Photocatalyst Test Standards

Applicable International Test Standards

Below is a list of the categories of photocatalyst tests offered by QIPS and the associated ISO standards. Click on any standard listed below for further details.

Air Purification

To date there are three published photocatalyst air-purification ISO methods, each dedicated to the removal of a different air-borne pollutant:

The standard sets out to measure the photocatalyst’s overall ability to remove the oxides of nitrogen, i.e. NOx.  Typical test conditions are given below:

Sample size5 cm x 10 cm and typically 5 mm thick
Suitable sample typeConstruction materials in flat sheet, board or plate shape; materials in honeycomb-form; plastic or paper materials if they contain ceramic microcrystals and composites.
Unsuitable sample typePowder or granular photocatalytic materials
RestrictionsApplicable to samples with photon efficiency of ca. < 1.5% (quantum efficiency of 0.5% reported for titania)
Sample pre-treatment5 hr UV irradiation at ≥ 1 mW/cm2
Test conditions3 L min-1 of 1 ppmv NO in air, adjusted to 50% RH; time: 5 hours; irradiance: 1 mW/cm2; temperature: 25 °C
Analytical methodChemiluminescence:  NO, NO2
Information returnedNet removal of nitrogen oxides (NOx) (= NO removed – NO2 formed) (mmol); and % NOx removed

Further Information

Nitric oxide is an important intermediate in the chemical industry and a major air pollutant produced by the combustion of substances in air, such as gasoline in automotives and fossil fuels in power stations.  In the absence of a catalyst NO is oxidised relatively slowly to nitric oxide by oxygen.  NO is used on a large scale in the manufacture of nitric acid, the bleaching of rayon, and as a stabiliser in the production of propene and methyl ether.  It is an important signalling molecule in most biological systems and, along with NO2, is associated with sick building syndrome and acid rain production.  Given its widespread commercial use and, maybe more importantly, its occurrence as a common air-borne, environmentally damaging pollutant, the removal of NO and its NOx counterpart, NO2, by semiconductor photocatalysis has resulted in the promotion of many commercial photocatalyst products, such as paint, tiles and paving stones, for their NOx removing ability.

The two key photocatalytic reactions whereby the titania-sensitised, photocatalytic oxidation of NO proceeds to nitric acid, via nitrous acid and a radical based mechanism [1-5] are:

nox-1-and-2

Recent work [1] reveals that the accumulation of nitric acid on the surface promotes its photocatalysed reaction with NO that generates the toxic product NO2, i.e.

nox-3

This can lead to an eventual steady state where the rate of NO removal is matched by the rate of NO2 production; which is clearly highly undesirable.  It follows that for any NOx-removing photocatalyst product to work effectively it is necessary that the HNO3 photogenerated, via reactions (1) and (2), must be removed at regular intervals, by rinsing with water, e.g. from rain or a damp cloth.

The standard sets out to measure the photocatalyst’s overall ability to remove the oxides of nitrogen, i.e. NOx, using a NO-containing air stream.  A measure of this ability is taken as the difference between the total NO removed (nNO) and NO2 generated (nNO2) during the irradiation period.  A typical data set generated, i.e. NO removed and NO2 generated, is illustrated below.  The hatched areas A and B are proportional to the amounts of NO adsorbed and photo-oxidised/removed, respectively.  Hatched area C is proportional to the amount of NO2 generated.  The key points are: 1 – start of contact with NO-containing feed, 2 – UV lights on and 3 – UV lights off and feed gas changed to zero calibration gas (i.e. air).

nox

References

  1. Y. Ohko, Y. Nakamura, N. Negishi, S. Matsuzawa and K. Takeuchi, J. Photochem. Photobiol., A, 205 (2009) 2.
  2. T. Ibusuki and K. Takeuchi, J. Mol. Catal., 88 (1994) 93.
  3. S. Devahasdin, C. Fan, K. Li and D.H. Chen, J. Photochem. Photobiol., A, 156 (2003) 161.
  4. N. Bowering, G.S. Walker and P.G. Harrison, Appl. Catal., B, 62 (2006) 208.
  5. M.M. Ballari, M. Hunger, G. Husken and H.J.H. Brouwers, Appl. Catal., B, 95 (2010) 245.

