Visible-Light-Enhanced Interactions of Hydrogen Sulfide with

Dec 19, 2011 - A systematic study of the iron hydroxide-based adsorbent for removal of hydrogen sulphide from biogas. Edoardo Magnone , Shin Dong Kim ...
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Visible-Light-Enhanced Interactions of Hydrogen Sulfide with Composites of Zinc (Oxy)hydroxide with Graphite Oxide and Graphene Mykola Seredych, Oluwaniyi Mabayoje, and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 Convent Avenue, New York, New York 10031, United States ABSTRACT: Composites of zinc(oxy)hydroxide graphite oxide and of zinc(oxy)hydroxide graphene were used as adsorbents of hydrogen sulfide under ambient conditions. The initial and exhausted samples were characterized by XRD, FTIR, potentiometric titration, EDX, thermal analysis, and nitrogen adsorption. An increase in the amount of H2S adsorbed/ oxidized on their surfaces in comparison with that of pure Zn(OH)2 is linked to the structure of the composite, the relative number of terminal hydroxyls, and the kind of graphene-based phase used. Although terminal groups are activated by a photochemical process, the graphite oxide component owing to the chemical bonds with the zinc(oxy)hydroxide phase and conductive properties helps in electron transfer, leading to more efficient oxygen activation via the formation of superoxide ions. Elemental sulfur, zinc sulfide, sulfite, and sulfate are formed on the surface. The formation of sulfur compounds on the surface of zinc(oxy)hydroxide during the course of the breakthrough experiments and thus Zn(OH)2 ZnS heterojunctions can also contribute to the increased surface activity of our materials. The results show the superiority of graphite oxide in the formation of composites owing to its active surface chemistry and the possibility of interface bond formation, leading to an increase in the number of electron-transfer reactions.

’ INTRODUCTION Hydrogen sulfide is a toxic gas that is naturally present in the atmosphere at low concentration as a result of anaerobic digestion or geothermal or volcano eruptions. When oxidized to SO2, it contributes to the formation of acid rain. Besides natural sources, H2S is also used in various industries either during processing or as a byproduct. It is naturally present in syngas or in digester gas. When released to the atmosphere, accidently or on purpose, besides causing long-term environmental damage, it can also cause death to humans owing to its very high toxicity. Various methods have been used to remove H2S from air1 3 or syngas.4 8 Although zeolites, modified alumina, or metal oxides have been applied mainly at elevated temperatures,9 11 under ambient conditions modified activated carbons are adsorbents of choice. Modification methods include impregnation with caustics or alkali metal salts,12,13 the incorporation of nitrogen functional groups,14 16 and the addition of alkaline earth oxides.17,18 The aim is to increase the surface basicity toward promoting H2S dissociation. In the presence of water, those ions are oxidized on the carbon surface either to sulfur3,12,19 or sulfuric acid.3,19 The high volume of pores is a must because the products of surface reactions have to be stored there.3,18 Recently it has been shown that the composites of graphite oxide with various metal oxides or MOF enhance the adsorption of species such as SO2,20 NH3,21 H2S,22,23 NO2,24,25 arsenate,26 and formaldehyde27 under ambient conditions. This is owing to the new porosity and new chemistry formed at the interface r 2011 American Chemical Society

between graphite oxide and an inorganic phase. Taking this into account and the fact the zinc oxide is known to be a very efficient adsorbent of H2S at high temperatures28,29 and a good adsorbent at low temperatures (25 300 °C),30 the composites of zinc(oxy)hydroxide and graphite oxide and graphene were synthesized.31 For the first time, such materials are investigated as hydrogen sulfide adsorbents under ambient conditions. The objective of this article is to evaluate their suitability for the removal of H2S and to derive the mechanism of surface interactions. The detailed surface/texture properties of the composites are described in detail in ref 31. For clarity of presentation, we report only selected surface features, which we judge as helpful in comprehending the processes taking place on the surfaces of these materials during H2S adsorption. The mechanism of the removal process is proposed by taking into account the new, unique surface properties of the composites and the conditions of the separation. The role of visible-light exposure is emphasized.

