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Selective Methane Functionalization via Oxyhalogenation over Supported Noble Metal Nanoparticles Vladimir Paunovic, Guido Zichittella, Patrick Hemberger, Andras Bodi, and Javier Pérez-Ramírez ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04375 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Catalysis

Selective Methane Functionalization via Oxyhalogenation over Supported Noble Metal Nanoparticles Vladimir Paunović,§,† Guido Zichittella,§,† Patrick Hemberger,‡ Andras Bodi,‡ and Javier Pérez-Ramírez*,† †

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences,

ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland ‡

Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, 5232

Villigen, Switzerland

1 ACS Paragon Plus Environment

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ABSTRACT This article investigates SiO2-supported Ru-, Pt-, Ir-, Rh-, and Pd-based catalysts (1 wt.% metal loading) as new catalytic systems for the oxychlorination and oxybromination of methane, both pivotal steps in the halogen-mediated production of commodities from natural gas, and gains insights into the structure-performance relationships and mechanism of these reactions as a function of the metal and the hydrogen halide. In-depth characterization of the equilibrated catalysts by X-ray diffraction, electron microscopy, Raman, and X-ray photoelectron spectroscopies evidences a fast restructuring of the starting oxide nanoparticles into metallic, metal silicide, or metal halide phases. The oxychlorination activity, which decreases in the order: Ru/SiO2 > Pt/SiO2 > Ir/SiO2 > Rh/SiO2 ≈ Pd/SiO2, is enhanced in the presence of metal oxides, while the activity order in oxybromination: Ru/SiO2 ≈ Ir/SiO2 ≈ Pd/SiO2 > Pt/SiO2 > Rh/SiO2 is less dependent on the phase composition. The highest selectivity to chloromethanes (78-83%) and bromomethanes (92-98.5%) at moderate methane conversion (20%), rivaling the performance of the best oxyhalogenation catalysts, is attained over Ir/SiO2, Rh/SiO2, and Pd/SiO2, and correlates with their ability to reduce and form metal halides. Catalyst propensity towards halogenation depends on the halide type, although it is less pronounced at higher loadings (up to 5 wt.%), while supporting over other inert carriers has marginal impact on the restructuring patterns. Finally, kinetic analysis coupled with detection of radical intermediates by operando photoelectron photoion coincidence spectroscopy indicates a significant role of gas-phase halogenation in methane activation via oxyhalogenation. In oxychlorination, the latter pathway has a similar contribution as surface activation for Pd/SiO2, Rh/SiO2, and Pt/SiO2, and major contribution for Ir/SiO2 and Ru/SiO2 catalysts, while in oxybromination it dominates for all the systems, limiting the potential to enhance the selectivity to mono- over dihalomethanes.


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KEYWORDS: gas-phase chemistry, metal nanoparticles, methane oxyhalogenation, operando photoelectron photoion coincidence spectroscopy, reaction mechanism, selective C-H activation, surface chemistry


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1. INTRODUCTION Methane, as the main component of natural gas (70-90%) and biogas (50-70%), is regarded as the key feedstock to sustain the manufacture of value added chemicals and liquid fuels until fully renewable production routes are technically and economically competitive.1–5 However, the need of centralized megaplants for commercial harvesting of methane, in combination to the scattered distribution of a significant fraction (30-60%) of the gas wells and the high transportation costs, limits the use of natural gas as a chemical feedstock and often results into environmentally harmful flaring and venting practices.1-7 Accordingly, there has been a great interest in developing catalytic processes aiming at the direct conversion of methane into valuable chemicals.1–6,8–12 Halogenation of methane into halomethane (CH3X; X = Cl, Br), a platform molecule with analogues upgrading pathways as those of methanol, enables activation of this alkane at moderate temperatures (< 833 K) and near ambient pressure, exhibiting a promising potential for the implementation in compact plants amenable to decentralization.5,13-19 The commercialization of this technology is primarily contingent on the complete recycling of hydrogen halide (HX), which is a stoichiometric byproduct generated in halogenation and CH3X coupling. An attractive complementary step to accomplish this goal is the catalytic oxyhalogenation of methane, which is conceptually analogue to the well-established industrial approach used to recycle HCl in the production of PVC from ethylene.20 Suppressing the carbon losses through the side combustion reactions is one of the core challenges in catalyst development for this process. Besides, reducing the formation of dihalomethane (CH2X2) is also sought after, although the latter product can be transformed into CH3X or valued hydrocarbons via hydrodehalogenation.19


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During the last decade, strategies based on redox-site isolation, supporting, or feeding an excess of halide were applied to minimize the production of carbon oxides (COx), unraveling different catalyst families such as phosphates, oxyhalides, oxides, and mixed-oxides. Notable selectivities (S) to halomethanes (CH3X + CH2X2) were achieved in methane oxychlorination, MOC, (S ≤ 88%) over LaOCl6,21 and CeO2,14 and methane oxybromination, MOB, (S = 75-96%) over CeO2,14,22 FePO4,15 (VO)2P2O7,15 LaVO4,23 and EuOBr16 at methane conversions of XCH4 = 20-30%. In contrast, studies on noble metal-based materials, which constitute benchmark catalysts in many methane activation processes,8,24-27 were limited to a few metal systems and lacked in-depth material characterization, preventing to establish structure-performance relationships. Zhou et al.28,29 reported total selectivity to halomethanes of S = 90% (XCH4 = 30%) over Rh/SiO2 and of S = 75-92% (XCH4 = 30-19.5%) over Ru/SiO2 catalysts in MOB, while Shalygin et al.30 obtained S = 83% (XCH4 = 13.5%) over K4Ru2OCl10/TiO2 in MOC. The mechanism of methane oxyhalogenation over the above listed systems is still disputed. Earlier studies on MOC over LaOCl, CeO2, and K4Ru2OCl10/TiO2 proposed purely surface driven formation of halomethanes, while theoretical work of Metiu et al. suggested the possibility of methyl radical formation over halogenated oxides, which may further react in the gas-phase.6,14,30,31 Nevertheless, kinetics analysis conducted over several oxide, phosphate, and oxybromide catalysts indicated that gas-phase halogenation of methane with halogen species generated on the catalyst surface might substantially contribute to the overall rate of MOC, while it constitutes the main reaction pathway in MOB.15,16,22,23,31 Only recently, our group verified the formation of methyl and bromine radicals over the state-of-the art (VO)2P2O7 and EuOBr catalysts in MOB using operando photoelectron photoion coincidence (PEPICO) spectroscopy, a cutting-edge technique enabling the detection of short-lived gaseous intermediates in


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non-catalytic and catalytic processes.33–38 Gas-phase alkane activation might curtail COx evolution by decoupling the methane-derived intermediates from the catalyst surface. However, the inherent kinetics of radical halogenations limits the possibility to reduce the selectivity to CH2X2 via catalyst engineering. Still, no study investigated the extent of radical chemistry in the oxyhalogenation for different types of HX through the combination of kinetic analysis and identification of the gaseous transient intermediates via operando techniques. Given their high potential to promote surface C-H bond activation and thus to minimize the impact of gas-phase pathways on the oxyhalogenation performance, the present study evaluates the catalytic behavior of silica-supported Ru-, Pt-, Ir-, Rh-, and Pd-based catalysts in MOC and MOB aiming to determine structure-performance relationships as well as the degree of the gas-phase chemistry contributions as a function of the metal and nature of the HX. In-depth characterization by X-ray diffraction, electron microscopy, Raman, and X-ray photoelectron spectroscopies evidences substantial catalyst restructuring during the reaction. Thereby, the highest selectivity to halomethanes is achieved over the catalysts displaying high propensity towards reduction and formation of metal halides. By complementing the kinetics analysis with operando PEPICO spectroscopy, we evidence the profound role of gas-phase chemistry in methane activation via oxyhalogenation and derive estimates of the relative radical concentrations as well as the contributions of the gas-phase pathways to methane activation as a function of the metal and the HX.

