Methanolic C-O Bond Activation on Pd(111): Coverage-Dependent Reaction

摘要

We find that the previously reported methanolic C-0 bond activation on Pd 111 toform adsorbed methyl and hydroxyl at 225 K occurs only for initial methanol coverages near 1 monolayer. At lower and higher coverages, methanol decomposes via the more traditional methoxy intermediate. Moreover, when methyl is formed, it is not observed to decompose until the temperature is raised to 400 K. This thermal behavior is quite unusual since CH3 has been observed to combine with H to desorb as CH4 below 300 K, dimerize to produce C2H6 below 200 K, or dehydrogenate to form CH2 and H below 200 K. In addition, we find that methyl undergoes sequential dehydrogenation as the temperature is increased to form CH2 at 400 K, CH at 500 K, and C at >500 K. We speculate that the coverage dependency is connected with the thermal stability of CH3 by a site-blocking mechanism, probably involving CO. Fischer-Tropsch synthesis, x-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS).