Revealing the crystal facet effect on N2O formation during the NH3-SCR over α-MnO2 catalysts.

The detailed atomic-level mechanism of the effect induced by engineering the crystal facet of α-MnO2 catalysts on N2O formation during ammonia-selective catalytic reduction (NH3-SCR) was ascertained by combining density functional theory (DFT) calculations and thermodynamics/kinetic analysis. The surface energies of α-MnO2 with specific (100), (110), and (310) exposed planes were calculated, and the adsorptions of NH3, NO, and O2 on three surfaces were analyzed. The adsorption energies showed that NH3 and NO molecules could be strongly adsorbed on the surface of the α-MnO2 catalyst, while the adsorption of O2 was weak. Moreover, the key steps in the oxidative dehydrogenation of NH3 and the formation of NH2NO as well as dissociation of NH2 were studied to evaluate the catalytic ability of NH3-SCR reaction and N2 selectivity. The results revealed that the α-MnO2 catalyst exposed with the (310) plane exhibited the best NH3-SCR catalytic performance and highest N2 selectivity, mainly due to its low energy barriers in NH3 dehydrogenation and NH2NO generation, and difficulty in NH2 dissociation. This study deepens the comprehension of the facet-engineering of α-MnO2 on inhibiting N2O formation during the NH3-SCR, and points out a strategy to improve their catalytic ability and N2 selectivity for the low-temperature NH3-SCR process.

Waste straw derived Mn-doped carbon/mesoporous silica catalyst for enhanced low-temperature SCR of NO.

This work proposed a new strategy for the high value utilization of waste straw, in which a Mn-doped carbon/mesoporous silica composite catalyst was prepared by simultaneous utilization of carbon and silicon source from straw for low-temperature denitration. The results showed that the NO conversion rate reached 93% at 180℃ for the composite catalyst with Si/C mass ratio of 35% (Mn/ACMS (35%)). This was significantly higher than those of the activated carbon catalyst (Mn/AC) and mesoporous silica catalyst (Mn/MS), i.e., 58% and 50%, respectively. The SEM images showed that mesoporous silica nanoparticles were dispersed evenly on the carbon surface to form composite materials. XPS results indicated that Mn/ACMS (35%) catalyst showed higher content of chemically adsorbed oxygen (Oα) and Mn4+ (54.67% and 46.81%) than Mn/AC catalyst (34.38% and 17.49%) and Mn/MS catalyst (32.71% and 30.18%), which was responsible for the improved catalytic activity. Moreover, NH3-TPD results revealed that Mn/ACMS (35%) had high surface acidity of 6.47 mmol·g-1, significantly higher than Mn/AC catalyst of 1.51 mmol·g-1, which was beneficial for adsorbing NH3. H2-TPR results suggested that Mn/ACMS (35%) catalyst had much higher H2 consumption of 1.32 mmol·g-1 than Mn/AC and Mn/MS catalyst, suggesting better redox performance. The results demonstrated that the straw derived Mn-doped carbon/mesoporous silica composite catalyst can be a potential material for low-temperature denitration.

Hierarchical porous biochar from plant-based biomass through selectively removing lignin carbon from biochar for enhanced removal of toluene.

This study proposed a simple and green air oxidation (AO) method to prepare hierarchical porous biochar by selectively removing lignin carbon from biochar after the pyrolysis of plant-based biomass, based on the fact that the thermal decomposition temperature in air between lignin carbon and cellulose/hemicellulose carbon was different. Three kinds of biomass with different lignocellulose contents were used, including walnut shell, cypress sawdust and rice straw. The results found that AO treatment could effectively improve the pore structure of the three biochar. The specific surface area of WCO-4, CCO-4 and RCO-4 was 555.0 m2/g, 418.7 m2/g and 291.9 m2/g, respectively, which was significantly higher than those of WC (319.5 m2/g), CC (381.7 m2/g) and RC (69.6 m2/g), respectively. Among these, walnut shell biochar with air oxidation (WCO) had higher surface area of 555.0 m2/g and mesopore volume of 0.116 cm3/g, this was related to its high content of lignin, which could facilitate the formation of mesopores by AO treatment with high selectivity. The toluene adsorption capacity of WCO reached 132.9 mg/g, which increased by 223.4% from that without AO treatment. The kinetics study indicated that the diffusion rates of toluene molecule were improved due to the increased mesopores volume of biochar and micropores also play an important role in the adsorption of toluene. The results demonstrate that AO treatment is a promising method to develop hierarchical porous structure for lignocellulose-rich plant-based biomass with low cost and environmental-friendly, which greatly enhanced the toluene adsorption capacity.

