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Theoretical study on the Si-doped graphene as an efficient metal-free catalyst for CO oxidation
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Theoretical study on the Si-doped graphene as an efficient metal-free catalyst for CO oxidation Yanan Tang a,b,∗ , Zhiyong Liu c , Xianqi Dai a,b,c , Zongxian Yang c , Weiguang Chen a,b , Dongwei Ma d , Zhansheng Lu c a
Department of Physics and Electronic Science, Zhengzhou Normal University, Zhengzhou, Henan 450044, People’s Republic of China Quantum Materials Research Center, Zhengzhou Normal University, Henan 450044, People’s Republic of China c College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China d College of Physics and Electrical Engineering, Anyang Normal University, Anyang, Henan 455000, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 24 March 2014 Accepted 27 April 2014 Available online xxx Keywords: The first-principle methods Si-graphene CO oxidation Metal-free catalyst
a b s t r a c t The reduction of noble metal used in carbon monoxide (CO) oxidation remains a challenge. Based on the first-principle methods, the geometry, electronic structure and catalytic properties of Si-doped graphene (Si-graphene) are investigated. The Si adatom has smaller adsorption energy on pristine graphene as compared with that of the Si dopant in graphene. The large atomic radii of Si dopant in graphene can induce the local surface curvature and modulate the electronic structure through inducing the charge redistribution. Besides, the metal-free Si-graphene can weaken the CO adsorption and facilitates the O2 adsorption, thus enhancing the catalytic activity for CO oxidation. It is found that the preadsorbed O2 molecule on the Si-atom can greatly enhance the interaction with CO molecule. Moreover, the complete CO oxidation reactions on the Si-graphene include a two-step process of the Eley–Rideal (ER) reactions, in which the first step has a low energy barrier of 0.43 eV and the second step exhibits an even negligible energy barrier of 0.07 eV. The results validate the reactivity of catalysts on the atomic-scale and initiate a clue for fabricating metal-free catalysts with low cost and high activity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The CO oxidation prototype as a textbook example of catalytic reaction provides us the richness and classic heterogeneous catalysts and plays an important role in solving the growing environmental problems [1]. It is significance to seek suitable support with a high activity and lower cost, which can effectively disperse the catalysts and realize the low-temperature oxidation of CO. Various kinds of carbon materials have been examined as catalyst supports including carbon black, carbon nanotubes (CNT) [2] and graphitized carbon materials [3]. Graphene, as a result of its unique two-dimensional (2D) monolayer structure of sp2 hybridized carbon [4], exhibits many unique chemical and physical properties, such as the superior electrical conductivity [5], thermal conductivities [6], structural flexibility [7], ultrathin thickness, high surface-to-volume ratio [8] and chemical stability [9]. Metal
∗ Corresponding author at: Department of Physics and Electronic Science, Zhengzhou Normal University, Zhengzhou, Henan 450044, People’s Republic of China. Tel.: +86 371 65501661; fax: +86 371 65501661. E-mail address: [email protected] (Y. Tang).
nanoparticles on the graphene sheet have unusually high catalytic activity as electrocatalyst in fuel cells [10–14]. Recent experimental [10] and theoretical [15] results demonstrated that small Pt clusters have better catalytic activity on graphene. Yoo et al. found that the catalytic properties of sub-nano-Pt clusters can be turned via the interface interactions between the graphene and the Pt atoms [16]. Generally, the interaction between the pristine graphene (prigraphene) and the supported metal atoms or clusters is rather weak due to the strong sp2 binding between carbon atoms in the graphene sheet [17]. In comparison, the chemical doping has been proved to be an effective approach to tailor the electrical properties and chemical activities of graphene [18], thus modifying the functional characters of graphene as a support has a wide variety of applications in sensors [19], batteries [20], and catalysts [21]. Both theoretical calculation and detailed experiments shown that the substitutional dopants, e.g., B [22,23], P [24], N [25] and Si [26] atoms, have been used to substitute sp2 -hybridized carbon frameworks in graphene or carbon nanotubes (CNTs). The decoration of graphene with non-metal dopants (NMDs) can control the size and degree of catalyst dispersion, as well as the stability of catalyst particles to promote catalytic performance [27–30]. For example, Kim
http://dx.doi.org/10.1016/j.apsusc.2014.04.189 0169-4332/© 2014 Elsevier B.V. All rights reserved.
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and Jhi [31] found that the Pt nanoparticles deposited on B-doped (BG) and N-doped graphene (NG) have enhanced CO tolerance. Recently, our works shown that the highly stable Pt catalysts supported on the graphene (with different NMDs) substrates exhibit good catalytic activity for CO oxidation [32]. Besides, a single atom Pt embedded graphene systems have been used as examples for the chemical reaction of CO oxidation [17]. However, these most common Pt and Pt-based materials as catalysts have several problems including limited natural resources, high cost, toxic CO-like intermediate species [33], which prevents the development of fuel cells for large-scale commercial applications [34]. Therefore, the metal-free catalysts as good candidates for green chemistry with low emission and an efficient use can be expected, due to they are friendly to environment and exhibits good thermal conductivity [35]. As known, unsaturated atoms with lower coordination often act as active sites [36]. The strong binding between vacancies and impurities would be stable enough to be utilized in catalytic reaction [37]. In the periodic table, Si exhibits semiconductor property due to its similar electron configurations with C (2s and 2p). Besides, the Si atom is more inclined to provide free electrons into the electron system of graphene and is known to strongly prefer the sp3 like bonding. Here, it is essential for exploring the doped Si atom into graphene catalysts with lower cost and better catalytic activity for CO oxidation, in order to satisfy the further practical application. In this paper, we perform first-principles computations to explore the geometry, electronic structure and magnetic properties of the Si-doped graphene (Si-graphene), which is then used as the reactive surface for the CO oxidation. It is found that the Si-graphene exhibits high catalytic activity for the CO oxidation. The catalytic process is likely to proceed with the Eley–Rideal (ER) reaction at the starting point and is followed by the ER reaction, which provides a valuable reference for designing of metal-free graphene catalysis.
