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Reactivity of steam in exhaust gas catalysis. part II: Sintering and regeneration of Rh and PtRh catalysts in propane oxidation
A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control I11 Studies in Surface Science and Catalysis, Vol. 96 1995 Elsevier Science B.V.
73
R E A C T I V I T Y OF STEAM IN E X H A U S T GAS CATALYSIS. PART II : SINTERING AND R E G E N E R A T I O N OF Rh AND PtRh CATALYSTS IN P R O P A N E O X I D A T I O N
J. Barbier Jr. and D. Duprez Laboratoire de Catalyse en Chimie Organique URA CNRS 350 40 av. du Recteur Pineau , 86022 POITIERS Cedex FRANCE Tel : (33) 49 45 39 98.
ABSTRACT PtRh catalysts were prepared on different supports composed of A1203, CeO2, ZrO2 and NiA1204. The variations of the activities in propane oxidation and steam reforming were used to obtain some indication concerning the surface state of these catalysts after thermal treatments at high temperature in an oxidizing and in a reducing medium. Cyclopentane hydrogenolysis was also carried out to observe the changes in the rhodium surface state. Platinum was the metal which catalysed the direct oxidation of propane while rhodium was the key-component in steam reforming catalysis. The treatment at 800~ in an oxidizing medium induces a very important decrease of Rh area linked to the fact that rhodium in its oxidized form (Rh 3+) can diffuse into the support. This phenomenon is more marked on A1203 and, to a lesser extent on CeO2-A1203, than on the other supports. On the other hand, oxidative treatments lead to an increase of the particle size of platinum, and temporarily to an enhancement of the oxidation activities. After a treatment in a reducing medium at high temperature (T>700~ the steam reforming activities can be recovered by extraction of rhodium from the support, with two exceptions however : Rh/CeO2-A1203 and PtRh/ZrO2 catalysts. This treatment has pratically no impact on platinum activities in oxidation except for the catalysts supported on NiA1204 which are deactived. Cyclopentane hydrogenolysis confirm all the above results.
1. INTRODUCTION PtRh catalysts are COlmnonly used in catalytic converters for eliminating pollutants (CO, hydrocarbons, NOx) from exhaust gases [1]. In the first part of our w o r k [2], we investigated, on P t ( l w t . - % ) , Rh(0.2wt.-%) and
74 Pt(lwt%)Rh(0.2wt%) catalysts supported on A1203 and CeO2-A1203, carbon monoxide and propane oxidation by oxygen (direct oxidation), by steam (water gas shift and steam reforming) and by a mixture of oxygen and steam (oxy-WGS and oxy-steam reforming). Steam can be considered a cor-eactant of oxidation during rich-phases (lean in O2) [3-5]. In oxy-steam conversion of propane, we showed (fig. 1) that propane oxidation was catalyzed by platinum (between 200 and 350~ while rhodium was the key-component in the catalysis of steam reforming (between 350 and 600~ Ceria was an excellent promotor of steam reactions [3, 6], particularly when this reaction was carried out in the presence of oxygen. Therefore, the steam reforming activity is an excellent indicator of the rhodium surface state since the activity systematically decreases when the metallic rhodium area decreases [7]. On the other hand, oxidation activity is a more complex indicator of platinum surface state because there exists an optimum dispersion [8, 9]. Metal sintering constitutes a very significant cause of loss in catalyst activity owing to the reduction in the metallic area [10,11 ]. The aim of this work is to study the sintering of Rh and PtRh catalysts by means of oxidation and steam reforming activities in propane conversion. Changes in the steam reforming activity have been compared with those obtained in cyclopentane hydrogenolysis, a reaction extremely sensitive to the state of Rh in the catalyst [12]. C3H8 conv. (%) 100 -
El E
80 Direct oxidation
[]
60
.
