Nassim Bouchoul

Houcine Touati

Elodie Fourré

Jean-Marc Clacens

Isabelle Batonneau- Gener

Catherine Batiot-Dupeyrat
email: catherine.batiot.dupeyrat@univ-poitiers.fr

Plasma-catalysis coupling for CH 4 and CO 2 conversion over mesoporous macroporous Al 2 O 3 : influence of the physico-chemical properties

Macro-mesoporous alumina, calcined at different temperatures (from 400 to 800 °C), were tested in dry reforming of methane reaction under plasma discharge at a fixed deposited power (P = 8 W) with a ratio CO2/CH4 = 2. Besides alumina materials, the one possessing the lowest surface area (72 m 2 g -1 ) exhibited the highest activity: Methane conversion reaches 31.3 % against 23.4, 22.6 and 26.8 % for alumina calcined at 400, 600 and 800 °C, respectively, suggesting that the active sites generated into the mesoporosity of alumina (Al2O3 400) are not accessible to reactants under the plasma discharge. Reactant transformation is promoted with the alumina possessing the highest number of acid and basic sites and a correlation is proposed.

It is believed that CH4 and CO2 adsorption are improved over acid and basic sites at the surface of alumina, favoring the production of syngas.

Introduction

For many years, plasma catalysis has been developed in different application fields such as air pollution control [START_REF] Barakat | Oxidation of isopropanol and acetone adsorbed on TiO2 under plasma generated ozone flow: gas phase and adsorbed species monitoring[END_REF], ammonia synthesis [START_REF] Hong | Plasma catalysis as an alternative route for ammonia production: status, mechanisms, and prospects for progress ACS Sustain[END_REF], reforming reactions [START_REF] Puliyalil | A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels[END_REF], CO2 hydrogenation [START_REF] Dębek | A review on plasma-catalytic methanation of carbon dioxide -Looking for an efficient catalyst[END_REF][START_REF] Mikhail | Electrocatalytic behaviour of CeZrOx-supported Ni catalysts in plasma assisted CO2 methanation[END_REF], catalyst regeneration [START_REF] Jia | New routes for complete regeneration of coked zeolite[END_REF], as listed in "the 2020 plasma catalysis roadmap" [START_REF] Bogaerts | The 2020 plasma catalysis roadmap[END_REF].

The combination of plasma with a catalyst for the reaction of dry reforming of methane (DRM) has been investigated to overcome the main drawbacks of the two separated techniques: catalyst deactivation by coke deposition at high temperature due to the high endothermicity of the reaction and a poor selectivity achieved with non-thermal plasmas [START_REF] Neyts | Understanding plasma catalysis through modelling and simulation -A review[END_REF][START_REF] Zhang | Conversion of greenhouse gases into syngas via combined effects of discharge activation and catalysis[END_REF][START_REF] Kraus | Investigation of mechanistic aspects of the catalytic CO2 reforming of methane in a dielectric-barrier discharge using optical emission spectroscopy and kinetic modeling[END_REF][START_REF] Sentek | Plasma-catalytic methane conversion with carbon dioxide in dielectric barrier discharges[END_REF][START_REF] Khoja | Recent developments in non-thermal catalytic DBD plasma reactor for dryreforming of methane[END_REF]. In the hybrid system, catalysts similar to those used in conventional thermal catalytic processes, such as Ni/SiO2, Ni/Al2O3 or Pt/Al2O3, Pd/Al2O3 [START_REF] Ray | Dry reforming of methane in DBD plasma over Ni-based catalysts: influence of process conditions and support on performance and durability[END_REF][START_REF] Mei | Plasma-catalytic reforming of biogas over supported Ni catalysts in a dielectric barrier discharge reactor: effect of catalyst supports[END_REF][START_REF] Cheng | Carbon dioxide reforming of methane over Ni/Al2O3 treated with glow discharge plasma[END_REF], were inserted in the plasma discharge region. In some studies, the presence of reduced Ni species in the discharge showed a small positive effect in comparison with plasma only [START_REF] Tu | Plasma-catalytic dry reforming of methane in an atmospheric dielectric barrier discharge: Understanding the synergistic effect at low temperature[END_REF][START_REF] Aziznia | Comparison of dry reforming of methane in low temperature hybrid plasma-catalytic corona with thermal catalytic reactor over Ni/γ-Al2O3[END_REF], while other authors obtained a decrease in conversions of CH4 and CO2 [START_REF] Tu | Dry reforming of methane over a Ni/Al2O3 catalyst in a coaxial dielectric barrier discharge reactor[END_REF].

It is unanimously accepted that the presence of a solid in the plasma region modifies significantly the discharge, particularly if the material is conductive [START_REF] Puliyalil | A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels[END_REF]. Jo et al. [START_REF] Jo | Effect of the electric conductivity of a catalyst on methane activation in a dielectric barrier discharge reactor[END_REF] proposed that the electric field is weakened by the electric conductivity of the solid, reducing the electron density and thereby lowering the reactants conversion. Moreover, Kim et al. [START_REF] Kim | Enhancing C-H Bond Activation of Methane via Temperature-Controlled, Catalyst-Plasma Interactions[END_REF] demonstrated that the bulk temperature strongly influences the C-H bond activation of methane, CH4 conversion increasing more than 8 fold in the presence of Ni/Al2O3 but only at temperature higher than 630 K.

In the different published papers, the authors used a specific catalyst composition and comment on the beneficial effect of the coupling leading to synergetic effect (in some cases). In a review, Chung [START_REF] Chung | Review of catalysis and plasma performance on dry reforming of CH4 and possible synergistic effects[END_REF] listed different catalysts used to perform the DRM reaction coupled with plasma and the authors concluded that plasma and catalyst interactions depend on many parameters such as operating temperature, basicity of the catalyst, power input, dielectric constant, reactor geometry and catalyst packing method.

