J. Phys. Chem. C 2010, 114, 6527–6533
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Interactions of Arsine with Nanoporous Carbons: Role of Heteroatoms in the Oxidation Process at Ambient Conditions Mykola Seredych,† John Mahle,‡ Gregory Peterson,‡ and Teresa J. Bandosz*,† Department of Chemistry, The City College of New York, 160 ConVent AVenue, New York, New York 10031, and Edgewood Chemical Biological Center, 5183 Blackhawk Road, APG, Maryland 21010 ReceiVed: December 16, 2009; ReVised Manuscript ReceiVed: March 3, 2010
Two carbons obtained from polymers containing no ash and commercial wood based carbons, as received and modified with nitrogen, were tested as adsorbents of arsine in dynamic conditions at room temperature. The chemical and structural features of the initial and exhausted carbons were analyzed by energy dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analyses, adsorption of nitrogen, and sorption of water. It was found that heteroatoms present on the surface of the carbons studied, namely oxygen, nitrogen, and sulfur, catalyze arsine oxidation mainly to arsenic tri- and pentoxide and/or in the formation of arsenic sulfides. When the surface has a high degree of hydrophilicity and water is present in the system it blocks these active centers resulting in the negligible arsine removal capacity. On the other hand, on a relatively hydrophobic surface with active nitrogen species arsine is adsorbed and oxidized to arsenic oxide. A small quantity of adsorbed water converts arsenic oxide to arsenic acid, which migrates to small pores releasing the centers for further adsorption and surface reactions. The process proceeds until all pores are filled/blocked, disallowing for further adsorption/reaction of arsine molecules. Introduction Arsine from the gas phase has to be removed owing to its toxicity, ability to damage semiconductor surfaces, and its poisoning action on catalysts. Separation processes have typically focused on its removal from either air or syngas.1–6 The most common method used to remove arsine (and other high volatality arsenic containing compounds) is adsorption. Usually for this purpose, complex adsorbents based either on alumina or activated carbons with oxides of such metals as copper lead, chromium, nickel, silver, manganese, or zinc are used. Copper oxide and metallic copper on carbons have been identified as the best adsorbents to separate arsine from syngas where either copper arsenite or arsenic is formed on the surface.1–3 Adsorbents based on copper were also applied to remove arsine from air.6 For this purpose cuprous oxide, Cu2O, was used as a catalyst for oxidation of arsine to arsenic trioxide. Since separation of arsine from air often happens at ambient conditions, which means in the presence of moisture, water was identified as either promoting or inhibiting adsorption of arsine on activated carbons with cuprous oxides deposited on the surface. In its promoting action, at low humidity (less than 30%), its presence is able to almost double the amount adsorbed from 270 to over 400 mg/g.6 On the other hand, at high water vapor concentration pore filling by water limits AsH3 adsorption. Thus far the role of carbon surface chemistry in the removal of arsine has not been extensively investigated. Quinn and coworkers1 and Hickey and Wiig6 suggested that surface oxygen groups are the active centers for arsine adsorption and reaction, but the mechanisms have never been further elaborated. Recently it has been found that oxygen and sulfur on the surface of highly * To whom correspondence should be addressed. Phone: (212) 650-6017. Fax (212) 650-6107. E-mail:
[email protected]. † The City College of New York. ‡ Edgewood Chemical Biological Center.
oxidized carbons participate in oxidation reactions in which arsine is converted either to arsenic trioxide or arsenic pentoxide.7 Moreover, arsenic sulfides can also be formed on the surface when sulfur functional groups exist. The objective of this work is to further investigate the effects of heteroatoms such as oxygen, sulfur, and nitrogen on the reactive adsorption of arsine. For this purpose, polymer-derived carbons with significant quantities of oxygen or sulfur on the surface were chosen as well as low ash, wood-based commercial carbon, with and without incorporated nitrogen functionalities. Adsorption of arsine was studied in dry and humid conditions to analyze the link between capacity and surface reaction to the chemistries and porosities of the adsorbents. Experimental Section Materials. The polymer-based carbons were formed from poly(4-styrenesulfonic acid-co-maleic acid), sodium salt, and poly(sodium 4-styrene-sulfonate) by carbonization at 800 °C. They are referred to as C-1 and C-2, respectively. The detailed procedure is described in refs 8 and 9. Another carbon was wood-based BAX-1500 supplied by Mead Westvaco, herein referred to as BAX. A subsample of this material was treated with urea according to the procedure described elsewhere and heat-treated at 950 °C in order to incorporate nitrogen functional groups.10 The sample obtained in this way is referred to as BAXU. The samples exposed to arsine at dry/humid conditions are designated with an -ED/EM suffix. Methods. AsH3 Breakthrough Dynamic Test. Microscale arsine breakthrough was conducted with use of commercially available glass tubes 0.63 mm o.d., 3.8 mm i.d. with a glass frit fused at the wall (Dynatherm Sampling Tube, CDS Analytical Inc.). This system utilizes a small adsorbent sample size, approximately 5-15 mg. Operation was controlled by three stream selector valves. The desired level of humidity was established by mixing water-saturated and dry (-40 °C dew
10.1021/jp911890c 2010 American Chemical Society Published on Web 03/12/2010
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Figure 1. AsH3 breakthrough curves in dry (ED) and humid conditions (MD).
