CH4 Oxidation Research Articles

Tailoring SnO2 structures for enhanced Pd/SnO2 catalytic activity in low-concentration methane oxidation

Tin dioxide (SnO2) nanorods, nanosheets, and nanoparticles were synthesized to explore the influence of morphology on the catalytic performance of Pd/SnO2 in CH4 oxidation. Among these, Pd/SnO2 nanorods exhibited superior activity, achieving the earliest onset of methane conversion (T10% at 227 °C) and complete conversion (T100% at 400 °C), compared to nanosheets and nanoparticles. The structure–activity study indicates that the nanorod morphology not only enhances the availability of active oxygen species and increases the Pd2+/Pd4+ ratio, but also lowers the reduction temperature of Pd-O-Sn moieties. These features collectively enhance the oxidation of CH4. This study highlights the critical role of SnO2 morphology in optimizing the catalytic efficiency of Pd/SnO2, providing valuable insights for designing catalysts aimed at low-concentration CH4 oxidation.

Electrons redistribution of palladium-copper nanoclusters boosting the direct oxidation of methane to methanol

The direct oxidation of methane to methanol (DOMM) at mild conditions is an enormous attractive yet highly challenging, which is known as the “Holy Grail” reaction. In this work, we prepare the PdxCuy NCs supported on mesoporous sulfur-carbon materials and investigate their catalytic DOMM performance. The Pd14Cu1 NCs exhibits the excellent DOMM performance with the productivity of 34.3 mmol g−1 h−1 and the selectivity of 92.3 % at 150 °C when H2/O2 as active oxygen species, which was obviously superior to other PdCu catalysts. The enhanced performance of Pd14Cu1 NCs in DOMM is attributed to the non-crystalline atomic arrangement of tiny cluster structures and the synergistic electronic effect between Pd and Cu atoms, which can reduce the desorption temperature for methane, improve the in-situ generation of H2O2, and provide more active oxygen. The time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that a radical-involved reaction pathway on Pd14Cu1 NC. This work provides the understanding for synergistic effect dual metallic and helps to design highly efficient and selective catalysts for the direct oxidation of CH4.

Photosynthesis of CH3OH via oxygen-atom-grafting from CO2 to CH4 enabled by AuPd/GaN

The direct co-conversion of methane and carbon dioxide into valuable chemicals has been a longstanding scientific pursuit for carbon neutrality and combating climate change. Herein, we present a photo-driven chemical process that reforms these two major greenhouse gases together to generate green methanol and CO, two high-valued industrial chemicals. Isotopic labeling and control experiments indicate an oxygen-atom-graft occurs, wherein CO2 transfers one O into the C–H bond of CH4 via photo-activated interfacial catalysis with AuPd nanoparticles supported on GaN. The photoexcited AuPd/GaN interface effectively orchestrates the CH4 oxidation and the CO2 reduction producing 13.66 mmol g−1 of CH3OH yield over 10 h. This design provides a solid scientific basis for the photo-driven oxygen-atom-grafting process to be further extended to visible light region.

Oxidation of lean methane by dielectric barrier discharge plasma and Mn–Ce catalyst

The synergistic dielectric barrier discharge plasma and Mn-Ce catalysts are employed to facilitate the low temperature and highly efficient oxidation of lean methane. The effect of the catalyst alone, plasma alone, and their synergistic effect on CH4 conversion are experimentally analyzed. It shows that the CH4 conversion rate under plasma catalysis is more than three times that of catalyst alone and plasma alone, with a maximum of 66.7% at 375 °C and 21 W, revealing a significant synergistic effect of plasma catalysis. The influence of Mn/Ce mass ratios (11Mn/Ce and 45Mn/Ce) and operational conditions (CH4/O2 molar ratio, preheating temperature, and discharge power) on CH4 conversion by plasma catalysis are also investigated. The CH4 conversion rate increases with the decrease of the CH4/O2 molar ratio and the increase in preheating temperature and discharge power. Meanwhile, the Mn/Ce mass ratio affects the CH4 oxidation and the synergistic effect of plasma catalysis. The oxidation activity of the 11Mn/Ce catalyst is mostly affected by the power at the preheating temperature below 325 °C. However, the 45Mn/Ce catalyst is affected by both preheating temperature and power. In addition, as the discharge power increases, the CH4 conversion rate exhibits a linear increase for 11Mn/Ce but exponential for 45Mn/Ce.

