Ru- and Rh-Based Catalysts for CO₂ Methanation Assisted by Non-Thermal Plasma

Abstract

The need to reduce the concentration of CO₂ in the atmosphere is becoming increasingly necessary since it is considered the main factor responsible for climate change. Carbon Capture and Utilization (CCU) technology offers the opportunity to obtain a wide range of chemicals using this molecule as a raw material. In this work, the catalytic Non-Thermal Plasma (NTP)-assisted hydrogenation of CO₂ to CH₄ (methanation reaction) in a Dielectric Barrier Discharge (DBD) reactor was investigated. Four different Ru- and Rh-based catalysts were prepared starting from γ-Al₂O₃ spheres, characterized and tested in both thermal and NTP-assisted methanation under different operating conditions. The experimental tests evidenced the very positive effect of the NTP application on the catalytic performance, highlighting that for all the catalysts the same CO₂ conversion was reached at a temperature 150 °C lower with respect to the conventional thermal reaction. Among the prepared catalysts, the bimetallic ones showed the best performance, reaching a CO₂ conversion of 97% at about 180 °C with a lower energy consumption with respect to similar catalysts present in the literature.

1. Introduction

Carbon dioxide (CO₂) is a product of many human activities, from daily to industrial ones. The need to reduce its concentration in the atmosphere is becoming increasingly necessary since it is considered the main factor responsible for climate change. In particular, global CO₂ emissions reached their highest level in history in 2021 [1]. This anomalous growth was linked to the high increment in energy demand after the COVID-19 crisis, which, despite the increase of energy from renewable sources, has also lead to a return to coal, which accounted for over 40% of the overall CO₂ emissions in 2021 [2]. In recent years, various sectors have moved towards greener methods, thus limiting their emissions in terms of CO₂. However, this may not be enough to limit global warming and it may also be necessary to combine these methods with systems for CO₂ capture. By geologically confining the captured CO₂, many drawbacks, such as uncontrolled release in the atmosphere, high energy consumption and high costs, may occur [3]. Carbon Capture and Utilization (CCU) technology, besides reducing the CO₂ concentrations in the atmosphere, also offers the opportunity to obtain a wide range of chemicals using this molecule as a raw material [4,5]. Recently, another novel technology, the Power-to-Gas (PtG) approach, has begun development for the generation of hydrogen fuel from excess electricity from renewable sources [6]. Since the storage and use of hydrogen require expensive infrastructure, converting it into methane can represent a better solution. Therefore, a combination of the two aforementioned technologies, CCU and PtG, may result in the “Power-to-Methane” process, obtaining “green” methane through the Sabatier reaction (Equation (1)).

$${\mathrm{CO}}_2+4{\mathrm{H}}_2\leftrightarrows {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\to (\Delta {{\mathrm{H}}^0}_{298\mathrm{k}}=-165.12\mathrm{kJ}/\mathrm{mol})$$

(1)

The conventional methanation process is usually carried out in the temperature range 300–500 °C, and under these reaction conditions several side reactions may occur. The need for a high temperature is also linked to the strong stability of the C–O bond, which makes the activation of the CO₂ molecule challenging for any catalyst. The most used catalysts for CO₂ methanation are the Ni-based ones due to their good catalytic performance, as well as their cheapness and availability. The amount of Ni in the catalytic formulation is usually within the range 10–20%wt, which is deposited on a support with high porous volume and a good surface area for obtaining a good dispersion. According to recent works [7,8,9], the best supports for Ni-based catalysts are ZrO₂ and CeO₂ because, compared to Al₂O₃, TiO₂ and SiO₂, they have a larger number of oxygen vacancies which can enable great amounts of CO₂ adsorption and therefore make CO₂ activation easier. Additionally, noble metals, such as ruthenium and rhodium, are very active and selective in the CO₂ methanation process, thanks to their low activation temperature and their high reducibility and stability. In this case, the amount of noble metal deposited on the support is particularly important, both from a catalytic and an economic point of view since noble metals are expensive. In several studies, Ru-based catalysts have shown a good catalytic activity and selectivity to CH₄ if the metal is loaded in the range 0.5–4%wt [10,11,12,13]. This low amount of Ru makes these catalysts very promising for a potential industrial CO₂ methanation application. Additionally, Rh-based catalysts, by using a small amount of metal in the catalytic formulation, are active and selective in the methanation process. In particular, their catalytic activity and selectivity mainly depend on the OSC (Oxygen Storage Capacity) of the support and on Rh particle sizes; CO₂ methanation efficiency is favoured on smaller Rh particles when dispersed on supports with a lack of oxygen ion lability, and vice versa on supports with high oxygen ion lability [14,15,16,17]. However, besides finding high-performance catalysts, the main challenge of the considered process consists of lowering the operating temperatures and pressures, which cause high operating and investment costs.

