ČSN ISO 22197-4 - Jemná keramika (speciální keramika, speciální technická keramika) - Zkušební metoda pro účinnost čištění vzduchu pomocí polovodičových fotokatalytických materiálů - Část 4: Odstraňování formaldehydu

3.6 dark condition

test condition with no light irradiation by the light source for testing and room lighting3

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Symbols

For the purposes of this document, the following symbols apply.

f

flow rate of test gas converted into that at the standard state (0 °C and 101,3 kPa) (l/min)

ϕF

volume fraction of formaldehyde at the reactor exit (µl/l)

ϕF0

supply volume fraction of formaldehyde (µl/l)

ϕFD

volume fraction of formaldehyde at the reactor exit under dark conditions (µl/l)

nF

removal quantity, by test piece, of formaldehyde (µmol)

RF

removal percentage, by test piece, of formaldehyde (%)

Principle

This document concerns the development, comparison, quality assurance, characterization, reliability and design data generation of photocatalytic materials. The method described is intended to obtain the air-purification performance of photocatalytic materials by exposing a test piece to model polluted air under irradiation by UV light. Formaldehyde (HCHO) is chosen because it is a typical indoor air pollutant that causes the so-called sick-building syndrome. The test piece, placed in a flow-type photoreactor, is activated by UV irradiation, and adsorbs and oxidizes gas-phase formaldehyde to form carbon dioxide (CO2) and other oxidation products. The air purification performance is determined from the net amount of formaldehyde removed by the test piece (μmol). The simple adsorption of HCHO by the test piece (not due to photocatalysis) is evaluated by tests in the dark. However, some test pieces absorb formaldehyde very strongly, and it is not always possible to attain a stable concentration of formaldehyde in the designated time of test. The photocatalytic activity can depend on physical and chemical properties of pollutants mainly due to the adsorption process involved. For a better evaluation of air purification performance of photocatalytic materials, it is recommended that one or more suitable test methods as provided in the other parts of the ISO 22197 series are combined.

Apparatus

Test equipment

The test equipment enables a photocatalytic material to be examined for its pollutant-removal capability by supplying the test gas continuously, while providing photoirradiation to activate the photocatalyst. It is the same as that used in the test method for the removal of nitric oxide (see ISO 22197-1) and consists of a test gas supply, a photoreactor, a light source and pollutant-measurement equipment. Since low concentrations of pollutants are to be tested, the system shall be constructed with materials of low absorption and resistant to UV radiation, e.g. acrylic resin, stainless steel, glass and fluorocarbon polymers. An example of a test system is shown in Figure 1.

Key

1

test gas supply

9

four-way valve

2

air compressor

10

photoreactor

3

air-purification system

11

test piece

4

standard gas (pollutant)

12

air-tight optical window

5

pressure regulator

13

light source

6

mass-flow controller

14

analyser

7

humidifier

15

vent

8

gas mixer

 

 

Figure 1 — Schematic diagram of the test equipment

a) For flat test pieces

b) For filter-type test pieces

 

test piece length l1

test piece width l2

air layer thickness lg

 

 

99,0 ± 1,0 mm

49,0 ± 1,0 mm

5,0 ± 0,5 mm

 

Key

1

test gas inlet

6

auxiliary plate

2

buffle

7

test piece (flat-type)

3

air-tight optical window

8

test gas outlet

4

flow channel

9

test piece holder

5

height-adjusting plate

10

test piece (filter-type)

Figure 2 — Cross-sectional views of photoreactor

Test gas supply

The test gas supply provides air polluted with model contaminant at a predetermined concentration, temperature and humidity, and supplies it continuously to the photoreactor. It consists of flow regulators, a humidifier, gas mixers and so on. The flow rate of each gas should be within 5 % of the designated value, which is easily attained by using thermal mass-flow controllers with knowledge of temperature and gas type at calibration in accordance with ISO 6145-7. The expression of gas flow rate in this document is that converted to the standard state (0 °C and 101,3 kPa). Typical capacities of flow controller for pollutant gas, dry air and wet air are 200 ml/min, 2 000 ml/min and 2 000 ml/min, respectively. The standard formaldehyde gas in a cylinder, normally balanced with nitrogen, shall have a volume fraction of about 20 µl/l.

Photoreactor

The photoreactor holds a planar test piece within a 50-mm-wide trough, with its surface parallel to an optical window for photoirradiation. The reactor shall be fabricated from materials that adsorb little test gas and withstand irradiation of near-UV light. The test piece shall be separated from the window by an air layer 5,0 mm ± 0,5 mm thick. The test gas shall pass only through the space between the test piece and the window. This gap shall be accurately set up according to the thickness of the test piece, for example by using height-adjusting plates with different thicknesses, as shown in Figure 2 a). When a filter-type material is tested, an alternative type of test-piece holder shall be used, which holds the test piece while allowing the test gas to pass through the cells of the filter under irradiation [Figure 2 b)]. Quartz or borosilicate glass that absorbs minimal light at wavelengths longer than 300 nm shall be used for the window.

Light source

The light source shall provide UV irradiation within a wavelength range of 300 nm to 400 nm. Suitable sources include the so-called black light (BL) and black light blue (BLB) fluorescent lamps, with a maximum at 351 nm, as specified in ISO 10677. The test piece shall be irradiated uniformly through the window by the light source. If testing filter-type photocatalysts, the light source shall irradiate one face of the test piece. A light source that requires warming up shall be equipped with a shutter. The distance between the light source and the reactor shall be adjusted so that the UV irradiance (300 nm to 400 nm) at the sample surface is 10 W/m2 ± 0,5 W/m2. The irradiance along the length of the test piece shall also be constant within ±5 %. The UV irradiance shall be measured with a radiometer which conforms to ISO 10677. The reactor shall be shielded from external light if necessary.

Analytical system

The test gas shall be sampled using a sampling cartridge, pump and flow controller, as specified in ISO 16000-3. The concentration of formaldehyde shall be determined by 2,4-dinitorphenylhydrazine-derivatised high-performance liquid chromatography (DNPH-HPLC). The reagents, equipment and procedure as specified in ISO 16000-3 shall be used. Other analytical methods that give equivalent or better performance can be used.

Test piece

The test piece shall be a flat material or a filter-type 49,0 mm ± 1,0 mm wide and 99,0 mm ± 1,0 mm long. It may be cut to these dimensions from a larger bulk material or coated sheet, or may be specially prepared for the test by coating a pre-cut substrate. The thickness of the test piece shall ideally be less than 5 mm, in order to minimize the contribution from the side faces. If thicker test pieces are to be tested, the side faces shall be sealed with an inert material before testing. The filter-type test piece shall not be thicker than 20 mm.

Procedure

General aspects

The test procedure consists of pretreatment of the test piece, an adsorption process in the dark and measurements of removal of formaldehyde under photoirradiation. An example of the concentration change of formaldehyde during the test is shown in Figure 3. Some test pieces will possibly not give accurate removal of formaldehyde due to lower photocatalytic activity. In these cases, loading of formaldehyde per test piece may be reduced following the procedure in Clause 10.

Key

X

time (min)

Y

concentration of formaldehyde (μl/l)

1

irradiation start

2

irradiation stop

O

outlet

I

inlet

Figure 3 — Typical trace of formaldehyde concentration during the test operation

Pretreatment of test piece