In situ identification of reaction intermediates and mechanistic understandings of methane oxidation over hematite: A combined experimental and theoretical study

Article abstract of DOI:10.1021/jacs.0c07179

Effective methane utilization for either clean power generation or value-added chemical production has been a subject of growing attention worldwide for decades, yet challenges persist mostly in relation to methane activation under mild conditions. Here, we report hematite, an earth-abundant material, to be highly effective and thermally stable to catalyze methane combustion at low temperatures (<500 °C) with a low light-off temperature of 230 °C and 100% selectivity to CO2. The reported performance is impressive and comparable to those of precious-metal-based catalysts, with a low apparent activation energy of 17.60 kcal mol-1. Our theoretical analysis shows that the excellent performance stems from a tetra-iron center with an antiferromagnetically coupled iron dimer on the hematite (110) surface, analogous to that of the methanotroph enzyme methane monooxygenase that activates methane at ambient conditions in nature. Isotopic oxygen tracer experiments support a Mars van Krevelen redox mechanism where CH4 is activated by reaction with a hematite surface oxygen first, followed by a catalytic cycle through a molecular-dioxygen-assisted pathway. Surface studies with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal the evolution of reaction intermediates from a methoxy CH3- O-Fe, to a bridging bidentate formate b-HCOO-Fe, to a monodentate formate m-HCOO-Fe, before CO2 is eventually formed via a combination of thermal hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) processes. The elucidation of the reaction mechanism and the intermediate evolutionary profile may allow future development of catalytic syntheses of oxygenated products from CH4 in gas-phase heterogeneous catalysis.

Full text of DOI:10.1021/jacs.0c07179

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Article
In situ Identification of Reaction Intermediates and
Mechanistic Understandings of Methane Oxidation over
Hematite: A Combined Experimental and Theoretical Study
Yulian He, Facheng Guo, Ke R. Yang, Jake A. Heinlein, Scott M. Bamonte, Jared J.
Fee, Shu Hu, Steven L. Suib, Gary L. Haller, Victor S. Batista, and Lisa D. Pfefferle
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Sep 2020
Downloaded from pubs.acs.org on September 16, 2020
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In situ Identification of Reaction Intermediates and
Mechanistic Understandings of Methane Oxidation
over Hematite: A Combined Experimental and
Theoretical Study
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Yulian He1,2†, Facheng Guo2,3†, Ke R. Yang2,3*, Jake A. Heinlein , Scott M. Bamonte , Jared J.
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Fee , Shu Hu , Steven L. Suib , Gary L. Haller , Victor S. Batista2,3*, Lisa D. Pfefferle1*
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1Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286,
USA.
2Energy Sciences Institute, 810 West Campus Drive, Yale University, West Haven, CT, 06516, USA.
3Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA.
4Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060, USA.
ABSTRACT:
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Effective methane utilization for either clean power generation or value-added chemical
production has been a subject of growing attention worldwide for decades, yet challenges persist
mostly in relation to methane activation under mild conditions. Here, we report hematite, an earth-
abundant material, to be highly effective and thermally stable to catalyze methane combustion at
low temperatures (< 500°C) with a low light-off temperature of 230°C and 100% selectivity to CO2.
The reported performance is impressive and comparable to those of precious-metal-based
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catalysts, with a low apparent activation energy of 17.60 kcal·mol . Our theoretical analysis
shows that the excellent performance stems from a tetra-iron center with an antiferromagnetically
coupled iron dimer on the hematite (110) surface, analogous to that of the methanotroph enzyme
methane monooxygenase that activates methane at ambient conditions in nature. Isotopic oxygen
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tracer experiments support a Mars van Krevelen redox mechanism where CH is activated by
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reaction with a hematite surface oxygen first, followed by a catalytic cycle through a molecular-
dioxygen-assisted pathway. Surface studies with in situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal the
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evolution of reaction intermediates from a methoxy CH -O-Fe, to a bridging bidentate formate b-
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HCOO-Fe, to a monodentate formate m-HCOO-Fe, before CO is eventually formed via a
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combination of thermal hydrogen-atom transfer (HAT) and proton-coupled electron transfer
(PCET) processes. The elucidation of the reaction mechanism and the intermediate evolutionary
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profile may allow future development of catalytic syntheses of oxygenated products from CH in
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gas-phase heterogeneous catalysis.
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INTRODUCTION
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Methane is the main constituent of natural gas (> 90%), increasingly exploited as a popular
alternative to the conventional fossil fuels since the shale revolution.1-2 This perspective is largely
driven by both the significant economic benefits of utilizing rather inexpensive and abundant CH4
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gas, and the rationality of partially reducing greenhouse gas emissions since methane produces
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the lowest amount of CO per unit energy produced. A natural-gas-powered vehicles (NGVs)
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program launched in Europe is an example of initiatives for using natural gas as a fuel in place of
conventional gasoline, or diesel. However, the environmental merits of the NGVs are jeopardized
if unburned CH is released by the exhaust since methane is a dangerous greenhouse gas with
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a global warming potential 86 times higher than CO over 20 years. Therefore, there is an urgent
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need for remediation of unburned CH to fulfill the strict emission regulations of combustion at low
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temperatures (< 500 - 550°C), including approaches like catalytic methane combustion (CMC),
as described in equation (1).4-5 CMC also has great potential as an alternative to conventional
flame combustion in green power generation processes, such as in the natural-gas-powered gas
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turbines, where the operation temperatures are much lower and fewer harmful air pollutants such
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as NO , CO, and SO , are produced.
x
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퐶퐻 (푔) + 2푂 (푔)→퐶푂 (푔) + 2퐻 푂(푔)
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°
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훥퐻 (298퐾) = ― 803 푘퐽·푚표푙
푟
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Methane activation is one of the main obstacles for its any oxidation reaction, be it methane to
methanol or catalytic combustion, due to the high degree of structural symmetry and low polarity
of the methane molecule, which renders both nucleophilic and electrophilic attacks on CH rather
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challenging.2, 7 The most common activation pathway is through thermal, homolytic C–H bond
cleavage using heterogeneous catalysts, where the hydrogen atoms can transfer to neighboring
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atoms in auxiliary reactions thus promoting the dissociative adsorption of CH via the so-called
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thermal hydrogen-atom transfer (HAT) process.8-9 So far, palladium-based catalysts have
attracted the most attention for the CMC reaction with effective conversion at relatively low
temperatures (below 550°C). However, their high cost, low water poisoning resistivity, reduction
at high temperature and redispersion upon re-oxidation have motivated the search for alternatives
based on earth-abundant elements, such as single metal oxides, spinels, and perovskites.4, 10-11
The variable oxidation states of the transition metals in these oxides enable a redox cycle in the
catalytic oxidation reactions by the release and restoration of the lattice oxygens. However, very
few of them are reported capable of activating methane initially, thus hindering any subsequent
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conversion. Notable exceptions are supported CuO,12 spinel oxide Co O , MnO , perovskite
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LaSrCuO ,1 etc. Most of these materials, however, despite some promises, have not been as
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economically attractive as that of Pd-based systems due to their complicated catalyst structures,
high catalyst mass loadings, small space velocity, or extremely lean fuel feed conditions. In
addition, the understandings of their mechanisms of CH activation remain less explored and
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controversial, making them worthwhile to investigate as they would be of great value not only for
the CMC reaction but also for the broader methane conversion community.
