Abstract · Web viewPlasma-assisted dry reforming of methane (DRM) was performed in a dielectric barrier discharge (DBD) reactor. The effect of different packing materials including

Plasma-assisted catalytic dry reforming of methane (DRM) over metal-organic frameworks (MOFs)-based catalysts

Reza Vakilia, Rahman Gholamia, Cristina E. Sterea, Sarayute Chansaia, Huanhao Chena, Stuart M. Holmesa, Yilai Jiaob, Christopher Hardacrea, Xiaolei Fana,[footnoteRef:1] [1: Corresponding author, Email address: [email protected]]

a School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

Abstract

Plasma-assisted dry reforming of methane (DRM) was performed in a dielectric barrier discharge (DBD) reactor. The effect of different packing materials including ZrO2, UiO-67 MOF and PtNP@UiO-67 on plasma discharge was investigated, showing that ZrO2 suppressed the plasma generation while UiO-67 improves it due to its porous nature which favours the formation of filamentary microdischarges and surface discharges. The improved plasma discharge increased the conversion of CH4 and CO2 by about 18% and 10%, respectively, compared to the plasma-alone mode. In addition, the distribution of hydrocarbon products changed from dominant C2H6 in the plasma-alone mode to C2H2 and C2H4 in the UiO-67 promoted plasma-assisted DRM. The UiO-67 MOF was stable in plasma, showing no significant changes in its properties under different treatment times, discharge powers and gases. Pt nanoparticles (NPs) on UiO-67 improved plasma-assisted DRM, especially the selectivity due to the presence of surface reactions. Due to the dehydrogenation of hydrocarbons over Pt NPs, the selectivity to hydrocarbons decreased by 30%, compared to the UiO-67 packing. In situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) was carried out to probe the surface reactions on PtNP@UiO-67 catalyst, showing the decomposition of surface formats to CO and C2H4 dehydrogenation over the metallic Pt. The PtNP@UiO-67 catalyst showed good reusability in the plasma-assisted DRM, and H2 production was improved by high CH4/CO2 molar ratio and low feed flow rate.

Keywords: Non-thermal plasma, Catalytic dry reforming of methane, Metal-organic frameworks (MOFs), Dielectric barrier discharge (DBD) reactor, in situ DRIFTS

Introduction

The ever increasing emission of greenhouse gases (GHGs), such as carbon dioxide (CO2) and methane (CH4), is the main reason for global warming. According to the Intergovernmental Panel on Climate Change (IPCC) in 2014, CO2 and CH4 accounted for 76% and 16% of the global human-caused emissions, respectively [1-3]. Therefore, the dominating GHGs need to be treated effectively and efficiently to avoid the consequences of the GHGs effect. For CO2, geological storage is one of the options to decrease its level in the atmosphere. However, this process is energy-intensive, and thus costly. In addition, there are many uncertainties with regard to long-term storage of CO2 in geological formations [4]. An alternative and more preferable way of reducing the amount of GHGs due to human actions is the conversion of them into other useful chemicals, enabling the circular economy.

So far, different catalytic processes, including dry reforming of methane (DRM) [5], CO2 hydrogenation to methane and methanol [6, 7] and reverse water gas shift (RWGS) [8, 9], have been proposed for CO2 utilisation (i.e. CO2 conversion to valuable chemicals). DRM (Eq. 1) is an attractive process from both environmental and industrial points of view due to the simultaneous conversion of two GHGs to syngas (i.e. H2 + CO) which is the platform for synthesising various chemicals and fuels, such as ammonia, methanol and dimethyl ether (DME) [5, 10-12].

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The catalysts employed for DRM are either supported noble metal catalysts (e.g. rhodium (Rh) and platinum (Pt)) or transition metal catalysts (e.g. nickel (Ni)). Although transition metal Ni catalysts are known as the most common catalysts for methane reforming, their use is associated with severe coke deposition, and hence rapid catalyst deactivation. Compared to the transition metal catalysts, noble metals show the relatively high catalytic activity and resistance to coke formation [13, 14]. For example, García-Diéguez et al. [15] compared the activity and stability of the supported Pt catalyst on Al2O3 with the Ni analogue in DRM reaction. Compared to the Ni catalyst, CO2 conversion was improved by ca. 10.5% over the Pt catalyst at 700 C. Conversely, due to coke deposition, Ni catalyst showed a decrease in CO2 conversion by 24% after 14 h of reaction, while no significant changes in the catalytic activity was observed for Pt catalyst. However, noble metals are less favourable for industrial use, due to their high cost and limited availability. Hence, efforts have been devoted to improve the activity and coke-resistance of Ni catalysts such as the development of bimetallic catalysts, in which noble metals are partially combined with nickel such as PtNi catalysts [16, 17].

