Methane Decomposition Using Metal-Assisted Nanosecond Laser

Article pubs.acs.org/JPCC

Methane Decomposition Using Metal-Assisted Nanosecond LaserInduced Plasma at Atmospheric Pressure Z. 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 a Q-switched Nd:YAG laser, focused on the metal catalysts including Ni, Fe, Pd, and Cu 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 characterization of the components. This indicates that methane is strongly decomposed within the metal-assisted laser-induced plasma, leading to the subsequent recombination and the production of heavier hydrocarbons. The dominant species, including propane, ethane, and ethylene, have been identified examining different metallic catalysts. The dissociation rate, conversion ratio, selectivity, and yield of products are strongly dependent on the metal target and plasma characteristics.

1. INTRODUCTION Methane is the principal component of the natural gas reservoirs which are a greatly under-utilized resource for gaseous (CNG) and liquid (LNG) fuels.1 Moreover, methane is one of the greenhouse gases affecting global warming that is an imperative environmental concern.2,3 Hence, the methane conversion into useful hydrocarbons is a significant issue to be extensively investigated.3−7 Different methods have so far been used to decompose methane molecules. The pyrolysis method refers to the decomposition of the hydrocarbons by heat or the plasma exposure without the addition of oxygen or an oxygencontaining reactant. The reaction is endothermic, and at high enough temperatures it decomposes to form hydrogen, ethane, ethylene, propylene, acetylene, and solid carbon. Methane plasma pyrolysis has received considerable attention for a long time and has also been thoroughly considered for the synthesis of higher hydrocarbons.8,9 On the other hand, oxidative coupling of methane (OCM) is a direct methane conversion to more valuable products. The process is highly exothermic, and the temperature increases during the reaction process. Hence, the rate and selectivity of the products are strongly affected.10 The OCM process is able to turn methane directly into ethane, ethylene, and acetylene. Most previous studies of methane activation are based on the OCM reactions with the metal catalysts. The heterogeneous catalytic OCM has been the major subject of a large body of research activities. This usually faces low selectivity, high energy cost, and low conversion efficiency. To conquer these problems, plasma technology has been recently used to decompose methane.4 The utilization of suitable catalysts in plasma can improve the conversion efficiency as well as the © XXXX American Chemical Society

selectivity of the desired products to such an extent that this method is accredited as an efficient alternative of the thermal method.11 For the catalytic decomposition, several metallic samples, i.e. Ni, Fe, Co, Pd, Mo, Cu, and Mn, have been exploited as catalysts.12−15 It is well-known that Fe, Co, and Ni have partially filled 3d orbitals to facilitate the dissociation of the hydrocarbon molecules through partially accepting electrons. This interaction along with back-donation from the metal into the unoccupied orbital in the hydrocarbon molecule changes the electronic structure of the adsorbed molecule leading to the dissociation.16 Copper is among nontransition metals whose 3d shell is completely filled. This catalyst is employed to yield only the amorphous carbon.17 Therefore, it is believed that Cu is an inactive element during the decomposition process of the traditional catalytic system.18,19 This takes for granted that the laser exhibits its ability to decompose methane without CO 2 generation in the combustion free process.20 Wang et al.21 investigated the dissociation of methane in the intense laser field using an amplified ultrafast Ti:sapphire laser at ∼800 nm coupled to a time-of-flight (TOF) mass spectrometer. This gives rise to the dissociation proceeding a stepwise mechanism by the gradually increase of the laser intensity. Moreover, Gondal et al.3 reported the photodissociation of methane into higher hydrocarbons using a high power pulsed ultraviolet laser at 355 nm under ambient conditions without catalyst contribution. Zerr et al.22 studied decomposition of alkanes such as Received: August 26, 2014 Revised: November 14, 2014

A

dx.doi.org/10.1021/jp508634d | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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methane, ethane, and octane. Sharifi et al.23 explained the highpower laser ionization-dissociation of CH4 at various femtosecond laser intensities as high as 1015 W/cm2 where no catalyst was employed. Furthermore, Wu et al.24 practically studied the fragmentation pattern of CH4 due to femtosecond laser shots. Wang et al.25 investigated the methane dissociation using femtosecond laser shots at 800 nm using the laser fluence ∼1014 W/cm2. Suzuki et al.26 demonstrated the formation of formaldehyde from photo-oxidation of methane over a molybdena-silica catalyst at ∼500 K using UV irradiation. Hill et al.27 reported photoinduced reactions of methane on the molybdena-silica surface based on the adsorption during UV exposure. When it heats up to a certain temperature (∼470 K), a considerable amount of ethylene, ethane, and hydrogen were produced as well as small amounts of C3 and C4 alkenes and alkanes. Furthermore, Reyhani et al.7 have shown the methane decomposition during Pd-assisted laser-induced plasma in the controlled chamber at various pressures using a nanosecond pulsed Nd:YAG laser at 1064 nm. Real time LIBS has revealed the decomposition by altering the plasma parameters at manometric pressures 1−10 mbar. It attests that the plasma creates higher hydrocarbons during the methane decomposition and the successive recombination process. Despite this, thermal (noncatalytic) decomposition of methane occurs at very high temperatures (>1200 °C) because of strong C−H bonds. However, different transition metal catalysts such as Ni, Fe, and Co have been inspected to reduce the optimal temperature required for the thermal decomposition.28,29 In fact, the catalyst benefits from losing a C−H bond during the surface adsorption. The major drawback associated with the use of metal catalysts is related to their rapid deactivation due to active sites blocking by means of carbon deposits.30 Fortunately, pulsed laser activates the blocking surface of the metal catalyst shot by shot, generating excessive nanocatalysts with efficient large contact area to react with methane molecules. Here, the nanosecond laser at 1064 nm was employed in order to decompose methane in the controlled chamber at atmospheric pressure based on the metal-assisted induced plasma. Different metal targets such as Ni, Fe, Cu, and Pd were examined. Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC) instruments were applied to detect and analyze the obtained compounds. It was shown that the decomposition without oxygen content at low equilibrium temperature generates higher hydrocarbon components regarding CO2 free reaction. To our best knowledge, there are a few reports available to investigate the effects of catalysts in nanosecond laser-induced plasma during methane decomposition and the subsequent generation of heavier hydrocarbons. Furthermore, this work describes an experiment on the border between plasma chemistry and catalysis. The conversion of photons to thermal energy (inducing the plasma) as well as generation of metal nanoparticles are coupled, and both processes influence chemical conversion of methane.

