Synthesis of a Triangle-Fused Six-Pointed Star and Its Electrocatalytic CO2 Reduction Activity.

As electrocatalysts, molecular catalysts with large aromatic systems (such as terpyridine, porphyrin, or phthalocyanine) have been widely applied in the CO2 reduction reaction (CO2RR). However, these monomeric catalysts tend to aggregate due to strong π-π interactions, resulting in limited accessibility of the active site. In light of these challenges, we present a novel strategy of active site isolation for enhancing the CO2RR. Six Ru(Tpy)2 were integrated into the skeleton of a metallo-organic supramolecule by stepwise self-assembly in order to form a rhombus-fused six-pointed star R1 with active site isolation. The turnover frequency (TOF) of R1 was as high as 10.73 s-1 at -0.6 V versus reversible hydrogen electrode (vs RHE), which is the best reported value so far at the same potential to our knowledge. Furthermore, by increasing the connector density on R1's skeleton, a more stable triangle-fused six-pointed star T1 was successfully synthesized. T1 exhibits exceptional stability up to 126 h at -0.4 V vs RHE and excellent TOF values of CO. The strategy of active site isolation and connector density increment significantly enhanced the catalytic activity by increasing the exposure of the active site. This work provides a starting point for the design of molecular catalysts and facilitates the development of a new generation of catalysts with a high catalytic performance.

Constructing Heterointerfaces in Dual-Phase High-Entropy Oxides to Boost O2 Activation and SO2 Resistance for Mercury Removal in Flue Gas.

The low O2 activation ability at low temperatures and SO2 poisoning are challenges for metal oxide catalysts in the application of Hg0 removal in flue gas. A novel high-entropy fluorite oxide (MgAlMnCo)CeO2 (Co-HEO) with the second phase of spinel is synthesized by the microwave hydrothermal method for the first time. A high efficiency of Hg0 removal (close to 100%) is achieved by Co-HEO catalytic oxidation at temperatures as low as 100 °C and in the atmosphere of 145 µg m-3 Hg0 at a high GHSV (gas hourly space velocity) of 95,000 h-1. According to O2-TPD and in situ FT-IR, this extremely superior catalytic oxidation performance at low temperatures originates from the activation ability of Co-HEO to transform O2 into superoxide and peroxide, which is promoted by point defects induced from the spinel/fluorite heterointerfaces. Meanwhile, SO2 resistance of Co-HEO for Hg0 removal is also improved up to 2000 ppm due to the high-entropy-stabilized structure, construction of heterointerfaces, and synergistic effect of the multicomponents for inhibiting the oxidation of SO2 to surface sulfate. The design strategy of the dual-phase high-entropy material launches a new route for metal oxides in the application of catalytic oxidation and SO2 resistance.

Nafion-Mediated Proton Transfer Facilitates the Electroreduction of SO2 to H2S.

The electrocatalytic reduction of SO2 to produce H2S is a critical approach for achieving the efficient utilization of sulfur resources. At the core of this approach for commercial applications lies the imperative need to elevate current density. However, the challenges posed by high current density manifest in the rapid depletion of protons, leading to a decrease in SO2 partial pressure, consequently hampering the generation and separation of H2S. Here, we demonstrate an effective solution to alleviate the problem of insufficient supply of protons by employing Nafion polymer as the proton conductor to modified Cu catalysts surface, creating a proton-enriched layer to boost H2S generation. It was observed that Nafion shortens the hydrogen bonds with water molecules in the electrolyte via its sulfonic acid groups, benefiting the proton transfer and consequently increasing the proton density on the electrode surface by 5-fold. With the Nafion-modified catalyst, the H2S partial current density and separation efficiency reached 205.9 mA·cm-2 (1.01 mmol·cm-2·h-1) and 87.8%, which were 1.34 and 1.22 times that on unmodified Cu, respectively. This work highlights the practicality of fabricating a proton conductor via ionic polymer for the control over product selectivity in pH-sensitive reactions under high current density.

Edge-Enriched Molybdenum Disulfide Ultrathin Nanosheets with a Widened Interlayer Spacing for Highly Efficient Gaseous Elemental Mercury Capture.

