The dehydration mechanism of Na and K birnessites: a comprehensive multitechnique study

Abstract

The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions (bir_A) to complete dehydration, and the involved mechanisms were scrutinized. Density Functional Theory (DFT) simulations were employed to derive structural models for lamellar A_0.33MnO₂·xH₂O (A = Na⁺ or K⁺, x = 0 or 0.66), subsequently compared with the experimental results obtained for Na_0.30MnO₂·0.75H₂O and K_0.22MnO₂·0.77H₂O materials. Thermal analysis (TGA-DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, and Near Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) measurements were conducted for both birnessites. Dehydration under vacuum, annealing, or controlled relative humidity were considered. Results indicated that complete birnessite dehydration was a two-stage process. In the first stage, water removal from the interlayer of fully hydrated birnessite (bir_A) down to a molar H₂O/A ratio of ∼2 (bir_B) led to the progressive shrinkage of the interlayer distance (3% for Na birnessite, 1% for K birnessite). In the second stage, water-free (bir_C) domains with a shorter interlayer distance (20% for Na birnessite, 10% for K birnessite) appeared and coexisted with bir_B domains. Then, bir_B was essentially transformed into bir_C when complete dehydration was achieved. The vibrational properties of bir_A were consistent with strong intermolecular interactions among water molecules, whereas partially dehydrated birnessite (bir_B) showed a distinct feature, with 3 (for Na-bir) and 2 (for K-bir) vibrations that were reproduced by DFT calculations for organized water into the interlayer (x = 0.66). The study also demonstrated that the electronic structure of Na birnessite depends on the interlayer water content. The external Na⁺ electronic level (Na 2p) was slightly destabilized (+0.3 eV binding energy) under near ambient conditions (bir_A) compared to drier conditions (bir_B and bir_C).