Single Silver Adatoms on Nanostructured Manganese Oxide Surfaces: Boosting Oxygen Activation for Benzene Abatement

Catalytic oxidation of volatile organic compounds over supported noble metal and single atom catalysts: A review

Volatile organic compounds (VOCs) exhausted from industrial processes are the major atmospheric pollutants, which could destroy the ecological environment and make hazards to human health seriously. Catalytic oxidation is regarded as the most competitive strategy for the efficient elimination of low-concentration VOCs. Supported noble metal catalysts are preferred catalysts due to their excellent low-temperature catalytic activity. To further lower the cost of catalysts, single atom catalysts (SAC) have been fabricated and extensively studied for application in VOCs oxidation due to their 100 % atom-utilization efficiency and unique catalytic performance. In this review, we comprehensively summarize the recent advances in supported noble metal (e.g., Pt, Pd, Au, and Ag) catalysts and SAC for VOCs oxidation since 2015. Firstly, this paper focuses on some important influencing factors that affect the activity of supported noble metal catalysts, including particle size, valence state and dispersion of noble metals, properties of the support, metal oxide/ion modification, preparation method, and pretreatment conditions of catalysts. Secondly, we briefly summarize the catalytic performance of SAC for typical VOCs. Finally, we conclude the key influencing factors and provide the prospects and challenges of VOCs oxidation.

Regulating the Electronic Metal-Support Interaction of Single-Atom Ruthenium Catalysts for Boosting Chlorobenzene Oxidation

Developing highly active single-atom catalysts (SACs) with excellent chlorine resistance for efficient oxidation of harmful chlorinated volatile organic compounds (CVOCs) is a great challenge. Tuning the electronic metal-support interaction (EMSI) is viable for promoting catalytic performances of SACs. Herein, an effective strategy of modulating the EMSI in Ru1/CeO2 SACs by thermal treatment control is proposed, which distinctly enhances the activities of the catalyst for chlorobenzene (CB) oxidation and chlorine conversion, accomplishing total CB degradation at nearly 260 °C. Detailed characterization and theoretical calculations reveal that the EMSI induces electron transfer from Ru to CeO2, optimizing the coordination and electronic structure of single-atom Ru and accordingly facilitating the adsorption and activation of CB. Moreover, the surface lattice oxygen (Olatt) at the Ru-O-Ce interface is demonstrated as the critical reactive oxygen species, the mobility and reactivity of which are also prompted by the EMSI, leading to the boosted conversion of reaction intermediates. This work sheds light on the effect of EMSI regulation on CVOC catalytic oxidation and provides guidance on fabricating high-efficiency SACs for environmental catalysis.

State of the Art and Challenges in Complete Benzene Oxidation: A Review

Increased levels and detrimental effects of volatile organic compounds (VOCs) on air quality and human health have become an important issue in the environmental field. Benzene is classified as one of the most hazardous air pollutants among non-halogenated aromatic hydrocarbons with toxic, carcinogenic, and mutagenic effects. Various technologies have been applied to decrease harmful emissions from various sources such as petrochemistry, steel manufacturing, organic chemical, paint, adhesive, and pharmaceutical production, vehicle exhausts, etc. Catalytic oxidation to CO2 and water is an attractive approach to VOC removal due to high efficiency, low energy consumption, and the absence of secondary pollution. However, catalytic oxidation of the benzene molecule is a great challenge because of the extraordinary stability of its six-membered ring structure. Developing highly efficient catalysts is of primary importance for effective elimination of benzene at low temperatures. This review aims to summarize and discuss some recent advances in catalyst composition and preparation strategies. Advantages and disadvantages of using noble metal-based catalysts and transition metal oxide-based catalysts are addressed. Effects of some crucial factors such as catalyst support nature, metal particle size, electronic state of active metal, redox properties, reactivity of lattice oxygen and surface adsorbed oxygen on benzene removal are explored. Thorough elucidation of reaction mechanisms in benzene oxidation is a prerequisite to develop efficient catalysts. Benzene oxidation mechanisms are analyzed based on in situ catalyst characterization, reaction kinetics, and theoretical simulation calculations. Considering the role of oxygen vacancies in improving catalytic performance, attention is given to oxygen defect engineering. Catalyst deactivation due to coexistence of water vapor and other pollutants, e.g., sulfur compounds, is discussed. Future research directions for rational design of catalysts for complete benzene oxidation are provided.

