Graphene quantum dots trap metal atoms for catalysis

An international team of researchers has developed a technique that may transform chemical catalysis by greatly increasing the number of single transition-metal atoms that can be loaded onto a carbon carrier. Led by chemical and biomolecular engineer Haotian Wang of Rice University's Brown School of Engineering and Yongfeng Hu of the University of Saskatchewan, Canada, the researchers report their novel technique in a paper in Nature Chemistry.

The technique uses graphene quantum dots (GQD), particles of the super-strong 2D carbon material 3–5nm in size, as anchoring supports. These facilitate the loading of single transition-metal atoms at a high density, but with enough space between the atoms to avoid clumping.

The researchers proved the value of this technique by making a GQD-enhanced nickel catalyst. In a reaction test, this novel catalyst showed a significant improvement in the electrochemical reduction of carbon dioxide, compared with a lower nickel-loading catalyst.

According to Wang, expensive noble metals like platinum and iridium are widely studied by the single-atom catalyst community, with the goal of reducing the mass needed for catalytic reactions. But these metals are hard to handle and typically make up a small portion – 5–10% by weight or less – of the overall catalyst, including supporting materials.

By contrast, the Wang lab achieved transition-metal loadings in an iridium single-atom catalyst of up to 40% by weight. That equates to three to four spaced-out iridium atoms per 100 carbon substrate atoms, as iridium atoms are much heavier than carbon atoms.

"This work is focused on a fundamental but very interesting question we always ask ourselves: How many more single atoms can we load onto a carbon support and not end up with aggregation?" said Wang, whose lab focuses on the energy-efficient catalysis of valuable chemicals.

"When you shrink the size of bulk materials to nanomaterials, the surface area increases and the catalytic activity improves. In recent years, people have started to work on shrinking catalysts to single atoms to present better activity and better selectivity. The higher loading you reach, the better performance you could achieve.

"Single atoms present the maximum surface area for catalysis, and their physical and electronic properties are very different compared to bulk or nanoscale systems. In this study, we wanted to push the limit of how many atoms we can load onto a carbon substrate."

He noted that the synthesis of single-atom catalysts has up to now been either a 'top-down' or 'bottom-up' process. The first requires making vacancies in carbon sheets or nanotubes for metal atoms, but because these vacancies are often too large or not uniform, the metals can still aggregate. The second involves annealing metal and other organic precursors to 'carbonize' them, but the metals still tend to cluster.

The new process takes a middle approach by synthesizing GQDs functionalized with amine linkers and then pyrolyzing them with the metal atoms. The amines crosslink with the metal ions and keep them spread out, maximizing their availability to catalyze reactions.

"The maximum appears to be about 3–4 atomic percent using this approach," Wang said. "Future challenges include how to further increase the density of single atoms, ensure high stability for real applications and scale up their synthesis processes."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.