The catalytic hydrogenation of CO 2 at the surface of a metal hydride and the corresponding surface segregation were investigated. The surface processes on Mg 2NiH 4 were analyzed by in situ X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS) and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). CO 2 hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that for the CO 2 methanation reaction, the dissociation of H 2 molecules at the surface is not the rate controlling step but the dissociative adsorption of CO 2 molecules on the hydride surface. © the Owner Societies 2012.