Preparation Of Cu-based Wgs Catalyst From Layered Double Hydroxides As Precursors

The water-gas shift reaction(WGS:CO + H2O(?)CO2 + H2,?H298K =-41.1 kJ mol-1)is an important industrial chemical process and has been widely applied for the ammonia synthesis,Fischer-Tropsch synthesis and methanol synthesis.Copper is one of the most active components for WGS,however,it easily suffers from deactivation due to the sintering of copper metal particles.Therefore,the development of well-dispersed and stable copper catalyst is highly desirable.In this thesis,various supported copper catalysts have been prepared from copper-containing layered double hydroxides(LDHs)as precursors,with an aim to develop a highly-dispersed copper catalyst with good activity and stability for the WGS reaction.The as-synthesized LDHs precursors and the resulted catalysts were characterized by various techniques including inductively coupled plasma(ICP),N2 physical adsorption,thermogravimetric analysis(TG),powder X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),temperature-programmed reduction(H2-TPR),and N2O chemisorption,and their catalytic activity and stability for the WGS reaction were investigated.The main results obtained are as follows:(1)Cu catalysts supported on different oxides were prepared by the calcination and reduction of Cu-containing LDHs including 30%Cu/Al LDH,30%Cu/Mg3AI LDH,30%Cu/Zn3Al LDH and 30%Cu/Zn3Cr LDH.It is found that upon calcination and reduction of the LDHs precursors,particularly 30%Cu/Mg3Al LDH,very small Cu nanoparticles were obtained,which could be attributed to the reduction of highly dispersed Cu2+ in Mg(Cu,Al)O solid solution.The activity of the LDHs-derived Cu catalysts for the WGS reaction was 30%Cu/Mg3Al?30%Cu/Zn3Al>30%Cu/Zn3Cr>30%Cu/Al.The high activity of 30%Cu/Mg3Al was attributed to its high CuO surface area and Cu dispersion,while the interaction between metallic Cu and ZnO on 30%Cu/Zn3Al might be responsible for the WGS activity.(2)By tuning the chemical compositions of Cu-Mg-Al LDHs,various catalysts with different Cu contents(10?40 wt%)and(Cu2+ + Mg2t)/Al3+,molar ratios(1?4)were obtained.It is found that upon calcination Cu-Mg-Al LDHs were converted to Mg(Cu,AI)O mixed metal oxides,where both Cu2+ and Al3+ were incorporated into the MgO framework to form a solid solution;reduction of Mg(Cu,Al)O gave highly dispersed and uniform Cu metal particles.The Cu dispersion was as high as 22-78%and the size of Cu nanoparticles varied from 1.5 to 5.0 nm depending on the chemical compositions.The WGS activity of the Cu-Mg-Al catalysts increased with the increase of CuO surface area,and the 30%Cu/Mg2AI catalyst showed the highest Cu0 surface area and the highest activity.The optimized 30%Cu/Mg2Al catalyst showed superior activity,thermal stability,and long-term stability than the commercial Cu/ZnO/Al2O3 catalyst.The characterization on the spent catalysts showed that the LDHs-Cu nanoparticles remained highly dispersed,suggesting that the Mg(Al)O-supported Cu nanoparticles are stable and possess good resistant against sintering.(3)By tuning the chemical compositions of Cu-Zn-Al LDHs,various catalysts with different Cu contents(10?40 wt%)and(Cu2+ + Zn2+)/Al3+molar ratios(0.5-4)were obtained.It is found that upon calcination Cu-Zn-Al LDHs were converted to a mixture of CuO,ZnO,and/or ZnAl2O4;after reduction,the Cu dispersion was 20-48%and the size of Cu nanoparticles varied in the range of 2.0-6.0 nm depending on the chemical compositions.The WGS activity of the Cu-Zn-Al catalysts increased with the increase of CuO surface area,and the 30%Cu/Zn1Al catalyst showed the highest CuO surface area and the highest activity.The optimized 30%Cu/Zn1Al catalyst also showed superior activity and stability than the commercial Cu/ZnO/Al2O3 catalyst and even the 30%Cu/Mg2Al catalyst.