Effects of potassium on the reaction pathway of C 3H 7 species over Mo 2C/Mo (1 0 0)

Abstract

The adsorption and surface reactions of propyl iodide on clean and potassium-modified Mo 2C/Mo(1 0 0) surfaces have been investigated by thermal desorption spectroscopy (TPD), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) in the 100–1200 K temperature range. This work is strongly related to the better understanding of the catalytic effect of Mo 2C in the conversion of hydrocarbons. Potassium was found to be an effective promoter: it induced the rupture of C–I bond in the adsorbed C 3H 7I even at 100 K. The extent of C–I bond scission varied approximately linearly with the concentration of K coverage at the adsorption temperature of 100 K. As revealed by HREELS and TPD measurements the primary products of the dissociation are C 3H 7 and I. The former one was stabilized by potassium and underwent dehydrogenation and hydrogenation to give propene and propane. The desorption of both compounds is reaction-limited process. A fraction of propyl groups was converted into di-σ-bonded propene, which was stable up to ∼380 K. The coupling reaction of propyl species was also facilitated by potassium and resulted in the formation of hexane and hexene with T p ∼ 230–250 K. Hydrogen was released with T p = 390 K, indicative of a desorption limited process. The effect of potassium was explained by the extended electron donation to adsorbed propyl iodide in one hand, and by the direct interaction between potassium and I on the other hand. This was reflected by the shift of the desorption of potassium from the coadsorbed layer at and above 1.0 ML to higher temperature, and by the coincidal T p values (∼700 K) of potassium and iodine. The formation of KI was also supported by the appearance of a loss feature at 650 cm −1 in the HREEL spectra attributed to a phonon mode of KI.