Sodium Rivals Silver as Single-Atom Active Centers for Catalyzing Abatement of Formaldehyde

Promotional catalytic activity and reaction mechanism of Ag-modified Ce0.6Zr0.4O2 catalyst for catalytic oxidation of ammonia

Ce_1-xZr_xO₂ composite oxides (molar, x = 0-1.0, interval of 0.2) were prepared by a cetyltrimethylammonium bromide-assisted precipitation method. The enhancement of silver-species modification and catalytic mechanism of adsorption-transformation-desorption process were investigated over the Ag-impregnated catalysts for low-temperature selective catalytic oxidation of ammonia (NH₃-SCO). The optimal 5 wt.% Ag/Ce_0.6Zr_0.4O₂ catalyst presented good NH₃-SCO performance with >90% NH₃ conversion at temperature (T) ≥ 250°C and 89% N₂ selectivity. Despite the irregular block shape and underdeveloped specific surface area (∼60 m²/g), the naked and Ag-modified Ce_0.6Zr_0.4O₂ solid solution still obtained highly dispersed distribution of surface elements analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) (mapping), N₂ adsorption-desorption test and X-ray diffraction (XRD). H₂ temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that Ag-modification enhanced the mobility and activation of oxygen-species leading to a promotion on CeO₂ reducibility and synergistic Ag⁰/Ag⁺ and Ce⁴⁺/Ce³⁺ redox cycles. Besides, Ag⁺/Ag₂O clusters could facilitate the formation of surface oxygen vacancies that was beneficial to the adsorption and activation of ammonia. NH₃-temperature programmed desorption (NH₃-TPD) showed more adsorption-desorption capacity to ammonia were provided by physical, weak- and medium-strong acid sites. Diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the activation of ammonia might be the control step of NH₃-SCO procedure, during which NH₃ dehydrogenation derived from NH_x-species and also internal selective catalytic reduction (i-SCR) reactions were proposed.

Designing of 3D MnO2-graphene catalyst on sponge for abatement temperature removal of formaldehyde

The Mn-based catalysts, with low cost and high activity, are believed to be the effective composites for eliminating in-door formaldehyde (HCHO), while the powdered form nanosized catalysts are hardly to apply for practical application. Herein, hetero-structure of nanosheets manganese oxide (MnO₂) encapsulating N-doping graphene sphere (GS) were deposited in network-like sponge for constructing 3D catalyst. The prepared MnO₂-GS-Sponge composite catalyst exhibited excellent performance for removing HCHO at room temperature compared with GS and commercial MnO₂. The MnO₂-GS with larger specific surface area (209.1 m²·g^-1) was dispersed evenly in 3D network of sponge, which facilitated exposing more activate sites and achieving fast transport kinetics accelerating catalytic reaction for converting 97.1 % of 100 ppm of HCHO continuously to CO₂ for 120 h. Moreover, rely on the chemisorption of amino groups on N-doping GS surface, HCHO could be enriched even at low concentrations and efficient elimination (from 1000 ppb to12 ppb, at 35 ℃ in 48 h). The average oxidation state and infrared spectra analysis suggested that abundant oxygen vacancies on MnO₂-GS-Sponge could be identified as surface-active sites of converting HCHO into the intermediates of dioxymethylene and formate. This work might inspire the designing 3D composite material for potential application in other fields of environmental engineering or energy industrial.

Assembled Organoruthenium(II) for Formaldehyde Decomposition and Hydrogen Production

