RESEARCH HIGHLIGHTS
Catalysis Soaking up the atmosphere
Combining carbon dioxide sorbents with gold nanoparticle catalysts makes manufacturing ultrapure hydrogen gas easier than ever
Pure hydrogen (H2 ) is an important chemical widely used in the
chemical industry, many semiconductor fabrication processes, as well as in Polymer Electrolyte Membrane (PEM) fuel cells. Almost all of the H2
gas generated today comes from the steam
reforming of natural gas at oil refineries. However, this process also produces trace amounts of carbon monoxide (CO) byproduct, which limits the application of H2
and can ‘poison’ or destroy the
delicate catalysts used in the manufacture of semiconductor and state-of-the-art fuel cells. Researchers led by Ziyi Zhong and Jizhong Luo from the A*STAR Institute of Chemical and Engi- neering Sciences in Singapore1
have now developed a material
that purifies H2 dioxide (CO2
gas by catalytically converting CO to carbon ) while simultaneously removing excess CO2
— an
approach that enables CO removal down to the parts-per-million (ppm) level. Although several methods exist for H2
purification, the pref-
erential oxidation (PROX) reaction is often favored by fuel cell designers because it can be adapted for use in small, on-board reactors. In the PROX system, a mixture of H2
, CO and oxygen
gases passes over a metal catalyst located on a ceramic support (see image). This sets off a complex series of oxidation reac- tions that consume CO, which generates various by-products including CO2
. Currently, gold nanoparticles are garnering attention as PROX
catalysts because they are active below 100°C; lower temperatures enable more selective CO oxidation and are safer for vehicle appli- cations. One problem with these catalysts, however, is their inabil- ity to lower CO concentrations below 100 ppm. Previous studies have suggested that the reason CO2 catalysts is because CO2 Removing CO2
gradually deactivates these
binds to the catalyst surface as carbonate. from the gas mixture with a solid-state sorbent
material is one way to enhance PROX reactions and lower CO concentrations to the single ppm levels needed for H2
fuel cells.
However, the challenge faced by Zhong and co-workers was that most common inorganic CO2
sorbents are incompatible
with gold nanoparticles — their high working temperatures decrease the effectiveness of CO oxidation and destabilize the tiny metallic particles. The team chose a novel porous material
A*STAR RESEARCH OCTOBER 2011– MARCH 2012 A combined catalyst/carbon dioxide (CO2 )-sorbent system (middle)
that removes carbon monoxide (CO) contaminants from hydrogen gas (H2
) may soon be part of on-board fuel cells. CO2 removed, reaction enhanced
CO + O2 + H2
Au/α-Fe2O3 catalyst
Ultrapure H2
Preferential oxidation of CO
known as APTES/SBA-15 for their sorbent because it has a robust silica structure and contains amine groups that readily react with free CO2
at low temperatures. Further experiments
revealed that APTES/SBA-15 sorbents boosted CO removal by an average 10% over unprotected gold PROX nanocatalysts. Optimizing the layered arrangement of catalysts and sorbents gas from 2,000 ppm
in the reactor lowered the CO levels in H2
to 25 ppm. Zhong says that he expects even better performance in the future. “There is still plenty of room for development of better CO2
sorbents and catalysts for this process,” says Zhong. on Au/α-Fe2 O3 ■
1. Ng, J. W. D., Zhong, Z., Luo, J. & Borgna, A. Enhancing preferential oxidation of CO in H2
APTES/SBA-15 CO2 -sorbent. International Journal of Hydrogen Energy 35, 12724–12732 (2010). catalyst via combination with
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© 2010 Elsevier
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