The role of metal/oxide interfaces for long-range metal particle activation during CO oxidation

Green Syngas from Photothermal Catalytic Cellulose Steam Reforming on Ni/SiO2 Nanocatalysts: Synergy of La3+ Promotion and Ni-O Photoactivation

Replacing fossil fuels by renewable biomass enables green syngas production in an effort to achieve carbon neutrality and sustainable circular processes. Here, an inexpensive catalyst of Ni nanoparticles supported on SiO2 modified by La3+ (Ni/La0.10-S) is presented, exhibiting exceptional H2 and CO production rates (4051.4 and 2467.8 mmol gcatalyst -1 h-1, respectively) with 7.7% light-to-fuel efficiency in cellulose steam reforming (SR), merely under focused illumination. Excellent performance is mainly attributed to photothermal catalysis resulting from the strong solar absorption and high photothermal conversion by the Ni nanoparticles. The mitigation of tar and char formation significantly benefits from the H2O involvement in the reaction, which is substantially improved by La3+ addition enhancing H2O sorption. Remarkably, the illumination exerts mere photoactivation during reaction, which is primarily attributed to the pronounced activation of Ni─O bonds at the catalyst surface. Particularly, the photoactivation of the Ni-O-La moieties, in combination with O species replenishment by H2O, makes Ni/La0.10-S superior to Ni/SiO2. The synergy of La3+ promotion and Ni-O photoactivation poses a promising strategy for efficient photothermal catalytic cellulose SR to green syngas.

Interfacial Catalysis at Atomic Level

Heterogeneous catalysts are pivotal to the chemical and energy industries, which are central to a multitude of industrial processes. Large-scale industrial catalytic processes rely on special structures at the nano- or atomic level, where reactions proceed on the so-called active sites of heterogeneous catalysts. The complexity of these catalysts and active sites often lies in the interfacial regions where different components in the catalysts come into contact. Recent advances in synthetic methods, characterization technologies, and reaction kinetics studies have provided atomic-scale insights into these critical interfaces. Achieving atomic precision in interfacial engineering allows for the manipulation of electronic profiles, adsorption patterns, and surface motifs, deepening our understanding of reaction mechanisms at the atomic or molecular level. This mechanistic understanding is indispensable not only for fundamental scientific inquiry but also for the design of the next generation of highly efficient industrial catalysts. This review examines the latest developments in atomic-scale interfacial engineering, covering fundamental concepts, catalyst design, mechanistic insights, and characterization techniques, and shares our perspective on the future trajectory of this dynamic research field.

In Situ Probing Dynamic Exsolution of Fe0 from Perovskite under Graded Potentials

Metal exsolution from perovskite oxides offers a promising approach to developing highly active catalysts. Voltage-driven exsolution achieves a faster rate and better dispersion of the nanoparticles. However, the impact of the external voltage on the dynamic of metal exsolution remains unclear. In this work, we utilized spatially resolved in situ X-ray photoelectron spectroscopy and photoemission electron microscopy to systematically investigate the effect of varied potentials on exsolution within one sample, where spatially graded potentials were applied on a La0.6Sr0.4FeO3-δ electrode. The dynamic characterizations reveal that the exsolution process includes pre-reduction and metal exsolution stages, with exsolution rate and degree modulated by the local surface potential. Potentials more negative than the critical potential at approximately -1.05 V can enhance oxygen vacancy formation and increase the exsolution rate and content of Fe0. This work establishes a direct correlation between the local surface potential and exsolution process and highlights the critical role of oxygen vacancies in voltage-driven exsolution.

Morphology-electronic effects in ultra-model nanocatalysts under the CO oxidation reaction: the case of ZnO ultrathin films grown on Pt(111)

The study of the surface morphology and interface of metal-oxides is crucial for understanding the behavior of these model systems as nanocatalysts. Besides, understanding the interplay between morphology, stability, and reactivity is crucial for designing efficient catalysts. Here, we investigated the stability and dewetting of ZnO ultrathin films on Pt(111) under CO oxidation conditions. For films <1 monolayer (ML), CO-induced dewetting occurs at the metal-oxide interface or defects. The morphology, dependent on thickness, influences reactivity. (6 × 6) structures show greater CO binding and structural changes compared to (4 × 4) structures, which exhibit resilience due to Zn-OH formation. ZnO electronic properties, as revealed by Auger spectroscopy and scanning tunneling spectroscopy (STS) investigations, vary with thickness. Low-thickness films [<2 monolayers (ML)] exhibit metallic-like behavior, possibly due to Zn-Pt interaction, while thicker films show n-type semiconductor behavior with a bandgap opening (EBG = 0.9 eV at 2 ML). DFT calculations of the local density of states (LDOS) as a function of ZnO thickness confirm the thickness-dependent electronic structure, with 0.3 ML films having a higher LDOS near the Fermi level than 1 ML films. These findings highlight the critical role of ZnO morphology in determining its stability and reactivity which opens up avenues for designing efficient and more stable ZnO-based nanocatalysts for a wide range of chemical reactions, including CO oxidation and CO2 hydrogenation.

