US20090180941A1 - Deactivation resistant photocatalysts - Google Patents

Deactivation resistant photocatalysts Download PDF

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US20090180941A1
US20090180941A1 US12/302,626 US30262607A US2009180941A1 US 20090180941 A1 US20090180941 A1 US 20090180941A1 US 30262607 A US30262607 A US 30262607A US 2009180941 A1 US2009180941 A1 US 2009180941A1
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photocatalyst
tio
nanocrystallites
fluid
diameter
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Thomas Henry Vanderspurt
Treese Hugener-Campbell
Norberto O. Lemcoff
Stephen O. Hay
Wayde R. Schmidt
Joseph J. Sangiovanni
Zissis A. Dardas
Di Wei
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Carrier Corp
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Carrier Corp
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Assigned to CARRIER CORPORATION reassignment CARRIER CORPORATION CORRECTIVE ASSIGNMENT TO CORRECT THE MISSPELLED ASSIGNOR NAME: LOMCOFF, NORBERTO O. PREVIOUSLY RECORDED ON REEL 021893 FRAME 0413. ASSIGNOR(S) HEREBY CONFIRMS THE CORRECT SPELLING OF ASSIGNOR NAME IS LEMCOFF, NORBERTO O.. Assignors: WEI, DI, HUGENER-CAMPBELL, TREESE, SANGIOVANNI, JOSEPH J., DARDAS, ZISSIS A., VANDERSPURT, THOMAS HENRY, HAY, STEPHEN O., LEMCOFF, NORBERTO O., SCHMIDT, WAYDE R.
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/39—Photocatalytic properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/88—Handling or mounting catalysts
    • B01D53/885—Devices in general for catalytic purification of waste gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/40—Catalysts, in general, characterised by their form or physical properties characterised by dimensions, e.g. grain size
    • B01J35/45—Nanoparticles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/70—Catalysts, in general, characterised by their form or physical properties characterised by their crystalline properties, e.g. semi-crystalline
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/70—Catalysts, in general, characterised by their form or physical properties characterised by their crystalline properties, e.g. semi-crystalline
    • B01J35/77—Compounds characterised by their crystallite size
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/30—Treatment of water, waste water, or sewage by irradiation
    • C02F1/32—Treatment of water, waste water, or sewage by irradiation with ultraviolet light
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/72—Treatment of water, waste water, or sewage by oxidation
    • C02F1/725—Treatment of water, waste water, or sewage by oxidation by catalytic oxidation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/80—Type of catalytic reaction
    • B01D2255/802—Photocatalytic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/66—Pore distribution
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2305/00—Use of specific compounds during water treatment
    • C02F2305/10—Photocatalysts

Definitions

  • the present invention relates generally to purification devices having photocatalysts. More specifically, the present invention relates to air purification devices having deactivation resistant photocatalysts.
  • Photocatalytic Oxidation is a technology used for elimination or reduction of the level of contaminants in a fluid, like air or water, using the chemical action of light.
  • UV ultraviolet
  • UV-PCO Ultraviolet Photocatalytic Oxidation
  • Semiconductors have a sufficiently wide band gap energetic enough to activate water or surface hydroxyls thus creating .OH radicals and electrons have been used in purification systems for elimination of organic contaminants.
  • These materials include, but are not limited to, titanium dioxide (TiO 2 ), zirconium dioxide (ZrO 2 ), zinc oxide (ZnO), calcium titanate (CaTiO 3 ), tin (stannic) dioxide (SnO 2 ), molybdenum trioxide (MoO 3 ), and the like.
  • titanium dioxide (TiO 2 ) is among the most widely-used of the semiconductor photocatalysts because of its chemical stability, relatively low cost, and an electronic band gap that is suitable for photoactivation by UV light.
  • Buildings, vehicles, aircraft, ships and the like may utilize air purification systems to improve the quality of indoor air thus enabling decreased ventilation, create an improved environment, or both.
  • the quality of indoor air is achieved through air purification using either aerosol removal or gaseous contaminant removal technologies.
