Suppressing deactivation in magnesium oxide for direct air capture via sodium nitrate–potassium nitrate promotion

Herein, we investigate low-loading alkali nitrates to suppress magnesium oxide (MgO) deactivation and enable ambient-temperature direct air capture in humid air. MgO prepared via precipitation–calcination was impregnated with 1 wt% sodium nitrate (NaNO 3 ; denoted NaMgO), 1 wt% potassium nitrate (KNO 3 ; denoted KMgO), or a binary mixture of 1 wt% NaNO 3 and 1 wt% KNO 3 (NaKMgO). Screening at total nitrate loadings of 1 wt% and 3 wt% showed no improvement beyond 1 wt% in uptake or adsorption–desorption reversibility; therefore, 1 wt% was selected for detailed evaluation. Humidity screening at 25 °C under 1 vol% carbon dioxide (CO 2 ) in air showed that 50% relative humidity (RH) maximized uptake by bare MgO (1.94 mmol g −1 in 60 min), and these conditions were used for sorbent comparison. At 25 °C and 50% RH, 1st cycle CO 2 adsorption/desorption amounts after 60 min were 1.94/1.93 (MgO), 1.66/1.61 (NaMgO), 1.80/1.60 (KMgO), and 1.37/1.36 (NaKMgO) mmol g −1 . After 20 cycles (25 °C adsorption; N 2 desorption to 400 °C at 10 °C min −1 with a 30 min hold), the CO 2 uptakes decreased from the 1st to 20th cycle as 1.94 → 0.46 (MgO; 76.3% drop), 1.66 → 0.16 (NaMgO; 90.4% drop), 1.80 → 0.12 (KMgO; 93.3% drop), and 1.37 → 0.84 mmol g −1 (NaKMgO; 38.7% drop), with NaKMgO stabilizing from the 4th cycle at ~0.85–0.90 mmol g −1 . In situ X-ray diffraction and differential scanning calorimetry results demonstrated that NaNO 3 –KNO 3 exhibited transitions at approximately 221 °C–223 °C and a nitrate crystallinity loss at 400 °C, consistent with a mobile, eutectic-like nitrate film wetting MgO. With 400 ppm CO 2 at 50% RH, NaKMgO maintained 1.72–1.74 mmol g −1 over 10 cycles. Thus, binary NaNO 3 –KNO 3 enables stable ambient-temperature CO 2 capture by MgO with regeneration at ≤400 °C. • Binary NaNO 3 –KNO 3 additive (1 wt% each) suppresses MgO deactivation in humid DAC. • Relative humidity (RH) screening at 25 °C shows that 50% RH maximizes MgO uptake (1.94 mmol g −1 ). • NaKMgO sorbent stabilizes at 0.85–0.90 mmol g −1 over 20 cycles with regeneration at 400 °C and 1 vol% CO 2 . • Under 400 ppm CO 2 and 50% RH, NaKMgO sorbent maintains a stable uptake of 1.72–1.74 mmol g −1 over 10 cycles.