Magnetically tunable selectivity in methane oxidation enabled by Fe-embedded liquid steel catalysts

Synthesis strategies

Preparation of Fe–LMS, Fe@Ga–In, Fe@Ga–Sn and Fe@Ga

The synthesis Fe–LMS should be carried out in a nitrogen ambiance all through to forestall oxygen oxidation.

Gallium (99.999%) was first melted at 50 °C in a beaker. Subsequently, 7 g molten gallium steel was blended with 2 g indium powder (99.999%) and 1 g tin powder (99.999%), adopted by vigorous stirring at 160 °C for five h. Completely different quantities of iron powder (0.1–2 g, 99.999%) have been added into the liquid steel alloy adopted by guide stirring till the iron powder was solely dissolved. Then, 100 µl of 5% hydrochloric acid was added to maintain the alloy in a liquid state. The obtained liquid steel catalyst Fe–LMS was saved in an oxygen-free container.

After adjusting the feeding ratio to Fe:Ga:In to 0.1:7:3 (w/w/w/), the aforementioned course of was repeated to acquire Fe@Ga–In. After adjusting the feeding ratio to Fe:Ga:Sn to 0.09:7:1 (w/w/w/), the aforementioned course of was repeated to acquire Fe@Ga–Sn. After adjusting the feeding ratio to Fe:Ga to 0.1:10 (w/w) and adjusting the temperature to 200 °C, the aforementioned course of was repeated to acquire Fe@Ga.

Preparation of Fe–LMS@H₂O

First, 10 mg of ready Fe–LMS was added to twenty ml of deionized water, and the combination was then positioned in a room-temperature water bathtub and agitated ultrasonically for 30 min till the answer grew to become turbid. The synthesis of Fe–LMS@H₂O doesn’t require a protecting ambiance. Fe–LMS@H₂O is a suspension, and the ready Fe–LMS@H₂O should be used instantly to keep away from sedimentation.

Catalytic efficiency analysis

Methane oxidation by Fe–LMS with no magnetic area

Oxidation of CH₄ was carried out in a stainless-steel Teflon-lined autoclave with a quantity of 100 ml. Sometimes, 10 mg catalyst Fe–LMS, 20 ml deionized water and 5 ml H₂O₂ (30%) have been added to the autoclave. Hydrochloric acid was added dropwise till the pH of the answer approached 4. The autoclave was flushed 3 times with methane after which pressurized with methane to the specified pressures (0.5–3.0 MPa CH₄, 99.999%). The response proceeded for 1 h at room temperature. The autoclave with obtained merchandise was cooled in ice water for 20 min previous to evaluation. Liquid merchandise have been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by way of the reactor’s exhaust valve and picked up in a fuel bag, which is subsequently transferred to the GC for evaluation.

Methane oxidation by Fe–LMS with a magnetic area

The CH₄ oxidation response was carried out in a closed high-pressure autoclave. The high-pressure reactor was positioned between two parallel everlasting magnets, and the depth of the magnetic area was managed by an exterior distance-adjustment machine. About 10 mg catalyst, 20 ml deionized water and 5 ml H₂O₂ (30%) have been added to the autoclave. Hydrochloric acid was added dropwise till the answer pH approached 4. The autoclave was flushed 3 times after which pressurized with methane to the specified stress (0.5–3.0 MPa CH₄, 99.999%). The response combination was left at room temperature for 1 h. The magnetic area will be regulated over the vary 0–1,200 G. The autoclave with obtained merchandise was cooled in ice water for 20 min previous to evaluation. Liquid merchandise have been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by way of the reactor’s exhaust valve and picked up in a fuel bag, which is subsequently transferred to the GC for evaluation.