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Recently, iron oxides have shown promise as active catalysts for methane conversion reactions
as they are analogous in functionality to the active sites in the enzyme methane monooxygenase,
which converts methane to methanol in nature.14-15 Over the centuries, iron oxides have gained
their reputation in the catalysis community as an earth-abundant, nontoxic, and excellent
performing catalyst for many important industrial processes, such as the Fischer-Tropsch
synthesis, the Haber process, and the water-gas shift reaction.16-17 However, their potential in
methane conversion via heterogeneous catalysis has been quite limited to partial oxidation
reactions such as to methanol18 or to formaldehyde , for instance, over Fe/ZSM-5 catalysts,
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+
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where the active center was found to be the FeO oxide species.
The complete oxidation
route of methane as well as the fundamental activation mechanism is whereas studied to a much
lesser extent.
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Here, we find that hematite (α-Fe O ), the most thermodynamically stable form of iron oxides, is
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a highly effective catalyst for the complete oxidation of methane with excellent stable performance
below 500°C in both nano and bulk forms, a low light-off temperature of 230°C, and 100%
selectivity to CO . From a molecular level, our theoretical analysis shows the catalytic active
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center is a tetra-iron structure with an antiferromagnetically coupled iron dimer on the (110)
surface of hematite, analogous to that of the iron dimer active site of the soluble methane
monooxygenase enzyme. In addition to its interest for CMC, the (110) facet of hematite has been
reported to be an effective catalyst for other important oxidation processes such as water21 and
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CO oxidation , so we anticipate our findings would be of interest for these oxidative reactions as
well beyond methane oxidation. We report the study of the individual roles of lattice oxygens and
oxygens from the gas phase based on isotopic oxygen tracer experiments to elucidate the
reaction mechanism. We find a Mars van Krevelen type of redox mechanism with participation of
the surface oxygen sites, supported by both experimental and theory. In addition, in situ diffused
reflectance infrared Fourier transform spectroscopy (DRIFTS) on a designed 2D hematite catalyst
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with a large surface area shows vibrational signatures of surface intermediates, revealing a
reaction pathway that evolves from methoxy to bridging bidentate formate, to monodentate
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formate, and finally to CO as the temperature is increased. Density functional theory (DFT)
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modeling supports such a reaction pathway with favorable energetic changes via both thermal
HAT and proton-coupled electron transfer (PCET) processes as well as calculated IR frequencies
of the proposed intermediate surface species consistent with our in situ observations. This
reported combination of experimental and theoretical analysis allows elucidation of the full
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mechanism of CH oxidation over hematite, where a pre-activation process first occurs by reaction
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with the surface lattice oxygens at lower temperatures, followed by a catalytic cycle through a
molecular-oxygen-assisted pathway at higher temperatures.
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EXPERIMENTAL SECTION
All chemicals used here were purchased from Sigma-Aldrich with purity ≥ 97%. Ultrahigh purity
(UHP) grade gases were purchased from Airgas.
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Synthesis of hematite nanosheets. The synthesis of 2D hematite is described in detail in our
previous work.16, 23 Typically, a rigid 2D material CuO nanosheet obtained from a soft templating
process was used to hard template the growth of the ultrathin graphene-like hematite nanosheets
via a redox reaction at the interface. The template was later removed by excessive washing with
ammonium hydroxide and the precipitate was annealed at 400°C for 30 minutes to form hematite
nanosheets. The powder was finely crushed using mortar and pestle before further
measurements.
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Methane catalytic combustion. CMC reactions were performed using a flow reactor set up as
schematically shown in Fig. S1. Mass flow controllers and k-type thermocouples were used to
control gas flow rates and temperature, respectively. Typically, 30.0 mg of catalyst was loaded
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into a straight tube quartz reactor and heated from 100–500°C at 10°C·min . The total volumetric
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gas flow rate Q was 100 mL·min and the inlet feed composition was 5% CH (WHSV = 10000
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mL·g ·h ), 20% O , and 75% Ar. The reaction was normally repeated in triplicate with standard
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deviation noted in the figures. To quantify the conversion, the outlet gas composition was
compared to the respective measurements of the pre-reaction steady state mass spectrometer
signal. Using the equation below (equation (2)), the conversion was calculated at each point.
Measurements were taken approximately every 3 seconds. The operating pressure was 1 atm
and no change in Argon signal was observed indicating that the pressure in the chamber remained
constant. The reaction stoichiometry was calculated as suggested by equation (3)(4), where PP
stands for partial pressure and R.S.F is the relative sensitivity factor of the mass spectrometer for
a specific mass to charge ratio. The stability test was performed at 500°C for over 10 hours under
proportional-integral-derivative mode instead of ramping.
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푀푒푎푠푢푟푒푑 푋 푠푖푔푛푎푙
%
푋 푐표푛푣푒푟푠푖표푛 = (1 ―
) × 100%
(2)
(3)
푃푟푒 ― 푟푒푎푐푡푖표푛 푋 푠푖푔푛푎푙
휂푂2
휂푂2(푟푒푎푐푡푒푑) %푂 푐표푛푣푒푟푠푖표푛 × 푄
2
푂2
=
=
휂퐶퐻4 휂퐶퐻4(푟푒푎푐푡푒푑) %퐶퐻 푐표푛푣푒푟푠푖표푛 × 푄
4
퐶퐻4
휂퐻2푂 푃푃 (퐻 푂)/푅.푆.퐹(퐻 푂)
2
2
=
(4)
휂퐶푂2 푃푃 (퐶푂 )/푅.푆.퐹(퐶푂 )
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Isotopic tracer experiments. Temperature-programmed isotopic oxygen exchange (TPIOE)
experiments and isotopic oxygen tracer experiments were performed using the same reactor set
up as the methane catalytic combustion reaction. The isotopic oxygen balanced with He with a
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concentration ratio of O : He = 1: 24 was purchased from Cambridge Isotope Laboratories, Inc.
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All parameters were kept the same except for changes in the inlet feed compositions: O : He:
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Ar = 1: 24: 75 for TPIOE while CH4 : 18O : He = 1:4:96 molar ratios for the tracer experiment,
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respectively. For both reactions, 30.0 mg of catalyst was used and the total flow rate was 50
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mL·min so the WHSV of CH = 10000 mL·g ·h , the same as the combustion experiment.
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CH -Temperature-Programmed Reduction (TPR). CH -TPR was performed using the same
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reactor set up. 5% CH (WHSV = 10000 mL·g ·h , the same as the combustion experiment) and
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95% Ar were introduced to 30.0 mg of hematite nanosheets and heated to 500°C at 10°C·min .
