Photochemical conversion of CO2 to CO by a Re complex: theoretical insights into the formation of CO and HCO3− from an experimentally detected monoalkyl carbonate complex

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Microtubular cellulose-derived kapok fibre as a solid electron donor for boosting photocatalytic H2O2 production over C-doped g-C3N4 hybrid complexation

Cellulose continues to play an important and emerging role in photocatalysis, and its favourable properties, such as electron-rich hydroxyl groups, could enhance the performance of photocatalytic reactions. For the first time, this study exploited the kapok fibre with microtubular structure (t-KF) as a solid electron donor to enhance the photocatalytic activity of C-doped g-C₃N₄ (CCN) via ligand-to-metal-charge-transfer (LMCT) to improve hydrogen peroxide (H₂O₂) production performance. As confirmed by various characterisation techniques, the hybrid complex consisting of CCN grafted on t-KF was successfully developed in the presence of succinic acid (SA) as a cross-linker via a simple hydrothermal approach. The complexation formation between CCN and t-KF results in the CCN-SA/t-KF sample displaying a higher photocatalytic activity than pristine g-C₃N₄ to produce H₂O₂ under visible light irradiation. The enhanced physicochemical and optoelectronic properties of CCN-SA/t-KF imply that the LMCT mechanism is crucial in improving photocatalytic activity. This study promotes utilising the unique t-KF material's properties to develop a low-cost and high-performance cellulose-based LMCT photocatalyst.

Chemical modification of dimethylpolysiloxane for enhancement of CO2 binding enthalpy

The intermittent increase in CO₂ concentration in the atmosphere is a serious problem that contributes to climate change; the combustion of fossil fuels produces the majority of CO₂, and technology is needed to capture it efficiently. Various CO₂ capture materials have been developed so far. Membrane separation, in particular, has an advantage over other capture technologies due to its ease of use. Poly(dimethylsiloxane) (PDMS) has been widely used as a membrane material for CO₂ capture because of its high gas permeability. However, despite their high CO₂ permeance, PDMS membranes are still in their infancy, especially regarding CO₂ selectivity due to the weak interaction between CO₂ and PDMS. Here we evaluated the CO₂ interaction with the PDMS chain at the atomic scale and attempted to improve the CO₂ affinity of the PDMS chain using density functional theory (DFT). Specifically, we substituted elements in the Si-O framework with other elements and substituted the methyl groups with other chemical groups, and incorporated metallic elements such as Mg and Ti. All the chemical modifications by main group elements resulted in physisorption, but chemisorption of CO₂ was observed in PDMS incorporating metallic elements. Since several modes of CO₂ binding were observed in PDMS with incorporated metal elements, the binding enthalpy and binding mode were analyzed. As a result of various chemical modifications, it was found that introducing earth metal elements into PDMS was the most effective way to enhance the interaction between PDMS and CO₂.