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The catalytic combustion of methane at low temperature under lean conditions was investigated over bimetallic palladium-platinum catalysts supported on alumina. Pd-Pt catalysts with constant 2 wt.% metal loading and varying compositions in Pt and Pd were prepared by successive impregnations of the metal salts. The catalysts were characterised by powder X-ray diffraction, transmission electron microscopy/electron dispersion X-ray spectroscopy (TEM/EDX), volumetry of H2 chemisorption, FTIR study of CO adsorption and temperature-programmed oxidation (TPO). In the absence of water added to the feed, the methane conversion over Pd-rich bimetallic catalysts (Pt/Pt + Pd molar ratios less than 0.3) was found to be the same as that of the reference Pd/Al2O3 catalyst. Interestingly, under wet conditions, these bimetallic catalysts exhibited an improved performance with respect to Pd/Al2O3. This effect was found to be maintained upon mild steam ageing. An interaction between both metals was suggested to explain the enhanced activity of bimetallic catalysts. This was confirmed by TPO experiments indicating that formation and decomposition of PdO is affected upon Pt addition even for very low amounts of Pt. The adsorption of CO on reduced catalysts studied by FTIR revealed new types of adsorbed CO species, suggesting again an interaction between two metals.

A series of Pd/Al 2 O 3 catalysts with a wide range of mean Pd particle sizes (ca. 2–30 nm in diameter) was prepared by using various precursors (H 2 PdCl 4 , Pd(NO 3 ) 2 and Pd(AcAc) 2 ) and pre-treatments. The mean particle size of reduced samples was determined by H 2 chemisorption. The catalytic activity in methane oxidation under lean burn conditions was measured. The oxidation of reduced samples was studied at 300 °C. The extent of oxidation was found to decrease with increasing mean particle size. While small particles ( 15 nm) proceeded via a two-step process, being first fast and then slow. The decomposition of oxide species was studied by temperature-programmed experiments under vacuum. Two distinct oxidised species with different stability were evidenced depending on the particle size. Oxidised species in larger particles were found of lower stability than in smaller ones. A correlation between the existence of distinct types of oxide species and catalytic properties in methane oxidation was discussed.

A commercial NOx-storage catalyst for gasoline applications containing platinum, palladium and rhodium together with Ba/CeO2/Al2O3 has been sulfated on the engine bench at 390 and 510 °C with nominal exposures of 0.3 and 1.3 g sulphur/l catalyst. Exposures of 0.3 g/l are too low to have a notable impact on the catalytic performance. At 390 °C, the sulphur is quantitatively adsorbed as opposed to 510 °C. Sulphur is mainly adsorbed at the inlet side of the catalyst thereby shielding the downstream region. Desulfation on synthetic gas bench at 700 °C leads to a removal of 50% of the sulphur. The residual sulphur is more evenly distributed along the length of the catalyst compared to the sulphur profile in the sulfated catalyst. This leads to an improvement of the NOx-activity at the inlet side and a drop in NOx-performance at the outlet side after desulfation. Results from SEM–EDX, TPD, TPR and IR suggest that the barium component is selectively poisoned by the sulphur while alumina and ceria are not affected significantly. The insufficient desulfation of the catalyst is almost entirely due to the incomplete sulphur removal from the barium sites. In the desulfated state the IR and TPD-data suggest the presence of barium carbonate species of low thermal stability together with bulk carbonate. After sulfation the IR data indicate the presence of different sulphate species, both bulk and surface phases. Thermal ageing and sulphur poisoning affect different parts of the operational temperature window of the NOx-storage catalyst. While thermal stress alone mainly affects low temperature NOx-performance, sulphur poisoning mainly reduces high temperature activity. The results of this work help to identify strategies to improve both the sulfur resistance of the catalyst and the operational conditions for a better management of the NOx-trap system.

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH4 traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H2S (100 vol. ppm). Their catalytic properties were compared to those of Pd/Al2O3 and Pt/Al2O3. In the absence of H2S in the feed, Pt/SnO2 appears as a very promising catalyst for CH4 oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al2O3. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO2 is slightly less resistant than Pd/Al2O3. In the presence of H2S, Pt/SnO2 catalysts are rapidly and almost completely poisoned, comparably to Pd/Al2O3, and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO2 sites active in CH4 oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO2.

The selective catalytic reduction (SCR) of nitric oxide by propene under lean-burn conditions over Ir/Al 2 O 3 and Ir/SiO 2 was studied. Irrespective of the support, Ir supported catalysts were found to be active in the reduction of NO above 673 K, Ir/SiO 2 being the most active. For both catalysts, the activity in the reduction of NO into N 2 was shown to be strongly enhanced after their exposure at 873 K under the reaction mixture. In spite of varying initial Ir dispersions depending on the metal content and the support, Ir sintered after activation in the reaction mixture, which led to the same final Ir dispersion. An in situ FTIR study of both catalysts at various temperatures under the reaction mixture and under CO or NO used separately was carried out. It was shown that, under the reaction mixture, the surface of initially fully oxidised Ir particles progressively reduced with increasing temperatures into partially reduced Ir δ + surface species allowing the adsorption of both CO and NO ( ν CO at 2070–2050 cm −1 and ν NO at 1870 cm −1 ). Additional species (formate, acetate, nitrate) formed only on the Al 2 O 3 support surface but these species are thought not to participate into the NO reduction.