Cation-Induced Self-Assembly of α-MnO2 Nanowires into High-Purity Self-Standing Three-Dimensional Network Aerogels for Catalytic Decomposition of Carcinogenic Formaldehyde at Ambient Temperature

Formaldehyde (HCHO), a ubiquitous gaseous pollutant in indoor environments, threatens human health under long-term exposure, necessitating its effective elimination. Due to its advantages in enhancing mass transfer and effectively exposing active sites, aerogels with a three-dimensional (3D) interconnected network structure are expected to achieve efficient and stable decomposition of HCHO at ambient temperature. However, how to realize the self-assembly of transition metal oxides to construct high-purity 3D network aerogels is still a huge challenge. Herein, the cation-induced self-assembly strategy was developed to construct high-purity self-standing 3D network manganese dioxide aerogels. The interaction between cations and the surface groups of nanowires is crucial for successful self-assembly, which leads to the cross-winding of nanowires with each other, forming a 3D-structured network. The K+-induced 3D-MnO2 exhibited excellent catalytic performance for HCHO, which could continuously and steadily decompose HCHO into CO2 and H2O at ambient temperature. Thanks to the 3D interconnected network structure, on the one hand, it provides a large specific surface area and porosity, reducing mass transfer resistance and promoting the adsorption of HCHO and O2 molecules. On the other hand, it is more important to fully expose the active sites, which can generate more surface active oxygen species and achieve effective recycling and regeneration. Importantly, 3D-MnO2 has a strong ability to capture and activate water molecules in the atmosphere, which could be further involved in the replenishment of the consumed hydroxyl groups. This study proposes a strategy for self-assembly of transition metal oxides through cation-induction, which provides a new catalyst design approach for the room temperature decomposition of VOCs.

Improving benzene catalytic oxidation on Ag/Co3O4 by regulating the chemical states of Co and Ag

Silver (9 wt.%) was loaded on Co3O4-nanofiber using reduction and impregnation methods, respectively. Due to the stronger electronegativity of silver, the ratios of surface Co3+/Co2+ on Ag/Co3O4 were higher than on Co3O4, which further led to more adsorbed oxygen species as a result of the charge compensation. Moreover, the introducing of silver also obviously improved the reducibility of Co3O4. Hence the Ag/Co3O4 showed better catalytic performance than Co3O4 in benzene oxidation. Compared with the Ag/Co3O4 synthesized via impregnation method, the one prepared using reduction method (named as AgCo-R) exhibited higher contents of surface Co3+ and adsorbed oxygen species, stronger reducibility, as well as more active surface lattice oxygen species. Consequently, AgCo-R showed lowest T90 value of 183°C, admirable catalytic stability, largest normalized reaction rate of 1.36 × 10-4 mol/(h·m2) (150°C), and lowest apparent activation energy (Ea) of 63.2 kJ/mol. The analyzing of in-situ DRIFTS indicated benzene molecules were successively oxidized to phenol, o-benzoquinone, small molecular intermediates, and finally to CO2 and water on the surface of AgCo-R. At last, potential reaction pathways including five detailed steps were proposed.

Emerging single-atom catalysts in the detection and purification of contaminated gases

Single atom catalysts (SACs) show exceptional molecular adsorption and electron transfer capabilities owing to their remarkable atomic efficiency and tunable electronic structure, thereby providing promising solutions for diverse important processes including photocatalysis, electrocatalysis, thermal catalysis, etc. Consequently, SACs hold great potential in the detection and degradation of pollutants present in contaminated gases. Over the past few years, SACs have made remarkable achievements in the field of contaminated gas detection and purification. In this review, we first provide a concise introduction to the significance and urgency of gas detection and pollutant purification, followed by a comprehensive overview of the structural feature identification methods for SACs. Subsequently, we systematically summarize the three key properties of SACs for detecting contaminated gases and discuss the research progress made in utilizing SACs to purify polluted gases. Finally, we analyze the enhancement mechanism and advantages of SACs in polluted gas detection and purification, and propose strategies to address challenges and expedite the development of SACs in polluted gas detection and purification.

Promoted catalytic performance of Ag-Mn bimetal catalysts synthesized through reduction route

VOCs can exert great harm to both human and environment, and catalytic oxidation is believed to be an effective technique to eliminate these pollutants. In this paper, Ag-Mn bimetal catalysts with 10 wt.% of silver were synthesized using doping, impregnation, and reduction methods respectively, and then they were applied to the catalytic oxidation of benzene. Through series of characterizations it showed that the loading of silver using reduction method significantly resulted in improved physico-chemical properties of manganese oxides, such as larger surface area and pore volume, higher proportion of surface Mn3+ and Mn4+, stronger reducibility and more active of surface oxygen species, which were all beneficial to its catalytic activity. As a result, the Ag-Mn catalysts synthesized by reduction method showed a lower T90 value (equals to the temperature at which 90% of initial benzene was removed) of 203°C. Besides, both the used and fresh Ag-Mn catalysts synthesized by reduction method showed preferable stability in this research.

