Partial Oxidation of Methane to Synthesis Gas Using

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DOI: 10.1002/cctc.201801030 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

Partial Oxidation of Methane to Synthesis Gas Using Supported Ga-Containing Bimetallic Catalysts and a Ti-Promoter David A. Kriz,[a] Quddus A. Nizami,[b] Junkai He,[c] Tahereh Jafari,[c] Yanliu Dang,[c] Peter Kerns,[a] Andrew G. Meguerdichian,[c] Steven L. Suib,*[a, c] and Partha Nandi*[b] A series of bimetallic Ga-containing materials using TiO2 and TiO2-promoted SiO2 supports have been prepared. Rhodium, palladium, and platinum have been used as additional metals in this system. The materials are characterized and used as catalysts for the partial oxidation of methane into synthesis gas (H2 and CO). The presence of a low quantity of titanium in the form of anatase TiO2 was shown to improve the overall activity of catalytic methane oxidation and to strongly increase the

selectivity of partial oxidation products over the total oxidation of methane to carbon dioxide and water. Particular attention is paid to the formation of gallium-metal alloys on the surface of the catalyst supports. Rh-Ga-Ti-SiO2 was found to be the most active and selective catalyst, giving 89 % conversion of methane and 99 % selectivity to synthesis gas at 750 8C, as well as exhibiting catalytic activity and preferential conversion to partial oxidation products at temperatures as low as 350 8C.

Introduction Methane is the main component of natural gas. According to the International Energy Agency (IEA), international production rate of natural gas reached a record high, while demand is projected to continue to increase.[1] Abundant deposits of natural gas exist, but many are located in remote areas of the world and therefore transportation of these materials over long distances is often required. There is therefore significant interest in developing methods for converting methane to liquid chemicals, such as methanol or larger hydrocarbons, which are more easily transported. One such method involves the conversion of methane into syngas, a mixture of hydrogen and carbon monoxide, which in turn is a feedstock for FischerTropsch hydrocarbon synthesis and an intermediate step in methanol synthesis from methane. Syngas is mainly produced from methane by steam reformation, carbon dioxide reformation, autothermal oxidation, and partial oxidation [Eqs. (1)– (4)].[2,3] Syngas Production from Methane:

[a] D. A. Kriz, P. Kerns, Prof. S. L. Suib Unit 3060 Department of Chemistry University of Connecticut Storrs CT 06269 (USA) E-mail: [email protected] [b] Q. A. Nizami, Dr. P. Nandi Corporate Strategic Research ExxonMobil Research and Engineering Annandale NJ 08801 (USA) E-mail: [email protected] [c] J. He, Dr. T. Jafari, Y. Dang, A. G. Meguerdichian, Prof. S. L. Suib Institute of Materials Science University of Connecticut Storrs CT 06269 (USA) Supporting information for this article is available on the WWW under https://doi.org/10.1002/cctc.201801030

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Steam reforming CH4 þ H2 O ! CO þ 3H2

DH ¼ 206 kJ=mol

ð1Þ

Dry reforming CH4 þ CO2 ! 2CO þ 2H2

DH ¼ 247 kJ=mol

ð2Þ

Autothermal reforming x CH4 þ O2 þ ð1 x ÞH2 O ! CO þ ð3 x ÞH2 2

ð3Þ

DH ¼ 206 242x kJ=mol ð0 < x < 1Þ Partial oxidation 1 CH4 þ O2 ! CO þ 2H2 2

DH ¼ -36 kJ=mol

ð4Þ

Of these, the partial oxidation route has distinct advantages including a lower energy requirement, lower process temperature, and the capability to adjust the H2:CO product ratio. However, the conversion of methane into valuable products remains difficult due to the high CH3-H bond dissociation energy (436.3 kJ/mol).[4] Improved heterogeneous catalysts are thus called for. Our approach involves the use of bimetallic galliumcontaining catalysts on titanium dioxide and titanium-promoted silicon oxide supports. A wide variety[4] of supported transition metal catalysts have been tested for the methane partial oxidation reaction, including other bimetallic systems including the noble metals rhodium, palladium and platinum. In many cases, an increase in catalytic activity attributed to the formation of an alloy has been demonstrated in similar materials.[5–10] Gallium is chosen as an alloy component in our system due to its demonstrated capability to bind methane