Redox and Kinetic Properties of Composition-Dependent Active Sites in Copper-Exchanged Chabazite for Direct Methane-to-Methanol Oxidation.

The CH4 oxidation performance of Cu-chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier-transform infrared, and X-ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CH3OH selectivity and Cu-efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu2+ and [CuOH]+ present at low Si/Al ratio and Cu loading. Formation of two distinct [Cu2(µ-O)]2+ moieties at higher Si/Al ratio or Cu loading forces these trends into the opposite direction. Operando electron paramagnetic resonance and ultraviolet-visible spectroscopy show that the apparent activation energy of monomeric Cu active species decreases with increasing Si/Al ratio, whereas the one of dimeric centers is unaffected.

Aromatic compound 2-acetyl-1-pyrroline coordinates nitrogen assimilation and methane mitigation in fragrant rice

Rice paddy has been the main source of anthropogenic methane (CH4) emissions, with significant variations among rice varieties. 2-Acetyl-1-pyrroline (2-AP) is the key component of the pleasant aroma in fragrant rice. Here, we show that fragrant rice is metabolically active in nitrogen assimilation and exhibits high levels of 2-AP and that CH4 fluxes at the booting stage and cumulative emissions are 25.5% and 14.8% lower, respectively, in fragrant rice paddies compared with nonfragrant rice paddies. Three precursors involved in 2-AP synthesis-proline, glutamic acid, and ornithine-are identified as crucial nitrogen compounds that significantly promote CH4 oxidation in the rhizosphere. Augmenting 2-AP synthesis, either through foliar spraying or by utilizing CRISPR-Cas9 technology to generate knockout lines of BETAINE ALDEHYDE DEHYDROGENASE 2 gene, effectively enhances CH4 oxidation and reduces CH4 fluxes. Our findings reveal that the 2-AP metabolic pathway coordinates the carbon/nitrogen cycle to improve nitrogen assimilation along with high 2-AP levels and mitigate CH4 emissions in paddy ecosystems.

Photocatalytic oxidation of methane to C1 oxygenates promoted by Fe−N−Ti electron bridge

Highly efficient oxidation of methane (CH4) to liquid oxygenates with O2 over non-noble heterogeneous catalysts under mild conditions remains a huge challenge owing to the ultra-inert C−H bond of CH4 molecule. In this work, we have successfully constructed photocatalysts of N-Fe2Ti3O9/TiO2 by thermal treatment of a bimetallic amino-functionalized metal-organic framework, i.e., Fe-NH2-MIL-125, aiming at photocatalytic oxidation of CH4 to one-carbon (C1) oxygenates using O2 at room temperature. Impressively, the optimized photocatalyst of 10 wt%N-Fe2Ti3O9/TiO2 exhibited superior activity in photocatalytic oxidation of CH4 with a formation rate of C1 liquid oxygenates of 5897 μmol·gcat.–1·h–1. The selectivity of the primary products (CH3OH and CH3OOH) was up to 91.7 % over 10 wt%N-Fe2Ti3O9/TiO2. Mechanistic studies demonstrated that the presence of Fe−N−Ti electron bridge could trigger robust charge redistribution in 10 wt%N-Fe2Ti3O9/TiO2 to form electron-deficient Fe sites and electron-rich Ti sites. The formed electron-deficient Fe sites could oxidize H2O molecules to ·OH radicals. Meanwhile, electron-rich Ti sites greatly enhanced the chemisorption and activation of CH4 molecules, thereby synergistically promoting the generation of C1 liquid oxygenates.

