TiO2 Sorbents under

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Catalysis and Kinetics

High-Performance Arsine Removal Using CuOx/ TiO2 Sorbents under Low Temperature Conditions Xueqian Wang, Hongqi Huang, Qiqi Zhou, Ping Ning, Jinhuan Cheng, Yilong Lin, Langlang Wang, and Yibing Xie Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00448 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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GRAPHIC

T represents reaction temperature.

High-Performance Arsine Removal Using CuOx/TiO2 Sorbents under Low Temperature Conditions Xueqian Wang, Hongqi Huang, Qiqi Zhou, Ping Ning*, Jinhuan Cheng, Yilong Lin, Langlang Wang, Yibing Xie Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China.

ABSTRACT In this work, copper-based titanium sorbents (CuOx/TiO2) synthesized by sol-gel method were used to remove arsine (AsH3) at low temperatures. The Cu content, calcination temperature, pH, oxygen concentration, and reaction temperature were the five key factors that were determined and analyzed in this work. The optimum breakthrough adsorption capacity for arsine was promising, and had the value of 534.3 mg/g for the Cu content of 20 wt.%, calcination temperature of 400 °C, pH of 10, oxygen concentration of 2% and reaction temperature of 120 °C. The chemical and structural features of the initial and arsine-exposed sorbents were analyzed using Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD) analysis, temperature-programmed desorption of carbon dioxide (CO2-TPD), Fourier transform

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infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It was found that the contents of anatase and rutile TiO2 in sorbents could be regulated by changing the pH of sorbents. Additionally, at pH of 10, the sorbent exhibited the highest specific surface area, a higher number of basic sites, whereas TiO2 mainly existed in mixing of crystal material with anatase TiO2 in dominating structure, which showed a significant promoting effect on the adsorption of arsine. Furthermore, CuO was the primary active component on the surface of sorbent, while As2O3 and H2O were the main products of arsine oxidation. As the reaction temperature increased, the preference for the formation of As2O5 was observed.

1. INTRODUCTION The toxicity of arsine (AsH3), which is a colorless and combustible gas, is seven times higher than the Lewisite.1 AsH3 is a by-product, which is formed during various processes, such as the smelting of arsenic-containing ferrosilicon, and burning of fossil fuels.2,3 A long-term exposure to AsH3 may cause severe health hazards, such as serious damage to lungs, respiratory tract, bronchus, and kidney, whereas the long-term exposure also results in high risk of cancer and even death.4-6 Furthermore, the most important factor in the poisoning of catalysts during synthesis process is the existence of many impurities, such as sulfur, phosphorus, arsenic, and cyanide.7,8 Previously, Air Products & Chemical Inc., USA reported that arsenic-containing species, especially arsine (AsH3), was a potential catalyst poison, even at very low concentrations.9 In order to maintain the activity of catalysts, even the traces of AsH3 should be removed.10 Once the catalyst has been poisoned, there is no practical method for its regeneration.11 Therefore, the removal of arsine from synthesis gas is an important issue for the catalytic process. In recent years, the transition metal oxides or sulfide based catalysts, which mainly consist of copper, lead, manganese and nickel,9,12-16 have been used as the oxidants to convert AsH3 to As(III) or As(V) .17-19 Among them, the copper species are known for their efficient arsenic adsorption. It was found that, on the surface of ACS Paragon Plus Environment

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alumina or activated carbon modified by copper oxide (CuO), arsine was deposited in the form of Cu3As, while elemental arsenic existed under non-oxidizing conditions.15 Haacke et al. reported that CuO/CrO3/carbon sorbent used the reduction of Cu(II) and Cr(VI) to produce metallic Cu and Cr(III), and enhanced the oxidation capacity of arsine in an inert atmosphere.17 These reports showed that CuO can oxidize AsH3 even though the molecular oxygen was absent during the process. Hickey et al. found that an oxidizing sorbent, which could adsorb 270 mg/g arsine, was obtained under dry conditions and used to oxidize arsine to arsenic trioxide by CuO.19 Quinn et al. reported that CuO/carbon provided higher activity for the oxidation of arsine than other metallic oxides, such as PbO, MnO2, and AgO.7 Additionally, copper oxide possessed the advantages of easy preparation, low toxicity and low cost. Palladium (Pd) is an excellent sorbent for the removal of AsH3, Hg, and H2Se from fuel gas at elevated temperatures,20-30 and has attracted significant research attention recently. However, palladium is not yet widely employed for arsine capture due to the small number of gasification facilities in the United States, as well as the need for further demonstration of the technology. Of various supported metal oxide and metal sorbents, copper-based sorbents have proved to be the most suitable for AsH3 removal from a syngas feed. At present, the sorbents for AsH3 adsorption tend to operate in the temperature range of 140 - 300 °C.7,20-23,31 However, it was found that even at modest temperatures (30 – 40 °C), some reduction to metallic copper (Cu) occurred.7 Meanwhile, AsH3 was oxidized to arsenic trioxide by cupric oxide. Furthermore, activated carbon modified with sulfonated cobalt phthalocyanine (CoPcS) and Cu(NO3)2 was able to efficiently adsorb AsH3 at 60 °C, and exhibited an AsH3 adsorption capacity of 35.7 mg/g.32 Petit et al. adsorbed AsH3 using sulfur-containing carbon at room temperature, which showed a promising capacity of 539 mg/g for arsine until the exhaustion of the sorbent (the inlet stream was 4% arsine in nitrogen).10 These reports showed that arsine could be removed at low temperatures. Most industrial exhaust contain large quantities of combustible gas, thus it will be safer to adsorb AsH3 at low temperatures. Furthermore, elevated temperatures can damage the sorbent such as make the sorbent ACS Paragon Plus Environment

