Environ. Sci. Technol. 2006, 40, 1603-1608
Application of Gold Catalyst for Mercury Oxidation by Chlorine Y O N G X I N Z H A O , † M I C H A E L D . M A N N , * ,† JOHN H. PAVLISH,‡ BLAISE A. F. MIBECK,‡ GRANT E. DUNHAM,‡ AND EDWIN S. OLSON‡ Chemical Engineering Department, University of North Dakota, Grand Forks, North Dakota 58202-7101, and Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9108
This paper discusses a recent study of mercury catalytic oxidation by chlorinating reagents. Gold was chosen as the catalyst because of its reluctance to chemisorb some gases such as O2, NO, H2O, and SO2. This property, as demonstrated in this study, is instrumental to mercury oxidation by circumventing some undesired inhibitory reactions such as OH + NO + M f HONO + M and OH + SO2 + M f HOSO2 + M, which were recognized under homogeneous situations at high temperatures. In comparison to Cl2, HCl showed weak oxidizing capability but appreciable inhibition in mercury oxidation by Cl2, probably through the competition of active sites with Cl2. Overall, the mercury catalytic oxidation by Cl2 on gold catalyst surfaces was viable, reaching 40-60% in this study under temperatures of 448-498 K, where the thermal decomposition of formed Hg2+ was effectively avoided.
Introduction Mercury is listed as a hazardous and toxic pollutant under Title III of the 1990 Clean Air Act Amendments (CAAA) for its environmental and health impacts. According to the U.S. Environmental Protection Agency’s (EPA) report to Congress, coal-fired boilers were concluded as the primary stationary sources of mercury emissions into the atmosphere, contributing approximate 48 tons annually or about one-third of total anthropogenic emissions. On December 14, 2000, the EPA decided that it was appropriate and necessary to regulate mercury emissions from coal-fired boilers. This decision was further implemented on March 15, 2005, by the first-ever Clean Air Mercury Rule (CAMR), through which mercury emissions from newly constructed and currently existing coal-fired utility boilers would be capped at specified nationwide levels. This rule, combined with the EPA’s Clean Air Interstate Rule (CAIR), will reduce emissions in two phases. In the first phase, due by 2010, emissions will be reduced by taking advantage of cobenefit reductions (i.e., mercury reductions achieved while reducing sulfur dioxide (SO2) and nitrogen oxides (NOx) under CAIR). The second phase would cut mercury to 15 tons by 2018, an overall reduction of nearly 70%. * Corresponding author phone: (701)777-3852; fax: (701)777-3773; e-mail:
[email protected]. † Chemical Engineering Department, University of North Dakota, Grand Forks, North Dakota 58202-7101. ‡ Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9108. 10.1021/es050165d CCC: $33.50 Published on Web 02/01/2006
2006 American Chemical Society
For coal-fired boilers, mercury leaves the high-temperature combustion zone as elemental mercury, and some transforms into oxidized forms as the temperature cools during postcombustion (1-3). Relative to Hg0, the Hg2+ compounds, such as HgCl2, in coal flue gases are less volatile and weakly to strongly soluble in water and can, therefore, be captured and removed in conventional air pollutant control devices (APCDs). Three control technologies for utility boilerssactivated carbon injection, wet flue gas desulfurization (FGD), and FGD dryerssshow promise to meet emission regulations (4). However, the efficiency of carbon injection is low for Hg0, and none of the current FGD systems can effectively capture Hg0. Thus, if mercury control targets are to be met, methods oxidizing Hg0 into Hg2+ in the flue gas from coal-fired power plants must be developed. In this study, the specific goal was to investigate the potential catalytic oxidation of Hg0 by chlorinating reagents. One of the advantages of using catalysts is that since mercury oxidation occurs at low temperatures, the thermal decomposition of formed Hg2+ back into its elemental form can be slowed or avoided (5). Heterogeneous oxidation of Hg with inorganic catalysts, such as CuO, Fe2O3, TiO2, and V2O5, was thought to promote Hg0 conversion to HgCl2 (6-12). One advantage of using inorganic catalysts is that the reaction can take place at low temperatures where the thermal decomposition of formed HgCl2 can be avoided. However, as in the case with homogeneous reactions, the inhibitory effects of NO and SO2 are still observed in some cases (3, 6, 13). This posed a question of how to effectively alleviate the inhibitory effects of NO and SO2. In the past decade, the gold catalyst applications, like oxidation of CO (14) and reduction of NO (14, 15), have begun to garner attention for their unique catalytic property (16). Gold has long been regarded as inert, and bulk gold surfaces do not chemisorb many molecules easily (14, 17). This inertial property may be instrumental for alleviating the inhibitory effects of NO and SO2. On the other hand, Baldeck and Kalb (18) found that gold can be quantitatively collected with elemental mercury from stack gas by direct amalgamation, and the capture was not influenced by the corrosive or reducing properties of substances such as SO2, H2S, and sulfuric acid mist. The resulting amalgam, probably Au2Hg3, can be desorbed by heating the gold. These findings thus became the basis for investigating the capability of gold to catalytically oxidize Hg0 with chlorine gas at elevated temperatures in this study.
