CO2 Diffusion Research Articles

Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco.

Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up-regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement.

Carbonation curing behavior and performance improvement of recycled coral waste concrete

Carbonation curing can improve material properties and reduce the concrete industry's carbon emissions. This study used powder prepared from waste coral to replace ordinary Portland cement (0, 10, and 20 %). After initial curing for 24 h, the mortar was cured by sealing and carbonation. The dilution effect of coral powder substantially reduces the mechanical properties under sealing curing. Under carbonation curing, the coral powder helps develop carbonation and CO2 diffusion, reducing strength loss and increasing resistivity. XRD and Fourier transform infrared spectroscopy analysis results showed that carbonation transforms hydration products and clinker phases into different types of CaCO3, such as calcite and aragonite. The novelty of this study is that it proposes a method that organically combines carbonation curing and marine waste reuse. Provide a sustainable solution to the problem of material shortage in marine engineering construction. Moreover, the carbon footprint can be considerably reduced by replacing cement materials, improving carbonation curing efficiency, and promoting CO2 uptake.

Effect of in-situ emulsification on CO2/N2-based cyclic solvent injection process for enhancing heavy oil recovery and CO2 storage

Surfactant-assisted CO2/N2-based CSI (SA CO2/N2-CSI) process emerges as a strategic solution to migrate the high energy consumption and carbon intensity associated with traditional thermal operations for heavy oil recovery. However, the interphase mass transfer makes the emulsification behavior in this process complex. In this study, the effect of in-situ emulsification on CO2/N2-CSI is assessed using a sandpack model and microchannel chip. Mesoscopically, SA CO2/N2-CSI outperforms conventional CO2/N2-CSI in terms of both oil recovery and CO2 storage, owing to the enhanced CO2 diffusion, oil solubilization, and foamy oil stability due to emulsification. Microscopically, the impact of emulsifier injection rate on oil removal efficiency in the dead-end pores becomes negligible when the injection rate exceeds 50 nL/min, providing insights to the emulsification mechanisms underlying the optimal pressure depletion rate (9 kPa/min) in SA CO2/N2-CSI. This study demonstrates the applicability of the SA CO2/N2-CSI in fostering sustainable heavy oil production and CO2 storage.

Exploring the Effect of Moisture on CO2 Diffusion and Particle Cementation in Carbonated Steel Slag

The study of the mechanisms affecting the preparation parameters of carbonated steel slag is of great significance for the development of carbon sequestration materials. In order to elucidate the mechanism of the influence of moisture on CO2 diffusion and particle cementation in steel slag, the effects of different water–solid ratios and water contents on the mechanical properties, carbonation products, and pore structure of steel slag after carbonation were investigated. The results show that increasing the water–solid ratio of steel slag can control the larger initial porosity and improve the carbon sequestration capacity of steel slag, but it will reduce the mechanical properties. The carbonation process relies on pores for CO2 diffusion and also requires a certain level of moisture for Ca2+ dissolution and diffusion. Increasing the water content enhances particle cementation and carbonation capacity in steel slag specimens; however, excessive water hinders CO2 diffusion. Reducing the water content can increase the carbonation depth but may compromise gelling and carbon sequestration ability. Therefore, achieving a balance is crucial in controlling the water content. The compressive strength of the steel slag with suitable moisture and initial porosity can reach 118.7 MPa, and 217.2 kg CO2 eq./t steel slag can be sequestered.

Effect of internal moisture content (IMC) on the CO2 sequestration efficiency of hollow natural fiber (HNF)-reinforced reactive magnesia cement (RMC) composites