2. EXPERIMENTAL 2.1. Catalyst Preparation. Bulk RuO2, IrO2, and PdO were obtained by calcination of RuCl3·nH2O (ABCR, 99.9%), IrCl3·nH2O (ABCR, 99.9%), and Pd(NO3)2·nH2O (Aldrich,


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ACS Catalysis

99.9%), respectively, in static air at 873 K for 5 h using a heating rate 5 K min-1. PtO2 (ABCR, 99.95%) and Rh2O3 (ABCR, 99.9%) were used as received. SiO2 (≤ 75 m, Evonik, SIPERNAT® 320, ≥ 97.0%), -SiC (0.4-0.6 mm, SICAT, 99%), MgO (~20 nm, Strem Chemicals, ≥ 99%), and -Al2O3 (5-100 m, Sasol, PURALOX® SCFa 140, ≥ 98%) were calcined as described above and stored under vacuum (50 mbar) at 298 K prior to their use as carriers in the catalyst preparation. Supported noble metal catalysts were prepared by incipient wetness impregnation. The metal precursors, RuCl3·nH2O, [Pt(NH3)4]Cl2·nH2O (Aldrich, 98%), IrCl3·nH2O, RhCl3·nH2O (ABCR, 38.5-45.5% Rh), and Pd(NO3)2·nH2O were dissolved in a volume of deionized water equal to the pore volume of the carrier in appropriate amounts to achieve the desired loading (1, 2, or 5 wt.%) calculated on metal basis. The precursor solutions were added dropwise to the carrier and the resulting materials were mixed at room temperature for 1 h, then dried under vacuum (50 mbar) at 373 K for 12 h, and finally calcined in static air at 873 K for 5 h using a heating rate of 5 K min-1. The catalysts are referred to as M/C-metal%, where M denotes the specific metal, i.e., Ru, Pt, Ir, Rh, or Pd, C represents the carrier, i.e., SiO2, MgO, SiC, or Al2O3, and metal defines the weight percentage of metal, which is omitted for the materials with 1 wt.% loading. It is important to note that M does not imply the phase composition of the catalyst, but only denotes the metal present in the catalyst composition. 2.2. Catalyst Characterization. N2 sorption at 77 K was performed in a Micromeritics TriStar II analyzer. Samples (ca. 0.25 g for bulk and ca. 0.15 g for supported catalysts) were evacuated to 50 mbar at 573 K for 12 h prior to the measurement. The Brunauer-Emmett-Teller (BET) method was applied to calculate the specific surface area, SBET. The pore volume, Vpore, was determined from the amount of N2 adsorbed at a relative pressure of p/p0 = 0.98. Powder X-ray diffraction (XRD) was measured in a PANalytical X’Pert PRO-MPD diffractometer with


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Bragg-Brentano geometry by applying Ni-filtered Cu K radiation ( = 1.54060 Å). The data were recorded in the 10-70° 2 range with an angular step size of 0.017° and a counting time of 2.04 s per step. The average size of the respective crystallites was estimated from the peak broadening by using the Scherrer equation with a dimensionless shape factor of K = 0.9. Temperature-programmed reduction with hydrogen (H2-TPR) was conducted in a Micromeritics Autochem II 2920 unit equipped with a thermal conductivity detector. The sample (0.1 g) was loaded in a U-shaped quartz reactor between two plugs of quartz wool and pretreated in He (20 cm3 min-1) at 573 K for 1 h. The analysis was then performed in 5 mol.% H2 in N2 (20 cm3 min-1) by heating up the catalyst in the range from 323-700 K at 10 K min-1. High-resolution transmission electron microscopy (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and elemental mapping with energy-dispersive X-ray spectroscopy (EDXS) were conducted on a FEI Talos microscope operated at 200 kV. All samples were dispersed as dry powders onto lacey carbon coated nickel grids. The particle size distribution was obtained by examining over 200 nanoparticles. Raman spectroscopy was performed on a WITec CRM200 confocal Raman system using a 532 nm diode laser with 20 mW power, a 100× objective lens with numerical aperture NA = 0.9 (Nikon Plan) and a fiber-coupled grating spectrometer (2400 lines per mm), giving a spectral sampling resolution of 0.7 cm-1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics Quantum 2000 X-ray photoelectron spectrometer using monochromatic Al K radiation, generated from an electron beam operated at 15 kV, and equipped with a hemispherical capacitor electron-energy analyzer. The solids were analyzed at an electron take-off angle of 45° and a pass energy of 46.95 eV. In order to suppress sample charging during analysis an electron and an ion neutralizer were operated simultaneously. Remaining charging


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effects were corrected by referencing all spectra to oxygen present in silica (O 1s at 532.65 eV).39 The elemental concentrations were quantified based on the measured photoelectron peak areas (O 1s, Si 2p, Cl 2p, Br 3d, and Pd 3d) after Shirley background subtraction using PHI-MultiPak software and the built-in relative sensitivity factors, which are corrected for the system transmission function. The Pd 3d 5/2 and 3/2 peaks were deconvoluted into components corresponding to metallic Pd (335.4 and 340.9 eV), PdO (336.4 and 341.7 eV), PdBr2 (337.1 and 342.6 eV), PdCl2 (337.9 and 343.4 eV), and PdO2 (337.9 and 343.4 eV).40-42 The latter two components were differentiated based on the chlorine content in the sample. 2.3. Catalyst testing. Methane oxyhalogenation (CH4 + HX + O2), methane halogenation (CH4 + X2), and hydrogen halide oxidation (HX + O2) were performed at ambient pressure in a continuous-flow fixed-bed reactor setup described elsewhere.16 Briefly, the gases: CH4 (PanGas, purity 5.0), HBr (Air Liquide, purity 2.8, anhydrous), HCl (Air Liquide, purity 2.8, anhydrous), O2 (PanGas, purity 5.0), Cl2 (PanGas, purity 2.8), Ar (PanGas, purity 5.0; internal standard), and He (PanGas, purity 5.0; carrier gas) were fed using digital mass-flow controllers (Bronkhorst) to the mixing unit equipped with pressure indicator, and Br2 (ABCR, 99%) was dosed using a syringe pump (Nexus 6000, Chemyx) with a water-cooled glass syringe (Hamilton, 1 cm3) coupled to a vaporizer operated at 343 K. A quartz reactor (internal diameter, di = 8 mm) was loaded with the catalyst (weight, Wcat, 1 g in oxyhalogenation, 0.5 g in HCl oxidation, and 0.03 g in HBr oxidation; particle size, dp, 0.4-0.6 mm), which was well-mixed with quartz particles (Thommen-Furler, dp = 0.2-0.3 mm) to ensure a constant bed volume (Vbed = 2 cm3), and placed in a homemade electrical oven. Non-catalytic gas-phase methane halogenation was performed over inert quartz particles (Vbed = 2.5 cm3). A K-type thermocouple fixed in a coaxial quartz thermowell with the tip positioned in the center of the catalyst bed was used to monitor the


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temperature during the reaction. Prior to testing, the bed was heated in a He flow to the desired temperature (T = 423-833 K) and allowed to stabilize for at least 30 min before the reaction mixture was fed at desired total volumetric flow (FT = 15-200 cm3 min-1). All flow units correspond to standard temperature and pressure (STP) conditions, i.e., 273 K and 1 bar. Carbon-containing compounds (CH4, CH3Br, CH2Br2, CH3Cl, CH2Cl2, CO, and CO2) and Ar were quantified on-line via a gas chromatograph equipped with a GS-Carbon PLOT column coupled to a mass spectrometer (GC-MS, Agilent GC 6890, Agilent MSD 5973N). Quantification of Cl2 and Br2 at the reactor outlet was performed by their absorption in an impinging bottle filled with 0.1 M KI solution (X2 + 3I- → I3- + 2X-) followed by iodometric titration

(Mettler

Toledo

G20

Compact

Titrator)

of

the

formed

triiodide

(I3- + 2S2O32- → 3I- + S4O62-) with 0.01 M sodium thiosulfate solution (Aldrich, 99.99%). The conversion of methane in oxyhalogenation, XCH4, the conversion of hydrogen halide in hydrogen halide oxidation, XHX, the reaction rate expressed with respect to the reactant i, ri, (i: CH4, HX), the selectivity, Sj, and the yield, Yj, of product j (j: CH3X, CH2X2, CO, or CO2), and the error of the carbon mass balance, C, were calculated using eqs 1-6, respectively, in which niinlet and nioutlet are the molar flows of the reactant i at the reactor inlet and outlet, respectively, and njoutlet is the molar flow of product j at the reactor outlet. The error of the carbon mass balance was less than 5% in all experiments. (1) (3)

(2)


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ACS Catalysis

(4)

(5)

(6) The evaluation of the dimensionless moduli based on the criteria of Carberry, Mears, and Weisz–Prater confirmed that all the catalytic tests were performed in the absence of mass and heat transfer limitations.43 2.4. Operando Photoelectron Photoion Coincidence Spectroscopy. Photoelectron photoion coincidence (PEPICO) spectroscopy experiments during methane oxyhalogenation over Pd/SiO2 catalyst were performed using the PEPICO end station at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (Figure 1).38 The gases: CH4, HBr or HCl, O2, and Ar were fed to a tubular reactor (dr = 1 mm) made of SiC and placed in the source chamber. The catalyst (Wcat = 0.05 g) was deposited on the inner walls of the reactor by wash-coating using a suspension of the solid in ethanol (Merck, absolute), followed by drying in air at 573 K for 1 h. Two ring electrodes connected to a DC power supply (Voltcraft) were applied to heat the reactor resistively. The reactor temperature was monitored by a type C thermocouple attached to the outside reactor wall at the middle point between the electrodes. The temperature of the flowing gas was calibrated against that measured at the outside wall and applied heating power in an independent experiment, which have shown that these deviations are in the range of ±30 K. This calibration was used to recalculate the reaction temperature in the oxyhalogenation experiments. The pressures at the reactor inlet and in the source chamber surrounding its outlet were


