1、Chinese Journal of Catalysis 35 (2014) 462467 催化学报 2014年 第35卷 第4期 | available at journal homepage: Article Theoretical study of the crystal plane effect and ionpair active center for CH bond activation by Co3O4nanocrystals Yanggang Wanga, Xiaofeng Yanga,b, Linhua Hua, Yadong Lia, Jun Lia,*aDepart
2、ment of Chemistry, Tsinghua University, Beijing 100084, China b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China ARTICLE INFO ABSTRACT Article history: Received 29 December 2013 Accepted 20 January 2014 Published 20
3、April 2014 Methane has attracted extensive interest in recent years due to its potential application as a replacement of oil and a feedstock for valuable chemicals. Due to the large CH bond energy, the conversion of methane into useful products has been a challenge. In the present study, density fun
4、ctional theory (DFT) calculations were performed to study the activation of the CH bond of methane on the (001) and (011) planes of Co3O4, which showed that CH4 activation on Co3O4 nanocrystalswas fairly easy with only small energy barriers (less than 1.1 eV). Surface CoO ion pairs are the active si
5、te for CH bond activation, where the two ions provide a synergistic effect for the activation of the strong CH bond and yield surface CoCH3 and OH species. The Co3O4(011) surface is shown to be more reactive for CH bond activation than the Co3O4(001) surface, which is consistent with previous experi
6、mental results. Our results suggest that methane oxidation on Co3O4 nanocrystalshas strong crystal plane effect and structure sensitivity and the ionpair active center plays a significant role in activating the strong CH bond. 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.P
7、ublished by Elsevier B.V. All rights reserved.Keywords: CH bond activation Methane conversion Crystal plane effect Microkinetic analysis Ionpair active center 1. Introduction Methane is the main constituent in natural gas and it is abundant in nature. The conversion of methane to the more useful met
8、hanol or other liquid fuels has attracted extensive interest in energy production and transportation fuels for several decades 1. Transition metal and metal oxide catalysts have been widely studied in the past decades because of their catalytic properties in alkane transformations 2. The CH bond act
9、ivation is generally the rate determining step (RDS). However, new strategies have still to be developed for the efficient, economical, and direct conversion of methane to more valuable products. The main difficulty is to achieve a highly selective activation of the enormously strong CH bond, which
10、has a bond dissociation energy of 4.5 eV 3. Therefore, the design and synthesis of new catalysts for CH bond splitting is an important topic in modern chemistry. These catalysts can lead to the economical conversion of methane and other lowcarbon alkanes into industrially useful products. Recently,
11、advances in the research of nanomaterials have given new opportunities for designing and screening well defined catalysts for catalytic oxidation reactions 48. Reducible metal oxides, due to their distinct redox properties, have been extensively studied for various catalytic reactions such as CO oxi
12、dation and alkane oxidation 911. Among these oxides, spinel cobalt oxide (Co3O4) has been reported to be a promising transition metal oxide catalyst for methane catalytic oxidation 1214. Recently, we reported the controllable synthesis of * Corresponding author. Tel: +861062797472; Email: This work
13、 was supported by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932401) and the National Natural Science Foundation of China (21221062, 10979031). DOI: 10.1016/S18722067(14)600437 | http:/ | Chin. J. Catal., Vol. 35, No. 4, April 2014 Yanggang Wang et al. /
14、Chinese Journal of Catalysis 35 (2014) 462467 463 different shapes of Co3O4 nanocrystals by a hydrothermal process with a cobalt hydroxide precursor 15,16. The conversion and temperatureprogrammed reduction (TPR) results for methane combustion showed that the nanocrystal (112) and (011) planes were
15、much more reactive for methane conversion than the (001) plane. Many previous literature reports have speculated that the high oxidation capacity of the Co3O4 surfaces can be attributed to the presence of octahedrally coordinated Co species, which can easily convert between trivalent Co3+and divalen
16、t Co2+by the redox process 8,1722. However, knowledge about the active sites for methane conversion on Co3O4 nanocrystals is still lacking due to its complex surface structure. Therefore the mechanism of CH bond activation is still not known. Few studies have looked at the crystal plane effect in su
17、rface oxidation reactions. Here the crystal plane effect refers to the observation that the different surfaces exposed by Co3O4 nanocrystals exhibit different reactivities in CH4 activation. In this paper, a comparative density functional theory (DFT) investigation was performed on CH bond activatio