The test piece, put in a flow-type photoreactor, is activated by UV illumination, and adsorbs and oxidizes gas-phase acetaldehyde to form carbon dioxide (CO2) and other oxidation products.  Typical test conditions are given below:

Sample size5 cm x 10 cm and typically 5 mm thick
Suitable sample typeConstruction materials in flat sheet, board or plate shape; structured filter materials including honeycomb-form; woven and non-woven fabrics; plastic or paper materials if they contain ceramic microcrystals and composites.
Unsuitable sample typePowder or granular photocatalytic materials
Sample pre-treatment16-24 hr UV irradiation at ≥ 1.5 mW/cm2
Test conditions1 L min-1 of 5 ppmv Acetaldehyde in air, adjusted to 50% RH; time: 3 hours; irradiance: 1 mW/cm2; temp: 25 °C
Analytical methodAcetaldehyde: GC-FID
Information returnedAmount of acetaldehyde removed (mmol) and % acetaldehyde removed

Further Information

Acetaldehyde occurs widely in nature, since it is produced by plants as part of their metabolism and also during the ripening process.  It is also a product of combustion (wood, oil, petrol and diesel) and so is a constituent of car exhaust fumes and tobacco smoke.  It is a significant industrial chemical which is used in the manufacture of acetic acid, perfumes, flavours, aniline dyes, plastics and synthetic rubber.  It is a cancer suspect agent, an irritant and large doses can cause death by respiratory paralysis.  It is an important indoor air pollutant as it is released by building materials such as polyurethane foams, adhesives, coatings and inks.  Along with formaldehyde and other volatile organic carbons, i.e. VOCs, such as toluene, it is associated with sick building syndrome.

The photocatalytic oxidation of acetaldehyde has been well studied [1-3] using titania photocatalysts, although the reaction pathway, and the major intermediates, are still the subject of debate [4].  Recent work indicates that it is first oxidised to acetic acid and then to formic acid, formaldehyde (the acids being adsorbed onto the surface of the titania) and then, finally to CO2 [4], i.e.

acetaldehyde-1-2-and-3

A typical data set generated, i.e. acetaldehyde removed and carbon dioxide generated, is illustrated below.  The hatched areas B and B’ are proportional to the amounts of acetaldehyde removed and carbon dioxide generated, respectively.  The key points are the start of contact with the acetaldehyde feed (t=0), UV lights on (↓) and UV lights off (↑).

acetaldehyde

References

  1. M.L. Sauer and D.F. Ollis, J. Catal., 158 (1996) 570.
  2. Y. Ohko, D.A. Tryk, K. Hashimoto and A. Fujishima, J. Phys. Chem. B, 102 (1998) 2699.
  3. E. Obuchi, T. Sakamoto and K. Nakano, Chem. Eng. Sci., 54 (1999) 1525.
  4. B. Hauchercorne, D. Terrens, S. Verbruggen, J. A. Martens, H. Van Langenhove, K. Demeestere and S. Lenaerts, Appl. Catal., B, 106 (2011) 630.

In the photocatalytic oxidation of toluene the major initial product appears to be benzaldehyde which is then subsequently oxidised to benzoic acid and eventually to carbon dioxide and water, provided the reaction intermediates do not adsorb so strongly to the surface of the titania as to render it photo-inactive.  Typical test conditions are given below:

Sample size5 cm x 10 cm and typically 5 mm thick
Suitable sample typeConstruction materials in flat sheet, board or plate shape; structured filter materials including honeycomb-form; woven and non-woven fabrics; plastic or paper materials if they contain ceramic microcrystals and composites.
Unsuitable sample typePowder or granular photocatalytic materials
Sample pretreatment16-24 hr UV irradiation at ≥ 1.5 mW/cm2
Test conditions0.5 L min-1 of 1 ppmv Toluene in air adjusted to 50% RH; time: 3 hours; irradiance: 1 mW/cm2; temperature: 25 °C
Analytical methodToluene: GC-FID
Information returnedAmount of toluene removed (mmol) and % toluene removed

Further Information

Toluene is a widely used chemical feedstock and industrial solvent.  As a solvent, it is used in paints, paint thinners, silicone sealants, printing inks, glues, resins and disinfectants.  It is also used as an octane booster in gasoline fuels. In industry it is also used in the manufacture of: benzoic acid, benzaldehyde, explosives, dyes and many other organic compounds.  Toluene is toxic, and is one of the VOCs associated with sick building syndrome.

