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CeO2–ZrO2 catalysts
Catalysis Today 45 (1998) 179±183
CO oxidation on Pd/CeO2±ZrO2 catalysts Elena Bekyarova1, Paolo Fornasiero, Jan KasÏpar*, Mauro Graziani Dipartimento di Scienze Chimiche, UniversitaÁ di Trieste, via L.Giorgieri, 1, 34127 Trieste, Italy
Abstract The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for puri®cation of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO2±Pd on the kinetics of CO oxidation over Pd/CeO2±ZrO2. Under reducing-to-moderately oxidizing conditions, a zero-order O2 pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO2±ZrO2 support, as determined from temperatureprogrammed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior. # 1998 Elsevier Science B.V. All rights reserved. Keywords: CO oxidation; CeO2±ZrO2 solid solution; Redox properties
1.
Introduction
The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for puri®cation of motor exhaust gases have been widely investigated in recent years. Despite this intensive work, the exact mechanism by which CeO2 promotes the TWCs is not well understood and multiple effects were ascribed to this component. Perhaps, the most important of these is the oxygen storage capacity (OSC) of CeO2 which is of technical importance in TWCs, because it acts as an oxygen partial-pressure regulator, keeping the reductant/oxidant ratio in the exhaust close to the stoichiometric value. High surface area of the CeO2-based support is very important, since the redox processes essentially occur on the surface, at exhaust temperatures (770 K). Accordingly, thermal stability of the *Corresponding author. Fax.: +33-40-6763903; e-mail: [email protected] 1 Bulgarian Academy of Sciences, Sofia, Bulgaria.
CeO2 surface area is a desirable property for these materials. Recent investigations have shown that addition of ZrO2 to CeO2 improves the thermal stability. In addition, the impregnation of ZrO2 in the CeO2 lattice strongly promotes bulk reduction at low temperatures [1] and it was shown that, among Rh/CeO2±ZrO2 solid solutions [2], those containing 40±60 mol% of CeO2 gave the highest degree of support reduction and the lowest reduction temperature. Accordingly, we have chosen Ce0.6Zr0.4O2 as a support for Pd-based catalysts in our study. At present, Pd is a metal of choice in the TWCs due to its ability to promote HC and CO removal during a cold start of the engine. Several investigations of precious metal (PM)loaded catalysts showed that the kinetics of CO oxidation is affected by the presence of ceria. The CeO2-induced changes in the kinetics lead to enhancement of the CO oxidation activity and have been often interpreted on the basis of a mechanism involving a reaction between CO adsorbed on PM and the surface oxygen from the CeO2 [3±5]. CO oxidation was
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suggested to occur on new sites at the PM/cerium interface [6]. It is well known that the favoured mechanism of CO oxidation over PM (Rh, Pt, Pd) is the interaction between adsorbed O2 and CO (Langmuir±Hinshelwood). The kinetics of the reaction has been examined, and a ®rst-order in O2 and inverse ®rst-order mechanism in CO found [3]. Under moderately oxidising or net reducing conditions, addition of cerium oxide to the catalysts results in changes in CO oxidation kinetics including suppression of the CO inhibition effect, decreasing sensitivity of the reaction rate to the gas-phase oxygen concentration and a reduced apparent activation energy [5]. It is proposed that these changes are due to the CeO2-mediated mechanism, which become more important: (i) for small PM particles [4]; (ii) at low temperatures [4,5]; (iii) at high CO partial pressures [4,5]; and (iv) at low O2 partial pressures [4]. According to Oh and Eickel [5], the kinetics in a strongly oxidising environment over low-loaded Rh/Al2O3 catalysts are not signi®cantly affected by the presence of ceria. Finally, it should be noted that the activity of CeO2-based catalysts is a function of the surface morphology [6,7], which depends on the synthesis of the catalysts. This makes comparison of the different studies dif®cult. 2.
Experimental
Ce0.6Zr0.4O2 was synthesised by a citrate route from a water solution [8]. Metal impregnation was carried out by the incipient wetness method using aqueous solutions of Pd(NO3)2. Nominal Pd loading was 0.5 wt%. Temperature-programmed reduction (TPR) and oxygen-uptake measurements were carried out as previously described [8]. CO oxidation was carried out in a ¯ow microreactor. Typically, 50 mg of catalyst were loaded between two layers of granular quartz which acted as a pre-heater. The analyses were carried out by on-line gas liquid chromatograph using a Porapak Q wide bore capillary column. 3. 3.1.