BaMgAl10O17 (BMA) solid has been prepared by a sol–gel process: after calcination at 1200 °C a regular structure is obtained along with BET area of 13 m2g−1. Unexpectedly this solid is active in the catalytic combustion of methane, and its activity is comparable to the activity of other hexaaluminates containing transition metal ions. This activity may be due to oxygen ion diffusion through the mirror plane in relation with a charge compensation mechanism induced by the presence of Mg2+ ions. The introduction of Mn ions does not alter the structure but leads to the same phenomenon due to the presence of Mn2+ ions. A minor amount of Mn3+, revealed by hydrogen TPR and by reflectance spectroscopy, is responsible for an enhanced oxidation activity and a lowering of the activation energy. Such BMA materials may have interesting properties as oxidation catalyst.

Three lanthanide cuprates Ln 2 CuO 4 (Ln = La, Pr and Nd) have been studied in the CO + NO reaction. Under reaction conditions, a reduction of the cuprate was observed, with formation of metallic copper, which constitutes the active species in the reaction. After stabilisation, the resulting activity is lower when La in La 2 CuO 4 is replaced by Pr or Nd. This decrease in activity has been related to a different reducibility of the cuprates, as shown by TPR under H 2 . When La is substituted by Pr or Nd, a higher temperature of formation of the copper active sites should induce the growth of bigger copper particles, which could be at the origin of the observed difference in activity. To cite this article: S.D. Peter et al., C. R. Chimie 7 (2004) .

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH 4 traces under lean-burn conditions was studied in the presence or the absence of water or H 2 S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H 2 S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H 2 S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H 2 S-poisoned Pd/Al 2 O 3 catalyst was studied by TPD in O 2 /He. Poisoning species decompose above 873 K as SO 2 and O 2 in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O 2 /He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H 2 S-poisoned Pd/Al 2 O 3 . A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.

This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed.

To find a method for estimating the surface area of ceria–zirconia in composite three-way catalysts, successive O2/H2/O2 chemisorption measurements were performed at room temperature on model Pd and/or Rh/Ce0.63Zr0.37O2 catalysts previously reduced at 573 K by hydrogen, according to a methodology developed for ceria or ceria–alumina supported metal catalysts. The quantity adsorbed during the first oxygen chemisorption was found to correspond to the reoxidation of several layers of the support. The number of layers increases when the catalysts are aged at 1323 or 1423 K, which clearly indicates the great mobility of bulk oxygen in CeO2–ZrO2 supports, as already shown in the literature. After a subsequent H2 chemisorption, the titration by oxygen of the hydrogen adsorbed on metal particles and on the support leads to a lower quantity of chemisorbed oxygen which is closely related to the surface area of ceria–zirconia. This relationship was observed in fresh and aged catalysts and was used to calculate a mean calibration coefficient of 5.1 μatom O m−2. Thus, contrary to temperature-programmed reduction measurements, this method of quantification by oxygen of the spill-over hydrogen can be used to estimate the surface area of ceria–zirconia in composite catalysts in which CeO2–ZrO2 is one of the components of the support. This latter point is illustrated with model Pd and PdRh catalysts deposited on a ‘ceria–zirconia + alumina’ support.

This paper reports the possibility of associating Pd with another noble metal (rhodium) which is able to form a thermally stable oxide to extend toward higher temperatures the stability range of PdO. Bimetallic Pd–Rh/alumina catalysts were prepared and compared to reference Pd/alumina and Rh/alumina solids. The synthesis of the bimetallic catalysts was carried out by either co-impregnation or stepwise impregnation of alumina from solutions of RhCl 3 and H 2 PdCl 4 . When submitted to TPO/TPD cycles, Pd 0.75 Rh 0.25 /alumina catalysts obtained by the co-impregnation method exhibited oxidation and decomposition peaks at the same temperatures as those for reference Pd/alumina samples. The amounts of oxygen involved at each step were also comparable for the two solids and lower than the expected stoichiometric atomic ratio O/Pd=1. Conversely, Pd 0.75 Rh 0.25 /alumina catalysts obtained by the stepwise impregnation method exhibited improved stability of the PdO oxide, the maximal decomposition temperature being increased from 730 to 800°C. In addition, the amounts of oxygen involved at each step were also increased and exceeded the value O/Pd=1. As shown by TPR, Rh 2 O 3 and PdO particles on the surface of catalysts obtained by the co-impregnation technique behave like the respective monometallic catalysts. When the catalyst was elaborated by stepwise impregnation, some Rh 2 O 3 and PdO oxide particles were in strong interaction and were simultaneously reduced at room temperature in a H 2 /He medium.

