The adsorption of acidic gaseous pollutants on metal and nonmetallic surface studied by first-principles calculation: A review

Recent advances of SO₂, NO_x, H₂S and CO₂ adsorption on metal and nonmetallic surfaces by first-principles calculation are reviewed, and the common adsorption properties and calculation methods are summarized.     

Recent progress in applications of graphene oxide for gas sensing: A review

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO₂, H₂, NH₃, H₂S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.     

Surface,Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂, H₂S, NO_x, CO₂, CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.     

过渡金属掺杂的g-GaN吸附Cl2和CO气体分子的第一性原理研究

基于密度泛函理论的第一性原理计算, 系统研究了类石墨烯氮化镓(g-GaN)和掺杂过渡金属原子(TM)的 g-GaN 对 Cl₂和CO气体分子的吸附行为。结果表明, Cl₂和 CO在本征 g-GaN上的吸附均为物理吸附, 2个体系的吸附能均为正值, 表明体系不稳定。相反, Cl₂和 CO在 Fe和 Co掺杂的 g-GaN上吸附时的吸附能为负值, 且吸附能较小, 表明吸附体系稳定。通过分析态密度、电荷密度差和能带结构等性质, 可以得出结论:过渡金属原子的引入能有效增强气体分子与 g-GaN之间的相互作用。     

A Soft Copper(II) Porous Coordination Polymer with Unprecedented Aqua Bridge and Selective Adsorption Properties

Herein, the synthesis, crystal structure, and full characterization of a new soft porous coordination polymer (PCP) of ([Cu₂(dmcapz)₂(OH₂)]DMF_1.5)_n ( 1 ) formulation, which is easily obtained in the reaction of CuX₂ (X=Cl, NO₃) salts with 3,5‐dimethyl‐4‐carboxypyrazole (H₂dmcapz) is present. Compound 1 shows a copper(II) dinuclear secondary building unit (SBU), which is supported by two pyrazolate bridges and an unprecedented H₂O bridge. The dinuclear SBUs are further bridged by the carboxylate ligands to build a diamondoid porous network. The structural transformations taking place in 1 framework upon guest removal/uptake has been studied in detail. Indeed, the removal of the bridging water molecules gives rise to a metastable evacuated phase ( 1 b ) that transforms into an extremely stable porous material ( 1 c ) after freezing at liquid‐nitrogen temperature. The soaking of 1 c into water allows the complete and instantaneous recover of the water‐exchanged material ( 1 a′ ). Remarkably, 1 b and 1 c materials possess structural bistability, which results in the switchable adsorptive functions. Therefore, the gas‐adsorption properties of both materials have been studied by means of single‐component gas adsorption isotherms as well as by variable‐temperature pulse‐gas chromatography. Both materials present permanent porosity and selective gas‐adsorption properties towards a variety of gases and vapors of environmental and industrial interest. Moreover, the flexible nature of the coordination network and the presence of highly active convergent open metal sites confer on these materials intriguing gas‐adsorption properties with guest‐triggered framework‐breathing phenomena being observed. The plasticity of Cu^II metal center and its ability to form stable complexes with different coordination numbers is at the origin of the structural transformations and the selective‐adsorption properties of the studied materials.     

过渡金属掺杂的g-GaN吸附Cl2和CO气体分子的第一性原理研究

基于密度泛函理论的第一性原理计算,系统研究了类石墨烯氮化镓(g-GaN)和掺杂过渡金属原子(TM)的g-GaN对Cl₂和CO气体分子的吸附行为。结果表明,Cl₂和CO在本征g-GaN上的吸附均为物理吸附,2个体系的吸附能均为正值,表明体系不稳定。相反,Cl₂和CO在Fe和Co掺杂的g-GaN上吸附时的吸附能为负值,且吸附能较小,表明吸附体系稳定。通过分析态密度、电荷密度差和能带结构等性质,可以得出结论：过渡金属原子的引入能有效增强气体分子与g-GaN之间的相互作用。     

A fiber optic sensor with a metal organic framework as a sensing material for trace levels of water in industrial gases

