Effect of Mg addition on acid-base characteristics in spinels structures

In this work, two spinels type catalysts families Ni_1-xMg_xAl₂O₄ (x = 0.01/0.05/0.1, NiMg_x) and Mg_1-xNi_xAl₂O₄ (x = 0.05/0.1/0.2, MgNi_x) were synthetized by the citrate method with the aim of generating Ni metal particles over a basic support. These catalysts were characterized by different techniques such as X-ray diffraction (XRD), specific surface area (S_BET), temperature programed reduction (TPR), temperature programed desorption of CO₂ (TPD-CO₂) and x-ray photoelectron spectroscopy (XPS). The synthesis method was appropriate, since pure structures were obtained with a suitable surface area. NiMg_x catalysts presented the highest surface area values, about the double of MgNi_x. These catalysts also showed a higher reducibility with maxima around 800 ºC, which was 200 ºC less than those of MgNi_x and, therefore, the higher Ni⁰ content at the surface. TPD-CO₂ results evidenced that basicity of samples increased with the increase in Mg content. The presence of basic strong sites was observed for MgNi_x while NiMg_x only presented weak and medium basic sites. These results showed that NiMgx catalysts could be easily used in reactions where medium basic sites are needed.