P. Malet

The selection of experimental conditions in temperature-programmed reduction experiments

The effect of experimental conditions on temperature-programmed reduction (t.p.r.) profiles has been studied. In the case of t.p.r. curves that include several reduction steps an inadequate selection of these parameters leads to distorted profiles; these distortions may produce a complete loss of resolution for the different reduction steps. A criterion to select appropriate experimental conditions through a simple parameter that relates heating rate (β), sample size (S0) and hydrogen feed to the reactor (FC0), is given. Finally, the conditions required to obtain reliable activation energies for the reduction processes from the variation in temperature of the t.p.r. maxima with β, without making any previous assumptions about the reduction kinetics, are discussed.

Hexacyanoferrate-interlayered hydrotalcite

Abstract Hexacyanoferrate(II) and hexacyanoferrate(III) hydrotalcites have been synthesized by exchange of originally existing carbonate ions at pH = 4.5. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and X-ray absorption spectroscopies, nitrogen adsorption for specific surface area and porosity assessment, and their thermal stability has been characterized by Differential Thermal Analysis and Thermogravimetric Analysis. It has been found that during exchange hexacyanoferrate(II) is partially oxidized to hexacyanoferrate(III), a constant oxidation percentage being already attained after 3 h stirring during synthesis. Hexacyanoferrate(III) ions, however, are partially reduced during the same preparation process, but the reduction degree increases with the stirring time. Decomposition takes place in three consecutive partially overlapped steps: dehydration, decomposition of the interlayer cyanide ligands and dehydroxylation of the brucite-like layers. Upon calcination at 600 °C, a mixture of an AlMgO solid solution is formed, together with MgFe 2 O 4 spinel.

Thermal behaviour of Zn–Cr layered double hydroxides with hydrotalcite-like structures containing carbonate or decavanadate

Hydrotalcite-like compounds containing ZnII and CrIII in the brucite-like layers have been prepared, containing carbonate or decavanadate in the interlayer, with the formulae [Zn0.67Cr0.33(OH)2](CO3)0.165· 1.34H2O and [Zn0.67Cr0.33(OH)2](CO3)0.165· 1.34H2O, respectively. The samples have been characterized by XRD, XAS, UV-VIS, FTIR and Raman spectrocopies, while the specific textuies of the samples were assessed by nitrogen adsorption, and reducibilities were studied by temperature-programmed reduction. A similar characterization study has been carried out on samples obtained after calcination of the parent samples in air at increasing temperatures. Thermal decomposition of the ZnCr hydrotalcite containing carbonate leads to oxidation of CrIII to CrO42– species at intermediate calcination temperatures, ZnII cations then retaining their original octahedral coordination; at higher calcination temperatures ZnO and ZnCr2O4 are formed. However, the presence of interlayer decavanadate species inhibits the CrIII→ CrO42– oxidation during thermal decomposition of the vanadate-containing hydrotalcite, owing to preferential formation of vanadates. In this case the final product is a mixture of Zn pyrovanadate, Zn2V2O7, and spinel ZnCr2O4, together with a phase consisting of a mixture of partially depolymerized decavanadate to β-VO3– chains, together with monomeric VO43– species.

Effect of consecutive and alternative oxidation and reduction treatments on the interactions between titania (anatase and rutile) and copper

Titania (anatase and rutile)-supported copper systems have been prepared by a conventional impregnation technique (2.5% CuTi atomic ratio). The effect of consecutive oxidation/reduction/ oxidation and reduction/oxidation treatments at 770 K on the final materials has been studied by X-ray diffraction, transmission electron microscopy, temperature-programmed reduction, and visible-ultraviolet (diffuse reflectance) spectroscopy, as well as assessment of the texture of the solids by nitrogen adsorption at 77 K. Clustered CuO, detected by XRD, is formed on anatase, together with dispersed Cu2+ species, that are dominant on rutile. On this support, migration of copper species into the bulk of the support crystallites takes place, leading to unreactive copper species, and so, hydrogen consumption during reduction is lower in rutile-supported systems than the expected value to reduce all Cu2+ species to the metallic state.

Layered Ni(II)-Zn(II) hydroxyacetates. Anion exchange and thermal decomposition of the hydroxysalts obtained

Layered Ni,Zn hydroxyacetates have been prepared by hydrothermal methods and have been used for anion exchange of the original acetate anions by chloride, bromide, carbonate, nitrate, sulfate, and phosphate. Exchange was complete in all cases. The solids have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analysis (thermogravimetric and differential), FT-IR spectroscopy and electron microscopy; XAS spectra were also recorded in some cases. Thermal decomposition in air leads to removal of interlayer water at 150–200 °C. Mixed NiO–ZnO oxides are formed at higher temperatures. Chloride, sulfate, and phosphate salts are also formed at intermediate calcination temperature, and phosphate remains even after calcination at 1000 °C.