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sintering, as well as decompose or volatilize chemisorbed species formed on the sorbent surface. To date present, less attention has been paid on purification of arsine at low temperature. Titanium oxide (TiO2) is a famous n-type semiconductor, and has been extensively used in gas purification due to its low cost, environmentally friendly nature, and excellent catalytic activity.33 TiO2 catalysts loaded with metals have been studied to remove HCN, Hg0, SO2, NOx and other industrial waste gases, and have exhibited excellent performance.34-37Copper supported on TiO2 presented wonderful properties and has been widely in CO oxidation38,39, NOx decomposition37, steam reforming and methanol dehydrogenation40. Xu et al. proposed that interactions between CuO and anatase TiO2 carriers could affect the crystalline phase transition and reduction behavior of TiO241. Solyman et al. also pointed out that CuO could interact

with

TiO2

to

promote

redox-initiated

polymerization

of

methyl

methacrylate42. However, the removal of arsine by CuO/TiO2 sorbents prepared by the sol-gel method is rarely reported in literature. In this paper, a series of CuOx/TiO2 sorbents, prepared by the sol-gel method, were applied for adsorbing AsH3. The effects of key parameters, including Cu content, calcination temperature, pH, oxygen concentration, and reaction temperature were investigated. Subsequently, the adsorption mechanism for arsine was closely examined. The surface structure and the chemical properties of sorbents were characterized using Brunauer–Emmett–Teller (BET) method, temperature-programmed desorption of carbon dioxide (CO2-TPD), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). 2. EXPERIMENTAL 2.1. Materials. Cu(NO3)2·3H2O (99.9%; Aldrich) was dissolved in deionized water, which was defined as the Solution A. The anhydrous citric acid, anhydrous ethanol, glacial acetic, butyl titanate (Ti(OC2H5)4; 97%; Aldrich) were subsequently introduced into the mixture and stirred rigorously. The resulting mixture was termed as the Solution B. ACS Paragon Plus Environment

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The Solution A was added to Solution B uniformly and slowly, and the mixture was hydrolyzed to form the sol. Afterwards, concentrated nitric acid was added to dissolve the sol. Then, concentrated ammonia water was used to adjust the pH value (pH=2, 3, 5, 7, 9, 10, and 11), which was measured using a pH meter (PHS-3C). The resulting solution was converted to a gel when the temperature was increased from 30 °C to 70 °C at the rate of 5 °C/h for about 8 h. The gel was subsequently dried in an oven at 110 - 120 °C for 3 - 5 days until it became a black dry gel, which was then calcined in a muffle furnace at the desired experimental temperatures (250, 300, 400 and 500 °C) for 4 - 6 h. Finally, the samples were cooled down to ambient temperature, and sieved through 40 - 60 mesh. This way, the desired sorbents were obtained. The amounts of butyl titanate, deionized water, glacial acetic acid, concentrated nitric acid, and anhydrous ethanol were in the volume ratio of 1:1:1:2:5, respectively. The total metal mass percentage was calculated using the equation (Cu/(Cu+Ti)) for each sorbent. The Cu content was varied from 10 wt.% to 25 wt.%, where wt.% means the mass percentage. The molar amount of anhydrous citric acid was the sum of the molar amounts of Cu and Ti. 2.2. AsH3 breakthrough dynamic test. Figure 1. shows the schematic diagram of the experimental setup used for arsine adsorption. The adsorption experiments were conducted in a quartz column at atmospheric pressure. The column had the inner diameter and length of 9 mm and 60 mm, respectively. In a typical test, a small amount of sorbent (200 mg) was placed in the column quartz glass tube, and fastened using the quartz cotton. The total flow rate of the modelled gas stream was 200 mL/min, which was composed of 220 mg/m3 AsH3 in N2 and O2 (0 - 2%). Firstly, they went through a mixer, and then, introduced into the quartz column reactor. The gas flow was controlled using a mass flow meter, which was calibrated with soap film flowmeter. By changing the oxygen content from 0 - 2%, AsH3 breakthrough curves for the removal of AsH3 were obtained in the dynamic test between the temperature range of 30 - 120 °C. The concentration of AsH3 was measured using silver diethyledithiocarbamate spectrophotometric method. The "breakthrough time" was defined as the time, at ACS Paragon Plus Environment

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which AsH3 removal efficiency was below 90% according to the breakthrough curves. Furthermore, the AsH3 exhaust gas was treated using acidic potassium permanganate solution. The adsorption capacity of AsH3 was calculated using Equation (1) under various conditions of the breakthrough curves. t

X = (QC 0 t − Q ∫ Cdt ) / m

(1)

0

where X is the adsorption capacity, mg/g; Q denotes the gas flow rate, m3/min; t represents the adsorption time, min; C0 and C represent the inlet and outlet mass concentration of AsH3, respectively, mg/m3; m is the weight of the adsorbent, g. 2.3. Sorbent Characterization. The

surface

areas

of

the

sorbents

were

characterized

using

the

Brunauer-Emmett-Teller (BET) method. The pore volume and average pore diameters were determined from the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method. The XRD patterns of the sorbents were obtained on a D/MAX-2200 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ= 0.15406 nm) operating at 20 - 60 kV and 2 - 50 mA. The CO2-TPDs were measured using a Quantachrome TPRWin v3.52. The CuOx/TiO2 sorbents were exposed to a flow of CO2 for 1 h, after which, the sorbents were heated in helium atmosphere from 50 to 800 °C at the rate of 10 °C/min to desorb CO2 (CO2-TPD). The FTIR spectra were manufactured using a thermo Nicolet Corp., USA unit having the model Magna-IR170. The X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Physical Electronics PHI5600 spectrometer using an Al-Kα anode X-ray source with the photo energy of hv = 1486.6 eV. The O1s, Cu2p, Ti2p and As3d binding energies were calibrated using C1s peak (BE = 284.8 eV) as standard. Scanning electron microscope (SEM) was performed on a XL30ESEM-TMP scanning electron microscope (PHILIPS-FEI, Netherlands). 3. CHARACTERIZATION of SORBENTS


