Ind. Eng. Chem. Res. 2009, 48, 625–631
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Study of I-/I2 Poisoning of Fe2O3-Based Catalysts for the H2SO4 Decomposition in the Sulfur-iodine Cycle for Hydrogen Production Vincenzo Barbarossa,† Sergio Brutti,*,‡ Bruno Brunetti,§ Maurizio Diamanti,† and Giuseppe Ricci† ENEA-Research Center “Casaccia”, 00123 S. M. di Galeria, Rome, Italy, Chemistry Department, UniVersita` di Roma “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy, and ISMN-CNR, Sezione Roma 1, P.le A. Moro 5, 00185 Rome, Italy
The poisoning effect of I-/I2 mixtures on ferrous oxide based catalysts was investigated. These catalysts were used in the sulfuric acid thermal decomposition that is the highest endothermic step in the sulfur-iodine thermochemical cycle for hydrogen production by water splitting. This decomposition reaction needs a temperature as high as 1100 K to occur with a convenient thermodynamic yield for SO2 formation, and it is affected by kinetic limitations. Therefore only the use of a suitable catalyst allows for a large decrease in the H2SO4 decomposition temperature and attaining reaction yields close to the thermodynamic limits. I2 and HI present even in traces in the sulfuric acid feeding stream could lead to the poisoning of the catalyst used for the decomposition process and must therefore be minimized. In this study, two Fe2O3 catalysts supported on quartz wool and on alumina were used in the temperature range 873-1073 K in ordinary pressure conditions. The SO2 formation rates were measured before and after the catalyst poisoning. Kinetics measurements and scanning electron microscopy (SEM) analysis show that I-/I2 contamination reduced the catalytic activity by modifying its surface properties. 1. Introduction The sulfur-iodine cycle is currently considered one of the most promising processes for hydrogen production, as an alternative to methane reforming and water electrolysis. This cycle operates at moderate-high temperatures (298-1200 K) and is a CO2 free method for hydrogen production by water splitting. The reaction scheme of the cycle includes the following three reactions: I2(l) + SO2(g) + 2H2O(l) f 2HI(aq) + H2SO4(aq)
(1)
2HI(g) f H2(g) + I2(g)
(2)
H2SO4(g) f H2O(g) + SO2(g) + 1⁄2O2(g)
(3)
The so-called Bunsen reaction (1) is the oxidation of sulfur dioxide to sulfuric acid and the reduction of molecular iodine to hydrogen iodide. The next two reactions are the thermal decompositions of the two acids generated in reaction 1. The Bunsen reaction proceeds spontaneously in the temperature range 293-373 K; the HI decomposition is slightly endothermic and takes place in the temperature range 673-973 K. The H2SO4 decomposition is a strongly endothermic process and needs temperatures as high as 1100 K to give a suitable thermodynamic SO2 formation rate; moreover, it is affected by kinetic limitations. Therefore the use of a suitable catalytic material allows us a great decrease in the H2SO4 decomposition temperature reaching yields close to the thermodynamic limits.1-4 Various catalysts have been employed in the H2SO4 decomposition since the early work of Dokiya et al.5 These authors studied various oxides of Si, Al, Zn, Cu, Fe, Ni, Co, Mn, Cr, V, and Ti and reported that iron(III) oxide exhibited the highest catalytic activity almost constant for more than 100 h of operation. For all these catalysts (metal oxides), the main * To whom correspondence should be addressed. E-mail:
[email protected]. † ENEA-Research Center “Casaccia”. ‡ Universita` di Roma “La Sapienza”. § ISMN-CNR.
mechanism for decrease of the catalytic activity seems to be related to sulfate formation6 and to specific surface area modifications.7,8 Another candidate catalyst for H2SO4 decomposition is platinum, supported on various oxides such as SiO2 and Al2O3. Ginosar at al.9 reported a study on the stability of Pt/ZrO2, Pt/ Al2O3, and Pt/TiO2; for the latter, the authors reported that after 250 h of operation the SO2 formation yield, in mole percent, decreased of about 26% respect to its initial level. In this case, formation of volatile sulfate and oxide molecules with Pt loss has been indicated as the main mechanism for deactivation of this catalysts. Beside the sulfuric decomposition step in the SI cycle the “separation problem” is a key factor for a successful industrial implementation (see as an example ref 3). Indeed the products of reaction 1 (the so-called Bunsen solution) are a mixture of HI, H2SO4, and H2O and therefore need to be separated in order to be recycled in the high temperature decomposition steps of the two acids. According to the relevant literature, the separation step is accomplished by phase separation techniques.3 Indeed under the condition of coexistence in water of HI, H2SO4, and an excess amount of I2, the two acids are separated into 2 liquid phases (the so-called sulfuric solution and iodidric solution). However in the sulfuric acid solution a residual quantity of the HIx specie has been observed.10 A preliminary study on the effects of I2 as contaminant in the H2SO4 gas was reported by Norman et al.6 who found a decrease in the catalytic performance of a Pt/TiO2 catalyst. The removal of I2 source apparently restored the original catalytic activity. The aim of this work is to characterize the poisoning effects on an iron oxide (Fe2O3) catalyst, supported on two different materials namely quartz wool and alumina pellets, used for H2SO4 to SO2 decomposition in the temperature range 873-1073 K. This paper is the fourth of a series of research works1,2,4 focused on the decomposition of the sulfuric acid at high temperature carried out by the authors in the frame of the Italian national research project TEPSI.