point) air streams. A single, thermostated water bath was used to control the temperature of the adsorbent bed and a water contactor. The chemical stream composition was delivered from a pressurized gas mixture prepared from 4% arsine in nitrogen (Matheson Gas Products Inc.). The transient chemical response of the system was recorded with an FTIR (Nicolet Thermoelectron). Total flow to the bed was 20 mL. All tubing was 1.59 mm o.d., 0.76 mm i.d. in order to minimize dead time response, while not offering excessive pressure drop. Adsorption of Water. The automated gravimetric apparatus to measure water adsorption isotherm data was comprised of three subsystems: (1) the vapor-phase concentration control, (2) the adsorbed-phase measurement, and (3) the temperaturecontrol systems. The operation of all three subsystems is
computer-controlled. A double pass contactor, using an aluminum oxide thimble, was used to generate a saturated water stream at a fixed temperature. Dry air was mixed with the saturated air to generate a known humidity and is directed to a microbalance (Cahn D-200, Thermo Inc.) with 1.0 µg sensitivity and 100 mg range. The weighing basket was enclosed in a thermostated chamber that had an additional capability of high temperature, 300 °C, operation. Heating of the adsorbent for preconditioning was controlled by regulating the current NiCr wire. The basket temperature was measured with a thermocouple. During the preconditioning step nitrogen purge was used. All other streams were air. SEM/EDX. SEM images were obtained with a Zeiss Supra 55 instrument. The instrument has a resolution of 5 nm at 30 kV. Scanning was performed on sample powder previously dried. Similarly, EDX analyses were performed with the same instrument, on dried samples. From EDX analyses, the content of elements on the surface was calculated and the maps of the elements derived. Thermal Analysis. TG curves were obtained with a TA Instruments thermal analyzer. The initial samples were exposed to an increase in temperature of 10 deg/min while the nitrogen flow rate was held constant at 100 mL/min. pH. The surface pH of the initial samples was measured. About 0.1 g of the carbon powder was stirred for 10 h with 5 mL of deionized water and then the pH of the suspension was recorded. Adsorption of Nitrogen. Nitrogen isotherms were measured at -196 °C with an ASAP 2010 instrument (Micromeritics). Prior to each measurement, initial and exhausted samples were outgassed at 120 °C to vacuum 10-4 Torr. Approximately 0.1 g of sample was used for these analyses. From the isotherms the surface area, SBET, was calculated (Brunauer-Emmet-Teller, BET method). The volume of pores smaller than 10 Å, V C-1 > BAX. Oxygen and nitrogen species on the surface of carbons were found as activators of oxygen leading to the formation of superoxides ions,16,17 leading to the oxidation of arsine to arsenic oxides. Thus BAX-U carbon has the highest capacity, as it contains the most nitrogen functional groups. The high surface activity of C-2 can be linked to the presence of a significant quantity of sulfur on the surface of this carbon. Although we do not have a direct proof we hypothesize that sulfur has a similar effect to that of oxygen and nitrogen. Support for this sulfur catalytic effect in forming active oxygen is the high oxidizing power of carbons with incorporated sulfur in other surface processes as, for instance, in reactive adsorption on ammonia.10,18 A small quantity of sulfur in C-1 carbon is likely responsible for its higher capacity than that of BAX, even though the former is more acidic. The effects of arsenic adsorption are seen by FTIR, Figure 4. Owing to the high level of aromatization, the only carbons which could be analyzed with this technique are BAX and BAXU. For all samples the band at 1585 cm-1 represents the aromatic ring stretching mode vibrations of CdC bonds.19 Stretching vibrations of CsO are visible at 1070 cm-1 and are attributed either to phenol, ether, and ester groups in the carbon structure or stretching vibrations of P-O-P in polyphosphate owing to a small content of phosphorus in BAX.20,21 The bands at 1700 cm-1 represent CdO vibrations in carboxylic groups. A band at 1170 cm-1 is linked to the stretching vibration CsO bond. The effects of oxidation are seen as new bands at about 900
and 800 cm-1 which represent As(III) and As(V), respectively.22 An additional effect of exposure to arsine in moist conditions on the BAX-U sample is a significant increase in the intensity of the bands at 1570 and 1140 cm-1. Because oxidation of arsine is a strongly exothermic process,6 it is possible that as a result of a released heat the carbon matrix was oxidized. The presence of arsenic oxides is also seen on the X-ray diffraction patterns (Figure 5). Sharp peaks at 2θ 13.8°, 27.8°, 32,3°, and 35.3° for BAX-U-ED represent the crystalline form of arsenic trioxide.23 On C-2-ED at 2θ 18.2°, arsenic pentoxide is detected, which suggests a strong oxidizing power of this carbon. It is interesting that on BAX-U-EM the peaks representing arsenic oxides are not present. In fact, in the presence of moisture, it is instead expected that those oxides are converted to arsenic acids.24 An indication of the presence of arsenic acids is seen on the pKa distributions (Figure 6). Even though in dry conditions arsenic oxides could be formed, during wet titration, all of those oxides are expected to be present as acids. The peaks at pKa 2.7, 7, and 11 represent pKa of arsenic acid (pKa 2.19, 6.94, and 11.5).24 It is interesting that those peaks are clearly visible for the samples run in dry conditions. At humid conditions arsenic acid could result in a partial oxidation of the carbon matrix. The presence of various species removed from the carbon surface at different temperatures is seen on DTG curves as peaks representing the weight loss (Figure 7). In comparison with the initial samples, an increased weight loss between 150 and 600 °C is noticed after exposure to arsine. In the case of C-1ED and C-2-ED samples a well-defined broad peak with maximum at 400 °C is seen. It may represent sublimation of arsenic trioxide, As2O3 (sublimates at 312 °C25), from pores of
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Figure 7. DTG curves in nitrogen for the samples before and after arsine adsorption. Figure 6. pKa distributions for the samples before and after arsine adsorption.
