Catalytically Active Single-Atom Sites Fabricated from Silver Particles

Streamlined synthesis of iron and cobalt loaded MCM-48: High-performance heterogeneous catalysts for selective liquid-phase oxidation of toluene to benzaldehyde

Hydrothermal synthesis of MCM-48 molecular sieves featuring the incorporation of both iron and cobalt with Si/M ratios of 20, 40 and 80 (where M represents either iron or cobalt) was performed using tetraethyl orthosilicate as the silica source and cetyltrimethylammonium bromide as a template. To gain a comprehensive understanding of the synthesized materials, these were thoroughly characterized using various techniques, including XRD, XPS, UV-Vis (DRS), FT-IR, N2 adsorption-desorption analysis, SEM with EDX, TEM, TGA and NH3-TPD analysis. XRD analysis revealed the presence of well-ordered MCM-48 structure in the metal-incorporated materials, while XPS and UV-Vis DRS confirmed the successful partial incorporation of metal ions precisely in their desired tetrahedral coordination within the framework. To assess their catalytic performance, we studied the activity and selectivity of these catalysts in liquid phase oxidation of toluene using tert-butyl hydroperoxide as the oxidant. Under optimized conditions, employing a 6% (w/w) Fe-MCM-48 (40) catalyst and maintaining a toluene to oxidant molar ratio of 1:3 at 353 K in a solvent-free environment for 8 h, the oxidation reaction resulted in the formation of benzaldehyde (88.1%) as the major product and benzyl alcohol (11.9%) as the minor product.

Combining First-Principles Modeling and Symbolic Regression for Designing Efficient Single-Atom Catalysts in the Oxygen Evolution Reaction on Mo2CO2 MXenes

In this study, we address the significant challenge of overcoming limitations in the catalytic efficiency for the oxygen evolution reaction (OER). The current linear scaling relationships hinder the optimization of the electrocatalytic performance. To tackle this issue, we investigate the potential of designing single-atom catalysts (SACs) on Mo2CO2 MXenes for electrochemical OER using first-principles modeling simulations. By employing the Electrochemical Step Symmetry Index (ESSI) method, we assess OER intermediates to fine-tune the activity and identify the optimal SAC for Mo2CO2 MXenes. Our findings reveal that both Ag and Cu exhibit effectiveness as single atoms for enhancing OER activity on Mo2CO2 MXenes. However, among the 21 chosen transition metals (TMs) in this study, Cu stands out as the best catalyst for tweaking the overpotential (ηOER). This is due to Cu's lowest overpotential compared to other TMs, which makes it more favorable for the OER performance. On the other hand, Ag is closely aligned with ESSI = ηOER, making the tuning of its overpotential more challenging. Furthermore, we employ symbolic regression analysis to identify the significant factors that exhibit a correlation with the OER overpotential. By utilizing this approach, we derive mathematical formulas for the overpotential and identify key descriptors that affect the catalytic efficiency in the electrochemical OER on Mo2CO2 MXenes. This comprehensive investigation not only sheds light on the potential of MXenes in advanced electrocatalytic processes but also highlights the prospect of improved activity and selectivity in OER applications.

Synergistic effect of photo-thermal oxidation for a low concentration of HCHO over Bi3+-TiO2/MnFeOx catalysts at ambient temperature

Formaldehyde (HCHO) has been one of the important air pollutants, and the effective removal of HCHO at ambient temperature has been a big challenge. In this work, the synergistic effect of photo-thermal oxidation with Bi³⁺-TiO₂/MnFeO_x for a low concentration of HCHO was investigated. MnFeO_x was synthesized by the complexation method (CM) and co-precipitation (CP), and TiO₂ with Bi³⁺ doping supported on MnFeO_x was prepared by using the hydrothermal method to obtain a higher oxidation performance. The results demonstrated an excellent oxidation activity of MnFeO_x (CM) for HCHO at ambient temperature, attributed to the morphology effect (large surface areas and small crystal sizes), the large absorption of oxygen, and the interaction and oxygen vacancy formed between MnO₂ and FeO_x. Although Bi³⁺-TiO₂/MnFeO_x showed a similar result as MnFeO_x at 48 h, the oxidation activities for HCHO were improved prominently under photo-thermal oxidation at 12 h. The improvement was ascribed to the synergistic effect of Bi³⁺-TiO₂ and MnFeO_x with surface adsorbed oxygen, and more generated reactive oxygen species on the surface. In particular, 2 wt% Bi³⁺-TiO₂/MnFeO_x displayed the highest activity (90.2%) and good stability (5 cycles), and the HCHO average conversion was increased from 46.2 to 58.2% at 12 h. The feasible oxidation mechanism and reaction pathway were also interpreted. This work provides a new insight for the development of photocatalysts supported on transition metal oxides to oxidize HCHO at ambient temperature.