10.1021/ie800064z CCC: $40.75 2009 American Chemical Society Published on Web 12/02/2008
626 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
Figure 1. Apparatus A for the SiO2/Fe2O3 experiments (R H2SO4 reservoir; D decomposition reactor; G1, G2 bunsen traps; IC ionic chromatograph).
2. Experimental Details The experimental study has been carried out by studying the effects onto the catalytic activity of two different catalysts for the sulfuric acid decomposition at high temperature. The catalyst was iron(III) oxide supported in one case onto quartz wool fibers and in the other on alumina pellets. The experimental apparatuses used for the Fe2O3/SiO2 (apparatus A) and Fe2O3/Al2O3 (apparatus B) studies were slightly different and are described in the following subsections as well as the catalysts synthesis methods and characterization. The morphological analyses of the catalysts were carried out by means of scanning electron microscopy (SEM) using an Oxford Instruments LEO 1450VP apparatus. The surface elemental analyses were carried out by an energy dispersive spectroscopy (EDS) X-ray detector coupled to the SEM instrument. The surface areas were determined by single point BrunauerEmmett-Teller (BET) measurements by using a Monosorb Quantachrome instrument. 2.1. Apparatus A: The Fe2O3/SiO2 Study. Apparatus A is schematically shown in Figure 1. The main components are the following: the H2SO4 feeding system, the decomposition reactor, and the out-coming gas collector system. During the experiments all the connections between the aforementioned components of the apparatus have been constantly kept at 630 K to prevent any H2SO4 condensation. H2SO4 (98%, Carlo Erba RS) was fed into the decomposition reactor by a N2 flux (F1) that flowed through a reservoir (R). This consists of a Pyrex tube filled with quartz wool and liquid sulfuric acid. The Pyrex tube has been kept at 573 K, in order to fix the sulfuric acid partial pressure. Flux F1 ranged between 25 and 100 sccm. The high temperature reactor (D) was a quartz tube (400 mm length, 36 mm internal diameter), inserted into an outer alumina tube. During experiments the reactor is inserted into a Carbolite furnace (TZF model) equipped with a proportional, integral, and derivative (PID) temperature controller (201 type). A K-type thermocouple in contact with the outer surface of the alumina tube was used to monitor the temperature during the sulfuric acid decomposition. The low flux and the high length/diameter ratio of the reactor, allows us to neglect a possible radial temperature gradient into the furnace that can be reasonably assumed to be isothermal. The N2-H2SO4 flux residence time into the high temperature furnace has been controlled using an additional N2 flux (F2)
Figure 2. Apparatus B for the Al2O3/Fe2O3 experiments. (a) N2 gas cylinder. (b) Gas flow controller. (c) Pressure gauges. (d) H2SO4 (liquid) automatic dispenser. (e) High temperature reactor. (f) Water trap. (g) NaOH trap. (h) Gas flow sensor. (i) O2 concentration sensor.