Results and discussion Characterisation and redox behaviour of Pd-loaded and metal-free Ce0.6Zr0.4O2
The Ce0.6Zr0.4O2 obtained has a BET surface area of 70 m2 gÿ1. The N2 adsorption isotherm at 77 K is of
Fig. 1. TPR profiles of (1) fresh Ce0.6Zr0.4O2, (2), sample from run 1 oxidised at 700 K, (3) fresh Pd/Ce0.6Zr0.4O2 and (4) sample from run 3 subjected to four consecutive TPR/oxidation treatments at 700 K.
type IV with H3 hysteresis according to IUPAC classi®cation [9], which is characteristic of a welldeveloped mesoporous texture. The Raman spectrum (not reported) of Ce0.6Zr0.4O2 features a strong peak centred around 475 cmÿ1, which is characteristic of a cubic ¯uorite phase (space group Fm3m). However, small peaks at 313 and 146 cmÿ1 are observed, suggesting an attribution to a t00 phase according to the classi®cation by Yashima et al. [10]. Upon Pd-loading, no signi®cant variation of either texture or phase nature could be detected by N2 physisorption or Raman spectroscopy. The redox behaviour of metal-free and Pd/ Ce0.6Zr0.4O2 was investigated by means of TPR and oxygen-uptake measurements. The TPR results are presented in Fig. 1. The TPR pro®le of the fresh Ce0.6Zr0.4O2 features a strong reduction peak at 834 K and a shoulder at ca. 720 K. The relatively low intensity of this shoulder, compared to the main reduction peak, suggests its attribution to some kind of sample inhomogeneity such as partial tetragonalisation not detectable from the broad XRD patterns of the fresh samples, rather than to a low temperature surface-reduction process as in CeO2 [11]. In fact, the reduction in the bulk of the solid solution is strongly promoted by insertion of ZrO2 into the CeO2 lattice. This makes the reduction of the surface, and that in the bulk, indistinguishable by TPR, since they both occur
E. Bekyarova et al. / Catalysis Today 45 (1998) 179±183
almost at the same temperature [8,12]. Distortion of the oxygen sublattice as detected by Raman and EXAFS spectroscopies, which increases the oxygen mobility in the bulk, was invoked to explain this result [13]. Accordingly, computer simulation studies showed comparable energy for surface and bulk reduction [14]. Generally speaking, CeO2 extensively sinters during the TPR experiment to a ®nal surface area <10 m2 gÿ1. This leads to a complete loss of the OSC at low temperatures [12]. To investigate these effects, the sample was oxidised at 700 K and then subjected to a TPR. The peak at 834 K shifts to a higher temperature, in contrast to the shoulder at 720 K, which is not shifted. Subsequent redox cycling did not modify the TPR behaviour further. Oxygen uptakes of 17±18 ml gÿ1 were measured in the oxidation at 700 K carried out after the TPR, which corresponds to a Ce4/Ce3 reduction extent of 77±81%. By loading Ce0.6Zr0.4O2 with Pd, the reduction behaviour is strongly modi®ed. Fresh Pd/Ce0.6Zr0.4O2 features two peaks, at 360 and 805 K (Fig. 1, trace 3). The former peak could be associated with reduction of the palladium oxide precursor; however, the magnitude of this peaks in comparison to the relatively low metal loading suggests that a concomitant partial reduction of the support is occurring. Remarkably, the peak at 805 K which should be attributed to the reduction in the bulk, appears shifted to a lower temperature as compared to unloaded Ce0.6Zr0.4O2. After oxidation at 700 K, the following TPR features a peak at 500 K with a shoulder at 480 K. Repetitive reduction/oxidation cycles shift the peak to ca. 580 K, the ®nal TPR pro®le being shown in the trace 4 of Fig. 1. Note that the shift of the reduction to low temperatures, in the presence of Pd, is found in both, fresh and redox aged samples. This indicates that the kind of the interaction between the noble metal and the support, responsible for the improved reduction behaviour, does not depend on the extent of surface area. The oxygen uptake at 700 K does not change signi®cantly compared to the bare support indicating that only the kinetics rather than thermodynamics of the reduction process is affected by the supported Pd. Note that the contribution of the Pd/PdO redox couple to the OSC is relatively small ( 0:5 ml gÿ1 cat ) compared to the overall amount of O2 exchanged. Summarising, it appears that Pd-loaded Ce0.6 Zr0.4O2 shows the following important characteristics
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with respect to CO oxidation: (i) an initial high surface area; (ii) a high OSC value, which is closely associated with the reduction behaviour of the support; (iii) a high degree of bulk reduction at low temperatures, and (iv) a strong effect of the supported Pd on the redox behaviour which is not deactivated by redox ageing. 3.2.
