The Effects Of Crystl Phase And Facet Of Rh Catalyst On The Ethanol Formation From Syngas

Rh-based catalysts have been widely inveatigated owing to the high ethanol selectivity for the ethanol synthesis from syngas,and the crystal phases and crystal facets structure of the catalyst is critical for the catalytic performance in heterogeneous catalysis.Up to now,the experimental and theoretical studies about the Rh-based catalyst exclusively focus on the FCC crystal phase;whereas there are scarce researches on the ethanol formation from syngas on HCP Rh crystal phase,and the catalytic performance of HCP Rh for the ethanol synthesis from syngas is still unclear,further,the effects of Rh crystal phase on the catalytic performance of ethanol formation also remains uncertain without the comparisons between FCC and HCP Rh crystal phase.More importantly,with the altering of preparation methods,reaction conditions and catalyst size,the phase transition between HCP and FCC may occur.Thus,based on the only research about ethanol synthesis from syngas on FCC Rh catalyst,the key issues in the syngas conversion to ethanol cannot be clarified:Why Rh has to be the base material for ethanol formation from syngas?What is the uniqueness of FCC Rh catalyst for ethanol synthesis from syngas?What is the selectivity-controlled and activity-controlled step of ethanol formation from syngas over the Rh catalysts?Thus,it is still significant to clear the effect of crystal phase and facet of Rh catalys on the ethanol synthesis from syngas.In this paper,density functional theory calculation was applied to investigate the mechanism of ethanol formation from syngas over the different HCP and FCC Rh facets（HCP Rh（0001）,（10-10）,（10-12）and FCC Rh（100）and（111））,illuminating the microscosmic nature of the FCC Rh catalyst has been widely applied to the ethanol formation from syngas,clarifying the crystal phase and facet structure sensitivity of Rh catalyst for ethanol synthesis from syngas;obtaining the key steps that controlling the activity and selectivity of ethanol formation on Rh catalyst,and provide the theoretical clues about the precise adjustment of catalytic structure and effective regulation of catalytic performance in the preparation of Rh base catalyst.The main conclusions can be obtained as follows:1.The effect of crystal phase and facets structure of Rh catalyst on the formation of favorable CH_x（x=1-3）monomer was revealed:（1）CO preferS to be hydrogenation to CHO instead of direct dissociation to form CH_x species on both HCP and FCC Rh catalyst;（2）CH is the favorable CH_x monomer irrespective of HCP or FCC Rh facets,however,the preferential pathway for CH formation is dependent on the various facet structures:the dissociation of CHOH and CHO contribute to CH formation over the（0001）and（10-10）facets,the dissociation of CHO is the favorable pathway for CH formation over（10-12）and（100）facets,and the dissociation of CHOH is the major pathway for CH formation on（111）facet;（3）the CH formation rates on different facets are calculated using microkinetic modeling,suggesting that FCC Rh catalyst not only exhibite higher intrinsic activity than HCP Rh catalyst for CH formation,but also expose more active factes;（4）The CH formation is more favorable than methanol formation on the 62.36%HCP and 81.78%FCC Rh facets,therefore,Rh catalyst can provide sufficient CH species to participate in the reactions with CHO/CO and contribute to the formation of C₂ oxygenations and ethanol.In general,considering the catalytic activity for CH formation and the exposure proportion of active facet,compared with HCP Rh,FCC Rh exhibit higher catalytic activity and selectivity for CH formation.2.Starting from the CH monomer,the influence of structures of Rh catalysts on the formation mechanism and relative selectivity and activity of ethanol was clarified:（1）the related reactions of ethanol formation on the（10-10）facets mainly occur at the step site,and the favorable route for ethanol formation is CH+CHO+H→CH₂+CHO+H→CH₃+CHO+H→CH₃CHO+H→CH₃CH₂O+H→C₂H₅OH,while the favorable route for ethanol formation on the stepped（10-12）and flatted（0001）,（100）and（111）facets is CH+CHO+H→CHCHO+H→CH₂CHO+H→CH₃CHO+H→CH₃CH₂O+H→C₂H₅OH;（2）the formation rate of ethanol is calculated,the results indicate that the formation rate of CH species is correlated with that of ethanol,namely,CH formation is the activity-controlled step of ethanol formation on Rh catalyst.Meanwhile,compared to HCP Rh,FCC Rh presents the higher intrinsic activity for the ethanol formation and the exposure of much denser active facets;（3）combining with microkinetic modeling,the relationship between the selectivity of ethanol and the structure of Rh facets was clarified:（10-12）facet（covering 37.74%of HCP crystal phase）has the highest selectivity of ethanol（96.5%）;secondly,the selectivity of ethanol on the（111）facet（covering 66.95%of FCC crystal phase）is 10.5%;while the selectivity of ethanol is lower and C₂ hydrocarbons is the dominant products on the other facets;（4）the results of microkinetic modeling revealed that the selectivity-controlled step of ethanol on Rh catalyst is the coupling of CH_x to form C₂ hydrocarbons,therefore,it is possible to improve the selectivity of ethanol by changing the activation energy of CHO insertion and/or CH_x coupling to boost the formation of ethanol and/or suppress the formation of C₂ hydrocarbons with the addition of promoter and supporter.3.It is clarified that the microscopic nature of Rh catalyst can be widely applied to the ethanol formation from syngas:compared to other metal catalysts（Cu,Co,Ni,Ru）,the moderate CH formation activity on Rh catalyst results in a large amount of undissociated CHO exist on the Rh facets and facilitates CHO undergoes the formation of C-C bond;meanwhile,it can provide sufficient CH species to promote the formation of C₂ oxygenates and ethanol,and improve the selectivity of ethanol;additionally,combined with the results of microkinetic modeling,we proposed that the selectivity of ethanol could be improved when the CHO concentration on the catalyst surface was slightly higher than CH,whereas the higher or lower CH formation activity would affect the coverage of CH or CHO on the surface and promote the formation of hydrocarbons or methanol,respectively.