’ EXPERIMENTAL SECTION Materials. Zinc (oxy)hydroxy composites either with graphite oxide (GO) or with graphene (Gr) (supplied by METSS Corporation (Westerville, OH)) were prepared by the precipitation of zinc hydroxide Received: November 1, 2011 Revised: November 30, 2011 Published: December 19, 2011 1337

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Figure 1. H2S breakthrough curves measured under dry (ED) and wet (EPM) conditions and SO2 concentration curves measured on the materials studied. in a graphene phase dispersed in ZnCl2. The details of the composite preparation are described elsewhere.31 The obtained composites were extensively washed with distilled water until neutral pH was obtained and no traces of chloride ions were found. A separated gel phase was dried at 100 °C over 48 h. Zinc hydroxide was prepared in the same way, without the GO or Gr additives in ZnCl2 solution. Zinc hydroxide is referred to as ZnH, and the composites are referred to as ZnGO and ZnGr following the type of graphene-based phase used. Methods. H2S Breakthrough Dynamic Test. To determine the breakthrough capacities of the materials studied, dynamic breakthrough tests were performed at room temperature. In a typical test, a flow of H2S diluted with either dry or moist air was passed through a fixed bed of adsorbent with a total inlet flow rate of 500 mL/min for H2S with an initial concentration of 1000 ppm. The adsorbent’s bed volume was about 1.5 cm3. The concentration of H2S in the outlet gas was measured using an electrochemical sensor (Multi-Gas Monitor, RAE system). The adsorption capacity of each adsorbent was calculated in milligrams per gram of the material by integrating the area above the breakthrough curve. Before the H2S test was performed in moist air, the adsorbent bed was prehumidified for 2 h by exposing it to air flow with 75% relative humidity. After the breakthrough tests, all samples were exposed to a flow of carrier air (400 mL/min) to impose the desorption of the target gas and thus to evaluate the strength of its retention. Suffixes ED and EPM are added to the names of the samples after exposure to gas under dry and moist conditions, respectively. Sorption of Nitrogen. N2 isotherms were measured at 196 °C using an ASAP 2050 Xtended pressure sorption analyzer (Micromeritics, Norcross, GA). Prior to each measurement, initial and exhausted samples were outgassed at 120 °C to a vacuum of 10 4 Torr. The surface area is SBET (Brunauer Emmet Teller method), the micropore volume is Vmic (calculated using the Dubinin Radushkevich approach),32 the mesopore volume is Vmes, and the total pore volume is Vt (calculated from the last point of the isotherms on the basis of the volume of nitrogen adsorbed). The volume of the mesopores Vmes represents the difference between the total pore and micropore volumes. The relative microporosity was calculated as the ratio of Vmic to Vt. FT-IR Spectroscopy. Fourier transform infrared (FT-IR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance (ATR) method. The spectrum was generated and collected 32 times and corrected for the background noise. The experiments were done on the powdered samples, without KBr addition. Thermal Analysis. Thermogravimetric (TG) curves were obtained using a TA Instruments thermal analyzer. The initial and exhausted samples were exposed to an increase in temperature (10 °C/min) while the nitrogen flow rate was held constant (100 mL/min). From the TG curves, differential TG (DTG) curves were derived.

X-ray Diffraction. X-ray diffraction (XRD) measurements were conducted using standard powder diffraction procedures. Adsorbents were ground with methanol in a small agate mortar. The mixture was smear-mounted and then analyzed by Cu Kα radiation generated in a Phillips X’Pert X-ray diffractometer. A standard glass slide was run for the background. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode where the equilibrium pH is collected. Subsamples of the initial and exhausted materials (∼0.100 g) were added to NaNO3 (0.01 M, 50 mL) and placed in a container maintained at 25 °C overnight for equilibrium. During the titration, the suspension was continuously saturated with N2 to eliminate the influence of atmospheric CO2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant starting from the initial pH of the material suspension up to pH 11. The experimental data was transformed into a proton binding curve, Q, representing the total number of protonated sites.33,34 These curves were deconvoluted using the SAIEUS procedure,33 and the pKa distributions for the species present on the surface were obtained. Elemental Analysis. The content of sulfur was evaluated in a commercial testing laboratory (Micro-Analysis, Willmington, DE) using the CHNS method. SEM/EDX. Scanning electron microscopy images were obtained using a Zeiss Supra 55 VP with an accelerating voltage of 5.00 kV. Scanning was performed in situ on a powder sample. SEM images with energy-dispersive X-ray (EDAX) analysis were obtained at 10 000 magnification.

