Synchronization in oscillatory surface reactions on single crystal surfaces

In oscillatory surface reactions on single crystal surfaces the partial pressure variations that accompany the oscillations in the reaction rate represent a global interaction between the local oscillators. The NO+CO reaction on Pt(100) exhibits both synchronized and unsynchronized oscillatory behavior depending on the substrate phase. Unsynchronized oscillations occur on the 1×1 phase but on the hex phase one finds synchronized oscillatory behavior. The mechanism leading to synchronization in the oscillations on Pt(100)‐hex can be traced back to a critical dependence of the 1×1⇄hex phase transition on the partial pressures of NO and CO. At both ends of the temperature window for rate oscillations one finds well‐defined transitions to a stationary reaction rate. These bifurcations are discussed in terms of a transition from synchronized to unsynchronized behavior. In particular, the occurrence of deterministic chaos in connection with a Feigenbaum scenario is interpreted as being due to such a transition

Synchronization and spatiotemporal self‐organization in the NO+CO reaction on Pt(100). I. Unsynchronized oscillations on the 1×1 substrate

The oscillatory NO+CO reaction on Pt(100) has been investigated in the 10−6 mbar range using photoemission electron microscopy (PEEM) as a spatially resolving method. The existence ranges for kinetic oscillations have been mapped out in (pCO,T)‐parameter space with fixed pNO=4×10−6 mbar. Kinetic oscillations occur within a partial pressure range of 0.8NO/p CO <1.9. In the lower lying of two temperature windows for oscillatory reaction behavior, the oscillations proceed unsynchronized on a 1×1 substrate without exhibiting macroscopic rate variations. Instead, one observes spatiotemporal pattern formation which has been studied in detail. These patterns are dominated by periodic wave trains, which become unstable at lower temperatures, giving rise to spiral waves and irregularly shaped reaction fronts. With decreasing temperature, the front velocity increases, while simultaneously the spatial periodicity of the wave trains becomes larger. In agreement with theoretical predictions by a three‐variable model, the local oscillations terminate at the upper T boundary via a Hopf bifurcation and at the lower T boundary via a bifurcation of the saddle‐loop type

Synchronization and spatiotemporal self‐organization in the NO+CO reaction on Pt(100). II. Synchronized oscillations on the hex‐substrate

The NO+CO reaction exhibits sustained rate oscillations on Pt(100) under conditions where the surface is mostly hex‐reconstructed. These rate oscillations have been investigated in the 10−6 mbar range using photoemission electron microscopy as a spatially resolving method. During the rate oscillations which appear in a temperature‐window ranging from 490 to 478 K, the surface reacts in a spatially homogeneous way. At the upper T‐boundary of the oscillatory range, the oscillations develop via a Feigenbaum scenario leading from chaotic small amplitude oscillations at high T to regular period‐1 oscillations at lower T. At the lower T‐boundary of the oscillatory range, at T=478 K, target patterns appear, causing the collapse of the amplitude of the rate oscillations. As the temperature is lowered further, the parallel wave trains become increasingly irregular. Spiral waves form, and finally one observes only local reaction outbursts. A model for the synchronization mechanism in the rate oscillations is proposed based on the 1×1⇔hex‐phase transition, while the origin of the chaotic oscillations in this reaction system is discussed in terms of a transition from unsynchronized to synchronized oscillations

Delay-Induced Chaos in Catalytic Surface Reactions: NO Reduction on Pt(100)

Deterministic chaos has been observed in the NO+CO and NO+H2 reactions on Pt(100). A mathematical model is proposed that explains the origin as being due to delays in the response of a population of reacting adsorbate islands globally coupled via the gas phase. The dynamical equations of this model yield a sequence of period-doubling bifurcations resulting in chaos

Kinetic hindrance during the initial oxidation of Pd(100) at ambient pressures

The oxidation of the Pd(100) surface at oxygen pressures in the 10^-6 to 10^3 mbar range and temperatures up to 1000 K has been studied in-situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions we either observe a thin sqrt(5) x sqrt(5) R27 surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K.Comment: 4 pages including 4 figures, Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

The developmental toxicity of complex silica-embedded nickel nanoparticles is determined by their physicochemical properties

Complex engineered nanomaterials (CENs) are a rapidly developing class of structurally and compositionally complex materials that are expected to dominate the next generation of functional nanomaterials. The development of methods enabling rapid assessment of the toxicity risk associated with this type of nanomaterial is therefore critically important. We evaluated the toxicity of three differently structured nickel-silica nanomaterials as prototypical CENs: simple, surface-deposited Ni-SiO2 and hollow and non-hollow core-shell Ni@SiO2 materials (i.e., ~1–2 nm Ni nanoparticles embedded into porous silica shells with and without a central cavity, respectively). Zebrafish embryos were exposed to these CENs, and morphological (survival and malformations) and physiological (larval motility) endpoints were coupled with thorough characterization of physiochemical characteristics (including agglomeration, settling and nickel ion dissolution) to determine how toxicity differed between these CENs and equivalent quantities of Ni2+ salt (based on total Ni). Exposure to Ni2+ ions strongly compromised zebrafish larva viability, and surviving larvae showed severe malformations. In contrast, exposure to the equivalent amount of Ni CEN did not result in these abnormalities. Interestingly, exposure to Ni-SiO2 and hollow Ni@SiO2 provoked abnormalities of zebrafish larval motor function, indicating developmental toxicity, while non-hollow Ni@SiO2 showed no toxicity. Correlating these observations with physicochemical characterization of the CENs suggests that the toxicity of the Ni-SiO2 and hollow Ni@SiO2 material may result partly from an increased effective exposure at the bottom of the well due to rapid settling. Overall, our data suggest that embedding nickel NPs in a porous silica matrix may be a straightforward way to mitigate their toxicity without compromising their functional properties. At the same time, our results also indicate that it is critical to consider modification of the effective exposure when comparing different nanomaterial configurations, because effective exposure might influence NP toxicity more than specific “nano-chemistry” effects

New type of microengine using internal combustion of hydrogen and oxygen

Microsystems become part of everyday life but their application is restricted by lack of strong and fast motors (actuators) converting energy into motion. For example, widespread internal combustion engines cannot be scaled down because combustion reactions are quenched in a small space. Here we present an actuator with the dimensions 100x100x5 um^3 that is using internal combustion of hydrogen and oxygen as part of its working cycle. Water electrolysis driven by short voltage pulses creates an extra pressure of 0.5-4 bar for a time of 100-400 us in a chamber closed by a flexible membrane. When the pulses are switched off this pressure is released even faster allowing production of mechanical work in short cycles. We provide arguments that this unexpectedly fast pressure decrease is due to spontaneous combustion of the gases in the chamber. This actuator is the first step to truly microscopic combustion engines.Comment: Paper and Supplementary Information (to appear in Scientific Reports