Tailoring the Electronic Metal-Support Interactions in Supported Silver Catalysts through Al modification for Efficient Ethylene Epoxidation.

Metal-modified catalysts have attracted extraordinary research attention in heterogeneous catalysis due to their enhanced geometric and electronic structures and outstanding catalytic performances. Silver (Ag) possesses necessary active sites for ethylene epoxidation, but the catalyst activity is usually sacrificed to obtain high selectivity towards ethylene oxide (EO). Herein, we report that using Al can help in tailoring the unoccupied 3d state of Ag on the MnO2 support through strong electronic metal-support interactions (EMSIs), overcoming the activity-selectivity trade-off for ethylene epoxidation and resulting in a very high ethylene conversion rate (~100 %) with 90 % selectivity for EO under mild conditions (170 °C and atmospheric pressure). Structural characterization and theoretical calculations revealed that the EMSIs obtained by the Al modification tailor the unoccupied 3d state of Ag, modulating the adsorption of ethylene (C2H4) and oxygen (O2) and facilitating EO desorption, resulting in high C2H4 conversion. Meanwhile, the increased number of positively charge Ag+ lowers the energy barrier for C2H4(ads) oxidation to produce oxametallacycle (OMC), inducing the unexpectedly high EO selectivity. Such an extraordinary electronic promotion provides new promising pathways for designing advanced metal catalysts with high activity and selectivity in selective oxidation reactions.

Modulating the Electronic States of Pt Nanoparticles on Reducible Metal-Organic Frameworks for Boosting the Oxidation of Volatile Organic Compounds.

The adsorption and activation of pollutant molecules and oxygen play a critical role in the oxidation reaction of volatile organic compounds (VOCs). In this study, superior adsorption and activation ability was achieved by modulating the interaction between Pt nanoparticles (NPs) and UiO-66 (U6) through the spatial position effect. Pt@U6 exhibits excellent activity in toluene, acetone, propane, and aldehyde oxidation reactions. Spectroscopic studies, 16O2/18O2 kinetic isotopic experiments, and density functional theory (DFT) results jointly reveal that the encapsulated Pt NPs of Pt@U6 possess higher electron density and d-band center, which is conducive for the adsorption and dissociation of oxygen. The toluene oxidation reaction and DFT results indicate that Pt@U6 is more favorable to activate the C-H of toluene and the C═C of maleic anhydride, while Pt/U6 with lower electron density and d-band center exhibits a higher oxygen dissociation temperature and higher reactant activation energy barriers. This study provides a deep insight into the architecture-performance relation of Pt-based catalysts for the catalytic oxidation of VOCs.

A novel iron sulfide phase with remarkable hydroxyl radical generation capability for contaminants degradation.

The hydroxyl radical (·OH) stands as one of the most potent oxidizing agents, capable of engaging in non-selective and instantaneous reactions with contaminants in water. Herein, we present a novel iron sulfide phase (S-FeS) characterized by an unprecedented structure, accompanied by its remarkable hydroxyl radical generation capability and contaminant degradation efficiency surpassing that of the conventional metastable iron sulfide phase, namely, the Mackinawite (FeS). In comparison to FeS, S-FeS exhibits enhanced degradation kinetics and higher efficacy in the removal of methylene blue, ciprofloxacin, and trivalent arsenic. Utilizing density functional theory (DFT) calculations, we postulate the mechanism for the exceptional contaminant degradation performance of S-FeS to be attributed to the increased exposure of the highly reactive (110) crystal facets. This research uncovers a new metastable phase that expands the polymorphisms within the iron sulfide family and showcases its capability for driving the oxygen reduction reaction.

Oxophilic Ce single atoms-triggered active sites reverse for superior alkaline hydrogen evolution.

