Xingchen Liu

Insight into the structure and energy of Mo27SxOy clusters

Oxygen incorporated molybdenum sulfide (MoS2) nanoparticles are highly promising materials in hydrodesulfurization catalysis, mechanical, electric, and optical applications. We report a systematic theoretical study of the successive oxidation reactions of the Mo27SxOy nanoparticles and the reaction network, along with the electronic structure changes caused by the oxygen substitution. The replacement of surface sulfur by oxygen atoms is thermodynamically favorable. Our results indicate that the oxidation on the S edge with 100% and 50% coverage is more favored than on the Mo edge. Meanwhile, it is found that the oxidation on the S edge with 100% coverage has similar replacement ability by oxygen as on the S edge with 50% coverage. Thus, sulfur coverage does not play an important role in the oxidation on the S edge. Further comparison shows that the oxidation on corner sites is more favorable than on edge sites. In addition, the replacement of the bulky sulfur on the Mo edge is equally as favored as those of sulfur on the S edge. This work provides important information on the thermodynamics of the Mo27SxOy nanoparticles, and gives new insights into the mechanism of the oxidation of MoS2 and the sulfidation of MoO3.

Tuning Gold Nanoparticles with Chelating Ligands for Highly Efficient Electrocatalytic CO2 Reduction

Capped chelating organic molecules are presented as a design principle for tuning heterogeneous nanoparticles for electrochemical catalysis. Gold nanoparticles (AuNPs) functionalized with a chelating tetradentate porphyrin ligand show a 110-fold enhancement compared to the oleylamine-coated AuNP in current density for electrochemical reduction of CO2 to CO in water at an overpotential of 340 mV with Faradaic efficiencies (FEs) of 93 %. These catalysts also show excellent stability without deactivation (<5 % productivity loss) within 72 hours of electrolysis. DFT calculation results further confirm the chelation effect in stabilizing molecule/NP interface and tailoring catalytic activity. This general approach is thus anticipated to be complementary to current NP catalyst design approaches.

How far away are iron carbide clusters from the bulk

Combining the basin hopping structure searching algorithm and density functional theory, the iron carbide clusters, FexCy (x ≤ 8 and y ≤ 8), and clusters with various stoichiometries (Fe2nCn, Fe3nCn, FenC2n, FenC3n and FenC4n (n = 1-7), Fe5nC2n, and Fe4nCn (n = 1-5)) are predicted. The stable structures of iron rich carbide clusters are composed of C-C dimers or single C atoms on the surface of the clusters, which are remarkably different from their corresponding bulk structures, where the carbon atoms are atomically distributed in the iron matrix. The most stable carbon rich clusters are highly diverse in topology (bowl, basket, plane, shoe, necklace, etc.) with long carbon chains. The Bader charge analysis shows that the size effect on iron carbide clusters is an electronic tuning. Large carbon-rich clusters appear even under low carbon chemical potentials, whereas small iron-rich clusters are only energetically stable in high carbon chemical potentials, which indicates that changing the carbon chemical potential can tune the morphology (size and stoichiometry) of the iron carbide clusters. These results may help us understand the catalytic activity of iron and iron carbides in reactions such as the Fischer-Tropsch synthesis and the carbon nanotube formation process.