Preparation Of Cobalt-based Catalysts And The Catalytic Performances On Methane Dry Reforming Reaction

Dry reforming of methane(DRM)to synthesis gas is an efficient resource for two greenhouse gases,reduces atmospheric pollution,improves ecological benefits and alleviates growing environmental problems.The resulting H2 and CO can also be used in Fischer-Tropsch synthesis to produce high value-added chemical products such as methanol.Cobalt-based catalysts are widely used in DRM because of their low price,resistance to carbon build-up and better oxygen affinity.However,it suffers from sintering and carbon accumulation at higher temperatures,which seriously affects its activity and stability at high temperatures.Therefore,it is important to develop efficient and stable Co-based catalysts for DRM reactions.In this paper,M-Co bimetallic catalysts consisting of cobalt-based catalysts and a series of second metal M(M=Fe,Cu)were prepared using nitrogen-doped carbon nanotubes and hollow magnesium oxide as carriers.The catalytic performance of the different catalysts was investigated by a fixed-bed reactor.The effects of nitrogen species and content,the morphology and size of the hollow magnesium oxide and the second metal M to Co ratio on the catalyst performance were investigated.The catalysts were characterised using XRD,BET,TEM,H2-TPR,XPS,CO2-TPD and Raman techniques,and it was found that:(1)Nitrogen doping promoted the formation of Co3O4,inhibited the reduction of Co oxides and enhanced the interaction between Co species and CNTs.The nitrogen-doped Co/N-CNTs possessed more carbon defects.The nitrogen doping provides more defects in the carbon nanotubes and promotes the formation of defect sites,which leads to an increase in the disorder of the catalyst and improves the catalytic activity of methane dry reforming.Both pyridinic nitrogen content and carbon defect concentration showed a good positive correlation with CH4 conversion.The synergistic effect between pyridinic nitrogen and carbon defects greatly improved the CH4 and CO2 conversions of the Co/N-CNTs catalysts.(2)The hydrothermal synthesis of Co-MgO catalysts promoted the production of Co3+.The reduction temperature of Co-MgO catalysts shifted towards the high temperature region,when the interaction between Co and the carrier became stronger.With the Co-MgO catalyst using urea as precipitant,the position of the CO2 absorption peak shifts towards the high temperature and the CO2 uptake reaches its maximum.At this point the Co-MgO catalyst has the largest number of basic sites and the increase in basic sites accelerates the adsorption of CO2,thus increasing the resistance to sintering and catalytic activity at higher reaction temperatures.As the ratio of glucose to metal salts gradually increases,the size of the synthesised hollow spheres gradually decreases and the catalytic activity of the DRM reaction gradually decreases,and the decrease in hollow MgO size is positively correlated with the dry reforming activity of methane.(3)When a second metal,Fe,was added to the Co-MgO catalyst,the catalytic activity of the prepared Fe-Co-MgO catalyst was higher during the methane CO2 reforming reaction.The addition of Fe promoted further facilitation of Co3+production.In contrast,the addition of Cu decreased Co3+.The reduction temperature was higher in the Fe-Co-MgO catalyst and the interaction between Co and the carrier was stronger.The strong interaction between the carrier and the active component further enhances the activity of the catalyst.The Fe-MgO catalyst alone had very low activity,achieving the highest CO2 conversion of 96.6%and the highest methane conversion of 98.8%at 800℃ at an Fe/Co ratio of 2/8.However,the CH4 and CO2 conversions of the 3Fe-7Co-MgO catalyst decreased slightly when the Fe and Co ratio was further increased.This suggests that the addition of a moderate amount of Fe increases the dry reforming activity of methane to some extent.The Co-MgO and 2Fe-8Co-MgO catalysts did not show any deactivation during the stability test,and after 70 h of reaction,the CH4 and CO2 conversions of the Co-MgO and 2Fe-8Co-MgO catalysts remained at the initial 98.4%and 98.8%,and the CO2 conversions remained at the initial 92.4%and 92.7%.The addition of iron lowered the H2/CO ratio inhibiting methane cracking.