Substoichiometric La0.8MnO3-based nanocomposites for PGM-free activation of CH4: Ni or Cu? Surface or bulk?

Methane is a renewable fuel when derived from biogas upgrading or by CO2 capture and utilization, with the possibility of fully replacing natural gas in the already established gas infrastructure and engines. However, being a potent greenhouse gas, its use as fuel demands a sustainable activation procedure, to convert any residual traces of unburned CH4 into CO2, a challenging reaction especially at near-stoichiometric conditions typical of Three-Way Catalytic systems (TWC). To avoid expensive noble metals, we developed cheaper alternative catalysts: A-site deficient LaMnO3-based nanocomposites incorporating Ni and Cu. These elements were introduced either as B-site dopants in the perovskite lattice or deposited via Ammonia-driven Deposition-Precipitation (ADP) on the perovskite surface, in both cases yielding metal nanoparticles (NPs) after a reductive treatment. Characterization encompassed various techniques: XRD, N2 physisorption, SEM-EDX, XPS, H2-TPR, HAADF STEM-EDX. CH4 oxidation to CO2 under stoichiometric O2 served as the probe reaction, as model for TWC conditions in methane-fed engines. The LaMn-perovskite itself displayed a promising activity thanks to favourable morphological and redox features (50 % and 90 % CH4 conversion at 581 °C and 678 °C, respectively); further improvements could be obtained upon Ni and Cu loading. The best performances were achieved by: i) the catalyst prepared by Ni-ADP (50 % and 90 % CH4 conversion at 517 °C and 635 °C), featuring high Ni surface concentration and NPs density and a synergy with the perovskite support, as opposed to the low Ni surface concentration on B-site doped samples; ii) the reduced Cu-doped catalyst (50 % and 90 % CH4 conversion at 508 °C and 645 °C), thanks to the high activity of reduced Cu species, the low NPs size and good metal-support interaction, in contrast to the strong NPs agglomeration of the Cu-ADP catalyst. Among the two most active catalysts, a better long-term stability was retained by the Cu-doped sample. This work laid the foundation for the development of alternative CH4 oxidation catalysts at the stoichiometric air-to-fuel ratio, cheaper than commercial Pd-based ones.