Ion Transport at Polymer-Argyrodite Interfaces

Solid-state electrolytes, particularly polymer/ceramic composite electrolytes, are emerging as promising candidates for lithium-ion batteries due to their high ionic conductivity and mechanical flexibility. The interfaces that arise between the inorganic and organic materials in these composites play a crucial role in ion transport mechanisms. While lithium ions are proposed to diffuse across or parallel to the interface, few studies have directly examined the quantitative impact of these pathways on ion transport and little is known about how they affect the overall conductivity. Here, we present an atomistic study of lithium-ion (Li+) transport across well-defined polymer-argyrodite interfaces. We present a force field for polymer-argyrodite interfacial systems, and we carry out molecular dynamics and enhanced sampling simulations of several composite systems, including poly(ethylene oxide) (PEO)/Li6PS5Cl, hydrogenated nitrile butadiene rubber (HNBR)/Li6PS5Cl, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/Li6PS5Cl. For the materials considered here, Li-ion exhibits a preference for the ceramic material, as revealed by free energy differences for Li-ion between the inorganic and the organic polymer phase in excess of 13 kBT. The relative free energy profiles of Li-ion for different polymeric materials exhibit similar shapes, but their magnitude depends on the strength of interaction between the polymers and Li-ion: the greater the interaction between the polymer and Li-ions, the smaller the free energy difference between the inorganic and organic materials. The influence of the interface is felt over a range of approximately 1.5 nm, after which the behavior of Li-ion in the polymer is comparable to that in the bulk. Near the interface, Li-ion transport primarily occurs parallel to the interfacial plane, and ion mobility is considerably slower near the interface itself, consistent with the reduced segmental mobility of the polymer in the vicinity of the ceramic material. These findings provide insights into ionic complexation and transport mechanisms in composite systems, and will help improve design of improved solid electrolyte systems.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Fabrication of High Surface Area Fe/Fe3 O4 with Enhanced Performance for Electrocatalytic Nitrogen Reduction Reaction

The development of high-efficient and large-scale non-precious electrocatalysts to improve sluggish reaction kinetics plays a key role in enhancing electrocatalytic nitrogen reduction reaction (NRR) for ammonia production under mild condition. Herein, Fe3 O4 and Fe supported by porous carbon (denoted as Fe/Fe3 O4 /PC-800) composite with a high specific surface area of 1004.1 m2  g-1 was prepared via a simple template method. On one hand, the high surface area of Fe/Fe3 O4 /PC-800 provides a large area to enhance N2 adsorption and promote more protons and electrons to accelerate the reaction, thereby greatly improving the dynamics. On the other hand, mesoporous Fe/Fe3 O4 /PC-800 provides high electrochemically active surface area for promoting the occurrence of catalytic kinetics. As a result, Fe/Fe3 O4 /PC-800 exhibited significantly enhanced NRR performance with an ammonia yield of 31.15 μg h-1  mg-1 cat. and faraday efficiency of 22.26 % at -0.1 V vs. reversible hydrogen electrode (RHE). This study is expected to provide a new strategy for the synthesis of catalysts with large specific area and pave the way for the foundational research in NRR.

Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM

The nanoparticle (NP) redox state is an important parameter in the performance of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Here, the compositional evolution of individual CoNPs (6-24 nm) in terms of the oxide vs metallic state was investigated in situ during CO/syngas treatment using spatially resolved X-ray absorption spectroscopy (XAS)/X-ray photoemission electron microscopy (X-PEEM). It was observed that in the presence of CO, smaller CoNPs (i.e., ≤12 nm in size) remained in the metallic state, whereas NPs ≥ 15 nm became partially oxidized, suggesting that the latter were more readily able to dissociate CO. In contrast, in the presence of syngas, the oxide content of NPs ≥ 15 nm reduced, while it increased in quantity in the smaller NPs; this reoxidation that occurs primarily at the surface proved to be temporary, reforming the reduced state during subsequent UHV annealing. O K-edge measurements revealed that a key parameter mitigating the redox behavior of the CoNPs were proximate oxygen vacancies (Ovac). These results demonstrate the differences in the reducibility and the reactivity of Co NP size on a Co/TiO2 catalyst and the effect Ovac have on these properties, therefore yielding a better understanding of the physicochemical properties of this popular choice of FTS catalysts.

