Operando Surface Studies on Metal-Oxide Interfaces of Bimetal and Mixed Catalysts

NiO/MnO2coated on carbon felt as an electrode material for supercapacitor applications

Transition metal oxides have demonstrated excellent capability for charge storage when used in supercapacitor electrodes. This study undertook the hydrothermal synthesis of bimetallic nickel and manganese oxide (NiO/MnO2) on a carbon-felt (CF) substrate. NiO/MnO2/CF electrode was characterized and examined in a three-electrode system in a potassium hydroxide electrolyte. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge analyses revealed Faradaic behavior during charge storage, a specific capacity of 1627 F g-1, and a stability of 96.8% after 5000 consecutive charge-discharge cycles. Subsequent investigations were conducted in a two-electrode system for constructing a symmetrical supercapacitor using the NiO/MnO2/CF electrode. The energy and power densities were determined as 43Wh kg-1and 559 W kg-1. Additionally, the stability of the constructed supercapacitor device was examined over 5000 consecutive cycles, verifying a 92% stability through charge-discharge cycles. Finally, the fabricated supercapacitor was utilized to power an LED lamp, successfully maintaining the illumination for 53 s.

Metal-oxide nanocatalysts for spontaneous sequestration of endocrine-disrupting compounds from wastewater

The quest for a good life, urbanization, and industrialization have led to the widespread distribution of endocrine-disrupting chemicals (EDCs) in water bodies through anthropogenic activities. This poses an imminent threat to both human and environmental health. In recent years, the utilization of advance materials for the removal of EDCs from wastewater has attracted a lot of attention. Metal-oxide nanocatalysts have emerged as promising candidates due to their high surface area, reactivity, and tunable properties, as well as enhanced surface properties such as mesoporous structures and hierarchical morphologies that allow for increased adsorption capacity, improved photocatalytic activity, and enhanced selectivity towards specific EDCs. As a result, they have shown extraordinary efficacy in removing a wide range of EDCs from aqueous solutions, including pharmaceuticals, agrochemicals, personal care items, and industrial chemicals. This study give insight into the unique physicochemical characteristics of metal-oxide nanocatalysts to effectively and efficiently remove harmful endocrine-disrupting chemicals (EDCs) from wastewater. Focusing on the advances in the synthesis, and properties of metal-oxide nanocatalysts, and insight into understanding the fundamental mechanisms underlying the adsorption and degradation of EDCs on metal-oxide nanocatalysts using advanced characterization techniques such as spectroscopic analysis and electron microscopy. The findings of the study present metal-oxide nanocatalysts as a good candidate for the spontaneous sequestration of EDCs from wastewater is an intriguing approach to mitigating water pollution and safeguarding public health and the environment.

Atomic-Scale Friction and Adhesion at Ambient Pressure

In this Perspective, we present the recent advancement and the prospects of atomic-scale friction and adhesion measurements across the pressure gap between ultrahigh vacuum and ambient pressure environments using variable-pressure atomic force microscopy (VP-AFM). We introduce the VP-AFM that enables nanotribological studies under various gas conditions with partial pressure ranging from UHV (1.0 × 10-10 mbar) to 1 bar. We highlight the frictional behaviors of ultrananocrystalline diamond surface in oxygen and water gas environments, as well as the chemical states probed with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The atomic scale degradation processes of MA(CH3NH3)PbBr3, which is an organic-inorganic hybrid perovskite (OHP) investigated with VP-AFM are introduced. Finally, we discuss the potential works on catalytic model systems including bimetallic Pt3Ni(111) and TiO2(110) and the future perspective of nanotribology under ambient conditions.

Using In situ Transmission Electron Microscopy to Study Strong Metal-Support Interactions in Heterogeneous Catalysis

Precisely controlling the microstructure of supported metal catalysts and regulating metal-support interactions at the atomic level are essential for achieving highly efficient heterogeneous catalysts. Strong metal-support interaction (SMSI) not only stabilizes metal nanoparticles and improves their resistance to sintering but also modulates the electrical interaction between metal species and the support, optimizing the catalytic activity and selectivity. Therefore, understating the formation mechanism of SMSI and its dynamic evolution during the chemical reaction at the atomic scale is crucial for guiding the structural design and performance optimization of supported metal catalysts. Recent advancements in in situ transmission electron microscopy (TEM) have shed new light on these complex phenomena, providing deeper insights into the SMSI dynamics. Here, the research progress of in situ TEM investigation on SMSI in heterogeneous catalysis is systematically reviewed, focusing on the formation dynamics, structural evolution during the catalytic reactions, and regulation methods of SMSI. The significant advantages of in situ TEM technologies for SMSI research are also highlighted. Moreover, the challenges and probable development paths of in situ TEM studies on the SMSI are also provided.

