High or Low Coordination: Insight into the Active Site of Pt Nanoparticles toward CO Oxidation.

The catalytic activity of metal nanoparticles (NPs) is highly dependent on the coordination environment of the surface sites. Understanding the role of different sites in reactions is essential for gaining insights into catalytic activity and the precise design of catalysts. Herein, we used first-principles calculation-based kinetic Monte Carlo simulations to investigate correlations between different sites on Pt NPs in CO oxidation reactions. Low-coordinated (LC) sites favor the CO adsorption and reaction, whereas the oxygen mainly adsorbs on high-coordinated (HC) sites and diffuses to LC sites for reaction at low temperatures. Compared with step-dominated and terrace-dominated structures, the step-terrace structures exhibit higher activities. This reveals that the catalytic performance is not simply determined by the sites where the reaction occurs but is dramatically affected by the kinetic synergies between different sites. A proper way to optimize the activity of Pt catalysts is to balance the LC and HC sites.

An antioxidation strategy based on ultra-small nanobubbles without exogenous antioxidants.

Antioxidation is in demand in living systems, as the excessive reactive oxygen species (ROS) in organisms lead to a variety of diseases. The conventional antioxidation strategies are mostly based on the introduction of exogenous antioxidants. However, antioxidants usually have shortcomings of poor stability, non-sustainability, and potential toxicity. Here, we proposed a novel antioxidation strategy based on ultra-small nanobubbles (NBs), in which the gas-liquid interface was employed to enrich and scavenge ROS. It was found that the ultra-small NBs (~ 10 nm) exhibited a strong inhibition on oxidization of extensive substrates by hydroxyl radicals, while the normal NBs (~ 100 nm) worked only for some substrates. Since the gas-water interface of the ultra-small NBs is non-expendable, its antioxidation would be sustainable and its effect be cumulative, which is different to that using reactive nanobubbles to eliminate free radicals as the gases are consumptive and the reaction is unsustainable. Therefore, our antioxidation strategy based on ultra-small NB would provide a new solution for antioxidation in bioscience as well as other fields such as materials, chemical industry, food industry, etc.

Identifying the morphology of Pt nanoparticles for the optimal catalytic activity towards CO oxidation.

The morphology of nanoparticles (NPs) is crucial for determining their catalytic performance. The dramatic changes in the morphology of metal NPs during reactions observed in many in situ experiments pose great challenges for the identification of the geometry for optimal catalytic activities, which arouses the controversial understanding of the reaction mechanism. In this work, taking CO oxidation as a model reaction, we coupled a multiscale structure reconstruction model with kinetic Monte Carlo simulations to study the catalytic performance of the Pt NPs with changing morphology and reaction conditions. Through the quantitative analysis of contour plots for turnover frequencies, we show that the NPs with more well-coordinated sites exhibit optimal activity under CO-rich conditions at higher temperatures, while the reactivity of NPs with more low-coordination sites is optimal under O2-rich conditions at lower temperatures. Further analysis indicates that the competitive adsorption of CO and O2 plays the key role, in which the structure with optimal activity has a closer CO and O coverage. This work not only reconciles the controversy of the active geometry in the experiments, but offers an efficient method to guide the rational design of high-performance catalysts.

Reply to the 'Comment on "Real-time atomistic simulation of the Ostwald ripening of TiO2 supported Au nanoparticles"' by V. P. Zhdanov, Nanoscale, 2022, 14, DOI: 10.1039/D1NR05352C.

Dr Zhdanov argues that the model in our Nanoscale paper cannot prove the Au dimer as the primary species in the Ostwald ripening of Au nanoparticles on TiO2. We would like to point out that the relative stability of the monomer and dimer of Au has been greatly changed on the substrate compared to the gas phase. Thus, the conclusion of the original paper is solid, as proved by not only the simulations but also the further density functional theory calculations, and relevant discussions in the main text.

Unveiling the Au Surface Reconstruction in a CO Environment by Surface Dynamics and Ab Initio Thermodynamics.

Surface reconstruction changes the atomic configuration of the metal surface and thus alters its intrinsic physical and chemical properties. Recent in situ experiments have shown a variety of surface reconstructions under reaction conditions, but how to effectively predict and characterize these structures remains challenging. Herein, we combine a DFT-based kinetic Monte Carlo simulation method and ab initio thermodynamics to explore the low-energy configurations of metal surface reconstructions, which takes the surface dynamics under the reactive environment into account. We systematically simulate 13 Au surfaces ((100), (110), (111), (210), (211), (221), (310), (311), (320), (321), (322), (331), and (332)) in the CO environment and identify 19 candidate reconstruction patterns driven by CO adsorption. The breakup of the original surfaces is attributed to the lateral interactions among the nearest-neighboring adsorbates. This work provides an efficient approach to unveil the reconstructed metal surface structures in reactive environments for guiding the experiments.

Investigation of the Stability and Hydrogen Evolution Activity of Dual-Atom Catalysts on Nitrogen-Doped Graphene.

