RESUMEN
Oxygen exchange reaction on mixed conducting oxide is a critical reaction for many applications, yet measuring its rate constant remains poorly reliable by standard techniques. Here, a new technique that adapts the conductivity relaxation measurements on porous ceramics is proposed. Using a simple image analysis tool, it is possible to accurately determine the grain size distribution of the porous oxide, which is used in a new relaxation model that integrates relaxation times over that distribution. With such a model, it is possible to fit relaxation transients with the oxygen exchange reaction rate constant kchem as the only fitting parameter. With such rigidity, the output values of kchem are not sensitive to the fitting procedure, which does not require optimization. The model is proven to be applicable to various mixed conducting oxides and to a wide range of microstructures, yielding a remarkably low residual for all the porous ceramics considered. The procedure uses porous ceramics, therefore the derived kinetics are representative of ceramics used in real applications such as fuel cells, sensors, or catalysis.
RESUMEN
In this paper, evidence is provided that the model reaction of aqueous dihydroxyacetone (DHA) conversion is as sensitive to the TiO2 catalysts' basicity as to their acidity. Two parallel pathways transformed DHA: while the pathway catalyzed by Lewis acid sites gave pyruvaldehyde (PA) and lactic acid (LA), the base-catalyzed route afforded fructose. This is demonstrated on a series of six commercial TiO2 samples and further confirmed by using two reference catalysts: niobic acid (NbOH), an acid catalyst, and a hydrotalcite (MgAlO), a basic catalyst. The original acid-base properties of the six commercial TiO2 with variable structure and texture were investigated first by conventional methods in gas phase (FTIR or microcalorimetry of pyridine, NH3 and CO2 adsorption). A linear relationship between the initial rates of DHA condensation into hexoses and the total basic sites densities is highlighted accounting for the water tolerance of the TiO2 basic sites whatever their strength. Rutile TiO2 samples were the most basic ones. Besides, only the strongest TiO2 Lewis acid sites were shown to be water tolerant and efficient for PA and LA formation.