18、n at various possible active sites on the Co3O4(001) and (011) surfaces. By comparing the energetics of the different reaction pathways for CH bond activation, we demonstrated that the activity of CH bond cleavage exhibited distinct dependence on the exposed crystal plane and active sites. The natur
19、e of the high activity of CH4 conversion on Co3O4 nanocrystals is discussed. A microkinetic model was used to analyze the activity at the different sites and to estimate the reaction temperature of CH bond activation. Our results are consistent with previous experimental observations. 2. Computation
20、al details The theoretical calculations were carried out by using DFT and the DMol3 program (version 4.0) developed by Accelrys, Inc. 2325. The generalized gradient approximation (GGA) with the PBE functional was chosen to describe the exchangecorrelation effects 26. The localized doublenumerical ba
21、sis sets with polarization functions (DNP) were used to describe the valence orbitals of the atoms. Relativistic effective core potentials (ECP) were used to replace the core electrons of Co 22,24. In the geometry optimization, the convergence criterion was set as 1104eV, 1103eV/, 5103 for energy, e
22、nergy gradient, and geometry, respectively. A Fermi surface smearing of 0.2 eV and a realspace cutoff of 4.5 were used. In the geometry relaxation calculations, the spinpolarization KohnSham formalism was used for the selfconsistent field (SCF) iterations and molecular symmetry was not enforced. In
23、all the calculations, stoichiometric supercell models were used. All the slabs were periodically repeated with a vacuum spacing of 15 between the images in the direction perpendicular to the surface. Twodimensional Brillouin integration was performed with an 8 8 8 kpoint mesh for the bulk crystal an
24、d a 4 4 1 kpoint mesh for the slabs. Larger kpoint meshes were also tested, and the total energy showed nearly converged behavior. The transition states were obtained using the complete LST/QST method, which used a linear synchronous transit (LST) calculation followed by repeated conjugate gradient
25、(CG) minimization and quadratic synchronous transit (QST) maximization until a transition state was located 27. The convergence criterion for each transition state was set at 0.05 eV/. Furthermore, a harmonic vibrational analysis was performed to make sure that the transition state has only one imag
26、inary frequency. The zero point energy (ZPE) was included in all energetics. The structural parameters, magnetic moments, and band gap of bulk Co3O4 were first calculated to evaluate the computational methods used in this work. The calculated results of bulk Co3O4 listed in Table 1 were close to the
27、 experimental data and were also in good agreement with previous computational studies, indicating that the selected exchangecorrelation functional and computational approach were appropriate. The Co3O4(001)p(11) (a = 5.79 , b = 5.79 ) and Co3O4(011)p(11) (a = 8.19 , b = 5.79 ) surface slab models c
28、leaved from an ideal bulk spinel structure were used to model the Co3O4 planes. Due to the special structure of the spinel lattice, there are two types of surface structure for each plane for different cleaved depths. In the present study, we chose the most stable slab model with the lower surface e
29、nergy for the (001) and (011) faces, which is shown in Fig. 1. Investigations on the stability of the different surface models can be found in previous theoretical studies 28,29. There are two types of oxygen ions on each surface, denoted respectively as Ooand Otin Fig. 1, which differ in the way th
30、ey are bonded in the lattice structure. Otis bonded to one Co2+ion and 13 Co3+ions, while all atoms near Ooare Co3+ions. Both the Otand Ooions on the (001) surface are threefold coordinated, while on the (011) surface the Otions are twofold coordinated and Ooions are threefold coordinated. Table 1 C
31、alculated and experimental data for bulk Co3O4. Property This work Previous work Exp. Lattice constant () 8.19 8.17 20, 8.12a8.09 32 Magnetic moments (B) 2.58/Co2+2.63 28, 2.53a /Co2+3.02/Co2+ 33Electronic gap (eV) : 2.20 : 2.06 28, 1.67a: 1.882.00b: 1.82 : 1.7528, 1.23aXX: 1.441.52baCalculated data
32、 with the DFT + U method 30,31. bGap range from previous experimental studies 32,3437. Co2+Co3+OtOoCo3+Co2+OtOo(a) Top view-(001)(b) Side view-(001)(c) Top view-(011)(d) Side view-(011)Fig. 1. Surface model of the Co3O4 crystal (001) and (011) planes. The ions of the top layers are shown in a ball a
33、nd stick model. The sublayer Co2+ions are also shown in a ball and stick model. Blue, Co atom; red, O atom. 464 Yanggang Wang et al. / Chinese Journal of Catalysis 35 (2014) 462467 3. Results and discussion 3.1. CH4 physisorption on the Co3O4 (001) and (011) surfaces As the Co2+sites are located in
