Abstract: Zero emission nested-loop (ZEN) reforming provides a scalable, eco-friendly process to produce high quality hydrogen at a relatively low operating cost. In one embodiment, a ZEN system comprises a reactor, a regenerator, and a photocatalytic reformer. During operation, the reactor receives a gas mixture and outputs hydrogen and catalyst adsorbed with carbon dioxide. The gas mixture is methane, steam, or hydrogen. Next, the regenerator receives the catalyst adsorbed with carbon dioxide and outputs carbon dioxide and desorbed catalyst. Next, the photocatalytic reformer receives carbon dioxide output by the regenerator and outputs methane and oxygen. The reactor receives at least some of the methane output by the photocatalytic reformer. By recycling methane in this way, the need for additional methane to fuel the system is reduced. The ZEN reforming system provides a novel technique to convert greenhouse gas emissions and carbon dioxide into oxygen and reusable methane gas.