The surface area and pore characteristics of the catalysts were obtained from N2 adsorption/desorption analysis at −196°C using a Quantachrome Quadrasorb SI-MP. Before the N2 physisorption, the catalysts were degassed at 300°C for 5 hours. The surface area was determined by the Brunauer-Emmett-Teller equation in the 0.05 to 0.35 partial pressure range. The pore volume and average pore diameter were determined by the Barrett-Joyner-Halenda method from the desorption branches of the isotherms.

XPS spectra of the catalysts were recorded on a scanning X-ray microprobe (Axis Ultra, Kratos Analytical Ltd.) using Al Kα radiation (1486.7 eV). All the binding energies were calibrated using the C 1s peak (binding energy = 284.8 eV) as standard.

NH3-TPD experiments were performed using a quadrupole mass spectrometer (HPR-20, Hiden Analytical Ltd.) to record the signal of NH3 [mass/charge (m/z) ratio = 15 for NH]. Before TPD experiments, the samples (150 mg) were pretreated at 400°C in a flow of 20 vol % O2/N2 (50 ml min−1) for 0.5 hours and cooled down to room temperature. The samples were then exposed to a flow of 1% NH3/N2 (50 ml min−1) at 50°C for 1 hour, followed by Ar purging for 1 hour. Last, the temperature was raised to 600°C in Ar flow at a rate of 10°C min−1.

Raman spectra were measured on a LabRAM HR 800 Raman spectrometer using a 532-nm laser as the excitation source. The spectrometer was equipped with an in situ reaction cell (PIKE Technologies), in which samples can be heated to 500°C in a gas flow. In our experiments, all catalysts were heated from room temperature to 400°C under an oxygen flow and then the Raman spectra were acquired.

The 51V solid-state NMR experiments were performed at 11.7 T on a Bruker Avance III 500 spectrometer with a resonance frequency of 131.6 MHz, using a 1.9-mm HX double-resonance probe at a spinning rate of 40 kHz. The NMR spectra of 51V were acquired using a Hahn-echo pulse sequence with a π/2 pulse width of 1.5 μs. For our samples with 1 wt % V2O5 loading, 60,000 scans with a 0.3-s recycle delay were used. The 51V chemical shift was referenced to V2O5 at −610 ppm.

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