Tunable Shape Microwave Synthesis of Zinc Oxide Nanospheres and

Publication Date (Web): March 28, 2012 ... Metal oxides and metal oxide mixtures are commonly used for adsorption processes as ...... from coal gas by...
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Tunable Shape Microwave Synthesis of Zinc Oxide Nanospheres and Their Desulfurization Performance Compared with Nanorods and Platelet-Like Morphologies for the Removal of Hydrogen Sulfide Hector F. Garces,†,‡ Anais E. Espinal,∥ and Steven L. Suib*,†,§ †

Institute of Materials Science, 97 North Eagleville Road, University of Connecticut, Storrs, Connecticut 06269-3136, United States Center for Clean Energy and Engineering, 44 Weaver Road, University of Connecticut, Storrs, Connecticut 06269-5233, United States § Department of Chemistry, 55 North Eagleville Road, Unit 3060, University of Connecticut, Storrs, Connecticut, 06269-3060, United States ∥ United Technologies Research Center, 411 Silver Lane, East Hartford, Connecticut 06108-1127, United States ‡

S Supporting Information *

ABSTRACT: A tunable shape microwave synthesis of ZnO nanospheres in a cosolvent mixture is presented. The ZnO nanospheres material is investigated as a desulfurizing sorbent in a fixed bed reactor in the temperature range 200−400 °C and compared with ZnO nanorod and platelet-like morphologies. Fresh and sulfided materials were characterized by X-ray diffraction, BET specific surface area, pore volume, scanning electron microscopy, X-ray energy dispersive spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The tunable shape microwave synthesis of ZnO presents a high sulfur sorption capacity at temperatures as low as 200 °C, which accounts for three and four times the other preparations presented in this work, and reached 76% of the theoretical sulfur capacityat 300 °C.

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MOx(s) + x H 2S(g) → MSx(s) + x H 2O(g)

INTRODUCTION Hydrogen sulfide (H2S), carbon disulfide (CS2), carbonyl sulfide (COS), and dimethyl sulfide (DMS) are common impurities found in natural gas, chemical feedstocks, coal gas,1−3 and anaerobic digested gas (ADG).4 Removal of these contaminants is required to avoid corrosion, environmental problems, deactivation of catalyst, poison of electrodes containing noble metals, and damage of equipment and downstream processes.,5−8 In particular, attention is devoted to remove H2S because od its hazardous, corrosive, toxic, and pyrophoric character.2,9 Adsorption, among other techniques, is one of the most studied methods for the removal of H2S because of its efficiency, reliability, and wide range of working temperatures.2,5,9 Metal oxides and metal oxide mixtures are commonly used for adsorption processes as desulfurization sorbents due to the broad range of temperatures and compositions where they effectively removed H2S.6,10,11 Metal oxide waste materials in a pelletized form containing significant quantities of reactive metal oxides have also been evaluated.12 The desulfurization potential of some of these metal oxides has been studied by the free-energy minimization method,10 and the thermodynamic analysis is reported for characteristic fuels and gasification mixtures.10,11 In general, the basic desulfurization reaction for metal oxides is represented as follows.5,13 © 2012 American Chemical Society

(1)

where MOx and MSx are the initial metal oxide sorbent and the metal sulfide after the reaction, respectively. Zinc oxide and zinc-based sorbents are very attractive for the removal of H2S2,5,11−16 because of their high equilibrium constant for sulfidation, thermal stability of sorbents and their sulfides, nonpyrophoric character, and the high level of sulfur removal that can be achieved.5,9 However, zinc oxides tend to be reduced to metallic zinc at temperatures above 700 °C, reducing its performance for hot desulfurization processes. Low-temperature development of solid materials for H2S sorption can be advantageous for improving energy efficiency and reducing operating cost.9,16−18 These sorbents can find applications in low-temperature hydrogen-rich streams, such as in polymer electrolyte (PEM) fuel cells,19 where small amounts of sulfur compounds poison electrodes and decrease cell performance. Different ZnO-based sorbents are reported in the literature for the removal of H2S.2,5 Some ZnO nanopreparations seem to have a higher degree of success during the removal of H2S and Received: November 8, 2011 Revised: March 26, 2012 Published: March 28, 2012 8465

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Scheme 1. Schematic Representation of the Experimental Setupa

GC: gas chromatograph, FPD: flame photometric detector, MFC: mass flow controller, TC: thermocouple, 3-WV: 3-way valve, CV: check valve, M: manometer. a

subsequent metal sulfide conversion.5 Desulfurization performance with morphological features, like porosity and surface chemistry, has already been studied for zeolites20 and activated carbons. 21 However, to evaluate, study, and correlate morphological changes due to sulfidation of different ZnO sorbents and to understand the deactivation mechanism during desulfurization process are still important for engineering ZnO sorbents with high sulfur capacity. This work explores the synthesis, characterization, and desulfurization behavior of three ZnO sorbents with different morphologies for the removal of H2S. A novel synthesis method using microwave irradiation in a cosolvent medium to control the total vapor pressure and composition during the synthesis presents a remarkable efficiency for the removal of H2S at temperatures as low as 200 °C. In the present microwave synthesis, the shape of the nanospheres can be wellcontrolled by modulating the mol fraction of the precursor concentrations. Several characterization techniques were employed to analyze the structural changes due to sulfidation of the metal oxides. Morphological changes to the sorbent resulting from conversion to sulfided phases are also presented as well as sulfur sorption capacities.

