Influence of the surface structure on the adsorption of hydrogen on platinum,as studied by field emission probe-hole microscopy

The adsorption of hydrogen on platinum was investigated with a field emission microscope, equipped with a probe-hole assembly to enable adsorption studies on individual emitter regions. Adsorption of hydrogen is markedly face-specific. At 95 K and a hydrogen equilibrium pressure smaller than 2 × 10^?9 Torr the work function decreased strongly on the (111) face but increased on the (110) and (210) regions. Three different adsorption states were observed: β-hydrogen which desorbed above 300 K, α-hydrogen which desorbed around 230 K and a very weakly bound γ-state with a maximum heat of adsorption of 6 $\text{kcal}\text{mole}$. The α- and γ-states caused a decrease, the β-state an increase of the work function. The results show that the relative contribution of these three states and their heat of adsorption depend strongly on the crystal face. The β-state appeared to be absent on a smooth (111) plane. Hydrogen bound in the αstate has a relatively high heat of adsorption on the (111) region. A model has been proposed for the nature of the sites on the different surfaces involved in the adsorption of hydrogen.