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3.1. N2-BET analysis and AsH3 breakthrough adsorption capacity at various pH values. Table 1. presents the parameters of porous structure and AsH3 breakthrough adsorption capacity (90 °C) for various samples under different pH conditions (AsH3 breakthrough capacity was calculated according to the raw breakthrough curve (Figure 10.)). The sample with pH=10 showed the optimum AsH3 breakthrough adsorption capacity of 397.9 mg/g, whereas on the whole, the alkaline samples were superior to acidic samples. The results showed that, compared with the samples with pH7 were larger. However, in terms of the pore width, there was no obvious difference among them, indicating that the pore structure of samples with pH>7 was more developed. Moreover, the sample with pH=10 exhibited the largest specific surface area and pore volume, which were beneficial in improving the physical adsorption of arsine that may be related to a higher proportion of anatase TiO243,44. Figure 2 shows the pore size distribution of the samples with different pH. The majority of pores in these sorbents fell in the range of 2.0 - 5.0 nm, indicating that all of them had mesoporous structures. Compared to other samples, the distribution in mesopores range of the sample with pH=10 was relatively narrow, which implied that its pores were more uniform.45 Furthermore, from the parameters of porous structure and pore size distribution of the sample with pH=2, it was inferred that the structure was destroyed when the acidity and basicity of samples were intensely excessive. 3.2. XRD analysis. The X-ray diffraction patterns for samples with different pH values and a control sample (only CuOx, no TiO2) before the removal of arsine and those of samples with pH=10 after the adsorption of arsine at different reaction temperatures are presented in Figures 3a, and 3b, respectively. It can be seen from Figure 3a that the well-defined diffraction peaks at 2θ=27.5, 36.1, 41.2, 54.3, 56.6 and 69.0° were typical of the rutile TiO2 (PDF#73-2224), whereas the two small sharp peaks at 2θ=35.6 and 38.7° were ascribed to CuO phase (PDF#72-0629). Additionally, alkaline sorbents had one obvious diffraction peak at 2θ =25.1°, which was assigned to anatase TiO2 ACS Paragon Plus Environment

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(PDF#71-1166). Compared to acidic sorbents, the anatase TiO2 peak of alkaline sorbents was sharper, especially when the pH was 10. These results indicated that TiO2 coexisted with anatase and rutile TiO2 phases in these sorbents. Liu et al. pointed out that mixing the crystal material with anatase and rutile titania in an appropriate ratio exhibited higher catalytic activity than pure anatase or rutile titania.46 In addition, the sample with pH=10 contained the mixing of crystal material with anatase TiO2, which was the dominating structure. Based on the experimental results, it can be said that that CuO/TiO2 (anatase) has more outstanding oxidation capacity and activity than CuO/TiO2 (rutile),47 especially TiO2 existed in a mixing crystal form with anatase TiO2 in dominating structure, and as reported that both the reduction behavior and the crystalline-structure transition of the TiO2 support were related to the interactions between TiO2 (anatase) and CuO41. Therefore, it enhanced the oxidation of arsine when pH is 10. In addition, it was found that the rutile TiO2 was the dominating structure when pH was neutral and acidic. An increase in the intensity of acidic sorbents’ rutile TiO2 diffraction peak was also observed, which suggested that the crystallinity of rutile TiO2 was better. In particular, when pH was 3, the crystallinity of rutile TiO2 was quite excellent, indicating that its AsH3 removal capacity was superior to neutral or even to some alkaline sorbents. As mentioned in the previous BET analysis results, excessive acidity and alkalinity would destroy the structure of sorbents. The results also showed that the crystallinity of rutile TiO2 and anatase TiO2 was undesirable when the acidity and basicity of samples were exceedingly strong. Furthermore, it was interesting that, in spite of high adsorption capacity, the formation of these CuO crystallites were not well-defined on their surfaces. For better conclusion, a control sample with CuOx (no TiO2) was prepared for parallel experiment. Compared with the control sample, it was found that the diffraction peak of CuO on CuOx/TiO2 sorbents weakened dramatically, manifesting that copper oxide was highly dispersed on the surface of the TiO2 support. According to experimental results and previous researches,41,42 it came to the conclusion that the interactions between the TiO2 and CuO species made the active component dispersibility better, as a result of an unique structure, thus facilitated the performance. ACS Paragon Plus Environment

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The X-ray diffraction patterns for the samples with pH=10 studied after the removal of arsine at different reaction temperatures are presented in Figure 3b. There were many well-defined diffraction peaks (such as, 13.8°, 28.0°, 32.4°, 35.4°, and 59.6°), all of which were associated with the presence of arsenic trioxide (PDF#72-1333).48,49 In addition, no peak for Cu3As2 and As2O5 was found, which indicated that either they were not formed on their surfaces, or their concentration was too low to be detected. Therefore, based on the results of XRD analysis, it can be concluded that the main oxidation product was As2O3 during the adsorption of arsine. 3.3. CO2-TPD analysis. Figure 4 shows the CO2-TPD profiles of the samples with pH=5, pH=7 and pH=10. The peak area corresponds to the amounts of basic sites, while the desorption temperature represents the strength of basic sites of the sorbents.50,51 It could be seen that the CO2-TPD spectra of samples exhibited three desorption peaks in the temperature range of 175.5 - 216.5 °C (weak basic sites), 409.7 - 464.1 °C (moderate-to-strong basic sites), and 634.0 - 680.6 °C (strong basic sites). According to the CO2-TPD test results, the desorption peak area of the sample with pH=7 was not obvious, indicating that the basic sites were quite few in number in the sample. After properly adjusting the pH of sorbents, it was found that the amounts and strengths of basic sites improved, which were beneficial to the adsorption of acid gas (AsH3). The peak area of the sample with pH=10 was found to be more than 4 times that of the sample with pH=7, while the peak area of the sample with pH=5 was 2 times larger than that with the pH=7, implying that the amounts of basic sites in the samples increased, especially at the pH of 10. Therefore, an enhanced arsine adsorption capacity was observed at the pH of 10. 3.4. FT-IR analysis. In order to further confirm the adsorption mechanism of AsH3, the changes in characteristic peaks of the samples with pH=10 before and after the adsorption of arsine were investigated using FT-IR spectra (see Figure 5). After the adsorption of arsine, the peak of 1440 cm-1 was observed for the fresh adsorbent, which was attributed to the surface OH groups.52,53 The result showed that the chemisorption of ACS Paragon Plus Environment

