A higher resolution magnetic bottle photoelectron spectrometer for the study of the electronic structure of size-selected metal clusters is presented. The initial study on Fe n Ϫ ͑nϭ3-24͒ is reported at a photon energy of 3.49 eV. The photoelectron spectra of these clusters exhibit sharp features throughout the size range. The spectra for Fe 3-8Ϫ show large size dependence with many resolved features. The spectra for Fe 9-15 Ϫ exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe 16 Ϫ , then again at Fe 19 Ϫ and Fe 23 Ϫ . These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H 2 . Extended Hückel molecular orbital ͑EHMO͒ calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe 9 when body-centered cubic ͑bcc͒ structures are assumed for Fe 9-15 and a similarly close-packed structure with a bcc Fe 15 core for the larger clusters. The EHMO calculations allow a systematic interpretation of the sharp photoelectron spectral features in Fe 9-15 Ϫ and reproduced the abrupt spectral change taking place from Fe 15Ϫ to Fe 16 Ϫ . Most importantly, the reactivity changes of the clusters with H 2 are successfully explained based on the detailed electronic structures of the clusters, as revealed from the photoelectron spectroscopy ͑PES͒ spectra and the theoretical calculations. The calculations also correctly predict the existence of magnetism in these clusters and yield reasonable values for the cluster magnetic moments.

It is challenging to introduce pendent sulfonic acid groups into modularly built crystalline porous frameworks for intrinsic proton conduction. Herein, we report the mechanoassisted synthesis of two sulfonated covalent organic frameworks (COFs) possessing one-dimensional nanoporous channels decorated with pendent sulfonic acid groups. These COFs exhibit high intrinsic proton conductivity as high as 3.96 × 10(-2) S cm(-1) with long-term stability at ambient temperature and 97% relative humidity (RH). In addition, they were blended with nonconductive polyvinylidene fluoride (PVDF) affording a series of mixed-matrix membranes (MMMs) with proton conductivity up to 1.58 × 10(-2) S cm(-1) and low activation energy of 0.21 eV suggesting the Grotthuss mechanism for proton conduction. Our study has demonstrated the high intrinsic proton conductivity of COFs shedding lights on their wide applications in proton exchange membranes.

ABSTRACT:We present a theoretical study of the structural evolution of small minimum energy platinum clusters, using density functional theory (DFT). Three growth pathways were identified. At the subnanoscale, clusters with triangular packing are energetically most favorable. At a cluster size of approximately n ϭ 19, a structural transition from triangular clusters to icosahedral clusters occurs. A less energetically favorable transition from triangular clusters to fcc-like clusters takes place at around n ϭ 38. Ionization potentials, electron affinities, and magnetic moments of the triangular clusters were also calculated. Understanding the structures and properties will facilitate studies of the chemical reactivity of Pt nanoclusters toward small molecules.

A palladium phosphide electrocatalyst supported on carbon black (PdP2@CB) shows efficient water splitting in both alkaline and neutral electrolytes. Significantly lower overpotentials are required for PdP2@CB (27.5 mV in 0.5 m H2SO4; 35.4 mV in 1 m KOH; 84.6 mV in 1 m PBS) to achieve a HER electrocatalytic current density of 10 mA cm−2 compared to commercial Pt/CB (30.1 mV in 0.5 m H2SO4; 46.6 mV in 1 m KOH; 122.7 mV in 1 m PBS). Moreover, no loss in HER activity is detectable after 5000 potential sweeps. Only 270 mV and 277 mV overpotentials are required to reach a current density of 10 mA cm−2 for PdP2@CB to catalyze OER in 1 m KOH and 1 m PBS electrolytes, which is better OER activity than the benchmark IrO2 electrocatalyst (301 mV and 313 mV to drive a current density of 10 mA cm−2). 1.59 V and 1.72 V are needed for PdP2@CB to achieve stable water splitting catalytic current density of 10 mA cm−2 in 1 m PBS and 50 mA cm−2 in 1 m KOH for 10 h, respectively.

We report observation of new prominent peaks in the Ti x C yanion mass spectra from laser vaporization experiments involving a pure Ti target with a CH 4 -seeded He carrier gas. Both photoelectron spectroscopy and density functional calculations were performed to obtain structural and bonding information for the new prominent anion clusters, including Ti 3 C 8 -, Ti 4 C 8 -, Ti 6 C 13 -, Ti 7 C 13 -, Ti 9 C 15 -, and Ti 13 C 22 -. For each cluster, we optimized several structures, evaluated their electron affinities (EAs), and simulated their single particle density of states (DOS). The calculated EAs and DOS of the different structures were then compared with the experimental photoelectron data. Good agreement between the experiments and calculations was found for the lowest energy isomers considered in each case. We found that three factors, i.e., the C 2 dimer, cubic framework, and layered structures, play essential roles in determining the structures and chemical bonding of the titanium carbide clusters. A growth pathway from Ti 3 C 8 to Ti 13 C 22 with Ti 6 C 13 , Ti 7 C 13 , and Ti 9 C 15 as intermediates is proposed and discussed.