Synthèse et caractérisations d'oxydes de molybdate pour des applications environnementales et/ou énergétiques

Development of green advanced processes for energy production and/or environmental protection is of prime importance. For this purpose, transition metal ternary oxides have been synthesized in the laboratory by a coprecipitation method. These are AMoO4 bifunctional molybdate particles (A = Co, Ni and Cd), of which we have sought to master the key parameters that make it possible to modulate the sizes and or shapes while ensuring reproducibility of the synthesis process. Several characterization techniques (DRX, FTIR, MEB, AFM, EDS) were used to access different characteristic parameters: crystal structure, lattice parameters, the crystallites size and strain. Measurements highlighted differences between the three molybdates concerning the mesh parameters and the existence of micro-stresses (extension or compression) as well as of defects and vacancies. This difference also concerned the surface roughness (mean square height values of a few tens of nanometers) as well as the size (from a hundred nanometers to tens micrometers) and the shape (agglomerated nano-rods for CoMoO4, nano-sheets for NiMoO4 and spheroids for CdMoO4). These features have been used to illustrate the combined multi-scale structural effects on the photo-catalytic performance of AMoO4 ternary systems. Beforehand, the optical properties of these compounds were evaluated by UV-Visible diffuse reflectance spectrophotometry (DRS). The optical band gaps were estimated by fitting the experimental measurements by the Kubelka-Munk model: 2.8, 2.4 and 3.7 eV in the cases of Co, Ni and Cd respectively. The concept of band diagram (EC and EV energy levels), associated with that of electrochemical potential (electronegativity and free energy of electrons), made it possible to establish an energy diagram verifying the adequacy between the potential energies (EC and EV) involved and the redox potentials of the couples H2O/•OH, OH-/•OH et O2/•O2‐ and therefore the potential of these ternary oxides to be efficiently used in the field of photocatalysis.The photo-catalytic tests, carried out on methylene blue (BM) considered as a dye (pollutant) model, were very conclusive with a degradation efficiency, under only one-hour irradiation, of order of 87% with NiMoO4 and of the order of 52% for CoMoO4. For the latter, the simplified model of Langmuir-Hinshelwood, proposed by Vollbrecht was suitable for fitting the experimental data, while it is the pseudo second order Nelson-Yoon model which proved to be the more suitable for nickel. The values of the kinetic constants then being much higher than those estimated from the simplified Langmuir model, sign of higher degradation process speed, in disagreement with other works relating to the same AMoO4 compounds.The reuse of molybdate particles in successive photo-catalytic cycles has shown that the degradation rates of MB under the effect of NiMoO4 remain quite remarkable despite the observed decrease: 80%, 74%, 65% and 55% from the second cycle to the last one. Under identical conditions, CoMoO4, showed a slower decrease in the photo-degradation efficiency: 47% and 46% for the second and third cycles against 52% for the first.