Sorbent Characterization. X-ray diffraction analysis was performed to evaluate the crystallinity of the ZnO in the fresh state and the formation of zinc sulfide after sulfidation. The fresh ZnO and sulfided adsorbent were analyzed in an Ultima IV Rigaku X-ray diffractometer (Cu Kα radiation). Diffraction patterns were obtained in the range of 5−90 2θ degrees at a scan rate of 2° min−1. Specific surface areas were determined by the Brunauer− Emmett−Teller (BET)22 method for the as-prepared ZnO sorbent and the resultant material sulfided at 300 °C in a Quantachrome Autosorb IQ. The total pore volumes of the samples were also determined using N2 adsorption. Degas pretreatments for BET and pore volume analyses were performed at 120 °C. Thermogravimetric (TGA) and differential scanning calorimetric analyses were done in a SDT Q500 (TA InstrumentsWaters) for the as-prepared material and the sorbent sulfided at 300 °C. Mass changes were measured at 5 °C min−1 from 50 to 800 °C under N2 and O2 with a purge rate of 40 and 60 mL min−1, respectively. Before the start of the analysis, the sample was held for 5 min at 50 °C. Raman spectroscopy analysis was performed using a Renishaw Ramanscope System 2000 instrument linked to a Leica microscope. A 514 nm Ar+ laser was used as an excitation source. Powder samples were analyzed after being deposited on a glass slide with four scans for 40 s for each sample. Scanning electron microscopy (SEM) was performed in a LEO/Zeiss DSM 982 Gemini field-emission scanning electron microscopy (FESEM) with a Schottky emitter operating at 2.0 kV with a beam current of 1.0 mA. The sample was dispersed in EtOH and deposited on AuPd-coated silicon chips. Chemical mapping and quantitative analysis of the samples were determined by energy-dispersive X-ray analysis (EDX) in an FEI Quanta ESEM 250 operated at 20.0 kV with X-ray spectra acquired and processed with an Ametek Genesis Apex 4. Hightemperature scanning electron microscopy (HTSEM) was performed in an FEI Quanta ESEM 250. A sample was formed as a pellet of ∼6.0 mm diameter and 1.20 height in an aluminum crucible and loaded in a heating stage assembly for dynamic experiments with specific temperatures and heating times. A heating stage was used to control the sample temperature up to 400 °C, and the sample was observed with the use of the FEI electron microscope. Sulfidation Reactor. The sorption experiments in a temperature range from 200 to 400 °C employed a vertical fixed-bed tubular reactor made of quartz with an internal diameter of 2 mm. In each experiment, the sorbent was heated to the analysis temperature with a He down flow of 36 SCCM and held for 1 h before the beginning of the sulfidation. The

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EXPERIMENTAL SECTION Zinc Oxide Synthesis. The ZnO sorbents used in desulfurization studies were prepared using three different methods. (ZnAcac)MW was synthesized with microwave irradiation. A mixture of 0.5 g of zinc acetyl acetonate (Zn(acac)2), 4.672 mL of methanol (MetOH), and 7.328 mL of 1-hexanol was introduced to a glass microwave vessel, capped, and placed in a microwave cavity (Biotage-Initiator). The mixture was stirred in the microwave cavity for 20 min. Afterward, microwave heating was applied to the mixture for 1 h at 160 °C. Then, the material was cooled, washed with ethanol (EtOH) and water, and dried at room temperature. (ZnAcac)HT was synthesized by dissolving 0.15 g of Zn(acac)2 in 12 mL of EtOH and adding 540 μL of tetrabutyl ammonium bromide (TBABr). The mixture was stirred in a Teflon linear for 20 min and autoclaved at 160 °C for 24 h in a conventional oven. The material was washed with EtOH and water and dried at room temperature. (Zn-NO3)HT was synthesized by dissolving 1.7426 g of zinc nitrate (Zn(NO3)2) in 10 mL of water and adding 0.774 g of potassium hydroxide (KOH). The linear that contains the dispersion was placed in an autoclave and mixed in a rotary oven for 2 h. Then, the material was treated in an autoclave at 120 °C for 24 h with conventional heating while being rotated. After the synthesis, the material was washed with double deionized water (DDW) and dried at room temperature. 8466

