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Cryptomelane has been widely applied as catalyst in oxidation reactions due to its excellent redox properties and low cost. Here, a novel one-pot hydrothermal synthesis using a potassium permanganate aqueous solution as precursor and ethanol as reducing agent has successfully been developed to obtain cryptomelane nano-oxides. This synthetic route makes it possible to control the amount of potassium incorporated into the structure of the cryptomelane by selecting the appropriate synthesis temperature and ethanol initial concentration. Taking advantage of this approach, the effect of potassium concentration on the structural stability and reducibility of the cryptomelane, which are poorly discussed in the literature, has been studied. We have observed that samples with low content of potassium (∼11%) show high conversions of CO to CO2 especially at low temperatures. The lower activity of the samples with high K contents (∼16%) can be ascribed to the beneficial effect of K on the structural stability of cryptomelane in detriment of labile oxygen on cryptomelane surface.

Redox and catalytic performance in total methane oxidation of a nanostructured ceria–terbia catalyst supported on magnesia is presented and compared to that of a pure ceria catalyst supported on MgO. The investigated material, Ce0.5Tb0.5Ox (3% mol.)/MgO, features several remarkable properties: a quite low total molar loading of the two lanthanide elements, high reducibility as well as very high oxygen storage capacity at low temperatures and higher catalytic activity than MgO-supported CeO2. In terms of lanthanide atomic content, the catalytic performance of Ce0.5Tb0.5Ox (3% mol.)/MgO largely improves compared to that of bulk type ceria and ceria–magnesia solid solutions. Such a behavior implies proper optimization of the usage of lanthanide elements. A second contribution to atomic economy in the catalyst design relates to the fact that the novel formulation demonstrates a good stability in the redox and catalytic performance against very high temperature treatments. An investigation of the structure of both the fresh and high temperature-aged catalysts at the atomic scale, by means of complementary aberration corrected microscopy techniques, reveals the occurrence of a variety of highly dispersed, exotic, lanthanide-containing nanostructures, which span from isolated, atomically dispersed, Ln species to nanometer-sized CeTbO2−x patches, extended CeTbO2−x bilayers and 3D CeTbO2−x nanoparticles. Nanoanalytical results evidence the mixing of the two lanthanides at atomic levels in these nanostructures. The combined effects of nanostructuring, mixing of the lanthanides at the atomic level and interaction with the MgO oxide are the roots of the improvement in functional, redox and catalytic properties of the novel Ce0.5Tb0.5Ox (3% mol.)/MgO catalyst.

Ni/CeO 2 /YSZ and Ni/Ce 0.15 Zr 0.85 O 2 have been investigated as catalysts for the dry reforming of methane at 750 °C. Ni was incorporated by the impregnation method. The supports were previously activated by using a thermo-chemical protocol consisting on a severe reduction (H 2 /Ar) at 950 °C followed by a mild oxidation (O 2 /He) at 500 °C. According to TPR results, this protocol leads to the development of unique redox properties in the case of the CeO 2 /YSZ oxide. Two types of CO 2 + CH 4 (1:1) mixtures (helium-diluted and undiluted) were used to feed the reactor. When using the Ni/Ce 0.15 Zr 0.85 O 2 catalyst with undiluted feed, the reactor became plugged by coke. By contrast, Ni/CeO 2 /YSZ behaved as an active and stable catalyst even under the most severe operation conditions. The characterization of the spent Ni/CeO 2 /YSZ using TGA, TEM, Raman and XPS spectroscopy revealed that only a limited amount of graphitic carbon, in form of nanotubes, was formed. No evidences of deactivating carbonaceous forms were obtained. The singular redox properties of the activated CeO 2 /YSZ oxides are proposed as a key for designing Ni catalysts highly stable in reforming processes.

