Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths
MM Hasan and S Dey and N Nafsin and J Mardinly and PP Dholabhai and BP Uberuaga and RHR Castro, CHEMISTRY OF MATERIALS, 28, 5163-5171 (2016).
Surface energy is a key parameter to understand and predict the stability of catalysts. In this work, the surface energy of MgAl2O4, an important base material for catalyst support, was reduced by using dopants prone to form surface excess (surface segregation): Y3+, Gd3+, and La3+. The energy reduction was predicted by atomistic simulations of spinel surfaces and experimentally demonstrated by using microcalorimetry. The surface energy of undoped MgAl2O4 was directly measured as 1.65 +/- 0.04 J/m(2) and was reduced by adding 2 mol % of the dopants to 1.55 +/- 0.04 J/m(2) for Y-doping, 1.45 +/- 0.05 J/m(2) for Gd-doping, and 1.26 +/- 0.06 J/m(2) for La-doping. Atomistic simulations are qualitatively consistent with the experiments, reinforcing the link between the role of dopants in stabilizing the surface and the energy of segregation. Surface segregation was experimentally assessed using electron energy loss spectroscopy mapping in a scanning transmission electron microscopy image. The reduced energy resulted in coarsening inhibition for the doped samples and, hence, systematically smaller particle sizes (larger surface areas), meaning increased stability for catalytic applications. Moreover, both experiment and modeling reveal preferential dopant segregation to specific surfaces, which leads to the preponderance of 111 surface planes and suggests a strategy to enhance the area of desired surfaces in nanoparticles for better catalyst support activity.
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