The state-of-the-art alkaline hydrogen evolution catalyst of united ruthenium single atoms and small ruthenium nanoparticles has sparked considerable research interest. However, it remains a serious problem that hydrogen evolution primarily proceeds on the less active ruthenium single atoms instead of the more efficient small ruthenium nanoparticles in the catalyst, hence largely falling short of its full activity potential. Here, we report that by combining highly oxophilic cerium single atoms and fully-exposed ruthenium nanoclusters on a nitrogen functionalized carbon support, the alkaline hydrogen evolution centers are facilely reversed to the more active ruthenium nanoclusters driven by the strong oxophilicity of cerium, which significantly improves the hydrogen evolution activity of the catalyst with its mass activity up to -10.1 A mg-1 at -0.05 V. This finding is expected to shed new light on developing more efficient alkaline hydrogen evolution catalyst by rational regulation of the active centers for hydrogen evolution.

Volatile organic compounds (VOCs) emissions from internal floating-roof tank in oil depots in Beijing: Influencing factors and emission reduction strategies analysis.

The internal floating-roof tank is the main type of storage tank for refined oil products. The volatile organic compounds (VOCs) emission from the internal floating-roof tank plays a dominant role in the unorganized emission source of the oil depot. In this study, we selected six typical oil depots in Beijing to investigate VOC emission characteristics from the tank top vent hole using infrared imaging technology and flame ionization detector (FID). The results reveal that infrared thermal imager is efficient in quickly identifying the emission level of the tank discharge point. The ambient temperature and wind speed have a direct effect on sealing loss, the turnover can greatly influence the wall hanging loss, and the concentration of VOCs emitted from the tank top vent hole is negatively correlated with liquid height. Furthermore, the influence of accessories type of the internal floating-roof tank on the concentration of VOCs emission from the top vent hole is also studied when other parameters remain unchanged, and find the floating deck type and sealing mode have a significant influence on their VOCs emissions, of which the combination of pontoon type floating deck and secondary seal are effective in controlling the concentration of VOCs emitted from the tank top vent hole. Finally, based on our experimental results, several feasible emission reduction strategies are proposed in terms of source prevention and process control in order to achieve the fine management of the whole process. This paper provides important technical support and policy thoughts for VOCs emission control during oil storage.

Boosting N2O Catalytic Decomposition by the Synergistic Effect of Multiple Elements in Cobalt-Based High-Entropy Oxides.

Nitrous oxide (N2O) has a detrimental impact on the greenhouse effect, and its efficient catalytic decomposition at low temperatures remains challenging. Herein, the cobalt-based high-entropy oxide with a spinel-type structure (Co-HEO) is successfully fabricated via a facile coprecipitation method for N2O catalytic decomposition. The obtained Co-HEO catalyst displays more remarkable catalytic performance and higher thermal stability compared with single and binary Co-based oxides, as the temperature of 90% N2O decomposition (T90) is 356 °C. A series of characterization results reveal that the synergistic effect of multiple elements enhances the reducibility and augments oxygen vacancy in the high-entropy system, thus boosting the activity of the Co-HEO catalyst. Moreover, density functional theory (DFT) calculations and the temperature-programmed surface reaction (TPSR) with isotope labeling demonstrate that N2O decomposition on the Co-HEO catalyst follows the Langmuir-Hinshelwood (L-H) mechanism with the promotion of abundant oxygen vacancies. This work provides a fundamental understanding of the synergistic catalytic effect in N2O decomposition and paves the way for the novel environmental catalytic applications of HEO.

Effective Nitric Oxide Reduction Over Core-Shell Cu-SSZ-13@meso-MOx Catalysts with Significant Catalytic Activity and Hydrothermal Stability.

In this study, Cu-SSZ-13 zeolites are successfully coated with mesoporous metal oxides (CeO2 , Fe2 O3 , TiO2 and SiO2 ) and prepared as core-shell Cu-SSZ-13@meso-MOx samples with hierarchical pore structure. Compared with Cu-SSZ-13, the catalytic activity and hydrothermal stability of the Cu-SSZ-13@meso-MOx have been greatly improved. The promoting effect of meso-MOx shell on the catalytic activity of Cu-SSZ-13 is mainly attribute to the presence of both microporous and mesoporous structures, the formation of more active isolated Cu2+ ions and more L-acid sites, and the synergistic effect between Cu-SSZ-13 and meso-MOx interaction. Moreover, all the Cu-SSZ-13@meso-MOx catalysts maintain their activities to a greater extent after hydrothermal aging. The core-shell structure may play a protective role and greatly improve the hydrothermal stability of Cu-SSZ-13. More L-acidic sites and more isolated Cu2+ ions are preserved under the protection of mesoporous structure.

