Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina‐supported niobium oxide (Nb₂O₅/Al₂O₃) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier‐transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy‐dispersive X‐ray (EDX) analysis indicated that the Nb₂O₅ monolayer domains were highly dispersed over alumina at low Nb₂O₅ loadings, such as 5 wt %, and no Brønsted acid sites were presents. The coverage of Nb₂O₅ monolayer domains over Al₂O₃ increased with increasing Nb₂O₅ loading and almost‐full coverage was obtained at a loading of 16 wt %. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb? O(H)? Nb), which were formed at the boundaries between the domains of the Nb₂O₅ monolayer, acted as thermally stable Brønsted acid sites.     

On-line mass spectrometric analysis of hydrothermal reactions for biomass model sample containing sulfur compounds

Biomass gasification using supercritical water is a promising way to produce hydrogen gas. However, this method might release toxic heteroatomic compounds. It is therefore important to clarify reaction pathways for efficiently obtaining hydrogen gas and suppressing environmental burden. L-cysteine was adopted for a test reagent containing sulfur and determination of the sulfur compound reaction pathways was studied by Li⁺-ion attachment mass spectrometry. It was found that H₂S, CO, CO₂, SO, SO₂ and SO₃ gasses were released at high concentrations in the gas phase during the hydrothermal reaction. By adding Ca(OH)₂ as alkali, the pathway of these gasses were, however, suppressed into the liquid phase so that the toxic emissions to the gas phase could be avoided.     

Dominant parameters for maximum velocity induced by body-force models for plasma actuators

This study investigates the relationship between body-force fields and maximum velocity induced in quiescent air for development of a simple body-force model of a plasma actuator. Numerical simulations are conducted with the body force near a wall. The spatial distribution and temporal variation of the body force are a Gaussian distribution and steady actuation, respectively. The dimensional analysis is performed to derive a reference velocity and Reynolds number based on the body-force distribution. It is found that the derived Reynolds number correlates well with the nondimensional maximum velocity induced in quiescent conditions when the center of the Gaussian distribution is fixed at the wall. Additionally, two flow regimes are identified in terms of the Reynolds number. Considering the variation of the center of gravity of force fields, another Reynolds number is defined by introducing a new reference length. The nondimensional maximum velocity is found to be scaled with the latter Reynolds number, i.e., the maximum induced velocity in quiescent conditions is determined from three key parameters of the force field: the total induced momentum per unit time, the height of the center of gravity, and the standard deviation from it. This scaling turns out to be applicable to existing body-force models of the plasma actuator, despite the force distributions different from the Gaussian distribution. Comparisons of velocity profiles with experimental data validate the results and show that the flow induced by a plasma actuator can be simulated with simple force distributions by adjustment of the key body-force parameters.