Insights into memory effect mechanisms of layered double hydroxides with solid-state NMR spectroscopy

Abstract

Layered double oxides (LDOs) can restore the parent layered double hydroxides (LDHs) structure under hydrous conditions, and this “memory effect” plays a critical role in the applications of LDHs, yet the detailed mechanism is still under debate. Here, we apply a strategy based on ex situ and in situ solid-state NMR spectroscopy to monitor the Mg/Al-LDO structure changes during recovery at the atomic scale. Despite the common belief that aqueous solution is required, we discover that the structure recovery can occur in a virtually solid-state process. Local structural information obtained with NMR spectroscopy shows that the recovery in aqueous solution follows dissolution-recrystallization mechanism, while the solid-state recovery is retro-topotactic, indicating a true “memory effect”. The amount of water is key in determining the interactions of water with oxides, thus the memory effect mechanism. The results also provide a more environmentally friendly and economically feasible LDHs preparation route.

Photochemical Ammonia Synthesis over Reduced Polyoxovanadate-Intercalated Defective ZnAl Layered Double Hydroxide Nanosheets

“…The power X-ray diffraction (PXRD) patterns of the as-synthesis LDHs display the classical characteristic diffractions of (003), (006), (009), and (110) that are ascribed to rhombohedral LDH structures. In detail, the 2θ of the (110) peak in the LDH samples displays a slight shift with Cu doping, indicating that the Cu dopant was successfully incorporated into the ZnAl LDH (Figure b) . A series of POVs show the high crystallinity and good phase purity as confirmed by PXRD analysis.…”

“…In detail, the 2θ of the (110) peak in the LDH samples displays a slight shift with Cu doping, indicating that the Cu dopant was successfully incorporated into the ZnAl LDH (Figure 2b). 18 A series of POVs show the high crystallinity and good phase purity as confirmed by PXRD analysis. The diffraction of the (003) plane of POV/0.5%-ZnAl shifted to lower angles than that of 0.5%-ZnAl LDH, indicating that the POVs were successfully intercalated into the 0.5%-ZnAl LDH (Figure 2b,c and Figure S4).…”

“…The peak at 74.5 eV is assigned to Al 2p (Figure b). The five peaks in the Cu 2p spectrum at around 932.9/952.8 and 934.7/955 eV are indexed to the Cu + and Cu 2+ of Cu 2p 3/2 and Cu 2p 1/2 , respectively (Figure c) . Furthermore, compared to Cu 2p peaks in 0.5%-ZnAl LDH, the obvious shifts in the Cu 2p peaks of all as-prepared materials could be observed, which is in favor of promoting the essential charge transfer (Figure d) .…”

“…In comparison with the POVs, 0.5%-ZnAl LDH, and POV/0.5%-ZnAl samples, the N 2 fixation activity of V 34 /0.5%-ZnAl reached 310.6 μmol h –1 g –1 , which was evidently enhanced compared to either the V 34 (22.8 μmol h –1 g –1 ) or 0.5%-ZnAl (44.2 μmol h –1 g –1 ). The 0.5%-ZnAl LDH was found to provide chemically adsorbed and activated sites for N 2 fixation. , What is more, the POVs as photoinduced electron donors play vital roles in the nitrogen photofixation. Similarly, such photo-driven N 2 fixation catalysts can be comparable to Wang et al’s Bi 4 O 5 Br/ZIF-8 with 327.3 μmol h –1 g –1 , Ji et al’s Bi 2 Sn 2 O 7 with 231 μmol h –1 g –1 , Yao et al’s amine-inserted MIL-68­(In) with 140.3 μmol h –1 g –1 , and so on.…”

“…Layered double hydroxides (LDHs), some of famous 2D materials, are composed of easily controllable 2D supermolecular-stratified structures with edge-sharing MO 6 octahedra and interlayer anions with an alternating stack. LDHs possess outstanding applications in the field of photocatalysis on account of their flexible tunability of structure and chemical composition. − On the one hand, defect engineering in LDH materials, such as Cu-ZnAl LDH nanosheets, has attracted great attention as a means of providing a large number of sites for activating reactants . Specifically, oxygen vacancies in LDHs have been proven as active sites for the chemisorption of N 2 molecules, optimizing the reaction kinetics and lowering the reaction energy barrier as confirmed by theoretical and experimental results. − On the other hand, the adjustable interlayer spacing of LDHs allows the interlayer anions to be exchanged by a wide variety of anionic species (CO 3 2– , dicarboxylic acid, organic ionic liquids, etc.)…”