2. Model and calculation method The spin-polarized density function theory (DFT) calculations are performed using the Vienna ab initio simulation package (VASP) [38,39]. To improve the calculation efficiency, core electrons are replaced by the projector augmented wave (PAW) pseudopotentials [40]. Both the van der Waals density functional (vdW-DF) [41,42] and the Perdew, Burke, and Ernzernhof (PBE) [43] to describe the effects of dispersion interactions between the adsorption of molecules and graphene surfaces. The C 2s2p, Si 3s3p and O 2s2p states are treated explicitly as valence electrons. The graphene sheet is represented using a hexagonal supercell containing 32 carbon atoms, with a p (4 × 4) structure in the x–y plane. An energy cut off of 400 eV is used for the plane wave expansion and the convergence criterion for the electronic self-consistent iteration is set to 10−5 eV. The energy of an isolated atom or molecules is simulated using a cubic cell of 15 A˚ × 15 A˚ × 15 A˚ with one atom putting inside. The Brillouin zone (BZ) integration is sampled using a 3 × 3 × 1 centered Monkhorst-Pack (MP) grid and a -centered MP grid of 15 × 15 × 1 is used for the final density of states (DOS) calculations. ˚ which is The calculated lattice constant of graphene is 2.47 A, slightly larger than the experimental value of 2.46 A˚ [44]. The optimized C C bonds are 1.43 A˚ and then the graphene supercell based on the calculated lattice constant. In the calculations, the entire C and the doped or adsorbed molecules are allowed to relax in all directions. Bader charge analysis [45] is used to evaluate the atomic charges and electrons transfer in the reactions. The climbing image nudged elastic band method (CI-NEB) [46–48] is employed to investigate the saddle points and minimum energy paths (MEP) for the diffusion and dissociation of reaction gases on the Si-graphene surface. The distance between the graphene sheet and its images
˚ which leads to negligible interactions between the is set to 15 A, graphene sheets and their mirror images. According to the frequency calculations, the structures with no imaginary frequency correspond to the stable configurations which can be chosen as the initial state (IS) and final state (FS) in the reactions, while those with one imaginary frequency correspond to the transition states (TS). Six to twelve images are inserted in between the IS and FS, and the spring force between adjacent images is set to 5.0 eV A˚ −1 . Images are optimized until the forces on each atom are less than 0.02 eV A˚ −1 . The substituting of a single Si atom into the center of the carbon lattice is shown in Fig. 1(a). In order to ensure the most stable configuration of adsorbed molecules on the Si-graphene surface, we have performed a scan in energy of the adsorbed molecules on the Si-graphene at the possible adsorption sites, which including the top sites of Si atom and C atoms, the first neighboring top site of Si C bond, second, third and fourth neighboring top site of C C bonds in same carbon ring. The adsorption (or coadsorption) energy (Eads ) of one or two atoms or molecules, (A, representing Si, CO, O2 , CO2 or O) on a substrate (B, representing the pri-graphene or Si-graphene) is calculated using the expression: Eads = EA + EB − EAB
(1)
3. Results and discussion 3.1. Geometric and electronic property of Si-graphene The geometric structure of Si-graphene is shown in Fig. 1(a), where a Si atom is located on top of the single vacancy site, forming three bonds with the nearest C atoms. The Si dopant into graphene has a much larger Eads (7.41 eV) at the vacancy site as compared with that of the Si adsorption on the pri-graphene (0.54 eV), since the Si dopant forms strong covalent bonds with the under-coordinated C atoms due to the breaking of the C C bonds of the pentagon [49,50]. Besides, the Si adatom has larger Eads on the bridge site of the C C bond (0.54 eV) than that on the top site of carbon atom (0.33 eV) (Si atom is not adsorbed on the hollow site). The small energy difference mean that the Si adatom can easily diffuse on the pri-graphene and the corresponding pathway diffuses is from a bridge site passing through the top site and then to next bridge site. With the same electronic configuration as the C atom, the Si atom preserves its sp3 like bonding character. The Si dopant induces the local surface curvature of graphene sheet and makes the stretched C atoms deviates from the carbon sheet with a height ˚ The Si dopant with larger atomic radii protrude outside of 0.30 A. ˚ and the from the graphene plane with the dopant height of 1.20 A, ˚ which is much larger than that of the formed Si C bonds is 1.77 A, ˚ Hence, the distortion likely C C bond in the pri-graphene (1.42 A). changes some of the sp2 -like character to some covalently reactive sp3 -like character. For the defect graphene with single vacancy (SV-graphene), the total magnetic moment is determined by the contribution of the localized sp2 dangling bond states and bond states. The SV defect breaks the symmetry between the sublattices and the corresponding spin-up and spin-down channels become asymmetric, resulting in the formation of local magnetic moment near the vacancy-defect site (2.0 B ), as shown in Fig. 2(a). To gain more insight into the origin of the high stability of the Si-graphene, we investigated the DOS plots as shown in Fig. 2(a). The SV-graphene states have been strongly altered when a Si atom is doped in the system. The broadened Si states overlap with the total DOS (TDOS) of Si-graphene around the Fermi level (EF ), suggesting that the strong interaction between Si atom and adjacent C atoms. This yields three Si C covalent bonds and one bond, and thus the system exhibits nonmagnetic. Besides, the TDOS at EF is nearly zero, which is similar
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Fig. 1. Top and side views of the geometric structures for (a) Si-graphene and (b) O2 on Si-graphene. Black, blue and red balls represent C, Si and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
to that of the pri-graphene [32]. Since the Si dopant can saturate the three dangling bonds of the undercoordinated carbon atoms around the vacancy, leaving the fourth electrons as the electron of the missing carbon atom. Next, we investigate here the charge density difference (CDD) of the optimized configuration of Si-graphene as depicted in Fig. 3(a), and the corresponding contour lines in plots are drawn at 0.03 e/A˚ 3 intervals, where the solid (dashed) lines denote the charge accumulation (depletion). It is found that the much more pronounced charge density has been redistributed between the Si and C atoms around the vacancy site, and indicates that stronger covalent bonding between the Si and C atoms. The electrons are mainly located within the C atoms rather than mainly located on the Si atom, which means that the transferred electrons move from the Si dopant to the neighboring C atoms, due to the Si atom has smaller electronegativity as compared with that of the carbon atom. Therefore, the Si dopant can modify the electronic structure and chemical reactivity of graphene sheet through inducing the local charge redistribution and the deviation of the perfect sp2 hybridization in the graphene sheet. 3.2. Adsorption of gas molecules on Si-graphene Based on the most stable structure of Si-graphene substrate, we considered the possible adsorption sites (including the Si atom and its nearest C atom) and different adsorption patterns (including side-on and end-on) in order to find out the energetically favorable configurations for each adsorbate (including the O2 , CO, CO2 and O). The most stable configuration of O2 adsorption on Si-graphene is shown in Fig. 1(b), the O2 parallel to the graphene surface by forming two chemical bonds with Si atom (side on). As shown in Table 1, the adsorption energy of this configuration is 1.59 eV, which is 1.04 eV more favorable than that of the end-on configuration. ˚ Moreover, the two formed Si O bond lengths are 1.70 and 1.76 A, respectively. There is about 0.66 electrons charge transfer from the embedded Si-atom to the 2* states of O2 and subsequently lead to ˚ thus the original the elongation of the O O bond from 1.23 to 1.52 A, double bond of O2 is transformed to a single Si O bonds. Compared
Fig. 2. Spin-resolved total density of states (TDOS) and partial DOS (PDOS) (spinup labeled with ↑ and spin-down labeled with ↓) for (a) Si-graphene and (b) O2 on the Si-graphene system. The solid and dotted lines represent the TDOS of SVgraphene (or O2 on Si-graphene) and Si-graphene, the dashed and dash dotted lines represent the LDOS of Si atom and the adsorbed O2 , respectively. The vertical dotted line denotes the Fermi level.
