methane decomposition using metal-assisted nanosecond laser-induced plasma at atmospheric pressure

Methane Decomposition Using Metal-Assisted Nanosecond Laser-Induced Plasma at Atmospheric PressureZ. Ghorbani,† P. Parvin,*,† A. Reyhani,‡ S. Z. Mortazavi,‡ A. Moosakhani,† M. Maleki,† and S. Kiani§

†Physics Department, Amirkabir University of Technology, P. O. Box 15875-4413, Tehran, Iran‡Physics Department, Faculty of Science, Imam Khomeini International University, P. O. Box 34149-16818, Qazvin, Iran§Department of Polymer Engineering & Color Technology, Amirkabir University of Technology, P. O. Box 15875-4413, Tehran, Iran

ABSTRACT: Methane decomposition has been extensively investigated using aQ-switched Nd:YAG laser, focused on the metal catalysts including Ni, Fe, Pd, andCu within the controlled chamber to verify the effect of catalyst, plasma properties,and yield and selectivity of the products. Fourier transform IR spectroscopy(FTIR) and gas chromatography (GC) are employed to support the character-ization of the components. This indicates that methane is strongly decomposedwithin the metal-assisted laser-induced plasma, leading to the subsequentrecombination and the production of heavier hydrocarbons. The dominantspecies, including propane, ethane, and ethylene, have been identified examiningdifferent metallic catalysts. The dissociation rate, conversion ratio, selectivity, andyield of products are strongly dependent on the metal target and plasmacharacteristics.

1. INTRODUCTION

Methane is the principal component of the natural gasreservoirs which are a greatly under-utilized resource forgaseous (CNG) and liquid (LNG) fuels.1 Moreover, methane isone of the greenhouse gases affecting global warming that is animperative environmental concern.2,3 Hence, the methaneconversion into useful hydrocarbons is a significant issue tobe extensively investigated.3−7

Different methods have so far been used to decomposemethane molecules. The pyrolysis method refers to thedecomposition of the hydrocarbons by heat or the plasmaexposure without the addition of oxygen or an oxygen-containing reactant. The reaction is endothermic, and at highenough temperatures it decomposes to form hydrogen, ethane,ethylene, propylene, acetylene, and solid carbon. Methaneplasma pyrolysis has received considerable attention for a longtime and has also been thoroughly considered for the synthesisof higher hydrocarbons.8,9

On the other hand, oxidative coupling of methane (OCM) isa direct methane conversion to more valuable products. Theprocess is highly exothermic, and the temperature increasesduring the reaction process. Hence, the rate and selectivity ofthe products are strongly affected.10 The OCM process is ableto turn methane directly into ethane, ethylene, and acetylene.Most previous studies of methane activation are based on theOCM reactions with the metal catalysts. The heterogeneouscatalytic OCM has been the major subject of a large body ofresearch activities. This usually faces low selectivity, high energycost, and low conversion efficiency. To conquer theseproblems, plasma technology has been recently used todecompose methane.4 The utilization of suitable catalysts inplasma can improve the conversion efficiency as well as the

selectivity of the desired products to such an extent that thismethod is accredited as an efficient alternative of the thermalmethod.11

For the catalytic decomposition, several metallic samples, i.e.Ni, Fe, Co, Pd, Mo, Cu, and Mn, have been exploited ascatalysts.12−15 It is well-known that Fe, Co, and Ni havepartially filled 3d orbitals to facilitate the dissociation of thehydrocarbon molecules through partially accepting electrons.This interaction along with back-donation from the metal intothe unoccupied orbital in the hydrocarbon molecule changesthe electronic structure of the adsorbed molecule leading to thedissociation.16 Copper is among nontransition metals whose 3dshell is completely filled. This catalyst is employed to yield onlythe amorphous carbon.17 Therefore, it is believed that Cu is aninactive element during the decomposition process of thetraditional catalytic system.18,19

This takes for granted that the laser exhibits its ability todecompose methane without CO2 generation in thecombustion free process.20 Wang et al.21 investigated thedissociation of methane in the intense laser field using anamplified ultrafast Ti:sapphire laser at ∼800 nm coupled to atime-of-flight (TOF) mass spectrometer. This gives rise to thedissociation proceeding a stepwise mechanism by the graduallyincrease of the laser intensity. Moreover, Gondal et al.3

reported the photodissociation of methane into higherhydrocarbons using a high power pulsed ultraviolet laser at355 nm under ambient conditions without catalyst contribu-tion. Zerr et al.22 studied decomposition of alkanes such as

Received: August 26, 2014Revised: November 14, 2014

Article

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methane, ethane, and octane. Sharifi et al.23 explained the high-power laser ionization-dissociation of CH4 at various femto-second laser intensities as high as 1015 W/cm2 where no catalystwas employed. Furthermore, Wu et al.24 practically studied thefragmentation pattern of CH4 due to femtosecond laser shots.Wang et al.25 investigated the methane dissociation usingfemtosecond laser shots at 800 nm using the laser fluence∼1014 W/cm2. Suzuki et al.26 demonstrated the formation offormaldehyde from photo-oxidation of methane over amolybdena-silica catalyst at ∼500 K using UV irradiation. Hillet al.27 reported photoinduced reactions of methane on themolybdena-silica surface based on the adsorption during UVexposure. When it heats up to a certain temperature (∼470 K),a considerable amount of ethylene, ethane, and hydrogen wereproduced as well as small amounts of C3 and C4 alkenes andalkanes.Furthermore, Reyhani et al.7 have shown the methane

decomposition during Pd-assisted laser-induced plasma in thecontrolled chamber at various pressures using a nanosecondpulsed Nd:YAG laser at 1064 nm. Real time LIBS has revealedthe decomposition by altering the plasma parameters atmanometric pressures 1−10 mbar. It attests that the plasmacreates higher hydrocarbons during the methane decomposi-tion and the successive recombination process.Despite this, thermal (noncatalytic) decomposition of

