Process intensification in gas-to-liquid reactions: plasma promoted Fischer-Tropsch synthesis for hydrocarbons at low temperatures and ambient pressure

1.1 Significance

With the development of modern society, the demand for energy increases significantly. Fossil fuels, including coal, petroleum, and natural gas that contain high percentages of carbon, are non-renewable resources. Since the industrial revolution in the 18th and 19th centuries, fossil fuels have been playing an important role in the world energy market. The reserves of fossil fuels are being depleted much faster than new ones are formed in nature. Also, the production and use of fossil fuels have been major environmental concerns. The CO₂ emissions resulting from energy production are a serious global environmental problem. A global movement toward the generation of renewable energy for sustainable development is therefore under way to meet the increased global energy needs.

The conversion of natural gas, coal bed gas, and biomass into clean liquid fuels is an attractive option to unlock large gas reserves to the fuel industry. There are two main gas-to-liquid technologies: the methanol to gasoline (MtG) process and the Fischer-Tropsch synthesis for production of hydrocarbons.

1.2 Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) was discovered in the 1920s by two German scientists; Franz Fischer and Hans Tropsch [1, 2]. In recent years, interest has increased significantly in applying the Fischer-Tropsch process for the production of clean fuels [3]. It is a chemical reaction between CO and H₂ for the formation of hydrocarbons over catalysts at higher pressure and various temperatures. Generally, the Fischer-Tropsch process is operated in the temperature range of 150–300°C. Higher reaction temperatures lead to faster reactions and higher conversion rates but tend to favour methane production. As a result, the reaction temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favours the formation of higher hydrocarbons in the process.

A variety of catalysts can be used for the Fischer-Tropsch process. Group VIII metals are the most commonly used catalysts for FTS. Cobalt is the second most popular metal used in FTS catalysts after Fe, which is due to its low cost [4] compared to other group VIII metals except for Fe. Cobalt is also favoured for its high activity [5] and higher resistance towards reoxidation of the active metal to the inactive metal oxide by water [4, 6, 7]. Co also has good selectivity to alcohols or linear hydrocarbons from short (C₂-C₄) to very long (C₂₀-C₆₀) chains [5, 6, 8–12]. At conditions favouring chain-growth, space-time yields on supported cobalt catalysts are traditionally considered independent of cobalt dispersion and of support identity [13, 14]. Cobalt remains in the metallic state under FT conditions, whereas iron’s composition changes during the FT reaction [15]. Ernst et al. [9] concluded that a completely reduced cobalt remains in the metallic state during CO/H₂ reaction by in situ EXAFS.

1.3 Dielectric-barrier discharge (DBD) plasma

In physics and chemistry, plasma is defined as an electrically neutral ionized gas. It is considered to be a fourth state of matter because of its unique properties. Plasma is constituted by particles in constant interaction including photons, electrons, positive and negative ions, atoms, free radicals, and excited and non-excited molecules [16].

A DBD is one commonly used method for producing a non-equilibrium plasma at atmospheric pressure, which is an effective tool to generate energetic electrons. The temperature of the energetic electrons is from 10,000 to 100,000 K, while the actual gas temperature remains near ambient temperature. Through electron-impact ionization, dissociation, and excitation of the source gases, active radicals and ionic and excited atomic and molecular species are generated, which can initiate plasma chemical reactions. The fundamental aspects of non-equilibrium plasma phenomena have been discussed in the literature [17–20].

