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Nickel (Ni) has been used as an active metal component catalyst for attaining high conversion in the tri-reforming of methane (TRM) reaction, which produces synthesis gas as a product. Impregnation of Ni within the multi-metal MOF (MM-MOF) materials results in enhanced catalytic activity for TRM. In this work, we have studied the properties of a Ni-cluster impregnated within a metal (Ce or Ti) doped UiO-66 MOF using density functional theory (DFT) calculations. Different amounts of cerium (Ce) or titanium (Ti) metals are doped (16.67% and 33.3%) in the UiO-66 structure, and different sizes of Nix clusters (x = 4–16) are impregnated in the pores of the MM-MOF structure. With increasing Ce-substitution, the MM-MOF shows better improvement in the Nix binding energy per Ni atom compared to Ti-substitution. Furthermore, projected density of state (PDOS) calculations reveal that while a charge localization occurs at the Ce site, no such charge localization is observed at the Ti site. Our results demonstrate that the strong binding of Ni clusters not only depends on the electron rearrangement but also depends on the structural stability of MOF materials. These atomic insights may be helpful in the synthesis of future stable and sustainable, catalytic hybrid MOF materials for energy and environmental applications. [ABSTRACT FROM AUTHOR]

A closure for the Ornstein-Zernike equation is presented, applicable for fluids of charged, hard spheres. From an exact, but intractable closure, we derive the radial distribution function of nonlinearized Debye-Hückel theory by subsequent approximations, and use the information to formulate a new closure by an extension of the mean spherical approximation. The radial distribution functions of the new closure, coined Debye-Hückel-extended mean spherical approximation, are in excellent agreement with those resulting from the hyper-netted chain approximation and molecular dynamics simulations, in the regime where the latter are applicable, except for moderately dilute systems at low temperatures where the structure agrees at most qualitatively. The method is numerically more efficient, and more important, convergent in the entire temperature-density plane. We demonstrate that the method is accurate under many conditions for the determination of the structural and thermodynamic properties of homogeneous, symmetric hard-sphere Coulomb systems, and estimate it to be a valuable basis for the formulation of density functional theories for inhomogeneous or highly asymmetric systems. [ABSTRACT FROM AUTHOR]

The classical Voorn-Overbeek thermodynamic theory of complexation and phase separation of oppositely charged polyelectrolytes is generalized to account for the charge accessibility and hydrophobicity of polyions, size of salt ions, and pH variations. Theoretical predictions of the effects of pH and salt concentration are compared with published experimental data and experiments we performed, on systems containing poly(acrylic acid) (PAA) as the polyacid and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) or poly(diallyldimethyl ammonium chloride) (PDADMAC) as the polybase. In general, the critical salt concentration below which the mixture phase separates, increases with degree of ionization and with the hydrophobicity of polyelectrolytes. We find experimentally that as the pH is decreased below 7, and PAA monomers are neutralized, the critical salt concentration increases, while the reverse occurs when pH is raised above 7. We predict this asymmetry theoretically by introducing a large positive Flory parameter (= 0.75) for the interaction of neutral PAA monomers with water. This large positive Flory parameter is supported by molecular dynamics simulations, which show much weaker hydrogen bonding between neutral PAA and water than between charged PAA and water, while neutral and charged PDMAEMA show similar numbers of hydrogen bonds. This increased hydrophobicity of neutral PAA at reduced pH increases the tendency towards phase separation despite the reduction in charge interactions between the polyelectrolytes. Water content and volume of coacervate are found to be a strong function of the pH and salt concentration. [ABSTRACT FROM AUTHOR]

We demonstrate periodic nanostructures emerged in homogeneouslycross-linked thin gels made of strongly charged polymers. Free-energyfunctional of polyelectrolyte gels with density and charge inhomogeneitiesis developed and numerical energy minimization is performed over candidatenanostructures. The two-dimensional calculations indicate the possibilityof realizing lamellar, square, solvent-rich hexagonal, and polymer-richhexagonal phases, by a fine interplay of polymer and solvent characteristics.Nonlinearities in network elasticity and electrostatic energy arefound to be the deciding factor in the thermodynamic selection ofnanostructures. In particular, the nonlinear network elasticity suppressesthe stability of solvent-rich hexagonal nanostructures as solventquality increases, which results in exchange of stabilities betweennanostructures and leads to stable nanostructures of square symmetryin two-dimensional gels. The phase transition between nanostructuresby changes in environmental stimuli suggests a possible means to controlbulk properties of polyelectrolyte gels. [ABSTRACT FROM AUTHOR]