  • photocatalysis is a proven technology that provides for the removal of gaseous airborne substances such as volatile organic compounds (hereinafter “VOCs”) including toluene and formaldehyde from the air supply.
  • VOCs volatile organic compounds
  • Photocatalytic air purifiers utilize a substrate or cartridge containing a photocatalyst, usually a titanium oxide based material, that interacts with airborne oxygen and water molecules to form hydroxyl radicals when placed under an appropriate light source, typically an ultraviolet (hereinafter “UV”) light source.
  • UV ultraviolet
  • the hydroxide radicals attack the contaminants thereby initiating oxidation reactions that convert the contaminants into less harmful compounds, such as water and carbon dioxide.
  • Titanium dioxide is the most stable oxide form of the transition metal titanium.
  • TiO 2 is mostly ionic material composed of Ti +4 cations and O ⁇ 2 anions.
  • powder form TiO 2 is white and is widely-used in industry to give whiteness to paint, paper, textiles, inks, plastics, toothpaste, and cosmetics.
  • crystalline form TiO 2 principally exists as one of three different polymorphic forms: rutile, anatase, and brookite.
  • the two more common polymorphic forms of TiO 2 , rutile and anatase have a tetragonal crystal structure, while the less-common brookite form of TiO 2 has an orthorhombic crystal structure.
  • the anatase form of TiO 2 which is a low temperature form, has been reported to have the greatest photocatalytic activity of the three polymorphic forms of TiO 2 when exposed to UV light. This may be due to a wider optical absorption gap and a smaller electron effective mass in the anatase form that leads to higher mobility of the charge carriers. Anatase is converted to rutile at temperatures above about 600° C. where it is accompanied by crystallite growth and a significant loss of surface area.
  • the rutile and anatase crystalline structures each have six atoms per unit cell.
  • the anatase form is a body-centered structure and its conventional cell contains two unit cells (i.e., 12 atoms).
  • titanium atoms are arranged in the crystal structure in such a way that neighboring octahedral units share edges and corners with each other.
  • four edges of every octahedral unit are shared edges, as compared within the rutile structure, in which two edges of every octahedral unit are shared edges.
  • Degussa Aeroxide TiO 2 P25 (Degussa Technical Information TI 1243, Titanium Dioxide P25 as Photocatalyst, March, 2002, Degussa Corporation; Business Line AEROSIL, Parsippany, N.J. 07054) consists of about 80% by weight 20 nm anatase TiO 2 crystals and 20% by weight larger, about 40 nm, rutile crystals. On exposure to UV light, electron hole separation can occur. Anatase with a strap gap of 3.20 eV requires higher energy, 385 mm photon, than rutile, 2.95 eV or 420 nm.
  • the hole at the surface takes the form of a hydroxyl radical (.OH) that is a stronger oxidizing agent than ozone or chlorine.
  • .OH hydroxyl radical
  • the electron on the surface can form active oxygen species through the reduction of dioxygen, perhaps through the formation of superoxide ion, O 2 ⁇ and then by its further reduction to peroxide dianion, O 2 ⁇ 2 than can on protonation yield hydrogen peroxide.
  • Hydrogen peroxide is believed to be the principal agent of remote photocatalytic oxidation (PCO), which describes the oxidation of substances that are very close to, but not in direct physical contact with, photoactive TiO 2 .
  • PCO remote photocatalytic oxidation
  • P25 crystallites have an average crystallite size of about 20 nm and a BET surface area of about 50 m 2 /gram.
  • BET stands for the well known method of Brunauer, Emmett, and Teller, (J.A.C.S. 60 (1938) 309.) surface science to calculate surface areas of solids by physical adsorption of gas molecules. This has been automated to a certain degree by instruments like the Micromeritics® 2010.
  • Table 1 provides a comparison of average crystallite size with various measures of surface area, including the anatase and rutile forms of TiO 2 .
  • Deactivation of the photocatalyst limits the effectiveness of photocatalytic air purifiers, and can occur reversibly or irreversibly.
  • the photocatalysts in air purification systems become deactivated, the systems become less efficient. Maintenance is required in order to clean, repair, and replace equipment. This results in increased operating expenses associated with the air purification systems.
