Hélène Praliaud

Surface and bulk properties of Cu-ZSM-5 and Cu/Al2O3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

Abstract 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.

Catalytic combustion of methane over palladium supported on alumina and silica in presence of hydrogen sulfide

Abstract The catalytic combustion of methane is performed over palladium supported on alumina and silica in presence or not of hydrogen sulfide. In the case of alumina, a treatment at 873 K under the sulfur-free mixture leads to an increase in the catalytic activity. The activation is instantaneous in the case of silica. The presence of sulfur strongly deactivates both solids and the extent of deactivation is independent on the support. However, the alumina support lowers the rate of deactivation because of the trapping of sulfate species. In the case of the silica support infrared spectroscopy shows that the poisoning species would be a sulfate group bonded to the palladium surface atoms; in addition, this technique allows to determine the variations of the amount of sulfur deposited after poisoning and after thermal treatment. The extent of regeneration under nitrogen and hydrogen is correlated with the stability of the poisoning species.

Influence of the nature of the support on the reducibility and catalytic properties of nickel: evidence for a new type of metal support interaction

Abstract Various nickel catalysts deposited on classical supports (Si0 2 , A1 2 0 3 , MgO) and on less conventional materials (TiO 2 , ThO 2 , CeO 2 , ZrO 2 , Cr 2 0 3 ) were prepared under conditions in which the strong metal support interaction of the Pt/TiO 2 -type does not occur. Their catalytic properties for ethane hydrogenolysis and carbon monoxide hydrogenation and the reducibility of the nickel phase deduced from magnetic measurements were investigated. Activity toward ethane hydrogenolysis per unit area varies by two orders of magnitude when the nature of the support is changed. A correlation between activity and nickel reducibility is demonstrated and interpreted in terms of geometric effects of dilution due to the presenceof unreduced surface-nickel species. These dilution effects are shown to play an important role in the case of the CO + H 2 reaction. However, additional effects leading to changes in apparent activation energy (not yet well understood but possibly linked to the carbon monoxide coverage) result in a poorer correlation between activity, selectivity toward C 2+ and nickel reducibility. These correlations lead to an interest in designing further experiments to study what can be termed the redox metal support interaction, which should not be confused with the strong metal support interaction.

Alkali addition to silica-supported palladium: Infrared investigation of the carbon monoxide chemisorption

Abstract Alkali nitrates (lithium, sodium, potassium, caesium) are added to a Pd n + /SiO 2 precursor. After reduction under hydrogen the addition leads to a moderate increase in the metallic particle size but to drastic changes in the infrared spectrum of carbon monoxide adsorbed on palladium. A direct interaction between the alkaline ion and carbon monoxide bonded to palladium is invoked. The extent of the interaction increases with the reduction temperature. The particular behaviour of lithium-doped solids is discussed.

Nitrogen dioxide effect in the reduction of nitric oxide by propane in oxidizing atmosphere

The role of nitrogen dioxide in the selective reduction of NO by propane over a Cu-MFI zeolite is investigated. NO2 and NO reductions were carried out under similar conditions of reaction. In the presence of oxygen, the reduction of NO by C3H8 does not differ significantly from that of NO2. In the absence of oxygen, the reduction of NO2 by propane occurs with a partial decomposition of the nitric dioxide molecule. Such a decomposition leads to the formation of oxygen, which is responsible for the increase in catalytic activity by comparison with the same reaction performed with NO. NO2 formed and released in the gas phase during the reduction of NO by propane in the presence of oxygen does not play a predominant role in the catalytic process.

Physico-chemical properties of potassium-promoted Ni/SiO2 catalysts

Abstract K-promoted Ni/SiO 2 catalysts have been prepared by adding KNO 3 to the Ni/SiO 2 precursor and subsequent hydrogen reduction. They were studied by XPS, infrared spectroscopy and magnetic methods. Chemisorbed hydrogen is strongly interacting with Ni surface atoms of K-promoted nickel catalysts, whereas oxygen penetrates deeply into the nickel particles with the formation of bulk NiO. Evidence of an increase of the electron density of the metallic nickel due to the presence of potassium is given. CO is adsorbed on nickel in a bridge bonded form. K is mainly present in the K + form and a Ni-O-K surface complex is probably formed. This complex is concentrated on the nickel surface into patches or islands. Ni interacting with the K-containing phase is accessible to hydrogen adsorption and not to CO adsorption. The hypothesis of Ni(111) arrangements of the free nickel atoms has been considered and no experimental evidence of a direct interaction between the promoter and the adsorbed CO was found.

