22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium

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Plasma-Pd/γ-Al203 catalytic system for methane, toluene and propene oxidation: effect of temperature and plasma input power

T. Pham Huu1, 2, P. Da Costa3, S. Loganathan1 and A. Khacef1

1 GREMI-UMR 6744, CNRS-Université d'Orléans, 14 rue d’Issoudun, P.O. Box 6744, FR-45067 Orléans Cedex 02, France

2 Institute of Applied Material Science, Vietnam Academy of Science and Technology, 1 Mac Dinh Chi, HCMC, Vietnam 3 Sorbonne Universités, UPMC Paris 6, Institut Jean Le Rond d’Alembert, CNRS UMR 7190, 2 place de la gare de

ceinture, FR-78210 Saint Cyr l’école, France

Abstract: A pulsed non-thermal plasma and 1 wt% Pd/γ-Al2O3 catalyst was used to investigate the CH4, C3H6, and C7H8 oxidation in air. Effects of temperature and specific input energy on the VOCs conversion were studied. The plasma-catalyst interaction revealed the benefit effect on the VOCs oxidation even at low temperature leading to high CO2 selectivity. The synergistic effect of combining plasma with catalyst in one-stage configuration is observed only for toluene.

Keywords: non-thermal plasma, Pd/γ-Al203, synergistic effect, methane, propene, toluene, oxidation

1. Introduction

Volatile organic compounds (VOCs) emitted from various industrial and domestic processes are important sources of air pollution and therefore, become a serious problem for damaging the human health and the environment. The well-established technologies for VOCs abatement, thermal and catalytic oxidation [1], require a thermal energy that makes them unsuitable and energetically expensive for the treatment of moderate gas flow rates containing low VOC concentrations. As an alternative to the catalytic oxidation, there have been extensive researches on using non-thermal plasmas (NTP) to remove various types of gas-phase hazardous pollutants over the last two decades [2–5]. However, NTP technology has a disadvantage like undesirable by-products formation such as ozone, aldehyde, and NOx. More recently, effective use of NTP on air pollution control is possible by exploiting its inherent synergistic potential through coupling with heterogeneous catalyst [6-11]. The combination of NTP with catalyst can be made in either single stage (plasma driven-catalyst) or two-stage configuration (plasma-assisted catalyst). Compared to conventional NTP alone, the effectiveness of these systems have been demonstrated in terms of energy efficiency, products selectivity and carbon balance.

In this work, the oxidation of methane, propene and toluene in air at atmospheric pressure was investigated using a Dielectric Barrier Discharge (DBD) reactor combined or not with γ-Al2O3 and 1 wt% Pd/γ-Al2O3 catalysts. Results are reported as a function of temperature, specific input energy (SIE), and position of the catalyst in the reactor (IPC, In Plasma catalysis and PPC Post-Plasma Catalysis).

2. Experimental The plasma reactor is a cylindrical DBD gives the

possibility to combine the catalyst with plasma reactor in single stage (IPC) or two-stage (PPC) configuration as shown in Fig. 1. Fig. 1. Schematic overview of the plasma-catalysis hybrid reactor: (a) In-Plasma Catalysis (IPC) and (b) Post-Plasma Catalysis (PPC).

The plasma reactor was powered by a pulsed sub-microsecond high voltage generator delivering an output HV up to 20 kV into 0.5 µs pulses (FWHM) at a maximum frequency of about 200 Hz. Electrical characterization of plasma was performed by current-voltage measurements using a Tektronix P6015A HV probe (attenuation ratio 1000:1) and Pearson 4001current probe (10 ns rise time), respectively. The energy consumption of the plasma reactor was evaluated through the specific input energy (J/L) which is the energy deposited per unit volume of gas in the reactor and is given by SIE = (Ep.f)/Q) (Ep is the discharge pulse energy, f the pulse frequency, and Q the total gas flow rate). Experiments were conducted by maintaining Ep

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constant as 13 mJ, the corresponding maximum SIE is 148 J/L.

Palladium supported catalysts (0.5 and 1 wt%) were prepared by wet impregnation method of γ-Al2O3 beads (1.8 mm; Sasol Germany GmbH). The catalysts crystal structure was examined by XRD pattern (Bruker D5005 diffractometer, Cu Kα radiation). The atoms chemical states in the catalyst surface were investigated by XPS (AXIS Ultra DLD spectrometer, Kratos Analytical). The surface area/porosity measurements were evaluated by the multipoint BET and BJH methods. Finally, Pd metal loading was determined by ICP-OES using an ACTIVA spectrometer (Horiba Jobin Yvon).

