Decomposition of H2SO4 by Direct Solar Radiation

Meccanica e Industriale, UniVersita` di Roma Tre, Via della Vasca NaVale 79, ... and ENEA-Research Center “Casaccia”, 00060 S. M. Di Galeria, Roma...
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Ind. Eng. Chem. Res. 2007, 46, 6393-6400

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Decomposition of H2SO4 by Direct Solar Radiation Sergio Brutti,*,† Giovanni De Maria,† Giovanni Cerri,‡ Ambra Giovannelli,‡ Bruno Brunetti,§ Patrizia Cafarelli,| Elvio Semprin,| Vincenzo Barbarossa,⊥ and Antonio Ceroli⊥ Dip. di Chimica, UniVersita` di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy, Dip. di Ingegneria Meccanica e Industriale, UniVersita` di Roma Tre, Via della Vasca NaVale 79, 00146 Roma, Italy, ISMN-CNR, Sezione Roma 1, P.le A. Moro 5, 00185 Roma, Italy, ISMN-CNR, Sezione Montelibretti, Via Salaria Km 29,300, Montelibretti, Italy, and ENEA-Research Center “Casaccia”, 00060 S. M. Di Galeria, Roma, Italy

The sulfur-iodine cycle is one of the most promising thermochemical cycles for hydrogen production. Its coupling with a solar energy primary source is a great challenge to achieve efficient and economically competitive H2 production. Within this cycle, the decomposition of sulfuric acid plays a key role, with this process being the most energy-demanding reaction step. In this paper, a combined computational and experimental study of the decomposition at high temperature of H2SO4 to SO2 is presented. The scope of this paper is to present new information and data about the experimental high-temperature decomposition of sulfuric acid carried out in a solar reactor in view of a possible industrial exploitation of this reaction. Starting from a new complete thermodynamic modeling of the process, carried out by investigating the effect of the pressure and the temperature on the SO2 conversion rates, the study of the high-temperature decomposition of H2SO4 by direct solar radiation using a Fe2O3-based catalyst was carried out for the first time. The modeling and experimental results obtained are discussed together with the available literature. In summary, SO2 conversion yields close to thermodynamic predictions were obtained in the temperature range 1050-1200 K at a starting sulfuric acid partial pressure of p ) 0.61 bar. Introduction Hydrogen is one of the most promising future energy carriers: public and private interests in many different industrial fields (transport technologies, metallurgy, energy production, and chemical industries) are continuously increasing so that a new “hydrogen-age” is expected to flourish in the near future. However, H2 production and storage systems still encounter various technical and economical problems. Hydrogen can be produced by fuel reforming or direct water splitting at high temperature or by means of electrolysis.1-6 However, only the second approach leads to a CO2-free H2 production. Other candidate processes to obtain large-scale H2 production could be the use of indirect thermochemical cycles that accomplish the split of H2O into gaseous hydrogen and oxygen by means of a heat source (i.e., solar7,8 or nuclear primary sources) or a combination of heat and electrolysis in hybrid cases. Thermochemical cycles are necessary, owing to the present practical unfeasibility of the direct thermal decomposition of water that requires extremely high temperatures (.2000 K). More than 100 different cycles have been proposed since the late 1960s,4 but these found negligible application outside research and development (R&D) laboratories. Between these, the sulfur-iodine (S-I) is one of the most promising and is currently the object of basic and applied research all over the world. The S-I cycle consists of three main reactions: * To whom correspondence should be addressed. E-mail: [email protected]. † Universita` di Roma “La Sapienza”. ‡ Universita` di Roma Tre. § ISMN-CNR, Sezione Roma 1. | ISMN-CNR, Sezione Montelibretti. ⊥ ENEA-Research Center “Casaccia”.

