Efficient Selective Catalytic Reduction of NO by Novel Carbon-doped

Article pubs.acs.org/est

Efficient Selective Catalytic Reduction of NO by Novel Carbon-doped Metal Catalysts Made from Electroplating Sludge Jia Zhang,† Jingyi Zhang,† Yunfeng Xu,† Huimin Su,† Xiaoman Li,† Ji Zhi Zhou,† Guangren Qian,*,† Li Li,‡ and Zhi Ping Xu*,†,‡ †

School of Environmental and Chemical Engineering, Shanghai University, No. 333 Nanchen Rd., Shanghai 200444, P. R. China ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Queensland 4072, Australia

‡

S Supporting Information *

ABSTRACT: Electroplating sludges, once regarded as industrial wastes, are precious resources of various transition metals. This research has thus investigated the recycling of an electroplating sludge as a novel carbon-doped metal (Fe, Ni, Mg, Cu, and Zn) catalyst, which was different from a traditional carbon-supported metal catalyst, for effective NO selective catalytic reduction (SCR). This catalyst removed >99.7% NO at a temperature as low as 300 °C. It also removed NO steadily (>99%) with a maximum specific accumulative reduced amount (MSARA) of 3.4 mmol/g. Gas species analyses showed that NO removal was accompanied by evolving N2 and CO2. Moreover, in a wide temperature window, the sludge catalyst showed a higher CO2 selectivity (>99%) than an activated carbon-supported metal catalyst. Structure characterizations revealed that carbon-doped metal was transformed to metal oxide in the sludge catalyst after the catalytic test, with most carbon (2.33 wt %) being consumed. These observations suggest that NO removal over the sludge catalyst is a typical SCR where metals/metal oxides act as the catalytic center and carbon as the reducing reagent. Therefore, our report probably provides an opportunity for high value-added utilizations of heavy-metal wastes in mitigating atmospheric pollutions.

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effectively.9 It seems that there is no research on recycling electroplating sludge as the catalyst for NOx removal. NOx could be effectively removed by selective catalytic reduction (SCR) with metal catalyst using NH3, H2 and hydrocarbon as reducing agent.10−14 While, SCR using carbon as reducing agent also receives considerable attention in treating NOx these days.15 Compared with traditional NH3− SCR, carbon/char-SCR owns the advantages of low cost, having little problem with duct deterioration and reductant slip.16 In general, noble metal containing catalyst has a high activity in NOx-SCR,17 but the high price of noble metal has driven researchers to develop low-cost transition metal catalysts, such as carbon supported Fe, Co, Ni and Cu.15,18 Active carbon supported Cr and Cu catalysts,19,20 and char supported Fe, Cu and Co catalysts all showed activity in NOxSCR,21−24 in which carbon material functioned as the support and reducing agent.15,18 Note that in some reports, carbon doped/coated catalysts were synthesized, while carbon was only used as the catalyst support and NH3 as the reducing agent.25,26 Since electroplating sludge contains various metal species and organic matters, its pyrolysis can generate metal/metal oxide/

INTRODUCTION

With the development of modern industrialization, there are increasing amounts of electroplating sludges produced as heavy-metal wastes after the treatment of electroplating wastewaters.1 For example, China yearly generates more than 100 000 tons of electroplating sludges,2 which is featured by high enrichment of metals (Ni, Cu, Zn, Fe, and Cr etc.) as well as organics (surfactants etc.). Currently electroplating sludges are disposed mainly by landfilling and brick-making.3,4 Unfortunately, the former would cause potential pollutions to the ecoenvironment and occupy the land, and the latter is usually accompanied by high risk of heavy metal volatilization at high temperatures. Thus, it is more desirable to recycle electroplating sludges properly so as to reduce the secondary pollution and save the limited metal resource. Nowadays, applications of thermally treated electroplating sludge are continuously reported. After being calcined at 1000 °C, it can be used to prepare black inorganic pigments;5 Ni-, Cr-, Cu-, and Fe-containing electroplating sludge can yield magnetic ferrite at temperatures over 800 °C or with the help of thermal plasma.6,7 It is also mixed with red mud, the mixture of which is calcined at 550 °C to produce catalyst for propane oxidation.8 All these examples show that the electroplating sludge is recyclable. In our previous report, we found that pyrolyzed electroplating sludge, a mixture of various metals/ metal oxides and carbons, decomposed the greenhouse gas SF6 © XXXX American Chemical Society

