] Telephone: +86-22-23503824 (O) Fax: +86-22-23502458

Abstract: We have studied the effect of CH4 dissociation and partial oxidation on carbon deposition resistance on oxygen-defective anatase TiO 2 supported Rh (Rh4 /Ana-Ov) and rutile TiO 2 supported Rh (Rh4 /Rut-Ov) by the density functional theory. We computed the adsorption structure of CH x species, and the adsorption of CHx species on Rh4 /Rut-Ov was stronger than that on Rh4 /Ana-Ov. On the Rh4 /Ana-Ov surface, the activation energy barrier of C-H bond cleavage involved in methane decomposition was higher than that on Rh4 /Rut-Ov, which means the Rh4 /Ana-Ov was favored to carbon deposition resistance. Additionally, the dominant process of methane partial oxidation was CH2 *+O*→CH2O*, where the energy barrier on Rh4 /Ana-Ov is lower relative to Rh4 /Rut-Ov. This implies that the oxidation reaction was also favorable to the carbo n deposition resistance on Rh4 /Ana-Ov. As the strong carbon deposition resistance effect on Rh4 /Ana-Ov, a high activity for syngas formation can be expected. The energetic span model analysis showed that the apparent activation energy was beneficial to the partial oxidation of methane on Rh4 /Ana-Ov compared to that of Rh4 /Rut-Ov. Moreover, the selectivity of CO was high and up to 90% by using the micro-kinetic model analysis, which was consistent with the experiment results.

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1. Introduction The major methods of preparing syngas from methane are steam reforming (STR) (Eq. (1)); dry reforming (DR) (Eq. (2)) and partial oxidation (POM) (Eq. (3)):

CH4+H2O  CO+3H2

H298  206 kJ/mol

(1)

CH4+CO2  2CO+2H2

H298  247 kJ/mol

(2)

1 CH4+ O2  CO+2H2 2

H298  36 kJ/mol

(3)

The steam reforming is dominant approach for syngas production and widely used for many years1 . But the reaction is highly endothermic and energy intensive, and the H2 /CO ratio (H2 /CO=3) is high, which is unsuitable for further reaction such as Fischer- Tropsch process2-3 . Dry reforming process caused a lower H2 /CO ratio (H2 /CO=1) which corresponds to the requirement of FischerTropsch process4 . However it is also highly endothermic and has carbon deposition. Different from the STR and the DR, the POM is mild exothermic and react at lower temperature. Many experimental5-7 and theoretical researches for POM have been investigated. The researchers have studied various catalysts and carriers for partial oxidation of methane. Zhu2 et al. studied the feasibility of POM by thermodynamic and kinetic analysis, and discussed the effects of the initial CH4 /O 2 ratio, temperature and pressure on the POM reaction. It is recognized that Rh catalysts performed best among the noble metals as its high activity8 , stability against volatilization, resistance to carbon deposition9 . Donazzi10 et al. investigated kinetic study of POM over Rh/α-Al2 O3 catalyst and found that the Rh catalysts have a high activity for POM. Mateos-Pedrero11 et al. studied the performances of Rh/TiO 2 catalysts in POM. The results indicated that the type of TiO 2 determined the performances of supported Rh catalysts and CH4 conversion and selectivity were improved on anatase support. Maestri12 et al. elucidated the dominant reaction pathways in POM on Rh, which indicated that CHx * is consumed via oxidative dehydrogenation. Carbon deposition is one of the disadvantages of methane activation which deactivated the catalyst and reduced catalytic efficiency. In general, at least two aspects should be taken into account to avoid the carbon deposition: on the one hand, increasing the C-H bond broken energy barrier involved in CH4 ; on the other hand, feasibility removal of carbon deposition by oxygen or other oxidation agent and inhibit CO disproportionate reaction. In the experimental side, Claridge13 et al. investigated the carbon deposition during the POM and found that the rate of carbon formation following the order: Ni>Pd>Rh>Ir. Moreover, the principal route for carbon formation was the decomposition of methane. In the theoretical side, many calculations have been studied for the 2


