Q. G. Jiangab,
Z. M. Ao*c,
S. Lib and
Z. Wen*a
aKey Laboratory of Automobile Materials (Jilin University), Ministry of Education, School of Materials Science and Engineering, Jilin University, Changchun 130022, China. E-mail: wenzi@jlu.edu.cn
bSchool of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
cCentre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney, PO Box 123, Broadway, Sydney, NSW 2007, Australia. E-mail: zhimin.ao@uts.edu.au
First published on 7th April 2014
The oxidation of CO molecules on Al-embedded graphene has been investigated by using the first principles calculations. Both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) oxidation mechanisms are considered. In the ER mechanism, an O2 molecule is first adsorbed and activated on Al-embedded graphene before a CO molecule approaches, the energy barrier for the primary step (CO + O2 → OOCO) is 0.79 eV. In the LH mechanism, O2 and CO molecules are firstly co-adsorbed on Al-embedded graphene, the energy barrier for the rate limiting step (CO + O2 → OOCO) is only 0.32 eV, much lower than that of ER mechanism, which indicates that LH mechanism is more favourable for CO oxidation on Al-embedded graphene. Hirshfeld charge analysis shows that the embedded Al atom would modify the charge distributions of co-adsorbed O2 and CO molecules. The charge transfer from O2 to CO molecule through the embedded Al atom plays an important role for the CO oxidation along the LH mechanism. Our result shows that the low cost Al-embedded graphene is an efficient catalyst for CO oxidation at room temperature.
As a novel form of carbon, graphene16 is a promising matrix to support metal atoms to realize new catalysts due to its outstanding electrical,17 mechanical18 and thermal properties.19 Moreover, the large surface-to-volume ratio also benefits for graphene as a support for heterogeneous catalysts. However, the interaction between pristine graphene and CO molecule is rather weak due to the inert nature of the graphene sheet.20 Theoretically, Au-, Fe-, Cu-, Pt- and Si-embedded graphenes were predicted to be highly active catalysts for CO oxidation,21–25 where the energy barriers for the rate limiting step are 0.31 eV for Au-embedded,21 0.58 eV for Fe-embedded,22 0.54 eV for Cu-embedded,23 0.59 eV for Pt-embedded,24 and 0.57 eV for Si-embedded graphene,25 respectively. Among these embedded graphenes, the Au-embedded graphene shows the best performance for CO oxidation. However, the adsorption energy of CO molecule (Ead_CO = −1.53 eV) is larger than that of O2 molecule (Ead_O2 = −1.34 eV) on Au-embedded graphene,21 which indicates that CO molecule has priority to adsorb on the Au-embedded graphene and may prevent the oxidation reaction. Therefore, further exploiting efficient graphene-based catalyst for CO oxidation is necessary. It has been reported that the reaction energy barrier is proportional to the adsorption energy of adsorbed molecules.6 For example, the reaction mechanism for CO oxidation on Au- and Cu-embedded graphene is the same, and the co-adsorption energy of CO and O2 molecules on Au-embedded graphene (Ead = −1.82 eV)21 is much smaller than that on Cu-embedded graphene (Ead = −3.29 eV),23 corresponding to a smaller energy barrier for the rate limiting step on Au-embedded graphene. Our previous studies show that Al-embedded graphene is a highly active catalyst for H2O dissociative adsorption26 and is a promising sensor material for CO detection.20 In addition, our calculated co-adsorption energy of CO and O2 molecules on Al-embedded graphene (Ead = −1.95 eV) is similar to that on Au-embeded graphene. Therefore, we expect that graphene embedded with the low-cost metal Al may be also highly active as a catalyst for the CO oxidation.
In this work, we will study the reaction mechanism of the CO oxidation on Al-embedded graphene through first principles calculations. The energy barriers for the reaction are discussed and the corresponding mechanisms are analyzed through understanding the electronic structures.