various sizes. A small peak at about 200 °C clearly seen on the DTG curve for C-1-ED likely represents decomposition of H3AsO4, leading to As2O3.25 That acid can be formed since at ambient conditions, a small quantity of water is always present on the surface of carbons with functional groups. Similar weight loss patterns are noticed for BAX-ED and BAX-U-ED. Differences in intensities of the peaks exist which are related to the amount of arsine adsorbed/converted to arsenic trioxide. For the samples run in humid conditions, besides water at about 120 °C, three other peaks are revealed with maxima at about 180, 300, and 400 °C. They are visible on BAX-U-EM since the amount adsorbed was high. The first peak is assigned to a partial dehydration of H3AsO4 resulting in H4As2O7. Then
H4As2O7 decomposes (206 °C25) to As2O5, which sublimates at about 400 °C. The shoulder seen at about 315 °C can also represent a decomposition of arsenic pentoxide (about 315 °C25). Formation of these species was indicated from other analyses and should be more pronounced for carbons with strong oxidizing power such as BAX-U. In fact for that carbon a slight increase in weight loss at temperatures higher than 600 °C is noticed, which might represent sublimation of arsenic (617 °C25), formed from decomposition of arsenic pentoxide. The broad peak for C-2-ED can also represent the contribution from the decomposition of arsenic sulfides which can also be formed in the reaction of arsine with sulfonic groups of this carbon. Their boiling/sublimation temperatures are between 300 and 500 °C.25 The EDX analysis indicated a significant increase in the surface sulfur content of this sample (Table 3).
Interactions of Arsine with Nanoporous Carbons
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6533 governs the amount adsorbed. The active centers attracting arsine are functional groups containing heteroatoms. When the surface has a high degree of hydrophilicity and water is present in the system, the active centers are blocked, resulting in a negligible arsine removal capacity. On the other hand, on a relatively hydrophobic surface with active nitrogen species, arsine is adsorbed and oxidized to arsenic oxide. A small quantity of adsorbed water converts the oxide to arsenic acid, which migrates to small pores releasing the centers for further adsorption and surface reactions. The process proceeds until all pores are filled/blocked for the arsine molecules. Acknowledgment. This work was supported by ARO grant W911NF-05-1-0537. References and Notes
Figure 8. Water adsorption isotherms on the samples studied.
An interesting finding is the relatively high amount of arsine adsorbed for BAX-U when water is present in the system. For all other samples water has a detrimental effect, resulting in no arsine capacity for C-1 and C-2 samples. To investigate this phenomenon the water adsorption isotherms were measured on the samples (Figure 8). Both C-1 and C-2 have high amounts of water adsorbed at low pressure, which indicates a high degree of surface polarity. This explains the lack of capacity for arsine removal when water is present. Its adsorption blocks the active centers/polar sizes, which consists of heteroatoms and entrances to small pores, and thus the surface is not accessible for AsH3. This phenomenon to a lesser extent also occurs for BAX although, based on the shape of the isotherms, its surface is much less hydrophilic. The surface of BAX-U is the most hydrophobic of all carbons studied. On this carbon, water present in the challenge gas weakly competes with arsine. Oxidation of arsine by active oxygen results in the formation of arsenic oxides. Owing to the presence of some water, arsenic acid can form. That acid likely migrates to small pores causing their gradual filling. This results in a simultaneous release of the active centers for adsorption/oxidation of other arsine molecules. This process, based on the results obtained (Table 2, Figure 2), continues until all pores are filled by the oxidation products. Conclusions These results showed that surface chemistry of carbons, and not porosity, plays a dominant role in the reactive adsorption of arsine in which it is oxidized mainly to arsenic trioxide. The presence of surface groups with the ability to activate oxygen
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