Received: May 15, 2014 Revised: September 1, 2014 Accepted: September 5, 2014

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dx.doi.org/10.1021/es502391y | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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catalyst mass (specific accumulative reduced amount, SARA, mmol/g) at a certain time point, each NO removal profile was treated by the method reported in our previous papers (SI Figure S2).9 Characterization. XRD patterns of sludge catalysts were recorded on a Rigaku D/max RBX X-ray diffractometer with Cu−Kα radiation (λ = 0.154 nm). Scanning rate was 6 o/min in 2θ range of 5−80 o. Metal contents of solids were analyzed by completely dissolving 0.010 g sample in 50 mL 5% HNO3 and the metal concentrations were determined with inductively coupled plasma optical-emission spectrometer (ICP-AES) (Model Prodidy, Leeman). Carbon weight percentages in catalysts were determined by CHN element analyzer (Jena EA). Transmission electron microscopy (TEM) images were obtained on a JEOL 1010 electron microscope operating at an acceleration voltage of 100 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL 2100 electron microscope operating at an acceleration voltage of 200 kV.

carbon mixture, which would be a potential substitution for metal catalyst in NOx-SCR. Therefore, the objectives of the present research were to (1) examine the feasibility and efficiency of an Fe,Ni-rich electroplating sludge-derived catalyst for NO-SCR under various conditions; (2) reveal the influence or role of carbon on NO removal; and (3) understand the catalytic mechanism of NO removal over the electroplating sludge-derived metal/metal oxide/carbon hybrid catalyst based on the structural changes before and after NO removal test.

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EXPERIMENTAL SECTION Catalyst Preparation. Electroplating sludge (10 g, Shanghai Xinsheng Electroplating Co., Ltd.) was first washed in water (100 mL) via vigorous stirring for 1 h, and then collected by filtration and dried at 105 °C overnight. Element analysis revealed that the washed sludge contained 19.0% Fe, 10.8% Ca, 9.5% Ni, and 8.7% C (other components were listed in Supporting Information (SI) Table S1), and the rest element was mainly oxygen as these metals existed in the form of oxide/ hydroxide. The dried solid was then pyrolyzed at 700 °C for 2 h under argon, and milled to the particle size between 100 and 150 meshes in air. After pyrolysis, there was 2.33 wt % carbon in the sludge, which was denoted as sludge catalyst hereafter. In some cases, 5 wt % active carbon (AC) powder was physically mixed with 95 wt % sludge catalyst before and after the NO removal test. To compare with the sludge catalyst, FeNiO was prepared by precipitating ferric and nickel nitrate in NaOH solution to mimic the main metals in the electroplating sludge. The precipitate was collected, washed and pyrolyzed at 700 °C for 2 h. Similarly, a mixture of AC and FeNiO was made by mixing 5 wt % AC with salt solutions, followed by precipitation and activation. In addition, metallic FeNi was also mixed with 5 wt % AC to make a metal/AC catalyst. These solids were all milled into particles with the size from 100 to 150 meshes. Experiment Setup and NO Removal. The experimental setup was showed in SI Figure S1. Briefly, the catalyst was filled in the middle part of a quartz reactor with an inner diameter of 8 mm, with a packing density being approximately 1.27 g/cm3. At first, a flow of Ar flushed the reaction system for 30 min to clean residual air. Then NO (990 ppm, balanced with Ar) was introduced into the reactor at a flow rate of 30.0 mL/min for 30 min to stabilize gas flow. The outlet stream was diluted with Ar by mass flowmeters (D07−7B, D07−19B Beijing Seven-star Electronics Co., Ltd.) to gain an NO concentration of 51.2 ppm. NO concentration was detected by Chemiluminescence NO-NO2−NOx Analyzer (Thermo Scientific 42i). When 0.64 g of catalyst was used, gas hourly space velocity (GHSV) was about 3600 h−1 (= 30.0 × 60/(0.64/1.27)). To compare the NO removal efficiency with the traditional NH3−SCR process, we applied the sludge catalyst to remove NO (990 ppm of NO, 990 ppm of NH3 and 3% O2, balanced with Ar) under similar conditions. After the detected NO concentration became steady (51.2 ppm), tube furnace started to increase the temperature to 250− 750 °C and then keep at that temperature for testing. NO concentration was monitored in every minute by NO-NO2− NOx Analyzer during the whole process. At some time points, concentrations of CO, CO2 and N2 in the evolved stream were detected by standard calibration method using GC (GC 7900). Data Acquisition for Lifetime of Sludge Catalyst in NO SCR. In order to get accumulative reduced NO amount per unit

