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Performance of Modified LaxSr1-xMnO3 Perovskite Catalysts for NH3 Oxidation: TPD, DFT and Kinetic Studies Dong Wang, Yue Peng, Qilei Yang, Shangchao Xiong, Junhua Li, and John C. Crittenden Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01352 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Performance of Modified LaxSr1-xMnO3 Perovskite Catalysts for NH3
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Oxidation: TPD, DFT and Kinetic Studies
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Dong Wanga, c, Yue Penga, c*, Qilei Yanga, Shangchao Xionga, Junhua Lia, c, and John
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Crittendenb
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a
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of Environment, Tsinghua University, Beijing, 100084, China
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b
State Key Joint Laboratory of Environment Simulation and Pollution Control, School
School of Civil and Environmental Engineering and the Brook Byers Institute for
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Sustainable Systems, Georgia Institute of Technology, 800 West Peachtree Street, Suite
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400 F-H, Atlanta, Georgia, 30332-0595, United States
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c
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and Equipment, Beijing, 100084, China
National Engineering Laboratory for Multi Flue Gas Pollution Control Technology
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*Corresponding author.
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Phone: +86 010 62782030
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E-mail address:
[email protected] (Yue Peng)
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Abstract
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The modified perovskites (LaxSr1-xMnO3) were prepared using the selective
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dissolution method for the selective catalytic oxidation (SCO) of NH3. We found that
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more Mn4+ cations and active surface oxygen species formed on the catalysts surface
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with increasing the dissolution time (dis). The 1h-dis catalyst exhibited excellent NH3
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conversion, and it performed well in the presence of SO2 and H2O. The 10h-dis and
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72h-dis catalysts produced considerable N2O and NO at high temperatures, while they
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were not detected from the fresh catalyst. Both temperature-programmed experiments
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and density functional theory calculations proved that NH3 strongly and mostly bonded
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to the B-site cations of the perovskite framework rather than A-site cations: this
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framework limited the bonding of SO2 to the surface. The reducibility increased
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superfluously after more than 10 h of immersion. The adsorptions of NH 3 on Mn4+
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exposed surface were stronger than that on La3+ or Sr4+ exposed surfaces. The selective
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catalytic reduction, non selective catalytic reduction and catalytic oxidation reactions
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all contributed to NH3 conversion. The formed NO from catalytic oxidation preferred
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to react with -NH2/-NH to form N2/N2O.
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Graphical Abstract
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1. Introduction
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Selective catalytic reduction (SCR) technology is the state-of-art method removing
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NOx from power plant and industrial boilers flue gas.1-7 However, the 100 % efficiency
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of the SCR installation cannot be achieved, because the NH3/NOx inlet ratios usually
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kept lower than 0.8 to minimize the NH3 slip. So far, the NH3 slip has become an
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important factor limiting the improvement of atmospheric quality. One solution for
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improving SCR performance of NOx abatement is increasing the inlet NH3/NOx feed
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ratio in the SCR reactor and adding a layer of catalyst that can selectively oxidize
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residual NH3 to N2 in the presence of excess O2, such as a selective catalytic oxidation
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(SCO) catalyst.8, 9 The SCO process is located downstream of the SCR catalysts, and
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low temperature operation of the SCO is often required. Therefore, many efforts have
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been made in order to develop catalysts working at relatively low temperature and low
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cost.10-13 Based on R&D experiences with NH3-SCR catalysts, the high oxidation
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ability at low temperature is determined by catalysts redox properties and number of
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surface-active oxygen. Surface acidity is another critical factor for catalysts capturing
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NH3 from flue gas.14-16 In addition to these chemical features, resistance to fouling by
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SO2 and H2O and thermal stability, are necessary for the catalysts to perform well in
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practice.
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Perovskite oxides (ABO3) exhibit excellent redox property, hydrothermal stability
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and low cost. Furthermore, perovskite oxides have considerable number of options
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when we consider the various elements that were used for A- and/or B-site. The
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substitution leads to the improvement of surface defects and variable oxidation states
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of the B-site.17,
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Perovskites usually possess low surface area compared with other metal oxides
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catalysts. Furthermore, their native surfaces are preferentially occupied by A-site such
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However, they faced some defects in environmental catalysis.
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as La, Sr and Ba.19 If A-site cations were selectively removed in part leaving the redox-
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active B-site cations were exposed, then the catalyst performance could be improved
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significantly.
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Recently, we applied a simple method using acid solutions on the as-prepared
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perovskite oxides.20-22 It could selectively dissolve A-site cations from the perovskite
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oxides and leave B-site cations exposed on the outermost of catalyst surface. This
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allowed us to tune the ratios of La/Sr and the percentage of La+Sr on Sr doped LaMnO3
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or LaCoO3 catalysts for the NOx storage and reduction for toluene catalytic combustion.
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Further, the framework of perovskites can be also modified (perovskite or simple metal
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oxides) using different acid density and immersion time. However, there are still some
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problems that need to be clarified before it can be used in SCO reaction. The resistance
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of the catalyst to H2O and SO2 should be evaluated; the adsorption of NH3 on catalyst
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surface should be studied; and, the reaction process and/or kinetic features should be
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elucidated.
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In this work, we prepared different perovskite catalysts by the selective dissolution
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method for SCO and test their performance in terms of temperature, H2O and SO2
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concentrations. We used both chemical characterizations and density functional theory
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(DFT) calculations to elucidate their catalytic properties and NH3 adsorption behaviors.
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We also performed kinetic studies on the catalysts during different gaseous
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concentrations to elucidate the NH3 oxidation process in the presence excess of O2.
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2. Experimental Section
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2.1 Catalyst Preparation
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The LaxSr1-xMnO3 catalyst was synthesized by a hydrothermal method as indicated
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in our previous references. 29 This sample without HNO3 treatment was denoted as the
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“fresh” catalyst. After crushing the fresh catalyst into powder (
