Alkali promotion on cobalt: Surface analysis of the effects of potassium on carbon monoxide adsorption and Fischer-Tropsch reaction

Abstract

A surface study is made of CO adsorption and the Fischer-Tropsch (CO hydrogenation) reaction on Co foils with K precoverages of from 0 to 0.8 monolayers (ML, 1 ML = saturation K coverage ≈ 5 × 10 14 K/cm 2). X-ray photoemission (XPS) and Auger spectroscopy (AES) are used to characterize the adsorbed surface species. The adsorption and reaction occur in an atmospheric-pressure microreactor coupled to the surface analysis vacuum chamber by a rapid, valveless sample-transfer mechanism. After CO exposure (548 K, 100 kPa, 10 s) core levels are seen corresponding to molecular and dissociated CO. Molecular CO 1s levels exhibit a K-dependent binding energy decrease attributed to a surface-to-molecule charge transfer enhanced by the alkali. Potassium enhances the amount of CO adsorbed and thermally stabilizes the adsorbed molecule. Surface analysis after Fischer-Tropsch reaction (523–548 K, 100 kPa total reactant pressure) characterizes the amount and nature of the C deposited. The results are compared with earlier measurements on Fe surfaces. For identical reaction conditions, clean Co shows much less deposited C than clean Fe. It also forms surface carbidic C deposits much less readily than Fe, the C deposited being predominantly graphitic. Potassium predosing of the surfaces enhances the amount of C deposited, attributable to its enhancement of the CO adsorption step. It also changes the nature of the C, inducing carbidic C. When the carbidic C is induced (by a K precoverage of ∼ 0.7 ML), the amount of C deposited by reaction actually decreases, compared to that produced by reactions on surfaces with less K, an effect not previously observed during K-promoted reactions on Fe or Ni. The reactivity of carbidic C to hydrogen on both Co and Fe is reduced by K, helping to explain the lower methanation rate that has been observed after K promotion of Fischer-Tropsch catalysts.