Venkatesan V. Krishnan

An Examination of Carbonaceous Deposits in Direct-Utilization SOFC Anodes

The deposition, stability, and function of carbonaceous films formed by exposing porous yttria-stabilized zirconia (YSZ) anodes in YSZ-based solid oxide fuel cells (SOFCs) to n-butane at elevated temperatures was studied using a combination of four-probe conductivity, impedance spectroscopy, and cell polarization measurements. The carbonaceous deposits were found to have high electronic conductivity and to be relatively stable for steam-to-carbon ratios as high as 3.75. Comparison of the performance of cells in which carbon films were used as the sole current collector in the anode with anodes containing both Cu and carbon films indicated that in the latter case, the carbon layer plays an important role in providing electronic conductivity near the three-phase boundary.

Encapsulation studies of hydrogen on cadmium exchanged zeolite rho at atmospheric pressure

Abstract Temperature programmed diffusion (TPDi) has been used to study the encapsulation of hydrogen in cadmium exchanged Cs-rho zeolite. The amount encapsulated after 2 h has been observed to be about 71 μmol/g at 50°C and 1 atm. This amount is over 30 times the amount of hydrogen encapsulated with NaX or NaA at 37°C for the same time and pressure. Upon increasing the encapsulation temperature to 100°C, the amount encapsulated increased to 161 μmol/g (2 h of encapsulation). At 200°C, the encapsulate is about 620 μmol/g, for the same pressure and time. With increasing temperature, more than one peak is seen in the TPDi spectra, revealing the availability of more than one site 1 for the encapsulation. 3 peaks are observed in TPDi spectra for the encapsulation at 200°C - at 107, 295 and 345°C. Large encapsulated amounts of hydrogen arise from blocking effects caused by the presence of cations (cadmium and/or cesium). Experiments for encapsulation of hydrogen on H-rho (hydrogen exchanged zeolite rho) show negligible uptake of hydrogen, proving that the presence of either the cadmium ion (5.05 Cd2+ per unit cell) or the cesium ion (1.87 Cs+ per unit cell) or both is directly responsible for the encapsulation of hydrogen. For encapsulation at 200°C, possible migration of the encapsulate among the sites seems to occur with encapsulation time. This could explain the relative changes in the intensities of the 3 peaks in the TPDi spectra for the encapsulation at 200°C.

Encapsulation of hydrogen in cadmium-exchanged zeolite rho; temperature-programmed diffusion studies

Cadmium-exchanged zeolite rho encapsulates 161 µmol g–1 of hydrogen at 100 °C and 620 µmol g–1 at 200°C, (in 2 h and at 1 atm), the highest ever observed for any zeolite at 1 atm; hydrogen-exchanged zeolite rho, however, shows negligible uptake of hydrogen, providing evidence that the cadmium ion is directly responsible for hydrogen encapsulation.

Mathematical modeling of transient diffusion and adsorption of cyclopropane in NaX, Ni/NaX and Eu/NaX zeolites

Abstract The transient uptake of cyclopropane gas in NaX zeolite under isothermal conditions has been simulated by a mathematical model. The model has been curve-fit to the experimental data obtained in a stainless steel microreactor with a small bed of catalyst (NaX zeolite). Intra-crystalline diffusion of cyclopropane gas was assumed to play a significant role in the uptake of the adsorbate gas in the zeolite. Based upon this assumption, the response to a switch from a stream of pure non-adsorbing argon to one of 0.5% cyclopropane/argon (a step function) was simulated by assuming an effective intracrystalline diffusivity of the cyclopropane/argon mixture and also taking into account the CSTR conditions in the microreactor. The Langmuir adsorption isotherm was also used to explain the adsorption of the cyclopropane gas in the active sites of the zeolite catalyst. From the simulation results and subsequent curve-fit of the experimental data, the effective diffusivity of the cyclopropane/argon was estimated to be about 2 × 10 −11 cm 2 /s. The diffusivity of the cyclopropane in Ni/NaX was estimated to be about 2 × 10 −12 cm 2 /s and 3 × 10 −12 cm 2 /s for the Eu/NaX system. These values of the diffusion coefficient seem reasonable in comparison to the results of diffusion coefficients obtained by similar methods such as Zero Length Chromatography and gravimetric techniques for other organic components in different zeolites. A non-isothermal model taking into account the heat of adsorption of cyclopropane and the activation energy of diffusion of the gas mixture was also formulated to observe any possible temperature rise in the catalyst bed. The temperature of the bed rose not more than 2°C, causing very little changes in the effective diffusivity of the gas mixture, thereby justifying the assumption of isothermality of the uptake process, according to the modeling data.