By depositing ceria on the surface of yttrium stabilized zirconia (YSZ) nanocrystals and further activation under high-temperature reducing conditions, a 13% mol. CeO2/YSZ catalyst structured as subnanometer thick, pyrochlore-type, ceria-zirconia islands has been prepared. This nanostructured catalyst depicts not only high oxygen storage capacity (OSC) values but, more importantly, an outstandingly stable redox response upon oxidation and reduction treatments at very high temperatures, above 1000 degrees C. This behavior largely improves that observed on conventional ceria-zirconia solid solutions, not only of the same composition but also of those with much higher molar cerium contents. Advanced scanning transmission electron microscopy (STEM-XEDS) studies have revealed as key not only to detect the actual state of the lanthanide in this novel nanocatalyst but also to rationalize its unusual resistance to redox deactivation at very high temperatures. In particular, high-resolution X-ray dispersive energy studies have revealed the presence of unique bilayer ceria islands on top of the surface of YSZ nanocrystals, which remain at surface positions upon oxidation and reduction treatments up to 1000 degrees C. Diffusion of ceria into the bulk of these crystallites upon oxidation at 1100 degrees C irreversibly deteriorates both the reducibility and OSC of this nanostructured catalyst.

A Ni/CeO2/ZrO2 catalyst with improved redox properties has been washcoated onto a honeycomb cordierite monolith in the form of a nonconventional alumina-catalyst layer, just a few nanometers thick. In spite of the very low active phase loading, the monolith depicts outstanding performance in dry reforming of methane, both in terms of activity, with values reaching the thermodynamic limit already at 750 °C, even under extreme Weight Hourly Space Velocities (WHSV 115−346 L·g−1·h−1), as well as in terms of stability during prolonged Time on Stream (TOS 24−48 h).

A new procedure to determine, with very high accuracy, the lattice parameter of individual supported nanoparticles of interest in catalytic system is presented, checked and applied to ceria supported Palladium catalysts. The procedure is based on the analysis of double diffraction effects present in High Resolution Electron Microscopy (HREM) images recorded in top view conditions. For nanoparticles about 5 nm in diameter accuracies in the order of ±0.2% are demonstrated by using a method in which intensity profiles of Digital Diffraction Patterns obtained from experimental HREM images are fitted to a set of Lorentzian-type functions. The new method allows characterizing the distribution of lattice parameter of nanoparticles in systems which are out of range for XRD, as it is the case of catalysts with very low loadings or those in which the particle size distribution is dominated by very small ( 773 K). Likewise, in Pd/CeO2 catalysts reduced at low temperatures (

A series of Ce–Zr mixed oxides with different composition have been characterized and used as catalysts in the catalytic wet oxidation (CWO) of phenol. The addition of Zr significantly enhanced the redox and catalytic activity of ceria. However, a catalytic activity drop occurred for zirconium contents higher than 80%, which is related with a ZrO 2 segregation observed in Zr-rich oxides. The effects of the temperature of reaction, the oxygen partial pressure and the phenol/catalyst ratio on the activity for the oxidation of phenol and the selectivity to CO 2 were also investigated. The values for CO 2 selectivities were greatly influenced by the formation of a carbonaceous deposit adsorbed on the catalyst surface. The results indicated that these C-deposits can be totally oxidized by selecting appropriate operating conditions, thus obtaining catalytic systems with a very promising performance for phenol abatement. A mechanism is proposed to interpret the role of the carbonaceous deposits in the oxidation of phenol as a function of the reaction conditions.

MnOx–CeO2 and Pt(2%)/MnOx–CeO2 catalysts were prepared and investigated in the catalytic wet oxidation (CWO) of phenol. The reaction was carried out in a high-pressure autoclave reactor operating at 140 °C and oxygen partial pressure of 2 MPa. The effect of a reduction pre-treatment at 350 °C on the catalytic performance of both, the platinum catalyst and the bare oxide was studied. Higher activities were found for the reduced catalysts, independently of the presence or absence of Pt. Formation of carbonaceous deposits on the catalytic surface and leaching of manganese was confirmed as the two major deactivation mechanisms occurring in CWO of phenol. The amounts of deposits formed and manganese leached were higher in the case of reduced samples. Different techniques (XRD, HREM, HAADF, EDS, ICP) were used to characterise the chemical and structural changes experimented by the catalysts during reaction. The behaviour of reduced catalysts was found to be associated to the presence of MnO. The formation of this manganese oxide phase during the reduction pre-treatment significantly influenced the catalytic performance and stability of the catalysts under operation conditions.

