A Spectroscopic Study of the Dissolution of Cesium Phosphomolybdate and Zirconium Molybdate by Ammonium Carbamate

摘要

Through a combination of Raman spectroscopy, multi-element NMR spectroscopy and chemical analysis, the differences between the action of carbonate and carbamate as agents for dissolving Cs3PMo12O40⋅xH2O(s) (CPM) and ZrMO2O7(OH)2(H2O)2(s) (ZM) have been elucidated. Alkaline H2NCO− 2/HCO3 −/CO3 2− solutions, derived from the dissolution of ammonium carbamate (NH4H2NCO2; AC), dissolve CPM by base hydrolysis of the PMo12O40 3− Keggin anion, ultimately forming [MoO4]2− and PO4 3− when excess base is present. If the initial concentration of H2NCO2 −/HCO3 −/ CO3 2− is lowered, base hydrolysis is incomplete and the dissolved species include [Mo7O24]6− and [P2Mo5O23]6−, and undissolved solid Cs3PMo12O40, Cs x NH7− x PMo11O39, and Cs x NH6− x Mo7O24 remain. Na2CO3 solutions dissolve Cs3PMo12O40 through a similar mechanism, but the dissolution rate is much lower. We attribute this difference to the different buffering effects of H2NCO2 −/HCO3 −/CO3 2− and CO3 2−/HCO3 − solutions, and the instability of carbamic acid, the protonated form of H2NCO2 − (which rapidly decomposes into NH3 and CO2). The ability of NH3 to produce NH4 + and OH−, together with the evolution of CO2 gas, drive the reaction forward. Low temperature measurements under conditions where pure H2NCO2 − is kinetically stable, allowed the rates of dissolution of CPM by H2NCO2 − and CO3 2− to be compared directly, confirming the faster dissolution by H2NCO2 −. Compared to CPM, the dissolution of ZM by H2NCO2 −/HCO3 −/CO3 2− is a much slower process and is driven by the formation of soluble Zr IV -carbonate complexes and MoO4 2−. The driving force for the dissolution of ZM is the superior complexing ability of carbonate over carbamate; consequently solutions containing a higher carbonate concentration dissolve ZM faster.