Hydrogen production process

A hydrogen production process comprising splitting water into hydrogen and oxygen, wherein

• the process comprises decomposing sulfuric acid into water, sulfur dioxide and oxygen through a reaction represented by the following formula (X1) by using solar thermal energy,
• the reaction represented by the following formula (X1) includes elementary reactions represented by the following formulae (X1-1) and (X1-2),
• at least a part of the elementary reaction of the following formula (X1-1) is performed using thermal solar energy,
• at least a part of the elementary reaction of the following formula (X1-2) is performed using additional thermal energy provided by combustion of hydrogen added in a reaction vessel used for performing the elementary reactions of formula (X1-1) and /or formula (X1-2), and the equilibrium of reaction represented by the following formula (X1-2) is shifted to the sulfur dioxide production side by reaction of the remaining added hydrogen after the combustion, and oxygen generated by conversion of SO3 to SO2, and wherein
• the heating temperature by said additional thermal energy is higher by 10° C. or more than the heating temperature by the solar thermal energy, and
• at least a part of said additional thermal energy is generated in the reaction vessel used for performing the elementary reactions of formula (X1-1) and/or formula (X1-2), and the reaction of the remaining added hydrogen after the combustion and the oxygen generated by conversion of SO3 to SO2 occurs in the reaction vessel used for performing the elementary reaction of formula (X1-2):
• H2SO4→H2O+SO2+1/2O2 (X1)
• H2SO4→H2O+SO3 (X1-1)
• SO3→SO2+1/2O2 (X1-2).