Reaction Mechanisms for CO Catalytic Oxidation by N2O on Fe

Jul 18, 2012 - Depletion NO x Made Easy by Nitrogen Doped Graphene. Xilin Zhang , Zhansheng Lu , Yanan Tang , Dongwei Ma , Zongxian Yang. Catalysis Le...
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Reaction Mechanisms for CO Catalytic Oxidation by N2O on FeEmbedded Graphene Sippakorn Wannakao,†,‡,§ Teeranan Nongnual,†,‡,§ Pipat Khongpracha,†,‡,§ Thana Maihom,†,‡,§ and Jumras Limtrakul†,‡,§,* †

Laboratory for Computational and Applied Chemistry, Department of Chemistry, Faculty of Science and Center of Nanotechnology, Kasetsart University Research and Development Institute, Kasetsart University, Bangkok 10900, Thailand ‡ Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand § NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart University, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: Catalytic conversion of hazardous gases can solve many of the environmental problems caused by them. We performed a density functional theory (DFT) study with the Perdew−Burke−Ernzerhof (PBE) functional to investigate the CO oxidation by using N2O as an oxidizing agent over an iron-embedded graphene (Fe-Graphene) catalyst. The N2O molecule was first decomposed on the Fe site yielding the N2 molecule and an Fe−O intermediate, which was an active species for the CO oxidation. The activation energy for the N2O decomposition step was predicted to be 8 kcal/mol. According to the population analysis, the graphene acted as both the electron withdrawing and donating support to assist the charge transfer between the Fe atom and the probe molecules, which are important for the reaction. The reaction was found to be less facile when the Fe site was first covered by the CO which has a higher adsorption energy than that of the N2O (−10.0 vs −33.6 kcal/mol). The reaction proceeded via a concerted transition structure and required an activation energy of 19.2 kcal/mol when the CO was prior adsorbed. Thus, control of the adsorbing molecules over Fe-Graphene might be a key factor for the activity of the catalyst. With the higher catalytic activities of Fe-embedded graphene compared to other typical catalysts, this may open new avenues in searching for oxidation of CO at an economical cost.

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INTRODUCTION The production of poisonous gases from the combustion of fuel, vehicles, and industrial processes is considered to be one of the big current environmental issues to be solved. Among those gases, carbon monoxide (CO) and nitrous oxide (N2O) have been considered to be harmful gases that are emitted from an automobile exhaust. The N2O has been recognized as being one of the contributors to global warming since its global warming potential (GWP) is 300 times greater than that of carbon dioxide (CO2) and is also a cause of ozone depletion.1,2 Even though transformation of N2O and CO to the nonharmful gases, N2 and CO2 is thermodynamically favorable, the high activation energy of an uncatalyzed process has limited the reaction.3 The catalytic conversion of the CO and N2O into N2 and CO2 by one catalyst seems to be the most promising and important way for solving environmental, economic, and social problems. The reaction of oxygen transfer from N2O to CO can be catalyzed by various transition metal cations in the gas phase, which has been widely studied both experimentally and theoretically.3−16 Nitrous oxide is considered as an O-donor molecule because of its low N2-oxygen affinity of 40 kcal/mol.17 © 2012 American Chemical Society

The reaction mechanism of the oxygen transfer was proposed in two elementary reactions, which are as follows: the decomposition of N2O on the metals and followed by the oxidation of CO as presented in eqs 1 and 2, where the metal cation is noted as M+: - The decomposition of N2O: N2O + M+ → MO+ + N2

(1)

- The oxidation of CO: MO+ + CO → M+ + CO2

(2)

Among the metal cations in the gas phase, Fe+ has a good catalytic activity toward these reactions. Kappes and Staley18 have shown that only Fe+ can oxidize the CO, while Ti+, Zr+, V+, Nb+, and Cr+ cannot exhibit such activity. Moreover, the N2O decomposition on the Fe+ generating FeO+ intermediate can occur with no energy barrier by an ion-beam technique.19 The FeO+ intermediate can also oxidize several organic Received: April 12, 2012 Revised: July 15, 2012 Published: July 18, 2012 16992

