Lanthanide ions are added to which increases catalytic activity
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Answer:
in metals with low reactivity
Explanation:
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Answer:
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.
Keywords: SAPO-18-incorporated Cu catalyst; lanthanide doping; selective catalytic reduction; NH3-SCR; hydrothermal stability
1. Introduction
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]. Selective catalytic reduction of NH3 (NH3-SCR) over different catalysts has been extensively studied. The SCR of NOx with ammonia is one of the most widely applied emission control technologies [4,5]. The key issue of such a NH3-SCR technology is the availability of high-performance catalysts [6]. In recent decades, the V2O5−WO3/TiO2 catalysts show high catalytic activities and SO2 poisoning resistance, and are widely used in the NH3-SCR reaction [7]. These catalyst systems, however, 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]. Therefore, it is urgent to develop catalysts with good hydrothermal resistance, wide active windows, and high nitrogen selectivity.
In the past two decades, the zeolites with large and medium pore sizes (such as zeolite Y, zeolite beta and ZSM-5) have received much attention [10,11]. The most extensive research has focused on the copper ion-exchanged ZSM-5, which showed good catalytic activity for direct NO decomposition and NH3-SCR reaction [12]. However, a big problem of this catalyst is its poor hydrothermal stability [8]. Nowadays, 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]. The AEI-structure is similar to the CHA-structure. Previously, 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 SAPO-18-supported Cu catalysts, however, have drawbacks of hydrothermal deterioration, narrow activity windows, and nonselective NH3 oxidation which produces NOx and hence restricts further commercial applications [16]. Since lanthanide oxides possess excellent oxygen storage capacity and unique redox properties [17], lanthanide-doped catalysts have been widely investigated for various reactions. Samojeden et al. found that the sample of Fe-modified vermiculite doped with Ho showed better activity as compared with the Cu-modified vermiculite sample [18].
In this work, lanthanide (La, Ce, Nd, Gd, Tb, Ho or Lu)-doped Cu-SAPO-18 samples were prepared by the ion-exchange method. Effects of lanthanide doping on NH3-SCR activity and hydrothermal stability of Cu-SAPO-18 were investigated. The XRD, BET, 27Al NMR, NH3-TPD, and in situ DRIFTS-NH3 adsorption were used to determine the changes in textural properties and acid density of Cu-SAPO-18 possibly induced by the doping of lanthanides. In addition, 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.