Hydrogen Production via Photolytic Oxidation of Aqueous Sodium

Jun 1, 2010 - AND ALI T-RAISSI. University of Central Florida, Florida Solar Energy Center, .... Corresponding author phone: (321) 638-1505; fax: (321...
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Environ. Sci. Technol. 2010, 44, 5283–5288

Hydrogen Production via Photolytic Oxidation of Aqueous Sodium Sulfite Solutions CUNPING HUANG,* CLOVIS A. LINKOUS,† OLAWALE ADEBIYI,‡ AND ALI T-RAISSI University of Central Florida, Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, Florida 32922-5703

Received December 15, 2009. Revised manuscript received March 30, 2010. Accepted May 4, 2010.

Sulfur dioxide (SO2) emission from coal-burning power plants and refinery operations has been implicated as a cause of acid rain and other air pollution related problems. The conventional treatment of SO2-contaminated air consists of two steps: SO2 absorption using an aqueous sodium hydroxide solution, forming aqueous sodium sulfite (Na2SO3), and Na2SO3 oxidation via air purging to produce sodium sulfate (Na2SO4). In this process, the potential energy of SO2 is lost. This paper presents a novel ultraviolet (UV) photolytic process for production of hydrogen from aqueous Na2SO3 solutions. The results show that the quantum efficiency of hydrogen production can reach 14.4% under illumination from a low pressure mercury lamp. The mechanism occurs via two competing reaction pathways that involve oxidation of SO32- to SO42- directly and through the dithionate (S2O62-) ion intermediate. The first route becomes dominant once a photostationary state for S2O62- is established. The initial pH of Na2SO3 solution plays an important role in determining both the hydrogen production rate and the final products of the photolytic oxidation. At initial solution pH of 9.80 Na2SO3 photo-oxidation generates Na2SO4 as the final reaction product, while Na2S2O6 is merely a reaction intermediate. The highest hydrogen production rate occurs when the initial solution pH is 7.55. Reduction in the initial solution pH to 5.93 results in disproportionation of HSO3- to elemental sulfur and SO42- but no hydrogen production.

1. Introduction The Clean Air Act requires the U.S. EPA to set National Ambient Air Quality Standards for six common air pollutants including sulfur dioxide (SO2). Environmentally, SO2 can cause acid deposition that leads to acidification of lakes and rivers and damage to tree foliage and agricultural crops (1-3). Compounding its impacts on the environment, there are multiple sources of SO2 emissions, fossil fuel combustion being a primary contributor. A number of technologies have been employed for the capture and disposal of SO2 from a flue gas stream. Those processes consist mainly of an absorption step and an oxidation step. A typical example is the use of a dilute aqueous sodium hydroxide (NaOH) solution to absorb SO2 from flue gas, forming an aqueous * Corresponding author phone: (321) 638-1505; fax: (321) 6381010; e-mail: [email protected]. † Current address: Department of Chemistry, Youngstown State University, 1 University Plaza, Youngstown, OH 44555-3663. ‡ Current address: Nucor Steel Arkansas, Melt Shop Metallurgist/ Chemical Engineering. 10.1021/es903766w

