Porous Metal-organic Frameworks Research Articles

A novel core–shell bimetallic ZrAl-MOF simultaneously boosting electrostatic attraction and ion exchange to eliminate excessive fluoride

A porous metal–organic framework (MOF) is fabricated via the solvothermal approach and employed for the removal of fluoride ions from water.

Host molecules inside metal-organic frameworks: host@MOF and guest@host@MOF (Matrjoschka) materials.

The controllable encapsulation of host molecules (such as porphyrin, phthalocyanine, crown ether, calixarene or cucurbituril organic macrocycles, cages, metal-organic polyhedrons and enzymes) into the pores of metal-organic frameworks (MOFs) to form host-in-host (host@MOF) materials has attracted increasing research interest in various fields. These host@MOF materials combine the merits of MOFs as a host matrix and functional host molecules to exhibit synergistic functionalities for the formation of guest@host@MOF materials in sorption and separation, ion capture, catalysis, proton/ion conduction and biosensors. (This guest@host@MOF construction is reminiscent of Russian (Matrjoschka) dolls which are nested dolls of decreasing size placed one inside another.) In this tutorial review, the advantages of MOFs as a host matrix are presented; the encapsulation approaches and general important considerations for the preparation of host@MOF materials are introduced. The state-of-the-art examples of these materials based on different host molecules are shown, and representative applications and general characterization of these materials are discussed. This review will guide researchers attempting to design functional host@MOF and guest@host@MOF materials for various applications.

Computational study on adsorption and separation of H2/CH4/CO2 in double-walled metal-dipyrazole frameworks

The density functional theory (DFT) and molecular simulation techniques were employed to comprehensively study the structural characteristics and adsorption and separation properties of H2/CH4/CO2 in six novel double-walled dipyrazole metal-organic frameworks (MOFs), BUT-53 to BUT-58 (BUT: Beijing University of Technology). The six new MOFs were characterized using numerical Monte Carlo methods, based on crystal structures optimized through DFT calculations. The results reveal that six MOFs exhibit large specific surface area (1068 – 1780 cm2·g-1), high porosity (0.27 – 0.44 cm3·g-1), and suitable pore size (4.34 – 8.15Å), which are advantageous for gas adsorption and separation. Grand canonical Monte Carlo (GCMC) simulation was implemented to study gas adsorption and separation properties at both 77 and 298K. The simulation results demonstrate that BUT-57 possesses the highest storage capacity for pure component gas, while BUT-53 owns the best separation properties for binary mixture gases. Meanwhile, we propose an approach to enhance the selectivity for CO2 by augmenting the availability of nitrogen sites in porous MOFs, thereby offering some insights for related experimental investigations.

Enhanced phosphate removal from water via hierarchically porous metal-organic frameworks

Enhanced phosphate removal from water via hierarchically porous metal-organic frameworks

Rational design and synthesis of atomically precise nanocluster-based nanocomposites: a step towards environmental catalysis.

Atomically precise metal nanoclusters (NCs) and metal-organic frameworks (MOFs) possess distinct properties that can present challenges in certain applications. However, integrating these materials to create new composite functional materials has gained significant interest due to their unique characteristics through a range of applications, particularly in catalysis. Considering MOFs as hosts and NCs as guests, several synergistic effects have been observed in composites, particularly in environmental catalytic reactions. However, the precise role of encapsulated NCs within the MOF pore structure is still in its infancy. Besides, stabilizing NCs, whether through intact ligands or without ligands via the MOF host, presents challenges that are currently being investigated. This feature article reviews recent advancements in the synthesis of NC@MOF composites, focusing on cutting-edge strategies for selecting MOFs and the roles of NC ligands, as well as characterization and catalytic applications.

Low-Temperature Lithium Metal Batteries Achieved by Synergistically Enhanced Screening Li+ Desolvation Kinetics.

