Abstract ：

Reducing the ever-growing level of CO2 in the atmosphere is critical for the sustainable development of human society in the context of global warming. Integration of the capture and upgrading of CO2 is, therefore, highly desirable since each process step is costly, both energetically and economically. Here, we report a CO2 direct air capture (DAC) and fixation process that produces methane. Low concentrations of CO2 (∼400 ppm) in the air are captured by an aqueous solution of sodium hydroxide to form carbonate. The carbonate is subsequently hydrogenated to methane, which is easily separated from the reaction system, catalyzed by TiO2-supported Ru in the aqueous phase with a selectivity of 99.9% among gas-phase products. The concurrent regenerated hydroxide, in turn, increases the alkalinity of the aqueous solution for further CO2 capture, thereby enabling this one-of-its-kind continuous CO2 capture and methanation process. Engineering simulations demonstrate the energy feasibility of this CO2 DAC and methanation process, highlighting its promise for potential large-scale applications. © 2024 Chinese Chemical Society. All rights reserved.

Keyword ：

catalytic mechanism hydrogenation CO2 activation sodium hydroxide CO2 capture and methanation process Ru/TiO2 carbonate

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Abstract ：

The long-standing contradiction between low-temperature activity and high-temperature stability is one of the difficulties in catalytic combustion of low-concentration methane. The traditional Pd-CeO2 catalyst system has been applied to the oxidation of methane with low concentrations. However, the problem of sintering at high temperatures still exists. In this work, we prepared the Pt-modified Pd-CeO2 nanowires (NW) sample (in which the actual Pt, Pd, and Ce contents were 0.12, 0.86, and 9.8 wt%, respectively) using the one-pot reverse-micelle emulsion method. It was found that Pt-Pd-CeO2NW@SiO2 showed the highest low-temperature catalytic activity at a space velocity of 20,000 mL/(g h) and the best water resistance and high-temperature stability in the combustion of methane. The T50 % and T90 % (the temperatures for achieving methane conversions of 50 and 90 %) were 298 and 342 degrees C, respectively, methane reaction rate at 270 degrees C was 0.49 mu mol/(gcat s), and turnover frequency (TOF) at 270 degrees C was 0.198 s-1 over Pt-Pd-CeO2NW@SiO2; whereas over Pd-CeO2NW@SiO2 (in which the actual Pd and Ce contents were 0.82 and 10.6 wt%, respectively), the T50 % and T90 % were 360 and 420 degrees C, respectively, methane reaction rate at 270 degrees C was 0.074 mu mol/(gcat s), and TOF at 270 degrees C was 0.032 s- 1. The introduction of the highly dispersed Pt to Pd-CeO2NW@SiO2 could effectively increase the PdOx sites of unsaturated coordination through the electron-donating interaction of the Pt with PdO, which played an important role in activating the C-H bonds in methane. In addition, the unique structure of encapsulation also rendered the Pt-Pd-CeO2NW@SiO2 sample to possess good water resistance and thermal stability in methane combustion. We are sure that the present work provides a possibility for developing the catalysts with stable catalytic and water-resistant performance at low and high temperatures in the combustion of methane.

Keyword ：

Methane combustion Water resistance Ceria nanowire Platinum doping Supported palladium catalyst

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Three-dimensionally ordered macroporous (3DOM) Ce0.7Zr0.3O2-supported 0.7 Zr 0.3 O 2-supported Pd (xPd/3DOM x Pd/3DOM Ce 0.7 Zr 0.3 O 2 , x = 0.97, 1.49, and 2.10 wt%) catalysts were fabricated using the polymethyl methacrylate (PMMA) microspheretemplating and polyvinyl alcohol (PVA)-protected reduction methods. It was found that the x Pd/3DOM Ce 0.7 Zr 0.3 O 2 samples with surface areas of 31.9-38.2 m2/g 2 /g possessed a high-quality 3DOM architecture, with the Pd nanoparticles (NPs) being uniformly dispersed on the 3DOM Ce 0.7 Zr 0.3 O 2 surface. Among all of the samples, 1.49 Pd/3DOM Ce 0.7 Zr 0.3 O 2 showed the best CH4 4 combustion activity: The T 10% , T 50% , and T 90% (the temperatures at CH4 4 conversions = 10, 50, and 90%) were 269, 330, and 380 degrees C, respectively; specific reaction rate at 260 degrees C was 115.0 x 10-- 6 mol/(gPd Pd s), and turnover frequency at 260 degrees C (TOFPd) Pd ) was 28.8 x 10-- 3 s-1.-1 . The good catalytic performance of 1.49 Pd/3DOM Ce 0.7 Zr 0.3 O 2 was associated with the well-defined 3DOM structure, high Pd NP dispersion, rich adsorbed oxygen species, and strong interaction between Pd NPs and 3DOM Ce 0.7 Zr 0.3 O 2 . Interestingly, the 1.49 Pd/3DOM Ce 0.7 Zr 0.3 O 2 sample also exhibited superior water-tolerant performance. The product analysis results after the in situ introduction of isotopic water demonstrated that the excellent water resistance of 1.49 Pd/3DOM Ce 0.7 Zr 0.3 O 2 was due to the interaction of oxygen and water to form the peroxy hydroxyl species on the sample surface after the water pretreatment, which was beneficial for the combustion of methane.

