Studies On Active Sites Of Cu Based Catalysts For Methanol Synthesis From Syngas

Methanol,as an important basic industrial chemicals and clean renewable energy carrier,is with a worldwide demand over 100 million tons per year.Currently,the dominating catalyst for methanol synthesis from syngas is Cu/Zn O/Al₂O₃ which shows high activity and selectivity.Moreover,the Cu/Ce O₂ catalyst with Ce O₂ as the support displays a promising prospect in methanol synthesis at lower temperature.Despite great efforts have been carried out on methanol synthesis from syngas,the understanding of the reaction mechanism and active sites for methanol synthesis are still in controversy,due to the complexity of catalyst structures and methanol synthesis reaction.This thesis devotes to reveal the reaction mechanism for methanol synthesis and the structure of active sites,so as to provide a theoretical basis for the design and development of methanol synthesis catalysts with high performance.The structures of active sites of Cu/Zn O/Al₂O₃ and Cu/Ce O₂ catalysts and mechanisms for methanol synthesis were investigated under the real reaction conditions using kinetics,chemical titration and a series of in-situ techniques（in situ X-ray diffraction（in-situ XRD）,in-situ diffuse reflectance infrared fourier transform spectroscopy（in-situ DRIFTS）,in-situ X-ray photoelectron spectroscopy（in-situ NAP-XPS）and in-situ transmission electron microscopy（in-situ TEM））,and the dynamic evolution of catalyst structures in response to the reaction atmosphere was revealed.The main results are as follows:1.The composition of syngas has a significant effect on the structure and performance of Cu/Zn O/Al₂O₃ catalyst.The introduction of CO₂ into CO-H₂ mixture increases the rates of methanol formation by nearly an order of magnitude.On the one hand,the CO₂ changes the hydrogenation reaction pathway of methanol synthesis from syngas,and promotes the WGS reaction approaches to equilibrium.On the other hand,the CO₂ changes the oxygen chemical potential in syngas thus drives the Cu/Zn O_xclusters to undergo atomistic rearrangement,allowing the maximum formation of Cu⁰-Zn²⁺active sites.These dynamic rearrangements of Cu/Zn O_x clusters in response to the changes in the composition of syngas are reversible.2.A series of Cu/Zn O/Al₂O₃-Ce O₂ catalysts with different Ce O₂ contents were prepared,and the effects of Ce O₂ on the structure and activity of Cu/Zn O/Al₂O₃catalysts were studied systemically.With an increase in Ce O₂ content（0.0-5.9 wt%）,the rates of CH₃OH formation increase first and then decrease,and reach the maximum at 3.0 wt%Ce O₂.When the content of Ce O₂ is less than 3.0 wt%,the Ce O₂ can be completely reduced to Ce³⁺during methanol synthesis.Based on thermodynamics analysis,the reduced Ce³⁺can react with Zn O to form metallic Zn at 493K(Ce₂O₃+Zn O=2Ce O₂+Zn,（35）G_{493 K}<0).The increase in number of metallic Zn is conducive to the dynamic atomic rearrangement of the catalyst surface which results in the well dispersion of Zn atoms on metallic Cu surface,and enriching the Cu-Zn O interface sites.When the content of Ce O₂ exceeds 3.0 wt%,unreduced Ce O₂ on surface is unfavourable for the reduction of Zn O.3.The effects of Ce O₂ content and contact extent of Cu and Ce O₂ clusters on the structure and activity of Cu/Ce O₂ catalysts were investigated,and the active sites for CO hydrogenation to CH₃OH was demonstrated.The Cu components in surface region of Cu/Ce O₂ catalysts are present in the form of metallic state,while Ce³⁺and Ce⁴⁺coexist during methanol synthesis.The Cu⁰-Ce³⁺sites act as active sites for the direct hydrogenation of CO to CH₃OH,and the number of active sites is determined by the content of Ce O₂ and the contact extent between Cu and Ce O₂ clusters.The reactive intermediates species of formyl（HCO*）and formate（HCOO*）are co-adsorbed on the Cu⁰-Ce³⁺sites,and these two hydrogenation reaction pathways occurr concurrently during CO hydrogenation to methanol.4.The effects of syngas composition on Cu/Ce O₂ catalyst were investigated.These results show that the active sites of Cu/Ce O₂ catalyst have dynamic chemical properties.The dynamic changes of Cu/Ce O₂ during methanol synthesis highlight the strong correlation between composition of syngas and chemical state of the catalyst surfaces.The CH₃OH formation rates,number of surface Ce³⁺atoms and isothermal adsorption of CO are single-valued functions of the operating CO₂/CO ratios.The oxygen chemical potential in syngas,which is determined by the CO₂/CO ratio,drives the redox circle of Ce⁴⁺/Ce³⁺on Cu/Ce O₂ catalyst,and then determines the density of Cu⁰-Ce³⁺active sites.The establishment of the connection between surface structure of catalysts and reactants has important implications for a comprehensive understanding of catalysis,due to the fact that coexistence of oxidizing and reducing reactant molecules on catalyst surfaces is general.