The Impact Of Core-shell Construction NiO@TiO₂ On The Stability Of Complex Catalyst In ReSER Hydrogen Production

Hydrogen is both the petrochemical raw material and the clean energy carrier.Reactive sorption enhanced reforming(ReSER)is the new efficient hydrogen production technology,uses nano-CaO as CO2 sorbent,couples calcium cycle to methane steam reforming process.It has obvious advantages,which improves methane conversion and hydrogen concentration,reduces reaction temperature and shortens the process.The complex catalyst has both catalytic and asorption capacities still have the inactivation problem during reaction-regeneration cycles.This paper aimed to improve the stability of complex catalyst,prepared core-shell construction NiO@TiO2 as precursor,studied its relationship to the enhancement of catalytic activity and stability of NiO@TiO2-CaO/Al2O3 and its CO2 adsorption performance,has important theoretical and practical significance for industrial application of ReSER technology.First of all,Ni(OH)2 was prepared by urea hydrolyzation,core-shell construction NiO(core)@TiO2(shell)was prepared by adsorption phase technique,and then wet mixed with nano CaO and alumina sol to form NiO@TiO2-CaO/Al2O3.NiO crystal size of 28.5 nm was achieved by optimizing urea hydrolysis time and adsorption phase reaction time.The morphology,microstructure,Nickel dispersibility and reducibility of NiO@TiO2-CaO/Al2O3 were characterized by means of SEM?BET?XRD?H2-TPD ? H2-TPR.The result showed that core-shell construction NiO@TiO2 made complex catalyst more abundant pore structure,and enhanced the interaction between the Nickel and the carrier.Comparing to NiO-CaO/Al2O3,the surface area,average pore diameter and pore volume of NiO@TiO2-CaO/Al2O3 increased by 16.0%?35.4%and 73.1%?respectively,crystal size of NiO decreased by 23.6%and dispersion improved by 23%.Result proved that successfully prepared core-shell construction NiO@TiO2 can effectively increase the number of active site in the complex catalyst.Secondly,the catalytic activity of NiO@TiO2-CaO/Al2O3 was evaluated on the laboratory-scale fixed-bed reactor.The result showed that,under the same reaction conditions and to reach the same methane conversion and hydrogen concentration,carbon space velocity of NiO@TiO2-CaO/Al2O3 can reach up to 1200 h-1.The test results indicated that the enhancement of catalytic activity was result of the increase of surface area,which increased the dispersion of nickel,and the augment of average pore diameter,which accelerated the diffusion rate of reaction gas in the pore.Then,the catalytic stability of complex catalysts was evaluated under the same carbon space velocity.The result showed that,NiO@TiO2-CaO/Al2O3 had no degradation in activity,its exit hydrogen concentration remained above 90%and methane conversion stayed above 85%during 36 reaction-regeneration cycles,while NiO-CaO/Al2O3 deactivated rapidly during 9 reaction-regeneration cycles,demonstrating the significant improvement of the stability of NiO@TiO2-CaO/Al2O3.In addition,stability of NiO@TiO2-CaO/Al2O3 with different initial crystal size of nickel oxide was compared.The result showed that reducing the initial nickel core size within the core-shell construction NiO@TiO2 is benefit for improving the stability.Reasons were analyzed that,on the one hand,NiO@TiO2 fixed the nickel core in the shell,avoid them move on the carrier and grew up,thus maintain the nickel core size;on the other hand,NiO@TiO2 played a role in structural support of the complex catalyst,and slow down the decrease of surface area caused by the consumption of porous Al2O3 carrier that contacted and reacted with CaO,and thus improved the microstructure stability of complex catalyst.Finally,the adsorption performance of NiO@TiO2-CaO/Al2O3 was evaluated.The result showed that,the presence of NiO@TiO2 can lower the decomposition temperature by 12 ?,and had little influence on the first adsorption rate and adsorption capacity,but the decay of adsorption capacity still exist.Therefore,to improve the adsorption performance,this paper prepared complex catalyst NiO@TiO2-CaO@TiO2/Al2O3 with double core-shell construction NiO@TiO2 and CaO@TiO2.The result showed that,the presence of CaO@TiO2 quicken the highest decomposition rate by 44.3%,hasten the first adsorption rate by 87.6%,enlarged the adsorption capacity by 40.0%and enhanced the adsorption capacity stability.The result of this paper had great significance to the industrial application of the complex catalyst in the ReSER process.