Phosphorus-Induced One-Step Synthesis of NiCo2S4 Electrode Material for Efficient Hydrazine-Assisted Hydrogen Production.

Rational control of the reaction parameters is highly important for synthesizing active electrocatalysts. NiCo2S4 is an excellent spinel-based electrocatalyst that is usually prepared through a two-step synthesis. Herein, a one-step hydrothermal route is reported to synthesize P-incorporated NiCo2S4. We discovered that the inclusion of P caused formation of the NiCo2S4 phase in a single step. Computational studies were performed to comprehend the mechanism of phase formation and to examine the energetics of lattice formation. Upon incorporation of the optimum amount of P, the stability of the NiCo2S4 lattice was found to increase steadily. In addition, the Bader charges on both the metal atoms Co and Ni in NiCo2S4 and P-incorporated NiCo2S4 were compared. The results show that replacing S with the optimal amount of P leads to a reduction in charge on both metal atoms, which can contribute to a more stable lattice formation. Further, the electrochemical performance of the as-synthesized materials was evaluated. Among the as-synthesized nickel cobalt sulfides, P-incorporated NiCo2S4 exhibits excellent activity toward hydrazine oxidation with an onset potential of 0.15 V vs RHE without the assistance of electrochemically active substrates like Ni or Co foam. In addition to the facile synthesis method, P-incorporated NiCo2S4 requires a very low cell voltage of 0.24 V to attain a current density of 10 mA cm-2 for hydrazine-assisted hydrogen production in a two-electrode cell. The free energy profile of the stepwise HzOR has been investigated in detail. The computational results suggested that HzOR on P-incorporated NiCo2S4 was more feasible than HzOR on NiCo2S4, and these findings corroborate the experimental evidence.

Alumina-Based Bifunctional Catalyst for Efficient CO2 Fixation into Epoxides at Atmospheric Pressure.

The quest toward sustainability and decarbonization demands the development of methods for efficient carbon dioxide capture and utilization. The nonreductive CO2 fixation into epoxides to prepare cyclic carbonates has gained attention in recent years. In this work, we report the development of guanidine hydrochloride-functionalized γ alumina (γ-Al2O3), prepared using green solvents, as an efficient bifunctional catalyst for CO2 fixation. The resulting guanidine-grafted γ-Al2O3 (Al-Gh) proved to be an excellent catalyst to prepare cyclic carbonates from epoxides and CO2 with high selectivity. The nitrogen-rich Al-Gh shows increased CO2 adsorption capacity compared to that of γ-Al2O3. The as-prepared catalyst was able to carry out CO2 fixation at 85 °C under atmospheric pressure in the absence of solvents and external additives (e.g., TBAI or KI). The material showed negligible loss of catalytic activity even after five cycles of catalysis. The catalyst successfully converted many epoxides into their respective cyclic carbonates under the optimized conditions. The gram-scale synthesis of commercially important styrene carbonates from styrene oxide and CO2 using Al-Gh was also achieved. Density functional theory (DFT) calculations revealed the role of alumina in activating the epoxide. This activation facilitated the chloride ion to open the ring to react with CO2. The DFT studies also validated the role of alumina in stabilizing the electron-rich intermediates during the course of the reaction.

Uncovering Heavy Actinide Covalency: Implications for Minor Actinide Partitioning.