Methane-cycling microbial communities from Amazon floodplains and upland forests respond differently to simulated climate change scenarios

Seasonal floodplains in the Amazon basin are important sources of methane (CH4), while upland forests are known for their sink capacity. Climate change effects, including shifts in rainfall patterns and rising temperatures, may alter the functionality of soil microbial communities, leading to uncertain changes in CH4 cycling dynamics. To investigate the microbial feedback under climate change scenarios, we performed a microcosm experiment using soils from two floodplains (i.e., Amazonas and Tapajós rivers) and one upland forest. We employed a two-factorial experimental design comprising flooding (with non-flooded control) and temperature (at 27 °C and 30 °C, representing a 3 °C increase) as variables. We assessed prokaryotic community dynamics over 30 days using 16S rRNA gene sequencing and qPCR. These data were integrated with chemical properties, CH4 fluxes, and isotopic values and signatures. In the floodplains, temperature changes did not significantly affect the overall microbial composition and CH4 fluxes. CH4 emissions and uptake in response to flooding and non-flooding conditions, respectively, were observed in the floodplain soils. By contrast, in the upland forest, the higher temperature caused a sink-to-source shift under flooding conditions and reduced CH4 sink capability under dry conditions. The upland soil microbial communities also changed in response to increased temperature, with a higher percentage of specialist microbes observed. Floodplains showed higher total and relative abundances of methanogenic and methanotrophic microbes compared to forest soils. Isotopic data from some flooded samples from the Amazonas river floodplain indicated CH4 oxidation metabolism. This floodplain also showed a high relative abundance of aerobic and anaerobic CH4 oxidizing Bacteria and Archaea. Taken together, our data indicate that CH4 cycle dynamics and microbial communities in Amazonian floodplain and upland forest soils may respond differently to climate change effects. We also highlight the potential role of CH4 oxidation pathways in mitigating CH4 emissions in Amazonian floodplains.

Analysis of methane emission characteristics and environmental response in natural wetlands

Wetlands are the most important natural source of methane (CH4) emissions. Apparent wetland CH4 emissions depend largely on the balance between CH4 production and oxidation. Wetland plants play an important role in all processes. However, direct observations and side-by-side comparisons of CH4 emissions from wetlands with different vegetation types are lacking at the global scale. There is still a lack of clarity in characterizing the environmental response to CH4 emissions from wetlands with different vegetation types. Based on the FLUXNET-CH4 dataset and the Global Standard Soil Database, we analyzed the environmental response characteristics of CH4 emissions and clarified the important role of plants in CH4 emissions. We categorized wetlands into vascular plant wetlands (VPWs) and moss plant wetlands (MPWs). VPWs CH4 emissions were typically higher and more heterogeneous, whereas MPWs CH4 emissions were lower and less variable. When the water table is below the surface, VPWs still have high CH4 emission rates, while MPWs have CH4 emission rates close to zero. In addition, the temperature sensitivity of CH4 emissions from VPWs (Q10 = 2.26) is significantly higher than that of MPWs (Q10 = 1.71). Solar radiation was the main factor affecting CH4 emissions from VPWs, and temperature was the main factor affecting CH4 emissions from MPWs. Finally, our results of structural equation model suggested that soil and water environment, temperature, and plants all contribute to wetland CH4 emissions to varying degrees. These findings highlight the important role of wetland vegetation in CH4 emissions, which should be seriously considered to reduce uncertainty in the assessment of wetland CH4 emissions.

Automated Skeleton Network Generation for ReaxFF Molecular Dynamics Simulations of Hydrocarbon Fuel Pyrolysis and Oxidation via a Rate-Based Algorithm.

In this study, we present an automated approach of rate-based skeleton network generation for ReaxFF MD simulation (RxMD-SN) for deriving the reaction kinetic mechanism of large hydrocarbon fuels in pyrolysis and oxidation from large-scale ReaxFF MD simulations. The approach contains the statistical calculation of reaction rate constants and the generation of skeleton reaction networks using a rate-based algorithm. The RxMD-SN method takes advantage of reaction flux ranking at a small time interval in terms of temporal reaction rate to extract the core reaction networks, which allows for keeping the rare reaction events that may be dominant in a certain period of the reaction network. The kinetic models derived from ReaxFF MD simulation in CH4 oxidation can reproduce what was obtained in the ReaxFF MD simulation, which demonstrates the capability of RxMD-SN in capturing the global reaction kinetics. An evaluation of reaction rate constants indicates that close kinetic parameters are shared for n-octane oxidation of similar reaction classes, shared oxidation reactions of CH4 against n-heptane, and shared pyrolysis reactions of the RP-3 surrogate fuel against n-heptane. This capability of RxMD-SN is particularly beneficial in meeting the challenges in characterizing the oxidation reaction kinetics of large hydrocarbon molecules. RxMD-SN approach is potentially a general approach in chemical kinetics modeling on the basis of ReaxFF MD simulations.

