Kinetic study of air treatment by photocatalytic paints under indoor radiation source: Influence of ambient conditions and photocatalyst content

Journal Pre-proof Kinetic study of air treatment by photocatalytic paints under indoor radiation source: influence of ambient conditions and photocatalyst content F. Salvadores, O.M. Alfano, M.M. Ballari

PII:

S0926-3373(20)30109-0

DOI:

https://doi.org/10.1016/j.apcatb.2020.118694

Reference:

APCATB 118694

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 September 2019

Revised Date:

31 December 2019

Accepted Date:

25 January 2020

Please cite this article as: Salvadores F, Alfano OM, Ballari MM, Kinetic study of air treatment by photocatalytic paints under indoor radiation source: influence of ambient conditions and photocatalyst content, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118694

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Kinetic study of air treatment by photocatalytic paints under indoor radiation source: influence of ambient conditions and photocatalyst content F. Salvadores, O.M. Alfano, M.M. Ballari*

INTEC (Universidad Nacional del Litoral and CONICET) Ruta Nacional Nº 168. Km. 472.5

Phone: 0054 342 4511595 Ext. 1057

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e-mail: [email protected]

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3000 Santa Fe, Argentina

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* Corresponding author

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Graphical abstract

Highlights

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 Intrinsic kinetic model for indoor air decontamination by photocatalytic paints  Minimum irradiation level needed to observe photocatalytic reaction  Determination of useful wavelength range according to photocatalyst bandgap  New parameter to correlate active area with TiO2 amount and agglomeration in paint  Quantum efficiency decrease with higher amount of photocatalyst in paint

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Abstract

Photocatalytic building materials constitute a promising technology to control air pollution,

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although intrinsic kinetics is needed for prediction of decontamination processes. Herein, the kinetic study of acetaldehyde oxidation was carried out applying photocatalytic paints under normal indoor illumination source and different ambient conditions. An intrinsic kinetic model

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for the main contaminant and reaction intermediates was proposed as a function of the Local Superficial Rate of Photon Absorption, which was evaluated for paints containing different

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amounts of carbon doped TiO2. A novel parameter to account for the relationship between photocatalytic active area, TiO2 amount and particles agglomeration in the paints was proposed.

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Also, based on the model predictions and the photocatalyst bandgap energy a spectral radiation analysis was performed determining the maximum wavelength at which the photocatalyst is active. Finally, the quantum and photonic efficiencies were calculated and the effect of operating

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conditions on these efficiencies was analyzed.

Symbols used A

av [1/cm]

Reactor irradiated area

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AR [cm2]

Radiation absorption fraction

Active surface area per unit volume of the reactor

C [mol/cm3]

Molar concentration

esa [Einstein/cm2/s]

Local superficial rate of photon absorption

E [J]

Radiant energy

Fq

Radiation fraction from indirect sources

f

Factor related with the photocatalytically active surface area

K [cm3/mol]

Adsorption equilibrium constant 2

Length of the lamp

nw

Unitary vector normal to the wall

P [W]

Lamp emission power

Q [cm3/s]

Air flow rate

q [Einstein/cm2/s]

Local net radiation flux

q [Einstein/cm2/s]

R

Local radiation flux vector Diffuse reflectance

r [mol/cm2/s]

Superficial reaction rate

rL [cm]

Radius of the lamp

T

Diffuse transmittance

vair [cm/s]

Air flow velocity

x [cm]

Position vector

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LL [cm]

Greek letters

Kinetic parameter (units depends on the specific parameter)

δ

Kinetic parameter (units depends on the specific parameter)

η

Efficiency

Φ [mol/Einstein]

Primary quantum yield

λ [nm]

Wavelength

ϕ [rad]

Spherical coordinate

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θ [rad]

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α

Spherical coordinate

Subscripts A

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acr

Acetaldehyde Acrylic plate

AR

Relative to the reactor area

dir

Direct

eff

Effective

F

Formaldehyde

ind

Indirect

max

Maximum

min

Minimum 3

p

Photonic

paint

Relative to photocatalytic paint

pp

Relative to pseudo-paint

q

Quantum

TiO2

Relative to titanium dioxide

VR

Relative to the reactor volume

W

Water

w

Relative to reactor wall

wR

Relative to the reactor width

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Normalized property

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norm

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Special symbols Average value



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Average value over wavelengths

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Abbreviations

Extended Superficial Diffuse Emission

FID

Flame Ionization Detector

GRG

Generalized Reduced Gradient

LSRPA PP

Root Mean Square Error Scanning Electron Microscope

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UV

Local Superficial Rate of Photons Absorption Pseudo-paint

RMSE SEM

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E-SDE

VOC

Ultraviolet light

Volatile Organic Compound

Keywords: Photocatalytic Paints; Air Decontamination; Acetaldehyde; Kinetic Study; Efficiencies

1.

Introduction 4

Numerous volatile compounds present in indoor environments can adversely affect the wellness and health of people. According to the U.S. Environmental Protection Agency [1] indoor air pollutants can cause from short- to long-term health issues, including irritation of the eyes, nose, and throat, headaches, respiratory diseases (asthma), heart disease and cancer. Acetaldehyde is one of the problematic Volatile Organic Compounds (VOCs) that can be accumulated in indoor environments causing some of the health issues mentioned above: it is irritant, toxic and a probable carcinogen [2]. This pollutant is generally released by burning

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reactions or by several construction materials in homes.