’ RESULTS AND DISCUSSION The measured H2S breakthrough curves on our adsorbents are presented in Figure 1. Different slopes of the curves suggest variations in the kinetics of the process and/or in surface reaction mechanisms. Apparently on zinc hydroxide different surface reactions take place under dry and moist conditions. The H2S concentration gradually increases, suggesting the involvement of various adsorption/reaction centers. Whereas for the composite containing graphene, ZnGr, the shape of the curve is similar to that for zinc (oxy)hydroxide, especially under moist conditions, the curves obtained for ZnGO are much steeper, which suggests the fast kinetics of surface reactions and/or surface-activated processes. On all materials studied, the adsorption of hydrogen sulfide is very strong, which is demonstrated by steep desorption curves. Nevertheless, some SO2 was released during 1338

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Table 1. H2S Breakthrough Capacity, Amount of Water Preadsorbed, and Surface pH Values for the Initial Samples and for Samples Exposed to H2S under Various Conditions

sample ZnH-ED ZnH-EPM ZnGO-ED

H2S breakthrough

H2O

outlet SO2

capacity

adsorbed

concentration

mg/g

3

mg/cm

49.0

13.8

139.2

41.1

pH

mg/g

ppm

in

exh

0.3

7.30 6.47

13.8

0.1

7.30 6.54

99.2

17.3

0.0

7.43 7.08

ZnGO-EPM ZnGr-ED

211.6 50.8

36.6 10.5

36.5

0.2 0.1

7.43 7.06 7.35 6.85

ZnGr-EPM

149.7

31.4

13.1

0.0

7.35 6.83

the breakthrough experiments. The SO 2 emission curves (Figure 1) show the highest concentration of SO2 released on ZnH under dry conditions. The emission starts when the H2S breakthrough is first recorded. The addition of moisture visibly suppresses the SO2 concentration. On the composites, SO2 is released much earlier than the first H2S concentrations are recorded, which suggests a much higher oxidation activity of their surfaces than that for Zn(OH)2. Interestingly, no SO2 was detected on ZnGO under dry conditions and on ZnGr under moist conditions. The extent of changes in the surface pH of the materials before and after exposure to H2S (Table 1) suggests that the most acidic compounds are formed on ZnH and the least acidic compounds are formed on ZnGO. The surface pH of the latter changed only slightly even though a large amount of H2S was adsorbed. The acidic pH of ZnH after exposure to hydrogen sulfide is in agreement with the highest concentrations of SO2 measured on this material. The calculated capacities expressed in milligrams per unit mass and per unit volume of the bed are listed in Table 1 along with the amount of water adsorbed during the prehumidification process. Even though there is no linear correlation between the amount of water preadsorbed and the capacity,23 the presence of water in the system enhances the performance by up to 3-fold.19,30 This effect is the most pronounced for ZnGr. Under either moist or dry conditions the composites outperform the zinc (oxy)hydroxide. It is important to mention that GO and graphene were found to be nonadsorbing materials. The synergetic effect of the composite formation is seen in Figure 2, where the measured and hypothetical breakthrough capacities calculated by assuming the physical mixture of the composite components are compared. The synergetic effect is much more visible for the composite with GO than for that with graphene. Placing the results obtained in perspective, the H2S removal capacities measured at room temperature on Centaur catalytic carbon,19 oxidized and then reduced carbon,35 SBA-15 with ZnO nanoparticles,36 nanocomposites of GO with Zr(OH)4,23 and HKUST22 were 60, 150, 436, 50, and 200 mg/g, respectively. This above-mentioned synergetic effect must be linked to the differences in the surface chemistries and porosity of both composites. As indicated elsewhere,31,37 in the case of ZnGO a new phase, a zinc acetate-like compound (Zn(OH)1.58(CH3COO)0.42 3 0.31 H2O), is formed as a result of the zinc hydroxide reaction with the carboxylic acids present on the edges of the graphene layers. Apparently, a high density of carboxylic groups enables this reaction where the OH groups of a zinc hydroxide phase are partially replaced with a carboxylic acid moiety.

Figure 2. Comparison of the measured and hypothetical (based on the chemical composition) H2S breakthrough capacities.

