Development of LnMnO_3+σ perovskite on low temperature Hg⁰ removal

Abstract

LnMnO_3+σ (Ln = La, Pr, Nd, Sm, Eu, Gd or Dy) perovskites synthesized by sol-gel method were employed for gaseous elemental mercury (Hg⁰) removal from coal-fired flue gas. Characterization results revealed the structure of the perovskites presented a phase transition process from rhombohedral system to O- and O'-orthorhombic structure with the change of A-site rare earth elements. The perovskites showed satisfactory Hg⁰ removal capacity in a narrow temperature range of 100–150°C. NdMnO_3+σ with an O-O’ orthorhombic structure presented the best Hg⁰ removal performance, which markedly depends on four factors: crystal structure, oxygen vacancy density, Mn⁴⁺/Mn³⁺ ratio and surface element segregation. The Hg⁰ removal mechanism was illustrated based on the mercury temperature programmed desorption experiment and X-ray photoelectron spectroscopy characterization. Both chemisorption and catalytic oxidation played a role in the Hg⁰ removal process. Chemisorption dominated the Hg⁰ removal, due to the slow catalytic oxidation rate at low temperature. This work preliminarily established the relation between the structure of rare earth manganese perovskite and Hg⁰ removal performance.

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 (Hg_p); vapor-phase elemental mercury (Hg⁰) and vapor-phase oxidized mercury (Hg²⁺) (Lopez-Anton et al., 2010). Hg_p 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 NO_x (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 NO_x 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.