Fabrication Of Oxygen-vacancy-rich Metal Oxide Catalyst And Its Composite Fiber For Hydrogen Production

As a recognized clean energy,hydrogen（H₂）energy is of great significance to alleviate the energy crisis and achieve the goal of“carbon peak and carbon neutrality”.Traditional H₂ production technology from fossil fuel has many by-products and high carbon emissions,which is unsustainable,severely limiting its large-scale application.High-efficiency H₂ production from formaldehyde（HCHO）solution with high hydrogen content from a wide range of sources through nano-catalytic reforming technology is considered as an effective mean for sustainable H₂ obtainment.However,the technology of catalytic HCHO reforming into H₂ still faces two technical bottlenecks in practical applications that need to be addressed:（1）The existing HCHO reforming catalysts have low efficiency,poor stability and uncontrollable selectivity;（2）The powdery catalyst with light specific gravity is easy to lose after reaction,which makes it has a low recovery rate and difficult to stably recycle.Therefore,it is crucial to research and develop a highly efficient,stable and recyclable catalytic HCHO reforming system for H₂ production.Focusing on the above problems,two aspects of research are carried out in this thesis.Firstly,to solve the problem of low efficiency and poor selectivity of catalytic HCHO reforming into H₂,we designed and prepared structurally stable oxygen-vacancy-rich WO₂ and Co@CoO nano-catalysts.The electronic structure,lattice strain,and local chemical environment of catalyst are regulated based on oxygen vacancies,and the molecular adsorption and activation effects can also be enhanced by oxygen vacancies,which realize the high-efficiency HCHO reforming to produce H₂.Secondly,to solve the problems of low recovery rate and poor stability cycle of powder catalyst during actual application,we skillfully utilize the technical characteristics of textile fiber such as good flexibility,surface processability and chemical stability,select carbon fiber（CF）and spinning carbon nanofiber（CNF）as stable carriers,and combine with nanomaterial immobilization technologies such as electrospinning,to prepare nanocomposite catalytic fibers loaded with WO₂ and Co@CoO,which greatly improve the recyclability of the catalyst,effectively solve the problem of poor recycling performance of the powder catalyst,and successfully construct a highly efficient and recyclable HCHO reforming H₂production system.The main research contents are as follows:（1）In order to achieve efficient and low-cost catalytic HCHO reforming H₂production,we combine hydrothermal method and calcination treatment to prepare tungsten dioxide nanorod（WO₂ NRs）catalyst,which show excellent catalytic H₂production activity from HCHO solution at room temperature without precious metals,the H₂ production rate are even higher than most precious metal catalysts.Experimental results showed that the oxygen vacancies on the WO₂ NRs surface derived from oxygen coordination-unsaturated W sites play a decisive role in the catalytic H₂ production from HCHO,and its content can linearly control the H₂production rate.It is found that oxygen vacancies not only provide abundant adsorption sites for reactant molecules,but also can effectively activate oxygen（O₂）to make it as a special catalyst to synergistically catalyze the H₂ production from HCHO.The co-catalytic mechanism of oxygen vacancy activating O₂ is:WO₂ activates the O₂ adsorbed on the coordination unsaturated site into superoxide radical anion（O₂·^-）by electron transfer through oxygen vacancy,then O₂·^-attacks the C-H bond in the HCHO and obtain H to transform into per oxyradicals（·OOH）,and then combine with H in H₂O to become H₂ and O₂.This process greatly accelerates the cleavage of C-H bonds,resulting in a significant increase in the H₂production rate,and the TOF value reaches 173 h^-1 at room temperature.（2）In view of the disadvantage that the H₂ production from HCHO catalyzed by WO₂ NRs requires Na OH assistance,we prepare Co@CoO heterointerface catalysts by anchoring the reduced metallic Conanoparticles on the CoO matrix via the in-situ controlled reduction of CoO nanosheets,which achieve the high-efficiency catalytic H₂production from HCHO at room temperature without precious metals and auxiliary agents.Experimental and theoretical calculation indicate that the in-situ reduction process induces the Co@CoO to generate a large number of oxygen vacancies-rich Co-CoO interface sites,which act as the catalytic active center in H₂ production from HCHO,where the Coand the CoO sites are responsible for C-H bond cleavage in HCHO and O-H bond activation in H₂O,and the resulting two H atoms are rapidly combined to produce H₂.Importantly,it is found that the lattice strain degree and oxygen vacancy concentration on CoO,as well as the charge transfer across the Co-CoO interface are all linearly correlated to the catalytic activity towards the reforming of HCHO to produce H₂.Optimal catalytic activity is achieved by the sample reduced at 350°C,Co@CoO-350,which exhibits the most Co-CoO interfaces,the most oxygen vacancies,a lattice strain of 5.2%in CoO and highest H₂ production TOF of 50.4 h^-1 at room temperature.（3）In order to solve the problems of high loss rate and poor stability of powder catalysts in actual catalytic applications,and effectively improve the recyclability,we use the technical characteristics of textile fiber materials such as good flexibility and high chemical stability,and adopt electrospinning,impregnation and hydrothermal method combines two high-efficiency H₂ production catalysts,WO₂ and Co@CoO,with carbon fibers and spun carbon nanofibers with abundant surface defect sites and large specific surface area to construct a series of nanocomposite catalytic fibers loaded with WO₂ and Co@CoO.Experimental results show that the three loading methods have almost no effect on the structure of catalyst itself,but compared to the agglomeration occurred in impregnation and hydrothermal method,the catalyst are only uniformly dispersed and supported in the composite fiber prepared by the electrospinning method.Moreover,studies on stability and recyclability have shown that WO₂/CNF and Co@CoO/CNF composite fibers prepared by electrospinning have better bonding strength and loading stability due to the catalyst particles being immobilized on the spun carbon nanofiber matrix in an inlaid structure.Therefore,by coupling the flexibility and recyclability of fiber,the recyclability of catalyst is dramatically improved,and the large number of adsorption sites provided by the fiber support can effectively increase the activity,thereby constructing an efficient and stable H₂ production system from HCHO reforming.In conclusion,to address the two technical bottlenecks of low activity and poor cycle stability of existing HCHO reforming catalysts.On one hand,this thesis explores and designs oxygen vacancy-rich WO₂ and Co@CoO catalysts,and optimizes the catalyst structure based on oxygen vacancies,combined with the adsorption and activation effect of oxygen vacancies on the reactants,effectively improves the activity of H₂ production by HCHO reforming.On the other hand,electrospinning and other technologies are used to load the catalyst on fiber carriers to prepare nanocomposite catalytic fibers.The stable loading effect of the fiber matrix on the catalyst is used to effectively solve the problem of poor cycle stability of the powder catalyst in actual use.Finally,an efficient,stable and recyclable catalytic H₂ production system is constructed,which realize the technological cross-innovation integration of catalytic and textile field,and provide new guiding ideas and solutions for sustainable large-scale H₂ production.