Approaches for Understanding and Controlling Interfacial Effects in Oxide-Supported Metal Catalysts

Unraveling the Dynamic Properties of New-Age Energy Materials Chemistry Using Advanced In Situ Transmission Electron Microscopy

The field of energy storage and conversion materials has witnessed transformative advancements owing to the integration of advanced in situ characterization techniques. Among them, numerous real-time characterization techniques, especially in situ transmission electron microscopy (TEM)/scanning TEM (STEM) have tremendously increased the atomic-level understanding of the minute transition states in energy materials during electrochemical processes. Advanced forms of in situ/operando TEM and STEM microscopic techniques also provide incredible insights into material phenomena at the finest scale and aid to monitor phase transformations and degradation mechanisms in lithium-ion batteries. Notably, the solid-electrolyte interface (SEI) is one the most significant factors that associated with the performance of rechargeable batteries. The SEI critically controls the electrochemical reactions occur at the electrode-electrolyte interface. Intricate chemical reactions in energy materials interfaces can be effectively monitored using temperature-sensitive in situ STEM techniques, deciphering the reaction mechanisms prevailing in the degradation pathways of energy materials with nano- to micrometer-scale spatial resolution. Further, the advent of cryogenic (Cryo)-TEM has enhanced these studies by preserving the native state of sensitive materials. Cryo-TEM also allows the observation of metastable phases and reaction intermediates that are otherwise challenging to capture. Along with these sophisticated techniques, Focused ion beam (FIB) induction has also been instrumental in preparing site-specific cross-sectional samples, facilitating the high-resolution analysis of interfaces and layers within energy devices. The holistic integration of these advanced characterization techniques provides a comprehensive understanding of the dynamic changes in energy materials. This review highlights the recent progress in employing state-of-the-art characterization techniques such as in situ TEM, STEM, Cryo-TEM, and FIB for detailed investigation into the structural and chemical dynamics of energy storage and conversion materials.

Quantifying Interface-Performance Relationships in Electrochemical CO2 Reduction through Mixed-Dimensional Assembly of Nanocrystal-on-Nanowire Superstructures

Fine-tuning the interfacial sites within heterogeneous catalysts is pivotal for unravelling the intricate structure-property relationship and optimizing their catalytic performance. Herein, a simple and versatile mixed-dimensional assembly approach is proposed to create nanocrystal-on-nanowire superstructures with precisely adjustable numbers of biphasic interfaces. This method leverages an efficient self-assembly process in which colloidal nanocrystals spontaneously organize onto Ag nanowires, driven by the solvophobic effect. Importantly, varying the ratio of the two components during assembly allows for accurate control over both the quantity and contact perimeter of biphasic interfaces. As a proof-of-concept demonstration, a series of Au-on-Ag superstructures with varying numbers of Au/Ag interfaces are constructed and employed as electrocatalysts for electrochemical CO2-to-CO conversion. Experimental results reveal a logarithmic linear relationship between catalytic activity and the number of Au/Ag interfaces per unit mass of Au-on-Ag superstructures. This work presents a straightforward approach for precise interface engineering, paving the way for systematic exploration of interface-dependent catalytic behaviors in heterogeneous catalysts.

Exploring the Facet-Dependent Structural Evolution of Pt/CeO2 Catalysts Induced by Typical Pretreatments for CO Oxidation

Atomic-scale insights into the interactions between metals and supports play a crucial role in optimizing catalyst design, understanding catalytic mechanisms, and enhancing chemical conversion processes. The effects of oxide support on the dynamic behavior of supported metal species during pretreatments or reactions have been attracting a lot of attention; however, very less systematic integrations are carried out experimentally using real catalysts. In this study, we here utilized facet-controlled CeO2 as examples to explore their influence on the supported Pt species (1.0 wt %) during the reducing and oxidizing pretreatments that are typically applied in heterogeneous catalysts. By employing a combination of microscopy, spectroscopy, and first-principles calculations, it is demonstrated that the exposed crystal facets of CeO2 govern the evolution behavior of supported Pt species under different environmental conditions. This leads to distinct local coordinations and charge states of the Pt species, which directly influence the catalytic reactivity and can be leveraged to control the catalytic performance for CO oxidation reactions.

Metal Atom-Support Interaction in Single Atom Catalysts toward Hydrogen Peroxide Electrosynthesis

Single metal atom catalysts (SACs) have garnered considerable attention as promising agents for catalyzing important industrial reactions, particularly the electrochemical synthesis of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (ORR). Within this field, the metal atom-support interaction (MASI) assumes a decisive role, profoundly influencing the catalytic activity and selectivity exhibited by SACs, and triggers a decade-long surge dedicated to unraveling the modulation of MASI as a means to enhance the catalytic performance of SACs. In this comprehensive review, we present a systematic summary and categorization of recent advancements pertaining to MASI modulation for achieving efficient electrochemical H2O2 synthesis. We start by introducing the fundamental concept of the MASI, followed by a detailed and comprehensive analysis of the correlation between the MASI and catalytic performance. We describe how this knowledge can be harnessed to design SACs with optimized MASI to increase the efficiency of H2O2 electrosynthesis. Finally, we distill the challenges that lay ahead in this field and provide a forward-looking perspective on the future research directions that can be pursued.

