Diffusion Of N2 Research Articles

The interface electric field accelerated charge transfer kinetics of 1D/0D TiO2/Bi2Sn2O7 Z-scheme heterojunction with close contact interface significantly improves photothermal catalytic N2 reduction

The slow kinetics of charge transfer and weak redox ability hindered the practical application of photocatalytic N2 reduction. Therefore, in this article, Bi2Sn2O7 nanoparticles were grown in situ on the surface of TiO2 nanorods using a solvothermal method to form a 1D/0D TiO2/Bi2Sn2O7 (TBSO-15) Z-scheme heterojunction with a tight contact interface. The results of density functional theory calculations (DFT) and Kelvin probe force microscopy (KPFM) indicated that the difference in Fermi levels led to spontaneous rearrangement of charges at the heterojunction interface and the formation of a strong interfacial electric field (IEF). IEF provided a powerful driving force for the spatial separation of photogenerated charges, accelerating charge transfer dynamics. Secondly, the Z-scheme migration pathway of photo-generated charges between TiO2 and Bi2Sn2O7 enhanced the redox ability of TBSO-15 and formed a spatially separated redox center. In addition, molecular dynamics simulations (MD) showed that the photothermal effect significantly promoted the dynamic diffusion of N2 and H2O, accelerating the chemical reaction kinetics. Therefore, the synergistic effect of the two significantly improved the performance of Photothermal catalytic N2 reduction (PTC-NRR). After simulating solar irradiation for 2 h, the PTC-NRR performance of TBSO-15 achieved 327.25 μmoL·g−1 in pure water, which was 19.7 times that of TiO2 and 4.2 times that of Bi2Sn2O7, respectively. This study provides a reference for the synergistic effect of charge space transfer dynamics and photothermal effect to promote PTC-NRR.

Modeling Study of Enhanced Coal Seam Gas Extraction via N2 Injection Under Thermal-Hydraulic-Mechanical Interactions.

N2 injection into coal seams can effectively enhance the gas flow capacity in the late stage of pumping, thereby improving the recovery rate and recovery efficiency of low coalbed methane (CBM). To reveal the thermodynamic flow coupling relationship between the reservoir and the gas phase and its transportation mechanism in the process of thermal N2 injection, a mathematical coupling model of N2 injection that considers the deformation of the coal seam, fluid transportation, and temperature change was established. The influence of the seepage heat transfer effect of the coal seam under the effect of N2 injection on the CH4 extraction rate was investigated using this model. Results indicate that the action mechanism of N2 injection in coal seams includes increasing seepage, promoting flow, and displacing gases. A higher initial coal seam temperature results in a smaller thermal expansion and deformation of the coal skeleton during thermal N2 injection and less pronounced coal permeability increase. A larger initial coal seam permeability results in more favorable N2 diffusion, which strengthens the flow-promoting effect on CH4. The effect of gas injection pressure on CH4 recovery is greater than that of the gas injection temperature: High pressure promotes N2 seepage, carries CH4 flow, and increases the CH4 diffusion effect, whereas higher temperature promotes the desorption of adsorbed gas in the coal seam and improves the recovery rate.

Insights on influence of polymer molecular weight on gas sorption, transport and plasticization in glassy 6FDA-DAM polyimide membranes

