Particle distribution in a microporous material: theoretical concept.

Particle distribution and exchange equilibria in a microporous host material, built up of equivalent particle sites, which are grouped in larger subsets, are described. Simplified descriptions evolve from the exact formulae in the thermodynamic limit. We find that, for example, a single zeolite A nanocrystal consisting of about 1000 pseudo-unit-cells fixes a lower limit for the use of the approximate formula describing particle distribution. A rational selectivity coefficient, which is approximately constant over the whole exchange range, only results if a single zeolite crystal consists of one million pseudo-unit-cells or more, or if a sufficiently large number of smaller crystals is considered. On the basis of the statistical particle distribution model, a closed, simple formula for the ion-exchange isotherm is then derived, which is valid for systems involving a variable number of coupled-exchange reactions. Its similarity to the Langmuir isotherm is discussed. The theory on ion-exchange equilibria is used to derive formulae for the change of free-energy, enthalpy, and entropy occurring in coupled ion-exchange reactions. The findings, though applicable to virtually any particle exchanging system with the structural properties described above, are applied to zeolite A, since this material can be treated as a nearly ideal model. The results derived can straightforwardly be used to evaluate experimental data quantitatively, since the common inequivalence of the host sites can be taken into account.

Water splitting with silver chloride photoanodes and amorphous silicon solar cells.

A thin silver chloride layer deposited on a conducting support photocatalyzes the oxidation of water to O(2) in the presence of a small excess of silver ions in solution. The light sensitivity in the visible part of the spectrum is due to self-sensitization caused by reduced silver species. Anodic polarization reoxidizes the reduced silver species. To test its water splitting capability, AgCl photoanodes as well as gold colloid modified AgCl photoanodes were combined with an amorphous silicon solar cell. The AgCl layer was employed in the anodic part of a setup for photoelectrochemical water splitting consisting of two separate compartments connected through a salt bridge. A platinum electrode and an amorphous silicon solar cell were used in the cathodic part. Illumination of the AgCl photoanode and the amorphous Si solar cell led to photoelectrochemical water splitting to O(2) and H(2). For AgCl photoanodes modified with gold colloids an increased photocurrent, and consequently a higher O(2) and H(2) production, were observed.

A Naphthalene-Like Si1010- Unit in the Novel ∞2 [Si2030- ] Planar Anion of Sr13 Mg2 Si20.

Two interpenetrating 2∞ [Si2030- ] polyanions with naphthalene-like Si1010- building blocks (see picture) characterize the"nonclassical" Zintl phase Sr13 Mg2 Si20 , which is formed from the elements at 1230-1240 K. The ecliptical stacking of the Si1010- units leads to one-dimensional conductivity along the stacking direction.