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also gives calculated energy gaps that are in good agreement with experimental results.25,32,41−44 A graphene sheet was generated in a supercell hexagonal periodic box composed of 4 × 4 repeated unit cells, which has been proven to be sufficient for investigating the reaction mechanisms in this study.32,34 The Fe atom was placed at a single vacancy site in the center of the graphene sheet due to the high Fe diffusion barrier of the defect compared to the perfect graphene.33 After the Fe atom was loaded on the graphene sheet, the supercell was reoptimized by scanning the length of the periodic box along the graphene plane with an interval of 0.01 Å. In order to avoid interaction by the mirror image, the distance between the graphene sheets was set at 20 Å. The Brillouin zone integration was calculated with 3 × 3 × 1 k sampling points for the geometry optimizations in order to gain an accuracy on the graphene sheet. The global orbital cutoff was set in fine quality (4.6 Å). The population analysis was performed by the Hirshfeld charge analysis method.45 Structures of transition states were confirmed by one imaginary frequency that corresponded with the reaction coordinate. Attempts have been made to estimate the energies of reaction pathways and transition states with a cluster model, using B3LYP with the dispersion correction proposed by Grimme46 (DFT-D3) using Gaussian09 package.47 Different quantum cluster sizes of Fe-doped graphene for the key steps of the reaction, i.e., the N2O decomposition and CO oxidation have been performed (cf. Supporting Information S1). We have found that the energy barriers of the transition states decrease as the size of the graphene cluster increases. By using the largest Fe-doped graphene cluster model, which consists of 63 carbon atoms surrounding the Fe active center, the energy barriers for the N2O decomposition and the CO oxidation of 11.2 and 21.7 kcal/mol, respectively. These values are only slightly (2−3 kcal/mol) higher than the ones obtained by our periodic PBE calculations. Therefore, despite the inability to perform periodic B3LYP or similar calculations, we believe that for this chemical reaction our PBE results are reliable.

molecules, for example, CH4, C2H6, and C6H6, forming their alcohol derivatives.4−9 Even though the Fe+ is very active for such reactions, it is still far away from the real-world catalytic converter since reactions need to be carried out under the condition of a mass spectrometer. Finding heterogeneous catalysts that still exhibit the activity for the reaction is promising for applications. Metal loading to the support could be one of those. Graphene, a one layer sheet of carbon atoms which has unique properties, is considered as the new generation of electronic materials.20,21 Due to its large surface area, graphene can be used as support for the heterogeneous catalysis. However, a perfect surface of graphene sheet weakly interacts with transition metal atoms.22−27 A single vacancy of graphene is the simplest point defect that can be observed by experiments.28−31 Foreign atoms, such as transition metals, can be put into the single vacancy of graphene and used as an active site for chemical reactions. Krasheninnikov et al.25 have studied the structure, bonding, and magnetism properties of transition metal atoms (Sc−Zn, Pt, and Au) doped on a vacancy of graphene. In addition, they also pointed out that the migration barriers of metal atoms on a single vacancy of graphene were quite high compared to those of perfect graphene. This observation confirms that metals-embedded graphene would be stable enough to be utilized in catalysis applications. For the CO oxidation reaction, some metals have been loaded to the graphene sheet and are being studied for the CO oxidation with oxygen, for example, Au,32 Fe,33 and Cu.34 Moreover, metal clusters on graphene sheet provide lower activation energies than those of nonsupported metal clusters.35−37 It is noted that graphene might modify those metal clusters electronic properties. Among those, Feembedded graphene (Fe-Graphene) is a low cost catalyst that can proceed the reaction under a low temperature condition. Thus, the graphene sheet could be one candidate for supporting the Fe atom without losing its catalytic activity. Although many metals embedded on graphene were studied, the role of the support has never been mentioned before. In this work, we employed the periodic density functional theory calculation for studying the oxidation of CO by using N2O decomposition on Fe-Graphene as an oxidizing catalyst. To the best of our knowledge, no previous study has been made of Fe-Graphene for N2O decomposition. This work shows the possibility for utilizing Fe-Graphene for reducing environmentally harmful gases. This is very important for the development of an automobile catalytic converter that can clean up more than one gas by a single process.