  • the present disclosure provides a purification device having deactivation resistant photocatalysts and deactivation resistant photocatalysts.
  • an air purification device having a porous photocatalyst for removing at least a portion of gaseous volatile organic compounds from an air stream in the presence of light.
  • a method of purifying an air stream includes passing the air stream over a photocatalyst sufficient to oxidize at least a portion of the volatile organic compounds in the air stream.
  • FIG. 1 is an air treatment device.
  • FIG. 2 is an illustration of a laboratory flat plate Intrinsic Rate Reactor (IRR).
  • IRR Intrinsic Rate Reactor
  • FIG. 3 illustrates the longevity of various TiO 2 based photocatalysts in the presence of 90 ppb hexamethyldisiloxane.
  • FIG. 4 illustrates the distribution profile of pore sizes for photocatalysts of the present disclosure as compared with other photocatalysts.
  • FIG. 5 illustrates effects of hexamethyldisiloxane concentrations on the deactivation rate of siloxane-resistant catalyst 2UV 27.
  • siloxanes arise primarily from the use of certain aerosol-based personal care products, such as hairspray, or dry cleaning fluids.
  • siloxanes can also be generated through the use of room temperature vulcanization (RTV) silicone caulks, adhesives, and the like.
  • RTV room temperature vulcanization
  • the deactivation of photocatalysts by such siloxanes can occur through a number of mechanisms such as, but not limited to, the direct physical blockage of the active sites of the photocatalysts and/or by preventing the VOCs from interacting with the active agent.
  • the photocatalyst is titanium dioxide, including suitably doped titanium dioxide TiO 2 supporting about a monolayer of another material like tungsten oxide or nanosized metal crystallites, as well as zinc oxide, tin oxide or other photocatalytic materials.
  • the present disclosure also contemplates the use of photocatalytic mixed metal oxides, an intimate mixture of nano-crystalline photocatalytic oxides and other oxides, such as, but not limited to titanium dioxide, zinc oxide or tin oxide.
  • titania photocatalysts such as Degussa P25 (Deanna C. Hurum, Alexander G. Agrios, and Kimberly A. Gray, J. Phys Chem. B, 107 (2003) 45454549) can be deactivated by certain airborne contaminants that upon oxidation leave a non-volatile deposit on the catalyst surface.
  • silicon compounds like siloxanes are most prevalent of these materials.
  • the subject of the present disclosure are photocatalysts rendered deactivation resistant by their porous morphology.
  • the photocatalysts have a pore structure with low mass transfer resistance and resists blockage by deposits.
  • This pore structure preferably comprised of cylindrical pores, having the majority of the surface area with pores that are 5 nm in diameter or larger and at least 200 m 2 surface area/cm 3 of skeletal volume of the aggregate photocatalyst has pores that are 6 nm in diameter or larger.
  • the overall distribution of pore size in the aggregate photocatalyst has a mode of 10 nm or greater, where mode is used to mean the most frequently occurring number or size in a set. This pore structure results in photocatalysts that are resistant to deactivation by environmental contaminants such as siloxane.
  • the porosity or pore structure of the photocatalyst can be characterized by its BET (Stephen Brunauer, P. H. Emmett, and Edward Teller, Journal of the American Chemical Society, Vol. 60, 1938, PP 309-319.) surface area, SA, and pore size distribution (PSD). These can be determined using the Micromeritics® ASAP 2010 instrument or its equivalent with its accompanying software packages that included BJH (Barrett, Joyner and Halenda, 1951) analysis for mesopore adsorption and pore size distribution. It is preferred that a mode of this pore size distribution is 10 nm or larger as illustrated in FIG. 4 .
  • the photocatalyst of the present disclosure shows that surprisingly the rate of activity loss expressed as % of initial single pass efficiency lost per hour does not decrease with an increase in BET SA as might be expected. Also, the rate of activity loss does not correlate with the surface area in pores smaller than 4 nm. However the rate of activity loss decreases, that is, the life expectancy of the catalyst increases with the SA in pores greater than or about equal to 6 nm in diameter.
