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Density Functional Theory Analysis of Metal/Graphene Systems As a Filter Membrane to Prevent CO Poisoning in Hydrogen Fuel Cells Deborah J. D. Durbin and Cecile Malardier-Jugroot* Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada, K7K 7B4 ReceiVed: October 11, 2010; ReVised Manuscript ReceiVed: NoVember 9, 2010
Hydrogen fuel cells are a very promising potential replacement for internal combustion engines. However, their current use is limited by carbon monoxide poisoning of the platinum anode catalyst that occurs when CO enters the cell in the H2 feed gas. A novel new solution to this problem is the addition of a metal/ graphene filter membrane exterior to the cell. This membrane will remove CO from the feed gas, allowing reduced loading of the expensive Pt catalyst and increasing cell lifetime. In the current work, density functional theory (DFT) was used to analyze graphene membranes containing nickel, copper, platinum, and iridium/ gold atoms. The binding energy of the metal to the graphene was measured for a lone system and in the presence of CO and H2 to predict its durability. The binding energy of CO and H2 to metal was also measured to estimate its efficiency. All systems were analyzed using natural bond orbitals (NBOs). It was found that copper is a poor choice for use in membranes in all respects. Nickel systems show the most promise: they have a consistent metal/graphene binding energy when feed gas molecules are introduced. In addition, although CO binding is strong to Ni, Pt, and Ir/Au, nickel systems show the weakest interaction with H2. NBO analysis of these systems shows that metal orbitals are the most involved in bonding. Introduction Current global energy production relies primarily on fossil fuels (36% oil, 28% coal, 24% natural gas).1 This leads to unsustainable energy practices that have significant negative environmental ramifications due to their limited resources and the environmental harm associated with drilling for and burning fossil fuels. Fuel cells are an attractive replacement for fossil fuel combustion because they are more efficient and produce notably fewer contaminants.2,3 Fuel cells are electrochemical cells that convert chemical energy into electrical energy. Every electrochemical cell consists of two electrodes (anode and cathode). In fuel cells, the anodic reaction is either direct oxidation of hydrogen or methanol or, occasionally, indirect oxidation via a reforming step; the cathodic reaction is oxygen reduction. The hydrogen oxidation reaction (HOR) occurring at the anode usually takes place on a platinum catalyst by reactions R1 and R2 with the overall reaction R3. The subscript “ads” indicates hydrogen atoms that have adsorbed to the platinum surface.4,5
2Pt + H2 f 2[Pt-Hads]
(R1)
[Pt-Hads] f Pt + H+ + e-
(R2)
H2 f 2H+ + 2e-
(R3)
There are six main types of fuel cells. PEM (proton-exchange membrane or polymer electrolyte membrane) fuel cells in particular are very important because they are often considered * To whom correspondence should be addressed. Phone: (613) 541-6000, ext 6046. Fax: (613) 542-8612. E-mail:
[email protected].
the most likely for portable commercial applications. PEMFCs use a proton-conducting polymer membrane as the electrolyte and operate at relatively low temperatures around 80 °C. They produce water by the reaction of hydrogen with oxygen. Hydrogen is taken up by the anode, where it is oxidized to protons at the electrocatalyst by R3. The produced protons then migrate through the proton-conducting membrane to the cathode, where they combine with reduced oxygen to form water by reaction R4. The produced water exits the cell and can either be released directly into the atmosphere or recycled back into the cell as a humidifying agent.4
/2O2 + 2H+ + 2e- f H2O
1
(R4)
PEMFCs show significant promise as a replacement for internal combustion engines in automobiles due to their light weight, low operating temperature, and absence of corrosive material. This idea has come into practice in the last four decades with the invention of fuel stacks, which are used in hydrogen vehicles to produce an automobile that is 20% more fuel efficient and 10% more energy efficient than the gasoline-powered equivalent with a 120% better power-to-weight ratio.6,7 One of the major problems facing fuel cell use is carbon monoxide (CO) poisoning of the anode catalyst. Carbon monoxide is formed during fuel refining and then travels in the H2 feed gas into the fuel cell. Once in the cell, CO chemisorbs to the platinum catalyst much more strongly than H2. Therefore, it occupies binding sites that should be used for hydrogen oxidation. Even a very small amount of CO can have a great impact on efficiency.8,9 Many techniques have been investigated to overcome this great obstacle in fuel cell development including air bleeds,10,11 addition of an oxygen evolving species12,13 and, more recently, CO-tolerant catalysts.14-24 Some progress has been made, but the ideal solution is still to remove
10.1021/jp109758t 2011 American Chemical Society Published on Web 12/21/2010
