Exposed Metal Sites Research Articles

CoCorrole-Functionalized PCN-222 for Carbon Monoxide Selective Adsorption.

The high risk of CO poisoning justifies the need for indoor air quality control and warning systems based on the detection of low concentrations (ppm-ppb) of CO. Cobalt corrole complexes selectively bind CO vs. O2, CO2, N2, opening new fields of applications. By combining the CO chemisorption properties of cobalt corroles with the known sorption capacity of MOFs, we hope to obtain high performance sensing materials for CO detection. In addition, the exposed metal sites of MOFs lead to CO2 physisorption, allowing the co-detection of CO and CO2. In this work, PCN-222, a stable Zr-based MOF made from Ni(TCPP) with natural vacancies, has been used as a porous matrix for the grafting of electron-poor metallocorroles. The materials were characterized by powder XRD, SEM and optical microscopy, BET analyses and gas adsorption measurements at 298 K. No degradation of the crystalline structure of PCN-222 was observed. At 1 atm, the adsorbed CO(g) volumes measured for the best materials were 12.15 cm3 g-1 and 14.01 cm3 g-1 for CoCorr2@PCN-222 and CoCorr3@PCN-222 respectively, and both materials exhibited high CO chemisorption and selectivity against O2, N2, and CO2 at low pressure due to the highest energy of the chemisorption process vs physisorption.

Highly effective Pt-Pd/ZSM-22 catalysts prepared by the room temperature electron reduction method for the n-hexadecane hydroisomerization

The development of highly effective bifunctional catalysts for n-hexadecane hydroisomerization is still essential to produce second-generation biodiesel. Herein, a Pt-Pd/ZSM-22-G (abbreviated as Pt-Pd/Z22-G) bimetallic catalyst was prepared by employing a room temperature electron reduction (RTER) method with glow discharge as the electron source. As a contrast, a series of Pt/Z22-H, Pd/Z22-H and Pt-Pd/Z22-H catalysts were prepared by the conventional hydrogen reduction method. The Pt-Pd/Z22-G catalyst reveals more exposed metal sites, larger CMe/CH+ values and an enhanced distribution of Pt-Pd(111) facets compared with the Pt/Z22-H, Pd/Z22-H and Pt-Pd/Z22-H catalysts. These modifications are originated from the stronger electron interactions and the smaller metal nanoparticles because of the effects of highly energetic reducing electrons. The n-hexadecane hydroisomerization results show that the iso-hexadecane yield over the Pt-Pd/Z22-G catalyst is 82.9%, which is the highest among four investigated catalysts in this work. This phenomenon occurs because more exposed Pt-Pd(111) facets and larger CMe/CH+ ratios are beneficial for the adsorption and hydrogenation of iso-alkene intermediates at metal sites to increase the iso-alkanes yield based on density functional theory (DFT) calculations. Furthermore, the iso-alkanes yield over the Pt-Pd/Z22-G catalyst also keeps steady after long-term tests for 120 h because of the limited metal aggregation and carbon deposition.

Impedimetric identification of heteroMOFs modified porous nickel foam self-supports as robust electrodes for CA 19-9 pancreatic tumor marker in human serum

An impedance-based electrochemical immunosensor for direct and binder-free target recognition of carbohydrate antigen (CA 19-9) tumor biomarker for the early diagnosis of pancreatic cancer was reported. This study uses an in situ solvothermal process to develop a simple method for the one-pot construction of Ni-Fe heteroMOFs (HFNMOFs,) self-supported on conductive nickel foam (NicFm). Utilizing HFNMOFs@NicFm developed in this study, the electrochemical performance of the biosensor was studied for the first time. The as-prepared mixed MOFs decorated NicFm electrodes displayed potential electrochemical outputs. They demonstrated the potential for developing binder-free non-enzymatic immunosensors with wide linear limits, high selectivity, stability, and excellent sensitivity. The bifunctional MOFs exhibit improved electrochemical conductivity owing to the highly exposed metal sites and hierarchical porosity. The changes in the charge transfer resistance present a linear response against increasing target concentrations. The immunosensor developed in this study exhibited its detection limit of 2.56 U/mL. Further, the bioelectrodes produced were validated against real human spiked serum with good results of percentage recovery, indicating the immunosensor is well-suitable to be tested in real-time analysis.

