Langmuir 1994,10, 717-722
717
Influence of the Reduction/Evacuation Conditions on the Rate of Hydrogen Spillover on Rh/CeOz Catalysts S. Bernal,. J. J. Calvino, and G. A. Cifredo Departamento de Qulmica Inorgbnica, Facultad de Ciencias, Universidad de Ciidiz, Apartado 40, Puerto Real, Cbdiz, Spain
A. Laachir and V. Perrichon Institut de Recherches sur la Catalyse, CNRS, 2, Avenue Albert Einstein, 69626 Villeurbanne Ckdex, France
J. M. Herrmann Laboratoire de Photocatalyse, Catalyse et Environnement, URA-CNRS, Ecole Centrale de Lyon, B.P. 163,69131 Ecully Ckdex, France Received April 26, 1993. I n Final Form: December 22, 199P The hydrogen chemisorptionwas studied on a Rh/CeOz catalyst after a reduction/evacuationtreatment at 773 K. In addition to volumetric and TPD-HZmeasurements, magnetic susceptibility and semiconductivity experiments were performed to determine the Ce3+ content and the electron concentration, respectively,in order to discriminatethe contribution of the support from the total hydrogen adsorption. By comparison with previous results obtained after reduction/evacuation at 623 K, the reduction at 773 K induces a strong decrease of the apparent H/Rh value determined at 295 K. The variations of the Ce3+ content calculated from magnetism indicate a very limited hydrogen chemisorption on the support at 295 K. It increasessignificantlyupon heating under Hz at 473 K. The conductivitydata are in good qualitative agreementwith magneticmeasurements. Theseresultsshowthat the main effectof increasingthe reduction temperature from 623 to 773 K is to slow down the hydrogen spillover rate on the support. In agreement with previous suggestions,they indicate that hydrogen chemisorption on the rhodium is not suppressed after reduction at 773 K as it could be expected for a metal under the classical SMSI state.
Introduction A key factor in determining the catalytic behavior of the supported metal catalysts is the dispersion of the metallic phase. It is well-known that the chemisorption of probe molecules, typically hydrogen, is one of the techniques more widely used for determining the metal dispersion. However,this method has serious limitations when the adsorbate strongly interacts with the support. This is the case of ceria, an oxide that can chemisorblarge amountsof hydrogen.I4 Since ceriais a major component of the so-called three way catalysts (TWC), a very important industrial catalyst extensively used in the antipollution device of motor vehicles: it seemed interesting to us to investigate in detail the interaction of Hz with Rh/CeO2 model catalysts. In this way, some further knowledge about the chemistry of this system, and, hopefully, some improvement of the methods currently applied for characterizing ceria-containing noble metal catalysts would be obtained. Consequently to the capability of ceria to chemisorb H2,l-l the volumetric adsorption studies on Rh/CeOz catalysts very often lead to apparent H/Rh ratios much AbetractpublishedinAdua~ce ACSAbstnrcts, February 1,1994. (1)Fierro, J.L. G.; Soria, J.; Sam, J.; Rojo, J. M. J. Solid State Chem. 1987,66,154. (2) Fbjo,J. M.; Sam, J.; Soria, J. A,; Fierro, J. L. G. Z . Phys. Chem. (Munich) 1987,162,407. 0
(3) Laachir, A.; Perrichon, V.; Badri, A.; Lamotta,J.; Catherine, E.; Lavalley, J. C.; El Fallah, J.; Hilaire, L.; Le Normand, F.; QuBmerB, E.; Sauvion, G. N.; Touret, 0.J. Chem. SOC.,F a ~ d a Trans. y 1991,87,1601. (4) B e d ,S.; Calvino, J. J.; Cifredo, G. A.; Gatica, J. M.; PBrez Om& J. A.; Pintado, J. M. J. Chem. SOC.,Faraday Trans. 1993.89.3499. (5) Oh, S. H.;Eickel, C. C. J. Catal. 1988,112, 543.
