Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation

Visualizing Surface-Subsurface Cu Atom Exchange at the FeO/Pt(111) Surface Induced by CO Adsorption at 150 K

Structural evolution of solid catalyst surfaces induced by direct exposure to reaction gas has been extensively studied and is well understood. However, whether and how subsurface atomic structures are affected by the reaction atmosphere require further exploration. In this work, our results confirm that Cu clusters supported on FeO/Pt(111) (Cun/FeO/Pt) transform into surface CuCO complexes (CuCO/FeO/Pt) with exposure to CO at 78 K. Surprisingly, Cu clusters on Pt(111) buried under monolayer FeO film (FeO/Cun/Pt) can also transform into surface CuCO complexes on FeO/Pt(111) upon CO adsorption at 150 K. The place exchange of surface and subsurface Cu atoms at the FeO/Pt(111) surface can be mediated by exposing to CO at 150 K and keeping in ultrahigh vacuum at 300 K, alternatively. Calculation results reveal that CO adsorption induces restructuring of the FeO film above the Cu clusters, generating a diffusion channel for Cu atoms to pass through the FeO film and form surface CuCO, while Cu atoms remaining at the FeO-Pt interface are more thermodynamically favored without CO. Our work suggests that buried subsurface atoms may be involved in strong restructuring processes driven by reaction gas, which could strongly influence the catalytic performance.

Electron Structure Tuned Oxygen Vacancy-Rich AuPd/CeO2 for Enhancing 5-Hydroxymethylfurfural Oxidation

The design of high activity catalyst for the efficiently conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) gains great interest. The rationally tailoring of electronic structure directly affects the interaction between catalysts and organic substrates, especially molecular oxygen as the oxidant. This work, the bimetallic catalysts AuPd/CeO2 were prepared by the combining method of chemical reduction and photo-deposition, effectively concerting charge between Au and Pd and forming the electron-rich state of Au. The increasing of oxygen vacancy concentration of CeO2 by acidic treatment can facilitate the adsorption of HMF for catalysts and enhance the yield of FDCA (99.0 %). Moreover, a series of experiment results combining with density functional theory calculation illustrated that the oxidation performance of catalyst in HMF conversion was strongly related to the electronic state of interfacial Au-Pd-CeO2. Furthermore, the electron-rich state sites strengthen the adsorption and activation of molecular oxygen, greatly promoting the elimination of β-hydride for the selective oxidation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to FDCA, accompanied with an outgoing FDCA formation rate of 13.21 mmol ⋅ g-1 ⋅ min-1 at 80 °C. The perception exhibited in this research could be benefit to understanding the effects of electronic state for interfacial sites and designing excellent catalysts for the oxidation of HMF.

Selective Orbital Coupling: An Adsorption Mechanism in Single-Atom Catalysis

Quantitative understanding of the chemisorption on single-atom catalysts (SACs) by their electronic properties is crucial for the catalyst design. However, the physical mechanism is still under debate. Here, the CO catalytic oxidation on single transition metal (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni) dopants is used as a theoretical model to explore the correlations between the characteristics of electronic structures and the chemisorption on SACs. For these metal dopants, their atomic d orbitals form several nondegenerate and localized electronic states that are found to be selectively coupled with the π* orbital of the adsorbed O2, which we defined as selective orbital coupling. Based on the selective orbital coupling, we find that the alignment between the selected d state and the π* state determines the bond strength, regardless of the electron occupation number of the selected d states; the electron transfer to form M-O bonding can be provided by the support. Such electron transfer can be related with the electronic metal-support interaction. We attribute the origin of the chemisorption mechanism to the coexistence of the localized orbital of the single transition metal and the continuous energy band of the Au support. Finally, we illustrate how this mechanism dominates the variation trend of the reaction barriers. Our results unravel a fundamental adsorption mechanism in SAC systems.