Lithium metal anode is desired by high capacity and low potential toward higher energy density than commercial graphite anode. However, the low-temperature Li metal batteries suffer from dendrite formation and dead Li resulting from uneven Li behaviors of flux with huge desolvation/diffusion barriers, thus leading to shortlifespan and safety concern. Herein, differing from electrolyte engineering, a strategy of delocalizing electrons with generating rich active sites to regulate Li+ desolvation/diffusion behaviors are demonstrated via decorating polar chemical groups on porous metal-organic frameworks (MOFs). As comprehensively indicated by theoretical simulations, electrochemical analysis, in situ spectroscopies, electron microscope, and time-of-flight secondary-ion mass spectrometry, the sieving kinetics of desolvation is not merely relied on pore size morphology but also significantly affected by the ─NH2 polar chemical groups,reducing energy barriers for realizing non-dendritic and smooth Li metal plating. Consequently, the optimal cells stabilize forlong lifespan of 2000 h and higher average Coulombic efficiency, much better than the-state-of-art reports. Under a lower negative/positive ratio of 3.3, the full cells with NH2-MIL-125 deliver a high capacity-retention of 97.0% at 0.33 C even under -20°C, showing the great potential of this kind of polar groups on boosting Li+ desolvation kinetics at room- and low-temperatures.

Cadmium(II)-Organic Frameworks with the Polynuclear Unit: Dimensionality Control and Luminescence Response to Pyridine

New porous metal-organic frameworks (MOF) [Cd7(Btdc)7(Bpa)2(Dmf)2(H2O)2] · 15Dmf · 2H2O (I) and [Cd7(Btdc)7(Bpe)2(Dmf)2] · 15Dmf · 3H2O (II) (H2Btdc is 2,2’-bithiophene-5,5’-dicarboxylic acid, Bpa is 1,2-bis(4-pyridyl)ethane, Bpe is 1,2-bis(4-pyridyl)ethylene, and Dmf is N,N-dimethylformamide) are synthesized under solvatothermal conditions. The structures and compositions of the compounds are determined by single-crystal X-ray diffraction (XRD) (CIF files ССDС nos. 2364290 (I) and 2364289 (II)) and confirmed by powder XRD, elemental analysis, thermogravimetry, and IR spectroscopy. Compound I has a 2D structure based on the heptanuclear discrete building unit {Cd7} with the linear structure. Compound II is a 3D MOF in which the {Cd7} building units are linked into a continuous chain motif due to additional interactions. The formation of discrete or continuous chains is directly related to the nature of the N-donor bridging ligand (Bpe or Bpa). Compounds I and II have open structures with the accessible volume about 50%. The solvate molecules are replaced by thiophene, benzene, and pyridine, and the luminescence properties of the prepared adducts are studied. Luminescence quenching in the presence of thiophene and an increase in the luminescence intensity in the presence of pyridine accompanied by a change in the quantum yield by 4–5 times are shown.

Defect-Engineered Metal-Organic Frameworks as Bioinspired Heterogeneous Catalysts for Amide Bond Formation.

The synthesis of amides from amines and carboxylic acids is the most widely carried out reaction in medicinal chemistry. Yet, most amide couplings are still conducted using stoichiometric reagents, leading to significant waste; few synthetic catalysts for this transformation have been adopted industrially due to their limited scope and/or poor recyclability. The majority of catalytic approaches focus on a single activation mode, such as enhancing the electrophilicity of the carboxylic acid partner using a Lewis acid. In contrast, nature effortlessly forges and breaks amide bonds using precise arrays of Lewis/Brønsted acidic and basic functional groups. Drawing inspiration from these systems, herein we report a simple defect engineering strategy to colocalize Lewis acidic Zr sites with other catalytically active species within porous metal-organic frameworks (MOFs). Specifically, the combination of pyridine N-oxide and Zr open metal sites within the defective framework MOF-808-py-Nox produces a heterogeneous catalyst that facilitates amide bond formation with broad functional group compatibility from amines and carboxylic acids, esters, or primary amides. Extensive density functional theory (DFT) calculations using cluster models support that the formation of a hydrogen-bonding network at the defect sites facilitates amide bond formation in this material. MOF-808-py-Nox can be recycled at least five times without losing significant crystallinity, porosity, or catalytic activity and can be employed in continuous flow. This defect engineering strategy can be potentially generalized to produce libraries of catalytically active MOFs with different combinations of colocalized functional groups.

