Thermochemical Reactions of Simple Alcohols and Aldehydes on Co(0001)

Author:Chen Jun

Supervisor:chen mao zuo yang xue ming


Degree Year:2019





The transition metal cobalt(Co)has been widely used in the Fischer-Tropsch synthesis(FTS)that converts a mixture of CO and H2 gases into liquid transportation fuels.In order to make a better FTS catalyst and control the reaction rate and selectivity,it is of great importance to understand the adsorption and reaction mechanisms of molecules related to FTS on Co surfaces.While,temperature programmed desorption(TPD)method is useful in studying the adsorption and reaction of molecules on surfaces.Using TPD method we have studied the adsorption and decomposition of several molecules(methanol(CH3OH),ethanol(C2H5OH)and acetaldehyde(CH3CHO)),which are the products of FTS,on the Co(0001)surface in order to gain further understanding of FTS on Co surfaces.The dissociation of CO on the surfaces of catalysts is the important initial step of FTS.It has been proposed that the dissociation of CO on the Co(0001)surface follows the CO insertion mechanism in which CH3CHO is the important intermediate.Previous study of C2H5OH decomposition on Co(0001)has supported the positive effect of CH3 group on the C end of CO moiety in C-O bond cleavage,whereas,the effect of H atoms remains unclear.We have investigated the decomposition of C2H5OH and CH3CHO on the Co(0001)surface using TPD method.Our results show that the C-O bond scission requires both CH:3 group and H atom attached to the C atom of CO moiety.Otherwise,the C-C bond breaking is more feasible if only CH3 is attached to the C end.Furthermore,we find the surface adsorbed H atoms have significant effect in promoting the C-O bond scission in the decomposition of CH3CHO by inhibiting the dehydrogenation of CH3CHO.In order to gain further knowledge of the effect of CO on the reaction of CH3CHO on Co(0001)surface,we investigated the adsorption and reaction of CH3CHO on the clean and CO pre-covered Co(0001)surfaces.We found that the chemisorbed CO molecules inhibit the decomposition of CH3CHO,which is disadvantageous to the CO insertion mechanism of FTS.Interestingly,we also found that the CO molecules adsorbed on the bridge/hollow sites of Co(0001)surface initiate the polymerization of CH3CHO into linear chain polyacetaldehyde effectively,however,the polymerization of CH3CHO on the clean Co(0001)surface is inef ficient and the product is paraldehyde.At the last,we investigated the H2 production from the decomposition of CH3OH on Co(0001)surface,which is one of reverse processes of FTS and plays vital roles in industrial H2 production and H2 fuel cell.The influence of various oxide states on the CO(0001)surface to H2 production efficiency has also been studied.The chemisorbed O atoms on the Co(0001)surface were found to promote and tune the rate of H2 production from CH3OH decomposition,while the oxides of Co(0001)reduce the selectivity to H2,which should be avoid when the Co(0001)surface is used to catalyze the production of H2.