Study On Catalytic Performance Of Substituted Hexaaluminate Catalysts For The Carbon Dioxide Reforming Of Methane

Study on Catalytic Performance of Substituted Hexaaluminate Catalysts for the Carbon dioxide Reforming of MethaneThe catalytic process of CO2 reforming of CH4 to synthesis gas has attracted many researchers for the utilization of the greenhouse gases. This reaction produces to synthesis gas with a H2/CO ratio of about 1, adequate for hydroformylation and carbonylation reactions as well as for both methanol and Fischer-Tropsch syntheses. The noble metals (Rh, Ru, Pt and Pd, etc) have been employed successfully as highly active catalysts for CO2 reforming of CH4 to synthesis gas. Although these noble metals tend to resist severe carbon deposition, they are not developed commercially for economic reasons. Improvement and development of non-noble metal catalysts that can prevent coke formation have been seriously attempted. Suitable supports have to be resistant to the high temperature applied and they have to maintain the metal dispersion of the catalyst during operation. And hexaaluminate oxides show high thermal resistance and stable structure under the high temperature. So we choose the hexaaluminate oxides as the catalysts for the carbon dioxide reforming of methane for our work.A series of hexaaluminate LaXAl11O19-δ(X= Mn, Fe, Co, Ni, Cu) catalysts have been prepared, in which the transition metal X as active component is inlayed in the hexaaluminate lattices by substituting part of Al ions. The catalytic properties are studied by XRD, DRS-UV-Vis, TPR, XPS, TEM, SEM and TGA technology. It was found that the ionic condition of transition metal X in the hexaaluminate lattices are Mn3+, Fe3+, Co2+, Ni2+and Cu2+. The LaNiAl11O19-δcatalyst demonstrates the best carbon resistance, activity and stability, so we choose LaNiAl11O19-δcatalyst as base catalyst to modifiy for carbon dioxide reforming of methane to synthesis gas.A series of La0.8M0.2NiAl11O19-δ(M= Ca, Mg, Sr) catalysts have been prepared and evaluated regarding their catalytic performance for CO2 reforming of CH4. More Ni ions locate in octahedral sites, while less Ni ions locate in tetrahedral sites in the crystal of these hexaaluminates due to the modification of M in the samples. In fact, the Ni ions in the octahedral sites can easily be reduced to active Ni0. So the addition of M can promote more Ni2+ to be reduced to Ni0 from the hexaaluminates. At the same time, the ionic radius of the alkaline earth metal M can reduce the particle size of the metallic Ni and improve its dispersion in these catalysts after reduction. The catalyst stability is also strongly influenced by type of carbon depositions on the catalyst surface. All these effects promote the catalytic activity of the hexaaluminate for this topic reaction. The catalytic activity shows the following sequence: La0.8Ca0.2NiAl11O19-δ> La0.8Mg0.2NiAl11O19-δ> La0.8Sr0.2NiAl11O19-δ.CO2 reforming of CH4 is studied over hexaaluminate Lao.8ZrxNiAl11O19-δ(x= 0, 0.02,0.04,0.06,0.08, and 0.10) catalysts. The partial substitution of La by Zr leads to a modification of the catalytic performance. The La0.8Zr0.04NiAl11O19-δshows the best catalytic stability, and the lowest amount of carbon deposition. A close correlation is observed between substitution degree (x) and the Ni3+/Ni2+ratios, which determines the Ni0 dispersion. The catalytic stability shows the following sequence:x=0