Support composition effect on the structures, metallic sites formation, and performance of Ni-Co-Mg-Al-O composite for CO2 reforming of CH4

Abstract

The relationship among the bulk structure, Ni/Co reduction, site formation, and the catalytic performance of Ni-Co-Al-Mg-O composite synthesized by co-precipitation method was studied for CO2 reforming of CH4 (DRM). Three catalysts were prepared by changing the Mg/Al ratio of the composite from 0.3 to 2. The catalysts were denoted as CopCat-Mg/Al-0.3, CopCat-Mg/Al-1, and CopCat-Mg/Al-2. ICP-MS, XRD, Al-NMR, XPS, TEM, and XANES (Mg, Al, Ni/Co K-edge) were utilized to characterize the crystalline structure, the coordination number, degree of spinel inversion, and the Ni /Co reduction of the catalysts. Results showed that the bulk structure of the calcined precipitates changed from spinel-dominating to the co-existence of spinel and MgO solid solution phases as the Mg/Al ratio changed from 0.3 to 2. In the spinel-dominating structures, Ni and Co reduction was found to be difficult, and a small number of metallic sites with larger nanoparticles were generated after reduction. As the MgO solid-phase became significant, the reduction of Ni and Co was enhanced, and more metallic sites of smaller sizes were formed. These metallic sites performed differently for the activation of CH4 and CO2, as the catalyst with the lowest Mg/Al ratio (<0.5) experienced carbon deposition and produced synthetic gas of low H2/CO ratio, which led to lower catalytic activity and quicker catalyst deactivation during DRM. In contrast, the catalyst with the highest ratio of Mg/Al suppressed carbon formation, maintained a higher catalytic activity and stability due to the improved basicity that promoted the quick conversion of carbon to CO.