Many papers have been published on the removal of toluene via its photocatalytic mineralisation [1-7].  In the absence of water vapour the photoreaction quickly stops due to the inhibition of the hydroxyl regeneration process and the accumulation of reaction products, such as benzoic acid.  In the presence of water vapour this deactivation process can be much slower, depending on how readily the reaction intermediates, such as benzoates, are adsorbed.  For example Schiavello et al reported that whereas Merck TiO2 exhibited a stable photocatalytic activity, Degussa P25 continuously deactivated upon illumination [5].  The following reactions summarise the photocatalytic oxidation of toluene [7]:

toluene-1-and-2

A typical data set generated, i.e. toluene removed, is illustrated below.  The hatched area B is proportional to the amount of toluene removed.  The key points are the start of contact with the acetaldehyde feed (t=0), UV lights on (↓) and UV lights off (↑).

toluene

References

  1. V. Augugliaro, S. Coluccia, V. Lodo, L. Marchese, G. Martra, L. Parmisano and M. Schiavelllo, Appl. Catal., B, 20 (1999) 15.
  2. A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa and J. Soria, J. Catal., 202 (2001) 413.
  3. H. Einaga, S. Futamura and T. Ibusuki, Appl. Catal., B, 38 (2002) 215.
  4. M. C. Blount and J.L. Falconer, Appl. Catal., B, 39 (2002) 39.
  5. G. Marci, M. Addamo, V. Augugliaro, S. Coluccia, E. Garcia-Lopez, V. Loddo, G. Martra, L. Parmisano and M. Schiavello, J. Photochem. Photobiol. A, 160 (2003) 105.
  6. J.M. Coronado and J. Soria, Catal. Today, 123 (2007) 37.
  7. O. Debono, F. Thevenet, P. Gravejat, V. Hequet, C. Raillard, L. Lecoq and N. Locoge, Appl. Catal., B, 106 (2011) 600.

For the best evaluation of air purification performance of photocatalytic materials, it is recommended by ISO to combine one or more of these test methods.

These methods are designed to obtain the air-purification performance of photocatalytic materials by exposing a test piece to model polluted air, in a photoreactor,  under illumination by ultraviolet (UV) light. The concentration(s) of the analyte(s) of interest are monitored regularly and the concentration versus time data profile(s) are then processed so as to provide one or more measures of the efficiency of the test piece to remove photocatalytically the air-pollutant under test.

Further Information

In all these air pollution tests the flow rate is normalised for STP and dry gas conditions and the flow ratecorrected for the water vapour present.

Other air-purification ISO standards are almost at the publication stage (e.g. for: formaldehyde and methyl mercaptan), and QIPS plans to also offer these tests in the near future.

Water Purification

These International Standards describe test methods for determining the water purification performance of photocatalytic materials, formed on, or attached to, another material surface for the purpose of decomposing, and thus eliminating the pollutants in water, using photocatalytic performance, thus relating to waste-water treatment.

Using artificial ultraviolet (UV) radiation, the photocatalytic activity of surfaces by degradation of the dye molecule methylene blue (MB) in aqueous solution, which is in contact with the potentially photocatalytically active surface, is determined, with the overall result being the decolourization of the solution.  The high molar absorptivity of MB+ ensures a striking and easily measured colour change, from blue to colourless, upon photobleaching of the dye by the semiconductor photocatalyst under study.  The structure of MB is illustrated below.

methylene-blue-structure

Typical test conditions are given below:

Sample size10 cm x 10 cm and typically 5 mm thick
Suitable sample typeSurfaces covered with photocatalytic coatings
Unsuitable sample typePowder or granular photocatalytic materials; porous materials.
Sample pretreatment24-72 hr UV irradiation at > 1 mW/cm2
Test conditionsAbs0 (665 nm, 1 cm cell) = 0.74; pH 5.5; time: ≤ 3 hours; irradiance: 1 ± 0.05 mW/cm2; temperature: 23 ± 2 °C
Analytical methodMethylene blue: UV-vis spectrophotometry
Information returnedDegradation rate (mol/m2/h) and the associated photonic efficiency, ξMB (%)

Further Information

Methylene blue (MB+) is a highly popular test pollutant in semiconductor photocatalysis used in the assessment of such key features as: new photocatalytic materials, photoreactors and light sources.  The test method is applicable to evaluation of water purification performance, and less appropriately for the evaluation of self-cleaning activity, of surfaces covered with photocatalytic coatings.

The overall photomineralisation of MB+ by semiconductor photocatalysis is summarised by the following reaction:

methylene-blue-1

Some disadvantages stem from underlying assumptions in this standard, and obviously, it is highly desirable to ensure the conditions are such that these assumptions are likely to hold.  With this in mind, we have tightened up some of the specified conditions to improve repeatability and reproducibility.  The ISO standard assumes that the MB+ used is of a high purity.  However, εMB values quoted in the literature span a wide range which is most likely a reflection of the different purities of commercial sources of MB [1].  Furthermore, the kinetics of MB+ photobleaching are highly dependent, upon the MB+ concentration [2].

This difficulty of MB+ purity is rectified by making up the MB+ reaction solution to a defined absorbance at 665 nm in a 1 cm cell (0.74), as opposed to a defined concentration as in the original standard.  Finally, the initial pH of the reaction solution is not stipulated in the current ISO test, but will be set at pH 5.5, since significant deviation from such set pH will alter the amount of MB+ adsorbed which will alter the measured rate and so the calculated value of ξMB.

A typical set of results for test samples with (●) and without (○) a photocatalyst coating are illustrated below.  The solid line is the calculated maximum allowable decay curve observable under standard conditions without a significant mass transfer contribution to the final calculated value of the photonic efficiency, ξMB.

methylene-blue

References

  1. S. Mowry and P.J. Ogren, J. Chem. Educ., 76 (1999) 970.
  2. J. Tschirch, R. Dillert and D. Bahnemann, J. Adv. Ox. Technol., 11 (2008) 193.

This method is used to determine the water purification performance of photocatalytic materials by exposing a specimen to water, polluted with DMSO, under illumination of ultraviolet (UV) light.  Water purification by photocatalytic reaction is based on the formation of active oxygen, and since DMSO reacts with active oxygen, the water purification performance is determined based on the decrease in concentration of DMSO. Typical test conditions are given below:

Sample size10 cm x 10 cm and typically 5 mm thick
Suitable sample typePhotocatalytic materials formed on, or attached to, another material surface for the purpose of decomposing, and thus eliminating the pollutants in water
Unsuitable sample typePowder or granular photocatalytic materials; porous materials.
Sample pretreatment≥ 5 hr UV irradiation at 2 mW/cm2
Test conditions10 mg L-1 DMSO at 500 mL min-1; time: 5 hours; irradiance: 2 mW/cm2; temperature: 20-25 °C
Analytical methodDMSO: HPLC
Information returnedConcentration change of DMSO and its half-life in the photoreactor

Further Information

There are many types of pollutants in water, but since the mechanism of water purification by photocatalytic reaction is based on the formation of active oxygen from the photocatalyst activated by UV irradiation, which oxidizes and decomposes the pollutants in water, it is possible to evaluate the water purification performance by measuring the ability of active oxygen to form from the activated photocatalyst in water.  Therefore, dimethylsulfoxide (DMSO) is chosen as an indicator that upon rapid reaction with hydroxyl radicals gives methanesulfonic acid (MSA) as a product of the photocatalytic reaction, and, ultimately, sulphuric acid [1,2], via a methanesulfinate (MSI) intermediate.  The overall photocatalytic process can therefore be summarised as follows:

dmso-1-2-and-3

Note: it is also possible some intermediate level of formaldehyde may be generated as is known to occur during the reaction of DMSO with hydroxyl radicals.

