J. Siera

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...

A comparative study of the behaviour of single-crystal surfaces and supported catalysts in NO reduction and CO oxidation over Pt-Rh alloys

The use of Pt–Rh-based three-way catalysts for automotive exhaust-gas control stimulated us to study various Pt–Rh alloy surfaces: well defined single-crystal surfaces and SiO2-supported catalysts. For the sake of comparison data were also obtained for pure Pt and Rh single-crystal surfaces and catalysts. The surface composition and chemical properties have been studied using AES, FEM, TDS and XPS. The surface composition was studied under various experimental conditions, both in vacuum and in the presence of the relevant gases, NO, O2, CO and H2. We discuss the effects of the surface structure and the surface composition on the chemical properties of the Pt–Rh alloys. The surface composition varies easily with changing experimental conditions (temperature and gas-phase composition). Clean Pt–Rh alloy surfaces are enriched with Pt after high-temperature annealing (T 1000 K). However, the surface composition of Pt-rich Pt–Rh alloys is almost bulk-like following low-temperature equilibration (T≈ 800 K). Adsorbates can easily induce segregation of Rh or Pt to the surface, and the chemical properties of the surfaces are changed accordingly. In particular, the NO dissociation, which is often mentioned as the first step in the NO reduction with CO or hydrogen, is extremely sensitive to both the structure and the composition of the surface. For the pure metals the influence of the surface structure is different for Pt and Rh. The NO reduction and CO oxidation reactions were studied at low pressures on the single-crystal surfaces and at atmospheric pressure on the supported catalysts. We show that the chemical properties of the supported catalysts can be understood on the basis of the single-crystal results.

The composition of the (111) and (100) surfaces of a Pt0.25-Rh0.75 single crystal

Abstract The surface compositions of Pt-Rh(100) and Pt-Rh(111) single-crystal surfaces were studied as a function of annealing temperature. It was found that both surfaces were enriched in platinum compared with the bulk platinum concentration. Relatively long annealing periods are required to reach the equilibrium concentration at low temperatures (

Dynamic behavior of a Pt0.25Rh0.75(100) single crystal surface during NO + H2 reaction

Abstract The catalytic reduction of NO by H 2 on a Pt 0.25 Rh 0.75 (100) alloy single crystal was investigated by monitoring the surface species while the reaction was proceeding. Adsorption of NO at 500 K formed an O-covered surface with p(3 × 1) structure. Enrichment of Rh on the surface induced by adsorption of O was observed, and a Rh-rich alloy surface forms the p(3 × 1)-O faster. The nature of the p(3 × 1) overlayer is discussed. The NO + H 2 reaction formed a c(2 × 2) overlayer of N(a). The hydrogenated adsorbate NH x (a) was observed under the presence of gas-phase H 2 . The hydrogenation reaction of N(a) to NH x (a) was reversible, and faster than complete hydrogenation to NH 3 (g).

Mechanism of the ammonia formation from NO-H2. A model study with Pt-Rh alloy single crystal surfaces

The reduction of NO by H2 was studied over three different Pt-Rh single crystal surfaces, i.e. Pt-Rh(111), (410) and (100). The adsorption and dissociation of NO was studied by HREELS, LEED, XPS, AES and TDS. It was found that the dissociation of NO and its reaction with H2 is very surface structure sensitive. The selectivity towards nitrogen and the dissociation activity increases in the same order, i.e. Pt-Rh(111) < (100) < 410). Nitrogen atoms were easily hydrogenated at 400 K in hydrogen to NHx (x = 1 or 2) on the surface. A model is proposed in which the selectivity of the NO-H2 reaction over Pt-Rh surfaces is determined by the relative amounts of hydrogen, NO and nitrogen adatoms on the surface.

Dynamical behaviour of PtRh(100) alloy surface during dissociative adsorption of NO and reaction of NO with H2

Abstract When a clean Pt-Rh(100) alloy surface was exposed to NO at T > 440 K, the LEED pattern changed sequentially as p(1 × 1) → c(2 × 2) → c(2 × 2) + p(3 × 1) → p(3 × 1), where the c(2 × 2) pattern appeared immediately after the exposure to NO. In contrast to this, the appearance time for the p(3 × 1) depends strongly on the initial Rh concentration on the surface adjusted by annealing. When the p(3 × 1) surface was exposed to H2 by mixing H2 into NO gas, the AES intensity of O(a) decreased and of N(a) increased markedly and the LEED pattern changed from p(3 × 1) to c(2 × 2). These results suggest that N(a) has equal affinity to Pt and Rh atoms so that the N(a) does not distinguish the Pt and Rh sites on the alloy surface. On the other hand, O(a) makes a stronger bond with Rh atoms so that Rh atom segregation onto the surface is induced. By reacting randomly distributed Rh atoms on the Pt-Rh(100) surface with oxygen, a surface compound in a p(3 × 1) arrangement is built on the surface.

Dynamical behaviour of PtRh(100) alloy surface upon NO dissociation and NO + H2 reaction

When a clean Pt-Rh(100) alloy surface was exposed to NO at T > 440K, the LEED pattern changed sequentially as p(1 × 1) → c(2 × 2) → c(2 × 2) + p(3 × 1) → p(3 × 1), where the c(2 × 2) pattern appeared immediately after the exposure to NO. Contrary to this, the formation of the p(3 × 1) needs some interval time which depends strongly on the initial Rh concentration of the alloy surface as adjusted by annealing in vacuum. When the p(3 × 1) surface was exposed to H 2 by adding H 2 to the NO gas, the AES intensity of O(a) decreased and while that of N(a) increased markedly. At the same time, the LEED pattern changed from p(3 × 1) to c(2 × 2). These results suggest that N(a) has equal affinity to the Pt and Rh sites on the alloy surface so that it is difficult for N(a) to distinguish the Pt and Rh atoms. On the other hand, O(a) prefers to make a stronger bond with Rh sites and prolonged exposure induces Rh surface segregation. The reaction of Rh atoms with O(a) on the Pt-Rh(100) surface yields a surface compound of p(3 × 1) structure.