Vincent Perrichon

Reduction of CeO2 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and X-ray photoelectron spectroscopy measurements

The reduction of CeO2 by hydrogen has been studied from 300–1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV–VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO2 samples were used with B.E.T. surface areas of 115 and 5 m2 g–1, respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the CeIII photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO1.83. Temperatures higher than 923 K were required to reduce the ceria further. The surface CeIII content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17 000 cm–1 band in the UV–VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO1.83 led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum.

Reduction of cerias with different textures by hydrogen and their reoxidation by oxygen

Successive reduction steps of CeO2 particles by hydrogen between 300 and 1070 K have been followed by temperature-programmed reduction (TPR) and in situ magnetic measurements on several samples with different BET surface areas. The nature of the phases present in cerias reduced between 670 and 1270 K was determined by X-ray analysis. Finally, reoxidation by oxygen or air was studied at room temperature for all the reduced samples.Magnetic and TPR results show a direct relationship between the degree of reduction and the BET surface area. Indeed, for most of the samples, the degree of reduction at 620–670 K determined by magnetism corresponded to the creation of one layer of Ce3+ ions at the surface of the ceria. A similar relationship between the BET surface area and the extent of reduction was established using the area of the low-temperature TPR composite peak, the maximum of which was found to be constant at 810 K.When the reduction progresses further into the bulk, two main phases were evidenced: first, and expanded cubic CeO2 –x phase derived from the initial ceria by a dilatation of the whole structure and, for deeply reduced samples, the hexagonal Ce2O3 phase. A new intermediate phase, cubic Ce2O3, was also observed on samples reduced at 1070–1170 K.Complete reoxidation by oxygen occurs at room temperature, for all reduction percentages below ca. 60 %, i.e. as long as the reduced phase remained in the cubic form. When the hexagonal Ce2O3 phase has been formed, the reoxidation cannot be completed at 294 K.

Thermal stability of a high surface area ceria under reducing atmosphere

Abstract The influence of a reducing atmosphere on the thermal stability of CeO2 was studied on a high surface area ceria sample (115 m2 g−1) treated 2 h under hydrogen or carbon monoxide at various temperatures between 673 and 1123 K. Comparative experiments were done under air or vacuum, and also in presence of water or carbon dioxide in order to estimate the relative importance of the reduction products. An important decrease of the specific surface area was observed between 850 and 1000 K under air, vacuum, carbon monoxide or water pressure, the residual BET area being below 10 m2 g−1 at 1100 K. Under hydrogen, the same loss of surface was obtained at ca. 150 K lower temperatures. In all cases, the first step was the elimination of the microporosity followed by the growth of the crystallites. The peculiar influence of hydrogen was related to the high concentration of lattice oxygen vacancies created during the reduction of the bulk. In the case of carbon monoxide atmosphere which also reduces the sample, the carbonate species formed during the reduction are eliminated from the bulk at higher temperatures, which explain the better resistance to sintering compared to hydrogen. This was confirmed by a treatment under carbon dioxide atmosphere which was found to preserve the specific surface area better because of the stabilization of the carbonate species on the ceria.

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.

Effect of support material on the catalytic combustion of diesel soot particulates

Several potential support materials were mixed with soot and subsequently the combustion temperature of the soot in 15% O2 was determined. Al2O3 and SiO2 showed no catalytic effect, TiO2 and ZrO2 lowered the soot combustion temperature with 80–90 K, whereas CeO2, La2O2CO3 and V2O5 (reference catalyst) showed a substantial activity for soot combustion. After poisoning with sulfur dioxide, the catalytic effect of CeO2 was strongly inhibited, whereas La2O2CO3 and V2O5 retained their high activity.