that ranged between 425 and 500 sccm. As the total resulting N2 flux was 525 sccm, in these conditions the overall residence time in the decomposer (D) was 46 s. Considering that only a portion of the entire quartz tube was filled with the catalyst particles the effective residence time of the reagents flow in the catalyzed reactor was approximately estimated as 10 s. The gas flowing from the high temperature reactor was a mixture of N2, H2SO4, SO3, SO2, O2, and H2O. These gaseous products were trapped into two Bunsen reactors connected in series (G1, G2): both of these reactors contain an I2/I- aqueous solution. G1 was heated at 353 K, whereas G2 was kept at room temperature. By simply measuring the increase of I- concentration in G1, it is possible to derive the original moles of SO2 produced during the H2SO4 high temperature decomposition. The SO4) concentration in the G1 and G2 reactors is the total amount of SO2 and SO3 moles produced in sulfuric acid decomposition. From these data by means of simple stoichiometric calculations, it is possible derive the yield of the decomposition reaction: % SO2 )
nSO2 ∆[I-] × 100 ) × 100 nSO2 + nSO3 2[SO) 4]
(eq1)
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. 2.2. Apparatus B: The Fe2O3/Al2O3 Study. The experimental setup for the Fe2O3/Al2O3 experiments is a typical flow reactor apparatus. A schematic representation of the experimental device in presented in Figure 2. The flow reactor is fed by a gaseous carrier gas (N2) regulated by a gas-flow controller (b) and a liquid solution of H2SO4 (96 wt %) introduced by an automatic dispenser (Dosimat 776, Metrohom Italiana) (d). The flow reactor (e) is a quartz cylinder (height 35 cm, internal diameter 3 cm) inserted in an high temperature furnace. The catalyst was packed in the middle portion of the cylinder (height 17 cm) between two area filled by alumina pellets (height 9 cm each). The isothermal section of the high temperature reactor has been verified experimentally to extend for more than 22 cm therefore including the entire portion filled by the catalyst.
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Figure 3. Alumina supported iron oxide catalyst. The SEM micrograph shows the cross section of the alumina spherical pellets. The impregnation of the iron oxide lead to the formation of an external iron rich egg-shell (A and B areas) surrounding a pure alumina core (C area). The iron concentration in the external and intermediate A shells was larger than in the B areas.
The products flowing from the reactor (e) are bubbled into a water trap (f) and flowed over a NaOH trap (g) in order to separate N2/O2 gaseous products from the H2O and sulfur based products and unreacted reagents. The occurrence of the reaction is monitored by two independent methods: (h) a gas flow meter sensor (Honeywell AWM2300V) (total products N2-O2 flow, Φp,tot) and (i) a gaseous oxygen concentration sensor (Electrovac SO-A0-960), % O2. The pressure along the reaction line, before and after the reactor, was monitored by (c) two vacuum gauges. The experimental setup was monitored and controlled online by a remote computer by using a LabView based routine [http:// www.ni.com/labview/]. The SO2 formation rate can be derived independently by the measured Φp,tot and % O2 data by simple equations. Indeed the SO2 formation rate, % SO2, is given by % SO2 )
2(Φp,tot - ΦN2)(p ⁄ RTamb) ΦH2SO4M
× 100
(eq2)
and
(
2 % SO2 )
) ( )
% O2 p Φ 1 - % O2 N2 RTamb × 100 ΦH2SO4M
(eq3)
from the gas flow and oxygen concentration measurements, respectively, being ΦN2 the N2 flow in milliliters per minute, p the mean pressure in the reactor, Tamb the ambient temperature, ΦH2SO4 the H2SO4(l) flow in milliliters per minute, M the H2SO4(l) molarity, and R the gas constant. It is to be noted that owing to the oscillating flow regime caused by the bubbling of the products flow in the water trap the relevant SO2 formation rate values were used as internal check whereas the corresponding data derived by the oxygen concentration sensor, that showed a larger signal-to-noise ratio, were adopted. 2.3. Catalysts Synthesis and Characterization. The catalysts were prepared following the procedures reported in refs 1 and 4, respectively. In the case of the silica supported iron oxide, the Fe2O3 coating had been synthesized by precipitation from a HNO3 acidic Fe(NO3)3 solution using NH3, directly on the quartz wool. The Fe(OH)3 nH2O colloid precipitate on SiO2 was then heated and converted to Fe2O3: a three-step heating procedure (2 h at 100 °C, 3 h at 200 °C, and 3 h 500 °C) was used to reduce the sintering and the size of the oxide particles. The general morphology of the obtained silica fibers coated by hematite particles was similar to that reported and illustrate in ref 1. A
