INTERFERING EFFECTS IN THE MEASUREMENT OF BTEX DEPOLLUTION IN AIR BY PHOTOCATALYTIC MATERIALS Alberto Strini and Elisa Bossi ITC, Consiglio Nazionale delle Ricerche, San Giuliano Mil., Italy Abstract In the determination of the activity of photocatalytic materials towards the degradation of air pollutants there are several potential interfering effects that could degrade the precision of the measurement. In this work it was investigated the effect of variations of irradiance, relative humidity, temperature and oxygen concentration in the determination of BTEX degradation in air by TiO 2 (Degussa P25) using a stirred flow photoreactor. The obtained results indicate the critical parameters to be controlled during a catalytic depollution measurement. The measures reported in this work relate to pure titanium dioxide samples but the results can be used as starting point in the definition of research and normalized measurement methods. 1. INTRODUCTION The increased interest in the research and commercialization of photocatalytic air depolluting materials [1, 2, 3] for building applications has stimulated the development of analytical methods and standardized test procedures for the assessment of the performance of these innovative multifunctional materials. The measurement of the depolluting properties of photocatalytic materials is usually done exposing a sample to an artificial atmosphere containing the target pollutant inside an irradiated photoreactor. This latter can be a sealed, batch type catalytic reactor or a continuous flow photoreactor. The depollution activity is typically inferred measuring the pollutant concentration decrease in the artificial atmosphere as consequence of the photocatalytic activity of the sample. The measurement process is particularly critical because, if the material must be tested under typical ambient conditions, very low pollutant concentrations are usually involved (in the ppb-ppm range) and the catalytic material activity must be determined under controlled conditions (e.g. irradiance, temperature and humidity). The experimental conditions must be defined and then controlled during the measurement process. It is quite important to estimate the precision required in the control of each experimental parameter because there is always a tradeoff between the required final precision of the measurement and the resource (e.g. cost, time, praticity) available to implement the measurement itself. The determination of the potential errors due to the 117
variations in key experimental conditions is very useful for identify the critical parameters to be tightly controlled from the less critical ones that allows less severe constraints. In this work it was measured the potential errors due to variations in four fundamental experimental parameter (irradiance, temperature, humidity, oxygen concentration) in the measurement of the photocatalytic depollution of an air mixture of benzene, toluene ethylbenzene and o-xylene (BTEX) mixture by a pure titanium dioxide sample. Although the data in this work are referred to the reactivity of this specific catalyst and for different catalytic materials are possible different behaviour, the reported results can be useful as a starting point for the estimation of the precision required in the control of some experimental parameters during the measurement of air depollution properties of other photocatalytic materials. 2. MATERIALS AND METHODS 2.1 Photocatalytic samples The measurements in this work were carried out using pure titanium dioxide catalyst samples, prepared with nanometric anatase (Degussa P25). Each sample was prepared suspending 65 mg of P25 titania in about 10 ml of deionized water. After manual mixing, the suspension was poured into a 9 cm diameter Petri dish and the water was allowed to evaporate. The resulting film on the Petri surface was then thermally treated in oven (1 h at 150 C). 2.2 Photocatalytic activity measurements The photocatalytic activity was measured with a 2 L stirred photochemical reactor irradiated with an array of 12 fluorescent UV-A lamps (Philips PL-S/10). The catalytic activity is defined in this work as: C ACT = O RATE C R (1) where O RATE is the oxidation rate as mass of pollutant degraded per sample surface area and time (µg m -2 h -1 ), C R is the pollutant concentration in the reactor chamber during the photocatalytic process and C ACT is the catalytic activity in (µg m -2 h -1 )/(µg m -3 ) or, briefly, (m h -1 ). The detailed procedure and experimental setup were described elsewhere [4]. For all experiments it was measured the photocatalytic degradation of a mixture of benzene, toluene, ethylbenzene and o-xylene (BTEX). The reference measuring conditions were a photocatalytic reactor internal air temperature of 25 C, and a 100 ml/min supply air flow with 50% relative humidity and 20.9 % oxygen concentration. The nominal BTEX concentration in the supply air was 100 ppb, obtained with a 4 ml/min flow from a 2.5 ppm standard cylinder (Sapio, Italy). 3. RESULTS 3.1 Irradiance The irradiance is one of the most important parameter of a photocatalytic process. The effects of slight irradiance variations around a set-point value (500 µw cm -2 ) were measured with a series of four determinations using an about 40 µw cm -2 step. As expected, variations 118
