Silica-Silver Nanocomposites as Regenerable Sorbents for Hg0

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Silica-Silver Nanocomposites as Regenerable Sorbents for Hg0 Removal from Flue Gases Tiantian Cao, Zhen Li, Yong Xiong, Yue Yang, Shengming Xu, Teresa M. Bisson, Rajender Gupta, and Zhenghe Xu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01701 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Environmental Science & Technology

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Silica-Silver Nanocomposites as Regenerable Sorbents for Hg0

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Removal from Flue Gases

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Tiantian Cao,† Zhen Li,† Yong Xiong,† Yue Yang,† Shengming Xu,†,* Teresa Bisson,‡ Rajender

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Gupta‡ and Zhenghe Xu†,╫, ‡,*

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†

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

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╫

Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China

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‡

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada

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TOC Art

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ACS Paragon Plus Environment


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ABSTRACT: Silica-silver nanocomposites (Ag-SBA-15) are a novel class of multi-functional

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materials with potential applications as sorbents, catalysts, sensors and disinfectants. In this work,

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an innovative yet simple and robust method of depositing silver nanoparticles within the channels

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of SBA-15 matrix was developed. The synthesized Ag-SBA-15 was found to achieve a complete

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capture of Hg0 at temperatures up to 200 °C. Silver nanoparticles on the SBA-15 were shown to be

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the critical active sites for the capture of Hg0 by the Ag-Hg0 amalgamation mechanism. At 1%

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Hg0 breakthrough, an Hg0 capture capacity as high as 13.2 mg·g-1 was achieved by

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Ag(10)-SBA-15, which is much higher than that achievable by existing Ag-based sorbents and

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comparable with that achieved by commercial activated carbon. Even after exposure to more

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complex simulated flue gas flow for 1 hour, the Ag(10)-SBA-15 could still achieve an Hg0

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removal efficiency as high as 91.6% with a Hg0 capture capacity of 457.3 µg·g-1. More

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importantly, the spent sorbent could be effectively regenerated and reused without

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noticeable performance degradation over five cycles. The excellent Hg0 removal capability

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combined with a simple synthesis procedure, strong tolerance to complex flue gas environment,

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great thermal stability and outstanding regeneration capability make the Ag-SBA-15 a promising

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sorbent for practical applications to Hg0 capture from coal-fired flue gases.

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KEYWORDS: coal-fired power plants, elemental mercury removal, silver nanoparticles, thermal

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reduction

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1. INTRODUCTION

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Mercury is one of the most hazardous environmental contaminants because of its volatility,

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persistence and bioaccumulation with subsequent conversion to hypertoxic methylmercury (MeHg),

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which has caused considerable worldwide concerns in recent years.1, 2 In October 2013, more than

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140 nations approved a legally binding treaty-the Minamata Convention on Mercury, sponsored by

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the United Nations Environment Programme (UNEP) to combat mercury emission into the

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environment.3, 4 Coal-fired power plants are one of the primary anthropogenic sources (~35%) of

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mercury emission to the environment due to inevitable utilization of large quantities of coal as the

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main source of electricity generation in developing countries such as China and India.5 As one of

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the largest mercury emitters, China promulgated a national air pollutants emission standard (GB

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13223-2011) for coal-fired power plants, in which the concentration of mercury and its compounds

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must be restricted to be below 30 µg·m-3.6

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In general, mercury in flue gases of coal-fired power plants exists in three different forms:

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elemental mercury (Hg0), gaseous oxidized mercury (Hg2+) and particle-bound mercury (HgP).7-9

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Most Hg2+ and HgP can be removed from the flue gases by the existing wet flue gas desulfurization

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(WFGD) device and electrostatic precipitator (ESP), respectively, before their emission.10,

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However, Hg0 is the most challenging form of mercury to capture and often released into the

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atmosphere directly, due to its extremely low solubility in water and high volatility.12, 13 To meet the

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increasingly stringent mercury emission control legislations, tremendous efforts have been devoted

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to developing various efficient technologies for Hg0 removal from coal combustion flue gases,

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mainly through catalytic oxidation or sorbent injection. To date, the most promising technology is

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injection of activated carbon (AC), usually impregnated with chlorine, bromide, iodine or sulfur.14

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The injection of AC-based sorbents has been commercially employed by some power plants.15, 16

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One major drawback of using AC-based sorbents is the inability of these sorbents to be regenerated

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and therefore used usually only once, incurring high operating costs. In addition, the halides used to

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modify AC inevitably degrade the performance of downstream WFGD and ESP,16, 17 contaminate 3


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the fly ash for concrete manufacture and may cause secondary pollutions.4, 15 To this end, it is

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extremely important to develop regenerable and recyclable sorbents for Hg0 capture.