Transition metal sulfides have exhibited remarkable advantages in gaseous elemental mercury (Hg0) capture under high SO2 atmosphere, whereas the weak thermal stability significantly inhibits their practical application. Herein, a novel N,N-dimethylformamide (DMF) insertion strategy via crystal growth engineering was developed to successfully enhance the Hg0 capture ability of MoS2 at an elevated temperature for the first time. The DMF-inserted MoS2 possesses an edge-enriched structure and an expanded interlayer spacing (9.8 Å) and can maintain structural stability at a temperature as high as 272 °C. The saturated Hg0 adsorption capacities of the DMF-inserted MoS2 were measured to be 46.91 mg·g-1 at 80 °C and 27.40 mg·g-1 at 160 °C under high SO2 atmosphere. The inserted DMF molecules chemically bond with MoS2, which prevents possible structural collapse at a high temperature. The strong interaction of DMF with MoS2 nanosheets facilitates the growth of abundant defects and edge sites and enhances the formation of Mo5+/Mo6+ and S22- species, thereby improving the Hg0 capture activity at a wide temperature range. Particularly, Mo atoms on the (100) plane represent the strongest active sites for Hg0 oxidation and adsorption. The molecule insertion strategy developed in this work provides new insights into the engineering of advanced environmental materials.

Systematic control technologies for gaseous pollutants from non-ferrous metallurgy.

Air pollutant emissions represent a critical challenge in the green development of the non-ferrous metallurgy industry. This work studied the emission characteristics, formation mechanisms, phase transformation and separation of typical air pollutants, such as heavy metal particles, mercury, sulfur oxides and fluoride, during non-ferrous smelting. A series of purification technologies, including optimization of the furnace throat and high-temperature discharge, were developed to collaboratively control and recover fine particles from the flue gas of heavy metal smelting processes, including copper, lead and zinc. Significant improvements have been realized in wet scrubbing technology for removing mercury, fluoride and SO2 from flue gas. Gas-liquid sulfidation technology by applying H2S was invented to recycle the acid scrubbing wastewater more efficiently and in an eco-friendly manner. Based on digital technology, a source reduction method was designed for sulfur and fluoride control during the whole aluminum electrolysis process. New desulfurization technologies were developed for catalytic reduction of the sulfur content in petroleum coke at low temperature and catalytic reduction of SO2 to elemental sulfur. This work has established the technology for coupling multi-pollutant control and resource recovery from the flue gas from non-ferrous metallurgy, which provides the scientific theoretical basis and application technology for the treatment of air pollutants in the non-ferrous metallurgy industry.

Controllable Disordered Copper Sulfide with a Sulfur-Rich Interface for High-Performance Gaseous Elemental Mercury Capture.

Copper sulfide (CuS) has received increasing attention as a promising material in gaseous elemental mercury (Hg0) capture, yet how to enhance its activity at elevated temperature remains a great challenge for practical application. Herein, simultaneous improvement in the activity and thermal stability of CuS toward Hg0 capture was successfully achieved for the first time by controlling the crystal growth. CuS with a moderate crystallinity degree of 68.8% showed a disordered structure yet high thermal stability up to 180 °C. Such disordered CuS can maintain its Hg0 capture activity stable during longtime test at a wide temperature range from 60 to 180 °C and displayed strong resistance to SO2 (6%) and H2O (8%). The significant improvement can be attributed to the synergistic effect of a moderately crystalline nature and a unique sulfur-rich interface. Moderate crystallinity guarantees the thermal stability of CuS and the presence of abundant defects, in which copper vacancy enhances significantly the Hg0 capture activity. The sulfur-rich interface enables CuS to provide plentiful highly active Sx2- sites for Hg0 adsorption. The interrelation between structure, reactivity, and thermal stability clarified in this work broadens the understanding toward Hg0 oxidation and adsorption over CuS and provides new insights into the rational design and engineering of advanced environmental materials.

Formation of sulfur oxide groups by SO2 and their roles in mercury adsorption on carbon-based materials.

