A grand unified model (GUM) is developed to achieve fundamental understanding of rich structures of all 71 liganded gold clusters reported to date. Inspired by the quark model by which composite ...particles (for example, protons and neutrons) are formed by combining three quarks (or flavours), here gold atoms are assigned three 'flavours' (namely, bottom, middle and top) to represent three possible valence states. The 'composite particles' in GUM are categorized into two groups: variants of triangular elementary block Au
(2e) and tetrahedral elementary block Au
(2e), all satisfying the duet rule (2e) of the valence shell, akin to the octet rule in general chemistry. The elementary blocks, when packed together, form the cores of liganded gold clusters. With the GUM, structures of 71 liganded gold clusters and their growth mechanism can be deciphered altogether. Although GUM is a predictive heuristic and may not be necessarily reflective of the actual electronic structure, several highly stable liganded gold clusters are predicted, thereby offering GUM-guided synthesis of liganded gold clusters by design.
The structures of the metal nanoparticles are crucial for their catalytic activities. How to understand and even control the shape evolution of nanoparticles under reaction condition is a big ...challenge in heterogeneous catalysis. It has been proved that many reactive gases hold the capability of changing the structures and properties of metal nanoparticles. One interesting question is whether water vapor, such a ubiquitous environment, could induce the shape evolution of metal nanoparticles. So far this question has not received enough attention yet. In this work, we developed a model based on the density functional theory, the Wulff construction, and the Langmuir adsorption isotherm to explore the shape of metal nanoparticle at given temperature and water vapor pressure. By this model, we show clearly that water vapor could notably increase the fraction of (110) facets and decrease that of (111) facets for 3–8 nm Cu nanoparticles, which is perfectly consistent with the experimental observations. Further investigations indicate the water vapor has different effects on the different metal species (Cu, Au, Pt, and Pd). This work not only helps to understand the water vapor effect on the structures of metal nanoparticles but also proposes a simple but effective model to predict the shape of nanoparticles in certain environment.
Oxidative dispersion has been widely used in regeneration of sintered metal catalysts and fabrication of single atom catalysts, which is attributed to an oxidation-induced dispersion mechanism. ...However, the interplay of gas-metal-support interaction in the dispersion processes, especially the gas-metal interaction has not been well illustrated. Here, we show dynamic dispersion of silver nanostructures on silicon nitride surface under reducing/oxidizing conditions and during carbon monoxide oxidation reaction. Utilizing environmental scanning (transmission) electron microscopy and near-ambient pressure photoelectron spectroscopy/photoemission electron microscopy, we unravel a new adsorption-induced dispersion mechanism in such a typical oxidative dispersion process. The strong gas-metal interaction achieved by chemisorption of oxygen on nearly-metallic silver nanoclusters is the internal driving force for dispersion. In situ observations show that the dispersed nearly-metallic silver nanoclusters are oxidized upon cooling in oxygen atmosphere, which could mislead to the understanding of oxidation-induced dispersion. We further understand the oxidative dispersion mechanism from the view of dynamic equilibrium taking temperature and gas pressure into account, which should be applied to many other metals such as gold, copper, palladium, etc. and other reaction conditions.
Characterization and control of the shape of nanoparticles has a primary importance in nanoscience and nanotechnology since most of the physical and chemical properties are shape-dependent. In recent ...years, many in situ experimental observations have shown that metal nanoparticles can change their shapes and structures dramatically and reversibly under reactive gas conditions. However, despite the experimental achievements, the precise theoretical prediction of this kind of shape evolution is still a challenging and demanding task. In this work, using CO@Pt as a benchmark, we develop a multiscale structure reconstruction model to quantitatively illuminate the equilibrium geometries of metal nanoparticles at given temperature and gas pressure. This model perfectly reproduces the experimental results and explains some intriguing phenomena, including the CO-induced breakup of Pt surfaces. The shape evolution results of Pt, Pd, Cu, and Au nanoparticles under CO and NO gas environments are presented. Our study provides useful guidelines for improving and developing real catalysts.
Metal nanoparticles (NPs) dispersed on a high‐surface‐area support are normally used as heterogeneous catalysts. Recent in situ experiments have shown that structure reconstruction of the NP occurs ...in real catalysis. However, the role played by supports in these processes is still unclear. Supports can be very important in real catalysis because of the new active sites at the perimeter interface between nanoparticles and supports. Herein, using a developed multiscale model coupled with in situ spherical aberration‐corrected (Cs‐corrected) TEM experiments, we show that the interaction between the support and the gas environment greatly changes the contact surface area between the metal and support, which further leads to the critical change in the perimeter interface. The dynamic changes of the interface in reactive environments can thus be predicted and be included in the rational design of supported metal nanocatalysts. In particular, our multiscale model shows quantitative consistency with experimental observations. This work offers possibilities for obtaining atomic‐scale structures and insights beyond the experimental limits.
