HYDROGEN EXCHANGE IN NA-BIRNESSITE: A TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION ANALYSIS

Manganese oxide minerals having the birnessite-type layer structure occur in a wide variety of geological settings, including soils, Mn nodules, and rock varnishes. Mn oxides readily participate in cation-exchange and oxidation-reduction reactions, and because they typically form as coatings and fine-grained aggregates with large surface areas, even small quantities can significantly affect the chemical composition and behavior of sediments and associated aqueous systems. Additionally, because of their great chemical activity, synthetic birnessite-like phases are being extensively studied as possible catalysts, cation-exchange agents and battery materials.

Of critical importance to the possible usage of birnessite as a cathodic material in solid-state batteries is its capacity to incorporate H ions during battery discharge. In this study, we examined the exchange of H for Na in birnessite in real time through synchrotron X-ray diffraction. Na-birnessite powders were exposed to HCl solutions (pH 3) at room temperature in a quartz capillary flow-through cell for approximately 1 hr, by which time H exchange was complete. Data were collected with an imaging plate, which allowed full-pattern diffraction patterns to be obtained every 2 minutes.

Rietveld analyses of these time-resolved diffraction experiments confirm the proposition that endmember hexagonal H-birnessite appears and grows concomitant with the replacement of Na by H in the triclinic starting material (Drits et al. 1997). Hexagonal H-birnessite first was observed after 20 minutes of solution flow and increased in abundance (as revealed by refined weight fractions) linearly with time at the expense of the triclinic birnessite, which was not discernible after 60 minutes. Substitution of H into triclinic Na-birnessite was manifested by systematic decreases in the a, b, c, and beta lattice values beginning 10 minutes after fluid flow was initiated, contributing to a volume decrease of approximately 1%. Refined occupancies of the octahedral Mn in the hexagonal H-birnessite suggest that some Mn cations occupy the interlayer.