Selective catalytic reduction of NOx by ammonia over phosphate-containing Ce0.75Zr0.25O2 solids

• ZP/CZ catalyst presents over 90% NOx conversion with a high space velocity of 3 × 105 h−1 at 250–450 °C.
• The pre-recombination of phosphate and zirconium ions reduces the strong interaction between phosphate and cerium ions.
• The oxygen vacancies facilitate the nitrites/nitrates formation and decomposition on catalyst.
• Ammonium nitrites/nitrates are the important intermediates for NH3-SCR reaction of the catalyst.

A zirconium phosphate @ Ce0.75Zr0.25O2 (ZP/CZ) catalyst was synthesized by impregnating zirconium phosphates on Ce0.75Zr0.25O2 microspheres. Catalysts with different structures were also synthesized by various methods to illustrate the structure advantage of ZP/CZ catalyst in selective catalytic reduction of NOx by ammonia (NH3-SCR). The catalysts were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), BET surface area, Infrared (IR) spectra, Ultraviolet-visible (UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (TPR) and diffuse reflectance infrared Fourier transformed spectra (DRIFTS) of NH3/NOx adsorption. ZP/CZ catalyst presents excellent catalytic activity at 250–450 °C with a good N2 selectivity under high space velocity of 300,000 h−1. Firstly, the zirconium phosphate provides adsorption sites for ammonia on the surface of the catalyst and the cerium sites in the core act as the redox sites for NO oxidation. The pre-combination of zirconium and phosphate can reduce the strong interactions between phosphates and cerium ions. Therefore, the mobility of surface lattice oxygen on Ce0.75Zr0.25O2 catalyst is retained, which is essential to obtain a catalyst with high NH3-SCR activity at low temperatures. Moreover, NH4NO3 has proved to be an important intermediate for the NH3-SCR reaction on the surface of ceria based catalysts. The ZP/CZ catalyst preserves plenty of oxygen vacancies, facilitating the Ce4+ → Ce3+ redox circles and therefore promotes the nitrites/nitrates formation and desorption on catalyst. Two distinct adsorption sites for NH3 and NOx result in a close contact between ads-NH3/NH4+ and ads-NO3−/NO2− species on ZP/CZ catalyst, which can react with NO rapidly with the participation of active surface lattice oxygen.

The close contact of ads-NH3/NH4+ and ads-NO3−/NO2− species on ZP/CZ catalyst can react with NO rapidly with the participation of active surface lattice oxygen, leading to a high activity of catalyst.Figure optionsDownload as PowerPoint slide