Experimental Procedures To evaluate the performance of gold catalysts on mercury oxidation, a bench-scale system consisting of a quartz reactor, a temperature-controlled furnace, a chemical conditioning unit, and a mercury analytical instrument was constructed as shown in Figure 1. The reactor (about 22 mm i.d.) was vertically placed in the furnace, which brought the reaction temperature to the desired value. A rough surface gold catalyst was first prepared by placing a Teflon-coated quartz filter in a low-pressure argon-gold sputtering apparatus. The coating was applied at a rate of about 0.3 mg of gold/cm2 filter surface area. A piece of filter, containing around 1 mg of gold, was subsequently cut to the reactor diameter and placed in the reactor. Usually, the gas stream containing only N2 and Hg0 was sent into the reactor first. The total amount of mercury in coal-derived exhausts is typically less than 10 ppbv. Consequently, mercury does not affect the concentration of other species that participate in its own oxidation because VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Application of catalyst and chlorine at 473 K.
FIGURE 1. Bench-scale system for catalytic oxidation of Hg0. all are present at much higher concentrations (3). In our experiments, a relatively high Hg0 concentration, around 55 µg/m3, was employed to reduce the experimental errors caused by the sensitivity of the analytical instrument. The effluent Hg0 was continuously monitored. After the effluent concentration of Hg0 was more than 90% of the influent, indicating that Hg0 adsorption onto the gold was nearing saturation, other gaseous constituents such as SO2, NO, Cl2, H2O, O2, and CO2 were brought into contact with the gold catalyst and Hg0, depending upon the specified test conditions. All of the gases were preheated prior to entering the reaction cell, and the real-time reaction temperature was monitored by sliding a thermocouple to the location where the catalyst was positioned. The total flow rate, matching the flow into the mercury analytical analyzer, Semtech 2000, was maintained at 3.5 ppm. The commercial Semtech Hg 2000 mercury analyzer (Semtech Metallurgy AB, Lund, Sweden) is essentially a portable Zeeman-modulated coldvapor atomic absorption (CVAA) apparatus that is designed to measure only Hg0. The Semtech analyzer uses Zeemaneffect background correction by applying a modulated magnetic field to a mercury lamp to minimize interferences from the presence of SO2, hydrocarbons, and fine particulates in the flue gas sample. Even so, before proceeding to Semtech 2000, the sample stream was sent through a conditioning unit where SO2 and H2O were removed. The level of oxidation and/or adsorption percentage was estimated according to eq 1.