Insight into the Coordination Environment of Ru Single-Atom for Dry Reforming of Methane

Dry reforming of methane (DRM), a pivotal process for converting greenhouse gases into syngas is demanding rationally designed catalysts with high stability and ideal catalytic performance for industrial applications due to its stability of reactant molecules and characteristic of carbon deposition. However, the mechanistic understanding of how the coordination environment of the metal in a single-atom catalytic system may influence the catalytic performance remains limited. In this work, high- and low-coordinating Ru-based (RuHC and RuLC) catalysts with distinct Ru-O coordination numbers are prepared using one-pot and two-step methods. The difference in the stability (12.3% and negligible deactivation during 20 h test for RuLC and RuHC catalysts respectively) and selectivity (0.57 and 0.37 of H2/CO ratio) brought by the coordination environment signified the structure-function relationship of single-atom catalysts in DRM. The impact of the structure on the properties is systematically investigated by thorough structural and operando characterization as well as density functional theory (DFT) calculation. The findings contribute to the optimal design of single-atom catalysts for DRM, offering a theoretical basis for industrial catalyst development and the potential to improve the process's environmental impact.

Probing the Metal/Oxide Interface of IrCoCeOx in N2H4·H2O Decomposition: An Experimental and Computational Study

Understanding the structure of a functional catalyst is crucial to disclosing the complexity of heterogeneous processes and improving their efficiency. Herein, coprecipitated cobalt-ceria (CoCeOx) oxides doped with Ir (IrCoCeOx) were synthesized and used to assess the performances of metal/oxide interfaces in the N2H4·H2O decomposition performed in aqueous NaOH. Kinetic experiments in batch showed that CoO is the active phase of CoCeOx and that the copresence of Ir and Co (IrCoCeOx) enhanced H2 productivity. A comprehensive characterization (X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy) combined with robust computational modeling based on the density functional theory was employed to attribute the IrCoCeOx performance enhancement to the Ir/CoO metal/oxide interface, the active site of the reaction. On these sites, the improved H2 productivity in the presence of aqueous NaOH was studied operando through modulated excitation-attenuated total reflectance infrared coupled with phase sensitive detection. The formation of surface Co-hydroxyl and -imido groups at the Ir/CoO interface induced the preferential breakage of the N-H bond of N2H4·H2O, favoring the production of H2.

Enhanced Hot Electron Flow and Catalytic Synergy by Engineering Core-Shell Structures on Au-Pd Nanocatalysts

The synergistic catalytic performances of bimetallic catalysts are often attributed to the reaction mechanism associated with the alloying process of the catalytic metals. Chemically induced hot electron flux is strongly correlated with catalytic activity, and the interference between two metals at the atomic level can have a huge impact on the hot electron generation on the bimetallic catalysts. In this study, we investigate the correlation between catalytic synergy and hot electron chemistry driven by the electron coupling effect using a model system of Au-Pd bimetallic nanoparticles. We show that the bimetallic nanocatalysts exhibit enhanced catalytic activity under the hydrogen oxidation reaction compared with that of monometallic Pd nanocatalysts. Analysis of the hot electron flux generated in each system revealed the formation of Au/PdOx interfaces, resulting in high reactivity on the bimetallic catalyst. In further experiments with engineering the Au@Pd core-shell structures, we reveal that the hot electron flux, when the topmost surface Pd atoms were less affected by inner Au, due to the concrete shell, was smaller than the alloyed one. The alloyed bimetallic catalyst forming the metal-oxide interfaces has a more direct effect on the hot electron chemistry, as well as on the catalytic reactivity. The great significance of this study is in the confirmation that the change in the hot electron formation rate with the metal-oxide interfaces can be observed by shell engineering of nanocatalysts.

Investigating Degradation Mechanisms in PtCo Alloy Catalysts: The Role of Co Content and a Pt-Rich Shell Using Operando High-Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy

Low Pt-based alloy catalysts are regarded as an efficient strategy in achieving high activity for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, the desired durability for the low Pt-based catalysts, such as the Pt1Co3 catalyst, has still been considered a great challenge for PEMFCs. In this study, we investigate sub-2.5 nm PtxCoy alloy catalysts with varying Co content and Pt1Co3@Pt core-shell (CS) nanostructure catalysts obtained through a simple displacement reaction. The Pt1Co3@Pt_H catalysts showed a high mass activity (MA) of 1.46 A/mgPt at 0.9 V and 14% MA loss after 10k accelerated degradation test (ADT) cycles, which suggested the improved stability compared with Pt1Co3 catalysts (52% MA loss). To clarify the degradation mechanism, operando high-energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) was applied in addition to conventional advanced measurement techniques, including operando conventional XAS, to analyze the electronic state and structure changes during operation potentials. We found that introducing Co improves the catalysts' activity mainly from the strain effect, but an excessive amount of Co leads to increased Pt-oxidation, which accelerates the degradation of the catalysts. The Pt1Co3@Pt_H catalyst shows high tolerance to Pt-oxidation, benefiting both the stability and activity. Our findings demonstrate an in-depth understanding of the degradation mechanism and the importance of designing PtCo CS nanostructures with optimal Co content for enhanced performance in PEMFCs.