A series of Ce/Mn oxides has been characterized using a variety of complementary transmission electron microscopy techniques to obtain nanometer scale information on the structure, chemical composition, and oxidation states of the different phases present in this extremely complex system. The phases have been identified to be α-Mn2O3, β-MnO2, mixed-valence manganese oxides (Mn3O4 and Mn5O8), and Ce−Mn mixed oxides in different proportions depending on the Ce/Mn atomic ratio. A correlation between the nanostructure of this composite system and its catalytic activity for the wet oxidation of phenol, as a model contaminant, is proposed.

Different Ce Mn composites have been used as catalyst for the Catalytic Wet Oxidation (CWO) of Phenol under different operating conditions in the range of 80–160 °C and 0.5–2 MPa P O 2 . High mineralization performances were observed by operating at moderate temperatures (130–160 °C) and oxygen partial pressures (1–2 MPa). The mechanism proposed for the mineralization process includes the initial polymerization of phenol with the rapid formation of a carbonaceous deposit on the catalytic surface (favoured by the presence of Mn), which is later oxidized to CO 2 and H 2 O (the richest Ce catalysts being more active in this process). At milder conditions, i.e. temperatures below 110 °C, no mineralization was observed and only the first process operates producing the irreversible polymer deposition until the total activity loss of the catalyst for phenol elimination.

Temperature–chemical environment–oxidation state diagrams are proposed as an alternative way of reporting on the redox behaviour of ceria-based catalytic materials. In these diagrams, the changes occurred in the reduction degree of the investigated materials are plotted against temperature for different chemical atmospheres (5%O 2 /He, He, 5%H 2 /Ar, and 5%CO/He). Compared to more conventional parameters like oxygen storage capacity, these diagrams allow us to gain a much wider view of the redox response of the investigated system. As an example, the diagrams are used to discuss the redox properties of two different ceria–terbia mixed oxide samples.

A series of Ce–Mn composites with different compositions was prepared by co-precipitation. All samples were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). According to their size and chemical composition, two different kinds of particles were observed: (a) small crystallites of Ce–Mn solid solution ( 50 nm). These results were used to explain differences between the surface and bulk compositions of the samples as well as the specific surface area. Furthermore, from XPS studies, the presence of various Mn species with different oxidation states could be stated. This fact might clarify the exceptional catalytic behaviour of the samples for the catalytic wet oxidation of toxic organic compounds. Copyright  2004 John Wiley & Sons, Ltd.

Benzene hydrogenation and n -butane hydrogenolysis reactions have been tested on a series of Rh/La 2 O 3 /SiO 2 catalysts, their catalytic properties being compared with those of a Rh/SiO 2 sample studied as reference. This investigation has revealed that both activity and selectivity can be controlled by the procedure followed to prepare the catalysts. In particular, the influence of preparative conditions such as the lanthana loading, method of introduction of the metal and promoter (either consecutive or by co-impregnation) and temperature of calcination (873 or 1173 K) of the intermediate La 2 O 3 /SiO 2 phase onto which the noble metal is later deposited, have been analyzed. The differences in the catalytic behavior have been correlated with the La 2 O 3 /SiO 2 interactions, leading to a different degree of surface hydroxylation and different Rh particle crystallinity.

The potential applicability of high-resolution electron microscopy (HREM), in combination with image analysis and image simulation tools, to retrieve structural information from nanometre-sized particles present in oxide-supported metal and oxide catalysts is analysed. Specifically, the possibilities and limitations of this technique to determine features such as the size, morphology and chemical nature of the particles, their surface structure and their structural relationship with the support are considered through the discussion of several examples. The interpretation of a series of HREM images of Pt and Rh catalysts supported on cerium oxides after treatments under different redox environments illustrates the case of highly dispersed metals. In addition, the results obtained in this study provide an approximate picture of the evolution of metal– support interaction effects in this family of catalysts, which is closely related to three-way catalysts (TWCs). The results of a nanostructural investigation of two catalyst systems, one consisting of MgO-supported neodymia clusters and the second of vanadium– magnesium oxide also supported on MgO, provide the examples for supported oxide catalysts. These find application in oxidation reactions. For the former, the growth of neodymia in the form of rounded patches in a parallel orientation relationship with the support has been observed. For the vanadia-containing catalysts, the formation of a weakly ordered MgV2O4 spinel surface phase on the MgO support crystallites, after exposure to typical reaction conditions in the oxidative dehydrogenation of propane, has been confirmed. The structural relationship at the spinel/MgO interface has also been established. Copyright © 2000 John Wiley & Sons, Ltd.

