Portable photoreactor for on-site
measurement of the activity of photocatalytic
surfaces
Francesco Pellegrino
a*, Marco Zangirolami
b, Claudio
Minero
aand Valter Maurino
a*
a Chemistry Department and NIS Inter-departmental Centre, University of Torino, Via P.
Giuria 7, Torino, 10125, Italy
b Fonderia Mestieri S.r.l., 10093, Collegno (TO), Italy
KEYWORDS: Photoreactor; Surface; Nitric Oxide; TiO2 Nanoparticles
Abstract
Photocatalytic building materials, asphalts for road paving, pigments and coatings have been shown to be a promising remediation technique for air pollutants such as NOx and VOC’s. An open issue is the durability of the air cleaning characteristic in time of deployed material. Current ISO standards for photocatalytic activity testing are time consuming and cannot be used directly on-site to follow the activity of the material surface in time. The present work describes a new portable system for assessing the photo-catalytic activity of the surface of a material. The system is composed of a continuously stirred continuous flow photoreactor with a very low volume chamber (15 mL) including a 4 cm diameter opening. This opening is equipped with a gasket system that assure very good seal when the reactor lay on the surface to be tested. The gaseous phase is mixed with a UV transparent turbine. The surface is uniformly irradiated with an adjustable LED source. A gaseous stream containing a substrate (e.g. NO) is fluxed in the photoreactor. The outflow from the reactor is monitored with the proper sensor (Photoionization detector for VOC, electrochemical sensor for NOx).
The portability and the low volume of the reaction chamber allow to carry out very fast measurements (few minutes) on-site, with several advantages in terms of time, costs, making very easy the durability control of deployed photocatalytic materials, also on rough surfaces. Here we report the description of the photoreactor and its characterization by using anatase TiO2 films as active materials and NO as substrate.
Introduction
Nitrogen oxides (NOx, which refers to NO and NO2) are among the most harmful
atmospheric pollutant gases. They are emitted by combustions, like for example in coal plants and automobile engines. Moreover, they contribute to several other pollution problems like acid rains, nitrogen pollution in water, smog and greenhouse effect [1, 2, 3]. There is, therefore, a great interest in reducing the concentration of NOx especially
in urban areas [4]. The ability of TiO
2 photocatalysis for NOx abatement was extensively
demonstrated and the mechanism of NOx transformation was studied in depth [4, 5, 6, 7, 8]. The NO photooxidation proceeds through a surface charge transfer complex and
not by a direct reaction with free/trapped valence band holes [4].
In this context, commercial photocatalytic materials are finding increasing usage throughout the world for air pollution abatement, with the market’s total being projected at over $3 billion for 2020 [9]; the bulk of this use is in the form of building
materials and road asphalts. Examples are self-cleaning and NOx/VOC abating glass, paint and concrete materials [10, 11]. It follows that there is a real need for novel,
accessible and economical testing methods for assessing the efficacies of such materials and their durability once deployed [12, 13]. Both the manufacturers and end-users of
such materials are in urgent need of reliable, reproducible, fast, on-site and, possibly, traceable measurement methods of photocatalytic activity. ISO testing standard methods for photocatalytic activity are slow, costly, require dedicated technical support and cannot be used on-site. A typical ISO 22197-1 (2016) test requires a six hours’ irradiation run of the tested sample. To test a deployed surface, a test specimen must be sampled and delivered to the laboratory [14, 15]. Recently, to overcome these
indicator inks (paiis), that can be used on-site and don’t require dedicated apparatuses [16, 17]. These are based on redox indicators dyes. The ink is applied on the
photocatalytic surface and its change of color/discoloration is a measurement of the photoactivity of the surface. However, color analysis to quantitatively assess the activity is feasible only on flat surfaces, like glass windows, and not on rough surfaces like concrete or facades. Moreover, the irradiation of these paiis is generally carried out with solar light, allowing only semi-quantitative evaluations. Finally, the results of dye tests are significantly influenced by various factors such as the dye sensitization of catalyst particles, the absorption spectral overlap between dyes and photocatalysts in the visible region, the electrostatic interaction (attractive or repulsive), and the properties of dye degradation intermediates. In general, the dye decolorization efficiency was poorly correlated with the dye mineralization efficiency [18]. Similar problems affect methods
based on contact angle measurement [19].
The present work describes a portable continuously stirred continuous flow photoreactor system able to measure the photo-catalytic activity of a surface in a fast and reliable manner. The internal volume of the reactor chamber is only 15 mL, assuring a very short transient (under 100 seconds) when incoming substrate gas flow is 100-500 sccm. A 4 cm diameter opening in the reaction chamber assures the contact with the tested surface. This opening is equipped with a gasket system that assure very good seal when the reactor lay on the surface to be tested, thus allowing on-site measurements directed to assess the durability of deployed photocatalytic materials. The gaseous phase is mixed with a UV transparent turbine. The surface is uniformly irradiated with an adjustable LED source. A gaseous stream containing a substrate (e.g. NO) is fluxed in the photoreactor. The outflow from the reactor is monitored with the proper sensor (Photoionization detector for VOC, electrochemical sensor for NOx).
