Platinum‐Catalysed Selective Aerobic Oxidation of Methane to Formaldehyde in the Presence of Liquid Water

Abstract The aerobic, selective oxidation of methane to C1‐oxygenates remains a challenge, due to the more facile, consecutive oxidation of formed products to CO2. Here, we report on the aerobic selective oxidation of methane under continuous flow conditions, over platinum‐based catalysts yielding formaldehyde with a high selectivity (reaching 90 % for Pt/TiO2 and 65 % over Pt/Al2O3) upon co‐feeding water. The presence of liquid water under reaction conditions increases the activity strongly attaining a methane conversion of 1–3 % over Pt/TiO2. Density‐functional theory (DFT) calculations show that the preferential formation of formaldehyde is linked to the stability of the di‐σ‐hydroxy‐methoxy species on platinum, the preferred carbon‐containing species on Pt(111) at a high chemical potential of water. Our findings provide novel insights into the reaction pathway for the Pt‐catalysed, aerobic selective oxidation of CH4.


Section I: Catalyst synthesis and characterization
The catalysts were prepared by incipient wetness impregnation of platinic acid in deionized water on titanium dioxide (Sigma Aldrich; Rutile; SBET = 44.5 m 2 /g), or alumina (γ-Al2O3; Alfa Aesar, 3 Micron APD Powder, LOT: K27Y013, SBET = 73.9 m 2 /g) as a support. The support was contacted with an aqueous solution of platinic acid (H2PtCl6, Sigma Aldrich) to obtain 10 wt.-% platinum on the support. The solid was dried, calcined at 400 o C (air flow rate: 48 mln/min/g) and subsequently reduced for 5 hrs at 400 o C in flowing hydrogen (48 mln/min/g).
The elemental composition of the materials was determined using inductively coupled plasma-optical emission spectrometry (ICP-OES). The nanoparticles were imaged using transmission electron microscopy (TEM; FEI Tecnai G2 T20 TEM, operating at 200 kV). Specimens for TEM analysis were prepared by casting one-drop of a colloidal suspension in acetone onto 3-mm carbon-coated copper grids. These were then air dried under ambient conditions. The particle size distribution and the average particle size of the nanosized particles representing platinum was determined by measuring between 400-550 nanoparticles using ImageJ® software.
The phase composition of the catalyst samples was determined using powder X-ray diffraction (XRD) on a Bruker D8 ADVANCE diffractometer (Co-Kα radiation: λ = 1.789 Å, 35 kV, 40mA). The spent catalysts were characterized using Fourier transform infrared spectroscopy (FTIR) on a Perkin Elmer Spectrum 100 FTIR Spectrometer in the range 650-4000 cm -1 with a resolution of 1 cm -1 . 3.2 ± 1.5 (3.6 ± 3.5) 2.1 ± 0.9 (6.1 ± 4.5) a : based on H2-chemisorption; b : estimated from = 113 (%) ; c : from TEM-measurement Section II: Catalyst testing Figure S.1 shows the process flow diagram of the trickle bed reactor, whilst Figure S.2 shows the crosssectional schematic drawing of the reactor tube. The reactor consisted of a quartz tube (length:38 cm; I.D.=12 mm packed with ca. 1.5 g of the pelletized and sieved catalyst (100-150 µm) in its centre. The void space on top of the catalyst was packed with silicon carbide particles (dp ~300µm, obtained from Colbern Abrasives cc, Parow, South Africa). The catalyst and the silicon carbide were held in place with 2 small glass wool plugs on either end. The quartz tube was placed inside a 19.05 mm O.D. stainless steel tube. A quartz-sheathed thermocouple was placed at the centre of the catalyst bed for temperature measurement. The reactor is enclosed in an aluminium heating furnace controlled with multiple heating zones each controlled by thermocouples on the outside of the reactor. The isothermal zone in the reactor was ca. 10 cm. Argon flowed pressure-controlled through the annular space between the two tubes at the same pressure as inside the quartz tubing. The bottom of the reactor rested on a bed of silicon carbide (dp ~ 300 µm; bed length 15.5 cm) to ensure evaporation of the liquid dripping out of the catalyst bed (the argon flow rate was typically set at ca. 2-3 times the total flow rate, including steam, through the catalyst bed). The temperature of the bottom zone was adjusted depending on the water flow rate to achieve smooth operation.
The reactor effluent passed a heated expansion valve placed directly under the reactor to reduce the pressure to atmospheric pressure and was transferred to the on-line GC (Ttransfer line = 180 o C), where it was injected over a heated 6-way valve. The effluent passed a condenser (operating at room temperature) and was vented into the vent line of the walk-in fume hood.
The products were analysed on a GC-FID (Agilent 6890N) equipped with a two-step methanation reactor (PolyArc TM , Activated Research Company) and an FID. The Polyarc™ reactor is designed to enable calibration free analysis of all carbon containing compounds via an FID by first oxidizing the organic samples eluting out of a GC column with air to carbon dioxide and then reducing the carbon dioxide with hydrogen to methane before passing it onto the FID [S1]. The products were separated on a HP-PLOT Q PT capillary column (0.32 mm diameter, 30 m length, 20 μm stationary phase film thickness polystyrene-divinylbenzene; Agilent Technologies).
A five layer Pt(111) slab with a 15 Å vacuum layer was used in this study. For the geometry optimizations, all the atoms were allowed to relax except the bottom two layers of the slabs. Surface structures were optimized with a maximum force of 0.01 eV/Å applying dipole correction in the direction perpendicular to the surface (SCF < 10 -5 eV). The optimized structures represent local minimums on the potential energy surface as confirmed by a vibrational analysis. The vibrational modes were obtained allowing only the atoms of the adsorbate to move by 0.015 Å.
The obtained energies were referenced to that of the bare surface and the molecules CH4, O2, H2, H, OH, H2O, methanol, methanediol and formaldehyde, which were optimized by placing each molecule in a 15 Å x 16 Å x 17 Å box (plane wave cut-off energy: 1000 eV; Gaussian smearing, σ = 0.005 eV; Gamma-centered k-point grid: 1x1x1; maximum allowable force: 0.01 eV/Å). The obtained bond distances and vibrational modes of the gas phase molecules were compared with experimentally determined values [S10-S12].
The adsorption energy as reported in Tables S3-S5 was determined relative to H2O, O2 and CH4 in the gas phase and normalized with respect to the size of the unit cell taking the (2x2) unit cell as a basis: The most stable structure at a particular condition can be obtained by considering the Gibbs free energy of a particular structure at that condition. The Gibbs free energy associated with a structure was calculated as: the Gibbs free energy associated with structure (slab + adsorbate) : the electronic energy of the structure (slab + adsorbate) : the electronic energy of the bare slab ℎ : the enthalpy correction due to vibration of the adsorbate : the entropy correction due to vibration of the adsorbate After determining the Gibbs free energy of structures on the surface, surface phase diagrams of species on Pt(111) were determined in the presence of water and O2 and in the presence of water, O2, and CH4. Numerous configurations can be obtained considering different species, vacant sites, and different adsorption geometries. Hence, only 63 configurations containing O, OH and H2O were considered for the surface phase diagram on Pt(111) exposed to water and O2 (see Table S.2) A much more limited additional set (10 different configurations) was considered when dealing with methyl and methoxy species on the surface was investigated to construct a surface phase diagram on Pt(111) exposed to water, O2 and CH4 (see Table S.3 and S.4).