Structural Characterization of Mg/Al hydrotalcite-like Compounds and their Thermal Stability

Hydrotalcite-like compounds, repersented by the formula [M$^{2+}_{\mathrm{1-x}}$M$_{\mathrm{x}}^{3+}$(OH)$_{2}$]X$_{\mathrm{x/n}}^{\mathrm{n-}}$ $\cdot$ nH$_{2}$O (M$^{2+}=$ Ni$^{2+}$, Mg$^{2+}$, Cu$^{2+}$,etc; M$^{3+}=$Al$^{3+}$, Fe$^{3+}$, etc; X$^{\mathrm{n-}}=$CO$_{3}^{2-}$, NO$_{3}^{-}$, etc) possess the brucite-like layers [Mg(OH)$_{2}$] with positive charge and anionic compounds in the interlayer to form neutral materials. Catalytic effects to decompose NO$_{\mathrm{x}}$ from automobile exhaust were highly related with the difference of M$^{2+}$ and thermal stability because the catylists locate are about 200 $\sim$ 500${^\circ}$. In this paper, Mg-Al-Cu and Mg-Al-Ni hydrotalcite-like compounds were characterized by XRD and FT-IR spectra and the thermal stability were analyzed by TGA and DTA. Even though they both have the typical diffraction peaks of hydrotalcites, but their interlayer spaces are different. Some weak chemical bonds were observed to be formed in Mg-Al-Ni hydrotalcites by FT-IR. Mg-Al-Ni hydrotalcite-like compound degraded at lower temperature, by contrast, Mg-Al-Cu hydrotalcite has the better structural stablilty and thermal stability.