Molybdenum sulfide-based materials for mixed alcohols synthesis and hydrogen evolution

Zeng, Feng; Palkovits, Regina (Thesis advisor); Mayrhofer, Karl J. J. (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Molybdenum sulfide is a promising non-noble metal-based material for mixed alcohols synthesis and electrochemical hydrogen evolution. Synthesis of molybdenum sulfide and tailoring its structure by facile and scalable methods are of great importance for its practical applications in catalysis. In this research, molybdenum sulfide was synthesized by two different facile and scalable wet chemical methods. With the first method, amorphous molybdenum sulfide was prepared by precipitation of (NH4)6Mo7O24 and NaS2. With the second method, cobalt-promoted molybdenum sulfide was prepared by the sulfurization of cobalt-promoted molybdenum oxide with KSCN under hydrothermal conditions. These materials were used as a catalyst or pre-catalyst for mixed alcohols synthesis and electrochemical hydrogen evolution. In Chapter 2, the amorphous molybdenum sulfide as a precursor for mixed alcohols synthesis was investigated. A varying amount of potassium was incorporated to prepare a series of potassium-promoted molybdenum sulfides containing MoS2 and KMoS2 phases. Under proper reaction conditions, the optimized nanostructured multilayer catalyst with well contacted MoS2 and KMoS2 phase achieved high liquid oxygenate (mainly alcohols) selectivity and yield of 29.1-32.7% and 7.9-10.6%, respectively, yet with good stability. The formation of C3+ alcohols was enhanced due to the intensified carbon chain growth benefiting from the well contacted MoS2 and KMoS2 phases. In Chapter 3, the cobalt-promoted molybdenum sulfide for CO conversion to mixed alcohols was studied. By tailoring the Co/(Co + Mo) ratio, a series of cobalt-promoted molybdenum sulfide containing MoS2, Co-Mo-S, and CoS2 can be obtained. Further promotion with potassium can activate the materials for mixed alcohols synthesis. By optimizing the Co/(Co + Mo) ratio and the reaction conditions, high liquid oxygenate (mainly alcohols) selectivity and yield of 70.4% and 10.6% were obtained with enhanced C3+ alcohols production and good stability. The enhancement was ascribed to the enhanced carbon chain growth due to the effective synergism between unpromoted MoS2, Co-Mo-S, and CoS2, and the potassium-promoted KCoMoS or KMoS2 species. In Chapter 4, the amorphous molybdenum sulfide as a precursor for electrochemical hydrogen evolution was investigated. The amorphous molybdenum sulfide was annealed at elevated temperatures to obtain varying molybdenum sulfide structures. Carbon nanotubes were also incorporated to increase electric conductivity and tailor the nature of the active sites. By a combination of structure modification and carbon nanotubes promotion, the synergy between active sites and electric conductivity possessing high exposure/availability of active sites, high intrinsic activity, and high electric conductivity enabled high hydrogen evolution performance with a low overpotential of 154 mV and a low Tafel slope of 31 mV dec−1 together with a good stability. In Chapter 5, the application of cobalt-promoted molybdenum sulfide for electrochemical hydrogen evolution was studied. The cobalt-promoted molybdenum sulfide containing MoS2, Co-Mo-S, CoS2, and CoMoO4 possessed different exposure/availability of active sites, intrinsic activity, and electric conductivity. Carbon nanotubes were also added to increase the electric conductivity and tailor the nature of the active sites. A medium Co/(Co + Mo) ratio together with carbon nanotubes promotion allowed a hydrogen evolution activity with a low overpotential of 210 mV and a low Tafel slope of 44 mV dec−1 due to the high exposure/availability of active sites, high intrinsic activity, and high electric conductivity. However, the catalysts are not stable for long-term hydrogen evolution.In summary, molybdenum sulfide-based materials were prepared by facile and scalable methods. By tailoring the structure of molybdenum sulfide with proper promotions, the materials can be activated for mixed alcohols synthesis and electrochemical hydrogen evolution.