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molecules and aid in the initial dissociation of the CH bond.[11,12] The reaction of methane with gallium is shown to occur by oxidative addition to form surface methyl and hydroxide or CH3MH moieties.[13,14] Ga-species have been shown to promote the recombination and desorption of dihydrogen from alkanes and is therefore a useful material for alkane aromatization via hydrogenation.[15]. Choudhary et al. have shown conversions of methane with alkanes or methanol over gallium-zeolite catalysts to the form of higher alkyl chains and aromatic hydrocarbons.[16,17] Gallium also plays a critical role in BP’s Cyclar process, wherein a continuously-regenerated gallium-doped catalyst, tailored for size selectivity, steadily converts short-chain hydrocarbons into aromatics. In lieu of a titania support, Chen et al. demonstrated a strong increase in the stabilization of Rh-adsorbed CO induced by the addition of a small amount of a Ti promoter (0.0025–0.01 wt%) on their silica-supported catalyst.[18] Therefore, the demonstrated capability for gallium to activate methane and generate dihydrogen from hydrocarbons, combined with the stabilization of CO from a trace-level Ti-promoter led us to expect the materials

presented herein to be good catalysts for the partial oxidation of methane to synthesis gas. Although the present focus of methane conversion by chemists is largely motivated by the current increase in use of natural gas from fossil fuel resources, methane can also be obtained from the decomposition of biomass and organic waste materials. The technologies developed for methane conversion are therefore also able to contribute to sustainable carbon-neutral energy production systems which continue to grow.[19]

Results Figure 1A shows the X-ray diffraction (XRD) patterns for TiO2supported materials. The diffraction peaks correlate to a pure anatase phase of the support material, while the Ti-SiO2supported materials are X-ray amorphous (Figure 1B). An additional peak manifests in the diffraction patterns of both Pdcontaining materials at 348 2-theta, which correlates to the

Figure 1. XRD patterns of (A) TiO2-supported materials and (B) Ti-SiO2-supported materials. (C) Raman spectra of the catalysts. (D) DSC results of the catalysts.

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(101) planes of tetragonal PdO. Raman spectra are shown in Figure 1C. All six materials exhibit a Raman shift at 150 cm1 corresponding to the Eg transition of anatase. The TiO2supported materials exhibit three additional signals at 400, 515, and 640 cm1 corresponding to B1g, A1g and Eg transitions. Due to the relatively much lower Ti concentration present in the TiSiO2-supported materials, only the strongest signal at 150 cm1 is observable but is sufficient to confirm the presence of anatase TiO2 in these samples. Differential scanning calorimetry (DSC) results (Figure 1D) for the as-synthesized samples show transitions between 25 and 100 8C corresponding to endothermic melting of gallium and its formation of an alloy with another metal present on the surface of the material. From these data, enthalpies of melting (DHmelt) were obtained. Comparing these values to that of the bulk gallium[20] (81.097 J/ g) allows us to gain some insight to the distribution of metals, the effect of particle size, and the interaction of the metals with the support materials., as summarized in Table 1. Surface textural properties of the materials as determined by N2 adsorption-desorption studies are outlined in Table 1. Surface areas were determined using the Brunauer-EmmettTeller (BET) method, while pore size metrics were calculated using the Barrett-Joyner-Halenda (BJH) analysis method. Pd-GaTiO2, Pt-Ga-TiO2, and Rh-Ga-TiO2 had surface areas of 165, 144, and 154 m2/g, respectively. All of the Ti-SiO2-supported materials had relatively higher surface areas of 196, 237, and 266 m2/g for Pd-Ga-Ti-SiO2, Pt-Ga-Ti-SiO2, and Rh-Ga-Ti-SiO2, respectively. A similar trend was observed for the average pore sizes of the materials. Pd-Ga-TiO2, Pt-Ga-TiO2, and Rh-Ga-TiO2 had relatively smaller average pore diameters of 9.62, 9.45, and 9.27 nm, respectively, versus Pd-Ga-Ti-SiO2, Pt-Ga-Ti-SiO2, and Rh-Ga-TiSiO2, which had average pore diameters of 13.0, 12.9, and 12.3 nm, respectively. The loading amount of the noble metals and Ga on each catalyst was determined using X-ray fluorescence (XRF), as summarized in Table 1. Scanning electron micrographs (SEM) of the materials reveal the surface morphology of the materials (Figure 2). The TiO2 support materials are characterized by roughly 50 nm plateletlike particles which aggregate to form micron-sized clusters. The materials appear highly porous but poorly ordered. The presence of the catalytic metals on the surface of the support is not readily observable. The Ti-SiO2 supported materials consist of much smaller nanoparticles, on the order of 10 nm, which coalesce to form much larger, micron-sized aggregates. On the surface of the support, 100 nm particles of another phase can be observed.