Industrial gases such as nitrogen, oxygen, argon, and helium are easily contaminated with water during production, transfer and use, because there is a high volume fraction of water in the atmosphere (approximately 1.2% estimated with the average annual atmospheric temperature and relative humidity). Even trace water (<1 parts per million by volume (ppmv) of H₂O, dew point < −76 °C) in the industrial gases can cause quality problems in the process such as production of semiconductors. Therefore, it is important to monitor and to control trace water levels in industrial gases at each supplying step, and especially during their use. In the present study, a fiber optic gas sensor was investigated for monitoring trace water levels in industrial gases. The sensor consists of a film containing a metal organic framework (MOF). MOFs are made of metals coordinated to organic ligands, and have mesoscale pores that adsorb gas molecules. When the MOF, copper benzene-1,3,5-tricarboxylate (Cu-BTC), was used as a sensing material, we investigated the color of Cu-BTC with water adsorption changed both in depth and tone. Cu-BTC crystals appeared deep blue in dry gases, and then changed to light blue in wet gases. An optical gas sensor with the Cu-BTC film was developed using a light emitting diode as the light source and a photodiode as the light intensity detector. The sensor showed a reversible response to trace water, did not require heating to remove the adsorbed water molecules. The sample gas flow rate did not affect the sensitivity. The obtained limit of detection was 40 parts per billion by volume (ppbv). The response time for sample gas containing 2.5 ppmvH₂O was 23 s. The standard deviation obtained for daily analysis of 1.0 ppmvH₂O standard gas over 20 days was 9%. Furthermore, the type of industrial gas did not affect the sensitivity. These properties mean the sensor will be applicable to trace water detection in various industrial gases.     

Sensing Characteristics of a Graphene‐like Boron Carbide Monolayer towards Selected Toxic Gases

By using first‐principles calculations based on density functional theory, we study the adsorption efficiency of a BC₃ sheet for various gases, such as CO, CO₂, NO, NO₂, and NH₃. The optimal adsorption position and orientation of these gas molecules on the BC₃ surface is determined and the adsorption energies are calculated. Among the gas molecules, CO₂ is predicted to be weakly adsorbed on the graphene‐like BC₃ sheet, whereas the NH₃ gas molecule shows a strong interaction with the BC₃ sheet. The charge transfer between the molecules and the sheet is discussed in terms of Bader charge analysis and density of states. The calculated work function of BC₃ in the presence of CO, CO₂, and NO is greater than that of a bare BC₃ sheet. The decrease in the work function of BC₃ sheets in the presence of NO₂ and NH₃ further explains the affinity of the sheet towards the gas molecules. The energy gap of the BC₃ sheets is sensitive to the adsorption of the gas molecules, which implies possible future applications in gas sensors.     

Accurate van der Waals force field for gas adsorption in porous materials

An accurate van der Waals force field (VDW FF) was derived from highly precise quantum mechanical (QM) calculations. Small molecular clusters were used to explore van der Waals interactions between gas molecules and porous materials. The parameters of the accurate van der Waals force field were determined by QM calculations. To validate the force field, the prediction results from the VDW FF were compared with standard FFs, such as UFF, Dreiding, Pcff, and Compass. The results from the VDW FF were in excellent agreement with the experimental measurements. This force field can be applied to the prediction of the gas density (H₂, CO₂, C₂H₄, CH₄, N₂, O₂) and adsorption performance inside porous materials, such as covalent organic frameworks (COFs), zeolites and metal organic frameworks (MOFs), consisting of H, B, N, C, O, S, Si, Al, Zn, Mg, Ni, and Co. This work provides a solid basis for studying gas adsorption in porous materials. © 2017 Wiley Periodicals, Inc.     

An Ultra-Microporous Metal–Organic Framework with Exceptional Xe Capacity

Molecular confinement plays a significant effect on trapped gas and solvent molecules. A fundamental understanding of gas adsorption within the porous confinement provides information necessary to design a material with improved selectivity. In this regard, metal–organic framework (MOF) adsorbents are ideal candidate materials to study confinement effects for weakly interacting gas molecules, such as noble gases. Among the noble gases, xenon (Xe) has practical applications in the medical, automotive and aerospace industries. In this Communication, we report an ultra-microporous nickel-isonicotinate MOF with exceptional Xe uptake and selectivity compared to all benchmark MOF and porous organic cage materials. The selectivity arises because of the near perfect fit of the atomic Xe inside the porous confinement. Notably, at low partial pressure, the Ni–MOF interacts very strongly with Xe compared to the closely related Krypton gas (Kr) and more polarizable CO₂. Further ¹²⁹Xe NMR suggests a broad isotropic chemical shift due to the reduced motion as a result of confinement.     