Across the actinide period, the stability of the trivalent oxidation state predominates in the heavy actinides, making their chemical nature close to that of rare earth elements. The resemblance in their chemistry poses difficulties in separating heavy actinides from lanthanides, which is a vital separation in the minor actinide partitioning process. Actinide contraction has conventionally implied electrostatic actinide-ligand interactions among the heavy actinides. The present study challenges this conventional understanding and reveals increasing covalency in the actinide-ligand bond across Am to Cf. Complexes of Am, Cm, Bk, and Cf have been examined for their electronic structure with a focus on the nature of their interactions with different ligands within the framework of density functional theory, where the relativistic effects have been incorporated by using zero-order regular approximation and spin-orbit coupling. The choice of ligands selected for this study facilitates the effect of the donor atom as well as denticity to be accounted for. Hence, heavy actinide complexes of the N- and O-donor ligand dipicolinic acid, S and O mixed donor ligands of the Cyanex type, and an octadentate ligand N, N, N' N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine have been optimized and evaluated. In each case energy decomposition analysis has been used to explicitly decompose the metal-ligand interaction energy into components which have then been analyzed. Irrespective of the hard-soft characteristics of donor atoms or the denticity of the ligands, steadily increased covalency has been observed across Am to Cf. Inspection of the ligand highest energy occupied molecular orbitals and metal orbitals sheds light on the origin of the unexpected covalency. An overall increase in bonding and also the orbital contribution along the Am-Cf series is clearly due to the enhancement in covalency, which is complementary to the orbital degeneracy induced covalency proposed very recently by Batista and co-workers.

Strong influence of weak hydrogen bonding on actinide-phosphonate complexation: accurate predictions from DFT followed by experimental validation.

Among the varied classes of weak hydrogen bond, the CHO type is one of immense interest as it governs the finer structures of biological and chemical molecules, hence determining their functionalities. In the present work, this weak hydrogen bond has been shown to strongly influence the complexation behaviour of uranyl nitrate [UO2(NO3)2] with diamyl-H-phosphonate (DAHP) and its branched isomer disecamyl-H-phosphonate (DsAHP). The structures of the bare ligands and complexes have been optimized by density functional theory (DFT) calculations. Surprisingly, despite having the same chemical composition the branched UO2(NO3)2·2DsAHP complex shows a remarkably higher stability (by ∼14 kcal mol-1) compared to the UO2(NO3)2·2DAHP complex. Careful inspection of the optimized structures reveals the existence of multiple CHO hydrogen-bonding interactions between the nitrate oxygens or U[double bond, length as m-dash]O oxygens and the α-hydrogens in the alkyl chains of the ligands. Comparatively stronger such bonds are found in the UO2(NO3)2·2DsAHP complex. The binding free energies associated with the complexes are computed and favoured superior binding energetics for the more stable UO2(NO3)2·2DsAHP complex. Calculations involving diisoamyl-H-phosphonate (DiAHP) and its complexes have also been performed. Theoretical predictions are experimentally tested by carrying out the extraction of U(vi) from nitric acid media using these ligands. DAHP, DsAHP and DiAHP are synthesised, characterised by NMR and evaluated for their physicochemical properties viz. viscosity, density and aqueous solubility. It was experimentally discovered that indeed DsAHP complexation with uranyl nitrate is more favoured. H-phosphonates are generically classified as acidic extractants owing to the formation of an enol tautomer at lower acidities, hence complexing the metal ion by proton exchange. Our experiments interestingly reveal a neutral ligand characteristic for DsAHP alone which is generically an acidic extractant. Furthermore, the enol tautomer of H-phosphonates that governs their extraction profiles at low acidities is also explored by DFT and the anomalous pH dependent complexation trend of DsAHP could be successfully explained. The extractions of Pu(iv) and Th(iv) have also been carried out in addition to U(vi). Solvent extraction behaviour of Am(iii) was also studied with all three ligands; the positive binding energies computed for the Am(iii) complexation corroborate with our experimental results on the poor extraction of Am(iii).

Influence of Branching on the Conformational Space: Case Study of Tri- sec-butyl Phosphate Using Matrix Isolation Infrared Spectroscopy and DFT Computations.

The conformational analysis of long chain phosphates poses a serious challenge due to the presence of rotationally flexible multiple alkyl groups. Tri- sec-butyl phosphate (TsBP) is an interesting example, in which branching can be expected to influence the conformational landscape. To solve the conformational problem of TsBP systematically, the conformations of model dimethyl- sec-butyl phosphate (DMsBP), a molecule possessing a single secondary butyl strand, were analyzed. On the basis of the analysis of the energy profile of DMsBP, a few conformational bunches were eliminated. The presence of branched methyl group appears to completely influence the conformational space of TsBP and as a result, the number of conformations is drastically reduced in comparison to its structural isomer, tri- n-butyl phosphate (TBP). B3LYP level of theory in association with 6-311++G(d,p) basis set was used for computing all the conformer geometries. Experimentally, the conformations of TsBP were studied using infrared spectroscopy by trapping the molecule in N2 and Ar matrixes at low temperatures, which were correlated well with the computational results.

