The reduction of copper oxide to metallic copper by hydrogen gas has been a popular chemistry experiment for generations.
It is not without its problems however. Hydrogen is extremely flammable and explosive over a wide range of concentrations. A common problem is that the air in the apparatus is not properly flushed out so that an explosive mixture remains which is detonated when a flame is applied to the end to burn off the excess hydrogen.
This microscale method uses a small amount of hydrogen only and in a very small space so there is no danger of explosion.
