Oxidation of propene from air by atmospheric plasma-catalytic hybrid system

The pulsed dielectric barrier discharge (DBD) combined with the palladium supported on alumina beads, was investigated for propene (C3H6) removal from air. The effects of thermal-catalysis, plasma-catalysis (in-plasma catalysis 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), 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.


INTRODUCTION
Volatile organic compounds (VOCs) are an important category of air pollutants and therefore, they have become a serious problem, damaging the human health and the environment in general.][3] As an alternative to the conventional VOCs removal techniques, the atmospheric 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.5][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 (postplasma catalysis).Such a combination helps to bring down the disadvantages of both catalytic and plasma treatments. 7,8n this study, the dielectric-barrier discharge (DBD) reactor, combined to 1 wt.% of Pd/Al 2 O 3 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 conversion.Systematic investigations were carried out in order to select the optimal positioning of the catalyst regarding the plasma discharge.

Catalyst preparation
The catalysts based on palladium supported on alumina beads (1 wt.% of Pd/Al 2 O 3 ) were prepared by the impregnation method as described in details elsewhere. 8The 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 N 2 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 determined by the multipoint BET (Brunauer-Emmett-Teller) method.Mesoporosity was evaluated 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 Committee on Powder Diffraction Standards (JCPDS) cards supplied by the International Centre for Diffraction Database (ICDD).The average crystallite sizes of Al 2 O 3 support and Pd-supported catalysts were estimated using the Scherrer equation: where d i is the mean size of the ordered (crystalline) domains of (i) Al 2 O 3 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 (3d 5/2 + 3d 3/2 ) = 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 deposition) 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 E d = E p Q -1 f, in which, E p 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 conducted maintaining the constant discharge pulse energy E p at about 80 mJ and in varying the 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 temperature 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 N 2 adsorption/desorption analysis show that both the BET surface area and the total pore volume increases with the alumina sphere diameter, corresponding to the mesoporous materials.The XRD patterns suggest the formation of the alumina phase with the presence of the characteristic peaks for γ-Al 2 O 3 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 Pd 2+ 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 temperatures, as PdO is quite stable and does not easily change the oxidation state of a metal. 9XPS results also showed the formation of palladium species in a higher oxidation state, probably PdO 2 (338 eV), inducing the formation of new interfacial sites for the oxidation reaction. 10ig. 2 shows the typical FTIR spectra illustrating the plasma and plasma--catalysis processing of air-C 3 H 6 mixture at 150 °C an energy deposition of 55 Fig. 2. Typical FTIR spectra for plasma and plasma-catalytic processing of air-C 3 H 6 mixture (150 °C, 55 J L -1 ).
PLASMA-CATALYTIC OXIDATION OF PROPENE 645 J L -1 .In addition to CO and CO 2 , the other detected gaseous carbon-containing compounds are formaldehyde (HCHO), formic acid (HCOOH), and nitric acid (HNO 3 ).At higher temperature, the nitric acid decomposition leads to the formation of NO and NO 2 .
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 CO 2 , H 2 O and CO which are the reaction products.For 1 wt.% of Pd/Al 2 O 3 catalyst, the temperature of total propene oxidation was drastically reduced to 250 °C leading to CO 2 and H 2 O.
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 propene could be converted by plasma at low temperature.However, the reaction by-products were HCHO, HCOOH, CO and O 3 .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, • O 2 , 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- 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 propene using plasma discharge (with and without catalyst) exhibits a lower threshold temperature and the reactions take place at room temperature.Larger conversion efficiencies were observed with the plasma-catalysis systems at any temperature, 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 propene 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, O 3 and NO x .
Fig. 4-7 show the amounts of CO, CO 2 , 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.At low temperature (<100 °C), CO selectivity is quite similar for IPC and PPC configurations.At higher temperature, CO concentration drastically increased in the case of plasma-alone and slightly decreased when plasma was combined to catalyst over two configurations.We observe that the addition of the catalyst to the plasma in both IPC and PPC configurations increased the CO 2 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 oxidation of propene, decrease when the catalyst is combined with plasma.We obs-Fig. 5. CO 2 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 .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 palladium supported on alumina beads (1 wt.% of Pd/Al 2 O 3 ) 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 conversion processes could be explained by the specific plasma-induced interactions between plasma reactive species (O 3 , 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.

Fig. 3 .
Fig. 3. Thermal, plasma, and plasma-catalytic conversion efficiency of C 3 H 6 in air as a function of temperature (E d = 148 J L -1 ).