Formaldehyde decomposition is not only an attractive method for hydrogen production, but also a potential approach for gaseous formaldehyde removal. In this research, we prepare some assembled organoruthenium through coordination reaction between Ru(p-Cymene)Cl₂ and bridge-linking ligands. It is a creative approach for Ru(p-Cymene)Cl₂ conversion into heterogeneous particles. The rigidity of bridge-linking ligand enables assembled organoruthenium to have highly ordered crystalline structure, even show clear crystal lattice with spacing of 0.19 nm. XPS shows the N-Ru bond are formed between bridge-linking ligand and Ru(p-Cymene)Cl₂ . The assembled organoruthenium has high abundant active sites for formaldehyde decomposition at low temperature. The reaction rate could increase linearly with temperature and formaldehyde concentration, with a TOF of 2420 h^-1 at 90 °C. It is promising for gaseous formaldehyde decomposition in wet air or nitrogen. Formaldehyde conversion is up to 95 % over Ru-DAPM is 4,4'-diaminodiphenylmethane at 90 °C in air. Gaseous formaldehyde decomposition is a two-steps process under oxygen-free condition. Firstly, formaldehyde dissolve in water, and be converted into hydrogen and formic acid through formaldehyde-water shift reaction. Then intermediate formic acid will further decompose into hydrogen and carbon dioxide. We also find formaldehyde decomposition is a synergetic catalysis process of oxygen and water in moist air. Oxygen is conducive to formic acid desorption and decomposition on the active sites, so assembled organoruthenium exhibit slightly higher conversion for formaldehyde decomposition in moist air. This work proposes a distinctive method for gaseous formaldehyde decomposition in the air, which is entirely different from formaldehyde photocatalysis or thermocatalysis oxidation.

Recent progresses on single-atom catalysts for the removal of air pollutants

The booming industrialization has aggravated emission of air pollutants, inflicting serious harm on environment and human health. Supported noble-metals are one of the most popular catalysts for the oxidation removal of air pollutants. Unfortunately, the high price and large consumption restrict their development and practical application. Single-atom catalysts (SACs) emerge and offer an optimizing approach to address this issue. Due to maximal atom utilization, tunable coordination and electron environment and strong metal-support interaction, SACs have shown remarkable catalytic performance on many reactions. Over the last decade, great potential of SACs has been witnessed in the elimination of air pollutants. In this review, we first briefly summarize the synthesis methods and modulation strategies together with the characterization techniques of SACs. Next, we highlight the application of SACs in the abatement of air pollutants including CO, volatile organic compounds (VOCs) and NOx, unveiling the related catalytic mechanism of SACs. Finally, we propose the remaining challenges and future perspectives of SACs in fundamental research and practical application in the field of air pollutant removal.

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.

Distinct Role of Surface Hydroxyls in Single-Atom Pt1/CeO2 Catalyst for Room-Temperature Formaldehyde Oxidation: Acid-Base Versus Redox

The development of highly efficient catalysts for room-temperature formaldehyde (HCHO) oxidation is of great interest for indoor air purification. In this work, it was found that the single-atom Pt₁/CeO₂ catalyst exhibits a remarkable activity with complete removal of HCHO even at 288 K. Combining density functional theory calculations and in situ DRIFTS experiments, it was revealed that the active O_latticeH site generated on CeO₂ in the vicinity of Pt²⁺ via steam treatment plays a key role in the oxidation of HCHO to formate and its further oxidation to CO₂. Such involvement of hydroxyls is fundamentally different from that of cofeeding water which dissociates on metal oxide and catalyzes the acid-base-related chemistry. This study provides an important implication for the design and synthesis of supported Pt catalysts with atom efficiency for a very important practical application-room-temperature HCHO oxidation.

Modulating the Electronic Metal-Support Interactions in Single-Atom Pt1 -CuO Catalyst for Boosting Acetone Oxidation

The development of highly active single-atom catalysts (SACs) and identifying their intrinsic active sites in oxidizing industrial hazardous hydrocarbons are challenging prospects. Tuning the electronic metal-support interactions (EMSIs) is valid for modulating the catalytic performance of SACs. We propose that the modulation of the EMSIs in a Pt₁ -CuO SAC significantly promotes the activity of the catalyst in acetone oxidation. The EMSIs promote charge redistribution through the unified Pt-O-Cu moieties, which modulates the d-band structure of atomic Pt sites, and strengthens the adsorption and activation of reactants. The positively charged Pt atoms are superior for activating acetone at low temperatures, and the stretched Cu-O bonds facilitate the activation of lattice oxygen atoms to participate in subsequent oxidation. We believe that this work will guide researchers to engineer efficient SACs for application in hydrocarbon oxidation reactions.

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.