Adsorption of O2 on the Preferred -O-Au Sites of Small Gold Oxide Clusters: Charge-dependent Interaction and Activation

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O2 and the impact of charge, remain unanswered. Here, we have utilized an improved genetic algorithm program coupled with the DFT method to systematically search for the structures of Au1-5Ox-/+/0 (x = 1-4) and calculated binding interactions between Au1-5Ox-/+/0 (x = 1-2) and O2, aiming to determine the active sites and to elucidate the impact of different charge states in gold oxide systems. The results revealed that the reactivity of all three kinds of small gold oxide clusters toward O2 is strongly site-dependent, with clusters featuring an -O-Au site exhibiting a preference for adsorption. The charges on small gold oxide clusters significantly impact the interaction strength and the activation degree of adsorbed O2: in the case of anionic cluster, the interaction between O2 and the -O-Au sites leads to a chemical reaction involving electron transfer, thereby significantly activating O2; in neutral and cationic clusters, the adsorption of O2 on their -O-Au sites can be viewed as an electrostatic interaction. Pointedly, for cationic clusters, the highly concentrated positive charge on the Au atom of the -O-Au sites can strongly adsorb but hardly activate the adsorbed O2. These results have certain reference points for understanding the gold oxide interfaces and the improved catalytic oxidation performance of gold-based systems in the presence of atomic oxygen species.

Decomposition of methanol-d4 on Rh nanoclusters supported by thin-film Al2O3/NiAl(100) under near-ambient-pressure conditions

The decomposition of methanol-d4 (CD3OD) on Rh nanoclusters grown by the deposition of Rh vapors onto an ordered thin film of Al2O3/NiAl(100) was studied, with various surface-probe techniques and largely under near-ambient-pressure (NAP) conditions. The results showed a superior reactivity of small Rh clusters (diameter < 1.5 nm) exposed to CD3OD at 5 × 10-3-0.1 mbar at 400 K; the gaseous production of CO and D2 from decomposed methanol-d4 per Rh surface site on the small Rh clusters with diameters of ∼1.1 nm was nearly 8 times that on large ones with diameters of ∼3.5 nm. The promotion of reactivity with decreased cluster size under NAP conditions was evidently greater than that under ultrahigh vacuum conditions. Moreover, the concentration of atomic carbon (C*; where * denotes adsorbate)-a key catalyst poisoner-yielded from the dissociation of CO* from dehydrogenated methanol-d4 was significantly smaller on small clusters (diameter < 1.5 nm). The NAP size effect on methanol-d4 decomposition involved the surface hydroxyl (OH*) from the little co-adsorbed water (H2O*) that was dissociated at a probability dependent on the cluster size. H2O* was more likely dissociated into OH* on small Rh clusters, by virtue of their more reactive d-band structure, and the OH* then effectively promoted the O-D cleavage of methanol-d4, as the rate-determining step, and thus the reaction probability; on the other hand, the OH* limited CO* dissociation on small Rh clusters via both adsorbate and lateral effects. These results suggest that the superior properties of small Rh clusters in both reactivity and anti-poisoning would persist and be highly applicable under "real-world" catalysis conditions.

Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

One-dimensional single atom arrays on ferroelectric nanosheets for enhanced CO2 photoreduction

Single-atom catalysts show excellent catalytic performance because of their coordination environments and electronic configurations. However, controllable regulation of single-atom permutations still faces challenges. Herein, we demonstrate that a polarization electric field regulates single atom permutations and forms periodic one-dimensional Au single-atom arrays on ferroelectric Bi4Ti3O12 nanosheets. The Au single-atom arrays greatly lower the Gibbs free energy for CO2 conversion via Au-O=C=O-Au dual-site adsorption compared to that for Au-O=C=O single-site adsorption on Au isolated single atoms. Additionally, the Au single-atom arrays suppress the depolarization of Bi4Ti3O12, so it maintains a stronger driving force for separation and transfer of photogenerated charges. Thus, Bi4Ti3O12 with Au single-atom arrays exhibit an efficient CO production rate of 34.15 µmol·g-1·h-1, ∼18 times higher than that of pristine Bi4Ti3O12. More importantly, the polarization electric field proves to be a general tactic for the syntheses of one-dimensional Pt, Ag, Fe, Co and Ni single-atom arrays on the Bi4Ti3O12 surface.

A MXene@AgAu@PDA nanoplatform loaded with AgAu nanocages for enhancing catalytic activity and antibacterial performance

With the rapid development of social industrialization, environmental problems seriously threaten people's health, especially water pollution. Therefore, there is an urgent need to construct a multifunctional nanoplatform for different scenarios. Two-dimensional MXene@AgAu@PDA nanosheets loaded with AgAu bimetallic nanocages have been prepared by a one-step method. First, the in situ generated MXene@Ag is used as an auxiliary template, and then HAuCl4 and dopamine are added for in situ redox-oxidizing polymerization reactions to obtain AgAu catalytic nanocages and the protective polydopamine (PDA) layer which can improve the stability and biocompatibility. MXene and PDA have excellent photothermal conversion ability while hollow AgAu nanocages have strong absorption in the near-infrared region and a local surface plasmonic resonance effect. In comparison to the catalytic reaction rates under dark and room temperature conditions, the catalytic kinetic rate of MXene@AgAu@PDA nanosheets under near-infrared irradiation increases from 0.13 to 0.69 min-1 mg-1. Density functional theory (DFT) is used to study the electron transfer behavior between AgAu nanocages and MXene nanosheets, and the mechanism of the enhanced catalytic reaction rate is analyzed. Besides, due to its Ag ions and photothermal coupling antibacterial properties, 40 μg mL-1 MXene@AgAu@PDA nanosheets inactivates nearly all E. coli and S. aureus after irradiation with near-infrared light for 6 min.

