Evaluation of Photocatalysis for Gas-Phase Air Cleaning— Part 1 ...

Evaluation of Photocatalysis forGas-Phase Air Cleaning—Part 1: Process, Technical,and Sizing ConsiderationsDean T. Tompkins, PhD, PE Benjamin J. LawnickiMember ASHRAE

Walter A. Zeltner, PhD Marc A. Anderson, PhD

4791 (RP-1134)

©2005, American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. (www.ashrae.org). Reprinted by permission from ASHRAETransactions, Volume 111, Part 2. The material may not be copied nor distrib-uted in either paper or digital form without ASHRAE’s permission.

ABSTRACT

The results of a literature survey and an engineering anal-ysis are presented that evaluate the process, technical, andsizing considerations of photocatalytic oxidation (PCO) as acommercial technology to treat (i.e., remove) low-levelcontaminants in feed streams of air. PCO uses the energy ofphotons from light sources to activate a catalyst. Upon acti-vation, adsorbed gases, particularly molecular oxygen (O2),water vapor (H2O), and contaminant species, can participatein surface-mediated reactions that, under appropriate oper-ating conditions, can produce and desorb product species,notably carbon dioxide (CO2) and H2O. Approximately 60organic compounds have been studied in heterogeneous gas-phase PCO, while only a few inorganic compounds have beenstudied. With regard to commercial-scale applications, thepresent challenge is to design photocatalytic treatment devicesthat have low pressure drop, make efficient use of light, andemploy a stable catalyst that can be readily regenerated if itbecomes poisoned or deactivated. Presently, gas-phase heter-ogeneous photocatalysis will likely find application wherein(1) the gas-phase contaminants have been identified in a feedstream and demonstrated to be removed when the feed ispassed through the PCO device and (2) reaction by-productshave been studied and not found to be of concern.

INTRODUCTION

Over the past quarter century, construction practices havefavored the design of tight buildings and residences to reduceenergy costs associated with the conditioning of building air.However, in the absence of sufficient ventilation to removevolatile organic compounds (VOCs), it is possible that theirconcentration can increase if sources that generate VOCs are

present (e.g., new carpet, smoking, cooking, etc.). Thisconcern is exacerbated due to the significant percentage oftime people spend in indoor environments. Indoor air contam-ination by undesired VOCs is a possible causative agent of thesick building syndrome. The 1989 OSHA workplace time-weighted average permissible exposure limits (TWA-PEL) forexposure to VOCs during a five-day work week are in therange of 1-500 ppm in air (Plog 1988).

One approach to achieve air that contains lower concen-trations of gas-phase contaminants is to employ ultraviolet(UV) catalytic technology or PCO. PCO has gained attentionin recent years as an approach to treat gaseous pollutants. Theoxidant used in PCO is the 20% of oxygen present in air.Because the concentration of oxygen in air is so much largerthan the (total) concentration of gaseous indoor air pollutants,it is not necessary to employ additional oxidants such as ozone(O3) or hydrogen peroxide (H2O2).

In this paper, the results of a literature survey and an engi-neering analysis are presented that evaluate the process, tech-nical, and sizing considerations of PCO as a technology totreat contaminants in feed streams of air.

PHOTOCATALYTIC OXIDATION

Semiconductor materials provide solid surfaces that caninfluence both the chemical reactivity of a wide range ofadsorbates and the ability to initiate, propagate, and terminatelight-induced oxidation-reduction (redox) reactions, or PCO.Upon photoexcitation of several semiconductors, simulta-neous redox reactions occur. The conversion often accom-plishes either a specific, selective oxidation or a completeoxidative degradation of an organic reactant in which thecarbon bound in the reactant species is converted to carbon

60 ©2005 ASHRAE.

D.T. Tompkins and W.A. Zeltner are associate scientists at the Water Science and Engineering Laboratory and M.A. Anderson is a professorin the Department of Civil and Environmental Engineering, University of Wisconsin—Madison. B.J. Lawnicki is a process engineer, IntelMassachusetts, Hudson, Mass.

dioxide. Molecular oxygen (O2) is typically the oxidizingagent as has been demonstrated unmistakably in a few gas/solid reactions. Incident photons from light sources thatinitiate these reactions are in a wavelength region varyingfrom the visible (~500 nm) to high-energy range of the UVspectrum (~250 nm), depending upon the band-gap of thephotocatalyst. The semiconductor material that acts as thephotocatalyst is often stable to the photolysis conditions,particularly when a metal oxide is employed. Though photo-catalytic reactions can occur in both gas and liquid phases, thefocus of this paper is on those reactions where gas-phasespecies are adsorbed and then react on solid surfaces, referredto as heterogeneous (gas/solid) photocatalysis (Serpone andPelizzetti 1989).

Interest in photoinduced redox reactions originated withthe observations of Fujishima and Honda (1972). Theydemonstrated that water could be split via a redox reactionupon illuminating a single-crystal semiconductor electrodemade of titanium dioxide (TiO2) to which a small electro-chemical bias had been applied. These observations promptedextensive work focusing on the production of hydrogen fromwater as a means of solar energy conversion. It soon becameapparent that novel redox reactions of organic and other inor-ganic substrates (Fox 1983, 1991; Fox et al. 1991; Fox andDulay 1993) could also be induced by band-gap irradiation ofa variety of semiconductor materials varying in size from clus-ters and colloids to powders and large single crystals. Scien-tific interest in these chemical redox reactions has alsodeveloped within the last two decades because of the use ofphotoexcited semiconductor dispersions in chemistries thatcan find application in environmental protection (Alpert et al.1991, Bahnemann et al. 1991; Blake et al. 1991; Matthews1987; Ollis 1985; Schiavello 1988).

BASIS OF PHOTOCATALYSIS

During the past quarter century, increasing research focushas been directed to the study of reactions on the illuminatedsemiconductor surface of metal oxides and sulfides consistingmainly of titanium dioxide (TiO2), zinc oxide (ZnO), zirco-nium dioxide (ZrO2), tungsten tri-oxide (WO3), and cadmiumsulfide (CdS). These materials have a moderate band-gap(1-3.3 eV) between their valence and conduction bands. Whenthe semiconductor is illuminated by photons of energy hvequal to or greater than the band-gap Eg, the semiconductorabsorbs the photons. This process excites the valence-band(VB) electrons to the conduction band (CB), creating highlyreactive electron ( )-hole ( ) charge carriers. Whenthese charge carriers successfully migrate to the solid surfacewithout recombining, the electrons and holes may undergoelectron-transfer processes with adsorbates of suitable redoxpotentials (Figure 1). Adsorbed water can react with holes toform highly reactive hydroxyl radicals ( ), which, in turn,can oxidize organic compounds. Under certain reaction condi-tions, organic reactants can be completely oxidized (or miner-alized) to form carbon dioxide, water, etc., as final products.

The equations below describe a possible reaction pathway.The steps are represented as a series of reactions using TiO2 asthe semiconductor:

eCB–

hVB+

OH•

Figure 1 Steps in photoelectrochemical mechanisms insolid semiconductor: [1] light energy hv greaterthan band gap energy Eg [2] excites electron fromvalence band to conduction band; [3] valence-band hole that successfully migrates to surfaceinitiates oxidative pathway, [4] conduction-bandelectron that successfully migrates to surfaceinitiates reduction reaction, [5] valence-bandhole and conduction-band electron recombine inthe bulk material, [6] valence-band hole andconduction-band electron recombine on thesurface; [7] trapping of valence-band hole at asurficial titanol group, [8] trapping ofconduction-band electron in surficial bond.Adapted from Linsebigler et al. (1995).

ASHRAE Transactions: Research 61

where ads = species adsorbed on semiconductor. A moredetailed description of these processes along with character-istic times for charge-carrier generation (fs), charge-carriertrapping (100 ps to 10 ns), charge-carrier recombination (10 to100 ns), and interfacial charge transfer (100 ns to ms) isreported in Hoffman et al. (1995). The quantum efficiency isdetermined by two critical processes—competition betweencharge-carrier recombination and trapping (picoseconds tonanoseconds) followed by the competition between recombi-nation of trapped charge carriers and interfacial charge trans-fer of charge carriers to reactants (since H2O and O2 arereactants) (microseconds to milliseconds) (Hoffmann et al.1995).

The first published report of photoreactivity was made byRenz (1921). However, heterogeneous photocatalysis studiesonly began in earnest in 1971 with the work of Teichner’sgroup (Gravelle et al. 1971; Formenti et al. 1971) and Stone’sgroup (Bickley et al. 1973). These studies provided the earlyvision for numerous potential applications of heterogeneousPCO. Studies involving gas-phase heterogeneous PCO arerelatively few in number compared with the published studiesof the photocatalytic treatment of compounds in aqueousmedia (Matthews 1982; Ollis et al. 1991; Serpone andPelizzetti 1989). However, there is increasing interest in gas-solid heterogeneous PCO because of the potential applicationto contaminant control in indoor environments, including resi-dences, office buildings, factories, aircraft, and spacecraft.The promise held by heterogeneous PCO lies in the moderateoperating conditions required to initiate redox reactions. Thesemiconductors mentioned above have proven to be effectivephotocatalysts for performing surface-mediated reactions inwhich many organics are converted to CO2 and H2O (Peral etal. 1997). For example, the two predominant photocatalystcompounds in the literature, TiO2 and ZnO, operate under thefollowing favorable circumstances (Peral et al. 1997):

1. Photoactivation by near UV (wavelength [λ] range is~300-370 nm) illumination, which allows for considerationof solar as well as artificial photon sources such as fluores-cent lights.