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RESULTS AND DISCUSSION The embedded Fe atom, which locates on the atomic vacancy of the graphene sheet, can give significant stress on the FeGraphene structure, leading to an overestimated interaction to any probe molecules. To avoid this problem, the cell parameters along the graphene plane, a and b, were concurrently reoptimized by scanning their lengths from 9.87 Å to 9.93 Å with an interval of 0.01 Å. As a result, the cell parameters at a = b = 9.91 Å were found to be the minimum in energy. After the cell reoptimization, the supercell hexagonal periodic box was fixed at 9.91 × 9.91 × 20.0 Å3 for investigating the reaction mechanism on this catalyst. The optimized structure of the Fe-Graphene presents the atomic distance between the Fe atom and its neighboring carbon atoms of 1.77 Å. The Hirshfeld population analysis shows partial charges: +0.254e on the Fe atom, −0.053e on each neighboring C atom, and −0.010e on the other C atoms. This result indicates that the electron density on the Fe atom can transfer to the graphene sheet which acts as an electron withdrawing support. In addition, this can suggest that the Fe atom is activated to be more electrophilic by its support. The electrophilic and nucleophilic Fukui function plot of the Fe-Graphene confirms that the Fe atom exhibits a strong value in electrophilic and nucleophilic properties (see Figure 1). The frontier molecular

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METHODOLOGY The periodic calculations were carried out using the spin unrestricted density functional theory (DFT) with the Perdew−Burke−Ernzerhof (PBE) functional38, implemented in DMol3 code.39,40 The double numerical basis set including dpolarization functions (DND) and the DFT semicore pseudopotentials (DSPPs) were employed. The DND basis set corresponds to a double-ζ quality basis set with d-type polarization functions and is comparable to 6-31G(d) Gaussian basis sets. By the DSPP scheme, all-electron calculations were performed for the C, N, and O atoms, and a relativistic effect included effective potential was used to represent core electrons of the Fe atom. This method has been found to be appropriate to describe this type of interaction in several relevant studies involving carbon-based materials such as carbon nanotubes and 16993

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Figure 1. Electrophilic (left) and nucleophilic (right) Fukui functions of the Fe-Graphene calculated by GGA:PBE/DND level of theory.

Figure 2. Frontier molecular orbitals of the Fe-Graphene, the contour value is ±0.03 eÅ−3. Electron depletion and electron accumulation are shown in yellow and blue, respectively.

orbitals, HOMO and LUMO, dominantly locate on the Fe atom (see Figure 2). The LUMO illustrates the strong dorbitals characteristics of the Fe atom allowing an incoming electron to occupy this state. All of these observations demonstrate that the Fe atom might be the active site for adsorbing an electrophile probe molecule. According to the possible situation in which two reactant gases, nitrous oxide and carbon monoxide, are completely mixed, the reaction mechanisms are divided into two pathways depending on the order of adsorption on the Fe site. 1. Stepwise Reaction by the Adsorption of Nitrous Oxide Followed by Carbon Monoxide. The reaction mechanism is proposed in stepwise reactions, which are the decomposition of N2O and followed by the oxidation of CO as presented in eqs 3 and 4: - The adsorption and decomposition of N2O: N2O + Fe‐Graphene → N2 + O − Fe‐Graphene

Figure 3. Energy profile of the N2O decomposition on the FeGraphene calculated by PBE/DND level of theory. All energies are reported in kcal/mol relative to the reactants, namely, Fe-Graphene, N2O, and CO and distances are in angstrom units.

(3)

- The adsorption and oxidation of CO: CO + O − Fe‐Graphene → CO2 + Fe‐Graphene

electron densities between the whole structure and its two isolated parts, which are the N2O and the Fe-Graphene. The definition; Δρ(ij) = ρ(ij) − ρ(i) − ρ(j), is considered and implied to both resonance character and the transfer of an electron between fragments in the system after binding. The PEDD plot shows the charge transfer from the N2O to the FeGraphene and also the reduction of electron density at the N O bond. The PEDD plot agrees well with the results of the Hirshfeld population analysis. The adsorption of the N2O on the Fe-Graphene is then compared to that on the bare Fe+ cation in the gas phase. The N2O can be adsorbed on the Fe-Graphene, which is weaker than in the Fe+ cation with its adsorption energy of −15.0 kcal/ mol.3 This is caused by the higher electrophilic properties in the Fe+ cation. The N2O, which is attached to the Fe-Graphene, is then decomposed via the transition structure, TS1. The energy profile of the decomposition is summarized in Figure 4. While the Fe···O distance is shortened from 2.10 Å to 1.82 Å, the N− O bond length is lengthened from 1.21 to 1.43 Å. This step requires an activation energy of 8.0 kcal/mol. The O atom closer to the Fe atom can accumulate more electrons by the significant increase of its negative charge from −0.029e to