Catalytic measurements
To investigate the participation of the support in the CO oxidation, the activity of Pd-loaded and metal-free Ce0.6Zr0.4O2 was investigated in a ¯ow reactor. Prereduction of M/CeO2 improves the activity on the CO oxidation [15]. Accordingly, Pd/Ce0.6Zr0.4O2 was reduced at 500 K before the catalytic experiments. As shown in Table 1, addition of Pd to the Ce0.6 Zr0.4O2 signi®cantly improves the activity as compared to the metal-free sample. All the T10% values (temperature for a 10% CO conversion) are, in fact, lowered by 70±90 K at all the reaction conditions investigated, e.g. rich, stoichiometric and lean. Remarkably, equal T10% are measured under rich and stoichiometric conditions for both, the Pd-loaded and metal-free catalyst. A CO oxidation rate of ÿ1 is measured at 500 K over Pd/ 8 10ÿ6 mol gÿ1 cat s Ce0.6Zr0.4O2, which is comparable to the value ÿ1 reported by Yu Yao [3] for 9:1 10ÿ6 mol gÿ1 cat s Pd/CeO2/Al2O3 at 523 K. The activation energies for CO oxidation on metalfree and Pd loaded samples are given in Table 1. Presence of Pd leads to high Ea under reducing and stoichiometric conditions and low Ea under oxidising condition compared to Ce0.6Zr0.4O2. It should be noted however, that due to the different activities, the apparent Ea were obtained in different ranges of temperatures which does not allow to make a direct comparison. The Ea observed on Pd/Ce0.6Zr0.4O2 under rich/stoichiometric conditions are rather high when compared to those of a Pd wire (125 kJ/mol). Generally speaking, a decrease in Ea is observed with CeO2-containing catalysts as compared to bulk PM or PM supported on CeO2-free supports [3,5], under the conditions (stoichiometric and reducing) for which we have established Ea somewhat higher to that of a Pd wire. The differences can be ascribed to the difference in surface nature of the supports. Oh and Eickel [5] have used an alumina support impregnated with ceria, while
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Fig. 2. CO conversion on (a) Ce0.6Zr0.4O2 at (1) CO: O 1 : 2, (2) CO : O1 : 1 and (3) CO : O2 : 1; and (b) Pd/Ce0.6Zr0.4O2 at (1) CO : O1 : 2, (2) CO : O1 : 1 and (3) CO : O2 :1. Partial pressures as reported in Table 1.
the support in our investigation is a CeO2±ZrO2 solid solution. The CO conversion vs. temperature curves for Ce0.6Zr0.4O2 (Fig. 2, Table 1) indicate a slow increase of conversion with the rise in temperature. The CO conversion curves for bulk Pd typically show the lightoff phenomena [3]. Accordingly, those observed on Pd/Ce0.6Zr0.4O2 (Fig. 2) can be considered as composed of both types of curves. Both differences, in the type of conversion curves and in Ea, suggest that CO oxidation occurs at different sites on metal-free and Pd-loaded Ce0.6Zr0.4O2. The kinetics of CO oxidation con®rm this hypothesis. Note that we have investigated the dependence of the reaction rate on the O2 partial pressure primarily under stoichiometric/oxidising conditions. Strongly reducing conditions, such as those employed previously [16], are rarely met under practical TWC applications. Table 1 CO oxidation activity of CeO6Zr0.4O2 and Pd/Ce0.6Zr0.4O2 Catalyst
Reaction conditions a
Ea b
T10% c (K)
Ce0.6Zr0.4O2
rich stoichiometric lean rich stoichiometric lean
87 97 176 163 175 82
598 600 587 533 533 495
Pd/Ce0.6Zr0.4O2
a
pCO23 torr; rich conditions: pO2 6 torr; stoichiometric conditions: pO2 11.5 torr; lean conditions: pO2 23 torr. b Measured in the range of temperatures 500±600 and 500±530 K, respectively, for Ce0.6Zr0.4O2 and Pd/Ce0.6Zr0.4O2. c Temperature of a 10% CO conversion.
Fig. 3. CO oxidation rate on Pd/Ce0.6Zr0.4O2 at 493 K. The lines are added only as a guide for the eye.