A great advantage of DBDs over other discharges is that the average energy of electrons can be influenced by changing either the gas pressure (or gas density) or gap width. Thus, in a simple way, the chemical process under investigation can be optimized. From the standpoint of enhancing the energy efficiency and improving the product selectivity, it is of interest to apply a catalyst in the plasma to realize the catalytic process at low temperature and ambient pressure. Kogelschatz et al. [21, 22] recently reviewed novel applications of DBD. Extensive research works on greenhouse gas conversion to produce carbon and hydrogen [23], oxygenates [24–26], synthesis gas (H₂+CO) [27–29] and higher hydrocarbons [30–32] by the combination of plasma and zeolite catalysts have been reported. Eliasson and Kogelschatz [33] studied the hydrogenation of CO₂ to methanol in a DBD with and without a catalyst in the DBD gap. Comparison of experiments shows that DBD can effectively lower the temperature range of optimum catalyst performance. The interaction between plasma and catalyst is more complicated. Van Durme and co-workers [34] reported some physical-chemical effects of the catalyst on the plasma such as change of discharge type and changes in the distribution of accelerated electrons. Other effects include the generation of new reactive species as reported by Roland et al. [35]. They concluded that short-lived oxidising species are formed in the pores of porous materials when they are exposed to plasma. It was reported that plasma can influence catalyst properties in a discharge, and it may also enhance the dispersion of active catalyst components as well as the oxidation state of the material [34–36].

1.4 Process intensification (PI) and integrated process intensification (IPI)

Chemical processing technologies involving commodity chemicals and fuels based on fossil fuels (i.e., oil refineries and petrochemical plants) are well established. In compliance with the centralised feedstock availability, these production facilities are also centralised and hence for economic viability, they require the benefit of ”economies of scale”, i.e., centralised very large production facilities [37]. Due to the unavailability of a suitable technology that does not have the burden of economies of scale, large quantities of oil-field gases are flared causing environmental damage without any benefit. Furthermore, unlike oil and gas technologies, renewable energy production from biomass and biomass waste require small-scale-distributed production facilities in compliance with the distributed nature of the feedstock availability [37–40].

In the first instance, such a small scale but sustainable technology can be achieved by using PI [41–44]. Further sustainability can be gained by IPI [45–48], in which intensified unit operations are integrated within the production plant as well as on a global scale so as to have a totally holistic process necessary to combat global warming. PI represents a new process synthesis philosophy based on the understanding of momentum, heat and mass transfer operations in confined spaces ranging from millimetre-to-nanometre taking into account multiple-interactions between reactants, products, catalysts, reactor and thermodynamic and processing conditions. PI can be achieved in several ways, including through micro-reactor technology [41–44] or the use of a suitable PI-intensification field (such as electric or radiation field) or by reducing the diffusion path for reactants/products or by increasing the surface area for catalytic or adsorptive processes [43].

Another technique for process intensification is the application of high pressure, which is particularly suitable for small-scale reactors or in food processing as a means of achieving bacterial cell rupture at low temperatures [43]. However, in some cases, the application of high pressure is necessary for thermodynamic reasons to achieve reasonable levels of conversions as in FT synthesis. Hence, any method that reduces pressure should be deemed to be PI as high pressure operations are highly capital intensive especially at high temperatures.

1.5 Novelty of the technique

The DBD plasma technique infact utilises both capillary flow and the above cited PI-fields. To our knowledge, the Fisher-Tropsch synthesis over porous catalysts promoted by a non-thermal plasma at low temperatures and ambient pressure has not been reported in the literature. The purpose of this study was to explore the feasibility of process intensified Fischer-Tropsch synthesis (PIFTS) over porous Co/Cu-based catalysts promoted by a DBD at ambient conditions. This is an alternative way to produce liquid hydrocarbons at low temperature and ambient pressure. The present study demonstrates that a non-thermal plasma-assisted Fisher-Tropsch process can reach higher CO conversions at low temperature and ambient pressure compared with conventional ones at relatively higher temperatures and higher pressure. Plasma-promoted FTS operates more efficiently at ambient pressure, which is different from the conventional FTS. The carbon formation on the surface of catalysts was strongly inhibited by the application of plasma. Non-thermal plasma showed a unique ability to improve the metal dispersion on the catalyst as well as reduce the metal particle size to less than 10 nm at low temperatures.