The incorporation of minor hollow natural fibers (HNF) can significantly accelerate the CO2 diffusion in reactive magnesia cement (RMC). The CO2 diffusivity in HNF is highly related to its internal moisture content (IMC), which subsequently affects the carbonation process of HNF-RMC, but this is outside the scope of the existing studies. Therefore, this work investigates the effect of IMC on carbonation, strength, and microstructure of HNF-RMC composites. HNF-RMC mortar specimens are dried to different levels of nominal IMC (0–100%), and cured under high CO2 concentration for carbonation. Following the curing, the matrix composition was characterized with varying techniques (QXRD, TGA, FTIR, and acid digestion) for quantifying the CO2 sequestration; the compressive strength at different depths was determined with uniaxial compressive test; the microstructure was examined with SEM/ESEM. The results show that the carbonation depth is increased by three to four times by reducing the IMC to 0%. The effects of varying IMC on local carbonation and strength are distinct in the shallow (0–20 mm) and middle/deep region (20–100 mm). In the shallow region, inadequate CO2 dissolution is the limiting factor, these performances decreased with the increasing desiccation. In the deeper regions, moisture absorption by HNF decreased the CO2 diffusivity, hence the CO2 availability is the limiting factor, and these performances increased with the desiccation. These observations suggest that the understanding that carbonation maximizes at a medium moisture level is not valid in HNF-RMC–controlling the moisture can be important for improving the CO2 sequestration and consequent mechanical strength.

Toward more accurate estimates of carbon emissions from small reservoirs

AbstractBecause of their abundance and high emissions rates, small reservoirs (< 0.01 km2) can be important emitters of the greenhouse gases carbon dioxide and methane. However, emissions estimates from small reservoirs have lagged those of larger ones, and efforts to characterize small reservoir emissions have largely overlooked variations in emissions pathways, times, and locations. We intensively sampled four small reservoirs in Georgia, USA, during the summer to quantify the contribution and spatiotemporal variability of different emissions pathways (CO2 and CH4 diffusion, CH4 ebullition). We used these data to evaluate the efficiency and accuracy of different sampling schemes. Every emissions pathway was dominant in one reservoir on one sampling day, and excluding ebullition caused misestimation between −89% and −15% of the total flux. Sampling only once daily caused misestimation between −78% and 45%, but sampling twice or just after dawn (07:00 h) reduced error. Sampling four or fewer locations caused misestimation between −85% and 366%, and our results indicated that 6–20 sampling locations may be needed for reasonable accuracy. The floating aquatic macrophyte Wolffia sp. (duckweed) appeared to exert control over emissions variability, and the consequences of not accounting for variability were greater in a duckweed‐covered reservoir. Our results indicate that sampling only at 10:00 h (modal sampling time of prior efforts) may lead to the erroneous conclusion that reservoirs with high photosynthetic biomass are CO2 sinks rather than sources. Improving estimation accuracy by accounting for within‐reservoir variation in emissions will facilitate more strategic management of these abundant, anthropogenic ecosystems.

Enhancing carbonation and thermal insulation of reactive magnesium oxide cement (RMC)-based 3D printable pastes with cenospheres

Reactive magnesium oxide cement (RMC) has been proposed as a sustainable alternative to ordinary Portland cement (OPC) due to its lower production temperature and ability to permanently sequester carbon dioxide (CO2) through carbonation. However, mixtures with only RMC can have significantly reduced CO2 absorption capacity due to limited CO2 diffusion, thereby compromising the performance of RMCs. This paper demonstrates the adoption of cenospheres, an industrial waste, as a lightweight filler in RMC-based 3D printable composites to enhance carbonation and improve thermal insulation. This work investigates the effect of incorporating different dosages of cenospheres on the rheological properties, compressive strength, thermal conductivity, and microstructural development of the RMC mixtures using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The study reveals that substituting 30 % of RMC with cenospheres not only improves the printability and carbonation of the composite but also results in a 16 % reduction in density, achieving 1.72 g/cm³, and reduces the thermal conductivity by 59 % compared to the control without cenospheres to 0.63 W/(m·K). The findings provide valuable insights for developing lightweight 3D printable RMC-based composites.