The removal of toluene by thermoscatalytic oxidation using CeO2-based catalysts:a review

Volatile organic compounds (VOCs) pose a serious threat to human health and the ecological environment. Thermal catalytic oxidation based on cerium dioxide based (CeO2-based) catalysts is widely used in the degradation of toluene. However, new problems and challenges such as how to reduce the energy consumption during catalytic oxidation, improve the anti-poisoning performance of catalysts, and enhance the multi-species synergistic catalytic ability of catalysts continue to emerge. On this basis, we systematically summarize the current status of research progress on the thermocatalytic oxidation of toluene based on CeO2-based catalysts. Firstly, we summarized the rules on how to improve the catalytic performance and anti-poisoning performance of CeO2-based catalysts; Secondly, we discussed the effect of light reaction conditions on the thermal coupled catalytic oxidation of toluene; In addition to this, we explored the current status of synergistic multi-pollutant degradation, mainly of toluene; Finally, we summarized the mechanism of catalytic oxidation of toluene by combining theoretical simulation calculations, in-situ infrared analyses, and other means. We present the promising applications of CeO2-based catalysts in the catalytic oxidation of toluene, and hope that these summaries will provide an important reference for the catalytic treatment of VOCs.

Ultrathin LayCoOx Nanosheets with High Porosity Featuring Boosted Catalytic Oxidation of Benzene: Mechanism Elucidation via an Experiment-Theory Combined Paradigm

Designing transition-metal oxides for catalytically removing the highly toxic benzene holds significance in addressing indoor/outdoor environmental pollution issues. Herein, we successfully synthesized ultrathin LayCoOx nanosheets (thickness of ∼1.8 nm) with high porosity, using a straightforward coprecipitation method. Comprehensive characterization techniques were employed to analyze the synthesized LayCoOx catalysts, revealing their low crystallinity, high surface area, and abundant porosity. Catalytic benzene oxidation tests demonstrated that the La0.029CoOx-300 nanosheet exhibited the most optimal performance. This catalyst enabled complete benzene degradation at a relatively low temperature of 220 °C, even under a high space velocity (SV) of 20,000 h-1, and displayed remarkable durability throughout various catalytic assessments, including SV variations, exposure to water vapor, recycling, and long time-on-stream tests. Characterization analyses confirmed the enhanced interactions between Co and doped La, the presence of abundant adsorbed oxygen, and the extensive exposure of Co3+ species in La0.029CoOx-300 nanosheets. Theoretical calculations further revealed that La doping was beneficial for the formation of oxygen vacancies and the adsorption of more hydroxyl groups. These features strongly promoted the adsorption and activation of oxygen, thereby accelerating the benzene oxidation processes. This work underscores the advantages of doping rare-earth elements into transition-metal oxides as a cost-effective yet efficient strategy for purifying industrial exhausts.

Accumulation of Ag(0) Single Atoms at Water/Mineral Interfaces during Sunlight-Induced Reduction of Ionic Ag by Phenolic DOM

Silver (Ag) undergoes a complex and dynamic Ag+/Ag0 cycle under environmental conditions. The Ag+ → Ag nanoparticles (AgNPs) transformation due to the combined actions of sunlight, O2, and dissolved organic matter has been a well-known environmental phenomenon. In this study, we indicate that this process may be accompanied by a pronounced accumulation of Ag(0) single atoms (Ag-SAs) on the minerals' surfaces. According to spherical aberration-corrected scanning transmission electron microscopy and high-energy-resolution X-ray adsorption fine structure analyses, humic acid (HA) and phenol (PhOH) can induce Ag-SAs accumulation, whereas oxalic acid causes only AgNPs deposition. Ag-SAs account for more than 20 wt % of total Ag(0) on the γ-Al2O3 surfaces during HA- and PhOH-mediated photolysis processes. HA also causes Ag-SAs to accumulate on two other prevalent soil minerals, SiO2 and Fe2O3, and the fractions of Ag-SAs are about 15 wt %. Our mechanism studies suggest that a phenolic molecule acts as a reducing agent of Ag+ and a stabilizer of Ag-SAs, protecting Ag-SAs against autocatalytic nucleation.

Electron Donation from Boron Suboxides via Strong p-d Orbital Hybridization Boosts Molecular O2 Activation on Ru/TiO2 for Low-Temperature Dibromomethane Oxidation

Low-temperature catalytic oxidation is of significance to the degradation of halogenated volatile organic compounds (HVOCs) to avoid hazardous byproducts with low energy consumption. Efficient molecular oxygen (O2) activation is pivotal to it but usually limited by the insufficient electron cloud density at the metal center. Herein, Ru-B catalysts with enhanced electron density around Ru were designed to achieve efficient O2 activation, realizing dibromomethane (DBM) degradation T90 at 182 °C on RuB1/TiO2 (about 30 °C lower than pristine Ru/TiO2) with a TOFRu value of 0.055 s-1 (over 8 times that of Ru/TiO2). Compared to the limited electron transfer (0.02 e) on pristine Ru/TiO2, the Ru center gained sufficient negative charges (0.31 e) from BOx via strong p-d orbital hybridization. The Ru-B site then acted as the electron donor complexing with the 2π* antibonding orbital of O2 to realize the O2 dissociative activation. The reactive oxygen species formed thereby could initiate a fast conversion and oxidation of formate intermediates, thus eventually boosting the low-temperature catalytic activity. Furthermore, we found that the Ru-B sites for O2 activation have adaptation for pollutant removal and multiple metal availability. Our study shed light on robust O2 activation catalyst design based on electron density adjustment by boron.