Strong Metal-Support Interaction Facilitated Multicomponent Alloy Formation on Metal Oxide Support

Multicomponent alloy (MA) contains a nearly infinite number of unprecedented active sites through entropy stabilization, which is a desired platform for exploring high-performance catalysts. However, MA catalysts are usually synthesized under severe conditions, which induce support structure collapse and further deteriorate the synergy between MA and support. We propose that a strong metal-support interaction (SMSI) could facilitate the formation of MA by establishing a tunnel of oxygen vacancy for metal atom transport under low reduction temperature (400-600 °C), which exemplifies the holistic design of MA catalysts without deactivating supports. PtPdCoFe MA is readily synthesized on anatase TiO2 with the help of SMSI, which exhibits good catalytic activity and stability for methane combustion. This strategy demonstrates excellent universality on various supports and multicomponent alloy compositions. Our work not only reports a holistic synthesis strategy for MA synthesis by synergizing unique properties of reducible oxides and the mixing entropy of alloy but also offers a new insight that SMSI plays a vigorous role in the formation of alloy NPs on reducible oxides.

Systematic Characterization of Electronic Metal-Support Interactions in Ceria-Supported Pt Particles

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides.

Phase Segregation in PdCu Alloy Nanoparticles During CO Oxidation Reaction at Atmospheric Pressure

Bimetallic nanoparticle (NP) catalysts are widely used in many heterogeneous gas-based reactions because they often outperform their monometallic counterparts. During these reactions, NPs often undergo structural changes, which impact their catalytic activity. Despite the important role of the structure in the catalytic activity, many aspects of how a reactive gaseous environment affects the structure of bimetallic nanocatalysts are still lacking. Here, using gas-cell transmission electron microscopy (TEM), it is shown that during a CO oxidation reaction over PdCu alloy NPs, the selective oxidation of Cu causes the segregation of Cu and transforms the NPs into Pd-CuO NPs. The segregated NPs are very stable and have high activity for the conversion of CO into CO2 . Based on the observations, the segregation of Cu from Cu-based alloys during a redox reaction is likely to be general and may have a positive impact on the catalytic activity. Hence, it is believed that similar insights based on direct observation of the reactions under relevant reactive conditions are critical both for understanding and designing high-performance catalysts.

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO₂) in H₂ oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO₂) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO₂, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO₂ support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Novel nano spinel-type high-entropy oxide (HEO) catalyst for hydrogen production using ethanol steam reforming

Catalyst sintering caused by high temperature operating conditions during ethanol steam reforming (ESR) is a common issue for traditional catalysts with low Tammann temperature metals. In recent years, high entropy oxides have been popular in thermal catalysis due to their special thermodynamics and kinetics characteristics, which is expected to be a suitable approach for enhancing catalyst stability. This paper first reports the application of HEO in ESR and the characterization. The results exhibited a nano structure (CoCrFeNiAl)₃O₄ HEO with a spinel-phase and was successfully synthesized by a polyol hydrothermal precipitation-calcination method. An abundance of oxygen vacancies were formed, and were further enriched in a hydrogen atmosphere as the M-O bond opened. Interestingly, its self-reorganization featured the rendered the metals spilling out of the HEO bulk phase as active species for hydrogen production during ESR, whereas the isolated metal cation randomly dissolved into the parent metal oxide cell again after the reaction instead of agglomerating over the catalyst surface. This gave the (CoCrFeNiAl)₃O₄ a large number of dispersed active sites, as well as a high thermal stability. In addition, 81% of the hydrogen yield as well as 85% of H₂ selectivity were achieved at 600 °C. This research might offer possibilities for the development of thermal catalytic hydrogen production under high temperature conditions such as steam reforming.

Diversity of platinum-sites at platinum/fullerene interface accelerates alkaline hydrogen evolution

Membrane-based alkaline water electrolyser is promising for cost-effective green hydrogen production. One of its key technological obstacles is the development of active catalyst-materials for alkaline hydrogen-evolution-reaction (HER). Here, we show that the activity of platinum towards alkaline HER can be significantly enhanced by anchoring platinum-clusters onto two-dimensional fullerene nanosheets. The unusually large lattice distance (~0.8 nm) of the fullerene nanosheets and the ultra-small size of the platinum-clusters (~2 nm) leads to strong confinement of platinum clusters accompanied by pronounced charge redistributions at the intimate platinum/fullerene interface. As a result, the platinum-fullerene composite exhibits 12 times higher intrinsic activity for alkaline HER than the state-of-the-art platinum/carbon black catalyst. Detailed kinetic and computational investigations revealed the origin of the enhanced activity to be the diverse binding properties of the platinum-sites at the interface of platinum/fullerene, which generates highly active sites for all elementary steps in alkaline HER, particularly the sluggish Volmer step. Furthermore, encouraging energy efficiency of 74% and stability were achieved for alkaline water electrolyser assembled using platinum-fullerene composite under industrially relevant testing conditions.