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.

This work reports on a high resolution electron microscopy (HREM) study of a series of Rh/CeO2 catalysts prepared from both Rh(NO3)3 and RhCl3 metal precursors. Our attention is focused on the influence of the reduction/reoxidation conditions on the metal particle size distribution. From the analysis of both experimental and computer simulated HREM images, we were able to conclude that the truncated cubo–octahedron model describes well the morphology of the rhodium microcrystals grown on ceria. Likewise, we have used the computer simulation techniques to estimate the lowest size limit for HREM detection of the rhodium particles in Rh/CeO2 catalysts. In our case, the smallest metal particles are well above the detection limit, and therefore, the reported size distributions account well for the actual ones. Three different (A–C) size distribution curves have been built up. From their analysis, we conclude that metal dispersions estimated from the mean size metal particle (Dd) can significantly deviate from the more accurate values determined from the so-called type C distribution curves (Dm). Reduction treatments up to 973 K induce a progressive sintering of the metal, the effect becoming much stronger upon reduction at 1173 K. We have also found that metal dispersion is more sensitive to the reduction treatment in Rh(Cl)/CeO2 catalysts. Among the reoxidation temperatures investigated by us, rhodium redispersion could only be observed at the highest temperatures: 1073 K or 1173 K. By contrast, reoxidation at 773 K causes significant metal sintering effects.

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.

In this work, the influence of the preparation procedure on the chemical and microstructural properties of a series of Rh/La 2 O 3 /SiO 2 catalysts has been investigated. The catalysts were characterized by using HREM, H 2 and CO volumetric adsorption and FTIR spectroscopy of chemisorbed CO. From our study, significant effects were observed for the following preparative variables: (a) the lanthana loading; (b) the way of introducing the metal and promoter, either consecutive (lanthana first and then the rhodium salt) or by co-impregnation; and (c) the calcination temperature (873 K or 1173 K) applied for preparing the La 2 O 3 /SiO 2 intermediate phase onto which the noble metal was deposited.

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.

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.

Platinum SiO 2 sonogel has been compared with the corresponding xerogel and an impregnated catalyst. The thermal behavior, the FTIR spectra and the pyridine adsorption show that Lewis acidity is present in the impregnated sample. In the sonogel, Lewis acidity turns to be very low and insensitive to desorption temperature. In the xerogel, both Lewis and Bronsted acidity was observed. In each sample, reduction in situ (without hydrogen flow) of the metal phase is determined by the OH content. Platinum is highly reduced in the sonogel, partially reduced in the xerogel and very slightly reduced in the impregnated sample.

6 páginas, 10 figuras, 1 tabla, The main current theories dealing with the crystalline structures of the fluorite related rare earth oxides, including those corresponding to reduced oxides, one based on the distribution of the coordination defect inside the fluorite structure, and the other which proposes the establishment of modular sequences constituted by modules with fluorite structure, are presented and comparatively explored in detail. Our in-depth analysis of both approximations indicates that they in fact provide smart and efficient rationalizations of the currently known intermediate rare earth oxides structures. We prove however that the strict application of the principles and rules proposed by each theory does not yield unique and unambiguous results for most of the members of the homologous series, as it has been claimed up to now. Moreover, the controversy about the reliability of these two, apparently different and competing, theories is definitely clarified and the exact equivalence of their structural predictions is demonstrated. Finally, we propose new extra systematization rules, not considered up to now in neither of these theoretical approaches, to overcome the observed limitations to properly rationalize the structure of this so technologically important family of oxides., We acknowledge financial support from MICINN-FEDER (Proj. Ref: MAT2008-00889-NAN and CSD2009-00013) and Junta de Andalucia (TEP217 and FQM334).

The influence of the preparation method on the microstructure and catalytic behavior of Rh and Ni dispersed on TiO2-SiO2 aerogels is investigated.

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.

Sol-gel methods have been recognized as interesting procedures to prepare catalysts. The versatility of the sol-gel techniques allows control of the texture, composition, homogeneity and structural properties of solids, and makes possible production of tailored materials such as dispersed metals, oxidic catalysts and chemically modified supports. In this work, different approaches, described in the literature, to prepare catalysts by the sol-gel methods, including both aerogel and xerogel active solids, are reviewed. In parallel, some of the most relevant catalytic reactions carried out with sol-gel catalysts are evaluated.