These nano-sized features are more easily seen in the transmission electron microscopy (TEM) images (Figure 3), and the high porosity of the materials is apparent. The blank dots on the surfaces of mesoporous substrates observed from the TEM images are attributed to the noble metal nanoparticles. The scanning transmission electron microscopy (STEM) and EDS mapping images of the Rh-Ga-Ti-SiO2 sample after CH4 oxidation reaction are shown in Figure 4. All the measured elements are evenly distributed throughout the material. The merged image (Figure 4B) displays the homogenous intermixing of Rh and Ga. The even elemental distribution and lack of visible discrete metal particles in the TEM and EDS maps indicates that the alloyed material exists as a thin layer distributed over the surface of the material. Secondary-ion mass spectrometry (SIMS) results of Ga and Rh for the fresh Rh-Ga-Ti-SiO2 material are presented in Figures 5A and 5B, respectively, both displayed uneven distribution for the selected area. However, after the CH4 oxidation reaction at 750 8C, Ga and Rh are found uniformly intermixed and homogeneously distributed in the tested area, indicating the formation of metal alloys. Further, hydrogen temperature programmed desorption (H2-TPR) experiments (Figure 5E) exhibit a similar trend, where reduction of the material occurs at a lower temperature for the Ti-SiO2 supported materials versus the TiO2 supported analogues. The lower reduction temperatures correspond with the lower melting enthalpies calculated from the DSC data and indicates a more reactive, facile alloy transition in the Ti-SiO2 materials. The Pt-containing materials exhibit no reduction peak. The inherently low melting point of gallium also results in greatly increased Brownian motion in the catalytically active surface which we believe leads to enhanced interaction between the catalyst and reactant components. Although the melting point is low, the low vapor pressure of gallium keeps the catalyst activity stable at the elevated temperatures required for the reaction. The percent methane conversion (Figure 5F) was calculated by measuring the CH4 concentration at the outlet and comparing it to the reactor input concentration. The conversion of methane generally trended up as a function of temperature for all materials. At higher temperatures, the Ti-SiO2 supported materials were significantly more active than the TiO2-supported materials. The outlet gas composition analysis, shown in Figure 6, shows the selectivity toward hydrogen, carbon monoxide, water, and carbon dioxide for each material. Comparison of the TiO2-supported and Ti-SiO2-supported materials shows that not only were the Ti-SiO2 (Figure 6B, D, F) materials more active, but also significantly more selective

Table 1. Nitrogen sorption, DHmelt, and chemical composition of catalysts. Sample

Surface area [m2/g]

Average pore diameter [nm]

DHmelt [J/g]

DHmelt vs bulk metal [%]

Noble metal atomic ratio [%]

Ga atomic ratio [%]

PdGa-TiO2 PtGa-TiO2 RhGa-TiO2 PdGa-Ti-SiO2 PtGa-Ti-SiO2 RhGa-Ti-SiO2

165 144 154 196 237 266

9.62 9.45 9.27 13.0 12.9 12.3

49.1 71.87 59.76 39.59 71.24 45.79

61.3 89.7 74.6 49.4 88.9 57.2

3.6 2.2 4.5 3.8 1.6 3.3

4.6 4.5 5.3 4.6 3 2.3

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Figure 2. SEM images of (A) Rh-Ga-TiO2, (B) Rh-Ga-Ti-SiO2 (C) Pd-Ga-TiO2, (D) Pd-Ga-Ti-SiO2, (E) Pt-Ga-TiO2 and (F) Pt-Ga-Ti-SiO2.

Figure 3. TEM images of (A) Pt-Ga-TiO2, (B) Pd-Ga-TiO2, (C) Rh-Ga-TiO2, (D) Pt-Ga-Ti-SiO2, (E) Pd-Ga-Ti-SiO2, and (F) Rh-Ga-Ti-SiO2.

