Silicene catalysts for CO2 hydrogenation: the number of layers controls selectivity

Exploring the catalytic performance of ligand-functionalized Cu-BTC paddlewheels in carboxylative cyclization of propargyl alcohol with CO2: DFT and SISSO insights

The M06-L functional with the 6-31G(d,p) and SDD ECP basis sets, was used to investigate the structure and electronic properties of defective linker-coordinated paddlewheel complexes (Cu-BTC(L1-L4)) in the catalytic conversion of propargyl alcohol (PA) and CO2 into cyclic carbonate. Two catalytic processes are proposed based on the different PA adsorption modes at the Cu center. The reaction proceeds via adsorption by the hydroxyl group in two sequential steps: PA/CO2 activation and cyclization. This pathway is proposed as the dominant process in the Cu-BTC and Cu-BTC(L1-L3) systems. However, only Cu-BTC(L3) and Cu-BTC(L4), which exhibit stronger electron back-donation compared to the other systems, effectively promote the catalytic process via PA adsorption through its alkyne bond. In this latter mode, the reaction proceeds through three consecutive steps: PA/CO2 activation, ring closure, and H-transfer. Compared to pristine Cu-BTC, Cu-BTC(L3) and Cu-BTC(L4) are proposed as more efficient catalysts for the carboxylative cycloaddition of CO2 with PA. The rate-determining step for the reaction on these two systems is the PA/CO2 activation via the latter mechanism. This step has an activation free energy of 16.7 kcal/mol and 15.0 kcal/mol for the Cu-BTC(L3) and Cu-BTC(L4). The SISSO model reveals the role of the Cu center in activating PA and stabilizing the generated intermediate, thereby lowering the activation free energy for PA/CO2 activation.

Transition Metal Anchored Novel Holey Boron Nitride Analogues as Single-Atom Catalysts for the Hydrogen Evolution Reaction

The increasing global energy demand and environmental pollution necessitate the development of alternative, sustainable energy sources. Hydrogen production through electrochemical methods offers a carbon-free energy solution. In this study, we have designed novel boron nitride analogues (BNyne) and investigated their stability and electronic properties. Furthermore, the incorporation of transition metals (TM) at holey sites in these analogues was explored, revealing their potential as promising electrocatalysts for the hydrogen evolution reaction (HER). The inclusion of transition metals significantly enhances their structural stability and electronic properties. The TM-anchored BNynes exhibit optimal Gibbs free energy changes (ΔGH) for effective HER performance. Additionally, the favorable alignment of d-band centers near the Fermi level supports efficient hydrogen adsorption. Machine learning models, particularly the Random Forest model, have also been employed to predict ΔGH values with high accuracy, capturing the complex relationships between material properties and HER efficiency. This dual approach underscores the importance of integrating advanced computational techniques with material design to accelerate the discovery of effective HER catalysts. Our findings highlight the potential of these tailored boron nitride analogues to enhance electrocatalytic applications and improve HER efficiency.

p-block germanenes as a promising electrocatalysts for the oxygen reduction reaction

The oxygen reduction reaction (ORR), a pivotal process in hydrogen fuel cells crucial for enhancing fuel cell performance through suitable catalysts, remains a challenging aspect of development. This study explores the catalytic potential of germanene on Al (111), taking advantage of the successful preparation of stable reconstructed germanene layers on Al (111) and the excellent catalytic performance exhibited by germanium-based nanomaterials. Through first-principles calculations, we demonstrate that the O2 molecule can be effectively activated on both freestanding and supported germanene nanosheets, featuring kinetic barriers of 0.40 and 0.04 eV, respectively. The presence of the Al substrate not only significantly enhances the stability of the reconstructed germanene but also preserves its exceptional ORR catalytic performance. These theoretical findings offer crucial insights into the substrate-mediated modulation of germanene stability and catalytic efficiency, paving the way for the design of stable and efficient ORR catalysts for future applications.

A first-principles study of electro-catalytic reduction of CO2 on transition metal-doped stanene

Employing first-principles calculations based on density functional theory, this work examines the activity of 3d transition metal-doped stanene for electro-catalytic CO2 reduction through the first two electron transfer steps to CO. Our results related to CO2 activation, the first and a crucial step of the reduction process revealed that, among the entire 3d transition metal row studied, only Ti- and Fe-doped stanene can bind and significantly activate the CO2 molecule, while the rest of the TM single atoms are inert in activating the molecule. The activation of the CO2 molecule on Ti- and Fe-doped stanene has been observed in the presence of water as well. In addition, the formation of OCHO has been observed to be energetically preferred over COOH formation as a reaction intermediate, indicating the preference for the formate path of the reduction reaction. Furthermore, despite the strong adsorption of H2O on the catalyst surface, the presence of water seems to enhance CO2 adsorption on the catalysts, contrary to what has been observed recently in graphene-based catalysts. Finally, our difference charge density and the Bader charge calculations reveal that the ability of Ti- and Fe-doped stanene in activating the CO2 molecule and their potential catalytic activity for CO2 reduction is to be attributed to the charge transfer between the catalyst and the molecule, providing new insights into the rational design of 2D catalysts beyond graphene.