Metal-Support Interactions in Heterogeneous Catalysis: DFT Calculations on the Interaction of Copper Nanoparticles with Magnesium Oxide

Oxide supports play an important role in enhancing the catalytic properties of transition metal nanoparticles in heterogeneous catalysis. How extensively interactions between the oxide support and the nanoparticles impact the electronic structure as well as the surface properties of the nanoparticles is hence of high interest. In this study, the influence of a magnesium oxide support on the properties of copper nanoparticles with different size, shape, and adsorption sites is investigated using density functional theory (DFT) calculations. By proposing simple models to reduce the cost of the calculations while maintaining the accuracy of the results, we show using the nonreducible oxide support MgO as an example that there is no significant influence of the MgO support on the electronic structure of the copper nanoparticles, with the exception of adsorption directly at the Cu-MgO interface. We also propose a simplified methodology that allows us to reduce the cost of the calculations, while the accuracy of the results is maintained. We demonstrate in addition that the Cu nanowire model corresponds well to the nanoparticle model, which reduces the computational cost even further.

Insight into the Mechanism for Catalytic Activity of the Oxygen/Hydrogen Evolution Reaction on a Dual-Site Catalyst

The dual-site catalysts consisting of two adjacent single-atom sites on graphene have exhibited promising catalytic activity of the electrochemical oxygen/hydrogen evolution reaction (OER/HER). However, the electrochemical mechanisms of the OER/HER on dual-site catalysts have still been ambiguous. In this work, we employed density functional theory calculations to study the catalytic activity of the OER/HER with a O-O (H-H) direct coupling mechanism on dual-site catalysts. Specifically, these element steps should be classified into two categories: a step evolving proton-coupled electron transfer (PCET step) that needs to be driven by electrode potential and a step without PCET (non-PCET step) that occurs naturally under mild conditions. Our calculated results show that both the maximal free energy change (ΔG_Max) contributed by the PCET step and the activity barrier (E_a) of the non-PCET step must be examined to evaluate the catalytic activity of the OER/HER on the dual site. Importantly, it is a basically inevitable negative relationship between ΔG_Max and E_a, which would play a critical role in guiding the rational design of effective dual-site catalysts for electrochemical reactions.

Noble Metal Nanoparticles Decorated Boron Nitride Nanotubes for Efficient and Selective Low-Temperature Catalytic Reduction of Nitric Oxide with Carbon Monoxide

Parallel to CO₂ emission, NO_x emission has become one of the menacing problems that seek a simple, durable, and high-efficiency deNO_x catalyst. Herein, we demonstrated simple syntheses of platinum group metal nanoparticle-decorated f-BNNT (PGM = Pd, Pt, and Rh, and f-BNNT stands for -OH-functionalized boron nitride nanotubes) as a catalyst for efficient and selective reduction of NO by CO at low-temperature conditions. PGM/f-BNNT with a low amount of noble metal nanoparticles (0.7-0.8 wt %) presents very efficient catalytic activity for NO reduction as well as CO oxidation during their removal process. The removal efficiencies of NO and CO with Pd/f-BNNT, Pt/f-BNNT, and Rh/f-BNNT catalysts were investigated under various temperatures, flow rates, and reaction times, respectively. For most cases, NO catalytic reduction with CO reaction was >99% at a temperature as low as ∼200 °C. The catalyst robustness and efficiency were also verified by presenting almost 100% conversion of NO using a Rh/f-BNNT catalyst, which was aged under humid air at 600 and 700 °C for 24 h, respectively. The synergic effect of the catalytic efficacy of the well-dispersed noble metal nanoparticles and the excellent surface properties of BNNT are reasons for the high selectivity and catalytic property at a low temperature. On the basis of this investigation, we demonstrated that the noble metal nanoparticle-decorated f-BNNT catalysts are possible to save expensive PGM catalysts, such as Pt, Pd, and Rd, as much as 100 times while presenting similar or better catalytic performance for simultaneous NO and CO removals.

Ordered Mesopore Confined Pt Nanoclusters Enable Unusual Self-Enhancing Catalysis

As an important kind of emerging heterogeneous catalyst for sustainable chemical processes, supported metal cluster (SMC) catalysts have received great attention for their outstanding activity; however, the easy aggregation of metal clusters due to their migration along the substrate's surface usually deteriorates their activity and even causes catalyst failure during cycling. Herein, stable Pt nanoclusters (NCs, ∼1.06 nm) are homogeneously confined in the uniform spherical mesopores of mesoporous titania (mpTiO2) by the interaction between Pt NCs and metal oxide pore walls made of polycrystalline anatase TiO2. The obtained Pt-mpTiO2 exhibits excellent stability with well-retained CO conversion (∼95.0%) and Pt NCs (∼1.20 nm) in the long term water-gas shift (WGS) reaction. More importantly, the Pt-mpTiO2 displays an unusual increasing activity during the cyclic catalyzing WGS reaction, which was found to stem from the in situ generation of interfacial active sites (Ti3+-Ov-Ptδ+) by the reduction effect of spillover hydrogen generated at the stably supported Pt NCs. The Pt-mpTiO2 catalysts also show superior performance toward the selective hydrogenation of furfural to 2-methylfuran. This work discloses an efficient and robust Pt-mpTiO2 catalyst and systematically elucidates the mechanism underlying its unique catalytic activity, which helps to design stable SMC catalysts with self-enhancing interfacial activity in sustainable heterogeneous catalysis.