larger than unity.”1o In accordancewith several authors, hydrogen chemisorption on bare ceria is an activated process, which starts to be observed at around 473 K.394** In the presence of highly dispersed rhodium, because of the occurrence of spillover,the process takes place at much lower temperature, even at room temperature.@-1° We have observed in ref 10 that upon increasing the reduction/ evacuation temperature from 623 to 773 K,the apparent H/Rh value, as determined from volumetric chemisorption experiments, at 295 K, strongly decreases. In agreement with some earlier results from the literaturel2Js dealing with the influence of the surface hydroxyl concentration on the spillover rate, it was suggested in ref 10 that the blocking of the spillover process rather than the deactivation of the metal was the main factor responsiblefor the results above. This interpretation for the loss of chemisorptive capability of the Rh/CeOz catalysts is different from that suggested from some earlier studies,14-16 according to which ceria supported metal catalysts exhibit (6) Katzer, J. R.; Sleight, A. W.; Gajardo, P.; Edward, M.; Gleaon, F.; McMillan, S. Discuss. Faraday Soc. 1982,72,121. (7) Trovarelli,A; Dolcetti, G.; Leitenburg, C.; Kaepar, J.; Finetti, P.; Santoni, A. J. Chem. SOC.,Fa~dcry!lYane. 1992,88,1311. (8) Bemal, S.; Calvino, J. J.; Cifredo, G. A.; Rodrlguee-hquierdo,J. M.; Perrichon, V.; Laachir, A. J. Catal. 1992,137, 1. (9) Bernal, S.; Calvino, J. J.; Ciedo, G. A.; Rodrlguea-hquierdo,J. M.; Pemchon, V.; Laachir,A. J. Chem. Soc., Chem. Commun. 1992,460. (10) Bernal, 5.;Botana, F. J.; Calvino J. J.; Cauqui, M. A.; Cifredo, G. A.; Jobacho, A.; Pintado, J. M.; &edzquierdo, J. M. J. Phys. Chem. 1993,97,4118. (11) Tournayan, L.; Marcilio, N. R.; Frety, R.Appl. Catal. 1991,78,
31. (12) Bond, G. C. Stud. Surf. Sci. Catal. 1983,17, 1. (13) Belzunegui, J. P.; Rojo, J. M.; Sam,J. J. Chem. SOC.,Faraday Trans. 1989,85,4287. (14) Barrault,J.;Alouche,A.; Paul-Boncour, V.; Hilaire, L.;PercheronGuegan, A. Appl. Catal. 1989,46, 269.
0743-7463/94/2410-0717$04.50/0 Q 1994 American Chemical Society
Bernal et al.
718 Langmuir, Vol. 10, No. 3, 1994
the so-calledSMSI-likeeffect. As is well-known,this type of effect, which has been widely investigated on titania supported metal catalysts,"-lg is characterizedby a strong chemical deactivation of the metal, associated with which some covering of the metal particles by the reduced support occurs. In this work the investigationof the hydrogen interaction withRh/CeO~catalysts has been carried out by combining conventional volumetric adsorption and TPD-H2 studies with magnetic balance and electronic conductivity measurements. By using these latter techniques,we have been able to obtain some further very useful information about the incorporation of hydrogen to the support. The results reported here provide some additional evidence to the interpretation suggested in ref 10. Experimental Section The metal loading of our Rh/CeO2 catalyst was 2.9% by weight. It was prepared by the incipient wetness impregnation technique from an aqueous solution of Rh(N0s)S. The ceria used, CeOzHSA, with a purity of 99.9%, was a high surface area sample, 130 m2&, supplied by RhBne-Poulenc Minerale Fine. After the impregnation treatment, the sample was dried in air, at 383 K, for 10 h, and further stored in a desiccator until its "in situ" reduction. The surface area of the sample was not significantly modified by the impregnation treatment. The same can be said for the reduction in flowing hydrogen at 623 K. On the contrary the surface area of the catalyst reduced at 773 K significantly decreased final S B ~62: m2@. Some additional characterization data concerning our Rh/CeOz sample are given elsewhere.*JO Hydrogen volumetric adsorption measurements were carried out in a conventional high vacuum system (P < 1 X 1W Pa) equipped with a capacitance gauge, MKS Baratron, Model 220 BHS. In the case of the conventional volumetric experiments, at 295 K, the time elapsed between successive adsorption measurements at increasing Hz pressures was routinely 20 min. The standard procedure for reducing the catalyst consisted of heating about 200 mg of the supported precursor, prepared as indicated above, in a flow of H2 (flow rate, 60 cma-min-l), at a heating rate of 10 Ksmin-l, up to the reduction temperature, either 623 or 773 K. The catalyst was held for 1 h at this temperature; it was further pumped off at the reduction temperature for 1h, and fiially cooled under high vacuum. The two catalysts described above will be hereafter referred to as Rh/CeOz-623 and Rh/CeOz-773, respectively. The TPD-HZstudies reported in this work were carried out in an experimental device equipped with a thermal conductivity detector (TPD-TC), similar to that described in ref 20. The water formed along the TPD experiment, if any, was eliminated before the TC detector by passing the gases coming out from the reactor