Hideki Hirano

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.

Phonon dispersion in monolayer graphite formed on Ni(111) and Ni(001)

Abstract The phonon dispersion relations of monolayer graphite on Ni(111) and on Ni(001) were measured by using electron energy loss spectroscopy. Both samples gave almost the same results. The data were analysed with a force constant model in a slab geometry, and it was revealed that the vertical bond bending force constant and the bond twisting force constant were greatly softened. These force-constant changes are comparable with the previously reported case of monolayer graphite on (111) surfaces of transition-metal carbides. However, the interaction between the Ni substrate and the graphite overlayer is not so strong as that on the carbide (111) surface. The similar phonon structures between graphitic layers on Ni(111) and on Ni(001) suggest that both substrates have similar charge transfer capability.

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.

A reason for the structure-insensitive catalytic activity of Ni(100) and Ni(111) surfaces for the methanation reaction of CO

Abstract The structure-insensitive methanation reaction, CO + 3H 2 → CH 4 + H 2 O, on Ni(111) and Ni(100) surfaces (1, 2), is rationalized on the basis of the structure of carbidic carbon intermediates. Accumulation of carbidic carbon intermediates on the Ni(100) surface results in a (2 × 2)p4g overlayer, and its hydrogenation proceeds at a rate almost equal to that of a steady-state methanation reaction. In contrast to the carbide overlayer on the Ni(100) surface, the LEED pattern of the carbide overlayer on the Ni(111) surface is too complex to be solved. A single-domain carbide on a Ni(111) surface accidentally obtained by the segregation of carbon allowed us to deduce the structure of the carbide overlayer on the Ni(111) surface. It was shown that the carbide overlayer on Ni(111) has exactly the same arrangement of carbon atoms as that of the (2 × 2)p4g structure on the Ni(100) surface. In addition, the carbide overlayer undergoes decomposition at 685 K on Ni(100), Ni(110), and Ni(111) surfaces. Therefore, we conclude that the accumulation of carbidic intermediates creates an identical surface carbide on Ni(100) and Ni(111) surfaces. This may be a reason for the structure-insensitive catalysis, because the methanation may be catalyzed by this surface carbide on Ni(100) and Ni(111) surfaces.

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.

Off-normal emission of N2 produced by desorption mediated reaction of NO on Pd(110) surface

Abstract NO molecules adsorbed on a Pd(110) surface undergo desorption with maxima at approximately 495 K at low coverage but two desorption peaks at approximately 370 K and 495 K at high coverage. Desorption of NO at 495 K mediates the emission of N2 and N2O. However, desorption of NO at 370 K produces neither N2 nor N2O. NO molecules desorbed at 495 K obey cos θ towards the 〈001〉 direction, whereas the N2 molecules emitted at 495 K have a very sharp spatial distribution 37° off-normal to the surface in the 〈001〉 direction. The emission of N2 has a full width at half maximum (FWHM) of 19° which can be represented by cos46(θ − 37). The spatial distributions of N2 and NO in the 〈110〉 direction, however, obey cos4.2θ and cos1.6θ, respectively.

The Reduction of Nitric Oxide by Hydrogen Over Pt, Rh and Pt-Rh Single Crystal Surfaces

Abstract The reduction of NO by hydrogen has been studied over Pt 0.25 - Rh 0.75 (100), (111), (410), Rh(100) and Pt(100) single crystal surfaces in the 10 mbar range. The surfaces were analysed using AES and LEED. Both the activity expressed as conversion after a constant reaction time and selectivity depend strongly on the surface structure and composition. The activity for the (100) surfaces decreases in the order Pt(100) ≥ Pt-Rh(100)〉Rh(100). The activity of pure Rh is drastically enhanced by alloying with 25% Pt. The selectivity towards N 2 for the (100) surfaces decreases in the order Rh(100)〉Pt-Rh(100)〉Pt(100) at a temperature of 575K and Rh(100)〉Pt(100)〉Pt-Rh(100) at 520K. The activity for the alloy surfaces decreases in the order Pt-Rh(100)〉Pt-Rh(410)〉Pt-Rh(111), and the selectivity towards N 2 formation decreases in the order Pt-Rh(410)〉Pt-Rh(100)〉Pt-Rh(111), at 520K and 575K. The differences in selectivity and activity can be understood on the basis of the relative concentrations of N, NO and H on the various surfaces.

Formation of a c(2 × 2) overlayer of nitrogen on Pd(100) by NO + CO reaction or NO + H2 reaction

Abstract A nitrogen overlayer (N(a)) was synthesized on Pd(100) by catalytic reduction of NO by CO or H 2 in the total pressure range of 10 −7 −10 1 Torr at crystal temperatures higher than 500 K. The process of the formation of N(a) was monitored by AES, LEED, HREELS and TDS. N(a) formed a c (2 × 2) superlattice, and desorbed at 600–700 K. N(a) was hydrogenated by gas-phase H 2 . HREELS revealed the reversible formation of NH x (a) species on the surface.