P. D. Cobden

The NO-H2 reaction over Pt(100) Oscillatory behaviour of activity and selectivity

The NO-H2 reaction has been studied over a Pt(100) single crystal surface as a function of temperature and partial pressures of the reactants. The activity as well as the selectivity, shows oscillatory behaviour under isothermal conditions from 420 K to 520 K. The oscillations observed for the formation rates of N2 and NH3 are out of phase with those found for the formation rate of N2O. These observations are in line with recently proposed mechanisms for the formation of N2, NH3 and N2O.

Mechanisms of the various nitric oxide reduction reactions on a platinum-rhodium (100) alloy single crystal surface

The reduction of nitric oxide with hydrogen was studied over a Pt0.25-Rh0.75(100) alloy surface used as a model catalyst for the automotive three-way catalyst. This paper emphasizes the mechanisms of the different reactions leading to the products dinitrogen, ammonia and nitrous oxide. For this purpose the reaction was studied under various experimental conditions including reactivity measurements both in the 10−7 mbar range under steady-state conditions and in the 10 mbar range with varying NO/H2 ratio. In addition, the thermal decomposition of NO and the reactions of NO + NH3 were investigated. 15NO and 15NH3 were used in order to gather additional information concerning the mechanisms of the formation reactions of the various N-containing products. The surface was characterized by using low-energy electron diffraction. Auger electron spectroscopy and thermal desorption spectroscopy. The main conclusions emerging from these studies are: (a) N2 can be formed by combination of 2 N adatoms in the whole temperature range used (350–1300 K) provided that sufficient N adatoms are available. (b) Below 600 K the main contribution to N2 formation is via NOads + Nads → N2+ Oads. At higher temperatures the dominant mechanism is 2Nads → N2. (c) N2O and NH3 are formed via Nads + NOads → N2O, and Nads + 3Hads → NH3 the contributions of which respectively decre increase with increasing temperature, (d) The selectivities to N2, NH3 and N2O are determined by the relative concentrations of NOads, Nads and Hads which vary with the experimental conditions such as the temperature.

Oscillatory behaviour of the reduction of nitric oxide by ammonia over the Pt(100) single-crystal surface: the role of oxygen, comparison with the NOH2 reaction and a general reaction mechanism for NO reduction by NH3 over Pt

Abstract The reaction between NO and NH 3 over a Pt(100) single-crystal surface was studied at low pressures (1.5 × 10 −6 −6 × 10 −5 Torr), at temperatures between 420 and 485 K, and with NO/NH 3 ratios ranging from 0.1 to 4.3. It is shown that this reaction shows oscillatory behaviour. Besides single-peaked rate oscillations also various multiple-peaked and aperiodic oscillations were observed. The influence of temperature, NO/NH 3 ratio, pressure and the addition of oxygen on the oscillations were assessed. In order to elucidate more details concerning the mechanism of the reduction reaction, labelled compounds ( 15 NH 3 and 18 O 2 ) were used. Based on the results obtained, a model for the reaction mechanism is proposed which can explain all experimental observations. The model involves the formation of NO islands on the surface. The reaction between NO and ammonia proceeds rapidly at the boundaries between the NO islands, with parts of the crystal where vacancies and NH 3 fragments are present. It was found that oxygen plays a minor role at the temperatures and pressures used, but that it is not just a spectator molecule. Addition of oxygen at up to 83% of the reaction mixture does not stop the oscillations observed. A short comparison between the NOH 2 and the NONH 3 reaction is given. Furthermore, it cannot be ruled out that the reversible (5 × 20) ↔ (1 × 1) surface reconstruction, which is known to take place upon NO adsorption on Pt(100), plays a role in the appearance of oscillations, but, as will be shown, this is not necessarily the case. A model for the oscillations is proposed in which surface vacant sites are the key factors determining the oscillating behaviour of the NO reduction by NH 3 or H 2 .

Oscillatory reduction of nitric oxide with hydrogen over Pt(100)

The reaction between NO and H2 was studied over the Pt(100) single crystal surface. Under suitably chosen parameters (partial pressures, flow rate, etc.) and under isothermal conditions, the reaction exhibited sustained temporal oscillations in reaction rate as seen by the variation in product and reactant partial pressure measured by a quadrupole mass spectrometer. The oscillations observed for the formation rates of N2 and NH3 are out of phase with those found for the formation rates of N2O. Systematic small changes in the H2 partial pressure cause the system to move from a single period oscillation via a sequence of period doublings to oscillations which were aperiodic or chaotic in nature. Analysis of this data through power spectra and correlation integrals identified it as being chaotic, and not noisy in nature, and these calculations also revealed that the minimum number of variables determining the physical properties of the system to be 3. On the basis of this and careful analysis of the differen...

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...

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).

Atomic nitrogen on steps: A fast x-ray photoelectron spectroscopy study of the NO uptake on Rh(533), Rh(311), and Rh(111)

The interaction of NO with a flat and two stepped Rh(111) surfaces was studied at different temperatures by monitoring in situ the evolution of the O 1s and N 1s spectra during NO exposure using fast high resolution x-ray photoelectron spectroscopy. The O 1s and N 1s intensities and binding energies were used to fingerprint the types of adsorbed species and to monitor the changes in their coverage and adsorption sites. From the O 1s intensity during uptake on Rh(533) it was determined that the NO initial sticking coefficient is temperature independent between 330 and 490 K. The N 1s spectra revealed the consecutive appearance of two atomic nitrogen species. By comparing the N 1s spectra on Rh(533) with those measured on Rh(111) and Rh(311) these species were attributed to adsorption on terrace sites, NT, and step sites, NS. NT is the only species formed in the initial stage of adsorption, whereas the NS species appears later on. This finding indicates that the NO dissociation occurs on terrace adsorption ...

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.