The reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates. The resonance Raman data of intermediate I, which could only be stabilized ...at −95 °C, indicates that it is a Fe(II)–CO2 2– adduct and is followed by an another intermediate II at −80 °C where the bound CO2 in intermediate I is protonated to form a Fe(II)–COOH species. While the initial protonation can be achieved using weak proton sources like MeOH and PhOH, the facile heterolytic cleavage of the C–OH bond in intermediate II requires strong acids.
Reactivity as well as selectivity are crucial in the activation and electrocatalytic reduction of molecular oxygen. Recent developments in the understanding of the mechanism of electrocatalytic O2 ...reduction by iron porphyrin complexes in situ using surface enhanced resonance Raman spectroscopy coupled to rotating disc electrochemistry (SERRS-RDE) in conjunction with H/D isotope effects on electrocatalytic current reveals that the rate of O2 reduction, ∼104 to 105 M–1 s–1 for simple iron porphyrins, is limited by the rate of O–O bond cleavage of an intermediate ferric peroxide species (FeIII–OOH). SERRS-RDE probes the system in operando when it is under steady state such that any intermediate species that has a greater rate of formation relative to its rate of decay, including the rate determining species, would accumulate and can be identified. This technique is particularly well suited to investigate iron porphyrin electrocatalysts as the intense symmetric ligand vibrations allow determination of the oxidation and spin states of the bound iron with high fidelity. The rate of O2 reduction could be tuned up by 3 orders of magnitude by incorporating residues in the catalyst design that can exert “push” or “pull” effects, that is, axial phenolate and thiolate ligands and distal arginine residues. Similarly the rate of O–O bond cleavage can be enhanced by several orders of magnitude upon incorporating a distal Cu site and installing the active site in a hydrophobic protein environment in synthetic models and biosynthetic protein scaffolds. The selectivity, however, is solely determined by the site of protonation of a ferric peroxide (FeIII–OOH) intermediate and can be governed by installing preorganized second sphere residues in the distal pocket. The 4e–/4H+ reduction of O2 entails protonation of the distal oxygen of the FeIII–OOH species, while 2e–/2H+ reduction requires the proximal oxygen to be protonated. Mechanistic investigations of CO2 reduction by iron porphyrins reveal that the rate-determining step is the C–O bond cleavage of a FeII–COOH species analogous to the O–O bond cleavage step of a FeIII–OOH species in O2 reduction. The selectivity, resulting in either CO or HCOOH, is determined by the site of protonation of this species. These similarities suggests that the chemical principles governing the rate and selectivity of reduction of small molecules like O2, CO2, NO x , and SO x may be quite similar in nature.
Iron porphyrin complexes with second-sphere distal triazole residues show a hydrogen evolution reaction (HER) catalyzed by the Fe(I) state in both organic and aqueous media, whereas an analogous ...iron porphyrin complex without the distal residues catalyzes the HER in the formal Fe(0) state. This activation of the Fe(I) state by the second-sphere residues lowers the overpotential of the HER by these iron porphyrin complexes by 50%. Experimental data and theoretical calculations indicate that the distal triazole residues, once protonated, enhance the proton affinity of the iron center via formation of a dihydrogen bond with an Fe(III)–H– intermediate.
Reduction of CO2 to value-added chemicals is a logical way of fixing the rising levels of CO2. Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in ...this chemistry is sensitivity of these low-valent metal sites to more abundant O2. Since O2 is a stronger oxidant than CO2 and isolated from the obvious competitive inhibition of CO2, partial reduction of O2 leads to formation of reactive oxygen species like O2 – and H2O2, which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O2 despite a higher driving force for the latter.
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•LULC analysis shows mainly built-up land expansion consumes urban peripheral wetland.•Infrastructural activity and urban developmental projects are the active agents of wetland ...shrinkage.•Markov Transition Probability Matrix has shown only 39 percent of wetland area will remain in 2025.•Simulated 2025 wetland surface shows extensive outlier loss.•The absence of institutional coordination, financial support, and land use regulations are the main determinants of policy failure.
The usefulness and need for wetland ecosystems are in general, manifold. Nonetheless, their current situation in many parts of the world is truly a matter of concern, both in terms of biodiversity as well as human well-being. While policy development and decision-making are vital, there is also a great need to understand the wetlands transition process, taking into account measures for their conservation. In an attempt towards such an understanding, this study analyses the eco-social transformation of the East Kolkata Wetland (EKW). As a primary step to examine the patterns and drivers of wetland change in the EKW, land cover changes have been quantified. In addition, the significance of the driving factors has been adjudged and modelled using Wetland Shrinkage Monitoring (WSM) model. The outcome shows that wetland shrinkage largely determined by proximity forces of urban growth. While the Markov transition indicates that 46% out of 38km2 wetland tends to alter to other classes, wetland transition 2025 points out that almost 9km2 area is at critical risk. In addition to these findings, the study ascertains that a decent functioning of the local authorities and a comprehensive land use planning are indispensable to curb wetland degradation.
