As a result of the powerful quantum confined space effects and multiple modes of very tiny atomic sizes, metal nanoclusters (NCs) that consist of thiolate-protected noble metals, such as silver (Ag) and gold (Au) have core sizes less than 3 nm, have emerged as a class of "metallic molecules" with a wide range of molecule-like optical, magnetic, and electronic properties. To investigate well-defined nanocatalysts, especially ligand-passivated metal nanoclusters, great strides have been achieved in the efficient synthesis of atomically precise nanoparticles. Methods of synthesis such as bottom-up growth, top-down approach, ligand engineering, and the interconversion system are covered in this overview. Such clearly defined metal nanoclusters have demonstrated considerable promise in catalysis research and have evolved into a distinct class of model catalysts. Focusing on the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER), this article attempts to outline current developments in nanoclusters of molecular metal employed in electrocatalytic reactions. The paper also highlights the connection between the catalytic mechanism's structure and performance as well as examines the potential effects of metal clusters' sizes, metal core structures, charges, ligands, and metal-ligand binding patterns on their electrocatalytic activity. Future research opportunities and challenges are also proposed.

Efficient catalysts play a pivotal role in advancing eco-friendly water treatment strategies, particularly in the removal of diverse organic contaminants found in water-petroleum sources. This study addresses the multifaceted challenges posed by contaminants, encompassing a spectrum of heavy metals such as As, Cd, Cr, Mn, Mo, Ni, Pb, Sb, Se, and Zn alongside pollutants like oily water (OIW), total suspended solids (TSS), chemical oxygen demand (COD), dyes, and pharmaceuticals, posing threats to both aquatic and terrestrial ecosystems. Herein, we present the synthesis of biogenically derived Mn@NiO nanocomposite (NC) photocatalysts, a sustainable methodology employing an aqueous Rosmarinus officinalis L. extract, yielding particles with a size of 36.7 nm. The catalyst demonstrates exceptional efficacy in removing heavy metals, achieving rates exceeding 99–100% within 30 min, alongside notable removal efficiencies for OIW (98%), TSS (87%), and COD (98%). Furthermore, our photodegradation experiments showed remarkable efficiencies, with 94% degradation for Rose Bengal (RB) and 96% for methylene blue (MB) within 120 min. The degradation kinetics adhere to pseudo-first-order behavior, with rate constants of 0.0227 min⁻¹ for RB and 0.0370 min⁻¹ for MB. Additionally, the NC exhibits significant antibiotic degradation rates of 97% for cephalexin (CEX) and 96% for amoxicillin (AMOX). The enhanced photocatalytic performance is attributed to the synergistic interplay between the Mn and NiO nanostructures, augmenting responsiveness to sunlight while mitigating electron-hole pair recombination. Notably, the catalyst demonstrates outstanding stability and reusability across multiple cycles, maintaining its stable nanostructure without compromise. Graphical Abstract