methane occurs at very high temperatures (>1200 °C) becauseof strong C−H bonds. However, different transition metalcatalysts such as Ni, Fe, and Co have been inspected to reducethe optimal temperature required for the thermal decom-position.28,29 In fact, the catalyst benefits from losing a C−Hbond during the surface adsorption. The major drawbackassociated with the use of metal catalysts is related to their rapiddeactivation due to active sites blocking by means of carbondeposits.30 Fortunately, pulsed laser activates the blockingsurface of the metal catalyst shot by shot, generating excessivenanocatalysts with efficient large contact area to react withmethane molecules.Here, the nanosecond laser at 1064 nm was employed in

order to decompose methane in the controlled chamber atatmospheric pressure based on the metal-assisted inducedplasma. Different metal targets such as Ni, Fe, Cu, and Pd wereexamined. Fourier transform infrared spectroscopy (FTIR) andgas chromatography (GC) instruments were applied to detectand analyze the obtained compounds. It was shown that thedecomposition without oxygen content at low equilibriumtemperature generates higher hydrocarbon components regard-ing CO2 free reaction. To our best knowledge, there are a fewreports available to investigate the effects of catalysts innanosecond laser-induced plasma during methane decomposi-tion and the subsequent generation of heavier hydrocarbons.Furthermore, this work describes an experiment on the

border between plasma chemistry and catalysis. The conversionof photons to thermal energy (inducing the plasma) as well asgeneration of metal nanoparticles are coupled, and bothprocesses influence chemical conversion of methane.

2. THEORYThe molecular chemical bonds are usually broken with a singlelaser shot using an intense femtosecond laser field up to 1014

W/cm2.21 Nanosecond IR laser at 1064 nm is seldom able todecompose most of molecules according to the photo-dissociation mechanism. The Nd:YAG laser provides 1.16 eVphoton energy, not to be sufficient to detach hydrogen atoms

from C−H bonds. The energy required to dissociate thehydrogen from the molecule is ∼4.5 eV. It is equivalent to a UVphoton energy which is supplied by the excimer lasers, higherharmonics of dye, or solid state lasers. In fact, the directexcitation of electronic transitions of methane requires a UVphoton energy above 8.9 eV (∼140 nm).31 Methane UVphotodissociation is believed to provide a synthetic route to thehigher hydrocarbons based on the recombination process.32

The laser excitation turns methane molecules into methyleneand methyl radicals, which will further react together toproduce C2 and C3 hydrocarbons such as ethane C2H6,ethylene C2H4, and propylene C3H6.FTIR analysis attests that the methane absorption lines are

located at ∼1300 and ∼3000 cm−1. Therefore, it demonstratesthat the optical beams at 7.6 and 3.3 μm are able to dissociateC−H bonds directly via multiphoton excitation and dissocia-tion processes. Similarly, the molecules may simultaneouslyabsorb ∼27 and ∼12 coherent photons via MPA/MPDmechanisms for C−H fragmentation, respectively. Accordingto the molecular energy scheme, methane is disintegrated intwo ways. (i) The photodissociation by proper coherentphotons via multiphoton absorption and (ii) the collisionaldecomposition using the energetic species within plasma basedon electron impact dissociation. The mechanisms above dealwith the drastic difference of the collisional cross sections.As a consequence, the conductive (metallic) or non-

conductive (isolator) nature of catalysts is essential in thecase of laser-induced plasma on solid. For metallic samples, theelectrons of the conduction band absorb laser photons and theexcess energy of the electrons is dissipated by the collisionswith the material lattice. Direct absorption of the laser energyby the ions in the lattice is prevented according to the dielectricscreening. The ablated material expands at supersonicvelocities, producing a shock wave which propagates from thesurface toward the surrounding atmosphere. The plasma plumecontinues to absorb energy from the laser within the pulseduration based on the inverse Bremsstrahlung absorption (IB)process. The plasma heating goes on during laser irradiation.The species are excited, leading to the plasma emissionafterward. The ablated material which contains free neutralatoms, ions, molecular fragments, and free electrons expands atsupersonic velocities.33 Interactions of these species withmolecules cause loosening of the bonding energy, resulting inthe decomposition eventually.Material ablated into the plasma may be in the form of

particles (either fresh, or melted and cooled) as well as atomsand/or molecules. In fact, laser ablation is governed by a varietyof distinct nonlinear mechanisms. Once the laser beamilluminates the sample, the mass leaves the surface in theform of electrons, ions, atoms, molecules, clusters, and particles,each of the processes separated in time and space.34,35

However, during methane decomposition in plasma, theablated catalysts are mostly metal NPs such that methanemolecules can attach to them, with subsequent catalysis of thedecomposition to CH2 and then recombination to highercompounds.36,37 The method involves the condensation ofatoms/molecules and cluster formation (with or without anychemical reactions) during the fast expansion of the vapor/plasma plume generated in front of a target. The time ofnucleation and size and composition of clusters depend on thetype of material, the laser parameters, and the ambientmedium.38

The Journal of Physical Chemistry C Article

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A focused pulsed laser beam on the target results in the rapidtemperature rise (>1011 K/s), with maintenance of thestoichiometry of the nanoparticles. High-energy atoms andions in the laser-induced plasma plume create a high surfacemobility which accounts for nanoparticles’ generation. Duringthe laser beam interaction with the target surface material, thereare various processes leading to atoms and clusters and dropletexpulsion from the target surface. The most common case isthe nanoparticle production by the pulsed laser ablation.39−41