An emerging alternative for air decontamination is the oxidation by heterogeneous

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photocatalysis. The most employed photocatalyst is the anatase form of the titanium dioxide (TiO2) working with ultraviolet radiation. In order to extend the range of wavelengths to activate

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the process and to enhance solar or visible light driven photocatalysis, different photocatalyst modification methods were investigated [3–10]. One of the TiO2 modifications is the carbon

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doping that reduces the photocatalyst bandgap energy, extends the absorption range to the visible light region, and increases the active surface area promoting the pollutant adsorption.

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The combination of TiO2 photocatalyst with construction materials with self-cleaning properties and air de-polluting potential is being increasingly investigated for outdoor uses [11– 14]. Although less frequent, applications of photocatalysts in building materials for indoor use

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have also been developed [15–19].

One of the most employed materials in construction is the wall paint or coating with aesthetic and protecting functions. Lately, several studies have been published that apply photocatalytic TiO2 in different paint formulations. These works have analyzed the following aspects: i) the air purification employing several model contaminants [18,20–25]; ii) the antimicrobial properties

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through bacteria and fungi inactivation studies [26–29]; and iii) the self-cleaning function observing colorant bleaching over the irradiated paint [30–33]. On the other hand, it is important to determine the specific wavelength at which these paints are really active. For example, Galenda et al. [34] have analyzed the ability of three commercially available photocatalytic paints for the methyl red and methylene blue bleaching under different light sources. It was concluded that under visible radiation the paints are scarcely active. Several works have investigated the kinetics of the acetaldehyde photocatalytic oxidation [35– 39]. These studies have applied the Langmuir–Hinshelwood kinetic model, which has parameters 5

depending on the radiation flux. The issue with this model is that by changing the light source, or even the distance at which the photoreactor is situated, there are modifications in the Langmuir– Hinshelwood kinetic parameters that cannot be estimated a priori. Thus, the usefulness of the determined kinetic parameters is limited to reproducing exactly the same experimental conditions at which they were calculated. In Salvadores et al. [40] the kinetic analysis of the photocatalytic degradation of acetaldehyde was carried out proposing a complete intrinsic model based on the photocatalytic oxidation mechanism of the pollutant and including the dependence with the Local Superficial Rate of Photons Absorption (LSRPA) by the photocatalyst.

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In this study, indoor wall paints were formulated employing different amounts of a commercial photocatalytic carbon doped TiO2 in replacement of the usual paint pigments. The air

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de-polluting capability of these paints was evaluated under a normal indoor light source, employing acetaldehyde as the model pollutant and varying the working conditions, like relative

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humidity, incident radiation flux and inlet contaminant concentration. A kinetic study was carried out solving the mass and radiation balance equations in the photoreactor and applying

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mechanistically derived kinetic expressions corresponding to the main and secondary pollutants as a function of the LSRPA. The new insight of this work is the evaluation of the LSRPA by

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photocatalytic paints containing different amounts of carbon doped TiO2 for which the optical properties of the paints had to be determined. For the mass balance resolution, a novel parameter was proposed that relates the photocatalytically active area and the TiO2 amount and

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agglomeration in the different paint formulations. Also, an effective LSRPA was considered to take into account the photocatalyst activation rate since under determined operating conditions the reaction does not occur under a minimum value of absorbed radiation. On the other hand, the correlation between the experimental data and the model predictions, together with the bandgap energy reported in the literature, has allowed to determine the useful wavelength range emitted by

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the employed lamps for the carbon doped TiO2. The importance of intrinsic kinetics is based on the fact that it can be applied to predict the photocatalytic air decontamination under other operating conditions or even for others photoreactor configurations. This is crucial for selecting the optimal light source and reacting conditions or for the scaling up process. Finally, the photonic and quantum efficiencies were calculated, which relate the pollutant reaction rate with the total radiation flux that reaches the catalytic surface and with the rate of absorbed photons, respectively. Through these parameters, the influence of the photocatalyst amount used for the paint formulations and the operating conditions was analyzed. 6

2.

Material and methods

2.1

Photocatalytic paints preparation and coatings characterization

To elaborate the paints, titanium dioxide KRONOClean 7000 containing carbon as doping agent was used in replacement of the normal paint pigments. The employed TiO2 is a powder with 87.5% anatase and 15 nm particle size. According to the manufacturer, the BET surface of

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this catalyst is greater than 250 m2/g. A more detailed characterization of this photocatalyst was done by Tobaldi et al. [41].

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Three different paints (12TiO2-C, 14TiO2-C and 18TiO2-C) varying the TiO2 quantity (12, 14 and 18% w/w, respectively) were elaborated maintaining the total amount of solids constant in

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36% w/w, i.e. photocatalytic TiO2 plus extender CaCO3. The employed resin is an aqueous dispersion of a styrene-acrylic copolymer (BasfAcronal® RS 723 sa) and the dispersant agent is a

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sodium salt solution of the polycarboxylic acid (BasfPolysal® BA). The composition of the prepared paints is shown in Table 1. A pseudo-paint (PP-noTiO2-C) was also elaborated, with the

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same components of the photocatalytic paints but without the TiO2. The TiO2 and the CaCO3 were hand milled in order to reduce and homogenize the particle sizes and dried at 110 °C for 30 minutes to eliminate the moisture that could be contained. To

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elaborate the paints, the photocatalyst is first added to the solution of distilled water and dispersing agent, while is mixed at about 300 r/min. Then the extender is incorporated to complete the solid matrix of the paint. Finally, the resin is added to the mixture. To test the good dispersion of the particles, the fineness of grind of the liquid paint was measured (Table 1). The paint application was made with an aerograph on acrylic plates of 9.4 × 20 cm. The plates

