Unravelling the surface chemical characteristics and nanostructure of MgO/NiO catalyst using positronium probe: positron annihilation lifetime and age momentum correlation study

Abstract

MgO/NiO catalyst samples with varying NiO loading (5, 10, 20 and 40 wt%) were prepared by mechanical mixing followed by annealing at 800 °C for 1.5 hours. The grain size and crystallinity of the catalyst samples were determined using X-ray powder diffraction. The triplet (ortho) state of positronium (o-Ps) has been used to characterize the nanostructure (nano-dimensional open spaces) and surface chemical characteristics of the grains. The availability of these vacant spaces as well as the surface characteristics of grains are crucial for the catalytic activity of a catalyst. Positron annihilation lifetime spectroscopy measurements showed two o-Ps lifetime components (τ3 and τ4) in both pure MgO as well as NiO powders indicating the presence of two types of open spaces. In the case of the catalyst samples, o-Ps component, τ3, is observed to disappear most probably due to spin-conversion of o-Ps in the presence of paramagnetic Ni2+ at MgO grain surfaces in the form of a NiO/MgO solid solution layer. This phenomenon has been confirmed by measuring positron age dependent line shape S(t)-parameter using the positron age momentum correlation (AMOC) technique. The study indicates the presence of a NiO/MgO solid solution layer on the surface of MgO grains along with other nanostructural changes as a result of mechanical mixing followed by the annealing process.