Lately, many studies in the literature have focused on the use of non-thermal plasma (NTP) in the methanation processes [18]. Additionally, NTP, known as non-equilibrium plasma, can produce a variety of active species, such as electrons, ions, excited neutrons and low-temperature radicals useful for activating the carbon dioxide molecule at temperatures close to ambient. NTP-assisted catalytic processes are characterized by fast start-up and shutdown and can be carried out at atmospheric pressure and temperature, thus requiring lower energy costs [19]. Moreover, the application of plasma in catalytic systems generates several advantages, as this activates the gas molecules and modifies the surface of the catalyst, generating new active sites for adsorption and, in this way, creating new reaction paths [20]. In particular, the combination of NTP and catalysis can significantly enhance CO₂ conversion and CH₄ selectivity, respectively, from values lower than 15% up to values around 70% for both parameters [21]. Most of the studies have focused on Dielectric Barrier Discharge (DBD)-type plasma; in these studies, the applied voltage varied in the range 7–20 kV, with a power ranging from 3 to 15 W [22,23]. From a comparison between conventional and NTP-assisted methanation, Biset-Peiró et al. [24] found that the use of NTP allowed the same results of conversion and selectivity, but at lower temperatures, to be obtained using a Ni/γ-Al₂O₃ catalyst. Additionally, Xu et al. [25] reported that, at the same temperature, CO₂ conversion under NTP conditions is more than six times higher that under the thermal conditions. In plasma-assisted processes, the support has a greater influence on the catalytic performance, as reported by Nizio et al. [26,27]; the use of support with low/medium basicity can lead to enhanced results, since the kinetically relevant step is the adsorption of CO₂ leading to adsorbed CO and adsorbed O. However, by employing noble metals, further particular reaction paths can occur. Xu et al. [28] revealed that the synergy of NTP and Ru species plays a key role in the catalytic activity of CO₂ methanation.

Starting from a study of the literature, in this work, different mono- and bimetallic Ru- and Rh-based catalysts were prepared starting from γ-Al₂O₃ spheres (1–2 mm), also aiming to investigate the effect of the impregnation order on the catalytic activity. The prepared catalysts were tested both in the conventional thermal and in the NTP-assisted (using a DBD reactor) methanation reactions under two different weight hourly space velocity (WHSV) values. The experimental tests evidenced the very positive effect of the NTP application on the catalytic performance, highlighting that for all the catalysts the same CO₂ conversion was reached at a temperature 100 °C lower compared to the conventional thermal reaction. Among the prepared catalysts, the bimetallic ones showed the best performance, reaching a CO₂ conversion of 97% at about 180 °C with a lower energy consumption with respect to similar catalysts present in the literature.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. Physical Characteristics

The results of Hg intrusion porosimetry and N₂ @ 77K physisorption are reported in Figure 1 and Figure 2. as specific pore volume vs. pore size, and total adsorbed volume vs. P/P₀, respectively.

As can be seen from Figure 1, and particularly from the increasing slope of the left part of the graph in correspondence with the smaller pore size, there is a considerable portion of the pores which could not be evaluated by the porosimeter; hence, a highly micro- and mesoporous behaviour can be supposed for all the catalysts. This supposition is confirmed by the adsorption–desorption isotherms reported in Figure 2. The isotherms relating to the loaded catalysts were all of type IV and presented a type H3 hysteresis due to the presence of both micropores and mesopores. It is important to note that the addition of the active species reduced the amount of micropores; in fact, the height of the isotherm curve at low values of P/P₀ (left side of the diagram) decreased after Rh and Ru deposition on the support (blue curve), which is particularly visible on the bimetallic catalysts (grey and orange curves). Nevertheless, it can be stated that all the catalysts kept the mesoporous behaviour of the support.

A summary of all the main characteristics of the catalysts is reported in

Table 1. Surface area, pore volume and average pore diameter.

	N2 @77K Physisorption			Hg Intrusion Porosimetry
Sample Name	Surface Area (BET), m2/g	Mesopores Volume, cm3/g	Average Mesopore Diameter, nm	Average Pore Diameter, nm	Total Pore Volume, cm3/g
γ-Al2O3	271.000	0.390	4.300	13.600	0.243
Rh-γAl2O3	246.000	0.410	4.870	13.860	0.367
Ru-γAl2O3	242.000	0.390	5.700	14.400	0.378
RhRu-γAl2O3	148.000	0.410	7.410	12.070	0.482
RuRh-γAl2O3	232.000	0.460	4.600	11.620	0.436

:

The Hg intrusion porosimetry showed that the average pore diameter increased with respect to the bare spheres after the deposition of a single active phase, and that the further deposition of the second one led to its decrease. The total pore volume, however, increased with the deposition of the active phases. The Brunauer–Emmett–Teller analysis highlighted a decrease of the catalysts’ surface area when compared to the bare support. Furthermore, a sensible increase of the mesopore volume, as well as of the average mesopore diameter, can be noted.

