Syngas production via microwave-assisted dry reforming of methane

Energy-e


Introduction
Sustainable development in chemical processes includes the utilization of renewable energy resources.Unfortunately, fossil-based fuels are still the dominant primary source of energy.Our dependency on the fossil fuels increases the anthropogenic emission of carbon dioxide, which is the major part of the greenhouse gases [1].The global carbon dioxide concentration in the atmosphere has already surpassed 400 ppm according to the recent data from the Earth System Research Laboratory (ESRL) at the National Oceanic & Atmospheric Administration (NOAA) [2].The report of the Intergovernmental Panel on Climate Change (IPCC, 4th assessment report 2016), declares the safe level of CO 2 concentration to be below 350 ppm.
Dry reforming of methane (DRM, reaction (1)) presents one of possible ways to valorize CO 2 and convert it to a useful product: the synthesis gas also known as syngas.Compared to other methods for making syngas from methane (steam reforming or partial oxidation), DRM offers lower production ratio of H 2 and CO.This is beneficial for the production of liquid hydrocarbons, considering the subsequent treatment of syngas, via Fischer-Tropsch synthesis.
Nevertheless, due to the high thermal stabilities of CH 4 and CO 2 (high bond-dissociation energies of CH 3 eH and C]OeO are 435 and 532 kJ/mol, respectively), the DRM presents a difficult, highly endothermic process.It requires high operating temperatures, e.g., 700-1000 °C, and low pressures to reach desirable conversion levels of CH 4 and CO 2 [3][4][5][6].Nikoo et al. [7] studied the thermodynamics of DRM focusing on the aspect of carbon formation.They state that DRM reaction to form syngas is the favored reaction, particularly at a temperature above 727 °C.That reaction is typically accompanied by the simultaneous reverse water-gas shift reaction (RWGS, reaction (2)).On the other hand, there are important side reactions which are responsible for the carbon deposition and affect the production ratio of H 2 and CO 2 , such as methane decomposition (reaction (3)) and carbon dioxide disproportionation, the so-called Boudouard reaction (reaction (4)).At higher temperatures, reaction (3) is more likely responsible for the carbon formation and reaction (4) tends to occur below 527 °C and can be influenced by equilibrium limitations at the higher temperature [7]. (2) (3) A variety of metal-based catalysts, including noble (Pt, Pd, Rh, Ru, and Ir) and transition metals (Co, Ni, and Fe), have been studied in the DRM reaction [4,8,9].Ni-based catalysts are one of the most frequently studied transition metal catalysts, due to their high reactivity with methane, low-cost and large abundance.However, thermodynamically inevitable carbon deposition on active surfaces of Ni decreases the performance of the catalyst during the process [10,11].
The DRM is an energy demanding but environmentally important process.Conventional electrical heating is the most widely used energy source in the studies of DRM.However, supplying the required heat with conventional heating is challenging and energetically inefficient, since the whole system including gas streams must be heated.On the other hand, microwave-susceptible catalysts and/or catalyst supports can be selectively heated by microwaves in an energy-efficient way.
The microwave heating mechanism changes depending on the composition of the materials.In this regard, the proper material selection is crucial.If the material of interest is not/or moderate microwave-susceptible, a mixture of this material with microwave-susceptible materials, e.g., silicon carbide (SiC) and carbon, can be prepared in order to reach the desired reaction temperature.The detailed information on the interaction between microwaves and solid materials can be found elsewhere [18][19][20].
Several researchers have performed the DRM reaction under microwave heating [21][22][23][24].Fidalgo et al. [21] studied microwave-assisted dry reforming of methane over activated carbon.Here, activated carbon acted as catalyst and microwave-susceptible material.The DRM reaction accompanied by simultaneous CO 2 gasification of carbon reaction ( + ⇌ C CO CO 2 ) was carried out at temperatures 700-800 °C.After 6 h operation (CH 4 :CO 2 = 1, WHSV = 320 mL/g/h, 800 °C), the conversions of CH 4 and CO 2 dropped by 20% and 30%, respectively.This was due to the non-recovered active centers of Ni catalyst during the DRM process.Fidalgo et al. also observed that CO 2 gasification of carbon support was more pronounced in the microwave heating.This was confirmed by the weight loss of the catalytic bed, in situ carbon gasification with CO 2 .The same group [23] also studied the Ni/Al 2 O 3 catalyst in the microwave-assisted dry reforming of methane.Due to the low microwave-susceptibility of Ni/Al 2 O 3 catalyst itself, the group prepared a mechanical mixture of Ni/Al 2 O 3 with a microwave-susceptive materialcarbon.The addition of carbon improved both the heating of the fixed-bed and conversions of CH 4 and CO 2 .One can therefore speak about carbon bi-functionality in the system, as a microwave-susceptible material and as a catalyst.
Recently, Gangurde et al. [25] synthesized a ruthenium-doped SrTiO 3 perovskite catalyst and applied it in the DRM reaction.The synthesized Ru/SrTiO 3 showed high microwave susceptibility and stable catalytic behavior.As a continuation, the same group [26] studied a range of mechanical mixtures of Ru/SrTiO 3 and different commercial and relatively cheap nickel supported metal oxides, in order to improve the overall microwave heating properties.In this work, we studied microwave-assisted DRM on Pt/C and Ni/Al 2 O 3 catalysts, and two silicon carbide-based mechanical mixtures, i.e.Ni/Al 2 O 3 -SiC and Ni/SiC.The above catalysts were tested under varying microwave power, different CH 4 :CO 2 vol% feed ratios and space velocities in order to determine the best candidate and optimum operating conditions.