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In situ X-ray Diffraction (XRD). In situ XRD diffraction patterns were collected through Thermo-
ARL X’TRA using Cu Kα radiation equipped with an Anton-Parr XRK-900 in situ reactor chamber.
The same composited gas with CH :O = 1:4 balanced with Ar was introduced over 30.0 mg of
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the catalyst with a consistent space velocity as the catalytic measurements. Data analysis was
performed using PDXL-2 Rigaku software.
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In situ DRIFTS spectroscopy. The in situ FTIR studies were performed on a Nicolet 6700 FT-
IR along with a DRIFTS chamber from Pike technologies. A ZnSe window was used for the
reaction and the data was collected by a liquid nitrogen cooled MCT detector at 256 scans and 2
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cm resolution. For the measurements, 20 mg of iron oxide nanosheets was placed in a ceramic
crucible and pretreated at 130°C in argon for 1 hour in the DRIFTS chamber to remove any
surface impurities. The composited gas with CH :O =1:4 balanced with Ar was introduced with
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similar space velocities as the catalytic combustion experiment, and the background spectra was
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taken before temperature ramping. The temperature was then increased at 10°C·min and
multiple spectra were taken over 30 min for each temperature.
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Calculations. All DFT calculations were performed with Vienna Ab initio Simulation Package24-27
(VASP) version 5.4.1 based on periodic boundary spin polarized DFT+U calculation. We used
Jmol version 14.30.228 as the visualization approach and Adobe After Effect version 17.0.6 for
animation. The Perdew-Burke-Ernerhof exchange-correlation functional29 was used in
conjunction with the projected augmented-wave method30-31 to describe the electron-ion
interactions. To properly describe the highly correlated 3d electrons in Fe ions, we used the
DFT+U implemented in VASP following Dudarev’s approach32 to add on-site potentials to the d
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electrons of Fe. A value of 4.0 eV was used for the U = U - J parameter of Fe and 0.0 eV for all
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other elements according to Rollmann’s first-principles calculations.33 The Gaussian smear was
used with the smearing parameter σ = 0.1 eV. The energy convergence criterion was set to be
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10 eV per unit cell. For the optimization of bulk hematite structure, the cutoff energy of the plane
wave basis was chosen to be 450 eV. A Monckhorst-Pack type k-point grid of 3×3×3 was chosen
for the bulk structure optimization. Both the unit cell and atomic positions were allowed to relax
during the geometry optimization. The geometry optimization of bulk structure was finished when
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the energy difference between two consecutive calculations is less than 10 eV.
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The hematite (110) facet model was generated from a 2 × 2 × 1 super cell of the optimized α-
Fe O bulk structure. For all the geometry optimization of slabs and molecules, we increased the
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cutoff energy of the plane wave basis to 520 eV. The force convergence criterion was set to be
at 0.01 eV·Å-1 and the maximum optimization step-size was at 0.4 Bohr. Hematite is an
antiferromagnetic system in a rhombohedral structure. The spins of Fe3+ ions are parallel within
the (111) plane, and anti-parallel in other adjacent planes (Fig. S15).34 The antiferromagnetic
property of hematite is modeled based on spin-polarized DFT+U theory. The reduction/oxidization
states of Fe are described by different initial magnetic moment parameters (Table S2) based on
Liao’s benchmark.35 The atomic boundary is defined by the Wigner-Seitz radius. A 3 × 1 × 3
Monckhorst-Pack type k-point grid was used to sample the Brillouin zone during the geometry
optimization. The unit cell with dimensions of 8.76 Å × 25.12 Å × 13.83 Å with rich oxygen
termination was used to model the α-Fe O (110) facet, an additional H parking slab with the same
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dimensions as the reaction site unit cell was also implemented in the calculations for the pre-
activation process to avoid the reaction slab from over-reduction. The reaction slab contains four
layers of Fe in total, the bottom two layers were frozen at their bulk positions while the top two
layers, as well as the molecules from the gas phase, were allowed to relax during geometry
optimizations.
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We also used the nudged elastic band (NEB) method36 to get the hydrogen transition states’
potential energy surface. NEB employed total 6 intermediate geometries with force convergence
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criterion set to be at 0.05 eV·Å . The relative electronic energy ΔE were calculated according to
equation (5)
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Δ퐸 = ∑퐸푝푟표푑푢푐푡 ― ∑퐸푟푒푎푐푡푎푛푡
(5)
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where ∑퐸푝푟표푑푢푐푡 and ∑퐸푟푒푎푐푡푎푛푡 stands for the total electronic energies of products and reactants,
respectively.
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The frequency calculations were performed with VASP as well and all of the parameters were
-5
kept the same for the optimization except for a higher energy convergence criterion at 10 eV per
unit cell. A step-size of 0.015 Bohr was used in the finite different calculations to obtain the
numerical hessian in frequency calculations.
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All coordinates of optimized structures can be found in the supporting VASP output compressed
file. Other characterization details on Electron Microscopies, X-ray Photoelectron Spectroscopy,
and N -physisorption can be found in the Supporting Information.
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RESULTS AND DISCUSSIONS
Structure composition
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The 2D hematite nanosheets are synthesized through a hard-templating wet-chemical route as
extensively described in our previous work.16, 23 They resemble a graphene-like structure, Fig. 1a,
with an ultrathin thickness of 4-7 nm from our previous microscopic studies. X-ray diffraction
patterns of the nanosheets match well with the hematite standard (PDF Card No.: 00-001-1053),
with notable peak broadenings caused by small crystallite sizes, Fig. 1b. The 2D hematite
2
-1
nanosheets possess a high surface area value of 160 ± 5.5 m ·g (measured by N -physisorption,
2
see characterization details in the supporting information) and a small average particle size of
37.3 ± 1.3 nm.
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Catalytic performance
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In our CMC experiments, 5% CH (weight hourly space velocity (WHSV) = 10000 mL·g ·h ), 20%
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O , and 75% Ar composited gases were flown over 30.0 mg of 2D hematite catalysts. Detailed
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descriptions on the reaction conditions and reactor setups can be found in the method section
and supporting information (Fig. S1). The catalytic performance is shown in Fig. 1c in yellow
squares, with a light-off temperature as low as 230°C, a maximum CH conversion of 80% at
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450°C and 100% selectivity to CO : no by-products such as H , CO, HCHO, HCOOH, and CH OH
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were detected in the complete mass spectra shown in Fig. S2. A self-dehydration behavior (Fig.
