10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Using Aerosol Photoemission (Ape) for On-Line Process Monitoring of Gas-Phase Particle Modifications
JANNIS RÖHRBEIN, Alfred Weber, Clausthal University of Technology
Abstract Number: 211 Working Group: Materials Synthesis
Abstract The surface properties of particulate materials are influencing various powder properties such as flowability, dispersibility, compressibility and general chemical properties like catalytic activity. These characteristics are of significant interest for handling and characterization of those materials. The surfaces of gas-borne particles can be modified in various ways: beside a chemical oxidation, a condensation process might be used to deposit material on their surface. If these condensation processes are to be investigated, it is essential not to change the particle environment during probing.
There are various systems to characterize particle properties such as size, morphology and mass with different degrees between on-line and off-line measurement. However, for the characterization of surface properties of nanoparticles rather involved techniques such as electron spectroscopy (e.g. XPS (X-ray Photoelectron Spectroscopy) or MIES (Metastable Impact Electron Spectroscopy)) have to be employed. These methods suffer from limitations when it comes to (nearly) in situ measurements, i.e. analysis of the particles in the original gas environment, since the electron spectroscopy requires vacuum conditions. When for instance the effect of condensation on the particle surface is to be investigated, vacuum conditions will heavily affect the surface state of the particles due to (partial) evaporation. Additionally, the particle transfer from the synthesis reactor to the vacuum analysing system could lead to sampling artefacts and may be far too complex for process monitoring.
Within this work, a system is presented, which operates at ambient pressure and is based on the photoelectric effect: Singly pre-charged aerosol particles (previously classified by a DMA) are irradiated with light of different wavelengths from an adjustable monochromatic source. The photoelectric effect may remove an electron leading to uncharged particles. Since this method does not require vacuum conditions, the particles can be analysed in the process environment with a simple on-line technique. In addition, in the photoemission measurements morphological effects are taken into account using particles classified according to their mobility with a DMA which is basically given by the particle projection area. Due to the normalization of the photoemission activity to the available surface area the apparatus used here was named Surface-Normalised Aerosol PhotoEmission Sensor (SN-APES).
There already exists extensive, interesting work about Aerosol Photoemission (APE), but the application of APE for process control and monitoring has hardly been addressed. Our approach is the usage of APE for process monitoring of the coating of titanium dioxide with nickel oxide and the diffusion of nickel into the crystal structure of titanium. For this purpose, a commercial photocatalyst (Degussa P25) was dispersed with 1 wt% nickel nitrate in an aqueous solution and spray dried with an atomizer. After drying, the aerosol passed a tube furnace at a temperature representing the process parameter to be investigated. The variation of the product properties with process temperature was analyzed with off-line methods such as TEM, XRD, FTIR and EDX.
By employing aerosol photoemission, it was possible to observe this process on-line, which thereby allows the production of particle material with very defined properties. It was established that the change of the photo-emission behavior of the particles can be related to the surface modification. It turned out that aerosol photoemission, while technically simple, is a very powerful tool for gas phase process monitoring.