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RESULTS AND DISCUSSION Temperature-Dependent NO Removal by Sludge Catalyst. Figure 1A shows NO removal profile over sludge catalyst at 300, 450, and 550 °C with GHSV = 3600 h−1. Surprisingly, 99.7 ± 0.2% of NO was removed for a period of time (defined as the breakthrough time) in this temperature range, and then removal percentage quickly decreased. The breakthrough time was about 220, 680, and 720 min at 300, 450, and 550 °C for NO removal >99.7%. It seemed that NO removal capacity, for example, the time for >99.7% NO removal, not the NO removal percentage, increased with increasing reaction temperature from 300 to 550 °C. This temperature-dependent NO removal style is in sharp contrast to what has been reported for many oxide catalysts, where NO removal percentage increased with the reaction temperature.27,28 In addition, the activity of the sludge catalyst is comparable to reported catalysts (such as CeO2-doped CNT) in terms of the temperature for nearly 100% NO removal,27 but much better than that using V2O5-doped AC.28 When the reaction temperature was raised to 750 °C, NO removal capacity slightly decreased, compared to that at 450 °C (Figure 1B). The breakthrough time at 750 °C was 1460 min, slightly less than 1490 min at 450 °C. This thus suggests that the sludge catalyst had a wide temperature window (300−750 °C) for the effective NO removal. In contrast, many catalysts only had a narrower temperature window, such as in the range of 300−450 °C.27,28 A wide temperature window is very critical for NO removal since local temperature in a reactor could exceed the furnace temperature due to the exothermal process. From this point of view, the sludge catalyst could be a suitable candidate for NO removal. Accumulative NO removal amount per unit catalyst mass (specific accumulative removal amount, SARA, mmol/g) was further estimated until NO removal 99.7% for about 680 min at 450 °C (GHVS = 3600 h−1), while the removal decreased to 50% within just 60 min. B



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repeatable, as two MSARA values were almost the same under the same conditions (such as 450 °C). Therefore, our findings (Figures 1) indicate that (1) 990 ppm of NO could be almost completely removed by the sludge catalyst for a period of time in a wide temperature range (300− 750 °C), where the breakthrough time was dependent on the temperature. (2) MSARA of the sludge catalyst was determined by the reaction temperature and GHSV, and a temperature in 450−750 °C with GHSV at 1800−3600 h−1 could give rise to a quite similar MSARA (3.0 mmol/g), showing a steady NO removal in a wide temperature window; (3) the highest MSARA (3.27−3.35 mmol/g) could be obtained at 450 °C and 1800 h−1. This reaction temperature is fairly close to that using other best catalysts reported elsewhere.28 Effect of Carbons. As shown in Figure 1A and 1B, the sludge catalyst seemed to suddenly lose the activity after the breakthrough, which is substantially different from catalyst that gradually lost its activity.29 It is our belief that carbon materials in the sludge catalyst play a very important role in NO removal. As shown in Figure 2A, AC only removed 30−50% NO and the synthetic mixed oxide (FeNiO) showed little activity in NO removal at 750 °C. The mixed catalyst (95% FeNiO+5% AC) removed 80−90% NO in the first 10 min and then the removal decreased to 50% within 60 min. In a striking contrast, the sludge catalyst and metallic FeNi+AC catalyst both removed >99.7% NO for some time (Figure 1B and 2A, SI Figure S4)

