Paul K. T. Liu

A study by in situ techniques of the thermal evolution of the structure of a Mg–Al–CO3 layered double hydroxide

Abstract Several in situ techniques have been used to investigate the thermal evolution of the structure of a Mg–Al–CO 3 layered double hydroxide (LDH) under an inert atmosphere. Based on the results of the study, a model is proposed to describe the structural evolution of the Mg–Al–CO 3 LDH. According to this model as the temperature is increased, loosely held interlayer water is lost in the temperature range of 70–190°C, but the LDH structure still remains intact. The OH − group, likely in a Al–(OH)–Mg configuration, begins to disappear at 190°C, and is completely lost at 280°C; a gradual transformation of the LDH structure begins in the same range of temperatures. The OH − group, likely in a Mg–(OH)–Mg configuration, begins to disappear at 280°C and is completely lost at 405°C; a gradual degradation of the LDH structure is observed in the same range. Although some CO 3 2− loss is observed at lower temperatures, its substantial loss begins at 410°C, and is completed at 580°C. At these temperatures the material becomes an amorphous metastable, mixed solid oxide solution.

Characterization of hydrogen-permselective microporous ceramic membranes

Abstract A series of Si-modified membranes were prepared by chemical vapor deposition using 40 A γ-alumina tubular membranes as supports. Their hydrogen permeance ranged from 0.028 to 17.6 m3/m2 h atm and H2/N2 selectivity (permeance ratio) ranged from 12.6 to 72 at 600°C. The selectivity to isobutane ranged from 40 to 240 at 300°C and could be higher at a higher temperature. Compared with similar membranes documented in the literature. these membranes exhibited “order-of-magnitude” improvement in the permeance while maintaining a moderate selectivity. These membranes could be ideal for industrial gas separations and catalytic reactions handling a large volume of streams. Hindrance diffusion through micropores (i.e.,5 A) and Knudsen diffusion through larger pores were suggested separation mechanisms for these modified membranes. These mechanisms coupled with hypothetical pore size distributions were tested satisfactorily with a wide range of permeation behaviors delivered by a series of membranes with different microporous structures. Specifically, they explained the subtle permeation difference between hydrogen and helium, and the relative contribution between hindrance and Knudsen diffusion for nitrogen. Separations of gas mixtures containing hydrogen were confirmed similar to the ideal separations determined by single components. The modified membranes were thermally stable at 600 °C. The hydrothermal stability test indicated that the membrane structure approached a new steady state immediately after exposing to moisture.

Characterization of ceramic membranes I. Thermal and hydrothermal stabilities of commercial 40 Å membranes

The thermal stability (up to 640°C) and hydrothermal (up to 90% of steam) stability of commercial microporous ceramic membranes with an ≈40 A pore diameter were characterized by dynamic capillary condensation porometry and gas permeation. The pore size of the membrane remained relatively constant under the thermal treatment for up to 110 h. Tne N2 gas permeance increased from ≈60 to ≈120 m3/m2 h atm, likely attributed to the removal of adsorbed moisture during storage. The pore size increased to≈50 and ≈65 A under hydrothermal treatment at 640°C with a stream containing 5 and 90% steam, respectively. The largest changes in membrane structures occurred within the first few hours of treatment. The corresponding increases in the gas permeance correlated well with the enlargement in the pore diameter, indicating the porous structure based upon the ratio of tortuosity to porosity likely remained constant during the hydrothermal treatment.

Characterization of ceramic membranes II. Modified commercial membranes with pore size under 40 Å

Abstract A methodology was developed for characterizing a series of membranes with pore size ⩽40 A. These membranes were modified from commercial microporous γ-Al 2 O 3 membranes (40 A) by pore size reduction. SEM/EDAX analysis showed a modified layer with a thickness of ≈1.5 μm deposited within the existing porous structure. A flow-weighted pore size distribution analyzer was used to determine pore size versus flow contribution for the pore size ranging from 40 A down to ≈15 A. Size exclusion with selected gaseous molecules was employed to determine pore size 40 to ⩽5 A at 25°C was investigated in detail. Selectivity decreased slightly along with the pore size reduction in the Knudsen regime (i.e., 40 A down to ≈20 A), then increased dramatically in the molecular sieving regime. The unexpected decrease of the selectivity in the Knudsen regime was likely to have resulted from the contribution of surface diffusion of nitrogen.

Water Gas Shift Reaction with A Single Stage Low Temperature Membrane Reactor

Palladium membrane and Palladium membrane reactor were developed under this project for hydrogen separation and purification for fuel cell applications. A full-scale membrane reactor was designed, constructed and evaluated for the reformate produced from a commercial scale methanol reformer. In addition, the Pd membrane and module developed from this project was successfully evaluated in the field for hydrogen purification for commercial fuel cell applications.