Article pubs.acs.org/EF
Improved Arsine Removal Efficiency Over MnOx Supported Molecular Sieves Catalysts via Micro-Oxygen Oxidation Yilong Lin, Xueqian Wang, Jiming Hao, Ping Ning,* Guangfei Qu, Yixing Ma, and Langlang Wang Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China ABSTRACT: With the aim of improving the arsine (AsH3) removal efficiency, 5A molecular sieves were modified by the transition metal oxides and used as catalysts. MnOx catalysts supported on 5A molecular sieves (Mn-5A) were prepared by an impregnation method and subsequently tested toward the catalytic oxidation of AsH3 under micro-oxygen conditions at 150 °C. The effects of the metal oxide types that supported, the impregnate concentration of manganese nitrate, the calcination temperature, the oxygen content, and the fixed bed reaction temperature on the AsH3 removal efficiency were studied. The optimum catalyst was prepared by a solution impregnation method with a 0.5 mol·L−1 Mn2+ aqueous solution and subsequently calcined at 500 °C. This catalyst showed the highest activity toward the oxidation of AsH3 among the samples prepared herein (100% conversion) over a good low-temperature operation window (120−160 °C) and under low oxygen concentration (0− 2.0%) conditions. The majority presence of Mn4+ active species is believed to be one predominant factor accounting for the excellent AsH3 removal efficiency of this catalyst. The dominant active Mn4+ species of Mn-5A is found to be a mixture of MnO2, Mn3O4, and Mn2O3. BET area analysis coupled with EDS measurement show that after oxidizing AsH3 with O2, the resulting As2O5 seems to block the surface of Mn-5A leading to a decrease of the activity. XPS data indicates that the oxidation state of MnOx species on Mn-5A was not affected after reaction. So the main deterioration mechanism of Mn-5A on AsH3 removal under our reaction condition is mainly due to physical blocking effect. According to the characterizations results, the Mn-5A catalyst possessed a higher number of active sites than the rest of samples and formed active intermediates during reaction, both factors leading the highly efficient removal of AsH3 under low-temperature micro-oxygen conditions.
1. INTRODUCTION Arsenic pollution is a worldwide issue.1 While the presence of high levels of arsenic in waste waters and contaminated lands has been pointed out by numerous research studies,1−6 gaseous arsenic emissions have not received adequate attention, being easily overlooked despite their toxicity.4 Nearly all arsenic is converted to arsine (AsH3) under the reducing environment required for the gasification process.7 The growing industrialization of China has increased the AsH3 emissions, for example, from the ore smelting process. AsH3 is also produced as a byproduct in metal extraction processes, or as a residue of burning fossil fuel and coal.7−10 Even at low concentrations, AsH3 is toxic to the organism, and may cause damage to the kidney.4,11,12 The unstable nature of AsH3 facilitates its diffusion and conversion into other arsenic species, which can subsequently affect organisms and pollute the environment. Thus, AsH3 emissions into the atmosphere should be avoided. Gas−solid reactions allow efficient removal of AsH3.13 The gas−solid system can directly remove AsH3 by stabilizing it into stable arsenic, thereby avoiding subsequent migration and transformation of arsenic species. From the practical and economical viewpoints, two methods are available for the removal of AsH3. First, AsH3 can be removed via adsorption on activated carbon or potassium permanganate.14,15 However, these materials are not suitable for AsH3 adsorption since they suffer from an As re-emission problem or secondary pollution issue of the absorption liquid. Additionally, AsH3 can be removed with significantly higher removal efficiencies via catalytic oxidation. In this sense, 5A molecular sieves are promising support materials owing to their high crush strength and high-temperature tolerance characteristics. With regard to © XXXX American Chemical Society
the oxidation products, AsH3 can be converted into oxidized arsenic, As(III), or As(V) species. The development of efficient catalysts that can catalyze the oxidation of AsH3 under appropriate conditions at low cost has become very important. Oxide-supported noble metals and transition metal oxides have been used for the catalytic oxidation of AsH3.7,16,17 In this sense, Pt catalysts have been recently employed for the removal of AsH3.18−20 Precious metal catalysts exhibit excellent activity toward the oxidation of AsH3, although they suffer from a number of issues (e.g., high cost, fast deactivation, and high operation temperature) that hinder their practical applications. Transition metal oxides (e.g., CuO, CrO, ZnO, and Al2O3) are more promising catalysts with similar catalytic performance toward the oxidation of AsH3 than precious metal catalysts. Among them, CuO−Al2O3 mixed oxide catalysts have been extensively studied because of their high activity and low toxicity. In this sense, Quinn et al. reported on CuO−Al2O3 mixed oxides prepared by coprecipitation with very high activity toward the oxidation of AsH3.21 Manganese oxide (MnOx) has a large number of applications in numerous scientific fields.22 As a significant transition metal oxide, MnOx is a low-cost material with excellent lowtemperature oxidation activity and durability. According to characterization data, the good catalytic performance of MnOx has originated from the high oxidation state of Mn and the presence of reactive surface oxygenated species.23−25 Despite Received: May 26, 2017 Revised: August 10, 2017 Published: August 14, 2017 A
DOI: 10.1021/acs.energyfuels.7b01477 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels the importance of this topic, the specific role of Mn remains unclear, and, to the best of our knowledge, this material has not been used for the removal of AsH3 so far. Considering the good performance of transition metals toward the removal of AsH3 and the urgent need for catalysts efficiently operating under low temperature and micro-oxygen conditions, we developed herein a MnOx catalyst supported on 5A molecular sieves (denoted as Mn-5A). The obtained catalyst showed high activity over a relatively wide temperature window. The physical−chemical properties, phase structures, and surface properties of the catalysts were characterized by N2 adsorption−desorption, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) with the aim to reveal the role of the Mn additive.