compact and homogeneous film of iron oxide was obtained as coating of the silica fibers. The SiO2 sample was characterized by SEM-EDS and BET techniques in order to study the catalyst morphology and surface features. The nominal iron content of the SiO2 supported catalyst as synthesized was 48 wt %, and the corresponding surface area measured by BET experiments was 21.7 m2/g. The Fe2O3 catalyst supported on Al2O3 pellets was synthesized following a similar procedure. 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 porous Al2O3 round shaped pellets by adding liquid NH3 to a Fe(NO3)3 aqueous solution. An image of the Fe2O3 catalyst supported on Al2O3 pellets is shown in Figure 3 together with a SEM micrograph of the cross section of a catalyst supporting spherical pellet. The iron oxide penetration in the alumina extended not only on the surface of the pellet but also within the inner pores. The impregnation of the iron oxide lead to the formation of an external iron rich “egg-shell” (A and B areas) surrounding a pure alumina core (C area). The iron concentration in the external and intermediate A shells was higher than in the B areas. The starting iron content estimated by the EDS data was 1.9 wt %. The surface area of the as synthesized catalyst measured by BET experiment was 118.8 m2/g. 3. Results and Discussion The Fe2O3/Al2O3 and Fe2O3/SiO2 as already verified in previous papers by the authors1,4 promote the sulfuric acid decomposition reaction. The effect of the catalyst has been newly verified by using the experimental apparatus described in the previous section. The well-known sigmoid behavior of SO2 formation yield in function of the temperature for both the catalysts is reported in Figure 4 where the experimental formation rate of SO2(g) obtained with and without the use of catalysts are shown. The average H2SO4 starting partial pressure was 0.1 and 0.6 bar for Fe2O3/SiO2 and Fe2O3/Al2O3, respectively, while the contact time in the catalyzed zone of the reactor was 10s for both catalysts. A microkinetic analysis of the reported results goes beyond the scope of this paper and is therefore omitted. It will be published elsewhere. Both catalysts show SO2 formation rates close to the thermodynamic predictions with complete decomposition starting from 973 and 1073 K for Fe2O3/SiO2 and Fe2O3/Al2O3, respectively, due to the different sulfuric acid partial pressure.
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Figure 4. Experimental formation rate to SO2(g) measured with and without the use of the two Al2O3/Fe2O3 and SiO2/Fe2O3 catalysts.
Figure 5. Experimental formation rate to SO2(g) measured in subsequent runs. The experimental conditions were as follows: (a) standard decomposition run, T ) 993 K, Φ(H2SO4(l)) ) 0.05 mL/min, Φ(N2(g)) ) 30 standard mL/min; (b) catalyst annealing step, T ) 993 K, Φ(N2(g)) ) 0.05 standard mL/min; (c) poisoning step T ) 993 K, Φ(H2SO4 50 wt %-KI 1 wt %-H2O solution) ) 0.05 mL/min, Φ(N2(g)) ) 30 standard mL/min.
When both the catalysts experienced the poisoning procedure, the SO2 formation rate undergoes a decrease over the whole temperature range. 3.1. Poisoning of Fe2O3/Al2O3. In the Fe2O3/Al2O3 experiments, the poisoning has been studied by submitting the system to a sequence of three different process steps: (a) a standard decomposition step with 96 wt % H2SO4 solution at 720 °C; (b) an annealing step in which the catalyst is cleaned from all the volatile components at 993 K; (c) a poisoning step in which a solution made of H2SO4 (50 wt %), KI (1 wt %), and H2O is introduced in the reactor at 993 K. Mixing H2SO4 and KI causes the inverse Bunsen reaction that leads to solid I2. The feeding solution was the liquid part of the mixture, free of solid I2 particles, but violet colored for I2 in solution. Our poisoning mixture therefore contains H2SO4, I-, K+, and I2. These step where repeated subsequently by the following sequence a-b-a-c-a-b-a. During step a, the reaction was monitored by measuring continuously the overall SO2 formation rates. As shown in Figure 4 the Fe2O3/Al2O3 catalyst at 993 K is able to promote the reaction up to 50% of the thermodynamic yield. Similar formation rate values can be attained without the use of a catalyst (Al2O3 only) at temperatures higher by 100-120 degrees, approximately. The results of the study of the poisoning of the Fe2O3/Al2O3 catalyst by I-/I2 contamination are presented in Figure 5. It is to be noted that fluctuations in the experimental results are observed. However we would like to stress that the experimental settings were chosen to test the process in condition suitable for a real demonstrator implementation. As a consequence, the experimental reactor was not a small capillary laboratory-scale system but a cylinder of 4 cm in diameter heated in an high
temperature furnace for his entire length (35 cm) in which a large flow or reagents was constantly fed. Owing to this the establishing of a steady state conditions in such experimental system is very complex and transient time are expected. During the poisoning, the high temperature equilibria of the H2SO4, I-, and I2 reagents feeding stream in the reactor is expected to be very complex. Owing to this, the measured oxygen concentration data during the poisoning are of difficult interpretation and cannot be reliably used to derive the reaction yield. However as the scope of this paper is the study of the possible permanent/unpermanent effects of the I3-/H2SO4 treatment on the catalyst, we always compared the reaction yields measured before and after the poisoning when the feeding stream was simply a N2/H2SO4 mixture, therefore completely neglecting the data during all the c steps. The comparison of the first two steps a reported in Figure 5 shows a significant lowering of the catalytic activity with the time. Indeed in both cases the formation rate apparently is reduced by 30% in 7-9 h. However this effect is reversible as verified in the sequence a-b-a, and the original