Methane oxidation by Fe–LMS@H₂O with magnetic area switching

The CH₄ oxidation response was carried out in a closed high-pressure autoclave. The high-pressure reactor was positioned between two parallel everlasting magnets, and the depth of the magnetic area was managed by an exterior distance-adjustment machine. First, 2 ml Fe–LMS@H₂O resolution, 20 ml deionized water and 5 ml H₂O₂ (30%) have been added to the autoclave. Hydrochloric acid was added dropwise till the answer pH approached 4. The autoclave was flushed 3 times after which pressurized with methane to the specified stress (2.0 MPa CH₄, 99.999%). The response combination was left at room temperature for 1 h. Exams have been performed underneath magnetic fields of 0, 50, 100, 150, 175, 200, 210, 250, 300, 400 and 500 G. The autoclave with obtained merchandise was cooled in ice water for 20 min previous to evaluation. Liquid merchandise have been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by way of the reactor’s exhaust valve and picked up in a fuel bag, which is subsequently transferred to the GC for evaluation.

Methane oxidation by Fe–LMS with extra CO and making use of a magnetic area

The CH₄ oxidation response was carried out in a closed high-pressure autoclave. The high-pressure reactor was positioned between two parallel everlasting magnets, and the depth of the magnetic area was managed by an exterior distance-adjustment machine. About 100 µl catalyst, 19 ml deionized water and 1 ml H₂O₂ (30%) have been added to the autoclave. Hydrochloric acid was added dropwise till the pH of the answer approached 4. The autoclave was flushed 3 times after which pressurized with CO to the specified stress (0.1–1.0 MPa), and a couple of MPa CH₄ was then added into the response system. The response proceeded for 1 h at room temperature, underneath 0 or 500 G. The obtained merchandise have been cooled in ice water for 10 min previous to evaluation. Liquid merchandise have been quantified by ¹H and ¹³C NMR spectroscopy. Fuel merchandise have been quantified by GC.

Reproducibility of liquid product conversion

First, 100 µl recent catalyst, 20 ml deionized water and 5 ml H₂O₂ (30%) have been added to the autoclave, which was flushed 3 times with deionized water after which pressurized with 2.0 MPa methane. Hydrochloric acid was added dropwise till the pH of the answer approached 4. The response was carried out whereas switching the magnetic area on–off 11 instances at 1-h intervals. The catalyst was washed with hydrochloric acid (pH 4) after every 1-h response to take away the oxidation movie on the catalyst. The reactants and washed catalyst have been then positioned again into the autoclave for the subsequent response. The obtained merchandise have been cooled in ice water for 10 min previous to evaluation. Liquid merchandise have been quantified by ¹H and ¹³C NMR spectroscopy.

Characterization strategies

X-ray diffraction measurements have been recorded on a Rigaku Miniflex-600 diffractometer utilizing Cu Kα radiation (λ = 0.15406 nm) with a step dimension of 0.02° and a counting time of 0.5 s. Transmission electron microscopy photographs have been recorded on a Hitachi H-7700 operated at 100 kV. Scanning electron microscopy photographs have been recorded on a Supra 40. A Quantum Design MPMS3 was used for magnetic second testing. Elemental evaluation was carried out by inductively coupled plasma atomic emission spectrometry utilizing an Optima 7300 DV spectrometer. Liquid merchandise have been quantified by NMR spectroscopy. Measurements have been performed on a Bruker Avance-Ⅲ 400 spectrometer. ¹H NMR spectra have been recorded with a 2-s recycle delay, for 64 scans, utilizing dimethyl sulfoxide as an inner customary. ¹³C NMR spectra have been recorded with a 10-s recycle delay, for two,048 scans. Gaseous merchandise have been quantified by a GC geared up with a 5-Å molecular sieve, a Porapak Q 80/100 mesh, and SE-30 and HP-Al₂O₃/S columns utilizing helium (ultrahigh purity) as service fuel.

In situ electron microscopy and corresponding atomic-level EDS mapping

Aberration-corrected HAADF-STEM photographs and corresponding EDS maps have been recorded on a FEI-Titan Cubed Themis G2 300 STEM. Frozen pattern rods have been used to load samples, permitting for cooling with liquid nitrogen throughout testing. Earlier than electron microscopy imaging, the samples have been subjected to magnetic fields of 0 G and 500 G and frozen with liquid nitrogen for 10 min to repair the construction.