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Noble metals including silver, gold, platinum and palladium can capture Hg0 by the reversible

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amalgamation process, which has been used not only as the mercury sorbent but also as the sensors

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for the detection of trace levels of mercury in gases.18, 19 Silver, for instance, can amalgamate with

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Hg0 at temperatures of coal-fired flue gases (typically around 150oC).19 The captured Hg0 can be

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readily released through thermal treatment at higher temperatures, providing a means to produce

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regenerable sorbent as an alternative to AC-based sorbents. To make use of silver effectively and

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economically, it is crucial to increase the silver surface area, generally by forming nanoparticles

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which were shown to possess tremendous superiority than silver films not only for higher Hg0

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capture rate and capacity, but also for a stronger tendency to capture Hg0 capture at higher

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temperatures.19 At present, a variety of approaches have been employed to incorporate silver

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nanoparticles on the surface of different porous materials, such as natural chabazite,20-23 carbon

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nanotubes24 and graphene oxide.25 As reported in our previous publications, chabazite-supported

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silver nanocomposites prepared by ion exchange and thermal annealing achieved a complete Hg0

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capture at temperatures below 250oC and could be regenerated repeatedly without noticeable

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performance deterioration.20, 21, 23 Carbon nanotube-silver composites synthesized by wet-chemistry

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and thermal reduction also showed excellent capture of Hg0 at temperatures below 150oC, which

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could be used as either an Hg0 sorbent or mercury trap.24 Graphene oxide supported nanosilver

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sorbent synthesized by direct reduction in solution also possessed a similar capability of mercury

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removal and sorbent regeneration.25 However, most of these silver-based sorbents suffer such

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shortcomings as relatively low Hg0 capture capacity, significant silver losses in the synthesis

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process, complicated synthesis procedures and/or difficulties in mass production.

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Herein, we develop an innovative, yet simple and robust route for the preparation of a high

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efficiency and high capacity mercury sorbent (Ag-SBA-15) by employing SBA-15 as the support

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material, silver ammonia complex as the silver source and subsequent thermal reduction in inert 4


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atmosphere as shown in Scheme 1. Due to its extremely large specific surface areas (600~1000

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m2·g-1), uniform mesopores with plentiful Si-OH active bonds on the surface, tunable pore sizes

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(5~30 nm), favorable mass transfer performance and relatively high thermal and mechanical

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stability, SBA-15 has been comprehensively explored as catalysts, catalyst support and

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adsorbents.26-28 These unique features of SBA-15 make it a potential scaffold to provide abundant

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sites and spaces for loading of silver nanoparticles and facilitate large-scale molecular diffusion and

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transportation during application and regenration. Silver ammonia complex, known as Tollens

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reagent, has been widely used as silver source to synthesize silver nanoparticles. In the process of

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silver ion reduction, the ammonia ligand contributes to stabilizing the growth and preventing the

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aggregation of silver nanoparticles.29-33 More importantly, the silver ions are reduced by thermal

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reduction in inert atmosphere instead of common H2 atmosphere or air atmosphere at higher

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temperatures, which is simpler and more robust without use of any reducing agent and significant

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silver losses. To the best of our knowledge, this approach for depositing silver nanoparticles on the

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SBA-15 matrix has not yet been reported in literature.

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Scheme 1. Illustration of fabrication process of silica-silver nanocomposites (Ag-SBA-15) and its

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use as regenerable sorbents for Hg0 removal: (a) the pure SBA-15 matrix with plentiful Si–OH

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groups; (b) SBA-15 loaded with Ag precursor; (c) SBA-15 loaded with Ag nanoparticles by thermal

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reduction; (d) its use for Hg0 removal by amalgamation and regeneration from (d) to (c) by thermal

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treatment. 5



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The possible application mode of the sorbent for Hg0 control from coal-combustion flue gases

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is shown in the Supporting Information (SI) Scheme S1 and TOC Art. The regenerable mercury

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sorbent can be injected directly downstream of the ESP, where the vast majority of the fly ash has

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already been removed. The spent sorbent is then collected by a relatively small fabric filter (FF)

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with small size fugitive fly ash for regeneration by mild thermal treatment and recycle for mercury

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capture. The concentrated Hg0 vapor released in the regeneration process is condensed to liquid

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mercury for safe disposal.34 It is interesting to note that the fugitive fly ash captured with the spent

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sorbent by FF helps dilution of the sorbent for better distribution during the injection.

