Promotional effect of gold added to palladium supported on a new mesoporous TiO2 for total oxidation of volatile organic compounds

Gold and palladium were supported on a mesoporous TiO₂ for total oxidation of volatile organic compounds (VOCs). Mesoporous high surface area titania support was synthesised using of Ti(OC₂H₅)₂ in the presence of CTMABr surfactant. After removing the surfactant molecules, 0.5 or 1.5 wt% of palladium and 1 wt% of gold were precipitated on the support by, respectively, wet impregnation and deposition–precipitation methods. The activity for toluene and propene total oxidation of the prereduced samples follows the same order: 0.5%Pd-1%Au/TiO₂ > 1.5%Pd/TiO₂ > 0.5%Pd/TiO₂ > 1%Au-0.5%Pd/TiO₂ > 1%Au/TiO₂ > TiO₂. Moreover, a catalytic comparison with samples based on a conventional TiO₂, shows the catalytic advantage of the mesoporous TiO₂ support. The promotional effect of gold added to palladium could be partly explained by small metallic particles (TEM), but meanly by metallic particles made up of Au-rich core with a Pd-rich shell. Moreover, the hydrogen TPR profile of 0.5%Pd-1%Au/TiO₂ shows only the signal attributed to small PdO particles. Gold also implies a protecting effect of the support under reduction atmosphere. Operando diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was carried on and allowed to follow the VOCs oxidation and the formation of coke molecules, but also a metallic electrodonor effect to the adsorbed molecule which increases in the same order as the activity for oxidation reaction. The presence of coke after test was also shown by DTA–TGA by exothermic signals between 300 and 500 °C and by EPR (g = 2.003).     

Investigation of the Intrinsic Activity of ZrxCe1−xO2 Mixed Oxides in the CO + NO Reactions: Influence of Pd Incorporation

The intrinsic activity of various Zr _x Ce_1–x O₂ mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of Zr _x Ce_1–x O₂ depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr_0.5Ce_0.5O₂ being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO₂ and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a bifunctional mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr_0.25Ce_0.75O₂ being the most resistant to deactivation.     

Catalytic Removal of Toluene in Air over Co–Mn–Al Nano-oxides Synthesized by Hydrotalcite Route

Co–Mn–Al hydrotalcite type solids were synthesized as precursors of catalysts for the total oxidation of toluene in air. For the as-prepared solids, XRD measurements indicate the coexistence of hydrotalcite and MnCO₃ phases. When calcination is performed at 500 °C, different mixed oxides are found as a function of Co:Mn molar ratio and preparation method, and very high specific surface areas were obtained for the Co–Mn solids. The comparison of catalytic activities in the presence of calcined hydrotalcites with those in the presence of calcined hydroxides evidences the superiority of the first oxides due to their higher reducibility. Co–Mn–Al nano-oxides synthesized using hydrotalcite type solids as precursors, are very promising candidates for the substitution of noble metal based solids.     

Methanol oxidation on LaCo mixed oxide supported onto MCM-41 molecular sieve

Lanthanum cobaltate LaCoO_x supported onto MCM-41 mesoporous molecular sieve was prepared by in-situ oxidative decomposition of mixed LaCo citrate complexes inside the mesopores of this support. The prepared materials were characterized by EDX, EPR and UV-Vis DRS techniques as well as by N₂-BET measurements. The nanosized LaCoO_x particles within the mesopores of MCM-41 matrix contains cobalt atoms in lower than Co(III) average oxidation state. Also, the supported cobaltate does not form short-range order species of LaCoO₃ but presents as the highly disordered, oxygen deficient Co oxide nanophase. Catalytic activity in MeOH oxidation was tested using both conventional fixed-bed flow system and “operando” mode of DRIFT technique. The very high activity of MCM-41-supported cobaltate is not only due to highly dispersed LaCoO₃ phase but also because of rather low oxidation state of Co.     

Nickel nanoparticles inserted in tBuONa matrix deposited on alumina Part II Thermal treatment and nickel content effects on their stabilty and catalytitic activity

Nanoparticles (1 nm–3 nm) of metallic nickel supported on alumina (4.3% Ni–17.9% Ni w/w) were prepared from a colloidal precursor inserted in an organic matrix. Their structural and stability properties have been studied by X-Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) and Thermal Gravimetric Analysis (TGA). Benzene hydrogenation at atmospheric pressure in the temperature range of 75 °C–200 °C was used as a test reaction of their catalytic capability. The thermal stability of the particles depended on the nature of the reactive atmosphere. Thus, a growth in size (up to around 20 nm) is observed under H₂ flow at 350 °C or during benzene hydrogenation but not under air flow at 300 °C. The growth may be due to the coalescence of the metal particles during the heating and decomposition of the stabilizing organic matrix. Under oxidative atmosphere, stable nickel oxide particles, firmly attached to the support, are formed. The catalysts pre-treated under H₂/350 °C were active and stable in benzene hydrogenation. The observed activities depended on the reaction conditions and nickel composition.