Oxygen vacancy-engineered titanium-based perovskite for boosting H2O activation and lower-temperature hydrolysis of organic sulfur.

Modulation of water activation is crucial to water-involved chemical reactions in heterogeneous catalysis. Organic sulfur (COS and CS2) hydrolysis is such a typical reaction involving water (H2O) molecule as a reactant. However, limited by the strong O-H bond in H2O, satisfactory CS2 hydrolysis performance is attained at high temperature above 310 °C, which is at the sacrifice of the Claus conversion, strongly hindering sulfur recovery efficiency improvement and pollution emissions control of the Claus process. Herein, we report a facile oxygen vacancy (VO) engineering on titanium-based perovskite to motivate H2O activation for enhanced COS and CS2 hydrolysis at lower temperature. Increased amount of VO contributed to improved degree of H2O dissociation to generate more active -OH, due to lower energy barrier for H2O dissociation over surface rich in VO, particularly VO clusters. Besides, low-coordinated Ti ions adjacent to VO were active sites for H2O activation. Consequently, complete conversion of COS and CS2 was achieved over SrTiO3 after H2 reduction treatment at 225 °C, a favorable temperature for the Claus conversion, at which both satisfying COS and CS2 hydrolysis performance and improved sulfur recovery efficiency can be obtained simultaneously. Additionally, the origin of enhanced hydrolysis activity from boosted H2O activation by VO was revealed via in-depth mechanism study. This provides more explicit direction for further design of efficacious catalysts for H2O-involved reactions.

A review of whole-process control of industrial volatile organic compounds in China.

Volatile organic compounds (VOCs) play an important role in the formation of ground-level ozone and secondary organic aerosol (SOA), and they have been key issues in current air pollution prevention and control in China. Considerable attention has been paid to industrial activities due to their large and relatively complex VOCs emissions. The present research aims to provide a comprehensive review on whole-process control of industrial VOCs, which mainly includes source reduction, collection enhancement and end-pipe treatments. Lower VOCs materials including water-borne ones are the keys to source substitution in industries related to coating and solvent usage, leak detection and repair (LDAR) should be regarded as an efficient means of source reduction in refining, petrochemical and other chemical industries. Several types of VOCs collection methods such as gas-collecting hoods, airtight partitions and others are discussed, and airtight collection at negative pressure yields the best collection efficiency. Current end-pipe treatments like UV oxidation, low-temperature plasma, activated carbon adsorption, combustion, biodegradation, and adsorption-combustion are discussed in detail. Finally, several recommendations are made for future advanced treatment and policy development in industrial VOCs emission control.

Adsorption and membrane separation for removal and recovery of volatile organic compounds.

Volatile organic compounds (VOCs) are a crucial kind of pollutants in the environment due to their obvious features of severe toxicity, high volatility, and poor degradability. It is particularly urgent to control the emission of VOCs due to the persistent increase of concentration and the stringent regulations. In China, clear directions and requirements for reduction of VOCs have been given in the "national plan on environmental improvement for the 13th Five-Year Plan period". Therefore, the development of efficient technologies for removal and recovery of VOCs is of great significance. Recovery technologies are favored by researchers due to their advantages in both recycling VOCs and reducing carbon emissions. Among them, adsorption and membrane separation processes have been extensively studied due to their remarkable industrial prospects. This overview was to provide an up-to-date progress of adsorption and membrane separation for removal and recovery of VOCs. Firstly, adsorption and membrane separation were found to be the research hotspots through bibliometric analysis. Then, a comprehensive understanding of their mechanisms, factors, and current application statuses was discussed. Finally, the challenges and perspectives in this emerging field were briefly highlighted.

Emission characteristics, risk assessment and scale effective control of VOCs from automobile repair industry in Beijing.

