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Supported Pt catalysts have been extensively investigated for CO2 hydrogenation with methane and CO as the principal products. In this work, a Pt/In2O3 catalyst prepared with decomposition precipitation was tested for CO2 hydrogenation to methanol. The Pt/In2O3 catalyst exhibited a highly improved activity towards CO2 hydrogenation, with methanol selectivity of ca. 100% at temperatures below 225 °C, 74% at 275 °C and 54% at 300 °C, respectively, compared to the pure In2O3 catalyst. This represents the highest methanol selectivity reported on Pt catalysts for CO2 hydrogenation. The stability of the Pt/In2O3 catalyst at elevated temperatures has also been higher than that of the pure In2O3 catalyst, indicated by the methanol formation rate decreasing only to 95% of the initial rate after 5 h in the reaction stream and remaining largely constant thereafter. In contrast, the pure In2O3 catalyst loses 20% of the initial methanol formation rate after 9 h in the reaction stream. The characterization of the catalysts confirms that the Pt nanoparticles are well dispersed on In2O3 with a particle size below 3 nm. The strong metal–support interaction (SMSI) between Pt and In2O3 improves the stability of the catalyst and prevents the over-reduction of In2O3. The synergy between the supported Pt nanoparticles and In2O3 balances the hydrogen activation and the density of the surface oxygen vacancies in In2O3, resulting in the high activity for CO2 hydrogenation and enhanced stability of the Pt/In2O3 catalyst.

In this work, the structural effect of the plasma decomposed Ni/ZrO2 catalyst and the thermally calcined Ni/ZrO2 catalyst on the formation and the reactivity of the carbon formed from methane decomposition was investigated. The plasma prepared Ni/ZrO2 catalyst possesses smaller Ni particle size with Ni (1 1 1) as the principally exposed facet, which favors the formation of the carbon with enhanced reactivity towards carbon dioxide. The Ni (1 1 1) facet benefits the formation of carbon nanotubes (CNTs) attached to Ni nanoparticles at the top and suppresses the formation of onion-like carbon which encapsulates Ni active sites. Besides, CNTs on the plasma-treated catalyst show smaller diameter sizes with lower graphitization degree, which are easier to be converted. Therefore, the plasma prepared catalyst presents a higher carbon resistance. The present work well explains the improved activity and stability of the plasma prepared Ni/ZrO2 catalyst for CO2 reforming of methane.

CO methanation on Ni/CeO2 has recently received increasing attention. However, the low-temperature activity and carbon resistance of Ni/CeO2 still need to be improved. In this study, plasma decomposition of nickel nitrate was performed at ca. 150°C and atmospheric pressure. This was followed by hydrogen reduction at 500 °C in the absence of plasma, and a highly dispersed Ni/CeO2 catalyst was obtained with improved CO adsorption and enhanced metal-support interaction. The plasma-decomposed catalyst showed significantly improved low-temperature activity with high methane selectivity (up to 100%) and enhanced carbon resistance for CO methanation. For example, at 250°C, the plasma-decomposed catalyst showed a CO conversion of 96.8% with high methane selectivity (almost 100%), whereas the CO conversion was only 14.7% for a thermally decomposed catalyst.

The ceria-zirconia solid solution is an excellent support of nickel catalysts for CO2 reforming of methane. In this work, we further improve the activity of the ceria-zirconia solid solution supported Ni catalyst via decomposition of nickel precursor by dielectric barrier discharge (DBD) plasma at ca.150 °C, followed by hydrogen reduction at 500 °C in the absence of DBD plasma. Compared to the catalyst prepared by the conventional thermal decomposition, the plasma decomposed catalyst possesses smaller Ni nanoparticles with mainly exposed Ni(111) lattice fringe. The plasma decomposed catalyst shows higher reducibility, which helps to form more active Ni0 species after hydrogen reduction. It also exhibits more basic sites and oxygen vacancies, promoting the activation of CO2. Additionally, the plasma decomposed catalyst tends to form highly reactive carbon, from methane decomposition, the initial reaction of CO2 reforming, towards the reaction with carbon dioxide. This leads to an enhanced coke resistance for CO2 reforming.