The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al 2 O 3 under lean-burn conditions (1000 vpm NO, 2000 vpm C 3 H 6 , 500 vpm CO, 10 vol.% O 2 ) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O 2 was found to be necessary to activate Ir/Al 2 O 3 while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir 0 reduced surface sites ( ν CO at 2060 cm −1 ) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO 2 particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.

For SCR of NO the study of Ir/Al2O3 solids shows the importance of the activation procedure under mixtures containing CO (NO–C3H6–CO–O2 or NO–CO–O2). The selective reductant remains C3H6, however. The activation goes with an iridium particles sintering without Ir loss.

The objective of this work was to prepare La2Cu1−xFexO4 substituted perovskites and to measure their catalytic activity in the CO+NO reaction. Although no real mixed oxide structure was formed, a synergetic effect was observed between copper and iron. The catalytic activity increased by a factor of 3 when the iron content reached 30%, i.e. for the LaCu0.7+LaFe0.3 ternary system. This activity enhancement has been attributed to an increase in the surface concentration of metallic copper which is formed under reaction conditions and can be considered as the main active species in the CO+NO reaction. To improve the activity, La, Cu and/or Fe oxides were prepared onto a MgAl2O4 spinel support in order to increase the number of surface copper sites. A reaction of the MgAl2O4 support with lanthanum to form LaAlO3 was observed. No Cu/Fe synergetic effect was obtained for LaCu0.7+LaFe0.3/MgAl2O4, the most active catalyst being the system La2Cu /MgAl2O4. It seems that for the latter, the support’s effect on the dispersion of copper was the most pronounced.

Measurements of the oxygen storage capacity (OSC) were performed on several aged PtRh/CeO2–Al2O3 industrial catalysts, using different techniques, i.e., TPR experiments, O2 and H2/O2 chemisorption. The obtained OSC values were compared to the catalytic activity measured at 723 K in the simultaneous elimination of CO, C3 hydrocarbons and NO. The best correlation was found for the OSC measured by O2 chemisorption at 298 K on the solids previously reduced at 573 K and which represents the most reactive oxygen atoms of the catalyst surface.

Mechanical mixtures consisting of a catalyst (Pt/SiO2 or Pt/SiO2–Al2O3) and supports of varying acidity (hydrotalcite, SiO2, SiO2–Al2O3, and ZSM-5 zeolite) were tested for the selective reduction of NO by C3H6. A certain degree of support acidity appears to favour N2 selectivity, but if there are too many acid sites, carbon deposition becomes extensive and leads to catalyst deactivation.

Pd and Pt catalysts supported on alumina, tin(IV) oxide and tin(IV) oxide grafted on alumina were prepared, characterised and tested with respect to the low-temperature combustion of methane after reduction in H2 and ageing under reactants at 600°C. In the case of Pd, the use of SnO2 or SnO2-based supports led to catalysts slightly less active than Pd/Al2O3. In contrast, SnO2 was found to strongly promote the oxidation of methane over Pt catalysts with respect to Pt/Al2O3, even after ageing under reactants. When Pt was supported on SnO2 grafted on Al2O3, the activity was found at most similar to or, after ageing, lower than Pt/Al2O3. This negative effect was discussed, being partly related to the sintering of SnO2 under reactants observed by FTIR and XRD.

During the selective reduction of NOx under lean-burn conditions with Pt/SiO2 solids, a particle size dependency has previously been observed. Furthermore, under the reactant mixture up to 773 K, the Pt particles sinter at the same time as the solids are activated. In fact, the size dependency is linked to the strength of the Pt–O bond and to the ease of NO dissociation.

Platinum-based catalysts can be used for the selective reduction of NOx in lean burn conditions, but they form undesirably high quantities of N2O. In this study conducted under stationary and transient conditions, we attempted to better understand the mechanism of SCR (NO) over Pt. We found that N2 selectivity increased with contact time, that significant quantities of N2 were formed when N2O was used as the reactant, and that N2O was formed more quickly than N2 when NO was used as the reactant. These results led us to propose a kinetic model in which adsorbed N2O is an intermediate for NO reduction into N2.

Pd/Al2O3 catalysts prepared by the impregnation of alumina free of chlorine with H2PdCl4 and Pd(NO3)2 precursors were characterised and their catalytic activity in methane oxidation was measured under oxygen-rich conditions. The presence of residual chlorine ions either originating from the precursor or from impregnation with HCl was shown to strongly inhibit the conversion of methane. Residual chlorine ions could be slowly removed under reaction conditions in the form of HCl. This was observed by in situ measurement of HCl departure and catalytic activity. Experimental data were explained by a simple model in which residual chlorine would block PdO surface active sites during the reaction. The resulting catalyst, free of chlorine ions, was similar to the one prepared from the nitrate precursor in terms of dispersion and activity. Chlorine-free catalysts exhibited slow deactivation with time on stream, which was tentatively attributed to the slow conversion of the active PdO phase into a less active Pd(OH)2 phase. This interpretation was supported by the poisoning effect of water vapour observed upon addition of water to the feed while CO2 had no influence. Regeneration of the catalyst could be achieved by purging in dry carrier above 500°C, which could be ascribed to the decomposition of Pd(OH)2 into PdO.

