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Nitrogen-doped penta-graphene as a superior catalytic activity for CO oxidation
Accepted Manuscript Nitrogen-doped penta-graphene as a superior catalytic activity for CO oxidation Ranganathan Krishnan, Shiuan-Yau Wu, Hsin-Tsung Chen PII:
S0008-6223(18)30188-X
DOI:
10.1016/j.carbon.2018.02.064
Reference:
CARBON 12900
To appear in:
Carbon
Received Date: 5 January 2018 Revised Date:
12 February 2018
Accepted Date: 14 February 2018
Please cite this article as: R. Krishnan, S.-Y. Wu, H.-T. Chen, Nitrogen-doped penta-graphene as a superior catalytic activity for CO oxidation, Carbon (2018), doi: 10.1016/j.carbon.2018.02.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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We performed a systematic study of CO oxidation on the nitrogen-doped penta-graphene by utilizing spin-polarized density functional theory calculations. The results manifest that the nitrogen-doped penta-graphene displays very high catalytic
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low-temperature CO oxidation.
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activity and is more competitive with many metal-based and carbon-base catalysts for
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Nitrogen-doped Penta-Graphene as a Superior Catalytic Activity for CO Oxidation
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Ranganathan Krishnan, Shiuan-Yau Wu, and Hsin-Tsung Chen*
Department of Chemistry, Chung Yuan Christian University, Chungli District, Taoyuan
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City, 32023, Taiwan
*Corresponding author. E-mails: [email protected] (H.-T.C.); Tel: +886-3-265-3324
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Abstract: We performed a systematic study of carbon monoxide oxidation on the nitrogen-doped penta-graphene by utilizing spin-polarized density functional theory calculations. The
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possible reaction pathways via both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms were illustrated. The results manifest that the nitrogen-doped penta-graphene displays very high catalytic activity and is more competitive with many metal-based and carbon-based catalysts for low-temperature CO oxidation by reason of the very small
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energy barrier of the rate-limiting step (there is no activation energy for the ER mechanism and only 0.33 eV for the LH mechanism). This study opens a new avenue for
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the design of 2-D graphene-based materials and provides valuable clues for developing
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catalytic activity carbon materials for low-temperature CO oxidations.
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1. Introduction Catalytic activity of carbon monoxide (CO) oxidation has attracted much interest over the past few decades because of its settling the rising environmental issue caused by CO emission from vehicle exhaust, industrial processes, low pollution applications1-5 and
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its important role removing CO poisoning from hydrogen fuel.6-12 The developing heterogeneous catalysts for CO oxidation process becomes more critical.13-17 Previous studies have been made by using metals (Au-Pd,18-20 Au,21-22 Pt,8, 23-24 Ru, Rh, Os, Ir,24 Pd,11,
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and Ag26) and metal-oxides (CeO2-,27-30 TiO2-,31 MnO2,32 and AuO2-based
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materials33) as catalysts for CO oxidation. However, this metal-based catalyst is relatively rare, very expensive and usually required at high temperature for efficient operation. It is
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necessary to develop alternative non-precious catalyst to replace the metal-based catalyst. Reducing the use of metal is a major target in this work for the new technology. The discovery of carbon-based materials such as fullerene, carbon nanotube, graphene, graphdiyne, and hierarchical 3-D carbon materials has spread out exciting opportunities for the design of robust catalysts for large-scale practical applications because of their lightweight, high surface area, and unique electronic properties. Metals
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decorated on the carbon-based materials for example, Au-embedded graphene,34 Ptembedded graphenes,35 Fe-embedded graphene,36 have been investigated to be activity for CO oxidation. A novel class of metal-free catalysts based on carbon materials, which has been demonstrated as effective catalysts, could dramatically diminish the cost.37-45
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Recently, Wang et al. have proposed a new two-dimensional carbon allotrope called penta-graphene (PG), which only consists of pentagonal configurations, has mechanically,
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dynamically and thermally stable by a theoretical study.46 The variables of PG and its derivatives are anticipated to have spacious applications in the fields of catalysts, nanomechanics, and nanoelectronics. These advantages make PG and its derivatives could be potential materials for CO catalytic oxidation. Doping nitrogen atoms in the carbon-based materials, which creates an electron-deficient C+ atom on the adjacent nitrogen atom, has been to be the approach to increase the catalytic performance.37, 42, 47-51 The N-doped penta-graphene (penta-CN2) is also thermally, mechanically, and dynamically stable found by Wang et al.52 To the best of our knowledge, no investigation on CO oxidation catalyzed by the N-doped penta-graphene has been explored. 3
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In the present work, we performed a comprehensive investigation of CO oxidation on nitrogen-doped penta-graphene by spin-polarized first-principles calculations. The possible mechanisms of CO oxidation including Eley-Rideal (ER) and LangmuirHinshelwood (LH) mechanisms are discussed. The calculation results proposed that N-
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doped PG sheet could be a low-cost and better catalytic metal-free catalyst for CO oxidation.