From typical silicon-rich biomass to porous carbon-zeolite composite: A sustainable approach for efficient adsorption of CO2.

Focusing on the high-valued utilization of the widespread silicon-rich waste biomass, a sustainable route by simultaneous utilization of carbon and silicon from silicon-rich rice husk was proposed in this work. Specifically, porous carbon-zeolite composite with hierarchical porous structure of micro/meso pores (carbon) and ultra-microporous pores (Na-X zeolite) was in situ prepared by a facile one-pot method. The obtained porous carbon-zeolite composite (PC2-Z) had a higher yield of 67.66% compared to the porous carbon without silicon (PC2) of 43.33%. Moreover, due to the high ultra-micropore volume of the PC2-Z sample (up to 0.181 cm3/g), it exhibited high dynamic CO2 adsorption capacity of 1.81 mmol/g and CO2/N2 selectivity of 9.80 (1 bar), which were higher than 1.67 mmol/g and 7.01 (1 bar) for PC2, respectively. PC2-Z also showed good regeneration efficiency above 99% after ten cycles. Furthermore, the economic and energy consumption assessment of this utilization route was conducted. Overall, a facile one-pot route was developed to prepare highly efficient composite absorbents from silicon-rich biomass, which can be widely used in different environmental applications.

Air oxidation coupling NH3 treatment of biomass derived hierarchical porous biochar for enhanced toluene removal.

In this study, hierarchical porous biochar was prepared from poplar sawdust by air oxidation coupling with NH3 treatment for the removal of toluene. The results showed that the mesopore volume of the sample with air oxidation (PS‒O2) increased significantly to 0.263 cm3/g from the blank sample (PS, 0.053 cm3/g). This could be attributed to the selective removal of the lignin carbon by air oxidation to develop mesopores in biochar. Following further NH3 treatment (PS‒O2‒NH3), the basic surface chemistry on biochar was improved due to increased basic N-containing groups and decreased acidic O-containing groups, together with the micropore volume also increased to 0.231 cm3/g from 0.186 cm3/g of PS‒O2. The formation mechanism of hierarchical porous structure of biochar was also discussed. The adsorption capacity of PS‒O2‒NH3 for toluene reached 218.4 mg/g at the initial concentration of 820 mg/m3, which was 383.2% higher than that of PS. The adsorption isotherm study indicated that the adsorption process of toluene was monolayered and the maximal adsorption capacity of PS‒O2‒NH3 for toluene could reach as high as 476.2 mg/g. The results demonstrated that air oxidation coupling NH3 treatment is a highly effective method for the preparation of hierarchical porous biochar for enhancing toluene adsorption performance.

Preparation of a novel nitrogen-containing graphitic mesoporous carbon for the removal of acid red 88.

A novel nitrogen-containing graphitic mesoporous carbon was prepared through MnO-templated method using polyacrylonitrile (PAN) as both carbon and nitrogen sources, and MnCO3 as both template and catalyst precursors. The effects of preparation conditions on the physicochemical properties of obtained samples were systematically investigated. The results showed that as the decrease of the weight ratios of PAN and MnO (2:1-1:4), the increase of carbonization temperature (700-900 °C) and pre-oxidation temperature (180-200 °C), the samples had higher specific surface area, mesopores volume and ratios, up to 507 m2/g, 0.824 cm3/g and 96.83%, respectively. Moreover, the prepared samples presented relatively high graphitic degree and nitrogen contents (~2.21%). The adsorption capacity for acid red 88 (AR88) was as high as 309 mg/g, which were dramatically affected by the mesoporous properties and C- and N-containing groups on the surface of prepared carbon. The rich graphic carbon and pyridine-N in mesoporous carbon generated π-π dispersion and electrostatic interaction with AR88, respectively, which jointly were responsible for the adsorption process. The results of the isotherm and kinetic studies indicated that the AR88 adsorption on mesoporous carbon could be well depicted using Langmuir model and pseudo-2nd-order model.

