Spectromicroscopy of pulses transporting alkali metal in a surface reaction

Abstract

The NO + H 2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatiotemporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10 À7 and 10 À6 mbar pressure range, with the potassium coverage varying between y K = 0.05 and y K = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation:coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium.

Excitability in the H2+O2 reaction on a Rh(110) surface induced by high coverages of coadsorbed potassium

“…Nearly exactly the same behavior of mass transport with pulses has been observed before in the NO + H 2 reaction on a K-promoted Rh(110) surface. [8][9][10] Increasing the temperature from 550 K to 580 K leads initially to the same development as at lower temperature but already after a few seconds the pulses disintegrate leading to a disordered state. This sequence is illustrated in Fig.…”

“…2(a), look rather similar to the target patterns observed in the NO + H 2 reaction over both, unpromoted and K-promoted Rh(110) single crystal surfaces. [8][9][10]18 There are also a number of differences. In contrast to the NO + H 2 reaction, the present system exhibits a pronounced front instability as demonstrated by the PEEM images in Figs.…”

“…In several studies, the effect of the addition of alkali metals to pattern forming reaction systems was explored. [4][5][6][7][8][9][10] Alkali metals play an important role in heterogeneous catalysis where they are used as so-called electronic promoters. 11 Well known examples are the ammonia synthesis after HaberBosch and the Fischer-Tropsch synthesis.…”

“…6,13 In the excitable system Rh(110)/NO + H 2 , the addition of potassium gave rise to the new phenomenon of pulses transporting potassium. [8][9][10] The analysis showed that the chemical attraction between K and O was not sufficient to explain the mass transport but that, instead, the repulsive interactions between coadsorbed a) E-mail: imbihl@pci.uni-hannover.de. potassium and atomic nitrogen are presumably responsible for pulses transporting potassium.…”

“…potassium and atomic nitrogen are presumably responsible for pulses transporting potassium. 8,10 In this report, we revisit the system Rh(110)/K/O 2 + H 2 but in contrast to previous studies where the K coverage was moderate with K ≈ 0.08, we focused on the behavior at high K coverages with K > 0.16; i.e., close to the saturation coverage of a monolayer at K = 0.21.…”