from C−H bonds. The energy required to dissociate the hydrogen from the molecule is ∼4.5 eV. It is equivalent to a UV photon energy which is supplied by the excimer lasers, higher harmonics of dye, or solid state lasers. In fact, the direct excitation of electronic transitions of methane requires a UV photon energy above 8.9 eV (∼140 nm).31 Methane UV photodissociation is believed to provide a synthetic route to the higher hydrocarbons based on the recombination process.32 The laser excitation turns methane molecules into methylene and methyl radicals, which will further react together to produce 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 demonstrates that the optical beams at 7.6 and 3.3 μm are able to dissociate C−H bonds directly via multiphoton excitation and dissociation processes. Similarly, the molecules may simultaneously absorb ∼27 and ∼12 coherent photons via MPA/MPD mechanisms for C−H fragmentation, respectively. According to the molecular energy scheme, methane is disintegrated in two ways. (i) The photodissociation by proper coherent photons via multiphoton absorption and (ii) the collisional decomposition using the energetic species within plasma based on electron impact dissociation. The mechanisms above deal with the drastic difference of the collisional cross sections. As a consequence, the conductive (metallic) or nonconductive (isolator) nature of catalysts is essential in the case of laser-induced plasma on solid. For metallic samples, the electrons of the conduction band absorb laser photons and the excess energy of the electrons is dissipated by the collisions with the material lattice. Direct absorption of the laser energy by the ions in the lattice is prevented according to the dielectric screening. The ablated material expands at supersonic velocities, producing a shock wave which propagates from the surface toward the surrounding atmosphere. The plasma plume continues to absorb energy from the laser within the pulse duration based on the inverse Bremsstrahlung absorption (IB) process. The plasma heating goes on during laser irradiation. The species are excited, leading to the plasma emission afterward. The ablated material which contains free neutral atoms, ions, molecular fragments, and free electrons expands at supersonic velocities.33 Interactions of these species with molecules cause loosening of the bonding energy, resulting in the decomposition eventually. Material ablated into the plasma may be in the form of particles (either fresh, or melted and cooled) as well as atoms and/or molecules. In fact, laser ablation is governed by a variety of distinct nonlinear mechanisms. Once the laser beam illuminates the sample, the mass leaves the surface in the form 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, the ablated catalysts are mostly metal NPs such that methane molecules can attach to them, with subsequent catalysis of the decomposition to CH2 and then recombination to higher compounds.36,37 The method involves the condensation of atoms/molecules and cluster formation (with or without any chemical reactions) during the fast expansion of the vapor/ plasma plume generated in front of a target. The time of nucleation and size and composition of clusters depend on the type of material, the laser parameters, and the ambient medium.38

2. THEORY The molecular chemical bonds are usually broken with a single laser shot using an intense femtosecond laser field up to 1014 W/cm2.21 Nanosecond IR laser at 1064 nm is seldom able to decompose most of molecules according to the photodissociation mechanism. The Nd:YAG laser provides 1.16 eV photon energy, not to be sufficient to detach hydrogen atoms B



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A focused pulsed laser beam on the target results in the rapid temperature rise (>1011 K/s), with maintenance of the stoichiometry of the nanoparticles. High-energy atoms and ions in the laser-induced plasma plume create a high surface mobility which accounts for nanoparticles’ generation. During the laser beam interaction with the target surface material, there are various processes leading to atoms and clusters and droplet expulsion from the target surface. The most common case is the nanoparticle production by the pulsed laser ablation.39−41 Regarding the plasma chemistry, the bonding energy is ranging 3−6 eV for most of hydrocarbons and the decomposition mainly ascribes to electron impact dissociation and ionization.42 The former is exhibited by dissociation of molecules 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 collect enough vibrational energy needed for the dissociation. Such processes efficiently contribute to decomposing gases such as N2, CO2, H2, and CO. In contrast, the dissociation through electronic excitation takes place just after a single collision stimulated by direct electron impact. The primary process can proceed through different intermediate steps of intramolecular transitions.43 The kinetics is proposed based on electron impact reactions of methane without catalytic contributions. The corresponding threshold energies are listed in Table 1.42

Figure 1. Typical sketch of a non-Maxwellian EDF of elecrons in laserinduced plasma and typical methane decomposition cross section due to electron impact collisions.

Table 2. The conversion of methane molecules to heavier species in the presence of catalyst deals with the dissociation and recombination reactions.45,46

3. EXPERIMENTAL SECTION Figure 2 depicts the experimental schematic for methane decomposition by laser having nanosecond duration. It consists of an irradiation chamber, high vacuum systems, conducting and focusing optics, and laser pulse diagnostics. The homemade cross type chamber was made up of stainless steel with six outlets, of which four are allocated for the windows. An ARcoated BK7 window with 5 cm diameter was employed to traverse the laser beam into the chamber. In order to monitor the dominant characteristic absorption peaks using FTIR, a couple of spectral broadband (500−20000 cm−1) AR-coated ZnSe windows were situated perpendicular to the axis along the BK7 window.7 Furthermore, an AR-coated BK7 window was placed at the top of the chamber in order to observe the plasma plume location. Antireflection coated borosilicate Crown glass (AR-coated BK7 glass window) gives high quality optical glass where spectral transmission is ranging 0.4−1.4 μm (Vis−NIR). The metallic Ni, Cu, Fe, and Pd targets were separately inserted in the holder inside the chamber in front of the laser beam. A Balzer vacuum valve was connected to the chamber in order to maintain 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 valve as long as the methane pressure in the cell reaches ∼1 bar. A pulsed Q-switched Nd:YAG laser (1064 nm, 100 mJ per pulse, 10 ns duration, 5 Hz repetition rate) was employed to generate a laser beam. A focal lens (f = 15 cm) was fixed between the laser output and the front window of the chamber to enhance the beam on the metal targets. The surfaces of Ni, Cu, Fe, and Pd targets were manually well polished after each exposure before the next trial to ensure the impurity levels, providing nearly identical exposure conditions. Methan grade 4.5 (99.995%) from Linde Co. was used during whole irradiation. The laser treated gas samples were scanned 20 times by a Fourier transform IR spectrometer (PerkinElmer−Spectrum 65) to characterize the functional groups. Furthermore, gas chromatography (Agilent 7890A) was applied for the quantitative analysis of the stable hydrocarbon compounds being generated during the methane decomposi-

Table 1. Typical Methane Threshold Energies for the Electron Impact Reactions42 Process

Reaction

Threshold energy (eV)

Vibrational excitation

CH4 + e → CH4 (υ24) + e CH4 + e → CH4 (υ13) + e CH4 + e → CH3 + H + e CH4 + e → CH2 + H2 + e CH4 + e → CH + H2 + H + e CH4 + e → CH4+ + 2e CH4 + e → CH3+ + H + 2e

0.162 0.361 9 10 11 12.6 14.3

Dissociation

Ionization

The electron energy distribution of laser-induced plasma spreads over a few electronvolts. Hence, the multiple electron impact collisions may dissociate or ionize the molecules. Furthermore, the utilization of metallic catalyst drastically decreases the electron energy that is required for the methane decomposition. The non-Maxwellian electron energy distribution of laser-induced plasma is typically ranging 1−5 eV while the threshold energy for dissociation requires greater than 9 eV. Hence, the plasma seldom dissociates methane by itself. Figure 1 depicts the non-Maxwellian energy distribution function (EDF) of electrons in laser-induced plasma and the typical methane decomposition cross section due to an energetic electron collision, which is a function of energy too.44 Table 2 proposes the kinetics generating various species during the laser-induced plasma formation in the presence of metal catalyst. In fact, the energetic metal species within the plasma are initially ablated from the targets. Those contribute to decomposing the methane molecules while the catalysts are partners to enhance the process, leading to the formation of CH3 radical and H2 after recombination.7 The species initiate the recombination reactions to react with one another to produce C2 and C3 hydrocarbons and hydrogen according to C



