Design Of Metal-Oxide Interfaces For CO₂ Hydrogenation

The large-scale use of fossil fuels has made carbon(CO2)emissions a global concern.In 2019,the global annual CO2 emissions have reached 33 billion tons.Realizing the utilization of CO2,as carbon resources,provides opportunities and challenges for chemistry community.In recent years,the conversion of CO2 and green hydrogen(H2)into methanol has received broad attention.Methanol can be used not only as a platform molecule for higher value-added chemicals,but also as a storage medium for hydrogen energy.There is a consensus in the academic community that the active sites of CO2 hydrogenation on metal/oxide catalysts are related to metalsupport interactions(MSIs).The dynamic evolution of catalyst in hydrogen/reaction gas modifies the geometric and electronic structures of the metal sites,conducive to the formation and conversion of intermediates.The improvement of catalytic performance is based on the understanding of active sites.However,the complexity under reaction conditions poses huge challenges,and higher requirements for precise preparation methods are also proposed nowadays.In this doctoral dissertation,we select classic Cu/ZnO and Pd/Ga2O3 systems and construct oxide modified metal nanoparticles catalysts using atomic layer deposition(ALD).ZnO/Ga2O3 prepared by ALD has unique properties such as high dispersion and amorphous.It is significantly different from bulk oxides in terms of redox properties,and has strong interactions with metal nanoparticles,thereby participating in the active site composition as much as possible in relatively small amounts.We have combined microscopy and spectroscopy characterization techniques to systematically study the structure and performance of catalysts under real conditions.The main research results are as follows:1.We constructed an inverse ZnO/Cu model catalyst and systematically studied its active sites for CO2 hydrogenation to methanol.First a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using ALD.ZnO decoration boosted methanol space time yield(STY)to 877 gMeOHkgcat-1h-1 with～80%selectivity at 220℃.High pressure in situ X-ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen-deficient ZnO ensembles instead of forming CuZn metal alloys.By modeling various potential active structures,density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies,rather than stoichiometric interfaces,Cu and CuZn alloys were essential to catalytic activation.2.We designed Cu-highly dispersed ZnO interfaces and observed stable ZnO single sites under reaction conditions,realizing higher specific mass activity for CO2 hydrogenation to methanol.We firstly deposited ZnO ensembles on SiO2 by ALD.Xray diffraction(XRD)and ultraviolet visible absorption(UV-Vis)confirmed that the size of ZnO ensembles can be adjusted from atomic dispersed to nanoparticles.Then Cu atoms were selectively deposited onto ZnO/SiO2.In situ XAS confirmed that after reduction at 250℃,copper atoms aggregate into particles of 3 nm while ZnO remains highly dispersed.Diffuse reflectance infrared spectroscopy(DRIFTS)confirmed that there is a strong interaction between highly dispersed ZnO and Cu,and the oxidation state of Cu increased.We found that the methanol STY of Cu-highly dispersed ZnO interface reached 21240 gMeOHkgCu-1h-1,which is higher than literature reported ones.After increasing ZnO sizes,the specific activity increased slowly.3.We found that low-temperature reactive metal-support interactions(RMSIs)exist in an inverse Ga2O3/Pd catalyst.Here we coated Pd nanoparticles by atomically thick Ga2O3 using ALD,to enable the initiation of RMSIs at a much lower temperature of approximately 250℃.State-of-the-art microscopic and in situ spectroscopic studies disclose that low-temperature RMSIs initiate the formation of rarely reported Ga-rich PdGa alloy phases,distinct from the Pd2Ga phase formed in traditional Pd/Ga2O3 catalysts after high-temperature reduction.In the CO2 hydrogenation reaction,the Ga2O3-PdGa interfaces impressively boost the formation of methanol and dimethyl ether approximately 5 times higher than that of Pd/Ga2O3.In situ infrared spectroscopy reveals that the Ga2O3-PdGa interfaces greatly favor formate formation as well as its subsequent hydrogenation,thus leading to high productivity.