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.

Noble Metal Single-Atom Catalysts for the Catalytic Oxidation of Volatile Organic Compounds

Volatile organic compounds (VOCs) are detrimental to the environment and human health and must be eliminated before discharging. Oxidation by heterogeneous catalysts is one of the most promising approaches for the VOCs abatement. Precious metal catalysts are highly active for the catalytic oxidation of VOCs, but they are rare and their high price limits large-scale application. Supported metal single-atom catalysts (SACs) have a high atom efficiency and provide the possibility to circumvent such limitations. This Review summarizes recent advances in the use of metal SACs for the complete oxidation of VOCs, such as benzene, toluene, formaldehyde, and methanol, as well as aliphatic and Cl- and S-containing hydrocarbons. The structures of the metal SACs used and the reaction mechanisms of the VOC oxidation are discussed. The most widely used SACs are noble metals supported on oxides, especially on reducible oxides, such as Mn₂ O₃ and TiO₂ . The reactivity of most SACs is related to the activity of surface lattice oxygen of the oxides. Furthermore, several metal SACs show better reactivity and improved S and Cl resistance than the corresponding nanocatalysts, indicating that SACs have potential for application in the oxidation of VOCs. The deactivation and regeneration mechanisms of the metal SACs are also summarized. It is concluded that the application of metal SACs in catalytic oxidation of VOCs is still in its infancy. This Review aims to elucidate structure-performance relationships and to guide the design of highly efficient metal SACs for the catalytic oxidation of VOCs.

Metal-Support Interactions of Single-Atom Catalysts for Biomedical Applications

The development of single-atom catalysts (SACs) has become a rapidly growing research field. It is a critical challenge to understand the interactions between the single-atom metal active sites and the support materials. Recently, original research reports of SACs in biomedical applications have emerged in the literature, yet this topic has seldom been reviewed. Here, this review focuses on the latest advances in single-atom catalysis for biomedical applications and highlights the keys for the design of SACs, such as understanding the interactions between metals and supports and classifying various enzyme-like activities. This review helps bridge the knowledge of multiple disciplines and provides prospects regarding the development of SACs for biomedicine.

Using the Interaction between Copper and Manganese to Stabilize Copper Single-atom for CO Oxidation

The interaction between Cu and Mn has been used to immobilize the Cu single-atom on MnO₂ surface by redox-driven hydrolysis. Comprehensive structure and property characterizations demonstrate that the existence of an Cu-Mn interaction on the catalyst surface can effectively restrain the aggregation of Cu single atoms and improve carbon monoxide (CO) oxidation activity. The interaction of forming the Cu-O-Mn entity is beneficial for CO catalytic activity as the migration of reactive oxygen species and the coordination effect of active centers accelerate the reaction. In particular, 3%-Cu₁ /MnO₂ shows an oxygen storage capacity (OSC) value (342.75 μmol/g) more than ten times that of pure MnO₂ (27.79 μmol/g) and has high CO catalytic activity (T_90% =80 °C), it can maintain CO conversion of 95 % after 15 cycles. This work offers a reliable method for synthesizing Cu single-atom catalysts and deepens understanding of the interaction effect between single transition metal atoms and supports that can improve the catalytic activity of CO oxidation.

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods for efficient Wacker oxidation of styrene derivatives

Single-atom catalysts provide a pathway to elucidate the nature of catalytically active sites. However, keeping them stabilized during operation proves to be challenging. Herein, we employ cryptomelane-type octahedral molecular sieve nanorods featuring abundant manganese vacancy defects as a support, to periodically anchor single-atom Ag. The doped Ag atoms with tetrahedral coordination are found to locate at cation substitution sites rather than being supported on the catalyst surface, thus effectively tuning the electronic structure of adjacent manganese atoms. The resulting unique Ag–O–MnO_x unit functions as the active site. Its turnover frequency reaches 1038 h⁻¹, one order of magnitude higher than for previously reported catalysts, with 90% selectivity for anti-Markovnikov phenylacetaldehyde. Mechanistic studies reveal that the activation of styrene on the ensemble site of Ag–O–MnO_x is significantly promoted, which can accelerate the oxidation of styrene and, in particular, the rate-determining step of forming the epoxide intermediate. Such an extraordinary electronic promotion can be extended to other single-atom catalysts and paves the way for their practical applications.