Tip-activated single-atom catalysis: CO oxidation on Au adatom on oxidized rutile TiO2 surface

Single-atom catalysis of carbon monoxide oxidation on metal-oxide surfaces is crucial for greenhouse recycling, automotive catalysis, and beyond, but reports of the atomic-scale mechanism are still scarce. Here, using scanning probe microscopy, we show that charging single gold atoms on oxidized rutile titanium dioxide surface, both positively and negatively, considerably promotes adsorption of carbon monoxide. No carbon monoxide adsorption is observed on neutral gold atoms. Two different carbon monoxide adsorption geometries on gold atoms are identified. We demonstrate full control over the redox state of adsorbed gold single atoms, carbon monoxide adsorption geometry, and carbon monoxide adsorption/desorption by the atomic force microscopy tip. On charged gold atoms, we activate Eley-Rideal oxidation reaction between carbon monoxide and a neighboring oxygen adatom by the tip. Our results provide unprecedented insights into carbon monoxide adsorption and suggest that the gold dual activity for carbon monoxide oxidation after electron or hole attachment is also the key ingredient in photocatalysis under realistic conditions.

Reconfigurable Manipulation of Oxygen Content on Metal Oxide Surfaces and Applications to Gas Sensing

Oxygen vacancies and adsorbed oxygen species on metal oxide surfaces play important roles in various fields. However, existing methods for manipulating surface oxygen require severe settings and are ineffective for repetitive manipulation. We present a method to manipulate the amount of surface oxygen by modifying the oxygen adsorption energy by electrically controlling the electron concentration of the metal oxide. The surface oxygen control ability of the method is verified using first-principles calculations based on density functional theory (DFT), X-ray photoelectron spectroscopy (XPS), and electrical resistance analysis. The presented method is implemented by fabricating oxide thin film transistors with embedded microheaters. The method can reconfigure the oxygen vacancies on the In2O3, SnO2, and IGZO surfaces so that specific chemisorption dominates. The method can selectively increase oxidizing (e.g., NO and NO) and reducing gas (e.g., H2S, NH3, and CO) reactions by electrically controlling the metal oxide surface to be oxygen vacancy-rich or adsorbed oxygen species-rich. The proposed method is applied to gas sensors and overcomes their existing limitations. The method makes the sensor insensitive to one gas (e.g., H2S) in mixed-gas environments (e.g., NO2+H2S) and provides a linear response (R2 = 0.998) to the target gas (e.g., NO2) concentration within 3 s. We believe that the proposed method is applicable to applications utilizing metal oxide surfaces.

Clustering-Evolved Frontier Orbital for Low-Temperature CO2 Dissociation

In this study, single Ni2 clusters (two Ni atoms bridged by a lattice oxygen) are successfully synthesized on monolayered CuO. They exhibit a remarkable activity toward low-temperature CO2 thermal dissociation, in contrast to cationic Ni atoms that nondissociatively adsorb CO2 and metallic Ni ones that are chemically inert for CO2 adsorption. Density functional theory calculations reveal that the Ni2 clusters can significantly alter the spatial symmetry of their unoccupied frontier orbitals to match the occupied counterpart of the CO2 molecule and enable its low-temperature dissociation. This study may help advance single-cluster catalysis and exploit the unexcavated mechanism for low-temperature CO2 activation.

