Supervised Machine-Learning-Based Determination of Three-Dimensional Structure of Metallic Nanoparticles

Tracking the structure of heterogeneous catalysts under operando conditions remains a challenge due to the paucity of experimental techniques that can provide atomic-level information for catalytic metal species. Here we report on the use of X-ray absorption near-edge structure (XANES) spectroscopy and supervised machine learning (SML) for refining the 3D geometry of metal catalysts. SML is used to unravel the hidden relationship between the XANES features and catalyst geometry. To train our SML method, we rely on ab initio XANES simulations. Our approach allows one to solve the structure of a metal catalyst from its experimental XANES, as demonstrated here by reconstructing the average size, shape, and morphology of well-defined platinum nanoparticles. This method is applicable to the determination of the nanoparticle structure in operando studies and can be generalized to other nanoscale systems. It also allows on-the-fly XANES analysis and is a promising approach for high-throughput and time-dependent studies

Monoethanolamine Adsorption on TiO₂(110): Bonding, Structure, and Implications for Use as a Model Solid-Supported CO₂ Capture Material

We have studied the adsorption of monoethanolamine (MEA, HO­(CH₂)₂NH₂), a well-known CO₂ capture molecule, on the rutile TiO₂(110) surface using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy measurements indicate that MEA adsorbs with the oxygen atom of the hydroxyl group and the nitrogen atom of the amine group bonded to adjacent 5-fold coordinated Ti-sites (Ti­(5f)) in the Ti-troughs, leading to a saturation coverage of 0.5 ML at room temperature. Density functional theory calculations confirm that this adsorption configuration is the most stable one with an adsorption energy of 2.33 eV per MEA molecule. The bonding of MEA to TiO₂(110) is dominated by local donor–acceptor bonds between the oxygen and nitrogen atoms of the MEA molecule and surface Ti­(5f) sites. Hydrogen bonds between adjacent MEA molecules stabilize the adsorption structure at saturation coverage. The implications of this bonding configuration for the use of MEA/TiO₂(110) as a model CO₂ capture material will be discussed

Accurate, Uncertainty-Aware Classification of Molecular Chemical Motifs from Multimodal X‑ray Absorption Spectroscopy

Accurate classification of molecular chemical motifs from experimental measurement is an important problem in molecular physics, chemistry, and biology. In this work, we present neural network ensemble classifiers for predicting the presence (or lack thereof) of 41 different chemical motifs on small molecules from simulated C, N, and O K-edge X-ray absorption near-edge structure (XANES) spectra. Our classifiers not only achieve class-balanced accuracies of more than 0.95 but also accurately quantify uncertainty. We also show that including multiple XANES modalities improves predictions notably on average, demonstrating a “multimodal advantage” over any single modality. In addition to structure refinement, our approach can be generalized to broad applications with molecular design pipelines

Surface Proton Transfer Promotes Four-Electron Oxygen Reduction on Gold Nanocrystal Surfaces in Alkaline Solution

Four-electron oxygen reduction reaction (4e-ORR), a key pathway in energy conversion, is preferred over the two-electron reduction pathway that falls short in dissociating dioxygen molecules. Gold surfaces exhibit high sensitivity of the ORR pathway to its atomic structures. A long-standing puzzle remains unsolved: why the Au surfaces with {100} sub-facets were exceptionally capable to catalyze the 4e-ORR in alkaline solution, though limited within a narrow potential window. Herein we report the discovery of a dominant 4e-ORR over the whole potential range on {310} surface of Au nanocrystal shaped as truncated ditetragonal prism (TDP). In contrast, ORR pathways on single-crystalline facets of shaped nanoparticles, including {111} on nano-octahedra and {100} on nanocubes, are similar to their single-crystal counterparts. Combining our experimental results with density functional theory calculations, we elucidate the key role of surface proton transfers from co-adsorbed H₂O molecules in activating the facet- and potential-dependent 4e-ORR on Au in alkaline solutions. These results elucidate how surface atomic structures determine the reaction pathways via bond scission and formation among weakly adsorbed water and reaction intermediates. The new insight helps in developing facet-specific nanocatalysts for various reactions

Key Structure–Property Relationships in CO₂ Capture by Supported Alkanolamines

Heterogeneous interfaces exhibit remarkable material properties resulting from their structural motifs, the judicious placement of functional chemical groups, etc. It has been a long-standing challenge to manipulate and design interface structures at the atomic level to achieve new functionalities. Here, we demonstrate that by modifying the length of the backbone in alkanolamines one can control the packing density of organic monolayers adsorbed on rutile TiO₂ and the interaction strength between their amine functional group and the substrate. As a result, we observed strikingly different activities in CO₂ capture by the amine functional group of different alkanolamines on TiO₂(110). Synchrotron photoelectron spectroscopy at near-ambient CO₂ pressures showed that adsorbed 2-amino-1-ethanol (monoethanolamine, MEA) is inactive, whereas the amine group in 3-amino-1-propanol (3AP)/TiO₂(110) readily reacts with and captures CO₂. Our results suggest that the geometry of the interface plays a decisive role in the reactivity of adsorbed functionalized organic molecules, such as solid-supported alkanolamines for CO₂ capture

Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by <i>in Situ</i> X‑ray Absorption Fingerprints

Zero-strain electrodes, such as spinel lithium titanate (Li_4/3Ti_5/3O₄), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. On the other hand, this same property makes it challenging to probe their structural changes during the electrochemical reaction. Herein, we report <i>in situ</i> studies of lithiation-driven structural transformations in Li_4/3Ti_5/3O₄ via a combination of X-ray absorption spectroscopy and <i>ab initio</i> calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small variation in the crystal lattice during lithiation, pronounced structural transformations occur in Li_4/3Ti_5/3O₄, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations. This work highlights the unique capability of combining <i>in situ</i> core-level spectroscopy and <i>first-principles</i> calculations for probing Li-ion intercalation in zero-strain electrodes, which is crucial to designing high-performance electrode materials for long-life batteries