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oxygen might have taken place during the reaction. Additionally, another peak at 2340 cm-1 disappeared, which was ascribed to CO2.54 This was due to the reason that the porous structures captured CO2 from air during the cooling process after calcination. The bands at 802, 1050 and 1620 cm-1 were visible for all sorbents after the adsorption at different reaction temperatures. The peaks at 802 and 1050 cm-1 were ascribed to the presence of As(III) from arsenic trioxide,10 which indicated that the oxidation reaction of arsine occurred and the formation of As2O3 took place. An increase in the intensity was also observed with the increase in temperature, suggesting that high temperature promoted the oxidation of arsine. For the fresh sorbents, another peak, centered at 1620 cm-1, was observed, and assigned to physisorbed H2O molecules,50 which was beneficial to the oxidation of arsine.10 After the adsorption, the intensity of peak increased, which implied that water produced during the process. Meanwhile, the result indicated the presence of oxidation reaction of arsine again. 3.5. XPS analysis. The contents of elements on the surface of samples with pH=10 before and after the adsorption of arsine are listed in Table 2 (in weight percentage ‘wt.%’, and atomic percentage ‘at.%’). The arsenic content on the surface of the sample after arsine adsorption was as high as 72.52 wt.%, which may be associated with the high arsine breakthrough adsorption capacities. Figure 6 presents the analysis of O1s, Cu2p, and Ti2p XPS spectra of the samples with pH=10 before and after the adsorption of arsine, as well as As3d after adsorption at different reaction temperatures. As noted in Figure 6(a), three peaks of O1s centered at 529.9, 531.6 and 532.9 eV were observed, and were attributed to the presence of lattice oxygen (denoted as Oα), chemisorbed oxygen or hydroxyl oxygen (denoted as Oβ), and adsorbed molecular water (denoted as Oγ), respectively.55-57 With the increase in reaction temperature, the proportion of Oα/O decreased from 57.96% to 21.36%, while that of Oβ/O increased from 35.99% to 75.36%, which was related to the lattice oxygen involved in the oxidation reaction of arsine to form oxidation products (AsOx). These results showed that the lattice oxygen was responsible for the ACS Paragon Plus Environment

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oxidation of AsH3. Furthermore, it was found that the proportion of Oγ/O was enhanced with the increase in reaction temperature, indicating that high temperature promoted the oxidation of arsine. However, the proportion of Oγ/O reached its maximum at 90 °C (and not at 120 °C), which was connected with the evaporation of water molecules at high temperature. These results suggested that the lattice oxygen on the surface of sorbents played a major role in the adsorption of AsH3. Cu2p XPS spectra of CuOx/TiO2 before and after the reaction are displayed in Figure 6(b). The peaks of Cu2p centered at around 932.6, 932.8, 934.6, 941.9, 944.5 and 953.7 eV were observed. The peak centered around 932.6 eV before the reaction corresponded to Cu2O. After the reaction, the peak disappeared, which implied the presence of Cu2O on the fresh sorbent surface, and that Cu2O was involved in the reaction. The peak centered at 932.8 eV could be ascribed to the existence of copper element, while the proportion of copper increased from 6.76% to 12.08% with the increase in temperature from 30 °C to 120 °C. The peaks at 934.6, 941.9, 944.5 and 953.7 eV could be attributed to CuO, whose content decreased from 83.92% to 77.01%, suggesting that CuO and Cu2O took part in the reduction and oxidation process, while the conversion of Cu(II) or Cu(I) to elemental Cu was directly related to the oxidation of AsH3. Additionally, it was evident that CuO was the predominant product with a considerably high proportion. Based on these results, it can be said that CuO and Cu2O coexisted in the CuOx/TiO2 structures as active components for the adsorption of arsine, and that CuO may play a major role in the oxidation of arsine. Figure 6(c) presents Ti2p XPS spectra of the samples with pH=10 before and after the adsorption of arsine. The peaks of Ti2p1/2 and Ti2p3/2 were centered at approximately 458.8 and 464.6eV, respectively, indicating that titanium existed in the oxidation state of Ti(IV)O2, and that the reduction peak of Ti4+ to Ti3+ did not appear in the sorbent.58 This result was consistent with the XRD results discussed previously. It can be concluded that TiO2 itself may not participate in the reaction, but the interactions between CuO and TiO2 promoted oxidation of AsH3. As shown in Figure 6(d), the XPS results also showed the As3d XPS spectra of CuOx/TiO2 after the reaction. At 30 °C, the peak centered at 45.4 eV, which was ACS Paragon Plus Environment

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assigned to As(III).59-61 It is worth noticing that there was no peak of As(V) in the spectra, indicating that either As2O5 was not produced at 30 °C or was too small in quantity to be detected. The As3d spectra of exhausted samples at 60, 90, and 120 °C were also divided into two peaks: one located at 45.4 eV was attributed to As2O3, while the other located at 46.1 eV was ascribed to As2O5.62 It was clear that As2O5 began to emerge when the temperature rose to 60 °C, while the amount of As2O5 increased from 9.33% to 27.21% as the temperature increased from 60 °C to 120 °C. It may be due to the reason that an increase in reaction temperature provided more activation energy, due to which, either the As(III) was further oxidized to As(V) or AsH3 was directly oxidized to As2O5. Therefore, As2O3 was the dominant oxidation product at all the temperatures accompanied by the formation of some As2O5 at high temperature. 3.6. SEM analysis. The surface morphology of the CuOx/TiO2 samples were investigated by scanning electron microscopy (SEM). Figure 7 shows the pre-reaction SEM images of the CuOx/TiO2 sorbents with pH=5, 7, and 10, as well as the sample with pH=10 after reaction, respectively. As presented in the Figure 7a, 7b and 7c, the surfaces of three samples were all relatively smooth. However, the surface pore structure of the sample of pH=10 (Figure 7c) was more developed than that of the samples of pH=5 (Figure 7a) and pH=7 (Figure 7b), which was consistent with the BET and XRD results. It could be found from Figure 7d that the number of pores of CuOx/TiO2-ED greatly reduced, and there were a large number of white particles on the surface which might be a kind of non-conductive oxides attached to the surface after the reaction. Combined with the previous analysis, it could be deduced that it was the AsOx adsorbed on the surface of the sample resulting in crystal agglomeration, pore blocking and the decrease of adsorption sites, as a consequence of the adsorption of AsH3 on the CuOx/TiO2 sorbent was suppressed. Hence, although there was continuous supply of oxygen in the reaction process, the limit of the adsorption reaction still existed. 4. EXPERIMENTAL RESULTS ACS Paragon Plus Environment