CeO 2 –ZrO 2 solid solutions have been prepared by precipitation of the corresponding hydroxides. They were calcined and aged at various temperatures and characterised by X-ray diffraction measurements, BET and TPR measurements as well by their activity in methane combustion. The Ce 0.67 Zr 0.33 O 2 solid exhibited the best thermal stability, the highest oxygen mobility and the best catalytic activity after ageing at 1000°C. It has been used to support active phases like platinum and manganese oxide. The fresh Pt/Ce 0.67 Zr 0.33 O 2 catalyst was much more active than the corresponding Pt/Al 2 O 3 solid although a deactivation on stream was observed in the 200–500°C temperature range. Nevertheless, the promoting effect of the Ce 0.67 Zr 0.33 support disappeared after ageing at 1000°C. In the case of MnO x supported onto the Ce 0.67 Zr 0.33 O 2 solid solution the activity of the fresh solid is similar to that of the MnO x /Al 2 O 3 catalyst. After ageing at 1000°C, the solid solution is decomposed, the BET area dramatically decreased and the catalytic properties almost disappeared. As far as temperature applications exceeding 1000°C are concerned, the CeO 2 –ZrO 2 solid solutions are not suitable supports for the catalytic combustion of methane.

Platinum was deposited on alumina and zirconia, subjected or not to addition of barium or yttrium, respectively. Surface acidic properties were probed by ammonia adsorption followed by FT-IR spectroscopy and temperature-programmed desorption (TPD) using an original inductive set-up for fast heating of the sample. The Pt/alumina sample exhibits 0.34 acid site per nm 2 . The addition of barium (14.8 wt%) to this catalyst neutralises the hydroxyl groups located at the surface of alumina and therefore leads to the complete eradication of Lewis acid sites. Considering the Pt/zirconia sample, the nature and density of surface acid sites are very close to what was determined for Pt/alumina. No Bronsted acidity is detected and additions of 2.03 and 3.42 wt% yttrium lead to a concomitant decrease of the number (and of the strength) of the acid sites in order to reach densities of 0.16 and 0.10 per nm 2 , respectively.

During selective reduction of NOx under lean-burn conditions, a Pt particle size dependency has previously been observed with various supports. In this study, we have examined the influence of various parameters over a large range of initial metal dispersion: nature of the support (silica or alumina), support porosity, presence of impurities (particularly chlorine or sulfur), nature of the platinum precursor salt, and Pt particle size distribution. Furthermore, we have considered the mean particle size after sintering under the reactant mixture up to 773 K. Of the factors considered, only the Pt dispersion is of key importance. The intrinsic activity increases with decreasing dispersion (measured initially or after reaction) for each of the main reactions: reduction of NO into N2 or N2O, oxidation of NO into NO2, or oxidation of C3H6 into CO2. The dispersion does not clearly affect the selectivity.

Successive O2/H2/O2 chemisorption measurements were performed at room temperature over Pt/CeO2 catalysts of different BET surface areas after reduction at 573 K under H2. It has been determined that about 4 μatom of oxygen per m2 of ceria is required to fill the oxygen vacancies created by the reduction in agreement with the value deduced from the TPR profiles. The same value was also obtained from the H2/O2 titration, which shows that oxygen is able to titrate the hydrogen which has spilled over the ceria surface via the metallic particles. Thus, the consistency of the three techniques demonstrates the validity of this method of measuring both the total ceria surface area and the ceria surface in contact with the precious metals. The application of this methodology to the case of Pt and Pt–Rh /CeO2–Al2O3 model three-way catalysts leads to a set of ceria surface areas deduced from O2 chemisorption and O2 titration much lower than those obtained by TPR. This behaviour has been attributed to the presence of chlorine ions originating from the precursor and to the formation of stabilised Ce3+ ions at the CeO2/Al2O3 interface during reduction. After an ageing treatment at 1273 K under N2 + 10% H2O, an even greater difference was observed between the TPR ceria surface areas and those deduced from the chemisorption method. The results indicate an increased fraction of ceria without metal due to sintering and to the segregation between ceria and the metal particles.

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.

