Methanol steam reforming catalysts derived by reduction of perovskite-type oxides LaCo1-x-yPdxZnyO3±δ

Abstract

Methanol steam reforming (MSR) catalysts are derived from perovskite-type oxides LaCo1-x-yPdxZnyO3±δ by reductive pretreatment. The unsubstituted LaCoO3±δ (LCO) and LaCo1-x-yPdxZnyO3±δ (Co substituted with Pd and/or Zn) are synthesized by a citrate method and characterized by different techniques. The perovskite-type oxides exhibit a rhombohedral crystal structure and a comparable surface area (≈8.5 (±2) m2 g-1). The temperature-programmed reduction (TPR) shows low (100°C < T < 450°C) and high (T > 450°C) temperature reduction events that correspond to partial and complete reduction of the non-rare-earth metal ions, respectively. At high temperatures, Pd-Zn alloy nanoparticles are formed exclusively on Pd-and Zn-containing LaCo1-x-yPdxZnyO3±δ, as evident from high angular annular dark-field scanning transmission electron microscopy (HAADF-STEM). The CO2-selective MSR performance of the catalysts strongly depends on the reductive pretreatment temperature, catalyst composition (i.e., the Pd:Zn molar ratio and the degree of Co substitution) and reaction temperature. Only LaCo1-x-yPdxZnyO3±δ catalysts show a low-temperature CO2 selectivity maximum between 225 and 250°C, while all catalysts present similar high-temperature selectivity maxima at T > 400°C. The former is missing on LCO, LaCo1-xPdxO3±δ or LaCo1-yZnyO3±δ. Pd-Zn nanoparticles facilitate Zn(OH)2 and Co(OH)2 formation exclusively on LaCo1-x-yPdxZnyO3±δ, as evident from in situ XRD under steam atmosphere. This indicates the important role of Pd-Zn nanoparticles in the low-temperature CO2 selectivity, which is improved from 0 to 76% at 225°C on LCO and LaCo0.75Pd0.125Zn0.125O3±δ, respectively. The high-temperature CO2 selectivity is governed by the bulk catalyst composition and the occurrence of reverse water gas shift reaction.