{Propene oxidation from air by atmospheric plasma-catalytic hybrid system} J. Serb. Chem. Soc. 83 (5) 641–649 (2018) UDC 547.313.3+66.094.3:533.9:544.478+546.98 JSCS–5101 Original scientific paper 641 Oxidation of propene from air by atmospheric plasma-catalytic hybrid system THIEN HUU PHAM1, HA MANH BUI2* and AHMED KHACEF3 1Institute of Applied Material Science, Vietnam Academy of Science and Technology, 01 Thanh Loc 29 St., District 12, Ho Chi Minh City 700000, Vietnam, 2Department of Environmental Sciences, Sai Gon university, 273 An Duong Vuong St., District 5, Ho Chi Minh City 700000, Vietnam and 3Research team in GREMI Laboratory, the National centre for Scientific research, University of Orleans, 14 Issoudun St., 45067 Orleans Cedex 02, France (Received 14 October, revised 19 November, accepted 27 December 2017) Abstract: The pulsed dielectric barrier discharge (DBD) combined with the pal- ladium supported on alumina beads, was investigated for propene (C3H6) rem- oval from air. The effects of thermal-catalysis, plasma-catalysis (in-plasma cat- alysis and post-plasma catalysis), and plasma-alone on the propene removal were compared. Results are presented in the terms of C3H6 removal efficiency, energy consumption, and by-products production. Temperature dependence studies (20–250 °C) show that in all conditions of input plasma energy density explored (23–148 J L-1), the plasma-catalysis systems exhibit better propene conversion efficiencies than the thermal catalysis at low temperature (60% at 20 °C). Plasma-alone treatment has a similar effectiveness compared to plasma-catalysis at room temperature, but it leads to the formation of high by-products concentrations. It appears that in the plasma-catalyst system, C3H6 removal was the most efficient, whatever was the configuration used, and it was helpful to minimize by-products formation. Keywords: non-thermal plasma; C3H6 oxidation; palladium catalyst. INTRODUCTION Volatile organic compounds (VOCs) are an important category of air pol- lutants and therefore, they have become a serious problem, damaging the human health and the environment in general. The well-established technologies for the removal of VOCs, namely thermal and catalytic oxidation, require a substantial supply of thermal energy (200–800 °C) and are not well adapted and also are energetically expensive in the case of the moderate gas flow rates containing low VOCs concentrations.1–3 * Corresponding author. E-mail: manhhakg@sgu.edu.vn https://doi.org/10.2298/JSC171014012P 642 PHAM, BUI and KHACEF As an alternative to the conventional VOCs removal techniques, the atmo- spheric non-thermal plasma (NTP) technology received the increasing interest during the last decades for the removal of dilute VOCs from many sources. The main advantage of these non-equilibrium plasmas consists of the ability to generate high energy electrons, while keeping the background gas close to room temperature. Thus, a highly reactive environment is created without spending energy on the gas heating, as in thermal processes. However, NTP technology for indoor air treatment has the disadvantage, because it produces the undesirable by-products such as ozone, aldehyde, and NOx.4–6 To overcome the by-products formation and to increase the energy efficiency, NTP could take the advantage of its synergetic effect through the combination with heterogeneous catalysts. This combination can be either single-stage (in-plasma catalysis) or two stages (post- plasma catalysis). Such a combination helps to bring down the disadvantages of both catalytic and plasma treatments.7,8 In this study, the dielectric-barrier discharge (DBD) reactor, combined to 1 wt.% of Pd/Al2O3 beads catalyst, were investigated for the removal of propene from air at the atmospheric pressure. Reported results, as a function of the gas temperature and the input plasma energy density, consider the catalyst effect, the plasma effect and the plasma-catalyst effect on the efficiency of propene con- version. Systematic investigations were carried out in order to select the optimal positioning of the catalyst regarding the plasma discharge. EXPERIMENTAL Catalyst preparation The catalysts based on palladium supported on alumina beads (1 wt.% of Pd/Al2O3) were prepared by the impregnation method as described in details elsewhere.8 The aqueous solution of tetraaminepalladium(II) nitrate (5 wt.