Steam reforming
[] [] El
9149 I
40 20
00
[]
,..--.~...~_~ .m 0 ~!,
200
[] [] I
I
300 400 Temperature (~
I
500
600
Fig 1 9 Oxy-steam reforming o f propane on PtRh/CeO2-AI203
75 2. E X P E R I M E N T A L
2.1 Catalysts Five different supports were used : (i) a gamma-alumina (120m2 g-l) which was impregnated with an aqueous solution of cerium nitrate to obtain (ii) a 12wt.-%CeO2-A1203 after calcination (100m2 g-l); (iii) a zirconia (40m2 g-l) supplied by Degussa, (iv) a support prepared by coimpregnation of a Ni(NO3)2 and an Al(NO3)3 aqueous solution on an alumina (200m2 g-l) so as to obtain after calcination (1000~ air, 24h) a 8.5wt.-%NiA1204-A1203 support and this support is impregnated with an aqueous solution of cerium nitrate to obtain (v) a 12wt.-%CeO2-8.5wt.-%NiA1204-A1203 support after calcination. These supports (A1203, CeO2-A1203, ZrO2, NiA1204-A1203 and CeO2-NiA1204-AI203) were crushed and sieved to 0.1-0.2 mm. They were used to prepare two series of catalysts by impregnation or coimpregnation with aqueous solutions of rhodium chloride and chloroplatinic acid. The catalysts were dried at 120~ then calcined at 500~ under an air flow and prereduced in H2 at 450~ The metal loadings were Rh(0.2wt.-%) and Pt(lwt.-%)Rh(0.2wt.-%). A Pt(lwt.-%) on gamma-altmaina (120m 2 g-l, 0.10.2mm) supplied by I.F.P. (French Institute of Petroleum) was also used for certain experiments. Table 1: Catalyst co m positions Pt Symbolic (wt.-%) Name 1 Pt/A Rh/A 0 0 Rh/CeA 1 PtRh/A 1 PtRh/CeA 1 PtRh/Z 1 PtRh/ANi PtRh/CeANi 1
Rh
Support
(wt.-%) 0 0.2 0.2 0.2 0.2 0.2 0.2 0.2
A1203 A1203 A1203 +CeO2 (12wt.-%) A1203 A1203 +CeO2(12wt.-%) ZrO2 A1203+A1204Ni(8.5%) A1203+A1204Ni(8.5%)+CEO2(12%)
76 2.2 Catalytic reaction
Before each run the catalyst samples were heated in air for l h at 450~ Propane oxidation was carried out in a flow reactor under the following conditions : (i) catalyst bed : 40mg diluted in 360 mg of eordierite (0.1-0.2 mm). (ii) Feed gas (in vol-%) : C3H8, 0.4 ; O2, 0.8; N2, 98.8. (iii) Gas flow-rate : 380em 3 min-1 (volume space velocity : 250,000h-1). Temperature-programmed reactions were carried out from 150 to 900~ using a 4~ min-1 temperature ramp (1 atm.). Analyses were carried out by gas chromatography : CO2, CH4 and C3H8 on Porapak Q (0.7m, 1/4 in.; 25~ or 100~ cartier gas HE), CO, O2, N2 and CH4 on molecular sieve 5A (0.4m, 1/4 in.; 25~ carrier gas H2), H2 on molecular sieve 5A (lm, 1/4 in.; 25~ carrier gas N2). C3H8 conversions were determined from the mass balance of carbon-containing products and verified by the disappearance of the C3H8 peaks in the ehromatograms. The main reactions considered here were direct oxidation (1), C3H8 steam reforming (2) and W.G.S. (3). C3H8 + 502 . . . . . > 3CO2 + 4H20 (1) C3H8 + 3H20 . . . . . > 3CO + 7H2 (2) CO + H20 . . . . . > CO2 + H2 (3) Reactions (2) and (3) occurred via the water produced in reaction (1) : even in the absence of steam in the inlet gases, typical light-off curves like those in Fig. 1 were obtained. Specific activities and activation energies in oxidation and steam reforming were determined at low conversion. Specific activities were calculated at 200~ for direct oxidation and at 300~ for steam reforming. 2.3 Sintering and regenerating conditions
The oxy-steam reforming reaction was carried out on : - flesh catalysts (450~ 02 (3vol.-%); lh)) (FRESH) - oxidized catalysts (650~ or 800~ 02 (3vol.-%); lh) (OX650 or
ox800), - oxidized (800~
or 900~
O2 (3vol.-%); lh) and then reduced catalysts (700, 800 H2 (3vol.-%); lh)(RED700 or 800 or 900).
77
2.4 Cyclopentane hydrogenolysis This model reaction was carried out "in situ" on flesh or sintered catalysts (OX800 and RED900). The catalysts were prereduced in H2 (lh, 300~ 30cm3 min-1). Cyclopentane hydrogenolysis was performed in a pulse flow reactor under the following conditions : (i) catalyst bed : 40mg diluted in 360mg of cordierite (0.10.2 mm); (ii) cyclopentane injection : 11 lamole per pulse; (iii) hydrogen flow rate: 30cm3min-1; (iv) temperature range 170-330~ Under these conditions, the only reaction product was n-pentane analyzed by gas chromatography on a reoplex 400 column (2m, 1/8 in., 50~ carrier gas H2). The specific activities in cyclopentane hydrogenolysis of catalysts were calculated at 200~
3. RESULTS AND DISCUSSIONS
3.1 Rh/A catalyst Figure 3 shows the light-off curves of Rh/A after different treatments. On this catalyst, the activities in steam reforming cmmot be determined because the two regions (oxidation mad steam refonning) are not sufficiently discrete : Rh being a poor oxidation catalyst, the two reactions occur pratically at the same temperature. Increasing the severity the treatment in 02 induces a very important deactivation of the catalyst (FRESH, OX650 and OX800 in Fig.2). At low temperatures (T<650~ an oxidizing atmosphere leads only to the total oxidation of rhodium into Rh203 without creating a significant decrease of the surface active area [10, 13]. Above 650~ the treatment leads to a decrease of the accessible surface of rhodium on A1203, mainly linked to the fact that rhodium in its oxidized form (Rh3+) can diffuse easily inside the alumina matrix [10, 13-18]. However this diffusion of rhodium ions in alumina to form a "diffuse oxide phase" seems to be limited to a subsurface layer of about 20 A [ 10].