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The main objective of this paper is to develop a superior regenerable sorbent for Hg0 emission

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control from coal-fired power plant flue gases using a simpler and greener synthesis method. The

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physical and chemical properties of the sorbent were determined by various characterization

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methods. The Hg0 breakthrough, Hg0 capture capacity and temperature-programmed desorption of

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Hg0 were determined. To determine the applicability of the synthesized sorbent to mercury capture

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in real flue gas environment, the Hg0 removal capability was also determined using the sorbent

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exposed to more stringent simulated flue gases. Meanwhile, the regeneration and recyclability of

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the sorbent were evaluated. It should be noted that the silica-silver nanocomposites have many other

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important potential applications as mercury trap,35 catalysts,36, 37 sensors and disinfectants.

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2. EXPERIMENTAL SECTION

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Materials and Preparation. All materials used in this study are provided in the SI. The

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procedures for synthesis of silica-silver nanocomposites, Ag-SBA-15 are shown in Scheme 1 (a-c).

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Typically, taking 10 wt.% Ag loaded SBA-15 as an example, 0.52 g AgNO3 was dissolved in 3 mL

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deionized water, followed by dropwise addition of 10 wt% ammonium hydroxide solution to form a

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colorless silver ammonia solution. Added into the silver ammonia solution was then 3 g synthesized

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SBA-15 (shown in Scheme S2). The resulting mixture was sonicated for 2 h and placed in a dark 6


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environment at ambient temperature overnight, followed by drying directly in an air-circulated oven

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at 60 °C. The resulting solid samples were pulverized and sieved by 100-mesh screen. The particles

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smaller than 100 mesh were thermally reduced at 240 °C for 4 h under inert gas flow. The main

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reduction reactions are shown in SI.29 The final product was referred to as Ag(x)-SBA-15, where x

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represents the percent mass of Ag in Ag(x)-SBA-15. For comparison, alternative methods were

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used to prepare SBA-15 supported silver samples and are described in the SI in detail.

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Characterization. The physicochemical properties of the sorbent were determined using X-ray

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diffraction (XRD), nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy

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(XPS), inductively coupled plasma mass spectrometry (ICP-MS) and transmission electron

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microscopy (TEM). More details on these characterization methods are given in the SI.

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Mercury Breakthrough Test. The mercury breakthrough test, also called mercury pulse

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injection test, is a convenient and widely used method to determine the mercury removal

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performance of the sorbent under a short contact time condition. A detailed description of mercury

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breakthrough tests can be found in previous publications.21-24, 38, 39 The schematic diagram of the

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experimental setup is shown in Figure S1. The assembly includes a lab-scale fixed-bed reactor, a

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vertical furnace and a real time on-line Hg0 analyzer (Lumex-Marketing JSC, RA-915 M,

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sensitivity: 0.1 µg·m-3). In a typical test, 30 mg of sorbent on approximately 5 mg silica wool were

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placed on a sieve plate located in the middle of a 3 mm ID and 550 mm length quartz tube. The

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quartz tube was enclosed in a small vertical tube furnace, which was used to adjust the test

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temperature. A high-purity N2 flow at 50 mL·min-1 was used as the carrier gas. A precise amount of

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Hg0-saturated air (200 µL, 4 times larger than the values reported in literature21, 23, 24) was injected

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into the carrier gas from Port 1. The gold beads (GB) trap was used for preconcentration of Hg0 that

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broke through the sorbents. The Hg0 captured by the GB trap was then thermally released by a rapid

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heating. The released Hg0 was detected using the on-line mercury analyzer. The value of Hg0

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breakthrough was determined by the ratio of the Hg0 released from the GB trap to the total Hg0

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injected. 7



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Mercury Capture Capacity Test. As shown in Figure S2, an Hg0 permeation tube was

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immersed in the thermostatic water bath to produce a constant Hg0 vapour, which was transported

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by pure N2 carrier gas. The Hg0 concentration was 531±1 µg·m-3 with a N2 flow rate of 50 mL·min-1.

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A precisely weighed amount of sorbent (30 mg) was placed in the quartz tube reactor. In this paper,

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the capacity represents the mass of Hg0 captured by the unit mass of sorbent once the Hg0

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breakthrough reaches only 1%, which is more stringent than the conditions adopted by most

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reported sorbents.

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Mercury Removal Test in Simulated Flue Gases. The sorbents were exposed to continuous

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simulated flue gases to further investigate the potential of practical Hg0 removal capability. Figure

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S3 displays the schematic diagram of the experimental set up. Placed in the quartz tube reactor was

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15 mg sorbent. The simulated flue gases consisted of 5% O2, 15% CO2, 4% H2O, 200 ppm NO, 600

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ppm SO2, 20 ppm HCl and 125±0.5 µg·m3 Hg0 (approximately four times higher than the Hg0

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concentration of real flue gas) with balance gas of N2. The simulated flue gas composition was

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chosen based on literature values and typical flue gas compositions from coal-fired power plants.25,

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34, 40, 41

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removal performance of Hg0 by the sorbents. The total flow rate was set at 1.0 L·min-1

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corresponding to a space velocity of approximately 260,000 h-1, which is much higher than the

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typical space velocity of sorbents in the actual flue gases of coal-fired power plants.

