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

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 2. Experimental Volatile organic compounds (VOCs) emitted from The plasma reactor is a cylindrical DBD gives the various industrial and domestic processes are important possibility to combine the catalyst with plasma reactor in sources of air pollution and therefore, become a serious single stage (IPC) or two-stage (PPC) configuration as problem for damaging the human health and the shown in Fig. 1. 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 Fig. 1. Schematic overview of the plasma-catalysis [6-11]. The combination of NTP with catalyst can be hybrid reactor: (a) In-Plasma Catalysis (IPC) and (b) Post- made in either single stage (plasma driven-catalyst) or Plasma Catalysis (PPC). two-stage configuration (plasma-assisted catalyst). Compared to conventional NTP alone, the effectiveness The plasma reactor was powered by a pulsed of these systems have been demonstrated in terms of sub-microsecond high voltage generator delivering an energy efficiency, products selectivity and carbon output HV up to 20 kV into 0.5 µs pulses (FWHM) at a balance. maximum frequency of about 200 Hz. Electrical In this work, the oxidation of methane, propene and characterization of plasma was performed by current- toluene in air at atmospheric pressure was investigated voltage measurements using a Tektronix P6015A HV using a Dielectric Barrier Discharge (DBD) reactor probe (attenuation ratio 1000:1) and Pearson 4001current combined or not with γ-Al2O3 and 1 wt% Pd/γ-Al2O3 probe (10 ns rise time), respectively. The energy catalysts. Results are reported as a function of consumption of the plasma reactor was evaluated through temperature, specific input energy (SIE), and position of the specific input energy (J/L) which is the energy the catalyst in the reactor (IPC, In Plasma catalysis and deposited per unit volume of gas in the reactor and is PPC Post-Plasma Catalysis). 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

O-19-2 1 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 Fig. 2. CH4 conversion as a function of temperature (IPC corresponding to a VVH of about 15 000 h-1. Initial versus PPC): SIE = 148 J/L. concentrations of the VOCs have been fixed as 1000 ppm. End-products detection and quantification were carried Fig. 3 shows the propene conversion for thermal, out using a FTIR (Nicolet 6700, Thermo-Scientific) plasma, and plasma-catalysis (IPC and PPC) systems. It equipped with a liquid nitrogen cooled MCT detector and can be seen that the conversion of propene by thermal 10 m gas path cell. catalysis has a threshold temperature of~130 °C and Steady-state activity of the catalyst, methane (CH4), increases steeply with increasing the temperature and propene (C2H6) and toluene (C7H8) conversion rates, reaches complete conversion at 250 °C. Plasma were measured in the presence and in the absence of processing of propene, with and without catalyst, exhibits catalyst for the temperature between 22 and 500 °C. The much lower threshold temperature and the conversion uncertainty was determined by repeating each experiment take place from room temperature. Plasma-catalysis, both four times. After reproducibility, we can conclude that IPC and PPC, have shown more propene conversions than the experimental relative uncertainty error is less than 5% the thermal catalysis for all temperature studied. The for all cases studied. The CH4, C3H6 and C7H8 most striking difference is at 150 °C, both IPC and PPC conversion rate was defined by Eq. 1, where VOC design have shown 62% propene conversion. Under the similar the considered molecule and the brackets refer to the operating conditions conventional thermal catalysis has concentrations. shown only 9% propene conversion.

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휏푉푉푉 푉푉푉 푖푖 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 Fig. 3. Plasma, catalytic, and plasma-catalytic (IPC and plasma-catalytic system compared to thermal catalytic PPC) conversion of propene as a function of temperature experiments for the highest specific input energy used. (SIE = 23 J/L). Although the difference is weak, the IPC configuration seems to be more efficient compared with PPC. In all At temperature lower than 150 °C, IPC exhibits higher cases, the reaction using Pd/γ-Al2O3 catalyst becomes propene conversion efficiency as compared to PPC more selective in CO2 formation than the reaction in configuration. Whilst the conversion efficiencies are plasma alone and, at high temperature O3 and HNO3 quite different, the nature of by-products observed are disappears in favour of NOx. At high temperature, the similar (CO, CH2O, CH2O2, CH3NO3, O3, and NOx). catalyst itself leads to 100% conversion and the plasma is For SIE = 87 J/L, the total C3H6 conversion was achieved no more needed and can be switched-off. Moreover, at 100 °C as compared to 250 °C for conventional thermal performance was slightly better in IPC configuration. catalysis. The increase in energy deposition enhances the

