Kinetics and catalytic efficiency of Pennisetum Purpureum-mediated Cu2O nanocatalyst for reductive decolourization of methyl orange and congo red

Synthetic dyes such as methyl orange (MO) and Congo red (CR) pose significant environmental and health risks, including diarrhoea, dysentery, skin irritation and acute respiratory infections. Herein, we report a facile and sustainable green synthesis of Cu₂O nanoparticles (NPs) using stem extract of Pennisetum purpureum (Super Napier grass), an abundant agricultural waste, and evaluate their catalytic performance for dye reduction in aqueous media. The as-prepared Cu₂O NPs were characterised by TEM, XRD and XPS, confirming spherical morphology with an average particle size of 20.3 nm, a cubic crystalline phase (14.1 nm crystallite size) and dominant Cu + species. UV–visible spectroscopy revealed ~90% reduction of both dyes. Kinetic analysis showed pseudo-first-order behaviour, with faster reduction of MO than CR due to fewer reducible azo groups. The catalyst exhibited high apparent mass-normalized catalytic activity, indicating excellent catalytic efficiency. A high negative zeta potential (−57.4 mV) and TGA/DTGA analyses confirmed good stability. The catalyst was tested to be reusable for both MO and CR dyes. These findings demonstrate that green-synthesised Cu₂O NPs are efficient, stable and reusable catalysts for catalytic reductive decolourization.