Materials and preparations
In this study, commercial SiC (SiCat, Germany), Pt/C (Sigma-Aldrich), Ni/Al 2 O 3 , (10 wt% Ni) (Johnson Matthey, UK) and the in-lab prepared Ni-impregnated SiC (denoted as Ni/SiC) were used.The Ni (10 wt%) impregnation on SiC was performed through an aqueous solution of Ni(NO 3 ) 2 •6H 2 O. SiC, as microwave-susceptible support, was soaked in the solution for one day at room temperature, and dried in an oven at 120 °C in air.Then, the catalysts were calcined in a muffle furnace at 500 °C for 5 h.In order to prevent compaction of the fixedbed, the fine powders of all aforementioned materials, were first pelletized, and then crushed and ground to 250-480 μm.Mechanical mixtures of the Ni/Al 2 O 3 and SiC were prepared with a weight ratio of 1:1.The prepared mixtures were then directly used in the reaction tests without any further pretreatment.

Microwave heating system
A custom-designed mono-mode microwave applicator (SAIREM), previously described in [25,27], includes a solid-state microwave generator with the maximum power of 400 W. The microwave generator operates at the spectral band of 2.4-2.5 GHz, which enables the finetuning by changing the microwave field frequency in the available 100 MHz range, in increments of 0.1 MHz.Rectangular air-filled aluminum waveguides (WR340) support the TE 10 mode.The microwave applicator also includes three sliding short circuits (SAIREM) to adjust the resonant frequency of the mono-mode cavity.Prior to the start of each experiment, the sliding short circuits were set manually to minimize the microwave reflection (lower than 10% of the forward power).For this purpose, the network analyzer (Agilent ENA series E5071) was employed [28][29][30] to measure reflected power within the frequency range of 2.4-2.5 GHz with 0.1 MHz steps.Once the optimum frequency was determined, the microwave generator was connected to the applicator.

Temperature measurements
The temperature measurement in microwave heating systems presents a challenging task.Thermocouples cannot be used because they interfere the microwave field.Since DRM reaction takes place at elevated temperatures, i.e., 600-800 °C, optical fiber sensors also cannot be employed due to the temperature limitations (usually circa 250 °C for the commercial sensors).Therefore, non-contact temperature measurement techniques, e.g., infrared thermal cameras, have to be applied.In this work, two different types of infrared thermal cameras were used for the temperature measurementssee Table 1.The accuracy of the measurement was ± 2 °C or ± 2% of the reading.After reaching the steady-state temperature, the average temperature of the area occupied by the catalyst was calculated from the outer surface temperature of the quartz reactor, using the software of the cameras.