 2010 American Chemical Society

Published on Web 06/01/2010

Na2SO3 solution. Before disposal the solution must be purged with air to oxidize Na2SO3 into Na2SO4. Alternatively, oxidation of an aqueous Na2SO3 solution can be carried out for the production of high purity clean hydrogen fuel. The oxidation can occur via either an electrochemical or photochemical process. Direct photocatalytic oxidation of SO2-contaminated air has proven to be unsuccessful due mainly to the deactivation of the photocatalysts used. Katagiri and Matsubara (4, 5) have reported the electro-oxidation of SO32- to produce S2O62- in the presence of copper ions. They have observed the catalytic effect of copper ions at relatively low cell potential at platinum and gold electrodes. Obviously, this process requires costly noble metals. Other issues, including catalyst deactivation and electrolyzer configurations, make the process highly complicated. An aqueous Na2SO3 solution can be photocatalytically oxidized, whereas water is reduced to hydrogen in a colloidal solution of photocatalysts such as cadmium sulfide (CdS) and zinc sulfide (6, 7) with platinum metal as a cocatalyst. The quantum yield of hydrogen production in a 0.88 M Na2SO3 (pH 9.5) solution was about 7% with 2.4 wt % Pt loaded on CdS catalyst (6). Since the semiconductor material CdS is known to be a hazardous substance, there are safety concerns with regard to the disposal of the photocatalyst. Another drawback of the photocatalytic process is the intrinsic deactivation of photocatalysts. Sayama et al have reported (8) copious amounts of hydrogen production during irradiation of SO32solutions by a high-pressure mercury lamp. In this approach no photocatalyst was needed and hydrogen evolution quantum yields obtained were as high as 4.6%. Because of wide spectral distribution and a high temperature effect, a high-pressure UV lamp is considered to be a low efficiency light source that leads to lower quantum efficiencies. We found that an aqueous Na2SO3 solution can be oxidized to Na2SO4, while water is reduced to hydrogen using a lowpressure mercury lamp. In this paper we present the reaction pathways as well as kinetics of the photo-oxidation of Na2SO3 solutions. If NH4OH is used to replace NaOH solution this process can produce not only hydrogen, but also an ammonium sulfate-based fertilizer (10).

2. Experimental Section Reagent grade Na2SO3 · H2O, (Aldrich) and sodium dithionate, Na2S2O6, (Pfaltz and Bauer) were obtained and used for the photolytic reactions without further purification. UV-vis spectral measurement was performed with a UV-vis light spectrophotometer (Shimadzu UV-2401). A 36 W germicidal lamp (Atlantic Ultraviolet) was employed as the light source. UV light intensity measurements were performed using an IL 1700 research radiometer/photometer (International Light, Inc.). A cyclic flow system for the Na2SO3 photolytic reaction is shown in (9). The total volume of solution used for each run was 1500 mL. In the kinetic measurements, prior to the UV irradiation, the solution was first purged with high purity argon gas for approximately 2.5 h to remove O2 dissolved in the solution. In order to simulate the flue gas treatment process in the presence of air, during the rest of the experiments no argon gas purging was carried out. In these cases hydrogen yields were less than those obtained when oxygen was purged. The continuous in situ pH measurements were carried out using a calibrated Oakton 1100 pH meter. A high performance liquid chromatograph (HPLC) (Dionex DX-500) equipped with an AD20 absorbance detector was used to determine concentrations of SO32-, SO42- and S2O62anions in solutions, and each recorded HPLC result is the average of two measurements. Hydrogen evolution was VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Kinetics of hydrogen production via UV photolysis of 0.045 M aqueous Na2SO3 solution at initial pH 7.652, T ) 50 °C, solution volume ) 1500 mL. measured volumetrically and the purity of the generated gas was determined using a gas chromatograph (Shimadzu 14B) equipped with a molecular sieve 5 Å packed bed column and both thermal conductivity and flame ionization detectors.

