Insights into the Morphology Effect of Ceria on the Catalytic Performance of NiO-PdO/CeO2Nanoparticles for Thermo-oxidation of n-C7Asphaltenes under Isothermal Heating at Different Pressures

Abstract

The overall objective of this study is to evaluate the effect of morphologies (C, cubic; O, orthorhombic; and S, spherical) of ceria in the catalytic activity for n-C7 asphaltene thermo-oxidation. In this way, cubic (C-CeO2), orthorhombic (O-CeO2), and spherical (S-CeO2) ceria nanoparticles were synthesized using the hydrothermal method and doped with 1.0% in a mass fraction of Ni and Pd oxides by incipient wetness impregnation. The catalytic activity of the systems was evaluated through non-isothermal and isothermal thermogravimetric analyses at different pressures. The non-isothermal thermogravimetric results evidenced an increase in the n-C7 asphaltene mass as the temperature increases between 100 and 230 °C for all systems and operating conditions. At 3.0 MPa, n-C7 asphaltene gains 3.6, 5.8, and 3.2% for O-CeO2, C-CeO2, and S-CeO2, respectively, while at 6.0 MPa, there is an increase of 7.3, 10.4, and 5.9% for the same systems, respectively. The Ni and Pd phases increase the amount of oxygen chemisorbed in all systems in the order S-NiPdCe < O-NiPdCe < C-NiPdCe and reduce the temperature required for the total n-C7 asphaltene decomposition at temperatures lower than 200 °C at 6.0 MPa in the same order. Also, isothermal thermogravimetric analysis demonstrates that the asphaltene conversion increased with ceria-based nanocatalysts in the increasing order S-CeO2 < O-CeO2 < C-CeO2. Cubic CeO2 doped with Ni and Pd presents the highest yield, reaching 100% conversion at 170 °C and 6.0 MPa at 60 min. The data confirm that the presence of Ni and Pd facilitate the transference oxygen vacancies between the bulk and surface of the nanocatalyst, and therefore, the catalytic activity is enhanced. The catalytic activity, in turn, is improved by the presence of {110}, {100}, and {111}, oxygen vacancies, and Ce3+ and NiO-Ce species. This work reveals the benefits of structured cubic ceria nanocatalysts as a promising support for nickel- and palladium-containing materials for n-C7 asphaltene decomposition.