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CO Oxidation Prefers Eley–Rideal or Langmuir– Hinshelwood Pathway: Monolayer vs. Thin Film of SiC Sinthika Sinthika, Surya Teja Vala, Yoshiyuki Kawazoe, and Ranjit Thapa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11384 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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ACS Applied Materials & Interfaces

CO Oxidation Prefers Eley–Rideal or Langmuir–Hinshelwood Pathway: Monolayer vs. Thin Film of SiC S. Sinthika#, 1, Surya Teja Vala#, 2, Y. Kawazoe3,4, and Ranjit Thapa*1, 2 1 2

SRM Research Institute, SRM University, Kattankulathur 603203, Tamil Nadu, India

Department of Physics and Nanotechnology, SRM University, Kattankulathur 603203, Tamil Nadu, India 3

New Industry Creation Hatchery Center (NICHe), Tohoku University, Sendai, Japan 4

#

Thermophysics Institute, Siberian Branch, Russian Academy of Sciences, Russia

these authors contribute equally to this work

ABSTRACT Using first principles approach, we investigated the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer and thin film structures and substantiated its catalytic activity towards CO Oxidation. 2H-SiC monolayer being planar is quite stable and has moderate binding with O2 while CO interacts physically thus Eley Rideal (ER) mechanism prevails over Langmuir Hinshelwood (LH) mechanism with an easily clearable activation barrier. Contrarily 2H-SiC thin film that exhibits non-planar structure allows moderate binding of both CO and O2 on its surface thus favoring LH Mechanism over ER. Comprehending these results leads to better understanding of the reaction mechanisms involving structural contrast. Weak overlapping between the 2pz (C) - 3pz (Si) orbital of the SiC-ML system has been found to be the primary reason to revert the active site toward sp3 hybridization, during interaction with the molecules. In addition, the influences of graphite and Ag-111 substrates on the CO oxidation mechanism were also studied and it is observed that ER mechanism is preserved on SiC/G system, while CO oxidation on SiC/Ag-111 system follows LH mechanism. The calculated Sabatier activities of the SiC catalysts show that the catalysts are very efficient in catalyzing CO oxidation. KEYWORDS: Adsorption, Oxidation, Interface, Density Functional Theory, Electronic Structure

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1. INTRODUCTION Carbon monoxide (CO) has high affinity to react with haemoglobin, making it a highly toxic gas for living beings. Chief sources for CO include transportation, power plants, industrial and domestic activities.1 CO oxidation is a process to reduce the emission of toxic CO gas from automobile exhaust and also to prevent poisoning of fuel cell catalysts.2-5 CO oxidation is often used as a probe reaction to study catalysis.6, 7 On a catalytic surface, CO oxidation reaction involves CO adsorption, O2 adsorption/dissociation, the reaction between CO and O2 (or dissociated O) 8-10and desorption of CO2. An extensive number of noble metal based catalysts Au, Pt, Ru, Rh, Pd, and Cu have been proposed and investigated by many researchers for the preferential oxidation of carbon monoxide.11-14 Platinum is most commonly used in catalytic converters for the oxidation of toxic pollutants released by the vehicles. Most of these metal-based catalysts suffer from many drawbacks that include cost, abundance, and durability where a major loss of catalytic activity occurs. This is even problematic when it comes to CO poisoning in fuel cells, where the Pt/C catalyst can longer catalyze some vital reactions like H2 splitting at the anode.15,16 Metal-free catalysts are considered to be the best replacements to Pt catalysts as they are relatively more abundant as well as cheaper than Pt while being resistant to catalytic poisoning.17 Introduction of dopants like N, B, Fe or Cu into the structures of graphene or CNTs enhances its chemical activity making it a better catalyst.18-21 Recent studies show that electron doped h-BN sheets are effective catalysts for CO oxidation.22 Surface structure and particle size of a catalyst also play vital roles in determining the activity of a catalyst toward the CO oxidation. For instance, extended gold surfaces that are considered to be extremely noble in their bulk form are able to effectively catalyze CO oxidation when the particle size is reduced to nanometers. The changes in the electronic structure, the lower coordination of gold atoms in the nanoscale regime and special 2


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geometries of gold nanoclusters synergistically enhance the reactivity and decide the reaction mechanism by which CO oxidation occurs.11The dependence of catalytic activity on the size, structure and shape are also observed in other metal catalysts.23 Recently, it has been observed that the size effects are prominent in metal-free catalysts also, with the reactivity being structure-selective and the catalytic properties differing significantly from monolayers to nanotubes.24-25 Moreover, it has also been proposed theoretically and verified experimentally that the presence of an underlying substrate can significantly alter the catalytic properties of an ordinarily inert material. The enhancement in activity has been linked to a number of factors including electron tunnelling, polaron-like distortions and mixing of substrate-overlayer orbitals26-28. Silicon carbide has emerged as a novel metal-free catalyst for many important reactions. Theoretically, it has been suggested that the catalytic potential of SiC nanotubes is dependent on the tube diameter and the presence of defects.29-31 Experimental studies also ascertain that SiC nanotubes are efficient in catalyzing the reduction of H2S in S.32 It has also been revealed experimentally that porous SiC exhibits excellent catalytic activity toward CO oxidation.33 To consider SiC as a CO oxidation catalyst, an atomic-level understanding of the adsorption energies, reaction intermediates, barrier energies, and reaction energies is anticipated. Also, we need to be clear about the preferred mechanism, either Eley-Rideal (ER) or Langmuir-Hinshelwood (LH) for CO oxidation based on the topology of SiC surfaces. In this work, we investigated in detail the dissimilarity in the electronic and chemical properties of wurtzite silicon carbide (2H-SiC) monolayer (henceforth named as SiC-ML) and thin film (henceforth named as SiC-TF) structures. The weak overlap between Si (3pz) and C (2pz) results in the SiC-ML surface being able to bind O2 strongly, at the same time being tolerant towards CO. The SiC-TF surface, on the other hand has unsaturated bonds at 3