2. Computational methods
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This study employed the spin-polarized first-principles computations based on the density-functional theory in framework of the plane-wave performed by the Vienna ab initio simulation package (VASP).53-54 The generalized gradient approximation with
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Perdew–Burke–Ernzerhof functional (GGA-PBE) was adopted.55 The basis set of plane wave was truncated at a kinetic cutoff energy of 500 eV, which allowing the total energy converges to smaller than 1 × 10-4 eV. The Monkhorst–Pack 5 × 5 × 1 k-point mesh was sampled in the Brillouin zone integration.56 The vacuum space of supercell in the zdirection was set to be 20 Å, which avoids interactions between periodic images. The
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formation energies (Ef) were predicted by using the equation (1). Ef = (EN-PG + EC) – (EPG + EN)
(1)
Where EN-PG, and EPG are the energy of N-doped PG containing with one nitrogen atom, and EC and EN are the energy of carbon and nitrogen atom, respectively. The adsorption
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energy (Eads) was calculated by the equation (2). Eads = Etol – Egas – EN-PG
(2)
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,where Etol, Egas, and EN-PG, the energy of the adsorbed molecule on the N-doped PG, the isolated molecule, and the N-doped PG, respectively. The climbing image nudged elastic band
(NEB) was performed to search the minimum energy pathway (MEP) for CO oxidation.57-58 In addition, the local density of state (LDOS) and Bader charge analysis5960
were carried out to explain the interaction between adsorbed species and N-doped PG.
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3. Results and discussion 3.1 Geometric stability and electronic properties of N-doped PG sheet First, we explored the electronic structure of N-doped penta-graphene sheet in which one
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C atom of the outermost C=C bond is substituted by a N atom. The model of N-doped penta-graphene containing of 23 carbon atoms and 1 nitrogen atom was optimized and depicted Figure 1a. The cell parameter of N-doped penta-graphene was calculated to be 7.204 Å with the C=N and C=C bond lengths of 1.383 (1.546 for C-N bond) and 1.344
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(1.543 for C-C bond) Å (see Figure 1), respectively. Doping nitrogen atoms in the pentagraphene sheet creates an electron-deficient C+ atom on adjacent nitrogen atom resulting
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in the formation of a polar C+-N- bond. The calculated formation energy is 2.49 eV which could be overcome in an actual experimental system. Moreover, the 3D iso-surface structure of electron density different map for the N-doped PG was calculated as shown in Figure 1b, the hybridization of the electron density between the N atom and neighboring C atom of the outermost C-N bond indicates the strong binding between N and C atom. The Bader charge calculations show that the net charge transfer the charge
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transfer from C to N atom in the C-N bond leading to the negative charge of -1.76 |e| on N atom and positive charge of 1.19 |e| on C atom. The local density of state for the Ndoped PG depicted in Figure 1c, which also show the strong interaction between N and neighboring C atom by the presence of the overlapped 2p states resulting in the
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hybridization of Nsp2 and Csp2. Furthermore, the band gap of the N-doped PG was calculated to be 0.60 eV along the Γ point at the GGA-PBE level while the band gap of
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penta-graphene is calculated to be 2.43 eV. The N substitution merely results in the formation of a new conduction band minimum crosses the Fermi level that shows n-type semiconductor behavior, see Figure S1 in Supporting Information. The results indicate that the N-doped PG could exhibit unexpected catalytic for CO oxidation.
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a
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Figure 1. Top and side view for (a) the optimized configurations of N-doped penta-graphene; (b) charge density deferent map of N-doped penta-graphene in which blue and yellow represent charge depletion and accumulation, respectively; (c) local density of states (LDOS) projected on N2p orbitals (red line) and C2p
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orbitals (black line) of the C+-N- bond. The Fermi level is set to zero. Atomic color code: gray for carbon and blue for nitrogen.
3.2 Adsorption behavior of CO and O2 on N-doped PG sheet
The adsorption behavior of O2 and CO molecules on the N-doped penta-graphene surface was illustrated. To search the most energetically stable adsorption of the O2 and CO
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molecules, these species were placed above the C=N and C=C sites at a proper distance
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during calculation of geometric optimization as shown in Figure 2.
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Figure 2. (a and a’) optimized configurations of end-on and side-on O2 adsorption. (b and b’) electron density difference maps of end-on and side-on O2 adsorption. (c and c’) Describes the LDOS projected onto isolated O2 (red line), adsorbed O2 (black line), N atom (blue line) and C atom (violet line).
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For O2 adsorption, two species of adsorbed O2 on the N-doped PG can be illustrated, see Figure 2. The most energetically favorable structure of O2 at the C=N site was found to
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be the end-on structure with forming a new O–C chemical bond of 1.536 Å and the N–O distance of 2.568 Å, see Figure 2a. The adsorption energy was predicted to be -0.92 eV. The optimized O–O bond distance of the end-on O2 species is 1.322 Å (see Figure 2a), which are increased from the O–O bond distance of 1.242 Å for isolated O2 molecule. The elongation of O–O bond suggesting an effective weakening of the O–O bond due to the reduction of adsorbed O2 species. The Bader charge analysis showed in Figure 2b indicates that charge transfer of -0.44 |e| from the polar C+-N- bond of the sheet to the adsorbed O2 evaluates the O2 reduction process. As illustrated in Figure 2c, the local density of states (LDOS) calculation reveals that the O2-2π* anti-bonding orbital is 7
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occupied below the Fermi level when the O2 species adsorbed on the N-doped PG. For the C=C site, the adsorbed O2 species is the side-on structure with forming two C–C chemical bonds of 1.441 Å and 1.440 Å, see Figure 2a’. The adsorption energy was computed to be -1.56 eV. The optimized O–O bond length of the side-on O2 species is
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increased to 1.517 Å. The more charge transfer (-1.39 |e|) from the C=C bond of the sheet to adsorbed O2 species and stronger hybridization between the adsorbed O2 species and the C=C bond leads to much elongation of O–O bond of the side-on O2 adsorption. For the CO adsorption, the computed adsorption energy of -0.42 eV which is obviously 0.50
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eV higher than that of the end-on O2 species in energy. The predicted C–O bond of CO adsorption is slight elongation from 1.144 Å to 1.174 Å, see Figure S2a. Therefore, O2 is
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more favorable to occupy on the N-doped PG surface forming the pre-adsorbed O2 surface when the two reactant gases (O2 and CO) are injected synchronously. The minimum energy path of O2 adsorption and reduction on both C=N and C=C sites of the N-doped PG surface has been mapped out and illustrated in Figure 3. The NEB calculation has demonstrated that there is nearly no activation energy (~0.01 eV) for the O2 reduction process on the C=N site while it requires to overcome an activation energy
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barrier of 0.26 eV on the C=C site. The results demonstrated that O2 could not be adsorbed and reduced on the C=C site when the C=N site exists. Therefore, we didn't
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proceed with the calculation of CO oxidation process on the C=C site.