A general Energy-Efficient strategy for optimizing CO2 Capture: Designing and harnessing the rapid adsorption kinetics of Amine-Impregnated adsorbents

This study aims to optimize the performance of amine-impregnated CO2 adsorbents for both direct air capture (DAC) and point source CO2 capture (PSC) applications. The main objective of this study is to investigate the relationship between the weight fraction of amine loading and the efficiency of CO2 uptake on the designated support, aiming to find a straightforward method to design effective amine support materials. To this end, a composite material denoted as PEI/CNT@ZIF-8 has been developed. This composite is comprised of polyethylenimine (PEI), multi-walled carbon nanotubes (CNTs), and zeolitic imidazolate frameworks (ZIF-8). CNTs offer several advantages for enhancing CO2 uptake, including rearranging the ZIF-8 particles to facilitate PEI loading, creating strong electrostatic interaction for amine anchoring, and establishing CO2 diffusion channels to improve CO2 capture kinetics. In DAC scenarios, with a CO2 concentration of 400 ppm balanced with N2, the 25.0 wt% PEI/CNT(1)@ZIF-8(5) demonstrates superior CO2 uptake, achieving 4.08 mmol g−1 within a one-hour interval by harnessing the rapid kinetic region and exhibiting a low desorption energy of 55.8 kJ mol−1 for releasing CO2. In PSC scenarios, with a CO2 concentration of 15 vol% under both dry and wet conditions, the 37.5 wt% PEI/CNT(1)@ZIF-8(5) exhibits excellent CO2 uptake of 2.21 and 2.58 mmol g−1, respectively, at 50 ℃. The findings suggest that adjusting the balance between CO2 uptake and adsorption kinetics by considering the amine weight fraction can optimize the performance of solid amine CO2 adsorbents across diverse operational conditions. Furthermore, rather than relying on a single support material, this study reveals the potential of combining CNTs with materials such as SiO2 nanoparticles, Al2O3, or metal–organic frameworks (MOFs) to enhance CO2 capture performance. This approach holds promise as an effective and straightforward strategy.

Properties of multiple Lewis acid sites in alkali metal-exchanged chabazites probed by CO adsorption

Carbon monoxide adsorption on alkali-metal exchanged chabazites (M-CHA, where M = Li, Na, K) was investigated across various Si/Al ratios. The study reveals significant insights into the adsorption behavior, including the persistence of cationic preferences with decreasing Si/Al ratios and the existence of multiple-center interactions involving alkali-metal cations and CO. Results show that for high-silica M-CHA zeolites, CO adsorption is effectively described by single and dual adsorption site models, with cation preferences varying by type. In low-silica zeolites, cation positions are primarily influenced by the aluminum distribution and Coulombic interactions. However, the propensity for single-site cation positions (Si/Al→∞) is preserved to a certain degree. The most noticeable example is the small difference between SIII’ occupancies (cations in 8-membered ring windows) in Na-CHA-2 and K-CHA-2 (0.80 vs. 0.85) that strongly influences the rate of diffusion of CO in the M-CHA-2 samples. While FT-IR spectra of high-silica zeolites can be accurately described using cation site stabilities, interaction energies, and CO stretching frequencies, predicting spectra of low-silica chabazites requires a statistical approach and/or molecular dynamics simulations at the DFT level. The findings demonstrate that the dynamical behavior of adsorbates changes dramatically between different alkali metal-exchanged chabazites, highlighting the complex nature of CO adsorption at multiple Lewis acid sites.

Tobacco aquaporin NtAQP1 and human aquaporin hAQP1 contribute to single cell photosynthesis in Synechococcus.

Aquaporins are H2O-permeable membrane protein pores. However, some aquaporins are also permeable to other substances such as CO2. In higher plants, overexpression of such aquaporins has already led to an enhanced photosynthetic performance due to improved CO2 mesophyll conductance. In this work, we investigated the effects of such aquaporins on unicellular photosynthetically active organisms, specifically cyanobacteria. Overexpression of aquaporins NtAQP1 or hAQP1 that might have a function to improve CO2 membrane permeability lead to increased photosynthesis rates in the cyanobacterium Synechococcus sp. PCC7002 as concluded by the rate of evolved O2. A shift in the Plastoquinone pool state of the cells supports our findings. Water permeable aquaporins without CO2 permeability, such as NtPIP2;1, do not have this effect. We conclude that also in single cell organisms like cyanobacteria, membrane CO2 conductivity could be rate limiting and CO2-porins reduce the respective membrane resistance. We could show that besides the tobacco aquaporin NtAQP1 also the human hAQP1 most likely functions as CO2 diffusion facilitator in the photosynthesis assay.