S2) was also observed prior to the onset of the reaction near 100°C, which is likely associated
with the dehydration of the surface hydroxyl groups from the hematite structure. The combustion
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휂푂2
휂퐻2
푂
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reaction proceeded with a stoichiometric ratio of 휂퐶퐻4 = 휂퐶푂2 = 2 as calculated by equation (3)&(4)
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in the Methods section and shown in Fig. S3, consistent with the stoichiometric complete oxidation
of methane. In a blank test without any catalyst, no activity was observed in this temperature
range (Fig. S4). The plateau of activity above 400°C is caused by the amount of catalyst used
under such a high WHSV and can be mitigated by increasing the catalyst mass loadings (see
light-off curve in Fig. S5), accompanied by a simultaneous shift of half-conversion temperatures
(T ) towards lower values as shown in the Fig. 1c (inset), or by lowering the total flow rate/WHSV
50
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to increase the catalyst contact time (data not shown). However, the lower catalyst loading, 30.0
mg, and this sufficiently high flow rate/WHSV were chosen as more appropriate parameters for
kinetic analyses to avoid diffusion limitations. We note that the catalytic performance with such a
-1 -1
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low mass loading/high WHSV, corresponding to a specific reaction rate of 62.00 mmol·g ·s at
T , is truly outstanding for a simple metal oxide structure. Most previously reported specific rates
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of similar unsupported oxide systems like NiCo O and MnO ,
are almost one order of
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magnitude lower than that of this work. It even outperforms many supported Pd catalysts,
bimetallic Pd/Pt and Au/Pd systems in terms of specific rate and apparent activation energy (vide
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infra).38-39 A comprehensive comparative table with some typical catalytic systems and the
justifications of such comparison between supported and non-supported systems are given in
Table S1.
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As a control, bulk hematite with only one-thirtieth surface area (5.38 ± 0.35 m ·g ) of that of the
2D structure and an average particle size of 1.16 ± 0.11 µm was also tested, giving a much lower
activity (up to 11% at 500°C) and a significantly higher light-off temperature (~400°C) upon the
same mass loading of 30.0 mg, Fig. 1c. However, when increasing its mass loading to reach a
similar total surface area, 900.0 mg of bulk hematite showed comparable activity to the 2D
nanosheets, Fig. S6, implying that hematite is reactive in both bulk and nano forms. While for the
interest of surface studies, 2D hematite is more attractive overall due to its large surface area and
high density of active sites per unit mass.
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In situ XRD analyses performed across the entire reaction temperature range barely show
changes in either the diffraction patterns or the peak broadenings, Fig. 1b, implying excellent
preservation of the chemical composition and particle size of the hematite nanosheets during the
CMC reaction. In addition, the nanomorphology was kept almost intact from post-reaction
TEM/SEM analyses, Fig. S7, and X-ray photoelectron spectroscopy (XPS) measurements barely
showed any changes at Fe 2p before and after the reaction, Fig. S8, where the oxidation state of
Fe and the surface atomic ratio of Fe : O = 2 : 3 remained consistent. Furthermore, the stability
was also maintained when the reaction was brought to 500°C rapidly (within 6 min) and kept
constant for over 10 hours upon 30.0 mg catalyst loading, Fig. 1d. While under this fast
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temperature increase, the CH conversion first soared up to 94%, then dropped to below 80%,
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and finally, as the temperature stabilized, the conversion equilibrated at 80%. This overshoot is
likely due to the heat released from the combustion reaction itself accumulated in such a short
period of time, resulting in a higher local temperature and thus higher conversion than that from
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the ramping test. After the steady state was reached, the activity was well-maintained over 10
hours with negligible drop.
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The apparent activation energy was calculated by constructing an Arrhenius plot from isothermal
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experiments at various temperatures with CH conversion below 10% to avoid diffusion limitations.
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The apparent activation energy calculated from the slope of the Arrhenius plot is 17.60 ± 1.34
-1
kcal·mol , Fig. S9, which is significantly lower than a typical metal oxide catalyst, for which the
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apparent activation energy falls between 19-35 kcal·mol . This value is even comparable to that
of most reported Pd based catalysts, whose apparent activation energies are in the 17-20
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kcal·mol range for crystalline PdO and in the 40-45 kcal·mol range for metallic Pd.
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Justification of such a low apparent activation energy requires investigation into the reaction
fundamentals from in situ spectroscopies, in conjunction with theoretical analyses at a molecular
level, as discussed in the following sections.
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Fig. 1 a) The TEM image shows the ultrathin structure of hematite nanosheets; b) The diffraction patterns of hematite nanosheets
agree well with the hematite standard (PDF Card No.: 00-001-1053). In situ XRD analyses on hematite nanosheets at different reaction
temperatures show no obvious changes both in the diffraction patterns and peak broadenings during the entire combustion experiment;
c) CH
4
conversion of hematite nanosheets vs bulk upon 30.0 mg as a function of temperature in CMC experiment with 5% CH
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(WHSV
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= 10000 mL·g ·h ) and 20% O
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standard deviations, the averaged conversions and T50 values are labeled on top of each column; d) The stability test of 30.0 mg of
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hematite nanosheets at 500°C over 10 hours with negligible activity drop.
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Reaction model
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The reaction models of metal oxide-based catalysts remain controversial in the CMC community,
as well as more broadly in hydrocarbon combustion studies. Four types of reaction models are
typically proposed as schematically illustrated in Fig. S10, including: 1. the Langmuir-Hinshelwood
(LH) model, where the surface reaction occurs through the dissociative/molecular adsorption of
oxygen and adsorption of methane; 2. the Eley-Rideal (ER) model, where the reaction occurs
between dissociatively adsorbed oxygen and gaseous methane; 3. the Mars van Krevelen (MvK)
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redox model, where the reaction occurs through the alternating reduction and oxidation of the
catalyst surface; and 4. the two-term model (TT), where oxidation on the catalyst surface takes
8
place by two routes, via the lattice oxygen and via the adsorbed oxygen. The controversy is
mainly resulting from the complexity introduced by the multiple oxygen sources of both the metal
oxide lattices and the molecular dioxygen from the gas phase. To elucidate the reaction model,
we designed isotopic oxygen tracer experiments that resolve the contributions from the different
oxygen sources.