Metal-organic framework-based materials for solar-driven interfacial evaporation

As a sustainable and green approach, solar-driven interfacial water evaporation (SIE) can recover clean water from diverse water resources such as seawater and wastewater by converting solar energy into heat energy, paving the way to addressing the increasingly severe water shortage. In recent years, metal–organic frameworks (MOFs) have been applied for constructing the evaporator for SIE, considering their merits such as large specific surface area, tunable component and structure, and high porosity. More importantly, MOFs materials have expanded their application in the synthesis of porous carbon, metal oxides/sulfides/carbides, and MOFs composites, which not only may preserve the merits of MOFs but also possess other functions. In this review, the latest developments regarding the solar evaporators designed by MOFs-based materials (including MOFs, MOFs-derivatives, and MOFs composites) is summarized and systematically discussed. The roles of MOFs-based materials in evaporation systems and the emerging applications of the evaporation systems are also highlighted. Ultimately, the challenges and perspectives of the MOFs-based materials used for designing of SIE systems are also presented.

(Invited) group 4 Metal-Based Metal–Organic Frameworks and Their Roles in Electrocatalysis and Electrochemical Energy Storage

Metal–organic frameworks (MOFs) with ultrahigh surface areas, interconnected porosity and intra-framework chemical functionality have been considered as attractive platforms to incorporate high-density and highly accessible active sites for electrocatalysis and electrochemical energy storage. However, the poor stability in water and electrically insulating nature of most MOFs strongly restrict the design and direct use of structurally robust MOFs in aqueous electrolytes. The development of stable group 4 metal-based MOFs, such as Zr(IV)-based MOFs (Zr-MOFs), has allowed the use of MOFs in aqueous media while preserving their structural integrity, but the poorly conducting nature of these highly robust MOFs still limits their performances in electrochemical applications.This talk will highlight a few recent examples from our research group on the use of chemically stable Zr-MOFs and their isostructural Ce(IV)-based MOFs in various electrochemical applications, with the main focus on the material-design strategies to overcome the challenges and the roles of such highly porous and stable MOFs in aqueous electrochemical systems. For example, with the redox-active and catalytically active sites, e.g., coordinated iridium ions, immobilized in the rigid Zr-MOF, electrons can be transported through the redox-hopping pathway during electrochemical operations, which is sufficient for electrocatalytic reactions that do not require a high current density such as electrochemical sensors (see panel (a) in the Figure). Further developing the nanocomposite composed of the redox-active group 4 metal-based MOF and conducting nanocarbons such as carbon nanotubes can facilitate the interparticle electronic conduction, allowing the use of such stable MOF-based materials in high-current electrocatalysis and charge storage (see panel (b) in the Figure).On the other hand, the rigid and porous Zr-MOFs that are electrically insulating and “non-catalytic” can also play a role as the thin-film coating on top of the active electrocatalyst to modulate the microenvironment near the underlying surface of the electrocatalyst and thus adjust the selectivity of complicated electrochemical reactions. The underlying electrocatalytic material with a high electrical conductivity and a high catalytic activity, and the highly porous MOF capable of turning the microenvironment near the electrocatalyst, can thus be designed separately. Following this concept, our recent work demonstrated that by coating a thin film of an anionic sulfonate-grafted Zr-MOF on top of the active electrocatalyst for electrochemical sensing, both the reaction rate for the targeted reactant as well as the selectivity toward the targeted cationic reactant against undesired anionic reactants can be remarkably enhanced (see panel (c) in the Figure). Such highly porous Zr-MOF coatings can even outperform the conventionally used Nafion thin films. In addition, as demonstrated in another example, such anionic and highly stable sulfonate-functionalized Zr-MOFs can also be used to disperse and align the cationic monomers during the oxidative polymerization of aniline in strong acids, which can largely improve the pseudocapacitance of the resulting polyaniline for the use in aqueous supercapacitors, even in the presence of the electrochemically inactive MOF inside (see panel (d) in the Figure). Findings here suggest that the chemically robust and porous Zr-MOFs, even without electrical conductivity and catalytic activity, can still play diverse roles to enhance the performances in electrochemical applications. Figure 1

Electrochemical Capacitance and Synthetic Pore Modification in 2D Conductive Metal-Organic Framework

