Unraveling Temperature-Dependent Plasma-Catalyzed CO2 Hydrogenation

Hydrogenation of carbon dioxide to value-added chemicals and fuels has recently gained increasing attention as a promising route for utilizing carbon dioxide to achieve a sustainable society. In this study, we investigated the hydrogenation of CO2 over M/SiO2 and M/Al2O3 (M = Co, Ni) catalysts in a dielectric barrier discharge system at different temperatures. We compared three different reaction modes: plasma alone, thermal catalysis, and plasma catalysis. The coupling of catalysts with plasma demonstrated synergy at different reaction temperatures, surpassing the thermal catalysis and plasma alone modes. The highest CO2 conversions under plasma-catalytic conditions at reaction temperatures of 350 and 500 °C were achieved with a Co/SiO2 catalyst (66%) and a Ni/Al2O3 catalyst (68%), respectively. Extensive characterizations were used to analyze the physiochemical characteristics of the catalysts. The results show that plasma power was more efficient than heating power at the same temperature for the CO2 hydrogenation. This demonstrates that the performance of CO2 hydrogenation can be significantly improved in the presence of plasma at lower temperatures.


INTRODUCTION
The rising concentration of CO 2 in the atmosphere is a pressing global issue due to its significant and long-term greenhouse effect, which has resulted in catastrophic phenomena, such as climate change, arctic sea ice decline, and ocean acidification.To mitigate greenhouse gas emissions and produce chemicals with a low or even zero carbon footprint, it is critical to directly utilize CO 2 in conjunction with the CO 2 capture process.−3 Various approaches such as electrocatalysis, plasma catalysis, photocatalysis, and thermal catalysis have been explored to convert CO 2 to methane and CO. 1,4−12 The most widely accepted mechanism for CO 2 methanation reaction (R1) is the combination of a reverse water−gas shift (R2) and an exothermic CO methanation (R3). 13 The selectivity for CH 4 formation decreases with an increasing reaction temperature, as shown in Figure 1.This is because the reverse water−gas shift reaction, which produces CO, becomes more favorable at higher temperatures.In addition, catalyst deactivation caused by coke formation on the catalyst surface is a well-known problem, especially under hightemperature conditions. 15,16As a result, hydrogenation of CO 2 at low temperatures is preferable.Nonthermal plasma (NTP) is a promising way to achieve optimal CO 2 hydrogenation performance by lowering the reaction temperature while boosting the CH 4 yield and CO 2 conversion.
−26 NTPs are capable of generating highly reactive species and electrons that facilitate thermodynamically unfavorable reactions under mild conditions.−35 Additionally, the fast on/ off switching of plasma processes, makes them suitable for integration with irregular and intermittent renewable energy sources, such as solar and wind power. 13,36−42 These catalysts have been shown to be effective in promoting the CO 2 methanation reaction at low temperatures and pressures.
Bacariza et al. investigated the performance of Ni-based zeolites (USY zeolites with varying Si/Al ratios) and Ni supported on commercial alumina in an atmospheric pressure dielectric barrier discharge (DBD) reactor. 43They found that the CO 2 conversion and CH 4 selectivity increased with increasing power input for all tested catalysts.This is consistent with previous research, which has shown that plasma catalysis can enhance the rate of CO 2 methanation.Zeng et al. examined nickel−alumina catalysts in various reaction conditions and discovered that Ni-based catalysts resulted in higher CO 2 conversion and CH 4 yield than Cu and Mn catalysts. 44They also found that the addition of Ar to the feed gases can significantly improve the performance of the plasma-catalytic process.Parastaev et al. compared the performance of CeZrO 4 -supported Co and Cu catalysts in CO 2 hydrogenation using a DBD reactor. 45They discovered that the Co-based catalyst was significantly more active and selective for CH 4 than the Cu-based catalyst.The increased number of H 2 adsorption sites may have contributed to its increased activity as the Co content rose.Interestingly, the authors confirmed that CO 2 methanation under plasma conditions primarily involves CO hydrogenation on Co particles and is unaffected by the support.The study of CO 2 methanation in DBD plasmas has revealed that the overall effectiveness of the process is significantly influenced not only by the catalyst used but also by the operating conditions.Thus, a direct and comprehensive comparison of the various catalysts and their impacts on plasma processes is required.Despite recent advances in plasma catalysis, there is still limited information about the rational design of effective hydrogenation catalysts for this process.Additionally, there has been relatively little research on how reaction temperature affects plasma-catalytic hydrogenation of CO 2 .This is an important area of research that could lead to the development of more efficient and cost-effective CO 2 methanation processes.