Co-Ti Bimetallic Complex-Induced Phase Modulation of Co@Black TiO2 for Catalytic Hydrogenation of Cinnamaldehyde

Transition-metal-doped black titania, primarily in the anatase phase, shows promise for redox reactions, water splitting, hydrogen generation, and organic pollutant removal, but exploring other titania phases for broader catalytic applications is underexplored. This study introduces a synthetic approach using a Co-Ti bimetallic complex bridged by a 1,10-phenanthroline-5,6-dione ligand as a precursor for the synthesis of cobalt-doped black titania [Co@L2N@b-TiO2]. The synthesis involves precise control of pyrolysis conditions, yielding a distinct structure dominated by the rutile phase over anatase, with active cobalt encapsulated within a nitrogen-doped graphitic layer, primarily as Co0 rather than CoII and CoIII. The synthesized material is employed for the selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) under industrially viable conditions. The efficiency and selectivity of Co@L2N@b-TiO2 was compared with other catalysts, including cobalt-doped rutile TiO2 (Co@r-TiO2), anatase TiO2 (Co@a-TiO2), and black titania (Co@b-TiO2) as well as materials pyrolyzed under different atmospheres and temperatures, materials with phenanthroline ligands, and materials lacking any ligands. The superior performance of Co@L2N@b-TiO2 is attributed to its high surface area, stable Co0 within the nitrogen-doped graphitic layer, and composition of rutile and anatase phases of TiO2 and Ti2O3 (referred to as RAT), along with the synergistic interaction between RAT and Co0. These factors significantly influence the efficiency and selectivity of COL over hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL), indicating potential for broader applications beyond catalysis, particularly in designing of black titania-based materials.

Strongly-Interacted NiSe2/NiFe2O4 Architectures Built Through Selective Atomic Migration as Catalysts for the Oxygen Evolution Reaction

The interactions between the catalyst and support are widely used in many important catalytic reactions but the construction of strong interaction with definite microenvironments to understand the structure-activity relationship is still challenging. Here, strongly-interacted composites are prepared via selective exsolution of active NiSe2 from the host matrix of NiFe2O4 (S-NiSe2/NiFe2O4) taking advantage of the differences of migration energy, in which the NiSe2 possessed both high dispersion and small size. The characteristics of spatially resolved scanning transmission X-ray microscopy (STXM) coupled with analytical Mössbauer spectra for the surface and bulk electronic structures unveiled that this strongly interacted composite triggered more charge transfers from the NiSe2 to the host of NiFe2O4 while stabilizing the inherent atomic coordination of NiFe2O4. The obtained S-NiSe2/NiFe2O4 exhibits overpotentials of 290 mV at 10 mA cm-2 for oxygen evolution reaction (OER). This strategy is general and can be extended to other supported catalysts, providing a powerful tool for modulating the catalytic performance of strongly-interacted composites.

Oxygen Impurity-Tuned Structure and Adhesion Properties of the Cu/SiO2 Interface

The properties of the Cu/SiO2 interface usually deteriorate in the complex atmospheric environment, which may limit its performance and application in the engineering. Using the reactive molecular dynamics method, we investigate how the mechanical behaviors of the Cu/SiO2 interface change as it interacts with oxygen impurities. The interfacial oxidation degree could be enhanced as O2 penetrates into the interface area. This makes the interfacial structure disordered and is not conducive to the survival of Cu-O-Si bondings, which reduces the tensile and shear strengths of the interface. To improve the abrupt bonding property change at the interface and modify the interfacial adhesion properties, O impurities are introduced at the Cu interstitial sites near the interface. By doing so, the interface strength can be significantly enhanced due to the production of typical O-Cu-O bondings while the regular interfacial structure is retained. Meanwhile, the interfacial oxidation also changes the tensile failure site and shearing sliding mode of the interface, i.e., from inside the oxide to between oxide and Cu. The findings of this work may not only advance the understanding of interaction mechanism between oxygen impurities and the Cu/SiO2 interface but also provide new insights into optimizing the bonding properties of the metal/oxide interface.