in irradiance give high variations in the measured catalytic activities (Figure 1). The error associated to irradiance variations measured in this experiment can be as high as 0.2 percent of the measured catalytic activity per µw cm -2 in irradiance variation. This translates to a 5% of catalytic activity measurement error with a 5% of lamp optical power variation in the adopted conditions. The errors due to irradiance variations can negatively affect the measurement repeatability because of the optical power drift associated with lamp aging and the differences found in each lamp, even if coming from the same production lot. If the relationship between irradiance and catalytic activity is known in advance (for a given catalyst and target pollutant) it is possible to keep it in account and refer the measurement to a standardized irradiance level [5]. In this case it is possible to trace the dayto-day irradiance variations due to lamp aging and lamp substitutions with a photodiode radiometer, improving the measurement repeatability. Given the sensitivity of the measured catalytic activity to the actual irradiance, in order to comparing results from different labs it is however mandatory to use sources with the same irradiation spectra and to measure the irradiance at the sample surface keeping in account the possible differences in the radiometers spectral sensitivity. Figure 1: Catalytic activity measured with irradiance variations 3.1 Temperature The effects of slight temperature deviation from the 25 C standard set-point were measured with a series of five determinations carried out at different temperatures using a 0.5 C step. The UV-A irradiance at the sample surface in the five tests was 655±10 µw cm -2. The results are reported in Figure 2, with a linear fitting superimposed to the experimental data. The data obtained in this experiment indicate an overall increase in the measured catalytic activity with the rise of the photoreactor internal air temperature. For the worst measured case (toluene) the error potentially associated to temperature variations can be about 5% of the measured catalytic activity value per degree of temperature variation. 119
Figure 2: Catalytic activity measured with air temperature variations 3.2 Humidity The effect of relative humidity variations from the 50% set-point was measured with four determinations taken at 5% R.H. interval. The results are reported in Figure 3. with a linear fitting superimposed to the data. The UV-A irradiance at the sample surface in the four tests was 590±8 µw cm -2. In this case it was observed a different behaviour for each compound, with a maximum dependence measured for toluene and benzene with a 2% of catalytic activity variation for each percent point of R.H. variation. Figure 3: Catalytic activity measured with supply air R.H. variations 120
These results suggests the needs for a good R.H. control in the artificial air generator. In this specific case, a ±2% variation in the R.H. level could cause a ±4% error in the catalytic activity determination for some compounds. 3.3 Oxygen concentration The effects of variations in the artificial air oxygen concentration around the 20.9 % standard atmospheric level was measured with measured with a series of four determinations carried out at different concentrations using a 2% step. The results are reported in Figure 4, with a linear fitting superimposed to the experimental data. The UV-A irradiance at the sample surface in the four tests was 535±12 µw cm -2. The results does not indicate a remarkable effects of the oxygen concentrations in these conditions, suggesting that the control of the exact percentage of oxygen in the supply air is not a critical issue. Figure 4: Catalytic activity measured with supply air oxygen concentration variations 4. CONCLUSIONS The effects of the variation of four key experimental parameters were measured using a pure TiO 2 sample in conditions comparable to those found in real ambient applications. The irradiation, temperature and the relative humidity appears to be critical parameters that should be precisely controlled in order to obtain reproducible results. The oxygen concentration (around the 20.9% standard air value) appears to be a not critical parameter, and slight variations can be tolerate. ACKNOWLEDGEMENTS This work was supported by Production System Department of CNR, subproject Nonconventional construction technologies and materials for the control of pollution in the built environment. 121
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