Coexisting multi-states in catalytic hydrogen oxidation on rhodium

Abstract

Catalytic hydrogen oxidation on a polycrystalline rhodium foil used as a surface structure library is studied by scanning photoelectron microscopy (SPEM) in the 10−6 mbar pressure range, yielding spatially resolved X-ray photoemission spectroscopy (XPS) measurements. Here we report an observation of a previously unknown coexistence of four different states on adjacent differently oriented domains of the same Rh sample at the exactly same conditions. A catalytically active steady state, a catalytically inactive steady state and multifrequential oscillating states are simultaneously observed. Our results thus demonstrate the general possibility of multi-states in a catalytic reaction. This highly unusual behaviour is explained on the basis of peculiarities of the formation and depletion of subsurface oxygen on differently structured Rh surfaces. The experimental findings are supported by mean-field micro-kinetic modelling. The present observations raise the interdisciplinary question of how self-organising dynamic processes in a heterogeneous system are influenced by the permeability of the borders confining the adjacent regions.

Pattern Formation in Catalytic H2 Oxidation on Rh: Zooming in by Correlative Microscopy

“…The reaction modes observed at the present conditions for the homogeneous Rh(7 5 5) and Rh(4 3 2) domains can be understood in view of recent studies in which different reaction modes of H 2 oxidation were simultaneously observed on adjacent crystallographically different regions. 16 The Rh(7 5 5) surface (ROI 1) is characterized by [1 1 1]-type terraces in combination with nonkinked [1 0 0]-type step edges. The Rh(4 3 2) surface (ROI 2) is also characterized by [1 1 1]-type terraces but combined with kinked [2 1 0]-type step edges.…”

“… 13 − 16 An illustration of the oscillation cycle and further details of the mechanism of the oscillations are presented in the Supporting Information ( Figure S2 ) and have been extensively discussed previously. 13 − 16 For Rh, the activation energy of subsurface oxygen formation strongly varies with the surface roughness, with the kink sites playing a particular role. 39 The Rh(4 3 2) surface exhibits kinked step edges, which accelerates the formation/depletion of subsurface oxygen compared to the nonkinked Rh(7 5 5) surface.…”

“…This is illustrated in atomic ball models in Figure d. The structural differences lead to differing kinetic behavior: as recently shown, the formation and depletion of subsurface oxygen on the Rh­( h k l ) surfaces and the resulting enabling or inhibition of dissociative hydrogen adsorption may lead to self-sustaining oscillations in H 2 oxidation, with the activation energy of the oxygen incorporation governing the oscillation frequency. − An illustration of the oscillation cycle and further details of the mechanism of the oscillations are presented in the Supporting Information (Figure S2) and have been extensively discussed previously. − For Rh, the activation energy of subsurface oxygen formation strongly varies with the surface roughness, with the kink sites playing a particular role . The Rh(4 3 2) surface exhibits kinked step edges, which accelerates the formation/depletion of subsurface oxygen compared to the nonkinked Rh(7 5 5) surface.…”

“…The interaction between active sites in a heterogeneous catalytic reaction may lead to spatial nonuniformities of the local reaction rates, which may also vary in time. − Such pattern formation has often been observed for reactions with oscillatory or multistable kinetics, such as H 2 or CO oxidation, NO reduction, or NO 2 reduction on noble metals, − and has been a subject of longstanding studies and several reviews. − Recently, single-crystal studies were extended to more realistic systems, and a number of effects were detected that influence the spatio-temporal structures: multifrequential oscillations, both in μm- and nm-sized systems, frequency transformation by grain boundaries and by atomic rows, coexisting multi-states and nm scale reaction pacemakers . These previous studies have demonstrated the plethora of novel effects that may occur in surface reactions.…”

“…To address these challenges, it is necessary to significantly improve the lateral resolution, preferentially combined with chemical sensitivity. Still, conventional modeling approaches have typically been focused on either mean-field kinetic modeling well applicable to extended homogeneous systems ,, or Monte Carlo simulations on the traditional atomic length and time scales. , To combine both approaches for the simulation of processes on mesoscopic heterogeneous surfaces is another challenge in studying surface reactions.…”