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 Al3+ could be isomorphously substituted by Sn4+ 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 SnO2 phase along with NiO and NiAl2O4 phases from NiAlSn-LDH calcined above 900 degrees C. On the other hand, a mixture of nonstoichiometric Co-spinel and Co2SnO4 inverse spinel phases was noticed from CoAlSn-LDH. The TPR profiles of NiAl-LDH and its calcined products exhibited peaks for the reduction of Ni2+ species existing in different chemical environments while an additional peak for the reduction of Sn4+ --> 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 Co2+-Co3+ (Co3O4-like) species (region I, between 250 and 450 degrees C), Co3O4-like species containing Al3+ or CoAl2O4-like species containing Co3+ (region II, 500-550 degrees C) and Co2+-Al3+ (CoAl2O4-like) species(region III, above 550 degrees 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.