(4)

The N2O adsorbs on the Fe atom in the N = N−O···FeGraphene adsorbing direction with the adsorption energy of −10.0 kcal/mol as shown in the N2O-ADS (Figure 3), because the strong partial negative charge on the O atom is attracted to the electrophilic site. The Fe···O atomic distance is 2.10 Å, while the N−O bond length is slightly elongated from 1.19 to 1.21 Å. The Hirshfeld charge analysis (see Table 1) shows that an electron on the Fe atom is gained with the decrease of its positive charge from +0.254e to +0.139e, while an electron on the O atom is lost with the decrease of its negative charge from −0.119e to −0.029e. From the molecular charge point of view, an electron on the N2O molecule is withdrawn by the FeGraphene, from a neutral state to be a positive charge of +0.182e. Therefore, the difference of the partial charges after the adsorption reveals that an electron transfers from the N2O to, not only the Fe atom of −0.115e, but also the graphene support of −0.067e. In other words, the graphene support can significantly assist in withdrawing an electron from the N2O molecule. The partial electron density difference (PEDD) plots, as shown in Figure 4, are calculated from the difference of the 16994

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Table 1. Hirshfeld Atomic Charge of Considered Atoms during the Reaction Pathways charges of the systems/e atoms

isolated systems

N2O-ADS

TS1

INT-FeO

INT-FeO−CO-ADS

TS2

CO2-ADS

Fe O N1 N2 Sum(N2O) C O1 Sum(CO) Sum(Graphene)

0.254 −0.115 0.198 −0.083 0.000 0.724 −0.722 0.002 −0.254

0.139 −0.029 0.221 −0.011 0.182

0.156 −0.129 0.095 −0.041 −0.075

0.201 −0.252

0.125 −0.250

0.120 −0.230

0.149 −0.088

0.117

0.170 −0.057 0.113 −0.003

0.341 −0.059

0.051

0.159 −0.060 0.099 0.026

0.118 −0.082 0.036 −0.153

−0.321

−0.081

−0.343

CO-ADS

CO-N2O-ADS

TS-con

0.107 −0.086 0.198 −0.064 0.048 0.105 −0.093 0.012 −0.167

0.085 −0.144 0.107 −0.023 −0.061 0.136 −0.076 0.060 −0.085

Figure 4. Electron density difference between the N2O adsorption state and the Fe-Graphene catalyst. The contour value was ±0.005eÅ−3 (the green region gains more electrons and the yellow less).

−0.129e, whereas an electron on the Fe atom is dramatically withdrawn by the increase of its positive charge from +0.139e to +0.295e. This charge transfer from the Fe atom back to the O atom might occur from the oxygen transfer from N2O to the Fe active site, forming the O−Fe-Graphene intermediate. Moreover, the graphene sheet became less negative. In a molecular point of view, one can say that an electron was transferred from Fe-Graphene back to the N2O to break the N−O bond. After the dissociation of the N2O, the N2 molecule can easily desorb from the O−Fe-Graphene using the low desorption energy of 1.7 kcal/mol. The O−Fe-Graphene intermediate (INT-FeO, cf Figures 3 and 5) shows the O−Fe bond length of 1.62 Å, with the relative energy of −37.2 kcal/mol. As compared to the TS, an electron transferring from the FeGraphene doubles the negative charge on the O atom from −0.129e to −0.252e, while the positive charge on the Fe atom is increased from +0.156e to +0.201e, and the charge on the graphene is reversed from the negative charge of −0.081e to the positive charge of +0.051e. This observation indicates that the oxygen atom can strongly pull the charge back from the FeGraphene support due to its strong electronegativity. The CO molecule can be oxidized by the O−Fe-Graphene intermediate due to not only the strong negative charge on the O atom of the intermediate which can attract the C atom of the CO, but also the Fe atom which can receive an electron and can stabilize the intermediate. The adsorption of the CO molecule on both O and Fe atoms of the O−Fe-Graphene was found with the adsorption energy of −10.5 kcal/mol (see Figure 5). The Fe···C and O···C distances are 1.98 Å and 2.31 Å, respectively, while the Fe−O bond length is slightly elongated from 1.62 to 1.67 Å. The molecular positive charge of the CO is increased from neutral to +0.099e, whereas the atomic positive