Copper- and manganese-substituted barium hexaaluminates were prepared by sol–gel method from metal alkoxides. The preparation conditions strongly influence the textural properties of the solids obtained. Manganese and copper occupy different crystallographic sites in the hexaaluminate structure: Mn3+ ions are located in octahedral sites, while Cu2+ enters only tetrahedral positions. The Cu sites are intrinsically more active than Mn sites for methane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate matrix: manganese substitution for aluminium is possible up to ≈3 Mn per unit cell, while copper substitution is limited to about 1.3 Cu per unit cell. The catalytic activity of the Mn-substituted barium hexaaluminates increases with Mn substitution, the optimum composition being obtained when about 3 Mn ions per unit cell are incorporated, as regards the activity and the resistance to ageing at 1200°C in the presence of steam. This is related not only to the Mn content, but also to the higher Mn3+/Mn2+ ratio when the amount of manganese introduced increases, as shown by TPR and Auger parameter measurements. The catalytic activity can be correlated with the fraction of reducible manganese species.

The interaction of CO with two aerogel solids have been investigated by in situ FTIR spectroscopy in order to obtain experimental data on the mechanism of formation of the formate species which can be considered as the intermediate adsorbed species in the methanol synthesis from CO/H2. The adsorption of CO on ZrO2 and ZnO/ZrO2 aerogel solids was studied in the temperature range 298–623 K. At temperatures lower than 373 K, CO is mainly reversibly adsorbed on a cationic site M1, leading to an IR band at 2192 cm−1 on ZrO2 and 2183 cm−1 on ZnO/ZrO2. It is shown that this adsorption follows the Langmuir's model on both solids. At higher temperatures (T>400 K) formate species are detected characterized on ZrO2 by IR bands at 2967, 2882, 1576 1382 and 1367 cm−1. It is shown that the formation of this species: (a) decreases the number of cationic sites M1 which adsorb CO at temperatures lower than 373 K and (b) creates a new cationic site M2. This is interpreted by a mechanism of formation of the formate species involving two sites of the surface: an OH group and a cationic site M1. The CO is first adsorbed on the cationic site M1, followed by a reaction with an OH group to produce a formate species. This last species is adsorbed on the cationic site M1 and a new cationic site M2 is formed.

The influence of a chlorinated precursor on the redox properties of ceria was studied by comparing two Pt/CeO2–Al2O3 catalysts prepared from hexachloroplatinic acid and platinum acetylacetonate. In absence of chlorine, a dispersion value higher than 100% has been measured by irreversible hydrogen chemisorption. To interpret this result, the interaction of H2 with the support and the two catalysts were studied by magnetic measurements. The variations of the Ce3+ content have given evidence that different redox processes may occur. The hydrogen spillover is clearly observed in presence of platinum with the phenomenon being more pronounced for the ex-acetylacetonate catalyst. The limited amount of hydrogen irreversibly adsorbed at 294 K on the support for this free chlorine reduced catalyst is sufficient to explain the excessive value obtained for the platinum dispersion. The results show a fixation of chlorine on the supported ceria phase which inhibits the mobility of hydroxyl species responsible for the hydrogen spillover. Moreover, independently to the presence of chlorine, a reduction at 573 K seems to stabilise a significant fraction of Ce3+ ions against reoxidation under air at room temperature. This was not the case for bulk ceria and probably originates from ceria/alumina interactions occurring after reduction.

Copper oxide has been deposited onto high surface area magnesium aluminate spinel prepared from alumina and magnesium nitrate. The catalytic properties of such a solid have been investigated in methane combustion. At the laboratory scale a very good activity is observed (light-off of 530°C) and no CO is detected. Aging at 1000°C under water vapour has no influence on activity. The previous catalyst has been washcoated on monolith and tested on a rig either with methane or synthetic natural gas at very high GHSV under conditions close to those of a gas turbine. In that case also, a good activity was observed.

The selective reduction of NO by C 3 H 8 is performed on copper-based catalysts, fresh and hydrothermally Cu-MFI with various Si/Al ratios and copper contents, Cu on conventional nonzeolitic supports with various Cu loadings. In the literature it is already known that the C 3 H 8 oxidation by NO occurs mainly on isolated copper ions. However, the sites responsible for the C 3 H 8 oxidation by O 2 are less studied. This note shows that nonisolated copper ions are active for the C 3 H 8 oxidation by O 2 . With or without NO in the feed, bulk oxides are more active for the C 3 H 8 -O 2 reaction than for the C 3 H 8 -NO (or NO 2 ) reaction. Furthermore, NO inhibits the hydrocarbon combustion by O 2 .

The surface composition of model and commercial bimetallic PtRh catalysts supported on ceria–alumina was tentatively determined by FTIR spectroscopy using the successive adsorption of NO at 473 K and CO at 298 K method which was previously applied to the case of PtRh/Al2O3 catalysts. The study of monometallic model catalysts shows that the adsorption of NO at 473 K on rhodium gives an intense band at 1912 cm−1 which is not modified by the presence of ceria and therefore can be used to quantify the number of surface rhodium atoms. However, the chemisorption properties of platinum toward the subsequent adsorption of CO at 298 K are strongly modified and the quantification of the surface platinum atoms could not be directly measured. However, their number could be indirectly obtained by combining the NO adsorption for rhodium and hydrogen volumetric adsorption for the total number of surface metal atoms, assuming that NO adsorption does not induce an important segregation of rhodium for bimetallic particles. Thus, for the fresh bimetallic model catalyst, the same composition was obtained at the surface and in the bulk. After aging at 1273 K, no rhodium was detected at the surface. This absence of surface rhodium atoms was confirmed by the data obtained from the direct adsorption of CO on platinum at room temperature, which gave the same number of surface atoms as hydrogen chemisorption. Similar results have been obtained with two commercial three-way catalysts. In particular, after aging at 1273 K, rhodium was practically not detected on the surface.

