Soft Atoms Enhance Hydrogen Purification

March 26, 2009

Materials with highly polarizable surfaces promise better performance.

The technology choice for purifying hydrogen won’t be hard, hope researchers at Northwestern University, Evanston, Ill. They have developed porous materials that use “soft” atoms to provide high selectivity in separating hydrogen from gas mixtures. The honeycomb-like structures rely on polarization instead of pore size for their effectiveness — and thus provide a new separation method, they say. “Soft atoms are large atoms with large electronic clouds around them which can be affected (sloshed around easily) by the application of electric fields. These are like large rubber balloons,” explains Mercouri Kanatzidis, professor of chemistry at the school. “We achieve high polarizability by using large heavy atoms (e.g., lead, germanium and tellurium).” “Most known porous materials are oxides. Oxides are hard,” he adds.

Figure 1 -- Better separation: Hydrogen (white) passes quickly
through the pore while carbon dioxide (red and black) interacts
with the walls.
Source: Northwestern University.

“These soft-wall atoms like to interact with other soft molecules passing by, slowing them down as they pass through the membrane. Hydrogen, the smallest element, is a ‘hard’ molecule. It zips right through while softer molecules like carbon dioxide and methane take more time.” (See Figure 1.) Separation takes place at “a convenient temperature range,” he notes — between 0°C and room temperature. In addition, the chalcogenide frameworks have the potential to resist the sulfur poisoning that can occur in other separation materials. Kanatzidis and Gerasimos Armatas, postdoctoral research associate, created materials based on germanium-rich chalcogenide networks with soft highly polarizable surfaces. In 2008 laboratory-scale trials, the high-surface-area materials provided CO₂/H₂ and CH₄/H₂ selectivities of about 50:1, which, as far as Kanatzidis knows, is the best so far achieved for such separations. The germanium, lead and tellurium material boasted about four times better selectivity than conventional materials made of lighter elements like silicon, oxygen and carbon. “Our materials could be used very effectively as membranes for gas separation. We have demonstrated their superior performance,” he says. Their selectivity can be further improved by increasing the soft character of the chalcogenide frameworks, he adds. Nitrogen purging at 30°C can remove adsorbed CO₂ and CH₄, enabling the material to serve for many separation cycles. At this point, however, the materials produced are delicate, admits Kanatzidis, and only prove the concept. So, the researchers are working to develop new versions that are more robust, and to construct good membranes from them.