Langmuir 1985,1, 245-250
NMR signal was no longer detected; this result confirms that the NMR spectra represent adsorbed molecules and not those in the gas phase. Slight shifts of the resonances relative to an external Me4Si standard (7.0 ppm for the adsorbed benzene, compared with 7.2 ppm for neat benzene, and 1.0 ppm for cyclohexane, compared with 1.4 ppm for neat cyclohexane) probably indicate susceptibility differences. The surface concentrations of benzene and cyclohexane determined by the NMR data are plotted as a function of time of reaction in Figure 2A. The gas-phase concentrations determined by gas chromatography in the same experiment are shown in Figure 2B. The sums give the total number of benzene and cyclohexane molecules in the systems, as shown in Figure 2C. The reaction is zero order in benzene partial pressure; the slopes of the lines for benzene and cyclohexane in Figure 2C both give a rate of 2.7 (fO.l) X 10" molecules/min, which corresponds to a rate of 9.6 X lo4 molecules of benzene converted per total rhodium atom per second. The mass balance does not close exactly, as shown by the early data for total detected benzene and cyclohexane (Figure 2C). At times