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process of CH4 dissociation (CH4 →CH3 →CH2 →CH→C)14-16 . The method for controlling carbon deposition included improving the dispersion of active metal17 and increasing carbon oxidation to resistant carbon formation18-19 . Zhang20 et al. reported a density functional theory (DFT) calculations of POM on Ni (111) surface alloying by Pt. They studied the process of CH 4 dissociation and identified that the likely main routes for CO formation on Ni (111) are CH+O→CHO→CO+H. However, the theoretical calculation of CHx species oxidation process for carbon deposition resistance is poor. Thus it is important to figure out the oxidation mechanism of partial oxidation of methane. Most recently, our theoretical study indicated that the barriers of C-H cleavage in the first two steps of CH4 dissociation on K pre-adsorbed Ni4 /Al2 O3 are lower than that on clean Ni4 /Al2 O3 . That remains the activity of steam- methane reforming. On the contrary, the barriers of C-H cleavage in last two steps of CH4 dissociation on K pre-adsorbed Ni4 /Al2 O3 are higher than that on clean Ni4 /Al2 O 3 . Thus K will relieve the carbon deposition21 . Inspired by the experimental finding, the activity of POM catalyzed by Rh is strongly controlled by the TiO 2 type, and Anatase TiO 2 supported Rh is more active relative to the Rutile TiO 2 11 . In the present work, we used the DFT calculation to analysis the CH4 dissociation on anatase and rutile supported Rh cluster. We compared the adsorption properties and the reaction activity of Rh cluster on different type of TiO2 . Moreover, the CHx species oxidation mechanism on Rh cluster supported on anatase and rutile was investigated. In this paper, we discussed the properties of carbon deposition resistance. Our present calculations clearly show that the defective Anatase- TiO2 supported Rh catalyst was more favorable for the carbon deposition resistance than that of Rutile-TiO 2 , which results in a higher activity for POM.

2. Computational Details and Models The DFT+U calculation used the Vienna ab initio simulation package (VASP)22-23 was introduced to analyze the properties in transition metal oxides. All the calculations are utilized the generalized gradient approximation (GGA-PW91)24 as the exchange-correlation functional and employed a plane wave basis with the kinetic cutoff energy of 400 eV. There is a strong Coulomb repulsion on metal ions and the DFT method lack of description. Thus the DFT+U method25 evaluated the on-site coulomb interactions in the localized d orbital and exchange interactions by adding an effective Hubbard-U parameter to express the repulsion between electrons on the same orbitial. Researchers set different values of U for the Ti 3d orbital and discussed the effects of the U parameter26-27 . In this work, the value of Hubbard-U parameter was set to 4 eV for Ti atom28-29 . Brillouin zone integration was sampled using Monkhorst-Pack scheme, and the k-points was set as

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3×3×1. The transition state (TS) was determined by nudged Elastic band (NEB) method 30-32 , ensuring that the force less than 0.035 eV and confirmed by the frequency analysis. An Rh4 /TiO 2 modeling catalyst, both anatase and rutile structures, used the symmetric periodic slab models in this work. Our previous study had built the model of Pd4 cluster on oxygen defective anatase (101) surface and rutile (110) surface 33 . We obtained the anatase TiO 2 structure (a=3.80Å, c=9.60Å, c/a=2.53) model by cleaving the bulk in the (101) surface, which contains four Ti atoms and eight O atoms in a unit cell, and get a 10.21 Å×11.33 Å lattice (p(2×3)). For rutile TiO 2 structure, a 12.99 Å×8.88 Å lattice (p(2×2)) was gained by cleaving the bulk in the (110) surface. The vacuum layer was set as 25 Å. In this study we investigated the structures of Rh4 cluster on TiO 2 with an oxygen vacancy (Ov) (named as Rh4 /Ana-Ov and Rh4 /Rut-Ov respectively). The oxygen defective surfaces were made by removing one bridging oxygen atom from the anatase (101) and the rutile (110) surface, respectively. We use the model with 71 atoms including 24 Ti atoms and 47 O atoms both anatase and rutile to ensure the same density of oxygen vacancies and the adsorbed Rh4 cluster in this work. The

adsorption

energy

Eads

was

calculated

using

the

formula

of Eads  Eadsorbate/ substrate   Eadsorbate  Esubstrate  . In this definition, Eadsorbate / substrate refers to the total energy of adsorbate–catalyst system, Eadsorbate and Esubstrate refer to the energies of adsorbate species and free substrate, respectively. The more negative the Eads, the stronger the adsorption is. We calculated the activation barrier (Ea) and the reaction energy on the basis of the following expression: Ea  ETS  EIS and E  EFS  EIS , where EIS , ETS , and EFS represent the energies of initial state (IS),

transition state (TS), and final state (FS), respectively. Besides, we introduced the zero point energy 1 (ZPE) into the activation energy as: ZPE   hvi , where v i was the real frequency of the system. i 2

The PDOS calculation was used the k-points of 7×7×1. And the occupied d-band center was Ef

calculated by equation:   c d

 E

d

( E )dE

 Ef



d

, where ρd is the projected density of states (PDOS) of the ( E )dE



d-band of surface atoms, and Ef is the Fermi level energy.