Moreover, the epoxy and OH groups existing on the basal planes were found to be involved in the chemical bonds between zinc (oxy)hydroxide with a hexagonal structure and the graphene based components. This resulted in the change in the reactivity/ pKa distribution of the bridging and terminal hydroxides.20,38 The comparison of the proton binding curves for the initial and exhausted samples is presented in Figure 3a. The negative values on the proton binding curves represent the proton release related to surface acidic groups. The titration was done only over a narrow pH range to avoid reactions of zinc (oxy)hydroxide with titrants. As seen, ZnH is the most acidic material and ZnGr is the least acidic material in our experimental window. The distributions of pKa for the species present on the surface are collected in Figure 3b. We link the first peak at pKa ∼7 to the acidic groups related to the bridging hydroxyls and the second peak to the acidity of terminal hydroxyls.39 For the exhausted ZnH, the number of the latter groups decreased, especially for the sample run under dry conditions with a visible increase in their acidic strength. The enhanced activity of terminal hydroxyl groups was also indicated in the literature in reactions with SO2,20 NO2,40 or warfare agents.41 The results show that the terminal groups consist of one-third of all hydroxyl groups of the ZnH sample. The distributions of acidic groups on the surfaces of the composites differ from those for ZnH. For ZnGr, the bridging and terminal hydroxyl groups are revealed at about 7.1 and 10.2, respectively. Their amounts are much smaller than those for ZnH, and a decrease in their number, especially for bridging groups, is greater than the decrease expected on the basis of the 20% graphene-phase addition. Nevertheless, their ratio is similar to that in ZnH, which suggests the same chemistry of the inorganic phase. The only visible difference between the exhausted ZnH and ZnGr samples is that the acidity of the terminal hydroxyls shifts. Their number after the test run in dry air significantly decreases. The numbers of bridging groups for both composites are similar; however, for ZnGr, these groups appear to be slightly less acidic than those on ZnH. On the distributions for ZnGO new species are detected at pKa values of between 9 and 10. We link them to the OH groups in the vicinity of the graphene-based phase and/or to those associated with the oxygen bonding of two phases of the composites.20,21,31 The ratio of terminal OH to bridging OH in ZnGO is 0.8, which indicates differences in the zinc (oxy)hydroxide morphology 1339

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Figure 3. (a) Proton binding curves and (b) pKa distributions for the materials before and after H2S adsorption.

compared to that phase in ZnH and ZnGr. The activity of the terminal groups in surface reactions, as indicated above, can explain the higher capacity of this composite compared to that of ZnH. Indeed, their number significantly decreases for the sample run under moist conditions on which the high capacity was measured. The results indicate that under dry conditions the groups related to new interface chemistry are somehow involved because their acidity decreased. Wang and co-workers in their study of the catalytic activity of zinc oxide deposited within the pores of SBA-15 toward gas-phase desulfurization indicated the importance of surface dispersion.36 Moreover, the importance of an “active surface area” and nanodispersion of ZnO was also pointed out in the review paper of Samokhvalov and Tatarchuk.30 X-ray diffraction patterns measured on the initial and exhausted samples clearly show the presence of crystalline phases of zinc sulfite and zinc sulfide in the case of the ZnH sample exposed to H2S (Figure 4). In this figure, the new zinc (hydroxy)acetate

phase mentioned above is also visible on the surface of ZnGO. Although the signals from ZnS are expected by taking into account the chemical nature of our material,30 ZnSO3 must be the result of the surface oxidation reactions.19,42,43 A discussion of the possible reaction paths is presented below. Interestingly, no crystalline species were found on the surface of ZnGO after exposure to H2S. This suggests that the zinc (oxy)hydroxide phase in this material behaves differently when in contact with hydrogen sulfide. Moreover, the evidence that zinc acetate is involved in surface reactions is clearly seen by the disappearance of the diffraction peaks representing this species. The lack of a crystalline phase upon modification of titania with sulfate ions was also reported by Gomez and co-workers.44 The amount of sulfur present in the samples was measured using elemental analysis about 4 weeks after the completion of the breakthrough tests, and it was compared to the amount expected on the basis of the breakthrough experiments (Table 2). 1340

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Table 2. Comparison of Detected Sulfur in Our Samples to That Expected on the Basis of Breakthrough Experiments sample ZnH-EPM ZnGO-EPM ZnGr-EPM

Figure 4. X-ray diffraction patterns for the initial and exhausted samples.

