Gudmund Smedler

Sulphur dioxide interaction with NOx storage catalysts

The effect of SO2 on the NOx storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al2O3 NOx storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO2 to a synthetic lean exhaust gas caused deactivation of the NOx storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NOx storage capacity was found to be proportional to the total SO2 dose that the catalyst had been exposed to. SO2 was found to be accumulated in the catalyst as sulphate.

The development of a model capable of predicting diesel lean NOx catalyst performance under transient conditions

Abstract Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.

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.

Influence of the platinum-support interaction on the direct reduction of NOx under lean conditions

Catalysts containing Pt supported on SiC, Al2O3 and ZSM-5 were prepared and studied for NOx reduction by C3H6 in Oz excess under transient (temperature ramps) and steady-state conditions. The maximum NOx reduction activity in the heating ramp experiments was similar for Pt/SiC and Pt/ZSM-5, while Pt/Al2O3 showed higher maximum activity. Both N-2 and N2O formation was observed for all catalysts, although the respective amounts varied with the investigated system. Highest Nz selectivity was observed for Pt/Al2O3. When the NOx reduction activity was studied under steady-state conditions the activity of Pt/Al2O3 decreased substantially (mainly due to a loss in N-2 production). Pt/ZSM-5 became somewhat more selective towards Na production whereas the activity and selectivity of Pt/SiC remained at about the same values as far the heating ramp experiments. Adsorbed species on the surface of the different catalysts were investigated using in-situ FTIR in order to obtain information about the reaction mechanisms. The adsorption of species on Pt/SiC was negligible, while a number of absorption bands were observed for Pt/Al2O3 (N and C containing species, and -NCO) and Pt/ZSM-5 (HC).

An Experimental and Kinetic Modelling Study for Methane Oxidation over Pd-based Catalyst: Inhibition by Water

The water inhibition of methane oxidation over a bimetallic Pt-Pd on CeO2–Al2O3 catalyst was investigated and the experimental data were used to develop a kinetic model, consisting of only three reaction steps. In the model, the water effect was assigned to the adsorption of H2O on surface sites, as well as to the formation and accumulation of surface hydroxyl groups. These two effects were accounted by the model, which could well describe the experimental data obtained under various conditions.Graphical Abstract

The effect of water on methane oxidation over Pd/Al2O3 under lean, stoichiometric and rich conditions

In this study, the effect of oxygen concentration and the presence of water on methane oxidation were examined over a Pd/Al2O3 catalyst. The physicochemical properties of the catalyst were investigated in detail using BET, XRD, STEM, O2-TPO and CH4-TPR. Ramping experiments from 150 to 700 °C were conducted using rich, stoichiometric and lean gas mixtures in the absence and presence of water. It was found that increasing the oxygen concentration in a dry atmosphere resulted in higher methane oxidation activity, which can be connected to the facilitation of palladium oxide formation. The TPO data showed that only minor amounts of PdO up to 700 °C were decomposed; however, in the stoichiometric and rich reaction mixture, PdO was still decomposed because of the oxygen limitation. This fact resulted in a “negative activation” during cooling, with increased activity because of palladium re-oxidation. Moreover, methane steam reforming and water gas shift reactions were important reactions under rich conditions over the metallic palladium sites. A significant inhibiting effect of water on the Pd-catalyst with loss of methane activity was found. Interestingly, the inhibition effect was much greater using high oxygen concentration in the gas mixture (500 ppm CH4, 8% O2, 5% H2O) than that at lower oxygen levels (800–1200 ppm) and we propose that the hydroxyl species formation, which blocks the active sites, are facilitated by a large oxygen excess. In addition, the re-oxidation of palladium occurring during the cooling ramp in dry feed using rich and stoichiometric gas mixtures was also significantly suppressed in the presence of a large amount of water. Thus, water impedes the oxidation of palladium, which significantly deactivates the Pd catalyst.

Study of methane oxidation over alumina supported Pd–Pt catalysts using operando DRIFTS/MS and in situ XAS techniques

Graphical Abstract Abstract Methane oxidation over Pd–Pt/ model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd–Pt nanoparticles are present only on catalysts subjected to elevated temperature () whereas calcination at lower temperature (500 ) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd–Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd–Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.

The Development of a Model Capable of Predicting Diesel Lean NOx Catalyst Performance Under Transient Conditions

Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics in isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.