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during the desulfurization studies differ from one another in the direction of the preferential growth, crystallinity, and crystallite size. Table 1 presents the BET surface area and the total pore volume of the fresh sorbent and the materials after sulfidation

analytical system comprised an SRI 8610C GC equipped with a flame photometric detector (FPD) (Scheme 1). A GS-GasPro capillary column (30 m × 0.32 mm I.D.) was used for the separation. A 10-port sampling valve with an electronic actuator made automatic injections into the GC every 10 min. Tubing and fittings were stainless steel throughout. In each experiment, ∼0.1 g of ZnO sorbent was placed in the reactor supported by quartz wool. A furnace with PID control held the temperature constant in the reactor. The thermocouple was placed at the top of the ZnO bed. Mass flow controllers (MFCs) (MKS model 1479A, 20 SCCM ± 1% full scale, Alicat Scientific, 200 SCCM full scale ± 0.2% full scale) were used to control flow rates, feed, and composition. A certified gas mixture (Scott Specialty Gases, 1% H2S in He) was used for the sulfidation experiments. The weight hourly space velocity was fixed at ∼24 L h−1 g−1. Sulfur Sorption Capacity. The sulfur sorption capacity at breakthrough for the material sulfided at different temperatures was determined5,23 and given by eq 2 ⎛ g sulfur ⎞ SC⎜ ⎟ ⎝ 100 g sorbent ⎠ ⎡ M = (WHSV) × ⎢ × ⎣ Vmol

∫0

t

⎤ (C in − Cout)dt ⎥ ⎦

Table 1. BET Surface Area and Pore Volume of the Fresh ZnO and Sulfided ZnO at 300 °C fresh sorbent

sulfided sorbent

sample

BET surface area (m2/g)

total pore volume (cm3/g)

(ZnAcac)MW

52

0.277

(ZnAcac)HT

14

0.148

(Zn-NO3)HT

14

0.118

sample (ZnAcac)MW300S (ZnAcac)HT300S (Zn-NO3)HT300S

BET surface area (m2/g)

total pore volume (cm3/g)

15

0.149

5

0.059

3

0.053

at 300 °C. An “S” in the sample ID signifies that the sample is sulfided. For example, (ZnAcac)MW is the fresh zinc oxide sorbent prepared with microwave irradiation before sorption studies. (ZnAcac)MW-300S is the zinc oxide sorbent after sulfidation at 300 °C. The adsorption isotherms and the pore size distribution for the fresh (ZnAcac)MW and sulfided (ZnAcac)MW-300S are presented in Figure 2. Isotherms and pore size distribution for samples (ZnAcac)HT and (Zn-NO3) before and after sulfidation at 300 °C are presented in the Supporting Information (Figures S1−S4). The contribution to the total pore volume of the mesopore region is much higher for (ZnAcac)MW than for the preparations of (ZnAcac)HT and (Zn-NO3)HT, where macropres contribute the most to the total pore volume. After sulfidation at 300 °C, the decrease in the mesopore contribution is significant for (ZnAcac)MW-300S (Figure 2b); instead (ZnAcac)HT-300S and (Zn-NO3)HT-300S did not show large variations (Supporting Information, Figures S2 and S4). The ZnO sorbent prepared under microwave heating (ZnAcac)MW has a surface area 73% larger than the sorbents prepared under conventional heating (ZnAcac)HT, (ZnNO3)HT. The pore volume of (ZnAcac)MW is 47 and 57% higher than the (ZnAcac)HT and (Zn-NO3)HT samples, respectively. Conventional heating was also used to prepare a sample with the same synthesis conditions as (ZnAcac)MW but using a heating oven (ZnAcac)HT‑2 instead of a microwave instrument. X-ray diffraction patterns, SEM images, and BET surface area are presented and compared with the microwave preparation (Supporting Information, Figure S5). X-ray diffraction data for both syntheses differ from one another in the preferential growth along the ⟨002⟩ direction, crystallite size, and morphology. Improvement in the BET surface area with microwave synthesis can be seen after comparison of the nitrogen sorption results for the microwave preparation ((ZnAcac)MW) and the same synthesis under hydrothermal conditions ((ZnAcac)HT‑2). The hydrothermal synthesis has a BET surface area of 32 m2 g−1 compared with the microwave synthesis that has a BET surface area of 52 m2 g−1. Morphology is different for both syntheses with high agglomeration and large particle sizes for the hydrothermal preparation (Supporting Information, Figure S5c,d). Nanospheres are obtained under microwave synthesis. Similar results can also be observed after changing the ratio of methanol to 1-hexanol in the

(2)

where WHSV is the weight hourly space velocity in mL h−1 g−1, M is the atomic weight of sulfur (32 g mol−1), Vmol is the molar volume in L mol−1 under standard conditions of 298 K and 1 atm (24.5 L mol−1), Cin and Cout are the inlet and outlet concentrations (%), respectively, and t is the breakthrough time (BT) in hours. The BT is defined as the time when the outlet concentration reached 50 ppm. The outlet concentration, Cout, in eq 2 was defined as 50 ppm, which is much higher than the detection limit of the FPD used in this study for H2S detection.

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RESULTS The diffraction patterns for the different ZnO sorbents are shown in Figure 1. The diffraction peaks correspond to the hexagonal ZnO zincite phase (ICSD no. 01-070-8072). No additional phases were detected. The starting materials used

Figure 1. X-ray diffraction patterns of fresh ZnO at different synthesis conditions (a) (ZnAcac)MW, (b) (ZnAcac)HT, and (c) (Zn-NO3)HT. Diffraction peaks correspond to zincite (ICSD no. 01-070-8072). 8467

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Figure 2. Adsorption−desorption isotherm and pore size distribution of (a) (ZnAcac)MW and (b) (ZnAcac)MW-300S.