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CO from the H2 feed gas before it reaches the catalyst. Therefore, the present project focuses on the development of external graphene membranes that contain metal nanoparticles capable of capturing CO before it enters the fuel cell. If these membranes can be successfully produced they will allow significantly lower loading of the expensive noble metal catalyst. This will decrease the overall production cost of fuel cells while increasing efficiency and lifetime. In this project, density functional theory (DFT) was used to study CO binding on different metals and alloys as well as evaluate the durability of the metal particles on a graphene layer. DFT is a powerful modeling tool, which uses quantum theory to study the electronic structure of matter. DFT is useful for large systems because it solves Schro¨dinger’s equation using electron density instead of wave functions. This drastically decreases computation time, which is important for large systems.25 In addition, DFT has already been extensively used for modeling projects in the area of catalysis including many involving fuel cells. For instance, DFT was used to investigate a more effective catalyst for the oxygen reduction reaction in polymer electrolyte membrane fuel cells (PEMFC)26 and to determine the mechanism of fuel oxidation in solid oxide fuel cells (SOFC).27 DFT is an effective modeling tool for this project because it can accommodate the electronics of large atoms (such as the nickel atoms employed in this study and in the aforementioned SOFC studies). In addition, DFT experiments have been shown to link well to experimental results.28
Computational Methods All simulations were performed with density function theory (DFT) with the B3LYP hybrid functional29 and the Lanl2MB basis set.30,31 This basis set uses STO-3G for first row atoms and the Los Alamos effective core potential plus MBS on sodium to bismuth. It has been previously shown to produce accurate results for comparable systems.32 An effective core potential was necessary in these simulations in order to handle the large number of electrons present in transition metals. All results were obtained with the Gaussian 03 software package33 on a Mac OS X Leopard 10.6.2 system. The pure graphene surface is composed of 14 graphene rings with terminal hydrogen atoms. This system was geometrically optimized before one metal atom (or two in the case of alloys) was placed above the surface and the entire system reoptimized (Figure 1). This complete optimization of the system accounts for geometrical changes of the grapheme surface due to the presence of the metal particle. This was done for all four metal/ graphene systems before the systems were analyzed separately with CO and H2 gas (Figure 2). The binding energy of the metal to the graphene surface was then measured in the absence of feed gas and in the presence of CO and H2 by eqs E1 and E2, respectively. In eq E2, gas refers to either CO or H2. Finally, the binding energy of each feed gas component to the metal/ graphene system was calculated by eq E3.
BEmetal-graphene ) Emetal-graphene - Egraphenee - Emetal (E1)
Figure 1. Visual representation of a nickel/graphene system indicating the atom numbers. The box displays a magnification of the active center.
BEmetal-graphene ) Emetal-graphene-gas - Egraphene - Emetal-gas (E2) BEmetal-graphene-gas ) Emetal-graphene-gas - Emetal-graphene (E3) The metal/graphene systems were further analyzed by natural bond orbital (NBO) analyses. Natural bond orbitals provide similar information to molecular orbitals (MOs). However, while MOs describe the location of electrons over an entire system, NBOs localize electronic information onto atomic orbitals (s, p, d, f). This allows better quantization of interactions and so increases ease of analysis.34,35 Results and Discussion It has been previously shown that both CO and H2 bind strongly to platinum36-40 with CO binding being the source of catalyst poisoning in fuel cells. Therefore, in order to select the metal center for the membrane, preliminary studies were performed to calculate the binding energy of platinum and surrounding metals (columns 8-11 of the Periodic Table) to CO and H2 without interaction with graphene. Using these simulations, the most promising metals for the active site of the membrane were selected. After platinum, nickel and iridium showed the strongest binding to CO with nickel having the weakest H2 binding. Therefore, nickel and platinum were selected for further analysis. Cobalt, copper, silver, and gold were found to have weak binding to CO; a copper/graphene system was further analyzed for comparison to the platinum and nickel systems. Finally, the effectiveness of alloys was analyzed with an iridium/gold alloy. This alloy was chosen because it contains elements with a similar electronic structure to platinum that, when combined, should have a comparable global electron population. Analysis found that this alloy has high binding energy with CO. Initial analysis looked at the binding energy of the metals to a graphene surface (Figure 3). A strong metal/graphene binding energy is necessary to produce a durable membrane. In addition, introduction of a graphene surface alters the electronic configuration of the system, which can lead to stronger metal/CO binding. This has proved especially effective when doped graphene surfaces are used.41 Of the four systems analyzed (Ni,
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Figure 2. Visual representation of metal/graphene systems with nickel (top) and iridium/gold (bottom) in the presence of CO (left) and H2 (right).