Metal-Metal Oxide Catalytic Interface Formation and Structural Evolution: A Discovery of Strong Metal-Support Bonding, Ordered Intermetallics, and Single Atoms

In-depth investigation of metal-metal oxide interactions and their corresponding evolution is of paramount importance to heterogeneous catalysis as it allows the understanding and maneuvering of the structure of catalytic motifs. Herein, using a series of core/shell metal/iron oxide (M/FeO_x, M = Pd, Pt, Au) nanoparticles and through a combination of in situ and ex situ electron and X-ray investigations, we revealed anomalous and dissimilar M-FeO_x interactions among different systems under reducing conditions. Pd interacts strongly with FeO_x after high-temperature reductive treatment, featured by the formation of Pd single atoms in the FeO_x matrix and increased Pd-Fe bonding, while Pt transforms into ordered PtFe intermetallics and Pt single atoms immediately upon the coating of FeO_x. In contrast, Au does not manifest strong bonding with FeO_x. As a proof of concept of tailoring metal-metal oxide interactions for catalysis, optimized Pd/FeO_x demonstrates 100% conversion and 86.5% selectivity at 60 °C for acetylene semihydrogenation.

Design of a Metal/Oxide/Carbon Interface for Highly Active and Selective Electrocatalysis

Sustainable energy-conversion and chemical-production require catalysts with high activity, durability, and product-selectivity. Metal/oxide hybrid structure has been intensively investigated to achieve promising catalytic performance, especially in neutral or alkaline electrocatalysis where water dissociation is promoted near the oxide surface for (de)protonation of intermediates. Although catalytic promise of the hybrid structure is demonstrated, it is still challenging to precisely modulate metal/oxide interfacial interactions on the nanoscale. Herein, we report an effective strategy to construct rich metal/oxide nano-interfaces on conductive carbon supports in a surfactant-free and self-terminated way. When compared to the physically mixed Pd/CeO₂ system, a much higher degree of interface formation was identified with largely improved hydrogen oxidation reaction (HOR) kinetics. The benefits of the rich metal-CeO₂ interface were further generalized to Pd alloys for optimized adsorption energy, where the Pd₃Ni/CeO₂/C catalyst shows superior performance with HOR selectivity against CO poisoning and shows long-term stability. We believe this work highlights the importance of controlling the interfacial junctions of the electrocatalyst in simultaneously achieving enhanced activity, selectivity, and stability.

Periodic structural changes in Pd nanoparticles during oscillatory CO oxidation reaction

Nanoparticle (NP) catalysts are ubiquitous in energy systems, chemical production, and reducing the environmental impact of many industrial processes. Under reactive environments, the availability of catalytically active sites on the NP surface is determined by its dynamic structure. However, atomic-scale insights into how a NP surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Using operando transmission electron microscopy, we show that Pd NPs exhibit periodic round-to-flat transitions altering their facets during CO oxidation reaction at atmospheric pressure and elevated temperatures. This restructuring causes spontaneous oscillations in the conversion of CO to CO₂ under constant reaction conditions. Our study reveals that the oscillatory behavior stems from the CO-adsorption-mediated periodic restructuring of the nanocatalysts between high-index-faceted round and low-index-faceted flat shapes. These atomic-scale insights into the dynamic surface properties of NPs under reactive conditions play an important role in the design of high-performance catalysts.

CeO2 Supported Gold Nanocluster Catalysts for CO Oxidation: Surface Evolution Influenced by the Ligand Shell

Monolayer protected Au nanocluster catalysts are known to undergo structural changes during catalytic reactions, including dissociation and migration of ligands onto the support, which strongly affects their activity and stability. To better understand how the nature of ligands influences the catalytic activity of such catalysts, three types of ceria supported Au nanoclusters with different kinds of ligands (thiolates, phosphines and a mixture thereof) have been studied, employing CO oxidation as model reaction. The thiolate-protected Au25/CeO2 showed significantly higher CO conversion after activation at 250 °C than the cluster catalysts possessing phosphine ligands. Temperature programmed oxidation and in situ infrared spectroscopy revealed that while the phosphine ligands seemed to decompose and free Au surface was exposed, temperatures higher than 250 °C are required to efficiently remove them from the whole catalyst system. Moreover, the presence of residues on the support seemed to have much greater influence on the reactivity than the gold particle size.

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO 2 Catalysts

Aiming at knowledge‐driven design of novel metal–ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well‐defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO₂ catalyst during CO oxidation. In situ X‐ray photoelectron spectroscopy and operando X‐ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low‐temperature activity is associated with sub‐oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi‐technique operando approach for establishing structure–activity relationships of technical catalysts.

