Kinetics of the decomposition and hydrogen reduction of nitric oxide on niobium, nickel and platinum filaments at high temperatures and low pressures

Abstract

NO decomposition kinetics were studied with ultra-high vacuum techniques on niobium, nickel and platinum wire filaments at high temperatures with pressures between ∼1 and 100 µN m^–2. Niobium readily decomposed NO with first-order kinetics, nitrogen and oxygen being deposited on the surface, resulting in a progressive loss of filament activity. Nitrogen was desorbed from nickel, which otherwise behaved similarly to niobium. Excess H₂ affected the kinetics of NO reaction on these metals mainly by reaction with and subsequent removal of surface contaminants rather than by a direct reaction with NO.

Platinum did not observably decompose NO at low pressures except in the presence of excess H₂. Above ∼1300 K the rate appeared to be determined by H₂ atomisation, but at lower temperatures a reaction between H₂ and NO may have been occurring. The rate-limiting step for this reaction may be the decomposition of H₂ and addition of H atoms to adsorbed NO. The pre-exponential term for this reaction indicated that the transition state was mobile on the surface.