Remediation of Alizarin Yellow GG from aqueous sources by Quasi-UiO-66; Box-Bencken design optimization and adsorption cherctization

This study develops a defect-engineered quasi-UiO-66 MOF adsorbent with enhanced performance for removing the organic dye Alizarin Yellow GG. Introducing structural defects and controlled thermal treatment to generate exposed metal sites significantly improved adsorption capacity, providing insights for designing advanced MOF adsorbents. This research successfully developed a controlled thermal partial ligand elimination. This process introduced structural modifications, forming additional open zirconium sites and hierarchical pores within the framework. These unique features contributed to the superior adsorption performance of the Q-UiO-66 adsorbent towards the organic dye Alizarin Yellow GG, exhibiting an unprecedented uptake capacity of 398.2 mg.g−1. Comprehensive characterization techniques, including FT-IR, FE-SEM, BET, XRD, and elemental analyses, were employed to understand the adsorbent’s properties. Furthermore, the optimization of the adsorption process was done using the Box-Behnken design approach, and the adsorption behavior was well-described by the Langmuier-Freundlich isotherm and pseudo-second-order kinetic models. The findings highlight the significant potential of this novel, defect-engineered Q-UiO-66 MOF adsorbent for effective dye removal applications.

Smartphone-assisted portable amperometric immunosensor based on CoMn-LDH for the visualized detection of tylosin

The precise and rapid analysis of antibiotics containing in food stuffs is essential for guaranteeing the safety of food products and drinking water. Thus, the sensitive determination of antibiotics is highly urgent. Herein, CoMn-LDH derived from bimetallic CoMn-ZIF was utilized as the sensitive material and ink for modifying the screen-printed electrode (SPE) to manufacture the visualized amperometric immunosensor to detect tylosin (TYL) with the smartphone assistance. Benefiting to the features of CoMn-ZIF, the achieved CoMn-LDH exhibited large specific surface area and pore size, abundant oxygen vacancies, rich and exposed metal sites, high functionality, and core–shell nanostructure with thin shell thickness. Thus, CoMn-LDH illustrated the high adhesive force with the SPE, the improved affinity toward the TYL-targeted antibody and fast electronic response. Thereby, the manufactured CoMn-LDH-based amperometric immunosensor displayed the wide linear range from 1.0 fg mL−1 to 1.0 ng mL−1 with an ultralow detection limit of 0.89 fg mL−1 toward TYL. Also, the developed amperometric immunosensor has demonstrated the good sensing ability of high selectivity, good reproducibility and stability, and accepted regenerated performance, as well as the great potential for the electronic visualized analysis of TYL containing in diverse real samples. This work has provided a new amperometric immunosensing strategy for the efficient analysis of antibiotics in food samples through the visual way.

Nanopore-regulated in situ polymerization for synthesis of homogeneous heparan sulfate with low dispersity

The biological activities of heparan sulfate (HS) are intimately related to their molecular weights, degree and pattern of sulfation and homogeneity. The existing methods for synthesizing homogeneous sugar chains of low dispersity involve multiple steps and require stepwise isolation and purification processes. Here, we designed a mesoporous metal-organic capsule for the encapsulation of glycosyltransferase and obtained a microreactor capable of enzymatically catalyzing polymerization reactions to prepare homogeneous heparosan of low dispersity, the precursor of HS and heparin. Since the sugar chain extension occurs in the pores of the microreactor, low molecular weight heparosan can be synthesized through space-restricted catalysis. Moreover, the glycosylation co-product, uridine diphosphate (UDP), can be chelated with the exposed metal sites of the metal-organic capsule, which inhibits trans-cleavage to reduce the molecular weight dispersity. This microreactor offers the advantages of efficiency, reusability, and obviates the need for stepwise isolation and purification processes. Using the synthesized heparosan, we further successfully prepared homogeneous 6-O-sulfated HS of low dispersity with a molecular weight of approximately 6 kDa and a polydispersity index (PDI) of 1.032. Notably, the HS generated exhibited minimal anticoagulant activity, and its binding affinity to fibroblast growth factor 1 was comparable to that of low molecular weight heparins.