Measuring and directing charge transfer in heterogenous catalysts

Precise control of charge transfer between catalyst nanoparticles and supports presents a unique opportunity to enhance the stability, activity, and selectivity of heterogeneous catalysts. While charge transfer is tunable using the atomic structure and chemistry of the catalyst-support interface, direct experimental evidence is missing for three-dimensional catalyst nanoparticles, primarily due to the lack of a high-resolution method that can probe and correlate both the charge distribution and atomic structure of catalyst/support interfaces in these structures. We demonstrate a robust scanning transmission electron microscopy (STEM) method that simultaneously visualizes the atomic-scale structure and sub-nanometer-scale charge distribution in heterogeneous catalysts using a model Au-catalyst/SrTiO₃-support system. Using this method, we further reveal the atomic-scale mechanisms responsible for the highly active perimeter sites and demonstrate that the charge transfer behavior can be readily controlled using post-synthesis treatments. This methodology provides a blueprint for better understanding the role of charge transfer in catalyst stability and performance and facilitates the future development of highly active advanced catalysts.

Revealing the Origin of Low-Temperature Activity of Ni-Rh Nanostructures during CO Oxidation Reaction with Operando TEM

In bimetallic heterostructured nanoparticles (NPs), the synergistic effect between their different metallic components leads to higher catalytic activity compared to the activity of the individual components. However, how the dynamic changes through which these NPs adopt catalytically active structures during a reaction and how the restructuring affects their activity are largely unknown. Here, using operando transmission electron microscopy, structural changes are studied in bimetallic Ni-Rh NPs, comprising of a Ni core whose surface is decorated with smaller Rh NPs, during a CO oxidation reaction. The direct atomic-scale imaging reveals that, under O2 -rich conditions, Ni core partially transforms into NiO, forming a (Ni+NiO)-Rh hollow nanocatalyst with high catalytic activity. Under O2 -poor conditions, Rh NPs alloy with the surface of the core to form a NiRh-alloy surface, and the NPs display significantly lower activity. The theoretical calculations indicate that NiO component that forms only under O2 -rich conditions enhances the activity by preventing the CO poisoning of the nanocatalysts. The results demonstrate that visualizing the structural changes during reactions is indispensable in identifying the origin of catalytic activity. These insights into the dynamic restructuring of NP catalysts under a reactive environment are critical for the rational design of high-performance nanocatalysts.

Tuning the Electronic and Steric Interaction at the Atomic Interface for Enhanced Oxygen Evolution

The two-dimensional surface or one-dimensional interface of heterogeneous catalysts is essential to determine the adsorption strengths and configurations of the reaction intermediates for desired activities. Recently, the development of single-atom catalysts has enabled an atomic-level understanding of catalytic processes. However, it remains obscure whether the conventional concept and mechanism of one-dimensional interface are applicable to zero-dimensional single atoms. In this work, we arranged the locations of single atoms to explore their interfacial interactions for improved oxygen evolution. When iridium single atoms were confined into the lattice of CoOOH, efficient electron transfer between Ir and Co tuned the adsorption strength of oxygenated intermediates. In contrast, atomic iridium species anchored on the surface of CoOOH induced inappreciable modification in electronic structures, whereas steric interactions with key intermediates at its Ir-OH-Co interface played a primary role in reducing its energy barrier toward oxygen evolution.

Effect of MnO2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO₂ polymorphs (α-, β-, and ε-MnO₂) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO₂ shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T₉₀ = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO₂ (0.135 molecule_CO·nm^-2·s^-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO₂ and α-MnO₂, respectively. The specific reaction rate order (β-MnO₂ > ε-MnO₂ > α-MnO₂) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO₂ were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

In Situ Studies of Single-Nanoparticle-Level Nickel Thermal Oxidation: From Early Oxide Nucleation to Diffusion-Balanced Oxide Thickening

High-temperature oxidation mechanisms of metallic nanoparticles have been extensively investigated; however, it is challenging to determine whether the kinetic modeling is applicable at the nanoscale and how the differences in nanoparticle size influence the oxidation mechanisms. In this work, we study thermal oxidation of pristine Ni nanoparticles ranging from 4 to 50 nm in 1 bar 1%O₂/N₂ at 600 °C using in situ gas-cell environmental transmission electron microscopy. Real-space in situ oxidation videos revealed an unexpected nanoparticle surface refacetting before oxidation and a strong Ni nanoparticle size dependence, leading to distinct structural development during the oxidation and different final NiO morphology. By quantifying the NiO thickness/volume change in real space, individual nanoparticle-level oxidation kinetics was established and directly correlated with nanoparticle microstructural evolution with specified fast and slow oxidation directions. Thus, for the size-dependent Ni nanoparticle oxidation, we propose a unified oxidation theory with a two-stage oxidation process: stage 1: dominated by the early NiO nucleation (Avrami-Erofeev model) and stage 2: the Wagner diffusion-balanced NiO shell thickening (Wanger model). In particular, to what extent the oxidation would proceed into stage 2 dictates the final NiO morphology, which depends on the Ni starting radius with respect to the critical thickness under given oxidation conditions. The overall oxidation duration is controlled by both the diffusivity of Ni²⁺ in NiO and the Ni in Ni self-diffusion. We also compare the single-particle kinetic curve with the collective one and discuss the effects of nanoparticle size differences on kinetic model analysis.