Figure 1. Schematic diagram of the experimental setup used for the catalytic tests. (1) O2 cylinder; (2) AsH3 (100 ppm AsH3 in N2) cylinder; (3) rotameter; (4) temperature controller; (5) mixer; (6) fixed bed unit for AsH3 with thermostatic apparatus; (7) AsH3 absorption liquid; (8) potassium permanganate solution.
2. MATERIALS AND METHODS 2.1. Catalyst Preparation. 5A molecular sieves (Nan Kai University, Tianjin, China) were used as a catalytic support. A series of modified 5A molecular sieve catalysts with varying MnOx molar loading amount were prepared by impregnation method. Each 5A molecular sieve sample was washed three times with distilled water to remove soluble impurities and subsequently dried at 105 °C for 12 h (denoted as 5A). 5A molecular sieve (2.0 ± 0.1 g) was impregnated with 10 mL of 0.50 mol·L−1 Pb(NO3)2, Co(NO3)2, Ce(NO3)3, Ni(NO3)2, or Mn(NO3)2 aqueous solutions at room temperature, respectively. The suspension was stirred for 24 h, followed by subsequently drying at 105 °C for 12 h, and calcined at 500 °C for 6 h to finally obtain samples denoted as Pb-5A, Co-5A, Ce-5A, Ni-5A, and Mn-5A, respectively. 2.2. Catalyst Characterization. An Agilent 725 simultaneous ICP-OES with radially viewed plasma was used for the elemental analysis. The Agilent 725 ICP-OES features a custom-designed CCD detector, which provides true simultaneous measurement with comprehensive wavelength coverage from 167 to 785 nm for fast, precise measurements of all elements of interest. A multispot NOVA2000e N2 adsorption meter (Quanta Chrome Corp.) was used to obtain the N2 adsorption−desorption isotherms at 77.35 K. The Brunauer−Emmett−Teller (BET) surface areas of the catalysts were calculated from the N2 isotherms. The pore volume and average pore diameters were determined from the desorption branch of the isotherms by the Barrett−Joyner−Halenda (BJH) method. The XRD patterns of the materials were recorded on an X-ray powder diffractometer (Bruker D8 Advance) equipped with Cu Kα radiation at a scanning speed of 0.4° min−1 (10−80°) operating at a voltage of 40 kV and with an applied potential current of 40 mA. The XPS spectra were collected on an America Thermo ESCALAB 250Xi device equipped with an X-ray source and operated with an Al Ka anode and a photoenergy (hυ) of 1486.6 eV. The Mn 2p and As 3d binding energies were calibrated by using the C 1s (BE = 284.8 eV) as a standard. The obtained spectra were deconvolved by using Lorentzian−Gaussian functions with a Shirley background. 2.3. Catalytic Activity Tests. The gas fixed-bed reactor used for the catalytic tests is shown in Figure 1. The catalytic activities of the samples (40−60 mesh) were measured in a quartz columnar fixed-bed reactor (8 mm inner diameter) at 120−160 °C. The reaction conditions, which simulated an industrial waste gas stream, were as follows: 1000 mg of catalyst, 200 ppm of AsH3, 1.0 vol % O2 (balance N2), and a total gas flow rate of 100 mL·min−1. The concentration of AsH3 was measured by diethyl dithiocarbamic acid Ag spectrophotometry. All the catalytic data were collected after an induction period of 30−60 min at the selected temperature in order to achieve steadystate reaction conditions. The AsH3 removal efficiency was calculated by the following equation:
AsH3 removal efficiency (%) = 1 −
[AsH3]in − [AsH3]out [AsH3]in
where AsH3in and AsH3out are the concentrations of AsH3 measured at the inlet and outlet of the reactor, respectively. The “breakthrough time” was defined as the time at which the AsH3 outlet concentration reached 10% of the inlet concentration in the breakthrough curves.
