New Mechanisms for the Activation and Desorption of Molecules at Surfaces

Abstract

Many chemical reactions occurring on the surfaces of solid materials appear to proceed only under high pressures of the gaseous reactants but not at low pressures (< 10⁻⁴ Torr), despite favorable thermodynamics. This lack of reactivity at the low pressures where ultrahigh vacuum (UHV) surface science techniques are operable is known loosely as the pressure gap in the reactivity in heterogeneous catalysis [2.1,2]. Our group proposed that an origin of the pressure gap is the presence of a barrier to dissociative chemisorption of at least one of the reactants upon collision with the surface [2.3–6]. Since it is the translational or internal energy of the incident molecule that is important in surmounting this barrier and not the surface temperature, the rate of the reaction is limited by the flux of incident molecules with energies above the energy of the barrier. High pressures simply increase the absolute number of high energy molecules, thereby increasing the reaction rate sufficiently for the products to be detected.