SSCI-VIDE+ATARI:CARE+CDS:PGE; International audience; Pd was introduced by exchange in non-dealuminated and dealuminated mordenites. After activation under oxygen at 500 degrees C, NO adsorption revealed the presence of isolated Pd2+ ions and Pd/PdO particles. Two types of Pd ions were shown: the first one would be located in the small cavities and leads to the formation of dinitrosyl complexes, whereas the second one, probably sitting in the main channels, is responsible for the obtainment of mononitrosyl complexes. Pd/PdO particles are active in the direct combustion of CH4 by oxygen and poorly active in the selective reduction (SCR) of NO. Pd ions are active for the NO SCR; the ions in the main channels are thought to be more active than the ions in the small cavities for accessibility reasons. In the case of the non-dealuminated mordenite catalyst, an activation under reactants is observed. Moreover, the activity is increased by the addition of 10 vol% water which is the opposite of the classical behavior of metal-loaded zeolites. Both phenomena are attributed to a migration of Pd ions from hidden to accessible sites. For the dealuminated sample, the observed deactivation seems to be due to a migration of isolated Pd ions into PdO particles located outside the zeolite matrix. (C) 1998 Academic Press.

For fresh and aged Pt/Al2O3 catalysts, TPD of oxygen is fairly well related to the noble metal surface areas and to the catalytic activity in butane combustion, whereas for aged Pt/Ba−Al2O3 solids, the catalytic activity is still preserved despite a tremendous sintering of the metallic phase and seems to be connected to a surface barium superoxide.

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al2O3, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H2, O2) and on their DeNOx activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu2+ and Cu0, while Cu+ is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation. The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu+ ions; the isolated Cu2+ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N2 are characterized by the presence of isolated Cun+ ions beside the additional influence of the zeolitic framework; after the formation of Cu+ ions the redox cycles are reversible but, after the formation of Cu0, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions.

In order to achieve a good activity with respect to methane combustion, a good route would be via catalysts incorporating transition metals. For devices operating at high temperatures, such as gas turbines, substituted hexa-aluminates present the advantage of good thermal stability when compared to impregnated Al2O3. For the purpose of this study, manganese-substituted hexa-aluminates were prepared either by the sol–gel method or by carbonate precipitation. Surfaces were analysed by soft x-ray emission, namely low-energy electron-induced x-ray spectroscopy (LEEIXS), and by photoelectron spectroscopy, namely x-ray photoelectron spectroscopy. The results on samples incorporating increasing amounts of manganese show the existence of an upper limit to the number of atoms substituted at the catalyst surface. This limit is about 2.7 manganese ions per unit cell, for samples prepared either by the sol–gel or by the carbonate precipitation technique and then calcinated at 1200°C. The interpretation of the Auger parameter variations shows that the relative amount of Mn in oxidation state III increases when the surface amount of Mn increases, as well as with the Mn substitution level. This fact can be related to an increased activity for methane conversion. © 1998 John Wiley & Sons, Ltd.

A sol–gel method using Ba and Al isopropylates and iron nitrate has been used to synthesise barium hexaaluminate partially substituted with iron. After calcination under oxygen at 1200°C the β-alumina structure was obtained. Formation of the mixed BaFexAl12-xO19 phase occurred for x=1–4. XRD measurements showed a good crystallinity of the structure and expansion of unit cell parameters due to the presence of larger Fe3+ ions substituting Al3+ ones in octahedral sites only. Mossbauer spectroscopy revealed that Fe3+ ions are present in four different octahedral sites slightly distorted. Catalytic activity in methane combustion showed that an optimum was obtained for solid containing 2 Fe ions per unit cell: the increase of the amount of introduced iron was counterbalanced by the decrease of specific surface area. Intrinsic activities have been calculated for the four solids in both the fresh and aged states. It is observed that increasing iron content increases relative activities in the same ratio as the populations of iron located in two sites as deduced from Mossbauer spectroscopy. It is then tentatively assumed that activity is attributed to octahedrally coordinated Fe3+ ions in some specific sites.

La2CuO4 is an active catalyst for the reduction of NO by CO. Under reaction conditions, the catalyst exhibits an activation which results in a lowering of the light‐off temperature by 80°C. XRD, TEM and EDX analysis carried out after the catalytic test indicate that the mixed oxide has been reduced to form a La2O3, Cu binary system. It seems that metallic copper species are the most active sites in the CO + NO reaction.

