Fabrication of molten nitrate/nitrite dual-phase four-channel hollow fiber membranes for nitrogen oxides separation

Molten salt-ceramic dual-phase membranes have great prospects in the selective separation of nitrogen oxides. A gas-tight KNO₃–Ba_0.5Sr_0·5Co_0·8Fe_0·2O_3-δ (KNO₃-BSCF) four-channel hollow fiber membrane was developed in this study. A wide operating temperature window of 350–500 °C with high nitrogen oxides permeance was achieved. The maximum nitrogen oxides permeance reached 9.2 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹ under the condition of 1% NO₂, 1% CO₂ and 20% O₂ at 500 °C. The presence of nitrogen oxides was able to suppress the reaction of BSCF oxide with CO₂ to form carbonates. Insights on the transport mechanism of nitrogen oxides transport were provided and the existence of nitrite route for nitrogen oxides transport was firstly demonstrated by experiments. An 1800 h long-term nitrogen oxides permeation was running on the KNO₃-BSCF hollow fiber membrane while the permeance increased by a factor of ~40 (5.2 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹ to 2.2 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ at 450 °C) due to the partial decomposition of nitrate into nitrite. This nitrite route was also verified in KNO₃/KNO₂-BSCF hollow fiber membrane constructed by preloading 30 wt% nitrite in molten salt.