In addition to the innovation and development of DRM catalysts, the development of novel processes for catalytic DRM is also being considered to improve the process. Non-thermal plasma (NTP) is a promising alternative to the traditional thermal activation of the reactions, especially thermodynamically limited ones [18, 19]. In a NTP, energetic electrons, accelerated by an electric field, collide with bulk gas molecules, causing bond breaking of the molecules and formation of highly reactive species (e.g. ions and free radicals). Since the electron mass is very low (<10−30 kg [20]), the NTP does not cause significant temperature rise for the bulk gas (by only few degrees). Accordingly, although the temperature of electrons can be at 104–105 K, the bulk gas temperature remains close to the ambient [21]. The non-equilibrium character of NTP is promising to activate many thermodynamically limited reactions at ambient temperature such as CO2 hydrogenation and CH4 oxidation, as well as DRM [22, 23]. NTP-assisted DRM has been demonstrated by different types of plasma such as dielectric barrier discharges (DBDs) [24, 25], corona discharges [26], glow discharges [27] and gliding arcs [28]. In general, NTP promoted non-catalytic gas-phase DRM is effective for producing syngas. However, the selectivity of the NTP-enabled gas-phase reactions towards the syngas is generally low. By combining NTP activation with heterogeneous catalysts, known as plasma-catalysis, not only the selectivity of the reaction but also the energy efficiency of the system can be improved appreciably [29]. For example, in the plasma-assisted catalytic DRM over K-Ni/Al2O3 catalyst, the H2 yield and total energy efficiency (i.e. the amount of the converted CH4 and CO2 per the used electrical energy) were improved up to 27% and 24%, respectively, compared to the plasma-alone case [30]. The observed improvement in the system resulted from the interaction between the plasma and catalyst which can modify both physical and chemical properties of the catalyst and plasma. For instance, Tu et al. [31] reported that the presence of TiO2 catalyst within a DBD reactor (N2 as the discharge gas) enabled spatially confined microdischarges, and hence transition to surface discharges on the catalyst. This caused a modification of the electron energy distribution in the plasma enhancing the concentration of higher energy electrons. In addition, the property of the catalyst can also be affected by the plasma discharges. For example, the oxidation state of Mn2O3 catalysts changed from +2 (Mn2O3) to co-existence of +2 and +3 (Mn3O4) after the plasma-assisted catalytic toluene oxidation in a DBD reactor. This low-valent Mn2O3 showed an improved oxidation capability, promoting toluene decomposition (i.e. the toluene conversion was measured as 88% and 51% for the plasma reaction with and without catalyst, respectively) [32]. These studies show the influences of the plasma on the catalyst and vice versa, as well as the improved catalysis.

To date, different catalysts (or packing materials) including ferroelectrics [33], Al2O3 [34], zeolites [35] and ceramic foams [36], have been used in the plasma-assisted DRM, showing that the reaction mostly occurred in the gas-phase rather than on the surface. For example, Krawczyk and co-workers showed that the plasma-alone could promote DRM in the gas phase in a DBD reactor with good CO2 and CH4 conversions of 35.7% and 19.5%, respectively, while the addition of different packing materials (e.g. Al2O3 and zeolites) only improved the conversions by <10% [34]. The findings suggest that, in the plasma-assisted DRM, chemical reactions predominantly occur in the gas phase, being influenced mainly by plasma properties. Accordingly, the influence of packing materials on the plasma-assisted DRM has been associated with their influence on properties of plasma. Ferroelectric materials with high dielectric constants (e.g. BaZr0.75T0.25O3, ε = 149) can improve the electric field, and thus electron energy distribution in the discharge zone, promoting the conversion of CH4 and CO2 [33]. Common porous materials used in plasma-catalysis as packing, such as zeolites and Al2O3, decrease the conversion of CH4 and CO2 in the plasma-assisted DRM due to a reduction in the discharge volume, and thus plasma generation. However, they change the system selectivity to hydrocarbons due to their porosity and morphology. For instance, compared to the plasma-alone DRM, zeolite 3A packing decreased the conversion of CH4 by about 75% in a DBD reactor at a discharge power of 30 W, while the selectivity to light hydrocarbons (C2H2 and C2H4) was remarkably enhanced by 86% because of the shape-selectivity incurred by zeolite 3A (0.3 nm pore size) [35]. It is worth mentioning that some studies show the significant improvement in CO2 and CH4 conversions in the plasma-catalytic DRM by supported metal catalysts. However, it is not always the case. For instance, Zeng et al. [37] reported an increase of 42% in CH4 conversion by Ni/Al2O3 catalysts in the plasma-catalytic DRM compared to the plasma-alone case, while Wang et al. [38] reported a decrease in the conversion by different catalysts (e.g. Cu/Al2O3, Au/Al2O3 and Pt/Al2O3) due to the suppression of plasma discharge. The inconsistent findings demonstrate the complexity of the plasma-catalysis system and the effect of catalysts on the reaction. Accordingly, the use of an appropriate catalyst, which can improve the plasma properties and selectivity to the target products, is of great importance for improving the performance of the plasma-assisted DRM. Most catalysts employed by the studies so far for the plasma-assisted DRM are conventional catalysts developed for the thermal systems. The bespoke design of new catalysts with novel features may be beneficial to further improve the process.

Metal-organic-frameworks (MOFs) are versatile porous materials for various applications, especially gas adsorption and separation [39, 40] and catalysis [41], due to their physical and chemical characteristics. Heterogeneous catalysis using UiO MOF-based catalysts (UiO for Universitetet i Oslo) has attracted great research interest due to the good thermal and chemical stability of UiO MOFs [42, 43]. Catalysts based on UiO MOFs have shown good performance in the conventional catalytic systems by thermal activation, such as liquid phase hydrogention and oxidation and gas phase carbon monoxide oxidation (CO) [42, 44]. In addition to their good stability, UiO MOFs have shown a good capability in CO2 and CH4 adsorption (ca. 17 mmol g−1 for CO2 and 6 mmol g−1 for CH4 at 25 C) [45], which may benefit the adsorption-enhanced CO2 conversion [30, 46]. It has also been reported that the use of porous materials, compared to non-porous materials, in a DBD plasma reactor can improve the performance of the system. Holzer and co-workers showed that, in comparison to non-porous Al2O3 packing, CO2 selectivity over porous Al2O3 was improved by ca. 33% in the plasma-assisted CO oxidation due to the extended residence time for intermediates in the discharge zone [47]. Considering the harsh conditions of thermal DRM (>500 C), catalysts based on UiO MOFs are unlikely to survive, and therefore NTP activation can be a suitable alternative to enable the use of MOF catalysts for DRM

Herein, we report the plasma-assisted DRM over UiO-67 MOF and its derivative catalyst in a DBD reactor. Comparative studies using different operating modes of plasma-alone, thermal and plasma-assisted catalysis were carried out to understand the interactions between the plasma and catalyst. The effect of packing materials (e.g. UiO-67 and ZrO2) on the plasma property was compared. Additionally, Pt nanoparticles (NPs) supported on UiO-67 MOF (i.e. PtNP@UiO-67) was also explored to improve the NTP-assisted DRM. In order to elucidate the existence of the surface reactions on PtNP@UiO-67, in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) characterisation was performed. The stability of the PtNP@UiO-67 catalyst was examined through multiple plasma on-off cycles, and the used catalysts were characterised by various techniques to evaluate the effect of plasma on their properties.