For one molecule (CO or O2) adsorbed on graphene, the adsorption energy Ead is determined by,
Ead = Emolecule/graphene − (Egraphene + Emolecule) | (1a) |
For two molecules (CO and O2) co-adsorbed on graphene, the co-adsorption energy Ead is determined by,
Ead = Emolecules/graphene − (Egraphene + ECO + EO2) | (1b) |
To determine the possibility of Al aggregation, which is a problem when the concentration of the metals atoms is high,37 the diffusion of the Al atom to its neighbouring position on graphene is investigated (see Fig. 1a). It is found that the corresponding diffusion barrier for Al atom is 2.98 eV. Since a surface reaction at ambient temperature may occur only when the energy barrier is smaller than the critical barrier of Ecbar = 0.91 eV,38 the Al atom here is rather stable. In addition, the binding energy of Al on graphene is −5.60 eV, which is stronger than the cohesive energy of Al element of −3.39 eV per atom.39 Therefore, the Al dopant can disperse on graphene quite stably without clustering problem.
To investigate the oxidation of CO on Al-embedded graphene, the adsorptions of CO and O2 on Al-embedded graphene are first calculated, respectively. Fig. 2a shows the most stable configuration of a CO molecule adsorbed on Al-embedded graphene. It shows that the CO molecule is on the top of the embedded Al atom and vertical to graphene surface. Chemical bond is formed between the C atom in CO and the Al atom, indicating chemical adsorption of the CO molecule on Al-embedded graphene, which also agrees with the reported result.20 The corresponding adsorption energies and structure data of the most stable configurations for CO and O2 adsorption on Al-embedded graphene are listed in Table 1. The adsorption energy of a CO molecule on Al-embedded graphene Ead = −0.83 eV, and ∼0.15e is transferred from the CO molecule to the Al-embedded graphene based on Hirshfeld analysis. Fig. 2b shows the PDOS and orbitals of a free CO molecule. All orbitals are labelled and the 2π* anti-bond orbital is fully empty since it locates far above Fermi level, which is consistent with the literature results for CO molecule.5 The other three orbitals are fully occupied. In addition, the 5σ orbital with two electrons at the Fermi level state mainly locates on the C atom, indicating that CO always acts as a donor when its C atom pointing to the graphene surface.40 This agrees with the above Hirshfeld analysis result. Fig. 2c and d show PDOS and orbitals of the adsorbed CO molecule on Al-embedded graphene, respectively. Compared with the free CO molecule, the 5σ spin down peak is significantly depressed in the CO adsorbed on Al-embedded graphene due to the transferring of electrons from CO to Al atom. Because CO-2π* is close to the Fermi level, there is some electrons transferred from Al atom to CO-2π* state and CO-2π* state is partial filled as shown in Fig. 2c and d, which leads to the slight elongation of the C–O bond from 1.14 Å in free CO molecule to 1.15 Å.
Ead (eV) | h (Å) | l (Å) | Q (e) | |
---|---|---|---|---|
Free CO | — | — | 1.14 | — |
Adsorbed CO | −0.83 | 2.06 | 1.15 | 0.15 |
Free O2 | — | — | 1.23 | — |
Adsorbed O2 | −1.57 | 1.86 | 1.43 | −0.31 |
For the O2 adsorption on Al-embedded graphene, the O2 molecule is adsorbed on Al atom much stronger with an Ead of −1.57 eV and the O–O bond is slightly titled with the graphene sheet (see IS in Fig. 3a and d). To assess the stability of O2 molecule on graphene, we also studied the dissociative adsorption of an O2 molecule on Al-embedded graphene as shown in Fig. 3a. After dissociative adsorption, one O atom adsorbs on Al atom while the other O atom co-binds with the Al atom and a neighbouring C atom at FS state as shown in Fig. 3a. After LST/QST and NEB calculations, the energy barrier for the dissociative adsorption of an O2 molecule on the Al-embedded graphene is 1.66 eV > Ecbar = 0.91 eV,38 which indicates that the adsorbed O2 molecule is stable on Al-embedded graphene without dissociation at room temperature. The possibility of CO dissociation on Al-embedded graphene is also considered. Our DFT calculations show that binding energy of C–O bond (Eb = 11.57 eV) is much stronger than that of O–O bond (Eb = 6.36 eV). Thus CO should be more difficult to be dissociated. As seen in Fig. 3b, all the orbitals of free O2 molecule are labelled while the 2π* anti-bond orbital is half filled, which is consistent with the literature results for O2 molecule.41 When one O2 molecule is adsorbed on Al-embedded graphene, based on Hirshfeld analysis, there are about 0.31e to be transferred from the Al-embedded graphene to O2 molecule, which occupies the initial empty component of the O2–2π* orbital. This can be confirmed by two new peaks presence for O2–2π* orbital after its adsorption on the Al-embedded graphene (see Fig. 3c), which also leads to the elongation of the O–O bond from 1.23 Å in free O2 molecule to 1.43 Å. The hybridization between the Al atom and O2–2π* orbitals is observed near Fermi level from the PDOS (see Fig. 3c). The above results reveal that Al-embedded graphene has strong interactions with O2 and CO (corresponding Ead are −1.57 and −0.83 eV, respectively), but the adsorption of O2 is much stronger. The adsorbed O2 is efficiently activated, which facilitates the further oxidation of CO molecules.
