Magnus Hagström

Chemical analysis of individual alkali-containing aerosol particles : Design and performance of a surface ionization particle beam mass spectrometer

A mobile particle beam mass spectrometer has been developed to measure the alkali metal content in individual submicron aerosol particles. The instrument employs an aerodynamic inlet system for efficient sampling of particles into vacuum, and the detection of individual particles is based on decomposition and surface ionization on a hot platinum surface. A boxlike design of the hot ionizing surface is shown to limit problems associated with particle bounce effects and incomplete ionization, and the decomposition/ionization process is not sensitive to detailed particle properties. High transmission efficiencies and quantitative determination of the alkali metal content in individual particles with diameters down to 14 nm are demonstrated. Experiments with particles doped with alkali salt show that the size range may be extended down to a few nanometers after further improvements of the inlet system. High size resolution can be achieved with the instrument for particle sizes down to tens of nanometers, as illustrated by the detection of multiply charged particles passing through a DMA. The robustness of the instrument makes it suitable for field measurement applications, and the technique is demonstrated at a 12 MW biomass combustion facility. The performance of the instrument and further refinements of the technique are discussed, and potential applications in field and laboratory studies are outlined.

High Pressure Desorption of K+ from Iron Ammonia Catalyst – Migration of the Promoter Towards Fe Active Planes

The thermal desorption of potassium ions from industrial iron catalysts was studied in situ in the wide pressure range of 10−8–10 bar of Ar, N2 and synthesis gas mixture of N2:3H2. While high activation energy of 284 ± 1 kJ/mol, for K+ was determined for the catalyst precursor, in the reaction conditions it drops down to 231 ± 5 kJ/mol, corresponding well to that found for iron single crystals in UHV studies. The results are rationalized in terms of potassium migration from oxide storage phases towards the iron facets developed during the catalyst activation.

Desorption kinetics at atmospheric pressure: alkali interactions with rhodium and steel surfaces

Abstract The kinetics for alkali ion emission from rhodium and stainless steel surfaces are studied with field reversal (FR) technique under atmospheric conditions. Rapid electric FR outside the surface disturbs the surface population of alkali metal atoms by retarding or accelerating the desorbing ions, and the flux of ions from the surface is monitored to determine the rate constants for desorption. The rhodium surface is oxidized at surface temperatures below 1373 K, and the oxides decompose at higher temperatures. This phase transition is clearly observed as a change in the desorption kinetics for alkali ions, with a higher alkali stability on the oxidized surface compared to the metallic surface. The kinetics for Na + , K + and Cs + desorption from Rh in air are characterized, and rate constants in the range 10 −2 –10 3 s −1 are obtained at surface temperatures of 990–1710 K. Rate constants for Na + and K + desorption from stainless steel are determined in the temperature range 920–1450 K. The heterogeneous character of the Rh and steel surfaces is reflected in the observed desorption kinetics that show effects of combined desorption and diffusion steps on the surfaces. The results confirm that the FR technique can be used as an in situ probe, capable of monitoring phase transitions and kinetics on hot surfaces at elevated pressure.

In situ characterization of an iron catalyst by potassium ion desorption and electron emission measurements

The surface conditions of an industrial iron catalyst were monitoredin situ by work function measurements and measurements of thermal desorption of potassium ions. Changes in activation energy for potassium ion desorption and in work function values during catalyst activation and deactivation are discussed in terms of the potassium coverage and chemical composition of the catalyst surface.

A Novel Particle Trap Impactor for Use with the Gas-Quenching Probe Sampling System

A novel particle trap impactor has been developed for use with the gas-quenching probe in order to exclude solid particles from entering into the probe during sampling of gaseous metallic species in fluidized bed combustion conditions. The impactor must be small in size (Øimpactor ≤ Øprobe = 45 mm) but capable of collecting a relatively large amount of particles at elevated temperatures. As the first step, the impactor was designed, constructed, and tested at room temperature for KCI aerosol particles. The impactor with a nozzle of 0.95 mm in diameter, in combination with the orifice-to-jet diameter ratio of 1.5 and the ratio of the jet-to-plate spacing to jet diameter at 1.4 yielded a sharp cutoff curve with a maximum collection efficiency of about 0.9 and a √Stk50 value of about 0.22. In addition, the collection efficiency of the impactor was compared with the particle removal efficiency of a filter of the same type as the filter previously used with the gas-quenching probe. The difference from the comparison is very small, indicating that the impactor can be used to replace the filter to prevent fly ash particles from entering the gas-quenching probe in fluidized bed combustion conditions.