through a little zeolite column. The gases, N-50 type (99.9990%), from SEO, were further purified by passing them through either a series of deoxo and zeolite traps (H2, Ar). The TPD experiments were carried out under the following condiheating rate, 10 K-min-'. The tions: Ar flow rate, 60 ~mS.min-~; sample weight was typically 200 mg. The magnetic susceptibility measurements were carried out by means of a Faraday microbalance. Details concerning the equipment and the calibration procedures are given elsewhere.ss21 All the susceptibility values, x , where corrected for the ferromagnetic impurities (6-12 ppm). The susceptibility of the initial sample was -0.18 X 10-8 emu CGS (to be multiplied by 12.56 to (15) Cunningham, J.; OBrien, S.;Sam,J.; Rojo, J. M.; Soria, J. A.; Fierro, J. L. G. J. Mol. Catal. 1990,67, 379. (16) Binet, C.; Jadi, A.; Lavalley, J. C.; Boutonnet-Kizling, M.J.Chem. SOC., Faraday Tram. 1992,88,2079. (17) Tauetar, S. J.; Fung, S.C.; Garten, R. L. J. Am. Chem. SOC. 1978,
100,170. (18) Haller, G.; Reeaeco, D. E. Adu. Catal. 1989, 36, 173. (19) Belzunegui, J. P.; Sanz, J.; Rojo, J. M. J. Am. Chem. SOC.1992, 114,6749. (20) Blanchard, G.; Charcoeset, H.; Forieeier, M.; Matray, F.; Tournayan, L. J.Chromatogr. Sci. 1982,20, 369. (21) Perrichon, V.; Candy, J. P. J. Cotal. 1984,89, 93.
obtain SI units in m8.g1) which is the expected value for a diamagnetic ceria sample containing only Ce4+ ions. As a result of the hydrogen treatmenta, the susceptibility of the sample became paramagnetic. This change was totally attributed to the formation of Cea+ ions, the magnetic contribution of rhodium being neglected (maximum value to be expected 3 X 1O-eemu CGS at 298 K). The extent of the reduction undergone by ceria was estimated on the basis of the reaction 2Ce0,
+ H,
-
Ce,O,
+ H20
by using for Ce20athe Curie-Weiss law determined previously
x = (4.8 X 10")/(T + 160) The semiconductivityof the catalyst was investigated by using a cell of the static type, specially designed to study the electronic interaction between two solid phases in intimate contact.s7 The catalyst was placed between two platinum electrodes under a constant compressionof 106Pa for good electricalcontact between particles without modifying the texture. The temperature of both electrodes was given by two soldered thermocouples whose wires were also used as conductors for electrical measurements. The electrical resistance of the samples were measured according to the range investigated, with a Kontron multimeter (Model DMM 4021) or a digital teraohmmeter (Guildine Instruments, Model 9520). The power samples behaved as massive conductors and the electricalconductivity was calculated from the equation u = t/RS, where R is the electrical resistance measured, t is the thickness of the powder (proportional to the amount of solid employed), and S is the cross-sectionalarea of the electrode (diameter = 1.00 cm). Conductivity measurements carried out on powders do not provide, in general, characteristic bulk values for the material. However, the relative variations of u as a function of a selected parameter such as the temperature or the nature and the partial pressure of a gaseous reactant can yield characteristic values identical to those obtained on single crystals and with other techniques such as thermogravimetry. This was exemplified on titania." In the present work, electrical conductivity measurements are particularly adapted to follow in situ, in the presence of hydrogen, the relative variations of conductivity as a function of a chosen parameter. Thus, the results can be considered qualitative or semiquantitative and are indicativeof the variations of the free electron density within the sample. The catalyst was reduced "in situ". The sample (0.580 g) was first evacuated at room temperature for 1h, in the presence of two liquid nitrogen traps. A pressure of 300 Torr H2 was subsequently introduced and the sample was heated up to 773 K a t a heating rate of 10 Ksmin-1. Rhodium nitrate was reduced and the catalyst surface was cleaned up, the decomposition products being eliminated by the liquid nitrogen traps. The conductivity cell was subsequently isolated under hydrogen, whereas the traps and the gas line were evacuated. After eliminationby pumping of the reduction products, fresh hydrogen was introduced in the gas line and in the cleaned traps, which were reconnected to the conductivity cell. This operation was without effect on the electrical conductivity of Rh/Ce02 and a second reduction period in hydrogen at 773 K for 1 h was performed. Electrical conductivity indicated that the total reduction of the catalyst had already been achieved within 1h. The electrical conductivity measurements correspond to that of ceria%because the amount of dispersed rhodium is lower than the percolation threshold, which prevents the existence of a preferencial pathway of the electrical current between the two electrodes via rhodium particles.