Harnessing Strong Metal-Support Interaction to Proliferate the Dry Reforming of Methane Performance by In Situ Reduction

The strong bonding at the interface between the metal and the support, which can inhibit the undesirable aggregation of metal nanoparticles and carbon deposition from reforming of hydrocarbon, is well known as the classical strong metal-support interaction (SMSI). SMSI of nanocatalysts was significantly affected by heat treatment and reducing conditions during catalyst preparation.the heat treatment and reduction conditions during catalyst preparation. SMSI can be weakened by the decrement of metal-doped sites in the supporting oxide and can often deactivate catalysts by the encapsulation of active sites through these processes. To retain SMSI near the active sites and to enhance the catalytic activity of the nanocatalyst, it is essential to increase the number of surficial metal-doped sites between nanometal and the support. Herein, we propose a mild reduction process using dry methane (CH₄/CO₂) gas that suppresses the aggregation of nanoparticles and increases the exposed interface between the metal and support, Ni and cerium oxide. The effects of mild reduction on the chemical state of Ni-cerium oxide nanocatalysts were specifically investigated in this study. As a result, mild reduction led to form large amounts of the Ni³⁺ phase at the catalyst surface of which SMSI was significantly enhanced. It can be easily fabricated while the dry reforming of methane (DRM) reaction is on stream. The superior performance of the catalyst achieved a considerably high CH₄ conversion rate of approximately 60% and stable operation up to 550 h at a low temperature, 600 °C.

Interface Engineering Between Multi-Elemental Alloy Nanoparticles and a Carbon Support Toward Stable Catalysts

Multi-elemental alloy (MEA) nanoparticles have recently received notable attention owing to their high activity and superior phase stability. Previous syntheses of MEA nanoparticles mainly used carbon as the support, owing to its high surface area, good electrical conductivity, and tunable defective sites. However, the interfacial stability issue, such as nanoparticle agglomeration, remains outstanding due to poor interfacial binding between MEA and carbon. Such a problem often causes performance decay when MEA nanoparticles are used as catalysts, hindering their practical applications. Herein, an interface engineering strategy is developed to synthesize MEA-oxide-carbon hierarchical catalysts, where the oxide on carbon helps disperse and stabilize the MEA nanoparticles toward superior thermal and electrochemical stability. Using several MEA compositions (PdRuRh, PtPdIrRuRh, and PdRuRhFeCoNi) and oxides (TiO₂ and Cr₂ O₃ ) as model systems, it is shown that adding the oxide renders superior interfacial stability and therefore excellent catalytic performance. Excellent thermal stability is demonstrated under transmission electron microscopy with in situ heating up to 1023 K, as well as via long-term cycling (>370 hours) of a Li-O₂ battery as a harsh electrochemical condition to challenge the catalyst stability. This work offers a new route toward constructing efficient and stable catalysts for various applications.

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.

Model Catalysis with HOPG-Supported Pd Nanoparticles and Pd Foil: XPS, STM and C2H4 Hydrogenation

A surface science based approach was applied to model carbon supported Pd nanoparticle catalysts. Employing physical vapour deposition of Pd on sputtered surfaces of highly oriented pyrolytic graphite (HOPG), model catalysts were prepared that are well-suited for characterization by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the HOPG substrate before and after ion-bombardment, and of Pd/HOPG before and after annealing, revealed the number of “nominal” HOPG defects (~ 10¹⁴ cm⁻²) as well as the nucleation density (~ 10¹² cm⁻²) and structural characteristics of the Pd nanoparticles (mean size/height/distribution). Two model systems were stabilized by UHV annealing to 300 °C, with mean Pd particles sizes of 4.3 and 6.8 nm and size/height aspect ratio up to ~ 10. A UHV-compatible flow microreactor and gas chromatography were used to determine the catalytic performance of Pd/HOPG in ethylene (C₂H₄) hydrogenation up to 150 °C under atmospheric pressure, yielding temperature-dependent conversion values, turnover frequencies (TOFs) and activation energies. The performance of Pd nanocatalysts is compared to that of polycrystalline Pd foil and contrasted to Pt/HOPG and Pt foil, pointing to a beneficial effect of the metal/carbon phase boundary, reflected by up to 10 kJ mol⁻¹ lower activation energies for supported nanoparticles.