The results on Pd/Ce0.6Zr0.4O2 are presented in Fig. 3. It should be noted that, on Ce0.6Zr0.4O2 (the results are not reported in the ®gure), the CO oxidation rate is approximately of zero order in O2 and of 0.7 order in CO. No changes in the kinetics were observed for broad partial pressure ranges of O2 (5±35 torr) and CO (6±29 torr). At 5-torr CO pressure, the reaction is of 0.8 order in O2 for the whole investigated range of O2 pressure (5± 35 torr). At 11 torr CO pressure, the O2 order is 0.4, while at 22 torr of CO partial pressure a zero order in O2 is observed for the whole range of O2 pressure (5± 35 torr). Also the dependence of the reaction rate on the CO pressure changes as it can be observed in
E. Bekyarova et al. / Catalysis Today 45 (1998) 179±183
Fig. 3. The relatively constant values of reaction rate observed at pO2 10 torr indicate that, at these low O2 partial pressures, there is a zero-order dependence on the CO pressure, while at higher O2 pressures a positive CO dependence is observed. The above ®nding can be explained by assuming that, at high CO partial pressures, the reaction is dominated by a second CeO2±ZrO2 mediated mechanism, as has been proposed by other authors for CeO2 [3±5,7,17]. At high CO pressures, the Pd surface is covered with CO which, in turn, inhibits O2 adsorption, as a result of which oxygen migration from the CeO2-based support becomes responsible for CO2 formation. In this case, Ce3/Ce4 oxidation by the oxygen from the gas phase takes place. It is noteworthy that we observe the support-mediated reaction at an O2 partial pressure which is remarkably high compared to 0.3 torr reported for Pd/CeO2 by Bunluesin et al. [16]. We suggest that the high ef®ciency of the Ce3/Ce4 redox couple in the Ce0.6Zr0.4O2 makes large amounts of weakly adsorbed oxygen available at the support surface [18]. This oxygen is then able to compete with the gaseous O2 even at relatively high O2 pressures. At low CO pressures, competition of O2 on the Pd surface may occur, and lead to a typical positive pressure dependence observed for the Pd. Note that, on Pd/ ZrO2, no evidence for oxygen transfer from the support were found except when metal alloys were employed as catalyst precursors [19]. 4.
Conclusions
In summary, our investigation of CO oxidation on Pd/Ce0.6Zr0.4O2 shows that, at a relatively modest CO pressure and under stoichiometric-to-moderately oxidising conditions, the CeO2-mediated mechanism is favoured, a fact that can be attributed to the improved redox performance of the Ce0.6Zr0.4O2 support in comparison to CeO2. In contrast, Yu Yao [3] and Oh and Eickel [5] have established that under oxidising conditions the kinetics are not signi®cantly affected by the presence of CeO2. Although Yu Yao has reported the kinetics to
183
be nearly independent of oxygen pressure in the presence of excess O2 for reduced Pd/CeO2/Al2O3, he has noticed instability of the reduced state. Acknowledgements CNR (Roma), MURST 40% (Roma) and UniversitaÁ di Trieste are acknowledged for ®nancial support. One of us (Elena Bekyarova) thanks the University of Trieste for a grant within the ``Legge Aree di Con®ne'' Programme. References [1] G. Balducci, P. Fornasiero, R. Di Monte, J. KasÏpar, S. Meriani, M. Graziani, Catal. Lett. 33 (1995) 193. [2] P. Fornasiero, R. Di Monte, G. Ranga Rao, J. KasÏpar, S. Meriani, A. Trovarelli, M. Graziani, J. Catal. 151 (1995) 168. [3] Y.F. Yu Yao, J. Catal. 87 (1984) 152. [4] T. Bunluesin, H. Cordatos, R.J. Gorte, J. Catal. 157 (1995) 222. [5] S.H. Oh, C.C. Eickel, J. Catal. 112 (1988) 543. [6] C. Hardacre, R.M. Ormerod, R.M. Lambert, J. Phys. Chem. 98 (1994) 10901. [7] A. Tschope, W. Liu, M. Flytzani-Stephanopoulos, J.Y. Ying, J. Catal. 157 (1995) 42. [8] P. Vidmar, P. Fornasiero, J. KasÏpar, M. Graziani, J. Catal. 171 (1997) 160. [9] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Sieminiewska, Pure Appl. Chem. 57 (1985) 603. [10] M. Yashima, H. Arashi, M. Kakihana, M. Yoshimura, J. Am. Ceram. Soc. 77 (1994) 1067. [11] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [12] P. Fornasiero, G. Balducci, R. Di Monte, J. KasÏpar, V. Sergo, G. Gubitosa, A. Ferrero, M. Graziani, J. Catal. 164 (1996) 173. [13] G. Vlaic, P. Fornasiero, S. Geremia, J. KasÏpar, M. Graziani, J. Catal. 168 (1997) 386. [14] G. Balducci, J. KasÏpar, P. Fornasiero, M. Graziani, M.S. Islam, J.D. Gale, J. Phys. Chem. B 101 (1997) 1750. [15] C. Serre, F. Garin, G. Belot, G. Maire, J. Catal. 141 (1993) 9. [16] T. Bunluesin, E.S. Putna, R.J. Gorte, Catal. Lett. 41 (1996) 1. [17] G.S. Zafiris, J. Gorte, J. Catal. 86 (1991) 86. [18] E.S. Putna, J.M. Vohs, R.J. Gorte, Catal. Lett. 45 (1997) 143. [19] A. Baiker, D. Gasser, J. Lenzner, A. Reller, R. Schloegl, J. Catal. 126 (1990) 555.