The higher Hg0 concentration represents worse conditions for us to better evaluate the

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3. RESULTS AND DISCUSSION

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Characterization of Sorbents. The small-angle X-ray scattering (SAXS) patterns of the

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synthesized SBA-15 and Ag-SBA-15 nanocomposites are shown in Figure S4a. All samples

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exhibit three distinct scattering peaks assigned to (100), (110) and (200) reflections of the

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P6mm space group, indicating a highly ordered 2D-hexagonal symmetry mesostructure.26, 27

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The results also suggest that loading of Ag nanoparticles leads to no obvious change to the 8


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mesostructure of SBA-15 matrix. Comparing Ag-SBA-15 to pure SBA-15 patterns, there is

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a slight decrease in the intensity of the peaks, which can be attributed to a pore-filling effect

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of Ag nanoparticles in the channels, reducing the X-ray scattering to a certain extent.

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According to IUPAC definition, N2 adsorption–desorption isotherms of SBA-15 and

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Ag-SBA-15 feature typical type-IV isotherms with a distinct H1 hysteresis loop (Figure S4b). The

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similar hysteresis loops imply not only the characteristics of mesoporous materials but also that the

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SBA-15 mesoporous structure remained intact after introducing the Ag nanoparticles. The surface

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areas and pore properties as calculated by BET methods are summarized in Table S1. It is evident

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that pure SBA-15 is an excellent support, with an extremely high specific surface area of 836 m2·g-1,

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large pore volume of 1.07 cm3·g-1 and pore size of 7.72 nm. The loading of Ag nanoparticles within

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small channels of SBA-15 matrix led to a significant decrease in the specific surface area and a

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small reduction in pore volume, accompanied by a slight increase in the average pore size for

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Ag-SBA-15 nanocomposites. The small increase in pore size could be attributed to the block small

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pore channels, which led to an apparent increase in the average pore size. The similar phenomenon

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was reported in previous studies.36, 37, 42 Compared to other sorbents, the specific surface area and

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pore volume of the resultant composites are still sufficient for Hg0 removal. As shown in Table S1,

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a decrease in Ag dispersion from 40.32% to 23.86% with increasing Ag loading from 5 wt% to 10

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wt% was observed. Such decrease corresponded well with the observed increase in the size of Ag

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nanoparticles, as shown in Figure S7.

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Figure S4c shows the XRD patterns of SBA-15 and Ag-SBA-15. The broad band ranging from

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2θ=10° to 32° is the characteristic peak of SBA-15 amorphous silica matrix. In addition, the other

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four diffraction peaks for Ag(10)-SBA-15 at approximately 38.1°, 44.3°, 64.4° and 77.5°

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corresponding to (111), (200), (220) and (311) crystallographic planes of cubic Ag (JCPDS card no. 9



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04-0783) confirm the presence of Ag nanoparticles loaded on the SBA-15 matrix. In the case of

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Ag(5)-SBA-15, there is only a typical peak of Ag at 38.1°, which can be attributed to the relatively

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low amount of silver nanoparticles, in an agreement with previous reports.36

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Figure S4d is the Ag3d XPS spectra for SBA-15 and Ag-SBA-15. As anticipated, the pristine

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SBA-15 shows no Ag3d band. In contrast, both Ag(5)-SBA-15 and Ag(10)-SBA-15 have two sharp

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peaks at 374.8 eV and 368.8 eV attributed to Ag3d5/2 and Ag3d3/2, indicating successful loading of

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elemental silver on SBA-15 matrix. By calculating the peak areas of Ag(5)-SBA-15 and

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Ag(10)-SBA-15, the silver content in Ag(5)-SBA-15 is approximately half than that in

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Ag(10)-SBA-15. In addition, we determined silver content in digested solutions of the synthesized

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sorbents using ICP method (see details on page 5 of Supporting Information). Within the

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experimental errors, the measured silver loading values as shown in Table S4 match the theoretical

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silver loading values as designed for all the sorbent.