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In this study, we designed and developed a temperature controlled DBD reactor, enabling us to investigate chemical reactions at different temperatures and across different reaction modes: plasma alone, thermal catalysis, and plasma catalysis.We investigated plasma-catalytic CO 2 hydrogenation over the M/SiO 2 and M/Al 2 O 3 (M = Co, Ni) catalysts.We also conducted the same reaction using thermal catalysis for comparison.The influence of reaction temperature on the performance of supported Ni-and Co-based catalysts in plasma-catalytic CO 2 hydrogenation was evaluated in terms of CO 2 conversion, yields, and selectivity of gaseous products to get new insights into the temperature on plasma-catalyzed CO 2 hydrogenation.

EXPERIMENTAL SECTION
2.1.Experimental System.Figure 2 shows the plasma reactor system used for the plasma CO 2 hydrogenation experiments.More details on the experimental setup can be found in our previous studies. 46The DBD reactor was powered by an AC high voltage power supply with a voltage output of 10 kV.The discharge power was determined using the Lissajous figure method and kept constant at 39 W using a homemade real-time plasma power control system.The discharge gap was fixed at 1.5 mm.The feed gas was a mixture of Ar, H 2 , and CO 2 gases with a mole ratio of 5:4:1, and the total flow rate was fixed at 69.2 mL min −1 .All gases used were 99.999% pure.The DBD reactor was positioned inside a furnace to allow for the exploration of CO 2 hydrogenation in three different modes at the same reaction temperature: plasma alone, thermal catalysis, and plasma catalysis.
We prepared 30 wt % M/Al 2 O 3 (M = Co, Ni) and 30 wt % M/SiO 2 catalysts using an impregnation approach.Al 2 O 3 or SiO 2 powders were used as catalyst supports, and nitrate salts (Alfa Aesar) were used as metal sources.The DBD reactor was filled with quartz wool and 0.4 g of the granular catalyst.In this study, only a portion (∼20 vol %) of the plasma discharge was packed with the catalysts.For thermal-catalytic and plasmacatalytic CO 2 hydrogenation experiments, 0.4 g of the catalyst was packed in the DBD reactor at a gas hourly space velocity (GHSV) of 10380 mL h −1 g −1 .All of the catalysts were reduced in the same DBD reactor using 20 vol % H 2 -80 vol % Ar (50 mL/min, 40 W, 40 min) prior to thermal-catalytic and plasma-catalytic CO 2 hydrogenation experiments.The CO 2 hydrogenation reaction reached a steady state after about 1 h of continuous operation.Gas products were analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with dual detectors (thermal conductivity detector and flame ionization detector) was used.Each measurement was repeated three times.It is important to note that CO and CH 4 were the major products in this work, accounting for more than 99% of the total products.Light alkanes such as C 2 H 6 , C 3 H 8 , and C 4 H 10 were produced in such small quantities that their gas yields were assumed to be zero.Unsaturated hydrocarbons were not found.
X-ray diffraction (XRD) patterns of the catalysts were recorded using a Rigaku D−Max 2400 diffractometer equipped with a Cu Kα radiation source, operating in the 2θ range from 20 to 80°.To assess the reducibility of the catalysts, we conducted temperature-programmed reduction of H 2 (H 2 -TPR) measurements using an automated chemisorption system (Quantac Chrome ChemBET 3000).In the TPR analysis, each sample was initially heated to 400 °C in helium at a flow rate of 20 mL min −1 for 1 h.Subsequently, the sample was gradually cooled to 150 °C, purged with hydrogen for 30 min, and then reintroduced helium for 1 h.The BET surface areas of the catalysts were determined by N 2 adsorption at −196 °C by using the Micrometrics ASAP 2020 instrument (USA).