Figure 1. NO removal at different temperature (A) GHSV = 3600 h−1; (B) GHSV = 1800 h−1 (a: 95% sludge catalyst +5% AC); (C) NO MSARA under different temperature (°C).

The maximum SARA (MSARA) of the sludge catalyst in a particular condition was obtained when SARA did not change with time, as shown in Figure 1C. Clearly, MSARA was affected by the reaction temperature and GHSV. At the same GHSV, MSARA increased with increasing temperature until 550 °C. At 3600 h−1, MSARA increased from 1.25 to 3.19 mmol/g when the reaction temperature increased from 300 to 550 °C. However, MSARA slightly decreased to 2.82−3.04 mmol/g when the reaction temperature further increased to 750 °C. As expected, MSARA decreased with an increasing GHSV at the same temperature. At 300 °C, MSARA was 0.54 and 1.25 mmol/g at 14400 and 3600 h−1. Similarly, MSARA decreased from 3.27 to 3.35 to 2.88−2.98 mmol/g when GHSV increased from 1800 to 3600 h−1 at 450 °C. Thus, MSARA seemed to reach the highest value (3.2−3.4 mmol/g) at 450−550 °C and 1800−3600 h−1. It is worth pointing out that NO concentration measurement and consequently deduced MSARA were

Figure 2. (A) NO removal by different agent; (B) NO MSARA under different residual carbon contents. C



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Table 1. Gaseous Species at Different Reaction Temperature (ppm) (GHSV = 14400 h−1) R.T 300 400 500 600

°C °C °C °C

NO

N2

CO2

CO

N2O

Nt

Ot

51.20 15.56 − − −

− 18.99 23.02 25.47 26.12

− 13.38 22.64 24.20 24.10

− 0.18 1.95 0.51 1.62

− 1.30 − − −

51.20 56.04 46.06 50.94 52.24 51.34 ± 4.17

51.20 43.80 47.23 48.91 49.82 47.44 ± 2.43

“−” stand for not detected. Total N (Nt) = NO + 2N2 +2N2O; Total O (Ot) = NO + 2CO2 + CO. In theory, Nt = Ot = initial NO concentration (51.2 ppm).