I2 + SO2 + 2H2O f 2HI + H2SO4

(1)

2HI f H2 + I2

(2)

H2SO4 f SO2 + H2O + 1/2O2

(3)

The first step, reaction 1, is the so-called Bunsen reaction in which I2 and SO2 are combined in aqueous solution to produce H2SO4 and HI. These acids are thereafter decomposed according to reactions 2 and 3, and finally, the resulting I2 and SO2 are recycled in the Bunsen section. Reactions 2 and 3 need temperatures as high as approximately 700 and 1200 K, respectively. The decomposition of sulfuric acid plays a key role: it is the most energy-demanding reaction step and takes place at the highest temperature of the whole cycle. From a qualitative chemical point of view, the decomposition of sulfuric acid can be simply split into three consecutive subreactions:

H2SO4 (l,aq) ) H2SO4 (g)

(4)

H2SO4 (g) ) SO3 (g) + H2O (g)

(5)

SO3 (g) ) SO2 (g) + 1/2O2 (g)

(6)

Reaction 4 is the sublimation of the acid, while reactions 5 and 6 are the gaseous dissociation and the decomposition of the H2SO4 molecule into SO3 and SO2, respectively. The most recent thermodynamic modeling of the hightemperature equilibria of H2SO4 (g) has been presented in ref 9. However, the formation of SO2 as the ultimate product at high temperature of the decomposition of H2SO4 was only mentioned and not fully investigated. Experimentally, the decomposition of H2SO4 or SO3 (g) has been studied by several authors,10-15 and it is known that this

10.1021/ie070245l CCC: $37.00 © 2007 American Chemical Society Published on Web 09/05/2007

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reaction takes place under kinetic control unless an appropriate catalyst is used:15-17 in the literature, a number of inorganic catalysts have been tested.18-24 Phenomenological kinetic equations for the overall process have been reported in the literature, but a detailed description of the reaction mechanism is missing. On the contrary, a number of experimental and computational studies about the thermal dissociation of the SO3 gaseous molecule (reaction 6) have been reported,11,14 confirming that this step is the kinetic bottleneck of the overall process. The investigation of the direct photodissociation mechanism of sulfuric acid by photofragmentation mass spectrometry or other spectroscopic methods is missing as well as accurate experimental measurements of its visible-UV (V-UV) spectra (the state-of-the-art estimate of this spectra is reported in ref 25). The V-UV spectra and the photodissociation of the similar SO3 molecule were investigated by Burkholder and McKeen and Hintze et al.26,27 and Thelen and Huber,28 respectively. In summary, this molecule is almost transparent to the visible radiation and was found to undergo direct photodissociation at a UV wavelength of 193 nm through an excited not-bonding electronic state. However, this dissociation channel is almost 200 kJ mol-1 higher in energy compared to the thermal dissociation way.11 On the other hand, Lupfert and Funken12 reported the study of the dissociation of sulfuric acid in a reactor heated by direct irradiation with solar lamps. These authors found that, at a concentrated light power of 300-900 kW m-2, the gaseous decomposition of sulfuric acid carried out without catalyst is apparently promoted by a factor of 2-14 in speed. In this work, we report our recent results about the investigation of the sulfuric step of the S-I cycle. This paper is directly related to our previous works, i.e., refs 9 and 15, and reports a combined experimental and modeling study of the hightemperature reaction. In particular, we present (1) a thermodynamic equilibrium study of the decomposition of H2SO4 and (2) the experimental investigation of the H2SO4 decomposition reaction by direct solar radiation using a Fe2O3 catalyst and a concentrated solar furnace. Thermodynamic Modeling of the H2SO4 Decomposition The thermodynamic modeling was obtained by means of the ThermoCalc software29 on the basis of a specific database previously assessed for the S-O-H system9 in order to predict the most useful thermodynamic states to promote the overall process. This database was assembled by taking the most recent and accurate thermodynamic tabulation available in the literature for all the involved species. For the condensed phases (H2SO4 (l), H2O (l)), data were taken from ref 30; it is to be noted that, in all simulations, the activity coefficients of the condensed phases were fixed to unity. Although arbitrary, this assumption is, in our opinion, reasonable by considering that the starting compositions in all the thermodynamic simulations were fixed to H/S/O ) 2:1:4. In view of this, and considering that H2SO4 (l) vaporizes congruently, the possible stable liquid solution could only be formed by pure sulfuric acid. For what concerns the gas phase, in addition to the wellknown H2SO4 (g),31 SO3 (g),32 SO2 (g),32 O2 (g),30 H2O (g),30 SO (g),30 and H2S (g),30 several minor gaseous species (H, HO, HSO, SOH, HO2, HS, H2, H2SO, HSOH, H2O2, H2S2, O, O3, S, S2, S3, S4, S5, S6, S7, and S8) were included in the database. Their thermodynamic properties were taken from ref 30. Furthermore, the gaseous coordination complexes SO3(H2O), SO3(H2O)2, SO3(H2O)3, H2SO4(H2O), and H2SO4(H2O)2 have also been considered.9 These last gaseous complexes are strongly bonded coordination adducts: they have been identified using