Results and Discussion Volumetric Adsorption and TPD Studies. Table 1 summarizes the results of the volumetric study of the hydrogen chemisorption on the ceria supported rhodium catalysts reduced at 623 and 773 K. In addition of data determined from the ordinary adsorption experiments at 295 and 191K, Table 1includes the results obtained from some complementary volumetric studies consisting of-
Hydrogen Chemisorption on RhlCeOa Catalyst
Langmuir, Vol. 10, No. 3,1994 719
Table 1. Hydrogen Chemirorption on Rh/CeOrHSA Catalysts apparent WRh ratio preparation chemisorption volumetric adsorption TPD treatment conditions redn 773 K + evac 773 K 191 K 0.82 0.47 295 K 0.75 0.73 0.84 296 K (20h) 450 K 2.30 473 K 2.2b redn 623 K + evac 623 K 191 K 1.41 3.97 295 K 0 The experiment consisted of heatingat 473 K and further cooling to 295 K always under H2 (PH, = 250 Ton).
heating the catalysts under hydrogen ( P H250 ~ Torr), for 1h, at several increasing temperatures up to 473 K and cooling them to 295 K,always under hydrogen. This latter type of experiment has allowed us to investigate the influenceof the temperature on the chemisorptivebehavior of our catalysts. As we have already noted,1° the apparent H/Rh ratio determined at 295 K for Rh/CeO2-773 (H/Rh = 0.75) is much smaller than that estimated for the catalyst reduced at 623 K (H/Rh = 3.97). If these data are compared with those obtained from the low temperature (191 K) adsorption studies, we may note that the H/Rh ratio remains constant for Rh/CeO2-773, whereas for Rh/CeO2-623 K we obtain a much lower value: 1.41 instead of 3.97. It has been proposed in refs 10and 22 that at 191K the spillover rate would be negligible, and therefore, that the H/Rh values determined from the low temperature chemisorption studies would directly estimate the metal dispersion. If it is so, the results in Table 1would indicate that an important fractionof the metal microcrystalsremain active after reduction at 773 K. This, in turn, would lead us to the conclusion that the main difference between the catalysts reduced at 623 and 773 K is the contribution of the spillover to the apparent H/Rh value determined at 295 K. This contribution would be negligible in the case of the catalysts reduced at 773 K. High resolution transmission electron microscopy (HRTEM) studies in refs 10 and 23-26 would also be in agreement with this proposal. Furthermore, the reduction treatment at 773 K induces a notable decrease of the catalyst surface area from 130 to 62 m2&. Inherent to this support sintering there would be some metal loss via an encapsulation process. In our opinion, this additional effect also reinforces our suggestion above. Some further evidence supporting our interpretation has been obtained from the thermally activated chemisorption studies reported in Table 1. Actually, we may conclude from the comparison of the results obtained for Rh/CeOz-623 and Rh/Ce02-773that the spillover rate is much slower in the latter case, since it is necessary to heat the catalyst well above 295 K to observe a significant increase of the amount of chemisorbed hydrogen. This contrasts with the observation reported for Rh/CeO2623,9%he saturation of which is almost complete at room temperature. (22) B e d , 5.;Calvino, J. J.; Cifredo, G. A.; Jobacho, A.; RodrwezIzquierdo, J. M. Proceedings of the 2nd Internutionul Conference on Rare Earthe; Beijing (China),1991;J.Rare Earths (Special Issue), 1991, 2, 838. (23) B e d , 8.; Botana, F. J.; Garcia, R.; Kang, Z.; Upez, M.L.;Pan, M.;Ramhez, F.;Rod&ez-Izquierdo, J. M.Catal. Today 1988,2,663. (24) Pan,M.;Cowley,J. M.; Garch, R. Micron Microec. Acta 1987,18,
166. (26) Pan,M.Doctoral Thesis,Arizona Stab University, 1991. (26) Cifredo, G. A. Dodoral Thesis, University of CBdiz, 1992.
t300
500
700
900
1100
1300
T (K)
Figure 1. TPD-Ha traces corresponding to the Rh/CeOl catalyst reduced at 773 K, and further submitted to a series of different treatments (a-d). The trace for CeOa reduced at 773 K and further cooled to 295 K in flowing H2,e, is also included for comparison. (For more details about the various treatments see the text.)