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Figure S5 and 1 show typical TEM and HRTEM images of the pristine SBA-15 and

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Ag(10)-SBA-15 nanocomposites. As shown in Figure S5, SBA-15 features a mesoporous structure

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of well-ordered hexagonal symmetry. Such structure is in line with the results of SAXS and N2

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sorption measurements. In the case of Ag(10)-SBA-15, a handful of spherical Ag nanoparticles

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(black dots) with an average diameter of ~24 nm are visible on the external surface of SBA-15 as

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shown in Figure 1a, while the long-range ordered mesoporous structure of SBA-15 is intact even

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with the incorporation of nanoparticles. The results of EDX analysis at location (1) confirm that the

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observed nanoparticles are indeed Ag nanoparticles as anticipated. It should be noted that the Cu

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peaks in Figure 1g are from the copper grid that supports the samples. Figure 1b, 1d and 1c, 1e are

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STEM dark field and bright field images of Ag(10)-SBA-15 at the same location, respectively. The

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silver of higher atomic number than silicon is shown as the bright spots on the STEM dark field 10


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image and as the black spots on the STEM bright field image. A large number of Ag

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nanoparticles with sizes (diameters) ranging from 8~24 nm are clearly seen on the SBA-15 matrix.

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From the magnified image in the inset of Figure 1d and in comparison with the results in Figure 1f,

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1g and 1h, smaller size of Ag nanoparticles in the range of 1 to 7 nm is found in the channels of

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SBA-15 matrix. The Ag nanoparticle size distribution of Ag(10)-SBA-15 is determined using

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imaging analysis on the TEM and STEM micrographs. The results in Figure S7a show a mean

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particle size of 5.8 nm. The insets in Figures 1g and 1h show clear lattice fringes of Ag

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nanoparticles with d=0.24 nm and 0.20 nm, which are attributed to the (111) and (200)

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crystallographic planes of cubic Ag, respectively. As for Ag(5)-SBA-15, the size of Ag

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nanoparticles are relatively uniform with the largest size of only 6 nm and a mean diameter of 2.4

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nm as shown in Figures S6 and S7b, respectively. In conclusion, Ag nanocomposites are shown to

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be successfully fabricated on SBA-15 by our innovative, yet simple and robust approach.

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Mercury Removal Performance of Sorbents. Mercury Breakthrough. Hg0 breakthrough tests

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were conducted to determine the Hg0 removal performance of the sorbent under a short contact time

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condition that better represents the real sorbent injection application environment. As shown in

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Figure 2, the common commercial activated carbon (referred to as CAC herein) at 50 °C is able to

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capture Hg0 with 28.3% Hg0 breakthrough. In this case, Hg0 is mainly physically adsorbed on the

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CAC due to numerous active sites, including functional groups on its large surface area. The

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Hg0 breakthrough increases significantly as the test temperature increases, reaching 67.2% at

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150 °C, which is the typical coal-fired flue gas temperature. The results suggest that CAC is not a

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suitable sorbent for Hg0 emission control. In comparison, the commercial activated carbon (Norit

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Darco Hg, referred to as NoricAC herein) manufactured specifically for mercury capture shows

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better Hg0 removal performance, possessing only 3.3%, 9.8% and 17.5% Hg0 breakthrough at 11



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100 °C, 150 °C and 200 °C, respectively.14 The pristine SBA-15 has a poor Hg0 removal

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performance with a minimum Hg0 breakthrough of 88.5% even at 50 °C, as anticipated. The limited

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Hg0 removal capability of pristine SBA-15 can be attributed to the absence of active sites despite its

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large but inert specific surface area, as in the case of carbon nanotubes.24

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Synthesis of silver nanoparticles on SBA-15 greatly improved Hg0 removal performance. Both

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Ag-SBA-15 composites show almost complete Hg0 capture at temperatures up to 200 °C, greatly

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superior to NoricAC as shown in Figure 2. The dramatic improvement in Hg0 removal by

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Ag-SBA-15 can be attributed to the key role of silver nanoparticles in Ag-SBA-15. As is well

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known, silver nanoparticles can efficiently capture Hg0 by Ag-Hg0 amalgamation, which is a more

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stable and strong interaction than physical adsorption. Previous reports on silver-based sorbents

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demonstrated Hg0 capture by the amalgamation mechanism.19, 21, 23 At higher temperatures,

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Ag-SBA-15 composites become less effective in capturing Hg0. At 250 °C, for example

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approximately 80% and 90% of Hg0 breaks through the packed bed of Ag(10)-SBA-15 and

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Ag(5)-SBA-15, respectively. A further increase in the temperature to 300 °C led to a complete

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Hg0 breakthrough for both Ag-SBA-15 nanocomposites. The negligible Hg0 capture at

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300 °C provides an effective means to regenerate Ag-SBA-15 sorbents by thermal treatment

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under or above this temperature.