microwave spectroscopy33,34 and matrix isolation Fourier transform infrared spectroscopy (FTIR).35-38 Six independent modeling runs at constant pressures were performed (10-3, 10-2, 0.1, 1, 10, 100 bar), by calculating the equilibrium thermodynamic states in the temperature range 600-2000 K with 50 K intervals (fixed composition ratios H/S/O ) 2:1:4). The conventional reference starting condition of all the thermodynamic modeling was 1 mol of liquid H2SO4 at 298 K. The simulation results are shown in Figure 1parts a-d. In parts a and b of Figure 1, the SO3 (g) and SO2 (g) conversion factors are reported. The conversion factors are the ratios between the molar fraction of the gaseous species of interest and the sum of the molar fractions of all the gaseous sulfur-containing species. In Figure 1c, the total heat necessary to produce 1 mol of SO2 (g) at the computed temperature and pressure values, starting from H2SO4 (l) at 298 K, is given. In Figure 1d, the SO2 (g) molar production for a hypothetical reactor of V ) 1 L is presented: these data, referred to in the following as volumetric efficiency, have been derived by multiplying the SO2 conversion efficiency (Figure 1b) by the starting H2SO4 (g) concentration, normalized for the resulting equilibrium total volume, for all the isobaric simulations. The volumetric efficiency is an interesting evaluation of the quantitative efficiency of each pressure-temperature condition. Because each produced mole of SO2 corresponds to a standard volume of 24.47 L of H2 (ideally produced at 100% yield via reactions 1 and 2), the volumetric efficiency can be directly related to the effective production of hydrogen by a simple multiplication. Furthermore, if a nominal contact time in the reactor to produce such quantity of SO2 is assumed, the yield of the sulfuric section of a S-I cycle in terms of obtained hydrogen production rate can be evaluated (see also the next section, Results and Discussion). As already mentioned, the overall process can be schematically described by a sequence of three main reactions (see above, reactions 4, 5, and 6); the thermodynamic modeling confirms this picture. Other minor reactions, such as H2SO4 (g) ) SO3(H2O) (g) or H2SO4 (g) + H2O (g) ) H2SO4(H2O) (g), that take place in particular thermodynamic conditions9 are predicted to give a negligible contribution to the overall process in the present cases. In particular, the concentration of the gaseous adduct SO3(H2O) is estimated to be always 80%) and volumetric (mol of SO2(Vreactor ) 1 L) > 10-2 mol) efficiencies and minimize the operating temperature (T < 1300 K) and the energetic cost (∆H < 500 kJ molSO2-1), the most favorable thermodynamic conditions could be found in the pressure-temperature window defined between the following states: (A) 1 bar, 1100 K; (B) 1 bar, 1200 K; and (C) 10 bar, 1200 K. Within this window, states A and C are energetically less favorable of 6% and 15%, respectively, compared to state B; on the other hand, the volumetric efficiencies for states A and B are smaller by 1 order of magnitude compared to that for state C. Experimental Details The experimental apparatus is presented in Figure 2 and consists of four parts: the 1.5 kW solar furnace (Figure 2a), the high-temperature reactor (Figure 2b), the H2SO4 + N2 inlet feeding system and the reaction product collector system. The used solar furnace39 consists of a 4 m2 planar movable mirror (heliostat), a parabolic mirror of 1.5 m in diameter (concentrator), and an automatic system for the independent two-angle heliostat motion (sun tracker). The fraction of the total radiation concentrated on the reactor can be varied by an automatic movable sliding door between the heliostat and the parabolic mirror. The high-temperature reactor was a cylindrical quartz reactor (50 cm long and 4.5 cm in diameter, see Figure 2b) filled with quartz rings and Al2O3 pellets covered by the catalyst. The liquid H2SO4 (96%, Carlo Erba RS) reagent was supplied into the reactor directly by means of a quartz tube (mean H2SO4 flow ) 0.08 mL min-1) and mixed with a controlled N2 flow (25 standard mL min-1). The liquid H2SO4 dropped constantly from an external reservoir through a glass capillary