2. The catalyst materials are generally safe—TiO2 is found intoothpastes and house paints, and both TiO2 and ZnO areused as a sunblock agent.

3. The oxygen source is O2 in air, which is present in enor-mous stoichiometric excess (210,000 ppm) compared with1-500 ppm oxidizable contaminants for lightly contami-nated air.

4. Catalyst activity appears to be maintained at water vaporconcentrations that are typical of humidified indoor envi-ronments.

PCO OF GAS-PHASE ORGANIC COMPOUNDS

Approximately 60 organic compounds have been studiedin heterogeneous gas-phase PCO. Forty-three of the moresalient organic compounds of concern in indoor environments

are listed below along with published PCO scientific investi-gations of them.

1. 1,1,1-trichloroethane (CHCl2CH2Cl) – d’Hennezel andOllis (1997); d’Hennezel (1998); Isidorov et al. (1997)

2. 1,3-butadiene (H2C:CHHC:CH2) – Obee and Brown(1995)

3. 1,4-dioxane (OCH2CH2OCH2CH2) – d’Hennezel andOllis (1997); d’Hennzel (1998)

4. 1-butanol (CH3(CH2)3OH)– Peral and Ollis (1992); Blakeand Griffin (1988)

5. 1-butene (C4H8) – Cao et al. (1999)

6. 2-hexene (C6H12) – Ohno et al. (1998)

7. 2-propanol (isopropanol, C3H8O) – Ait-Ichou et al. (1985);Alberici and Jardim (1997); Bickley et al. (1973); Cunning-ham and Hodnett (1981); Larson et al. (1995); Ohko et al.(1997, 1998a); Wentworth and Chen (1994)

8. acetaldehyde (CH3CHO) – d’Hennezel and Ollis (1997);Obuchi et al. (1999, 1998b); Sauer and Ollis (1996); Shifuet al. (1998); Sopyan et al. (1994, 1996a)

9. acetic acid (CH3CO2H) – Muggli and Falconer (1999);Sclafani et al. (1988)

10. acetone (CH3COCH3) – Alberici and Jardim (1997); Peraland Ollis (1992); Sauer and Ollis (1994); Shifu et al. (1998);Vorontsov et al. (1997b, 1999); Yu et al. (1998); Zorn et al.(1999)

11. benzene (C6H6) – Atkinson and Schmann (1989);d’Hennezel and Ollis (1996, 1997); d’Hennezel et al.(1998); Einaga et al. (1999); Fu et al. (1995); Jacoby et al.(1996); Larson and Falconer (1997); Sauer et al. (1995);Seuwen and Warneck (1994); Sitkiewitz and Heller (1996)

12. butyraldehyde (CH3(CH2)2CHO) – Peral and Ollis (1992)

13. carbon dioxide (CO2) – Anpo et al. (1995) and Kaneco(1999) reported on the “reduction” of CO2

14. carbon monoxide (CO) – Anderson et al. (1996); Linsebig-ler et al. (1996b); Vorontsov et al. (1997a, 1998b)

15. chloroform (CHCl3) – Alberici and Jardim (1997); Albericiet al. (1998)

16. dichloromethane (CH2Cl2) – Alberici et al. (1998)

17. diethyl ether ((C2H5)2O) – Vorontsov et al. (1997b)

18. dimethoxymethane (C3H8O2) – Alberici and Jardim (1997)

19. dimethylmethylphosphonate (DMMP, (CH3)2CH3PO2) –Obee and Satyapal (1998)

20. ethanol (C2H5OH) – Cunningham et al. (1974); Kennedyand Datye (1998); Muggli et al. (1996, 1998); Nimlos et al.(1996); Sauer and Ollis (1996); Vorontsov et al. (1997b)

21. ethylene (H2C:CH2) – Fu et al. (1996a); Obee and Hay(1997); Sirisuk et al. (1999); Yamazaki et al. (1999); Zornet al. (2000)

22. formaldehyde (HCHO) – Noguchi et al. (1998); Obee(1996); Obee and Brown (1995); Peral and Ollis (1992)

62 ASHRAE Transactions: Research

23. iso-octane ((CH3)2CH(CH2)4CH3) – Alberici and Jardim(1997)

24. isopropyl alcohol (IPA, (CH3)2CHOH) – Brinkley andEngel (1998a, 1998b)

25. isopropylbenzene (C9H12) – Alberici and Jardim (1997)

26. methane (CH4) – Dreyer et al. (1997); Okabe et al. (1997)

27. methanol (MeOH, CH3OH) – Alberici and Jardim (1997);d’Hennezel and Ollis (1997); Liu et al. (1985)

28. methyl acrylate (CH2:CHCOOCH3) – d’Hennezel andOllis (1997)

29. methyl chloroform (C2H3Cl3) – Alberici and Jardim (1997)

30. methyl ethyl ketone (MEK, CH3COCH2CH3) – Albericiand Jardim (1997); d’Hennezel and Ollis (1997)

31. methyl isopropyl ketone (C5H10O) – Alberici and Jardim(1997)

32. methyl tert-butyl ether (MTBE, (CH3)3COCH) – d’Henne-zel and Ollis (1997)

33. methylene chloride (CH2Cl2) – Alberici and Jardim (1997);d’Hennezel and Ollis (1997)

34. m-xylene (C8H10) – Peral and Ollis (1992)

35. propene (C3H6) – Pichat et al. (1979)

36. propionaldehyde (C2H5CHO) – Takeda et al. (1995, 1997)

37. pyridine (C5H5N) – Alberici and Jardim (1997); Sampath etal. (1994)

38. t-butyl methyl ether (C5H12O) – Alberici and Jardim (1997)

39. tetrachloroethylene (PCE, Cl2C:CCl2) – Alberici et al.(1998); Alberici and Jardim (1997); Hung and Yuan (1998);Li et al. (1998); Yamazaki-Nishida et al. (1996)

40. toluene (C6H5CH3) – Blanco et al. (1996); d’Hennezel et al.(1998); Ibrahim and de Lasa (1999); Ibusuki and Takeuchi(1986); Li et al. (1998); Luo and Ollis (1996); Méndez-Román and Cardona-Martínez (1998); Obee (1996); Obeeand Brown (1995)

41. trichloroethylene (TCE, CHCl:CCl2) – Alberici et al.(1998); Alberici and Jardim (1997); Anderson et al. (1993);Annapragada et al. (1997); Buechler et al. (1999); Driessenand Grassian (1998); Driessen et al. (1998a, 1998b); Dibbleand Raupp (1990, 1992); Hung and Mariñas (1997a,1997b); Hwang et al. (1998); Jacoby et al. (1994); Kim et al.(1996, 1998); Liu et al. (1997); Luo and Ollis (1996);Nimlos et al. (1993); Phillips and Raupp (1992); Wang et al.(1998a, 1998b); Wang and Hsieh (1998); Yamazaki-Nishida et al. (1993, 1995)

42. vinyl acetate (CH3COOCH:CH2) – d’Hennezel and Ollis(1997)

43. xylene (C8H10) –Blanco et al. (1996); d’Hennezel and Ollis(1997)

Other—investigations of multiple VOCs—Lichtin et al.(1996); Obee and Hay (1999)

Table 1 lists some organic compounds that have beenstudied in photocatalysis.

Table 1. Photocatalytic Investigations: Some Organic Compounds of Interest in Indoor Air Applications


PCO OF SELECT ORGANIC COMPOUNDS

Formaldehyde and Acetaldehyde

Noguchi et al. (1998) investigated the photocatalyticdegradation of gaseous formaldehyde and acetaldehyde usinga flat plate (TiO2 thin-film photocatalyst) photoreactor oper-ated in plug flow. The TiO2 thin film was placed on soda limeglass and prepared from an STS-21 suspension (40 wt%anatase TiO2, pH 8.5, 20-nm particle diameter, 50 m2 g-1

surface area) by the spin coating method. After spin coating,the film was calcined at 450°C for 1 h. The thickness of thetranslucent anatase TiO2 film was 1.7 µm. Ultraviolet lightwas provided by an Hg-Xe lamp (Hayashi Tokei, Luminar Ace210) and passed through a 365-nm band-pass filter. Ultravioletlight intensity on the film surface was 1 mW cm-2. Experi-ments were conducted at room temperature (22°C) and a rela-tive humidity of 40%. Reactant concentration was between 30and 2000 ppmv (parts per million by volume). Decompositionwas initiated by UV illumination of TiO2 film. No traces offormaldehyde in the reactor were observed after 80 min ofreaction. The expected stoichiometric CO2 concentration wasgenerated. The Langmuir-Hinshelwood (L-H) modelprovided the quantitative kinetics for the rate of the unimolec-ular surface reaction. The surface rate constant, k, values were0.19 and 0.16 µmol min-1 and the apparent adsorption coeffi-cient, Kapp, values were 0.51 and 0.21 µmol dm-3 for formal-dehyde and acetaldehyde, respectively. These results suggestlittle difference in the k values that directly correspond to thereactivity of each compound. However, the Kapp values, whichare related to the adsorption strength of each compound ontoTiO2, are significantly different—formaldehyde affinity forTiO2 surface sites was ca. 2.5 times larger than that of acetal-dehyde. This suggests that the differences in decompositionrates can be attributed, in part, to the differences in adsorptionstrength. Here, in a mid-range of reactant concentrations (30-2,000 ppmv), the reaction kinetics are mass transfer limited.