Using a zeolite A, Jiang et al. [START_REF] Jiang | Plasma methane conversion using dielectric-barrier discharges with zeolite A[END_REF] showed that CH4 and CO2 conversions were reduced, but the formation of carbon black was inhibited. Metal-organic-frameworks (MOFs) was successfully used by Vakili et al. [START_REF] Vakili | Plasma-assisted catalytic dry reforming of methane (DRM) over metalorganic frameworks (MOFs)-based catalysts[END_REF] in the plasma-assisted dry reforming of methane, the improved CH4 and CO2 conversion compared to the plasma-alone mode was attributed to the favorable generation of filamentary micro discharges and surface discharges with the highly porous material.

An important point is that the combination of plasma with simple oxides possessing no thermal activity exhibited higher performances compared with plasma only: a significant improvement in CH4 and CO2 conversion over Al2O3 and La2O3/Al2O3 was observed when coupled with plasma at 573 K [START_REF] Yap | Catalyst assisted by non-thermal plasma in dry reforming of methane at low temperature[END_REF]. A higher CO2 conversion and a lower CH4 conversion were reported by Zhang et al. [START_REF] Zhang | The simultaneous activation of methane and carbon dioxide to C2 hydrocarbons under pulse corona plasma over La2O3/γ-Al2O3 catalyst[END_REF] using La2O3/Al2O3 under pulse corona plasma. Michielsen et al. [START_REF] Michielsen | Altering conversion and product selectivity of dry reforming of methane in a dielectric barrier discharge by changing the dielectric Packing Material[END_REF] have shown that -Al2O3 and γ-Al2O3 are beneficial for CO2 and CH4 conversion compared to other oxides such as ZrO2, SiO2, or BaTiO3. In a recent paper [START_REF] Bouchoul | Plasmametal oxides coupling for CH4-CO2 transformation into syngas and/or hydrocarbons, oxygenates[END_REF] we studied the dry reforming of methane by non-thermal plasma coupled with different metal oxides: BaO, La2O3, ZnO, CaO, -Al2O3, MgO, γ-Al2O3, TiO2 and CeO2 and it was observed that the CO2 and CH4 conversions were enhanced when the permittivity of solid was reduced, the best results being obtained with γ-Al2O3.

In this context the main objective of the present study was to investigate the influence of physico-chemical properties of different alumina oxides compared to a commercial gamma alumina. The aim was to correlate the physico-chemical properties of the solid to the CH4 and CO2 transformations in a single plasma reactor at a fixed deposited power.

Experimental part

Experimental setup

The reaction was performed in a coaxial dielectric barrier discharges (DBD) reactor (Fig. 1) at room temperature and atmospheric pressure. The reactor is an alumina tube with 4mm internal and 6 mm external diameter. A stainless steel rod (diameter: 1mm) is centered inside the alumina tube and set as the high voltage electrode while a copper electrode (100 mm long) is wrapped around it and set as the ground electrode.

Helium, CH4 and CO2 were flown at a total flow rate of 40 mL min -1 (residence time of 1.6 s) with a ratio CO2/CH4 of 2 and a constant concentration in He (75 %).

The material (alumina) filled the entire plasma zone. Powders were sieved before experiments in the range: 355-650 µm, since we showed in a previous study that it corresponds to an optimized grain size to perform the reaction under plasma discharge [START_REF] Bouchoul | Plasma-Catalytic dry reforming of CH4 over calcium oxide: Catalyst structural and textural modifications[END_REF]. The advantage of using small grains has also been highlighted by Kasinathan et al. for the CH4 conversion into hydrocarbons over MgO/Al2O3 [START_REF] Kasinathan | Plasma-Enhanced Methane Direct Conversion over Particle-Size Adjusted MOx/Al2O3 (M = Ti and Mg) Catalysts Plasma[END_REF] and by Zhou et al. [START_REF] Zhou | DBD Plasma-ZrO2 catalytic decomposition of CO2 at Low Temperatures[END_REF] for the decomposition of CO2 on ZrO2.

A sinusoidal AC supply of power was applied across the electrode (TG1010A Aim-TTi, Thurlby Thandar Instruments Brand). The discharge power was calculated through the figure of Lissajous and fixed at 8 watts, while the frequency was kept constant at 800 Hz. The voltage was varied from 13.2 to 14.3 kV depending on the nature of the material in order to fix the deposited power at 8W. The applied voltage was measured with high voltage probes (PMK, model PHV4-2757) using an oscilloscope (waveRunner 62 Xi, Lecroy).

An Ametek ®, Solartron Analytical apparatus was used to determine dielectric constants of oxides: a sinusoidal voltage of 50 V was applied with a scan from 0,1 Hz to 1 MHz. The oxides were pressed at 5 tons for measurements, producing wafers of 2 mm thick that allows to neglect the contribution of air permittivity. Before analysis, wafers were placed in an oven at 100 °C to avoid water adsorption.

Al2O3 synthesis and characterization

Mesoporous macroporous alumina were prepared according to the procedure inspired by Z-Y.

Yuan et al. [START_REF] Yuan | Facile and Generalized Preparation of Hierarchically Mesoporous-Macroporous Binary Metal Oxide Materials[END_REF]: 10.42 g of cetyltrimethylammonium bromide (CTMABr) were dissolved in 93.93 g of deionized water. The suspension was stirred at 40 °C for 1 h before the addition of 24.12 g of aluminum isopropoxide, under stirring. The resulting gel was further agitated for 3 h at 40 °C. The obtained gel was placed in a closed polypropylene bottle at 80 °C for 24 h, under static conditions. After a cooling down at room temperature, the suspension was filtered and washed with water (200 ml, 2 times). The resulting paste was dried at 100 °C overnight.

The obtained powder was washed again with 200 ml of water, and then dried overnight at 100 °C. Finally, the obtained alumina was calcined at 400, 600 or 800 °C for 6 h (heating rate: 2 °C min -1 ). Characterization of the materials was carried out before and after reaction under plasma.