Improved activity and significant SO2 tolerance of Sb-Pd-V oxides on N-doped TiO2 for CB/NOx synergistic degradation

The synergistic degradation of VOCs and NO_x that were emitted from the incineration of municipal and medical wastes by a single catalyst is challenging, due to the poor activity at low temperatures, and the SO₂ poisoning on the active sites. Herein, N-doped TiO₂ (N-TiO₂) was used as the support for designing a highly efficient and stable catalyst system for CB/NO_x synergistic degradation even in the presence of SO₂. The prepared SbPdV/N-TiO₂ catalyst, which presented excellent activity and tolerance to SO₂ in the CBCO + SCR process, was investigated by a series of characterizations (such as XRD, TPD, XPS, H₂-TPR and so on) as well as DFT calculations. The electronic structure of the catalyst was effectively modulated after N doping, resulting in effective charge flow between the catalyst surface and gas molecules. More importantly, the adsorption and deposition of sulfur species and reaction transient intermediates on active centers were restrained, while a new N adsorption center for NO_x was provided. Abundant adsorption centers and superior redox properties ensured smooth CB/NO_x synergistic degradation. The removal of CB mainly follows the L-H mechanism, while NO_x elimination follows both E-R and L-H mechanisms. As a result, N doping provides a new approach to develop more advanced anti-SO₂ poisoning CB/NO_x synergistic catalytic removal systems for extensive applications.

Single-Atom Catalysts in Environmental Engineering: Progress, Outlook and Challenges

Recently, single-atom catalysts (SACs) have attracted wide attention in the field of environmental engineering. Compared with their nanoparticle counterparts, SACs possess high atomic efficiency, unique catalytic activity, and selectivity. This review summarizes recent studies on the environmental remediation applications of SACs in (1) gaseous: volatile organic compounds (VOCs) treatment, NO_x reduction, CO₂ reduction, and CO oxidation; (2) aqueous: Fenton-like advanced oxidation processes (AOPs), hydrodehalogenation, and nitrate/nitrite reduction. We present the treatment activities and reaction mechanisms of various SACs and propose challenges and future opportunities. We believe that this review will provide constructive inspiration and direction for future SAC research in environmental engineering.

Abrading-Induced Breakdown of Ag Nanoparticles into Atomically Dispersed Ag for Enhancing Antimicrobial Performance

Silver is among the most essential antimicrobial agents. Increasing the efficacy of silver-based antimicrobial materials will reduce operating costs. Herein, we show that mechanical abrading causes atomization of Ag nanoparticles (AgNPs) into atomically dispersed Ag (AgSAs) on the surfaces of an oxide-mineral support, which eventually boosts the antibacterial efficacy considerably. This approach is straightforward, scalable, and applicable to a wide range of oxide-mineral supports; additionally, it does not require any chemical additives and operates under ambient conditions. The obtained AgSAs-loaded γ-Al₂O₃ inactivated Escherichia coli (E. coli) five times as fast as the original AgNPs-loaded γ-Al₂O₃. It can be utilized over 10 runs with minimal efficiency loss. The structural characterizations indicate that AgSAs exhibit a nominal charge of 0 and are anchored at the doubly bridging OH on the γ-Al₂O₃ surfaces. Mechanism studies demonstrate that AgSAs, like AgNPs, damage bacterial cell wall integrity, but they release Ag⁺ and superoxide substantially faster. This work not only provides a simple method for manufacturing AgSAs-based materials but also shows that AgSAs have better antibacterial properties than the AgNPs counterpart.

Low-temperature degradation of toluene over Ag-MnOx-ACF composite catalyst

Volatile organic compounds (VOCs) have caused a serious threat to the atmosphere and human health. Therefore, it is of great significance to exploit effective catalytic materials for the safe and effective catalytic elimination of VOCs. Herein, Ag-MnO_x-ACF composite catalysts were constructed via a two-step impregnation strategy and used for catalytic toluene degradation. A remarkable low-temperature catalytic activity (T₁₀₀= 50℃), excellent stability, as well as CO₂ selectivity (80%) were achieved over the Ag-MnO_x-ACF catalyst. A series of characterizations indicated that the unique manganese defects structure of birnessite phase manganese oxide played an essential role for toluene oxidation, which was conducive to generating surface adsorbed oxygen. The higher ratio of Mn³⁺/Mn⁴⁺, abundant surface adsorbed oxygen and highly dispersed Ag species were determined to significantly facilitate toluene degradation. The mechanism of efficient degradation of toluene at low temperature was proposed. O₃ and H₂O molecules were activated via surface hydroxyl and Mn defects on Ag-MnO_x-ACF to produce sufficient •OH, enhancing the degradation performance of toluene. We provide a new idea for the catalytic oxidation of benzene VOCs at low even room temperatures.

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.

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.