ExperimentalChemicals, synthesis and characterisation of materials

Terephthalic acid (BDC), ZrCl4, platinum(II) acetylacetonate (Pt(acac)2) and 4,4’-biphenyldicarboxylic acid (BPDC) were purchased from Arcos. Benzoic acid and zirconium oxide (ZrO2) were purchased from Sigma-Aldrich. N,N’-dimethylformamide (DMF) was obtained from Fischer Scientific. All chemicals were used as received, with no further purification.

UiO-67 MOFs were synthesised by a microwave-assisted method described elsewhere [45], using ZrCl4 , benzoic acid and 4,4’-biphenyldicarboxylic acid (BPDC) as the starting materials and DMF as solvent. The as-synthesised materials are subject to a workup procedure involving washing and activation as described in the previous work [45]. PtNP@UiO-67 catalysts were prepared by the wetness impregnation method in which Pt(acac)2 was used as Pt precursors. More information about the preparation of PtNP@UiO-67 is described elsewhere [44].

X-ray diffraction (XRD) of materials was carried out on a Rigaku Miniflex diffractometer using CuKα1 radiation (λ = 0.15406 nm, 30 kV, 15 mA). The measurement was performed over a range of 4° < 2θ < 45° in 0.05 step size at a scanning rate of 1° min−1. Scanning electron microscopy (SEM) was undertaken using a FEI Quanta 200 ESEM equipment using a work distance of 8‒10 mm and an accelerating voltage of 20 kV. All samples were dispersed in ethanol and dropped onto SEM grids, followed by the gold coating using an Emitech K550X sputter coater under vacuum (1×10−4 mbar) before SEM. Transmission electron microscopy (TEM) imaging was performed using a Philips CM20 operating at 200 kV. Samples were dispersed in ethanol and dropped onto carbon grids and dried prior to imaging. Nitrogen (N2) physisorption on materials at −196.15 °C was carried out using a Micromeritics ASAP 2020 analyser. Prior to the N2 adsorption, samples (~100 mg) were pretreated by degassing at 200 °C under vacuum overnight. The surface area and total pore volume of the materials were calculated based on the Brunauer-Emmett-Teller (BET) method and at relative pressure P/P0 of 0.99, respectively. The Pt content of the developed catalysts was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo iCAP 6000 SERIES). Before ICP-OES analysis, samples were digested in nitric acid solution overnight then solutions were analysed by ICP for the quantitative determination of the Pt content. It was measured that 2 wt.% Pt species were present in the catalyst.

NTP-assisted DRM

Fig. 1 shows a schematic diagram of the experimental rig. A DBD reactor consisting of two coaxial quartz tubes was used in this study. The outer electrode was an aluminium foil wrapped on the external surface of the outer tube (6 mm OD), and the inner electrode was a tungsten rod (0.5 mm) fixed at the centre of the inner tube (2 mm OD). The outer electrode was connected to a high voltage power supply and the inner electrode was grounded. The length of discharge zone was 20 mm, and the discharge gap was ~2 mm between the two tubes. The reactor was loaded with 70 mg of packing pellets (250‒400 µm) and sandwiched by quartz wool. To generate the plasma, variable voltages (6‒10 kVpk–pk, Vpk–pk: peak to peak voltage) at an optimised AC sine wave frequency of 30 kHz were applied across the discharge gap. The applied voltage was measured using a high voltage probe. All electrical signals were recorded using a digital oscilloscope (Rohde&Schwarz, HMO1002 series). For the reactions with a packing, the packing material was treated using Ar plasma at the discharge power of 11 W before the reaction.

Fig. 1. Schematic diagram of (a) the experimental rig and (b) the DBD plasma-catalytic reactor.

Diluted CH4 and CO2 (5,000 ppm CH4 in Argon (Ar) balance and 5,000 ppm CO2 in Argon balance) were fed into the DBD reactor at different flow rates (50–100 ml min−1) and CH4/CO2 molar ratios (0.5–1.5). The gas products were analysed by an in-line gas chromatography (PerkinElmer, Clarus 580 GC) equipped with the flame ionisation detector (FID) and thermal conductivity detector (TCD). Mass spectrometry (MS) of the outlet gas was also performed using an HPR20 QIC mass spectrometer (Hiden Analytical). During the experiments, the spectrometer continuously monitored the ion currents at a mass-to-charge ratio (m/e) of 36, 16, 28, 2 and 44, corresponding to signals of Ar, CH4, CO, H2 and CO2, respectively. All calculations regarding the reactant conversion (X), product yield (Y) and product selectivity (S) are defined in the Supporting Information (SI).

Catalytic DRM by thermal activation

The thermal catalytic DRM was carried out in a quartz tubular reactor (9.0 mm ID), placed inside a programmable furnace at atmospheric pressure, with a flowrate of 100 ml min−1 (5,000 ppm CO2, 5,000 ppm CH4 balanced by Ar). 70 mg of the pelletised catalyst (i.e. PtNP@UiO-67) were packed into the reactor and sandwiched between quartz wool. Prior to the reaction, the catalyst was treated for 1 h in a reducing environment in 10 vol.% H2 in Ar at a total flowrate of 100 ml min−1 at 280 °C. The reaction temperature was ramped from room temperature to 400 °C at a heating rate of ca. 8 °C min−1. The bed temperature was measured and recorded by a K type thermocouple adjacent to the catalyst bed. After each run, the furnace was turned off automatically to allow the reactor to cool down to room temperature under Ar flowed at 100 ml min−1.

In situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) characterisation

In situ DRIFTS characterisation of DRM over the PtNP@UiO-67 catalyst under plasma was measured using a Bruker Tensor 70 FTIR spectrometer (resolution of 4 cm−1) using a specifically designed in situ flow cell with the plasma generated in the catalyst bed using a modified Spectra Tech Collector II DRIFTS accessory [19]. The catalyst was reduced in a reducing environment with H2/Ar flow (10 vol.% H2, the total flowrate of 50 ml min−1) under plasma at room temperature. The spectrum of the reduced sample was used as the background reference for the subsequent plasma-assisted DRM reaction. The reaction was performed using a mixture gas of CH4 (5,000 ppm) and CO2 (5,000 ppm) in a total flow rate of 50 ml min−1 balanced with Ar. The plasma generator was an a.c. power source (PVM500 model) and the electrical parameters were monitored using an oscilloscope (Tektronix TBS1062), connected to the DRIFTS cell through a high-voltage probe (Tektronix, P6015). The applied voltage was 6 kV at a frequency of 23 kHz.

Results and discussionStability of UiO MOF in NTP

NTPs have been shown to be to maintain a MOF’s stability, even under challenging conditions, for example in the presence of water as reported for Cu-HKUST-1 during the NTP-activated water gas shift reaction [19]. In this work, the stability of UiO-67 MOF in plasma was assessed as well after the treatment with 5 vol.% CH4 in Ar, 5 vol.% CO2 in Ar and pure Ar flowed at 100 ml min−1 at a discharge power of 11 W for 2 h. Figs 2a–2c present the XRD and SEM analysis of the post-plasma treated UiO-67 MOFs in comparison to the as-synthesised material and show comparable X-ray diffraction patterns and morphologies. As seen in Fig. 2d, after the plasma treatments, the treated UiO-67 MOFs show an increased porosity in comparison to the pristine UiO-67, as evidenced by BET analysis (Table S1), e.g. total pore volume: 0.87 cm3 g−1 for the fresh UiO-67 versus 0.9 cm3 g−1 for the Ar plasma treated UiO-67. Since deterioration of crystallinity and morphology was not observed for the plasma-treated MOFs, the improved N2 adsorption ability can be attributed to the plasma-assisted desolvation of DMF and ethanol, as well as the removal of uncoordinated monocarboxylate ligands from the framework. By comparing the relevant micropore and mesopore volumes of the fresh MOF and plasma-treated MOFs, as shown in Table S1, the absolute increment of the micropore volume is higher than that of the mesopore volume, suggesting the desolvation which renders more micropores available. Similar effect by Ar plasma on HKUST-1 MOF was also reported previously [19]. The effect of the plasma discharge power the treatment time on the stability of UiO-67 was also investigated, and the two parameters showed no significant effect on the crystallinity of the treated materials (Fig. S1).

Fig. 2. (a) XRD patterns; SEM images of UiO-67 samples (b) before and (c) after the Ar plasma treatment (at 11 W for 2 h); (d) N2 adsorption isotherms of UiO-67 samples after 2 h plasma treatment under various gases at the discharge power of 11 W.

Effect of packing materials on NTP-assisted DRM

In this study, diluted CO2 and CH4 with Ar balance was used since the presence of the dilution inert gases such as Ar and He can enhance plasma chemistry. The addition of dilution gases lowers the breakdown voltage, enabling the plasma initiation at relatively low voltages (e.g. the breakdown voltage decreased from 3.4 kV to 3 kV by increasing the helium mole fraction from 0.7 to 0.8 in a CO2/CH4/He mixture [48]) and the efficient use of the applied power for conversing the reactants [49]. Additionally, the plasma-excited Ar species can also transfer the energy to CO2 and CH4 molecules via inelastic collisions, promoting their dissociations and consequently conversions [48, 50]. Fig. 3 shows the electrical signals in the discharge zone at frequency of 30 kHz and the voltages of 6 kVpk–pk and 10 kVpk–pk, when different packing materials were placed in the discharge zone. As shown, the total current of discharges in absence of the packing (i.e. empty tube) is quasi-sinusoid with numerous superimposed current pulses. These current pulses correspond to filamentary microdischarges, generated over the dielectric surface and extended across the discharge gap. When the reference ZrO2 packing was used, the intensity of current pulses and plasma generation reduced due to the reduction of the void fraction in the discharge gap for generating filamentary microdischarges. Accordingly, with the reference ZrO2 packing, the discharge mode changed from the filamentary microdischarges to a combination of spatially limited microdischarges and predominant surface discharges on the surface of packing [31]. Interestingly, placing UiO-67 pellets in the discharge zone increased the intensity of current pulses, suggesting the improved plasma generation (i.e. the improved discharge, Fig. 3). MOFs-enhanced plasma generation in the DBD reactor can be attributed to (i) the filamentary microdischarges are maintained due to highly porous nature of MOFs (which does not reduce the available void space) and (ii) the formation of surface discharges on MOFs due to the high surface area of UiO-67. Findings of this work are in line with the previous report [51], in which a microscope-intensified charge coupled device (ICCD) camera was used to observe the plasma generation on porous packing (i.e. quartz-wools with different pore sizes of 2–50 μm) in reference to an empty tube (in a DBD reactor with CH4/CO2 gas flow), showing the improved plasma discharge facilitated by porous materials. It should be mentioned that the formation of microdischarges inside the pores of dielectric materials has been reported previously. Zhang et al. [52] showed the theoretical plasma generation in porous materials with a pore size of 10 µm and different dielectric constants. For the low dielectric constant (ε = 25), the formation of microdischarges was observed inside pores. However, by increasing the dielectric constants (ε > 300), the plasma generation was most pronounced in the sheath and was negligible inside pores due to the polarisation of both sidewalls of the pores. It was concluded that the most commonly used porous materials in catalysis (e.g. Al2O3 and SiO2), which have lower dielectric constants (ε = 9 and 4.2, respectively), should allow the microdischarge formation inside their pores. Since UiO MOF has a low dielectric constant (ε = 1.73 [53]), it is hypothesised that the microdischarges can be generated inside its pores (pore size = 1.6 nm).

