The study explores the synthesis of Co3O4 nanomaterials from extracts of three microalgae—Spirulina platensis, Chlorella vulgaris, and Haematococcus pluvialis—using a sustainable approach. The nanomaterials were synthesized by calcining the extracts at different temperatures (450, 650, and 800 °C). The resulting Co3O4 nanomaterials exhibited various morphologies, including octahedral, nanosheet, and spherical forms, with structural defects and surface segregation of phosphorus and potassium. These elements significantly enhanced the catalytic activity for CO oxidation. When normalized by specific surface area, the microalgae-derived catalysts outperformed a commercial benchmark catalyst. In situ studies revealed differences in oxygen mobility and carbonate formation during the reaction. The findings provide insights into the development of new synthesis strategies for highly active Co3O4 nanocatalysts, which could be used in automotive catalytic converters, petrochemical refineries, and fuel cells to reduce CO emissions and improve environmental sustainability.The study explores the synthesis of Co3O4 nanomaterials from extracts of three microalgae—Spirulina platensis, Chlorella vulgaris, and Haematococcus pluvialis—using a sustainable approach. The nanomaterials were synthesized by calcining the extracts at different temperatures (450, 650, and 800 °C). The resulting Co3O4 nanomaterials exhibited various morphologies, including octahedral, nanosheet, and spherical forms, with structural defects and surface segregation of phosphorus and potassium. These elements significantly enhanced the catalytic activity for CO oxidation. When normalized by specific surface area, the microalgae-derived catalysts outperformed a commercial benchmark catalyst. In situ studies revealed differences in oxygen mobility and carbonate formation during the reaction. The findings provide insights into the development of new synthesis strategies for highly active Co3O4 nanocatalysts, which could be used in automotive catalytic converters, petrochemical refineries, and fuel cells to reduce CO emissions and improve environmental sustainability.