As catalytic particles go, smaller is generally better. The tinier the particle, the larger its surface-to-volume ratio and the larger the proportion of active sites available to reactants. To maximize that surface-area advantage, reaction engineers commonly deposit catalysts as nanoparticles on larger, inert support particles.
Heat is often used alongside catalysts to speed reactions. Unfortunately, under prolonged heat exposure, nanoparticles can migrate about the support surface, collide, and glom onto one another, which can diminish their reactivity or even deactivate them altogether. That’s what Yadong Li (Tsinghua University, Beijing) and his colleagues initially observed when they began heating nanoparticles of palladium deposited on carbon–nitrogen supports. As the researchers now report, however, when they continued heating the catalyst, something surprising happened: The nanoparticles began vanishing from the group’s electron microscopy images. High-resolution techniques revealed that Pd was still present on the supporting substrate, but now in atomized form. (In the scanning transmission electron microscopy image shown here, individual Pd atoms are indicated with circles.)
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