The first topic of this talk is focused on the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst (SAAC) Pd/Cu(111) [1]. The hydrogen spillover on the Cu(111) surface from the Pd site was successfully observed in real-time using infrared reflection absorption spectroscopy (IRAS) at 80 K. The observed chemical shifts of Pd 3d5/2 in X-ray photoelectron spectra (XPS) indicate that H2 is dissociated and adsorbed at the Pd site initially. The “two-step” chemical shifts of the Pd 3d5/2 binding energy have been observed. The proposed mechanism of the hydrogen dissociation and spillover processes includes (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 at the Pd site increases up to three, and (iii) the hydrogen atoms will spill over onto the Cu surface. Systematic DFT calculations revealed the adsorption configurations during the hydrogen adsorption and spillover processes on the Pd/Cu(111) surface and their chemical shifts of Pd 3d.
The second topic is the activation, spillover and reaction of hydrogen on the MoS2 basal surface using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). In the Mo 3d core-level, S 2p core-level, and valence-band photoelectron spectra of a bare MoS2 surface, little change was observed in the energy shift before and after exposure to hydrogen gas at 300 K. On the other hand, on the Pd-deposited MoS2 surface, each peak in the photoelectron spectra (Mo 3d, S 2p, and valence-band) shifted to a lower binding energy with 0.1 eV. When hydrogen gas was evacuated, the peak energy remained lower compared with that before hydrogen gas exposure. The Pd 3d XPS spectra changed upon hydrogen gas exposure, which can be interpreted as the adsorption of dissociated hydrogen atoms on the Pd sites. These results indicate that the dissociation of molecular hydrogen and the adsorption of atomic hydrogen occur on the Pd-deposited sites on MoS2, and thereafter hydrogen atoms spillover onto the MoS2 surface.
On the other hand, by heating the MoS2 surface in hydrogen above 600 K, the present AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed (hydrodesulfurization). 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. Mulliken charge analysis indicates that the core-level shifts are caused by an increase in the number of electrons in Mo and S atoms around the sulfur vacancy compared to the pristine surface.

[1] W. Osada et al., Phys. Chem. Chem. Phys., 24, 21705 (2022).
[2] F. Ozaki et al., Applied Surface Science, 593, 153313 (2022).
[3] F. Ozaki et al., to be submitted (2023).