The past decade has seen considerable growth in the development of materials for fuel cell electrodes, and there is a desire for active electrocatalysts derived from base metals instead of noble ...metals. Fuels cells that consume H2 and O2 require catalysts to cleave these reactants, with the oxygen reduction reaction (ORR) — either 4H+/4e− reduction to 2H2O or 2H+/2e− reduction to H2O2 — being particularly challenging. The ORR is efficiently performed by certain metalloenzymes, and understanding the links between their structure and function aids the design of molecular ORR electrocatalysts. These bio-inspired catalysts exhibit good activity relative to previous synthetic systems and, furthermore, have provided mechanistic insights relevant to synthetic and enzymatic catalysts. This Review covers recent developments in homogeneous and heterogeneous molecular ORR catalysis, placing emphasis on reaction mechanisms and the factors governing rates and selectivities.Electrocatalysts for the oxygen reduction reaction are important components of energy technologies such as fuel cells. The study of molecular catalysts affords mechanistic insights that further the development of robust, active and energy-efficient systems. This Review describes state-of-the-art metal complexes that operate either in solution or immobilized on an electrode.
We present an example of host–guest complexes of atomically precise noble metal nanoparticles with cucurbit7uril (CB) in water, specifically concentrating on Ag29(LA)12 (where LA is α-lipoic acid), ...a well-known red luminescent silver cluster. Such host–guest interactions resulted in enhanced luminescence of about 1.25 times for the modified system, compared to the parent cluster. We extended our study to cyclodextrins (CDs), where about 1.5 times enhanced luminescence was estimated compared to the parent cluster. The formation of supramolecular complexes was confirmed using high-resolution electrospray ionization mass spectrometry (HRESI MS) and nuclear magnetic resonance spectroscopy. Molecular docking and density functional theory calculations supported our experimental results and showed that while CB formed inclusion complexes by encapsulation of one of the LA ligands of the cluster, CD formed supramolecular adducts by interaction with the cavity built by the ligands on the cluster surface. The complexation was favored by geometrical compatibility. Consequently, these superstructures are labeled as Ag29LA12∩CB n and Ag29LA12@CD n (n = 1–3), where ∩ and @ indicate the inclusion complex and supramolecular adduct, respectively. Solution-phase Ag29LA12@CD n complexes were employed to detect dopamine (10 nM). Luminescent Ag29LA12@CD n and Ag29LA12∩CB n complexes in water could be potential candidates for organic pollutant sensing and biomedical applications.
The feasibility of a hydrogen-based economy relies very much on the availability of catalysts for the hydrogen evolution reaction (HER) that are not based on Pt or other noble elements. Significant ...breakthroughs have been achieved with certain first row transition metal complexes in terms of low overpotentials and large turnover rates, but the majority of reported work utilized purified and deoxygenated solvents (most commonly mixtures of organic solvents/acids). Realizing that the design of earth abundant metal catalysts that operate under truly ambient conditions remains an unresolved challenge, we have now developed an electronically tuned Co(III) corrole that can catalyze the HER from aqueous sulfuric acid at as low as −0.3 V vs NHE, with a turnover frequency of 600 s–1 and ≫107 catalytic turnovers. Under aerobic conditions, using H2O from naturally available sources without any pretreatment, the same complex catalyzes the reduction of H+ with a Faradaic Yield (FY) of 52%. Density functional theory (DFT) calculations indicate that the electron density on a putative hydride species is delocalized off from the H atom into the macrocycle. This makes the protonation of a Co(III)-H– species the rate determining step (rds) for the HER consistent with the experimental data.
The growth in rice consumption has either slowed down or become negative in all the continents although additional per capita demand for rice exists in Africa. The future additional rice demand will ...mainly come from the population growth in Africa and Asia. The production growth during the past led by growth in yield has helped in meeting rice demand around the globe. However, Africa continues to lag far behind other continents in achieving the desired level of yield. Additional research efforts are needed in all continents for the production of varieties with higher yield potential and desired grain quality with more emphasis on the African continent. The global demand for rice will be about 584 million tons or less towards 2050. With positive technological developments in the rice research arena, the shape of rice agriculture will change towards 2050. Asia may lose about 5 million ha and Africa gain about 10 million ha of rice land by 2050.
The self‐assembled structures of atomically precise, ligand‐protected noble metal nanoclusters leading to encapsulation of plasmonic gold nanorods (GNRs) is presented. Unlike highly sophisticated DNA ...nanotechnology, this strategically simple hydrogen bonding‐directed self‐assembly of nanoclusters leads to octahedral nanocrystals encapsulating GNRs. Specifically, the p‐mercaptobenzoic acid (pMBA)‐protected atomically precise silver nanocluster, Na4Ag44(pMBA)30, and pMBA‐functionalized GNRs were used. High‐resolution transmission and scanning transmission electron tomographic reconstructions suggest that the geometry of the GNR surface is responsible for directing the assembly of silver nanoclusters via H‐bonding, leading to octahedral symmetry. The use of water‐dispersible gold nanoclusters, Au≈250(pMBA)n and Au102(pMBA)44, also formed layered shells encapsulating GNRs. Such cluster assemblies on colloidal particles are a new category of precision hybrids with diverse possibilities.
GNRs inside: Atomically precise nanoclusters were assembled on surface‐reconstructed plasmonic gold nanorods (GNRs), resulting in octahedral composite crystals. pMBA=p‐mercaptobenzoic acid.
