Development of LnMnO3+σ perovskite on low temperature Hg0 removal
Graphical abstract
Introduction
Atmospheric pollution caused by mercury is gaining high attention and coal combustion contributes as one of the main sources of mercury emissions. The mercury in coal-fired flue gas mainly exists in three species: particle-bound mercury (Hgp); vapor-phase elemental mercury (Hg0) and vapor-phase oxidized mercury (Hg2+) (Lopez-Anton et al., 2010). Hgp and Hg2+ can be controlled using existing air pollution control devices. However, it is difficult to remove Hg0 due to its volatile and insoluble characteristics (Pavlish et al., 2003). Adsorption and catalytic oxidation are main methods for the removal of Hg0 (Cao et al., 2021; Yang et al., 2020). Adsorption method is achieved through adsorbent injection technology. Modified activated carbon is the most commonly-used adsorbent at commercial level. However, it has the drawbacks of high cost and adverse impact of fly ash recovery (Brown et al., 2000). The development of more economical mercury-removing adsorbents and oxidation catalyst should be given attention.
Manganese oxides have been extensively utilized for Hg0 removal, due to their environmentally benign nature and for being economical (Qiao et al., 2009; Xu et al., 2015). Therefore, modification of manganese oxide for catalytic processes has attracted much attention. The valence state of manganese and surface adsorbed oxygen species in manganese oxide are important for the removal of Hg0. Perovskite materials have been widely studied and applied in heterogeneous catalysis owing to their mixed valence states of cations, lattice defects and high oxygen mobility, which are favorable for the removal of Hg0 (Royer et al., 2014). Perovskite structure oxides have a general molecular formula ABO3. The A site cation is generally a rare earth element, while the B site is commonly occupied by a transition metal cation and is the main catalytical active site. In the field of environmental catalysis, rare earth manganese perovskites have proven to be excellent catalysts, such as for the oxidation of CO and the removal of NOx (Zhu and Thomas, 2009). Whilst, there are still limited investigations on the application of perovskite in Hg0 removal. Xu et al. (2016a, 2016b, 2016c) studied the Hg0 removal performance of LaMnO3 as catalyst carrier or active component and concluded that higher surface concentration of active adsorbed oxygen species and Mn4+ result in better Hg0 removal performance. Zhou et al. (2016) improved the Hg0 removal performance of LaMnO3 by doping with Sr at A site to increase the surface concentration of adsorbed oxygen and Mn4+. The doping of other elements at A and B sites was also investigated by Yang et al. (2018). They proposed that the surface manganese concentration is more important for catalysis than the adsorbed oxygen concentration. Apart from these studies, few researches systematically investigated the relationship between the rare earth manganese perovskite structure and Hg0 removal performance.
Although the A-site cation in rare earth perovskites is catalytically inactive, the properties of A-site cation will affect the electronic state of the B-site cations and lattice defect, which is related to the redox property of perovskite oxides (Nitadori et al., 1988). Rare earth elements have similar chemical properties with different ionic radius and charge densities. The comparison of different lanthanides as A-site cation can facilitate the investigation of the influence of subtle structural changes on catalytic activity. Baiker et al. (1994) found that the nature of different rare earth ions at the A site strongly affects redox properties of perovskites. Raj (1980) established relationship between the activation energy of NOx catalytic decomposition and the lattice parameters of different rare earth perovskites. On the other hand, Zhang et al. (2018) compared the Hg0 catalytic oxidation activity of LnMnO3 (Ln = La, Ce or Pr) but emphasized on the influence of reaction conditions on activity. In present work, we studied the effect of rare earth elements-substitution at A-site on structure, surface property and Hg0 removal activity of LnMnO3 perovskite.
Herein, LnMnO3 (Ln = La, Pr, Nd, Sm, Eu, Gd or Dy) perovskite-type oxides were synthesized by using the sol-gel method for the removal of Hg0. Comprehensive structure characterizations of the LnMnO3 were performed to compare surface and bulk properties. The effect of temperature and gas components were investigated to check the practical application. The Hg0 removal mechanism was discussed on the basis of mercury desorption experiment and characterization of fresh and used perovskites oxide. The structural and property factors that affect the Hg0 removal performance are demonstrated. LnMnO3 perovskites exhibit satisfactory Hg0 removal capacity at low temperature.
Section snippets
Samples preparation
LnMnO3 was synthesized by ethylenediamine tetraacetic acid (EDTA)-citric acid (CA) sol-gel method (Zhou et al., 2006). The stoichiometric amount of Mn(NO3)2 and Ln(NO3)3 (Ln = La, Pr, Nd, Sm, Eu, Gd, or Dy) were dissolved in deionized water. The citric acid and EDTA were dissolved in NH3 solution, and then mixed with metal nitrate solution under stirring. The molar ratios of total metal ions, citric acid and EDTA were 1:2:1. The pH value of the mixed solution was adjusted around 6 by adding
Structure and morphology
The perovskite structure synthesized at different temperatures was investigated. The TG tests of the gel precursors in air atmosphere are presented in Fig. 1a and the thermogravimetric curves are immensely similar. The main weight loss occurs before 500°C and can be attributed to the complete decomposition of nitrate and organic component (Huang et al., 2000). Another slight weight loss will occur at the temperature varies from 650 to 800°C depending on samples. To alleviate the Jahn–Teller
Conclusions
In this study, the Hg0 removal performance of lanthanide manganese perovskites LnMnO3 (Ln = La-Dy) was investigated. The crystal structure of the LnMnO3+σ perovskites from La to Dy underwent a R-O-O' transition process with decreasing excess oxygen content. NdMn exhibited the O-O' transition state structure with a higher Mn4+/Mn3+ ratio and oxygen vacancy density. LnMnO3+σ perovskites presented satisfactory Hg0 removal capacity at low temperature of 100-150°C and NdMnO3+σ exhibited the best
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51778229) and the Fundamental Research Funds for the Central Universities (No. JKB012015019).
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