It is well-known that spinning of defect-free asymmetric polyimide hollow fibers is promoted by using high polymer molecular weight. On the other hand, effects of polymer molecular weight on microstructure and separation performance of dense film 6FDA-based polyimide membranes relevant to sheath layers of composite membranes are less well-explored. In this work, gas sorption, diffusion and permeation of CO2, N2 and CH4 for 6FDA-DAM dense films are considered for 45 kDa and 176 kDa weight average molecular weights. Such molecular weights are relevant for practical hollow fiber spinning of membranes. Earlier results for polystyrene samples show similar trends in dual mode sorption model results like those noted here for the 6FDA-DAM polyimide case; however, effects on permeation like those considered here were not addressed for the polystyrene case. Here we also address actual pure and mixed gas CO2 and CH4 permeation and show higher free volumes, higher gas permeability but lower 50:50 CO2/CH4 selectivity associated with the higher molecular weight sample. Analysis using the self-consistent dual-mode sorption and transport models help understand the observed trends. Our results suggest denser packing of polymer segments exists in the Henry's law environment of the low molecular weight sample. Moreover, the higher polymer segmental packing in the Henry's law environment suppresses CO2 plasticization with a dual-mode microstructure controlling separation performance and plasticization behavior for this important member of the 6FDA polyimide family. We show evidence of effects of charge transfer complexes as possible main features in these trends. These dense film results help guide future work on dense sheath layers on composite hollow fiber membranes.

Experimental and numerical simulation study on enhanced oil recovery by N2-Assisted water huff-n-puff in a tight oil reservoir

The water huff-n-puff (WHP) method is widely used for improving oil recovery in tight reservoirs. However, the poor flow conductivity and low sweep efficiency of the matrix restrict the oil recovery performance of this method. Herein, N2-assisted water huff-n-puff (NAWHP) was proposed to improve oil recovery under reservoir conditions. First, the feasibility of NAWHP for tight reservoirs was investigated by comparing the enhanced oil recovery (EOR) performance with that of WHP and N2 huff-n-puff (NHP). The core flooding experimental results revealed that NAWHP facilitated oil recovery improvement, outperforming the existing practice of water or N2 huff-n-puff injection. Thereafter, with the aid of nuclear magnetic resonance (NMR) technology, the application of NAWHP could significantly enhance oil recovery in fractured tight cores, which mainly increased oil recovery from micro- and nanopores in the matrix. Specifically, for submicron pores and micropores, after three cycles of NAWHP, the recovery efficiency increased by 21.35% and 36.19%, respectively, while the recovery only increased by 14.34% and 24.8%, respectively, during WHP. For nanopores and micro/nanopores, the use of WHP increased the total oil recovery by only 4.71%, while the use of NAWHP increased the total oil recovery threefold. Finally, a series of numerical simulations were conducted to explore the effects of different parameters on oil recovery by NAWHP. The results illustrated that a high N2–water ratio (NWR) positively affected the overall oil recovery, but the higher the NWR was, the more difficult to extract the oil below the injection layer. By injecting 13,000 m3 of N2, the maximum swept volume was achieved for the matrix, and water filled the major fractures after injecting 11,000 m3, effectively accomplishing this task. Increasing the soaking time positively influenced oil recovery, N2 diffusion within the matrix led to an even distribution over 50 days, and water imbibition caused a significant increase in the oil saturation in hydraulic fractures within 30 days. The application of NAWHP increased the number of huff-n-puff cycles of water from a maximum of 3–5.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene.

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Transformation from Knudsen diffusion to facilitated diffusion for CO2 in confined mass transfer channels of polyimide mixed matrix membranes

Incorporation of porous materials in a polymeric membrane is an effective strategy for improving the gas permeability of the resulting mixed matrix membranes (MMMs). However, effective improvement of selectivity of membrane toward CO2 separation is still a significant challenge due to the insufficient CO2 capture performance of porous materials. In this study, polyimide (PI)-based MMMs with enhanced CO2 capture performance were prepared for CO2 separation. First, an ionic liquid (IL) with a high affinity for CO2 was immobilized in the inner pore structures of two types of mesoporous silica (SiO2) molecules to form IL@SiO2 composites. These composites were used as nanofillers to construct the MMMs. The stable immobilization of IL inside the pores of the mesoporous SiO2 indicated that the IL@SiO2 and MMMs both exhibited higher adsorption selectivity for CO2. The gas solubility coefficient of the MMMs was positively proportional to their maximum gas adsorption capacity, which was determined by the gas adsorption capacity of the PI polymer and nanofillers. More interestingly, the diffusion coefficients of different gases in the MMMs were closely related to the confined mass transfer channels introduced by the nanofillers. The diffusion of CO2 in the confined mass transfer channels of MMMs was facilitated and N2 diffusion was slightly inhibited, which increased the diffusion selectivity of the resulting MMMs for CO2. Combined with Maxwell–Wagner–Sillar model calculations for gas permeation, it was confirmed that IL immobilization transformed the diffusion mode of CO2 in the confined mass transfer channels of the MMMs from Knudsen diffusion to facilitated diffusion. However, the immobilization of excessive ILs in the mesopores blocked the mass transfer channels of the MMMs, resulting in lower gas permeability and decreased CO2 separation selectivity.