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its top layer amounting to strong adsorption of O2 and moderate adsorption of CO. This contrast in the adsorption properties of SiC-ML and SiC-TF leads to differences in the mechanism by which CO oxidation occurs in both surfaces: SiC-ML prefers ER mechanism, while SiC-TF prefers LH. Here we also discuss the substrate-induced effects on CO oxidation over a SiC-ML surface and observe that in the presence of a metallic substrate, SiC-ML is able to bind CO, and the preferred mechanism switches over from ER to LH, while a graphite substrate preserves the ER mechanism of CO oxidation. 2. COMPUTATIONAL DETAILS Theoretical calculations were performed using spin-polarized density functional theory as implemented in the Vienna ab-initio simulation package (VASP).34 The generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE)

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functional was

employed to describe the exchange and correlation effects and the potentials of the atoms were described by the projected augmented wave (PAW) 36 method. In order to incorporate the effects of long-range van der Waals attraction, the Grimme’s method (DFT-D2) was used along with PBE functional (denoted as PBE+D).37 A 3x3 supercell of SiC containing 18 carbon atoms was used in simulating the SiC monolayer. Four layers of silicon and carbon were chosen to model a thin film of SiC and the last layer was kept fixed to represent the bulk. For simulating SiC monolayer on graphite substrate, a 4x4 SiC supercell on three layer of graphite-001 was considered (henceforth named SiC/G). To model SiC monolayer on a metal substrate, a 4x4 SiC supercell on four layer 4x4 Ag-111 slab was chosen (henceforth named SiC/Ag-111), which ensured minimal lattice mismatch. The last two layers of the Ag were fixed in all calculations to represent the bulk metal. A vacuum thickness of 15 Å was used in the direction perpendicular to the plane to avoid interaction between periodic images. It was found that plane wave cut-off energy of 500 eV was sufficient to give well-converged results. Brillouin zone sampling was done using a 9x9x1 Monkhorst-Pack grid. All the 4


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structures were relaxed until the total energy converged to less than 10-5 eV per atom and the maximum force converged to lower than 0.001 eV/Å. The adsorption energy of the reactants on the surface was calculated by subtracting the total energy of the molecule placed far from the surface (8 Ǻ) from the total energy of the molecule adsorbed on the surface. The free energy profile is constructed using ∆G = ∆E – T∆S + ∆ZPE, where ∆E is the total energy change obtained from DFT calculations, ∆S denotes the entropy change and ∆ZPE is the change in the zero point energies. TS of free molecules are obtained from ref 38, while TS of the adsorbates and ZPE of the free molecules and adsorbates are estimated from our DFT calculations considering vibrational frequencies of the molecules in the harmonic approximation.39 3. RESULTS AND DISCUSSION 3.1. Structural Analysis. We first compare the optimized geometry of bulk 2H-SiC with that of monolayer SiC and a (001) C-terminated four-layer slab. The bottom layer of the four-layer slab was fixed and passivated with hydrogen to saturate the broken bonds and to ensure that the last layer resembles the bulk SiC (see inset of Figure1b). The optimized lattice parameter of bulk SiC was found to be a=b= 3.116 Å. The SiC-001 retained the lattice parameter similar to bulk after optimization (a=b= 3.119 Å), whereas the SiC-ML lattice parameter is found to be slightly smaller (a=b= 3.094 Å). Furthermore, the bond length of Si-C varied from 1.882 Å in bulk to 1.897 Å in SiC-TF and 1.787 Å in SiC-ML. The results of geometry optimization are in agreement with previous theoretical results by Bekaroglu et al.,40 who obtained a lattice parameter of 3.091 Å and 3.094 Å respectively for bulk SiC and SiC-ML respectively using PW91 functional. Also, the Si-C bond length was calculated to be 1.896 Å in bulk SiC and 1.786 Å in 2D SiC-ML. Atoms in bulk SiC are sp3 hybridized with more p character in their

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hybrid orbitals. On the other hand, atoms of SiC-ML are sp2 hybridized, having more s character in their hybrid orbitals and hence stronger bonds, which is the reason for the reduced Si-C bond lengths in SiC-ML. Also, Bader charge analysis indicates that the amount of charge transferred from each Si to the more electronegative C is 2.61e in the case of bulk SiC and 2.51e in SiC-ML. In bulk SiC, each C is tetrahedrally coordinated with four Si neighbors, while each C in SiC-ML, is three-fold coordinated. Thus, the average charge transferred per bond is smaller in the case of bulk SiC resulting in weaker and elongated Si−C bonds as compared to SiC-ML. In the case of SiC-TF, the unsaturated top C atoms are less negative (-2.01 e) as they withdraw electrons from only three neighboring Si atoms. All other C atoms of SiC-TF withdraw 2.56 e of charge from the neighboring Si atoms. In order to gain more insight on the electronic properties of monolayer and thin film SiC, the density of states (DOS) of the two systems are plotted and shown in Figure 1. The density of states of the SiC monolayer (see Figure 1a) resembles that of the bulk SiC, indicating the semiconducting nature of the system, with a band gap of 2.19 eV, while the SiC-TF surface has a band gap of 2.59 eV. A notable difference in the DOS between SiC-ML and SiC-TF is the presence of localized surface states in the band gap of SiC-TF (see Figure 1b) arising from the dangling bonds of unsaturated Si atoms in the topmost layer. This suggests that the SiC-TF could be chemically more reactive in interacting with incident molecules. 3.2. Adsorption of Reactants. Before investigating the CO oxidation mechanism, the adsorption of the reactants CO and O2 were studied on the two silicon carbide structures Monolayer and thin film. In the case of SiC-ML, it was found that molecular O2 prefers to adsorb parallel to the Si-C bond, in the side-on fashion with an adsorption energy(Ead) of 0.51 eV/-0.65 eV (PBE/PBE+D). The O2 bond length is 1.50 Å and the O2 surface distance is 1.74 Å (from Si) and 1.53 Å (from C). The strong tendency of SiC-ML toward O2 adsorption 6


