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 LaCo_1−x−yPd_xZn_yO_3±δ by reductive pretreatment. The unsubstituted LaCoO_3±δ (LCO) and LaCo_1−x−yPd_xZn_yO_3±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 LaCo_1−x−yPd_xZn_yO_3±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 LaCo_1−x−yPd_xZn_yO_3±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, LaCo_1−xPd_xO_3±δ or LaCo_1−yZn_yO_3±δ. Pd–Zn nanoparticles facilitate Zn(OH)₂ and Co(OH)₂ formation exclusively on LaCo_1−x−yPd_xZn_yO_3±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 LaCo_0.75Pd_0.125Zn_0.125O_3±δ, respectively. The high-temperature CO₂ selectivity is governed by the bulk catalyst composition and the occurrence of reverse water gas shift reaction.