Catalytic combustion of hydrogen for mitigating hydrogen risk in case of a severe accident in a nuclear power plant: study of catalysts poisoning in a representative atmosphere

Applied Catalysis B: Environmental 47 (2004) 47–58

Catalytic combustion of hydrogen for mitigating hydrogen risk in case of a severe accident in a nuclear power plant: study of catalysts poisoning in a representative atmosphere Franck Morfin a , Jean-Christophe Sabroux b , Albert Renouprez a,∗ a

Institut de Recherche sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France b Institut de Radioprotection et de Sˆ ureté Nucléaire, DPEA/SERAC, Centre d’Etudes de Saclay, B.P. No. 68, 91192 Gif-sur-Yvette Cedex, France Received 11 February 2003; received in revised form 8 May 2003; accepted 23 July 2003

Abstract In case of a severe (beyond design basis) accident in a nuclear power plant, a large amount of hydrogen could be generated by reaction of water of the primary coolant circuit with the fuel rods inside the reactor pressure vessel, and eventually released into the air-filled reactor building. For mitigating the risk of an explosion within the containment, a catalytic combustion of this hydrogen is considered as one of the most efficient counter-measure. The difficulty which is to be overcome is a possible poisoning of the catalyst by fission products and other components released by the damaged core, notwithstanding the fact that most of them enter the containment building as non-reactive large oxide particles. The main vapors which are suspected to have an inhibiting or poisoning effect are indeed di-iodine and methyl iodide, both potentially present in the containment atmosphere. We report on the possible effect of these molecules on Pt, Pd and Pt–Pd model catalysts at lower temperatures and somewhat higher iodine or iodide concentrations, as compared to inferred catalyst operational parameters in a reactor building during a severe accident. In these particular experimental conditions, platinum is substantially poisoned by both vapors. On the other hand, palladium, about 400 times less active than platinum, is much less altered by I2 and ICH3 vapors. A marked beneficial effect was found by alloying the two noble metals: the alloys show only a threefold decrease in activity with respect to platinum, and undergo a much weaker deactivation. © 2003 Elsevier B.V. All rights reserved. Keywords: Passive Autocatalytic Recombiners (PAR); Hydrogen risk; Nuclear Power Reactor; Severe accident in PWR; Hydrogen catalytic combustion

1. Introduction On the morning of March 28, 1979, the 880 MWe Unit 2 reactor at the Three Mile Island (TMI) nuclear power plant in Pennsylvania suddenly overheated, as a response to a loss of feedwater. Equipment failures and human errors did converge to cause a partial (>40%) core meltdown [1,2]. Consequently, due to the oxidation of the fuel rods by the water of the primary coolant circuit, a hydrogen “bubble” formed in the reactor pressure vessel, leaked through a relief valve and a broken rupture disk, and eventually accumulated at the top of the air-filled containment building. The hydrogen–oxygen mixture exploded 10 h after the onset of the accident. Fortu-

∗

Corresponding author. E-mail address: [email protected] (A. Renouprez).

0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.07.001

nately, mainly because the deflagration-generated pressure spike did not exceed 2 bar (gauge pressure), no containment failure occurred, and the fission products release was minimal, with no measurable early health effects on the population nor impact on the environment [3]. Since the very beginning of the industrial development of pressurized water reactors (PWR), the possibility of an ignition of ternary mixtures containing hydrogen, air and water vapour was indeed considered, and thoroughly described by Shapiro and Moffette [4]. They showed that, hydrogen being released with a large amount of steam from the primary circuit in case of a loss-of-coolant accident, the lower flammability limit of the mixture in the containment is significantly above the H2 = 4.1% limit in air (Fig. 1). Nevertheless, overheating of the reactor core above 1200 ◦ C in the presence of steam can induce a runaway

48

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58 100

0

Containment atmosphere limits in case of severe accidents

20˚C 10

90

20

80

30

70

40

80˚C

Detonation (assumed)

