The aim of the project is the development of new active, selective and robust catalysts for syngas (H2+CO) production by the catalytic partial oxidation of methane (CH4-CPO). The research program will be focused on the preparation, the characterization and the study of catalytic activity of nanostructured ZrO2-supported Ni systems that showed in preliminary tests a promising activity and stability for CH4-CPO. Ni/ZrO2 samples at various Ni loading (0.5-5.0 wt%) will be prepared by using ZrO2 support with different morphology and characterized by the combined use of different chemical-physical techniques (XRD, UV-vis, SEM, FTIR, Raman, XPS and porosimetry measurements). Catalytic study will be performed in a flow apparatus at atmospheric pressure in steady state conditions with a CH4+O2 mixture, by changing feed ratio, reaction temperature, contact time and catalyst pre-treatment.
The comparison between the results of characterization and those of catalytic studies is aimed at understanding the relationship between composition, structure and catalytic properties of the materials. Beside this, characterization results on Ni/ZrO2 catalysts will be compared with those of unsupported Ni in order to clarify the nature of the metal-support interaction and its role in stabilizing the nano-structures against sintering in the reaction conditions.
The following points will deserve special attention:
i) the role played by preparation methods and activation conditions (temperature, atmosphere) on the dispersion of the active phase;
ii) the effect of Ni particle size and ZrO2 crystallographic modifications on the catalytic performances
iii) the conditions that lead to deactivation and the structural properties of deactivated species;
iv) the structural and morphological properties of the systems after catalytic tests.
Some theoretical calculation will be used to understand the nature of the possible transition states and intermediates involved in the reaction pathway.
Nickel supported systems (mainly on Al2O3) are active for CH4-CPO, although they suffer from deactivation due to coke deposition. ZrO2 has been considered a good alternative to Al2O3 as a support for Ni. The Ni/ZrO2 preparation method influenced the phase composition, particle size, and catalytic properties of the Ni active component, and affects the resistance to carbon formation. However, a full understanding of the microscopic process occurring at the surface of the Ni particles or at the interface between particles and support is still lacking. Thus, the effect of preparation method and activation procedure (atmosphere and temperature) on the size and reactivity of metal nanoparticles need a deeper investigation. To achieve this information, a wide variety of surface and bulk techniques, applied in our unit or in collaboration with the ISMN-CNR unit, will be useful to characterize new Ni-based supported catalysts and to establish a correlation between structure (composition, dispersion, nature and concentration of active centers) and catalytic activity. X-ray diffraction (XRD) will contribute to assess the phase composition of the support and the mean particle size of the Ni phase (oxide or metal), to be compared with those obtained by FE-SEM images; UV-vis spectroscopy will investigate electronic features and FTIR will give information on dispersion, coordinative unsaturations and redox properties of the supported species and their reactivity with suitable probe-molecules (CO and NO); Raman Spectroscopy will elucidate the molecular structure of supported species, adsorbates and types of carbonaceous species that can form on the catalyst surface during the reactions as well as the nature of the support; X-ray Photoelectron Spectroscopy (XPS) will determine the oxidation state and the surface composition; Temperature Programmed Reduction (TPR) will clarify the redox properties of the supported Ni species and H2/O2 titration will evaluate dispersion of the metal particles.
Within this context, zirconia-supported Ni catalysts, Ni/ZrO2 systems, at different Ni loading, by using ZrO2 with specific morphological and textural properties as support will be investigated and tested for the CH4-CPO reaction for syngas production under different conditions (temperature, time of stream, pre-activation..). Theoretical calculations will be useful to investigate the mechanism of the CH4-CPO reaction with its intermediates and transition states. Potential energy profiles can successfully be employed to calculate reaction rates and to perform a comparison between different possible mechanisms.