Fig. 3. Electrical signals in a DBD plasma reactor at the frequency of 30 kHz and the voltage of (a) 6 kVpk–pk and (b) 10 kVpk–pk with different packing materials in the discharge zone (CH4/CO2 = 5,000 ppm/5,000 ppm, total flow rate = 100 ml min−1 balanced with Ar).

NTPs in the absence of a packing material can promote DRM significantly in the gas phase without a heat source, resulting in the conversion of CH4 and CO2 at 44% and 35%, respectively (at power of 11 W). By adding different packing materials in the discharge zone of the NTP system, both the conversion and selectivity was found to be improved as expected (Fig. 4). When UiO-67 packing was used, the conversion of CH4 and CO2 was enhanced by about 18% and 10%, respectively, compared to plasma-promoted gas phase reactions. The findings are in good agreement with Gallon and co-workers’ work where the plasma-alone mode could promote the DRM reaction, and the addition of quartz wool increased the conversion of CH4 by about 22% at a discharge power of 11 W, due to the improved plasma generation [35]. Conversely, the use of ZrO2 packing reduced the conversion slightly which was attributed to the reduction in plasma generation. In order to have a better comparison between different packing materials, two criteria, i.e. total energy efficiency (E, Eq. 2) and synergy capacity (SC, Eq. 3), were used [54]. In Eq. 3, superscripts pc, p and c represent the CH4 conversion obtained from plasma-catalyst, plasma-alone and catalyst-alone (i.e. thermal activation) systems, respectively. The effect of the packing materials on the system is reflected by comparing the synergy capacity values (SC, as shown in Table S2), in which the case of ZrO2 presents a negative SC value of −1.8%, whereas UiO-67 MOFs gives 7.8%.

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Fig. 4. Effect of DBD packing on (a) energy efficiency and conversions of CO2 and CH4, (b) selectivity to CO, H2 and hydrocarbons, (c) selectivities to C2 and C3 hydrocarbons and (d) yields of CO and H2 (at a discharge power of 11 W, feed flow rate of 100 ml min−1 and CH4/CO2 molar ratio of 1).

In this study, light hydrocarbons (e.g. C2 and C3) were also produced from the plasma-assisted DRM. As seen in Fig. 4b, the selectivity to C2–C3 hydrocarbons was 29% for the plasma-alone case (i.e. the plasma-promoted gas-phase reactions), and it decreased to 20.5% when UiO-67 was packed in the discharge zone, suggesting the effect of the packing on the methane coupling reactions in the gas phase.

Scheme 1 illustrates the possible pathways for plasma-assisted DRM [55-57], which are initiated by plasma dissociation of CH4 and CO2 (Table S3, Eqs. S9–S17) to produce CHx (x = 1, 2 and 3) and O radicals. Recent simulation results revealed that electron-induced dissociation of CH4 leads to 79% CH3 radical formation and only 15% and 5% CH2 and CH radicals, respectively [58]. The excited Ar species (i.e. Ar*) can participate in the dissociation of CO2 and CH4 by the Penning dissociation phenomenon [59]. Ionisation of Ar requires a higher electron energy of 15.7 eV than that needed for its excitation (i.e. 11.5 eV) [60]. Therefore, in plasma, Ar is mostly excited to its metastable state rather than being ionised. Due to the higher energy of the metastable Ar than the dissociation energy of CH4 and CO2 (4.5 eV and 5.5 eV, respectively [61]), the inelastic collisions between them lead to the Penning dissociation of CH4 and CO2, and hence the formation of CH3 and O radicals according to Eqs. S18 and S19 [59]. After the generation of the reactive radicals, various reactions (Eqs. S20–S30) can proceed in the gas phase, from which syngas and light hydrocarbons are formed. Previous studies on the plasma-assisted non-oxidative methane coupling showed that the electron energy of the NTP system played a crucial role in the selectivity, and the degree of hydrocarbon ionisation depended on the input energy. Specifically, a low electron energy (<6 eV) promotes the formation of ethane (C2H6) and propane (C3H8) while a high electron energy >13 eV encourages the selective formation of acetylene (C2H2). Therefore, in a DBD reactor where the electron energy is 5–10 eV, C2H6 is the primary hydrocarbon produced [62]. As shown in Fig. 4c, C2H6 was measured as the dominant hydrocarbon in the plasma-alone system in this work. Placing ZrO2 in the discharge zone weakened the plasma discharge, and thus supressed the electron energy, alleviating the electron dissociation of C2H6 in to C2H4 (Eq. S30). Accordingly, C2H6 continued to react with O, H and OH radicals to form C2H5 radical (Eqs. S25–S27) which was later recombined with CH3 radical to form C3H8 (Eq. S28). Compared to the plasma-alone system, the system using the ZrO2 packing enhanced the selectivity to C3H8 from 4% to 8.5% (Fig. 4c). Interestingly, the use of UiO-67 packing changed the distribution of hydrocarbon products significantly with C2H2 and C2H4 as the dominant products (i.e. the selectivity to C2H2/C2H4, C2H6 and C3H8 was measured as 11%, 8.5% and 1.5%, respectively). It agrees well with the observed improvement in plasma discharge which promoted the reactions (Eqs. S29 and S30) towards C2 hydrocarbons production. It is noteworthy that the conversion of CH4 was greater than that of CO2 in the plasma-assisted DRM, whereas in the thermal system the opposite is true (as seen in Fig S2). It might be associated with either their different dissociation energies (4.5 eV for CH4 and 5.5 eV for CO2 [61]) or CO2 re-production through water gas shift (WGS) reaction.