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

Hydrogen collisions

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

CH3 + H2 → CH4 + H CH3 + H → CH2 + H2 CH2 + H2 → CH3 + H CH2 + H → CH + H2 H + H + CH4 → H2 + CH4

Molecular collisions CH4 CH4 CH3 CH3 CH3 CH3 CH3 CH3

+ + + + + + + +

CH2 → CH3 + CH3 CH → C2H4 + H CH3 + CH4 → C2H6 + CH4 C2H6 → C2H5 + CH4 C2H5 + CH4 → C3H8 + CH C2H4 → C2H3 + CH4 C2H3 → C2H2 + CH4 C2H3 + CH4 → C3H6 + CH

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

attendance of the metal target. This lucidly demonstrates that metal species contribute to efficient methane decomposition. Furthermore, GC and FTIR analyses are employed to determine the hydrocarbon formation rates and the methane decomposition accordingly. 4.1. GC Analysis. We have analyzed the content of an irradiated cell using gas chromatography. The analyzer was employed after gas sampling, and subsequent experiments were carried out at atmospheric pressure for the untreated and laser irradiated targets after 6000 shots, equivalent to 20 min of exposure. Figure 3(a−d) depicts the corresponding chromatograms of the different catalysts. Components such as H2, C2H2, C2H4, C2H6, C3H6, and C3H8 are well identified to be the major compounds, as anticipated given in Table 2, even though here the catalytic functions are crucial too. This analysis accredits the event of methane decomposition, demonstrating the distribution and yield of the products. Methane conversion, selectivity, and yield of products were obtained using GC data to verify the influence of catalytic species on the molecular decomposition. Consequently, a number of hydrocarbons (and hydrogen) species appear in plasma during multiple laser shots. Each reactant describes the fraction that is converted to any final product. The selectivity of a particular compound defines the amount of reactant that is converted to a certain product. The yield explains the actual number of components produced relating the theoretical values.16 The methane conversion X, selectivity S, and yield Y of hydrocarbons and hydrogen species are given in eqs 1−5.4,47 The coefficient x in eq 2 denotes the stoichiometric coefficient.

tion. After exposure and the gas sampling, the generated compounds were taken from the chamber and injected to the GC instrument through a microneedle. The instrument is configured to analyze gas in 7 min, having a detection level of 100 ppm. The system has five valves and three detectors. The FID channel is used to analyze the hydrocarbons from C1 to C5, while C6/C6+ components are back-flushed and measured as one peak at the beginning of the analysis. The first TCD channel (reference gas: He) is employed to analyze the certain gases, which may include CO2, CO, O2, and N2. The third one is the second TCD channel (third detector, on the side, with reference 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 an irradiation chamber filled with CH4 to create a methane environment at atmospheric pressure. The experiments were carried out by examining four different pure metal samples such as Ni, Fe, Pd, and Cu metallic targets to verify the catalytic effects on the methane decomposition and product yield. At first, the laser irradiation of the methane in the filled chamber was carried out without the metal target. No plasma radiation was detected. The microplasma takes place when a Q-SW Nd:YAG laser focuses on the target. Subsequently, the metal species appear in the induced plasma, leading to the decomposition events.7 In fact, the laser was unable to decompose methane molecules in the absence of the metal target. In this case, metal NPs cannot be generated during laser ablation to enter into laser-induced plasma. Conversely, the plasma is well formed and the nanoparticles appear in the induced plasma, leading to the efficient decomposition at the

XCH4 (%) = D

moles of CH4 consumed × 100 moles of CH4 introduced

(1)



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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.

⎛ moles of desired component Cx Hy ⎞ SCxH y (%) = x⎜ ⎟ × 100 moles of CH4 consumed ⎝ ⎠ (2)

Y (%) =

XCH4 (%) × S (%) 100

(3)

Furthermore, yield and selectivity for hydrogen byproduct are given as below47 SH 2

⎛ moles of H produced ⎞ 2 (%) = 0.5⎜ ⎟ × 100 ⎝ moles of CH4 consumed ⎠

⎛ moles of H produced ⎞ 2 YH2 (%) = 0.5⎜ ⎟ × 100 moles of CH ⎝ 4 introduced ⎠

Figure 4. Selectivity of compounds after laser exposure of methane in the presence of the various metal targets after 6000 laser shots (data taken from GC). Selectivity of propane species is dominant using Ni catalyst.

(4)

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

(5)

Figure 4 displays the selectivity of products after laser exposure (6000 shots) in the presence of a variety of metal targets. It indicates that the propane, ethylene, ethane, and ethylene obtain the maximum selectivity using Ni, Fe, Pd, and Cu targets, respectively. Moreover, Table 3 summarizes the E



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into hydrogen atoms, those can migrate into the bulk metal crystallite and, in some cases, react with the solid to form metal hydrides.52 The unique behavior of Pd is highlighted because this catalyst gains a high affinity to adsorb the hydrogen atoms after methane decomposition to generate hydrogen molecules and solid carbons accordingly.52−54 In fact, hydrogen gas generation is a measure of Pd catalytic activation, as expected during the experiments here. 4.2. FTIR Analysis. Figures 6−9 represent FTIR spectra of the CH4 gas at 1000 mbar before and after laser irradiation for typical 3000, 6000, and 9000 successive shots examining various metals, i.e, Ni, Fe, Cu, and Pd, as the catalysts. The metallic species in plasma may act to facilitate the plasma generation at lower laser pulse energies by creation of a relatively higher initial electron density. In fact, the catalyst contributes to enhance chemical reactions of methane decomposition as well as plasma initiation to drop the required electron energy for decomposition according to the kinetic scheme in Table 1. The characteristic peaks of methane locate over the spectral range of 1200−1400 cm−1 and ∼3000 cm−1, corresponding to the bending and stretching vibration of the methane 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 number of peaks appearing as the fingerprint over 600−1200 cm−1. The spectral line at 628 cm−1 is attributed to the acetylenic C−H bend over 600−650 cm−1 while the other peak is located at ∼729.5 cm−1. One of the absorption peaks of acetylene is centered at ∼730 cm−1 too. The characteristic peak at ∼949.5 cm−1 is related to the C−H bond in alkene, which is situated over the 650−1000 cm−1 spectral region. The absorption peak at 949 cm−1 is related to the ethylene, and the other one exhibiting around 1500−1700 cm−1 corresponds to the CC stretch modes of carbon groups. Furthermore, the spectral absorption over 2100−2200 cm−1 and 3200−3400 cm−1 indicates a triple bond CC and acetylenic stretching C−H in alkynes, respectively.30,55−57 The existence of whole bonds after laser irradiation indicates the formation of heavier hydrocarbons such as alkane, alkene, and alkyne. The exposure time in the interval of 10−30 min equivalent to 3000−9000 laser shots does not imply the creation of further product species; however, it may enhance the corresponding absorption line amplitudes, as illustrated typically in Figure 10. In summary, with the number of pulses, the concentrations of product molecules increase, and there is no sign that additional compounds form. This accordingly gives rise to populated components with shot numbers. Figure 11 depicts the absorption peaks in terms of laser shots. A notable rise appears proportional to the laser dose. The catalyst activity in the course of the methane decomposition was studied accordingly. In the attendance of each metal catalyst, the corresponding dissociation rates are determined using FTIR according to following formalism:

Table 3. Selectivities (%) of Products Using Various Targets C2H6 C2H4 C2H2 C3H8 C3H6 H2

Ni

Fe

Pd

Cu

3.24 14 0.72 96.54 2.15 33.26

7.34 22.03

32.09 5.01

22.49 28.11

8.26 2.62 45.43

15.04

2.24 0.56 42.17

41.12

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

Ni catalyst. Pd and Ni are active catalysts for methane decomposition.29 The decomposition of CH4 on oriented Pd surfaces and nanostructured Pd catalysts is well understood.48−51 Dissociative chemisorption of CH4 occurs as low as 400 K and accelerates rapidly with increasing temperature, leaving carbidic surface carbon behind. Evidence for the existence of CHx species (x = 1−3) on Pd could not be found in experimental studies, and it was concluded that they were very short-lived on Pd at the temperatures investigated here.48,49 Theoretical studies confirmed that the remaining CH bonds of the initially formed CH3 are broken in a fast reaction cascade because the coordinative stabilization of C on Pd surfaces improves with decreasing number of H ligands.50,51 As a consequence, combinative reactions between CHx intermediates are largely suppressed. C2H6 was the only hydrocarbon observed in appreciable amounts in one of these studies,49 which is consistent with CH3 being the most stable fragment.50,51 Similarly, Table 4 tabulated the yield of compounds generated by use of various catalysts. Table 4. Yields (%) of Products Using Various Targets Ni C2H6 C2H4 C2H2 C3H8 C3H6 H2

0.04 0.2 0.01 1.40 0.031 0.48

Fe

Pd

Cu

0.1 0.3

0.7 0.1

0.27 0.33

0.11 0.036 0.63

0.33

0.02 0.01 0.5

0.9

However, the major products are propane, ethane, ethylene, and hydrogen while other components are comparatively negligible. This is in agreement with the previous work on methane decomposition as well.7 The yield of hydrogen is maximum in the presence of Pd targets too. This arises from the fact that palladium is an optimal catalyst for the methane decomposition and hydrogen generation. The recommended hydrogenation catalysts for alkane’s reactions are Ni and Pd. Because of the small size of hydrogen relative to the other molecules, after adsorbing on metal surfaces and dissociating

nN = n0e−ωN

(6)

where n0 and nN denote the initial molecule concentration and those after N-pulse irradiation. N and ω are the number of shots and the dissociation rate per pulse, respectively.58,59 Equation 6 is basically used for MPD analysis, and the decomposition data were fitted to the experimental curve. Here, the decomposition of methane molecules in terms of successive laser shots is modeled in virtue of the exponential equations. F



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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.

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

The exponential decay is linearly approximated for small decomposition 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 nN = n0[1 − Γ(1 − e−ω)]N = n0(1 − ΓR )N

nN ≃ n0(1 − Γω)N 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, regardless of the type of molecule, the dissociation process has a number of parameters in common, as given in eqs 6 and 7. The basis of the equations is the obliteration of a chemical component (or molecules) in terms of number of laser shots.59,60 Even though the exponential relationship is initially used for the multiple photodissociations, the experimental data can be

(7)

where ω is a unitless parameter implying the dissociation events and R = 1 − e−ω denotes the dissociation rate per pulse. The parameters ω and then R are determined from experimental data. G



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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 copper target.

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 palladium target.

fitted by means of the least-square method for catalytic decomposition. Here, this model was examined for decomposition in laser-induced plasma. Figure 12 illustrates the exponential dependence of the nN/n0 ratio versus the number of laser shots having 100 mJ/pulse, 10 ns, and 5 Hz applied for various metal targets. The slope is an indicative of catalysts activity for the methane decomposition. This varies with different metallic catalysts such that it gives out the highest dissociation rate for the Pd target. The following order is obeyed for examining catalysts:

Figure 13 displays the comparison of dissociation rates of various catalysts. The greatest dissociation rate is achieved with the palladium target, and the smallest rate is obtained using the copper catalyst. The greatest dissociation rate of methane has been 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 hydrocarbon components. The absence of ethane is likely due to the overlap with the methane characteristic line to be unable to identify those peaks.7,55,61 The creation of an absorption band ranging 720−750 cm−1 corresponds to the bending modes of the CH2

ωPd > ω Ni > ωFe > ωCu H



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

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

Figure 13. Methane dissociation rate using various metallic catalysts.

approves that characteristic absorption spectra of propane are located at 748, 1500, and 3000 cm−1. Thus, C3H8 absorption bonds coincide with those bands of CH4 and C2H2 in the FTIR spectra.55,58 On the other hand, the carbon deposited on the ZnSe windows is obviously visible after 9000 laser shots. This is taken for granted as alternative evidence of the methane decomposition during metal-assisted induced plasma deposition.7 4.3. Properties of Methane Decomposition. In fact, the methane conversion and dissociation rate ω are proportional to laser dose for each catalyst. The fluence exceeds a threshold value to create the plasma to be on the order of several J/cm2 for nanosecond laser duration. The plasma formation initiates near the surface when the energy deposited on the target

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

chain (quartet or higher). This illustrates the formation of residual CH2 chains after methane decomposition, as suggested in Table 2. Finally, those molecules were attached to one another to create C3H8. The vibrational energy in NIST I