In situ X-ray 3D CT

In situ X-ray 3D CT was carried out at beamline BL07W of the Nationwide Synchrotron Radiation Laboratory. The pattern holder, containing the nickel grid, was transferred to the chamber of a transmission mushy X-ray microscope, the place an elliptical capillary condenser targeted the mushy X-ray beam onto the cells for statement. Within the pattern chamber, the magnetic area is adjusted by controlling the gap between the pure magnet and the pattern holder. For the era of 3D volumes, the cells have been rotated from −60° to +60°, capturing a steady collection of 121 projected photographs at 1° intervals with a 2-s publicity time. X-ray energies is 706 eV and 715 eV (protecting the Fe L3 edge) have been used. Alignment of the lean collection was carried out utilizing XMController, and 3D CT reconstruction was carried out utilizing XMReconstruction.

In situ Mössbauer spectroscopy

Mössbauer spectroscopy measurements have been performed utilizing a Wissel MR-2500 spectrometer. For the Fe–LMS pattern underneath a 500-G magnetic area, the sphere power on the pattern location was adjusted by putting a pure magnet exterior the measurement chamber. Every pattern weighed 100 mg, and measurements have been carried out at room temperature in a vacuum surroundings. The spectral vary was set to ±12 mm s⁻¹, with a measurement length of 24 h.

ESR

ESR was carried out on the Regular Excessive Magnetic Subject Amenities, Excessive Magnetic Subject Laboratory, utilizing the next parameters: temperature, 173 Okay; energy, 0.01 mW; central area, 7,000 G; sweep width, 14,000 G; modulation frequency, 100 kHz; modulation amplitude, 2.00 G. The samples have been frozen at 173 Okay for 10 min underneath magnetic fields of 0 G and 500 G, respectively, and the iron powder customary samples have been frozen at 0 G and 173 Okay for 10 min earlier than testing.

NAP-XPS measurements

NAP-XPS measurements have been carried out utilizing a system situated at Shanghai Tech College. This method was manufactured by SPECS Floor Nano Evaluation (Supplementary Fig. 28). The power consists of a most important chamber, a preparation chamber and a load–lock chamber. The evaluation chamber is supplied with a PHOIBOS NAP hemispherical electron vitality analyser, a microfocus monochromatized Al Kα X-ray supply with a beam diameter of 300 μm, a SPECS IQE-11A ion gun and an infrared laser heater. Fe₁–LMS (1.1 wt%) was dropped onto a clear silicon wafer and dried at room temperature. Having put in the pattern within the evaluation chamber, high-purity CH₄ was fed into the chamber as much as a stress of 0.4 mbar. After accumulating the C 1s spectra, a flask containing 30% H₂O₂ resolution was linked to the XPS testing chamber (see Supplementary Fig. 29 for pictures). The detrimental stress inside the chamber ensured the managed evaporation of H₂O₂ into the ambiance, and a brand new collection of C 1s spectra have been acquired. The above outcomes have been recorded because the dispersed state. We then changed the iron content material to present a focus of 9.8 wt%, repeated the above experiment, and recorded the outcomes because the aggregated state.

It must be famous that because of the stringent stress necessities of NAP-XPS, the testing situations symbolize a compromise in contrast with these of the particular catalytic experiments. Beneath, we offer an in depth justification for these compromises:

1. (1)

   The first goal of NAP-XPS is to seize response intermediates. Given the method’s qualitative nature and operational constraints, direct introduction of a liquid-phase surroundings was unfeasible. As a substitute, we used a 0.13-mbar H₂O₂ (30% resolution) vapour ambiance to approximate the H₂O₂ liquid surroundings current within the precise catalytic response. Though this substitution differs from the precise response situations, it permits the identification of key intermediates underneath operando situations.