Single atom catalysts (SACs) have received a lot of attention in recent years for their high catalytic activity, selectivity, and atomic utilization rates. Two-dimensional N-doped graphene has been widely used to stabilize transition metal (TM) SACs in many reactions. However, the anchored SAC could lose its activity because of the too strong metal-N interaction. Alternatively, we studied the stability and activity of dual-atom catalysts (DACs) for 24 TMs on N-doped graphene, which kept the dispersion state but had different electronic structures from SACs. Our results show that seven DACs can be formed directly compared to the SACs. The others can form stably when the number of TMs is slightly larger than the number of vacancies. We further show that some of the DACs present better catalytic activities in hydrogen evolution reaction (HER) than the corresponding SACs, which can be attributed to the optimal charge transfer that is tuned by the additional atom. After the screening, the DAC of Re is identified as the most promising catalyst for HER. This study provides useful information for designing atomically-dispersed catalysts on N-doped graphene beyond SACs.

Facet-Dependent Oxidative Strong Metal-Support Interactions of Palladium-TiO2 Determined by In Situ Transmission Electron Microscopy.

The strong metal-support interaction (SMSI) is widely used in supported metal catalysts and extensive studies have been performed to understand it. Although considerable progress has been achieved, the surface structure of the support, as an important influencing factor, is usually ignored. We report a facet-dependent SMSI of Pd-TiO2 in oxygen by using in situ atmospheric pressure TEM. Pd NPs supported on TiO2 (101) and (100) surfaces showed encapsulation. In contrast, no such cover layer was observed in Pd-TiO2 (001) catalyst under the same conditions. This facet-dependent SMSI, which originates from the variable surface structure of the support, was demonstrated in a probe reaction of methane combustion catalyzed by Pd-TiO2 . Our discovery of the oxidative facet-dependent SMSI gives direct evidence of the important role of the support surface structure in SMSI and provides a new way to tune the interaction between metal NPs and the support as well as catalytic activity.

In Situ Resolving the Atomic Reconstruction of SnO2 (110) Surface.

Understanding surface reconstruction of nanocrystals is of great importance to their applications, however it is still challenging due to lack of atomic-level structural information under reconstruction conditions. Herein, through in situ spherical aberration corrected scanning transmission electron microscopy (STEM), the reconstruction of nanocrystalline SnO2 (110) surface was studied. By identifying the precise arrangements of surface/subsurface Sn and O columns through both in situ bright-field and high-angle annular dark-field STEM images, an unexpected added Sn2O model was determined for SnO2 (110)-(1 × 2) surface. The protruded Snδ+ of this surface could act as the active sites for activating O2 molecules according to our density functional theory (DFT) calculations. On the basis of in situ observation of atomic-level reconstruction behaviors and DFT calculations, an energy-driven reconstruction process was also revealed. We anticipate this work would help to clarify the long-standing debate regarding the reconstruction of SnO2 (110) surface and its intrinsic property.

In situ identification of the metallic state of Ag nanoclusters in oxidative dispersion.

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.

Structure reconstruction of metal/alloy in reaction conditions: a volcano curve?

Recent in situ works have shown extensive evidence of the dramatic and reversible structure reconstructions of metal and alloy materials in reaction conditions. The reconstructions are of primary interest because they could lead to alternative catalytic mechanisms during real reactions. However, how the catalyst structure evolves under the pressures relevant to industrial applications (>1 atm) is so far unexplored. In our recent works, we have developed multiscale theoretical models to give reliable and precise predictions of the equilibrium shapes of metal nanoparticles and of the segregation properties of alloy surfaces at a given temperature and gas pressure. The theoretical predictions have been successfully used in interoperations of various in situ experimental observations. In this work, we applied these methods to study the detailed structural information of metal NPs and of bimetallic alloys at the temperature from 300 to 1000 K and the gas pressure from 10 to 107 Pa. The results show, in some cases, both the gas-induced shape change and the gas-induced segregation change are maximized when the gas adsorption is 'just right'. The fraction of the low-coordinated sites of the metal NP shows a volcano-like curve with pressure at a constant temperature. A similar volcano shape could also be found in the plot of the environmental segregation energy as functions of temperature and pressure. The similar gas effects at low pressure and at high pressure indicate the structural information obtained in laboratory environments (<1 atm) could be of use to understanding the catalysts structure reconstruction in industrial conditions (>1 atm).

In situ manipulation of the active Au-TiO2 interface with atomic precision during CO oxidation.

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO2) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO2 interface with the epitaxial rotation of gold nanoparticles on a TiO2 surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO2 interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.

Understanding the Chemical Insights of Staple Motifs of Thiolate-Protected Gold Nanoclusters.

Improving the fundamental understanding of the basic structures of ligand-protected gold nanoclusters is essential to their bottom-up synthesis as well as their further application explorations. The thiolate ligands that cover the central metal core in staple motifs are vital for the stability of the gold clusters. However, the knowledge about the geometrical and bonding characters of the thiolate ligands has not been fully uncovered yet. In this work, density functional theory calculations and molecular orbital analysis are applied to show that the Au atoms in the thiolate ligands are hypervalent. The chemical insights of the linear SAuS configuration as well as the lengthened AuS bond by combining the 3-center 4-electron (3c-4e) model and the well-recognized valence shell electron pair repulsion theory are revealed. Valence bond formulations of the motifs are given to provide more chemical insights, for example, the resonant structures, to show how the thiolate motif forms one covalent bond and one dative covalent bond with the Au core. This work provides a thorough understanding of the structure and bonding pattern of thiolate ligands of Au nanoclusters, which is important for the rational design of ligands-protected Au nanoclusters.

