Stefan Johansson

Catalytic ignition in the COO2 reaction on platinum: experiment and simulations

Abstract Catalytic ignition temperatures for the CO+O2 reaction on a Pt wire, and the corresponding transient ignition curves, have been measured and analyzed theoretically for nonflammable reactant mixtures in Ar and He at atmospheric pressure. At very low relative CO concentrations (

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.

Nanofabricated model catalysts; Manufacturing and model studies

Modern micro- and nano-fabrication methods provide a new avenue to prepare controlled model catalysts, which realistically mimic real supported catalysts. Such catalysts consist of 2D arrays of active catalysts deposited on active or inactive support materials. Particle size, shape, separation and support can be systematically varied. Such structures are easily accessible to scanning probe imaging and surface analysis techniques. This approach to build 2D analogues of supported catalysts is illustrated by Pt particles in the size range 10–500 nm deposited on alumina and ceria. The manufacturing technique, electron beam lithography and ‘lift-off’, is described. Its current limitations and future perspectives are discussed briefly.

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.

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).

Microreactor for studies of low surface area model catalysts made by electron-beam lithography

A flow reactor for evaluation of low surface area, model catalysts has been designed. The aim is to measure catalytic activity at atmospheric pressure on nanofabricated model catalysts, primarily made by the electron-beam lithography (EBL) technique. This is a challenge because of the low attainable active surface area on such model catalysts, due to the serial processing in EBL manufacturing. The interest in using EBL-nanofabricated model catalysts derives from the high level of control over the catalyst geometry that is obtainable. In order to evaluate the performance of the reactor for such catalysts, two different sample types were studied with regard to their activity for CO oxidation, during a heating ramp. One sample was an EBL-microfabricated Pt/CeOx model catalyst with Pt disks 700 nm in diameter, arranged in ordered arrays, with a total active Pt area of about 0.4 mm2. The other sample was a Pt wire with a diameter of 0.1 mm and a length of about 1 mm. The light-off traces of these samples were ...

Mitigation of voltage collapse caused by armature current protection

The importance of field current limiter behaviour during voltage instabilities is generally known. A field current limiter will weaken the system by introducing extra reactance. A tripping of the generator by an armature overcurrent relay or the activation of an armature current limiter will severely cripple the power system which often causes the breakdown of the system voltages. One way to alleviate the influence of the armature current protection during the instability is to make small changes in the active power production of the generator and thereby fully utilize the capability of the generator. Depending on the location of the overloaded generator, different actions can be taken to support the critical area as long as sufficient transmission capacity and that active and reactive power reserves are available remotely. This active power rescheduling may also alleviate the influence of a field current limiter. Some simulations are shown for a power system with a radial structure.

Avoiding voltage collapse by fast active power rescheduling

Abstract Basic ideas for a method where the active power production is rescheduled in an automatic (fast) way to increase the loadability of the power system during a voltage instability are presented. Active power production is a parameter that is controllable during this instability phase and it may have a positive influence on the system vulnerability to collapse, especially when current limitations of the generators are involved. Depending on the strength of the system, two major objectives can be distinguished: to strengthen a local area from collapsing or to avoid an increase of the voltage depressed area.

Theta-lifts of Maaß waveforms

Let O be ein arbitrary order in an indefinite quaternion division algebra over ℚ. If O 1 is the group of elements in O with norm equal to 1, and H the complex upper half-plane, then X O := O 1\H is a compact Riemann surface. Furthermore, let Г0(d) \(\subseteq\) S L 2(ℤ) be the Hecke congruence group of level d. Then X d := Г0(d)\H is a non-compact Riemann surface with finite volume. Let △ be the hyperbolic Laplace-operator on H. In certain situations, it is known that it is possible to relate the spectral resolutions of automorphic Laplacians in the compact case to the non-compact case. In this paper, we give an explicit construction of such correspondence in the case of Maas waveforms. The construction uses Siegel theta functions and generalizes the one in [8]. Furthermore, we prove that the theta-lifts commute with Hecke operators. Finally, we investigate to what extent the lifted forms are newforms or not.

Photo-assisted processes for improved catalytic activity and selectivity of environmentally harmful emissions

Photoenhanced activity of several supported noble metal/metal oxide combinations has been explored. Significant activity is observed for supported Pt and Pd samples. Specifically, the so called light-off temperatures for CO oxidation are found to decrease upon illumination. Evidence for a photocatalytic non-thermal mechanism rather than a photo-assisted thermal process is found through the wavelength dependence for CO oxidation on a Pt/alumina catalyst. The difficulty of separating real photocatalytic from other photo-assisted effects in the case of porous samples is discussed.