Interaction of Hydrogen with Supported Ru Catalysts: High Pressure in Situ NMR Studies

An in situ 1H nuclear magnetic resonance (NMR) technique has been employed to study the interaction of hydrogen with silica-supported ruthenium catalysts in the temperature range 300-473 K and at pressures of 10 −5-760 Torr. In addition to an upfield NMR peak (called α) occurring at ∼−60 ppm and observed at low pressures ( P < 100 Torr), we have noted the emergence of a second peak (called β) occurring at ∼−30 ppm at elevated pressures ( P > 100 Torr). These two hydrogen populations represent adsorbed hydrogen interacting with Ru metal particles, as indicated by the observed Knight shifts. The α hydrogen species exists in the form of less mobile hydrogen species α 1, or a mobile hydrogen species α M with heats of adsorption determined by microcalorimetry to be about 70 kJ/mol and 40 kJ/mol, respectively. The heat of adsorption of the β species was determined to be approximately 10 kJ/mol. Hydrogen adsorption isotherms were obtained from the NMR peak intensities calibrated by a gadolinium-doped lanthanum hydride internal reference. A higher dispersion of metal particles correlated with a larger amount of the weakly adsorbed β hydrogen species. The H/Ru surface ratio was observed to be in excess of 5 at elevated pressures (>500 Torr). The high H/Ru surface was due to the adsorption of large amounts of β hydrogen.