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4.1. Effect of Cu content on AsH3 adsorption. The effect of contents of active component of Cu (10, 15, 20 and 25 wt.%) on the removal of AsH3 was investigated (at conditions of: pH=7, T=90 °C and 2% O2). The results are presented in Figure 8. It could be seen that with the increase in Cu content of the sorbents from 10 wt.% to 20 wt.%, the AsH3 removal efficiency enhanced significantly. This result indicated that the increase in Cu content had a promoting effect on the capture efficiency of AsH3. However, when Cu content was further increased to 25 wt.%, the adsorption capacity of AsH3 rapidly decreased. Among these sorbents, the sorbent with 20 wt.% content of Cu showed the best performance, and was able to maintain an adsorption efficiency of at least 90% for AsH3 for more than 7.5 h. 4.2. Effect of calcination temperature on AsH3 adsorption. The calcination temperature is a very critical factor, which will have an important influence on the pore structure of sorbents, composition of the active component and the structure of the carrier. Therefore, the effect of different calcination temperatures (250, 300, 400 and 500 °C) of the sorbents on the removal of AsH3 were evaluated (at conditions of: pH=7, T=90 °C, and 2% O2), and the results are shown in Figure 9. As shown in Figure 9, when the calcination temperature of the sorbent increased from 250 to 400 °C, both the AsH3 removal efficiency and the breakthrough time enhanced obviously. However, any further increasing the calcination temperature from 400 to 500 °C, the AsH3 removal efficiency decreased significantly. Based on the characterization results of XRD analysis, when calcination temperature was 400 °C, the crystal phase of the CuO was visible, and there were not many desired peaks. The results showed that copper oxide was produced, and that the most of impurities were broken down at this calcination temperature. Tseng et al also reported that CuO and Cu2O could be formed simultaneously when the calcination temperature was about 400 °C.63 In addition, it was found that Cu existed mainly in the form of CuO, accompanied by the formation of a small amount of Cu2O (from the results of Cu 2p XPS spectra). Furthermore, the XRD diffraction peak of TiO2 was ACS Paragon Plus Environment

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evident at 400 °C, indicating that TiO2 was completely crystallized at this temperature. When the calcination temperature was further increased (beyond 400 °C), the ratio of anatase to rutile TiO2 increased,64,65 which was unfavorable for the catalytic oxidation performance of the sorbents. 4.3. Effect of pH of sorbents on AsH3 adsorption. The pH value determines the pore structure and the chemical properties of the sorbents to some extent. The effect of different pH values (2, 3, 5, 7, 9, 10 and 11) on the removal of AsH3 was studied (under conditions of: T=90 °C, and 2% O2), and the results are shown in Figure 10. The results showed that the adsorption capacity of various samples can be ranked in the following descending order: pH=10＞pH=11＞ pH=3＞pH=9＞pH=5＞pH=7＞pH=2. These results indicated that the pH of sorbents had a significant influence on both the removal efficiency of AsH3 and the breakthrough time. The sample with pH=10 reached the maximum AsH3 breakthrough adsorption capacity of about 397.9 mg/g and 31 h when pH＞7, while the one with pH=3, reached the optimum value of approximately 279.7 mg/g and 20.5 h when pH＜7. Furthermore, it was found that the alkaline condition was preferred over neutral and acidic conditions for the removal of arsine. Based on the BET analysis, the sample with pH=10 possessed the largest specific surface area, the best developed pore structure, and the most uniform particle size, which contributed to its optimal performance for AsH3 adsorption. In addition, the sorbent with pH=10 exhibited more basic sites (as was evident from the results of CO2-TPD), which was conducive to the adsorption of acid gas (AsH3). Based on the characterization results of XRD analysis, the phase of TiO2 was the mixing crystal, while the anatase TiO2 was the dominating structure when pH＞7. However, rutile TiO2 was the dominating structure when pH≤7. The crystal structure of CuOx/TiO2 (rutile) is more stable than that of anatase, though CuOx/TiO2 (anatase) has a higher activity.46 Therefore, the performance of alkaline sorbents for oxidizing arsine was better than the neutral and acidic sorbents. The anatase TiO2 of the sorbent with pH=10 had the highest intensity, and promoted the oxidation of arsine significantly.


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When pH＜7, it was found that the crystallinity of rutile TiO2 was first-ranked for the sorbent with pH=3, and the diffraction peak intensity of anatase was extremely weak. Therefore, the sorbent with pH=3 exhibited higher arsine adsorption performance when pH＜7, however, it was not as good as the sorbent with pH=10. Based upon these results, the optimum pH value was selected to be 10. 4.4. Effect of oxygen concentration on AsH3 adsorption. Figure 11 exhibits the effect of different oxygen concentrations (0, 0.5, 1.0 and 2.0 %) on the removal of AsH3 for CuOx/TiO2 sorbents (under conditions of: T=90 °C, and pH=10). As illustrated in Figure 11, the AsH3 removal efficiency significantly improved with the increase in oxygen concentration from 0 to 2.0%. Therefore, 2.0% O2 was chosen as the optimum oxygen concentration for arsine removal. In addition, in the absence of O2, the breakthrough time of AsH3 adsorption was approximately 1h and the AsH3 removal efficiency decreased to 51.3% at 3h, indicating the arsine adsorption mechanism was mainly the chemical oxidation process. Based upon the XRD, XPS and FT-IR results, it can be concluded that the oxidation of AsH3 occurred, while As2O3 and H2O formed on the surface of the sorbents. The lattice oxygen on the sorbents was activated to trigger the oxidation of AsH3, whereas the resulting O vacancies were replenished by O2. The whole adsorption process was mainly the chemisorption. Therefore, enough oxygen concentration was vital for efficient adsorption of AsH3. 4.5. Effect of reaction temperature on AsH3 adsorption. Considering that arsine may decompose in the presence of sorbents when the reaction temperature is too high,66 the present study aimed to remove arsine at low temperatures. Therefore, in this experiment, temperatures of 30, 60, 90 and 120 °C were selected to study the effect of reaction temperature on the removal of AsH3 (under the conditions of: 2% O2, and pH=10). Figure 12 shows that the arsine adsorption efficiency increased with the increase in temperature. The corresponding breakthrough adsorption capacity increased from 75.0 mg/g at 30 °C to 534.3 mg/g at 120 °C. In general, the higher reaction temperature, more beneficial it is to the chemical reaction, which also implied that the oxidation of AsH3 was dominant. ACS Paragon Plus Environment