Finally, to better understand the dispersion of the active phases on the support, a SEM-EDX analysis of the catalysts was carried out (Figure 3, Figure 4, Figure 5 and Figure 6).

As can be clearly seen from Figure 3, Figure 4, Figure 5 and Figure 6, for both the monometallic catalysts and the bimetallic ones, a homogeneous deposition of the active phases on the whole surface of the support was obtained.

2.1.2. Chemical Characteristics

To evaluate the effective loading of the active phases on the support, a H₂-Temperature Programmed Reduction analysis was performed. The results are reported in Figure 7 and in

Table 2. Catalysts’ theoretical and experimental H₂ consumption.

Catalyst Name	Theorical Hydrogen Uptake, mol	Experimental Hydrogen Uptake, mol
Ru-γγAl2O3	7.66 × 10−4	7.40 × 10−4
Rh-γAl2O3	7.64 × 10−4	7.35 × 10−4
RhRu-γAl2O3	2.70 × 10−3	2.40 × 10−3
RuRh-γAl2O3	2.70 × 10−3	2.75 × 10−3

.

From the above reported figure, it can be seen that (i) the monometallic Rh-based catalyst is characterized by the presence of a wide reduction area with two main peaks at 160 and 210 °C, and (ii) the monometallic Ru-based catalyst is characterized by a single peak centred at 110 °C. These results are in good agreement with the literature [29,30]. The bimetallic catalysts are characterized by the presence of two peaks, at lower and higher temperatures, due to a synergic effect of the two metals.

To evaluate the theorical hydrogen consumption for both Ruthenium and Rhodium, the oxides Rh₂O₃ and Ru₂O₃ and the correspondent reduction reactions (Equations (2) and (3)) were considered.

Rh₂O₃ + 3H₂ → 2Rh + 3H₂O,

(2)

Ru₂O₃ + 3H₂ → 2Ru + 3H₂O,

(3)

As can be seen from Table 2, the theorical and experimental hydrogen consumption are comparable; hence, the active phase loading was verified.

2.2. Activity Tests

2.2.1. Thermal Methanation Tests

All four catalysts were tested under the same conditions and the results were evaluated in terms of conversion, selectivity and yield evaluated according to the formulas below:

$$X_{C{O}_2}=\frac{n_{C{O}_2}^{in}-n_{C{O}_2}^{out}}{n_{C{O}_2}^{in}}$$

(4)

$$S_{C{H}_4}=\frac{n_{C{H}_4}}{n_{C{O}_2}^{in}-n_{C{O}_2}^{out}}$$

(5)

$$Y_{C{H}_4}=\frac{n_{C{H}_4}}{n_{C{O}_2}^{in}}$$

(6)

in which n_i are the moles of the component “i”.

The results relevant to the experimental tests performed at the two WHSV are reported in the following figures (Figure 8 and Figure 9) for all the catalysts.

As can be clearly seen from Figure 8 and Figure 9, the bimetallic catalysts showed the best results both in terms of CO₂ conversion and CH₄ yield in all the investigated temperature ranges. By deeply analysing these figures, one more difference is observable. While for the higher space velocity there was a remarkable difference between all the catalysts’ results, with the RuRh-γAl₂O₃ obtaining a CO₂ conversion (which corresponds to the CH₄ yield being the selectivity equal to 1) of about 80% at 320 °C, the results at lower space velocity showed that the bimetallic catalysts had very similar behaviour, with a maximum of 84% in terms of both CO₂ conversion and CH₄ yield. It is important to highlight that the selectivity followed the pattern of the equilibrium curve at low temperatures and kept high values even at higher temperatures.

Among the bimetallic catalysts, the RuRh-γAl₂O₃ catalyst was chosen to compare the results obtained at the two space velocities in terms of CH₄ yield (Figure 10).

Figure 10 shows that tests performed at a higher space velocity (orange curve) obtained the same results at temperatures slightly lower than the lower space velocity one (blue curve), but the latter has a slight increase in terms of CH₄ yield obtaining a maximum CH₄ yield of 84%vol. at 320 °C. However, at higher temperatures it can be seen that the higher space velocity tests produced a higher quantity of CH₄.