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Figure 1. (a) TEM micrographs of Ag(10)-SBA-15 nanocomposites and EDX spectrum (inset) of

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point 1; (b, d) STEM dark field images of Ag(10)-SBA-15 and (c, e) corresponding to STEM bright

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field images; (f, g, h) HRTEM images of Ag(10)-SBA-15 with the inset showing the selected Ag

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nanoparticles. 13



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Breakthrough (%)

100

80

60

SBA-15 CAC Ag(5)-SBA-15 Ag(10)-SBA-15 NoricAC

40

20

0 50

100

150

200

250

300

Temperature (oC) 277 278

Figure 2. Results of mercury breakthrough tests as a function of temperature for CAC, NoricAC,

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SBA-15, Ag(5)-SBA-15 and Ag(10)-SBA-15.

280 281

Mercury Capture Capacity. The sorbents were exposed to continuous Hg0 flow to determine

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Hg0 capture capability. In this paper, the Hg0 capacity is more strictly defined as the amount of Hg0

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adsorbed per mass of sorbent when Hg0 breakthrough reaches only 1% (compared to 20% or 100%

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as reported in other publications). The results in Figure 3 show a negligible Hg0 capture by the

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pristine SBA-15, confirming the poor performance on Hg0 removal shown in the breakthrough tests.

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In contrast, Ag(5)-SBA-15 and Ag(10)-SBA-15 nanocomposite sorbents show Hg0 capture

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capacities as high as 5.9 and 13.2 mg·g-1 under the identical test conditions. The nearly double

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mercury capture capacity by Ag(10)-SBA-15 of double silver content than that by Ag(5)-SBA-15

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confirms the critical role of silver nanoparticles in mercury capture by amalgamation. To further

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illustrate the importance of silver loading, the Hg removal efficiency of Ag on Ag(5)-SBA-15 and

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Ag(10)-SBA-15 was calculated to be 120 and 133 mg Hg0/ g Ag, respectively. It is evident that Hg0

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capacity is positively correlated to the amount of Ag on Ag(5)-SBA-15 and Ag(10)-SBA-15.

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Furthermore, increasing the dispersion would lead to more Ag active sites on the surface for a given 14


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load of Ag. However, Ag(5)-SBA-15 with much higher dispersion showed an inferior efficiency.

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The high Hg0 removal capacity is therefore attributed to more Ag active sites. As shown in Figure

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S8b, a binding energy shift of Ag3d bands towards lower binding energy by 0.3 eV was observed

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for the spent Ag(10)-SBA-15 sample. This shift indicates the formation of Ag-Hg amalgam. It is

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interesting to note mercury capture capacity by silver nanoparticles supported on a natural mineral

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chabazite, Ag-MC resulted in a 0.9 mg·g-1 Hg0 capture capacity at 1% Hg0 breakthrough.21 Clearly,

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SBA-15 is a superior support than natural chabazite due to its larger surface areas, and larger and

301

more uniform channel for better mass transfer. Such structural difference allows smaller size and

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highly dispersed Ag nanoparticles to be formed on SBA-15. Reducing the size of Ag nanoparticle

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on the support is known to greatly improve the rate and capacity of Hg0 capture.19 Although it

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would be interesting to compare this value with the theoretical capacity of mercury capture based

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on sliver loading and particle sizes of the loaded silver, it is important to note that our mercury

306

capture capacity of 13.2 mg·g-1 represents the amount of Hg0 captured at 1% Hg0 breakthrough

307

threshold, which is very different from the amount of mercury that could be captured at dynamic

308

equilibrium of 0% capture or 100% Hg0 breakthrough point, which makes the comparison

309

extremely difficult if not impossible. Compared with other related sorbents such as Ag-beads (0.2

310

µg·g-1), Au-beads (2.9 µg·g-1),24 Ag nanoparticle-based magnetic chabazite (13.3 µg·g-1),22, 23 Ag

311

nanoparticle-based magnetic HZSM-5 (22.3 µg·g-1),9 and AgMC (0.9 mg·g-1),21 Ag-SBA-15 is

312

shown to be a more efficient and promising sorbent for Hg0 capture from coal-fired flue gases.

313

Furthermore, the Ag-SBA-15 can be considered as a mercury trap for many other applications,

314

which will be reported in a separate paper.

