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).