2 O-19-2 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 Fig. 5. FTIR spectra for catalytic and plasma-catalysis the specific input energy. processing of air-toluene mixture at room temperature for SIE = 148 J/L (1 wt% Pd/γ-Al2O3 catalyst).

Fig 4. CO concentration from oxidation of C3H6 as a function of temperature (SIE = 23 J/L). Fig. 6. Plasma, catalytic, and plasma-catalytic (IPC and PPC) conversion of toluene as a function of temperature In the case of toluene oxidation, the situation is quite (SIE = 23 J/L). different. We observed huge toluene desorption from the catalyst surface (porous γ-Al2O3 support) at low The synergistic effect of combining plasma with temperature (< 100 °C) when the catalyst was placed in catalyst in one-stage configuration (IPC) is shown in the plasma discharge (IPC), while for the PPC Fig. 7 for toluene removal in air at 150 and 200 °C. From configuration toluene desorption was not observed. Fig. 5 left to right, the contributions to the toluene conversion shows a typical FTIR spectra illustrating the toluene are from plasma alone and from thermal catalysis alone. desorption from the catalyst surface in IPC configuration These are combined to give the resultant effect that one during plasma discharge at room temperature. This might predict if the two effects were additive (~37% at phenomenon, which is much more pronounced at high- 150 °C, and 83% at 200 °C). The actual toluene removal energy deposition, is presented by negative conversion achieved by plasma-catalytic reactor is ~59% at 150 °C (Eq. 1) reported in Fig. 6 (the outlet toluene concentration and ~93% at 200 °C show a synergistic gain of a factor of is higher than the initial one). ~2 and ~1.2, respectively. At higher temperature, there is In IPC system above 100 °C, compared to PPC, higher a much less marked difference between the conversion toluene conversion is observed. Depending on SIE, about achieved thermally and by plasma catalysis indicating that 10-30% more toluene conversion is evidenced. synergism is dependent on the operating temperature. Therefore, it can be suggested that, in IPC configuration; Under over experimental conditions, no synergistic the catalyst could be activated by high-energy species effects were observed for methane and propene removal. produced by the plasma and the in-situ decomposition of These results are in agreement with the literature [8, 9]. ozone could increase the toluene conversion. The As reported by Whitehead [11], for dichloromethane, increase in SIE not only increases the toluene conversion, synergetic effects have not been observed. Authors but also leads to the better CO2 selectivity. The concluded that the synergistic effects do not unanimously improvement of the toluene conversion in the IPC system occur in plasma catalysis processes. It depends on several by the increasing the specific input energy also leads to a factors such as nature of catalysts, types of plasma better CO2 selectivity, as compared to PPC system. reactors, operating conditions and nature of the pollutants.

O-19-2 3 [7] A.H. Than Quoc, T. Pham Huu T. Le Van, J.M. Cormier and A. Khacef. Catal. Today, 176, 474 (2011) [8] A. Baylet, P. Marécot, D. Duprez, X. Jeandel, K. Lombaert and J.M. Tatibouët. Appl. Catal. B: Environm., 113, 31 (2012) [9] H. Lee, D.H. Lee, Y.H. Song, W.C. Choi, Y.K. Park and D.H. Kim, Chem. Engng. J., 259, 761 (2015) [10] Z. Jia, A. Vega-Gonzales A.M. Ben Amar, K. Hassouni, S. Tieng, S. Touchard, A. Kanaev and X. Duten. Catal. Today, 208, 82 (2013) [11] J.C. Whitehead. Pure Appl. Chem., 82, 1329 (2010)

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.

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