Diffusion of alloying elements during high temperature oxidation in a low-cost third generation Ni-based single crystal superalloy

Diffusion of alloying elements in a low-cost third generation Ni-based single crystal superalloy during oxidation at high temperature was investigated. The evolution of oxide layers depended on dominant diffusion of alloying elements at different stages of oxidation. Outward diffusion of Cr, Co and Ni was dominant at initial stage of oxidation, resulting in the formation of (Ni, Co)O and Cr2O3. Diffusion of Al and Ta was restricted in γ/γ' area, but dominant in γ'-free layer. Due to outward diffusion of other elements, passive enrichment of refractory elements, Re, W, Mo and Ta included, caused the formation of mixed oxides layer. Finally, selective oxidation of Al led to the formation of (Ni, Co)Al2O4 and Al2O3 layer.

Effect of hollow natural fiber (HNF) content on the CO2 diffusion, carbonation, and strength development of reactive magnesium cement (RMC)-based composites

Reactive magnesia cement (RMC) is an emerging class of green cement that hardens by sequestering CO2. However, CO2 diffusion into RMC is restricted to a few millimeters by the carbonation-induced dense microstructure on the outer layer, which severely slows down the strength growth and CO2 sequestration. To address this issue, this work employed hollow natural fibers (HNFs) to facilitate CO2 diffusion into the deep regions of RMC. The effects of HNFs contents on the mechanical strength development, holistic porosity, CO2 sequestration, CO2 diffusivity, and microstructure of RMC were investigated through different techniques. The findings revealed that the compressive strength could be more than doubled with the addition of adequate sisal fiber. Moreover, the CO2 sequestration and diffusivity could be continuously enhanced with the increasing HNFs content. However, overdosage of HNFs could induce a higher porosity and additional defects, which slightly compromises the mechanical strength. Finally, the durability of HNFs in simulated RMC and Portland cement (PC) environment was compared by accelerated aging test, showing that the alkaline-induced deterioration of HNFs could be almost eliminated in RMC. Therefore, this preliminary study reinforces the function of RMC as a carbon reservoir and lays the foundation for the large-scale utilization of HNFs in RMC.

Oxygen-enriched carbon molecular sieve hollow fiber membrane for efficient CO2 removal from natural gas

Natural gas is considered one of the most attractive low carbon energy sources while the separation of CO2 is a crucial process for the natural gas purification. Herein, cellulose-based carbon molecular sieve (CMS) hollow fiber membranes with high CO2 affinity capability were fabricated through a post-treatment strategy of hydrogen-induced reduction followed by oxygen-doped modification (H-O treatment), and show good potential for high pressure natural gas purification. The CMS membranes were reduced by H2 at high temperatures (500–600 °C) to form active sorption sites in the carbon matrix, and subsequently chemically absorbed O2 to generate oxygen-containing functional groups, which are favorable for CO2 adsorption and diffusion. After being treated at 600 °C (CMS-600), the CMS membrane presented a high CO2 adsorption capacity of 62.78 cm3/g (298 K). As a result, the CO2 permeability was enhanced from 264 to 1125 Barrer, with approximately 400 % improvement. Moreover, mixed gas separation performances (CO2/N2 and CO2/CH4) were investigated at high pressure and impurities feedings. The CMS membrane exhibited a remarkable CO2/CH4 separation factor of 104 when operated at 20 bar and 500 ppm heptane existed.

Comparison of the effect of topology type and linker composition of zeolitic imidazolate framework fillers on the performance of mixed matrix membranes in CO2/N2 separation