Low-Temperature Combustion of Toluene over Cu-Doped SmMn2O5 Mullite Catalysts via Creating Highly Active Cu2+-O-Mn4+ Sites

Catalytic combustion of volatile organic compounds (VOCs) at low temperatures is still an urgent issue to be solved. Herein, low-temperature combustion of toluene over Cu-doped SmMn₂O₅ mullite catalysts via creating highly active Cu²⁺-O-Mn⁴⁺ sites has been originally demonstrated. Cu-doped SmMn₂O₅ mullite catalysts exhibited 90% conversion of toluene at 206 °C and displayed robust stability even in the presence of water. It has been demonstrated that Cu doping created Cu²⁺-O-Mn⁴⁺ active composite sites that were more exposed after removing surface Sm species via acid-etching. Benefiting from this, the redox and oxygen activation ability of catalysts was significantly enhanced. The consumption of benzaldehyde and benzoic acid as intermediate species and the CO₂ generation ability were apparently promoted, which were the direct reasons for the enhanced low-temperature combustion of toluene. This work provides novel ideas for the development of high-performance catalysts for low-temperature VOC combustion, which has great industrial application prospects.

Reduced graphene oxide as an effective promoter to the layered manganese oxide-supported Ag catalysts for the oxidation of ethyl acetate and carbon monoxide

The layered manganese oxide (δ-MnO₂)-supported reduced graphene oxide (rGO)-promoted silver catalysts (xAg- yrGO/δ-MnO₂; x and y are the Ag and rGO contents (wt%), respectively) were prepared via a polyvinyl alcohol-protected reduction route. Physicochemical properties of these materials were determined using the numerous techniques, and their catalytic activities were evaluated for the oxidation of CO and ethyl acetate. It is found that the loading of rGO as an electron transfer promoter could significantly strengthen the metal-support interaction (SMSI) between Ag and δ-MnO₂ and increase specific surface area of the sample, hence improving catalytic performance of the sample. Activity evaluation reveals that 1Ag- 1.0rGO/δ-MnO₂ showed the best catalytic activity and the lowest apparent activation energy (E_a), giving a T_90% of 140 °C and an E_a of 42.7 kJ/mol for CO oxidation, and a T_90% of 160 °C and an E_a of 39.8 kJ/mol for ethyl acetate oxidation at space velocity (SV) = 60,000 mL/(g h). The good performance of 1Ag- 1.0rGO/δ-MnO₂ was associated with its high Mn³⁺/Mn⁴⁺ or O_ads/O_latt molar ratio, good low-temperature reducibility, and strong SMSI between Ag and δ-MnO₂. The in situ DRIFTS characterization demonstrates that the carbonate and acetate species were the main intermediate products in CO and ethyl acetate oxidation over 1Ag- 1.0rGO/δ-MnO₂, respectively. The 1Ag- 1.0rGO/δ-MnO₂ sample was not significantly altered in physicochemical property after 55 h of stability test, but its activity decreased in the presence of water vapor, especially such an effect on ethyl acetate oxidation was more obvious, which was possibly due to the competitive adsorption of water and reactants on the catalyst surface.

Construction of K and Tb Co-doped MnO2 nanoparticles for enhanced oxidation and detoxication of organic dye waste

Developing low-cost and efficient materials for dye pollutant removal under mild condition remains a great challenge. Here K⁺ and Tb³⁺ co-doped porous MnO₂ (K-Tb-MnO₂) nanoparticles with tailored properties including crystal structure, surface area and catalytic activity have been synthesized. Experimental results reveal that K-Tb-MnO₂ nanoparticle has higher specific surface area, Mn³⁺ content and surface oxygen vacancies than pristine MnO₂ nanoparticle and single-doped MnO₂ materials, showing the uniqueness of dual-doped metal ions. Using methyl blue (MB) as a model pollutant, its removal efficiency by K-Tb-MnO₂ nanoparticles within 5 min is 93.6%, which is 18, 8.3, and 2.9 times higher than that of MnO₂, K-MnO₂, and Tb-MnO₂ nanomaterials, respectively. Oxalic acid triggered MnO₂ material dissolving assay and FT-IR spectrum suggested that remarkable performance of K-Tb-MnO₂ nanoparticle toward MB removal can be attributed to a combined effect of adsorption (16% MB removal) and catalytic degradation (84% MB removal). Moreover, K-Tb-MnO₂ nanoparticle mediated MB degradation is demonstrated to be a combination of non-radical oxidation by Mn³⁺ and radical-participated degradation, with ¹O₂ as the main species. And the intermediates and pathways of MB degradation were studied by liquid chromatography-mass spectrometry. Importantly, cell viability experiment suggests that the toxicity of MB dye could be efficiently alleviated after the treatment with K-Tb-MnO₂ nanoparticle. These results demonstrate the great potential of the novel K-Tb-MnO₂ particles to be used as a highly effective nanomaterials to reduce the risk of dye wastes toward the environment and human health.