Figure 3. Scanning electron microscopy of fresh ZnO and sulfided ZnO at 300 °C. Fields (a) (ZnAcac)MW, (b) (ZnAcac)MW-300S, (c) (ZnAcac)HT, (d) (ZnAcac)HT-300S, (e) (Zn-NO3)HT, and (f) (Zn-NO3)HT-300S.

the electromagnetic radiation into heat directly. The texture of the sorbent after sulfidation (ZnAcac)MW-300S, (ZnAcac)HT300S, and (Zn-NO3)HT-300S compared with the fresh sorbents (ZnAcac)MW, (ZnAcac)HT, and (Zn-NO3)HT decreased by 71, 64, and 78%, respectively, from their initial surface area values (Table 1). SEM images are shown in Figure 3 for the fresh zinc oxide and the sorbent after sulfidation at 300 °C. Fields a−f exemplify changes in the surface morphology. EDX spectroscopy

precursor mixture for the synthesis under microwave irradiation. The formation of nanospheres with higher surface area is obtained (Supporting Information, Figure S8f). When using conventional heating, the yield was low compared with microwave heating, suggesting incomplete decomposition of Zn(acac)2 and low nucleation rates and growth. Heat and mass transfer limitations may be the cause. Conventional heating creates a temperature gradient within the sample as compared with microwave heating, which transforms 8468

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Figure 4. Thermogravimetric analysis of ZnO (a) (ZnAcac)MW-300S, (ZnAcac)HT-300S, and (Zn-NO3)HT-300S under N2 and (b) (ZnAcac)MW300S, (ZnAcac)HT-300S, and (Zn-NO3)HT-300S under O2.

mapping after sulfidation is presented in the Supporting Information (Figure S6). TGAs under N2 and O2 for the samples sulfided at 300 °C are presented in Figure 4a,b respectively. The TGA analyses for fresh ZnO sorbents (ZnAcac)MW, (ZnAcac)HT, and (ZnNO3)HT are presented in the Supporting Information (Figure S7). Higher weight change is observed in the microwave preparation as compared with the hydrothermal synthesis. The sulfided samples at 300 °C show comparable weight losses (Figure 4a) under inert atmosphere until the start of the reduction of ZnO to zinc metal (550 °C). The TGA for the sulfide sorbent under O2 (Figure 4b) presents different time frames for the oxidation of the converted ZnS and the formation of intermediate compounds in the temperature range of 600 to 700 °C. X-ray diffraction analyses performed on the samples after sulfidation at 200, 300, and 400 °C are shown in Figure 5a−c. Sulfidation at any temperature leads to partial transformation of the former ZnO hexagonal phase (JCPDS no. 36-1451) into a blend of hexagonal (JCPDS no. 36-1450) and cubic (JCPDS no. 5-566) ZnS phases. Sulfidation of (ZnAcac)MW below 300 °C produces a high proportion of the secondary phases of ZnS among the sorbents evaluated. Contributions from both crystalline forms can be seen in the lines at the bottom of the pattern that represent the position and intensity of the reflections. The breakthrough curves are presented in Figure 6. The BT is defined as the time where the outlet concentration reaches 50 ppm. Samples sulfided at 200 and 300 °C for the (ZnAcac)HT material have BT times of 1.40 h [(ZnAcac)HT-200S] and 4.71 h [(ZnAcac)HT-300S], respectively. Sulfidation of (Zn-NO3)HT at 200 and 300 °C have BT times of 0.6 h [(Zn-NO3)HT-200S] and 3.55 h [(Zn-NO3)HT-300S]. The values for both ZnO sorbents [(ZnAcac)HT and (Zn-NO3)HT] are low, as much as half of the results obtained after sulfidation of (ZnAcac)MW at 200 and 300 °C (4.74 and 9.63 h, respectively). H2S removal at 400 °C for samples (ZnAcac)HT and (Zn-NO3)HT increased as compared with (ZnAcac)MW, which is much lower than sulfidation at 300 °C. Sulfur sorption capacities are determined according to eq 2 and presented in Figure 7. A quantitative analysis of the sulfur in the sorbents after sulfidation at 300 °C for the three different samples by EDX supports this calculation. In total, 15 areas for the samples sulfided at 300 °C were scanned, and the elemental composition was obtained.

The results are averaged and presented in Table S2 (Supporting Information). These results by EDX for the quantitative analysis are consistent and close to the values obtained after the breakthrough. Earlier breakthroughs are directly related to the low ability to remove sulfur. Samples (ZnAcac)HT and (Zn-NO3)HT over the entire range of sulfidation remove more H2S as the temperature increases. This trend is not observed for (ZnAcac)MW, where an optimum might be obtained in between 300 to 400 °C.