Thermal stability and catalytic performance tests
Thermal stability tests of the materials were performed with microwave heating at 800 °C and 120 mL/min N 2 flow for 6 h, under atmospheric pressure, using a quartz tube (8 mm  the fixed-bed (catalyst particle diameter = 250-480 μm, length = 15 mm).Then, selected materials were tested on catalytic performance in the DRM process.Due to the lack of autonomous control and restricted working hours in our laboratory, the catalytic performance trials were run for maximum 6 h.To perform a standard test, the equimolar mixture of CH 4 and CO 2 with a total flow of 100 mL/min (WHSV of 11,000 mL/g/h) was fed to the reactor.The concentration changes of the reactants and products (CH 4 , CO 2 , H 2 , and CO) were measured with the gas chromatograph (Agilent 7890B GC System).Thermal stability of the Pt/C catalyst was studied under different microwave power inputs.The test was performed with three different batches of the catalyst and repeated three times per batch.Each experiment was carried out under a flow of 100 mL/min N 2 , while the microwave input power was increased until the required DRM reaction temperatures, e.g., 600, 700 and 800 °C were reached.Fig. 1 shows the surface temperatures of the quartz reactor wall at different microwave power inputs measured by the infrared thermal camera (FLIR, A655sc).

Results and discussions
The desired temperatures of 600, 700 and 800 °C were reached with a 30, 35 and 45 W microwave power inputs, respectively, see Fig. 1(a).Reproducible heating results were obtained; however, the overall catalytic bed temperature distribution was not homogeneous, see Fig. 1(b).The right side of the catalytic bed had higher temperature than the rest of the catalytic area.This is due to the electromagnetic field distribution inside the microwave cavity as well as Pt/C catalytic bed.The electromagnetic field distribution is directly related to the dielectric properties of the material under test.This non-homogenous temperature distribution was explained in detail in the previous studies via experimental and numerical analysis [30].Shortly speaking, the high-values of dielectric constant and loss factor affected significantly the electric field distribution inside the microwave cavity.Hence, the highest electric field intensity of the standing wave and consequently highest temperatures were located in the right part of the fixed-bed.
The performance of the Pt/C catalyst was tested with a CH 4 /CO 2 feed ratio of 1 at different temperatures, i.e., 600, 700 and 800 °C at a WHSV of 11000 mL/g/h.Three different batches were prepared and tested for the three temperatures in order to ensure the reproducibility of the test.The conversions of CH 4 and CO 2 at different temperatures and corresponding ratios of H 2 /CO and supplied power inputs are presented in Fig. 2.
It is clear that both conversions of CH 4 and CO 2 increase with temperature.In particular, at 600 °C the conversions of CH 4 and CO 2 were similar, see Fig. 2(a).However, at higher temperatures the conversion of CO 2 is slightly higher than CH 4 .Similar behavior reported by Fidalgo et al. [21], where a 10% difference between CO 2 and CH 4 conversions was observed at 800 °C, 320 mL/g/h, after 5 h.The described difference can be caused by either (i) catalyst deactivation which reduces the CH 4 conversion or (ii) in situ carbon gasification  ) and/or Boudouard reaction ( ⇌ + CO C CO 2 ).The ratios of H 2 and CO in the product gas are close to 1 at the three temperatures concerned, see Fig. 2(b).The large error bars in the conversion of CH 4 and CO 2 and the ratios of H 2 and CO, especially at 600 and 800 °C, can be explained by the continuous fluctuation in the supplied microwave power, see Fig. 2(b).For example, in order to keep the temperature at 600 °C, the supplied microwave power needed to be continuously reduced.This effect could be explained by the carbon deposition at 600 °C being higher than its in situ consumption.This is because the carbon formation through the Boudouard reaction ( ⇌ + 2 CO C CO 2 ), the hydrogenation of carbon mon- oxide ( 2 ), and the hydrogenation of carbon dioxide ( 2 ) reactions are favored at lower temperature (527 °C) [7].One has to remember that carbon is a good microwave absorbing material.Hence, the temperature increases with carbon deposition at the constant supplied microwave power.On the other hand, the effect was the opposite at 800 °C.In this case, the supplied microwave power had to be increased, in order to keep the temperature at 800 °C.This is mainly due to the in situ carbon gasification with CO 2 , which results in an increase in the required power at 800 °C.The temperature was roughly constant for 1-h test at 700 °C.This is mainly because the both aforementioned effects are roughly compensated at this temperature.
