The foundation for the volatile compound analysis necessary for this project is based on the procedure described in ISO 22197-1 for the determination of the photocatalytic air purifying capacity on the basis of the degradation of nitrogen monoxide as a typical air pollutant. A test specimen is hereby placed in a flow-through photoreactor and the surface is activated through UV radiation, so that gaseous nitrogen monoxide is adsorbed and, in the ideal case, converted into nitrogen dioxide and nitrate. A continuous laminar gas flow over the sample is realized via the reactor geometry as well as through a 5-mm gap. Further sections of the ISO 22197 relate to the photocatalytic degradation of acetaldehyde, toluene, formaldehyde and methyl mercaptan (Parts 2-5) and should, whererever possible and necessary, also be taken into account in the assessment of the overall system.
For the validation of existing testing technology for photocatalyst reactors as well as for the conception of a future photoreactor, we initially conducted literature research. The aim was to unite existing reactors which work on the basis of photocatalytically active materials and to evaluate them with regard to their suitability for practical use. This overview of photocatalysis reactors (flat-plate, fluidized-bed, annular and other reactors) was used to create a basic concept for the best-possible interpretation of the requirements of the ISO Standard 22197-1, to parameterize the necessary properties and to conceptualize the modular design of the reactor/air purifier (module) which was to be newly designed.
The fundamental objectives for the new reactor concept, taking into account the requirements of current and future photocatalysis test standards, were:
- Reduction of air pollution through released hydrocarbons whilst avoiding the occurrence of secondary emissions (zero-emission situation)
- Design and selection of materials to enable a long-term environmentally-friendly and resource-saving operation
In accordance with the specifications, we examined potentially critical materials, whose application in the reactor was planned (e.g. seals, circuit boards), with regard to their own emissions. The aim was to exclude, from the very start, those materials whose own emissions could either contaminate the exhaust air from the reactor or impair its functionality.
Within the framework of the investigations, we established that whilst many materials for the planned test reactor are suitable for the standardized determination of degradation rates (NO, VOCs), they are, however, due to their - in part unavoidable - high blank values only suitable to a limited extent for the determination of possible degradation products. We thereupon developed an alternative concept in which the flat-plate reactor was replaced by a compact stainless-steel housing which largely avoids the use of sealing materials (mini-cell). The periphery was largely preserved or adapted in this concept.
Test chambers in accordance with ISO16000-9 are usually used in the characterization of primary and secondary emissions from building materials and consumer materials, as well as other items used indoors. The avoidance of high-emission substances automatically becomes a concept. As a consequence, such test chambers are particularly well-suited as regards the detection of minor or temporarily occurring emissions of by-products. The complete air purifier or purification stages can be tested therein; this also includes the emission of undesirable particles.
For the assessment of the fundamental performance capability of the newly developed photoreactor, we performed benchmarking using the following reactor variants:
1. Modular flow-through reactor (mini-cell) based on ISO 22197-1
2. Modular flat-plate reactor in accordance with ISO 22197-1
3. Test chambers based on ISO 16000-9 for the examination of complete units
Based on the current data situation, the developed approaches promise comparable or higher levels of efficiency compared to other known systems. In addition, there is less tendency to form by-products, and lower energy consumption through the use of LED technology. The cost-effective implementation of PVD coatings on three-dimensional surfaces must, however, also be successful in the long term. Furthermore, the tendency to form by-products still offers potential for improvement, as does the effectiveness of the applied catalysts. in a follow-up project in collaboration with the industry, we have already developed an air purifier on the basis of the project results.
Additional information can be found on our project partner’s website, the Fraunhofer Institute for Surface Engineering and Thin Films IST:
Topic website “Photocatalytic air purification“
Topic website “Nitrogen oxide depletion“
Fraunhofer Institute for Wood Research 
