Ceramic honeycombs based on homogeneous composites of zirconia and iron oxide are formed using polymer-based coextrusion for testing in thermochemical reactors to generate CO for renewable fuels. The honeycomb substrates possess controlled surface areas and are processed using zirconia with 3 and 8 mol % yttria additions to investigate the influence of surface area and oxygen conductivity of the substrate on the CO generation properties. CO generation was tested using a gas chromatography mass spectrometer and a laboratory scale thermochemical reactor capable of precisely controlling temperature and gas conditions. Results showed that reaction temperature and reactant gas flow rate effect CO generation. The yttria content of the zirconia support phase was also found to have a significant impact on the long-term CO generation, improving iron oxide conversion from 41 to 58%. Yttria content did not markedly impact the short-term reaction properties. Increasing the surface area of the substrates, from 2.6 up to 8.5 cm(2), did not result in improvements in CO generation within the resolution of the test equipment. The substrates reacted by two distinct mechanisms, an initial, spontaneous surface reaction that changed over time to a diffusion-based mechanism utilizing reaction material from the bulk. These substrate systems exhibit the high reactivity necessary for large-scale thermochemical reactors, while being based on common materials.