Wang et al. [31] stated that the decomposition of CH 4 happens above 557 °C, while the disproportionation of CO ( ⇌ + 2 CO C CO 2 ) below 700 °C.The deposition of carbon generated hot spots in different areas of the catalytic fixed-bed.Due to the lack of uniformity in the temperature distribution, the production of carbon will be higher in the areas with lower temperatures [31,32].Furthermore, the reduction on the catalytic-bed volume was observed after the DRM experiments.It was difficult to measure the weight-loss due to the problems associated with removing the catalyst from the quartz reactor.The variation in the catalytic volume might indicate that carbon of the catalyst was reacted.Both factors generate a change in the heating pattern.
In order to determine the catalyst stability, a 6-h test at 700 °C (CO 2 :CH 4 = 1, WHSV = 5500 mL/g/h, 101 kPa) was performed to evaluate the conversion loss with time.The conversions of CH 4 and CO 2 , the ratios of H 2 /CO and the supplied microwave power are presented in Fig. 3.The first two hours presented similar conversions for CH 4 and CO 2 .However, both conversions of CH 4 and CO 2 started to decrease and differ from each other for the rest of the experiment (CH 4 : from 87% to 70%, and CO 2 : from 86% to 74%), see Fig. 3(a).This result might mean that the non-homogeneous carbon formation can generate a significant deactivation of Pt catalyst.Moreover, a side reaction may take place in between the produced carbon and CO 2 via in situ carbon gasification, + ↔ C CO 2CO 2 , which results in higher CO 2 conversions.This result can be also observed in Fig. 3(b) due to the reduction in the H 2 /CO ratio which might indicate that more CO was produced.In terms of the supplied microwave power input (see Fig. 3(b)) a slight decrease over the studied period is observed.This is because the less conversion of CH 4 and CO 2 requires less energy input.In the work of Fidalgo et al. [21], the conversions of CH 4 and CO 2 dropped drastically to 40 and 45%, respectively, at the same condition with a lower space velocity (CO 2 :CH 4 = 1, WHSV = 920 mL/g/h, activated carbon: Filtracarb FY5).It should be also highlighted that different kind of carbon formation during DRM process can play a significant role [33].On one hand, the deposited carbon acts as a microwave absorbing medium, which leads to the power reduction.On the other hand, more active carbon can be simultaneously consumed in the process via carbon gasification with CO 2 .

Nickel on alumina, Ni/Al 2 O 3
Nickel supported catalysts are widely used for the DRM reaction as alternative to noble metals supported catalysts, and this is mostly due to their high activity, availability and low cost [23,34].For that reason, we subjected this catalyst to the microwave heating test, during which the temperature linearly increased with increasing microwave power inputs (data not shown).Unfortunately, even with the maximum microwave power input, i.e. 400 W, the achieved temperature was circa 400 °C, which is far from the temperature levels required to perform the DRM reaction.This is due to poor dielectric properties of the Al 2 O 3 support, which make it slightly microwave-transparent.Higher temperatures can be achieved with higher microwave power input because the dielectric loss of the Al 2 O 3 support increases with temperature from 0.06 to 0.2 linearly between 250-600 °C [35] but these temperatures are still not enough to run the DRM reactions.Therefore, a mixture of Ni/Al 2 O 3 with a microwave-susceptible material, e.g., silicon carbide, must be used in order to reach the desired operating conditions for the DRM, i.e. 600-800 °C.Similar conclusion was achieved by Fidalgo et al. [23].

