Reduction Kinetics of Ag2MoO4 by Hydrogen

Abstract

powders were mechanically (ball) milled with the objective of obtaining a Ag 2 MoO 4 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 Ag 2 MoO 4 . It was found that Ag 2 MoO 4 is reduced in three steps, e.g., Ag 2 MoO 4 to MoO 3 and Ag followed by MoO 3 to MoO 2 and finally MoO 2 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 Ag 2 O to Ag, 64 kJ/ mol, the latter was found to be lower than the corresponding value for MoO 3 to MoO 2 under isothermal conditions, 124 kJ/mol. It was found that the presence of metallic silver obtained from the initial reduction step of Ag 2 MoO 4 not only lowers the energy barrier but the reducing temperature of the subsequent reaction steps (i.e., MoO 3 fi MoO 2 fi Mo).

Temperature‐programmed reduction of silver(I) oxide using a titania‐supported silver catalyst under a H2 atmosphere

“…This study was done with the motivation to determine the reduction model that accurately describes reduction of Ag 2 O to Ag using the Ag/TiO 2 catalyst. Studies on Ag 2 O reduction models were also reported previously at lower heating rates such as 5–11 K/min and 2.5–30 K/min . Thus, this study provides further insights into the Ag 2 O reduction model over higher heating rates (20–50 K/min).…”

“…Computation of P(x) value via this simplification is valid only for x values between 9 and 174, which are generally applicable to TPR conditions. By substituting Equation (12) into Equation (9), one can get: g α ð Þ = AEe − x Rβ 674:567 + 57:421x− 6:055x 2 −x 3 1; 699:066x + 841:655x 2 + 49:313x 3 −8:02x 4 − x 5 ð13Þ g(α) can be derived from f(α) as given in Table 1. By substituting appropriate g(α) into Equation (13) based on the correct reduction mechanism and with the use of x = E/RT substitution, α can be derived as an explicit function of T. Derivative of α with respect to T, dα/dT, would be the mathematical function of TPR patterns.…”

“…For instance, several temperature-programmed reduction (TPR) kinetic analyses of reduction of metal oxides such as cobalt oxide, chromium oxide, iron oxide, vanadium oxide, and silver(I) oxide were reported in the literature. [8][9][10][11][12][13][14] Tiernan et al [15] reported that the reduction mechanism of metal oxide is influenced by particle size in which larger metal oxide particles were noted to be reduced via the phase boundary mechanism while smaller metal oxide particles were noted to be reduced via the uniform internal reduction mechanism. Various kinetic results on metal oxide reduction such as Fe 2 O 3 were also reviewed by Pineau et al [16] in which the numerous variations in reduction behavior were also noted to be affected by powdered sample preparation, its physicochemical properties, and TPR procedures.…”

“…[17,18] As for silver(I) oxide, its current study is more established in its thermal decomposition as compared with H 2 -based reduction. The majority of the works on Ag 2 O thermal decomposition [19][20][21][22][23][24] were reported on its autocatalytic nature and nucleation mechanism while H 2 -based Ag 2 O reduction experiments by Jeli c et al, [13] Juarez and Morales [12] also reported likewise.…”

“…Therefore, a kinetic study on the reduction of metal oxide samples would elucidate more on the role and dominance of such elementary steps involved in the reduction process. For instance, several temperature‐programmed reduction (TPR) kinetic analyses of reduction of metal oxides such as cobalt oxide, chromium oxide, iron oxide, vanadium oxide, and silver(I) oxide were reported in the literature . Tiernan et al reported that the reduction mechanism of metal oxide is influenced by particle size in which larger metal oxide particles were noted to be reduced via the phase boundary mechanism while smaller metal oxide particles were noted to be reduced via the uniform internal reduction mechanism.…”