A series of Co3O4/HZSM-5 catalysts with different Co3O4 loadings were prepared by incipient wetness impregnation followed by thermal calcination or dielectric barrier discharge (DBD) plasma decomposition. Their catalytic performances for the complete combustion of methane were evaluated from 300 to 700 °C. The plasma decomposed samples exhibit much higher catalytic activity than the calcined ones. Among all the samples tested, the plasma decomposed catalyst with 20 wt.% loading of Co3O4 possesses the highest catalytic activity with T10 (10% CH4 conversion) at 300 °C and T100 (complete conversion) at 550 °C. The excellent catalytic activity of the DBD plasma decomposed samples would attribute to well-dispersed Co3O4, enhanced low-temperature reducibility, high content of lattice oxygen and adsorbed oxygen species, as well as the low formation tendency of the intermediate of Co-formate.

A Ni/CeO2 catalyst was prepared via decomposition of nickel precursor by gas discharge plasma, followed by hydrogen reduction thermally. The activity of the obtained catalyst reaches the highest level towards CO2 methanation with methane selectivity above 99 % at reaction temperatures lower than 300 °C. For example, the CH4 formation rate at 275 °C is 100.3 μmol gcat−1 s−1, higher than the reported catalysts at the same reaction temperature. Characterization results indicate that the plasma decomposition leads to an interfacial structure where Ni atoms bind with O atoms from ceria. A strong metal-support interaction is caused. Rich interfacial Ni-CeO2 sites are thus formed with excellent redox property. The unique interfacial structure confines small nickel particles on the ceria surface, exposing more metallic Ni as active sites for splitting H2. Therefore, the plasma prepared Ni/CeO2 catalyst shows balanced active sites for H2 splitting and CO2 activation, improving low temperature catalytic activity significantly.

Dielectric barrier discharge (DBD) plasma was applied for the decomposition of nickel nitrate under air at ∼150 °C. Calcination (at 500 °C) under air and hydrogen reduction (at 500 °C) thermally were then conducted. The obtained Ni/SiO 2 catalyst was investigated for steam reforming of methane (SRM) at the reaction temperatures of 600, 700 and 800 °C with a very low steam to methane molar ratio (H 2 O/CH 4 = 0.5), in order to test the coke resistance of the catalyst. The DBD plasma prepared catalyst possesses an average particle size of 5.5 nm, much smaller than 15.3 nm of the thermally calcined one. The smaller catalyst size can lead to an enhanced coke resistance over the plasma prepared catalyst. The carbon nanotubes obtained on the used plasma prepared catalyst are very different from those formed on the calcined catalyst. The plasma decomposition is excellent for further investigation of effect of the catalyst structure beyond the nickel catalyst.

CO2 methanation has been a hot topic because of its important application in the spacecraft and potential utilization of carbon dioxide. Nickel catalyst is active for this reaction. However, its activity still needs to be improved. Dielectric barrier discharge (DBD) plasma, initiated at ambient condition and operated at ∼150 °C, has been employed in this work for decomposition of nickel precursor to prepare Ni/MgAl2O4. The plasma decomposition results in high dispersion, unique structure, enhanced reducibility of Ni particles and promoted catalyst-support interaction. An improved activity of CO2 methanation with a higher yield of methane has been achieved over the plasma decomposed catalyst, compared to the catalyst prepared thermally. For example, the methane yield of the plasma prepared catalyst is 71.8% at 300 °C but it is 62.9% over the thermal prepared catalyst. The catalyst characterization confirmed that CO2 methanation over the DBD plasma prepared catalyst follows pathway of CO methanation.

Dry reforming of methane (DRM) over Ni catalysts has attracted increasing attentions. However, the catalytic activity needs to be improved. Herein Ni/ZrO2 catalyst was prepared via decomposition of nickel precursor under influence of dielectric barrier discharge (DBD) plasma at ~150 °C. Such prepared catalyst has mainly exposed Ni (111) facets, smaller metal particles and more tetragonal zirconia with increased oxygen vacancies. All lead to significantly improved activities in the temperature range 550–800 °C, compared to the calcined catalyst. For example, the hydrogen yield over the plasma prepared catalyst attains 50.38% at 650 °C, while it is 36.57% with the calcined catalyst.

The structure of Ni catalyst has a significant effect on the growth and reactivity of carbon nanofibers (CNFs) formed from methane decomposition. We present here a detailed study on this structure effect with Ni/MgAl2O4 as model catalyst. Different catalyst structures were obtained via dielectric barrier discharge (DBD) plasma and thermal decomposition. For the thermal decomposed catalyst, a complex combination of microfacets presents with a higher content of defects. A diffusion interfacial region is formed due to the migration of Ni atoms into the support. However, a higher concentration of close-packed plane is observed for the DBD decomposed catalyst with less defects and a clean interface between Ni particle and support. The CNFs over the DBD decomposed sample show smaller diameter and narrower distribution with a smaller slope of graphene layers but enhanced activity toward O2 and CO2, indicating better coke resistance for methane conversion.

