Temperature-programmed desorption studies of methanol and formic acid decomposition on copper oxide surfaces

We have obtained temperature-programmed desorption data for methanol and formic acid adsorption on bulk powders of CuO and Cu₂O. Methanol adsorption on CuO at 300 K results in CO₂, H₂ and H₂O desorption at 550 K indicating formate decomposition; this decomposition temperature is very close to that obtained from the decomposition of formate produced by formic acid adsorption. No significant desorption was observed from vacuum-annealed Cu₂O following exposure to methanol due to the formation of a copper metal film at the surface. However, formic acid was adsorbed on this surface decomposing at significantly lower temperature, 485 K, than on CuO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Iron antimony oxide selective oxidation catalysts – an inelastic neutron scattering study

The inelastic neutron scattering spectra of allyl iodide (3-iodopropene, CH₂=CHCH₂I) and allylpalladium chloride, and allyl iodide dosed onto activated iron(III) oxide and iron antimonate catalysts at room temperature have been determined to characterise the adsorbed allyl species. The spectra are energy loss vibrational spectra in the range 16–4000 cm⁻¹. Allyl iodide is not decomposed on the surface and interacts through the localised C = C bond, more strongly with iron antimonate than with iron(III) oxide. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Effect of nitric oxide on the photocatalytic oxidation of small hydrocarbons over titania

The catalytic UV photo-oxidation of NO in the absence and presence of ethane, ethene, propane, propene, and n-butane over TiO₂ in the presence of excess oxygen was studied in the temperature range 21–150 °C. It was confirmed in our system that in the absence of hydrocarbon NO was photocatalytically oxidised by oxygen to NO₂ over TiO₂ and was strongly absorbed. Both NO and hydrocarbon could be simultaneously photo-converted with the conversion varying considerably with both NO and hydrocarbon concentration and the nature of the hydrocarbon. In some instances the presence of NO in the feed gas enhanced hydrocarbon oxidation via reactions involving NO₂ that is a powerful oxidant. The extent of this effect depended on the relative strengths of adsorption on TiO₂ of the reactants and products. To reduce surface coverage of hydrocarbon most reactions were run at 150 °C, and it was shown that at this temperature NOx adsorbed on titania could be reduced by photogenerated hydrocarbon surface species to N₂O and N₂ under these conditions. The formation of N₂ was confirmed using ¹⁵NO with helium as carrier gas. By contrast, at room temperature in the presence of propene NO was converted to NO₂.     

Electrochemical promotion in a monolith electrochemical plate reactor applied to simulated and real automotive pollution control

The novel monolithic-type electrochemical promoted reactor (MEPR), filled with thick Rh and Pt coated films on thin yttria-stabilized zirconia (YSZ) plates, has been tested in simulated and real automotive exhaust gas. Ethylene oxidation and NO reduction by C₂H₄ in presence of oxygen were investigated in laboratory scale while the automotive bench testing was performed in the exhaust of a diesel engine. In all cases electropromotion was achieved, an advance which in addition to their compact and simple design, makes electropromoted units quite promising for automotive exhaust pollution control.     

Palladium based sorbents for high temperature arsine removal from fuel gas

The use of Pd-based sorbents for high temperature removal of AsH₃ from gasified coal was investigated using a simulated gas feed. A sorbent consisting of 5 wt% Pd on alumina beads has been tested for AsH₃ removal from synthetic fuel gas (CO, CO₂, H₂) at 204 and 288 °C. Arsenic uptake was found to be essentially linear with exposure time and considerably higher than that for unpromoted alumina beads. Arsenic loadings in excess of 7 wt% were achieved, though the sorbent is unlikely to be saturated at this loading. As the arsenic loading on the sorbent increased a PdAs₂ phase was identified in the XRD pattern. The adsorption of arsine on palladium has potential implications for catalysts, electrodes, and membranes for the separation of hydrogen from fuel gas.     

Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO_x trap catalyst using different reductants. H₂ was found to be more effective at regenerating the NO_x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO_x storage catalyst is sulphated in SO₂ under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H₂/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H₂ reduced sample was exposed to a rich condition in a gas mixture containing CO₂ at 650 °C, the storage activity can be recovered. Under these rich conditions the S²⁻ species becomes less stable than the CO₃²⁻, which is active for storing NO_x. Samples which were lean aged in air containing 60 ppm SO₂ at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO₂ is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO_x storage.     

Insights into surface reactivity: formic acid oxidation on Cu(110) studied using STM and a molecular beam reactor

Using a combination of STM and molecular beam reactor data we summarise some important features of a model reaction (formic acid oxidation on Cu(110)) which is of general significance to surface reactivity and to catalysis. Three such features are highlighted here. The first concerns the role of weakly held species (possibly physisorbed) in surface reactions. These species, although of very short lifetime on the surface, can, nevertheless, diffuse over long distances to “find” a sparse distribution of active sites. Thus a very low coverage of oxygen on the surface of Cu(110) increases the sticking probability of all the formic acid molecules which strike the surface to high value (0.82), even though the clean surface is relatively unreactive. The important concept here is the “diffusion circle” or “collection zone” which represents the area of surface visited by the molecule in its short sojourn in the weakly held state. The second theme concerns the concept of the “flexible surface”. We show that the involvement of surface atoms in reactions directs the structure and reactivity for a particular reaction. For formic acid oxidation the liberation of Cu atoms during the removal of oxygen as water leads to gross restructuring of the surface and can lead to “compression” of one reactant (the oxygen in this case) into a lower area, higher local coverage, unreactive state (the c(6×2) oxygen structure). Thirdly, and finally, it is proposed that, for many surface reactions, the surface acts in an analogous way to a solvent, supporting a “dissolved” (highly mobile and fluxional) phase of intermediates at low coverage, which crystallise out above a critical coverage (the 2D “solubility limit”). This revised version was published online in July 2006 with corrections to the Cover Date.     

Metal sorbents for high temperature mercury capture from fuel gas

We have determined the Hg removal capacities of Pt and Pd supported on alumina at a range of different metal loadings from 2 to 9 wt% using Hg vapour in a simulated fuel gas feed. In the temperature range studied (204-388 °C) Pd proved far superior to Pt for Hg removal. The Hg removal capacity for both Pt and Pd increased with metal loading, though decreased with sorbent temperature. A shift in the 2θ position of the Pd XRD diffraction peak from 82.1 to 79.5 after Hg adsorption at 204 °C was consistent with the formation of a solid solution of Hg in Pd.     

Electronic,chemisorption and catalytic properties of Pd overlayers on Ru(100) probed by CO and O2 adsorption and reaction

We have employed UHV surface sensitive techniques together with CO and oxygen adsorption and reaction to link the chemical, electronic and structural properties of ultra-thin Pd films vapour-deposited on Ru(100). At low Pd coverage the properties of the Pd overlayer are considerably modified relative to the bulk-like properties exhibited by thick Pd films. A Pd coverage of 6 ML seems to mark a critical transition to bulk-like Pd behaviour. The strongest modifications in the Pd overlayer occur for coverages up to 2 ML and are exemplified by a reduction in the CO desorption temperature, a low DOS at Fermi level,E _f, and a lower activity toward CO oxidation.