We investigated the densities of the hydrogen radical generated by various catalyst materials such as W, Pt, Ni, Mo, Ta and 5%Re-W. The hydrogen radical densities were measured with a hydrogen radical monitoring system. By using Mo instead of W as the catalyst, it was possible to reduce the temperature by about 300 K and still achieve the same density of hydrogen radical production. The Arrhenius plots of hydrogen radical density versus catalyst temperature were obtained in order to calculate the activation energies of hydrogen radical generation for the different catalysts. The activation energies of hydrogen radical generation for W and Mo were 2.53 and 2.36 eV, respectively. (C) 2009 Elsevier B.V. All rights reserved.