Table 1 ˚ bond length of adsorbate The adsorption energy (Eads , eV), adsorption height (h, A), ˚ the number of electrons transferred from the Si-graphene substrate to the (L, A), adsorbate (q, e) of the most stable configuration for CO and O2 on the Si-graphene. Adsorbate
Eads
h
L
q
CO O2
0.28 1.59
1.95 1.73
1.16 1.52
0.15 0.66
Please cite this article in press as: Y. Tang, et al., Theoretical study on the Si-doped graphene as an efficient metal-free catalyst for CO oxidation, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.189
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(a)
Si
C
C
(b)
O
O
Si C
Fig. 3. The valence charge density difference plots of the (a) Si-graphene and (b) O2 on Si-graphene, respectively. Contour lines in plots are drawn at about 0.03 e/A˚ 3 intervals, and the solid (dashed) lines denote the charge accumulation (depletion).
˚ without the O2 adsorption, the with the Si C bond length (1.77 A) Si C bond is elongated from 1.77 to 1.82 A˚ after the O2 adsorption, since the transferred charge from the Si atom to O2 will weaken the Si–C interaction. This result means that the Si dopant is benefit for saturating the dangling-bonds of carbon atoms and enhancing the adsorption of O2 molecule. As shown in Fig. 2(b), it is found that the electrons mainly accumulate on O2 , where O2 -2* states are half filled. The broadened Si states strongly hybridize with O2 -2* , 5, 1 and 4 orbitals around the EF and also overlap with the TDOS. Besides, the spin-up and spin-down channels become asymmetric and the hybridization between Si and O atoms induce the whole system of magnetic moment (1.30 B ) due to the increase in the number of unpaired electrons. Electronic structure, which fundamentally determines the physical and chemical properties of a system, is directly related to the geometry of the gas molecules on graphene. We investigate here the CDD as depicted in Fig. 3(b) of the optimized configurations of the O2 molecule on the Si-graphene system. The corresponding contour lines in plots are drawn at 0.03 e/A˚ 3 intervals. Unlike the sp2 hybridization in the graphene with each carbon atom bonded with three other carbon atoms, in the Si-graphene system, the Si dopant bonds with three neighboring carbon atoms and establishes the sp3 like hybridization. It is found that the charge redistribution after the adsorption of O2 on the Si-graphene, where the solid (dashed) lines
Fig. 4. The minimum energy profiles and the configurations of different states for CO oxidation reaction on the Si-graphene, (a) CO + O2 with the ER reaction and (b) CO + Oads with the ER reaction. Red, blue and black balls represent O, Si and C atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
denote the charge accumulation (depletion). The electrons deplete from the vicinity of the Si C atoms and accumulate in the vicinity of the O O bond or the O Si interface. This result indicates that the transferred electrons move from the Si dopant to the adsorbed O2 . Hence, the partly transferred electrons of Si dopant are used to saturate the three dangling-bond states of carbon atoms and the rest are transferred to the adsorbed gas molecule. For the adsorption of CO on Si-graphene, the end-on configuration has adsorption energy of 0.28 eV, which is greatly less favorable than of O2 molecule. The Si CO distance is 1.95 A˚ and the C O ˚ Compared with the length of CO elongate from 1.14 to 1.16 A. ˚ with the O2 adsorption, the CO adsorpSi C bond length (1.82 A) ˚ because the tion has less effect on the Si C bond length (1.78 A), adsorbed CO has smaller adsorption energy and gains few electrons. Moreover, the calculated result shown that a peroxy-type O1–O2–C–O complex (intermediate states, MS) is formed above the Si-graphene surface, as shown in Fig. 4(a). The distance between ˚ where one of the O1–O2–C–O complex and Si-graphene is 1.58 A, O-atom of adsorbed O2 escapes from the Si-atom and binds with CO molecule. The adsorption energy of O1–O2–C–O complex on the Si-graphene is 2.87 eV, which is larger than that of individual
Please cite this article in press as: Y. Tang, et al., Theoretical study on the Si-doped graphene as an efficient metal-free catalyst for CO oxidation, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.189
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Y. Tang et al. / Applied Surface Science xxx (2014) xxx–xxx Table 2 Structural parameters of the CO oxidation on the Si-graphene at each state along the MEP (a) CO + O2 → OOCO → CO2 + Oads , (b) CO + O → CO2 , IS, TS, MS and FS are displayed in Fig. 4(a) and (b). IS
TS
MS
FS
(a) dSi–O1 dO1–O2 dC–O2 dC–O
1.70 1.52 3.12 1.14
1.70 1.41 1.91 1.15
1.70 1.48 1.37 1.21
1.57 3.14 1.17 1.18
(b) dSi–O1 dC–O1 dC–O
1.57 2.47 1.15
1.60 2.00 1.16
2.91 1.18 1.17
gas molecule, resulting in the CO molecule is energetically favorable to be attached to the nearest C-atom after a preadsorbed O2 molecule on the Si-atom. In addition, the adsorption energy of CO2 (0.11 eV) on the Si-graphene is much smaller than that of O atom (5.38 eV), suggesting that a produce CO2 molecule is more easily desorbed from the Si-graphene and a chemisorbed atomic O (Oads ) can proceed for the CO oxidation. These results reveal that the Sigraphene can strongly bind the reactants (O2 and O atom), and release the CO2 product easily, which may facilitate the catalytic processes of CO oxidation. 