Effect of pendant ester groups on gas transport property of 6FDA-BAPP/6FAP copolyimide membrane

Despite the various methods that have been developed, controlling the microstructure of polyimide membranes to achieve excellent separation performance based on target gases remains a challenge. One such method involves rearranging polyimides containing ortho-hydroxyl groups into polybenzoxazoles at temperatures above 400 °C, which can effectively distort the backbones and improve gas diffusion. However, this approach is limited by the extremely high conversion temperatures required and low conversion degrees achieved. The esterification process of the polyimide, which contains ortho-hydroxyl groups to the imide ring, represents a simple and effective approach to enhance the membrane's competitiveness in gas separation without undergoing thermal rearrangement at high temperatures. The polyimide was prepared by the polycondensation of 4,4'-(Hexafluoro isopropylidene) diphthalic anhydride (6FDA), 2,2-double (4,4-amino phenoxy) propane (BAPP), and 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (6FAP). Microstructures and gas transport coefficients were analyzed when the esterification of 6FDA-BAPP/6FAP was conducted using acetic anhydride and caproic anhydride. In CO2/N2 separation, a-6FDA-BAPP/6FAP exhibits CO2 permeability of 1279.41 ± 66.93 Barrer and perm-selectivity of 40.80 ± 2.27, owing to the significantly improved affinity to CO2. The membrane of c-6FDA-BAPP/6FAP exhibit the further improved CO2 permeability and deteriorated perm-selectivity, due to a leveling effect on the diffusion of CO2 and N2 caused by the pendant caproates with longer alkyl chains. The results confirmed that the gas separation performance of polyimide membrane containing ortho-hydroxyl groups is sensitive to the pendant group.

Thermal stability of Al3BC3 powders under a nitrogen atmosphere

Abstract The thermal stability of Al3BC3 powders under nitrogen was studied. AlN–BN composites were generated during the nitridation of Al3BC3. Possible reaction mechanisms responsible for the thermal decomposition of Al3BC3 powders were discussed. The relatively weaker Al–C bonds in Al3BC3 promoted the fast diffusion of Al and the generation of AlN–BN layers inhibited the deeper nitridation, thus the thermal decomposition was governed by surface reaction. The formed nitrides resulted in a volume change and cracked the resulting layers as the reactions progressed, facilitating the diffusion of N2 and enhanced the decomposition of Al3BC3. The intensive reaction involving Al3BC3 and N2 could be attributed to the prolonged reaction time at high temperature and continued escape of vaporized Al and B. This result contributes to a theoretical basis of high-temperature application of Al3BC3 under nitrogen.

Insights into oxidation and nitridation of alumina-forming FeCrAl alloy after supercritical water exposure: Effect of pre-corrosion on high-temperature damage behavior

This work investigated the corrosion characteristics of FeCrAl alloy in supercritical water (SCW), and focused on the effect of SCW pre-corrosion on the alloy oxidation and nitridation at 900 °C. FeCrAl alloy formed outer Fe-rich oxides (mainly Fe2O3) and inner Fe-Cr-Al oxides during SCW exposure. SCW pre-corrosion made FeCrAl alloy exhibit different morphologies and phase compositions in the same medium. SCW pre-corrosion promoted the deposition of Ni and Mo elements from the autoclave, filling the gaps in flaky oxides by forming Ni- and/or Mo-rich products in air and N2. Moreover, SCW pre-corrosion avoided the appearance of accelerated nitridation zones in N2. However, pre-corrosion caused the preferential diffusion of N2 and air. Outward-diffused Cr ions during SCW exposure reacted with O impurities in N2 to form Cr2O3 that easily dissociated N2, leading to the development of nitridation towards the alloy interior.

