Oxidation of asphaltenes adsorbed onto NiO nanoparticles

Differential thermogravimetry (DTG) and differential scanning calorimetry (DSC) plots help identifying reaction zones and enable activation energy calculations. Recently, Nassar et al. [1] and [2] reported major shifts in the DTG combustion peaks and reaction zones between virgin and adsorbed asphaltenes onto commercial metal oxide nanoparticles. They attributed the accompanying reduction in activation energy to a significant catalytic role played by the nanoparticles, especially for NiO nanoparticles. It should be noted that in these experiments only monolayer adsorption from toluene model solutions was encountered. More recently, our group reported multilayer adsorption of asphaltenes from heavy oils onto in situ prepared and commercial NiO nanoparticles [3]. Contrary to the previous literature, the thermal behavior of these asphaltenes revealed a surface role entailing an enhanced exposure of adsorbed asphaltenes to the oxidant stream. In this work, we critically re-evaluated the claim of catalytic effect of nanoparticles [1], [2], [4] and [5] and provide an experimental protocol which demonstrates a surface effect.

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► In situ prepared NiO nanoparticles show high potency as adsorbent and additive.
► Sequential combustion of adsorbed layers model is more founded.
► Activation energy, Ea, calculations reflect lower values for physically adsorbed species.
► Chemically adsorbed species react at high T with higher Ea.
► Role of NiO nanoparticles is surface exposure not catalytic.