Role of Coadsorbates in Shaping the Reaction Pathways of Alkyl Fragments on Co Surfaces

Understanding the elementary surface reaction network of the Co-catalyzed F-T synthesis (FTS) reaction is of great interest. In this Article, we have studied the surface reactions of CHx(a) prepared by CH3I dissociation on clean and on H(a) and CO(a) precovered Co(0001) surfaces using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). CH3I and CHx(a) species exhibit a vacant surface sites-dependent decomposition behavior on the Co(0001) surface. The CH3(a) and CH(a) fragments are dominant CHx fragments upon CH3I dissociation on Co(0001) at 100 K. Upon heating, the CH3(a) species undergoes the hydrogenation reaction to CH4, the self-coupling reaction to C2H6, and the decomposition reaction to CH(a), while the CH(a) species undergoes the hydrogenation reaction to CH4, the dehydrocyclization reaction to graphitic carbon, and the decomposition reaction to atomic carbon. The coadsorbed H(a) species on Co(0001) does not affect the speciation of CHx fragments formed by CH3I dissociation at 100 K and greatly enhances the hydrogenation reaction probability of both CH3(a) and CH(a) species at the expense of decomposition reaction probability. The coadsorbed CO(a) species on Co(0001) stabilizes the CH2(a) species that undergoes the hydrogenation reaction to CH4 and the insertion reaction to CH3CH2(a) followed by H elimination and the hydrogenation reaction, respectively, to C2H4 and C2H6. The methanation reactivity of various CHx(a) fragments follows an order of CH3(a) > CH2(a) > CH(a). These results establish the elementary surface reaction networks of CHx(a) fragments on Co(0001) and reveal the shaping effect of coadsorbed H(a) and CO(a) on the reaction pathways of CHx(a) fragments, which greatly broaden the fundamental understanding of the complex Co-catalyzed FTS reaction.