Barium hexaaluminates partially substituted by copper have been synthesized by a sol–gel method. Formation of the parent BaAl12O19 and the mixed BaCuAl11O18.5 catalysts occurred during calcination at 1200°C. XRD showed a good crystallinity of the structure and expansion of the unit cell parameter due to the presence of larger mathrmCu2+ ions in place of Al 3+ in tetrahedral sites. The presence of copper strongly enhances the catalytic activity in methane combustion, which should be due to the Cu2+ species evidenced by FTIR of adsorbed CO and by TPR. The introduction of a second copper ion in the structure was not possible, however, as revealed by the formation of copper (II) aluminate in addition to the Cu-substituted hexaaluminate phase.

The selective reduction of NO by C3H8 is performed on copper-based catalysts: Cu/Al2O3, Cu/SiO2, Cu/SiO2–Al2O3 solids, fresh and hydrothermally-treated Cu-MFI with various Si/Al ratios. For all the Cu-MFI solids and for the non-zeolitic supported-copper solids with low copper loadings, O2 promotes the reduction of NO. The C3H8–NO reaction rate correlates with the number of accessible and isolated Cun+ ions deduced from the infrared spectroscopy of adsorbed CO. When only one type of sites is detected, the FTIR spectroscopy of adsorbed CO allows an estimation of the copper dispersion.

SSCI-VIDE+ATARI:CARE+CDS:PGE; International audience; Pd-exchanged MFI-type zeolites containing 3.7 and 0.7 framework aluminium atoms per unit cell (corresponding to Si/Al ratios of 25 and 131) were found active in the selective reduction of nitrogen monoxide in the presence of excess oxygen. Upon steaming at 800 degrees C, both catalysts exhibited the total loss of their catalytic activity in the reduction of NO. Such a behaviour was ascribed to the complete aggregation of Pd ions into large metal particles on the external surface of the zeolite crystals. Both supports, although maintaining their crystallinity, are shown to experience extended dealumination upon steaming. Although the loss of Pd exchange capacity could partially explain the Pd migration and sintering, a mechanism involving the formation of mobile Pd hydroxyl entities condensing into PdO particles outside the zeolite crystallites is preferred. (C) 1997 Elsevier Science B.V.

The accessible surface area of cerium oxide was measured on model ceria-alumina supported Pt and/or Rh catalysts submitted to an hydrothermal treatment at 1273 K (10% H2O in N2). Two distinct methods, developed for fresh catalysts, were utilized. They are based respectively on the hydrogen temperature programmed reduction (TPR) profiles and the infrared spectra of the specific adsorption of CO2 on the hydroxyl groups of alumina. After aging, the TPR curves have become flattened and it is difficult to have a good reliability in the deconvolution of the peak corresponding to the ceria surface reduction. On the support alone, the results of both methods are in agreement and indicate a decrease of the ceria surface area by a factor close to 4, whereas the total surface area is only divided by 2. It appears that cerium oxide sinters more rapidly than alumina. For the metallic catalysts, the calculated ceria surface areas values depend on the hypothesis done on the oxidation state of the precious metals before the TPR. If reliable results can be obtained in the case of rhodium with ORh=1.5, in presence of platinum, the mean oxidation state of platinum OPt=1 results in underestimated values. The ceria surface areas deduced from the infrared results are higher than those deduced from TPR. Although the former are uncertain, they are in good agreement with a lower coverage of the alumina surface by sintered cerium oxide particles.

The accessible metallic area measured by hydrogen chemisorption on model ceria–alumina supported platinum or(and) rhodium catalysts has been compared to the results obtained in cyclohexane aromatization activity tests. The benzene formation was studied at 573 K after reduction by hydrogen at the same temperature. The turnover frequency, calculated on the basis of hydrogen chemisorption data, was found to depend on the type of support, ceria or alumina, in interaction with the precious metal particles. However, for the catalysts supported on ceria–alumina, the activity per metallic platinum or rhodium site was almost the same for both fresh catalysts or after hydrothermal aging at 1273 K. The possible use of the cyclohexane aromatization activity measurement for the determination of the metallic area of three-way catalysts is examined. Compared to hydrogen chemisorption, this method presents the advantage of a better sensitivity for measuring very low dispersions of aged catalysts, typically dispersions inferior to 3%. In the discussion, the possible influence of hydrogen spill-over effects and of the connected role of chlorine are considered. Although not directly studied, the possible effect of sulphur present in commercial catalysts aged in vehicles is also discussed.

