Theoretical and Practical Aspetcts of the Catalytic Pattern of Nanostructured MnCeOx Systems for Environmental Applications

The technological development of human beings over the last few centuries has led them to produce great quantities of atmospheric pollutants. Nowadays, large areas of the globe find themselves facing pollution phenomena that undermine people's health, causing serious damage to the respiratory system, as well as to the ecosystems. Catalytic oxidation technologies proved to be successful in preventing or mitigating harmful gaseous emissions, but their large-scale exploitation is bound to the development of efficient and economical catalytic systems. Transition Metal Oxide catalysts, and in particular those based on Mn, have demonstrated excellent oxidative capacities in various applications such as the mineralization of toxic organic compounds from wastewater (CWAO), the oxidative detoxification of gas exhausts, the selective NOx reduction (SCR), the synthesis of bio-fuels and fine chemicals, showing performance comparable or even superior to the traditional noble metal systems. Among these, the catalytic oxidation of carbon monoxide has a great value both from a scientific and technological point of view: in fact, it is considered a model reaction, which can provide basic information on the reactivity of heterogeneous catalysts but at the same time, many are the possible fields of application, ranging from the purification of industrial streams (e.g., PROX) to the development of ambient temperature operating systems for indoor or mobile applications (e.g., gas masks, gas sensors). In this context, the work of this thesis is aimed at analyzing the practical and theoretical aspects of the oxidation of carbon monoxide on MnCeOx catalysts. To achieve this goal, a series of samples with different Ce content was synthesized by the redox precipitation method and tested by means of Temperature Programmed Catalytic Reaction (TPCR) in the CO oxidation reaction. From a systematic comparison of the bulk (e.g., XRD, Raman, BET) and superficial (XPS) characterization data, with the results of catalytic activity, it was possible to identify the Mn(IV) sites as main responsible for the marked oxidation activity of the MnCeOx systems. Subsequently, the empiric reaction kinetics and mechanistic issues of the CO oxidation reaction were assessed on a model MnCeOx (i.e., M5C1) catalyst. The numerous mechanistic evidences collected, led us to the formulation of a Langmuir-Hinshelwood reaction mechanism and the relative kinetic equation. The last goal of this work was to obtain a reaction model able to predict the reactivity pattern of the studied catalyst in a wide range of experimental conditions in terms of temperatures (293-533K), reagents pressure (p0CO-p0O2, 0.00625-0.025 atm) and CO/O2 ratio (λ0, 0.25-4.0).