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were weighted before and after the painting of each side to ensure that a comparable amount of paint per area was deposited on the different plates (Table 1). Previously to the air decontamination test, the photocatalytic coating was cured exposing it

under fluorescent visible lamps for a period of time between 5 and 8 hours. With this procedure, the paint organic compounds that surround the photocatalytic particle were oxidized, allowing the interaction between the air contaminant and the TiO2 present in the paint [42]. A Scanning Electron Microscope (SEM) JEOL JSM-35C and a Transmission Electron Microscope (TEM)

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JEOL JEM-2100 Plus were used for the visualization at 2000× and at 30000×, respectively, of the coatings before and after the curing process. To determine the superficial composition of the paints, X-ray photoelectron spectroscopy (XPS) analysis was performed using a Specs Multitechnique instrument equipped with dual Xray source Mg/Al and a PHOIBOS 150 hemispherical analyser operating in fixed analyser transmission (FAT) mode. The spectra were obtained with a step energy of 30 eV and the Mg anode operated at 100 W. Pressure during measurements was less than 2 × 10−9 mbar. In addition, the diffuse reflectance and transmittance of the paint coatings on the acrylic plates

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were spectrally measured from 300 nm to 800 nm. A spectroradiometer Optronic OL Series 750 with an integrating sphere OL 740-70 covered with polytetrafluorethylene was used [40,43,44].

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These experimentally determined optical properties are needed to calculate the fraction of

Photocatalytic reactor and experimental procedure

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2.2

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radiation absorbed by the photocatalytic paint film.

The experimental arrangement to assess the photocatalytic capability of the paints consists of a

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continuous flat photoreactor where the coated acrylic pieces are placed between the reactor walls. The dimensions of the photocatalytic reactor are 20 cm long, 10 cm wide and 0.2 cm thick on each side of the painted plate, and it is irradiated with seven fluorescent visible radiation lamps,

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typically used for indoor illumination, on both sides (Fig. 1). Further details of the reactor can be found in [40].

Acetaldehyde stabilized in nitrogen (300 ppm) is used as model pollutant. Dry and clean air flow is split into a gas washer bottle with the purpose of adjusting the humidity level. The total air stream is then used to dilute the acetaldehyde gas flow till reaching the desired contaminant

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concentration to feed the reactor. The experiments were performed at isothermal conditions selecting a representative room temperature of 25±1°C. The inlet and outlet concentrations of acetaldehyde and the reaction intermediates are analyzed through gas chromatography with a flame ionization detector (FID). A radiometer IL1700 with a visible radiation sensor SED033/F/W (400-1064 nm) and a UV radiation sensor SED005/WBS320/W (250 - 400 nm) was used to measure the total incident radiation flux. The spectral energy emission of the fluorescent lamps GE F4T5/CW was determined with a fiber optic spectrometer of Ocean Optics (Fig. 2). Optical filters were used to achieve different levels of incident radiation. The diffuse 8

transmittance of the filters was measured in the spectroradiometer Optronic OL Series 750 (Fig. 2). The pivot operating conditions used during the tests were 16.67 cm3/s for the gas flow rate, 2.1 × 10-10 mol/cm3 for the inlet acetaldehyde concentration, 50% for the gas relative humidity and 58.8 × 10-4 W/cm2 for the incident radiation flux within the total wavelength range lamp emission on the centre of the reactor window. For each one of the paints elaborated by varying the catalyst amount, three different sets of experimental runs were carried out maintaining the pivot working conditions and changing, one at a time, the relative humidity, the irradiation level and the inlet

Theoretical models

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2.3

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concentration of acetaldehyde.

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2.3.1 Optical properties

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Based on measured optical properties of diffuse reflectance and transmittance of the paintacrylic-paint system (Rpaint,acr,paint and Tpaint,acr,paint) and the acrylic plate (Racr and Tacr), and using

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the equations developed in Ballari et al. [43], the diffuse reflectance (Rpaint) and diffuse transmittance (Tpaint) of one layer of the paint coating are calculated:

Rpaint,acr,paint,Tacr, -Tpaint,acr,paint, Racr,

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Rpaint, =

Eq. (1)

2 2 Tpaint,acr,paint,Tacr,  -Tpaint,acr,paint, Racr, +Tacr,

    

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2   Tacr, R  Rpaint,acr,paint, -Rpaint,  1-Rpaint,  Racr, + 1-R  Rpaint, acr, paint,   Tpaint, = 2 T R Racr, + acr, paint, 1-Racr, Rpaint,

Eq. (2)

Then, the fraction of radiation absorption (Apaint) is determined as:

Apaint, = 1-Rpaint, -Tpaint,

Eq. (3)

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To take into account the absorbed radiation fraction by the photocatalyst, Equation (4) is proposed since some compounds of the paints (other than TiO2) can absorb radiation that is not used for the photocatalytic reaction: ATiO2 ,norm, = Apaint,norm,  App,norm,

Eq. (4)

where ATiO2,norm,λ, Apaint,norm,λ and App,norm,λ (cm2/g) are the normalized radiation absorption fraction

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per unit of weight of the deposited paint and per unit of superficial area of the TiO2, paint, and

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pseudo-paint (PP-noTiO2-C), respectively.