efficiency of the catalyst can be recovered by annealing the catalyst for few hours. Indeed in step b, the catalyst is submitted to a thermal cycle (298-993-298 K in 2-3 h) that probably cleans its surface by all the volatile species. The occurrence of this phenomena has been verified at different temperatures in the T range 873-1173 K. After the poisoning step at 993 K the activity of the Al2O3/ Fe2O3 catalyst was found remarkably lowered, leading to a SO2 formation rate that is the half of the value measured before the poisoning. However at this stage, the poisoning is apparently reversible. Indeed in the isothermal decomposition step a that follows the c step, the formation rate increases, and in 5 h, it reaches values similar to those expected form the prepoisoning trend. Finally, the second annealing step b apparently restores the catalyst that shows after this step activities comparable to the starting preannealing values. Before further discussion of the results, it is important to spent some words about the possible effect of the additional pollutant K+ in the feeding stream, beside the I3- and the H2SO4 species, on the morphological degradation of the catalyst surface. From a thermodynamic point of view as K2SO4 is a refractory compound,11 its formation is expected to occur immediately when the liquid feeding stream enters the high temperature reactor. To confirm this hypothesis we performed a thermodynamic modeling by the Thermocal software.12 A thermodynamic system constituted by H2SO4 (50 wt %), KI (1 wt %), and H2O has been modeled at 900, 1000, and 1100 K and p ) 1 bar. The relevant thermodynamic functions of all the condensed and volatile species of the H-S-O-I-K system were retrieved from the databases sub94 provided with the Thermocalc software.12 At equilibrium conditions the potassium sulfate is expected to be the unique stable condensed phase: its formation is predicted to be quantitative. All the other constituent elements form a variety of gaseous molecules such as H2O, SO3, SO2, O2, I, I2, and IOH. In our experiments the liquid feeding stream, before the interaction with the catalyst, flows in a portion of the reactor kept at high temperature and filled by inert alumina pellets, as already discussed in section 2.2. As a consequence the formation of the potassium sulfate is expected to occur in this section of the reactor and not on the catalyst surface. The SEM-EDS data confirmed that the catalyst was not contaminated by any traces of potassium. Owing to this we believe that potassium does
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Figure 6. SO2 formation rate vs H2SO4 flow rate for fresh and poisoned catalysts (P ) poisoned catalyst; NP) not-poisoned catalyst).
not contribute to the degradation of the catalyst activity in the decomposition of sulfuric acid and that the only active poisoning agents are the iodine-containing gaseous species in the feeding stream. The SEM-EDS analysis of the poisoned catalyst sample showed that no contamination due to both iodine or potassium occurred and moreover that the morphology of the samples submitted to the two different sequences a-b-a and a-b-a-c were indistinguishable and apparently unaltered in comparison with the as-synthesized samples. The BET measurements showed that the surface area decrease from 118 ( 11 to 14.1 ( 2.3 and 13.2 ( 4.5 m2/g, respectively, for the as synthesized, the a-b-a, and the a-b-a-c catalyst. The largest reduction of the surface is observed between the fresh and the used samples (-88%) whereas between the a-b-a and the a-b-a-c samples the surface area is apparently similar. In summary the present data confirm that the Al2O3/Fe2O3 catalyst is poisoned by I-/I2 contamination and its catalytic activity is considerably reduced. However this effect is apparently reversible and a simple annealing (regeneration) restores the starting catalytic activities. Moreover it is to be noted that during the online monitoring of the high temperature decomposition of H2SO4 without poisoning a progressive lowering of the catalytic activities was observed. Also, this effect is apparently reversible. 3.2. Poisoning of Fe2O3/SiO2. In the experiments on the Fe2O3/SiO2 catalyst, the poisoning was obtained by feeding the reactor with a H2SO4/HI mixture 10:1 mole to mole initial ratio. As previously noted, the mixing of H2SO4 and HI causes the inverse Bunsen reaction to occur with production of gaseous SO2 and solid I2. Also in this case, the feeding solution was the liquid portion of the mixture, free of solids I2 particles, but violet colored for I2 in solution. Therefore our poisoning mixture contains H2SO4, HI, and I2. It is to be noted that in this case the poisoning solution was much more concentrated compared to the Fe2O3/Al2O3 experiments. The reactor temperature, during the poisoning process, was kept at 923 K, the liquid mixture flow rate was 0.5 mL/min, and the operation time was 40 min. The effect of poisoning was evaluated by measuring the H2SO4 to SO2 molar yield, with the same catalyst, before and after the poisoning step. The analytic procedure for I- and SO) determinations and further details about the external gas collector system are reported elsewhere.1 The effect of poisoning is clearly shown in Figure 6 where the SO2 formation yield at constant temperature of 973 and 1073 K is reported for various flow rate of sulfuric acid.
