The influence of water in methane oxidation catalysts based on Pd supported on CeO2 and CeO2-containing materials

The purpose of this PhD thesis is the study of catalytic methane oxidation over Pd-based catalysts supported on different CeO2-based oxides for their application on natural gas fuelled vehicles (NGVs). This research work has been carried out with the financial support from Ford Motor Company (Dearborn, MI, USA) under the University Research Project award "Three-Way Catalyst Materials for Compressed Natural Gas Vehicles". Pd supported on CeO2-based oxides are recognized as the most promising catalytic materials for methane oxidation. The objective of the thesis work was to address the study of new catalytic materials supported on different CeO2-based oxides and prepared by single-step solution combustion synthesis (SCS). Particular attention was dedicated to the study of water poisoning, a key issue in designing catalytic materials for NGVs, as water is known to strongly deactivate Pd-based catalysts. The first part of the work was dedicated to the investigation of the catalytic behavior of Pd/CexZr1-xO2 catalysts for CH4 oxidation in lean atmosphere: the performance of each sample was evaluated both in temperature programmed and steady-state conditions in the presence and in the absence of steam and compared with analogues compositions prepared by traditional incipient wetness technique (IW). In order to explain the difference in terms of catalytic activity between SCS and IW samples, a relevant part of the work was focused on the study of their redox properties, and in particular to the characterization of PdO-Pd-PdO phase transformation. The results indicate that solution combustion synthesized Pd/CexZr1-xO2 catalysts are not only more active than their IW counterparts, but also display an improved resistance to hydrothermal ageing. The results also highlighted the beneficial use of Ce0.75Zr0.25O2 mixed oxide to reduce the deactivation observed in presence of water vapor. Once the effectiveness of solution combustion synthesis to prepare active and stable Pd-based catalysts was assessed, other supports were investigated. Pd supported on CeO2-SiO2 and CeO2-Al2O3 mixed oxides were prepared to study the effect of SiO2 and Al2O3 addition on the redox properties and catalytic activity of Pd/CeO2. Finally, the most promising materials were tested also in stoichiometric conditions, varying oxygen/methane ratio in the feed, with the aim to evaluate the effect of gas feed composition on their catalytic performance and obtain preliminary results on their catalytic behavior as three-way-catalysts.