Photocatalytic oxidation of gas-phase Hg⁰ on the exposed reactive facets of BiOI/BiOIO₃ heterostructures

Highlights

• The removal efficiency of the as-prepared BiOI/BiOIO₃ photocatalyst reached up to 98.53% and 100% under visible light and UV irradiation respectively.

• The exposure mechanism of the dominant reactive facets of BiOI/BiOIO₃ was analyzed in detail.

• The mechanism of Hg⁰ oxidation by BiOI/BiOIO₃ was proposed.

Abstract

Hetero layered BiOI/BiOIO₃ composites were fabricated by a simple hydrothermal method. The prepared BiOI/BiOIO₃ heterostructures were characterized and evaluated for gas-phase Hg⁰ oxidation. The photocatalytic activity of the BiOI/BiOIO₃ heterostructures was detected under LED light and UV irradiation. Meanwhile, the cycle experiments were carried out to certify the stability of BiOI/BiOIO₃ composites. The results showed that BiOI/BiOIO₃ photocatalysts were of excellent photocatalyic ability with great efficient in oxidizing Hg⁰. The Hg⁰ removal efficiency reached up to 98.53% and 100% under LED light and UV irradiation respectively when the mole ratio of BiOI/BiOIO₃ was 3:1. The photocatalytic reaction mechanism was elaborated in detail.

Introduction

Mercury emitted from coal combustion seriously damages to the environment, and as a toxic pollutants, it can harm to human health through the food chain biological accumulation. United States Environmental Protection Agency (EPA) survey has concluded that coal fired power plant is the largest source of mercury emissions, accounting for 1/3 of anthropogenic mercury emissions [1]. And the control of mercury emissions has caused enough attention [2]. China’s ministry of environmental protection issued a new national regulation for coal-fired power station in 2011, in which mercury emission controlling standard was mentioned for the first time, and the limiting value of mercury emission is less than 0.03 mg/m³ [3]. Hg exists in three chemical forms in coal combustion flue gas: elemental mercury (Hg⁰), oxidized mercury (Hg²⁺), and particle-bound mercury (Hg^p) [4]. Mercury removal from flue gas largely depends on the form of mercury in the flue gas. Hg²⁺ is water-soluble, so it can be removed by wet scrubbers and the removal efficiency can reach up to 90% [5], [6]. Hg^p can be easily removed by electrostatic precipitator (ESP) or fabric filter (FF) [7]. But it is difficult for the existing air pollution control devices(APCDs) to remove Hg⁰ because of its volatility, insolubility and chemical stability [5], so it is a great challenge to remove Hg⁰ from flue gas at the coal combustion power stations. The key is to oxidize Hg⁰ into Hg²⁺ in the flue gas. Recently, the research progress of photocatalytic oxidation removing Hg⁰ gives us a new perspective [8]. Compared with other methods, photocatalytic oxidation method is of the highest efficiency without secondary pollution and it has recyclable characteristics. Therefore, Hg⁰ removal from coal combustion flue gas has great potential development [9].

Photocatalytic in the direct use of solar energy to solve the two major issues of environmental pollution and energy shortage shows potential application prospects. In all of this, semiconductor photocatalysts have been applied to energy conversion, photocatalysis remediation and organic synthesis [10], [11], [12], [13], [14]. The traditional TiO₂ photocatalyst is widely recognized semiconductor for photocatalytic oxidation because of its high chemical stability, high specific surface area, low cost, strong oxidation ability and photocatalytic activity [15]. However, TiO₂ can only be excited by ultraviolet (UV) irradiation, which accounts for merely about 4% of the total solar radiation, what’s more, the rapid recombination of photogenerated electron-hole pairs (e⁻–h⁺) causes poor quantum yield [16]. Hence many researchers have tried to improve the photocatalytic oxidation under visible light, and to improve the efficiency of photocatalytic oxidation, which included metal or nonmetal ion doping [17], [18], [19], [20], surface modification [17], [18], combining with other semiconductors or metal oxides [19], [20], etc. However, to some extent, these methods have not achieved the desired photocatalytic efficiency of mercury removal.

Bi-based layered semiconductors were found to possess a high photocatalytic activity[21]. The heterolayered BiOIO₃ possesses an excellent photocatalytic activity among all of these Bi-based layered semiconductors, which is mainly due to the internal polar field and the heterolayered structure. Both features can effectively separate the e⁻–h⁺ at the BiO₆ and IO₃ pyramids [22]. In our previous work [23], the pristine BiOIO₃ photocatalyst was fabricated and studied the effect of pH value on its morphology and crystal structure, and we discussed its photocatalytic performance. However, the relatively wide band gap(3.13 eV) of BiOIO₃ reduces its photoactivity under visible light. So we used BiOI with a narrow band gap (1.75 eV) to dope with BiOIO₃ to narrow the band gap of BiOI/BiOIO₃ compounds which can also fabricate the heterostructures, increasing the response to visible light region and separating the photogenerated e⁻-h⁺. The photoactivity experiments of BiOI/BiOIO₃ compounds were verified under ultraviolet (UV) light and visible light to remove mercury. Related research has been conducted in previous work [24], in which we used Bi(NO₃)₃·5H₂O and KIO₃ as the raw materials and added carbon spheres (CSs) into the solution to reduce I⁵⁺ to I⁻ directly, resulting in the formation of BiOI and carbon-doped simultaneously. In the previous work, we mainly emphasized the importance of carbon. The doped carbon element and produced BiOI can bring dramatic changes in their structural, optical, electronic and chemical properties, and the CSs acted as a photosensitizer. However the fabrication process was relatively cumbersome. In this study, we improved the fabrication process, which was very simple and was of low cost. We studied the effects of exposed reactive facets on photocatalytic activity and discussed the mechanism of mercury removal from two aspects of band gap and internal structure of the sample. The physical and chemical properties of the as-prepared BiOI/BiOIO₃ compounds were characterized with a series of techniques, such as UV–vis DRS, PL, XRD, XPS, SEM and TEM. The as-prepared BiOI/BiOIO₃ compounds were applied to photocatlytic oxidation of gaseous elemental mercury under ultraviolet (UV) light and visible light.