Figure 3. Binding energy of the metal atom to the graphene surface alone and in the presence of CO or H2.
Cu, Pt, Ir/Au), copper showed the weakest binding to the graphene surface; the iridium/gold alloy showed the strongest binding. This trend was consistent for lone metal/graphene systems and for the metal/graphene systems in the presence of feed gas components (CO or H2) as shown in Figure 3. For platinum and iridium/gold, binding of gas to the metal caused a weaker metal/graphene binding, with H2 having the greatest effect. Nickel and copper systems showed significantly less variation in the metal/graphene binding energy when feed gas components were introduced to the system. A natural bond orbital (NBO) analysis of the above systems is shown below (Figures 4 and 6-8). [In the case of iridium/ gold systems, only the iridium orbitals are shown in order to simply the plots. The gold atom had a significantly smaller interaction with both the graphene surface and the feed gases (Figure 2), so it is not reported. Furthermore, the principle quantum numbers reported on the plots are for row 4 transition metals (Ni, Cu). For the row 6 metals (Pt, Ir), n ) 5 or 6 (not 3 or 4).] Figure 4 shows the NBO analysis of the metal orbitals for all metal/graphene systems when no feed gas molecules are present. Lower level s, p and d orbitals are omitted in order to simplify the figures; these orbitals showed very little orbital occupancy change. Figures 6-8 show only the metal orbitals for nickel, platinum, and iridium/gold systems. Copper systems
are omitted because the differences in their orbital occupancies are insignificant in agreement with their low metal/graphene binding energies. All plots show the change in orbital occupancy between the full metal/graphene (or metal/graphene/gas) system and the individual graphene and metal (or metal/gas) systems. This change represents the difference in orbital occupancy due to the interaction between the graphene support and the metal in the absence and presence of CO and H2 for the orbital specified. A positive change in orbital occupancy indicates that the orbital under investigation has a higher occupancy in the complete system; a negative change indicates that the orbital has a higher occupancy in the separate systems. In all cases, the metal contributes most of the electrons involved in the interaction of the metal with the graphene surface. Therefore, the largest changes in NBO occupancy are found in the metal orbitals, particularly in the exterior d-orbitals. Much smaller changes were observed in orbitals of the graphene surface or gas molecules. The largest contributions from the graphene surface were observed in the carbon 2pz orbitals. This suggests that the electronic density involved in the conjugation of the graphene surface is also interacting with the metal. This trend has been previously observed for similar systems.32 Within the metal orbitals, the most significant contribution to the metal/graphene system is from the dz2 orbital for the
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Figure 4. Natural bond orbital analysis of metal/graphene systems containing Ni, Cu, Pt, or Ir/Au in the absence of feed gas molecules. Only metal orbitals are displayed.
Figure 5. Visual representations of molecular orbitals with energy near 3.4 eV for nickel/graphene systems in the absence (a) and presence of CO (b) and H2 (c). A prominent dx2-y2 orbital is observed around the nickel atom.
Figure 6. Natural bond orbital analysis of nickel/graphene systems in the absence and presence of CO and H2. Only nickel orbitals are displayed.
iridium/gold system and the dx2-y2 orbital for the nickel and platinum systems. These findings suggest that the stated metal
orbitals are primarily responsible for the metal/graphene interaction. This observation corresponds well with the location of the
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Figure 7. Natural bond orbital analysis of platinum/graphene systems in the absence and presence of CO and H2. Only platinum orbitals are displayed.