A typical data set generated, i.e. DMSO removed vs. irradiation time, is illustrated below.  The insert diagram is a log plot of the data in the main diagram, revealing the first order nature of the kinetics of DMSO removal by semiconductor photocatalysis and from which a first order rate constant, k1, and so half-life can be calculated.

dmso

References

  1. M. Mori, K. Tanaka, H. Taoda, M. Ikedo and H. Itabashi, Talanta, 70 (2006) 169.
  2. M.N. Abellan, R. Dillert, G. Gimenez and D. Bahnemann, J. Photochem. Photobiol., A, 202 (2009) 164.

Self Cleaning

These International Standards specify test methods for the determination of the self-cleaning performance of materials containing a photocatalyst or which have photocatalytic films on the surface, and which are usually made from semiconducting metal oxides such as titanium dioxide.

This test method evaluates the self-cleaning performance of a photocatalytic material by simultaneously evaluating the decomposition of the organic substance and change of water affiliation.   An organic material (oleic acid, C18H34O2) is applied to the surface and the change in the wettability of the semiconductor substrate, as measured via its water droplet contact angle, is then monitored as a function of UVA irradiation time.  Typical test conditions are given below:

Sample size10 cm x 10 cm and typically 5 mm thick
Suitable sample typeMaterials that contain a photocatalyst or have photocatalytic films on the surface
Unsuitable sample typePowder or granular materials; permeable, rough, or highly hydrophobic surfaces.
Sample pretreatment≥ 24 hr UV irradiation at 2 mW/cm2
Test conditionsDip-coating: 60 cm min-1 in 0.5 % oleic acid in n-heptane; dried at 70 °C for 15 min; time: until complete; irradiance: 1 ± 0.1 mW/cm2; temperature: 23 ± 5 °C; humidity: 40-70% RH
Analytical methodWater droplet: Contact angle
Information returnedInitial and final contact angle; total irradiation time to achieve a superhydrophilic state

Further Information

If it is not possible to cut a product into 10 cm x 10 cm square pieces, then the test piece may have a different shape or size, as long as its shape and size allow the measurement of the contact angle at five different points.

In the ISO test, once all of the oleic acid has been destroyed via the photocatalytic mineralization of the organic top layer, i.e.

contact-angle-1

it is then expected that the water droplet contact angle will be reduced to ≤ 5° for most photocatalytic materials used in self-cleaning systems [1].

A typical data set generated, i.e. water droplet contact angle, θ, vs. irradiation time, is illustrated below.

contact-angle

References

  1. J.T. Yates, Surf. Sci., 603 (2009) 1605.

Using artificial ultraviolet (UV) radiation, the photocatalytic activity of surfaces by degradation of the dye molecule methylene blue (MB) in aqueous solution, which is in contact with the potentially photocatalytically active surface, is determined, with the overall result being the decolourization of the solution. The high molar absorptivity of MB+ ensures a striking and easily measured colour change, from blue to colourless, upon photobleaching of the dye by the semiconductor photocatalyst under study. The structure of MB is illustrated below.

methylene-blue-structure

Typical test conditions are given below:

Sample size10 cm x 10 cm and typically 5 mm thick
Suitable sample typeSurfaces covered with photocatalytic coatings
Unsuitable sample typePowder or granular photocatalytic materials; porous materials.
Sample pretreatment24-72 hr UV irradiation at > 1 mW/cm2
Test conditionsAbs0 (665 nm, 1 cm cell) = 0.74; pH 5.5; time: ≤ 3 hours; irradiance: 1 ± 0.05 mW/cm2; temperature: 23 ± 2 °C
Analytical methodMethylene blue: UV-vis spectrophotometry
Information returnedDegradation rate (mol/m2/h) and the associated photonic efficiency, ξMB (%)

Further Information

Methylene blue (MB+) is a highly popular test pollutant in semiconductor photocatalysis used in the assessment of such key features as: new photocatalytic materials, photoreactors and light sources. The test method is applicable to evaluation of water purification performance, and less appropriately for the evaluation of self-cleaning activity, of surfaces covered with photocatalytic coatings.

