Bora Seo

Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in the Hydrogen Evolution Reaction

Metal sulfide-based nanostructured materials have emerged as promising catalysts for hydrogen evolution reaction (HER), and significant progress has been achieved in enhancing their activity and durability for the HER. The understanding of nanoscale size-dependent catalytic activities can suggest critical information regarding catalytic reactivity, providing the scientific basis for the design of advanced catalysts. However, nanoscale size effects in metal sulfide-based HER catalysts have not yet been established fully, due to the synthetic difficulty in precisely size-controlled metal sulfide nanoparticles. Here we report the preparation of molybdenum sulfide (MoS2) nanoparticles with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm, and their size-dependent catalytic activity in the HER. Using density functional theory (DFT) calculations, we identified the most favorable single-, double-, and triple-layer MoS2 model structures for the HER, and calculated elementary step energetics of the HER over these three model structures. Combining HER activity measurements and the DFT calculation results, we establish that the turnover frequency of MoS2 nanoparticles in the HER increases in a quasi-linear manner with decreased layer numbers. Cobalt-promoted MoS2 nanoparticles also exhibited similar HER activity trend. We attribute the higher HER activity of smaller metal sulfide nanoparticles to the higher degree of oxidation, higher Mo-S coordination number, formation of the 1T phase, and lower activation energy required to overcome transition state. This insight into the nanoscale size-dependent HER activity trend will facilitate the design of advanced HER catalysts as well as other hydrotreating catalysts.

Nature of Rh Oxide on Rh Nanoparticles and Its Effect on the Catalytic Activity of CO Oxidation

Surface oxide layers formed on transition metal catalysts are well known as one of the controlling factors in enhancing or suppressing the catalytic activity of metal catalysts. We investigated the growth of a surface oxide layer on two sizes of Rh metal nanoparticles (NPs) as a function of UV–ozone (UV/O3) dosing as well as how the oxide layer formed on the surface of the Rh NPs affects the catalytic activity for CO oxidation. Monodisperse Rh NPs were synthesized via one-pot polyol reduction using poly(vinylpyrrolidone) as a capping agent. Varying the concentration of the Rh precursors controlled the size of the NPs. The changes that occurred as a function of UV/O3 dosing were characterized using X-ray photoelectron spectroscopy, which showed that the oxidation state increased with increasing surface modification time. The catalytic activity and activation energy of the two-dimensional Rh NPs arrays were measured as the UV/O3 exposure time increased. Our reaction studies indicate that the turnover rate of CO oxidation on the Rh NPs is enhanced as the quantity of the surface oxide layer formed during UV/O3 surface treatment increases, indicating that the oxides grown on the surface of the Rh metal are catalytically active. These results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide.Graphical Abstract

Recent advances in unveiling active sites in molybdenum sulfide-based electrocatalysts for the hydrogen evolution reaction

Hydrogen has received significant attention as a promising future energy carrier due to its high energy density and environmentally friendly nature. In particular, the electrocatalytic generation of hydrogen fuel is highly desirable to replace current fossil fuel-dependent hydrogen production methods. However, to achieve widespread implementation of electrocatalytic hydrogen production technology, the development of highly active and durable electrocatalysts based on Earth-abundant elements is of prime importance. In this context, nanostructured molybdenum sulfides (MoSx) have received a great deal of attention as promising alternatives to precious metal-based catalysts. In this focus review, we summarize recent efforts towards identification of the active sites in MoSx-based electrocatalysts for the hydrogen evolution reaction (HER). We also discuss recent synthetic strategies for the engineering of catalyst structures to achieve high active site densities. Finally, we suggest ongoing and future research challenges in the design of advanced MoSx-based HER electrocatalysts.