As seen, visible discrepancies exist between the expected sulfur contents, assuming that all H2S is retained in the surface, and the measured sulfur contents. This assumption in fact is not completely true because some SO2 was released during the breakthrough experiments. The results suggest that some H2S or sulfur, if present, could get further oxidized on the surface and desorbed before elemental analysis is performed. The percentage of undetected sulfur is highest for ZnGr (66%), signifying either the strongest oxidizing power and/or the weakest adsorption of formed SO2. The highest percentage of adsorbed sulfur is retained on the surface of ZnGO (60%), which can be considered to be a valuable asset of this material when applied as a sorbent. The results obtained on the composites containing ZnO are different than those obtained on the composites of GO with

Selemental analysis (wt %)

SH2S breakthrough (wt %)

6.57

13.1

11.87 4.70

19.9 14.1

Zr(OH)4.23 In the latter, only a slight difference between the measured and expected sulfur content was observed. Different behaviors of the systems addressed in this article indicate the differences in the surface activities of Zn(OH)2 and Zr(OH)4. Although zirconium oxide may exhibit some photoactivity when excited by UV light,45,46 zinc oxide is known to be photoactive in visible light.47,48 This activity can explain the oxidation of sulfur during the storage period. Changes in the chemistry of zinc (oxy)hydroxides and the composites exposed to H2S are seen in the FTIR spectra (Figure 5). Bands at 3455 and 3500 cm 1 seen on the ZnH spectrum represent hydroxyl stretching vibrations of the molecules in the unit cell.49 In the region between 700 and 1600 cm 1, all bands originate from the OH group vibrations and appear at 715, 830, 920, 1040, 1390, and 1500 cm 1. The two peaks observed at 1390 cm 1 with small shoulders at 1360 and 1500 cm 1 with a splitting at 1560 cm 1 are assigned to deformation vibrations of O H bonds bound to zinc hydroxide.50,51 The peak at 1500 cm 1 can also represent stable forms of chemisorbed oxygen.51 The bands at 830 and 1040 cm 1 correspond to the Zn OH bending mode, whereas the band at 920 cm 1 is assigned to the out-of-plane bending mode. The strong band at 715 cm 1 originates from librational modes of the OH groups or corresponds to librational modes of the H2O molecules.49 After exposure to H2S, the intensity of the bands between 700 and 1600 cm 1 representing hydroxyl groups significantly decreased, suggesting the involvement of these groups in the reactive adsorption process, which was also indicated by potentiometric titration results (Figure 3). The bands from sulfides are not expected to be seen owing to their low intensity. Even though the vibrations from SO32 at about 920 cm 1 overlap with those representing OH groups, after H2S adsorption the relative intensities of these peaks visibly increased.52 For the zinc hydroxide measured under dry conditions and for the zinc (oxy)hydroxide/graphene composite exposed to H2S under moist conditions, the intensity of the band at 1390 cm 1 visibly increases, indicating the formation of the asymmetrical stretching vibration of OdSdO, and can be related to the weakly bound SO252 molecules or the formation of sulfate.44 The ZnGO bands at 1500 and 1390 cm 1 can also be linked to the COO stretching mode of a zinc acetate-like structure as a result of the formation of the new Zn(OH)1.58(CH3COO)0.42 3 0.31H2O crystallographic phase, or Zn OH, or chemisorbed oxygen as described above.51,53 The same peaks are found for the ZnGr composite with diminished intensity at 1500 and 1390 cm 1. The formation of a carboxylo-carbonate structure in this case is not considered owing to the absence of carboxylic groups in graphene.31 Interestingly, the spectra for the exhausted ZnGO composites do not show any marked changes, in spite of the highest amount of H2S adsorbed. A plausible explanation, consistent with X-ray diffraction patterns, is that either highly dispersed ZnS or amorphous sulfur is deposited on the surface of this material.30 However, the visible changes are 1341

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Figure 5. FTIR spectra of the initial and exhausted samples.

