J.M. Rodríguez-Izquierdo

Reversibility of hydrogen chemisorption on a ceria-supported rhodium catalyst

Abstract This work reports on some new aspects of the chemistry of hydrogen-ceria systems. It is shown that, at room temperature, in the presence of highly dispersed rhodium, ceria chemisorbs large amounts of hydrogen. As deduced from magnetic measurements carried out in situ , this spillover process leads to the reduction of ceria to an extent of 21% of the total amount of cerium ions present in the sample, which is roughly equivalent to the complete surface reduction of the oxide. It is found that over a highly hydroxylated sample the reduction of ceria induced by the spillover process is partly reversible even at 295 K. If the sample is pumped off at 773 K, the initial oxidation state of ceria is almost completely recovered. Both the rate and extent of hydrogen chemisorption on ceria were found to be sensitive to the specific pretreatment applied to the catalyst. Over bare ceria, hydrogen chemisorption at 298 K was negligible, temperatures as high as 473 K being necessary to activate the process. In contrast to the rhodium-containing catalyst, over pure ceria the desorption of hydrogen leads to a much larger extent to water formation, thus revealing a deeper irreversible reduction of the oxide.

Behaviour of rare earth sesquioxides exposed to atmospheric carbon dioxide and water

Abstract The behaviour of the rare earth sesquioxides, when exposed to atmospheric water and carbon dioxide under ordinary conditions of temperature and pressure, is reviewed. Data corresponding to eleven samples from seven different oxides systematically investigated by X-ray diffraction (XRD), IR spectroscopy, thermogravimetry (TG) and temperature-programmed decomposition (TPD), are reported. The BET surface areas of all these samples are also given. In accordance with the results summarized, upon exposure to air, the lanthanide sesquioxides all become hydrated and carbonated in bulk. However, the rate and intensity of these processes, as well as the nature of the resulting phases, are not the same for the whole group of oxides. Furthermore, the characterization carried out on the oxides aged in air suggests that throughout the lanthanide series at least three types of behaviour can be distinguished. In some cases, the behaviour has been found to depend on the origin of the samples, which prevents the generalization of definite sequences of reactivity for the whole series of oxides. Also, the use of a single technique, particularly XRD, does not allow one to establish the nature of the aged phases, or even to determine whether or not alterations in the bulk of the starting oxides have occurred.

Study of some aspects of the reactivity of La2O3 with CO2 and H2O

The behaviour of three different samples of La2O3 on exposure to atmoshperic CO2 and H2O and the influence of the origin is discussed. The thermal evolution of the samples, stabilized in air for months or even years, has been studied by TG, TPD, IR spectroscopy and X-ray diffraction. BET surface areas of the samples were determined from the corresponding nitrogen adsorption isotherms at 77 K. In all three samples, hydration and carbonation occur in bulk. In accordance with our results, lanthana samples stabilized in air would consist of lanthanum hydroxide, La(OH)3, partially carbonated, La2(OH)6−3x (CO3)x,(x⪝1). When hexagonal phases of La2O3, obtained by calcining, at 1130 K, the samples stabilized in air were re-exposed, hydration and carbonation levels similar to those observed in the stabilized samples were reached after less than 24 h. Some lanthana samples were soaked in water, at 298 K, and then dried at 380 K. In this way, the evolution of the oxide when treated under similar conditions to those used in both impregnation and ion exchange techniques for preparation of supported metal phases, could be investigated.

High-resolution electron microscopy investigation of metal–support interactions in Rh/TiO2

High-resolution electron microscopy (HREM), combined with digital image processing and computer simulation techniques, has been used to carry out a microstructural characterization of some Rh/TiO2 catalysts reduced at 473, 773 or 823 K. A sample resulting from the reduction of the catalyst at 773 K, reoxidized with flowing O2 at 673 K and finally reduced at 473 K was also investigated. In contrast to earlier literature reports, the HREM images recorded here suggest the existence of some definite structural relationships between both the rutile and anatase titania microcrystals and the rhodium particles grown on them. Though some micrographs have been interpreted in terms of slightly reduced Magneli phases, our HREM study suggests that the major microstructural feature of the high temperature (773 and 823 K) reduced samples is the formation of an amorphous reduced titania phase, associated with which metal decoration occurs. Some metal sintering is also observed. The reoxidation treatment, although inducing reversal of the decoration process leads to larger metal particles than those of the starting low-temperature reduced catalysts. Note, also, that twinned metal particles are more often observed in the catalyst resulting from the mild reduction of the reoxidized sample.

Cerium-terbium mixed oxides as alternative components for three-way catalysts: A comparative study of Pt/CeTbOx and Pt/CeO2 model systems

Abstract By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbOx catalyst is compared to that of a classic model TWC system: Pt/CeO2. The results reported here show that the redox efficiency of the Pt/CeTbOx catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.