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Figure 4. (A) HAADF and (B–F) element mapping images of Rh-Ga-Ti-SiO2 after CH4 oxidation.

Figure 5. SIMS profiles of (A) Ga and (B) Rh for Rh-Ga-Ti-SiO2 before CH4 oxidation. SIMS profiles of (C) Ga and (D) Rh for Rh-Ga-Ti-SiO2 after CH4 oxidation. (E) H2-TPR profiles for TiO2 and Ti-SiO2-supported materials. (F) Percent methane conversion using supported bimetallic catalysts at 300–750 8C.

toward partial oxidation products when compared to the TiO2 materials (Figure 6A, C, E), which favored the formation of total ChemCatChem 2018, 10, 4300 – 4308

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oxidation products carbon dioxide and water. Among the TiO2supported catalysts, only the Rh-containing material generated

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Figure 6. Composition of the gas stream at the outlet of the partial methane oxidation reactor using (A) Rh-Ga-TiO2, (B) Rh-Ga-Ti-SiO2, (C) Pd-Ga-TiO2, (D) PdGa-Ti-SiO2, (E) Pt-Ga-TiO2, and (F) Pt-Ga-Ti-SiO2.

significant quantities of partial oxidation products, but only at temperatures exceeding 550 8C. A comparison of the Ti-SiO2supported materials shows that the Pd-Ga-Ti-SiO2 required higher reactor temperatures above 450 8C to generate partial oxidation products. The Rh-Ga-Ti-SiO2 and Pt-Ga-Ti-SiO2 materials produce these products at 350 and 400 8C, respectively. At lower temperatures, any methane conversion observed yields CO2 and H2O. ChemCatChem 2018, 10, 4300 – 4308

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A summary of the highest methane conversion achieved and the selectivity to partial oxidation products CO and H2 is shown in Table 2. For each material, selectivity to partial oxidation was favored at higher temperatures if they were formed at all throughout the experiment. Among the TiO2supported materials, the Pt-Ga-TiO2 material exhibited the highest methane conversion (52 %) and partial oxidation product selectivity (82 %) at 650 8C. This is followed by the

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Table 2. Maximum methane conversion and partial oxidation product selectivity. Sample

RhGa-TiO2 PdGa-TiO2 PtGa-TiO2 RhGa-TiSiO2 PdGa-TiSiO2 PtGa-TiSiO2

Max. conversion [%]

Temperature [8C]

CO selectivity at max. conversion [%]

44 43 52 89

750 750 650 750

46 28 82 99

80

700

93

71

750

94

RhGa-TiO2 material which showed a maximum conversion of 44 % and selectivity of 46 % at 750 8C, and PdGa-TiO2 which showed maximum conversion of 43 % and only 28 % selectivity at the same temperature. Among the TiO2-supported materials, the activity and selectivity of the materials fall in the order PtGa-TiO2 > Rh-Ga-TiO2 > Pd-Ga-TiO2. Although all of the Ti-SiO2 materials significantly outperform the TiO2-supported materials in terms of both conversion and selectivity, this same trend is in activity is not observed. In this case, the most active and selective material is Rh-Ga-Ti-SiO2, which was capable of 89 % conversion and 99 % selectivity at 750 8C. This is followed by Pd-Ga-Ti-SiO2 which gave 80 % conversion and 93 % selectivity at 700 8C, and the Pt-Ga-Ti-SiO2 gave 71 % conversion and 94 % selectivity at 750 8C. In this set of materials, the activity and selectivity fall in the order Rh-Ga-Ti-SiO2 > Pd-Ga-Ti-SiO2 > PtGa-Ti-SiO2. During experimentation, CO chemisorption was attempted to determine the number of active sites on the catalyst. These tests proved difficult, yielded inadequate data, and therefore made the reliable calculation of turnover numbers for methane conversion not possible.