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).

High-resolution transmission electron microscopy has been applied to the characterization of the Rh/Sm2O3 system. The comparison between Rh/CeO2 and Rh/Sm2O3 catalysts suggests a model of behavior that would be operative for metals supported on rare-earth sesquioxides. The model assumes that a significant dissolution of the support takes place during the impregnation step. As a consequence, the metal would be present in a highly dispersed state after the drying process, forming a mixed phase with the support material, which explains why metal crystallites are only observed after reduction followed by heat treatment at high temperature.

On the basis of detailed previous knowledge about the correlation existing between the Oxygen Storage Capacity (OSC) of ceria–zirconia mixed oxides, the nature of redox pretreatments and the nanostructure of this family of complex materials, it has been possible to design a synthetic strategy allowing one to prepare ceria–zirconia and ceria–yttria-doped zirconia materials, with total ceria contents below 20%, featuring OSC values higher than those observed for catalysts which incorporate supported noble metal particles in their formulation, especially in the very low, 373–473 K, temperature range. These novel, noble-metal free and highly reducible, ceria–zirconia materials allow a much more efficient usage of ceria as a component of oxygen storage formulations, which is nowadays considered one of the key innovation targets in the field of lanthanide oxide based catalysts.

Summary Tetraethoxysilane and tetrabutylorthotitanate were used as precursors to obtain a series ofTiO 2 -SiO 2 gels. The use of ultrasound, and the type of drying treatment followed to obtain catalytic materials, play a significant role in the dispersion of Ti atoms. The structural properties of these gels, studied by XRD, XANES and FTIR spectroscopy can be related to their acidity. Thus, the trends of activities for tert-butanol decomposition to isobutene are in tight connection both with the TiO 2 content and with the observed levels of Ti dispersions. Rh and Pt supported catalysts, prepared from several TiO 2 -SiO 2 gels, present higher activities and selectivities for n-butane hydrogenolysis than other preparations used as references.

Three different TiO2-SiO2 gels (Xerogel, Carbogel and Aerogel) are more active acid catalysts than other reference samples used here. As deduced from FTIR, XRD and XANES studies, the structural properties of these gels are quite different to each other, thus revealing the strong influence of the drying treatment. It is found that the degree of Si-O-Ti linking and the surface acidity follows the same trend (X 〉 C 〉 A). We conclude that supercritical drying at 600 K and 190 bars can induce Ti leaching followed by redeposition in the narrower pores of the gel. These effects modify both the textural and surface chemical properties of the resulting material.

In the presence of rhodium, the H2–CeO2 interaction is to a large extent reversible, simple evacuation of the reduced sample has been demonstrated to promote reoxidation of the support; this explains some puzzling effects of the reduction/evacuation treatments on the actual oxidation state of ceria in Rh–CeO2 catalysts.

The decomposition of 1-propanol and 2-propanol over a series of eight lanthanide oxides (La2O3, CeO2, Pr6O11, Sm2O3, Eu2O3, Dy2O3, Ho2O3 and Yb2O3) is studied. The activity data were obtained in a flow reactor at normal pressure. The alcohol partial pressure was around 69KPa (520 Torr). The conversions were generally lower than 10%. Dehydrogenation to carbonylic products and dehydration to propylene were the only reactions here observed. For both alcohols, the differences of activity among the eight oxides, though significant, were within one order of magnitude. Contrary to suggested in the literature no simple relationship was found between the position of 4/element in the series and the catalytic activity of the corresponding oxides. No definite patterns of apparent activation energies or selectivity data were observed either. When the reactivities of 1-propanol and 2-propanol towards dehydration are compared, significantly lower values are found for the former alcohol. The origin of this influence of the alcohol structure, as well as the chemical meaning of the observed linear relationships between Ea and log A, are discussed. © 1983, R. Oldenbourg Verlag, Munchen

The preparation of lanthana supported rhodium catalysts, with 1% and 10% metal loading, was investigated. The impregnation of lanthana induces nucleation on the support of a crystalline hydroxycarbonate phase, La(OH)CO 3 , which can represent up to 80% of the original lanthanum oxide. The study of the reduction process shows that the behaviour of lanthana support is very complex. Following the occurrence of carbonation phenomena on lanthana, the nature of the phase acting as support after the reduction treatment depends not only on the reduction conditions, but also on the previous treatment of the sample to be reduced. Reduction conditions allowing the preparation of true lanthanum oxide supported rhodium catalysts can be established.