Numerical Investigation of the Evolution of Gas and Coal Spontaneously Burned Composite Disaster in the Goaf of Steeply Inclined Coal Seam

As the coal mine gets deeper and the stopes’ structures become more complex, gas and coal spontaneously burned composite disaster seriously threatens the efficient operation of coal mines. To study the interaction process and disaster-causing mechanism of gas and coal spontaneous combustion (GCSC), this paper establishes a numerical model to study the influence of drilling location/pressure and N2 injection on the evolution of gas and coal spontaneously burned composite disaster in the goaf. The simulation shows that in the central part of the goaf, a combined area of gas and coal combustion poses a possibility of spontaneous combustion calamity, and the length of the compound disaster area is about 20 m. The methane (CH4) explosion zone and the dioxygen(O2) temperature rise zone do not overlap in the air entrance roadway and return air roadway, indicating that there is no risk of compound disasters. The optimal nitrogen (N2) injection rate for this working face is 2000 m3/h, and the N2 port should be located 25 m profound into the goaf, which can effectively drive the diffusion of N2 and narrow the O2 zone’s breadth. The findings have considerable engineering applications for revealing the evolution process, risk assessment and control for GCSC compound disasters in coal mines.

High nitrogen and argon diffusion in cyclic olefin copolymer foil versus temperature

Measurements of nitrogen and argon diffusion coefficient in cyclic olefin copolymer (COC) have been performed at different temperatures ranging between 21 °C and 101 °C. Optical transparent COC foils have been prepared with 10 μm thickness and total surface of about 25 cm2. The foils have exhibited high purity in composition. The morphology of the COC surface has been investigated by Scanning Electron and Atomic Force Microscopies. The room temperature measured diffusion coefficients, of about 6.56 × 10−3 cm2/s and 6.30 × 10−3 cm2/s for N2 and Ar, respectively, have been obtained with a home-made experimental set-up by measuring the gas pressure gradient reduction applied to the two faces of COC thin foils versus the time. The foils show a significant increase of the N2 and Ar diffusion coefficients with the temperature. The native amorphous nature, the obtained nano-porous structure, the damage suffered by the thin foils of COC under high applied pressure are responsible of the evaluated high diffusion coefficients for nitrogen and argon gases. The experimental apparatus to measure the diffusion coefficients vs. temperature, the obtained results, the COC sheets microstructure, the measurements comparison with the literature data and possible COC applications in biomedicine are presented and discussed.

N2 Inhibition Characteristics of Coal Spontaneous Combustion in Goaf: Experimental Study and Numerical Analysis

ABSTRACT To investigate the inhibition characteristics of coal spontaneous combustion in goaf under different nitrogen injection conditions, the oxidation and the heat release characteristics of coal bodies with easy spontaneous combustion were characterized by using adiabatic oxidation experiment. A comparative study on the evolution of the coal spontaneous combustion risk zone (CSCZ) in goaf was carried out by using numerical simulation. The results indicate that the oxygen consumption rate of coal bodies with the rise of temperature in a power function. The exothermic characteristics of coal decrease with the increase of N2 addition. The location and rate of N2 injection restrict the expansion process of CSCZ, which makes the maximum width of CSCZ shrink, the oxygen attenuation rate, N2 diffusion intensity and inerting area increase. When the N2 injection position is 40 m away from the working face and the N2 injection rate is 450 m3/min, the critical ideal value of inhibition coal spontaneous combustion can be achieved.