Mixed matrix membranes (MMMs) combine the high separation performance of porous materials with the processibility of polymers and so possess potential for carbon capture from CO2-containing gas streams. Zeolitic imidazolate frameworks (ZIFs) are promising candidates as molecular-sieve fillers in MMMs due to their tunability and ease of synthesis. We have compared four ZIFs, all as nanoparticles of similar sizes (ca. 400 nm), as MMM fillers, to investigate the effects of ZIF structure and chemistry on MMM performance of pure gas (CO2, N2) permeation under the same conditions. The chosen ZIFs include two that exhibit strong CO2 adsorption (hybrid ZIF-7/COK-17 and ZIF-94) and two that have higher pore volumes but weaker CO2 interactions (ZIF-8 and a hybrid ZIF-11/ZIF-71). The hybrid ZIF-7/COK-17 and ZIF-94 are structurally related to ZIF-7 (rhombohedral sod topology) and ZIF-8 (cubic sod), respectively, via partial or complete substitution of benzimidazole or 2-methylimidazole by 4,5-dichloroimidazole or 4-methyl-5-imidazolecarboxaldehyde, while the hybrid ZIF-11/ZIF-71 has the rho topology but the same composition as the ZIF-7/COK-17 hybrid. In the first part of the comparative study, MMMs based on two types of commercial polymers, Matrimid®5218 and PEBAX-MH1657, were prepared containing the ZIF-7/COK-17 hybrid and also with ZIF-94. ZIF-94 shows much better compatibility with the polymers, forming homogeneous dispersions at all loadings attempted (≤35 % wt%) whereas the hybrid shows inhomogeneity above 12 wt% in each case. At 12 wt% loading, both fillers show an increase in CO2 permeability at 1.2 bar and 293 K compared to the pure membrane (in PEBAX, this increases from 49.5 to 60 and 68 Barrer) which is the result of increased solubility compensating for decreased diffusivity, and this improvement in permeability continues to increase at the higher levels of loading possible with ZIF-94. ZIF-7/COK-17 in PEBAX show higher selectivity, achieving a calculated CO2/N2 selectivity up to 70. Further investigation of CO2 and N2 permeation on MMMs with the four ZIFs at 12 wt% in PEBAX-MH1657 showed a clear distinction between the ZIF-94 and ZIF-7/COK-17 MMMs (which show higher membrane solubilities but lower diffusivities) compared to ZIF-8 and ZIF-11/ZIF-71 MMMs. At the loading chosen, the CO2 permeability increase achieved by the four ZIFs over PEBAX-MH1657 increases in the order ZIF-11/-71, ZIF-7-COK-17 (ca. 60 Barrer) < ZIF-94 (68) < ZIF-8 (81), reflecting the complex interplay between CO2 solubility (increasing with interaction strength) and diffusivity (increasing with available cage and window size). The calculated CO2/N2 selectivity is highest for the hybrid ZIF-7/COK-17 membrane (70), which is attributed to molecular sieving effects in the rhombohedral sod structure.

Development of an autonomous on-site dissolved inorganic carbon analyzer using conductometric detection

BackgroundThe increase in anthropogenic CO2 concentrations in the Earth's atmosphere since the industrial revolution has resulted in an increased uptake of CO2 by the oceans, leading to ocean acidification. Dissolved Inorganic Carbon (DIC) is one of the key variables to characterize the seawater carbonate system. High quality DIC observations at a high spatial-temporal resolution is required to improve our understanding of the marine carbonate system. To meet the requirements, autonomous DIC analyzers are needed which offer a high sampling frequency, are cost-effective and have a low reagent and power consumption. ResultsWe present the development and validation of a novel analyzer for autonomous measurements of DIC in seawater using conductometric detection. The analyzer employs a gas diffusion sequential injection approach in a “Tube In A Tube” configuration that facilitates diffusion of gaseous CO2 from an acidified sample through a gas permeable membrane into a stream of an alkaline solution. The change in conductivity in the alkaline medium is proportional to the DIC concentration of the sample and is measured using a detection cell constructed of 4 hollow brass electrodes. Physical and chemical optimizations of the analyzer yielded a sampling frequency of 4 samples h−1 using sub mL reagent volumes for each measurement. Temperature and salinity effects on DIC measurements were mathematically corrected to increase accuracy. Analytical precision of ±4.9 μmol kg−1 and ±9.7 μmol kg−1 were achieved from measurements of a DIC reference material in the laboratory and during a field deployment in the southwest Baltic Sea, respectively. SignificanceThis study describes a simple, cost-effective, autonomous, on-site benchtop DIC analyzer capable of measuring DIC in seawater at a high temporal resolution as a step towards an underwater DIC sensor. The analyzer is able to measure a wide range of DIC concentrations in both fresh and marine waters. The achieved accuracy and precision offer an excellent opportunity to employ the analyzer for ocean acidification studies and CO2 leakage detection in the context of Carbon Capture and Storage operations.