Enhanced Performance of Supported Ternary Metal Catalysts for the Oxidation of Toluene in the Presence of Trichloroethylene

Chlorinated volatile organic compounds (CVOCs), even in small quantities, can cause Pt-based catalyst poisoning. Improving the low-temperature chlorine resistance of catalysts is of vital importance for industrial application, although it remains challenging. Considering actual industrial production, a TiO2-supported ternary metal catalyst was prepared in this work to study the catalytic oxidation of multicomponent VOCs (toluene and trichloroethylene (TCE)). Among all of the samples, PtWRu/TiO2 and PtWCr/TiO2 exhibited the best catalytic performance for toluene oxidation. In the mixed VOC oxidation, the PtWCr/TiO2 sample showed the best catalytic activity for toluene combustion (a toluene conversion of 90% was achieved at 258 °C and a space velocity of 40,000 mL g−1 h−1, and the specific reaction rate and turnover frequency at 215 °C were 44.9 × 10−6 mol gPt−1 s−1 and 26.2 × 10−5 s−1). The PtWRu/TiO2 sample showed the best catalytic activity for TCE combustion (a TCE conversion of 90% was achieved at 305 °C and a space velocity of 40,000 mL g−1 h−1, and the specific reaction rate and turnover frequency at 270 °C were 9.0 × 10−6 mol gPt−1 s−1 and 7.3 × 10–5 s−1). We concluded that the ternary metal catalysts could greatly improve chlorine desorption by increasing the active lattice oxygen mobility and surface acidity, thus reducing chlorinated byproducts and other serious environmental pollutants. This work may serve as a reasonable design reference for solving more practical industrial production emissions of multicomponent VOCs.

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.

Enhancement in catalytic performance of birnessite-type MnO2-supported Pd nanoparticles by the promotional role of reduced graphene oxide for toluene oxidation

The birnessite-type MnO2 (δ-MnO2) supported reduced graphene oxide (rGO)-promoted palladium (xPdyrGO/δ-MnO2) samples were prepared using the polyvinyl alcohol-protected reduction method. Physicochemical properties of the catalysts were determined by means of...

Excellent catalytic oxidation performance on toluene and benzene over OMS-2 with a hierarchical porous structure synthesized by a one-pot facile method: modifying surface properties by introducing different amounts of K

The formation of K–O–Mn bond weaken the bond of Mn–O–Mn which increases oxygen species mobility leading to excellent catalytic oxidation performance over OMS-2 by introducing different amounts of K.

Insight into the Improvement Effect of Nitrogen Dopant in Ag/Co3O4 Nanocubes for Soot Oxidation: Experimental and Theoretical Studies

Different doping amounts of N-doped Ag/Co₃O₄ nanocubes were synthesized for the first time for catalytic soot oxidation. The N-doped sample exhibited remarkably improved catalytic activity, of which the maximum decrease in temperature for 90% soot conversion was almost 40 ℃. Characterization results analyzed by TEM, XPS, EPR, H₂-TPR, O₂-TPD, etc. revealed that the incorporation of N atoms can alter the electronic structure, leading to the generation of more oxygen vacancies and enhancement of lattice oxygen mobility. Meanwhile, larger surface area, rugged morphology and promoted reducibility also contribute to the performance improvement. DFT calculations on the differential charge density, Gibbs free energy, etc. were performed to investigate the intrinsic reasons on an atomic level. Due to the relatively higher electronegativity, N dopant could be an electron-appealing center to promote efficient electron transfer, resulting in the redistribution of charge density and formation of conductive Co-N bonds. This variation in electronic structure favors lowering the formation energy of oxygen vacancies and facilitating the activation of the lattice oxygen originated from the highly hybridized Co-O bonds, which ultimately reduces the activation barriers for reactants/intermediates and accelerates the reaction kinetics. This study evidenced that N doping could be an effective strategy to promote catalytic soot oxidation.

SO2 Poisoning Mechanism of the Multi-active Center Catalyst for Chlorobenzene and NOx Synergistic Degradation at Dry and Humid Environments

The performance of fresh (PdV/TiO₂), sulfur poisoned (Used-S and Used-H), and regenerated (Used-R_S and Used-R_H) multi-active center catalysts for chlorobenzene catalytic oxidation and selective catalytic reduction (CBCO + SCR) reaction is investigated. The reaction on the catalyst surface is blocked after sulfur poisoning owing to the occupation and deposition of catalyst active centers (mainly Pd centers) by PdSO₄ (and/or PdS in a dry environment) and NH₄HSO₄ species, especially the CBCO process. Sulfates (mainly NH₄HSO₄) on the sulfur poisoned catalyst surface are partially decomposed after 400 °C thermal regeneration, while the deactivation caused by the formation of PdSO₄ species is irreversible. Density functional theory calculation results show that in the PdSO₄ and NH₄HSO₄ generation paths, each step of the elementary reaction has just a small energy barrier to overcome, and the stability of the product for each elementary reaction increases gradually. Even worse, SO₂ is easily combined with H₂O gas molecules to form H₂SO₃ in a humid environment, and the energy barrier for conversion of SO₃^2- to SO₄^2- is just 0.041 eV. The two oxygen vacancies (VO_x-1 or TiO_x-1) provide adsorption sites for CBCO + SCR reaction gas molecules but do not exhibit adsorption properties for SO₂, which gives a possible idea for optimization of sulfur resistance. The present work is favorable for further synergistic removal of CB/NO_x by the catalyst for anti-SO₂ poisoning modification and application in the manufacture industry.

