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May 27, 2016 - Advances in methane conversion processes. Bingwen Wang , Sandra Albarracín-Suazo , Yomaira Pagán-Torres , Eranda Nikolla. Catalysis ...
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A Viewpoint on Direct Methane Conversion to Ethane and Ethylene Using Oxidative Coupling on Solid Catalysts Brittany Lancaster Farrell, Valentina Omoze Igenegbai, and Suljo Linic* Department of Chemical Engineering, University of Michigan, NCRC Building 28, 2800 Plymouth Road, Ann Arbor, Michigan 48109, United States

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use molecular oxygen as the oxidant, there have also been a few publications on using alternative, mild oxidants such as sulfur12 as well as employing organometallic complexes operating in a liquid phase at lower temperatures.13 Although these reports are scientifically interesting, the reported yields of C2 products were relatively low and the processes are in general considered to be more costly. Furthermore, there have been studies that evaluate the cointegration of OCM with subsequent processing of gases in the product stream (e.g., by reforming, hydrogenation, or oligomerization reactions).14 For example, the cointegration of OCM with reforming of unconverted methane using the CO2 byproduct to produce syngas has been discussed.15 Siluria technologies claimed to have demonstrated a commercially viable OCM process.16 Although the information about the specific details of the process is restricted, it has been suggested that the process employs nanowire catalysts grown on biological substrates which are able to drive OCM at relatively low temperatures of 473−573 K.17,18 Methane and ethane recycling is used to further improve the ethylene yields.18 Although these multistage systems could potentially improve the commercial viability of the process, improving the single-pass C2 yields remains a critical challenge. We note that a number of review articles are available that discuss extensively the numerous catalytic systems that have been studied for OCM.14,18−21 A summary of the published single-pass methane conversion and C2 selectivity data for various tested OCM catalysts obtained either in packed bed or membrane plug flow reactors is shown in Figure 1. The dashed lines in the figure mark the C2 yields of 20% and 30%. The shaded area represents the techno-economic target for commercialization of OCM, suggesting that in order for an OCM process to be economically viable, single-pass C2 yield and selectivity need to be greater than 30 and 90% respectively.22 Although these techno-economic targets are highly cited in the OCM literature, it is important to keep in mind that these were published in a 1989 report that considered methane to olefins processes using undiluted feed streams of methane and air. An obvious conclusion from Figure 1 is that developing efficient catalysts for the OCM process that approach the technoeconomic targets has been challenging. One reason for the low reported C2 yields on almost all OCM catalysts (Figure 1) is that under high-temperature reaction conditions, the formation of the undesired products (mainly CO, CO2, and solid graphitic carbon) is thermodynamically favorable. Figure 2a shows the calculated change in the Gibbs free energy at 1073 K and 1 atm for several reactions that can occur in the process. Although the ΔGs for the formation of the C2 products

ver the last several decades, extensive research has focused on the direct conversion of methane to alcohols or higher hydrocarbons. Research in this field has intensified in recent years due to the increased production of methane from shale gas and tight oil,1,2 leading to the reduction in the cost of methanerich natural gas compared to crude oil.3 Moreover, much of the natural gas that is recovered as a byproduct of oil recovery is considered “stranded”, and because it cannot currently be utilized economically, it is combusted in flares releasing the greenhouse gas CO2 into the atmosphere.2 Current processes that convert methane to alcohols or olefins are complex and costly as they require the intermediate step of reforming methane to synthesis gas (CO and H2).2,4 Although there are clear environmental and potentially financial incentives to convert methane directly to more valuable products, there are no available commercial-scale processes. In this Viewpoint (this is not a review or a research article), we analyze certain aspects of a methane upgrading process of potential interest, the oxidative coupling of methane (OCM). We focus on pointing out the inherent obstacles in the development of the catalytic OCM technology by analyzing thermodynamic and kinetic features of the process. We also discuss some potential advantages and promising research directions for integrating selective OCM catalysts into oxygenion conducting membrane reactors. We note that our goal is not to provide a comprehensive review of the OCM field. Rather, it is to present a viewpoint on a very small sliver of the fast developing field and to support this viewpoint by analyzing previous reports in the field. Although we fully understand that the separations of the desired products from the product stream is an important component of the cost of any methane upgrading technology, our focus is on chemical catalysis. Catalytic oxidative coupling of methane (OCM) was first reported in early 1980s by Keller and Bhasin and Hinsen and Baerns.5,6 The proposed mechanism for this process involves the extraction of a hydrogen atom from a methane molecule by an oxygen atom on the surface of a catalyst.7 The remaining methyl radical (*CH3) is released from the surface of the catalyst and couples with another methyl radical in the gas phase to form ethane.8 An extraction of another hydrogen atom from methane by the OH group on the catalyst surface results in the formation and desorption of water and the completion of the catalytic cycle. Since the initial reports of the process, the majority of research efforts have focused on identifying catalysts that maximize the yield of ethane and ethylene (the C2 products).9 These studies have shown that mixed metal oxide catalysts, such as Li/MgO and Mn/Na2WO4/SiO2, operating at temperatures between 943 and 1223 K, in general give the highest reported C2 yields.9−11 In addition to the reports discussing heterogeneous catalysts that © 2016 American Chemical Society