Impregnation of γ-Al 2 O 3 by chromium(III) ions leads to surface chromate species after calcination under oxygen at 773 K. This solid is very active in methane combustion, however selectivity towards CO 2 is not complete because some CO is also produced even in the presence of a large excess of oxygen. To prevent the formation of CO, subsequent ultra-low palladium deposition (0.05–0.2 wt% Pd) has been performed on the chromate supported alumina. In this case the activity is unchanged but full selectivity towards CO 2 is now observed. For this bimetallic catalyst, palladium is present in the form of large PdO particles, which do not participate in the conversion of methane to CO 2 , mainly due to their size, however, they promote the oxidation of CO to CO 2 . The presence of H 2 S in the feed leads to a severe deactivation of the Pd/Al 2 O 3 catalyst owing to the formation of sulfate groups at the surface of PdO particles. On the contrary, the activity of the Cr/Al 2 O 3 catalyst was not dramatically affected, but selectivity towards CO was strongly improved. In the presence of H 2 S the chromate groups of the Cr/Al 2 O 3 are reduced to Cr 3+ ions probably in the form of Cr 2 O 3 . For the mixed catalyst containing both palladium and chromium, H 2 S addition has a limited effect on the global catalytic activity, whereas the formation of CO is fully suppressed. An electronic interaction between chromium oxide and palladium oxide is postulated leading to a weakening of the Pd–S bond of the adsorbed sulfate.

A La(MnRh)O3.15catalyst, containing 1 wt% Rh, was prepared by a citrates/polyacrylamide gel method, leading to a sample with a specific surface area of 27 m2/g after calcination at 700°C. This catalyst shows high activity in three-way catalysis reactions for the simultaneous reduction of NO and oxidation of CO and C3H6. However, the addition of 10 vol.% steam slightly deactivates the catalyst, mainly for NO reduction. Large amplitude oscillations of the feed streams between oxidizing and reducing compositions do not deactivate the sample: these variations in the composition are compensated for by the high mobility of the over-stoichiometric oxygen, which is available to oxidize CO under a reducing feed stream and is easily replenished by O2or NO in the presence of an oxidizing feed stream. This mobility corresponds to the reversible structural change, at low temperature, between hexagonal La(MnRh)O3.15and orthorhombic La(MnRh)3.00, leading to an oxygen storage capacity of 0.15 mole [O] per mole of catalyst.

Barium hexaferrite BaFe12O19 and iridium-containing barium hexaferrites have been prepared by the citrates gel method. Their catalytic activity in methane combustion has been evaluated. BaFe12O19 is an efficient catalyst for this reaction, and the introduction of iridium in the hexaferrite structure does not improve this activity. Mossbauer spectroscopy suggests that a part of the iridium ions are incorporated in the hexaferrite structure, however in crystallographic sites where they cannot interact with the gas phase. Infrared study of CO adsorption reveals the presence of two types of iridium particles in the surface: small Ir particles, in strong interaction with the hexaferrite structure, and some larger Ir particles which were not incorporated into the lattice.

A series of palladium-substituted La2CuO4, corresponding to the formula La2Cu1 −xPdxO4 (x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m2/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C3H6 decreased markedly when increasing the palladium content, the activity of catalysts La2Cu0.9Pd0.1O4 and La2Cu0.8Pd0.2O4 being comparable to that of a Pt-Rh/CeO2–Al2O3 catalyst for NO reduction, and higher for CO and C3H6 oxidation. All the La2Cu1 − x PdxO4 catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd0 ions atomically dispersed, surrounded by Cu+ and Cu2+ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La2Cu1 −xPdxO4 structure.

Monometallic and bimetallic catalysts based on palladium and copper deposited on a spinel carrier have been investigated in the catalytic combustion of methane. Great differences were found in catalytic activity, according to the sequence Pd/MgAl2O4>CuO−Pd/MgAl2O4>Pd−CuO/MgAl2O4>CuO/MgAl2O4. They were explained by changes in surface composition of the catalysts. In the case of bimetallic catalysts the metallic surface is preferentially enriched in copper, which acts as a diluting agent for the Pd atom ensembles. As a consequence, the adsorption of reactants is limited and the catalysts so obtained behave like copper slightly doped with palladium.

Platinum was deposited onto pure alumina as well as on alumina modified by addition of 7 and 15 wt.-% barium. The alumina hydroxyl groups were neutralized by barium addition and the surface of the modified support had a basic character. The introduction of barium decreased the accessible platinum area because of a sintering of the metal particles associated with a partial coverage of the platinum crystallites by barium containing species. In addition, the platinum surface atoms were electron-enriched. As a consequence the catalytic activity of alumina supported platinum was lowered in the presence of barium. Nevertheless, the turn-over numbers were not strongly modified by the barium additive, suggesting that the catalytic properties of accessible platinum atoms were not sensitive to the electronic effect induced by barium addition.

The reduction of NO by CO is performed over palladium supported on activated carbons and on a graphite of lower surface area. The presence of very low amounts of Pd gives active and stable catalysts towards the NO CO reaction and the contribution of the surface carbon atoms to the process is negligible, the presence of CO lowering the consumption of the carbon support by NO. In the absence of any other reductant in the gas phase, NO is reduced by reaction with the carbon atoms. In the presence of an excess of O 2 (net-oxidizing NO CO O 2 mixtures), the consumption of the carbon supports by O 2 is important. The reactions of NO with the C atoms are also inhibited, NO reacting with CO but O 2 reacts with both reductants, CO and C.