Regarding the plasma chemistry, the bonding energy isranging 3−6 eV for most of hydrocarbons and thedecomposition mainly ascribes to electron impact dissociationand ionization.42 The former is exhibited by dissociation ofmolecules through both vibrational and electronic excitations.The vibrational states usually deal with the multiple quanta.The dissociation occurs as a nondirect multistep process,containing energy exchange between molecules to collectenough vibrational energy needed for the dissociation. Suchprocesses efficiently contribute to decomposing gases such asN2, CO2, H2, and CO. In contrast, the dissociation throughelectronic excitation takes place just after a single collisionstimulated by direct electron impact. The primary process canproceed through different intermediate steps of intramoleculartransitions.43 The kinetics is proposed based on electron impactreactions of methane without catalytic contributions. Thecorresponding threshold energies are listed in Table 1.42

The electron energy distribution of laser-induced plasmaspreads over a few electronvolts. Hence, the multiple electronimpact collisions may dissociate or ionize the molecules.Furthermore, the utilization of metallic catalyst drasticallydecreases the electron energy that is required for the methanedecomposition. The non-Maxwellian electron energy distribu-tion of laser-induced plasma is typically ranging 1−5 eV whilethe threshold energy for dissociation requires greater than 9 eV.Hence, the plasma seldom dissociates methane by itself. Figure1 depicts the non-Maxwellian energy distribution function(EDF) of electrons in laser-induced plasma and the typicalmethane decomposition cross section due to an energeticelectron collision, which is a function of energy too.44 Table 2proposes the kinetics generating various species during thelaser-induced plasma formation in the presence of metalcatalyst.In fact, the energetic metal species within the plasma are

initially ablated from the targets. Those contribute todecomposing the methane molecules while the catalysts arepartners to enhance the process, leading to the formation ofCH3 radical and H2 after recombination.7 The species initiatethe recombination reactions to react with one another toproduce C2 and C3 hydrocarbons and hydrogen according to

Table 2. The conversion of methane molecules to heavierspecies in the presence of catalyst deals with the dissociationand recombination reactions.45,46

3. EXPERIMENTAL SECTIONFigure 2 depicts the experimental schematic for methanedecomposition by laser having nanosecond duration. It consistsof an irradiation chamber, high vacuum systems, conductingand focusing optics, and laser pulse diagnostics. The home-made cross type chamber was made up of stainless steel with sixoutlets, of which four are allocated for the windows. An AR-coated BK7 window with 5 cm diameter was employed totraverse the laser beam into the chamber. In order to monitorthe dominant characteristic absorption peaks using FTIR, acouple of spectral broadband (500−20000 cm−1) AR-coatedZnSe windows were situated perpendicular to the axis along theBK7 window.7 Furthermore, an AR-coated BK7 window wasplaced at the top of the chamber in order to observe the plasmaplume location. Antireflection coated borosilicate Crown glass(AR-coated BK7 glass window) gives high quality optical glasswhere spectral transmission is ranging 0.4−1.4 μm (Vis−NIR).The metallic Ni, Cu, Fe, and Pd targets were separately insertedin the holder inside the chamber in front of the laser beam. ABalzer vacuum valve was connected to the chamber in order tomaintain high vacuum and block the atmospheric pressure.7

The chamber was evacuated twice using a rotary pump. Then,methane flow was fed into the chamber using the needle valveas long as the methane pressure in the cell reaches ∼1 bar. Apulsed Q-switched Nd:YAG laser (1064 nm, 100 mJ per pulse,10 ns duration, 5 Hz repetition rate) was employed to generatea laser beam. A focal lens ( f = 15 cm) was fixed between thelaser output and the front window of the chamber to enhancethe beam on the metal targets. The surfaces of Ni, Cu, Fe, andPd targets were manually well polished after each exposurebefore the next trial to ensure the impurity levels, providingnearly identical exposure conditions.Methan grade 4.5 (99.995%) from Linde Co. was used

during whole irradiation. The laser treated gas samples werescanned 20 times by a Fourier transform IR spectrometer(PerkinElmer−Spectrum 65) to characterize the functionalgroups. Furthermore, gas chromatography (Agilent 7890A) wasapplied for the quantitative analysis of the stable hydrocarboncompounds being generated during the methane decomposi-

Table 1. Typical Methane Threshold Energies for theElectron Impact Reactions42

Process ReactionThreshold energy

(eV)

Vibrational excitation CH4 + e → CH4 (υ24) + e 0.162CH4 + e → CH4 (υ13) + e 0.361

Dissociation CH4 + e → CH3 + H + e 9CH4 + e → CH2 + H2 + e 10CH4 + e → CH + H2 + H + e 11

Ionization CH4 + e → CH4+ + 2e 12.6

CH4 + e → CH3+ + H + 2e 14.3

Figure 1. Typical sketch of a non-Maxwellian EDF of elecrons in laser-induced plasma and typical methane decomposition cross section dueto electron impact collisions.


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tion. After exposure and the gas sampling, the generatedcompounds were taken from the chamber and injected to theGC instrument through a microneedle. The instrument isconfigured to analyze gas in 7 min, having a detection level of100 ppm. The system has five valves and three detectors. TheFID channel is used to analyze the hydrocarbons from C1 to C5,while C6/C6

+ components are back-flushed and measured asone peak at the beginning of the analysis. The first TCDchannel (reference gas: He) is employed to analyze the certaingases, which may include CO2, CO, O2, and N2. The third oneis the second TCD channel (third detector, on the side, withreference gas: N2), which is only allocated to analyze hydrogen.The samples must contain no water or hydrocarbons above C9.

4. RESULTS AND DISCUSSION

The laser beam was focused on the target in the middle of anirradiation chamber filled with CH4 to create a methaneenvironment at atmospheric pressure. The experiments werecarried out by examining four different pure metal samples suchas Ni, Fe, Pd, and Cu metallic targets to verify the catalyticeffects on the methane decomposition and product yield. Atfirst, the laser irradiation of the methane in the filled chamberwas carried out without the metal target. No plasma radiationwas detected. The microplasma takes place when a Q-SWNd:YAG laser focuses on the target. Subsequently, the metalspecies appear in the induced plasma, leading to thedecomposition events.7 In fact, the laser was unable todecompose methane molecules in the absence of the metaltarget. In this case, metal NPs cannot be generated during laserablation to enter into laser-induced plasma. Conversely, theplasma is well formed and the nanoparticles appear in theinduced plasma, leading to the efficient decomposition at the

attendance of the metal target. This lucidly demonstrates thatmetal species contribute to efficient methane decomposition.Furthermore, GC and FTIR analyses are employed to

determine the hydrocarbon formation rates and the methanedecomposition accordingly.