Received: April 16, 2016 Published: May 27, 2016 4340

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moles of carbon in each compound are divided by the total moles of carbon in the system. The data shows that at low O2/CH4 ratios (i.e., near stoichiometric ratios for the production of the C2 compounds), solid graphitic carbon is the favorable product. As the O2/CH4 ratio increases, the thermodynamically favored product becomes CO, and eventually CO2 at high O2/CH4. We note that there is no thermodynamic preference for C2 products at any conditions. The conversion−selectivity data in Figure 1 also shed light on another critical problem associated with the design of efficient OCM catalysts. The data shows that any catalyst operated at high methane conversions exhibited low C2 selectivity and that only the catalysts operated at low methane conversion achieved high C2 selectivity. This behavior is typical for catalytic processes where there is significant sequential chemical conversion of the desired products to the undesired, thermodynamically stable products. In this particular case, the rates of the oxidative conversion of ethane and ethylene, leading to the deeper oxidation products, are relatively high.23 While the number of “synthesize and test” studies of OCM on various oxide catalysts is large, the number of rigorous experimental studies of the kinetics of OCM reactions on these oxides is rather limited.23−25 These kinetic studies are critical to quantify the relevant importance of various chemical conversion pathways involved in OCM and to provide us with a direction for potential improvements in the process. The main reasons for the lack of rigorous experimental kinetics studies is associated with the (1) difficulties to control the local catalyst temperatures as the deep oxidation reactions are highly exothermic, (2) the deposition of carbon on the catalyst and low thermal stability of some oxide catalysts that lead to the rapidly changing nature and numbers of active centers, and (3) the influence of rapid, freeradical, gas-phase reactions. One very detailed experimental kinetic study was performed by Stansch et al., who analyzed the OCM kinetics on a La2O3/CaO catalyst.23 Based on their experimental results, the authors developed a kinetic model using the reaction network in Scheme 1. While the model is relatively simple, it can capture the experimental observables, and it has also been found to sufficiently describe experimental data observed on other oxide OCM catalysts.26,27 In addition to reporting the kinetic parameters (e.g., apparent pre-exponential factors and apparent activation barriers) for the reactions in Scheme 1, Stansch et al. also showed that the reactions leading to the undesired COx (CO and CO2) products exhibited approximately first-order dependence on the partial pressure of O2, whereas the reactions leading to the desired C2 products showed ∼1/2-order dependence on O2.23 These findings have been corroborated by Tiemersma et al., where a similar dependence of C2 and COx production rate on the O2 partial pressure was observed from kinetic studies on a Mn/Na2WO4/ SiO2 catalyst.28 This kinetic information suggests that higher C2 selectivity should be achieved at relatively low partial pressures of oxygen. We use the kinetic data and model reported by Stansch et al. as the basis for some of our analysis below. To summarize the discussion in previous paragraphs, there are significant thermodynamic and kinetic obstacles to the design of catalytic systems that could convert CH4 to C2 products with high C2 yields. The thermodynamic obstacle is the energetic preference for solid carbon, CO, and CO2, while the kinetic obstacle manifests itself in high rates of the sequential reactions leading to the undesired products and the observed higher reaction orders with respect to O2 for undesired compared to desired chemical reactions.

Figure 1. Published methane conversion and C2 selectivity data for metal oxide catalysts (all catalysts are oxides, however oxygen has been omitted in the formulas to save space). Circles indicate that the reaction was performed in a packed bed reactor and diamonds indicate that the reaction was performed in a membrane reactor. Data from refs 29−43.