3. Results and Discussion 3.1. Kinetics of Photolytic Oxidation of Aqueous Na2SO3. The photo-oxidation of aqueous Na2SO3 solution was conducted at 50 °C. A 0.045 M Na2SO3 solution was irradiated using a standard low-pressure Hg vapor UV lamp (λ ) 254 nm) and the volume of hydrogen produced was measured as a function of irradiation time. Prior to UV light irradiation, the solution pH (9.80) was adjusted to 7.65 by dropwise addition of H2SO3 solution. The concentrations of SO32-, SO42and S2O62- were determined using an HPLC, with samples taken before and after the photolytic process. Each datum point was the average of two measurements, and the results were used to calculate material balance for the aqueous phase. Figure 1 depicts hydrogen production for a period of 21.1 h of UV light exposure. The hydrogen production episode can be divided into three stages as follows: (1) t ) 0-2.0 h, the rate of H2 production is constant (2.7 mL/min); (2) t ) 2.0-14 h, the H2 rate gradually drops to 1.0 mL/min; 3) t ) 14-21.1 h, the production rate gradually drops off to zero. During the first interval the solution pH increases from the initial value of 7.65 to 9.50. Solution pH begins to gradually drop in the second interval. Finally, at the border between intervals II and III, pH drops dramatically to 3.5 even though hydrogen production has already ended. The reaction intermediate detected during the stage II photolytic process (at t ) 8.5 h) was Na2S2O6. However, after t ) 21.3 h, Na2SO4 was the only product found, indicating that SO32- ions were completely converted to SO42- ions. This result shows that although Na2S2O6 was present as a reaction intermediate, it was eventually converted to Na2SO4. This result agrees with our previous observation of the photolytic oxidation of aqueous ammonium sulfite solution ((NH4)2SO3) (10). Material balance calculations were carried out to confirm overall reaction stoichiometry. The results are shown in Table 1. It is noted that the as received Na2SO3 contained about 3.6 mol percent Na2SO4 due to the oxidation of sodium sulfite. The theoretical amount of hydrogen production from aqueous Na2SO3 solution was calculated assuming the hydrogen produced is an ideal gas at 1.0 atm and 22.5 °C. Table 1 shows that the SO42- produced accounts for all but 3.5% of the SO32- consumed, while H2 production was 10.32% of stoichiometry. To further verify the formation and oxidation of S2O62ions, the photolytic oxidation of aqueous sodium dithionate (Na2S2O6) solution (0.025M) was conducted using the same photoreactor system. The UV light radiation was applied for 3.0 h, and only 20 mL of hydrogen evolved in the process. 5284

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This result implies that hydrogen production from S2O62photo-oxidation is kinetically slow. HPLC analyses of the solution before and after the experiment show that S2O62ions did completely convert to SO32- and SO42- ions. This finding confirms the validity of the reaction mechanism proposed above. The pH change during the UV photolysis of 0.025 M aqueous Na2S2O6 solution is given in ref 10. The solution pH drops precipitously from 8.50 to about 2.50 within 10 min of UV dosage. The similarity of the pH data for these two reacting systems provides additional support that S2O62is indeed an intermediate formed during SO32- photolysis. 3.2. Effect of Initial Solution pH. The initial pH of the solution was adjusted by adding either HCl or sodium carbonate (Na2CO3). Figure 2 illustrates hydrogen evolution as a function of radiation time during the photo-oxidation of 0.025 M aqueous Na2SO3 solution at various initial pH values. The results shown indicate that the hydrogen production rate increases as the solution’s initial pH decreases. However, at an initial solution pH of 5.93, no H2 was produced. Rather, the formation of fine sulfur particles was observed. Figure S1 in the Supporting Information (SI) shows the rate of H2 evolution at the reaction onset as a function of the solution’s initial pH. Solution pH changes during the photo-oxidation are depicted in SI Figure S2. The effect of initial solution pH is attributed to the different sulfur compounds in the solution. There are basically three sulfur species that are present in an aqueous Na2SO3 solution: sulfite ions (SO32-), bisulfite ions (HSO3-), and sulfurous acid (H2SO3). Their pH dependent equilibrium concentrations in the solution can be determined and are given in Section 3.4 of this paper. Therefore, the H2 production rates measured at each pH value represent the oxidation rates of the corresponding sulfur species. 3.2.1. Initial pH > 10.96. When the pH of the solution is greater than 10.96 the solution contains nearly 100% SO32ions. As shown in Figure 2 and SI Figure S1, a solution with high initial pH is not favorable for hydrogen production as compared to that of lower pH, suggesting that the reaction of SO32- to SO42- at this pH range is slower than that at low pH. 3.2.2. Initial pH e 5.93. When initial pH of the solution was lower than or equal to 5.93, no hydrogen was formed from the photo-oxidation of aqueous Na2SO3 solution. During the course of the photolytic process, the pH of the solution dropped precipitously to about 2.00 soon after the UV light reaction commenced. No hydrogen production was observed. Instead, a milky solution containing sulfur particles was formed. In this case, the equilibrium between H2SO3 and HSO3- may play an important role. At pH of 5.93, about 90% of the solution are comprised of bisulfite (HSO3-) ions among the three sulfur species. When pH drops to 2.00, 60% of the sulfur species in the solution are H2SO3 and 40% HSO3-. Therefore, formation of elemental sulfur implies that the mechanism of photolysis of Na2SO3 solution under low initial pH consists mainly of the photo-disproportionation of HSO3and H2SO3. The overall reactions for sulfur formation can be expressed as follows: o + 23HSO3 ) S + 2SO4 + H2O + H

