Determining the hydration energetics on carbon-supported Ru catalysts: An adsorption calorimetry and density functional theory study

Abstract

Fundamental knowledge on the energetics at the interface between a water layer and a metal catalyst is essential so as to understand the roles that water can play in the synthesis, activation, and regeneration of noble metal-based catalysts. Here, we report the direct measurement of the enthalpy of water adsorption (Δhads) on activated carbon (C) and activated C-supported ruthenium (Ru) nanoparticles, which is a promising catalyst as applied to the hydrogenation/hydrodeoxygenation (HDO) of oxygenates (phenolics, aldehydes, etc.). Specifically, the near-zero coverage enthalpy of water adsorption on a C-supported Ru catalyst is −75.3 ± 0.4 kJ/(mol water), suggesting favorable water–metal binding. This is much more exothermic than that on C, which has an enthalpy of adsorption of −50.3 ± 1.3 kJ/(mol water). Despite the favorable initial binding, the magnitudes of enthalpies of water condensation on C and Ru-C indicate that overall, their surfaces are both hydrophobic. Moreover, the experimentally-measured near-zero coverage water adsorption enthalpy at the Ru sites is in very good agreement with our density functional theory based calculations. At low coverages, we obtain a water binding energy of −61.7 kJ/(mol water), which increases to −78.1 kJ/(mol water) at saturation. Complementary results are also obtained from a thermal analysis, which employed a thermogravimetric analysis–mass spectrometry (TG-MS), a spectroscopic investigation using ex situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and a morphological evaluation with transmission electron microscopy (TEM). We point out that in carbon-supported metal catalysts, such as Ru-C, a strong hydration at near-zero coverage and relative weak water-surface interactions occur upon saturation. Such heterogeneity is essential and crucial for catalytic hydrogenation/HDO reactions that involve balanced interactions among the water-rich reactant mixture and nonpolar organic products.