Definition of Parameters.
To evaluate the performances of CO 2 hydrogenation, the conversion (C) of CO 2 was defined by eq 1 The selectivity (S) and yield (Y) of CO and CH 4 were calculated using eqs 2−5 (5) The carbon balance (B carbon ) and H 2 /CO 2 molar ratio were defined as follows: H CO H input (mol) CO input (mol)   3.2.N 2 Adsorption−Desorption Analysis.The nitrogen adsorption−desorption isotherm measurements were used to characterize the texture of the catalysts and supports.The pore-size distribution and specific surface area of the studied supports and catalysts were determined by using the Barrett− Joyner−Halenda (BJH) and Brunauer−Emmett−Teller (BET) methods, respectively.The pore volume and specific surface area of the catalysts were smaller than those of the corresponding supports (Table 1), which can be explained by the crystallite particles of active metal species covering the pores and tunnels in the support.In addition, Ni-based catalysts displayed higher adsorption values than Co-based catalysts, and silica-supported catalysts had larger BET surfaces and pore volumes than alumina-supported catalysts.5 shows the CO 2 conversions of the hydrogenation processes over the M/SiO 2 and M/Al 2 O 3 (M = Co, Ni) catalysts at a reaction temperature of 350 °C.The addition of catalysts improved the CO 2 conversion in both thermal-and plasma-catalytic processes, with the CO 2 conversions across all catalysts in plasma-catalytic processes being higher than those in thermal-catalytic processes.This suggests that plasma− catalyst coupling is beneficial for CO 2 hydrogenation.One possible explanation for this is that CO 2 is excited by the DBD plasma before adsorption onto the catalyst surface in the plasma-catalytic process.This lowers the energy barrier for conversion to intermediates compared to thermal catalysis, where an elevated temperature is required to activate the adsorbed surface CO 2 molecules.Additionally, the catalysts with the SiO 2 support (Ni/SiO 2 and Co/SiO 2 ) exhibited significantly better CO 2 conversion than those with the Al 2 O 3 support (Ni/Al 2 O 3 and Co/Al 2 O 3 ), which may be linked to the larger BET surface area of the catalysts with the SiO 2 support.Specifically, the CO 2 conversion of Ni/SiO 2 was 61.5% in plasma catalysis, which was significantly higher than the CO 2 conversion of Ni/SiO2 in thermal catalysis (47.4%).The CO 2 conversion of Co/SiO 2 was also higher in plasma catalysis (65.8%) than in thermal catalysis (51.0%).In contrast, the CO 2 conversion of Ni/Al 2 O 3 and Co/Al 2 O 3 was similar in plasma catalysis (42.4 and 36.2%) and thermal catalysis (32.1 and 26.0%).These results suggest that the plasma−catalyst coupling is more effective for catalysts with the SiO 2 support than for the catalysts with the Al 2 O 3 support.As shown in Table 1, the BET surface areas of Ni/SiO 2 and Co/SiO 2 are 133.4 and 128.5 m 2 g −1 , respectively, which is significantly higher than the BET surface area of Ni/Al 2 O 3 (105.5 m 2 g −1 ) and Co/Al 2 O 3 (97.6 m 2 g −1 ).The larger surface area of the catalysts with the SiO 2 support provides more active sites for the CO 2 conversion reaction.
Figure 6 summarizes the yields and selectivity of CH 4 and CO during plasma-and thermal-catalytic CO 2 hydrogenation processes at a reaction temperature of 350 °C.Remarkably, all the catalysts except Co/Al 2 O 3 demonstrated high CH 4 selectivity (>90%) during thermal-catalytic processes, although the Co/Al 2 O 3 catalyst significantly enhanced the CH 4 selectivity from 0 to 59% under the same conditions compared to the case without using a catalyst.In comparison to thermal catalysis, the CH 4 selectivity of plasma-catalytic hydrogenation processes decreased across all catalysts.For example, the CH 4 selectivity using Co/Al 2 O 3 slightly dropped from 59 to 52%, as shown in Figure 6.The largest drop occurred over Ni/Al 2 O 3 , where the CH 4 selectivity decreased from 94.3 to 48.6%.In contrast, the highest CO selectivity (notably 96.6%) was obtained in the plasma alone process.Among the studied catalysts, the Co/Al 2 O 3 catalyst had the best CO selectivity in the thermal-catalytic mode, while the Ni/Al 2 O 3 catalyst demonstrated the highest CO selectivity in the plasma-catalytic process.In comparison to the results obtained from thermal catalysis, the CO selectivity over all of the catalysts increased significantly during plasma catalysis.For instance, using Ni/ Al 2 O 3 and Ni/SiO 2 , the CO selectivity was increased by a factor of 957 and 294%, respectively.These findings indicate that the presence of plasma generated a cascade of reactive species, which altered the reaction pathway.The DBD plasma activated the CO 2 molecules, primarily increasing the level of CO production.Furthermore, the plasma may activate and convert the produced CH 4 through CH 4 decomposition or dry reforming of methane (DRM), 49 reducing overall selectivity toward CH 4 .
As noted in Figure 6c, SiO 2 -supported catalysts exhibited a higher yield of CH 4 than did Al 2 O 3 -supported catalysts.In both thermal catalysis and plasma catalysis modes, the CH 4 yields over Co/SiO 2 and Ni/SiO 2 were very similar.However, there was no significant difference in plasma-catalytic processes with and without a catalyst (the plasma alone case), as shown in Figure 6d.The CO yields increased in the order: Co/Al 2 O 3 < Co/SiO 2 < Ni/SiO 2 < plasma alone < Ni/Al 2 O 3 .