Loss of microbial diversity increases methane emissions and arsenic release in paddy soils

Microorganisms are vital to the emission of greenhouse gases and transforming pollutants in paddy soils. However, the impact of microbial diversity loss on anaerobic methane (CH4) oxidation and arsenic (As) reduction under flooded conditions remains unclear. In this study, we inoculated microbial suspensions into natural As-contaminated paddy soils using a dilution approach (untreated, 10−2, 10−4, 10−6, 10−8 dilutions) to manipulate microbial diversity levels. The results revealed that the 10−4 and 10−6 dilutions resulted in the highest CH4 emissions (97.0 μmol and 102.3 μmol) compared to untreated groups (27.6 μmol). However, anaerobic CH4 oxidation was not observed in 10−4 dilution groups and higher dilutions, suggesting the loss of diversity inhibited the natural reduction of CH4. Moreover, the porewater As concentration in the dilution groups was 1.8–8.2 times greater than in the untreated groups. The loss of microbial diversity promoted the reductive dissolution of iron (Fe) minerals bearing As, leading to increased concentrations of Fe(II) and dissolved organic carbon (DOC), which further enhanced As release (Fe(II), R = 0.9, p < 0.001) (DOC, R = 0.8, p < 0.001) from soil to porewater. However, CH4-dependent As(V) reduction was almost entirely inhibited under diversity loss. The decline in microbial diversity increased the relative abundances of methanogens (e.g., Methanobacterium and Methanomassiliicoccus), Fe(III)/As(V)-reducing bacteria (e.g., Bacillus, Clostridium_sensu_stricto_10, and Geobacter), and the related functional genes (i.e., mcrA and Geo). These findings suggest that microbial diversity is critical for specialized soil processes, highlighting the detrimental effects of biodiversity loss on CH4 emissions and As release in As-contaminated paddies.

Dielectric Barrier Plasma Discharge Exsolution of Nanoparticles at Room Temperature and Atmospheric Pressure.

Exsolution of metal nanoparticles (NPs) on perovskite oxides has been demonstrated as a reliable strategy for producing catalyst-support systems. Conventional exsolution requires high temperatures for long periods of time, limiting the selection of support materials. Plasma direct exsolution is reported at room temperature and atmospheric pressure of Ni NPs from a model A-site deficient perovskite oxide (La0.43Ca0.37Ni0.06Ti0.94O2.955). Plasma exsolution is carried out within minutes (up to 15min) using a dielectric barrier discharge configuration both with He-only gas as well as with He/H2 gas mixtures, yielding small NPs (<30nm diameter). To prove the practical utility of exsolved NPs, various experiments aimed at assessing their catalytic performance for methanation from synthesis gas, CO, and CH4 oxidation are carried out. Low-temperature and atmospheric pressure plasma exsolution are successfully demonstrated and suggest that this approach could contribute to the practical deployment of exsolution-based stable catalyst systems.

Spatial variation of methane oxidation and carbon dioxide sequestration in landfill biogeochemical cover

