Oxidative steam reforming of ethanol over MxLa2-xCe1.8Ru0.2O7-delta (M = Mg, Ca) catalysts: effect of alkaline earth metal substitution and support on stability and activity

Abstract

Alkaline earth metal substitutions on the A-site of pyrochlore oxide M<INF>x</INF>La<INF>2-x</INF>Ce<INF>1.8</INF>Ru<INF>0.2</INF>O<INF>7-delta</INF> (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 Mg<SUP>2+</SUP> and Ca<SUP>2+</SUP> ions affects the oxidation states of Ce<SUP>4+/3+</SUP> and Ru<SUP>n+</SUP> ions and creates oxygen vacancies, which leads to enhanced catalytic activity and reduced ethylene selectivity. A long-term stability test showed optimized catalysts Mg<INF>0.3</INF>La<INF>1.7</INF>Ce<INF>1.8</INF>Ru<INF>0.2</INF>O<INF>7-delta</INF> and Ca<INF>0.2</INF>La<INF>1.8</INF>Ce<INF>1.8</INF>Ru<INF>0.2</INF>O<INF>7-delta</INF> with S<INF>H<INF>2</INF></INF> = 101(1)% and S<INF>H<INF>2</INF></INF> = 91(2)% under OSRE conditions. The initial operation temperatures were lower than that of the unsubstituted catalyst La<INF>2</INF>Ce<INF>1.8</INF>Ru<INF>0.2</INF>O<INF>7-delta</INF>. Catalysts supported on La<INF>2</INF>Zr<INF>2</INF>O<INF>7</INF> showed stable OSRE/ATR performance and low carbon deposition compared to catalysts supported on Al<INF>2</INF>O<INF>3</INF>. We ascribe the enhanced activity to well-dispersed alkaline earth metal and Ru ions in a solid solution structure, synergistic effects of (Mg, Ca)<SUP>2+</SUP>/Ce<SUP>3+/4+</SUP>/Ru<SUP>n+</SUP> ions, and a strong catalyst-support interaction that optimized the ethanol conversion and hydrogen production.