(1)

o + 5H2SO3(aq) ) 2S2O26 (aq) + S + 4H (aq) + 3H2O

∆E0 ) 0.115 V vs NHE

(2)

3.2.3. 10.96 > Initial pH > 5.93. Aqueous Na2SO3 solutions with initial pH in this range are comprised of both SO32- and HSO3- ions, and H2 evolution results from the photooxidation of these two ions. Data for the variations in solution pH vs UV dosage (SI Figure S2) indicate that in the first few minutes of irradiation the pH of the solutions with initial pH of 8.88 and 9.92 increase rapidly to 10.76. A similar result was

TABLE 1. Material Balance for the Photolysis of 0.045 M Aqueous Na2SO3 Solution T = 50 °C, Radiation Time = 21.33 h, Initial Solution pH Was Adjusted to 7.65 by Adding H2SO3 ionic species 2-

SO3 SO42gaseous product H2 a

initial (mmol) (t ) 0.00 h)

final (mmol) (t ) 21.33 h)

difference (mmol)

deviation (%)

63.41 2.40 theoretical H2 (mL)a 1550

0.00 63.59 actual H2 (mL) (t ) 21.33 h) 1390

63.41 61.19 difference (mL) 160

3.50 error (%) 10.32

The theoretical H2 production is calculated based on the moles of SO32- present.

FIGURE 2. Effect of initial solution pH on H2 formation via photolytic oxidation of 0.025 M Na2SO3 aqueous solutions (initial solution pH: (a) 7.53 (SO32- + HCl); (b) 8.88 (SO32- + HCl); (c) 9.92 (original Na2SO3 solution); (d) 10.96 (SO32-:CO32- ) 1:0.05); (e) 11.15 (SO32-:CO32- ) 1:0.125); (f) 11.38 (SO32-:CO32- ) 1:0.5); (g) 11.93 ((SO32-:OH- ) 1:0.10); (h) 5.93 (SO32- + HCl)).

FIGURE 4. Hydrogen production as a function of UV dosage (initial pH ∼ 7.55, circulating flow reaction system). rates are the same for both 0.025 and 0.05 M Na2SO3 solutions with initial solution pH less than 7.55. As mentioned before, the primary sulfur species in the aqueous Na2SO3 solution are SO32-, HSO3-, and H2SO3. The concentrations of these three species can be determined from the chemical equilibrium calculations using eqs 3, 4, and 5 (11, 12), and ideal solution assumption. + H2SO3 ) HSO3 + H log[HSO3 ]/[H2SO3] ) -1.97 + pH (3) + 22HSO3 ) SO3 + H log[SO3 ]/[HSO3 ] ) -7.26 + pH (4) 2[H2SO3] + [HSO3 ] + [SO3 ] ) total solution concentration (5)

FIGURE 3. Hydrogen production as a function of UV dosage (initial pH ∼ 9.80, circulating flow system). observed for a 0.045 M solution with initial pH of 7.652 (Figure 1). The rapid increase in the pH of the solution is clearly not attributable to temperature change (resulting from the radiative heating effect) because solutions with varying pH levels did not respond in like manner. Similar to Figure 1, the solution pH slightly decreases as the reaction times increase in the first stage and then, in the second stage, pH dramatically drops to about 3.0 and no hydrogen is produced. 3.3. Reaction Pathways. Two concentrations and two pH levels of aqueous Na2SO3 solutions (0.025 and 0.05 M) were investigated to interpret the photo-oxidation of Na2SO3 solutions. The target solution pH was obtained by adding HCl to aqueous Na2SO3 solutions. Figures 3 and 4 depict the extent and rate of hydrogen production from 0.05 and 0.025 M Na2SO3 solutions at initial solution pH (7.55 and 9.80). The variation of solution pH as a function of UV irradiation time is depicted in SI Figures S3 and S4. Table 2 summarizes the initial rate of H2 production for four Na2SO3 solutions with two concentrations and two pH levels, and shows that the initial hydrogen production rate at pH ∼ 9.8 for a 0.05 M Na2SO3 solution is 1.4 mL/min, about twice that for a 0.025 M solution (0.7 mL/min). However, initial H2 production