Preparation of Ce-MnOx Composite Oxides via Coprecipitation and Their Catalytic Performance for CO Oxidation

Ce-MnOx composite oxide catalysts with different proportions were prepared using the coprecipitation method, and the CO-removal ability of the catalysts with the tested temperature range of 60-140 °C was investigated systematically. The effect of Ce and Mn ratios on the catalytic oxidation performance of CO was investigated using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), H2 temperature programmed reduction (H2-TPR), CO-temperature programmed desorption (CO-TPD), and in situ infrared spectra. The experimental results reveal that under the same test conditions, the CO conversion rate of pure Mn3O4 reaches 95.4% at 170 °C. Additionally, at 140 °C, the Ce-MnOx series composite oxide catalyst converts CO at a rate of over 96%, outperforming single-phase Mn3O4 in terms of catalytic performance. With the decrement in Ce content, the performance of Ce-MnOx series composite oxide catalysts first increase and then decrease. The Ce MnOx catalyst behaves best when Ce:Mn = 1:1, with a CO conversion rate of 99.96% at 140 °C and 91.98% at 100 °C.

Rare-earth metal-N6 centers in porous carbon for electrocatalytic CO2 reduction

Single-atom catalysts fabricated using rare earth elements have emerged for electrocatalytic carbon dioxide reduction, but they need to be studied systematically and intensively. Herein, density functional theory was employed to determine the electrocatalytic CO₂ reduction activity of rare earth-N₆ porous carbon (Re = Ce, Nd, Sm, Eu, Gd, Tb, Er, Tm, Yb, and Lu) single-atom catalysts. The results revealed that the binding energy of the rare-earth atoms to the N₆C monolayers in the ten studied Re-N₆C monatomic catalysts is much more negative than the cohesion energy of the bulk rare-earth metal, which makes rare-earth atoms stably dispersed in the N₆C skeleton. CO is the primary chemical product of electrocatalytic CO₂ reduction by Ce, Eu, and Lu. The primary product of the six monatomic species, i.e., Nd, Sm, Tb, Er, Tm, and Yb, is HCOOH. The dominant product of Gd is CH₄. The limiting potentials of these catalysts are in the range of 0.31-0.786 V and their overpotentials are in the range of 0.06-0.707 V, all of which are relatively low, showing that they are potential and promising electrocatalysts for CO₂ reduction. Subsequently, Eu-N₆C was experimentally synthesized and used for electrocatalytic CO₂ reduction to obtain CO products, and the overpotential showed good agreement with the theoretically calculated values.

Tuning the Hydroxyl Density of MXene to Regulate the Electrochemical Performance of Anchored Cobalt Phthalocyanine for CO2 Reduction

Precise electronic state regulation through coordination environment optimization by metal-support interaction is a promising strategy to facilitate catalysis reaction, while the limited density of functional groups in the bulk substrate restricts the regulation degree. Herein, different sizes of Ti₃C₂T_x MXene with hydroxyl (-OH) terminal including the MXene layer (ML-OH, 3 μm), the MXene nanosheet (MNS-OH, 600 nm), and the MXene quantum dot (MQD-OH, 8 nm) were prepared to anchor CoPc, and the effect of -OH density on the performance of electrochemical CO₂ reduction was systematically investigated. Notably, a linear relationship was established by plotting reactivity vs hydroxyl density. With the highest -OH density, CoPc/MQD-OH exhibits a superior Faradaic efficiency for CO formation (FE_CO) of ∼100% at -0.9 to -1.0 V vs RHE and a high FE_CO of >90% over a wide potential window from -0.8 to -1.4 V. The mechanism exploration shows that the axial coordination interaction of the -OH terminal with Co increases the electron density of the Co site, thus promoting the adsorption and activation of CO₂. This work provides a new insight into designing of molecular catalysts with high efficiency and tunable structure for other electrochemical conversions.

Zr-MOF-Induced Smart Accumulation Enables Surface-Enhanced Raman Spectroscopic Detection of Dioxin at ppt Level in Food Samples

The fast and economical detection of trace polychlorinated dibenzo-p-dioxins (PCDDs) in food samples by current mass spectrum-based methods is hindered by tedious sample preparation and bulky & expensive analytical instruments. Surface-enhanced Raman spectroscopy (SERS) successfully detects many organic pollutants in foods but not dioxins because the employed metal nanoparticles weakly adsorb hydrophobic PCDDs. Herein, we report the detection of PCDDs in milk with SERS for the first time using a bifunctional substrate consisting of Au nanoparticles embedded in a zirconium-based metal-organic framework shell (AuNP/Zr-MOF). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), as the most toxic PCDD, is detected as low as 1.2 parts per trillion (ppt) in real milk samples with massive interfering substances in 30 min, which is the lowest among all reported methods. The aromatic rings of Zr-MOF promote the smart accumulation of TCDD through π-π interactions, and Au-Cl interactions drive TCDD onto Au surfaces. Zr-MOF shells with pore sizes of 12.7 and 20 Å block the accessibility of larger interfering molecules. A one-step apparatus and protocol are established to be superior to traditional methods in terms of time and cost. This work provides new insight into a rational screening method for the detection of persistent organic pollutants in a real sample matrix.

