Fixation of nitrogen with cavitation.

The sonochemical fixation of nitrogen to ammonia was investigated by sonolysis of liquids with nitrogen/hydrogen gas mixtures passing through them. The maximum rate (4 nmol min-1 W-1) was found in water irradiated with 900 kHz ultrasound with gas of a mole fraction 0.6 of nitrogen and the lowest temperature (278 K). Some traces of ammonia were found in the absence of external hydrogen gas, as hydrogen atoms are also formed in sonolysis of water. Thermodynamic calculations indicate that some ammonia formation should occur at the "hot spot" temperatures present in collapsing cavities. The decrease in the rate with bulk temperature suggests that kinetics, rather than thermodynamics, is the limiting condition for sonochemical synthesis of ammonia. Ammonia can be produced in organic media, but at a lower rate. A substantial portion of the ammonia in the experiments with alkanes exited with the sparging gas and was trapped in a dilute HCl solution.