Scheme 1. Main reaction pathways in plasma-assisted catalytic DRM. Products and intermediates are shown in black (or green) and gray, respectively. Metastable Ar is shown in blue.

Investigations were also conducted to study the influence of the input discharge power on the three systems for DRM (Fig. S4). The results show that the plasma system with UiO-67 packing outperformed the other two systems (regarding the conversions and production of syngas) at all input powers. The increase of the input power led to an approximately linear increase in the conversion of CH4 and CO2 (Figs. S2a and S2b). This is expected since an increase in the plasma power (at a constant frequency) increases the electric field and electron density and promotes and the dissociation of reactants, which contribute to the intensification of DRM reaction [21].

Effect of PtNP@UiO-67 catalysts on plasma-assisted DRM

In order to understand the effect of DRM catalysts on the plasma-assisted DRM, the performance of DRM was compared under three different systems, i.e. the plasma-alone (gas phase reactions), catalyst-alone (by thermal activation) and plasma-catalyst system. Pt NPs catalyst was employed in this work due to its good performance in catalytic DRM (by thermal activation). TEM analysis of the fresh PtNP@UiO-67 catalyst (Fig. S5a) shows that the reduced Pt NPs dispersed well on UiO-67 with an average particle size of 2.1 nm.

The thermal DRM over the PtNP@UiO-67 catalyst was carried out in a plug flow reactor at 400 C. As depicted in Fig. S2, conversions of CO2 (7%) and CH4 (5%) were low in the thermal catalytic DRM due to the thermodynamic limitation of DRM at low temperatures (higher temperatures at >400 °C were not attempted due to the thermal stability limitation of UiO-67 MOF according to TGA [45]). As shown in Fig. 5a, by combining the PtNP@UiO-67 catalyst with plasma, the performance of DRM regarding conversions was further improved to 56% for CH4 and 43% for CO2, respectively (the effect of the catalyst reduction method on the NTP-assisted catalysis was found insignificant, as shown in Fig. S3), which may be associated with the surface reactions (Eqs. S13–S17), assisted by Pt NPs. This enhancement in CH4 dissociation resulted in an increase in (i) CHx radicals and (ii) H2 production (Eq. S20). In comparison with the plasma system using the UiO-67 packing, the plasma-PtNP@UiO-67 system suppressed the reactions towards hydrocarbons and reduced the selectivity to C2–C3 hydrocarbons by ca. 30% (Fig. 5b). The suppression of hydrocarbon production may be attributed to the dehydrogenation of C2H6 and C2H4 (Eqs. S29–S30) over Pt NPs [63], increasing the selectivity to H2 in the system (Fig. 5b). On the other hand, the selectivity to CO decreased by ca. 11% in comparison with the plasma system using the UiO-67 packing. Since the CO2 conversion did not change considerably by adding PtNP@UiO-67 catalyst (i.e. CO2 conversion was measured as 39% and 43% for the UiO-67 and PtNP@UiO-67 packings, respectively), it is hypothesised that CO2 was re-produced by WGS reaction. As a result, the PtNP@UiO-67 catalyst increased the H2/CO molar ratio from 0.85 to 1.1, compared to the UiO-67 packing (Fig. 5c). These findings demonstrate that the PtNP@UiO-67 catalyst intensified the dissociation and dehydrogenation reactions. Fig. 5a shows an increase in E from 0.48 mmol kJ−1 in the plasma-alone system to 0.6 mmol kJ−1 in the plasma-catalyst system, showing the performance improvement due to the synergy between the catalyst and plasma, which was also evidenced by the changes in SC (Table S2).

Apart from the activity and selectivity of the used catalysts, the carbon deposition during the plasma-assisted catalytic DRM, associated with methane coupling reactions, was assessed as well. The system under study did not experience carbon deposition, which was evidenced by the insignificant colour change of the catalyst bed and the reactor wall, as shown in Fig. S6. Additionally, the carbon balance (Bcarbon (%), Eq. S8) of the system using the UiO-67 and PtNP@UiO-67 packing were measured as ~94% and ~93%, respectively, suggesting the insignificant carbon deposition in the systems under study. The carbon deposition on the UiO-67 packing was also evaluated at higher concentrations of CH4 and CO2 as reported in Table S4. The result indicated that, by increasing the concentrations of CO2 and CH4 from 5,000 ppm (0.5%) to 15,000 ppm (1.5%), the carbon balance slightly reduced from ~94% to ~92%, respectively. It worth noting that the obtained values for the carbon balance at the different concentrations of CO2 and CH4 are in line with the previous studies (with pure CH4/CO2 mixtures), in which the carbon balance of >90% was obtained [18, 64]. These findings confirm the ability of plasma to suppress the carbon deposition on the catalyst. The crystallinity of the materials after the reaction was checked by XRD, showing the stability of the UiO-67 packing under plasma at the different concentrations of CO2 and CH4 (Fig. S1c).

