Design And Preparation Of Highly Stable Cu-Based Catalysts And Study On Reaction Performance Of CO₂ Hydrogenation To Methanol

Cu-based catalysts have been widely utilized in a series of industrially reactions owing to their perfect catalytic performance and inexpensive production cost.Esepcially,a variety of research has been done in methanol synthesis process from CO₂ hydrogenation over Cu-based catalysts.However,Cu-based catalysts are prone to grow into larger particles owing to their intric property during the reaction process.Up to now,several studies have been done to investigate the substantial reason for Cu particle growth and the result illustrated that the growth of copper particle on catalysts could be attributed to two main mechanisms.One mechanism is the migration of entire particles across the support followed by their coalescence.And the other mechanism is Ostwald ripening effect.However,both of these mechanisms would lead to the decrease of active sites,which would cause catalyst rapid deactivation.Herein,developing a Cu-based catalyst to retard metal particle growth was significant on both basic research and industrial production.Hence,in order to restrain Ostwald ripening effect,a modified deposition-precipitation method was proposed to prepare homo-dispersed CuZnO/SiO₂catalyst.Besides,a series of SiO₂ supported Cu-based catalysts with different metal support interaction were designed to investigate the effect of strong metal support interaction on catalytic performance.The main conclusions of this thesis are as follows:?1?The homo-dispersed CuZnO/SiO₂ catalyst was prepared by rotary evaporation assisted deposition-precipitation method.In order to clarify the impact of heterogeneities on the catalytic stability,other modified deposition-precipitation methods were applied to prepare other catalysts with different particle size distribution.From the results of the catalytic reaction evaluation,homo-dispersed CuZnO/SiO₂ catalyst displayed better catalytic stability.The XRD,TEM and XPS et al.characterizations indicated that the existence of bulk CuO on catalysts significantly impacted catalytic stability meanwhile broader particles size distribution on catalyst led to a higher deactivation rate.Herein,it was proposed that Ostwald ripening could be restrained by the homo-dispersed Cu particles on CuZnO/SiO₂ catalyst and therefore the catalyst prepared by rotary evaporation assisted deposition-precipitation method displayed better stability.?2?The CuZnO/SiO₂ catalysts with different metal-support interaction?Cu/ZnO?were prepared by the conventional precipitation method where the precipitation sequences were altered and they were denoted as Nor-CZS,Co-CZS,Re-CZS respectively.All the catalysts were tested under the same condition to clarify the effect of different metal-support interaction on their catalytic performance.The evaluation results suggested that both of the CO₂ conversion stability and methanol yield stability was promoted by the improvement of metal-support interaction?Cu-ZnO?.Hence the Re-CZS catalyst exhibited the best catalytic stability due to its highest metal-support interaction.However,it was observed that the higher metal-support between Cu and ZnO might resulted in lower methanol-selectivity and therefore the Re-CZS catalyst exhibited the poorest methanol selectivity at the initial reaction stage.The FT-IR and XRD results suggested that altering the precipitation sequense showed negligible influence on the existing state of Cu species in catalysts?in the form of CuO?,however,the TPR,XPS et al.characterization results proved that the precipitation sequences could dramatically alter the metal-support interaction?Cu-ZnO?.As for Re-CZS catalyst,partial ZnO leached from bulk phase and migrated to the surface due to the strong metal-support interaction.As a result,the methnol selectivity over Re-CZS catalyst increased with time on stream.Meanwhile,it was found that the Ostwald ripening effect played a crucial role in particle growth at the intial reaction stage and hence the Nor-CZS and Co-CZS exhibited similar deactivation rate though the latter possessed higher metal-support interaction.