Catalytic reactions on solid catalysts consist of adsorption of reactants, surface reactions, and desorption of adsorbed products. These steps must cooperate well to realize efficient catalysis. In this talk, I will illustrate the beautiful surface chemistry behind efficient catalysis with surface chemistry of FTS reaction on Co(0001) as an example. Co-based catalysts are active and stable in catalyzing the FTS reaction. We studied elementary surface reactions on Co(0001) pretreated with the syngas reactant at different conditions with high resolution photoemission spectroscopy. Elementary surface reactions on Co(0001) were observed to sensitively depend on the reaction pressure and temperature. Under the typical FTS reaction condition (4 bar and 490 K), Co(0001) surface is metallic and covered with carbidic carbon, hydrocarbon fragments, atomic oxygen and hydroxyl groups. Experimental evidence was provided for both CO direct dissociation to form carbidic carbon species and hydrogen-assisted CO dissociation to form HCO species. Both CO activation reactions can occur facilely already at 4 bar and 300 K. However, at elevated temperatures, the active carbidic carbon species for FT synthesis tends to transform into the inactive graphitic carbon species on Co(0001) that poisons the Co surfaces. We found that subsurface atomic oxygen on Co(0001) can stabilize the active carbidic carbon species and quench the transformation process. Thus during the FTS reaction over Co surfaces, the reactivity of various types of atomic carbon, oxygen and hydrogen species formed on Co surfaces by CO and H<sub>2</sub> dissociation that dynamically maintain a delicate balance to empower Co catalysts the long-term stability. CO dissociates at Co surface to produce the active carbidic carbon and surface atomic oxygen species. Surface atomic oxygen species on Co surfaces can diffuse into the subsurface region to form subsurface oxygen species capable of quenching the transformation of carbidic carbon into graphitic carbon, and the atomic hydrogen species can suppress the accumulation of subsurface oxygen species and the subsequent oxidation of Co catalysts. These results reveal a self-anticoking mechanism of cobalt surface by subsurface oxygen in FT synthesis and exemplify the beautiful surface chemistry behind heterogeneous catalysis..

Beautiful surface chemistry behind efficient catalysis

254th National Meeting (2017)

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