Figure 3. Minimum energy paths calculated by NEB calculation for O2 reduction process on (a) the C=N site and (b) the C=C site of N-doped penta-graphene.
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3.3 Reaction mechanisms for CO oxidation on N-doped PG Generally, there are two kinds of well-known traditional pathways named Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms for the CO oxidation. The ER mechanism is the CO gas-phase molecule approaches to react with the O2 adsorbate
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directly. Whereas the CO and O2 molecules are co-adsorbed on the surface before reacting for the LH mechanism. In the current study, each type of mechanisms was systematically studied for comparison.
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3.3.1 Eley-Rideal (ER) mechanism.
First, we investigated CO oxidation process by ER mechanism. Since the ER mechanism
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was beginning from the end-on O2 pre-adsorption on the C=N site of the N-doped PG. The process of CO oxidation occurs via two Eley-Rideal (ER) reactions. (1) The gasphase CO molecule interacts with the end-on O2 adsorption directly leading to the formation of CO2(gas) and the remaining atomic O(ads) by the reaction of O2 (ads) + CO (gas) → O(ads) + CO2(gas). (2) Consequent CO oxidation takes place by the reaction of O(ads) + CO(gas) → CO2(gas). It should be noticed that the ER mechanism for CO oxidation by the
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pre-adsorbed side-on O2 on the C=C site has been studied previously and the energy barrier of the rate determining step is 0.65 eV.37
Figure 4 displays the minimum energy path for the reaction of O2 (ads) + CO (gas) → O(ads) +
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CO2(gas). The reaction starts from the O2 pre-adsorption, the physisorption of CO is placed at the distance of 3.035 Å above the pre-adsorbed O2 as the initial state (IS), see Figure
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S2 in Supporting Information. Then the gas phase CO molecule moves to approach the end-on O2 adsorption and directly extracts one oxygen atom to produce the final state (CO2 + O) without any activation energy barrier as shown in Figure 4, with a greatly exothermic reaction energy of -4.89 eV. The calculation result demonstrates that this process proceeds both kinetically and thermodynamically preferred. One should be noted that CO2 is easily desorbed because of the small adsorption energy of 0.08 eV. The staying atomic O(ads) strongly adsorbed on the N-doped PG sheet with the adsorption energy of -2.52 eV. In the secondary CO oxidation process, the additional CO molecule approaches to react with the remaining O species forming a weak CO2 adsorption. The 9
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distance between the C of CO and O(ads) is 2.924 Å and the Csheet–O is 1.210 Å. As shown in Figure 5, this process needs to overcome an activation energy barrier of 0.33 eV with an exothermic of 0.69 eV while those are 0.65 and 1.31 eV for pristine PG.37 The CO2 is easily desorbed from the N-doped PG sheet because of the interaction between CO2 and
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the sheet is weak (0.08 eV). The N-doped PG sheet reforms its incipient structure after the CO2 molecule is released from the sheet and can be used for the next cyclic CO oxidation. The computed parameters of initial states, transition states and final states in
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Figures 4 and 5 are listed in the Supporting Information.
Figure 4. Minimum energy path of ER mechanism calculated by NEB calculation for CO oxidation process on the C=N site of N-doped penta-graphene.
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Figure 5. Energy diagram of ER mechanism for CO oxidation reaction on N-doped penta-graphene
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including the initial state (IS), the transition state (TS) and the final state (FS).
3.3.2 Langmuir-Hinshelwood(LH) mechanism. The process of CO oxidation via LH mechanism was also studied as illustrated Figure 6. First, we investigated the energetically most favorable structure of O2 and CO co-
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adsorption on the N-doped PG. The most stable co-adsorption geometry of O2 and CO is depicted in Figure S2 of Supporting Information. The predicted co-adsorption energy is -
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1.01 eV which is slightly larger than that (-0.92) of O2 adsorption. The co-adsorption of O2 and CO is taken for initial state (IS). In the IS, the C–O and O–O bonds of coadsorbed CO and O2 are computed to be 1.144 and 1.340 Å, respectively. Then the adsorbed CO moves towards the adsorbed O2 by overcoming an energy barrier of only 0.33 eV (TS1) indicting that it is kinetically favored and the metastable peroxo-type (OOCO) of the intermediate state (MS) is formed. Compared with the pristine PG, it is close to that (0.31 eV) on the C=C site.37 The reaction process is thermodynamically favored and computed to be exothermic by -2.10 eV. The peroxo-type of intermediate 11
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state proceeds to decompose the first CO2 and adsorbed O species. The process step is exothermic by -2.59 eV with overcoming a very low energy barrier of 0.20 eV (TS2) suggesting this step is both thermodynamically and kinetically preferred. However, the second process on the pristine PG needs to pass a very high energy barrier of 1.93 eV.37
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The CO2 molecule is rapidly desorbed from the N-doped PG surface according to the small adsorption energy of 0.08 eV. Similarly, the remaining adsorbed O species proceeds to react with a second CO molecule producing the second CO2 via ER
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mechanism as mentioned above.
