Electrochemistry of Layered Hydroxides and Related Materials

LDHs of Al, Ni, Zn, and other metals are composed of rigid layers where anions and water molecules can move. Typical LDHs can be represented by the formula [Mj m (OH)J[A -U mH20, where M is a divalent metal cation, M could be a tri-valent metal cation or a mix of trivalent and divalent metal cations, and A represents an interlayer anion or mix of anions. Both divalent and trivalent cations are located at the center of octahedral composed of OH ions, M(OH)g and M (OH)g units share edges forming two-dimensional layers, whereas the interlayer space incorporates anions (and water molecules) to maintain electroneutrality. These materials exhibit a stable lamellar structure while 0.2 x 0.5, and the interlayer A anions can be freely exchanged by foreign anions (Khan and O Hare, 2002). 

FIGURE 6.5 CVs at different scan rates recorded at Co-Al LDH attached to graphite electrode in contact with 0.10 M NaOH. Potential scan rate (a) 520 (b) 5 mV/s. 

Zn-Al LDHs can be prepared by electrodeposition from aqueous solutions of the nitrates of such metals (Yarger et al., 2008). Cathodic depositions, conducted at room temperature without stirring over noble metal-coated electrodes, were achieved by reducing nitrate ions to generate hydroxide ions on the working electrode, the optimal potential being -1.65 V vs. AgCl/Ag in 4 M KCl. The overall reactions involved are 

The electrochemistry of LDH-attached species has received relatively little attention compared with that of zeolite-associated species. The electrochemistry of LDH-supported bis(2-mercapto-2,2-diphenyl-ethanoate) dioxomolybdate(VI) complex, Mo O2( TI A)2l (TEA = O2CC(S)Ph2) is representative, however, of the capabilities of electrochemical methods for obtaining information on this type of system. The mineralogical composition of the used Zn-Al hydrotalcite was Zn2 2oA1o 79(OH)g(N03)o 79 H2O. X-ray absorption spectroscopy and other techniques (Cervilla et al., 1994a), suggested that the complex, which, in solid phase exhibits 

FIGU RE 6.7 Repetitive CV at LDH Mf/ O/ I BAljil IjOll-inodified paraffin-impregnated graphite electrodes immersed into 0.50 M phosphate buffer, pH 7.0. Potential scan rate, 20 mV/sec. 

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