Role des supports, evolution en cours de preparation des catalyseurs metal/oxyde de lanthanide

The preparation of two ceria - supported rhodium catalysts has been studied by means of XRD, TG, TPD-MS and TPR-MS. The two cerium dioxides used here, from different origins, showed BET surfaces areas of 19 m 2 g −1 and 110 m 2 g −1 . According to the TPR-MS study the Rh(NO 3 ) 3 /CeO 2 samples,the precursor reduction takes place at slightly lower temperature over the ceria sample with higher surface area. The preparation of rhodium supported on free carbonate ceria samples requires reduction temperatures higher than approximately 600 K. The reduction of the support has been found to occur at about 873 K. The temperature at which this latter process is observed, as well as the intensity of this reaction,was found to be independent both of the ceria sample investigated and of the presence of supporting rhodium. The study by XRD of the reduced and unreduced samples suggests that associated with the reduction treatment in flow of H 2 , some sintering of the support occurs. In accordance with our resuIts,upon reduction at 773 K, changes of the surface area from 110 m 2 g −1 to 20 m 2 g −1 can be observed.

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.

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.

The present work deals with the preparation of some lanthana, samaria and ytterbia supported rhodium catalysts. The three oxides used as starting supports are actually hydrated and carbonated in bulk; the intensity of the processes, and even the nature of the resulting phases are quite different from each other. It has been found that the impregnation process induces further carbonation of lanthana, to such an extent that the carbonated phase becomes the major component of the support. No similar effects could be detected on samaria and ytterbia. From the study of the reduction step, experimental conditions which allow the preparation of true rare earth oxide supported rhodium catalysts have been devised. In order to achieve this, reduction temperatures ranging from 723 K (La 2 O 3 ) to 623 K (Yb 2 O 3 ) were found to be necessary. In the case of lanthana the phases resulting from the use of lower reduction temperatures were also studied. It can be concluded from the present work that the behaviour as support of lanthanide oxides, particularly in the case of lanthana, is even more complex than had been presumed earlier.

Data corresponding to the catalytic decomposition of 2-propanol over La 2 O 3 , Eu 2 O 3 and two samples of Yb 2 O 3 (denoted (a) and (b)) are reported. Yb 2 O 3 (a) was prepared by the same method as La 2 O 3 and Eu 2 O 3 . Yb 2 O 3 (b), which was obtained by grinding Yb 2 O 3 (a), showed quite different textural properties from Yb 2 O 3 (a). The specific activity, with reference to 1 m 2 of the Brunauer-Emmett-Teller surface area, towards both dehydrogenation and dehydration of the alcohol is found to be about 12 times higher for Yb 2 O 3 (b) than for Yb 2 O 3 (a). When the specific activities of the three 4f oxides prepared in the same way are compared, the differences are smaller than those observed between Yb 2 O 3 (a) and Yb 2 O 3 (b). On the basis of these results some correlations relating to the recently reported catalytic behaviour of lanthanide oxides are discussed.

The catalytic activity of eight lanthanide oxides (La 2 O 3 , CeO 2 , Pr 6 O 11 , Sm 2 O 3 , Eu 2 O 3 , Dy 2 O 3 , Ho 2 O 3 and Yb 2 O 3 ) prepared in our laboratory was studied. The test reactions were the decomposition of ethanol and 2-pentanol. These data and those previously reported for the decomposition of 1-butanol and 2-butanol were used to check the existence of correlations in the catalytic behaviour of the 4f oxides. Three different parameters, reaction rate, fractional selectivity and activation energy, were used to describe the behaviour of the catalysts. The validity of some general correlations involving the properties of Ln 3+ ions (Ln ≡ lanthanide) which have been proposed in the literature are discussed on the basis of the results obtained.