Figure 6. Energy diagrams of LH mechanism for CO oxidation reaction on N-doped penta-graphene including the initial state (IS), the transition states (TS1 and TS2), the intermediate state (MS), and the final state (FS).
3.3.3 Evaluation of N-doped penta-graphene catalyst Compared with metal-based catalysts and N-doped carbon materials, the energy barrier of the rate-limiting step is barrierless for the ER mechanism and 0.33 eV the LH 12
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mechanism on the N-doped PG which is more competitive or much lower than those on Au (211) (0.59 and 0.65 eV),21 Au (111) (1.97 eV),26 Au29 nanocluster (0.69 eV),22 and Au–Pd core–shell nanoparticles (0.37, 0.10, 0.59 and 0.49 eV for the Au38, Au32/Pd6, Pd32/Au6 and Pd38 clusters)61, Fe-embedded graphene (0.58 eV),62 Pt-embedded graphene
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(0.41-0.77 eV for LH, 0.72 to 1.07 for ER),63 Cu-embedded graphene (0.54 eV),64 Auembedded graphene (0.31 eV),65 N-graphene (0.69, 0.19 eV),66 P-graphene (0.54, 1.11 eV),66 N-doped carbon nanotube (0.45 ~ 0.58 eV)38 and 0.65 eV (0.31 eV for LH mechanism) for pristine PG.37 In addition, we also examine the catalytic properties of
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penta-graphene with higher N-doping. We apply the same computation procedures for O2 adsorption and CO oxidation via ER mechanism on the C20N4 penta-graphene which has
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4 N atoms substituting C atoms. The calculations (see Figure S3 of Supporting Information) show that the catalytic properties of higher N-doping penta-graphene (C20N4) are consistent with those of the single N-doing penta-graphene (C23N1). Therefore, our results indicate that the N-doped penta-graphene would exhibit better catalytic active for low-temperature CO oxidation which is expected to be verified by
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experiments.
In conclusion, we have carried out a comprehensive study of the catalytic oxidation reaction of CO activity on N-doped penta-graphene by means of spin-polarized DFT
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computations. We found that the end-on chemisorbed O2 on the C=N site is the active species for CO oxidation. Through ER and LH mechanisms, the active O2 further reacts
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with CO to produce CO2 molecule. The very small energy barrier and high exothermicity of the rate-limiting step (barrierless for the ER mechanism and 0.33 eV for the LH mechanism) indicates both mechanisms are preferred to occur at low temperature and the activation energy is more competitive with many metal-based and carbon-base catalysts. The cyclic catalytic reaction occurs energetically and the N-doped penta-graphene can regain to its incipient geometry for the next CO oxidation. Our calculation results demonstrate that that N-doped penta-graphene without noble metals is an effective catalyst for low-temperature CO oxidation. This work might provide valuable clues to
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developing low-cost and highly catalytic carbon-based materials for low-temperature CO oxidations.
Supporting Information
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Calculated band structures of pristine penta-graphene and N-doped penta-graphene (Figure S1), optimized configurations and adsorption energies for CO adsorption, atomic O adsorption, CO2 adsorption, CO adsorbed on O2 pre-adsorption, O2 and CO coadsorption, and CO adsorbed on O pre-adsorption (Figure S2). The optimized O2 adsorption on the C20N4 penta-
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configuration including adsorption energies for
graphene and calculation minimum energy path of ER mechanism for CO oxidation
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process on the C20N4 penta-graphene (Figure S3). The parameters of optimized structures and energy barriers for CO oxidation via ER and LH on N-doped penta-graphene (Table S1).
Acknowledgment
H.-T.C. thanks the Ministry of Science and Technology (MOST) under Grant Numbers
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MOST 106-2113-M-033-003, MOST 105-2113-M-033-008, and MOST 104-2113-M033-010, Chung Yuan Christian University (CYCU), and National Center for Theoretical Sciences (NCTS), Taiwan, for supporting this work and the use of facilities at the
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39. Wu, P.; Du, P.; Zhang, H.; Cai, C., Graphdiyne as a metal-free catalyst for lowtemperature CO oxidation. Phys. Chem. Chem. Phys 2014, 16, 5640-5648. 40. Sinthika, S.; Kumar, E. M.; Surya, V. J.; Kawazoe, Y.; Park, N.; Iyakutti, K.; et al., Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study. Sci Rep. 2015, 5, 17460-13. 41. Xiong, W.; Du, F.; Liu, Y.; Perez, A.; Supp, M.; Ramakrishnan, T. S.; et al., 3-D Carbon Nanotube Structures Used as High Performance Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 15839-15841. 42. Fellinger, T. P.; Hasché, F.; Strasser, P.; Antonietti, M., Mesoporous NitrogenDoped Carbon for the Electrocatalytic Synthesis of Hydrogen Peroxide. J. Am. Chem. Soc. 2012, 134, 4072-4075. 43. Tang, Y.; Allen, B. L.; Kauffman, D. R.; Star, A., Electrocatalytic Activity of Nitrogen-Doped Carbon Nanotube Cups. J. Am. Chem. Soc. 2009, 131, 13200-13201. 44. Hu, X.; Wu, Y.; Zhang, Z., CO oxidation on metal-free nitrogen-doped carbon nanotubes and the related structure–reactivity relationships. J. Mater. Chem. 2012, 22 (30), 15198-15205. 45. Wu, P.; Du, P.; Zhang, H.; Cai, C., Graphdiyne as a metal-free catalyst for lowtemperature CO oxidation. Phys. Chem. Chem. Phys. 2014, 16 (12), 5640-5648. 46. Zhang, S.; Zhouc, J.; Wang, Q.; Chen, X.; Kawazoe, Y.; Jena, P., Penta-graphene: A new carbon allotrope. Proc. Natl. Acad. Sci. 2015, 112, 2372-2377. 47. Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L., Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. science. 2009, 323 (5915), 760-764. 48. Yu, D.; Zhang, Q.; Dai, L., Highly efficient metal-free growth of nitrogen-doped single-walled carbon nanotubes on plasma-etched substrates for oxygen reduction. J. Am. Chem. Soc. 2010, 132 (43), 15127-15129. 49. Hu, X.; Liu, C.; Wu, Y.; Zhang, Z., Density functional theory study on nitrogendoped carbon nanotubes with and without oxygen adsorption: the influence of length and diameter. New J. Chem. 2011, 35 (11), 2601-2606. 50. Khosousi, Z. A.; Zhao, L.; Pálová, L.; Hybertsen, M. S.; Reichman, D. R.; Pasupathy, A. N.; et al., Segregation of Sublattice Domains in Nitrogen-Doped Graphene. J. Am. Chem. Soc. 2014, 136, 1391−1397. 51. Gao, F.; Zhao, G.-L.; Yang, S.; Spivey, J. J., Nitrogen-Doped Fullerene as a Potential Catalyst for Hydrogen Fuel Cells. J. Am. Chem. Soc. 2013, 135, 3315 - 3318. 52. Zhang, S.; Zhou, J.; Wang, Q.; Jena, P., Beyond graphitic carbon nitride: nitrogen-rich penta-CN2 sheet. J. Phys. Chem. C. 2016, 120 (7), 3993-3998. 53. Kresse, G.; Furthmuller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169-11186. 54. Kresse, G.; Furthmuller, J., in Vienna Ab-initio Simulation Package Edition edn. 2006. 55. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 56. Monkhorst, H. J.; Pack, J. D., Special points for Brillonin-zone integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1976, 13, 5188-5192. 17