Electrically conductive 2D metal-organic frameworks (MOFs) are a distinct class of materials known for their crystalline extended structures, porosity, and inherent electrical conductivity resulting from the in-plane conjugation system and out-of-plane π interaction, rendering them promising candidates for active electrode materials in electrochemical energy storage devices. Here, we investigated the charge storage mechanisms of a functionalized Cu-MOF, wherein the pore structure was tailored to enhance polarity. This synthetic modification induced a consequential alteration in its band structure, ultimately impacting its electrochemical performance. Through a comparative analysis with its isostructural parent MOF, our study aimed to elucidate the influence of pore modification on electrochemical capacitance. Our findings demonstrated an increased capacitance under aqueous condition compared to the parent MOF, primarily attributed to the facilitated diffusion of electrolyte ions into and within the MOF pores. While the utilization of 2D conductive MOFs holds promise for advancing energy storage technologies, offering versatile attributes including tunable structures, high surface area, and redox-active moieties, these results underscore the potential of systematic pore engineering to enhance electrochemical capacitance. Our study contributes to the ongoing investigation of electrode materials for supercapacitors, emphasizing the critical interplay between structure and function. We anticipate that our discoveries can stimulate further exploration in advanced materials for electrochemical energy storage applications and drive continued progress in the field of MOF-based electrode materials.

In Situ Phase Transformation-Enabled Metal-Organic Frameworks for Efficient CO2 Electroreduction to Multicarbon Products in Strong Acidic Media.

The electrochemical CO2 reduction reaction (CO2RR) has been acknowledged as a promising strategy to relieve carbon emissions by converting CO2 to essential chemicals. Despite significant progresses that have been made in neutral and alkaline media, the implementation of CO2RR in acidic conditions remains challenging due to the harsh conditions, especially in producing high-value multicarbon products. Here, we report that Cu-btca (btca = benzotriazole-5-carboxylic acid) metal-organic framework (MOF) nanostructures can act as a stable catalyst for the CO2RR in an acidic environment. The Cu-btca MOF undergoes phase transformation and morphology evolution during electrolysis, forming a stable porous Cu-btca MOF network. The resultant MOF network exhibits excellent selectivity toward ethylene and multicarbon products with Faradaic efficiencies of 51.2% and 81.9%, respectively, in a strong acidic electrolyte with a flow cell at 300 mA/cm2. Mechanism studies uncover that the Cu-btca MOF network can limit the proton reduction to suppress hydrogen evolution and maintain high local *CO concentration to promote CO2RR. Theoretical calculations suggest that two adjacent Cu sites in the Cu-btca MOF provide a favorable microenvironment for carbon-carbon coupling, facilitating the multicarbon production. This work reveals that rational structure control of MOFs can enable highly selective and efficient CO2 electroreduction to multicarbon products in strong acidic conditions toward practical applications.

High-temperature carbon dioxide capture in a porous material with terminal zinc hydride sites.

Carbon capture can mitigate point-source carbon dioxide (CO2) emissions, but hurdles remain that impede the widespread adoption of amine-based technologies. Capturing CO2 at temperatures closer to those of many industrial exhaust streams (>200°C) is of interest, although metal oxide absorbents that operate at these temperatures typically exhibit sluggish CO2 absorption kinetics and instability to cycling. Here, we report a porous metal-organic framework featuring terminal zinc hydride sites that reversibly bind CO2 at temperatures above 200°C-conditions that are unprecedented for intrinsically porous materials. Gas adsorption, structural, spectroscopic, and computational analyses elucidate the rapid, reversible nature of this transformation. Extended cycling and breakthrough analyses reveal that the material is capable of deep carbon capture at low CO2 concentrations and high temperatures relevant to postcombustion capture.

Phytic Acid Functionalized Hierarchical Porous Metal-Organic Framework Microspheres for Efficient Extraction of Uranium from Seawater.

Uranium is a critical resource in the development of nuclear energy, and its extraction from natural seawater has been identified as a potential solution to address uranium resource shortages. In this study, hierarchical meso-/micro- porous metal-organic framework functionalized with phytic acid (PA) microspheres (denoted H-UiO-66-PA) are rationally synthesized for efficient uranium extraction. This unique design allowed for a large adsorption capacity and fast adsorption kinetics, making it a promising material for uranium extraction. Due to the hierarchically porous structure, H-UiO-66-NH2 materials not only provide more sites for grafting PA, and thus enhance the adsorption capacity, but also facilitate the rapid diffusion of uranyl ions which can quickly and effectively access the interior of the H-UiO-66-PA adsorbent. Therefore, the obtained H-UiO-66-PA can achieve an impressive absorption efficiency of 6.76mgg-1 in actual seawater within 15 days. Meanwhile, the H-UiO-66-PA possesses a good adsorption capacity for uranyl ions in the five adsorption/desorption cycles. This work proposes a feasible strategy for constructing functional hierarchical porous MOFs for uranium removal.