Figure 2. (a) Change in Gibbs free energy for the reactions that can occur in an OCM reactor at 1073 K and 1 atm, where Cs is solid graphitic carbon. (b) Thermodynamic carbon product selectivity for a reactor at 1073 K with minimized Gibbs free energy as a function of the O2/CH4 ratio in the feed.

are negative, indicating that these products are thermodynamically favorable at the reaction conditions, the formation of CO and CO2 is even more energetically downhill. The data in Figure 2b show the equilibrium carbon product selectivity at 1073 K and 1 atm as a function of the O2/CH4 ratio calculated by minimizing the Gibbs free energy of the system. The carbon product selectivity is calculated on a per carbon atom basis, where the 4341

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low mixing of reactants and products. One way to achieve low mixing is to use plug flow reactors. To capture the outcome of the potential impact of using a plug flow reactor on the selectivity and yield of C2 products, we developed a plug flow reactor model using the reaction network, rate equations, and kinetic parameters provided by Stansch et al. for the La2O3/CaO catalyst.23 This relatively simple model, which considered the 10 chemical reactions shown in Scheme 1, allows us to shed light on the reactions that have the largest impact on the OCM yield and selectivity. We note that the model neglects possible formation of carbon deposits. We will comment on this issue later in this document. The data in Figure 3a,b show the mole fractions of different products as a function of space time in a plug flow packed bed reactor obtained using the model. We define the space time as the mass of the catalyst divided by the mass flow rate of methane at the reactor inlet. The reactor is modeled at 1073 K and 1.1 atm total pressure with an inlet O2/CH4 molar flow rate ratio of 0.25 (stoichiometric feed for ethane formation). Under these conditions, on the basis of the kinetic model (using the kinetic parameters reported for La2O3/CaO catalyst), we conclude that the maximum achievable C2 selectivity is ∼57%, with a methane conversion of ∼29%, and C2 yield of 16%. These results are consistent with the experimental data obtained for a La2O3/CaO catalyst and in line with the measurements for other catalysts as suggested by the data in Figure 1.44 Rather than focusing on details, it is more instructive to analyze the general behavior of the system. The model shows that O2 is activated and consumed very rapidly at these high temperatures. After the rapid oxygen

Scheme 1. OCM Reaction Network Presented by Stansch et al.23 Reproduced with Permission from Ref 23. Copyright 1997 American Chemical Society

Let us analyze these kinetic issues. In general, to solve the problem of sequential reactions limiting the yields of preferred products, it is desired to use reacting systems that operate with

Figure 3. Packed bed plug flow reactor model results at 1073 K, total pressure of 1 atm, and a CH4/O2 ratio of 0.25 obtained using the kinetic parameters from Stansch et al.23 (a) Component molar fraction of each reactant and product. (b) Methane conversion and C2 selectivity and yield. (c) Sensitivity coefficients for the yield and selectivity for each reaction in the reaction network. 4342

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Figure 4. Schematic illustration of a dense plug flow membrane reactor with integrated ionically conducting methane-side catalyst and an optional airside catalyst.

methane conversion leads to lower C2 selectivity). This kinetic analysis is consistent with the arguments presented by Labinger and Ott45 who suggested that the main cause for the low C2 yields was that the rates of hydrogen-atom abstraction from C2 products (C2H6 and C2H4) are higher than the rates of abstraction from methane on most tested catalysts. Based on similar kinetic arguments, an upper limit in C2 yield of 28−30% has been predicted for OCM.46,47 Since these correlations in the rates of C−H bond activation in methane, ethane, and ethylene seem to apply for all catalysts tested to far, it is difficult to envision that a “miracle”, conventional heterogeneous catalyst could be designed that would significantly deviate from these previously observed conversion-selectivity trends (Figure 1). In addition to the above-discussed observations that the C2 selectivity (and yield) in the OCM process is compromised by the fact that most materials that activate C−H bonds in methane also activate efficiently the C−H bonds in ethane and ethylene, the yield is also affected by the measured first-order dependence on the O2 partial pressure for the deep oxidation processes compared to the 1/2 -order O2 dependence for desired C2 forming reaction.23 This kinetic information suggests that a better outcome could be achieved if the process is operated at low partial pressures of O2. One strategy to accomplish this objective is to use a plug flow membrane reactor, where methane and oxygen enter the reactor in separated streams, and the membrane allows for the selective permeation of oxygen from the oxygen side to the methane side along the length of the reactor.48 At the high temperatures at which OCM is operated, the choice of membrane is mostly limited to dense solid oxide membranes that selectively conduct the O2− ions. These membranes are attractive because oxygen can be used directly from air without the need for expensive separation equipment because the membranes allow only the selective passage of O2− ions across the membrane leaving behind nitrogen and other minor air components.48 A schematic illustrating the operation of a dense plug flow solid oxide membrane with an integrated O2− conducting catalyst on the methane-side is shown in Figure 4. An air-side catalyst may be included to aid the dissociation of oxygen molecules to O2− ions. Previous reactor modeling studies have shown that membrane reactors can give improved C2 selectivities and yields compared to conventional packed bed reactors.49−51 To further estimate the difference in the performance of a dense solid oxide membrane reactor compared to the packed bed reactor, we have modeled a plug flow membrane reactor, using kinetic parameters identical to those used above. In addition to the chemical transformations in Scheme 1, in this model oxygen (specifically O2−) was allowed to diffuse inside the reactor along the lengths of

