Design And Catalytic Performance Of Nanocarbon Supported Atomically Dispersed Metal Catalysts For CO Oxidation

In the process of industrial exhaust gas and automobile exhaust emissions,a large amount of carbon monoxide would be produced,as a colorless,odorless and toxic gas,which causes serious problems to the environment and human health.In addition,the hydrocarbon reforming reaction also generate a small amount of CO in the industrial hydrogen production process,which causes great difficulties for energy purification and further utilization.Removing carbon monoxide through oxidation of CO and oxygen is the most effective solution at present.Traditional carbon monoxide oxidation catalysts are supported noble metal(Pt-based)catalysts.However,Pt nanoparticle catalysts have poor CO oxidation activity due to strong CO adsorption at low temperatures.Although it has been reported that the oxidation activity is improved by introducing other components to modify Pt particles,the amount and cost of noble metals are still high.Therefore,the design of low-cost,high-utilization and efficient oxidation catalysts is urgently needed.In this context,this dissertation mainly constructs atomically dispersed metal catalysts and achieves the precise regulation of metal catalytic materials at sub-nanometer scale by designing novel defect-rich graphene supported metal catalytic materials,and shows excellent catalytic performance in specific catalytic reactions to maximize metal utilization and reactivity.Meanwhiles,the reaction mechanisms were deeply studied,combined with a variety of spectroscopic characterization and theoretical calculations.The main contents are summarized as follows:(1)By regulating the metal loading and method,a series of Pt catalysts anchored on nanodiamond/graphene(ND@G)hybrid supports,namely Pt single atoms,fully exposed Pt clusters and Pt nanoparticles,were constructed to reveal the intrinsic relationship between CO oxidation activity and Pt species structure.In the CO oxidation reaction,fully exposed platinum clusters have superior activity compared to typical platinum nanoparticles and single atoms,with a TOF of 3.5 s-1 at 30℃.Combined with temperature programmed experiments,kinetic measurements and theoretical calculations,it is shown that the Pt nanoparticle catalyst shows low CO oxidation activity due to the strong adsorption of CO at low temperature,which makes it difficult for O2 to be activated on the surface.The fully exposed Pt cluster has a weak CO adsorption capacity,which can achieve the simultaneous adsorption and activation of CO and O2,thereby promoting the low-temperature CO oxidation performance.In addition,single-atom catalysts exhibit low CO oxidation activity due to their difficulty in co-adsorbing and activating of CO and O2 at a single site.This work constructs a special fully exposed cluster active structure to maximize metal utilization and reactivity,which provides scientific evidence for the design of efficient heterogeneous catalysts at sub-nanometer scale.(2)Industrially prepared crude hydrogen contains trace CO impurities,which would poison the noble metal catalyst involved in the hydrogenation reaction and inhibit its activity,so it is a great challenge to improve the CO tolerance of noble metals and directly use crude hydrogen for catalytic hydrogenation.First of all,we found that in the hydrogenation reaction system of aromatic nitro compounds involved in high-purity hydrogen,the fully exposed Pt clusters show excellent catalytic performance in the hydrogenation reaction of nitrobenzene and nitrostyrene by promoting the co-adsorption and activation of nitrobenzene and hydrogen due to their ensemble sites and electron-deficient state.In addition,due to the weak adsorption of CO for fully exposed Pt clusters,the catalyst achieves CO-resistant catalytic hydrogenation in the nitrobenzene hydrogenation reaction in the presence of 1000ppm CO.In the nitrostyrene hydrogenation reaction,it exhibits efficient selective hydrogenation performance with more than 99%selectivity,which can reduce the cost of product purification.This work provides a new pathway to use crude hydrogen as low-cost hydrogen source for hydrogenation reactions with high efficiency and chemoselectivity.(3)By introducing a second component of atomically dispersed Fe on the ND@G,a fully exposed PtFe bimetallic cluster was constructed by Pt atomic clusters bonding with adjacent Fe atoms to maximize the metal utilization and the number of Pt-Fe interface sites,thereby improving the catalytic performance of the PROX reactions.In the PROX reaction,the fully exposed PtFe clusters achieved complete CO removal and 100%CO selectivity at 30℃,and showed optimal low-temperature oxidation activity compared with Pt/ND@G catalysts and currently reported catalysts.In addition,the catalytic performance was stable for more than 100 hours at 30℃.Combined with in-situ XPS,CO-DRIFTS experiments and theoretical calculations,it is shown that the fully exposed PtFe cluster can achieve non-competitive adsorption of CO and oxygen,and sufficient Pt-Fe interface sites can promote the oxygen adsorption activation on unsaturated Fe atoms and CO adsorption on Pt clusters,so as to quickly occur CO oxidation reaction and obtain excellent catalytic performance.This work provides a new direction for the design and development of efficient and stable sub-nanometer metal catalytic materials.(4)Atomically dispersed and fully exposed Cu clusters were prepared by electrostatic adsorption and further reduction pretreatment.Compared with Cu single atoms or Cu nanoparticles,fully exposed Cu cluster catalysts exhibit higher catalytic activity and high-temperature stability in CO oxidation reactions.In the reaction of excess oxygen,CO oxidation can occur at room temperature and still exhibit high activity and stability under the response test of the catalyst to dynamic variations of the load.Through detailed structural characterization and experimental studies,it is shown that the fully exposed Cu cluster structure bonded by Cu-C exists in the form of Cu+species with electron-deficiencies and sub-nanometer ensemble sites,which can be used as bifunctional active sites with strong adsorption and low activation energy for CO and O2,so that improve CO oxidation activity.Fully exposed Cu+clusters is firstly applied to CO oxidation reactions in this work,providing guidance for the development of inexpensive and efficient transition metal catalysts.