2.1 Catalyst preparation

The supported Co/Cu catalysts used in the experiment were prepared by the incipient wetness method using copper nitrate (Sigma Aldrich, Gillingham, Dorset, UK) and cobalt nitrate (Sigma Aldrich, Gillingham, Dorset, UK) solutions. An aluminosilicate (Al₂SiO₅) ceramic foam monolith was received from SELEE Corporation, Hendersonville, NC, USA. The monolith had pore size of 20 ppi. The monolith was immersed in 45 ml of cobalt nitrate solution (20%, w/w) for 2 h, and then it was evaporated at 100°C for 24 h to remove water. After that, the sample was immersed in 45 ml of copper nitrate solution (20%, w/w) for 2 h, and then the sample was evaporated at 100°C for 24 h to remove water. The above operation was repeated twice to obtain a total catalyst loading of 35% (w/w) with Co/Cu=1 (weight ratio). After each impregnation, the catalyst was baked in air at 350°C for 2 h. The catalyst was crushed into 2–3 mm particles for further use. The prepared catalyst was then reduced in situ at 350°C in a hydrogen flow of 100 ml/min for 12 h prior to the reaction. It is important to make sure that the support aluminosilicate particles are not fully covered (hence crushing of the monolith into particulates) with the catalyst so that the catalyst does not act as an electrical conductor.

2.2 Catalyst characterization

Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used in the characterisation of the catalysts.

Scanning electron microscopy (JSM-5300LV, JEOL Ltd., Tokyo, Japan) incorporated with EDX spectroscopy was used to investigate the morphology and the microstructure of the supported Cu/Co catalyst samples. EDX was used to analyze the surface composition of those catalyst samples. The catalyst structure was analysed by a PANalytical X’PERT Pro Diffractometer (PANalytical B. V., Almelo, The Netherlands) using CuKα1 source (λ=1.540598 Å). The scan range (2θ) is from 5 to 120°. The fine structure of the supported Cu/Co catalyst samples were analyzed with a Philips CM100 TEM (Philips/FEI Corporation, Eindhoven, Holland), and the images were collected using an AMT CCD camera.

2.3 Catalytic reactions

Figure 1 shows a schematic diagram of the set-up of the plasma assisted FTS. A tubular coaxial glass reactor was designed and constructed to incorporate HV electrodes for plasma generation. It consisted of two coaxial glass tubes, one inside the other. The wall thickness of the glass tubes was 1.5 mm. The outer glass tube had a 32 mm inner diameter and was 300 mm in length. The inner tube had a 17 mm outer diameter. Hence the gap between the two tubes was 7.5 mm. The reactor was fitted with a pressure gauge and a pressure relief valve to control the reaction pressure.

Figure 1

Schematic diagram of the set-up of the plasma promoted FTS.

A stainless steel mesh was wrapped around the outer surface of the external glass tube and the other mesh was inserted inside the inner glass tube. A high-voltage power supply could feed between 0 and 150 W into the discharge reactor by adjusting the amplitude of the applied voltage. An alternating sinusoidal high voltage of up to 20 kV amplitude (peak-to-peak) and about 20 kHz frequency was applied to the stainless steel mesh electrodes. The wall power consumed by the plasma system was measured by a plug-in power meter. The plasma power dissipated in the discharge was calculated by integrating the product of voltage and current.

The discharge was maintained in an annular discharge gap of 7.5 mm in radial width and 175 mm in length, giving a discharge volume of about 100 ml. The discharge volume of the reactor was filled with 100 ml of the porous Cu/Co-based catalyst (20 g) with a particle size of 2–3 mm. Prior to the plasma reaction, the catalyst was reduced at 350°C in a hydrogen flow of 50 ml/min for 12 h. After catalyst reduction, the furnace was switched off and the reactor was allowed to cool down to room temperature.