3.3. The CO oxidation reaction on Si-graphene There are two well-established reaction mechanisms for the adsorbed CO and O2 , namely the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms [51,52]. For the ER mechanism, the gas-phase CO molecule approaches the already-activated O2 . The LH mechanism involves the coadsorption of CO and O2 molecules before reaction. Generally, the gas molecule adsorption ability determines the reaction pathways on the catalyst [17]. Due to the much larger adsorption energy of O2 (1.59 eV) as compared with that of CO (0.28 eV) on the Si-graphene, the Si atom could be dominantly covered by O2 if a CO/O2 mixture is injected as the reaction gas. Besides, the elongated O O bond of O2 adsorbed on the Si-graphene, and the preadsorbed O2 molecule on Si-graphene can greatly enhance the interaction with CO molecule. The above results indicate that the adsorbed O2 is efficiently activated for the catalytic oxidation CO reaction. Meanwhile, the catalytic process of CO oxidation on Si-graphene through the LH mechanism is almost impossible or proceeds with great difficulty. Therefore, we will study the favorable reaction processes of CO oxidation on the Si-graphene through ER mechanism. For the ER reaction, the atomic configurations at each state along the reaction pathways are displayed in Fig. 4(a), and the corresponding structural parameters for IS, TS, MS and FS are shown in Table 2(a). The configuration with a physisorbed CO nearby the preadsorbed O2 on the Si-graphene is selected as the IS. In this structure, the distance between CO and O2 is 3.12 A˚ and the bond ˚ respectively. When the CO lengths of CO and O2 is 1.14 and 1.52 A, molecule approaches the activated O2 , the O O bond is elongated ˚ Further the to 1.41 A˚ and the CO–O2 distance is decreased to 1.91 A. system reaches the TS, the corresponding energy barrier of TS along the reaction pathway is estimated to be 0.43 eV, where a peroxotype O1–O2–C–O complex is formed over the Si atom, and the bond ˚ respectively. lengths of O2 and CO are elongated to 1.48 and 1.21 A, Passing over the MS, the reaction processes undergo without an energy barrier. The O O bond is broken and a CO2 molecule is formed, leaving an atomic Oads adsorbed on the Si atom. The formed CO2 molecule has a very weak interaction on the Si-graphene substrate (0.11 eV), thus it can easily desorb from the reactive site at room temperature.
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In the following steps, we check whether the atomic Oads is active for CO oxidation through the ER reaction. As shown in Fig. 4(b), the configuration with a CO molecule more than 2.47 A˚ away from the preadsorbed Oads (5.38 eV) on the Si atom is chosen as the IS. The FS is simply set to the configuration with a CO2 adsorbed on the Si-graphene. When the carbon atom of CO approaches the Oads , a TS with the CO–Oads distance of 2.00 A˚ is formed, the corresponding structural parameters are shown in Table 2(b). It is found that the ER process has a small energy barrier (Ebar2 = 0.07 eV), which is much smaller than the energy barrier for the first CO oxidation. Hence, the complete CO oxidation reactions on Pt/Si-graphene include a two-step process: the ER reaction (CO + O2 → OOCO → CO2 + Oads ) as a starting step, followed by the ER reaction (CO + Oads → CO2 ). It is found that the reaction processes of CO oxidation on Sigraphene is different from those on the Pt-graphene [17] and Pt/Si-graphene [32], because of the adsorption energy of O2 is much larger than that of CO and the preadsorbed O2 molecule on the Si-atom can greatly enhance the adsorption of CO molecule. Besides, the relatively low energy barrier for the catalyzed reaction of CO oxidation on Si-graphene (0.43 eV, 0.07 eV), which is smaller than that on the Pt-graphene (0.58 eV) and approximate the energy barrier on the Pt/Si-graphene (0.41 eV), thus the energy values of metal-free Si-graphene are quite close to those processes using noble metal Pt catalyst. In general, the CO oxidation with a reaction barrier of smaller than 0.5 eV is expected to occur at room temperature [53]. Hence, the sequential processes of CO oxidation on the Si-graphene are more likely to proceed rapidly because of the low activation barriers involved. These results would give a suggestion that the Si-graphene is an efficient and high activity of the metal-free catalyst for CO oxidation reaction. 4. Conclusion Using the first-principles calculations, we have explored the possibility of Si-doped graphene as a potential metal-free catalyst for CO oxidation by O2 adsorption. The Si dopants can tune the flexible character by the changing surface curvature and improve the chemical reactivity of the graphene sheet due to the inducing local charge redistribution. The catalytic process is likely to proceed with the ER reaction at the starting point for CO oxidation by O2 with a low activation barrier (0.43 eV). Then, the leaving Oads can be pulled away from the Si atom by the subsequent CO (CO + Oads → CO2 , 0.07 eV). The energy barriers of CO oxidation are quite close to those processes using noble metal Pt catalyst. Hence, these results indicate that the Si-graphene for CO oxidation exhibits high efficiency and activity in the catalytic process, which open a clue for fabricating metal-free catalyst in energy-related devices. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174070, 11347186, 11304084 and 11147006), and the Science Fund of Educational Department of Henan Province (Grant Nos. 14B140019, 14A140015, 14A140010 and 13A140347). References [1] H.J. Freund, G. Meijer, M. Scheffler, R. Schlogl, M. Wolf, CO oxidation as a prototypical reaction for heterogeneous processes, Angew. Chem. Int. Ed. 50 (2011) 10064–10094. [2] L. Li, Y. Xing, Electrochemical durability of carbon nanotubes in noncatalyzed and catalyzed oxidations, J. Electrochem. Soc. 153 (2006) A1823–A1828. [3] Y. Zhou, R. Pasquarelli, T. Holme, J. Berry, D. Ginley, R. O’Hayre, Improving PEM fuel cell catalyst activity and durability using nitrogen-doped carbon supports: observations from model Pt/HOPG systems, J. Mater. Chem. 19 (2009) 7830–7838.