78 Conv. C3H8 (%) 100
o~e
:
j'
80" 60
40
///
2o! 0 350
%m-%,=--.--="::e t:~
// ,._-.--r
40O
450
dL
/'
//
'/"0/'
~, =
I
~
" ox6so I
d
:
,
500 550 600 650 Temperature (~
. ,<~D~oo I ." 700
750
800
Fig 2. 9Effects o f thermal treatments on the activities o f Rh/A Catalyst in propane conversion (0. 4%C31-18 + O.8%02)
After treatment in a reducing medium, the catalytic activity can be recovered progressively by extraction of the rhodium from the alumina [10, 13, 16]. But contrary to the surface rhodium oxide (Rh203), the Rh3+ ions contained in the "diffuse oxide phase" are difficult to reduce at low temperature (<500~ [13,15]. Recently Beck et al. [19] have showed that rhodium deactivation, in oxidizing atmosphere, could be due to the formation, at the catalyst surface, of an unusual rhodium oxide which is very difficult to reduce. Whatever the cause of deactivation ("diffuse oxide phase" or refractory rhodium oxide), high temperature treatments in H2 (900~ in Fig.3) are required to regenerate the catalyst.
3.2 Rh/CeA catalyst Figure 3 compares the light-off temperatures determined on Rh/A and Rh/CeA catalysts. CeO2 is known to stabilize noble metals [18] and probably limits here the rhodium migration into the support. Nevertheless, contrary to what was observed on Rh/A, a reducing treatment increases the deactivation of Rh/CeA. The most probable explanation is that a RhCexOy mixed oxide could be
79 formed at high temperature in a reducing medium [20, 21]. This mixed oxide should be inactive both in oxidation and in steam reforming.
Light-off Temp. (~ 700"/ B 65O
RhYA m Rh/CeA
~
[ ,-
6OO 55O 5OO 450 400 / FRESH
OX800 RED900 Thermal treatments
Fig 3. 9Light-off temperatures o f RhlA et RhlCeA after thermal treatment.
3.3 PtRh/A and PtRh/CeA catalysts The presence of Pt increases the oxidation activity so that the two regions (oxidation and steam reforming) are now clearly separated as shown in Fig. 1. The activities in oxidation at 200~ (fig. 4) and steam reforming at 300~ (fig 5) are determined for the two bimetallic catalysts before and after thermal treatments. These figures show the following points : (i) Fresh catalysts : in accordance with the literature, CeO2 is a promotor of steam reforming [5] and an inhibitor of C3H8 oxidation [2, 8, 9] (ii) OX800 catalysts "this treatment induces an enhancement of oxidation activities and a strong deactivation in steam reforming more particularly on A1203. An oxidizing treatment (02 or NO) at high temperature leads to a significant increase in the size of the platinum particles [22], which favors the oxidation activity [8, 9]. On the other hand, this treatment induces a surface segregation [22-24] depending essentially on the atomic ratio between Rh and Pt. Kacimi and Duprez [ 11 ] have shown, by isotopic exchange techniques, that in a PtRh/A1203 bimetallic series treated at 900~ in 1%O2 there was a definite enrichment in rhodium for an atomic content of Rh/Pt+Rh greater than 45%. Below this content, the opposite phenomenon occurs because most of the
80 rhodium ions migrate on (and probably in) the support to give the non-reducible form of rhodium that is inactive in catalysis [11 ]. (iii) CeO2 stabilizes the rhodium : the activity ratio between PtRh/CeA and PtRh/A in steam reforming after oxidizing treatment was close to 100 while it was only 2 in the fresh catalysts. (iv) the reducing treatment induces no significant changes in the oxidation activities. In fact, the measurement of the surface composition of a PtRh/A1203 catalyst which has been submitted to treatment in a reducing medium (H2 or CO) at high temperature shows a moderate increase of the size of the crystallites without any striking surface enrichment up to 1000~ [22, 23]. But contrary to the Rh/CeA catalyst, this treatment leads to a regeneration of rhodium in the PtRh/CeA catalyst. When both Rh and Pt are present, the tendency to form PtRh bimetallic particules instead of RhCexOy is favored.
Ox. Activity (mol h- 1 g- 1) 10-1 - ~, D m
S t e a m ref. Activity (mol h-1 g-1)
PtRh/A PtRh/CeA
m m
PtRh/A PtRh/CeA
10 -2
10 -a
10 -4
10-5 FRESH
OX800 RED900 Treatments
Fig 4 " Oxidation activities of PtRh catalysts: Effects of thermal treatments
FRESH
OX800 R E D 9 0 0 Treatments Fig 5 : Steam reforming activities PtRh catalysts: Effects of thermal treatments
81
3.4 Cyclopentane hydrogenolysis Figure 6 represents the curves "conversion" vs "temperature" for the fresh Pt/A, Rh/A, Rh/CeA, PtRh/A and PtRh/CeA catalysts. Table 2 gives the activities calculated at 200~ for the fresh (Xf200) and treated catalysts (X2oo). Rh being much more active than Pt, the activities X20o can be used to observe the rhodium deactivation. The results corroborate those obtained in steam reforming of propane : (i) a very important deactivation in hydrogenolysis after an oxidizing treatment at 800~ more particularly on alumina, (ii) a regenerative effect of the reducing medium for all the catalysts except for the Rh/CeA catalyst. Conv. C5H10 (%) 100 A Pt/A o Rh/A 80 v PtRh/A 9 Rh/CeA 6O g PtRh/CeA
/
/
40
/
20 ~
170
i
190
.
