https://doi.org/10.1016/j.cej.2022.135394
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X-ray absorption spectroscopy (XAS) at Ni K-edge (8.3 keV) were recorded at the B18 beamline of the Diamond Light Source, UK, using Si (1 1 1) double crystal monochromator. The catalysts were pelletised with boron nitride. All samples were collected in fluorescence mode, while standards were collected in transmission. Ni foil spectra were simultaneously collected to samples and standards to enable energy calibrations. Data processing was carried out using Demeter open source software package, version 0.9.26, with XAS spectra processing (normalization and background subtractions) and extended X-ray absorption fine structure (EXAFS) fitting conducted within the Athena and Artemis programs, respectively.
XPS data were acquired using a Kratos Axis SUPRA using monochromated Al kα (1486.69 eV) X-rays at 12 mA emission and 15 kV HT (180 W) and a spot size/analysis area of 700 × 300 µm. The instrument was calibrated to gold metal Au 4f (83.95 eV) and dispersion adjusted to give a BE of 932.6 eV for the Cu 2p3/2 line of metallic copper. Ag 3d5/2 line FWHM at 10 eV pass energy was 0.544 eV. Source resolution for monochromatic Al Kα X-rays is ∼ 0.3 eV. The instrumental resolution was determined to be 0.29 eV at 10 eV pass energy using the Fermi edge of the valence band for metallic silver. Resolution with charge compensation system on < 1.33 eV FWHM on PTFE. High-resolution spectra were obtained using a pass energy of 20 eV, step size of 0.1 eV and sweep time of 60 s, resulting in a line width of 0.696 eV for Au 4f7/2. Survey spectra were obtained using a pass energy of 160 eV. Charge neutralisation was achieved using an electron flood gun with filament current = 0.38 A, charge balance = 2 V, filament bias = 4.2 V. Successful neutralisation was adjudged by analysing the C 1s region wherein a sharp peak with no lower BE structure was obtained. Spectra have been charge corrected to the mainline of the carbon 1 s spectrum (adventitious carbon) set to 284.8 eV. All data was recorded at a base pressure of below 9 × 10-9 Torr and a room temperature of 294 K. Data was analysed using CasaXPS v2.3.19PR1.0. Peaks were fit with a Shirley background before component analysis. Ni core-lines were fit using the Gupta and Sen multiplet approximation for free ion Ni2+ [36], while satellites were fit using an LA(1.53, 253) lineshape. Ceria3+/4+ ratio was determined according to the fits derived by Romeo et al. [37].
Ni dispersion was calculated by CO chemisorption performed on Micromeritics Autochem II 2920 analyser. Typically, 100 mg of as-synthesised catalyst was reduced at 550 °C for 30 min at a heating rate of 10 °C min−1 in a U-shape quartz tube. The temperature was further increased to 600 °C under pure He for 60 min to remove the absorbed H2. 1% CO/He and He were used for the loop gas and carrier gas, with pulse chemisorption measurement conducted at 35 °C.
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