Development of lanthanum-based heterogeneous catalysts for sustainable hydrogen production

Aqueous phase reforming (APR) of waste biomass, such as glycerol, offers a route to sustainable hydrogen production. The development of new stable and active catalysts for APR is required, due to the harsh hydrothermal conditions (200-250 ℃, 15-50 bar), which results in a lack of catalyst stability. The performance of a series of 1 wt.% Pt/LaMO3 (where M = Al, Cr, Mn, Fe, Co, Ni) for the APR of glycerol in a batch reactor was studied and compared to a standard 1 wt.% Pt/ γ-Al2O3. Several perovskite-based catalysts (where M = Al, Cr, Ni) had higher glycerol conversion (13-19%; TOF = 442-652 h-1) and hydrogen formation rates (275.2-294.3 µmol min-1 g-1cat) than 1 wt.% Pt/γ-Al2O3 (10%; TOF = 246 h-1; H2 formation rate = 259.5 µmol min-1 g-1cat). All of the perovskite catalysts showed higher lactic acid selectivity (15-39%) when compared to the standard catalyst (4%). This is attributed to the different hydrogen producing pathways promoted by the perovskite-based catalyst, rather than the reforming reaction. The Pt/LaMO3 catalysts (where M = Al, Cr, Ni) were found to be catalytically stable over three cycles, whereas the Pt/LaMnO3 suffered from severe deactivation. Characterisation of the post-reaction catalysts showed most of the perovskite catalysts undergo phase transformation to hexagonal LaCO3OH and the respective M oxide, with Pt particle migration and growth. The exception being LaCrO3, which was found to be structurally stable, albeit with surface enrichment of Cr. Leaching of the M-site (Mn (75.2%) and Co (66.1%)) led to a significant loss in support surface area and uncontrolled migration of Pt nanoparticles, causing deactivation. Although, LaAlO3 could be stabilised by the addition of base (KOH), as predicted by Pourbaix diagrams.

The activity and stability of Pt/LaAlO3 under glycerol APR is also influenced by the initial phase purity of LaAlO3. Calcination of the support at 700 °C produces the LaAlO3 perovskite phase and an amorphous lanthanum carbonate phase, evidenced by LaB6 internal standard studies, IR, and X-ray PDF. Higher temperature calcination at 900 and 1100 °C produce the phase pure LaAlO3, with a larger particle size. Catalysts comprised of phase pure LaAlO3 were notably more active, with a support calcination temperature of 1100 °C resulting in 20.4% glycerol conversion (TOF 686 h-1) in a 2 h batch reaction. Regardless of LaAlO3 phase purity, the catalysts eventually transform into Pt/LaCO3OH-AlO(OH) during the reaction. The rate of perovskite phase decomposition strongly influenced the final catalyst performance, with the initially phase impure LaAlO3 decomposing too quickly to facilitate Pt redistribution. This is further elucidated with studies using simulated reaction products; organic acid products (lactic acid), in the absence of CO2, facilitated La leaching and loss of crystallinity. A carbonate source (CO2) is essential to limit La leaching and form stable Pt/LaCO3OH-AlO(OH).

The formation and consequent stability of crystalline LaCO3OH under APR conditions led to work towards the development of a high surface area LaCO3OH/KIT-6 nanocomposite. Nanoparticle La2O3 species were supported within the KIT-6 matrix, resulting in a drop in surface area (680 to 384 m2 g-1) and pore volume (0.780 to 0.188 cm3 g-1). The aqueous hydrothermal conditions required to synthesise LaCO3OH and for glycerol APR are unsuitable for KIT-6 and nanoparticle La2O3/KIT-6, with collapse of the pore network occurring even at 100 °C. Optimisation of the synthesis of LaCO3OH and related La2O2CO3 phases from La2O3 was undertaken. Carbonate concentration was found to be the most important factor in the synthesis of LaCO3OH from either La2O3 or La2O2CO3, with the molar ratio (NH¬4)2CO3 5:1 La needed to minimise by-products. The inter-relation of crystalline La species in the La2O3-H2O-CO2 ternary phase system was further elucidated.