I
i
210
u.
i
i
230 250 270 Temperature (~
&
A A
&
i
290
i
310
330
Fig 6" Cyclopentan Hydrogenolysis on fresh catalysts.
CATALYSTS
Xf 200
Pt/A Rh/A PtRh/A Rh/CeA PtRh/CeA
1.25 42.5 35.0 11.25 22.5
X200 OX800 0.011 0.0034 0.1 0.23 1.1
RED900 0.009 4.3 7.0 0.11 3.4
Table 2 9 Cyclopentan hydrogenolysis activities (pmol C5Hlo per pulse per gram at 200~ on fresh and treated catalysts
82
3.5 Other supports In order to stabilize noble metals during rich or lean excursions at high temperature, the role o f three other supports have been studied. Tables 3 and 4 give the activities in oxidation and steam reforming of propane of PtRh bimetallic catalysts. CATALYSTS
PtRh/CeA PtRh/Z PtRh/ANi PtRh/CeANi
FRESH
mmol 8-1 h-1 0.076 1.2 0.6 0.6
Table 3 9Oxidation activities (at 200~
CATALYSTS
1.56
1.78 0.26 0.6
RED900
mmol g-1 h-1 4.2 0.54 0.06 0.3
of fresh and treated catalysts
FRESH
mmol ~;-1 h-1 PtRh/CeA PtRh/Z PtRh/ANi PtRh/CeANi
OX800
mmol g-1 h-1 9.0 1.8 16.0 9.0
OX800
mmol g-1 h-1 0.12 0.48 0.072 0.18
Table 4 9Steam reforming activities (at 300~
RED900
mmol g-1 h-1 0.39 0.024 0.26 0.54
of fresh and treated catalysts
In an oxidizing medium, zirconia seems to be the best support for stabilizing noble metals, there are low changes of the oxidation and steam reforming activities. But the steam reforming activity is strongly affected by a reducing treatment. During the rich or lean excursions, the two catalysts containing NiA1204 seem to present a very good stability of Rh with a minimal loss of steam reforming activity after regeneration in H2. But for those catalysts, platinum oxidative activity is very affected by a reducing treatment.
4. CONCLUSION
In A1203 supported catalysts, both ceria and platinum limit the deactivation of rhodium at high temperatures in an oxidizing medium. Nevertheless, while the rhodium in PtRh/CeA can be regenerated by a reducing treatment, Rh/CeA catalyst cannot be regenerated. Zirconia is a very efficient support for stabilization of metals and particularly rhodium in an oxidizing medium but the latter is deactived by a
83 reducing treatment. On the contrary, rhodium on NiAI204 presents a good stability both in an oxidizing and in an reducing atmosphere. But platinum is deactivated on this support after a reducing treatment.
ACKNOWLEDGEMENTS
This work was supported by the "Groupement de Recherche sur les Catalyseurs de Post-combustion" (IFP, ADEME, PIRSEM, CNRS). Thanks are due to IFP and ADEME for a grant (J.B.).
REFERENCES
7 8 9 10 11
12 13 14 15 16 17 18 19
K.C. Taylor, in A. Crucq et A. Frennet (editors), Catalysis and Automotive Pollution Control, Proc. 1st. Int. Symp., CAPOC 1, Brussels, Sept 8-11, 1986, Stud. Surf. Sci. Catal., Vol.30, Elsevier, Amsterdam (1987), 97. J. Barbier Jr. and D. Duprez, Appl. Catal. B : Environmental, 3, (1993), 61. J. Barbier Jr. and D. Duprez, Appl. Catal. A : General, 85, (1992), 89. J.C. Schlatter, SAE Techn. Pap. Ser. n ~ 780199. G. Kim, Ind. Eng. Chem. Prod. Res. Dev., 21, (1982), 267. J.C. Schlatter and P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev., 24, (1985), 43. D. Duprez, Appl. Catal., 82, (1992), 111. Y.-F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev., 19, (1980), 293. Y.-F. Yu Yao., J. Catal., 87, (1984), 152. D.D. Beck and C.J. Carr, J. Catal., 144, (1993), 296. S. Kacimi et D. Duprez, in A. Crucq (editor), Catalysis and Automotive Pollution Control II, Proc. 2nd. Int. Symp., CAPOC 2, Brussels, Sept 1013, 1990, Stud. Surf. Sci. Catal., Vol.71, Elsevier, Amsterdam, (1991), 581. G. Leclercq and R. Maurel, Bull. Soc. Chim. Fr., (1971), 1234. H.C. Yao, S. Japar and M. Shelef, J. Catal., 50, (1977), 407. H.C. Yao, H.K. Stepien and H.S. Gandhi, J. Catal., 61, (1980), 547. R.H. Harmnerle and C.H. Wu, SAE Techn. Pap. Ser. n ~ 840549. J.C. Summers and L.L. Hegedus, Ind. Eng. Chem. Prod. Res. Dev., 18, (1979), 318. T.H. Ballinger and J.T. Yates Jr., J. Phys. Chem., 95, n~ (1991), 1694. B. Harrison, A.F. Diwell and C. Hallet, Plat. Met. Rev., 32, (1988), 73. D.D. Beck, T.W. Caperhart, C. Wong, and D.N. Belton, J. Catal. 144, (1993), 311.