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DISCUSSION The ZnO materials produced under different synthesis conditions present structural differences, as can be seen from the heights and widths of the X-ray diffraction peaks (Figure 1). The synthesis in organic media [(ZnAcac)MW, (ZnAcac)HT)] leads to smaller crystals as compared with the aqueous synthesis [(Zn-NO3)HT]. The intensity of the ⟨002⟩ diffraction peaks reflects the growth habit for the different preparations. This peak is prominent for (ZnAcac)HT, suggesting higher growth rates along the ⟨002⟩ direction as compared with (ZnAcac)MW and (Zn-NO3)HT. Zinc acetyl acetonate is a stable chelate complex, and the mechanism for ZnO formation through decomposition of (Zn(acac)2) is known.24−26 Some research uses cationic surfactants during the synthesis to prevent agglomeration and to tune the particle size and morphology.27 The formation of ZnO does not go through the dehydration−condensation reaction of Zn(OH)2, which is the case in the synthesis (Zn-NO3)HT, where metal salts are used as starting precursors. Tetra butyl ammonium bromide (TBABr) used during the synthesis of (ZnAcac)HT preferentially adsorbes on the prism planes28,29 of ZnO and leaves the polar faces exposed, allowing further crystal growth along the c direction. Microwave irradiation is known for providing rapid and homogeneous heating, uniform nucleation and growth conditions, and nanomaterials with small sizes.30,31 The present approach for the preparation of ZnO with microwave heating [(ZnAcac)MW] uses a cosolvent mixture and monitors the total vapor pressure of the mixture (PT) during the synthesis (Supporting Information, Figure S8a,c,e). The mol fraction of each component in the mixture was changed to obtain a PT from about 4 to 12 bar. The final product evolves from fibershape morphology to round morphology (Supporting Information, Figure S8b,d,f), leading to a nanomaterial with higher surface area. 8469

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Figure 6. Breakthrough curves for the different zinc oxide sorbents evaluated.

Figure 7. Sulfur sorption capacities of zinc oxide after sulfidation at different temperatures.

has also been reported, and a round morphology with a high degree of agglomeration is observed.33 The morphology is similar to our findings for the low vapor pressure cosolvent synthesis. Semicircles with flattened axes (Supporting Information, Figure S8d) and quasi-spherical particles (Supporting Information, Figure S8f) are obtained when the mol fraction of methanol is increased during the synthesis. In general, solvents with lower polarity produce rods with high aspect ratios.32 The use of a cosolvent mixture of methanol and 1-hexanol has not been reported yet. When methanol with a lower boiling point (65 °C) is added to 1-hexanol (boiling point 155 °C), the saturation vapor pressure of the solvent mixture increases. The quasi-spherical nanoparticles obtained at high mol fraction of methanol in the cosolvent mixture are consistent with previous reports when methanol is used alone during the synthesis.33 However, this synthesis produces nanoparticles that do not agglomerate. Zinc oxide is a polar crystal with different growth rates along different crystal directions.32,34,35 In solvents with high polarities and using (Zn(acac)2) as a precursor, the growth along the [002] direction has been shown to be enhanced, resulting in rod-like morphologies. Lower polarity solvents lead to more equi-axed ZnO crystals.32 Our findings with methanol-1-hexanol system differ from previous reports. A less anisotropic growth rate is obtained when a high mol fraction of methanol in relation to 1-hexanol was used during the synthesis. The final ZnO

Figure 5. X-ray diffraction patterns of fresh ZnO and ZnO after sulfidation at different temperatures (a) (ZnAcac)MW, (b) (ZnAcac)HT, and (c) (Zn-NO3)HT.

Saturation vapor pressure of the solvent plays a crucial role in the morphology of the obtained crystals prepared through solvothermal synthesis. High saturation vapor pressure solvents lead to the formation of aggregates of nanoparticles due to ZnO nuclei being limited under high pressure. When solvents with low saturation vapor pressure are used, larger ZnO crystals are obtained.32 During the microwave synthesis of (ZnAcac)MW, low mol fraction of methanol during the preparation leads to the formation of aggregates composed of fiber-like morphology (Supporting Information, Figure S8b). Fiber growth occurs in both directions by expanding from a central nucleus. Synthesis of ZnO in 1-hexanol with inorganic precursors has also been reported. The morphology of the synthesized material is composed of nanoneedles with an end tip as small as 20 nm.33 The use of methanol during the synthesis of ZnO 8470