3. RESULTS AND DISCUSSION 3.1. Effect of Metal Oxides Types Supported to 5A Molecular Sieves on the AsH3 Removal Activity. Various types of transition metal oxides including PbOx, CoOx, CeOx, NiOx, and MnOx, were supported on 5A molecular sieves by impregnating 5A molecular sieves with their nitrate salts (impregnation concentration = 0.5 mol·L−1). As indicated above, five modified 5A molecular sieves with different oxides were studied (Pb-5A, Co-5A, Ce-5A, Ni-5A, and Mn-5A), and their catalyst activities were compared by AsH3 dynamic removal capacity tests. The breakthrough times of the above catalysts are plotted in Figure 2. Depending on the catalysts, the breakthrough curves showed different profiles and efficiencies toward the removal of AsH3. Despite being prepared under similar conditions, the Pb-5A, Co-5A, and Ce-5A samples showed very poor activity toward
Figure 2. Breakthrough times for the different metal oxide catalysts supported on 5A molecular sieves. Reaction conditions: impregnation concentration = 0.5 mol·L−1; reaction temperature = 150 °C; calcination temperature = 500 °C; [AsH3] = 200 ppm; and [O2] = 1.0 vol % (balance N2).
(1) B
DOI: 10.1021/acs.energyfuels.7b01477 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
min. With the aim to further understand these experimental results and evaluate the performance of the catalysts, the actual Mn metal loadings of the catalysts were measured by inductively coupled plasma (ICP). In Table 1, as the impregnation concentration of manganese nitrate increased from 0.2 mol·L−1 to 0.8 mol·L−1, the manganese element loading amount increased linearly. The samples of Mn-5A with better activities, which are impregnated with 0.5 mol·L−1 and 0.6 mol·L−1 Mn2+, had Mn loading amount of 12.15 and 12.65 wt %, respectively. The loading amount of Mn in these two samples is almost the same, so the removal efficiency performances of these two samples are very similar. Excessive loading amount of Mn had a negative effect on the AsH3 removal capacity of Mn-5A, which might be assigned to the blocking of the micropores of 5A molecular sieves by too much Mn loading. From the practical point of view, the optimized loading amount of Mn on 5A is ∼12 wt % with an impregnation concentration of ∼0.5 mol·L−1. 3.3. Influence of the Calcination Temperature. The calcination temperature, which may influence the formation of active species, the BET area, and the mechanical strength, is a crucial parameter for the preparation of highly active catalysts. The influence of the calcination temperature (400−600 °C) on the oxidation performance of Mn-5A is summarized in Figure 4.
the removal of AsH3 as compared to Ni-5A and Mn-5A. Thus, these three metal oxide catalysts were not suitable for the removal of AsH3. In contrast, Ni-5A and Mn-5A showed relatively high breakthrough times and good AsH3 removal capacities for the purification process of AsH3 at temperature of 150 °C were obtained, which may be ascribed to much better low temperature oxidation ability of NiOx and MnOx than that of PbOx, CoOx, and CeOx.26−30 The MnOx is considered as a low-temperature catalyst in many reactions, such as CO conversion and NOx abatement. Therefore, the resulting Mn5A in our study is found to show the best catalytic performance. 3.2. Influence of the Impregnation Concentration Manganese Nitrate. The loading amount of manganese nitrate on Mn-5A is another factor influencing the performance of the Mn-5A catalyst toward the removal of AsH3. Figure 3
Figure 3. Effect of the Mn(NO3)2 concentration on the AsH3 removal efficiency. Reaction conditions: reaction temperature = 150 °C; calcination temperature = 500 °C; [AsH3] = 200 ppm; and [O2] = 1.0 vol % (balance N2).