Mesoporous Ag@WO3 core-shell, an investigation at different concentrated environment employing laser ablation in liquid

In this study, silver-tungsten oxide core-shell nanoparticles (Ag-WO3 NPs) were synthesized by pulsed laser ablation in liquid employing a (1.06 µm) Q-switched Nd:YAG laser, at different Ag colloidal concentration environment (different core concentration). The produced Ag-WO3 core-shell NPs were subjected to characterization using UV-visible spectrophotometry, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive spectroscopy, electrical analysis, and photoluminescence PL. The UV-visible spectra exhibited distinct absorption peaks at around 200 and 405 nm, which attributed to the occurrence of surface Plasmon resonance of Ag NPs and WO3 NPs, respectively. The absorbance values of the Ag-WO3 core-shell NPs increased as the core concentrations rose, while the band gap decreased by 2.73-2.5 eV, The (PL) results exhibited prominent peaks with a central wavelength of 456, 458, 458, 464, and 466 nm. Additionally, the PL intensity of the Ag-WO3-NP samples increased proportionally with the concentration of the core. Furthermore, the redshift seen at the peak of the PL emission band may be attributed to the quantum confinement effect. EDX analysis can verify the creation process of the Ag-WO3 core-shell nanostructure. XRD analysis confirms the presence of Ag and WO3 (NPs). The TEM images provided a good visualization of the core-spherical shell structure of the Ag-WO3 core-shell NPs. The average size of the particles ranged from 30.5 to 89 (nm). The electrical characteristics showed an increase in electrical conductivity from (5.89 × 10-4) (Ω cm)-1 to (9.91 × 10-4) (Ω cm)-1, with a drop in average activation energy values of (0.155 eV) and (0.084 eV) at a concentration of 1.6 μg/mL of silver.

Significance of Epitaxial Growth of PtO2 on Rutile TiO2 for Pt/TiO2 Catalysts

TiO2-supported Pt species have been widely applied in numerous critical reactions involving photo-, thermo-, and electrochemical-catalysis for decades. Manipulation of the state of the Pt species in Pt/TiO2 catalysts is crucial for fine-tuning their catalytic performance. Here, we report an interesting discovery showing the epitaxial growth of PtO2 atomic layers on rutile TiO2, potentially allowing control of the states of active Pt species in Pt/TiO2 catalysts. The presence of PtO2 atomic layers could modulate the geometric configuration and electronic state of the Pt species under reduction conditions, resulting in a spread of the particle shape and obtaining a Pt/PtO2/TiO2 structure with more positive valence of Pt species. As a result, such a catalyst exhibits exceptional electrocatalytic activity and stability toward hydrogen evolution reaction, while also promoting the thermocatalytic CO oxidation, surpassing the performance of the Pt/TiO2 catalyst with no epitaxial structure. This novel epitaxial growth of the PtO2 structure on rutile TiO2 in Pt/TiO2 catalysts shows its potential in the rational design of highly active and economical catalysts toward diverse catalytic reactions.

Renaissance of Strong Metal-Support Interactions

Strong metal-support interactions (SMSIs) have emerged as a significant and cutting-edge area of research in heterogeneous catalysis. They play crucial roles in modifying the chemisorption properties, interfacial structure, and electronic characteristics of supported metals, thereby exerting a profound influence on the catalytic properties. This Perspective aims to provide a comprehensive summary of the latest advancements and insights into SMSIs, with a focus on state-of-the-art in situ/operando characterization techniques. This overview also identifies innovative designs and applications of new types of SMSI systems in catalytic chemistry and highlights their pivotal role in enhancing catalytic performance, selectivity, and stability in specific cases. Particularly notable is the discovery of SMSI between active metals and metal carbides, which opens up a new era in the field of SMSI. Additionally, the strong interactions between atomically dispersed metals and supports are discussed, with an emphasis on the electronic effects of the support. The chemical nature of SMSI and its underlying catalytic mechanisms are also elaborated upon. It is evident that SMSI modification has become a powerful tool for enhancing catalytic performance in various catalytic applications.

A single site ruthenium catalyst for robust soot oxidation without platinum or palladium

The quest for efficient non-Pt/Pd catalysts has proved to be a formidable challenge for auto-exhaust purification. Herein, we present an approach to construct a robust catalyst by embedding single-atom Ru sites onto the surface of CeO2 through a gas bubbling-assisted membrane deposition method. The formed single-atom Ru sites, which occupy surface lattice sites of CeO2, can improve activation efficiency for NO and O2. Remarkably, the Ru1/CeO2 catalyst exhibits exceptional catalytic performance and stability during auto-exhaust carbon particle oxidation (soot), rivaling commercial Pt-based catalysts. The turnover frequency (0.218 h-1) is a nine-fold increase relative to the Ru nanoparticle catalyst. We further show that the strong interfacial charge transfer within the atomically dispersed Ru active site greatly enhances the rate-determining step of NO oxidation, resulting in a substantial reduction of the apparent activation energy during soot oxidation. The single-atom Ru catalyst represents a step toward reducing dependence on Pt/Pd-based catalysts.

