Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

238 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSFig. 31. Apparatus for the study of oxygen exchange with a circulation system. 1, reaction vessel;2, circulation pump; 3, valve box; 4, cold traps; 5, gas sampling ampoule; 6, cold trap; 7, manometricvalves; 8 to 12, gas storage bulbs; 14, gas burette; 16, adsorption vessel; 24, oxygen purificationsystem. From ref. 245.the order with respect to oxygen will vary from zero (at high coverage) to 1 (atlow coverage). Similarly if atomic oxygen is involved, the order will vary from zeroto 0.5.When exchange with lattice oxygen takes place (heterolytic exchange), thekinetic equations become more complicated. The overall fraction of '*O in the gasphase (a) decreases, i.e. du/dz is negative. The kinetic data can be represented bythe following equationdu= -K,,+$ (--)dzwhere $ = 8K3/(K2 +2K3)2. K = Kl + K, + K3 are the exchange coefficients forhomolytic exchange (K,X heterolytic exchange involving a single oxygen atomfrom the catalyst (K,) and heterolytic exchange involving two oxygen atoms fromthe catalyst (K3). y = C,*,- C34. Heterolytic exchange involving one or two oxygenatoms can be distinguished by the fact that C34 increases with time, (dC,,/dz =- 0),if one oxygen atom is involved and decreases with time (dC,,/dr -= 0) if twooxygen atoms are involved.(c) Radioactive isotopesCarbon-14, the radioactive isotope of carbon, has been widely used in mechanisticstudies with organic hydrocarbons but not in kinetic studies. 14C decays toI4N with the emission of a beta particle and has an extremely long half life (5568 It30 years)246. The emitted radiation can be detected by an ionisation chamber,