50

50

% % m m ea Stea

60

St

AiA ir % r%

70˚C

85˚C

40

60

Deflagration

30

90˚C 70

20

80

10

90

0

100 100

90

80

70

60

50

40

30

20

10

0

100˚C

Hydrogen % Hydrogen % Fig. 1. Flammability limits of hydrogen–air–steam mixtures (after [4], with additions). The triangular diagram is valid at 1 atm total pressure, for a steam-saturated atmosphere (100% relative humidity). Thus, the deflagration (experimental) and detonation (assumed) limits are not isothermal. Note that no deflagration can occur in such a steam-saturated atmosphere above 85 ◦ C, whatever the H2 /air ratio.

oxidation of the zircaloy (an alloy of zirconium and tin) composing the fuel cladding. The 29,660 kg of zircaloy present in the pressure vessel of a French 1400 MWe PWR could yield a mean 18.2% H2 in a dry atmosphere [5]. Indeed, a large amount of energy is produced by this exothermic oxidation, raising the temperature and resulting in a self-acceleration of the in-vessel core melt down: Zr + 2H2 Ovap → ZrO2 + 2H2 , H = −617 kJ mol−1 Up to 15,000 m3 STP of hydrogen can be released within a few hours during this critical step, including the destruction of the core support structure and control rods, creating, together with the melted fuel rods, the so-called corium slumping down into the lower head of the pressure vessel. Later, other phenomena in case of pressure vessel failure, the corium-concrete interaction and water radiolysis can produce an equivalent amount of additional hydrogen. However, these late phenomena have a much slower kinetics, lasting a few weeks to a few months [6,7]. Several solutions have been considered in order to avoid an explosion, and thus to mitigate the so-called hydrogen risk in nuclear power plants. Among them, a deliberate ignition system, using electric spark igniters [7,8], is already installed in some American boiling water reactors (BWR) [9]. Also, the injection of an inert gas such as CO2 or N2

seriously limits the risk of an explosion, but has the disadvantage to increase the overall pressure in the containment building [10,11]. Finally, these systems can be associated with a catalytic combustion device; this is now considered as the safer stand-alone solution, since the system is completely passive, with no need of any external power supply and human intervention. Such Passive Autocatalytic Recombiners (PAR) are already at an industrial stage of development [12,13]. For example, the recombiners marketed by Framatome ANP—formerly Siemens [6,8]—or by the Atomic Energy of Canada Ltd. [14] consist of a 0.1–1 m3 metallic housing, including the inlet opening, the catalyst insert and a convection shaft (or “chimney”). The active phase is made up of a series of metallic plates covered by a thin film of alumina-supported platinum and/or palladium. The system is fed at the bottom by natural convection, thanks to the large amount of heat produced by the catalytic combustion. One of the major problems which was to be addressed prior to the installation of PAR in the nuclear power plants, is the possible poisoning of the catalyst by fission products and reactor structure materials which enter the containment building under the form of vapors and particulate matter. This paper presents a study of the poisoning, by these chemical substances, of model alumina-supported Pd and Pt catalysts, which have a composition similar to that of the industrial recombiners.