We have also carried out some parallel TPD-H2 experiments. Figure 1shows the traces recorded for the Rh/ CeO2 catalyst reduced with flowing H2 at 773 K,for 1h, and subsequently treated as follows: (a) The sample was heated in a flow of Ar at 773 K,for 1h, and cooled to 295 K,also in flowing Ar. (b) The treatment was similar to part a, but the sample was cooled in a flow of inert gas up to 191K,then, it was treated for 1h, at 191K,with flowing Ha; argon was passed again through the sample for 15 min, at 191K;and finally, the TPD experiment was run. (c) The sample resulting from part a was further treated with flowing H2 for 1h, at 295 K,cooled to 191 K also in a flow of H2, and flushed with Ar for 15 min, at 191K.(d) The experiment was similar to part c except the temperature of treatment with flowing hydrogen: 450 K. The heating of the sample from 191to 295 K took place freely by removing the cold trap. The correspondingpart of the TPD diagrams, not shown in Figure 1, did not provide any significant information. However, by using this procedure, the stabilization of the TC detector after switching from H2 to Ar was carried out with the catalyst at 191 K,thus minimizing the likely loss of chemisorbed hydrogen that can occur at 295 K.a10 We have carried out some TPD experiments parallel to those reported in Figure 1 in which mass spectrometry has been used as analytical technique. They have shown that hydrogen evolution from the Rh/CeO2-773 catalyst only takes place as H2, i.e. there is no significant contribution of H20 to the total amount of desorbed hydrogen.% A quite similar observation has already been noted for the behavior of bare ceria reduced at 773Ke4 This means that upon integration of the traces depicted in Figure 1, an estimate of the total amount of chemisorbed hydrogen can be obtained. Table 1accounts for these results. Since the Rh/CeOz catalyst reduced at 773 K and further flushed with Ar at the same temperature does not retain any significant amount of hydrogen, Figure la, the integrated traces in Figure 1would diredtly give us an estimate of the amount of hydrogen chemisorbedby the catalyst asa result of the treatments which followed the reduction/evacuation at 773 K.
Bernal et al.
720 Langmuir, Vol. 10, No.3, 1994 In accordance with Table 1,data determined from both volumetric and TPD experiments are in a fairly good agreement. The TPD study confirms that our Rh/CeOz catalyst can chemisorb, at 191K,a significant amount of hydrogen, thus showing, as already discussed in ref 10, that no strong inhibition of the adorptive properties is induced on rhodium by the reductiontevacuation treatment at 773 K. No SMSI effect like that described for M/Ti02 catalystal7J8 would operate in the present case. This contrasts with the conclusion drawn from some recent studies dealing with different ceria supported metal catalysts.lC16 In accordance with the evolution of TPD traces b-d in Figure 1, the variation of the amount of chemisorbed hydrogen with the temperature of hydrogen treatment is similar to that deduced from the volumetric studies. Indeed, Figure 1shows that, in contrast with that reported earlier for the catalyst reduced at 623 K?Jo*22over the samplereduced at 773 K,the spillover process only occurs to a significant extent at temperatures well above 295 K. Several authorsl2J3have reported on the influence of the surface hydroxyl species on the spillover rats. This has also been confirmed by some previous studies dealing with the hydrogen chemisorption on Rl1/Ce02.~Accordingly, in the present case, the reduction/evacuation treatment at 773 K would lead to a support surface strongly dehydroxylated,thus explaining the slow spillover rate at 295 K. Some earlier FTIR4Jo*m and 'H NMRl studies of both Rh/CeOzlo and Ce021*49nhave shown that the evacuation treatment at 773 K leads to highly dehydroxylated ceria surfaces. The evolution undergone by the TPD-Hz diagrams as a function of the hydrogen treatment indicates the occurrence of a number of unresolved peaks. If the spectrum in Figure Id is compared with that reported for bare ceria in Figure le, we may conclude that the major peak is strongly shifted downward for Rh/CeO2-773. This shows, as already discussed in ref 9, that rhodium plays an important role in determining the desorption temperature of the hydrogen chemisorbed on ceria. In accordance with ref 28, hydrogen desorption from supported rhodium would be expected to occur below 473 K. However, as deduced from Table 1, an important fraction of the hydrogen desorbed from our RWCeOa773 in Figure Id proceeds from the support. This means that there is no neat separation of the contributions of H2 desorption from both the metal and the support. However, if the position of the main peak in Figure Id is compared that of the diagram recorded for Rh/Ce0~-623,2~ a shift from 473 to 383 K can be observed when going from the higher reduction temperature (773 K)to the lower one (623 K). This would indicate that both direct and back spillover are slower on the catalyst reduced at 773 K. A further element of complication of the TPD diagrams in Figure 1proceeds from the interconversion of hydrogen chemisorbed forms during the TPD run.22In effect, traces b and c in Figure 1 show that, in addition to a peak at around 373 K, which might in principle be assigned to hydrogen desorbed from the metal,28 there is a broad feature at considerably higher temperature, above 473 K, very probably correspondingto hydrogen desorbed through the metal from the support. Since curves b and c correspond to hydrogen treatments at 191 and 295 K respectively, Le. adsorption conditions at which there is no significant contribution of the spillover, the traces b (27) Li, C.; SaLata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc., Faraday Tram. I 1989,86,1451. (28) Chin, A. A.; Bell,A. T. J. Phye. Chem. 1983,87, 3482.