Two mechanisms for CO oxidation have been established: Eley–Rideal (ER) mechanism and Langmuir–Hinshelwood (LH) mechanism.21–25,42,43 In the ER mechanism, the pre-adsorbed O2 molecule is activated by charge transferring with the substrate and an intermediate product forms when a free CO molecule approaching. In the LH mechanism, O2 and CO molecules co-adsorb on the embedded graphene before the reaction happening. Since O2 has stronger adsorption energy (−1.57 eV) than that of CO on Al-embedded graphene (−0.83 eV), the adsorption of O2 on Al-embedded graphene has higher priority, and thus the ER mechanism seems to be more favourable. Although the adsorption energy difference between CO and O2 is relatively large, the lower co-adsorption energy of CO and O2 molecules (−1.95 eV) indicates that there is a certain probability of having O2 and CO co-adsorbed on Al atom as discussed in literatures.21,23,24 Therefore, both mechanisms will be investigated in this work.
In order to determine the preferred mechanism for CO oxidation, the first reaction step of both ER and LH mechanisms is studied, respectively. If the ER mechanism is favourable, the physisorbed CO molecule will react with the pre-adsorbed O2 molecule to form OOCO intermediate (shown as the TS in Fig. 4a). If the LH mechanism is favourable, the physisorbed CO molecule will co-adsorb on Al atom with O2 molecule with a small energy barrier. The energy profile for the ER mechanism is shown in Fig. 4a. The configuration of physisorbed CO perpendicular to the graphene surface was selected as the initial state IS after considering all possible adsorption positions. When one CO molecule approaches the activated O2, after overcoming an energy barrier of 0.79 eV, one O–Al bond is broken, the O with dangling bond would form a covalent bond with the C atom in the CO molecule (see TS in Fig. 4a). Then the C atom of the CO binds with one C atom closed to the Al dopant and a peroxo-type OOCO complex is formed over the Al atom (see FS in Fig. 4a). This process is exothermic and releases energy of 1.28 eV. Fig. 4b shows the energy profile for the CO molecule co-adsorbed on Al atom with O2 molecule, where the CO molecule is titled towards the Al atom at TS state. After overcoming a small energy barrier of 0.03 eV, the CO molecule co-adsorbs on Al atom at FS state as shown in Fig. 4b. This process is exothermic with releasing energy of 0.18 eV.
The energy barriers above are considered at zero Kelvin. To take into consideration the effect of temperature, free energy change (ΔG) between IS and TS is considered as the corrected energy barrier E′bar and can be determined by ΔG = ΔH − TΔS, where ΔH is the change in enthalpy, T is the room temperature (298.15 K), and ΔS is the change in entropy. It is known that ΔH = (ΔU + PΔV), ΔU = (ΔEtot + ΔEvib + ΔEtrans + ΔErot) and ΔS = ΔSvib + ΔStrans + ΔSrot, where ΔU is the change of internal energy, ΔEtot is the total electronic energy change obtained from DFT calculations, the subscript vib, trans and rot indicate the components from vibration, translation and rotation, respectively. All the above terms can be calculated with vibrational frequency calculations in Dmol3 code. The corrected energy barriers at room temperature for each step in Fig. 4 are: E′bar = 0.95 eV in Fig. 4a, and E′bar = 0.16 eV in Fig. 4b. This result is reasonable since the physisorbed CO molecule prefers to desorb from graphene at higher temperature, which makes the reaction more difficult to occur. Since a surface reaction at room temperature may occur only when Ebar < Ecbar = 0.91 eV,38 the reaction for the co-adsorption of CO and O2 molecules in Fig. 4b will happen at room temperature, the LH mechanism is thus focused in the following.