under the same conditions. The high activity of the sludge catalyst thus must be related to the special carbon/metal/metal oxide hybrid species and these species are at least different from a simple combination of metal oxide and AC. In this connection, the relation was obtained between MSARA vs retained carbon amount in the sludge catalysts after reactions under various conditions (Figure 2B). NO MSARA decreased almost linearly with the retained carbon amount (R2 > 0.98). The linear relationship thus reveals that carbon material in the sludge catalyst is tightly relevant to NO removal. The carbon decreased in the sludge catalyst along with NO removal (Figure 2B), demonstrating that carbon material directly involved in NO removal. Therefore, we dynamically monitored the outlet gaseous composition for 30 min from 300 to 600 °C. As listed in Table 1, NO was completely removed except for the case at 300 °C due to too high GHSV. The main resultant gaseous species were N2 and CO2, with trace amounts of N2O (only at 300 °C) and CO. More interestingly, total [N] (Nt) was 46−56 ppm, with an average of 51.34 ppm (Table 1), very close to the inlet NO concentration (51.20 ppm). Unexpectedly, total [O] (Ot) was not constant, but increased with the reactor temperature. The average Ot was 47.44 ppm, obviously lower than the inlet NO concentration (51.20 ppm), which may be related to the chemical changes of the sludge catalyst, as discussed below. Having found the relationship between carbon amount and MSARA, we then tested the efficiency of the sludge catalyst (95 wt %) + AC (5 wt %) in NO removal. As shown in Figures 1B and 1C, the breakthrough time was extended from 1560 to 2140 min and the MSARA increased from 3.04 to 4.07 mmol/g at 750 °C. These data obviously indicate that AC facilitates NO removal by about 30%, probably through reacting with part of added AC over the interface contacts between AC and metals/ metal oxides. In addition, the sludge catalyst showed a higher NO removal activity than NH3−SCR under similar operation conditions (SI Figure S5), in which carbon seemed to be consumed by oxygen. Microstructural Transformation of Sludge Catalyst. Figure 3 displays the XRD patterns of the sludge catalyst before and after catalytic reactions. As-pyrolyzed sludge catalyst was featured with two diffraction peaks, with the d values being 0.207 and 0.179 nm (metallic FeNi), respectively. Such an XRD pattern is well matched with that of Fe0.64Ni0.36 (PDF #47− 1405),30 and carbon-combined FeNi.31 Note that FeNi was the main metals in the sludge catalyst. However, the sludge contained other metals, such as Mg, Cu, Zn and Cr (SI Table S1). Since the original electroplating sludge contained various amounts of surfactants, pyrolysis at 700 °C may reduce metal cations to metal. Meanwhile, some surfactants may transform to carbon (2.33 wt %).32

Figure 3. XRD patterns of sludge catalysts before and after NO removal tests. “sludge-450 °C-2h” and “sludge-450 °C” stand for 700 °C-activated sludge catalyst reacting for 2 h and until very low activity in NO-SCR at 450 °C, respectively.

As shown in TEM images, small particles (10−50 nm in size, Figure 4A) were surrounded by many curved sheets (Figure 4B). These two components, further enlarged in the HRTEM image (Figure 4D), were found to be graphite-like carbon (Figure 4E) and metallic phase (Figure 4G), and interconnected into carbon−metal hybrid structure. Figure 4E shows the carbon species to be regular fringes with a spacing of 0.34 nm, which is the typical layer distance of graphite-like carbons and further evidenced by the point marked as (0002) in SAED (Figure 4F).33 The high resolution TEM image (Figure 4G) shows an ordered pattern with the plane distances being 0.20 (111) and 0.18 nm (200), which is well matched with the XRD pattern (Figure 3) and the SAED pattern in Figure 4F.31,34 In addition, a weak peak with the d value of 0.282 nm could be identified and attributed to the small amount of FeNi oxide (Figure 3), which is probably corresponding to the marked point (X) in Figure 4F. In terms of chemical composition of the sludge catalyst, EDX demonstrates that the sludge catalyst contained Fe, Ni, and C as well as a small amount of O (Figure 4C), meaning that there would be some metal−oxygen species, as the XRD and SAED patterns indicated. After reactions at 450 and 750 °C, the metallic phase seemed to transform into mixed metal oxides. As shown in Figure 3, the XRD pattern of sludge catalyst completely changed, with the main diffraction peaks very similar to NiFe2O4 (PDF #10− 0325),35 and two d values was 0.294 and 0.251 nm. Interestingly, 2-h catalytic reaction changed the sludge catalyst D



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carbons that could not be involved at 300 °C could be involved at 450 or 550 °C. This hypothesis, for example, activity difference of carbon species, has been further supported by the test using mixed sludge catalyst/AC (Figure 1B and 1C). When the sludge is physically mixed with AC, some more carbon− metal/metal oxide contact points are generated to use more carbon for NO removal. However, only a small part of AC is involved, as discussed below. Mechanism of NO SCR via Sludge Catalyst. According to above analyses, NO SCR by the sludge catalyst could be expressed in eq 1: Me

2NO(g) + C(s) ⎯→ ⎯ N2(g) + CO2 (g)