Investigating the impact of TiO2 crystalline phases on catalytic properties of Ru/TiO2 for hydrogenolysis of polyethylene plastic waste

A series of TiO₂-supported Ru catalysts with different TiO₂ crystalline phases was synthesized and employed for the hydrogenolysis of polyethylene (PE). CO chemisorption, high-angle annular dark-field-scanning transmission electron microscopy, temperature-programmed reduction, and CO-Fourier transform infrared spectroscopy suggested that the degree of strong metal-support interactions (SMSIs) varied depending on the type of the TiO₂ phase and the reduction temperature, eventually influencing the catalysis of PE hydrogenolysis. Among the synthesized catalysts, Ru/TiO₂ with the rutile phase (Ru/TiO₂-R) exhibited the highest catalytic activity after high-temperature reduction at 500 °C, indicating that a certain degree of SMSI is necessary for ensuring high activity in PE hydrogenolysis. Ru/TiO₂-R could be successfully employed for the hydrogenolysis of post-consumer plastic wastes such as LDPE bottles to produce valuable chemicals (liquid fuel and wax) in high yields of 74.7%. This work demonstrates the possibility of harnessing the SMSIs in the design and synthesis of active catalysts for PE hydrogenolysis.

Nanoscale Ni-NiO-ZnO Heterojunctions for Switchable Dehydrogenation and Hydrogenation through Modulation of Active Sites

Catalysts consisting of metal-metal hydroxide/oxide interfaces are highly in demand for advanced catalytic applications as their multicomponent active sites will enable different reactions to occur in close proximity through synergistic cooperation when a single component fails to promote it. To address this, herein we disclosed a simple, scalable, and affordable method for synthesizing catalysts consisting of nanoscale nickel-nickel oxide-zinc oxide (Ni-NiO-ZnO) heterojunctions by a combination of complexation and pyrolytic reduction. The modulation of active sites of catalysts was achieved by varying the reaction conditions of pyrolysis, controlling the growth, and inhibiting the interlayer interaction and Ostwald ripening through the efficient use of coordinated acetate and amide moieties of Zn-Ni materials (ZN-O), produced by the reaction between hydrazine hydrate and Zn-Ni-acetate complexes. We found that the coordinated organic moieties are crucial for forming heterojunctions and their superior catalytic activity. We analyzed two antagonistic reactions to evaluate the performance of the catalysts and found that while the heterostructure of Ni-NiO-ZnO and their cooperative synergy were crucial for managing the effectiveness and selectivity of the catalyst for dehydrogenation of aryl alkanes/alkenes, they failed to enhance the hydrogenation of nitro arenes. The hydrogenation reaction was influenced by the shape, surface properties, and interaction of the hydroxide and oxide of both zinc and nickel, particularly accessible Ni(0). The catalysts showed functional group tolerance, multiple reusabilities, broad substrate applicability, and good activity for both reactions.

Fabricating Uniform TiO2-CeO2 Solid Solution Supported Pd Catalysts by an In Situ Capture Strategy for Low-Temperature CO Oxidation

Support properties regulation has been a feasible method for the improvement of noble metal catalytic performance. For Pd-based catalysts, TiO₂-CeO₂ material has been widely used as an important support. However, due to the considerable discrepancy in the solubility product constant between titanium and cerium hydroxides, it is still challenging to synthesize a uniform TiO₂-CeO₂ solid solution in the catalysts. Herein, an in situ capture strategy was constructed to fabricate a uniform TiO₂-CeO₂ solid solution as supports for an enhanced Pd-based catalyst. The obtained Pd/TiO₂-CeO₂-iC catalyst possessed enriched reactive oxygen species and optimized CO adsorption capability, manifesting a superior CO oxidation activity (T₁₀₀ = 70 °C) and stability (over 170 h). We believe this work provides a viable strategy for precise characteristic modulation of composite oxide supports during the fabrication of advanced noble metal-based catalysts.

Theoretical Calculations on Metal Catalysts Toward Water-Gas Shift Reaction: a Review

Water-gas shift (WGS) reaction offers a dominating path to hydrogen generation from fossil fuel, in which heterogeneous metal catalysts play a crucial part in this course. This review highlights and summarizes recent developments on theoretical calculations of metal catalysts developed to date, including surface structure (e. g., monometallic and polymetallic systems) and interface structure (e. g., supported catalysts and metal oxide composites), with special emphasis on the characteristics of crystal-face effect, alloying strategy, and metal-support interaction. A systematic summarization on reaction mechanism was performed, including redox mechanism, associative mechanism as well as hybrid mechanism; the development on chemical kinetics (e. g., molecular dynamics, kinetic Monte Carlo and microkinetic simulation) was then introduced. At the end, challenges associated with theoretical calculations on metal catalysts toward WGS reaction are discussed and some perspectives on the future advance of this field are provided.