4.1. GC Analysis. We have analyzed the content of anirradiated cell using gas chromatography. The analyzer wasemployed after gas sampling, and subsequent experiments werecarried out at atmospheric pressure for the untreated and laserirradiated targets after 6000 shots, equivalent to 20 min ofexposure. Figure 3(a−d) depicts the corresponding chromato-grams of the different catalysts. Components such as H2, C2H2,C2H4, C2H6, C3H6, and C3H8 are well identified to be the majorcompounds, as anticipated given in Table 2, even though herethe catalytic functions are crucial too. This analysis accredits theevent of methane decomposition, demonstrating the distribu-tion and yield of the products.Methane conversion, selectivity, and yield of products were

obtained using GC data to verify the influence of catalyticspecies on the molecular decomposition. Consequently, anumber of hydrocarbons (and hydrogen) species appear inplasma during multiple laser shots. Each reactant describes thefraction that is converted to any final product. The selectivity ofa particular compound defines the amount of reactant that isconverted to a certain product. The yield explains the actualnumber of components produced relating the theoreticalvalues.16 The methane conversion X, selectivity S, and yield Yof hydrocarbons and hydrogen species are given in eqs 1−5.4,47The coefficient x in eq 2 denotes the stoichiometric coefficient.

= ×X (%)moles of CH consumedmoles of CH introduced

100CH4

44 (1)

Table 2. Proposed Kinetics of Laser-Induced Plasma for Methane Dissociation

Electron impact collisions Hydrogen collisions Molecular collisions

CH4 + e → CH3 + H + e CH3 + H2 → CH4 + H CH4 + CH2 → CH3 + CH3

CH4 + e → CH2 + H2 + e CH3 + H → CH2 + H2 CH4 + CH → C2H4 + HCH4 + e → CH + H2 + H + e CH2 + H2 → CH3 + H CH3 + CH3 + CH4 → C2H6 + CH4

CH3 + e → CH2 + H + e CH2 + H → CH + H2 CH3 + C2H6 → C2H5 + CH4

CH3 + e → CH + H2 + e H + H + CH4 → H2 + CH4 CH3 + C2H5 + CH4 → C3H8 + CHCH2 + e → CH + H + e CH3 + C2H4 → C2H3 + CH4

CH + e → C + H + e CH3 + C2H3 → C2H2 + CH4

CH3 + C2H3 + CH4 → C3H6 + CH

Figure 2. Experimental schematic for methane decomposition using metal target-assisted nanosecond laser-induced plasma within the controlledchamber.


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= ×⎛⎝⎜

⎞⎠⎟S x(%)

moles of desired component C H

moles of CH consumed100x y

C H4

x y

(2)

=×

YX S

(%)(%) (%)

100CH4

(3)

Furthermore, yield and selectivity for hydrogen byproduct aregiven as below47

= ×⎛⎝⎜

⎞⎠⎟S (%) 0.5

moles of H producedmoles of CH consumed

100H2

42

(4)

= ×⎛⎝⎜

⎞⎠⎟Y (%) 0.5

moles of H producedmoles of CH introduced

100H2

42

(5)

Figure 4 displays the selectivity of products after laserexposure (6000 shots) in the presence of a variety of metaltargets. It indicates that the propane, ethylene, ethane, andethylene obtain the maximum selectivity using Ni, Fe, Pd, andCu targets, respectively. Moreover, Table 3 summarizes the

corresponding selectivity of all products using examiningcatalysts. Figure 5 exhibits the yield of products accordingly.It is lucid that production of propane and ethylene using nickeltarget, ethane, and propane in the presence of palladium target,ethylene, and propane with Fe target and ethane and ethylenewith copper target obtains the highest yields. According toexperiments, the hydrogen production is notably generated inthe attendance of Pd, while propane content is dominant using

Figure 3. Typical GC spectra of (a) the untreated methane gas and the laser treated targets (b) Ni, (c) Fe, (d) Pd, and (e) Cu after 6000 shots,demonstrating the distribution of the generated compounds.

Figure 4. Selectivity of compounds after laser exposure of methane inthe presence of the various metal targets after 6000 laser shots (datataken from GC). Selectivity of propane species is dominant using Nicatalyst.


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Ni catalyst. Pd and Ni are active catalysts for methanedecomposition.29 The decomposition of CH4 on oriented Pdsurfaces and nanostructured Pd catalysts is well under-stood.48−51 Dissociative chemisorption of CH4 occurs as lowas 400 K and accelerates rapidly with increasing temperature,leaving carbidic surface carbon behind. Evidence for theexistence of CHx species (x = 1−3) on Pd could not befound in experimental studies, and it was concluded that theywere very short-lived on Pd at the temperatures investigatedhere.48,49 Theoretical studies confirmed that the remaining CHbonds of the initially formed CH3 are broken in a fast reactioncascade because the coordinative stabilization of C on Pdsurfaces improves with decreasing number of H ligands.50,51 Asa consequence, combinative reactions between CHx inter-mediates are largely suppressed. C2H6 was the only hydro-carbon observed in appreciable amounts in one of thesestudies,49 which is consistent with CH3 being the most stablefragment.50,51 Similarly, Table 4 tabulated the yield ofcompounds generated by use of various catalysts.