Grafting nanometer metal/oxide interface towards enhanced low-temperature acetylene semi-hydrogenation

Metal/oxide interface is of fundamental significance to heterogeneous catalysis because the seemingly “inert” oxide support can modulate the morphology, atomic and electronic structures of the metal catalyst through the interface. The interfacial effects are well studied over a bulk oxide support but remain elusive for nanometer-sized systems like clusters, arising from the challenges associated with chemical synthesis and structural elucidation of such hybrid clusters. We hereby demonstrate the essential catalytic roles of a nanometer metal/oxide interface constructed by a hybrid Pd/Bi₂O₃ cluster ensemble, which is fabricated by a facile stepwise photochemical method. The Pd/Bi₂O₃ cluster, of which the hybrid structure is elucidated by combined electron microscopy and microanalysis, features a small Pd-Pd coordination number and more importantly a Pd-Bi spatial correlation ascribed to the heterografting between Pd and Bi terminated Bi₂O₃ clusters. The intra-cluster electron transfer towards Pd across the as-formed nanometer metal/oxide interface significantly weakens the ethylene adsorption without compromising the hydrogen activation. As a result, a 91% selectivity of ethylene and 90% conversion of acetylene can be achieved in a front-end hydrogenation process with a temperature as low as 44 °C.

Metal/oxide interface is of fundamental significance to heterogeneous catalysis. Here, the authors construct a nanometer Pd/Bi₂O₃ interface by grafting Pd clusters onto Bi₂O₃ clusters and demonstrate its essential roles in the low-temperature semi-hydrogenation of acetylene.

In Situ Raman Observation of Oxygen Activation and Reaction at Platinum-Ceria Interfaces during CO Oxidation

Understanding the fundamental insights of oxygen activation and reaction at metal-oxide interfaces is of significant importance yet remains a major challenge due to the difficulty in in situ characterization of active oxygen species. Herein, the activation and reaction of molecular oxygen during CO oxidation at platinum-ceria interfaces has been in situ explored using surface-enhanced Raman spectroscopy (SERS) via a borrowing strategy, and different active oxygen species and their evolution during CO oxidation at platinum-ceria interfaces have been directly observed. In situ Raman spectroscopic evidence with isotopic exchange experiments demonstrate that oxygen is efficiently dissociated to chemisorbed O on Pt and lattice Ce-O species simultaneously at interfacial Ce³⁺ defect sites under CO oxidation, leading to a much higher activity at platinum-ceria interfaces compared to that at Pt alone. Further in situ time-resolved SERS studies and density functional theory simulations reveal a more efficient molecular pathway through the reaction between adsorbed CO and chemisorbed Pt-O species transferred from the interfaces. This work deepens the fundamental understandings on oxygen activation and CO oxidation at metal-oxide interfaces and offers a sensitive technique for the in situ characterization of oxygen species under working conditions.

Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso-Scale Aggregates

Operando characterization of working catalysts, requiring per definitionem the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure, composition and adsorbed species. Frequently applied operando techniques are discussed, including X-ray absorption spectroscopy, near ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy. In contrast to these area-averaging spectroscopies, operando surface microscopy by photoemission electron microscopy delivers spatially-resolved data, directly visualizing catalyst heterogeneity. For thorough interpretation, the experimental results should be complemented by density functional theory. The operando approach enables to identify changes of cluster/nanoparticle structure and composition during ongoing catalytic reactions and reveal how molecules interact with surfaces and interfaces. The case studies cover the length-scales from clusters via nanoparticles to meso-scale aggregates, and demonstrate the benefits of specific operando methods. Restructuring, ligand/atom mobility, and surface composition alterations during the reaction may have pronounced effects on activity and selectivity. The nanoscale metal/oxide interface steers catalytic performance via a long ranging effect. Combining operando spectroscopy with switching gas feeds or concentration-modulation provides further mechanistic insights. The obtained fundamental understanding is a prerequisite for improving catalytic performance and for rational design.

Resolving multifrequential oscillations and nanoscale interfacet communication in single-particle catalysis

In heterogeneous catalysis research, the reactivity of individual nanofacets of single particles is typically not resolved. We applied in situ field electron microscopy to the apex of a curved rhodium crystal (radius of 650 nanometers), providing high spatial (~2 nanometers) and time resolution (~2 milliseconds) of oscillatory catalytic hydrogen oxidation, to image adsorbed species and reaction fronts on the individual facets. Using ionized water as the imaging species, the active sites were directly imaged with field ion microscopy. The catalytic behavior of differently structured nanofacets and the extent of coupling between them were monitored individually. We observed limited interfacet coupling, entrainment, frequency locking, and reconstruction-induced collapse of spatial coupling. The experimental results are backed up by microkinetic modeling of time-dependent oxygen species coverages and oscillation frequencies.