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can be understood from the orbital-projected density of states and the band projected charge density (in the range -2 eV to 0 eV, where only the pz peaks are present) of a unit cell of SiCML, shown in Figure 2. Even though the px and py orbitals of Si and C atoms are involved in forming the in-plane strong σ bonds (see Figure 2a and 2b), the valence pz electrons do not overlap considerably (see Figure 2c and 2d). The pz states near Fermi level (EF) in the valence band are dominated mainly by carbon, while the states near EF in the conduction band are dominated by the silicon peaks. The weak 2pz (C) - 3pz (Si) overlap implies that these orbitals are readily available for the system to revert to sp3 hybridization. Hence when SiC-ML interacts with an incoming O2 molecule, one Si and one C atoms that are attached to O2 are pulled out of the SiC plane, which indicate some sp3 bonding feature.41 This strong adsorption can be attributed to the transfer of electrons from the SiC surface to O2, which is clearly revealed from the partial density of states (PDOS) plot of O2 and SiC-ML after adsorption, as shown in Figure 3a. The 2π* states of O2 become partially occupied acquiring electrons from Si and C, indicating strong adsorption. Bader charge analysis indicates that the amount of charge transferred to the O2 molecule is 1.24 e. On the other hand, a CO molecule interacts physically with SiC-ML, with a small adsorption energy of 0.074 eV/-0.068 eV at the C site and 0.097 eV/-0.023 eV at the Si site. The PDOS of CO adsorbed on SiC-ML, shows that the CO molecular levels are unaffected and retain their discrete nature of peaks. The higher CO tolerance of the SiC surface indicates that the SiC-ML surface is not poisoned easily by CO molecules. In the case of O2 adsorbed on C-terminated SiC –TF (Figure 3b), there is a larger amount of charge transfer from the surface p states to the O2 2π* orbital, resulting in a stronger adsorption (Ead = -0.92eV/-1.12eV) in comparison to the O2 adsorption over monolayer SiC. The Bader charge on the O2 molecule is -0.638 e. Because of the unsaturated electronic surface states, the SiC -TF is able to interact with CO molecule also with slightly 7



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higher adsorption energy of 0.061eV/-0.117eV. The adsorption energies of O2 and CO and the Bader charges on the reactant molecules of post adsorption state are tabulated in Table1. The PDOS of CO and SiC-TF after adsorption is shown in Figure 3c. A magnified view of the DOS near the Fermi level is shown in the inset, which clearly proves that the CO molecule’s states are broadened and the π* states become slightly occupied. Again, Bader charge confirms the charge transfer, with CO acquiring 0.046 e from the surface. More physical insight on the nature of interaction of O2 and CO with the SiC surfaces can be obtained using the charge density difference (∆ρ), defined as: ∆ρ = ρ(R-SiC) – ρ (SiC) – ρ(R), where

(1)

ρ(R-SiC), ρ(SiC) and ρ(R) are the individual charge densities of the reactant (O2 or CO) adsorbed system, clean SiC surface, and isolated reactant molecule respectively. The resulting plots of charge density difference (CDD) isosurfaces are shown in Figure 4. The blue and pink lobes represent charge accumulation and charge depletion respectively and the isovalue is set to 0.001 e/Bohr3. From Figure 4a and 4c, it is evident that SiC-ML and SiCTF surfaces undergo the highest charge redistribution after O2 adsorption, indicating strong tendency to adsorb O2. As can be seen from Figure 4b, the CO molecule does not interact with SiC-ML, with the electron density lobes of CO being unaffected after adsorption. But the interaction of CO with SiC-TF is enough to cause charge redistribution in the molecule, as is evident from Figure 4d. 3.3. Co-adsorption of reactants. In order to investigate the reaction mechanism by which CO oxidation proceeds on both the surfaces, it is required to assess the bonding behavior when both the adsorbate bind to the surface. SiC-ML interacts very weakly with CO, suggesting that ER mechanism is most likely to occur on these surfaces. On the contrary, coadsorption of O2 and CO is plausible on SiC-TF surface. When an O2 molecule is adsorbed

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on the SiC-TF surface, a CO molecule can adsorb at a neighboring site. Relaxed geometry shows that the O2-CO interaction is so strong that the CO molecule pulls an O atom of O2 toward itself, and the O-O bond elongates to 2.11 Å, forming an intermediate O*+OCO*, where * denotes the adsorbed state. The partial density of states of O-O and CO adsorbed on the SiC-TF is shown in Figure 3d. From a magnified view of the plot near the Fermi level, it is apparent that the molecular orbitals of both CO and O2 hybridize with the surface states, confirming the co-adsorption of reactants. 3.4. Reaction Mechanisms. Next, the mechanism of CO oxidation is explored in detail. Since SiC-ML and SiC-TF are able to bind O2 strongly, CO oxidation by ER mechanism can be expected to occur at both surfaces. The free energy profile of CO oxidation on SiC-ML and SiC-TF are shown in Figure 5a and 5b respectively. Initially, 2CO molecules, an O2 molecule, and the surface are taken to be sufficiently far from each other without any interaction. O2 adsorption on the surface is taken to be the next step, which is exothermic by 0.20 eV. In step 3, a CO molecule is made to react with this adsorbed O2 to form the first CO2 molecule. The activation barrier (Ea) required for this step is calculated by plotting constrained potential energy curves, fixing the CO–O2 distance and relaxing the entire system as shown in the inset of Figure 5a. The Ea is 0.16 eV, which is quite small indicating that CO oxidation via ER mechanism on SiC monolayer can take place at near-room temperatures. The overall reaction is exothermic by -3.3 eV. The remaining oxygen atom is adsorbed in an epoxy fashion, which reacts almost spontaneously with another CO to form the next CO2 molecule, with reaction energy of -1.78 eV. On SiC-TF surface, the reaction profile is similar, with all the steps being downhill in energy (see Figure 5b). A significant difference in the reaction pathway is that after O2 adsorption, an incoming CO molecule prefers to adsorb on a neighboring surface site instead of reacting with the adsorbed O2. Hence at close distances from O2, the CO molecule does not 9



