Direct thermal decomposition of hydrocarbons into solid carbon and hydrogen is performed by a process and a device. The process comprises preheating a hydrocarbon gas stream to a temperature between 500° C. and 700° C. and injecting the pre-heated hydrocarbon gas stream into the reactor pool of a liquid metal reactor containing a liquid media; forming a multi-phase flow with a hydrocarbon gas comprising hydrogen and solid carbon at a temperature between 900° C. and 1200° C.; forming a carbon layer on the free surface of the liquid media made up of solid carbon particles which are then displaced into at least one carbon extraction system and at least one recipient for collecting them; and, at the same time, the gas comprising hydrogen leaves the reactor pool through a porous rigid section, being collected at a gas outlet collector from where the gas comprising hydrogen finally leaves the liquid metal reactor.