Facile and efficient amine@AD-SiO2 adsorbents from coal fly ash: Comparison of various amines for CO2 adsorption capacity and cyclic stability

Solid amine adsorbent is recognized as an efficient CO2 capture route for globe climate change mitigation. The pore structure of nano supports generally plays a crucial role in the CO2 adsorption performance of solid amine adsorbents; however, the involving of costly and toxic pore-expanding agents limited their scalable application. Herein, an azeotropic-distillation nano-SiO2 (AD-SiO2) support was recycled from coal fly ash (CFA) using the CO2-assisted precipitation and n-butanol AD methods, and possessed a super pore volume of 2.52 cm3·g−1 and an average pore size of 36.6 nm. After impregnating the triethyltetramine, tetraethylenepentamine, pentaethylenehexamine (PEHA), or polyethyleneimine, the synthesized amine@AD-SiO2 adsorbents achieved the excellent adsorption capacity of 3.50–6.13 mmol·g−1 and the fast adsorption kinetics, owing to the superior pore structure of AD-SiO2 support. Based on the viscosity, TGA and liquid-NMR characterization, we explained that the large amine molecule would block the CO2 diffusion channel and thus restrict the active sites, while the small amine molecule would evaporate during the regeneration process. Inspiringly, the PEHA@AD-SiO2 adsorbent possessed the excellent cyclic stability with merely 0.18 % decay per cycle and achieved a final CO2 uptake of 4.24 mmol·g−1 over 50 cycles. This study notably improved the long-term adsorption capacity of solid amine adsorbents via the pore-expanded AD-SiO2 support from CFA, thereby is a low-carbon technology for large-scale industrial CO2 capture.

Effects of reactive MgO on durability and microstructure of cement-based materials: Considering carbonation and pH value

Reactive MgO is a type of eco-friendly material with a lower carbon footprint, as it is produced by calcination of magnesite (MgCO3) at 750 ℃, much less than that (i.e. 1450℃) needed for calcination of (CaCO3) to produce clinker for Portland cement. Carbonation can enhance the mechanical properties of reactive MgO-based cementitious materials but will lower the pH value, leading to the neutralization of cement-based materials and further deterioration in rebar passivation. In this study, varying amounts of reactive MgO (0%, 10%, 20%, and 30%) were incorporated into mortars as cement replacements. By adjusting the duration of CO2 curing (i.e. carbonation curing) and standard curing, it was found that the curing regime involving 36 hours of carbonation curing plus 26.5 days of standard curing led to a higher pH and compressive strength of cement mortars. The pore size distribution was improved as the internal pores of the mortar were filled with carbonation products, which hindered the reaction between CO2 and alkaline hydration products inside specimens and was conducive to maintaining the alkalinity within the mortar. Therefore, the chloride ion penetration resistance and the mechanical properties of mortar were enhanced. SEM and EDS characterization results indicate that the excessive MgO did not fully participate in carbonation and continuously hydrated to generate Mg(OH)2. The expansion of Mg(OH)2 led to crack formation inside the mortar and reduced its strength, which accelerated CO2 diffusion and further reduced the pH value of reactive MgO-based cementitious materials.

Stabilizing Diluted Active Sites of Ultrasmall High-Entropy Intermetallics for Efficient Formic Acid Electrooxidation.

The poisoning of undesired intermediates or impurities greatly hinders the catalytic performances of noble metal-based catalysts. Herein, high-entropy intermetallics i-(PtPdIrRu)2FeCu (HEI) are constructed to inhibit the strongly adsorbed carbon monoxide intermediates (CO*) during the formic acid oxidation reaction. As probed by multiple-scaled structural characterizations, HEI nanoparticles are featured with partially negative Pt oxidation states, diluted Pt/Pd/Ir/Ru atomic sites and ultrasmall average size less than 2 nm. Benefiting from the optimized structures, HEI nanoparticles deliver more than 10 times promotion in intrinsic activity than that of pure Pt, and well-enhanced mass activity/durability than that of ternary i-Pt2FeCu intermetallics counterpart. In situ infrared spectroscopy manifests that both bridge and top CO* are favored on pure Pt but limited on HEI. Further theoretical elaboration indicates that HEI displayed a much weaker binding of CO* on Pt sites and sluggish diffusion of CO* among different sites, in contrast to pure Pt that CO* bound more strongly and was easy to diffuse on larger Pt atomic ensembles. This work verifies that HEIs are promising catalysts via integrating the merits of intermetallics and high-entropy alloys.