ABSTRACT The study investigated the spatial variation of potential methane (CH4) oxidation and residual carbon dioxide (CO2) sequestration in biogeochemical cover (BGCC) system designed to remove CH4, CO2, and hydrogen sulfide (H2S) from landfill gas (LFG) emissions. A 50 cm x 50 cm x 100 cm tank simulated BGCC system, comprising a biochar-amended soil (BAS) layer for CH4 oxidation, a basic oxygen furnace (BOF) slag layer for CO2 and H2S sequestration, and an upper topsoil layer. Synthetic LFG was flushed through the system in five phases, with each corresponding to different compositions and flow rates. Following monitoring, the system was dismantled, and samples were extracted from different depths and locations to analyze spatial variations, focusing on moisture content (MC), organic content (OC), pH, and electrical conductivity (EC). Additionally, batch tests on selected samples from BAS and BOF slag layers were performed to assess potential CH4 oxidation and residual carbonation capacity. The aim of study was to evaluate the BGCC's effectiveness in LFG mitigation, however this study focused on assessing spatial variations in physico-chemical properties, CH4 oxidation in the BAS layer, and residual carbonation in the BOF slag layer. Findings revealed CH4 oxidation in the BAS layer varied between 22.4 and 277.9 µg CH4/g-day, with higher rates in the upper part, and significant spatial variations at 50 cm below ground surface (bgs) compared to 85 cm bgs. The BOF slag layer showed a residual carbonation capacity of 40-49.3 g CO2/kg slag, indicating non-uniform carbonation. Overall, CH4 oxidation and CO2 sequestration capacities varied spatially and with depth in the BGCC system.

Oxygen Species Involved in Complete Oxidation of CH4 by SrFeO3-δ in Chemical Looping Reforming of Methane.

Compared with conventional methane reforming technologies, chemical looping reforming (CLR) has the advantages of self-elimination of coke, a suitable syngas ratio for certain down-stream processes, and a pure H2 or CO stream. In the reduction step of CLR, methane combustion has to be inhibited, which could be achieved by designing appropriate oxygen carriers and/or optimizing the operating conditions. To gain a further understanding of the combustion reaction, methane oxidation by perovskite (SrFeO3-δ) at 900 °C and 1 atm in a pulse mode was investigated in this work. The oxygen non-stoichiometry of SrFeO3-δ prepared by a Pechini-type polymerizable complex method is 0.14 at ambient conditions, and it increases to 0.25 and subsequently to 0.5 when heating from 100 to 900 °C in argon that contains 2 ppmv of molecular oxygen. The activation energies of the first and second transitions are 294 and 177 kJ/mol, respectively. The presence of 0.99 vol.% hydrogen in argon significantly reduces the amount CO2 produced. At a pulse interval of 10 min, the amount of CO2 produced in the absence of hydrogen is one order of magnitude greater than that in the presence of hydrogen. In the former case, the amount of CO2 produced dramatically decreases first and then gradually approaches a constant, and the oxygen species involved in methane combustion can be partially replenished by extending the pulse interval, e.g., 82.5% of this type of oxygen species is replenished when the pulse interval is extended to 60 min. The restored species predominantly originate from those that reside in the surface layer or even in the bulk.

Reaction and deactivation mechanisms of a CeIn/HBEA catalyst with dual active sites for selective catalytic reduction of NOx by CH4

Methane selective catalytic reduction (CH4-SCR) technology has shown promise for controlling NOx emissions from natural gas generator sets. This study presents a highly efficient Ce30In15/HBEA catalyst featuring dual active sites, Al-O–(InO+)-Si site for CH4 activation and ceria oxygen vacancy (Ce3+-□) site for NOx activation. However, the CH4-SCR activity of this catalyst decreased in the presence of H2O, SO2, C3H6, and CO2. The inhibitory effects on CH4-SCR activity followed by the sequence: H2O > SO2 > C3H6 > CO2. In particular, H2O combined with Al-O–(InO+)-Si to form inactive In(OH)3-zz+ species and occupied Ce3+-□, preventing Ce4+-NO3–/NO2– formation. In/Ce sulfate formation via the reaction between SO2 and In/Ce active sites hindered CH4 and NOx adsorption and activation. The formed carbon deposits covered the active sites on the catalyst surface due to incomplete C3H6 oxidation. In comparison, the dual active sites were almost unaffected by CO2. Overall, this study provides a basis for the rational design of novel catalysts for CH4-SCR.