PCO OF SELECT GAS-PHASEINORGANIC COMPOUNDS

In contrast to the number of heterogeneous PCO investi-gations with organic compounds, only a few inorganic gas-phase compounds have been studied with PCO. These studiesinclude:

1. ammonia (NH3) – Mozzanega et al. (1979); Toshiaki et al.(1997, 1998a, 1998b); Cant and Cole (1992)

2. hydrogen sulfide (H2S) – Canela et al. (1998)

3. nitrogen oxides (NOx, N2O) – Anpo et al. (1991, 1997),Hoshi et al. (1998); Ichihashi et al. (1997); Kudo andNagayoshi (1998); Murata et al. (1998); Nishikata (1997);Wang et al. (1997); Cant and Cole (1992)

4. ozone (O3) – Ohtani et al. (1992); Ohtani et al. (1993)

5. sulfur oxides (SOx) – Fukumori (1998)

It is readily apparent that a significantly greater number ofexperiments have been performed and reported on the PCO oforganic compounds (those containing carbon and hydrogenatoms and sometimes oxygen) than inorganic compounds. Aprimary reason for this observation is that most of the PCOstudies into the 1990s were focused on treating trace organiccompounds in waste water. Many of these compounds werepresent because of the use of organic solvents (e.g., ketones,alcohols, organic acids, chlorinated hydrocarbons). Then,beginning in the late 1980s and continuing to the present, therehas been an increase in the PCO studies of compounds presentin the gas phase. Second, it has been generally reported thatorganic compounds containing carbon, hydrogen, and oxygencan, under conditions of complete oxidation, form water vaporand carbon dioxide. However, it is also generally reported thatinorganic compounds, in particular those containing Cl, S, andN can form by-products, including acid gases and oxides ofsulfur and nitrogen. In the event of formation of these by-prod-ucts, further treatment (e.g., adsorption, catalytic treatment,etc.) downstream of the PCO device may be warranted.

CATALYST FORMULATIONS

The nature of a photocatalyst is also important in deter-mining the rate and efficiency of PCO. The anatase form oftitanium dioxide has the desirable properties of being chemi-cally stable, readily available, and active as a catalyst forphoto-oxidation processes. The 3.2 eV band gap of TiO2matches the output of a wide variety of readily available arti-ficial light sources or lamps. (High photon efficiencies arereported occasionally, particularly when VOC concentrationsare high [and so not readily applicable to indoor air treatment]and when some chlorinated VOCs are treated. In the lattercase, high photon efficiencies are attributed to the formation ofhighly reactive chlorine radicals during the reaction.) Consid-erable work has been directed toward modifying TiO2 and test-ing other semiconductors to identify methods to increaseprocess efficiency and to improve the overlap of the absorptionspectrum of the photocatalyst with the solar spectrum. Varioustypes of photocatalysts are discussed below.

Titanium Dioxide

Titanium dioxide (TiO2 or titania) and modified forms,including different commercially available forms, heat treatedmaterials, and materials prepared by a range of techniquesincluding sol-gel chemistry (Negishi et al. 1995; Saitoh andFukayama 1996; Zorn et al. 2000) and other methods (Sopyanet al. 1996b) comprise the vast majority of photocatalystknowledge. Titania is largely obtained by either purchasingcommercially available powder forms or by synthesis usingsol-gel chemistry or flame formation techniques. Results fromone investigation suggest a much higher rate of carbon dioxideformation with the sol-gel-derived form versus that obtainedwith the commercially available forms (Kominami et al.2000). Sopyan et al. (1996a) reported higher photocatalyticactivity for a UV-illuminated TiO2 thin film vs. a TiO2 powder


form, as indicated by high total quantum yields. The higheractivity of the sol-gel-derived form of TiO2 photocatalysts isattributed to its high crystallinity and large surface area. Tita-nium dioxide is also one of the most stable photocatalystsbecause it does not photo-corrode, as does CdS.

Metal and Metal Ion Doping ofMetal and Mixed-Metal Oxides

Metal ions have been introduced into the titanium dioxidelattice to modify the properties as presented in the followingarticles: Al (Muradov et al. 1996), Ga (Vergnon et al. 1978),Nb (Vergnon et al. 1978), and W (Muradov et al. 1996). Noblemetals, primarily platinum, have been deposited onto thesurface of titanium dioxide to enhance catalytic activity(Courbon et al. 1981, 1984, 1985; Fu et al. 1995; Linsebigleret al. 1996a).

Table 2 contains a list of some photocatalyst formula-tions. A large percentage of reported investigations usecommercially available forms of particulate catalyst. In somecases, the particulate catalysts are used directly (e.g., fixed- orfluidized-bed reactors). Often the particulate catalyst isdispersed in a liquid and then deposited onto supports—metal,ceramics, monoliths, glass, among others—as a thin film witha thickness varying between 0.1 and 2 µm and sometimesthicker. Other investigators (Zorn et al. 2000) employ sol-gelchemistry to synthesize materials that contain metal oxides asprimary particles (2-10 nm in size) suspended in water or alco-

hol. Thereafter the suspensions can be deposited onto supportsvia spin-coating, dip-coating, or spraying techniques. A widevariety of other semiconductors and other materials have beentested for photocatalytic activity, including:Al2O3 (alumina), Fe2O3 – Casado et al. (1990)V2O5, ZnO, ZrO2 (zirconia), SnO2, Sb2O4, CeO2, WO3 –

Herrmann et al. (1981); Pichat et al. (1979)Sn/SbO2 – Pichat et al. (1979) Poly (p-phenylene) – Toth et al. (1995)SiO2/TiO2 and ZrO2/TiO2 – Fu et al. (1996c)

SUPPORTED PHOTOCATALYSTS

Most experimental work in gas-phase photocatalysis isconducted with the photocatalyst deposited onto appropriatematerials called supports. Titanium dioxide has beensupported by a variety of surfaces including:

• glass (including fibers) – Kim et al. (1996); Trivedik(1994); Zorn et al. (2000)

• glass rings – Zorn et al. (1999, 2000); Sirisuk (2003)• silica (SiO2) – Mune et al. (1996); Sauer and Ollis

(1996); Takeda et al. (1995)• polymer – Takeuchi (1996)• thin films – Kim et al. (1996); Negishi et al. (1995); Sai-

toh and Fukayama (1996)• zeolite, alumina – Takeda et al. (1995) • carbon – Takeda et al. (1995); Ishihara and Furutsuka

(1996)

Table 2. Some Photocatalyst Formulations


• paper – Matsubara et al. (1995)

• metal oxides and ceramics – Sauer and Ollis (1996);Takeuchi (1996)

PCO CONVERSION AND MECHANISMS

Extent of Oxidation

Alberici and Jardim (1997) investigated the gas-phasephotocatalytic destruction of 17 VOCs over illuminated TiO2under plug-flow reactor conditions of 200 ml min-1 flow rate,23% relative humidity, 21% oxygen, and an organiccompound concentration range of 400-600 ppmv. At steadystate, high conversion yields were obtained for trichloroethyl-ene (99.9%), isooctane (98.9%), acetone (98.5%), methanol(97.9%), methyl ethyl ketone (97.1%), t-butyl methyl ether(96.1%), dimethoxymethane (93.9%), methylene chloride(90.4%), methyl isopropyl ketone (88.5%), isopropanol(79.7%), chloroform (69.5%), and tetrachloroethylene(66.6%). Unfortunately, the photodegradation of isopropyl-benzene (30.3%), methyl chloroform (20.5%), and pyridine(15.8%) was not particularly efficient. However, conversion(i.e., removal of target reactant species) was demonstrated forall 17 compounds tested.

Reaction Mechanisms

Numerous individual reactions are involved in this three-step process—initiation, propagation, and termination—which results in rather complex reaction mechanisms beingproposed. Examples of some key conclusions of reactionmechanism studies are provided below.

Toluene

The benzyl radical is suggested as the key species thatleads to formation of benzyl alcohol and benzaldehyde(Blanco et al. 1996; Sitkiewitz and Heller 1996). Toluenedegradation rate is enhanced by the simultaneous presence anddegradation of tricholoethylene (TCE); the increased rate isassumed to be due to the formation of active chlorine radicals(d’Hennezel and Ollis 1997). Prechlorination of the TiO2surface by use of aqueous HCl is shown to enhance the toluenereaction rate; it is proposed that Ti/Cl– surface species can trapphotogenerated holes and degrade some organic pollutantsthrough the production of chlorine radicals (d’Hennezel1998).