Decomposition of methanol-d4 on a thin film of Al2O3/NiAl(100) under near-ambient-pressure conditions

We have studied the decomposition of methanol-d4 on thin film Al2O3/NiAl(100) under near-ambient-pressure conditions, with varied surface-probe techniques and calculations based on density-functional theory. Methanol-d4 neither adsorbed nor reacted on Al2O3/NiAl(100) at 400 K under ultrahigh vacuum conditions, whereas they dehydrogenated, largely to methoxy-d3 (CD3O*, * denoting adsorbates) and formaldehyde-d2 (CD2O*), on the surface when the methanol-d4 partial pressure was increased to 10-3 mbar and above. The dehydrogenation was facilitated by hydroxyl (OH* or OD*) from the dissociation of little co-adsorbed water; a small fraction of CD2O* interacted further with OH* (OD*) to form, via intermediate CD2OOH* (CD2OOD*), formic acid (DCOOH* or DCOOD*). A few surface carbonates were also yielded, likely on the defect sites of Al2O3/NiAl(100). The results suggest that alumina not only supports metal clusters but also participates in reactions under realistic catalytic conditions. One may consider accordingly the multiple functions of alumina while designing ideal catalysts.

Time-Resolved Formation and Operation Maps of Pd Catalysts Suggest a Key Role of Single Atom Centers in Cross-Coupling

An approach to the spatially localized characterization of supported catalysts over a reaction course is proposed. It consists of a combination of scanning, transmission, and high-resolution scanning transmission electron microscopy to determine metal particles from arrays of surface nanoparticles to individual nanoparticles and individual atoms. The study of the evolution of specific metal catalyst particles at different scale levels over time, particularly before and after the cross-coupling catalytic reaction, made it possible to approach the concept of 4D catalysis-tracking the positions of catalytic centers in space (3D) over time (+1D). The dynamic behavior of individual palladium atoms and nanoparticles in cross-coupling reactions was recorded with nanometer accuracy via the precise localization of catalytic centers. Single atoms of palladium leach out into solution from the support under the action of the catalytic system, where they exhibit extremely high catalytic activity compared to surface metal nanoparticles. Monoatomic centers, which make up only approximately 1% of palladium in the Pd/C system, provide more than 99% of the catalytic activity. The remaining palladium nanoparticles changed their shape and could move over the surface of the support, which was recorded by processing images of the array of nanoparticles with a neural network and aligning them using automatically detected keypoints. The study reveals a novel opportunity for single-atom catalysis─easier detachment (capture) from (on) the carbon support surface is the origin of superior catalytic activity, rather than the operation of single atomic catalytic centers on the surface of the support, as is typically assumed.

Single-atom catalysis enabled by high-energy metastable structures

Owing to limited degrees of freedom, the active sites of stable single-atom catalyst (SAC) often have one structure that is energetically much lower than other local-minimum structures. Thus, the SAC adopts one lowest-energy structure (LES) with an overwhelmingly larger proportion than any other high-energy metastable structure (HEMS), and the LES is commonly assumed to be solely responsible for the catalytic performance of an SAC. Herein, we demonstrate with SACs anchored on CeO₂ that the HEMS of an SAC, even though its proportion remains several orders of magnitude lower than the LES throughout the catalytic reaction, can dictate catalysis with extraordinary activity arising from its unique coordination environment and oxidation state. Thus, we unravel the key role of HEMS-enabled catalysis in single-atom catalysis, which shakes the common assumption in the studies of SACs and urges new developments in both experiment and theory to identify and exploit catalysis via HEMSs.

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

The adsorption of single Au atom and nucleation on γ-Al2O3 surfaces

Single-atom catalysts (SACs) in heterogeneous catalysts have attracted increasing attention and the adsorption and nucleation of single atom on the surface are closely related to the performance of the catalyst. The present work employed density functional theory calculations to examine the adsorption of single Au atom and nucleation on γ-Al₂O₃ surfaces at the atomic level. The effect of surface hydroxyls group on the adsorption and nucleation of single Au atom on γ-Al₂O₃ surfaces is explored. It was found that the spillover reactions of surface hydroxyls H atoms with the deposited Au_- are not available on the hydroxylated surface. The interaction of Au to the clean surface is the stronger than to the hydroxylated surface. The even-odd alternations of Au_x and weak binding of single Au atoms to γ-Al₂O₃ leads to large even-numbered Au cluster on the surface. Density of states and electron density difference analysis show that the electronic structure of Au/γ-Al₂O₃ is quite different from the reported Cu and Pd on Al₂O₃.

