TEM measurements were performed on a JEM-2100F instrument operated at 200 kV to characterize the morphology of the 10Al2O3-Pd catalyst. Scanning TEM (STEM) characterization of 8FeOx-Pd catalysts and Pd/Al2O3 samples with various particle sizes was carried out on an aberration-corrected high-angle annular dark-field STEM instrument at 200 kV (JEM-ARM200F, University of Science and Technology of China). Elemental mapping using EDS was performed on the same equipment. The Pd contents in various catalysts were analyzed by an inductively coupled plasma atomic emission spectrometer.

The DRIFTS CO chemisorption measurements were performed on a Nicolet iS10 spectrometer equipped with a mercury-cadmium-telluride detector and a low-temperature reaction cell (Praying Mantis Harrick). After loading a sample into the cell, it was first calcined in 10% O2 in Ar at 150°C for 1 hour followed by a reduction in 10% H2 in Ar at 150°C for another 1 hour. After cooling the sample to room temperature under a continuous flow of Ar, a background spectrum was collected. Subsequently, the sample was exposed to 10% CO in Ar at a flow rate of 20 ml/min for about 30 min until saturation. Next, the sample was purged with Ar at a flow rate of 20 ml/min for another 30 min to remove the gas-phase CO, and then, the DRIFT spectrum was collected with 256 scans at a resolution of 4 cm−1.

In situ x-ray photoemission spectroscopy (XPS) measurements were conducted at the BL10B beamline photoemission end station at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Briefly, the beamline is connected to a bending magnet and covers photon energies from 100 to 1000 eV. The end-station consists of four chambers, i.e., analysis chamber, preparation chamber, quick sample load lock chamber, and a high pressure reactor. The analysis chamber, with a base pressure of <5 × 10−10 torr, is connected to the beamline and equipped with a VG Scienta R3000 electron energy analyzer and a twin anode x-ray source. The high pressure reactor houses a reaction cell where the samples can be treated with different gases up to 20 bars and simultaneously heated up to 650°C. After sample treatment, the reactor can be pumped down to high vacuum (<10−8 torr) for sample transfer. In the current work, the samples were first treated with the flowing 10% H2 in Ar (20 ml/min) at 150°C for 0.5 hour at ambient pressure in the high pressure reactor. Next, the samples were transferred to the analysis chamber for XPS measurements in the Pd 3d region without exposing to air. The Al 2p binding energy at 74.3 eV was used as internal reference (40).

The Pd dispersions were determined by CO pulse chemisorption, which were conducted using a Micromeritics AutoChem II chemisorption analyzer at room temperature. Before CO pulse, the catalysts were first calcined in 10% O2 in He at 200°C for 1 hour followed by a reduction in 10% H2 in He at 200°C for 2 hours. Then, the catalysts were cooled to room temperature in He, and CO pulses were introduced to the catalyst surface using 10% CO in He (41). A stoichiometry of CO:Pd = 1 for dispersion calculations was assumed according to literature.

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