As seen in Table S4, although an increase in the concentration of CO2 and CH4 reduced the conversions, the energy efficiency of the system increased (i.e. by increasing the concentration of CO2 from 0.5% to 10%, the conversion decreased from 39% to 19.5%, while the energy efficiency of the system increased by ca. 40%). This can be explained by the improved conversion of CO2 and CH4 in the feed flow. On the other hand, as discussed above, an increase in the concentration of CO2 and CH4 leads to an increase in the breakdown voltage, which can be used to initiate the plasma with the improved discharge and mean electron energy. For example, Ramakers et al. [49] reported that the discharge in pure CO2 was relatively low (i.e. the low intensity of current peaks) compared to a CO2/Ar mixture (vol.% = 5/95). Therefore, the Ar dilution in the feed flow influence the reaction considerably which should be assessed carefully for further development.

Fig. 5. Effect of different catalysts on (a) conversions and energy efficiency, (b) H2/CO molar ratio and (c) selectivity of CO, H2 and hydrocarbons during the plasma-assisted catalytic DRM (at a power of 11 W, feed flow rate of 100 ml min−1 and CH4/CO2 molar ratio of 1).

In order to reveal the surface reactions, in situ DRIFTS was carried out using the PtNP@UiO-67 catalyst. In situ DRIFTS characterisation of plasma-assisted catalytic DRM was performed at 6 kV (the application of the exact voltage and frequency used by the DBD reactor to the DRIFTS rig is challenging due to the possibility of arcing).

Fig. 6a shows in situ DRIFTS spectra when the reduced catalyst was exposed to the feed gas flow under the plasma-off condition. The peaks related to the gas-phase CH4 and CO2 can be easily seen at wavenumbers of 3015 cm−1, 1,305 cm−1 and 2,360 cm−1, respectively. The adsorption of CO2 on the UiO-67 surface caused the formation of various surface species including monodentate and bidentate carbonates (at 1,515 cm−1 and 1,550 cm−1, respectively) [65]. By ignition of the plasma (i.e. plasma on), a wide peak ranging from 1,900 cm−1–2,100 cm−1 was observed, corresponding to CO adsorbed on metallic Pt surfaces (Fig. 6b). In addition, relatively small peaks at 2,115 cm−1 and 2,150 cm−1, which belong to gaseous CO, were observed after the plasma ignition, showing the initiation of DRM reaction [66].

Fig. 6. in situ DRIFTS spectra of (a) whole adsorbed species, (b) CO adsorbed and (c) carbonates and formates adsorbed on PtNP@UiO-67 catalyst during plasma-assisted DRM.

More importantly, the ignition of plasma improved the formation of surface carbonates. As seen in Fig. 6c, after 1 min of the plasma ignition, new vibrational bands at 1,590 cm−1and 1,550 cm−1, which are characteristic of formates and carbonates, respectively, were measured [65]. Simultaneously, absorbed CO on the metallic Pt appeared at 1,900–2,100 cm−1. The results showed that the dissociated CO2 was adsorbed on the solid surface in the form of carbonates with different degree of coordination, and then decomposed to formates. The adsorbed formates further decomposed to CO and OH on Pt NPs [67], confirming the existence of surface reactions on the PtNP@UiO-67 catalyst in the plasma-assisted DRM. 5 min after the plasma ignition, the formation of formates, carbonates and CO were stabilised, showing that the plasma-assisted DRM reached the steady-state. In addition to the decomposition of the formates on metallic Pt NPs, the bands appearing at 1,415 cm−1 and 1,610 cm−1 revealed the dehydrogenation of C2H4 on the catalyst surface (Fig. 6c). The vibrational band at 1,415 cm−1 is assigned to π–bonded or σ–bonded CH2=CH2 [68, 69], showing that the produced C2H4 in the gas-phase reactions (Eqs. S23 and S30) was adsorbed on the catalyst surface during the plasma-assisted DRM. Subsequently, the adsorbed C2H4 was dehydrogenated to C2H3 intermediates (observed at 1,610 cm−1) on Pt NPs. The peak intensity of the adsorbed C2H3 was weak as it was rapidly dehydrogenated to C2H2 [69]. The vibrational band of gas-phase C2H2 should appear at 3,315 cm−1 and 3,226 cm−1; however, it could not be detected due to the overlapping with C-H vibrational band of CH4. Therefore, in situ DRIFTS measurements clearly proved that PtNP@UiO-67 intensified the surface reactions in the plasma-assisted DRM.

The PtNP@UiO-67 catalyst is stable in the plasma-assisted DRM system, as evidenced by its activity in the repeatability test (Fig. 7). Fig. 6a depicts MS signals attributed to CH4, CO, H2 and CO2 in four successive plasma on-off cycles. The measured conversions of CH4 and CO2 showed no signs of decline during the experiments (Fig. 7b). The used catalyst was characterised by SEM and XRD, showing that the morphology (Fig. S7a) and crystallinity (Fig. S7b) of the PtNP@UiO-67 catalyst remained intact after the cyclic plasma-assisted DRM. TEM analysis of the used catalyst (Fig. S5) shows that the sintering of Pt NPs was prevented as well (the avarage size of 2.4 nm).

Fig. 7. (a) Mass spectroscopy (MS) results and (b) reactant conversions of the plasma on-off cycles during the plasma-assisted DRM using the PtNP@UiO-67 packing (feed flow rate: 100 ml min−1, discharge power: 11 W, CH4/CO2 molar ratio = 1).

