Ammonia Uptake from Ambient Air by Protonic Layered Metal Oxide as a Cause of the Gradual Degradation of Its Swelling Reactivity

Protonic layered transition metal oxides readily undergo significant swelling and exfoliation into unilamellar nanosheets upon contact with aqueous solutions of amines and other basic compounds. Despite the crucial importance of this reactivity, it often degrades over time, leading to a loss of swelling and exfoliation capabilities. However, the cause of this degradation has remained unclear. In this study, we systematically investigated the conditions that lead to the loss of swelling ability, using protonic layered perovskite crystals of HCa₂Nb₃O₁₀·1.5H₂O as a representative example. The crystals were stored under various conditions, including ambient air, sealed environments, and dry/humid conditions, both in the dark and under light. Their reactivity with dimethylaminoethanol (DMAE) was then examined. We found that only the crystals exposed to ambient air for more than 10 days lost their swelling ability, whereas those stored under other conditions retained their reactivity. Based on these results, we speculated that ammonia, the most abundant alkaline component in the atmosphere, is absorbed from the air via an acid-base interaction. This hypothesis was confirmed through XRD and FTIR characterization. We conclude that trace amounts of ammonia (0.02-0.12 ppm) in the atmosphere intercalate into the acidic interlayer galleries and are responsible for the degradation in reactivity. Furthermore, the original reactivity can be easily restored by acid treatment, which replaces the intercalated ammonium ions with protons, thereby recovering the swelling capability. These findings resolve a long-standing question regarding the degradation of protonic layered metal oxides and provide a practical method for restoring their native properties.