Abstract
Lanthanide (La, Ce, Nd, Gd, Tb, Ho or Lu)-doped Cu-SAPO-18 samples were prepared using the ion-exchange method. Physicochemical properties of the samples were systematically characterized by a number of analytical techniques, and the effects of lanthanide doping on catalytic activity and hydrothermal stability of the Cu-SAPO-18 catalysts for the NH3-SCR reaction were examined. It is shown that the doping of lanthanide elements could affect the interaction between the active components (copper ions) and the AEI-structured SAPO-18 support. The inclusion of some lanthanides significantly slowed down hydrolysis of the catalyst during hydrothermal aging treatment process, leading to an enhanced catalytic activity at both low and high temperatures and hydrothermal stability. In particular, Ce doping promoted the Cu2+ ions to migrate to the energetically favorable sites for enhancement in catalytic activity, whereas the other lanthanide ions exerted little or an opposite effect on the migration of Cu2+ ions. Additionally, Ce doping could improve hydrothermal stability of the Cu-SAPO-18 catalyst by weakening hydrolysis of the catalyst during the hydrothermal aging treatment process. Ce doping increased the catalytic activity of Cu-SAPO-18 at low and high temperatures, which was attributed to modifications of the redox and/or isolated Cu2+ active centers.
Highlights
Nitrogen oxides (NOx ) are one of the main pollutants to the atmosphere
The decrease in NO conversion might be caused by the reaction of partial amount of NH3 with O2 to form NOx, which reduced the conversion of nitrogen oxides
It is interesting to note that the Ce-Cu-SAPO-18 and Tb-Cu-SAPO-18 catalysts showed higher activities than the Cu-SAPO-18 catalyst in the whole temperature range, which might be due to the synergistic effect
Summary
Nitrogen oxides (NOx ) are one of the main pollutants to the atmosphere. NOx is emitted from fossil fuels burning, nitric acid and plating industries, and automotive vehicles [1,2], in which NOx emission from vehicle exhaust has become a major source of air pollutants [3]. Catalysts 2020, 10, 336 widely used in the NH3 -SCR reaction [7] These catalyst systems, have some disadvantages, for example, generation of NOx due to oxidation of ammonia at high temperatures, narrow activity window, and low hydrothermal resistance [8,9]. It is urgent to develop catalysts with good hydrothermal resistance, wide active windows, and high nitrogen selectivity. Copper-based zeolites with small pore structures (such as Cu-SSZ-13 and Cu-SAPO-34), especially the CHA structures, have attracted much attention in eliminating NOx pollution since they possess good hydrothermal aging stability and NH3 -SCR performance [13,14]. Our team found that Cu-SAPO-18 possessed excellent performance for the SCR of NOx , excellent hydrothermal aging stability and poisoning hydrocarbons resistance [15]. The interaction between copper and lanthanide was clarified by the XPS characterization, and reduction behaviour of the copper species in the catalyst was measured by the H2 -TPR technique
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