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

2. Experimental 2.1. Determination of the nature of the potential poisons During a severe accident, most of the components evaporating from the fuel (UO2 + fission products + minor actinides), the control rods (e.g. Ag–In–Cd), the core structure materials (mainly stainless steel) and the fuel cladding (Zr alloy) are under the form of oxides. The concentration of the various elements in the core (the “core inventory”) at any time during a fuel cycle in a reactor is accurately known. But the composition of the aerosol produced by an accidental meltdown depends on the vaporization yield of each of these elements, and on the chemical reactions taking place within the primary circuit, which have to be determined experimentally [15,16]. For this purpose, a separate experimental facility— dubbed H2-PAR [17], not described here—had been installed at the Cadarache nuclear research center. In a 7.5 m3 Terphane® polyester film tent, accommodating an industrial prototype catalytic recombiner, a charge of 25 elements is vaporized in an induction furnace, in a steam-saturated atmosphere at 85 ◦ C. The charge is representative of a nuclear core inventory, including 60% of UO2 , smaller quantities of Fe, Zr, Cr, Ni, etc. and non-radioactive elements as surrogates of fission products (e.g. 0.03% of 127 I). An analysis of the aerosols shows a very low concentration of the refractory oxides (UO2 , ZrO2 , Y2 O3 , etc.) and a comparative increase of those which have a larger vaporization yield. Finally, as compared to a blank run, no significant decrease of the recombiner efficiency could be observed in the presence of these aerosols in the H2-PAR experimental containment [18]. All the elements, except iodine, are under the form of ca. 3 ␮m particles. This coarse size distribution and the low cumulated concentration (<100 mg m−3 ) of potential poisons such as Cd, Se or Te, compared to the overall active surface developed by the catalyst, explains the innocuousness of the aerosols [19]. Several experimental and numerical simulations of a severe accident, however, indicate that iodine, mainly entering the containment as CsI [15], can be present under the molecular form I2 at higher concentrations than in the H2-PAR tent, i.e. up to 5 ppm (2.2 × 10−4 mol m−3 ). Moreover, in the later stage of the accident sequence, iodine can also react with organic compounds such as wall paints in the containment to form methyl iodide ICH3 [20]. Therefore, our own experiments aim at determining to what extent classical supported noble metal catalysts are actually poisoned by iodine and iodine compounds. 2.2. Catalytic reaction The catalytic oxidation of hydrogen by air was performed in a fixed-bed, flow reactor. Hydrogen and air streams are delivered through mass-flow controllers to a mixer, necessary to generate an homogeneous mixture of the two gases.

49

If necessary the gaseous mixture is water saturated at 90 ◦ C (70 kPa H2 O) before its admission into the reactor. To take place in a representative atmosphere, this reaction was carried out under high oxygen and low hydrogen partial pressures (lean-burn conditions). The oxygen pressure was kept constant, ca. 21 kPa if the gaseous mixture is not water saturated, while that of hydrogen was varied between 0 and 1 kPa. To study the effect of iodine, a saturator containing iodine flakes, kept at the required temperature by a thermostatic water bath, produces iodine vapour in accordance with the vapour pressure versus temperature relation [21]: log PT3/2 = 16.945 −

3464 T

(1)

where P is the vapour pressure in Pa and T is the absolute temperature. The saturator is swept by dry air, which in turn is mixed with the main gas flow before the entrance of the catalytic reactor. At the exit of the reactor, a NaOH trap scrubs the remaining iodine, then the gas is dehydrated. The hydrogen concentration in the reaction products is measured by a LEYBOLD UL 200 (H2 and He) mass spectrometer. The experimental set-up is shown in Fig. 2. For the experiments of poisoning by ICH3 , iodine is replaced by a cylinder-fed gas flow containing 1000 ppm of methyl iodide in nitrogen. The reaction is carried out below 420 K in a mixture containing 0.3% of H2 in air. In that case, the CO and CO2 concentrations in the reaction products are determined by two COSMA Beryl-100 infrared analyzers. 2.3. The catalysts The alumina-supported Pd, Pt and bimetallic catalysts were prepared using acetyl-acetonates precursors. The salts are dissolved in toluene at 350 K and ␥-alumina (RhˆonePoulenc, SPH 557, 310 m2 g−1 ) is added to the solution. The suspension is stirred at ambient temperature for 24 h, filtered and dried under vacuum at 350 K; the organic ligands are removed by calcination under flowing oxygen at 600 K. A reduction of the catalysts is then performed under flowing hydrogen at 600 K. The chemical analysis, performed by ICP atomic emission spectroscopy, shows that 80% of the precursors are fixed on the support. The chemical compositions of the four catalysts prepared are given in the first two columns of Table 1. As shown recently [22], a simultaneous crystallization of Pd and Pt(acac)2 leads to mixed salts with the same structure as the Pd(acac)2, in the Pd100 to Pd20 atomic concentration range. This peculiarity is certainly favorable to the formation of bimetallic particles. This point was verified by analytical microscopy, using a JEOL JEM 2010 instrument equipped with an EDX probe. An analysis performed on large zones leads to a Pd/Pt ratio identical to the value measured by chemical analysis. On the contrary, if all the particles do contain the two metals, the mean deviation of the

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F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

Fig. 2. Schematic diagram of the catalytic test bench. The catalytic reactor accommodates a few grams of powdered catalyst.