Keyword ：

Supported palladium catalyst Water resistance Ceria-zirconia solid solution Methane combustion Three-dimensionally ordered macropore

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Nitrate or nitrite-dependent anaerobic methane oxidation (n-DAMO) is a microbial process that links carbon and nitrogen cycles as a methane sink in many natural environments. This study demonstrates, for the first time, that the nitrite-dependent anaerobic methane oxidation (nitrite-DAMO) process can be stimulated in sewer systems under continuous nitrate dosing for sulfide control. In a laboratory sewer system, continuous nitrate dosing not only achieved complete sulfide removal, but also significantly decreased dissolved methane concentration by similar to 50 %. Independent batch tests confirmed the coupling of methane oxidation with nitrate and nitrite reduction, revealing similar methane oxidation rates of 3.68 +/- 0.5 mg CH4 L-1 h(-1) (with nitrate as electron acceptor) and 3.57 +/- 0.4 mg CH4 L-1 h(-1) (with nitrite as electron acceptor). Comprehensive microbial analysis unveiled the presence of a subgroup of the NC10 phylum, namely Candidatus Methylomirabilis (n-DAMO bacteria that couples nitrite reduction with methane oxidation), growing in sewer biofilms and surface sediments with relative abundances of 1.9 % and 1.6 %, respectively. In contrast, n-DAMO archaea that couple methane oxidation solely to nitrate reduction were not detected. Together these results indicated the successful enrichment of n-DAMO bacteria in sewerage systems, contributing to approx. 64 % of nitrite reduction and around 50 % of dissolved methane removal through the nitrite-DAMO process, as estimated by mass balance analysis. The occurrence of the nitrite-DAMO process in sewer systems opens a new path to sewer methane emissions.

Keyword ：

Nitrate dosing Nitrite-dependent anaerobic methane oxidation (nitrite-DAMO) Anaerobic methane oxidation Sewer Dissolved methane removal Sulfide control

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This study investigated biochar addition to improve the performance and stability of anaerobic membrane bioreactor (AnMBR) to resist different oil contents in food wastewater treatment. Results show that an increase in oil content in synthetic food wastewater from 2 to 4 g/L of oleateNa aggravated AnMBR performance in terms of methane (CH 4 ) production and membrane fouling as showed by an obvious increase in transmembrane pressure (TMP). The reduced performance was caused by the accumulation of volatile fatty acids (VFAs) to inhibit the growth and activity of Methanobacterium in response to increased oil content. Nevertheless, biochar addition improved AnMBR resistance to against oil increase. The dominance of Methanobacterium and their potential mutualism with genus T78 were enhanced by biochar to sustain CH 4 yield. Moreover, biochar reduced the production of extracellular polymeric substance (EPS) to alleviate membrane fouling.

Keyword ：

Biochar Oleate-Na Oil content Food wastewater Anaerobic membrane bioreactor

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The reverse water-gas shift (RWGS) reaction holds promise for producing high value-added CO from CO2, serving as a crucial intermediate to feedstock for producing various hydrocarbons through Fischer-Tropsch synthesis reactions. Metallic catalysts often yield significant methane production as a by-product during operation, whereas non-metallic oxide catalysts offer potential for high CO selectivity. We prepare Al doped CeO2 nanorod by atomic layer deposition (ALD). The incorporation of Al creates more oxygen vacancies on the surface, resulting in increased Frustrated Lewis Pair (FLP) sites. Particularly, the CO yield of ALD-modified CeO2 was enhanced by 134 % compared to that of pristine CeO2 nanorods (CeO2-NR) at 500 degrees C, with CO selectivity reached 100 %. Density Functional Theory (DFT) calculations reveal a lower energy barrier for H2 dissociation facilitated by the formation of FLP sites, potentially leading to heightened activity in the Reverse Water Gas Shift (RWGS) reaction.

