The Effect Of Active Composition Micro-scale And Reaction Conditions About Cu Catalysts On Reaction Of Ethanol Synthesis From Syngas

Syngas conversion provides an attractive route to produce ethanol in industry. Rh is a catalyst for its higher selectivity on the production of ethanol,however, its cost is too high so that its application is restricted. Currently, the cheap, easily available Cu-based catalysts have been widely used in one-carbon reaction, wherein, the catalytic performances of Cu-based catalysts are closely related to their corresponding microstructure.Due to the unexpected catalytic properties compared to bulk materials, Cu cluster can exhibit remarkable catalytic activity of chemical reactions. The properties of catalyst depend on its strcture, thus, it is effective to improve the productivity and slelectivity of ethanol formation by changing the particle size of active component.In this work, the effect of particle size and reaction conditions about Cu catalysts on reaction of ethanol synthesis from syngas has been systematically investigated using the density functional theory calculations and ab initio thermodynamics method. Firstly, the adsorption and activation of CO and H2 atdifferent coverage over Cu(100) surface and the corresponding equilibrium phase diagrams under different temperatures and partial pressures have been systematically investigated under the gas phase and liquid paraffin conditions,respectively; then, the major species of CHx(x=1~3) intermediates and the formation of C2 oxygenates have been systematically investigated to explore the effect of copper particle size on the catalytic activity, and further to facilitate the design of improved catalysts for syngas conversion. The main conclusions are as follows:(1) CO dominantly presents the molecular adsorption state on Cu catalyst in different reaction conditions, including solvent, temperature and pressure:a) The solvent effect not only improve the stability of CO adsorption at low coverage, but also can weaken the lateral repulsive interactions between CO molecules at high coverage, which stabilize CO adsorption configurations.b) The stepwise adsorption energies of CO decrease with the increasing of CO coverage from 1/12 ML to the saturated adsorption 8/12 ML in gas phase and liquid paraffin, but the decrease trend of adsorption energies has difference due to the reaction conditions.c) Under the ultrahigh vacuum(UHV) conditions, the molecular adsorption CO starts to desorb in a narrow temperature range of 700~800 K in gas phase;however, in liquid paraffin, the molecular adsorption CO starts to desorb in a broad temperature range of 680~1130 K.(2) For the adsorption of H2 on Cu catalysts with different reaction conditions, H2 dominantly exists in the form of dissociated H atoms rather than H2 molecule.a) Owing to solvent effect, more H2 molecules in liquid paraffin become the dissociative adsorption, and H atoms in liquid paraffin are more stable.b) With the increasing of pressure, H2 dissociative adsorption becomes difficult. H2 is the dissociative adsorption when the coverage is less than or equal to 3/12 ML in gas phase and 6/12 ML in liquid paraffin.c) Under the ultrahigh vacuum(UHV) conditions, the adsorbed H starts to desorb in a narrow temperature range of 300~400 K in gas phase, namely, H2 molecules can spontaneously dissociate into H atoms in a narrow temperature range of 300~400 K in gas phase; however, in liquid paraffin, more dissociative adsorption of H2 molecules regions exist, and H2 molecules start to dissociate into H atoms in a broad temperature range of 200~500 K.(3) The different size of Cu clusters has been used to represent the effect of particle size about Cu catalysts on the reaction of ethanol synthesis from syngas.The major species of CHx(x=1~3) intermediates and the formation of C2 oxygenates from syngas on different particle size of Cu clusters have been obtained. Meanwhile, the mechanisms of ethanol formation from C2 oxygenate on different particle size of Cu clusters are also obtained.a) The formation pathway of CHx(x=1~3) changes with the particle size ofCu clusters. For CH formation, CH is dominantly formed by the direct C-O bond scission of CHO on both Cu13 and Cu55 clusters; however, the pathway of CO+2H→CHO+H→CHOH→CH+OH is mainly responsible for CH formation on Cu38 cluster. For CH2 formation, CH2 prefers to be produced by the C–O bond cleavage of CH2 O on Cu13 cluster, whereas, with respect to CO+H species,the pathway of CO+3H→CHO+2H→CH2O+H→CH2OH→CH2+OH is the most favorable for CH2 formation on both Cu38 and Cu55 clusters. However, for CH3 formation, the pathway of CO+3H→CHO+2H→CH2O+H→CH3O→CH3+O is dominantly responsible for CH3 formation on all Cu clusters.b) The major species of CHx(x=1~3) intermediates are also different on Cu clusters: all CHx(x=1~3) species are abundant on Cu13 cluster; however, on Cu38 cluster, only CH2 is the major CHx species; CH2 and CH3 species are the main existence form of CHx on Cu55 clusters; wherein, CH2 species is always the major species of CHx on Cu clusters.c) Interestingly, the formation mechanism of C2 oxygenates on different paticle size of Cu clusters are the same way, that is, CHx prefers to form CHxCO by CO insertion, CHx(x=1~3)+CO→CHxCO. Considering the effect of other reactions related to CHx, we can conclude that CHCO and CH3 CO are the main C2 oxygenates on Cu13 cluster, however, the major species of C2 oxygenates on Cu38 or Cu55 clusters are CH2 CO or CH3 COspecies, respectively.d) The mechanisms of ethanol formation from C2 oxygenate on differentparticle size of Cu clusters are also obtained. Ethanol formation goes through the process of CHCO+5H→CHCHO+4H→CH2CHO+3H→CH3CHO+2H→CH3CH2O+H→CH3CH2OH or CH3CO+3H→CH3CHO+2H→CH3CH2O+H→CH3CH2OH on Cu13 cluster; CH3CH2 OH formation on Cu38 cluster goes through the process of CH2CO+4H→CH2CHO+3H→CH3CHO+2H→CH3CH2O+H→CH3CH2OH; whereas, on Cu55 cluster, CH3 CO successive hydrogenate to produce CH3CH2 OH goes through the pathway of CH3CO+3H→CH3COH+2H→CH3CHOH+H→CH3CH2OH.(4) The effect of copper particle size on the catalytic selectivity has been elucidated. First, the selectivity of CHx(x=1~3) species is decreased with the increasing of the copper particle size; as a result, methanol(CH3OH) becomes the main product from syngas. Then, the effect of methane(CH4) on the formation of C2 oxygenates increase with the copper particle size, namely, the selectivity of C2 oxygenates is reduced by CH4 formation, which can further affect the production of ethanol.(5) The basic researches about the effect of particle size and reaction conditions about Cu catalysts on the reaction of ethanol synthesis from syngas have been carried out by using the density functional theory calculations and ab initio thermodynamics method, the results can prove the effects of Cu particle size and reaction conditions on the catalytic performance, which will reveal the physical and chemical essence for the highly efficient catalytic reaction system.Moreover, it can also give a theoretical guidance and new clues for the catalyst modification and preparation.