The specific surface areas were obtained according to the BET procedure. The nitrogen adsorption-desorption isotherms were carried out using a Micromeritics Flowsorb II 2300 apparatus at -77 K. The commercial -alumina was purchased from Alfa Aesar (CAS: 1344-28-1). It was calcined before use at 400 °C for 6h (heating rate: 2 °C min -1 ) and noted -Al2O3.

The surface acidity was measured by temperature programmed desorption of ammonia (NH3-TPD). NH3-TPD experiments were carried out using a homemade apparatus equipped with a TCD detector. Prior to the measurements, about 80 mg of sample were pretreated by heating at 200 °C for 1 h under helium with a ramp of 5 °C min -1 , and, after cooling for 5 min to 30 °C, ammonia adsorption was carried out. A flow of 30 mL min -1 containing 5 vol % NH3 in helium was passed over the sample during 45 minutes at 30 °C. Physically adsorbed ammonia was removed by purging with helium at 50 °C for 3 h. The NH3-TPD was carried out by increasing the temperature linearly from 50 to 600 °C with a heating rate of 5 °C min -1 and a helium flow rate of 30 mL min -1 .

The amount of acidic sites on the catalyst was calculated from the desorption amount of NH3, which was determined by measuring the areas of the desorption profiles.

In a previous work, we have shown that the amount of CO2 adsorbed in the monolayer is directly linked to the basic site number present at the material surface [START_REF] Célerier | Glycerol dehydration to hydroxyacetone in gas phase over copper supported on magnesium oxide (hydroxide) fluoride catalysts[END_REF]. CO2 monolayer content was calculated from the analysis of CO2 adsorption isotherm by BET model. Adsorption equilibrium data were measured at 25 °C using a symmetrical SETARAM microbalance. 40 mg of samples were out-gassed under secondary vacuum at 350 °C for 12 h and cooled down to 25 °C prior to the sorption measurements. The temperature of the system was kept constant during analysis by a water circulation in the double walls of the analysis tubes. The CO2 pressure was then increased step by step in order to obtain the entire adsorption isotherm. For each uptake, the equilibrium was reached when the mass recorded versus time and the pressure were stable. Thermogravimetric analysis were performed with a TA instruments SDTQ600 analyzer under a 50 mL min -1 flow of air to 900 °C (10 °C min -1 ).

Thermogravimetric analysis coupled with mass spectrometry (TGA-MS) were performed on a SDT Q600 apparatus from TA Instruments coupled by a heated capillary column with a QGA mass spectrometer from Hiden. The samples were heated up to 800 °C (heating rate: 10 °C min - 1 ) under an air flow (50 mL min -1 ).

The catalysts were characterized by X-Ray Diffraction (XRD) using a Siemens D-5005 diffractometer with CuK = 1.5417 Å, operated at 40 kV and 30 mA. The diffraction patterns were recorded in the 2 range of 10-90 ° with a step interval of 0.02 ° and period of 1 s.

Gas phase analysis and calculation

The gas analyses were conducted on-line by gas chromatography. A flame ionization detector allowed the identification of oxygenated products, while a thermal conductivity detector identified the hydrogen, carbon monoxide, carbon dioxide and methane. All the reactions lasted 1 hour.

Each experiment was repeated 3 times, changing the catalyst and cleaning the inner electrode each time, resulting in a margin error of ± 3 %.

Textural properties of alumina

The BET specific surface area, BJH desorption cumulative pores volume and average pore width are gathered in Table 1. As expected, the alumina materials synthesized in the laboratory possess a large surface area compared to the commercial alumina. As the calcination temperature increases from 400 to 800 °C, the SBET decreases from 313 to 261 m 2 g -1 . The pore volumes are low especially for the gamma alumina with the lowest surface area. Nitrogen adsorption desorption isotherms and pore size distributions are shown in Fig. 2. According to the IUPAC classification, the prepared alumina exhibit type IV isotherm with H2 hysteresis loop, typical of mesoporous materials with a complex porosity. The commercial γ-Al2O3 exhibits a loop of type IIb, indicating the presence of macropores, the porosity being not completely filled by N2 condensation [START_REF] Thommes | Physisorption of Gases, with Special Reference to the Evaluation of Surface Area and Pore Size Distribution[END_REF]. The pore size distribution (Fig. 2) confirms the presence of large pores for the commercial alumina (dV/dD pore volume maximum around 400 Åaverage pore diameter of 142 Å), while for the materials synthesized in the laboratory, this maximum is around 50 Å (average pore diameter from 42 to 72 Å).

The SEM images (Fig. 3) show the presence of abundant pores whatever the calcination temperature 400, 600 or 800°C.

XRD patterns of -Al2O3, Al2O3 400, Al2O3 600 and Al2O3 800 are presented in Fig. 4. They all show the representative diffraction peaks of gamma alumina (ICDD 00-010-0425).

Physical and electrical discharge characteristics of alumina packed plasma reactor

Substantial alterations are observed for the Lissajous Q-V curves of the four alumina oxides (Fig. 5). Note that a peak voltage of 13.2kV was required to obtain a deposited power of 8W for Al2O3 600, while 14.3 kV was used for -Al2O3. The charges accumulated at the surface of the dielectric material and the capacitances slightly differ between materials. Lines BC and DA correspond to the plasma-off phase, the slope of these lines gives Ccell (capacitance of the dielectric and of the gap). The discharge-on is represented by lines AB and CD, the slope of these lines corresponding to the effective capacitance (Table 2). The differences in capacitances prove that the electrical properties of the alumina oxides are different, whereas similar grain size was used (355-650 µm).

Note that the lowest values were obtained for the commercial alumina material, which also has the smallest surface area. The effective capacitance is supposed to depend on the spatial distribution of the discharge across the discharge gap. The higher value obtained for the prepared γ-Al2O3 can be explained by the expansion of the discharge at the surface of the alumina material [START_REF] Jo | Methane activation using noble gases in a dielectric barrier discharge reactor[END_REF].

