PEM fuel cells are highly sensitive to the presence of carbon monoxide (CO) impurities in hydrogen gas (H₂), since CO adsorbs strongly on precious metals—such as the platinum commonly used in fuel cell anodes. However, depending on the reforming model, the hydrogen stream may initially contain almost 10% CO (dry basis). Therefore, one of the most important requirements for successful operation of a fuel cell is the reduction of the CO concentration to <10 ppm (99.99% conversion) to protect the Pt anode from CO poisoning and deactivation.

The objective of PROX is high CO conversion to CO₂ without excessive hydrogen oxidation (to water) via a catalyst. Recent studies have shown that gold nanoparticles less than five nanometers wide can effect the PROX reaction under mild temperature and pressure conditions. Unfortunately, gold nanoparticles tend to lose their catalytic activity after a few hours of use.

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 colleagues set out to design an improved catalyst for PROX reactions.

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 achieved localized structural characterization of their catalyst at atomic scale using high-resolution transmission electron microscopy (HR-TEM) and three-dimensional electron tomography. 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.

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. However, heating to >300 °C reversed the alloying process and restored the catalyst’s activity.

...we have successfully prepared an Au/CuO/SBA-15 catalyst via a nanoengineering approach. This includes the surface modification of SBA-15 with APTES molecules and the DP method for Au and Cu deposition. A two-step pretreatment process, which generates an active phase consisting of very small Au or Au_xCu_1−x particles in the immediate vicinity of highly dispersed CuO layer on SBA-15, results in a highly active catalyst for the PROX reaction. Three roles have been identified for the CuO layer: (i) minimizing the Au particle size and (ii) stabilizing them and (iii) facilitating the activation of molecular oxygen.

This metal−oxide bi-particle structure is highly active for PROX reaction. It is found to be superior to both the Au/SBA- 15 and the CuO/SBA-15 catalysts. However, this type of catalyst deactivates easily at both room temperature and high reaction temperatures, because CuO is reduced to Cu₂O and Cu under the reducing atmosphere, and the Cu phase further migrates to Au particles and combine/dissolve in them.

Although we cannot exclude that some other factors may also lead to the deactivation of the catalysts, for example, the growth of the Au particles; however, in the early stage of the reaction at low temperatures, the main reason for the deactivation of the catalyst is due to the alloying of Au and Cu. This alloying process can be reversed via the oxidation treatment process. At high reaction temperatures (≥100 °C), in addition to the combination of Au with Cu, the agglomeration of Au particles also needs to be suppressed to prevent the deactivation of the catalyst. We believe these fundamental findings will play an important role in new catalyst design and development as well as deepen our understanding of the catalytic behaviors.

—Li et al.

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

Resources

• Li, X., Fang, S. S. S., Teo, J., Foo, Y. L., Borgna, A. et al. (2012) Activation and deactivation of Au–Cu/SBA-15 catalyst for preferential oxidation of CO in H₂-rich gas. ACS Catalysis 2, 360–369 doi: 10.1021/cs200536a

• Shore, L. and Farrauto, R. J. (2010) PROX catalysts. Handbook of Fuel Cells. doi: 10.1002/9780470974001.f302019