Catalyst characterization

The resonance frequency was set to 156.5 MHz, the spinning speed to 12 kHz, and a relaxation delay of 1 s was applied.Ammonia temperature-programmed desorption (NH3-TPD) measurements were performed on a BSD-Chem C200 chemisorption analyzer (Beijing BSD Instrument Technology Co., Ltd.). Before the test, approximately 50 mg of the catalyst sample was weighed and placed in a quartz tubular reactor. The sample was pretreated at 300 oC for 30 min under a high-purity Ar flow of 30 mL·min-1, then cooled to 50 oC.

Subsequently, the sample was exposed to a 10 vol% NH3/He mixture for 1 h for adsorption. Afterward, the gas was switched to high-purity He and purged at the same flow rate for 30 min to remove physically adsorbed NH3. The temperature was then increased from 50 oC to 700 oC at a heating rate of 10 oC·min-1 under a He atmosphere.

The desorption signal was monitored using a thermal conductivity detector (TCD), yielding an NH3-TPD profile over the temperature range 100-700 oC.Hydrogen temperature-programmed reduction (H2-TPR) was conducted using the BSD-Chem C200 chemisorption analyzer (Beijing BSD Instrument Technology Co., Ltd.). Approximately 50 mg of the catalyst sample was placed in a quartz tubular reactor and pretreated at 300 oC for 30 min under a high-purity Ar flow (30 mL·min-1), then cooled to 50 oC.

The temperature-programmed reduction was performed in a 10 vol% H2/Ar mixture at a heating rate of 10 oC·min-1 from 50 oC to 800 oC. The signal change in the 200-800 oC range was recorded using a thermal conductivity detector (TCD) to obtain the H2-TPR profile.Nitric oxide temperature-programmed desorption (NO-TPD) tests were also conducted using the BSD-Chem C200 chemisorption analyzer (Beijing BSD Instrument Technology Co., Ltd.).

Approximately 50 mg of catalyst sample was loaded into a quartz tubular reactor and pretreated at 300 oC for 30 min under a high-purity Ar flow (30 mL·min-1), then cooled to 50 oC. The sample was exposed to a 10% NO/He atmosphere for 1 h for adsorption, after which the gas was switched to high-purity He and purged at the same flow rate for 30 min to remove physically adsorbed NO. Subsequently, the temperature was increased from 50 °C to 600 °C at a heating rate of 10 °C·min-1 under a He atmosphere, and the signal change in the range of 50-600 °C was simultaneously recorded using a thermal conductivity detector (TCD) to obtain the NO-TPD profile.Sulfur dioxide temperature-programmed desorption (SO2-TPD) was performed using a Thermo Fisher Antaris IGS infrared gas analyzer.

Approximately 50 mg of the catalyst sample was placed in a quartz tubular reactor and pretreated at 300 °C for 30 min under a high-purity Ar flow of 30 mL·min-1, then cooled to 50 °C. The sample was then exposed to a 2000 ppm SO2/Ar atmosphere for 1 h for adsorption, after which the gas was switched to high-purity Ar and purged at the same flow rate for 30 min. Subsequently, the temperature was increased from 50 °C to 800 °C at a heating rate of 10 °C·min-1 under an Ar atmosphere.

The signal change in the 50-800 °C range was recorded with the IGS detector to obtain the SO2-TPD profile.The specific surface area and pore structure of the samples were characterized using N2 isothermal adsorption-desorption measurements on a Micromeritics ASAP 2460 physisorption analyzer. Before the measurements, the samples were degassed under vacuum at 300 oC for 10 h. N2 adsorption-desorption isotherms were collected in the relative pressure (P/P0) range of 0-1.0.

The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the pore-size distribution was derived from the desorption branch based on the Barrett-Joyner-Halenda (BJH) model.The chemical composition (oxide species and content) of the catalysts was analyzed by X-ray fluorescence spectroscopy (XRF) using a Rigaku ZSX Primus III+ spectrometer. Approximately 300 mg of the catalyst was placed in a 2 cm-diameter sample ring, and the measurement was performed with a scanning time of 10 min.X-ray photoelectron spectroscopy (XPS) of the catalysts was performed on a Shimadzu AXIS SUPRA+ XPS system, using Al Kα radiation (hν = 1486.6 eV) as the excitation source.

All binding energies were calibrated by referencing the C 1s peak of adventitious carbon at 284.80 eV for charge correction. The obtained spectra were fitted using Avantage software, with the full width at half maximum (FWHM) of all peaks constrained to the range 0-3.5 eV.Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) of the catalysts was recorded on a Shimadzu UV-3600 spectrophotometer. Before measurement, an appropriate amount of BaSO4 powder was pressed into the sample holder, and its surface was leveled to serve as the reference background.

Subsequently, a suitable quantity of the catalyst sample was evenly spread over the BaSO4 surface and gently pressed to form a smooth layer. Spectra were collected over the wavelength range of 200-800 nm, with a scan duration of approximately 6 min.In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments were conducted on a Thermo Scientific Nicolet iS50 FT IR spectrometer equipped with a Harrick TDZ4 WS diffuse reflectance cell.

An appropriate amount of catalyst sample was uniformly spread in the sample holder over a KBr bed. The sample was first pretreated at 500 oC under a high-purity Ar flow for 30 min, then cooled to the desired temperature to collect the background spectrum. To investigate the surface-adsorbed species and reaction pathways, four sequential gas-exposure experiments were designed: (1) Exposing the sample to a mixture of 500 ppm NO + 5% O2 for 30 min, purging with Ar for 10 min, and then introducing 500 ppm NH3.

(2) Exposing the sample to 500 ppm NH3 for 30 min, purging with Ar for 10 min, and then introducing a mixture of 500 ppm NO + 5% O2. (3) After pretreatment, background spectra were collected at 200 oC, 300 oC, 400 oC, and 500 oC. Subsequently, a mixture of 500 ppm NH3 and 500 ppm NO + 5% O2 was introduced, and spectra were recorded at 200 oC, 300 oC, 400 oC, and 500 oC, respectively.

(4) SO2 was first introduced at a concentration of 200 ppm for 30 min for adsorption, followed by purging with Ar for 20 min.