Nanoporous Ti3C2O2 monolayer towards high permeance and selectivity for CO2/N2 separation: CO2-MXene interaction tuning

MXene, as a new gas separation membrane, has good stability and suitable pore topological structures to mitigate the greenhouse effect caused by excessive CO2 emission. However, the large quadrupole and polarizability of CO2 make it a strong interaction with MXene, thus leading to a strong solubility and weak diffusivity of CO2. Here, we design nanoporous Ti3C2O2 monolayers (YmXn) with exposed C atoms that could tune CO2-Ti3C2O2 interaction so as to improve gas separation performance by density functional theory and molecular dynamics simulation. The scope of “Cut-off” size is obtained by analyzing the gas separation state from un-permeation to permeation. Results reveal that nanoporous Ti3C2O2 membranes achieve both outstanding CO2 selectivity and permeance. The best-performance Y3X3-Ti possesses CO2 permeance of 1.01 × 10-4 and 2.56 × 10-5 mol s−1 m−2 Pa−1 for equimolar gas mixtures and flue gases, respectively, and both the CO2/N2 selectivity reach 100 %. The gas separation mechanism is dominated by diffusion selectivity rather than solution selectivity in nanoporous Ti3C2O2. The dynamic processes of gas permeation, including the center of mass, mean square displacement, and radial distribution function of the gases, are precisely investigated. The electrostatic repulsion between the C atoms in the channel of the membrane and CO2 follows the sequence Y3X3-C > Y2X4 > Y3X3-Ti, which is the key to regulating gas diffusion. Results of this work highlight that tuning gas-membrane interaction and designing “Cut-off” size are effective strategies to achieve excellent CO2 separation performance, and shed new light on the intrinsic mechanism in high-performance gas separation membranes.

Anthropogenic dissolved organic matter accumulation fuels greenhouse gas diffusive emissions in urban lakes along trophic state levels

Lakes are an important reservoir of dissolved organic matter (DOM) and also emit large amounts of greenhouse gases (GHGs), such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Yet, the linkages between quantity and quality of DOM and GHGs emissions are still not well understood. Hence, we investigated how DOM optical properties mediated dissolved GHGs levels and diffusive fluxes along a trophic state level in subtropical urban lakes, China. Our study showed that anthropogenic protein-like DOM (tryptophan- and tyrosine-like components) dominated the DOM pools, comprising 62% of total fluorescence DOM (FDOM). Partial least squares path modeling (PLS-PM) showed that the direct contribution of relative abundance of DOM to CO2 and CH4 diffusion in the highly-eutrophic state was higher than in the mesotrophic and lightly/moderately-eutrophic states. In particular, protein-like DOM primarily accounted for CO2 variations (44%-49%), while humic- and protein-like DOM contributed comparably to CH4 variations (38%-40% vs. 36%-40%). Furthermore, in the lightly/moderately- and highly-eutrophic states, nutrients contributed to DOM, CO2 and CH4 in both direct and indirect ways, while phytoplankton (chlorophyll a, Chl-a) negatively and directly affected carbon emissions. However, only nitrogen content directly contributed to N2O production and diffusion in the mesotrophic and lightly/moderately-eutrophic states. We concluded that protein-like DOM enrichment had the potential to trigger CO2 and CH4 diffusion and this process was largely mediated by nutrient levels. N2O diffusion was found to be mainly influenced by nitrogen loadings and forms. Our findings highlight the urgency for effective strategies to reduce anthropogenic DOM inputs and control nutrient levels to reduce GHGs diffusion in highly human-impacted aquatic systems.