Anisotropic Porosity and Interface Synergy Enhanced Gas Permselectivity in Heterolayer Metal-Organic Framework Membrane.

The pursuit of sustainable, carbon-free separation technology hinges on the efficient separation of gas mixtures with high separation factors and flow rates, i. e. high permselectivity. However, achieving this objective is arduous due to the meticulous engineering at the angstrom scale and intricate chemical manipulation required to design the pores within membranes. To address this challenge, a proof-of-concept for an anisotropic porous membrane has been devised. Employing a meticulous step-by-step methodology, two distinct porous metal-organic frameworks (MOFs) are integrated to form a monolithic anisotropic membrane. By harnessing pore anisotropy (3.4 to 6 Å) aligned with the gas permeation direction and a unique interface characterized by cross-linked pores derived from the two distinct MOFs, this membrane transcends the performance limitations inherent in the individual MOF membranes (~45 % enhanced selectivity). This approach not only sheds light on the heterolayer membrane design strategy but also elucidates the intricate CO2/N2 permselectivity relationship inherent in the interface structure.

Enhanced Carbon Dioxide Capture from Diluted Streams with Functionalized Metal-Organic Frameworks.

Capturing carbon dioxide from diluted streams, such as flue gas originating from natural gas combustion, can be achieved using recyclable, humidity-resistant porous materials. Three such materials were synthesized by chemically modifying the pores of metal-organic frameworks (MOFs) with Lewis basic functional groups. These materials included aluminum 1,2,4,5-tetrakis(4-carboxylatophenyl) benzene (Al-TCPB) and two novel MOFs: Al-TCPB(OH), and Al-TCPB(NH2), both isostructural to Al-TCPB, and chemically and thermally stable. Single-component adsorption isotherms revealed significantly increased CO2 uptakes upon pore functionalization. Breakthrough experiments using a 4/96 CO2/N2 gas mixture humidified up to 75% RH at 25 °C showed that Al-TCPB(OH) displayed the highest CO2 dynamic breakthrough capacity (0.52 mmol/g) followed by that of Al-TCPB(NH2) (0.47 mmol/g) and Al-TCPB (0.26 mmol/g). All three materials demonstrated excellent recyclability over eight humid breakthrough-regeneration cycles. Solid-state nuclear magnetic resonance spectra revealed that upon CO2/H2O loading, H2O molecules do not interfere with CO2 physisorption and are localized near the Al-O(H) chain and the -NH2 functional group, whereas CO2 molecules are spatially confined in Al-TCPB(OH) and relatively mobile in Al-TCPB(NH2). Density functional theory calculations confirmed the impact of the adsorbaphore site between of two parallel ligand-forming benzene rings for CO2 capture. Our study elucidates how pore functionalization influences the fundamental adsorption properties of MOFs, underscoring their practical potential as porous sorbent materials.

Hydrogel-embedded vertically aligned metal-organic framework nanosheet membrane for efficient water harvesting

Highly porous metal-organic framework (MOF) nanosheets have shown promising potential for efficient water sorption kinetics in atmospheric water harvesting (AWH) systems. However, the water uptake of single-component MOF absorbents remains limited due to their low water retention. To overcome this limitation, we present a strategy for fabricating vertically aligned MOF nanosheets on hydrogel membrane substrates (MOF-CT/PVA) to achieve ultrafast AWH with high water uptake. By employing directional growth of MOF nanosheets, we successfully create superhydrophilic MOF coating layer and pore channels for efficient water transportation to the crosslinked flexible hydrogel membrane. The designed composite water harvester exhibits ultrafast sorption kinetics, achieving 91.4% saturation within 15 min. Moreover, MOF-CT/PVA exhibits superior solar-driven water capture-release capacity even after 10 cycles of reuse. This construction approach significantly enhances the water vapor adsorption, offering a potential solution for the design of composite MOF-membrane harvesters to mitigate the freshwater crisis.