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2×10−1 bar and 2×10−8 bar, respectively, and the effective pressure in the reaction zone was estimated to be in the range of 1–4×10−2 bar based on the previous studies and by taking into account the flow rate, temperature, and the reactor diameter applied in the experiments.33 The above described reactor configuration minimizes the quenching of short-lived reaction intermediates, which may be desorbed from the catalyst or generated in the gas-phase, by high dilution of the feed and relatively low pressure inside the reactor and in the source chamber surrounding its outlet. The central part of the molecular beam leaving the reactor was skimmed and fed in the analysis chamber operated at 2×10−9 bar. The sample was ionized by collimated synchrotron VUV radiation, which was dispersed by a 150 mm–1 grating working in grazing incidence, and further monochromatized by a rare gas filter (PanGas, 25 mol.% Ar in Ne 5.0) operating at 1 × 10−2 bar over an optical length of 10 cm, which suppresses the higher order radiation in the 9-14 eV range. A MgF2 filter was used to suppress radiation above 10.6 eV quantitatively in the case of grating orders in the 9-10.6 eV photon energy range. The photoions and photoelectrons generated upon photoionization are accelerated vertically in opposite directions by a constant field of 250 V cm–1 and velocity map imaged onto two delay-line anode detectors (Roentdek, DLD40) in delayed coincidence. The signals of neutral and radical species were recorded at the following mass-to-charge ratios (m:z) and photon energies (hv): CH4 (m:z = 16; hv = 12.6 eV), O2 (m:z = 32; hv = 12.6 eV), HCl (m:z = 36, 38; hv = 13.1 eV), HBr (m:z = 80, 82; hv = 12.3 eV), CH3Cl (m:z = 50, 52; hv = 11.4 eV), CH3Br (m:z = 94, 96; hv = 10.6 eV), Cl2 (m:z = 70, 72, 74; hv = 11.6 eV), Br2 (m:z = 158, 160, 162; hv = 10.6 eV), CH3• (m:z = 15; hv = 10 eV), Cl• (m:z = 35, 37; hv = 13.1 eV), and Br• (m:z = 79, 81; hv = 12.3 eV). The photon energies for each experiment were selected so that they are above the neutral ionization energy of the targeted species but below its dissociative photoionization


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threshold (e.g., CH3X + hν → CH3+ + X + e−) yielding a fragment ion of the same m:z. This suppresses fragmentation quantitatively and ensures that the detected ions stem exclusively from the photoionization of the neutral or radical species with the same m:z. The false coincidence background was subtracted in the time-of-flight mass spectra to obtain the presented peak integrals. The methyl photoionization cross section was estimated as 6 Mb at 12.3 eV.44 To account for the losses of CH3• signal at this high photoionization energies, test measurements in which methyl radicals were prepared from various precursors were conducted. These showed that ca. 25% of the total methyl ion signal is recorded at 12.3 eV photon energy. In the absence of experimental absolute photoionization cross sections, the Br• photoionization cross section at this photon energy is estimated to be 50-100 Mb, based on the calculations of Lin and Saha,45 the experimental results of Benzaid et al.,46 and the Kr cross section of 45 Mb at similar excess energies above the ionization energy.47 Because of the higher ionization energy of Br•, almost all of the bromine photoelectrons may be detected. Assuming constant photoion detection efficiency and a Br•:CH3• ratio of 1:x in the mass spectrum, this corresponds to a concentration ratio of 1:35x–1:70x in the ionization region. Further, assuming a unit mass discrimination factor,48 this tentatively translates to the same concentration ratio at the reactor exit.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Fresh Catalysts. The textural and structural properties of the fresh silica-supported noble metal-based catalysts were assessed by N2 sorption (Table 1), powder X-ray diffraction, and Raman spectroscopy (Figure 2). SiO2 was chosen as s suitable carrier due to its high surface area, favoring high metal dispersion, lack of geometric and electronic support-active phase interactions, which allow to study the intrinsic catalytic


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performance of metal active phases, and its inertness towards undesired reactions with halomethanes.22 It is important to note that the catalyst codes only reflect the metal present in the catalyst, and do not imply its phase composition. The specific surface areas of the silicasupported catalysts were in the range of 146-175 m2 g-1. The XRD patterns of Ru/SiO2, Ir/SiO2, Rh/SiO2, and Pd/SiO2 displayed the reflections of the respective metal oxides. In contrast, Pt/SiO2 exhibited only the diffraction lines corresponding to the metallic phase, which can be explained by thermal decomposition of PtO2 into Pt at the temperatures used in the final calcination step.49 The average crystallite size of the identified phases as determined by Scherrer equation were in the range of 6-18 nm (Table 1). The observations on the identified phases were corroborated by Raman spectroscopy, which evidenced the characteristic bands of crystalline RuO2 (510 cm-1 and 625 cm-1),49 IrO2 (556 cm-1),50 Rh2O3 (273 cm-1 and 423 cm-1),51 and PdO (634 cm-1)52 in the respective supported catalysts. The spectra of Pt/SiO2 catalyst displayed a peak at 154 cm-1, which suggests that, in addition to the metallic Pt identified by XRD, an amorphous PtOx phase is likely present in the material.52 3.2. Evaluation of Catalysts in Methane Oxyhalogenation. The performance of the noble metal-based catalysts in MOC and MOB was investigated in a relatively broad range of conditions, comprising the variations of temperature, space velocity, and feed HX concentration (Figure S1). To ensure that the performance could be related to the restructured catalysts after the oxyhalogenation reactions (vide infra), the activity and product distribution were evaluated over the materials equilibrated under the reaction environment for over 8 h. The reaction rates expressed with respect to the mass of active metal (eq 3) and measured at 753 K (Figure 3a), reflected the activity differences between the catalysts. In particular, the activity in MOC decreased in the order Ru/SiO2 > Pt/SiO2 > Ir/SiO2 > Rh/SiO2 ≈ Pd/SiO2, which is comparable to


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the behavior of the respective bulk catalysts (Figure S2). In MOB, the rates changed in the following order: Ru/SiO2 ≈ Ir/SiO2 ≈ Pd/SiO2 > Pt/SiO2 > Rh/SiO2. This activity ranking is consistent with the one of the respective bulk catalysts, expect for the Pd-based system that significantly enhances its activity when supported on SiO2. Notably, the metal-based activities of supported catalysts were ca. 2 orders higher compared to those of the bulk materials. The product distribution over the catalysts was compared at ca. 20% methane conversion that was achieved by adjusting the space velocity at a constant temperature of 753 K (Figure 3b). The trends observed are representative of virtually all conditions investigated, as shown in Figure 4 and Figure S1. Supporting of noble metals nanoparticles over SiO2 carrier led to a significant decrease in the production of carbon oxides (COx) and concomitant increase in the generation of halomethanes (CH3X + CH2X2) compared to the corresponding bulk catalysts (Figure S2). Thereby, CH2X2 is produced at a significantly lower selectivity compared to CH3X at moderate methane conversions. As already indicated in the introduction, the former product can be selectively (≈ 90%) hydrodehalogenated over metal-based catalysts into the latter or oligomerized into higher hydrocarbons,19 thus virtually eliminating any halogen loss that oxyhalogenation reactions might cause. The process of hydrodehalogenation is beyond the scope of our study, but, as proposed in the literature decades ago and commercially applied today, it is absolutely possible to catalytically dehydrogenate propane,53 generally found together with methane in natural gas or formed upon coupling of halomethane, to produce on-demand H2 and propene in order to sustain polyhalomethane reforming. The total selectivity to chloromethanes decreased

in

the

order

Pd/SiO2 (84%) > Rh/SiO2 (78%) ≈ Ir/SiO2 (78%) > Ru/SiO2 (70%) > Pt/SiO2 (40%), and that to bromomethanes

followed

a

similar

trend:


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Pd/SiO2 (98.5%) > Rh/SiO2 (93%) ≈ Ir/SiO2 (92%) > Ru/SiO2 (83%) ≈ Pt/SiO2 (83%).

