Single atoms shine as catalysts

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.)

The researchers used density functional theory to piece together what happened. The high temperatures—around 900 °C—essentially vaporized the nanoparticles, and the gas-phase atoms were individually recaptured at nitrogen-rich sites on the support material. Each Pd atom was tethered to the surface by four sturdy Pd–N bonds, which stabilized the dispersion when the catalyst was cooled back to room temperature.

In test reactions involving the conversion of acetylene to ethylene—an important step in the industrial preparation of polyethylene—the atomically dispersed catalysts substantially outperformed their nanoparticle counterparts; they delivered higher conversion rates at lower operating temperatures. Importantly, the heat-driven atomization technique isn’t limited to Pd. The researchers found that gold and lead could be atomized in the same manner. (S. Wei et al., Nat. Nanotechnol., 2018, doi:10.1038/s41565-018-0197-9 .)