Methane, propene, and toluene oxidation was performed in a continuous flow gas fixed to 1 L/min corresponding to a VVH of about 15 000 h-1. Initial concentrations of the VOCs have been fixed as 1000 ppm.

End-products detection and quantification were carried out using a FTIR (Nicolet 6700, Thermo-Scientific) equipped with a liquid nitrogen cooled MCT detector and 10 m gas path cell.

Steady-state activity of the catalyst, methane (CH4), propene (C2H6) and toluene (C7H8) conversion rates, were measured in the presence and in the absence of catalyst for the temperature between 22 and 500 °C. The uncertainty was determined by repeating each experiment four times. After reproducibility, we can conclude that the experimental relative uncertainty error is less than 5% for all cases studied. The CH4, C3H6 and C7H8 conversion rate was defined by Eq. 1, where VOC design the considered molecule and the brackets refer to the concentrations. 𝜏𝑉𝑉𝑉 = [𝑉𝑉𝑉]𝑖𝑖− [𝑉𝑉𝑉]𝑜𝑜𝑜

[𝑉𝑉𝑉]𝑖𝑖 (1)

3. Results and discussion

Experiments were designed so that the effect of thermal catalysis, plasma-catalysis, and plasma conversion alone could be compared. As reported in Fig. 2, the methane conversion by plasma reached a maximum of 67%. In that case, the observed main products were CO, CO2, O3, and HNO3. When plasma is combined with 1 wt% Pd/γ-Al2O3 catalyst, it exhibits higher catalytic activity at low-temperature for both IPC and PPC configurations. CH4 conversion improvements by a factor 3 at 300 °C and 1.4 at 350 °C were obtained in plasma-catalytic system compared to thermal catalytic experiments for the highest specific input energy used. Although the difference is weak, the IPC configuration seems to be more efficient compared with PPC. In all cases, the reaction using Pd/γ-Al2O3 catalyst becomes more selective in CO2 formation than the reaction in plasma alone and, at high temperature O3 and HNO3 disappears in favour of NOx. At high temperature, the catalyst itself leads to 100% conversion and the plasma is no more needed and can be switched-off. Moreover, performance was slightly better in IPC configuration.

Fig. 2. CH4 conversion as a function of temperature (IPC versus PPC): SIE = 148 J/L.

Fig. 3 shows the propene conversion for thermal, plasma, and plasma-catalysis (IPC and PPC) systems. It can be seen that the conversion of propene by thermal catalysis has a threshold temperature of~130 °C and increases steeply with increasing the temperature and reaches complete conversion at 250 °C. Plasma processing of propene, with and without catalyst, exhibits much lower threshold temperature and the conversion take place from room temperature. Plasma-catalysis, both IPC and PPC, have shown more propene conversions than the thermal catalysis for all temperature studied. The most striking difference is at 150 °C, both IPC and PPC have shown 62% propene conversion. Under the similar operating conditions conventional thermal catalysis has shown only 9% propene conversion. Fig. 3. Plasma, catalytic, and plasma-catalytic (IPC and PPC) conversion of propene as a function of temperature (SIE = 23 J/L).

At temperature lower than 150 °C, IPC exhibits higher propene conversion efficiency as compared to PPC configuration. Whilst the conversion efficiencies are quite different, the nature of by-products observed are similar (CO, CH2O, CH2O2, CH3NO3, O3, and NOx). For SIE = 87 J/L, the total C3H6 conversion was achieved at 100 °C as compared to 250 °C for conventional thermal catalysis. The increase in energy deposition enhances the

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conversion efficiency and decreases the difference between IPC and PPC configuration.

Fig. 4 shows the CO concentration produced by plasma and plasma-catalysis as a function of temperature. At low temperature (< 100 °C), CO concentrations are quiet similar for IPC and PPC configurations. At higher temperature, CO concentration linearly increased in case of plasma-alone and slightly decreased when plasma was combined to a catalyst. We observed that at 150 °C, compared to thermal catalysis, the addition of the catalyst to the plasma increased the CO2 selectivity to about 85-90%. At a given temperature, the amounts of by-products such as CH2O and CH2O2 produced from the partial oxidation of C3H6 decrease in both IPC and PPC systems and could be drastically reduced by increasing the specific input energy. Fig 4. CO concentration from oxidation of C3H6 as a function of temperature (SIE = 23 J/L).