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Furthermore, plasma-catalytic hydrogenation yielded more CO than the thermal-catalytic process across all catalysts.The high CO yield using plasma catalysis further confirms that the reactive species present in plasma primarily activated CO 2 molecules to produce CO, which is in good agreement with the high CO selectivity shown in Figure 6b.Table 2 shows that the carbon balance is very close to 100% for both thermal-and plasma-catalytic CO 2 hydrogenation using Co-and Ni-based catalysts at 350 °C.This indicates that the overall selectivity toward CO and CH 4 is very high at this temperature.
Figure 7 shows the CO 2 conversions over M/SiO 2 and M/ Al 2 O 3 (M = Co, Ni) at varying reaction temperatures using plasma catalysis.The results show that the CO 2 conversion over the SiO 2 -supported Co and Ni catalysts remained nearly constant when increasing the reaction temperature between 350 and 500 °C.However, the CO 2 conversion over Al 2 O 3supported catalysts increased significantly when increasing the temperature.Among all of the catalysts tested, the highest CO 2 conversion (67.8%) was obtained using Ni/Al 2 O 3 at a reaction temperature of 500 °C.
Figure 8 presents the effect of the reaction temperature on the production of the main gases products (CO and CH 4 ) using different catalysts in the plasma catalysis system.The selectivity and yields of CH 4 increased for all of the catalysts except Co/SiO 2 when the reaction temperature was increased from 350 to 500 °C.Notably, the CH 4 selectivity increased substantially from 48.6 to 72.4% over Ni/Al 2 O 3 , whereas the CO selectivity dropped from 50.3 to 22.4%.Similarly, the CH 4 selectivity achieved using the Co/SiO 2 catalyst dropped from 71 to 67%, while the CO selectivity slightly increased from 26 to 29%.
Table 3 shows the carbon balances of plasma-catalytic CO 2 hydrogenation using different catalysts at reaction temperatures of 350 and 500 °C.Although the carbon balance slightly decreased as the reaction temperature increased, it remained above 99%.Table 4 presents a summary of the results obtained from various catalysts tested for plasma-assisted CO 2 methanation in DBD reactors.The maximum levels of CO 2 conversion and CH 4 yield obtained in this work were comparable to those in previous studies conducted under similar conditions.

CONCLUSIONS
This work investigated the plasma-catalytic CO 2 hydrogenation over M/SiO 2 and M/Al 2 O 3 (M = Co, Ni) catalysts at different reaction temperatures.The results show that the CO 2 conversions in plasma-catalytic processes were higher than those in thermal-catalytic processes across all catalysts, indicating a plasma-catalytic synergy.In addition, the catalysts with SiO 2 support performed significantly better than the Al 2 O 3 -supported metal catalysts in terms of CO 2 conversion in both plasma-catalytic and thermal-catalytic modes at a reaction temperature of 350 °C.This is likely due to the higher reducibility and larger specific surface areas of the SiO 2supported catalysts.The catalytic performance of SiO 2supported catalysts was hardly affected by increasing the reaction temperature from 350 to 500 °C.However, the activity of Al

Figure 2 .
Figure 2. Schematic representation of the experimental setup.

