XPD and XANES Studies of Ce0.9Zr0.1O2 Nanocatalysts under Redox and Catalytic CH4 Oxidation Conditions

Abstract

The aim of this work is to take further insight into the structural stability of cerium-zirconium catalysts under reducing, oxidizing and reaction conditions. In situ synchrotron based techniques (XANES and XPD) were used in order to determine the stability of crystal structure and the oxidation state of cerium cations in the reaction conditions prevailing in the catalytic studies. In situ XPD studies in 5 mol % H2 atmosphere revealed that no structural changes occur until 870 °C. At this temperature, the sample synthesized with glycine, which presents lower crystallite agglomeration, segregated a small quantity of a reduced phase. On the contrary, the solid synthesized with lysine, with more agglomerated crystallites, does not show structural changes in all the temperature range. Reoxidation treatments in 5 mol % O2 revealed that at 750 °C the segregated phase disappears and that the original cubic structure is restored. In situ XANES studies in the Ce LIII absorption edge indicate that under catalytic reaction conditions, the degree of reduction of Ce4+ is low, allowing the occurrence of methane oxidation. The solid is capable to deliver the oxygen of its structure when no oxygen is fed into the reactor until the 50% of cerium cations become reduced, triggering the deactivation process. Therefore, it is clear for these studies that the Ce4+ to Ce3+ ratio in the lattice is governing the catalytic behavior of the solid. The catalytic results collected during in situ XANES experiments are in excellent agreement with our previous catalytic studies performed with a laboratory fixed bed reactor.