Fluorescence sensor based on Methionine-Modified silver nanoparticles located on Fe-BTC metal–organic framework (Meth-AgNPs@Fe-BTC) for trace detection of fenitrothion pesticide in aqueous samples

This research introduces a new “turn-on mode” fluorescence sensor for the detection of fenitrothion (FNT) pesticide in various samples. The sensor is constructed using a porous metal–organic framework (Fe-BTC) as a template to locate silver nanoparticles (AgNPs) and methionine amino acid (Meth). Methionine acts as a bridge, facilitating the interaction between FNT and AgNPs, which subsequently results in the release of AgNPs from the composite structure. The physicochemical properties of the synthesized Meth-AgNPs@Fe-BTC composite were analyzed by Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDAX), Transmission electron microscopy (TEM), and elemental mapping (MAP) analysis. The sensing system is based on tracking the fluorescence of the synthetic composite in such a way that the intensity of the fluorescence of the composite increases in the presence of different concentrations of fenitrothion (FNT). The effective parameters on the sensor signal, including composite dosage, pH, sonication and reaction time were investigated and optimized. The calibration graph, under optimal conditions, exhibited linearity in the concentration range of 2–95 nM for FNT, with a limit of detection of 1.9 nM. The suggested sensor was successfully validated by analyzing FNT in several real water samples and fruit juices. This research presents a significant technical achievement in the development of a fluorescence sensor for the detection of FNT, offering a sensitive and reliable method for environmental monitoring and public health preservation.

Steering diffusion selectivity of chemical isomers within aligned nanochannels of metal-organic framework thin film

The movement of molecules (i.e. diffusion) within angstrom-scale pores of porous materials such as metal-organic frameworks (MOFs) and zeolites is influenced by multiple complex factors that can be challenging to assess and manipulate. Nevertheless, understanding and controlling this diffusion phenomenon is crucial for advancing energy-economic membrane-based chemical separation technologies, as well as for heterogeneous catalysis and sensing applications. Through precise assessment of the factors influencing diffusion within a porous metal-organic framework (MOF) thin film, we have developed a chemical strategy to manipulate and reverse chemical isomer diffusion selectivity. In the process of cognizing the molecular diffusion within oriented, angstrom-scale channels of MOF thin film, we have unveiled a dynamic chemical interaction between the adsorbate (chemical isomers) and the MOF using a combination of kinetic mass uptake experiments and molecular simulation. Leveraging the dynamic chemical interactions, we have reversed the haloalkane (positional) isomer diffusion selectivity, forging a chemical pathway to elevate the overall efficacy of membrane-based chemical separation and selective catalytic reactions.

Positional Isomerism: A Novel Paradigm for Enhancing Iodine Adsorption in Functionalized Metal-Organic Frameworks.

Porous metal-organic frameworks (MOFs) have shown great potential as adsorbents for capturing radioiodine, a major fission product generated during the reprocessing of nuclear fuel. However, studies exploring the correlation between the structure of MOFs and iodine uptake capacity remain notably rare. In this study, we introduce a new strategy for enhancing the iodine adsorption efficiency of MOFs by strategically varying the position of functional groups on the organic linkers. Employing ligand-functionalized UiO-67 MOFs, our findings reveal that ortho-amino substitution of UiO-67-o-NH2, proximal to the node of the dicarboxylate linker, markedly accelerates adsorption kinetics of iodine vapor in comparison to meta-amino substitution of UiO-67-m-NH2, where the amino groups are oriented away from the node. In contrast, UiO-67-m-NH2 exhibits a higher adsorption capacity of 2.19 g/g, compared to 1.91 g/g for UiO-67-o-NH2, attributable to its higher porosity. Furthermore, a competitive I2/H2O vapor adsorption study demonstrated that UiO-67-o-NH2 exhibits faster adsorption kinetics and higher selectivity for iodine in the presence of water vapor compared to UiO-67-m-NH2. Additionally, the crucial influence of positional isomerism on enhancing iodine adsorption has been corroborated through Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. These analyses reveal that the nitrogen atom positioned at the ortho site demonstrates a stronger affinity for iodine molecules compared to the nitrogen atom at the meta site, thereby improving adsorption kinetics.