The feed gases, CO and H₂ purchased from BOC, (BOC Industrial Gases, Surrey, UK), were introduced into the reactor from high-pressure cylinders via mass flow controllers (MFCs), admitting a total gas flow of 20–100 ml/min. A back pressure valve at the inlet of the DBD reactor was used to regulate the pressure in the reactor up to a maximum pressure of 6 bar. The preselected temperature of the reactor could be maintained by a tubular furnace.

The reaction products were analysed online using a Varian 450-GC (Varian, Inc., California, USA). The GC is equipped with 2 ovens, 5 columns and 3 detectors (1 TCD and 2 FID). One oven houses 3 columns [one HAYESEP Q 0.5 m×1/8"×2.0 mm Ultimetal (#CP81073), one HAYESEP T 0.5 m×1/8"×2.0 mm Ultimetal (#CP81072) and one Molsieve 13X 1.5 m×1/8"×2.0 mm Ultimetal (#CP81071)] to detect permanent gases. The second oven houses a CP-SIL 5 CB column (25 m, 0.25 mm, 0.4 μm, #CP7709) for hydrocarbons and a CP-WAX 52 CB column (25 m, 0.32 mm, 1.2 μm, #CP7763) for alcohols. The mass balance of the reaction was obtained by adding a controlled flow of nitrogen (20 ml/min) as reference gas to the exit of the reactor in order to monitor the change of volume flow due to the reaction. All results are reported in mole percent.

The conversion of CO is defined as:

The conversion of H₂ is defined as:

The product selectivity is defined as:

(i=CO₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₁₀, C₄H₈, C₄H₁₀).

The selectivity to C₅₊ hydrocarbons was calculated from the carbon balance of the reactions.

3.1 Catalyst characterization

The fresh and used catalyst samples were characterized by SEM, EDX, XRD and TEM. Figure 1A and B show the images of the supported Co/Cu catalyst structure before reaction and after plasma promoted FTS, respectively.

It is clear from Figure 2A and B that the metal particle size had reduced after the plasma reaction. Furthermore, EDX results of those catalyst samples demonstrated that the fresh catalyst surface was mainly dominated by Cu prior to the reaction. After plasma reaction, the amount of Co and Cu at the catalyst surface was almost the same, indicating that Co atoms can migrate to the surface of the catalyst in the reaction under plasma conditions. These results demonstrate that a non-thermal plasma can improve metal dispersion on the surface of a catalyst, which is very useful in catalyst preparation and catalyst modification at low temperature and ambient pressure.

Figure 2

SEM images of supported Cu/Co catalyst samples. (A) fresh catalyst; (B) after reaction in plasma.

XRD results (Figure 3) show that Co₃O₄ and CuO were present in the catalyst before reduction with H₂. Only Cu and Co metals were present in the catalyst sample after reduction in a H₂ flow of 50 ml/min at 350°C for 12 h. The results also revealed that Cu and Co were detected by XRD in the two used catalyst samples after conventional and plasma FTS reaction.

Figure 3

XRD histogram of Co/Cu catalyst supported on aluminosilicate before reduction and after reaction with and without plasma.

Figure 4 shows the TEM images of supported Cu/Co catalyst samples. A notable difference is in particle size among fresh catalyst (Figure 4A), the used catalyst in conventional FTS (Figure 4B) and in plasma-assisted FTS (Figure 4C). Catalyst particles in the used catalyst after plasma-assisted FTS appeared to be much smaller (Figure 4C) than those in the fresh one, which confirms the results from SEM. Furthermore, it is important to note that the particle size of the catalyst used in conventional FTS did not show any change compared to the fresh sample. These SEM, EDX, XRD and TEM results suggest that plasma can be used in catalyst preparation as a technique to enhance the metal distribution on the catalyst support. They also demonstrate that plasma modified the metal particle size to approximately less than 10 nm in the plasma promoted FTS.

Figure 4

TEM images of supported Cu/Co catalyst samples. (A) Fresh catalyst; (B) catalyst used in conventional FTS; and (C) catalyst used in plasma-assisted FTS.

