 Sabatier's principle suggests that for hydrogen evolution, a plot of rate constant versus hydrogen adsorption energy should result in a volcano, but this is not observed when oxide-covered metals are excluded. The reaction rate is determined by several factors and not just Sabatier's principle. The case of nickel is given special attention because it is a 3D metal and its orbitals are compact, making it a poor catalyst for hydrogen adsorption. This article was authored by Paula Quino, Fernanda Juarez, Elizabeth Santos, and others.