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The CMC reaction was again conducted by flowing isotopic O instead of O without disturbing
2 2
other parameters. The oxygen kinetic isotopic effect could be neglected as the reaction order with
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respect to oxygen is zero. The resulting isotopic tracer mass spectra are shown in Fig. 2a, with
a similar light-off behavior as that of the ordinary combustion experiment. However, most notably,
with the reaction initiated at 230°C, C O was produced first, followed by H216O, C16,18O , H218O,
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and finally C O in a distinct order (Fig. 2a inset). Note that during the entire tracer experiment,
2
there was negligible amount of 16,18O species detected in the gas phase, suggesting no exchange
2
of lattice 16O to the gas phase in the temperature range of our experiment. Nevertheless, metal
oxides are known for undergoing oxygen-exchange at gas-solid interfaces at elevated
temperatures. For methane combustion in particular, several authors have proposed the
occurrence of an oxygen-exchange reaction prior to the increase of catalytic activity.43 An
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additional control experiment using TPIOE test, however, showed the lattice oxygen O diffusion
to the gas phase did not start below 400C, Fig. S11. These findings suggest that the oxygen-
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exchange behavior occurs in fact later than the reaction initiation in our case, so the detected O
component in the products must only come from the lattice solid phase. This observation is
consistent with the numerical analysis of the mass balance, showing that initially (T< 385°C) the
ratio 16O/ O > 1 in all oxygen-containing products, then O/ O ~ 1 at T~ 385°C, and finally
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O/ O < 1 at T> 385°C. This observed trend, in conjunction with the order for product formation
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mentioned above, strongly implies a predominant active role of the lattice oxygen at the reaction
initiation of the reaction, ruling out both the LH and ER mechanisms, where the oxygen from the
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gas phase 18O would be the preferred oxidizing agent. Additionally, the prioritized formation of
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been preferentially formed, instead. Therefore, the results are most consistent with a MvK redox
mechanism with participation of the surface lattice oxygen as the initial oxidation step, forming
lattice-oxygen-containing products C O and H216O. Then the catalyst surfaces are re-oxidized
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by molecular dioxygen O from the gas phase, producing a mixture of isotopic products including
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including PdO44 and Co O spinel catalysts. Note that the deconvolution of reaction models
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using oxygen isotopes becomes more complex and difficult at temperature > 400°C due to the
oxygen exchange between the gas and solid phase, so we only focus our analyses before such
exchange predominates.
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A control experiment of a TPR reaction, where only CH and Ar were flown, also supported a MvK
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redox mechanism (Fig. 2b). In this test, hematite showed the ability of activating CH in the
4
absence of molecular dioxygen to produce CO , which is not expected for many other catalysts
2
2
where CH activation must occur through a molecular-dioxygen-assisted pathway, again
4
suggesting the participation of lattice oxygen in methane activation. A similar self-dehydration
behavior was observed again here prior to the onset of the reaction (Fig. S12). More information
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can be gleaned when comparing the C O channel from the isotopic tracer experiment and the
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CO channel from the CH -TPR, Fig. 2b. Note that in this comparison, both channels only reflect
2
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the interactions between the lattice oxygens and CH . For both scenarios, a multi-stage activation
4
behavior was observed that is likely due to CH reacting with different lattice oxygen made
4
available by the increased lattice oxygen diffusion as the temperature was increased. In the first
activation stage (circled in blue), both channels appeared similar with comparable initiation
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36
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38
39
40
temperatures (230C) and partial pressures (yields), presumably associated with the similar
reaction between the active surface oxygens and the adsorbed CH . While after the consumption
4
of the first-generation surface oxygens, the reaction kinetics deviated. In the case of CH -TPR,
4
the yields were much lower in the absence of external oxygen sources, as hematite was gradually
0
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reduced into magnetite by reacting with CH , as examined by XRD analysis, Fig. S13. With
4
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342
additional oxygen from the gas phase, the catalyst surfaces can be quickly re-oxidized following
the MvK redox mechanism, so the hematite composition as well as the active centers can be well-
maintained at all times during the combustion experiment, thus resulting in higher yields. Notably,
in both scenarios, this observed multi-activation stage behavior is highly consistent in terms of
temperature dependence, suggesting a similar interaction of methane with surface lattice oxygens
regardless of the presence of molecular dioxygen, further supporting the active participation of
lattice oxygen in the entire CMC reaction.
3
3
3
3
3
43
44
45
46
47
We also note the leveling off of the C16O channel and the much faster rates of producing 18O
3
3
3
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3
48
49
50
51
52
2
16
18
containing species at higher temperatures, compared to the O counterparts, particularly C O
2
versus C O (Fig. 2a), with the ratio of O/ O < 1. An estimation of the 16O mass balance at
1
6
16
18
2
16
-5
450°C using the ideal gas law gave a total O consumption on the magnitude of 10 mol, taking
-4
up only a small percentage (~2.6%) of the total lattice oxygen (~10 mol) loaded. Therefore, the
16
353
354
level off of the C O channel and the lower producing rates are likely not caused by the intrinsic
2
concentration constraints, but more likely resulting from the slower reaction kinetics of lattice
oxygen 16O participation compared to the molecular oxygen 18O at high temperatures. All
discussions above point to a MvK redox mechanism with an initial lattice oxygen dominated
participation and an increase of molecular oxygen participation as temperature increases.
3
3
3
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Fig. 2 a) Species evolution profile of hematite nanosheets in the isotopic oxygen tracer experiment with a gas feed composition of
: 18
-1
: He = 1:4:96, WHSV = 10000 mL·g ·h-1 for CH
. He was used instead of Ar as the carrier gas here as the purchased 18
O
2
CH
4
O
2
4
was balanced by He gas initially. b) Multi-stage activation behaviors between lattice oxygens and CH
experiment) or without (CH -TPR) additional oxygen in the gas phase. The gas feed for CH TPR is 5% CH
WHSV = 10000 mL·g ·h for CH . The first stage activation is similar in both cases but the rate deviates afterwards given the difference
4
with (refers to isotopic
4
4
4
balanced by Ar with a
-1
-1
4
in ability to maintain the active hematite structure.
3
65
Reaction intermediates
3
3
3
66
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68
Surface intermediates were probed by in situ DRIFTS measurements (Fig. 3), where IR signals
were collected over the hematite nanosheet surfaces in the temperature range of 25 - 300°C
under CMC reaction conditions. Multiple positive peaks were detected in the 3580-3750 cm-1
-
1
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range (Fig. S14a) and 1632 cm (Fig. 3b) above 100°C, corresponding to the desorption of water
46
(or -OH groups) formed upon self-dehydration and methane combustion. Additionally, the paired
-
1
3
3
3
71
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73
negative peaks at 2300-2400 cm (Fig. S14b), detected above 200°C, are typical of the P, Q, and
47
R branches of the CO in the gas phase. Strong characteristic antisymmetric stretching ν (CH)
2
as
-1
and bending δ(CH) signals for free CH molecules (Fig. 3) were observed at 3017 cm and 1305
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cm , respectively, with intensities decreasing with temperature, indicative of CH consumption.
4
All these observations confirm once again the complete oxidation of CH to CO and H O, with no
4
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2
CO signal detected.
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-1
-1
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4
Fig. 3 In situ DRIFTS spectra with species assignments labeled on top, WHSV of CH = 10000 mL·g ·h a) C–H stretching of free
3
CH
of free CH
splitting is characteristic of a bidentate formate transition to monodentate.
4
, methoxy and formate species together with the combination modes of formate; b) Fingerprint region showing the C–H bending
4
and methoxy as well as the paired carbonyl (COO) stretching of formate. The peak splitting of the anti-symmetrical carbonyl
3
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-1
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The paired peaks at ~2960 cm and ~2875 cm along with the band at 1437 cm detected above
150°C correspond, respectively, to the characteristic ν(CH) asymmetric, symmetric stretch, and
bending upon formation of methoxy species (–OCH ).46, 49 Notably, the intensity of the bending
3
peak first increased with temperature, then decreased, and finally disappeared at 300°C (Fig. 3b).