84 20 21 22 23 24
J.C. Summers and S.A. Ausen, J. Catal., 58, (1979), 131. F.L. Williams and M. Boudart, J. Catal., 33, (1973), 43 8. I. Onal, in C.H. Bartholomew and J.B. Butt (editors), Catalyst deactivation 1991, Stud. Surf. Sci. Catal., Vol 68, Elsevier, Amsterdam, (1991), 621. W.B. Williamson, H.S. Gandhi, P. Wynblatt, T.J. Truex and R.C. Ku, AIChE Symp. Ser., n~ 76, (1980), 212. F.C.M.J.M. Van Delft, B.E. Nieuwenhuys, J. Siera and R.M. Wolf, Iron and Steel Institute of Japan International, n~ 29, (1989), 550.
73
R E A C T I V I T Y OF STEAM IN E X H A U S T GAS CATALYSIS. PART II : SINTERING AND R E G E N E R A T I O N OF Rh AND PtRh CATALYSTS IN P R O P A N E O X I D A T I O N
J. Barbier Jr. and D. Duprez Laboratoire de Catalyse en Chimie Organique URA CNRS 350 40 av. du Recteur Pineau , 86022 POITIERS Cedex FRANCE Tel : (33) 49 45 39 98.
ABSTRACT PtRh catalysts were prepared on different supports composed of A1203, CeO2, ZrO2 and NiA1204. The variations of the activities in propane oxidation and steam reforming were used to obtain some indication concerning the surface state of these catalysts after thermal treatments at high temperature in an oxidizing and in a reducing medium. Cyclopentane hydrogenolysis was also carried out to observe the changes in the rhodium surface state. Platinum was the metal which catalysed the direct oxidation of propane while rhodium was the key-component in steam reforming catalysis. The treatment at 800~ in an oxidizing medium induces a very important decrease of Rh area linked to the fact that rhodium in its oxidized form (Rh 3+) can diffuse into the support. This phenomenon is more marked on A1203 and, to a lesser extent on CeO2-A1203, than on the other supports. On the other hand, oxidative treatments lead to an increase of the particle size of platinum, and temporarily to an enhancement of the oxidation activities. After a treatment in a reducing medium at high temperature (T>700~ the steam reforming activities can be recovered by extraction of rhodium from the support, with two exceptions however : Rh/CeO2-A1203 and PtRh/ZrO2 catalysts. This treatment has pratically no impact on platinum activities in oxidation except for the catalysts supported on NiA1204 which are deactived. Cyclopentane hydrogenolysis confirm all the above results.
1. INTRODUCTION PtRh catalysts are COlmnonly used in catalytic converters for eliminating pollutants (CO, hydrocarbons, NOx) from exhaust gases [1]. In the first part of our w o r k [2], we investigated, on P t ( l w t . - % ) , Rh(0.2wt.-%) and
74 Pt(lwt%)Rh(0.2wt%) catalysts supported on A1203 and CeO2-A1203, carbon monoxide and propane oxidation by oxygen (direct oxidation), by steam (water gas shift and steam reforming) and by a mixture of oxygen and steam (oxy-WGS and oxy-steam reforming). Steam can be considered a cor-eactant of oxidation during rich-phases (lean in O2) [3-5]. In oxy-steam conversion of propane, we showed (fig. 1) that propane oxidation was catalyzed by platinum (between 200 and 350~ while rhodium was the key-component in the catalysis of steam reforming (between 350 and 600~ Ceria was an excellent promotor of steam reactions [3, 6], particularly when this reaction was carried out in the presence of oxygen. Therefore, the steam reforming activity is an excellent indicator of the rhodium surface state since the activity systematically decreases when the metallic rhodium area decreases [7]. On the other hand, oxidation activity is a more complex indicator of platinum surface state because there exists an optimum dispersion [8, 9]. Metal sintering constitutes a very significant cause of loss in catalyst activity owing to the reduction in the metallic area [10,11 ]. The aim of this work is to study the sintering of Rh and PtRh catalysts by means of oxidation and steam reforming activities in propane conversion. Changes in the steam reforming activity have been compared with those obtained in cyclopentane hydrogenolysis, a reaction extremely sensitive to the state of Rh in the catalyst [12]. C3H8 conv. (%) 100 -
El E
80 Direct oxidation
[]
60
.