The presence of SO2 display significant effect on the mercury (Hg) adsorption ability of carbon-based sorbent. Yet the adsorption and oxidation of SO2 on carbon with oxygen group, as well as the roles of different sulfur oxide groups in Hg adsorption have heretofore been unclear. The formation of sulfur oxide groups by SO2 and their effects on Hg adsorption on carbon was detailed examined by the density functional theory. The results show that SO2 can be oxidized into SO3 by oxygen group on carbon surface. Both C-SO2 and C-SO3 can improve Hg adsorption on carbon site, while the promotive effect of C-SO2 is stronger than C-SO3. Electron density difference analyses reveal that sulfur oxide groups enhance the charge transfer ability of surface unsaturated carbon atom, thereby improving Hg adsorption. The experimental results confirm that surface active groups formed by SO2 adsorption is more active for Hg adsorption than the groups generated by SO3.

Low-temperature Hg0 abatement by ionic liquid based on weak interaction.

Low-temperature gaseous elemental mercury (Hg0) abatement is an objective demand in industrial flue gas treatment. In this work, we proposed a new approach for Hg0 capture via weak interaction of ionic liquids. Ionic liquids with varied anions (1-butyl-3-methylimidazolium thioacetate ([Bmim][ThAc]), 1-butyl-3-methylimidazolium diethyldithiocarbamate ([Bmim][DTCR]), and 1-butyl-3-methylimidazolium ethylxanthate ([Bmim][EX])) were designed and synthesized. The interaction energies between ionic liquids and elemental mercury were proved to be positively related to mercury removal efficiency, revealing that the electrostatic interaction derived physical adsorption from anions is the dominant factor affecting mercury removal performance. [Bmim][ThAc] with the largest anionic electrostatic interaction energy showed the best mercury abatement performance, achieving a Hg0 removal efficiency of over 98% and an adsorption capacity of 10.66 mg/g at 50 °C. The influence of temperature and the results of mercury temperature-programmed desorption (Hg-TPD), X-ray photoelectron spectroscopy (XPS) further confirmed that the ionic liquid combines with elemental mercury through physical adsorption. The work provides a new perspective on designing high-efficiency sorbents for mercury removal at low temperature.

On-line detection and kinetic study of selenium release during combustion, gasification and pyrolysis of sawdust.

An on-line analysis system was firstly developed to quantitatively measure the temporal concentrations of selenium in the flue gas directly. Then the selenium release during air combustion, CO2/argon gasification, and argon pyrolysis of sawdust was systematically studied using the on-line analysis system, based on the inductively coupled plasma optical emission spectroscopy. The peak of selenium concentration in the flue gas ranges from 0.38 to 1.76 mg∙Nm-3 with change of reaction temperature and atmosphere. The overall activation energy for selenium release is 75.3 kJ∙mol-1 in air combustion, 102.4 kJ∙mol-1 in CO2/argon gasification, and 81.9 kJ∙mol-1 in argon pyrolysis, respectively. The results show that the combustion atmosphere contributes to the selenium release more than that in gasification and pyrolysis. The promotion effect of chlorine on selenium release under combustion environment was one to three times higher than that under gasification and pyrolysis atmosphere. Thermodynamic equilibrium calculation showed that selenium oxides were the main gaseous selenium species in combustion, while the dominant gaseous selenium species were H2Se (g) and Se (g) under gasification/pyrolysis condition. The selenium release was increased with different degrees by additive chlorine species, mainly because of the formation of SeCl2 (g). The role of chlorine in selenium transformation has been provided in the proposed reaction pathways of selenium release, based on the new findings using on-line analysis system. The selenium species retained in sawdust can be transformed into selenium oxide (SeO2, SeO, corresponding to the combustion condition) and selenium hydride (H2Se, corresponding to the gasification/pyrolysis conditions).

DFT and Experimental Studies on the Mechanism of Mercury Adsorption on O2-/NO-Codoped Porous Carbon.