Effect of binder on CO2, CH4, and N2 adsorption behavior, structural properties, and diffusion coefficients on extruded zeolite 13X

The effect of inorganic binder-based extrusion (Silica sol, Bentonite, Attapulgite, and SB1) in the selective adsorption of CO2, CH4, and N2 on zeolite 13X in the context of flue gas carbon capture and natural gas purification has been studied to reduce CO2 emissions. The effect of extrusion with binders was examined by adding 20 wt% of the mentioned binders to pristine zeolite and the results were analyzed by four approaches; (i) the effect on structural properties was analyzed by XRD patterns followed by Williamson–Hall (W–H) plot, FESEM images, and BET surface area. In addition, the mechanical strength of the shaped zeolites was measured by crush resistance testing; (ii) the effect on the adsorption capacity for CO2, CH4, and N2 were measured by volumetric apparatus up to 100 kPa; (iii) the impact on binary separation (CO2/CH4 and CO2/N2) were investigated; (iv) the influence on diffusion coefficients were estimated by micropore and macropore kinetic model. The results showed that the presence of a binder can cause reductions in BET surface area and pore volume, indicating partial pore blockage. It was found that the Sips model had the best adaptability to the experimental isotherms data. The trend of CO2 adsorption was 13X > pseudo-boehmite > bentonite > attapulgite > silica, in which the adsorption capacity reached 6.02, 5.60, 5.24, 5.00, and 4.71 mmol/g, respectively. Among all samples, silica was found the most suitable binder for CO2 capture in terms of selectivity, mechanical stability, and diffusion coefficients.

Engineering hydrophobic-aerophilic interfaces to boost N2 diffusion and reduction through functionalization of fluorine in second coordination spheres.

Ammonia is a crucial biochemical raw material for nitrogen containing fertilizers and a hydrogen energy carrier obtained from renewable energy sources. Electrocatalytic ammonia synthesis is a renewable and less-energy intensive way as compared to the conventional Haber-Bosch process. The electrochemical nitrogen reduction reaction (eNRR) is sluggish, primarily due to the deceleration by slow N2 diffusion, giving rise to competitive hydrogen evolution reaction (HER). Herein, we have engineered a catalyst to have hydrophobic and aerophilic nature via fluorinated copper phthalocyanine (F-CuPc) grafted with graphene to form a hybrid electrocatalyst, F-CuPc-G. The chemically functionalized fluorine moieties are present in the second coordination sphere, where it forms a three-phase interface. The hydrophobic layer of the catalyst fosters the diffusion of N2 molecules and the aerophilic characteristic helps N2 adsorption, which can effectively suppress the HER. The active metal center is present in the primary sphere available for the NRR with a viable amount of H+ to achieve a substantially high faradaic efficiency (FE) of 49.3% at -0.3 V vs. RHE. DFT calculations were performed to find out the rate determining step and to explore the full energy pathway. A DFT study indicates that the NRR process follows an alternating pathway, which was further supported by an in situ FTIR study by isolating the intermediates. This work provides insights into designing a catalyst with hydrophobic moieties in the second coordination sphere together with the aerophilic nature of the catalyst that helps to improve the overall FE of the NRR by eliminating the HER.

Ultramicroporous polyureas synthesized with amines and 1,1′-Carbonyldiimidazole and their CO2 adsorption

Microporous polyureas can be highly stable, but isocyanates or phosgene are normally used for the synthesis. Here, it was postulated and demonstrated that 1,1′-carbonyl diimidazole (CDI) could be used for the synthesis. By reacting tetrakis(4-aminophenyl)methane with CDI, a series of new polyureas with ultramicropores (pores <0.7 nm) were synthesized. To tailor thermal properties and porosity, the ratio of tetraamine-to-CDI and the reaction temperature were varied. The CO2 adsorption capacities (with values up to 0.74 mmol/g at 0.15 bar/273 K and 1.91 mmol/g at 1 bar/273 K) were ascribed to the ultramicroporosity. The CO2-based Dubinin-Radushkevich surface areas reached 395 m2/g (at 273 K), while the N2-based BET surface areas (at 77 K) were small. The apparent CO2-over-N2 selectivity was also high for the polymers at 273 K with estimated values of 31–92 for 15/85 v/v mixtures of CO2 and N2. This high selectivity was ascribed to the kinetic hindrance of N2 diffusion. It was noted that one of the polymers changed color irreversibly upon heating. In conclusion, it was shown that CDI and amines could be used to synthesize ultramicroporous polyureas, and that these polymers can exhibit irreversible thermochromism. This thermal effect was attributed to the electron-rich urea moieties, aromatic units, and conjugation.

