Design And Modulation Of Ruthenium Cluster-based Catalysts For Efficient Reverse Water Gas Shift Reaction

The conversion of carbon dioxide and renewable hydrogen into carbon monoxide through reverse water gas shift(RWGS)reaction holds promise for carbon utilization and the development of green H2 economy.However,the large energy consumption in the conventional RWGS catalysis undermines the realization of carbon-neutrality.In comparison,low-temperature RWGS catalysis increases energy and cost efficiencies.In addition,cascade catalysis of low-temperature RWGS and well-established CO hydrogenation provides an avenue for the production of liquid hydrocarbons and methanol under mild conditions.However,it remains a crucial challenge to develop lowtemperature RWGS catalysts with high activity,selectivity,and stability.Ruthenium(Ru)clusters are potential candidates for efficient RWGS catalysis,as they not only inherit the excellent activity of Ru in CO2 hydrogenation reactions,but are also able to selectively convert CO2 to CO.Nevertheless,because of their high surface energy and low Tammann temperatures,Ru clusters are vulnerable to rapid deactivation through sintering,making it difficult to exhibit long-term durability.To address these shortcomings,this thesis demonstrated highly stable Ru cluster-based catalysts constructed by oxide coating strategy in efficient and stable low-temperature RWGS system.The following studies have been conducted in the dissertation:(1)In chapter 2,we reported the design of a sandwich-like RWGS catalyst by encapsulating Ru clusters inside hollow silica shells.The spatially confined structure prevented the sintering of Ru clusters while the permeable silica layer allowed the diffusion of gaseous reactants and products.This catalyst with reduced particle sizes inherited the strong hydrogenation ability of Ru in hydrogenation of CO2 and exhibited nearly 100%CO selectivity as well as superior stability at 200-400℃.This universal encapsulation strategy could be easily extended to the preparation of sintering-resistant catalysts based on other metal nanoparticles and clusters.The ability to selectively produce CO from CO2 at relatively low temperatures paves the way for the production of value-added fuels from CO2 and renewable H2.(2)Having established the fact that the sandwiched structure design greatly improved the sintering resistance of Ru clusters,the use of a chemically active support is expected to further enhance the catalytic reactivity.Therefore,in chapter 3,we proposed a SiO2-protected calcination strategy for the fabrication of the reducible TiO2 hollow sphere with Ru clusters embedded in the inner surfaces to boost the performance of lowtemperature RWGS catalysis.Thanks to an effective metal-support contact,Ru clusters were thermally stable against sintering during high-temperature pretreatment.The multicore@hollow structure could exhibit a complete selectivity to CO formation in a wide temperature range(200-400℃)and reach a record CO2 conversion rate of 11.2 mol·gRu1·h-1.In addition,control experiments clearly revealed that the confined space of the silica played a critical role in stabilizing Ru clusters against sintering.Our study holds potential for furthering the application of Ru cluster catalysts,shedding light on the rational design of low-temperature RWGS catalysts with high activity,selectivity and stability.