consumption which leads to the coupling reactions, the methane conversion and the C2 selectivity and yield are relatively stable. However, it needs to be pointed out that at longer space times (not shown on the graph), the C2 selectivity and yield eventually decrease due to the continued steam reforming of the C2 products to form CO and H2. At very large space times, the product distribution equilibrates to a mixture of unreacted methane, CO, H2, CO2, and water. To shed light on particular chemical transformations involved in the OCM sequence that have the highest impact on the C2 yield and selectivity, we performed a sensitivity analysis on the reaction network in Scheme 1. For each reaction in the network, a sensitivity coefficient (Si) was calculated using the following expression: ⎛ ∂Z C2 ⎞ ⎛ ki ⎞ ⎛ δZ C2 ⎞ ⎛ ki ⎞ ⎟⎟ ≈ ⎜ ⎟⎟ Si = ⎜ ⎟ ⎜⎜ ⎟ ⎜⎜ ⎝ ∂ki ⎠k ⎝ Z C2 ⎠ ⎝ δki ⎠k ⎝ Z C2 ⎠ j

j

where ZC2 is the overall yield or selectivity to the C2 products, and ki is the rate constant for the particular reaction in the network. The sensitivity coefficient Si is the normalized measure of the impact of the rate of the ith chemical transformation on the overall outcome of the OCM reaction network without changing anything else in the system. A positive sensitivity coefficient indicates that the change in yield or selectivity (δZC2) has the same sign as δki (e.g., the yield increases as ki increases), whereas a negative sensitivity coefficient indicates that δZC2 has the opposite sign as δki. In the sensitivity analysis, the change in a particular reaction’s rate (δki) was 0.1% of the value reported by Stansch et al. for the La2O3/CaO catalyst, while all the other kinetic parameters were kept constant. Data in Figure 3c show the sensitivity coefficients for each reaction in the reaction network of Scheme 1 in a packed bed plug flow reactor. The data show that the C2 yield is most dependent on the rate of ethane production from methane and oxygen, followed by the rate of the oxidation of ethane to ethylene. Not surprisingly, the sensitivity analysis shows that more optimal catalysts should achieve high rates of C−H bond activation in methane, which is the prerequisite for the formation of *CH3 radicals and their coupling to form ethane, and concurrently low rates of C−H activation in ethane, which leads to subsequent formation of ethylene and its rapid reactions to the COx products. Judging by the reported experimental C2 selectivity−conversion data in Figure 1, there is a clear and expected correlation among all tested catalysts where the catalysts that activate C−H bonds in methane more readily are also more active in activating C−H bonds in ethane (i.e., higher 4343

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Figure 5. Membrane plug flow reactor model results at 1073 K, total pressure of 1 atm, and an overall CH4/O2 ratio of 0.25 at the highest space time (end of the reactor) obtained using the kinetic parameters from Stansch et al.23 (a) Component molar fraction of each reactant and product. (b) Methane conversion, C2 selectivity, and C2 yield. (c) Sensitivity coefficients for the yield and selectivity for each reaction in the reaction network.

