Multistep reaction processes in epoxide formation from 1-chloro-2-methyl-2-propanol on Ag(110) revealed by TPXPS and TPD experiments
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abstract
Synchrotron-based temperature-programmed X-ray photoelectron spectroscopy (TPXPS) in combination with temperature-programmed desorption (TPD) has been used to track C-Cl scission in the reaction of 1-chloro-2-methyl-2-propanol (Cl-tert-BuOH) on oxygen-containing Ag(110) surfaces. The results show that the oxygen pre-coverage strongly influences the cleavage of the C-Cl bond. At low coverages, C-Cl scission of chloro-t-butoxide intermediates begins at 200 K and isobutylene oxide (IBO) appears with a peak temperature of 235 K; at higher coverages, the onset of C-Cl scission is shifted upward by 50 K and the IBO peak by 80 K. Quantitative models for the surface reaction kinetics were developed from the experimental data. These show that the reaction of adsorbed intermediates does not occur by an S N2 process that releases IBO directly into the gas phase. Instead, C-Cl scission deposits organic intermediates or products on the surface, and the appearance of IBO in the gas-phase lags the appearance of atomic chlorine on the surface. For the lower temperature channel, the rate of IBO evolution in TPD is influenced by the kinetics of both C-Cl scission and molecular TBO desorption. At higher temperatures, surface diffusion processes to open surface sites limit the rate of IBO production. Comparison with results for chlorine diffusion into silver suggests that this is the relevant diffusion process.