Thermal stability and selective nitridation of Cr2AlC in nitrogen at elevated temperatures

The thermal stability and selective nitridation of Cr2AlC under a high–temperature nitrogen atmosphere were studied. Cr2AlC began to decompose beyond 1073 K in N2, and the activation energy was estimated to be 108.93 kJ·mol−1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr2AlC and the process was dominated by a surface reaction. The final products were AlN, Cr7C3 and Cr3C2, and the weaker Cr–Al bonds in Cr2AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N2 and thus accelerated the reaction. Finally, the intensive reaction involving Cr2AlC and N2 could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum.

Highly efficient photocatalytic nitrogen fixation on bio-inspired triphase interface with improved diffusion of nitrogen

Low solubility and slow diffusion of nitrogen (N2) in water are significant impediments for enhancing the efficiency of fixation nitrogen into ammonia (NH3). Herein, we have prepared bio-inspired hierarchical assembly of carbonized loofah [email protected]/Au ([email protected]/Au) to improve diffusion of nitrogen by immobilization of hydrophilic BiOBr-OV/Au onto hydrophobic CL sponge. The hydrophobic CL sponge provides an optimized directional channel for N2 diffuse to the triphase interface N2 (gas)-H2O (liquid)-BiOBr-OV/Au (solid), which supplies generous N2 for the photocatalytic reaction and replaces the slow diffusion of N2 through the liquid phase. Continuous delivery of N2 can effectively capture generated electrons, thereby preventing the recombination of generated electron-hole and promoting N2 activation. The modification of AuNPs can broaden the photoresponse range of BiOBr-OV, increase oxygen vacancies concentration and enhance N2 adsorption and activation. [email protected]/Au shows an impressive in nitrogen fixation performance with high selectivity and recyclability of 2.64 mmol·gcat−1·h−1, that is 10.2 and 8.0 times compared to [email protected] and BiOBr-OV/Au, respectively. Our work affords a convenient method for the construction of bio-inspired triphase catalysts with a directional N2 transport channel to enhance the efficiency of nitrogen fixation, which also holds great promise for the application in other reactions involving gas consumption.

The gas permeability properties of poly(vinyltrimethylsilane) treated by low‐temperature plasma

AbstractThe results of one‐sided treatment of polyvinyltrimethylsilane (PVTMS) films using low‐temperature air plasma are presented. The treatment was carried out at 25°C in air at a pressure in the reaction chamber of 10–25 Pa and a processing time of 10–60 s. The surface of the plasma‐treated PVTMS films was analyzed by X‐ray photoelectron spectroscopy and atomic force microscopy and the contact angle values determined. The effect of the plasma conditions, such as the treatment time and the pressure in the reactor chamber, on the transport properties of the modified membrane for He, O2, N2, CO2, and CH4 were investigated in detail. The permeability of all plasma‐treated PVTMS films for the studied gases except for He was found to decrease depending on the modification conditions, leading to an increase in selectivity for such pairs as He/CH4, CO2/CH4, and O2/N2. The selectivity of the modified PVTMS achieved a maximum value when the films were treated with plasma for 20–40 s at a chamber pressure of 15–20 Pa. The effective diffusion coefficients of the studied gases were obtained experimentally. It was shown that the volumetric diffusion of gases, particularly O2 and N2, in PVTMS before as after plasma treatment is described by the classical diffusion equations. The theoretical approaches to the influence of boundary conditions on selective gas transfer through polymeric membrane are considered. To estimate the prospects for the long‐term use of membranes based on modified PVTMS, the stability of the parameters gas permeability and selectivity of the modified samples was investigated over 9 months. It was shown that the modification of PVTMS by the method of low‐temperature plasma in air is a promising method for expanding the spectrum of membrane processes based on existing commercial membranes.

