Study On The Active Site Structures Of Cu/ZnO Plate Model Catalyst For Co₂ Hydrogenation To Methanol

Since Cu/ZnO/Al₂O₃ catalyst is a typical industrial catalyst for methanol synthesis from syngas,CO₂ hydrogenation to methanol based on Cu/ZnO has been investigated extensively,including the construction of highly efficient catalysts and the study of active site structures.The Cu/ZnO catalyst exhibits high catalytic activity due to the synergistic effect of Cu and ZnO.However,there is still controversy about the active site structure of Cu/ZnO-based catalysts for CO₂ hydrogenation to methanol.In this paper,the regularly structured and simple Cu/ZnO plate model catalyst?CZ?was prepared to simplify the structure of Cu/ZnO catalyst.The catalytic performance was evaluated under realistic reaction conditions.The effects of Cu loading amount on the catalytic performance of CZ catalysts were studied preliminarily.The effects of Cu nanoparticle?NP?morphology and the chemical states of Cu,Zn species,induced by different reducing atmosphere,on the catalytic performance of CZ catalyst with 1wt%Cu?1CZ?were investigated.The active site structures of the catalyst were revealed based on these studies.1)ZnO plate was synthesized by hydrothermal method,and a series of CZ catalysts with varying Cu loading amount were prepared by simple impregnation method.The catalysts were evaluated for CO₂ hydrogenation to methanol at P=3 MPa,temperature T=260³00^oC and GHSV=3600 mL·g^-1·h^-1.The microstructure of catalysts can be modulated by increasing the Cu loading,leading to different performance for CO₂ hydrogenation to methanol.Among them,CZ catalyst with 5 wt%Cu exhibited the highest CO₂ conversion?9.8%?and CH₃OH yield?4.3%?.Three different active site structures,namely Cu-ZnO in direct contact interface,the ZnO_x-Cu interface caused by Zn migration and the ZnO_x-Cu interface formed by ZnO_x migration to separated Cu NPs,have been proposed according to TOF_CO2,TOF_CH3OHH3OH and SEM ananlysis.Based on further CO₂-TPD characterization,it is speculated that Cu NPs covered by migrated ZnO_x play an important role in the formation of CH₃OH.2)1CZ catalyst was reduced in different atmosphere at 300 ^oC,then evaluated for CO₂hydrogenation to methanol.The results indicate that in situ tuning of Cu NP morphology can be realized through changing the reducing atmosphere.5%H₂/Ar reduction induced the formation of small irregular spherical Cu NPs covered with a large amount of migrated Zn species,promoting CO₂ conversion and methanol synthesis.However,5%CO/Ar reduction induced the generation of ellipsoidal Cu NPs with medium size and a low amount of migrated Zn species,leading to poor catalytic performance for CO₂ hydrogenation.CO₂ hydrogenation to methanol displays an apparent Cu NP morphology dependency over the Cu-ZnO based catalysts.3)1CZ catalyst was reduced in different atmosphere at 400 ^oC to modulate the chemical states of key species,which was evaluated for both RWGS reaction and CO₂ hydrogenation to methanol reaction.The desorption peak?₁ in CO₂-TPD can be attributed to the adsorption on Cu-ZnO interface in direct contact,and the corresponding adsorbed CO₂ species is beneficial to the formation of CO and methanol.The desorption peak?₂ can be attributed to the adsorption of CO₂ on the ZnO_x-Cu interface,and the corresponding adsorbed CO₂ species only favors the formation of methanol.On the one hand,the chemical states of Cu and Zn were changed by the high temperature reduction while ZnO was partially reduced forming ZnO_x and even Cu₃Zn alloy,and the ZnO_x-Cu interface formed by the migration of Zn species to Cu surface weakened CO₂ adsorption and activation,resulting in a lower methanol yield.On the other hand,high-temperature reduction generated more defects on the surface of the catalyst,which enhances CO₂ adsorption and activation by Cu-ZnO interface in direct contact,and results different activities in RWGS reaction after reduction in different atmosphere.