Figure 7. SEM micrograph of the Fe2O3/SiO2 catalyst (a) before and (b) after the poisoning process.
After poisoning, a decrease of the catalyst’s efficiency of about 20% is observed: this reduction was apparently irreversible. The catalyst was submitted to tentative “regeneration procedures” in which an air or N2 streams were flowed at 973 or 1073 K. After these regeneration steps the catalyst was submitted to isothermal high temperature sulfuric acid decomposition experiments at 1073 or 973 K. However even after a long time exposition (5 h) of the catalyst to the regeneration process, the catalyst activity was never restored to the prepoisoning value. The SEM micrographs confirmed that the poisoning leads to a morphological degradation of the catalyst surface features. In Figure 7, two SEM micrographs of the Fe2O3/SiO2 catalyst before and after the poisoning are shown. The iron oxide film before the poisoning was continuous and compact, completely covering the support fibers. After the poisoning procedure, the Fe2O3 film is damaged and partially removed form the fibers surface: it resulted apparently formed by micrometric particles (white spots) with irregularly rounded shapes, close to each other. On the other hand the EDS analysis of the catalyst after the poisoning procedure did not detect any trace of sulfur or iodine on the fibers or on the iron oxide particles. The BET measurements confirmed that the poisoning dramatically alters the morphology of the surface by reducing the exposed area. Indeed the BET surface area decreased in comparison to the as synthesized sample from 21.7 to 18 and 2.0 m2/g for the used catalyst before and after the poisoning. The relative reduction of the surface area in comparison to the pristine catalyst was -17% and -90% in the two cases, used and used-poisoned sample, respectively.
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Apparently the main mechanism responsible for the catalyst detriment originates from structural modifications on the catalyst surface. It is important to note that we did not observe the formation of any sulfate phase on the catalyst surface by SEMEDS analysis and XRD (X-ray diffraction, Phillips X′Pert Pro diffractometer using a Cu KR wavelength). This experimental evidence was expected and it is in agreement with the sulfate decomposition temperatures reported by Tagawa.13 Moreover during the high temperature sulfuric acid decomposition experiments no traces of iron was revealed in the reactor downstream during the poisoning and in the subsequent decomposition experiments by atomic absorption analysis. These experimental evidence suggested us that the catalyst surface apparently does not undergo chemical degradation reactions transforming the Fe2O3 active sites in chemically different inactive particles, due to the iodine poisoning. Most probably, the decrease of the catalytic activity is due to a morphological change of the iron oxide particles surface, e.g. smaller active surface area due to catalyst crystal dimensions growth. This hypothesis can be checked by calculating the variation of the gas volume that forms a monolayer on the surface by using a simple Langmuir model. Within the Langmuir assumption (i.e., (1) equivalent adsorption surface sites, (2) no interaction between the adsorbing gas molecules and those adsorbed, and (3) monolayer absorption), the adsorption isotherm is written by the following usual equation:14 θ (eq4) (1 - θ) where p is the pressure of the adsorbing gas, θ is the surface coverage. and b is a constant. The surface coverage θ can be calculated by the equation θ ) V/Vm where V is the adsorbed gas volume and Vm gas volume that forms a monolayer on the surface. The Langmuir isotherm can be rewritten as pb )
p B p ) + V Vm Vm
(eq5)
where B ) 1/b. Let us consider the following simplified scheme for the reaction mechanism derived from the gaseous reaction path15,16 by considering SO3(g) adsorption. H2SO4(g) ) SO3(g) + H2O(g)
(4)
SO3(g) ) SO3(ads)
(5)
SO3(ads) ) SO2(g) + 1⁄2O2(g)
(6)
The key step is the latter, i.e. the reduction of sulfur trioxide to sulfur dioxide released as gaseous molecule. It is correct to underline that within our hypothesis this last step is a simplification of the real mechanism. Indeed from a microkinetic point of view, the last step should be split into two separate reactions, e.g. SO3(ads) ) SO2(g) + O(ads) and 2O(ads) ) O2(g) or others. However any O(ads) association-desorption reaction is probably very fast and therefore all the really occurring reactions can be approximated with reaction 6. It is important to underline that here our purpose is not to propose and discuss the detailed reaction mechanism but, rather based on a reasonable microkinetic model, to verify our hypothesis for the catalyst poisoning. Starting form the previous expression (eq 5), we have pSO3 VSO3ads
)
pSO3 Vm
+
B Vm
(eq6)
Table 1. Gas Volume Forming a Monolayer on the Catalyst Surface (Vm) and Gas Volume Forming a Monolayer on 1 g of Catalyst for Nonpoisoned catalyst (NP) and for Poisoned (P) catalysta catalyst
T (K)
Vm (cm3)
Vmg (cm3/g)
NP (17 g) NP (7.6 g) NP (17 g) NP (7.6 g) P (9.4 g) P (9.4 g)
973 973 1073 1073 973 1073
474 228 2489 1369 242 878
28 30 146 180 25 93
a The mass values associated to the different catalysts correspond to the actual amount of material tested.