While the membrane reactor shows elevated yield and selectivity to C2 products compared to packed bed reactors, it is important to note that due to the slower kinetics of the process, induced by the slow oxygen diffusion step, it requires a larger volume. This will not only impact the overall cost of the reactor, but the gas phase reactions will also likely be more significant. In addition, the lower local oxygen partial pressures will almost inevitably result in elevated rates of carbon-induced catalyst deactivation. The analysis above indicates that membrane reactors can theoretically offer higher selectivity to C2 products and higher C2 yields. However, the published data for membrane reactors in Figure 1 shows that the reported C2 yields are not substantially higher than the best-performing packed bed reactors. We believe that it is due to the fact that there are significantly fewer studies of OCM membrane reactors compared to packed bed reactors and that most of these studies have not used optimized membrane designs. In our opinion, to determine the full potential of the membrane reactor technology, two critical research directions need to be considered: (i) developing membrane materials with improved O2− flux compared to the current state-of-the-art (this in principle should enhance methane conversion and decrease the required reactor volume); and (ii) developing selective OCM catalysts that can be integrated with the membrane materials in the reactor design. There are two main types of solid oxide membranes, those with ionic conductivity (and very low electronic conductivity), which are generally used in solid oxide fuel cells, and those with mixed ionic and electronic conductivity (MIEC). As discussed previously, the rates of the O2− transport through most solid oxide membranes are relatively low compared to the rates of

the tube with the diffusion rates previously reported for conventional solid oxide membrane, such as La0.6Sr0.4Co0.2Fe0.8O3‑δ (LSCF).52 The data in Figure 5a,b show the mole fractions for each product as well as the C2 selectivity, C2 yield, and methane conversion as a function of space time in the membrane plug flow reactor model. The conditions are assumed to be identical to those discussed above for the packed bed plug flow reactor model with the temperature of 1073 K and 1.1 atm total pressure. Since in the model, oxygen was added to the system throughout the length of the reactor, a fair comparison with the packed bed plug flow reactor can be made only for the space time where the total O2/CH4 ratios and the total catalyst mass are identical for both systems. These conditions are met for the last data point in Figure 5a,b. The data in Figure 5b show that under these conditions, the C2 selectivity reaches 85%, at CH4 conversion of 45%, and the C2 yield is 38%. It is clear that the membrane reactor offers significantly higher selectivity to C2 products compared to the PFR reactor. The conversion of methane in the membrane reactor is also higher due to the fact that the C2 products require less oxygen consumption than the unselective COx products. The main reason for the improved performance of the membrane reactor is a lower partial pressure of oxygen on the methane side, which lowers the rates of the chemical transformations leading to the combustion products (∼ first-order O2 dependence) more than those leading to the C2 products (∼1/2order dependence on O2). Not surprisingly, the sensitivity analysis in Figure 5c showed that the C2 yield is most dependent on the rate of the oxygen flux into the reactor, followed by the rate of ethane production (i.e., methane activation), and finally the rate of ethylene oxidation. 4344

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conductivity52 and could potentially be explored as methane-side catalyt in electrochemically pumped OCM membrane reactors. Optimal catalysts for OCM membrane reactor systems not only need to have high selectivity to C2 products, but they also need to be integrated with the solid oxide membrane and operated at membrane reactor conditions. In a membrane reactor system, oxygen is transported through the solid oxides as O2−, and in order for the transfer of O2− between the membranes and catalysts to occur, it is desirable to have an ionically conductive catalyst. Furthermore, to ensure uninhibited flow of the O2− ions, it is important to have large areas of close contact between the membrane and the catalyst. To achieve the contact area required for the O2− transfer, it is typically necessary to fabricate the catalysts in solid oxide membrane systems at high temperatures (>1273 K) where the catalyst is sintered to the membrane layer. At these high temperatures, solid-state reactions can occur between the membrane and catalyst materials which can change the properties of the materials.57,58 Furthermore, if membrane and catalyst materials with dissimilar coefficients of thermal expansion are used, the membrane can crack and leak, compromising its performance.55 In addition to these requirements, the catalyst material must also be designed to withstand the high methane and ethylene concentrations that are present in a membrane reactor. As mentioned above, high ratios of methane and ethylene to oxygen at high temperatures can (and will) result in carbon deposits that form on the catalyst and degrade its activity.59 It is difficult to envision a scenario where carbon deposition will not be an issue in any membrane reactor design. However, it is important to point out that these problems can be partially solved by stopping the reaction and removing carbon through high-temperature oxidation. In conclusion, direct methane upgrading using oxidative coupling is a vibrant field that so far has not yielded commercially viable catalytic technology. The obstacles to the development of the viable catalysts are (i) thermodynamic, where CO2 or solid carbon are the thermodynamically preferred products, and (ii) kinetic, where the overoxidation reaction channels are readily accessible under the high-temperature conditions. We performed a simple kinetic analysis demonstrating that due to these thermodynamic and kinetic constraints, it is difficult to envision a catalytic system operated in packed bed reactors that could reach techno-economic targets for OCM. We note that our analysis was performed on a limited set of kinetic data, and it would be very beneficial to perform more rigorous experimental kinetic measurements on more OCM catalysts. Based on our analysis, it appears that the kinetic constraints can be partially alleviated by employing membrane reactors, which have been comparatively less explored. It is important to develop membrane materials that can conduct O2− with high rates as well as selective O2− conducting OCM catalysts that can be integrated with the membranes.