However, the major products are propane, ethane, ethylene,and hydrogen while other components are comparativelynegligible. This is in agreement with the previous work onmethane decomposition as well.7 The yield of hydrogen ismaximum in the presence of Pd targets too. This arises fromthe fact that palladium is an optimal catalyst for the methanedecomposition and hydrogen generation. The recommendedhydrogenation catalysts for alkane’s reactions are Ni and Pd.Because of the small size of hydrogen relative to the othermolecules, after adsorbing on metal surfaces and dissociating

into hydrogen atoms, those can migrate into the bulk metalcrystallite and, in some cases, react with the solid to form metalhydrides.52 The unique behavior of Pd is highlighted becausethis catalyst gains a high affinity to adsorb the hydrogen atomsafter methane decomposition to generate hydrogen moleculesand solid carbons accordingly.52−54 In fact, hydrogen gasgeneration is a measure of Pd catalytic activation, as expectedduring the experiments here.

4.2. FTIR Analysis. Figures 6−9 represent FTIR spectra ofthe CH4 gas at 1000 mbar before and after laser irradiation fortypical 3000, 6000, and 9000 successive shots examiningvarious metals, i.e, Ni, Fe, Cu, and Pd, as the catalysts. Themetallic species in plasma may act to facilitate the plasmageneration at lower laser pulse energies by creation of arelatively higher initial electron density. In fact, the catalystcontributes to enhance chemical reactions of methanedecomposition as well as plasma initiation to drop the requiredelectron energy for decomposition according to the kineticscheme in Table 1. The characteristic peaks of methane locateover the spectral range of 1200−1400 cm−1 and ∼3000 cm−1,corresponding to the bending and stretching vibration of themethane molecules, respectively.30,55 Regarding the spectra,several bonds appear after exposure in the presence of targets.In the presence of whole targets, the spectra contain a numberof peaks appearing as the fingerprint over 600−1200 cm−1. Thespectral line at 628 cm−1 is attributed to the acetylenic C−Hbend over 600−650 cm−1 while the other peak is located at∼729.5 cm−1. One of the absorption peaks of acetylene iscentered at ∼730 cm−1 too. The characteristic peak at ∼949.5cm−1 is related to the C−H bond in alkene, which is situatedover the 650−1000 cm−1 spectral region. The absorption peakat 949 cm−1 is related to the ethylene, and the other oneexhibiting around 1500−1700 cm−1 corresponds to the CCstretch modes of carbon groups. Furthermore, the spectralabsorption over 2100−2200 cm−1 and 3200−3400 cm−1

indicates a triple bond CC and acetylenic stretching C−Hin alkynes, respectively.30,55−57 The existence of whole bondsafter laser irradiation indicates the formation of heavierhydrocarbons such as alkane, alkene, and alkyne. The exposuretime in the interval of 10−30 min equivalent to 3000−9000laser shots does not imply the creation of further productspecies; however, it may enhance the corresponding absorptionline amplitudes, as illustrated typically in Figure 10. Insummary, with the number of pulses, the concentrations ofproduct molecules increase, and there is no sign that additionalcompounds form. This accordingly gives rise to populatedcomponents with shot numbers. Figure 11 depicts theabsorption peaks in terms of laser shots. A notable rise appearsproportional to the laser dose.The catalyst activity in the course of the methane

decomposition was studied accordingly. In the attendance ofeach metal catalyst, the corresponding dissociation rates aredetermined using FTIR according to following formalism:

= ω−n n eNN

0 (6)

where n0 and nN denote the initial molecule concentration andthose after N-pulse irradiation. N and ω are the number ofshots and the dissociation rate per pulse, respectively.58,59

Equation 6 is basically used for MPD analysis, and thedecomposition data were fitted to the experimental curve. Here,the decomposition of methane molecules in terms of successivelaser shots is modeled in virtue of the exponential equations.

Table 3. Selectivities (%) of Products Using Various Targets

Ni Fe Pd Cu

C2H6 3.24 7.34 32.09 22.49C2H4 14 22.03 5.01 28.11C2H2 0.72C3H8 96.54 8.26 15.04 2.24C3H6 2.15 2.62 0.56H2 33.26 45.43 41.12 42.17

Figure 5. Yield of compounds after laser exposure of methane in thepresence of the various metal targets after 6000 laser shots (data takenfrom GC). Yield of propane species is dominant using Ni catalyst.

Table 4. Yields (%) of Products Using Various Targets

Ni Fe Pd Cu

C2H6 0.04 0.1 0.7 0.27C2H4 0.2 0.3 0.1 0.33C2H2 0.01C3H8 1.40 0.11 0.33 0.02C3H6 0.031 0.036 0.01H2 0.48 0.63 0.9 0.5


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The exponential decay is linearly approximated for smalldecomposition rates. In general, the logarithm of (nN/n0)follows a linear relationship with number of pulses.Let us assume N = 3000 Pulse and ω ≃ 4 × 10−5; then ωN

≪ 1 and eq 6 can be rewritten in the form of nN = n0 (1 − ωN),which is demonstrated to be a linear equation.If the irradiated volume Vo and the cell volume Vc differ such

that Γ= Vo/Vc < 1, then eq 6 can be given as

= − Γ − = − Γω−n n e n R[1 (1 )] (1 )NN N

0 0 (7)

where ω is a unitless parameter implying the dissociation eventsand R = 1 − e−ω denotes the dissociation rate per pulse. Theparameters ω and then R are determined from experimentaldata.

For ω ≪ 1, we find R ≃ ω; that is the case here (typically ω≃ 4 × 10−5).Hence, eq 7 can be rearranged to

ω≃ − Γn n (1 )NN

0

The binomial theorem is valid for ω ≪ 1 such that nN = n0(1 −ωN) for Γ ≃ 1, which is equivalent to the simplified eq 6.Experimental investigations of the dissociation of the

numerous polyatomic molecules have shown that, regardlessof the type of molecule, the dissociation process has a numberof parameters in common, as given in eqs 6 and 7. The basis ofthe equations is the obliteration of a chemical component (ormolecules) in terms of number of laser shots.59,60

Even though the exponential relationship is initially used forthe multiple photodissociations, the experimental data can be

Figure 6. FTIR spectra of untreated and treated methane gas by Nd:YAG laser after 3000, 6000, and 9000 laser shots in the presence of nickel target.