315

Mercury Temperature-Programmed Desorption (Hg-TPD). In order to investigate the potential

316

of regeneration, Hg0 release characteristic from spent Ag(10)-SBA-15 sorbent was determined in a

317

Hg-TPD experiment. The test was carried out over a temperature range of 30~550 °C at the heating

318

rate of 10 °C·min-1 in N2. As shown in Figure S9, there is only one apparent peak emerged on the

319

Hg-TPD curve for Ag(10)-SBA-15. This desorption peak is attributed to decomposition and release 15



Page 16 of 23

320

of mercury amalgamated with nanosilver.21, 24 The Hg0 starts to desorb at approximately 140 °C

321

and reaches a peak at approximately 215 °C, implying that the sorbent can still retain a relatively

322

high Hg0 removal efficiency at the real coal-fired flue gas temperature (150 °C). Above 215 °C, the

323

sorbent possesses poor Hg0 capture capability, which agrees with the breakthrough test results in

324

Figure 2. At 300 °C, more than 93% of adsorbed Hg0 has been released. In addition, Ag(5)-SBA-15

325

and Ag(10)-SBA-15 exhibit similar thermal desorption characteristics. The peak temperature of

326

desorption of 220 °C for Ag(5)-SBA-15 is similar to 215 °C for Ag(10)-SBA-15. The result implies

327

that temperatures at or slightly above 300 °C (holding a certain time) are sufficient for sorbent

328

regeneration.

Mercury Capacity (mg/g)

14

13.2

12 10 8 5.9

6 4 2 0.9

0

0

SBA-15

Ag(5)-SBA-15 Ag(10)-SBA-15

AgMC

329 330

Figure 3. Mercury capture capacities on SBA-15, AgMC,21 Ag(5)-SBA-15 and Ag(10)-SBA-15 at

331

150 C when Hg0 breakthrough reaches only 1%.

o

332 333

Mercury Capture in Flue Gases. In order to evaluate the potential for practical application in

334

coal-fired power plants, the Ag(10)-SBA-15 sorbent was exposed to continuous flue gases at

335

different temperatures for 1 hour. The simulated flue gas composition was 5% O2, 15% CO2, 4% 16


Page 17 of 23


336

H2O, 200 ppm NO, 600 ppm SO2, 20 ppm HCl and 125±0.5 µg·m3 Hg0 (about four times higher

337

than the Hg0 concentration of real flue gases in industry) with balance gas of N2. The total flow rate

338

was set at 1.0 L·min-1 with a space velocity of approximately 260,000 h-1, which is higher than the

339

typical space velocity of sorbents in real flue gases of coal-fired power plants. As shown in Figure

340

4, Ag(10)-SBA-15 exhibits the highest Hg0 removal efficiency of 95.5% and largest capture

341

capacity of 478.6 µg·g-1 in 1 hour at 50 °C. At the typical real coal-fired flue gas temperature of

342

150 °C, Ag(10)-SBA-15 still possesses as high as 91.6% Hg0 removal efficiency at 457.3 µg·g-1

343

mercury loading capacity. Compared to the test in pure N2 environment (shown in Figure

344

S10), there is only a minimal decrease in Hg0 capture capability, strongly implying the

345

suitability and robustness of the sorbent in a complex flue gas environment. The Hg0

346

captured by Ag(10)-SBA-15 over 5-min exposure in flue gases at 150 °C was calculated to

347

be 40.2 ppm by weight (equivalent to 96% Hg0 removal efficiency). This represents a

348

significantly higher Hg0 capture capacity than any other Ag-based sorbents such as Ag

349

nanoparticle-based magnetic chabazite (~16 ppb),22, 23 AgMC (~137 ppb)21, Ag-based magnetic

350

graphene oxide (4.5 ppm)25 over the same exposure time. Ag(10)-SBA-15 also outperforms many

351

carbon-based sorbents (see Table S2 for the summary of sorbents on Hg0 removal). For a mercury

352

concentration of 10 µg·m-3 (elemental mercury accounts for 40%) in a coal-fired flue gas, an

353

injection rate of 24 kg typical commercial activated carbon per actual million cubic meter flue gas is

354

needed to achieve a removal efficiency of 80%. In comparison, the injection rates of

355

Ag(10)-SBA-15 are conservatively estimated to be 0.48 kg per million cubic meter flue gas.19, 23,

356

43-45

357

would be more economical compared to commercial activated carbons although the initial

358

investment in fresh sorbent is higher as shown in Table S3. After conducting a large number of

359

screening synthesis as shown in Figure S11, S12 and Table S4, we identified key parameters to

360

fabricating the sorbent of the highest efficiency by forming relatively smaller sizes of silver

361

particles (less than 24 nm) using Ag(NH3)2NO3 as silver precursor at 10 wt.% silver loading and

More importantly, Ag(10)-SBA-15 can be regenerated and reused. Therefore Ag(10)-SBA-15

17



362

Page 18 of 23

thermal reduction in the inert gas atmosphere.

363

Regeneration performance. The regeneration and recyclability of Ag(10)-SBA-15 sorbent are

364

significant measuring criteria for practical applications. The spent Ag(10)-SBA-15 exposed to flue

365

gases was regenerated by thermal treatment in a N2 flow environment for approximately 20 min at

366

300 °C. This regeneration temperature was determined based on the results in Figure 2 and S9.