Mixture of Ni/Al 2 O 3 and different SiC polytypes
Silicon carbide, SiC, is a good candidate as a catalyst support material, due to their high thermal conductivity, high mechanical strength, low specific weight, and chemical inertness [36].Among all aforementioned good properties, SiC is also a well-known microwave-susceptible material.In this SiC can be heated easily with microwave due to its semiconductivity [37].As a catalyst support and microwave absorber, two different commercial polytypes of SiC, namely 6H-SiC and 3C-SiC were investigated.Before the mechanical mixing of Ni/Al 2 O 3 and SiC polytypes, we first investigated the heating performance of the SiC polytypes alone.The heating profiles of two different fresh batches of 6H-SiC are presented in Fig. 4. It is clear that 6H-SiC requires high microwave power inputs in order to achieve the desired temperatures.In addition, the continuous microwave heating caused thermal runaways, that were observed approximately at 500 °C for two different batches, see Fig. 4(a).It has been reported that the dielectric loss of SiC at the 2.45 GHz is increased from 1.71 to 27.99 at 20 °C and 695 °C, respectively [37].This exponential increment in the dielectric loss leads to thermal runaway.When the thermal runaways occurred, microwave generator was immediately stopped to avoid the damage of the quartz reactor tube.After the runaway had occurred and the reactor had been cooled down to room temperature, the same batches of the catalyst were heated up again.Unexpectedly, the microwave power input necessary to reach 500 °C was now reduced by ca.80%, see Fig. 4(b).The desired temperatures for DRM experiments, i.e., 600, 700 and 800 °C were reached with the microwave power inputs of 40, 60 and 80 W, respectively.Also, the overall fixed-bed temperature distribution changed significantly after the thermal runaway process, see the insets in Fig. 4. The temperature distribution was more homogeneous before the runaway.
Fig. 4(b) depicts the heating profile of 3C-SiC.In this polytype, no thermal runaways were observed during several heating tests.It is clear that 3C-SiC presents a better heating behavior and the temperature reaches up to 950 °C with only a 50 W microwave power input.The above described behavior could be explained with the electron mobility of these different SiC polytypes.Roschke et al. [38] studied and developed the electron mobility models in the three most important silicon carbide polytypes, namely, 4H-, 6H-, and 3C-SiC.Regarding their work, 3C-SiC has higher electron mobility compared with 4H-SiC and 6H-SiC polytypes (650, 950, and 420 cm 2 /V/s, respectively) [38].Apparently, the thermal runaway partially transformed the 6H-SiC polytype into the 4H-SiC and 3C-SiC polytypes (mixture of polytypes), which have more electron mobility than the 6H-type.
After studying the heating performances of SiC polytypes, Ni/Al 2 O 3 were mixed mechanically with 6H-SiC and 3C-SiC by 50/50 wt% and subjected to microwave heating for the thermal stability test.The heating results are presented in Fig. 5.The thermal runaway of 6H-SiC was still present even in the mechanical mixture with Ni/Al 2 O 3 , see Fig. 5(a).After the thermal runaways, the desired temperatures, i.e. 700 and 800 °C, were reached with relatively low microwave power inputs in the range of 30−65 W, see Fig. 5(b).As mentioned before, this is due to the phase changes of 6H-SiC.Since this phase transformation occurred partially, more thermal runaways were observed in the subsequent heating trials.In the case of mechanical mixture of 3C-SiC and Ni/Al 2 O 3 , see Fig.The conversions of CH 4 and CO 2 at 800 °C, the corresponding H 2 / CO ratios, and the supplied microwave power inputs are presented in Fig. 6.During the stability test, the conversions of CH 4 and CO 2 were stable and no catalyst deactivation was observed.After 6 h, the average conversions of CH 4 and CO 2 were ca.90% at 800 °C and WHSV of 11,000 mL/g/h, see Fig. 6(a).Fidalgo et al. [23] also achieved the same CH 4 conversion at the same temperature but with a lower space velocity (WHSV: 1500 mL/g/h), while using a mechanical mixture of Ni/Al 2 O 3 and activated carbon, FY5.However, they achieved higher CO 2 conversion (97%), and they claimed that this was due to the in situ carbon gasification ( + ⇌ C CO CO 2 ).The thermodynamic calculations at 800 °C resulted in CH 4 and CO 2 conversions of 96.3% and 88.7%, respectively.
The fluctuations in conversions, see Fig. 6, are mainly due to the microwave power fluctuations necessary to maintain the desired temperature.There was also hot-spot formation, which altered the conversions of CH 4 and CO 2 .Nevertheless, these results indicate that the mechanical mixture of 3C-SiC and Ni/Al 2 O 3 is a good candidate for the DRM process.The same catalyst mixture was also tested under the conventional thermal heating at 800 °C.The tests showed (see the Supplementary information) a fast catalyst deactivation observed during the first hour, and the conversions of CH 4 and CO 2 reaching a value of 79% during the second and third hour.The conventional heating experiment was performed for 3 h only, because the pressure increased inside the reactor reaching 202 kPa, which was the maximum allowable pressure for the experimental set-up.Such behavior was only seen in the conventional heating.It could be attributed to the coke formation when the samples were subjected to the conventional heating.

