Systematic engineering of Bi-promoted Pd/Al2O3 catalysts: from optimizing synthesis protocols to postharvest preservation of Capsicum annuum L

Abstract

In this work, a bimetallic 0.3Bi:Pd/Al₂O₃ catalyst was developed via sequential impregnation (Pd first, followed by Bi) on cordierite/γ-Al₂O₃ monoliths for the selective aerobic oxidation of glucose to gluconic acid. Systematic optimization of the Bi:Pd molar ratio, support and catalyst calcination temperatures, H₂ reduction temperature, and impregnation sequence established clear structure–activity relationships. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) revealed the formation of a Pd–Bi solid solution with uniformly dispersed nanoparticles (~ 3.5 nm), and X-ray photoelectron spectroscopy (XPS) indicated a small positive shift (~ 0.3 eV) in the Pd 3d binding energy upon Bi incorporation, tentatively consistent with Pd–Bi electronic interaction. Under optimized conditions (60 °C, pH 9.0, O₂ atmosphere), the catalyst achieved near-complete glucose conversion with > 99% selectivity toward gluconic acid, and maintained high performance over 30 consecutive reaction cycles with only a minor decline to 95% conversion. The synthesized sodium gluconate was further evaluated for postharvest preservation of chili peppers (Capsicum annuum L.). Using a Comprehensive Quality Index (CQI) model integrating six quality indicators, a 0.1% (w/v) sodium gluconate (SG) treatment was identified as the hormetic optimum, which significantly delayed senescence by reducing respiratory intensity and upregulating peroxidase (POD) activity, thereby preserving vitamin C and chlorophyll levels over a 21-day storage period.