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Revealing CO2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO₂) activation. Despite extensive reactivity benchmarks, a correlation between CO₂ conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO₂(g). The CO₂ dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO₂ conversion to renewable energy sources during C₁ chemical reactions.

Facet-Controlled Cu2O Support Enhances Catalytic Activity of Pt Nanoparticles for CO Oxidation

The metal-support interaction plays a crucial role in determining the catalytic activity of supported metal catalysts. Changing the facet of the support is a promising strategy for catalytic control via constructing a well-defined metal-support nanostructure. Herein, we developed cubic and octahedral Cu₂O supports with (100) and (111) facets terminated, respectively, and Pt nanoparticles (NPs) were introduced. The in situ characterizations revealed the facet-dependent encapsulation of the Pt NPs by a CuO layer due to the oxidation of the Cu₂O support during the CO oxidation reaction. The CuO layer on Pt at cubic Cu₂O (Pt/c-Cu₂O) significantly enhanced catalytic performance, while the thicker CuO layer on Pt at octahedral Cu₂O suppressed CO conversion. The formation of a thin CuO layer is attributed to the dominant Pt-O-Cu bond at the Pt/c-Cu₂O interface, which suppresses the adsorption of oxygen molecules. This investigation provides insight into designing high-performance catalysts via engineering the interface interaction.

Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI

The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10, and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence, and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster size. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e., 200 K above the Hüttig temperature that indicates the thermodynamic stability limit.

Tunable Interfacial Electronic Pd-Si Interaction Boosts Catalysis via Accelerating O2 and H2O Activation

Engineering the interfacial structure between noble metals and oxides, particularly on the surface of non-reducible oxides, is a challenging yet promising approach to enhancing the performance of heterogeneous catalysts. The interface site can alter the electronic and d-band structure of the metal sites, facilitating the transition of energy levels between the reacting molecules and promoting the reaction to proceed in a favorable direction. Herein, we created an active Pd-Si interface with tunable electronic metal-support interaction (EMSI) by growing a thin permeable silica layer on a non-reducible oxide ZSM-5 surface (termed Pd@SiO₂/ZSM-5). Our experimental results, combined with density functional theory calculations, revealed that the Pd-Si active interface enhanced the charge transfer from deposited Si to Pd, generating an electron-enriched Pd surface, which significantly lowered the activation barriers for O₂ and H₂O. The resulting reactive oxygen species, including O₂ ^-, O₂ ^2-, and -OH, synergistically facilitated formaldehyde oxidation. Additionally, moderate electronic metal-support interaction can promote the catalytic cycle of Pd⁰ ⇆ Pd²⁺, which is favorable for the adsorption and activation of reactants. This study provides a promising strategy for the design of high-performance noble metal catalysts for practical applications.

Metal Oxide-Supported Metal Catalysts for Electrocatalytic Oxygen Reduction Reaction: Characterization Methods, Modulation Strategies, and Recent Progress

The sluggish kinetics of the oxygen reduction reaction (ORR) with complex multielectron transfer steps significantly limits the large-scale application of electrochemical energy devices, including metal-air batteries and fuel cells. Recent years witnessed the development of metal oxide-supported metal catalysts (MOSMCs), covering single atoms, clusters, and nanoparticles. As alternatives to conventional carbon-dispersed metal catalysts, MOSMCs are gaining increasing interest due to their unique electronic configuration and potentially high corrosion resistance. By engineering the metal oxide substrate, supported metal, and their interactions, MOSMCs can be facilely modulated. Significant progress has been made in advancing MOSMCs for ORR, and their further development warrants advanced characterization methods to better understand MOSMCs and precise modulation strategies to boost their functionalities. In this regard, a comprehensive review of MOSMCs for ORR is still lacking despite this fast-developing field. To eliminate this gap, advanced characterization methods are introduced for clarifying MOSMCs experimentally and theoretically, discuss critical methods of boosting their intrinsic activities and number of active sites, and systematically overview the status of MOSMCs based on different metal oxide substrates for ORR. By conveying methods, research status, critical challenges, and perspectives, this review will rationally promote the design of MOSMCs for electrochemical energy devices.