Figure 7. FTIR spectra of untreated and treated methane gas by Nd:YAG laser after 3000, 6000, and 9000 laser shots in the presence of iron target.


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fitted by means of the least-square method for catalyticdecomposition. Here, this model was examined for decom-position in laser-induced plasma. Figure 12 illustrates theexponential dependence of the nN/n0 ratio versus the numberof laser shots having 100 mJ/pulse, 10 ns, and 5 Hz applied forvarious metal targets. The slope is an indicative of catalystsactivity for the methane decomposition. This varies withdifferent metallic catalysts such that it gives out the highestdissociation rate for the Pd target. The following order isobeyed for examining catalysts:

ω ω ω ω> > >Pd Ni Fe Cu

Figure 13 displays the comparison of dissociation rates ofvarious catalysts. The greatest dissociation rate is achieved withthe palladium target, and the smallest rate is obtained using thecopper catalyst. The greatest dissociation rate of methane hasbeen obtained using the empirical data to be ω = 1.19 × 10−4

for the Pd target.After irradiation, the absorption bands of propane C3H8 and

ethane C2H6 obviously disappear in the FTIR spectra; however,the GC spectra reveals the footprints of those hydrocarboncomponents. The absence of ethane is likely due to the overlapwith the methane characteristic line to be unable to identifythose peaks.7,55,61 The creation of an absorption band ranging720−750 cm−1 corresponds to the bending modes of the CH2

Figure 8. FTIR spectra of untreated and treated methane gas by Nd:YAG laser after 3000, 6000, and 9000 laser shots in the presence of coppertarget.

Figure 9. FTIR spectra of untreated and treated methane gas by Nd:YAG laser after 3000, 6000, and 9000 laser shots in the presence of palladiumtarget.


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chain (quartet or higher). This illustrates the formation ofresidual CH2 chains after methane decomposition, as suggestedin Table 2. Finally, those molecules were attached to oneanother to create C3H8. The vibrational energy in NIST

approves that characteristic absorption spectra of propane arelocated at 748, 1500, and 3000 cm−1. Thus, C3H8 absorptionbonds coincide with those bands of CH4 and C2H2 in the FTIRspectra.55,58 On the other hand, the carbon deposited on theZnSe windows is obviously visible after 9000 laser shots. This istaken for granted as alternative evidence of the methanedecomposition during metal-assisted induced plasma deposi-tion.7

4.3. Properties of Methane Decomposition. In fact, themethane conversion and dissociation rate ω are proportional tolaser dose for each catalyst. The fluence exceeds a thresholdvalue to create the plasma to be on the order of several J/cm2

for nanosecond laser duration. The plasma formation initiatesnear the surface when the energy deposited on the target

Figure 10. Typical spectra of enhancement of the corresponding absorption line amplitudes with various metal targets to show the effect of laserdose on the breakage of the bonds.

Figure 11. Absorption peak versus laser shots at the bands of (a) 628cm−1 (acetylenic C−H bend), (b) 729.5 cm−1 (C2H2), and (c) 949.38cm−1 (C2H4).

Figure 12. Obliteration of methane as a function of laser shots andcorresponding dissociation rates of various metal catalysts at 1000mbar using the characteristic band of 1300 cm−1.

Figure 13. Methane dissociation rate using various metallic catalysts.


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enforces the metal ablation proportional to the reverse of thelatent heat of vaporization Lv. The threshold fluence, Fth, isgiven by Fth (J/cm2) = ρLvα

1/2Δt1/2 where ρ, α, and Δtascertain the target density (kg/cm2), thermal diffusivity (cm2/s), and laser duration (s), respectively. If the laser fluence isbelow Fth, then no evaporation occurs.34,36,62 At high laserfluence, sufficiently above the threshold, a high-temperatureplasma is formed and the laser energy is absorbed effectivelywithin the plasma, leading to serious plasma heating. Thefluence threshold critically depends on the sample propertiesand, consequently, differs from one material to another.62

Regarding the above equation, Fth values of various targets suchas palladium, iron, nickel, and copper are determined to be2.04, 2.30, 2.72, and 4.62 J/cm2, respectively. The thresholdfluence is the minimum energy required for the evaporationand subsequent ablation of the metallic sample, as the low Fthleads to the high ablation rate and larger amounts of evaporatedmaterial are given to the dense plasma. For instance, palladiumtakes the smallest value among the other available metal targetsthat gives rise to the more ablated species available in theplasma plume. This results in high methane conversionaccompanying higher electron density that correlates with thegreater collision rates following higher dissociation rates duringthe electron-impact process. As a consequence, Fth of palladiumis the smallest value corresponding to the maximum electrondensity, because more ablated species are added to theplasma.36

For metals with lower ionization energy, Ei, such as Ni andCu, the plasma temperature, Te, notably rises due to the smalleramount of laser energy imparted to ablate the target.Consequently, more accessible energy is available to heatplasma according to the IB absorption, leading to the higherplasma temperature. As a result, the plasma temperature isinversely correlated to the ionization energy of the metal target.The ionization energies required for Ni, Cu, Fe, and Pd aregiven to be 737.1, 745.5, 762.5, and 804.4 kJ/mol, respectively.Hence, the electron density is remarkably high for the metalshaving low threshold fluence (Fth). Despite the plasmatemperature being strongly dependent on the electronicproperties of metals, the electron density mainly arises fromthe thermal characteristics, such as latent heat of vaporization,thermal diffusivity, and density. The plasma temperature is onthe order of 10000 K using plasma optical emissionspectroscopy (OES) while the electron density is on theorder of 7 × 1017 cm−3.36 Moreover, the ratio of absorptioncoefficient to reflectance α/R for Pd is greater than the others.This ratio for Pd, Ni, Fe, and Cu is denoted to be 9.1, 8.4, 7.8,and 8.6, respectively. This illustrates that the absorption of laserpulse energy by Pd is prominent compared with the othertargets. This is accredited to the fact that the abundance ofpalladium species leads to the denser plasma, contributing tofurther dissociation and methane conversion. Other works alsoreported that Pd is more active with respect to Ni and Fecatalysts, during methane thermal decomposition.53,54 Finally,the methane conversion ratio correlates the dissociation rateand is reciprocally proportional to the product of Fth × Ei,emphasizing the dual effect of plasma temperature and densityon X and ω. Figure 14 illustrates dissociation rate versusproduct Fth × Ei for various metal catalytic targets using laser-induced plasma.The methane conversion percentage XCH4

