Sample gas: air ofclass 10 in a clean room
- concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm
- Sox concentration: 40 to 60 ppb;
Stocker volume: 80 liters;
Light source: low-pressure mercury lamp (with peaks at 184 nm and 254 nm);
Photocatalyst: TiO₂;
The way of carrying photocatalyst on the light source: TiO₂was coated onto the surface of the mercury lamp to a thickness of 50 nm by sol-gel method;
Means for trapping and removing acidic and basic gases: an ion exchange fiber (anion-type) or a fibrous active charcoal;
Fabrication of the ion exchange fiber: anion exchange fiber: graft polymerization of fibrous polypropylene was carried out by exposing to an electron beam of 20 Mrad under nitrogen and then immersing in a solution consisting of hydroxystyrene monomer and isoprene. Quaternary amination of the reaction product afforded an anion exchange filter;
Measurement of the contact angle: a method for measuring the contact angle of a water drop (manufactured by Kyowa Kaimen Kagaku Inc., CA-DT type);
Concentration of non-methane hydrocarbons in the stocker: was measured by gas chromatography (GC) method;
Concentration of SOx in the stocker: was measured by solution conductivity method;
Identification of the hydrocarbons adsorbed on the wafer: GC/MS method;
Wafer stored in the stocker: a highly pure 5-inch silicon wafer was cut into pieces of 1 cm×8 cm and pretreated as stated below prior to introduction into the stocker;
Pretreatment of the wafer: washing with a detergent and alcohol on a clean bench in the clean room, followed by UV/O₃cleaning. The wafer was exposed to UV radiation under the conditions allowing O₃evolution.
Results:
(1) The Contact Angle on the Wafer

FIG. 23 shows the contact angle (in degrees) as the function of the number of days when an ion exchange fiber was employed. InFIG. 23, the symbol-◯-represents the combination of an ion exchange fiber and a photocatalyst (present invention), the symbol-●-represents a control without ion exchange fiber and the symbol-▴-represents a control without photocatalyst.

The control without photocatalyst showed the contact angle of 20 degrees at 20 hours after and 40 degrees at 60 hours after, thus no preventive effect was observed on the increase in the contact angle. The measured values as long as the 350th day are shown inFIG. 23. When an ion exchange fiber was disposed at the inlet in the manner as shown inFIG. 22, the apparatus could maintain a high performance without deterioration with time.

The control applying a fibrous active charcoal in place of an ion exchange fiber for trapping and removing acidic and basic gases showed the results similar to those shown inFIG. 23 and exhibited an increase in the contact angle of below 2 degrees after 300-day operation.

(2) Organic Compounds in the Stocker

The concentrations of non-methane hydrocarbons and SOx as well as the identification of the hydrocarbons adsorbed on the wafer (presence or absence of the identified components) are shown in the table 3. The results obtained by using a control having an ion exchange fiber without photocatalyst or a control having a photocatalyst without ion exchange fiber are also shown in the table 3.

TABLE 3


storage		hydrocarbon

period		conc. in the	deposition on	SOxconc. in
(days)	conditions	air (ppm)	the wafer	the air (ppb)

1	invention	<0.1	absent	<1
	no photocatalyst	1.2˜1.5	present	<1
	no ion exchange	<0.1	absent	5˜7
	fiber
10	invention	<0.1	absent	<1
	no photocatalyst	1.2˜1.5	present	<1
	no ion exchange	<0.1	absent	5˜7
	fiber
100	invention	<0.1	absent	<1
	no photocatalyst	1.2˜1.5	present	<1
	no ion exchange	<0.1	absent	10˜15
	fiber
300	invention	<0.1	absent	<1
	no photocatalyst	1.2˜1.5	present	<1
	no ion exchange	0.5˜0.8	absent	15˜20
	fiber

In table 3, the hydrocarbon deposited on the wafer was phthalic ester such as DOP.

Example 21

The same apparatus as that used in the example 21 was used, except that a photocatalyst was employed and an ion exchange fiber was omitted. The number of days required for a 5 degree increase in the contact angle was observed as the function of the SOx concentration by using a sample gas adjusted to a SOx concentration of 1 to 50 ppb. Other experimental conditions were same as those used in Example 20.