For CO oxidation with LH mechanism, there are several steps and also intermediate produces (MS) for the oxidation procedure as reported.21,23,24 For each reaction step, e.g. from IS to MS in Fig. 5a, there is also a transition state, the corresponding atomic structures before (IS) and after oxidation (FS), and MS are given in Fig. 5a. The configuration of co-adsorbed CO and O2 on the Al-embedded graphene is taken as the initial state (IS in Fig. 5a) before oxidation based on the discussion above, where CO and O2 are titled and parallel to the graphene surface, respectively. At TS state, one Al–O bond elongates from 1.86 to 2.38 Å and the new C–O bond is formed. After overcoming an energy barrier of 0.32 eV, OOCO complex (MS in Fig. 5a) is formed while one O–Al bond is broken. This reaction continues with the formation of one CO2 molecule (MS2) after overcoming an energy barrier of 0.18 eV (see Fig. 5b), and the CO2 molecule is physically adsorbed in MS2 structure with an adsorption energy of −0.16 eV. Since the energy released in this step (2.34 eV) can sufficiently surmount the adsorption energy, the desorption of the first produced CO2 molecule would be efficient. Note that the OOCO configuration in MS remains at TS2 with slight adjustments of bond length and bond angle. The remaining O atom will react with another CO to produce CO2 after overcoming an energy barrier of 0.10 eV based on our calculation (see Fig. 5c), similar to the reported result on Au-embedded graphene.21 The energy released in this step (2.60 eV) can also overcome the adsorption energy of CO2 (−0.40 eV) in FS structure and the desorption of the second CO2 molecule would be efficient as well. The energy profile for the formation of the first CO2 molecule is shown in Fig. 5d where the rate limiting energy barrier is quite low (0.32 eV) for the formation of OOCO complex. Therefore, CO molecules can be easily oxidized on Al-embedded graphene at room temperature, indicating that Al-embedded graphene is an excellent catalyst for the CO oxidation. The catalyst efficiency is the second highest among present catalysts except Au-embedded graphene (0.31 eV (ref. 21)).
The corrected energy barriers at room temperature for each step in Fig. 5 are: E′bar = 0.35 eV for step1, E′bar = 0.12 eV for step2, and E′bar = 0.19 eV for step3. The energy barriers for step1 and step3 slightly increase at room temperature compared with those at zero kelvin. The reaction time for each step in Fig. 5 at room temperature is estimated by the following equation,
![]() | (2) |
To gain more insight into the origin of the high activity of the Al-embedded graphene, the Hirshfeld charge transfer and PDOS near the adsorbates along the LH reaction path are shown in Fig. 5 and 6, respectively. O2 obtains 0.362e while CO loses 0.163e by Hirshfeld charge analysis for IS structure in Fig. 5a. The initial empty component of the O2-2π* orbital is partially filled while the CO-2π* orbital is nearly empty (see Fig. 6a) due to the electron transformation during the adsorption. Therefore, there is a tendency of charge transfer from the electron-efficiency O2 to the electron-inefficiency CO, which will facilitate the bond formation of O2 and CO. At TS state, the Hirshfeld charge of O2 molecule is 0.287e while that of CO molecule is 0.094e in Fig. 5a, which indicates that there is charge transfer from O2 to CO. The CO–2π* orbital is more filled and splitted as shown in Fig. 6b. After overcoming the energy barrier, there is more charge transfer from O2 to CO, where the Hirshfeld charge of O2 molecule is 0.237e while that of CO molecule is 0.049e for MS structure in Fig. 5a. The O2–2π* orbital is significantly depressed while the CO–2π* orbital is half filled in Fig. 6c. The above discussions show that the Al-embedded graphene mediate the charge distribution of O2 and CO molecules, where the charge transfer from O2 to CO plays a key role for the OOCO formation. Moreover, the states of C–O in CO molecule and O1–O2 (labelled in Fig. 5a) in O2 molecule interact with each other, strengthening C–O2 bond while weakening O1–O2 bond, as indicated by the superposition of the C–O and O1–O2 states at TS2 (see Fig. 6d), which will facilitate the formation of CO2 molecule.
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