Probing ensemble effects in surface reactions. 3. Cyclohexane adsorption on clean and bismuth-covered Pt(III)

Article abstract of DOI:10.1021/j100339a058

The adsorption and chemistry of cyclohexane on clean and Bi-covered Pt(111) surfaces have been studied with thermal desorption mass spectroscopy (TDS), deuterium labeling, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The thermal experiments show that, on clean Pt(111), cyclohexane is adsorbed in three molecular states with desorption temperatures of approx. 236 (monolayer), approx. 181 (second monolayer), and approx.154 K (multilayer). There is little isotopic exchange in the molecularly desorbing cyclohexane. Benzene is formed at approx. 236 K on the clean Pt(111) surface is a product of the dehydrogenation of adsorbed cyclohexane. This process is 13-fold slower for perdeuterated cyclohexane. An analysis of quantitative XPS data and the effect of Bi poisoning on the chemisorption of cyclohexane shows that an ensemble of about five Pt atoms is necessary to chemisorb C₆H₁₂. The dehydrogenation of adsorbed cyclohexane is poisoned at very low bismuth coverages (θ$-Βi$/<0.1) according to mechanism dominated by steric, site-blocking effects.