Lastly, Pt contamination of active sites caused by impurities is also a critical degradation mechanism for Pt-based catalysts. The most common contaminant within Pt-based catalysts is CO, which is typically a by-product of reforming natural gas. A small amount of CO within the fuel cell inlet gas is absorbed into the Pt's catalytically active sites, blocking the passage of H 2 contact sites and reducing the catalyst's activity. But this also reveals that a noble-metal catalyst with good durability should also have a low CO absorption performance to maintain its long durability within the fuel cell .