composition of individual particles with respect to the mean stoichiometry is of the order of 30%. The distribution of particle diameters has been measured for each catalyst on 400 particles, either on extractive replica or by direct observation; a surface weighted mean diameter was calculated from the histograms by the expression:
3 nd ds =
2 (2) nd On the basis of the electron microscopy photographs showing their regular shape, one can assume that these supported clusters, as largely admitted, have a cubo-octahedral shape. One can thus relate the mean diameter ds to the dispersion D, which is the ratio of the number of surface atoms to the total number of atoms in the particle: D=6

M NA ρads

(3)

where M is the atomic mass, NA is the Avogadro number, ρ is the mass density of the particle, and a is the surface area occupied by an atom on the particle surface [23]. For a fcc metal, the surface area occupied by one atom is calculated by assuming an equal proportions of the (1 1 1), (1 0 0) and (1 1 0) planes on the particle surface. This leads to a = 8.07 Å2 for Pt [23] and a = 7.9 Å2 for Pd [24]. For bimetallic catalysts, we have used the atomic proportions of each metal present in the solid to calculate the lattice parameters. However, since the lattice parameters of these two metals are very close, the possible error due to Pd enrichment by surface segregation is lower than 2% [25]. The determination of the dispersion, deferred in the fifth column of Table 1, is indispensable to evaluate the chemisorptive properties and the catalytic activity of the metal surface. Taking into account the type of reaction under study, hydrogen chemisorption experiments were thus performed.

Table 1 Characteristics of the various catalysts prepared for this study Chemical analysis (wt.%) Pd

Atomic composition

Mean particle diameter (nm)

Dispersiona (%)

Pt

0.92 1.41 0.2 a

1.78 1.38 1.41

Pd100 Pt100 Pd65 Pt35 Pd20 Pt80

2.3 2.5 3.9 2.5

49 44 29 45

Chemisorbed hydrogen (ml g−1 metal) Total

Irreversible

104 73 51 79

66 39 26 41

H/total metal atom

Stoichiometry (H/surface metal atom)

0.62 0.67 0.32 0.64

1.27 1.52 1.10 1.42

Dispersion is defined as the ratio of the number of metal atoms of surface on the total number of metal atoms (given by EM).

Adsorbed hydrogen volume [ml STP / g Pt]

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

51

120

Reversible + irreversible adsorption

100

80

Irreversible adsorption

60

40

Reversible adsorption 20

0 0

5

10

15

20

25

Hydrogen pressure [kPa] Fig. 3. Hydrogen adsorption isotherm of the Pt catalyst at 330 K. The strongly adsorbed hydrogen is given by the difference between the measurement of the weakly plus strongly adsorbed hydrogen (dark circles) and the measurement of weakly adsorbed hydrogen (empty circles).

Adsorbed hydrogen volume [ml STP / g metal]

Adsorption isotherms were determined by volumetry, using a MKS 510 pressure gauge; the whole equipment is placed in a thermostatic vessel at 330 K to avoid temperature fluctuations. In this facility, the samples are re-reduced at 700 K under hydrogen and evacuated with a turbo-molecular pump at 700 K before the adsorption experiment. All the isotherms have the same shape, shown in Figs. 3 and 4, with a linear section ranging from 2.5 up to 25 kPa.

Following the well-accepted procedure, although somewhat arbitrary, a total adsorbed volume is deduced from an extrapolation of the linear part to zero pressure. If one subtracts to these values the amount evolved by evacuation at 300 K [24] and the amount absorbed in bulk palladium under the form of hydride [26,27], one obtains the number of H atoms per metal atom in the particle displayed in the eighth column of Table 1, frequently considered as

120

Reversible + irreversible adsorption

100

80

Irreversible adsorption 60

40

Reversible adsorption (Pd20Pt80)1,6% / Al2O3

20

Alumina support 0 0

5

10

15

20

Hydrogen pressure [kPa] Fig. 4. Hydrogen adsorption isotherm of the Pd20 Pt80 catalyst at 330 K.