Steam reforming of methane normally requires extra steam feed in order to inhibit the carbon formation. A catalyst with enhanced coke resistance is always helpful to reduce the use of steam and to save the energy. Dielectric-barrier discharge (DBD) plasma, initiated at ambient condition, was applied for the decomposition of nickel precursor to prepare Ni/SiO2 for steam reforming of methane. The catalyst prepared this way shows an intrinsically low activity with small size, leading to a good balance between the carbon formation and the carbon gasification by steam at low H2O/CH4 ratio. An enhanced coke resistance has been thus achieved.

In this work, the bimetallic Pd catalysts containing Ag, Cu, Co and Ni were prepared via the glow discharge plasma treatment followed by calcination thermally. The X-ray photoelectron spectroscopy (XPS) analysis confirms that the plasma treatment can reduce Pd2+ effectively. The calcination of the plasma formed Pd clusters enhances the formation of PdO that was identified as the active species for methane combustion. Compared to the conventional catalysts, all the plasma prepared samples show an enhancement in the activity at the same temperature. Among the investigated catalysts, the Pd–Ag/HZSM-5 sample exhibits the best activity for methane combustion.

Cluster models were used to represent the β-type cationic sites of the protonated beta zeolite (H-BEA) and the loading of PdO on these sites. The properties of these clusters and the cleavage of methane C H bond over these clusters were studied using density functional theory (DFT) method. The stability of H-BEA was enhanced due to the formation of hydrogen bonds. After PdO loading, the Pd atom bonds to four oxygen atoms among which three H-BEA framework oxygen atoms are included to form an approximate planar structure with Pd in the centre. This structure is very similar to that of bulk PdO. The acidic proton of H-BEA and the oxygen atom of PdO participate in the cleavage of methane C H bond, indicating that PdO is the active species for the activation of methane. Over the clusters constructed in the present work, the calculated energy barriers for the cleavage of methane C H bond are in the region between 17.54 and 21.02 kcal mol −1 .

The glow discharge plasma treated Ni/Al2O3 catalyst showed an excellent anti-coke property for CO2 reforming of methane. Characterizations using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), transmission electron microscopy (TEM), and CO adsorbed infrared spectroscopy (IR) were conducted to investigate the structure and reactivity of the plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane. It confirms that the plasma treatment of Ni precursor at room temperature followed by calcination thermally has a significant influence on the surface characteristics of the active phase. The plasma treated catalyst contains high concentration of close packed plane with improved Ni dispersion and enhanced Ni-alumina interaction, which lead to high catalytic activity and excellent resistance to formations of filamentous carbon and encapsulating carbon.

In this work, characterizations using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirm that argon glow discharge plasma can effectively reduce the Al 2 O 3 supported Ir catalyst without the use of any conventional reducing chemicals. Such plasma-reduced Ir/Al 2 O 3 catalyst shows a significantly enhanced dispersion. High conversions have been achieved on this plasma-reduced Ir/Al 2 O 3 catalyst for CO 2 reforming of methane.

Argon glow discharge plasma was applied for drying the impregnated Ni/SiO2 catalyst instead of the drying thermally. Such plasma treated catalyst significantly inhibits the coke formation from methane decomposition. The methane-derived carbon shows an improved reactivity against CO2. This can result in a better balance of coke formation and gasification by CO2 when the plasma-treated Ni/SiO2 catalyst is used for the CO2 reforming of methane.

Silica supported Ni catalyst is highly active for the CO 2 reforming of methane but it has poor stability due to coke formation. In this work, a glow discharge plasma was applied for the decomposition of nickel nitrate on the SiO 2 support, followed by thermal calcination in air. The plasma treatment enhances the interactions between the Ni particles and the silica and significantly improves the Ni dispersion. The plasma-treated Ni/SiO 2 catalyst exhibits comparable activity to the Ni/SiO 2 catalyst prepared by the thermal method without plasma treatment. The coke resistance of the Ni/SiO 2 catalyst is significantly enhanced by the plasma treatment.