Figure 8. Natural bond orbital analysis of iridium/gold/graphene systems in the absence and presence of CO and H2. Only iridium orbitals are displayed.
metal atom above the graphene surface. Nickel and platinum atoms are located directly between two carbon atoms. Therefore, a d-orbital that contains multiple lobes capable of interacting with multiple carbon atoms is the most involved (Figure 5a). In contrast, iridium is more localized over one carbon atom. Therefore, the dz2 orbital, which only has one lobe capable of interacting with the graphene surface, is most involved in metal/ graphene bonding. Overall, the largest changes in orbital occupancy between the separate metal and graphene systems and full metal/graphene systems are observed for iridium/gold. This is followed by the nickel and platinum systems, which show similar changes in orbital occupancy, and the copper systems, which show almost no change in orbital occupancy. This trend is consistent with the strength of metal/graphene binding: systems with stronger metal/graphene binding have larger changes in orbital occupancy between the separate metal and graphene systems and the combined metal/graphene system. When the metal orbitals of all systems are observed in the presence and absence of CO and H2 (Figures 6-8), the nickel/ graphene system (Figure 6) shows a distinct trend in orbital
occupancy change for all nickel systems: in every nickel/ graphene system, the 3s, 3dxz, 3dyz, and 3dz2 orbitals show a decrease in orbital occupancy from the separate (nickel/graphene and gas) systems to the complete nickel/graphene/gas system, while the 4px, 4py, 4pz, and 3dx2-y2 orbitals show an increase in occupancy; the occupancy change of the 3dx2-y2 orbital is the most significant in every system. This suggests that the nickel/ graphene bond is largely dependent on this orbital. This conclusion is also in agreement with visualizations of the molecular orbitals for nickel systems (Figure 5). In these images, the nickel 3dx2-y2 orbital shows a strong interaction with two carbon atoms for both the lone nickel/graphene system and the nickel/graphene/H2 system. When CO is present, the nickel 3dx2-y2 orbital only interacts with one carbon atom. This is consistent with the lower binding energy observed for the nickel/ graphene/CO system compared to the other nickel systems. The consistent changes in orbital occupancy for all nickel systems are consistent with the similar nickel/graphene binding energies for all nickel systems. In contrast, in the platinum and iridium/gold systems the orbitals involved in metal/graphene binding vary between the
Analysis of Metal/Graphene Systems metal/graphene, metal/graphene/CO, and metal/graphene/H2 systems (Figures 7 and 8). This is consistent with the variable binding energies observed for these systems. In the iridium/ gold systems, the lone system shows a very large increase in the occupancy of the iridium 5dz2 orbital when iridium/gold binds to the graphene surface. However, there is only a very small change in the occupancy of this orbital in the presence of CO and H2. Since iridium/gold binds much more strongly to the graphene surface when CO and H2 are absent, this suggests that the iridium 5dz2 orbital is primarily responsible for metal/ graphene binding. From the aforementioned analyses, it appears that nickel/ graphene systems are the most promising for use in fuel cells. Although iridium/gold and platinum systems have the strongest metal/graphene binding energy in the absence of feed gas particles, the nickel/graphene system shows the most consistent metal/graphene binding when feed gas is introduced. However, for a system to be effective as a CO-capture membrane in hydrogen fuel cells, it must also bind CO strongly and H2
J. Phys. Chem. C, Vol. 115, No. 3, 2011 813 weakly. Therefore, CO will be captured before entering the fuel cell, while H2 will pass through the membrane unaffected. The binding of CO and H2 to all metal systems is shown in Figure 9. These binding energies show that CO binds more strongly to the metals than H2 in all systems. The copper system shows weak CO-binding and so is not a promising system. However, CO binds strongly to the nickel, platinum, and iridium/gold systems. Of these systems, both platinum and iridium/gold have significant binding to H2. Therefore, the nickel/graphene system is the most promising for an external CO-capture membrane in hydrogen fuel cells. Natural bond orbital analyses were performed for all systems. Results for the high-order metal s, p, and d orbitals, and the gas orbitals are displayed in Figures 10 and 11. There is a distinct trend observed in all carbon and oxygen orbitals; there is a consistent decrease in the occupancy of the 2s and 2pz carbon orbitals and an increase in occupancy of the 2px and 2py carbon orbitals when CO is added to the metal/graphene system.
Figure 9. Binding energy of CO and H2 to Ni-, Cu-, Pt-, and Ir/Au-graphene systems.