(1)

C is the special carbon species, and Me the catalytic metals in the sludge catalyst, such as main elements FeNi, and trace elements Cu and Cr (SI Table S1). The reaction temperature of NO and C is usually over 700 °C.36 However, with the sludge catalyst, eq 1 took place at a temperature as low as 300 °C (Figure 1). It is proposed that the catalytic reaction between NO and carbon by metal species involves metal oxidation by NO and subsequent reduction of the resulting oxide by carbon.15,18 In eq 1, one carbon atom reacts with two NO molecules, which is very close to the slope of the line (1.63) in Figure 2B. This further supports the involvement of carbons in NO removal, probably in the following way: Cn − MeOx + NO → Cn − MeOx − NO

(2)

2Cn − MeOx − NO → Cn − MeOx − O + Cn − MeOx − N2O

Cn − MeOx − N2O → Cn − MeOx − O + N2

(3) (4)

2Cn − MeOx − O → Cn − 1 − MeOx + Cn − MeOx + CO2

(5)

Cn − MeOx − O → Cn − 1 − MeOx + CO

(6)

Cn − MeOx − O → Cn − MeOx + 1

(7)

Since the sludge catalyst (Cn-MeOx) mainly contained metallic FeNi before catalytic reactions (Figure 3), thus x ≈ 0. Note that other transition metals, such as Ni, Cu and Cr in the sludge catalyst (SI Table S1) could also function as the catalytic center.15 As shown in SI Figure S6, NO is first adsorbed (eq 2). Part of adsorbed NO would desorb at 200 °C (SI Figure S7). Other adsorbed NO then dissociates and releases N2 (eqs 3 and 4). N2O was only observed in the case at 300 °C (Table 1), together with some N left in various N-oxide species on the catalyst surface (SI Figure S6). The left O oxidizes C into CO2/ CO (eqs 5 and 6) and metal to metal oxide (eq 7). Consequently, the freshly formed carbon−metal/metal oxide hybrids (e.g., Cn‑1-MeOx in eq 6 and Cn-MeOx+1 in eq 7) can be used again in eqs 2−7, so that more carbons and metals are oxidized to CO2 and metal oxides. This mechanism suggests 3fold functions of metallic FeNi: adsorbent for NO (eq 2), transient reservoir of active O (eqs 3 and 4), and reducing agent (eqs 5−7). These functions would be also reflected in the test for FeNi+AC catalyst with ∼100% NO removed (SI Figure S4). According to these reactions, NO removal by the sludge catalyst is a typical process of selective catalytic reduction. When the available carbon is consumed with metals being oxidized to metal oxide (Figure 3), its activity for further NO

Figure 4. (A-G) TEM (A,B), EDX (C), HRTEM (D, E, G), and SAED (F) of the sludge catalyst; TEM (H) and HRTEM (I) after catalytic test at 450 °C. Point X in SAED pattern (F) is attributed to the metal oxide.

phase, which contained both FeNi and NiFe2O4 phases (Figure 3). This change indicates the transformation of partial metal to mixed metal oxide, probably using NO as the oxidizing reagent. The oxidization thus consumed some oxygen from NO, leading to a decreased total oxygen in the outlet stream (Table 1). Furthermore, Figure 4H indicates that there was only little amount of carbons left surrounding the metal/metal oxide structure after the catalytic test. The formed metal oxide had an ordered plane pattern with distances between planes being 0.30 and 0.26 nm (Figure 4I), which was also in good agreement with the XRD pattern (Figure 3). During NO removal, carbon materials in the catalyst were consumed to generate CO2 and CO. The amount of carbons that could be consumed is believed to be dependent on the reaction temperature. At 300 °C, the consumed carbon (1.10 wt % out of 2.33 wt %) was much less than that at 450 °C (1.98 wt % out of 2.33 wt %). Some carbons seemed to be more active than the others in the reaction with NO, so that the E