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2.3.2 Radiation field model

qw,  x   nw qw,  x  

2π

 I  x,   n

w

d

Eq. (5)

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0

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The spectral local net radiation flux on the reactor wall (qw,) is defined by [45]:

in which nw is the normal unitary vector that points to the reactor wall and I  is the radiation

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intensity associated to a beam of rays carrying energy of wavelength λ in the direction of the unit vector Ω corresponding to the solid angle Ω. In terms of the adopted coordinate system (Fig. 3) and making use of the extended, superficial, diffuse emission (E-SDE) source model [46], Eq. (5) can be formulated as: max max

   

P sin 2 cos d d 2π rL LL

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qw,λ  x  

min

min

Eq. (6)

2

where ϕmin, ϕmax, θmin and θmax are the limiting vision angles of the rays, Pλ is the spectral emission power of the lamp, and rL and LL are the radius and the length of the lamp, respectively. For the photoreactor used in the experiments, direct (qdir) and indirect (qind) radiation fluxes reach one coating layer coming from seven lamps placed on each side of the photoreactor. The

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direct flux comes from the lamps ahead to the deposited paint, while the indirect one comes from behind, crossing the other side paint layer and the acrylic that serves as support (Fig. 3). Thus, the total incident radiation flux for one side paint layer is calculated as [40]:

qp aint,   x    qdir,  ,i  x   qind,  ,i  x    qw, ,i  x  Tw, 1  Fq,  7

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i 1

i 1

Eq. (7)

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where Tw,λ is the spectral transmittance of the reactor wall and Fq,λ is the transmitted and reflected radiation fraction by the other side paint layer and support. Fq,λ is calculated by applying a

2 2 Racr,Tpaint,  Racr,  Rpaint,Tpaint,  Tacr,Tpaint,  Tacr, Rpaint, Tpaint, 2 1  Racr, Rpaint,   Tacr,2  Rpaint, 

Eq. (8)

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2

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Fq, 

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radiative flux balance in the three layer system:

es,a  x  =qp aint,   x  ATiO2 , max



min

es,a  x  d 

max



min

Eq. (9)

qp aint,   x  ATiO2 , d

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esa  x  =

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Finally, the spectral LSRPA and the total (polychromatic) LSRPA are calculated as:

Eq. (10)

where ATiO2,λ is ATiO2,norm,λ multiplied by the specific paint load on the acrylic plates (Table 1). To be able to calculate the useful total radiation flux or the total LSRPA it is important to define the

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integration limits λmin and λmax within which the photoreaction occurs. In this work, the lowest emission wavelength of the lamps is 310 nm, which sets the λmin for the reaction. On the other hand, the bandgap energy of the photocatalyst defines the maximum wavelength at which the electrons and holes are generated. Several works [41,47,48] have established that the bandgap energy of the KRONOClean 7000 titanium dioxide carbon doped is 3.21 or 3.30 eV (corresponding to 386 nm or 376 nm, respectively), while Pierpaoli et al. [49] have determined a much lower bandgap energy of 2.65 eV (λ=468 nm). The emission of the lamp used in this work

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is negligible between 376 and 386 nm (Fig. 2), so the upper limit of 386 nm will be considered for calculations, as well as the 468 nm limit.

2.3.3 Kinetic equations

The main intermediates of the acetaldehyde photocatalytic oxidation to carbon dioxide are formaldehyde and formic acid [50-52]. However, formic acid was not detected in gas phase with the adopted analytical method, so that formaldehyde is the only stable intermediate considered

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throughout the kinetic study.

The kinetic expressions for acetaldehyde and formaldehyde species can be derived from the

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reaction mechanism considering the following assumptions: a) the mass action law for the elemental reaction rates, b) the micro steady state approximation for the unstable or radical

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species, c) Langmuir adsorption isotherms with active site competition between water and the stable gaseous species, and d) the photocatalyst activation rate by radiation is equal to the

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wavelength-averaged primary quantum yield ( Φ  ) multiplied by an effective total LSRPA. The following equations can be derived for the reaction rates of acetaldehyde and formaldehyde [40]:

1 3CW CA  2 3CA  2 4CF   2 1  K WCW  K ACA  K FCF 

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rA  

rF 

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  2 1  K W CW  K ACA  K FCF  1 a   1 Φ es,eff  1 1  K W CW  K ACA  K FCF  1CW 

Eq. (11)

3CA - 4CF 1CW  2 3CA  2 4CF   2 1  K W CW  K ACA  K FCF  Eq. (12)

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  2 1  K W CW  K ACA  K FCF  1 a   1 Φ es,eff  1 1  K W CW  K ACA  K FCF  1CW 

where CA, CF and CW (in mol/cm3) are the gas phase concentrations of acetaldehyde, formaldehyde and water, respectively; KA, KF and KW (in cm3/mol) are their respective adsorption equilibrium constants; i (i=1 to 4) are kinetic parameters defined as groups of the elemental a reaction constants proposed in [40]; and es,eff is the effective total LSRPA that makes possible the

photocatalytic oxidation, which is calculated by: 12

max a s,eff

e

=

 e   e  a s,

a s,min,

 d

Eq. (13)

min

This equation is proposed to take into account that under certain conditions a minimum LSRPA ( esa,min ) has to be achieved for the photoreaction to take place, as some authors have also shown [53,54].

