The Adsorption And Decomposition Of Water And Ammonia Molecules On FCC Cobalt Surfaces:A Theoretical Study

The production of ultra-clean transportation fuels from biomass through the Fischer–Tropsch synthesis(FTS)reaction,has attracted great attention in recent years.The cobalt-based catalyst has been used as a common catalyst in FTS,because of its high activity,low water-gas shift activity and high selectivity to linear hydrocarbons.Presently,available studies have demonstrated that the byproduct water in FTS reaction and the impurity gases in the syngas(e.g.NH3)can deactivate the cobalt catalyst,while the mechanism of catalyst deactivation remains to be explored so far.Herein,we have studied the adsorption and dissociation of water and ammonia molecules on cobalt catalyst surfaces,by means of first-principles calculations,to reveal the mechanism of cobalt catalyst deactivation.The results are concluded as following:Firstly,water molecule weakly binds to the surfaces and it is feasible to desorption from the clean surfaces.The presence of oxygen atom on Co surfaces has a prominent promotion effect on the dissociation of water into hydroxyl,especially Co(110)is identified as the most active one.The analysis of microscopic decomposition behaviors demonstrates that molecular water adsorbs dissociatively with the aid of pre-adsorbed oxygen atom,forming OH radicals chemisorbed on the considered surfaces.Distinctively,the presence of hydroxyl hinders the dissociation of water molecule.The results demonstrate that hydroxyl produced by the dissociation of water molecules may be the cause of cobalt catalyst deactivation.This study is also elaborates the general fact that the dissociation of water molecule is structure-sensitive on metals.Secondly,the interaction of NH_x(x = 0-3)species with the Co surfaces becomes stronger with its further dehydrogenation,and the existence of oxygen atom shows different influences on NH_x(x = 3-1)dissociation on the three surfaces.Specifically,pre-adsorbed O atom significantly promotes the stepwise dehydrogenation of ammonia on Co(110)surface,probably giving rise to N atom strongly binding with the surface.Comparatively,the NH dissociation appears to be the rate-determining step on O-covered Co(111)and Co(100)surfaces,due to the highest energy barrier.Present results demonstrate that the intermediates of N and NH produced in ammonia dehydrogenation are likely responsible for cobalt catalyst deactivation in the excess of oxygen atom.