G.A. Cifredo

Hydrogen chemisorption on ceria: influence of the oxide surface area and degree of reduction

The chemisorption of hydrogen on two ceria samples (CeO2-BS, 4 m2 g–1; CeO2-SM, 56 m2 g–1) reduced at temperatures ranging from 623 to 1173 K has been studied by Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption followed by thermal conductivity (TPD-TC) and mass spectrometry (TPD-MS). The concentration of the oxygen vacancies created by the reduction treatments was determined by using a combination of O2 pulses and temperature-programmed oxidation. According to our TPD-MS study, hydrogen can be desorbed from ceria as both H2(reversible adsorption) and H2O (irreversible adsorption), the relative contribution of these two forms depending on the reduction temperature. For samples reduced at 773 K or higher temperatures, H2 was the only desorption product. From this observation, some earlier TPD-TC and TPR-TC results could be better understood. Upon reduction at 773 K, the amount of H2 chemisorbed per mole of CeO2 was ten times larger for CeO2-SM than for CeO2-BS. Likewise, the molar chemisorptive capability of CeO2-SM strongly decreased (45 times) with the reduction temperature. No simple relationship could be observed between the amount of chemisorbed hydrogen and the total concentration of oxygen vacancies in the oxide. In contrast to earlier results on the contribution of a massive bronze-like phase when chemisorbing H2 at 195–500 K, the results reported here show that the hydrogen chemisorption on reduced ceria is a surface-related process. Furthermore, the highest value for the hydrogen chemisorption we have obtained, 7.1 H atom nm–2(BET), suggests a pure surface process.

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.

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.

Influence of reduction treatment on the structural and redox behaviour of ceria, La/Ce and Y/Ce mixed oxides

A methodology based on reoxidation at 298 K with O2(5%)/He pulses followed by quantitative TPO runs allowed us to study the redox state reached by CeO2 and Ln3+ -containing ceria‐based mixed oxide samples Ln3+:La3+ or Y3+) after their reduction in a flow of hydrogen. The temperature range covered in the reduction treatments was from 623 up to 1223 K. During the severe reduction treatments hexagonal sesquioxide phases segregate from all the samples. The reduction temperature at which segregation occurs follows the trend La/Ce < Y/Ce < CeO2. The TPO and H2-TPD traces of reduced samples which showed evidence of segregation in XRD exhibit characteristic features which can also be used as an indication of this effect. The discussion is focussed on the overall and detailed understanding of the reoxidation process and on the analysis of the H2-TPD profiles.

XPS analysis and microstructural characterization of a Ce/Tb mixed oxide supported on a lanthana‐modified transition alumina

In the framework of a research project aimed at developing alternative materials for application in three-way catalysis, this work reports on the preparation and characterization of a ceria-terbia mixed oxide supported on a lanthana-modified transition alumina. This complex multicomponent oxide system has been characterized by combining x-ray diffraction (XRD), high-resolution electron microscopy (HREM), temperature-programmed desorption mass spectrometry (TPD-MS), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Following a dry impregnation procedure, the lanthana-containing phase was deposited first, the ceria-terbia oxide being dispersed on the modified alumina in a subsequent step. The structural and chemical characterization studies carried out on the La 2 O 3 /Al 2 O 3 binary system provide no evidence of a crystalline lanthana-containing phase. Neither lanthanum oxide nor the phases resulting from its aging in air could be identified. Likewise, no lanthana-alumina mixed oxide phases could be deduced from analysis of the HREM micrographs, selected-area electron diffraction (SAED) or XRD patterns. As deduced from the H 2 O and CO 2 TPD studies, the chemisorptive properties of the La 2 O 3 /Al 2 O 3 are significantly different from those of the alumina and the bulk lanthana. We conclude, accordingly, that the supported lanthana consists of a highly dispersed oxide in strong interaction with the alumina support. By contrast, after deposition and further calcination of the cerium-terbium nitrate precursors, the presence of fluorite microcrystals of average size 5 nm could be identified both by XRD and HREM. The XPS studies carried out on the alumina support, La 2 O 3 /Al 2 O 3 and CeTbO x /La 2 O 3 /Al 2 O 3 , as well as a series of reference systems consisting of aged-in-air lanthana, CeLaO x and CeTbO x mixed oxides, have revealed the presence in the ternary system (CeTbO x /La 2 O 3 /Al 2 O 3 ) of at least two different types of chemical environments for the lanthanum ions. With the help of the La 3d 5/2 spectra recorded for the different reference systems, we conclude that these two environments correspond to highly dispersed lanthana strongly interacting with OH- and/or CO 3 2- groups, and to La 3+ species incorporated in the Ce/Tb mixed oxide phase.

