  Metal nano-aerogels combine a large surface area, a high structural
stability, and a high catalytic activity towards a variety of chemical
reactions. The performance of such nanostructures is underpinned by the
atomic-level distribution of their constituents. Yet monitoring their
sub-nanoscale structure and composition to guide property optimization remains
extremely challenging. Here, we synthesized Pd nano-aerogels from a K2PdCl4
precursor and two different NaBH4 reductant concentrations in distilled water.
Atom probe tomography reveals that the aerogel is poly-crystalline and that
impurities (Na, K) are integrated from the solution into grain boundaries. Ab
initio calculations indicate that these impurities preferentially bound to the
Pd-metal surface and are ultimately found in grain boundaries forming as the
particles coalesce during synthesis, with Na atoms thermodynamically
equilibrating with the surrounding solution and K atoms remaining between
growing grains. If controlled, impurity integration, i.e. grain boundary
decoration, may offer opportunities for designing new nano-aerogels.