Mass production of thermally stable Pt single-atom catalysts for the catalytic oxidation of sulfur dioxide

Owing to the 100% atom utilization and the potential in bridging the gap between homogeneous catalysis and heterogeneous catalysis, single-atom catalysts (SACs) have doubtlessly obtained broad attention from both academia and industry.

Water-induced gaseous formaldehyde decomposition using ruthenium organic crystalline particles

Novel ruthenium organic crystalline particles are prepared for providing two distinctive approaches for formaldehyde decomposition: catalytic oxidation or water-induced formaldehyde decomposition.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

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.

General and Efficient Synthesis of Two-Dimensional Monolayer Mesoporous Materials with Diverse Framework Compositions

Two-dimensional (2D) mesoporous materials have received substantial research interest due to their highly exposed active sites and unusual nanoconfinement effect. However, controllable and efficient synthesis of 2D mesoporous materials and investigation of their intrinsic properties have remained quite rare. Herein, a general and effective surface-limited cooperative assembly (SLCA) method enabled by leveling precursor solutions on KCl crystals via centrifugation is employed to conveniently synthesize two-dimensional (2D) monolayer mesoporous materials with different compositions. This novel strategy is performed in a manner similar to spin coating, not only enabling generation of ultrathin mesostructured composite film on KCl particles and recycling excessive precursor solution but also providing favorable solvent annealing environment for the film to form ordered mesostructures. Taking monolayer mesoporous Ce_0.8Zr_0.2O₂ solid solutions as a sample, they display ultrathin nanosheet morphology with a thickness of ∼20 nm, highly open porous structure, and easily accessible active sites of surface superoxide species. Upon decoration of 2D mesoporous Ce_0.8Zr_0.2O₂ nanosheets with Pt nanoparticles, the obtained catalyst exhibits superior catalytic activity and stability toward CO oxidation with a low onset temperature of 30 °C and a 100% conversion temperature of 95 °C, which are 35-70 °C lower than those for their counterpart materials, namely, three-dimensional (3D) mesoporous Pt/Ce_0.8Zr_0.2O₂. Moreover, their TOF_Pt value is ∼11.3 times higher than that of 3D mesoporous Pt/Ce_0.8Zr_0.2O₂. Characterizations based on various techniques indicate that such an outstanding catalytic performance is due to the ultrashort distance (20 nm) of mass diffusion, highly exposed active sites, rich surface-chemisorbed oxygen, and the synergistic effect between the Ce_0.8Zr_0.2O₂ matrix and Pt species.

A review of the hot spot analysis and the research status of single-atom catalysis based on the bibliometric analysis

The bibliometric method was used to analyze the development trend and research hotspots in past 10 years since the concept of single-atom catalysis was proposed in 2011. This article can provide some guidance for future research of SACs.

A Universal Lab-on-Salt-Particle Approach to 2D Single-Layer Ordered Mesoporous Materials

The advantages of existing ordered mesoporous materials have not yet been fully realized, due to their limited accessibility of in-pore surface and long mass-diffusion length. A general, controllable, and scalable synthesis of a family of two-dimensional (2D) single-layer ordered mesoporous materials (SOMMs) with completely exposed mesopore channels, significantly improved mass diffusion, and diverse framework composition is reported here. The SOMMs are synthesized via a surface-limited cooperative assembly (SLCA) on water-removable substrates of inorganic salts (e.g., NaCl), combined with vacuum filtration. As a proof of concept, the obtained CeO₂ -based SOMMs show superior catalytic performance in CO oxidation with high conversion efficiency, ≈33 times higher than that of conventional bulk mesoporous CeO₂ . This SLCA is a promising approach for developing next-generation porous materials for various applications.

Mechanochemical redox: a calcination-free process to support CoMnOx catalysts

A calcination-free process (mechanochemical redox loading method) to load CoMnO_x onto various supports for efficient and stable CO oxidation.

