6 research outputs found
Electrochemical Behavior of Chemical Vapor Deposited Protective Aluminum Oxide Coatings on Ti6242 Titanium Alloy
The electrochemical behavior at room temperature in a neutral sodium chloride aqueous solution of four types of metallorganic chemical vapor deposited aluminum oxide coatings on commercial Ti6242 titanium alloy was investigated. Polarization and electrochemical impedance curves revealed that porosity free, amorphous alumina coatings provide a two order of magnitude improvement of the corrosion resistance with regard to the bare alloy. Crystallized alumina as well as amorphous AlO(OH) only slightly improve the corrosion resistance of Ti6242. It was demonstrated that metallorganic chemical vapor deposition processed amorphous alumina is a highly promising solution to the protection of titanium alloys against corrosion in salt environments
Synthesis of nanostructured materials in nonaqueous solvents
Dans la synthĂšse des matĂ©riaux catalytiques, deux aspects orientent en particulier les recherches actuelles: le contrĂŽle de taille afin dâobtenir des nanoparticules finement divisĂ©es et le contrĂŽle prĂ©cis de la morphologie. Dans cette Ă©tude, nous avons utilisĂ© les milieux non aqueux pour Ă©laborer des matĂ©riaux catalytiques Ă base dâoxydes des mĂ©taux de transition (Ti, Zr, Mn). Trois mĂ©thodologies ont Ă©tĂ© employĂ©es pour la prĂ©paration des solides Ă base de TiO2. La synthĂšse dans les nitrates fondus dopĂ©s avec diffĂ©rents anions permet un boncontrĂŽle des propriĂ©tĂ©s texturales des matĂ©riaux et de la nature des plans cristallins exposĂ©s. Les prĂ©parations utilisant le nitrate dâammonium stabilisĂ© avec des molĂ©cules organiques azotĂ©es conduit Ă des solides prĂ©sentant des tailles et des formes de particules variables en fonction de la nature du stabilisateur. Le traitement Ă reflux dans des solvants organiques polaires conduit Ă lâobtention des supports TiO2 de grandes surfaces spĂ©cifiques. Pour la valorisation catalytique de ces matĂ©riaux, deux rĂ©actions appliquĂ©es au domaine de la photocatalyse ont Ă©tĂ© employĂ©es: la production dâH2 par dĂ©shydrogĂ©nation du mĂ©thanol etlâoxydation photocatalytique de lâacide formique. La rĂ©action modĂšle de dĂ©composition delâisopropanol a Ă©tĂ© utilisĂ©e comme test de caractĂ©risation des propriĂ©tĂ©s acido-basiques de nossystĂšmes. La technique de nitrates fondus a Ă©tĂ© Ă©galement appliquĂ© avec succĂšs pourlâobtention en une seule Ă©tape des catalyseurs supportĂ©s Mn-Zr et Mn-Ti. Ces catalyseursmontrent une excellente activitĂ© pour lâoxydation totale de composĂ©s organiques volatils(COV).Two major challenges are particularly important for current research in the catalytic materials synthesis: preparation of finely divided particles and control of their shape. In this work we used non aqueous solvents to prepare catalytic materials containing transition metals oxides (Ti, Mn, Zr). Three techniques were applied to prepare TiO2-based solids. Reactions in molten alkali metal nitrates doped with different anions offered good control of textural properties and of the nature of exposed facets of nanocrystalline particles. Synthesis in molten ammonium nitrate stabilized with organic nitrogen-containing compounds provided TiO2 solids with variable size and shape as a function of stabilizer used. The reflux treatment in polar organic solvents leads to titania supports having high specific surface area. Two reactions from the field of photocatalysis were employed to valorize the obtained catalysts: H2 production by methanol dehydrogenation and photocatalytic oxidation of formic acid. Isopropanol decomposition has been used to determine the acid-base properties of the solids. Molten nitrate technique was then successfully applied in order to obtain in one-step highly dispersed Mn-Zr and Mn-Ti supported oxide catalysts. These systems showed excellent activity for the total oxidation of volatile organic compounds (VOC)
SynthÚse de matériaux nano structurés dans des solvants non aqueux
Two major challenges are particularly important for current research in the catalytic materials synthesis: preparation of finely divided particles and control of their shape. In this work we used non aqueous solvents to prepare catalytic materials containing transition metals oxides (Ti, Mn, Zr). Three techniques were applied to prepare TiO2-based solids. Reactions in molten alkali metal nitrates doped with different anions offered good control of textural properties and of the nature of exposed facets of nanocrystalline particles. Synthesis in molten ammonium nitrate stabilized with organic nitrogen-containing compounds provided TiO2 solids with variable size and shape as a function of stabilizer used. The reflux treatment in polar organic solvents leads to titania supports having high specific surface area. Two reactions from the field of photocatalysis were employed to valorize the obtained catalysts: H2 production by methanol dehydrogenation and photocatalytic oxidation of formic acid. Isopropanol decomposition has been used to determine the acid-base properties of the solids. Molten nitrate technique was then successfully applied in order to obtain in one-step highly dispersed Mn-Zr and Mn-Ti supported oxide catalysts. These systems showed excellent activity for the total oxidation of volatile organic compounds (VOC).Dans la synthĂšse des matĂ©riaux catalytiques, deux aspects orientent en particulier les recherches actuelles: le contrĂŽle de