Hydrogen production thermochemical cycles, based on the recirculation of sulfur-based compounds, are among the best suited processes to produce hydrogen using concentrated solar power. The sulfuric acid decomposition section is common to each sulfur-based cycle and represents one of the fundamental steps. A novel direct solar receiver-reactor concept is conceived, conceptually designed and simulated. A detailed transport phenomena model, including mass, energy and momentum balance expressions as well as suitable decomposition kinetics, is described adopting a finite volume approach. A single unit reactor is simulated with an inlet flow rate of 0.28 kg/s (corresponding to a production of approximately 11 kg(H2)/h in a Hybrid Sulfur process) and a direct solar irradiation at a constant power of 143 kW/m(2). Results, obtained for the high temperature catalytic decomposition of SO3 into SO2 and O-2, demonstrate the effectiveness of the proposed concept, operating at pressures of 14 bar. A maximum temperature of 879 degrees C is achieved in the reactor body, with a corresponding average SO2 mass fraction of 27.8%. The overall pressure drop value is 1.7 bar. The reactor allows the SO3 decomposition into SO2 and O-2 to be realized effectively, requiring an external high temperature solar power input of 123.6 kJ/mol(SO2) (i.e. 123.6 kJ/mol(H2)). (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.