Enhancing CO2 reduction through the catalytic effect of a novel silicon haeckelite-inspired 2D material

We propose a novel 2D material based on silicon haeckelite (Hck), whose structure contains a silicon atom arranged in a periodic pattern of pentagons and heptagons. Stacking the two layers gives rise to a planar geometry of the layers that compose it. This new structure presents a semiconductor character with a band gap of 0.17 eV. Furthermore, we studied CO2 reduction using molecular hydrogen to form formic acid, carbon monoxide, formaldehyde, methanol, and methane. All these have been studied theoretically at the Grimme D3BJ corrected TPSS/def2-SVP level. A massive biflake containing 132 Si atoms was used to model the Hck surface. According to the results, CO2 capture with Hck is a spontaneous step; in contrast, the same process for silicene mono- and bi-flakes studied previously was endergonic. After the capture of CO2, the addition of H2 to the substrate passes through an intermediate containing a Si-H bond. The formation of Si-H intermediates is the origin of the catalytic effect, facilitating H2 dissociation and acting as the hydrogen atom donor for the substrate. These intermediates are transformed by adding hydrogen atoms and losing water molecules, producing formic acid and formaldehyde as the most probable products, with rate-controlling steps of 29.2 and 27 kcal mol-1, whose values were less than those exhibited by the silicene biflake. This means that the silicon haeckelite biflake presents better catalytic activity than the silicene biflake. The results show that the novel 2D silicon hackelite material has remarkable potential for CO2 capture and reduction. The theoretical analysis of this innovative 2D structure provides valuable insights into the potential applications of silicene-based materials.

The marriage of Xenes and hydrogels: Fundamentals, applications, and outlook

Hydrogels have blossomed as superstars in various fields, owing to their prospective applications in tissue engineering, soft electronics and sensors, flexible energy storage, and biomedicines. Two-dimensional (2D) nanomaterials, especially 2D mono-elemental nanosheets (Xenes) exhibit high aspect ratio morphology, good biocompatibility, metallic conductivity, and tunable electrochemical properties. These fascinating characteristics endow numerous tunable application-specific properties for the construction of Xene-based hydrogels. Hierarchical multifunctional hydrogels can be prepared according to the application requirements and can be effectively tuned by different stimulation to complete specific tasks in a spatiotemporal sequence. In this review, the synthesis mechanism, properties, and emerging applications of Xene hydrogels are summarized, followed by a discussion on expanding the performance and application range of both hydrogels and Xenes.

Theoretical study of p-block metal–nitrogen–carbon single-atom catalysts for the oxygen reduction reaction

A ‘double-peak’ ORR volcano plot is found for the p-block metal–nitrogen–carbon catalysts by considering both pristine and OH* self-modifying sites.

BiOCl/group-IV Xene bilayer heterojunctions: stability and electronic and photocatalytic properties

Vertical van der Waals heterojunctions (HJs) composed of a photocatalytic star material BiOCl monolayer and group-IV Xene monolayer (silicene, germanene etc.) were studied by using first-principles calculations. Formation energy analysis and molecular dynamics simulation show that the BiOCl/Xene bilayer HJs can exist stably up to room temperature. Owing to evident charge redistribution and accumulation occurring between the bilayers, electron-hole puddles form and charge carrier transfer and separation occur in the HJs, which are beneficial to the improvement of photocatalytic performance. The HJ energy bands maintain the Dirac cones with almost linear dispersion curves, suggesting low effective mass and high mobility of carriers, and can be effectively tuned by strain. Our results show that the BiOCl/Xene bilayer HJs with high separation efficiency and high mobility of carriers and strain-adjustable bandgaps provide varieties in the functionalities of 2D van der Waals HJs and show great potentials in photocatalytic applications.

Two-dimensional gersiloxenes with tunable bandgap for photocatalytic H2 evolution and CO2 photoreduction to CO

The discovery of graphene and graphene-like two-dimensional materials has brought fresh vitality to the field of photocatalysis. Bandgap engineering has always been an effective way to make semiconductors more suitable for specific applications such as photocatalysis and optoelectronics. Achieving control over the bandgap helps to improve the light absorption capacity of the semiconductor materials, thereby improving the photocatalytic performance. This work reports two-dimensional -H/-OH terminal-substituted siligenes (gersiloxenes) with tunable bandgap. All gersiloxenes are direct-gap semiconductors and have wide range of light absorption and suitable band positions for light driven water reduction into H2, and CO2 reduction to CO under mild conditions. The gersiloxene with the best performance can provide a maximum CO production of 6.91 mmol g-1 h-1, and a high apparent quantum efficiency (AQE) of 5.95% at 420 nm. This work may open up new insights into the discovery, research and application of new two-dimensional materials in photocatalysis.