CO adsorbed infrared spectroscopy study was conducted in this work in order to better understand the significantly improved anti-coke performance of Ni/Al2O3 catalyst obtained via argon glow discharge plasma treatment. The present study revealed a significant decrease of linear to bridge (L/B) adsorbed CO for glow discharge plasma treated Ni/Al2O3, compared to that for untreated Ni/Al2O3, indicating an enhancement of close packed plane concentration. This structure change leads to lower methane turnover frequency (TOF) and better balance of carbon formation-gasification, resulting in better anti-coke property of Ni/Al2O3 for CO2 reforming of methane.

Ni/Al2O3 catalyst was first treated by argon glow discharge plasma followed by calcination in air. The catalyst prepared this way exhibits an improved low-temperature activity for carbon dioxide reforming of methane, compared to the catalyst prepared without plasma treatment. The catalyst characterization using XRD, chemisorption and TEM analyses show that the plasma treatment followed by calcination thermally induces a generation of specific nickel species on the support. This kind of “plasma” metal species is highly dispersed on the support and can remain stable during reforming reactions. The average size of the “plasma” metal particles is ca. 5 nm. The plasma treatment can also enhance the anti-carbon deposition performance of the catalyst. The formation of carbon species that is responsible for catalyst deactivation can be inhibited. The catalyst stability is therefore improved.

Methane is one of the most stable molecules. The first C–H bond activation of methane is a crucial step in methane conversion. Once the first C–H bond was broken, the remaining C–H bonds could be very easily activated. In this work, DFT study of the first C–H bond activation of methane over PdO/HZSM-5 catalyst for methane combustion was conducted. The 7T(6, 1) and 6T cluster models, correspond to the α- and β-sites of ZSM-5 that take 85–100% of the total metal sites in the zeolite, were chosen for the study. The physisorbed state of methane and the transition state geometries of the first C–H bond dissociation were obtained. The study confirms that the oxygen in PdO directly involves in methane activation that means PdO is the active Pd species. The proton in the zeolite promotes the methane activation via the formation of H 2 O with OH, formed during methane activation over PdO. The barrier energy was also calculated. The activation energy of methane dissociation over 6T1Al and 7T1Al-PdO/HZSM-5 is 98 and 94 kJ/mol, respectively, while the activation energy over 6T2Al and 7T2Al-PdO/HZSM-5 decreases 16 and 22 kJ/mol, compared to that over 6T1Al and 7T1Al clusters. This suggests more acidic sites would reduce the activation energy for methane activation.

Thermogravimetric (TG) analysis confirms that glow discharge plasma treatment has a remarkable influence on coke formation during the non-oxidative aromatization of methane over a Mo-Fe/HZSM-5 catalyst. The plasma treatment favors formation of active carbon species for methane aromatization.

An experimental investigation was conducted to convert methane in the presence of dimethyl ether (DME), using dielectric barrier discharge (DBD) at atmospheric pressure and 100 °C. The co-feed of DME induces an increase in methane conversion, from 18.22% to 38.46%. The major products include C2 and C3 hydrocarbons, methoxy-containing oxygenates (methyl ethyl ether (MEE), dimethoxymethane (DMM), and dimethoxyethane (DMET)) and syngas (CO and H2). The selectivity of products changes with the CH4/DME feed ratio. The larger the CH4/DME feed ratio, the higher the selectivity of light hydrocarbons. The highest selectivity of all the oxygenates is observed for a CH4/DME feed ratio of 1:1.

PdO/Al2O3 catalysts prepared by glow discharge plasma treatment followed by thermal calcination show a much higher dispersion and a better catalytic activity for methane combustion at relatively low temperatures. The dispersion of palladium active species by such plasma prepared catalysts is 29.7%, 5.4 times higher than that of conventional catalysts. XPS analysis indicates that a surface enrichment of Pd active species (PdO) has been achieved after plasma treatment. The surface atomic composition of PdO of plasma prepared catalysts reaches 10.5%. XRD characterization also confirms a well‐crystallized PdO phase present on the plasma prepared catalyst. The light‐off temperature of the plasma prepared catalyst is 370°C, 50°C lower than that obtained from the conventional catalyst.

A novel plasma reduction and calcination (PR&C) method has been recently developed. Upon the experimental investigations on methane conversions, including partial oxidation, methane combustion, NO reduction by methane and CO2 reforming, the catalysts prepared by this PR&C method exhibit a remarkable enhancement in the dispersion, low-temperature activity and stability. A plasma-enhanced acidity has also been addressed, which would play an important role in the improved dispersion. The PR&C method is leading to a better preparation of supported catalysts, esp., those for methane conversion.