This is likely to be the result of the increased CH adsorption to form -OCH as temperature rises,
4
3
balanced with the simultaneous increase of the turnover rate to consume –OCH . While
3
-1
-1
388
389
390
interestingly, the paired peaks at ~2960 cm and ~2875 cm did not follow such a trend, but
instead, intensified as temperature increased. These peaks above 200°C are more likely
associated with the ν(CH) and the combination modes of ν (COO) and ν (COO) from a formate
as
s
3
91
species on metal oxide surfaces, formed by the subsequent oxidation of the methoxy
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intermediates.50-51 The detection of the characteristic carbonyl stretching signals ν (COO) and
as
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52
ν (COO) at ~ 1555 cm and 1335 cm also supports formation of formate. In addition, the
s
difference between the ν (COO) and ν (COO) peak centers was found to be approximately 220
as
s
-1
cm , typical of a bidentate formate configuration and distinguishable from other species such as
surface carbonates.52 More interestingly, the peak at 1555 cm-1 was separated into two
components when the temperature further increased above 250°C. This was observed previously
in the transition from a bidentate formate to a monodentate formate due to the repulsion among
the bidentate configurations at high concentration by Bell et al.50, 52 These findings strongly imply
formation of stable formate intermediate species on the surface of hematite nanosheets in the
CMC reaction, with intermediate evolution from a methoxy, to a bidentate formate, and to a
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00
01
02
monodentate formate before CO is finally formed as the temperature increases.
2
Table 1| Comparisons of experimental and theoretical vibrational frequencies of important reaction intermediates
-1
Theoretical (cm-1)a
Species
Vibrational Modes
Experimental (cm )
Temperature (℃)
휈as(CH)
3017
3100
CH
4
25 - 300
훿(CH)
1305
1284
휈
as(CH)
2930 - 2985
2850 - 2900
1437
2986
CH
3
–O–Fe
휈
s
(CH)
2921
150 - 250
150 - 300
훿(CH)
휈(CH)
1463, 1447 &1424b
2985
2930 - 2985
2850 - 2900
1555
휈
휈
휈
as(COO) + 휈
s
(COO)
(COO)
2903
b-HCOO–Fe
m-HCOO–Fe
as(COO)
1557
s
(COO)
1335
1345
휈(CH)
2930 - 2985
2850 - 2900
1572
2976
휈
휈
휈
as(COO) + 휈
s
2799
250 - 300
as(COO)
1606
s
(COO)
1189
1193
4
03
a. Theoretical harmonic frequencies;
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05
b. These three vibrational modes were ascribed to be distinct CH bending signals of methoxy species with small frequency
difference, they may be all described by the broad peak at 1437 cm-1 observed experimentally.
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DFT calculations allowed us to validate our experimental frequency assignments at the molecular
level. Structural models of the proposed intermediate species were built on a hematite (110)
surface (vide infra), and all vibrational modes were carefully examined. Table 1 provides a
detailed comparative analysis of experimental and theoretical vibrational bands. The IR vibrational
4
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07
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9
0
frequencies of free CH were used as reference for theory-based calculations to properly compare
4
the theoretical and experimental findings, and their values matched well so no scaling was
required for the theoretical model. The theoretical calculations were found to be highly consistent
with our experimental IR assignments. These results strongly support our analyses of the reaction
intermediates, as described above. Animations for all the calculated vibrational modes along with
the frequencies can be found in the supporting electronic materials.
414
415
4
16
Reaction mechanism
4
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17
18
19
Based upon the successful identifications of reaction intermediates in situ and their order of
formations as a function of temperature, we applied density functional theory to rationalize the
reaction mechanism of CH oxidation over hematite at an atomic level, where the activation
4
420
421
422
process first occurs with participation of the surface lattice oxygen, followed by a full catalytic
cycle, with molecular dioxygen from the gas phase involved. The hematite (110) surface was
chosen as the primary model since it is the most pronounced facet in the XRD pattern (Fig. 1b),
and it has been reported to be the most reactive plane for many catalytic oxidation reactions such
as water21 and CO oxidation.22 Most importantly, this facet possesses antiferromagnetic diiron
oxo cores analogous to the diiron active site in the soluble methane monooxygenase enzyme,
4
4
4
4
4
23
24
25
26
27
15
where CH is activated to produce CH OH in nature, although with a different Fe oxidation state
4
3
and coordination environment, Fig. S15.
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Like most polar surfaces of metal oxides, the (110) surface of hematite is covered with hydroxyl
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groups due to dissociatively chemisorbed water molecules, which hinder the activation of CH by
4
blocking the surface active sites.53 At elevated temperatures, the surface hydroxyl groups are
eliminated by self-dehydration, an essential step for opening up the covalency of active sites and
0
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9
0
enable CH adsorption. As we mentioned previously, this dehydration step was experimentally
4
observed in our system prior to the onset of the combustion reaction. After self-dehydration (Fig.
S16), the model surface consists of two types of oxygens on the first two atomic layers, including
a 3-coordinate O and a 2-coordinate O with a ratio of 2:1, and two types of 5-coordinate irons
434
435
3c
2c
4
4
4
36
37
38
Fe , ⅔ of which are bridged by two O while the rest are connected by one O and one O . The
5c 3c 3c 2c
two adjacent Fe5c atoms bridged by two O3c possess opposite magnetic moments.34 More
electronic structure details can be found in calculation methods and Fig. S15.
4
4
4
4
4
4
4
39
40
41
42
43
44
45
The complete oxidation process of CH involves four hydrogen atom and eight electron transfers.
4
We first considered the adsorption of CH to the surface to simulate the reaction with dominant
4
participation of lattice oxygen at lower temperatures, Fig. 4. For clarity, we only show the reaction
evolutionary profile at the proposed tetra-iron active core. An additional slab was used as a
reservoir in conjunction to model H atom abstraction, as shown and justified in Fig. S17, to avoid
over-reduction of the reaction site. Interestingly, unlike many scenarios where lower-coordinate
atoms are typically more reactive,53-54 we find the most favorable adsorption site is on the two
adjacent O3c atoms (2.58 Å away from each other, the closest O–O distance on the surface),
inducing formation of the methoxy intermediate (Fig. 4, from (1) to (2)), with an adsorption energy
of -1.04 eV, and a C–O bond length of 1.44 Å. The comparison to binding energies for CH4
adsorption on Fe and O is provided in supporting information (Fig. S18). The stability is due to
446
447
4
4
4
4
4
48
49
50
51
52
2c
the favorable O–O distance of the two adjacent O3c atoms as well as the antiferromagnetic
characteristic of the bridged diiron couple, which favors dissociation of the C–H bond by keeping
the total spin of the system intact (zero), therefore lowering the adsorption barrier. In this case, a
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possible CH activation pathway is through homolytic bond cleavage by direct thermal HAT.