seen on the FTIR spectra of ZnGr exposed to H2S. The bands at 1500 and 1390 cm 1 are much more intense than those before adsorption, and new bands at 940 and 1040 cm 1 appear. Because the formation of new OH groups in the zinc (oxy)hydroxide phase is rather unexpected, we link these bands to the formation of zinc sulfite/sulfate and/or the oxidation of the graphene phase by the products of surface reactions.44,52 Therefore, these bands can also represent C O stretching bands of epoxide and carboxyl groups.51 In the case of the composite with GO, the graphene-based phase was already strongly oxidized and all active sites were occupied by oxygen. Thus, the SO2 that formed on this material was not able to change the chemistry of that phase. Thermal analysis is another method used to evaluate the changes in surface chemistry or the products of reactive adsorption.3 DTG curves measured in nitrogen are presented in Figure 6. For ZnH, three main peaks centered at 150, 200, and about 450 °C are visible. They are assigned to the weight loss associated with the removal of physically adsorbed water, the dehydration of Zn(OH)2, and its dehydroxylation, respectively. The composite of zinc hydroxide with Gr does not reveal the presence of sizable amounts of physically adsorbed water, owing to the hydrophobicity of Gr. The peak from the dehydration of the zinc hydroxide phase is present at the same position and with the same intensity as that for ZnH. The continuous weight loss between 200 and 600 °C is associated with the removal of

oxygen-containing groups existing on the surface of the Gr component.54 At temperatures of between 550 and 800 °C, the dehydroxylation of zinc hydroxide takes place. In the case of ZnGO, the peak representing dehydration at about 200 °C overlaps with another intense peak at 225 °C, which we link to the removal of water associated with the Zn(OH)1.58(CH3COO)0.42 3 0.31 H2O phase.31 Between 225 and 400 °C, the decomposition of these species and/or the species in which both zinc and oxygen from epoxy groups are involved likely takes place. At temperatures higher than 400 °C, weight loss related to the dehydroxylation of the zinc hydroxide phase is revealed. Another weight loss seen as a small peak at about 650 °C might be linked to the removal of OH groups linked to both the distorted layers and the zinc (oxy)hydroxide structures formed at the interface. Then, for both ZnGO and ZnGr weight loss occurring at over 800 °C is assigned to the reduction of zinc oxide by the carbonaceous phase. Exposure to H2S significantly changes the weight loss patterns. The peak at a temperature of less than 100 °C is linked to the removal of water and/or physically adsorbed SO2. We do not assign this peak to H2S because the breakthrough experiments indicated its strong adsorption (Figure 1). Interestingly, on pure zinc (oxy)hydroxide exposed to H2S the peak at about 150 °C is more intense than that for the initial sample, and its position and intensity suggest that the same species are removed when the experiments are run under dry or moist conditions. For ZnH-ED, 1342

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Figure 6. DTG curves in nitrogen for the initial and exhausted samples.

the number of species removed at about 210 °C dramatically increased, although no effect was noticed for ZnH-EPM. However, the peak related to the dehydroxylation of Zn(OH)2 shifts to much higher temperatures after exposure to H2S, especially for the sample run under moist conditions. The number of decomposed species represented by that peak slightly increases. On the basis of the chemistry of our systems and the results discussed above, we assigned the peaks at 150 and 200 °C to weakly and strongly adsorbed SO2,55 respectively. The latter can refer to ZnSO3. More of these species under dry conditions than those under wet conditions can be explained by the absence of water. If H2S is oxidized to SO2 on the surface, then dry conditions are apparently in favor of this process. The peak on the DTG curves located between 400 and 700 °C for the exhausted samples must be related to the combined effect of the dehydroxylation and decomposition of zinc sulfate (expected at 600 °C).30,56 The