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Usually, in the reaction rate equations (Eqs. 11 and 12), the terms including the concentrations of acetaldehyde and formaldehyde can be neglected in comparison to the humidity term because

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the water gas phase concentration is several orders of magnitude higher than the pollutants concentrations. In addition, the square root terms in Eqs. (11) and (12) can be expanded as Taylor

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series in which the higher order terms of the series can be neglected for the case of low radiation absorption by the photocatalyst (as will be demonstrated below). Then, the simplified equations

1  K W CW

a es,eff

1  CA   2CF  a es,eff 1  K W CW

in which:

 3 Φ 2

2 

4 3

Eq. (14)

Eq. (15)

Eq. (16)

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1 

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rF 

1CA

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rA  

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for the superficial reaction rates of acetaldehyde and formaldehyde can be expressed as:

Eq. (17)

2.3.4 Mass balances in the photoreactor

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Considering that the reaction is free of external and internal mass transfer limitations, a onedimensional acetaldehyde mass balance along the reactor can be expressed as:

vair

dCA  x   av  rA  x, y  w  x  R dx  C  x A a  R f paint  1 A es,eff  x, y  VR 1  K W CW

wR

Eq. (18)

 x

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dCF  x   av  rF  x, y  w  x  R dx 1 CA  x    2CF  x   a A  R f paint   es,eff  x, y  VR 1  K W CW

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vair

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A similar equation can be written for the formaldehyde:

 x

re

wR

Eq. (19)

where vair (cm/s) is the air velocity, x (cm) represents the coordinate along the reactor, and av (1/cm) is the active photocatalytic surface area per unit volume of the reactor. The latter can be

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expressed as a relation between the reactor irradiated area (AR), the reactor volume (VR) and a proposed parameter fpaint, which takes into account the specific surface of the photocatalyst, the amount of TiO2 in the paint formulation and its particles agglomeration.

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The boundary conditions to solve Eqs. (18) and (19) are:

Eq. (20)

CF  x  0   0

Eq. (21)

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CA  x  0   CA,in

2.4

Photonic and quantum efficiencies

The photonic efficiency correlates the radiation flux that reaches the paint with the reaction rate, while the quantum efficiency takes into account the absorbed radiation that is effectively used on the photocatalytic reaction by the TiO2 inside the paint formulation. For this reason, the

14

effective total LSRPA is used (Eq. 13). These efficiencies can be determined with the following equations [43]:

rA max



AR

Eq. (22)

qw,   x  Tw, d

min

rA a s,eff

e



AR

 x

AR

rA

AR

max

 e   x   e  a s,

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ηq,A =

AR

a s,min,

 d AR

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min

Eq. (23)

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ηp,A =

The average acetaldehyde reaction rate in the reactor is calculated by:

=

Q  CA,in - CA,out 

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AR

AR

Eq. (24)

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rA

where Q (cm3/s) is the flow rate of the gas in the reactor and CA,out can be determined

3.

Results and discussion

Paint coatings images

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3.1

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experimentally or predicted by the kinetic model.

Fig. 4 shows the images obtained with Scanning Electron Microscope (SEM) and with Transmission Electron Microscope (TEM) for the coatings of the paints and the pseudo-paint. Figs. 4a and 4b correspond to SEM images of the same photocatalytic paint (18TiO2-C) before and after the coating were cured, respectively, while Figs. 4g and 4h correspond to TEM images of the same samples. In the latter it can be seen that the particles of TiO2 protrude from the coating surface. It could be stated that the resin surrounding the photocatalytic particles (see Fig. 4g) is partially degraded during the curing process. Figs. 4 b-d show the decrease of the TiO2 15

content in the paints from 18% w/w to 12% w/w, while Fig. 4e corresponds to the pseudo-paint that does not contain TiO2. As the amount of CaCO3 increases and the amount of TiO2 decreases, the number and extension of the surface cracks enhances, being the PP-noTiO2-C the coating that exhibits the largest ones. In the latter, the CaCO3 particles are covered by the resin, even though the sample was previously exposed to light. Finally, a side view of the paint 18TiO2-C is shown in Fig. 4f. It is observed that the thickness of the paint coating is almost constant, presenting a value of 11.2±1.6 μm for a deposited specific load of 1×10-3 g/cm2.

Radiation field

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3.2

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The calculated normalized spectral radiation absorption fractions for each prepared paints and for the pseudo-paint are shown in Fig. 5. First it can be noticed that the fraction of radiation

-p

absorbed by the paint film increases as the TiO2 amount in the formulations rises. Also, it is clear that the paints absorb a larger fraction of radiation at shorter wavelengths (300-400 nm) than in

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the visible radiation spectrum (400-800 nm). In addition it can be seen that the components present in the pseudo-paint can also absorb radiation. So, Eq. (4) was used to account for the

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radiation absorbed by TiO2.

The total LSRPA ( esa ) was computed employing two upper integration limits in Eq. (10) corresponding to the two different bandgap energies of the carbon doped TiO2 found in the

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literature, i.e. 386 and 468nm. Radiation with a lower energy than the bandgap, i.e. corresponding to larger wavelengths, cannot excite the photocatalyst to produce the electron/hole pair. Fig. 6 a shows the width average total LSRPA ( es

wR

) along the photoreactor for the three paints and

λmax of 386 nm. The LSRPA calculated with a maximum wavelength of 468 nm is shown in the

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Electronic Supplementary Material (Fig. ESM1) and its value is much higher than the one using the 386 nm limit. This is because the lamp shows two peaks at ca. 404 nm and ca. 435 nm (Fig. 2) that are not taken into account for the 386 nm integration limit. In Fig. 7 the total average incident radiation flux for one coating layer ( qp ain t a total average LSRPA by the photocatalyst ( es

