Cu-Plate ZnO Model Catalyst For CO₂ Hydrogenation To Methanol:the Studies Of Active Site

In the recent years,climate change and ocean acidification resulting from the emission of greenhouse gases have received widespread attention in the environmental and energy fields.CO₂ as the main greenhouse gas,researchers have carried out extensive research on the conversion and utilization of CO₂ to alleviate the greenhouse gas emissions.Since Methanol is an important bulk chemical in the chemical industry and also can be used directly as alternative energy or considered as a potential storage medium for renewable energy in the energy field.Thus,CO₂ hydrogenation to methanol has attracted wide attention.Cu/Zn O-based catalysts are the most extensively investigated in CO₂ hydrogenation to methanol.Despite the intensive research,the active site and reaction mechanism of Cu/Zn O catalyst remain debated.In addition,the RWGS reaction accompanying the methanol synthesis process can reduce methanol selectivity and the generated water also accelerate catalyst sintering.Therefore,the exploring and understanding the active site of methanol synthesis is very important to obtain stable and high methanol yield catalyst.A lot of research indicates that Zn Ox species migrated from bulk Zn O support to Cu nanoparticules?NPs?during activation in H₂ can generate Zn Ox-Cu NP-Zn O interface which is critical to methanol formation,probably the critical active site for CO₂ hydrogenation to methanol.In this thesis,Cu supported on plate Zn O?Cu/plate Zn O?model catalyst and Cu doped plate Zn O?plate Zn O:Cu?model catalyst were prepared to simulate both Zn Ox-Cu NP-Zn O interface and direct contact Cu-Zn O interface.The structure of model catalysts was characterized,and the catalytic performance of the model catalysts for RWGS reaction and CO₂ hydrogenation to methanol was evaluated.The structure-activity relationship between Cu-Zn O interface structure and catalytic performance,and CO₂ hydrogenation reaction route were analyzed,and then the active site and reaction mechanism of Cu/Zn O-based catalyst for CO₂ hydrogenation to methanol were further revealed.1)A series of doped Zn O:XCu catalysts and 1Cu/Zn O catalysts were prepared for RWGS reaction,the RWGS reaction performance over the direct contact Cu-Zn O interface and the Zn Ox-Cu NP-Zn O interface was revealed.EPR and Raman results indicate that Cu²⁺replaces lattice Zn²⁺sites and interstitial Zn sites?Zn_i?,which brings more intrinsic defects.Cu doping adjustes the basicity of the catalyst,the CO formation rate is related to the amount of CO₂ desorption from the medium strong basic sites.The catalytic performance reveales that direct contact Cu-Zn O interface displays inferior RWGS reaction reactivity at reaction temperature lower than 500?compared with Zn Ox-Cu NP-Zn O interface;however,it is more stable at reaction temperature higher than 500?,Cu NPs of Zn Ox-Cu NP-Zn O interface may sinter,then,Zn O:1Cu has superior catalytic performance than that of 1Cu/Zn O.2)Based on the above investigation,plate Zn O:1Cu model catalysts calcined in N₂ and dry air and 1Cu/plate Zn O model catalyst calcined in dry air were constructed,denoted as Zn O:1Cu-N₂,Zn O:1Cu-air and 1Cu/Zn O-Air,respectively,and evaluated for the hydrogenation of CO₂ to methanol.Under the reaction conditions P=3 MPa,H₂:CO₂:N₂=selectivity followed by Zn O:1Cu-Air,and Zn O:1Cu-N₂ had the worst catalytic performance.SEM,XRD and in-situ XPS results showed that no Cu NPs over Zn O:1Cu-N₂ model catalyst were found after reduction,therefore only direct contact Cu-Zn O interface was formed;however,doped Cu over Zn O:1Cu samples segregated to form Cu NPs during calcination in air and followed reduction.In-situ XPS,FTIR and Raman studies confirmed the migration of Zn species of 1Cu/Zn O-Air led to the formation of Zn Ox-Cu NP-Zn O interface which has more oxygen defects and exhibits higher CO₂ adsorption and activation ability,as confirmed by CO₂-TPD.CO/H₂ and CO₂/H₂ performance evaluations suggested that the main carbon source of methanol synthesis is CO₂ over direct contact Cu-Zn O interface and Zn Ox-Cu NP-Zn O interface.But the produced CO over Zn Ox-Cu NP-Zn O interface was easier to be further hydrogenated to methanol,leading to higher methanol selectivity and yield.Moreover,as the pressure increases,the methanol synthesis through RWGS+CO hydrogenation route over Zn Ox-Cu NP-Zn O interface is suppressed.The above research shows that the selectivity of Cu/Zn O-based catalysts to catalyze the hydrogenation of CO₂ to methanol is low.On the one hand,the CO produced over the direct contact Cu-Zn O interface is difficult to be further hydrogenated to methanol;On the other hand,Zn Ox-Cu NP-Zn O interface has high RWGS catalytic performance,but the generated CO can not be further converted to methanol entirely.This means that to further improve the selectivity of Cu/Zn O-based catalysts to catalyze CO₂hydrogenation to methanol,it is necessary to understand the active site structure of methanol and CO from a more microscopic point of view,which puts forward a higher requirment for the research,design and preparation of efficient catalyst structures.