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Theoretical study on the Si-doped graphene as an efficient metal-free catalyst for CO oxidation Yanan Tang a,b,∗ , Zhiyong Liu c , Xianqi Dai a,b,c , Zongxian Yang c , Weiguang Chen a,b , Dongwei Ma d , Zhansheng Lu c a
Department of Physics and Electronic Science, Zhengzhou Normal University, Zhengzhou, Henan 450044, People’s Republic of China Quantum Materials Research Center, Zhengzhou Normal University, Henan 450044, People’s Republic of China c College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China d College of Physics and Electrical Engineering, Anyang Normal University, Anyang, Henan 455000, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 24 March 2014 Accepted 27 April 2014 Available online xxx Keywords: The first-principle methods Si-graphene CO oxidation Metal-free catalyst
a b s t r a c t The reduction of noble metal used in carbon monoxide (CO) oxidation remains a challenge. Based on the first-principle methods, the geometry, electronic structure and catalytic properties of Si-doped graphene (Si-graphene) are investigated. The Si adatom has smaller adsorption energy on pristine graphene as compared with that of the Si dopant in graphene. The large atomic radii of Si dopant in graphene can induce the local surface curvature and modulate the electronic structure through inducing the charge redistribution. Besides, the metal-free Si-graphene can weaken the CO adsorption and facilitates the O2 adsorption, thus enhancing the catalytic activity for CO oxidation. It is found that the preadsorbed O2 molecule on the Si-atom can greatly enhance the interaction with CO molecule. Moreover, the complete CO oxidation reactions on the Si-graphene include a two-step process of the Eley–Rideal (ER) reactions, in which the first step has a low energy barrier of 0.43 eV and the second step exhibits an even negligible energy barrier of 0.07 eV. The results validate the reactivity of catalysts on the atomic-scale and initiate a clue for fabricating metal-free catalysts with low cost and high activity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The CO oxidation prototype as a textbook example of catalytic reaction provides us the richness and classic heterogeneous catalysts and plays an important role in solving the growing environmental problems [1]. It is significance to seek suitable support with a high activity and lower cost, which can effectively disperse the catalysts and realize the low-temperature oxidation of CO. Various kinds of carbon materials have been examined as catalyst supports including carbon black, carbon nanotubes (CNT) [2] and graphitized carbon materials [3]. Graphene, as a result of its unique two-dimensional (2D) monolayer structure of sp2 hybridized carbon [4], exhibits many unique chemical and physical properties, such as the superior electrical conductivity [5], thermal conductivities [6], structural flexibility [7], ultrathin thickness, high surface-to-volume ratio [8] and chemical stability [9]. Metal
∗ Corresponding author at: Department of Physics and Electronic Science, Zhengzhou Normal University, Zhengzhou, Henan 450044, People’s Republic of China. Tel.: +86 371 65501661; fax: +86 371 65501661. E-mail address: [email protected] (Y. Tang).
nanoparticles on the graphene sheet have unusually high catalytic activity as electrocatalyst in fuel cells [10–14]. Recent experimental [10] and theoretical [15] results demonstrated that small Pt clusters have better catalytic activity on graphene. Yoo et al. found that the catalytic properties of sub-nano-Pt clusters can be turned via the interface interactions between the graphene and the Pt atoms [16]. Generally, the interaction between the pristine graphene (prigraphene) and the supported metal atoms or clusters is rather weak due to the strong sp2 binding between carbon atoms in the graphene sheet [17]. In comparison, the chemical doping has been proved to be an effective approach to tailor the electrical properties and chemical activities of graphene [18], thus modifying the functional characters of graphene as a support has a wide variety of applications in sensors [19], batteries [20], and catalysts [21]. Both theoretical calculation and detailed experiments shown that the substitutional dopants, e.g., B [22,23], P [24], N [25] and Si [26] atoms, have been used to substitute sp2 -hybridized carbon frameworks in graphene or carbon nanotubes (CNTs). The decoration of graphene with non-metal dopants (NMDs) can control the size and degree of catalyst dispersion, as well as the stability of catalyst particles to promote catalytic performance [27–30]. For example, Kim
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and Jhi [31] found that the Pt nanoparticles deposited on B-doped (BG) and N-doped graphene (NG) have enhanced CO tolerance. Recently, our works shown that the highly stable Pt catalysts supported on the graphene (with different NMDs) substrates exhibit good catalytic activity for CO oxidation [32]. Besides, a single atom Pt embedded graphene systems have been used as examples for the chemical reaction of CO oxidation [17]. However, these most common Pt and Pt-based materials as catalysts have several problems including limited natural resources, high cost, toxic CO-like intermediate species [33], which prevents the development of fuel cells for large-scale commercial applications [34]. Therefore, the metal-free catalysts as good candidates for green chemistry with low emission and an efficient use can be expected, due to they are friendly to environment and exhibits good thermal conductivity [35]. As known, unsaturated atoms with lower coordination often act as active sites [36]. The strong binding between vacancies and impurities would be stable enough to be utilized in catalytic reaction [37]. In the periodic table, Si exhibits semiconductor property due to its similar electron configurations with C (2s and 2p). Besides, the Si atom is more inclined to provide free electrons into the electron system of graphene and is known to strongly prefer the sp3 like bonding. Here, it is essential for exploring the doped Si atom into graphene catalysts with lower cost and better catalytic activity for CO oxidation, in order to satisfy the further practical application. In this paper, we perform first-principles computations to explore the geometry, electronic structure and magnetic properties of the Si-doped graphene (Si-graphene), which is then used as the reactive surface for the CO oxidation. It is found that the Si-graphene exhibits high catalytic activity for the CO oxidation. The catalytic process is likely to proceed with the Eley–Rideal (ER) reaction at the starting point and is followed by the ER reaction, which provides a valuable reference for designing of metal-free graphene catalysis.
