Cross-section TEM investigation of quasicrystalline catalysts prepared by aqueous NaOH leaching

Abstract

The Al–Cu–Fe quasicrystal (QC) has been shown to reveal high catalytic activity and thermal stability for steam reforming of methanol (SRM: CH₃OH + H₂O → 3H₂ + CO₂), after leaching with NaOH aqueous solution. To investigate the origin of the high catalytic performance, cross-sectional transmission electron microscopy (TEM) observations of the catalyst particles were examined. By leaching with NaOH aq., a homogeneous composite structure, composed of Cu and Fe and/or their oxides or hydroxides, is formed at the outer layer of catalyst particles and the QC phase remains in the inner area. The homogeneous dispersion of Fe and/or the oxide/hydroxide in the composite suppresses sintering of Cu particles. In contrast, for the catalyst prepared from crystalline ω-phase (Al₇₀Cu₂₀Fe₁₀), the original phase completely collapses and, instead, a complicated composite structure is formed. The inhomogeneity results in the formation of Cu-enriched regions at the rim of the catalyst particles. We suggest that the high catalytic performance of the Al–Cu–Fe QC catalyst prepared by leaching with NaOH aq. is due to the formation of a homogeneous composite region at the outer layer of the catalyst particles.