Obtenção de perovskitas La1-x(Ce, Sr)xNiO3-(Delta) para geração de gás de síntese a partir da tri-reforma do metano

Abstract

Perovskites type-oxides LaNiO3 (La), La0.95Sr0.05NiO3 (LaSr) and La0.95Ce0.05NiO3 (LaCe) were synthesized by the polymer precursor method, also known as Pechini method. The synthesized polymer resins were separated into two fractions, totaling six samples, which were submitted to two different temperature programs (700 °C and 800 °C with pre-calcination) to obtain the oxides. In order to understand the influence of the insertion of Ce4+ and Sr2+ in perovskite type structures, the oxides were characterized by the techniques of Thermogravimetric Analysis (TGA), X-Ray Fluorescence (XRF), X-ray Diffraction (XRD), Nitrogen adsorption (BET), Scanning Electron Microscopy coupled with Dispersive Energy X-ray Spectroscopy (SEM-EDS), Temperature-programmed reduction (TPR) and, finally, evaluated with regards to catalytic activity in the Tri-Reform of Methane (TRM). By the XRD technique it was possible to verify LaNiO3 phase formation in a practically pure form only for LaCe catalysts at both calcination temperatures; for LaSr7 and LaSr8 a formation of a Ruddlesden-Popper type structure, La2NiO4, was observed, which can arise from the decomposition of LaNiO3 and La2Ni2O5, being more stable than ABO3 type oxides. The most significant difference was noted between La7 and La8, where the change in the calcination process resulted in phase elimination of lanthanum oxycarbonate (La2O2CO3) verified at 700 °C, since the two-stage calcination possibly prevented reactivity of lanthanum oxide (La2O3) with carbon dioxide (CO2) from the volatilization of the organic portion of the precursor polymer, favoring the formation of La2NiO4 and LaNiO3 structures. Only the catalysts calcined at 800 °C were evaluated in the tri-reform reactions, as they showed higher amounts of perovskite and spinel phases. The results of the catalytic tests showed that all the catalysts were active for the TRM, having similar conversions for ethane. In addition to presenting higher conversion of methane, LaCe8 showed to be more active for carbonic gas conversions, probably due to the redox cycle caused by cerium oxide that reacts with CO2, generating oxygen species which causes the coke gasification deposited on the active surface, causing minor deactivation of the catalyst, higher conversion of methane and CO production. LaSr8 obtained the second-best result, possibly caused by the strontium action in the ABO3 structure, generating greater oxygen mobility and formation of SrCO3 phases that favor the elimination of coke, providing a longer catalyst life. La8 presented a conversion slightly below the others for methane, which can be suggested that the formation of La2O2CO3 phases have helped in the greater stability and conversion of methane, but with a faster deactivation by coqueifation. Finally, suitable H2/CO ratios were obtained for the synthesis of C2-C4 hydrocarbons, methanol and dimethyl ether, and the possibility of conversion of synthesis gas to electricity through high temperature fuel cells.