Benzene

It has been shown that benzene is less susceptible tophotocatalytic oxidation than toluene (d’Hennezel and Ollis1997). Wallington et al. (1988) have studied chlorine radicaland hydroxyl radical reactivities with toluene and benzene and

found that toluene is 13 times more reactive than benzene withchlorine radicals, but much closer reactivities between tolueneand benzene are observed with only hydroxyl radicals present.The difference could partially explain why the presence ofchlorine does not enhance the degradation rate of benzene(d’Hennezel et al. 1998). D’Hennezel et al. (1998) haveproposed two routes for benzene degradation:

1. (i) Direct hole oxidation followed by reaction of the result-ing radical cation either(ii) with a surface basic OH group, or(iii) with an adsorbed water molecule and subsequentdeprotonation to yield phenol, the major intermediatedetected.

2. radical addition to yield a cyclohexadienyl radical.Addition of oxygen to this radical and elimination of theradical produces phenol by analogy with what hasbeen determined in water.

Formaldehyde

Noguchi et al. (1998) investigated the photocatalyticdegradation of gaseous formaldehyde (and acetaldehyde)using a TiO2 thin film photocatalyst. Although CO2 is the finaloxidation product, intermediates and/or by-products wereextracted from the TiO2 surface. After 75% of the formalde-hyde reacted, the product distribution was CO2 (80%) as a gasand formic acid (HCOOH) (20%), which remained on thesurface as a partial oxidation by-product. Similarly for PCO ofacetaldehyde, the product distribution was CO2 (50%) as a gasand HCOOH (20%), acetic acid (CH3COOH) (25%), andother species (5%) as partial oxidation by-products. Withprolonged UV illumination, all the intermediates and by-prod-ucts were oxidized to CO2. Peral and Ollis (1992) and Aguadoet al. (1994) have described the major oxidative and reductive

OH•

HO2

•


processes in the photodegradation of formaldehyde. Theprocess can be written as

COMPETITIVE ADSORPTION

It is recognized that there will be several gaseous contam-inants present in indoor air environments. Successfully treat-ing all or nearly all these contaminants with PCO reactors willrequire knowledge of the competitive adsorption process.That is, each contaminant, in addition to water and oxygen, ina mixture of several gas-phase species will have its own affin-ity for adsorption to sites on a photocatalytic surface. Obee andBrown (1995) and Obee and Hay (1999) have studied thecompetitive absorption of compounds on the surface of aphotocatalyst. Recall the aforementioned study of Noguchi etal. (1998) where the larger affinity of formaldehyde for TiO2surface sites over that of acetaldehyde resulted in higher reac-tion rates for formaldehyde, suggesting that the differences indecomposition rates can be attributed, in part, to the differ-ences in adsorption strength.

REACTOR DESIGN

Several articles address topics relevant to the design ofreactors for photocatalytic processes including nonconcen-trating reactors (Takeuchi 1995), kinetic modeling (Sauer andOllis 1996), ceramic monoliths (Sauer and Ollis 1996), andfield tests (Ren et al. 1995). In recent years, successful labo-ratory demonstration of heterogeneous PCO for processingwaste streams of gas contaminants has resulted in industrialinterest. There are several configurations of gas-solid hetero-geneous PCO reactors; a few are presented here.

Supported Powder Layer

A feed stream of gas is passed vertically downwardthrough a powdered photocatalyst supported on a porous glassfrit, which is illuminated from above (Figure 2). The reactionrate is reported to increase linearly with the thickness ofpowder layer (0 mm-4 mm) and is independent of catalystdepth greater than 4.5 mm, presumably corresponding tocomplete photon absorption (Courbon et al. 1973). Thepowder layer reactor was employed in the early studies ofTeichner’s group. Other investigators (Blake and Griffin[1988], Dibble and Raupp [1990], Peral and Ollis [1992], andPeral et al. [1997]] have also employed this design.

Annular

A light source located along the centerline illuminates thecatalyst-filled reactor (Figure 3). In studies with acetone it wasobserved that at low catalyst loading the reaction ratedecreases because of insufficient catalyst to capture all thelight, while with high loadings of catalyst, high concentrationslead to excessive opacity. In the latter case, the rate began to

Figure 2 Diagram of powder layer photocatalytic reactor.Design employed by Teichner’s group. Adaptedfrom Formenti et al. (1971).

Figure 3 Annular reactor where do is lamp diameter, Di isreactor inner wall diameter, and Do is reactorouter wall diameter. Design adapted from Ayoub(1986) and Fu et al. (1995).


diminish because an increasing fraction of the reactor flow isoperating with insufficient or no light (Peral et al. 1997).

Fluidized Bed

Fluidized bed reactors pass a gas feed stream verticallythrough a transparent container filled with catalyst (Figure 4),promoting mixing between the gas and catalyst particles. Illu-mination is from the side and there is a particle disengagementsection near the top. Bench-scale laboratory studies with fluid-ized bed photoreactors have been investigated for ammoniaoxidation (Cant and Cole 1992) and photocatalytic destructionof trichloroethylene (TCE) in humidified air (Dibble andRaupp 1992). In the latter study, catalysts suitable for fluidi-zation were prepared via sol-gel impregnation of silica gel(0-63 g TiO2/g silica gel) of 250-450 nm diameter. The appar-ent quantum efficiencies were reported between 2% and 8%,with a maximum of 13%. Dibble and Raupp (1992) cite advan-tages of the fluidized bed configuration to include excellentreactant-catalyst contacting, high volumetric flow rates, andlow pressure drop, the latter attribute being exceedinglyimportant in applications of high throughput.

Honeycomb Monolith

The honeycomb monolith provides a low-pressure dropcatalyst configuration, typically providing pressure drops thatare much lower than those associated with packed or fluidizedbeds (Peral et al. 1997). One challenge for the monolith photo-reactor design is providing sufficient illumination to the cata-lyst. Some practitioners employ end-on illumination (Suzukiet al. 1991). Observations from studies with a photocatalyticmonolith for oxidation of odors demonstrated that the kineticsof disappearance of each of several odor compounds wasnearly first order (Suzuki et al. 1991). Sauer and Ollis (1994)confirmed the observations of Suzuki’s group using an illumi-nated TiO2-coated monolith, where reactor performance forthe oxidation of acetone was modeled sufficiently with Lang-muir-Hinshelwood kinetics. Later, Luo and Ollis (1996)analyzed the behavior of the photocatalytic monolith includ-ing the influence of various flow regimes—developing(entrance region), undeveloped, and developed—on reactorperformance. Using rate parameters for various oxygenates, itappears that appreciable conversions occur for small channelReynolds numbers (1-50); single channel experiments supportthe analysis (Luo and Ollis 1996).

Other Configurations

Since photocatalyst prepared either from sol-gel chemis-try or as suspensions of powders can be deposited on a varietyof surfaces, many other photoreactor geometries and config-urations are possible. The liquid-solid photocatalysis litera-ture cites the application of immobilized photocatalyst onfiberglass mesh, glass beads that are solid (for packed beds) orhollow (for oil slick treatment), coiled glass tubes (for totalorganic carbon analyzers), and within pillared clays and

zeolites. Catalyst-coated optical fibers for liquid or gas treat-ment have been analyzed (Marinangeli and Ollis 1977, 1980,1982; Peill and Hoffmann 1995, 1996). Aguado et al. (1992)describe an experimental device to conduct solid-liquid-gasphotocatalytic studies. Discussion would not be completewithout mentioning the ubiquitous packed-bed designwherein photocatalyst is packed around light sources within areactor housing or shell. (Note: the packed-bed design can bethought of as an annular design.) To date, companies that arecommercializing PCO technology for gas-phase applicationshave generally adapted the packed-bed or thin-film design.

Ibrahim and de Lasa (1999) investigated the photocata-lytic conversion of toluene in a newly designed batch photo-reactor in which TiO2 is supported on a filter mesh with goodcontact with near UV light and air. The photoreactor isdesigned with special features including a venturi sectionand a heated perforated plate. The heated perforated plateminimizes water adsorption on the mesh and, consequently,water effects on the reaction rate. The system performance isexamined for different toluene concentrations and twohumidity levels.