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The

selectivity to halomethanes on Pd/SiO2, Rh/SiO2, and Ir/SiO2 rivals that of the best oxyhalogenation catalysts, which were mostly developed by our group (Figure 4). In particular, Pd/SiO2 catalyst achieved a very high selectivity to chloromethanes (95%) and bromomethanes (98.5%) at methane conversions of ca. 10 and 20%, respectively, which exceeds the top-performing region of the previously reported catalytic systems. This catalyst was further assessed in long term tests at moderate methane conversion, which evidenced its stable catalytic behavior for over 42 h in both MOC and MOB (Figure 5). 3.3. Characterization of the Equilibrated Catalysts. X-ray diffractograms and Raman spectra of the catalysts equilibrated for 8 h in methane oxyhalogenation highlighted substantial structural changes (Figure 2). These comprised of i) the reduction of the starting oxides into respective metallic phases, as observed for Rh/SiO2 and Pd/SiO2 after MOC, and Ir/SiO2 after both MOC and MOB, ii) the formation of metal silicides that were identified over Ir/SiO2 after MOC and Ru/SiO2 after both MOB and MOC, and iii) the generation of metal halide phases that was observed for Ir/SiO2 catalyst equilibrated in MOB, and Rh/SiO2 and Pd/SiO2 catalyst equilibrated in both MOC and MOB (Figure 2). The aforementioned catalyst transformations were also consistent with the changes in the H2-TPR reduction profiles of the equilibrated catalysts with respect to the fresh materials (Figure S3). In the case of Pt/SiO2, partial reoxidation of Pt into PtO2 was observed in the diffractogram of the catalyst equilibrated in MOC. Besides, the latter material was the only one displaying a band (1377 cm-1) that can be associated with formation of coke.54 Notably, while the activity is generally higher for the oxide-containing catalysts in MOC, and much less dependent on the phase composition in MOB, the selectivity to halomethanes in these


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two reactions is strongly correlated with the presence of oxygen-deficient metallic, silicide, and particularly metal halide phases (Figure 6), whereby the latter are particularly evident in Raman spectra. Herein, a peak detected at 224 cm-1 in the spectra of Ir/SiO2 catalyst after MOB could be associated with Ir-Br bonds (Figure 2b).55 Similarly, the presence of RhBr3 and RhCl3 in Rh/SiO2 was evidenced from XRD and Raman spectra, in which the bands at < 200 cm-1 can be associated with Rh-Br and those at 316 cm-1 < 200 cm-1 with Rh-Cl bond vibrations.55 The XRD and Raman spectra of Pd/SiO2 equilibrated for 8 h and for > 40 h on-stream were essentially identical, indicating that after initial and relatively fast phase transformations, the catalyst structure remained unchanged. Thereby, the peak at 275 cm-1 in Pd/SiO2 equilibrated in MOC that can be associated with Pd-Cl bonds indicates the presence of chloride phases in addition to the metallic palladium in this sample.56 On the other hand, the diffractograms of Pd/SiO2 catalysts equilibrated in MOB displayed the reflections that could not match any of the reference phases (Figure 2a). Nevertheless, Raman bands at 179 and 239 cm-1 that can be associated with stretching vibrations of Pd-Br bond suggest that these likely originate from a bromide phase,57 as further corroborated by electron microscopy analysis (Figure 7) and X-ray photoelectron spectroscopy (Figure 8). The scanning transmission electron microscopy (STEM) of the fresh catalyst coupled to elemental mapping indicated the bimodal particle size distribution centered around ca. 5 nm and 14 nm, where the latter particle diameter is consistent with the crystallite size analysis determined from XRD (Table 1). The high resolution transmission electron microscopy (HRTEM) indicated that the interlattice distances of the nanoparticles (2.6 Å) corresponded to those of the most prominent PdO(011) planes. Notably, the average particle diameter was significantly reduced during relatively short exposure to MOC and MOB, although a small fraction of the bigger particles remained (Figure 7b,d). Thereafter, the size of


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nanocrystals was essentially unaltered during further reaction (Figure 7c,e). The small nanoparticles size and drifting caused by the SiO2 carrier made the determination of the interplanar distances in the equilibrated catalysts challenging. Still, the fringes characteristic of Pd(111) planes could be observed in the catalyst equilibrated in MOC (Figure 7c), while in the material equilibrated in MOB, the observed interplanar separation of ca. 3.1 Å (Figure 7d) was consistent with the diffraction peak at 29 degrees 2 (Figure 2a), likely originating from a brominated phase. X-ray photoelectron spectroscopy (XPS) was applied to shed light on the surface structure of the Pd/SiO2 catalyst (Figure 8). The deconvolution of the Pd 3d core level spectra of the fresh material indicated that in addition to the major contribution from PdO phase, a small fraction of metallic Pd also existed (Figure 8a). The portion of metallic Pd increased and that of PdO substantially decreased upon the exposure to the oxyhalogenation environment. Moreover, Pd 3d, as well as Cl 2p (Figure 8b) and Br 3d (Figure 8c) spectra strongly suggested the presence of Pd halides in the equilibrated catalysts. Pd 3d spectra also imply more severe reduction and halogenation during MOB compared to MOC, which is in a good agreement with the lower combustion propensity observed in the former reaction (Figure 3b). The spectra of the catalysts equilibrated in oxyhalogenation for over 40 h closely resembled to those acquired after 8 h, corroborating the previous observations on stable catalytic behavior and structural rearrangements. 3.4. Loading and Supporting Effects. The best performing Pd-based catalytic system was chosen to study the impact of the metal loading and carrier type on the activity and product distribution in methane oxyhalogenation. In this regard, Pd/SiO2-2% and Pd/SiO2-5%, with 2 and 5 wt.% palladium loading, respectively, as well as Pd/SiC, Pd/MgO, and Pd/Al2O3 catalysts with


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1 wt.% of palladium were synthesized and evaluated in MOC and MOB. Similar to the benchmark Pd/SiO2 catalyst, the X-ray diffractograms of the freshly prepared materials revealed the presence of PdO peaks, which were accompanied by the well-defined reflections stemming from the respective supports in the case of Pd/SiC, Pd/MgO, and Pd/Al2O3 catalysts (Figure 9a). The presence of the oxide phase was also inferred from the respective Raman bands centered at ca. 633 cm-1 in the SiO2 supported samples and ca. 646 cm-1 over other carriers (Figure 9b). The only exception to this was Pd/SiC, in which no contributions from the palladium phases could be detected, likely due to a strong background scattering stemming from the support. The evaluation of Pd/SiO2-2% and Pd/SiO2-5% in MOC showed almost no variation in the metal-based reaction rate and a slight increase in selectivity to chloromethanes with increasing Pd loading (Figure 10 and Figure S4). In contrast, the specific activity in MOB decreased almost linearly and the selectivity to bromomethanes was reduced more substantially at higher metal loadings. Supporting palladium over different carriers induced some differences in the catalytic

performance.

In

particular,

the

rate

of

MOC

decreased

in

the

order:

Pd/Al2O3 > Pd/SiO2 ≈ Pd/SiC > Pd/MgO, while in MOB, the reactivity trend was different: Pd/SiO2 > Pd/SiC > Pd/MgO > Pd/Al2O3. The selectivity patterns of SiC and MgO supported catalysts in MOB and MOC were comparable to those over SiO2, while supporting over Al2O3 carrier led to a marked COx formation. The latter result can be rationalized by the strong propensity of the bare Al2O3 support to oxidize halocarbons, in contrast to SiO2, MgO, and SiC, which are rather inert in these side reactions.22 Similar to the benchmark Pd/SiO2 catalysts, the XRD analysis of Pd/SiO2-2% and Pd/SiO2-5% equilibrated in MOC revealed the reflections of metallic Pd and PdO (Figure 9a). Consistently, the bands corresponding to Pd-O bond vibrations were also present in the Raman spectra of the


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samples with higher Pd content (Figure 9b). These results suggest that within the studied range of loadings, SiO2 supported Pd catalyst display a similar structure after equilibration in MOC, which along with the comparable crystallite size (Table 1), correlates well with their almost identical metal-based activity and product distribution patterns (Figure 10). In contrast, Pd/SiO2-2% and Pd/SiO2-5% catalysts equilibrated under MOB conditions show substantial differences compared to the benchmark Pd/SiO2 (Figure 9). In particular, they displayed the reflections of metallic Pd, which were absent at 1% palladium loading, while their Raman spectra also indicated the presence of oxide phases. Moreover, the stretching band of Pd-Br bond (170 cm-1) could not be detected in the Pd/SiO2-5% catalyst after its use in MOB. It could be inferred that the reducibility of the catalyst as well as its potential to brominate under MOB conditions ceased at higher palladium content. This can rationalize the observed increase in carbon oxides production at higher palladium loadings. Additional insights into the structural changes at higher palladium loadings were acquired from microscopy analysis of Pd/SiO2-5% catalyst (Figure 11). The fresh material displayed a relatively broad particle-size distribution centered around 6 nm with significant portion of particles greater than 10 nm. Consistent with XRD, lattice fringes associated with PdO(011) planes were observed. After the use in MOC and MOB, spherical palladium particles were formed, with most of the Pd crystals reducing their size down to ca. 2 nm. Nevertheless, the catalyst equilibrated in MOB also displayed particles larger than those in the fresh material, implying catalyst sintering. Furthermore, surface structural rearrangements of the Pd/SiO2-5% catalyst were monitored by XPS spectroscopy (Figure 12). The deconvolution of Pd 3d core level spectra evidenced a smaller contribution of the PdBr2 component and a significant fraction of PdO and PdO2. This, along with the lower intensity of Br 3d peak and pronounced Pd-O band