Keyword ：

Atomic layer deposition Frustrated Lewis Pairs CO2 hydrogenation RWGS reaction

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Ni-based catalysts doped with Fe, Co, Cr, and Cu were successfully synthesized via an interesting impregnation method with EDTA for the catalytic decomposition of methane. XRD, N 2 sorption, SEM, H 2 -TPR, and TPO were accomplished for catalyst characterization. Among the Ni-doped catalyst series, the Ni-Cu catalyst carried out better toward the rest of the Ni catalysts, and a rising trend in the CH 4 conversion from 61 to 86 % was perceived with a rise in temperature in the range of 575 - 650 degrees C, which was associated with its high surface area (104.3 m 2 . g -1 ) and good low-temperature reducibility. Furthermore, the maximum CH 4 conversions of 28, 86, 81, and 78 % were acquired over the 50Ni-xCu/SiO 2 .MgO (x = 2.5, 5, 7.5, and 10 wt%) at 650 degrees C with a GHSV = 24000 mL h -1 g cat1 and CH 4 :N 2 = 3:17, respectively. The optimal specimen (5 wt% Cu) with a slow decline in activity depicted better durability in the stability test at 675 degrees C for 300 min compared to the quick deactivation of other Cu loadings within 150 min of reaction initiation. SEM micrograph of spent specimens with diverse Cu loadings indicated that the generated carbon is whisker type.

Keyword ：

SiO 2 .MgO-supported Ni catalyst Hydrogen production Methane decomposition Promoters

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CH 4 + NH 3 + NO x + N 2 O coupling system was proposed for high-effective simultaneous catalytic reduction of both NO and N 2 O from flue gas generated by nitric or adipic acid industries. The Fe-SSZ-13 catalyst performed both elegant activity for NO x and N 2 O at 400-600 degrees C. Based on experimental studies, the active Fe species were illustrated and the transforming of Fe species during the N 2 O decomposition ( de N 2 O) and selective catalytic reduction (SCR) processes was clarified. It found that de N 2 O could cause the main active sites of [Fe-O 2 -Fe] 2+ transform to [Fe-O-Fe] 2+ and consequently inhibited the NH 3 -SCR process over the Fe-SSZ-13 catalyst. Meanwhile, the alpha O* generated via N 2 O decomposition would strengthen NO oxidation to NO 2 , which is advantageous to NH 3 -SCR. More importantly, the functions of CH 4 for de N 2 O and NH 3 -SCR were also investigated. CH 4 could work as reductant to convert N 2 O over the Fe-SSZ-13. CH 4 could also facilitate the reduction of NO; however, NH 3 has stronger activity to NO than that of CH 4 . And CH 4 was easily oxidized by O 2 rather than N 2 O or NO, which significantly limit the CH 4 -SCR for N 2 O with present of O 2 . At the same time, CH 4 can also accelerate alpha O* consumption that somehow enhance deN 2 O efficiency. Moreover, the consumption of alpha O* by CH 4 prevent the transforming of [Fe-O 2 -Fe] 2+ to [Fe-O-Fe] 2+ , and CH 4 play an important role on turnover rates of [Fe-O 2 -Fe] 2+ and [Fe-O-Fe] 2+ active sites, guaranteeing both satisfactory purifying efficiency for NO and N 2 O. This work reveals the positive promoting effect of CH 4 on simultaneous reduction of NO x and N 2 O, and proposes a potential catalyst for implementation.

Keyword ：

N2O decomposition Coupling reaction Methane Mechanisms NH3-SCR

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Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

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Ni-based catalysts with different promoters, such as Fe, La, Zr, Ce, and Ca, were prepared using the one-step sol-gel method and tested for their performance in the combined reforming of methane using steam and oxygen. The catalysts were characterized by XRD, SEM, TPO, BET, and TPR analysis. The consequences revealed that deposited carbon was lower on the surface of the 10% Ni-3% Fe-MgOAl2O3 catalyst in oxygen-combined CH4 reforming. The Fe2O3-doped sample depicted the highest CH4 conversion, more than 65%, and showed a higher catalytic stability for O-2 reforming of methane. In addition, introducing steam into the feeds benefited the performance of catalysts due to lower coke formation, and the H-2/CO ratio was changed from 1.5 to 3 at 700 degrees C. It is concluded that the high catalytic efficiency of the Fe2O3-doped sample in combined reforming of methane was associated with its high reducibility and more reduction of NiO species.

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