Effect of chlorine in the formation of PtRe alloys in PtRe/Al2O3 catalysts

Abstract The role of chlorine and the thermal pretreatments of the samples in PtRe alloying during the reduction of Pt Al 2 O 3 and PtRe Al 2 O 3 systems have been examined by using the reaction of n -butane hydrogenolysis with the aid of temperature-programmed reduction (TPR) experiments. The presence of chlorine modifies only slightly the TPR profiles, and rhenium reduction is always catalyzed by the presence of platinum. This effect depends strongly on the hydration degree of the support, while it is almost independent of the chlorine content of the sample. However, chlorine greatly decreases the catalytic activity of both Pt and PtRe catalysts in n -butane hydrogenolysis. Selectivity to methane (measured as C 3 C 1 ratio) is close to 1 for the platinum samples, while in the PtRe systems it remains close to 0.5 for the chlorine-free catalyst after all the pretreatments, but changes from 0.5 to 1.0 when the chlorinated catalyst is predried at increasing temperatures, indicating differences in the degree of PtRe alloying. Chlorination/dechlorination experiments confirm the role of chlorine on the above results and point to the existence of oxychlorinated Pt and/or Re species that may strongly interact with the progressively dehydrated Al 2 O 3 support, leading to higher metallic dispersions and lower PtRe alloying in the chlorinated catalysts after hydrogen reduction.

Spectroscopic characterisation and photochemical behaviour of a titanium hydroxyperoxo compound

In order to obtain more information about the generation of peroxide species during water photo-cleavage using M/TiO2(M = noble metal) systems in which an O2 evolution is normally not observed, a titanium hydroxyperoxo compound has been prepared by the reaction of TiCl4 with H2O2. The structure and the thermal behaviour of this compound have been examined using TG-MS, XRD and TPD techniques. XPS, i.r., e.s.r. and u.v.-visible reflectance spectroscopy have been used to characterise the types of peroxo species present in this sample. The photochemical decomposition of this compound, which leads to O2 evolution, has been studied compared to a TiO2(Degussa P25) sample. A model is proposed to explain the lack of O2 evolution during water photo-cleavage on M/TiO2 systems where peroxo species, similar to those observed in the titanium hydroxyperoxo compound, become stabilised against photodecomposition.

Sodium-doped V2O5/TiO2 systems: An XRD, DTA, TG/DTG, IR, V-UV, TPR, and XANES study

Abstract Vanadia-titania samples doped with different amounts of sodium have been prepared by impregnation of titania (P-25) with aqueous solutions of ammonium vanadate and calcination at 770 K, and have been characterized by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier Transform-infrared spectroscopy, visible-ultraviolet (diffuse reflectance) spectroscopy, temperature-programmed reduction, and X-ray absorption near-edge structure spectroscopy. The results obtained indicate that doping the vanadia-titania system with increasing amounts of sodium leads initially to formation of VNaO compounds with a Na/V ratio close to 0.5 that are reduced at a higher temperature than V 2 O 5 , formed in the absence of sodium. If the amount of sodium is increased up to 3%, formation of Na 3 VO 4 has been found.

MoO3/M9O systems: Effect of preparation method on their physicochemical properties

Abstract MoO 3 /MgO systems have been prepared and characterized by X-ray diffraction, specific surface area and porosity measurements, visible-ultraviolet (diffuse reflectance) and Fourier transform infrared spectroscopies, differential thermal analysis, determination of the surface isoelectric point, and temperature-programmed reduction to analyze how the preparation method and pretreatments regulate their surface properties. Formation of MgMoO 4 on the surface of the support under certain experimental conditions stabilizes the surface of the support, thus avoiding its sintering during calcination at high temperature; in addition, reduction of Mo(VI) species in this case is more difficult, taking place at temperatures higher than those for bulk or supported molybdena.

Surface structure and reactivity of molybdena–titania catalysts prepared by different methods

MoO3–TiO2 samples have been prepared from aqueous solutions of ammonium heptamolybdate and titania (by conventional impregnation and equilibrium adsorption) and by mechanically mixing both oxides (with and without further hydrothermal treatment). The samples have been characterized by X-ray diffraction, specific surface area assessment, Fourier-transform infrared (FTIR) and ultraviolet–visible diffuse refectance (UV–VIS DR) spectroscopies and temperature-programmed reduction (TPR). Surface properties have been studied following the adsorption of probe molecules by FTIR spectroscopy. Dispersed phases are formed preferentially in samples prepared by equilibrium adsorption and in physical mixtures submitted to hydrothermal treatment; in all other cases crystalline MoO3 is mainly formed. Dispersed phases are more easily reduced. They have been identified as surface molybdenyl species bonded to Ti cations through bridging oxygens, and their overall coordination depends on the degree of surface hydration. These species predominate quantitatively on the catalyst surfaces and have been found to be strong Lewis and Bronsted acids. They are also more easily reduced than bulk MoO3 under TRP conditions, and very active as oxidizing agents for methanol.