(%) and thedissociation rate (ω) are taken from GC and FTIR analyses,

respectively. Furthermore, the metal properties, such as Fth, Ei,Fth × Ei, and α/R are tabulated in Table 5. It is obvious that the

maximum methane conversion takes place in the presence ofthe palladium target whose dissociation rate is the largestamong the four examined catalysts. Furthermore, thecorresponding threshold fluence Fth is the smallest one.However, the relative hydrogen yield is high and the solidcarbon deposit is notable.7,53,54

Note that methane conversion, yields, and activities arerelatively low to be intrinsic as the nature of laser-inducedplasma decomposition. Despite the fact that the dissociationrate of methane in the presence of metal target is on the orderof 10−5, in the event of more energetic laser shots (200−400mJ/pulse), the plasma properties will significantly rise, leadingto the greater decomposition rate.

4.4. Proposed Model for the Methane Decomposi-tion. In summary, there is an alternative efficient process todecompose CH4 based on laser-induced plasma even withrelatively low photon energy. In laser-induced plasma, thedesired molecules face an intense photon flux, energeticelectrons, as well as ion species. In this case, the directphotoexcitation and ionization do not certainly take place. Thedirect photodissociation during nanosecond laser shots occursif the laser line matches the molecules transitions. This is notthe case here because methane absorption lines are centered inthe VUV spectral range. The intense femtosecond (modelocked) laser shots may induce virtual states, creating a propersituation for the MPA/MPD process due to its inherentbroadband property according to the Fourier transform limit.Here, the nanosecond laser shot with a sufficient energy densityis focused on the solid, and then laser-induced plasma can beformed. At initial instants, the atomic and molecular structuresof the samples are broken and heated up based on the

Figure 14. Dissociation rate versus product Fth × Ei for various metalcatalytic targets using laser-induced plasma.

Table 5. Catalytic Properties, i.e. XCH4(%), ω, Fth, Ei, FthEi,

and α/R Ratio of Methane, after 6000 Laser Shots Using aQ-SW Nd:Yag Laser Having a 10 ns, 100 mJ/pulse at 1000mbar

Catalyst Pd Ni Fe Cu

Methane Conversion (%) (taken fromGC)

2.18 1.44 1.38 1.20

Dissociation rate ω (×10−5) (takenfrom FTIR)

11.9 8.07 6.45 3.99

Threshold fluence Fth (J/cm2) 2.04 2.30 2.72 4.62

Ionizatin energy Ei (kJ/mol) 804.4 737.1 762.5 745.5FthEi 1641 1695 2074 3444Absorption coefficient to reflectanceratio α/R

9.1 8.4 7.8 8.6


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successive energetic electron collisions with atoms/moleculesvia a thermionic effect, causing vaporization of a small fractionof the material. This contains free neutral atoms, ions,molecular fragments, and free electrons. Afterward, theincoming energy of the same laser pulse attains hightemperature plasma of ∼10,000 K, in which the vaporizedspecies are excited to the upper states to deactivatesubsequently by emitting electromagnetic radiations. Whenthe laser irradiance is adequately high to induce a plasmaplume, the leading edge of the pulse rapidly heats, melts, andvaporizes material into a layer just above the surface. Part of thelaser energy contributes to heat up the plasma plume tocontinue absorbing energy from the laser during the pulseduration.33 The plasma properties over the target surface areenhanced by the inverse Bremsstrahlung absorption (IB)during collisions among other atoms and ions, electrons, andgas species.34,62 The conductive nature of the given metal isstrongly relevant to the mechanism dealing with the laser-induced plasma initiation.33,35,63 In the meantime, the plasmacreates a shock wave which spreads from the surface to thesurrounding medium33,34 to centrifuge the particles out of thetarget surface. Here, the sequential events are proposed thatgive rise to the hydrocarbon dissociation and recombination bycatalytic assistance within laser-induced plasma. Figure 15illustrates the sequential steps for the generation of higherhydrocarbons and hydrogen gas. Those include plasmaformation, metallic NP generation, methane dissociation, andthe recombination of species to form higher hydrocarbons C2and C3 with the efficient contribution of NP catalytic surfaces.Furthermore, the methane molecules are initially adsorbed

on the metal surface of the catalyst nanoparticles, resulting inthe formation of chemisorbed carbon species and the release ofgaseous hydrogen after the decomposition process in plasma.64

The catalytic decompositions are based on the d orbitals of themetallic target transferring electronic charge into theantibonding levels of the adsorbate, facilitating dissociation.65

The induced plasma rarely decomposes methane regardingthe electron impact collisions because of the large discrepancyof electron distribution and sufficient energy required fordecomposition.5 The conditions would be different when thecatalyst attends in the plasma region. The vibrationally excitedspecies are taken into account as the active sites on the NPsgenerated in plasma having the minimum internal energy toenhance the catalytic reactions. Numerous studies have beendevoted to enhance the dissociative adsorption in the catalyticreactions by elevating the vibrational energy of reactants. Oncethe dissociative adsorption is accelerated, the production ratecan be greatly enhanced regarding the activated recombinationprocess. Figure 16 summarizes the typical steps of reactions toexplain the vibrationally excited species which assist thedissociative adsorption. Because of the higher internal energyof vibrational states, the activation barrier requires that chemicaladsorption could be reduced from Ea to (Ea − ET), where Eaand ET ascertain the activation barrier of chemisorption forground state reactant and the threshold energy for theformation of vibrational reactant through electron-impactreactions, respectively. This arises from laser-induced plasma.For solid−gas catalytic reactions, part of the residual reactionenergy might be transferred to the desorbed products asvibrational excitation. Methane molecules in the vibrationalstate demonstrate much higher catalytic activity than those inthe ground state.42