2. Model and calculation method The spin-polarized density function theory (DFT) calculations are performed using the Vienna ab initio simulation package (VASP) [38,39]. To improve the calculation efficiency, core electrons are replaced by the projector augmented wave (PAW) pseudopotentials [40]. Both the van der Waals density functional (vdW-DF) [41,42] and the Perdew, Burke, and Ernzernhof (PBE) [43] to describe the effects of dispersion interactions between the adsorption of molecules and graphene surfaces. The C 2s2p, Si 3s3p and O 2s2p states are treated explicitly as valence electrons. The graphene sheet is represented using a hexagonal supercell containing 32 carbon atoms, with a p (4 × 4) structure in the x–y plane. An energy cut off of 400 eV is used for the plane wave expansion and the convergence criterion for the electronic self-consistent iteration is set to 10−5 eV. The energy of an isolated atom or molecules is simulated using a cubic cell of 15 A˚ × 15 A˚ × 15 A˚ with one atom putting inside. The Brillouin zone (BZ) integration is sampled using a 3 × 3 × 1 centered Monkhorst-Pack (MP) grid and a -centered MP grid of 15 × 15 × 1 is used for the final density of states (DOS) calculations. ˚ which is The calculated lattice constant of graphene is 2.47 A, slightly larger than the experimental value of 2.46 A˚ [44]. The optimized C C bonds are 1.43 A˚ and then the graphene supercell based on the calculated lattice constant. In the calculations, the entire C and the doped or adsorbed molecules are allowed to relax in all directions. Bader charge analysis [45] is used to evaluate the atomic charges and electrons transfer in the reactions. The climbing image nudged elastic band method (CI-NEB) [46–48] is employed to investigate the saddle points and minimum energy paths (MEP) for the diffusion and dissociation of reaction gases on the Si-graphene surface. The distance between the graphene sheet and its images
˚ which leads to negligible interactions between the is set to 15 A, graphene sheets and their mirror images. According to the frequency calculations, the structures with no imaginary frequency correspond to the stable configurations which can be chosen as the initial state (IS) and final state (FS) in the reactions, while those with one imaginary frequency correspond to the transition states (TS). Six to twelve images are inserted in between the IS and FS, and the spring force between adjacent images is set to 5.0 eV A˚ −1 . Images are optimized until the forces on each atom are less than 0.02 eV A˚ −1 . The substituting of a single Si atom into the center of the carbon lattice is shown in Fig. 1(a). In order to ensure the most stable configuration of adsorbed molecules on the Si-graphene surface, we have performed a scan in energy of the adsorbed molecules on the Si-graphene at the possible adsorption sites, which including the top sites of Si atom and C atoms, the first neighboring top site of Si C bond, second, third and fourth neighboring top site of C C bonds in same carbon ring. The adsorption (or coadsorption) energy (Eads ) of one or two atoms or molecules, (A, representing Si, CO, O2 , CO2 or O) on a substrate (B, representing the pri-graphene or Si-graphene) is calculated using the expression: Eads = EA + EB − EAB
(1)
3. Results and discussion 3.1. Geometric and electronic property of Si-graphene The geometric structure of Si-graphene is shown in Fig. 1(a), where a Si atom is located on top of the single vacancy site, forming three bonds with the nearest C atoms. The Si dopant into graphene has a much larger Eads (7.41 eV) at the vacancy site as compared with that of the Si adsorption on the pri-graphene (0.54 eV), since the Si dopant forms strong covalent bonds with the under-coordinated C atoms due to the breaking of the C C bonds of the pentagon [49,50]. Besides, the Si adatom has larger Eads on the bridge site of the C C bond (0.54 eV) than that on the top site of carbon atom (0.33 eV) (Si atom is not adsorbed on the hollow site). The small energy difference mean that the Si adatom can easily diffuse on the pri-graphene and the corresponding pathway diffuses is from a bridge site passing through the top site and then to next bridge site. With the same electronic configuration as the C atom, the Si atom preserves its sp3 like bonding character. The Si dopant induces the local surface curvature of graphene sheet and makes the stretched C atoms deviates from the carbon sheet with a height ˚ The Si dopant with larger atomic radii protrude outside of 0.30 A. ˚ and the from the graphene plane with the dopant height of 1.20 A, ˚ which is much larger than that of the formed Si C bonds is 1.77 A, ˚ Hence, the distortion likely C C bond in the pri-graphene (1.42 A). changes some of the sp2 -like character to some covalently reactive sp3 -like character. For the defect graphene with single vacancy (SV-graphene), the total magnetic moment is determined by the contribution of the localized sp2 dangling bond states and bond states. The SV defect breaks the symmetry between the sublattices and the corresponding spin-up and spin-down channels become asymmetric, resulting in the formation of local magnetic moment near the vacancy-defect site (2.0 B ), as shown in Fig. 2(a). To gain more insight into the origin of the high stability of the Si-graphene, we investigated the DOS plots as shown in Fig. 2(a). The SV-graphene states have been strongly altered when a Si atom is doped in the system. The broadened Si states overlap with the total DOS (TDOS) of Si-graphene around the Fermi level (EF ), suggesting that the strong interaction between Si atom and adjacent C atoms. This yields three Si C covalent bonds and one bond, and thus the system exhibits nonmagnetic. Besides, the TDOS at EF is nearly zero, which is similar
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Fig. 1. Top and side views of the geometric structures for (a) Si-graphene and (b) O2 on Si-graphene. Black, blue and red balls represent C, Si and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
to that of the pri-graphene [32]. Since the Si dopant can saturate the three dangling bonds of the undercoordinated carbon atoms around the vacancy, leaving the fourth electrons as the electron of the missing carbon atom. Next, we investigate here the charge density difference (CDD) of the optimized configuration of Si-graphene as depicted in Fig. 3(a), and the corresponding contour lines in plots are drawn at 0.03 e/A˚ 3 intervals, where the solid (dashed) lines denote the charge accumulation (depletion). It is found that the much more pronounced charge density has been redistributed between the Si and C atoms around the vacancy site, and indicates that stronger covalent bonding between the Si and C atoms. The electrons are mainly located within the C atoms rather than mainly located on the Si atom, which means that the transferred electrons move from the Si dopant to the neighboring C atoms, due to the Si atom has smaller electronegativity as compared with that of the carbon atom. Therefore, the Si dopant can modify the electronic structure and chemical reactivity of graphene sheet through inducing the local charge redistribution and the deviation of the perfect sp2 hybridization in the graphene sheet. 3.2. Adsorption of gas molecules on Si-graphene Based on the most stable structure of Si-graphene substrate, we considered the possible adsorption sites (including the Si atom and its nearest C atom) and different adsorption patterns (including side-on and end-on) in order to find out the energetically favorable configurations for each adsorbate (including the O2 , CO, CO2 and O). The most stable configuration of O2 adsorption on Si-graphene is shown in Fig. 1(b), the O2 parallel to the graphene surface by forming two chemical bonds with Si atom (side on). As shown in Table 1, the adsorption energy of this configuration is 1.59 eV, which is 1.04 eV more favorable than that of the end-on configuration. ˚ Moreover, the two formed Si O bond lengths are 1.70 and 1.76 A, respectively. There is about 0.66 electrons charge transfer from the embedded Si-atom to the 2* states of O2 and subsequently lead to ˚ thus the original the elongation of the O O bond from 1.23 to 1.52 A, double bond of O2 is transformed to a single Si O bonds. Compared
Fig. 2. Spin-resolved total density of states (TDOS) and partial DOS (PDOS) (spinup labeled with ↑ and spin-down labeled with ↓) for (a) Si-graphene and (b) O2 on the Si-graphene system. The solid and dotted lines represent the TDOS of SVgraphene (or O2 on Si-graphene) and Si-graphene, the dashed and dash dotted lines represent the LDOS of Si atom and the adsorbed O2 , respectively. The vertical dotted line denotes the Fermi level.
