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 Cs(3)PMo(12)O40 center dot xH(2)O(s) (CPM) and ZrMO2O7(OH)(2)(H2O)(2)(s) (ZM) have been elucidated. Alkaline H2NCO2-/HCO3-/CO32- solutions, derived from the dissolution of ammonium carbamate (NH4H2NCO2; AC), dissolve CPM by base hydrolysis of the PMo12O403- Keggin anion, ultimately forming [MoO4](2-) and PO43- when excess base is present. If the initial concentration of H2NCO2-/HCO3-/CO32- is lowered, base hydrolysis is incomplete and the dissolved species include [Mo7O24](6-) and [P2Mo5O23](6-), and undissolved solid Cs3PMo12O40, CsxNH7-xPMo11O39, and CsxNH6-xMo7O24 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-/CO32- and C-3(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 CO32- to be compared directly, confirming the faster dissolution by H2NCO2-. Compared to CPM, the dissolution of ZM by H2NCO2-/HCO3-/CO32- is a much slower process and is driven by the formation of soluble Zr-IV-carbonate complexes and MoO42-. 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.