The overall photomineralisation of MB+ by semiconductor photocatalysis is summarised by the following reaction:

methylene-blue-1

Some disadvantages stem from underlying assumptions in this standard, and obviously, it is highly desirable to ensure the conditions are such that these assumptions are likely to hold. With this in mind, we have tightened up some of the specified conditions to improve repeatability and reproducibility. The ISO standard assumes that the MB+ used is of a high purity. However, εMB values quoted in the literature span a wide range which is most likely a reflection of the different purities of commercial sources of MB [1]. Furthermore, the kinetics of MB+ photobleaching are highly dependent, upon the MB+ concentration [2].

This difficulty of MB+ purity is rectified by making up the MB+ reaction solution to a defined absorbance at 665 nm in a 1 cm cell (0.74), as opposed to a defined concentration as in the original standard. Finally, the initial pH of the reaction solution is not stipulated in the current ISO test, but will be set at pH 5.5, since significant deviation from such set pH will alter the amount of MB+ adsorbed which will alter the measured rate and so the calculated value of ξMB.

A typical set of results for test samples with (●) and without (○) a photocatalyst coating are illustrated below. The solid line is the calculated maximum allowable decay curve observable under standard conditions without a significant mass transfer contribution to the final calculated value of the photonic efficiency, ξMB.

methylene-blue

  1. References
  2. S. Mowry and P.J. Ogren, J. Chem. Educ., 76 (1999) 970.
  3. J. Tschirch, R. Dillert and D. Bahnemann, J. Adv. Ox. Technol., 11 (2008) 193.

Light Sources

All the ISO tests utilised make reference to the use of a suitable UV light source.  However, typically three different types are recommended – two types of BLB and a xenon arc lamp.

All the ISO tests utilised make reference to the use of a suitable UV light source.  However, typically three different types are recommended – two types of BLB and a xenon arc lamp.  The relative light intensity vs. wavelength profiles for (a – solid) a xenon arc lamp, (b – dashed) a BLB light source with λmax (other than peak at 365 nm) at ca. 351 nm and (c – bold) a BLB light source with λmax (other than peak at 365 nm) at ca. 368 nm are illustrated below.

light-sources

The specification of a suitable UV light source is paramount to repeatability and reproducibility of the standard tests reported, and since the photocatalytic efficiency of a system depends upon the spectral distribution and radiant intensity of the light source, only one type of BLB, namely one with a europium-doped fluoroborate phosphor is used in our testing.  In order to evaluate photocatalytic materials for use in sunlight however, xenon arc lamps are used.

At QIPS we can measure the emission spectrum and UV irradiance (units: mW/cm2) of your submitted light source using a calibrated spectroradiometer, and compare and contrast it to that of other light sources.

Further Information

Black light fluorescent lamps are made as normal white light fluorescent tubes except that only one phosphor is used, and the glass envelope has a blue filter which ensures that most of the light emitted is UVA light. The two types of BLB arise because there are two types of phosphor, namely: (i) europium doped fluoroborate, which produces a UV emission peak ranging from 368-371 nm and a band width of 20 nm and (ii) lead-doped barium silicate with a UV emission peak ranging from 350-353 nm and a band width of 40 nm [1].

To produce solar simulated UV light (and ensure the significant visible component is removed) we use a UG5 filter (cuts out most light between 400-650 nm [2]) in combination with a WG320 filter (cuts out light below 300 nm [3]).  The overall emission has been shown to be a very good fit to the actual solar UV emission spectrum.  The removal of most of the visible light produced by the Xe arc lamp is also a very important feature, since the standards are designed for UV absorbing systems only.

We use a calibrated spectroradiometer, for measuring the UV irradiance.

The relative light intensity vs. wavelength profiles for (a – solid) a xenon arc lamp, (b – dashed) a BLB light source with λmax (other than peak at 365 nm) at ca. 351 nm and (c – bold) a BLB light source with λmax (other than peak at 365 nm) at ca. 368 nm are illustrated below.

light-sources

References

  1. http://en.wikipedia.org/wiki/Black_light; accessed 01/05/2012.
  2. http://www.optical-filters.com/ug5.html; accessed 01/05/2012.
  3. http://www.heliosoptical.net/html/schott_wg_320__colorless_uv_fi.html; accessed 01/05/2012.