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product exhibits a fiber-like morphology when a small amount of methanol is used during the synthesis. These findings might suggest that 1-hexanol, with a less polar character that methanol and with its long hydrocarbon chain, is not only preventing agglomeration of the final ZnO but also suppressing the interaction of the polar solvent along the [002] direction, restraining the anisotropic growth of ZnO and leading to a material with low aspect ratio. The texture of the material changed considerably. High loss of surface area and pore volume due to sulfidation at 300 °C is observed for all ZnO preparations (Table 1). The low-surfacearea sorbents [(ZnAcac)HT, (Zn-NO3)HT] presented about the same change. However, examining the pore size distributions for samples (ZnAcac)HT and (Zn-NO3)HT shows that the macropore range still contributes the most to the total pore volume of the material (Supporting Information, Figures S2 and S4), suggesting low interaction between the H2S gas and the solid adsorbate, leaving some of the H2S without reaction. A different observation is seen for (ZnAcac)MW, where the mesopore contribution to the total pore volume decreased by about half of the initial value. The gas−solid interaction is much stronger, resulting in more conversion of oxide to sulfide. This might suggest that low-temperature desulfurization with ZnO is favored in a microporous−mesoporous material, where surface interactions are stronger, leading to faster reactions and diffusion processes. Macroporous nanomaterials still can have some sulfur retention ability, but the majority of the molecules do not interact with the sorbent active sites. The material to be removed moves throughout large channels in the sorbent with low or no interaction with the sorbent surface active sites, leading to low conversion of oxide to sulfide. Morphology evolution after sulfidation for samples (ZnAcac)HT-300S and (Zn-NO3)HT-300S (Figure 3d,f) shows sintering-type morphology where the final mixture of oxide and sulfide agglomerates. The resultant material still keeps some characteristic features of the initial oxide. The low conversion of oxide to sulfide suggest that the loss of surface area might be principally due to the analysis temperature instead of the conversion of oxide to sulfide (Supporting Information, Figure S6b, EDX mapping). Sample (ZnAcac)MW still preserves the round-type morphology that the initial oxide exhibits, and the loss of surface area and change in morphology must be due to high conversion of metal oxide to sulfide (Supporting Information, Figure S6a, EDX mapping). The metal oxide to metal sulfide conversion can be seen in the sequence of X-ray diffraction patterns in Figure 4a−c for all sorbents. Poor conversion is obtained below 300 °C for (ZnAcac)HT and (Zn-NO3)HT, as indicated by the low strength for the ZnS reflections as compared with (ZnAcac)MW. Higher metal sulfide conversion is observed for all sorbents at 400 °C. This does not reflect the BT for sample (ZnAcac)MW-400S, which is obtained when the outlet concentration reaches 50 ppm. Conversion of oxide to sulfide continues after the breakthrough, but the material loses the ability to remove H2S efficiently at concentrations lower than 50 ppm. This can be explained by the sulfur breakthrough characteristic curves that show that the same H2S inlet concentration must have different slopes because of the different nature of the sorbents. The microwave preparation has a round morphology. The sorption and reaction processes can be explained following a radial mode for fouling the sorbent.36 Sulfidation is a surface-controlled process. As the sulfidation of the material proceeds, H2S must diffuse farther into the material to reach the unreacted solid

phase.36,37 Molecules of H2S that cannot diffuse to the core of the solid or find active surface sites in the outer edge of the pellet leave the bed. Even though the concentration is higher than 50 ppm, the sorbent is still able to remove sulfur to some extent, but the breakthrough threshold has already been surpassed. This explains the higher strengths of ZnS observed in the X-ray diffraction pattern for sulfidation at 400 °C (ZnAcac)MW-400S). The amount of sulfur that can be removed for a metal oxide is determined by the equilibrium constant for sulfidation.10 High sorbent utilization at temperatures as low as 500 °C is difficult to obtain. Previous breakthrough for (ZnAcac)HT and (Zn-NO3)HT and low sulfur sorption capacity below 300 °C (Figures 6 and 7) might suggest low diffusion rates of H2S due to the crystallite size, pore size distribution, and morphology of the hydrothermal preparations. The sorption at 400 °C is higher for (ZnAcac)HT and (Zn-NO3)HT than for (ZnAcac)MW, which may account for the organic components that remain in (ZnAcac)MW after the synthesis. These organic byproducts suffer pyrolysis at 400 °C, leaving carbonaceous deposits and blocking paths for H2S to diffuse through the solid sorbent core. Raman spectroscopy (Supporting Information, Figure S10) confirmed the carbon content of (ZnAcac)MW-300S with characteristic peaks between 1300 and 1600 cm−1. The loss of weight for the fresh sorbents under N 2 corresponds to the removal of adsorbed water (Supporting Information, Figure S7a,b). The microwave preparation loses ∼4 wt % compared with the hydrothermal preparations, which account for ∼1.5 wt.% loss up to 800 °C. The analysis under O2 presents similar characteristics. This suggests that under these synthesis conditions ZnO is not reduced below 700 °C and preserves the structure without losing oxygen or presenting volatilization of zinc due to the high temperatures. When the sulfided material at 300 °C is evaluated under N2 (Figure 4a), the differences in the spectrum up to 300 °C are associated with weight losses. TGAs with O2 for the three sorbents after sulfidation at 300 °C are represented in Figure 4b. The reaction of ZnS with oxygen has been reported to proceed via two alternative pathways:38 Pathway 1 ZnS(s) +

3 O2(g) → ZnO(s) + SO2(g) 2

ZnO(s) + SO2(g) +

1 O2(g) → ZnSO4(s) 2

(3)

(4)

Pathway 2 ZnS(s) + 2O2(g) → ZnSO4(s)

(5)