Infrared determination of the accessible metallic surface of supported palladium containing ceria

Abstract The dispersion of palladium supported on alumina has been measured by volumetric chemisorptions of hydrogen and carbon monoxide and the adsorption of carbon monoxide has been followed by infrared spectroscopy. A linear relationship has been found between the overall integrated absorbance of the bands due to carbon monoxide bonded to palladium and the metallic surface accessible to gases. When volumetric measurements do not allow determination of the metallic surface owing to the chemisorption by the support itself, for instance in the case of ceria containing solids, the spectroscopic method seems suitable. The hypotheses involved are discussed. Ceria increases and stabilizes the dispersion of palladium.

Influence of the preparation method on the selective reduction of nitric oxide over Cu-ZSM-5. Nature of the active sites

Abstract Copper ions were introduced in a Na-ZSM-5 zeolite by using exchange, impregnation and precipitation procedures. The catalytic activity of the three solids was measured in the reduction of NO by propane in the presence of oxygen (0 to 10 vol.-%) in the 423–773 K temperature range. At a given temperature the activity for nitrogen formation remarkably increased with the oxygen content in the 0.5–2 vol.-% range, then slightly decreased for higher amounts of oxygen. For an oxygen content exceeding 0.5 vol.-% and regardless of the reaction temperature, the Cu-ZSM-5 sample prepared by exchange was the most active catalyst, the catalytic activity being expressed per gram of introduced copper. The adsorption of carbon monoxide followed by FT-IR spectroscopy suggested that copper was present as isolated Cu+ ions in the three catalysts. The dispersion of cuprous ions was the highest for the sample prepared by the exchange process. The catalytic activity appeared related to a high dispersion of isolated Cu+ ions in the MFI framework.

Oxidation state and surface segregation of Mo ions in titania catalysts evidenced by ESR, UV and ESCA techniques

Abstract Incorporation of Mo ions into a TiO 2 matrix has been studied as a function of preparation procedure and heat treatment conditions, using ESCA, UV and ESR spectroscopies. Oxidation state, coordination geometry and incorporation of Mo ions into the TiO 2 particles have been considered. For anastase samples, UV and ESCA techniques show that surface and bulk concentrations of Mo are about identical, with the majority of Mo being in the form of Mo 6+ and a significant quantity existing as Mo 5+ in distorted octahedral symmetries. ESR spectroscopy only detects substitutional ions designated Mo s 5+ in very small amounts (∼1% of the 0.5% Mo introduced). For rutile, heated at 800°C in air, ESCA technique shows that the surface concentration is enriched in Mo in the +6 oxidation state by a factor 10 compared to the bulk, whereas UV spectroscopy shows that Mo is mainly incorporated in the bulk in the form of Mo 5+ presenting two different environments of the distorted octahedral type. ESR techniques detect substitutional Mos in very small amounts (∼1% of the 0.5% Mo introduced). For rutile samples, directly prepared by the flame method, the surface concentration of Mo is about twice that of the bulk, Mo ions being mainly in a +6 oxidation state at the surface and +5 in the bulk. Strong surface enrichment (× 10–20) occurs after heating anastase or rutile at 800°C in air. Rutile samples turn out to be composed of MoO 3 islands at the surface of TiO 2 doped with Mo(V) ions in Mo ensembles or clusters occluded in the TiO 2 .

Sulfur poisoning and regeneration of palladium-based catalysts. Part 1.—Dehydrogenation of cyclohexane on Pd/Al2O3 and Pd/SiO2–Al2O3 catalysts

The catalytic activity of palladium deposited on alumina and silica–alumina for the dehydrogenation of cyclo-hexane was studied. The introduction of sulfur-containing compounds, thiophene or hydrogen sulfide into the feed, was investigated.The Pd/Al2O3 catalyst was less active than platinum deposited on alumina for cyclohexane aromatization in the absence of any poisoning additive. In addition, the thio-tolerance is smaller for palladium than for platinum. The decontamination by the sulfur-free feed was also less efficient in the case of alumina supported palladium. However, regeneration under pure hydrogen was more effective for the Pd/Al2O3 sample than for the Pt/Al2O3 one.When palladium was supported on silica–alumina instead of alumina, the deactivation by coking was more intense for sulfur-free reactants. Nevertheless the thio-tolerance was strongly improved in comparison with the Pd/Al2O3 catalyst. In contrast to iridium and platinum supported on silica–alumina, previously studied under similar conditions in the literature, the removal of thiophene or hydrogen sulfide from the feed allowed a total recovery of the catalytic activity of the Pd/SiO2–Al2O3 sample. The sample regeneration was observed after treatment under flowing hydrogen. Such behaviour is discussed in terms of changes of coke precursor reactivity in the presence of adsorbed sulfur.