where pSO3 is the starting partial pressure of sulfur trioxide in the reactor and VSO3ads is the adsorbed SO3(g) volume that can be calculated by the equation: VSO3ads ) VSO2 ) VSO3% SO2
(eq7)
pSO3/VSOads 3
From the linear fitting of the vs pSO3 plot an estimate for the Vm value can be derived. Data calculated by applying this procedure to the Fe2O3/SiO2 experimental results at the temperatures of 973 and 1073 K are summarized in Table 1. By comparing the results obtained at 973 and 1073 K, an increase of the Vmg value in the fresh catalyst is observed at the higher temperature, as well as in the case of the poisoned catalyst. This effect could be a clue of a temperature activation of gas adsorption. As previously shown, the poisoning of catalyst result in a decrease of SO2 formation yield and as we can see in Table 1, the poisoning of the catalyst leads to a decreased gas adsorption capability. At a temperature of 973 K, the gaseous volume of SO3 that forms a monolayer is 29 cm3/g of catalyst before poisoning, while after poisoning this value drops to 25 cm3/g. The effect is most remarkable at the temperature of 1073 K when Vmg reduces from a medium value of 163 to 93 cm3/g. These data confirm that the main effect of the I-/I2 poisoning is a reduction of the surface area available to SO3 adsorption. 4. Conclusions In this work, the poisoning effects due to I2/I- contamination on an iron oxide (Fe2O3) catalysts used for H2SO4 to SO2 decomposition in the temperature range 873-1073 K were investigated. Two different catalysts were tested: in both cases the catalytic active material was iron oxide whereas the supporting inert matrix were SiO2 and Al2O3, respectively. In both cases the I2/I- contamination led to a decrease of the catalytic activity of about 20-50% of the starting rate depending on the iodine concentration in the poisoning stream. It is to be noted that the decrease of the catalyst activity was observed to be reversible in the case of the Fe2O3/Al2O3 experiments and irreversible in the Fe2O3/SiO2 case. Furthermore, it is to be noted that the morphological effect of the poisoning that was evidenced by the SEM micrographs in the case of the silica-based catalyst was apparently absent in the case of the alumina-supported sample. Moreover the poisoning does not lead to apparent precipitation of other chemical species. In the case of the silica supported catalyst the altered morphology of the catalyst surface caused the large reduction of the surface area in comparison to the not-poisoned catalyst. This remarkably different behavior may be due to the different concentration of the I3- species in the reactants stream during the poisoning process. Indeed in the case of the alumina supported catalyst, the poisoning agent was 1 wt % of the aqueous sulfuric acid feeding mixture, whereas in the case of silica supported catalyst, it was 10 wt %.