LIGHT SOURCES AND IRRADIANCE

Activating a photocatalyst requires energy from a lightsource that is greater than the band-gap. Some suitable lightsources are listed in Table 3. For example, Noguchi et al.(1998) employed UV light provided by an Hg-Xe lamp and

Figure 4 Fluidized bed PCO reactor. Design employedand adapted by Dibble and Raupp (1992).


passed through a 365-nm band-pass filter. UV light irradianceon the film surface was 1 mW cm-2. Fluorescent light sourceswith the energy needed to activate photocatalysts are those thatemit predominantly in the near-UV or UVA (320-400 nm).Light sources and the electronic and magnetic ballasts thatpower them are practical for use in commercial-scale PCOreactors. Here, the so-called black light (or “BL”) and blacklight blue (or “BLB”) lamps, which are designed to emit in theUVA spectrum, can find application. One can expect that theirradiance (mW cm-2) emitted from a light source will dimin-ish with illumination time. Indeed, manufacturers of fluores-cent light sources often rate a useful life varying between5,000 and 10,000 h. During the continuous operation of a hot-cathode (i.e., low-pressure Hg vapor lamp) fluorescent lightsource, the gradual thinning of the tungsten filament can leadto complete failure of the bulb. The decrease in light irradiancewith illumination time will affect the performance of photo-catalytic reactors.

In general, the rates of photocatalytic reactions depend onthe light irradiance. For illumination by a UV source at irra-diances in excess of one sun equivalent, reaction rates increasein proportion to the square root of the irradiance. (The reactionrate does not increase indefinitely but eventually becomesconstant. Transport limitations in photoreactors that utilizeextremely intense light sources [> 100 suns, as in solar concen-trators] cause the rate to level off [Bahnemann et al. 1994].)For irradiances below one sun equivalent, reaction ratesincrease approximately linearly with the irradiance (Obee andBrown 1995). One sun equivalent is defined at 4.1 × 1015

photons cm-2 s-1 for photons with wavelengths shorter than

385 nm (Raupp and Junio 1993). A linear dependence wasobserved between irradiance and the rate of photocatalyticoxidation of ethylene over titania pellets (Yamazaki et al.1999) and in the photocatalytic decomposition of gaseous 2-propanol (Ohko et al. 1997).

The square root dependence of the reaction rate arises asa consequence of the enhanced rate of recombination of photo-generated electrons and holes that occurs when the photocat-alyst is subjected to high irradiance UV light (Hoffmann et al.1995). Aguado et al. (1994) reported that the photocatalyticdegradation of formic acid over certain types of titaniamembranes obeys a square root dependence on light irradi-ance. Mills and Morris (1993) studied the photomineralizationof 4-chorophenol and reported that the reaction rate is depen-dent on I0.74, a power dependence that lies in the transitionregion between the exponents expected at low irradiance (I1.0)and high irradiance (I0.5).

CATALYST LIFE/FOULING

It has been reported that photocatalysts can be fouledand poisoned or deactivated, thereby decreasing their perfor-mance (Table 4). For example, during the PCO of toluene andbenzene, a yellow discoloration of the catalyst surface wasobserved with the gradual deactivation of the catalyst(d’Hennezel et al. 1998). It has been shown that tolueneoxidizes rapidly over UV-irradiated TiO2 to form intermedi-ate products that are strongly adsorbed and much less reac-tive than toluene (Larson and Falconer 1997). This study alsodemonstrated that benzaldehyde and benzyl alcohol react 10to 30 times faster than toluene. That observation promptedthe investigators to conclude that these compounds are not

Table 3. Light Sources Employed in Photocatalytic Reactors


likely to be the stable intermediate products formed duringPCO of toluene (Larson and Falconer 1997). Therefore,these authors postulated that the unidentified yellow materialrecovered from used TiO2 could result from adsorption ofless reactive intermediates (Larson and Falconer 1997). Ayellowish discoloration of TiO2 during PCO is mentioned byseveral investigators both for toluene (Ibusuki and Takeuchi1986; Larson and Falconer 1997) and for benzene (Fu et al.1995; Jacoby et al. 1996; Sitkiewitz and Heller 1996). Inanother study (Wolfrum et al. 1997), it was demonstrated thathexamethyldisilazane (HMDS) rapidly deactivated a cata-lyst. In another PCO study with toluene, Alberici and Jardim(1997) detected catalyst (TiO2) deactivation when the reactorwas operated under steady-state plug-flow reaction condi-tions of 200 ml min-1, 23% relative humidity, 21% oxygen,and a toluene concentration range between 400 and 600ppmv. However, illuminating the catalyst in the presence ofhydrogen peroxide restored the catalyst activity.

Muggli et al. (1988) identified the concentrations ofspecies on a TiO2 surface during photocatalytic oxidation(PCO) of ethanol. Ethanol and its partial oxidation intermedi-ates (acetaldehyde, acetic acid, formaldehyde, and formicacid) were reported to be on the catalyst surface, and theirconcentrations depended on the feed concentrations of etha-nol, O2, and water. The rate of PCO was greater initially thanat steady state for all experimental conditions, and the initial

deactivation was ascribed in part to the accumulation of acetal-dehyde on the surface.

EFFECT OF WATER VAPOR

Water vapor molecules compete with other feed streamgases, notably reactant species and molecular oxygen, for siteson the photocatalyst surface. The effect of water vapor inphotocatalytic processes reported in several investigations ispresented in Table 5. Summaries from selected PCO studiesexamining the effect of water vapor are described here.

Benzene

The presence of water vapor in a feed stream containingbenzene enhances its PCO over TiO2 (Table 5). High initialconversion followed by a decrease over time is observed forthree different inlet water concentrations—0, 1000, 8000 mgm-3 (0, 1342, 10,800 ppmv)—and conversion stabilized afterthree hours of reaction at 20%, 36%, and 40% conversion,respectively (d’Hennezel et al. 1998). Other investigationsconfirm the finding that the presence of water vapor is neces-sary for continued high benzene conversion (Fu et al. 1995,Sitkiewitz and Heller 1996).

O-xylene

Ameen and Raupp (1999) reported that the reversibledeactivation rate during gas-solid heterogeneous PCO of

Table 4. Catalyst Fouling and Deactivation


airborne dilute o-xylene over near-UV irradiated TiO2 cata-lyst depended on o-xylene concentration and the relativehumidity of the reactant stream. The deactivation decreasedlinearly with decreasing o-xylene concentration andincreasing relative humidity. The concentration of o-xyleneand aromatic partial oxidation intermediates on the titaniasurface are significantly higher for conditions of low rela-tive humidity than for conditions of high humidity. Experi-ments performed at low relative humidity favored thebuildup of o-xylene and o-toluic acid on the catalyst surface.Regeneration of the catalyst deactivated during the photo-catalytic oxidation process operated at high humidityrequires much shorter catalyst treatment time than for thecatalyst deactivated by photocatalytic oxidation operated atlow humidity. The observations of Ameen and Raupp (1999)suggest that hydroxyl radicals play a significant role in boththe oxidation and the regeneration processes.

Trichloroethylene (TCE)

Annapragada et al. (1997) studied the gas-solid heteroge-neous photocatalytic oxidation of TCE in humid airstreams ina bench-scale annular photocatalytic reactor operated underpartial vacuum. Reduction of the operating pressure at fixedfeed conditions and molar feed rate significantly enhancedPCO performance as measured by the observed TCE conver-sion. Higher conversions were obtained in spite of a reductionin the residence time accompanying the lower pressure oper-

ation. The greatest enhancements in the TCE destruction effi-ciency occurred for low TCE feed concentrations and highwater vapor levels (Dibble and Raupp 1992). The performanceenhancement appears to be linked to reduction in the absolutewater vapor concentration and competition between TCE andwater vapor for adsorption sites on the catalyst.

Formaldehyde, Toluene, and 1,3-Butadiene

Obee and Brown (1995) examined the effects of humidityand trace (sub-ppmv) contaminant levels on the oxidationrates of formaldehyde, toluene, and 1,3-butadiene. The eval-uation also included variations in UV irradiance and flow resi-dence time. Irradiances from inexpensive mercury fluorescentlamps vary between 1 and 10 mW cm-2. The data indicated thereaction was first-order for the three reactants at the sub-ppmvlevel. An important finding was that competitive adsorptionbetween water and trace (sub-ppmv) contaminants has asignificant effect on the oxidation rate. The dependencies ofhumidity and contaminant concentrations on the oxidationrates are explained as resulting from competitive adsorptionon available hydroxyl adsorption sites and on changes inhydroxyl radical concentrations.

Ethylene

Fu et al. (1996a) investigated the photocatalytic degrada-tion of ethylene in airstreams over the temperature range 30-110°C using a packed bed reactor containing sol-gel-derived

Table 5. Water Vapor Effect on a Photocatalytic Reaction


TiO2 or platinized TiO

2 particulates. Results from this study

indicate that the reactivity of ethylene is enhanced withincreasing temperature and that the fraction of ethylene thatreacts is stoichiometrically oxidized to CO

2 under all operat-

ing conditions. The effect of raising the temperature has beenascribed to decreasing adsorption of water for both types ofcatalysts, as well as an increase in conventional heterogeneouscatalytic reactions occurring on the Pt/TiO

2 catalyst. In addi-

tion, platinizing the TiO2 photocatalyst and increasing the

content of water vapor in the gaseous feed streams bothdecrease the rate of photocatalytic oxidation of ethylene. Theactivation energies for the photocatalytic and heterogeneouscatalytic oxidation of ethylene were determined to be 13.9-16.0 kJ mol-1 and 82.8 kJ mol-1, respectively.