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S0008-6223(18)30188-X
DOI:
10.1016/j.carbon.2018.02.064
Reference:
CARBON 12900
To appear in:
Carbon
Received Date: 5 January 2018 Revised Date:
12 February 2018
Accepted Date: 14 February 2018
Please cite this article as: R. Krishnan, S.-Y. Wu, H.-T. Chen, Nitrogen-doped penta-graphene as a superior catalytic activity for CO oxidation, Carbon (2018), doi: 10.1016/j.carbon.2018.02.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Table of content graphic
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We performed a systematic study of CO oxidation on the nitrogen-doped penta-graphene by utilizing spin-polarized density functional theory calculations. The results manifest that the nitrogen-doped penta-graphene displays very high catalytic
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low-temperature CO oxidation.
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activity and is more competitive with many metal-based and carbon-base catalysts for
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Nitrogen-doped Penta-Graphene as a Superior Catalytic Activity for CO Oxidation
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Ranganathan Krishnan, Shiuan-Yau Wu, and Hsin-Tsung Chen*
Department of Chemistry, Chung Yuan Christian University, Chungli District, Taoyuan
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City, 32023, Taiwan
*Corresponding author. E-mails: [email protected] (H.-T.C.); Tel: +886-3-265-3324
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Abstract: We performed a systematic study of carbon monoxide oxidation on the nitrogen-doped penta-graphene by utilizing spin-polarized density functional theory calculations. The
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possible reaction pathways via both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms were illustrated. The results manifest that the nitrogen-doped penta-graphene displays very high catalytic activity and is more competitive with many metal-based and carbon-based catalysts for low-temperature CO oxidation by reason of the very small
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energy barrier of the rate-limiting step (there is no activation energy for the ER mechanism and only 0.33 eV for the LH mechanism). This study opens a new avenue for
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the design of 2-D graphene-based materials and provides valuable clues for developing
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catalytic activity carbon materials for low-temperature CO oxidations.
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1. Introduction Catalytic activity of carbon monoxide (CO) oxidation has attracted much interest over the past few decades because of its settling the rising environmental issue caused by CO emission from vehicle exhaust, industrial processes, low pollution applications1-5 and
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its important role removing CO poisoning from hydrogen fuel.6-12 The developing heterogeneous catalysts for CO oxidation process becomes more critical.13-17 Previous studies have been made by using metals (Au-Pd,18-20 Au,21-22 Pt,8, 23-24 Ru, Rh, Os, Ir,24 Pd,11,
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and Ag26) and metal-oxides (CeO2-,27-30 TiO2-,31 MnO2,32 and AuO2-based
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materials33) as catalysts for CO oxidation. However, this metal-based catalyst is relatively rare, very expensive and usually required at high temperature for efficient operation. It is
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necessary to develop alternative non-precious catalyst to replace the metal-based catalyst. Reducing the use of metal is a major target in this work for the new technology. The discovery of carbon-based materials such as fullerene, carbon nanotube, graphene, graphdiyne, and hierarchical 3-D carbon materials has spread out exciting opportunities for the design of robust catalysts for large-scale practical applications because of their lightweight, high surface area, and unique electronic properties. Metals
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decorated on the carbon-based materials for example, Au-embedded graphene,34 Ptembedded graphenes,35 Fe-embedded graphene,36 have been investigated to be activity for CO oxidation. A novel class of metal-free catalysts based on carbon materials, which has been demonstrated as effective catalysts, could dramatically diminish the cost.37-45
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Recently, Wang et al. have proposed a new two-dimensional carbon allotrope called penta-graphene (PG), which only consists of pentagonal configurations, has mechanically,
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dynamically and thermally stable by a theoretical study.46 The variables of PG and its derivatives are anticipated to have spacious applications in the fields of catalysts, nanomechanics, and nanoelectronics. These advantages make PG and its derivatives could be potential materials for CO catalytic oxidation. Doping nitrogen atoms in the carbon-based materials, which creates an electron-deficient C+ atom on the adjacent nitrogen atom, has been to be the approach to increase the catalytic performance.37, 42, 47-51 The N-doped penta-graphene (penta-CN2) is also thermally, mechanically, and dynamically stable found by Wang et al.52 To the best of our knowledge, no investigation on CO oxidation catalyzed by the N-doped penta-graphene has been explored. 3
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In the present work, we performed a comprehensive investigation of CO oxidation on nitrogen-doped penta-graphene by spin-polarized first-principles calculations. The possible mechanisms of CO oxidation including Eley-Rideal (ER) and LangmuirHinshelwood (LH) mechanisms are discussed. The calculation results proposed that N-
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doped PG sheet could be a low-cost and better catalytic metal-free catalyst for CO oxidation.