This work reports on the characterization of two Rh/La 2 O 3 catalysts with 10% metal loading, reduced at 623 K and 723 K, respectively. The chemisorption studies showed that H/Rh ratios were much larger than those determined for CO/Rh. This has been interpreted as due to the occurrence of hydrogen spill over. The study of the catalysts by XRD and HRTEM revealed that no rhodium containing phases other than the metal are present in the reduced phases. The XRD and HRTEM results also showed that the metal particles are well dispersed with amean size not larger than 5 nm. These results, which suggest that the metal particles are buried into the support, are discussed in relation to the mechanisms involved in the process of decoration/encapsulation of the rhodium crystallites.

This work reports on the chemical and microstructural characterization of a series of Rh/CeO 2 catalysts. The volumetric adsorption and FTIR spectroscopic studies show the occurrence of considerable changes in the chemistry of the H 2 -(Rh/CeO 2 ) system upon increasing the reduction temperature from 623 to 773 K. However, the H/Rh values determined from hydrogen isotherms at 191 K suggest that the rhodium microcrystals remain chemically active after reduction at 773 K. The major effect of the high-temperature reduction treatment would be the blocking of the spillover process, much slower on the catalysts reduced at 773 K. In agreement with this, no dramatic effect of the reduction temperature on the catalytic activity for benzene hydrogenation has been found

A detailed high resolution electron microscopy investigation of different Rh/lanthana catalysts has been carried out. Samples prepared by wet impregnation using either aqueous or acetone solutions of rhodium nitrate and metal deposition by evaporation under high vacuum have been studied. The nanostructural evolution of the support and the metal phase through the different preparation steps has been tracked. Thus, the effects of reduction treatments in a wide range of temperatures (473–973 K) have been taken into consideration. Scanning electron microscopy images evidence an intense dissolution of the lanthanide sesquioxide crystallites during the impregnation step with the rhodium nitrate aqueous solution. Such an effect could not be detected in the catalyst prepared using an acetone solution of the same metal precursor. On its hand, HREM indicates the presence of a highly dispersed metal phase in the catalyst prepared in water after reduction at low temperatures. Increasing the reduction temperature leads to a significant sintering of rhodium present both on the surface and the bulk of the support. Decorated rhodium particles are clearly visible in the three catalysts in the whole range of reduction temperatures. The drastic structural rearrangements which take place in the support during the reduction treatment and support dragging during sintering, are most likely the driving force for the decoration of the metal particle surfaces.

Thermal analysis, energy dispersive spectroscopy, X-ray diffraction, high-resolution electron microscopy, X-ray photoelectron spectroscopy, and CO 2 adsorption have been applied to study a series of La 2 O 3 /SiO 2 samples with lanthana loadings ranging from 1.6 to 75 wt%. The results are interpreted on the basis of a lanthana–silica interaction model which assumes that patches of an amorphous La–Si–O phase, embedded in the silica matrix, modify the support surface in a progressive way, leading to a full coverage for 37.5% La 2 O 3 . This mixed silicate is soluble in acid media and its properties can be satisfactorily explained considering a structure which has a short-range order resembling that of crystalline La 2 Si 2 O 7 . This behaviour differs from that described in the La 2 O 3 /Al 2 O 3 system, more extensively studied in the literature, and from previous studies on La 2 O 3 /SiO 2 in which the formation of La 2 O 3 particles on the silica surface was proposed. This contribution serves as a basis for understanding the additional promoting effect of lanthana in catalysts consisting of metals dispersed on modified silica supports.

Ultrasound can be used in sol-gel synthesis as a source of energy which may modify the structural and chemical properties of the resulting gels. Pt/SiO 2 sonogel catalysts were obtained and characterized by X-ray diffraction and small angle X-ray scattering. It was found that the aging and polymerization mechanisms are altered by ultrasound. The catalysts were then tested in phenylacetylene hydrogenation, showing a high selectivity toward styrene.

The redox behaviour of a Ce0.68Zr0.32O2 mixed oxide is reversibly modified by alternating high temperature (1223 K) reduction with either mild (823 K) or high temperature (1223 K) re-oxidation treatments.

A new method to compare the oxygen buffering capacity (OBC) of catalysts shows that Ce–Tb mixed oxide presents an exceptional capability for attenuating fast oscillations of oxygen partial pressure.