AR

AR

) (Fig. 7a) and the

) (Fig. 7b) within the useful wavelengths

considering the higher reported bandgap energy (i.e. λmax = 386 nm) are shown for the different filters used between the lamps and the reactor window. The same results for λmax=468 nm are 16

reported in the Electronic Supplementary Material (Fig. ESM2). The first difference between these two limits is that qw

AR

for the 468 nm limit is almost one order of magnitude higher than

for the 386 nm limit. This difference in irradiation level will then affect the value of the radiation reaching the photocatalytic coating layer and the calculation of LSRPA. Also, when the filters are used, due to the variations in transmittance under different wavelengths (Fig. 2), the reduction of the irradiation level is different for both maximum wavelengths considered. For example, 46.5% of the radiation is transmitted by the lightest filter for the 386 nm limit, while this value is 67.2%

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for the 468 nm limit. In addition, for the 386 nm limit the LSRPA is a higher fraction of the total incident radiation flux because of the higher radiation absorption that presents the TiO2 in the UV

ro

compared to the visible spectrum range (Fig. 5). Finally, analyzing the effect of the TiO2 amount in the formulated paints in Fig. 7a, it can be observed that for the same irradiation level (i.e. the AR

), when the photocatalyst amount is increased in the paint, qp ain t

-p

same qw

AR

decreases. The

latter can be explained by the reduction of the indirect radiation flux coming from the other side

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of the coating when the TiO2 quantity in the paint is increased. However, an increase of the

3.3

Kinetic model results

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photocatalyst amount results in a higher total LSRPA (Fig. 7b).

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The species mass balances, Eqs. (18) and (19), were solved numerically with the Euler method and, using the Generalized Reduced Gradient (GRG) non-linear regression algorithm, the kinetic model parameters were estimated from the experimental values of the acetaldehyde and formaldehyde concentrations at the reactor outlet. Firstly, the highest reported bandgap energy corresponding to 386 nm was employed for the calculations. It should be noted that to calculate

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a a according to Eq. (13), the minimum LSRPA ( es,min ) was established at the highest es,eff

irradiation level used at which the observed reactor outlet acetaldehyde concentration is equal to the inlet one. In Table 2 the estimated kinetic parameters values, including the jointly estimated a parameters 1×fpaint, and the empirical values of es,min for each paint are shown employing

max=386 nm to calculate the effective LSRPA. Fig. 8 shows the kinetic model predictions against the outlet concentration values measured experimentally, resulting in a total Root Mean Square Error (RMSE) of 5.74% for acetaldehyde and 9.97% for formaldehyde. 17

The same analysis was done employing as integration limit max=468 nm for the calculation of the effective LSRPA. However, the kinetic model predictions are not suitable when the irradiation level is changed (see Fig. ESM3 of the Electronic Supplementary Material). Thus, the experimental results are better explained when a maximum useful wavelength of 386 nm is considered for the kinetic study. It should be highlighted that the maximum wavelength to activate the carbon doped TiO2 is almost identical to the corresponding one of the pristine TiO2. Despite of that, the acetaldehyde degradation by a paint formulated with 18 % w/w of undoped

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KRONOClean 7050 TiO2 was lower than the current tested paint with the same amount of carbon doped TiO2 KRONOClean 7000 under the same irradiation conditions of this work. While under the pivot conditions the carbon doped TiO2 paint showed 55.3% of acetaldehyde conversion, the

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paint formulated with pristine TiO2 presented 38.8%. In fact, KRONOClean 7000 has higher surface area which would contribute to adsorption of reactants onto the surface, and also shows

-p

radiation absorbance under visible light. However, this does not result in a lower bandgap according to the calculations of the optical bandgap energy applying the Kubelka – Munk method

re

indicating the photocatalyst modification could occur in the surface, so it would not be actually doped [47].

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Fig. 8a shows that as the irradiation level increases, the outlet concentration of acetaldehyde decreases. This is explained by Eq. (14) in which the reaction rate is proportional to the effective LSRPA. On the other hand, the formaldehyde concentration achieves a maximum for an

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intermediate level of radiation and is lower for higher irradiation levels. This is because the formaldehyde reaction rate depends not only on LSRPA but also on the acetaldehyde concentration (Eq. 15). Also, it should be noted that the photocatalytic reaction did not occur for the lowest irradiation level tested for the paints. So, the hypothesis of a minimum LSRPA to make possible the photooxidation reaction is experimentally confirmed. To validate that the

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a experimental determination of es,min by applying discrete reduction of the irradiation level is an a enough accurate method, the es,min for each paint was estimated as an extra kinetic parameter

with the nonlinear regression method. It was verified that the estimated values (not shown here) are slightly larger than the experimentally determined ones shown in Table 2. The outlet contaminant concentrations for the diverse paints varying the relative humidity level between 10 and 60% (CW =1.21 – 7.25 × 10-7 mol/cm3) are shown in Fig. 8b. As a general trend, it can be seen that when the relative humidity rise, the outlet concentrations are higher. 18

This is because the water molecules present in the air compete with the acetaldehyde and formaldehyde for the active sites of the catalyst, reducing the reaction possibility of the pollutants. The effect of changes on the inlet pollutant concentration can be seen in Fig. 8c. For the acetaldehyde it is observed that, within the experimental error, a pseudo first-order reaction rate with respect to the inlet pollutant concentration takes place. On the other hand, the measured outlet concentration of formaldehyde remains almost constant for the variations in the experimental conditions, while the model predicts a slightly increment of the outlet formaldehyde

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concentration as the inlet acetaldehyde concentration rises.

