Reduction Kinetics of Ag2MoO4 by Hydrogen

Ag2O and MoO3 powders were mechanically (ball) milled with the objective of obtaining a Ag2MoO4 phase for subsequent reduction with hydrogen gas. A thermogravimetric unit was used to follow the course of the reduction process aiming at the chemical reaction as the rate controlling step. Isothermal reduction experiments were performed on the individual oxides to establish a ground for the reduction parameters of silver molybdate. Consequently, a nonisothermal treatment was used on Ag2MoO4. It was found that Ag2MoO4 is reduced in three steps, e.g., Ag2MoO4 to MoO3 and Ag followed by MoO3 to MoO2 and finally MoO2 to Mo. By assuming a shrinking core model, the calculated activation energies for the first and second reduction steps were 69 and 29 kJ/mol, respectively. While the former value is in good agreement with the activation energy obtained from the isothermal reduction of Ag2O to Ag, 64 kJ/mol, the latter was found to be lower than the corresponding value for MoO3 to MoO2 under isothermal conditions, 124 kJ/mol. It was found that the presence of metallic silver obtained from the initial reduction step of Ag2MoO4 not only lowers the energy barrier but the reducing temperature of the subsequent reaction steps (i.e., MoO3 → MoO2 → Mo).