CO oxidation on Pd/CeO2±ZrO2 catalysts Elena Bekyarova1, Paolo Fornasiero, Jan KasÏpar*, Mauro Graziani Dipartimento di Scienze Chimiche, UniversitaÁ di Trieste, via L.Giorgieri, 1, 34127 Trieste, Italy
Abstract The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for puri®cation of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO2±Pd on the kinetics of CO oxidation over Pd/CeO2±ZrO2. Under reducing-to-moderately oxidizing conditions, a zero-order O2 pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO2±ZrO2 support, as determined from temperatureprogrammed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior. # 1998 Elsevier Science B.V. All rights reserved. Keywords: CO oxidation; CeO2±ZrO2 solid solution; Redox properties
1.
Introduction
The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for puri®cation of motor exhaust gases have been widely investigated in recent years. Despite this intensive work, the exact mechanism by which CeO2 promotes the TWCs is not well understood and multiple effects were ascribed to this component. Perhaps, the most important of these is the oxygen storage capacity (OSC) of CeO2 which is of technical importance in TWCs, because it acts as an oxygen partial-pressure regulator, keeping the reductant/oxidant ratio in the exhaust close to the stoichiometric value. High surface area of the CeO2-based support is very important, since the redox processes essentially occur on the surface, at exhaust temperatures (770 K). Accordingly, thermal stability of the *Corresponding author. Fax.: +33-40-6763903; e-mail: [email protected] 1 Bulgarian Academy of Sciences, Sofia, Bulgaria.
CeO2 surface area is a desirable property for these materials. Recent investigations have shown that addition of ZrO2 to CeO2 improves the thermal stability. In addition, the impregnation of ZrO2 in the CeO2 lattice strongly promotes bulk reduction at low temperatures [1] and it was shown that, among Rh/CeO2±ZrO2 solid solutions [2], those containing 40±60 mol% of CeO2 gave the highest degree of support reduction and the lowest reduction temperature. Accordingly, we have chosen Ce0.6Zr0.4O2 as a support for Pd-based catalysts in our study. At present, Pd is a metal of choice in the TWCs due to its ability to promote HC and CO removal during a cold start of the engine. Several investigations of precious metal (PM)loaded catalysts showed that the kinetics of CO oxidation is affected by the presence of ceria. The CeO2-induced changes in the kinetics lead to enhancement of the CO oxidation activity and have been often interpreted on the basis of a mechanism involving a reaction between CO adsorbed on PM and the surface oxygen from the CeO2 [3±5]. CO oxidation was
0920-5861/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0920-5861(98)00212-0
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suggested to occur on new sites at the PM/cerium interface [6]. It is well known that the favoured mechanism of CO oxidation over PM (Rh, Pt, Pd) is the interaction between adsorbed O2 and CO (Langmuir±Hinshelwood). The kinetics of the reaction has been examined, and a ®rst-order in O2 and inverse ®rst-order mechanism in CO found [3]. Under moderately oxidising or net reducing conditions, addition of cerium oxide to the catalysts results in changes in CO oxidation kinetics including suppression of the CO inhibition effect, decreasing sensitivity of the reaction rate to the gas-phase oxygen concentration and a reduced apparent activation energy [5]. It is proposed that these changes are due to the CeO2-mediated mechanism, which become more important: (i) for small PM particles [4]; (ii) at low temperatures [4,5]; (iii) at high CO partial pressures [4,5]; and (iv) at low O2 partial pressures [4]. According to Oh and Eickel [5], the kinetics in a strongly oxidising environment over low-loaded Rh/Al2O3 catalysts are not signi®cantly affected by the presence of ceria. Finally, it should be noted that the activity of CeO2-based catalysts is a function of the surface morphology [6,7], which depends on the synthesis of the catalysts. This makes comparison of the different studies dif®cult. 2.
Experimental
Ce0.6Zr0.4O2 was synthesised by a citrate route from a water solution [8]. Metal impregnation was carried out by the incipient wetness method using aqueous solutions of Pd(NO3)2. Nominal Pd loading was 0.5 wt%. Temperature-programmed reduction (TPR) and oxygen-uptake measurements were carried out as previously described [8]. CO oxidation was carried out in a ¯ow microreactor. Typically, 50 mg of catalyst were loaded between two layers of granular quartz which acted as a pre-heater. The analyses were carried out by on-line gas liquid chromatograph using a Porapak Q wide bore capillary column. 3. 3.1.
