  Metastable nanostructures are kinetically trapped in local energy minima
featuring intriguing surface and material properties. To unleash their
potential, there is a need for non-equilibrium processes capable of stabilizing
a large range of crystal phases outside thermodynamic equilibrium conditions by
closely and flexibly controlling atomic reactant composition, spatial
temperature distribution and residence time. Here, we demonstrate the capture
of metastable pseudo-binary metal oxides at room temperature with scalable
combustion-aerosol processes. By a combination of X-ray diffraction, electron
microscopy and online flame characterization, we investigate the occurrence of
metastable CoCu$_2$O$_3$ with controlled crystal size (4-16 nm) over
thermodynamically stable CuO and Co$_3$O$_4$. Immediate practical impact is
demonstrated by exceptional sensing and catalytic performance for air pollutant
detection (e.g., 15 parts-per-billion benzene). This approach can be extended
to various binary, ternary and high entropy oxides with even more components to
access novel materials also promising for actuators, energy storage or solar
cells.
