Major and minor reactions in Fischer-Tropsch syntehsis on cobalt catalysts

摘要

Minor reactions,accompanying the major reactions for building straight-cahsin of aliphatic hydorcarbons form the reactants CO and H_2 on ehsurface of cobalt catalysts,can contribute substantially to the understanding of the regime of fischer-Tripsch synthesis.This goal affords precise mass balances,precise determination of product composition and consistent kinetic schemes for obtaining the right kinetic coefficients.Teh concept of self-organizaiton of the Fischer-Tropsch regime is established from time dependence of activity,selectivity and catalyst structure.A process of thermodynamically controlled restruturing/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically,disproponating on-plane sites into sites of lower coordination (on-top sites0 and higher coordination (in-hole sites0.These differetn sites appear to collaborate in teh Fischer-Tropsch regime,with steps of coordination chemistry(comparable to those of transition metal complexes) on on-top sites and dissociation (specifically of CO) on in-hold sites and further in principle suppressed reactions on on-plane sites.thsi concept is developed and illustrated here iwth the results of several investigations such as tracing of activity and selectivity during the initial episodes of synthesis,experiments with added (~14C-labeled) olefins and variation of synthesi sparameters to see their specific influences.as minor reactions of coordination chemistyr on on-top sites,reversible CH_2 cleavage from alkyl chains,CO insertion and ethene insertion are visualized.On on-plne sites CO methanation,olefin hydrogenation and olefin double bond shift are noticed,but much inhibited.As compared to Fischer-Tropsch on iron catalysts,the common Fischer-Tropsch principle appears to be the inhibition of chain desorption to allow for growth reactions of the adsorbed chains.Minor reactiosn and detailed kinetics on rion and cobalt catalysts differ bisically.