Table 1 ˚ bond length of adsorbate The adsorption energy (Eads , eV), adsorption height (h, A), ˚ the number of electrons transferred from the Si-graphene substrate to the (L, A), adsorbate (q, e) of the most stable configuration for CO and O2 on the Si-graphene. Adsorbate
Eads
h
L
q
CO O2
0.28 1.59
1.95 1.73
1.16 1.52
0.15 0.66
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(a)
Si
C
C
(b)
O
O
Si C
Fig. 3. The valence charge density difference plots of the (a) Si-graphene and (b) O2 on Si-graphene, respectively. Contour lines in plots are drawn at about 0.03 e/A˚ 3 intervals, and the solid (dashed) lines denote the charge accumulation (depletion).
˚ without the O2 adsorption, the with the Si C bond length (1.77 A) Si C bond is elongated from 1.77 to 1.82 A˚ after the O2 adsorption, since the transferred charge from the Si atom to O2 will weaken the Si–C interaction. This result means that the Si dopant is benefit for saturating the dangling-bonds of carbon atoms and enhancing the adsorption of O2 molecule. As shown in Fig. 2(b), it is found that the electrons mainly accumulate on O2 , where O2 -2* states are half filled. The broadened Si states strongly hybridize with O2 -2* , 5, 1 and 4 orbitals around the EF and also overlap with the TDOS. Besides, the spin-up and spin-down channels become asymmetric and the hybridization between Si and O atoms induce the whole system of magnetic moment (1.30 B ) due to the increase in the number of unpaired electrons. Electronic structure, which fundamentally determines the physical and chemical properties of a system, is directly related to the geometry of the gas molecules on graphene. We investigate here the CDD as depicted in Fig. 3(b) of the optimized configurations of the O2 molecule on the Si-graphene system. The corresponding contour lines in plots are drawn at 0.03 e/A˚ 3 intervals. Unlike the sp2 hybridization in the graphene with each carbon atom bonded with three other carbon atoms, in the Si-graphene system, the Si dopant bonds with three neighboring carbon atoms and establishes the sp3 like hybridization. It is found that the charge redistribution after the adsorption of O2 on the Si-graphene, where the solid (dashed) lines
Fig. 4. The minimum energy profiles and the configurations of different states for CO oxidation reaction on the Si-graphene, (a) CO + O2 with the ER reaction and (b) CO + Oads with the ER reaction. Red, blue and black balls represent O, Si and C atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
denote the charge accumulation (depletion). The electrons deplete from the vicinity of the Si C atoms and accumulate in the vicinity of the O O bond or the O Si interface. This result indicates that the transferred electrons move from the Si dopant to the adsorbed O2 . Hence, the partly transferred electrons of Si dopant are used to saturate the three dangling-bond states of carbon atoms and the rest are transferred to the adsorbed gas molecule. For the adsorption of CO on Si-graphene, the end-on configuration has adsorption energy of 0.28 eV, which is greatly less favorable than of O2 molecule. The Si CO distance is 1.95 A˚ and the C O ˚ Compared with the length of CO elongate from 1.14 to 1.16 A. ˚ with the O2 adsorption, the CO adsorpSi C bond length (1.82 A) ˚ because the tion has less effect on the Si C bond length (1.78 A), adsorbed CO has smaller adsorption energy and gains few electrons. Moreover, the calculated result shown that a peroxy-type O1–O2–C–O complex (intermediate states, MS) is formed above the Si-graphene surface, as shown in Fig. 4(a). The distance between ˚ where one of the O1–O2–C–O complex and Si-graphene is 1.58 A, O-atom of adsorbed O2 escapes from the Si-atom and binds with CO molecule. The adsorption energy of O1–O2–C–O complex on the Si-graphene is 2.87 eV, which is larger than that of individual
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Y. Tang et al. / Applied Surface Science xxx (2014) xxx–xxx Table 2 Structural parameters of the CO oxidation on the Si-graphene at each state along the MEP (a) CO + O2 → OOCO → CO2 + Oads , (b) CO + O → CO2 , IS, TS, MS and FS are displayed in Fig. 4(a) and (b). IS
TS
MS
FS
(a) dSi–O1 dO1–O2 dC–O2 dC–O
1.70 1.52 3.12 1.14
1.70 1.41 1.91 1.15
1.70 1.48 1.37 1.21
1.57 3.14 1.17 1.18
(b) dSi–O1 dC–O1 dC–O
1.57 2.47 1.15
1.60 2.00 1.16
2.91 1.18 1.17
gas molecule, resulting in the CO molecule is energetically favorable to be attached to the nearest C-atom after a preadsorbed O2 molecule on the Si-atom. In addition, the adsorption energy of CO2 (0.11 eV) on the Si-graphene is much smaller than that of O atom (5.38 eV), suggesting that a produce CO2 molecule is more easily desorbed from the Si-graphene and a chemisorbed atomic O (Oads ) can proceed for the CO oxidation. These results reveal that the Sigraphene can strongly bind the reactants (O2 and O atom), and release the CO2 product easily, which may facilitate the catalytic processes of CO oxidation. 