Elementary Surface Reactions Of Fischer-Tropsch Synthesis On Cobalt Surface

Fischer-Tropsch synthesis(FTS),catalytically transforming the syngas(CO+H2)into higher hydrocarbons,such as gasoline,diesel,and kerosene,is an important industrial route to produce clean fuels and chemicals.Although extensively studied,the reaction mechanisms of FT synthesis are still debated due to the complexity.CHx(a)(x=1-3)species are the key surface intermediates of catalysts during the FTS and their complex surface reaction networks consisting of hydrogenation,dehydrogenation and coupling reactions are responsible for the product distributions of the FTS.Under working conditions,CHx(a)is difficult to be detected due to the short lifetime on the surface.In this thesis,we controllably prepared various CHx(a)species via thermal decompositions of CH3I and CH2I2 on Co(0001)under ultra-high vacuum(UHV)condition.Reactivity of various CHx(a)species,as well as influences of coadsorbed H(a)and CO(a)on their reactivity,were studied.Meanwhile,adsorption and reaction of CO and H2 on different crystal facets of a Co polycrystalline foil were also studied.The main results are summarized as the following:(1)Reactivity of CHx(a)species prepared by CH3I dissociation on clean,H(a)and CO(a)precovered Co(0001)surfaces were studied using thermal desorption spectroscopy(TDS)and X-ray photoelectron spectroscopy(XPS).CH3I and CHx(a)species exhibit an vacant surface sites-dependent decomposition behavior on Co(0001)surface.The CH3(a)and CH(a)fragments are dominate CHX fragments upon CH3I dissociation on Co(0001)at 100 K.Upon heating,the CH3(a)species undergoes hydrogenation reaction to CH4,self-coupling reaction to C2H6,and decomposition reaction to CH(a),while the CH(a)species undergoes hydrogenation reaction to CH4,dehydrocyclization reaction to graphitic carbon,and decomposition reaction to atomic carbon.The methanation reactivity of various CHx(a)fragments follows an order of CH3(a)>CH2(a)>CH(a).Co-adsorbed 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.Co-adsorbed 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 hydrogenation reaction respectively to C2H4 and C2H6.(2)Reactivity of CHx(a)species prepared by CH2I2 dissociation on clean,H(a)and CO(a)precovered Co(0001)surfaces were studied using TDS and XPS.CH2I2 and CH2(a)species exhibit an vacant surface sites-dependent decomposition behavior on Co(0001)surface.Upon adsorption on clean Co(0001)surface at 110 K,CH2I2 undergoes stepwise decomposition reactions to produce carbon adatoms,CH(a)and CH2(a)species at small coverages,and chemisorbs both dissociatively and molecularly at large coverages.Upon heating,CH2(a)species facilely undergoes surface reactions to produce CH4,C3H6,and C2H4 in gas phase and CH(a)species on the surface at low temperatures.CH(a)species undergoes surface reactions to produce CH4 in gas phase and C2H2(a)species on the surface at higher temperatures,and both CH(a)and C2H2(a)species undergo further surface reactions to produce H2 in gas phase and carbon species on the surface.Coadsorbed H adatoms and CO molecules were found to strongly affect surface chemistry of CH2I2 and the resulting CHx(a)species on Co(0001)via suppressing the decomposition reactions and promoting the carbon-carbon bond coupling reactions.(3)Carbidic and graphic carbon species on Co(0001)were prepared by controlled C2H4 decomposition and catbidic carbon-to graphic carbon transformation reaction,and their influences on reactivity of CHx(a)were studied.At a constant total carbon coverage,the probability of coupling reaction of CHx(a)species is higher on Co(0001)surface with co-existing carbidic and graphic carbon than on Co(0001)surfaces dominantly with carbidic carbon or dominantly with graphic carbon.The carbidic carbon species was observed to exhibit both geometric and electronic effects,which stabilizes the CH2(a)species active for the chain propagation reactions,while the graphitic carbon species mainly exhibits a geometric effect and occupies the surface sites for CH4 formation,which decreases the methanation reaction probability and increases the chain-growth reaction probability.(4)Adsorption and reaction of CO and H2 on a Co polycrystalline foil with Co(10-12)and Co(0001)microfacets were studied using PEEM/LEEM,Auger electron spectroscopy(AES),XPS and TDS.Adsorption and reaction of CO and H2 on the cobalt polycrystalline surface depend on the pressure and form different kinds of surface species.H2 exposures induce segregation of bulk oxygen species in Co polycrystalline foil,which passivates the surface.Under the same condition,Co(10-12)micro facets are more reactive than Co(0001)microfacets.The above results establish the elementary surface reaction networks of CHx(a)fragments on Co(0001)and reveal the shaping effect of co-adsorbed H(a),CO(a)and carbon species on the reaction pathways of CHx(a)fragments,and preliminarily demonstrate the facet effect of Co surfaces on the reactivity toward CO and H2,which greatly broaden the fundamental understanding of complex Co-catalyzed FTS reaction.