The dielectric constant measurements of the alumina materials were low (2-4) compared to those reported in the literature for and -alumina (generally between 9 and 10) [START_REF] Robertson | High dielectric constant oxides[END_REF][START_REF] Wang | Streamer propagation in a packed bed plasma reactor for plasma catalysis applications[END_REF][START_REF] Taheraslani | Coupling of CH4 to C2 Hydrocarbons in a Packed Bed DBD Plasma Reactor: The Effect of Dielectric Constant and Porosity of the Packing[END_REF]. These differences can be explained from the different measurement methodology followed. In this work, dielectric constants were measured experimentally, while published data were determined mathematically, as a sum of electronic and lattice contributions in most cases. The differences could come from the interfacial polarizations originating from sub-grain boundary and grain boundary barriers [START_REF] Lin | Origin of high dielectric performance in fine grain-sized CaCu3Ti4O12 materials[END_REF], which is not taken into account when the data come from calculation.

Influence of the type of alumina for CH4 and CO2 conversion

The alumina materials, freshly calcined at 400 °C and sieved in the range 355-650 µm, were introduced into the reactor, filling completely the plasma zone. The deposited power was adjusted at 8 W (13.2-14.3 kV and 800 Hz). The conversion of methane and carbon dioxide are plotted versus the surface area in Fig. 6. The highest conversions were obtained with the commercial alumina, possessing the lowest surface area: 72 m 2 g -1 , methane conversion reached 31.3 % against 23.4, 22.6 and 26.8 % for alumina calcined at 400, 600 and 800 °C, respectively.

From these results, it seems that the active sites localized into the mesoporosity of alumina (Al2O3 400, 600 and 800) are not accessible to reactants under the plasma discharge. The accessibility of the porosity of solid is a key point in DBD plasma conditions. In fact, the formation of micro-discharges is possible only when the pores diameter is larger than the Debye length value, which lies between 100 nm to 1µm. This value depends on the electron density and temperature of the plasma streamer. A. Bogaerts group showed that plasma can diffuse in pores of about 50 nm, corresponding to the upper limit size of mesopores, but to a certain extent and in a very limited time period [START_REF] Zhang | Can plasma be formed in catalyst pores? A modeling investigation[END_REF][START_REF] Zhang | Propagation of a plasma streamer in catalyst pores[END_REF]. Nevertheless, in a study on VOC removal by nonthermal plasma using a γ-Al2O3, Roland et al. [START_REF] Holzer | Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds, Part 1. Accessibility of the intra-particle volume[END_REF] suggested the stabilization of short-lived oxidant species on the surface and their diffusion from gas phase into the pores of 7.4 nm (diameter). Moreover, based on EPR spectroscopy, the authors [START_REF] Roland | Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds, Part 3. Electron paramagnetic resonance (EPR) studies of plasma-treated porous alumina[END_REF] proposed the formation of an Al-O-O  aluminium peroxyl group via oxygen present in the gas atmosphere. The results obtained in the present study showed experimentally that the coupling of plasma with catalyst depends strongly on the nature of surface active species (presence of OH) and in a lower extend to the amount of active sites as soon as there are located in small pores of oxides. Results in Table 3 reveals that the hydrogen yield and selectivity towards C2H6 are maximum for the commercial γ-Al2O3 compared to the other Al2O3 materials. Oxygenates were formed only when using the commercial alumina, the main products being formaldehyde > acetone > methanol > acetaldehyde > ethanol. However, the amount of oxygenates remains very low. The experimental conditions (excess of CO2), not favorable for oxygenates production, were chosen to avoid carbon deposition [START_REF] De Bie | The dominant pathways for the conversion of methane into oxygenates and syngas in an atmospheric pressure dielectric barrier discharge[END_REF]. The formation of oxygenates was also reported by Li et al. [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF] using SiO2 coupled with plasma in an excess of CO2, they obtained mainly methanol, ethanol and acetic acid. In our study, the carbon balance is always lower than 100% especially in the presence of alumina with the highest surface area, suggesting that other products, not analyzed by GC, are formed. They could be hydrocarbons other than C2, because insignificant amount of ethylene and acetylene were detected. The formation of volatile oxygenated products such as ethers, peroxides or carboxylic compounds could also be produced, along with carbon deposition and/or product condensation at the surface of the material or in the setup lines. DRM was studied by different groups using Al2O3 as packing material (Table 5). Important differences in terms of reactant conversion and selectivity to products are observable. These differences could be explained by the reactor design (gap, volume of plasma discharge…) and process conditions. Note that comparison between non-packing and packing reactor makes no sense if the gas flow is not adjusted in order to maintain the contact time at the same value. This aspect is unfortunately rarely taken into account in many papers. Nevertheless from the reported data, it appears that high deposited power promotes the selectivity to CO [START_REF] Gallon | Effects of reactor packing materials on H2 production by CO2 reforming of CH4 in a dielectric barrier discharge[END_REF][START_REF] Song | Synthesis gas production via dielectric barrier discharge over Ni/ γ-Al2O3 catalyst[END_REF][START_REF] Long | CO2 reforming of CH4 by combination of cold plasma jet and Ni/ γ-Al2O3 catalyst[END_REF]. Michielsen et al. [START_REF] Michielsen | Altering conversion and product selectivity of dry reforming of methane in a dielectric barrier discharge by changing the dielectric Packing Material[END_REF] show that -Al2O3 material exhibits a slightly higher activity than γ-Al2O3,

Characterization of alumina materials before and after reaction under plasma

The BET specific surface area, BJH desorption cumulative pores volume and average pore width for the catalysts after reaction under plasma are gathered in Table 1. We observe a slight decrease of the surface area after plasma, except for Al2O3 400, while pore volumes remain constant. The pore size increases for -Al2O3 and Al2O3 800 and remains stable for the two other solids. TGA-TDA analysis reported in Fig. 10 exhibits mainly two weight loss in the temperature ranges 100-300 °C and 300-600 °C, corresponding respectively to hydroxyl and carbon elimination. TGA coupled with mass spectrometry analysis (Fig. 11) confirms that H2O is eliminated at low temperature while carbon oxidation proceeds at the highest temperature (analysis of CO2). The values of weight loss are also reported in Table 4 for the alumina materials before and after plasma. It appears that the amount of -OH groups differs significantly from one material to another one. As expected a higher calcination temperature (from 400 to 800 °C) leads to dehydroxylation. The lowest amount of hydroxyl is obtained with the commercial alumina due to its lowest surface area. After plasma, the amount of OH has significantly decreased for the commercial gamma alumina, from 3.3 % before plasma to 1.1 % after plasma. It indicates that dehydroxylation occurred under plasma, which could be related to the selectivity into oxygenates (Table 3) with the commercial alumina. The weight loss in the temperature range 300-600 °C is low after reaction indicating that carbon deposition is limited under the experimental conditions used in this study for the four alumina samples. Indeed the excess of CO2 used in this study avoid carbon deposition by methane cracking.