Locating Gases in Porous Materials: Cryogenic Loading of Fuel‐Related Gases Into a Sc‐based Metal–Organic Framework under Extreme Pressures

An alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single‐crystal of a porous metal–organic framework, is demonstrated to have considerable advantages over other gas‐loading methods when investigating host–guest interactions. Specifically, loading the metal–organic framework Sc₂BDC₃ with liquefied CO₂ at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH₄ at 3–25 kbar demonstrates hyperfilling of the Sc₂BDC₃ and two high‐pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.     

Scorpionate‐Type Coordination in MFU‐4l Metal–Organic Frameworks: Small‐Molecule Binding and Activation upon the Thermally Activated Formation of Open Metal Sites

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU‐4l frameworks. Upon thermal treatment of MFU‐4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are generated selectively. Cu^I‐MFU‐4l prepared in this way was stable under ambient conditions and showed fully reversible chemisorption of small molecules, such as O₂, N₂, and H₂, with corresponding isosteric heats of adsorption of 53, 42, and 32 kJ mol^?1, respectively, as determined by gas‐sorption measurements and confirmed by DFT calculations. Moreover, Cu^I‐MFU‐4l formed stable complexes with C₂H₄ and CO. These complexes were characterized by FTIR spectroscopy. The demonstrated hydride transfer to electrophiles and strong binding of small gas molecules suggests these novel, yet robust, metal–organic frameworks with open metal sites as promising catalytic materials comprising earth‐abundant metal elements.     

Discovery of Complex Binding and Reaction Mechanisms from Ternary Gases in Rare Earth Metal–Organic Frameworks

Understanding the selectivity of metal–organic frameworks (MOFs) to complex acid gas streams will enable their use in industrial applications. Herein, ab initio molecular dynamic simulations (AIMD) were used to simulate ternary gas mixtures (H₂O-NO₂-SO₂) in rare earth 2,5-dihydroxyterephthalic acid (RE-DOBDC) MOFs. Stronger H₂O gas-metal binding arose from thermal vibrations in the MOF sterically hindering access of SO₂ and NO₂ molecules to the metal sites. Gas-gas and gas-linker interactions within the MOF framework resulted in the formation of multiple secondary gas species including HONO, HNO₂, NOSO, and HNO₃⁻. Four gas adsorption sites were identified along with a new de-protonation reaction mechanism not observable through experiment. This study not only provides valuable information on competitive gas binding energies in the MOF, it also provides important chemical insights into transient chemical reactions and mechanisms.     

A DFT study on the structural and electronic properties of small toxic gases on B- and Al- doped C<Subscript>20</Subscript> fullerene

The structural and electronic properties of semiconducting BC₁₉ and AlC₁₉ heterofullerenes as adsorbents for toxic small gas molecules (H₂S and SO₂) are determined by DFT. Structural parameters, energy gaps, natural population analysis, partial density of state, dipole moments, and vibrational frequencies were extracted. The adsorption process and sensitivity to the gases are increased by doping with B or Al. The results show that AlC₁₉ is the most sensitive structure. The good sensing of AlC₁₉ is related to high charge transfer upon gas adsorption. Adsorption of the H₂S on the BC₁₉ has negligible effects on the electronic properties, to be categorized as “harmless adsorption”. H₂S is weakly adsorbed on BC₁₉ and AlC₁₉. The H₂S and SO₂ molecules act as electron donating and electron withdrawing molecules, respectively. Notably, the adsorption processes are highly exothermic. In general, BC₁₉ is more reactive than C20 and AlC₁₉ is the most reactive cage. This provides a theoretical basis to fabricate B- and Al-doped C20-based gas sensors.     