The concentrations of HSO32- and SO32- at pH 7.55 and 9.80 are listed in Table 2. The concentrations of H2SO3 at these two pH levels are very low as compared to either HSO3- or SO32- and can be neglected. At pH 9.80, SO32- accounts for 99.6% of total species present and, therefore, the rate of hydrogen production is attributed mostly to SO32- photooxidation. The results in Table 2 show that the initial rate of hydrogen evolution increases twofold from a solution concentration of 0.025 to 0.050 M, indicating that the photooxidation of SO32- is a first order reaction with respect to SO32- concentration at very low concentration levels. When initial solution pH 7.55 HSO3- makes up 34% and SO32- 66% of the total species presented. As shown in Table 2, initial H2 production rates for both 0.025 and 0.05 M solutions are the same (1.9 mL/min), and are higher than those at pH 9.80. The result indicates that hydrogen evolution rates are independent of total Na2SO3 concentrationsa zero order reaction at low concentrations. Apparently, due to coexistence of SO32- and HSO3- at pH 7.55, hydrogen is produced by photo-oxidation of both SO32- and HSO32- ions. Comparing distributions of SO32- and HSO3- ions at these two pH levels we can conclude that an increase in the concentration of HSO3- increases the hydrogen evolution rate. Results from Table 2 also indicate that the HSO3- concentration effect is more significant at lower concentration of Na2SO3 (0.025 M) VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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than that at higher concentration (0.05 M). The rate of hydrogen generation increases from 0.74 mL/min to 1.91 mL/min for 0.025 M solution, but it only increases from 1.40 to 1.91 mL/min for 0.050 M solution when initial solution pH decreases from 9.80 to 7.55. 3.3.1. Discussions of Reaction Pathways of Photo-Oxidation of SO32- and HSO3- Ions. Based on the results derived from Table 2 and kinetic study (Figure 1) we found that in addition to the two primary routes of SO32- photo-oxidation discussed previously, that is, SO32- oxidation to SO42- and S2O62-, reaction pathways of an aqueous Na2SO3 solution should also include mechanisms involving photo-oxidation of HSO3- ions. Combining results from Table 2 and Figures 3 and 4, the photo-oxidation pathways of aqueous Na2SO3 solution can be constructed. Solution pH changes during the photo-oxidation processes can be obtained from SI Figures S3 and S4. Based on these results, several major reaction pathways and associated redox potentials are given as follows (4, 5, 10): SO32- photo-oxidation to SO42-: 2-* irradiation: SO23 + hν f SO3

(6)

+ Oxidation: S2O2-* + 2OH- f 2SO2+ 2e6 4 + 2H

(16) reduction: 2H2O + 2e- f H2 + 2OH-

(17)

overall: S2O26 + 2H2O + hν f + 2SO24 + H2 + 2H (pH decreases)

2 2∆E0 ) -0.22 - 0.0591 pH + 0.0295log[SO24 ] /[S2O6 ] (19)

S2O62- photodisproportionation to form SO32- and HSO3-: 2-* irradiation: S2O26 + hν f S2O6

(20)

reduction: S2O2-* + 2e- f 2SO26 3

(21)

+ 22oxidation: 2SO2+ 2e3 + H2O f SO3 + SO4 + 2H (22)

overall: S2O26 + H2O + hν f + 2 SO23 + SO4 + 2H (pH decreases)

+ 2OH- f SO2oxidation: SO2-* 3 4 + H2O + 2e

-

reduction: 2H2O + 2e f H2 + 2OH

-

(7)