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removal becomes very low. When this metal oxide was mixed with 5% AC, it could be reused in NO-SCR, but with a much lower activity than the inherent carbon in the sludge catalyst (SI Figure S8). As previously pointed out, some AC contributes to eqs 5 and 6 at a temperature as low as 250−300 °C while others can only join the reactions at higher temperatures. It is our hypothesis that the distance between carbon and metal/metal oxide catalytic center determines the involvement degree of carbon materials. When carbon atoms are very close to metal/metal oxide, they could join the reactions at lower temperatures. Increasing the reaction temperature could thermally move carbon atoms faster for a longer path, so that some carbons that are apart from the metal/metal oxide and not available for eqs 5 and 6 at lower temperatures could now take part in the reactions. This hypothesis could explain why the time for >99.7% NO removal was extended from 300 to 550 °C (Figure 1A). It seems that the metals are first oxidized or temporarily store the active ‘O’ after adsorption and dissociation of NO. Then some of the resulting oxide is reduced by carbon. When the available carbon is exhausted and metallic FeNi is oxidized, the sludge catalyst loses its activity in NO-SCR. More importantly, the effective NO SCR has also been resulted from the structure advantage of the sludge catalyst. The metal particles are covered with high-efficient carbons during the activation (Figure 4), providing an effective microinterface full of active sites for NO reactions (SI Figure S6). This structure is totally different from a mere mixture of metal oxide and AC (Figure 3), and highlights efficient carbon−metal/metal oxide hybrid species. As presented above, the sludge catalyst with 2.33 wt % C (1.94 mmol/g) could react with 3.0−3.4 mmol/g NO, that is, 80−90% C was used for eq 5. However, when sludge was added with 5 wt % AC, the obtained MSARA was only 4.07 mmol/g (Figure 1C), although the theoretical value should be up to 13.8 mmol/g. The low usage of added AC is because majority of physically mixed carbons are too far away from the metal/ metal oxide catalytic center to be used in the reactions. In comparison, metals and organic materials are highly dispersed with each other in the sludge, and thus pyrolysis makes carbon and metal species fairly close to each other to form carbon/ metal/metal oxide hybrids. The sludge catalyst seems to be stable in air at the room temperature as various milled catalysts showed the similar activity in NO-SCR (Figure 1). However, high temperature would facilitate carbon oxidation with O2 over the sludge catalyst as the MSARA was smaller in the presence of O2 in the stream (SI Figure S9). In addition, CO2 selectivity (= CO2(ppm)/[CO(ppm) + CO2(ppm)]) is compared between the sludge catalyst and FeNiO + 5% AC (SI Figure S10). CO2 selectivity of FeNiO + 5% AC decreased from 100% to 50% when the reaction temperature increased from 450 to 750 °C. By comparison, CO2 selectivity was retained at >99.5% when using the sludge catalyst at this temperature range. According to eqs 5 and 6, a high CO2 selectivity would result in a higher NO MSARA. Thus, for the first time, the electroplating sludge has been made into a novel carbon/metal/metal oxide hybrid catalyst for effective NO SCR. In practice, the used sludge catalyst could be regenerated by redispersing in organic wastewater to trap organic wastes and subsequently pyrolyzing. Our findings may open a good opportunity for management and recycling of heavy metal wastes.

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ASSOCIATED CONTENT

* Supporting Information S

Supplementary data associated with this article. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(G.Q.) Phone: 86-21-66137758; fax: 86-21-56333052; e-mail: [email protected]. *(Z.P.X.) Phone: 61-7-33463809; fax: 61-7-33463973; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by National Nature Science Foundation of China No. 21107067, No. 51174132, No. 21477071 National Major Science and Technology Program for Water Pollution Control and Treatment 2009ZX07106-01, 2008ZX0742-002 and program for Innovative Research Team in University (No.IRT13078). We also thank Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation, and China Scholarship Council.

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Efficient Selective Catalytic Reduction of NO by Novel Carbon-doped

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