Catalytic activity of porous manganese oxides for benzene oxidation improved via citric acid solution combustion synthesis

Various manganese oxides (MnO_x) prepared via citric acid solution combustion synthesis were applied for catalytic oxidation of benzene. The results showed the ratios of citric acid/manganese nitrate in synthesizing process positively affected the physicochemical properties of MnO_x, e.g., BET (Brunauer-Emmett-Teller) surface area, porous structure, reducibility and so on, which were in close relationship with their catalytic performance. Of all the catalysts, the sample prepared at a citric acid/manganese nitrate ratio of 2:1 (C2M1) displayed the best catalytic activity with T₉₀ (the temperature when 90% of benzene was catalytically oxidized) of 212℃. Further investigation showed that C2M1 was Mn₂O₃ with abundant nano-pores, the largest surface area and the proper ratio of surface Mn⁴⁺/Mn³⁺, resulting in preferable low-temperature reducibility and abundant surface active adsorbed oxygen species. The analysis results of the in-situ Fourier transform infrared spectroscopy (in-situ FTIR) revealed that the benzene was successively oxidized to phenolate, o-benzoquinone, small molecules (such as maleates, acetates, and vinyl), and finally transformed to CO₂ and H₂O.

Enhanced catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr2+ contents in the tunnels

Cryptomelane-type manganese oxides (OMS-2) have been intensively investigated for application in the catalytic oxidation of carcinogenic benzene, and doping metal ions in the OMS-2 tunnels are widely used for modifying its catalytic activity. In this study, we reported a novel strategy of enhancing catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr²⁺ concentration in the tunnels. The catalytic activity result revealed that an obvious decrease (△T₅₀ = 27 °C and △T₉₀ = 37 °C) in T₅₀ and T₉₀ (corresponding to benzene conversions at 50 % and 90 %, respectively; initial benzene concentration was 2000 mg m^-3; contact time was 1.5 s) had been observed by increasing the Sr²⁺ concentration in the OMS-2 tunnels. The origin of Sr²⁺ doping effect on catalytic activity was theoretically and experimentally investigated by CO temperature-programmed reduction, ¹⁸O₂ isotope labeling, and density functional theory calculations. The result confirmed that increasing Sr²⁺ concentration in the tunnels not only promoted the lattice oxygen activity, but also facilitated the generation of more oxygen vacancy defects, thus considerably improving the catalytic activity of OMS-2.

A perspective on oxide-supported single-atom catalysts

Single-atom catalysts (SACs) can not only maximize the metal atom utilization efficiency, but also show drastically improved catalytic performance for various important catalytic processes. Insights into the working principles of SACs provide rational guidance to design and prepare advanced catalysts. Many factors have been claimed to affect the performance of SACs, which makes it very challenging to clarify the correlation between the catalytic performance and physicochemical characteristics of SACs. Oxide-supported SACs are one of the most extensively explored systems. In this minireview, some latest developments on the determining factors of the stability, activity and selectivity of SACs on oxide supports are overviewed. Discussed also are the reaction mechanisms for different systems and methods that are employed to correlate the properties with the catalyst structures at the atomic level. In particular, a recently proposed surface free energy approach is introduced to fabricate well-defined modelled SACs that may help address some key issues in the development of SACs in the future.

Boosting Toluene Combustion by Engineering Co-O Strength in Cobalt Oxide Catalysts

Exploring active and low-cost transition metal oxides (TMOs) based catalysts for volatile organic compounds (VOCs) abatement is vital for air pollution control technologies. Since 18 oxygen atoms are required for the complete mineralization of a toluene molecule, the participation of a large amount of active oxygen is a key requirement for the catalytic oxidation of toluene. Here, toluene degradation was improved by weakening the Co-O bond strength on the surface of cobalt oxide, so as to increase the amount of active oxygen species, while maintaining the high stability of the catalyst for toluene combustion. The bond strength of Co-O and the amount of surface active O₂ was regulated by tuning the pyrolysis temperature. The catalyst's redox ability and surface oxygen species activity are improved due to the weakening of the Co-O bond strength. It has been demonstrated that active oxygen plays a crucial role in boosting toluene combustion by engineering Co-O strength in cobalt oxide catalysts. This work provides a new understanding of the exploration and development of high-performance TMO catalysts for VOCs abatement.