Engineering Heterogeneous Catalysis with Strong Metal-Support Interactions: Characterization, Theory and Manipulation

Strong metal-support interactions (SMSI) represent a classic yet fast-growing area in catalysis research. The SMSI phenomenon results in the encapsulation and stabilization of metal nanoparticles (NPs) with the support material that significantly impacts the catalytic performance through regulation of the interfacial interactions. Engineering SMSI provides a promising approach to steer catalytic performance in various chemical processes, which serves as an effective tool to tackle energy and environmental challenges. Our Minireview covers characterization, theory, catalytic activity, dependence on the catalytic structure and inducing environment of SMSI phenomena. By providing an overview and outlook on the cutting-edge techniques in this multidisciplinary research field, we not only want to provide insights into the further exploitation of SMSI in catalysis, but we also hope to inspire rational designs and characterization in the broad field of material science and physical chemistry.

Catalytic Activity of Ni Based Materials Prepared by Different Methods for Hydrogen Production via the Water Gas Shift Reaction

Water gas shift reactions (WGS) were evaluated over Ni/CeO2 and Ni/CeSmO catalysts for hydrogen production. The effects of catalyst preparation method and Sm loading were investigated. The Ni/ceria and Ni/CeSmO catalysts were synthesized by combustion, sol gel and sol gel-combustion method. After WGS tests, the catalysts were determined the carbon deposition by thermogravimetric analysis. The thermogravimetric analysis and temperature programmed NH3 desorption showed that addition of Sm promoter made higher the weak acid sites and lower the amount of carbon deposition than the unpromoted catalyst due to it being easily removed. CO chemisorption result indicated that Ni/Ce5%SmO catalyst prepared by combustion method has the highest Ni metal dispersion and metallic surface area compared to the other catalysts. The enhancement of WGS activity of this catalyst is due to more surface active sites being exposed to reactants. Furthermore, H2-temperature programmed reduction analysis confirmed an easiest reduction of this catalyst. This behavior accelerates the redox process at the ceria surface and enhances the oxygen vacancy concentration. The catalytic activity measurements exhibited that the optimum Sm loading was 5% wt. and the best catalyst preparation was the combustion method. The high surface area and small crystallite size of the 5%Ni/Ce5%SmO (combustion) catalyst resulted in sufficient dispersion, which closely related to the WGS activity of the catalyst.

Illustrating new understanding of adsorbed water on silica for inducing tetrahedral cobalt(II) for propane dehydrogenation

Highly dispersed metal sites on the surface of silica, achieved from immobilization of metal precursor within hydroxyl groups, has gained increasing attention in the field of heterogeneous catalyst. However, the special role of adsorbed water derived by hydroxyl groups on the silica is generally ignored. Herein, a new understanding of adsorbed water on the formation of highly dispersed tetrahedral Co(II) (T_d-cobalt(II)) sites is illustrated. It is indicated that sufficient adsorbed water induces the transformation of precursor of Co(NO₃)₂ into intermediate of [Co(H₂O)₆]²⁺. Subsequently, [Co(H₂O)₆]²⁺ makes the highly dispersed T_d-cobalt(II) sites to be available during direct H₂-reduction process. A systematic characterization and DFT calculation prove the existence of the adsorbed water and the importance of the intermediate of [Co(H₂O)₆]²⁺, respectively. The as-synthesized catalyst is attempted to the propane dehydrogenation, which shows better reactivity when compared with other reported Co based catalysts.

Universal properties of metal-supported oxide films from linear scaling relationships: elucidation of mechanistic origins of strong metal–support interactions

General principles of Strong Metal–Support Interactions (SMSI) overlayer formation have been elucidated using predictive models derived from ultrathin (hydroxy)oxide films on transition metal substrates.

Atomic-Scale Engineering of CuOx-Au Interfaces over AuCu Single-Nanoparticles

A face-centered tetragonal (fct) AuCu particle with a size of 7.1 nm and an Au/Cu molar ratio of 1/1 was coated by a silica shell of 6 nm thickness. Segregation of Cu atoms from the metal particle under an oxidative atmosphere precisely mediated the CuO_x-Au interfacial structure by simply varying the temperature. As raising the temperature from 473 to 773 K, more Cu atoms emigrated from the AuCu particle and were oxidized into CuO_x layers that grew up to 0.8 nm in thickness. Simultaneously, the size of the Au-rich particle lowered moderately while the crystalline structure transformed from the fct phase into the face-centered cubic (fcc) phase. The CuO_x-Au interface shifted from the CuO_x monolayer bound to Au single-atoms to Au@CuO_x core-shell geometry, while the catalytic activity for CO oxidation at 433 K decreased dramatically. Moreover, a sharp loss in activity was observed as the crystal-phase transition occurred. This change in catalytic performance was ascribed to the geometrical configuration at the interfacial sites: the synergetic effect between the fct-AuCu particle and CuO_x monolayer contributed to the much higher activity, whereas the fcc-AuCu/Au particle weakened its interaction with the thicker CuO_x layer and thus decreased the activity.