nozzle into the feeding line. The lower part of the reactor, where the feeding line releases the reagents, was constantly heated by electric resistances at ∼573 K in order to avoid H2SO4 recondensation. The H2SO4-N2 mixture flowed in the preheated zone (573 K) and then in the hot zone, where the decomposition occurred. At 1000 K, the total gas flow inside the reactor is estimated to be 200 mL min-1. The central part of the reactor was heated by direct irradiation through a circular quartz window (surface ≈ 2 × 10-3 m2): this portion was entirely surrounded by a refractory oxide in order to improve the thermal homogeneity. The assumed “hot zone” (HZ), that is, the volume of the reactor directly exposed to the solar radiation through the optical window, corresponds to a cylindrical volume of about V ) 80 cm3 and 5 cm in height. The maximum concentrated solar power transferred to the cylindrical reactor through the quartz window was calculated to be ∼750 kW m-2 by considering the overall power of the furnace and the effective surface of the reactor exposed to the radiation. The solar energy was captured in the reactor by the catalyst-supporting alumina pellets: at constant irradiation, the temperature rises up to a steady-state condition where reirradiation and the other heat losses equilibrate with the direct concentrated solar irradiation. The energy is transferred to the gas flow by direct contact with the pellets and the reactor walls. Temperatures in the hot zone were measured by means of a Pt-Pt (Rh, 10%) thermocouple. The thermocouple was not directly exposed to the solar irradiation. It was inserted in a protective quartz tube placed along the vertical axis of the reactor. This tube was shielded from direct irradiation by the catalyst-supporting alumina pellets. In Figure 2c, the vertical temperature profiles in the central portion of the reactor are shown. A large temperature gradient was observed in the hot zone both across the horizontal section and along the vertical direction. Apparently, a mean temperature gradient of ∼150 K was observed from the optical window to the opposite quartz wall of the reactor in the same cylindrical

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section. On the contrary, only a smooth decrease of ∼50 K is observed on the vertical dimension within the hot zone: outside it, the temperature gradient increases significantly also in this direction. Apparently, the mean temperature calculated in the whole hot zone surrounded by the refractory oxide is equal to the temperature measured in its middle point. Therefore, in the sulfuric acid decomposition experiments, only a single thermocouple has been employed, placed in the middle of the cylindrical section in front of the quartz window (see Figure 2b). The resulting gas flux flowing outside the high-temperature solar reactor was formed by N2, H2SO4, SO3, SO2, O2, and H2O. These gaseous products were bubbled and trapped in a Bunsen reactor that acts as product-collector system. It is an I2/Iaqueous solution heated at 343 K (70 °C). In the Bunsen reactor, the sulfuric anhydride immediately reacts with water, to give H2SO4 (aq), and SO2 molecules react immediately with I2, producing I- and SO4). By simply measuring the increase of I- concentration in the Bunsen reactor, it is possible to derive the original mol of SO2 produced during the H2SO4 hightemperature decomposition. The measured SO4) concentration corresponds to the total amount of SO2 and SO3 mol produced in the sulfuric acid decomposition. The total amount of sulfuric acid passed through the reactor during the decomposition experiment was also checked by the volumetric acid reduction in the external feeding reservoir. From these data, by means of simple stoichiometric calculations, it is possible derive the yield of the decomposition reaction:

% SO2 )

∆[I-] 2[SO4)]

× 100 )

nSO2 nSO2 + nSO3

Figure 3. Experimental SO2 conversion rate obtained in the H2SO4 decomposition experiments by direct solar radiation; starting pressure of H2SO4 ) 0.61 bar; mean contact time ) 7 s. Gaseous decomposition rates are obtained from Yilmaz et al.14 data for the SO3 gaseous decomposition without catalyst in the same conditions compared to our experiments; H2SO4 (g) conversion to SO3 (g) has been assumed as complete at T > 800 K. Thermodynamic data are calculated at p ) 0.61 bar.

× 100

Sulfur compounds and iodide concentrations were determined using a Metrohm 761 compact ionic chromatograph, with an anion column (Metrosep A SUP 4). The eluant solution was 1.8 mM in Na2CO3 and 1.7 mM in NaHCO3. In the hightemperature experiments, the total sulfuric acid flow rate supplied to the reactor ranged between 48 mmol/h and 121 mmol/h, and the average reactor temperature ranged between 871 and 1179 K. Each run lasted ∼32 min: the mean contact time of the reagent in the hot zone was 7 s, and the average H2SO4 starting pressure was 0.61 bar. The starting pressure of H2SO4 and the contact time have been calculated for each experiment from the measured total liquid sulfuric acid volume consumed, the flow rate of N2, and the total time. The total pressure in the reactor was, in all cases, 1 bar. In all the experiments, a Fe2O3 catalyst supported on Al2O3 pellets was used. It has been synthesized following the same procedure discussed for the Fe2O3-SiO2 catalyst used in ref 15. The iron oxide deposition was obtained by annealing at high temperature in air (3 h at 573 K + 3 h at 773 K) a Fe(OH)3‚ (H2O)x colloid directly precipitated on the Al2O3 pellets by adding liquid NH3 to a Fe(NO3)3 aqueous solution. The iron content on the inert Al2O3 support was ∼0.3 wt % Fe. Results and Discussion Experimental data are reported in Figure 3 in the form of the measured SO2 conversion rate versus the temperature, in comparison with the thermodynamic predictions at p ) 0.61 bar, the kinetic estimates for the gaseous reaction, and the experimental data taken from ref 15 for the not-catalyzed reaction. Gaseous decomposition rates were calculated by using Yilmaz et al.14 kinetic equations and the CKS code40 (H2SO4

Figure 4. Catalyst lifetime efficiency: experimental SO2 conversion rates at constant temperature.

starting pressure ) 0.61 bar, contact time ) 7 s, and temperatures ) 800-1300 K): H2SO4 (g) conversion to SO3 (g) has been assumed to be complete at T > 800 K. In the present experimental condition, the reaction kinetics is strongly promoted in comparison with the gas-phase decomposition and the data from electric furnace experiments without catalysts. Solar furnace data are close to thermodynamic rates: for T > 950 K, thermodynamic conversion rates are achieved at temperatures higher than ∼50 Κ compared to the equilibrium data. The effect of the iron oxide in the promotion of the decomposition of the sulfuric acid at high temperature was expected on the basis of our previous studies reported in ref 15. Indeed, in that study, we verified that Fe2O3 deposited on quartz wool strongly promoted the reaction. Conversion rates of 25, 50, and 75% are obtained at temperatures lower than 250, 200, and 150 K, respectively, in comparison with Yilmaz et al.14 predictions. It is to be noted that our data are somewhat scattered: this could be due to the temperature gradients in the HZ and to the possible loss of activity of the catalyst. Indeed, after 25 h of use, the Fe2O3-Al2O3 catalyst in the hot zone was sintered, particularly close to the optical window. In the other part of the reactor, the catalyst was apparently unaltered. On the whole, the catalyst activity was found to decrease slightly in the course of the experimental campaign. This effect is presented in Figure 4, where the conversion rates to SO2 measured at constant temperatures are plotted against the time. In ∼20 h, the conversion rates measured at 1073 and 1123 K reduced by about 15-20%.

