Mechanochemical reactions in Na2CO3–M2O5 (M = V, Nb, Ta) powder mixtures: Influence of transition-metal oxide on reaction rate
Abstract We have studied the mechanochemical reactions between Na 2 CO 3 and the transition-metal oxides M 2 O 5 (M = V, Nb, Ta), giving special attention to the course of the individual reaction and to the influence of the transition-metal oxide on the reaction rate. A common mechanism links the studied reactions and is characterized by the formation of an intermediate amorphous carbonato complex, in which the CO 3 2– ions that are present initially in the Na 2 CO 3 , coordinate to the transition-metal cation. In a subsequent milling stage the carbonato complex decomposes, leading to the formation of the final binary compounds, i.e., NaVO 3 , NaNbO 3 and NaTaO 3 . Quantitative experimental results, based on the Rietveld refinement method and thermogravimetric analyses, showed that the reaction rate follows the order Na 2 CO 3  + V 2 O 5  > Na 2 CO 3  + Ta 2 O 5  > Na 2 CO 3  + Nb 2 O 5 . By analyzing the acid–base properties of the participating reagents, we found that the experimental observations agree with the acid–base interaction mechanism, which means that the larger the acidity of the transition-metal cation involved, the faster and more complete the mechanochemical reaction. Finally, it was found that only partial decomposition of the carbonato complex, even after a prolonged mechanochemical treatment, can be expected in reactions characterized by lower acid–base potential.