2∆E0 ) -0.11 + 0.0295log[SO24 ]/[SO3 ]

2-

SO3

(23)

HSO3- photo-oxidation to S2O62-:

(8)

2overall: SO23 + H2O + hν f SO4 + H2(pH independent) (9)

(18)

-* irradiation: 2HSO3 + 2hν f 2HSO3

(24)

+ 2oxidation: 2HSO-* + 2e3 f S2O6 + 2H

(25)

reduction: 2H + + 2e- f H2(g)

(26)

2overall: 2HSO3 + 2hν f S2O6 + H2(g)(pH independent) (27)

(10) - 2 ∆E0 ) 0.455 + 0.0295log[S2O26 ]/[HSO3 ]

(28)

2-

photo-oxidation to S2O6 :

HSO3- photo-oxidation to SO42-:

2oxidation: 2SO23 f S2O6 + 2e

-

reduction: 2H2O + 2e f H2 + 2OH

-* irradiation: HSO3 + hν f HSO3

(29)

+ 2oxidation: HSO+ 2e3 + H2O f SO4 + 3H

(30)

reduction: 2H + + 2e- f H2(g)

(31)

(11)

-

(12)

overall: 2SO23 + 2H2O f S2O26 + H2 + 2OH (pH increases)

(13)

2- 2 ∆E0 ) 0.026 + 0.0591 pH + 0.0295log[S2O26 ]/[SO3 ] (14)

S2O62- photo-oxidation to form SO42-: 2-* irradiation: S2O26 + hν f S2O6

(15)

overall: HSO3 + H2O + hν f + SO24 + H2(g) + H (pH decreases)

(32)

∆E0 ) -0.1175 - 0.0295 pH + 0.0295log[SO24 ]/[HSO3 ] (33)

3.3.2. Hydrogen Formation From 0.025 and 0.05 M Na2SO3 at pH 9.80 (Figure 3 and SI S3). As mentioned before, at initial pH of 9.80 the solution consists of 99.6% SO32- and 0.4% HSO3- ions (see Table 2). The reaction proceeds primarily via oxidation of SO32- ions. It is noted that the

TABLE 2. HSO3- and SO32- Concentrations and H2 Production Rates from 0.025 and 0.05 M Aqueous Na2SO3 Solutions Na2SO3concentration (M) pH 7.55 percentage pH 9.80 percentage

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9

0.025 2-

SO3

-

0.05 2-

-

(M)

HSO3 (M)

H2 rate (mL/min)

SO3 (M)

HSO3 (M)

H2 rate (mL/min)

