In situ kinetics and mechanism of furan decomposition and desorption with CO formation on Pd(111)

The work presented here uses kinetic studies under conditions of varying temperature, coverage, and isotopic substitution to elucidate the energetics and mechanism of furan decomposition and desorption on Pd(111). Laser-induced thermal desorption with Fourier transform mass spectrometry detection is used as a sensitive tool for measuring the in situ kinetics of furan decomposition simultaneously with the formation of CO. The kinetics of both furan loss and CO formation have been studied for 0.17 and 0.32 Langmuir exposures of furan on Pd(lll) at 100 K. This corresponds to about 2 and 4% of a monolayer and approximately 15 and 30% of a saturation monolayer. respectively. Total observed furan loss follows two processes involving a competition between desorption and decomposition. The kinetic factors for each process are extracted yielding an activation barrier of approximately 60 kJ/mol and preexponential factor of 10(7) s(-1) for decomposition, and an activation barrier of approximately 95 kJ/mol and preexponential factor of 10(14) s(-1) for desorption. The parameters for decomposition show a substantial coverage dependence at significantly less than a saturation monolayer, whereas those for desorption are relatively independent of coverage. The kinetic parameters for CO formation match those for the decomposition of furan, indicating that the rate for both reactions is limited in the same processes. Results from coverage effects and isotopic labeling experiments suggest that the rate-determining step is not simple alpha-C-H bond cleavage.