4
According to that process (see details in Fig. S19a), the lone-pair of electrons in the O3c can
conjugate with Fe as a valence tautomeric transition to form oxygen-centered radical species,
5c
analogous to the commonly reported active site for thermal activation of CH via HAT in oxides
4
0
1
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9
gas-phase catalysis. The indirect HAT pathway by metal mediation seems less likely, although
-1
the surface irons are unsaturated, as formation of the Fe–C bond (-376.3 ± 28.9 kJ·mol ) indirectly
-1
459
460
is much less favorable than that of a C–O bond (-1076.38 ± 0.67 kJ·mol ) following the direct
route.55
4
61
PCET process, where the electron and proton are transferred to different locations, was also
analyzed as a mechanism to vacate the O site for subsequent HAT activation of the adsorbed
462
463
3c
methane backbone. Both HAT and PCET can be rationalized with valence tautomerism. The
underlying dependence of such dichotomic electron transfer processes was explained in great
detail elsewhere by Usharani, et al.56 where H-abstraction occurred mostly by PCET for those
MO–H bonds (M = transition metal), and by HAT for C–H bonds in similar iron oxo complexes.
Consequently, the proton adsorbed on the O3c can be transferred to a neighboring oxygen by
PCET with a negligible energy change of -0.02 eV (Fig.4, from (2) to (3)) and the calculated energy
barrier (0.85 eV) of such transfer process is sufficiently low for thermal catalysis (Fig. S20). The
4
4
4
4
4
4
4
64
65
66
67
68
69
70
transfer leaves the O unoccupied and therefore facilitates the second C–H bond dissociation by
3c
471
472
HAT to form a formaldehyde-like moiety (Fig. 4, from (3) to (4)), with a reduced C–O bond length
of 1.23 Å characteristic of a double bond, C=O. The energy cost of this step (0.54 eV) is due to
the rearrangement of Fe–O bonds necessary for the geometry change. After another PCET
process (Fig. 4, from (4) to (5), with energy change of -0.37 eV), the O site is regenerated, with
4
4
4
4
4
73
74
75
76
77
3c
a lone-pair of electrons on O that reacts with the formaldehyde moiety through a nucleophilic
3c
attack (Fig. 4, from (5) to (6) with energy change of -0.39 eV), cross-linking the two O to form an
3c
acetal-like structure O–CH –O as during formaldehyde hydration when forming methylene
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glycol.57 The detailed proposed electron transfer process is provided in supporting information
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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2
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3
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6
(Fig. S19a). The O–CH –O bridging species is a precursor for formation of bidentate formate (Fig.
2
rd
4, from (6) to (7)), after the 3 HAT process on the carbon backbone. This process is very
favorable, with an energy decrease of -1.71 eV, due to the structural stability of the bidentate
0
1
2
3
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9
0
intermediates which makes it easy to be probed by in situ DRIFTS measurements.
4
4
4
4
83
84
85
86
As the bidentate formate accumulates on the surface, transition from a bidentate bridging
configuration to a monodentate coordination is expected due to replusion,52 with an energy
change of -1.30 eV (Fig. 4, from (8) to (9)). The transition forms an oxygen vacancy that can be
quickly replenished by either a neighboring lattice oxygen diffusion (Fig. 4, from (8) to (9a), ΔE =
-1.15 eV) or molecular dioxygen (Fig. 4, from (8) to (9b), ΔE = -1.30 eV), both of which lead to
487
488
16
formation of C O but different H O species eventually. The lattice oxygen diffusion is consistent
2
2
with the observed multi-stage activation behavior, as discussed earlier in Fig. 2b. After the 4th
4
4
4
4
4
4
4
89
90
91
92
93
94
95
HAT process and PCET (Fig. 4, from (9) to (11)), CO and H O are formed, leaving oxygen
2
2
vacancies on the surface. Remarkably, the C O was detected much earlier than H216O in the
16
2
isotopic tracer experiment, which is consistent with the desorption of CO being more favorable
2
(Fig. 4, from (9) to (10), with an energy change of -0.09 eV) than the desorption of water (Fig. 4,
from (11) to (12), with an energy change of +0.23 eV), which may cause water retention on the
surface and therefore belated detection, as commonly observed in many other systems such as
58
496
497
supported PdO catalysts. Another observation is that the pre-activation of CH over hematite by
4
participation of surface lattice oxygens leads to reduction of surface Fe(III) to Fe(II) as well as
formation of O vacancies (Fig. 4, from (1) to (12)). Therefore, in the absence of external oxygen
source to replenish the vacancies, further reduction of the hematite to magnetite will occur, as is
4
4
5
98
99
00
the case in the CH TPR experiment (Fig. 2b).
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Fig. 4 Proposed CH
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pre-activation process via lattice oxygen on hematite governing at low temperatures with calculated relative
electronic energies according to equation (5). The overall reaction described here is hematite (110)(s) + CH
(110)–O vacancy(s) + CO (g) + H O(g). HAT and PCET stand for thermal hydrogen-atom transfer and proton-coupled electron transfer
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(g) + O (g) ⟶ hematite
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process, respectively. The labeled C–O bond lengths of 1.44 Å and 1.23 Å are characteristic of a single bond and double bond,
respectively. The corresponding H atom parking situation for each model can be found in the supporting information Fig. S17.
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Formation of oxygen vacancies has been shown in many previous reports to facilitate both lattice
oxygen diffusion and molecular dioxygen adsorption,59 both processes will promote catalytic
activity. Here, in Fig. 5, we evaluate the transition to the molecular-dioxygen-assisted regime at
higher temperatures. With the oxygen vacancies formed after the pre-activation process, the
adsorption of molecular oxygen becomes more favorable with a calculated adsorption energy of
-1.76 eV (Fig. 5, from ① to ②). A peroxide bond is proposed to form, which assists the activation
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of CH by a similar HAT process as proposed above to form methoxy species with an energy
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decrease of -0.37 eV (from ② to ③). The proposed activation pathway involving detailed electron
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transfer process is provided in supporting information (Fig. S19b). In this scenario, the total energy
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decrease for the dissociative adsorption of CH is -2.13 eV (from ① to ③), which is much lower
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than that of the pre-activation process (-1.04 eV) involving the surface lattice oxygen, consistent
with the observation that as the vacancies accumulate at higher temperatures, the 18O isotope-
containing products become dominant in our isotopic tracer experiment. In contrast, at lower
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temperatures, such vacancy structures are fewer so CH is primarily activated by lattice oxygen.
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In addition, the second HAT process (from ③ to ④) via molecular oxygen assistance is
calculated to be more favorable (-1.40 eV) than when assisted by lattice oxygen (+0.54 eV) due
to the more favored geometry with fewer Fe–O bonds breaking and the higher oxidation power of
OOH over just the surface lattice O. The subsequent activation pathway (from ④ to ⑨) is
analogous to that proposed in the pre-activation stage with similar intermediate evolutions and
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energy changes, leading to formation of isotope-containing products C16,18O , H O, and C O .
