Reaction mechanism of elemental mercury oxidation to HgSO4 during SO2/SO3 conversion over V2O5/TiO2 catalyst

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Abstract

Experiments and density functional theory calculations were conducted to uncover the reaction chemistry of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst. The results show that SO2 promotes Hg0 oxidation over V2O5/TiO2 catalyst with the assistance of oxygen. The promotional effect is dependent on the reaction temperature, and is associated with the bimolecular reaction between Hg0 and SO3 over V2O5/TiO2 catalyst. SO2 can be oxidized to SO3 which has high oxidation ability for Hg0 oxidation. SO2/SO3 conversion proceeds through a three-step reaction process in the sequence of SO2 adsorption → SO2 oxidation → SO3 desorption. SO2 oxidation presents an activation energy barrier of 223.84 kJ/mol. HgSO4 species is formed from the bimolecular reaction between Hg0 and SO3 over V2O5/TiO2 catalyst. Hg0 oxidation by SO3 over V2O5/TiO2 catalyst occurs through three reaction pathways, which are energetically favorable for HgSO4 formation. SO2* → SO3* is identified as the rate-determining step of HgSO4 formation. During Hg0 oxidation by SO3 over V2O5/TiO2 catalyst, HgSO4 desorption is a highly endothermic reaction process and requires a higher external energy. The proposed skeletal reaction network can be used to well understand the reaction mechanism of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst.

Introduction

Mercury emission from stationary combustion sources has caused a series of human and ecosystem health problems because of its toxicity and bioaccumulation property [1]. Recently, a more stringent limited value of 1 µg/m3 has been suggested to reduce mercury emission from the stationary combustion source. Moreover, the Minamata Convention on Mercury, an international convention, entered into force in August 2017. Consequently, mercury abatement in stationary combustion sources is an urgent task, which is dependent on the understanding of mercury transformation chemistry [2].

Mercury removal efficiency of stationary combustion sources depends on mercury speciation in flue gas [3]. Mercury species in flue gas includes elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp) [4]. Hg0 is very difficult to remove using the conventional air pollutant control devices (APCD) because of its low solubility and high volatility [5,6]. However, Hg2+ can be easily removed by the wet flue gas desulfurization (WFGD) system [7,8]. Therefore, mercury can be effectively removed from flue gas using WFGD if Hg0 is oxidized to Hg2+.

Catalytic oxidation of Hg0 to Hg2+ is a cost-effective technology to control mercury emission from flue gas [9,10]. The selective catalytic reduction (SCR) devices have been widely equipped for NO reduction. Meanwhile, Hg0 can be catalytically oxidized to Hg2+ in the SCR devices. However, Hg0 oxidation in the channels of honeycomb SCR catalyst (V2O5 species dispersed on TiO2 support) is controlled by a series of complicated chemical reactions. Therefore, the fundamental understanding of reaction mechanism that controls Hg0 oxidation in the SCR devices is very important for mercury emission control.

In recent years, some studies have been conducted to investigate the reaction mechanism of Hg0 oxidation over V2O5/TiO2 catalyst [11], [12], [13], [14]–15]. These studies mainly focus on the Hg/Cl, Hg/Br and Hg/O reaction sub-mechanisms. SO2 is one of the important acid flue gas components and can be oxidized to SO3 over V2O5-based SCR catalyst [16]. SO3 produced from SO2 oxidation can react with Hg0 to form HgSO4 species [17], and HgSO4 is one of the dominant oxidized mercury species [18,19]. Obviously, HgSO4 formation over SCR catalyst is dominated by the Hg0/SO3 reaction mechanism. However, to date, no attempts have been performed to understand the heterogeneous Hg0/SO3 reaction mechanism over SCR catalyst. In addition, the detailed coupling reaction process of Hg0 oxidation and SO2/SO3 conversion over SCR catalyst remains unknown.

In this work, experiments were carried out to investigate Hg0 oxidation over V2O5/TiO2 catalyst, and mercury species produced from Hg0 oxidation was identified using the temperature-programmed desorption (TPD) method. Density functional theory (DFT) calculations were performed to understand the coupling reactions of SO2/SO3 conversion and Hg0 oxidation to HgSO4 over V2O5/TiO2 catalyst. Finally, a comprehensive reaction network was proposed to uncover the reaction mechanism of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst. To the best of the authors’ knowledge, this is the first study exploring the heterogeneous Hg0/SO3 reaction mechanism over V2O5/TiO2 catalyst.

Section snippets

Experimental methods

It is well-known that the V2O5 content of commercial SCR catalyst is 1 wt%. Therefore, 1 wt% V2O5/TiO2 catalyst was synthesized using the impregnation method. The required amount of NH4VO3 was dissolved in oxalic acid solution at room temperature. The molar ratio of NH4VO3: oxalic acid was 1:1. Subsequently, TiO2 nanoparticles were added to the above mixed solutions under the magnetic stirring condition. After impregnation for 24 h, water of the slurry was vaporized at 90 °C. The solid sample

Hg0 oxidation experiments

Experiments were first conducted to investigate Hg0 oxidation over V2O5/TiO2 catalyst in the reaction atmosphere (SO2 + O2 + CO2 + N2). The transient measurements of both Hg0 and Hg2+ at the outlet of the catalyst were used to distinguish between adsorption and oxidation, as shown in Fig. 1. It can be seen that SO2 weakly promotes Hg0 oxidation in the presence of oxygen. It was reported that Hg0 deposits as HgSO4 [18,25]. Moreover, SO3 produced from SO2 oxidation shows high oxidation ability

Conclusions

The reaction chemistry of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst was investigated using experiments and density functional theory calculations. SO2 shows a promotional effect on Hg0 oxidation over V2O5/TiO2 catalyst with the assistance of oxygen. The promotional effect of SO2 depends on the reaction temperature. At the lower temperatures (150–200 °C), the promotional effect increases with increasing reaction temperature, whereas at the higher temperatures (200–400 °C),

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Key Research and Development Program of China (2018YFC1901303), Fundamental Research Funds for the Central Universities (2019kfyRCPY021), China Postdoctoral Science Foundation (2018M640697), and Program for HUST Academic Frontier Youth Team (2018QYTD05). The authors also thank the Analytical and Testing Center in Huazhong University of Science and Technology for the measurement.

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