Experimental
Reagents and Materials
The photocatalytic tests were carried out on two different commercial TiO2 powders:
Cristal ACTiV PC105 (anatase) was provided from CRISTAL, P25 (80% anatase, 20% rutile) by Evonik Industries, Germany.
TiO
2Films Preparation
The photocatalytic activity is evaluated on TiO2 thin film produced starting from
powders and using a pyrex glass as a support, using the following procedure:
1. Production of the TiO2 paste: TiO2 powders, PVP, ethanol, acetylacetone, acetic
acid and Triton-X 100;
2. Spin coating (520 rpm/min) of the paste on the pyrex support (round pyrex glass with 4.7 cm of diameter);
3. Heat treatment at 450°C for 1 hour in order to remove all the organic matter. The resulting films have thickness around 2 micrometers. Thick films assure the independence of the photoactivity from the amount of deposited material, having nearly infinite optical thickness in the UV.
Results and discussion
Photoreactor design
The choice of a continuously stirred continuous flow photoreactor Vs a plug flow photoreactor, like that used in the ISO 22197-1:2016, was done on the basis of a previous detailed analysis of these two configuration. The continuously stirred, continuous flow reactor allows the extrapolation of the intrinsic photocatalytic activity of the tested surface, thus excluding the contribution of the mass transfer resistance across the gaseous boundary layer on the solid surface [8].
The 15 mL reaction chamber and the rotating fan were built by casting a UV transparent PMMA polymer (PLEXIGLAS GS, Evonik, transmittance > 90% on a
thickness of 8 mm till to 320 nm). In Figure 1 and Figure 2 are reported a perspective view and a 3D rendering of the chamber.
Figure 1. Perspective view of the reaction chamber with the fan system and the LED source. See text for details.
With reference to Figure 1, the radiating source (3) lies outside the reaction chamber (1), while the fan (7) lies in the optical path of the LED source (3), allowing the reduction of both the sizes of the reaction chamber (1) and the flow rate of the gaseous fluid, decreasing the filling transient of the chamber under 100 seconds (500 sccm of inlet gas flow).
With reference to Figures 1 and Figure 3, it is possible to note that the LED source (3) was realized with an array of six UV LED (model LZ4-04UV00 Led Engin, with the emission peak centered at 365 nm). The LEDs are mounted on an aluminum printed circuit board containing also a constant current power supply (5). The LEDs (4) are arranged at suitable distances with respect to the reactive surface of the material to guarantee a homogeneous irradiation (±10%). The LEDs emission is controlled with a closed loop feedback with a control photodiode (6). Incident UV radiation can be regulated from 5 to 150 W m-2. The fan (7) rotates at 1-2 mm over the surface to be
tested, and the rotating speed can be continuously adjusted from 500 to 10000 rpm. The fan rotor (8) is equipped with holes (10), to allow a swirling motion of the gaseous fluid coming from the inlet pipe (14), with respect to the reactive surface of the material and with radial blades (11), to drag in rotation the gaseous fluid. The stator (9) is coaxial with respect to the rotor (8). The stator (9) comprises radial blades (12) shaped in order assure a turbulent motion of the gas inside the chamber. In this way, a pressure difference is created, necessary to force the gas to perform several circulations through the holes (10), before outflowing towards the outlet pipe (15). The inlet and outlet pipes (14, 15) are connected to the stator (9).
With reference to Figures 1, Figure 3A and Figure 4 it is possible to note that the reaction chamber (1) delimited by the bell shaped stator (9) during the test is laying over the reactive surface of the material. The stator (9) comprises a gasket (2) to allow a good seal with the reactive surface to be tested. The gasket sealing the reaction chamber is made of closed cells EPDM foam. A photograph of the seal is reported in Figure 5.
Figure 3. A) perspective view of the reaction chamber; B and C) perspective view of the radiating system, of the ventilation means and of the reaction chamber
The system allows to:
- improve the distribution and regulate the intensity of incident radiations;
- decrease the volume of the reaction chamber in order to decrease the necessary gas flow and decrease the duration of the filling transient as well as the response time of
the photoreactor (a typical measure is complete in 30 minutes, instead of 6 hours needed by the ISO 22197-1:2016 standard);
- regulate the turbulent regime inside the reaction chamber, in order decrease the thickness of the gaseous boundary layer and allow the extrapolation of the intrinsic photocatalytic activity of the surface [8].
Figure 5. Photograph of the bottom opening of the photoreactor with the EPDM gasket seal.
The finite elements analysis of the fluid dynamic field showed the occurrence of a turbulent regime already with a fan speed of 300 rpm, namely more than one meter per second of absolute speed, enough to avoid a laminar regime. Further details on the photoreactor design are reported elsewhere [20].
