Oxygen Vacancy Promoted Ammonia Synthesis With Ruthenium-based Catalysts

The development of modern society is essentially dependent on reliable and efficient energy sources.From the beginning of the industrial revolution to the end of the 18th century,fossil fuels have proven to be a stable source of energy and are widely used.However,with the rapid development of industrial technology,human demand for fossil energy has grown rapidly,resulting in over-exploitation and utilization of fossil energy.Due to the limited and unsustainable use of fossil energy reserves,humans have to switch to renewable energy sources such as solar,wind and water.Among them,solar energy,as one of the most promising alternative energy sources,has received wide attention from researchers.Solar energy is cheap and easy to obtain,and the reserves are abundant.Therefore,if solar energy can be captured and stored in the form of chemical energy,it is of great significance to global environmental and energy issues.Semiconductor photocatalytic technology can directly use solar energy to drive chemical reactions to prepare high value-added chemicals,including photocatalytic hydrogen production,CO2 reduction,and nitrogen fixation.Among them,photocatalytic nitrogen fixation is an emerging field of recent photocatalysis research.This is because the traditional ammonia industry relies on the Haber-Bosch process driven by fossil energy,which usually requires catalyst Fe/Ru and high temperature and high pressure reaction conditions,while the photocatalytic process enables the conversion of nitrogen to ammonia under mild conditions without the consumption of fossil energy.Semiconductor photocatalysis has an ideal nitrogen fixation path:the photocatalyst generates excited state electrons under illumination,and then is injected into the nitrogen adsorbed on the surface of the catalyst,and N2 is reduced to NH3 by coupling protons.However,the current photocatalytic nitrogen fixation efficiency is still far lower than industrial synthetic ammonia and has no commercial value.Based on this,how to design a more efficient catalyst to enhance the efficiency of photocatalytic nitrogen fixation is of great strategic significance for promoting national technological development.In recent years,a large number of articles have reported that surface oxygen vacancies can significantly enhance the photocatalytic reactivity of the catalyst.Oxygen vacancies have coordination-unsaturated metal atoms and electron-rich properties,which not only directly participate in molecular activation,but also selectively adsorb metal ions.If oxygen vacancies are introduced into the semiconductor catalyst carrier,it is possible to increase the surface dispersion of the supported Ru nanocrystals and adjust the electronic structure of the Ru active center,thereby promoting the ammonia synthesis efficiency.Therefore,this paper studies the effect of ruthenium-based catalysts and oxygen vacancies on the photocatalytic synthesis of ammonia by regulating the carrier oxygen vacancies.The purpose of this study is to reveal the mechanism of action of oxygen vacancy-Ru on catalytic ammonia synthesis,and provide new ideas for designing and developing high-efficiency photocatalysts.This paper contains the following two parts:1.The ruthenium-supported niobium pentoxide containing oxygen vacancy was used as the research model to study the effect of oxygen vacancy on the promotion of nitrogen molecule adsorption and activation,as well as the effect on hydrogen overflow and ammonia synthesis rate.Through the photocatalytic activity test of ammonia synthesis and the in-situ infrared adsorption experiment of nitrogen molecules,we confirmed that the carrier of Ru/Nb2O5-x catalyst containing oxygen vacancy is rich in electrons,and the carrier electrons can be efficiently transferred to ruthenium to enhance the capacity of Ru-activated nitrogen.Next,through hydrogen temperature-programmed reduction and hydrogen temperature-programmed desorption experiments,we further analyzed the cleavage of hydrogen molecules and the overflow process of hydrogen atoms on the carrier.The results showed that the cracked hydrogen produced hydrogen overflow on the catalyst,which effectively alleviated the hydrogen poisoning of ruthenium active center and provided more adsorption sites for nitrogen molecules.This work analyzed the effect of oxygen vacancy on ammonia synthesis efficiency from the molecular level,which has important guiding significance for the establishment of efficient ammonia synthesis system.2.Inspired by metal plasmon resonance effect,we took WO2.72 with a large number of oxygen vacancies and a special structure as the research object to synthesize RU/WO2.72 and Ru/WO3-x catalysts containing a small number of oxygen vacancies.We first tested the rate of ammonia synthesis by Ru/WO2.72 and Ru/WO3-x catalysts and found that the synthetic ammonia efficiency of Ru/WO2.72 catalyst was higher than that of Ru/WO3-x catalyst at near room temperature.Subsequently,we analyzed the amount of photogenerated electrons and the transfer process on the catalyst,and performed in-situ infrared test of the adsorption process of nitrogen molecules on the catalyst.It was confirmed that Ru/WO2.72 was electron rich due to a large number of oxygen defects,and that the plasma resonance effect under illumination could also produce more free electrons,thus effectively facilitating the activation of nitrogen molecules.In addition,through in-situ infrared tests of molecular adsorption of hydrogen and deuterium,we found that the special structure of WO2.72 and the presence of a large number of oxygen vacancies are conducive to cracking hydrogen overflow and binding with activated nitrogen,so as to achieve highly efficient photocatalytic synthesis of ammonia.This study maximizes the effective utilization of the carrier,and provides a new idea for the design of high-performance ammonia catalyst.