Joong Gill Choi

Kinetic analysis of supported Ni-catalyzed CO2/CH4 reactions using photoacoustic spectroscopy

CO2 photoacoustic spectroscopy was employed to analyze kinetically the CO2/CH4 reaction catalyzed by 14 wt% Ni/Al2O3 and 14 wt% Ni/TiO2. The catalytic reaction was carried out in the temperature range of 673-923 K at various partial pressures of CO2 and CH4 (40 Torr total pressure) in a closed-circulating reactor system. The CO2 photoacoustic signal, measured by using a differential photoacoustic cell, was recorded as a function of reaction time. Under these conditions, Al203 and TiO2 used as supports do not promote the reaction as noted by the lack of changes in the CO2 photoacoustic signal. Reactions run in the presence of H2-reduced supported Ni catalysts are associated with significant time dependent changes in the CO2 photoacoustic signal, while processes carried out in the presence of unreduced catalysts do not. Changes in the CO2 photoacoustic signal at early reaction times provide precise data for the rate of CO2 disappearance. The rate of CO2 disappearance is observed to increase with increasing temperature in the range of 673-923 K. Apparent activation energies for CO2 consumption were calculated to be 15.4 kcal mol(-1) for the Ni/Al2O3- and 14.3 kcal mol(-1) for the Ni/TiO2-catalyzed reactions. Reaction orders, determined from initial rates of CO2 disappearance at 873 K, were found to be 0.48 in CH4 and 0.45 in CO2 for the Ni/Al2O3-promoted process, and 0.38 in CH4 and 0.32 in CO2 for the Ni/TiO2-catalyzed reaction. The results of this effort were compared with those reported previously and were used to construct a mechanism for the low pressure CO2/CH4 reaction.

Photoluminescence characteristics of coupled CdSe∕ZnS quantum dots on self-assembled silica nanospheres

The photoluminescence (PL) characteristics of the self-assembled silica nanospheres containing coupled CdSe∕ZnS core-shell quantum dots (QDs) on the surface of the sphere were studied. The degree of QD-coupling and the distribution of the coupled QDs were controlled by generating spatially separated QD-binding sites on the surface of the spheres and varying the QD concentration during the attachment process. When the sizes of QD spots on the surface probed by atomic force microscopy gradually increased, the band gap energy shifted to lower energy via strong Forster energy transfer between nearby QDs, and when UV-exposure time increased, the band gap energy shifted to slightly higher energy, accompanied by a large enhancement in the emission intensity. Moreover, the Bragg diffraction of the self-assembled crystal structure of the spheres affected the PL characteristics of QDs by shifting the entire emission spectra toward longer wavelength as the incident angles were increased.

An acoustic based, laser induced breakdown method for determining the sizes and concentrations of nanoparticles

An acoustic, laser induced breakdown (LIBD) method has been developed for determining the sizes and concentrations of nano-sized colloids. The size distributions from a mixture of aquatic nanoparticles are determined by measuring amplitudes of plasma shockwave that are fitted to multi-curve Gaussian distributions displayed in separate peaks with positions corresponding to particle sizes and areas being proportional to concentrations.

Nondestructive Photoacoustic Measurement of Doping Densities in Bulk GaAs

The frequency-dependent photoacoustic (PA) signals and phase spectra of GaAs samples with different doping densities are measured and analyzed. In particular, we find that the magnitude differences of the PA signal and phase between intrinsic GaAs and doped GaAs materials decrease linearly with the doping density in the frequency region of nonradiative bulk recombination. The results show that the PA spectroscopic technique can be used to measure doping density in bulk semiconductors with reasonable accuracy.

Electrophoretron: a new method for enhancing resolution in electrokinetic separations.

Two capillaries, each of which have different surface preparations on their inside walls, are joined together to form a closed loop, and electrodes are placed inside the two capillaries. When the loop is filled with liquid and a potential difference is applied between the two electrodes, a circulating flow of liquid is established inside the loop because the resistance to flow is unequal in going from one electrode to another in a clockwise versus a counterclockwise direction. Consequently, a sample injected into this device, which we call an electrophoretron, repeatedly circulates between the two electrodes and the capillary separation column becomes effectively one of unlimited length. On each cycle the separation between analytes with different mobilities increases, thus enhancing resolution of analytes having nearly the same mobilities. The operation of a prototype electrophoretron is demonstrated.

Michelson interferometric detection for optoacoustic spectroscopy

Abstract We have described a Michelson interferometeric optophone which can be used as a sensitive microphone. A Michelson interferometer is constructed with a pellicle beam splitter attached on the wall of the optoacoustic cell. Periodic variation of the pressure inside the cell alter the path difference of the reflected beams, thereby changing the optical interference pattern. The optoacoustic signal can be detected by monitoring the change in the intensity of the interference fringe. This method shows a linear response to small signals and a comparable sensitivity to the conventional microphone.

PHOTOACOUSTIC MEASUREMENTS FOR CATALYTIC EFFECTS OF PEROVSKITE ELECTRODES IN A SEALED-OFF CO2 DISCHARGE TUBE

To investigate catalytic activities of perovskite-type electrodes in a sealed-off CO2 laser cavity, we studied the dissociation and reformation reactions of CO2 during electric discharge by monitoring the concentration change of CO2 as a function of time. A photoacoustic spectroscopy detection was introduced to measure the amount of CO2 in a closed discharge system. The catalytic activities in the dissociation of CO2 were demonstrated and compared for both Cu and La0.7Sr0.3CoO3 cathodes. The discharge with the gas mixture of CO2:N2:He=1:1:8 was performed under nine combinations of different conditions at pressures of 10, 20, and 30 Torr and discharge currents of 3, 4, and 5 mA. Within 1 min after the discharge, the concentration of CO2 reduced drastically by as much as 60% for both cathodes. In the case of the Cu cathode, the amounts of CO2 in the tube had not recovered even after 30 min, while with the perovskite cathode the CO2 contents recovered slowly up to 90%–95% within 5 min and then remained almos...