L. Tournayan

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.

Platinum-rhenium-alumina catalysts: III. Catalytic properties

Abstract The activity of PtRe catalysts on α- or γ-alumina has been determined for various reactions. In no case is the activity of bimetallic catalysts the sum of the activity of platinum and rhenium of which the catalyst is composed. Curves of activity as a function of the composition of the catalyst exhibit one or two maxima in benzene hydrogenation, benzene-deuterium exchange, cyclopentane, and butane hydrogenolysis, those maxima being more or less pronounced according to the reaction and the support. Conversely, the rate of 1,1,3-trimethyl cyclohexane dehydrogenation decreases when the percentage of rhenium is increased. A pronounced analogy has been established between, on the one hand, the changes of activity with bimetallic composition, and, on the other hand, with the position of the metal in the Periodic Table for the monometallic period Pt, Ir, Os, Re deposited on alumina.

Platinum-rhenium-alumina catalysts: II. Study of the metallic phase after reduction

Abstract Two percent ( Pt + Re ) Al 2 O 3 catalysts prepared by co-impregnation of Al2O3 with H2PtCl6 and Re2O7 and generally reduced by hydrogen at 500 °C were mainly investigated by thermogravimetry (H2O2 cycles at 25 °C), electron microscopy, and infrared spectroscopy (chemisorption of CO at 25 °C). The greater reducibility in the presence of Pt of oxygen chemisorbed by Re, and the vCO frequency shifts of the PtCO and ReCO species against the percentage of Re indicated a strong interaction between both metals. Most probably Pt and Re are alloyed. The H2O2 cycles at 25 °C allowed the determination of the overall dispersion of the (Pt + Re) phase while the optical density of the infrared band of CO adsorbed on platinum gave some information about the surface concentration of Pt in the (Pt + Re) phase.

Platinum-rhenium/alumina catalysts: I. Investigation of reduction by hydrogen

Abstract Pure Re2O7 when heated in a hydrogen atmosphere, sublimes before reduction. However, when Re2O7 is mixed with Pt or Pd or Re powder, instead of volatilization complete reduction to metallic Re occurs below 200 °C. A strong activation by Pt of the reduction of Re2O7 also takes place on Al2O3 samples coimpregnated with H2PtCl6 and Re2O7. With such catalysts, the reduction of Re2O7 to metallic Re under 1 atm pressure of hydrogen seems to be complete at a rather low temperature.

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.

Promotion of hydrogen uptake in cerium dioxide: The role of iridium

Abstract The interaction of hydrogen with ceria and iridium-ceria catalysts was studied at isothermal conditions and by temperature-programmed methods. Independent of the final temperature of reduction, hydrogen uptake during TPR was promoted by the presence of iridium in iridium-ceria catalysts. Partially reduced ceria was capable of storing hydrogen. The hydrogen storage ability increased with the reduction temperature of the ceria. It was also promoted by the presence of iridium. Various hypotheses, including the formation of Ir-Ce clusters at low temperature and hydrogen spillover, are considered to explain the observations.

Study of the formation and removal of carbonaceous deposits on platinum - ruthenium alumina supported bimetallic catalysts

Abstract Alumina supported mono and bimetallic catalysts based on platinum and ruthenium were studied for the conversion of n-heptane at 400 and 500°C. The combustion of the 0.1 to 0.3 wt % of resulting carbonaceous deposits was followed by means of thermal programmed oxidation (TPO), that is by simultaneous quadrupole analysis of the oxygen consumption and the carbon dioxide evolution. No significant variation of the molecular ratio of oxygen consumed to carbon dioxide evolved was observed, either with the temperature during a single TPO experiment or with the ruthenium to platinum atomic ratio in the catalyst. On the other hand, the lowest temperature necessary to complete the combustion of the carbonaceous deposits was found to be much smaller for the ruthenium monometallic catalyst than for the platinum mono or platinum - ruthenium bimetallic catalysts.

Influence of the reduction temperature on the properties of silica-supported nickel catalysts

The reduction of silica-supported nickel catalysts with hydrogen at temperatures higher than 873–923 K modifies the properties of the metallic particles. These modifications have been analysed through catalytic activity and selectivity variations, magnetic measurements, controlled atmosphere X-ray diffraction, temperature-programmed oxidation and Fourier-transform infrared spectroscopy, with carbon monoxide as a probe molecule. The modifications are not due to the sintering of nickel particles but seem to arise from an interaction between nickel and fractions of partially reduced carrier, both in the bulk and at the surface of the metallic particles.

Low temperature gasification of carbonaceous materials by air, as catalysed by calcium

Abstract The catalytic effect of calcium during air gasification of carbonaceous materials at mild temperature is maximum when the calcium is ion exchanged on their surface. A pyrolysis or/and HNO 3 oxidation treatment performed before the cation exchange operation enables to modify the exchangeability of the carbonaceous material and the catalytic effect of the calcium.