2. Computational methods
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This study employed the spin-polarized first-principles computations based on the density-functional theory in framework of the plane-wave performed by the Vienna ab initio simulation package (VASP).53-54 The generalized gradient approximation with
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Perdew–Burke–Ernzerhof functional (GGA-PBE) was adopted.55 The basis set of plane wave was truncated at a kinetic cutoff energy of 500 eV, which allowing the total energy converges to smaller than 1 × 10-4 eV. The Monkhorst–Pack 5 × 5 × 1 k-point mesh was sampled in the Brillouin zone integration.56 The vacuum space of supercell in the zdirection was set to be 20 Å, which avoids interactions between periodic images. The
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formation energies (Ef) were predicted by using the equation (1). Ef = (EN-PG + EC) – (EPG + EN)
(1)
Where EN-PG, and EPG are the energy of N-doped PG containing with one nitrogen atom, and EC and EN are the energy of carbon and nitrogen atom, respectively. The adsorption
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energy (Eads) was calculated by the equation (2). Eads = Etol – Egas – EN-PG
(2)
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,where Etol, Egas, and EN-PG, the energy of the adsorbed molecule on the N-doped PG, the isolated molecule, and the N-doped PG, respectively. The climbing image nudged elastic band
(NEB) was performed to search the minimum energy pathway (MEP) for CO oxidation.57-58 In addition, the local density of state (LDOS) and Bader charge analysis5960
were carried out to explain the interaction between adsorbed species and N-doped PG.
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3. Results and discussion 3.1 Geometric stability and electronic properties of N-doped PG sheet First, we explored the electronic structure of N-doped penta-graphene sheet in which one
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C atom of the outermost C=C bond is substituted by a N atom. The model of N-doped penta-graphene containing of 23 carbon atoms and 1 nitrogen atom was optimized and depicted Figure 1a. The cell parameter of N-doped penta-graphene was calculated to be 7.204 Å with the C=N and C=C bond lengths of 1.383 (1.546 for C-N bond) and 1.344
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(1.543 for C-C bond) Å (see Figure 1), respectively. Doping nitrogen atoms in the pentagraphene sheet creates an electron-deficient C+ atom on adjacent nitrogen atom resulting
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in the formation of a polar C+-N- bond. The calculated formation energy is 2.49 eV which could be overcome in an actual experimental system. Moreover, the 3D iso-surface structure of electron density different map for the N-doped PG was calculated as shown in Figure 1b, the hybridization of the electron density between the N atom and neighboring C atom of the outermost C-N bond indicates the strong binding between N and C atom. The Bader charge calculations show that the net charge transfer the charge
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transfer from C to N atom in the C-N bond leading to the negative charge of -1.76 |e| on N atom and positive charge of 1.19 |e| on C atom. The local density of state for the Ndoped PG depicted in Figure 1c, which also show the strong interaction between N and neighboring C atom by the presence of the overlapped 2p states resulting in the
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hybridization of Nsp2 and Csp2. Furthermore, the band gap of the N-doped PG was calculated to be 0.60 eV along the Γ point at the GGA-PBE level while the band gap of
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penta-graphene is calculated to be 2.43 eV. The N substitution merely results in the formation of a new conduction band minimum crosses the Fermi level that shows n-type semiconductor behavior, see Figure S1 in Supporting Information. The results indicate that the N-doped PG could exhibit unexpected catalytic for CO oxidation.
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Figure 1. Top and side view for (a) the optimized configurations of N-doped penta-graphene; (b) charge density deferent map of N-doped penta-graphene in which blue and yellow represent charge depletion and accumulation, respectively; (c) local density of states (LDOS) projected on N2p orbitals (red line) and C2p
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orbitals (black line) of the C+-N- bond. The Fermi level is set to zero. Atomic color code: gray for carbon and blue for nitrogen.
3.2 Adsorption behavior of CO and O2 on N-doped PG sheet
The adsorption behavior of O2 and CO molecules on the N-doped penta-graphene surface was illustrated. To search the most energetically stable adsorption of the O2 and CO
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molecules, these species were placed above the C=N and C=C sites at a proper distance
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during calculation of geometric optimization as shown in Figure 2.
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Figure 2. (a and a’) optimized configurations of end-on and side-on O2 adsorption. (b and b’) electron density difference maps of end-on and side-on O2 adsorption. (c and c’) Describes the LDOS projected onto isolated O2 (red line), adsorbed O2 (black line), N atom (blue line) and C atom (violet line).