The remarkable effect of alkali earth metal ion on the catalytic activity of OMS-2 for benzene oxidation

Cryptomelane-type octahedral molecular sieve (OMS-2) is one of the most promising catalysts for VOCs oxidation, and introduction of metal ions in OMS-2 tunnel is widely used for tailoring its catalytic activity. Here, different types of alkali earth metal ions with the same X/Mn atomic ratio of approximately 0.012 (X represents Mg²⁺, Ca²⁺, and Sr²⁺) were successfully introduced into OMS-2 tunnel by a one-step redox reaction. The catalytic test showed that introducing alkali earth metal ions into tunnels had a considerable effect on the catalytic performance of OMS-2 for benzene oxidation. The Sr²⁺ doped OMS-2 catalyst exhibited the better catalytic activity compared with those of Mg-OMS-2 and Ca-OMS-2 samples, and was also superior to a commercial 0.5% Pt/Al₂O₃ catalyst, as evidenced by its low reaction temperatures of T₅₀ = 200 °C and T₉₀ = 223 °C (corresponding to benzene conversions at 50% and 90%, respectively). The origin of the considerable effect of alkali earth metal doping on the catalytic activity of OMS-2 catalysts was experimentally and theoretically investigated by an ¹⁸O₂ isotopic labeling experiment, CO temperature-programmed reduction, O₂ temperature-programmed oxidation, and density functional theory calculations. The greatest catalytic activity of Sr-OMS-2 compared with those of Mg-OMS-2 and Ca-OMS-2 samples was attributed to its highest lattice oxygen activity as well as its largest surface area. By introducing alkali earth metal ions into the OMS-2 tunnel, we developed a low-cost and highly efficient catalyst that could be used as alternative to noble metal catalysts.

Atomically Dispersed Manganese on a Carbon-Based Material for the Capture of Gaseous Mercury: Mechanisms and Environmental Applications

A novel, atomically dispersed carbon-based sorbent was synthesized by anchoring manganese atoms with N atoms for the capture of gaseous elemental Hg (Hg⁰). Oxygen atoms were also introduced into the synthesis process to adjust the oxidizing ability of the Mn atoms. High-valence Mn (Mn⁴⁺) anchored by the O and N atoms (Mn-O/N-C) in the carbon-based materials provided more exposed active sites. The mercury removal efficiency of the composite exceeded 99%. The composite with a Mn loading of 0.9 wt % exhibited high affinity for Hg⁰, and the capacity for Hg⁰ adsorption within 275 min at room temperature reached 16.95 mg·g^-1. The Mn utilization was ∼56.61%, which is much larger than that of reported Mn-based oxide sorbents. The atomic-level distribution of Mn was well evidenced by aberration-corrected high-angle annular darkfield scanning transmission electron microscopy. Density functional theory calculations were conducted to evaluate the energy for adsorption of Hg⁰ on Mn-O/N-C. The results indicated that the amount of N and O atoms in the Mn coordination environment determined the Hg⁰ adsorption energy, and the presence of five optimized Mn adsorption structures in Mn-O/N-C was confirmed by Hg temperature-programmed desorption analysis. These materials may be utilized for mercury removal from disposal sites with high concentrations of mercury, broken mercury-containing lamps, or mercurial thermometers. The strategy of atomic dispersion during synthesis of the materials and adjusting the oxidizing ability in the single-atom strategy may be helpful for the development of environmentally benign functional materials.

Optimization of Cu catalysts for nitrophenol reduction, click reaction and alkyne coupling

Earth-abundant nanocatalysts are actively searched to replace expensive noble metal catalysts for a number of essential processes.

Deep insight into the catalytic removal mechanism of a multi-active center catalyst for chlorobenzene: an experiment and density functional theory study

Catalytic oxidation is a promising dioxin purification removal technique due to high efficiency and low consumption, as well as no secondary pollution.

Supported Noble-Metal Single Atoms for Heterogeneous Catalysis

Single-atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single-atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble-metal-based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure-activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single-atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.

Role of Structural Defects in MnOx Promoted by Ag Doping in the Catalytic Combustion of Volatile Organic Compounds and Ambient Decomposition of O3

Manganese oxides are prominent candidates for the catalytic oxidation of volatile organic compounds (VOCs) or ambient decomposition of O₃ individually. Here, we compared various preparation methods to create a defect-enriched Ag-MnO_x nanocomposite that exhibits a remarkably multifunctional activity in VOC combustion and ozone decomposition. Ag⁺ ions were well-dispersed in the microtunnels of Ag-MnO_x-H via hydrothermal replacement of the original K⁺ ions; this catalyst's benzene combustion efficiency (T_90% = 216 °C at a space velocity of 90 000 mL h^-1 g^-1) was comparable to that of typical noble metal catalysts. Moreover, the decomposition of ozone over the Ag-MnO_x-H catalyst (space velocity = 840 000 mL h^-1 g^-1) under a relative humidity of 60% was above 90%, indicating that it is a promising material for ozone elimination in practical application. The local structure results indicated that silver incorporation via the hydrothermal method facilitates the formation of nonstoichiometric defects in the MnO_x matrix. The large number of active oxygen species related to O vacancies appeared to play critical roles in VOC combustion; moreover, the oxygen vacancies originating from O defects were also critical in O₃ abatement. This work provides multifunctional catalysts for VOC combustion and ambient O₃ decomposition and may assist with the rational design of MnO_x catalysts for application in various conditions.