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Figure 5. SEM micrograph of the surface of the Fe2O3-Al2O3 catalyst after the use in the high-temperature decomposition of H2SO4. The Fe2O3 polycrystalline aggregates on the surface (white amorphous particles) are probably originated by coalescence of the starting dispersed film during the high-temperature reaction, therefore leading to a partial exposition of the Al2O3 surface (regular cubes below the iron oxide particles).

On passing, it is of some interest to report that the energetic efficiency of the used cylindrical reactor, i.e., the ratio between the total energy nominally transferred to the reactor by irradiation and the effective energy captured by the system and used to promote the decomposition of the sulfuric acid, is estimated to be (0.6 ( 0.4)%. However, as expected, this average efficiency value resulted from a temperature-dependent trend that ranged from 1.8% at 800 K to 0.5% at 1200 K due to reirradiation of the HZ through the optical window. The catalyst morphology was investigated by BrunauerEmmett-Teller (BET) measurements (surface area and porosity), SEM (scanning electron microscopy), and EDS (energydispersive spectroscopy) analysis of the catalyst surface before and after the experiments. As expected, the specific surface area of the catalyst decreases, after 25 h of use, from 13.11 to 3.99 m2 g-1 and the porosity decreases from 0.015 to 0.004 cm3 g-1. In Figure 5, a micrograph of the surface of the catalyst after its usage is shown. SEM-EDS micrographs have been obtained using an Oxford Instruments LEO 1450VP. The Fe2O3 particles are apparently concentrated on the surface in large micrometric aggregates formed by smaller submicrometric grains. These large aggregates are inhomogeneously spread on the surface: well-formed Al2O3 cubes can be easily observed in the uncovered portion of the surface. The coalescence of the Fe2O3 particles to give aggregates could be responsible for the overall reduction of the porosity, surface area, and catalytic activity. In Figure 6, a further extended comparison of the experimental results with the literature data is shown. In this figure, the estimated contact time, τc,90%-T, in the hot zone to achieve a SO2 conversion factor of 90% of the corresponding thermodynamic yield (see Figure 3) is presented. The thermodynamic contact time estimates for the gaseous decomposition were obtained by using the CKS code40 and the kinetic equation taken from ref 14. Contact time estimates from (1) direct solar reactor, (2) electric furnace with Fe2O3 catalyst (see ref 15), and (3-5) Lupfert and Funken data were obtained by assuming a simple kinetic model. The model is based on two postulates: (i) the dissociation of H2SO4 (g) to SO3 (g) is instantaneous and does not affect the kinetics of the process; (ii) the kinetic path includes

Figure 6. Calculated contact time to achieve a SO2 conversion rate equal to 90% of the corresponding temperature-dependent thermodynamic rate (see Figure 3) (sqm ) square meters).

