2.3. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku
D/Max-2550 diffractometer equipped with a SolX detector — Cu Kα radiation
with wavelength of λ=1.5418 Å. SEM images were recorded
on SUPRA 55 operated with an acceleration voltage of 200 kV. Energy
dispersive X-ray (EDX) analysis was carried out in an electron microscope
coupled to an X-sight analyzer operated at 200 kV. Nitrogen sorption
isotherms were obtained at 77 K on a Micromeritics ASAP2020 Gas
Sorption and Porosimetry system. Samples were normally prepared for
measurement after degassing at 423 K under vacuum until a final pressure
of 1×10−3 Torr was reached. Temperature-programmed desorption
of NH3 (NH3-TPD) was carried out on FINESORB-3010 equipped
with a thermal conductivity detector. The samples were first outgassed
under 823 K for 1 h before the measurement. After cooling to 373 K, the
samples were saturated in an NH3 stream (5% in Ar) for 1 h and consequently
treated in Ar (30 ml/min) for 2 h for removing physisorbed
NH3. Finally, the TPD profile was determined by increasing temperature
from 373 K to 873 K with a heating rate of 5 k/min while recording NH3
desorption with a thermal conductivity detector. UV–vis spectroscopy
was recorded with a U-4100 UV–vis-NIR spectrometer. The measurement
parameters were scan speed 300 nm/min, slit width (UV–vis)2.0 nm. X-ray photoelectron spectroscopy (XPS) data was collected on
an ESCALAB 250 X-ray photoelectron spectroscopy, using Mg Kα
X-ray as the excitation source. All binding energies were referenced to
C1s of 284.6 eV.