Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID4AS INTERFACE 237Recently gas chromatographic techniques have been developed which allow theseparation of the various hydrogen modifications and their subsequent detectionby a thermal conductivity cell. Hydrogen, deuterium and hydrogen deuteride havebeen separated on alumina columns at 77" K using heli~rn~~'.~~~ or neonz3'carrier gas. Coating with ~hromia~~' or ferric oxide238 was used to achieve completeequilibration between the ortho and para isomers of hydrogen and deuterium.Separation of ortho- andpara-hydrogen has been achieved on alumina241 and 13Xmolecular sieve242 at 77-90" K using a helium carrier gas.(b) Oxygen isotopesThe use of oxygen-18 for the study of catalytic reactions was pioneered byWinter243 who obtained kinetic results with a number of oxide systems. Morerecently, Russian workers have used oxygen exchange to assist in the study ofcatalytic oxidation and the subject has been reviewed by B0reskov~~4.Two types of oxygen exchange are possible: homolytic exchange, i.e. exchangebetween two adsorbed molecules on the surface, and heterolytic exchange, i.e.exchange between lattice oxygen and adsorbed oxygen"02+ 1602 + 2180(ads)+2160(ads) 2180160O(1attice) + 2' 'O(ads) =: "O(1attice) + 0''O(ads)The rate of reaction is followed by measuring the changes in intensity of the peakscorresponding to masses 32(160,), 34(l6O1*O) and 36("02) in a mass spectrometer.Winter243 used a static system with a capillary leak to the mass spectrometer.If the rate of exchange is high, it is more satisfactory to carry out the exchangein a circulation system to eliminate errors due to diffusion limitations.The circulation system used by Dzisjak et ~ 1.~~' is shown in Fig. 31. It circulatesgas at the rate of half to one litre per hr with a circulation volume of about one litre.10 ml samples are removed for analysis and the oxygen pressure can be kept constantduring the experiment.In the absence of exchange with the catalyst (heterolytic exchange), the rate ofhomolytic exchange is given bywhere z is the reaction time (sec), C:4, C3O4, C3, are the fractional concentrationsof 160180 at equilibrium, zero time and time z respectively, and K = ZS/N,. Zis the overall number of molecules which have already exchanged (molecules .cm-2.sec-'), S is the surface area (cm2) and Ng is the overall number of oxygenmolecules in the system. Further evidence about the rate limiting step can beobtained from the oxygen pressure dependence. If molecular oxygen is involved,References pp. 270-278