  • the purification device, 20 comprises a filter 22 , a photocatalyst 24 , and a UV lamp 26 .
  • Filter 22 removes particulates and optionally has adsorption properties with a preference for siloxanes.
  • the deactivation resistant photocatalyst 24 has crystallites of less than 14 nanometers (nm) in diameter with at least 200 m 2 surface area/cm 3 of skeletal volume in cylindrical pores of 5 nm in diameter or larger, with the mode of the pore size distribution 10 nm or more.
  • a laboratory flat plate intrinsic rate reactor 8 there is provided a laboratory flat plate intrinsic rate reactor 8 .
  • the reactor 8 has a VOC supply 1 and a VOC mass flow controller 2 .
  • the reactor 8 has a nitrogen supply 3 that feeds in to a water bubbler 4 , and then to a moist nitrogen mass flow controller 5 .
  • Reactor 8 also has an oxygen supply 6 and oxygen mass flow controller 7 .
  • Reactor 8 has a machined aluminum block 9 , which has a bed 10 for the catalyst-coated slide 11 .
  • Reactor 8 has glass beads 12 , 13 , that serve to mix and distribute gas.
  • a UV transparent window 17 is positioned above the catalyst coated slide 11 to seal the reactor.
  • the gas atmosphere within the reactor 8 is analyzed by gas analyzer 14 .
  • the reactor has an exit gas flow meter (not shown).
  • Reactor 8 has a first UV-A lamp 18 and a second UV-A lamp 19 . The height of the lamps may be adjusted by the lamp height adjustment 16
  • Exemplary embodiments of the nanocrystalline TiO 2 having a high surface area and large pore structure according to the present disclosure were tested and compared for deactivation rates to Degussa P25 TiO 2 , and the results are provided in Example 1 below.
  • the conventional BET-specific surface area measurement units of m 2 /g are used for convenience.
  • 1′′ by 3′′ slides were coated with an aqueous suspension of nanocrystalline TiO 2 and allowed to dry.
  • the TiO 2 coating was sufficient to absorb about 100% of the incident light when used in the intrinsic rate reactor according to FIG. 2 .
  • This reactor is a flat plate photocatalytic reactor having UV illumination that is provided by two black-light lamps (SpectroLine XX-15A).
  • the spectral distribution was symmetrical about a peak intensity located at about 352 nm and extended from 300 nm to 400 nm.
  • the illumination intensity was varied by adjusting the distance between the lamp and the titania-coated slide. UV intensity at the reactor surface was measured by a UVA power meter.
  • High-purity nitrogen gas passed through a water bubbler to set the desired humidity level.
  • the contaminants were generated either from a compressed gas cylinder, such as propanal/N 2 , or from a temperature controlled bubbler.
  • An oxygen gas flow was then combined with the nitrogen and contaminant flows to produce the desired carrier gas mixture (15% oxygen, 85% nitrogen).
  • the titania-coated slides were placed in a well, measuring 1′′ by 18′′ that was milled from an aluminum block.
  • the well was then covered by a quartz window that was about 96% UVA transparent. Gaskets between the quartz window and aluminum block created a flow passage above the titania-coated slides.
  • the flow passage had a 1′′ width and a 2 mm height.
  • Contaminated gas entered the reactor by first passing through a bed of glass mixing beads. Next, the gas flow entered a 1′′ by 2 mm entrance region of sufficient length (3′′) to produce a fully-developed laminar velocity profile. The gas flow then passed over the surface of the titania-coated slides. Finally, the gas passed through a 1′′ by 2 mm exit region (3′′ long) and the second bed of glass beads before exiting the reactor.
  • the longevity of various TiO 2 based photocatalysts was determined in the presence of 90 ppb hexamethyldisiloxane, using the intrinsic rate reactor of FIG. 2 .
  • the deactivation rate of the photocatalyst was determined by the slope of a straight line that represents the catalyst performance during its initial stages of operation.
  • the value for P25 represents the average of several tests.
  • the rate of photocatalytic activity loss decreases as the surface area in pores greater than or about equal to 6 nm becomes larger.