Conclusion

The dry reforming of methane reaction was performed by coupling plasma with different types of alumina materials, while the granulometry was fixed in the range: 355-650 µm. Physical characterization of the discharge through the Lissajous Q-V figures exhibited significant differences for the four alumina oxides. The lowest effective capacitance being obtained for the commercial alumina. As expected, excess of CO2 (CO2/CH4 = 2) leads to a low carbon deposition, as evidenced by TGA analysis after reaction. We showed that besides γ-Al2O3 materials, the highest conversions were obtained with the commercial alumina, possessing the lowest surface area: 72 m 2 g -1 . Methane conversion reaches 31.3 % against 23.4, 22.6 and 26.8 % for alumina exhibiting surface area of 313, 301 and 261 m 2 g -1 respectively. These results suggest that the active sites generated into the mesoporosity of alumina synthesized in the laboratory are not accessible to reactants under the plasma discharge. A correlation between the number of acid and basic sites is clearly visible. The results obtained in the present study confirmed experimentally that the coupling of plasma with catalyst depends strongly on the nature of surface active species (presence of OH) but in a lower extend to the amount of active sites as soon as there are located in small pores of oxides.

  The conversion, yield, selectivity and energy efficiency were defined as: Conversion (%) of CH4 and CO2 = 100  mole of CH4 (or CO2) converted / mole of CH4 (or CO2) in the feed Yield in H2 (%) = 100  mole of H2 / 2 x (mole of CH4) introduced Selectivity to CnHy (%) = 100  n  mole of CnHy / (mole of CH4 converted + mole of CO2 converted) Selectivity to CO (%) = 100  mole of CO / (mole of CH4 + mole of CO2) converted Carbon Balance: CB (%) = 100  (mole of CO +∑ n  mole of CnHy) / (mole of CH4 converted + mole of CO2 converted) 3 Results and discussion

Figure 1 :Figure 2 :Figure 3 :Figure 4 :

 1234 Figure 1: Plasma reactor scheme

Figure 5 :Figure 6 :Figure 7 :Figure 8 :Figure 9 :Figure 10 :Figure 11 :

 567891011 Figure 5: Lissajous figures of the CH4/CO2/He DBD with alumina materials and typical Lissajous figure of DBD

Table 1 .

 1 Characterization of alumina materials, S BET, pore volume and pore size before and after plasma

		S BET a (m 2 .g -1 )	Pore volume b (cm 3 .g -1 )	Pore size c (Å)
	Alumina	Before	After	Before	After	Before	After
		plasma	plasma	plasma	plasma	plasma	plasma
	γ-Al2O3	72	55	0.2	0.2	142	165
	Al2O3 400	313	334	0.6	0.7	61	61
	Al2O3 600	301	275	0.7	0.7	72	71
	Al2O3 800	261	165	0.3	0.6	42	104
	a. Specific surface area calculated by the BET method.
	b. Total pore volume determined at P/P0=0.99.
	c. Average pore width deduced from the BJH method using the desorption branch.

Table 2 .

 2 Physical characterization of the materials dielectric constant and resulting capacitances of the reactor filled with the different alumina materials

		Dielectric constant	Capacitance
	Alumina	800 Hz	1 MHz	C dielectric (pF)	C effective (pF)
	γ-Al2O3	3.07	2.94	9	22
	Al2O3 400	4.11	3.30	13	32
	Al2O3 600	3.79	3.23	16	39
	Al2O3 800	3.97	2.90	10	25

Table 3 .

 3 Influence of the nature of alumina on product selectivity and hydrogen yield (average

	value, reaction time: 30 minutes), grain size: 355-650 µm, P = 8 W, total flow: 40 mL min -1 ,
				CO2/CH4 = 2, He: 75 %.
	catalyst	H2 yield (%)	Selectivity (%) CO C2H6 Oxy.*	CB (%)
	γ-Al2O3	12.0	70	8.8	2.3	83
	Al2O3 400	7.8	71	6.7	0.0	78
	Al2O3 600	8.4	69	8.8	0.0	77
	Al2O3 800	7.8	65	6.4	0.0	72
	* oxy. = oxygenates

Table 4 .

 4 Mass loss from TGA analysis, before and after plasma (1 h, grain size: 355-650 µm, P = 8 W, total flow: 40 mL•min -1 , CO2/CH4 = 2, He: 75 %)

			Mass loss (%)
	Oxide	before plasma	after plasma
		100-300°C 300-600°C 100-300°C 300-600°C
	γ-Al2O3	3.3	1.7	1.1	3.2
	Al2O3 400	5.1	2.6	5.8	4.7
	Al2O3 600	6.0	2.5	3.2	3.0
	Al2O3 800	3.7	1.7	3.6	4.1

Table 5 .