a single reversible step, i.e., SO3 (g) ) SO2 + 1/2O2 (g).41 Within the frame of this simplified model the reaction rate, r, is given by the equation r ) kd[SO3] - kr[SO2][O2]1/2, where kd and kr are the kinetic constants for the direct and reverse reaction, respectively, and [SO3], [SO2], and [O2] are the concentrations of the reagents and the products. By imposing the microkinetic reversibility constraint for the overall reaction, the two kinetic constants for the direct and reverse steps are related through the thermodynamic equilibrium constant, Kc ) kd/kr. In summary, the proposed model simplifies the gaseous reaction path, in which the SO3 (g) ) SO2 (g) + O (g) is the kinetic bottleneck, by imposing an overall thermodynamic reversibility constraint, therefore neglecting all the other possible kinetic steps (see ref 14). It is important to underline that the above-reported model describes only phenomenologically the reaction kinetics of the process, neglecting important features of the effective mechanism such as the role of the N2 molecules in the unimolecular dissociation of SO3, the adsorption reaction over the surfaces, the formation of possible catalytic surface intermediates, and the role of the gaseous water. We would like to stress that we avoid proposing a more meaningful reaction mechanism in consideration of the aforementioned problems of our experi-

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mental setup, in particular concerning the temperature gradients in the hot zone and the limited number of data points. However, present data are suitable for a qualitative comparison with the other literature sources in order to get insights about the effective role of the radiation in the promotion of the reaction. A moredetailed study is in progress in our laboratories concerning the kinetics of the heterogeneously assisted decomposition of sulfuric acid. The equation of the reaction rate reported above cannot be analytically solved, but its temperature-dependent solution, kd ) A e-Ea/RT (A ) pre-exponential factor, Ea ) apparent activation energy), can be optimized numerically starting from the experimental data sets (starting H2SO4 concentration, contact time, temperature, and final SO3, SO2, and O2 concentrations). By fitting the experimental data, the values of the preexponential factors and the apparent activation energies have been optimized. We performed the same analysis also to the data reported in ref 15 for the sulfuric acid decomposition carried out in an electrical furnace by using a Fe2O3 on SiO2 catalyst. The two solutions are kd ) (9.7 × 104) e-125000/RT (s-1) and kd ) (1.0 × 104) e-90900/RT (s-1), for the solar reactor and the electric furnace, respectively. Starting from the kinetic constant values obtained from the previous equations and, in the case of the Lupfert and Funken data, taken from ref 12, it is straightforward to calculate numerically the τc,90%-T by fixing the starting H2SO4 partial pressure (0.1 bar for Lupfert data and 0.61 bar in the other cases) and temperature. As shown in Figure 6, in the case in which a Fe2O3 catalyst is used (direct solar radiation or electric furnace), a strong decrease of the temperatures necessary to obtain similar τc,90%-T is predicted. For example, for a τc,90%-T ) 20 s, temperature reductions from about 1225 to 1050 and 925 K are estimated for the solar furnace and the electric furnace cases, respectively, in comparison with the gaseous decomposition predictions. The analysis of the possible effect of the direct solar radiation on the reaction yield needs a more detailed discussion. If kinetic predictions for the gaseous decomposition are compared with those derived from Lupfert and Funken determinations12 a “photocatalytic” effect is evident: it increases at larger irradiated powers. However, apparently the τc,90%-T derived from the present direct solar radiation data appears in all cases larger at the same temperature compared to the electric furnace estimates. This evidence suggests that, in the case of the Fe2O3-assisted reaction, the photocatalytic effect is apparently lost. However, the difference could be due also to other effects not directly related to the use of direct solar radiation. In particular, the use of a different catalyst support (SiO2 and Al2O3 for the electric and the solar furnaces, respectively) and the consequent different surface morphologies are probably the main reasons responsible for the decrease in the overall reaction yield. In view of this, owing to the overlap of different concurrent effects, present determinations do not allow us to go further ahead in the speculation about the reason for this different behavior, in terms of reaction mechanism. The experimental data can be compared with the thermodynamic modeling results reported in the previous section. The performed modeling suggests a thermodynamic window that balances the concurrent effects of the reported stoichiometric and volumetric efficiencies and the energetic costs. This favorable thermodynamic region can be found approximately within the pressure-temperature area defined by the following limiting points: 1 bar, 1100 K; 1 bar, 1200 K; and 10 bar, 1200 K. Owing to the kinetic barrier that controlled the reaction, apparently thermodynamic rates can be achieved only if the