Effect of Interlayer Anions on NiFe Layered Double Hydroxides for Catalytic Ozone Decomposition.

NiFe layered double hydroxides (NiFe-LDH) exhibited an outstanding performance and promising application potential for removing ozone. However, the effect of interlayer anions on ozone removal remains ambiguous. Here, a series of NiFe-LDH with different interlayer anions (F-, Cl-, Br-, NO3-, CO32-, and SO42-) were prepared to investigate the effect of the interlayer anion on ozone removal for the first time. It was found that the interlayer anions are a key factor affecting the water resistance of the NiFe-LDH catalyst under moist conditions. NiFe-LDH-CO32- exhibited the best water resistance, which was much better than that of NiFe-LDH containing other interlayer anions. The in situ DIRFTS demonstrates that the carbonates in the interlayer of NiFe-LDH-CO32- will undergo coordination changes through the interaction with water molecules under moist conditions, exposing new metal sites. As a result, the newly exposed metal sites could activate water molecules into hydroxyl groups that act as active sites for catalyzing ozone decomposition. This work provides a new insight into the interlayer anions of LDH, which is important for the design and development of LDH catalysts with excellent ozone removal properties.

Glassy State Hydroxide Materials for Oxygen Evolution Electrocatalysis.

Hydroxides are the archetype of layered crystals with metal-oxygen (M-O) octahedron units, which have been widely investigated as oxygen evolution reaction (OER) catalysts. However, the better crystallinity of hydroxide materials, the more perfect octahedral symmetry and atomic ordering, resulting in the less exposed metal sites and limited electrocatalytic activity. Herein, a glassy state hydroxide material featuring with short-range order and long-range disorder structure is developed to achieve high intrinsic activity for OER. Specifically, a rapid freezing point precipitation method is utilized to fabricate amorphous multi-component hydroxide. Owing to the freezing-point crystallization environment and chaotic M-O (M = Ni/Fe/Co/Mn/Cr etc.) structures, the as-fabricated NiFeCoMnCr hydroxide exhibit a highly-disordered glassy structure, as-confirmed by X-ray/electron diffraction, enthalpic response, and pair distribution function analysis. The as-achieved glassy-state hydroxide materials display a low OER overpotential of 269mV at 20mA cm-2 with a small Tafel slope of 33.3mV dec-1, outperform the benchmark noble-metal RuO2 catalyst (341mV, 84.9mV dec-1) . Operando Raman and density functional theory studies reveal that the glassy state hydroxide converted into disordered active oxyhydroxide phase with optimized oxygen intermediates adsorption under low OER overpotentials, thus boosting the intrinsic electrocatalytic activity.

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity.

The implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface-induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA-15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched-row structure in softer Pt nanoparticles, which was identified by microscopic and surface-sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt-CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity

AbstractThe implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface‐induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA‐15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched‐row structure in softer Pt nanoparticles, which was identified by microscopic and surface‐sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt−CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Atomically precise Cu41 clusters as model catalysts: Open metal sites matter