N2 fixation is less sensitive to changes in soil water content than carbon and net nitrogen mineralization

Biogeochemical processes catalyzed by soil microorganisms depend on the soil water content (SWC). Yet, little is known about the sensitivity of non-symbiotic N2 fixation to changes in SWC in comparison to carbon (C) and net nitrogen (N) mineralization. Therefore, we determined the rates of N2 fixation, C mineralization, and net N mineralization in soils at five SWCs that were created by water addition to the field-moist soil (rewetting), resulting in SWCs of 20, 40, 60 and 80% of the soil water holding capacity (WHC) which correspond to 14, 28, 42, and 57% water-filled pore space (WFPS). The soils originated from an arid, semiarid, Mediterranean, and humid site located in the Coastal Cordillera of Chile. The N2 fixation rate was measured immediately after the adjustment of the SWC and additionally after a five-day pre-incubation period. We found that the rates of all three processes where higher in the rewetted soils compared to the field-moist soils, but the sensitivity to changes in SWC differed between the processes. N2 fixation tended not to increase with increasing SWC beyond 20% WHC. Furthermore, the N2 fixation rate increased faster after rewetting in the soils of the semiarid and Mediterranean zone than in the soil of the humid zone. In contrast, C mineralization reached the highest rate at 80% WHC in the soil of the humid zone and at 60% and 80% WHC in the soils of the other three climate zones. The net N mineralization rate was highest at 40% and 60% WHC in the soils of the semiarid and Mediterranean zone, and was significantly decreased at 80% WHC compared to 60% WHC. Our study shows, first, that N2 fixation is less sensitive to changes in SWC than C and net N mineralization and tends to be already at its maximum in a given soil at low SWC, i.e., 20% WHC. This is likely because the diffusion of N2 in soil does not increase with the SWC, in contrast to the diffusion of organic solutes which are the substrate of C and N mineralization. Second, our results indicate that the N2 fixation rate increases more quickly in response to rewetting of soils of the more arid climate zones than of the humid zone, which might indicate a microbial adaptation to the climate conditions. The results have important implications since they suggest that the N2 fixation rate is likely less affected than the C and net N mineralization rates by decreases in the SWC that might occur more frequently in the future due to climate change.

Infrared Spectroscopic Study of Methane Ice, Pure and in Mixtures with Polar (H2O) and Nonpolar (N2) Molecules.

Mid-infrared studies of fundamental modes of ices of pure CH4 and its mixtures with polar (H2O) and nonpolar (e.g., N2) molecules are essential in order to learn the state of aggregation and thermal history of ices present in the interstellar medium and outer solar system bodies. Such data will be useful in the interpretation of observational data from the James Webb Space Telescope. Using an ultrahigh vacuum apparatus, we conducted reflection–absorption infrared spectroscopy measurements in the mid-IR range of pure methane ice and methane-containing ice mixtures of interest to interstellar and solar system ice chemistry, e.g., with H2O and N2 molecules. We found that nuclear spin conversion (NSC) in solid methane and its crystalline structures is affected—in different ways—by the presence of H2O and N2. Specifically, we found a relationship between the thickness and the solid-state ordering transformation in methane thin films. This new study of the NSC of pure CH4 ice and of the CH4:H2O ice mixture at 7 K is carried out in relation to the segregation of H2O using the ν1 and ν2 IR inactive modes of methane. The diffusion of N2 and CH4 in the CH4:N2 ice mixture with temperature cycling has been studied to obtain the relationship between IR features and the state of aggregation of the ice.