Edward Jobson

Low temperature catalytic activity of cobalt oxide and ceria promoted Pt and Pd: -Influence of pretreatment and gas composition

The influence of pretreatment, gas composition and metal (Ce or Co) oxide promotion on the low-temperature CO and C3H6 oxidation activity over alumina-supported Pt and Pd has been studied. The monolith catalysts have either been preoxidised in O2/N2 Or prereduced in H2/N2 prior to evaluation with respect to light-off performance, using either net oxidising or net reducing CO/C3H6/O2/N2 gas mixtures. Compared with unpromoted Pt, promotion with preoxidised ceria or cobalt oxide enhances the low-temperature activity significantly and lowers the light-off temperatures by about 60-70 degrees C for both CO and C3H6. Prereduction of a cobalt-oxide catalyst (without precious metals) gives a dramatically improved performance compared with a preoxidised catalyst in terms of light-off and overall conversion. Prereduction of metal oxide promoted Pt and Pd can shift the light-off temperatures for CO and C3H6 by up to 100 degrees C toward lower temperatures compared with preoxidised samples. When using gas mixtures containing both CO and C3H6, the conversion of CO always starts at lower temperatures than the conversion of C3H6 The catalysts have been characterised by temperature-programmed desorption (TPD) of carbon monoxide, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements (BET). The reduced cobalt containing samples adsorb large amounts of CO. The high activity over the catalysts containing prereduced cobalt oxide is suggested to be due to the presence of reduced cobalt-oxide sites on the surface of those samples.

SO2 promoted oxidation of ethyl acetate, ethanol and propane

The effect of SO2 addition on the oxidation of ethyl acetate, ethanol, propane and propene, over Pt/gamma -Al2O3 and Pt/SiO2 has been investigated. The reactants (300-800 vol, ppm) were mixed with air and led through the catalyst bed. The conversions below and above light-off were recorded both in the absence and in the presence of 1-100 vol. ppm SO2. For the alumina-supported catalyst, the conversion of ethyl acetate, ethanol and propane was promoted by the addition of SO2, while the conversion of propene was inhibited. The effect of SO2 was reversible, i.e. the conversion of the reactants returned towards the initial values when SO2 was turned off. However, this recovery was quite slow. The oxidation of propane was inhibited by water, both in absence and presence of SO2. For the silica-supported catalyst no significant effect of SO2 could be observed on the conversion of ethyl acetate, ethanol or propane, whereas the conversion of propene was inhibited by the presence of SO2. In situ FTIR measurements revealed the presence of surface sulphates on the Pt/gamma -Al2O3 catalyst with and after SO2 addition. It is proposed that these sulphate groups enhance the oxidation of propane, ethyl acetate and ethanol by creating additional reaction pathways to Pt on the surface of the Pt/gamma -Al2O3 catalyst.

The effect of a changing lean gas composition on the ability of NO2 formation and NOx reduction over supported pt catalysts

Three model catalysts (Pt/Al2O3, Pt/TiO2, Pt/V2O5/TiO2) were examined in regard to their NO2 formation ability under a changing lean gas composition. The results show that the NO to NO2 oxidation function as well as the NOx reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al2O3 and Pt/TiO2, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V2O5/TiO2.

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NOx in lean exhausts is by using NOx trap catalysts. This paper addresses storage of NOx on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NOx when NO2 was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO2 and forming NO and CO2. Propene inhibited the reaction between NO2 and CO and therefore gave rise to larger NOx storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NOx at low temperatures.

Response of metal-oxide-silicon carbide sensors to simulated and real exhaust gases

Field effect devices based on catalytic metal-oxide-silicon carbide (MOSiC) structures can be used as high temperature gas sensors. The devices are sensitive to hydrocarbons and hydrogen and can be ...

Reduction of NOx stored at low temperatures on a NOx adsorbing catalyst

Isothermal storage and reduction of NO2 with CO, C3H6 and H2 as reducing agents on a lean NOx adsorber was investigated by temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies. The reduction of NOx was clearly favoured with H2 as reducing agent. Carbon monoxide and C3H6 showed fairly low reduction of NOx. The NOx reduction at low temperatures with H2 as reducing agent was found to be effective, clearly much more effective than for CO.

Molecule sieving catalysts for NO reduction with hydrocarbons in exhaust of lean burn gasoline and diesel engines

Abstract NO oxidation catalysts with small pores and low hydrocarbon oxidation activity owing to molecular sieving were prepared by dispersing platinum in the pores of ferrierite and chabazite zeolites. These small pore platinum zeolite crystals were physically mixed with large pore silver mordenite zeolite crystals and evaluated as HC-SCR catalysts for selective catalytic reduction of NO in a synthetic gas mixture doped with octane or isooctane, mimicking exhaust from lean burning engines with unburned hydrocarbons. A synergetic effect on N2 formation and hydrocarbon efficiency was obtained in these physical zeolite mixtures. In the pores of the small pore zeolite where the diffusion of NO, O2 and NO2 molecules with small kinetic diameters was rapid, NO was effectively oxidized by the platinum into NO2, while the small window apertures suppressed the diffusion and reaction of the hydrocarbon. The silver plated large pore zeolite catalyzed the reaction between the NO2 formed in the small pore zeolite and the hydrocarbon. The importance of molecular sieving was demonstrated in experiments with permutation of pore sizes and catalytic functions.

Spatially Resolved Effects of Deactivation on Field-Aged Automotive Catalysts

Four samples from each of two field-aged catalysts subjected to different field test conditions were investigated. The light-off and conversion performance of each sample was measured in a synthetic exhaust flow reactor system. Time-resolved laser IR spectroscopy was used to investigate the catalyst behaviour under transient conditions. Significant differences in light-off temperatures and transient conversion performance between the samples was observed. The samples taken from the inlet side of the monolith were more deactivated than the corresponding ones from the outlet. However, samples taken from peripheral positions always showed better performance than samples originating from the centre. For the covering abstract of the conference see IRRD 852385.

Improved light-off performance by using transient gas compositions in the catalytic treatment of car exhausts

The influence of transient changes in the exhaust gas composition on the low-temperature activity of a Pt/CoOx/Al2O3, catalyst and a commercial three-way catalyst has been studied. Periodic O2 and CO pulses caused well-controlled perturbations in the composition of a synthetic car exhaust which was found to affect the overall performance of both catalysts. The presence of transients shifted the conversion starts of CO and propene as well as their light-off temperatures towards lower temperatures. The improved performance is suggested to origin from the combined effect of rich (CO) and lean (O2) pulses that cause perturbations in the adsorbate composition on the active sites at temperatures below light-off.

Induced low temperature catalytic ignition by transient changes in the gas composition

The effect of gas composition changes on the low temperature activity for supported platinum model catalysts has been studied. By introducing well-controlled periodic O2 pulses to a simple diluted gas mixture of CO and O2, a substantial improvement of the low temperature oxidation activity was observed. The reason for low activity on noble metals at low temperatures is often attributed to self-poisoning by CO. The improved catalytic performance observed is proposed to origin from the transients causing a surface reactant composition that is favourable for the reaction rate, i.e. lower degree of self-poisoning. This was also confirmed by in situ Fourier transform infrared spectroscopy in combination with mass spectrometry measurements, which gave evidence for the existence of a strong interplay between the gas phase concentration and the adsorbate composition for these catalysts.