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3. Results 3.1 TiO2 supported tetrahedral Rh4 cluster In order to explore the effect of the different structure of TiO 2 on CH4 partial oxidation, the Rh4 cluster on anatase (101) and rutile (110) respectively were investigated in this study. In this work, we calculated the adsorption characteristics of Rh4 cluster on both perfect and defective Titania surfaces. On perfect TiO 2 surface, the Rh4 cluster was tetrahedron with the bond length of 2.42~2.53 Å (see Fig.S1 in the Supporting Information). The adsorption energies of Rh4 were -3.66 eV (on anatase) and -3.52 eV (on rutile) respectively. Because of the strong interaction with substrates, the Rh4 clusters adsorbed on the surface with defect were distorted and the Rh-Rh bond lengths were elongated. The optimized Rh4 cluster on anatase (101) oxygen vacancy surface was shown as Fig.1 (a). The calculated Rh-Rh bond distances were at the range of 2.49 Å~2.67 Å, while the average RhRh bond of free Rh4 cluster is 2.50 Å34 . And the Rh atom binds with O atom with the bond lengths of 2.24 Å and 2.23 Å. The rutile (110) supported Rh4 structure was different from the anatase, in which Rh-Rh bond lengths were 2.46 Å~2.50 Å (see Fig.1 (b)). At the same time, the Rh-O bond lengths were 2.04 Å, 2.15 Å and 2.24 Å respectively. Most of Rh-Rh bond lengths of Rh4 /Ana-Ov were longer than that of Rh4 /Rut-Ov. The stability of Rh4 on TiO 2 surface was analyzed by Bader charge scheme35-36 . The Bader charge analysis results (shown as Table 1) shown that the Bader valence of Ti atoms binding with Rh cluster on anatase increased by +0.12e and +0.25e, the charge transferred to Rh cluster so that the Rh4 cluster was electronegative. Moreover, the Bader valence of Ti atoms binding with Rh cluster on rutile decreased by -0.12e and -0.04e and the Rh cluster was electropositive. Considering the electrostatic interaction of Ti and Rh, the Rh4 cluster on anatase surface was more stable than that on rutile surface.

Table 1 The Bader charge and the d-band center of Rh on anatase and rutile TiO 2 Ana Bader valence (e) Rh1 Rh2 Rh3 Rh4 Average

-0.070 -0.057 -0.19 +0.015 —

Rut d-band center (eV) -1.04 -2.00 -1.48 -2.00 -1.62

Bader valence (e)

5


+0.11 -0.031 -0.079 -0.036 —

d-band center (eV) -1.68 -1.59 -0.88 -1.40 -1.39


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Fig.1 Top view and side view of the structures of TiO2 supported Rh4 cluster, (a) Rh4 /Ana-Ov; (b) Rh4 /Rut-Ov.

Moreover, we have also studied the Rh cluster size and the oxygen-defect constants on the calculation results. For the Rh cluster size effect, we also calculated the adsorption energies of single Rh atom and Rh13 cluster on defective anatase (101) and rutile (110) surface (the structure was shown as Fig.S2 in the Supporting Information). The adsorption energies of single Rh atom were 4.56 eV (on anatase) and -3.84 eV (on rutile), while that of Rh13 on anatase and rutile were -5.43 eV and -4.18 eV respectively. It indicated that three sizes of Rh cluster were all adsorbed stronger on Ana-Ov than that on Rut-Ov. This is generally agreement well with our previous DFT study of Pd 4 on anatase and rutile TiO 2 for the semi- hydrogenation of acetylene37 . The adsorption energies of Rh4 cluster on one oxygen vacancy anatase (101) and rutile (110) surface were -5.14 eV and -4.03 eV. On the two-Ov anatase and rutile surface, the adsorption energies of Rh4 cluster were -6.37 eV and -5.24 eV respectively, which adsorbed stronger than that on one-Ov and perfect surface. The results was consistent with our previous DFT study of Pd4 cluster on perfect and defect TiO 2 surface.37 Anyhow, the binding strength of Rh cluster on anatase is always stronger than that of rutile regardless of the cluster size and oxygen-defect, so one may expected that the present result in this study can be extend to these systems. The present work was focused on the differences between anatase and rutile, and the effect of oxygen stoichiometry and the Rh cluster size on the parameters of intermediates and elementary of POM will be studied in our further research. 3.2 CH4 dissociation 6


