The continuous increase in atmospheric CO₂ concentration underscores the urgent need for scalable and energy-efficient carbon removal technologies. This study demonstrates, for the first time, the implementation of a tailored Direct Air Capture (DAC) concept integrated within large-scale microalgae raceway reactors, enabling direct CO₂ uptake from ambient air without external gas supply. A 600 m² reactor operated continuously with Scenedesmus sp. maintained stable productivity (12 g m⁻² day⁻¹) under extreme carbon limitation (TIC ≈ 20 mg L⁻¹, pH ≈ 10). Fine-bubble aeration in the sump achieved nearly complete CO₂ removal from air streams, while passive absorption across the raceway and paddlewheel sections provided almost all the carbon required for biomass growth. The overall CO₂ removal efficiency reached 95%, confirming the reactor’s operation as a functional bio-DAC system. The estimated energy demand for air bubbling (≈ 2.9 kWh kg⁻¹ CO₂) is comparable to or below that of current engineered DAC technologies. This approach establishes a low-cost, renewable-compatible, and scalable pathway for atmospheric CO₂ capture that couples negative-emission performance with biomass production, laying the groundwork for decentralized, carbon-negative biotechnological systems contributing to global greenhouse-gas mitigation. • Large-scale microalgae raceways directly capture CO₂ from ambient air. • Sump-based aeration achieves up to 95% CO₂ removal efficiency. • pH-driven carbonate balance governs atmospheric CO₂ absorption and fixation. • Bio-DAC integrates carbon capture with renewable biomass production. • Demonstrated low-energy (≈ 2.9 kWh kg⁻¹ CO₂) pathway for negative emissions.

The direct conversion of CH 4 and CO 2 into syngas garners significant attention as a promising strategy for greenhouse gas utilization. However, the development of highly active and stable catalysts remains a significant challenge. Here, Ni-based catalysts supported on Al 2 O 3 and promoted with alkaline–earth metals (Ca, Mg, Sr, and Ba) are prepared via an impregnation method and systematically characterized using various analytical techniques. The catalytic performance is evaluated in the combined steam and dry reforming of methane. Among the investigated promoters, Sr notably enhances CH 4 and CO 2 adsorption capacities and improves the reducibility of Ni active sites. The catalyst containing 9 wt% Sr exhibits the smallest Ni particle size and the highest catalytic activity. Sr doping modifies surface basicity, oxygen vacancies, and metal–support interactions, thereby enhancing CO 2 adsorption and Ni dispersion. However, excessive Sr loading causes the formation of Sr–aluminate phases (SrAl 2 O 4 and SrAl 4 O 7 ) that negatively affects the catalytic performance. In-situ diffuse reflectance infrared Fourier transform spectroscopy analysis clearly demonstrates improved adsorption of CH 4 and CO 2 and formation of carbonate species, providing insight into the surface reaction mechanism. Kinetic studies reveal that the lower apparent activation energy is attributed to enhanced surface reactivity. Long-term stability tests demonstrate steady CH 4 and CO 2 conversions over 500-h without detectable carbon deposition. Overall, Sr incorporation stabilizes Ni particles, suppresses sintering, and preserves active surface area, thereby markedly improving the catalytic performance. • Ca, Mg, Sr, Ba were added to Ni/Al 2 O 3 for combined steam and dry reforming of CH 4 . • Sr enhanced adsorption of CH 4 and CO 2 and improved reducibility of Ni active sites. • Ni-9Sr/Al 2 O 3 exhibited the highest activity, achieving 94 % CH 4 and 66 % CO 2 conversion. • Effect of Sr promoter was due to Ni dispersion, oxygen vacancies, and basicity. • The overall reaction mechanism was also comprehensively evaluated.

In this study, 15 wt% Ni/Al₂O₃ catalysts promoted with Fe, Mn, and Cr were synthesized through a mechanochemical route and evaluated for the combined dry and steam reforming of methane reaction. Structural and physicochemical properties were examined using XRD, BET, TPR, TPO, and FESEM analyses. The catalysts exhibited high BET surface areas ranging from 220.82 to 233.6 m²/g and mesoporous structures with pore sizes of 2–10 nm. The incorporation of Cr notably improved the catalyst’s performance, delivering CH₄ and CO₂ conversions of 66.7 % and 33.5 %, respectively, with excellent stability (H₂/CO ∼ 2.1) maintained over 7 h of reaction at 700 °C. TPO results revealed that both Mn and Cr minimized carbon deposition by improving Ni dispersion and enhancing carbon oxidation, leading mainly to filamentous carbon, as confirmed by FESEM images. Increasing Cr loading from 3 % to 7 % significantly improved activity and minimized carbon formation, attributed to oxygen-rich lattice sites promoting carbon oxidation. Upon raising the GHSV from 6000 to 24,000 mL/g cat .h, CH₄ conversion in combined reforming decreased, falling from 85.2 % to 68.6 %. Increasing the steam content in the CH₄:CO₂ feed enhanced CH₄ conversion but slightly suppressed CO₂ conversion, owing to the stronger oxidizing ability of steam. • Fe, Mn, Cr-promoted 15 %Ni/Al₂O₃ catalysts boost SCO₂-MRe process efficiency. • Chromium incorporation promoted lattice oxygen formation in the catalyst structure. • Cr(7)-NiAl showed highest activity with improved methane conversion and lower carbon buildup. • Steam’s high stability and oxidizing power vs CO₂ lowers carbon formation. • Effect of operating conditions on catalyst performance was studied.