Figure 5. Energy profile of the CO2 formation step of the CO oxidation reaction with N2O on the Fe-Graphene catalyst.

charge of the Fe atom is decreased from +0.201e to +0.125e. This result shows that an electron of the CO molecule can transfer to the Fe atom. The CO2 formation can easily occur via the transition state, TS2, by the low activation energy of 4.0 kcal/mol. This transition state shows the O atom moving from the O−FeGraphene to the CO molecule. The O···C atomic distance is decreased from 2.31 Å to 2.19 Å, while the Fe···C distance and the Fe−O bond length are insignificantly changed. Finally, the CO2 is formed with its intermolecular distance to the FeGraphene of 2.13 Å, which can desorb from the Fe-Graphene with the desorption energy of 9.6 kcal/mol. The transformation of the N2O and CO reactants to the highly stable products of CO2 and N2 is calculated to be an exothermic reaction with a total energy change of −80.2 kcal/ mol. This value acceptably agrees with the experimental value of −87.0 kcal/mol.18 The desorption of the CO2 product is the rate limiting step due to its higher energy barrier during the reaction pathway. The transition structure of the CO 2 formation in this work using an activation energy of only 4.0 kcal/mol is more favorable than that was found in the literature with an activation energy of 25.6 kcal/mol.33 Furthermore, Zhou et al.48 have investigated the adsorption of gases on metal embedded graphene. For O2, a significant elongation of the O− O bond has been observed which is due to the interaction of the oxygen molecule with the Ti- and Au-doped graphene materials. As the O−O covalent bond is weakening, the oxygen becomes more basic, increasing its ability to promote reactions such as the CO oxidation. O2-activated graphene doped with Ti 16995

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that the graphene support can assist for an electron backdonation in its doped metal. The partial electron density difference plots of the CO adsorption are illustrated in Figure 7.

or Au might therefore be an alternative material for CO oxidation. 2. Concerted Reaction by the Coadsorption of Carbon Monoxide and Nitrous Oxide. The reaction mechanism is proposed by the adsorption of the CO followed by the N2O. The coadsorption of these two reactants results in the concerted reaction of the oxygen transfer as presented in eqs 5−7. - The adsorption of the CO followed by the coadsorption of the N2O: CO + Fe‐Graphene → CO − Fe‐Graphene

(5)

N2O + CO − Fe‐Graphene → N2O − CO − Fe‐Graphene

Figure 7. Electron density difference between the N2O adsorption state and the Fe-Graphene catalyst.

(6)

- The oxygen transfer: The electron accumulation zone in green color covering the C and Fe atoms clearly shows not only an electron transfer to the Fe atom, but also an electron back-donation to the C atom. The back-donation is supported by an electron from the graphene as shown in its electron depression zone in yellow color. The plots agree well with the results of the Hirshfeld population analysis. The N2O is then coadsorbed on the CO−Fe-Graphene with the coadsorption energy of −36.3 kcal/mol. Upon the coadsorption of the N2O molecule, the overall system is more stable by only 2.6 kcal/mol. This may be due to the fact that there is a lack of the coordination site of the Fe atom and the strong adsorption of the CO (and the electrophilicity of Fe may be reduced by those electron accumulations between the C and Fe atoms). The coadsorption N2O−CO−Fe-Graphene presents the intermolecular distances of O···C and O···Fe with the same length of 3.12 Å and the unperturbed Fe−C bond length of 1.86 Å. The population analysis shows that an electron on the N2O is slightly lost from the neutral state to the positive charge of +0.048e. The electron transfers to the CO− Fe-Graphene, increasing its partial charge of the CO, Fe, and graphene by −0.024e, −0.010e, and −0.014e, respectively. This result indicates that an electron on the N2O can transfer to not only its binding neighbors of the CO and the Fe atom, but also to the graphene support. The reduction of the N2O concertedly takes place via the oxygen transfer to the CO, forming the N2 and CO2 molecules. The transition step presents the O atom detaching from the N atom and then binding to the C atom with the activation energy of 19.2 kcal/mol. The intermolecular distance of O···C is dramatically shortened from 3.12 to 2.46 Å, while the N−O bond length is stretched to 1.49 Å. In addition, the Fe atom may play a role in attracting the O atom by shortening the O···Fe atomic distance from 3.12 to 1.84 Å. The vibration frequency mode of the transition state clearly illustrates and confirms the trajectories of the oxygen transfer. The concerted reduction pathway is calculated to be an exothermic reaction with a total energy change of −102.8 kcal/mol. The graphene support plays an important role in the chemical properties for the Fe-doped graphene. The positive charge of +0.254e of the Fe atom in the Fe-Graphene, representing an unoccupied d-orbital, can be lowered to +0.139e or +0.117e by an electron transfer from the adsorption of N2O or CO, respectively. This result suggests that the Fe atom in the Fe-Graphene can only act as an electron withdrawing group, either attacked by an electrophile N2O or