Discussion The introduction of a trace amount of anatase TiO2 onto a porous SiO2 support for MGa alloy catalysts has been shown to greatly improve activity and selectivity in the partial oxidation of methane to CO and H2 versus a pure anatase support. In a similar system, Rh has been shown to stabilize CO molecules on solid surfaces,[18] while Ga is known to activate CH bonds in methane and other hydrocarbons, and to contribute the recombination of surface -H species to desorb molecular hydrogen.[15–17] Rh-Ga-Ti-SiO2, the most active material, also showed the strongest exothermic transition in the DSC study (Figure 1D), indicating the formation of a Rh-Ga alloy may play a critical role in the system. The as-synthesized materials include metal and gallium as separate particles on the surface but form a uniform mixture upon heating under methane partial oxidation conditions. Additional factors influencing the activity of the catalyst may be the surface textural properties and the morphologies of the materials. In general, the TiO2-supported materials had somewhat lower surface areas (144–165 m2/g) compared to the ChemCatChem 2018, 10, 4300 – 4308

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Ti-SiO2-supported materials (196–266 m2/g), but also much lower average pore sizes and overall lower total pore volumes (~ 9.4 nm vs 13 nm, and 0.35 cc/g vs 0.75 cc/g, respectively). The particle size of the more active material support is also smaller by an order of magnitude, perhaps improving dispersion of catalytically active sites. In general, the TiO2-supported materials had higher DHmelt values compared to Ti-SiO2 materials. This can be explained by taking into account the relatively lower pore size values obtained from N2-sorption experiments. The smaller pore sizes increase the degree to which individual metal particles are confined in discrete locations prior to alloy formation, resulting in a necessary increase in energy required to promote alloying. Similarly, the relatively sharper transitions indicate a narrower particle size distribution which would be expected with smaller pores in the support material. The Ti-SiO2 supported materials, on the other hand, have relatively lower DHmelt values. These data show that the larger pore sizes are more conducive to the formation of gallium-metal alloys in this system. An alternative explanation for the decrease in DHmelt is that the alloy formation is incomplete. However, TEM mapping data shows that the metals are intimately mixed. DSC studies provide insight to the behavior of the metals on the support surface upon heating, specifically with respect to gallium-metal alloy formation. In general, DHmelt is decreased in the Ti-SiO2 supported materials versus the TiO2 supported materials. This is likely due to properties of the support such as pore size and the distribution of the metal particles influencing the alloying process upon heating of the material. Specifically, we believe that the pore size in the Ti-SiO2 material is more suitable for alloy formation. A similar effect was observed by Unruh et al., where DHmelt of indium was substantially affected by the porosity of porous glass substrates.[21] It is interesting to note that the two most active materials, Rh-Ga-Ti-SiO2 and PdGa-Ti-SiO2 both showed the greatest decrease in DHmelt compared to bulk Ga, indicating that alloy formation is critical to these systems.

Conclusion A set of bimetallic Ga-containing materials using TiO2 and Tipromoted SiO2 supports have been synthesized and characterized. Rhodium, palladium, and platinum have been incorporated into these systems to form catalytically active M-Ga alloys on the support surface. The materials, in particular the Tipromoted materials, are active and selective in the partial oxidation of methane to synthesis gas (H2 and CO) at temperatures between 300–750 8C. The trace level of titanium in the catalyst was shown to not only improve the overall activity of catalytic methane oxidation but also strongly increased the selectivity toward partial oxidation products versus carbon dioxide and water. Overall, Rh-Ga-Ti-SiO2 was found to be the most active and selective catalyst, giving 89 % conversion of methane and 99 % selectivity to synthesis gas at 750 8C, and showed activity for the partial oxidation reaction at temperatures as low as 350 8C.