Establishing tungsten carbides as active catalysts for CO2 hydrogenation

Molybdenum carbides are promising catalysts for the reverse water-gas shift (RWGS) reaction, and we aim to understand if similar performance can be observed across the library of transition metal carbides. Although tungsten and molybdenum carbides exhibit similar catalytic properties for hydrogenation reactions, tungsten carbide has not been thoroughly evaluated for CO₂ hydrogenation. We hypothesize that the extreme synthesis conditions necessary for carburizing tungsten can cause sintering, agglomeration, and carbon deposition, leading to difficulty evaluating the intrinsic activity of tungsten carbides. In this work, tungsten is encapsulated in silica to preserve particle size and demonstrate correlations between the active phase composition and RWGS performance.

A Study of Support Effects for the Water-Gas-Shift Reaction over Cu

The water–gas-shift (WGS) reaction was studied on a series of supported Cu catalysts in which the MgAl2O4 (MAO) support was modified by depositing ZnO, CeO2, Mn2O3 and CoO using Atomic Layer Deposition (ALD). Addition of Cu by one ALD cycle gave rise to catalysts with nominally 1-wt% Cu. A 1.1-wt% Cu/MAO catalyst prepared by ALD exhibited twice the dispersion but ten times the WGS activity of a 1-wt% Cu/MAO catalyst prepared by impregnation, implying that the reaction is structure sensitive. However, Cu catalysts prepared with the ZnO, CeO2, and Mn2O3 films showed negligible differences from that of the Cu/MAO catalyst, implying that these oxides did not promote the reaction. Cu catalysts prepared on the CoO film showed a slightly lower activity, possibly due to alloy formation. The implications of these results for the development of better WGS catalysts is discussed.

Metal-Metal Bond Umpolung in Heterometallic Extended Metal Atom Chains

Understanding the fundamental properties governing metal-metal interactions is crucial to understanding the electronic structure and thereby applications of multimetallic systems in catalysis, material science, and magnetism. One such property that is relatively underexplored within multimetallic systems is metal-metal bond polarity, parameterized by the electronegativities (χ) of the metal atoms involved in the bond. In heterobimetallic systems, metal-metal bond polarity is a function of the donor-acceptor (Δχ) interactions of the two bonded metal atoms, with electropositive early transition metals acting as electron acceptors and electronegative late transition metals acting as electron donors. We show in this work, through the preparation and systematic study of a series of Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, and Ni; dpa = 2,2'-dipyridylamide; OTf = trifluoromethanesulfonate) heterometallic extended metal atom chain (HEMAC) complexes that this expected trend in χ can be reversed. Physical characterization via single-crystal X-ray diffraction, magnetometry, and spectroscopic methods as well as electronic structure calculations supports the presence of a σ symmetry 3c/3e- bond that is delocalized across the entire metal-atom chain and forms the basis of the heterometallic Mo2-M interaction. The delocalized 3c/3e- interaction is discussed within the context of the analogous 3c/3e- π bonding in the vinoxy radical, CH2CHO. The vinoxy comparison establishes three predictions for the σ symmetry 3c/3e- bond in HEMACS: (1) an umpolung effect that causes the Mo-M interactions to become more covalent as Δχ increases, (2) distortion of the σ bonding and non-bonding orbitals to emphasize Mo-M bonding and de-emphasize Mo-Mo bonding, and (3) an increase in Mo spin population with increasing Mo-M covalency. In agreement with these predictions, we find that the Mo2···M covalency increases with increasing Δχ of the Mo and M atoms (ΔχMo-M increases as M = Cr < Mn < Fe < Co < Ni), an umpolung of the trend predicted in the absence of σ delocalization. We attribute the observed trend in covalency to the decreased energic differential (ΔE) between the heterometal d z 2 orbital and the σ bonding molecular orbital of the Mo2 quadruple bond, which serves as an energetically stable, "ligand"-like electron-pair donor to the heterometal ion acceptor. As M is changed from Cr to Ni, the σ bonding and nonbonding orbitals do indeed distort as anticipated, and the spin population of the outer Mo group is increased by at least a factor of 2. These findings provide a predictive framework for multimetallic compounds and advance the current understanding of the electronic structures of molecular heteromultimetallic systems, which can be extrapolated to applications in the context of mixed-metal surface catalysis and multimetallic proteins.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Semi-quantitative design of synergetic surficial/interfacial sites for the semi-continuous oxidation of glycerol

