CO2 and H2 adsorption on 3D nitrogen-doped porous graphene: Experimental and theoretical studies

Density functional theory study of doped coronene and circumcoronene as anode materials in lithium-ion batteries

Density functional theory calculations are carried out to investigate the adsorption properties of Li+ and Li on twenty-four adsorbents obtained by replacement of C atoms of coronene (C24H12) and circumcoronene (C54H18) by Si/N/BN/AlN units. The molecular electrostatic potential (MESP) analysis show that such replacements lead to an increase of the electron-rich environments in the molecules. Li+ is relatively strongly adsorbed on all adsorbents. The adsorption energy of Li+ (Eads-1) on all adsorbents is in the range of - 42.47 (B12H12N12) to - 66.26 kcal/mol (m-C22H12BN). Our results indicate a stronger interaction between Li+ and the nanoflakes as the deepest MESP minimum of the nanoflakes becomes more negative. A stronger interaction between Li+ and the nanoflakes pushes more electron density toward Li+. Li is weakly adsorbed on all adsorbents when compared to Li+. The adsorption energy of Li (Eads-2) on all adsorbents is in the range of - 3.07 (B27H18N27) to - 47.79 kcal/mol (C53H18Si). Assuming the nanoflakes to be an anode for the lithium-ion batteries, the cell voltage (Vcell) is predicted to be relatively high (> 1.54 V) for C24H12, C12H12Si12, B12H12N12, C27H18Si27, and B27H18N27. The Eads-1 data show only a small variation compared to Eads-2, and therefore, Eads-2 has a strong effect on the changes in Vcell.

Molecularly dispersed nickel complexes on N-doped graphene for electrochemical CO2 reduction

In this work, new hybrid catalysts based on molecularly dispersed nickel complexes on N-doped graphene were developed for electrochemical CO2 reduction (ECR). Nickel(II) complexes (1-Ni, 2-Ni), and a new crystal structure ([2-Ni]Me), featuring N4-Schiff base macrocycles, were synthesized and investigated for their potential in ECR. Cyclic voltammetry (CV) in NBu4PF6/CH3CN solution demonstrated that the nickel complexes bearing N-H groups (1-Ni and 2-Ni) showed a substantial current enhancement in the presence of CO2, while the absence of N-H groups ([2-Ni]Me) resulted in an almost unchanged voltammogram. This indicated the necessity of the N-H functionality towards ECR in aprotic media. All three nickel complexes were successfully immobilized on nitrogen-doped graphene (NG) via non-covalent interactions. All three Ni@NG catalysts exhibited satisfactory CO2-to-CO reduction in aqueous NaHCO3 solution with the faradaic efficiency (FE) of 60-80% at the overpotential of 0.56 V vs. RHE. The ECR activity of [2-Ni]Me@NG also suggested that the N-H moiety from the ligand is less important in the heterogeneous aqueous system owing to viable hydrogen-bond formation and proton donors from water and bicarbonate ions. This finding could pave the way for understanding the effects of modifying the ligand framework at the N-H position toward fine tuning the reactivity of hybrid catalysts through molecular-level modulation.

One-Step Synthesis of 3D Pore-Structured Adsorbent by Cross-Linked PEI and Graphene Oxide Sheets and Its Application in CO2 Adsorption

In this study, a one-step method of polyethylenimine (PEI) cross-linking graphene oxide (GO) was used to prepare a 3D pore-structured adsorbent with abundant amine groups for chemisorption of CO₂. The cross-linking of PEI with GO sheets and the vacuum freeze-drying step are the keys to the formation of the 3D pore structure. The results of characterization analysis revealed that the as-prepared adsorbent had a 3D porous structure rich in amine groups. Besides, the adsorption/desorption test showed that the prepared adsorbent has excellent and stable adsorption performance, and the maximum CO₂ adsorption capacity is 2.18 mmol/g at 343 K and 10 vol % CO₂. Moreover, the adsorption kinetics analysis indicated that the adsorption process was dominated by homogeneous adsorption, and the adsorbent had a strong affinity with CO₂. Finally, the correlation analysis shows that the kinetic constants obtained by the Avrami model simulation can be effectively used for the actual CO₂ adsorption process design.