Table 2. Magnetic Balance Study of the E2 Interaction with Rh/CeOr773 x , emu(CGS). treatment l3-' O % Ces+ unreduced precursor/ 0.15 1.5 support evacuated for 15 h precursor reduced at 773 K with 1.18 11.1 flowing H2 and further evacuation at 773 K for 1 h HZtreatmenta (PH,= 300 Torr) at 295 K for t=5min 1.19 11.2 t=lh 1.27 11.9 t=Eh 1.37 12.9 Hs treatments (fir5 300 Torr) for 1 h, at the temperatures below, followed by cooling to 295 K under Ha 323 K 1.46 13.8 1.98 18.7 423 K 2.00 18.8 473 K evacuation treatments at the temperaturea below for 1 h 295 K 1.86 17.8 1.44 16.1 373 K 0.90 11.9 473 K 0.67 11.5 623 K 0.58 11.5 773 K 0.52 11.5 873 K 0.45 11.0 973 K Magnetic susceptibilitydata at 295 K, except for the evacuation treatments. In this latter caw the measurementa were carried out at the evacuation temperature.
and c would be interpreted as an indication of hydrogen transfer from the metal to the support occurred as the temperature was increased during the TPD run. Magnetic Balance Study. Some earlier studies dealingwith bare Ce023sand Rh/CeOz8v9catalystshave shown that the magnetic balance is a powerful tool for investigating hydrogen and CO adsorption on ceria containing catalysts. In the present case, we have used this technique to track the transfer of hydrogen to the support. Table 2 summarizesthe results obtained from this study. The experimental procedure was similar to that used in the volumetric and TPD studies reported above. As seen in Table 2, upon reduction/evacuation at 773 K, the percentage of cerium ions present in the sampleas Ce(II1) is equal to 11.1%. Since our TPD study has shown that the catalyst reduced and further evacuated at 773 K does not retain significant amounts of chemisorbed hydrogen, the reduction level reported above (11.1%) should correspond to the so-called irreversible reduction of ceria leading to the formation of H20 and corresponding to the If the catalyst pretreated creation of oxygen vacancie~.~*~ in this way is subsequently treated with H2 ( P H300 ~ Torr) at several different conditions, the variations of the magnetic susceptibility corresponding to that of the concentration of paramagnetic Ce(II1) ions constitute an interesting way of determining the hydrogen Chemisorption on the support. In accordance with Table 2, at room temperature, the hydrogen chemisorption on the support takes place very slowly; even after 15 h of treatment, the ceriareduction level slightly increases from 11.1to 12.9%, which is equivalent to a variation of the amount of chemisorbed hydrogen expressed as an apparent H/Rh ratio of 0.36. This observation is in a fairlygood agreement with the volumetric and TPD studies reported above, as well as the conductivity measurements to be discussed (29) Badri, A.; Lamotte, J.; Lavalley, J. C.;Laachir, A.; Perrichon, V.; Touret, 0.;Sauvion, G. N.;Qu6m6r6, E. Eur. J. Solid State Znorg. Chem. 1991,28,445.
Hydrogen Chemisorption on RhlCe02 Catalyst
Langmuir, Vol. 10, No. 3,1994 721
r-R
2.8
vac.