Qualitatively identifying the dominant catalytic site for each step of a semi-continuous reaction and semi-quantitatively correlating such different sites to the catalytic performance is of great significance toward the integration of multiple well-optimized sites on a heterogeneous catalyst. Herein, a series of structurally defined TiOx-based catalysts were synthesized to provide a feasible approach to investigate the aforementioned issues using the semi-continuous oxidation of glycerol as a model reaction. Detailed investigations have verified the simultaneous presence of two kinds of Pt active sites: 1) Negatively charged Pt bound to the oxygen vacancies of modified TiOx in the form of Ptδ--Ov-Ti3+ sites and 2) metallic Pt (Pt0 site) located away from the interface. Meanwhile, the proportion of surficial and interfacial sites varies over this series of catalysts. Combined in situ FTIR experiments revealed that the reaction network was well-tuned via a site cooperation mechanism: The surficial Pt0 sites dissociatively adsorb the OH group of glycerol with a monodentate bonding geometry and the Ptδ--Ov-Ti3+ sites dissociate the C=O bond of the aldehyde group in a bidentate form. Furthermore, CO-FTIR spectroscopy confirmed a correlation between the reaction rate/product selectivity and the fraction of surficial/interfacial sites. A rational proportion of surficial and interfacial sites is key to enabling a high yield of glyceric acid. The most active catalyst with 32% surface sites and 68% interfacial sites exhibited 90.0% glycerol conversion and 68.5% GLYA selectivity. These findings provide a deeper understanding of the structure-activity relationships using qualitative identification and semi-quantitative analysis.

Modeling surface spin polarization on ceria-supported Pt nanoparticles

In this work, we employ density functional theory simulations to investigate possible spin polarization of CeO₂-(111) surface and its impact on the interactions between a ceria support and Pt nanoparticles. With a Gaussian type orbital basis, our simulations suggest that the CeO₂-(111) surface exhibits a robust surface spin polarization due to the internal charge transfer between atomic Ce and O layers. In turn, it can lower the surface oxygen vacancy formation energy and enhance the oxide reducibility. We show that the inclusion of spin polarization can significantly reduce the major activation barrier in the proposed reaction pathway of CO oxidation on ceria-supported Pt nanoparticles. For metal-support interactions, surface spin polarization enhances the bonding between Pt nanoparticles and ceria surface oxygen, while CO adsorption on Pt nanoparticles weakens the interfacial interaction regardless of spin polarization. However, the stable surface spin polarization can only be found in the simulations based on the Gaussian type orbital basis. Given the potential importance in the design of future high-performance catalysts, our present study suggests a pressing need to examine the surface ferromagnetism of transition metal oxides in both experiment and theory.

Overturning CO2 Hydrogenation Selectivity with High Activity via Reaction-Induced Strong Metal-Support Interactions

Encapsulation of metal nanoparticles by support-derived materials known as the classical strong metal-support interaction (SMSI) often happens upon thermal treatment of supported metal catalysts at high temperatures (≥500 °C) and consequently lowers the catalytic performance due to blockage of metal active sites. Here, we show that this SMSI state can be constructed in a Ru-MoO₃ catalyst using CO₂ hydrogenation reaction gas and at a low temperature of 250 °C, which favors the selective CO₂ hydrogenation to CO. During the reaction, Ru nanoparticles facilitate reduction of MoO₃ to generate active MoO_3-x overlayers with oxygen vacancies, which migrate onto Ru nanoparticles' surface and form the encapsulated structure, that is, Ru@MoO_3-x. The formed SMSI state changes 100% CH₄ selectivity on fresh Ru particle surfaces to above 99.0% CO selectivity with excellent activity and long-term catalytic stability. The encapsulating oxide layers can be removed via O₂ treatment, switching back completely to the methanation. This work suggests that the encapsulation of metal nanocatalysts can be dynamically generated in real reactions, which helps to gain the target products with high activity.

Interspersing CeOx Clusters to the Pt-TiO2 Interfaces for Catalytic Promotion of TiO2-Supported Pt Nanoparticles

We propose an interface-engineered oxide-supported Pt nanoparticle-based catalyst with improved low-temperature activity toward CO oxidation. By wet-impregnating 1 wt % Ce on TiO₂, we synthesized hybrid oxide support of CeO_x-TiO₂, in which dense CeO_x clusters formed on the surface of TiO₂. Then, the Pt/CeO_x-TiO₂ catalyst was synthesized by impregnating 2 wt % Pt on the CeO_x-TiO₂ supporting oxide. Pt-CeO_x-TiO₂ triphase interfaces were eventually formed upon impregnation of Pt on CeO_x-TiO₂. The Pt-CeO_x-TiO₂ interfaces open up the interface-mediated Mars-van Krevelen CO oxidation pathway, thus providing additional interfacial reaction sites for CO oxidation. Consequently, the specific reaction rate of Pt/CeO_x-TiO₂ for CO oxidation was increased by 3.2 times compared with that of Pt/TiO₂ at 140 °C. Our results demonstrate a widely applicable and straightforward method of catalytic activation of the interfaces between metal nanoparticles and supporting oxides, which enabled fine-tuning of the catalytic performance of oxide-supported metal nanoparticle classes of heterogeneous catalysts.

