A catalyst system including at least one metal and an oxide support, said oxide support including at least one of Al2O3, MnxOy, MgO, ZrO2, and La2O3, or any mixtures thereof; said catalyst being suitable for catalyzing at least one reaction under supercritical water conditions is disclosed. Additionally, a system for producing a high-pressure product gas under super-critical water conditions is provided. The system includes a pressure reactor accommodating a feed mixture of water and organic matter; a solar radiation concentrating system heating the pressure reactor and elevating the temperature and the pressure of the mixture to about the water critical temperature point and pressure point or higher. The reactor is configured and operable to enable a supercritical water process of the mixture to occur therein for conversion of the organic matter and producing a high-pressure product fuel gas.