Catalytic combustion of diesel soot particles on copper catalysts supported on TiO2. Effect of potassium promoter on the activity

We have prepared a TiO2 supported copper catalyst and studied the effect of potassium on its activity in the oxidation of soot particles. The catalysts, with a K/Cu atomic ratio varying between 0 and 2, were calcined at 1073 K. They were characterized by BET surface area measurements, X-ray diffraction and temperature-programmed reduction under hydrogen. The catalytic activity was measured in a microbalance by means of temperature-programmed oxidation in air or argon. The catalytic activity of copper was enhanced by the presence of potassium. This effect was attributed to the formation of mixed KTi oxides which inhibit the sintering of the TiO2 support and thus increases the surface area of the catalyst. Although a redox mechanism can explain the catalytic combustion, no correlation could be established between the reducibility of the different solids and their activity in soot combustion.

Palladium–ceria catalysts: reversibility of hydrogen chemisorption and redox phenomena

The interactions of hydrogen with palladium–ceria catalysts have been studied by volumetric and magnetic susceptibility measurements. The ratio Hirr : Pdtotal, where Hirr stands for irreversibly adsorbed hydrogen, lies in the range 2.5–3.5; this result is ascribed to hydrogen spillover from palladium to ceria. The uptake of hydrogen by ceria leads to its complete surface reduction at room temperature, whereas bulk reduction begins at 473 K. Magnetic measurements show that: (i) a large amount of hydrogen is reversibly adsorbed on ceria, (ii) evacuation at 373 K induces the reverse migration of hydrogen (back spillover), which leads to the reoxidation of Ce3+ ions. The reoxidation ratio depends on the outgassing temperature wheras the amount of irreversibly formed Ce3+ increases with the reduction and the outgassing temperatures. However, the estimation of the amount of hydrogen retained by ceria from the quantity of Ce3+ formed does not allow determination of the metal fraction exposed (MFE) by hydrogen adsorption at 373 K.

Temperature-programmed reduction: limitation of the technique for determining the extent of reduction of either pure ceria or ceria modified by additiv

Abstract The classical temperature-programmed reduction technique using a thermally controlled detector and a water vapour trap did not permit the quantification of the extent of reduction of unsupported ceria with a high surface area. During temperature-programmed reduction of pure ceria with hydrogen, not only is water formed; carbon monoxide and carbon dioxide desorbing from the sample are also able to reach the thermally controlled detector and contribute to the variations in conductivity of the actual reduction mixture. When ceria is modified by impregnation with alkaline nitrate, followed by calcination at 673 K, NO x , compounds are also formed and contribute both to hydrogen uptake and to variations in gas conductivity. A further complication is caused by the storage of some hydrogen in ceria, below 773 K, followed by the release of hydrogen above this temperature. Temperature-programmed oxidation of the reduced samples is an alternative way to measure the extent of ceria reduction.

Influence of high temperature treatments under net oxidizing and reducing conditions on the oxygen storage and buffering properties of a Ce0.68Zr0.32O2 mixed oxide

Abstract TPR, TPO, O2 pulses and magnetic balance experiments have been performed to study the effect of two different aging treatments, in oxidizing atmosphere and in reducing conditions, on the redox behavior of a Ce0.68Zr0.32O2 mixed oxide. It has been observed that, although the samples resulting from both treatments have similar textural properties, their redox behavior is significantly different. In particular, aging in H2 improves the oxygen storage capacity (OSC) at low temperatures by decreasing the temperature at which reduction takes place. Also, the oxygen buffering capacity (OBC) is enhanced after aging under reducing conditions. In addition, the study of the re-oxidation of the samples reveals that the rate of the O2 uptake increases if compared with the fresh sample or that aged in air. High resolution electron microscopy and Raman spectroscopy data have also been used to obtain information on the possible origin of such effects.

NO reduction by CO over aluminate‐supported perovskites

Well crystallised La2CuO4 and LaMO3 perovskites were studied in the CO + NO reaction. Whereas for LaMO3 solids (M = Cr, Mn, Co and Ni) the activity decreased after reaction at 650 °C, the opposite was observed for LaFeO3 and La2CuO4 leading to the most active catalysts. Their activity was even more enhanced when supported onto magnesium aluminate of 60 m2 g−1.