CO2 Reforming of CH4 over Pt/ZrO2 Catalysts Promoted with La and Ce Oxides

Abstract

The CO2 reforming of CH4 was studied over cerium- and lanthanum-promoted Pt/ZrO2 catalysts at 1073 K and a CH4 : CO2 ratio of 2 : 1. The high ratios of CH4 : CO2 were used in an attempt to accelerate deactivation of the catalyst and make a faster comparison of the different samples. The addition of Ce resulted in a significant improvement in the stability, with no decrease in either CH4 or CO2 conversion observed. The promotion with La also resulted in an increase in the stability of the catalyst as well as an increase in the initial activity. The mechanism of reaction has been studied on the promoted catalysts, and it has been shown that it is not altered by the presence of the promoters. The long-term activity of the catalyst is dependent upon the balance between the rate of CH4 decomposition and the rate of dissociation of CO2. The results of pulse studies demonstrate that addition of the cerium results in an increase in the reducibility and oxygen transfer ability of the support. Temperature-programmed oxidation studies demonstrated that although the total amount of carbon deposits on the Ce-promoted catalyst is not lower than that on the unpromoted one, these deposits are eliminated at much lower temperature, indicating the ability of the catalyst to clean the active sites. On the other hand, the TPO studies on the La-doped catalyst showed a much lower extent of carbon deposition. Interestingly, the ability of La to inhibit carbon formation is only observed in the presence of CO2. Under pulses of pure CH4, the La-promoted catalyst was the one which showed the highest tendency for carbon deposition. X-ray diffraction and BET area characterization of this catalyst showed that the promoter increases the thermal stability of the support. Finally, it was shown that the promoters also retard metal particle growth during reaction at high temperature. Smaller Pt particles result in an increase in the interfacial area and subsequent increase in the removal of the carbon from the metal. The increase in the efficiency of the cleaning mechanism resulted in catalysts that were stable for the dry reforming reaction under severely deactivating conditions (i.e., 1073 K, CH4 : CO2 ratio=2 : 1) for over 20 h.