Mechanism of Hg(0) oxidation in the presence of HCl over a commercial V2O5-WO3/TiO2 SCR catalyst

Mercury removal using various modified V/Ti-based SCR catalysts: A review

Growing levels of mercury pollution has made countries urgently need a suitable mercury treatment technology. Among various technologies, heterogeneous oxidative mercury removal via different modified V/Ti-based SCR catalysts is considered as a promising approach due to excellent economic value and removal efficiency. Although various related modification experiments have been worked in recent years, the research on the performance, including activity and resistance, and mechanism of catalysts still needs to be improved, so it is necessary to summarize these experiments to guide further work. This article will review many modifications start from the V/Ti catalyst. Not only the performance of these catalysts, but also a lot of speculation about the mercury removal mechanism are include in our research. In addition, the characteristics of some modified catalysts have been linked with their oxidation mechanism and structural changes by comparing many studies, and finally attributed to some special properties of the corresponding modifiers. We expect this study will clarify the research progress of modified V/Ti-based SCR catalysts in mercury removal, and guide future modification so that some properties of the catalyst can be improved in a targeted manner.

A review on removal of mercury from flue gas utilizing existing air pollutant control devices (APCDs)

Mercury is a highly toxic heavy metal pollutant. It is of great significance to develop cost-effective mercury pollution control technologies of coal-fired flue gas. Among various mercury from flue gas removal methods, the application of existing air pollution control devices (APCDs) to remove mercury from flue gas is one of the most valuable methods because it doesn't need to install additional mercury removal equipment, reducing the cost of mercury removal. This review summarizes the recent progress of mercury from flue gas removal by APCDs (e.g., SCR denitration device, WFGD system and dust removal device). SCR denitration device can achieve partial removal of mercury in flue gas through combined with WFGD system, but easy inactivation and poor sulfur/water/heavy metals resistance of SCR catalyzers are still the main problems. WFGD systems can remove most of Hg²⁺ (80%-95%), but have low treatment ability for Hg⁰. Various oxidants can effectively oxidize Hg⁰ into Hg²⁺. However, traditional oxidants have high prices and secondary pollution due to the formation of by-products. Fabric filters (FFs), electrostatic precipitators (ESPs) and hybrid fabric filters (HFs) can all control the emission of mercury in the flue gas to a certain extent, especially can effectively remove most of Hg^P and part of Hg²⁺, but has low removal capacity for Hg⁰. Compared with ESP, FF has better capture efficiency for Hg²⁺ and Hg⁰, and a combination of ESP and FF, that is HF, can effectively improve the mercury removal capacity.

Recent advances in layered double hydroxides (LDHs) derived catalysts for selective catalytic reduction of NOx with NH3

In recent years, layered double hydroxides (LDHs) derived metal oxides as highly efficient catalysts for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) have attracted great attention. The high dispersibility and interchangeability of cations within the brucite-like layers make LDHs an indispensable branch of catalytic materials. With the increasingly stringent and ultra-low emission regulations, there is an urgent need for highly efficient and stable low-medium temperature denitration catalysts in markets. In this contribution, we have critically summarized the recent research progress in the LDHs derived NH₃-SCR catalysts, including their ability for NO_x removal, N₂ selectivity, active temperature window, stability and resistance to poisoning. The advantages and defects of various types of LDHs-derived catalysts are comparatively summarized, and the corresponding modification strategies are discussed. In addition, considering the importance of the catalyst's resistance to poisoning in practical applications, we discuss the poisoning mechanism of each component in flue gases, and provide the corresponding strategies to improve the poisoning resistance of catalysts. Finally, from the perspective of practical applications and operation cost, the regeneration measures of catalysts after poisoning is also discussed. We hope that this work can give timely technical guidance and valuable insights for the applications of LDHs materials in the field of NO_x control.

