John L. Fulton

Rubidium ion hydration in ambient and supercritical water

X‐ray absorption fine structure (XAFS) measurements and analyses are presented for Rb+ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 °C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well‐defined hydration shell around the cation even at 424 °C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 A is also observed under supercritical conditions. The reduction in the number of water‐ion bonds is analogous to the reduction in the amount of water–water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters‐of‐hydration under supercritical conditions may also be in part due to formation of contact‐ion pairs.

Sintering-resistant Single-Site Nickel Catalyst Supported by Metal-Organic Framework

Developing supported single-site catalysts is an important goal in heterogeneous catalysis since the well-defined active sites afford opportunities for detailed mechanistic studies, thereby facilitating the design of improved catalysts. We present herein a method for installing Ni ions uniformly and precisely on the node of a Zr-based metal-organic framework (MOF), NU-1000, in high density and large quantity (denoted as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM is demonstrated to be an efficient gas-phase hydrogenation catalyst upon activation. The structure of the active sites in Ni-AIM is proposed, revealing its single-site nature. More importantly, due to the organic linker used to construct the MOF support, the Ni ions stay isolated throughout the hydrogenation catalysis, in accord with its long-term stability. A quantum chemical characterization of the catalyst and the catalytic process complements the experimental results. With validation of computational modeling protocols, we further targeted ethylene oligomerization catalysis by Ni-AIM guided by theoretical prediction. Given the generality of the AIM methodology, this emerging class of materials should prove ripe for the discovery of new catalysts for the transformation of volatile substrates.

An X-ray absorption fine structure study of copper(I) chloride coordination structure in water up to 325°C

Abstract X-ray absorption fine structure (XAFS) spectroscopy was used to measure the Cl − and H 2 O coordination structure about Cu 1+ in water at temperatures up to 325°C including the coordination numbers, symmetry, distances and the amount of bond disorder. The linear dichloro Cu 1+ species, [CuCl 2 ] − , is especially stable and it is predominant from 100°C to 325°C in the presence of excess Cl − . Even for solutions with 2.0 m NaCl, only the dichloro Cu 1+ species is observed with no evidence of higher Cl − coordination. There is no evidence of hydration waters in the first-solvation shell of this dichloro-species.

Quantitatively Probing the Al Distribution in Zeolites

The degree of substitution of Si(4+) by Al(3+) in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and (27)Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.

The ion pairing and hydration structure of Ni2+ in supercritical water at 425 °C determined by x-ray absorption fine structure and molecular dynamics studies

The ion pairing structure of Ni(Br)2 solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy. These first measurements of the average bulk structure show that approximately one Br− counterion is associated with each Ni2+. The Ni2+-to-Br− distance of 2.40 A is very accurately determined and the strength of this interaction, as indicated by the Debye–Waller factor (σ2=0.009 A2), shows that the bromine anion is very tightly bound to the nickel cation under these supercritical conditions. In addition to the onset of ion pairing interactions, there is also a dramatic transition in the hydration structure. Results show a loss of about 50% of the waters in the first shell upon going from ambient to a hydrothermal condition of 425 °C and 690 bar. Finally, we use molecular dynamics simulations with refined intermolecular potentials to directly calculate XAFS spectra that are shown to quantitatively reproduce the experimental results for ...

Probing the Hydration Structure of Polarizable Halides: A Multiedge XAFS and Molecular Dynamics Study of the Iodide Anion

A comprehensive analysis of the H(2)O structure about aqueous iodide (I(-)) is reported from molecular dynamics (MD) simulation and X-ray absorption fine structure (XAFS) measurements. This study establishes the essential ingredients of an interaction potential that reproduces the experimentally determined first-solvation shell of aqueous iodide. XAFS spectra from the iodide K, L(1), and L(3) edges were corefined to establish the complete structure of the first hydration shell about aqueous iodide. Further, we have utilized molecular dynamics simulations employing both DFT (+dispersion) and empirical polarizable interaction potentials to generate an ensemble of structures that were directly compared to the XAFS data. Our results indicate that DFT-MD simulations provide a description of the molecular structure that is more consistent with the XAFS experimental data. The experimental data yield approximately 6.3 water molecules located at I-H and I-O distances of 2.65 and 3.50 Å, respectively. The differences in the two interaction potentials can be traced to the treatment of the electronic charge density in the vicinity of the iodide. The empirical polarizable interaction potential yields a significantly higher induced dipole for the aqueous iodide than the DFT study. The lower induced dipole moment from the DFT simulation produces a higher coordination number and leads to a more symmetric solvation environment than that produced by the empirical polarizable interaction potential. Furthermore, the hydrogen bonding of second-shell water with the first-shell water establishes a strong ordering of the water about the iodide surface.

Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

Results of ab initio molecular dynamics (AIMD) simulations (density functional theory+PBE96) of the dynamics of waters in the hydration shells surrounding the Zn(2+) ion (T approximately 300 K, rho approximately 1 gm/cm(3)) are compared to simulations using a combined quantum and classical molecular dynamics [AIMD/molecular mechanical (MM)] approach. Both classes of simulations were performed with 64 solvating water molecules ( approximately 15 ps) and used the same methods in the electronic structure calculation (plane-wave basis set, time steps, effective mass, etc.). In the AIMD/MM calculation, only six waters of hydration were included in the quantum mechanical (QM) region. The remaining 58 waters were treated with a published flexible water-water interaction potential. No reparametrization of the water-water potential was attempted. Additional AIMD/MM simulations were performed with 256 water molecules. The hydration structures predicted from the AIMD and AIMD/MM simulations are found to agree in detail with each other and with the structural results from x-ray data despite the very limited QM region in the AIMD/MM simulation. To further evaluate the agreement of these parameter-free simulations, predicted extended x-ray absorption fine structure (EXAFS) spectra were compared directly to the recently obtained EXAFS data and they agree in remarkable detail with the experimental observations. The first hydration shell contains six water molecules in a highly symmetric octahedral structure is (maximally located at 2.13-2.15 A versus 2.072 A EXAFS experiment). The widths of the peak of the simulated EXAFS spectra agree well with the data (8.4 A(2) versus 8.9 A(2) in experiment). Analysis of the H-bond structure of the hydration region shows that the second hydration shell is trigonally bound to the first shell water with a high degree of agreement between the AIMD and AIMD/MM calculations. Beyond the second shell, the bonding pattern returns to the tetrahedral structure of bulk water. The AIMD/MM results emphasize the importance of a quantum description of the first hydration shell to correctly describe the hydration region. In these calculations the full d(10) electronic structure of the valence shell of the Zn(2+) ion is retained. The simulations show substantial and complex charge relocation on both the Zn(2+) ion and the first hydration shell. The dipole moment of the waters in the first hydration shell is 3.4 D (3.3 D AIMD/MM) versus 2.73 D bulk. Little polarization is found for the waters in the second hydration shell (2.8 D). No exchanges were seen between the first and the second hydrations shells; however, many water transfers between the second hydration shell and the bulk were observed. For 64 waters, the AIMD and AIMD/MM simulations give nearly identical results for exchange dynamics. However, in the larger particle simulations (256 waters) there is a significant reduction in the second shell to bulk exchanges.

Defining active catalyst structure and reaction pathways from ab initio molecular dynamics and operando XAFS: Dehydrogenation of dimethylaminoborane by rhodium clusters

We present the results of a detailed operando XAFS and density functional theory (DFT)-based ab initio molecular dynamics (AIMD) investigation of a proposed mechanism of the dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh(4) cluster catalyst. Our AIMD simulations reveal that previously proposed Rh structures, based on XAFS measurements, are highly fluxional, exhibiting both metal cluster and ligand isomerizations and dissociation that can only be accounted for by examining a finite temperature ensemble. It is found that a fluxional species Rh(4)(H(2)BNMe(2))(8)(2+) is fully compatible with operando XAFS measurements, suggesting that this species may be the observed catalyst resting state. On the basis of this assignment, we propose a mechanism for catalytic DMAB dehydrogenation that exhibits an energy barrier of approximately 28 kcal/mol.

Thin fluoropolymer films and nanoparticle coatings from the rapid expansion of supercritical carbon dioxide solutions with electrostatic collection

Abstract Application of nanometer thick fluoropolymer films onto metal and semiconducting substrates is described. In the first step, nanometer-sized polymer particles are generated by a process of homogeneous nucleation during the rapid expansion of supercritical fluid solutions. These gas-phase particles are then charged as they are being formed by application of a high voltage to the expansion nozzle. In this way the charged nanoparticles can be collected on a solid surface forming uniform coatings with thicknesses from tens of nanometers to several micrometers thick. Supercritical carbon dioxide solutions of three different fluoropolymers were used to generate different types of coatings. This represents a ‘green’ process for film deposition. A further unique aspect of this process is that the small charged nanoparticles can be deposited to electrically conducting microscopic regions with a spatial resolution better than 50 nm.

Hydrated Structure of Ag(I) Ion from Symmetry-Dependent, K- and L-Edge XAFS Multiple Scattering and Molecular Dynamics Simulations

Details of the first-shell water structure about Ag(+) are reported from a corefinement of the K- and L(2)-edge multiple scattering signal in the X-ray absorption fine structure (XAFS) spectra. Detailed fits of the Ag K-edge data that include the contributions from multiple scattering processes in the hydrated ion structure cannot distinguish between models containing tetrahedral symmetry versus those containing collinear O-Ag-O bonds. However, we show that the multiple scattering oscillations at the L(2)-edges have distinctly different phase and amplitude functions than at the K-edge. These phase and amplitude functions depend not only on the symmetry of the multiple scattering paths but also on the nature of the final state electronic wave function probed by the dipole-allowed transition. Hence the multiple scattering portions of K- and L(2)-edge spectra provide independent measurements of the local symmetry--not a redundant measurement as is commonly believed. On the basis of the enhanced information content obtained by the simultaneous assessment of both the K- and L(2)-edges, we report that the hydrated Ag(+) structure contains five or six water molecules in the first shell with a significant number of nearly collinear and 90 degrees O-Ag-O bond angles. Finally, the K- and L(2)-edge spectra are used to benchmark the hydration structure that is generated from both DFT-based and classical molecular dynamics simulations. Simulated first-shell structures are compared to the experimental structures.