Figure (a) shows the schematic representation of the recipe of synthesis and nature of atomic arrangement on the HEA nanoparticle; each constituent of HEA is present on surface of the nanoparticle, allowing adsorbate molecule to land over the surface of the nanoparticles and turn to CO 2 . A unique recipe has been applied to prepare a five-element HEA catalyst using low-cost materials transition elements for this purpose; Fe, Ni, Co, Ga and Zn. The elements were melted together in an argon environment to obtain an ingot and then milled at extremely low temperatures (-123 K) to avoid oxidation and contamination of the nanocrystalline HEA. The XRD pattern (Figure ) reveals single-phase FCC nanoparticles are being synthesized by this route. The size and shape of as-prepared nanoparticles were obtained from bright-field TEM micrograph (Figure )) and the average size is found to be ~12 ±3 nm. Their surface is pristine, as shown in the HRTEM image of a nanoparticle in Figure . It is to be noted that there are five different elements in single HEA nanoparticle, and therefore, the homogeneity (element distribution) has been confirmed by elemental mapping using EDS attached to the TEM. Figure (f) shows elemental maps for a nanoparticle, revealing that all elements are homogeneously distributed in the single-phase materials. The HEA nanoparticles are further investigated for heterogeneous gas-solid reaction in the unsupported state. The CO-oxidation reaction is carried out under stoichiometry conditions, the details of gas mixture containing 10% carbon monoxide, 10% Oxygen and remaining Nitrogen.