reduction of inlet Hg0, % )
Hginitial0 - Hgfinal0 Hginitial0
× 100% (1)
Results and Discussion Effect of Cl2. The adsorption of Hg0 on gold surfaces has been demonstrated in some commercial applications. DeBerry et al. (19) developed a gold-coated Nafion reactive membrane. The initial feasibility tests showed very good adsorption characteristics for elemental mercury. Mibeck et al. (20) established a conversion system with gold-coated sand to oxidize mercury in which mercury adsorption was also identified. Regarding Hg0 adsorption on gold surfaces, Butler et al. (21) believed that the interaction of mercury vapor with gold proceeds in two steps. Initial chemisorption of a monolayer of mercury on the gold was followed, at sufficiently high mercury partial pressures, by the subsequent adsorption of additional layers. The formation of these additional layers of mercury was necessary for the initiation of the amalgamation process. Once the amalgamation process started, there was considerable movement of both 1604
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mercury and gold atoms in the gold film, leading to the formation of a vermiform amalgam structure of the approximate composition Au2Hg3. One important consideration is whether Cl2 is also adsorbed onto the gold surfaces where the interaction subsequently occurs (Langmuir-Hinshelwood mechanism) or whether only one of the reacting species adsorbs, with the other coming into contact by kinetic collision from the gas phase (Eley-Rideal mechanism). The adsorption of Cl2 on gold at 120 and 500 K was examined by Kastanas and Koel (22). They found that chlorine desorbed in two peaks: a molecular desorption peak at 640 K and an atomic desorption peak at 790 K. In Mibeck et al.’s (20) research with goldcoated sand, the adsorption of Cl2 on gold was further supported by the continued reaction when Cl2 is removed from the gas stream; that is, Hg0 continuously reacted with adsorbed Cl2 on the gold surfaces. Another issue of whether the adsorption of Cl2 on gold surfaces is eventually dissociative also arises. Specifically, one would like to know if a stable chemisorbed molecular species is formed or if dissociative adsorption does occur. Kastanas and Koel (22) found that the dissociative adsorption of Cl2 was capable of producing Cl atoms on the gold surfaces, even at 120 K, which is just above the temperature required to form physisorbed Cl2 multilayers. No stable state of molecular Cl2 chemisorbed on the surfaces was detected. In addition, no evidence of surface chloride formation or chlorine diffusion to the crystal bulk was observed. However, how chlorine dissociates on gold surfaces is not yet clear. The first group of data was obtained to examine the roles of a gold catalyst and Cl2 in Hg0 oxidation. Results are illustrated in Figure 2. The first column was obtained with no Cl2 addition to the gas stream. Hg0 (balanced with N2) flowed over the catalyst, with the effluent reaching over 90% within 10 min. The second column, also serving as the baseline, was achieved in the absence of gold catalyst. In this testing case, Hg0 and 3 ppm Cl2 simultaneously flowed through the reactor. There was no appreciable change, as expected, on the effluent concentration of Hg0. For column three, Hg0 first continuously flowed through the catalyst until the effluent concentration of Hg0 exceeded 90% of the influent. When 3 ppm Cl2 was brought into the gas stream, the effluent concentration was lowered to 50% of the influent. As can be seen, Hg0 oxidation can be realized only in the presence of both Cl2 and gold catalyst at such a low reaction temperature of 473 K. The effects of Cl2 concentration and reaction temperature on Hg0 oxidation in the presence of gold catalyst (only Cl2, Hg0, and N2 in the flue gas) were subsequently evaluated. After the effluent concentration of Hg0 was more than 90% of the influent, Cl2 was introduced into the reactor. The resulting data for two different concentrations of Cl2 are illustrated in Figure 3, which shows 40-60% Hg0 being transformed to its oxidized form in the temperature range
FIGURE 3. Effect of Cl2 concentration on mercury oxidation. of 448-498 K. With the increases of temperature and chlorine concentration, more Hg0 was oxidized. At higher temperatures, the time required for the effluent concentration of Hg0 to exceed 90% of the influent was reduced, implying that less Hg0 was adsorbed onto the gold catalyst. However, this did not result in less Hg0 being oxidized by Cl2 at higher temperatures, as indicated in Figure 3. While the direct measurement of oxidized mercury was not taken in this study, it was shown by Mibeck et al. (20) that a large amount of formed Hg2+ was immediately liberated from gold surfaces when Cl2 was added to a Hg0-saturated gold-coated sand. A quantitative evaluation of the binding capabilities of Hg0 and Hg2+ onto gold surfaces based on Mibeck et al.’s results was inconclusive because of the condensation of Hg2+ in the sampling system. However, more than 95% of the formed Hg2+ was constantly detected in the flue gas, indicating that the binding of HgCl onto gold is not as strong as that of Hg0 (20). Mechanisms involving atomic and molecular chlorine have been proposed by many researchers (3, 5, 9, 23-28). Eqs 2-4 show one potential pathway, with Hg0 first reacting with atomic chlorine, while eqs 5 and 6 show a second pathway, with Hg0 first reacting with molecular chlorine. Pathway 1: Hg0 first reacts with atomic chlorine.
Hgad + Clad f HgClad
(2)
HgClad + Clad f HgCl2ad
(3)
HgClad + Cl2ad f HgCl2ad + Clad
(4)
Pathway 2: Hg0 first reacts with molecular chlorine.