N2O − CO − Fe‐Graphene → N2 + CO2 + Fe‐Graphene

(7)

The CO strongly adsorbs on the Fe-Graphene in the OC···Fe-Graphene adsorbing direction (Figure 6) with the adsorption energy of −33.6 kcal/mol, which is much stronger than that of the N2O adsorption (−10.0 kcal/mol).

Figure 6. Energy profile of the concerted reaction pathway of the CO oxidation with N2O on the Fe-Graphene catalyst.

The Fe···C intermolecular distance is 1.86 Å, while the C−O bond length is slightly lengthened from 1.14 to 1.16 Å. The Hirshfeld charge analysis shows a significant gain of the electrons on the Fe and C atoms with the decrease of their positive charge from +0.254e to +0.117e and from +0.724e to +0.118e, respectively, while an electron on the O atom is dramatically lost with the decrease of its negative charge from −0.722e to −0.082e. From the molecular charge point of view, an electron on the CO molecule is slightly donated to the FeGraphene, from the neutral state to be the positive charge of +0.036e. This observation is in accordance with the adsorption of CO on the Cu, Ag, and Au embedded graphene.48 The difference of the partial charges after the adsorption reveals that an electron transfers from the CO molecule to the FeGraphene and, surprisingly, strongly back-donates to π electronic states of the carbon atom. The back-donation is supported by the graphene support with the decrease of its negative charge from −0.321e to −0.153e. This result suggests 16996

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a nucleophile CO. In contrast, the negative charge of −0.254e of the graphene in the Fe-Graphene can be not only decreased to −0.321e by an electron transfer from the adsorption of N2O, but also increased to −0.153e by its electron back-donation to the adsorption of CO. This result suggests that the graphene support can either act as an electron withdrawing group or donating group, depending on the electronic properties of a probe molecule. This donor−acceptor ability was also shown in the N2O reduction step. In order to understand the electron transferability between probe molecules and the Fe-Graphene, the electronic structures are investigated by the plots of partial density of states (PDOS) as shown in Figure 8. Significant peaks are marked by Peaks 1−

This result suggests that the bare Fe-Graphene catalyst is highly selective for the CO adsorption, leading to the coadsorption of N2O and then to the concerted oxygen transfer pathway. In order to improve the reaction activity by decreasing the influence of the selectivity of CO, two solutions can be suggested, one, loading one reactant at a time, beginning with the N2O for its reduction and followed by the CO for its oxidation. However, it is quite impractical to feed two reactants one-by-one in tandem. Moreover, the excess CO from its oxidation by the O−Fe-Graphene, can adsorb on the bare FeGraphene obtained by the oxidation itself. The other solution is that the contribution of the N2O should dominate over the CO in their mixed feed. The high-loading of N2O can not only adsorb on the bare Fe-Graphene catalyst improving the reaction rate in the stepwise pathway, but also eliminate the strong CO adsorption on Fe-Graphene via the concerted pathway. Even in a low concentration of CO, the N2O has no adsorption to the O−Fe-Graphene with the adsorption energy of −3.0 kcal/mol, allowing the oxidation site to be active only to the CO. In addition, two gaseous products, N2 and CO2, have weak adsorption of both the bare Fe-Graphene catalyst and O−Fe-Graphene (The adsorption energies of the N2 and CO2 on the Fe-Graphene are −20.2, −9.6 kcal/mol and on the O−Fe-Graphene are −1.7, −2.0 kcal/mol, respectively). Even N2 adsorption on the Fe-Graphene is stronger than that of the N2O, The Fe site was already oxidized when N2 was found. These results indicate that the reaction hardly inhibits the reaction by these products. The feasible properties of the combination of metal and carbon nanostructure can be taken into account in order to improve the catalytic activity. The Fe-Graphene presents a good catalytic active site of the Fe atom and the electronic support by the graphene sheet. The proposed pathways are theoretically possible with the overall activation energy of 9.6 kcal/mol for the CO2 desorption step via the stepwise mechanism, and 19.2 kcal/mol for the oxygen transfer step via the concerted mechanism. The reaction activity might be improved by controlling the contribution of the N2O in the mixed feed in order to overcome the strong selectivity. However, the activation barrier of 19.2 kcal/mol is not high enough to make the reaction significantly slower. From these results, the Fe-Graphene could be a candidate material to reduce the nitrous oxide with the carbon monoxide, converted to environmental-friendly gases at a mild condition.