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Four elementary steps (CH4 *→CH3 *+H*, CH3 *→CH2 *+H*, CH2 *→CH*+H*, CH*→C*+H*) involved in methane dissociation were discussed in the present study. In order to investigate the difference between Rh4 /Ana-Ov and Rh4 /Rut-Ov for methane decomposition, the adsorption energy of IS (Eads,IS ) and the final state (FS) (Eads,FS ), activation energy barrier (Ea) , reaction energy (∆E) of CH4 dissociation reaction and the bond lengths of the transition states were listed in Table 2.

Table 2 Adsorption energy of IS (Eads,IS ) and FS (Eads,FS ), Activation energy barrier (Ea), Reaction energy (∆E) of CH4 dissociation reaction and the bond lengths of the transition states. The ZPE was provided in brackets.

Reactions

CH4 *→CH3 *+H* CH3 *→CH2 *+H* CH2 *→CH*+H* CH*→C*+H*

Bond

Eads,IS

Eads,FS

Ea

∆E

(eV)

(eV)

(eV)

(eV)

Rh4 /Ana-Ov

-0.36

-5.26

0.38 (0.27)

0.10

1.58

Rh4 /Rut-Ov

-0.31

-5.11

0.31 (0.20)

0.05

1.57

Rh4 /Ana-Ov

-2.37

-6.68

0.91 (0.80)

0.52

1.68

Rh4 /Rut-Ov

-2.60

-7.82

0.66 (0.57)

-0.01

1.63

Rh4 /Ana-Ov

-4.71

-10.21

1.20 (1.08)

-0.64

1.53

Rh4 /Rut-Ov

-4.79

-10.51

0.51 (0.40)

-0.89

1.38

Rh4 /Ana-Ov

-7.25

-10.01

0.96 (0.85)

0.30

1.44

Rh4 /Rut-Ov

-7.83

-10.98

0.57 (0.47)

-0.03

1.39

Substrates

length of TS (Å)

CH4 *→CH3 *+H* The most stable adsorption geometry of CH4 was shown as Fig.2. The distances of C-Rh were 2.41 Å and 2.45 Å, respectively. Two C-H bond of CH4 on Rh4 were elongated to 1.13 Å which were activated from free CH4 molecule (bond length is 1.096 Å). The adsorption energy of CH4 on Rh4 /Ana-Ov was closed to that on Rh4 /Rut-Ov. This indicated a physical adsorption of CH4 on Rh4 . The van der Waals correction for adsorption using vdw-DF method was discussed at the energetic span model parts. The CH4 dissociate into CH3 * and H* (shown as Fig.2). The co-adsorption energy of CH3 * and H* on Rh4 /Ana-Ov is -5.26 eV, which was similar to that on Rh4 /Rut-Ov (-5.11 eV). The first step of CH4 dissociation was endothermic and the reaction energy on the Rh4 /Ana-Ov and Rh4 /Rut-Ov are both lower than 0.10 eV. The bond length of C-H of the transition state (as shown in Fig.2) was 1.58 Å for Rh4 /Ana-OV. Due to the strong adsorption, the calculated activation energy 7



