Effect of Sn Incorporation on the Thermal Transformation and Reducibility of M(II)Al-Layered Double Hydroxides [M(II) = Ni or Co]

Our recent study on the incorporation of Sn in the lattice of MgAl-layered double hydroxides (LDHs) indicated that about 30 atom % of Al 3+ could be isomorphously substituted by Sn 4+ to form a new MgAlSn ternary LDH. In the present study, similar NiAlSn- and CoAlSn-LDHs were synthesized by a coprecipitation method. The influence of Sn on the thermal transformation and redox properties of NiAl- and CoAl-LDHs and their thermally derived products were investigated by X-ray powder diffraction (XRD), thermogravimetry/differential thermal analyses (TG/DTA), and temperature-programmed reduction (TPR) methods. The thermal transformation and reducibility of NiAlSn-LDH were different from that of the CoAlSn-LDH. Sn crystallized out as a SnO 2 phase along with NiO and NiAl 2 O 4 phases from NiAlSn-LDH calcined above 900 °C. On the other hand, a mixture of nonstoichiometric Co-spinel and Co 2 SnO 4 inverse spinel phases was noticed from CoAlSn-LDH. The TPR profiles of NiAl-LDH and its calcined products exhibited peaks for the reduction of Ni 2+ species existing in different chemical environments while an additional peak for the reduction of Sn 4+ → Sn o was observed in the Sn-containing counterparts. The Sn incorporation greatly enhanced the reducibility of Ni-containing phases. The CoAl- and CoAlSn-LDH and their calcined products exhibited complex TPR profiles. At least three different reduction regions were identified. They were assigned to the reduction of Co 2+ −Co 3+ (Co 3 O 4 -like) species (region I, between 250 and 450 °C), Co 3 O 4 -like species containing Al 3+ or CoAl 2 O 4 -like species containing Co 3+ (region II, 500−550 °C) and Co 2+ −Al 3+ (CoAl 2 O 4 -like) species (region III, above 550 °C). In contrast to that observed in the Ni-containing analogues the reducibility of Co species in these samples was found to decrease upon Sn incorporation.