Results and discussion Characterisation and redox behaviour of Pd-loaded and metal-free Ce0.6Zr0.4O2
The Ce0.6Zr0.4O2 obtained has a BET surface area of 70 m2 gÿ1. The N2 adsorption isotherm at 77 K is of
Fig. 1. TPR profiles of (1) fresh Ce0.6Zr0.4O2, (2), sample from run 1 oxidised at 700 K, (3) fresh Pd/Ce0.6Zr0.4O2 and (4) sample from run 3 subjected to four consecutive TPR/oxidation treatments at 700 K.
type IV with H3 hysteresis according to IUPAC classi®cation [9], which is characteristic of a welldeveloped mesoporous texture. The Raman spectrum (not reported) of Ce0.6Zr0.4O2 features a strong peak centred around 475 cmÿ1, which is characteristic of a cubic ¯uorite phase (space group Fm3m). However, small peaks at 313 and 146 cmÿ1 are observed, suggesting an attribution to a t00 phase according to the classi®cation by Yashima et al. [10]. Upon Pd-loading, no signi®cant variation of either texture or phase nature could be detected by N2 physisorption or Raman spectroscopy. The redox behaviour of metal-free and Pd/ Ce0.6Zr0.4O2 was investigated by means of TPR and oxygen-uptake measurements. The TPR results are presented in Fig. 1. The TPR pro®le of the fresh Ce0.6Zr0.4O2 features a strong reduction peak at 834 K and a shoulder at ca. 720 K. The relatively low intensity of this shoulder, compared to the main reduction peak, suggests its attribution to some kind of sample inhomogeneity such as partial tetragonalisation not detectable from the broad XRD patterns of the fresh samples, rather than to a low temperature surface-reduction process as in CeO2 [11]. In fact, the reduction in the bulk of the solid solution is strongly promoted by insertion of ZrO2 into the CeO2 lattice. This makes the reduction of the surface, and that in the bulk, indistinguishable by TPR, since they both occur
E. Bekyarova et al. / Catalysis Today 45 (1998) 179±183
almost at the same temperature [8,12]. Distortion of the oxygen sublattice as detected by Raman and EXAFS spectroscopies, which increases the oxygen mobility in the bulk, was invoked to explain this result [13]. Accordingly, computer simulation studies showed comparable energy for surface and bulk reduction [14]. Generally speaking, CeO2 extensively sinters during the TPR experiment to a ®nal surface area <10 m2 gÿ1. This leads to a complete loss of the OSC at low temperatures [12]. To investigate these effects, the sample was oxidised at 700 K and then subjected to a TPR. The peak at 834 K shifts to a higher temperature, in contrast to the shoulder at 720 K, which is not shifted. Subsequent redox cycling did not modify the TPR behaviour further. Oxygen uptakes of 17±18 ml gÿ1 were measured in the oxidation at 700 K carried out after the TPR, which corresponds to a Ce4/Ce3 reduction extent of 77±81%. By loading Ce0.6Zr0.4O2 with Pd, the reduction behaviour is strongly modi®ed. Fresh Pd/Ce0.6Zr0.4O2 features two peaks, at 360 and 805 K (Fig. 1, trace 3). The former peak could be associated with reduction of the palladium oxide precursor; however, the magnitude of this peaks in comparison to the relatively low metal loading suggests that a concomitant partial reduction of the support is occurring. Remarkably, the peak at 805 K which should be attributed to the reduction in the bulk, appears shifted to a lower temperature as compared to unloaded Ce0.6Zr0.4O2. After oxidation at 700 K, the following TPR features a peak at 500 K with a shoulder at 480 K. Repetitive reduction/oxidation cycles shift the peak to ca. 580 K, the ®nal TPR pro®le being shown in the trace 4 of Fig. 1. Note that the shift of the reduction to low temperatures, in the presence of Pd, is found in both, fresh and redox aged samples. This indicates that the kind of the interaction between the noble metal and the support, responsible for the improved reduction behaviour, does not depend on the extent of surface area. The oxygen uptake at 700 K does not change signi®cantly compared to the bare support indicating that only the kinetics rather than thermodynamics of the reduction process is affected by the supported Pd. Note that the contribution of the Pd/PdO redox couple to the OSC is relatively small ( 0:5 ml gÿ1 cat ) compared to the overall amount of O2 exchanged. Summarising, it appears that Pd-loaded Ce0.6 Zr0.4O2 shows the following important characteristics
181
with respect to CO oxidation: (i) an initial high surface area; (ii) a high OSC value, which is closely associated with the reduction behaviour of the support; (iii) a high degree of bulk reduction at low temperatures, and (iv) a strong effect of the supported Pd on the redox behaviour which is not deactivated by redox ageing. 3.2.
