Anders Törncrona

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.

Investigations of NOx storage catalysts

NOx storage catalysts are used to reduce nitrogen oxides from lean-burn vehicles. The nitrogen oxides are stored in the catalyst during lean conditions and subsequently released and reduced during short periods of rich conditions. In the present study, we systematically investigate the sequence of elementary steps in the NOx reduction cycle, and the extent to which these steps influence the maximum NOx, reduction potential of the catalyst. As a model system, we use barium oxide as the NOx, storing compound in a Pt/Rh/Al2O3 system. Kinetics of NO oxidation, NO and NO2 adsorption, NO and NO2 release and reduction are studied under controlled conditions with systematic variations of temperature, gas composition, and storing/release times. The transient experiments comprise a storing phase using a lean NO/C3H6/O2/N2 gas mixture, and a regenerating phase where the O2 now is turned off. Experimentally, a significant amount of NOx is found to be stored in the Ba-containing material. A maximum in NOx storage is observed around 380 degrees C. For most of the experiments, there are clear NO and NO2 desorption peaks upon switching from the storing to the regeneration phase. TPD studies of NO and NO2 reveal a significant difference between prereduced and pre-oxidised samples where the former produce predominantly N2 and N2O at around 200 degrees C while NO and O2 desorb from the latter around 500 degrees C. In situ FTIR spectra show nitrate peaks in the region 1300-1400 cm(-1) when NOx is stored under lean conditions.

Sol-gel coating of alumina fibre bundles

Abstract A procedure for coating alumina fibre bundles with fibrillar boehmite particles is described. Fibrillar boehmite particles, having a specific surface area of 150 m2/g, were adsorbed onto the surface of the alumina fibres. The surface of the alumina fibres was first charge reversed by adsorption of a charge reversing agent. Polyvinylsulphate (PVS) proved to be an excellent charge reversing agent, whereas citric acid and Tiron (3,5-pyrocatecholdisulphonic acid disodium salt) were unable to reverse the surface charge of the alumina fibres. By using polyvinyl-sulphate (PVS), it was possible to adsorb an arbitrary number of layers of fibrillar boehmite particles. The coating of the alumina fibre bundles resulted in an increase in specific surface area from 0.09 m2/g for the uncoated fibres, to 0.7 m2/g for alumina fibres coated with four layers of fibrillar boehmite particles. The adsorbed boehmite particles were fixed onto the alumina fibre surface by calcination at 550 °C for 120 min, during which the boehmite was transformed into γ-Al2O3. The coated alumina fibre bundles were characterized by krypton adsorption and scanning electron microscopy (SEM).

Preparation of ceria-coated composite silica particles

A method for preparing particles with highly dispersed ceria by a stepwise procedure is described. The surface of silica particles was modified by aluminate and coated with an interlayer of titanina. CeOSO4 was deposited on the titania coated silica particles. The sulfur in the dispersed ceria was removed by calcining the particles in air at 800°C for 30 min. A highly dispersed ceria was obtained when using titania-coated silica particles with diameters of 80, 240 and 500 nm. The prepared products were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), UV spectroscopy and electrophoresis.

Preparation of a novel fibrous catalyst support

A method for preparing wash-coated silica fibres is described. Colloidal silica particles (22 nm) were deposited onto the surface of 2 µm amorphous silica fibres, surface-modified by deposition of a thin film of amorphous silica with a high density of silanol groups and with the charge reversed by adsorption of a water-soluble cationic polymer. This deposition resulted in an increase in specific surface area from 0.9 m2 g–1 for the fibre to 40 m2 g–1 for the fibre with deposited silica particles. Hydrothermal treatment at 750 °C in 100% steam for 24 h fixed the silica particles onto the fibre surface and made them coalesce into a strong porous structure. During this process, the specific surface area decreased from 40 to 30 m2 g–1. Particles of fibrillar boehmite were deposited onto the surface of silica fibres. The boehmite particles were fixed by thermal treatment at 550 °C, during which the boehmite was transformed into γ-Al2O3. The specific surface area of the alumina wash-coated silica fibres was 28 m2 g–1. Wash-coated fibres were characterized by nitrogen adsorption-desorption analysis, scanning and transmission electron microscopy (SEM and TEM).

Light-off performance over cobalt oxide- and ceria-promoted platinum and palladium catalysts

Monolith catalysts containing Co, Ce, Pt and Pd supported on alumina were prepared and tested with respect to low-temperature activity for oxidation of CO and propene. The catalysts were either pre-oxidised or pre-reduced prior to evaluation with respect to light-off performance, using net oxidising and net reducing CO/C3H6/O2/N2 gas mixtures. Promotion of Pt and Pd with cobalt or cerium oxide, favoured the low temperature activity significantly. Pre-reduction of Co- and Ce-promoted noble metals shifted the conversion starts of CO and propene toward lower temperatures compared with pre-oxidised samples. Pre-reduction of cobalt oxide, without Pt or Pd, yielded a dramatic improvement of the low-temperature catalytic performance compared with pre-oxidation of the said oxide. The catalysts were characterised by temperature programmed desorption of CO and specific surface area measurements. The high activity over the pre-reduced cobalt containing catalysts is suggested to be due to the presence of reduced cobalt oxide sites on those samples.