 5 Performance of plasma + alumina processes

	Plasma type	CH4/CO2	Flow (mL.min -1 )	P (W) T (°C) packing shape	size	Conversion (%) CH4 CO2 CO HC Selectivity (%)	comments	Ref.
	DBD	1	33	19	240	-Al2O3 beads	1-2 mm	36.5 21.9 19	43	P= 1.2 bar, 24%	11
												C2H6
	DBD	1	50	100	room -Al2O3 sphere	2-2.24 mm 33	22.5 72.2 18.5	26
	DBD	1	50	100	room γ-Al2O3 sphere	2-2.24 mm 32	14	70.1 18.0	26
	DBD	1	50	35	room γ-Al2O3 pellets	500-850µm 22.5 7.5	32.5 17	CB=92%, 9% C2H6 43
	DBD	1	30	130	room γ-Al2O3 beads	1-2 mm	50.3 30	50	18	44
	Plasma	0.7	16660	770	room γ-Al2O3 n.i.	n.i.	45	30	86	n.i.	45
	jet
	n.i.: not indicaded

Acknowledgements

The authors grateful acknowledge the ANR for the financial support of the PRC program VALCO2PLAS and the financial support from the European Union (ERDF) and "Région Nouvelle Aquitaine".

suggesting that other effects than surface area or dielectric constant explain the results. The authors propose that the surface acidity of the material may lie at the base of this observation.

In order verify this hypothesis, the influence of the number of acid and basic sites per unit surface of the alumina materials on CH4 and CO2 conversion was verified (Fig. 7). The highest activity is obtained with the commercial alumina exhibiting the highest number of acid and basic sites. Note that the acidity of γ-Al2O3 is associated with the ionic nature of the Al-O bond [START_REF] Busca | Chapter Three -Structural, Surface, and Catalytic Properties of Aluminas[END_REF]. More specifically the acid sites would be localized closed to the -OH groups of the surface [START_REF] Liu | DRFT-IR Studies of the Surface of γ-Alumina[END_REF]. The role of acid sites in methanol production was proposed by Chawdhury et al. [START_REF] Chawdhury | promising plasma-catalytic approach towards single-step methane conversion to oxygenates at room temperature[END_REF]. The authors obtained higher selectivity to oxygenates using Cu/γAl2O3 catalyst presenting more acid sites than Fe/γAl2O3 in the plasma-catalytic partial oxidation of methane. Also, the presence of basic sites favors the reactant conversion (Fig. 8). Indeed, the basicity of alumina is associated with the presence of surface O -ions as counterpart of acid sites [START_REF] Medema | Isomerization of butene over alumina[END_REF]. These basic sites adsorb carbon dioxide leading to an increase of CO2 conversion and also methane conversion [START_REF] Kundu | Experimental investigation of alumina and quartz as dielectrics for a cylindrical double dielectric barrier discharge reactor in argon diluted methane[END_REF]. The effect of a stronger basicity, increasing CO2 and CH4 conversions was suggested by Wang et al. [START_REF] Wang | Non-thermal plasma-enhanced dry reforming of methane and CO2 over Ce promoted Ni/C catalysts[END_REF] investigating a series of Ce-promoted nickel catalysts (NiCexC). Moreover, Wen et al. [START_REF] Wen | Decomposition of CO2 using pulsed corona discharges combined with catalyst[END_REF] proposed that strong CO2 adsorption on γ-Al2O3 plays an important role in CO2 decomposition under corona discharges, favoring CO production. Using diffuse scattering IR spectroscopy Gadzhieva [START_REF] Gagdzhieva | Methane adsorption and plasma-assisted catalytic conversion on the surface of γalumina[END_REF] showed that CH4 adsorption sites were formed by molecular and dissociative mechanisms under the action of the electric-discharge plasma at the surface of γ-Al2O3.

So according to the results presented here and based on works of Chawdhury et al. [START_REF] Chawdhury | promising plasma-catalytic approach towards single-step methane conversion to oxygenates at room temperature[END_REF] and D. Li [START_REF] Li | Direct conversion of CO2 and CH4 into liquid chemicals by plasma-catalysis[END_REF] possible reaction pathways for CH4 and CO2 transformation into CO and H2 is proposed in Fig. 9. The reaction takes place in gas phase under plasma discharge by electron-induced dissociation of reactants.

Surface reactions are also involved, adsorption of methane can be followed by the cleavage of a C-H bond to produce surface CHx species. It is also possible to propose the adsorption of CHx formed in gas phase under plasma. CO2 can also be activated directly in the gas phase or after adsorption leading to CO formation and a radical O able to react with CHx to produce CO.


Thomas Planès

Éric Larose

Ludovic Margerin

Vincent Rossetto

Decorrelation and

come


The Qiāng language occupies a privileged place among Sino-Tibetan languages spoken in China. It is one of the few languages in the area to have been uninterruptedly studied in the complexity of its dialects for almost a century by many non-Chinese and Chinese scholars, including some of the most eminent and prolific Sino-Tibetanists in China, such as Sūn Hóngkāi and Huáng Bùfán.

Linguistic fieldwork on Qiāng was pioneered by Wen Yu in the 1930s (Wen 1940, 1941) and the analysis of his data was set forth by Chang Kun in the 1960s (Chang 1967, 1968). In the 1950s, a study of Qiāng dialects became a team effort of young linguists of the Chinese Academy of Sciences. Their work has had a lasting effect on the field, for they produced the lion's share of available data on Qiāng dialects to date (e.g. Huáng Bùfán 1987, 1994, 2000; Liú 1981, 1984, 1997, 1998, 1999; Sūn 1962, 1981, 1988).

Since the 1990s, native Qiāng linguists and non-Chinese scholars have joined the field, adding native command of research data and new methodology and insights (Evans 2001, 2004; Huáng Chénglóng 1994, 1997, 2000, 2003; LaPolla 2003a, 2003b; LaPolla and Huáng 2003). 1 As a result of these research efforts, recent years witnessed the publication of several in-depth descriptions of various Qiāng dialects (Liú 1998; Evans 2001; LaPolla and Huáng 2003; Huáng and Zhōu 2006). Huáng Bùfán and Zhōu Fāchéng's work, the subject of this review, stands out from these publications, among others because of the applied goals it pursues.

The collaboration between Huáng Bùfán and Zhōu Fāchéng began in the late 1980s as part of a larger project aimed to develop a writing system for the Qiāng language. The writing of this book commenced in 1993, after the writing system was officially acknowledged by the Chinese government. The authors focused on the dialect that had served as the basis for the writing system, the Qúgǔ 曲谷 village variety of the Yǎdū 雅都 subdialect of Northern Qiāng.