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in the Raman spectra (Figure 9b), indicated that the catalyst preserved its oxidic character, which consequently led to a more pronounced formation of COx (Figure 10b). Conversely, the surface structure of Pd/SiO2-5% catalyst equilibrated in MOC was comparable to that of the benchmark Pd/SiO2, in line with their more similar performance (Figure 10b). In contrast to Pd/SiO2 (Figure 2), the diffractograms of Pd/MgO, Pd/SiC, and Pd/Al2O3 catalysts equilibrated in MOB exhibited the reflections of metallic Pd (Figure 9a). However, the Raman spectra of Pd/MgO and Pd/Al2O3 also displayed the stretching vibrations of Pd-Br bonds (Figure 9b), similarly to the benchmark catalyst. The diffractograms of catalysts supported over MgO and Al2O3 carriers indicated the presence of both Pd and PdO after MOC, wherein the oxide phase appeared to be the least abundant in the least active Pd/MgO catalyst. Besides, weak Raman bands in the region of 300 cm-1 suggest the formation of palladium chloride in the case of these carriers, similar to the benchmark catalyst. 3.5. Insights in the Reaction Mechanism. Previous studies on oxide-, phosphate-, and oxyhalide-based catalysts suggested that methane activation in oxyhalogenation may involve the surface catalyzed oxidation of HX into gaseous halide species such as molecular halogen (X2) and halogen radical (X•), which can activate methane via gas-phase radical reactions.22,23,31,33 The feasibility of the latter pathway is corroborated by the fact that non-catalytic methane bromination (CH4 + Br2) and methane chlorination (CH4 + Cl2) can proceed at high-enough rates in the typical temperature window where the respective oxyhalogenation reactions occur (Figure S5). Nevertheless, other studies have proposed that the formation of halomethanes in oxychlorination over catalysts such as LaOCl and K4Ru2OCl10/TiO2 predominantly proceeds on the catalyst surface,6,30 while the theoretical work by Metiu et al. suggests that halogen doped CeO2 surface might facilitate C-H bond activation and formation of CH3•.32 Hence, it is of


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significant importance to address the role of both surface and gas-phase reactions in methane activation via oxyhalogenation over noble metal-based catalysts, particularly as the latter systems are known for their ability to activate both methane and HX and thereby can allow the occurrence of both mechanistic pathways.8,24-27,58 The first insights into the mechanism were obtained by comparing the rates of methane oxyhalogenation with those of HX oxidation over the catalysts at constant temperature and feed concentrations of HX and oxygen (Figure 13). An equal or higher activity in HX oxidation with respect to oxyhalogenation was observed over all the catalysts in MOB and over Ir/SiO2 and Ru/SiO2 catalysts in MOC, which indicates that the production of molecular halogen might be sufficient to completely sustain the formation of halomethanes by gas-phase halogenation. Notably, the rates of HBr oxidation were ca. 2 orders of magnitude higher than those in MOB, providing a strong hint that molecular bromine is a highly abundant intermediate in the oxybromination system. Nevertheless, chlorine production over Pd/SiO2, Rh/SiO2, and Pt/SiO2 was lower compared to the rate of methane consumption in oxychlorination, but still in a comparable range, suggesting that both the surface-mediated and gas-phase activation of C-H bonds might be possible. The higher activity of Ru/SiO2 with respect to other catalysts in MOC and with respect to its activity in MOB is consistent with the high propensity of this catalytic system to oxidize HCl into Cl2 (Figure 13). The latter is likely the rate-limiting step in oxychlorination in contrast to oxybromination, wherein the gas-phase reaction of methane and bromine appears to control the overall rate. Consistent with these observations, the fraction of CH3X and CH2X2 over the most selective Rh/SiO2, Ir/SiO2, and Pd/SiO2 catalysts in both oxyhalogenation reactions is very similar to that in the respective non-catalytic methane halogenations in the lower conversion regime (Figure S5). However, at higher conversions the


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deviations in product distribution between catalytic oxyhalogenation and non-catalytic halogenation become more pronounced, particularly in MOC. This can be caused by side combustion pathways in oxyhalogenation, but may be also the sign of more influential surface contributions to C-X bond formation. Additional hints on the mechanism of methane oxyhalogenation were obtained by comparing the metal-based rates of the Pd/SiO2 catalysts with different palladium loadings (Figure 10a). The latter decreased almost linearly with an increase in palladium content in MOB, suggesting that methane activation is decoupled from the surface and predominantly proceeds in the gas-phase. In contrast, the rates remained essentially unaltered in MOC, indicating that they are likely controlled by surface-driven steps. Nevertheless, the latter processes might not only involve C-H bond scission and C-Cl bond formation, but also the catalytic oxidation of HCl to Cl2, which has been previously proposed as rate limiting.31 To further elucidate the role of gas-phase methane activation, operando photoelectron photoion coincidence (PEPICO) spectroscopy was conducted over Pd/SiO2, which was identified as the most selective catalyst in both MOC and MOB (Figures 3 and 4). Additionally, this system was particularly interesting to unravel potential gas-phase contributions in MOC, as it exhibits the lowest activity in HCl oxidation (Figure 13) and is thus the least prone to facilitate methane activation with gaseous halogen species. PEPICO spectroscopy provides high energy resolution, high dynamic range, and fragment-free ionization, enabling isomer-specific detection of highly reactive intermediates in the gas-phase.33-38 This technique has been recently demonstrated as a powerful tool to identify the formation of gaseous radical species participating in methane oxybromination.33 In a typical experiment, the reactive gaseous mixture was fed over the Pd/SiO2 catalyst deposited on the walls of a SiC reactor, which was placed in the source


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chamber of the PEPICO end station and heated resistively (Figure 1). The central part of the molecular beam leaving the reactor, which comprises products, transient intermediates, and remaining reactants is skimmed and expands into the ionization chamber. Herein, the gaseous species are photoionized by the monochromatic vacuum ultraviolet (VUV) radiation, resulting in photoelectrons and photoions, which are detected in delayed coincidence. The PEPICO microreactor operates at low methane conversions (< 5%) enabling to derive intrinsic kinetic information. Consistent with the steady-state experiments on methane oxyhalogenation and noncatalytic methane halogenation (Figure S5), neither dihalomethanes nor carbon oxides were observed during the PEPICO experiments under these differential conditions. Operando PEPICO experiments confirmed that, in addition to the reactants and halomethane, methyl radicals (CH3•) with characteristic mass to charge ratio (m:z) of 15 were formed over the catalyst in both MOC and MOB (Figures 14a,b and Figures S6-S9). The intermediates were detected at photon energies (hv) of 10 eV, at which neither of the species present in the reaction medium could yield methyl ions through side dissociative ionization processes. Moreover, the evolution of bromine radicals (Br•) was evidenced in MOB, while chlorine radicals (Cl•) could barely be detected in MOC. However, molecular chlorine was evidenced under all reaction conditions, and small concentrations of Cl• radicals were detected when only HCl and O2 were fed to the reactor (conditions corresponding to HCl oxidation, Figure S7). Based on the latter results, the absence of Cl• radical detection in MOC can be rationalized by its higher reactivity, i.e., lower stability compared to Br•, which makes its concentration rather low in the system.18 Although the precise quantification of radicals concentrations goes well beyond the present capabilities of PEPICO spectroscopy due to the lack of experimental photoionization cross sections as well as reliable internal standards, the analysis of the photoion cross-sections of


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ACS Catalysis

methyl and bromine radicals at 12.3 eV enabled to estimate that Br• are ca. one order of magnitude more abundant than CH3•, which is in line with the higher reactivity of the latter intermediates. In addition to the detection of transient intermediates, PEPICO spectroscopy revealed a correlation between the relative concentrations of halomethane and CH3• (Figures 14c,d), indicating that these radical intermediates might play an important role in methane functionalization via oxyhalogenation. As a matter of fact, the generation of CH3• was dependent on the concentration of HX and dropped to zero in the absence of this reactant. Still, contributions to methane activation from surface-driven pathways cannot be excluded, particularly in the case of methane oxychlorination where the latter might co-exist with gas-phase reactions in a similar extent. In fact, based on the rates of halogen evolution in HX oxidation and methane oxyhalogenation, it can be estimated that gas-phase pathways contribute to >90% to the rate in MOB. In MOC, gas-phase halogenation could have at least a comparable contribution as the surface-catalyzed routes Pd/SiO2, Rh/SiO2, and Pt/SiO2 catalysts, while for Ir/SiO2 and particularly Ru/SiO2 it likely plays the major role, similarly to MOB.

