Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations.

Sulfur vacancy on an MoS2 basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS2 basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.

Carbon Nitride Loaded with an Ultrafine, Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction.

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO3 and BaLa4 Ti4 O15 .

Elucidation of the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst Pd/Cu(111).

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d5/2 indicate that H2 is dissociated and adsorbed at the Pd site. In addition, a "two-step" chemical shift of the Pd 3d5/2 binding energy was observed, indicating two types of hydrogen adsorption states at the Pd site. The proposed mechanism of the hydrogen dissociation and spillover processes is as follows: (i) a hydrogen molecule is dissociated at a Pd site, and the hydrogen atoms are adsorbed on the Pd site; (ii) the number of hydrogen atoms on the Pd site increases up to three; and (iii) the hydrogen atoms will spill over onto the Cu surface.