Study On Carbon Dioxide Reforming Of Methane To Synthesis Gas Over Low Nickel Content Substituted Hexaaluminate Catalysts

In recent years, CO2 reforming of methane to synthesis as a new approach of CH4 conversion and CO2 utilization has gained growing attention from academia and industry. Traditional supported nickel catalysts have also been reported to be effective catalyst for the title reaction and have been studied extensively. However, they suffer from serious deactivation due to carbon deposition, nickel particle sintering and phase transformation, resulting in rapid decrease in activity and lifetime. Therefore, it is very important to improve the state of nickel catalyst in order to overcome these disadvantages. The LaNiAl11O19-δcatalyst prepared exhibits rather high catalytic activities for CO2 reforming methane and also possesses essential advantages over traditional supported nickel catalysts in resistance to carbon deposition, and it is believed that metallic nickel (Ni0) is the active component for this reaction. In order to investigate the action of metallic Ni0 reduced from the lattices and Ni2+ located in the lattices during the process of the crystalline formation and the reaction, a series of catalysts, such as LaNixAl11O19-δ,La1-xPrxNiAl11O19-δ, and hexaaluminate La0.8A0.2 NiAl11O19-δcatalysts (A: rare earth metal), were prepared. Their catalytic activities for this reaction were investigated and their structures were characterized in detail. The behavior of carbon deposition on the Ni substituted hexaaluminate catalysts(LaNi Al11O19-δ) studied also. Some significative results are obtained.It is found that the unique MP phase can be formed when the ratio of Ni/La (mol) of inserted Ni2+ions is higher than 0.5. This part of Ni2+ions, playing very important role in the stability of the MP crystals, is very difficult to be reduced from the lattices. At the same time, when the ratio of Ni/La (mol) of inserted Ni2+ions is lower than 0.5, the Ni2+ locate in tetrahedral sites at first. When the mount of inserted Ni2+ions is higher, more Ni2+ locate in octahedral sites. This may be one of the reasons for the catalytic activity of these catalysts.Based on the experimental data, we analyzed the main reactions, which may take place under the experimental conditions and assumed the mechanism of CO2 with CH4 over the hexaaluminate LaNiAl11O19-δcatalyst in detail. At low temperature CH4→C+2H2 reaction is predominant. However, at high temperature CH4+CO2→2CO+2H2 is main reaction. While the main reactions of CO2 with CH4 over hexaaluminate LaNiAl11O19-δcatalyst is CH4+CO2→2CO+2H2.A series of Pr-doped hexaaluminate La1-xPrxNiAl11O19-δ(x=0,0.2,0.4,0.6,0.8,1.0) as catalysts for carbon dioxide reforming of methane were prepared. These catalysts were studied in more detail and it was found that the addition of Pr can promote the interactions of the Ni ions and the La ions and reduce the particle size of the metal and improve the dispersion of the Nio on these catalysts after reduction and more Ni2+ can be reduced to Nio from the crystal structure of hexaaluminates. A series of A-modified hexaaluminates, La0.8A0.2NiAl11O19-δ(A = Ce,Pr,Nd,Sm) as new catalysts for carbon dioxide reforming of methane to synthesis gas, were prepared by decomposition of nitrates and calcination at high temperature. Nickel ions as active component were inlayed in the hexaaluminate lattices to substitute part of Al ions. The series of catalysts exhibited significantly catalytic activity and ability of restraining carbon deposition at high temperature, for instance at 750 oC for 2h. The experimental results indicate that the catalytic properties of the La0.8Pr0.2NiAl11O19 are more optimal than others. These catalysts were studied in more detail and it was found that the active nickel phase strongly depends on the interactions of the Ni ion and the La ion, which are related to the additives.The reduced Ni-based hexaaluminates exhibited rather high activity of decomposition of methane. It is concluded that there are three kinds of deposited carbon: Cα,Cβand Cγon the surface of the catalyst after the top reaction. And based on the analysis of XPS, the Cαis attributed to carbide carbon and Cβand Cγare attributed to graphitic carbon. At the same time , the decomposition of methane is the main reason for the deposited carbon on the surface of the catalysts in the top reaction. Finally, the author proposed that a mechanism of carbon deposition and elimination in the reaction over the hexaaluminates catalyst. It is assumed that, in the case of hexaaluminates catalyst (eg.LaNiAl11O19-δ), as a first step the active H is absorbed on the catalyst after reduction. Then there is··O···H exist on the surface of the hexaaluminates catalyst. During the CH4-CO2 refoming reaction, it promotes the activated dissociation of CO2, which produce the active oxygen. Whereafter the active oxygen eliminates the active carbon species which is formed on the metallic nickel surface via the activation of CH4 before converting to graphite carbon.