Steam reforming
[] [] El
9149 I
40 20
00
[]
,..--.~...~_~ .m 0 ~!,
200
[] [] I
I
300 400 Temperature (~
I
500
600
Fig 1 9 Oxy-steam reforming o f propane on PtRh/CeO2-AI203
75 2. E X P E R I M E N T A L
2.1 Catalysts Five different supports were used : (i) a gamma-alumina (120m2 g-l) which was impregnated with an aqueous solution of cerium nitrate to obtain (ii) a 12wt.-%CeO2-A1203 after calcination (100m2 g-l); (iii) a zirconia (40m2 g-l) supplied by Degussa, (iv) a support prepared by coimpregnation of a Ni(NO3)2 and an Al(NO3)3 aqueous solution on an alumina (200m2 g-l) so as to obtain after calcination (1000~ air, 24h) a 8.5wt.-%NiA1204-A1203 support and this support is impregnated with an aqueous solution of cerium nitrate to obtain (v) a 12wt.-%CeO2-8.5wt.-%NiA1204-A1203 support after calcination. These supports (A1203, CeO2-A1203, ZrO2, NiA1204-A1203 and CeO2-NiA1204-AI203) were crushed and sieved to 0.1-0.2 mm. They were used to prepare two series of catalysts by impregnation or coimpregnation with aqueous solutions of rhodium chloride and chloroplatinic acid. The catalysts were dried at 120~ then calcined at 500~ under an air flow and prereduced in H2 at 450~ The metal loadings were Rh(0.2wt.-%) and Pt(lwt.-%)Rh(0.2wt.-%). A Pt(lwt.-%) on gamma-altmaina (120m 2 g-l, 0.10.2mm) supplied by I.F.P. (French Institute of Petroleum) was also used for certain experiments. Table 1: Catalyst co m positions Pt Symbolic (wt.-%) Name 1 Pt/A Rh/A 0 0 Rh/CeA 1 PtRh/A 1 PtRh/CeA 1 PtRh/Z 1 PtRh/ANi PtRh/CeANi 1
Rh
Support
(wt.-%) 0 0.2 0.2 0.2 0.2 0.2 0.2 0.2
A1203 A1203 A1203 +CeO2 (12wt.-%) A1203 A1203 +CeO2(12wt.-%) ZrO2 A1203+A1204Ni(8.5%) A1203+A1204Ni(8.5%)+CEO2(12%)
76 2.2 Catalytic reaction
Before each run the catalyst samples were heated in air for l h at 450~ Propane oxidation was carried out in a flow reactor under the following conditions : (i) catalyst bed : 40mg diluted in 360 mg of eordierite (0.1-0.2 mm). (ii) Feed gas (in vol-%) : C3H8, 0.4 ; O2, 0.8; N2, 98.8. (iii) Gas flow-rate : 380em 3 min-1 (volume space velocity : 250,000h-1). Temperature-programmed reactions were carried out from 150 to 900~ using a 4~ min-1 temperature ramp (1 atm.). Analyses were carried out by gas chromatography : CO2, CH4 and C3H8 on Porapak Q (0.7m, 1/4 in.; 25~ or 100~ cartier gas HE), CO, O2, N2 and CH4 on molecular sieve 5A (0.4m, 1/4 in.; 25~ carrier gas H2), H2 on molecular sieve 5A (lm, 1/4 in.; 25~ carrier gas N2). C3H8 conversions were determined from the mass balance of carbon-containing products and verified by the disappearance of the C3H8 peaks in the ehromatograms. The main reactions considered here were direct oxidation (1), C3H8 steam reforming (2) and W.G.S. (3). C3H8 + 502 . . . . . > 3CO2 + 4H20 (1) C3H8 + 3H20 . . . . . > 3CO + 7H2 (2) CO + H20 . . . . . > CO2 + H2 (3) Reactions (2) and (3) occurred via the water produced in reaction (1) : even in the absence of steam in the inlet gases, typical light-off curves like those in Fig. 1 were obtained. Specific activities and activation energies in oxidation and steam reforming were determined at low conversion. Specific activities were calculated at 200~ for direct oxidation and at 300~ for steam reforming. 2.3 Sintering and regenerating conditions
The oxy-steam reforming reaction was carried out on : - flesh catalysts (450~ 02 (3vol.-%); lh)) (FRESH) - oxidized catalysts (650~ or 800~ 02 (3vol.-%); lh) (OX650 or
ox800), - oxidized (800~
or 900~
O2 (3vol.-%); lh) and then reduced catalysts (700, 800 H2 (3vol.-%); lh)(RED700 or 800 or 900).