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods efficiently catalyses Wacker oxidation of styrene derivatives.

Pt/MnO2 Nanoflowers Anchored to Boron Nitride Aerogels for Highly Efficient Enrichment and Catalytic Oxidation of Formaldehyde at Room Temperature

The catalytic room temperature oxidation of formaldehyde (HCHO) is widely considered as a viable method for the abatement of indoor toxic HCHO pollution. Herein, Pt/MnO₂ nanoflowers anchored to boron nitride aerogels (Pt/MnO₂ -BN) were fabricated for the catalytic room temperature oxidation of HCHO. The three-dimensional Pt/MnO₂ -BN aerogels demonstrated superior catalytic activity as a result of the improved diffusion of the reactant molecules within the porous structure. Furthermore, the porous aerogels displayed excellent HCHO adsorption capacities, which promote a rapid HCHO gas-phase concentration reduction and a subsequent complete oxidation of the adsorbed HCHO. The combined adsorption and oxidation properties of the Pt/MnO₂ -BN aerogels enhance the oxidative removal of HCHO. The optimized Pt/MnO₂ -BN demonstrated excellent catalytic activity toward HCHO (200 ppm) at room temperature, achieving a 96 % formaldehyde conversion after 50 min.

Silver Single-Atom Catalyst for Efficient Electrochemical CO2 Reduction Synthesized from Thermal Transformation and Surface Reconstruction

We report an Ag₁ single-atom catalyst (Ag₁ /MnO₂ ), which was synthesized from thermal transformation of Ag nanoparticles (NPs) and surface reconstruction of MnO₂ . The evolution process of Ag NPs to single atoms is firstly revealed by various techniques, including in situ ETEM, in situ XRD and DFT calculations. The temperature-induced surface reconstruction process from the MnO₂ (211) to (310) lattice plane is critical to firmly confine the existing surface of Ag single atoms; that is, the thermal treatment and surface reconstruction of MnO₂ is the driving force for the formation of single Ag atoms. The as-obtained Ag₁ /MnO₂ achieved 95.7 % Faradic efficiency at -0.85 V vs. RHE, and coupled with long-term stability for electrochemical CO₂ reduction reaction (CO₂ RR). DFT calculations indicated single Ag sites possessed high electronic density close to Fermi Level and could act exclusively as the active sites in the CO₂ RR. As a result, the Ag₁ /MnO₂ catalyst demonstrated remarkable performance for the CO₂ RR, far surpassing the conventional Ag nanosized catalyst (Ag_NP /MnO₂ ) and other reported Ag-based catalysts.

Radical-Assisted Formation of Pd Single Atoms or Nanoclusters on Biochar

Supported single atom or nanocluster catalysts have been widely studied due to their excellent catalytic properties. Many methods to prepare such catalysts start with constructing defects on supports, and the main focus is to improve dispersion and stability of the active sites. This paper for the first time reports a radical-assisted method to prepare single atom or nanocluster Pd on a biochar. The char was prepared by pyrolyzing walnut shell at 600°C under N₂, and Pd was loaded on the char by impregnating with palladium acetate in toluene under an oxygen-free atmosphere. It is found that there are three types of radicals in the fresh char (F-Char-600), two of them may adsorb/bond with O₂ or Pd²⁺ resulting in decreases in the char's radical concentration. The Pd on F-Char-600 for 24 h impregnation are single atoms (0.1-0.3 nm, 2%) and nanoclusters (0.3-1.2 nm, 98%), which grow larger (0.3-4 nm, 100%) for 84 h impregnation. The Pd on N₂ purged O₂-adsorbed-char (N-O-Char-600) is much larger in size. The bond between Pd and char is probably C-Pd in F-Char-600 or C-O-Pd in N-O-Char-600.

