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

The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions (bir) to complete dehydration, and the involved mechanisms were scrutinized. Density Functional Theory (DFT) simulations were employed to derive structural models for lamellar AMnO·HO (A = Na or K, = 0 or 0.66), subsequently compared with the experimental results obtained for NaMnO·0.75HO and KMnO·0.77HO 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) down to a molar HO/A ratio of ∼2 (bir) led to the progressive shrinkage of the interlayer distance (3% for Na birnessite, 1% for K birnessite). In the second stage, water-free (bir) domains with a shorter interlayer distance (20% for Na birnessite, 10% for K birnessite) appeared and coexisted with bir domains. Then, bir was essentially transformed into bir when complete dehydration was achieved. The vibrational properties of bir were consistent with strong intermolecular interactions among water molecules, whereas partially dehydrated birnessite (bir) 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 ( = 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) compared to drier conditions (bir and bir).