% of Pd, Strem Chemicals) and alumina beads (1 mm diameter, SASOL) were stirred in the rotary evaporator for 3 h at 50 °C, under atmospheric pressure. Then, the sample catalyst was dried for 12 h at 120 °C and calcined for 4 h at 500 °C at a heating rate of about 3°C per min under air flow. Catalysts characterization Surface area/porosity measurements were conducted using a Micromeritics ASAP 2010 apparatus with N2 as the sorbate at –196 °C. All the samples were outgassed prior to analysis at 300 °C under vacuum (5×10-3 Torr) for 3 h. The total specific surface areas were deter- mined by the multipoint BET (Brunauer–Emmett–Teller) method. Mesoporosity was eva- luated by the Barret–Joyner–Halenda (BJH) method. X-Ray powder diffraction (XRD) analyses were conducted by a Bruker D5005 powder diffractometer scanning, using CuKα radiation. The samples were scanned at a rate of 0.02° per step in the 2θ range of 4–80° (scan time = 2s per step). The applied voltage and current were 50 kV and 35 mA, respectively. Diffraction patterns were assigned using Joint Com- mittee on Powder Diffraction Standards (JCPDS) cards supplied by the International Centre for Diffraction Database (ICDD). The average crystallite sizes of Al2O3 support and Pd-sup- ported catalysts were estimated using the Scherrer equation: PLASMA-CATALYTIC OXIDATION OF PROPENE 643 i cos λ β θ = k d where di is the mean size of the ordered (crystalline) domains of (i) Al2O3 or/and PdO and Pd, which may be smaller or equal to the grain size, k (0.9) is the shape factor, λ (0.154 nm) is the X-ray wavelength, β is the line broadening at half maximum intensity (FWHM) in rad and θ is the Bragg angle. Chemical states of the atoms in the catalyst surface were investigated by the X-ray photoelectron spectroscopy (XPS) on an AXIS Ultra DLD spectrometer produced by Kratos Analytical, operating with Al (Kα) radiation. XPS data were calibrated using the binding energy of C 1s (284.6 eV) as the standard. The XPS core level spectra were analyzed with a fitting routine, which decomposes each spectrum into individual, mixed Gaussian–Lorentzian peaks using a Shirley background subtraction over the energy range of the fit. The surface composition was calculated from the integrated peaks, using empirical cross-section factors for XPS (C 1s = 1, O 1s = 2.93, Al 2p = 0.54, Pd 3d (3d5/2+ 3d3/2) = 16.04). Plasma-catalysis system The plasma reactor we used is a cylindrical DBD shown in Fig. 1. That configuration gives the possibility to combine the catalyst with the plasma reactor in two different ways: by introducing the catalyst in the discharge zone (in-plasma catalysis, IPC) or by placing the catalyst downstream the discharge zone (post-plasma catalysis, PPC).The plasma reactor was powered by a pulsed sub-microsecond high voltage generator delivering HV amplitude (up to 20 kV) at frequency up to 200 Hz. The electrical characterization of plasma (energy depo- sition) was performed by the current and voltage measurements. The discharge pulse energy was measured with a capacitive circuit. The energy deposition in the plasma reactor (J L-1) is given by Ed = EpQ -1f, in which, Ep is the discharge pulse energy, f the pulse repetition rate, and Q is the gas flow rate at standard conditions (25 °C and 1 atm). The experiments were con- ducted maintaining the constant discharge pulse energy Ep at about 80 mJ and in varying the Fig. 1. Schematic view of the plasma-catalyst reactor: a) post- -plasma catalysis; b) in-plasma catalysis. 644 PHAM, BUI and KHACEF pulse repetition frequency in the range 30–190 Hz, and as a consequence the desired energy deposition ranged from 23 to 148 J L-1. Experimental conditions The propene oxidation was performed in a continuous flow fixed-bed reactor in the tem- perature range 20–250 °C. The total flow through the catalyst bed was kept at 1 L min-1, leading a weight hourly space velocity of about 15000 h-1. The initial propene concentration was fixed at about 1000 ppm. The reactant and reaction products were analyzed in-situ using the FTIR spectrometer (Nicolet 6700, Thermo-Scientific). RESULTS AND DISCUSSION The N2 adsorption/desorption analysis show that both the BET surface area and the total pore volume increases with the alumina sphere diameter, corres- ponding to the mesoporous materials. The XRD patterns suggest the