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discrepancy in the magnitude of the number of species decomposed in the experimental window and those expected on the surface based on the breakthrough results can be explained by the presence of zinc sulfide, detected using XRD analysis, which is expected to decompose at temperatures higher than 1000 °C.56 Although similar weight loss patterns to those for the ZnH series of samples are found for the exhausted ZnGr composites, weakly adsorbed SO2 is detected only in a very small amount on the latter materials. A new peak at about 350 °C is linked to the formation of oxygen groups on the surface of the graphene phase.54 The support for this is in the FTIR results, suggesting the oxidation of graphene. The results indicate that on the surface of ZnGr exposed to H2S under dry or wet conditions zinc sulfate is formed. However, the composite with graphite oxide shows a very different pattern of weight loss. Here the intense peak related to the dehydration of Zn(OH)2 and the decomposition of zinc acetate decreases significantly in intensity. The peak related to sulfite is intense, especially under dry conditions. Interestingly, the peak assigned to the decomposition of sulfate is not present. This suggests that on this material significant numbers of sulfides are formed and they are apparently in the amorphous form. The peak at about 200 °C, which was linked to the dehydration of Zn(OH)2, decreases significantly in its intensity for the exhausted ZnGO samples, and a shoulder at about 300 °C appears on the DTG curve for the sample run under dry conditions. Even though dehydroxylation is expected at about 500 °C, the number of species removed from the surface of ZnGO-ED represented by that peak increased compared to the number of species in the initial sample. This pattern suggests that water, OH, and O centers of Zn(OH)2 take part in surface reactions and that they accept sulfur/oxysulfur species. The involvement of those OH centers is also seen in the potentiometric titration results. On the basis of the results presented above, the mechanism of surface reactions under ambient conditions is proposed. This mechanism takes into account the photocatalytic activity of zinc oxide47,48 and zinc sulfide.57 Even though the carbonaceous surface with incorporated basic oxygen groups was shown to be an oxygen activator,58,59 the results indicate that the inorganic phase and/or products of surface reactions are the major participants in the oxidation reactions. The support for this is the discrepancy in the sulfur content compared to the expected content based on the breakthrough experiments (Table 2). It is plausible to assume that if photocatalytic reactions take place and a very active sample is stored under visible light then the oxidation of sulfur species can take place on the surface. Moreover, the autocatalysis by the products of surface reactions can also contribute to this.30 This oxidation is expected to take different paths on the samples run under dry conditions where water does not screen/affect the active phase of zinc (oxy)hydroxide, zinc sulfate, and zinc sulfide formed in the reactions. Even though water apparently helps in acid base reactions and the capacities measured under wet conditions are 2 to 3 times higher than those under dry conditions, the speciation of the products apparently differs. In these surface processes, the graphene-based phase of ZnGO plays a significant role. Owing to its chemical bonds with zinc (oxy)hydroxide and the conductivity of distorted graphene layers,31,60 it helps in electron transfer to activate oxygen. Positive effects of the carbonaceous phase on the photocatalytic activity were also found in its composites with titania, where an increase in the activity was 1343

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Figure 7. Schematic representation of surface processes.

linked to unspecified weak interactions between the activated carbon surface and TiO2.47,61 The enhanced photoactivities in visible light of the mixed-phase carbon-containing titanium oxides were also attributed to the modification of mixed anatase and rutile phases by carbon.62 Obviously, the graphene phase has much less activity than does GO because it is not bonded to the zinc (oxy)hydroxide phase and does not modify the lattice to the same extent. The X-ray diffraction patterns for our materials showed that the zinc (oxy)hydroxide phase in ZnGO is much more amorphous than those in ZnH and ZnGr (Figure 4). Moreover, ZnGO has a higher contribution of terminal OH groups than do the other materials (Figure 5). A lack of bonds between graphene and zinc (oxy)hydroxide does not exclude the participation of this phase in electron transfer, although this effect is expected to be smaller than in the case of ZnGO. It is also likely that the electron hole pair (e H+) formed upon exposure to light is more spatially separated as redox-active sites on the composites than on pure ZnH.62 This leads to the more efficient generation of reactive oxygen species as superoxide ions (O2 ), atomic oxygen, O , OH*, and HO2*. In the formation of the two latter species, terminal OH groups and the water of hydration can be involved. This is supported by our experimental results. In the case of GO, its functional groups are also expected to participate in oxygen activation42,43,58 and thus in the enhanced surface activity. Functional groups of activated carbons, such as the quinone type, were indicated as participating in direct electron transfer from sulfide to the surface.43 The participation of COOH moieties in redox reactions also cannot be ruled out.35 These groups can become active in Gr when it is oxidized by SO2 or by the formed radicals during the breakthrough test, although their effect is expected to be limited. Interesting quantum chemical insight into H2S oxidation on the carbon surface was presented by Khavryuchenko and co-workers.42 They indicated that the carbonaceous matrix acts as a source of spin density. Owing to its large spatial size compared to the reagents, the charge and spin density are delocalized and potentially spinactive species can interact indirectly. It is likely that OH* radicals formed from consumed terminal groups lead to the formation of SO32 and thus sulfates as shown for ZnH and ZnGr. Interestingly, this effect is not noticed on ZnGO even though the relative number of terminal groups is the greatest. It is likely that the formation of sulfides and sulfur in the form of bulky sulfur polymers on the surface of this material prevents the extensive involvement of terminal OH groups in the oxidation reactions. Moreover, in