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enforces the metal ablation proportional to the reverse of the latent heat of vaporization Lv. The threshold fluence, Fth, is given by Fth (J/cm2) = ρLvα 1/2Δt1/2 where ρ, α, and Δt ascertain the target density (kg/cm2), thermal diffusivity (cm2/ s), and laser duration (s), respectively. If the laser fluence is below Fth, then no evaporation occurs.34,36,62 At high laser fluence, sufficiently above the threshold, a high-temperature plasma is formed and the laser energy is absorbed effectively within the plasma, leading to serious plasma heating. The fluence threshold critically depends on the sample properties and, consequently, differs from one material to another.62 Regarding the above equation, Fth values of various targets such as palladium, iron, nickel, and copper are determined to be 2.04, 2.30, 2.72, and 4.62 J/cm2, respectively. The threshold fluence is the minimum energy required for the evaporation and subsequent ablation of the metallic sample, as the low Fth leads to the high ablation rate and larger amounts of evaporated material are given to the dense plasma. For instance, palladium takes the smallest value among the other available metal targets that gives rise to the more ablated species available in the plasma plume. This results in high methane conversion accompanying higher electron density that correlates with the greater collision rates following higher dissociation rates during the electron-impact process. As a consequence, Fth of palladium is the smallest value corresponding to the maximum electron density, because more ablated species are added to the plasma.36 For metals with lower ionization energy, Ei, such as Ni and Cu, the plasma temperature, Te, notably rises due to the smaller amount of laser energy imparted to ablate the target. Consequently, more accessible energy is available to heat plasma according to the IB absorption, leading to the higher plasma temperature. As a result, the plasma temperature is inversely correlated to the ionization energy of the metal target. The ionization energies required for Ni, Cu, Fe, and Pd are given to be 737.1, 745.5, 762.5, and 804.4 kJ/mol, respectively. Hence, the electron density is remarkably high for the metals having low threshold fluence (Fth). Despite the plasma temperature being strongly dependent on the electronic properties of metals, the electron density mainly arises from the thermal characteristics, such as latent heat of vaporization, thermal diffusivity, and density. The plasma temperature is on the order of 10000 K using plasma optical emission spectroscopy (OES) while the electron density is on the order of 7 × 1017 cm−3.36 Moreover, the ratio of absorption coefficient 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 laser pulse energy by Pd is prominent compared with the other targets. This is accredited to the fact that the abundance of palladium species leads to the denser plasma, contributing to further dissociation and methane conversion. Other works also reported that Pd is more active with respect to Ni and Fe catalysts, during methane thermal decomposition.53,54 Finally, the methane conversion ratio correlates the dissociation rate and is reciprocally proportional to the product of Fth × Ei, emphasizing the dual effect of plasma temperature and density on X and ω. Figure 14 illustrates dissociation rate versus product Fth × Ei for various metal catalytic targets using laserinduced plasma. The methane conversion percentage XCH4 (%) and the dissociation rate (ω) are taken from GC and FTIR analyses,

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

respectively. Furthermore, the metal properties, such as Fth, Ei, Fth × Ei, and α/R are tabulated in Table 5. It is obvious that the Table 5. Catalytic Properties, i.e. XCH4 (%), ω, Fth, Ei, FthEi, and α/R Ratio of Methane, after 6000 Laser Shots Using a Q-SW Nd:Yag Laser Having a 10 ns, 100 mJ/pulse at 1000 mbar Catalyst

Pd

Ni

Fe

Cu

Methane Conversion (%) (taken from GC) Dissociation rate ω (×10−5) (taken from FTIR) Threshold fluence Fth (J/cm2) Ionizatin energy Ei (kJ/mol) FthEi Absorption coefficient to reflectance ratio α/R

2.18

1.44

1.38

1.20

11.9

8.07

6.45

3.99

2.04 804.4 1641 9.1

2.30 737.1 1695 8.4

2.72 762.5 2074 7.8

4.62 745.5 3444 8.6

maximum methane conversion takes place in the presence of the palladium target whose dissociation rate is the largest among the four examined catalysts. Furthermore, the corresponding threshold fluence Fth is the smallest one. However, the relative hydrogen yield is high and the solid carbon deposit is notable.7,53,54 Note that methane conversion, yields, and activities are relatively low to be intrinsic as the nature of laser-induced plasma decomposition. Despite the fact that the dissociation rate of methane in the presence of metal target is on the order of 10−5, in the event of more energetic laser shots (200−400 mJ/pulse), the plasma properties will significantly rise, leading to the greater decomposition rate. 4.4. Proposed Model for the Methane Decomposition. In summary, there is an alternative efficient process to decompose CH4 based on laser-induced plasma even with relatively low photon energy. In laser-induced plasma, the desired molecules face an intense photon flux, energetic electrons, as well as ion species. In this case, the direct photoexcitation and ionization do not certainly take place. The direct photodissociation during nanosecond laser shots occurs if the laser line matches the molecules transitions. This is not the case here because methane absorption lines are centered in the VUV spectral range. The intense femtosecond (mode locked) laser shots may induce virtual states, creating a proper situation for the MPA/MPD process due to its inherent broadband property according to the Fourier transform limit. Here, the nanosecond laser shot with a sufficient energy density is focused on the solid, and then laser-induced plasma can be formed. At initial instants, the atomic and molecular structures of the samples are broken and heated up based on the J



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successive energetic electron collisions with atoms/molecules via a thermionic effect, causing vaporization of a small fraction of the material. This contains free neutral atoms, ions, molecular fragments, and free electrons. Afterward, the incoming energy of the same laser pulse attains high temperature plasma of ∼10,000 K, in which the vaporized species are excited to the upper states to deactivate subsequently by emitting electromagnetic radiations. When the laser irradiance is adequately high to induce a plasma plume, the leading edge of the pulse rapidly heats, melts, and vaporizes material into a layer just above the surface. Part of the laser energy contributes to heat up the plasma plume to continue absorbing energy from the laser during the pulse duration.33 The plasma properties over the target surface are enhanced by the inverse Bremsstrahlung absorption (IB) during collisions among other atoms and ions, electrons, and gas species.34,62 The conductive nature of the given metal is strongly relevant to the mechanism dealing with the laserinduced plasma initiation.33,35,63 In the meantime, the plasma creates a shock wave which spreads from the surface to the surrounding medium33,34 to centrifuge the particles out of the target surface. Here, the sequential events are proposed that give rise to the hydrocarbon dissociation and recombination by catalytic assistance within laser-induced plasma. Figure 15 illustrates the sequential steps for the generation of higher hydrocarbons and hydrogen gas. Those include plasma formation, metallic NP generation, methane dissociation, and the recombination of species to form higher hydrocarbons C2 and 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 in the formation of chemisorbed carbon species and the release of gaseous hydrogen after the decomposition process in plasma.64 The catalytic decompositions are based on the d orbitals of the metallic target transferring electronic charge into the antibonding levels of the adsorbate, facilitating dissociation.65 The induced plasma rarely decomposes methane regarding the electron impact collisions because of the large discrepancy of electron distribution and sufficient energy required for decomposition.5 The conditions would be different when the catalyst attends in the plasma region. The vibrationally excited species are taken into account as the active sites on the NPs generated in plasma having the minimum internal energy to enhance the catalytic reactions. Numerous studies have been devoted to enhance the dissociative adsorption in the catalytic reactions by elevating the vibrational energy of reactants. Once the dissociative adsorption is accelerated, the production rate can be greatly enhanced regarding the activated recombination process. Figure 16 summarizes the typical steps of reactions to explain the vibrationally excited species which assist the dissociative adsorption. Because of the higher internal energy of vibrational states, the activation barrier requires that chemical adsorption could be reduced from Ea to (Ea − ET), where Ea and ET ascertain the activation barrier of chemisorption for ground state reactant and the threshold energy for the formation of vibrational reactant through electron-impact reactions, respectively. This arises from laser-induced plasma. For solid−gas catalytic reactions, part of the residual reaction energy might be transferred to the desorbed products as vibrational excitation. Methane molecules in the vibrational state demonstrate much higher catalytic activity than those in the ground state.42

Figure 15. Sequential steps for the generation of the higher hydrocarbons 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.