formation of the alumina phase with the presence of the characteristic peaks for γ-Al2O3 phase. The XRD analysis also confirms a small Pd metal peak to be present along with the major PdO peaks. However, the XPS analyzes shows only the Pd2+ peak, corresponding to the PdO phase, probably because the amount of the exposed Pd metal is too small to be picked up by XPS. Thus, it can be expected that the exchange or equilibration should occur on the surface of PdO at lower tempe- ratures, as PdO is quite stable and does not easily change the oxidation state of a metal.9 XPS results also showed the formation of palladium species in a higher oxidation state, probably PdO2 (338 eV), inducing the formation of new inter- facial sites for the oxidation reaction.10 Fig. 2 shows the typical FTIR spectra illustrating the plasma and plasma- -catalysis processing of air-C3H6 mixture at 150 °C an energy deposition of 55 Fig. 2. Typical FTIR spectra for plasma and plasma-catalytic processing of air–C3H6 mixture (150 °C, 55 J L -1). PLASMA-CATALYTIC OXIDATION OF PROPENE 645 J L-1. In addition to CO and CO2, the other detected gaseous carbon-containing compounds are formaldehyde (HCHO), formic acid (HCOOH), and nitric acid (HNO3). At higher temperature, the nitric acid decomposition leads to the form- ation of NO and NO2. Before comparing the effect of the thermal catalysis, plasma-catalysis (IPC and PPC), and plasma alone on the propene removal, the preliminary studies with alumina beads were performed. In the absence of plasma, the alumina beads show a high activity at 450 °C and above. The propene conversion over alumina alone was about 60 % with CO2, H2O and CO which are the reaction products. For 1 wt.% of Pd/Al2O3 catalyst, the temperature of total propene oxidation was drastically reduced to 250 °C leading to CO2 and H2O. The effect of the energy deposition on propene conversion was studied as function of temperature in the range 23–148 J L-1. The results indicated that pro- pene could be converted by plasma at low temperature. However, the reaction by-products were HCHO, HCOOH, CO and O3. In the plasma-catalyst system, the interaction of the catalyst active phase, with the reactive species produced by the plasma, changed the catalyst activity by the increasing of the conversion efficiency and the decrease of the concentration of by-products. In some cases, the plasma-catalyst system in IPC configuration is better than the plasma-catalyst system in PPC configuration. This could be explained by the interaction of the catalyst active phase with the reactive species produced by plasma (•O, •OH, •O2, etc.) in IPC configuration. In this study, only the data obtained at 148 J L–1 will be presented. Fig. 3 shows the comparisons between the thermal and the plasma-catalysis for the removal of propene from air using both configurations: in-plasma cat- Fig. 3. Thermal, plasma, and plasma-catalytic conversion efficiency of C3H6 in air as a function of temperature (Ed = 148 J L -1). 646 PHAM, BUI and KHACEF alysis and post-plasma catalysis. We can note that the removal of propene by the thermal catalysis has a threshold temperature of 150 °C and increases steadily with the temperature reaching 100 % removal at 250 °C. The processing of pro- pene using plasma discharge (with and without catalyst) exhibits a lower thres- hold temperature and the reactions take place at room temperature. Larger con- version efficiencies were observed with the plasma-catalysis systems at any tem- perature, over the range 20–150 °C as illustrated in Fig. 3. At room temperature, the plasma-alone and the plasma-catalysis (IPC and PPC) exhibit 60 % of pro- pene conversion, compared to 0 % for thermal-catalysis. While the conversion efficiencies are quite similar, the nature and the amounts of the end-products observed are different. The total propene conversion was achieved at 100°C (IPC and PPC) and 200 °C (plasma alone) leading to the production of the by-products such as CO, HCHO, HCOOH, O3 and NOx. Fig. 4–7 show the amounts of CO, CO2, HCHO and HCOOH respectively, produced in the case of the plasma alone and the plasma-catalysis (IPC and PPC) processing of the air-propene mixture as a function of temperature. Fig. 4. CO selectivity according to temperature at input density energy 148 J L-1. At low temperature (<100 °C), CO selectivity is quite similar