Theoretical investigation of M@Pb122- and M@Sn122- Zintl clusters (M = Lrn+, Lun+, La3+, Ac3+ and n = 0, 1, 2, 3).

The positions of lawrencium (Lr), lutetium (Lu), actinium (Ac) and lanthanum (La) in the periodic table have been a controversial topic for quite some time. According to studies carried out by different groups with their justifications, these elements may potentially be placed in the d-block, p-block or all four in a 15 element f-block. The present work looks into this issue from a new perspective, which involves encapsulation of these four elements into Zintl ion clusters, Pb122- and Sn122-, followed by the determination of the structural, thermodynamic and electronic properties of these endohedral M@Pb122- and M@Sn122- clusters (M = Lrn+, Lun+ with n = 0, 1, 2, 3) using first principles based density functional theory (DFT). These parameters are compared with similar clusters encapsulated La3+ and Ac3+ ions in order to seek out similarities and differences to draw conclusions about their placement in the periodic table. For the first time the structural, energetic, and electronic properties of these metal atom/ion encapsulated Pb122- and Sn122- clusters have been investigated thoroughly. Structural parameters such as bond distances, geometry and symmetry, electronic properties viz. the density of states, the molecular orbital ordering, the electron localization function, bond critical point properties and charge distributions have been analyzed. Additionally, the thermodynamic property of the binding energy during the encapsulation process has also been calculated. All M@Pb12+ and M@Sn12+ (M = Lr and Lu) clusters form stable 18 bonding electron magic number systems with shell closing. They show negative values of binding energy and relatively large HOMO-LUMO energy gaps indicating the stability of such clusters. All the calculated parameters for Lr encapsulated clusters closely match with the corresponding calculated parameters of Lu encapsulated clusters, confirming the similarity between Lr and Lu metal atoms in various oxidation states, though their atomic ground state valence electronic configurations are different. The effect of spin orbit coupling has also been investigated using the ZORA approach. It is interesting to discover that La and Ac showed striking similarities to Lr and Lu with respect to all the properties investigated and have formed a stable 18-electron system.

Remarkable structural effects on the complexation of actinides with H-phosphonates: a combined experimental and quantum chemical study.

The structural effects of the carbon chain on the extraction of actinides by organo-phosphorus extractants have been examined experimentally and by computation. Branched butyl H-phosphonates and their linear chain isomer, n-butyl H-phosphonate (DBHP), were synthesised and characterised using IR, NMR and GC-MS techniques. Their physical properties viz. viscosity, density and aqueous solubility have been examined. DBHP, Di-iso-butyl H phosphonate (DiBHP) and Di-sec-butyl H phosphonate (DsBHP) were employed for the extraction of uranium and americium ions from nitric acid. 233U (α-tracer) and 241Am (γ-tracer) were employed as representative isotopes for the extraction of U and Am, respectively, and their distribution ratios (D) were obtained as a function of nitric acid concentration (0.01-8 M). Branching of the alkyl chain at the secondary carbon atom showed unexpected neutral extractant behaviour for DsBHP which is generally classified as an acidic extractant. The acid-dependent dual extraction mechanisms for the H-phosphonates have been examined both experimentally and through quantum chemical calculations. This dramatic effect can be partly attributed to the hindrance in the formation of the enol tautomer through the strengthening of the P[double bond, length as m-dash]OH hydrogen bonding in the DsBHP extractant. Density functional theory (DFT) based calculations were carried out to understand the complexation behaviour of actinides with the two extractants. Possible structures and binding affinities of actinides with H-phosphonates have been deduced from electronic structure calculations. Finally, the trends in distribution ratios were additionally explored and correlated with experimental observations for both metal ions.