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form CO2, but forms an (O* +OCO*) intermediate. This suggests that the CO oxidation on SiC-TF surface prefers the LH mechanism over ER as will be discussed in the next section. For the LH mechanism, the initial step in the reaction pathway is considered to be the case when 2CO, O2, and the surface are non-interacting. The first step in the case of SiC-ML (see Figure 5c) involves the co-adsorption of both reactants, which results in the formation of an O2*+CO* intermediate. This step is slightly endothermic by 0.031 eV, owing to the CO molecule’s inability to bind strongly with the surface. This is followed by the desorption of the first CO2 from the catalyst surface, which can be considered as the rate determining step, requiring an Ea of about 1.03 eV (the initial and final structures are shown in the insets of Figure 5c). This reaction is exothermic by -3.53 eV. The remaining O atom and the next CO molecule react via ER mechanism to form another molecule of CO2. The reaction energies of each step in the reaction pathway and the optimized structures are shown in detail in Figure 6a. In the case of LH mechanism on SiC-TF (see Figure 5d for free energy profile), however, the first step (co-adsorption) results in O2 bond elongation as mentioned earlier, forming O*+OCO*. Hence the first step is highly exothermic by -3.61 eV. The formation of CO2 from this intermediate is fairly easier with an Ea of 0.72 eV. The remaining O atom is adsorbed on a C site. The third step involves the co-adsorption of a second CO molecule and this residual O atom on neighboring C sites, which is again exothermic by -2.73 eV. The final endothermic step is the desorption of the second CO2 from the surface, which is uphill by 1.27 eV. More detailed information on the reaction energies and the optimized structures of each step in the reaction pathway are provided in Figure 6b. 3.5 Role of Support (Graphite vs. metal). Based on the above results, it can be concluded that SiC-ML can be used as an efficient catalyst for CO oxidation, with the ER mechanism

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prevailing over the LH mechanism. Experimentally, the epitaxial growth of SiC-ML is needed to be supported by a substrate. In order to comprehend the substrate-induced changes that influence the CO oxidation, we consider two different substrates (i) a graphite support and (ii) a metal support. The interaction of SiC-ML and a graphite substrate is weak as they are coupled by van der Waals forces.42 This can be clearly identified from the CDD plots shown in Figure 7a. Here the CDD (∆ρ) is defined as: ∆ρ = ρ(SiC/S) – ρ (SiC) – ρ(S),

(2)

where ρ(SiC/S), ρ(SiC) and ρ(S) are the charge densities of the SiC monolayer on the substrate (graphite or Ag-111), SiC-ML surface and the substrate respectively. The blue lobes indicate accumulation and the pink lobes denote depletion of charge. The isovalue is set at 0.0002 e/Bohr3 for the SiC/G surface. The pz states of the carbon atoms of the SiC-ML are almost unaffected by the presence of the graphite substrate (see Figure 7a). The density of states projected on the SiC-ML and the first layer of the graphite substrate are also shown in Figure 7b, which shows no indication of orbital mixing between the substrate and SiC overlayer. The adsorption of O2 and CO on SiC/G hence mimic that of a free-standing SiCML, with Ead(O2)= -0.14 eV/-0.76 eV. Here, the importance of including van der Waals correction to obtain accurate binding energies is clearly identified. The weak adsorption of CO onto the surface suggests that ER mechanism is favorable over LH. The overall free energy profile of CO oxidation via ER mechanism is shown in Figure 8a. All the intermediate steps are downhill in free energy and the activation energy for the formation of the first CO2 is again rather small which is quite similar to the free-standing SiC-ML case. In the case of SiC/Ag-111, however, the substrate-induced changes on the CO oxidation activity are more prominent as is evident from the CDD plot shown in Figure 7c The isovalue is set at 0.0009 e/Bohr3. The redistribution of charge at the interface provides

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evidence for the strong metal - SiC-ML interaction. The PDOS of the SiC-ML and the first layer of the Ag substrate are also shown in Figure 7d. Hybridization between the metal d states and the p states of SiC-ML can be identified in the DOS, demonstrating that the binding of SiC-ML with the metal substrate is rather strong. It has to be noted that, the C atoms share more of their charge with the Ag-111 surface as compared to the Si atoms, which is clearly observed from the CDD plot (see Figure 7c). This indicates that the Si atoms here are more active in capturing the incoming O2 molecule. The O2 molecule adsorbs at the Si site in an end-on fashion, with an adsorption energy of -2.10 eV/-2.27 eV. This strong adsorption can be attributed to the tunneling of electrons from the underlying substrate to the O2 molecule via the SiC monolayer.26 The adsorption of CO molecule is moderate at the Si site, with Ead = -0.22 eV/ -0.33 eV, indicating that the insulator overlayer hinders the bonding and back bonding between Ag-111 and CO molecule.26 The moderate CO adsorption energy of on SiC/Ag111 surface also implies that when a mixture of CO/O2 gas is let into the reaction chamber onto the catalytic surface, the CO molecules prefer to compete with O2 for the adsorption sites rather than reacting directly with adsorbed O2 via the ER mechanism. The adsorbed CO and O2 molecules can then interact on the surface to form CO2 via the LH mechanism. In short, the higher the CO binding with the surface, the more probable is the LH mechanism. This has been observed in metal surfaces that adsorb CO rather strongly like the Pt(111) surface and prefer LH mechanism. We have illustrated this in scheme S1 of the supporting information (SI). The free energy profile is shown in Figure 8b. The first step involves the co-adsorption of O2 and CO onto the SiC-ML on Ag surface, which is exothermic by -1.28 eV. The removal of first CO2 leaves behind atomic oxygen adsorbed on the top site of a silicon atom. The second CO molecule then reacts with this adsorbed oxygen atom forming another CO2 molecule.