3.2 FTS under conventional reaction conditions

First of all, for a better understanding of the reaction over a supported Cu/Co catalyst, we carried out the reaction at 230°C, reaction pressure of 6 bar, and a flow rate of 50 ml/min with a H₂/CO molar ratio of 0.33, 0.5 or 1. The results are depicted in Figure 5A and B.

Figure 5

Influence of H₂/CO molar ratio on FTS under conventional conditions. (A) Conversion; (B) selectivity. (Pressure: 6 bar; flow rate: 50 ml/min; wall temperature: 503 K.)

Figure 5A shows that an increase in the H₂/CO molar ratio in the feed gas leads to an increase in CO conversion. In the reaction, the H₂/CO molar ratio was lower than the standard ratio of 2 as lowering the ratio steers the reaction towards the production of short chain hydrocarbons and alcohols [15]. However, while steering the reaction towards these products, FTS activity with respect to CO conversion will be decreased. For the purpose of this research, the H₂/CO molar ratios selected were 1, 0.5 and 0.33. It is apparent that the H₂ conversion is at an almost identical level (87–89%) under all ratios tested.

Figure 5B reveals that the higher the H₂/CO molar ratio is, the higher the selectivity to CH₄ is in the reaction. This is due to the fact that the effective molar ratio of H₂ and CO reacted is higher than the feed ratio and CH₄ formation is favoured at high H₂/CO molar ratios. With an increase in H₂/CO ratio from 0.5 to 1, the selectivity to CH₄ was increased significantly from 42 to 19%. In contrast, changes in CO₂ selectivity do not follow a particular trend with an increase in H₂/CO molar ratio in the reaction. It is important to mention that the selectivity to higher hydrocarbons (C_2-4, C₅₊) decreases with an increase in H₂/CO molar ratio under conventional FTS reaction conditions.

3.3 Plasma-assisted Fischer-Tropsch synthesis

In the plasma-assisted Fischer-Tropsch synthesis over Cu/Co-based catalyst at low temperatures and ambient pressure, we can detect methane, carbon dioxide, water, and higher hydrocarbons including C₂ (ethylene, propylene), C₃ (propane, propene), C₄ (butylene, butane) and C₅₊ (other higher hydrocarbons) in the outlet gas stream.

3.3.1 Effect of plasma power

Plasma power is an important factor affecting FTS over a supported Cu/Co catalyst under plasma conditions. As the plasma power increases, more CO and H₂ molecules are activated and more reactive species are generated in plasma or on the surface of the catalyst. So, increasing the discharge power leads to a substantial increase in CO conversion over a supported Cu/Co catalyst in plasma. The results depicted in Figure 6A demonstrate that an increase in the plasma power from 50 to 90 W enhances the CO conversion apparently from 15 to 24%.

Figure 6

Influence of plasma power on FTS. (A) Conversion; (B) selectivity. (Pressure: 1 bar; flow rate: 50 ml/min; wall temperature: 373 K; H₂/CO in the feed: 1:1; frequency: 20 kHz.)

Figure 6B shows the influence of plasma power on the reaction selectivity over a supported Cu/Co catalyst. In terms of the effect of plasma power on the selectivity of FTS, one of the important observations was the absence of methane in the reaction under plasma conditions, which is different from the results obtained under conventional FTS conditions described in Figure 5B. It is suggested that the reaction channel between CO and H₂ over a Cu/Co-based catalyst could be modified in plasma. The reaction channel for CH₄ formation over the catalyst is strongly suppressed in plasma. When the plasma power was increased from 50 to 90 W, the selectivity to CO₂ increased from 5.1 to 19.6%. In contrast, the selectivity to higher hydrocarbons decreased from 94.9 to 80.4%. Higher plasma power favours the higher selectivity to CO₂, while lower plasma power is beneficial to higher selectivity to higher hydrocarbons over the supported Cu/Co catalyst under plasma conditions.