367

Higher temperature facilitates the Ag-Hg0 amalgam decomposition and promotes Hg0 release,

368

but may also bring about negative effect on the morphology of Ag nanoparticles (increasing

369

size) that may decrease the Hg0 removal performance.19 Therefore, 300 °C is considered an

370

appropriate temperature for sorbent regeneration. In this study, both the Hg0 breakthrough test in N2

371

flow and Hg0 removal efficiency and capture capacity test in flue gases were conducted to

372

determine the regeneration performance of the recycle sorbent.

95.5

94.1

Removal Efficiency (%)

478.6

Removal Efficiency Capacity

91.6

470.9

457.3 73.4

80

366.4

60

500

400

300

40

200

20

100

0

Capacity (µg/g)

100

0 50

100

150

Temperature (

200

)

373 374

Figure 4. Mercury removal efficiency and capture capacity of Ag(10)-SBA-15 exposed to flue

375

gases for 1 hour from 50~200 °C.

376 377

In Hg0 breakthrough tests, the results in Figure 5a show that the spent Ag(10)-SBA-15 sorbent 18


Page 19 of 23


378

can retain complete Hg0 capture at 150 °C after 5 regeneration/capture cycles, showing similar Hg0

379

removal performance to that of fresh Ag(10)-SBA-15. In addition, the spent sorbent was also

380

continuously heated at 300 °C for 200 min to investigate the influence of extended thermal

381

treatment (equivalent to 10 recycling tests theoretically). The results (the blue bar in Figure 5a)

382

show 100% of Hg0 removal efficiency by recycle Ag(10)-SBA-15, further illustrating the stability

383

of the spent sorbent to be repeatedly regenerated at 300 °C. Figure 5b shows the regeneration

384

performance of the spent Ag(10)-SBA-15 exposed to flue gases. After 5 cycles of regeneration, the

385

sorbent is still able to achieve higher Hg0 removal efficiency at higher capture capacity.

386

Compared to fresh sorbent, there is only a slight loss in Hg0 capture efficiency,

387

demonstrating excellent stability of the sorbent. Considering this excellent regeneration

388

performance along with simple synthesis procedures, higher Hg0 removal efficiency, larger

389

Hg0 capacity, appropriate operating temperature range, strong tolerance to complex flue

390

gases and excellent thermal stability, the Ag-SBA-15 is shown to be a promising sorbent for

391

practical application in Hg0 capture from coal-fired power plant flue gases. o

Cumulative time of 300 C regeneration (min)

(b)

20

40

60

80

100

40

20

80

60

40

20

0

0 1

392

2

3

4

5

6

500

400 200 min-Ag-SBA-15

60


200 min-Ag-SBA-15


100

80

100 Removal Efficiency Capacity

300

200

Capacity (µg/g)

o

Cumulative time of 300 C regeneration (min) 100 20 40 60 80

(a)

100

0 1

2

3

4

5

6

Regeneration Cycles

Regeneration Cycles

393

Figure 5. Regeneration test results for Ag(10)-SBA-15 at 150 °C: (a) Mercury breakthrough over 5

394

regeneration/capture cycles and the mercury breakthrough after continuous 200-min thermal

395

treatment at 300 °C (blue bar); (b) Mercury removal efficiency and capture capacity over 5

396

regeneration/capture cycles and the mercury removal efficiency and capture capacity after 19



397

continuous 200-min thermal treatment at 300 °C.

398 399

ASSOCIATED CONTENT

400

Supporting Information

401

Materials used in this work, SBA-15 preparation, reaction equations, characterization of the

402

sorbents, experimental setup scheme for Hg0 breakthrough test and capture capacity, TEM of

403

SBA-15 and Ag-SBA-15, comparison of Hg0 capture capability for different sorbents et al. are

404

provided. This material is available free of charge via the Internet at http://pubs.acs.org.

405

AUTHOR INFORMATION

406

Corresponding Author

407

* Prof. Zhenghe Xu, E-mail: [email protected]. Tel: +01-780.492.7667, Fax:

408

+01-780.492.2881.

409

* Prof. Shengming Xu, E-mail: [email protected]. Tel: +86-10-62773585(Fax).

410

Notes

411

The authors declare no competing financial interest.

412

ACKNOWLEDGEMENTS

413

The project was supported by the National Natural Science Foundation of China

414

(U1261204). We would like to thank Mr. Peng Li and Anqiang He at the University of

415

Alberta for the help of TEM analysis.

416

20


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417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461


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Silica-Silver Nanocomposites as Regenerable Sorbents for Hg0

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