Effect of Al2O3 doping on the structure and performance of an Al2O3/Fe2O3 catalyst for mercury oxidation

In this study, the thermal stability of a Fe₂O₃ catalyst for mercury oxidation was significantly improved by doping with Al₂O₃. After 1 hr, the catalyst doped with 10 wt.% Al₂O₃ still exhibited a mercury conversion efficiency of 70.9%, while the undoped sample even lost its catalytic activity. Doping with Al₂O₃ retarded the collapse of the catalyst mesoporous structure during high-temperature calcination, and the doped samples maintained a higher specific surface area, smaller pore size, and narrower pore size distribution. Transmission electron microscope images revealed that after calcination at 350°C, the average size of the catalyst grains in Fe₂O₃ was 23.4 nm; however, the corresponding values for 1%Al₂O₃/Fe₂O₃, 3%Al₂O₃/Fe₂O₃, and 10%Al₂O₃/Fe₂O₃ were only 13.3, 7.1, and 4.7 nm, respectively. Results obtained from X-ray diffraction and thermogravimetry coupled with differential scanning calorimetry confirmed that doping with Al₂O₃ also retards the crystallization of the catalysts at high temperature, constraining catalyst grains to a smaller size.

Catalytic conversion of mercury over Ce doped Mn/SAPO-34 catalyst: Sulphur tolerance and SO2/SO3 conversion

A series of Mn-Ce/SAPO-34 catalysts were prepared to study the catalytic oxidisation of elemental mercury (Hg⁰). Sulphur tolerance and SO₃ formation over the catalyst were studied further. Hg⁰ was transported by compressed air from PSA Cavkit. NO, SO₂, and NH₃ are standard gases, and H₂O is produced by gas carrying. Mn could incorporate into the cerium oxide lattice to form capping oxygen and well-dispersed high valance manganese ions after the addition of Ce, which was conducive to NO removal and Hg⁰ oxidisation. 9 Mn-9Ce showed the best performance regarding Hg⁰ conversion, achieving more than 92% Hg⁰ conversion efficiency at 50-300 °C. The sulphur resistance of the Mn-based catalyst was significantly improved after the addition of cerium due to the high affinity of Ce for SO₂, and the relative content of HgSO₄ was exceeded 72% on the 9 Mn-9Ce catalyst with SO₂; SO₃ formation over the 9 Mn-9Ce decreased by 17% compared with the 9 Mn. H₂O not only reduced the available active site, but also decreased the oxidation rate of SO₂. The active sites were preferentially occupied by NH₃ rather than Hg⁰ and SO₂, generated NH₄⁺ occupied cation vacancies. Therefore, both H₂O and NH₃ have inhibitory effects on Hg⁰ conversion and SO₃ formation.

A complete catalytic reaction scheme for Hg0 oxidation by HCl over RuO2/TiO2 catalyst

RuO₂-based catalysts have attracted great attention in mercury emission control region due to their outstanding catalytic activity and long-term stability. Quantum chemistry calculation was performed to uncover the atomic-scale reaction mechanism of Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst. The results indicate that Hg⁰ adsorption on RuO₂/TiO₂(110) surface is controlled by a weak chemisorption mechanism. The 5-fold coordinated surface Ru atom is identified as the active center for Hg⁰ adsorption. HgCl molecule serves as an intermediate connecting reactant state to product state. The weak interaction between HgCl₂ and catalyst surface is favorable for product desorption. HCl activation is an O-assisted surface reaction process in which HCl is oxidized into active Cl atom for Hg⁰ oxidation. The heterolytic cleavage of HCl molecule occurs without noticeable activation energy barrier. Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst proceeds through two independent reaction channels. The dominant reaction channel of Hg⁰ oxidation is identified as a four-step process. Finally, a complete catalytic cycle that can produce the correct stoichiometry was proposed to understand the heterogeneous reaction mechanism of Hg⁰ oxidation over RuO₂/TiO₂ catalyst. The catalytic cycle consists of HCl activation, mercury oxidation and surface reoxidation. Mercury oxidation is the rate-determining step of the catalytic cycle.

Mechanism of Mercury Adsorption and Oxidation by Oxygen over the CeO₂ (111) Surface: A DFT Study

CeO₂ is a promising catalytic oxidation material for flue gas mercury removal. Density functional theory (DFT) calculations and periodic slab models are employed to investigate mercury adsorption and oxidation by oxygen over the CeO₂ (111) surface. DFT calculations indicate that Hg⁰ is physically adsorbed on the CeO₂ (111) surface and the Hg atom interacts strongly with the surface Ce atom according to the partial density of states (PDOS) analysis, whereas, HgO is adsorbed on the CeO₂ (111) surface in a chemisorption manner, with its adsorption energy in the range of 69.9–198.37 kJ/mol. Depending on the adsorption methods of Hg⁰ and HgO, three reaction pathways (pathways I, II, and III) of Hg⁰ oxidation by oxygen are proposed. Pathway I is the most likely oxidation route on the CeO₂ (111) surface due to it having the lowest energy barrier of 20.7 kJ/mol. The formation of the HgO molecule is the rate-determining step, which is also the only energy barrier of the entire process. Compared with energy barriers of Hg⁰ oxidation on the other catalytic materials, CeO₂ is more efficient at mercury removal in flue gas owing to its low energy barrier.