3.3.2 Effect of operating pressure

Operating pressure is an important parameter influencing the FTS under conventional conditions. Higher operation pressure can enhance the CO conversion in conventional FTS according to chemical equilibrium calculation. Under plasma conditions, the variation of operating pressure can influence the characteristics of DBDs. The electron energy distribution in the discharge strongly depends on electric field and gas density ratio, E/n. At a certain temperature, gas density is proportional to the gas pressure. So, it is possible to affect plasma chemical reactions in a DBD reactor by changing the feed gas pressure. As presented in Figure 7A, increasing the operating pressure decreases the conversion of CO apparently. For example, increasing the operating pressure from 1 to 6 bar resulted in the decreasing of CO conversion significantly from 24% to 8%. Our further study revealed that the parallelograms of voltage-current Lissajous figures became narrower at higher pressures, indicating that the discharge behavior changes with pressure at constant power. It is apparent that reaction operating pressure has a profound effect on FTS over the catalyst under plasma conditions. In contrast to conventional FTS, CO conversion is higher in plasma assisted FTS at low operating pressures.

Figure 7

Influence of operating pressure on FTS. (A) Conversion; (B) selectivity. (Wall temperature: 373 K; flow rate: 50 ml/min; H₂/CO in the feed: 1:1; power: 90 W; frequency: 20 kHz.)

Figure 7B presents carbon selectivity of the reaction under plasma conditions. Increasing the operating pressure results in a significant decrease in the selectivity to CO₂. It is important to note that there was no CH₄ detected in the pressure range tested. The formation of higher hydrocarbons is significantly enhanced at higher operating pressures.

From the above results, it is clear that the average electron energy plays an important role in the reaction over a Cu/Co supported catalyst. At lower operating pressures, the average electron energy is higher than that at higher pressures, so favoring a high conversion of CO and higher selectivity to CO₂. A higher operating pressure reduces the mean electron energy, decreases the conversion of feed gases, and is beneficial to get a high selectivity to lighter hydrocarbons. In the reaction, a suitable average electron energy is required to obtain high selectivity to higher hydrocarbons. Therefore, it is possible to control the FTS activity and selectivity over a supported Cu/Co catalyst by optimizing operating pressure.

3.3.3 Effect of H₂/CO molar ratio

The influence of feed gas composition on the reaction under plasma conditions by using a supported Cu/Co catalyst is depicted in Figure 8A. When the molar ratio of H₂/CO increased from 0.5 to 2, the conversion of CO increased from 21% to 38%. Similar to the FTS under convention conditions, higher H₂/CO ratio in the feed gas results in higher CO conversion under plasma conditions.

Figure 8

Influence of H₂/CO molar ratio on FTS. (A) Conversion; (B) selectivity. (Pressure: 1 bar; wall temperature: 373 K; flow rate: 50 ml/min; power: 90 W; frequency: 20 kHz.)

Our further experimental results demonstrate that CO conversion also increases with an increase in H₂/CO ratio at higher operating pressure (3 bar and 6 bar) at various dissipated plasma power (50 W, 70 W, 90 W).

The product selectivity versus the molar ratio of H₂/CO in the reaction is depicted in Figure 8B. The major products are higher hydrocarbons and CO₂. Only a small amount of CH₄ formed (1.6%) at the H₂/CO molar ratio of 0.5. We did not detect CH₄ at higher H₂/CO molar ratios in the outlet gas stream. Increasing the H₂/CO molar ratio leads to a substantial increase in the selectivity to higher hydrocarbons, while it leads to a decrease in the selectivity to CO₂. When the ratio is increased from 0.5 to 2, the selectivity to higher hydrocarbons increased significantly from 65.4% to 88.2%. It is possible to enhance the formation of higher hydrocarbons, and at the same time, inhibit the formation of CH₄ and CO₂ by optimizing the H₂/CO molar ratio of the feed gases in the reaction.

