Predicting Catalytic Activity of Nanoparticles by a DFT-Aided Machine-Learning Algorithm

Catalytic activities are often dominated by a few specific surface sites, and designing active sites is the key to realize high-performance heterogeneous catalysts. The great triumphs of modern surface science lead to reproduce catalytic reaction rates by modeling the arrangement of surface atoms with well-defined single-crystal surfaces. However, this method has limitations in the case for highly inhomogeneous atomic configurations such as on alloy nanoparticles with atomic-scale defects, where the arrangement cannot be decomposed into single crystals. Here, we propose a universal machine-learning scheme using a local similarity kernel, which allows interrogation of catalytic activities based on local atomic configurations. We then apply it to direct NO decomposition on RhAu alloy nanoparticles. The proposed method can efficiently predict energetics of catalytic reactions on nanoparticles using DFT data on single crystals, and its combination with kinetic analysis can provide detailed information on structures of active sites and size- and composition-dependent catalytic activities

Extrapolating Energetics on Clusters and Single-Crystal Surfaces to Nanoparticles by Machine-Learning Scheme

A Bayesian linear regression scheme using a local structural similarity kernel as a descriptor is used to predict the energetics of atoms and molecules on nanoparticles. Examination of the binding energies of N, O, and NO with RhAu alloy single-crystal surfaces and particles indicates that regression models predict the binding energies on nanoparticles having diameters greater than 1.5 nm within an error range of 100–150 meV when the DFT data on single-crystal surfaces are used for training. By contrast, when the DFT data on small clusters are used for training, the regression models produce an error range of 200–400 meV. Kinetic analyses using the predicted energetics of the direct decomposition of NO on RhAu nanoparticles indicate that catalytic activity increases with a decrease in the particle diameter to 2.0 nm, whereas the activity drops when the diameter decreases to 1.5 nm. Detailed examinations of the free energy diagrams and the structures of active sites indicate that the drop in catalytic activity derives from the disappearance of active alloyed corner sites on the small nanoparticles as a result of Au segregation at the corners of narrow facets

What Makes the Photocatalytic CO₂ Reduction on N‑Doped Ta₂O₅ Efficient: Insights from Nonadiabatic Molecular Dynamics

Recent experimental studies demonstrated that photocatalytic CO₂ reduction by Ru catalysts assembled on N-doped Ta₂O₅ surface is strongly dependent on the nature of the anchor group with which the Ru complexes are attached to the substrate. We report a comprehensive atomistic analysis of electron transfer dynamics in electroneutral Ru­(di-X-bpy) (CO)₂Cl₂ complexes with X = COOH and PO₃H₂ attached to the N–Ta₂O₅ substrate. Nonadiabatic molecular dynamics simulations indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO₃H₂ groups, due to larger nonadiabatic coupling. Quantum coherence counteracts this effect, however, to a small extent. The COOH anchor promotes the transfer with significantly higher frequency modes than PO₃H₂, due to both lighter atoms (C vs P) and stronger bonds (double vs single). The acceptor state delocalizes onto COOH, but not PO₃H₂, further favoring electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition, in contrast to PO₃H₂, making the former show smaller turnover numbers in some cases. These theoretical predictions are consistent with recent experimental results, legitimating the proposed mechanism of the electron transfer. We emphasize the role of anchor stability, nonadiabatic coupling, and quantum coherence in determining the overall efficiency of artificial photocatalytic systems

What Makes the Photocatalytic CO₂ Reduction on N‑Doped Ta₂O₅ Efficient: Insights from Nonadiabatic Molecular Dynamics