Large-Pore Mesoporous CeO2 -ZrO2 Solid Solutions with In-Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NO_x ) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation-induced co-assembly process is employed to design ordered mesoporous Ce_x Zr_{1- x} O₂ (0 ≤ x ≤ 1) solid solutions by using high-molecular-weight poly(ethylene oxide)-block-polystyrene as the template. The obtained mesoporous Ce_x Zr_{1- x} O₂ possesses high surface area (60-100 m² g^-1 ) and large pore size (12-15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce_0.8 Zr_0.2 O₂ catalyst exhibits superior CO oxidation activity with a very low T₁₀₀ value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce_0.8 Zr_0.2 O₂ framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well-interconnected pore structure of the mesoporous Pt/Ce_0.8 Zr_0.2 O₂ catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Ce_x Zr_{1- x} O₂ -s with small pore size (3.8 nm), ordered mesoporous Pt/Ce_x Zr_{1- x} O₂ not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.

Supported ceria-modified silver catalysts with high activity and stability for toluene removal

Herein we fabricated the supported single-atom silver catalysts using an in situ molten salt method. The Mn₂O₃ nanowires supported single-atom silver catalyst (i.e., 0.06 wt% Ag/Mn₂O₃) exhibited excellent catalytic activity for toluene combustion, with the temperatures required for 50 and 90% of toluene conversions being 170 and 205 °C, respectively, at a space velocity of 40,000 mL/(g h). However, the toluene conversion at 205 °C quickly decreased from 90 to 30% within 2.5 h of on-stream reaction. Based on the various characterization results, we found that there were no aggregation of Ag particles, no change in crystal structure of the Mn₂O₃ nanowire support, and no carbon deposition on the catalyst surface, and the quick deactivation of 0.06 wt% Ag/Mn₂O₃ was mainly associated with the low oxygen activation ability. The proper CeO₂ addition to the 0.06 wt% Ag/Mn₂O₃ catalyst was found to not only improve the catalytic activity but also significantly enhance the stability of the catalyst. Toluene conversion at 195 °C over 0.63 wt% CeO₂-0.06 wt% Ag/Mn₂O₃ decreased by only 10% in 50 h of on-stream reaction. Because Ag and CeO₂ particles were highly dispersed on the Mn₂O₃ nanowire support, the oxygen species formed at the surface oxygen vacancies of CeO₂ could efficiently migrate to the active sites (i.e., the interface of Ag-Mn₂O₃) and replenish the surface reactive lattice oxygen species. Thus, the present single-atom silver catalyst is an alternative for commercial noble metal catalysts for the removal of VOCs.

Photo-mediated co-loading of highly dispersed MnOx-Pt on g-C3N4 boosts the ambient catalytic oxidation of formaldehyde

Exploration of effective metal/support combinations and new fabrication approaches is attractive in the catalytic oxidation of HCHO. In this study, we proposed graphitic carbon nitride (g-C3N4) as a non-metal oxide based support to co-load Pt and MnOx through room-temperature photodeposition and in turn applied for HCHO oxidation. Here, Pt was the active component, while MnOx was the cocatalyst to compensate the shortage of active oxygen on g-C3N4. g-C3N4 was found as a promising support for the high dispersion of Pt and MnOx. Well dispersed Pt nanoparticles with an average diameter of 1.8 nm were obtained, which were highly favorable for the loading of MnOx as MnOx-Pt/g-C3N4. Catalytic performance results indicated that the limited HCHO conversion over g-C3N4 and Pt/g-C3N4 was significantly promoted with the introduction of MnOx, with an optimum MnOx amount of 3.0 wt%. The developed catalysts remained highly stable for 30 h. The enhanced catalytic activity of MnOx-Pt/g-C3N4 was due to the increased number of active oxygen species with the introduction of MnOx and the efficient transfer of electrons from g-C3N4 to Pt. Compared to the traditional impregnation, photodeposition process avoids the application of H2 and high temperatures, scoring in favor of its green and safe nature. This study can concomitantly provide a new way for the design and fabrication of a non-metal oxide based support for the efficient HCHO catalytic oxidation and the application of the photocatalytic process in catalyst fabrication.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

Fabrication, characterization, and stability of supported single-atom catalysts

Strong metal–support interactions are key requirements for development of stable single-atom catalysts with pronounced catalytic activity.