Analyzing the influence of the paint composition on the air purification capability, the paint

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with the higher amount of photocatalyst (18TiO2-C) presents a higher acetaldehyde conversion than the other paints tested under the same experimental conditions. However, it should be

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considered that with the decrease of the TiO2 amount, and therefore a lower cost for the paint production process, satisfactory results can be also expected during the decontamination process.

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To study the effect of the extender on the photocatalytic performance, additional paints with lower amounts of CaCO3 were prepared remaining constant the amount of TiO2-C in 18 % w/w.

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No significant effect on the acetaldehyde degradation was observed when the CaCO3 was decreased from 18% w/w to 8% w/w.

In addition, it should be noted that after more than 140 hours of reaction applying the same

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photocatalytic paint coating, no photocatalyst deactivation was detected for the acetaldehyde degradation [55].

For a better understanding of the 1×fpaint jointly estimated parameters shown in Table 2, a linear function was fitted with the obtained values (Fig. 9). Replacing this linear function into the mass balances (Eqs. 18 and 19), predictions of the acetaldehyde conversion by photocatalytic

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paints containing other amounts of TiO2 can be performed. It should be noticed that this parameter presents the highest value for the lowest amount of TiO2 in the paint. Given that 1 represents a group of kinetic constants of elementary reactions, which should be constant for the three paints, then the different values of the joined parameter 1×fpaint correspond to the variation of fpaint. This indicates that increasing the amount of TiO2 in the paint, the degree of agglomeration of the photocatalytic particles also increases and thus the available active surface for reaction proportionally decreases. The XPS results confirm this last since the Ti/O ratio at the

19

paint coating surface (10 nm deep) increases from 0.015 to 0.019 when the TiO2-C amount in the paint was decreased from 18 to 12 % w/w. For lower amount of TiO2-C, smaller particles agglomerations are formed that are better distributed on the paint coating surface. On the other hand, in paints containing more photocatalyst (e.g. 18 %w/w) bigger and less agglomerates coexist, which are mainly under the surface and only a fraction of them is on the surface. Still the photocatalyst is absorbing radiation, so the Local Superficial Rate of Photon Absorption (LSRPA) is higher and contributing to the photocatalyst activation, but the available photocatalytic area at the paint surface in contact with the contaminant is lower. Also, the XPS

Efficiencies

-p

3.4

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Ti/O ratio is 0.011), which is in agreement with the TEM observation.

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analysis of 18TiO2-C coating before the irradiation curing revels less TiO2 onto the surface (the

The photonic and quantum efficiencies for the acetaldehyde and for the 14TiO2-C and 18TiO2-

re

C paints are computed according to Eqs. (22) and (23), respectively. To be able to observe the behavior of these efficiencies, a surface for variations in humidity and irradiation levels is

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calculated. For the average reaction rate Eq. (24) is used, in which the outlet acetaldehyde concentration was experimentally determined or theoretically calculated with the kinetic model. A good agreement between the experimental concentrations (dots) and the kinetic model

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predictions (surfaces) can be observed in Fig. 10. The photonic efficiency (Fig. 10a) presents a maximum with the incident irradiation level on the wall reactor that varies according to the gas water concentration. This result can be explained with the definition of photonic efficiency (Eq. 22), which makes use of average values. While the average radiation flux is reduced in the same proportion as the variation in irradiation level, the average reaction rate presents a different

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behavior at high irradiation levels. The acetaldehyde reaction rate along the reactor does not only change with the irradiation level but also with the pollutant concentration (see Eq. 14). Thus, the average reaction rate increases till certain value of incident radiation flux at which the pollutant is consumed very fast decreasing the reaction rate at the reactor exit. On the other hand, this maximum is not observed for the quantum efficiency (Fig. 10b) in which the lower the humidity and irradiation levels, the higher the quantum efficiency. It should be noted in Fig. 10b that a values of es,eff

AR

are different for the two formulations when the irradiation level is changed.

20

Also, comparing Figs. 10a with 10b, it can be observed that paint 18TiO2-C shows higher photonic efficiency, while paint 14TiO2-C exhibits superior quantum efficiency. The paint with a lower amount of photocatalyst or with a lower irradiation level is more efficient in terms of radiation use because the decrease of the rate of absorbed photons by the catalyst (denominator of Eq. 23) is lower than the decrease of the average oxidation rate (numerator of Eq. 23).

4.

Conclusions

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Photocatalytic paints for indoor air decontamination were formulated using different amounts of a carbon doped TiO2. Based on the photocatalytic reaction mechanism, an intrinsic kinetic

ro

study was developed employing acetaldehyde as the main pollutant and analyzing the formation of reaction intermediates. A minimum irradiation level was experimentally observed below

-p

which the photocatalytic reaction did not take place. So, the photocatalyst activation rate was postulated to be proportional to an effective local superficial rate of photons absorption. From the

re

resolution of the mass balances and the radiation field equations, the kinetic parameters were estimated employing the experimental reactor outlet concentrations. A new parameter (fpaint) was

lP

proposed to relate the photocatalytically active area with the amount of TiO2 in the diverse paint formulations. This parameter increases as the TiO2 amount in the paint formulation decreases, which can be explained by the agglomeration degree of the photocatalyst particles in the paint.