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However, the possibility of CO oxidation by ER mechanism on the SiC/Ag-111 surface also can’t be ruled out completely. Hence, we have also calculated the free energy profile for CO oxidation by ER mechanism on the SiC/Ag-111 surface. The barrier for the formation of first CO2 is found to be 0.2 eV. The free energy profile for CO oxidation by ER mechanism on the SiC/Ag-111 surface is given is Figure S1 of SI. The Sabatier activities (SA) can be used to provide a qualitative estimate of the efficiency of the catalysts. The activation energies of the rate determining step and the adsorption energies of reactants play key roles in determining the activity. The higher the SA, the better is the catalytic activity. The calculation steps are outlined in our previous work22. The Sabatier activities of the SiC catalysts are compared with a few other metal-based and metal-free catalysts in Table 2. It is noteworthy that the SiC-ML catalysts perform exceptionally well when the CO oxidation takes place by ER mechanism, with SA = 1.16. Also, the SiC-TF surface has a SA of -0.30, which is better than that of an extended Pt surface and is comparable to Ag13 nanoparticles. 3.6 Effects of van der Waal’s correction. In order to comprehend the importance of incorporating van der Waal’s dispersion correction, we have tabulated explicitly the free energies of the elementary steps of CO oxidation, with and without vdW correction in Table 3 (for the ER mechanism) and Table 4 (for the LH mechanism). The free energies are given with respect to the final step, 2CO2 + *. It can be seen that the effect of dispersion is more prominent in SiC/G because of the weak binding of SiC with the graphite substrate. Also, our calculations reveal that reaction barriers do not change significantly upon incorporating van der Waals dispersion correction, which is consistent with previous studies.47

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3.7 TER Mechanism. Recently, the Tri-molecular Eley Rideal mechanism (TER) is identified to be a possible mechanism by which CO oxidation occurs on a single Pd atom decorated on h-BN sheet48. In the TER mechanism, two CO molecules are co-adsorbed initially on the catalytic surface. An incoming O2 molecule is then activated by the two CO molecules to form an OCO-surface-OCO intermediate, which finally dissociates into two CO2 molecules. Hence it is evident that the catalytic surface should be capable of binding CO for TER to occur. Among the catalysts chosen in our study, the SiC/Ag-111 surface could satisfy this criterion, with Ead(CO)= -0.33 eV. However, the adsorption energy of the CO molecule in all cases is lesser than its free energy at 300 K. That is, the first step is slightly endothermic by 0.1 eV. Hence, we found that the adsorption of two CO molecules onto the surface is not strong enough to activate an incoming O2 molecule, form an intermediate structure and remain anchored to the surface. As a test calculation, when an O2 molecule is placed sufficiently near the two co-adsorbed CO molecules, the 2CO-O2 lateral interaction is stronger than the CO-surface bonds. The formation of 2CO2 is highly exothermic by -5.58 eV. The detail free energy profile is shown in Figure 9. Further, the far O2 molecule approaching towards the adsorbed CO molecules actually requires 0.14 eV of energy to overcome the barrier, to form CO2 molecules. Whereas the adsorption energy of O2 on SiC/Ag-111 is -2.27 eV, which implies that the O2 molecules would prefer to occupy the active sites near to the CO molecules. Hence it can be concluded that for the TER mechanism to prevail over LH, the surface should be able to bind CO with higher adsorption energy. 4. CONCLUSION We test the catalytic capabilities of a silicon carbide monolayer and thin film towards CO oxidation. The rather strong tendency of SiC-ML to interact with O2 and high CO tolerance suggests an exciting possibility of utilizing this system in the design of metal-free catalysts. We attribute the strong O2 binding tendency to the weak overlap of 2pz (C) - 3pz 14


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(Si) orbitals in SiC-ML. We also probe the mechanism by which CO oxidation occurs in detail and find that ER mechanism is preferred to LH and the reaction proceeds with an easily surmountable barrier of 0.16 eV. In the case of SiC-TF on the other hand, the unsaturated bonds at the surface ensure moderate binding of CO along with the strong binding of O2. CO oxidation hence proceeds via LH mechanism and the barrier for the formation of first CO2 is rather large (0.72 eV). We also investigate the importance of substrate-effects and observe that the preferred mechanism for CO oxidation switches over from ER to LH depending on whether SiC-ML is grown on a graphite substrate or on a metallic substrate. The Sabatier activities of the chosen systems were also calculated and it is found that the SiC-ML surface exhibits excellent catalytic activity towards CO oxidation via ER mechanism. In addition, the TER mechanism on the SiC/Ag-111 surface was also investigated. ASSOCIATED CONTENT Supporting Information Sketch illustrating the posibilty of LH mechanism on SIC/Ag-111 surface and free energy profile for CO oxidation by ER mechanism on SiC-Ag-111 surface. AUTHOR INFORMATION *Corresponding Author E-mail: [email protected], [email protected]; Tel. No.: +91-44-27417918, Fax: +91-44-27456702 Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS RT and SS thank the Science and Engineering Research Board (SERB), India for the financial support (Grant no: SB/FTP/PS028/2013). Authors thank SRM Research Institute, SRM University for providing a supercomputing facility and financial support. YK thanks the 15



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Russian Megagrant Project (No.14.B25.31.0030) “New energy technologies and energy carriers” REFERENCES: 1. Xu, J.; Deng, Y. Q.; Luo, Y.; Mao, W.; Yang, X. J.; Han, Y. F. Operando Raman Spectroscopy and Kinetic Study of Low-Temperature CO Oxidation on an α-Mn2O3 Nanocatalyst. J. Catal. 2013, 300, 225-234. 2.

Yoon, B.; Landman, U.; Habibpour, V.; Harding, C.; Kunz, S.; Heiz, U.; Moseler, M.; Walter, M. Oxidation of Magnesia-Supported Pd30 Nanoclusters and Catalyzed CO Combustion: Size-Selected Experiments and First-Principles Theory. J. Phys. Chem. C, 2012, 116, 9594-9607.

