Hydrogen chemisorption and mobility on Ru/SiO2, K/Ru/SiO2 and Ru-Ag/SiO2

Microcalorimetry and H-1 NMR have been used to investigate hydrogen chemisorption on Ru/SiO2, K/Ru/SiO2 and Ru-Ag/SiO2. Adding small amounts of K or Ag to Ru/SiO2 significantly reduces the amounts of hydrogen adsorbed and eliminates intermediate (similar to 50 kJ/mole) as well as weakly bound states (<10 kJ/mole) observed at higher pressures on Ru/SiO2. Initial heats of adsorption (similar to 90 kJ/mole) are not affected by the presence of K or Ag on Ru. These observations combined with earlier atomistic simulations with Ru-Ag suggest that defect-like sites play an important role in adsorption and desorption behavior of hydrogen on Ru/SiO2. Similar initial heats of adsorption and invariance of the H-1 NMR Knight shift on Ag or K addition imply that there is no electronic effect. Further, the mobility of hydrogen is also reduced in the presence of K promoter. The different theories of alkali promotion in the literature is discussed in the light of the observations made in this work. It is noted that the weakly bound, atomic hydrogen observed at higher pressures can play an important role in catalysis.