0
9 b
I
2.4
1
3
100
300
time (h)
Figure 2. Evolution of the electrical conductivity u (Wcm-l) of the WCeOp catalyst throughout the reduction treatmentat 773 K (& 300 Torr),and ita further evacuation at 773 K. The arrow R indicates the renewal of hydrogen atmosphere.
below. As can be deduced from Table 2, even after heating the catalyst for 1h at 323 K, and further cooling it to 295 K always under Hz, the increase of the amount of chemisorbed hydrogen is limited. It is necessary to heat up to 423 K to achieve an apparent saturation of the catalyst. This conclusion is supported by the very slight variation observed in the reduction level (from 18.7 % to 18.8%), upon increasing the treatment temperature up to 473 K. This latter reduction degree would correspond to an amount of chemisorbed hydrogen of H/Rh 1.6. If this amount is added to that determined from the volumetric measurements at 191 K, which we have assigned to hydrogen chemisorbed on the metal (H/Rh 0.8),we would obtain a total H/Rh value of 2.4, very close to that determinedfrom volumetricmeasurements. This suggests that, under the experimental conditions used here, the magnetic susceptibility measurements account for the total amount of Hz chemisorbed on ceria; i.e. there is no significant contribution of chemisorbed forms either implying no reduction of ceria or even disappearance of paramagnetic Ce(II1) species. In this latter respect, it would be noted that by analogy with that reported for Rh/TiOz catalysts,90*31the formation of diamagnetic Ce(111)-H species has been proposed in the literature.1*2*32 Also worth of noting, the treated with hydrogen Rh/CeOz773 catalyst was evacuated at several increasing temperatures including 773,873, and 973 K with no modification of the reduction degree. Consequently,it can be reasonably concluded that the catalyst evacuated at 773 K does not contain significant amounts of hydrogen chemisorbed species responsible for either the creation of paramagnetic Ce(II1) or its masking as Ce(II1)-H species. This observation, on the other hand, is in agreement with the TPD diagram in Figure l a commented on above. Conductivity Measurements. After 1h of reduction at 773 K the electrical conductivity u reaches a steady state (Figure 2) which is not modified upon prolonging the treatment for longer time. The conductivity level is rather high ( u = 1.85 X 1VP1*cm-l). This indicates that (30) Conem, J. C.; Malet, P.; Munuera, G.; S a m ,J.; Soria, J. J. Phys.
Chem. 1984,88,2986.
(31) MuBoz, A.; Gondez-Elipe, A. R.; Munuera, G.; Ehpinb, J. P.; Rives, V. Spectrochim. Acta 1987,43,1599. (32) Munuera, G.;Fernbdez, A.; Godez-Elipe, A. R. In Catalysis and Automotiue Pollution Control ZI; Crucq, A., Ed.; Elsevier: Amsterdam, 1991; p 207.
PH*
Figure 3. Electrical conductivity of the WCeOp catalyst vs hydrogen pressure plot ( u in i2l.cm-l; P H i~n Torr). The experiment was carried out at 296 K.
ceria has been reduced in two different ways: by reversibly chemisorbed hydrogen as well as by the formation of anionic vacancies Vw resulting from the elimination of two OH groups in close vicinity 20H- e H,O
+ 0% + Vo,
(1)
VOZ-represents an anionic vacancy with two electrons trapped. This is a neutral entity with respect to the solid. At 773 K, the vacancy loses one electron, which is promoted to the conductionband and, therefore, becomes positively charged with respect to the lattice:
Vo, e V+*
+ e-
(2)
V+wrepresents a singly ionized anionic vacancy.
When hydrogen is pumped out at 773 K, the electrical conductivity decreases as illustrated in Figure 2. This decrease is similartothat reported for severalother systems such as Pt, Rh, or Ni supported on TiOzss or NVCe0z.a In the present case, this support reoxidation can be attributed to the elimination as Hz of the chemisorbed hydrogen. A rather similar effect has already been observed with the help of a magnetic balances. There are less free electrons present in the conductionband; i.e. the concentration in Ce3+ions detected by magnetic susceptibility measurements has decreased. Both electrical conductivity and magnetic susceptibility measurements are giving concordant results. After evacuation at 773 K for 1h, the catalyst was cooled to room temperature under vacuum. The electrical conductivity was subsequently measured at steady state as a function of hydrogen pressure ranging between 10 and 300 Torr. The corresponding o-P(H2) plot is given in Figure 3. The variations are rather small but significant. The slight increase of u versus P(H2) can be ascribed to a hydrogen spillover effect.3s This process can be written (33) Herrmann, J. M. J. Catal. 1984,89, 404. (34) Herrmann, J. M.; Romaroeon, E.; Tempere, J. F.;Guilleux, M. F. Appl. Catal. 1989,53, 117. (35) Herrmann, J. M.; Pichat, P. In Spillover of Adsorbed Species; Pajonk, G. M., Teichner,S. J., Germain, J. E., Eds.; Elsevier: Amsterdam, 1983; p 77. (36) Laachir, A. Doctoral Thesis, Univeraity of Lyon, No.: 241-91, 1991. (37) Herrmann, J. M. In Lee Techniques Physiques Appliqubs B la Catalyse; Imelik, B., VBdrie, J. C., Eds.; Editions Technip: Paris, l98& Chapter 22, p 753.