chemical transformations at high temperatures. The rate of the O2− flux through the membranes is a function of the temperature, oxygen partial pressure difference across the membrane, membrane thickness, and the difference in the electrical potential across the membrane. It is well-established and not surprising that the O2− flux can be increased by increasing the operating temperature and the oxygen partial pressure difference across the membrane.52 In addition to these factors, the membrane thickness has a significant impact on the O2− flux where thinner membranes support higher fluxes. We note that most of the membrane reactors fabricated and tested for OCM so far have been supported by the membrane (i.e., the membrane itself provides the entire structure for the reactor). These designs require relatively thick membranes, reaching the thickness of ∼200 μm or more in most cases, which limits the flux of O2−. It is highly desirable to deploy solid oxide membrane reactors supported on an inert porous material which supports the reactor structure. Using this approach, structures with the oxide membrane thicknesses of 10−50 μm can be relatively easily fabricated. This can be accomplished using several different techniques including sputtering, spray deposition, electrochemical vapor deposition, slip casting, or screen printing.53 In addition, hollow fiber membrane reactors that have an improved surface area to volume ratio compared to disk-shaped or extruded tubular membranes have been developed.40,54 Another strategy to increase the O2− flux in solid oxide membrane reactors is to use electrical pumping to increase the electrical potential difference between the two sides of the membrane. In this scenario, the membrane reactor is set up as an electrochemical cell where an external energy input from a power source is used to drive the flux of electrons from one side of the membrane to another. To keep the charge balanced, the O2− ions are driven through the membrane in the opposite direction.48 It is clear that in this scenario the membranes that offer mixed ionic and electronic conduction are desired. Also, for such a system to be effective, electronically conducting catalysts should be included on the methane and oxygen side. A challenge in using such a setup for OCM is that most electronically conductive catalysts are metals which are chemically reactive and promote unselective deeper oxidation of methane.56 In an effort to circumvent this problem, mixtures of metal oxides and a relatively inert metal such as Au or an electronically conducting oxide (e.g., Bi2O3−Pr6O11) have been developed as a potential electron-conducting and selective catalysts for OCM.48 None of these systems have so far achieved appreciable C2 yields. It will take serious research effort to determine how practical these systems are and what materials are best-suited for each component in these systems especially on the methane-side. Beyond increasing the flux of O2− across the membrane, it is also important to focus on developing OCM catalysts that are specifically tailored for membrane reactor systems. Although it is possible to use the membrane material for both functions, the oxygen ion transport and the catalytic activation and conversion of methane, it has been reported that adding a specifically tailored selective OCM catalyst to the membrane results in higher yields.40,42,52 Despite these findings, only few catalysts that are compatible with membrane technology have been tested. Othman et al.40 demonstrated that by incorporating an ionically conducting and selective Bi1.5Y0.3Sm0.2O3−δ (BYS) catalyst into a LSCF hollow fiber membrane, the C2+ yield of up to 39% in a single-pass reactor was attained. However, this study was conducted using a highly diluted feed stream (10% methane in argon). BYS also has a considerable amount of electronic

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 734 647 7984. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from United States Department of Energy, Office of Basic Energy Science, Division of 4345

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Chemical Sciences (FG-02-05ER15686) and National Science Foundation (DMRF- 1436056). B.F. acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260. V.O.I. acknowledges support from the Nigerian Government under the PRESSID Scholarship Scheme.

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DOI: 10.1021/acscatal.6b01087 ACS Catal. 2016, 6, 4340−4346