0.0165 66.0 0.0249 99.6

0.0085 34.0 0.0001 0.4

1.9

0.033 66.0 0.0498 99.6

0.0170 34.0 0.0002 0.4

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0.7

1.4

main mechanism for hydrogen evolution from aqueous NaSO3 solution is the photochemical oxidation of SO32- ions while water is reduced to produce hydrogen. The in situ pH measurements (SI Figure S3) indicate that the solution pH drops slightly from 10.72 to 10.42 for both 0.025 and 0.050 M solutions. However, after a certain period of UV irradiation, the solution pH precipitously drops to about 4.00, and no hydrogen evolution occurs during this period: 827 and 1512 min for 0.025 and 0.05 M solutions, respectively. Based on reactions 8-11, oxidation of SO32- to form SO42- is not affected by solution pH because no proton or hydroxide ions are generated during the process. Accordingly, when pH is 9.80, the primary mechanism for photo-oxidation of aqueous Na2SO3 solution is photo-oxidation of SO32- to produce SO42ions. Since only 0.4% HSO3- ions exist in the solution, the slight pH drop during the photolysis (SI Figure S3) may primarily be attributed to the photo-oxidation of HSO3forming SO42- ions. 3.3.3. Hydrogen Production From 0.025 M Na2SO3 at pH 7.53 (Figure 4 and SI Figure S4). At initial solution pH of 7.53, the Na2SO3 solution contains a mixture of SO32- and HSO32ions, and hydrogen evolution results from the photooxidation of both SO32- and HSO32- ions. SI Figure S4 shows that solution pH remains nearly constant during the first stage of photoreaction (irradiation time 0-250 min). The independence of the solution’s pH at this stage, according to reactions 4 and 12, indicates that SO32- is oxidized to SO42(reaction 9), whereas HSO32- is oxidized to S2O62 (reaction 27). Both reactions are accompanied by water reduction for the production of hydrogen. 3.3.4. Hydrogen Production from 0.05 M Na2SO3 at pH 7.55 (Figure 4 and SI Figure S4). Hydrogen production for 0.05 M Na2SO3 solution at pH 7.55 is a complex process that can be divided into three stages as shown in Figure 4. The rate of hydrogen production for each stage is a constant, representing a single mechanism for the oxidation of one sulfur species. Corresponding to the hydrogen production curve, pH variation can also be divided into three stages (SI Figure S4): During the first stage pH remains approximately constant. The pH of the solution increases rapidly to its original value (∼11) at the second stage. In the third stage, pH drops dramatically to 3.30 and no hydrogen is produced. Note that the concentrations of SO32- and HSO3- ions at pH 7.55 are 0.033 and 0.017 M, respectively (Table 2). In this solution, oxidation of HSO3- can produce either S2O62(reaction 27) or SO42- (reaction 32). Reaction 32 leads to a drop in the solution pH, which is contrary to our experimental findings for stage “a” (SI Figure S4). Therefore, the mechanism for HSO3- oxidation during this stage must be reaction 27, that is, HSO3- oxidation to S2O62- ions. Considering photooxidation of SO32- ions, because pH remains invariant, the corresponding mechanism must also involve a pH independent process (reaction 12), that is, SO32- ions are converted into SO42- ions. With reference to the hydrogen production curve (Figures 3 and 4) and the discussion above, the HSO3- oxidation rate must be faster than that of SO32oxidation. The end point of stage “a” may represent the depletion of HSO3- ions. SO32- oxidation may also be involved in this stage. However, a large portion of SO32- ions remains unconverted. The rate of H2 evolution during this period is 2.0 mL/min. Comparing the pH change for 0.025 M Na2SO3 solution (SI Figure S3), photo-oxidation of SO32- ions shows a similar trend during the time period from t ) 0 to 200 min. The second stage, “b”, (SI Figure S4) may be the combination of two mechanisms mentioned in the first section: the oxidation of reacted SO32- ions to form S2O62ions (reaction 13) with pH increase and the oxidation of SO32to SO42-, a pH independent process. The average hydrogen evolution rate during the second stage is 1.5 mL/min, slightly lower than that of the first period “a”. The concentrations

TABLE 3. Maximum Hydrogen Production and Yields during Photo-Oxidation of Aqueous Sodium Sulfite Solutions concentration (M)/pH theoretical H2 formation (ml) experimental H2 production (ml) H2 yields Y(%) H2lost (mmol)

0.025/9.92

0.05/9.92

0.025/7.53

0.05/7.59

840

1680

840

1680

535

1270

532

1290

63.7 14

75.6 18

63.3 14

76.7 18

of SO42- and S2O62- continue to increase in this period (stage “b”) until the photo-oxidation of S2O62- to SO42- (reaction 18) takes over (stage “c”), which results in an abrupt drop in solution pH. The average rate of H2 evolution during this period is 0.1 mL/min, which is very close to the rate of photooxidation of pure Na2S2O6 solution (10). Note that the kinetic curves in Figure 1 and Figures 4 and 6 are very similar, although one used an aqueous H2SO3 solution and the other an HCl solution to adjust the initial solution pH. 3.4. Hydrogen Yields and Quantum Yields from PhotoOxidation of Aqueous Sodium Sulfite Solutions. In this paper, we define hydrogen yield (Y) as follows: Y(%) ) (experimental hydrogen produced/ theoretical hydrogen production) × 100

(34)

