In a jeweler's workshop, gold is never mixed with ruthenium: one is soft and shiny, the other refractory and brittle. But at the nanometer scale, these metals merge into a single alloy that, when heated, begins to separate like oil and water in a cooling broth. That's exactly the behavior researchers captured while studying particles smaller than a hundredth of a hair's thickness.
Initially, the nanoparticles are a homogeneous mixture of gold and ruthenium with a cubic atomic packing. When heated above 350 degrees Celsius, distinct regions form inside each particle: gold-rich (with a cubic lattice) and ruthenium-rich (with a hexagonal one). It's like how crystallites of different shapes assemble in solidifying wax. But the most interesting part happens when the particles are placed in a hydrogen-nitrogen mixture at atmospheric pressure, just like in industrial reactors for ammonia synthesis.
These voids and sharp facets on the particle surfaces appear only under high gas pressure. Calculations showed that hydrogen amplifies the difference in mobility between gold and ruthenium atoms, making the former leave faster and leave vacancies—'holes' left by departed atoms. This mechanism is known as the Kirkendall effect: if two metals diffuse at different rates, pores form at the interface.
Understanding these restructurings will help create catalysts that don't degrade during operation. After all, ammonia is needed for fertilizers, and its production currently consumes about 2% of the world's energy. If particles become more stable, the process will become cheaper and more environmentally friendly.
The Kirkendall effect was first observed in 1947 in brass: zinc diffused faster than copper, and pores formed at the contact point. Now the same mechanism has been found in nanoparticles for catalysis.