Sample gas: air ofclass 10 in a clean room
Concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm
SOx concentration: 1 to 50 ppb

The concentration of SOx was appropriately adjusted by regulating the passing speed of the gas through the ion exchange fiber for removing a part of SOx as desired.

Results

FIG. 24 represents the number of days required for a 5 degree increase in the contact angle versus the SOx concentration in the sample gas. It was obvious that the less was the concentration of sulfur oxides in a sample gas, the more was the number of days required for increase in the contact angle. In other words, the air was kept clean for longer.

FIG. 25 is a total ion chromatogram obtained by gas chromatography/mass spectrography of organic compounds in the air. The axis of abscissa x represents the mass of an ion and the axis of ordinate y represents a relative strength. 30 liters of sample air was flown into an adsorbent (TENAX-GR) at a rate of 0.5 liter/min. The adsorbent was heated in a device for concentration and introduction (manufactured by CHROMPACK Inc., CP4010 model) so as to desorb the adsorbed gas, which was cooled with liquid nitrogen, concentrated and measured in a gas chromatography/mass spectrometry device (manufactured by Shimazu Seisakusho Inc., QP-1100EX model).

(a) is a graph showing the results of the air of class 10,000 in a clean room. Non-methane hydrocarbons are present in a concentration of 1.1 to 1.3 ppm and SOx, NOx and NH₃are present in a concentration of 40 ppb, 30 ppb and 150 ppb (average concentration) respectively. Each peak represents the presence of an organic compound.

(b) is a graph showing the results of the air of class 10,000 in a clean room after treated by the apparatus of the present invention. It is obvious that the amount of organic compounds was drastically decreased.

The present invention, by producing microparticles of contaminants and concentrating them locally, can decompose the contaminants with a photocatalyst effectively, even when the contaminants are present at low concentration. Contaminants such as organic compounds in the air, even when they are present at low concentration, can be efficiently decomposed because they are made into microparticles and brought into contact with a photocatalyst in the concentrated form. As a result, the decomposition rate of organic compounds was improved. The photocatalyst is effective in decomposing oxidizable compounds including gaseous organic compounds such as phthalic ester, gaseous organosilicon compounds such as siloxane and basic gas such as ammonia. The present invention is particularly advantageous in view of the fact that gaseous organic compounds and gaseous organosilicon compounds are difficult to remove by photoelectronical charging or with a filter or an ion exchange fiber.

When a removal section utilizing a filter, an adsorbent, a photoelectron and an ion exchange resin is provided, other gaseous contaminants which are present together with organic compounds, including acidic gases such as SO₂, NOx, HCl, HF and the like as well as basic gases such NH₃, amines and the like can also be removed. In other words, a wide variety of contaminants ranging from gas to particles can be removed.

In addition, when acidic and basic gases are removed prior to the treatment with a photocatalyst, adverse effects on the photocatalyst by the acidic and basic gases can be avoided. In consequence, the photocatalyst can act reliably over a long period, thus allowing long-term operation.

By way of an illustrative application, the present invention was described mainly with reference to a clean room of semiconductor production factories.

The present invention are also suitable to other applications as set forth below:

(1) Air purification for public welfare, for example, purification of air in offices, buildings, houses, hospitals, hotels and the like;
(2) Purification of exhaust gases originating in various industries including sewer drains and waste disposal sites; purification of industrial atmosphere; as well as purification of exhaust gases originating in underground parking areas and tunnel ventilators; and
(3) Purification of air and gases including nitrogen, oxygen and the like for use in gas production units for feeding clean rooms, clean booths, clean tunnels, clean benches, safety cabinets, bioclean boxes, sterile rooms, path boxes, precious article stockers, transportation spaces, interfaces, air curtains, air knives, drying sections, production lines and the like in pioneer industries such as semiconductor industries, electronic industries, pharmaceutical industries, food industries, agricultural and forestry industries, medical industries, precision machinery industries and the like.

Public welfare was mentioned in consideration of the fact that gaseous contaminants present in the air may adversely affect human health producing so-called sick building syndrome.