Ni–Fe/Al 2 O 3 catalyst for partial oxidation of methane was prepared first by glow discharge plasma treatment following by calcinations thermally. Such prepared catalyst exhibits better performance over the catalyst prepared without such plasma treatment. The conversion of methane and the selectivity of CO and H 2 over the plasma treated catalyst are 97.44, 100 and 100% at 875 °C, while, at the same temperature, they are 90.09, 97.28 and 97.09%, respectively, over the catalyst prepared without plasma treatment. At the same methane conversion, the reaction temperature with the plasma treated Ni–Fe/Al 2 O 3 is at least 80 °C lower. TEM characterization shows that the catalytically active species after plasma treatment are still highly dispersed on the support. In addition, the plasma treated Ni–Fe/Al 2 O 3 catalyst also possesses better anti-carbon deposit ability.

A novel catalyst preparation for NO reduction by CH 4 has been conducted using a glow discharge plasma treatment followed by calcinations thermally. Such plasma prepared Pt/NaZSM-5 catalyst exhibits a high dispersion of metal active species. A remarkable improvement in the activity and stability, compared to the catalysts prepared conventionally, has been achieved. Especially, an excellent low temperature activity over the plasma-treated Pt/NaZSM-5 catalyst has been obtained. For example, the conventional 0.1 wt% Pt/NaZSM-5 catalyst shows no activity at temperatures below 673 K, while at 673 K, the NO conversion to nitrogen reaches 61.3% over the plasma-treated 0.1 wt% Pt/NaZSM-5 catalyst. The initiated temperature for the plasma-treated 0.1 wt% Pt/NaZSM-5 catalyst can be as low as 548 K.

A density functional theory (DFT) study has been conducted in this work to investigate the reaction mechanism of synthesis of oxygenates and higher hydrocarbons from methane (CH4) and carbon dioxide (CO2), using cold plasmas. The main dissociation routes of the reactants were analyzed. The feasibility of the formation of various products, including syngas, higher hydrocarbons, and oxygenates, was discussed. The DFT study confirmed that the major obstacle of the synthesis is the dissociation of the reactants CH4 and CO2. After the cold plasma has supplied the necessary energy for the dissociation of CH4 and CO2, syngas, higher hydrocarbons, and oxygenates can be then easily produced. The present DFT study also demonstrates that the plasma synthesis will normally lead to a formation of a mixture of syngas, higher hydrocarbons, and oxygenates.

Methane conversion in the presence of carbon dioxide was investigated under the conditions of dielectric-barrier discharge plasmas. Different from the previous investigations, the grounded electrode is covered by the dielectric material (quartz) in the present reactor design. The product contains gaseous hydrocarbons, syngas and oxygenates. No liquid hydrocarbons can be detected with the present reactor design. The oxygenates produced includes acetic acid, propanoic acid, ethanol and methanol. There exists an optimum feed ratio of CH 4 /CO 2 to make the maximum selectivity of the objective oxygenate. The highest selectivity of acetic acid was 5.2% achieved at CH 4 and CO 2 conversions of 64.3% and 43.1%, respectively.

A novel Pd/HZSM-5 catalyst was prepared first by glow discharge plasma treatment followed by calcination thermally. Such prepared catalyst shows a higher activity and an enhanced stability for methane combustion. The XRD characterization and XPS analysis confirm that the plasma preparation leads to a better preparation of PdO active species over the HZSM-5 support. Especially, a plasma-enhanced acidity has been achieved upon the FT-IR analysis. The enhanced acidity plays an important role in stabilizing the dispersed PdO active species on the zeolite support.

A novel Ni-Fe/Al2O3 catalyst for partial oxidation of methane has been prepared by glow discharge treatment followed by calcination thermally. The catalyst prepared exhibits better activity and selectivity.

A series of Sn-Cr binary oxide catalysts were prepared by a co-current co-precipitation method and tested for methane total oxidation. The binary oxide catalysts have much higher surface areas and catalytic activities for methane oxidation than pure SnO2. CrO x /SnO2 with a Cr/Sn atomic ratio of 3:7 displays the highest activity. Selected samples were subjected to temperature-programmed 18O isotope-exchange measurements. Both complete and partial heteromolecular 18O isotope exchange, as well as oxygen release, was observed for all catalysts. Reaction between CH4 and 18O2 under static conditions was performed to investigate the reaction mechanism and it was found that the total oxidation of methane over Sn-Cr binary oxide catalysts occurs via a redox cycle with the chromium ion in a high oxidation state as the active center. Oxygen mobility of the catalyst plays an important role in the total oxidation of methane, but too high a mobility leads to very high oxygen release and a reduction of the surface reoxidability. This causes a decrease in the catalytic oxidation activity.

