Development of new Ceria-based supported metal catalysts for synthesis Gas production via dry reforming of Methane

Abstract

The present PhD thesis investigates the effect of (a) CeO¬2 synthesis method and the chemical composition of metal oxides based on CeO2 (Ce1-xMxO2-δ: M = Pr3+, Ti4+) used as supports of Ni metal catalysts, (b) Ni and NiPt catalysts supported on Ce0.8Pr0.2O2-δ, and (c) Ni particle size (dNi, nm) over Ce0.8Ti0.2O2-δ-supported Ni catalysts, on the following parameters of catalytic behavior for the Dry Reforming of Methane (DRM: CH4 + CO2  2CO + 2H2): (i) the origin of carbon (CH4 versus CO2 reaction path) resulting in catalyst deactivation, (ii) the kinetic rate of carbon deposition via dissociation of CH4 and CΟ (transient kinetics of CH4 and CO dissociation), (iii) the participation of support lattice oxygen in the gasification (oxidation) of deposited carbon towards CO formation under DRM reaction conditions, and (iv) the concentration and reactivity (k, s-1) of active carbon-containing intermediate species derived from both CO2 and CH4 activation routes in the dry reforming of methane reaction (SSITKA experiments using 13CH4 and 13CO2 isotopes). Several transient and isotopic experiments using 13CΟ, 18Ο2, 13CH4, and 13CO2 were performed to achieve the present doctoral thesis’ goals and innovation. In addition, to better understand the lattice oxygen participation in the gasification rate of carbon under DRM reaction conditions as a function of Ni particle size, a detailed analysis of Ni particle size distribution was performed by the HRTEM technique. Characterization of the deposited carbon was performed by temperature-programmed oxidation (TPO) and TEM techniques. To investigate the formation of NiPt solid solution, H2-TPD and HRTEM/EDX were performed. The addition of 20 atom-% Pr3+ in CeO2 lattice resulted in the reduction of accumulated carbon by a factor of  2, while 0.5 wt% Pt addition on the 5 wt% Νi/Ce0.8Pr0.2O2-δ catalyst led to a decrease of deposited carbon by a factor of  60. On the other hand, incorporating 20 atom-% Ti4+ in CeO2 led to the remarkable reduction of accumulated carbon by  320 times, while CH4 conversion and H2 yield were improved. Furthermore, it was found that a decrease of the Ni particle size in the Ni/Ce0.8Ti0.2O2-δ catalytic system reduces significantly the rate of carbon deposition (deriving exclusively by CH4 decomposition). In addition, it has been proven that the rate of carbon removal by its reaction with labile support’s lattice oxygen is of extreme importance regardless of dNi (nm) under DRM reaction conditions (20% CH4, CH4/CO2 = 1, T= 750 oC).