Real-time atomistic simulation of the Ostwald ripening of TiO2 supported Au nanoparticles.

Ostwald ripening (OR), one of the major processes of nanoparticle sintering, is critical for the rational design of functional nanomaterials. However, the atomistic mechanism of OR has not been fully understood, because the characterization of interparticle transport of atoms in real-time is challenging by either experiments or theoretical simulations. Thus, current understandings are based on ad hoc assumptions about the OR mechanism, which have never been confirmed yet at the atomic scale. Herein, we realized all-atom kinetic Monte Carlo simulation of sintering of TiO2 supported Au nanoparticles (NPs) through the OR mechanism at millisecond timescales. We demonstrated that the "semi-spherical" assumption should be removed. The OR process was a stagewise process determined by different rate-determining steps, which is in contrast to the single-stage presumption. Au dimers, rather than monomers as generally assumed, were exchanged among different NPs. Besides, we proposed a new kinetic model for describing the determining rate of OR without presumptions. This work brings deeper insights into the atomistic OR mechanism and also paves the way for real-time monitoring of catalyst sintering at the atomic scale.

Theoretical study of CO2 hydrogenation on Cu surfaces.

CO2 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 CO2 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 CO2 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 CO2 hydrogenation, which will be helpful for the design of Cu-based nanocatalysts.

Visualizing H2O molecules reacting at TiO2 active sites with transmission electron microscopy.

Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium "active rows" to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water-gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.

Reshaping of Metal Nanoparticles Under Reaction Conditions.

The shape of metal nanoparticles (NPs) is one of the key factors determining their catalytic reactivity. Recent in situ TEM observations show that dynamic reshaping of metal NPs occurs under the reaction conditions, which becomes a major hurdle for fully understanding catalytic mechanisms at the molecular level. This Minireview provides a summary of the latest progress in characterizing and modeling the equilibrium shape of metal NPs in various reactive environments through the joint effort of state-of-the-art in situ environmental transmission electron microscopy experiments and a newly developed multiscale structure reconstruction model. The quantitative agreement between the experimental observations and theoretical modeling demonstrate that the fundamental mechanism of the reshaping phenomenon is driven by anisotropically changed surface energies under gas adsorption. The predictable reshaping of metal NPs paves the way for the rational design of truly efficient nanocatalysts in real reactions.

Atomic Mechanism in Layer-by-Layer Growth via Surface Reconstruction.

Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO2 (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO2 experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO2. A similar phenomenon was also observed in the MoO2 dissolution procedure.

Surface Segregation in CuNi Nanoparticle Catalysts During CO2 Hydrogenation: The Role of CO in the Reactant Mixture.

Surface segregation and restructuring in size-selected CuNi nanoparticles were investigated via near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at various temperatures in different gas environments. Particularly in focus were structural and morphological changes occurring under CO2 hydrogenation conditions in the presence of carbon monoxide (CO) in the reactant gas mixture. Nickel surface segregation was observed when only CO was present as adsorbate. The segregation trend is inverted in a reaction gas mixture consisting of CO2, H2, and CO, resulting in an increase of copper concentration on the surface. Density functional theory calculations attributed the inversion of the segregation trend to the formation of a stable intermediate on the nanocatalyst surface (CH3O) in the CO-containing reactant mixture, which modifies the nickel segregation energy, thus driving copper to the surface. The promoting role of CO for the synthesis of methanol was demonstrated by catalytic characterization measurements of silica-supported CuNi NPs in a fixed-bed reactor, revealing high methanol selectivity (over 85%) at moderate pressures (20 bar). The results underline the important role of intermediate reaction species in determining the surface composition of bimetallic nanocatalysts and help understand the effect of CO cofeed on the properties of CO2 hydrogenation catalysts.

Morphology evolution of fcc Ru nanoparticles under hydrogen atmosphere.

Tuning the morphology and structural evolution of metal nanoparticles to expose specific crystal facets in a certain reaction atmosphere is conducive to designing catalysts with a high catalytic activity. Herein, coverage dependent hydrogen adsorption on seven fcc Ru surfaces was investigated using density functional theory (DFT) calculations. The morphology evolution of the fcc Ru nanoparticles under the reactive environment was further illustrated using the multiscale structure reconstruction (MSR) model, which combines the DFT results with the Fowler-Guggenheim (F-G) adsorption isotherm and the Wulff construction. At constant pressure, the shape of a fcc Ru nanoparticle changes from a rhombic dodecahedron to a truncated octahedron with an increase of the temperature. More importantly, the desired Ru morphology, with abundant open facets, was predicted to occur at a high temperature and low pressure. Our results provide an insightful understanding of the reshaping of Ru nanoparticles during real reactions, which is crucial for its rational design for use as a nanocatalyst.