Development of MnOx base catalysts for the PROX process. Theoretical insights and practical application

The growing demand for sustainable energy carriers has positioned hydrogen at the center of the global energy transition. However, hydrogen produced by conventional hydrocarbon-reforming processes typically contains residual carbon monoxide (CO) hindering its direct use in proton exchange membrane fuel cells (PEM) due to the poisoning of Pt electrodes. Therefore, the abatement of CO represents a crucial purification step in hydrogen processing for PEM applications. Among the available H2 purification strategies, apart from methanation and membrane separation, the Preferential Oxidation of CO (PROX) has emerged over the past decades as a highly promising alternative, although the development of efficient, selective and low-cost catalysts represents the main scientific and technological challenge for the practical deployment of PEM technology. Therefore, aiming at bringing contribution in the field of H2 purification processes, this study offers a comprehensive investigation of the H2 and CO oxidation functionalities of a nanocomposite MnCeOx catalyst by a combined computational and experimental approach documenting its potential PROX behavior for PEM technology exploitation. The catalyst, synthesized by the redox-precipitation route, was extensively characterized, revealing the dominant role of surface Mn(IV) sites in governing a distinctive PROX behavior in the range of 293-423 K. In fact, catalytic tests reveal that this depends on the high CO oxidation activity of the MnCeOx catalyst at low temperature (∼293 K) and a substantial inactivity towards hydrogen oxidation at T<373 K. Complementary Density Functional Theory (DFT) calculations performed on a model Mn4O8 cluster provided theoretical support for the experimental observations, revealing different interaction pathways of Mn(IV) sites with CO and H2. These give rise to two separate oxidation cycles, characterized by a significant activation energy gap, which explains the observed selectivity toward CO oxidation. The experimental and theoretical insights were integrated into two simplified reaction mechanisms involving direct lattice oxygen abstraction and diatomic oxygen species respectively. The validity of these mechanistic interpretations was confirmed through the development of two kinetic models able to predict the CO and H2 oxidation activity of the MnCeOx catalyst in the range of 293-533K. Overall, this work contributes to the fundamental understanding of MnCeOx catalysts, elucidating the interplay between surface redox chemistry, oxygen mobility and catalytic selectivity. Beyond elucidating the molecular basis of PROX activity, the integration of computational and experimental insights establishes a robust methodological framework for the rational design of next-generation mixed-oxide catalysts for energy and environmental applications. Chapter 1 provides an overview of the global energy context highlighting the role of hydrogen as a sustainable energy carrier. It addresses the main hydrogen production routes, including Steam Methane Reforming (SMR), Partial Oxidation (POX) and Autothermal Reforming (ATR) of hydrocarbons and outlines the key purification strategies, namely Pressure Swing Adsorption (PSA), membrane separation and methanation required to meet hydrogen purity standards for hydrogen application. The chapter concludes with a review of recent advances in the preferential oxidation of CO describing the main classes of catalysts, their reaction mechanisms and the technological limitations that motivate further research. Chapter 2 focuses on materials and methods employed in this study. It describes the preparation of the MnCeOx catalyst and the suite of physico-chemical characterization techniques used to assess its properties, including X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), N2 physisorption, Laser Raman Spectroscopy (LRS), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), Temperature Programmed Reduction (TPR) and Temperature Programmed Desorption (TPD). The chapter also outlines the catalyst testing protocols, such as Temperature Programmed Catalytic Reaction (TPCR) experiments, and describes the computational approach adopted for Density Functional Theory (DFT) analysis. Chapter 3, devoted to results and discussion, begins with a detailed interpretation of the main physicochemical characterization data, providing a detailed interpretation of the structural, surface properties and redox behavior of the catalyst. Then, the second part of the chapter highlights the PROX performance of the catalyst under various conditions, in terms of activity, selectivity and stability under both kinetic regime and under real process conditions. Mechanistic insights from Density Functional Theory (DFT) calculations on a model Mn4O8 cluster and kinetic data of CO and H2 oxidation are then used to obtain simplified reaction mechanisms. Based on these evidences, finally, the chapter describes the development of suitable macrokinetic models able to predict the CO and H2 oxidation functionality of MnO2-based catalysts under a wide range of conditions, establishing quantitative correlations between structure, mechanism and reactivity and offering a rational basis for the interpretation and optimization of PROX performance.