Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

176 EXPERIMENTAL METHODS FOR FAST REACTIONSNickeldiscCollectorpumpMolecularbeamFig. 24. Use of a single molecular beam for following gas-solid reactions.tating disc and the molecules are adsorbed on to the surface. A small collector tubeis located directly above the disc such that a molecule evaporating from the surfacewhen it is under the tube is pumped off and passes down the tube to a mass spectrometerto be counted. The position of the tube can be varied relative to the impactpoint as shown by the dashed line. In the system chosen all the hydrogen wasadsorbed, and the rate of desorption (associated with an average retention time ofabout sec) followed a simple and reproducible first order kinetic law fromwhich it was possible to derive a frequency factor and an activation energy. Byusinga mixed molecular beam of hydrogen and deuterium it was found that noexchange occurred, suggesting an adsorption process in which the bonds of the originalmolecules are not completely broken. It would seem that this method hasconsiderable potential for measuring the kinetics of fast gas-solid reactions, includingthe study of single steps in heterogeneous catalytic reactions, although thereare bound to be difficulties in obtaining surfaces of known composition.ACKNOWLEDGEMENTIt is a pleasure to acknowledge stimulating discussions with Dr. D. B. Rorabacher.REFERENCES1 M. EIGEN AND L. DE MAEYER, Z. Elektrochem., 59 (1955) 986; M. EIGEN, Angew. Chem.(Intern. Ed. Engl.), 3 (1964) 1.2 M. EIGEN, L. DE MAEYER AND H.-CH. SPATZ, Ber. Bunsenges. Physik. Chem., 68 (1964) 19.3 R. G. W. NORRISH AND G. PORTER, Nature, 164 (1949) 658; G. PORTER, Proc. Roy. Soc.(London), A, 200 (1950) 284.4 G. PORTER AND M. W. WINDSOR, Discussions Faraday Soc., 17 (1954) 178.5 M. C. SAUER AND L. M. DORFMAN, J. Am. Chem. SOC., 86 (1964) 4218.6 F. J. W. ROUGHTON, Z. Elektrochem., 64 (1960) 3; also in ref. 15, p. 5.7 cf. N. F. RAMSEY, Molecular Beams, Oxford University Press, Oxford, 1956, Chapter 2.8 E. H. TAYLOR AND S. DATZ, J. Chem. Phys., 23 (1955) 1711.9 The study of fast reactions, Discussions Faraday SOC., 17 (1954).