Elemental Steps During the Catalytic Decomposition of no over Stepped Single Crystal Surfaces of Platinum and Ruthenium

Abstract

The interaction of NO with stepped single crystal surfaces of Pt and Ru field emitters has been studied. Pulsed Field Desorption Mass Spectrometry (PFDMS) is employed to perform time resolved measurements. It is found that the NO adsorption (p = 1.3×10 −5 to 6.7×10 −4 Pa, T = 523 to 605 K) occurs molecularly on the high index planes of Pt with (111) orientation of the terraces, whereas molecular as well as dissociative adsorption is found to occur on the high index planes with (001) orientation of the terraces. By varying the repetition rate of the field pulses from 1 Hz up to 1 kHz (corresponding to a field free reaction time between 1 s and 1 ms) kinetic data of the adsorption, thermal desorption and decomposition of NO are obtained. The rate parameters for the first order thermal desorption on stepped Pt(111) are: E d =139kJ/mol, τ 0 = 3×10 −14 s. The initial stages of NO dissociation on stepped Pt(001) follow a complex kinetic mechanism which can be understood on the basis of structural autocatalysis. On stepped Ru(001) the dissociation proceeds according to first order kinetics. In addition, the oxygen deposition and accumulation leads to strong oxidation of Ru. This is evidenced by desorption of RuO x 2+ ions (x up to 3). A field induced decomposition of NO on Pt is observed and leads to high ionic rates of N 2 O and N + 2 . No marked face specifity has been found so far for this reaction.