Nonlinear effects in the hydrogen/deuterium catalytic exchange reaction over Pt{100}

Abstract

The exchange reaction between H2 and D2 has been studied over Pt{100} as a function of surface temperature between 150 and 1100 K using molecular beam techniques. The reactive sticking probability, sr, shows strong hysteresis over this temperature range due to adsorbate-induced restructuring of the surface from hex-R to (1×1) structures. A nonlinear (1×1) island growth power law, rate ∝(θ hexH)4.0, plays a critical role in the dynamics of the exchange process. Hydrogen adsorption below ∼250 K induces incomplete (1×1) island growth. As the temperature is ramped upwards desorption leaves a clean (1×1) surface at ∼400 K, with a high sr(=0.21), but by ∼450 K the surface is converted to a defect-containing hex surface; the defects are characterized by a higher exchange activity than on the hex surface. At 650 K the defects are annealed away, and the surface exhibits the relatively low reactivity [sr=0.06+(4×10−5) Ts] of the stable Pt{100}-hex-R phase. On cooling from 1100 to 150 K, the hex-R structure remains until ∼250 K, when (1×1) island growth causes irreversible hydrogen adsorption to saturation, with a sharp decrease in sr. A kinetic model incorporating these features gives an excellent description of the data.