The Interaction of Hydrogen with Ru/MgO Catalysts

The energetics of the interaction of hydrogen with MgO-supported Ru catalysts (Ru/MgO, Cs–Ru/MgO) was studied by combining temperature-programmed (TP) techniques, volumetric hydrogen chemisorption, and microkinetic modeling. Various TP experiments (TP desorption, TP adsorption) were carried out, applying different H2 dosing procedures (isothermal flow experiments, pulse flow chemisorption, etc.). The interaction of H2 with a Ru/MgO catalyst can be rationalized as follows: hydrogen preferentially dissociates on low-coordinated defectlike sites (“portal”-mediated adsorption). These species migrate to high-energy sites and subsequently fill up via diffusion the intermediate-energy and then the low-energy sites. Furthermore, a direct adsorption pathway on the different Ru sites was identified. Results obtained by volumetric hydrogen chemisorption performed at different temperatures correlate well with the amount detected in the TP experiments. By means of microkinetic modeling, the peaks in the obtained TPD signal could be assigned to the desorption from different Ru single-crystal surfaces. On Cs–Ru/MgO, the dopant was found to block the portal sites suppressing the fast nonactivated adsorption path.