A range of alumina-supported platinum catalysts have been prepared and investigated for the selective reduction of nitrogen monoxide in the presence of a large excess of oxygen. Steady-state microreactor experiments have demonstrated that these catalysts are very active and selective for the reduction of nitrogen monoxide by propene at temperatures as low as 200°C. There does not appear to be a simple correlation between the activity for nitrogen monoxide reduction and the platinum surface area. Instead it is found that there is a very good inverse correlation between the maximum nitrogen monoxide reduction activity and the temperature. The most active catalysts for selective nitrogen monoxide reduction are those that generate activity at the lowest temperature. The technique of temporal analysis of products (TAP) has been used to obtain detailed mechanistic data about the selective nitrogen monoxide reduction reaction on an alumina-supported platinum catalyst. Using carbon monoxide, hydrogen or propene as reductant it has been demonstrated that the predominant mechanism for selective nitrogen monoxide reduction involves the decomposition of nitrogen monoxide on reduced platinum metal sites, followed by the regeneration of the active platinum sites by the reductant. In the decomposition step it has been shown that oxygen from nitrogen monoxide is retained on the surface of the platinum and blocks the surface for further adsorption/reaction of nitrogen monoxide; it has been observed that oxidised platinum catalysts are not active for the nitrogen monoxide reduction reaction. Under typical operating conditions, propene is a far more efficient reductant than either carbon monoxide or hydrogen. The greater efficiency of propene as a reductant is explained on the basis of the additional reducing power of the propene molecule, which can react with as many as nine adsorbed oxygen atoms, ensuring that 'patches' of reduced platinum are available for nitrogen monoxide adsorption/reaction. A small additional activity of reduced platinum in the presence of propene, which is not observed when carbon monoxide or hydrogen is used as reductant, has been explained on the basis of a second mechanism involving the carbon-assisted decomposition of nitrogen monoxide at sites on the reduced platinum adjacent to adsorbed carbon-containing moieties, believed to be fragments from adsorbed propene molecules. A model for the selective reduction of nitrogen monoxide on alumina-supported platinum catalysts is presented which is capable of explaining all the results obtained in this work and in the published literature on this subject.

Mechanism of the selective reduction of nitrogen monoxide on platinum-based catalysts in the presence of excess oxygen

Oscillatory reactions on single crystal surfaces

Carbon on transition metal surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NHx (x = 0–2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t−1), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH2 adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH2 fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade−1, which is explained by a rate-determining dimerization of NH2 fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH2 adsorbate.

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces.

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 07 eV) can be related to different geometric and electronic factors associated with this particular intermediate Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111)

DFT characterization of adsorbed NH(x) species on Pt(100) and Pt(111) surfaces.

Oscillations in the carbon monoxide oxidation on platinum surfaces observed by field electron microscopy

The adsorption states of hydrogen on Pt(100)-(hex) and (1×1) surfaces have been studied using TDS and HREELS in the temperature range of 170-330 K. Hydrogen atoms are shown to adsorb on the (1×1) surface in bridge and 4-fold hollow sites. On the (hex) surface hydrogen adsorption induces a partial lifting of the (hex) reconstruction with the consequent appearance of the bridge and 4-fold hollow states on (1×1) patches. Additional adsorption states appear due to the population of structural defects and patches of the (hex) phase.

V.V. Gorodetskii

Papers

Oscillations in the carbon monoxide oxidation on platinum surfaces observed by field electron microscopy

HREELS characterization of hydrogen adsorption states on the Pt(100)-(hex) and (1×1) surfaces

Hydrogenation of isolated atoms and small clusters of carbon on Pt(111) surface: HREELS/TDS studies

NH2 formation in the reaction of Hads with NO on the Pt(100)-(1 × 1) surface

Formation of mixed (NOads+COads)/(1×1) islands on the Pt(100)-(hex) surface