Self-Supported Nanoporous Au3 Cu Electrode with Enriched Gold on Surface for Efficient Electrochemical Reduction of CO2

The key to the electrochemical conversion of CO₂ lies in the development of efficient electrocatalysts with ease of operation, good conductivity, and rich active sites that fulfil the desired reaction direction and selectivity. Herein, an oxidative etching of Au₂₀ Cu₈₀ alloy is used for the synthesis of a nanoporous Au₃ Cu alloy, representing a facile strategy for tuning the surface electronic properties and altering the adsorption behavior of the intermediates. HRTEM, XPS, and EXAFS results reveal that the curved surface of the synthesized nanoporous Au₃ Cu is rich in gold with unsaturated coordination conditions. It can be used directly as a self-supported electrode for CO₂ reduction, and exhibits high Faradaic efficiency (FE) of 98.12 % toward CO at a potential of -0.7 V versus the reversible hydrogen electrode (RHE). The FE is 1.47 times that over the as-made single nanoporous Au. Density functional theory reveals that *CO has a relatively long distance on the surface of nanoporous Au₃ Cu, making desorption of CO easier and avoiding CO poisoning. The Hirshfeld charge distribution shows that the Au atoms have a negative charge and the Cu atoms exhibit a positive charge, which separately bond to the C atom and O atom in the *COOH intermediate through a bidentate mode. This affords the lowest *COOH adsorption free energy and low desorption energy for CO molecules.

Assessment of Ag Nanoparticles Interaction over Low-Cost Mesoporous Silica in Deep Desulfurization of Diesel

Chemical interactions between metal particles (Ag or Ni) dispersed in a low-cost MCM-41M produced from beach sand amorphous silica and sulfur compounds were evaluated in the deep adsorptive desulfurization process of real diesel fuel. N2 adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (STEM-EDX) were used for characterizing the adsorbents. HRTEM and XPS confirmed the high dispersion of Ag nanoparticles on the MCM-41 surface, and its chemical interaction with support and sulfur compounds by diverse mechanisms such as π-complexation and oxidation. Thermodynamic tests indicated that the adsorption of sulfur compounds over Ag(I)/MCM-41M is an endothermic process under the studied conditions. The magnitude of ΔH° (42.1 kJ/mol) indicates that chemisorptive mechanisms govern the sulfur removal. The best fit of kinetic and equilibrium data to pseudo-second order (R2 > 0.99) and Langmuir models (R2 > 0.98), respectively, along with the results for intraparticle diffusion and Boyd’s film-diffusion kinetic models, suggest that the chemisorptive interaction between organosulfur compounds and Ag nanosites controls sulfur adsorption, as seen in the XPS results. Its adsorption capacity (qm = 31.25 mgS/g) was 10 times higher than that obtained for pure MCM-41M and double the qm for the Ag(I)/MCM-41C adsorbent from commercial silica. Saturated adsorbents presented a satisfactory regeneration rate after a total of five sulfur adsorption cycles.

Effect of redox state of Ag on indoor formaldehyde degradation over Ag/TiO2 catalyst at room temperature

Ag/TiO₂ catalysts were prepared via in-situ synthesis and impregnation methods. The effect of redox state of Ag species on catalytic activity of Ag/TiO₂ catalysts was studied. The Ag-i-300 catalyst with partially oxidized state of Ag species shows superior catalytic activity, keeping HCHO removal efficiency at an extraordinary level of 100% during the 200 min's reaction. The Ag/TiO₂ catalysts were characterized by XPS, UV-Vis, BET, XRD, TEM, and in-situ DRIFTS technologies. XPS and TEM results exhibit that the partially oxidized state of Ag^δ+ (0 < δ < 1) and high dispersion of Ag species are beneficial for the oxidation of HCHO over Ag/TiO₂ catalysts. According to the above results, a reaction pathway for HCHO oxidation over Ag-i-300 catalyst was also proposed.

Nanopowder-Supported Ultra-Low Content Co–Rh Bimetallic Catalysts for Hydroformylation of Monoformyltricyclodecenes to Value-Added Fine Chemicals

The hydroformylation of monoformyltricyclodecenes (MFTD) to diformyltricyclodecanes (DFTD) was studied systematically. A series of 0.006 wt% Rh–0.006 wt% Co catalysts supported on commercially available nanopowders such as Al2O3, ZnO, TiO2 and CeO2 was prepared by the incipient wetness method and used to catalyse the hydroformylation of MFTD to DFTD. The 0.006 wt% Rh–0.006 wt% Co/ZnO catalyst showed the highest catalytic performance among the catalysts investigated, thus 41.8% DFTD yield with 100% DFTD selectivity could be achieved. This suggested that there may be a key role of the carrier on the catalytic performance in MFTD hydroformylation. Furthermore, the kinetic profiles for MFTD hydroformylation over the 0.006 wt% Rh–0.030 wt% Co/ZnO catalyst have been examined systematically to explore the effect of reaction temperature on the catalytic performance. These results collectively suggested that a particular reaction temperature might benefit MFTD hydroformylation. There may be some agglomeration of the active sites at higher reaction temperatures.