3. Tang, D.; Hu, C. DFT Insight into CO Oxidation Catalyzed by Gold Nanoclusters: Charge Effect and Multi-State Reactivity. J. Phys. Chem. Lett., 2011, 2, 2972-2977. 4. Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation. Science, 2008, 321, 1331-1335. 5. Liu, K.; Wang, A.; Zhang, T. Recent Advances in Preferential Oxidation of CO Reaction over Platinum Group Metal Catalysts. ACS Catal., 2012, 2, 1165-1178. 6.

Freund, H. J.; Meijer, G.; Scheffler, M.; Schlogl, R.; Wolf, M. CO Oxidation as a Prototypical Reaction for Heterogeneous Processes. Angew. Chem. Int. Ed. Engl. 2011, 50, 10064-10094.

7. McClure, S. M.; Lundwall, M.; Yang, F.; Zhou, Z.; Goodman, D. W. CO Oxidation on Rh/SiO2/Mo(112) Model Catalysts at Elevated Pressures. J. Phys. Chem. C, 2009, 113, 9688-9697. 8. Wang, Y.; Wu, G.; Yang, M.; Wang, J. Competition between Eley–Rideal and Langmuir– Hinshelwood Pathways of CO Oxidation on Cun and CunO (n = 6, 7) Clusters. J. Phys. Chem. C, 2013, 117, 8767-8773. 9.

Khedr, M. H.; Halim, K. S. A.; Nasr, M. I.; El-Mansy, A. M. Effect of Temperature on the Catalytic Oxidation of CO over Nano-Sized Iron Oxide. Mater. Sci. Engg. A, 2006, 430, 40-45.

10. Gong, X.-Q.; Raval, R.; Hu, P. General Insight into CO Oxidation: A Density Functional Theory Study of the Reaction Mechanism on Platinum Oxides. Phys. Rev. Lett. 2004, 93, 106104. 16


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11. Lopez, N.; Nørskov, J. K. Catalytic CO Oxidation by a Gold Nanoparticle: A Density Functional Study. J. Am. Chem. Soc. 2002, 124, 11262-11263. 12. Tang, D.; Chen, Z.; Hu, J.; Sun, G.; Lu, S.; Hu, C. CO Oxidation Catalyzed by Silver Nanoclusters: Mechanism and Effects of Charge. Phys. Chem.Chem. Phys., 2012, 14, 12829–12837. 13. Dobrin. S. CO Oxidation on Pt Nanoclusters, Size and Coverage Effects: a Density Functional Theory Study. Phys. Chem. Chem. Phys., 2012, 14, 12122–12129 14. Hsieh, Y.-C.; Zhang, Y.; Su, D.; Volkov, V.; Si, R.; Wu, L.; Zhu, Y.; An, W.; Liu, P.; He, P.; Ye, S.; Adzic, R. R.; Wang, J. X. Ordered Bilayer Ruthenium–Platinum CoreShell Nanoparticles as Carbon Monoxide-Tolerant Fuel Cell Catalysts. Nat. Commun., 2013, 4, Article number: 2466. 15. Greeley, J.; Markovic, N. M. The Road from Animal Electricity to Green Energy: Combining Experiment and Theory in Electrocatalysis. Energy Environ. Sci., 2012, 5, 9246–9256 16. Camara, G. A.; Ticianelli, E. A.; Mukerjee, S.; Lee, S. J.; McBreen, J. The CO Poisoning Mechanism of the Hydrogen Oxidation Reaction in Proton Exchange Membrane Fuel Cells. J. Electrochem. Soc. 2002, 149, A748-A753. 17. Xie, X.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. Low-Temperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature, 2009, 458, 746-749. 18. Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321-1326. 19. Sheng, Z.-H.; Gao, H.-L.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Synthesis of Boron Doped Graphene for Oxygen Reduction Reaction in Fuel Cells. J. Mater. Chem. 2012, 22, 390-395. 20. Li, Y.; Zhou, Z.; Yu, G.; Chen, W.; Chen, Z. CO Catalytic Oxidation on Iron-Embedded Graphene: Computational Quest for Low-Cost Nanocatalysts. J. Phys. Chem. C 2010, 114, 6250-6254. 21. Song, E. H.; Wen, Z.; Jiang, Q. CO Catalytic Oxidation on Copper-Embedded Graphene. J. Phys. Chem. C 2011, 115, 3678-3683. 22. Sinthika, S.; Kumar, E. M.; Thapa, R. Doped h-BN Monolayer as Efficient Noble MetalFree Catalysts for CO Oxidation: the Role of Dopant and Water in Activity and Catalytic De-poisoning. J. Mater. Chem. A 2014, 2, 12812-12820. 23. Koenigsmann, C.; Scofield, M. E.; Liu, H.; Wong, S. S. Designing Enhanced OneDimensional Electrocatalysts for the Oxygen Reduction Reaction: Probing Size- and 17