  • this linear relationship does not hold with the total BET surface area, or the surface area in pores greater than about 4 nm in diameter, as determined by N 2 adsorption and BJH analysis of this adsorption as performed by a Micrometrics® ASAP 2010 surface area determination unit.
  • the distribution of pore sizes for photocatalysts P25, UV139, and UV114 are shown as the relation of pore diameter, in nm (X-axis) and Specific Surface Area, in m 2 /g (Y-axis).
  • the photocatalysts with the lowest deactivation rates not only possess increased surface area in pores of greater than about 6 nm, but also the mode (i.e., most prevalent) pore size is about 10 nm or greater, and may be bimodal, as shown by the graph of pore size for UV114.
  • UV114 which has about 4.2 times the surface area in pores greater than about 6 nm as compared with P25, has a projected life that is at least 6 times longer than P25 when challenged by hexamethyldisiloxane at a concentration of 90 ppb, under the same UV illumination. Extrapolating these data to a time-averaged concentration of 2 ppb of siloxanes, and assuming that the deactivation rate is linear with respect to concentration of contaminants, UV114 should retain at least 20% of its initial activity after about 10,000 hours, while P25 would be projected to lose about 80% of its initial activity after only about 1,700 hours, under the same challenge of siloxanes. It is important to note that the catalyst with the highest total BET surface area, 2UV27 does not have the lowest deactivation rate.
  • Example 1 For Example 1, 1 ppm propanal was oxidized by UV-A light at 50% relative humidity, under conditions where about 20% of the propanal was initially oxidized.
  • the deactivation agent was 90 parts per billion (ppb) hexamethyldisiloxane.
  • the photocatalytic deactivation rate is proportional to the siloxane concentration
  • the activity of P25 in the presence of 90 ppb hexamethyldisiloxane, would be expected to drop to about 50% of its initial activity in about 24 hours.
  • the photocatalytic activity of P25 would be expected to drop to about 50% of its initial activity in 90 days.
  • the photocatalytic activity of UV114 would be expected to drop to about 50% of its initial activity after 550 days in the presence of 1 ppb of hexamethylsiloxane.
  • FIG. 5 illustrates the results of an experiment showing the effect of various hexamethyldisiloxane concentrations on the deactivation rate of a siloxane-resistant catalyst, 2UV27.
  • the abscissa, siloxane exposure time was normalized to a selected hexamethyldisiloxane level (90 ppb).
  • the linear scaling factor was equal to the exposure time multiplied by the hexamethyldisiloxane concentration divided by 90.
  • Each catalyst was exposed to a controlled level of hexamethyldisiloxane for various periods of time. Periodically, the photocatalytic activity, and hence the rate of deactivation, was determined at various times, using propanal as the probe gas.
  • the photocatalyst life increased by a factor of about 1.2 (ratio of normalized exposure time) over the linear increase corresponding to the ratio of hexamethyldisiloxane concentration (i.e., 2.65 equals 90 divided by 34), for a net increase in life of 3.18 times (i.e., 1.2 ⁇ 2.65).
  • ratio of normalized exposure time a factor of normalized exposure time
  • the inference from such data is that lowering the hexamethyldisiloxane concentration, as by using an adsorbent filter, for example, would result in a non-linear increase in photocatalyst life.

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20130180932A1 (en) * 2012-01-12 2013-07-18 Nitto Denko Corporation Transparent Photocatalyst Coating
US10071361B2 (en) * 2013-11-02 2018-09-11 Dräger Safety AG & Co. KGaA Filter material for the selective removal of siloxanes
US10537870B2 (en) * 2012-02-01 2020-01-21 Torrey Hills Technologies, Llc Methane conversion device

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Publication number Priority date Publication date Assignee Title
EP2043761A4 (fr) 2006-06-01 2011-11-30 Carrier Corp Photocatalyseurs résistant à la désactivation
WO2007143041A2 (fr) 2006-06-01 2007-12-13 Carrier Corporation Systèmes et procédés d'élimination de contaminants présents dans des courants de fluides
US9370167B2 (en) * 2012-10-18 2016-06-21 Otomik Products, Inc. Pet toy with squeaker mechanism

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