ur na

Based on bibliographic information of the photocatalyst bandgap and the emission spectrum of the lamp, two maximum wavelengths were selected for the calculation of the total LSRPA. According to the correlation between measured and predicted values of the contaminant and intermediate outlet concentrations, the maximum useful wavelength was determined to be 386 nm. The RMSE between model predictions and experimental outlet concentrations for the three

Jo

photocatalytic paints and different experimental conditions was 5.74% for acetaldehyde and 9.97% for formaldehyde. Additionally, the photonic and quantum efficiencies were calculated for the 14TiO2-C and 18TiO2-C paints, for variations in humidity and irradiation levels. The 18TiO2C paint showed the highest photonic efficiency, while the 14TiO2-C formulation exhibited the highest quantum efficiency. The results obtained in this work indicate that the role of the radiation should be studied in more detailed to be able to know the minimum radiation required for the photocatalytic process and the wavelengths that are effectively used by the chemical reaction. On the other hand, the photocatalytic paints are a feasible technology of air 21

decontamination to be developed on a larger scale, in which the development of an intrinsic kinetic model is an essential tool for the scaling up process.

Credit Author Statement

ro

of

Federico Salvadores, Orlando Mario Alfano and María de los Milagros Ballari designed the study. Federico Salvadores performed the experiments and data collection. Federico Salvadores, Orlando Mario Alfano and María de los Milagros Ballari analysed all the data and made theoretical calculations. Federico Salvadores and María de los Milagros Ballari wrote the main manuscript text and prepared the figures and tables. Orlando Mario Alfano and María de los Milagros Ballari obtained funding for the research. All authors reviewed the manuscript, made amendments and contributed with their expertise.

re

-p

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The

authors

are

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Acknowledgements

grateful

to

Universidad

Nacional

del

Litoral

(UNL,

Project

PIC50420150100009LI), Consejo Nacional de Investigaciones Científicas y Técnicas

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(CONICET, Project PIP-2015 0100093), and Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT, Project PICT-2015-2651) of Argentina for financial support. Thanks are given to ANPCyT for the purchase of the SPECS multitechnique analysis instrument (PME82003). Antonio Negro, Juan Andini, Claudio Passalía and Eduardo Vidal are acknowledged for their help during the experimental work at INTEC. Kronos is thanked for the provided

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photocatalyst. BASF Argentina is thanked for the provided resin and dispersant agent.

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Figures Air, acetaldehyde and formaldehyde outlet Photocatalytic paint coating Optical filter (when corresponds)

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Indoor light lamps

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Reactor wall Air and acetaldehyde inlet

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Figure 1: Schematic reactor configuration

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Figure 2: Normalized spectral emission of the lamps and diffuse transmittance of the filters

29

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Figure 3: Coordinate system used in the photoreactor

b)

c)

d)

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a)

30

f)

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e)

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TiO2-C 200 nm

Resin

200 nm

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g) h) Figure 4: SEM images (2000×): a) 18TiO2-C previously to the irradiation curing; b) 18TiO2-C; c) 14TiO2-C; d) 12TiO2-C; e) PP-noTiO2; f) 18TiO2-C (side view). TEM images (30000×): g) 18TiO2-C previously to the irradiation curing; h) 18TiO2-C

Figure 5: Normalized spectral fraction of radiation absorbed by the photocatalytic paints and pseudo-paint

31

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a)

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of

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three paints, 100% of irradiation level and λmax= 386 nm

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Figure 6: Reactor width average Local Superficial Rate of Photon Absorption (LSRPA) for the

32

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b)

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Figure 7: Correlations of the average incident radiation flux (λmax=386 nm) on the window

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a)

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reactor with: a) the radiation flux on the coated photocatalytic paint and b) LSRPA

33

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b)

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c) Figure 8: Experimental (symbols) and calculated with the model (lines) outlet concentrations of acetaldehyde and formaldehyde for different: a) irradiation levels, b) relative humidities and c)

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inlet concentrations of acetaldehyde

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Figure 9: Parameter 1×fpaint as a function of the amount of photocatalyst in the paint formulation

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a)

b)

Figure 10: Experimental (symbols) and theoretical (surface) efficiencies for paints 14TiO2-C and 18TiO2-C varying the humidity level and the incident radiation flux: a) photonic efficiency and b) quantum efficiency

35

Tables

14TiO2-C

18TiO2-C

Water

29.8

29.9

29.9

Extender (CaCO3)

23.9

21.9

17.9

Photocatalytic TiO2

12.0

14.1

17.9

Resin

33.4

33.4

33.3

Dispersing Agent

0.7

0.7

0.7

Fineness of grind [μm]

22

Deposited specific load [g/cm2] × 104

9.1

24

21

8.9

11.8

re

-p

Component [% w/w]

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12TiO2-C

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Table1. Paint composition and specific load on the acrylic plate

Table 2. Estimated kinetic parameters of the models and empiric minimum LSRPA Value

95 % C.I.

1.15 × 106 5.68 × 108

2.06 × 107

6.25 × 104

δ1× f12TiO2-C [cm3/Einstein]

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Name and units

δ1× f14TiO2-C [cm3/Einstein]

4.84 × 108

1.01 × 107

δ1× f18TiO2-C [cm3/Einstein]

3.28 × 108

6.70 × 106

7.58

0.04

a [Einstein/cm2/s] es,min,12TiO 2 -C

8.54 × 10-12

-

a [Einstein/cm2/s] es,min,14TiO 2 -C

1.50 × 10-11

-

a [Einstein/cm2/s] es,min,18TiO 2 -C

2.65 × 10-11

-

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KW [cm3/mol]

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δ2 [dimensionless]

36