Catalytic measurements
To investigate the participation of the support in the CO oxidation, the activity of Pd-loaded and metal-free Ce0.6Zr0.4O2 was investigated in a ¯ow reactor. Prereduction of M/CeO2 improves the activity on the CO oxidation [15]. Accordingly, Pd/Ce0.6Zr0.4O2 was reduced at 500 K before the catalytic experiments. As shown in Table 1, addition of Pd to the Ce0.6 Zr0.4O2 signi®cantly improves the activity as compared to the metal-free sample. All the T10% values (temperature for a 10% CO conversion) are, in fact, lowered by 70±90 K at all the reaction conditions investigated, e.g. rich, stoichiometric and lean. Remarkably, equal T10% are measured under rich and stoichiometric conditions for both, the Pd-loaded and metal-free catalyst. A CO oxidation rate of ÿ1 is measured at 500 K over Pd/ 8 10ÿ6 mol gÿ1 cat s Ce0.6Zr0.4O2, which is comparable to the value ÿ1 reported by Yu Yao [3] for 9:1 10ÿ6 mol gÿ1 cat s Pd/CeO2/Al2O3 at 523 K. The activation energies for CO oxidation on metalfree and Pd loaded samples are given in Table 1. Presence of Pd leads to high Ea under reducing and stoichiometric conditions and low Ea under oxidising condition compared to Ce0.6Zr0.4O2. It should be noted however, that due to the different activities, the apparent Ea were obtained in different ranges of temperatures which does not allow to make a direct comparison. The Ea observed on Pd/Ce0.6Zr0.4O2 under rich/stoichiometric conditions are rather high when compared to those of a Pd wire (125 kJ/mol). Generally speaking, a decrease in Ea is observed with CeO2-containing catalysts as compared to bulk PM or PM supported on CeO2-free supports [3,5], under the conditions (stoichiometric and reducing) for which we have established Ea somewhat higher to that of a Pd wire. The differences can be ascribed to the difference in surface nature of the supports. Oh and Eickel [5] have used an alumina support impregnated with ceria, while
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Fig. 2. CO conversion on (a) Ce0.6Zr0.4O2 at (1) CO: O 1 : 2, (2) CO : O1 : 1 and (3) CO : O2 : 1; and (b) Pd/Ce0.6Zr0.4O2 at (1) CO : O1 : 2, (2) CO : O1 : 1 and (3) CO : O2 :1. Partial pressures as reported in Table 1.
the support in our investigation is a CeO2±ZrO2 solid solution. The CO conversion vs. temperature curves for Ce0.6Zr0.4O2 (Fig. 2, Table 1) indicate a slow increase of conversion with the rise in temperature. The CO conversion curves for bulk Pd typically show the lightoff phenomena [3]. Accordingly, those observed on Pd/Ce0.6Zr0.4O2 (Fig. 2) can be considered as composed of both types of curves. Both differences, in the type of conversion curves and in Ea, suggest that CO oxidation occurs at different sites on metal-free and Pd-loaded Ce0.6Zr0.4O2. The kinetics of CO oxidation con®rm this hypothesis. Note that we have investigated the dependence of the reaction rate on the O2 partial pressure primarily under stoichiometric/oxidising conditions. Strongly reducing conditions, such as those employed previously [16], are rarely met under practical TWC applications. Table 1 CO oxidation activity of CeO6Zr0.4O2 and Pd/Ce0.6Zr0.4O2 Catalyst
Reaction conditions a
Ea b
T10% c (K)
Ce0.6Zr0.4O2
rich stoichiometric lean rich stoichiometric lean
87 97 176 163 175 82
598 600 587 533 533 495
Pd/Ce0.6Zr0.4O2
a
pCO23 torr; rich conditions: pO2 6 torr; stoichiometric conditions: pO2 11.5 torr; lean conditions: pO2 23 torr. b Measured in the range of temperatures 500±600 and 500±530 K, respectively, for Ce0.6Zr0.4O2 and Pd/Ce0.6Zr0.4O2. c Temperature of a 10% CO conversion.
Fig. 3. CO oxidation rate on Pd/Ce0.6Zr0.4O2 at 493 K. The lines are added only as a guide for the eye.