2. (2)

   As mentioned elsewhere (Supplementary Figs. 20–24), HCl serves to forestall full oxidation of the catalyst floor within the catalytic response. Nevertheless, throughout the NAP-XPS measurement timeframe, the oxidative capability of the H₂O₂ ambiance is inadequate to totally oxidize the catalyst floor; thus, its absence doesn’t alter the noticed response pathway. Moreover, because of the stress limitations of NAP-XPS, the concentrations of the first reactants (CH₄ and H₂O₂) have been already decrease than these within the catalytic course of. Introducing HCl would additional scale back reactant concentrations, adversely affecting the signal-to-noise ratio and the reliability of the measurements. Consequently, HCl was intentionally excluded from the NAP-XPS experiments to protect knowledge high quality.

In situ XAFS measurements

Fe Okay-edge (7,112 eV) XAFS spectra have been recorded on the 1W1B beamline of the Beijing Synchrotron Radiation Facility. The storage ring was operated at 2.5 GeV, with a most electron present of 250 mA. The arduous X-ray beam was monochromatized with a Si(111) double-crystal monochromator. For the elimination of higher-order harmonics, the X-ray was detuned by 30% for the Fe Okay-edge. The detection system consists of a 19-element germanium solid-state detector and a Lytle detector. In situ XAFS measurements have been carried out utilizing a do-it-yourself reactor (10 ml) with a vitreous carbon window (diameter, 6 mm; Supplementary Fig. 30). A catalyst-coated carbon-fibre paper (diameter, ∼5 mm) was sealed behind the window. Spectra have been collected in fluorescence mode. The spectrum of a iron steel foil was collected concomitantly for inner vitality calibration. Three spectra have been averaged for every dataset. The catalysts used within the measurements are Fe–LMS–1% and Fe–LMS–10%, with exact iron loadings of 1.1 wt% and 9.8 wt%, respectively. To research the affect of the self-absorption impact, check samples have been collected at incidence angles of 45° and 15°, respectively.

XAFS knowledge evaluation

Prolonged X-ray absorption tremendous construction (EXAFS) knowledge have been processed and analysed following customary procedures inside the ATHENA module carried out within the IFEFFIT library software program bundle. The Fe Okay-edge ok³-weighted χ(ok) knowledge within the ok-space have been Fourier-transformed to the actual house (R) with Hanning home windows (dok = 1.0 Å⁻¹) to separate out EXAFS contributions from completely different coordination shells. Efficient backscattering amplitudes and section shifts have been calculated utilizing the ab initio code FEFF8.054. For the FeGa pattern, a ok-rage of two.5–12.1 Å⁻¹ (Δok) was used and the curve fittings within the R house have been carried out inside the vary 1.0–3.0 Å (ΔR). The variety of unbiased factors was: N_ipt = 2Δok × ΔR/π = 2 × (12.1 − 2.5) × (3.0 − 1.0)∕π ≈ 12. The becoming ranges of all the opposite samples are listed beneath Supplementary Desk 2, and the numbers of their unbiased factors have been calculated equally; all of the samples had greater than 9 unbiased factors.

The Fourier-transformed curves for Fe–LMS, Fe–LMS–B, Fe–LMS–H₂O₂–B and Fe–LMS–H₂O₂–CH₄–B confirmed a large peak at 2.30 Å, which will be assigned to the mixture of two sorts of Fe–Ga coordination with completely different constructions, and a two-shell construction mannequin with two completely different sorts of Fe–Ga scattering paths was used for becoming. For Fe–LMS–H₂O₂ and Fe–LMS–H₂O₂–CH₄, apart from the huge peak at 2.30 Å from the Fe–Ga coordination, two distinct peaks at 1.50 Å and 1.58 Å come up resulting from Fe–O and Fe–C bonds. Consequently, a three-shell construction mannequin with one Fe–O/C path and two completely different Fe–Ga scattering paths was used to suit the information of each samples.