The theoretical H2 volume is calculated as: Na2SO3 concentration (mol/L) × total solution volume (l) × 24.3 (l/mol). The maximum experimental H2 production during the course of photo-oxidation of aqueous Na2SO3 solution is given in Table 3. The results show that hydrogen yields depend only on the concentration of Na2SO3, and are independent of the solution pH. At a concentration of 0.025 M, hydrogen yields are 63.7 and 63.3% for the solutions at pH of 9.92 and 7.53, respectively. At the concentration of 0.05 M, hydrogen yields are 75.6 and 76.7% for solution pH of 9.92 and 7.59, respectively. Apparently, as shown in Table 3, H2 yield is a function of the concentration of Na2SO3 but is independent of its initial solution pH. The higher the concentration of Na2SO3, the greater the hydrogen yields will be. UV-vis light spectral analyses (SI Figure S5) show that UV light photo-oxidation of Na2SO3 solution can completely convert Na2SO3 into the final product, Na2SO4. When 0.025 M Na2SO3 was subjected to the UV irradiation for 42 h (Spectrum (d) in SI Figure S5), the UV-vis spectrum of the resultant solution is basically identical to that of a 0.025 M Na2SO4 (Spectrum (c) in SI Figure S5). The result is in the agreement with the kinetic study shown in Figure 1 and Table 1. The hydrogen loss during the photo-oxidation process is due to the presence of oxygen in the solution. As mentioned in the Experimental Section, in order to simulate a flue gas treatment system these solutions were not purged with an inert gas to reduce the dissolved oxygen. Because the solution volumes are fixed, initial O2 content in the solutions is also roughly constant at ∼5 mmol. It is obvious that the higher the Na2SO3 concentration in solution, the lower the percentage of H2 consumed by dissolved oxygen, leading to higher hydrogen yields. There are two possible mechanisms involved in the hydrogen loss in the presence of oxygen. First, dissolved oxygen may oxidize SO32- ions to SO42- ions directly without producing hydrogen. Second, hydrogen evolved from the photo-oxidation can be oxidized by oxygen to form water. The stoichiometry of hydrogen loss is the same in either case. The quantum yield of hydrogen formation, φ(H2), is defined as follows (8): VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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φ(H2) )

2 · ν(H2) × 100% J

(35)

search Center (GRC) under contract No. NAG3-2751. We are grateful to Mr. Timothy Smith, NASA-GRC Program Manager.

Supporting Information Available Where, ν(H2) and J are the hydrogen evolution rate (mol/s) and the total moles of photons absorbed by Na2SO3 solution (Einstein/s), respectively. In this work, J was measured at a wavelength of 254 nm, as total radiation from the entire length of the low-pressure Hg vapor lamp. SI Table S1 shows the highest quantum yields of 14.4% for the aqueous Na2SO3 solution at 0.05 M concentration and pH 7.59, obtained at 100 min irradiation time. A 0.025 M solution with pH of 9.92 shows a quantum yield of only 5.4% at an irradiation time of 75.5 min. Higher quantum efficiencies are obtained at lower pH. SI Table S1 shows that the highest quantum yields result at UV dosages between 50 and 100 min. We noticed that quantum yields via photo-oxidation of aqueous Na2SO3 solution vary with the type of UV light source used. The photolytic oxidation of Na2SO3 solution subject to high-pressure mercury lamp gives φ(H2) in the range of 3.3-4.6% (8). In this experiment, the photo-oxidation was carried out using a low-pressure mercury lamp and the quantum efficiency is about three times greater than that from a high-pressure mercury lamp. Note that elevating the temperature of the solution and optimizing the concentration of the Na2SO3 in the solution can also increase the quantum yield of the process. Although the efficiency of photochemical oxidation of Na2SO3 is lower than that of an electrolytic process, the simplicity and lack of requirement for noble metals makes the photolytic process the lower operational cost option. Furthermore, this process is environmentally benign because it is capable of extracting hydrogen from low concentration SO32- solution, which is suitable for flue gas applications.

Acknowledgments Funding for this research has been provided by the National Aeronautics and Space Administration (NASA)sGlenn Re-

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Figures S1-S4 and Table S1 are available.This material is available free of charge via the Internet at http://pubs.acs.org.

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