Catalysis: Ready for a close-up

© EERE

The presence of carbon monoxide (CO) impurities in hydrogen gas
(H2) can have a detrimental impact on the performance of fuel
cells. Recent studies have shown that gold nanoparticles —
particles less than five nanometers wide — can catalytically
remove CO impurities from H2 under mild temperature and pressure
conditions. This breakthrough understanding has helped facilitate
the development of fuel-cell vehicles that use
‘onboard’ fuel processing technology. Unfortunately,
gold nanoparticles tend to lose their catalytic activity after a
few hours of use — and scientists need to overcome this
problem if gold nanoparticles are to be used.

Ziyi Zhong at the A*STAR Institute of Chemical and Engineering
Sciences, Ming Lin at the A*STAR Institute of Materials Research
and Engineering and co-workers have identified the subtle,
atomic-scale structural transformations that can activate and
de-activate gold nanoparticle catalysts, a finding that may lead to
longer-lasting hydrogen fuel cells.

The researchers set out to design an improved catalyst for
so-called preferential oxidation (PROX) reactions. This approach
transforms CO impurities into carbon dioxide (CO2) on a ceramic
support containing metal catalysts. Previously, the team found that
silica-based supports, called SBA-15, could boost CO removal by
selectively absorbing the CO2 by-product. The researchers took
advantage of another SBA-15 characteristic — a mesoporous
framework decorated by terminal amine groups — to engineer a
novel PROX catalyst.

First, the team used amine modification to disperse a mixture of
gold and copper(II) oxide (CuO) precursors evenly over the SBA-15
support. They then used heating treatment to generate gold and CuO
nanoparticles on the SBA-15 support. The numerous pores in SBA-15
and the CuO particles work together to hinder agglomeration of gold
nanoparticles — a major cause of catalyst
de-activation.

The team then achieved a near-unprecedented chemical feat:
localized structural characterization of their catalyst at atomic
scale, using high-resolution transmission electron microscopy
(HR-TEM) and three-dimensional electron tomography (see movie
below). These imaging techniques revealed that the active catalyst
sites — gold or gold–copper alloy nanoparticles in the
immediate vicinity of amorphous and crystalline CuO —
remained stable for up to 13 hours. However, the reducing
atmosphere eventually transforms CuO into copper(I) oxide and free
copper; the latter of which then alloys with the gold nanoparticles
and deactivates them. Fortunately, heating to >300°C
reversed the alloying process and restored the catalyst’s
activity.

“People working in catalysis are always curious about the
‘local structures’ of their materials,” says
Zhong. “Because the Au-CuO/SBA-15 catalyst is active at room
temperature, advanced characterization in our state-of-the-art
facilities is possible — though it takes great patience and
requires multidisciplinary collaboration.”

The A*STAR-affiliated researchers contributing to this research are
from the Institute of Chemical and Engineering Sciences and the
Institute of Materials Research and Engineering

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References:

Li, X., Fang, S. S. S., Teo, J., Foo, Y. L., Borgna, A. et al.
Activation and deactivation of Au–Cu/SBA-15 catalyst for
preferential oxidation of CO in H2-rich gas. ACS Catalysis 2,
360–369 (2012).