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.
Manganese dioxide nanomaterials have wide applications in many areas from catalysis and Li−ion batteries to gas sensing. Understanding the crystallization pathways, morphologies, and formation of defects in their structure is particularly important but still a challenging issue. Herein, we employed an arsenal of X-ray diffraction (XRD), scanning electron microscopy (SEM), neutron diffraction, positron annihilation spectroscopies, and ab initio calculations to investigate the evolution of the morphology and structure of α-MnO 2 nanomaterials prepared via reduction of KMnO 4 solution with C 2 H 5 OH prior to being annealed in air at 200−600 °C. We explored a novel evolution that α-MnO 2 nucleation can be formed even at room temperature and gradually developed to α-MnO 2 nanorods at above 500 °C. We also found the existence of H + or K + ions in the [1 × 1] tunnels of α-MnO 2 and observed the simultaneous presence of Mn and O vacancies in α-MnO 2 crystals at low temperatures. Increasing the temperature removed these O vacancies, leaving only the Mn vacancies in the samples.
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