Recent experimental studies demonstrated that photocatalytic CO₂ reduction by Ru catalysts assembled on N-doped Ta₂O₅ surface is strongly dependent on the nature of the anchor group with which the Ru complexes are attached to the substrate. We report a comprehensive atomistic analysis of electron transfer dynamics in electroneutral Ru­(di-X-bpy) (CO)₂Cl₂ complexes with X = COOH and PO₃H₂ attached to the N–Ta₂O₅ substrate. Nonadiabatic molecular dynamics simulations indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO₃H₂ groups, due to larger nonadiabatic coupling. Quantum coherence counteracts this effect, however, to a small extent. The COOH anchor promotes the transfer with significantly higher frequency modes than PO₃H₂, due to both lighter atoms (C vs P) and stronger bonds (double vs single). The acceptor state delocalizes onto COOH, but not PO₃H₂, further favoring electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition, in contrast to PO₃H₂, making the former show smaller turnover numbers in some cases. These theoretical predictions are consistent with recent experimental results, legitimating the proposed mechanism of the electron transfer. We emphasize the role of anchor stability, nonadiabatic coupling, and quantum coherence in determining the overall efficiency of artificial photocatalytic systems

Upward Shift in Conduction Band of Ta₂O₅ Due to Surface Dipoles Induced by N‑Doping

Density functional theory calculations were executed to clarify the mechanism of the experimentally observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta₂O₅, which is used as a photosensitizer in CO₂ reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of nondoped Ta₂O₅ and N-doped Ta₂O₅. Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta₂O₅, and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface is not high enough to compensate positive charges induced at the surface defects, the remaining positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta₂O₅, allowing photogenerated electrons to transfer from N-doped Ta₂O₅ to the catalytic active sites for CO₂ reduction as realized with Ru complex on the surface in experiment

Effects of Ta₂O₅ Surface Modification by NH₃ on the Electronic Structure of a Ru-Complex/N–Ta₂O₅ Hybrid Photocatalyst for Selective CO₂ Reduction

This work examined a Ru-complex/N–Ta₂O₅ (N–Ta₂O₅: nitrogen-doped Ta₂O₅) hybrid photocatalyst for CO₂ reduction. In this material, electrons are transferred from the N–Ta₂O₅ to the Ru-complex in response to visible light irradiation, after which CO₂ reduction occurs on the complex. N-doping is believed to produce an upward shift in the conduction band minimum (CBM) of the Ta₂O₅, thus allowing more efficient electron transfer, although the associated mechanism has not yet been fully understood. In the present study, the effects of NH₃ adsorption (the most likely surface modification following nitrification) were examined using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy data suggest that NH₃ molecules are adsorbed on the N–Ta₂O₅ surface, and it is also evident that the photocatalytic activity of the Ru-complex/N–Ta₂O₅ is decreased by the removal of this adsorbed NH₃. Calculations show that both the occupied and unoccupied orbital levels of Ta₁₆O₄₀(NH₃)_<i>x</i> clusters (<i>x</i> = 4, 8, 12, or 16) are shifted upward as <i>x</i> is increased. Theoretical analyses of Ru-complex/cluster hybrids demonstrate that the gap between the lowest unoccupied molecular orbital of the Ta₁₆O₄₀ moiety and the unoccupied orbitals of the Ru-complex in Ru-complex/Ta₁₆O₄₀(NH₃)₁₂ is much smaller than that in Ru-complex/Ta₁₆O₄₀. The highest occupied molecular orbital of [Ru-complex/Ta₁₆O₄₀]⁻ is evidently localized on the Ta₁₆O₄₀ moiety, whereas that of [Ru-complex/Ta₁₆O₄₀(NH₃)₁₂]⁻ is spread over both the Ta₁₆O₄₀ and Ru-complex. These results indicate that the NH₃ adsorption associated with N-doping can result in an upward shift of the CBM of Ta₂O₅. Additional calculations for Ta₁₆O_{40–<i>y</i>}(NH)_<i>y</i> (<i>y</i> = 2, 4, 6, 8, or 10) suggest that the substitution of NH groups for oxygen atoms on the Ta₂O₅ surface may be responsible for the red shift in the adsorption band edge of the oxide but makes only a minor contribution to the upward shift of the CBM