2.2.2. Non-Thermal-Plasma-Assisted Methanation Tests

All four catalysts were tested in the NTP-assisted reaction under the same operating conditions of the conventional tests in terms of space velocities. The results are reported in Figure 11 for the higher space velocity.

The data reported above highlighted the very positive effect of the NTP application: the same CO₂ conversion can be obtained at lower temperatures than the thermal process. Moreover, the reported data evidenced, apart from the high selectivity shown by all the catalysts, the best catalytic performance obtained by using the bimetallic catalysts, among which the RuRh-γAl₂O₃ showed the highest CO₂ conversion (about 72%). This result may be ascribable to the high surface area of the latter sample, coupled with the higher mesopore volume and lower pore radius. One more important result is that it was possible to obtain methane starting from 100 °C.

The results relevant to the experimental tests performed at the lower space velocity are reported in Figure 12.

The most interesting results were obtained in these conditions with bimetallic catalysts. Due to the operating temperature being considerably lower than the thermal process temperature, it was possible to reach CO₂ conversions much higher than the ones obtained in such processes which were limited by the thermodynamic equilibrium. In fact, the best catalyst was RuRh-γAl₂O₃, which approached the equilibrium values at a CO₂ conversion of about 97% at 190 °C, which resulted in a CH₄ yield of 95%.

2.2.3. Thermal/NTP-assisted Methanation Comparison

Finally, to better highlight the importance of the non-thermal-plasma-assisted methanation (Figure 13), a comparison between the best CH₄ yields of RuRh-γAl₂O₃ in both thermal and NTP-assisted tests at the lower space velocity is reported in Figure 6.

As previously stated, the ability to activate the CO₂ molecules at lower temperatures results in the possibility of reaching yields that would be impossible to reach due to thermodynamic limitations. Furthermore, it is also fundamental to note that it is possible to obtain CH₄ yields similar to the thermal ones at temperatures about 100 °C lower, which would result in a considerably lower energy expense.

2.3. Energy Considerations

It is important to consider the energy consumption of the whole non-thermal plasma assisted process and to compare it with those found in the literature. In order to make a careful comparison, the Specific Energy Input (SEI) index was taken into account and evaluated according to the following formula:

$$Specific Energy Input \left[\frac{kJ}{L}\right]=\frac{P\left[kW\right]}{F\left[L/s\right]}$$

(7)

The results of said comparison are reported in

Table 3. Present work catalysts’ SEI, CO₂ conversion and CH₄ production compared with the literature.

Catalyst Name	Operating Conditions	CO2 Conversion	SEI [kJ/L]	CH4 Produced [mmol/min]	Reference
0.5%Ru0.5%Rh-γAl2O3	Flow rate: 340 mL/min, T = 188.1 °C, H2:CO2 = 4; P = 55W	0.97	9.7	14.71	Present work
0.5%Rh0.5%Ru-γAl2O3	Flow rate: 340 mL/min, T = 190.1 °C, H2:CO2 = 4; P = 55W	0.88	9.7	13.35	Present work
0.5%Ru0.5%Rh-γAl2O3	Flow rate: 1 L/min, T = 217.6 °C, H2:CO2 = 4; P = 55W	0.73	3.3	32.56	Present work
0.5%Rh0.5%Ru-γAl2O3	Flow rate: 1 L/min, T = 208.3 °C, H2:CO2 = 4; P = 55W	0.7	3.3	31.23	Present work
15%Ni-CeZrO2	Flow rate: 200 mL/min, T = 170 °C, H2:CO2 = 7; P = 16W	0.76	4.8	8.14	[31]
Mn-Al2O3	Flow rate: 34.6 mL/min, T = 135 °C, H2:CO2 = 4; P = 35W	0.76	60	1.18	[32]
Ni-Al2O3	Flow rate: 69.2 mL/min, T = 150 °C, H2:CO2 = 4; P = 30W	0.48	26	1.47	[33]
Ru-γAl2O3	Flow rate: 80 mL/min, T = 250 °C, H2:CO2 = 7; P = 33W	0.18	25	0.14	[34]

.

As can be clearly seen from the table above, the specific energy input of the test conducted in the present work is considerably lower than those found in the literature and, when this input is slightly higher those in the literature, the CO₂ conversion and the CH₄ production are considerably higher. Hence, it can be stated that, although the energy efficiency of the present setup is still far from being optimized up to the industrial level, it is a good basis to develop further improvements in the energy efficiency of plasma-catalytic methanation systems.