As a general remark, water vapor on the surface of aphotocatalyst is generally regarded as fortuitous, as the waterreadily converts into hydroxyl radicals on reaction with photo-generated holes; recall the fundamental chemical reactionsinvolved in PCO as described earlier. The hydroxyl radicalsare active surface-bound oxidants, available to participate inredox reactions. Since the relative humidity in indoor environ-ments is typically maintained between 40% and 60% RH, theRH in a feed stream of air passing through a PCO reactor isacceptable. Further, water vapor is a reaction product of PCO.Because the inlet concentration of gas-species participating inthe reactions is low (≤1-10 ppmv), the amount of water vaporformed as a product of reaction will be low (<< 1% RH at roomtemperature). (Note: at 25°C and 1 atm, a 1% change in RHcorresponds to about 315 ppmv H2O.) Therefore, it isexpected that most applications of photocatalysis will notrequire pre- or post-humidification nor dessication.

REACTION BY-PRODUCTS

Reactions with No By-Product Formation

Under steady-state reaction conditions whereby titaniumdioxide is illuminated in a plug flow reactor at 200 ml min-1,23% relative humidity, 21% oxygen, and an organiccompound concentration range between 400 and 600 ppmv,Alberici and Jardim (1997) reported that no detectable by-products formed during the PCO of 17 VOCs (trichloroethyl-ene, isooctane, acetone, methanol, methyl ethyl ketone, t-butylmethyl ether, dimethoxymethane, methylene chloride, methylisopropyl ketone, isopropanol, chloroform, tetrachloroethyl-ene, isopropylbenzene, methyl chloroform, pyridine, andcarbon tetrachloride).

Reactions with By-Product Formation

The formation of undesired by-products (e.g., phosgene)is a significant concern that should be addressed by anycompany that plans to commercialize indoor air treatmentsystems that incorporate PCO. However, concentrations ofindividual VOCs in indoor air in homes and offices are typi-cally in the range of parts per billion. If only part of the VOC

load in indoor air reacts on a single pass through a PCO reac-tor, the concentrations of undesired by-products may well bebelow the detection limits of most analytical methods andbelow levels that would cause health concerns. Also, one mustrealize that there are challenges in testing a nearly limitlessnumber of combinations of VOCs at different concentrationsthat can be present in indoor air. These issues become moreimportant for specific industrial hygiene applications whereconcentrations of airborne contaminants can be orders ofmagnitude higher than in homes and commercial buildings. Inindustrial settings, though, the contaminated load is generallymuch better defined in terms of the types and concentrationsof contaminants that are present.

Alberici et al. (1998) utilized on-line mass spectrometryto monitor and identify the by-products and total mineraliza-tion products of TiO2/UV photocatalytic degradation of fourchlorinated volatile organic compounds (VOCs) includingtrichloroethylene (TCE), tetrachloroethylene (PCE), chloro-form, and dichloromethane. Several by-products weredetected including phosgene for TCE, PCE, and chloroform;dichloroacetyl chloride for TCE; and trichloroacetyl chloridefor PCE.

In studies with a photocatalytic reactor fed with a mixtureof TCE in air, it has been reported that a number of compoundsin the effluent are produced including dichloroacetyl chloride(DCAC), phosgene (COCl2), carbon monoxide (CO), molec-ular chlorine (Cl2), HCl, and CO2 (Nimlos et al. 1993). It wasproposed that the gas-solid heterogeneous PCO of TCE in airproceeds by two pathways—(1) via the production of theDCAC intermediate or (2) the direct oxidation of TCE to(photocatalytically) stable products (Nimlos et al. 1993). Thegeneration of phosgene was consistently observed during thegas-solid PCO of gas-phase TCE (Jacoby et al. 1994).However, other investigators have reported conditions underwhich phosgene was not observed (Yamazaki-Nishida et al.1996). In PCO studies with gas-phase TCE and PCE and theuse of sol-gel-derived porous pellets of TiO2, Yamazaki-Nishida et al. (1996) report producing undesirable chlorinatedby-products as chloroform (CHCl3) and carbon tetrachloride(CCl4). The researchers noted that the higher porosity andspecific surface area achieved with (TiO2) pellets fired atlower temperatures closely related to the formation of by-products. They state that with respect to the gas-phase heter-ogeneous PCO of TCE and PCE it is possible to develop astrategy for suppressing the formation of undesirable by-prod-ucts (as was reported by Nimlos et al. [1993]) via increasingthe oxygen mole fractions in the feed gas stream and/or usingTiO2 pellets fired at lower temperatures. Yamazaki-Nishida etal. (1996) purport that in employing these strategies, the PCOof TCE will favor a reaction pathway/mechanism initiated bythe OH radical rather than the Cl-radical-initiated reaction thatwas predominant in the systems of Nimlos et al. (1993).


OTHER PCO-RELATED TOPICS

Review articles provide a broad overview of variousaspects related to the physics, chemistry, engineering, andapplication of photocatalysis. Review articles that cover broadtopics mentioned here can be found in the following: solarprocesses (Takeuchi and Kutsuna 1996), disinfection(Murakami and Ishida 1996), indoor air quality (Murakamiand Ishida 1996; Sugawara 1996; Takaoka and Ebihara 1995),and environmental applications (Takeuchi and Kutsuna 1996).

Numerous survey articles elaborate on the many issuescomprising the principles and practice of PCO. Fu et al.(1996b) and Peral et al. (1997) review the application of heter-ogeneous photocatalysis for purification of air. Rao andNatarajan (1994) provide a review of the models in heteroge-neous photocatalysis. Hoffmann et al. (1995) provide a reviewof the environmental applications of semiconductor photoca-talysis. Linsebigler et al. (1995) discuss the principles, mech-anisms, and selected results of PCO.

As an aid to the more interested reader, a bibliography ofscientific articles concerning research, development, andpatents awarded in the application of heterogeneous photoca-talysis for treating air and aqueous feed streams is compiledand periodically reported by the National Renewable EnergyLaboratory (Blake 1999). This bibliography is a valuedresource for end-users that desire a condensed summary ofresearch articles in the field of heterogeneous photocatalysis.

PREDICTIVE MODELING—CONTAMINANT REMOVAL FROM A ZONE

Although it is an intuitive and unproven assumption, scal-ing up the aforementioned bench-top laboratory reactors forcommercial-scale applications of gas-solid photocatalysismay require conventional heterogeneous reactor configura-tions including fixed and fluidized beds, transport reactors,and monolith, among others. Whichever configuration isappropriate for a given application, the absolute reaction rateof contaminant removal must match the contaminant genera-tion and infiltration rate minus the exfiltration rate. Thisrequirement for efficient conversion processes must bebalanced against designs that achieve low-pressure drop, asignificant concern of PCO devices used for air purification inthe office, home, or factory. As such, it may be less importantto achieve high reactant conversion/oxidation on single passesthrough a commercial scale PCO device than is typical forother chemical processing and, in particular, dust removaltechnologies, where single-pass removal efficiencies can behigh. Furthermore, because PCO reactors operated for purifi-cation will recycle air multiple times, the absolute level ofconversion per pass through the reactor may be less important.Modeling the disappearance of a contaminant species via useof a photocatalytic reactor is introduced by describing thephysical conditions of an enclosure containing a PCO reactor.

Physical Model

Consider an enclosure (building or residence) as a struc-ture with porous walls. While the enclosure may have multiplezones, one can consider the enclosure as a single room or zonewith volume Vz. In the presence of a pressure differentialbetween the ambient and the zone, there is movement of(ambient) outside air (gas) into the zone with a volumetricflow rate of υi—called infiltration—and movement of the airfrom the zone to the ambient at the (same) rate of υi—calledexfiltration. Consider the scenario in which infiltration air iscomposed of one (gaseous) contaminant A in the parts permillion vapor (ppmv) range and the balance is air (N2:O2). Theinfiltrating contaminant has a concentration of CAi. Assumingthe air in the zone is well mixed, the concentration of thecontaminant in exfiltration air is CA. Additionally, consider asource(s) in the enclosure that uniformly generates thecontaminant in the zone at a rate of SA. Consider that the zonecontains a PCO treatment device or reactor, which is small inrelation to the size of the zone. The device has its own dedi-cated fan that forces zonal air through the device at a volumet-ric flow rate of υR. The rate of disappearance of thecontaminant from the air passing through the device is rA andrequires a mass of photocatalyst, W, and period of time, t, toeffect this removal. The contaminant enters the device at CAand leaves at CAeR.

Numerical Model

A mathematical model can be employed to determine thetime-varying concentration of the contaminant in the zone. Amolar balance on the zone is given as

(1)

The steady-state molar balance on the reactor is given by

(2)

KINETIC MODELING OF PCO REACTOR

The kinetics of PCO reactions can be determined byperforming experiments in which the reactors are operated ineither of two modes—recirculation (batch) or plug-flow(single-pass). Both approaches are described here. A kineticmodel is required to estimate the removal of a (contaminant)species by the treatment device. The fractional conversion, fA,is calculated as

(3)

where CA is the concentration of the species of interest (reac-tant) at a given time during the reaction and CA0 is its initialconcentration, PA is the pressure of the reactant and PA0 is itsinitial pressure, and nA are the moles of reactant and nA0 are itsinitial number of moles. Assuming ideal gas behavior, Equa-tion 3 makes use of the partial pressure of the reactant as

CAiυi CAeRυR CAυi– CAυR– S·AVz+ + Vz

dCA

dt---------- .=

CAυR CAeRυR– rAW– 0 .=

fA 1CA

CA0

---------– 1PA

PA0

---------– 1nA

nA0-------- ,–= = =


(4)

where R is the gas constant, T is the absolute temperature, andV is volume (assumed not to change depending on stoichiom-etry of the reaction).