Thermal evolution of a sample of La2O3 exposed to the atmosphere

Abstract Because the basic character of La 2 O 3 carbonation and hydration of this oxide may strongly modify its catalytic behaviour, a study of the regeneration conditions of a sample of La 2 O 3 exposed to atmospheric H 2 O and CO 2 for a long period of time have been carried out. TG-DTA data have shown that the thermal decomposition of La 2 O 3 in a flow of helium takes place in three well-defined steps: evolution of water in the first two steps, and CO 2 in the third could be observed by TPD. Stoichiometric calculations as well as IR spectroscopy and X-ray diffraction data corresponding to the initial sample and each of the phases formed after the three steps reported previously, suggest that exposure of lanthanum oxide to the air results in the formation of the hydroxide and an amorphous hydroxycarbonate phase. The regeneration of the hexagonal phase of La 2 O 3 occurs above 1020 K, after the third step, which seems to correspond to the decomposition of a dioxomonocarbonate phase.

Promoting effect of Lanthana in the hydrogenation of carbon monoxide over supported rhodium catalysts

Abstract The catalytic activity in the CO+H 2 reaction is reported for a number of supported rhodium catalysts including Rh/La 2 O 3 , Rh/SiO 2 , Rh/Al 2 O 3 , as well as ternary phases Rh/La 2 O 3 /SiO 2 with lanthana loadings ranging from 0.1% to 8.5%. The highest conversions to C 2 -oxygenates were found to occur over the lanthana containing catalysts. In the particular case of ternary catalysts a clear promoting effect of lanthana was observed, the highest activity and selectivity to oxygenates being found at quite low lanthana loading: 0.25%. Our studies of both characterization and catalytic activity suggest that the metal particles were coated by lanthana. According to the lanthanum and rhodium contents determined in the solutions resulting from the impregnation treatment, redissolution of lanthana during this step of the preparation of the catalysts was found to occur. An interpretation of the mechanism of coating of the metal crystallites by lanthana is presented.

Characterisation of rare earth oxide supported metal catalysts. Study of some ceria supported rhodium phases

Abstract The present work reports on the characterisation of several ceria supported rhodium catalysts. The selected reduction temperature were: 623 K and 773 K; the metal loading: 1% and 2.5%. The surface area of the two starting ceria samples used as support were: 20 m 2 .g −1 (LS) and 110 m 2 .g −1 (HS), respectively. The apparent H.Rh and CO/Rh were found to be quite different, which according to the results of the TPD study can be interpreted as due to the occurrence of strong adsorption of hydrogen on the support. Upon reduction at 773 K the adsorption capabilities are notably reduced. The effect is much stronger in the case of the phases based on HS ceria. For Rh/CeO 2 (LS) catalysts, the metal dispersions determined by HRTEM agree fairly well with those estimated from CO adsorption. By contrast, over Rh/CeO 2 (HS) catalysts, the values determined by TEM were found to be larger than those deduced from chemisorption. Taking into account that the reduction treatment leading to the Rh/CeO 2 (HS) induces strong sintering phenomena on the support and that this effect is negligible in the case of Rh/CeO 2 (LS), the results above have been interpreted assuming that the inhibition of the chemisorptive capacity of Rh/CeO 2 (HS) reduced at 773 K is primarily due to the encapsulation of the metal particles associated to the sintering occurred at the support. Reoxidation of the phases reduced at 773 K did not allow to recover the chemisorptive properties of the catalysts, which confirms that no classic SMSI occurs in the present case.

Metal-support interaction phenomena in rhodium/ceria and rhodium/titania catalysts: Comparative study by high-resolution transmission electron spectroscopy

Abstract The high resolution transmission electron microscopy (HRTEM) study of Rh/CeO 2 and Rh/TiO 2 catalysts reduced at temperatures ranging from 473 K to 773 K suggests that the metal-support interaction phenomena they show are different. In contrast to that found for Rh/TiO 2 , over Rh/CeO 2 no metal decoration effects could be observed. Likewise, the chemisorptive and catalytic behaviour of Rh/ CeO 2 does not exhibit drastic changes with the reduction temperature. The HRTEM images, however, reveal the occurrence of quite singular rhodium-ceria interaction phenomena.

Reducibility of ceria–lanthana mixed oxides under temperature programmed hydrogen and inert gas flow conditions

Abstract The present paper deals with the preparation and characterization of La/Ce mixed oxides, with La molar contents of 20, 36 and 57%. We carry out the study of the structural, textural and redox properties of the mixed oxides, comparing our results with those for pure ceria. For this aim we use temperature programmed reduction (TPR), temperature programmed desorption (TPD), nitrogen physisorption at 77 K, X-ray diffraction and high resolution electron microscopy. The mixed oxides are more easy to reduce in a flow of hydrogen than ceria. Moreover, in an inert gas flow they release oxygen in higher amounts and at lower temperatures than pure CeO2. The textural stability of the mixed oxides is also improved by incorporation of lanthana. All these properties make the ceria–lanthana mixed oxides interesting alternative candidates to substitute ceria in three-way catalyst formulations.