In the case of toluene oxidation, the situation is quite different. We observed huge toluene desorption from the catalyst surface (porous γ-Al2O3 support) at low temperature (< 100 °C) when the catalyst was placed in the plasma discharge (IPC), while for the PPC configuration toluene desorption was not observed. Fig. 5 shows a typical FTIR spectra illustrating the toluene desorption from the catalyst surface in IPC configuration during plasma discharge at room temperature. This phenomenon, which is much more pronounced at high-energy deposition, is presented by negative conversion (Eq. 1) reported in Fig. 6 (the outlet toluene concentration is higher than the initial one).

In IPC system above 100 °C, compared to PPC, higher toluene conversion is observed. Depending on SIE, about 10-30% more toluene conversion is evidenced. Therefore, it can be suggested that, in IPC configuration; the catalyst could be activated by high-energy species produced by the plasma and the in-situ decomposition of ozone could increase the toluene conversion. The increase in SIE not only increases the toluene conversion, but also leads to the better CO2 selectivity. The improvement of the toluene conversion in the IPC system by the increasing the specific input energy also leads to a better CO2 selectivity, as compared to PPC system.

Fig. 5. FTIR spectra for catalytic and plasma-catalysis processing of air-toluene mixture at room temperature for SIE = 148 J/L (1 wt% Pd/γ-Al2O3 catalyst). Fig. 6. Plasma, catalytic, and plasma-catalytic (IPC and PPC) conversion of toluene as a function of temperature (SIE = 23 J/L).

The synergistic effect of combining plasma with catalyst in one-stage configuration (IPC) is shown in Fig. 7 for toluene removal in air at 150 and 200 °C. From left to right, the contributions to the toluene conversion are from plasma alone and from thermal catalysis alone. These are combined to give the resultant effect that one might predict if the two effects were additive (~37% at 150 °C, and 83% at 200 °C). The actual toluene removal achieved by plasma-catalytic reactor is ~59% at 150 °C and ~93% at 200 °C show a synergistic gain of a factor of ~2 and ~1.2, respectively. At higher temperature, there is a much less marked difference between the conversion achieved thermally and by plasma catalysis indicating that synergism is dependent on the operating temperature.

Under over experimental conditions, no synergistic effects were observed for methane and propene removal. These results are in agreement with the literature [8, 9]. As reported by Whitehead [11], for dichloromethane, synergetic effects have not been observed. Authors concluded that the synergistic effects do not unanimously occur in plasma catalysis processes. It depends on several factors such as nature of catalysts, types of plasma reactors, operating conditions and nature of the pollutants.

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Fig. 7. Synergistic effect of plasma-catalytic hybrid system for toluene conversion. 4. Conclusions

Methane, propene and toluene oxidation in air by thermal-catalytic, plasma, and plasma-catalytic systems have been investigated in wide range of temperature and plasma input energy. Whatever the molecule studied, the plasma significantly enhanced the catalytic activity even at low temperature leading to high CO2/CO selectivity.

Performance of the system was slightly better when the catalyst was located in the discharge zone, i.e., IPC configuration. For instance, to achieve 100% conversion, IPC requires 100 °C whereas thermal catalysis process requires 250 °C. At room temperature and for given input energy, the catalyst was helpful in minimizing the by-products produced during plasma discharge (CO, CH2O, CH2O2, HNO2).

It is evidenced that 1 wt% Pd/γ-Al2O3 catalyst shows the synergistic effect on only toluene conversion at low temperature in in-plasma catalysis hybrid system. However, post-plasma catalysis system did not show synergistic effect. 5. Acknowledgments

This work was performed in the framework of international research network (GDRI) "Catalysis for polluting emissions aftertreatment, and production of renewable energies". The authors greatly acknowledge S. Gil and A. Giroir-Fendler from IRCELYON for assistance in catalyst analysis and fruitful discussions. 6. References [1] P. Gelin and M. Primet. Appl. Catal. B: Environ.,

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