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Based on the FT-IR and XPS analyses, the intensity of peaks of H2O and arsenic oxides (As2O3 and As2O5) increased with the increase in reaction temperature, indicating that an increase in the reaction temperature favored the catalytic oxidation of arsine. Furthermore, the activity of CuO and the conversion rate of Cu to CuO were accelerated, due to which, the adsorption rate of arsine was promoted. Therefore, the AsH3 breakthrough adsorption capacity improved with the increase in reaction temperature. 4.6. Regeneration. The regeneration performance of the sorbents is also an important indicator to evaluate the sorbents. Prior to regeneration tests, follow steps are acquired. First, the arsine-exposed sample was activated under a hot gas flow for three hours, then the sample was soaked and washed with deionized water for 4 - 5 times. Afterwards, the sample was placed in an electric blast oven to dry at 110 - 120 °C. Subsequently, the sample was put in the tube furnace and flushed with nitrogen. Considering that the material was calcined at 400 °C, the sample deserved to be accordant with the calcination temperature and was also heated to 400 °C at a rate of 10 °C/min, after the sample was held for 2 hours at 400 °C, it was then cooled to room temperature. Finally, the recycled material was obtained to perform the AsH3 adsorption regeneration test. The regeneration experimental results (under the conditions of: AsH3 inlet concentration 220 mg/m3, total gas flow 200 mL/min, oxygen content 2%, reaction temperature 120 °C) are shown in Figure 13. It was found the regenerated sample still had a high adsorption efficiency, but the adsorption breakthrough time was shortened to some extent. In addition, the adsorption breakthrough time of CuOx/TiO2-R1 dropped to about 25 hours, and it was further reduced to nearly 18 hours of the CuOx/TiO2-R2 correspondingly. It was concluded that the regenerated sorbent could restore approximately 60% adsorption capacity of the fresh sorbent after the first regeneration, demonstrating the CuOx/TiO2 had a reasonable adsorption regeneration capability for removing AsH3. 5. DISCUSSION 5.1. Mechanism for the AsH3 oxidation over CuOx/TiO2 sorbents. ACS Paragon Plus Environment

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In the absence of O2, the breakthrough time for the removal of AsH3 was around 1h, while the breakthrough time got significantly extended to 20 - 41 h in the presence of O2. These results indicated that the adsorption of arsine took place mainly through chemisorption. According to the characterization of the sorbents, the oxidation of AsH3 was primarily due to the reaction with lattice oxygen. The TiO2 may not participate in the reaction, but the interactions between CuO and TiO2 promoted the oxidation of AsH3. The active components included CuO and Cu2O, whereas CuO was the chief active component. In addition, As2O3 and H2O were the main products of the oxidation reaction of AsH3. Finally, the formation of As2O5 was favored with the increase in reaction temperature. Considering the above discussion and previous researches67,68, a mechanism for the adsorption of AsH3 over CuOx/TiO2 (at the pH of 10) at low temperatures was proposed. Initially, the lattice oxygen was generated through the active components (CuO, Cu2O) on the sorbent, and then, was activated in the presence of O2. Afterwards, AsH3 reacted with the lattice oxygen to produce As2O3 and H2O. As the reaction temperature increased, As2O3 or AsH3 was further oxidized to As2O5. Finally, the resulting O vacancies of the active component (CuO) were replenished by O2. In summary, the overall (macroscopic) reaction could be represented as follow equations. CuO → Cu + [O]

(2)

Cu2O → 2Cu + [O]

(3)

2AsH3 + 6[O] → As2O3 + 3H2O

(4)

The preference for the formation of As2O5 was observed with the increase in reaction temperature. As2O3 + 2[O] → As2O5

(5)

or 2AsH3 + 8[O] → As2O5 + 3H2O

(6)

O2 provided the metal oxide with lattice oxygen in the presence of oxygen. 2Cu + O2 → 2CuO

(7)

The main reaction can be summarized as follows. ACS Paragon Plus Environment

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At low temperatures：6CuO + 2AsH3 → As2O3 + 3H2O + 6Cu

(8)

At high temperatures：8CuO + 2AsH3 → As2O5 + 3H2O + 8Cu

(9)

In this way, gaseous AsH3 was transformed into solid As2O3 and As2O5, which are less toxic and easy to capture using a precipitator. 6. CONCLUSIONS In this study, the CuOx/TiO2 sorbents were prepared using the sol-gel method, and their AsH3 adsorption capacities were thoroughly evaluated. The optimum breakthrough adsorption capacity of AsH3 was found to be 534.3 mg/g. The contents of anatase and rutile TiO2 of sorbents could be regulated by changing the pH of sorbents, which had a vital influence on the removal of AsH3. According to the BET, XRD and CO2-TPD analyses, the sorbent with pH=10 exhibited the maximum specific surface area. The phase of TiO2 was the mixing crystal, while the anatase TiO2 was the dominating structure. Additionally, the number of basic sites (for alkaline sorbents) was about 4 times that of the neutral sorbent, which greatly promoted the oxidation and adsorption of AsH3. From the results of XRD, FT-IR and XPS analyses, it was concluded that CuO played a crucial role on the surface of sorbents. TiO2 support itself might not participate in the reaction, and the synergies effects between CuO and TiO2 played an important role in the oxidation of AsH3. The oxidation products were mainly As2O3 and H2O, while with the increase in reaction temperature, the formation of some As2O5 was also observed. In this way, AsH3 was efficiently oxidized to less toxic and easy to capture species (As2O3, As2O5) at low temperatures. The CuOx/TiO2 sorbents could be considered as promising materials for the removal of arsine.