Models based on power law rate expressions have alsobeen reported to provide adequate approximation of kineticdata for photocatalytic reactions (Fu et al. 1996b, 1996c; Zornet al. 1999). Models based on Langmuir-Hinshelwood rateexpressions have been used successfully to fit kinetic data ofphotocatalytic reactions (Dibble and Raupp 1990; Sabate et al.1991; Peral and Ollis 1992; Obee and Brown 1995; Sauer andOllis 1996; Obee and Hay 1997; Yamazaki et al. 1999; Zornet al. 1999).

There are two underlying assumptions when using theproposed modeling methods:

1. Under the experimental conditions, the reaction is not masstransfer limited so that the bulk and near-surface gas-phaseVOC concentrations are essentially the same.

2. The light irradiance on the catalyst surface should beuniform under experimental conditions. If not, the reactionrate constant k will be an average value for the average lightirradiance on the catalyst surface. To apply this averagereaction rate constant k obtained from experiments withlaboratory PCO reactors, it requires that the light irradianceon the catalyst surface remain essentially the same duringthe scaling-up process. In other words, for the proposedmodeling methods to be predictive, the average light irra-diance within laboratory PCO reactors should be essentiallythe same as that within the scaled-up (commercial-scale)reactors.

Recirculation (or Batch)Reactor Operation and Modeling

In one approach, PCO treatment devices are operated asrecirculation or batch reactors. Here, the reactors are placedwithin a gas-tight chamber with volume Vres. The chamber isbackfilled with a reactant gas with an overwhelming stoichi-ometric excess of oxygen (often air) as the balance. Then, aflow of reactant gases through the reactor is established. Aftera steady-state initial reactant gas concentration (CA0) is estab-lished, an experiment is initiated (t = 0) by illuminating thelight source(s). Sample volumes (<< volume of the chamber)of the gases in the chamber are taken periodically and analyzedwith appropriate analytical instruments (e.g., GC/FID, GC/TCD, GC/MS, FTIR, PID, etc.). An experiment is terminatedafter a considerable amount of the reactant gas has reacted,i.e., fA > 0.9. The concentration vs. time data obtained from theexperiment is analyzed according to an appropriate kineticmodel.

Power Law Model

Assuming that the PCO of reactant species follows apower law model, the reactor design equation is (Edwards1994)

(5)

In Equation 5, is the rate of disappearance of species A,kA is the reaction rate constant, and n is the reaction order. (Sincethe concentration of reactant gas occupying Vres decreases withreaction time, the term dCA/dt is negative. Therefore, a negativesign is required on the right-hand side of Equation 5.) Separat-ing variables and integrating Equation 5 gives

(6)

or, after rearranging,

(7)

Equation 7 can be solved for n = ½ (½-order model) asfollows:

(8a)

(8b)

Since Equation 8b is in the form of y = mx + b, the slopeof the line fitting the data can be used to find the reaction rateconstant (kA) for

(9)

where kA has units of mol1/2 L1/2 g-1 s-1. Equation 7 can also besolved for other reaction orders (n) including 1, 3/2, and 2; therate expression, reactor design equation, regression plots, andreaction rate constants are summarized in Table 6a.

Langmuir-Hinshelwood (LH) Model

Because PCO reactors involve interactions between thegas-phase contaminants and the surface of the photocatalyst,LH rate expressions can be expected to provide a very goodapproximation of the overall reaction kinetics. The LH modelsof photocatalytic reactions are based on a mechanism thatinvolves a reactant adsorbed on the surface of a catalyst. Asimple LH rate expression for a reaction that is first order withrespect to the fraction of the surface covered by adsorbedspecies A (θA) can be written as

PA

nA R T⋅ ⋅

V----------------------

nA0 R T 1 fA–( )⋅ ⋅ ⋅

V----------------------------------------------- PA0 1 fA–( ) ,⋅= = =

rA kACA

n Vres–

W-------------

dCA

dt---------- .= =

rA

kAW

Vres

-----------– td

t 0=

t t=

∫dCA

CA

n----------

CA0

CA

∫ CA

n–dCA

CA0

CA

∫= =

CA

n–dCA

CA0

CA

∫ kAW

Vres

-----------– t=

CA

1 2⁄–dCA

CA0

CA

∫CA

1 2⁄

1 2⁄------------

CA0

CA

CA

1 2⁄CA0

1 2⁄–

kAW

2Vres

-------------– t= = =

CA

1 2⁄ kAW

2Vres

------------- t– CA0

1 2⁄+=

kA 2Vres

W-------------slope ,–=


(10)

For this case, the rate constant kA typically has units ofmol g-1 s-1. The adsorption equilibrium constant for adsorbateA (KA) typically has the units of reciprocal concentration (e.g.,l-mol-1). Combining Equations 3 and 10 gives

(11)

Substitution of this LH rate expression into the designequation (Equation 5) and subsequent integration leads to thefollowing equation:

(12)

Linear regression analysis of the data in the form of a plotof ln(1 – fA) / fA versus t / fA is employed to determine the slopeand the intercept of the straight line that best fits the data. Theinitial estimates of the parameters of the LH model (kA and KA)can be calculated using the following equations:

(13a)

(13b)

Plug-Flow Operation and Modeling

In another approach, PCO reactors are operated in plugflow where the reactant species make a single pass through thereactor. Here, the independent variable is often the molar flowrate and is varied by either holding the inlet concentration(CA0) constant and varying the volumetric flow rate or viceversa. The dependent variable is the fractional conversion.

The power law kinetic models can arise as limiting formsof LH rate expressions. As above, each model type isdescribed, together with the linear form of the model used toobtain initial estimates of the kinetic parameters. Thesemodels are summarized in Table 6b. Nonlinear regressionanalyses of the kinetic data can be performed using severaldifferent kinetic models for the rate term in the design equa-tion.

rA– kAθAkAKACA

1 KACA+----------------------- .= =

Table 6a. Kinetic Models of PCO Batch Reactor Operation

Table 6b. Kinetic Models of PCO Plug Flow Operation

rA–kAKACA0 1 fA–( )

1 KACA0 1 fA–( )+--------------------------------------------- .=

1 fA–( )ln

fA------------------------

kAKAW–

Vres

---------------------t

fA---- KACA0+=

kA

VresCA0–

W intercept⋅--------------------------------- slope⋅=

KA

intercept

CA0

------------------------=


Power Law Model

Assuming that the photocatalytic oxidation of reactantspecies follows a power law model, the reactor design equa-tion is (Hill 1977)

(14)

In Equation 14, –rA is the rate of disappearance of speciesA, W is the mass of the catalyst, and FA0 is the molar flow rateof reactant A entering the reactor. For a kinetic model that is½ order in the limiting reactant, the rate expression is of theform,

(15)

where kA is the reaction rate constant (with typical units ofmol½ ·l½·s-1·g-1). When this rate expression (Equation 15) iscombined with the design equation (Equation 14) and theresult integrated, one obtains the following expression:

(16)

where FA0 is the initial molar flow rate. The task at hand is todetermine kA. The initial estimate for kA is obtained from theslope of a plot of versus W/FA0 using linear regressionanalysis. The initial estimate of the parameter kA is calculatedusing the following relation:

(17)

Note that the initial estimate of kA can then be used as astarting value of this parameter in a subsequent nonlinearregression analysis of the data based on this model. In a similarmanner, the expressions for the rate expression, reactor designequation, and regression plot to estimate kA for reactions thatare first-, 3/2-, and second-order can be determined (Table 6b).

Langmuir-Hinshelwood Models

Combining Equation 11 into Equation 14 and subsequentintegration and rearranging gives a reactor design equation of

(18)

Linear regression analysis of the data in the form of a plot

of versus is employed to determine

the slope and the intercept of the straight line that best fits the

data. The initial estimates of the parameters of the LH model

(kA and KA) can be calculated using the following equations:

kA = slope (19a)

(19b)

It is possible to develop a rate expression form in whicha reaction rate constant is independent of light irradiance.For example, Peral and Ollis (1992) employed an LH rateequation of the form,

(20)

which can be integrated using a plug flow model. Equation 20contains an intensity dependence power, α. Sirisuk (2003),among others, have performed experiments to estimate α.