The results on Pd/Ce0.6Zr0.4O2 are presented in Fig. 3. It should be noted that, on Ce0.6Zr0.4O2 (the results are not reported in the ®gure), the CO oxidation rate is approximately of zero order in O2 and of 0.7 order in CO. No changes in the kinetics were observed for broad partial pressure ranges of O2 (5±35 torr) and CO (6±29 torr). At 5-torr CO pressure, the reaction is of 0.8 order in O2 for the whole investigated range of O2 pressure (5± 35 torr). At 11 torr CO pressure, the O2 order is 0.4, while at 22 torr of CO partial pressure a zero order in O2 is observed for the whole range of O2 pressure (5± 35 torr). Also the dependence of the reaction rate on the CO pressure changes as it can be observed in
E. Bekyarova et al. / Catalysis Today 45 (1998) 179±183
Fig. 3. The relatively constant values of reaction rate observed at pO2 10 torr indicate that, at these low O2 partial pressures, there is a zero-order dependence on the CO pressure, while at higher O2 pressures a positive CO dependence is observed. The above ®nding can be explained by assuming that, at high CO partial pressures, the reaction is dominated by a second CeO2±ZrO2 mediated mechanism, as has been proposed by other authors for CeO2 [3±5,7,17]. At high CO pressures, the Pd surface is covered with CO which, in turn, inhibits O2 adsorption, as a result of which oxygen migration from the CeO2-based support becomes responsible for CO2 formation. In this case, Ce3/Ce4 oxidation by the oxygen from the gas phase takes place. It is noteworthy that we observe the support-mediated reaction at an O2 partial pressure which is remarkably high compared to 0.3 torr reported for Pd/CeO2 by Bunluesin et al. [16]. We suggest that the high ef®ciency of the Ce3/Ce4 redox couple in the Ce0.6Zr0.4O2 makes large amounts of weakly adsorbed oxygen available at the support surface [18]. This oxygen is then able to compete with the gaseous O2 even at relatively high O2 pressures. At low CO pressures, competition of O2 on the Pd surface may occur, and lead to a typical positive pressure dependence observed for the Pd. Note that, on Pd/ ZrO2, no evidence for oxygen transfer from the support were found except when metal alloys were employed as catalyst precursors [19]. 4.
Conclusions
In summary, our investigation of CO oxidation on Pd/Ce0.6Zr0.4O2 shows that, at a relatively modest CO pressure and under stoichiometric-to-moderately oxidising conditions, the CeO2-mediated mechanism is favoured, a fact that can be attributed to the improved redox performance of the Ce0.6Zr0.4O2 support in comparison to CeO2. In contrast, Yu Yao [3] and Oh and Eickel [5] have established that under oxidising conditions the kinetics are not signi®cantly affected by the presence of CeO2. Although Yu Yao has reported the kinetics to
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be nearly independent of oxygen pressure in the presence of excess O2 for reduced Pd/CeO2/Al2O3, he has noticed instability of the reduced state. Acknowledgements CNR (Roma), MURST 40% (Roma) and UniversitaÁ di Trieste are acknowledged for ®nancial support. One of us (Elena Bekyarova) thanks the University of Trieste for a grant within the ``Legge Aree di Con®ne'' Programme. References [1] G. Balducci, P. Fornasiero, R. Di Monte, J. KasÏpar, S. Meriani, M. Graziani, Catal. Lett. 33 (1995) 193. [2] P. Fornasiero, R. Di Monte, G. Ranga Rao, J. KasÏpar, S. Meriani, A. Trovarelli, M. Graziani, J. Catal. 151 (1995) 168. [3] Y.F. Yu Yao, J. Catal. 87 (1984) 152. [4] T. Bunluesin, H. Cordatos, R.J. Gorte, J. Catal. 157 (1995) 222. [5] S.H. Oh, C.C. Eickel, J. Catal. 112 (1988) 543. [6] C. Hardacre, R.M. Ormerod, R.M. Lambert, J. Phys. Chem. 98 (1994) 10901. [7] A. Tschope, W. Liu, M. Flytzani-Stephanopoulos, J.Y. Ying, J. Catal. 157 (1995) 42. [8] P. Vidmar, P. Fornasiero, J. KasÏpar, M. Graziani, J. Catal. 171 (1997) 160. [9] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Sieminiewska, Pure Appl. Chem. 57 (1985) 603. [10] M. Yashima, H. Arashi, M. Kakihana, M. Yoshimura, J. Am. Ceram. Soc. 77 (1994) 1067. [11] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [12] P. Fornasiero, G. Balducci, R. Di Monte, J. KasÏpar, V. Sergo, G. Gubitosa, A. Ferrero, M. Graziani, J. Catal. 164 (1996) 173. [13] G. Vlaic, P. Fornasiero, S. Geremia, J. KasÏpar, M. Graziani, J. Catal. 168 (1997) 386. [14] G. Balducci, J. KasÏpar, P. Fornasiero, M. Graziani, M.S. Islam, J.D. Gale, J. Phys. Chem. B 101 (1997) 1750. [15] C. Serre, F. Garin, G. Belot, G. Maire, J. Catal. 141 (1993) 9. [16] T. Bunluesin, E.S. Putna, R.J. Gorte, Catal. Lett. 41 (1996) 1. [17] G.S. Zafiris, J. Gorte, J. Catal. 86 (1991) 86. [18] E.S. Putna, J.M. Vohs, R.J. Gorte, Catal. Lett. 45 (1997) 143. [19] A. Baiker, D. Gasser, J. Lenzner, A. Reller, R. Schloegl, J. Catal. 126 (1990) 555.