1 O2(g) (6) 2 The three different preparations sulfided at 300 °C (ZnAcac)MW, (ZnAcac)HT, and (Zn-NO3)HT of ZnO present different regeneration behavior (Figure 4b). The different thermogravimetric response toward regeneration of the sulfided ZnO is influenced not only by the synthesis conditions of the former metal oxide used for the sulfidation studies but also by the morphology, particle size, surface area, and extent of sulfidation of the former oxide. The TGA under O2 for sorbent (ZnAcac)MW-300S with a round morphology and with a high degree of sulfidation might suggest that the initial weight loss is ZnSO4(s) → ZnO(s) + SO2(g) +

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Although (ZnAcac)MW was employed at 400 °C for sulfidation studies, higher efficiencies were obtained at temperatures of 200 and 300 °C for the ZnO nanospheres prepared by microwave irradiation. Heat treatment of sample (ZnAcac)MW from room temperature to 400 °C under low vacuum in real time, using an environmental scanning electron microscope (ESEM) with an incorporated heating stage, revealed that the metal oxide nanospheres preserved their morphology and did not sinter (Supporting Information, Figure S9a−f). Different results are obtained after heat treatment of the same material (ZnAcac)MW at 300 and 400 °C for 9 h under air without sulfidation. The round morphology is still preserved for the sample heat-treated at 300 °C with the formation of cavities in the former ZnO nanospheres (Supporting Information, Figure S9g). Loss of the spherical shape and larger amounts of cavities are clearly observed after calcination at 400 °C (Supporting Information, Figure S9h). These cavities might be produced by evolution of CO2, after decomposition of organic material that remains after the synthesis. There is also a high loss in surface area after calcination in air at 300 and 400 °C, 25 and 16 m2 g−1, respectively. Even though a decrease in surface area and poisoning due to carbon deposits decreased the sulfur capacity at 400 °C, the major effect might be attributed to sulfidation. The molar volume of ZnS is higher than the precursor ZnO, facilitating the agglomeration and pore closure of the nanospheres, decreasing the conversion and rate of reaction, with subsequent early breakthrough (Supporting Information, Figure S9i). Sintering, increase in molar volume, decrease in surface area, and fouling by carbon are principally responsible for the decrease in sulfur sorption capacity at 400 °C for sample (ZnAcac)MW. Other metal oxides can be used as desulfurizing sorbents as long as the reaction is thermodynamically favored.10,41 These solid gas reactions can be driven to nearly stoichiometric proportions. Oxides such as ZnO, Cu2O, CoO, MoO, and NiO to name a few have received the most attention because they can achieve desulfurization beyond 98% conversion for H2S in the temperature range of 400 to 600 °C. However, these oxides present several advantages and disadvantages, such as the tendency of Cu2O to reduce, the high volatility of ZnO, and the formation of carbonates for CoO. The extent to which sorbents are converted to their sulfide forms is influenced by factors such as stoichiometries, partial pressures, residence times, and kinetics limitations.41 Compared with highly efficient commercial ZnO sorbents reported in the literature5 and under similar desulfurization conditions, the present work at 300 °C where desulfurization with (ZnAcac)MW reaches a maximum is 10% more efficient for the removal of H2S. Other metal oxides have been used for lowtemperature desulfurization such as CoO, NiO, and MnO with relatively low H2S uptakes17 compared to the value obtained in this work. Similar H2S uptakes have been reported using zincbased sorbents16,42−44 or mixed metal oxide sorbents supported with an inert material.45 However, some of these involve several cycles of sulfidation and regeneration, high-temperature conditions, and special preparations, making the process timeconsuming and energy demanding. In general, under these particular sulfidation conditions, the microwave preparation shows higher sulfur sorption capacities at low temperature (200, 300 °C) for the removal of H2S than the ZnO powder prepared under conventional heating and presenting different morphologies and reaches 76% of the theoretical sulfur capacity with sulfidation at 300 °C. The value