Automobile repair is regarded as a typical domestic source of VOCs in China characterized by numerous sites, wide dispersion and intermittent VOCs emissions. It is of great importance to study and control VOCs from such activities. In this research, emission characteristics, risk assessment and scale effective control of VOCs from automobile repair in Beijing were studied. Results showed that coating spraying and baking were the main processes of VOCs and the major species determined were mostly oxygen-containing VOCs and aromatic hydrocarbons in the case of solvent-based coating usage. Meanwhile, alkanes were determined and accounted for 40 % of total VOCs emissions during the water-based coating spraying and baking. Generally, the total determined VOCs during the automobile repair processes were 1.06-1.27 mg/m3 and 2.93-53.46 mg/m3 for the usage of water-based and solvent-based paint, respectively. Health risk assessments indicated that the residents in the region about 30 m high within a radius of 20 m around the automobile repair plants might suffer from both serious non-carcinogenic and carcinogenic risk threats in the case of solvent-based coating usage in that the values of total hazard index (HI) represented by dichloropropane and acrolein were higher than 1 and the value of lifetime cancer risk (LCR) represented by dichloroethane was higher than 10-5. Besides, those in the region about 30 m high and within a wider radius of 340 m might suffer from carcinogenic risk threat with a certain probability (LCR > 10-6) no matter either solvent-based or water-based coatings were used. As for the scale control of VOCs from automobile repair, independent adsorption by activated carbon combined with mobile regeneration by catalytic combustion was also proposed as an efficient way.

Simultaneously Constructing Active Sites and Regulating Mn-O Strength of Ru-Substituted Perovskite for Efficient Oxidation and Hydrolysis Oxidation of Chlorobenzene.

Chlorinated volatile organic compounds (CVOCs) are a class of hazardous pollutants that severely threaten environmental safety and human health. Although the catalytic oxidation technique for CVOCs elimination is effective, enhancing the catalytic efficiency and simultaneously inhibiting the production of organic byproducts is still of great challenge. Herein, Ru-substituted LaMn(Ru)O3+ δ perovskite with Ru-O-Mn structure and weakened Mn-O bond strength has been developed for catalytic oxidation of chlorobenzene (CB). The formed Ru-O-Mn structure serves as favorable sites for CB adsorption and activation, while the weakening of Mn-O bond strength facilitates the formation of active oxygen species and improves oxygen mobility and catalyst reducibility. Therefore, LaMn(Ru)O3+ δ exhibits superior low-temperature activity with the temperature of 90% CB conversion decreasing by over 90 °C compared with pristine perovskite, and the deep oxidation of chlorinated byproducts produced in low temperature is also accelerated. Furthermore, the introduction of water vapor into reaction system triggers the process of hydrolysis oxidation that promotes CB destruction and inhibits the generation of chlorinated byproducts, due to the higher-activity *OOH species generated from the dissociated H2 O reacting with adsorbed oxygen. This work can provide a unique, high-efficiency, and facile strategy for CVOCs degradation and environmental improvement.

An ultrafast and facile nondestructive strategy to convert various inefficient commercial nanocarbons to highly active Fenton-like catalysts.

The Fenton-like process catalyzed by metal-free materials presents one of the most promising strategies to deal with the ever-growing environmental pollution. However, to develop improved catalysts with adequate activity, complicated preparation/modification processes and harsh conditions are always needed. Herein, we proposed an ultrafast and facile strategy to convert various inefficient commercial nanocarbons into highly active catalysts by noncovalent functionalization with polyethylenimine (PEI). The modified catalysts could be in situ fabricated by direct addition of PEI aqueous solution into the nanocarbon suspensions within 30 s and without any tedious treatment. The unexpectedly high catalytic activity is even superior to that of the single-atom catalyst and could reach as high as 400 times higher than the pristine carbon material. Theoretical and experimental results reveal that PEI creates net negative charge via intermolecular charge transfer, rendering the catalyst higher persulfate activation efficiency.

Engineering Ultrafine NiFe-LDH into Self-Supporting Nanosheets: Separation-and-Reunion Strategy to Expose Additional Edge Sites for Oxygen Evolution.