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Composition-Dependent Electrocatalytic Behavior in Noble Metal Nanowires. J. Phys. Chem. Lett. 2012, 3, 3385-3398. 24. Zhang, P.; Chen, X. F.; Lian, J. S.; Jiang, Q. Structural Selectivity of CO Oxidation on Fe/N/C Catalysts. J. Phys. Chem. C 2012, 116, 17572-17579. 25. Zhang, P.; Hou, X. L.; Mi, J. L.; Jiang, Q.; Aslan, H.; Dong, M. D. Curvature Effect of SiC Nanotubes and Sheets for CO2 Capture and Reduction. RSC Adv. 2014, 4 , 4899448999. 26. Shin, D.; Sinthika, S.; Choi, M.; Thapa, R.; Park, N. Ab Initio Study of Thin Oxide–Metal Overlayers as an Inverse Catalytic System for Dioxygen Reduction and Enhanced CO Tolerance. ACS Catal., 2014, 4, 4074-4080. 27. Lyalin, A.; Nakayama, A.; Uosaki, K.; Taketsugu, T. Functionalization of Monolayer hBN by a Metal Support for the Oxygen Reduction Reaction. J. Phys. Chem. C, 2013, 117, 21359-21370. 28. Uosaki, K.; Elumalai, G.; Noguchi, H.; Masuda, T.; Lyalin, A.; Nakayama, A.; Taketsugu, T. Boron Nitride Nanosheet on Gold as an Electrocatalyst for Oxygen Reduction Reaction: Theoretical Suggestion and Experimental Proof. J. Am. Chem. Soc., 2014, 136, 6542-6545. 29. Zhang, P.; Hou, X.; Mi, J.; He, Y.; Lin, L.; Jiang, Q.; Dong, M. From Two-Dimension to One-Dimension: the Curvature Effect of Silicon-Doped Graphene and Carbon Nanotubes for Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2014, 16, 17479-17486. 30. Zhou, X.; Tian, W. Q. Sensitivity of (5,5) SWSiCNTs and SWSiCNTs with Stone–Wales Defects toward Hazardous Molecules. J. Phys. Chem. C 2011, 115 , 11493-11499. 31. Zhao, J.-X.; Xiao, B.; Ding, Y.-H. Theoretical Prediction of the N−H and O−H Bonds Cleavage Catalyzed by the Single-Walled Silicon Carbide Nanotube. J. Phys. Chem. C 2009, 113, 16736-16740. 32. Pham-Huu, C.; Keller, N.; Ehret, G.; Ledoux, M. J. The First Preparation of Silicon Carbide Nanotubes by Shape Memory Synthesis and their Catalytic Potential. J. Catal. 2001, 200 , 400-410. 33. Shen, X.-N.; Zheng, Y.; Zhan, Y.-Y.; Cai, G.-H.; Xiao, Y.-H. Synthesis of Porous SiC and Application in the CO Oxidation Reaction. Mater. Lett. 2007, 61, 4766-4768. 34. Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mat. Sci. 1996, 6, 15-50. 35. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. 18


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36. Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979. 37. Grimme, S. Semiempirical GGA-type Density Functional Constructed with a LongRange Dispersion Correction. J. Comp. Chem. 2006, 27, 1787-1799. 38. Atkins, P.; Paula, J.; Atkins' Physical Chemistry, ed. W. H. Freeman and Company, New York, 8th edn, 2006, pp. 993– 1001. 39. Cramer, C. J. Essentials of Computational Chemistry Theories and Models, John Wiley & Sons, 2ndedn, 2005, pp. 355-365. 40. Bekaroglu, E.; Topsakal, M.; Cahangirov, S.; Ciraci, S. First-Principles Study of Defects and Adatoms in Silicon Carbide Honeycomb Structures. Phys. Rev. B 2010, 81, 075433(1-9). 41. Huda, M. N.; Yan, Y.; Al-Jassim, M. M. On the Existence of Si–C Double Bonded Graphene-Like Layers. Chem. Phys. Lett. 2009, 479, 255–258. 42. Ukpong, A. M. First Principles Study of van der Waals Heterobilayers. Comput. Condens. Matter 2015, 2, 1–10. 43. Nørskov, J. K.; Bligaard, T.; Hvolbæk, T.; Pedersen, F. A.; Chorkendorffc, IB.; Christensen, C. H. The Nature of the Active Site in Heterogeneous Metal Catalysis. Chem. Soc. Rev. 2008, 37, 2163-2171. 44. Kim, H. Y.; Kim, D. H.; Ryu, J. H.; Lee, H. M. Design of Robust and Reactive Nanoparticles with Atomic Precision: 13Ag-Ih and 12Ag-1X (X = Pd, Pt, Au, Ni, or Cu) Core-Shell Nanoparticles. J. Phys. Chem. C 2009,113, 15559–15564. 45. Lin, S.; Huang, J.; Gao, X.; A Cu (111) Supported h-BN Nanosheet: a Potential LowCost and High-Performance Catalyst for CO oxidation. Phys. Chem. Chem. Phys. 2015, 17, 22097-22105. 46. Sinthika, S.; Kumar, E. M.; Surya, V. J.; Kawazoe, Y.; Park, N.; Iyakutti, K.; Thapa, R.; Activation of CO And CO2 On Homonuclear Boron Bonds of Fullerene-Like BN Cages: First Principles Study. Sci. Rep. 2015, 5 ,17460 (1-12). 47. Liu, B.; Cheng, L.; Curtiss, L.; Greeley, J. Effects of van der Waals Density Functional Corrections on Trends in Furfural Adsorption and Hydrogenation on Close-Packed Transition Metal Surfaces. Surf. Sci. 2014, 622, 51-59. 48. Lu, Z.; Lv, P.; Xue, J.; Wang, H.; Wang, Y.; Huang, Y.; He, C.; Ma, D.; Yang, Z.; Pd1/BN as a Promising Single Atom Catalyst of CO Oxidation: A Dispersion-Corrected Density Functional Theory Study. RSC Adv., 2015, 5, 84381-84388

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Table 1 Adsorption Energies of Reactants and Bader charges after adsorption: -ve sign of Bader charge denotes the gaining of charge.

Surface

O2 Adsorption

CO Adsorption

Adsorption

Adsorption Energy Bader Charge

Energy (eV)

Bader Charge on

(eV)

on O2 (e) PBE

PBE + D

CO (e) PBE

PBE + D

SiC-ML

-0.51 -0.65

-1.24

0.074

-0.068

-0.04

SiC-TF

-0.92 -1.12

-0.64

0.061

-0.117

-0.046

SiC/G

-0.14 -0.76

-1.22

0.38

-0.14

-0.056

-2.1

-1.45

-0.22

-0.33

+0.647(C),

SiC/Ag-

-2.27

111

-1.01(O)

Table 2: Sabatier activities of various SiC based catalysts for CO oxidation compared with a few other metal and metal-free catalysts. Boron nitride nanotube denoted as BN-NT and stone wales denoted as SW. System

Ead (O2) (eV)