reagents flow in the reactor with contact times of ∼10-20 s at 1100-1200 K. If a thermodynamic rate contact time of 20 s is assumed, starting from the computed volumetric efficiency predictions, it is possible to calculate the yield of the sulfuric section of a S-I cycle in term of obtained hydrogen production rate. Within the limits of the aforementioned pressuretemperature window, the hydrogen production rates would range between 0.11 and 0.12 to 0.99 m3 h-1 (at 1 bar), for the states 1 bar, 1100 K; 1 bar, 1200 K; and 10 bar, 1200 K, respectively, for a reactor of V ) 1 L. Conclusions In this paper, a combined computational and experimental study of the decomposition at high temperature of H2SO4 to SO2 is presented. A thermodynamic modeling of the process was carried out by investigating the so-called stoichiometric, energetic, and volumetric efficiencies of the reaction. An experimental study of the high-temperature decomposition of H2SO4 by direct solar radiation is presented. This is the firstever reported study of this process carried out by using a solar furnace. A Fe2O3-Al2O3 (0.3 wt % Fe2O3) catalyst was used in the experiments, obtaining a satisfactory catalytic activity compared to the predicted conversion rates for the reaction without catalyst. Apparently, the use of direct solar radiation does not contribute to the promotion of the catalyzed reaction differently from what observed in the literature for the notcatalyzed case. Acknowledgment This research project has been carried out with the financial support of the “Ente Nazionale per l’Energia e L’Ambiente” (ENEA “Progetto Solare Termodinamico-Produzione di Idrogeno”), the “Italian National Research Council” (CNR-ISMN), and the Universities of Rome “La Sapienza” and “Roma Tre” in the frame of the TEPSI (Tecnologie e processi innovativi per affrontare la transizione e preparare il futuro del sistema idrogeno) project and in collaboration with the HYTHEC (Hydrogen THErmochemical Cycles) project. Thanks are due to Prof. G. Balducci, Prof. G. Gigli, and Dr. A.Ciccioli for the fruitful discussions and the help in the use of the Thermocalc software. Thanks are also due also to Dr. Daniela Ferro for the SEM analyses. Literature Cited (1) Bollinger, R. B.; Aaron T. M. Low Cost Hydrogen Production Platform. Proceedings of the 2002 US DOE Hydrogen Program Review, Golden, Colorado, 2002. (2) Bolton, J. R. Solar Photoproduction of Hydrogen. IEA Agreement on the Production and Utilization of Hydrogen, National Renewable Energy Laboratory, Golden, Colorado, 1996. (3) Bromberg, L.; Rabinovich, A.; Alexeev, N.; Cohn, D. R. Plasma Catalytic Reforming of Natural Gas. Proceedings of the 1999 US DOE Hydrogen Program Review, Golden, Colorado, 1999. (4) Brown, L. C.; Funk, J. E.; Showalter, S. K. High Efficiency Generation of Hydrogen Fuels Using Nuclear Power; Final Technical Report, Grant No. DE-FG03-99SF21888; U.S. DOE: Washington, DC, 2003. (5) Funk, J. E. Thermochemical Hydrogen Production: Past and Present. Int. J. Hydrogen Energy 2001, 26, 185. (6) Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, D. A solar chemical reactor for co-production of zinc and synthesis gas. Energy 1998, 23, 803. (7) Hahm, T.; Schmidt-Traub, H.; Lessmann, B. A cone concentrator for high-temperature solar cavity-receivers. Solar Energy 1999, 65, 33. (8) Meyer, A.; Ganz, J.; Steinfeld, A. Modelling of a novel hightemperature solar chemical reactor. Chem. Eng. Sci. 1996, 51, 3181.

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ReceiVed for reView February 15, 2007 ReVised manuscript receiVed July 9, 2007 Accepted July 10, 2007 IE070245L