Effect of the Ni/Al ratio of hydrotalcite-type catalysts on their performance in the methane dry reforming process

Hydrotalcite-type solids of the form NiAl-R, where R refers to the ratio of Ni to Al (R = 2, 3, 5, 8, and 10), were successfully synthesized following co-precipitation method at pH = 12. The obtained solids were calcined at 800 degrees C, except for NiAl-R-2 where calcination was performed at temperatures ranging between 300 and 800 degrees C. Following calcination, the resulting materials were evaluated for their catalytic activity and stability during the process of dry reforming of methane. Factors affecting the catalytic activity of the obtained materials such as the ratio R and calcination temperature were also studied. Prior to calcination, X-ray diffraction analyses clearly illustrated the typical hydrotalcite structure of the synthesized materials (when R <= 5). On the other hand, calcination at various temperatures prompted decomposition of all solids to form NiO, with exception to NiAl-R-2, which upon calcinations at 800 degrees C was decomposed to form NiO and a second phase spinel containing NiAl2O4. The chemical composition of the obtained solids was determined by atomic absorption spectroscopy. Further characterization was performed using several techniques, including: surface area measurements (SBET), scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. The reducibility of nickel species was studied via temperature-programmed reduction. The catalytic performance of the as-prepared samples was studied for dry reforming of methane under atmospheric pressure at temperatures ranging between 400 and 700 degrees C. The catalytic activity of the designed substances highlighted the importance of molar ratios i.e. Ni2+/Al3+ on the success of the overall dry reforming of methane process. The catalytic activity of the synthesized materials was also found to be directly proportional to the ratio of Ni/Al as well as the calcination temperature, with exception to NiAl-R-2 which was found to exhibit the highest activity of all. The latter observation was perhaps associated with the lower size of the crystalline particles in conjunction with the presence of a second phase containing NiAl2O4. In this study, it is shown that the calcination temperature has a significant effect on the catalytic property and the crystallite size of the metal.