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Present results does not allow to investigate in details the poisoning mechanism. However some preliminary conclusion about the overall effect of the I2/I- contamination can be drawn on the basis of our experimental evidence. Apparently the poisoning due to I2/I- can occur in the following two ways: the formation of a reversible passivation or an irreversible morphological degradation of the catalyst surface. The passivation occurs after “light-poisoning” procedures and can be apparently removed after some hours of operation at high temperature with pure H2SO4/N2 feeding stream and so the pristine catalyst activity is restored. On the other hand after “heavy-poisoning” treatments, the catalysts were apparently irreversibly corrupted. It is to be noted that in this second case even subsequent tentative high temperature treatments failed to regenerate the catalyst samples that, as a consequence of the deep irreversible contamination, showed a strongly altered surface morphology. Acknowledgment This research was financially supported by the TEPSI project (FISR 2003-Italian national research grants). Thanks are due to Dr. Daniela Ferro for the SEM-EDS experiments. Literature Cited (1) Barbarossa, V.; Brutti, S.; Diamanti, M.; Sau, S.; De Maria, G. Catalytic thermal decomposition of sulphuric acid in sulphur-iodine cycle for hydrogen production. Int. J. Hydrogen Energy 2006, 31, 883. (2) Brutti, S.; Bencivenni, L.; De Maria, G.; Barbarossa, V.; Sau, S.; Diamanti, D. Gas Phase Dissociation of Sulfuric Acid. J. Chem. Thermodyn. 2006, 38, 1292. (3) Le Duigou, A.; Borgard, J. M.; Larousse, B.; Doizi, D.; Allen, R.; Ewan, B. C.; Priestman, G. H.; Elder, R.; Devonshire, R.; Ramos, V.; Cerri, G.; Salvini, C.; Giovannelli, A.; De Maria, G.; Corgnale, C.; Brutti, S.; Roeb, M.; Noglik, A.; Rietbrock, P. M.; Mohr, S.; de Oliveira, L.; Monnerie, N.; Schmitz, M.; Sattler, C.; Orden Martinez, A.; de Lorenzo Manzano,
D.; Cedillo Rojas, J.; Dechelottef, S.; Baudouin, O. HYTHEC:An EC funded search for a long term massive hydrogen production route using solar and nuclear technologies. Int. J. Hydrogen Energy 2007, 3, 1516. (4) Brutti, S.; Brunetti, B.; Barbarossa, V.; Ceroli, A.; Cafarelli, P.; Semproni, E.; De Maria, G.; Giovannelli, A.; Cerri, G. Decomposition of H2SO4 by direct solar radiation. Ind. Eng. Chem. Res. 2007, 46, 6393. (5) Dokiya, M.; Kameyama, T.; Fukuda, K., III. An oxigen-evolving step through the thermal splitting of sulfuric acid. Bull. Chem. Soc. Jpn. 1977, 50, 2657. (6) Norman, J. H.; Mysels, K. J.; Sharp, R.; Williamson, D. Studies of the sulphur-iodine thermochemical water-splitting cycle. Int. J. Hydrogen Energy 1982, 7, 545. (7) Dobrovinskaya, N. A.; Dobkina, E. I.; Kuuznetsova, S. M. Development of a supported catalyst for decomposition of spent sulphuric acid. Russ. J. Appl. Chem. 2004, 77, 34. (8) Togawa, H.; Endo, T. Catalytic decomposition of sulphuric acid using metal oxides as the oxygen generating reaction in thermochemical water splitting process. Int. J. Hydrogen Energy 1989, 14, 11. (9) Ginosar, D. M.; Anderson, R. P.; Glenn, A. W. Stability of sulfuric acid decomposition catalyst for thermochemical water splitting cycles. 2005 AIChE Spring Meeting, Conference Proceedings, Cincinnati, OH, October 30-November 4, 2005. (10) Sakurai, M; Nakajima, H; Onuki, K; Ikenoya, K; Shimizu, S. Int. J. Hydrogen Energy 1999, 24, 603. (11) Powell, D. G.; Wyatt, P. A. H. J. Chem. Soc. A 1971, 22, 3614. (12) Thermo-Calc AB, Thermodynamic Databases. www.thermocalc. com, 1994. (13) Tagawa, H. Thermal decomposition temperatures of metal sulfates. Thermochim. Acta 1984, 80, 23. (14) Chorkendorff, I.; Niemantsverdriet, J. W. Concept of Modern Catalysis and Kinetics; Wiley-VCH: Weinheim, Germany, 2003. (15) Yilmaz, A.; Hindiyarti, L.; Jensen, A. D.; Glarborg, P. Thermal dissociation of SO3 at 1000-1400 K. J. Phys. Chem. A 2006, 110, 6654. (16) Hindiyarti, L.; Glarborg, P.; Marshall, P. Reactions of SO3 with the O/H radical pool under combustion conditions. J. Phys. Chem. A 2007, 111, 3984.
ReceiVed for reView January 15, 2008 ReVised manuscript receiVed September 22, 2008 Accepted October 26, 2008 IE800064Z
