CF2Cl2 decomposition over nanocrystalline MgO: evidence for long induction periods

CF(2)Cl(2) has been found to react with nanoscale MgO at 325 degrees C and higher temperatures. In excess of the halocarbon, the reaction results in the formation of MgF(2) as a predominant solid product, with CCl(4), and CO(2) formed as the main gaseous products. The kinetics of the process is characterized by a prolonged induction period, which is as long as 8.5 h at 325 degrees C. The length of the induction period decreases with temperature increase and becomes negligible at 500 degrees C. Complete CF(2)Cl(2) mineralization has been achieved in an excess of MgO at 450 degrees C. Detailed HRTEM and EDX analysis has shown that the induction period involves the formation of small amounts of magnesium halides on the oxide surface and results in its reconstruction leading to initial oriental ordering of the nanocrystals followed by substantial changes in the bulk composition of the nanoparticles. The reaction proved to be structurally sensitive. It has been found that deep fluoridation is possible only for nanoscale MgO samples. The use of samples with lower surface areas results in lengthening of the induction period and decrease of the reaction depth. The MgO transformation to MgF(2) has been found to result in a surface area decrease by more that an order of magnitude as a result of intense sintering of magnesium fluoride under the reaction conditions.