In this work, starch has been used to enhance the oxygenate formation directly from methane and carbon dioxide using dielectric-barrier discharges (DBDs). The use of starch inhibits the formation of liquid hydrocarbons and significantly increases the selectivity of oxygenates. Oxygenates produced include primarily formaldehyde, methanol, ethanol, formic acid, and acetic acid. The total selectivity is about 10–40% with conversion of methane and carbon dioxide of about 20%. Lower methane feed concentration favors the production of oxygenates, and higher feed flow rate leads to higher selectivity of oxygenates in the presence of starch.

We have previously reported an experimental investigation on synthesis of acetic acid directly from CH 4 and CO 2 via dielectric-barrier discharge. In this work, a DFT study was conducted using three hybrid DFT methods in order to understand the mechanism of such direct synthesis. It suggests that the synthesis is via two pathways with CO 2 − and CO as key intermediates. The energy requirement with CO 2 − pathway is much less than that with CO. The methyl radical formation and the dissociation of CO 2 are two limiting steps for the synthesis of acetic acid directly from CH 4 and CO 2 .

In this work, a direct conversion of methane in the presence of carbon dioxide using dielectric-barrier discharge plasmas has been conducted. The product includes syngas (H2 and CO), gaseous hydrocarbons (C2 to C5), liquid hydrocarbons (C5 to C11+), and oxygenates. The liquid hydrocarbons are highly branched, representing a high octane number, while the oxygenates mainly consist of series of alcohols and acids. A detailed analysis of product distribution has been performed under variable feed conditions with different reactor configurations. At the high CH4/CO2 feed ratio, the wider discharge gap (1.8 mm) is more favored for the formation of methanol and ethanol. For the production of acetic acid, the narrower discharge gap (1.1 mm) is better, especially, with the existence of after-glow zones. Conditions favored for the production of acetic acid are also good for the production of liquid fuels.

Experimental investigation on plasma methane conversion in the presence of carbon dioxide using dielectric-barrier discharges (DBDs) has been conducted. Zeolite A has been applied to inhibit the formation of carbon black and plasma-polymerized film during such plasma methane conversion. A co-generation of syngas, light hydrocarbons and liquid fuels has been achieved. The conversions and selectivities are determined by the CH4/CO2 feed ratio, residence time and input power. Compared to the use of zeolite X within the DBDs, plasma methane conversion with zeolite A leads to a higher selectivity of light hydrocarbons (C2–C4).

The effect of electrode materials on the co-generation of syngas and higher hydrocarbons from CO2 and CH4 using dielectric-barrier discharge has been investigated. The electrode materials tested here include aluminum, copper, steel, and titanium. For the feed of methane in the absence of CO2, the order of activity of methane conversion from high to low was Ti ≈ Al > Fe > Cu, while the order of the activity of CO2 conversion was Al > Cu > Ti > Fe for the case of CO2 feed without methane. Regarding the co-feed of methane and CO2, the titanium electrode shows the best activity for the conversions, while the other three materials show a similar performance for the conversions. The effect of dilution gas, helium, on the conversions has also been discussed.

Experimental investigation has been conducted to convert methane into higher hydrocarbons in the presence of carbon dioxide within dielectric-barrier discharge (DBD) plasmas. The objectives of cofeed of carbon dioxide are to inhibit carbon deposit and to increase methane conversion. The products from this plasma methane conversion include: (1) syngas (H2+CO), (2) gaseous hydrocarbons containing ethylene, acetylene, and propylene, (3) liquid hydrocarbons, (4) plasma-polymerized film, and (5) oxygenates. The selectivity of products is subject to the DBD plasma-reactive conditions and catalyst applied. The liquid hydrocarbons produced by this way are highly branched, which represents a better fuel production.

Direct plasma methane conversion to higher hydrocarbons using pure methane feed is almost impossible without using zeolite. In the presence of zeolite, such direct conversion leads to a very high yield of higher hydrocarbons. The use of zeolite not only inhibits the formation of non-selective carbon black and plasma-polymerized films but also significantly increases the selectivity of light hydrocarbons during plasma methane conversion with dielectric barrier discharge (DBD). The DBD plasma-used zeolite has been characterized by x-ray photoelectron spectroscopy (XPS) and suggests the DBD plasmas have led to reactions within the channel of zeolite. The present investigation gives us more supporting results for the proposed mechanism of plasma-promoted catalysis.