RESUMEN
Proton conduction in crystalline porous materials has received much attention from basic scientific research through to practical applications. Polyoxometalates (POMs) can efficiently transport protons because of their small superficial negative charge density. A simple method for enhancing proton conductivity is to introduce NH4+ into the crystal structure, because NH4+ can form hydrogen bonds and function as a proton carrier. According to these considerations, NH4+ was introduced into the porous structure of A2[Cr3O(OOCH)6(etpy)3]2[α-SiW12O40]·nH2O (A = Li, Na, K and Cs; etpy = 4-ethylpyridine) (I-A+) via topotactic cation exchange. The resulting compound, diammonium tris(4-ethylpyridine)hexaformatooxidotrichromium α-silicododecatungstate hexahydrate, (NH4)2[Cr3(CHO2)6O(C7H9N)3]2[α-SiW12O40]·6H2O, showed high proton conductivity and low activation energy under high relative humidity (RH), suggesting that protons migrate efficiently via rearrangement of the hydrogen-bonding network formed by the NH4+ cations and the waters of crystallization (Grotthuss mechanism). The proton conductivity and activation energy greatly decreased and increased, respectively, with the decrease in RH, suggesting that protons migrate as NH4+ and/or H3O+ under low RH (vehicle mechanism).
RESUMEN
Tantalum oxynitrides with various nitrogen contents were synthesized from Ta2O5 powder by nitridation under a flow of ammonia at 1123 K for various durations. X-ray powder diffraction, transmission electron microscopy, Ultraviolet-visible spectroscopy, energy-dispersive spectroscopy, elemental analysis and photocatalytic reaction were performed to investigate these samples. Selected-area electron diffraction analysis of the mixed crystalline phases of powder samples revealed that each particle had only one crystalline phase. This indicates that entire particles underwent a rapid structural transformation once their nitrogen content reached a critical value. We discovered a new intermediate crystalline phase of tantalum oxynitride, TaO(a > 1)N(b <1), appeared before the generation of the ß-TaON phase. The crystal structure of TaO(a > 1)N(b <1) is suggested to be monoclinic, with unit cell parameters of a = 5.1 Å, b = 35.6 Å, c = 5.4 Å and ß = 93.5°. The ratio of nitrogen to oxygen in the samples increased with increasing nitridation duration. The increasing rate is different in the different nitridation stage due to the different structure of the samples. Nitrogen entered the samples quickly during the initial 5 h of nitridation, and a monoclinic ß-TaON phase was formed. A mesoporous structure emerged in the nitrided particles during the phase transition, greatly increasing the surface area of the samples. The more the nitrogen entered one sample, the darker the color of it due to the narrower the band gap. H2 and O2 evolved by water splitting from the nitrided samples irradiated with visible light. Change in the evolution rate of H2 and O2 had a relation with the structure of the samples.
RESUMEN
Thermally driven migration of formate between two types of surface active sites on titanium oxide film supported on Pt(111) is successfully resolved by picosecond sum frequency generation (SFG) spectroscopy in combination with a laser-induced temperature jump technique. This kinetic process explains the in-phase thermal decomposition of formate adsorbed on these different surface active sites. The enthalpy difference (DeltaH) between these formate species is estimated to be ca. 3 kJ mol(-1).