AUTHOR INFORMATION Corresponding Author *E-mail:[email protected]. Tel/Fax: +86 13708409187. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFC0210500), National Natural Science Foundation of China (no. 51568027) and Candidates of the Young and Middle Aged Academic Leaders of Yunnan Province (2015HB012).

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with NH3 at low temperatures. Appl. Catal. A. 2007, 327, 261-269. (57) Dupin, J.C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS studies of metal oxides, hydroxides and peroxides. Phys. Chem. Chem. Phys. 2000, 2, 1319-1324. (58) He, C.; Wang, Y.; Cheng, Y.; Lambert, C.K.; Yang. R.T. Activity, stability and hydrocarbon deactivation of Fe/Beta catalyst for SCR of NO with ammonia. Appl. Catal. A. 2009, 368, 121-126. (59) Bahl, M.K.; Woodall, R.O.; Watson, R.L.; Irgolic, K.J. Relaxation during photoemission and LMM Auger decay in arsenic and some of its compounds. J. Chem. Phys. 1976, 64, 1210-1218. (60) Taylor, J.A. An XPS study of the oxidation of AlAs thin films grown by MBE. J. Vac. Sci. Technol. 1982, 20, 751-755. (61) Stec, W.J.; Morgan, W.E.; Albridge, R.G.; Wazer, J.R.V. Measured binding energy shifts of "3p" and "3d" electrons in arsenic compounds. Inorg. Chem. 1972, 11, 219-225. (62) Zhou, W.P.; Kibler, L.A.; Kolb, D.M. XPS study of irreversibly adsorbed arsenic on a Pt(111) electrode. Electrochim. Acta. 2004, 49 (27), 5007-5012. (63) Tseng, H.H.; Wey, M.Y.; Fu, C.H. Carbon materials as catalyst supports for SO2 oxidation: catalytic activity of CuO–AC. Carbon. 2003, 41, 139-149. (64) Pan, J.H.; Sun, D.D.; Lee, C. Effect of calcination temperature on the textural properties and photocatalytic activities of highly ordered cubic mesoporous WO3/TiO2 films. J. Nanosci. Nanotechnol. 2010, 10, 4747-4751. (65) Okeke, G.; Hammond, R.B.; Antony S.J. Effects of heat treatment on the atomic structure and surface energy of rutile and anatase TiO2 nanoparticles under vacuum and water environments. Chem. Eng. Sci. 2016, 146, 144-158. (66) Tamaru, K. The Decomposition of Arsine. J. Phys. Chem. 1955, 59, 777-780. (67) Granite E.J.; And H.W.P.; Hargis R.A. Novel Sorbents for Mercury Removal from Flue Gas[J]. Ind. Eng. Chem. Res. 1998, 39(4), 1020-1029. (68) Presto A.A.; Granite E.J. Survey of catalysts for oxidation of mercury in flue gas.[J]. Environ. Sci. Technol. 2006, 40(18), 5601-5609.


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Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1. The parameters of porous structure and AsH3 breakthrough adsorption capacity (90 °C) for 20 wt% of CuOx/TiO2 samples with different pH. Table 2. Surface content of elements from XPS analysis for 20 wt% of CuOx/TiO2 sample (pH=10) before and after arsine adsorption at 120 °C. Figure 1. Schematic diagram of the experiment. Figure 2. Pore size distributions of 20 wt% of CuOx/TiO2 samples with different pH. Figure 3(a) (b). X-ray diffraction patterns for 20 wt% of CuOx/TiO2 samples with different pH and a control sample (only CuOx, no TiO2), as well as 20 wt% of CuOx/TiO2 sample (pH=10) after arsine adsorption at different reaction temperatures, respectively. Figure 4. CO2-TPD spectra of 20 wt% of CuOx/TiO2 samples with different pH. Figure 5. FTIR for 20 wt% of CuOx/TiO2 sample (pH=10) before and after arsine adsorption at different reaction temperatures. Figure 6(a) (b) (c) (d). XPS spectra of 20 wt% of CuOx/TiO2 over the spectral regions of O1s, Cu2p, and As3d before and after arsine adsorption at different reaction temperatures, respectively. Figure 7. SEM images of CuOx/TiO2 sorbents with pH=5, 7, and 10, as well as the sample with pH=10 after reaction.(-ED represents after arsine adsorption.) Figure 8. Effects of Cu content on AsH3 adsorption over CuOx/TiO2 sorbents (pH=7, T=90 °C, and 2% O2). Figure 9. Effects of calcination temperature on AsH3 adsorption over 20 wt% CuOx/TiO2 sorbents (pH=7, T=90 °C, and 2% O2). Figure 10. Effects of pH of sorbents on AsH3 adsorption over 20 wt% CuOx/TiO2 sorbents (T=90 °C, and 2% O2). Figure 11. Effects of oxygen concentration on AsH3 adsorption over 20 wt% CuOx/TiO2 sorbents (T= 90°C, pH=10). Figure 12. Effects of reaction temperature on AsH3 adsorption over 20 wt% CuOx/TiO2 sorbents (pH=10). Figure 13. Regeneration performance of 20 wt.% CuOx/TiO2 sorbents (pH=10).


