Promotional catalytic activity and reaction mechanism of Ag-modified CeZrO catalyst for catalytic oxidation of ammonia.

CeZrO composite oxides (molar, x = 0-1.0, interval of 0.2) were prepared by a cetyltrimethylammonium bromide-assisted precipitation method. The enhancement of silver-species modification and catalytic mechanism of adsorption-transformation-desorption process were investigated over the Ag-impregnated catalysts for low-temperature selective catalytic oxidation of ammonia (NH-SCO). The optimal 5 wt.% Ag/CeZrO catalyst presented good NH-SCO performance with >90% NH conversion at temperature (T) ≥ 250°C and 89% N selectivity. Despite the irregular block shape and underdeveloped specific surface area (∼60 m/g), the naked and Ag-modified CeZrO solid solution still obtained highly dispersed distribution of surface elements analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) (mapping), N adsorption-desorption test and X-ray diffraction (XRD). H temperature programmed reduction (H-TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that Ag-modification enhanced the mobility and activation of oxygen-species leading to a promotion on CeO reducibility and synergistic Ag/Ag and Ce/Ce redox cycles. Besides, Ag/AgO clusters could facilitate the formation of surface oxygen vacancies that was beneficial to the adsorption and activation of ammonia. NH-temperature programmed desorption (NH-TPD) showed more adsorption-desorption capacity to ammonia were provided by physical, weak- and medium-strong acid sites. Diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the activation of ammonia might be the control step of NH-SCO procedure, during which NH dehydrogenation derived from NH-species and also internal selective catalytic reduction (i-SCR) reactions were proposed.