The success of a new writing system for a previously unwritten language, especially one with many highly divergent dialects, like Qiāng, largely depends on the extent to which overall speakers are familiar with the dialect that serves as the basis of the writing system. To be able to read and write, one should master the phonological system of the dialect in question. Moreover, the success of the system is contingent upon a sufficient number of reading materials and dictionaries written in the variety underlying the writing system. With these considerations in mind, Huáng and Zhōu's book has been conceived as a standard reference grammar of the Qúgǔ variety that serves to facilitate compilation of reading materials and reference books, to familiarize speakers of various Qiāng dialects with the dialect underlying the writing system, and to further promulgate the use thereof (p. 2). The ultimate aspiration of the authors is to promote the Qúgǔ variety to the status of common Qiāng, hence the book title. These goals aside, this book presents the reader with what is doubtless one of the most painstaking synchronic descriptions of one of a many Qiāng varieties.

The volume contains all usual parts of a linguistic description: an introductory chapter detailing the history and the sociolinguistic background of the studied language, and separate chapters on phonology and phonetics, grammar, sentence types and lexicon, in that order. In addition, one whole chapter is devoted to Qiāng dialects and the newly introduced writing system is illustrated by one appended text in the Qiāng orthography.

The volume is concluded by seven fully annotated Qiāng texts (in the International Phonetic Alphabet) and an extensive Chinese-Qiāng glossary arranged by semantic fields.

The book took more than a decade to complete. These years were devoted to repeated verification of data, addition of new materials, and extensive discussions between the authors and their language consultants. In addition, for one author, Huáng Bùfán (responsible for the analytic chapters), this book is the culmination of half a century of research on Qiāng, which for her started in the late 1950s as part of the Chinese Academy of Sciences survey and continued throughout the years. As the product of a fruitful collaboration between one of the most meticulous Chinese linguists and fieldworkers, Huáng Bùfán, and a young native Qiāng linguist with an extensive fieldwork experience, this book combines state of the art linguistic expertise with reliable command of ample data.

In many ways, the book heralds a new era in minority languages description in China, as it reaches levels of sophistication and detail that are rarely found in comparable studies by other authors, due to the sheer amount and the degree of familiarity of Huáng and Zhōu with the data, amassed through fifty years of research.

The volume is based on the speech of a large number of language consultants of different genders and age groups, to which native language feel of one of the two authors is adduced. This allows the authors not only to sketch a reliable and representative outline of the described variety, but also to make interesting observations on sound variation and change (p. 55). In addition, in the spirit of the latest guidelines for linguistic fieldwork (cf. Furthermore, their mastery of Qiāng allows the authors to reach a high degree of precision in analysis. For example, Huáng and Zhōu are able to distinguish between various meanings of Qiāng homophonous derivational morphemes and particles and gloss them accordingly (thereby avoiding the unfortunate practice common in Chinese linguistic descriptions to gloss function words and particles by their Chinese nearequivalents or under the underspecified label zhùcí 'particle'). They are able to reveal complex verb paradigms, obscured in Qiāng by assimilation, vowel harmony and syllable contraction. Idiosyncratic uses, such as patterns of co-occurrences of directional prefixes with verbs (p. 267), are explained at length. For example, the verb [ʔi-ɕi] 'die', contrary to what might be logically expected ('away from the speaker'), takes the directional prefixes 'inwards' or 'to the front'. Culturally specific lexicon is treated in a separate section (p. 278-281). All discussed phenomena are abundantly illustrated with example sentences, some taken from the stories appended to the end of the volume, thus assuring coherence between analytic sections and appended data. In sum, this thorough familiarity with the researched language makes this book a reliable and valuable source of information for further studies on Qiāng dialects, Qiangic languages and the Sino-Tibetan language-family at large.

To add some criticism, since no book is perfect, I would like to point out two minor shortcomings of this work: (1) occasional speculative etymologies; and (2) lack of historical background information.

(1) One possible by-product of the authors' high proficiency in the researched language, is that the authors at times appear to be rather unrestrained in their etymologizing attempts, as, for example, in their treatment of consonant finals.

Qiāng stands out from the languages of the area for its complex phonological system. Unlike most Sino-Tibetan languages, it has a large variety of consonant finals, including also consonant clusters. The Qúgǔ variety has a total of 28 simple consonant and 11 consonant cluster finals, thus far surpassing the usual 10 consonant final inventory in most languages of the region. 2 The authors' explanation of this phenomenon is that many consonant finals are due to the merging of two syllables into one as a result of reduction to schwa, devoicing and dropping of the unstressed vowel in the root of the second syllable (p. 31). 3 With their good command of the researched data, the authors are naturally in an advantageous position with respect to uncovering the etymologies of such merged words. In fact, the authors even posit the restoration of original affixation and compounding patterns obscured by the effects of syllable merging as one of the explicit goals for their analysis (p. 59).

The problem is that at the present state of knowledge, it is difficult, if not altogether impossible, to reliably distinguish between those cases where a consonant final is a result of the merging of two syllables into one; those, where it is an inherent part of the root (even though these are held to be exceedingly few in number, cf. Liú 1984: 47) 4 and those, where a consonant final is a derivational or a classifying suffix. (This explains the choice of the authors not to indicate the voiceless vowel in nouns.) Overall, to many Qiāng speakers, numerous words with consonant finals (even when resulting from a fairly transparent merger of two words into one) are already fully lexicalized and semantically indivisible (cf. LaPolla and Huáng 2003: 34). To propose etymologies for words with consonant finals is therefore a challenging problem, to which the authors propose creative solutions. However, some of the offered etymologies are, in my opinion, unorthodox to the extent of bordering on questionable.

