The main types of fibrous structures for catalyst (applications) are addressed, all the way from the nanoscale to the macroscale and back by combining different morphologies into advanced materials for today's environmental challenges. These materials show a very large potential but future research is needed to further expand the use of fibrous structures in industrial processes.

Abstract

The continuous development of advanced catalysts to increase process yield and selectivity is crucial. A high specific surface area and a good active phase dispersion are generally essential to create catalytic materials with a large number of active sites. Notably, materials with a fibrous morphology are appealing because of their large surface-to-volume ratio and flexibility. This contribution highlights the morphology of different types of fibrous structures currently under investigation, all the way from the nanoscale to the macroscale and back, where the distinction lies in the length and diameter of the fibers, as well as in the connection between the structures. Fibers with at least one submicron to nanoscale characteristic result in a higher yield, but can display practical usability issues when unbound. Therefore, fibrous structure catalysts with a balance between the small diameter and handleability are important for industrial viability. By combining different morphologies, the best of both nanomaterials and macroscopic integer materials can be combined into advanced catalytic materials. This overview showcases the large potential of these materials but makes clear that further research is needed to keep expanding the use and effectiveness of fibrous structures in catalysis.

Selective oxidation of hydrocarbons using molecular oxygen as sole oxidant under mild conditions remains a challenging task. In this context, metal-organic frameworks (MOFs) have been widely used in various oxidation reactions due to their porosity, high surface area and designability. However, the slow diffusion of substrates/products in micropores of three-dimensional (3D) bulk MOFs hinders the efficient catalytic performance of such materials. Herein an ultrathin two-dimensional (2D) porphyrin-based Zr-MOF nanosheet (Zr-TCPP) is synthesized through modulator-control strategy. Subsequently, various metal ions are anchored into the porphyrin ring by post synthesis modification to afford a series of 2D Zr-TCPP(M) (M=Mn, Fe, Co, Ni, Cu and Zn). Various structural characterization techniques indicate Zr-TCPP(M) is nanoflower structure with ultrathin nanoplate petals which provides fully exposed accessible active sites. Among them, Zr-TCPP(Fe) shows excellent catalytic performance in styrene epoxidation reactions and benzylic C-H oxidation reactions using O2 as sole oxidant under ambient temperature and pressure. The remarkable activity arises from high density of exposed porphyrin-Fe active sites, low diffusion barriers for substrates and products, as well as a similar homogeneous reaction space. Furthermore, Zr-TCPP(Fe) nanosheet is easily recycled by centrifugation and reused at least five times without significant loss of catalytic activity.

This review thoroughly analyses the fabrication process, structural, morphological characterization, different applications which highlights the role of QDs (carbon dots, sulfide QDs, and oxide QDs) within the QD/LDH heterostructure, serving as interlayer support-catalyst, mediator, semiconductor and sensitizer. These integrated heterostructures demonstrate superior performance in photocatalytic (PC) electrocatalytic (EC) and photoelectrochemical (PEC) water splitting, with enhanced long–term stability.

Abstract

Layered double hydroxides (LDHs) is a category of 2D materials that possess excellent physicochemical properties for enhancing photocatalytic (PC), electrocatalytic (EC), and photoelectrochemical (PEC) performances. However, pristine LDH encounters challenges like sluggish charge–carrier mobility, high rate of electron–hole recombination, low conductivity, and tendency to agglomerate, making them unsuitable for practical applications. Therefore, modifications such as composite preparations, co-catalyst integration, semiconductor coupling, and ternary heterostructure engineering have been explored to disclose new possibilities for LDHs in PC, EC, and PEC applications. In the realm of semiconducting materials aimed at enhancing LDH productivity, quantum dots (QDs) i. e., 0D materials have proven to be effective due to their advantages, including abundant reserves, affordability, and environmental friendliness. This review explores the role of QDs as interlayer support, co-catalyst, mediator, semiconductor, and sensitizer in QDs@LDH heterostructures to achieve superior photocatalytic activities. These QD-infused heterostructures also deliver improved EC and PEC water–splitting performance coupled with long–term stabilities. Additionally, this review delves into characterization techniques, intrinsic structural features, and designing of the QD@LDH heterostructures. Future scopes and challenges in constructing and cutting–edge theoretical anticipations of QD@LDH are also discussed. This review may be a guiding light to a sustainable approach to outperform QD-modified LDH for versatile catalysts.

Ammonia is one of the most important chemicals in the world because it is a feedstock for fertilizer production and, moreover, it has been proposed as a convenient storage media for renewable H2. Currently, it is produced by Haber-Bosch process, which entails harsh operation conditions and a high carbon footprint, rendering the process difficult to be run by renewable energy. Therefore, substituting the conventional Haber-Bosch process for other approach less energy-intensive and carbon-free is and urgent need. A milder process for ammonia synthesis will enable the implementation of smaller distributed plants more aligned with renewable energies. For this reason, there is a plethora of current research focusing on the development of suitable catalysts with higher activity and selectivity for nitrogen reduction and processes that will work under less severe operating conditions or even at ambient conditions. Some of the most relevant research approaches are here revised and compared.

