One of the methods I used to prepare Mg nanoparticles was using physical vapor deposition by subjecting pure Mg particles to laser beam in vacuum then collecting the vapor over copper plate, then scratching the Mg layer and fine grinding to get the nano-sized particles. But I think there are another methods. Good luck. 

Do you want to use magnesium nanoparticles to get hydrogen when exposed to them?

Best regards.

I want to produce biogas and methane using magnesium nanoparticles as they give rise to enzyme activity.

Magnesium nanoparticles will react with water. Therefore, it is cheaper to use magnesium salts.

Thank you Sir Yuri Mirgorod