N. M. H. Janssen

Non-linear behaviour of nitric oxide reduction reactions over metal surfaces

Chemical reactions carried out under strongly non-equilibrium conditions can result in a variety of interesting effects such as oscillations, chemical waves, kinetic phase transitions, bistability, instabilities and chaos - all of these effects being mathematically due to the strong non-linearities in the kinetic rate equations describing these chemical processes. The present article reviews the various types of non-linear behaviour observed in several NO reduction reactions over Pt-group metals. A large part of this review deals with oscillations and spatiotemporal pattern formation in the reaction over Rh surfaces: the reaction dynamics of this system has been realized on the microscopic, mesoscopic and macroscopic scales using field emission microscopy, photoemission electron microscopy and macroscopic rate measurements respectively. The various mechanisms of this and the other reactions reviewed in this article are also considered.

Kinetic oscillations and hysteresis phenomena in the NO+H2 reaction on Rh(111) and Rh(533): Experiments and mathematical modeling

Interesting kinetic phenomena, such as multiple steady states and kinetic oscillations recently found in the NO+H2 reaction over Rh(533) and Rh(111) single crystal surfaces in the 10−6 mbar pressure range have been studied by means of experiments and computer modeling. A mathematical model, consisting of five ordinary differential equations and taking into account the lateral interactions in the adlayer, has been developed for simulating the NO+H2/Rh(533) and NO+H2/Rh(111) reactions. The simulation results make it possible to explain in detail the underlying reasons for the experimentally observed complex dynamic behavior. In particular, the kinetic oscillations and their properties have been reproduced. It was found that accumulation of NHads species, which serves as an intermediate in the pathway of NH3 production, is an important step in the oscillatory mechanism. In addition, the same mathematical model is able to successfully reproduce the experimental data concerning temperature programmed desorptio...

The interaction of NO with stepped Rh surfaces

Abstract The dissociation of NO has been studied on a number of Rh single crystal surfaces consisting of (111) and (100) terraces by means of thermal desorption spectroscopy. The influence of both the surface structure and the presence of atomic nitrogen and atomic oxygen on the dissociation of NO have been investigated. The thermal decomposition of NO, which manifests itself by the formation of N 2 , is not surface structure dependent at saturation coverage. The presence of N ad and O ad inhibit the dissociation of NO. At an excess of O ad , NO is formed via recombination of N ad and O ad atoms. Atomic nitrogen is more stable on (100) terraces than on (111) terraces. No effect of steps on the N 2 and NO TDS spectra was observed.

Unusual behaviour of chemical waves in the NO + H2 reaction on Rh(111): long-range diffusion and formation of patches with reduced work function

Abstract Pattern formation in the NO + H 2 reaction on Rh(111) has been investigated in the 10 −6 mbar range using photoelectron emission microscopy (PEEM) as a spatially resolving method. Target patterns, spiral waves and irregular patterns are observed in a T -window of ∼20 K width at around 460 K, located in the transition range between the reactive and unreactive states of the surface. A new species characterized by a work function below that of the clean surface forms upon collision of two wave fronts. This new species is tentatively assigned to subsurface oxygen. The colliding wave fronts start to interact when they are still more than 100 μm apart, thus demonstrating the presence of a long-range diffusional coupling.

Non-linear behaviour in the NO-H2 reaction over Ir(110)

Abstract The dynamic behaviour of the NOH 2 reaction over Rh(111) was studied using mass spectrometry and Auger electron spectroscopy in the 10 −7 –10 −5 mbar total pressure range for NO:H 2 ratios between 1:5 and 1:30. On cooling the sample in a flow of the gases, an increase in the rate of formation of N 2 occurred between 500 and 450 K, which indicated the presence of an autocatalytic step in the reaction sequence. A heat-cool cycle of the sample actually produced a large hysteresis in the formation of all products: N 2 , NH 3 and H 2 O. This type of behaviour was also observed under similar conditions over the Rh(533) surface, as were regular macroscopic rate oscillations. During these oscillations, N 2 production was out of phase with that of NH 3 and H 2 O. The Rh(533) surface consists of four-atom wide (111) terraces separated by (100) steps. The steps seem to play an important role in the synchronisation of the oscillations, as only irregular macroscopic rate oscillations could be observed over Rh(111) under these conditions. Furthermore, the introduction of microscopic defects on the Rh(111) surface by means of Ar + ion sputtering led to an increase in the synchronisation of the oscillations, these defects are presumably playing a similar role to the steps on the Rh(533) surface.

Hysteresis and oscillations in the selectivity during the NO-H2 reaction over Rh(533)

The NO-H2 reaction over Rh(533) shows oscillatory behaviour at H2-rich mixtures in the 10−6 mbar pressure regime around 470 K. The selectivity changes periodically in time: the rate of N2 formation is out of phase with the NH3 and H2O formation rates. Accumulation of atomic N plays a central role in the oscillating behaviour. A comparison will be made with the NO-H2 reaction over Pt(100).

Oscillatory behaviour of the NOH2 reaction over Rh(533)

Abstract Waves moving over the surface of a Rh field emitter tip in an oscillatory way during the NO-H 2 reaction have been visualized earlier by field electron microscopy (FEM). An autocatalysis model has been proposed to describe the oscillatory behaviour of the reduction of NO by H 2 . To study the oscillatory behaviour and the effect of the surface structure in more detail a large Rh(100) surface and a large stepped Rh(533) surface, Rh[4(111) ∗ (100)], have been selected. The first results show that, in correspondence with the FEM experiments, rate oscillations could be observed over the Rh(533) surface in the 10 −6 mbar pressure regime around 470 K. No oscillatory behaviour was obtained on the Rh(100) surface under these conditions. The structure-sensitivity of the process is related to the large dependence of the Rh-N bond strength on the surface structure. Nitrogen desorbs at a much higher temperature from Rh(100) than from Rh(533).

Spatial distribution of N2 and NO desorbing from a Rh(533) surface

The spatial distribution of N2 and NO desorbing from Rh(533), Rh(S)-[4(111)×(100)], was measured along the [65 5] direction (along the steps) after adsorption of NO at 320 K. The N2 desorption at 450 K and 600 K showed a specific spatial distribution, that is, there was no peak either normal to the (111) terraces (θ=+14.4°) or normal to the (100) steps (θ=−40.3°), but a desorption maximum around θ=−15°. Integrated TPD spectra of N2 from the NO preadsorbed Rh(533) surface were very similar to those on Rh(111), but the spatial distribution of N2 from Rh(533) indicates desorption of N2 from the boundary between the (111) terraces and the vacant (100) steps. The spatial distribution of NO desorbing from the Rh(533) surface at 420 K obeyed cos θ, indicative of desorption normal to the crystal surface. Contrary to this, the spatial distribution of the desorption of NO molecules produced by the recombination reaction of adsorbed N and O desorbed at 550 K exhibits a peak intensity at θ=+15° (normal to the (111)...

Study of spatial pattern formation during the NO+H2/Rh(111) reaction by means of mathematical modeling

Recent investigations with the photoemission electron microscope showed the formation of spatial patterns (target patterns, spiral waves, disordered patterns) during the NO+H2 reaction over a Rh(111) single crystal surface. A five-variable mathematical model of the reaction-diffusion type has been developed to describe the experimental observations. A simplified version of this model was originally designed to explain the complex temporal behavior (e.g., oscillatory) found for the NO+H2 reaction on Rh(111). The simulation results successfully reproduce the main experimental findings and explain the underlying reasons for spatial pattern formation. In addition, the numerical studies predict a variety of self-organization phenomena which should be experimentally verified.