The free electrons in the plasma collide with the CH4 molecules physisorbed on the catalyst surface. The collisions distort the CH4 molecules from their tetrahedral configuration, thereby lowering the barrier to dissociate those molecules into the adsorbed methyl (CH3) species and hydrogen atoms. The electron collisions with the catalytic surface contribute to enhance the decomposition rate while adsorption events make the CH bond length looser than before detachment. The adsorbed CH 3 dissociates into CH and CH2 species accordingly, while those species recombine to form the adsorbed C2H2, C2H4, etc. In fact, the adsorbed C2H2 and K



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consequence, the subsequent multiple recombinations of CH bonds lead to the generation of higher hydrocarbon species. The FTIR and GC spectrographs verify the formation of alkane, alkene, and alkyne products, recording the corresponding abundance of each compound. The abundant products are propane, ethane, and ethylene with the assistance of the catalysts above. Pd and Ni are prone to function as the efficient catalysts for the decomposition in laser-induced plasma. Varying those metal targets, the dissociation rate and the conversion ratio of the methane drastically change. The generation of propane exhibits the highest yield and selectivity in the case of Ni catalyst and similar properties appear for ethane in the case of Pd. Furthermore, the maximum conversion ratio and the dissociation rate are determined to be 2.18% and 1.19 × 10−4 when dealing with the Pd target, respectively. In the meantime, the palladium catalyst dissolves the atomic hydrogen content after methane decomposition over the catalytic surface to generate excessive hydrogen. It gives the highest yield. The deposition of carbon on the windows and chamber walls is evidence of strong methane decomposition during laser irradiation. Eventually, the empirical data emphasizes that the methane conversion ratio strongly correlates to the inverse of the product of ablative threshold laser fluence and the ionization energy (Fth × Ei), exhibiting dual effects of plasma temperature and density on methane decomposition.

Figure 16. Schemes of the summary of major events: Proposed mechanism for the vibrationally excited species during laser beam irradiation to activate the catalytic reactions. R and R* represent gasphase reactants in ground and vibrational states, respectively. Ea is the activation barrier of chemisorption for R, and ET denotes the threshold energy for the formation of R* through electron-impact reaction (Ea − ET).42 Typical chemical conversions of CH4 into C2H4 and H2 are depicted too.

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CH species react together to dimerize or trimerize into a series of hydrocarbons such as C3H6 and C3H8.66 Microdischarges between catalyst particles may be partially responsible for the conversion and selectivity. It is believed that the nanocatalyst surface in contact with the plasma notably activates the reactions. The charged plasma species may lead to charge accumulation on the catalyst surface, which in turn alters the electrostatic potential and the work function of the catalyst, accordingly.42,47

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE α/R absorption coefficient to reflectance ratio CNG compressed natural gas GC gas chromatography FTIR Fourier transform IR spectroscopy Ea activation barrier of chemisorption for ground state reactant EDF energy distribution function of electron Ei ionization energy ET threshold energy for formation of vibrational reactant FID flame ionization detector Fth threshold fluence IB inverse bremsstrahlung LIBS laser-induced breakdown spectroscopy LNG liquefied natural gas MPA multiphoton absorption MPD multiphoton dissociation N number of laser shots nN/n0 ratio of molecule concentration after N-shots to initial concentration NPs nanoparticles ω dissociation rate per pulse OCM oxidative coupling of methane OES optical emission spectroscopy R gas-phase reactants in ground state R* gas-phase reactants in vibrational state S selectivity of species TCD thermal conductivity detector Y yield of species

5. CONCLUSION This work is an extension of our previous work on Pd-assisted laser-induced plasma decomposition of methane at manometric (sub atmospheric) pressures.7 The methane molecules are irradiated by a nanosecond Q-switched Nd:YAG laser in the controlled chamber using various catalysts, i.e. Pd, Ni, Fe, and Cu, at atmospheric pressure in order to investigate the corresponding catalytic activity. Despite the fact that photodissociation was previously reported by making use of femtosecond lasers, here the molecules are disintegrated according to the metal-assisted nanosecond laser-induced plasma to form higher hydrocarbons and hydrogen. The laser beam ablates the metallic target in methane atmosphere and creates transient and localized plasma over the surface containing NPs and free electrons. When the laser fluence is noticeably greater than the ablation threshold, the nanoparticles metal species are created. The NP catalysts in turn provide a large activation area to enhance the adsorption of methane molecules accompanying the multiple electron collisions in plasma. This would accelerate the decomposition rate significantly. Methane is rarely dissociated via direct electron impact collisions due to the discrepancy of EDF with the dissociation energy threshold. However, it most likely occurs due to skin catalytic reactions of the NPs. The latter loosens the CH bonds as well as enhancing the spillover events that finally give rise to the methane decomposition. As a L