3.3. The CO oxidation reaction on Si-graphene There are two well-established reaction mechanisms for the adsorbed CO and O2 , namely the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms [51,52]. For the ER mechanism, the gas-phase CO molecule approaches the already-activated O2 . The LH mechanism involves the coadsorption of CO and O2 molecules before reaction. Generally, the gas molecule adsorption ability determines the reaction pathways on the catalyst [17]. Due to the much larger adsorption energy of O2 (1.59 eV) as compared with that of CO (0.28 eV) on the Si-graphene, the Si atom could be dominantly covered by O2 if a CO/O2 mixture is injected as the reaction gas. Besides, the elongated O O bond of O2 adsorbed on the Si-graphene, and the preadsorbed O2 molecule on Si-graphene can greatly enhance the interaction with CO molecule. The above results indicate that the adsorbed O2 is efficiently activated for the catalytic oxidation CO reaction. Meanwhile, the catalytic process of CO oxidation on Si-graphene through the LH mechanism is almost impossible or proceeds with great difficulty. Therefore, we will study the favorable reaction processes of CO oxidation on the Si-graphene through ER mechanism. For the ER reaction, the atomic configurations at each state along the reaction pathways are displayed in Fig. 4(a), and the corresponding structural parameters for IS, TS, MS and FS are shown in Table 2(a). The configuration with a physisorbed CO nearby the preadsorbed O2 on the Si-graphene is selected as the IS. In this structure, the distance between CO and O2 is 3.12 A˚ and the bond ˚ respectively. When the CO lengths of CO and O2 is 1.14 and 1.52 A, molecule approaches the activated O2 , the O O bond is elongated ˚ Further the to 1.41 A˚ and the CO–O2 distance is decreased to 1.91 A. system reaches the TS, the corresponding energy barrier of TS along the reaction pathway is estimated to be 0.43 eV, where a peroxotype O1–O2–C–O complex is formed over the Si atom, and the bond ˚ respectively. lengths of O2 and CO are elongated to 1.48 and 1.21 A, Passing over the MS, the reaction processes undergo without an energy barrier. The O O bond is broken and a CO2 molecule is formed, leaving an atomic Oads adsorbed on the Si atom. The formed CO2 molecule has a very weak interaction on the Si-graphene substrate (0.11 eV), thus it can easily desorb from the reactive site at room temperature.
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In the following steps, we check whether the atomic Oads is active for CO oxidation through the ER reaction. As shown in Fig. 4(b), the configuration with a CO molecule more than 2.47 A˚ away from the preadsorbed Oads (5.38 eV) on the Si atom is chosen as the IS. The FS is simply set to the configuration with a CO2 adsorbed on the Si-graphene. When the carbon atom of CO approaches the Oads , a TS with the CO–Oads distance of 2.00 A˚ is formed, the corresponding structural parameters are shown in Table 2(b). It is found that the ER process has a small energy barrier (Ebar2 = 0.07 eV), which is much smaller than the energy barrier for the first CO oxidation. Hence, the complete CO oxidation reactions on Pt/Si-graphene include a two-step process: the ER reaction (CO + O2 → OOCO → CO2 + Oads ) as a starting step, followed by the ER reaction (CO + Oads → CO2 ). It is found that the reaction processes of CO oxidation on Sigraphene is different from those on the Pt-graphene [17] and Pt/Si-graphene [32], because of the adsorption energy of O2 is much larger than that of CO and the preadsorbed O2 molecule on the Si-atom can greatly enhance the adsorption of CO molecule. Besides, the relatively low energy barrier for the catalyzed reaction of CO oxidation on Si-graphene (0.43 eV, 0.07 eV), which is smaller than that on the Pt-graphene (0.58 eV) and approximate the energy barrier on the Pt/Si-graphene (0.41 eV), thus the energy values of metal-free Si-graphene are quite close to those processes using noble metal Pt catalyst. In general, the CO oxidation with a reaction barrier of smaller than 0.5 eV is expected to occur at room temperature [53]. Hence, the sequential processes of CO oxidation on the Si-graphene are more likely to proceed rapidly because of the low activation barriers involved. These results would give a suggestion that the Si-graphene is an efficient and high activity of the metal-free catalyst for CO oxidation reaction. 4. Conclusion Using the first-principles calculations, we have explored the possibility of Si-doped graphene as a potential metal-free catalyst for CO oxidation by O2 adsorption. The Si dopants can tune the flexible character by the changing surface curvature and improve the chemical reactivity of the graphene sheet due to the inducing local charge redistribution. The catalytic process is likely to proceed with the ER reaction at the starting point for CO oxidation by O2 with a low activation barrier (0.43 eV). Then, the leaving Oads can be pulled away from the Si atom by the subsequent CO (CO + Oads → CO2 , 0.07 eV). The energy barriers of CO oxidation are quite close to those processes using noble metal Pt catalyst. Hence, these results indicate that the Si-graphene for CO oxidation exhibits high efficiency and activity in the catalytic process, which open a clue for fabricating metal-free catalyst in energy-related devices. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174070, 11347186, 11304084 and 11147006), and the Science Fund of Educational Department of Henan Province (Grant Nos. 14B140019, 14A140015, 14A140010 and 13A140347). References [1] H.J. Freund, G. Meijer, M. Scheffler, R. Schlogl, M. Wolf, CO oxidation as a prototypical reaction for heterogeneous processes, Angew. Chem. Int. Ed. 50 (2011) 10064–10094. [2] L. Li, Y. Xing, Electrochemical durability of carbon nanotubes in noncatalyzed and catalyzed oxidations, J. Electrochem. Soc. 153 (2006) A1823–A1828. [3] Y. Zhou, R. Pasquarelli, T. Holme, J. Berry, D. Ginley, R. O’Hayre, Improving PEM fuel cell catalyst activity and durability using nitrogen-doped carbon supports: observations from model Pt/HOPG systems, J. Mater. Chem. 19 (2009) 7830–7838.
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