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.

Isolating Contiguous Ir Atoms and Forming Ir-W Intermetallics with Negatively Charged Ir for Efficient NO Reduction by CO

The lack of efficient catalysts with a wide working temperature window and vital O₂ and SO₂ resistance for selective catalytic reduction of NO by CO (CO-SCR) largely hinders its implementation. Here, a novel Ir-based catalyst with only 1 wt% Ir loading is reported for efficient CO-SCR. In this catalyst, contiguous Ir atoms are isolated into single atoms, and Ir-W intermetallic nanoparticles are formed, which are supported on ordered mesoporous SiO₂ (KIT-6). Notably, this catalyst enables complete NO conversion to N₂ at 250 °C in the presence of 1% O₂ and has a wide temperature window (250-400 °C), outperforming the comparison samples with Ir isolated-single-atomic-sites and Ir nanoparticles, respectively. Also, it possesses a high SO₂ tolerance. Both experimental results and theoretical calculations reveal that single Ir atoms are negatively charged, dramatically enhancing the NO dissociation, while the Ir-W intermetallic nanoparticles accelerate the reduction of the N₂ O and NO₂ intermediates by CO.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

Insight into the transient inactivation effect on Au/TiO2 catalyst by in-situ DRIFT and UV–vis spectroscopy

Au catalysts have drawn broad attention for catalytic CO oxidation. However, a molecular-level understanding of the reaction mechanism on a fast time-resolved scale is still lacking. Herein, we apply in situ DRIFTS and UV-Vis spectroscopy to monitor the rapid dynamic changes during CO oxidation over Au/TiO₂. A pronounced transient inactivation effect likely due to a structural change of Au/TiO₂ induced by the reactants (CO and O₂) is observed at the beginning of the reaction. The transient inactivation effect is affected by the ratio of CO and O₂ concentrations. More importantly, during the unstable state, the electronic properties of the Au particles change, as indicated by the shift of the CO stretching vibration. UV-Vis spectroscopy corroborates the structure change of Au/TiO₂ surface induced by the reactants, which leads to a weakening of the Au catalyst’s ability to be oxidized (less O₂ adsorption), resulting in the transient inactivation effect.

A molecular-level understanding of the Au-catalyzed CO oxidation on a fast time-resolved scale is still lacking. Here the authors monitor the rapid dynamic changes during CO oxidation over Au/TiO2 using in situ DRIFTS and UV-Vis spectroscopy, and reveal that the catalyst undergoes a surprising structural change at the beginning of the reaction.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Mn-doped single atom nanozyme composited Au for enhancing enzymatic and photothermal therapy

As an emerging technology, nanocatalytic medicine attracts much attention, especially the ones according to the enzymatic reaction by using excess H₂O₂ in the tumor. Among various candidates, single-atom catalyst (SAC) revealed unique and outstanding redox reaction performance, since the active sites consisting of single metal atoms may achieve the maximum utilization of metal atoms and emerge obviously amplified reaction rate. Here we developed an M-N_x (M = Mn, Zn) center-based SAC with a hollow structure by calcination of Mn²⁺-doped zeolitic imidazolate frameworks (ZIF-8), and PEGylation was applied to improve the hydrophilicity. According to the enzymatic reaction, the M-N_x (M = Mn, Zn) centers have an inherent peroxidase-like activity to catalyze over-expressed H₂O₂ in the weak acidic tumor microenvironment and generate a large amount of toxic reactive oxygen species (ROS) like hydroxyl radicals for therapy. To keep efficient therapeutic output, we integrated the hollow SAC with Au which could expend the glucose in tumor and supply H₂O₂ as the substrate of peroxidase-like activity. Better yet, Au may boost the photothermal effect of SAC and offer another non-invasive photothermal therapy (PTT) to promote the effect of tumor removal. This platform provided a new idea for the construction of more efficient peroxidase-like activity in tumor therapy.