APPROXIMATING THE SIZE OFA PCO REACTOR BASED ON QUANTUM YIELD

The quantum yield (φ) of forming a product species in alight-induced chemical reaction is the number of moleculesformed for each quantum of radiation absorbed. Often φ

cannot be measured accurately because of light reflection andscattering. This fact likely explains why φ is not reported inmost studies of heterogeneous photocatalysis. Nonetheless, itis an important metric of performance and is useful forcomparing reactors from various laboratories. Because thequantitative analysis of product formation can be difficult (dueto adsorption of product species on reactor surfaces and photo-catalytic surfaces), it is sometimes estimated from the amountof reactant that disappears. Therefore, the quantum yield canbe estimated by

(21)

where rs is the conversion rate on the surface of the catalyst, Asis the surface area, and the initial reaction rate is determinedvia

(22)

Consider the case where one desires to approximate thesize of a large (i.e., field-test or commercial-scale prototype)PCO reactor and one has available an estimate of φ basedeither on published data or on experiments with a bench-top(or small-scale) reactor that is configured and operated simi-larly to that of the desired prototype. Also, assume that an esti-mate of the average light irradiance I on the surface of thecatalyst is known, which would likely be obtained by measure-ment. Under these assumptions, estimating the surface (orreaction) conditions and the inlet feed conditions can providean estimate for As as shown here.

Surface (or Reaction) Conditions

The energy per photon is determined with

(23)

where h is Planck’s constant, c is speed of light, and λ is thewavelength of the light source’s peak emission. The photonflux Γ can be estimated by

W

FA0

---------dfA

rA–-------- .

fA,in

fA,out

∫=

rA– kACA

1 2⁄kACA0

1 2⁄1 fA– ,= =

1 fA– 1kACA0

1 2⁄

2------------------–

W

FA0

---------⋅=

1 fA–

kA2 slope⋅–

CA0

1 2⁄------------------------=

fA

1 fA–( )ln------------------------ kA

W

FA0 1 fA–( )ln----------------------------------

1

KACA0

----------------- .+=

fA

1 fA–( )ln------------------------

W

FA0 1 fA–( )ln----------------------------------

KA

1

CA0 intercept⋅--------------------------------------=

kA0

kA′

kA0I

I0

----⎝ ⎠⎛ ⎞α ,=

φmoles degraded

absorbed light irradiance-----------------------------------------------------------

rA0W–

rsAs

---------------- ,= =

rA0–kAKAPA0

1 KAPA0+-------------------------- .=

Ehc

λ------ ,=


(24)

and the conversion rate on the surface of the catalyst is givenby

(25)

where NA is Avogadro’s number.

Feed Conditions

Assuming ideal gas behavior, one can estimate the initialmolar flow (FA0) rate of species A with volumetric flow rate Qfand initial concentration CA0 as

(26)

The surface area of the reactor for complete removal ofspecies A can be estimated by

(27)

In contrast to this estimation approach, once the kineticmodel for a particular photocatalytic reaction over a particularcatalyst is known, the mathematical model describing theperformance of the reactor can be developed; recall, for exam-ple, Equation 18. The development of the model (Equation 18)requires not only the proper kinetic model but also a model forthe distribution of light irradiance within the reactor.

CONCLUSION

Numerous scientific investigations have elucidated theoperating conditions necessary to achieve complete mineral-ization of many organic compounds to final products of CO2and H2O. These reactions are achieved with only minimalrequirements—light sources, power supply, a stable photocat-alyst, and oxygen. Many of these reactions occur at or nearroom temperature and atmospheric pressure. The presence ofwater vapor has been shown to enhance reaction rates for manyreactant gases, often occurring due to the generation of surfacehydroxyl radicals. Additional factors affecting reactor perfor-mance include light irradiance, the presence of species caus-ing catalyst deactivation (e.g., species containing S and N),and competitive adsorption between individual species withina mixture of reactant gases. L-H and power-law kinetic modelshave demonstrated acceptable predictions of laboratory data.

With regard to commercial-scale applications, the presentchallenge is to design photocatalytic treatment devices thathave low pressure drop, make efficient use of light (i.e.,achieve high and relatively uniform light irradiances over thesurface of a catalyst that provides high quantum efficiencies),and employ a stable catalyst that can be readily re-activated ifit becomes deactivated.

With regard to research that can be conducted to furtheradvance the state-of-the-art in gas-phase heterogeneousphotocatalysis, the HVAC industry will benefit from effortsthat:

• Continue to examine the influence of water vapor oncompetitive adsorption processes occurring on photocat-alytic surfaces. (Here, the industry will benefit if testingis conducted within the range of relative humidity foundin building environments [40% to 60%].)

• Investigate low-level concentrations (ppmv and sub-ppmv) in the inlet feed streams, which are concentra-tions that are representative of indoor environments.

• Conduct investigations in which the inlet feed streamcontains multiple reactants (VOCs).

• Report reaction rate constants (here, the HVAC industrycan benefit if studies generate reaction rate constantsthat are reported in the peer-reviewed literature).

• Explore innovative approaches for getting light at thecatalyst—this includes but is not limited to the studies inwhich waveguide technology is addressed.

• Develop better photocatalysts that limit (or negate) by-product formation, enhance reaction rates, and increasethe rate of target specie adsorption onto the catalyst sur-face.

• Investigate the use of PCO in a mixed (multi) gas air-stream and monitor and report the reaction by-productsformed.

The review of the literature suggests the eventual need todevelop a standard method of test so that valid comparisonscan be made on reported performance metrics of photocata-lytic reactors. Presently gas-phase heterogeneous photocatal-ysis holds promise for penetrating niche markets. The earlymarkets for air treatment with PCO need not be limited toindoor or outdoor air treatment with PCO reactors within theHVAC system of buildings. For example, some applicationsinclude controlled atmospheres in produce cold storage roomsand floral display cases. In most cases, though, the applica-tions should be characterized by those wherein the feedstreams to be treated will contain low concentrations (≤1-10ppmv) of either a single gas, mixture of a few (two to four)gases, or a known and fairly consistent suite of chemicals at theinlet to the PCO reactor. By limiting the influent gases to aknown set of chemicals, possible by-product formation ofthese PCO reactions can be measured, quantified, and found tobe acceptable or otherwise.

ACKNOWLEDGMENTS

The authors of this report would like to thank several indi-viduals for their contributions. Prof. Michael Zorn of the Natu-ral Applied Sciences Department at the University ofWisconsin-Green Bay is recognized for his contributions tothe photocatalytic studies within our research group during hispost-doctoral visit to UW-Madison. Gratitude is extended toProf. Charles Hill of the UW-Madison and Dr. Akawat Sirisukof Chulalongko University (Bangkok, Thailand) for discus-sions pertaining to PCO reactor modeling. This project wassupported through ASHRAE research project 1134-RP.

ΓI

E--- ,=

rsφ Γ⋅NA

----------- ,=

FA0 QfCA0 .=

As FA0 rs⁄ .=


NOMENCLATURE

As = surface area, m2

c = speed of light (= 2.998 × 108 m s-1), m s-1

CA = concentration of (gas) species A in a zone and in exfiltration air, mol L-1

CA0 = initial concentration of (gas) species A, mol L-1

CAeR = concentration of (gas) species A exiting reactor and entering the zone, mol L-1

CAi = concentration of (gas) species A in infiltration air, mol L-1

E = energy per photon, J photon-1

Eg = band gap (energy), eV

fA = fractional conversion, dimensionless

FA0 = initial molar flow rate of species A, mol s-1

h = Planck’s constant (6.626 × 10-34), J s photon-1

intercept = intercept in regression plot, dependent on plot

I = irradiance, J s-1 m-2

I0 = initial irradiance, J s-1 m-2

I = average light irradiance, J s-1 m-2

kA = reaction rate constant of species A, dependent on n

= reaction rate constant (n-order model); independent of light irradiation, temperature-dependent, mol(1-n) Ln cm2a mW-a g-1 s-1

KA = equilibrium adsorption constant, Pa-1

n = reaction order—based on power law model, dimensionless

nA = moles of reactant species A, mol

nA0 = initial moles of reactant species A, mol

NA = Avogadro’s number (= 6.02 × 1023 molecules mol-1), molecules mol-1

PA = partial pressure of species A, Pa

PA0 = initial partial pressure of species A, Pa

Qf = volumetric flow rate, L s-1

rA = rate of disappearance of species A, mol s-1 g-1

rA0 = initial rate of disappearance of species A,mol s-1 g-1

rS = rate of conversion on surface of catalyst, mol s-1 m-2

R = gas constant, L Pa mol-1 K-1

slope = slope of regression plot, dependent on plot

SA = generation rate of (gas) species A, mol s-1 L-1

t = time, s

T = temperature, K

V = volume occupied by reactant species, L

Vres = volume of gas-tight reservoir in batch reactor operation, L

Vz = volume of zone, L

W = mass of photocatalyst, g

Greek Symbols

α = exponent of light dependence parameter, dimensionless

φ = quantum yield, dimensionless

λ = wavelength of light, m

υi = volumetric flow rate of infiltration air (outside air) into zone, L s-1

υR = volumetric flow rate of gas through PCO reactor, L s-1

ϖ = frequency of light, Hz

θA = surface coverage parameter, dimensionless

Γ = photon flux, mol s-1 m-2

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