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 environment 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. Totally different quantities of iron powder (0.1–2 g, 99.999%) had 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 tub and agitated ultrasonically for 30 min till the answer grew to become turbid. The synthesis of Fe–LMS@H₂O doesn’t require a protecting environment. 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 discipline

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%) had 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 had been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by means 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 discipline

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 discipline was managed by an exterior distance-adjustment system. About 10 mg catalyst, 20 ml deionized water and 5 ml H₂O₂ (30%) had 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 strain (0.5–3.0 MPa CH₄, 99.999%). The response combination was left at room temperature for 1 h. The magnetic discipline might 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 had been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by means 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 discipline 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 discipline was managed by an exterior distance-adjustment system. First, 2 ml Fe–LMS@H₂O resolution, 20 ml deionized water and 5 ml H₂O₂ (30%) had 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 strain (2.0 MPa CH₄, 99.999%). The response combination was left at room temperature for 1 h. Assessments had been performed beneath 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 had been quantified by ¹H and ¹³C NMR spectroscopy. The gas-phase merchandise are discharged by means 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 further CO and making use of a magnetic discipline

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 discipline was managed by an exterior distance-adjustment system. About 100 µl catalyst, 19 ml deionized water and 1 ml H₂O₂ (30%) had 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 strain (0.1–1.0 MPa), and a pair of MPa CH₄ was then added into the response system. The response proceeded for 1 h at room temperature, beneath 0 or 500 G. The obtained merchandise had been cooled in ice water for 10 min previous to evaluation. Liquid merchandise had been quantified by ¹H and ¹³C NMR spectroscopy. Fuel merchandise had been quantified by GC.

Reproducibility of liquid product conversion

First, 100 µl contemporary catalyst, 20 ml deionized water and 5 ml H₂O₂ (30%) had 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 discipline 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 had been then positioned again into the autoclave for the subsequent response. The obtained merchandise had been cooled in ice water for 10 min previous to evaluation. Liquid merchandise had been quantified by ¹H and ¹³C NMR spectroscopy.

Characterization strategies

X-ray diffraction measurements had been recorded on a Rigaku Miniflex-600 diffractometer utilizing Cu Kα radiation (λ = 0.15406 nm) with a step measurement of 0.02° and a counting time of 0.5 s. Transmission electron microscopy photos had been recorded on a Hitachi H-7700 operated at 100 kV. Scanning electron microscopy photos had 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 had been quantified by NMR spectroscopy. Measurements had been performed on a Bruker Avance-Ⅲ 400 spectrometer. ¹H NMR spectra had been recorded with a 2-s recycle delay, for 64 scans, utilizing dimethyl sulfoxide as an inner normal. ¹³C NMR spectra had been recorded with a 10-s recycle delay, for two,048 scans. Gaseous merchandise had been quantified by a GC outfitted 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 provider fuel.

In situ electron microscopy and corresponding atomic-level EDS mapping

Aberration-corrected HAADF-STEM photos and corresponding EDS maps had been recorded on a FEI-Titan Cubed Themis G2 300 STEM. Frozen pattern rods had been used to load samples, permitting for cooling with liquid nitrogen throughout testing. Earlier than electron microscopy imaging, the samples had 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 comfortable X-ray microscope, the place an elliptical capillary condenser centered the comfortable X-ray beam onto the cells for remark. Within the pattern chamber, the magnetic discipline is adjusted by controlling the space between the pure magnet and the pattern holder. For the technology of 3D volumes, the cells had been rotated from −60° to +60°, capturing a steady sequence of 121 projected photos at 1° intervals with a 2-s publicity time. X-ray energies is 706 eV and 715 eV (overlaying the Fe L3 edge) had been used. Alignment of the lean sequence was carried out utilizing XMController, and 3D CT reconstruction was carried out utilizing XMReconstruction.

In situ Mössbauer spectroscopy

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

ESR

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

NAP-XPS measurements

NAP-XPS measurements had been carried out utilizing a system positioned at Shanghai Tech College. This technique was manufactured by SPECS Floor Nano Evaluation (Supplementary Fig. 28). The ability consists of a predominant chamber, a preparation chamber and a load–lock chamber. The evaluation chamber is provided 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 strain of 0.4 mbar. After amassing the C 1s spectra, a flask containing 30% H₂O₂ resolution was related to the XPS testing chamber (see Supplementary Fig. 29 for images). The destructive strain throughout the chamber ensured the managed evaporation of H₂O₂ into the environment, and a brand new sequence of C 1s spectra had been acquired. The above outcomes had 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 ought to be famous that because of the stringent strain necessities of NAP-XPS, the testing circumstances 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 environment to approximate the H₂O₂ liquid surroundings current within the precise catalytic response. Though this substitution differs from the precise response circumstances, it allows the identification of key intermediates beneath operando circumstances.