4. CONCLUSIONS We performed the first comprehensive assessment of the structure-performance relationships of supported noble metal-based catalysts in methane oxyhalogenation as a function of the metal and the halide of choice and provided new insights in the mechanism of C-H bond activation over these materials. The specific activity and halomethanes selectivity of supported systems were substantially enhanced with regards to the respective bulk materials. The activity order was dependent

on

the

type

of

HX

used

and

changed

as

follows:


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Ru/SiO2 > Pt/SiO2 > Ir/SiO2 > Rh/SiO2 ≈ Pd/SiO2

Page 26 of 52

in

MOC

and

Ru/SiO2 ≈ Ir/SiO2 ≈ Pd/SiO2 >Pt/SiO2 > Rh/SiO2 in MOB. The selectivity to halomethanes was higher in MOB compared to MOC, although the trends in product distribution were very similar. Ir/SiO2, Rh/SiO2, and Pd/SiO2 were identified as the most selective catalysts, which compete or outperform the best reported catalytic materials. The oxyhalogenation environment led to the transformation of supported metal oxide nanoparticles into metallic, metal silicide, or halogenated phases, testifying the dynamic nature of the catalyst surfaces. While the rates in MOC were mostly higher on the systems comprising oxides and much less dependent on the phase composition in MOB, selectivity to halomethanes in both reactions was strongly linked with the catalyst propensity towards reduction and formation of metal halides. The latter transformations were more pronounced in MOB compared to MOC, consistent with lower carbon oxides generation in the former reaction. Besides, the reducibility of the palladium-based catalysts in MOB decreased at higher loadings and was less affected by the carrier type. The gathered structure-performance relationships complemented with kinetic analyses and detection of highly reactive methyl radicals by operando PEPICO spectroscopy conducted over Pd/SiO2 indicated that the catalyst might act as a source of reactive molecular and radical halogen species, which activate methane in the gas-phase. Although the formation of C-X bonds on the catalyst surface cannot be excluded, their generation via gas-phase halogenation contributes significantly to the process. Based on the rates of methane oxyhalogenation and halogen evolution via HX oxidation, gas-phase halogenation has a comparable contribution as the surface activation over Pd/SiO2, Rh/SiO2, and Pt/SiO2 in MOC, while it plays the main (>90%) role over Ir/SiO2 and Ru/SiO2, as well as for all studied systems in MOB. These findings highlight the promising potential of noble metal-based catalysts in methane oxyhalogenation in view of highly


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selective halomethane production systems. In addition, they elucidate the inevitable impact of gas-phase halogenation in catalytic oxyhalogenation, which inherently constrains any possible enhancement of mono- over di-substituted products by tailoring the catalyst surface. In a broader context, these results testify the relevance of unraveling the potential role of gas-phase chemistry in C-H bond activation over heterogeneous catalysts when considering the scope of performance improvement through catalyst design.

ASSOCIATED CONTENT Supporting Information. Supplementary information associated with this article, containing additional catalytic and characterization data, can be found in the online version.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions § These

authors contributed equally to this work. The manuscript was written through

contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Vladimir Paunović: 0000-0001-6630-1672 Guido Zichittella: 0000-0002-7062-8720. Patrick Hemberger: 0000-0002-1251-4549. Andras Bodi: 0000-0003-2742-1051. 27 ACS Paragon Plus Environment

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Javier Pérez-Ramírez: 0000-0002-5805-7355.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by ETH research grant (ETH-04 16-1) and the Swiss Federal Office of Energy (contract no. SI/501269-01). The authors acknowledge Dr. Frank Krumeich, Dr. Sharon Mitchell, and Evgeniya Vorobyeva, all from ETH Zurich, for performing microscopy analysis and Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich for granting access to its facilities. Dr. Roland Hauert from Empa, Dübendorf, and Nicola Carrara from ETH are acknowledged for conducting X-ray photoelectron spectroscopy analysis and part of the catalytic tests, respectively.


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REFERENCES (1) Horn, R.; Schlögl, R. Methane Activation by Heterogeneous Catalysis. Catal. Lett. 2015, 145, 23-39. (2) Olivos-Suarez, A. I.; Szécsényi, À.; Hensen, E. J. M.; Ruiz-Martinez, J.; Pidko, E. A.; Gascon, J. Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities. ACS Catal. 2016, 6, 2965-2981. (3) Kondratenko, E. V.; Peppel, T.; Seeburg, D.; Kondratenko, V. A.; Kalevaru, N.; Martin, A.; Wohlrab, S. Methane Conversion into Different Hydrocarbons or Oxygenates: Current Status and Future Perspectives in Catalyst Development and Reactor Operation. Catal. Sci. Technol. 2016, 7, 366-381. (4) Taifan, W.; Baltrusaitis, J. CH4 Conversion to Value Added Products: Potential, Limitations and Extensions of a Single Step Heterogeneous Catalysis. Appl. Catal. B 2016, 198, 525-547. (5) Lin, R.; Amrute, A. P.; Pérez-Ramírez, J. Halogen-Mediated Conversion of Hydrocarbons to Commodities. Chem. Rev. 2017, 117, 4182-4247. (6) Podkolzin, S. G.; Stangland, E. E.; Jones, M. E.; Peringer, E.; Lercher, J. A. Methyl Chloride Production from Methane over Lanthanum-Based Catalysts. J. Am. Chem. Soc. 2007, 129 , 2569-2576. (7) J.

Tollefson.

'Flaring'

Wastes

3.5%

of

World's

Natural

Gas.

Nature

2016,

DOI:10.1038/nature.2016.19141. (8) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative. Science 1998, 280, 560-564. (9) Bedard, J.; Hong, D. Y.; Bhan, A. Co-Processing CH4 and Oxygenates on Mo/H-ZSM-5: 2. 29 ACS Paragon Plus Environment

ACS Catalysis 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 58 59 60

Page 30 of 52

CH4-CO2 and CH4-HCOOH Mixtures. Phys. Chem. Chem. Phys. 2013, 15, 12173-12179. (10) Guo, X.; Fang, G.; Li, G.; Ma, H.; Fan, H.; Yu, L.; Ma, C.; Wu, X.; Deng, D.; Wei, M.; Tan, D.; Si, R.; Zhang, S.; Li, J.; Sun, L.; Tang, Z.; Pan, X.; Bao, X. Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen. Science 2014, 344, 616-619. (11) Grundner, S.; Markovits, M. A. C.; Li, G.; Tromp, M.; Pidko, E. A.; Hensen, E. J. M.; Jentys, A.; Sanchez-Sanchez, M.; Lercher, J. A. Single-Site Trinuclear Copper Oxygen Clusters in Mordenite for Selective Conversion of Methane to Methanol. Nat. Commun. 2015, 6, 7546-7555. (12) Farrell, B. L.; Igenegbai, V. O.; Linic, S. A Viewpoint on Direct Methane Conversion to Ethane and Ethylene Using Oxidative Coupling on Solid Catalysts. ACS Catal. 2016, 6, 4340-4346. (13) Lin, R.; Ding, Y.; Gong, L.; Dong, W.; Wang, J.; Zhang, T. Efficient and Stable Silica-Supported Iron Phosphate Catalysts for Oxidative Bromination of Methane. J. Catal. 2010, 272, 65-73. (14) He, J.; Xu, T.; Wang, Z.; Zhang, Q.; Deng, W.; Wang, Y. Transformation of Methane to Propylene: A Two-Step Reaction Route Catalyzed by Modified CeO2 Nanocrystals and Zeolites. Angew. Chem. Int. Ed. 2012, 51, 2438-2442. (15) Paunović, V.; Zichittella, G.; Moser, M.; Amrute, A. P.; Pérez-Ramírez, J. Catalyst Design for Natural-Gas Upgrading through Oxybromination Chemistry. Nat. Chem. 2016, 8, 803-809. (16) Paunović, V.; Lin, R.; Scharfe, M.; Amrute, A. P.; Mitchell, S.; Hauert, R.; Pérez-Ramírez, J. Europium Oxybromide Catalysts for Efficient Bromine Looping in Natural Gas


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Valorization. Angew. Chem. Int. Ed. 2017, 56, 9791-9795. (17) Svelle, S.; Aravinthan, S.; Bjrøgen, M.; Lillerud, K. P.; Kolboe, S.; Dahl, I. M.; Olsbye, U. The Methyl Halide to Hydrocarbon Reaction over H-SAPO-34. J. Catal. 2006, 241, 243-254. (18) Lorkovic, I. M.; Sun, S.; Gadewar, S.; Breed, A.; Macala, G. S.; Sardar, A.; Cross, S. E.; Sherman, J. H.; Stucky, G. D.; Ford, P. C. Alkane Bromination Revisited: "Reproportionation" in Gas-Phase Methane Bromination Leads to Higher Selectivity for CH3Br at Moderate Temperatures. J. Phys. Chem. A 2006, 110, 8695-8700. (19) Ding, K.; Derk, A. R.; Zhang, A.; Hu, Z.; Stoimenov, P.; Stucky, G. D.; Metiu, H.; McFarland, E. W. Hydrodebromination and Oligomerization of Dibromomethane. ACS Catal. 2012, 2, 479-486. (20) Vajglová, Z.; Kumar, N.; Eränen, K.; Peurla, M.; Murzin, D. Y.; Salmi, T. Ethene Oxychlorination over CuCl2/g-Al2O3 Catalyst in Micro- and Millistructured Reactors. J. Catal. 2018, 364, 334-344. (21) Peringer, E.; Podkolzin, S. G.; Jones, M. E.; Olindo, R.; Lercher, J. A. LaCl3-based Catalysts for Oxidative Chlorination of CH4. Top. Catal. 2006, 38, 211-220. (22) Paunović, V.; Zichittella, G.; Mitchell, S.; Hauert, R.; Pérez-Ramírez, J. Selective Methane Oxybromination over Nanostructured Ceria Catalysts. ACS Catal. 2018, 8 (1), 291-303. (23) Paunović, V.; Artusi, M.; Verel, R.; Krumeich, F.; Hauert, R.; Pérez-Ramírez, J. Lanthanum Vanadate Catalysts for Selective and Stable Methane Oxybromination. J. Catal. 2018, 363, 69-80. (24) Gélin, P.; Primet, M. Complete Oxidation of Methane at Low Temperature over Noble Metal Based Catalysts: A Review. Appl. Catal. B 2002, 39, 1-37.