Possibilities of mercury removal in the dry flue gas cleaning lines of solid waste incineration units

Dry methods of the flue gas cleaning (for HCl and SO2 removal) are useful particularly in smaller solid waste incineration units. The amount and forms of mercury emissions depend on waste (fuel) composition, content of mercury and chlorine and on the entire process of the flue gas cleaning. In the case of high HCl/total Hg molar ratio in the flue gas, the majority (usually 70-90%) of mercury is present in the form of HgCl2 and a smaller amount in the form of mercury vapors at higher temperatures. Removal of both main forms of mercury from the flue gas is dependent on chemical reactions and sorption processes at the temperatures below approx. 340 °C. Significant part of HgCl2 and a small part of elemental Hg vapors can be adsorbed on fly ash and solid particle in the air pollution control (APC) processes, which are removed in dust filters. Injection of non-impregnated active carbon (AC) or activated lignite coke particles is able to remove mainly the oxidized Hg(2+) compounds. Vapors of metallic Hg(o) are adsorbed relatively weakly. Much better chemisorption of Hg(o) together with higher sorbent capacity is achieved by AC-based sorbents impregnated with sulfur, alkali poly-sulfides, ferric chloride, etc. Inorganic sorbents with the same or similar chemical impregnation are also applicable for deeper Hg(o) removal (over 85%). SCR catalysts convert part of Hg(o) into oxidized compounds (HgO, HgCl2, etc.) contributing to more efficient Hg removal, but excess of NH3 has a negative effect. Both forms, elemental Hg(o) and HgCl2, can be converted into HgS particles by reacting with droplets/aerosol of poly-sulfides solutions/solids in flue gas. Mercury captured in the form of water insoluble HgS is more advantageous in the disposal of solid waste from APC processes. Four selected options of the dry flue gas cleaning with mercury removal are analyzed, assessed and compared (in terms of efficiency of Hg-emission reduction and costs) with wet methods and retrofits for more efficient Hg-removal. Overall mercury removal efficiencies from flue gas can attain 80-95%, depending on sorbent type/impregnation, sorbent surplus and operating conditions.

Removal of gas-phase Hg0 by Mn/montmorillonite K 10

Mn/montmorillonite K 10 (Mn/MK10) prepared by an impregnation method was studied to remove Hg⁰ in simulated coal-fired flue gas. ​4% Mn/MK10 was the optimal sample with outstanding Hg⁰ removal efficiency over the temperature range of 100–400 °C.

Role of NO in Hg(0) oxidation over a commercial selective catalytic reduction catalyst V2O5-WO3/TiO2

Experiments were conducted in a fixed-bed reactor that contained a commercial catalyst, V2O5-WO3/TiO2, to investigate mercury oxidation in the presence of NO and O2. Mercury oxidation was improved by NO, and the efficiency was increased by simultaneously adding NO and O2. With NO and O2 pretreatment at 350°C, the catalyst exhibited higher catalytic activity for Hg(0) oxidation, whereas NO pretreatment did not exert a noticeable effect. Decreasing the reaction temperature boosted the performance of the catalyst treated with NO and O2. Although NO promoted Hg(0) oxidation at the very beginning, excessive NO counteracted this effect. The results show that NO plays different roles in Hg(0) oxidation; NO in the gaseous phase may directly react with the adsorbed Hg(0), but excessive NO hinders Hg(0) adsorption. The adsorbed NO was converted into active nitrogen species (e.g., NO2) with oxygen, which facilitated the adsorption and oxidation of Hg(0). Hg(0) was oxidized by NO mainly by the Eley-Rideal mechanism. The Hg(0) temperature-programmed desorption experiment showed that weakly adsorbed mercury species were converted to strongly bound ones in the presence of NO and O2.