25

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F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

dispersion measured by chemisorption, when stoichiometry at the surface is assumed to be unity. Alternatively, considering the geometric dispersion measured by EM, one can deduce by dividing this ratio (column 8) by the dispersion (column 5), the stoichiometries of hydrogen adsorption shown in column 9. These values are systematically larger than unit for all samples, a phenomenon reported by many authors, in the case of supported Pt catalysts [27,28]. For Pd or Pd-rich catalysts, one finds a lower stoichiometry, in spite of a total adsorbed volume almost equal to that found on platinum. This can be ascribed to the larger fraction of the total adsorbed hydrogen evolved below 300 K on this metal, than on platinum. In any case, these chemisorption experiments have shown that the preparation conditions lead to a clean metal surface, able to normally chemisorb hydrogen.

3. Results and discussion 3.1. Reactivity in the absence of poisons For each reaction temperature and each catalyst, a first step is the determination of the domain into which the flow of reactants is high enough to avoid reaction rate control by diffusion. Fig. 5 clearly shows that, as the temperature is increased for example from 363 to 400 K, it can be necessary to increase the flow by nearly an order of magnitude, depending on the activity of the metal. Also, one should note that, because of the large difference of reactivity of Pd and

Pt, one was obliged to select different reaction temperatures, namely 40–50 ◦ C higher for the palladium catalyst than for the three samples containing platinum. Figs. 6–8 report the catalytic activities, or turnover frequencies (TOF), defined as the number of hydrogen molecules consumed (equal to the number of molecules of water produced) per unit of time and surface metallic atom (value determined by EM) for fixed temperature and hydrogen concentration. It was verified that the reaction is indeed of the first order with respect to hydrogen and zero order with respect to oxygen in our conditions of large oxygen excess and in the temperature interval considered. The consequence is that the activity, in s−1 , is a linear function of [H2 ], the H2 concentration, and becomes: A = k[H2 ]

(4)

where the pseudo-rate constant k is expressed in m3 mol−1 s−1 . The pseudo-rate constants reported in Table 2 show that platinum is about 400 times more active than palladium. Concerning the two alloys, their activities are similar, two orders of magnitude higher than that of pure palladium and only three times lower than that of pure platinum. These observations are at first sight surprising, because, as predicted by theory and measured by LEIS [25], the surface of the Pd65 Pt35 particles is almost entirely covered by non-reactive Pd atoms which segregate from the bulk. Conversely, the second layer is Pt enriched with respect to the mean composition. Therefore, a plausible explanation would be that not only the surface layer but also the first two layers,

Fig. 5. Diffusion limitations as a function of the reaction temperature, Pd catalyst, with 0.37% H2 . The reaction rate is defined as dnH2 /mcat dt.

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

53

0.12 403 K 0.1

383 K 365 K

Activity [s-1]

0.08

343 K 0.06

0.04

0.02

0 0

0.05

0.1

0.15

0.2

0.25

H2 concentration [mol.m-3] Fig. 6. Activity of the Pd catalyst from 343 up to 403 K.

which altogether contain an equal amount of the two types of atoms, contribute to the reaction. The Arrhenius plot of rate constants in Fig. 9 yields the apparent activation energies reported in Table 2 for the four catalysts. These values are of the order of magnitude as those reported by Ertl [29] on Pt (1 1 1) or by Engel and Kuipers [30] on Pd (1 1 1).

3.2. Reactivity in the presence of iodine 3.2.1. Molecular iodine Fig. 10 shows the variation, as a function of time on stream, of the activity of the various catalysts in the presence of di-iodine vapour. Palladium and platinum behave rather differently. After an initial decrease during the first 5 min,

10 365 K 354 K

Activity [s-1]

8

343 K 334 K

6

313 K 4

2

0 0

0.05

0.1

0.15

0.2 -3

H2 concentration [mol.m ] Fig. 7. Activity of the Pt catalyst from 313 up to 365 K.