Nickel on 3C-SiC, Ni/SiC
Even though the mechanical mixture of 3C-SiC and Ni/Al 2 O 3 can be used in DRM process, the temperature homogeneity must be improved further.This can be done by removing Al 2 O 3 form the mixture, since alumina has relatively low thermal conductivity and poor dielectric properties compared to 3C-SiC.For that reason, we prepared a nickelimpregnated 3C-SiC catalyst and subjected it to the DRM performance tests.
The catalytic performance of Ni/SiC was tested with a CH 4 /CO 2 feed ratio of 1 at 700 and 800 °C at a WHSV of 11,000 mL/g/h.Three different batches were prepared and tested at the desired temperatures, in order to ensure the reproducibility of the test.The conversions of CH 4 and CO 2 at different temperatures, the corresponding H 2 /CO ratios, and the supplied power inputs are presented in Fig. 7.The XRD characterization of the Ni/SiC was presented as supplementary information.
Compared to the mechanical mixture of 3C-SiC and Ni/Al 2 O 3 , the same conversions of CH 4 and CO 2 and slightly higher ratios of H 2 /CO were observed.In terms of the supplied microwave power inputs (see   , the results were similar to the mechanical mixing experiment; however, the fluctuations of the supplied power during the test were lower.Due to the higher conversions obtained, the 6-h stability test was conducted at 800 °C, see Fig. 8.It is clear that the conversion of CO 2 is significantly higher than the conversion of CH 4 .This could be explained by the simultaneous carbon gasification with CO 2 .These differences in the conversions of CH 4 and CO 2 were less pronounced in the case of the mechanical mixture of 3C-SiC and Ni/Al 2 O 3 , because the presence of Al 2 O 3 delayed the carbon formation.