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For O2 adsorption, two species of adsorbed O2 on the N-doped PG can be illustrated, see Figure 2. The most energetically favorable structure of O2 at the C=N site was found to
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be the end-on structure with forming a new O–C chemical bond of 1.536 Å and the N–O distance of 2.568 Å, see Figure 2a. The adsorption energy was predicted to be -0.92 eV. The optimized O–O bond distance of the end-on O2 species is 1.322 Å (see Figure 2a), which are increased from the O–O bond distance of 1.242 Å for isolated O2 molecule. The elongation of O–O bond suggesting an effective weakening of the O–O bond due to the reduction of adsorbed O2 species. The Bader charge analysis showed in Figure 2b indicates that charge transfer of -0.44 |e| from the polar C+-N- bond of the sheet to the adsorbed O2 evaluates the O2 reduction process. As illustrated in Figure 2c, the local density of states (LDOS) calculation reveals that the O2-2π* anti-bonding orbital is 7
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occupied below the Fermi level when the O2 species adsorbed on the N-doped PG. For the C=C site, the adsorbed O2 species is the side-on structure with forming two C–C chemical bonds of 1.441 Å and 1.440 Å, see Figure 2a’. The adsorption energy was computed to be -1.56 eV. The optimized O–O bond length of the side-on O2 species is
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increased to 1.517 Å. The more charge transfer (-1.39 |e|) from the C=C bond of the sheet to adsorbed O2 species and stronger hybridization between the adsorbed O2 species and the C=C bond leads to much elongation of O–O bond of the side-on O2 adsorption. For the CO adsorption, the computed adsorption energy of -0.42 eV which is obviously 0.50
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eV higher than that of the end-on O2 species in energy. The predicted C–O bond of CO adsorption is slight elongation from 1.144 Å to 1.174 Å, see Figure S2a. Therefore, O2 is
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more favorable to occupy on the N-doped PG surface forming the pre-adsorbed O2 surface when the two reactant gases (O2 and CO) are injected synchronously. The minimum energy path of O2 adsorption and reduction on both C=N and C=C sites of the N-doped PG surface has been mapped out and illustrated in Figure 3. The NEB calculation has demonstrated that there is nearly no activation energy (~0.01 eV) for the O2 reduction process on the C=N site while it requires to overcome an activation energy
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barrier of 0.26 eV on the C=C site. The results demonstrated that O2 could not be adsorbed and reduced on the C=C site when the C=N site exists. Therefore, we didn't
b
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proceed with the calculation of CO oxidation process on the C=C site.
Figure 3. Minimum energy paths calculated by NEB calculation for O2 reduction process on (a) the C=N site and (b) the C=C site of N-doped penta-graphene.
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3.3 Reaction mechanisms for CO oxidation on N-doped PG Generally, there are two kinds of well-known traditional pathways named Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms for the CO oxidation. The ER mechanism is the CO gas-phase molecule approaches to react with the O2 adsorbate
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directly. Whereas the CO and O2 molecules are co-adsorbed on the surface before reacting for the LH mechanism. In the current study, each type of mechanisms was systematically studied for comparison.
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3.3.1 Eley-Rideal (ER) mechanism.
First, we investigated CO oxidation process by ER mechanism. Since the ER mechanism
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was beginning from the end-on O2 pre-adsorption on the C=N site of the N-doped PG. The process of CO oxidation occurs via two Eley-Rideal (ER) reactions. (1) The gasphase CO molecule interacts with the end-on O2 adsorption directly leading to the formation of CO2(gas) and the remaining atomic O(ads) by the reaction of O2 (ads) + CO (gas) → O(ads) + CO2(gas). (2) Consequent CO oxidation takes place by the reaction of O(ads) + CO(gas) → CO2(gas). It should be noticed that the ER mechanism for CO oxidation by the
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pre-adsorbed side-on O2 on the C=C site has been studied previously and the energy barrier of the rate determining step is 0.65 eV.37
Figure 4 displays the minimum energy path for the reaction of O2 (ads) + CO (gas) → O(ads) +
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CO2(gas). The reaction starts from the O2 pre-adsorption, the physisorption of CO is placed at the distance of 3.035 Å above the pre-adsorbed O2 as the initial state (IS), see Figure
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S2 in Supporting Information. Then the gas phase CO molecule moves to approach the end-on O2 adsorption and directly extracts one oxygen atom to produce the final state (CO2 + O) without any activation energy barrier as shown in Figure 4, with a greatly exothermic reaction energy of -4.89 eV. The calculation result demonstrates that this process proceeds both kinetically and thermodynamically preferred. One should be noted that CO2 is easily desorbed because of the small adsorption energy of 0.08 eV. The staying atomic O(ads) strongly adsorbed on the N-doped PG sheet with the adsorption energy of -2.52 eV. In the secondary CO oxidation process, the additional CO molecule approaches to react with the remaining O species forming a weak CO2 adsorption. The 9
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distance between the C of CO and O(ads) is 2.924 Å and the Csheet–O is 1.210 Å. As shown in Figure 5, this process needs to overcome an activation energy barrier of 0.33 eV with an exothermic of 0.69 eV while those are 0.65 and 1.31 eV for pristine PG.37 The CO2 is easily desorbed from the N-doped PG sheet because of the interaction between CO2 and
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the sheet is weak (0.08 eV). The N-doped PG sheet reforms its incipient structure after the CO2 molecule is released from the sheet and can be used for the next cyclic CO oxidation. The computed parameters of initial states, transition states and final states in
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Figures 4 and 5 are listed in the Supporting Information.
Figure 4. Minimum energy path of ER mechanism calculated by NEB calculation for CO oxidation process on the C=N site of N-doped penta-graphene.
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Figure 5. Energy diagram of ER mechanism for CO oxidation reaction on N-doped penta-graphene
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including the initial state (IS), the transition state (TS) and the final state (FS).
