Exploring the Sub-nanoscale Structure of Cobalt Molybdenum Sulfide and the Role of a Cobalt Promoter in Catalytic Hydrogen Evolution

Abstract

Cobalt-promoted molybdenum sulfide (CoMoS) is known as a promising catalyst for H2 evolution reaction and hydrogen desulfurization reaction. This material exhibits superior catalytic activity as compared to its pristine molybdenum sulfide counterpart. However, revealing the actual structure of cobalt-promoted molybdenum sulfide as well as the plausible contribution of a cobalt promoter is still challenging, especially when the material has an amorphous nature. Herein, we report, for the first time, on the use of positron annihilation spectroscopy (PAS), being a nondestructive nuclear radiation-based method, to visualize the position of a Co promoter within the structure of MoS at the atomic scale, which is inaccessible by conventional characterization tools. It is found that at low concentrations, a Co atom occupies preferably the Mo-vacancies, thus generating the ternary phase CoMoS whose structure is composed of a Co-S-Mo building block. Increasing the Co concentration, e.g., a Co/Mo molar ratio of higher than 1.12/1, leads to the occupation of both Mo-vacancies and S-vacancies by Co. In this case, secondary phases such as MoS and CoS are also produced together with the CoMoS one. Combining the PAS and electrochemical analyses, we highlight the important contribution of a Co promoter to enhancing the catalytic H2 evolution activity. Having more Co promoter in the Mo-vacancies promotes the H2 evolution rate, whereas having Co in the S-vacancies causes a drop in H2 evolution ability. Furthermore, the occupation of Co to the S-vacancies leads also to the destabilization of the CoMoS catalyst, resulting in a rapid degradation of catalytic activity.

Sulfur Vacancy-Engineered Co3S4/MoS2-Interfaced Nanosheet Array for Enhanced Alkaline Overall Water Splitting

“…In the high-resolution Mo 3d spectrum (Figure e) of Co 3 S 4 /MoS 2 , the spin–orbit splitting peaks are observed at 232.8 and 228.9 eV, corresponding to Mo 4+ 3d 3/2 and 3d 5/2 , respectively. ,, In S v -Co 3 S 4 /MoS 2 , a shift to lower binding energy is discovered for both Mo- and Co-related peaks compared with Co 3 S 4 /MoS 2 , suggesting a higher electron density at the metal sites. Regarding the high-resolution XPS S 2p region (Figure f), the peaks centered at 161.9 and 163.9 eV correspond to the S 2p 3/2 and S 2p 1/2 of the Co–S bands, respectively. , Additionally, the peaks at 163.0 and 164.7 eV are characteristic of the S 2p 3/2 and S 2p 1/2 of the Mo–S bands, respectively. , These results verify the presence of incorporated S defects in S v -Co 3 S 4 /MoS 2 , leading to the decreased valence states of the metal sites. Moreover, the electron-rich low-valence-state metal sites may be beneficial to the electrochemical catalytic reactions (such as HER) in alkaline conditions …”

“…43,44 Additionally, the peaks at 163.0 and 164.7 eV are characteristic of the S 2p 3/2 and S 2p 1/2 of the Mo−S bands, respectively. 24,39 These results verify the presence of incorporated S defects in S v -Co 3 S 4 /MoS 2 , leading to the decreased valence states of the metal sites. Moreover, the electron-rich low-valence-state metal sites may be beneficial to the electrochemical catalytic reactions (such as HER) in alkaline conditions.…”

“…This observation suggests an increased concentration of dangling bonds originating from the S vacancies in S v -Co 3 S 4 /MoS 2 . ,− Based on inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis (Tables S1 and S2), the weight ratio of Co, Mo, and S in S v -Co 3 S 4 /MoS 2 is determined to be 1.35:0.84:1, which is higher than that of Co 3 S 4 /MoS 2 (1.09:0.65:1). This result further implies the existence of sulfur vacancies. ,,− The valence states of Co, Mo, and S in Co 3 S 4 /MoS 2 and S v -Co 3 S 4 /MoS 2 were analyzed by X-ray photoelectron spectroscopy (XPS). The high-resolution XPS Co 2p spectra of Co 3 S 4 /MoS 2 (Figure d) exhibit four main peaks at 778.8 eV (Co 3+ 2p 3/2 ), 781.4 eV (Co 2+ 2p 3/2 ), 793.9 eV (Co 3+ 2p 1/2 ), and 797.9 eV (Co 2+ 2p 1/2 ), which are consistent with previously reported Co 3 S 4 .…”

Roles of the Comproportionation Reaction in SO2 Reduction Using Methane for the Flexible Recovery of Elemental Sulfur or Sulfides

“…37 The Mo atoms at the edge of the surface combine with the neighboring promoter Co atoms to form CoMoS. 38,39 Therefore, CoMoO of CoMo(O)/Al 2 O 3 reacts with H 2 S to form CoMoS.…”

Construct of an Electrodeposited Cobalt–Molybdenum Film and Evaluation of Its Efficiency in Hydrogen Evolution

“…Because Co (d7) has a low hydrogen adsorption strength and Mo (d5) has a high hydrogen adsorption strength, the two alloys can draw on each other’s strengths to increase hydrogen adsorption strength and catalytic activity. − In addition, binary catalysts consisting of Mo and Co elements have been used as cathodes for hydroelectrolysis. They have demonstrated good catalytic properties due to either crystalline structure modification − or electronic formation. − Besides, the self-supporting electrode allows for better contact between the catalyst and the reactant on the surface. − …”