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Abstract
Over the past few decades, numerous heterogeneous catalytic systems have been found to exhibit a rich variety of nonlinear activity including reaction rate oscillations and spatiotemporal pattern formation. As such, the nonlinear dynamics promoting this complex behavior have been the focus of many experimental and theoretical investigations, motivated as much by the goal of understanding the origins of complex behavior as by the potential of exploiting such behavior to improve system performance. To that end, the present work is concerned with the manipulation and characterization of the dynamic behavior of two model surface reactions, CO oxidation and the reduction of NO by NH3 , both on Pt surfaces. While the dynamics of these systems have been investigated previously, the impact of this experimental work lies in the ability to address reaction dynamics on a local length scale, thus providing a unique perspective on various fundamental catalytic processes.
With regard to the control aspect of this topic, local manipulation of surface reactivity was accomplished using an external forcing technique designed to locally dose reactant onto the catalyst surface at variable frequencies and concentrations. This technique was further paired with imaging ellipsometry (EMSI) such that the surface response to forcing could be monitored under reaction conditions in real time. Local control over surface reactivity was demonstrated in the CO oxidation system, where low-reactive, CO-poisoned surface regions could be selectively promoted to a high-reactive, O-covered state using local O2 perturbations. Besides monostable reactivity states, forcing was also used to generate localized metastable, excitable, and oscillatory structures on an otherwise stable surface. The emergence of such localized structures highlights the effect chemical reaction can have on the catalyst surface, evident here through a long-term surface memory observed at the forcing site. Based on the spatial arrangement, as well as the thermal and temporal stability of the modified surface at the forcing site, subsurface O was identified as a potential source for the observed local patterning.
Another dynamic behavior considered by this work is the ability of local structures, such as those created by forcing, to propagate throughout the heterogeneous system via gas-phase coupling. Although this coupling mode has been implicated in a number of catalytic systems, the details of this mechanism are lacking due to experimental limitations involved with creating and detecting localized behavior both on the surface and in the gas-phase simultaneously. To address these challenges, we utilize a system of spatially isolated chemical oscillators in the form of individual catalyst grains on a polycrystalline Pt surface. These local oscillators were visualized on the surface using EMSI, and in the case of the CO oxidation system, were found to exhibit remarkably slow coupling behavior given the vacuum operating conditions. Scanning quadrupole mass spectrometry (SQMS) provided additional insight into this behavior, as clouds of oscillating reactant were identified up to 250 μm away from oscillating grains, suggesting that grains separated by twice this length should be able to communicate, a result supported indirectly by EMSI experiments.
The complexity of gas-phase coupling dynamics was further illustrated in the reduction of NO by NH3 on polycrystalline Pt. In particular, we focused on the role gas-phase communication plays in the development of mixed-mode oscillations by examining the system from three distinct levels, simultaneously. These viewpoints included monitoring the activity of adsorbates directly on the surface using EMSI, locally (∼100 μm from the surface) in the gas-phase using SQMS, and globally (10 cm from the surface) through an additional mass spectrometer. Discrepancies between each of these data streams emphasize the point that local gas-phase gradients do exist near the reactive surface, as gas-phase coupling does not necessarily lead to instantaneous and uniform changes throughout the reactor system, even under vacuum conditions where gas-phase communication is generally expected to be most efficient.