Electrochemical nitrogen reduction reaction (NRR) is considered as an important strategy for ammonia using green energy. Single atom catalysts (SACs) with a metal atom as active site have been shown its advantages to high ammonia yield and low energy consumption. In this work, we proposed a new concept of SACs with the capacity of d-p orbital hybridization. These computationally designed SACs contained a metal and non-metal pair anchored on g-C3N4 slab. Among these SACs studied in the present work, RuB@g-C3N4 catalyst performed the best activity for NRR with low onset potential of -0.33 V by a two-step scanning computations using density functional theory (DFT). Moreover, suppressing effectively to the competitive hydrogen evolution reaction (HER) also make it more selective. It was further revealed that the hybridization of p-d orbitals between metal and nonmetal could regulate the electronics of active sites and facilitate the adsorption and charge transfer of adsorbed N2 molecules resulting in enhanced catalytic performance. This work demonstrated an alternative way to further enhance the catalytic activity of SACs by introducing a non-metal atom.

Oxidation reactions catalyzed by O2-dependent enzymes are gaining increasing interest in the chemical industry due to their potential to provide a more selective, benign and sustainable alternative to the conventional chemical oxidation methods. O2-dependent enzymes, like oxidases and oxygenases, catalyze a versatile range of oxidative reactions using only molecular oxygen as oxidant. However, their practical application on larger scale has been limited up to this point, primarily due to factors like their low catalytic rates combined by a narrow substrate spectrum and low stability. Nonetheless, in recent years, enzyme engineering studies have made significant progress in addressing these challenges and moving O2-dependent enzymes closer towards industrial utilization. In this review, we aim to provide a concise overview of the most recent engineering approaches on O2-dependent enzymes. We will highlight recent studies that have targeted various aspects of O2-dependent enzymes including, activity, selectivity, stability, and substrate spectrum with a focus on engineering studies where the engineered enzymes catalyze synthetically valuable reactions.

Density Functional Theory is used to unravel the mechanism of the nitrobenzene to aniline reduction, catalyzed by dioxomolybdenum (VI) dichloride. The use of pinacol as an oxoaccepting reagent and the production of only acetone and water as byproducts, signals a novel and environmentally friendly way to add value to the oxygen-rich biomass-derived polyols. The reaction proceeds through three consecutive cycles, each one responsible for one of the three reductive steps needed to yield aniline from nitrobenzene, with  nitrosobenzene and hydroxylamine as intermediates. Each cycle regenerates the catalyst and releases one water and two acetone molecules. The mechanism involves  singlet/triplet state crossings, a crucial feature in polyoxomolibdate catalyzed processes. The role of the Mo-coordinated water as the provider of the mysterious protons needed to reduce the nitro group, was revealed. The disclosure of this challenging mechanism and its rate limiting step can contribute to the design of more effective Mo(VI) catalysts.

Tethered tungsten-alkylidenes bearing azoimido ligands (M≡Nγ-Nβ=NαR) are synthesized, characterized, and tested as initiators for ring expansion metathesis polymerization (REMP). While these ligands are typically unstable and prone to dinitrogen loss, this work demonstrates that tethered alkylidene complexes bearing azoimido ligands are stable enough to be REMP initiators. Moreover, they are more efficient, long-lived, and stereoselective than their corresponding imido derivatives (M≡NR). Density Functional Theory (DFT) analysis of the azoimido complexes provides insight into their unusual stability.

Designing robust catalysts for low-temperature oxidation is pertinent to the development of advanced combustion engines to meet increasingly stringent emissions limitations. Oxidation of CO, hydrocarbon and NO pollutants over platinum-group catalysts suffer from strong inhibition due to their competitive adsorption, while coinage metals are generally slow at activating O2. Through computational screening we discovered a PdCu alloy catalyst that completely oxidizes CO below 150 °C without inhibition by NO, propylene or water. This is attributed primarily to geometric effects and the presence of CO bound to Pd sites within the Cu-rich surface of the PdCu alloy. We demonstrate that the novel PdCu catalyst can be used in tandem with a PtPd catalyst to achieve sequential, inhibition-free, complete oxidation of CO in a two-bed system, while also achieving 50% NO conversion below 120 °C. Moreover, neither water nor propylene adversely affect the low temperature CO oxidation activity.

The utilization of photo-induced processes in C–H functionalization via radical pathways has emerged as a highly promising strategy for the preparation and modification of complex organic compounds. While current methods for generating carbon-centred radicals from C–H bonds primarily focus on hydridic C–H bonds to yield nucleophilic radical species, the reactivity and potential applications of electrophilic radicals derived from protic C–H bonds remain largely unexplored. In this review, we aim to shed light on the seminal findings regarding the activation of protic C(sp3)–H bonds while also showcasing noteworthy examples of this radical formation process. Mechanistically diverse modes of activation are discussed, unified by proton-coupled electron transfer (PCET) concepts.