34、the subsurface layer, only the Co3+sites on the surface layer were considered for CH4 adsorption. The adsorption configurations are shown in Fig. 2. On the (001) and (011) surfaces, the C atom was located at 2.76 and 2.57 above the surface, respectively. The CH bond lengths in the adsorption configu
35、ration on both surfaces were unchanged with respect to the calculated value for free CH4 molecule, which was 1.10 (close to the experimental value of 1.09 ), indicating that little interaction existed between CH4 and the Co3O4 surfaces. The CH4 adsorption energy was calculated using the formula Eads
36、orption = (Eslab+adsorbate Eslab Eadsorbate), where Eslab is the energy per surface unit cell of the slab model containing n bulk unit cells, Eslab+adsorbate is the energy of the adsorption complex including those of the relaxed surface and adsorbed CH4, and Eadsorbate is the energy of the isolated
37、CH4 molecule. The calculated CH4 adsorption energies were 0.11 and 0.08 eV for the (001) and (011) planes, respectively, implying that the CH4 molecule was only weakly physisorbed on the Co3O4 surfaces. 3.2. CH4*CH3+*H decomposition on Co3O4(001) and (011) surfaces Many previous studies on the CH4 o
38、xidation mechanism proposed that the first CH bond activation is the key step for CH4 conversion 2,38. Generally, the first CH bond cleavage can be divided into *CH3 species binding to one surface site and H* species binding to an adjacent site. As an example, Psofogiannakis et al. 38 reported CH sc
39、ission accompanied by *CH3 and *H binding to two adjacent Pt sites. In the present study, the Co3O4 surfaces contained both Co ions and O ions. Although CH4 cannot chemisorb at the surface Co site, we found that CH4 can be activated at CoO ion pairs by charge polarization of the CH bond, leading to
40、the formation of CoCH3 and OH species. To understand the activation nature of the CH bond on Co3O4 nanocrystals, the transition states of the elementary steps were searched using DFT calculations on the Co3O4(001) and (011) model surfaces. On each surface, two reaction pathways for CH bond activatio
41、n were investigated, for where the proton ended up either on the Otsite or Oosite. To provide the mechanistic details of the first CH bond activation, we considered the thermodynamic reaction energies Er of the CH splitting and the kinetic barrier energies Ea. The calculated results are shown in Tab
42、le 2. The positive reaction energy indicated that the oxidation process is endothermic, where a negative value corresponds to an exothermic process following the thermodynamic convention. Despite the high CH3H(g) bond dissociation energy (4.5 eV), the Co3O4 nanocrystal surfaces exhibit remarkable re
43、activity for the first CH cleavage with rather low barriers (less than 1.1 eV). The geometries of the reactants, transition states (TS), and products at each CoO pair site are given in Figs. 3 and 4. The bond distances are given in Tables 3 and 4. Compared to the configurations of the reactants, the
44、 transition states on both surfaces have a similar configuration transition that accompanies the elongation of the CH and CoO bonds and the approaching of the C and Co atoms. Finally, these transition processes led to the production of CoCH3 and OH species. Therefore, it can be inferred that the for
45、mation of surface CoC and OH bonds decreases the barriers of CH bond scission and also weakens the surface CoO bonds. Table 2 shows that the energy barriers for the first CH (a) Top view-(001)(b) Side view-(001)(c) Top view-(011)(d) Side view-(011)Fig. 2. The calculated CH4 adsorption configurations
46、. Table 2 Reaction energies Er and activation energies Ea for the first CH bond splitting on the Co3O4 (001) and (011) surfaces. Zero point energy is included. Surface type Ea at CoOt(eV) Er at CoOt (eV) Ea at CoOo (eV) Er at CoOo (eV) (001) 0.87 0.16 0.98 0.19 (011) 0.81 0.62 1.10 0.13 ReactantTSPr
47、oductTS(a) Pathway I at CoOt(b) Pathway II at CoOoProductFig. 3. Optimized configurations of reactants, transition states (TS), and products on the Co3O4(001) surface. ReactantTS ProductTS(a) Pathway I at CoOt(b) Pathway II at CoOoProductFig. 4. Optimized configurations of reactants, transition stat
48、es (TS), andproducts on the Co3O4(011) surface. Yanggang Wang et al. / Chinese Journal of Catalysis 35 (2014) 462467 465 bond activation at the CoOtpair are, respectively, 0.11 and 0.29 eV lower than at the CoOopair on the (001) and (011) surfaces, suggesting that the CoOtpairs on both the (001) and (011) surfaces are more reactive than the CoOopairs. Therefore, the CoOtpairs are the active sites for CH bond activation on the Co3O4 surfaces. In particular, the CoOtpairs are expected to be highly active for
