Insights into Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxide catalysts for NH3-SCR reaction

Abstract

The presence of alkali metal/alkaline earth metal in fuel gas is one of the important reasons for the deactivation of catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR). In this work, the Na+/Ca2+ poisoning mechanisms of Zr-modified and unmodified La-Mn perovskite oxides for NH3-SCR reaction were studied. It is found that the deposition of Na+ into LaMnO3 (LM) catalyst has a much larger effect than that of Ca2+, while such a result is opposite for the Na+/Ca2+-deposited La0.8Zr0.2MnO3 (ZLM) catalysts. The deposition of Na+/Ca2+ compounds can block the pores and affect the adsorption and activation of NH3 on the catalysts. After Zr modification, the catalyst with a higher Mn4+ content shows a better redox property and increased acid sites. These acidic sites can trap Na+, thereby delaying the catalyst poisoning and causing a better activity of ZLM catalyst than LM catalyst. The deposition of Ca2+ in the form of CaCO3 on the catalyst surface leads to a serious pore blockage, showing a poor resistance to Ca2+. After regeneration, the pores of the catalyst are dredged, and the active sites originally occupied by Na+/Ca2+ are released, resulting in an recovery of catalyst activity. The recovery of adsorbed oxygen and Mn4+ content is the key factor for returning the SCR activity.