Catalytic hydrogen oxidation on rhodium: regular and irregular self-sustaining oscillations on a nm-scale

Abstract

Spatio-temporal phenomena related to the oscillating behavior in catalytic reactions have been studied for many years as interesting features in different surface reactions. Aside from purely academic interest, they also have applicability in real catalytic systems,where systems exhibiting oscillations may have improved effciency. The catalytic hydrogen oxidation is a technologically important reaction which is applied particularly in the context of fuel cell technology, where it can be used to generate green energy and therefore is one of the possible technologies to counteract the anthropogenic-driven climate change. Additionally, the catalytic hydrogen oxidation can be used for catalytic heat production, elimination of hydrogen via catalytic recombination and hydrogen sensors. Newly, the phenomenon of multifrequential self-sustaining oscillations on μm-sized domains of polycrystalline Rh foil was observed in this reaction. In the present work, the studies of the oscillating mode of hydrogen oxidation on rhodium is extended from the μm- to the nm-scale. Using the field ion microscopy (FIM) and field emission microscopy (FEM), the catalytic hydrogen oxidation is studied on a Rh nanotip with [110]-orientation. Such a nanotip serves as a well-defined model for a real catalytic nanoparticle. Emphasis was put on studying the bistability and kinetic instabilities on the nm-scale. Kinetic phase diagrams were constructed which elucidate the conditions where the catalytic hydrogen oxidation on Rh can be found in a steady state of low activity, high activity and bistability. On the same Rh nanotip, oscillations in the catalytic hydrogen oxidation were studied. It was shown that these oscillations can be synchronized over the whole Rh tip apex. Fast moving reaction fronts were observed and a kinetic phase diagram constructed, which illustrated under which conditions oscillations can occur. At constant oxygen pressure, the frequency of oscillations was found to depend on both the hydrogen pressure and temperature. At certain conditions, however, irregular oscillations can be observed, where no particular frequency can be attributed to the observed process and different regions of the tip apex are no longer synchronized. Such irregular oscillations over a continuous parameter range were not yet observed for the catalytic hydrogen oxidation on Rh. These irregular oscillations show indications of deterministic chaotic spatio-temporal behavior. This was quantified by the calculations of trajectories of the measured time-series and calculation of the correlation dimension and the approximate entropy from these experimental data.