1 .
XRD Analysis.Figure 3 presents the XRD patterns of the fresh catalysts and their respective supports.The Al 2 O 3 support showed three main diffraction peaks at 2θ of 67.0, 45.9, and 37.6°, which are attributed to the cubic alumina crystallite structure (JCPDS 10-425).The SiO 2 support showed only one broad peak centered at around 22°( JCPDS 29-0085).The Co-and Ni-based catalysts exhibited

Figure 3 .
Figure 3. XRD patterns of the fresh catalysts and supports (club symbol Co 3 O 4 , Heart symbol NiO).
clear diffraction peaks corresponding to Co 3 O 4 (JCPDS 42-1467) and NiO (JCPDS 1-75-197), respectively.The average crystallite sizes of the catalysts were determined using the Scherrer equation and are summarized in Table 1.The Nibased catalysts had smaller crystal sizes than the Co-based samples, and among all of the catalysts investigated, Ni/Al 2 O 3 showed the smallest average crystal size (29.7 nm).

3. 3 .
Temperature-Programmed Reduction.The reducibility of the catalysts was determined by using H 2 -TPR experiments, and the results are shown in Figure 4.The Ni/ SiO 2 catalyst showed a single reduction peak centered at 430 °C, corresponding to the reduction of NiO particles.The welldefined reduction peak of the Ni/SiO 2 catalyst suggests that all of the reducible Ni oxides reacted near 430 °C.In comparison, the Co/SiO 2 catalyst showed a typical feature of Co oxides supported on amorphous SiO 2 .The peaks located at 340 and 430 °C correspond to the reduction of Co 3 O 4 to Co 2+ species and Co 2+ to metallic Co, respectively.It should be noted that the reduction of Co 2+ was reflected by several superimposed peaks, indicating the different interaction strengths between Co 2+ species and the SiO 2 support.The TPR profile of Ni/ Al 2 O 3 showed a peak beginning at 370 °C, corresponding to the reduction of bulk NiO, and other peaks above 500 °C, attributed to the reduction of NiO x species or inert Ni-spinel aluminates that react significantly with the alumina support. 47,48Since this temperature was higher than the calcination or reaction temperature in this study, it was not taken into consideration.By contrast, the Co/Al 2 O 3 catalyst exhibited two overlapped peaks at 370 and 430 °C, corresponding to the reduction of Co 3 O 4 to Co 2+ species and Co 2+ to Co 0 , respectively.The TPR results indicated that Ni and Co species interacted more strongly with the Al 2 O 3 support than SiO 2 , resulting in a broader reduction pattern in the TPR results, particularly at high temperatures.When the active metal species Ni and Co are compared, the reduction of Co 3 O 4 occurred at a lower temperature than that of NiO.Furthermore, the transformation of Co 3 O 4 to Co occurred over a broad temperature range, resulting in a wider reduction peak for Co-based catalysts.Overall, the Co-based catalysts (Co/SiO 2 and Co/Al 2 O 3 ) reduced at lower temperatures than the Ni-based ones (Ni/SiO 2 and Ni/Al 2 O 3 ), suggesting a weak interaction between Co species and supports.Similarly, the SiO 2 -supported catalysts (Ni/SiO 2 and Co/SiO 2 ) reduced at

Table 1 .
Textural Properties of the M/SiO 2 and M/Al 2 O 3 (M = Co, Ni) Catalysts and Supports

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lower temperatures than the Al 2 O 3 -supported catalysts (Ni/Al 2 O 3 and Co/Al 2 O 3 ).Based on the TPR results, the catalyst reducibility was in the following order: Co/Al 2 O 3 < Ni/Al 2 O 3 < Ni/SiO 2 < Co/SiO 2 .3.
2 O 3 -supported catalysts significantly increased when the reaction temperature was increased to 500 °C.Overall, this study demonstrated that the use of plasma with SiO 2 -supported Co and Ni catalysts could significantly enhance the performance of hydrogenation of CO 2 to CH 4 at low temperatures.Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, 63755 Alzenau, Germany; Email: guoxing.chen@iwks.fraunhofer.deXin Tu − Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool L69 3GJ, U.K.; orcid.org/0000-0002-6376-0897;Email: xin.tu@ liverpool.ac.uk