The utilization of O2 and NO in flue gas to activate the raw porous carbon with auxiliary plasma contributes to an effective mercury (Hg)-removal strategy. The lack of in-depth knowledge on the Hg adsorption mechanism over the O2-/NO-codoped porous carbon severely limits the development of a more effective Hg removal method and the potential application. Therefore, the generation processes of functional groups on the surface during plasma treatment were investigated and the detailed roles of different groups in Hg adsorption were clarified. The theoretical results suggest that the formation of functional groups is highly exothermic and they preferentially form on a carbon surface, and then affect Hg adsorption. The active groups affect Hg adsorption in a different manner, which depends on their nature. All of these active groups can improve Hg adsorption by enhancing the interaction of Hg with a surface carbon atom. Particularly, the preadsorbed NO2 and O3 groups can react directly with Hg by forming HgO. The experimental results confirm that the active groups cocontribute to the high Hg removal efficiency of O2-/NO-codoped porous carbon. In addition, the mercury temperature-programmed desorption results suggest that there are two forms of mercury present on O2-/NO-codoped porous carbon, including a carbon-bonded Hg atom and HgO.

Co9S8 nanoparticles-embedded porous carbon: A highly efficient sorbent for mercury capture from nonferrous smelting flue gas.

In this study, a novel Co9S8 nanoparticles-embedded porous carbon (Co9S8-PC) was designed as an effective reusable sorbent for Hg0 capture from smelting flue gas. Some flue gas components can create more active sites on Co9S8-PC for Hg0 adsorption, but compete with Hg0 for the same sulfur sites over nano Co1-xS/Co3S4 (CoS) and Co1-xS/Co3S4 embedded porous carbon (CoS-PC), which can be ascribed to the difference in crystal structure between Co9S8 and Co1-xS/Co3S4. Therefore, Co9S8-PC shows much better Hg0 capture ability than CoS and CoS-PC under smelting flue gas. O2, SO2 and HCl improve Hg0 adsorption on Co9S8-PC mainly through creating Co3+ site, but H2O has neglectable effect on Hg0 capture. Co9S8-PC shows a remarkably large Hg0 adsorption capacity of 43.18 mg/g, which is greatly higher than the representative metal sulfides for Hg0 removal from smelting flue gas. During Hg0 adsorption, Co3+ is the primary site to directly interact with Hg0, and the adsorbed mercury exists as HgS. Co9S8-PC exhibits an excellent recyclability for capturing Hg0, which is mainly assigned to the replenishment of consumed Co3+ site by O2, SO2 and HCl. Therefore, Co9S8 nanoparticles-embedded porous carbon is an efficient, sustainable and highly recyclable sorbent for Hg0 recovery from smelting flue gas.

Experimental and theoretical study of kinetic and mechanism of hydroxyl radical-mediated degradation of sulfamethazine.

Hydroxyl radical (•OH)-based advanced oxidation technologies (AOTs) is an effective and clean way to remove sulfonamide antibiotics in water at ambient temperature and pressure. In this study, we systematically investigated the degradation kinetics of sulfamethazine (SMT) by •OH with a combination of experimental and theoretical approaches. The second-order rate constant (k) of SMT with •OH was experimentally determined to be 5.27 ± 0.06 × 109 M-1 s-1 at pH 4.5. We also calculated the thermodynamic and kinetic behaviors for the reactions by density functional theory (DFT) using the B3LYP/6-31G*. The results revealed that •OH addition pathways at the methylene (C4) site on the pyridine ring and the ortho sites (C12 and C14) of the amino group on the benzene ring dominate the reaction, especially C14 site on the benzene ring accounted for 43.95% of SMT degradation kinetics. The theoretical k value which was calculated by conventional transition state theory is 3.96 × 109 M-1 s-1, indicating that experimental observation (5.27 ± 0.06 × 109) is correct. These results could further help AOTs design in treating sulfonamide during wastewater treatment processes.

Temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust.

The temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust was studied using an on-line analysis system. The arsenic release amount in combustion atmosphere was higher than that in CO2 gasification and argon pyrolysis. The derived values of activation energy followed the order: combustion < gasification < pyrolysis. Furthermore, the enhancement effect of chlorine species on arsenic release percentage in air combustion was also higher than that in gasification and pyrolysis conditions. The total proportion of arsenic release in combustion with additive chlorine is bigger than the case in gasification and pyrolysis, especially when 20% chlorine is added. According to equilibrium analysis, arsenic oxides were identified as the main gaseous arsenic species and their formation were decreased in the oxygen-deficient environment, mainly accounting for lesser arsenic release proportion in gasification and pyrolysis than combustion. The release of arsenic was promoted to a different extent with additive chlorine, mainly caused by the AsCl3 (g) formation. By the findings of the experiments and theoretical analyses, the possible reaction pathways and release mechanisms of arsenic species were proposed.

Development of O2 and NO Co-Doped Porous Carbon as a High-Capacity Mercury Sorbent.

In this study, a novel Hg0 adsorption strategy based on nonthermal plasma and porous carbon was proposed and tested. The O2 and NO in flue gas were used to activate porous carbon with auxiliary plasma. The plasma significantly increased the functionalities on the carbon surface, and it has a negligible effect on the textural properties of porous carbon. The O2/NO co-doped porous carbon was used to remove elemental mercury (Hg0). The sample functionalized by plasma in 4% O2 and 200 ppm NO (balanced with N2) for 3 min exhibited superior Hg0 adsorption ability, which could be assigned to the formation of a large amount of C═O, C-NO, and C-NO2. O2, NO, and HCl have a positive effect on Hg0 adsorption, whereas SO2 and H2O have an inhibitory effect on Hg0 removal. The equilibrium Hg0 adsorption capacity of optimal O2/NO co-doped porous carbon was found to be 12315 µg/g, which was far greater than that of brominated activated carbon (1061 µg/g). Density functional theory was used to investigate the mechanism responsible for Hg0 adsorption. C═O and C-NO improved the interaction of Hg0 with neighboring carbon sites. C-NO2 could react with Hg0 by forming HgO.

Design of O2/SO2 dual-doped porous carbon as superior sorbent for elemental mercury removal from flue gas.

A porous carbon was synthesized via hydrothermal carbonization and CO2 activation. O2 and SO2 were successfully co-doped onto carbon surface by applying non-thermal plasma technique. Porous carbon possessing excellent textural properties is effective to adsorb the radicals generated by plasma. Plasma promotes the adsorption of O2 and SO2 on carbon surface with the formation of abundant CO, C-S and C-SOx (x = 1-3) groups. The O2/SO2 dual-doped porous carbon was utilized to adsorb elemental mercury (Hg0) from the flue gas of coal combustion. The Hg0 adsorption ability of the O2/SO2 dual-doped porous carbon is closely related with the concentrations of O2 and SO2 for plasma treatment and the treatment time. The optimal O2/SO2 dual-doped porous carbon exhibited far greater Hg0 adsorption capacity than a commercial brominated activated carbon. Density functional theory was employed to understand the Hg0 adsorption mechanism at the molecular level. CO, C-S and C-SOx (x = 1-3) groups enhanced the interaction of Hg0 with surface carbon atom. The activity of them for enhancing Hg0 adsorption is in the order of C-SO2 > CO > C-S > C-SO > C-SO3. Porous carbon can be activated by plasma in flue gas containing O2 and SO2, and used as superior sorbent for Hg0 removal.

Oxygen-Rich Porous Carbon Derived from Biomass for Mercury Removal: An Experimental and Theoretical Study.

A porous carbon was synthesized by the combination of freeze-drying and CO2 activation from starch. Nonthermal plasma was employed to quickly produce oxygen functional groups on a porous carbon surface. The plasma treatment has a negligible effect on the textural properties of the porous carbon. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses suggested that the plasma treatment significantly increased the amount and promoted the evolution of oxygen groups on surface. The unique pore structure of porous carbon was proven favorable to effective oxygen loading. The elemental mercury (Hg0) adsorption ability of the oxygen enriched porous carbon was tested. The results indicated that the oxygen-rich porous carbon constitutes an effective sorbent for Hg0 removal. The excellent textural properties, surface atomic oxygen concentration, and the type of oxygen group are the three key factors for realizing high Hg0 removal performance. Density functional calculations were performed to understand the effect of oxygen groups on Hg0 adsorption. Carbonyl and ester groups are beneficial for Hg0 adsorption, whereas epoxy, carboxyl, and hydroxyl groups inhibit Hg0 adsorption. Plasma treatment enhances Hg0 adsorption by increasing the amount of ester and carbonyl groups on surface.