The NO and NO2 concentrations in the outflow of the photoreactor were monitored by
miniature electrochemical gas sensors (Alphasense mod NO-A4 and NO2-A43F, www.alphasense.com). The sensors signals were digitalized by a 16 bit AD/DA converter (National Instrument mod 6002 USB, http://www.ni.com/it-it/support/model.usb-6002.html). The temperature and humidity were monitored by a digital miniature thermohygrometer sensor (Sensirion SHT75, https://uk.rs-online.com/web/p/temperature-humidity-sensors/1691853/). The gaseous streams were generated by portable 1 and 10 liters gas cylinder containing NO in N2 (100 ppmv
certified gas mixture, Praxair) and air (zero grade, praxair), by using an array of three mass flow controllers (NO, dry air and humidified air). The mass flow controllers were controlled through the DA channels of the AD/DA converter. The control software was written by using LabView 2016 (National Instruments).
Photoreactor characterization
The effect of the fan speed on the photoconversion of NO is shown in Figure 6 and Figure 7, where is reported an example of photocatalytic test carried out on the TiO2
P25 film. The increase in NO photoconversion increasing the fan speed is due to the decrease in the boundary layer thickness, resulting in a reduction of the mass transfer resistance. The complete treatment of the CSTR theory and the model used for the NO photodegradation is reported in a previous work [8]. The low volume of the reactor allows to carry out a test in no more than 20 minutes.
Figure 6. Time profiles of NO, NO2 and NOx concentrations during the photocatalytic test carried out on TiO2 P25 at 1 ppmv NO initial concentration, 60% relative humidity, temperature 25 °C, 144 W m-2 of
radiant power at 365 nm, 500 sccm inlet gas flow.
Figure 7. LEFT: plot of the NO degradation rate vs the fan speed: RIGHT: linearization of the results using a plot of the inverse of the NO degradation rate vs the inverse of the fan speed. The plots refer to the same measurement: 1 ppmv NO nominal initial concentration, 60% relative humidity and 144 W m-2 for the material P25.
From the extrapolation of the graph reporting the inverse of the NO degradation rate vs the inverse of the fan speed (Figure 7), it is possible to obtain the intrinsic photoactivity at infinite fan speed, where the thickness of the boundary layer is null and no resistance at the mass transfer occur [8].
In Figure 7 are reported the extrapolated NO degradation rates for P25 and PC105 films as a function of inlet substrate concentration, incident radiant power and relative humidity. The trends are in good agreement with the behaviors reported in literature [21, 22, 23, 24], and can be successfully described by the quadratic kinetic model
developed by Minero and Vione [21]. This approach, extensively validated [23], takes
into account the influence of the photon flux and substrate adsorption on the photocatalytic rate, where the Langmuir-Hinshelwood model is not adequate. In Figure 8, the NO photocatalytic conversion has: A) a saturative trend as a function of substrate concentration: a common behavior for several photocatalytic reactions, due to the limit of carriers’ photo-generation rate under a specific irradiance [21, 24, 25, 26]; B) a
dependence from the square root of the incident radiant power, as already observed and described by Minero and Vione [21]; C) a decrease with the increase of the relative humidity, due to the competition for the active sites given by water. All the photocatalytic runs were carried out in triplicate, and the relative standard deviation of the measurements were all under 5%.
Figure 8. A) Trend of the photocatalytic NO degradation for the two considered materials as a function of inlet NO concentration (incident radiant power 22 W m-2, 60% relative humidity); B) Trend of the photocatalytic NO degradation for the two considered materials as a function of the square root of the incident radiant power (I) (NO inlet concentration 1 ppmv, 60% relative humidity); C) Trend of the photocatalytic NO degradation for the two considered materials as a function of the relative humidity
(incident radiant power 22 W m-2, NO inlet concentration 1 ppmv). For all experiment the temperature was set at 25 °C.
Conclusion
The new portable photoreactor described here allows fast, reliable and on-site assessment of the photo-catalytic activity of the surface of a material, also on rough surfaces. The system is composed of a continuously stirred continuous flow photoreactor with a very low volume chamber (15 mL) including a 4 cm diameter opening equipped with a gasket system that assure very good seal when the reactor lay on the surface to be tested. The regime in the reactor is maintained turbulent with a UV transparent fan, and the extrapolation of the substrate conversion rate per unit area at infinite fan speed allows to obtain the intrinsic photocatalytic activity of the material, depurated from mass transfer resistances. The surface is uniformly irradiated with an adjustable LED source. A gaseous stream containing a substrate (e.g. NO) is fluxed in the photoreactor. The outflow from the reactor can be monitored with the proper sensor (Photoionization detector for VOC, electrochemical sensor for NOx).
The portability and the low volume of the reaction chamber allow to carry out very fast measurements (few minutes) on-site, with several advantages in terms of time, costs, making very easy the durability control of deployed photocatalytic materials, also on rough surfaces.
Acknowledgements
Dedication. The authors dedicate this work to the memory of professor E. Pelizzetti (16 February 1944- 25 July 2017) – University of Torino, Italy - for his pioneering research in heterogeneous photocatalysis, which inspired many of the papers cited in this work. FP and VM acknowledge funding from SETNanoMetro, EU Project, FP7-NMP-2013_LARGE-7. Project number: 604577
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