Oxidative steam reforming of ethanol over M_{: X}La_{2- x}Ce_1.8Ru_0.2O_{7- δ} (M = Mg, Ca) catalysts: Effect of alkaline earth metal substitution and support on stability and activity

Alkaline earth metal substitutions on the A-site of pyrochlore oxide M_xLa_2-xCe_1.8Ru_0.2O_7-δ (M = Mg, Ca) were studied as catalyst materials for oxidative/autothermal steam reforming of ethanol (OSRE/ATR). The as-prepared oxides were synthesized by a combustion method and characterized using powder X-ray diffraction (PXRD), and X-ray photoelectron and absorption spectroscopy (XPS and XAS). PXRD Rietveld analysis and elemental analysis (ICP-AES) support the formation of a pyrochlore-type structure (space group Fd3m) with a distorted coordination environment. The substitution of Mg²⁺ and Ca²⁺ ions affects the oxidation states of Ce^4+/3+ and Ruⁿ⁺ ions and creates oxygen vacancies, which leads to enhanced catalytic activity and reduced ethylene selectivity. A long-term stability test showed optimized catalysts Mg_0.3La_1.7Ce_1.8Ru_0.2O_7-δ and Ca_0.2La_1.8Ce_1.8Ru_0.2O_7-δ with S_H2 = 101(1)% and S_H2 = 91(2)% under OSRE conditions. The initial operation temperatures were lower than that of the unsubstituted catalyst La₂Ce_1.8Ru_0.2O_7-δ. Catalysts supported on La₂Zr₂O₇ showed stable OSRE/ATR performance and low carbon deposition compared to catalysts supported on Al₂O₃. We ascribe the enhanced activity to well-dispersed alkaline earth metal and Ru ions in a solid solution structure, synergistic effects of (Mg, Ca)²⁺/Ce^3+/4+/Ruⁿ⁺ ions, and a strong catalyst-support interaction that optimized the ethanol conversion and hydrogen production.