722 Langmuir, Vol. 10, No.3, 1994
Rh,-H
+ 0% ~iRh, + OH- + e-
Bernal et al. (3) 4
0% represents an anion of ceria which can support a hydrogen atom in its protonated state (OH- group) and e- represents a free electron of conduction. In accordance witheq 3, inherent to the hydrogen spilloverprocess, there would be an increase of the concentration of free electrons in the surface of the support. This effect would be responsible for the overall increase of the conductivity we measure experimentally. It has been observed that the magnetic susceptibility slightly increased when H2 was introduced. If x increases, this means that Ice3+]increases because the equilibrium Ce4++ e- F! Ce3+
0
z b
3
(4) 100
Equation 4 indicates that if [Ce3+lincreases, it is because of an increase in free electron concentration in ceria. This is in good qualitative agreement with the results of electrical conductivity which demonstrated an electron transfer to the support because of the hydrogen spillover. Also in agreement with these observations, the FTIR spectroscopy has shown that hydrogen spillover on Rh/ CeO2-773 leads to the formation of OH group~.~O~36 As already reported in the literat~re,'~J~ and confirmed by the electrical conductivity measurements on Pt/Ti02,36 the degree of surface hydroxylation of the reduced catalyst plays an important role in determining the spillover rate, the process being favored by the presence of 0-H groups. The influence of the temperature on the extent of the hydrogen spilloverhas also been investigated. The reduced catalyst was successively heated at 373,423,473, and 573 K, and further cooled to room temperature always under hydrogen;fiially, the electricalconductivitywas measured. The results are presented in Figure 4. There is a significant difference between the initial state (295 K)and the first sequence of heating at 373 K. The following u values practically do not differ. It seemsthat the most important effect in the extension of hydrogen spillover detected by electrical conductivity occurs during the first heating at 373 K and reaches completion at temperatures above 373 K. These results are in agreement with volumetric adsorption, TPD studies, and magnetic susceptibility measurements discussed above. Final Remarks. In the present work the hydrogen interaction with a Rh/CeOa catalyst reduced at 773 K has been investigated by combining conventional volumetric and TPD studies with magnetic balance and electric conductivity measurements. The results reported here have been shown to be consistent to each other. In
300
T H ("1 Figure 4. Variation of the electrical conductivity (a) of Rh/
CeOz as a function of the treatmenttemperature (TH) under Hz (PH300 Torr). The catalyst was always cooled to 296 K, under hydrogen, before carrying out the conductivity measurements.
accordance with them, the reduction treatment at a temperature as high as 773 K does not seem to induce the classic SMSI effect,17-19 suggested earlier in the literaturelPl6 for a number of M/Ce02 catalysts. As the volumetric and TPD studies suggested, and the magnetic balance and electric conductivity measurements have confirmed, upon reduction at 773 K the chemisorptive capability of the metal crystallites does not seem to be drastically suppressed. The main difference between the sample reduced/evacuated at 773 K and that treated at 623 K9being the spilloverrate, much slower on the catalyst reduced at the highest temperature. Our results also suggest that the major component of the hydrogen chemisorbed forms on ceria leads to its reduction. This would be consistent with hydrogen species mainly chemisorbed on oxygen ions, with minor contributions of adspecies involving cationic centers either Ce(II1) or Ce(IV). The formation of hydroxyl species upon adsorption and Rh/CeOz catalystsl09M of H2 on both bare ceria3*4*27 has already been shown by FTIR spectroscopy.
Acknowledgement. The authors thank Mr. J. Disdier for carrying out the electrical conductivitymeasurements. This work has been partly supported by CICYT,ref. PB920483. G.A.C., expresses his gratitude to the Junta de Andalucia for the financial support of his stay at the Institute de Recherche8 sur la Catalyse (CNRS),Villeurbanne, France.