0.25

54

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58 8 383 K 365 K 6

Activity [s-1]

354 K 343 K 4

2

0 0

0.05

0.1

0.15

0.2

0.25

-3

H2 concentration [mol.m ] Fig. 8. Activity of the Pd20 Pt80 catalyst from 343 up to 383 K.

the activity of palladium reaches a constant level that, for the highest di-iodine concentration, 32 ppm, is still one third of the initial one. On the other hand, a continuous decrease is observed for platinum, and the activity becomes negligible after 3 h. The reactivity of the Pd65 Pt35 alloy is less affected by the presence of iodine than platinum, and even than pure pal-

ladium, since its residual activity is still 60% of the initial one. This phenomenon is comparable to the thioresistance of this bimetallic couple in the hydrogenation of aromatics observed by many authors [31]. A similar improvement of the resistance to sulfur poisons has been suggested for the Pt–Pd catalyst on alumina, as compared to the monometallic Pt or Pd catalysts, in the complete oxidation of methane at

Fig. 9. Arrhenius diagram of ln(k) vs. 1/T, with k in m3 mol−1 s−1 .

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58 Table 2 Pseudo-rate constants and activation energy for the oxidation at low hydrogen partial pressure Pseudo-rate constants (m3 mol−1 s−1 )

Pd/Al2 O3 Pt/Al2 O3 Pd65 Pt35 Pd20 Pt80

Then, Eq. (5) becomes: 1 bI
2 =1+ [I2 ] 1 − θI2 1 + (bj [j])nj

Activation energy (kJ mol−1 )

T = 343 K

T = 365 K

0.10 39 14 12

0.20 76 32 32

The fraction of active surface, free from iodine, should be proportional to the remaining activity A/A0 . If αI2 is the number of catalytic sites inhibited by one di-iodine molecule, in the case of a molecular adsorption of di-iodine, Eq. (6) becomes:   A0 1 bI2 1
= 1+ = [I2 ] (7) A αI2 (1 − θI2 ) αI2 1 + (bj [j])nj

36 41 40 44

The oxygen pressure was kept constant, 21 kPa, while that of hydrogen was varied between 0 and 1 kPa.

and A0 /A is an affine function of [I2 ]. In the case of a dissociative adsorption of iodine, Eq. (6) becomes:     bI2 A0 1 1
= = 1+ [I2 ] (8) A αI (1 − θI ) αI 1 + (bj [j])nj

low temperature [32]. Indeed, Pt–Pd aggregates deposited on acidic supports remain more active in the presence of H2 S or thiophene than the pure metals. To elucidate the mechanism of resistance to iodine of these catalysts, we have studied the variation of their activity as a function of the di-iodine concentration [I2 ], in order to know whether this compound is adsorbed under the molecular form or under the dissociative one. Assuming a Langmuir model, the fraction of surface free from iodine writes:
1 + (bj [j])nj
1 − θI2 = (5) 1 + bI2 [I2 ] + (bj [j])nj

√ and A0 /A is an affine function of [I2 ]. As shown in Fig. 11, A0 /A is an affine function of [I2 ] and the ordinate at the origin, 1/αI2 = 1. Thus, iodine is adsorbed under the molecular form and one adsorbed di-iodine molecule inhibits one catalytic site. Considering the large iodine molecule diameter, 0.53 nm, it can be easily understood that it would not penetrate below the first metal layer. One can, therefore, assume that iodine remains at the surface of the alloys but that atomic hydrogen and oxygen can diffuse to react on the Pt-rich second layer. The behaviour of pure platinum, which has a lower resistance to poisoning, is more difficult to understand; one notes that XPS experiments, however not performed under

where θ
I2 is the fraction of the active surface occupied by iodine ( θi = 1), j is an another adsorbed species, bi is the adsorption equilibrium constant of the i species, [i] is concentration of the i species in the gas phase, and nj is a coefficient equal to 1 for non-dissociative adsorption of the j species and 1/2 for dissociative adsorption of the j species.

15

Activity Pt and Pd65Pt35 /Al2O3 [s-1]

(6)

0.15

Pt1,78%/Al2O3, [I2] = 10 ppm 10

0.1

(Pd65Pt35)2,79%/Al2O3, [I2] = 64 ppm 5

0.05

Pd0,92%/Al2O3, [I2] = 10 ppm 0 0

500

1000

1500

2000

2500

3000

3500

Time on stream [s] Fig. 10. Activity of catalysts on stream containing di-iodine, at T = 383 K, with 0.3% H2 in air.