Unraveling the mechanism of assimilatory nitrate reduction and methane oxidation by Methylobacter sp. YHQ through dual N-O isotope analysis and kinetic modeling

Assimilatory nitrate reduction and methane (CH4) oxidation by bacteria play important roles in carbon (C) and nitrogen (N) biogeochemical cycles. Here, an investigation of enzymatic assimilatory nitrate reduction and CH4 oxidation by Methylobacter sp. YHQ from the wetlands is presented, specifically concentrating on N and oxygen (O) isotope fractionation with various initial nitrate and oxygen concentrations. The N enrichment factors (15εassimilation) increased from 4.2 ± 0.7‰ to 6.9 ±1.3‰ and the O isotope enrichment factors (18εassimilation) increased from 2.7 ± 0.9‰ to 4.7 ± 0.8‰ during nitrate assimilation when initial nitrate concentrations increased from 0.9 mM to 2 mM. Similar 18ε and 15ε values were observed at different oxygen concentrations. The values of 18ε and 15ε provided vital parameters for the assessment of assimilatory nitrate reduction via the Rayleigh equation approach. The ratios of O and N isotope enrichment factors (18ε:15ε)assimilation ranged from 0.64 ± 0.15 to 0.74 ± 0.18 during nitrate assimilation by Methylobacter sp. YHQ with Nas, which were different from (18ε:15ε)assimilation for assimilatory eukaryotic nitrate reductase (eukNR) from literature data. Thus, N and O isotope fractionation could be useful tools to distinguish eukNR from Nas during nitrate assimilation. Additionally, the rates of CH4 oxidation and nitrate reduction were evaluated with a reaction-based kinetic model, and it quantitatively described the enzymatic reactions of nitrate assimilation. Combining dual N-O isotope analysis with kinetic modeling provides new insights into the microbially driven C-N interactions.Graphical

Effects of anaerobic oxidation of methane (AOM) driven by iron and manganese oxides on methane emissions in constructed wetlands and underlying mechanisms

In recent years, anaerobic oxidation of methane with iron and manganese oxides as electron acceptors has been confirmed to exist in electrochemical constructed wetlands (CW), and this process is an important way to reduce methane emissions. However, the internal mechanisms of emission reduction, microbial ecological networks, and electron transfer mechanisms remain unclear. In this study, we found that AOM driven by iron and manganese oxides (Fe/Mn-AOM) reduced methane emissions by more than 50.86% in constructed wetland – microbial fuel cell (CW-MFC) systems. In iron and manganese systems, the relative abundance of microorganisms associated with methane metabolism (methanotrophic bacteria) increased significantly. AOM-related microorganisms cooperated with other microorganisms to form a robust and stable microbial ecological network. In the process of methane metabolism, there was an increase in the proportion of reverse methanogenic genes in iron and manganese systems, with the proportion of mcr genes being 1.75 and 1.79 times that of control systems. The key genes (cyc1 and cox11) that influence cytochrome C synthesis in AOM process were significantly enriched, resulting in enhanced electron transfer process. The coexistence of iron and manganese oxides enabled the simultaneous biochemical processes of CH4 oxidation and CO2 reduction within a single system. Additionally, the increase in humic acid also facilitated electron transfer, thereby significantly enhancing the removal efficiency of COD, TN, and TP in the CW-MFC systems. This study reveals the potential mechanism of Fe/Mn-AOM for CW-MFC methane emission reduction, which can provide a theoretical basis for the collaborative treatment of sewage and methane control in CWs, so as to achieve the maximum environmental benefits of CW.

Exploring the roles of atomic radius on methane oxidation in different M(Y, Sm, and Ni)-doped CeO2 catalysts using operando DRIFTS-MS and DFT computation

Ce-based oxides doped with different metal ions can greatly improve CH4 catalytic activity, but the relationship between the surface microstructure and its activity is not clear. In this article, M(Y, Sm, and Ni)-CeO2 catalyst prepared by aerosol method is used as the research object. CH4 is used as a molecular probe for the study. Operando DRIFTS-MS and DFT computation are used to verify the structure-activity relationship between the surface structure and activity of M-CeO2. Ni-CeO2 with the smallest atomic radius ratio has the best CH4 oxidation performance (550 °C) and excellent redox characteristics. Operando DRIFTS-MS results show that the NiO is more likely to adsorb CH4 and form the intermediates CH3*, gas-phase CO2, CO, and H2O at low temperatures. At high temperatures, Ni-OH, Ni-O-Ce, and CeO further react with CH4. Among them, the formation of M-Ov-Ce has a positive role in promoting the decomposition of O₂. Importantly, carbonate is the rate-determining step that affects the activation of M-Ov-Ce. Monodentate carbonates decompose more easily at low temperatures. These findings reveal a novel doping effect and provide a new idea for the design and development of Ce based catalysts with high CH4 removal ability