Insights into the ion-exchange properties of Zn(ii)-incorporated MOR zeolites for the capture of multivalent cations.

Understanding the properties of zeolites for cation exchange is important because the ion-exchange performance largely determines their suitability in applications such as catalysis and adsorptive separation. We synthesized a Zn(ii)-incorporated mordenite-framework aluminosilicate zeolite (Zn,Al-MOR), in which both Zn and Al are substituted in the framework, and studied its ion-exchange behavior for multivalent cations. For comparison, the original aluminosilicate mordenite (Al-MOR) was also synthesized with a composition adjusted to ensure that its charge density was similar to that of Zn,Al-MOR. While the incorporation of Zn(ii) led to a slower kinetic process, the selectivities and the exchange capacities toward multivalent cations (especially divalent cations) were significantly improved. Herein, we discussed the mechanism responsible for improving the ion-exchange performance in the presence of Zn(ii) and found that the incorporation of Zn(ii) led to a significant improvement in the ion-exchange temperature dependence of the MOR, which led to the ability to enhance ion-exchange capacity through temperature control during actual application. It was also revealed that the presence of Zn(ii) significantly improves selectivity and spontaneity toward the exchange of multivalent cations, Ni2+. Moreover, XRD and nitrogen-adsorption/desorption analyses revealed that the structures of the materials are maintained during the ion exchange, which is indicative of superior structural stability and tolerance to ion exchange.

Uranium speciation in coal bottom ash investigated via X-ray absorption fine structure and X-ray photoelectron spectra.

Similar to chromium contamination, the environmental contamination caused by uranium in radioactive coal bottom ash (CBA) is primarily dependent on the chemical speciation of uranium. However, the relationship between uranium speciation and environmental contamination has not been adequately studied. To determine the relationship between uranium speciation and environmental contamination, X-ray absorption fine structure (XAFS) and X-ray photoelectron spectra (XPS) analyses were performed to determine the uranium speciation in CBA exposed to different chemical environments and simulated natural environments. The leachability of the different forms of uranium in the CBA was studied via a simulated acid rain leaching experiment, and the results showed that 57.0% of the total uranium was leached out as U(VI). The results of a linear combination fit (LCF) of the X-ray absorption near edge structure (XANES) spectrum revealed that in the raw CBA, the uranium mainly occurred as U3O8 (71.8%). However, in the iron-rich particles, the uranium mainly occurred as UO2 (91.9%) after magnetic separation. Magnetite is a ubiquitous ferrous-bearing oxide, and it was effective for the sorption of U(IV). The result of FeSO4 leaching experiment indicated that 96.57% of total uranium was reduced from U(VI) to U(IV) when infiltrated with the FeSO4 solution for 6months. This result clearly demonstrated the changes in chemical valence of uranium in the coal ash and provided a conceptual principle for preventing uranium migration from ash to the surrounding soil and plants.

Increasing the ion-exchange capacity of MFI zeolites by introducing Zn to aluminosilicate frameworks.

MFI zeolites exchanged with various cations have gained a great deal of attention as catalysts. Increase in the ion-exchange capacity of zeolites can improve their catalytic properties by introducing more active sites; however, the ion-exchange capacity of MFI zeolites is limited by maximum aluminum content in the structure. To improve the ion-exchange capability of the MFI zeolites beyond the upper limit of the aluminosilicate MFI zeolites, we propose herein an approach to incorporate Zn(ii) in the zeolitic framework, because Zn in the framework sites generates two negative charges per atom. Using zincoaluminosilicate gels prepared via co-precipitation, organic-free synthesis of zincoaluminosilicate MFI zeolites was achieved. The obtained zincoaluminosilicate MFI zeolites had high Zn contents comparable to those in the initial zincoaluminosilicate gels with both Zn and Al in the zeolite framework. In contrast, the use of conventional sources of Si, Al, and Zn resulted in zeolites with extra-framework zinc oxide species. The obtained Zn-substituted MFI zeolites were shown to possess higher ion-exchange capacity compared to aluminosilicate MFI zeolites. It was also revealed that the zincoaluminosilicate MFI zeolites have high affinity for the divalent cation compared to the aluminosilicate analog, likely due to the two negative charges in close proximity. Because of these higher ion-exchange efficiencies, especially for divalent cations, the obtained zincoaluminosilicate MFI zeolites are expected to be efficient platforms for several important catalytic reactions.

