Formation and thermal stability of subsurface deuterium in Ni(110)

A unique feature of plasma-enhanced catalysis compared to thermal catalysis is the presence of reactive hydrogen radicals and ions in the gas phase above the catalyst surface. The uptake and subsequent thermal desorption of deuterium on a Ni(110) surface have been measured using incident gaseous D₂ molecules, D atoms, and D₂ ⁺ ions. Molecular D₂ exposures on Ni(110) at 90 K form adsorbed D adatoms at the surface, but do not populate subsurface deuterium binding states under UHV conditions. In contrast, such subsurface states on Ni(110) are readily populated at 90 K by incident D atoms and D₂ ⁺ ions. Subsurface D atoms recombine to desorb as D₂ gas in temperature programmed desorption (TPD) measurements to create new characteristic subsurface-derived D₂ thermal desorption peaks near 175 K for incident D atoms and 190–260 K, depending on the energy of the incident D₂ ⁺ ions used, along with a new peak at 435 K for incident ions. While there are small differences for adsorbed D adatoms on Ni(110) and Ni(111) surfaces regarding their thermal stability and subsequent D₂ TPD curves, the thermal stability and D₂ TPD peaks from subsurface D atoms are nearly the same for these two substrates. This information will be helpful for a fuller understanding of the role of subsurface hydrogen and its reactivity in hydrogenation for plasma-enhanced catalysis over Ni-based catalysts.