Throughout curve fittings, the amplitude discount issue S₀² was fastened at a price of 0.78 as decided by becoming the information for a iron foil. For the Fe–LMS pattern, the coordination numbers, interatomic distances (R) and vitality shifts (ΔE₀) for each Fe–Ga paths have been handled as adjustable parameters, and the vitality shifts (ΔE₀) for the 2 paths have been thought of to be the identical.

Dysfunction issue (Debye–Waller issue, σ²)

A single σ² worth is used for one or two Fe–Ga paths inside the similar pattern, set as an adjustable parameter. For Fe–LMS–B samples, the Fe–Fe path is ready as an adjustable parameter. To cut back the variety of adjustable parameters in subsequent pattern fittings, particularly for Fe–LMS–H₂O₂ and Fe–LMS–H₂O₂–CH₄, the place three paths are utilized, the σ² worth for the Fe–Fe path is ready to be the identical as that of Fe–LMS–B. The Fe–C/O paths within the remaining samples are set as adjustable parameters.

The precise variety of adjustable parameters, N_para = 5, was decrease than the utmost quantity N_ipt = 12. The entire different samples have been handled equally, and their precise numbers of adjustable parameters have been all decrease than their unbiased factors. The structural parameters obtained from becoming are listed in Supplementary Desk 1. The R issue obtained for each match just isn’t bigger than 0.031, indicating an excellent high quality of the becoming.

Self-absorption correction

Based mostly on iron mass fractions of 1% and 10% within the GaInSn alloy, self-absorption correction was utilized utilizing the self-absorption correction operate in Athena. The incident and exit angles have been set at 45°, reflecting the precise testing situations, and the fluo-μ(E) algorithm and troger-χ(ok) algorithm have been used for XANES and EXAFS corrections, respectively.

DFT calculations

A vacuum spacing of 10 Å was utilized in three instructions for all constructed fashions to forestall interactions between periodic photographs. All DFT calculations have been carried out with the Vienna Ab initio Simulation Package deal (VASP)³⁰. The exchange-correlation interactions have been handled utilizing the Perdew–Burke–Ernzerhof purposeful³¹ inside the generalized gradient approximation. The projector augmented wave³² was used to deal with the inert core electrons. Spin polarization was carried out in all calculations. To simulate the presence of an exterior magnetic area, noncollinear magnetic calculations have been carried out, with the route of spin second fastened alongside the constructive x axis, whereas permitting the magnitude of the magnetic moments to chill out. A plane-wave kinetic vitality cut-off of 400 eV was used all through all calculations. The Brillouin zone was sampled with a 1 × 1 × 1 Monkhorst–Pack ok-point mesh. The strongly localized 3d orbitals of iron have been handled by the DFT + U methodology with an efficient U worth of three eV (refs. ^33,34). The digital vitality convergence standards have been set to 10⁻⁵ eV, and the power convergence threshold was 0.02 eV Å⁻¹. The climbing-image nudged elastic band methodology³⁵ was used to determine transition state constructions. All optimized constructions have been confirmed as floor states (zero imaginary frequency) or transition states (one imaginary frequency) by performing vibrational frequency evaluation. Lengthy-range dispersion interactions have been included utilizing Grimme’s DFT-D3 methodology³⁶. Solvent results have been thought of by making use of an implicit solvation mannequin for water through the VASPsol module^37,38. Bader cost evaluation was carried out to quantify electron switch. PDOS calculations and orbital interplay analyses have been performed utilizing the VASPKIT toolkit³⁹.

AIMD simulations

The structural modifications and dynamic properties of the Fe_n–LMS have been investigated with AIMD simulations utilizing VASP. The identical computational settings have been used as within the static DFT calculations, together with convergence standards, plane-wave kinetic vitality cut-off, ok-point mesh, exchange-correlation purposeful, spin remedy, DFT-D3 dispersion correction, and the DFT + U correction. Canonical NVT ensemble and Nosé–Hoover thermostats⁴⁰ have been adopted at 298.15 Okay with a time step of 1 fs.