77
2.4 Cyclopentane hydrogenolysis This model reaction was carried out "in situ" on flesh or sintered catalysts (OX800 and RED900). The catalysts were prereduced in H2 (lh, 300~ 30cm3 min-1). Cyclopentane hydrogenolysis was performed in a pulse flow reactor under the following conditions : (i) catalyst bed : 40mg diluted in 360mg of cordierite (0.10.2 mm); (ii) cyclopentane injection : 11 lamole per pulse; (iii) hydrogen flow rate: 30cm3min-1; (iv) temperature range 170-330~ Under these conditions, the only reaction product was n-pentane analyzed by gas chromatography on a reoplex 400 column (2m, 1/8 in., 50~ carrier gas H2). The specific activities in cyclopentane hydrogenolysis of catalysts were calculated at 200~
3. RESULTS AND DISCUSSIONS
3.1 Rh/A catalyst Figure 3 shows the light-off curves of Rh/A after different treatments. On this catalyst, the activities in steam reforming cmmot be determined because the two regions (oxidation mad steam refonning) are not sufficiently discrete : Rh being a poor oxidation catalyst, the two reactions occur pratically at the same temperature. Increasing the severity the treatment in 02 induces a very important deactivation of the catalyst (FRESH, OX650 and OX800 in Fig.2). At low temperatures (T<650~ an oxidizing atmosphere leads only to the total oxidation of rhodium into Rh203 without creating a significant decrease of the surface active area [10, 13]. Above 650~ the treatment leads to a decrease of the accessible surface of rhodium on A1203, mainly linked to the fact that rhodium in its oxidized form (Rh3+) can diffuse easily inside the alumina matrix [10, 13-18]. However this diffusion of rhodium ions in alumina to form a "diffuse oxide phase" seems to be limited to a subsurface layer of about 20 A [ 10].
78 Conv. C3H8 (%) 100
o~e
:
j'
80" 60
40
///
2o! 0 350
%m-%,=--.--="::e t:~
// ,._-.--r
40O
450
dL
/'
//
'/"0/'
~, =
I
~
" ox6so I
d
:
,
500 550 600 650 Temperature (~
. ,<~D~oo I ." 700
750
800
Fig 2. 9Effects o f thermal treatments on the activities o f Rh/A Catalyst in propane conversion (0. 4%C31-18 + O.8%02)
After treatment in a reducing medium, the catalytic activity can be recovered progressively by extraction of the rhodium from the alumina [10, 13, 16]. But contrary to the surface rhodium oxide (Rh203), the Rh3+ ions contained in the "diffuse oxide phase" are difficult to reduce at low temperature (<500~ [13,15]. Recently Beck et al. [19] have showed that rhodium deactivation, in oxidizing atmosphere, could be due to the formation, at the catalyst surface, of an unusual rhodium oxide which is very difficult to reduce. Whatever the cause of deactivation ("diffuse oxide phase" or refractory rhodium oxide), high temperature treatments in H2 (900~ in Fig.3) are required to regenerate the catalyst.
3.2 Rh/CeA catalyst Figure 3 compares the light-off temperatures determined on Rh/A and Rh/CeA catalysts. CeO2 is known to stabilize noble metals [18] and probably limits here the rhodium migration into the support. Nevertheless, contrary to what was observed on Rh/A, a reducing treatment increases the deactivation of Rh/CeA. The most probable explanation is that a RhCexOy mixed oxide could be
79 formed at high temperature in a reducing medium [20, 21]. This mixed oxide should be inactive both in oxidation and in steam reforming.
Light-off Temp. (~ 700"/ B 65O
RhYA m Rh/CeA
~
[ ,-
6OO 55O 5OO 450 400 / FRESH
OX800 RED900 Thermal treatments
Fig 3. 9Light-off temperatures o f RhlA et RhlCeA after thermal treatment.
3.3 PtRh/A and PtRh/CeA catalysts The presence of Pt increases the oxidation activity so that the two regions (oxidation and steam reforming) are now clearly separated as shown in Fig. 1. The activities in oxidation at 200~ (fig. 4) and steam reforming at 300~ (fig 5) are determined for the two bimetallic catalysts before and after thermal treatments. These figures show the following points : (i) Fresh catalysts : in accordance with the literature, CeO2 is a promotor of steam reforming [5] and an inhibitor of C3H8 oxidation [2, 8, 9] (ii) OX800 catalysts "this treatment induces an enhancement of oxidation activities and a strong deactivation in steam reforming more particularly on A1203. An oxidizing treatment (02 or NO) at high temperature leads to a significant increase in the size of the platinum particles [22], which favors the oxidation activity [8, 9]. On the other hand, this treatment induces a surface segregation [22-24] depending essentially on the atomic ratio between Rh and Pt. Kacimi and Duprez [ 11 ] have shown, by isotopic exchange techniques, that in a PtRh/A1203 bimetallic series treated at 900~ in 1%O2 there was a definite enrichment in rhodium for an atomic content of Rh/Pt+Rh greater than 45%. Below this content, the opposite phenomenon occurs because most of the
80 rhodium ions migrate on (and probably in) the support to give the non-reducible form of rhodium that is inactive in catalysis [11 ]. (iii) CeO2 stabilizes the rhodium : the activity ratio between PtRh/CeA and PtRh/A in steam reforming after oxidizing treatment was close to 100 while it was only 2 in the fresh catalysts. (iv) the reducing treatment induces no significant changes in the oxidation activities. In fact, the measurement of the surface composition of a PtRh/A1203 catalyst which has been submitted to treatment in a reducing medium (H2 or CO) at high temperature shows a moderate increase of the size of the crystallites without any striking surface enrichment up to 1000~ [22, 23]. But contrary to the Rh/CeA catalyst, this treatment leads to a regeneration of rhodium in the PtRh/CeA catalyst. When both Rh and Pt are present, the tendency to form PtRh bimetallic particules instead of RhCexOy is favored.
