ALKALI PROMOTION ON COBALT - SURFACE-ANALYSIS OF THE EFFECTS OF POTASSIUM ON CARBON-MONOXIDE ADSORPTION AND FISCHER-TROPSCH REACTION

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. X-ray photoemission (XPS) and Auger spectroscopy (AES) are used to characterize the adsorbed surface species. 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, 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.