Insights into the effect of chlorine on arsenic release during MSW incineration: An on-line analysis and kinetic study.

The effect of chlorine on arsenic (As) release dynamics during municipal solid waste (MSW) incineration in a fluidized bed was studied on the basis of an on-line analysis system. This system can continuously and quantitatively measure the concentrations of trace elements in flue gas. Chlorine addition increases obviously the concentration of arsenic in flue gas, indicating a promoting effect of chlorine on arsenic release during MSW incineration. Based on the temporal concentration of arsenic in flue gas, the overall kinetic parameters of arsenic release during MSW incineration were calculated. A second-order kinetic law r(x) = 81.6e-66.9/RT (-1.05x2 - 0.01x + 1.03) was ascertained for arsenic release during MSW incineration without chlorine addition, and r(x) = 177.3e-65.3/RT (-0.68x2 - 0.43x + 1.08) for arsenic release with chlorine addition. Thermodynamic calculations were performed to predict the partitioning behavior of arsenic during MSW incineration. The addition of chlorine can not only compete with gaseous arsenic to react with mineral, but is also able to increase the volatilization of arsenic by forming volatile arsenic chlorides, thereby affecting the release kinetics of arsenic during MSW incineration.

Temporal measurements and kinetics of selenium release during coal combustion and gasification in a fluidized bed.

The temporal release of selenium from coal during combustion and gasification in a fluidized bed was measured in situ by an on-line analysis system of trace elements in flue gas. The on-line analysis system is based on an inductively coupled plasma optical emission spectroscopy (ICP-OES), and can measure concentrations of trace elements in flue gas quantitatively and continuously. The results of on-line analysis suggest that the concentration of selenium in flue gas during coal gasification is higher than that during coal combustion. Based on the results of on-line analysis, a second-order kinetic law r(x)=0.94e(-26.58/RT)(-0.56 x(2) -0.51 x+1.05) was determined for selenium release during coal combustion, and r(x)=11.96e(-45.03/RT)(-0.53 x(2) -0.56 x+1.09) for selenium release during coal gasification. These two kinetic laws can predict respectively the temporal release of selenium during coal combustion and gasification with an acceptable accuracy. Thermodynamic calculations were conducted to predict selenium species during coal combustion and gasification. The speciation of selenium in flue gas during coal combustion differs from that during coal gasification, indicating that selenium volatilization is different. The gaseous selenium species can react with CaO during coal combustion, but it is not likely to interact with mineral during coal gasification.

On-Line Analysis and Kinetic Behavior of Arsenic Release during Coal Combustion and Pyrolysis.

The kinetic behavior of arsenic (As) release during coal combustion and pyrolysis in a fluidized bed was investigated by applying an on-line analysis system of trace elements in flue gas. This system, based on inductively coupled plasma optical emission spectroscopy (ICP-OES), was developed to measure trace elements concentrations in flue gas quantitatively and continuously. Obvious variations of arsenic concentration in flue gas were observed during coal combustion and pyrolysis, indicating strong influences of atmosphere and temperature on arsenic release behavior. Kinetic laws governing the arsenic release during coal combustion and pyrolysis were determined based on the results of instantaneous arsenic concentration in flue gas. A second-order kinetic law was determined for arsenic release during coal combustion, and the arsenic release during coal pyrolysis followed a fourth-order kinetic law. The results showed that the arsenic release rate during coal pyrolysis was faster than that during coal combustion. Thermodynamic calculations were carried out to identify the forms of arsenic in vapor and solid phases during coal combustion and pyrolysis, respectively. Ca3(AsO4)2 and Ca(AsO2)2 are the possible species resulting from As-Ca interaction during coal combustion. Ca(AsO2)2 is the most probable species during coal pyrolysis.