Bioinspired Design of a Giant [Mn86] Nanocage-Based Metal-Organic Framework with Specific CO2 Binding Pockets for Highly Selective CO2 Separation

Adsorption-based removal of carbon dioxide (CO₂) from gas mixtures has demonstrated great potential for solving energy security and environmental sustainability challenges. However, due to similar physicochemical properties between CO₂ and other gases as well as the co-adsorption behavior, the selectivity of CO₂ is severely limited in currently reported CO₂-selective sorbents. To address the challenge, we create a bioinspired design strategy and report a robust, microporous metal–organic framework (MOF) with unprecedented [Mn₈₆] nanocages. Attributed to the existence of unique enzyme-like confined pockets, strong coordination interactions and dipole-dipole interactions are generated for CO₂ molecules, resulting in only CO₂ molecules fitting in the pocket while other gas molecules are prohibited. Thus, this MOF can selectively remove CO₂ from various gas mixtures and show record-high selectivities of CO₂/CH₄ and CO₂/N₂ mixtures. Highly efficient CO₂/C₂H₂, CO₂/CH₄, and CO₂/N₂ separations are achieved, as verified by experimental breakthrough tests. This work paves a new avenue for the fabrication of adsorbents with high CO₂ selectivity and provides important guidance for designing highly effective adsorbents for gas separation.     

Functional Group Modification and Bonding Characteristics of Ti3C2 MXene-Organic Composites from First-Principles Calculations

The unique physical structure and abundant surface functional groups of MXene make the grafted organic molecules exhibit specific electrical and optical properties. This work reports the results of first-principles calculations to investigate the composite systems formed by different organic molecular monomers, namely acrylic acid (AA), acrylamide (AM), 1-aziridineethanol (1-AD) and glucose, and Ti₃C₂ MXene saturated with different functional groups, namely −OH, −O and −F. The results show that the interaction between organic molecules and the MXene surface depends on the type of functional groups of the organic molecules, while the strength of the interaction is determined by the type of surface functional groups and the number of hydrogen bonds. The bare Ti₃C₂ and Ti₃C₂(OH)₂ can readily form strong chemical and hydrogen bonds with AA and AM molecules, leading to strong adsorption energy and a large amount of charge transfer, while the interaction between organic molecules and MXene saturated by −F or −O groups mainly exhibits physical interactions, accompanied by low adsorption energy and a small amount of charge transfer. This research provides theoretical guidance for the synthesis of high-performance MXene organic composite systems.     

Exceptional CO2 Adsorbing Materials under Different Conditions

In this article we discuss those materials that have recorded the highest adsorption capacities for the greenhouse gas CO₂ under ambient conditions as well as at different temperatures and pressures. For convenience, the materials have been categorized under four categories, viz., porous carbon, metal–organic, zeolite and mesoporous silica, and porous organic frameworks. It has been found that the gas adsorption property significantly relies on several factors such as high surface area and pore volume and the presence of N‐, O‐ and S‐containing moieties. The presence of a microporous structure and strong interaction between the CO₂ molecules with the framework through H‐bonding or dipole–quadrupole interactions facilitates adsorption of the gas.     

Microporous Magnesium and Manganese Formates for Acetylene Storage and Separation

Acetylene sorption of microporous metal formates M(HCOO)₂ (M=Mg and Mn) was investigated. Measurements of acetylene sorption at 196, 275, and 298 K showed a Type I isotherm with quick saturation at low pressures, and 50–75 cm³ g^?1 uptake at 1.0 atm. The single‐crystal X‐ray structure analysis of the acetylene‐adsorbed metal formates revealed that acetylene molecules occupy two independent positions in the zigzag channels of the frameworks with a stoichiometry of M(HCOO)₂?1/3C₂H₂, which is consistent with the gas sorption experiments. No specific interaction except van der Waals interactions between the adsorbed acetylene molecules and the walls of the frameworks was found. Sorption properties of other gases, including CO₂, CH₄, N₂, O₂, and H₂, were also investigated. When the temperature was increased to 298 K, the amount of adsorbed acetylene was still above 60 cm³ g^?1 for Mg(HCOO)₂ and 50 cm³ g^?1 for Mn(HCOO)₂, whereas the uptake of other gases decreased substantially. The microporous metal formates may thus be useful not only for the storage of acetylene but also its separation from other gases at room or slightly higher temperatures.     

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal–Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N₂, H₂, CO₂ and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.