Adsorption of Potassium on MoS2(100) Surface: A First-Principles Investigation

Potassium- (K-) promoted MoS2 catalyst is a very promising catalyst used for synthesis of mixed higher alcohols from syngas. Herein, periodic density functional theory calculations were performed to investigate the interaction of potassium with the Mo and S edges of the MoS2(100) surface. Both neutral K- and charged "K+"-promoted MoS2(100) systems at different sulfur coverages were studied. Our calculations indicate that the adsorbed K atom readily donates its single 4s valence electron to the MoS2 structure, and the neutral K and charged K+ systems show similar adsorption behavior. Isolated K atom/ion tends to maximize its interactions with the available S atoms on the edge surface, preferring the 4-fold S hollow site on fully sulfided Mo and S edges and the interstitial sites where K atom/ion binds with 2-4 S atoms on the edge surface. The presence of K atoms/ions affects the electronic and magnetic properties of the edge surface. As the K coverage increases, the average adsorption energy of the K atoms, surface work function, and amount of 4s electron transfer from the K atoms to the MoS2(100) surface all decrease, suggesting an increased metallization of the K adlayer. The tendency to form a chainlike K adlayer along the interstitial gap area of two edges is found. The K-K distances in the K chains of the adlayer are 3.2-3.7 angstrom, which is notably less than that of bulk K metal. Density of states analysis for the K-saturated MoS2(100) surface suggests enhanced involvement of broad K 3d states beginning just above the Fermi level. The K promotional effects on the selectivity of mixed alcohol synthesis from CO hydrogenation can be rationalized as an increase in the surface basicity due to the increasing surface electron charge donated by K doping. The adsorbed K atoms/ions also provide active sites that facilitate CO hydrogenation, block Mo and S edge sites for CO dissociation leading to hydrocarbon formation, and limit H-2 dissociative adsorption at the edge surface via the s-type electron repulsion from the transferred K 4s electron.