In-situ grown Co₃O₄ nanoparticles on wood-derived carbon with natural ordered pore structure for efficient removal of Hg⁰ from flue gas

Highlights

• A novel sorbent was obtained by in-situ growth of Co₃O₄ on wood vessel channel.

• The sorbent has ordered pore structure and uniform size of Co₃O₄ nanoparticles.

• The Hg⁰ removal efficiency is 97% at 200 °C with Co₃O₄ loading amount of 5%.

• The mechanism of the different effects of NO and SO₂ on Hg⁰ removal was studied.

Abstract

Sorbent morphology is significant to Hg⁰ removal performance due to its serious impacts on the number and availability of adsorption sites and the mass transfer of the Hg⁰ removal process. Given this, a novel sorbent was obtained by in-situ growth of Co₃O₄ nanoparticles on channel walls of wood vessel which has regular mesoporous channels. The effects of synthetic conditions of hydrothermal temperature and NH₄F concentration on sorbent morphology and Hg⁰ removal performance are investigated. Characterization results show that Co₃O₄ nanoparticles integrate a monolithic shell as synthesis temperature rises, while the nanoparticles grow radially with the increase of NH₄F concentration. Radial growth of nanoparticles results in a higher crystallinity. The sorbent synthesized at hydrothermal temperature of 90 °C with the NH₄F concentration of 0.05 M (denoted as T₉₀C_0.05) has uniform Co₃O₄ nanoparticle size, homogeneous dispersion and a larger specific surface area. Furthermore, T₉₀C_0.05 has a higher chemisorbed oxygen concentration and a higher Co³⁺/Co²⁺, which is of benefit to Hg⁰ removal process. Therefore, T₉₀C_0.05 has an excellent Hg⁰ removal efficiency of 97% with a high gas hourly space velocity of 180, 000 h⁻¹ at 200 °C and the Co₃O₄ loading is only wt.5%. The effects of compounded flue gas components on the Hg⁰ removal performance were also studied. NO can weaken the inhibition effect of NH₃ through the reaction between NO and NH₃ forming labile [NH₂NO]. In contrast, SO₂ can not weaken the inhibition effect of NH₃ due to the production coming from the reaction of SO₂ and NH₃ is stable at Hg⁰ removal temperature.

Introduction

Mercury emission from anthropogenic source receives widespread attention because of its toxicity, persistence and bioaccumulation, which has detrimental effects on human health and ecological environment [1,2]. Coal-fired boilers are considered to be one of the major sources of mercury emissions [3]. Mercury is often presented as oxidized mercury (Hg2+), particulate-bound mercury (HgP) and elemental mercury (Hg0) in coal combustion flue gas [4,5]. Hg2+ and Hgp in coal-fired flue gas can be removed by the wet flue gas desulfurization devices (WFGDs) and electrostatic precipitators (ESPs), respectively [6]. However, the removal of Hg0 is difficult due to its high volatility and insolubility in water [7]. Thus, the removal of Hg0 is the key to solve the increasingly serious mercury pollution problem.

In recent years, many researches have pointed that converting Hg⁰ oxidation and adsorption is a promising way to control the Hg⁰ emission from flue gas [8,9]. Up to date, various transition-metal oxides such as VO_x [10], MnO_x [11], TiO_x [12,13], CoO_x [14], and CeO_x [15,16] have been used as catalysts for Hg0 removal. Among them, CoO_x is considered to be a powerful catalyst due to its large Hg0 adsorption capacity and excellent redox performance [17]. Especially, Co₃O₄ with a unique redox couple of Co²⁺/Co³⁺, which can lead to an increased redox capacity, has been widely used in Hg⁰ removal process [18]. Mei et al. [19] synthesized a spinel Co3O4 and found that it had a good Hg0 removal activity of 72.3% at 623 K Co3O4 doped with some other metal such as Co-Cu [20], Co-Mn-Fe [21] have shown even better Hg0 removal efficiency.

It is worthwhile mentioning that lower the size of Co3O4 nanoparticles can provide more active sites and may further increase the oxidation and adsorption capacity [22]. Yi et al. [23] investigated the differences of the catalytic performance of nano and bulk Co₃O₄ and it showed that the removal efficiency of nano-sized Co3O4 (less than 1 μm) is 1.84 times higher than bulk Co3O4 (more than 10 μm). However, nanoparticles with a smaller size usually have higher surface energy which will lead to a serious agglomeration during synthesized process and it is difficult to obtain a high dispersion in nano-size [24]. To overcome this problem, it seems to be a better alternative to disperse and anchor the Co3O4 nanoparticles on a supporting matrix. Impregnation method has been widely used to disperse Co3O4 nanoparticles on support. Choi et al. [25] has pointed that Co₃O₄/SiO₂ synthesized by impregnation method has a higher N₂O removal performance when compared to unsupported Co₃O₄. It is due to that SiO₂ support can effectively prevent the agglomeration of Co₃O₄ nanoparticles. However, impregnation method is difficult to obtain uniform nanoparticles with high dispersity. The uneven distribution of active components on support is the main factor limiting removal performance.

In-situ growth of Co₃O₄ nanoparticles on the support surface is a feasible strategy, which can get a controllable morphology and a uniform particle size [26]. Supports such as nickel foam [27], alumina [28], graphene [29] and porous carbon [30] have been proposed. Lu et al. [31] researched the in-situ growth of Co₃O₄ on graphene to obtain uniform Co₃O₄ nanoparticles of 20–50 nm, which effectively solved the agglomeration problem resulting in a high electrochemical performance. Jiang et al. [32] synthesized a sample with a method of Co₃O₄ nanorods in-situ growth on Ni foam. It was found that the obtained sample can be in a controllable nanorod array structure by adjusting NH₄F concentration. Therefore, the in-situ growth of Co₃O₄ nanoparticles may obtain a stable and more efficient catalyst with a lower Co₃O₄ loading amount.

Wood carbon has plenty of vessel cell with long, through and ordered macroporous channels and twisted mesoporous channels. Furthermore, there are many pores on vessel walls resulting in a 3D connected pore structure [33,34]. Benefitting from the unique structural merits, ordered macroporous channels can reduce the mass transfer resistance, while the twisted mesoporous pore can effectively disperse the active components. Thus, it has been widely used in structural materials, cellulose manufacturing, flexible electronics, and energy storage applications [[35], [36], [37], [38], [39]]. Our previous work has shown that 30% Co₃O₄ loaded on wood vessel surface has a good Hg⁰ removal efficiency of 90% at 200 °C [40]. Therefore, the homogeneous growth of Co₃O₄ nanoparticles on the wall surface of the wood channels may stabilize Co₃O₄ nanoparticles and promote mass transfer so as to obtain better Hg⁰ removal efficiency.

In this work, in-situ growth of Co₃O₄ nanoparticles on wood carbon with a 3D pore structure was obtained by a hydrothermal method. The influence of hydrothermal condition on sample morphology and the Hg⁰ removal efficiency are investigated. Various characterization methods such as SEM, XRD, BET and XPS were used to study the physicochemical properties of the samples to establish a structure-activity relationship.