Deveopment of innovative catalysts free of platinum group metals (PGMs) and/or with recycled PGMs

This PhD thesis reports on the synthesis, characterization, and catalytic testing of a series of catalysts prepared using different supports and active phases. The active phases are derived from a leachate solution containing recycled Platinum Group Metals (PGMs) from spent diesel autocatalysts, where only platinum and/or palladium are present, or from commercial nickel precursors, with the aim of fully replacing platinum group metals. Platinum group metals are transition metals characterized by their excellent catalytic properties but also by their high cost, mainly due to their limited natural abundance. The primary objective of this work is the development of sustainable and innovative catalysts for the Dry Reforming of Methane (DRM) reaction, which use either recycled PGMs or are PGM-free. In the DRM process, two major greenhouse gases, methane (CH₄) and carbon dioxide (CO₂), react to form syngas, a mixture of H₂ and CO with an ideal H2/CO ratio equal to 1. However, the Reverse Water Gas Shift (RWGS) reaction occurs as a side reaction, in which part of the produced hydrogen reacts with CO₂ to form additional CO, thereby lowering the H₂/CO ratio. Chapter 1 introduces the DRM reaction, focusing particularly on its thermodynamic and kinetic aspects, as well as on the valorization of CO₂. Chapter 2 describes the synthesis procedures of the supports and catalysts. For many of the catalysts, both commercial supports and commercial metal precursors were considered as a benchmark. The investigated systems include: (i) a CeO₂-supported catalyst impregnated with a recycled solution containing Pt as the active phase; (ii) a CeO₂-supported catalyst impregnated with a recycled solution containing Pt and Pd; (iii) a Pt-Pd/CeO2 catalyst obtained by one-pot process using the recycled solution containing Pt and Pd as the metal precursors; (iv) a CeO₂-supported catalyst impregnated with Ni; (v) a Ca12Al14O33 (mayenite)-supported catalyst impregnated with Ni; (vi) a catalyst based on ZrO₂ nanopowders synthesized via a flame-spray method impregnated with Ni. Chapter 3 presents the characterization techniques employed, including Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES), Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, Temperature-Programmed Reduction (TPR), Temperature-Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, Scanning Electron Microscopy (SEM), equipped with Energy-Dispersive X-ray Spectroscopy (EDS) and Transmission Electron Microscopy (TEM). The methodologies adopted for catalytic testing and the type of reactor used are also described. Chapter 4 reports and discusses the results of both characterization and catalytic performance for each catalyst system. Specifically, the experimental results demonstrate that recycled single-metal Pt/CeO2 catalysts (1 wt% Pt) achieved excellent DRM conversions up to 96% for CO2 and 94% for CH4 at 850 °C with remarkable stability over 25 hours. 4Pt1Pd_CS-w catalyst showed DRM conversions up to 57 % for CO2 and 42 % for CH4 at 850 °C, dropping to 15% and 7% at 750 °C, and at 2% and 1% at 650 °C. The last step at 750 °C for 50 h shows good stability over time; the conversions of the reactants remain stable, and no decrease is observed. In contrast, the one pot synthesized platinum-palladium catalyst showed DRM conversions up to 87% for CO2 and 75% for CH4 at 850 °C, and 4% and 3.4% at 650°C, respectively. At 750 °C, the conversions are not stable, probably due to the fact that this is the activation temperature; in the first step, there is a decrease of CO2 and CH4 from 61% and 48 % to 27 % and 17%, and in the second step at the same temperature there is an increase, since it is going from a lower temperature to a higher temperature, conversions for CO2 and CH4 go from 18% and 12 % to 50 % and 25%. Among PGM-free alternatives, 10 wt% Ni supported on synthesized mayenite matched noble metal performance by maintaining 99% CO2 and 94% CH4 conversions post-stability testing, while flame-spray synthesized Ni/ZrO2 systems exhibited strong in-situ activation driven by progressive NiO reduction, reaching conversions up to 96% CO2 and 95% CH4 after 30 hours. Chapter 5 reports the conclusion of the PhD thesis.