Hgad + Cl2ad f HgCl2ad
(5)
Hgad + Cl2ad f Clad + HgClad
(6)
Modeling our reactor as a shallow bed gave an apparent activation energy of 7.2 kcal/mol-1 for the global reaction of Hg0 + Cl2 f HgCl2. In an attempt to identify the reaction pathways, the rate constant, k ) ATne-E/RT, for the case of 3 ppm Cl2 has been computed. As demonstrated in Table 1, the calculated k in global reaction 1 is of comparable magnitude to reactions 3 and 5 and extremely larger than that in reactions 2 and 4. Hence, on surfaces of gold, the key species oxidizing Hg0 is atomic Cl. That is, Hg0 first reacts with one atomic Cl to form HgCl, which, in turn, is oxidized into HgCl2 by another Cl. In essence, gold served as a media to dissociate the Cl2 molecule into atomic Cl, not to concentrate Cl2. Effect of NO. Research efforts have focused on the effects of NO in flue gases or simulated flue gases on Hg0 oxidation. Niksa et al. (3) indicated that the essential homogeneous
Hg0 oxidation sequence by chlorinating species at high temperature was a self-recycle process where the reaction first was initialized by atomic Cl. The formed HgCl, in turn, reacted with molecular Cl2 to generate HgCl2 with an associated regeneration of Cl. The inhibitory effect of NO on Hg0 oxidation was strong, and such an inhibition was mainly through the elimination of OH via the OH + NO + M f HONO + M reaction according to sensitivity analysis. As a comparison, a homogeneous reaction was carried out at temperature as high as 750 °C prior to this work (30). One of basic findings was that the oxidizing capability of Cl2 dramatically declined once NO was added into the gas stream with H2O being present, but there was little change in the absence of H2O. Therefore, it was thought that the inhibitory effect of NO on Hg0 oxidation through elimination of OH was actually associated with H2O when Cl2 served as the primary chlorine-containing species. In this study, the single factor effect of NO on Hg0 catalytic oxidation was first evaluated. As depicted in Figure 4, about 55 µg/m3 Hg0 was continuously added to the gas stream (balanced with only N2) at 9:02. The Hg0 effluent concentration increased from about 40 to about 50 µg/ m3 within 5 min. Then, 600 ppm NO was sent into the gas stream at 9:06 to identify its impact on Hg0 adsorption and/or oxidation. As seen from Figure 4, the effluent concentration of Hg0 immediately dropped to about 30 µg/m3, and the decline continued, reaching about 23 µg/m3 at 9:16. Subsequently, 8% H2O was added to the gas stream. Of interest was that the effluent concentration of Hg0 immediately recovered to about 50 µg/m3. Repeated tests under different conditions (Figure 5) demonstrated that such a phenomenon was duplicable. It was initially conjectured that the role NO played was to oxidize Hg0 into mercury nitrites or nitrates, while H2O functioned as an inhibitor to abate the oxidizing capability of NO. However, continued release of Hg0 when both NO and Hg0 were removed from the gas stream, as demonstrated in Figure 6, suggested that NO worked as a promoter to enhance Hg0 adsorption on gold surfaces regardless of its weak adsorption to gold surfaces (14, 16, 31). H2O has been reported to weakly adsorb on gold surfaces at low temperatures (14, 32). As to the pronounced decline of Hg0 oxidation/adsorption on gold surfaces resulting from H2O addition, there were two possible interpretations. First, the function of H2O was to hinder NO from adsorption on gold surfaces. However, no further evidence was available to support this interpretation as NO was reported to weakly adsorb on the gold surface. The second interpretation is that H2O was diminishing Hg0 adsorption. When only H2O was added to the gas flow, it was observed that the time required for the effluent Hg0 to reach 90% of the influent concentration was reduced. With the second interpretation, it was possible that H2O may also alter the Hg0-Cl2 reaction mechanism from Langmuir-Hinshelwood, having both Cl2 and Hg0 adsorbed, to Eley-Rideal with little to zero Hg0 being adsorbed. While it was recognized that NO was capable of inhibiting Hg0 oxidation through eliminating OH radicals (3), the experimental results (Figure 7) nevertheless