In the section on compounding (pp. 260-262), Huáng and Zhōu propose a set of classifying suffixes or "shape markers" (xíngzhuàng biāojì 形状标记). These markers are said to classify nouns by their associations with different shapes. For example, it is argued that the marker -q frequently appears on words denoting oval objects. This marker is explained as derived from the stem qa of the word [qapatʂ] 'head'. 5 The authors note that this stem is sometimes pronounced [qə], in which syllable the schwa can weaken and drop, leaving the initial qfree to attach to other words as a consonant final. The proposed examples include [zəq] 'tongue', [paq] 'heel ' and [ʁləq] 'tear' (the etymology of the first syllable of these presumed contractions is not indicated). More questionable examples of this oval shape marker are: [jimiaq] 'thumb', [dʐəq] 'dew', [ɕyaeːq] 'shadow' and even [ʐətɕʰaq] 'rabbit, hare'. An interesting side question is the relation of this presumably nominal marker -q to the final -q in some adjectives (p. 271), including also the word for 'oval', [dʑindʑaq] (but also [bumbaq] 'numb', [tsitstsaeq] 'sharp' or

[hatʂaq] 'raw'), which issue is regrettably unaddressed in the book.

Etymologies proposed in this section on compounding suggest many peculiar semantic categorizations in Qiāng. Rabbit is apparently perceived by the Qiāng as oval, wound (tsʰuejmi) as fruit-shaped (from -jmi 'fruit-like shape') and ears (ɲiku̥ ) as circular (from -ku̥ 'circular shape'). Apart from these presumably Qiāng-specific associations, Qiāng appears to share some of its semantic associations with Chinese. Take, for instance, the marker [-hae͂ ʴ] 'yellow ' (p. 262). This word can be used on its own in the meaning 'snivel, snot'. As a suffix attached to other words, it is explained as denoting the meaning 'yellow'. Some of the given examples are [Xʂaehae͂ ʴ] 'yellow', [Xɬəhae͂ ʴ] 'loess', [tsʰ uphae͂ ʴ] 'spittle', as well as [quaʴhae͂ ʴ] 'face'. While the word 'face' can potentially be associated with yellow (skin) for the Chinese, it is an interesting and, in my opinion, far from certain question whether the same semantic extension is valid for Qiāng.

(2) Given the primarily descriptive goals of this study, a synchronic description of the Qúgǔ variety to serve as a standard reference work on Qiāng, a historical outline detailing questions of the relationship of Qiāng with other Sino-Tibetan languages is obviously beyond its scope. At the same time, given the importance of this language (which lent its name to a subgrouping within the Sino-Tibetan: Qiangic languages) and given the expertise of Huáng Bùfán in the field of historical linguistics, it is a pity that these issues are not taken up. Some assumptions about previous stages of the Qiāng language can nonetheless be inferred from the descriptive exposition. Let us examine one example, with a possible bearing on the Qiangic subgrouping.

Qiangic languages are known to have a large inventory of existential or location verbs (cúnzài dòngcí 存 在 动 词 'existential verbs' and língyǒu dòngcí 领 有 动 词 'possession verbs' in Huáng and Zhōu's formulation, pp. 123-126), the use of which depends on the semantics of the referent (animate or inanimate, large or valuable) or on the nature of its location (for example, inside a container). Qiāng is no exception in this respect. The Qúgǔ variety has six possession verbs, of which one, [qaeːqe], is more general and, in the authors' analysis (p. 128), can normally replace any of the remaining five verbs. Based on this observation, the authors suggest that [qaeːqe] is the original possession verb and that other verbs are the products of later diversification. Regrettably, the authors do not specify whether they had other grounds for this assumption than just the semantic compatibility of this verb in modern Qiāng. For example, the fact that these existential and location verbs are generally not cognate among Qiangic languages may have also played a role.

All in all, the plethora of Qiāng data and the detailed analyses in this study raise important questions, to which often no clear-cut answers exist. The book is commendable for suggesting issues of broad significance and wide implications, proposing tentative solutions and supplying detailed evidence for further analysis and comparison with other languages.

It will take time to ascertain whether the Qiāng writing system, which motivated the writing of this book, will be a success, and whether the Qúgǔ variety can be effectively promoted to the status of lingua franca among the Qiāng. Irrespective of these two issues, there is no doubt that this book will stand the proof of time as a lasting contribution to Sino-Tibetan studies. For one author, Huáng Bùfán, this volume is the culmination of her 50-year long study of Qiāng dialects. I hope that at the same time, it unfolds a new chapter in her research, that of continued release, with the help of her younger colleagues, of valuable data and meticulous analyses painstakingly amassed in the past half a century of work.

  Review of Qiāngyǔ yánjiū 《羌语研究》 [A Study of Qiāng] by Huáng Bùfán 黄布凡 and Zhōu Fāchéng 周发成. 2006. Chéngdu 成都: Sìchuān Rénmín Chūbǎnshè 四川人民 出版社.

  Dixon 2007: 30), the authors detail the process of gathering data: names of linguistic consultants and the exact division of work (from whom, in what settings and what data were collected, with whom they were checked and analyzed, who were the story narrators etc.) (p. 58).

For an overview of previous linguistic work on Qiang and a complete bibliography of published works, seeLaPolla and Huáng (2003: 16-17).

Most Sino-Tibetan, Kam-Tai, Hmong-Mien and Austronesian languages have the following consonant finals: -p, -t, -k; -w, -j; -l, -r; -s, -h and a glottal stop (cf. Jacques 2004: 82).

This view is generally accepted in Qiang studies, cf.Liú 1984, Huáng 1991: 250-252 or LaPolla 2003a: 573.

In her 1991 outline of Qiangic languages (1991: 252), Huáng Bùfán tentatively proposes, on the basis of a comparative study of cognate words in rGyal-rong, Dàofú, Quèyù and Qiāng, nine consonant codas as common for their ancestor stage. These codas are: -p (-v), -t, -k (-X); -m, -n, -ŋ; -r, -l and -s.

[petʂ] or[patʂ] is in turn a marker for round object (p. 261). The proposed examples are, among others,[qapatʂ] 'head' and [dzuqpetʂ] 'baldhead'.