Hydrogen spillover is strictly what it feels like. Small steel nanoparticles anchored on a thermally steady oxide, like silica, comprise a significant class of catalysts, that are substances used to speed up chemical reactions with out being consumed themselves. The catalytic response normally happens on the reactive — and costly — steel, however on some catalysts, hydrogen atom-like equivalents actually spill from the steel to the oxide. These hydrogen-on-oxide species are referred to as ‘hydrogen spillover.’