3.3.2 Langmuir-Hinshelwood(LH) mechanism. The process of CO oxidation via LH mechanism was also studied as illustrated Figure 6. First, we investigated the energetically most favorable structure of O2 and CO co-
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adsorption on the N-doped PG. The most stable co-adsorption geometry of O2 and CO is depicted in Figure S2 of Supporting Information. The predicted co-adsorption energy is -
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1.01 eV which is slightly larger than that (-0.92) of O2 adsorption. The co-adsorption of O2 and CO is taken for initial state (IS). In the IS, the C–O and O–O bonds of coadsorbed CO and O2 are computed to be 1.144 and 1.340 Å, respectively. Then the adsorbed CO moves towards the adsorbed O2 by overcoming an energy barrier of only 0.33 eV (TS1) indicting that it is kinetically favored and the metastable peroxo-type (OOCO) of the intermediate state (MS) is formed. Compared with the pristine PG, it is close to that (0.31 eV) on the C=C site.37 The reaction process is thermodynamically favored and computed to be exothermic by -2.10 eV. The peroxo-type of intermediate 11
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state proceeds to decompose the first CO2 and adsorbed O species. The process step is exothermic by -2.59 eV with overcoming a very low energy barrier of 0.20 eV (TS2) suggesting this step is both thermodynamically and kinetically preferred. However, the second process on the pristine PG needs to pass a very high energy barrier of 1.93 eV.37
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The CO2 molecule is rapidly desorbed from the N-doped PG surface according to the small adsorption energy of 0.08 eV. Similarly, the remaining adsorbed O species proceeds to react with a second CO molecule producing the second CO2 via ER
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mechanism as mentioned above.
Figure 6. Energy diagrams of LH mechanism for CO oxidation reaction on N-doped penta-graphene including the initial state (IS), the transition states (TS1 and TS2), the intermediate state (MS), and the final state (FS).
3.3.3 Evaluation of N-doped penta-graphene catalyst Compared with metal-based catalysts and N-doped carbon materials, the energy barrier of the rate-limiting step is barrierless for the ER mechanism and 0.33 eV the LH 12
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mechanism on the N-doped PG which is more competitive or much lower than those on Au (211) (0.59 and 0.65 eV),21 Au (111) (1.97 eV),26 Au29 nanocluster (0.69 eV),22 and Au–Pd core–shell nanoparticles (0.37, 0.10, 0.59 and 0.49 eV for the Au38, Au32/Pd6, Pd32/Au6 and Pd38 clusters)61, Fe-embedded graphene (0.58 eV),62 Pt-embedded graphene
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(0.41-0.77 eV for LH, 0.72 to 1.07 for ER),63 Cu-embedded graphene (0.54 eV),64 Auembedded graphene (0.31 eV),65 N-graphene (0.69, 0.19 eV),66 P-graphene (0.54, 1.11 eV),66 N-doped carbon nanotube (0.45 ~ 0.58 eV)38 and 0.65 eV (0.31 eV for LH mechanism) for pristine PG.37 In addition, we also examine the catalytic properties of
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penta-graphene with higher N-doping. We apply the same computation procedures for O2 adsorption and CO oxidation via ER mechanism on the C20N4 penta-graphene which has
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4 N atoms substituting C atoms. The calculations (see Figure S3 of Supporting Information) show that the catalytic properties of higher N-doping penta-graphene (C20N4) are consistent with those of the single N-doing penta-graphene (C23N1). Therefore, our results indicate that the N-doped penta-graphene would exhibit better catalytic active for low-temperature CO oxidation which is expected to be verified by
4. Conclusion
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experiments.
In conclusion, we have carried out a comprehensive study of the catalytic oxidation reaction of CO activity on N-doped penta-graphene by means of spin-polarized DFT
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computations. We found that the end-on chemisorbed O2 on the C=N site is the active species for CO oxidation. Through ER and LH mechanisms, the active O2 further reacts
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with CO to produce CO2 molecule. The very small energy barrier and high exothermicity of the rate-limiting step (barrierless for the ER mechanism and 0.33 eV for the LH mechanism) indicates both mechanisms are preferred to occur at low temperature and the activation energy is more competitive with many metal-based and carbon-base catalysts. The cyclic catalytic reaction occurs energetically and the N-doped penta-graphene can regain to its incipient geometry for the next CO oxidation. Our calculation results demonstrate that that N-doped penta-graphene without noble metals is an effective catalyst for low-temperature CO oxidation. This work might provide valuable clues to
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developing low-cost and highly catalytic carbon-based materials for low-temperature CO oxidations.
Supporting Information
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Calculated band structures of pristine penta-graphene and N-doped penta-graphene (Figure S1), optimized configurations and adsorption energies for CO adsorption, atomic O adsorption, CO2 adsorption, CO adsorbed on O2 pre-adsorption, O2 and CO coadsorption, and CO adsorbed on O pre-adsorption (Figure S2). The optimized O2 adsorption on the C20N4 penta-
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configuration including adsorption energies for
graphene and calculation minimum energy path of ER mechanism for CO oxidation
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process on the C20N4 penta-graphene (Figure S3). The parameters of optimized structures and energy barriers for CO oxidation via ER and LH on N-doped penta-graphene (Table S1).
Acknowledgment
H.-T.C. thanks the Ministry of Science and Technology (MOST) under Grant Numbers
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MOST 106-2113-M-033-003, MOST 105-2113-M-033-008, and MOST 104-2113-M033-010, Chung Yuan Christian University (CYCU), and National Center for Theoretical Sciences (NCTS), Taiwan, for supporting this work and the use of facilities at the
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