The Journal of Physical Chemistry C x XCH4

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(20) Hanst, P. L.; Hanst, S. T. Gas Measurement in the Fundamental Infrared Region. In Air Monitoring by Spectroscopic Techniques; Sigrist, M. W., Ed.; John Wiley & Sons, Inc.: New York, 1994. (21) Wang, S.; Tang, X.; Gao, L.; Elshakre, M. E.; Kong, F. Dissociation of Methane in Intense Laser Fields. J. Phys. Chem. A 2003, 107, 6123−6129. (22) Zerr, A.; Serghiou, G.; Boehler, R.; Ross, M. Decomposition of alkanes at high Pressures and temperatures. High Pressure Research 2006, 26, 23−32. (23) Sharifi, M.; Kong, F.; Chin, S. L.; Mineo, H.; Dyakov, Y.; Mebel, A. M.; Chao, S. D.; Hayashi, M.; Lin, S. H. Experimental and theoretical investigation of high-power laser ionization and dissociation of methane. J. Phys. Chem. A 2007, 111, 9405−9416. (24) Wu, Z.; Wu, C.; Liang, Q.; Wang, S.; Liu, M.; Deng, Y.; Gong, Q. Fragmentation dynamics of methane by few-cycle femtosecond laser pulses. J. Chem. Phys. 2007, 126, 074311 (1−7). (25) Wang, C.; Song, D.; Liu, Y.; Kong, F. Pulse width effect on the dissociation probability of CH4+ in the intense femtosecond laser field. Chin. Sci. Bull. 2006, 51, 1269−1272. (26) Suzuki, T.; Wada, K.; Shima, M.; Watanabe, Y. Photoinduced partial oxidation of methane into formaldehyde on silica supported molybdena. J. Chem. Soc., Chem. Commun. 1990, 15, 1059−1060. (27) Hill, W.; Shelimov, B. N.; Kazaansky, V. B. Photoinduced reaction of methane with molybdena supported on silica. J. Chem. Soc. Faraday Trans.1 1987, 83, 2381−2389. (28) Calahan, M. In Electrocatalysis to Fuel Cells; Sandstede, G., Ed.; Univ. Washington Press: Seattle, WA, 1972; pp 189−204. (29) Shah, N.; Panjala, D.; Huffman, G. P. Hydrogen production by catalytic decomposition of methane. Energy Fuels 2001, 15, 1528− 1534. (30) Coates, J. Interpretation of Infrared Spectra, A Practical Approach. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons Ltd, Chichester, U.K., 2000; pp 10815−10837. (31) Derk, A. R.; Funke, H. H.; Falconer, J. L. Methane Conversion to Higher Hydrocarbons by UV Irradiation. Ind. Eng. Chem. Res. 2008, 47, 6568−6572. (32) Mordaunt, D. H.; Lambert, I. R.; Morley, G. P.; Ashfold, M. N. R.; Dixon, R. N.; Western, C. M.; Schnieder, L.; Welge, K. H. Primary product channels in the photodissociation of methane at 121.6 nm. J. Chem. Phys. 1993, 98, 2054−2065. (33) Pasquini, C.; Cortez, J.; Silva, L. M. C.; Gonzaga, F. B. Laser Induced Breakdown Spectroscopy. J. Braz. Chem. Soc. 2007, 18, 463− 512. (34) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley and Sons: New York, 2006. (35) Mortazavi, S. Z.; Parvin, P.; Reyhani, A.; Nozad Golikand, A.; Mirershadi, S. Effect of Laser Wavelength at IR (1064 nm) and UV (193 nm) on the Structural Formation of Palladium Nanoparticles in Deionized Water. J. Phys. Chem. C 2011, 115, 5049−5057. (36) Mortazavi, S. Z.; Parvin, P.; MousaviPour, M. R.; Reyhani, A.; Moosakhani, A. Moradkhani, Sh. Time-resolved evolution of metal plasma induced by Q-switched Nd:YAG and ArF-excimer lasers. Opt. Laser Technol. 2014, 62, 32−39. (37) Nedialkov, N. N.; Imamova, S. E.; Atanasov, P. A.; Berger, P.; Dausinger, F. Mechanism of ultrashort laser ablation of metals: molecular dynamics simulation. Appl. Surf. Sci. 2005, 247, 243−248. (38) Bäuerle, D. Laser chemical processing: an overview to the 30th anniversary. Appl. Phys. A: Mater. Sci. Process. 2010, 101, 447−459. (39) Muhammad Khan, T.; BiBi, T. Application of NS Pulsed Laser Ablation for Dense CdS Nanoparticles Deposition in Argon Atmosphere. SOP Transactions on Applied Physics 2014, 1, 48−54. (40) Stoffels, W. W.; Stoffels, E.; Ceccone, G.; Rossib, F. Laserinduced particle formation and coalescence in a methane discharge. J. Vac. Sci. Technol. A 1999, 17, 3385−3392. (41) Stafe, M.; Marcu, A.; Puscas, N. N. Pulsed Laser Ablation of Solids (Basics, Theory and Applications); Springer: Heidelberg, 2014. (42) Chen, H. L.; Lee, H. M.; Chen, S. H.; Chao, Y.; Chang, M. B. Review of plasma catalysis on hydrocarbon reforming for hydrogen

stoichiometric coefficient methane conversion

REFERENCES

(1) Lunsford, J. H. Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century. Catal. Today 2000, 63, 165−174. (2) Chao, Y.; Huang, C. T.; Lee, H. M.; Chang, M. B. Hydrogen production via partial oxidation of methane with plasma-assisted catalysis. Int. J. Hydrogen Energy 2008, 33, 664−671. (3) Gondal, M. A.; Yamani, Z. H.; Dastgeer, A.; Ali, M. A.; Arfaj, A. Photo-Conversion of Methane into Higher Hydrocarbons Using 355 nm Laser Radiation. Spectrosc. Lett. 2003, 36, 313−326. (4) Cho, W.; Baek, Y.; Moon, S.; Kim, Y. C. Oxidative coupling of methane with microwave and RF Plasma catalytic reaction over transitional metals loaded on ZSM−5. Catal. Today 2002, 74, 207− 223. (5) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K.; Nozaki, T.; Okazaki, K. Reaction mechanism of methane activation using nonequilibrium pulsed discharge at room temperature. Fuel 2003, 82, 2291−2297. (6) Li, X. S.; Zhu, A. M.; Wang, K. J.; Xu, Y.; Song, Z. M. Methane conversion to C2 hydrocarbons and hydrogen in atmospheric non thermal plasmas generated by different electric discharge techniques. Catal. Today 2004, 98, 617−624. (7) Reyhani, A.; Mortazavi, S. Z.; Parvin, P.; Mahmoudi, Z. Simultaneous laser induced breakdown spectroscopy and Pd-assisted methane decomposition at different pressures. Spectrochim. Acta, Part B 2012, 74−75, 124−130. (8) Chen, C. J.; Back, M. H.; Back, R. A. The Thermal Decomposition of Methane. I. Kinetics of the Primary Decomposition to C2H6+H2; Rate Constant for the Homogeneous Unimolecular Dissociation of Methane and its Pressure Dependence. Can. J. Chem. 1975, 53, 3580−3590. (9) Parvulescu, V. I., Magureanu, M., Lukes, P., Eds.; Plasma Chemistry and Catalysis in Gases and Liquids; Wiley-VCH: , Weinheim, Germany, 2012. (10) Kazemeini, M.; Mohammadi, A. R. Reactor Modeling of a NonCatalytic OCM Process. IAChE 2005, 2, 3−14. (11) Malik, M. A.; Malik, S. A.; Jiang, X. plasma reforming of natural gas to more valuable fuels. Journal of Natural Gas Chemistry 1999, 8, 166−180. (12) Biris, A. R.; Li, Z.; Dervishi, E.; Lupu, D.; Xu, Y.; Saini, V.; Watanabe, F.; Biris, A. S. Effect of hydrogen on the growth and morphology of single wall carbon nanotubes synthesized on a Fe− Mo/MgO catalytic system. Phys. Lett.A 2008, 372, 3051−3057. (13) Takenaka, S.; Ishida, M.; Serizawa, M.; Tanabe, E.; Otsuka, K. Formation of carbon nanofibers and carbon nanotubes through methane decomposition over supported cobalt catalysts. J.Phys.Chem.B 2004, 108, 11464−11472. (14) Chen, J.; Zhou, X.; Cao, L.; Li, Y. COx-free hydrogen and carbon nanofibers production by decomposition of methane on Fe, Co and Ni metal catalysts. Stud. Surf. Sci. Catal. 2004, 147, 73−78. (15) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. Methane decomposition into hydrogen and carbon nanofibers over supported Pd−Ni catalysts. J. Catal. 2003, 220, 468−477. (16) Dupuis, A. C. The catalyst in the CCVD of carbon nanotubes − a review. Prog. Mater. Sci. 2005, 50, 929−961. (17) O’Byrne, J. P.; Li, Z.; Tobin, J. M.; Larsson, J. A.; Larsson, P.; Ahuja, R.; Holmes, J. D. Growth of carbon nanotubes from heterometallic palladium and copper catalysts. J. Phys. Chem. C 2010, 114, 8115−8119. (18) Horita, T.; Yamaji, K.; Kato, T.; Sakai, N.; Yokokawa, H. Imaging of labeled gas movements at the SOFC electrode/electrolyte interfaces. Solid State Ionics 2004, 169, 105−113. (19) Liao, M. S.; Au, C. T.; Ng, C. F. Methane dissociation on Ni, Pd, Pt and Cu metal (111) surfaces -a theoretical comparative study. Chem. Phys. Lett. 1997, 272, 445−452. M