In conventional FTS, changes in CO₂ selectivity due to variation in H₂/CO molar ratio did not follow a particular trend and was attributed to the fact that the majority of CO₂ produced was as a result of the water-gas-shift (WGS) reaction. However, in plasma-assisted FTS, CO₂ selectivity decreases with an increase in H₂/CO molar ratio suggesting that CO₂ production could be a direct result of the plasma breaking down carbon monoxide molecules to produce CO₂ via the plasma-promoted Boudouard disproportion reaction (2CO=CO₂+C) on the surface of the catalyst. Another reaction channel for CO₂ formation could be a plasma-assisted WGS reaction (CO+H₂O=CO₂+H₂) under plasma conditions.

3.4 Mechanism of FTS over catalyst promoted by plasma

The FTS can be summarised by the following reactions:

Equations 1 and 2 describe the formation of hydrocarbons and methane, respectively. The main reaction is the formation of alkanes (CnH_2n+2):

Other products are alkenes (C_nH_2n) and alcohols (C_nH_2n+1OH):

Boudouard reaction: 2CO→C+CO₂

Catalyst reduction:

1. MxOy+y H₂ → yH₂O+xM

2. MxOy+y CO → yCO₂+xM.

Carbon monoxide can react with water vapour either present in the syngas or formed from equations 1 and 2 to form carbon dioxide and hydrogen as shown in equation 3.

From the point of the view of the mechanism under conventional FTS conditions, the Fischer-Tropsch reaction can be considered as a kind of polymerization. It contains 1) an adsorption and reactant activation step, 2) a chain initiation, 3) a propagation and 4) a chain growth termination. Fischer and Tropsch [1] supposed the dissociative adsorption of the carbon oxide on the metal atom, with formation of a carbide species. The insertion of the adsorbed dissociated hydrogen on this carbide generates the active CH₂- intermediate that is the key component for the propagation step. The growing alkyl chain can desorb from the metal by hydrogenation, forming hydrocarbons, mainly olefins. According to this mechanism, higher pressure favours the reaction. In order to promote the dissociative adsorption of carbon monoxide on the metal, and generate adsorbed dissociated hydrogen, higher temperature and higher pressure is necessary to overcome the energy barrier of the reaction.

To our knowledge, the plasma-assisted FTS over catalysts has not been attempted in the literature. On the basis of the above discussion, a FTS reaction mechanism promoted by non-thermal plasma could be proposed as shown in the following steps.

1. Plasma promoted H₂ dissociation, and plasma promoted CO dissociative adsorption on metal atom

   

   

   

2. Insertion of plasma dissociated hydrogen

   

3. Propagation

   

Under plasma conditions, the reaction is carried out via different routes from that under conventional FTS conditions. It is influenced apparently by plasma-activated species. Unlike the reaction mechanism under thermal conditions, the high energy electrons produced in non-thermal plasma can not only activate CO on the surface of the catalyst, but also activate hydrogen at low temperatures and ambient pressure. Because of the high dissociation energy of CO (11.16 eV), the plasma-promoted CO dissociative adsorption on the metal atom is the control step of FTS over the catalyst in plasma. The dissociative energy of H₂ is only 4.25 eV, indicating that H₂ can be dissociated easily in a DBD. The abundant plasma-activated hydrogen species generated in plasma can react with adsorbed and plasma-activated carbon monoxide to generate CH_x-M, which will influence the initiation, propagation, and chain growth termination steps of the reaction. It is also possible that the CO can react with the plasma-activated CO adsorbed on the catalyst, which can generate carbon-metal legend and CO₂. The carbon-metal legend can react with plasma-activated hydrogen to form CH_x-M. The carbon formation on the catalyst is strongly inhibited due to the formation of plasma-activated hydrogen. The reaction between oxygen species on the surface of the catalyst with CO can form CO₂, and with hydrogen can produce water.