Influence of the nature of the metal precursor salt on the redox behaviour of ceria in Rh/CeO2 catalysts

Abstract The hydrogen chemisorption on two Rh/CeO 2 catalysts prepared respectively from rhodium nitrate and rhodium chloride has been studied. The evolution of the ceria oxidation state with the series of treatments applied was monitored by means of a magnetic balance. Temperature Programmed Reduction and Oxidation (TPR/TPO) as well as Volumetric Adsorption techniques have also been used. Upon treating with H 2 , at 623 K, the Precursor/Support systems, the reduction level reached by ceria was 11.4% for the sample prepared from Rh(NO 3 ) 3 and much higher, 22.1%, for the chlorine containing catalyst. In the later case, the reduction degree varied very slightly with succesive evacuation/hydrogen adsorption cycles. This contrasts with the behaviour observed for the sample prepared from rhodium nitrate, for which the reduction process is to a much larger extent reversible. The results reported here for RhCl 3 /CeO 2 are interpreted as due to the substitution of the ceria lattice oxygen ions by Cl − , thus blocking the operation of both the direct and incorporation of Cl − into the support. This induces a much higher irreversible reduction of ceria as well as the blocking of both the direct and back spillover processes responsible for the reversiblity of the ceria reduction by hydrogen.

Preparation and characterization of a praseodymium oxide to be used as a catalytic support

This work reports on the preparation and characterization of a praseodymia sample to be used as a support for highly dispersed metals. The oxide sample was prepared by decomposing in a flow of helium, at 973 K, the phase obtained by precipitation with ammonia from a solution of Pr(NO3)3. The preparation conditions were established after investigating the thermal evolution of the precursor phase by thermogrammetric analysis, temperature-programmed decomposition-mass spectrometry, Fourier transform IR spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Data corresponding to the textural and chemical characterization studies carried out on the oxide are also presented and discussed.

Preparation of rhodium catalysts dispersed on TiO2SiO2 aerogels

Abstract TiO2 SiO2 aerogels have been used as supports to disperse rhodium. Three different ways have been followed for the preparations: (a) a classic TiO2 SiO2 aerogel, obtained from a mixture of alkoxides and water dissolved in ethanol, was impregnated with a rhodium nitrate solution; (b) a TiO2 SiO2 sonogel, obtained by hydrolysis of alkoxides in the absence of alcohol, under the action of ultrasound, was impregnated with a rhodium nitrate solution; and (c) a mixture of alkoxides and water, including rhodium nitrate in the reaction vessel, was exposed to the action of ultrasound, thus leading to a ternary Rh TiO2 SiO2 sonogel. The behaviour of these three catalysts has been compared with that of a Rh/TiO2 SiO2 system (d), obtained by conventional impregnation methods, starting from a commercial silica support. The samples prepared by impregnation of aerogels (a,b) present high levels of rhodium dispersion and show an increase of the catalytic activities in parallel with the pretreatment temperature in flow of hydrogen. On the contrary, the sample prepared including rhodium before gelling (c) presents a poor metallic dispersion, is not active for benzene hydrogenation, and has the capability of uptaking large amounts of hydrogen at ambient temperature. Both patterns of behaviour are greatly different from that characteristic of conventional Rh/TiO2 SiO2 catalyst (d).

HRTEM and TPO Study of the Behaviour under Oxidizing Conditions of some Rh/CeO2 Catalysts

A series of Rh/CeO2 catalysts reduced at temperatures ranging from 523 K to 1173 K has been investigated by O2 Pulses-TPO and HRTEM. Rhodium crystals can be fully oxidized to Rh2O3 by heating them at 773 K in flowing O2(5%)/He. The ceria oxidation state could also be determined in every case. Upon reduction at 1173 K, metal decoration and encapsulation phenomena do occur. This prevents the reoxidation of part of the rhodium crystals.