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(25) Wei, J.; Iglesia, E. Mechanism and Site Requirements for Activation and Chemical Conversion of Methane on Supported Pt Clusters and Turnover Rate Comparisons among Noble Metals. J. Phys. Chem. B 2004, 108, 4094-4103. (26) Stötzel, J.; Frahm, R.; Kimmerle, B.; Nachtegaal, M.; Grunwaldt, J. D. Oscillatory Behavior during the Catalytic Partial Oxidation of Methane: Following Dynamic Structural Changes of Palladium using the QEXAFS Technique. J. Phys. Chem. C 2012, 116, 599-609. (27) Pakhare, D.; Spivey, J. A Review of Dry (CO2) Reforming of Methane over Noble Metal Catalysts. Chem. Soc. Rev. 2014, 43, 7813-7837. (28) Wang, K. X.; Xu, H. F.; Li, W. S.; Zhou, X. P. Acetic Acid Synthesis from Methane by Non-Synthesis Gas Process. J. Mol. Catal. A 2005, 225, 65-69. (29) Yang, F.; Liu, Z.; Li, W. S.; Wu, T. H.; Zhou, X. P. Acetic Acid Synthesis from Methane by Non-Synthesis Gas Process. Catal. Letters 2008, 124, 226-232. (30) Shalygin, A.; Paukshtis, E.; Kovalyov, E.; Bal’zhinimaev, B. Light Olefins Synthesis from C1-C2 Paraffins via Oxychlorination Processes. Front. Chem. Sci. Eng. 2013, 7, 279-288. (31) Zichittella, G.; Paunović, V.; Amrute, A. P.; Pérez-Ramírez, J. Catalytic Oxychlorination versus Oxybromination for Methane Functionalization. ACS Catal. 2017, 7, 1805-1817. (32) Hu, Z.; Metiu, H. Halogen Adsorption on CeO2: The Role of Lewis Acid-Base Pairing. J. Phys. Chem. C 2012, 116, 6664-6671. (33) Paunović, V.; Hemberger, P.; Bodi, A.; López, N.; Pérez-Ramírez, J. Evidence of Radical Chemistry in Catalytic Methane Oxybromination. Nat. Catal. 2018, 1, 363-370. (34) Oßwald, P.; Hemberger, P.; Bierkandt, T.; Akyildiz, E.; Köhler, M.; Bodi, A.; Gerber, T.; Kasper, T. In Situ Flame Chemistry Tracing by Imaging Photoelectron Photoion


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Coincidence Spectroscopy. Rev. Sci. Instrum. 2014, 85, 25101-25112. (35) Tang, X.; Garcia, G. A.; Nahon, L. CH3+ Formation in the Dissociation of Energy-Selected CH3F+ Studied by Double Imaging Electron/Ion Coincidences. J. Phys. Chem. A 2015, 119, 5942-5950. (36) Osborn, D. L.; Hayden, C. C.; Hemberger, P.; Bodi, A.; Voronova, K.; Sztáray, B. Breaking Through the False Coincidence Barrier in Electron-Ion Coincidence Experiments. J. Chem. Phys. 2016, 145, 164202-164230. (37) Hemberger, P.; Custodis, V. B. F.; Bodi, A.; Gerber, T.; van Bokhoven, J. A. Understanding the Mechanism of Catalytic fast Pyrolysis by Unveiling Reactive Intermediates in Heterogeneous Catalysis. Nat. Commun. 2017, 8, 15946-15955. (38) Sztáray, B.; Voronova, K.; Torma, K. G.; Covert, K. J.; Bodi, A.; Hemberger, P.; Gerber, T.; Osborn, D. L. CRF-PEPICO: Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectroscopy for Reaction Kinetics Studies. J. Chem. Phys. 2017, 147, 13944-13954. (39) Yang, P. Z.; Liu, L. M.; Mo, J. H.; Yang, W. Characterization of PECVD Grown Porous SiO2 Thin Films with Potential Application in an Uncooled Infrared Detector. Semicond. Sci. Technol. 2010, 25 (4), 4-7. (40) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, 1992; pp.118. (41) Kumar, G.; Blackburn, J. R.; Albridge, R. G.; Moddeman, W. E.; Jones, M. M. Photoelectron Spectroscopy of Coordination Compounds. II. Palladium Complexes. Inorg. Chem. 1972, 11 (2), 296-300. (42) Grunthaner, P. J.; Grunthaner, F. J.; Madhukar, A. Chemical Bonding and Charge


ACS Catalysis 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 58 59 60

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Redistribution: Valence Band and Core Level Correlations for the Ni/Si, Pd/Si, and Pt/Si Systems. J. Vac. Sci. Technol. 1982, 20 (3), 680–683. (43) Kapteijn, F.; Moulijn, J. A. In Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2008; pp. 2030-2038. (44) Krüger, J.; Garcia, G. A.; Felsmann, D.; Moshammer, K.; Lackner, A.; Brockhinke, A.; Nahon, L.; Kohse-Höinghaus, K. Photoelectron-photoion coincidence spectroscopy for multiplexed detection of intermediate species in a flame. Phys. Chem. Chem. Phys. 2014, 16, 22791-22804. (45) Lin, D.; Saha, H. P. Partial photoionization cross section and resonance structure of Br. Phys. Rev. A 1999, 59, 3614-3621. (46) Benzaid, S.; Krause, M. O.; Menzel, A.; Caldwell, C. D. Structure and Dynamics of the 4pns,md Autoionizing Resonances between the 3P and 1S Thresholds in Atomic Bromine. Phys. Rev. A 1998, 57, 4420-4431. (47) Samson, J. A. R.; Stolte, W. C. Precision Measurements of the Total Photoionization Cross-Section of He, Ne, Ar, Kr, and Xe. J. Electron Spectrosc. Relat. Phenom. 2002, 123, 265-276. (48) Holzmeier, F; Fischer, I.; Kiendl, B; Krueger, A.; Bodi, A.; Hemberger, P. On the Absolute Photoionization Cross-Section and Dissociative Photoionization of Cyclopropenylidene. Phys. Chem. Chem. Phys. 2016, 18, 9240-9247. (49) Nur, A. S. M.; Funada, E.; Kiritoshi, S.; Matsumoto, A.; Kakei, R.; Hinokuma, S.; Yoshida, H.; Machida, M. Phase-Dependent Formation of Coherent Interface Structure between PtO2 and TiO2 and Its Impact on Thermal Decomposition Behavior. J. Phys. Chem. C 2017, 122, 662-669.


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(50) Korotcov, A. V.; Huang, Y. S.; Tiong, K. K.; Tsai, D. S. Raman Scattering Characterization of Well-Aligned RuO2 and IrO2 Nanocrystals. J. Raman Spectrosc. 2007, 38, 737-749. (51) Weber, W. H.; Baird, R. J.; Graham, G. W. Raman Investigation of Palladium Oxide, Rhodium Sesquioxide and Palladium Rhodium Dioxide. J. Raman Spectrosc. 1988, 19, 239-244. (52) Mollenhauer, J. A.; Davies, S. R.; Schmid, T. M.; Puhl, W.; Sampath, T. K.; Aydelotte, M. B.; Kuettner, K. E. Raman Investigation of Platinum Oxides. J. Raman Spectrosc. 1991, 22, 1-9. (53) Sattler, J. J. H. B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114, 10613-10653. (54) Sattler, J. J. H. B.; Beale, A. M.; Weckhuysen, B. M. Operando Raman Spectroscopy Study on the Deactivation of Pt/Al2O3 and Pt-Sn/Al2O3 Propane Dehydrogenation Catalysts. Phys. Chem. Chem. Phys. 2013, 15, 12095-12103. (55) Bennett, M. A.; Clark, R. J. H.; Milner, D. L. Far-Infrared Spectra of Complexes of Rhodium and Iridium with -Bonding Ligands. Inorg. Chem. 1967, 6, 1647-1652. (56) Wang, G.; Yu, X.; Cao, X.; Li, H.; Zhang, Z. Micro-Raman Spectroscopy of Pd–B/SiO2 Amorphous Alloy Catalyst. J. Raman Spectrosc. 2000, 31, 1051-1055. (57) Liu, X.; Huang, T. J.; Zhang, L.; Tang, B.; Zhang, N.; Shi, D.; Gong, H. Highly Stable, New, Organic-Inorganic Perovskite (CH3NH3)2PdBr4: Synthesis, Structure, and Physical Properties. Chem. Eur. J. 2018, 24, 4991-4998. (58) Moser, M.; Paunović, V.; Guo, Z.; Szentmiklósi, L.; Hevia, M. G.; Higham, M.; López, N.; Teschner, D.; Pérez-Ramírez, J. Interplay Between Surface Chemistry and Performance of


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Rutile-Type Catalysts for Halogen Production. Chem. Sci. 2016, 7, 2996-3005.