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

Table 1. The parameters of porous structure and AsH3 breakthrough adsorption capacity (90 °C) for 20 wt.% of CuOx/TiO2 samples with different pH. sample

Surface area(m2 /g)

Pore volume(cm3/g)

Pore width(nm)

AsH3 breakthrough adsorption capacity(mg/g)

CuOx/TiO2-pH=2

18.3

0.026

1.460

101.3

CuOx/TiO2-pH=3

74.9

0.077

3.552

279.7

CuOx/TiO2-pH=5

51.9

0.112

3.557

204.4

CuOx/TiO2-pH=7

96.5

0.101

3.545

101.7

CuOx/TiO2-pH=9

96.5

0.176

3.833

228.3

CuOx/TiO2-pH=10

122.7

0.203

3.587

397.9

CuOx/TiO2-pH=11

92.8

0.113

3.570

330.0


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Table 2. Surface content of elements from XPS analysis for 20 wt.% of CuOx/TiO2 sample (pH=10) before and after arsine adsorption at 120 °C. sample

Ti

O

Cu

As

(wt.%)

(at.%)

(wt.%)

(at.%)

(wt.%)

(at.%)

(wt.%)

(at.%)

20%CuO/TiO2

45.94

27.34

36.35

64.72

17.71

7.94

-

-

20%CuO/TiO2-ED

7.84

7.59

15.33

44.44

4.31

3.15

72.52

44.82

-ED represents after arsine adsorption.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

3 9

1 10 8 4

2

6

12

7

5

11

Figure 1. Schematic diagram of the experiment. (1) cylinder with AsH3 (220 mg/m3 AsH3 in N2); (2) cylinder with O2; (3) mass flowmeter; (4) mixer; (5) three-way valve; (6) heating furnace; (7) insulating layer; (8) sorbents; (9) temperature controller; (10) AsH3 tail gas absorber; (11) outlet measuring solution; (12) inlet measuring point.


Page 29 of 40

0.045

pH=2 pH=3 pH=5 pH=7 pH=9 pH=10 pH=11

0.040 3

Incremental Pore Volume(cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.1

1

10

100

Pore Width/nm

Figure 2. Pore size distributions of 20 wt.% of CuOx/TiO2 samples with different pH.


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1-anataseTiO2 2-rutile TiO2

1 2

32 3 2 4

2 2

3-CuO 4-Cu2O

2 Cu(no Ti)

Intensity/a.u.

pH=11 pH=10 pH=9 pH=7 pH=5

pH=3 pH=2

30

40

50

60

70

2θ/°

* *

*

*

*-As2O3

*

*

120°C

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

90°C

60°C 30°C 10

20

30

40

50

60

70

80

2θ/°

Figure 3(a) (b). X-ray diffraction patterns for 20 wt% of CuOx/TiO2 samples with different pH and a control sample (only CuOx, no TiO2), as well as 20 wt% of CuOx/TiO2 sample (pH=10) after arsine adsorption at different reaction temperatures, respectively.


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20

15

Signal/mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

pH=10 5

pH=5

pH=7 0 100

200

300

400

500

600

700

800

T/°C

Figure 4. CO2-TPD profiles of 20 wt.% of CuOx/TiO2 samples with different pH.


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2340

1620 1440

1050 802

120°C

90°C

Absorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

60°C

30°C

Fresh adsorbent

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers/cm

Figure 5. FT-IR for 20 wt.% of CuOx/TiO2 sample (pH=10) before and after arsine adsorption at different reaction temperatures.


Page 33 of 40

120°C

120°C Satellite peak

Lattice oxygen

Chemisorbed oxygen

CuO2p1/2

CuO2p1/2

CuO2p3/2

Cu2p1/2

90°C

Adsorbed water

90°C

Intensity/a.u.

60°C 60°C 30°C 30°C Fresh sorbent

Fresh sorbent

538

536

Cu2O2p1/2

534

532

530

526 970

528

960

950

940

Binding energy/eV (b)

Binding energy/eV (a)

120°C

Satellite peak Ti2p1/2

Intensity/a.u.

90°C

Ti2p3/2

60°C

30°C Fresh sorbent 474

472

470

468

466

464

462

460

458

456

Binding energy/eV (c)

120°C

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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As2O3

As2O5

Satellite peak

90°C

60°C 30°C 50

48

46

44

42

40

38

36

Binding energy/eV (d)

Figure 6(a) (b) (c) (d). XPS spectra of 20 wt.% of CuOx/TiO2 over the spectral regions of O1s, Cu2p, Ti2p before and after arsine adsorption, as well as As3d after adsorption at different reaction temperatures, respectively.


930

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(a)

(c)

pH=5

pH=10

Page 34 of 40

pH=7

(b)

(d)

CuOx/TiO2-ED

Figure 7. SEM images of CuOx/TiO2 sorbents with pH=5, 7, and 10, as well as the sample with pH=10 after reaction.(-ED represents after arsine adsorption.)


Page 35 of 40

100

10wt.% 15wt.% 20wt.% 25wt.%

95

arsine removal efficiency/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

90

85

80

75

0

1

2

3

4

5

6

7

8

9

t/h

Figure 8. Effect of Cu content on AsH3 adsorption over CuOx/TiO2 sorbents (pH=7, T=90 °C, and 2% O2).


Energy & Fuels

100

250°C 300°C 400°C 500°C

95


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

90

85

80

75 0

1

2

3

4

5

6

7

8

9

t/h

Figure 9. Effect of calcination temperature on AsH3 adsorption over 20 wt.% CuOx/TiO2 sorbents (pH=7, T=90 °C, and 2% O2).


Page 37 of 40

100

95


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

90

pH=7 pH=2 pH=3 pH=5 pH=9 pH=10 pH=11

85

80

75

0

5

10

15

20

25

30

t/h

Figure 10. Effect of pH of sorbents on AsH3 adsorption over 20 wt.% CuOx/TiO2 sorbents (T=90 °C, and 2% O2).


Energy & Fuels

100

90


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

80

70

0% 0.5% 1.0% 2.0%

60

50 0

5

10

15

20

25

30

35

40

t/h

Figure 11. Effect of oxygen concentration on AsH3 adsorption over 20 wt.% CuOx/TiO2 sorbents (T= 90°C, pH=10).


Page 39 of 40

100

30°C 60°C 90°C 120°C

95

arsine removal effciency/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

90

85

80

75 0

10

20

30

40

50

t/h

Figure 12. Effect of reaction temperature on AsH3 adsorption over 20 wt.% CuOx/TiO2 sorbents (pH=10).


Energy & Fuels

100

CuOx/TiO2 CuOx/TiO2-R1 arsine removal efficiency/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

CuOx/TiO2-R2

96

92

88

84 0

10

20

30

40

50

t/h

Figure 13. Regeneration performance of 20 wt.% CuOx/TiO2 sorbents (pH=10).


TiO2 Sorbents under

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