Understanding and application of metal-support interactions in catalysts for CO-PROX

Preferential oxidation of carbon monoxide (CO-PROX) plays a vital role in H₂ purification in the upstream systems of proton exchange membrane fuel cells (PEMFCs) for its high efficiency, low cost and practicability. The key to the application of CO-PROX is the design and preparation of catalysts, and the supported metal catalysts have been the mainstay after decades of development. The metal-support interaction (MSI), which acts as a bridge between the design of supported catalysts and atomic-level theoretical research, has triggered increasing attention. There is a growing body of literature that recognizes the importance of the MSI in heterogeneous catalysis. In this review, the impacts of the MSI including strong metal-support interactions and electronic metal-support interactions on the essential characteristics of supported single atom, nanocluster and nanoparticle catalysts, and therefore, on catalytic behaviors were discussed, respectively, primarily focusing on electron transfer, chemical bonding and the encapsulation of active sites induced by the MSI. We also presented an overview of how the MSI can be utilized to rationally design catalysts to meet target requirements such as high activity, selectivity or stability via appropriate selection and modification of support and active species. The perspectives of the future development for comprehensive understanding of the MSI were also proposed.

Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2-Hexanol over Au/ZrO2 Catalysts

2-Hexanol (2-HOL) is a versatile biomass-derived platform molecule for synthesis of liquid transportation fuels, lubricants, or detergents. Herein, a one-step preparation of 2-HOL using 5-hydroxymethylfurfural (HMF) as a substrate was reported for the first time. Several Au-based catalysts supported on different metal oxides were prepared to explore the relationship between carrier and catalytic activity. The results showed that the highest 2-HOL yield of 65.8 % was obtained at complete HMF conversion over the 5 %Au/ZrO₂ catalyst. The 5 %Au/ZrO₂ catalyst exhibited excellent durability after five consecutive recycling runs, while confirming its remarkable ring-opening hydrogenolysis on other biomass-derived furanics, furfural, with a total yield of 1-pentanol and 2-pentanol of 67.4 %. The distinguished ring-opening hydrogenolysis performance of the Au/ZrO₂ catalyst originated from a synergistic effect between the interfacial Au-O-Zr oxygen vacancies-induced Lewis acidic sites (activating C-OH/C=O bonds) and metallic Au (activating H₂ ). This work provides a possibility for producing 2-HOL from HMF with high yield, expanding the sustainable application of lignocellulosic biomass.

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.

Coadsorption Interfered CO Oxidation over Atomically Dispersed Au on h-BN

Similar to the metal centers in biocatalysis and homogeneous catalysis, the metal species in single atom catalysts (SACs) are charged, atomically dispersed and stabilized by support and substrate. The reaction condition dependent catalytic performance of SACs has long been realized, but seldom investigated before. We investigated CO oxidation pathways over SACs in reaction conditions using atomically dispersed Au on h-BN (AuBN) as a model with extensive first-principles-based calculations. We demonstrated that the adsorption of reactants, namely CO, O₂ and CO₂, and their coadsorption with reaction species on AuBN would be condition dependent, leading to various reaction species with different reactivity and impact the CO conversion. Specifically, the revised Langmuir–Hinshelwood pathway with the CO-mediated activation of O₂ and dissociation of cyclic peroxide intermediate followed by the Eley–Rideal type reduction is dominant at high temperatures, while the coadsorbed CO-mediated dissociation of peroxide intermediate becomes plausible at low temperatures and high CO partial pressures. Carbonate species would also form in existence of CO₂, react with coadsorbed CO and benefit the conversion. The findings highlight the origin of the condition-dependent CO oxidation performance of SACs in detailed conditions and may help to rationalize the current understanding of the superior catalytic performance of SACs.

Adatom Bonding Sites in a Nickel-Fe3 O4 (001) Single-Atom Model Catalyst and O2 Reactivity Unveiled by Surface Action Spectroscopy with Infrared Free-Electron Laser Light

Single-atom (SA) catalysis presently receives much attention with its promise to decrease the cost of the active material while increasing the catalyst's performance. However, key details such as the exact location of SA species and their stability are often unclear due to a lack of atomic level information. Here, we show how vibrational spectra measured with surface action spectroscopy (SAS) and density functional theory (DFT) simulations can differentiate between different adatom binding sites and determine the location of Ni and Au single atoms on Fe₃ O₄ (001). We reveal that Ni and Au adatoms selectively bind to surface oxygen ions which are octahedrally coordinated to Fe ions. In addition, we find that the Ni adatoms can activate O₂ to superoxide in contrast to the bare surface and Ni in subsurface positions. Overall, we unveil the advantages of combining SAS and DFT for improving the understanding of single-atom catalysts.