Ox. Activity (mol h- 1 g- 1) 10-1 - ~, D m
S t e a m ref. Activity (mol h-1 g-1)
PtRh/A PtRh/CeA
m m
PtRh/A PtRh/CeA
10 -2
10 -a
10 -4
10-5 FRESH
OX800 RED900 Treatments
Fig 4 " Oxidation activities of PtRh catalysts: Effects of thermal treatments
FRESH
OX800 R E D 9 0 0 Treatments Fig 5 : Steam reforming activities PtRh catalysts: Effects of thermal treatments
81
3.4 Cyclopentane hydrogenolysis Figure 6 represents the curves "conversion" vs "temperature" for the fresh Pt/A, Rh/A, Rh/CeA, PtRh/A and PtRh/CeA catalysts. Table 2 gives the activities calculated at 200~ for the fresh (Xf200) and treated catalysts (X2oo). Rh being much more active than Pt, the activities X20o can be used to observe the rhodium deactivation. The results corroborate those obtained in steam reforming of propane : (i) a very important deactivation in hydrogenolysis after an oxidizing treatment at 800~ more particularly on alumina, (ii) a regenerative effect of the reducing medium for all the catalysts except for the Rh/CeA catalyst. Conv. C5H10 (%) 100 A Pt/A o Rh/A 80 v PtRh/A 9 Rh/CeA 6O g PtRh/CeA
/
/
40
/
20 ~
170
i
190
.
I
i
210
u.
i
i
230 250 270 Temperature (~
&
A A
&
i
290
i
310
330
Fig 6" Cyclopentan Hydrogenolysis on fresh catalysts.
CATALYSTS
Xf 200
Pt/A Rh/A PtRh/A Rh/CeA PtRh/CeA
1.25 42.5 35.0 11.25 22.5
X200 OX800 0.011 0.0034 0.1 0.23 1.1
RED900 0.009 4.3 7.0 0.11 3.4
Table 2 9 Cyclopentan hydrogenolysis activities (pmol C5Hlo per pulse per gram at 200~ on fresh and treated catalysts
82
3.5 Other supports In order to stabilize noble metals during rich or lean excursions at high temperature, the role o f three other supports have been studied. Tables 3 and 4 give the activities in oxidation and steam reforming of propane of PtRh bimetallic catalysts. CATALYSTS
PtRh/CeA PtRh/Z PtRh/ANi PtRh/CeANi
FRESH
mmol 8-1 h-1 0.076 1.2 0.6 0.6
Table 3 9Oxidation activities (at 200~
CATALYSTS
1.56
1.78 0.26 0.6
RED900
mmol g-1 h-1 4.2 0.54 0.06 0.3
of fresh and treated catalysts
FRESH
mmol ~;-1 h-1 PtRh/CeA PtRh/Z PtRh/ANi PtRh/CeANi
OX800
mmol g-1 h-1 9.0 1.8 16.0 9.0
OX800
mmol g-1 h-1 0.12 0.48 0.072 0.18
Table 4 9Steam reforming activities (at 300~
RED900
mmol g-1 h-1 0.39 0.024 0.26 0.54
of fresh and treated catalysts
In an oxidizing medium, zirconia seems to be the best support for stabilizing noble metals, there are low changes of the oxidation and steam reforming activities. But the steam reforming activity is strongly affected by a reducing treatment. During the rich or lean excursions, the two catalysts containing NiA1204 seem to present a very good stability of Rh with a minimal loss of steam reforming activity after regeneration in H2. But for those catalysts, platinum oxidative activity is very affected by a reducing treatment.
4. CONCLUSION
In A1203 supported catalysts, both ceria and platinum limit the deactivation of rhodium at high temperatures in an oxidizing medium. Nevertheless, while the rhodium in PtRh/CeA can be regenerated by a reducing treatment, Rh/CeA catalyst cannot be regenerated. Zirconia is a very efficient support for stabilization of metals and particularly rhodium in an oxidizing medium but the latter is deactived by a
83 reducing treatment. On the contrary, rhodium on NiAI204 presents a good stability both in an oxidizing and in an reducing atmosphere. But platinum is deactivated on this support after a reducing treatment.
ACKNOWLEDGEMENTS
This work was supported by the "Groupement de Recherche sur les Catalyseurs de Post-combustion" (IFP, ADEME, PIRSEM, CNRS). Thanks are due to IFP and ADEME for a grant (J.B.).
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7 8 9 10 11
12 13 14 15 16 17 18 19
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