taille afin dâobtenir des nanoparticules finement divisĂ©es et le contrĂŽle prĂ©cis de la morphologie. Dans cette Ă©tude, nous avons utilisĂ© les milieux non aqueux pour Ă©laborer des matĂ©riaux catalytiques Ă base dâoxydes des mĂ©taux de transition (Ti, Zr, Mn). Trois mĂ©thodologies ont Ă©tĂ© employĂ©es pour la prĂ©paration des solides Ă base de TiO2. La synthĂšse dans les nitrates fondus dopĂ©s avec diffĂ©rents anions permet un boncontrĂŽle des propriĂ©tĂ©s texturales des matĂ©riaux et de la nature des plans cristallins exposĂ©s. Les prĂ©parations utilisant le nitrate dâammonium stabilisĂ© avec des molĂ©cules organiques azotĂ©es conduit Ă des solides prĂ©sentant des tailles et des formes de particules variables en fonction de la nature du stabilisateur. Le traitement Ă reflux dans des solvants organiques polaires conduit Ă lâobtention des supports TiO2 de grandes surfaces spĂ©cifiques. Pour la valorisation catalytique de ces matĂ©riaux, deux rĂ©actions appliquĂ©es au domaine de la photocatalyse ont Ă©tĂ© employĂ©es: la production dâH2 par dĂ©shydrogĂ©nation du mĂ©thanol etlâoxydation photocatalytique de lâacide formique. La rĂ©action modĂšle de dĂ©composition delâisopropanol a Ă©tĂ© utilisĂ©e comme test de caractĂ©risation des propriĂ©tĂ©s acido-basiques de nossystĂšmes. La technique de nitrates fondus a Ă©tĂ© Ă©galement appliquĂ© avec succĂšs pourlâobtention en une seule Ă©tape des catalyseurs supportĂ©s Mn-Zr et Mn-Ti. Ces catalyseursmontrent une excellente activitĂ© pour lâoxydation totale de composĂ©s organiques volatils(COV)
Comparison of CuxZnAlO mixed oxide catalysts derived frommulticationic and hybrid LDH precursors for methane total oxidationMonica
International audienceCuxZnAlO mixed oxide catalysts for methane total oxidation have been obtained by thermal activa-tion of layered double hydroxides (LDHs) precursors. LDHs have been prepared following two differentroutes: (i) conventional coprecipitation (pH = 8 or 10) and (ii) hybridation achieved by intercalation ofCu-containing anionic complexes ([Cu-EDTA]2â and [Cu-citrate]â) into host ZnAl-LDH. The mixed oxidecatalysts showed only ZnO phase at Cu < 5 at.% and a mixture of ZnO and CuO phases at higher Cu con-tent. CuO particles differently interacting with the oxide supports were revealed by TPR analysis. Theywere reduced at lower temperature but with a lower H2 consumption in the mixed oxides derived fromthe hybrids suggesting lower accessibility. The activity of the catalysts in the total oxidation of methanestrongly depended on the method of preparation of the LDH precursors and on the Cu content. Cu10ZnAlO (10 at.% Cu) derived from a coprecipitated LDH showed the highest intrinsic activity and a good stabilitysuggesting an optimum dispersion of the copper-containing active species. Among the catalysts derivedfrom the hybrid precursors, the higher activity of the Cu(EDTA)-ZnAl ones may be due to their higher Cucontent. The lower intrinsic activities and higher activation energies of this series of catalysts comparedto the coprecipitated ones may be due to the lower accessibility to the CuO active sites
Modified Catalysts and Their Fractal Properties
Obtaining high-area catalysts is in demand in heterogeneous catalysis as it influences the ratio between the number of active surface sites and the number of total surface sites of the catalysts. From this point of view, fractal theory seems to be a suitable instrument to characterize catalysts’ surfaces. Moreover, catalysts with higher fractal dimensions will perform better in catalytic reactions. Modifying catalysts to increase their fractal dimension is a constant concern in heterogeneous catalysis. In this paper, scientific results related to oxide catalysts, such as lanthanum cobaltites and ferrites with perovskite structure, and nanoparticle catalysts (such as Pt, Rh, Pt-Cu, etc.) will be reviewed, emphasizing their fractal properties and the influence of their modification on both fractal and catalytic properties. Some of the methods used to compute the fractal dimension of the catalysts (micrograph fractal analysis and the adsorption isotherm method) and the computed fractal dimensions will be presented and discussed
Band-Gap Engineering of Layered Perovskites by Cu Spacer Insertion as Photocatalysts for Depollution Reaction
A multi-step ion-exchange methodology was developed for the fabrication of Cu(LaTa2O7)2 lamellar architectures capable of wastewater depollution. The (001) diffraction line of RbLaTa2O7 depended on the guest species hosted by the starting material. SEM and TEM images confirmed the well-preserved lamellar structure for all intercalated layered perovskites. The UVâVis, XPS, and photocurrent spectroscopies proved that Cu intercalation induces a red-shift band gap compared to the perovskite host. Moreover, the UVâVis spectroscopy elucidated the copper ions environment in the Cu-modified layered perovskites. H2-TPR results confirmed that Cu species located on the surface are reduced at a lower temperature while those from the interlayer occur at higher temperature ranges. The photocatalytic degradation of phenol under simulated solar irradiation was used as a model reaction to assess the performances of the studied catalysts. Increased photocatalytic activity was observed for Cu-modified layered perovskites compared to RbLaTa2O7 pristine. This behavior resulted from the efficient separation of photogenerated charge carriers and light absorption induced by copper spacer insertion