Single-Cation Catalyst: Ni Cation in Monolayered CuO for CO Oxidation

It is vital to differentiate catalytic properties between cationic and metallic single atoms at the atomic level. To achieve this, we fabricated well-defined cationic Ni atoms snugged in and metallic Ni atoms supported on monolayered CuO. The Ni cations are chemically inert for CO adsorption even at 70 K but highly active toward O₂ dissociation at room temperature. The adsorbed O atoms are active to oxidize incoming CO molecules from the gas phase into CO₂, which follows the Eley-Rideal mechanism, in contrast to the Mars-van Krevelen mechanism on CuO-monolayer-supported metallic Ni atoms as well as our previously reported Au and Pt model catalysts. This study helps understand the chemistry of a supported single-metal cation, which is of great importance in heterogeneous catalysis.

Activity origin of boron doped carbon cluster for thermal catalytic oxidation: Coupling effects of dopants and edges

Catalytic oxidation plays important roles in energy conversion and environment protection. Boron-doped crystalline carbocatalyst has been demonstrated effective; however, the application potential of boron-doped amorphous carbocatalyst remains to be explored. For amorphous carbon material, finite-sized carbon clusters are the basic structural units, which exhibit unique activity due to edge and size effect. Herein, using sulfur dioxide (SO₂) and carbon monoxide (CO) oxidation as probe thermal-catalysis reactions, we found the distribution and reactivity of active sites in boron-doped carbon clusters are simultaneously determined by dopants and edges. According to comparisons of oxygen (O₂) chemisorption energy at different sites of symmetric and non-symmetric carbon cluster, the most active site is found to be the edge carbon atom with high electron donation ability, which can be accurately identified by electrophilic Fukui function. More importantly, the reactivity of boron-doped cluster is simultaneously influenced by doping configuration and the type of edge, based on which -O-B-O- configuration embedded into K-region edge (isolated carbon-carbon double bonds that do not belong to Clar sextet) is predicted to exhibit the highest reactivity among various boron doping configurations. This work clarifies unique activity origin of heteroatom-doped amorphous carbon materials, providing new insights into designing high-performance carbocatalysts.

Research Progress and Application of Single-Atom Catalysts: A Review

Due to excellent performance properties such as strong activity and high selectivity, single-atom catalysts have been widely used in various catalytic reactions. Exploring the application of single-atom catalysts and elucidating their reaction mechanism has become a hot area of research. This article first introduces the structure and characteristics of single-atom catalysts, and then reviews recent preparation methods, characterization techniques, and applications of single-atom catalysts, including their application potential in electrochemistry and photocatalytic reactions. Finally, application prospects and future development directions of single-atom catalysts are outlined.

Adjacent single-atom irons boosting molecular oxygen activation on MnO2

Efficient molecular oxygen activation is crucial for catalytic oxidation reaction, but highly depends on the construction of active sites. In this study, we demonstrate that dual adjacent Fe atoms anchored on MnO₂ can assemble into a diatomic site, also called as MnO₂-hosted Fe dimer, which activates molecular oxygen to form an active intermediate species Fe(O = O)Fe for highly efficient CO oxidation. These adjacent single-atom Fe sites exhibit a stronger O₂ activation performance than the conventional surface oxygen vacancy activation sites. This work sheds light on molecular oxygen activation mechanisms of transition metal oxides and provides an efficient pathway to activate molecular oxygen by constructing new active sites through single atom technology.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O₂/Ar and 10 vol % H₂/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au³⁺ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au³⁺ possesses the optimal activity with 1.8 × 10^-2 s^-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au³⁺ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au³⁺ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H₂ via constructing frustrated Lewis pairs with O.

Atomically dispersed Pb ionic sites in PbCdSe quantum dot gels enhance room-temperature NO2 sensing

Atmospheric NO2 is of great concern due to its adverse effects on human health and the environment, motivating research on NO2 detection and remediation. Existing low-cost room-temperature NO2 sensors often suffer from low sensitivity at the ppb level or long recovery times, reflecting the trade-off between sensor response and recovery time. Here, we report an atomically dispersed metal ion strategy to address it. We discover that bimetallic PbCdSe quantum dot (QD) gels containing atomically dispersed Pb ionic sites achieve the optimal combination of strong sensor response and fast recovery, leading to a high-performance room-temperature p-type semiconductor NO2 sensor as characterized by a combination of ultra-low limit of detection, high sensitivity and stability, fast response and recovery. With the help of theoretical calculations, we reveal the high performance of the PbCdSe QD gel arises from the unique tuning effects of Pb ionic sites on NO2 binding at their neighboring Cd sites.