represents the AsH3 removal efficiencies of Mn-5A catalysts containing varying concentrations of manganese nitrate (i.e., 0.2, 0.4, 0.5, 0.6, and 0.8 mol·L−1). As shown in Figure 3, the AsH3 removal efficiency increased with the impregnation concentration of manganese nitrate on Mn-5A from 0.2 mol· L−1 to 0.6 mol·L−1, the removal efficiency of AsH3 removal efficiency become increased proportionally. The breakthrough time was ca. 250 min for the catalyst prepared with a 0.2 mol· L−1 manganese nitrate solution, while this time increased to 500 min for the catalysts prepared with a 0.5 mol·L−1 solution. Thus, AsH3 removal capacity of the latter catalysts was twice that of the former. Different trends were observed at concentrations higher than 0.6 mol·L−1, while there were no significant differences between the catalysts prepared with 0.5 mol·L−1 and 0.6 mol·L−1 of Mn2+. Specifically, the AsH3 removal efficiency decreased for the material prepared with a 0.8 mol·L−1 solution. A 10% breakthrough time of 350 min was observed for Mn-5A-0.8 mol·L−1, much shorter than that of Mn-5A-0.6 mol·L−1, which has a 10% breakthrough time of 540
Figure 4. Effect of the calcination temperature of Mn-5A catalysts on the AsH3 removal efficiency. Reaction conditions: [Mn2+] = 0.5 mol· L−1; reaction temperature = 150 °C; calcination temperature = 500 °C; [AsH3] = 200 ppm; and [O2] = 1.0 vol % (balance N2).
Optimum results were obtained for the material calcined at is 500 °C (breakthrough time = 480 min). Calcination temperatures other than 500 °C led to significantly shorter reaction times. Specifically, calcination temperatures of 400 and 450 °C, that are below 500 °C, may result in the inadequate decomposition of Mn(NO3)2 precursor. So, the removal performance of the Mn-5A catalysts being annealed at 400 and 450 °C was poorer than that of Mn-5A being treated at 500 °C. As the annealing temperature increased above 500 °C, the 10% breakthrough time of the catalysts also decreased with
Table 1. Manganese Loading Amount of the Catalysts Prepared with Different Impregnation Concentrations of Manganese Nitrate Samples
1
2
3
4
5
concentration of manganese nitrate/mol·L−1 manganese element/wt %
0.2 4.67
0.4 8.75
0.5 12.15
0.6 12.85
0.8 16.35
C
DOI: 10.1021/acs.energyfuels.7b01477 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels reference to that of Mn-5A-500 °C. This may be attributed to the aggregation of catalysts at high temperatures, resulting in a loss of BET area. Therefore, the appropriate annealing temperature for Mn-5A catalysts is 500 °C according to our investigation. 3.4. Influence of the Fixed Bed Reaction Temperature. The reaction temperature is a significant factor affecting the adsorption. Generally, the reaction temperature is set to below 200 °C to avoid the AsH3 being decomposed into more toxic metal arsenic species.31 Since arsenic is unstable, toxic, and difficult to collect/dispose, we decided to avoid temperatures higher than 200 °C for safety reasons. Thus, the Mn-5A reaction studies were limited to fixed bed temperatures within the range of 120−180 °C. Blank experiment shows that without catalysts, the AsH3 cannot be abated at all temperatures below 180 °C (not shown here). As the Mn-5A catalyst being loaded on the fixed bed reactor, AsH3 started to be removed from the feed gas. However, as shown in Figure 5, Mn-5A showed initial
3.5. Influence of the Oxygen Content. The effect of the oxygen content in the reaction condition on the AsH3 removal efficiency was investigated at a range of 0−2.0 vol %, which are common values in real industrial tail gas. As shown in Figure 6,
Figure 6. Effect of the oxygen concentration on the AsH3 removal efficiency over Mn-5A catalyst. Reaction conditions: reaction temperature = 150 °C; calcination temperature = 500 °C; and [AsH3] = 200 ppm (balance N2).
in the absence of oxygen, the AsH3 removal efficiency of Mn5A decreased rapidly (by 90%) compared with the experiments carried out in the presence of oxygen, and Mn-5A deteriorated continuously as a function of reaction time. In this condition, the MnOx species on Mn-5A catalyst was sacrificed to oxidize AsH3. The AsH3 removal performance (i.e., removal capacity and breakthrough time) improved as the oxygen content increased. Thus, at an oxygen concentration of 2.0 vol %, the breakthrough time below 90% was extended to 660 min, 4 times longer than the catalytic performance at conditions without O2. This indicates that the concentration of gaseous O2 in the removal reaction of AsH3 by Mn-5A catalysts is one of the important factors that influences the lifetime of catalysts, and higher O2 concentration will lead to higher removal efficiency for the practical application in industry. The effect of the oxygen content in the reaction condition on the AsH3 removal efficiency was investigated at a range of 0− 2.0 vol %. However, the typical values of oxygen content are often