Although the significance of open metal sites for controlling metal nanoparticle catalysis have been well-documented, the direct identification of authentic active sites remains a grand challenge. In this work, we report a new couple of “isostructural” copper nanoclusters, namely, [Cu41Cl2(2-F-C6H4S)12(CF3COO)6(PPh3)6H19]2- (1H) and [Cu41(2,5-di-Methyl-C6H3S)12(BO3)3Cl3(PPh3)6H19] (2H), from which heavy dependence of catalytic activity on coordinatively unsaturated metal sites is directly visualized. Both clusters have been characterized by various advanced techniques including high resolution mass spectroscopy (MS), ultra-violet visible spectroscopy (UV-Vis), nuclear magnetic resonance (NMR), density functional theory (DFT) calculations and X-ray single crystal diffraction. The clusters adopt a core-shell arrangement, in which the Cu29 core is grown via the face-fusion of three Cu13 units. Notably, such Cu13-based growth mode is observed for the first time for copper nanoclusters. The main difference of the two Cu41 clusters lies in the presence or absence of two additional chlorides on the surface, which in turn heavily control the exposure of metal sites. 2H with more open metal sites exhibits, as envisioned, much higher activity than 1H in catalytic hydrogenation reaction (6-fold increase in rate constant). This work thus not only represent a mode system to directly visualize the true active sites of metal nanoclusters, but also provides the atomistic evidence on highlighting the importance of exposed metal sites for controlling performance of metal nanocatalysts.

Regulation of surface hydrophobicity and metal dispersion on Pt-based catalyst for the boosted hydrodeoxygenation of guaiacol into bio-hydrocarbons

Transformation of lignin-derived feedstocks into valuable hydrocarbons is of significant importance but still challenging. This work investigates the hydrodeoxygenation (HDO) of lignin compound guaiacol on Pt-based catalysts with improved surface hydrophobicity and metal dispersion for the first time. The efficiency of guaiacol hydrodeoxygenation into hydrocarbons over Nb2O5-modified Pt-based catalyst reaches 51 %, nearly doubling that over unmodified Pt/γ-Al2O3 catalyst (25 %). On the one hand, Nb2O5 causes the decrease of hydroxyl groups on catalyst, leading to the enhanced hydrophobicity of catalysts, which increases the reaction rate of guaiacol into hydrocarbons on unit of Pt metal sites. On the other hand, Nb2O5 strengthens the metal-support interaction, which minimizes the size of Pt nanoparticles, providing more exposed metal sites, thus leading to a higher hydrocarbons yield on catalyst. However, excess introduction of Nb2O5 leads to partial coverage of Pt metal sites, resulting in decreased yield of hydrocarbons. Furthermore, the routes of guaiacol HDO reaction over modified Pt-based catalysts are investigated. This work further illustrates the influence of catalyst surface properties on the HDO process of lignin compound, and thereby provides a promising approach for adjustment of catalyst surface properties in order to promote water-liberated reactions.

High‐Spin and Reactive Fe13 Cluster with Exposed Metal Sites

AbstractAtomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*4W4Fe13S12, displaying a Fe13 core with M−M bonds that has precedent only in main group and late metal chemistry. M13 clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*4W4Fe13S12 displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.

High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites.

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*4 W4 Fe13 S12 , displaying a Fe13 core with M-M bonds that has precedent only in main group and late metal chemistry. M13 clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*4 W4 Fe13 S12 displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.

Array-Based Clusters of Copper with Largely Exposed Metal Sites for Promoting Catalysis

Array-Based Clusters of Copper with Largely Exposed Metal Sites for Promoting Catalysis

Design Principles for Using Amphiphilic Polymers To Create Microporous Water.

Aqueous dispersions of microporous nanocrystals with dry, gas-accessible pores─referred to as "microporous water"─enable high densities of gas molecules to be transported through water. For many applications of microporous water, generalizable strategies are required to functionalize the external surface of microporous particles to control their dispersibility, stability, and interactions with other solution-phase components─including catalysts, proteins, and cells─while retaining as much of their internal pore volume as possible. Here, we establish design principles for the noncovalent surface functionalization of hydrophobic metal-organic frameworks with amphiphilic polymers that render the particles dispersible in water and enhance their hydrolytic stability. Specifically, we show that block co-polymers with persistence lengths that exceed the micropore aperture size of zeolitic imidazolate frameworks (ZIFs) can dramatically enhance ZIF particle dispersibility and stability while preserving porosity and >80% of the theoretical O2 carrying capacity. Moreover, enhancements in hydrolytic stability are greatest when the polymer can form strong bonds to exposed metal sites on the external particle surface. More broadly, our insights provide guidelines for controlling the interface between polymers and metal-organic framework particles in aqueous environments to augment the properties of microporous water.

