Distinguishing Anionic Species That Are Intercalated in Layered Double Hydroxides from Those Bound to Their Surface: A Comparative IR Study

Guest molecules in layered double hydroxides (LDHs) may be located both on the surface and in the interlayer gallery. In this work, a comparative study of various IR spectroscopic methods coupled with X-ray diffraction (XRD) is presented, which makes it possible to distinguish between the two possibilities. During the experimental work, first, CaAl-LDHs intercalated with nitrate/carbonate ions were transformed to d-gluconate- and benzoate-containing solids. Depending on the initial interlayer ion, the organic molecules may end up between the layers and/or on the surface of the LDH. The IR spectra of the specimens were recorded by using four different techniques, which are different in their penetration depth (photoacoustic spectroscopy (PAS), attenuated total reflectance (ATR), diffuse reflectance IR spectroscopy (DRIFTS), and wide angle diffuse reflectance (WA-DRIFTS)). By comparing the spectra obtained, the anionic species that are surface adsorbed to LDHs were proven to be distinguishable from those intercalated in the interlayer gallery. PAS spectra are shown to provide information about interlamellar and bulk compounds. The screening of the surface species could be performed using the WA-DRIFTS technique, while ATR and DRIFTS are capable of detecting both the surface and the bulk anions residing in the top few layers.