Molecular hydrogen is of critical importance in the chemical industry and might play vital role in future for benign and secure energy technologies. However, its generation from renewable sources, such as water and biomass without the addition of external energy (heat, light and electricity) remains difficult. Herein, we report that Group VIII and Group IB transition metal nanoparticles supported on magnesium oxide are active all-solid-state catalyst systems for external energy-free liberation of molecular hydrogen by aldehyde reforming in liquid water at ambient conditions. Based on experimental observation and DFT calculation, it is suggested that molecular oxygen as well as water can be readily reduced on Group IB metals via two-electron process to form peroxy radicals, while most of Group VIII metals such as Pt end up with sites creating oxo species but instead form hydroxyl radicals via four-electron processes. Therefore, by altering supporting metals, we are able to control the reaction pathways (reduction of aldehydes into H-2 or oxidation of aldehydes into CO2) due to the difference of oxygen reduction products. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.