Figure 10. Natural bond orbital analysis of metal/graphene/CO systems for systems containing Ni, Cu, Pt, and Ir/Au. Only the most involved orbitals are displayed.
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Figure 11. Natural bond orbital analysis of metal/graphene/H2 systems for systems containing Ni, Cu, Pt, and Ir/Au. Only the most involved orbitals are displayed.
Similarly, there is a consistent decrease in the occupancy of the oxygen 2pz orbital. All other oxygen orbitals show marginal changes in occupancy. The involvement of the carbon 2s orbital is particularly interesting because it is an interior orbital and so indicates strong bonding of CO to the metal. The greatest changes in occupancy of the CO orbitals when CO binds to a metal/graphene system occur in the iridium/gold system, followed by platinum then nickel and finally copper. This trend is consistent with the binding energy of CO to the metal. Therefore, greater changes in orbital occupancy are observed for systems with higher binding energy to CO. In the metal/graphene/H2 systems (Figure 11), both hydrogen atoms in H2 show a decrease in electron density from the lone H2 molecule to the metal/graphene/H2 system for all metals. This is expected because some electronic interactions contribute to the metal/H2 bond. In the metal orbitals, the greatest occupancy changes are observed in the iridium/gold system followed by platinum then nickel and finally copper. This is consistent with the trend observed for the binding energy of H2 to the metal where the system that binds H2 most strongly shows the greatest changes in orbital occupancy. This, again, suggests that greater changes in orbital occupancy are observed for systems with higher binding energy to the gas. Conclusions Density functional theory was used to model metal/graphene systems with nickel, copper, platinum, and iridium/gold in the absence and presence of CO and H2. These data were used to find a membrane with strong metal/graphene binding that binds CO strongly, while having minimal interaction with H2. This membrane could be placed outside a fuel cell to capture CO from the feed gas to prevent it from poisoning the anode catalyst. The metal/graphene binding energy was calculated for the lone system and the system in the presence of CO and H2. The binding energy of each feed gas component to the membrane was also computed. Systems were better understood by analyzing natural bond orbitals (NBOs). Nickel/graphene systems were found to be the most promising because, although the iridium/
gold and platinum systems have the strongest metal/graphene binding energy in the absence of gas particles, the nickel/ graphene system shows the most consistent metal/graphene binding when feed gas is introduced. All systems bind CO more strongly than H2. In particular, CO binds very strongly to nickel, platinum, and iridium/gold systems. However, both the platinum and iridium/gold systems have significant binding to H2 as well. Therefore, the nickel/graphene system is the most promising for an external CO-capture membrane in hydrogen fuel cells in this respect as well. Copper systems were found to be a poor choice for a filter membrane in all respects. NBO analysis of these systems showed that metal orbitals were the most involved in bonding. In particular, the dx2-y2 orbital was most involved in nickel and platinum systems, while the iridium dz2 orbital was most involved in the iridium/gold system. This is consistent with the location of the metal atom over the graphene surface. Furthermore, in general, systems with higher binding energies were found to have larger changes in orbital occupancy from the individual systems (metal and graphene or metal/graphene and gas) to full (metal/graphene or metal/graphene/gas) systems. Acknowledgment. This project was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Defense and Security Research Institute-Royal Military College Defense Academic Research (DSRI-RMC DAR) Programme, and the Royal Military College of Canada (RMCC). References and Notes (1) BP Statistical Review of World Energy, 2007. (2) Carrette, L.; Friedrich, K. A.; Stimming, U. ChemPhysChem 2000, 1, 162–193. ´ .; Nørskov, J. K.; Chorkendorff, I. (3) Davies, J. C.; Bonde, J.; Logado´ttir, A Fuel Cells 2004, 5, 429–435. (4) Padro´, C. E. G.; Keller, J. O. Hydrogen Energy. In Kirk-Othmer Encyclopedia of Chemical Technology; Othmer, K., Ed.; John Wiley & Sons Inc.: New York, 2006; Vol. 13, pp 837-866. (5) Schmidt, T. J.; Jusys, Z.; Gasteiger, H. A.; Behm, R. J.; Endruschat, U.; Boennemann, H. J. Electroanal. Chem. 2001, 501, 132–140. (6) Hoogers, G.; Thompsett, D. CATTECH 1999, 3, 106.
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