Lattice oxygen mobility and acidity improvements of NiO–CeO2–ZrO2 catalyst by sulfation for NOx reduction by ammonia

Different amounts of sulfates were introduced to NiO–CeO2–ZrO2 catalyst by a citric-aided sol–gel method for NOx reduction with ammonia. The catalysts were characterized by X-ray diffraction, Raman, H2 temperature programmed-reduction, NH3 temperature-programmed oxidation, DRIFTS of NH3/NO adsorption, oxygen storage capacity and activity measurements for selective catalytic reduction of NOx with ammonia. The results show that the interactions between sulfates and NiO–CeO2–ZrO2 solid solution lead to an enhanced Lewis acidity of the catalyst due to the electron withdrawing effect of sulfates mainly on Nin+. The addition of sulfates decreases the amount of readily available oxygen on the catalyst, but promotes the lattice oxygen mobility by the formation of Nin+OSn+ bond in CeO2–ZrO2 mixed oxides. The ammonia oxidation is inhibited by the reduced amount of surface active oxygen, while the adsorption of ammonia on the catalyst is promoted with the sulfate-derived acid sites. Meanwhile, the oxidation of nitrites to nitrates on the sulfated catalyst at the temperature interval for high NOx conversion is accelerated by the increased lattice oxygen mobility. These effects result in a high NH3-SCR activity and high selectivity to nitrogen over the sulfated NiO–CeO2–ZrO2 catalysts.

The acid sites arising from sulfates act as the main ammonia adsorption sites. The mobility of oxygen is promoted by sulfate to facilitate NO (nitrites) oxidation to NO2 (nitrates). In this way, a “fast SCR route” occurs more easily on the sulfated NiO–CeO2–ZrO2 catalyst.Figure optionsDownload high-quality image (102 K)Download as PowerPoint slideHighlights
► The modification of sulfate enhances the Lewis acidity of the catalyst.
► Ammonia oxidation is inhibited due to the reduced surface oxygen after sulfation.
► Ammonia adsorption is promoted on the sulfate-derived acid sites.
► Increased lattice oxygen mobility promotes the oxidation of nitrites to nitrates.