Reversible room temperature ammonia gas absorption in pore water of microporous silica-alumina for sensing applications.

Microporous silica and silica-alumina powders exhibit a reversible uptake and release of ammonia gas from water vapor containing gas mixtures at ambient temperature, with capacities of 0.9 and 2.0 mmol g-1 in the presence of 100 ppm and 1000 ppm NH3, respectively. The ammonia trapping mechanism was revealed using a combination of direct excitation 1H MAS, 1H-1H EXSY and 1H DQ-SQ NMR spectroscopy, indicating that the major part of the captured ammonia is blended in the hydrogen bonded water network in the pores of the adsorbent. A small fraction is irreversibly bound as result of protonation and chemisorption. While common ammonia adsorbents need thermal regeneration, microporous silica-alumina can be regenerated by sweeping with dry gas at ambient temperature, desorbing the physisorbed fraction together with occluded water. As carbon dioxide does not interfere with the ammonia absorption process, this reversible absorption process of ammonia gas at ambient temperature is particularly attractive for sensor applications.

Interfacial Water Drives Improved Proton Transport in Siliceous Nanocomposite Nafion Thin Films.

Nafion proton exchange membranes dehydrate when they are used in the gas phase and in high-temperature applications, such as fuel cells and (photo)electrolysis. Retaining a high level of membrane hydration under such conditions can be achieved by using inorganic fillers, but has never been demonstrated for thin films. Herein, several types of siliceous nanoparticles were incorporated for the first time into Nafion thin films. For composite Nafion materials, increased water uptake does not always induce increased proton conductivity. Here, increased water uptake did result in higher proton conductivity due to a synergistic effect within the composite film. The nanocomposites displayed a higher water uptake than could be expected based on the water uptake of the individual materials. Excess water present at the Nafion-filler interface was found to cause the proton conductivity of nanocomposite Nafion/Ludox AS-40 thin films to double compared with pristine Nafion at low relative humidity (from 2 to 4 mS cm-2 ). Knowledge about the properties of such interfaces will allow for the better design of self-humidifying nanocomposite Nafion membranes, films, and catalyst layers.