Density functional theory study of the adsorption of NO on CunX (n=2-8; X = Cu, K) clusters

Density functional theory calculations are performed to analyze the structure and stability of Cu and Cu-K clusters with 3 to 9 atoms. The results indicate that the stability of the clusters decreases after doping with a K atom. With the increase of cluster size, the stability of the clusters shows odd-even alternation. Cu-8 and Cu7K clusters exhibit the highest stability. Next, different adsorption sites are considered to investigate the geometry of CunNO and Cun-1KNO clusters. By calculating the adsorption energy and the HOMO-LUMO energy gap, it is determined that both types of reactions are exothermic processes, indicating stable adsorption of NO. Notably, the CunK clusters are more active (stronger adsorption) for NO than the Cu-n clusters. The most chemically active clusters among CunNO and Cun-1KNO clusters are Cu8NO and Cu7KNO clusters. Finally, electron transfer and Mayer bond order analysis of Cu8NO and Cu7KNO clusters reveal that the NO bond order decreases due to electron transfer when Cu/Cu-K clusters adsorb NO. In this process, the N atom is the electron donor and the Cu atom is the electron acceptor. Fundamental insights obtained in this study can be useful in the design of Cu/Cu-K catalysts.