Ag–K/MnO2 nanorods as highly efficient catalysts for formaldehyde oxidation at low temperature

A series of Ag–K/MnO₂ nanorods with various molar ratios of K/Ag were synthesized by a conventional wetness incipient impregnation method. The as-prepared catalysts were used for the catalytic oxidation of HCHO. The Ag–K/MnO₂ nanorods with an optimal K/Ag molar ratio of 0.9 demonstrated excellent HCHO conversion efficiency of 100% at a low temperature of 60 °C. The structures of the samples were investigated by BET, TEM, SEM, XRD, H₂-TPR, O₂-TPD and XPS. The results showed that Ag–0.9K/MnO₂-r exhibited more facile reducibility and greatly abundant surface active oxygen species, endowing it with the best catalytic activity of the studied catalysts. This work provides new insights into the development of low-cost and highly efficient catalysts for the removal of HCHO.

Ag–K/MnO₂ nanorods with appropriate K/Ag ratio demonstrated excellent catalytic activity for complete oxidation of formaldehyde.

Computational screening of a single transition metal atom supported on the C2N monolayer for electrochemical ammonia synthesis

The single Mo atom supported by C₂N layer is an effective electrocatalyst for NH₃ synthesis from N₂.

Confinement boosts CO oxidation on an Ni atom embedded inside boron nitride nanotubes

Because of the confinement effect, Ni embedded on the interior surface of BNNT exhibits a much higher catalytic activity for CO oxidation by comparing with that embedded in h-BN or on the outside surface of BNNT.

Theoretical investigation on the photocatalytic activity of the Au/g-C3N4 monolayer

Two-dimensional optical catalysis materials have a wonderful potential application. Here, a new two-dimensional material consisting of the supported single-atom Au on a graphite carbon nitride (g-C3N[Formula: see text] single layer has been designed and its electronic and optical properties have been characterized by density functional calculations. The bandgap of 1.82[Formula: see text]eV calculated by the hybrid functional HSE06 shows that the Au/g-C3N4 is an indirect semiconductor, and the electron can easily be excited from the single-atom Au to the bottom of the conduction band. This material therefore has relatively strong optical properties in the visible region. Moreover, the process of Au insertion into the cavity of g-C3N4 single layer is energy-favorable. This work may provide insights and a new avenue for fabricating supported Au catalysts with high stability.

A Theoretical Investigation on CO Oxidation by Single-Atom Catalysts M1/γ-Al2O3 (M=Pd, Fe, Co, and Ni)

Single‐atom catalysts have attracted much interest recently because of their excellent stability, high catalytic activity, and remarkable atom efficiency. Inspired by the recent experimental discovery of a highly efficient single‐atom catalyst Pd₁/γ‐Al₂O₃, we conducted a comprehensive DFT study on geometries, stabilities and CO oxidation catalytic activities of M₁/γ‐Al₂O₃ (M=Pd, Fe, Co, and Ni) by using slab‐model. One of the most important results here is that Ni₁/Al₂O₃ catalyst exhibits higher activity in CO oxidation than Pd₁/Al₂O₃. The CO oxidation occurs through the Mars van Krevelen mechanism, the rate‐determining step of which is the generation of CO₂ from CO through abstraction of surface oxygen. The projected density of states (PDOS) of 2p orbitals of the surface O, the structure of CO‐adsorbed surface, charge polarization of CO and charge transfer from CO to surface are important factors for these catalysts. Although the binding energies of Fe and Co with Al₂O₃ are very large, those of Pd and Ni are small, indicating that the neighboring O atom is not strongly bound to Pd and Ni, which leads to an enhancement of the reactivity of the O atom toward CO. The metal oxidation state is suggested to be one of the crucial factors for the observed catalytic activity.

A promising single atom catalyst for CO oxidation: Ag on boron vacancies of h-BN sheets

Due to “CO-Promoted O₂ Activation”, the termolecular Eley–Rideal (TER) mechanism is the most relevant one for CO oxidation over the SAC, Ag₁/BN.

Hydroformylation of Olefins by a Rhodium Single-Atom Catalyst with Activity Comparable to RhCl(PPh3 )3

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single-atom catalysts have the advantages of both homogeneous catalysts, such as "isolated sites", and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single-atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF-STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.