Optimization of the operational conditions for Ni/SiC catalyst
As described above, higher conversions and homogenous temperature distribution were obtained with Ni/SiC when compared to Pt/C and the mechanical mixture of Ni/Al 2 O 3 and 3C-SiC.For that reason, we continued to study the Ni/SiC catalyst at different feed ratios (CO 2 :CH 4 ) at 800 °C.It is expected that higher feed ratios (CO 2 :CH 4 ) might increase the catalyst stability because CO 2 can not only react with CH 4 but can also react with the carbon formed during the DRM process.The corresponding results of the conversions of CH 4 and CO 2 , H 2 /CO ratios and the supplied microwave power inputs are presented in Fig. 9.
As expected, the complete conversion of CH 4 was observed at higher ratios of CO 2 /CH 4 .On the other hand, the conversion of CO 2 (see Fig. 9(a)) was higher at the ratio of 1 but there was a slight decrease in the conversion of CO 2 over time.When the feed ratio of CO 2 :CH 4 was increased to 1.5 and 2, the conversion of CO 2 was constant for 6 h.Most of the methane was reacted, that produced a fixed amount of hydrogen, while there was an increase in the amount of CO because of the aforementioned carbon gasification with CO 2 .The difference in the H 2 / CO ratio for 1.5 and 2 could indicate that in order to maximize the lifetime of the catalyst a ratio of 2 would be beneficial, due to the reduction of the amount of carbon formed during the DRM reaction.In such case, we see higher conversion of CO 2 (> 20%) compared to the work of Xu et al. [34].(CO 2 :CH 4 :N 2 = 2:1:0.8,WHSV = 9944 mL/g/h, 11.8 wt% Ni, 101 kPa, 800 °C).Regarding the supplied microwave power input, its increase at higher ratios might be explained by the fact that two endothermic reactions are taking place at the same time, see Fig. 9(b).It can be concluded that the best results in terms of conversions and power consumption over the studied period was obtained with the ratio of 1.5.
To finalize the study of Ni/SiC, it is necessary to analyze the influence of the space velocity and determine the relation between the conversions and the reactants flow.This test was performed at 800 °C for two hours with a feed ratio CO 2 /CH 4 : 1.5.Due the limitations of the experimental set-up we studied WHSV's up to 24,000 mL/g/h.The results in terms of the conversion of CH 4 and CO 2 and corresponding microwave power inputs are presented in Fig. 10.A linear decrease in both CH 4 and CO 2 conversion was seen.Despite that decrease, 93% methane and 88% CO 2 conversions at the highest WHSV were observed.As can be seen in Fig. 10, larger WHSVs required slightly higher power in order to keep the temperature constant.This is logical and results from the larger volume treated.

Fig. 1 .
Fig. 1.(a) Heating test of Pt/C under a flow of 100 mL/min N 2 .(b) Temperature distribution of Pt/C on the quartz reactor at 15 W.

Fig. 2 .
Fig. 2. Catalytic performance of Pt/C at 600, 700 and 800 °C in terms of (a) CH 4 and CO 2 conversions (b) H 2 /CO ratios and supplied microwave power inputs.

Fig. 3 .
Fig. 3. Stability test of Pt/C at 700 °C in terms of (a) CH 4 and CO 2 conversions (b) H 2 /CO ratios and supplied microwave power inputs.
5(b), reproducible heating profiles were obtained in different batches, without any thermal runaways.The desired

Fig. 6 .
Fig. 6.Catalytic performance of the mechanical mixture of 3C-SiC and Ni/ Al 2 O 3 at 800 °C in terms of (a) CH 4 and CO 2 conversions (b) H 2 /CO ratios and supplied microwave power inputs.

Fig. 7 .
Fig. 7. Catalytic performance of Ni/SiC at 700 and 800 °C in terms of (a) CH 4 and CO 2 conversions (b) H 2 /CO ratios and supplied microwave power inputs.

Fig. 7 (
Fig.7(b)), the results were similar to the mechanical mixing experiment; however, the fluctuations of the supplied power during the test were lower.Due to the higher conversions obtained, the 6-h stability test was conducted at 800 °C, see Fig.8.It is clear that the conversion of CO 2 is significantly higher than the conversion of CH 4 .This could be explained by the simultaneous carbon gasification with CO 2 .These differences in the conversions of CH 4 and CO 2 were less pronounced in the case of the mechanical mixture of 3C-SiC and Ni/Al 2 O 3 , because the presence of Al 2 O 3 delayed the carbon formation.

Fig. 8 .
Fig. 8. Catalytic performance of Ni/SiC at 800 °C in terms of (a) CH 4 and CO 2 conversions (b) H 2 /CO ratios and supplied microwave power inputs.

Fig. 9 .
Fig. 9. Influence of the ratios of CO 2 :CH 4 in (a) conversions of CH 4 and CO 2 (b) H 2 /CO ratios and supplied microwave power inputs.

Fig. 10 .
Fig. 10.Influence of the space velocity on the conversion of (a) CH 4 and (b) CO 2 .

Table 1
in diameter) filled with Characteristics of infrared cameras.