One-Step Synthesis of Ultra-Small RhNPs in the Microreactor System and Their Deposition on ACF for Catalytic Conversion of 4–Nitrophenol to 4–Aminophenol

The rising demand for platinum-group metals, driven by their essential applications in catalysis, energy storage, and chemical conversion, underscores the need to identify new sources for their recovery. Waste solutions originating from industrial processes offer a promising alternative source of noble metals. However, due to their typically low concentrations, effective recovery requires a highly targeted approach. In this study, we present a synthetic waste solution containing trace amount of Rh(III) ions as both a medium for metal ion recovery and a direct precursor for catalyst synthesis. Using a bimodal water-ethanol solvent system, ultra-small rhodium nanoparticles were synthesized and subsequently immobilized onto activated carbon fibers (ACFs) within a microreactor system. The resulting Rh@ACF catalyst demonstrated high efficiency in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), serving as a model catalytic reaction. The Rh@ACF catalyst, containing 4.24 µg Rh per milligram of sample, exhibited notable catalytic activity, achieving 75% conversion of 4-NP to 4-AP within 1 h. Full conversion to 4-AP was also reached within 5 min, but requires extra NaBH₄ addition to the catalytic mixture.