Synthesis of vanadium metal-organic framework, characterization, and study of electrochemical properties for using in supercapacitor and oxygen evolution reaction

Transition-metal (V) based metal–organic framework drew a lot of attention for use in upcoming generation electrochemical energy storage devices (Supercapacitors) and energy production during oxygen evolution reaction (OER). The large surface area, the exposed metal sites, and structural controllability make MOFs preferred heterogeneous catalysts for various catalytic reactions. Due to their tunable structures and high porosity, MOFs have shown favorable applications as electrode materials for supercapacitors. This investigation synthesized V-MOF nanorods by hydrothermal and microwave methods. The NiCo2S4/V-MOF@rGO synthesized offered a maximum specific capacitance of 1844F/g at a current density of 1.0 Ag−1. Moreover, an asymmetric supercapacitor NiCo2S4/V-MOF@rGO//AC exhibits a high energy density of 800 Wh kg−1 at a power density of 61.10 W kg−1 and 98 % of its capacitance retention after 3000 cycles. In another part of this study, the V-MOF synthesized in the previous step showed an excellent electrocatalytic performance toward the OER. Furthermore, by compositing it with graphene sheets and doping it with phosphorus as a heteroatom, its electrocatalytic activity toward OER was further enhanced, showing160 mV overpotential at a current density of 10 mA cm−2 in a 3.0 M KOH after 1000 sweeps aqueous solution. This researches present a simple, efficient, and high-performance electrocatalyst for OER and active material for supercapacitors.

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Catalytic activation of peroxymonosulfate using MnO2@quasi-MOF for singlet oxygen mediated degradation of organic pollutants in water

The properties of metal organic frameworks (MOFs), surface area, porosity, and functionality make them an ideal material for heterogenous catalysis. We developed a MnO2@quasi-MOF (MnO2@q-MOF) catalyst by incorporating MnO2 into a MIL-53 (Fe) structure with reduction of KMnO4 to MnO2 followed by a mild heat treatment at 300 °C. MnO2@q-MOF showed higher exposed metal sites due to thermally induced decarboxylation and higher activity because of in situ MnO2 formation while preserving the porosity and crystalline structure of MIL-53 (Fe). We activated potassium peroxymonosulfate (PMS) using our MnO2@q-MOF catalyst for decomposition of methylene blue and HEPES in water. The MnO2@q-MOF catalyst outperformed both MIL-53 (Fe) and unsupported MnO2 in the degradation of dye and was reusable. The primary mechanism of PMS activation was revealed to be a singlet oxygen (1O2) mediated process.

Promoting effects of residual poly(vinyl alcohol) capping agent on the activity and chemoselectivity of Pt/Al2O3 for catalytic hydrogenation

Capping agents are key to the colloidal synthesis of size- and shape-controlled nanoparticles (NPs) for catalysis. The capping agents on the NP surface are generally removed after synthesis to expose the catalytically active sites. However, the total removal of the capping agents results in the loss of the ability of the capped NPs to regulate the adsorption mode of the reactants, which can, in some cases, decrease selectivity. Here, poly(vinyl alcohol) (PVA)-capped Pt/Al2O3 are prepared and the amount of PVA capping agent remaining on the NP surfaces is controlled by heat treatment to take full advantage of capped NPs. Compared to a surface-clean Pt catalyst, those covered in PVA residues having partially exposed metal sites show enhanced activity and selectivity for the hydrogenation of acetophenone to 1-phenylethanol. Detailed characterization reveals that the promoting effects of the residual PVA are attributed to the combined contributions of electronic modification of the Pt surface, selective exposure of highly active Pt defect sites, and steric hindrance with the aromatic ring of acetophenone. This study offers insights into the roles of residual capping agents in heterogeneous catalysis and provides a new strategy for enhancing the performance of nanocatalysts using capped NPs.