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Experimental Sample Preparation Preparation of Ga-TiO2. A 10 % Ga loading on titania was prepared by adding 2 g of elemental Ga to 18 g of TiO2 (HOMBIKAT TiO2, 8602 having a surface area of 850 m2/g. The material was calcined at 200 8C for 16 hours in air, cooled to room temperature, followed by subjecting to a tumbling Parr reactor at 150 8C for 4 hours. The material was subsequently used to prepare the following titaniasupported bimetallic systems with gallium. Preparation of Pd-Ga-TiO2. A 1 g portion of palladium nitrate hydrate (99.8 %) was transferred into a beaker and 3 mL of HNO3 (69–70 %, J. T. Baker) was added and the solution was mixed well. A 10 g portion of Ga-TiO2 was added to the solution and the mixture was stirred with a spatula for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight. Preparation of Pt-Ga-TiO2. A 1 mL portion of platinum tetraamine solution was transferred into a beaker. A 5 g portion of Ga-TiO2 was mixed with the Pt-containing solution for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight. Preparation of Rh-Ga-TiO2. A 1 mL portion of rhodium nitrate solution (10 % Rh) was transferred into a beaker. A 5 g portion of Ga-TiO2 was mixed with the Rh-containing solution for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight. Preparation of Ti-SiO2. A 20 g portion of silica (Davisil 646) was modified with titania by reacting 1 mL of Ti(OiPr)4. The material was calcined in air at 400 8C overnight. The resulting material was subsequently used to prepare the following bimetallic systems with titania-promoted silica supports. Gallium was added following the previously described procedure. Preparation of Pd-Ga-Ti-SiO2. A 1 g portion of palladium nitrate hydrate (99.8 %) was transferred into a beaker and 3 mL of HNO3 (69–70 %, J. T. Baker) was added and the solution was mixed well. A 10 g portion of Ti-SiO2 was added to the solution and the mixture was stirred with a spatula for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight. Preparation of Pt-Ga-Ti-SiO2. A 1 g portion of palladium nitrate hydrate (99.8 %) was transferred into a beaker and 3 mL of HNO3 (69–70 %, J. T. Baker) was added and the solution was mixed well. A 5 g portion of Ti-SiO2 was added to the solution and the mixture was stirred with a spatula for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight. Preparation of Rh-Ga-Ti-SiO2. A 1 mL portion of rhodium nitrate solution (10 % Rh) was transferred into a beaker. A 5 g portion of TiSiO2 was mixed with the Rh-containing solution for 15–20 minutes at room temperature. The material was calcined in air at 400 8C overnight.

obtained using a TA Instruments DSC 2920 and a temperature ramp rate of 10 8C/min in an Ar environment. H2-TPR was completed using a MKS e-Vision + residual gas analyzer (quadrupole mass spectrometer). A 100 mg portion of material was loaded into a 7 mm ID quartz tube between quartz wool plugs. This tube was housed inside a programmable tube furnace. A mixture containing 5 % H2 in argon was flowed through the packed bed at a rate of 50 sccm and the furnace temperature was ramped from room temperature to 400 8C. The outlet gas was continuously sampled to the MS via a capillary line. Raman analysis was completed using a Renishaw 2000 Raman microscope using a 514 nm laser source. SIMS measurements of the samples were done using a Hiden SIMS workstation. An IG-20 ion gun and the MAXIM quadrupole mass spectrometer for the detection of secondary positive ions and sputtered neutrals.

Methane Partial Oxidation Tests Methane oxidation catalysis was carried out by preparing a fixed bed catalyst by loading 50 mg of material into a 7 mm ID quartz tube between quartz wool plugs. The quartz tube was housed inside a tube furnace. The samples were degassed under flowing helium (50 sccm) for 1 hour prior to analysis, then cooled to ambient temperature. A mixture containing 5 % methane and 2.5 % oxygen in helium was flowed through the packed bed at 50 sccm (GHSV = 60,000 h1). The catalyst bed was allowed to equilibrate at temperatures between 300–750 8C for 35 minutes prior to sampling with a standardized on-line SRI 8610C gas chromatograph (GC) equipped with Haysep and molecular sieve columns as well as helium ionization and thermal conductivity detectors. All products were well separated, identified based on retention time in the GC column, and quantified using standardized integrated peak areas.

Acknowledgements We would like to acknowledge ExxonMobil for their support of this research. The TEM/STEM studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). The SEM images were acquired at the Bioscience Electron Microscopy Facility at the University of Connecticut, with financial support from NSF grant #1126100. We thank the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences under grant DE-FG02-86ER13622.A000 for partial support of this research.

Conflict of Interest The authors declare no conflict of interest.

Characterization XRD patterns were collected using a Rigaku Ultima IV X-ray diffractometer using CuKa radiation operating at 40 kV and 44 mA. N2 sorption experiments were carried out using a Quantachrome Autosorb-1-1C automated adsorption system. The powders were degassed for 6 hours prior to the measurements under vacuum. The surface areas were calculated using the BET method and pore sizes were determined using the BJH analysis method. The STEM and TEM images were collected using a Talos F200X microscope operating at 200 kV equipped with a Super-X SDD energy dispersive X-ray spectroscopy detector (EDS). DSC data was

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Keywords: Bimetallic catalyst · Gallium · Methane · Partial oxidation · Synthesis gas

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