Article

productionInteraction, integration, and prospects. Appl. Catal., B 2008, 85, 1−9. (43) Fridman, A. Plasma Chemistry; Cambridge University Press: New York, 2008. (44) Matte, J. P.; Lamoureux, M.; Moller, C.; Yin, R. Y.; Delettrez, J.; Virmont, J.; Johnston, T. W. Non-Maxwellian electron distribution and continuum X-ray emission in inverse Bremsstrahlung heated plasmas. Plasma Phys. Controlled Fusion 1988, 30, 1665−1689. (45) Bie, C. D.; Verheyde, B.; Martens, T.; Dijk, J. V.; Paulussen, S.; Bogaerts, A. Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge. Plasma Process. Polym. 2011, 8, 1033−1058. (46) Cho, W.; Beak, Y.; Park, D.; Kim, Y. C.; Anpo, M. The conversion of natural gas to higher hydrocarbons using a microwave plasma and catalysts. Res. Chem. Intermed. 1998, 24, 55−66. (47) Liu, C.; Mallinson, R.; Lobhan, L. Nonoxidative Methane Conversion to Acetylene over Zeolite in a Low Temperature Plasma. J. Catal. 1998, 179, 326−334. (48) Yarw-Nan, W.; Herman, R. G.; Klier, K. Dissociative adsorption of methane on Pd(6 7 9) surface. Surf. Sci. 1992, 279, 33−48. (49) Solymosi, F.; Erdö helyi, A.; Cserényi, J.; Felvégi, A. Decomposition of CH4 over supported Pd catalysts. J. Catal. 1994, 147, 272−278. (50) Paul, J.-F.; Sautet, P. Chemisorption and transformation of CHx fragments (x = 0−3) on a Pd(1 1 1) surface: a periodic density functional study. J. Phys. Chem. B 1998, 102, 1578−1585. (51) Zhang, C. J.; Hu, P. Methane transformation to carbon and hydrogen on Pd(1 0 0): pathways and energetics from density functional theory calculations. J. Chem. Phys. 2002, 116, 322−327. (52) Reyhani, A.; Mortazavi, S. Z.; Mirershadi, S.; Moshfegh, A. Z.; Parvin, P.; Nozad Golikand, A. Hydrogen Storage in Decorated Multiwalled Carbon Nanotubes by Ca, Co, Fe, Ni, and Pd Nanoparticles under Ambient Conditions. J. Phys. Chem. C 2011, 115, 6994−7001. (53) Reyhani, A.; Mortazavi, S. Z.; Akhavan, O.; Moshfegh, A. Z.; Lahooti, Sh. Effect of Ni, Pd and Ni−Pd nano-islands on morphology and structure of multi-wall carbon nanotubes. Appl. Surf. Sci. 2007, 253, 8458−8462. (54) Mortazavi, S. Z.; Reyhani, A.; Iraji zad, A. The effect of Pd addition to Fe as catalysts on growth of carbon nanotubes by TCVD method. Appl. Surf. Sci. 2008, 254, 6416−6421. (55) Hsu, C. P. S. Infrared spectroscopy. In Handbook of Instrumental Techniques for Analytical Chemistry; Settle, F. A., Ed.; Prentice Hall: Englewood Cliffs, NJ, 1997. (56) NIST electronic database at http://webbook.nist.gov/ chemistry/. (57) Mistry, B. D. A Handbook of Spectroscopic Data Chemistry (UV, JR, PMR, JJCNMR and Mass Spectroscopy); Oxford Book Company: Jaipur, India, 2009. (58) Sajad, B.; Parvin, P.; Silakhori, K.; Zamanipour, Z.; Hooshvar, M. Dissociation rate measurements for selectivity evaluation in SF6 selective dissociation by multiline vs. single line CO2 laser. J. Nucl. Sci. Technol. 2004, 41, 771−776. (59) Sajad, B.; Parvin, P.; Bassam, M. A. SF6 decomposition and layer formation due to excimer laser photoablation of SiO2 surface at gas− solid system. J. Phys. D: Appl. Phys. 2004, 37, 3402−3408. (60) Velikhov, E. P.; Letokhov, V. S.; Makarov, A. A.; Ryabov, E. A. Isotope separation by multiphoton dissociation of molecules with high-power CO2 laser radiation. I. Practical methods. Sov. J. Quantum Electron. 1979, 9, 179−186. (61) Heracleous, E.; Lemonidoua, A. A.; Lercher, J. A. Mechanistic features of the ethaneoxidative dehydrogenation by in situ FTIR spectroscopy over a MoO3/Al2O3 catalyst. Appl. Catal., A 2004, 264, 73−80. (62) Cabalin, L. M.; Laserna, J. J. Experimental Determination of Laser Induced Breakdown Thresholds of Metals under Nanosecond Q-Switched Laser Operation. Spectrochim. Acta, Part B 1998, 53, 723− 730.

(63) Dehghanpour, H. R.; Parvin, P. Excimer and Nd:YAG laserinduced SF6 decomposition at the vicinity of amorphous SiO2 glass. Appl. Phys. B: Lasers Opt. 2010, 101, 611−616. (64) Nuernberg, G. B.; Fajardo, H. V.; Mezalira, D. Z.; Casarin, T. J.; Probst, L. F. D.; Carreño, N. L. V. Preparation and evaluation of Co/ Al2O3 catalysts in the production of hydrogen from thermo-catalytic decomposition of methane: Influence of operating conditions on catalyst performance. Fuel 2008, 87, 1698−1704. (65) Kohl, D. The Role of Noble Metals in the Chemistry of Solidstate Gas Sensors. Sens. Actuators, B 1990, 1, 158−165. (66) Li, M. W.; Liu, C. P.; Tian, Y. L.; Xu, G. H.; Zhang, F. C.; Wang, Y. Q. Effects of Catalysts in Carbon Dioxide Reforming of Methane via Corona Plasma Reactions. Energy Fuels 2006, 20, 1033−1038.

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Methane Decomposition Using Metal-Assisted Nanosecond Laser

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