Nanoparticle Assembly as a Materials Development Tool

Nanoparticle assembly is a complex and versatile method of generating new materials, capable of using thousands of different combinations of particle size, shape, composition, and ligand chemistry to generate a library of unique structures. Here, a history of particle self-assembly as a strategy for materials discovery is presented, focusing on key advances in both synthesis and measurement of emergent properties to describe the current state of the field. Several key challenges for further advancement of nanoparticle assembly are also outlined, establishing a roadmap of critical research areas to enable the next generation of nanoparticle-based materials synthesis.

Efficient Role of Nanosheet-Like Pr2O3 Induced Surface-Interface Synergistic Structures over Cu-Based Catalysts for Enhanced Methanol Production from CO2 Hydrogenation

In a complex heterogeneous metal-catalyzed reaction process, unique cooperative effects between metal sites and surface-interface active sites, as well as favorable synergy between surface-interface active sites, can play crucial roles in improving their catalytic performances. In this work, a ZnO-modified Cu-based catalyst over defect-rich Pr₂O₃ nanosheets for high-efficiency CO₂ hydrogenation to produce methanol was successfully constructed. It was demonstrated that an as-fabricated nanosheet-like Cu-based catalyst presented several structural advantages including the formation of highly dispersive Cu⁰ sites and the coexistence of abundant defective Pr³⁺-V_o-Pr³⁺ structures (V_o: oxygen vacancy) and interfacial Cu-O-Pr sites. Combining structural characterization and catalytic reaction results with density functional theory calculations, it was clearly unveiled that the synergy between surface defective structures and Cu-Pr₂O₃ interfaces over the catalyst remarkably promoted the adsorption of CO₂ and CO intermediate, thus boosting the CO₂ hydrogenation simultaneously via both the formate intermediate pathway and the intense reverse water-gas shift reaction-derived CO hydrogenation pathway, along with a high space-time yield of methanol of 0.395 g_MeOH·g_cat^-1·h^-1 under mild reaction conditions (260 °C and 3.0 MPa). The study provides a new strategy to construct high-performance Cu-based catalysts for high-efficiency CO₂ hydrogenation to produce methanol and a deep understanding of the promotional roles of synergy between surface-interface active sites in the CO₂ hydrogenation.

Interfacial compatibility critically controls Ru/TiO2 metal-support interaction modes in CO2 hydrogenation

Supports can widely affect or even dominate the catalytic activity, selectivity, and stability of metal nanoparticles through various metal-support interactions (MSIs). However, underlying principles have not been fully understood yet, because MSIs are influenced by the composition, size, and facet of both metals and supports. Using Ru/TiO₂ supported on rutile and anatase as model catalysts, we demonstrate that metal-support interfacial compatibility can critically control MSI modes and catalytic performances in CO₂ hydrogenation. Annealing Ru/rutile-TiO₂ in air can enhance CO₂ conversion to methane resulting from enhanced interfacial coupling driven by matched lattices of RuO_x with rutile-TiO₂; annealing Ru/anatase-TiO₂ in air decreases CO₂ conversion and converts the product into CO owing to strong metal-support interaction (SMSI). Although rutile and anatase share the same chemical composition, we show that interfacial compatibility can basically modify metal-support coupling strength, catalyst morphology, surface atomic configuration, MSI mode, and catalytic performances of Ru/TiO₂ in heterogeneous catalysis.

Supports can largely affect the catalytic performance of metal nanoparticles, but the underlying principles are not yet fully understood. Here the authors demonstrate that metal-support interfacial compatibility of Ru/TiO₂ can critically control the metal-support interaction modes and the catalytic performances in CO₂ hydrogenation.

Understanding and controlling the formation of surface anion vacancies for catalytic applications

Systematic computational efforts aimed at calculating surface anion vacancy formation energies as important descriptors of catalytic performance are summarized.

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia–titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H₂-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia–titania supported Au. Among them, Au/Pr₁Ti₂O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO₂ and Au/PrO_x. The Au/Pr₁Ti₂O_x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Schematic illustration of H₂O activation on the Pr-TiO_x support and the following reaction with CO in the Au–oxide interface.

Tunable surface electric field of electrode materials via constructing Schottky heterojunctions for selective conversion of trash ions to treasures

Surface electric field of catalyst is widely recognized as one of the key points to boost catalytic activity. However, there is still a lack of convenient ways to tune the surface electric field to selectively boost the catalytically conversions of different types of reactants in specific catalytic reaction. Here, we introduce a concept-new method to tune the surface electric field of electrode materials by adjusting the number and density of heterojunctions inside. Both theoretical and experimental results prove that the well-designed surface electric field of an electrocatalyst plays a key role in facilitating pre-adsorption and/or activation of reactants for selective conversion of trash ions to useful products in hydrogen evolution reaction, oxygen evolution reaction and NO x - reduction reaction.