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barrier on Rh4 /Ana-Ov was much lower equal to 0.38 eV. The CH4 dissociation on Rh4 /Rut-Ov is very similar to that on the Rh4 /Ana-Ov, and the C-H bond length of TS was elongated to 1.57 Å, and the activation energy barrier was 0.31 eV. Obviously, the activation for the first steps for CH 4 dissociation on two kinds of TiO 2 surface is similar. CH3 *→CH2 *+H* The CH3 * was adsorbed at a top site on Rh4 /Ana-Ov and Rh4 /Rut-Ov as shown at Fig.2. The distance of C-Rh on anatase surface was 1.99 Å which is similar to that on rutile surface (2.00 Å). The adsorption of CH3 * on Rh4 /Rut-Ov was stronger than that on Rh4 /Ana-Ov (shown in Table 2). The calculated co-adsorption energy of CH2 * and H* on Rh4 /Ana-Ov was lower than that on Rh4 /Rut-Ov. The CH3 * decomposition is endothermic on Rh4 /Ana-Ov and the activation energy barrier is 0.91 eV with the reaction energy of 0.52 eV. Moreover, the dissociation on Rh4 /Rut-Ov was a thermal neutral reaction with the reaction energy of -0.01 eV, and the activation energy barrier was lower than that on Rh4 /Ana-Ov about 0.25 eV. Both the energy barrier and the reaction energy on Rh4 /Ana-Ov were higher than that on Rh4 /Rut-Ov. It indicated that the Rh4 /Ana-Ov was favor to resistant carbon deposition by decreased the decomposition of CH4 . The C-H bond lengths of TS were elongated to 1.68 Å on anatase and 1.63 Å on rutile respectively. It is necessary to analyze the energy barrier of the abo ve first two step of methane dehydrogenation. The energy barrier of the first step was 0.38 eV and 0.31 eV on Rh4 /Ana-Ov and Rh4 /Rut-Ov respectively, which was lower than that on Rh (111) surface with the coordination number (CN) of nine calculated by Kokalj et al. 38 . And the barrier of the second step was 0.91 eV on Rh4 /Ana-Ov and 0.61 eV on Rh4 /Rut-Ov, which was higher than that on Rh (111) surface with the CN of nine. However, the energy barrier was consistent with the regular of that on ad-atom model of Rh (111) surface whose CN was three38 . That might due to the CN of Rh4 in our model was small and also close to three. CH2 *→CH*+H* Shown as Fig.2, the CH2 * was preferred to adsorb at the bridge site on the Rh4 . The CH2 was adsorbed on Rh4 through C-Rh bond and the bond length were 1.91 Å, 2.08 Å on Rh4 /Ana-Ov, and the adsorption energy was -4.71 eV. The C-Rh bond lengths on Rh4 /Rut-Ov were 1.96 Å, 1.98 Å with the adsorption energy of -4.79 eV. The co-adsorption energies of CH* and H* on Rh4 /Ana-Ov and Rh4 /Rut-Ov were -10.21 eV and -10.51 eV respectively. The CH2 * dissociated through the C-H bond cleavage; the reaction on Rh4 /Ana-Ov overcome energy barrier of 1.20 eV and the C-H bond length of the transition states was 1.53 Å. On Rh4 /Rut-Ov, the energy barrier was lower than that on Rh4 /Ana-Ov and the bond length of C-H was 1.38 Å. The process of CH2 * decomposition was 8


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exothermic and the reaction on Rh4 /Rut-Ov release more energy than on Rh4 /Ana-Ov. It was more difficult to cleave C-H of CH2 * on Rh4 /Ana-Ov than on Rh4 /Rut-Ov. CH*→C*+H The CH* species was adsorbed at the three hollow site with the adsorption energies of -7.25 eV on Rh4 /Ana-Ov and -7.83 eV on Rh4 /Rut-Ov (see Fig.2). The co-adsorption of the products C* and H* on Rh4 /Ana-Ov and on Rh4 /Rut-Ov were calculated in this study and the data were shown in Table 2. It is obvious that the carbon was stronger adsorbed on Rh4 /Rut-Ov than on Rh4 /Ana-Ov. The dissociation of CH* process was endothermic by 0.30 eV on Rh4 /Ana-Ov, and the activation energy barrier was 0.96 eV. Moreover, the process on Rh4 /Rut-Ov was a thermal neutral reaction with the reaction energy of -0.03 eV and the activation energy was 0.57 eV. The C-H bond lengths of transition states were elongated to 1.44 Å on Rh4 /Ana-Ov and 1.39 Å on Rh4 /Rut-Ov.

Fig.2 The geometries of the initial state (IS), the transition state (TS) and the final state (FS) of CH4 dissociation reactions on the Rh4 /Ana and Rh4 /Rut surfaces.

From the energetic results of Table 2, we know that the adsorption energy of CHx * species increases as: CH4 *

Partial Oxidation of Methane on Anatase and Rutile Defective TiO2

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