Calculation of MoS2 slabs morphology descriptors from transmission electron microscopy data revisited. Case study of the influence of citric acid and treatment conditions on the properties of MoS2/Al2O3

Abstract

Molybdenum sulfide MoS2 slabs length L and stacking S obtained from the transmission microscopy (TEM) images are the major characteristics of hydrotreating sulfide catalysts, widely used as morphology descriptors. In this work we demonstrate that the formulae routinely used to estimate mean L and S should be revisited. To obtain physically meaningful values L and S, both should be calculated as surface weighted averages. Moreover, correlation between L and S must be taken into account. Using surface weighted values of mean L and S, MoS2 phase morphology was studied for a series of MoS2/Al2O3 catalysts. The effects of Mo loading, addition of citric acid and sulfidation by H2S/N2 or H2S/H2 mixtures at different temperatures from 350 to 550°C were studied. The catalysts were evaluated in hydrodesulfurization of thiophene (HDS) and studied by temperature programmed reduction (TPR). Thorough statistical treatment of TEM, HDS and TPR data suggests that the intrinsic properties of MoS2 active phase depend primarily on the treatment atmosphere and temperature rather than on the addition of citric acid. If the impregnated catalyst is dried at temperature above 120°C, citric acid allows moderate improvement of MoS2 morphology. However the edge-specific activity of supported sulfide remains unaffected.