# Elemental Mercury Removal over CeO\({}_{2}\)/TiO\({}_{2}\) Catalyst Prepared by Sol-Gel Method
## 1 Introduction  
Mercury (Hg), a severe global concern to human health, is derived from both natural sources and anthropogenic pollution [1; 2; 3]. The natural portion mainly comes from volcanic eruption, forest burning, geogenic activity, soil and vegetation, etc. As for the anthropogenic part, according to the latest Global Mercury Assessment published by United Nations Environment Programme (UNEP) in 2019, about 2200 tons of mercury is emitted into the air as a result of human activities each year [4]. This number is 20% higher than that reported in 2010. In comparison, most of the mercury pollution is released from anthropogenic activities, which results in a 450% increase of mercury concentration in the air. In terms of regional contributions, a declining trend has shown up in the anthropogenic mercury pollution in Europe and North America due to the effective implementation of mercury emission regulations. However, global mercury pollution is still serious because of the continuing growth of mercury emissions from developing countries. At present, nearly half (49%) of all anthropogenic mercury emission around the world occurs in Asia, especially in East and South-East Asia. Besides, South America and Sub-Saharan Africa are also facing serious mercury pollution problems. If the anthropogenic mercury results are classified by industry, it can be found that small-scale gold mining and the combustion of fossil fuels are the main sources, followed by non-ferrous metal production andcement production. Moreover, mercury emission from coal combustion makes up approximately 88% of all mercury emission from fossil fuel combustion. As a result, reducing mercury emission from coal combustion is of great significance for the global mercury pollution control. Mercury can be divided into three forms in the coal-fired boiler flue gas: elemental mercury (Hg\({}^{0}\)), oxidized mercury (Hg\({}^{2+}\)), and particulate-bound mercury (Hg\({}^{\text{P}}\)) [5; 6]. Among them, oxidized mercury and particulate-bound mercury can be effectively removed by air pollution control devices (APCDs), such as wet flue gas desulfurization (WFGD) and particulate matter control devices (PMCDs) [7; 8]. On the contrary, elemental mercury is difficult to be controlled directly and has attracted broad attention among laboratories [9]. In addition, the atmospheric lifetimes of Hg\({}^{2+}\) and Hg\({}^{\text{P}}\) are relatively short because of the high solubility and gravity, respectively. However, unlike other mercury species, gaseous Hg\({}^{0}\) can be carried globally before being deposited onto waters or lands. Hence, the excellent atmospheric transport ability of Hg\({}^{0}\) is another critical factor for assessing its environmental impact. Cerium (Ce) is the most abundant rare earth element. More importantly, due to the superior oxygen storage capacity and the existence of the Ce\({}^{3+}\)/Ce\({}^{4+}\) couple, high elemental mercury removal efficiency could be obtained by using the catalysts or sorbents with the addition of Ce [10; 11; 12]. According to our previous study, Ce modification could improve the Hg\({}^{0}\) removal performance of a honeycomb selective catalytic reduction (SCR) catalyst [8]. The preparation approach adopted in [8] was wet impregnation. A disadvantage of this approach is that it is hard to control the loading value of cerium oxides in the catalyst accurately and actively. Changing the concentration of the precursor (cerium nitrate) solution could only adjust the relative content of Ce. Besides, various catalysts and sorbents were synthesized by impregnation or ultrasonic-assisted impregnation to remove the elemental mercury from the flue gas. E.g., the Mo-Ce/cylindrical activated coke prepared by [PERSON] et al. [13], and the Mn-Ce modified wheat straw chars made by [PERSON] et al. In addition, coprecipitation method was employed by [PERSON] et al. to prepare the novel V/ZrCe\({}_{x}\) catalyst [14]. Different preparation methods could result in different performances of the catalysts.