Using elemental copper as a catalyst for a reaction.
Copper is a widely used catalyst for several transformations in organic chemistry. In most cases it is used as it’s salts, mainly copper(I)-iodide, but in some cases, as Ulmann reactions elemental copper could be also used (for more, see: https://youtu.be/UKyO2e-35xc).
An interesting fact about metal catalysis is, that in old times they had to investigate again every reaction in every 10-20-30 years that used metals as a catalyst. This was because in most cases only impurities of the used metal catalyzed the reaction and not the metal that they used in the reaction. What does this means?
If they used an e.g. zinc powder that was manufactured in 1895 for a reaction and it worked well it could be a result of the palladium, copper or any other impurities that were present in the zinc in low percent. However when the manufacturer improved it’s technology for the preparation of zinc powder and in 1910 it contained 1% of the previous impurities it often gave bad results for the same reaction.