due to a rapid regeneration of the sulfide sorbent through the formation of zinc oxide. The initial weight loss of (ZnAcac)MW300S must have contributions due to formation of CO2 from the carbon deposits detected in the sulfided sample by Raman spectroscopy (Supporting Information, Figure S10). The formation of CO2 from carbon deposits can be expected to occur in this time frame (580 °C). After this decrease in weight percentage, a weight gain occurs, showing the subsequent formation of sulfate (ZnSO4) and reaching a maximum rate of conversion at ∼650 °C. This mechanism has been reported39 and implies that zinc sulfate is formed according to reaction 4 from regenerated ZnO sorbent at the outer layer of the pellet, SO2 is emitted by sorbent after being regenerated at the inner layers of the pellet, and diffusion of O2 from the feeding gas occurs according to pathway 1. Sample (ZnAcac)HT prepared with a similar precursor as (ZnAcac)MW but under hydrothermal treatment shows the direct formation of ZnSO4 after oxidation of (ZnAcac)HT-300S through eq 5 and subsequent decomposition to ZnO (pathway 2). There is no evidence in the TGA analysis for (ZnAcac)HT-300S under O2 for the formation of CO2. However, amorphous carbon can be seen in the Raman spectroscopy (Supporting Information, Figure S11). Sample (ZnAcac)HT has lower sulfur capacity than the microwave-activated sample (ZnAcac)MW. Because the breakthrough is earlier and subsequent sulfidation is stopped, the carbon deposits must be low, and the sensitivity is not high enough to detect the weight loss due to the formation of CO2. The time frame for the formation of ZnSO4 and the start of oxidation of the portion of ZnS in the sample (T = 610 °C) is also close to the values obtained for (ZnAcac)MW-300S. Initial sulfate formation might be due to the high amount of ZnO available that was not converted to ZnS at the surface of the sorbent. Then, the SO2 released can easily find unreacted ZnO to form sulfate. Thermal behavior of (Zn-NO3)HT-300S only produces ZnS after oxidation (Figure 4b). The curve presents two decomposition rates. First, there is a high rate for conversion from sulfide to oxide from the outermost layers, then slows down as the last remaining ZnS transforms into ZnO. This behavior is typically observed in materials that suffer sintering after long periods of heating. In this case, some of the ZnS particles might be confined between clusters of ZnO particles (Figure 3f), and oxygen has difficulty in diffusing to form the former oxide. Because the process involves the gradual oxidation of external layers of ZnS36 as the reaction proceeds, it is more difficult for oxygen to diffuse within the core of the grains, and the oxidation is increasingly slow, decreasing the rate of ZnO formation (Figure 4b). The best desulfurization at low temperature for this work was obtained because of the minimal thermal sintering of ZnO at 300 °C and the high surface/volume ratio of the nanospheres of ZnO prepared by microwave irradiation, which enhanced the reactivity with H2S at low temperatures. Many Zn atoms are expected to be on the surface, directly available for the reaction with H2S, and the higher specific surface area compensates the slower sulfidation kinetics expected at low temperature. The desulfurization performance of the sorbent depends on both its nanomorphology and surface chemical composition. As previously reported, in the case of the nanospheres, the reaction proceeds deep into the bulk, suggesting that the crystal morphology also plays a role.40 X-ray diffraction confirms that many layers are reacting to the extent that bulk stoichiometric processes take place (Figure 5a). 8472

dx.doi.org/10.1021/jp210755t | J. Phys. Chem. C 2012, 116, 8465−8474

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Sciences of the U.S. Department of Energy under grant DEFG02-86ER13622.A080 for support of this work.

is as much as two times the values obtained for sulfidation of the materials prepared under hydrothermal synthesis at the same sulfidation temperature (300 °C), and as much as three and four times for sulfidation at 200 °C for (ZnAcac)HT and (Zn-NO3)HT, respectively. Additionally, the synthesis is fast (1 h) and produces a narrow size distribution of quasi-spherical particles (Figure 3a and Supporting Information, Figure S8f) that can be tuned by controlling the mol fraction of the components in the final mixture of solvents and do not suffer sintering, preserving the round morphology after sulfidation.

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CONCLUSIONS A simple and fast synthesis route for the formation of highly homogeneous quasi-spherical ZnO particles under microwave irradiation in a short time with high surface area has been presented, tuned, and evaluated for the removal of H2S. The material showed superior sulfur sorption capacity at temperatures as low as 200 °C with an optimum value around 300 °C. Decrease in sulfidation performance at high temperatures (400 °C) is associated with the formation of carbon deposits left from organic precursors used during the synthesis, which blocks paths for H2S to diffuse and suppresses or decreases the rate of ZnS formation. Even though sulfidation at low temperatures is kinetically slow, this sorbent has good sorption capacity at 200 °C with high conversion of oxide to sulfide. The observations mentioned above demonstrate that the mol fraction of the components during the synthesis has significant influence on the morphology, which can lead to materials with tunable properties.

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ASSOCIATED CONTENT

S Supporting Information *

Chromatography conditions used for the separation of H2S, EDX elemental analysis of the sulfided sorbents (ZnAcac)MW, (ZnAcac)HT, and (Zn-NO3) HT at 300 °C, adsorption isotherms, and pore size distribution for (ZnAcac)HT, (ZnNO3)HT, (ZnAcac)HT-300s, and (Zn-NO3)HT-300S are presented. X-ray diffraction patterns, surface area, and SEM images of the (ZnAcac)MW and (ZnAcac)HT‑2 are compared. Energydispersive X-ray spectroscopy mapping of (ZnAcac)MW-300S and (ZnAcac) HT -300S is shown. TGA under N 2 of (ZnAcac)MW, (ZnAcac)HT, and (Zn-NO3)HT and O2 (ZnAcac)MW-300S, (ZnAcac)HT-300S, and (Zn-NO3)HT-300S) is also presented. Total vapor pressure during the microwave synthesis of (ZnAcac)MW and corresponding SEM and surface area. High-temperature SEM of fresh (ZnAcac)MW zinc oxide heat-treated in real time, after calcination in air, and sulfided at 400 °C. Finally, Raman spectroscopy of (ZnAcac)MW-300S, (ZnAcac)HT-300S, and amorphous carbon as a reference are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 860 486-2797. Fax. 860-486-2981. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of 8473

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