0 4000

Activity Pd/Al2O3 [s-1]

Catalyst

55

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F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58

Fig. 11. Reciprocal of the remaining activity of Pd catalyst at equilibrium vs. [I2 ] and

the reaction conditions, show that both the alumina support and the metallic phase are partly covered by iodine. One can thus admit that in this competitive adsorption between the metal and the support, platinum is more covered by iodine than palladium. It should be outlined that, in the simulation experiments mentioned above (H2-PAR), no loss of activity could be observed with a commercial recombiner (mostly Pt on alu-

√

[I2 ].

mina). Actually, several possible reasons can be put forward to explain this difference [19]. • the potential poisons of the catalyst (among them, iodine), are in low concentrations as compared to the quantity of catalyst available on the catalyst sheets; • large iodine-bearing solid particles (1–3 ␮m, CsI, AgI, CdI2 , InI, etc.), which transport most of the iodine in the

Fig. 12. Remaining activity of catalysts on stream containing 6 ppm of methyl iodide, at T = 420 K with 0.3% H2 in air.

F. Morfin et al. / Applied Catalysis B: Environmental 47 (2004) 47–58 Table 3 Activity (s−1 ) at equilibrium at 420 K for two methyl iodide concentrations Concentration (ppm)

Pd

Pd65 Pt35

Pd20 Pt80

Pt

0 6 10

0.09 0.054 0.031

16.5 1.25 0.18

18.9 0.6 0.5

45 0 0

The oxygen pressure was 21 kPa and the hydrogen pressure was 0.3 kPa.

containment, cannot interact directly with the hot catalyst, because of the much smaller size of the pores of the alumina support material (3–20 nm); • thermophoretic effects [33] hinder vaporization of the particulate matter by direct contact with the red hot catalyst surface; • the operation temperature of actual recombiners (up to 900 ◦ C) is much higher than the temperature of our experiments; and • even in the case of a partial poisoning of the catalyst, the recombiner keeps an important efficiency thanks to the diffusion regime which is established at the beginning of the recombiner operation (not kinetically limited). 3.2.2. Methyl iodide The activity variations as a function of time on stream are measured for two methyl iodide concentrations, 6 and 10 ppm. The results for ICH3 = 6 ppm are shown in Fig. 12. For this concentration, the activity of pure platinum becomes negligible after 2000 s, whereas that of pure palladium, after a decrease of 40% during the first 1000 s, remains stable. The activity limits, reached after 6000 s, are reported in Table 3 for the two concentrations. As for molecular iodine, the alloys remain more active than the pure metals. Thus, under our experimental conditions, after an initial decrease of activity, the bimetallic catalysts remain sufficiently active during a very long period of time.

4. Conclusions This study aimed at determining to what extent hydrogen oxidation in catalytic recombiners would be sensitive to poisoning by di-iodine and methyl iodide. Poisoning of alumina-supported Pt and Pd catalysts, with respect to this reaction, is quantifiable in our experimental conditions. When extrapolated to the actual recombiner operation in a nuclear power plant, the loss of efficiency attributable to these poisons does not affect significantly the yield of hydrogen recombination. The same conclusion holds for most poisons potentially present in the atmosphere of a reactor building under severe accident conditions [18,19]. At last, our study emphasizes the remarkable resistance of Pd–Pt bimetallic catalysts to di-iodine and methyl iodide poisoning, even for concentrations far above those expected in the reactor building in case of a severe accident.

57

Much less attention has been paid by chemists and chemical engineers to the catalytic recombination of hydrogen and oxygen than to other catalytic combustions (e.g. methane, biogas, carbon monoxide, syngas) or conversions, not to mention the (photo)catalytic splitting of water into oxygen and hydrogen. With a current 1 million outlay for installing a complete array of recombiners in one reactor building (and, say, 400 power reactors in the world that could accommodate such an equipment), there is certainly a niche for additional basic research on optimization of recombiner performances, innocuousness, and resistance to poisoning or aging, that have not been carried out so far.

Acknowledgements The experimental results pertaining to the catalytic tests with molecular iodine originate from the thesis by Franck Morfin, granted jointly by EDF (Electricité de France) and IPSN.

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