Role of alkali promoters in K/MoS2 catalysts for CO-H2 reactions

The effect of alkali promoters on selectivity of CO-H2 reactions was studied for potassium-promoted MoS2 employing different potassium salts and pretreatment conditions (oxidized vs. fresh samples). Promoters assisted either chain growth of hydrocarbon products or alcohol formation. A good correlation was observed between pKa of the conjugate acid of each promoter and its spacetime yield of alcohol formation. Alcohol selective promoters such as K2CO3, KOH and K2S readily removed their counter anions under the reaction conditions to form a new potassium complex and spread themselves uniformly over MoS2. This complex appears to serve as an active site which adsorbs carbon monoxide molecularly and, at the same time, cover the majority of the MoS2 surface which is responsible for dissociative carbon monoxide adsorption and hydrogenation. Promoters for chain growth such as K2SO4 and KCl maintained their initial chemical states throughout the reactions and showed highly nonuniform lateral distributions. Thus, the promoters have a limited coverage over MoS2, yet modify the electronic state of MoS2 which interacts directly with carbon monoxide. Exposure of K2CO3- or KOH-promoted MoS2 to atmosphere for an extended period oxidized the catalyst and caused segregation of potassium into the bulk of MoS2 Thus, the most of MoS2 surface is now exposed, yet modified by potassium located in the subsurface region of MoS2. These modified catalysts promoted hydrocarbon chain growth without forming alcohols. The results demonstrate that the distribution of promoter is one of the primary factors determining its role in catalytic CO-H2 reactions.