the case of this material, owing to its quite amorphous structure, a ZnO/Zn(OH)2 ZnS/C visible-light-responsive photocatalyst can be formed. Even though zinc (oxy)hydroxide has not been studied as a photocatalyst itself, Zn Ti-layered double hydroxides were shown to be efficient visible-light catalysts.63 Their activity was attributed to a low band gap and a relatively high surface area (about 100 m2/g). A decrease in the HOMO LUMO gap in ZnO was also predicted when sulfur impurities were present in the zinc oxide structure.64 In the case of amorphous ZnO ZnS/C photocatalysts, the ZnO ZnS heterojunctions were linked to the increased photoactivity in visible light.57 Such heterojunctions may also exist in our materials, especially in ZnGO, where the number of sulfides formed during the duration of the breakthrough tests is expected to be the highest and the formed sulfur is the most resistant to oxidation. Besides this, the sulfonation of ZnO can also contribute to the increased activity, which was observed in the case of sulfonated titania where the S O bonds replaced the vacancies formed during the thermal dehydroxylation of TiO2.44 In our case, we refer to quasi-dehydroxylation that took place when the terminal groups participated in the formation of OH*. The visible-light photoactivity was also enhanced on zinc (oxy)sulfide composites.65 The schematic view of the surface processes on our materials discussed above is presented in Figure 7. The formation of elemental sulfur on the surface of the exhausted ZnGO sample and/or changes in its lattice structure are also suggested by a porosity study. After exposure to H2S, the surface of this composite increased 40% (from 54 m2/g for ZnGO to 76 m2/g for ZnGO-EPM) with a 12% increase in porosity. For ZnH and ZnGr exposed to H2S, only slight changes in porosity were noticed after exposure to H2S. The sulfur species formed on the surface of ZnGO-EPM are seen on the EDX element maps (Figure 8). The X-ray diffraction analysis showed 21.5 atomic % zinc, 15 atomic % oxygen, 6 atomic % sulfur, and 57.5 atomic % carbon on the surface of this material. A comparison of sulfur and zinc distributions indicates that besides ZnS, elemental sulfur is also the product of surface reactions. Apparently, a lot of oxygen is still present on the surfaces of the exhausted samples. Although some capacity of our adsorbents to retain hydrogen sulfide could be potentially regenerated by heat treatment to about 400 °C as suggested by TA analysis, such a treatment is also expected to change the materials’ properties (decomposition of surface functional groups, dehydration, and dehydroxylation). 1344

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Figure 8. EDX analysis of the ZnGO-EPM sample.

Nevertheless, for our target application of these materials, in gas mask filters the regeneration capability of an adsorbent bed is less important than that in large-scale industrial applications. For our purpose, the efficiency of the toxin retention is of paramount importance.

’ CONCLUSIONS The results collected in this article for the first time present the zinc (oxy)hydroxide/graphite oxide composite as a very efficient adsorbent of hydrogen sulfide, where the removal process is enhanced by the photocatalytic activity in visible light. An increase in the amount of H2S adsorbed/oxidized on the surface in comparison with the amount of pure Zn(OH)2 adsorbed is linked to the structure of the composite in which the zinc hydroxide component has more terminal groups than that in the bulk phase. These groups were found to participate in oxidation reactions likely via the photochemical path. The graphite oxide component, owing to chemical bonds with the zinc (oxy)hydroxide phase and conductive properties, helps in the electron transfer leading to oxygen activation. Moreover, functional groups of GO also participate in the formation of superoxide ions. Therefore, besides ZnS, elemental sulfur is also the product of surface reactions. On zinc (oxy)hydroxide, zinc sulfide, zinc sulfite, and zinc sulfate are formed on the surface, as on the composite with graphene. The graphene phase increases the electron transfer, but its effect is rather limited owing to the lack of oxygen activating functional groups and bonds with the zinc (oxy)hydroxide phase. The formation of sulfur compounds on the surface of zinc (oxy)hydroxide during the course of the breakthrough experiments and thus Zn(OH)2 ZnS heterojunctions can also contribute to the increased surface activity of our materials. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: (212) 650-6017. Fax: (212) 650-6107.

’ ACKNOWLEDGMENT This work was supported by ARO grant W911NF-10-1-0039, NSF collaborative grant 0754945/0754979, and PSC-CUNY grant 63098-0041.

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