Investigation On Preparation And Properties Of High-Performance Ni-based Oxygen Carriers In Chemical Looping Reforming For H₂ Generation

Chemical looping reforming for hydrogen generation could be a significant stride towards the sustainable development of a hydrogen economy.The utilization of biofuel glycerol and ethanol,two promising alternative fuel candidates,as feedstocks is capable of achieving envisaged negative carbon emissions in hydrogen production.Oxygen carriers play a pivotal role in the development of chemical looping technologies.Herein,we focus on Ni-based oxygen carriers and their three challenges,i.e.,dead time,coke deposition,and,active phase sintering,and expect to surmount them by the nanoconfinement effect and the lanthanide doping effect.We investigate the performance and deactivation mechanism of conventional Ni-based oxygen carriers,throwing light onto the R&D of high-performance oxygen carriers.We synthesize Ni-Al and NiW-Al oxygen carriers using a co-precipitation method and utilize fixed-bed and moving-bed reactors to evaluate their reactivity and stability.The reactivity test results show that there is a dead time in the fuel feed step in which no H2 is generated and that its duration depends crucially on the oxygen vacancies and Ni dispersion of oxygen carriers.Besides,coke deposition occurs in both dead time and reforming stages,resulting in decreases in NiO reduction degree and the reactivity of oxygen carriers.The stability test results exhibit that Ni sintering is responsible for the extension of the dead time and coke deposition amounts,significantly affecting the multi-cycle stability of chemical looping reforming.We intend to address the issue of Ni sintering using the nanoconfinement effect.We synthesize one-dimensional confinement oxygen carriers with lamellar structure(xNi-MMT).The experimental results exhibit that 20Ni-MMT possesses higher stability and reactivity than 20Ni-Al and that the dead time of 20Ni-MMT is close to that of 20Ni-Al.The characterization results show that the excellent stability of 20Ni-MMT stems from the limitation effect of the lamellar structure on Ni particle migration.Although the confinement effect decreases the reducibility of NiO,the high NiO dispersion of 20Ni-MMT contributes to NiO reduction rates,leading to a similar dead time between 20Ni-Al and 20Ni-MMT.We synthesize ordered mesoporous Ni/Al-MCM-41 and nanotube xCeNi/PSNT oxygen carriers with the two-dimensional confinement effect to improve anti-sintering capability and shorten the dead time.Also,we investigate the effect of preparation methods and Ce doping amounts on their performance.Compared with the one-dimensional confinement effect,the two-dimensional confinement effect could further decrease the freedom of Ni thermal migration,contributing to a high sintering resistance.Both the direct synthesis method and Ce doping could shorten the dead time and improve the reactivity.The oxygen carriers derived from the direct synthesis method possess high NiO dispersion,and Ce doping effectively separates NiO particles and increases oxygen vacancies.These features contribute to NiO reduction and coke deposition elimination.To further improve sintering resistance,we study the performance of LnNi@ZrO2(Ln=Ce,La,Yb,and Pr)nanocomposite oxygen carriers,which are featured by the three-dimensional confinement effect.In a 50-cycle test,CeNi@ZrO2 exhibits the best stability without conspicuous deactivation and the highest reactivity.The doping ions with large radii would generate the strong capability of inhibiting phase transformation,resulting in the high thermal stability of the nanocomposite structure.Lanthanide doping affects the Ni dispersion and oxygen mobility to different degrees,shortening the dead time of oxygen carriers.Moreover,compared with other samples,CeNi@ZrO2 shows the highest NiO dispersion and oxygen mobility,accounting for the highest coke resistance and the strongest water gas shift reactivity.