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After desorption of all products, the active center eventually goes back to the initial vacancy
structure (from ⑨ to ①) to restart the catalytic cycle. The overall electronic energy change (-7.64
eV) in the cycle is equivalent to the theoretical energy difference between reactants and products
in the CMC reaction, and it is consistent with the reported standard heat of reaction at 298K (-
8.32 eV), justifying the effective use of electronic energy change (ΔE) to approximate the enthalpy
change (ΔH) of the reaction. Notably, the order of formation of the proposed intermediates on
the surface along with their respective energetics is highly consistent with our experimental
observations in the in situ DRIFTS analyses both in the pre-activation process and the catalytic
cycle. A video of the animated illustration of the full reaction mechanism can be found in the
supporting electronic material.
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Fig. 5 Proposed catalytic cycle for the CMC reaction at higher temperatures via molecular-oxygen-assisted pathway with calculated
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relative electronic energies according to equation (5). The overall reaction described here is CH (g) + 2O (g) ⟶ CO (g) + 2H O (g).
The energy difference for this cycle equals to the theoretical heat released by the CMC reaction at 0K (-7.64 eV), and it’s close to
the standard heat of the reaction at 298K. The omitted two steps in the cycle are the water desorption process between ④ and ⑤
with an energy cost of + 0.23 eV, similar to that from ⑨ to ①, and an intended PCET process with negligible (-0.05 eV) energy
difference between ⑥ and ⑦ due to the size constraint of the computational model. The numbers are in the unit of eV.
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CONCLUSIONS
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We have found that hematite is an effective catalyst for methane activation, with great potential
as an alternative to precious metal catalysts for low temperature combustion of methane into CO2
and H O. We find that a MvK type redox mechanism with participation of the hematite surface
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oxygen is most consistent with our isotopic oxygen tracer experiments and theoretical analyses.
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At low temperatures, methane is activated by lattice oxygen rather than by molecular dioxygen
from the gas phase. The latter, however, plays a more significant role at higher temperatures,
forming a catalytic cycle upon vacancy-promoted molecular dioxygen adsorption. Reaction
intermediates were probed by in situ DRIFTS and analyzed by DFT calculations at the molecular
level. In summary, CH is first dissociatively adsorbed on the lattice oxygens O on a tetra-iron
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center with an antiferromagnetically coupled iron dimer, forming a methoxy CH –O species, which
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then transforms into an adsorbed formate intermediate through both thermal HAT and PCET. A
bridging bidentate formate is initially formed, followed by the transition to a monodentate
configuration. Finally, CO and H O are formed and desorbed, leaving oxygen vacancies on the
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surface, while other neighboring oxygens both from the lattice and the dioxygen from the gas
phase replenish the vacancies and therefore reconstruct the active center. At higher temperature,
CH is activated more favorably by O owing to the vacancy-promoted adsorption of molecular
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dioxygen, and a complete CMC catalytic cycle is proposed to form on this active vacancy structure.
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Hematite, and many other cheap metal oxides, have long been considered mostly as support
materials for metal catalysts in many important catalytic reactions, while less exploration of their
use as the active component and reaction mechanism has been published. Compared to metal
catalysts, many metal oxides can be more readily made into stable high surface area materials
that are beneficial for surface mechanistic studies, as conducted in this work. This is crucial not
only for elucidating the reaction chemistry, but also for guiding the next-generation of catalyst
design and optimization. In this study particularly, hematite demonstrates great potential in the
effective activation of the inert methane molecule at relatively low temperatures (< 500°C). The
formation of the stable reaction intermediates also implies future possibilities for upgrading
methane to value-added chemicals in heterogenous catalysis. One example could be to prevent
overoxidation of the methoxy and formate species to obtain methanol and formic acid by keeping
the reaction temperature lower than 230°C and extracting the intermediates with proton donors
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such as water. A hydrophobic surface diffusion layer may also help not only the adsorption of the
hydrophobic CH reactant but also the desorption of the hydrophilic products such as methanol
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and formaldehyde to avoid overoxidation. Such a direct route from methane to oxygenated
products would be highly desirable for industrial applications due to reduced costs incurred in
separation and multiple reaction steps.
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ASSOCIATED CONTENT
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The Supporting Information and Electronic Materials are available free of charge on the ACS Publication website at
DOI:.
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Flow Reactor Setup; Complete Mass Spectra; Reaction Stoichiometry; Blank control test; Light-off curve upon
a large mass loading; Comparison of typical catalytic systems in CMC; Bulk hematite with a large mass loading;
Post-reaction TEM/SEM analyses; XPS analyses; Arrhenius plot; Reaction models; TPIOE; Self-dehydration
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in TPR; CH
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-TPR; in situ DRIFTS spectra of water desorption and CO
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; Models of different hematite facets;
Self-dehydrated model surface; Hydrogen atom parking slabs; CH
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adsorption on different sites; Proposed
electron transfer processes; Calculated energy barrier of the PCET process; Magnetic moment
parameters.(PDF) Animated molecular vibrations video (MP4); Animated illustration of the proposed full
reaction pathway (MP4); Compiled VASP geometry output files of all calculations; (ZIP)
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AUTHOR INFORMATION
Corresponding authors:
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*Lisa D. Pfefferle: lisa.pfefferle@yale.edu
*Victor S. Batista: victor.batista@yale.edu
*Ke R. Yang: ke.yang@yale.edu
ORCID IDs:
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Yulian He: 0000-0002-8994-1979
Facheng Guo: 0000-0002-1856-2285
Shu Hu: 0000-0002-5041-0169
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Ke R. Yang: 0000-0003-0028-2717
Steven L. Suib: 0000-0003-3073-311X
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Victor S. Batista: 0000-0002-3262-1237
Lisa D. Pfefferle: 0000-0003-4760-631X
Author contributions
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† These authors contributed equally to this work.
Notes
The authors declare no competing interests.
ACKNOWLEDGEMENTS
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The authors acknowledge Dr. Michael Rooks at Yale YINQE facilities for providing access to and assistance with TEM
and SEM instruments. Thank Dr. Min Li at Yale West Campus Materials Characterization Core for assisting with the X-
ray Photoelectron Spectrometer. Y.H. thanks Yale Edward L. Barlow Funds for financial support. F.G., K.R.Y., and
V.S.B. acknowledge an allocation of high-performance computer time from the National Energy Research Scientific
Computing Center (NERSC) and support by the Air Force Office of Scientific Research (AFOSR) grant #FA9550-13-1-
0020. We also thank the Yale Center for Research Computing for guidance and use of the research computing
infrastructure. S.L.S thanks the U. S. Department of Energy Office of Basic Energy Sciences, Division of Chemical,
Biological and Geological Sciences under grant DE-FG02-86ER13622.A000 for support of this research.
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