Optimizing the Properties of Hybrids Based on Graphene Oxide for Carbon Dioxide Capture

The reduction of CO2 emissions and its elimination from the atmosphere has become one of the major problems worldwide, since CO2 is the main cause of the greenhouse effect and climate change. In recent years, a great number of carbonaceous materials that can be used as CO2 adsorbents have been synthesized. The strategy is usually to synthesize the materials and determine their adsorption capacity without studying previously the factors that influence this capacity. In this work, different properties of the adsorbents are analyzed to study their influence on the CO2 adsorption capacity. For this purpose, 10 adsorbents have been synthesized using different strategies and characterized with X-ray photoelectron spectroscopy, X-ray diffraction, and micro-Raman spectroscopy. The percentage of sp2 carbons, the position of the D + D' peak of the second-order Raman spectrum, the micropore volume, and the grain size of the C sp2 domains have been related to the amount of CO2 adsorbed by the adsorbents. The results confirm a linear relationship between the volume of the micropores and the CO2 uptake and it proves that CO2 retention is favored in those materials that, in addition to having a high volume of micropores, also have low grain size of C.

Theoretical Investigation of Carbon Dioxide Adsorption on Li+-Decorated Nanoflakes

Recently, the capture of carbon dioxide, the primary greenhouse gas, has attracted particular interest from researchers worldwide. In the present work, several theoretical methods have been used to study adsorption of CO2 molecules on Li+-decorated coronene (Li+@coronene). It has been established that Li+ can be strongly anchored on coronene, and then a physical adsorption of CO2 will occur in the vicinity of this cation. Moreover, such a decoration has substantially improved interaction energy (Eint) between CO2 molecules and the adsorbent. One to twelve CO2 molecules per one Li+ have been considered, and their Eint values are in the range from -5.55 to -16.87 kcal/mol. Symmetry-adapted perturbation theory (SAPT0) calculations have shown that, depending on the quantity of adsorbed CO2 molecules, different energy components act as the main reason for attraction. AIMD simulations allow estimating gravimetric densities (GD, wt.%) at various temperatures, and the maximal GDs have been calculated to be 9.3, 6.0, and 4.9% at T = 77, 300, and 400 K, respectively. Besides this, AIMD calculations validate stability of Li+@coronene complexes during simulation time at the maximum CO2 loading. Bader's atoms-in-molecules (QTAIM) and independent gradient model (IGM) techniques have been implemented to unveil the features of interactions between CO2 and Li+@coronene. These methods have proved that there exists a non-covalent bonding between the cation center and CO2. We suppose that findings, derived in this theoretical work, may also benefit the design of novel nanosystems for gas storage and delivery.

Porosity of Rigid Dendrimers in Bulk: Interdendrimer Interactions and Functionality as Key Factors

The porous structure of second- and third-generation polyphenylene-type dendrimers was investigated by adsorption of N₂, Ar, and CO₂ gases, scanning electron microscopy and small-angle X-ray spectroscopy. Rigid dendrimers in bulk are microporous and demonstrate a molecular sieve effect. When using CO₂ as an adsorbate gas, the pore size varies from 0.6 to 0.9 nm. This is most likely due to the distances between dendrimer macromolecules or branches of neighboring dendrimers, whose packing is mostly realized due to intermolecular interactions, in particular, π-π interactions of aromatic fragments. Intermolecular interactions prevent the manifestation of the porosity potential inherent to the molecular 3D structure of third-generation dendrimers, while for the second generation, much higher porosity is observed. The maximum specific surface area for the second-generation dendrimers was 467 m²/g when measured by CO₂ adsorption, indicating that shorter branches of these dendrimers do not provide dense packing. This implies that the possible universal method to create porous materials for all kinds of rigid dendrimers is by a placement of bulky substituents in their outer layer.