for IPC and PPC configurations. At higher temperature, CO concentration drastically inc- reased in the case of plasma-alone and slightly decreased when plasma was com- bined to catalyst over two configurations. We observe that the addition of the catalyst to the plasma in both IPC and PPC configurations increased the CO2 selectivity to about 60 %, when comparing to the thermal-catalysis at 150 °C. At a given temperature in the range of 20–250 °C, the concentrations of formaldehyde (HCHO) and formic acid (HCOOH), derived from the partial oxid- ation of propene, decrease when the catalyst is combined with plasma. We obs- PLASMA-CATALYTIC OXIDATION OF PROPENE 647 Fig. 5. CO2 selectivity according to temperature at input density energy 148 J L -1. Fig. 6. HCHO concentration according to temperature at input density energy 148 J L-1. Fig. 7. HCOOH concentration according to temperature at input density energy 148 J L-1. 648 PHAM, BUI and KHACEF erve that the concentrations of these by-products could be drastically reduced by increasing the plasma energy density. CONCLUSIONS In this research, the thermal catalysis, the plasma-catalysis (IPC and PPC), and the plasma-alone processing of the air–propene mixture were investigated as the function of temperature and of plasma energy deposition. Alumina and pal- ladium supported on alumina beads (1 wt.% of Pd/Al2O3) catalysts were used in combination with the sub-microsecond pulsed dielectric barrier discharge. The plasma-catalysis systems exhibit better propene conversion efficiencies than the thermal catalysis at low temperature. The plasma-alone treatment has a similar effectiveness to the plasma-catalysis at room temperature (up to 60 % propene conversion) but leads to the formation of the high concentration of by-products such as carbon monoxide, formaldehyde and formic acid. The total conversion of propene was achieved at 100 °C in plasma-catalysis case and 250 °C in catalysis alone case. It has been shown that at a given energy density, the plasma-catalyst was helpful in minimizing the by-products formation. The plasma-catalytic con- version processes could be explained by the specific plasma-induced interactions between plasma reactive species (O3, O, OH,…) and the catalyst active phase at low temperature, whereas at higher temperature the thermal activation of the catalyst becomes important, overtaking the contribution of the plasma-activated processes. Acknowledgements. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.99-2016.67. И З В О Д ОКСИДАЦИЈА ПРОПАНА У ВАЗДУХУ АТМОСФЕРСКИМ ПЛАЗМА-КАТАЛИТИЧКИМ СИСТЕМОМ THIEN HUU PHAM1, HA MANH BUI2 и AHMED KHACEF3 1 Institute of Applied Material Science, Vietnam Academy of Science and Technology, 01 Thanh Loc 29 St., District 12, Ho Chi Minh City 700000, Vietnam, 2 Department of Environmental Sciences, Sai Gon university, 273 An Duong Vuong St., District 5, Ho Chi Minh City 700000, Vietnam и 3 Research team in GREMI Laboratory, the National centre for Scientific research, University of Orleans, 14 Issoudun St., 45067 Orleans Cedex 02, France Метода пулсне диелектричне баријере (DBD), у комбинацији са паладијумом на носачу од алуминијумских перли, испитана је у циљу уклањања пропена из ваздуха. Поређени су ефекти термалне катализе, катализе плазмом и саме плазме на уклањнање пропена. Резултати су поређени у односу на ефикасност уклањања пропена, потрошњу енергије и стварање споредних проивода. Проучавање зависности од температуре (20– –250 °C) показало је да је у било којим испитиваним условима уклањање пропена помоћу катализе плазмом ефикасније од термалне катализе. Третман самом плазмом је на собној температури ефикасан слично као и катализа плазмом, али има превише споредних производа. Катализа плазмом се показала као најефективнија. (Примљено 14. октобра, ревидирано 19. новембра, прихваћено 27. децембра 2017) PLASMA-CATALYTIC OXIDATION OF PROPENE 649 REFERENCES 1. Z. Zhang, Z. Jiang, W. Shangguan, Catal. Today 264 (2016) 270 2. X. Zhu, X. Gao, R. Qin, Y. Zeng, R. Qu, C. Zheng, X. Tu, Appl. Catal., B: Environ. 170 (2015) 293 3. M. S. Kamal, S. A. Razzak, M. M. Hossain, Atmos. Environ. 140 (2016) 117 4. R. Zhu, Y. Mao, L. Jiang, J. Chen, Chem. Eng. J. 279 (2015) 463 5. X. Xu, J. Wu, W. Xu, M. He, M. Fu, L. Chen, A. Zhu, D. Ye, Catal. Today 281 (2017) 527 6. C. 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