Here, a strategy is reported to prepare Ni-Fe layered double hydroxide (NiFe-LDH) with abundant exposed edge planes for enhanced oxygen evolution reaction (OER). The edge-to-edge assembly of ultrafine NiFe-LDH directed by graphite-like carbon is performed through a one-step hydrothermal process to form self-supporting nanosheet arrays (named NiFe-LDH/C), in which ascorbic acid is employed as the carbon precursor to control both the platelet size and the assembly mode of NiFe-LDH. Benefiting from the unique structural engineering, NiFe-LDH/C can not only achieve a fast surface reconstruction into the highly active γ-phase structure, but also exposes abundant active edge sites, thus leading to a superior OER performance with the overpotential as low as 234 mV at a current density of 50 mA cm-2 . Furthermore, density functional theory (DFT) calculations reveal that the unsaturated Fe-sites and the bridge-sites connecting Ni and Fe atoms, which only exist on the edge planes of NiFe-LDH, are the main active centers responsible for promoting the intrinsic OER activity. This work provides a specific and valuable reference for the rational design of high-quality electrocatalysts through structural engineering for renewable energy applications.

Agar-stabilized sulfidated microscale zero-valent iron: Its stability and performance in chromate reduction.

Sulfidated microscale zero-valent iron (SmZVI) attracts much attention recently in remediation of contaminated groundwater, but whether polymer coating on SmZVI would impact on its reactivity and capacity is yet to be understood. In this work, SmZVI was prepared by milling mZVI with elemental sulfur, and its stability in agar solution was evaluated. The impact of polymer coating on SmZVI grains' capacity and reactivity for chromate reduction was then examined. Experimental results indicated that SmZVI having the best overall performance was attained by grinding mZVI with elemental sulfur at 0.05 S/Fe molar ratio for 10 h. SmZVI's stability can be substantially improved if dispersed in 2.0 g/L agar solution. Existence of agar films on the SmZVI grain (A-SmZVI) lowered the material's capacity for chromate reduction by 56%, and the associated reaction kinetics by 70.4%, as estimated by pseudo first-order reaction model using the early-stage experimental data. Analysis of XPS spectra of A-SmZVI post reaction with chromate indicated that multiple reductive species including Fe0, Fe(II), FeS, and S(-II) may have jointly participated in the redox reaction taking place on the A-SmZVI-water interface. Fitting of XPS data supported that S(-II) was oxidized to SO42-, S2O32-, and S0, in order of decreasing surface concentration.

Study of heterogeneous reaction of dimethyl sulfide on atmospheric-like particulate TiO2.

Dimethyl sulfide (DMS) related to solar radiation and greenhouse effect is one of the most important volatile sulfides and its' oxidation products are also important contributors to acid rain. It is of great importance to study the consumption and reactions of DMS in the atmosphere. In this work, atmospheric-like particulate TiO2 was selected to study the reaction mechanism of DMS on TiO2 with the purpose to explore the possible heterogeneous oxidation of DMS. The results showed that the heterogeneous reaction of DMS with TiO2 occurred under the condition of illumination, which is a first-order-like reaction with the rate constant K = 2.83 × 10-4/s, the initial reaction uptake coefficient and the steady reaction uptake coefficient indicated the occupation of products and by-products on the surface of TiO2. The heterogeneous reaction mechanism of DMS studied by aerosol time-of-flight mass spectrometry (ATOFMS) suggested that DMS underwent a series of complex chemical reactions with sulfate and various sulfur-containing gas products, in which hydroxyl radicals might play an important role.

Unraveling the adsorption and diffusion properties of hexamethyldisiloxane on zeolites by static gravimetric analysis.

Utilization of biogas from wastewater treatment plants (WWTPs) as a kind of renewable energy can effectively alleviate the escalating energy crisis. However, volatile methylsiloxanes (VMS) exist in biogas as impurities and hinder its efficient use. Adsorption is the main technology to achieve high-efficiency purification of VMS currently, yet studies on the adsorption processes and mechanisms are quite insufficient. Here, we use the static vapor adsorption method to investigate the adsorption performances and mechanisms of four typical zeolite adsorbents (Hbeta, SBA-15, MOR and MCM-41) towards hexamethyldisiloxane (L2), which is a representative of VMS. Results suggest the adsorption interaction of L2 and zeolite is closely related to the pore size of zeolite and follows the order of Hbeta > MCM-41 > SBA-15 > MOR. The adsorption rate constants of L2 on MCM-41 are larger than the others in the most relative pressure ranges. Besides, cyclic adsorption/desorption performances of L2 on MCM-41, SBA-15 and Hbeta show their super recycling properties. These studies would provide important information on designing an effective zeolite-L2 adsorption system, and are helpful to understand the deeper adsorption mechanisms of VMS on zeolites.

