Oxygen-lean syngas production utilizing CO2 premixed with H2, CH4 and C2H6 in a microwave-induced plasma

• CO 2 conversion approaches 100% with H 2 , CH 4 or C 2 H 6 co-feeds. • Hydrocarbon co-feeds yield dry, O 2 -lean syngas versus neat H 2 co-feeding. • Main pathways: RWGS, dry reforming (DRM/DRE) and CO 2 dissociation. Upgrading CO 2 into value-added products is considered a cornerstone of a future carbon-neutral economy. Although CO 2 utilization can contribute to closing the global carbon cycle, its high thermodynamic stability requires substantial energy input for activation. This study systematically compares CO 2 conversion and oxygen-lean syngas formation in a low-pressure (0.13 bar) microwave-induced plasma by co-feeding H 2 , CH 4 , or C 2 H 6 under fixed power and total flow. CO 2 conversion approaches ∼100% at high co-feed ratios, yielding CO- and H 2 -rich syngas. A constrained stoichiometric extent-of-reaction model is applied to reconstruct the dry product composition and quantify the relative contributions of global reaction pathways. The model indicates RWGS-dominated behaviour for H 2 co-feeding, whereas CH 4 and C 2 H 6 co-feeding increases the contribution of reforming routes, consistent with the experimentally observed suppression of molecular O 2 . Optical-emission-spectroscopy-based calculations show that OH rotational temperatures - used here as a proxy for the gas temperature in the emitting region - were 2.03-2.44∙10 3 K for CO 2 +H 2 , 2.24-2.44∙10 3 K for CO 2 +CH 4 , and 2.04-2.53∙10 3 K for CO 2 +C 2 H 6 . Where the C 2 Swan band was sufficiently intense, an additional vibrational temperature indicator was extracted to assess non-equilibrium effects. Overall, the results demonstrate that co-feeding hydrogen or hydrocarbons with CO 2 is an effective strategy to achieve near-complete CO 2 conversion while producing oxygen-lean syngas suitable for downstream applications.