demonstrated that the simultaneous addition of NO and H2O had little effect on Hg0 oxidation by Cl2. In particular, H2O, which was regarded to be diminishing Hg0 adsorption, did not dramatically affect the oxidizing capability of Cl2. Hence, it was inferred that the adsorption rate of Hg0 to gold surfaces was not an important indicator for Hg0 oxidation. In comparison with the homogeneous inhibitory effect resulting from NO introduction (3, 6), it was further conjectured that Hg0 oxidation was unaffected because Cl2 is strongly adsorbed on the gold surface, whereas NO was not, nor H2O. Effect of HCl. Meischen and Pelt (33) evaluated the effect of HCl on mercury oxidation with gold-coated sand. Their VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mercury Oxidation Reactions and Kinetic Parameters R1 R2 R3 R4 R5
reactions
A (cm3/(mol/s))
n
E (kcal/mol)
k at 473 K
source
Hg0 + Cl2 f HgCl2 (global) Hg0 + Cl2 f HgCl + Cl Hg0 + Cl f HgCl HgCl + Cl2 f HgCl2 + Cl HgCl + Cl f HgCl2
2.2 × 1019 1.39 × 1014 9.0 × 1015 1.39 × 1014 2.19 × 1018
0 0 0.5 0 0
7.2 34.0 0 1.0 3.1
1.0 × 1016 2.3 × 10-2 2.0 × 1017 4.8 × 1013 8.0 × 1016
calculation 3 3 29 29
FIGURE 4. Outcome for the addition of 8% H2O on mercury adsorption at 473 K.
FIGURE 5. Effects of 8% H2O and 600 ppm NO on mercury adsorption with gold at 473 K.
FIGURE 7. Effects of NO and H2O on mercury oxidation with gold at 473 K.
FIGURE 8. Effect of HCl on mercury oxidation with gold at 473 K.
FIGURE 9. Effect of SO2 on mercury oxidation with gold at 473 K. FIGURE 6. Release of Hg0 from gold surfaces after the removal of NO and Hg0 at 473 K. results showed that Hg0-saturated gold surfaces will interact with dilute gaseous HCl to enhance the oxidation of Hg0 most probably to mercury chloride. They speculated that the gold surfaces acted to concentrate the relatively diluted Hg0 and brought it into intimate contact with the chlorinating reagent, resulting in Hg0 oxidation and, in turn, the release of HgCl2 from the gold surfaces. However, the catalytic effect of gold on the reaction of Hg0 and Cl2 has not yet been 1606
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performed (33). In our study, the role of HCl in mercury oxidation was examined as well, with the result illustrated in Figure 8. Within the certainty of experimental error, HCl had slight effect on Hg0 oxidation. However, Hg0 oxidation by Cl2 was apparently decreased by the presence of HCl in the gas stream. In other words, the gold catalyst was somewhat deactivated by HCl. This implied that there was a competition for the active sites between HCl and Cl2 and that the actual oxidation effect of HCl, as determined under homogeneous conditions, was dramatically inferior to that of Cl2. On the other hand, as in homogeneous reactions, it is thought that mercury oxidation by HCl requires the
assistance of O2/O (3). However, it has been shown that bulk gold is inert to oxygen exposure (34). Bonding through the oxygen atoms does not occur (16). It is also unlikely that molecular oxygen could be an adsorbed/stable species on bulk gold. Although it has been speculated that molecular oxygen (and possibly dissociated) could be adsorbed on small gold particles, no direct experimental evidence has yet been found (14, 34). In Figure 8, one can also see that the HCl individually had little effect on mercury oxidation, even with the participation of O2. This does not agree well with Meischen and Pelt’s conclusions (33). Effect of SO2. Laudal et al. (35) examined the effect of SO2 on Hg0 oxidation by Cl2. Results indicated that the addition of SO2, if fly ash was not present, completely eliminated the effect of Cl2. Qiu et al. and Sterling et al. (13, 36) also indicated that SO2 had a large inhibitory effect on Hg0 oxidation mainly via the elementary reactions SO2 + OH f HOSO2 and SO2 + OH f SO3 + H to eliminate OH radicals. The effect of SO2 on Hg0 oxidation under a variety of conditions with the assistance of catalyst was evaluated in this study. Figure 8 indicates that SO2 had little impact on Hg0 catalytic oxidation. As other studies have shown, when the adsorption of SO2 on gold surfaces is examined, it was found that SO2 weakly adsorbed onto gold surfaces (bonding energy