The structural reconstruction of supported metal nanoparticles has been investigated, including changes in the active perimeter interface between the nanoparticle and the support using a developed multiscale model coupled with in situ spherical aberration‐corrected TEM experiments. The dynamic changes of the interface in reactive environments can be predicted and considered when designing supported metal catalysts.
Pd/CeO2 has attracted great attention owing to its unique activity for methane catalytic oxidation; however, the active sites for CH4 catalytic oxidation still remain elusive, which affects the ...comprehensive understanding of the catalytic mechanism. In this work, the structures of PdO x nanoparticles (NPs) loaded on octahedrons, cubes, and rods of nanocrystal CeO2 supports were systematically studied by Cs-corrected HRTEM/STEM, XPS, and Raman spectroscopy. Our results indicate that the Pd species on CeO2 supports are morphology-dependent: PdO NPs (Pd2+) on octahedrons, PdO x (x = 1–2) clusters (1–2 nm) on cubes, and dispersed Pd4+ ions on the CeO2 rods. Additionally, the chemical states of Pd can be tuned in oxidizing/reducing atmospheres via interactions between Pd and CeO2. Detailed studies reveal that the Pd2+ species are the active centers for the catalytic oxidation of methane. The activity of Pd0 could be ascribed to Pd2+ produced through the gradual oxidation of Pd0 during the CH4 oxidation. Further, Pd4+ in the CeO2 lattice is inactive for CH4 oxidation. In situ Fourier transform infrared spectroscopy results suggest that the mechanism of CH4 oxidation reaction on PdO x /CeO2 follows the Mars–van Krevelen mechanism, and adsorbed CO can be produced in CH4 decomposition over Pd2+ in the absence of gas-phase oxygen. As revealed by density functional theory calculations, the incomplete coordination of Pd2+ ions and adjacent oxygen atoms has excellent activity in cracking the C–H bond of CH4, which leads to high methane oxidation ability.
To control the shape and structure of a metal nanoparticle (NP) is a crucial strategy to improve its catalytic properties, but the understanding and quantitative description of the structure ...reconstruction of the catalysts under reaction conditions has not been achieved. Previous works are mostly focused on the single gas conditions, which is apparently not the case in the real catalytic reactions. In this work, a multiscale structure reconstruction model is established to describe the equilibrium structures of metal NPs in a mixed gas environment quantitatively. Taking NO and CO reaction as a model system, the structures of the Pd, Pt, and Rh NPs in a large range of temperature and pressure are fully presented. Moreover, we show the variation of P NO :P CO plays the critical role in determining the structures and therefore the number of active sites of the NPs at certain conditions. This work provides an efficient model to guide the future experiments in the real reactions.
Surface composition is a critical factor determining the chemical and physical properties of bimetallic alloys, which highly depends on the alloy surface segregation behaviors. Recent in situ ...experiments revealed the dynamic and reversible alloy segregation changes in reaction conditions, leading to distinctive catalytic performances. Thus, how to predict and control the surface composition during catalytic reactions becomes crucial for nanoalloy catalysts. However, the experimental results are mostly complicated and sometimes contradictory, which can hardly be understood by current theoretical models. Herein, we propose an environmental segregation energy (E eseg), which precisely describes the surface segregation behavior at a given temperature and gas pressure. Using CO as a probe, we reveal insights into the flexible alloy segregation trends under changing gas conditions and perfectly explain the existing experimental results. The surface composition evolutions of 72 binary alloys under various CO conditions are presented in the segregation maps. This work provides a guide to high-throughput prescreening of the desired surface composition under optimal conditions, which paves the way for tuning the alloy structure in real catalytic reactions.
CO
2
reduction has attracted extensive attentions for its wide applications in chemical engineering and green chemistry. As one of major commercial catalysts, Cu have been widely studied considering ...its low price and high catalytic efficiency. However, previous studies were mostly focused on the Cu(111) surface, while other surfaces were rarely studied. In this work, we employed the density functional theory calculations to fully investigate the adsorption of all intermediates and products of CO
2
hydrogenation on three low-index surfaces as Cu(111), Cu(100), and Cu(110), which have been reported as the main facets of Cu nanoparticles under reaction conditions. Besides, the reaction pathways were also discussed. Our results indicated CO
2
hydrogenation is preferred to adopt formate pathways on the Cu surfaces, while the COOH pathway is least favorable. Moreover, Cu(100) and Cu(110) surfaces have the comparable (even better) catalytic activities compared with Cu(111) surface. This study provides the fundamental data for the adsorption and reaction of CO
2
hydrogenation, which will be helpful for the design of Cu-based nanocatalysts.
•Pd segregation on surface of AuPd bimetallic nanoparticles under CO exposure.•Pd decorates the edges of the AuPd nanoparticles.•DFT calculation, a powerful tool for DRIFTS band assignments.
Combined ...Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy under-coordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant π-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites.
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