Lawrence Berkeley National Laboratory

The influence of A- and/or B-site doping of Ruddlesden-Popper perovskite materials on the crystal structure, stability, and dry reforming of methane (DRM) reactivity of specific A₂BO₄ phases (A = La, Ba; B = Cu, Ni) has been evaluated by a combination of catalytic experiments, in situ X-ray diffraction, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and aberration-corrected electron microscopy. At room temperature, B-site doping of La₂NiO₄ with Cu stabilizes the orthorhombic structure (Fmmm) of the perovskite, while A-site doping with Ba yields a tetragonal space group (I4/mmm). We observed the orthorhombic-to-tetragonal transformation above 170 °C for La₂Ni_0.9Cu_0.1O₄ and La₂Ni_0.8Cu_0.2O₄, slightly higher than for undoped La₂NiO₄. Loss of oxygen in interstitial sites of the tetragonal structure causes further structure transformations for all samples before decomposition in the temperature range of 400 °C-600 °C. Controlled in situ decomposition of the parent or A/B-site doped perovskite structures in a DRM mixture (CH₄:CO₂ = 1:1) in all cases yields an active phase consisting of exsolved nanocrystalline metallic Ni particles in contact with hexagonal La₂O₃ and a mixture of (oxy)carbonate phases (hexagonal and monoclinic La₂O₂CO₃, BaCO₃). Differences in the catalytic activity evolve because of (i) the in situ formation of Ni-Cu alloy phases (in a composition of >7:1 = Ni:Cu) for La₂Ni_0.9Cu_0.1O₄, La₂Ni_0.8Cu_0.2O₄, and La_1.8Ba_0.2Ni_0.9Cu_0.1O₄, (ii) the resulting Ni particle size and amount of exsolved Ni, and (iii) the inherently different reactivity of the present (oxy)carbonate species. Based on the onset temperature of catalytic DRM activity, the latter decreases in the order of La₂Ni_0.9Cu_0.1O₄ ∼ La₂Ni_0.8Cu_0.2O₄ ≥ La_1.8Ba_0.2Ni_0.9Cu_0.1O₄ > La₂NiO₄ > La_1.8Ba_0.2NiO₄. Simple A-site doped La_1.8Ba_0.2NiO₄ is essentially DRM inactive. The Ni particle size can be efficiently influenced by introducing Ba into the A site of the respective Ruddlesden-Popper structures, allowing us to control the Ni particle size between 10 nm and 30 nm both for simple B-site and A-site doped structures. Hence, it is possible to steer both the extent of the metal-oxide-(oxy)carbonate interface and its chemical composition and reactivity. Counteracting the limitation of the larger Ni particle size, the activity can, however, be improved by additional Cu-doping on the B-site, enhancing the carbon reactivity. Exemplified for the La₂NiO₄ based systems, we show how the delicate antagonistic balance of doping with Cu (rendering the La₂NiO₄ structure less stable and suppressing coking by efficiently removing surface carbon) and Ba (rendering the La₂NiO₄ structure more stable and forming unreactive surface or interfacial carbonates) can be used to tailor prospective DRM-active catalysts.