Behaviour of platinum nanoparticles under reducing and oxidising conditions using ESTEM

Abstract

Platinum nanoparticles are well‐known to be catalytically active in a wide variety of important chemical processes including hydrogen fuel cells 1 and diesel oxidation catalysts 2 . Platinum, as well as other nanoparticle catalysts are known to reduce their efficiency over time under operating conditions due to several mechanisms. The first method is via particle migration and coalescence where nanoparticles become mobile, collide and eventually form larger structures. Alternatively, the nanoparticles may undergo Ostwald ripening where large nanoparticles grow at the expense of smaller ones due to diffusion of less stable atoms across the support or in a gaseous phase. In either case, valuable catalytically active sites such as edge, corner and adatom sites may be lost as larger more stable facets take their place. As nanoparticles coalesce, more of the precious metal is locked away beneath the surface of the nanoparticles resulting in loss of active surface area and ultimately wasted metal. These processes have been studied with the help of atomic resolution transmission electron microscopy where heat combined with gas can be used to replicate real world operating conditions inside the microscope 3,4 . HAADF‐STEM with its Z contrast imaging capability is ideal for studying these nanoparticle growth mechanisms, particularly Ostwald ripening where small clusters/atoms are generally not visible in TEM. To further this important field of research, we investigate the differences in nanoparticle dispersion on two different supports before and after being exposed to a range of gases at different temperatures. We utilised the York JEOL 2200FS featuring double aberration correction and environmental TEM/STEM capability which has previously demonstrated single atom resolution in gas 5 . A MEMs chip holder from DENSsolutions was used for heating. To produce a simple model system to infer nanoparticle aging mechanisms on more complex industrial catalysts, we deposited platinum via magnetron sputtering onto a SiN X MEMs chip upon which graphite had previously been deposited (from ethanol suspension of graphite powder). Figure 1 shows an example of a graphite sheet loaded onto the SiNx MEMs chip. Figure 2 shows an example of the differences in dispersion before and after exposure to O 2 at elevated temperature for 6 hours. The image in Figure 2 is taken near a graphite sheet edge. The nanoparticles on the graphite are slightly larger than those on the SiN x on the freshly deposited sample but after the heat treatment in O 2 , the nanoparticles on the graphite are much larger. These results and environmental (S)TEM in general open new avenues into nanoparticle research using electron microscopy and have wide applications in chemical production, exhaust catalyst and future fuel cell design.