Role of Nickel Migration from Carbon Nanofibers Support to Active Phase in Tuning MoS2 Catalysts for Syngas Conversion to Higher Alcohols

Abstract

The role of nickel migration from carbon nanofiber (CNF) supports to the active MoS2 phase was systematically investigated for higher alcohol synthesis (HAS) from syngas. A series of K-promoted MoS2 catalysts with varying nickel loadings (0–5 wt %) was prepared on CNF supports derived from methane decomposition over a Ni-Cu/Al2O3 catalyst. Comprehensive characterization by N2 physisorption, XRF, XRD, TEM, HRTEM, SEM-EDX, catalytic testing, and DFT calculations revealed a volcano-type relationship between nickel loading and catalytic performance. At the optimal nickel loading of 1.5 wt %, the catalyst exhibited a total alcohol selectivity of 98.5% on a CO2-free basis (87.1% with CO2 included), while hydrocarbon selectivity was suppressed to only 1.5% (1.4% with CO2 included). The alcohol distribution was dominated by C3−C5 linear alcohols, with 1-propanol (25.4%), 1-butanol (20.0%), and 1-pentanol (10.8%) as major products, while methanol was completely suppressed. DFT calculations further revealed that among the considered 0/3, 1/3, 2/3, and 3/3 models, the 1/3 Ni-promoted edge model exhibits the most favorable combination of coordinatively unsaturated sites and high density of states near the Fermi level, correlating directly with the optimal experimental performance. Below optimal loading, the NiMoS phase resulted in moderate alcohol selectivity (65–90%) with significant hydrocarbon formation. Above optimal loading, excess nickel led to the formation of separate Ni phases and oxysulfide species that promoted undesired hydrogenation and water-gas shift reactions, shifting selectivity toward hydrocarbons (up to 42%) and shorter-chain alcohols. Bimetallic promotion with cobalt (KÑixCoMoS/CNF) produced a distinct product slate dominated by ethanol (30.7%) and methanol (17.5%), with a total alcohol selectivity of 90%, reflecting competitive edge-site occupation by cobalt. These findings establish that precise control of nickel loading is essential for maximizing the NiMoS phase and achieving superior alcohol selectivity with extended chain growth to C3−C5 products.