De-NOx

de-noxUrban airborne pollution treatment

Air pollution in the cities is a major concern and is regulated by EU. Joma International can offer a range of the products that can be utilised for air de-pollution including de-NOx applications. The doped MOX powders provide an effective photocatalytic solution for low levels of light and low levels of UV radiation, and can be particularly advantageous for indoors applications, enclosed areas (e.g. tunnels) and outdoors application in high latitude climates and areas where sun is frequently overcast with clouds.

Use of neutral and basic dispersions can provide an effective solution for manufacturing nano-TiO2 impregnated concrete, cement, bitumen & asphalt products used in cities infrastructure and that due to high surface coverage area (city squares, roads etc) may help to reduce air pollution by using energy of the sun. JOMA is actively engaged in collaborative research projects aimed at measuring and quantifying the effectiveness of photocatalytic technology for air depollution in real life situations.

 

NOx vs NO2, selectivity and the ISO standard

When discussing photocatalytic NOx removal, there is one pretty important point that not always comes across in marketing material, namely that of understanding what test results mean. Very bluntly put, it is easy to present numbers so that a material appears to be much better than it really is, and in such a way that the results are not easily comparable to other tests.

The first thing to look out for are tests in closed systems. If there is no source of pollution then any active material should eventually break it all down. Quotes of "more than 99% NOx removal" are basically meaningless since they give no real information beyond "there was some level of activity". For comparing NOx removal rates, there is a ISO test specifically designed to do so.

The second thing to look out for are 'modified' or 'variants' of that very ISO tests. It is one thing to simply do the measurements in shorter time, but quite another to increase the UV intensity, reduce the gas flow (thus increasing the time for breaking it down), or increase the area of the test sample without compensating for it when presenting results. Suffice to say, we have observed claimed breakdown rates that correspond to far above the theoretical maximum of the ISO test. 

The third and potentially most dangerous thing is that even if the ISO test is done correctly, results can still be very misleading. The test starts with 1000ppb of NO gas, which is first oxydised to NO2 before it can be degraded to nitrate. The activity can the be presented as % degradation of the input NO gas (what we call NO%), or as a % degradation of total NOx input to nitrates (what we call NOx%). Even if the radicals themselves are not selective, the surface will have different adsorption affinity for NO and NO2 molecules, giving different materials very different NO2/NO selectivities.

The critical point here is that accumulation of toxic NO2 contributes to a high NO% (while having no effect on the NOx%), and that it perfectly "ok" to report ISO activity only as NO%. A material with poor selectivty will actually be a net producer of toxic NO2 under typical conditions, and having a record high NO% rating only makes it do so faster.

The ideal material for NOx degradation should not just have a high NOx%, but also as high as possible NO2/NO selectivity, as seen by the NOx% : NO% ratio. Fortunately, we have developed just that.

 

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