Integrated geochemical and microbiological assessments of Astroni lakes reveals Campi Flegrei unrest signatures

Astroni volcano in the Campi Flegrei caldera (southern Italy) is a 2 km wide, densely vegetated tuff ring and hosting several volcanic structures, including tuff cones, scoriae cones, lava domes, and three small lakes. Geochemical data of waters and dissolved gases from the lakes, coupled with microbiological analyses on lake water and sediments, were used to shed light on the possible relationship between the lakes and the hydrothermal fluid circulation system as suggested by previous geophysical surveys. Water chemistry was dominated by solutes, mainly Na+ and HCO3−, deriving from fluids and CO2-rich gases typically found in discharges located at the periphery of hydrothermal-volcanic systems. Lago Grande (LG) lake showed an anoxic hypolimnion with abundant non-atmospheric dissolved gases, consisting of biogenic CH4 and CO2, the latter having a twofold origin, biogenic and hydrothermal. The occurrence of anaerobic methanotrophs coupled with the lack of hydrogenotrophic methanogenic archaea along the whole vertical profile of LG suggested that CH4 was mostly produced from degradation of abundant terrestrial organic matter within the deep lake sediments, and then consumed during its diffusion through the lake. Notwithstanding, the output rate of CH4 from LG surface was anomalously high relative to those commonly measured in lakes. Carbon dioxide from the hydrothermal source and produced by CH4 oxidation was partially fixed in the lake via the acetyl-CoA pathway. Accordingly, the CO2 fluxes from the LG surface were relatively low, in the range of those measured in volcanic lakes dominated by biogenic CO2. The dependence of the chemistry of the Astroni lakes on inputs from the Campi Flegrei hydrothermal system, besides on biogeochemical processes, offers a possible explanation for the anomalous increase of the LG water level occurred in the last years, which was not consistent with the recorded local rainfall but likely caused by an increasing hydraulic pressure related to the enhanced hydrothermal activity recorded at Campi Flegrei in the last decades. According to this hypothesis, the future evolution of the current volcanic unrest may govern the fate of the lake water level with important implications for the functioning of the precious Astroni ecosystem.

Low-temperature catalytic methane deep oxidation over sol-gel derived mesoporous hausmannite (Mn3O4) spherical particles

In this study, Mn3O4 spherical particles (SPs) were synthesized by the sol-gel process, after which they were thermally annealed at 400 °C, and comprehensively characterized. X-ray Diffraction (XRD) revealed that Mn3O4 exhibited a tetragonal spinel structure, and Fourier transformed infrared (FTIR) spectroscopy identified surface-adsorbed functional groups. Scanning electron microscopy (SEM) and the specific surface area analyses by Brunauer−Emmett−Teller (BET) revealed a porous, homogeneous surface composed of strongly agglomerated spherical grains with an estimated average particle size of ∼35 nm, which corresponded to a large specific surface area of ∼81.5 m2/g. X-ray photoelectron spectroscopy (XPS) analysis indicated that Mn3O4 was composed of metallic cations (Mn4+, Mn3+, and Mn2+) and oxygen species (O2−, OH− and CO32−). The optical bandgap energy is ∼2.55 eV. Assessment of the catalytic performance of the Mn3O4 SPs indicated T90 conversion of CH4 to CO2 and H2O at 398 °C for gas hourly space velocity (GHSV) of 72000 mL3 g−1 h−1. This observed performance can be attributed to the cooperative effects of the smallest spherical grain size with a mesoporous structure, which is responsible for the larger specific surface area and available surface-active oxygenated species. The cooperative effect of the good reducibility, higher ratio of active species (OLat/OAds), and results of density functional theory (DFT) calculations suggested that the total oxidation of CH4 over the mesoporous Mn3O4 SPs might take place via a two-term process in which both the Langmuir−Hinshelwood and Mars−van Krevelen mechanisms are cooperatively involved.