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.”