2. (2)

   As mentioned elsewhere (Supplementary Figs. 20–24), HCl serves to forestall full oxidation of the catalyst floor within the catalytic response. Nonetheless, through the NAP-XPS measurement timeframe, the oxidative capability of the H₂O₂ environment is inadequate to completely oxidize the catalyst floor; thus, its absence doesn’t alter the noticed response pathway. Moreover, because of the strain limitations of NAP-XPS, the concentrations of the first reactants (CH₄ and H₂O₂) had 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 had 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 had been carried out utilizing a home made 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 had been collected in fluorescence mode. The spectrum of a iron steel foil was collected concomitantly for inner vitality calibration. Three spectra had 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 analyze the affect of the self-absorption impact, take a look at samples had been collected at incidence angles of 45° and 15°, respectively.

XAFS knowledge evaluation

Prolonged X-ray absorption wonderful construction (EXAFS) knowledge had been processed and analysed following normal procedures throughout the ATHENA module applied within the IFEFFIT library software program package deal. The Fe Okay-edge ok³-weighted χ(ok) knowledge within the ok-space had been Fourier-transformed to the true house (R) with Hanning home windows (dok = 1.0 Å⁻¹) to separate out EXAFS contributions from totally different coordination shells. Efficient backscattering amplitudes and part shifts had 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 had been carried out throughout 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 had 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 might be assigned to the mixture of two sorts of Fe–Ga coordination with totally different buildings, and a two-shell construction mannequin with two totally different sorts of Fe–Ga scattering paths was used for becoming. For Fe–LMS–H₂O₂ and Fe–LMS–H₂O₂–CH₄, apart from the broad peak at 2.30 Å from the Fe–Ga coordination, two distinct peaks at 1.50 Å and 1.58 Å come up on account of Fe–O and Fe–C bonds. Consequently, a three-shell construction mannequin with one Fe–O/C path and two totally different Fe–Ga scattering paths was used to suit the info of each samples.

Throughout curve fittings, the amplitude discount issue S₀² was fastened at a worth of 0.78 as decided by becoming the info for a iron foil. For the Fe–LMS pattern, the coordination numbers, interatomic distances (R) and vitality shifts (ΔE₀) for each Fe–Ga paths had been handled as adjustable parameters, and the vitality shifts (ΔE₀) for the 2 paths had 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 throughout the similar pattern, set as an adjustable parameter. For Fe–LMS–B samples, the Fe–Fe path is about 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 about 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 had been handled equally, and their precise numbers of adjustable parameters had 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 shouldn’t be bigger than 0.031, indicating a very good 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 perform in Athena. The incident and exit angles had been set at 45°, reflecting the precise testing circumstances, and the fluo-μ(E) algorithm and troger-χ(ok) algorithm had 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 photos. All DFT calculations had been carried out with the Vienna Ab initio Simulation Bundle (VASP)³⁰. The exchange-correlation interactions had been handled utilizing the Perdew–Burke–Ernzerhof practical³¹ throughout the generalized gradient approximation. The projector augmented wave³² was used to deal with the inert core electrons. Spin polarization was applied in all calculations. To simulate the presence of an exterior magnetic discipline, noncollinear magnetic calculations had been carried out, with the path of spin second fastened alongside the optimistic x axis, whereas permitting the magnitude of the magnetic moments to loosen up. 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 had been handled by the DFT + U technique with an efficient U worth of three eV (refs. ^33,34). The digital vitality convergence standards had been set to 10⁻⁵ eV, and the power convergence threshold was 0.02 eV Å⁻¹. The climbing-image nudged elastic band technique³⁵ was used to determine transition state buildings. All optimized buildings had been confirmed as floor states (zero imaginary frequency) or transition states (one imaginary frequency) by performing vibrational frequency evaluation. Lengthy-range dispersion interactions had been included utilizing Grimme’s DFT-D3 technique³⁶. Solvent results had 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 had been performed utilizing the VASPKIT toolkit³⁹.

AIMD simulations

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