Preparation of amidoxime-functionalized mesoporous silica nanospheres (ami-MSN) from coal fly ash for the removal of U(VI).

It is usually difficult to control the microstructure of mesoporous silica materials using coal fly ash as raw materials. In this study, amidoxime-functionalized mesoporous silica nanospheres (ami-MSN) were prepared from coal fly ash using a novel interfacial cohydrolysis-condensation method in an alkane-aqueous system. Characterizations suggested a regular microstructure, high specific surface area (676 m2/g) as well as stable and uniformly distributed amidoxime groups in the ami-MSN framework. Furthermore, ami-MSN displays a high U(VI) removal capacity in sorption experiments (98.9% removal efficiency of 50 ppm U(VI) at a dosage of 600 mg/L). The sorption showed significant pH dependence. Introducing various cations and anions showed differing effects on sorption, which can be attributed to differing complexation abilities of ions/ami-MSN/U(VI). The sorption mechanism was also studied. In pursuit of the strategy of "treating wastewater with materials derived from waste," this work suggests that ami-MSN can be an effective and low-cost sorbent for U(VI) removal.

Fulvic acid anchored layered double hydroxides: A multifunctional composite adsorbent for the removal of anionic dye and toxic metal.

A novel multifunctional composite adsorbent which possesses the ability for anion exchange and toxic metal complexation has been synthesized by the hybridization of layered double hydroxides (LDH) and fulvic acid (FA) in this study. The results show that FA with lots of functional groups can be effectively and stably anchored on the surface of LDH through coagulation process without occupying the interlayer of LDH. Therefore, the anion exchange ability remains and the adsorption capacity of Orange II can reach 1.9mmol/g, which is almost as much as stoichiometric anion exchange capacity of pure LDH. Moreover, the composite adsorbent's adsorption capacity of Cu2+, Pb2+, Ni2+ and Cd2+ can also get to 2.25mmol/g, 0.98mmol/g, 0.99mmol/g and 0.16mmol/g respectively with an adsorption preference order of Cu2+>Pb2+>Ni2+>Cd2+. In addition, Orange II and toxic metals are able to be simultaneously removed by this composite adsorbent, and the adsorption of toxic metals can be enhanced by the synergetic adsorption of Orange II. Anion exchange with Cl- in LDH matrix accounts for the adsorption of Orange II, while the adsorption of toxic metal is mainly attributed to the complexation of carboxyl functional group derived from FA.

Novel synthesis of cyano-functionalized mesoporous silica nanospheres (MSN) from coal fly ash for removal of toxic metals from wastewater.

Trace amounts of toxic metals are usually difficult to be purified by conventional chemical precipitation or physical adsorption in wastewater. In this study, in order to realize high-value utilization of coal fly ash for wastewater purification, a novel method was applied to prepare high-performance mesoporous silica materials from coal fly ash. In comparison with a commonly used method, characterizations revealed that the new method obtained mesoporous silica nanospheres with uniformly distributed cyano groups (denoted by MSN), while the common method only obtained irregular sponge-like microstructure (denoted by ISM). Besides, MSN showed better hydrothermal stability, higher specific surface area (693m2/g) and more ordered mesopores from the comparison. Moreover, the sorption experiments of simulated wastewater suggested that MSN was better in removing toxic metals (Ni2+ and Cd2+) than ISM. For the practical wastewater from a battery plant, 2g/L dosage of MSN showed excellent performance for purification of trace amounts of various toxic metals (Ni, Cd, Mn, Zn, Hg and Pb), the concentration of which reduced to ppb level after MSN treated. The results suggested that MSN can be an effective and low-cost sorbent for removing various toxic metals from wastewater.