Atomically Dispersed Pt-Doped Co3 O4 Spinel Nanoparticles Embedded in Polyhedron Frames for Robust Propane Oxidation at Low Temperature

This study adopts a facile and effective in situ encapsulation-oxidation strategy for constructing a coupling catalyst composed of atomically dispersed Pt-doped Co₃ O₄ spinel nanoparticles (NPs) embedded in polyhedron frames (PFs) for robust propane total oxidation. Benefiting from the abundant oxygen vacancies and more highly valent active Co³⁺ species caused by the doping of Pt atoms as well as the confinement effect, the optimized 0.2Pt-Co₃ O₄ NPs/PFs catalyst exhibits excellent propane catalytic activity with low T₉₀ (184 °C), superior apparent reaction rate (21.62×10⁸ (mol g_cat ^-1 s^-1 )), low apparent activation energy (E_a = 17.89 kJ mol^-1 ), high turnover frequency ( 811×10⁷ (mol g_cat ^-1 s^-1 )) as well as good stability. In situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculations indicate that the doping of Pt atoms enhances the oxygen activation ability, and decreases the energy barrier required for CH bond breaking, thus improving the deep oxidation process of the intermediate species. This study opens up new ideas for constructing coupling catalysts from atomic scale with low cost to enhance the activation of oxygen molecules and the deep oxidation of linear short chain alkanes at low temperature.

Unravelling intrinsic descriptors based on a two-stage activity regulation of bimetallic 2D c-MOFs for CO2RR

The bimetallic 2D conductive MOFs of M₁Pc-M₂-O, possessing dual metal sites to realize flexible molecular-level structural modification, are brilliant catalysts for electrochemical CO₂ reduction. However, the bimetallic centers bring about the complex regulatory mechanism of catalytic activity and obscure principles for catalyst design. Herein, systematical theoretical investigation unravels intrinsic descriptors to design favorable M₁Pc-M₂-O catalysts based on the discovered coarse-fine two-stage activity regulation mechanism. The reaction site controls the M-COOH distance of the key intermediate and therefore affects the reaction kinetics for the first stage of coarse regulation. The other metal site influents the d-band center of the reaction site and thus constitutes the second stage of fine regulation. The coarse and fine regulation are related to the valence electrons (V), electronegativity (E), and bond length (L_M-N/O) between the metal and coordination atoms. The intrinsic descriptor ϕ = (4 × V_M1 × (E_M1 + E_N/O)/E_N/O + V_M2 × (E_M2 + E_N/O)/E_N/O) × L_M1-N/O (with a coefficient ratio of 4 : 1) was eventually established and correlated well with the reported experiments. On this basis, the favorable catalysts CoPc-Zn-O and CoPc-Co-O were located. The research results could contribute to the diversity of bimetallic 2D c-MOFs in CO₂RR.

Effect of Water Vapor on Oxidation Processes of the Cu(111) Surface and Sublayer

Copper-based catalysts have different catalytic properties depending on the oxidation states of Cu. We report operando observations of the Cu(111) oxidation processes using near-ambient pressure scanning tunneling microscopy (NAP-STM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The Cu(111) surface was chemically inactive to water vapor, but only physisorption of water molecules was observed by NAP-STM. Under O₂ environments, dry oxidation started at the step edges and proceeded to the terraces as a Cu₂O phase. Humid oxidation of the H₂O/O₂ gas mixture was also promoted at the step edges to the terraces. After the Cu₂O covered the surface under humid conditions, hydroxides and adsorbed water layers formed. NAP-STM observations showed that Cu₂O was generated at lower steps in dry oxidation with independent terrace oxidations, whereas Cu₂O was generated at upper steps in humid oxidation. The difference in the oxidation mechanisms was caused by water molecules. When the surface was entirely oxidized, the diffusion of Cu and O atoms with a reconstruction of the Cu₂O structures induced additional subsurface oxidation. NAP-XPS measurements showed that the Cu₂O thickness in dry oxidation was greater than that in humid oxidation under all pressure conditions.

Ultrafine oxygenophilic nanoalloys induced by multifunctional interstitial boron for methanol oxidation reaction

Interface construction is one of the most feasible approaches to optimize the physical and chemical properties of noble metal-based catalysts and consequently improve their catalytic performance. Herein, the design of effective reaction interfaces by bimetallic, trimetallic or polymetallic alloying has been extensively explored. In this research, metalloid boron (B) was alloyed within palladium-iridium (Pd-Ir) nanoalloy supported on nitrogen-doped graphene (NG) to promote the methanol oxidation reaction (MOR) in alkaline media. Being benefited from this, the optimum Pd7IrBx/NG catalyst exhibited enhanced EOR activity mass activity (1141.7 mA mg-1) and long-term stability (58.2 % current density retention rate after 500 cycles of cyclic voltammetry). The mechanism was further studied by electrochemical experiments and characterization, which highlighted that the multifunctional effect of electronic effect and strain effect and kinetic optimization induced by boron doping played a very positive role on MOR.

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.