Ead (CO) eV

Sabatier Activity

Pt(111)*43 Rh(111) *43

-1.0 -1.8

-1.25 -1.6

-1.0 LH -1.4 LH

13Ag-(Icosahedron)44 h-BN on Cu(111)45 Carbon doped h-BN22 Oxygen doped h-BN22

-0.65 -1.17 -1.91 -3.04

-0.88 -0.13 -0.31 -1.2

-0.23LH -0.63LH 1.31ER, 1.21ER, -0.61LH

B25N35 cage46 B27N33 cage46 B30N30 cage46 BN-NT with SW defect46

-2.98 -2.95 -2.62 -2.97

-0.5 -0.65 -0.38 -0.08

-1.8 LH -1.3 LH -0.61LH -2.60 LH

SiC-ML (This Work) SiC-TF(This Work)

-0.65 -1.12

-0.068 -0.117

1.16ER,-0.25 LH -0.30LH

*The S.A were calculated at T=600 K, p(O2)=0.33 bar, and p(CO)=0.67 bar, for all other cases, it was calculated at low temperature conditions (T=273 K, p(O2)=0.21 bar, and p(CO)=0.01)

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Table 3: Reaction free energies of the steps for CO oxidation via ER mechanism, with and without dispersion correction. The final step 2CO2+* is taken as reference.

Reaction Step (ER Mechanism)

Reaction free energy (∆G, eV) SiC-ML

SiC-TF

SiC/G

SiC/Ag111

PBE

PBE+D

PBE

PBE+D

PBE

PBE+D

PBE

PBE+D

2CO+O2+*

5.290

5.290

5.290

5.290

5.290

5.290

5.290

5.290

2CO+O2*

5.227

5.085

4.566

4.592

5.607

4.986

3.933

3.587

CO2+CO+O*

1.892

1.782

1.550

1.464

2.039

1.738

0.551

0.091

2CO2 + *

0

0

0

0

0

0

0

0

Table 4: Reaction free energies of the steps for CO oxidation via LH mechanism, with and without dispersion correction. The final step 2CO2+* is taken as reference.

Reaction (LH Mechanism)

Reaction free energy (∆G, eV) SiC-ML

SiC-TF

SiC/G

SiC/Ag111

PBE

PBE+D

PBE

PBE+D

PBE

PBE+D

PBE

PBE+D

2CO+O2+*

5.290

5.290

5.290

5.290

5.290

5.290

5.290

5.290

(O2···CO)*+CO

5.662

5.321

2.042

1.658

6.364

5.478

3.57

2.932

CO2+CO+O*

1.892

1.782

1.550

1.464

2.039

1.738

0.551

0.092

CO2+COO*

2.332

2.096

-0.948

-1.273

-

-

-

-

2CO2+*

0

0

0

0

0

0

0

0

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Figures and Captions

Figure 1. Density of states of (a) a monolayer of SiC and (b) thin film of SiC. The insets show the geometry optimized structures, with grey spheres representing carbon atoms, yellow spheres representing Si atoms and small sky blue sphere represent hydrogen atom. The red and green arrows denote the up and down spin states respectively.

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Figure 2. Partial density of states of a unit cell of SiC-ML projected onto the (a) px orbitals of Si and C, (b) py orbital of Si and C (c) pz orbitals of Si and C (d) band projected partial charge density calculated in the energy range -2 to 0 eV, with an isovalue of 0.003 e/Bohr3.

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Figure 3. Partial density of states of (a) O2 adsorbed on SiC-ML (b) O2 adsorbed on SiC-TF (c) CO adsorbed on SiC-TF and (d) O2 and CO coadsorbed on SiC-TF. A magnified view of the states near the Fermi level are shown in the insets of Figure (c) and (d).

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Figure 4. Charge density difference isosurfaces after the adsorption of (a) O2 on SiC-ML, (b) CO on SiC-ML, (c) O2 on SiC-TF and (d) CO on SiC-TF. The isovalue is set to 0.001 e/Bohr3.

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Figure 5. Free energy profile of (a) CO oxidation on the SiC-ML surface following the ER mechanism. The lower left inset shows the constraint PE curve employed to calculate the energy barrier for desorption of the first CO. The upper right inset shows the structure used to obtain the constrained potential energy curve. The reaction coordinate (Z) is taken to be the distance of the CO molecule from the adsorbed O2 molecule, (b) CO oxidation on the SiC-TF surface via the ER mechanism, (c) CO oxidation on SiC-ML following the LH mechanism, (d) CO oxidation on SiC-TF following the LH mechanism. The upper and lower insets of Figure (c) and Figure (d) show the initial and final structures considered for estimating the barrier energy for the desorption of the first CO2 from the surface. Ea denotes the activation barrier. In the inset figures the second CO molecule is not shown for simplicity.

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Figure 6. (a) The reaction energies and barrier energy during CO oxidation on SiC-ML via LH mechanism. (b) The reaction energies and barrier energy during CO oxidation on SiC-TF via LH mechanism. The structures for the corresponding reaction steps and transition states are shown in the inset.

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Figure 7. Charge density difference and projected density of states of (a) & (b) SiC-ML on graphite and (c) and (d) SiC-ML on Ag111 respectively. The isovalue is set at 0.0002 e/Bohr3 in (a) and 0.0009 e/Bohr3 in (c). The pink shaded plot indicates the DOS projected on the atoms constituting the SiC-ML and the blue plot indicates the DOS projected on the first layer of the graphite in Figure (b) and metal substrate in Figure (d).

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Figure 8. Free energy profile of (a) CO oxidation on the SiC-ML surface on a graphite substrate following the ER mechanism. The upper right inset shows the structure used to obtain the constrained potential energy curve. (b) CO oxidation on the SiC-ML surface on an Ag substrate following the LH mechanism. The upper and lower insets of Figure (b) show the initial and final structures considered for estimating the barrier energy for the desorption of the first CO2 from the surface. In the inset figures the second CO molecule is not shown for simplicity.

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Figure 9. Free energy profile of CO oxidation on the SiC/Ag-111 following the TER mechanism. The upper and lower insets show the initial and final structures considered for estimating the barrier for the formation of 2CO2 molecules.

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CO oxidation mainly follows either ER or LH mechanism depending on the surface type and the underneath support 199x84mm (300 x 300 DPI)


CO Oxidation Prefers the EleyâRideal or Langmuir ... - ACS Publications

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