The free electrons in the plasma collide with the CH4molecules physisorbed on the catalyst surface. The collisionsdistort the CH4 molecules from their tetrahedral configuration,thereby lowering the barrier to dissociate those molecules intothe adsorbed methyl (CH3) species and hydrogen atoms. Theelectron collisions with the catalytic surface contribute toenhance the decomposition rate while adsorption events makethe CH bond length looser than before detachment. Theadsorbed CH3 dissociates into CH and CH2 speciesaccordingly, while those species recombine to form theadsorbed C2H2, C2H4, etc. In fact, the adsorbed C2H2 and

Figure 15. Sequential steps for the generation of the higherhydrocarbons and hydrogen: (a) plasma information and IB, (b)methane dissociation based on metal NP catalytic activities, (c)recombination of species to generate C2 and C3 compounds.


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CH species react together to dimerize or trimerize into a seriesof hydrocarbons such as C3H6 and C3H8.

66 Microdischargesbetween catalyst particles may be partially responsible for theconversion and selectivity. It is believed that the nanocatalystsurface in contact with the plasma notably activates thereactions. The charged plasma species may lead to chargeaccumulation on the catalyst surface, which in turn alters theelectrostatic potential and the work function of the catalyst,accordingly.42,47

5. CONCLUSIONThis work is an extension of our previous work on Pd-assistedlaser-induced plasma decomposition of methane at manometric(sub atmospheric) pressures.7 The methane molecules areirradiated by a nanosecond Q-switched Nd:YAG laser in thecontrolled chamber using various catalysts, i.e. Pd, Ni, Fe, andCu, at atmospheric pressure in order to investigate thecorresponding catalytic activity. Despite the fact that photo-dissociation was previously reported by making use offemtosecond lasers, here the molecules are disintegratedaccording to the metal-assisted nanosecond laser-inducedplasma to form higher hydrocarbons and hydrogen.The laser beam ablates the metallic target in methane

atmosphere and creates transient and localized plasma over thesurface containing NPs and free electrons. When the laserfluence is noticeably greater than the ablation threshold, thenanoparticles metal species are created. The NP catalysts inturn provide a large activation area to enhance the adsorptionof methane molecules accompanying the multiple electroncollisions in plasma. This would accelerate the decompositionrate significantly. Methane is rarely dissociated via directelectron impact collisions due to the discrepancy of EDF withthe dissociation energy threshold. However, it most likelyoccurs due to skin catalytic reactions of the NPs. The latterloosens the CH bonds as well as enhancing the spillover eventsthat finally give rise to the methane decomposition. As a

consequence, the subsequent multiple recombinations of CHbonds lead to the generation of higher hydrocarbon species.The FTIR and GC spectrographs verify the formation of

alkane, alkene, and alkyne products, recording the correspond-ing abundance of each compound. The abundant products arepropane, ethane, and ethylene with the assistance of thecatalysts above. Pd and Ni are prone to function as the efficientcatalysts for the decomposition in laser-induced plasma.Varying those metal targets, the dissociation rate and the

conversion ratio of the methane drastically change. Thegeneration of propane exhibits the highest yield and selectivityin the case of Ni catalyst and similar properties appear forethane in the case of Pd. Furthermore, the maximumconversion ratio and the dissociation rate are determined tobe 2.18% and 1.19 × 10−4 when dealing with the Pd target,respectively. In the meantime, the palladium catalyst dissolvesthe atomic hydrogen content after methane decompositionover the catalytic surface to generate excessive hydrogen. Itgives the highest yield. The deposition of carbon on thewindows and chamber walls is evidence of strong methanedecomposition during laser irradiation.Eventually, the empirical data emphasizes that the methane

conversion ratio strongly correlates to the inverse of theproduct of ablative threshold laser fluence and the ionizationenergy (Fth × Ei), exhibiting dual effects of plasma temperatureand density on methane decomposition.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ NOMENCLATUREα/R absorption coefficient to reflectance ratioCNG compressed natural gasGC gas chromatographyFTIR Fourier transform IR spectroscopyEa activation barrier of chemisorption for ground state

reactantEDF energy distribution function of electronEi ionization energyET threshold energy for formation of vibrational reactantFID flame ionization detectorFth threshold fluenceIB inverse bremsstrahlungLIBS laser-induced breakdown spectroscopyLNG liquefied natural gasMPA multiphoton absorptionMPD multiphoton dissociationN number of laser shotsnN/n0 ratio of molecule concentration after N-shots to initial

concentrationNPs nanoparticlesω dissociation rate per pulseOCM oxidative coupling of methaneOES optical emission spectroscopyR gas-phase reactants in ground stateR* gas-phase reactants in vibrational stateS selectivity of speciesTCD thermal conductivity detectorY yield of species

Figure 16. Schemes of the summary of major events: Proposedmechanism for the vibrationally excited species during laser beamirradiation to activate the catalytic reactions. R and R* represent gas-phase reactants in ground and vibrational states, respectively. Ea is theactivation barrier of chemisorption for R, and ET denotes the thresholdenergy for the formation of R* through electron-impact reaction (Ea −ET).

42 Typical chemical conversions of CH4 into C2H4 and H2 aredepicted too.


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mailto:[email protected]

x stoichiometric coefficientXCH4

methane conversion

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methane decomposition using metal-assisted nanosecond laser-induced plasma at atmospheric pressure

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