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ACS Catalysis

Table 1. Characterization data of the samples. Metal SBETa / Vporeb / dXRDc / loading / m2 g-1 cm3 g-1 nm Catalyst wt.% fresh MOC fresh fresh MOB MOC MOB MOC MOB Ru/SiO2 1 164 151 168 0.99 0.99 0.99 RuO2, 18 RuO2, 18 / Ru, 26 RuO2, 17 / RuSi2, 18 Pt/SiO2 1 174 181 181 0.98 0.98 0.99 Pt, 11 Pt, 26 PtO2, 17 Ir/SiO2 1 146 181 180 0.98 0.98 1 IrO2, 6 Ir, 18 / IrSi, 27 IrO2, 11 Rh/SiO2 1 164 154 176 0.99 0.99 0.98 Rh2O3, 13 Rh2O3, 35 / RhCl3, 17 Rh, 31 / RhBr3, 27 Pd/SiO2 1 175 138/140d 160/121d 0.99 0.98 0.98 PdO, 15 Pd, 32 Pd/SiO2-2% 2 148 126 130 0.60 0.60 0.67 PdO, 11 Pd, 22 Pd, 26 Pd/SiO2-5% 5 146 139 126 0.56 0.68 0.6 PdO, 11 Pd, 29 Pd, 34 Pd/SiC 1 26 24 22 0.09 0.11 0.11 PdO, 19 Pd, 34 Pd, 34 Pd/MgO 1 34 8 14 0.15 0.03 0.07 Pd, 18 Pd, 34 Pd/Al2O3 1 148 138 138 0.40 0.43 0.42 Pd, 18 Pd, 34 a BET model. b Volume of N adsorbed at p/p = 0.98. c Scherrer equation. d After stability test. 2 0


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Figure 1. PEPICO reactor set-up for radicals detection in catalytic methane oxyhalogenation.


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ACS Catalysis

Figure 2. a) X-ray diffractograms and b) Raman spectra of the fresh and equilibrated SiO2-supported catalysts. Diffraction and Raman profiles of the fresh materials and catalysts equilibrated in methane oxychlorination (MOC) and oxybromination (MOB) are labeled using the color code given in the bottom panel in a). Reference diffraction patterns are shown as vertical lines below the measured diffractograms or filled symbols above them and are identified with

their

ICDD-PDF

numbers.

Conditions:

CH4:HX:O2:Ar:He = 6:6:3:4.5:80.5,

FT/Wcat = 6000 cm3 h-1 gcat-1, T = 753 K, P = 1 bar, and tos = 8, 42, or 43 h. 39 ACS Paragon Plus Environment

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Figure 3. a) Rate and b) selectivity to product j in methane oxychlorination (MOC, solid bars) and oxybromination (MOB, open bars) over the SiO2-supported catalysts. In b), the selectivity to halomethanes and carbon oxides are presented on the left and right panels of the plots, respectively. Product selectivities are determined at ca. 20% conversion of methane achieved by adjusting

the

space

velocity

CH4:HX:O2:Ar:He = 6:6:3:4.5:80.5,

at

constant

temperature.

FT/Wcat = 1800-12000 cm3 h-1 gcat-1,

Conditions:

T = 753 K,

and

P = 1 bar.


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ACS Catalysis

Figure 4. Total selectivity to a) chloromethanes and b) bromomethanes as a function of methane conversion in methane oxychlorination and oxybromination over SiO2-supported catalysts at various reaction conditions. As shown in the legend above the panels, the shapes of the symbols specify the reaction temperature and their interior provides the information on the feed composition and space velocity. The dashed blue lines denote the yield of halomethanes, whereas the blue areas indicate the typical performance regions of the benchmark catalysts comprising LaOCl,21 FeOx-CeO2,14 and FePO4 31 in oxychlorination, and EuOBr,16 CeO2/MgO,22 and LaV0.5O2.75 23 in oxybromination, which were evaluated in a similar range of reaction parameters.

Conditions:

CH4:HX:O2:Ar:He = 6:6-15:3:4.5:80.5,

FT/Wcat = 1800-12000 cm3 h-1 gcat-1, T = 613-833 K, and P = 1 bar.


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Figure 5. Stability b) oxybromination.

test

of

Pd/SiO2

Conditions:

catalyst

Page 42 of 52

in

methane

a) oxychlorination

CH4:HX:O2:Ar:He = 6:6:3:4.5:80.5,

and and

FT/Wcat = 6000 cm3 h-1 gcat-1, T = 753 K, and P = 1 bar.


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ACS Catalysis

Figure 6. Structure-performance

relationships

in

methane

oxychlorination

(left)

and

oxybromination (right) for the SiO2-supported catalysts. The color of the symbols interior indicates the rate of methane oxyhalogenation measured at 753 K, which is classified in three regions, indicating catalyst reactivity: low (light grey), medium (grey), high (black). On the other hand, the position of the symbol on the colored diamond map denotes the total selectivity to halomethanes determined at the same temperature and at ca. 20% methane conversion, which ranges from 40% (red) till 100% (dark blue), as defined by the color bar on the right. The lines arising from the symbols point to the phases identified by X-ray diffraction and Raman analysis in the catalyst samples equilibrated under the same conditions of the reported performance as specified in the caption of Figure 2, which vary from metal halides (top), to metallic (center), metal silicides (left and right), and metal oxides (bottom).


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Figure 7. STEM images, EDX maps, particle size distribution, and HRTEM images of Pd/SiO2 catalysts a) in fresh form and after equilibration in methane b, c) oxychlorination and d, e) oxybromination for b, d) 8, c) 43, and e) 42 h. The gray area indicates the fitted particle-size distribution. Equilibration conditions as reported in the caption of Figure 2. 44 ACS Paragon Plus Environment

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ACS Catalysis

Figure 8. a) Pd 3d, b) Cl 2p, and c) Br 3d core level X-ray photoelectron spectra of fresh and equilibrated Pd/SiO2 catalyst. Spectra related to the fresh material and catalysts equilibrated in methane oxychlorination (MOC) and oxybromination (MOB) are labeled using the color code given in the panel a). Equilibration conditions as reported in the caption of Figure 2.


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Figure 9. a) X-ray diffractograms and b) Raman spectra of the fresh and equilibrated supported palladium-based catalysts. Diffraction and Raman profiles of the fresh materials and catalysts equilibrated in methane oxychlorination (MOC) and oxybromination (MOB) are labeled using the color code given in the bottom panel in a). Reference diffraction patterns are shown as vertical lines below the measured diffractograms or filled symbols above them and are identified with their ICDD-PDF numbers. Equilibration conditions as reported in the caption of Figure 2. 46 ACS Paragon Plus Environment

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ACS Catalysis

Figure 10. a) Rate and b) selectivity to product j in methane oxychlorination (MOC, solid bars) and oxybromination (MOB, open bars) over supported palladium-based catalysts. In b), the selectivity to haloalkanes and carbon oxides are presented on the left and right panels of the plots, respectively. Product selectivities are determined at ca. 20% conversion of methane achieved by adjusting the space velocity at constant temperature. Conditions as reported in the caption of Figure 3.


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Figure 11. HRTEM images and particle size distribution of Pd/SiO2-5% a) in fresh form and after equilibration in methane b) oxychlorination and c) oxybromination. The gray area indicates the fitted particle-size distribution. Equilibration conditions as reported in the caption of Figure 2, tos = 8 h.


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ACS Catalysis

Figure 12. a) Pd 3d, b) Cl 2p, and c) Br 3d core level X-ray photoelectron spectra of fresh and equilibrated Pd/SiO2-5% catalyst. Spectra related to the fresh material and catalysts equilibrated in methane oxychlorination (MOC) and oxybromination (MOB) are labeled using the color code given in the panel a). Equilibration conditions as reported in the caption of Figure 2.


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Figure 13. Rate of methane oxyhalogenation as a function of the rate of HX oxidation in methane

a) oxychlorination

(MOC)

and

b) oxybromination

(MOB)

over

different

SiO2-supported metal catalysts. The dashed gray lines denote the points at which rates of oxyhalogenation

and

HX

oxidation

are

equal.

Conditions

for

MOB

and

MOC:

CH4:HX:O2:Ar:He = 6:6:3:4.5:80.5, FT/Wcat = 6000 cm3 h-1 gcat-1. Conditions for HX oxidation: HX:O2:He = 6:3:91, FT/Wcat = 6000 cm3 h-1 gcat-1 (X = Cl) or FT/Wcat = 200000 cm3 h-1 gcat-1 (X = Br). All rate measurements were performed at T = 753 K and P = 1 bar.


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ACS Catalysis

Figure 14. Representative mass spectra of different reactants, product, and intermediate radical species detected in methane a) oxychlorination and b) oxybromination over Pd/SiO2. Correlation between the peak areas of methyl halide and methyl radical in methane c) oxychlorination and d) oxybromination at different conditions. As shown in the legend above the panels c) and d), the shapes of the symbols specify the reaction temperature and their interior provides the information on the feed composition. The photon energies at which the signal of each chemical species were recorded are shown in Figures S6-S9. Conditions: CH4:HX:O2:Ar = 2:0-2:1:19-17, FT/Wcat = 26400 cm3 h-1 gcat-1, T = 673-913 K, and P = 2×10−2 bar.


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