Disclosed herein is a method for producing excess enthalpy by (a) either dispersing atomic metal ions or clusters on a support and reacting the metal ions with a chelating ligand or dispersing chelated atomic metal ions on a support and (b) pressurizing with hydrogen or deuterium to reduce the metal ion to a metal atom resulting in the growth of dispersed metal particles less than 2 nm in diameter on the support. During the particle growth, there is a growth period during which a critical particle size is reached and excess enthalpy is produced. The growth period is typically several days long.