The influence of operating parameters such as CH4/CO2 molar ratio and feed flow rate on the performance of the plasma-assisted DRM was studied over the PtNP@UiO-67 catalyst. Fig. 8a shows the conversions of CH4 and CO2 for different CH4/CO2 molar ratios at a constant discharge power of 11 W. The conversion of CH4 reduced from 67% to 42% by increasing CH4/CO2 ratio from 0.5 to 1.5, whereas the conversion of CO2 increased from 40 % to 62%. This is thought to be due to the relatively concentrated CO2 in the feed enhancing the conversion of CH4 due to increased oxygen radicals (due to the plasma-induced CO2 dissociation, Eq. S13) can attack CH4 molecules. Fig. S8 also shows that the selectivity towards H2 and CO was strongly influenced by the presence of CO2 in the feed. The highest selectivity to H2 and yield of H2 were obtained at the CH4/CO2 ratio of 1.5 (78% and 50%, respectively). By increasing the CH4/CO2 ratio, the generation of H and CHx radicals (Eqs. S9-S11, which are the main intermediates for H2 production) in the system was improved (due to the disassociation of CH4), and thus promoting the production of H2. Conversely, the selectivity to CO and CO yield decreased from 88% to 45% and from 48% to 26%, respectively, by increasing the CH4/CO2 ratio from 0.5 to 1.5 (Fig. S8). An increase in the CH4/CO2 ratio reduced the CO2 availability in the system, and hence the selectivity to CO and yield of CO. Accordingly, it is concluded that the H2/CO ratio can be controlled by adjusting the CH4/CO2 molar ratio in the feed gas.

Table 1 presents the influence of the total flow rate on the reaction at a constant power of 11 W and CH4/CO2 ratio of 1. Increasing the feed flow rate decreased the conversion of CH4 and CO2 because of the decrease in the residence time of the reactants in the discharge zone. By the decreasing the feed flow rate, the selectivity of reactants was almost constant while the yield of CO and H2 increased. The results suggest that a lower feed gas flow rate is more beneficial for improving the conversion of CH4 and CO2 and producing more H2 and CO.

Fig. 8. Effect of CH4/CO2 molar ratio on (a) conversion of CO2 and CH4 and (b) H2/CO molar ratio in the plasma-assisted DRM with the PtNP@UiO-67 catalyst (discharge power: 11 W, feed flow rate: 100 ml min−1).

Table 1

Effect of the feed flow rate on the plasma-assisted DRM with the PtNP@UiO-67 catalyst at a constant power of 11 W and the CH4/CO2 ratio of 1:1.

Feed flow rate

(ml min−1)

Conversion

(%)

Selectivity

(%)

Yield

(%)

H2/CO molar

ratio

CH4

CO2

H2

CO

H2

CO

50

66

51

61

66

42

40

1.01

100

56

43

62

64

34

31

1.1

200

39

27

63

68

28

29

0.95

Conclusion

In summary, plasma-assisted dry reforming of methane (DRM) was studied in a DBD reactor with different packing materials including ZrO2, UiO-67 MOF and PtNP@UiO-67 catalyst. Without the packing (i.e. plasma-alone), the NTP was very capable of promoting the gas-phase reactions (with CO2 and CH4 conversion of 35% and 44%, respectively). By using pellet packing of ZrO2 and UiO-67 in the DBD reactor, the plasma properties were altered. Specifically, in comparison with ZrO2, the highly porous UiO-67 improved the plasma generation, favouring the formation of filamentary microdischarges and surface discharges in the discharge zone, and thus the enhanced CO2 and CH4 conversions. Consequently, the improved plasma generation altered the distribution of hydrocarbon products from abundant C2H6 in the plasma-alone mode to C2H2 and C2H4 in the plasma-assisted DRM using the UiO-67 packing (i.e. the selectivity to C2H2 and C2H4 increased from 8.5% in the plasma-alone mode to 11% in the plasma-catalysis mode by UiO-67).

The UiO-67 framework was also stable under different plasma conditions. Accordingly, the effect of the PtNP@UiO-67 catalyst (2 wt.%) on the plasma-assisted DRM was studied, showing the comparatively best conversion (of CO2 and CH4) and selectivity to hydrogen. Under thermal activation at 400 °C, the PtNP@UiO-67 catalyst was not very active, giving insignificant conversions of CO2 and CH4 at <10%. The presence of Pt NPs promoted the dissociation of CH4 and CO2 on the catalyst surface in plasma, leading to an increase in H2 production. Furthermore, compared to UiO-67, the PtNP@UiO-67 catalyst decreased the selectivity to light hydrocarbons by ca. 30% which was attributed to the dehydrogenation of C2H6 and C2H4 over Pt NPs. In situ DRIFTS characterisation was performed to show the existence of surface reactions on Pt NPs, and the relevant findings indicated the decomposition of formates to CO as well as the dehydrogenation of C2H4 over the surface of PtNP@UiO-67 catalyst. The findings showed that the PtNP@UiO-67 catalyst intensified the surface reactions in plasma-assisted DRM, improving synergy capacity and total energy efficiency by ~51% and ~11%, respectively, compared to the UiO-67 packing.

The PtNP@UiO-67 catalyst also demonstrated a good stability in four successive plasma on-off cycles, showing a stable catalytic performance (concerning the CO2 and CH4 conversions), as well as the unaffected morphology and crystallinity (confirmed by SEM and XRD), respectively. Additionally, TEM analysis of the used catalyst also showed insignifiacnt Pt NPs sintering during the reaction. With the PtNP@UiO-67 catalyst, the influence of different operating parameters, such as the feed flow rate and CH4/CO2 molar ratio, on the plasma-assisted DRM was also investigated, showing the improved performance by decreasing the feed flow rate and the enhanced H2/CO ratio by increasing CH4/CO2 molar ratio.

Acknowledgement

RV acknowledges The University of Manchester President's Doctoral Scholar Award for supporting his PhD research. HC thanks the financial support from the European Commission Marie Skłodowska-Curie Individual Fellowship (748196) for his research.

Supporting information

Supplementary data associated with this article can be found in the online version at http://

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1

1

42

molesofCHconverted+molesofCOconverted

(mmolkJ)

Dischargepower(W)

-

=

E

444

CHCHCH

()

=-+

pcpc

SCXXX

1

422

CH+CO2H+2CO247 kJ mol

D

-

®=

o

H