Unprecedented Nonphotomediated Hole (h + ) Oxidation System Constructed from Defective Carbon Nanotubes and Superoxides.

Holes (h + ) on heterogeneous photocatalysts could act as important oxidative species or precursors for reactive oxygen species (ROS). However, due to the ultrafast recombination of photoinduced electrons and holes, a majority of carriers are consumed prior to surface reactions. Herein, we report an unprecedented nonphotomediated hole oxidation system constructed from carbon nanotubes (CNTs) and superoxides. This system exhibited high catalytic activity for the degradation of organic pollutants, which outperforms the classical oxidation processes in the remediation of actual wastewater and is comparable to that of the best single cobalt atom catalyst. Theoretical and experimental results reveal that the intrinsic defects with unpaired spins on CNTs served as adsorptive sites to activate superoxides. This is the first report on exploring the oxidation properties of nonphotomediated hole carriers on heterogeneous catalysts, which will be of broad interest for researchers in environmental remediation, chemical synthesis, and biological fields.

Remarkable MnO2 structure-dependent H2O promoting effect in HCHO oxidation at room temperature.

H2O is often critical in determining the activity and stability of metal oxide catalysts for HCHO oxidation; however, synthesis of metal oxide catalysts with super resistance to H2O remains a challenging. Herein, we synthesized Akhtenskite-type MnO2 catalyst with Mn-O-Mn stretching along MnO6 octahedra layers, which promotes the utilization of the associatively adsorbed H2O. The activity and stability of formaldehyde oxidation at room temperature enhanced in humid air. Diffuse-reflectance infrared Fourier transform (DRIRFT) spectroscopy was used to characterize the H2O adsorption and intermediate species. The associatively adsorbed H2O promotes the oxidation of formaldehyde to CO2 via the formic acid intermediate. The service life of MnO2 is prolonged due to formic acid generation. MnO2 gradually deactivates when formic acid accumulates and forms formate and hydrogen carbonate species. This study provides significant insights into the development of a high-efficiency MnO2 catalyst for formaldehyde oxidation in humid air, and the developed MnO2 catalyst is a promising candidate for application in practical formaldehyde elimination.

Effects of mesoporous silica particle size and pore structure on the performance of polymer-mesoporous silica mixed matrix membranes.

The fabrication of mixed matrix membranes (MMMs) has been regarded as an effective and economic approach to enhance the gas permeability and selectivity properties of conventional polymeric membranes for gas separation applications. However, the poor compatibility between polymeric matrix and inorganic filler in MMMs could lead to the generation of interfacial defects resulting in reduced gas selectivity. In this work, with the aim of studying the effect of particle size and pore structure of the filler on the performance of the resultant MMMs, nano/micro sized spherical mesoporous silicas with 2D/3D pore structure (MCM-41 and MCM-48) were synthesized and selected as fillers for the preparation of polydimethylsiloxane (PDMS)-based MMMs. The separation properties of the membranes prepared were characterized by permeability measurements for nitrogen and organic vapors (C3H6 and n-C4H10). Compared with microsized particles, nanosized fillers have better dispersion in the polymer matrix which could minimize the formation of non-selective microvoids around the particles, leading to higher vapor/N2 ideal selectivities of the MMMs, even at the high loading (15 wt%). Moreover, due to the conventional random packing orientation of the particles in the polymer, vapor permeation was severely hindered in the MMMs fabricated from mesoporous silica with 2D pore channels. The interface morphologies and gas diffusion paths in the MMMs have also been proposed. With an optimum loading of nanosized MCM-48 (3D pore structure), the vapor permeabilities and vapor/N2 ideal selectivities of the MMMs were shown to increase simultaneously, compared with the neat polymer membrane.