Figure 8. Partial density of states of N2O and CO adsorb on FeGraphene.

6. Any electronic state which is not shown in the PDOS has no peak in the range −5 to 5 eV, which means no contribution for the adsorption. The N2O and CO can donate their two electrons via σ-donation to the 3d-orbital of the Fe. In addition, the CO is also a very strong π-acceptor which has an antibonding π*2p molecular orbital. The 3d-orbital of Fe merges with the 2p-orbital of defect graphene at Peaks 1 and 2 around the Fermi level. These peaks, which are caused by their strong hybridization, are important for the activity to probe molecules. Low level electronic states of the conduction band are then considered to refer to the electron transferability. For the N2O−Fe-Graphene, the 3d-orbital of Fe at Peak 3 remains unchanged, but the 2p-orbital of N2O highly merges to the FeGraphene at Peaks 2 and 4. Moreover, Peak 4 also indicates that the N2O molecule lost its electrons. These electronic states refer to an electron transfer from the N2O to the Fe-Graphene. For the CO−Fe-Graphene, the Peak 3 disappeared, while the strong combinations are shown at Peaks 5 and 6. These peaks refer to the appearance of new π states between the d orbitals of Fe and p-orbitals of CO. The reaction selectivity is considered in both stepwise and concerted pathways. The N2O can adsorb on the Fe-Graphene with the adsorption energy of −10.0 kcal/mol while the CO can adsorb on the Fe-Graphene with the adsorption energy of −33.7 kcal/mol, which is more favorable than that on the O− Fe-Graphene with the adsorption energy of −10.5 kcal/mol.

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CONCLUSIONS The density functional theory using the PBE functional has been employed to investigate the oxidation of carbon monoxide using nitrous oxide as an oxidizing agent on Fe-Graphene. The graphene sheet acts as both the electron donor and acceptor surface depending on the reaction intermediates during the reaction pathway. Graphene is able to induce the positive charge of an Fe atom and, hence, make it ready to react with the N2O molecule. The charge transfer between the Fe atom and the N2O molecule was found to be important for the N2O decomposition step. The oxidation of CO by N2O on the catalyst (activation energy was 8.0 kcal/mol) was more favorable than the activation of O2 that was previously studied in literature for Fe- and Cu-embedded graphene catalysts (Ea ≈ 12−13 kcal/mol). After the N2O decomposition, the CO molecule easily reacts with the Fe−O intermediate having the low activation energy of 4.0 kcal/mol. The activity of the reaction was found to decrease when the CO molecule was first 16997

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The Journal of Physical Chemistry C

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adsorbed on the Fe-site of the catalyst. However, the activation energy is considered to be not high enough to inhibit the reaction. Our findings suggest that Fe-Graphene is one of the promising candidates for solving the environmentally harmful exhaust gases generated from vehicles and industrial wastes.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 and Table S1.. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +66 2562 5555 ext 2176. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by grants from the National Science and Technology Development Agency (NSTDA Chair Professor and NANOTEC Center of Excellence), the Thailand Research Fund (to J.L.), the Kasetsart University Research and Development Institute (KURDI), the Commission on Higher Education, Ministry of Education (“the National Research University Project of Thailand (NRU)” and “Postgraduate Education and Research Programs in Petroleum and Petrochemicals and Advanced Materials”), and by a grant under the Royal Golden Jubilee Ph.D. program from the Thailand Research Fund (to S.W.). The Graduate School, Kasetsart University, is also acknowledged.

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dx.doi.org/10.1021/jp3035192 | J. Phys. Chem. C 2012, 116, 16992−16998