3. Materials and Methods

3.1. Preparation of the Catalysts

The catalysts were prepared through the wet impregnation technique. The support, γ-Al₂O₃ in spheres (1–2 mm), was previously calcined at 600 °C and then dipped in a solution of the precursor salts, ruthenium acetylacetonate (C₁₅H₂₁O₆Ru) and rhodium nitrate hydrate (RhN₃O₃·xH₂O) for Ru- and Rh-based catalysts, respectively, in acetone for 15 min. After that, the catalysts were dried at 120 °C for 2 h, and finally calcined at 600 °C for 1 h. The bimetallic catalysts differ each other from the order of impregnation of the metals. The metal loading for both mono- and bi-metallic catalysts was of 0.5%wt for each active species.

3.2. Catalysts’ Characterization

The prepared samples were characterized by means of physicochemical techniques, such as Hg intrusion porosimetry, Electron Microscopy SEM-EDX and physisorption of N₂ @ 77 K, in order to assess the distribution and dimension of the pores, and to evaluate the distribution of the active phase, both superficial and volumetric. Furthermore, a Temperature-Programmed Reduction (TPR) was carried out to obtain qualitative and quantitative information; thus, the reduced species and the quantity of metal stored on the support. The TPR was conducted with a ramp of 10 °C/min until 500 °C, in presence of 5% H₂ in Ar.

3.3. Experimental Plant

The NTP-assisted tests on the catalysts were carried out in the experimental plant shown in Figure 14. A mass flow controller (MFC) battery was used to feed the gases (Ar, CO₂, H₂) to the quartz reactor, which was linked to the plasma generator through the two electrodes, high voltage and ground. A moisture trap was placed at the exit of the reactor to catch the water produced by the reaction. The composition of the outlet mixture was then analysed by a mass spectrometer. Both composition and temperature data were collected on a computer using specific software.

Two temperature probes were placed inside the reactor, one at the inlet and one at the outlet of the catalytic bed, to detect the temperature trend inside the catalyst. The probes were OPTOCON fibre optic temperature sensors, specifically designed to be inert to electromagnetic fields and in particular to high voltages. The reactor for the NTP-assisted tests was a DBD reactor; this is a quartz tube with an internal chamber where the internal electrode (high-voltage) is placed, an external metal covering mesh, which is the ground electrode, and between the two electrodes is the dielectric, the catalyst itself. The plasma generator induces the non-equilibrium discharge with an adjustable frequency between 20–60 kHz and a high voltage between 1–40 kV alternative current (AC). The catalytic tests were carried out using a feeding mixture of 48%H₂, 12%CO₂ and 40%Ar, and at two different WHSV, 1 NL/gcat·h and 6 NL/gcat·h, in order to verify if it would have any effect on the catalyst’s performance. These conditions were the same for the NTP-assisted and conventional methanation tests. The NTP catalytic tests were carried out by supplying a value of current between 0.1 and 0.6 A, with increments of 0.1 A, and the data were taken once the discharge reached a steady state because this situation also corresponded to a stable value of temperature and composition. Then, the thermal tests were carried out by considering the same temperature values registered for the NTP-assisted process in order to make a comparison between the two process modalities.

4. Conclusions

In this work, non-thermal-plasma-assisted CO₂ methanation has been studied. Four alumina-supported structured catalysts were prepared, two monometallic ones and two bimetallic ones, the latter prepared by varying the impregnation order of the metals. All the prepared catalysts were characterized and tested in the CO₂ hydrogenation reaction (methanation) both in a thermal plant and in a Dielectric Barrier Reactor (DBD) non-thermal plasma plant. The effect of different parameters (temperature and space velocity), as well as of the impregnation order for the bimetallic catalysts, were studied, and a comparison made between the thermal and the NTP-assisted processes.

A fundamental result of this work is the possibility of obtaining CH₄ yields comparable to the thermal yields at temperatures about 100 °C lower than the latter, granting the possibility of approaching the equilibrium line at higher values. The best catalytic activity (CH₄ yield = 95%vol.) was obtained at 188.1 °C with H₂:CO₂ ratio = 4 by using the catalyst RuRh-γAl₂O₃; the other bimetallic catalyst showed a slightly lower CH₄ yield (88%vol.) at about the same temperature (190 °C). These results were obtained with an SEI (Specific Energy Input) lower than the ones found in the literature by a whole order of magnitude while maintaining higher conversions and yields.

The results shown in this work have demonstrated the possibility of applying the plasma catalysis approach to CO₂ methanation, obtaining remarkable results at temperatures much lower than those of the traditional processes. Possible future improvements may involve the optimization of the reacting system and the study of the catalysts stability over time.