The Effect Of Structure Modulation About Rh-based Catalysts On Catalytic Reaction Of Ethanol Synthesis From Syngas

Ethanol as a high quality and clean energy has a good applicationforeground. Ethanol formation from syngas using Rh-based catalysts was theresearch center and development direction at present. It is necessary to clarifythe route, to make clear the key step during the reaction process, to obtain thereaction mechanism with the help of promoter and/or support and determinetheir roles during the ethanol synthesis. Exploring the new method based onethanol synthesis from gyngas is the problem to be solved in this field.In this study, ethanol synthesis from syngas is chosen as the target reactionto investigate the mechanism of ethanol synthesis over the single metal Rh(211)surface and Rh/TiO2catalyst with the help of TiO2support by using densityfunctional theory (DFT) method. By comparing, we can obtain the effects ofsurface structure and the support on the catalytic performance of ethanolsynthesis from syngas; Moreover, the effect of the second metal promoter inRh(211) and Rh/TiO2catalysts on the key step of ethanol synthesis have beeninvestigated. Lastly, considering the effect of CH4formation on the productivityand selectivity of ethanol synthesis from syngas as well as the cost of Rh-based catalyst, ethanol systhesis from methane-syngas via two-step reaction has beenconsidered on the quantitative Rh atoms doped into the cheap Cu catalystsurface in order to obtain the effect of Rh consumption on the catalyticperformance and the optimal Rh/Cu ratio over Rh-doped Cu catalyst.Conclusions in this work are as follows:(1) For the effect of catalyst surface structure on the catalytic performance,we have investigated the mechanism of ethanol synthesis from synthesis on thestepped Rh(211), and compared ethanol synthesis mechanisms betweenthestepped Rh(211) surface and flat Rh(111) surface. The mechanism on Rh(211)surface is: CH3formation via the route CO+3Hâ†'CHO+2Hâ†'CH2O+Hâ†'CH3+O, CHO insertion into CH3via the route CH3+CHOâ†'CH3CHOâ†'CH3CH2Oâ†'C2H5OH leads to ethanol. During this process, CH2O can behydrogenated via CH3O intermediate to CH3OH, however, CH3formation ismore easier than CH3OH formation; Meanwhile, CH3hydrogenation to CH4iscompetitive with the formation of C2oxygenates via CHO insertion into CH3.Compared to the flat Rh(111) surface, the stepped Rh(211) surface not onlychange the formation route of CH3and C2oxygenates, but also decrease thereaction barriers, suggesting that the stepped Rh(211) surface can exhibit thehigh activity and selectivity to ethanol synthesis from syngas. However, CH4formation is competitive with C2oxygenates formation, which decreases theselectivity and productivity of ethanol.(2) The second metal Mn doped MnRh(211) surface can decrease the activation barrier of CH4and C2oxygenates formations, which make C2oxygenates formation more favorable than CH4both kinetically andthermodynamically. Thus, metal promoter Mn can increase the activity andselectivity of C2oxygenates formation, further improves the selectivity ofethanol synthesis.(3) In order to clarify the role of support, we investigated the mechanism ofethanol synthesis on the supported Rh/TiO2catalyst, which is compared withthose on the flat Rh(111) and stepped Rh(211) surfaces. The mechanism onRh/TiO2catalyst is that: CO+3Hâ†'CHO+2Hâ†'CH2O+Hâ†'CH3Oâ†'CH3+O canform CH3, then, CH3via the route CH3+CHOâ†'CH3CHOâ†'CH3CH2Oâ†'C2H5OH leads to ethanol, During this process, CH3O can be hydrogenated toCH3OH, and CH3formation is easier than CH3OH formation; Meanwhile, CH3hydrogenation to CH4is competitive with C2oxygenates formation.Compared with the flat Rh(111) and stepped Rh(211) surfaces, Rh/TiO2catalyst can increase the catalytic activity of ethanol synthesis from syngas.Especially, TiO2support can promote CHO formation and increase the stabilityof CHO over the catalyst, which plays a vital role to improve the activity andselectivity of ethanol synthesis. However, CH4formation is still the controlfactor for the selectivity of C2oxygenates and ethanol on Rh/TiO2catalyst;Further, the second metal promoter Fe doped FeRh/TiO2catalyst can increasethe activation barrier of CH4formation, and suppress CH4formation, as a result,FeRh/TiO2catalyst can improve the selectivity of ethanol synthesis. (4) By investigating ethanol synthesis from methane-syngas via two-stepreaction on the Rh doped Cu-based catalyst surface, the moderate Rh doped intoCu catalyst surface can synthesize ethanol with the high activity and selectivity,in which the optimal Rh/Cu atom ratio is1/2. ethanol synthesis mechanism frommethane-syngas via two-step reaction is that: firstly, methane directlydissociation leads to CH3, then, CO insertion into CH3leads to CH3CO, furtherhydrogenation to ethanol, the corresponding route is that CH3+COâ†'CH3COâ†'CH3COHâ†'CH3CHOHâ†'C2H5OH.(5) The effect of catalyst structure on the catalytic performance of ethanolsynthesis from syngas on Rh-and Cu-based catalysts shows that the catalyticreaction mechanism is the foundation to elucidate the essential relationshipbetween the structure modulation of catalyst and the regulation of catalyticperformances. Only when the reaction mechanism of the catalytic reaction isillustrated clearly, the key step for catalytic reaction can be obtained. Thus, itcan be determined that which step is the key step of controlling reaction toachieve the regulation of catalytic performances, then, the structure modulationcan been further carried out. As a result, the accurate information between thestructure modulation and the regulation of catalytic performance can beemployed to provide the theoretical guidance for the screen and design of thehighly efficient catalysts.