Peter Thormählen

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.

A mechanistic study of low temperature CO oxidation over cobalt oxide

The CO oxidation over Co3O4 and Co3O4/Al2O3 has been investigated using flow reactor and in situ FTIR studies. Cobalt oxide shows very high activity even at room temperature. However, a gradual deactivation takes place during reaction. The deactivated catalyst shows the presence of two different carbonate species and one graphite-like species. A possible mechanism for the deactivation is discussed.

The influence of CO2, C3H6, NO, H2, H2O or SO2 on the low-temperature oxidation of CO on a cobalt-aluminate spinel catalyst (Co1.66Al1.34O4)

A preparation method for making a high temperature stable monolith catalyst, using a cobalt-rich cobalt-aluminate spinel (Co1.66Al1.34O4) as the active material, is proposed. This catalyst, which is known for being active for CO oxidation at low temperatures, was prepared and characterised by BET, SEM, XRD, XPS and CO-TPD. The catalyst was tested for its capacity to oxidise carbon monoxide using oxygen only and oxygen in combination with other compounds typically present in cold start exhausts from Otto engines, i.e. CO2, C3H6. NO, H2, H2O or SO2. When the catalytic activity was tested with only CO and O-2 present in the feed gas, complete conversion was reached at room temperature. When other compounds were present in the gas mixture, they inhibited the CO oxidation to various degrees. The degree of inhibition for the compounds investigated was found to be: SO2 > H2O > NO = C3H6 > H2 > CO2. The main reason for the loss of activity is suggested to origin from the compounds adsorption and formation of different species on the cobalt oxide surface, which seems to inhibit the reduction and/or re-oxidation process of the metal oxide surface and/or the adsorption of CO.

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.

Catalysis in the nm‐regime: manufacturing of supported model catalysts and theoretical studies of the reaction kinetics

We briefly review the methods employed to fabricate model supported nm catalysts, including wetness impregnation, vacuum vapor deposition, electron‐beam lithography, spin‐coating, and vesicle‐mediated deposition. Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are discussed as well. The attention is focused on such effects as reactant supply via the support, interplay of the reaction kinetics on different facets and edges, and adsorbate‐induced reshaping of catalyst particles.

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.

Simulations of the kinetics of rapid reactions on supported catalyst particles

Abstract Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are briefly reviewed. The attention is focused on such effects inherent for nanometer chemistry as reactant supply via the support, interplay of the reaction kinetics on different facets, and adsorbate-induced reshaping of catalyst particles. In addition, original Monte-Carlo simulations are presented illustrating the contribution of the facet edges to the reaction rate.

Monte Carlo simulations of the kinetics of catalytic reactions on nanometer-sized particles, with diffusion over facet boundaries

Using the Monte Carlo technique, we comprehensively investigate the steady-state kinetics of the 2A+B2→2AB reaction on a nanometer-sized catalyst particle. The attention is focused on the adsorbate-diffusion-mediated interplay of reaction kinetics occurring on different facets. This effect is explored for different ratios of the rate constants for adsorption, desorption, reaction, and diffusion. The results obtained demonstrate that the reaction kinetics can be uniquely different on nanometer particles compared to those on macroscopic single-crystal surfaces.

Periodic control for improved low-temperature catalytic activity

The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al2O3 model catalyst has been studied. By introducing well-controlled periodic O2 pulses to simple gas mixtures of CO or C3H6 (in N2), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate.

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.